JPH0714784A - Formation of thin crystal film controlled at unit of subatomic layer - Google Patents

Abstract

PURPOSE:To grow a thin crystal film digitally with a growth unit of subatomic weight even it the saturated adsorption is less than a unit molecular weight by removing unnecessary atoms or molecules, adsorbing to the surface of a substrate, therefrom and shifting the surface atoms by thermal excitation thereby flattening the surface. CONSTITUTION:At least a part of material molecules is adsorbed onto a substrate and residual molecules are discharged. A surface exciting means removes unnecessary molecules or atoms, adsorbed to the surface of the substrate, therefrom while simultaneously or subsequently raising the surface temperature of the substrate to a level allowing desired migration of adsorbed atoms thus migrating the surface atoms through thermal excitation and flattening the surface. These steps constitute one cycle and a layer less than one atomic layer is formed every cycle. This method allows digital growth at a growth unit of subatomic layer even if the saturated adsorption is less than single molecular layer.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a new method for producing a crystal thin film, which allows a crystal thin film having an arbitrary number of layers to be grown in units of subatomic layers of one atomic layer or less.

[0002]

2. Description of the Related Art In recent years in the development of electronic devices, especially semiconductor devices, in order to improve performance and develop new functions, it is necessary to improve the performance of the atomic layer unit such as superlattice device or quantum well device. Attention has been focused on the development of seed-controlled devices, and various approaches have been taken to investigate the measures. In order to realize such a device, it is necessary to control the crystal growth on an atomic layer basis. As a method for enabling such growth control, the atomic layer epitaxy (ALE: Atomic Layer) proposed by Suntra is proposed. Epita
xy) method is known. With this method, in principle, any number of layers can be grown in units of one atomic layer, so this method is attracting attention as an ideal method for growing crystals on an atomic scale, and the development of technologies related to this method is active. . In recent years, the ALE method has been applied as a growth method for binary compound semiconductors such as III-V semiconductors. Specifically, two kinds of molecules each containing one of the two kinds of atoms of the binary compound semiconductor are used, and the molecules are alternately irradiated to the substrate. The bond strength between the two types of atoms is stronger between different types than between the same types, and the irradiated molecule easily reacts with different types of molecules adsorbed on the base, and does not easily react with the same types of molecules adsorbed on the base. Therefore, when one of the raw material molecules is irradiated, the molecule of the same kind or its decomposed species is further adsorbed and saturated.
Unwanted molecules contained in the raw material molecules are released during the adsorption process. Such saturated adsorption is called a self-stop function. With this function, crystals can be grown in units of one atomic layer. Therefore, this ALE method is suitable for a method of manufacturing a compound semiconductor composed of two or more kinds of atoms.

On the other hand, in recent years, group IV semiconductors such as Si / Ge are expected to be applied to high speed transistors such as heterojunction bipolar transistors and modulation-doped field effect transistors, and strained superlattice optical devices. Crystal growth has been needed. However, since the principle of alternate adsorption found in the ALE method for compound semiconductors cannot be used with a single element material, there is no general principle for obtaining a monolayer, and it is difficult to obtain the saturated adsorption amount of a monolayer. Therefore, in the ALE method of a single element material, in the adsorption process of irradiating raw material molecules,
We can only expect that the raw material molecules or their decomposition species will be saturated by the adsorption amount of the monolayer. Of course, after that, unnecessary molecules contained in the monolayer saturated and adsorbed are removed by thermal excitation by heating or photoexcitation by light irradiation, or by irradiating other reactive molecules with the molecules and releasing them together. Is adopted. As a result, the desired atom grows further after the unwanted molecule is released. Therefore, the general procedure of the ALE method for a single element material is to repeat the cycle of (1) irradiation of raw material molecules, (2) removal of unnecessary molecules, and ideally perform monoatomic layer growth in each cycle. become. So far, Ge (C ₂ H ₅ ) ₂ H ₂ and Ge (C
Research on Ge ALE method using H ₃ ) ₂ H ₂ and S
Research on the ALE method of Si using iH ₂ Cl ₂ has been advanced, and in the process, it has been reported that the saturated adsorption amount is about a monolayer, which is a self-terminating function. That is, in the research using these raw material molecules, ALE growth has been realized in units of a monoatomic layer. However, this method has a problem that a trace amount of impurities such as C and Cl remain, and such a raw material molecule has a problem in terms of crystal purity.

Therefore, the inventor of the present invention first focused on a hydrogen compound molecule free from such a problem of impurities, and as a result of atomic level analysis, Si ₂ H ₆ can be applied as a raw material molecule of Si ALE. Potential J. Vac. Sc
i. Technol. B7, 1171 (1989). After that, research on ALE using a large number of hydrogen compound molecules including Si ₂ H ₆ has been conducted. However, although the adsorption amount is saturated, the saturated adsorption amount is less than a monolayer, which is a problem. . In addition, recently, a study using GeH ₄ has been reported, and it was shown that saturated adsorption occurs in about a single molecule. However, there still remain problems such as a narrow range of growth conditions. Thus, when a hydrogen compound molecule is used,
Most of them show a saturated adsorption phenomenon, but since the saturated adsorption amount is less than a monolayer, it is difficult to stack atoms flatly. Therefore, despite the advantage that impurity atoms are not included, , It is difficult to establish the ALE method.

The present invention has been made in view of the situation of the prior art as described above, and even if the saturated adsorption amount is less than a monomolecular layer, the amount (sub-atomic layer) is used as a growth unit for a digital unit. It is an object of the present invention to provide a new method of manufacturing a crystal thin film that can be grown and is controlled in units of sub atomic layers.

[0006]

In order to solve the above problems, the present invention provides a first step of adsorbing at least a part of raw material molecules on a substrate, and exhausting residual molecules that have not been adsorbed. In two steps, unnecessary molecules or atoms of the molecules or atoms adsorbed on the substrate surface by means of surface excitation are released from the substrate surface, and at the same time or subsequently, the temperature of the substrate surface is a temperature at which the desired adsorbed atoms can move. A plurality of steps including a third step of planarizing the surface by thermally exciting the surface atoms to move the surface atoms,
Provided is a method for producing a crystalline thin film controlled in sub atomic layer units, which is characterized in that the number of layers of one atomic layer or less is formed in each cycle.

[0007]

That is, the method for producing a crystal thin film according to the present invention will be described in detail. As described above, in the first step (A), a substrate on which the crystal thin film is formed is irradiated with a certain amount of raw material molecules, At least a part of the raw material molecules is adsorbed on the substrate. (B) In the second step, residual molecules that have not been adsorbed are exhausted. Then, as the (C) third step, for example, by means of surface excitation such as thermal excitation,
Of the molecules or atoms adsorbed on the substrate surface, unnecessary molecules or atoms are released from the substrate surface, and at the same time or subsequently, the temperature of the substrate surface is raised to a temperature at which the desired adsorbed atoms can move, and the substrate surface is kept constant. Thermal excitation is performed for a time, more preferably 1 msec or more, and the surface is flattened by the movement of surface atoms. Such (A) (B) (C)
Multiple steps including the three steps of
The number of layers of one atomic layer or less is formed in each cycle.

In this case, it is important that the third step (C) always includes thermal excitation by heating. This is because the surface atoms are moved by this thermal excitation. For example, when the saturated adsorption amount is 0.5 atomic layer, when the adsorption is performed on the completely flat surface in the first step, 0.5 atomic layer of the raw material molecules are adsorbed. When thermally excited in the third step, unnecessary atoms are released and adsorbed atoms move and gather to form a large terrace. The amount of terraces corresponds to 0.5 atomic layer, and there are still 0.5 atomic layer of unfilled sites.
In the following first step of the second cycle, the raw material molecules are adsorbed on the terrace or on the site of 0.5 atomic layer which is not filled in the first cycle. In the third step of the second cycle, atoms that are adsorbed in pieces in the second cycle mainly move, atoms on the terrace move on the terrace, and they fall from the edge of the terrace and are adsorbed on unfilled sites. To do. In this way, the outermost surface is flattened by the movement of atoms by thermal excitation, and when the saturated adsorption amount is 0.5 atomic layer, the flattened 1 atomic layer grows in two cycles.

The mechanism for flattening as described above is based on Si on the substrate.
The (100) plane was realized by the inventor of the present invention in a system using Si ₂ H ₆ as a raw material molecule. As a result of observing the period of surface atoms by reflection electron diffraction, it was found that the surface can be planarized at the same time when hydrogen, which is an unnecessary atom, is released by thermal excitation. Specifically, the saturated adsorption amount of Si ₂ H ₆ is about 0.45 atomic layer, and one atomic layer grows in about 2 cycles.

In the present invention, it is important to migrate surface atoms by thermal excitation of the surface as described above, and when unnecessary molecules are released by photoexcitation or electron beam excitation,
It is necessary to simultaneously or subsequently heat the surface by light irradiation or electron beam irradiation, or to thermally excite the surface simultaneously or subsequently by the second excitation means. The surface temperature of the saturated adsorption process of the first step is lower than the surface temperature of the third step. Therefore, when the first step is performed after the surface temperature reaches an appropriate level in the adsorption process, it is necessary to provide a period for controlling the surface temperature as the fourth step. In order to migrate the surface atoms by thermal excitation, only the vicinity of the surface needs to be excited. In order to efficiently excite only the surface, the surface may be irradiated with light or an electron beam. In particular, light having a short wavelength has a large absorption coefficient, and particularly light having a wavelength of 1 μm or less is absorbed in the vicinity of the surface and is suitable for heating the vicinity of the surface. Further, in the third step, in order to remove unnecessary atoms and migrate surface atoms, it is necessary to apply appropriate excitation energy to the surface and continue excitation for 1 msec or more. For this purpose, a laser capable of continuous oscillation may be used and the irradiation time may be adjusted appropriately. A large light output is required for heating by light irradiation, and a laser is also suitable in this respect. In particular, an ion laser can obtain a large optical output, is continuous oscillation, and has a light wavelength of 1 μm or less.
Optimal for thermal excitation of the surface. Further, according to the invention, even if the saturated adsorption amount is less than the monolayer, if the constant saturated adsorption amount is obtained in a wide range of conditions, the self-stop function can be obtained in the same wide range of conditions.

Examples will be shown below to describe the method of the present invention in more detail.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 of the accompanying drawings shows an example of an apparatus used in the manufacturing method of the present invention. In this device,
A sample (1) having a Si (100) surface on its surface is attached to the center of a vacuum container (2). The vacuum container (2) is constantly evacuated by the exhaust pump (3). For surface excitation in the second step, A
An r ⁺ ion laser (4) was used. The main wavelength range of the output light is 458 to 515 nm, and this light is applied to the surface of the sample (1) through the vacuum window (5). Disilane (Si ₂ H ₆ ) was used as the raw material molecule. The pressure of the disilane discharged from the cylinder (6) is controlled by the pressure reducing valve (7), and the sample surface is irradiated with the disilane through the nozzle (8). Whether the disilane was irradiated or not was controlled by the on / off valve (9). The sub atomic layer crystal growth procedure is as follows: (1) irradiation of raw material molecules for 10 seconds (first step), (2) exhaust of unnecessary gas for 10 seconds (second step), (3) laser irradiation for 10 seconds (third step). ), (4) 10 seconds of cooling (4th process). However, the sample was constantly heated by the heater (10) so that the surface temperature in the irradiation process of the first step was 300 ° C. Also,
In the third step, laser irradiation was performed so that the surface temperature became 700 ° C. The reason for the set value of the surface temperature in the third step will be described later. FIG. 2A shows the film thickness distribution when 840 cycles are repeated. For comparison, FIG. 2B shows a film thickness distribution of a film grown by continuously irradiating a laser and raw material molecules. Both distributions are normalized by the peak film thickness.
The distribution of b is proportional to the distribution of the power density of the laser, and has a Gaussian distribution. There is a flat portion in the distribution of a, and it can be seen that the self-stop function works because it is saturated to a certain saturated adsorption amount. The growth rate of the flat portion was 0.45 atomic layer / cycle.

Prior to this experiment, the process of subatomic layer crystal growth was observed by a method of observing the periodic structure of the surface called reflection type high-energy electron diffraction. As a result, it was found that (1) when the saturated adsorption surface was heated at a gradually higher temperature, hydrogen was released at 700 ° C., and the atoms could gather in a terrace shape. It was also found that (2) approximately 1 atomic layer can be grown in 2 cycles. This result shows that (a) the saturated adsorption amount is about 0.
As a result of the 5 atomic layer, (a) the surface migration of the atoms, and (c) the surface migration, the saturated adsorption amount is about 0.5.
It shows concretely the principle of the present invention that the surface can be planarized in two cycles even with an atomic layer. In the experiment of sub-atomic layer crystal growth using a laser, the laser was irradiated such that the surface temperature in the third step was 700 ° C. based on the present invention. The experimental results according to the present invention also show that the growth of about 0.5 atomic layer per cycle is obtained, and it can be seen that the constant saturated adsorption amount was repeated 840 times. That is, it can be seen that the flattening function by surface migration worked.

[0014]

According to the present invention, when the saturated adsorption amount is an amount corresponding to the fractional degree of the monoatomic layer, that is, when the saturated adsorption amount corresponds to 1 / n atomic layer, the reciprocal of n cycles is used. Atomic layer grows. Therefore, not only the crystal growth can be controlled in the unit of a single atomic layer, but in this case, the digital growth can be performed in the unit of the fractional order layer. Generally, if the saturated adsorption amount is a sub-atomic layer, digital growth can be performed with the sub-atomic layer as a unit. For example, when the saturated adsorption amount corresponds to 0.3 atomic layer, 0.9 atomic layer is grown in 3 cycles, and 1.8 atomic layer is grown in the next 6 cycles. In this case, it is not possible to grow in a unit of a single atomic layer, but crystal growth can be performed in a unit close to the amount of a single atomic layer, and application to atomic layer scale growth becomes possible. In this way, even for systems where it was conventionally impossible to stack atoms flatly because the saturated adsorption amount was less than a monoatomic layer, the path to atomic layer scale crystal growth was opened. It will be. At the same time, the oscillation wavelength is 6
When an Ar ⁺ ion laser of 00 nm or less is used, thermal excitation of the surface is optimally performed as described in the summary of the invention, but the beam radius of the oscillation light can be optically narrowed, so that a narrow range can be selectively selected. It is possible to draw and finely grow selectively.
Therefore, it is possible to fabricate a new superlattice element having a structure as shown in FIG. 3, which was very difficult in the past. As described above, according to the present invention, the applicable range of the atomic layer scale crystal thin film growth technique is expanded.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a configuration example of an apparatus used in an example.

FIG. 2 is a film thickness distribution diagram as an example.

FIG. 3 is a perspective view illustrating a superlattice element according to the present invention.

[Explanation of symbols]

1 sample 2 vacuum container 3 exhaust pump 4 Ar ⁺ ion laser 5 window 6 cylinder 7 pressure reducing valve 8 valve 9 nozzle 10 heater

Claims (6)

[Claims] 1. A first step of adsorbing at least a part of raw material molecules on a substrate, a second step of exhausting unadsorbed residual molecules, and a step of adsorbing molecules or atoms adsorbed on the substrate surface by means of surface excitation. Unnecessary molecules or atoms are detached from the substrate surface, and simultaneously or subsequently, the temperature of the substrate surface is raised to a temperature at which desired adsorbed atoms can move and thermally excited to move the surface atoms to flatten the surface. A method for producing a crystal thin film controlled in sub-atomic layer units, wherein a plurality of steps including the third step are defined as one cycle, and the number of layers of one atomic layer or less is formed in each cycle. 2. The method for producing a crystalline thin film according to claim 1, wherein a fourth step of lowering the substrate temperature is added. 3. The method for producing a crystal thin film according to claim 1, wherein light having a wavelength of 1 μm or less is used as the excitation means. 4. The method for producing a crystalline thin film according to claim 1, wherein a laser that continuously outputs is used as the exciting means. 5. An electron beam is used as the excitation means.
Alternatively, the method for producing a crystalline thin film according to item 2. 6. The method for producing a crystal thin film according to claim 1, wherein silane (SiH ₄ ), disilane (Si ₂ H ₆ ), germane (GeH ₄ ) is used as a raw material molecule.