JP2002124697A - Semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve a detection/emission characteristics related to a semiconductor device which performs light detection/light emission by electron transition between different energy levels. SOLUTION: A semiconductor device is provided where light is absorbed or emitted by electron transition between different energy levels using a quantum well layer 4. The transition of electron at the quantum well layer 4 is performed at a conduction band X valley where the effective mass of electron is larger than at a conduction band Γ valley. By constituting the quantum well layer 4 with the X valley where an effective density of states is large, the intensity and coefficient of light absorption is raised for improved light detection/emission characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor device and a method of manufacturing the same, and is particularly suitable for application to a semiconductor device forming an infrared light detecting element or a light emitting element.

[0002]

2. Description of the Related Art Conventionally, an infrared detector (for example, a quantum well infrared detector) based on a transition of electrons (or holes) between quantum levels or between a quantum level and a continuous level formed in a quantum well (quantum box). Quantum-Well Infrared Photodetecto
r: QWIP).

In a detector using such a quantum well,
Normally aluminum gallium arsenide (AlGaAs)
A potential barrier for forming a quantum well is formed using a conduction band / valley of a heterojunction / gallium arsenide (GaAs) heterojunction semiconductor material system, and a current corresponding to light intensity is detected. In this case, G
The aAs layer functions as a well layer and the AlGaAs layer functions as a barrier layer.

[0004] In the conduction band valley, the energy order of AlGaAs is higher than that of GaAs. The electrons in the Γ valley of GaAs obtained energy by light absorption are AlG
Excited to the Γ valley of aAs, where electron transfer occurs.

Conventionally, the conduction band valley has been mainly used as described above because the transition between the valence band and the conduction band has been used in light absorption or light emitting elements as a historical background. The conduction band valley is due to no change in momentum between the valence band and the valence band. The valley having the lowest conduction band among the valleys of the conduction band is referred to as a direct transition type semiconductor, and the rest is referred to as an indirect transition type semiconductor. The direct transition type semiconductor is desirable for an optical semiconductor device. And has been.

[0006]

However, when a conduction band valley is used in a light emitting device having a quantum well structure, the conduction band valley is one of the conduction bands in each material.
Since the conduction band has the smallest effective mass of electrons, the density of states that electrons can take (effective state density, which is proportional to the 3/2 power of the effective mass of electrons) is small. This means that the number of states in both the start state and the end state is relatively limited when electrons cause transition due to light absorption. Therefore, when the conduction band valley is applied to the quantum well structure, the intensity of light absorption cannot be increased due to the small effective state density, and the absorption coefficient cannot be increased.

When a conduction band valley is used for a light emitting device having a quantum well structure, a GaAs layer is formed of a well layer and AlGa.
Since the As layer serves as a barrier layer, electrons excited / emitted by light absorption from the GaAs layer, which is a well layer, travel through the AlGaAs layer having low mobility, and the current density due to the movement of electrons decreases. As a result, there has been a problem that good detection characteristics cannot be obtained.

The present invention has been made in order to solve such a problem.
In an infrared detector based on the transition of electrons (or holes) between continuous levels, large light absorption or light emission is performed using a conduction band having a high electron state density, and electrons excited / emitted by light absorption are performed. It is an object of the present invention to improve detection characteristics and improve reliability by running a vehicle in a layer having high mobility.

[0009]

Means for Solving the Problems The present inventors pay attention to the density of states that electrons can take in the transition of electrons in a quantum well structure, and perform the transition of electrons in a conductor valley having a large effective state density. It has been found that light absorption or light emission can be increased.

The semiconductor device of the present invention is directed to a semiconductor device that absorbs or emits light by transition of electrons between different energy levels using a quantum well layer. In this semiconductor device, the electron transition in the quantum well layer is performed in another conduction band valley having an effective mass of electrons larger than the conduction band valley.

[0011]

In the present invention, the quantum well layer is formed by using another conduction band valley having a large effective mass of electrons (for example, a conduction band X valley) instead of the conventional conduction band valley. Large light absorption or light emission can be performed using a conduction band having a high state density. Further, when the present invention is applied to light absorption, a large signal current can be taken out by running electrons as a signal in a valley where the electron mobility is high and the electron effective mass is small.

[0012]

An embodiment of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a schematic diagram illustrating a basic configuration of a photodetector 10 according to the present embodiment. FIG. 2 is a schematic diagram for explaining the principle of the photodetector 10 according to the present embodiment. In the following description, G
The photodetector 10 using a quantum well of an aAs / AlGaAs-based material will be described as an example.

As shown in FIG. 1, the basic structure of a photodetector 10 of the present embodiment is such that an n-GaAs layer 3 is provided on a GaAs substrate 1 via a buffer layer 2 and a quantum well is provided on the n-GaAs layer 3. Multiple quantum well layer 4 (i-GaAs / n-) in which an AlGaAs layer and a GaAs layer functioning as
AlGaAs × 50) is formed. On the multiple quantum well layer 4, an n-GaAs layer 5 is formed.

In the photodetector 10 of the present embodiment,
The composition ratio of Al in the AlGaAs layer is expressed as Al _x Ga
_{(1-x) When} represented by x of As, the composition ratio x is, for example, 0.8
As a value of the degree, an AlGaAs layer and a GaAs layer are combined. FIG. 2A is a characteristic diagram illustrating a dispersion relationship between the momentum and the energy of the conduction band of the photodetector 10 according to the present embodiment. Semiconductor materials have a dispersion relationship between electron momentum and energy due to the periodicity of the constituent atomic lattice arrangement. Since this dispersion relation originates from the periodicity of the lattice arrangement, it has some "valleys" reflecting the spatial symmetry of the lattice. This is Γ,
The valleys are X and L. FIGS. 2A and 2B show a Γ valley and an X valley among these valleys. In the photodetector 10 according to the present embodiment, focusing on the X valley,
The energy of the X valley of the conduction band of the lGaAs layer is configured to be lower than that of the Γ valley, and the energy of the X valley of the conduction band of the AlGaAs layer is set to be lower than that of the conduction band of the GaAs layer in the multiple quantum well layer 4. It is configured to be lower than the valley energy. Thereby, a quantum well can be formed by the X valleys of the conduction bands of both the AlGaAs layer and the GaAs layer.

In the photodetector 10 of this embodiment, the energy of the X valley of AlGaAs is increased by increasing the Al composition ratio in AlGaAs with respect to GaAs in order to form a quantum well in the X valley. Is smaller than the energy of the X valley.

FIG. 3A shows that the relationship between the energy of the X valley and the energy of the Γ valley in the conduction band is represented by AlGa according to the Al composition ratio.
It is a characteristic view showing that it depends on the Al composition ratio in the As layer.

S. Adachi, J. Appl. Phys. Vol. 58, No. 3, R1 (19
85) and HJLee, LYJuravel, a quoted there
According to nd JC Woolley, Phys. Rev. B21, 659 (1980), A
Al composition x dependency of each major conduction valley energy gap in lxGa1-xAs based material, at room temperature, Γ: 1.425 + 1.155x + 0.37x 2 X: 1.911 + 0.005x + 0.245x 2 L: 1.734 + 0.574x + 0 the .055x ^2. FIG. 3A shows the bottom of each valley based on the bottom of the Γ valley of GaAs using this dependency and the assumption that the conduction band discontinuity at the time of forming a heterogeneous junction = the band gap difference × 0.57. FIG.

As shown in FIG. 3A, the Al composition ratio x =
With the vicinity of 0.4 as a boundary, the magnitude relationship between the energies of the Γ valley and the x valley is reversed. In the photodetector 10 of the present embodiment, A
Since the AlGaAs layer having the l composition ratio x = 0.8 is used, the energy of the X valley can be made much smaller than the energy of the Γ valley as shown in FIG. About 5 eV can be secured.

On the other hand, FIG. 2B shows, for comparison, the valley of the GaAs layer in the conventional quantum well and the AlGa in the GaAs layer.
FIG. 4 is a characteristic diagram showing a dispersion relationship between momentum and energy of a conduction band with a valley of an As layer as a barrier layer. Here, x is a value of about 0.3, and as shown in FIG. 2B, the energy of the Γ valley is lower than the energy of the other conduction band valleys.
In this case, a quantum well is formed by the valleys of the conduction bands of both the AlGaAs layer and the GaAs layer.

When a transition between subbands of a quantum well or a quantum box is used, the conduction band (or valence band) is used for both the well layer and the barrier layer. Therefore, as long as the same conduction band valley is used, the momentum change during the transition is small. Not accompanied. Therefore, when the inter-subband transition is used, it is possible to use a valley other than the conduction band valley. Therefore, a quantum well can be formed by the valley of the conduction band of the AlGaAs layer and the GaAs layer as in the present embodiment.

FIG. 3 (b) shows the dependency of the effective state density on the Al composition x, which is based on the well-known dependence of the effective electron mass of each of the main conduction valleys on the Al composition x, summarized in the above-mentioned S. Adachi paper. FIG. As shown in FIG.
The effective state density in the conductor X valley of the GaAs layer is larger than the effective state density in the Γ valley. For this reason, by forming a quantum well by the conductor X valley as in the photodetector 10 of the present embodiment, it becomes possible to increase the intensity of light absorption and the absorption coefficient, thereby greatly improving the photodetection characteristics. It is possible to do. Further, as shown in FIG. 3B, the dependence of the effective state density on the Al composition is low in each conduction valley.
Even in the X valley, the change in the effective state density with respect to the composition ratio x is small. Therefore, even if the composition ratio x of Al in the AlGaAs layer is increased, the effective state density at the X valley can be maintained high.

In this embodiment, the present invention is applied to the photodetector 10
However, light absorption and light emission are reverse processes and are essentially the same, so even in a light-emitting element, by using a conductor X valley having a high effective state density, Luminous efficiency can be improved.

In the case of light absorption, it is necessary that electrons exist in the ground level of the quantum well or quantum box in advance. Therefore, the conduction band valley having a large density of states, which constitutes a quantum well or a quantum box, is preferably the lowest in energy among the main conduction valleys. FIG.
(A), as shown in FIGS. 3 (a) and 3 (b), Al
By using the X valleys by increasing the Al composition ratio in GaAs, it is possible to increase the effective state density and minimize the energy. in this case,
As shown in FIG. 3A, the Al composition x of the layer to be a quantum well or a quantum box needs to be approximately x> 0.4 or more. The multiple quantum well layer 4 can be formed by setting x as desired.

In the case of a light emitting element, particularly a laser, it is necessary to inject electrons into the upper level and make the number of electrons larger (inversion distribution) than the number of electrons in the lower level.
In this case, if the quantum well layer is not constituted by the conduction band valley having the lowest energy, before forming the population inversion,
It is conceivable that the energy is relaxed to another conduction band valley which is stable in terms of energy. Therefore, in the light emitting element, as in the case of the light absorbing element, the A of the layer serving as the quantum well or the quantum box is used.
The l composition x needs to be approximately x> 0.4 or more, and if it is more than x, the composition ratio x can be arbitrarily designed as needed.

In the photodetector 10 of the present embodiment thus configured, light is detected by applying a potential difference between the upper and lower n-GaAs layers 3 and the n-GaAs layer 5 and applying light to the multiple quantum well layer 4. Is irradiated, the n-GaAs layer 3 and the n-G
This is performed by detecting a current flowing between the aAs layers 5.

Here, it is assumed that light enters a quantum well formed by both conduction band X valleys. FIG. 1A and FIG.
As shown in (c), light absorption, that is, electron transition occurs selectively for light having a wavelength corresponding to the energy difference between subbands or between a subband and a continuous level. In the figure, for the sake of simplicity, the AlG starts from the bottom of the conduction band X valley of the GaAs layer.
Although it is shown as excitation to the conduction band X valley of the aAs layer,
Of course, the energy in the well layer is discrete with respect to the direction of confinement due to confinement.

At this time, as described above, since the effective mass at the conductor X valley is large, the intensity of light absorption or the absorption coefficient can be increased. The conduction band X valley of the GaAs layer is much energetically (0.5 eV
), The energy is unstable in terms of energy, and electrons that have transitioned due to interaction with phonons of the crystal are GaAs.
The conduction band of the layer will relax to the valley.

In the conductor Γ valley, as shown in FIG. 1C, when electrodes and the like are formed at both ends of the quantum well configured as described above and an electric field is applied, electrons drift. It becomes a current, and light can be detected from the current value. The electrons that have relaxed to the conduction band valley of the GaAs layer travel through the conduction band valley of the GaAs layer having high mobility, so that the current density can be increased. Therefore,
It becomes possible to increase the signal current due to the electrons excited by the light detection, and it is possible to detect even weak light with high accuracy.

In the conduction band {valley} of the GaAs layer, GaAs
Since the energy of the valley is lower than that of the AlGaAsΓ valley, the AlGaAs layer functions as a potential barrier as in the related art.
By setting the thickness of the lGaAs layer to such a thickness that a quantum well can be formed (about several nm), electrons can be transmitted through the AlGaAs layer by a tunnel phenomenon. Therefore, the AlGaAs layer does not hinder electron traveling, that is, current conduction.

On the other hand, in the conventional quantum well as shown in FIG. 2B, electrons travel in a conductor valley with low mobility, so that detection is particularly accurately performed when the light intensity is low. It is not possible.

As described above, in the photodetector 10 according to the present embodiment, electrons are excited by light absorption in the conductor X valley having a large effective state density of electrons, and electrons are excited by the tunnel current in the multiple quantum well layer 4. Can be performed in the conduction band valley where the electron mobility is high. Therefore, a conductor valley suitable for each of the excitation of electrons and the movement of electrons in the multiple quantum well layer 4 is used properly according to the function. Can be. Therefore, light detection can be performed with higher precision as compared with a conventional quantum well using a conduction band valley.

As described above, AlGaAs /
Although the case of a GaAs-based semiconductor has been described as an example, the present embodiment is characterized in that light absorption or light emission is performed using a conduction band valley having a larger effective state density than a conventionally used valley. Therefore, even if a material other than the GaAs / AlGaAs system is used, any material can be used as long as the above effects can be obtained. Although the description has been made mainly by light absorption, light absorption and light emission are mutually opposite processes and are essentially the same, and therefore, a quantum well or quantum box for light emission, A light-emitting element formed using a band X valley and utilizing a large state density is also included in the scope of the present invention.

Next, a specific configuration of the photodetector 10 according to the present embodiment and a method of manufacturing the same will be described with reference to FIG.

First, as shown in FIG. 4A, a normal metalorganic vapor phase epitaxy method using, for example, trimethylgallium, arsine, and trimethylaluminum on a normal GaAs (100) plane substrate 11 is performed. For example, the buffer layer 12 and the first n-GaAs layer 13 are grown.
Subsequently, for example, an i-GaAs layer having a thickness of about 50 nm and n-AlGaAs (Al composition of about 0.3 nm) having a thickness of about 3 nm.
8) Multi quantum well (MQW) with one layer as one cycle
The m-well) layer 14 is grown, for example, for 50 periods. afterwards,
The second n-GaAs layer 15 is grown to complete the crystal growth

Next, as shown in FIG. 4B, the optical coupling portion 16 for optical coupling is formed using, for example, a normal lithography technique and a dry etching technique using, for example, silicon tetrachloride and sulfur hexafluoride gas. To form

Next, as shown in FIG. 4C, the semiconductor portion between the adjacent elements is first formed using, for example, a normal lithography technique and a dry etching technique using silicon tetrachloride gas.
Is performed until the surface of the n-GaAs layer 13 is exposed. As a result, adjacent elements are separated.

Next, as shown in FIG. 4D, for example, by using a normal lithography technique and a vacuum deposition method,
The uGe / Ni ohmic electrodes 17 and 18 are formed. Thereby, the photodetector 10 is completed.

This manufacturing process exemplifies the configuration of an essential part of the photodetector 10 of the present embodiment, and includes, for example, an etching stop layer necessary for improving the controllability of the photocoupler forming process. However, an additional crystal layer may be included.

As described above, according to the present embodiment, the quantum well is formed by using the X valley having a larger effective state density than the conduction band Γ valley, thereby increasing the light absorption or light emission. A detection element and a light emitting element can be formed.

In the case of a photodetecting element, the electrons excited to the X valley of the GaAs layer are relaxed to the valley of the GaAs layer having a high mobility, so that the sensitivity at the time of detection can be increased. As a result, it is possible to accurately detect light even with weak light.

In this embodiment, AlGaAs / Ga
Although the embodiment using the As-based semiconductor has been described, other combinations of materials may be used as long as the same effects as described above can be obtained. Therefore, in principle, any combination of materials having a proper dispersion relationship between electron momentum and energy can be applied to the present invention within the range permitted by the crystal production technology.
That is, any semiconductor material that can continuously grow a crystal on a substrate can be applied to the present invention by performing electron transition using a conductor valley other than the conductor valley.

In the above-described embodiment, the photodetector has been described.
By making one of the s layers p-GaAs, pn
It is also possible to form a light emitting element using a junction.

[0043]

According to the present invention, it is possible to provide a semiconductor device capable of achieving further improvement in light absorption or light emission characteristics and capable of performing light absorption or light emission with high precision. Becomes

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a basic configuration of a photodetector according to an embodiment of the present invention.

FIG. 2 is a schematic diagram for explaining the principle of a photodetector according to one embodiment of the present invention.

FIG. 3 is a characteristic diagram showing the dependence of conduction valleys Γ, X, and L on Al composition.

FIG. 4 is a schematic view showing a method for manufacturing a photodetector according to an embodiment of the present invention in the order of steps.

[Explanation of symbols]

 Reference Signs List 1 GaAs substrate 2, 12 buffer layer 3, 5 n-GaAs layer 4, 14 multiple quantum well layer 10 photodetector 11 GaAs (100) plane orientation substrate 13 first n-GaAs layer 15 second n-GaAs layer 16 Optical coupling part 17 AuGe / Ni ohmic electrode

Claims (5)

[Claims] 1. A semiconductor device that performs light absorption or light emission by transition of electrons between different energy levels using a quantum well layer, wherein the transition of electrons in the quantum well layer is performed by an electron rather than a conduction band Γ valley. A semiconductor device having another effective conduction mass having a large effective mass. 2. The semiconductor device according to claim 1, wherein said quantum well layer comprises an AlGaAs layer as a well layer and a GaAs layer as a barrier layer. 3. The semiconductor device according to claim 2, wherein an Al composition ratio in said AlGaAs layer is 0.4 or more. 4. The semiconductor device according to claim 1, wherein said another conduction band valley is a conduction band X valley. 5. An electron excited from a conductor X valley of the AlGaAs layer to a conduction band X valley of the GaAs layer.
3. The semiconductor device according to claim 2, wherein the detection current is obtained by the light absorption by relaxing the As layer to the conduction band Γ valley and moving through the quantum transfer layer. 4.