JPH0235716B2 - - Google Patents

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention relates to a method for manufacturing germanium thin film crystals, and in particular to manufacturing germanium thin film crystals for use in semiconductor devices such as light emitting diodes, optical composite devices, and monolithic multifunctional integrated circuits. Regarding the method.

<Prior art> Conventionally, in manufacturing this type of germanium thin film crystal, a germanium layer is deposited in advance to a desired thickness on a dielectric substrate such as silicon (silicon surface coated with an oxide film). After that, the germanium layer was crystallized by being melted and solidified by irradiating a laser beam or an electron beam from one side to the other or by a zone melting method using linear heater heating. However, when crystallizing germanium using this method, when only the germanium layer is deposited on the substrate, in the case of heating and melting with a linear heater, the heater temperature, the substrate temperature, and the relative speed (movement) between the beam and the substrate are In the case of heating and melting with an energy beam, the beam power, substrate temperature, relative speed between the beam and the substrate, etc. must be strictly determined.

This is because, as shown in Figure 7a, when germanium is dissolved, its adhesion to the substrate (SiO ₂ ) is weak, so if the conditions deviate even slightly from the above conditions, sufficient crystal growth will not occur, or, as shown in Figure 7a. This is because germanium condenses and becomes liberated from the substrate as shown in b, and thin film crystallization cannot be achieved. In particular, in a method using a linear heater in which the area of molten germanium during heating crystallization is large, it becomes extremely difficult to crystallize a thin film over the entire substrate surface. In order to prevent such condensation, conventionally, as shown in Fig. 1, a method has been adopted in which layers 1 and 3 of a high-melting point metal such as tungsten are formed above and below a germanium layer 2 and then heated and crystallized. However, although crystallization by such a method is effective in preventing condensation of germanium, as shown in FIG. In order to epitaxially grow the gallium oxide layer 6, after crystallizing the germanium 2, the high melting point metal layer 3 shown in FIG. 1 is removed to expose the germanium layer 2, and then epitaxial growth is performed as shown in FIG. Layer 6 needs to be formed. At present, chemical etching is mainly used to remove the high melting point metal layer 3, but it is difficult to obtain a sufficiently flat surface of the germanium layer 2 after the layer 3 is removed. Therefore, the surface of the epitaxially grown layer 6 is also affected by the surface of the layer 2, making it impossible to obtain a sufficiently flat surface, resulting in the disadvantage that non-uniformity appears in the characteristics of the elements formed on the layer 6. Also, the third
As shown in the figure, a photocoupler element configured to receive an optical signal 8 emitted from a light emitting element 7 formed on a gallium arsenide layer 6 with a light receiving element 9 formed in advance on a substrate uses a high melting point metal that is opaque to the optical signal 8. Due to the existence of layer 1, in order for the optical signal 8 to effectively reach the light receiving element 9, it is necessary to reduce the thickness of layer 1 within a range that is effective for preventing condensation. The power of the arriving light was often insufficient to drive the light receiving element 9. Another method for preventing germanium condensation is to form a SiO ₂ B layer 10 with a thickness of about 1 μm on the germanium layer 2 as shown in FIG. There is a limit to the area of germanium that can prevent condensation.
Crystallization over a wide area was difficult.

<Problems to be Solved> The present invention was made in order to eliminate the above-mentioned drawbacks in the production of germanium thin film crystals. An object of the present invention is to provide a method for producing a germanium thin film crystal that can grow crystals over the entire surface of a substrate with high reproducibility and can produce a germanium thin film crystal with good surface flatness.

<Means for Solving the Problems> In order to solve the above-mentioned problems, one of the means of the present invention is to apply a material selected from the group consisting of boron nitride, aluminum nitride, beryllium oxide, and silicon carbide on a dielectric substrate. depositing a first layer of germanium on the first layer; depositing a second layer of germanium on the first layer;
depositing a third layer consisting of at least one selected from the group consisting of boron nitride, aluminum nitride, beryllium oxide and silicon carbide on the layer; and depositing the first, second and third layers. The method is characterized by comprising a step of partially heating the dielectric substrate along the entire dielectric substrate or the second layer.

Further, another means of the present invention includes the step of depositing, on the dielectric substrate, a first layer consisting of at least one selected from the group consisting of boron nitride, aluminum nitride, beryllium oxide, and silicon carbide;
depositing a second layer made of germanium on the layer; and depositing a third layer made of a high melting point metal that does not form an alloy with germanium at a temperature near the melting point of germanium on the second layer. and a step of partially heating the dielectric substrate on which the first, second, and third layers are deposited or along the second layer.

Another means of the present invention is to provide a first layer on a dielectric substrate made of a high melting point metal that does not form an alloy with germanium at a temperature near the melting point of germanium, and a first layer made of germanium on the first layer. depositing a second layer on the second layer; depositing a third layer comprising at least one selected from the group consisting of boron nitride, aluminum nitride, beryllium oxide, and silicon carbide; This method is characterized in that it is a step of heating the entire dielectric substrate on which the first, second, and third layers are deposited or partially heating the dielectric substrate along the second layer.

<Function> In the method for producing a germanium thin film crystal of the present invention, it is necessary to suppress condensation of germanium and sufficiently crystallize the thin film. For this purpose, it is considered that the following requirements must be satisfied.

Sandwiching the germanium layer between thin films of material with high surface tension (hereinafter referred to as protective films) in the form of a sandwich.

In general, when a liquid substance exists on a solid, it is well known whether the liquid substance is in a condensed state, that is, clumped together in a bead shape, or spread out, that is, well wetted on the solid surface. It depends on the magnitude relationship between the surface tension of the solid and the surface tension of the liquid substance. As shown in the inset of FIG. 7, the degree of wetting between the liquid substance 20 and the solid 21 can be evaluated as the contact angle θ. It can be said that the larger the contact angle θ, the smaller the degree of wetting, and the closer to a bead-like condensed state. In a completely condensed state, the contact angle θ is
It becomes 180 degrees. The graph in Figure 7 is the result of measuring the contact angle of germanium on various solid materials, and shows the contact angles of germanium on various solid materials, including boron nitride, aluminum nitride, beryllium oxide, and silicon carbide, which are used as protective films in the present invention, and tungsten, a high melting point metal. has been shown to have greater wettability, that is, less condensation, than other typical dielectrics (SiO ₂ ). Considering this qualitatively, a state where condensation is easy and wettability is poor is defined as (1) the area of the exposed part of the surface of the solid 21 is large, and the surface atoms of the solid 21 are not bonded with other substances and are suspended in the air; The state of unbonded surface electrons, that is, the state in which the solid 21 has a unique solid surface energy or surface tension; (2) the liquid substance 20 uses the action of surface tension to minimize its own liquid surface energy per unit area. It is in a state where it maintains a ball-like shape.
Therefore, when the solid surface energy is larger than the liquid surface energy in terms of unit surface area, the area of the exposed part of the surface of the solid 21 is larger than the surface area of the liquid substance as a whole system of solid and liquid substances. The larger the value, the smaller the total energy. That is, this liquid substance becomes condensed. Conversely, a state in which it is difficult to condense and easily wet is a state in which the solid surface energy is larger than the liquid surface energy in terms of unit area, and the exposed area of the surface of the solid 21 does not expand, but the solid The surface of 21 is covered with the liquid substance 20 and bonds between solid and liquid atoms occur, so that the solid surface (interfacial) energy is lower than in the unbonded state, and instead the liquid substance 2
0 spreads, and the energy of the entire system approaches the value defined by the liquid surface energy.

Therefore, in order to prevent condensation of molten germanium, it is important to contact germanium with a solid having a high surface energy, in other words, a high surface tension, as a protective layer.

As mentioned above, the magnitude of surface energy is a quantity that represents the strength of the tendency of surface atoms to seek bonds, and is a quantity that is directly related to the strength of the bonds of material atoms themselves. Cohesive energy is a macroscopically measurable physical constant that represents bond strength. FIG. 8 shows the cohesive energies of various substances, mainly in the crystalline state. The horizontal axis shows the ionicity of substances for ease of viewing, and lines connect substances with the same period on the periodic table. As is clear from FIG. 8, the substances used in the present invention, such as boron nitride and beryllium oxide, all have large cohesive energy, that is, surface energy (surface tension). Furthermore, high melting point metals such as tungsten generally have large cohesive energy.

In reality, the state of wetting may change depending on the degree of contamination of the surface of each solid or liquid, and this is the cause of the variation in the measurement of the contact angle in FIG. 7. Also, dirt,
Due to slight irregularities, etc., the surface is in a different state from the ideal surface, so it becomes completely wet (contact angle θ = 0 degrees) and completely condenses (contact angle θ = 180 degrees).
degree) is rare. Therefore, in order to sufficiently suppress the actual condensation of germanium,
As shown in the inset of Figure 7, not only one side (lower surface) is brought into contact with the protective film, but also a similar protective film is placed on the upper surface of the germanium before melting, which is the so-called sandwich structure. Therefore, it is considered necessary to increase the contact area.

The upper and lower protective layers must have a coefficient of linear expansion similar to that of germanium.

Even if the aggregation of molten germanium is suppressed, immediately after germanium solidifies (crystallizes), the temperature of the system is just below the melting point of germanium, and it is difficult to imagine that the sandwich structure will be destroyed by thermal strain during the cooling process to room temperature. It doesn't even become. Therefore,
It is necessary that the linear expansion coefficients of germanium and the protective film are similar values. For this reason, all materials used in the present invention were selected to have low thermal distortion.

The upper and lower protective layers must not react significantly with germanium even at temperatures near the melting point of germanium.

When the molten germanium reacts with the protective film, the interface between the germanium and the protective film generally becomes a rough surface with unevenness. It becomes a state where it is impossible to grow. In a conventional structure in which tungsten is used as the upper and lower protective layers, a small amount of reaction occurs at both the upper and lower interfaces between the protective layer and dungsten, so the synergistic effect of the roughness on both interfaces causes roughness, especially on the top surface of germanium. It became large and became a problem during epitaxial growth. Also, the third
When manufacturing an optical device with a structure similar to the one shown in the figure, it is necessary to make the germanium layer as thin as possible in order to increase the optical coupling efficiency. The synergistic effect of roughness increases significantly, and as a result, the roughness of the top surface of germanium increases.

Therefore, instead of using a substance such as boron nitride for both the upper and lower protective layers (Claim 1), it is possible to simply replace one of them with a substance such as boron nitride (Claim 2). and Section 3), since the interfacial reactions between these substances and germanium can be completely ignored, there is no synergistic effect of roughening of both interfaces, and the roughness caused by the reaction between high melting point substances such as tungsten and germanium is minimized. It can be suppressed to

Note that if metal is used as a part of the laminated structure in this way, it can be used as an electrode.

<Example> Examples of the present invention will be described below.

A substrate 5 whose surface is coated with a SiO ₂ film 4 is maintained at 600° C. and a boron nitride film 11 with a thickness of 70 nm is deposited by chemical vapor deposition using diborane (B ₂ H ₆ ) and ammonia (NH ³ ) as source gases. After depositing on this substrate, the substrate temperature was lowered in a vacuum of 1×10 ^-6 Torr.
A 400 nm thick germanium film 2 was deposited at 700° C. by electron beam heating evaporation. After undergoing such processing steps, a 70 nm boron nitride film 12 was formed on the germanium film 2 by chemical vapor deposition using diborane and ammonia as raw material gases at a substrate temperature of 600° C. as shown in FIG. 5A. A sample with the cross-sectional structure shown was obtained.

Next, the obtained sample is placed on a quartz plate 14 as shown in FIG. Crystallization was carried out at approximately 1200°C and maintained at approximately 770°C.
The germanium layer 2 is locally heated by the linear heater 13, and a portion 15 of the layer 2 directly under the heater 13 becomes molten. Therefore, (arrow C)
By sliding the substrate on the support stand 14 in the direction of the arrow at a speed of about 1 millimeter per second, the germanium melting portion 15 was moved on the substrate, causing zone melting, and the germanium layer 2 was sequentially melted.
It solidified into crystals. At the time of the state shown in FIG. 5B, 17 is a crystallized portion and 18 is an uncrystallized portion. Even in such a situation where part 15 of the germanium layer melts, the boron nitride layers 11 and 12 are stably maintained without any change, and it is possible to suppress the condensation of germanium in the boron nitride layer 15. One of the reasons for this is that boron nitride does not react with germanium near germanium's melting point of 1213°K and does not cause decomposition or evaporation. Another reason is that boron nitride has a high surface tension, so when it comes into contact with molten germanium, it wets the germanium well and keeps it stable in a thin layer. In addition, by forming boron nitride layers above and below the germanium layer,
In addition to increasing the area of the interface between germanium and boron nitride with good adhesion, the mechanically strong boron nitride material also had a synergistic effect, making it possible to create a stable thin film crystal. On the other hand, germanium and boron nitride have approximately the same coefficient of linear thermal expansion (6.8 to 5.7×10 ^-6 /℃) at temperatures between the melting point of germanium (1213°K) and room temperature, so they can be used immediately after crystallization. No cracking, peeling, etc. occurred in each layer even during the period from the temperature to room temperature. As stated above, the requirements for functioning sufficiently as a protective film to prevent germanium condensation are (1) not to react with germanium;
(2) It has a large surface tension, (3) It has a coefficient of linear thermal expansion similar to that of germanium, and (4) It has a certain degree of mechanical strength. In addition to boron nitride, aluminum nitride, beryllium oxide, and silicon carbide can be cited as substances that meet the above requirements. Therefore, it goes without saying that a similar effect can be obtained by appropriately combining these substances in addition to those in the above embodiments. In this example, after the crystallization of germanium, the boron nitride layer 12 on the germanium was removed by a reactive etching method using CF ₄ gas or C ₂ F ₆ gas, but the surface of the germanium layer after removal was flat. The epitaxially grown layer of gallium arsenide on germanium was similarly flat and had few crystal defects, resulting in a gallium arsenide light-emitting device with good reproducibility.

Furthermore, a boron nitride layer 11 under the germanium layer
Since it is transparent to the emission wavelength of the gallium arsenide element, the optical signal could effectively reach the silicon substrate 5.

In addition, as shown in Fig. 6, a germanium layer 2 with a thickness of 400 nm was formed by electron beam heating evaporation.
A sample with a laminated structure consisting of an aluminum nitride layer 19 with a thickness of about 50 nm formed on the top by a sputtering method using argon gas at 10 ^-2 Torr, and a tungsten layer 1 with a thickness of about 50 nm formed on the bottom by electron beam heating evaporation. When crystallized using the same zone melting method as in the above example, a germanium thin film crystal with good surface flatness could be obtained.

Furthermore, a gallium arsenide layer epitaxially grown on this germanium thin film also had good flatness.

In this example, a tungsten layer 1 is present below the germanium layer 2, which layer 1 can be used as an electrode for a device in germanium or even gallium arsenide. Further, depending on the purpose, a structure may be adopted in which a tungsten layer is formed on the germanium layer and an aluminum nitride layer is formed below.
It goes without saying that various changes and improvements can be made in addition to these embodiments without departing from the spirit of the invention. The substrate material is not limited to thermally oxidized silicon, but can be applied to a wide range of materials.

In the above examples, the sample with the laminated structure shown in FIG. 5A and the laminated structure shown in FIG.
An example of crystallizing the germanium layer using the zone melting method shown in FIG. It may be crystallized by rapidly raising the temperature to 1000°C, which is higher than the melting point of 986°C, and then cooling it.

<Effects of the Invention> As is clear from the above description, a structure consisting of a first layer made of boron nitride or the like, a second layer made of germanium, and a third layer made of boron nitride or the like can be formed on a desired substrate. After crystallizing germanium, a layer of boron nitride or the like on the germanium is removed by chemical etching or dry etching to obtain a semiconductor substrate having a flat germanium crystal on the surface. Furthermore, if one of the first or third layers is made of a high melting point metal such as tungsten depending on the purpose, this layer can be used as an electrode of the device. Also, if you want to create a structure that avoids contact between germanium and conductive substances,
The first and third layers may be composed only of an insulating material such as boron nitride.

On the other hand, when coupling an element formed in a substrate with an element formed on gallium arsenide on germanium using optical signals, it is easy to achieve the purpose by selecting an appropriate configuration considering the transparency of the layer. It can be achieved.

As an application of the present invention, by using gallium arsenide grown on a germanium layer, inexpensive gallium arsenide solar cells can be manufactured. Furthermore, a monolithic photocoupler device can be realized by combining a gallium arsenide light emitting device and a light receiving muntin in the silicon substrate. Furthermore, it has the advantage that it can be used to create monolithic multifunctional integrated circuits that combine opto-electronic integrated circuits in gallium arsenide and silicon.

[Brief explanation of drawings]

1 to 4 are explanatory diagrams showing the laminated structure of germanium thin films melted and heated in the conventional germanium thin film crystal production method, and FIG. 5A is melted and heated in the germanium thin film crystal production method of the present invention. An explanatory diagram of the laminated structure of one embodiment of a germanium thin film, FIG. 5B is an explanatory diagram showing a zone melting method for a germanium thin film having the laminated configuration of FIG. 5A, and FIG. 6 is a method for producing a germanium thin film crystal of the present invention. FIG. 7a is an explanatory diagram of molten germanium, FIG. 7b is an explanatory diagram showing condensation of germanium by heating, and FIG. FIG. 9 is a graph showing the relationship between the ionicity and cohesive energy of various substances. In the drawings, 1, 3...Tungsten (high melting point metal) layer, 2...Germanium layer, 4...Silicon oxide film substrate, 5...Silicon film substrate, 6...Gallium arsenide epitaxial layer, 11, 12 ...Boron nitride film, 13 ... Linear heater (carbon), 15
... Melted portion of germanium layer, 17... Crystallized portion, 18... Uncrystallized portion, 19... Aluminum nitride layer.

Claims (1)

[Claims] 1. A step of depositing on a dielectric substrate a first layer consisting of at least one selected from the group consisting of boron nitride, aluminum nitride, beryllium oxide, and silicon carbide; depositing a second layer of germanium on the second layer; and depositing a third layer of at least one selected from the group consisting of boron nitride, aluminum nitride, beryllium oxide, and silicon carbide on the second layer. a step of causing
A method for producing a germanium thin film crystal, comprising the step of heating the entire dielectric substrate on which the first, second and third layers are deposited or partially heating the dielectric substrate along the second layer. 2 depositing a first layer made of at least one selected from the group consisting of boron nitride, aluminum nitride, beryllium oxide, and silicon carbide on a dielectric substrate; depositing a first layer made of germanium on the first layer; a third layer made of a high melting point metal that does not form an alloy with germanium at a temperature near the melting point of germanium on the second layer;
and partially heating the dielectric substrate along the entire or second layer on which the first, second and third layers are deposited. A method for producing germanium thin film crystals. 3 Depositing a first layer made of a high melting point metal that does not form an alloy with germanium at a temperature near the melting point of germanium on a dielectric substrate, and depositing a second layer made of germanium on the first layer. a third layer comprising at least one selected from the group consisting of boron nitride, aluminum nitride, beryllium oxide and silicon carbide on the second layer;
and partially heating the dielectric substrate along the entire or second layer on which the first, second and third layers are deposited. A method for producing germanium thin film crystals.