JPH0526760B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a technology for epitaxial crystal growth of compound semiconductors. By employing the method of the present invention, it is possible to improve crystal quality, control crystal composition, and increase the degree of freedom in designing a crystal growth furnace.

(Prior art and its problems) In recent years, molecular beam epitaxial growth (MBE)
method: Molecular Beam Epitaxy) and organic metal thermal decomposition vapor phase growth method (MOCVD method: Metal
Orgenic Chemical Vapor Deposition) has been actively researched as it is effective in controlling the thickness of thin film epitaxial layers. The greatest feature of these crystal growth methods is that they enable the production of thin film epitaxy of several tens of angstroms or less. From this point,
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.

However, in heteroepitaxy of these ultrathin films, when the crystal growth temperature is increased, interdiffusion of host constituent elements occurs at the hetero interface, and doping impurities from the crystal layer intended for selective doping diffuse into adjacent layers. Problems arise when a predetermined structure cannot be obtained. Furthermore, which is a particular problem in the MBE method, when the crystal growth temperature is increased, degassing from the chamber wall occurs due to an increase in the temperature of the epitaxy chamber, resulting in a decrease in the purity of the crystal. Furthermore, as the growth temperature increases, the materials constituting the epitaxy chamber are limited to heat-resistant materials, which extremely limits the selection of materials for constituting the epitaxy chamber system.

However, there is a demand to lower the crystal growth temperature as much as possible. For example, a film with a thickness of several tens of Å
The superlattice structure, which consists of only the GaAs layer doped with Si and an undoped AlAs layer with a thickness of several tens of angstroms, does not generate the deep levels seen in the mixed crystal AlGaAs layer, so the electric field operated by a two-dimensional electric gas Although it is highly expected to be used as an electron supply layer for effect transistors, the superlattice structure has a strong photoluminescence (PL) intensity inherent to that structure unless the crystal growth temperature is about 550°C or lower. Phenomena such as "no crystallinity" were observed, and problems such as crystallinity were observed. In other words, to summarize, crystal growth at low temperatures is desirable in order to prevent interdiffusion at heterointerfaces, diffusion of doping impurities in selective doping, and contamination by residual impurities, but low-temperature growth causes problems with crystal quality. There are many cases.

(Object of the invention) The object of the present invention is to provide at least one
A semiconductor wafer containing crystal layers having a multilayer structure is grown by alternately stacking periodic Al _y Ga _1-y As and Al _x Ga _1-x As (x≠y, Al composition X = 0.2 to 1.0) layers. It is an object of the present invention to provide a crystal growth method that can reliably prevent problems such as interdiffusion at the hetero-interface of a multilayer structure and that can obtain high-quality epitaxial crystals.

(Structure of the Invention) The gist of the present invention is that at least one
Semiconductor wafers containing crystal layers with a multilayer structure in which periodic GaAs and AlGaAs (Al composition ,
Thereafter, the crystal growth method is characterized in that heat treatment is performed at a temperature of 570 to 630°C.

(Examples) Hereinafter, the present invention will be described in detail based on Examples.

Here, an example will be described in which a superlattice structure consisting of a Si-doped GaAs layer and an undoped AlAs layer is epitaxially grown.

FIG. 1 shows a cross-sectional view of a wafer containing the superlattice structure to be formed. 11 in Figure 1 is
The GaAs substrate, 12 will be called a buffer layer to avoid impurity contamination etc. from the GaAs substrate.
AlGaAs layer, 13 is a high purity GaAs layer, 14
is a superlattice layer made of ultrathin films of GaAs and AlAs.
The thickness of each layer in the examples is the buffer layer.
The AlGaAs layer 12 is 0.5 μm thick, and the high purity GaAs layer 13 is
1 μm and the superlattice layer 14 is 0.5 μm. 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. 2, the central portion 23 of the GaAs layer 21 constituting the layer structure of the superlattice layer 14 is doped with 3x10 ¹⁸ cm ^-3 of Si.
Undoped GaAs layer 2 made to sandwich this
It consists of 4. When the superlattice layer 14 shown in FIG. 2 is formed at a low temperature of 520°C or lower, Al _x Ga _1-x As (Al composition (ratio
A photoconductivity phenomenon called (persistent photoconductivity), which is considered to be correlated with deep levels, is also hardly observed. For this reason, in mixed crystal AlGaAs, a crystal layer with a high carrier concentration cannot be obtained even when doped with donor impurities, whereas in the superlattice structure, the carrier concentration can be easily increased.

On the other hand, when growing at temperatures higher than 520°C, the occurrence of DX centers or PPCs, which correspond to approximately 1/2 or more of the carrier concentration, began to be observed, and as the growth temperature rose, both increased and the amount of doping increased. Even if the carrier concentration is increased, the carrier concentration does not increase as expected. At present, the origin of the DX center or PPC is unknown, but the generation of these deep levels is due to the diffusion of Si donors into the AlAs layer 22, or the formation of Al and Ga at the interface between the AlAs layer 22 and GaAs layer 21. In any case, it is clear that mutual diffusion at the interface is involved, such as mutual diffusion, or even the simultaneous progress of both. As is clear from the above explanation, the superlattice grown at a temperature higher than 520°C does not have the superlattice structure shown in Figure 2, but rather due to interdiffusion of host constituent elements at the interface, diffusion of donor Si, or It is likely that both are occurring.

Therefore, in order to produce a superlattice structure as shown in FIG. 2, it is necessary to perform crystal growth at a low temperature of 520° C. or lower. However, when epitaxial growth is performed at such low temperatures, good crystallinity is not necessarily obtained. For example, strong PL intensity, which is considered to be unique to a superlattice structure, cannot be obtained, and changes in PL intensity over time are often observed, indicating a problem with crystal stability. Moreover, although it was mentioned above that the carrier concentration can be easily increased with a superlattice structure, this is also not sufficient.

That is, FIGS. 3 and 4 show the superlattice layers obtained when epitaxial wafers having the structure shown in FIG. 1 grown by MBE at a substrate temperature of 520°C or less were heat-treated in H ₂ for 30 minutes at various temperatures. showed changes in physical properties. Figure 3 shows the photoluminescence (PL) spectrum, and it can be seen that the PL peak wavelength changes and the PL intensity decreases significantly when heat treated at 650°C or higher. Recognize. The PL intensity of the heat-treated sample at 800°C is similar to that of a mixed crystal made with similar Si doping.
It is equivalent to AlGaAs, and it is clear that interdiffusion of Al and Ga progresses with heat treatment at 650°C or higher.

Furthermore, it is worth noting that in FIG. 3, when heat treatment is performed at 600°C or lower, the PL intensity increases as the heat treatment temperature increases. Figure 4 shows carrier concentration,
DX center concentration (figure a), and measured at 77K
PPC concentration (n ^PPC _77K ) and carrier concentration measured at 77K and 300K, respectively n _77K and n _300K (b
Figure) shows the dependence on heat treatment temperature T, and as the heat treatment temperature T rises, the carrier concentration increases even below about 650°C. However, Fig. 4b
Each carrier concentration is the amount of added Si (C _Si )
It has been standardized. Although both the DX center concentration and the PPC concentration increase even at low temperatures below 650°C, this amount is less than 1/10 of the carrier concentration and does not pose a practical problem. In Figure 4
The slight decrease in PPC concentration and increase in carrier concentration during low-temperature heat treatment below 650°C correspond well to the increasing trend of PL intensity during low-temperature heat treatment shown in FIG. The above results indicate that such low-temperature heat treatment improves crystallinity or activates inactive doping impurities.

Furthermore, according to the results of a detailed study of heat treatment temperature dependence, the upper limit of heat treatment temperature is approximately 630℃.
The lower limit of the heat treatment temperature at which heat treatment is effective is 570°C. Of course, if the heat treatment time is increased, it is natural that the effect of crystallinity improvement can be seen even below 570°C. Furthermore, instantaneous heat treatment at an upper limit temperature of about 700° C. for about 10 seconds does not significantly change the physical properties of the superlattice structure. However, it can be considered as normal heat treatment time
Heat treatment at a temperature in the range of approximately 570°C to 630°C for about 30 minutes will not damage the physical properties of the superlattice structure.
Furthermore, it is possible to improve crystallinity.

To summarize the results mentioned above, the period 100
Al _y Ga _1-y As and Al _x with at least one period less than Å
Ga _1-x As (x≠y, Al composition showed that a superlattice structure with excellent physical properties could be obtained with good reproducibility by growing crystals at temperatures below 520°C and then heat-treating them at 570-630°C.

The present invention has a remarkable effect on superlattices with a period of 100 Å or less, and when creating a superlattice with a period of more than 100 Å, it is not necessary to use low-temperature growth as in the present invention. However, these
Needless to say, the present invention is effective when creating a superlattice with a long period of 100 Å or more, or even if the period is long, when one material layer constituting the superlattice layer is several tens of Å or less than 100 Å. Nor.

In addition, in the embodiment, an example of a superlattice layer made of GaAs and AlAs was described, but the present invention generally relates to a superlattice layer made of GaAs and _AlAs.
Ga _1-y As and Al _x Ga _1-x As (x≠y, 0.2≦x≦1.0)
It is also effective for superlattice layers consisting of Furthermore, although the growth temperature and subsequent heat treatment temperature differ depending on the material, the method of the present invention of heat treatment at a higher temperature after low-temperature growth can be applied to superlattice layers made of other combinations of materials, and is effective.

(Effect of the invention) By applying this invention, at least one period of Al _y Ga _1-y As and Al _x Ga _1-x with a period of 100 Å or less
A crystal layer having a multilayer structure, that is, a superlattice layer, can be manufactured with good reproducibility by stacking layers of As (x≠y, Al composition X=0.2 to 1.0) alternately. That is, (1) Al _y Ga _1-y As and Al _x Ga _1-x As (Al composition X = 0.2 ~
1.0) Structural disturbances due to interdiffusion at layer interfaces are prevented. Moreover, (2) a low-temperature growth method is taken to prevent the problem of interdiffusion, but the problem of low crystal quality in this case can also be avoided. Furthermore, (3) when producing a superlattice layer, low-temperature growth is possible, so there is less contamination during crystal growth, and restrictions on heat-resistant materials, etc. in the growth apparatus are removed, increasing the degree of freedom in designing the growth apparatus.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing the cross-sectional structure of a wafer containing a superlattice layer used in experiments demonstrating the effects of the present invention, and Figure 2 is a schematic diagram corresponding to approximately one period of the superlattice layer in Figure 1. Band diagram, Fig. 3 is a graph showing the photoluminescence spectra before and after heat treatment, Fig. 4 a is a graph showing the dependence of the net ionized impurity concentration of the superlattice layer and the DX sensor concentration on the heat treatment temperature T, Fig. 4 b is the heat treatment temperature T for the carrier concentration and PPC concentration normalized by the added Si concentration (C _Si )
A diagram showing dependence. 11; GaAs substrate, 12; buffer AlGaAs
Layer 13; High purity GaAs layer 14; Superlattice layer 2
1; GaAs forming a superlattice layer, 22; AlAs forming a superlattice layer, 23; forming a superlattice layer
A region of GaAs doped with Si; 24; a region of GaAs forming a superlattice layer to which no impurities are added;

Claims (1)

[Claims] 1 At least one period of Al _y with a period of 100 Å or less
Ga _1-y As and Al _x Ga _1-x As (x≠y, Al composition X=0.2
~1.0) When growing a crystal layer with a multilayer structure by stacking layers alternately, first, the substrate temperature is
Grow below 520℃, then 570~630
A crystal growth method characterized by heat treatment at a temperature of °C.