JPH097947A - Molecular beam cell - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a molecular beam cell capable of keeping the evaporation amount of the molecular beam constant even if the raw material is consumed in the crucible. [Structure] In the molecular beam cell 22, the outer peripheral wall surface of the crucible 10 on the side where the heating source is not provided is covered with a good thermal conductor 16.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molecular beam cell.

[0002]

2. Description of the Related Art In the molecular beam epitaxy method, vapor generated from a molecular beam cell is taken out from a crucible opening in the form of a molecular beam on a substrate placed in an ultrahigh vacuum to form an epitaxial layer. Is the way. For this reason, the raw material (crystal constituent element) is contained in the crucible of the molecular beam cell, and the raw material is heated by heating the heating source provided on the outer periphery of the crucible. The heated raw material can be passed straight from the molecular beam cell in the form of molecular beam vapor.
Therefore, when forming an epitaxial layer on a substrate, the constituent elements (composition ratio) of the film can be arbitrarily selected by using the crystal constituent elements that make up the film as the supply source of each molecular beam cell. Can be done. Therefore, in order to form an epitaxial layer having excellent physical properties, it is necessary to accurately control the evaporation amount of the molecular beam from the molecular beam cell. Further, the evaporation amount of the molecular beam from the molecular beam cell depends on the temperature of the raw material, and therefore it is necessary to minimize the temperature change of the raw material in order to obtain a semiconductor crystal having excellent physical properties. As a molecular beam cell developed for such a purpose, there is a molecular beam cell having the structure shown in FIG. 2 (P
Rogless Crystal Growth Ch
aract, 1978, Vol. 2, pp. 15-3
2, Pergmon Press Ltd. reference).

FIG. 2 is a partial cross-sectional view for explaining the structure of a conventional molecular beam cell.

The molecular beam cell 39 has a cylindrical crucible 3
0, and a heating source (heater) 32, a reflector 34, and a shutter 36, which are provided so as to cover the entire outer peripheral wall surface thereof. This molecular beam cell 39 is
Since the entire outer peripheral wall surface of the crucible 30 is covered with the heating source 32, it is possible to make the raw material temperature uniform and control the evaporation amount of the molecular beam.

However, in the conventional molecular beam cell in which the heating source is provided on the entire outer peripheral wall surface of the crucible, when the epitaxial layer (also referred to as an epi layer) is formed on the substrate, the crystal composition distribution of the epi layer is improved. And in order to increase the growth rate, raw material 3
The opening portion of the crucible 30 from which 8 is vaporized and jetted is opened. Therefore, the temperature in the vicinity of the opening of the crucible is lower than the temperature of the center of the crucible 30 inside the cylinder. When the raw material 38 is supplied into the crucible 30 and the crucible 30 is heated under such a state, molecular beams are generated from the raw material 38 and fly to the opening of the crucible 30. Although this molecular beam obtains heat from the heating source 32, since the temperature of the opening is lower than that of the central part of the crucible 30, the heat of the molecular beam is rapidly removed, and as a result, the raw material is recondensed. There's a problem. This re-condensed raw material adheres to the inner wall surface near the opening of the crucible.

This phenomenon becomes more remarkable especially when a raw material such as gallium (Ga) having a low melting point and a low vapor pressure is evaporated. The raw material recondensed at this time is also called a droplet. When a droplet (raw material) adheres to the inner wall of the opening of the crucible 30, the temperature of the droplet is applied to the droplet and the heat quantity of the droplet itself becomes more active. It becomes easy to form a compound with other vaporized raw materials. Since the raw material that has become a compound generally has a higher vapor pressure than the raw material that has not become the original compound, it re-evaporates at a lower temperature. When this re-evaporated compound reaches the surface of the substrate and adheres to the surface of the substrate, crystals are abnormally grown at the time of film formation due to the influence of the compound and defects occur in the film. Therefore, crystals having desired physical properties cannot be obtained.

A molecular beam cell having a structure shown in FIG. 3 is a molecular beam cell developed to solve the above-mentioned problems.

FIG. 3 is a partial sectional view of a conventional molecular beam cell in which a heating source is locally provided.

As a molecular beam cell of this kind, for example, reference I
(Advice Electronics Series, Molecular Beam Epitaxy, published by Baifukan, 1994, p18, see “Structure of K cell”).

The molecular beam cell 40 includes a crucible 30, a heating source (heater) 32, a reflector 34, and a shutter 36.
And is constituted by. Further, various raw materials 38 for film formation are supplied into the crucible 30. This molecular beam cell 4
The feature of 0 is that the heating source 32 is locally provided near the opening of the crucible 30 in order to prevent the raw material 38 from adhering to the opening of the crucible 30. The reflector 34 is provided so as to surround the outer peripheral wall surface of the crucible 30, and serves to keep the temperature of the crucible 30 and the raw material 38 warm and to block the influence of temperature from the outside.

The shutter 36 is attached to the upper part of the crucible 30 on the opening side, and the shutter 38 is provided.
Thus, the opening of the crucible 30 can be opened and closed. That is, the shutter 36 is opened when the desired crystal constituent element is taken out as a molecular beam, and the shutter 36 is closed when not taken out.

[0012]

However, in the conventional molecular beam cell in which droplets are prevented, since the heating source 32 is locally provided in the vicinity of the opening of the crucible 30, the crucible 30 having the heating source 32 is provided. Thermal radiation causes a temperature gradient between the opening and the crucible portion away from the heating source.
That is, the temperature of the crucible portion of the crucible 30 farther from the heating source 32 becomes lower as the distance from the heating source 32 becomes higher than the temperature near the opening of the crucible 30 due to heat radiation. Therefore, when the raw material is supplied into the crucible 30,
There is a problem that the raw material temperature decreases as the raw material is consumed and reduced. Therefore, the evaporation amount of the molecular beam reaching the substrate changes every moment in a short period of time, which makes it difficult to form an epilayer having a crystal structure having a desired composition on the substrate surface.

Therefore, there has been a demand for a molecular beam cell capable of keeping the evaporation amount of the molecular beam constant even if the raw material is consumed in the crucible.

[0014]

Therefore, according to the molecular beam cell 40 of the present invention, the crucible, the heating source provided so as to cover a part of the outer peripheral wall surface near the opening of the crucible, and the reflector. In the provided molecular cell, the outer peripheral wall surface of the crucible on the side where the heating source is not provided is covered with a good thermal conductor. The thermally good conductor may be a high melting point metal foil or a sintered film containing a high melting point metal.

[0015]

According to the molecular beam cell of the present invention, the heating source is provided in the outer peripheral wall surface area of the crucible on the opening side, and the heating source is formed on the outer peripheral wall surface area on the other end side of the crucible opposite to the opening. A good thermal conductor is provided adjacent to the source or separate from the heating source. Therefore, in this molecular beam, the outer peripheral wall surface area other than the outer peripheral wall surface area of the crucible in which the heating source is provided is covered with a good thermal conductor. By providing a good thermal conductor in this way, the heat heated by the heating source is transmitted through the good thermal conductor with good heat conduction, and the outer peripheral wall surface region of the crucible other than the outer peripheral wall surface region of the crucible in which the heating source is provided is transferred. Because it warms
The temperature gradient between the temperature on the heating source side of the outer peripheral wall surface region and the temperature on the bottom face side of the crucible becomes small. Therefore, when the raw material is supplied into the crucible of the molecular beam cell, the outer peripheral wall surface region of the crucible facing the raw material supply portion is also covered with the good thermal conductor, so that the temperature near the opening is covered with the good thermal conductor. Although it becomes higher than the temperature of the outer peripheral wall surface area, the temperature of the outer peripheral wall surface area covered with the good thermal conductor becomes uniform. Therefore, the temperature of the raw material in the crucible located away from the heating source can be kept uniform. As a result, even if the raw material supplied into the crucible is consumed, the temperature of the raw material does not decrease, so that the evaporation amount of the molecular beam generated from the opening of the crucible can be controlled uniformly at all times.

[0016]

Embodiments of the molecular beam cell of the present invention will be described below with reference to the drawings. FIG. 1 merely schematically shows the shape, size and arrangement of each component so that the present invention can be understood.

FIG. 1 is a partially cutaway perspective view for explaining an embodiment of the molecular beam cell of the present invention.

The molecular beam cell 22 of this embodiment is a crucible 1
0, a heating source (heater) 12, a reflector 14, a good thermal conductor 16 and a shutter 18. The crucible 10 has an open end at one end and an opening 11 formed at the other end and a closed end at the other end, and has a generally cylindrical shape. A raw material 20 is stored in the bottom of the crucible 10. Molecular beam cell 22 of this embodiment
Has a structure similar to that of the conventional cell described with reference to FIG. 3 except that a good thermal conductor 16 is provided. In this embodiment, the good thermal conductor 16 is provided in a portion of the crucible 10 in the vicinity of the opening 11 away from the outer peripheral wall surface region, and the outer peripheral wall surface region of the crucible 10 is covered with the good thermal conductor 16.

The good thermal conductor 16 surrounds at least the periphery of the raw material 20 as one strip-shaped layer (or film) continuous to the outer peripheral wall surface region of the crucible 10 or as a plurality of strip-shaped layers (spaced apart from each other). Or as a film). In the embodiment shown in FIG. 1, an example in which it is provided as one continuous strip-shaped layer is shown. When provided as a plurality of belt-shaped layers, they may be provided separately from each other in the direction along the central axis of the crucible, that is, the tube axis, or may be provided apart from each other along the circumferential direction centered on the tube axis. It may be provided. Alternatively, unlike the case described above, the good thermal conductor 16 may be provided as a plurality of island-shaped regions on the outer peripheral wall surface of the crucible 10. In this embodiment, the material of the crucible 10 is, for example, well degassed in ultra-high vacuum, PBN (pyrolytic born nitri).
de). Note that a material such as quartz, molybdenum, or tantalum may be used instead of PBN.

Further, in this embodiment, as the material of the heating source 12 provided on the outer peripheral wall surface in the vicinity of the opening 11 of the crucible 10, for example, tungsten (W) or carbon (graphite, SiC whose surface is coated) is used. Including.) Is used.

The method of coating the outer peripheral wall surface region of the crucible 10 with the good thermal conductor 16 is preferably as follows.

As a first method, a high melting point metal foil is used as the good thermal conductor 16. This high melting point metal foil is made of, for example, stainless steel, tantalum, molybdenum, carbon, or the like. The outer peripheral wall surface of the crucible 10 is covered with these high melting point metal foils. At this time, it is preferable that the high melting point metal foil and the outer peripheral wall surface of the crucible be in close contact with each other as much as possible so that no void portion is formed to cover the outer peripheral wall surface area of the crucible 10.

In the second method, for example, carbon powder is used as the good thermal conductor 16 and the carbon powder is dissolved in a suitable solvent to prepare a paste containing carbon.
This paste is applied to the outer peripheral wall surface of the crucible 10 other than the heating source 12. Then, the paste is baked (baked) on the outer peripheral wall surface of the crucible 10 at a predetermined heating temperature. In this example, the baked carbon film is referred to as a sintered film.

In this embodiment, it is preferable that the region where the good thermal conductor 16 covers the outer peripheral wall surface of the crucible 10 is wider than at least the portion to which the raw material is supplied.

A reflector 14 is provided outside the crucible 10. As the material of the reflector 14, for example, molybdenum foil or molybdenum plate, tantalum foil, stainless plate, or the like is used. As described above, the reflector 14 includes the crucible 10 and the raw material 2.
It keeps the temperature 0 and also acts to block the influence of external temperature.

As can be understood from the above, since the heating source is conventionally provided locally near the opening of the crucible, the temperature on the heating source side and the bottom surface of the crucible are located in the outer peripheral wall surface area away from the heating source. Although a temperature gradient is generated between the heating source 1 and the side temperature, in this embodiment, the heat source 1 is provided by the good thermal conductor 16.
Since the outer peripheral wall surfaces of the crucibles 10 other than 2 are covered, heat is efficiently transferred to the crucible portion other than the heating source (the portion of the crucible remote from the heating source) by heat conduction, and the outer periphery away from the heating source. Since the temperature of the wall surface region is covered with a good thermal conductor, the temperature gradient can be made smaller than in the conventional case. Therefore, when the raw material is supplied into the crucible, the change in the evaporation amount of the molecular beam generated from the raw material is reduced, and an epitaxial layer having an excellent crystal structure can be formed. Further, in this embodiment, since the heating source is locally provided in the vicinity of the opening of the crucible as in the conventional molecular beam cell, even when the raw material having a low vapor pressure (for example, gallium (Ga)) is vaporized. Therefore, the raw material does not adhere to the inner wall of the opening of the crucible in the form of droplets.

[0027]

As is apparent from the above description, according to the molecular beam cell of the present invention, the outer peripheral wall surface of the crucible on the side where the heating source is not provided is covered with a good thermal conductor. is there. For this reason, even if the raw material in the crucible is consumed, the amount of change in the evaporation amount of the molecular beam is smaller than in the conventional case, so that a stable evaporation amount of the molecular beam can be obtained. Therefore, it becomes easier to control the crystal composition during film formation,
Excellent film formation becomes possible.

Further, in the molecular beam cell of the present invention, since the heating source is locally provided in the vicinity of the opening of the crucible as in the conventional case, the liquid droplet does not adhere to the inner wall of the opening of the crucible. . Therefore, the droplets combine with other substances to form a compound, and the compound is prevented from being mixed during the film formation to cause a film defect, so that a semiconductor crystal film (epi layer) having excellent physical properties can be obtained. can get.

[Brief description of drawings]

FIG. 1 is a partially cutaway perspective view schematically showing the structure of a molecular beam cell showing an example.

FIG. 2 is a partially cutaway perspective view schematically showing the structure of a conventional molecular beam cell.

FIG. 3 is a partially cutaway perspective view schematically showing the structure of a conventional molecular beam cell (where a heating source is locally provided).

[Explanation of symbols]

 10: crucible 11: opening 12: heating source (heater) 14: reflector 16: good thermal conductor 18: shutter 20: raw material 22: molecular beam cell

Claims (3)

[Claims] 1. A molecular beam cell comprising a crucible, a heating source provided so as to cover a part of the outer peripheral wall surface in the vicinity of the opening of the crucible, and a reflector, wherein the heating of the outer peripheral wall surface of the crucible is performed. A molecular beam cell, characterized in that the outer peripheral wall surface on the side where the source is not provided is coated with a good thermal conductor. 2. The molecular beam cell according to claim 1, wherein the thermally good conductor is a refractory metal foil. 3. The molecular beam cell according to claim 1, wherein the thermally good conductor is a sintered film containing a refractory metal.