JPH0712023B2 - Crystal structure - Google Patents

Description

TECHNICAL FIELD The present invention relates to a structure of a crystal of a III-V compound semiconductor, particularly a crystal layer including a superlattice layer.

[Conventional technology]

In recent years, the molecular beam epitaxial growth method (MBE method: Molecular
Beam Epitaxy) and pyrolysis of organic metals (MOCVD: Metal Orgnic Chemical Vapor Depositio)
n) has been actively studied because it is effective for controlling the thickness of the thin film epitaxial layer. The most important feature of these crystal growth methods is that they enable thin film epitaxy with a thickness of several 10Å or less. From this point, these crystal growth methods are most characteristic in producing hetero epitaxy of ultra-thin film structure such as superlattice and quantum well structure, and so-called selective doping crystal. We are enabling new devices. However, in such ultra-thin film multilayer crystals, the problem of mutual diffusion at the interface formed by different substances is always serious.

[Problems to be solved by the invention]

The ultra-thin film multi-layer crystal technology always has a problem of interdiffusion at the interface between different substances that form a multi-layered structure, and such interdiffusion occurs during crystal growth or during the process. Significantly reduce manufacturing yield. Therefore, it is necessary to solve the problem of interdiffusion at the interface between the layers in the ultrathin film multilayer structure crystal to which the present invention is directed.

The object of the present invention is to provide GaAs and Al _x Ga _1-x As (Al composition ratio x ≠ 0).
It is an object of the present invention to provide a superlattice crystal structure effective for preventing the problem of interdiffusion at a hetero interface of a multilayer structure in which layers are alternately stacked.

[Means for solving problems]

In the crystal structure of the present invention, a multilayer structure in which GaAs layers and Al _x Ga _1-x As (Al composition ratio x ≠ 0) layers are alternately stacked constitutes a superlattice layer, and the GaAs layers and Al _x Ga are formed. The boundary of the _1-x As layer is characterized by having an Al _y Ga _1-y N (Al composition ratio 0 <y ≦ 1) layer having a thickness of at least one molecular layer.

〔Example〕

 Hereinafter, the present invention will be described in detail based on examples.

Here, a GaAs layer doped with Si and Al _x Ga _1-x As (x =
An example in which a superlattice structure composed of 0.5) layers is epitaxially grown by the MBE method will be shown. A cross-sectional view of a wafer including a structure of a superlattice to be formed is shown in FIG. In FIG. 1, 11 is a GaAs substrate, and 12 is a buffer layer for avoiding impurity contamination from the GaAs substrate 11.
Al _x Ga _1-x As (x = 0.3) mixed crystal layer, 13 is a high-purity GaAs layer, 14 is a superlattice composed of GaAs ultra-thin film layer and Al _x Ga _1-x As (x = 0.5) ultra-thin film It is a layer. The thickness of each layer is 0.5 μm for the Al _x Ga _1-x As layer 12, which is a buffer layer, and _{1 for} the high-purity GaAs layer 13.
μm and the superlattice layer 14 are 0.5 μm, the thickness of the GaAs ultra-thin film layer constituting the superlattice layer 14 is 25 Å, and the thickness of the Al _x Ga _1-x As ultra-thin film layer is 20 Å. Has a thickness of 0.2 μm
Is.

The cross-sectional structure of the layer structure of the superlattice layer 14 for one cycle is shown in FIG. Ga that constitutes the superlattice layer as shown in FIG.
As the central part 24 of the ultra-thin film layer 21 and the region having a thickness of about 20Å is doped with Si at 3 × 10 ¹⁸ cm ^-3 , and the thickness of about 2.5Å made so as to sandwich it. Undoped GaA with
It consists of s layer 25. In addition, the structure that is the essence of the present invention is that the region of at least one molecular layer, that is, a region having a thickness of about 5Å between the Al _x Ga _1-x As ultrathin film layer 22 and the GaAs layer 25 is Al _x Ga _1-x.
It is the N layer 23. Other than the Al _x Ga _1-x N layer 23, there is no difference from normal MBE growth, but the Al _x Ga _1-x N layer 23
Cut off As source and grow with chemical MBE introduced NH ₃ .

The superlattice layer 14 shown in FIG. 2 is conventionally the Al _x Ga _1-x N layer 2
There was no such thing as setting up 3, but the Japanese Journal of Applied Physics
Part 2, Letter (Japanese Journal of Applied Physic
s Part2 Letters), Volume 22, L627 (1983) and Japanese Journal of Applied Physics Part 2, Letter (Japanese Journal of Ap
plied Physics Part2 Letters), Volume 24, Page L17 (1
985), a deep center called DX center that is universally observed in Al _x Ga _1-x As (Al composition ratio x ≥ 0.2) mixed crystals doped with donors such as Si, Te, and Se. The concentration of levels is extremely low, and the well-known PPC (Persistent Phot)
There is almost no photoconduction phenomenon, which is considered to correlate with deep level called ocondactivity). Therefore, in the mixed crystal Al _x Ga _1-x As, a crystal layer having a high carrier concentration cannot be obtained even if a donor impurity is doped, whereas in the superlattice structure, the carrier concentration can be easily increased. . Therefore, in the wafer having the sectional structure shown in FIG. 1, a high-density two-dimensional electron gas can be induced in the high-purity GaAs layer 13, and a high-speed operation two-dimensional electron gas field effect transistor (commonly called HEMT) Is realized.

However, in the conventional superlattice structure without the Al _x Ga _1-x N layer 23, it was observed that at high temperature growth of 520 ° C or higher, DX centers or PPCs corresponding to about 1/2 or more of the carrier concentration were generated. It begins to take, and as the growth temperature rises, both increase, and even if the doping amount is increased, the carrier concentration does not increase as expected. Currently DX Center or PPC
The origin of these is unknown, but it is expected that these deep levels occur due to the interdiffusion of Al and Ga at the interface between the Al _x Ga _1-x As layer 22 and the GaAs layer 21.

Further, the restrictions on the process temperature were severe even in the case of forming a device using the conventional superlattice structure in which the Al _x Ga _1-x N layer 23 was not provided. That is, FIG. 3 shows 520.
The photoluminescence (PL) spectrum of the superlattice layer is shown when an epitaxial wafer with a conventional superlattice layer grown by MBE at a substrate temperature of ℃ or less is heat-treated in H ₂ for 30 minutes at various temperatures. Yes, but 650 ℃
It can be seen that the PL peak wavelength is changed and the PL intensity is significantly reduced when the above heat treatment is performed.
The PL strength of the heat-treated sample at 800 ° C is equivalent to that of the mixed crystal AlxGa _1- xAs that does not have a multilayer structure made by similar Si doping. It is clear that diffusion is progressing [Japanese Journal of Applied Phys
ics Part2 Letters), Volume 24, page L17 (1985)].

On the other hand, in the superlattice layer having the structure in which the Al _x Ga _1-x N layer 23 of the present invention is provided, even if the substrate temperature during growth is 700 ° C.
The generation of X centers was as small as about 1/10 to 1/100 of the amount of Si added to the GaAs layer 24, and the epitaxial wafer having the superlattice structure layer of the present invention was kept in H ₂ at 800 ° C. for 30 minutes. The PL peak wavelength does not change and the PL intensity does not decrease even when subjected to the heat treatment of. Therefore, Al _x Ga
_{Although the} superlattice structure without the _1-xN layer 23 is destroyed by the heat treatment at about 650 ° C., the Al _x Ga _1-xN layer 2 of the present invention
By adopting the structure provided with 3, a superlattice structure that can withstand a heat treatment at 800 ° C can be obtained.

The thickness of the Al _x Ga _1-x N layer 23 does not substantially change the high-density two-dimensional electron gas concentration described above in the case of one or two molecular layers, and also in the sense of preventing mutual diffusion. A significant improvement can be achieved by including the Al _x Ga _1-x N layer 23 of one molecular layer or more. Further, the GaAs substrate 11 has a zinc blende type crystal structure. On the other hand, the Al _x Ga _1-x N layer 23 usually has a wurtz-type crystal structure, but epitaxial growth is currently possible up to 10 molecular layers. Thus, the thicker the Al _x Ga _1-x N layer 23 is, the higher the heat resistance is.

The mechanism by which the heat treatment effect is increased by inserting the Al _x Ga _1-x N layer 23 of the present invention is not clear, but it is thermodynamically predicted that nitride has higher thermal stability than arsenide. It is said that

Although the case where the Al composition ratio x = 0.5 has been described in the present embodiment, the Al composition ratio of the Al _x Ga _1-x As layer 22 is not limited to this value and may be non-zero, and the present invention is applied. Is valid.

〔The invention's effect〕

By applying the structure of the present invention, GaAs and Al _x Ga _1-x As (Al composition ratio x ≠
It is possible to fabricate a crystal layer having a multi-layered structure, that is, a superlattice layer, which has a multi-layered structure of 0) layers alternately, at a high temperature of about 700 ° C. with high reproducibility, and in the manufacturing process, a high temperature treatment of about 800 ° C. That is, Al _x Ga _1-x N of the present invention
By adopting a structure in which a layer is provided, GaAs and Al _x Ga _1-x As (Al composition ratio x
Distortion of the structure due to interdiffusion at the interface of the ≠ 0) layer is prevented, and the process temperature when manufacturing a device from such a superlattice wafer can be significantly increased.

The present invention has a remarkable effect on a superlattice having a period of 100 Å or less, and it is not always necessary to adopt the structure of the present invention when forming a superlattice having a period of more than 100 Å. However, it is said that the present invention is effective when a superlattice having a long period of 100 Å or more is created, or even when the period is long and one material layer constituting the superlattice layer or a few 10 Å or less such as 100 Å or less. There is no end.

Note that the present invention relates to a superlattice layer made of GaAs and Al _x Ga _1-x As, but a superlattice layer made of a combination of other materials is also inserted in a nitride layer superlattice interface similar to the structure of the present invention. As a result, it is possible to increase the growth temperature and further increase the heat treatment temperature during device fabrication.

[Brief description of drawings]

FIG. 1 is a schematic view showing a sectional structure of a wafer including a superlattice layer used in an experiment showing the effect of the present invention, and FIG. 2 is a sectional structure corresponding to almost one period of the superlattice layer in FIG. And FIG. 3 is a graph showing photoluminescence spectra before and after heat treatment in a superlattice layer having a conventional structure. 11 …… GaAs substrate 12 …… buffer Al _x Ga _1-x As layer 13 …… high-purity GaAs layer 14 …… superlattice layer 21 …… ultra-thin GaAs layer forming superlattice layer 22 …… superlattice layer Formed Al _x Ga _1-x As (Al composition ratio x = 0.
5) Ultra-thin layer 23 …… Al _x Ga _1-x N layer 24 …… Si-added region of GaAs forming superlattice layer 25 …… No impurities are added in GaAs forming superlattice layer Area of

Claims (1)

[Claims] 1. A multi-layer structure in which GaAs layers and Al _x Ga _1-x As (Al composition ratio x ≠ 0) layers are alternately stacked to form a superlattice layer, and the GaAs layers and Al _x Ga _{1-x are formed.} At the boundary of the As layer, Al _y Ga _1-y N (Al composition ratio 0 <y ≦
1) A crystal structure having a layer.