JP4854975B2 - Write / read method for optical semiconductor memory device - Google Patents

Description

  The present invention relates to a method for writing / reading an optical semiconductor memory device, and more particularly to a method for writing / reading information in an optical semiconductor memory device having quantum dots.

  Due to dramatic progress in information and communication technology, it is desired to construct a system that takes advantage of the characteristics of optical devices and electronic devices.

  The feature of the optical device is that it has a higher speed and higher possibility of parallel processing than an electronic device. Various storage devices (memory devices) using optical devices have been proposed so far, but none have been embodied yet.

  This is because even if memory cells are constructed from optical devices, it is important to use optical devices and electronic devices with their own characteristics in the current situation where the surrounding theoretical circuits and switching circuits are composed of electronic devices. Because it becomes. Also, there is no clear device concept of optical memory.

Since the transition energy between the ground levels of quantum dots (quantum boxes) has an extremely steep light absorption spectrum, and if light absorption saturation is strong, information can be written and read by 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 for 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, 1 bit can be stored in a 10 nm × 10 nm region, and about 1 terabit can be stored in a 1 cm ² region.

  Such a wavelength multiplexing memory is proposed in Shunichi Muto, Jpn. J. Appl. Phys. Vol. 34 (1995) pp. L210-L212.

  This wavelength division multiplexing memory is composed of an AlAs substrate whose main surface is inclined and a III-V group compound semiconductor layer formed thereon. A step is formed on the main surface of the AlAs substrate in two directions by inclination, GaAs is formed at the corner of the step, and an AlGaAs layer is formed around the step. As a result, GaAs surrounded by the AlGaAs layer becomes a quantum box. FIG. 30 shows a partial band diagram of one quantum box of this multi-wavelength memory and its surrounding layers.

  Then, in the band diagram of the quantum box, when the light having the same energy as the separation energy between the ground level of the valence band and the conduction band is irradiated, the conduction band from the ground level of the quantum box valence band Electrons transition to the ground level and electron-hole pairs are generated.

  Here, since electrons have a high tunnel probability and the X point band energy of AlAs is low, the electrons tunnel through the AlGaAs barrier and move outside the quantum dots. Electron movement is detected as a change in current.

  On the other hand, since it is difficult for holes to come out, only the holes are left in the quantum box. In this state, even if light of the same wavelength is irradiated, since there is a hole and strong absorption saturation occurs, light absorption hardly occurs.

  By using such a phenomenon, information can be written and read by light. If a plurality of quantum boxes having different sizes and shapes are used, the wavelengths for writing and reading differ for each quantum box, so that a large amount of information can be stored with multiple wavelengths. Thereby, a multiple wavelength memory is obtained.

  However, since such a multi-wavelength memory has a high probability that electrons coming out of the quantum box will return to the quantum box and recombine, the retention time is as short as about 1 to 10 ms. Moreover, it is necessary to detect whether absorption saturation exists by light absorption, and a specific structure for that purpose is not shown.

  The present invention has been made in view of such a problem, and it is an object of the present invention to provide a method for writing and reading an optical semiconductor memory device that can be practically used while further extending the recording retention time.

The above-described problem is that a quantum dot formed in a semiconductor layer is irradiated with light, and light having a wavelength corresponding to the energy difference between each ground level of the conduction band and the valence band of the quantum dot is irradiated. A method for writing and reading information in an optical semiconductor memory device, wherein information is written by causing burning, and the quantum dots are irradiated with light of the wavelength to read the information by a change in current flowing from the semiconductor layer. Is formed between an n-type semiconductor layer and a p-type semiconductor layer , a reverse bias voltage is applied to the n-type semiconductor layer and the p-type semiconductor layer at the time of writing, and the n-type semiconductor layer at the time of reading. This is solved by a method for writing and reading in an optical semiconductor memory device, wherein a reverse bias voltage is applied to the semiconductor layer and the p-type semiconductor layer.

The above-described problem is that a quantum dot formed in a semiconductor layer is irradiated with light, and light having a wavelength corresponding to the energy difference between each ground level of the conduction band and the valence band of the quantum dot is irradiated. Burns and writes information,
An optical semiconductor memory device writing / reading method for reading the information by the amount of light passing through the semiconductor layer by irradiating the quantum dots with light of the wavelength,
The semiconductor layer is formed between an n-type semiconductor layer and a p-type semiconductor layer , a reverse bias voltage is applied to the n-type semiconductor layer and the p-type semiconductor layer at the time of writing, and at the time of reading. This is solved by a write / read method for an optical semiconductor memory device in which a bias voltage is not applied or a forward bias voltage is applied to the n-type semiconductor layer and the p-type semiconductor layer.

  In a method for writing and reading in an optical semiconductor memory device, the method includes a plurality of quantum dots having different energy differences between the ground levels, and continuously reading light having a plurality of wavelengths corresponding to the energy differences when reading the information. The quantum dots are irradiated with a change in the pattern.

  In a method for writing and reading in an optical semiconductor memory device, the plurality of quantum dots having different energy differences between the ground levels, and a plurality corresponding to these energy differences when the information is written and the information is read It is characterized by simultaneously irradiating light of the wavelength. 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 quantum dots are arranged in the order in which the carriers move by light irradiation. It is characterized by writing information. Further, the plurality of quantum dots having different energy differences 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 carrier moving direction by light irradiation. This information is read out.

  In the write / read 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, there are a plurality of quantum dots 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 problem is solved by a method for writing and reading in an optical semiconductor memory device, wherein the quantum dots are irradiated to the quantum dots so that all of the quantum dots are in a hole burning state.

  According to the present invention, a semiconductor layer having quantum dots is sandwiched between first and second conductive films, an electric field is generated between the conductive films, and electrons emitted from the quantum dots by hole burning are difficult to return to the quantum dots. is doing.

  According to another aspect of the present invention, a wavelength-division multiplexed memory is formed by forming a carrier barrier semiconductor layer having quantum dots therein on the first conductivity type impurity-containing semiconductor layer. When burning occurs, the probability that electrons emitted from the quantum dots flow out to the first conductivity type impurity-containing semiconductor layer is high, and the probability that carriers flow into the quantum dots and recombination occurs is low.

  Information is written depending on whether the quantum dots are irradiated with light to cause hole burning, or not irradiated with light to cause hole burning. For example, the data is “1” when hole burning occurs, and the data is “0” when no hole burning occurs.

  The information written in this way is read out by irradiating light of the same wavelength, and “1” and “0” are changed by the change in the current flowing through the semiconductor layer by the light irradiation and the change in the amount of absorbed light. to decide.

  When data “0” is written in the quantum dot, an electron-hole pair is generated in the quantum dot by the readout light, and thus light absorption occurs in the quantum dot or electrons flow to the outside. . As a result, it is determined that 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 can be obtained by using the first conductivity type impurity semiconductor layer as an n-type semiconductor layer. Therefore, the electrons are difficult to return to the quantum dots due to the diffusion potential.

  When data “1” is written in the quantum dot, the electron-hole pair is not generated in the quantum dot by the light for reading, and thus the light is not absorbed by the quantum dot and the electron does not move outside. . As a result, it is determined that data “1” has been written.

  The light used for writing and reading information is limited to a wavelength corresponding to the energy difference between the ground level of the conduction band and the valence band of the quantum dot. For this reason, since the wavelength of light used for writing and reading differs by changing the size of the quantum dots, the number of bits of the storage element is determined by the number of the different sizes of the quantum dots.

  Information initialization is performed by injecting electrons into the quantum dots, for example, by applying an electric field or irradiating light to a 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 dots so that the energy barrier around the quantum dots is thick when storing or reading information.

  When a semiconductor layer including quantum dots is adopted as part of an active element such as a photodiode, bipolar transistor, or field effect transistor, an optical semiconductor memory element is configured thereby. By adopting an active element as a basic structure, it is possible to amplify a current flowing when reading information, or to convert a light amount into an electrical signal, and to read information easily.

  When multiple quantum dots are formed in the film thickness direction and their sizes are different, the information is written by changing the light energy over time or by mixing multiple light energies of different sizes. Either write. When the quantum dots are irradiated with light while changing energy, it is desirable to write in the order in which they are arranged in the carrier moving direction in order to prevent destruction of information on the already written quantum dots.

  In addition, when a plurality of quantum dots are formed in the film thickness direction and their sizes are different, it is preferable to read information by changing light energy with time. However, in order to prevent destruction of information of quantum dots that have not yet been read, it is desirable to write in the order aligned in the direction opposite to the carrier movement direction.

  As described above, according to the present invention, the semiconductor layer having quantum dots is sandwiched between the first and second conductive films, an electric field is generated between the conductive films, and the electrons emitted from the quantum dots by the hole burning are detected. It is difficult to return to quantum dots.

  According to another aspect of the present invention, a wavelength-division multiplexed memory is formed by forming a carrier barrier semiconductor layer having quantum dots therein on the first conductivity type impurity-containing semiconductor layer. When burning occurs, the probability that electrons emitted from the quantum dots flow out to the first conductivity type impurity-containing semiconductor layer is high, and the probability that carriers flow into the quantum dots and recombination occurs is low.

  In these inventions, quantum dots are formed in the semiconductor layer, and information is written by irradiating light to the hole to cause hole burning or not irradiating light to cause hole burning. Depending on whether or not hole burning occurs, data can be written.

  The written information is read out by irradiating light of the same wavelength, and “1” and “0” can be determined by the change in the current flowing through the semiconductor layer by the light irradiation and the change in the amount of absorbed light.

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

  Since information is written depending on whether or not hole burning occurs, information initialization can be performed by injecting electrons into the quantum dots.

  When a bias voltage is applied so as to thicken the energy barrier around the quantum dots during information storage or information reading, information destruction can be prevented.

  An optical semiconductor memory element is configured by adopting a semiconductor layer containing quantum dots as described above as a part of the active element, so that the current that flows when reading information is amplified or the amount of light is converted into an electrical signal. It becomes possible to read out information.

  When multiple quantum dots are formed in the film thickness direction and their sizes are different, when changing the energy and irradiating the quantum dots with light for writing, write in the order in which they are aligned in the carrier movement direction. Then, it is possible to prevent destruction of information on the already written quantum dots.

  In addition, when multiple quantum dots are formed in the film thickness direction and their sizes are different, writing in the order aligned in the direction opposite to the carrier movement direction causes information on the quantum dots that have not been read out yet. Can be prevented.

  Accordingly, embodiments of the present invention will be described below with reference to the drawings.

(First embodiment)
A manufacturing process of the optical semiconductor memory element according to the first embodiment of the present invention will be described with reference to FIGS. The group III-V semiconductor described below is grown using, for example, an MBE apparatus.

First, an n-type GaAs substrate 1 whose main surface is a surface whose main surface is slightly inclined from the (111) plane is prepared. Then, as shown in FIG. 1 (a), an n-type GaAs layer 2 having an impurity concentration of 1 × 10 ¹⁸ atoms / cm ³ is formed on the main surface of the GaAs substrate 1 to a thickness of 50 nm, and an n-type GaAs layer is further formed. An undoped Al0.5Ga0.5As layer 3 is formed on 2 with a thickness of 25 nm. Subsequently, an InAs layer 4 is formed on the Al0.5Ga0.5As layer 3 to a thickness of several atomic layers (hereinafter referred to as ML). Next, as shown in FIG. 1B, an undoped Al0.5Ga0.5As layer 3 is again grown to a thickness of 25 nm so that the InAs layer 4 is included. After the second Al0.5Ga0.5As layer 3 is formed, the InAs layer 4 changes into a droplet shape due to the difference in lattice constant between the Al0.5Ga0.5As layer 3 and the InAs layer 4. That is, the droplet-like InAs layer 4 becomes a quantum dot (quantum box) 4a. The diameter of the quantum dot 4a increases as the growth temperature increases. For example, the average size is about 15 nm at 450 ° C. There are a plurality of quantum tods 4a of the same size.

  When the quantum dots 4a are formed on a plane inclined several degrees from the (111) plane, the growth of the InAs layer 4 to be the quantum dots 4a is adjusted, as shown in FIG. There are a plurality of quantum dots 4a each having a different size. Basically, if the size and shape of the quantum dot 4a are different, the potential well width of the quantum dot 4a is different. As a result, the energy (wavelength) of light for generating electron-hole pairs by light absorption in the quantum dot 4a is different. Different.

Thereafter, as shown in FIG. 1C, a p-type Al0.5Ga0.5As layer 4 is grown on the undoped Al0.5Ga0.5As layer 3 to a thickness of 200 nm. The impurity concentration of the p-type Al0.5Ga0.5As layer 5 is 1 × 10 ¹⁸ atoms / cm ³ . The Al0.5Ga0.5As layer 5 becomes a carrier barrier layer.

  The formation of quantum dots is described in JY Marchin et al., Photoluminesence of Single InAs Quantum Dots Obtained by Self-Organized Growth on GaAs, Physical Review Letters Vol. 73, No. 5. 1994, pp. 716-719. The dot size and shape can be controlled by the growth temperature and the molar ratio of In and As.

  Then, as shown in FIG. 1D, an n-side electrode 1 a is formed on the bottom surface of the n-type GaAs substrate 1, and a p-side electrode 5 a is formed on the p-type Al 0.5 Ga 0.5 As layer 5.

  As a result, a basic structure of an optical semiconductor memory element called a wavelength multiplexing memory is completed.

  FIG. 3A shows the energy band of one quantum dot 4a and the surrounding semiconductor layer.

  When the quantum dots 4a are irradiated with light, as shown in FIG. 3B, electrons transition from the valence band (Ev) to the conduction band (Ec) to generate electron / hole pairs. The energy (wavelength) hv of the light is the same as the energy between the ground level of the valence band and the ground level of the conduction band (hereinafter referred to as separation energy). Even if light with an energy different from the separation energy is irradiated, electron-hole pairs are not generated. The light that contributed to the generation of the electron / hole pairs is absorbed and does not pass through the quantum dots 4a.

  By the way, in the energy band diagram, the undoped Al0.5Ga0.5As layer 3 is inclined by the diffusion potential between the n-type GaAs layer 2 and the p-type Al0.5Ga0.5As layer 5, so that Al0 with respect to the quantum dot 4a The energy barrier of the 5Ga0.5As layer 3 becomes thinner. As a result, the electrons that have transitioned to the ground level of the conduction band of the quantum dot 4a tunnel through the Al0.5Ga0.5As layer 3 and move to the n-type GaAs layer 2.

  The conduction band of the n-type GaAs layer 2 is spatially separated from the conduction band of the quantum dots 4a via an undoped Al0.5Ga0.5As layer 3. The band gap of the n-type GaAs layer 2 is smaller than the band gap of the Al0.5Ga0.5As layer 3, and the energy difference between them is large. This means that the quantum level of the conduction band of the quantum dots 4a and the GaAs layer 2 are energetically separated.

  Therefore, the probability that electrons that have reached the n-type GaAs layer 2 return to the quantum dot 4a via the Al0.5Ga0.5As layer 3, that is, the probability of recombination becomes extremely small.

  For example, if the energy difference between the conduction band or Fermi level EF of the n-type GaAs layer 2 and the ground level of the conduction band of the quantum dot 4a is about 0.6 eV, the retention time of holes in the quantum dot 4a is More than 1000 hours and longer than the conventional one, and operation at room temperature becomes possible.

  As a result, as shown in FIG. 3C, holes are confined in the ground level of the valence band of the quantum dots 4a.

  In this way, the phenomenon in which electrons that have transitioned to the ground level of the conduction band of the quantum dot 4a tunnel through the energy barrier and move to the outside, and only holes are left in the valence band of the quantum dot 4a. This is called burning.

  Next, writing and reading of information in the optical semiconductor memory element having the above structure will be described.

  Light used for writing and reading is emitted from the optical fiber F.

  First, when data “0” is written, the energy band state as shown in FIG.

  On the other hand, when data “1” is written, the quantum dot 4a is irradiated with light having energy corresponding to the separation energy of the quantum dot 4a. As a result, hole burning occurs in the quantum dots 4a as shown in FIG. As a result, data “1” is written.

  The electrons transferred to the conduction band of the n-type GaAs layer 2 by writing the data “1” return to the spatially and energy-separated quantum dots 4a and have a very low probability of recombination there. "Will 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.

  When reading the written data, the quantum dots 3a are irradiated with light having the same energy as the separation energy of the quantum dots 3a.

  When data “1” is written, there is no movement of electrons from the quantum dot 4a to the n-type GaAs layer 2 as shown in FIG. 3C, and the monitor M connected to the electrodes 1a and 5a The flowing current does not change. Therefore, the irradiated light is transmitted through the quantum dots 4a.

When data “0” is written, as shown in FIG. 3B, hole burning occurs in the quantum dots 4a due to light irradiation, and electrons move to the n-type GaAs layer 2 to monitor. Current will flow. In this case, if a large number of quantum dots having the same size are formed, the amount of electrons that move increases, so that the detection sensitivity increases. Further, an 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 an avalanche is easily generated by applying a high electric field here, thereby increasing the light receiving sensitivity. Good.

  As described above, the stored information can be read depending on the presence or absence of the current. However, since the reading cannot be performed a plurality of times, the storage needs to be refreshed after the reading. That is, when hole burning occurs by reading data “0”, a positive bias voltage is applied between the n-type GaAs layer 2 and the p-type Al0.5Ga0.5As layer 5 to inject carriers into the quantum dots 3a. Rewriting is performed after information is erased by causing recombination.

  By the way, the separation energy of the quantum dots 3 a differs depending on the diameter of the quantum dots 4 a and the growth temperature of the InAs layer 4. The presence of a plurality of quantum dots 4a having different diameters causes hole burning by irradiating the element with a plurality of lights having different wavelengths, and a plurality of information can be simultaneously written and read.

  Accordingly, 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 using light having different wavelengths, it is preferable that the amount of light of a specific wavelength irradiated to the quantum dots 4a at the time of reading is smaller than the light irradiated at the time of writing. This is because when the data “0” is read, a part of the plurality of quantum dots having the same diameter is hole-burned by reducing the amount of light. Therefore, it is possible to reduce the number of electrons emitted by hole burning entering quantum dots of other diameters and destroying information, thereby reducing or eliminating the number of refreshes.

  Note that although there are two electrons emitted from one quantum dot, only one electron is shown in the figure for easy understanding.

(Second Embodiment)
A method of reading data from 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 dots 4a are irradiated with light having the same energy as the separation energy of the quantum dots 4a.

  The energy band in which data “0” is written is as shown in FIG. In this state, when the quantum dots 4a are irradiated with light, 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. In this way, when the light absorptance of the specific wavelength is large, it is determined that the data “0” has been written.

  On the other hand, the energy band in which the data “1” is written is as shown in FIG. In this state, since holes exist in the valence band of the quantum dots 4a, no electron / hole pairs are generated even when irradiated with light. Therefore, the light transmittance in the quantum dots 4a Becomes larger. As described above, when the amount of light irradiated to the quantum dots 4a is small, it is determined that the data “1” has been written.

  In this example, information is monitored by a change in light absorption rate, and information is not monitored by current. Therefore, it is not necessary to move electrons from the quantum dot 4a to the n-type GaAs layer 2 when reading data "0". .

  In this case, in order to reduce the number of information refreshes, it is only necessary to make it difficult to generate a tunnel at the time of reading. Therefore, in order to make a tunnel with a barrier as shown in FIG. 3B 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, it is necessary to facilitate movement of electrons from the quantum dots 4a to the outside of the undoped Al0.5Ga0.5As layer 3. For this reason, it is preferable to apply a reverse bias voltage to the n-type GaAs layer 2 and the p-type Al0.5Ga0.5As layer 5.

  On the other hand, when reading stored information or holding information, the n-type GaAs layer 2 and the p-type Al0.5Ga0.5As layer 5 are irradiated with light without applying a bias voltage or applying a forward bias. According to this, at the time of reading information, as shown in FIGS. 4A and 4B, the inclination of the undoped Al0.5Ga0.5As layer 3 serving as a barrier becomes small and the barrier B becomes substantially thick. Therefore, in the state where data “0” is written, as shown in FIG. 4 (a), an electron / hole pair is generated by light irradiation, but the electron is quickly regenerated as a hole without tunneling through the barrier B. Since they are combined, the stored information is not destroyed. Of course, when data “1” is written, as shown in FIG. 4B, no electron / hole pair is generated by light irradiation, so the data is not lost.

  Even when data is held, if no voltage is applied between the n-type GaAs substrate 1 and the p-type Al0.5Ga0.5As layer 5 or a forward bias voltage is applied, the barrier B becomes thicker. Therefore, data loss 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 data “1” is written in the quantum dots 4a having separation energy corresponding to the wavelength having a small light absorption rate.

(Third embodiment)
The optical semiconductor memory device according to the third embodiment of the present invention has a transparent conductive layer (not shown) in ohmic contact with the undoped Al0.5Ga0.5As layer 3 including the quantum dots 4a in FIG. It is formed at a position sandwiching the 5As layer 3. For example, instead of at least one of the n-type GaAs layer 2 and the p-type Al0.5Ga0.5As layer 5, a transparent electrode made of ITO is formed.

  In the device having this structure, when an electric field E is applied in the lateral direction of the undoped Al0.5Ga0.5As layer 3 around the quantum dot 4a, an energy band as shown in FIG. 6A is obtained. When the quantum dot 4a is irradiated with light having an energy hν1 equal to the separation energy of the quantum dot 4a, hole burning occurs and data “1” is written as shown in FIG.

  When writing data “0”, it is selected not to irradiate the quantum dots 3 a with light.

  The method for reading 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, a metal layer made of WSi is formed to a thickness of, for example, 200 nm instead of the p-type Al0.5Ga0.5As layer 5 of the device shown in the first embodiment, and undoped Al0 is formed. A Schottky barrier is formed at the junction between the 5Ga0.5As layer 3 and the metal layer. The energy band is as shown in FIG. 7, and it can be seen that a Schottky barrier is formed between the metal layer 6 and the Al0.5Ga0.5As layer 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 the electrons hardly return to the quantum dots 4a. It becomes longer than the first embodiment.

  An example of an optical semiconductor memory element having a Schottky barrier is shown in FIG.

In this example, an n-type GaAs layer 8 having an impurity concentration of 5 × 10 ¹⁷ atoms / cm ³ is formed on a semiconductor insulating substrate 7 to a thickness of 50 nm, and a plurality of InAs diameters made of InAs are formed thereon. An undoped Al0.5Ga0.5As layer 3 enclosing the quantum dots 4a is formed to a thickness of 50 nm, and a gate electrode 9 made of WSi is formed thereon to a thickness of 200 nm. In this case, a Schottky barrier is formed at the junction between the gate electrode 9 and the Al0.5Ga0.5As layer 3.

  Further, the source electrode 10 and the drain electrode 11 are resistance-connected to the Al0.5Ga0.5As layer 3 at intervals on both sides of the gate electrode 9. The source electrode 10 and the drain electrode 11 are formed from 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 Al0.5Ga0.5As layer 3 is irradiated with light from the 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, and “1” is determined depending on the magnitude of the source / drain current. "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 spread of the depletion layer from the gate voltage 9, The fact that the drain current is larger than the state in which data “0” is written is used.

  As another method of reading information, a change in the source / drain current in the state where the light of a specific wavelength is not irradiated and in the state where the light is irradiated is applied while a voltage is applied to the gate electrode 9 to cause the source / drain current to flow. May be. In this case, when the drain current increases by irradiating with light of a specific wavelength, electrons move from the quantum dot 4a to the channel region and hole burning occurs, so that data “0” has been written. I understand that.

  The plurality of quantum dots 4a having different diameters have different wavelengths (energy) of light that cause hole burning, so that the data of each quantum dot 4a is read out by changing the wavelength of the irradiation light.

(Fifth embodiment)
An optical semiconductor memory element in which the semiconductor layer having the above quantum dots is provided in part of a bipolar transistor will be described with reference to FIG. The energy band of the optical semiconductor memory element is as shown in FIG.

In FIG. 9, a collector contact layer 13 made of n ⁺ -type GaAs with an impurity dose of 5 × 10 ¹⁸ atoms / cm ^{3 and} an n-type GaAs with an impurity dose of 5 × 10 ¹⁷ atoms / cm ³ are formed on a GaAs substrate 12. A collector layer 14 made of undoped GaAs, a base layer 16 made of p ⁺ -type GaAs with a dose of 5 × 10 ¹⁸ atoms / cm ^3, a second i layer 17 made of undoped AlAs, and a dose An emitter layer 18 made of n-type Al0.5Ga0.5As of 5 × 10 ¹⁷ atoms / cm ³ is sequentially stacked.

  Quantum dots 19 made of InAs are formed on the second i layer 17. Further, regarding the thickness of each layer, the collector contact layer 13 is 300 nm, the collector layer 14 is 200 nm, the first i layer 15 is 300 nm, the base layer 16 is 100 nm, the second i layer 17 is 100 nm, and the emitter layer 18 is 200 nm. It has become.

  In this bipolar transistor, the separation energy Eg 0 of the quantum dots 19 in the AlAsi layer 7 is wider than the band gap Eg 1 of the first i layer 15. In addition, the energy barrier by the second i layer 17 with respect to the base layer 16 is increased so that holes do not easily enter the quantum dots 19 from the base layer 16.

  In FIG. 9, reference numeral 20C indicates a collector electrode, 20B indicates a base electrode, and 20E indicates an emitter electrode.

  Next, a method for writing and reading information on the quantum dots in the bipolar transistor will be described below.

(I) Writing Method At the time of writing information, when a reverse bias voltage is applied between the emitter layer 18 and the base layer 16, the energy band becomes as shown in FIG. And the energy barrier B on the emitter layer 18 side of the quantum dots 19 becomes thin. For this reason, the movement of electrons by the tunnel from the quantum dot 19 to the emitter layer 18 is facilitated, and the data “1” is easily written. In this case, no reverse bias voltage is applied between the collector layer 14 and the base layer 16 or a forward bias voltage is applied to suppress the generation of current due to light absorption in the first i layer 15. Is preferred. Note that the data “0” is written by not irradiating light.

(Ii) Reading Method When reading information, a bias voltage is not applied between the emitter layer 18 and the base layer 16. As a result, the energy barrier B on the emitter layer 18 side of the quantum dot 19 becomes thick, so that even if an electron / hole pair is generated in the quantum dot 19 due to the irradiation of the reading light, the electrons hardly move to the emitter layer 18 side. Therefore, recombination becomes easy and hole burning does not occur.

  For this reason, the data “0” is not destroyed by the reading light. Note that the data “1” is that the quantum dot 19 is in the hole burning state, and therefore the data is not destroyed by light irradiation.

  In reading information, it is preferable to apply a reverse bias voltage between the collector layer 14 and the base layer 16 as shown in FIG. Thereby, when data “1” is written, the light for reading passes through the quantum dot 19 and is absorbed by the first i layer 15. This absorption generates electron / hole pairs in the first i layer 15, but electrons move to the collector layer 14, 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 amount of light absorption is large, or when the collector-base current is large, data “1” is written. In this case, holes move to the base layer 16, but a high hole barrier exists between the base layer 16 and the emitter layer 18, and holes are not easily tunneled. The data “0” is not changed to data “1” by moving to the quantum dot 19.

  When 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 is reduced. Due to this decrease, the photocurrent flowing through the first i layer 15 is small.

  Therefore, when the amount of light absorption is small, or when the collector-base current is small, data “0” is written.

  Since written information may change, a bias is not applied between the collector and the base, or a forward bias is applied so that no photocurrent flows. 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 varies depending on the difference in the amount of light absorption. Can be detected.

  According to this example, since reading and writing can be performed independently, refresh 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 band gap smaller than the separation energy of the quantum dots 19 to generate a photocurrent.

  Further, when the interband energy of the first i layer 15 is larger than the separation energy of the quantum dots 19, the change in the collector current and the light absorption rate are the same as shown in the first or second embodiment. Read data by change.

  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 increases, and the response speed increases. Thereby, it has gain with respect to light, and S / N ratio can be enlarged.

  In this embodiment, an npn bipolar transistor is used, but a quantum dot may be included in a part of the pnp bipolar transistor.

(Sixth embodiment)
Next, an optical semiconductor memory element in which a layer having quantum dots is formed on a part of a pin junction photodiode will be described with reference to FIGS. 12 (a) and 12 (b).

  12A and 12B, on a semi-insulating substrate 21 made of GaAs, an n-type GaAs layer 22 having a film thickness of 300 nm, an undoped In0.1Ga0.9As layer 23 having a film thickness of 100 nm, and a film thickness of 200 nm. The p-type AlAs layers 24 are sequentially formed. A plurality of quantum dots 25 made of GaAs or InGaAs having different sizes are formed in a region of 10 nm in the center of the p-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 impurity concentration of the n-type GaAs layer 22 is impurity ^{5 × 10 18 atoms / cm 3} , p -type AlAs layer 24 is ^{5 × 10 18 atoms / cm 3} .

  A pin photodiode having such quantum dots 25 has an energy band structure as shown in FIG. Since there are a plurality of quantum dots 25 having different sizes in the p-type AlAs layer 24, the wavelength of light for generating electron-hole pairs depends on the size of the quantum dots 25 as already described in the above embodiment. In addition, each quantum dot 25 absorbs only light having a wavelength corresponding to each separation energy.

  Moreover, in the quantum dot 25 that has absorbed light, hole burning occurs and holes are confined in the quantum dot 25. Therefore, when light of the same wavelength is irradiated again, the light is emitted from the quantum dot 25. Transparent.

  Here, since the undoped In0.1Ga0.9As layer 23 has a smaller energy band gap than the material of the quantum dots 25, the light transmitted through the quantum dots 25 is absorbed by the undoped In0.1Ga0.9As layer 23. Then, here, like a general pin photodiode, an electron / hole pair is generated, and a photocurrent flows. 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. In addition, 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 currents flowing through the p-side electrode 26 and the n-side electrode 27.

  Since the quantum dots 25 are in the hole burning state after the stored information is read out, refreshing is necessary.

(Seventh embodiment)
In the present embodiment, an optical semiconductor memory element in which quantum dots are formed at positions different from those in the sixth embodiment will be described.

In FIG. 13, on a semi-insulating substrate 31 made of GaAs, an n ⁺ -type GaAs layer 32 having a thickness of 300 nm, an undoped In0.1Ga0.9As layer 33 having a thickness of 100 nm, and a p-type AlAs layer 34 having a thickness of 200 nm. Are formed in order.

  Further, an undoped AlAs layer 35 is formed to a thickness of 50 nm in a region surrounded by the annular p-side electrode 26 on the p-type AlAs layer 34, and GaAs of different sizes is included 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 of the p-type AlAs layer 34 is 5 × 10 ¹⁸ atoms / cm ³ .

  In this embodiment, information is written and read in the same way as in the sixth embodiment.

(Eighth embodiment)
A 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.

  For example, a surface emitting semiconductor laser may be used as the light emitting element, and the pin photodiode of the sixth embodiment may be stacked thereon. The energy band diagram is as shown in FIG.

  An n-type cladding layer 41, an undoped active layer 42, and a p-type cladding layer 43 are sequentially stacked on a GaAs substrate (not shown), and a first reflective film (not shown) is formed below the GaAs substrate. A second reflective film (not shown) is formed on the p-type cladding layer 43, thereby constituting the semiconductor laser 39. Further, the optical semiconductor memory element 40 is formed on the second reflective film.

  For example, an In0.5 (Ga1-xAlx) 0.5P layer is used as the cladding layers 41 and 43, and an In0.5 (Ga1-yAly) 0.5P layer is used as the active layer 42 (x> y).

  The optical semiconductor memory element 40 on the p-type cladding layer 43 has the same layer structure as that of the sixth embodiment, and includes a p-type AlAs layer 24 having quantum dots 25, an undoped In0.1Ga0.9As layer 23, n It comprises a type GaAs layer 22.

(Ninth embodiment)
In the present embodiment, an optical semiconductor memory element using an npn junction phototransistor as a basic structure will be described as an example 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 formed in this order on a GaAs substrate 45, and an annular n-type is formed on the base layer 48. A type GaAs emitter layer 50 and an emitter electrode 51 are formed. A plurality of quantum dots 49 made of GaAs of different sizes are formed in part or all of the base layer 48. A collector electrode 52 is connected to the collector layer 46.

  In such an element, since the band gap of the subcollector layer is smaller than that of the quantum dot, light transmitted through the quantum dot is irradiated to the subcollector layer to form an electron / hole pair and a collector current flows. .

  When forming a bipolar transistor together on one GaAs substrate 45, two elements having the structure shown in FIG. 15A are formed, and one of the elements is formed on the base layer 48 as shown in FIG. When such a metal base electrode 53 is formed, the base layer 48 is shielded from light by the base electrode 53, so that the element functions as a normal bipolar transistor. As the bipolar transistor, a multi-emitter type 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 50 is formed of n-type AlAs. The energy band gap in this case is as shown in FIG.

  When the quantum dots 49 in the emitter layer 50 are in the hole burning state, the light irradiated 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 by light irradiation in the base layer 48, and the holes in the base layer 48 become excessive. The current changes.

  In this embodiment, since quantum dots are provided in the phototransistor, when hole burning occurs in the quantum dots, electrons and holes generated between the base layer and the collector layer by the light for reading are amplified. As a result, the photocurrent increases.

  In addition, in order to reset (initialize) the information of quantum dots in which hole burning has occurred, the amount of light with an energy equal to the separation gap of the quantum dots is increased to generate electrons generated 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, thereby resetting the information.

  For light-receiving elements such as photodiodes and phototransistors, if a large number of quantum dots of different sizes are formed, light with energy equal to the separation gap of each quantum dot is selected and writing of the desired quantum dot is performed. , Reading becomes possible. That is, there are as many storage cells as the number of quantum dots, and addressing corresponding to the wavelength of light becomes possible, so that the storage capacity can be greatly increased.

(Tenth embodiment)
In the present embodiment, an optical semiconductor memory element configured by applying quantum dots to a multi-emitter heterojunction bipolar transistor (HBT) will be described below.

In FIG. 18, on a GaAs substrate 61, a collector layer 62 made of n ⁺ -type GaAs with a thickness of 200 nm, a subcollector layer 63 made of undoped GaAs with a thickness of 100 nm, and p-type GaAs with a thickness of 50 nm. A base layer 64 and an emitter layer 65 made of n-type In0.5Ga0.5P having a thickness of 50 nm are formed. A plurality of emitter contact layers 67 and 68 made of n ⁺ -type GaAs with a thickness of 200 nm are formed on the emitter layer at intervals (two in the figure), and emitter electrodes 69 and 70 are formed thereon. Has been. 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 does not have a base electrode. When the voltages of the emitter electrodes 69 and 70 are equal, no current flows through the collector layer 62. When the voltages of the emitter layers 69 and 70 are different, the collector layer The operation is such that a current flows through 62. Note that 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, when reading the information stored in multiple quantum dots of different sizes or writing information to the quantum dots, light of a wavelength corresponding to the separation energy of each quantum dot is continuously scanned. Then, a plurality of quantum dots may be irradiated, or light having different wavelengths may be irradiated simultaneously. That is, since the separation energy of the quantum dots depends on the size, growth temperature, etc., information is written and read by irradiating each quantum dot with light having a specific wavelength. In the quantum dot, hole burning does not occur due to light having a wavelength other than the intrinsic wavelength. Therefore, no error occurs even when light of different wavelengths is irradiated simultaneously or continuously.

  FIG. 19A shows a light absorption saturation characteristic when light of different wavelengths is continuously scanned when reading stored information. In FIG. 19 (a), hole burning is caused by light irradiation in the quantum dots having the separation energy corresponding to the wavelength having a reduced light absorption coefficient. When hole burning occurs, the photocurrent increases due to electrons that have escaped 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. 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. Can do. From this, it can be seen which wavelength should be re-input to 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 memory cell corresponds to the optical semiconductor memory element of the above embodiment.

  In this case, as shown in FIG. 20, the first electric wirings E1 to E4 are arranged in the X-axis direction, the second electric wirings C1 to C4 are arranged in the Y-axis direction, and the electric wirings intersect. The two electrodes 73 and 74 of the memory cell 72 are connected in the vicinity of the portion. As these two electrodes, there are an n-side electrode and a p-side electrode or an emitter electrode and a collector electrode of the above-described optical semiconductor memory element.

  In addition, the light hv11 to hv44 irradiated when writing or reading the memory cells 72 is provided with optical wirings (not shown) arranged on the first and second electric wirings E1 to E4 and C1 to C4. To do. As the optical wiring, for example, there is an optical shutter arranged in a pixel in an image display device. When light hv11 to hv44 having different wavelengths is simultaneously irradiated onto the storage cell 72 and information is written or read at a time, all the storage cells 72 may be irradiated with one spot-like light.

  According to such wiring, since electrical wiring and optical wiring are used, efficient wiring is possible without interference of light and electrical signals.

  When electrons are injected into a quantum dot in which only holes are confined and the electrons and holes are recombined within the quantum dot, the hole burning of the quantum dot disappears and the data is reset to “0”. By injecting electrons into the quantum dots at once, the processing speed is increased and simplified.

  As a method for 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. In the case of a multi-emitter type HBT as shown in the tenth embodiment, when electrons are supplied to a quantum dot by passing a current between a plurality of emitters, hole burning does not occur in the quantum dot.

  As described above, when any of the first electric wirings E1 to E4 and any of the second wirings C1 to C4 is selected and a current is passed through the memory cell 72, the selected optical semiconductor memory element cannot be written. become. This means that an optical semiconductor memory element can be selected.

  If a memory cell is selected and written, light with a small spot diameter for selecting the memory cell becomes unnecessary, and light may be irradiated to a plurality of memory cells 72 at once.

(13th 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 an energy higher than the band gap Eg01 of the base layer including the quantum dots is irradiated, the accumulation of holes in the valence band of the base layer is promoted. As a result, the potential of the base layer decreases 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 quantum dot data is set to “0”. When a plurality of quantum dots exist in one base layer, the data of all quantum dots becomes “0” by the light irradiation.

  When a plurality of quantum dots having different sizes exist in one base layer, as shown in FIG. 21 (b), a quantum dot having the largest size is selected and is equal to or higher than the separation energy of the quantum dots and The base layer is irradiated with light in a wavelength band corresponding to energy 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 data of all quantum dots is set to “1”.

  Here, the reason why the largest quantum dot was selected is that the ground level of this quantum dot is smaller than the ground level of other quantum dots.

(14th Embodiment)
A plurality of quantum dots constituting the above-described 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 size in the surface direction of the quantum dots 82a to 82e is aligned. The size in the thickness direction may be varied. In this case, the quantum dots 82a to 82e are stacked in the order of the diameters, or the quantum dots 82a to 82e are stacked in the order of the diameters, or the quantum dots 82a to 82e are set so that the diameters are irregular. 82e may be stacked.

  As shown in FIG. 23, the size of the quantum dots 82a to 82e decreases as the growth temperature increases, and the density increases as the growth temperature increases. Thereby, the structure as shown in FIG. 22 is obtained by adjusting the growth temperature of the semiconductor constituting the quantum dots 82a to 82e.

  By controlling the number of sizes of the plurality of quantum dots 82a to 82e, the number of bits in one semiconductor layer 81 can be adjusted.

  A technique for changing the size of the quantum dots in this way is described in J. Oshinowo et. Al., Appl. Phys. Lett. 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 constituting the quantum dot. For example, in the process shown in FIGS. 1A to 1C, when the thickness of the InAs layer 4 that becomes the quantum dots 4a is changed, there is a relationship as shown in FIG. In FIG. 24, the supply amount of InAs is shown by the thickness of InAs.

  According to FIG. 24, it can be seen that the quantum dot size decreases as the supply amount of InAs is increased when the supply amount is 1.5 to 2.3 ML.

  In this way, the diameters of the plurality of quantum dots arranged in the film thickness direction are made different from each other, and an n-type GaAs substrate is used as the semiconductor substrate 80 and an AlAs or AlGaAs layer as the semiconductor layer 81 as shown in FIG. When InAs or InGaAs dots are used as the quantum dots 82a to 82e, and an electrode 83 in Schottky contact with the semiconductor layer 81 is formed on the semiconductor layer 81, a band diagram as shown in FIG. 25 is obtained. . In this case, the quantum dots 82a to 82e are stacked in order of increasing diameter.

  Information writing and reading methods of the optical memory element having this structure are as follows.

  First, a case where information is written will be described.

  When the uppermost quantum dot 82e is irradiated with light of a specific wavelength to generate electron-hole pairs and the electrons are emitted to the outside of the quantum dot 82e by the tunnel effect, the electrons are transferred between the electrode 83 and the semiconductor substrate 80. It moves to the semiconductor substrate 80 by the electric field. However, a voltage is applied between the electrode 83 and the semiconductor substrate 80, and the voltage of the semiconductor substrate 80 is higher than that of the electrode 83.

  When the lower quantum dot 82b is already irradiated with light and “1” is written before the electron reaches the semiconductor substrate 80, the electron moved from the uppermost quantum dot 82e is moved to the lower quantum dot 82e. There is an inconvenience that information in the lower quantum dot 82b is rewritten to “0” by recombination within 82b.

  Therefore, in the case of an optical memory element having quantum dots 82a to 82e having a multilayer structure, it is preferable to write information in order from the quantum dots on the lower potential side. In FIG. 22A, information is written sequentially from the side farthest from the semiconductor substrate 80.

  Next, a case where information is read will be described.

  Assuming that information “0” is written in the uppermost quantum dot 82e, when information is read by irradiating the quantum dot 82e with a specific wavelength, electrons are emitted from the quantum dot 82e to the outside. In this case, the electrons may recombine in the lower quantum dot 82b in which the information “1” is written, and the information may be rewritten to “0”.

  Therefore, in the case of an optical memory element having the quantum dots 82a to 82e having a multilayer structure, it is preferable to sequentially read information from the quantum dots on the higher potential side. In FIG. 22A, information is read sequentially from the side closest to the semiconductor substrate 80.

  However, after reading the information of all the quantum dots 82a to 82e, it is necessary to reset and rewrite all the quantum dots 82a to 82e. That is, a current is passed through the Schottky barrier in the forward direction, and all stored information is reset to “0” by the electronic 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. In this case, the reset is performed by passing a current in the forward direction of the pn junction.

  As shown in FIG. 26, when an undoped semiconductor layer 84 made of GaAs is interposed between an n-type semiconductor substrate 80 and a semiconductor layer 81 and a high electric field is applied to the undoped semiconductor layer 84, an avalanche is already described. By decay, electrons are amplified and a large signal can be obtained.

  Note that light to the optical memory element is irradiated from the side of the semiconductor layer 81 or from above using the electrode 83 made of a transparent conductive material.

(Fifteenth embodiment)
The following method is employed for writing and reading information in an optical memory having a quantum dot having a multilayer structure as shown in FIG.

  As shown in FIG. 27, the writing of information may be performed by simultaneously irradiating composite light of energy hv01 to hv05 equal to the transition energy of each quantum dot 82a to 82e, or a plurality of energy hv01 as shown in FIG. ˜hv05 light may be individually applied to each quantum dot 82a to 82e in time series. Since each quantum dot 82a to 82e can be written only by irradiation with light having the same energy as the transition energy, any method can be employed. However, when irradiating light in time series, as shown in the fourteenth embodiment, it is preferable to write information sequentially from the quantum dots on the lower potential side.

  Here, h is the Planck constant, and v is the light frequency.

  Information is read out by discriminating information “0” and “1” according to the amount of change in the current flowing through the semiconductor substrate 80. Therefore, as shown in FIG. 27, light of a plurality of energies hv01 to hv05 is individually and sequentially arranged in time series. This is performed by irradiating the quantum dots 82a to 82e. However, as described in the fourteenth embodiment, it is preferable to determine the irradiation order of light so 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 irradiated in order, the detected current flowing through the semiconductor substrate 80 and the electrode 83 changes in time series according to the light pulse irradiation timing. Is detected.

  The energy difference (transition energy) Eg01 between the conduction band and the valence band in the quantum dot is equal to hv01. Similarly, Eg02 is equal to hv02, Eg03 is equal to hv053, Eg04 is equal to hv04, and Eg05 is equal to hv015.

(Sixteenth embodiment)
In an optical memory element in which quantum dots are multi-layered, a structure as shown in FIG. 29A is adopted in order to prevent hole tunneling among electron / hole pairs generated in the quantum dot by light irradiation.

  29A, the same reference numerals as those in FIG. 22 denote the same elements.

  In this case, the second barrier layer 85 made of a material having a high band offset of the valence band Ec and a small band offset of the conduction band Ev is disposed on the low side of the externally applied electric field among the quantum dots 82a to 82e.

  For example, when the second barrier layer 85 is made of In0.5Ga0.5P, the band offset of the conduction band is 0.1 eV and the band offset of the valence band is 0.5 eV. In this case, an n-type GaAs substrate is used as the semiconductor substrate 80, an Al0.5Ga0.5As layer is used as the semiconductor layer 81 serving as the first barrier layer, InAs dots are used as the quantum dots 82a to 82e, and further, the semiconductor When the electrode 83 in Schottky contact with the layer 81 is formed from tungsten, the band diagram is as shown in FIG.

  As is clear from FIG. 29 (b), the holes remaining in the quantum dots 82a after the generation of the electron / hole pairs are blocked by the second barrier layer 85 and accumulated. There is no adverse effect. In addition, since the 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 surrounding semiconductor layers in the embodiments described above may be any of III-V semiconductors, II-IV semiconductors, Si, Ge, SiGe, etc. It is not limited to.

  In order to configure an optical semiconductor memory integrated circuit, not only the above-described 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 light is not irradiated to a semiconductor layer including quantum dots. In the case where such an element functions as a switching element, a light-shielding film may be formed in the semiconductor layer including quantum dots. For example, a light shielding film may be formed at a position indicated by a one-dot chain line on the semiconductor layer 17 of the element shown in FIG. 9 and used as a normal bipolar transistor.

It is sectional drawing which shows the formation method of the optical semiconductor memory element used for 1st Embodiment of this invention. It is sectional drawing which shows the state from which the magnitude | size of the quantum dot used for 1st Embodiment of this invention differs. It is an energy band diagram of the optical semiconductor memory element used for 1st Embodiment of this invention. It is an energy band diagram in the case of the information holding | maintenance of the optical semiconductor memory element based on 2nd Embodiment of this invention, or information reading. It is a figure which shows the relationship between the wavelength of the light at the time of the information reading of the optical semiconductor memory element concerning 2nd Embodiment of this invention, and the light absorptance of a quantum dot. It is an energy band diagram of the optical semiconductor memory element which concerns on 3rd Embodiment of this invention. It is an energy band diagram of the optical semiconductor memory element which has a Schottky barrier concerning a 4th embodiment of the present invention. It is sectional drawing which shows the optical semiconductor memory element which concerns on 4th Embodiment of this invention. It is sectional drawing which shows the optical semiconductor memory element which concerns on 5th Embodiment of this invention. The structural energy band diagram of the optical semiconductor memory element which concerns on 5th Embodiment of this invention is shown. The energy band diagram at the time of the writing of the information of the optical semiconductor memory element concerning 5th Embodiment of this invention at the time of reading is shown. Sectional drawing, the top view, and the energy band diagram which show the optical semiconductor memory element concerning 6th Embodiment of this invention are shown. It is sectional drawing which shows the optical semiconductor memory element which concerns on 7th Embodiment of this invention. It is an energy band diagram of the optical semiconductor memory element which concerns on 7th Embodiment of this invention, and the light emitting element connected to this. It is sectional drawing and the energy band diagram of the optical semiconductor memory element concerning 8th Embodiment of this invention. It is sectional drawing which shows the bipolar transistor comprised by covering the optical semiconductor memory element which concerns on 8th Embodiment of this invention with the light shielding film. It is an energy band diagram at the time of forming the quantum dot in the optical semiconductor memory element concerning 8th Embodiment of this invention in an emitter area | region. It is sectional drawing which shows the optical semiconductor memory element concerning 10th Embodiment of this invention. It is a light absorption saturation characteristic figure and branching sensitivity characteristic figure at the time of scanning continuously the light from which the wavelength used for the reading of the optical semiconductor memory element which concerns on 10th Embodiment of this invention, and writing differs. It is a circuit diagram which shows the optical semiconductor memory device based on 12th Embodiment of this invention. It is the energy band diagram which shows the state which cancels the hole burning of the optical semiconductor memory element based on 13th Embodiment of this invention, and the energy band diagram which shows the energy range which produces a hole burning. It is sectional drawing which shows the optical semiconductor memory element concerning 14th Embodiment of this invention. It is a figure which shows the size of the quantum dot of the optical semiconductor memory element shown in FIG. 22, the relationship between density, and growth temperature. It is a figure which shows the relationship between the size of a quantum dot, and InAs supply amount in the process of forming the quantum dot which consists of InAs of the optical semiconductor memory element concerning 14th Embodiment of this invention. It is an energy band diagram of the optical semiconductor memory element concerning 14th Embodiment of this invention. It is sectional drawing which shows the structure which interposed the undoped semiconductor layer for producing avalanche collapse to the optical semiconductor memory element concerning 14th Embodiment of this invention. It is an optical waveform diagram which shows simultaneously irradiating the several light from which the energy differs for the writing of the information to the optical semiconductor memory element concerning 15th Embodiment of this invention. FIG. 14 is an optical waveform diagram showing simultaneous irradiation with a plurality of lights having different energies for writing or reading information to / from the optical semiconductor memory element according to the fifteenth embodiment of the present invention; It is a wave form diagram which shows an example. It is sectional drawing which shows the optical semiconductor memory element concerning 16th Embodiment of this invention, and its energy band diagram. It is an energy band diagram of the conventional optical semiconductor memory element.

Explanation of symbols

2 n-type GaAs layer (semiconductor layer)
3 Al0.5Ga0.5As layer (semiconductor layer)
4a Quantum dot 5 p-type Al0.5Ga0.5As 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 In0.1 Ga0.9As 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 memory device 47 Subcollector layer 48 Base layer 49 Quantum dot 50 Emitter layer 64 Base layer 65 Emitter layer 66 Quantum dots 67 and 68 Emitter contact layer 81 Semiconductor layers 82a to 82e Quantum dots 85 Barrier layer

Claims (9)

Irradiating light to the quantum dots formed in the semiconductor layer, and irradiating 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, causing hole burning. Write information and
An optical semiconductor memory device writing / reading method for reading the information by a change in current flowing from the semiconductor layer by irradiating the quantum dots with light of the wavelength,
The semiconductor layer is formed between an n-type semiconductor layer and a p-type semiconductor layer , a reverse bias voltage is applied to the n-type semiconductor layer and the p-type semiconductor layer at the time of writing, and at the time of reading. A write / read method for an optical semiconductor memory device, wherein a reverse bias voltage is applied to the n-type semiconductor layer and the p-type semiconductor layer. Irradiating light to the quantum dots formed in the semiconductor layer, and irradiating 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, causing hole burning. Write information and
An optical semiconductor memory device writing / reading method for reading the information by the amount of light passing through the semiconductor layer by irradiating the quantum dots with light of the wavelength,
The semiconductor layer is formed between an n-type semiconductor layer and a p-type semiconductor layer , a reverse bias voltage is applied to the n-type semiconductor layer and the p-type semiconductor layer at the time of writing, and at the time of reading. A write / read method for an optical semiconductor memory device, wherein a bias voltage is not applied or a forward bias voltage is applied to the n-type semiconductor layer and the p-type semiconductor layer. A plurality of the quantum dots having different energy differences between the ground levels are included, and when reading the information, light of a plurality of wavelengths corresponding to the energy differences is continuously changed to irradiate the quantum dots writing and reading method for an optical semiconductor memory device according to any one of claims 1 to 2, characterized in that. It has a plurality of the quantum dots having different energy differences between the ground levels, and simultaneously irradiates light of a plurality of wavelengths corresponding to these energy differences when writing the information and reading the information. 3. The method of writing and reading data in an optical semiconductor memory device according to claim 1, 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, information is written to the quantum dots in the order in which the carriers move by light irradiation. 4. The method of writing and reading data in an optical semiconductor memory device according to claim 3 . The plurality of quantum dots having different energy differences between the ground levels are arranged in the film thickness direction. When reading the information, information is transferred to the quantum dots in the order opposite to the carrier moving direction by light irradiation. 4. The method of writing and reading data in an optical semiconductor memory device according to claim 3, wherein: Optical semiconductor memory device according to any one of claims 1 to 2, characterized in that initializing the information to produce an electric field to supply electrons to the quantum dot semiconductor layer having the quantum dots Write / read method. Writing and reading method for an optical semiconductor memory device according to any one of claims 1 to 2, characterized in that prohibits writing of the information by supplying a current to the semiconductor layer having the quantum dots. There are a plurality of quantum dots in the semiconductor layer, and the energy width is larger than the smallest energy difference between the ground levels among the plurality of quantum dots and has an energy width in a range smaller than the energy band gap of the semiconductor layer. 3. The method of writing and reading data in an optical semiconductor memory device according to claim 1, wherein the quantum dots are irradiated with light so that all of the quantum dots are in a hole burning state.

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