JPH0917966A - Optical semiconductor storage device, and its write and read method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To elongate the storage preservation time and put this device to practical use, concerning an optical semiconductor storage device which has quantum dots. SOLUTION: This device includes a first conductive layer 2, a semiconductor layer 3 made on the first conductive layer 2, a quantum dot 4a made within the semiconductor layer 3, and a second conductive layer 5 made on the semiconductor layer and has an electric field applied between itself and the first conductive layer 2.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical semiconductor memory device and a writing / reading method thereof, and more particularly to an optical semiconductor memory device having quantum dots and a method of writing / reading information in / from this optical semiconductor memory device.

[0002]

2. Description of the Related Art With the rapid progress of information and communication technology, it is desired to construct a system utilizing the characteristics of optical devices and electronic devices. The characteristics of optical devices are high speed and high possibility of parallel processing compared to electronic devices. Various storage devices (memory devices) using optical devices have been proposed so far, but none have been embodied.

Even if the memory cell is constructed from an optical device, it is possible to utilize the optical device and the electronic device with their respective characteristics under the present circumstances where the peripheral theoretical circuit and switching circuit are composed of the electronic device. This is an important issue. The reality is that there is no clear device concept for optical memory. Quantum dots (quantum boxes) have transition energy between ground levels that has an extremely sharp optical absorption spectrum, and if the absorption saturation of light is strong, it is possible to write and read information using light. It is considered to be used as an optical memory. Further, if the shape and size of the quantum dots are changed, the absorption wavelength of each quantum dot can be changed, and information can be stored at multiple wavelengths. If 1-bit information can be stored in one quantum dot, 10 nm x 10
It is possible to store 1 bit in the nm area and 1 terabit in the 1 cm ² area.

Such a wavelength division multiplexing memory is available from Shunichi M
Auto, Jpn. J. Appl. Phys. Vol. 34 (1995) pp. L210-L212
Have been proposed in. This wavelength multiplexing memory is composed of an AlAs substrate having a tilted main surface and a III-V group compound semiconductor layer formed thereon. A step is formed in two directions due to the inclination on the main surface of the AlAs substrate, GaAs is formed at the corner of the step, and an AlGaAs layer is formed around it. As a result, the GaAs surrounded by the AlGaAs layer becomes a quantum box.

FIG. 30 is a partial band diagram of one quantum box of this multi-wavelength memory and the layers around it.
become that way. In the band diagram of the quantum box, when light having the same energy as the separation energy between the ground level of the valence band and the conduction band of the conduction band is irradiated, the ground level of the valence band of the quantum box changes to the ground of the conduction band. Electrons transit to the level to generate electron-hole pairs.

Here, since electrons have a high tunnel probability and the X point band energy of AlAs is low, AlGaAs
Tunnel through the barrier and move out of the quantum dot. The movement of electrons is detected as a change in current. On the other hand, since holes are less likely to come out to the outside, only holes are left in the quantum box. In this state, even if light of the same wavelength is irradiated, strong absorption saturation occurs due to the existence of holes, so that light absorption hardly occurs.

By utilizing such a phenomenon, information can be written and read by light. Further, if a plurality of quantum boxes having different sizes and shapes are used, the wavelengths for writing and reading are different for each quantum box, so that it is possible to store a large amount of information at multiple wavelengths. Thereby, a multi-wavelength memory is obtained.

[0008]

However, in such a multi-wavelength memory, the retention time is 1 to 10 because the electrons emitted from the quantum box have a high probability of returning to the quantum box and recombining.
It is as short as ms. Further, it is necessary to detect whether or not absorption saturation is present by absorption of light, and a specific structure therefor is not shown.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a practical optical semiconductor memory device and a writing / reading method thereof, which can further lengthen the record holding time.

[0010]

[Means for Solving the Problems]
(c), as illustrated in FIG. 8, FIG. 9, FIG. 12, FIG. 22, etc., the first conductive layer 2 and the semiconductor layer 3 (17, 24) formed on the first conductive layer 2. , 81) and the quantum dots 4 formed in the semiconductor layer 3 (17, 24, 81).
a (19, 25, 82a to 82d) and the semiconductor layer 3
And a second conductive layer 5 formed on (17, 24, 81) and to which an electric field is applied between the first conductive layer 2 and the first conductive layer 2. .

The above-mentioned problems are as shown in FIG. 1 (c), FIG. 8, FIG.
As illustrated in FIGS. 12 and 22, the first semiconductor layer 2 containing the first conductivity type impurity and the carrier barrier semiconductor layer 3 (17, 2) formed on the first semiconductor layer 2.
4, 81) and the carrier barrier semiconductor layer 4a (19,
25, 82a to 82d) and a quantum dot formed in the optical semiconductor memory device.

In the optical semiconductor memory device, the quantum dots 4a are present in a plurality of different sizes. In the optical semiconductor memory device, the carrier barrier semiconductor layer 3 (24) is formed of an undoped semiconductor or a second conductivity type impurity-containing semiconductor.

In the optical semiconductor memory device, as illustrated in FIG. 1 or FIGS. 7 and 8, a metal layer 9 or a second layer which is in Schottky contact with the carrier barrier semiconductor layer 3 is formed on the carrier barrier semiconductor layer 3. Second containing conductive impurities
One of the semiconductor layers 5 is formed. In the optical semiconductor memory device, as illustrated in FIG. 3, the quantum level of the conduction band or valence band of the quantum dot 4a and the conduction band or valence band of the first semiconductor layer 8 are energetically It is characterized by being separated.

In the optical semiconductor memory device, as shown in FIG. 12, an undoped third semiconductor layer 23 is formed between the first semiconductor layer 22 and the carrier barrier semiconductor layer 24. Is characterized by. In the optical semiconductor memory device, as illustrated in FIG. 8, the carrier barrier semiconductor layer 3 having the quantum dots 4a is formed on a channel layer 8 of a field effect transistor.

In the optical semiconductor memory device, as illustrated in FIG. 9, the carrier barrier semiconductor layer 17 having the quantum dots 19 constitutes an emitter layer or a base layer of a bipolar transistor. .
The energy band gap of a part or the whole of the collector layer of the bipolar transistor is smaller than the energy difference between the ground level of the conduction band and the valence band of the quantum dot.

An undoped semiconductor layer is formed between the collector layer and the base layer of the bipolar transistor, and the energy band gap of the undoped semiconductor layer is between the conduction level and the valence band ground level of the quantum dot. It is characterized by being smaller than the energy difference. A plurality of emitter electrodes are formed on the emitter layer of the bipolar transistor, and a collector electrode is formed on the collector layer.

In the optical semiconductor memory device, as illustrated in FIG. 12, the carrier barrier semiconductor layer 24 having the quantum dots 25 constitutes an i layer of a pin junction type photodiode. . In the optical semiconductor memory device, as illustrated in FIG. 14, the carrier barrier semiconductor layer 24 including the quantum dots 25 is
It is characterized in that it is formed on the light emitting element 39.

In the optical semiconductor memory device, as illustrated in FIG. 22, a plurality of quantum dots 82a to 82e are present in the thickness direction of the carrier barrier semiconductor layer 81. In this case, the plurality of quantum dots 82a to 82e are characterized by different diameters in the film thickness direction. In addition, a barrier layer 85 that prevents movement of holes or electrons accumulated in the quantum dots 82a to 82e in the hole burning state is formed on a side of the quantum dots 82a to 82e opposite to the moving direction of the carriers. It is characterized by being

In the optical semiconductor memory device, as illustrated in FIG. 9, the carrier barrier semiconductor layer 17 is formed in a part of a plurality of active elements, and the carrier barrier semiconductor layers 17 of some of the active elements are formed. It is characterized by being covered with a light shielding film. In addition, the above-mentioned problem is to irradiate the quantum dots formed in the semiconductor layer with light, and to irradiate with light of a wavelength corresponding to the energy difference between each ground level of the conduction band and the valence band of the quantum dots. Method for writing information by causing hole burning to write information, and reading the information by irradiating the quantum dots with light of the wavelength and changing the current flowing from the semiconductor layer. Solve by.

Alternatively, the above-mentioned problem is that a quantum dot formed in a semiconductor layer is irradiated with light, and light having a wavelength corresponding to an energy difference between each ground level of a conduction band and a valence band of the quantum dot is irradiated. Of the optical semiconductor memory device characterized in that the information is read by irradiating the quantum dots to write information and irradiating the quantum dots with light of the wavelength and transmitting the light through the semiconductor layer. The problem is solved by the writing / reading method.

In the writing / reading method of the optical semiconductor memory device, the semiconductor layer is formed between an n-type semiconductor layer and a p-type semiconductor layer, and the n-type semiconductor layer and the p-type semiconductor layer are formed during the writing.
A reverse bias voltage is applied to the type semiconductor layer, and no bias voltage or a forward bias voltage is applied to the n-type semiconductor layer and the p-type semiconductor layer at the time of reading.

In the writing / reading method of the optical semiconductor memory device, the semiconductor layer having the quantum dots constitutes an emitter or a base of a bipolar transistor, and when reading the information, the semiconductor layer having the quantum dots is used as a base and a collector of the bipolar transistor. It is characterized in that a voltage of a magnitude that causes avalanche collapse by light irradiation is applied between them.

In the method of writing and reading in an optical semiconductor memory device, the semiconductor layer having the quantum dots constitutes an emitter or a base of a bipolar transistor, and in the method of writing and reading in an optical semiconductor memory device, when writing the information, A reverse bias is applied between the emitter and the base of the bipolar transistor, a bias voltage is not applied between the base and the collector, and a bias voltage is applied between the emitter and the base when reading the information. Instead, a reverse bias voltage is applied between the base and the collector.

In the writing / reading method of the optical semiconductor memory device, a plurality of quantum dots having different energy differences between the ground levels are provided, and when reading the information, a plurality of wavelengths corresponding to the energy differences are included. It is characterized in that light is continuously changed to irradiate the quantum dots. In the writing / reading method of an optical semiconductor memory device, the semiconductor layer having the quantum dots constitutes a part of a light receiving element, and when reading the information, there is a difference in spectral sensitivity characteristics due to a light irradiation wavelength of the light receiving element. It is characterized in that the information of the plurality of quantum dots is read out by a change.

In the writing / reading method of the optical semiconductor memory device, a plurality of quantum dots having different energy differences between the ground levels are provided, and when writing the information and reading the information, the energy difference is generated. It is characterized in that light of a plurality of corresponding wavelengths is simultaneously irradiated. in this case,
The plurality of quantum dots having different energy differences between the ground levels are arranged in the film thickness direction, and when writing the information, the information is written in the quantum dots according to the order of the direction in which carriers are moved by light irradiation. It is characterized by Further, the plurality of quantum dots having different energy levels between the ground levels are arranged in the film thickness direction, and when reading the information, the quantum dots are arranged in the order opposite to the moving direction of carriers by light irradiation. The information is read out.

In the writing / reading method of the optical semiconductor memory device, an electric field is generated in the semiconductor layer having the quantum dots to supply electrons to the quantum dots to initialize the information. In the writing / reading method of the optical semiconductor memory device, writing of the information is prohibited by passing a current through the semiconductor layer having the quantum dots. Alternatively, a plurality of quantum dots are present in the semiconductor layer, and an energy width in a range larger than the smallest energy difference between the ground levels among the plurality of quantum dots and smaller than the energy band gap of the semiconductor layer. This is solved by a writing / reading method for an optical semiconductor memory device, characterized in that the quantum dots are irradiated with the light that they have to bring all of the quantum dots into a hole burning state.

[0027]

[Operation] According to the present invention, a semiconductor layer having a quantum dot is sandwiched between first and second conductive films, and an electric field is generated between the conductive films, so that electrons emitted from the quantum dot are quantum-quantized by hole burning. It's hard to return to dots. Further, according to another aspect of the present invention, since the carrier barrier semiconductor layer having the quantum dots inside is formed on the first conductivity type impurity-containing semiconductor layer to form the wavelength multiplex memory, the hole multiplexing is performed by light irradiation. When the burning occurs, the electrons emitted from the quantum dots have a high probability of flowing out to the first conductivity type impurity-containing semiconductor layer, and the probability of carriers flowing into the quantum dots to cause recombination becomes low.

Information is written by irradiating the quantum dots with light to cause hole burning or by not irradiating light to cause hole burning. For example, the data is set to "1" when the hole burning is generated, and the data is set to "0" when the hole burning is not generated. The information written in this way is read by irradiating light of the same wavelength, and “1” and “0” are changed depending on the change of the current flowing in the semiconductor layer and the change of the absorption amount of the light by the light irradiation. to decide.

When data “0” is written in the quantum dots, electron-hole pairs are generated in the quantum dots by the reading light, so that light is absorbed in the quantum dots and electrons are emitted to the outside. It flows. As a result, it is determined that the data “0” has been written. When electrons are emitted from the quantum dots by light irradiation, the quantum level of the quantum dots and the energy of the first conductivity type impurity semiconductor layer are set by making the above-mentioned first conductivity type impurity semiconductor layer an n-type semiconductor layer. As a result, the electrons are difficult to return to the quantum dots due to the diffusion potential.

When data "1" is written in the quantum dot, electron-hole pairs are not generated in the quantum dot due to the reading light, so that the quantum dot absorbs light or moves the electron to the outside. There is no such thing. As a result, it is determined that the data “1” has been written. The light used for writing and reading information is limited to the wavelength corresponding to the energy difference between the ground level of the conduction band and the valence band of the quantum dot. Therefore, since the wavelength of light used for writing and reading differs depending on the size of the quantum dots, the number of bits of the storage element is determined by the number of different sizes of the quantum dots.

Information is initialized by injecting electrons into the quantum dots, for example, by applying an electric field or irradiating light to the semiconductor layer around the quantum dots. In order not to destroy the information, a bias voltage is applied to the impurity-containing semiconductor layer around the quantum dot so that the energy barrier around the quantum dot becomes thick when the information is stored or read.

When a semiconductor layer containing quantum dots is used as a part of an active element such as a photodiode, a bipolar transistor or a field effect transistor, an optical semiconductor memory element is constituted by this. By adopting an active element in the basic structure, it becomes possible to amplify the current flowing at the time of reading information or to convert the amount of light into an electric signal, which facilitates reading of information.

When a plurality of quantum dots are formed in the film thickness direction and the sizes thereof are different, the light energy is changed with time to write information, or a plurality of light energies having different sizes are mixed. Either write information at the same time. When the quantum dots are irradiated with light by changing the energy, it is desirable to write in the order arranged in the carrier movement direction in order to prevent the destruction of the information of the already written quantum dots.

When a plurality of quantum dots are formed in the film thickness direction and the sizes thereof are different, it is preferable to change the light energy with time and read the information. However, in order to prevent the destruction of the information of the quantum dots which has not been read yet, it is desirable to write in the order arranged in the direction opposite to the carrier movement direction.

[0035]

Embodiments of the present invention will be described below with reference to the drawings. (First Embodiment) A manufacturing process of an optical semiconductor memory device according to a first embodiment of the present invention will be described with reference to FIGS. 1 (a) to 1 (c).
The III-V semiconductors described below are grown using, for example, an MBE apparatus.

First, the main surface was slightly inclined from the (111) plane.
An n-type GaAs substrate 1 whose main surface is the surface is prepared. And
As shown in FIG. 1 (a), the impurity concentration on the main surface of the GaAs substrate 1 is increased.
Degree 1 × 10¹⁸atoms / cm^ThreeN-type GaAs layer 2 of 50 nm thick
And undoped Al on the n-type GaAs layer 2.
_0.5Ga_0.5The As layer 3 is formed to a thickness of 25 nm. Then, Al
_0.5Ga_0.5InAs layer 4 on As layer 3
er, hereinafter referred to as ML). Next, FIG.
As shown in (b), again undoped Al_0.5Ga_0.5As layer 3
To a thickness of 25 nm so that the InAs layer 4 is included therein.
You. Second Al_{0. Five}Ga_0.5After forming As layer 3, Al
_0.5Ga_0.5In due to the difference in the lattice constants of As layer 3 and InAs layer 4,
The As layer 4 changes into a droplet shape. That is,
The roplet-shaped InAs layer 4 is a quantum dot (quantum box) 4a.
become. As the growth temperature of the quantum dots 4a increases,
Diameter becomes large, for example, at 450 ℃, average size
Is about 15 nm. Quantum todd 4 of the same size
There are a plurality of a's.

When the quantum dots 4a are formed on a surface inclined by a few degrees from the (111) plane, the InAs that becomes the quantum dots 4a.
By adjusting the growth conditions of the layer 4, as shown in FIG. 2, there are a plurality of quantum dots 4a having different sizes for each step of the crystals which are inclined. Basically, the quantum dots 4a
If the size and shape of the quantum dots are different, the potential well widths of the quantum dots 4a are different, and thus the energy of light (wavelength) for generating electron-hole pairs by light absorption in the quantum dots 4a.
Is different.

Thereafter, as shown in FIG. 1C, the p-type Al _0.5 Ga _0.5 As layer 4 is formed on the undoped Al _0.5 Ga _0.5 As layer 3.
To a thickness of 200 nm. This p-type Al _0.5 Ga _0.5 As
The impurity concentration of the layer 5 is 1 × 10 ¹⁸ atoms / cm ³ . this
The Al _0.5 Ga _0.5 As layer 5 serves as a carrier barrier layer.
Regarding the formation of quantum dots, JY Marchin et
al., Photoluminesence of Single InAs Quantum Dots
Obtained by Self-Organized Growth on GaAs, Physic
al Review Letters Vol.73, No. 5. 1994, pp.716-719
The dot size and shape depend on the growth temperature, In
It can be controlled by the molar ratio of As and As.

Then, as shown in FIG. 1 (d), n-type Ga
An n-side electrode 1a is formed on the bottom surface of the As substrate 1 and p-type Al _0.5 G
A p-side electrode 5a is formed on the a _0.5 As layer 5. This completes the basic structure of an optical semiconductor memory device called a wavelength division multiplexing memory. The energy band of one quantum dot 4a and the semiconductor layer around it is shown in FIG. 3 (a).

When the quantum dots 4a are irradiated with light, electrons transit from the valence band (Ev) to the conduction band (Ec) to generate electron-hole pairs, as shown in FIG. 3 (b). It The energy (wavelength) hv of the light is the energy between the ground level of the valence band and the ground level of the conduction band (hereinafter,
It is the same as the separation energy). Irradiation with light having an energy different from the separation energy does not generate electron-hole pairs. Light that has contributed to the generation of electron-hole pairs is absorbed and does not pass through the quantum dots 4a.

By the way, in the energy band diagram, the n-type GaAs layer 2 and the p-type Al _0.5 Ga _0.5 As layer 5 are shown.
Since the undoped Al _0.5 Ga _0.5 As layer 3 is tilted by the diffusion potential between the two, the energy barrier of the Al _0.5 Ga _0.5 As layer 3 to the quantum dots 4a becomes thin. As a result, the electrons transitioned to the ground level of the conduction band of the quantum dot 4a are Al _0.5 Ga
_{The 0.5} As layer 3 is tunneled and moved to the n-type GaAs layer 2.

The conduction band of the n-type GaAs layer 2 is undoped.
It is spatially separated from the conduction band of the quantum dot 4a via the Al _0.5 Ga _0.5 As layer 3. The band gap of the n-type GaAs layer 2 is smaller than the band gap of the Al _0.5 Ga _0.5 As layer 3, and the energy difference between them is large. This means that the quantum level of the conduction band of the quantum dot 4a and the GaAs layer 2 are energetically separated.

Therefore, the probability of the electrons reaching the n-type GaAs layer 2 returning to the quantum dots 4a through the Al _0.5 Ga _0.5 As layer 3, that is, the recombination probability becomes extremely small. For example, n
Energy difference between the conduction band or Fermi level E _{F of the} GaAs type GaAs layer 2 and the ground level of the conduction band of the quantum dot 4a is about 0.6e.
If V is set, the retention time of holes in the quantum dots 4a is 1
It is more than 000 hours, which is longer than before, and operation at room temperature becomes possible.

As a result, as shown in FIG. 3 (c), holes are confined in the ground level of the valence band of the quantum dot 4a. As described above, a phenomenon in which an electron transiting to the ground level of the conduction band of the quantum dot 4a moves through the energy barrier to the outside and only holes are left in the valence band of the quantum dot 4a is generally referred to as a hole. It's called burning.

Next, writing and reading of information in the optical semiconductor memory device having the above structure will be described. Light used for writing and reading is emitted from the optical fiber F.
First, when writing data “0”, the state of the energy band as shown in FIG. 3A is set without irradiating light.

On the other hand, when writing data "1", the quantum dot 4a is irradiated with light having an energy corresponding to the separation energy of the quantum dot 4a. This causes hole burning in the quantum dots 4a, as shown in FIG. 3 (c). As a result, the data "1"
Has been written. The probability that the electrons moved to the conduction band of the n-type GaAs layer 2 by writing the data "1" return to the quantum dots 4a spatially and energy separated and recombine there is very small. "
Does not disappear.

When reading data, the following method is used. First, a reverse bias voltage is applied between the n-side electrode 1a and the p-side electrode 5a. Then, when reading the written data, the quantum dot 3a is irradiated with light having the same energy as the separation energy of the quantum dot 3a. When data "1" is written, it is shown in Fig. 3 (c).
As shown in, there is no movement of electrons from the quantum dots 4a to the n-type GaAs layer 2, and the current flowing through the monitor M connected to the electrodes 1a and 5a does not change. Therefore, the irradiated light passes through the quantum dots 4a.

When the data "0" is written, as shown in FIG. 3 (b), hole burning occurs in the quantum dots 4a due to light irradiation, and electrons move to the n-type GaAs layer 2. A current will flow through the monitor. In this case, if a large number of quantum dots of the same size are formed, the amount of electrons that move increases, so the detection sensitivity increases.
Further, even if the n ⁻ type GaAs layer is interposed between the n type GaAs layer 2 and the undoped Al _0.5 Ga _0.5 As layer 3 and a high electric field is applied to the n ⁻ type GaAs layer to easily cause avalanche, the light receiving sensitivity is increased. Good.

As described above, it is possible to read the stored information depending on the presence or absence of the current, but since the reading cannot be performed a plurality of times in succession, the memory needs to be refreshed after the reading. That is, if hole burning occurs due to the reading of data "0", the n-type GaAs layer 2 and the p-type Al are
_A positive bias voltage is applied between the _0.5 Ga _0.5 As layers 5 to inject carriers into the quantum dots 3a to cause recombination to erase information and then rewrite.

By the way, the separation energy of the quantum dots 3a varies depending on the size of the diameter of the quantum dots 4a and the growth temperature of the InAs layer 4. The presence of the plurality of quantum dots 4a having different diameters makes it possible to irradiate the element with a plurality of light having different wavelengths to cause hole burning, and simultaneously write and read a plurality of information. Therefore,
The number of groups of the quantum dots 4a divided by the difference in diameter becomes the number of bits of the optical storage element.

When reading is performed using light having different wavelengths, it is preferable that the quantity of light of a specific wavelength applied to the quantum dots 4a at the time of reading is smaller than the light irradiated at the time of writing. This means that when the data “0” is read out, by reducing the light quantity, some of the plurality of quantum dots having the same diameter undergo hole burning. Therefore, it is possible to reduce the number of electrons emitted by the hole burning that enter the quantum dots of other diameters and destroy the information, which can reduce or eliminate the number of refreshes.

The number of electrons emitted from one quantum dot is 2
However, in order to facilitate understanding, only one electron is shown in the figure. (Second Embodiment) A method of reading information from the data of the optical semiconductor memory element shown in the first embodiment by changing the light absorption rate will be described below.

When reading the written data, the quantum dot 4a is irradiated with light having the same energy as the separation energy of the quantum dot 4a. The energy band in the state where the data “0” is written is as shown in FIG. 3 (a). When the quantum dots 4a are irradiated with light in this state, electron-hole pairs are generated as shown in FIG. 3B, and the light is absorbed in the quantum dots 4a, so that the light transmittance is small. When the absorptance of the light of the specific wavelength is large as described above, it is determined that the data “0” has been written.

On the other hand, the energy band in the state where the data "1" is written is as shown in FIG. 3 (c). In this state, since holes exist in the valence band of the quantum dots 4a, electron-hole pairs do not occur even when irradiated with light. Therefore, the light transmittance of the light in the quantum dots 4a is high. Grows larger. As described above, when the amount of light emitted to the quantum dots 4a is small, it is determined that the data “1” has been written.

In this example, since the information is monitored by the change of the light absorption rate and the information is not monitored by the current, the electrons are moved from the quantum dots 4a to the n-type GaAs layer 2 when the data "0" is read. No need.
In this case, in order to reduce the number of times the information is refreshed, it is sufficient to make it difficult for a tunnel to occur during reading. Therefore, in order to create a tunnel at the barrier as shown in FIG. 3 (b) at the time of writing and make it difficult to tunnel the barrier at the time of reading, the following method may be adopted.

At the time of writing information, electrons are emitted by the quantum dots 4
It is necessary to facilitate the movement from a to the outside of the undoped Al _0.5 Ga _0.5 As layer 3. For this purpose, n-type GaAs layer 2 and p-type Al
Preferably, a reverse bias voltage is applied to the _0.5 Ga _{0 .5} As layer 5. On the other hand, when reading stored information or holding information, n
Do not apply to the type GaAs layer 2 and the p-type Al _0.5 Ga _{0 .5} As layer 5 a bias voltage, or irradiated with light while applying a forward bias. According to this, at the time of reading the information, as shown in FIG.
As shown in (b), the slope of the undoped Al _0.5 Ga _0.5 As layer 3 serving as a barrier becomes small and the barrier B becomes substantially thick.
Therefore, in the state in which the data “0” is written, as shown in FIG.
As shown in (1), electron-hole pairs are generated by light irradiation, but the electrons recombine with holes quickly without tunneling through the barrier B, so that stored information is not destroyed. Of course, when the data “1” is written,
As shown in (b), the electron irradiation does not generate electron-hole pairs, so the data is not erased.

Even when data is stored, if no voltage is applied or a forward bias voltage is applied between the n-type GaAs substrate 1 and the p-type Al _0.5 Ga _0.5 As layer 5, the barrier B Since it becomes thicker, the loss of data is prevented. When reading information from a storage element having a plurality of quantum dots 4a having different diameters, for example, the relationship between the wavelength and the light absorption spectrum as shown in FIG. 5 is examined. Then, it can be seen that the data “1” is written in the quantum dots 4a having the separation energy corresponding to the wavelength having a small light absorption rate. (Third Embodiment) An optical semiconductor memory device according to a third embodiment of the present invention is a transparent conductive layer (not shown) in ohmic contact with the undoped Al _0.5 Ga _0.5 As layer 3 including the quantum dots 4a in FIG. 1 (a). Are formed at positions sandwiching the undoped Al _0.5 Ga _0.5 As layer 3. For example, n-type GaAs layer 2 and p-type Al
_A transparent electrode made of ITO is formed in place of at least one of the _0.5 Ga _0.5 As layers 5.

In the device of this structure, the quantum dots 4a
Electric field E in the lateral direction of the undoped Al _0.5 Ga _0.5 As layer 3 around the
Is applied, an energy band as shown in FIG. 6 (a) is obtained. Then, light having an energy hν ₁ equal to the separation energy of the quantum dot 4a is applied to the quantum dot 4a.
6B, hole burning occurs and data "1" is written.

When writing data "0", it is selected not to irradiate the quantum dots 3a with light. The method of reading the stored information is the same as in the first and second embodiments, and will not be described. (Fourth Embodiment) In the fourth embodiment of the present invention, W is used in place of the p-type Al _0.5 Ga _0.5 As layer 5 of the device shown in the first embodiment.
Forming a metal layer of Si to a thickness of 200 nm,
A Schottky barrier was formed at the junction between the undoped Al _0.5 Ga _0.5 As layer 3 and the metal layer. The energy band is as shown in Fig. 7, and the metal layer 6 and Al _0.5 Ga _0.5 As
It can be seen that a Schottky barrier is formed between the layers 3.

Since the height of the Schottky barrier is about 0.7 eV with respect to n-GaAs, the potential difference between the n-type GaAs layer 2 and the Schottky barrier is large, and it is difficult for electrons to return to the quantum dots 4a. The holding time becomes longer than that in the first embodiment. FIG. 8 shows an example of an optical semiconductor memory device having a Schottky barrier. In this example, the semiconductor insulating substrate 7
N-type GaAs with an impurity concentration of 5 × 10 ¹⁷ atoms / cm ³
A layer 8 is formed to a thickness of 50 nm, and an undoped Al _0.5 Ga _0.5 As layer 3 containing a plurality of quantum dots 4a made of InAs and having different diameters is formed thereon to a thickness of 50 nm, and further thereon. The gate electrode 9 made of WSi is formed to a thickness of 200 nm. In this case, the gate electrode 9 and Al _0.5 Ga _0.5
A Schottky barrier is formed at the junction of the As layer 3.

Further, the source electrode 10 and the drain electrode 11 are resistively connected to the Al _0.5 Ga _0.5 As layer 3 at intervals on both sides of the gate electrode 9. Source electrode 10 and drain electrode 1
1 is formed of two metal layers of AuGe having a thickness of 20 nm and Au having a thickness of 200 nm. In such a semiconductor memory element, the Al _0.5 Ga _0.5 As layer 3 is irradiated with light from its side to write information.

On the other hand, when reading information, a negative voltage for reading is applied to the gate electrode 9 and a voltage is applied between the source electrode 10 and the drain electrode 11 to determine the magnitude of the source / drain current. Is used to determine "1" or "0". When hole burning occurs in the quantum dots 4a, that is, when data "1" is written in the quantum dots 4a, the presence of holes suppresses the depletion layer from spreading from the gate voltage 9. The fact that the drain current is larger than that in the state where the data “0” is written is used.

As another method of reading information, a voltage is applied to the gate electrode 9 to cause a source / drain current to flow, and a source / drain current of a state in which light of a specific wavelength is not irradiated You may investigate changes. In this case, when the drain current increases by irradiating with light of a specific wavelength, electrons move from the quantum dots 4a to the channel region to cause hole burning, so that data "0" is written. I understand.

A plurality of quantum dots 4a having different diameters are used.
Have different wavelengths (energy) of the light that causes the hole burning, the data of each quantum dot 4a is read by changing the wavelength of the irradiation light. (Fifth Embodiment) An optical semiconductor memory device in which the semiconductor layer having the quantum dots described above is provided in a part of a bipolar transistor will be described with reference to FIG. The energy band of the optical semiconductor memory device is as shown in FIG.

In FIG. 9, a collector contact layer 13 made of n ⁺ -type GaAs with an impurity dose amount of 5 × 10 ¹⁸ atoms / cm ³ and an impurity dose amount of 5 × 10 ¹⁷ are formed on the GaAs substrate 12.
collector layer 14 made of atoms / cm ³ of n-type GaAs, first i-layer 15 made of undoped GaAs, dose amount 5 × 10 ¹⁸
Base layer 16 made of p ⁺ type GaAs of atoms / cm ³ , second i layer 17 made of undoped AlAs, dose amount 5 × 10 ¹⁷
atoms / cm ³ of n-type Al _{0. 5} Ga _0.5 emitter layer 18 made of As
Are sequentially stacked.

Quantum dots 19 made of InAs are formed on the second i layer 17. Also, regarding the thickness of each layer,
Collector contact layer 13 is 300 nm, collector layer 14
Is 200 nm, the first i layer 15 is 300 nm, and the base layer 16 is
Is 100 nm, the second i layer 17 is 100 nm, and the emitter layer 1
8 is 200 nm. In this bipolar transistor, the separation energy Eg ₀ of the quantum dots 19 in the AlAs i layer 7 is wider than the band gap Eg _{1 of} the first i layer 15. Further, the energy barrier of the second i layer 17 with respect to the base layer 16 is increased, and holes are prevented from easily entering the quantum dots 19 from the base layer 16.

Reference numeral 20C in FIG. 9 is a collector electrode,
20B is a base electrode and 20E is an emitter electrode. Next, a method of writing and reading information of quantum dots in this bipolar transistor will be described below. (I) Writing Method When a reverse bias voltage is applied between the emitter layer 18 and the base layer 16 at the time of writing information, the energy band becomes as shown in FIG. 11 (a), and the energy of the second i layer 17 is changed. Becomes large, and the energy barrier B of the quantum dot 19 on the side of the emitter layer 18 becomes thin. Therefore, the electrons can easily move by the tunnel from the quantum dots 19 to the emitter layer 18, and the data “1” can be easily written. In this case, the reverse bias voltage is not applied between the collector layer 14 and the base layer 16 or the forward bias voltage is applied to suppress the generation of current due to light absorption in the first i layer 15. Is preferred. The writing of data "0" is performed by not irradiating light. (Ii) Reading method When reading information, the emitter layer 18 and the base layer 16
No bias voltage is applied between the two. As a result, the energy barrier B of the quantum dot 19 on the side of the emitter layer 18 becomes thicker, so that the quantum dot 1 is irradiated with the reading light.
Even if an electron-hole pair is generated in 9, the electrons are less likely to move to the side of the emitter layer 18 and recombination is facilitated, and hole burning does not occur.

Therefore, the data "0" is not destroyed by the reading light. The data "1" is not destroyed by light irradiation because the quantum dots 19 are in the hole burning state. When reading information, as shown in FIG. 11 (b),
It is preferable to apply a reverse bias voltage between the collector layer 14 and the base layer 16. Accordingly, when the data “1” is written, the reading light is transmitted through the quantum dots 19 and absorbed by the first i layer 15. Due to this absorption, electron-hole pairs are generated in the first i-layer 15,
The electrons move to the collector layer 14, the holes move to the base layer 16, and a photocurrent flows. The photocurrent increases the amount of current between the collector and the base.

Therefore, when the light absorption amount is large or the collector-base current is large, the data "1" has been written. In this case, the holes move to the base layer 16, but a high hole barrier exists between the base layer 16 and the emitter layer 18, and the holes are originally difficult to tunnel. The data “0” is not changed to the data “1” by moving to the quantum dot 19.

When the data “0” is written, the light for reading is absorbed by the quantum dots 19 and the amount of light reaching the first i layer 15 decreases. Due to this decrease, the photocurrent flowing through the first i layer 15 is small. Therefore, when the light absorption amount is small or the collector-base current is small, the data “0” is written. Since the written information may change, no bias is applied between the collector and the base, or a forward bias is applied to prevent photocurrent from flowing.

Therefore, according to this reading method, if a reverse bias is applied between the base and the collector, the amount of current flowing between the base and the collector differs due to the difference in the amount of light absorbed, and thus the difference in the amount of current. This makes it possible to detect stored information. According to this example, since reading and writing can be performed independently, refreshing is unnecessary.

Although the first i layer 15 may be omitted, in this case, a collector layer is formed of a semiconductor material having a bandgap smaller than the separation energy of the quantum dots 19 to generate a photocurrent. Let If the interband energy of the first i-layer 15 is larger than the separation energy of the quantum dots 19, the first
Alternatively, as in the second embodiment, data is read by changing the collector current or the light absorptance.

Furthermore, if a high voltage that causes avalanche collapse is applied between the base layer 16 and the collector layer 14, the sensitivity to changes in the amount of received light becomes high,
Response speed becomes faster. As a result, a gain is obtained for light, and the S / N ratio can be increased.
Although the npn bipolar transistor is used in this embodiment, a quantum dot may be included in a part of the pnp bipolar transistor. (Sixth Embodiment) Next, an optical semiconductor memory device in which a layer having quantum dots is formed in a part of a pin junction type photodiode will be described with reference to FIGS. 12 (a) and 12 (b).

In FIGS. 12 (a) and 12 (b), an n-type GaAs layer 2 having a film thickness of 300 nm is formed on a semi-insulating substrate 21 made of GaAs.
2. An undoped In _0.1 Ga _0.9 As layer 23 having a film thickness of 100 nm and a p-type AlAs layer 24 having a film thickness of 200 nm are sequentially formed. p
A plurality of quantum dots 25 made of GaAs or InGaAs having different sizes are formed in the 10 nm region of the center of the type AlAs layer 24.

An annular p-side electrode 26 is formed on the p-type AlAs layer 24, and an n-side electrode 27 is formed on the n-type GaAs layer 22. The n-type GaAs layer 22
Impurity concentration of 5 × 10 ¹⁸ atoms / cm ³ and p-type AlAs layer 24
The impurity is 5 × 10 ¹⁸ atoms / cm ³ . A pin photodiode having such quantum dots 25 is shown in FIG.
It has an energy band structure as shown in 2 (c). Since a plurality of quantum dots 25 having different sizes are present in the p-type AlAs layer 24, the wavelength of light for generating electron-hole pairs is set to the quantum dot 2 as already described in the above embodiment.
5 depending on the size of each quantum dot 25
Absorbs only the light of the wavelength corresponding to each separation energy.

Moreover, in the quantum dots 25 that have absorbed light, hole burning occurs and holes are confined therein, so that when light of the same wavelength is irradiated again, that light is It transmits through the quantum dots 25. Here, the undoped In _0.1 Ga _0.9 As layer 23 is a quantum dot 2
Since it has a smaller energy band gap than the material of No. 5, the light transmitted through the quantum dot 25 is undoped.
When absorbed by the In _0.1 Ga _0.9 As layer 23, an electron-hole pair is generated here, and a photocurrent flows, similarly to a general pin photodiode. The light has the same energy as the separation energy of the quantum dots 25.

Therefore, also in the element of this embodiment, data "1" or "0" is written depending on the presence or absence of light irradiation. Further, since the photocurrent varies depending on the presence / absence of holes in the quantum dots 25, the stored information can be read by measuring the current flowing through the p-side electrode 26 and the n-side electrode 27. Note that the quantum dots 25 are in a hole-burning state after reading the stored information, so that the quantum dots 25 need to be refreshed. (Seventh Embodiment) In this embodiment, an optical semiconductor memory device in which quantum dots are formed at positions different from those in the sixth embodiment will be described.

In FIG. 13, a semi-insulating group made of GaAs is used.
On the plate 31, n with a film thickness of 300 nm ⁺Type GaAs layer 32, film
Undoped In with a thickness of 100 nm_0.1Ga_0.9As layer 33, film thickness 20
A 0 nm p-type AlAs layer 34 is sequentially formed. Also, p
In the region surrounded by the annular p-side electrode 26 on the AlAs layer 34
Is an undoped AlAs layer 35 having a thickness of 50 nm,
In addition, in the AlAs layer 35,
A plurality of quantum dots 36 are formed.

Further, an n-side electrode 27 is formed on the n ⁺ type GaAs layer 22. The impurity concentration of the n-type GaAs layer 32 is 5 × 10 ¹⁸ atoms / cm ³ , and the impurity concentration of the p-type AlAs layer 34 is 5 × 10 ¹⁸ atoms / cm ³ . Also in this embodiment, the same information writing and reading as in the sixth embodiment is performed. (Eighth Embodiment) The light emitting element used for writing and reading information may be integrated with the optical semiconductor memory element of the sixth embodiment, and an example thereof will be described below.

As the light emitting element, for example, a surface emitting semiconductor laser may be used, and the pin photodiode of the sixth embodiment may be laminated thereon. The energy band diagram is as shown in FIG. An n-type clad layer 41, an undoped active layer 42, and a p-type clad layer 4 are formed on a GaAs substrate (not shown).
3 in order, a first reflective film (not shown) is formed under the GaAs substrate, and a second reflective film (not shown) is formed on the p-type cladding layer 43. Laser 3
9. Further, the optical semiconductor memory element 40 is formed on the second reflective film.

As the cladding layers 41 and 43, for example, In
_0.5 (Ga _1-x Al _x ) _0.5 P layer, as the active layer 42, for example, In
_{A 0.5} (Ga _1-y Al _y ) _0.5 P layer is used (x> y). The optical semiconductor memory device 40 on the p-type clad layer 43 has the same layer structure as that of the sixth embodiment, and has the p-type Al having the quantum dots 25.
As layer 24, undoped In _0.1 Ga _0.9 As layer 23, and n-type Ga
It is composed of an As layer 22. (Ninth Embodiment) In this embodiment, an optical semiconductor memory device using an npn junction type phototransistor as a basic structure will be described with reference to FIG.

In FIG. 15A, an n-type GaAs collector layer 46, an undoped GaAs subcollector layer 47, and a p-type AlAs base layer 48 are sequentially formed on the GaAs substrate 45, and on the base layer 48. An annular n-type GaAs emitter layer 50 and an emitter electrode 51 are formed. A plurality of quantum dots 49 made of GaAs having different sizes are formed in a part or all of the region of the base layer 48. A collector electrode 52 is connected to the collector layer 46.

In such a device, since the bandgap of the subcollector layer is smaller than that of the quantum dots, the light passing through the quantum dots is irradiated to the subcollector layer to form electron-hole pairs, and An electric current flows. By the way, when a bipolar transistor is also formed on one GaAs substrate 45, as shown in FIG.
When two elements having the above structure are formed and one of the elements is formed on the base layer 48 with a metal base electrode 53 as shown in FIG. 16, the base electrode 53 shields the light from the base layer 48. The device functions as a normal bipolar transistor. A multi-emitter type bipolar transistor may be used.

The quantum dots 49 may be formed in the emitter layer 50 instead of the base layer 48. In this case,
The base layer 48 is formed of p-type GaAs, and the emitter layer 5 is formed.
0 will be formed by n-type AlAs. The energy band gap in this case is as shown in FIG. Then, when the quantum dots 49 in the emitter layer 50 are in the hole burning state, the light emitted for reading passes through the emitter layer 50 and is applied to the base layer 48.
Since the energy band gap of the base layer 48 is smaller than the separation energy of the quantum dots 49,
Electron-hole pairs are generated in the base layer 48 by light irradiation, and the holes in the base layer 48 become excessive, so that the collector current changes.

In this embodiment, since the phototransistor is provided with quantum dots, when hole burning occurs in the quantum dots, the electrons and holes generated between the base layer and the collector layer by the reading light are generated. Because it is amplified
The photocurrent increases. Further, in order to reset (initialize) the information of the quantum dot in which the hole burning is occurring, the amount of light having the energy equal to the separation gap of the quantum dot is increased to generate electrons between the base layer and the collector layer. Can be supplied to the quantum dots and recombined. In addition, a large voltage can be applied across the quantum dots to cause recombination and thereby reset the information.

Also in the light receiving element such as the photodiode and the phototransistor, if a large number of quantum dots having different sizes are formed, the light having the energy equal to the separation gap of each quantum dot is selected and the desired quantum is selected. Dots can be written and read.
That is, there are as many memory cells as the size of the quantum dot,
Since addressing corresponding to the wavelength of light is possible,
The storage capacity can be greatly increased. (Tenth Embodiment) In this embodiment, a quantum dot is used as a multi-emitter heterojunction bipolar transistor (HB).
An optical semiconductor memory device configured by applying the above-mentioned T) will be described below.

In FIG. 18, on the GaAs substrate 61,
A collector layer 62 made of n ⁺ -type GaAs having a thickness of 200 nm
And a subcollector layer 63 made of undoped GaAs having a thickness of 100 nm, and a base layer 6 made of p-type GaAs having a thickness of 50 nm.
4 and an emitter layer 65 of n-type In _0.5 Ga _0.5 P having a film thickness of 50 nm. A film thickness of 20 on the emitter layer
An emitter contact layer 67 made of 0 nm n ⁺ type GaAs,
A plurality of (68 in the figure) 68 are formed at intervals, and emitter electrodes 69 and 70 are formed on them. A plurality of quantum dots 65 made of GaAs are formed in the emitter layer 65 with different sizes.

The impurity concentration of the collector layer 62, the base layer 64, the emitter layer 65, and the emitter contact layers 67 and 68 is 5 × 10 ¹⁸ atoms / cm ³ , respectively. This multi-emitter type HBT has no base electrode, and when the voltages of the emitter electrodes 69 and 70 are equal, no current flows through the collector layer 62, and the emitter layers 69 and
When the voltage of 0 is different, the current flows through the collector layer 62. The voltage applied to the emitter layers 69 and 70 is set higher than that of the collector layer 62. (Eleventh Embodiment) In the semiconductor device described above, stored information of a plurality of quantum dots having different sizes may be read,
When writing information to the quantum dots, you may continuously scan the light of the wavelength corresponding to the separation energy of each quantum dot and irradiate multiple quantum dots, or you can simultaneously emit light of different wavelengths. It may be irradiated. That is, since the separation energy of the quantum dots depends on the size, the growth temperature, etc., writing and reading of information is performed by irradiating each quantum dot with light having a wavelength unique to each quantum dot. Since hole burning does not occur in the quantum dots due to light having a wavelength other than the specific wavelength, no error occurs even when light having different wavelengths is simultaneously or continuously irradiated.

FIG. 19 shows the light absorption saturation characteristic when light of different wavelengths is continuously scanned when reading stored information.
(a). In FIG. 19A, the hole burning is caused by the light irradiation in the quantum dot having the separation energy corresponding to the wavelength where the light absorption coefficient is reduced. When hole burning occurs, the photocurrent increases due to the electrons that escape from the quantum dots.

Therefore, it is possible to write and read a large amount of information by continuously scanning light having different wavelengths. Further, the pin junction type photodiode has different light receiving sensitivity depending on the wavelength as shown in the spectral sensitivity characteristic of FIG. 19 (b). When quantum dots are formed on such an element, measure the relationship between the light absorption coefficient and the wavelength in advance and know the wavelength of the read information based on the value of the light absorption coefficient at the time of reading. You can From this, it is clear to which wavelength the light should be re-input for the element that needs to be refreshed. (Twelfth Embodiment) A large number of the memory cells described above may be arranged in a matrix in the X-axis direction and the Y-axis direction on one substrate. The optical semiconductor memory element of the above embodiment corresponds to the memory cell.

In this case, as shown in FIG. 20, the first electric wires E _{1 to} E ₄ are arranged in the X-axis direction and the second electric wires E _{1 to} E ₄ are arranged in the Y-axis direction.
Of the electric wirings C _{1 to} C ₄ of the memory cell 72, and the two electrodes 73, 7 of the memory cell 72 in the vicinity of the intersection of the electric wirings.
Connect to 4. The two electrodes include the n-side electrode and the p-side electrode of the optical semiconductor memory element described above, or the emitter electrode and the collector electrode.

Further, writing and reading of those memory cells 72 are performed.
Light hv to illuminate when taking out₁₁~ Hv ₄₄Is the first and second
Electric wiring of₁~ E_Four, C₁~ C_FourLight distribution arranged above
Place a line (not shown). As the optical wiring, for example, an image
There is an optical shutter arranged in a pixel in the display device. Of wavelength
Different light hv₁₁~ Hv₄₄Simultaneously irradiate the memory cell 72
If you want to write or read information collectively,
Irradiate the memory cell 72 with one spot of light.
Yes.

According to such wiring, since the electric wiring and the optical wiring are used, it is possible to carry out the wiring efficiently without the optical and electric signals interfering with each other. When electrons are injected into a quantum dot in which only holes are confined and electrons and holes are recombined in the quantum dot, hole burning of the quantum dot disappears and the data is reset to "0". By collectively injecting electrons into the QDs, the processing speed can be increased and simplified.

As a method of injecting electrons into the quantum dots, for example, there is a method of applying a forward bias voltage to the p-type semiconductor layer and the n-type semiconductor layer existing around the quantum dots.
Multi-emitter type HBT as shown in the tenth embodiment
In this case, when a current is passed between the plurality of emitters to supply electrons to the quantum dots, hole burning does not occur in the quantum dots.

As described above, when any one of the first electric wirings E _{1 to} E ₄ and the second wirings C _{1 to} C ₄ is selected and a current is passed through the memory cell 72, the selected optical semiconductor is selected. The storage element cannot be written. This means that the optical semiconductor memory element can be selected. When the memory cells are selected and written, the light having a small spot diameter for selecting the memory cells is not necessary, and it is sufficient to collectively irradiate the plurality of memory cells 72 with the light. (Thirteenth Embodiment) In this embodiment, an example in which data is reset by light irradiation is shown.

For example, in the bipolar transistor type memory cell of FIG. 21 (a), when light hv having energy equal to or higher than the band gap Eg _{01 of the} base layer including the quantum dots is irradiated, holes in the valence band of the base layer are irradiated. Accumulation is promoted, the potential of the base layer is lowered, and a current flows through the base layer. As a result, electrons are supplied into the plurality of quantum dots in the base layer, so that hole burning is eliminated and the data of the quantum dots is set to "0".
When multiple quantum dots are present in one base layer,
Data of all quantum dots is "0" by the light irradiation.
become.

When a plurality of quantum dots having different sizes are present in one base layer, as shown in FIG. 21 (b), the quantum dot having the largest size is selected and the separation energy of the quantum dots is equal to or larger than that. The base layer is irradiated with light having a wavelength band corresponding to an energy which is smaller than the energy band gap of the base layer. According to this, hole burning occurs in all quantum sizes in the base layer, and the data of all quantum dots are set to “1”.

Here, the largest quantum dot is selected because the ground level of this quantum dot is smaller than the ground levels of other quantum dots. (Fourteenth Embodiment) A plurality of quantum dots forming the above-mentioned optical semiconductor memory element may have a multilayer structure.

For example, as shown in FIG. 22, in the plurality of quantum dots 82a to 82e formed in the semiconductor layer 81 on the n-type semiconductor substrate 80, the quantum dots 82a to 8e.
The sizes in the surface direction of 2e may be made uniform, while the sizes in the thickness direction may be different. In this case, the quantum dots 82a to 82e may be stacked in the order of diameter, or
The quantum dots 82a to 82e may be stacked in ascending order of the diameter, or the quantum dots 82a to 82e may be stacked so that the size of the diameter becomes irregular.

The size of the quantum dots 82a to 82e becomes smaller as the growth temperature becomes higher as shown in FIG. 23, and the density thereof becomes higher as the growth temperature becomes higher. Thereby, the quantum dots 82a to 82e
2 by adjusting the growth temperature of the semiconductor
A structure like 2 is obtained. A plurality of quantum dots 82a-
By controlling the number of sizes of 82e, it is possible to adjust the number of bits in one semiconductor layer 81.

Regarding the technique for changing the size of the quantum dot as described above, J. Oshinowo et. Al., Appl. Phys. Let
t. 65 (11), 12 September 1994, pp.1421-1423. The size of the quantum dot can also be adjusted by changing the supply amount of the semiconductor forming the quantum dot. For example, in the steps shown in FIGS. 1 (a) to 1 (c),
If the thickness of the InAs layer 4 that becomes the quantum dots 4a is changed,
There is a relationship as shown in 4. In addition, in FIG. 24, the supply amount of InAs is shown by the thickness of InAs.

According to FIG. 24, the supply amount is 1.5 to 2.3.
It can be seen that up to ML, the size of the quantum dot becomes smaller as the supply amount of InAs is increased. In this way, the plurality of quantum dots arranged in the film thickness direction have different diameters, and as shown in FIG.
GaAs substrate is used and AlAs or AlGaAs is used as the semiconductor layer 81.
Layer, using InAs or In as the quantum dots 82a to 82e.
A band diagram as shown in FIG. 25 is obtained by using dots of GaAs and further forming an electrode 83 which is in Schottky contact with the semiconductor layer 81 on the semiconductor layer 81. In this case, the quantum dots 82a to 82e are stacked in descending order of diameter.

Writing information in the optical memory device of this structure,
The reading method is as follows. First, the case of writing information will be described. When the uppermost quantum dot 82e is irradiated with light of a specific wavelength to generate an electron-hole pair and the electron is emitted to the outside of the quantum dot 82e by the tunnel effect, the electron is generated between the electrode 83 and the semiconductor substrate 80. Is moved to the semiconductor substrate 80. However, a voltage is applied between the electrode 83 and the semiconductor substrate 80, and the semiconductor substrate 8
The voltage of 0 is higher than that of the electrode 83.

Before the electrons reach the semiconductor substrate 80, if the lower quantum dot 82b is already irradiated with light and "1" is written, the uppermost quantum dot 82e will be written.
There is an inconvenience that the electrons moved from are recombined in the quantum dots 82b below and rewrite the information of the quantum dots 82b below to “0”. Therefore, in the case of the optical memory device having the quantum dots 82a to 82e having the multilayer structure, it is preferable to write the information in order from the quantum dot with the lower potential side. In FIG. 22A, information is written in order from the side farthest from the semiconductor substrate 80.

Next, the case of reading information will be described. When information “0” is written in the uppermost quantum dot 82e, when the quantum dot 82e is irradiated with a specific wavelength to read the information, electrons are emitted from the quantum dot 82e to the outside. In this case, the electrons are recombined in the lower quantum dot 82b in which the information "1" is written,
The information may be rewritten to "0".

Therefore, the quantum dots 82a-8 of the multilayer structure
In the case of the optical memory device having 2e, it is preferable to read the information of the quantum dots on the higher potential side in order. FIG.
In 2 (a), information is read in order from the side closest to the semiconductor substrate 80. However, all quantum dots 82
After reading the information of a to 82e, it is necessary to reset and rewrite all the quantum dots 82a to 82e. That is, a forward current is applied to the Schottky barrier to reset all stored information to "0" by the electron current.

In this embodiment, the electrode 83 in Schottky contact is formed on the semiconductor layer 81, but a p-type semiconductor layer may be formed instead of the electrode 83. Reset in this case is performed by passing a current in the forward direction of the pn junction. Note that, as shown in FIG. 26, an n-type semiconductor substrate 80
When a high electric field is applied to the undoped semiconductor layer 84 by interposing the undoped semiconductor layer 84 made of GaAs between the semiconductor layer 81 and the semiconductor layer 81, electrons are amplified by avalanche collapse and a large signal can be obtained as described above. Become.

The light to the optical memory element is the semiconductor layer 8
Irradiation is performed from the side of 1 or from above by using the electrode 83 made of a transparent conductive material. (Fifteenth Embodiment) The following method is adopted when writing and reading information in an optical memory having quantum dots having a multilayer structure as shown in FIG.

To write information, as shown in FIG. 27, composite light having energies hv _{01 to} hv ₀₅ equal to the transition energies of the quantum dots 82a to 82e may be simultaneously irradiated, or as shown in FIG. , Multiple energies hv ₀₁ ~
The light of hv ₀₅ is individually time-sequentially used for each quantum dot 82a to 8a.
Irradiate 2e. Each quantum dot 82a can be obtained only by irradiation with light having the same energy as the transition energy.
Since any writing of ~ 82e is possible, either method can be adopted. However, when irradiating light in time series, the first
As shown in the fourth embodiment, it is preferable to write information in order from the quantum dot with the lower potential side.

Note that h is Planck's constant and v is the frequency of light. When reading information, information “0” or “1” is determined according to the amount of change in the current flowing through the semiconductor substrate 80.
As shown in FIG. 27, light of a plurality of energies hv _{01 to} hv ₀₅ is sequentially and sequentially time-sequentially applied to the respective quantum dots 82a to 82e.
By irradiating the However, as described in the fourteenth embodiment, it is preferable to determine the irradiation order of light such that information is sequentially read from the quantum dots on the higher potential side.

In this case, as shown in FIG. 28, when light pulses having different energies are sequentially irradiated, the detection current flowing through the semiconductor substrate 80 and the electrode 83 changes in time series according to the irradiation timing of the light pulses. As a result, the output is detected. The energy difference (transition energy) E between the ground level of the conduction band and the valence band in the quantum dot E
g ₀₁ is equal to hv ₀₁ , and similarly, Eg ₀₂ is hv ₀₂ ,
Eg ₀₃ is hv ₀₅₃ , Eg ₀₄ is hv ₀₄ , Eg ₀₅ is hv ₀₁₅
be equivalent to. (Sixteenth Embodiment) In an optical memory device having a multi-layered quantum dot, in order to prevent tunneling of holes among electron-hole pairs generated in the quantum dot by light irradiation, FIG.
The structure shown in 9 (a) is adopted.

In FIG. 29A, the same symbols as those in FIG. 22 indicate the same elements. In this case, each quantum dot 82a
~ 82e, the valence band Ec
A second barrier layer 85 made of a material having a high band offset and a small band offset of the conduction band Ev is arranged. For example, when the second barrier layer 85 is composed of In _0.5 Ga _0.5 P, the band offset of the conduction band is 0.
The band offset of 1 eV and the valence band is 0.5 eV.
In this case, an n-type GaAs substrate is used as the semiconductor substrate 80, and Al _0.5 Ga is used as the semiconductor layer 81 serving as the first barrier layer.
InAs as quantum dots 82a to 82e using _0.5 As layer
29B, when the electrode 83 which is in Schottky contact with the semiconductor layer 81 is formed of tungsten, the band diagram is as shown in FIG.

As is apparent from FIG. 29 (b),
The holes remaining in the quantum dots 82a after the generation of hole pairs are second
The barrier layer 85 prevents the tunnel from accumulating and thus does not adversely affect other layers. Moreover, since tunneling of electrons from the quantum dots 85a is not hindered, writing and reading of the optical memory element are easy. Other Embodiments The material of the quantum dots and the semiconductor layers around them in the above-described embodiments may be any of III-V group semiconductors, II-IV group semiconductors, Si, Ge, SiGe, and the like. However, the present invention is not limited to the above description of the embodiment.

In order to construct an optical semiconductor memory integrated circuit, not only the above-mentioned optical semiconductor memory element but also a switching element as its peripheral circuit is indispensable. Among the optical semiconductor memory elements described above, there is an element that operates as a switching element if the semiconductor layer including the quantum dots is not irradiated with light. When such an element is made to function as a switching element, a light shielding film may be formed on the semiconductor layer containing the quantum dots. For example, the semiconductor layer 1 of the device shown in FIG.
A light-shielding film may be formed at the position shown by the alternate long and short dash line on 7 and used as a normal bipolar transistor.

[0115]

As described above, according to the present invention, the semiconductor layer having the quantum dots is sandwiched between the first and second conductive films, an electric field is generated between the conductive films, and the quantum dots are removed from the quantum dots by hole burning. It makes it difficult for the emitted electrons to return to the quantum dots. Further, according to another aspect of the present invention, since the carrier barrier semiconductor layer having the quantum dots inside is formed on the first conductivity type impurity-containing semiconductor layer to form the wavelength multiplex memory, the hole multiplexing is performed by light irradiation. When the burning occurs, the electrons emitted from the quantum dots have a high probability of flowing out to the first conductivity type impurity-containing semiconductor layer, and the probability of carriers flowing into the quantum dots to cause recombination becomes low.

In these inventions, information is written by forming a quantum dot in a semiconductor layer and irradiating it with light to cause hole burning, or by not irradiating light to cause hole burning. Because
Data can be written depending on whether or not the hole burning has occurred. The written information is read by irradiating light of the same wavelength, and "1" and "0" can be determined by the change of the current flowing through the semiconductor layer and the change of the absorption amount of the light by the light irradiation.

The light used for writing and reading information is limited to the wavelength corresponding to the energy difference between the ground level of the conduction band and the valence band of the quantum dot. Therefore, by changing the size of the quantum dot. Since the wavelength of light used for writing and reading is different, the number of bits of the storage element can be determined by the number of different quantum dot sizes.

Since information is written depending on whether or not hole burning occurs, information can be initialized by injecting electrons into quantum dots.
When storing and reading information, applying a bias voltage so that the energy barrier around the quantum dots becomes thick can prevent the destruction of information. Since the optical semiconductor memory element is configured by adopting the semiconductor layer including the quantum dots as described above as a part of the active element, the current flowing at the time of reading information is amplified and the light quantity is converted into an electric signal. It becomes possible to read information easily.

When a plurality of quantum dots are formed in the film thickness direction and the sizes are different, when the energy is changed and the writing light is applied to the quantum dots, the quantum dots are arranged in the order in which they are arranged in the carrier movement direction. If you write it,
It is possible to prevent the destruction of the information of the already written quantum dots. In addition, if multiple quantum dots are formed in the film thickness direction and if they are of different sizes, writing is performed according to the order in which they are arranged in the direction opposite to the carrier movement direction. Can be prevented from being destroyed.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view showing a method for forming an optical semiconductor memory element used in the first embodiment of the present invention.

FIG. 2 is a cross-sectional view showing a state in which quantum dots used in the first embodiment of the present invention have different sizes.

FIG. 3 is an energy band diagram of an optical semiconductor memory device used in the first embodiment of the present invention.

FIG. 4 is an energy band diagram at the time of holding information or reading information from the optical semiconductor memory device according to the second embodiment of the present invention.

FIG. 5 is a diagram showing the relationship between the wavelength of light and the absorptance of light of the quantum dots when reading information from the optical semiconductor memory device according to the second embodiment of the present invention.

FIG. 6 is an energy band diagram of an optical semiconductor memory device according to a third embodiment of the present invention.

FIG. 7 is an energy band diagram of an optical semiconductor memory device having a Schottky barrier according to a fourth embodiment of the present invention.

FIG. 8 is a sectional view showing an optical semiconductor memory element according to a fourth embodiment of the present invention.

FIG. 9 is a sectional view showing an optical semiconductor memory element according to the fifth embodiment of the present invention.

FIG. 10 is a structural energy band diagram of an optical semiconductor memory device according to a fifth embodiment of the present invention.

FIG. 11 is an energy band diagram at the time of writing and reading information of the optical semiconductor memory device according to the fifth embodiment of the present invention.

FIG. 12 shows a sectional view, a plan view, and an energy band diagram showing an optical semiconductor memory element according to a sixth embodiment of the present invention.

FIG. 13 is a sectional view showing an optical semiconductor memory element according to a seventh embodiment of the present invention.

FIG. 14 is an energy band diagram of an optical semiconductor memory device and a light emitting device connected thereto according to a seventh embodiment of the present invention.

FIG. 15 is a cross-sectional view of an optical semiconductor memory device according to an eighth embodiment of the present invention and its energy band diagram.

FIG. 16 is a cross-sectional view showing a bipolar transistor configured by covering an optical semiconductor memory element according to an eighth embodiment of the present invention with a light shielding film.

FIG. 17 is an energy band diagram when quantum dots are formed in the emitter region in the optical semiconductor memory device according to the eighth embodiment of the present invention.

FIG. 18 is a sectional view showing an optical semiconductor memory element according to the tenth embodiment of the present invention.

FIG. 19 is a light absorption saturation characteristic diagram and a branching sensitivity characteristic diagram in the case where light of different wavelengths used for reading and writing information of the optical semiconductor memory device according to the tenth embodiment of the present invention is continuously scanned. is there.

FIG. 20 is a circuit diagram showing an optical semiconductor memory device according to a twelfth embodiment of the invention.

FIG. 21 is an energy band diagram showing a state in which the hole burning of the optical semiconductor memory device according to the thirteenth embodiment of the present invention is eliminated, and an energy band diagram showing an energy range in which the hole burning is generated.

FIG. 22 is a sectional view showing an optical semiconductor memory element according to the fourteenth embodiment of the present invention.

23 is a diagram showing the relationship between the size and density of quantum dots and the growth temperature of the optical semiconductor memory element shown in FIG.

FIG. 24 is a diagram showing the relationship between the quantum dot size and the InAs supply amount in the step of forming quantum dots made of InAs in the optical semiconductor memory device according to the fourteenth embodiment of the present invention.

FIG. 25 is an energy band diagram of an optical semiconductor memory device according to the fourteenth embodiment of the present invention.

FIG. 26 is a sectional view showing a structure in which an undoped semiconductor layer for causing avalanche collapse is interposed in an optical semiconductor memory device according to a fourteenth embodiment of the present invention.

FIG. 27 is an optical waveform diagram showing simultaneous irradiation with a plurality of lights having different energies for writing information to the optical semiconductor memory element according to the fifteenth embodiment of the present invention.

FIG. 28 is an optical waveform diagram showing simultaneous irradiation with a plurality of lights having different energies for writing or reading information to or from an optical semiconductor memory element according to the fifteenth embodiment of the present invention, and a flow chart of the semiconductor memory element. It is a wave form diagram which shows an example of change of electric current.

FIG. 29 is a sectional view showing an optical semiconductor memory element according to the sixteenth embodiment of the present invention, and an energy band diagram thereof.

FIG. 30 is an energy band diagram of a conventional optical semiconductor memory device.

[Explanation of symbols]

2 n-type GaAs layer (semiconductor layer) 3 Al _0.5 Ga _0.5 As layer (semiconductor layer) 4a quantum dot 5 p-type Al _0.5 Ga _0.5 As layer (semiconductor layer) 7 metal layer 8 n-type GaAs layer (semiconductor layer) 9 gate Electrode 16 Base layer 17 i layer 18 Emitter layer 19 Quantum dot 23 In _0.1 Ga _0.9 As layer 24 p-type AlAs layer (semiconductor layer) 34 p-type AlAs layer (semiconductor layer) 36 AlAs layer 37 Quantum dot 39 Semiconductor laser 40 Optical semiconductor Storage element 47 Sub-collector layer 48 Base layer 49 Quantum dot 50 Emitter layer 64 Base layer 65 Emitter layer 66 Quantum dot 67,68 Emitter contact layer 81 Semiconductor layers 82a to 82e Quantum dot 85 Barrier layer

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication location H01L 29/73 (72) Inventor Naoto Horiguchi 1015 Ueodachu, Nakahara-ku, Kawasaki-shi, Kanagawa Within Fujitsu Limited (72) Inventor Yoshihiro Sugiyama 1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture, Fujitsu Limited (72) Inventor Yoshiaki Nakata 1015, Kamedotachu, Nakahara-ku, Kawasaki City, Kanagawa Prefecture, Fujitsu Limited

Claims (31)

[Claims] 1. A first conductive layer, a semiconductor layer formed on the first conductive layer, quantum dots formed in the semiconductor layer, and a quantum dot formed on the semiconductor layer, An optical semiconductor memory device comprising: a second conductive layer to which an electric field is applied between the first conductive layer and the first conductive layer. 2. A first semiconductor layer containing a first conductivity type impurity, a carrier barrier semiconductor layer formed on the first semiconductor layer, and quantum dots formed in the carrier barrier semiconductor layer. An optical semiconductor memory device comprising: 3. The optical semiconductor memory device according to claim 2, wherein a plurality of the quantum dots are present with different sizes. 4. The optical semiconductor memory device according to claim 2, wherein the carrier barrier semiconductor layer is formed of an undoped semiconductor or a second conductivity type impurity-containing semiconductor. 5. A metal layer in Schottky contact with the carrier barrier semiconductor layer or a second semiconductor layer containing an impurity of a second conductivity type is formed on the carrier barrier semiconductor layer. The optical semiconductor memory device according to claim 2, which is characterized in that: 6. The quantum level of the conduction band or valence band of the quantum dot and the conduction band or valence band of the first semiconductor layer are energetically separated from each other. Alternatively, the optical semiconductor memory device according to item 5. 7. The optical semiconductor memory device according to claim 2, wherein an undoped third semiconductor layer is formed between the first semiconductor layer and the carrier semiconductor layer. 8. The optical semiconductor memory device according to claim 2, wherein the carrier barrier semiconductor layer is formed on a channel layer of a field effect transistor. 9. The optical semiconductor memory device according to claim 2, wherein the carrier barrier semiconductor layer constitutes an emitter layer or a base layer of a bipolar transistor. 10. The energy band gap of a part or all of the collector layer of the bipolar transistor is smaller than the energy difference between the ground level of the conduction band and the valence band of the quantum dot. 9. The optical semiconductor memory device described in 9. 11. An undoped semiconductor layer is formed between the collector layer and the base layer of the bipolar transistor, and an energy band gap of the undoped semiconductor layer is a ground level of a conduction band and a valence band of the quantum dot. 10. The optical semiconductor memory device according to claim 9, wherein the difference is smaller than the energy difference between them. 12. The optical semiconductor memory device according to claim 9, wherein a plurality of emitter electrodes are formed on the emitter layer of the bipolar transistor, and a collector electrode is formed on the collector layer. 13. The optical semiconductor memory device according to claim 2, wherein the carrier barrier semiconductor layer constitutes an i layer of a pin junction type photodiode. 14. The optical semiconductor memory device according to claim 2, wherein the carrier barrier semiconductor layer is formed on a light emitting element. 15. The optical semiconductor memory device according to claim 2, wherein a plurality of the quantum dots are present in a thickness direction of the carrier barrier semiconductor layer. 16. The optical semiconductor device according to claim 15, wherein the plurality of quantum dots have different diameters in the film thickness direction. 17. A barrier layer for preventing movement of holes or electrons accumulated in the quantum dots in the hole burning state is formed on a side of the quantum dots opposite to a moving direction of the carriers. The optical semiconductor device according to claim 15. 18. The carrier barrier semiconductor layer is formed on a part of a plurality of active elements, and the semiconductor layers of some of the active elements are covered with a light shielding film.
The optical semiconductor memory device described. 19. A hole is formed by irradiating a quantum dot formed in a semiconductor layer with light, and irradiating light with a wavelength corresponding to an energy difference between respective ground levels of a conduction band and a valence band of the quantum dot. A method for writing and reading information in an optical semiconductor memory device, which comprises causing burning to write information, and irradiating the quantum dots with light of the wavelength to read the information by a change in current flowing from the semiconductor layer. 20. The quantum dots formed in the semiconductor layer are irradiated with light, and the quantum dots formed in the semiconductor layer are irradiated with light having a wavelength corresponding to the energy difference between the ground level of the conduction band and the valence band of the quantum dots. A method for writing and reading information in an optical semiconductor memory device, characterized in that burning is performed to write information, and the quantum dot is irradiated with light of the wavelength to read the information according to the amount of light transmitted through the semiconductor layer. 21. The semiconductor layer is formed between an n-type semiconductor layer and a p-type semiconductor layer, and a reverse bias voltage is applied to the n-type semiconductor layer and the p-type semiconductor layer during the writing, 21. The writing / reading method of an optical semiconductor memory device according to claim 19, wherein a bias voltage is not applied to said n-type semiconductor layer and said p-type semiconductor layer at the time of reading, or a forward bias voltage is applied. . 22. The semiconductor layer having the quantum dots constitutes an emitter or a base of a bipolar transistor, and when reading the information, avalanche collapse is caused between the base and the collector of the bipolar transistor by light irradiation. 21. The writing / reading method of the optical semiconductor memory device according to claim 19, wherein a voltage having a magnitude that causes the above is applied. 23. The semiconductor layer having the quantum dots constitutes an emitter or a base of a bipolar transistor, and when writing the information, a reverse bias is applied between the emitter and the base of the bipolar transistor,
A bias voltage is not applied between the base and the collector, a bias voltage is not applied between the emitter and the base, and a reverse bias voltage is applied between the base and the collector when reading the information. 21. The write / read method of an optical semiconductor memory device according to claim 19, wherein: 24. A plurality of quantum dots having different energy differences between the ground levels are provided, and when reading the information, light of a plurality of wavelengths corresponding to the energy differences is continuously changed. 21. The write / read method of an optical semiconductor memory device according to claim 19, wherein the quantum dots are irradiated. 25. The semiconductor layer having the quantum dots constitutes a part of a light receiving element, and at the time of reading the information, a plurality of the plurality of the light receiving elements are changed due to a change in spectral sensitivity characteristic depending on a light irradiation wavelength of the light receiving element. 21. The writing / reading method of the optical semiconductor memory device according to claim 20, wherein the information of the quantum dots is read. 26. A plurality of quantum dots having different energy differences between the ground levels are provided, and when writing the information and reading the information, light having a plurality of wavelengths corresponding to these energy differences is provided. 21. The write / read method of an optical semiconductor memory device according to claim 19, wherein the irradiation is performed simultaneously. 27. The plurality of quantum dots having different energy levels between the ground levels are arranged in the film thickness direction, and when writing the information, the quantum dots are arranged in the order of the direction in which carriers move by light irradiation. 27. The writing / reading method of the optical semiconductor device according to claim 26, wherein information is written in the dots. 28. The plurality of quantum dots having different energy levels between the ground levels are arranged in the film thickness direction, and when reading the information, the quantum dots are arranged in the order opposite to the moving direction of carriers by light irradiation. 27. The writing / reading method of an optical semiconductor device according to claim 26, wherein information of the quantum dots is read. 29. The optical semiconductor memory device according to claim 19, wherein an electric field is generated in the semiconductor layer having the quantum dots to supply electrons to the quantum dots to initialize the information. Write and read method. 30. The write / read method of an optical semiconductor memory device according to claim 19, wherein writing of the information is prohibited by passing a current through the semiconductor layer having the quantum dots. 31. A plurality of quantum dots are present in a semiconductor layer,
The quantum dot is irradiated with light having an energy width larger than the smallest energy difference between the ground levels among the plurality of quantum dots and smaller than the energy band gap of the semiconductor layer. A method for writing and reading in an optical semiconductor memory device, characterized in that all of the above are put into a hole burning state.