JPH0654754B2 - Method for forming amorphous semiconductor film - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for forming an amorphous semiconductor film containing a compensating element such as silicon or germanium that reduces the localized density of states.

2. Description of the Related Art Conventionally, the plasma CVD method that can form a thin film at a relatively low temperature is most commonly used for forming an amorphous semiconductor film such as an amorphous silicon film such as a solar cell or an electrophotographic photoreceptor or an amorphous germanium film. ing. In this method, for example, when forming an amorphous silicon film, SiH ₄ (silane gas) or a mixed gas of SiH ₄ and H ₂ is used as a source gas, the substrate temperature is 200 to 350 ° C., and the gas pressure is 0. 1-10 To
In this method, high frequency power is applied between the substrates facing the electrodes while controlling to rr to generate plasma, and the silane gas is decomposed to form a film on the substrate. In this case, the deposition rate is as slow as about 3 μm / h, and it takes 6 hours or more to manufacture an electrophotographic photosensitive member requiring about 20 μm or more, which makes rapid mass production difficult. In addition, when the high frequency power is increased to increase the deposition rate, a large amount of powdery silicon is generated in the source gas due to a gas phase reaction, which causes a waste in the exhaust system of the manufacturing apparatus, which is a major problem in mass production. It was

Electron cyclotron resonance (EC) has been proposed as a means for solving the problems of the above plasma CVD apparatus.
It is a plasma CVD apparatus using R). (JP-A-56
No. 155535, JP-A-59-159167) FIG. 2 shows the basic configuration of this device. Reference numeral 1 is a plasma generation chamber, 2 is a film formation chamber, a plasma generation chamber 1 is connected to a waveguide 3 from a magnetron power source through a waveguide window 4, and a magnetic coil 5 is installed on the outer periphery thereof. is there. Also,
The outer periphery of the film forming chamber 2 is cooled by a water cooling pipe 6. A plasma excitation gas is introduced into the plasma generation chamber 1 through the first gas introduction port 7, and the plasma generation chamber 1 is microwaved.
The plasma is excited and the magnetic field is applied to cause electron cyclotron resonance in the plasma, and the diffusion magnetic field guides the plasma to the substrate 10 on the substrate holder 9 arranged in the film forming chamber 2 from the inlet 8. Second in the film forming chamber 2
A source gas is introduced through the gas inlets 11 and 11A and brought into contact with plasma to decompose the source gas 12 and deposit a film.

Since this method uses electron cyclotron resonance, discharge can be sustained even under a low pressure of 2 × 10 ^{−5 to} 0.1 Torr, and the energy used for film formation is applied to the substrate 10. The high-quality film can be obtained even when the substrate temperature is set to room temperature because it is given by the energy of the incident ions and collision of the ions on the film surface causes the film to honey. It has been shown that this method can form a good-quality insulating layer such as SiN or SiO ₂ or Si for a semiconductor process at a low temperature. It is considered that this is because only the very surface rises to a high temperature due to ion energy and breaks the Si—H bond.

Problems to be Solved by the Invention The CVD method using ECR as described above is not suitable for forming a uniform film over a large area because it uses microwaves and a magnetic field while it can deposit a film at a low temperature and a high speed. Further, the generation of silicon powder, which is a drawback of the CVD method, is still unknown.

In addition, it is an unknown state as to whether an amorphous semiconductor film containing hydrogen, which is a monovalent compensating element that reduces the localized density of states by the incidence of ions, can be formed.

It is an object of the present invention to realize a method for depositing an amorphous semiconductor film, which produces a small amount of powder and enables uniform film deposition even in a large area.

Further, a method of depositing a film, which uses a monovalent element such as hydrogen as an excitation gas and can form an amorphous semiconductor film containing hydrogen which is a monovalent compensating element which reduces the localized density of states by the incidence of ions, is realized. The purpose is to

Means for Solving the Problems Amorphous that decompresses the inside of a container in which a substrate to be deposited is placed and introduces gasified molecules, which is a raw material for a deposition film, into the container and deposits on the substrate. A method for forming a film, wherein a gas is pre-excited by using an electric discharge to bring an excited gas containing ions into contact with the raw material gas introduced into the container, and the raw material gas is radicalized to form a gas on the substrate. And the width of the sheath formed between the substrate and the plasma containing the ions and radicals (hereinafter referred to as Si _s ) is the mean free path of ions in the container.
For (λ), the gas pressure is such that Si _s <λ.

The pre-excited gas that is excited and ionized by the working electric discharge is introduced into the container in which the substrate is installed. At this time, the ions attract electrons and come into contact with the raw material gas to generate a large amount of radicals. These radicals diffuse to the surface of the substrate and react with the film material on the substrate to be deposited.

In addition, the lifetime of ions is relatively short, and energy is lost before they adhere to the substrate due to collision with neutral molecules existing in the sheath where plasma is not present. Therefore, it is considered that even a large amount of generated ions has a very small amount reaching the substrate, which slows down the deposition rate. Further, at this time, a large amount of gas phase reaction is induced by collision of ions and radicals in the gas phase to generate a polymer of the semiconductor material, and the powder is attached to the inside of the reaction container. Based on this idea, by controlling the gas pressure so as to reduce the width of the sheath, the deposition rate could be significantly improved, and the generation of powder in the reaction vessel could be significantly reduced. Further, by using a gas containing hydrogen as the excitation gas, a semiconductor film capable of reducing the localized density of states could be formed.

Embodiments Embodiments of the present invention will be described below with reference to the drawings.

Example 1 FIG. 1 shows the ion source used in the present invention. A gas containing hydrogen as an exciting gas is introduced into the ion source through the exciting gas inlet 13. Magnets 15 are arranged on the back and side surfaces of the plasma source 14 to create a cusp magnetic field 15a. The maximum magnetic field strength is about 2.3 KG, 1 KG inside the plasma source 14 and 20 G about 4 cm inside the wall. The plasma is generated by applying an AC voltage of 120 V to 250 V between the filament 16 and the anode (wall) 17 and discharging. The cusp magnetic field 15a prolongs the electron confinement time, so that stable discharge occurs even at 10 ^-2 to 10 ^-3 Torr. The plasma thus generated moves through the porous grid 18 to the reaction chamber 20 into which the raw material gas has been introduced from the raw material gas introduction port 19 by being guided by the diffusion magnetic field by the external magnetic field 21 to move the raw material gas (SiH ₄ ) Contact with and radicalize.

Since a negative bias of 20 V is applied between the reaction chamber and the substrate 22, cations in the plasma are accelerated to the substrate 22. At this time, the pressure in the reaction chamber 20 is 5 × 10 ^{−2 as the} source gas.
Exhaust and control by a pump 23 to × 5 × 10 ⁻⁴ Torr. The distance from the porous grid 18 to the substrate 22 is set to about 15 cm. The mean free path of the ions introduced at this time is 0.1 to 20 cm because the ionization rate of plasma is several%. The negative bias applied to the substrate 22 forms an ion sheath on the substrate surface. From the Langmuir-Child equation, the width of the ion sheath is proportional to I _O ^-1/2 for the ion saturation current (I _O ) and proportional to V _f ^3/4 for the negative bias voltage (V _f ). At 5 × 10 ^-4 Torr,
Ion sheath width when ^{_{I O = 30mA / cm 2,}} V f = -20V is about 0.2 mm. However, at 5 × 10 ⁻² Torr, the ion sheath current becomes about 1 mm at an ion saturation current I _O = 1 mA / cm ² . In this state, the amorphous silicon film deposited on the substrate has dark conductivity and photoconductivity (1.
4 μW / cm ² ) and the hydrogen content was determined from the absorbance of infrared absorption. The temperature of the substrate is controlled from room temperature to 350 ° C. by the heater 24, but the dependency of the substrate temperature is small.

It was also confirmed that the deposition rate at this time was as high as 20 μm / h.

Figure 3 shows the results of dark conductivity and photoconductivity. 31 in the figure
a and 31b are SiH ₄ and H _{2 for the} source gas and the excitation gas, respectively.
With a flow rate of SiH ₄ / H ₂ of 67%,
32a and 32b have a flow rate ratio of 34%.
Reference numeral 32b represents dark conductivity, and reference numerals 31a and 32a represent photoconductivity.
In the vicinity of 5 × 10 ^-2 Torr where the width of the ion sheath and the mean free path of ions are equal, the photoconductivity is far from the dark conductivity and the photosensitivity is poor, so that the film has poor film quality. Further, FIG. 4 shows that the amount of hydrogen contained in the amorphous silicon film can be controlled by the ratio of the flow rates of the source gas and the excitation gas at 5 × 10 ⁻⁴ Torr. 41 is hydrogen as an exciting gas 10
0% and 42 indicate the case where He / H ₂ = 0.24 was used as the excitation gas, and 40 to 14 atm% and 20 to 5 a, respectively.
It turns out that it can control up to tm%.

Further, under such a deposition condition, unlike the ordinary plasma CVD method, the amount of silicon powder generated in the deposition container is remarkably small, and even if the film is deposited to a film thickness of 20 μm or more required for the electrophotographic photoreceptor, the powder is not produced. Since the generation of the body is extremely small, the time required for the maintenance of the device is unnecessary and the productivity can be remarkably improved.

Although SiH ₄ was used as the source gas, other SiF ₄ , SiCl _4
In addition to silicon-based gases such as ₄ , Si ₂ H ₆ , Si ₂ F ₆ , SiH ₂ F ₂ , SiH ₃ F, and SiHCl ₃ , GeH ₄ , GeF ₄ , GeH ₂ F ₂ , Ge ₂ F ₆ , GeH
Amorphous germanium or amorphous silicon or germanium semiconductor film can be formed by using germanium-based gas such as ₃ F or GeHF ₃ .

Example 2 Next, the case where B ₂ H ₆ is added as a doping gas into the deposition container using the apparatus shown in FIG. 1 will be described.

In FIG. 5, 5 × 10 ⁻⁴ Torr, SiH ₄ / H ₂ was 34%, the amount of B ₂ H ₆ was changed, and the dark conductivity 51b and the photoconductivity 51a were measured as in FIG. On the other hand, 5 × 10 ^-2 Torr is 52
As indicated by b and 52a, the sensitivity is low and the amount of silicon powder generated is significantly increased.

Although a mixed gas of H gas, H, and He was used as the excitation gas, similar results can be obtained by mixing H gas with a rare gas such as Ar, Ne, or Xe.

As the doping material, it is clear that in addition to B, all gasifiable materials such as P, As and Ga can be applied.

EFFECTS OF THE INVENTION As described above, according to the present invention, an amorphous semiconductor film can be deposited at an extremely high deposition rate and with almost no powder generated in the reaction vessel. Further, an amorphous semiconductor film capable of controlling valence electrons can be easily obtained by using a monovalent element that reduces the localized density of states in the semiconductor film as an excitation gas.

As a result, a functional device using an amorphous semiconductor film such as an electrophotographic photoconductor or a solar electron can be manufactured more easily at a lower cost than the conventional CVD method.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of a deposition apparatus used in an embodiment of the present invention, FIG. 2 is a sectional view showing an example of a conventional ECR plasma CVD apparatus, and FIG. 3 is an amorphous structure according to the present invention. Graph showing dark conductivity and photoconductivity of the high-quality semiconductor film, FIG. 4 is a graph showing an example of measuring hydrogen concentration in the film, and FIG. 5 shows changes in dark conductivity and photoconductivity by B doping. It is a graph. 13 ... Gas inlet, 14 ... Plasma source, 15 ...
… Magnet, 16… Filament, 17… Anode, 18…
... Porous grid, 19 ... Raw material gas inlet, 20 ... Reaction chamber, 21 ... External magnetic field, 22 ... Substrate.

Claims (3)

[Claims] 1. An amorphous semiconductor film in which a container in which a substrate to be deposited is placed is decompressed, gasified molecules as a raw material for a film to be deposited are introduced into the container and deposited on the substrate. A method of forming, wherein the excited gas is pre-excited using an electrical discharge in a space adjacent to the container, the excited gas containing ions is contacted with the raw material gas introduced into the container, A step of radicalizing a raw material gas and depositing it on a substrate, wherein a width (referred to as Si _s ) of a sheath formed between the substrate and a plasma containing the ions and radicals is an average of the ions in the container. A method of forming an amorphous semiconductor film, characterized in that deposition is carried out at a gas pressure such that Si _s <λ for the free process (λ). 2. A gas containing silicon is used as a raw material gas,
The method for forming an amorphous semiconductor film according to claim 1, wherein a gas containing hydrogen is used as an exciting gas to deposit an amorphous silicon film containing hydrogen. 3. The pressure inside the container in which the substrate is placed is 5 × 10 ^-2.
The method for forming an amorphous semiconductor film according to claim 2, wherein: