JPH0584051B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a semiconductor thin film and a method for forming the same,
In particular, the present invention relates to an n-type semiconductor thin film made of silicon having high conductivity and a wide optical forbidden band, and a method for forming the same. Silicon thin films have been extensively studied recently, and their applications include solar cells, photosensitive drums, scanning circuits for image reading devices, and drive circuits for image display devices. Therefore, when such a silicon thin film is used as a window material for a solar cell, that is, a thin film with low incident light loss, the optical forbidden band (hereinafter referred to as Eopt) of the silicon thin film is
) is preferably as large as possible. Because light with energy greater than Eopt is
This is because the light is cut off by the silicon layer before reaching the photoactive layer and is not used effectively. Conventionally, it was difficult to form window materials using only silicon raw materials, so amorphous silicon carbide (hereinafter referred to as a-
SiCx) and amorphous silicon nitride (hereinafter a-SiNy) have been studied as wide gap windows, and boron-doped p-type a-SiCx
is used in some practical applications as a window material for heterojunctions. However, (i) a-SiCx and a-SiNy are inherently difficult to conduct electricity, and when these are used as window materials, problems arise due to low electrical conductivity. In order not to reduce electrical conductivity, a-SiCx or a-SiNy
In this case, it was not possible to increase Eopt by increasing the amount of C or N added, that is, by increasing the values of x and y. For example, in p-type a-SiCx, it is said that adjusting Eopt to 1.9 to 2.0 eV is optimal in terms of balance with electrical conductivity, but the electrical conductivity at this time is about 10 ^-7 s・cm ^-1 is still low, and acts as a resistance component in photoelectric conversion elements such as solar cells, which is disadvantageous for improving efficiency. (ii) Furthermore, a carbon source must be used as a raw material for a-SiCr formation. As a result, carbon is deposited inside the glow discharge chamber, and a new problem arises in that the deposited carbon must be removed.
Since the above-mentioned electrical conductivity is greatly affected by the carbon content, the composition of the silicon raw material and the carbon raw material must be strictly controlled, which complicates the process. The present invention has been made in view of the above points, and its purpose is to provide a semiconductor thin film made only of silicon raw material, which is n-type, has high conductivity, and has a wide Eopt and is suitable as a window material, and a method for forming the same. As a result of intensive studies from the above viewpoint, the present inventors found that Eopt increases in silicon thin films formed by glow discharge decomposition using disilane gas (Japanese Patent Application Laid-Open No. 59-25278), and furthermore, the formation of thin films. After studying the method, we found that when adding an n-type conductivity imparting substance to the disilane gas, the conductivity must be at least 1×10 ^-3 s・cm ^-1 or higher, and 1 s・cm ^-1 for high conductivity. They also found that a silicon thin film with an Eopt of 1.80 eV, and a wide one exceeding 2.0 eV, could be obtained, and the thin film of the present invention was completed. Furthermore, the inventors have discovered that a silicon thin film having the above characteristics can be obtained more accurately by performing the glow discharge decomposition operation at a specific film formation rate, and have completed the formation method of the present invention. That is, the present invention provides (1) a wide optical forbidden band exceeding 1.80 eV and A silicon semiconductor thin film with high conductivity of ×10 ^-3 s・cm ^-1 or higher. and (2) In a method of forming a thin film on a substrate by introducing a gas mixture consisting of disilane, an n-type conductivity imparting substance, and a diluent gas into a glow discharge chamber and decomposing it by glow discharge, the formation speed of the thin film is increased. A method for forming a silicon semiconductor thin film having a wide optical forbidden band and high electrical conductivity, characterized by suppressing the optical bandgap to 2 Å/sec or less. It provides: Hereinafter, the constituent elements of the present invention will be explained in detail. In the present invention, disilane is defined by the general formula
It is a substance where n=2 in a polysilane compound represented by SinH _2o+2 . Of course, high purity is preferable, but there is nothing wrong with containing higher-order substances such as trisilane, tetrasilane, etc. (n = 3, 4 in the general formula), which behave in the same way as disilane in glow discharge. do not have. Disilane can be synthesized physically by converting monosilane into disilane through silent discharge, but from the point of view of industrially producing solar cells, it is preferable to use a chemical method that can produce large amounts of stable quality. It is more preferable to use a synthetic method (for example, a method such as reducing hexachlorodisilane). Further, the n-type conductivity imparting substance is, for example, PH ₃
(phosphine) and AsH ₃ (arsine) group element compounds. The ratio of these substances to disilane is 0.1 to 5 parts by volume per 100 parts by volume of disilane. H ₂ ,
He, Ne, Ar, etc. are used. The total amount of diluent gas used for disilane is 10 parts by volume or more per part by volume of disilane. In addition, for an n-type conductive substance, the substance 1
Use 10 to 1000 parts by volume of diluent gas per part by volume. The silicon semiconductor thin film of the present invention having an optical forbidden band exceeding 1.80 eV and a conductivity of 1×10 ^-3 s cm ^{-1 or} more is produced by combining disilane as described above, an n-type conductivity imparting substance, and a diluent gas. It is easily formed by decomposing a three-part gas mixture by means of a conventional glow discharge, which is known per se, and depositing it on the substrate. The thin film obtained by the present invention is probably an n-type hydrogen-containing amorphous silicon thin film that partially contains a microcrystalline state. However, the n-type hydrogen-containing amorphous silicon thin film containing microcrystalline state is monosilane (SiH ₄ ).
It was previously known that it could be obtained only as a raw material, but its optical forbidden band was at most 1.80 eV.
This is clearly distinguished from the silicon thin film of the present invention. The present invention also provides a method for forming a silicon semiconductor thin film having a wide optical bandgap and high conductivity as described above, which comprises a mixture comprising the above-mentioned disilane, an n-type conductivity-imparting substance, and a diluent gas. In a method of forming a thin film on a substrate by supplying gas into a glow discharge chamber and decomposing it by glow discharge, the formation rate of the thin film is kept within a specific range of 2 Å/sec or less. When the formation rate of the thin film is increased beyond the above 2 Å/sec, Eopt becomes narrower or the conductivity becomes lower, as shown in Examples and Comparative Examples below.
The purpose of the present invention cannot be achieved. The rate of thin film formation can vary depending on various factors such as the shape of the discharge equipment, electrode structure, distance between electrodes, formation temperature, formation pressure, and gas flow direction, but especially the flow rate (supply amount) of disilane. By changing both parameters, ie, glow discharge energy, and glow discharge energy, the formation rate correspondingly changes greatly, and it is usually difficult to effectively control the formation rate within an optimal range in relation to this. However, the present inventors have determined that "glow discharge energy per unit mass of disilane supplied" [KJ/
By focusing on the factor ``g-Si ₂ H ₆ '' and using this factor as a parameter, we found that the thin film formation rate and the disilane supply amount (flow rate) are uniquely correlated. is 10KJ/g-Si ₂ H ₆
In the following cases, i.e. per unit mass of disilane,
We discovered the important fact that when glow discharge is performed by applying a discharge energy of 10 KJ or more, the rate of thin film formation is almost unaffected by the glow discharge energy and changes depending only on the flow rate of disilane. be. Based on this fact, the optimum supply amount of disilane can be easily determined as follows. That is, it is sufficient to apply glow discharge energy of 10 KJ or more per unit mass of disilane and measure each thin film formation rate for each amount of disilane supplied. The glow discharge energy per unit mass of disilane in the present invention is calculated using the following formula. Glow discharge energy (J/g-Si ₂ H ₆ ) = applied glow discharge power / (raw material gas flow rate) × (average molecular weight / 22.4
) x weight fraction of disilane
() For example, (i) disilane (Si ₂ H ₆ ) diluted to 10 vol % with helium (He) 0.02/min, (ii) phosphine (PH ₃ ) diluted to 1 vol % with hydrogen (H ₂ )
The energy when 100 W of glow discharge power is applied to the raw material mixed with 0.002/min and (iii) H ₂ 0.06/min is calculated as follows. Raw material gas flow rate = 0.02 + 0.002 + 0.06 = 0.082 (/
) Average molecular weight of raw material gas = {(62.2×2/82)+(4
×18/82)+(31×0.02/82)+(2×61.98/82)
≒3.914 (gr) Weight fraction of disilane (62.2×2/82)/3.914
≒0.388Substituting these into the above equation (), energy per unit mass of disilane = 100/0.082/60×
3.914/22.4×0.388=162×10 ³ [J/g−Si ₂ H ₆ ] Now, if 10 ³ J is expressed as KJ, the energy is
It can be expressed as 162KJ/g・Si ₂ H ₆ . In the present invention, the value of glow discharge energy per unit mass of disilane calculated by the above method is at least 10 KJ/g-Si ₂ H ₆ or more, preferably 50 to 500 KJ/g-Si ₂ H ₆ is added to form a thin film. Form. If a large amount of energy exceeding 500KJ/g-Si ₂ H ₆ is applied, the balance between the raw material gas and the discharge is likely to become unstable, making it difficult to form a stable thin film, and adding more energy than necessary is itself economically disadvantageous. In addition to this, a large and expensive glow discharge power source must be installed. on the other hand,
Although the thin film obtained by applying a glow discharge energy of 10 KJ/g-Si ₂ H ₆ or more, which is less than 50 KJ/g-Si ₂ H ₆ , tends to have a slightly lower conductivity, it sufficiently achieves the purpose of the present invention. be able to. In the present invention, the temperature for forming the thin film must be selected appropriately, but a range of 100 to 400°C is usually preferred. Eopt becomes large at low formation temperatures, but at 100℃
If the temperature is lower than that, the formation of a silicon thin film will not proceed effectively and the film will peel off or yellow amorphous silicon powder will be generated, making it impossible to achieve the object of the present invention. In addition, increasing the formation temperature improves the conductivity.
If the temperature is increased too much above 400°C, Eopt will decrease, so in the present invention, it is preferable to form a thin film at 400°C or lower. In addition, although the relationship between the thin film formation temperature and Eopt is shown in the example below, Eopt increases as the formation temperature decreases.
increases, and in this example it becomes 2.0 eV at 150°C. On the other hand, although the conductivity decreased slightly, it still maintained a high value of 9.1×10 ^-2 s·cm ^-1 even at 150°C. For the glow discharge decomposition operation of the present invention, a glow discharge decomposition method known per se may be applied as is. That is, a substrate made of glass, stainless steel, ceramics, or the like on which a semiconductor thin film is to be formed is placed in a reaction chamber capable of glow discharge. The substrate is heated and maintained at a temperature of 100 to 400°C under reduced pressure. Next, raw material gas is introduced into the reaction chamber, and the pressure in the chamber is adjusted to 0.5 to 5 Torr.
shall be. When forming a thin film by applying the above amount of glow discharge energy, the thin film formation rate is suppressed to 2 Å/sec or less. In practice, it is preferable to increase the flow rate of the raw material gas by increasing the flow rate of the diluent gas. In the above energy calculation formula, the flow rate is in the denominator, and as the flow rate is increased, the glow discharge power that can be applied can be increased accordingly.
This is because the permissible range of power variation becomes larger, and the discharge state becomes easier to manage. In addition to these advantages, as shown in Example 9, there is an effect of increasing the electrical conductivity. Hereinafter, the present invention will be explained in more detail with reference to Examples. Examples 1 to 5 A capacitively coupled high frequency plasma CVD ( Chemical Vapor Deposition ) apparatus having a glow discharge chamber having a substrate heating means, a vacuum evacuation means, a gas introduction means, and a parallel plate electrode in which a substrate can be installed. A 7059 glass substrate manufactured by Corning was installed in the substrate installation section. The substrate was heated to a temperature of 150° C. to 350° C. (thin film formation temperature) while evacuating to 10 ⁻⁷ Torr or less using an oil diffusion pump.
Disilane diluted with He (dilution ratio Si ₂ H ₆ /He=1/
9), PH ₃ of H ₅ dilution (dilution ratio PH ₃ /H ₂ = 1/99)
and H ₂ at 20 c.c./min, 2
cc min and a flow rate of 60 c.c./min into the glow discharge chamber, and the pressure was adjusted to 1 Torr. glow discharge power
Glow discharge was performed at 130W, that is, the glow discharge energy of the present invention was approximately 210KJ/g-Si ₂ H ₆ .
A silicon thin film was formed. The formation time was kept constant at 30 minutes. Table 1 shows the results of varying the formation temperature in the range of 150 to 350°C. The film thickness was measured using a stylus-type film thickness meter, and the quotient obtained by dividing this film thickness by the formation time was defined as the thin film formation rate. On the other hand, Eopt measures the light absorption coefficient α of a thin film with a spectrophotometer, then plots (αhν) ^1/2 against the energy of incident light (hν), extrapolates its linear part, and calculates the intercept with the hν axis. It was calculated as the value of To measure conductivity, form electrodes with a spacing of 200μ,
This was done by applying a voltage between the electrodes and measuring the current.

[Table] Comparative Example 1 Experiment similar to Example 5 (formation temperature 350°C) except that a thin film was formed by increasing the flow rates of disilane, PH ₃ and diluent gas so that the formation rate was 5 Å/sec. I went there. The conductivity of the obtained thin film is 2.7×
10 ^-2 s cm ^-1 , but Eopt was small at 1.74 eV.
No material having Eopt exceeding 1.80 eV, which is the object of the present invention, could be obtained. Comparative Example 2 A thin film was formed by increasing the flow rates of disilane, PH ₃ and diluent gas so that the formation rate was 5 Å/sec. Otherwise, the same experiment as in Example (forming temperature 150° C.) was conducted. The Eopt of the obtained thin film is
Although the electrical conductivity was 1.87 eV, the conductivity was small at 1.3×10 ⁻⁵ s·cm ⁻¹ and was not suitable for the purpose of the present invention. Examples 6 to 8 The same experiments as in Example 3 were conducted except that the glow discharge power was changed. The results are shown in Table 2.

[Table] In this example, the glow discharge energy starts from 160.
Even if it changes to 30KJ/g-Si ₂ H ₆ , its formation rate and Eopt hardly change, but the conductivity decreases by about 1/
300. Example 9 An experiment was conducted under the same glow discharge conditions as in Example 5, but with the flow rate of H ₂ as a diluent gas reduced to 40 c.c./min. Eopt of the obtained n-type silicon film
was 1.84 eV, which was unchanged from Example 5, but
The electrical conductivity was 1.8×10 ⁻¹ s·cm ⁻¹ , which was reduced to about 1/30 of that in Example 5. As described above, the silicon semiconductor thin film obtained in the present invention has a wide optical forbidden band exceeding 1.80 eV and also has a high conductivity of 1×10 ⁻³ s·cm ⁻¹ or more. Therefore, it is useful as a window material for semiconductor devices, particularly photoelectric conversion elements such as solar cells, and as a material for elements disposed on the light incident side of tandem junction elements. Furthermore, the thin film of the present invention having high electrical conductivity facilitates ohmic contact and is therefore useful as an electrode material for various semiconductor devices.

Claims (1)

[Claims] 1. A mixed gas consisting of disilane, an n-type conductivity-imparting substance, and a diluent gas is decomposed by glow discharge, and the thin film is formed on a substrate at a temperature of 100 to 400°C while keeping the formation rate to 2 Å/sec or less. formed in
Wide optical bandgap exceeding 1.80eV and 1×10 ^-3
An n-type conductive silicon semiconductor thin film with high conductivity of S cm ^-1 or higher. 2 A mixed gas consisting of disilane, an n-type conductivity-imparting substance, and a diluent gas is introduced into the glow discharge chamber and decomposed by glow discharge to form 100 to 400 particles on the substrate.
A method for forming an n-type conductive silicon semiconductor thin film having a wide optical forbidden band and high electrical conductivity, the method comprising: forming a thin film at a temperature of 2 Å/sec or less. 3. The method according to claim 2, wherein the thin film formation rate is kept below 2 Å/sec by controlling the supply amount of disilane in the mixed gas introduced into the glow discharge chamber.