JPH03159990A - Element having superlattice structure - Google Patents

Abstract

PURPOSE:To obtain an element having superlattice structure and useful for a device requiring a high-quality crystallinity by forming a buffer layer with a substance having a lattice constant larger than that of the substrate and alternately depositing specific substances having different lattice constant on the buffer layer. CONSTITUTION:A buffer layer composed of a substance A having a lattice constant aA is formed on a substrate composed of a substance S having a lattice constant aS(aA>aS). A substance B having a lattice constant aB(aB<aS) and the substance A are alternately deposited on the buffer layer to obtain the objective element having superlattice structure. The presence of the buffer layer composed of a substance having a lattice constant larger than the lattice constant of the substrate is effective in decreasing the influence of the compressive stress on the substance having larger lattice constant among the substances constituting the superlattice and improving the stability of the deposition. Accordingly, the controllability of the structure can be improved compared with the direct formation of a superlattice on the substrate.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a superlattice structure element used in various electronic devices, optoelectronic devices, etc. that require high quality crystallinity.

Conventional technology A superlattice is a material with an arrangement of atoms and molecules that does not exist in nature.It is artificially created to improve functions that cannot be achieved with conventional materials, or as an attempt to create new functions. Research and development has become increasingly profitable in recent years. In particular, gallium arsenide/7lumium arsenide (GaAs/Ae
Some devices have already been made using m-v group compound semiconductor superlattices such as (As-based). In the heterostack or superlattice structure of devices currently in practical use, the crystal lattice constants of the substrate and the superlattice, or of the substances that make up the superlattice, have extremely close values. 9. It is relatively easy to obtain a string that allows high-quality heteroevitaxial growth without producing crystal defects.

On the other hand, differences in lattice constants of a few percent or more have a non-negligible effect on crystal growth, but it is difficult to substitute them with other materials, so it is difficult to eliminate these strains. Various attempts have been made to create so-called strained superlattices in a complicated manner. This is an attempt to evitaxially elongate the lattice in a state where the lattice has some strain and without causing lattice relaxation.

Problems to be Solved by the Invention Various film methods are used to fabricate superlattices, but
The molecular beam epitaxy method (MBE method) and the MOCVD method have become mainstream in view of their good controllability. Depending on the purpose of the superlattice, it may be necessary to fabricate a superlattice structure with a very short period, for example, one having a period of about 1 to 20 times.

The so-called atomic layer epitaxy method (▲LX method) is considered to be most suitable for accurately controlling the short-period structure. A
The LE method utilizes the principle that if the substrate temperature is appropriate, the same atoms will not adhere to the atomic plane on which a bond has been formed, and a layer of different types of atoms can be formed.

However, when the ALE method is used to fabricate a superlattice, there is a problem that depending on the intended structure of the superlattice, an ideal ALE method that is formed layer by layer may not necessarily be obtained.

For example, the human substance has three layers. When trying to create a superlattice made of two layers of substance B, it is possible that ▲ actually has two layers instead of three. moreover,
The number of layers (III) to be formed relative to the planned layer thickness (Il1.) of ▲ also depends on the layer thickness of B. As an example, the straight line (B) in FIG. 2 shows the relationship between the actual number m of deposited layers of substance A (zinc telluride ZnTe) and the layer thickness of substance B (zinc sulfide Zn8). This is because when trying to form three layers of p ZnTe using the ALIC method when making a ZnTe-ZnS superlattice a certain length on material C (GaAs), the number of ZnTa layers actually deposited is affected by the layer thickness of ZnS. receive,
It shows that the thicker the ZnS layer, the more difficult it is for ZnTe to deposit. In order to improve the controllability of superlattice production, such a phenomenon is not desirable, and it is necessary to make the adhesion rate of each layer (number of layers/planned number of layers=m/mo) as uniform as possible.

The present invention solves the above-mentioned conventional problems, and aims to provide a superlattice structure element using a combination of materials having different lattice constants.

Means for Solving the Problems In order to achieve this object, in the present invention, a single layer of buffer is formed on a substrate, consisting only of substances having a larger lattice constant than that of the substrate among the substances constituting the superlattice, A superlattice is formed on one layer of this buffer.

The mechanism by which the deposition rate of the superlattice is affected by the structure of the superlattice during the fabrication of the working superlattice is not necessarily clear, but it is thought that the change is due to the difference in the lattice constant.

By forming the buffer layer with a material that has a larger lattice constant than that of the substrate, it is possible to reduce the effect of compressive stress on the material with a large lattice constant among the different materials that make up the superlattice, and improve the adhesion rate. can be stabilized. Therefore, it is possible to improve the controllability of the structure compared to the case where the superlattice is directly formed on the substrate.

Examples The present invention provides a structure that can be widely used to create a superlattice by combining materials with different lattice constants.
Here, the superlattice 'il-
A case where the semiconductor device is formed on a group V compound semiconductor substrate will be described with reference to the drawings.

FIG. 1 is a cross-sectional view of a superlattice structure element according to an embodiment of the present invention, and the straight line A in FIG.
Direction aB is the adhesion rate during crystal lengthening of a conventional superlattice structure element, and FIG. 3 is a schematic diagram of an apparatus used to fabricate a superlattice structure element in an embodiment of the present invention.

As shown in FIG. 1, the superlattice structure element according to the present invention has an I
-I-
Forming a buffer layer 2 consisting of M group compound substance ▲,
A substance B3 selected from the I-Vl group compounds is added thereto.
It has a structure in which material (4) and material (4) are alternately deposited. 5 shows a layer in which material B3 and material 7.\-noA4 are deposited alternately. Here, the substance S surrounding the substrate 1 is gallium arsenide (GaAs), indium arsenide (InP), or indium arsenide (
Substances A and B, which constitute the buffer layer and the superlattice layer formed thereon, are single crystals of group IV compound semiconductors such as InAs), and are each made of group H compounds such as sulfide. Zinc (ZnS), cadmium sulfide (C
These two materials are selected from among (IS), zinc selenide (ZnSe), and the like. Note that the substance S
, A and B, each lattice constant 2L
8. aA and aB must satisfy the relationship a, )a, )aB.

Next, a method of manufacturing a superlattice structure element according to an embodiment of the present invention will be explained using the apparatus shown in FIG.

1-V group compound semiconductor substrate, for example GaAs (a6
aAs=6. e 5 3 3 people) I-4 group compound semiconductor, such as ZnTe (a2, Te=6.10
37 people) Button Yoto ZnS (aznS =5.j$09
The superlattice consisting of 3A) is formed as follows. Z
The nTe-ZnS superlattice crystal has high purity under ultra-high vacuum and low temperature stability in non-equilibrium state. A molecular beam epitaxy device (MBE device) is used, considering the advantages of instantaneous switching of the molecular beam by shutter operation, such as the formation of a steep interface. FIG. 3 shows a schematic diagram of the MBE apparatus used to fabricate the superlattice structure element of the present invention. Only a certain long chamber is shown, and the load lock chamber, substrate moving mechanism, etc. are omitted. The GaAs substrate 24 set on the substrate holder 25 is maintained at a required temperature by the heating mechanism 23. The degree of vacuum in the chief chamber 21 is measured by an ionization vacuum gauge 27, and the residual gas is measured by a quadrupole mass spectrometer 26. The appearance of thin film crystals and crystals was observed using a reflection high-speed electron diffractometer (R
(HEED) 29, and the pattern is projected on the screen 28.

The inside of the long chamber 21 is heated to 1 Cr10 Torr level 1 using the exhaust system 22.
After evacuation and stabilizing the cells 31a to 310 containing the raw materials at a temperature at which a predetermined molecular beam intensity is obtained,
The temperature of the GaAs substrate 24 is raised to 600°C to remove the surface oxide film. At this time, the RHEED 9 page 7 pattern of sharp nuts is observed on the screen 28 to confirm that the thermal etching has been completed. Next, the substrate temperature is lowered to about 200-350°C depending on the purpose, and superlattice formation is started.

1. First, a single layer of material A (such as ZnTe) is formed. In this case, the ALE method or the MBF method may be used, but since the MBE method is faster in terms of time, the latter will be used as an example. The shutters 30a and sob'l of the cells 31a and 3lb each containing zinc (Zn) and ternore (Te) in the form of a single substance are simultaneously opened and closed after a predetermined time. The substrate temperature is 330°C, and the molecular beam intensities of Zn and Te are 2X10-76X 10-7 Tor, respectively.
About 1,500 people were deposited in one hour according to r (fusix monitor value by ionization vacuum gauge κ). Thereafter, the substrate temperature was lowered to 240 DEG C., and then the substrate was transferred to a certain length of superlattice in which material B and material Ai were alternately deposited. The molecular beam intensity is constant 11. The substances to be deposited are switched using the shutters 30a to 30C.
This is done by opening and closing. That is, during the deposition of ZnS, which is substance B, the shutter starter 10 of the cell 310 containing the ZnS raw material
The vane 30C is opened, the others are closed, and the ZnS shutter 300'Q is closed after a certain period of time. The molecular beam intensity of ZnS is 7 X I Cr 7 Torr. 8 Here, a ZnS compound raw material was used in consideration of handling in an ultra-high vacuum, but Zn and sulfur (S) individual raw materials or S
i It may also be supplied by cranking hydrogen sulfide (H2S) gas. Next, after an appropriate interval (1 to several seconds), the shutters SOa of the cells 31a and 31b containing Zn and Te raw materials are opened to deposit ZnTef, which is the substance A.
and 30b are alternately opened and closed a preset number of times. That is, when attempting to form n layers, each gate is opened and closed n times. This operation is repeated to alternately deposit ZnS as the material B and ZnTe as the material.

After forming one layer of ZnTe buffer as described above, ZnTe. The relationship between the number m of ZnTe layers constituting the superlattice and the number of ZnS layers when a -ZnS superlattice is formed is shown directly in FIG. 2 by a (A). The vertical axis is the actual ZnTe
The number of layers m is the planned number of layers m. It has been standardized. Does it also show if there is no buffer for comparison? 1. There is Kuno. For those without a single buffer layer, after thermal etching of the GILAS substrate, lower the substrate temperature to 240 °C.
This is a sample in which the superlattice was lengthened under similar conditions. As is clear from this figure, when a ZnTa-ZnS superlattice is directly grown to a certain extent on a GaAs substrate, the number of ZnTe layers decreases as the number of ZnS layers increases. This means that the number of adhesion layers is significantly reduced. On the other hand, when a single layer of ZnTe buffer was used, this tendency was improved and the ZnTa deposition rate was made uniform.

Above, we have described an example in which a buffer layer of ZnS is formed on a GaAs substrate, and a ZnTe layer and a ZnS layer are alternately deposited on the buffer layer to create a superlattice structure element.
There are many other combinations that yield good crystals. Hereinafter, the I-V group compound material forming the substrate is S, the I-M group compound material forming the buffer layer is ▲, and the 1-%'I group compound material forming the superlattice structure is ▲( (same as the material constituting the buffer layer) and the combination thereof will be described as B.

When GaAs is selected as the substrate S, the material ▲ is CdS
, CdSe or CdTe, substance B is ZnS
A good combination is When InP is selected as the substrate S, it is preferable to use a combination in which material A is ZnTe, CdSe or CdTe, and material B is ZnS or ZnSe. 1 InAI9 j or G on the substrate S
When arb is selected, material ▲ is CdTe, material B is OdS, ZnSe-! A good combination is ZnS or ZnS.

Effects of the Invention According to the present invention, when elongating a superlattice, a buffer layer is formed of a material having a lattice constant larger than that of the substrate among the materials that control the superlattice, and the superlattice is formed on the buffer layer. The elongated structure reduces the dependence of the deposition rate of substances with larger lattice constants on the superlattice structure, making it possible to realize superlattice structure elements necessary for devices that require high-quality crystallinity.

[Brief explanation of the drawing]

Figure 1 is a schematic cross-sectional view of a superlattice structure element according to an embodiment of the present invention, and Figure 2 shows the actual state of the material with a larger lattice constant when the superlattice is directly placed on a substrate with a length of 13 be/cm. How the number of layers is influenced by the number of layers of the material with which the bare material is assembled (B) and the effect of the buffer layer according to the present invention (A)
FIG. 3 is a schematic diagram of a molecular beam epitaxy apparatus used for manufacturing a superlattice structure element in an embodiment of the present invention. 1...Single crystal substrate (substrate), 2...Buffer single layer, 3...Substance B, 4...Substance A, 5...・A layer in which substance B and substance ▲ are deposited alternately.

Claims (4)

[Claims] (1) A buffer layer made of a substance A whose lattice constant a_A is larger than a_S is formed on a substrate made of a substance S whose lattice constant is a_S, and a buffer layer made of a substance A whose lattice constant a_B is smaller than a_S is formed on the buffer layer. A superlattice structure element formed by alternately depositing B and the substance A. (2) Substance S is GaAs, substance B is ZnS, and substance A is ZnTe, CdS, CdSe or CdTe.
The superlattice structure element according to claim 1. (3) Substance S is InP, Substance B is ZnS or Z
nSe, and substance A is ZnTe, CdSe or Cd
The superlattice structure element according to claim 1, which is Te. (4) The substance S is InAs or GaSb, and the substance A
is CdTe, and substance B is CdS, ZnS or Zn
The superlattice structure element according to claim 1, which is Se.