JP3560686B2 - Method for manufacturing silicon optical device - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a method for manufacturing a porous silicon optical device. More specifically, the present invention relates to a silicon LED in the visible region, a silicon LD, a silicon photoconductive device in the visible region, a silicon optical waveguide, a silicon optical resonator, and even an optoelectronic integrated device integrating them. New silicon light that is effective for improving the quantum efficiency of light-emitting and light-receiving devices using silicon as a basic material, narrowing the emission spectrum width, and increasing the speed of light emission, which is important for establishing the technical basis of optoelectronics. The present invention relates to a method for manufacturing an element.
[0002]
[Prior art and its problems]
As a silicon material that emits light in the visible region at room temperature, porous silicon, silicon clusters, silicon polymers, and the like are known. When comparing these materials in terms of luminous efficiency, controllability of process parameters in sample preparation, applicability of device technology such as enlargement and integration, development into optical devices, sample stability, etc. The best overall is porous silicon.
[0003]
This porous silicon is usually formed by an anodic oxidation method in which the surface of a single crystal silicon substrate is electrochemically etched in an HF aqueous solution. However, the porous silicon produced by this method and its luminescent properties are:
(1) It is difficult to obtain high luminous efficiency while maintaining uniformity of structural, optical, and mechanical properties.
(2) Since it is difficult to obtain a sample having a uniform optical property, the peak wavelength of the emission spectrum generally varies greatly depending on the wavelength of the excitation light.
(3) The width of the emission spectrum is wide (half width: 300 to 400 meV) regardless of the emission peak wavelength, and the color purity is insufficient.
(4) The decay process of light emission is non-exponential, and its lifetime includes several components from nanosecond to microsecond.
However, there are large restrictions and limitations in practical application as an element and in terms of an application range as an optical element.
[0004]
These problems are due to the structural and optical inhomogeneity of the porous silicon layer itself and the light emitting elements inside the porous silicon, which are associated with the technique of anodization. Therefore, it is difficult to dramatically improve the light-emitting characteristics of porous silicon only by changing the conditions of anodic oxidation, and some external effects that favorably affect the process of forming porous silicon and the properties of porous silicon. It is considered necessary to improve the controllability of the anodic oxidation method qualitatively.
[0005]
Looking at the current state of research and development from such a viewpoint, so far, almost no proposals for overcoming the above problems have been made except for the examination of the present application.
The present invention has been made in view of the above circumstances, and solves the conventional disadvantages, enables full-scale development of a silicon optical device using visible light-emitting porous silicon, and applies silicon. It is an object of the present invention to provide a method for manufacturing a new porous silicon optical device that can expand the field.
[0006]
[Means for Solving the Problems]
The present invention, as to solve the above problems, in the production of porous silicon devices anodizing the surface of the silicon substrate in an HF solution, apply a magnetic field to the silicon substrate in the course of the positive electrode oxidation process to provide a method of manufacturing a porous silicon optoelectronic device, wherein the this.
[0007]
[Action]
The present invention has been made in consideration of the fact that the formation of the porosity is controlled by the supply of carriers (holes) from a silicon substrate. That is, the silicon elution reaction at the silicon-HF solution interface is a process in which holes in the substrate are consumed. In the process of forming a porous body, the silicon elution reaction is locally and autonomously performed only at the tip of the pore. The present invention focuses on the following two points: “anodic oxidation using an external magnetic field”, “static anodization at a weak current level”, and With the combination of the two, the qualitative improvement of the light emission characteristics of the porous silicon optical device is realized.
[0008]
<Anodic oxidation by applying magnetic field>
One of the basic techniques of the present invention is to apply a uniform external magnetic field to a silicon substrate surface when anodizing the surface with a HF solution. This limits the direction of holes supplied to the silicon elution reaction at the tip of the pore.
In the formation of porous silicon, the pores preferentially advance in the [100] direction. For example, when anodic oxidation is performed by applying a magnetic field in a direction perpendicular to the surface of the silicon substrate having a plane orientation of (100), The holes moving in a direction different from the direction of the magnetic field among the holes from the hole toward the surface are deflected by the magnetic field, and are difficult to reach the tip of the pore. When a sufficiently strong magnetic field is applied, only holes traveling parallel to the magnetic field are preferentially supplied to the tip of the pore. Therefore, the pores always evolve in the direction perpendicular to the substrate, and a PS having a uniform structure is formed. At the same time, the hydrogen termination of the silicon atom bond on the pore surface is surely performed.
[0009]
As a result, high-quality porous silicon containing silicon microcrystals as light-emitting elements at a high spatial density, having a uniform structure, and having a low dangling bond density is formed. Thereby, the luminous efficiency of the porous silicon is significantly improved. In addition, the undesirable phenomenon that the emission spectrum changes depending on the wavelength of the excitation light, which is observed in ordinary porous silicon, is also eliminated.
[0010]
In the sense that holes are selectively supplied, the direction in which the magnetic field is applied does not necessarily have to be perpendicular to the substrate surface, and the same effect can be produced even when the magnetic field is horizontal to the substrate surface. Considering that the pores preferentially advance in the [100] direction, generally, the application direction of the magnetic field is set to an optimum condition according to the plane orientation of the substrate.
<Static anodization>
Another basic technique in the present invention is static anodization. Here, the static anodic oxidation refers to anodic oxidation of a silicon substrate having a high resistivity in a dark state at a current density as low as possible. That is, a substrate originally having a low hole density supplied by thermal excitation from the substrate to the interface of the HF solution is electrochemically etched at a low current density. According to this method, anodic oxidation proceeds under a low sample potential, and the disorder of the structure during the process of forming porous silicon can be minimized.
[0011]
The main effects of this method on the emission characteristics are narrowing of the emission spectrum width, reduction of the emission lifetime, and integration into a single component. Therefore, in order to improve the luminous efficiency, the sample is irradiated with light in an aqueous HF solution immediately after anodic oxidation (a tungsten lamp, a halogen lamp, or the like is used as a light source, and a filter is placed in front of the sample in some cases to suppress the wavelength of the irradiation light). Do). Thereby, high luminous efficiency can be obtained while maintaining a narrow luminous spectrum width and a short luminous life of a single component.
[0012]
<Combination of two basic technologies>
The above two basic technologies can be easily combined, and the effect of each can be further enhanced. That is, by performing static anodic oxidation under the application of a magnetic field, and by appropriately incorporating a light irradiation method, the characteristics of porous silicon as an optical element material are significantly improved.
[0013]
By the above means, while taking advantage of the features of the anodic oxidation method,
(1) It is structurally uniform and has high luminous efficiency.
(2) the peak wavelength of the emission spectrum does not depend on the wavelength of the excitation light,
(3) The emission spectrum is narrow and the color purity of the emission is high.
(4) The emission decay process is represented by an exponential function having a single lifetime component.
It is possible to obtain porous silicon exhibiting such characteristics.
[0014]
In addition, the general sample of the anodic oxidation method is as follows.
[0015]
【Example】
Example 1
FIG. 1 conceptually illustrates a magnetic field application anodic oxidation method as one of the embodiments. That is, a 50 wt% HF aqueous solution is applied to the surface of a p-type silicon wafer (specific resistance: 0.4 to 0.6 Ωcm, thickness: 200 μm) having a plane orientation (100) with an ohmic electrode (2) on the back. A constant current anodic oxidation treatment (current density: 30 mA / cm ² , time: 10 min) is performed in a mixed solution (mixing ratio: 1: 1) (3) of water and ethanol to form a porous silicon layer (4). At this time, the anodizing cell (5) is arranged between the magnetic pole pieces (6) as shown in FIG. 1, and anodizing is performed while applying a magnetic field in a direction perpendicular to the silicon substrate surface.
[0016]
The porous silicon sample thus produced was optically excited at room temperature (Ar or He-Cd laser was used as an excitation source), and the intensity and spectrum of photoluminescence (PL) were measured. The main measurement item is the dependence of the PL emission intensity on the magnetic field intensity.
FIG. 2 shows a measurement example of the PL spectrum. Although the peak wavelength of the PL emission spectrum does not depend on the magnetic field, the PL intensity sharply increases as the applied magnetic field intensity increases. When the magnetic field is 8 kG, the PL intensity is increased by about one digit as compared with a normal anodic oxidation sample to which no magnetic field is applied.
[0017]
Further, in a sample obtained by ordinary anodic oxidation in which no magnetic field is applied, the peak wavelength of the PL spectrum greatly changes depending on the wavelength of the excitation light. On the other hand, when a magnetic field was applied, the PL peak wavelength was unchanged whether the excitation light was an Ar laser (wavelength: 488 nm) or a He-Cd laser (wavelength: 325 nm). This demonstrates that the variation in the size of the light-emitting elements (silicon microcrystals) in the porous silicon is reduced, and that the size distribution in the depth direction is also uniform.
[0018]
According to the observation with a scanning electron microscope, the thickness of the porous silicon layer is constant regardless of the presence or absence of a magnetic field. It was more uniform.
Thus, it was confirmed that the application of a magnetic field resulted in an improvement in PL intensity and a uniform optical property through a change in the structure of porous silicon.
[0019]
Example 2
A low-current anodizing treatment is performed in a dark state on the surface of a p-type silicon wafer (specific resistance: 90 to 100 Ωcm, thickness: 200 μm) having a plane orientation (111) and an ohmic electrode on the back surface. The HF aqueous solution used had a mixture ratio of HF: ethanol: water = 2: 1: 2, the current density of the anodic oxidation was 1 mA / cm ² , and the time was 60 s. After anodic oxidation, light is introduced into the anodic oxidation cell through the quartz window, and irradiated onto the sample surface (light source: tungsten lamp of 500 W, light irradiation time: 3 min).
[0020]
The produced porous silicon sample was optically excited at room temperature (Ar or He-Cd laser was used as an excitation source), and the intensity and spectrum of PL were measured. Further, the _{N 2} laser used as a pulse excitation light source (wavelength: 337 nm), was also measured emission lifetime.
For comparison, a sample was prepared by anodizing the same substrate under normal conditions (medium current density), and the same measurement as above was performed. Specific anodizing conditions were as follows: anodizing current density: 20 mA / cm ² , anodizing time: 10 minutes, and light irradiation time after anodizing was 10 minutes.
[0021]
The PL spectrum at room temperature of the sample prepared by static anodic oxidation was as shown by the solid line in FIG. In FIG. 3, the PL spectrum of the sample obtained by the ordinary anodic oxidation method is also indicated by a dotted line. In both cases, a He-Cd laser having the same intensity (50 μW) was used as the excitation light. As can be seen from the results, in the sample obtained by the static anodic oxidation, the PL emission efficiency is high, and the spread of the PL spectrum is 170 meV in half-value width, which is narrowed to about half that of the normal sample.
[0022]
Further, from the measurement of the PL emission lifetime, it was confirmed that the narrowing of the PL spectrum resulted in a decrease in the emission lifetime and a single component.
Even with a direct transition compound semiconductor such as gallium arsenide (GaAs) generally used in a light emitting device, the PL spectrum has a half value width of about 60 meV. The results in FIG. 3 show that if the effects of the static anodization method and the application of a magnetic field are combined, a sharp PL spectrum and a high-speed response comparable to a direct transition type compound semiconductor can be obtained even in porous silicon. ing.
[0023]
【The invention's effect】
As described in detail above, by applying the effects of a magnetic field application, a static process, and light irradiation to the anodic oxidation method, the greatest problems in achieving an element of porous silicon (improvement of luminous efficiency, PL spectrum Narrowing the band, speeding up the luminous response and making it a single component).
[0024]
To improve the characteristics of porous silicon qualitatively and to realize a visible-range silicon light-emitting material with the same light-emitting characteristics as a direct-transition semiconductor, it has functions of light reception, light emission, waveguide, optical resonance, and optical amplification in the visible region. In order to enable a silicon optical integrated device having the above-mentioned structure, the application range of the present invention is very wide, and the development and ripple power are extremely large.
[Brief description of the drawings]
FIG. 1 is a view showing the concept of a magnetic field application anodic oxidation method.
FIG. 2 is a diagram showing the effect of an applied magnetic field during anodization on a PL emission spectrum.
FIG. 3 is a diagram showing a comparison between a PL spectrum (solid line) of porous silicon prepared by a static anodic oxidation method and a PL spectrum (dotted line) of porous silicon prepared by a normal anodic oxidation method.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Silicon wafer 2 Al ohmic electrode 3 Mixture 4 Porous silicon layer 5 Anodizing cell 6 Magnetic pole piece

Claims (1)

A method for manufacturing a porous silicon optical device, comprising: applying a magnetic field to the silicon substrate in the process of anodizing, wherein the surface of the silicon substrate is anodized in an HF solution.

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