JP2870061B2 - Super lattice structure element - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superlattice structure element using a superlattice structure requiring high quality crystallinity, such as various electronic elements and optoelectronic elements.

2. Description of the Related Art Superlattices have recently been researched and developed as an attempt to improve the functions of existing materials or to create new functions by artificially producing substances with atomic and molecular arrangements that do not exist in nature. It is becoming more and more popular. Especially GaAs / AlA
Some devices using a III-V compound semiconductor superlattice such as an s-system have already been made into devices. In the hetero-stacked structure or the superlattice structure practically used in these and other ways, the crystal lattice constant between the substrate and the superlattice or the materials constituting the superlattice have extremely close values, and the crystal defect Thus, a combination capable of high-quality heteroepitaxial growth without causing cracks is relatively easily obtained.

On the other hand, there is a difference of several percent or more in the lattice constant,
Although there are effects that cannot be ignored on crystal growth, various strains of so-called strained superlattices in which these strains are incorporated have been tried because other substances are difficult to substitute. This is intended to make the lattice epitaxially grow in a state where the lattice has a little strain and not to cause lattice relaxation.

Problems to be Solved by the Invention Various film forming methods are used for fabricating a superlattice, but MBE and MOCVD are predominant from the viewpoint of good controllability. Depending on the purpose, the superlattice structure may need to have a very short period, for example, about 10 to 20 °. For accurate control of such a short-period structure, stabilization and uniformity of the crystal growth rate are indispensable. In the case of the MBE method, the substrate temperature is kept constant, and the temperature of the cell containing the raw materials is precisely controlled to take measures such as keeping the molecular beam intensity constant. In addition, it is common to use a buffer layer to improve crystallinity. However, the present inventors have found a problem that the crystal growth rate depends not only on these growth conditions but also on the superlattice structure to be prepared itself, particularly the material of the buffer layer. For example, when a superlattice is formed of the substance A and the substance B, the growth rate of A differs depending on whether the buffer layer is formed of A or a third substance other than the above is used even under the same conditions. See Figure 1 for an example
The ZnTe-ZnS superlattice grown directly on the GaAs substrate by the MBE method and the B and C ZnTe-ZnS superlattices grown on the buffer layer made of a different material have different ZnTe-ZnS superlattices.
It shows how the growth rates of the layers are different. The horizontal axis indicates the shutter open time (arbitrary unit), and the vertical axis indicates the ZnTe deposition rate (arbitrary unit). All growth conditions are unified. Therefore, different ZnTe layer thicknesses mean different growth rates.

On the other hand, the buffer layer is not limited to anything, and the range of selection is naturally limited depending on the purpose of use, and a substance different from the substrate may be required in some cases. Furthermore, a pattern is formed as a base in the device configuration,
When it is necessary to grow a superlattice having the same structure on different materials, it is difficult to manufacture an intended device if the growth rate of each layer constituting the superlattice is different depending on the location.

An object of the present invention is to solve such a conventional problem and to provide a superlattice structure element having the same structure and the same thickness by making the growth rates of the respective layers of the superlattice formed on different materials uniform. It is assumed that.

Means for Solving the Problems Therefore, in the present invention, when a buffer layer is formed of a substance different from the substrate, the difference between the lattice constant of the substrate and the substrate is one.
%, By using a substance having a lattice constant of less than 1% so that the growth rate becomes uniform even when the portion for growing the superlattice directly on the substrate and on the buffer layer are present in the same step. It is.

Action The mechanism by which the growth rates of the respective layers during superlattice growth are equal if the lattice constants are substantially equal is not necessarily clear, but it is considered that the strain caused by the difference in the lattice constants is affecting. When compressive stress is applied, it tends to hinder the attachment of flying molecules or atoms, and when tensile stress is applied, it tends to assist in attachment. Therefore, even if the lattice constant is close, the same growth rate can be obtained on both the substrate and the buffer layer.

Embodiments The present invention can be widely used for producing a superlattice by a combination of substances having different lattice constants.Here, as an example, a superlattice made of a II-VI compound semiconductor is formed on a III-V compound semiconductor substrate. The case of forming will be described.

ZnTe (a _ZnTe = 6.1037) on a GaAs (a _GaAs = 5.6533Å) substrate
The superlattice composed of Å) and ZnS (a _ZnS = 5.4093Å) is formed as follows. ZnTe-ZnS-based superlattice crystal growth is high-purity under ultra-high vacuum, low-temperature growth in non-equilibrium state,
Considering the advantages such as a steep interface due to instantaneous switching by molecular beam shutter operation, a molecular beam epitaxy device (MBE device) is used. FIG. 2 shows a schematic configuration diagram of the MBE apparatus. Only a growth chamber portion is shown, and a load lock chamber, a substrate moving mechanism, and the like 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 gauge 7, and the residual gas is monitored by a quadrupole mass spectrometer 6. During the growth of the thin film crystal, reflection high-energy electron diffraction (RHEE)
D) Observe by 9 and the pattern is projected on the screen 8.

After evacuation of the inside of the growth chamber 1 to the order of 10 ⁻¹⁰ Torr by the exhaust system 2 and stabilization of the cells 11 a to 11 d containing the raw materials to a temperature at which a predetermined molecular beam intensity can be obtained, the temperature of the GaAs substrate 4 is reduced. Raise the temperature to 600 ° C to release the surface oxide film. At this time, RHE
The completion of this thermal etching is confirmed by observing sharp streaks in the ED pattern.
Next, lower the substrate temperature to approximately 200-350 ° C, depending on the purpose,
Start superlattice formation.

First, a ZnSe buffer layer (aZnSe = 5.6687 °) is formed. In that case, even with the ALE method
The MBE method can be used, but the latter is taken as an example because the MBE method is faster in terms of time. The shutters 10a and 10d of the cells 11a and 11d containing the raw materials of Zn and Se, respectively, are simultaneously opened and closed after a predetermined time. Substrate temperature 330 ° C, Zn
And Se molecular beam intensities are 2 × 10 ^-7 and 8 × 10 ^-7 Torr, respectively.
(Flux monitor value by ionization gauge) 1
Approximately 500 で was deposited in an hour. Since the substrate temperature at this time was too high to be used in the ALE method, the temperature was lowered to 240 ° C., and then, the superlattice growth was started. The molecular beam intensity remains constant. The material to be deposited is switched by opening and closing the shutters 10a to 10d. That is, during the ZnS deposition, the cell 11 containing the ZnS raw material was
The shutter 10c of c is opened, the others are closed, and after a certain time, the shutter 10c of ZnS is closed. ZnTe and Zn
The molecular beam intensity of S is 1 × 10 ⁻⁶ and 7 × 10 ⁻⁷ Torr. Here consider the handling of in ultrahigh vacuum, was used ZnS compound materials, Zn and S each single raw material or S H ₂ S
The gas may be supplied by cracking. Next, after an appropriate interval (1 to several seconds), the shutters 10a and 10b of the cells 11a and 11b containing the Zn and Te raw materials are opened and closed after a certain time. Repeat this operation until the ZnS
And ZnTe are alternately laminated.

As described above, the relationship between the number of stacked ZnTe layers constituting the superlattice and the time for opening the Zn and Te shutters when a ZnSe buffer layer is formed thereon and then a ZnTe-ZnS superlattice is formed thereon. As shown in FIG. For comparison, the case without the ZnSe buffer layer is also shown. The sample without a buffer layer was a sample in which the substrate temperature was directly lowered to 240 ° C. after thermal etching of the GaAs substrate, and a superlattice was grown under exactly the same conditions. As is clear from this figure, Zn
Te-ZnS superlattice grown directly on GaAs substrate and ZnS
It can be seen that the formation rate of the ZnTe layer is equal to that in the case where the e-buffer layer is used, and that the adhesion coefficient of ZnTe is made uniform.

The above description is based on a III-V compound semiconductor substrate (substance S).
The case where aAs is used, ZnTe (substance A) and ZnS (substance B) are used as the superlattice group II-VI compound semiconductor, and ZnSe (substance C) is used as the buffer layer has been described. It is desirable that the difference between the lattice constant of the substrate and the lattice constant of the buffer layer be as small as possible, but the difference that is practically acceptable is less than 1%. The following combinations are also possible, assuming that the difference in lattice constant is less than 1% and an existing substance is used.

Effect of the Invention According to the present invention, the superlattice growth rate can be made uniform on both the substrate and the buffer layer, so that the controllability of the superlattice structure is improved. The same structure can also be formed.

[Brief description of the drawings]

FIG. 1 shows that when a superlattice is grown, a buffer layer is present (A), there is no buffer layer (B), and the difference between the substances affects the actual number of layers of a substance having a larger lattice constant. FIG. 2 is a schematic configuration diagram of a molecular beam epitaxy apparatus used for fabricating a superlattice in the embodiment, and FIG. 3 is a diagram showing an effect of a buffer layer according to the present invention. 4 ... substrate.

Continuation of the front page (56) References JP-A-3-93221 (JP, A) JP-A-2-92899 (JP, A) JP-A-3-20096 (JP, A) JP-A-1-289257 (JP) JP-A-2-114648 (JP, A) JP-A-2-263798 (JP, A) JP-A-2-285630 (JP, A) JP-A-2-289499 (JP, A) 3-159990 (JP, A) JP-A-62-243144 (JP, A) JP-A-64-45113 (JP, A) JP-A-2-96384 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C30B 28/00-35/00 H01S 3/096 H01S 3/103 H01S 3/133 H01S 3/18-3/19

Claims (4)

(57) [Claims] To 1. A substrate made of a lattice constant a _S becomes substance S,
A buffer layer made of a substance C having a lattice constant whose difference from the lattice constant a _S is less than 1% is formed, and a lattice constant a _A is formed on the buffer layer from a substance _A having a relation of a _A > a _S. 1. A superlattice structure element comprising: layers of materials B having a relationship of a _S > a _B with a lattice constant of a _S > a _B. 2. The material S is GaAs, and the material A is ZnTe, Cd.
S, CdSe or CdTe, ZnS as substance B, ZnS as substance C
2. The superlattice structure element according to claim 1, wherein e is used. 3. The material S is InP, and the substance A is ZnTe, CdSe.
2. The superlattice structure element according to claim 1, wherein CdTe, ZnS or ZnSe as the substance B, and CdS as the substance C are used. 4. The superlattice structure element according to claim 1, wherein InAs or GaSb is used as the material S, CdTe is used as the material A, CdS, ZnSe or ZnS is used as the material B, and ZnTe is used as the material C.