JPH0524640B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor device, and particularly to a semiconductor optical device exhibiting optical phenomena such as electroluminescence.

[Conventional technology]

An electroluminescent device is a device that excites electrons by a method other than pure thermal radiation, resulting in the emission of light. This includes those that emit light due to in-band transition, and those that emit light due to intra-band transition. Since excitons are generated along with bands, in this specification, exciton transition is also considered as a type of interband transition. Below, we will discuss electroluminescence that utilizes interband transitions.

In semiconductors such as - group (GaAs, etc.), which have a relatively narrow band gap and are easy to form pn junctions by adding impurities, light-emitting diodes are known in which minority carriers are injected through the pn junction and recombined with majority carriers by light emission. ing. A semiconductor laser that emits coherent light can also be considered a type of light emitting diode.
Particularly in semiconductor lasers, not only homojunctions but also heterojunctions are increasingly used. In addition, molecular beam (MB) method organometallic (MO) CVD
Techniques for forming a so-called superlattice by methods such as the method are also known. Various techniques have also been proposed to increase the transition related to light emission.

In semiconductors with wide band gaps, such as - group semiconductors, it is generally difficult to control the p-type and n-type dielectric types. EL using these semiconductors
Known devices include those in which powdered EL materials such as zinc sulfide (ZnS) are solidified and sandwiched between electrodes. It is thought that the light emission phenomenon occurs when electrons are excited from a low energy level to a high energy level due to the action of a strong electric field, and then fall back to a low energy level. The luminous efficiency of such EL light emission depends on the square of the amplitude of the wave function before and after the transition related to light emission, and the transition probability of that transition. In other words, it depends on the number of states (carriers) and the likelihood of transitions.

In order to obtain strong light emission, in light emitting diodes (semiconductor lasers), minority carriers are injected across the pn junction, and amphoteric carriers are further confined in the active layer to cause radiative recombination. However, this method is difficult to apply to materials in which it is difficult to form a suitable pn junction. It is difficult to increase carrier density locally without using energy barriers. In an EL device using a powdered EL material, even if carriers are excited, the carriers are localized and it is difficult to increase the carrier density. Furthermore, undesired levels are likely to occur due to grain boundaries, etc., and it is not easy to make desired transitions occur preferentially.

Even in light absorption, when many levels coexist, it becomes difficult to perform absorption in a desired wavelength spectrum. When producing wavelength conversion elements, high-pass, low-pass, and other color filters using semiconductors, it is desirable to control the optical properties of the materials. It is also required that the manufacturing cost is not too high. For this reason, semiconductors are not often used in these devices, except for a few devices that utilize properties specific to semiconductors.

[Summary of the invention]

An object of the present invention is to provide a semiconductor optical device that can realize high transition efficiency near a potential barrier by using a potential barrier and an electric field.

When quantum wells for electrons and holes are formed on both sides of a potential barrier, the probability of transition between the valence band and the conduction band does not increase because the two quantum wells do not overlap locally. However, when a strong electric field is applied near the potential barrier, the wave function penetrates into the potential barrier, causing local overlap of the wave functions and increasing the transition probability.

The potential barrier can be formed by a heterojunction. Heterojunctions tend to be strained unless lattice matched, and strain tends to cause lattice defects and other unfavorable effects on optical devices, such as an increase in non-radiative recombination centers. However, even if materials with different lattice constants are used, stacking ultrathin films can increase flexibility against lattice mismatch. In particular, when the superlattice structure has a certain periodicity, the crystallinity and optical properties are significantly improved.

[Structure and operation of the invention]

By using a heterojunction of different materials in the bandgap, a potential barrier can be formed at the junction. The potential barrier for electrons and the barrier for holes may occur on the same side of a heterojunction (hereinafter referred to as type) or on different sides (hereinafter referred to as type). The invention uses a type of heterojunction.

FIG. 1A shows a band model of a type of heterojunction in the absence of an electric field (E=0). The vertical axis indicates energy e, and the horizontal width indicates location x. Substance A and substance B form a heterojunction, with substance A forming a potential barrier to electrons, and substance B forming a potential barrier to holes.
Substances A and B are both intrinsic or nearly intrinsic semiconductors. Free electrons existing in the conduction band CB of substance B cannot move to the left of the heterojunction because of the potential barrier formed by substance A. Valence band VB of similar substance A
Because of the potential barrier formed by substance B, the free holes existing in the heterojunction cannot move to the right. The potential barrier that limits the movement of such carriers has a height of at least kT (approximately 25 mV), preferably several kT,
More preferably, it has a height of about 10 kT (about 250 mV) or more. Since material A and material B are separated in location, there is almost no overlap in the wave functions of free electrons and free holes, and the value of the transition matrix is extremely small. Therefore, the valence band VB of substance A
Even if an energy relative to the energy difference E _gx between the material B and the conduction band CB is supplied from the outside, the probability that electrons will be excited from the valence band VB to the conduction band CB is small. Furthermore, even if a hole is generated in the valence band VB and an electron is generated in the conduction band CB by excitation or the like, the probability that the electron-hole will be recombined is low.

FIG. 1B shows a band model when a forward bias is applied to the heterojunction shown in FIG. 1A. That is, the polarity is positive on the substance A side with respect to the substance B side.

Both substance A and substance B are intrinsic or almost intrinsic, and the bending of the band (formation of a depletion layer) due to the sweeping of free carriers is negligible. Therefore, the electric field is shown as being generated uniformly. When electrons are generated in the conduction band, they are concentrated in a low potential area near the heterojunction due to the action of the electric field. Similarly, holes are also collected at a location near the heterojunction where the hole potential is low (the location where the energy e is high in the figure). The movement of these free carriers is due to drift due to the electric field, and occurs faster than the movement due to diffusion.

Furthermore, under the influence of the electric field, the wave functions of electrons and holes penetrate across the heterojunction and into the potential barrier.
For this reason, the wave functions overlap in the tail portion, and the transition probability (value of the transition matrix) increases.

In this way, the density of electrons and holes involved in transition increases, and the probability of transition increases, resulting in an increase in luminous efficiency.

Generation of free carriers may be achieved by a strong electric field near the heterojunction, or may be injected externally. For example, a heterojunction may be sandwiched between insulating layers on both sides, and soft breakdown may occur in the insulating layers.

As a material that forms a type of heterojunction that creates quantum wells for electrons and holes at separate locations, - group cubic or hexagonal materials are known. For example, substance A is ZnTe, and substance B is a ZnS-ZnSe mixed crystal system (including substances at both ends). Although it is preferable that substance A and substance B are single crystal, they may be polycrystalline. The electric field strength may be any strength as long as it achieves the desired effect, but typically
It is on the order of 10 ⁶ V/cm.

Examples of device structures are shown in FIGS. 1C and 1D. In FIG. 1C, a first layer 2 of material A and a second layer 3 of material B are laminated on a substrate 1, and insulating layers 5, 6
Electrodes 7 and 8 are provided on both sides via. At least one of the electrodes 7 and 8 (preferably the upper electrode 8) is made of a transparent electrode such as indium-stannic acid (ITO) or thin film gold in order to transmit light. A DC or AC voltage source 9 is connected between both electrodes.
Although the voltage source 9 forms an electric field, it can also serve as a carrier excitation function at the same time. The substrate 1 may be a - group semiconductor like the materials A and B, but may also be other materials such as Si, Ge, GaAs, and GaP. Preferably, the specific resistance of the substrate 1 is negligible compared to the specific resistance of the layers 2 and 3. If layer 2 is of the same material as substrate 1, layer 2 may be built into the substrate. The insulating layers 5 and 6 are made of, for example, SiO ₂ ,
It may be made of a known insulating material such as Si ₃ N ₄ , Al ₂ O ₃ or PIQ.

The configuration of FIG. 1D is similar to that of FIG. 1C except that the substrate 1 is omitted. After forming layers 2 and 3, the substrate may be etched off, and layers 5, 2, 3, 6 and 8 may be sequentially formed on electrode 7. When the substrate 1 is not used, there is a large cost advantage.

Carrier excitation may be performed by voltage source 9, but
This may be performed by applying high frequency energy or irradiating high energy electromagnetic waves (near ultraviolet rays, etc.).

When used as an optical element such as a wavelength conversion filter or a color filter, a carrier excitation means is not required.
The configuration of FIG. 1D is particularly suitable for back-illuminated, front-emitting modes of operation.

The above description can also be applied to the following embodiments unless otherwise specified.

When using a single heterojunction, there is a problem in terms of lattice matching, in addition to the fact that only one plane emits light, and if the lattice constants do not match. It is effective to solve this problem by repeatedly forming a large number of heterojunctions by forming a multilayer structure of ultra-thin films, each layer having a thickness of about several hundred Å or less.

FIGS. 2A and 2B show band models in the case of a multilayer structure. FIG. 2A shows the case without an electric field, and FIG. 2B shows the case with an electric field. Material A and material B are alternately stacked, and each layer of material A forms a quantum well for holes, and each layer of material B forms a quantum well for electrons. For example, material A is ZnTe, material B is
ZnS _x Se _1-x (x=0-1), and each layer has a thickness of several Å to several hundred Å. When the thickness of the layer is reduced, the quantum well will create a certain level due to boundary conditions. The ground levels of electrons and holes are shown by broken lines, respectively. In the absence of an electric field, the wave functions of these ground levels spread widely within the quantum well and hardly extend outside the quantum well. Therefore the conduction band (electron)
The wave function ψc and the wave function ψv of the valence band (hole)
There is only a slight spatial overlap with , and the transition probability is low. When an electric field is applied as shown in FIG. 2B, the wave functions for electrons and holes are concentrated at lower energy levels. When electron-hole pairs are generated, the electrons and holes drift to a region with a lower energy level. Because of the alternating quantum wells, free carriers generated within the multilayer are efficiently collected near the forward biased heterojunction. These high-density electron-holes recombine efficiently using the overlap of wave functions. Therefore, luminous efficiency is improved.

Now, when an electron positive photo pair is generated with light having an energy greater than the energy equivalent to E _g1 , light with an energy equivalent to E _gx is generated by recombination. In other words, it works as a wavelength conversion filter.

When a strong electric field causes a collision to generate electron-hole pairs, it works as an electric field EL device.

Also, if recombination is non-luminescent, long wavelength transmission (low pass) that absorbs holes with energy above a certain level
Works as a filter.

The ground state energy shown in Figure 2A is the physical property of substances A and B (electron affinity, etc.)
It depends not only on the boundary conditions of the quantum well but also on the boundary conditions of the quantum well. Therefore, the energy difference E _gx between levels can also be controlled by controlling the layer thickness or the like.

FIG. 2C shows an example of a device structure. substance A,
The structure is similar to that of FIG. 1D except that layers 2 and 3 of B are multilayered. A substrate as shown in FIG. 1C may also be used.

Multilayering of ultra-thin films is also very beneficial in improving the crystallinity of the layers. When there is lattice constant mismatch, lattice defects are likely to occur if the layers are thick, but if each layer is made ultra-thin, the crystal lattice of the material itself deforms and can absorb strain. For example, when an ultra-thin film of a hexagonal material is deposited on a cubic substrate, the hexagonal material adopts a cubic-like structure and matches its lattice. In this regard, the 17th ICPS (San Francisco, 1984)
The present inventors have made a report in the Communication of Applied Physics (March 1985), etc. (August 1985). This lattice self-alignment is particularly remarkable in the case of a superlattice formed in multiple layers according to a certain rule.

As methods for manufacturing such ultra-thin films or superlattices, the molecular beam method, MOCVD method, hot wall method, etc. can be used. Especially hot
The wall method is a preferable method in terms of both control and manufacturing cost. Regarding the hot wall method, see the third section.
Multiple molecular beam epitaxy (3rd MBE)
There are reports by the present inventors in the International Conference Abstracts (1984), etc.

Note that - group semiconductors generally have pn within a single material.
Although it is difficult to form a junction, there are some materials that can be given a certain type of conductivity by doping with impurities. For example, ZnTe can be p-type, and ZnS _x S _1-x (x=0-1) can be n-type.

FIG. 3 shows an example of a structure that utilizes the above fact.
Comparing with FIG. 2C, instead of the insulating layers 5 and 6, p
The difference is that a type ZnTe layer 11 and an n type ZnSSe layer 12 are used. A DC voltage source 9 is used as a power source. The p-type ZnTe layer 11 is the outermost layer of the superlattice stack.
It supplies holes to the ZnTe layer, and is an n-type
The ZeSSe layer 12 supplies electrons to the outermost ZnSSe layer of the superlattice stack. Each layer of superlattice 2, 3
By controlling the thickness of the layer, electrons and holes supplied to the outermost layer can be tunneled to the inner layer. The mechanism of light emission is the same as that explained in FIGS. 2B and 2C.

FIG. 4 shows an example of a band model when the composition is gradually changed within each layer using a mixed crystal. The device structure may be similar to that of FIG. 2C or the structure of FIG. 2C with a substrate added. Within layer A, the mixed crystal composition changes from right to left, and the band gap becomes wider. In layer B, on the other hand, the mixed crystal composition changes from right to left, and the bandcap becomes narrower.

The drift electric field is strengthened by changing the composition, and the carrier can move faster. Furthermore, since the band gap changes gradually, the absorption spectrum becomes the integral of absorption spectra of various mixed crystal compositions. It may be particularly effective for wavelength conversion filters that absorb white light and emit light of a single wavelength.

Note that it is naturally possible to use either layer A or layer B as a layer with a graded composition.

〔Effect of the invention〕

According to the present invention, a quantum well for electrons and a quantum well for holes can be formed in separate adjacent locations using a heterojunction, and by applying an electric field, the wave function of electrons (conduction band) and the wave function of holes (valence band) can be formed. ) with the wave function can be increased and the density can be increased, and the transition efficiency can be improved. In addition, high-speed operation is possible due to drift movement and reduction of associated capacitance.

[Brief explanation of drawings]

1A and 1B are diagrams showing the band model of a type of heterojunction; FIGS. 1C and 1D are cross-sectional views of a device structure using a single heterojunction; and FIGS. 2A and 2B are diagrams showing a band model of a type of heterojunction. Diagram showing band model of junction, 2nd
Figure C is a cross-sectional view of a device structure using multiple heterojunctions, Figure 3 is a cross-sectional view of a device structure equipped with electron-hole supply means, and Figure 4 is a diagram showing a band model of a heterojunction using graded layers. be.

Claims (1)

[Scope of Claims] 1. A semiconductor device comprising bonded intrinsic or nearly intrinsic first and second semiconductor layers, and when a voltage is applied to the first and second semiconductor layers, the first semiconductor layer layer so that the second semiconductor layer acts as a barrier for electrons in the conduction band of the second semiconductor layer and that the second semiconductor layer acts as a barrier for holes in the valence band of the first semiconductor layer. The first and second semiconductor layers are heterojunctioned so that most of the electrons in the conduction band of the semiconductor layer and the holes in the valence band of the first semiconductor layer are recombined between bands. Characteristic semiconductor optical device. 2. Claim 1, wherein the set of the first and second semiconductor layers is formed repeatedly.
The semiconductor optical device described in Section 1. 3. The semiconductor optical device according to claim 2, wherein each of the first and second semiconductor layers is formed of an ultra-thin film so as to form a superlattice.