Claims
- 1. A substrate and a silicon thin film deposited n said substrate, said thin film having an electrical conductivity of more than 5.times.10.sup.0 .OMEGA..sup.-1 cm.sup.-1 for a P type film and more than 1.times.10.sup.2 .OMEGA..sup.-1 cm.sup.-1 for an N type film, said film comprising primarily silicon atoms, 2 to 5 atm % of hydrogen, fluorine and a dopant impurity element, wherein at least about 80%, but less than 100%, of the film volume comprises microcrystalline grains, which grains are interspersed in an amorphous phase.
- 2. The combination according to claim 1, wherein the optical band gap of said film is higher than 1.3 eV.
- 3. The combination according to claim 1, wherein the average crystallite size is about 30 .ANG. to about 500 .ANG..
- 4. The combination according to claim 3, wherein the average crystallite size is about 150 .ANG. to about 500 .ANG..
- 5. The combination according to claim 1, wherein the dopant impurity element is at least one element selected from Group V of the Periodic Table.
- 6. The combination according to claim 1, wherein the dopant impurity element is at least one element selected from Group III of the Periodic Table.
- 7. The combination according to claim 1, wherein the activation energy on the basis of the electrical conductivity of said film is below about 0.2 eV.
- 8. Method of preparing a silicon thin film on a substrate in a plasma atmosphere using as a raw material gas silane (SiH.sub.4), fluorinated silane (SiH.sub.0-3 F.sub.4-1), or a combination thereof, and a dopant gas mixed with the raw material gas, said method comprising:
- diluting said mixed gas with hydrogen in a ratio of the diluting gas to the raw material gas of from 50:1 to 100:1; and
- applying an electric power to provide a plasma discharge power density of about 0.1 to about 0.5 W /cm.sup.2 at a reaction pressure of 5 to 10 torr, whereby the produced film contains 2 to 5 atm % of hydrogen, and at least about 80%, but less than 100%, of the film volume comprises microcrystalline grains, which grains are interspersed in an amorphous phase.
- 9. Method according to claim 8, wherein said dopant gas includes a gas comprising at least one element selected from Group III of the Periodic Table, or compounds thereof, and the resulting silicon thin film is of the P type.
- 10. Method according to claim 8, wherein said dopant gas includes a gas comprising at least one element selected from Group V of the Periodic Table, or compounds thereof, and the resulting silicon thin film is of the N type.
- 11. A substrate and a silicon thin film deposited on said substrate, said film comprising primarily silicon atoms, 2 to 5 atm % of hydrogen, and fluorine, wherein at least about 80%, but less than 100% of the film volume comprises microcrystalline grains having an average crystallite size from about 30 .ANG. to about 500 .ANG., which grains are interspersed in an amorphous phase.
- 12. The combination according to claim 11, wherein the average crystallite size is about 150 .ANG. to about 500 .ANG..
- 13. Method of preparing a silicon thin film on a substrate in a plasma atmosphere using as a raw material gas silane (SiH.sub.4), fluorinated silane (SiH.sub.0-3 F.sub.4-1), or a combination thereof, said method comprising:
- diluting said mixed gas with hydrogen in a ratio of the diluting gas to the raw material gas of from 50:1 to 100:1; and
- applying an electric power to provide a plasma discharge power density of about 0.1 to about 0.5 W/cm.sup.2 at a reaction pressure of 50 to 10 torr, whereby the produced film contains 2 to 5 atm % of hydrogen and at least about 80%, but less than 100% by volume of microcrystalline grains, which grains are interspersed in an amorphous phase.
Priority Claims (1)
| Number |
Date |
Country |
Kind |
| 55-143010 |
Oct 1980 |
JPX |
|
CROSS REFERENCES
This is a continuation-in-part of copending U.S. application Ser. No. 689,036, filed Jan. 7, 1985now abandoned, which is a continuation of U.S. application Ser. No. 562,688 filed Dec. 19, 1983, now abandoned, which is a continuation-in-part of U.S. application Ser. No. 394,996, filed June 11, 1982, now abandoned.
This invention relates to a silicon thin film and a method of producing the same, and particularly, to a low resistance silicon thin film which is formed on an appropriate substrate in a plasma atmosphere, and a method of producing the same.
It is well known to form a silicon thin film on an appropriate substrate in a plasma atmosphere, using as a raw material a mixture of silane (SiH.sub.4) and a dopant material. The conventional silicon thin film produced by such method is entirely amorphous. The amorphous film exhibits a halo pattern by X-ray diffraction, and such amorphous silicon thin film has an electrical conductivity of at most about 10.sup.-2 .OMEGA..sup.-1 cm.sup.-1 for the N type and about 10.sup.-3 .OMEGA..sup.-1 cm.sup.-1 for the P type. The activation energy which is estimated on the basis of the temperature dependence of the electrical conductivity is approximately 0.2 eV both for P type and N type films. Thus, it is difficult to characterize the amorphous thin film as a P.sup.+ or N.sup.+ type film which provides ohmic contact to metal and in which the Fermi level is adequately degenerated (for example, see the Philosophical Magazine, Vol. 33, p. 935, 1976). Especially, in the case of a P type film, as the electrical conductivity becomes higher, the optical band gap is sharply reduced (see the Physical Review, Vol. 19, p.2041, 1979). As a result, in a P-N or P-I-N junction structure for solar cells, due to the reduction of the, optical band gap of the P type layer, the light is absorbed by the P type layer before it arrives at the active region of the junction (P and N or P and I interface). Moreover, the difference of band gap between the P and I layers produces a heterojunction which causes smaller built-in potential and open circuit voltage. On the other hand, in an N type layer, the fill factor (curve factor of efficiency) decreases because of poor ohmic contact to metal and high series resistance. These facts mean that eventually the conversion efficiency of the light to electricity is lowered.
On the other hand, while polycrystalline thin films which are produced of silane (SiH.sub.4) as by a chemical vapor deposition have higher electrical conductivities, they have optical band gaps as low as 1.2 eV, which is lower than the optimum valve of about 1.5 eV which matches well with solar spectrum. Further, the grain boundaries between crystals not only act as recombination centers of electron-hole pairs, but also cause the leakage of current.
It is an object of the invention to provide a silicon thin film, which has a low electrical resistance and adequately wide optical band gaps, with the combined advantages of the amorphous silicon thin films and the polycrystalline silicon thin films.
It is a further object of the invention to provide a silicon thin film which has crystalline grains in a specific range of diameters, a high electrical conductivity and a wide optical band gap.
It is a still further object of the invention to provide a P type silicon thin film, which has a high electrical conductivity, a wide band gap, and an excellent doping efficiency, which has so far been difficult to obtain.
It is a still further object of the invention to provide an N type silicon thin film, which has a high electrical conductivity and an excellent doping efficiency, which also has so far been difficult to obtain.
It is a still further object of the invention to provide a method of producing a silicon thin film having a low resistance and a wide band gap on an appropriate substrate in a plasma atmosphere.
It is a still further object of the invention to provide a silicon thin film which has a specified proportion of microcrystalline grains in the amorphous substance thereof, has a high electrical conductivity and a wide optical band gap.
The silicon thin film according to this invention is composed primarily of silicon atoms, at least one element selected from the group of fluorine, chlorine, bromine and iodine, and hydrogen, as well as an impurity element. The silicon thin film is characterized by the regularity of the arrangement of atoms and by a microcrystalline substance interspersed in the amorphous layer. Especially, according to the present invention the content of hydrogen is in the range of from 0 to 8 atm % (atomic percentage), preferably from 2 to 5 atm % and the content of the microcrystalline grains is in the range of from 80 to 100% by volume.
In brief, the present silicon thin film comprises a substrate and a film deposited on the substrate, said film comprising primarily silicon atoms, 0 to 8 atm %, preferably 2 to 5 atm % of hydrogen, at least one element selected from the group consisting of fluorine, chlorine, bromine and iodine, and an impurity element, wherein 80 to 100% microcrystalline grains are interspersed in an amorphous phase.
Particularly, on X-ray diffraction, the usual amorphous silicon thin films which are prepared under a plasma atmosphere exhibit a wide and gently-sloping halo pattern and a spectrum which does not have any sharp peaks, while the polycrystalline silicon thin films which are prepared by chemical vapor deposition, high temperature annealing, etc. exhibit a clear and intensive peak which is derived from the silicon crystal lattice.
On the other hand, the present inventors have discovered that the silicon thin films having microcrystalline grains interspersed in the amorphous layer show a weak peak near Si(111) or Si(220) on the halo pattern, which is presumably derived from the silicon crystal lattice. The average crystallite size in the silicon thin film can be calculated from the breadth of a diffraction pattern at points of half maximum intensity using Scherrer's equation and it ranges from about 30 .ANG. to about 500 .ANG.. The microcrystalline substance in this range of crystallite size does not provide any optical interference with the light in the range of wave lengths involved in the solar radiation, and can only cause the electrical conductivity to increase.
It is deduced that the microcrystalline substance having the average crystallite size below about 30 .ANG. will hardly continue to exist and tend to lose the characteristics of a crystal and thus change into an amorphous substance, while the microcrystalline substance having the average grain diameter over about 500 .ANG. A will tend to change into a polycrystalline one, so that an interference of light occurs at the boundary between the amorphous phase and the crystallites, and thus, it would be impossible in these ranges to lower the electrical resistance without narrowing down the optical band gap. Thus, the most desirable range of average crystallite size for the silicon thin film to have a low electrical resistance and a wide optical bandgap is about 150 .ANG. to about 500 .ANG..
Further, the proportion of the microcrystalline substance in the amorphous substance can be estimated from the X-ray diffraction pattern on the basis of the height of the peak and the breadth at half maximum intensity. According to the present invention, as mentioned above, the proportion of the microcrystalline substance in the amorphous substance is in the range of from 80 to 100% by volume.
The inventors of the present invention have discovered that when the proportion of microcrystalline substance is over about 80% by volume and the content of hydrogen in the film falls below about 8 atm %, the crystallinity in the film is promoted to obtain a high electrical conductivity. The present invention is based on this novel finding.
The present invention may provide N type silicon thin films having an electrical conductivity of more than about 1.times.10.sup.2 .OMEGA..sup.-1 cm.sup.-1 and P type silicon thin films having an electrical conductivity of more than about 5.times.10.sup.0 .OMEGA..sup.-1 cm.sup.-1. On the other hand, when the proportion of the microcrystalline substance is below about 80% by volume, the electrical conductivity falls below 5.times.10.sup.0 .OMEGA..sup.-1 cm.sup.-1 for N type silicon thin films and 5.times.10.sup.-1 .OMEGA..sup.-1 cm.sup.-1 for P type silicon thin films.
It is deduced with the silicon thin film according to this invention that the existence of the microcrystalline substance in the amorphous layer may closely relate to the fact that the film has the combined excellent features of an amorphous silicon thin film such as an adequately wide optical band gap, and of a polycrystalline silicon thin film, such as a remarkably high electrical conductivity.
In this invention, various elements can be used as an impurity dopant. Particularly, when elements in Group V of the Periodic Table, such as phosphorus, arsenic, etc., are used, silicon thin films having the property of an N type semiconductor are obtained, while the use of elements in Group III of the Periodic Table, such as boron, aluminium, etc. will provide a silicon thin film having the property of a P type semiconductor. The former films are characterized by an electrical conductivity of more than about 1.times.10.sup.2 .OMEGA..sup.-1 cm.sup.-1, while the latter films are characterized by a conductivity of more than about 5.times.10.sup.0 .OMEGA..sup.-1 cm.sup.-1. Thus there can be produced silicon thin films having N type or P type conductivity, which are characterized by an activation energy on the basis of the electrical conductivity below about 0.2 eV, often below about 0.1 eV, a good doping efficiency, an adequately degenerated Fermi level, and an excellent ohmic contact to metal. Further, the silicon thin films according to this invention, either of N type or P type, can maintain an adequately wide optical band gap, and they have a considerably higher optical band gap value of about 1.3 eV to about 1.8 eV in comparison with the value of about 1.2 of the polycrystalline films. Especially, the P type thin films have the two combined excellent characteristics of high electrical conductivity and wide optical band gap, which could not have been obtained in the conventional films. These advantages substantiate the premise that the silicon thin films according to this invention are of a novel crystalline construction which is not completely amorphous and not completely polycrystalline, and particularly in which more than 80% microcrystalline grains are interspersed in an amorphous phase.
Next, the method of making the silicon thin film according to this invention will be explained. First, any one of silane (SiH.sub.4) or halogenated silane (SiH.sub.0-3 X.sub.4-1) (X represents a halogen element), or a gas mixture including two or more of these gases is diluted with hydrogen gas in a ratio of the diluting gas to the raw material gas of 50:1 to 100:1, and then a dopant gas is added to the diluted gas mixture. The sequence of mixing and dilution is not limited to this one. Electric power having a plasma discharged power density of 0.1 to 0.5 W/cm.sup.2 at a reaction pressure of 5 to 10 torr is applied to the gas mixture to produce a plasma condition, in which a film is formed on a substrate (consisting of glass, plastic, metal, etc.). Then, the impurity atoms acting as dopants are efficiently incorporated into a silicon network with four coordinations, so that a silicon thin film having a high electrical conductivity can be formed without decreasing the optical band gap. The purpose of diluting the silane (SiH.sub.4) with hydrogen in a high proportion of 50-100:1 is to control the film deposition rate under the applied low electrical power at a high reaction pressure.
Particularly, in order to produce the present silicon thin film, the ratio of dilution of hydrogen (SiH.sub.4 /H.sub.2) is controlled within the range of 1/50 to 1/100, the reaction pressure (film forming pressure) 5 to 10 torr and the electric power supplied (cathode electric power density) 0.1 to 0.5 W/cm.sup.2. If the silicon film is produced under a high electric power such as 0.8 to 1.6 W/cm.sup.2 and a low reaction pressure of less than 1 torr, particles with high energy such as cracked ionic species (SiH.sub.x.sup.+ and H.sup.+) and electrons impinge against the layer surface produced, so that the percent of microcrystalline substance is less than about 80%.
In contrast, according to the present invention, the film forming pressure is increased by about a factor of ten and the cathode electric power density is reduced below about 0.5 W/cm.sup.2 so as to bring about the reduction of impingement of ions and electrons and the increase of the ratio of the microcrystalline substance in the amorphous phase.
More particularly, one of silane or halogenated silane or a gas mixture including two or more of these gases is diluted with hydrogen gas in a ratio of the diluting gas to the raw material gas of more than 50:1, preferably in a ratio of 50:1 to 100:1. If the ratio of the dilution does not attain 50:1, the desired proportion of microcrystalline substance cannot be obtained. Namely, in order to promote the crystallinity of the silicon thin film it is required to increase the concentration of hydrogen on the surface of the film being manufactured. A ratio of hydrogen dilution over 50:1 can attain the objects of this invention.
The X-ray diffraction pattern from the film which has been produced under such conditions shows that microcrystalline grains are interspersed in the amorphous substance, and it is deduced that the existence of such microcrystalline grains remarkably reduces the resistance of the film while giving the film the optical properties of an amorphous film. According to the X-ray diffraction pattern, average crystallite size of such grains is in the range of about 30 .ANG. to about 500 .ANG., and especially in the range of about 150 .ANG. to about 500 .ANG..
US Referenced Citations (2)
| Number |
Name |
Date |
Kind |
|
4409134 |
Yamazaki |
Oct 1983 |
|
|
4433202 |
Maruyama et al. |
Feb 1984 |
|
Non-Patent Literature Citations (1)
| Entry |
| A. Matsuda et al, Jap. J. Appl. Phys., vol. 19, Jun. 1980, pp. L305-L308. |
Continuations (1)
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Number |
Date |
Country |
| Parent |
562688 |
Dec 1983 |
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Continuation in Parts (2)
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Number |
Date |
Country |
| Parent |
689036 |
Jan 1985 |
|
| Parent |
394996 |
Jun 1982 |
|
Patent Grant
5017308
