JPH06329495A - Method for molecular beam epitaxial growth and device using the same - Google Patents

Abstract

PURPOSE:To provide a method for MBE capable of lowering growth temperature in an extremely high degree of vacuum and attaining diversification of appli cable element, and a device using the method. CONSTITUTION:A target containing a raw material in a molten state set at a melting point or a temperature higher than the melting point is irradiated with laser beam, the raw material is evaporated in an atomic.molecular state and crystallized on a substrate. A device for molecular beam epitaxial growth comprises a heating source 51 for melting the target raw material, an evaporation source 30 for molten target composed of a sensing means for detecting the melting of the target and a temperature controlling means for maintaining the temperature of the molten target 36 constant, a laser transmission part 10 for sending laser beam, an optical means having an optical system 11 and a reflector 12 for introducing the emitted laser beam to the target. The sensing means and the temperature controlling means, for example, consist of a heating electric source 52, the heating source 51, a thermometer 53 and a controller 54.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a molecular beam epitaxial growth method (MBE) useful for forming a thin film of a compound semiconductor or the like.
Method) and an MBE device employing the same.

[0002]

BACKGROUND OF THE INVENTION The MBE method is a technology for forming a single crystal thin film in an ultrahigh vacuum. In the MBE method, the constituent elements of the crystal are placed in a crucible (molecular beam cell), heated in a vacuum, and the generated vapor is applied to the heated substrate in the form of a molecular beam, and the crystal is epitaxially grown there.

Since the MBE method is (A) an ultra-high vacuum apparatus, it can eliminate the influence of impurities such as residual gas and keep the substrate surface clean, and (B) a well-controlled evaporation source (generally K cell). To control the growth rate at the atomic layer level,
(C) Since the crystal growth is in a non-equilibrium state, it is possible to create a new material by selecting the constituent elements.
It is widely used for making electronic devices or optical devices using quantum effects, and for studying physical properties.

[0004]

However, the MBE method having such characteristics also has the following problems.

The first problem is that the apparatus has a large number of K cells controlled to a high temperature, so that the degree of vacuum is limited even in an apparatus requiring an ultra-high vacuum. To consume a large amount of liquid nitrogen to maintain it. Therefore, if a structure that does not have a high temperature even during crystal growth can be provided, the background vacuum degree can be kept low and not only can it contribute to the growth of high-purity crystals, but also the liquid It is possible to reduce the consumption of nitrogen.

The second problem is a substance having a high melting point, for example, 2
That is, tungsten having a melting point of 000 ° C. or higher cannot be evaporated even by the K cell, and carbon cannot obtain sufficient molecular beam strength.

The third problem is that it is difficult to control the evaporation amount of a substance having a high vapor pressure. For example, although phosphorus is an important group V source material, it is difficult to handle it by the MBE method, and although arsenic can be used as a composition element during the crystal growth of III-V group semiconductors, silicon can be processed by the MBE method. It is difficult to use as a dopant when grown.

MBE is to overcome these second and third problems.
The scope of application of the law is greatly expanded.

The fourth problem is to reduce the crystal growth temperature. The MBE method can control the growth temperature to be lower than that of other growth methods. However, when considering heteroepitaxy, it is necessary to further lower the temperature than the current state in order to alleviate thermal strain between the substrate and the crystal growth layer.

The present invention has been made under the background of such conventional technical problems, and an object thereof is to reduce the growth temperature in the state of ultra-high vacuum and further applicable elements. It is an object of the present invention to provide an MBE method capable of achieving diversification and a device using the MBE method.

[0011]

The present invention is directed to a molecular beam epitaxial growth method in which a raw material is evaporated in an ultrahigh vacuum and a crystal is grown on a heated substrate.
A melting point or a melting point of the raw material set to a temperature higher than the melting point is included, and the target is irradiated with a laser beam to evaporate the raw material in an atomic / molecular state and to grow crystals on the substrate.

Further, according to the present invention, in a molecular beam epitaxial growth apparatus for evaporating a raw material in an ultrahigh vacuum and growing a crystal on a heated substrate, a heating means for melting the target raw material and a sensor means for detecting the melting of the target raw material. ,
And a molten target evaporation source including temperature control means for holding the temperature of the melted target material constant, laser emitting means for emitting laser light, and optical means for guiding the emitted laser light to the target. It is characterized by

First, the points of interest of the present invention will be described. The following has been confirmed by the present inventors.

Since pulsed lasers generally produce outputs on the order of megawatts, focusing them on a target gives extremely high power and most substances dissolve and evaporate. Therefore, if this method is used instead of the heating evaporation source in the MBE method, the range of substances to which the MBE method can be applied is significantly expanded. However, what is important here is that the laser cannot be used as an evaporation source simply by focusing the laser on the target. That is, in order to use it as an evaporation source, it is desirable that the vaporized and scattered states are separated at the atomic / molecular level, but when the target is irradiated with a laser, particles of 1 to 5 μm are mixed depending on the target material. It evaporates and scatters in the state.

In order to show this, FIG. 5 shows a photograph of the deposition surface when a laser is irradiated with several kinds of substances as targets to deposit them on the substrate.

FIGS. 5A and 5B are photographs of deposited surfaces of tungsten and carbon, respectively. The surface of each of these is smooth and mirror-like. On the other hand, FIG.
(C) and (d) are photographs of deposited surfaces of aluminum and indium. In these, granular deposits are seen on the smooth surface of the base. This indicates that not only the target substance is vaporized in the form of atoms and molecules at the time of vaporization, but also solid substances of 1 to 5 μm are scattered at the same time.

Further, the same experiment was conducted for various substances,
The result of observing the deposition surface is shown in FIG. 6 in three stages. In the evaluation, if there is no deposit on the deposition surface and it shows a mirror surface condition, it is indicated as ○, small (maximum about 2 μm) deposit is recognized as △, and large (maximum about 4 μm) deposit is recognized as x. To do. FIG. 6 also shows the melting points of the various substances.

The results show that the states of evaporation and scattering differ depending on the nature of the substance. That is, a sublimable substance such as carbon or arsenic and a substance having a melting point of 2000 ° C. or higher form a deposition surface in a mirror state, and accordingly, atoms and
It can be seen that it is evaporated and scattered in the molecular state. For the other substances, the presence of deposits was recognized on the deposition surface, and therefore, it is shown that solid substances are evaporated and scattered in the coexistence state. Therefore, it is necessary to take some measures for the medium and low melting point substances.

Therefore, focusing on gallium, although gallium has a low melting point, no deposit is observed on the deposition surface. The reason may be that gallium has a low melting point of 30 ° C., and therefore gallium is completely melted during laser irradiation. Therefore, if the target is in a molten state, it is expected that the generation of solid matter can be prevented. Therefore, aluminum and indium were irradiated with a laser in a molten state, and the deposition surface was observed. The result is shown in FIG. 7. 7 (a) and 7 (a ') are photographs of the deposition surface when using a solid aluminum target and a molten target, respectively. 7 (b),
(B ') is a photograph of the deposition surface when using a solid target of indium and a molten target. From these photographs, it was confirmed that the substances in which deposits were found on the deposition surface when the solid target was used could prevent the generation of solids during deposition if they were in a molten state.

The present invention has been completed on the basis of such novel knowledge, and has a main feature in the MBE method involving laser irradiation in that a molten target is used for a specific substance. . The target in the molten state is set to a melting point or a temperature higher than the melting point, more preferably higher than the melting point and sufficiently lower in vapor pressure. The temperature range above the melting point is 20
-50 degreeC is preferable.

In the apparatus for carrying out the MBE method, heating means for melting the target raw material by resistance heating or electron beam heating, a sensor means for detecting whether or not the target raw material is melted, and a molten material are used. An evaporation source for a molten target, which is provided with a temperature control means for keeping the temperature of the target material constant, is required. Further, for a substance capable of excellent evaporation and scattering in a solid target state, an evaporation source can be provided separately from the evaporation target evaporation source. Then, the molten target and the solid target are simultaneously or alternately irradiated with laser light through appropriate optical means to perform controlled molecular beam epitaxial growth. Further, the apparatus of the present invention can be provided with a solid target evaporation source that evaporates the solid target, in addition to the molten target evaporation source.

[0022]

According to the present invention, there are the following operational effects.

(A) Maintaining a high degree of vacuum Since the target is vaporized by laser irradiation, a high temperature portion unlike the conventional K cell is not required, so that the degree of vacuum in the vacuum container is kept extremely high even during crystal growth. can do. In addition, since the melting target is set to a melting point or a temperature slightly higher than the melting point, the vapor pressure of the target can be made considerably low, and the pressure during crystal growth can be made lower than when using the K cell. can do.

(B) Expansion of applicable materials Not only refractory substances such as tungsten but also substances (for example, carbon or arsenic) for which it is difficult to control the molecular beam intensity by the K cell by using a laser as evaporation means are used. (When used as a dopant) can also be used in the MBE method. Then, for medium and low melting point substances such as aluminum and indium, by using these as a target in a molten state, it is possible to vaporize the atomic and molecular states by laser irradiation. As described above, not only the substances to which the conventional MBE method can be applied, but also the high melting point substances, carbon, arsenic, or other sublimable substances whose molecular beam intensity is difficult to control by the K cell are widely applied. Can be expanded.

(C) Reduction of growth temperature A major factor that regulates the crystal growth temperature is the surface diffusion phenomenon of elements. In the conventional MBE method, the surface diffusion is promoted by raising the substrate temperature to obtain a good crystal. Therefore, if the surface diffusion can be promoted by a method other than heating the substrate, the crystal growth temperature can be reduced. In the present invention, since the target is evaporated by laser irradiation, the evaporated substance reaches the substrate with high energy of 1 to 15 eV. This energy state is one to two orders of magnitude higher than the energy of the substance evaporated from the K cell. Then, the vaporized substance that has reached the substrate collides with the surface and releases energy, thereby locally heating the surface, and the surface diffusion is promoted. Therefore, even if the substrate temperature is lowered as compared with the conventional MBE method, the deposited layer having good crystallinity can be grown.

[0026]

1 is a schematic configuration diagram showing an example of an MBE apparatus to which the present invention is applied.

This MBE apparatus comprises a vacuum container 20 and a laser emitting section 10.

The vacuum container 20 has a susceptor 2 on one side.
1 is arranged, and the evaporation source 30 for the melting target and the evaporation source 40 for the solid target are arranged on the other side. The melting target evaporation source 30 and the solid target evaporation source 40 are each formed of a substantially cylindrical body, and are detachably formed on the main body of the vacuum container by upper vacuum flanges 35 and 45. In addition, the vacuum container 20
Can be brought to a high vacuum state by the vacuum pump 23.

The susceptor 21 is provided with a heater inside and is controlled to a constant temperature by a controller (not shown). A substrate 22 is placed on the susceptor 21.

The evaporation source 30 for the melting target is shown in FIG.
As shown in the enlarged view of FIG. 1, a crucible 32 for accommodating the target in a molten state is provided at the bottom. This crucible 32
Is heated by the heating power source 52 and the heating source 51. Resistance heating or electron beam heating is used as the heating source 51 (resistance heating in the illustrated example). Further, the melting target 36 is set in a predetermined temperature range by the thermometer 53 and the controller 54.

Further, a laser window 31 is formed on a side portion of the evaporation source 30 for the melting target at a position facing the laser emitting portion 10, and a laser emitting portion is provided on a side portion opposite to the laser window 31. A concave mirror 33 is provided which irradiates the crucible 32 with a laser beam emitted from the laser beam 10. The concave mirror 33 is driven by a concave mirror driving device 34 so that the laser beam scans the molten target 36. By scanning the laser beam in this manner, a target having a large area can be used, so that the scattering range of the evaporated substance can be expanded. As a result, it is possible to improve the uniformity of the film thickness of the deposited layer on the substrate 22 (see FIG. 1) and use a substrate having a large area.

Next, the temperature control of the melting target 36 will be described.

As a means for confirming the melting of the target, for example, there is a method of monitoring the change in the rate of temperature rise of the target raw material. FIG. 3 shows a temperature change when indium is heated with a constant power. That is,
The horizontal axis shows the heating time and the vertical axis shows the temperature. As is clear from FIG. 3, the temperature of the target rises at a substantially constant rate until it reaches the melting point of indium (157 ° C.),
When the melting point is reached, the heat of fusion is taken away, so the temperature becomes constant until the entire raw material melts, and the temperature rises again after the entire raw material melts. Therefore, when the temperature rise of the target stops and the temperature rises again, the thermometer 5
By monitoring with 3 and the controller 54, it is possible to know whether or not all the raw materials are melted.

In this embodiment, the melting target 36
The temperature of is different depending on the type of the target, but is controlled so as to be higher than the melting point of the target material and to have a sufficiently low vapor pressure, and is set to, for example, 20 to 50 ° C. higher than the melting point.

The solid target evaporation source 40 is shown in FIG.
As shown in enlarged form, a rotary target holder 42 is provided on the bottom. This target holder 42 is
It is rotationally driven intermittently by the motor unit 46. A plurality of solid targets 46 are placed apart from each other on the target holder 42, and by rotating the target holder 42 by a predetermined rotation angle, the necessary solid targets 46 are installed at the laser beam focusing position. It is configured.

A laser optical system 11 is provided on the optical path of the laser beam between the laser transmission unit 10 and the laser window 31.
A plurality of (three in this embodiment) reflecting mirrors 12 are installed between the laser optical system 11 and the laser window 41 of the solid target evaporation source 40.
The laser optical system 11 is a semitransparent mirror or a motor 1.
3 is composed of a movable mirror which can advance and retreat with respect to the optical path, and has a function of simultaneously guiding the laser beams to the targets 36 and 46 or alternately to the targets 36 and 46.

Next, the operation of the apparatus of this embodiment will be described.

First, a single target or a plurality of targets are selected according to the substance to be crystal-grown. At that time, it is checked in advance whether or not a particulate adhered matter is generated on the deposition surface when laser irradiation is performed, and a target in which the adhered matter is not generated is set in the solid target evaporation source 40 and the adhered matter is set. The target for which is generated is set in the evaporation source 30 for the molten target.

Next, the vacuum pump 23 is driven to reduce the pressure in the vacuum container 20 to an ultrahigh vacuum (10 ^-8 Pa or less). Then, the substrate 22 for growing the crystal deposition film is set on the susceptor 21, and the substrate 22 is heated to a predetermined temperature and held. When the laser emitting unit 10 is driven in this state and a laser beam is output, the laser beam is distributed to the evaporation sources 30 and 40 by the optical system 11 and irradiated toward the melting target 36 and the solid target 46. The target component is evaporated by this irradiation, and the substrate 2
At 2, epitaxial growth is performed.

The optical system 11 has a structure in which a semitransparent mirror or a beam cross section is divided into two parts when it is desired to supply each element at the same time when a binary compound is crystallized. Further, when it is desired to supply the raw materials alternately, the motor 13 is driven to move the mirror and alternately supply the laser beam to the evaporation source 30 or 40. Although not shown in the present embodiment, when it is desired to grow a compound of multiple elements, a laser beam can be supplied to each evaporation source by using a combination of a plurality of mirrors or a plurality of lasers. it can.

Next, an experimental example of the present invention will be described.
In this experimental example, a compound semiconductor, gallium arsenide, was epitaxially grown. As an MBE device,
Using the one shown in FIG. 1, as the target raw material,
Two types of gallium and arsenic were used. Since gallium is a low melting point metal, it was set in the evaporation source 30 for melting targets equipped with resistance heating, and arsenic was set in the evaporation source 40 for solid targets because it was a sublimable substance. An ArF excimer laser was used as the laser emitting unit 10. The pulse width and peak power of the excimer laser are
The laser light was 10 nsec and 200 MW, respectively, and the laser light was alternately introduced into the evaporation sources 30 and 40 by moving the mirror used in the optical system 11 in and out of the optical path. Then, the laser light incident from the laser windows 31 and 41 is focused on the target by a concave mirror having a focal length of 200 mm.
The laser light was applied at a ratio that the pulse number for gallium and arsenic was 1: 2.

On the other hand, the vacuum container 20 is decompressed to 1 × 10 ⁻⁷ Pa, a single crystal gallium arsenide is used as the substrate 22, the substrate 22 is set on the susceptor 21, and the heater is used at 200 ° C. to 400 ° C. Was set between ° C.

FIG. 8 (a) shows a photograph of the surface of gallium arsenide epitaxially grown at a substrate temperature of 300.degree.
For comparison, the same substrate temperature 300 is shown in FIG.
The photograph of the surface which carried out the epitaxial growth of the solid gallium arsenide as a target at ℃ is shown. Comparing these photographs, the deposited surface used as a target in the molten state of gallium is a mirror surface state, whereas a large number of particulate deposits are observed on the surface when solid gallium arsenide is used as a target. From this, it was confirmed that the method of the present invention is effective. Further, in the conventional MBE method, when gallium arsenide is epitaxially grown,
Although it was necessary to set the growth temperature to 500 to 700 ° C., it is also confirmed that the method of the present invention can achieve the reduction of the growth temperature because the present invention allows epitaxial growth at the growth temperature of 200 to 400 ° C. Was done.

Although one embodiment of the present invention has been described above, the present invention is not limited to this, and various modifications can be made within the scope of the gist of the present invention. For example, in the above-mentioned examples, each constituent element is used alone as a target when growing a compound crystal, but the compound itself may be used as a target. However, in the case of a substance that accompanies the generation of solid matter during evaporation, it is necessary to bring the target into a molten state.

Further, in the above embodiment, the single crucible 32 is used in the evaporation source 30 for the melting target. However, when there are a plurality of melting targets, a plurality of crucibles corresponding to the number may be installed. it can. In this case, the crucible using a rotary target holder or the like can be made movable as in the solid target evaporation source 40.

The present invention can be applied not only to the growth of compounds but also to the growth of elemental semiconductors such as silicon. However, since the melting point of silicon is as high as 1412 ° C., electron beam heating is effective as a means for melting the target. Further, silicon and arsenic are used as targets, and the silicon crystal is doped with arsenic by irradiating the arsenic evaporation source with laser at a necessary timing as the silicon is evaporated. Further, since the supply of the target can be controlled by the ON / OFF operation of the laser emission, doping having an extremely steep interface, for example, doping such as delta doping is also possible.

Furthermore, it is possible to combine the method of the present invention with the K cell, and thin film formation other than crystal growth,
For example, it can be applied to formation of electrodes of semiconductor devices, formation of anti-slopes of reflecting mirrors, formation of various protective films and the like.

[0048]

According to the present invention, there is provided an MBE method capable of reducing the growth temperature in an ultra-high vacuum state and further achieving diversification of applicable elements, and an apparatus using the MBE method. be able to.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an example of an MBE device to which the present invention is applied.

FIG. 2 is a schematic configuration diagram showing a melting target evaporation source of the MBE apparatus shown in FIG.

FIG. 3 is an explanatory diagram showing a state of temperature rise when indium is heated.

4 is a schematic configuration diagram showing a solid target evaporation source of the MBE apparatus shown in FIG. 1. FIG.

FIG. 5 is a photomicrograph of the surface of a thin film deposited on a substrate by irradiating various targets with a laser, where (a) is tungsten, (b) is carbon, (c) is aluminum, and (d) is indium. Indicates.

FIG. 6 is a diagram showing a state of a deposition surface and melting points of various targets other than the targets listed in FIG.

FIG. 7 is a micrograph of a surface of aluminum and indium using a target in a solid state and a molten state, (a) is solid aluminum, (a ′) is molten aluminum, (b) is solid indium, and (B ') shows the case of molten indium.

FIG. 8A is a micrograph of the surface of gallium arsenide targeting molten gallium and solid arsenic, and FIG.
FIG. 4 is a micrograph of the surface of gallium arsenide targeting solid gallium arsenide.

[Explanation of symbols]

 10 Laser Transmitting Section 20 Vacuum Container 21 Susceptor 22 Substrate 30 Evaporation Source for Melting Target 36 Melting Target 40 Evaporation Source for Solid Target 46 Solid Target

Claims (2)

[Claims] 1. A molecular beam epitaxial growth method of evaporating a raw material in an ultrahigh vacuum and growing a crystal on a heated substrate, the raw material being in a molten state set at a melting point or a temperature higher than the melting point as a target. A molecular beam epitaxial growth method characterized in that a target is irradiated with laser light to evaporate the raw material in an atomic / molecular state, and crystal is grown on a substrate. 2. A molecular beam epitaxial growth apparatus for evaporating a raw material in an ultrahigh vacuum to grow a crystal on a heated substrate, a heating means for melting the target raw material, a sensor means for detecting melting of the target raw material, and a melted target material. And an evaporation source for melting the target, which includes a temperature control means for keeping the temperature of the target material constant, a laser emitting means for emitting a laser beam, and an optical means for guiding the emitted laser beam to the target. Molecular beam epitaxial growth equipment.