JPH02277228A - Apparatus and method for molecular beam epitaxial crystal growth - Google Patents

Abstract

PURPOSE:To obtain a high-accuracy molecular beam epitaxial crystal growth apparatus by a method wherein a substrate is irradiated with molecular beams jetted from various molecular beam sources in a pulse manner of a short cycle. CONSTITUTION:The following are provided at the inside of a vacuum chamber 10: a molecular beam source cell 11; cell shutters 12, 13; a substrate shutter 14; a specimen holder 15 equipped with a heating mechanism; an electron diffraction apparatus 16; a quadrupole mass spectrometer 17. A shutter for cell use is provided with the two shutters for each cell; the first shutter 12 is a rotary disk, and slits are formed at equal intervals, a molecular beam from the cell 11 is stopped at shielding parts of the disk; the molecular beam is passed at the slot parts. A shuttering cycle is adjusted by the number of revolutions of the disk. The second shutter 13 is opened and shut according to the formation timing of a function layer in the same manner as ordinary shutters for cell use. Thereby, it is possible to adjust an intensity of the molecular beam with which a substrate is irradiated and a cycle to excite a migration of atoms adhered to the substrate independently of each other; it is possible to grow a thin-film crystal whose accuracy and quality are high.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a molecular beam epitaxial crystal growth apparatus and a growth method, and particularly to a high-precision forming apparatus and method for ultra-thin compound semiconductor films made of many kinds of elements. Regarding.

[Conventional technology]

Conventionally, in molecular beam epitaxial growth (MBE), various methods have been proposed to uniformly spray molecular beams from a single molecule beam source and form thin films on substrates with highly precisely controlled film thickness and composition. ing.

For example, JP-A-59-92 relates to a heater and its control method for uniformizing the temperature of a molecular beam source cell.
No. 996, JP-A-53-126262, and the like.

In addition, Japanese Patent Application Laid-Open No. 60-129126 and Japanese Patent Application Laid-open No. 60-576 are related to the improvement of the shutter structure to reduce fluctuations in the crucible temperature when the shutter of the molecular beam source cell is opened and closed.
No. 11, JP-A No. 63-58918, and the like.

Furthermore, there is a method of controlling molecular beam irradiation in a film forming process to improve the quality of deposited films, such as the flatness of the atomic arrangement at the hetero interface and the steepness of the junction, as disclosed in Japanese Patent Application Laid-Open No. 63-102222. No., Japanese Patent Publication No. 63-132421
No., Japanese Journal of Applied Physics, Vol. 24 (1985), pp. L417-L42
0 pages (Japanese Journal of Appl.
ied Physics, 24p, L417
~L420 (1985)), Japanese Journal of Applied Physics Vol. 25 (1986)
2007) Pages L868 to L870 (Japanese J
Internal of Applied Physics
, 24p, L868-L870 (1986)).

[Problem to be solved by the invention]

All of the above conventional techniques improve the control of the deposited film thickness and the flatness of the hetero interface, but their compositional reproducibility, M
No consideration is given to issues such as the formation of a mixed crystal film containing phosphorus and arsenic and the formation of a quantum wire structure, which are extremely difficult in the BE method.

An object of the present invention is to provide a highly accurate molecular beam epitaxial crystal growth apparatus and crystal growth method that solves the above problems.

Another object of the present invention is to reduce the frequency of maintenance of a shutter that is frequently opened and closed.

[Means to solve the problem]

The above purpose is achieved by irradiating the substrate with molecular beams ejected from various molecular beam sources in a short cycle pulse form. In order to
This can be easily achieved by providing two shutters and controlling the opening and closing of each shutter.

[Effect]

First, the term "layer" of the deposited film used in the present invention will be defined.

An arrangement of one atomic layer deposited on a substrate is called an "atomic layer."6 For example, a Ga atomic layer, eAs atomic layer, etc.

Stoichiometric composition of group ■ elements and group ■ elements
-metry) is called a "molecular layer".

For example, a GaAs molecular layer, an InP molecular layer, etc.

Next, in a superlattice structure, an ultra-thin film, etc., a layer deposited in several molecular layers or more and having one function is referred to as a "functional layer". For example, a quantum well layer, a barrier layer, a base layer, an emitter layer, etc.

Further, a layer comprising a plurality of functional layers and constituting an element device is referred to as a "structural layer"8. For example, it is a heterojunction bipolar transistor layer.

In the normal MBE method, for example, in order to form a GaAs functional layer, a substrate is simultaneously irradiated with an As molecular beam (Asa) and a Ga molecular beam in a large excess of the stoichiometric ratio. At this time, the attachment coefficient of the Ga molecular beam is approximately 1,
That is, almost all of the Ga molecular beams irradiated onto the substrate surface contribute to deposition. On the other hand, the adhesion coefficient of the As molecular beam is approximately 1 on the Ga atomic layer, but is O when there are no free Ga atoms, that is, the As molecular beam is deposited only on the Ga atomic layer.

Therefore, the deposition rate is determined by the intensity of the Ga molecular beam.

It is also known that surface migration of Ga on a substrate has a large effect on crystal quality.

In order to effectively effect the surface migration of Ga atoms, - After forming the functional layer, Ga is applied for several seconds to several tens of seconds.
By interrupting molecular beam irradiation and activating migration during that time, or by alternately irradiating Ga molecular beam and As molecular beam one atomic layer at a time, the adhesion of As on the Ga atomic layer is interrupted and Ga migration is activated. There are known methods for activating it.

If the ordered arrangement of atoms is determined by the activation of migration, the migration distance of Ga will be relatively increased and the crystallinity will be improved by decreasing the single molecule beam density and slowing down the deposition rate. Therefore, the inventor of the present invention tried to grow the crystal by decreasing the molecular beam intensity and slowing down the deposition rate, but no improvement in crystallinity (surface flatness) was observed. On the other hand, it was thought that two-dimensional nuclei would be generated and crystal growth would occur due to supersaturation of atoms adhering to (residing in) the substrate surface. In order to irradiate the molecular beam to a supersaturation degree for generating two-dimensional nuclei and to activate surface migration, in the process of forming one atomic layer, the molecular beam should be irradiated at a period shorter than that of forming one atomic layer. Irradiation in pulsed form will be effective.

By irradiating molecular beams in a pulsed manner, conventional MBE
The composition of the deposited film can be controlled by alternately irradiating the mixed crystal of phosphorus and arsenic with pulses of P molecular beams and As molecular beams, which was extremely difficult to do.

Furthermore, it becomes possible to control deposition of one atomic layer or less, making it easier to form a quantum well thin wire structure.

Next, pulsed molecular beam irradiation can be easily controlled by using a plurality of shutters for one molecular beam source cell. The first shutter opens and closes repeatedly over a short period of time to generate a pulsed molecular beam. Due to the short period, no temperature fluctuations in the molecular beam source crucible are observed. The second shutter is used to open and close in accordance with the formation time of one functional layer, similar to that used in normal MBE.

〔Example〕

FIG. 1 shows a schematic diagram of a vacuum chamber 10, which is the main part of the MBE apparatus according to the present invention.

Although not shown in the drawings, the MBE apparatus includes a vacuum chamber, a sample preparation chamber, an analysis chamber, and an exhaust system.

The exhaust system consists of an ion pump and a titanium getter pump.
The ultimate pressure of the vacuum chamber is 4 x 10'' Torr.The analysis room is equipped with an Auger electron spectrometer and a secondary ion mass spectrometer.

Inside the vacuum chamber 10, a molecular beam source cell 11 (l
la, llb, llc, 1id), cell shutter 12 (
12a, 12b, 12c, 12d).

13 (13a, 13b, 13c, 13d), substrate shutter 14. Sample holder with heating mechanism 15. Electron beam diffraction device 16. A quadrupole mass spectrometer 17 is equipped. The chamber 10 is made of stainless steel and equipped with an external baking heater and an internal liquid nitrogen cooling panel.The molecular beam source cell 11 is made of boron nitride with a capacity of 20 mfi and can be heated to a maximum of 1550°C by a surrounding tantalum heater.

The cell shutters are two shutters 12 per cell.
．． It has 13. The first shutter 12 is a rotating disk with slits provided at equal intervals.

The molecular beam from the cell 11 is stopped at the shielded portion of the disk, and the molecular beam passes through the slit portion. The shuttering cycle can be adjusted by changing the rotation speed of the disc. The second shutter 13 is opened and closed in accordance with the timing of formation of the functional layer, like a normal cell shutter.

The substrate shutter 14 is for shielding all molecular beams and contaminants irradiated onto the substrate.

The electron beam diffraction device 16 is used to detect the crystallinity of the deposited film on the spot, and is equipped with a 30 kV electron gun 16a and a fluorescent screen 1.
Consisting of 6b.

Example I Formation of GaAs-AQAs multiple quantum well molecular beam source cell l
High purity AQ, Ga, and A for la, llb, and llc, respectively.
Loaded with s. The substrate is a Cr-doped semi-insulating GaAs wafer manufactured by the horizontal Bridgman method, with a (001) plane, an etch pit density of 500a+1'', and a size of 50φ.

First, the inside of the vacuum chamber 10 is evacuated to a level of 10'''' Torr, firstly, the substrate temperature is heated to 600±5°C, and the A
The natural oxide film on the substrate surface was removed in an atmosphere of s-th molecular beam flux 4 x 1014/cd.S.

Next, the substrate temperature was set to 560+5°C, and 3000 GaAs buffer layers were deposited, followed by an AflAs barrier layer 51.
40 GaAs quantum well layers (18 molecular layers) and 40 GaAs quantum well layers (14 molecular layers) were deposited alternately.

Figure 2 shows the formation of an AQAs-GaAs multiple quantum well layer.
The first rotating disc-shaped shutters 12a and 12b of the AΩ and Ga molecular beam source cells, which indicate the opening/closing sequence of the cell shutters 12 and 13 and the timing of the molecular beam irradiated onto the substrate, are each operated at 600 rpm.

Rotate at 36Qrpm. This causes repeated opening and closing,
When forming the AQAs barrier layer, only the second shutter 13a of the AQ molecular beam source cell (heating temperature 1120°C) is opened, and the G
When forming the aAs well layer, a Ga molecular beam source cell (heating temperature 9
20° C.) only the second shutter 13b was opened. The molecular beam intensity of AP and Ga at these times is 6×1014/d
-5.

The first shutter 12c and the second shutter 13c of the As molecular beam source cell are always open.

In crystal growth using this process, the reflected electron beam intensity of electron beam diffraction repeatedly oscillates clearly, and flatness on the atomic scale can be achieved with good reproducibility.

In addition, in the photoluminescence evaluation at liquid nitrogen temperature of the crystal film created by the single-hydro method, the half-width of the 740 nm emission peak was approximately 4 meV, which is 172 to 1/3 that of the conventional single shutter method. A sharp emission waveform was obtained.

Example 2 Low-temperature growth of ARAs Using the apparatus used in Example 1, an AQGaAs layer was crystal-grown on a GaAs substrate by simultaneously irradiating AM and Ga molecular beams in a pulsed manner.

As a result, a good crystal without surface roughness could be grown at a substrate temperature of 550° C. or higher. Conventional AQ GaAs requires a substrate temperature of 650°C or higher to form a good crystal, which is approximately 100°C lower than the crystal growth rate of GaAs.
Due to the high temperature, impurities tend to diffuse and it is difficult to form a steep profile.

Example 3 (A Q A s) n/z (G a A s) i/z
Formation of quantum wire structure In the same manner as in Example 1, a GaAs single crystal substrate with a (001) plane tilted 2'' in the [110] direction was used, and Afl (10 Hz) and Ga (6 Hz) were used. ) pulsed molecular beams were applied alternately every 3 seconds.

As a result, quantum wires with a period of 8.2 nm as shown in FIG. 3 were formed.

This is because the migration of All atoms on the surface is effectively activated and can grow laterally in the plane in an orderly manner from the steps.

Example 4 Formation of In(i-x)GaxA 5yP(i-y) Mixed crystal An InP single crystal wafer was used as the substrate. In, Ga, As, and P were used as the hexamolecule beam source. Crystals were grown according to the sequence shown. Crucible temperature is In760
℃, Ga850℃, P210℃.

A s is 270°C. In particular, the opening and closing timings of the shuttering were controlled to be staggered so that the molecular beams of P and As would not meet in vacuum.

As a result, the crystal grew at a deposition rate of 0.3 μm/h, and its composition was determined by secondary electron mass spectrometry to be I no,
Furthermore, it is possible to adjust the mixed crystal ratio X+'j by controlling the crucible temperature of each molecular beam source cell 11 and opening/closing of the shutter 13. .

〔Effect of the invention〕

According to the present invention, the intensity of the molecular beam irradiated onto the substrate (two-dimensional nucleation density) and the period of migration excitation of atoms attached to the substrate can be adjusted independently, making it possible to grow thin film crystals with high precision and high quality. be.

In particular, the flatness of the hetero interface on the atomic scale has been improved by 2 to 3 times compared to the conventional method, and the process reproducibility (film thickness of 9 compositions) has been improved.
can also be improved. Furthermore, quantum wire structures and mixed crystals containing phosphorus and arsenic can be easily formed.

Furthermore, in conventional MBE, the shutter is opened and closed extremely frequently, resulting in poor durability and requiring maintenance that frequently breaks the high vacuum. By using two shutters for the same purpose, it can also contribute to reducing maintenance frequency.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of the apparatus according to the invention, FIG. FIG. 4 is a diagram showing a time sequence of molecular beam irradiation in an example of the present invention, and FIG. 3 is a schematic diagram of a quantum wire structure formed according to the present invention. 10... Vacuum chamber, 11... Molecular beam source cell, 1
2...First Cell Shutter, 13...Second Cell Shirt Diagram Bodhisattva 2 Nagi Badjiru Keja, Rest 11@8. To the bar ° - 1 tithe. Kaya 4 figure evaporation: Teru @ 113*'r line glue 4 Mushikanojin rL

Claims (1)

[Claims] 1. A method for epitaxially growing a compound semiconductor thin film by irradiating a substrate with a plurality of molecular beams in a high vacuum,
A molecular beam epitaxial crystal growth method characterized in that at least one of the molecular beams ejected from a molecular beam source cell is irradiated onto a substrate in an intermittent pulse form to form a molecular layer on the substrate. 2. Molecular beam epitaxial crystal growth in an apparatus for molecular beam epitaxial growth of a compound semiconductor thin film on a substrate in a high vacuum, characterized by having a plurality of shutters for one molecular beam source cell and a mechanism for controlling opening and closing of the shutters. Device.