JPH0458439B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] This invention relates to - the structure of epitaxial crystals of compound semiconductors, particularly superlattice layers;

[Conventional technology]

In recent years, molecular beam epitaxial growth (MBE)
method: Molecular Beam Epitaxy) and organic metal thermal decomposition vapor phase growth method (MOCVD method: Metal
Organic chemical vapor deposition (organic chemical vapor deposition) is being actively researched because it is effective in controlling the layer thickness of thin film epitaxial layers. The greatest feature of these crystal growth methods is that they enable thin film epitaxy with a thickness of several tens of microns or less. From this point of view, these crystal growth methods are most effective in producing heteroepitaxy of ultra-thin film structures such as superlattices and quantum well structures, and so-called selectively doped crystals.

[Problem that the invention seeks to solve]

However, in the heteroepitaxy of these ultrathin films, increasing the crystal growth temperature or heating during the device fabrication process causes interdiffusion of host constituent elements at the heterointerface, making it difficult to obtain the desired structure. Without it, the problem mentioned above will occur.

However, there is a demand for lowering the crystal growth temperature as much as possible or for lowering the process temperature. For example, a GaAs layer of several tens of angstroms doped with Si and an undoped layer of several tens of angstroms.
The superlattice structure consisting of AlGaAs layer has an Al composition ratio of
Since there is no generation of deep levels seen in mixed crystal AlGaAs layers with a temperature of 0.2 or higher, it is highly expected to be used as an electron supply layer for field effect transistors that operate with two-dimensional electron gas, but the crystal growth temperature is
Unless the temperature is about 550° C. or lower, phenomena such as the inability to obtain the high photoluminescence (PL) intensity inherent to the superlattice structure will be observed.
Furthermore, in the case of this superlattice structure, the upper limit of the process temperature is about 600°C or higher, which causes interdiffusion at the hetero interface, so it is difficult to withstand heat treatment (annealing) of 800°C or higher after ion implantation. In other words, in order to prevent interdiffusion at the hetero-interface, growth and processes must be performed at low temperatures.

The purpose of the present invention is to combine GaAs and AlGaAs (Al composition
0) An object of the present invention is to provide an epitaxial crystal structure that is effective in preventing the problem of interdiffusion at the heterointerface of a multilayer structure in which layers are stacked alternately.

[Means for solving problems]

The epitaxial crystal of the present invention is made of GaAs and
It has a multilayer structure in which layers of AlGaAs (Al composition .

〔Example〕

Hereinafter, this invention will be explained in detail based on examples.

Here, Si-doped GaAs layer and Al _0.5
An example will be described in which a superlattice structure consisting of a Ca _0.5 As layer is epitaxially grown.

FIG. 2 shows a cross-sectional view of a wafer containing the superlattice structure to be formed. In Figure 2, 1
1 is a GaAs substrate with a (100) crystal plane, 12 is a
An AlGaAs layer called a buffer layer for avoiding impurity contamination from the GaAs substrate 11, 13 a high purity GaAs layer, and 14 a superlattice layer consisting of an ultra-thin film of a GaAs layer and an Al _0.5 Ca _0.5 As layer. The thickness of each layer is 0.5 μm for the buffer layer AlGaAs layer 12, 1 μm for the high purity CaAs layer 13, and 0.5 μm for the superlattice layer 14.
The thickness of the GaAs layer constituting the superlattice layer 14 is 25 Å, and the thickness of the Al _0.5 Ca _0.5 As layer is 20 Å.

A band diagram for approximately one period of the layer structure of the superlattice layer 14 is conceptually shown in FIG. As shown in FIG. 3, a Si-doped GaAs layer 23 doped with 3×10 ¹⁸ cm ^-3 of Si is formed in a region with a thickness of approximately 20 Å at the center of the GaAs layer 21 constituting the superlattice layer. An undoped GaAs layer 24 having a thickness of about 2.5 Å is formed to narrow this.

Figure 1 is a cross section of this epitaxial layer structure, showing the GaAs forming the superlattice layer seen from the (110) plane.
This shows the atomic structure at the interface between the layer 21 and the Al _0.5 Ca _0.5 As layer 22, where the black circle indicates a Ga atom 31, the open circle ○ indicates an Al atom 32, and the square □ indicates a group V atom.
The As atom 34 and the black square mark (■) indicate the N atom 35, which is a group V atom.

The gist of this invention is that the Al _0.5 Ca _0.5 As layer 22 and the undoped GaAs layer 24 are in contact with each other, usually indicated by a □ mark.
As shown in FIG. 1, a large amount of N atoms 35 indicated by black circles are added to one layer of group V atoms, which should be composed of As atoms, and the layer is essentially an N atomic layer. The N atomic layer is inserted into the GaAs layer 2 constituting the superlattice layer 14.
1 or the epitaxial growth of the Al _0.5 Ca _0.5 As layer 22 is interrupted, and the Al _0.5 Ca _0.5 As layer 22 or
Although NH ₃ was introduced into the system when transferring to the GaAs layer 21, an N atomic layer can also be obtained by introducing N ₂ or N ₂ H ₄ .

According to the superlattice structure of this example, Japanese Journal of Applied Physics Part 2, Letter (Japanese Journal of Applied Physics Part 2)
Applied Physics Part2 Letters), Volume 22,
Page L627 (1983) and Japanese Journal of Applied Physics
Part 2, Letter (Japanese Journal of
Applied Physics Part2 Letters), Volume 24, L17
As shown in Page (1985), Si, Te,
The concentration of deep levels called DX centers, which are commonly observed in AlGaAs mixed crystals doped with donors such as Se (Al composition ratio
Photoconductivity, which is considered to be correlated with deep levels, is hardly observed, so in contrast to mixed-crystal AlGaAs, a crystal layer with a high carrier concentration cannot be obtained even when doped with donor impurities. With the superlattice structure, the carrier concentration could be easily increased.

In contrast, in conventional superlattice structures, when grown at high temperatures of 520°C or higher, the occurrence of DX centers or PPCs, which corresponds to about 1/2 or more of the carrier concentration, begins to be observed, and as the growth temperature increases, both Even if the amount of doping is increased, the carrier concentration does not increase as expected. At present, the origin of the DX center or PPC is unknown, but this may be due to the generation of deep levels as mentioned above.
It is expected that this is caused by mutual diffusion of Al and Ga at the interface between the AlGaAs layer and the GaAs layer.

Furthermore, there are also restrictions on process temperature when forming devices using conventional superlattice layers. In other words, Figure 4 shows an epitaxial wafer with a conventional AlGaAs/GaAs superlattice layer grown by MBE at a substrate temperature below 520°C in _H2 .
This shows the photoluminescence (PL) spectra of the superlattice layer when heat treated at various temperatures for 30 minutes, and the PL peak wavelength changes when heat treated at 650°C or higher. It can also be seen that the PL intensity has decreased significantly.
The PL intensity of the sample heat-treated at 800°C is equivalent to that of mixed crystal AlGaAs, which does not have a multilayer structure, and is made with similar Si doping. The progress is clear. (Japanese Journal of Applied Physics Part 2, Letter (mentioned above)
Physics Part 2 Letters), Volume 24, Page L17 (1985)).

On the other hand, in a superlattice layer with a structure in which a large amount of N atoms is added and a layer of N atoms is substantially provided as shown in this example, even if the substrate temperature during growth is 700°C, the DX center The occurrence of is as small as about 1/10 to 1/100 of the amount of Si added to the GaAs layer 23.
Furthermore, even when the epitaxial wafer with the superlattice structure layer of this example was heat-treated in H ₂ for 30 minutes at 800°C, the PL peak wavelength did not change, but the PL intensity decreased. Nor. Therefore,
The conventional superlattice structure without N atomic layer is 650
While the superlattice structure is destroyed by heat treatment at temperatures around 800°C, by adding a large amount of N atoms and essentially creating a structure with a layer of N atoms, a superlattice structure that can withstand heat treatment at 800°C can be obtained. .

The reason why the heat-resistant treatment effect can be obtained by adding a large amount of N atoms, that is, by essentially providing a layer of N atoms, is not necessarily clear, but
The N atomic layer is inserted into GaAs constituting the superlattice layer 14.
At the interface between the layer 21 and the Al _0.5 Ga _0.5 As layer 22, a large amount of Ca-N or Al-N bonds with high bond energy is formed, and this strong bond energy
It can be considered that Al interdiffusion is inhibited.

In this embodiment, the case where the Al composition ratio of the AlGaAs layer is 0.5 has been described, but it is not limited to this value and may be any value other than zero when applying the present invention.

In addition, we took an epitaxial layer formed on the (100) crystal plane as an example, but the GaAs layer 21 and Al _0.5 Ga _0.5
Since it is desirable to make the Ga-As-Al bonds at the interface of the As layer 22 into Ga-N-Al bonds as much as possible, a crystal plane with polarity was selected.
Therefore, it goes without saying that the effect is extremely high even on the (111) plane, but it is also possible to use Ca-N-Al near the interface on the (211) plane and other crystal planes that do not necessarily have polarity. These crystal planes can be selected since the bond concentration is considered to be high.

In the above embodiment, an example of a superlattice layer made of GaAs and Al _0.5 Ga _0.5 As was described, but by adopting the structure of the present invention for a superlattice layer made of a combination of other materials, the growth temperature can be reduced. Furthermore, it is possible to increase the heat treatment temperature during device fabrication. That is, by employing the method of the present invention, a superlattice layer structure with high thermal stability can be obtained.

〔Effect of the invention〕

By applying the structure of this invention, periodic
GaAs and AlGaAs with at least one period less than 100 Å
(Al composition High temperature processing becomes possible. In other words, by adding a large amount of N atoms and having a structure in which a layer of N atoms is substantially provided, the present invention has a structure in which GaAs and AlGaAs (Al composition), which has been a problem when fabricating a superlattice layer, are eliminated. X≠
0) Structural disturbances due to interdiffusion at layer interfaces are prevented, and the process temperature when manufacturing devices from such superlattice wafers can be significantly increased.

The present invention has a remarkable effect on superlattices with a period of 100 Å or less, and it is not necessarily necessary to employ the structure of the present invention when creating a superlattice with a period of more than 100 Å. However, the present invention is particularly effective when creating such a superlattice with a long period of 100 Å or more, or when one material layer constituting the superlattice layer has a length of several tens of Å or less, even if the period is long. Needless to say.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of the atomic arrangement at the interface of the superlattice layer as viewed from (100), which is a cross-section of the wafer. Figure 2 is a schematic diagram showing the cross-sectional structure of the wafer containing the superlattice layer of the present invention. is a schematic band diagram corresponding to approximately one period of the superlattice layer in FIG. 2, and FIG. 4 is a graph showing photoluminescence spectra before and after heat treatment in the superlattice layer of the conventional structure. 11...GaAs substrate, 12...Buffer
AlGaAs layer, 13... High purity GaAs layer, 14...
Superlattice layer, 21...GaAs layer, 22...AlGaAs
layer, 23... Si-doped GaAs layer, 24... undoped GaAs layer, 31... Ga atom, 32... Al atom, 34... As atom, 35... N atom.

Claims (1)

[Claims] 1 It has a multilayer structure in which GaAs and AlGaAs (Al composition Characteristic epitaxial crystal.