JPH0354853B2 - - 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 two 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 a manufacturing method for compound semiconductor devices, especially 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
A GaAs layer 2 functioning as a buffer layer is formed on the substrate 1, and an undoped GaAs layer 2 serving as a channel layer is further formed on the substrate 1.
Layer 3 is formed. And the GaAs layer 3
On top, 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 of the layer 4 there is formed an electron supply layer 4 with a high concentration of p-type impurities, such as n-Ga x Al 1-x As. A layer 5 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
are alloyed, and 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 a compound semiconductor device, the dependence of electron mobility in a 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, for example, 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.

To make intervalley scattering less likely to occur, the energy difference ΔE between the valleys in k-space must be large.

The energy gap _Eq 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 _Eq , taking the energy band structure of GaAs crystal as an example, the fifth
As shown in the figure. Moreover, the relationship between the effective mass m ^* and the energy band structure is 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 _q , and m ^* are naturally determined. . 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 _p・d ² . However, 1 and 1′ 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 of adjacent atoms, m _p 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 the vertical 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 (for example, WA Harrison,
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 changed. , 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 As mixed crystal, only the atomic arrangement has changed slightly, and the crystal structure has hardly changed, so the energy band structure does not show any major changes. .

However, when the lattice misalignment of the two types of layers is increased to about 1.5%, a large change in the energy gap _Eq 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 due to such lattice misalignment is reported in JAP 45 , No.
9, (1974) 3789.

From the above, by alternately stacking two types of compound semiconductor thin film layers with large lattice misalignment, the crystal structure or electronic arrangement can be changed over the entire growth layer.

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. In other words, 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 separated from the gate electrode on both sides of 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 laminating two 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 lattice constant is 0.3% or more and 6.5%.
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 Figure 1 has a crystal structure in which about 80 layers of InAs and AlAs are alternately stacked on a semi-insulating InP substrate 10, each with several atomic layers each.
Al _x In _1-x As compound semiconductor crystal multilayer thin film layer 11
It is formed as a channel layer of FET. The total thickness of the multilayer thin film layer 11 is 0.1 μm.
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 within the compound semiconductor crystal multilayer thin film layer 11 is disturbed, and the crystal structure in this region 14 becomes the same as that of the conventional Al _x In _1-x As mixed crystal. As a result, the energy band structure in this region was also the same as that of the conventional Al _x In _1-x As 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 has already been reported by N.
Published in 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, the fact that the crystal structure and energy band structure of _the compound semiconductor crystal multilayer thin film layer ₁₁ in FIG. This could be confirmed by conducting measurements such as As a result, as shown in Figure 2, the electron mobility in the high electric field region is lower than that of conventional Al _x In _1-x with the same composition.
The improvement was approximately three times that of the As mixed crystal case.

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 AlAs (5.662 Å) were used as constituent layers of the multilayer thin film layer.
Å) was used. In this case, the difference in lattice constant is
It is approximately 6.5%.

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 occur more easily 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 and the AlAs layer in the above embodiment, the above-mentioned difference in lattice constant of about 6.5% can be considered as a guideline for the upper limit of the difference in lattice constant.

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, as a compound semiconductor with a large lattice constant, in addition to InAs, GaSb (6.095 Å),
InSb (6.479Å), etc. In addition to AlAs, compound semiconductors with small lattice constants include
GaAs (5.654Å), GaP (5.451Å), InP (5.869Å)
and so on. A ternary 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 faster than in conventional compound semiconductor mixed crystals.

[Brief explanation of drawings]

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. ohmic junction source electrode and drain electrode, the channel layer is configured by alternately laminating two types of compound semiconductor thin film layers having different lattice constants, and each layer of the compound semiconductor thin film layer 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 6.5%.
A compound semiconductor device characterized by being in the following range.