JP6920673B2 - Method of forming a GaAs layer on the Ge (111) plane and tandem solar cell - Google Patents

Description

The present invention relates to a method for forming a GaAs layer on a Ge (111) plane, and in particular, a layer capable of suppressing the formation of rotating twins in a GaAs layer and providing a high-quality layer structure or a highly efficient solar cell structure. Regarding the structure formation method.

Although the tandem type solar cell is highly efficient, it tends to be expensive. For example, a Ge solar cell structure and a GaAs solar cell structure laminated on a Ge single crystal substrate are limited to space use. One of the reasons for the high cost is that the Ge substrate used is an expensive Ge (001) single crystal substrate.

On the other hand, as a method for producing a tandem solar cell without using a Ge single crystal substrate, a large particle size Ge polycrystalline film is formed on a glass substrate by an Al-induced crystallization method, and further, III is formed on this Ge film. A method for forming a −V group compound semiconductor has been proposed (Patent Document 1).

According to Non-Patent Document 1, the orientation of the large particle size Ge film formed on the glass substrate by the Al-induced crystallization method is the (111) direction. Therefore, in order to produce a high-efficiency tandem solar cell by the method of Patent Document 1, it is necessary to form a high-quality III-V compound semiconductor on the Ge (111) surface.

On the other hand, there is almost no research on the heteroepitaxy of group III-V compound semiconductors to Ge (111) because there was no need for it, but for Si of the same family as Ge, group III-V on Si (111). It is known that rotating twins are generated in heteroepitaxy of compound semiconductors (Non-Patent Documents 2 to 8). Since rotating twins cause deterioration of semiconductor quality or solar cell characteristics, their occurrence needs to be suppressed as much as possible, and in fact, III-V compound semiconductors on Si (111) In heteroepitaxis, suppression of the generation of rotating twins has been studied by various methods (Non-Patent Documents 4 to 8).

Therefore, the inventor of the present application tried to epitaxially grow GaAs on a Ge (111) substrate with one goal of establishing an inexpensive manufacturing method, and confirmed that rotating twins were always generated in the GaAs.

That is, when trying to manufacture or manufacture a tandem type solar cell composed of a Ge solar cell and a GaAs solar cell, conventionally, an expensive Ge (001) single crystal substrate is used, or a glass substrate is used to inexpensively obtain a Ge layer. There is a problem that one has to choose either to obtain a GaAs solar cell structure of which the quality is not always good, which contains rotating twins, although it can be produced.

JP 2013-138128

K. Toko, M. Kurosawa, N. Saitoh, N. Yoshizawa, N. Usami, M. Miyao, and T. Suematsu, Applied Physics Letters 101, 072106 (2012). M. Kitabatake, T. Kawasaki, and T. Korechika, Thin Solid Films 281, 17-19 (1996). K. Murata, N. B. Ahmad, Y. Tamura, M. Mori, C. Tatsuyama, and T. Tambo, Journal of Crystal Growth 301, 203-206 (2007). M. Deura, Y. Kondo, M. Takenaka, S. Takagi, Y. Nakano, and M. Sugiyama, Journal of Crystal Growth 312, 1353-1358 (2010). A. Proessdorf, F. Grosse, O. Romanyuk, W. Braun, B. Jenichen, A. Trampert, and H. Riechert, Journal of Crystal Growth 323, 401-404 (2011). H. Toyota, A. Okabe, T. Endoh, Y. Jinbo, and N. Uchitomi, Journal of Crystal Growth 378, 129 (2013). O. Morohara, H. Geka, Y. Moriyasu, and N. Kuze, Journal of Crystal Growth 378, 113 (2013). H. Suzuki, D. Ito, A. Fukuyama, and T. Ikari, Journal of Crystal Growth 380, 148-152 (2013).

The present invention has been made in view of the above, and an object of the present invention is to provide a method for obtaining a layered structure by epitaxially growing GaAs by suppressing the generation of rotating twins on the Ge (111) plane.
Another object of the present invention is to provide a technique for inexpensively obtaining a tandem type solar cell structure including a Ge solar cell and a GaAs solar cell.

The layer structure forming method according to claim 1 comprises epitaxially growing GaSb as a buffer layer on a Ge (111) plane and then epitaxially growing GaAs to suppress the generation of rotating twins in the GaAs layer. It is a feature.

According to the present invention, a GaAs on Ge (111) when the epitaxial growth, Ru can be suppressed the occurrence of rotational twins in GaAs.

It is a (224) X-ray diffraction pattern of GaAs epitaxially grown on a Ge (111) substrate by a molecular beam epitaxy method (MBE). The upper plot is the diffraction from the Ge substrate and the fundamental domain of GaAs, and the lower plot is the diffraction from the sub-domain of GaAs. The larger the difference between the upper and lower plots (logarithmic plot), the smaller the volume ratio of the rotating twin region. 1 (a) shows the case without the GaSb buffer layer, FIG. 1 (b) shows the case where the GaSb buffer layer diatomic layer (2ML) is provided (thickness is about 0.8 nm), and FIG. 1 (c) shows GaSb. The case where the buffer layer 20 atomic layer (20ML) is provided (thickness is about 8 nm) is shown. This is a configuration example of a tandem solar cell.

As a result of diligent studies, the inventor of the present application discovered that the generation of rotating twins in GaAs was suppressed by interposing GaSb in Ge (111), and the present invention was made based on this. Is.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. Here, when epitaxially growing GaAs on a Ge (111) substrate, an experiment was conducted in which GaSb was laminated with different thicknesses in between to examine the generation of rotating twins of GaAs.

Specifically, when MBE growing GaAs of 0.5 mm at a substrate temperature of 550 ° C. on a Ge (111) substrate, samples sandwiching 0, 2 ML and 20 ML GaSb buffer layers were prepared, respectively. Asymmetric (224) reflections from the normal growth region and the rotating twin region were measured for those samples.

Crystallinity was assessed by asymmetric (224) reflection in X-ray diffraction. The results are shown in FIG. The upper plot in each figure is the (224) reflection from the normal growth region (the sharp peak is the (224) reflection from the Ge substrate) and the lower plot is the (224) reflection from the rotating twin region. .. The larger the difference between the upper and lower plots (logarithmic plot), the smaller the volume ratio of the rotating twin region. From the figure, it can be seen that the rotation twin region (sub-domain) in GaAs is effectively suppressed by providing the GaSb buffer layer. When considering a solar cell, translucency is ensured if the thickness is up to about 100 nm, so that sufficient performance can be exhibited even with a buffer layer of about 10 nm as shown in FIG. 1 (c). Is.

In the above description, the GaSb buffer layer and the GaAs layer are grown by the molecular beam epitaxy method, but other growth methods such as the organic metal vapor phase growth method may be adopted.

As a configuration example of the tandem type solar cell, a stacking example of FIG. 2 can be mentioned. When manufacturing a tandem solar cell, not only a two-junction structure in which a GaAs solar cell structure is laminated on a Ge solar cell structure, but also a three-junction structure in which an InGaP solar cell structure is further added on a GaAs solar cell structure. May be provided.

As described above, since the GaAs epitaxial layer having a small number of rotating twin regions can be grown on the Ge (111) plane, the (111) oriented large particle size Ge polycrystalline film on the glass substrate also has a rotating twin region. A small number of GaAs epitaxial layers can be grown, and a highly efficient tandem solar cell can be obtained.

In the above example, it is an invention using GaSb as a buffer layer. That is,
A method for forming a layer structure, which comprises epitaxially growing GaSb on a Ge (111) plane and then epitaxially growing GaAs.
A method for forming a layer structure, which comprises forming a Ge layer on a glass substrate, then forming a buffer layer of GaSb, and then forming a GaAs layer.
A tandem type solar cell comprising a structure in which a GaAs layer is formed by sandwiching a GaSb buffer layer on a Ge (111) surface.
The AlSb in terms of the product layer defects generated energy may be employed as a buffer layer. That is, the invention can be as follows.
1. 1.
A layer structure forming method characterized by epitaxially growing an AlSb buffer layer on a Ge (111) plane and then epitaxially growing GaAs.
2.
A layer structure forming method, characterized in that a Ge layer is formed on a glass substrate, then a buffer layer of AlSb is formed, and then a GaAs layer is formed.
3. 3.
A tandem type solar cell comprising a structure in which an AlSb buffer layer is sandwiched on a Ge (111) surface and a GaAs layer is formed.

10 Glass substrate 11 n ^{+ type} Ge layer 12 n type Ge layer 13 p type Ge layer 14 p ^{+ type} Ge layer 21 p ^{+ type} GaSb buffer layer 31 n ^{+ type} GaAs layer 32 n type GaAs layer 33 p type GaAs layer 34 p ⁺ Shaped GaAs layer 41 n electrode 51 p electrode

Claims (1)

A method for forming a layer structure, which comprises epitaxially growing GaSb as a buffer layer on a Ge (111) plane and then epitaxially growing GaAs to suppress the generation of rotating twins in the GaAs layer.

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