JPH0328063B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a compound semiconductor device. More specifically, the present invention achieves an energy band structure different from that of existing so-called mixed crystal compound semiconductors by alternately stacking three types of compound semiconductor thin film layers with different lattice constants, thereby improving electron transfer under high electric field conditions. The present invention relates to a compound semiconductor device with a high degree of performance.

BACKGROUND OF THE INVENTION Epitaxial growth methods are generally used as methods for manufacturing compound semiconductor devices, particularly electronic devices, because of the ease of growing thin, uniform layers and controlling the composition ratio of component elements. Among these, a molecular beam epitaxial growth method (hereinafter referred to as "MBE growth method" for simplicity) is known as a technique that has recently attracted particular attention. For example, in Nikkei Electronics No. 308163 (1983) by WTTsang,
The MBE growth method and devices using thin film periodic structures are explained in detail.

By following this MBE growth method, the crystal growth rate can be controlled at the monatomic level (JPvan
der Ziel et al., J. Appl. Phys. 48 (1977) P3018), and furthermore, the composition of even a single atomic plane can be precisely controlled by using reflection electron diffraction (J.H.
Neave et al., Appl. Phys. A31, 1 (1983)). By using such an MBE method, a high electron mobility transistor (hereinafter referred to as
This makes it possible to manufacture HEMT (abbreviated as HEMT).

Note that microwave elements using conventional compound semiconductors are described in, for example, Japanese Patent Laid-Open No. 59-4085 and Japanese Patent Laid-Open No. 58-147169.

The HEMT structure shown in Figure 3 is a semi-insulating GaAs
The device has a substrate 1, on which is formed a GaAs layer 2 which functions as a buffer layer, and further on which is formed an undoped GaAs layer 3 which serves as a channel layer. Then, on the GaAs layer 3, an electron supply layer 4 with a high impurity concentration such as n-Ga _x Al _1-x As is formed, and in the center thereof, a large A layer 5 made of a semiconductor having electron affinity is provided, and a gate electrode 6 is formed on the layer 5.
Furthermore, the surface region 7 of the electron supply layer 4 sandwiching the layer 5 is alloyed, and the source and drain electrodes 8 are formed thereon.

In such a semiconductor device, the gate electrode 6
When an appropriate bias voltage is applied to the electron supply layer 4 and the channel layer 3, a two-dimensional electron gas 9 is formed on the channel layer 3 side at the interface between the electron supply layer 4 and the channel layer 3. As a result, a large amount of electrons flows through the channel layer 3, which has a thickness of several tens of angstroms near the interface, where there are few impurity ions. Therefore, 1 which limits the electron mobility
There is little impurity ion scattering, which is one of the major factors, and high mobility can be achieved.

Problems to be Solved by the Invention However, in such compound semiconductor devices, the dependence of electron mobility in the two-dimensional electron gas on the applied electric field strength is extremely large, and although high mobility can be achieved in the case of a low electric field, , in the case of a high electric field, its mobility decreases significantly. Such a phenomenon has been described, for example, by M. Inoue et al., JJAP 22 357
(1983). In addition, one example is the GaAs/n-Ga _x
The case of the Al _1-x As structure is shown by the dotted line in FIG.

Electron scattering mechanisms within the semiconductor under such high electric field application include intervalley scattering, impact ionization, and phonon (lattice vibration) scattering.
Therefore, semiconductor crystals generally used as channel layers in ultra-high frequency transistors are required to have the following improved characteristics.

In order to make intervalley scattering less likely to occur, the energy difference between the valleys in k-space is
E is large.

The energy gap E _g must be large to make impact ionization less likely to occur.

In order to reduce the loss of carrier electron kinetic energy due to phonon scattering, the effective mass
m ^* is small.

For parameters such as the energy difference ΔE between valleys and the energy gap E _g , taking the energy band structure of a GaAs crystal as an example, the fifth
As shown in the figure. Further, the effective mass m ^* has a relationship with the energy band structure as follows: 1/m ^* =1/〓 ² ·d ² E(K)/dk ² .

However, in conventional compound semiconductor devices, the compound semiconductor in each layer with a substantial thickness is sought to have a uniform composition structure, so the above-mentioned ΔE, E _g and m ^* are naturally determined. Ta. Therefore, there is a barrier to eliminating the above-mentioned scattering, and high electron mobility cannot be achieved in a state where a high electric field is applied.

Therefore, the present invention aims to provide a compound semiconductor device that suppresses the above-mentioned effects of electron scattering and has high electron mobility even when a high electric field is applied.

Means for Solving the Problems Therefore, the present inventor conducted various studies on the problem of electron scattering for the above purpose.

As can be seen from the above explanation, the compound semiconductor crystal used as the channel layer in ultra-high frequency transistors reduces scattering due to intervalley scattering and impact ionization when a high electric field is applied by changing its energy band structure. The mobility in a high electric field application state can be increased by increasing or decreasing the effective mass of electrons.

On the other hand, regarding the energy band structure of semiconductors,
According to LCAO theory, the off-diagonal matrix element V _LL ′ _n of the Hamiltonian, which is important when calculating the energy band structure, is expressed as V _LL ′ _n =η _LL ′ _n・〓 ² /m ⁰・d ² . However, l and l′ are the azimuthal quantum numbers of the outermost p-orbitals of adjacent atoms constituting the crystal, respectively,
Similarly, m is the magnetic quantum number, d is the internuclear distance between adjacent atoms, m ₀ is the electron mass, η _LL ′ _n is a coefficient depending on the crystal structure, 〓=h/2π (h: Planck's constant).

Also, the diagonal matrix elements ε _s ^c , ε _s ^a ,
ε _p ^c , ε _p ^a , etc. are relatively related to the term value of an isolated atom. However, ε _s ^c is related to the term value of the cation atomic S orbit of the polar compound semiconductor, and similarly, ε _s ^a is related to the term value of the anion atomic S orbit,
ε _p ^c is related to the term value of the cation atomic P orbital, and ε _p ^a is related to the term value of the anion atomic P orbital.

This established theory (e.g. WAHarrison,
Electronic Structure and the Properties of
Solids, 1980), by changing the crystal structure or atomic arrangement, that is, by changing the internuclear distance d of adjacent atoms in a compound semiconductor crystal as well as the type of adjacent atoms, the matrix element V _LL ′ _n of the Hamiltonian can be , ε _sa , ε _sc ^, ε _p ^a , ^and ε _p ^c can be changed. This means that the energy band structure of a compound semiconductor can be changed by changing the crystal structure or atomic arrangement.

On the other hand, as mentioned above, recent compound semiconductor crystal growth techniques include molecular beam epitaxial growth (MBE) and metal organic vapor phase epitaxy (abbreviated as MOCVD, MOVPE, etc.).
Control of the growth layer thickness has been improved to the level of a single atomic layer, and we have actually succeeded in stacking GaAs and AlAs alternately at the single atomic level (AC
Gossard, Thin Solid Films 57 , 3 (1979)).

However, when GaAs and AlAs are stacked alternately, the lattice mismatch is extremely small at about 0.3%.
Therefore, distortion of the crystal lattice caused by alternately stacking these two types of layers is small. This means that compared to the conventional Ga _x Al _1-x mixed crystal, only the atomic arrangement has changed slightly, and the crystal structure has hardly changed, and therefore the energy band structure does not show a large change.

However, when the lattice misalignment of the two types of layers is increased to approximately 1.5%, a change in the energy gap E _g has been observed (GCOsbourn et al., J.
Appl. Phys. 48 (1977) 3018). That is, when the lattice misalignment is relatively large like this, not only the atomic arrangement but also the crystal structure changes, and a relatively large change in the energy band structure is observed.

However, in the case of this report, the thickness of each layer is as thick as 100 to 200 Å, and compared to the distorted part of the crystal lattice near the interface between the two types of layers, the conventional crystal structure with an undistorted crystal lattice is observed. There are an extremely large number of areas that have .
Therefore, most of the energy band structure is dominated by the energy band structure resulting from the conventional crystal structure of each layer, and the contribution from distorted portions of the crystal lattice is small.
The fact that the distortion of the crystal lattice is limited to the vicinity of the interface between each layer is explained by JMBrown et al., A.
PL 43 (1983) 863. Furthermore, the direct result that the crystal structure changes due to distortion based on such lattice misalignment is reported in JAP 45 , No.
9, (1974) 3789.

However, when there are two types of layers, the difference in atomic arrangement between a crystal in which these layers are alternately laminated and a conventional mixed crystal is not large.

From the above, by alternately stacking three or more types of compound semiconductor thin film layers with large lattice mismatches, both the crystal structure and atomic arrangement throughout the growth layer can be significantly changed compared to the case of two types of layers. be able to.

Note that similar reports regarding semiconductor devices in which compound semiconductor thin film layers are alternately laminated include, for example,
JP-A-59-76468 or T. Yao, JJA
P. 22 (1983) L680, but they do not improve the energy band structure at all.

The present invention was made as a result of research based on such knowledge. That is, according to the present invention, a substrate, a channel layer formed on the substrate, a Schottky junction gate electrode formed on the channel layer, and a gate electrode on both sides of the gate electrode separated from the gate electrode. The channel layer includes an ohmic junction source electrode and a drain electrode formed on the channel layer, and the channel layer is configured by alternately stacking three types of compound semiconductor thin film layers having different lattice constants, and The thickness of each layer of the compound semiconductor thin film layer is within the range of 1 to 100 atomic planes, and the difference in the lattice constant is 0.3% or more and 13%.
A compound semiconductor device is provided that is characterized by being within the following range.

Function In the above-described compound semiconductor device, the compound semiconductors in each thin film layer forming the channel layer have different lattice constants and are extremely thin, so the energy band structure changes over a substantial portion of each layer. , to reduce scattering due to intervalley scattering or impact ionization during the electron transport process in the channel layer under high electric field application, or to reduce the effective mass of electrons.
Therefore, the electron mobility in the channel layer is maintained at a high level when a high electric field is applied.

EXAMPLES The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a cross-sectional view illustrating an embodiment of a compound semiconductor device according to the present invention. In addition, Figure 1 shows
An example in which the present invention is implemented as a field effect transistor (hereinafter abbreviated as FET) is shown.

The FET shown in FIG _. 1 has a crystal structure in which about 80 layers each of InAs, GaAs, and InSb are alternately stacked on a semi-insulating InP substrate ₁₀ , each with several atomic layers each _. An Sb _1-y compound semiconductor crystal multilayer thin film layer 11 is formed as a channel layer of the FET. The total thickness of the multilayer thin film layer 11 is
The InAs layer is 10 Å thick, the GaAs layer is 20 Å thick, and the InSb layer is 3.5 Å thick.
Note that such a multilayer thin film layer 11 is made of semi-insulating material.
It was formed on an InP substrate 10 using the MBE growth method.

Furthermore, on the surface of the multilayer thin film layer 11, AuGeNi
Ohmic junction electrodes 12 were deposited to form source and drain electrodes. Further, an Al shot key junction electrode 13 was deposited to form a gate electrode.

Au atoms are diffused into the compound semiconductor crystal multilayer thin film layer 11 by vapor deposition and alloying treatment when forming the AuGeNi ohmic electrode 12. As a result, the periodicity of the laminated structure in the compound semiconductor crystal multilayer thin film layer 11 is disturbed, and the crystal structure in this region 14 is the same as that of the conventional In _x Ga _1-x As _y Sb _1-y mixed crystal. I get used to it. As a result, the energy band structure in this region also changes from the conventional In _x Ga _1-x As _y
It became the same as the Sb _1-y mixed crystal, and no adverse effects occurred in forming an ohmic junction. A similar phenomenon in which the periodicity of the laminated structure of a compound semiconductor multilayer thin film layer is destroyed by such diffusion is as follows.
It has already been published in N. Holonyak et al., APL 39 (1981) 102, etc.

Source electrode 12 and drain electrode 1 shown in FIG.
2. Regarding gate electrode 13 etc., conventional
By using the conventional technology used to fabricate the FET structure, we were able to confirm that it functions as an FET.

Furthermore _, regarding the fact that the compound semiconductor crystal _multilayer thin film layer 11 in FIG. ₁ has a crystal structure and an energy band structure that are completely different from the _In This was confirmed by measurements such as line diffraction and optical absorption. As a result, as shown in Figure 2, the electron mobility in the high electric field region is lower than that of the conventional one with the same composition.
The improvement was approximately five times that of the In _x Ga _1-x As _y Sb _1-y mixed crystal.

In addition, when the thickness of each thin film layer of the compound semiconductor crystal multilayer thin film layer 11 was changed from 1 atomic plane to 100 atomic planes using the same structure as the above example, there was a difference compared to the conventional example. However, similar results were obtained. However, it is preferable that the thin film layers having the same composition have the same thickness throughout the multilayer thin film layer.

In the above example, InAs (lattice constant 6.058 Å) and GaAs (5.654 Å) were used as constituent layers of the multilayer thin film layer.
Å) and InSb (6.479 Å) were used. Based on InSb, which has the largest lattice constant, the difference in lattice constant from GaAs, which has the smallest lattice constant, is approximately 13%.
It is.

Generally, as the thickness of each layer of the compound semiconductor crystal multilayer thin film layer 11 increases, the tendency for each thin film layer to adopt the lattice constant inherent to the compound semiconductor crystal increases. It can be said that the thinner the layer, the weaker the tendency of each thin film layer to take on the lattice constant inherent in the compound semiconductor crystal. Therefore, it can be said that dislocations due to lattice misalignment are more likely to occur as the film thickness increases, assuming that the difference in lattice constant is constant. In other words, it can be said that the upper limit of lattice misalignment at which dislocations do not occur due to lattice misalignment depends on the film thickness.

That is, when the thickness of each layer of the compound semiconductor crystal multilayer thin film layer 11 is in the range of 1 atomic plane to 100 atomic planes, the thinner the thickness of each layer of the multilayer thin film layer 11, the larger the difference in lattice constant. As the thickness of each layer of the multilayer thin film layer 11 increases, it is necessary to reduce the difference in lattice constants. Therefore, considering the thicknesses of the InAs layer, GaAs layer, and InSb layer in the above example, the difference in the lattice constants described above is approximately 13%.
can be seen as the upper limit of the difference in lattice constants.

Note that the multilayer thin film layers are not limited to the combinations of the above-mentioned embodiments, and other combinations are also possible, and the difference in lattice constant is 0.3% or more, more preferably
If it is 1.5% or more, similar effects can be obtained although there are differences in degree. For example, in addition to InAs, GaSb (6.095Å) is a compound semiconductor with a large lattice constant.
In addition to GaAs, other compound semiconductors with small lattice constants include GaP (5.451Å), InP
(5.869Å) etc. A quaternary compound semiconductor multilayer thin film channel layer can also be formed by appropriately combining them.

Effects of the Invention According to the semiconductor device of the present invention, by changing the crystal structure or atomic arrangement of the compound semiconductor used in the channel layer of a conventional FET, the energy band structure is changed, and the semiconductor crystal under high electric field application is This makes it possible to reduce various types of scattering within the space and also to reduce the effective mass. Therefore, electron mobility under high electric field application is significantly faster than in conventional compound semiconductor mixed crystals.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of an FET implementing a compound semiconductor device according to the present invention, FIG. 2 is a graph showing measurement results of the dependence of electron mobility on applied electric field strength in the compound semiconductor device according to the present invention, The diagram is
Schematic cross-sectional diagram of a conventional high electron mobility transistor,
FIG. 4 is a graph showing measurement results of the dependence of electron mobility on applied electric field strength in a conventional high electron mobility transistor, and FIG. 5 is a diagram for explaining the energy band structure of a compound semiconductor. [Main reference number], 1...Semi-insulating GaAs substrate,
2...GaAs buffer layer, 3...GaAs channel layer, 4...electron supply layer, 5...layer made of a semiconductor containing a high concentration of p-type impurity and having a large electron affinity, 6...gate electrode, 7... Alloying region, 8... Source electrode, drain electrode, 9... Two-dimensional electron gas, 10... InP substrate, 11... Multilayer thin film layer, 12... Source electrode, drain electrode, 1
3...Gate electrode, 14...Mixed crystal region.

Claims (1)

[Claims] 1. A substrate, a channel layer formed on the substrate, a Schottky junction gate electrode formed on the channel layer, and a Schottky junction gate electrode formed on both sides of the gate electrode away from the gate electrode. The channel layer is formed by alternately stacking three types of compound semiconductor thin film layers having different lattice constants, and each of the compound semiconductor thin film layers has a The thickness is within the range of 1 to 100 atomic planes, and the difference in the lattice constant is 0.3% or more and 13%.
A compound semiconductor device characterized by being in the following range.