JP4169145B2 - Method for forming hemispherical silicon microcrystals - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for forming hemispherical silicon microcrystals used in electronic devices and optical devices utilizing the quantum size effect.
[0002]
[Prior art]
In recent years, research has been actively conducted to provide materials with electronic and optical functions based on quantum mechanical effects by arranging nanometer-sized silicon crystals on the substrate surface.
[0003]
A nanometer-sized silicon crystal is formed as follows. Si atoms are deposited on the SiO ₂ film by an LPCVD method (for example, References 1 to 3 described later) or a plasma CVD method (for example, Reference 4 described later). At this time, since the surface energy of Si is larger than that of SiO ₂ , the reaction proceeds in the direction of reducing the surface area of the entire Si film. That is, Si atoms are aggregated to form self-organized Si quantum dots on the SiO ₂ surface. Using this quantum dot, a resonant tunnel diode (for example, Reference 2), a single electron memory (for example, Reference 5 described later), and the like have been prototyped.
[0004]
In addition, the said literatures 1-5 are as follows.
Reference 1: A. Nakajima, Y. Sugita, K. Kawamura, H. Tomita, and N. Yokoyama, “Si quantum dot formation with low-pressure chemical vapor deposition”, Jpn. J. Appl. Phys. Part 1 35, L189 (1996).
Reference 2: M.M. Fukuda, K. Nakagawa, S .; Miyazaki, and M. Hirose, “Resonant tunneling through a self-assembled Si quantum dot”, Appl. Phys. Lett. 70, 2291 (1997).
Reference 3: S, Miyazaki, Y. Hamamoto, E. Yoshida, M. Ikeda, and M. Hirose, “Control of self-assembling formation of nanometer silicon dots by low pressure chemical vapor deposition”, Thin Solid Films 369, 55 (2000).
Reference 4: M.M. Otobe, H.C. Yajima, and S. Oda, Appl. Phys. Lett. 72, 1089 (1998).
Reference 5: S.M. Tiwari, F.A. Rana, H. Hanafi, A. Hartstein, E .; F. Crabbe, and K. Chan, “A silicon nanocrystals based memory”, Appl. Phys. Lett. 68, 1377 (1996).
[0005]
As a system other than the combination of Si and SiO ₂ as described above, hemispherical microcrystalline Si on an amorphous silicon (hereinafter referred to as a-Si) film is known although there are very few examples of research (documents described later) 6, 7). The documents 6 and 7 are as follows.
Reference 6: A. Sakai and T. Tatsumi, “Novel seeding method for the growth of resulting Si films with hemispherical grains”, Appl. Phys. Lett. 61, 159 (1992).
Reference 7: A. Sakai, T. Tatsumi, and K. Ishida, “Growth kinetics of Si hemispherical grains on clean amorphous-Si surfaces”, J. Am. Vac. Sci. Technol. A 11, 2950 (1993).
[0006]
Here, the outline of the production procedure reported in the above-mentioned document 6 will be described. According to this manufacturing procedure, first, an a-Si film having a thickness of 100 nm is formed on a Si substrate with a natural oxide film at room temperature by a molecular beam epitaxy (MBE) method. Next, while maintaining the substrate temperature at 450 ° C., Si atoms are blown for 50 seconds by MBE to form Si crystal nuclei. Further, annealing is performed at a substrate temperature of 510 ° C. for 6 minutes to promote aggregation and crystallization of Si atoms around the crystal nucleus. It has been reported that this method forms hemispherical microcrystalline Si on the surface of the a-Si film at a temperature lower than the temperature at which crystallization occurs in the a-Si film.
[0007]
[Problems to be solved by the invention]
Wherein the of SiO ₂ on the Si dots of the system, since it hardly controls the generation of crystal nuclei, there is a problem that can not be obtained Si dots of uniform size. Attempts have also been made to reduce variations in size between dots by precisely controlling conditions such as temperature, gas pressure, surface termination state, and the like (Reference 3). However, it has not yet reached an essential solution.
[0008]
If the height between the Si dots is not uniform, the following problem occurs. For example, when a resonant tunneling diode is manufactured in a state where the heights between Si dots are not uniform, variations occur in the voltage that gives the minimum and maximum tunnel currents for each dot. As a result, there is a problem that the sharp characteristic of the negative conductance that is a characteristic of the quantum effect device cannot be obtained.
[0009]
On the other hand, in the system of Si dots on the a-Si film, a narrower size distribution is realized than in the system on the SiO ₂ film. However, the average size of Si dots depends almost linearly on parameters such as the molecular beam flux and irradiation time for crystal nucleation and the annealing time for crystal growth. For this reason, precise control has been difficult. In addition, there is a problem that a high throughput required for manufacturing cannot be obtained due to the MBE method in an ultra-high vacuum. Therefore, a system in which the size of Si dots does not depend much on the annealing time and annealing temperature has been desired.
[0010]
The present invention solves the above problems and provides a method for forming hemispherical silicon microcrystals in which the size of Si dots does not depend on the annealing time or annealing temperature.
[0011]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the invention of claim 1 is characterized in that an amorphous silicon film having a density lower than that of the MBE method is deposited on a silicon substrate at room temperature by RF sputtering, and the deposited amorphous silicon film is heated from 600 ° C. A method of forming hemispherical silicon microcrystals is characterized in that hemispherical microcrystalline Si is formed on the surface of the amorphous silicon film by annealing in vacuum at a temperature of 850 ° C. According to a second aspect of the present invention, in the method for forming a hemispherical silicon microcrystal according to the first aspect, the amorphous silicon film is maintained in an amorphous state by setting the annealing temperature at 600 ° C. to 700 ° C. Hemispherical microcrystalline Si is formed on the surface of the silicon film.
[0012]
In the present invention, an amorphous silicon film deposited on a silicon substrate at room temperature is annealed in vacuum at a temperature of 600 ° C. to 850 ° C. by RF sputtering. As a result, hemispherical microcrystalline Si is formed on the surface of the amorphous silicon film in a self-organized manner by spontaneous nucleation without crystal nucleation. Further, by setting the annealing temperature to 600 ° C. to 700 ° C., hemispherical microcrystalline Si can be formed on the surface in a self-organizing manner while maintaining the amorphous state of the amorphous silicon film.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Next, a method for forming hemispherical silicon microcrystals according to an embodiment of the present invention will be described in detail. In Reference 6 described above, nucleation of Si atoms was necessary to obtain hemispherical microcrystalline Si of uniform size. Furthermore, as can be seen in the cross-sectional TEM image of Document 7, a part of the a-Si film may be eaten by the formation of hemispherical microcrystalline Si. Such a phenomenon suggests that the diffusion rate of Si atoms is not sufficient.
[0014]
The agglomeration of hemispherical microcrystalline Si on the surface of a-Si is closely related to the fact that the a-Si network is formed only by Si-Si bonds and is energetically more metastable than crystalline Si. is doing. In other words, it is important that the Si atom has a flexible structure that can easily recombine the bonds of the atoms. This is particularly noticeable on surfaces that are terminated in a vacuum rather than in bulk. This is because Si atoms have many unsaturated bonds, so that diffusion of Si atoms is easily promoted thermally.
[0015]
Inferring from its physical meaning, if an a-Si film having a density lower than that of the MBE method is produced, hemispherical microcrystalline Si is self-organized by spontaneous nucleation without nucleation. It is expected to form. In fact, when the RF sputtering method is used, an a-Si film having a lower density than the MBE method can be formed.
[0016]
In the method for forming hemispherical silicon microcrystals according to this embodiment, an amorphous silicon film deposited on a silicon substrate at room temperature is annealed in vacuum at a temperature of 600 ° C. to 850 ° C. by RF sputtering. Thereby, hemispherical microcrystalline Si is formed in a self-organizing manner on the surface of the amorphous silicon film. Further, by changing the annealing temperature from 600 ° C. to 700 ° C., hemispherical microcrystalline Si is formed on the surface in a self-organizing manner while maintaining the amorphous state of the amorphous silicon film.
[0017]
According to these methods for forming hemispherical silicon microcrystals, it is possible to form hemispherical microcrystalline Si having a uniform size by simply annealing the a-Si film.
[0018]
【Example】
Next, embodiments of the present invention will be described in detail with reference to the drawings. In this embodiment, hemispherical microcrystalline Si having a uniform size is formed on an a-Si film. First, as shown in FIG. 1, the hydrogen termination surface is exposed by treating the Si substrate 1 with dilute hydrofluoric acid. Next, an a-Si film 2 having a thickness of about 50 to 100 nm is formed on the Si substrate 1 at room temperature by an RF sputtering method using argon as a sputtering gas. Thereafter, the a-Si film 2 is annealed at a temperature of 540 ° C. or higher.
[0019]
Thereby, the hemispherical microcrystals Si3 having the same size are arranged on the surface of the a-Si film 2 in a self-organized manner. Most of the aggregation of Si atoms is completed in the first 20 minutes. However, about 2 hours of annealing time is required to reach perfect equilibrium. This means that the formation of hemispherical microcrystalline Si3 stops at a certain point.
[0020]
FIG. 2 shows an example of the size distribution of hemispherical microcrystalline Si formed after a sufficiently long annealing time. In FIG. 2, the part shown by the coarse dot shows the result of the present embodiment. It can be seen that the base radius of the hemispherical microcrystalline Si is distributed in a very narrow range of 44 to 52 nm and the height of 38 to 46 nm. This gives a narrower distribution than the result of Reference 6 described above for comparison.
[0021]
The minimum temperature at which the hemispherical microcrystalline Si is formed is 540 ° C., but an annealing temperature of 600 ° C. or higher is necessary to obtain a certain dot density. FIG. 3 shows the annealing temperature dependence of the size distribution. When the annealing temperature is between 600 ° C. and 850 ° C., the average value of the diameter of the bottom surface of the hemispherical microcrystalline Si is in the range of 90 to 97 nm, and the average value of the height is in the range of 35 to 43 nm. This means that the process is stable against temperature changes. In FIG. 3, the average values of the diameter and height when the circles A1 to A5 are 600 ° C., 690 ° C., 770 ° C., 850 ° C., and 930 ° C. are shown.
[0022]
In addition, the size variation is within ± 10%. In the samples with annealing temperatures of 600 ° C. and 690 ° C., the a-Si film remained in an amorphous state, but in the samples with 770 ° C., 850 ° C., and 930 ° C., recrystallization occurred. Therefore, in order to obtain hemispherical microcrystalline Si on the amorphous film, it is necessary to set the annealing temperature in the range of 600 ° C. to 700 ° C.
[0023]
From the above results, the average size and size variation of the hemispherical microcrystalline Si depend only very slowly on the process parameters such as annealing temperature and time, and are mainly a-Si deposited first. It was shown to be determined by the characteristics of the film. This is effective for self-organizing semi-spherical microcrystalline Si with a controlled size.
[0024]
The embodiments and examples of the present invention have been described in detail above, but the specific configuration is not limited to the present embodiments and examples, and the design can be changed without departing from the gist of the present invention. Are included in the present invention. For example, in this embodiment, it is assumed that the RF sputtering method is applied to the production of the a-Si film, but a magnetron sputtering method or the like can also be used.
[0025]
【The invention's effect】
As described above, in the present invention, an amorphous silicon film deposited on a silicon substrate at room temperature is annealed in vacuum at a temperature of 600 ° C. to 850 ° C. by RF sputtering. Accordingly, hemispherical microcrystalline Si can be formed on the surface of the amorphous silicon film in a self-organized manner by spontaneous nucleation without nucleation. At the same time, hemispherical microcrystalline Si of extremely uniform size can be formed.
[0026]
In the present invention, the annealing temperature is 600 ° C. to 700 ° C. Thus, hemispherical microcrystalline Si can be formed on the surface in a self-organizing manner while maintaining the amorphous state of the amorphous silicon film.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a cross-sectional structure of hemispherical microcrystalline Si formed on an a-Si film according to an embodiment of the present invention.
FIG. 2 is a histogram of the bottom radius and height of hemispherical microcrystalline Si.
FIG. 3 is a diagram showing the annealing temperature dependence of the size of hemispherical microcrystalline Si.
[Explanation of symbols]
1 Si substrate 2 a-Si film 3 Hemispherical microcrystalline Si

Claims (2)

An amorphous silicon film having a lower density than the MBE method is deposited on a silicon substrate at room temperature by RF sputtering,
By annealing the deposited amorphous silicon film at a temperature of 600 ° C. to 850 ° C. in vacuum,
A method of forming hemispherical silicon microcrystals, comprising forming hemispherical microcrystalline Si on the surface of the amorphous silicon film.   The hemispherical microcrystalline Si is formed on the surface of the amorphous silicon film while maintaining the amorphous state of the amorphous silicon film by setting the annealing temperature to 600 ° C to 700 ° C. A method of forming the hemispherical silicon microcrystal described.

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