JPH02263798A - Superlattice element - Google Patents

Abstract

PURPOSE:To epitaxially grow a superlattice in the thickness exceeding the critical film thickness of the single superlattice in the superlattice of ZnTe and ZnS by combining the superlattices of the same material having smaller and larger lattice constants than that of a substrate. CONSTITUTION:ZnTe and ZnS are alternately laminated on the GaAs substrate to produce a superlattice element. In this case, the superlattice I consisting of ZnTe and ZnS is formed on the substrate so that the average lattice constant is made smaller than the lattice constant of the substrate, the superlattice Il consisting of ZnTe and ZnS is formed on the superlattice I so that the average lattice constant is made larger than the lattice constant of the substrate, and the thicknesses of the superlattices I and II are selected so that the average lattice constant of the entire system consisting of the superlattices I and II is practically equalized with the lattice constant of the substrate to obtain the superlattice element.

Description

Detailed Description of the Invention Field of the Invention The present invention relates to various electronic devices, optoelectronic devices such as light emitting diodes, and other superlattice structure devices using a superlattice structure that require high quality crystallinity. .

Conventional technology Superlattice is a method of artificially creating materials with unusual atomic and molecular arrangements that do not exist in nature, and researching them in an attempt to improve the functions of conventional materials or create new functions. Development has been gaining momentum in recent years. Especially GaAs/
Some devices have already been manufactured using superlattices of ■-V group compound semiconductors such as AlAs. In the Peter stacked structure or superlattice structure that has been put into practical use, including these, the crystal lattice constants of the substrate and the superlattice, or the substances that make up the superlattice, have extremely close values, and crystal defects It is relatively easy to obtain a combination that allows high-quality heteroepitaxial growth without causing.

On the other hand, if there is a difference of several percent or more in lattice constants, this will have a non-negligible effect on crystal growth, but it is difficult to substitute other materials, so it is difficult to create a form that incorporates these strains. So-called strained superlattices are also observed at each pole. This is an attempt to grow epitaxially so that the lattice is in the form of a cane with some distortion and lattice relaxation does not occur.

Problems to be Solved by the Invention Reducing defects caused by differences in lattice constants is an important problem even in heteroepitaxial growth or superlattice growth using a single type of material, and this poses a number of constraints. There is a countermeasure that involves forming a buffer layer between the substrate and the film to be epitaxially grown on it and forming the necessary film after lattice relaxation has been completed, but this method requires a process called buffer layer formation. Furthermore, it is inappropriate in cases where direct contact between the substrate and the film is required for device functionality. on the other hand,
When the lattice constants differ, it is known that defects will not occur if the grown film thickness is kept below a certain thickness depending on the difference in lattice constants, but this critical film thickness is not necessarily theoretically determined. It is becoming clear that this is not something that can be determined and is influenced by growth conditions. Therefore, the method of keeping the film thickness below a certain thickness cannot necessarily be said to be effective, and poses problems such as being a constraint on the design of superlattices that pursue various functions.

In view of the above-mentioned problems, the present invention provides a superlattice structure element that achieves matching with the lattice of a substrate using only the material constituting the superlattice.

Means for Solving the Problems In order to solve the above-mentioned problems, the present invention provides superimposed semiconductors made of ■-■ group compound semiconductors, such as ZnTe and ZnS, which are a combination of a substance with a larger lattice constant and a substance with a smaller lattice constant than that of the substrate material. In the lattice, first, ZnTe is formed into a superlattice so that the average lattice constant is slightly smaller than that of the substrate.
A superlattice I in which each layer thickness of ZnS and ZnS is determined is formed on a GaAs substrate, and then a superlattice II is formed on the GaAs substrate so that the average lattice constant of the superlattice is slightly larger than that of the substrate. A superlattice structure element formed thereon is provided.

Effect By configuring a superlattice made of the same material system as a combination of lattice constants smaller and larger than the lattice constant of the substrate, it is possible to create a thin layer in which lattice matching with the substrate cannot necessarily be achieved with a superlattice of a single structure. Repeated superlattices have much better consistency and can be epitaxially grown to thicknesses that exceed the critical thickness of a single superlattice.

EXAMPLE GaAs is very suitable as a substrate for use in heteroepitaxial growth due to its quality and accumulated handling techniques. Although there are several combinations of II-Vl group compound semiconductors with lattice constants larger and smaller than that of the substrate, a specific example will be described here using a combination of ZnTe and ZnS as an example.

If we consider simply from the size of the lattice constant, Z n
It seems possible to achieve average lattice matching with the GaAs substrate by setting each T eN Z n S to an appropriate thickness, but quantum effects as a superlattice, such as when using potential well tunnels, etc. It is necessary to make each layer extremely thin, and in particular, when each layer requires several tens of atomic layers or less, perfect lattice matching cannot be achieved no matter how they are combined. Therefore, when making each layer thinner, the superlattice is divided into two parts, one whose average lattice constant is slightly smaller than that of the substrate, and the other whose average lattice constant is larger than that of the substrate.
It is necessary to obtain lattice matching as an overall average.

This situation is schematically shown in FIG. Average lattice constant of superlattice I d sIs Average lattice constant of superlattice ■ dsn and lattice constant of substrate d The size relationship of GaAs is ds+<d
amns<dsxx. Compared to a superlattice with a single structure consisting of alternating layers of ZnTe and ZnS, ds+ and ds2 are dGsll.
Since it is possible to obtain a value close to s even if the ZnTe and ZnS layers become thinner, the overall lattice constant of the superlattice and superlattice can be made much closer to that of the substrate.

ZnTe-ZnS superlattice crystal growth has been developed using the molecular beam epitaxy method, considering the advantages of high purity under ultra-high vacuum, low-temperature growth in non-equilibrium conditions, and steep interfaces due to instantaneous switching by shutter operation of the molecular beam. (MBE method) is used.

FIG. 2 shows a schematic configuration diagram of the MBE device. Only the growth chamber is shown, and the load-lock chamber, substrate movement mechanism, etc. are omitted.

The GaAs substrate 4 set on the substrate holder 5 is heated to a required temperature by the heating mechanism 3. The degree of vacuum in the growth chamber 1 is monitored by an ionization vacuum gauge 7, and the residual gas is monitored by a quadruple seed mass spectrometer 6. The state during thin film crystal growth is observed by reflection high-speed electron diffraction (RHEED) 9, and the pattern is projected on a screen 8.

The growth chamber 1 is evacuated to a level of 10” Torr using the exhaust system 2.
In addition, cells 11 a sl l b , l containing raw materials
1c is stabilized at a temperature at which a predetermined molecular beam intensity is obtained, and then the temperature of the GaAs substrate 4 is raised to 600° C. to remove the surface oxide film. At this time, by observing sharp streaks in the RHEED pattern, it is confirmed that this thermal etching has been completed. Next, the substrate temperature is lowered to approximately 250 to 350° C. depending on the purpose, and superlattice formation is started. The substances to be deposited are switched by opening and closing the shutters 10a, 10b, and 10c. That is, during ZnS deposition, the shutter 100 of the cell llc containing the ZnS raw material is opened and the others are closed, and after a certain period of time, the ZnS shutter 10c is closed and the Zn and Te
Shutter 10a of cells lla and flb containing raw materials
and open 10b. Repeat this operation to remove ZnS and Zn
Te and Te are laminated alternately. The thickness of each layer is controlled by the opening/closing time of each shutter, and the molecular beam intensity is kept constant.

Since the MBE method can control film formation with an accuracy of one atomic layer, it is possible to create a superlattice made of extremely thin layers. In the formation of the superlattice structure in Fig. 1, which consists of one molecular layer of ZnTe and three molecular layers of ZnS, zns, Zn+T
eEach molecular beam intensity is 2X1G-', 5X10
-', IXIG-'' under the conditions of Torr (flux monitor value using an ionization vacuum gauge), ZnTe,
Deposition times for Z n S were 3 seconds and 3.5 seconds, respectively. Similarly, to form a superlattice consisting of two molecular layers of ZnTe and three molecular layers of ZnS, the deposition time for each is 7.
seconds and 3.5 seconds. When superlattice material is deposited for 100 cycles and superlattice II is deposited for 264 cycles using these combinations, the average lattice constant is 5.8531, assuming that the height of strain is extremely small, which is 5.8531% for the GaAs substrate.
There is a slight mismatch of 0.0025% with respect to 1i533A, and even if the film thickness reaches the micron order, the influence of lattice relaxation can be avoided.

Effects of the Invention If the superlattice structure according to the present invention is used, it becomes possible to form a short-period superlattice while achieving lattice matching that cannot be achieved with a single-structure superlattice, thereby eliminating crystal defects caused by lattice distortion. It is possible to eliminate the generation of superlattices, create a superlattice of the necessary material directly on the substrate, and grow it to a thickness that exceeds the critical film thickness for a single superlattice. This increases the degree of freedom in superlattice structure design, making it particularly effective for device fabrication.

[Brief explanation of drawings]

FIG. 1 is a conceptual diagram for explaining a superlattice structure related to the present invention, and FIG. 2 is a schematic diagram of a molecular beam epitaxy apparatus used for producing a superlattice structure element in an example. 4...Substrate, 10a+10b, 10c...Shutter 11 asl l b+11c...Cell O containing raw materials Name of agent Patent attorney Shigetaka Awano 1 person Figure Figure

Claims (3)

[Claims] (1) In a superlattice structure element manufactured by alternately layering ZnTe and ZnS on a substrate made of GaAs, ZnTe and ZnS are layered on the substrate so that the average lattice constant is smaller than the lattice constant of the substrate. A superlattice I consisting of ZnTe and ZnS is formed on the superlattice I, and a superlattice II consisting of ZnTe and ZnS is formed on the superlattice I such that the average lattice constant is larger than the lattice constant of the substrate. A superlattice structure element, wherein the thickness of each of the superlattice I and the superlattice II is selected so that the average lattice constant of the entire system consisting of the superlattice II is approximately equal to the lattice constant of the substrate. (2) The superlattice structure element according to claim 1, wherein CdS is used in place of ZnTe. (3) The superlattice structure element according to claim 1, wherein CdSe is used in place of ZnTe.