Patent Application
20080008640
The present invention relates to a cluster-free amorphous silicon film which is free from large clusters having a size of 1 nm or more, and the production of the amorphous silicon film.
It is the highest priority issue in the 21st century to solve problems of expanding energy consumption and environmental destruction arising from economic development and population growth (so-called “trilemma”). Photovoltaic power generation is expected to play a large role in solving the problems, and therefore solar cells are needed to achieve higher efficiency and lower cost.
Heretofore, an amorphous silicon (hereinafter referred to as “a-Si:H”) thin film for use in a photoelectric conversion element for solar cells has been deposited, for example, in the following manner. A pair of flat-plate electrodes are disposed parallel to each other in a vacuum vessel, and a substrate is held by one of the flat-plate electrodes. After a silane gas is supplied into the vacuum vessel to set a desired degree of vacuum therein, a high-frequency power is fed to the other flat-plate electrode in opposed relation to the substrate-holding flat-plate electrode to generate a capacitively-coupled high-frequency discharge plasma, whereby an amorphous silicon thin film is deposited on a surface of the substrate. While the solar cell using an a-Si:H thin film is expected as a core of power-generating solar cells, light-induced degradation in an a-Si:H thin film deposited at a high rate remains as a long-standing major problem to be solved.
In this context, it was recently pointed out that Si microparticles (Si clusters) with a size about 10 nm or less, which are generated in a silane plasma used in depositing a-Si:H film, are likely to have a close relation to the light-induced degradation (see the following Non-Patent Publication 1). From this standpoint, a key to solving to the light-induced degradation problem is to clarify a growth mechanism of the Si clusters and quantitatively define a relationship between an amount of Si clusters to be incorporated in an a-Si:H film, and properties of the film, so as to develop a process of depositing a high-quality a-Si:H film at a high rate while suppressing Si clusters causing film degradation, based on the obtained knowledge.
In view of the above approach, based on a newly-developed on-site measurement technique for Si clusters, the inventors of this application clarified the growth mechanism of Si clusters in a silane plasma, and the relationship between growth suppression of Si clusters and a deposited film, in the Non-Patent Publication 1. Specifically, the inventors obtained experimental data showing that small clusters (about 0.5 nm), large clusters (about 1 to 10 nm) and particles (about 10 nm or more) coexist in a silane plasma during a nucleus formation stage, and the large clusters will grow with time, wherein the large cluster is mainly composed of a particle with an amorphous structure which comprises a primary component of silicon.
The deposition of a-Si:H on the substrate according to a silane gas plasma is caused by the following primary reaction.
[Primary Reaction]
SiH4+e→SiH3+H+e (minimum electron energy: 8.75 eV)
SiH4+e→SiH2+H2+e (minimum electron energy: 9.47 eV)
SiH4+e→SiH+H2+H+e
SiH4+e→Si+2H2+e
Further, the formation of a nucleus which will grow into a large cluster is primarily caused by creation and accumulation of a higher-order silane SixHn (x<5) based on the following secondary reaction.
[Secondary Reaction]
SiH2+SiH4→Si2H6
SiH2+Si2H6→Si3H8
SiH2+Si3H8→Si4H10
The Non-Patent Publication 1 further shows that a technique of combining respective effects of discharge modulation, electrode heating, gas flow and hydrogen radicals to suppress the growth of Si clusters has great potential as an effective measure. In a prototype solar cell using an a-Si:H thin film deposited through a Si-cluster-controlling plasma CVD process developed by the inventors (see the following Patent Publication 1), although a relatively high stabilized efficiency of 9% (equivalent to 2×1016 cm−3 in a light-induced defect density of this a-Si:H film) is obtained, a light-induced degradation phenomenon considered as a problem still occurs. In this respect, the plasma CVD process disclosed in the Patent Publication 1 has not reached a radical solution. The term “light-induced defect density” means a density of defects (unpaired electrons) in an a-Si:H film which are measureable by an electron spin resonance method, and newly developed due to irradiation of light having a spectrum and an intensity equivalent to those of solar light on earth.
As another technique of suppressing the incorporation of Si clusters in an a-Si:H thin film, the following Patent Publication 2 discloses a plasma treatment method of decomposing and reducing Si clusters generated in a plasma creation region while suppressing thermal deformation of a substrate and electrodes due to heating. Specifically, the plasma treatment method is intended for use with an apparatus designed such that a flat electrode and a substrate supported by an earth electrode connected to the ground is disposed in a face-to-face arrangement within a vacuum chamber supplied with a gas containing a deposition material. In the plasma treatment method, a high-frequency power generated by a high-frequency power feeder circuit is fed to the flat electrode to create a plasma between the flat electrode and the substrate so as to treat the deposition material, wherein a laser light is emitted to a plasma creation region to decompose Si clusters generated together with the plasma by energy of the laser light. Even in an a-Si:H thin film obtained by this method, a defect density is about 1015 cm−3 (this value is assumed to be an initial defect density, and equivalent to a light-induced defect density of about 2×1016 cm−3). As above, at present, there is no a-Si:H thin film having a light-induced defect density of less than 2×1016 cm−3. Thus, it is still awaited to clarify the relationship between Si clusters incorporated in an a-Si:H thin film and the light-induced degradation phenomenon.
[Patent Publication 1] JP 2002-299266 A
[Patent Publication 2] JP 2004-146734 A
[Non-Patent Publication 1] SHIRATANI, et al., “Growth Mechanism of Microparticles in Low-Pressure Silane Plasma”, School of Material Science, Japan Advanced Institute of Science and Technology, Summaries of 2001 1st School Forum “Basics and Applications of Silane-based CVD Process”, March/2002, p. 13-18
It is an object of the present invention to provide a cluster-free a-Si:H thin film which is practically produceable. It is another object of the present invention to clarify an upper limit of each film property achievable by Si-cluster suppression, and characteristics of a super-high-quality a-Si:H thin film obtained by the Si-cluster suppression. It is still another object of the present invention to figure out a further quantitative relation between an amount of large clusters incorporated in an a-Si:H thin film and properties of the film, while identifying a Si-cluster size having an impact on the film properties, and clarify a formation mechanism of nuclei of microparticles, so as to contribute to establishment of mass production techniques for a solar cell using a high-efficiency a-Si:H thin film free of light-induced degradation.
A cluster-free a-Si:H film of the present invention is characterized in that an in-film Si—H2 bond density is 10−2 atomic % or less, and an in-film volume fraction of large clusters is 10−1% or less. The term “in-film Si—H2 bond density” means a ratio of H2-bonded Si atoms to the entire Si atoms in an a-Si:H film, and the in-film Si—H2 bond density is proportional to an integrated intensity of an absorption spectrum component having a maximum absorption intensity around 2100 cm−1 in an infrared absorption spectrum of the a-Si:H film. These numerical values are measurement results obtained by a FTIR (Fourier transform infrared spectroscopy) and an ESR (electron spin resonance) method. In a-Si:H films based on conventional film-depositing techniques, the Si—H2 bond density and the volume fraction of large clusters have been 10−1 atomic % and 2×10−1% at best, respectively.
The cluster-free a-Si:H film of the present invention is produced by depositing, on a Si or glass substrate, a plasma flow of a silane gas or a disilane gas. Thus, the a-Si:H film (referred to occasionally as “Si film”), has prominent characteristics, such that: a light-induced defect density is reduced from 2×1016 cm−3 or more in conventional Si films to substantially zero, specifically, a value equal to or less than a detection sensitivity (3×1014 or less) of a detector; a stabilized efficiency (%), i.e., a light-energy conversion efficiency, is increased from 9% at the highest in existing Si films up to 14% or more; and a light-induced degradation rate, i.e., [(initial efficiency−stabilized efficiency)/initial efficiency]×100%, is reduced from 20% at the lowest in the existing Si films to substantially zero, specifically, a value equal to or less than a detection sensitivity (2% or less) of a detector.
The above cluster-free a-Si:H film is obtained by preventing large clusters from being incorporated in an a-Si:H film to be deposited, by means of suppressing the generation itself of large clusters, or removing generated large clusters, or a combining them. The first means for suppressing the generation itself of large clusters may include a technique of controlling an electron energy distribution in a VHF discharge, and a technique of diluting a discharge atmosphere with one or a combination of two or more selected from the group consisting of H2, Ar, He, Ne and Xe. The second means for removing generated large clusters may include a technique of removing generated large clusters from a discharge region by use of a gas flow-induced viscous force, a technique using a thermophoretic force (i.e., thermal migration force) based on a temperature gradient, a technique of exerting an electrostatic force, a technique of eliminating a gas stagnation region, and a technique of applying a repetitive on-off discharge and removing generated large clusters during the OFF period. In particular, large clusters with a size of several nm or more can be approximately fully removed from a discharge region by means of the thermophoretic force based on a temperature gradient. The incorporation of large clusters can be suppressed by means of the repetitive pulsed discharge, to an undetectable level even by an ultrasensitive photon-counting laser scattering method. Further, a filter for removing large clusters may be additionally provided so as to prevent large clusters from being incorporated in an a-Si:H film during deposition of silane plasma onto the substrate.
The cluster-free a-Si:H film of the present invention has prominent characteristics which are not an extension of those of the conventional Si cluster-reduced a-Si:H film, and can eliminate 90% or more of large clusters which have existed in the conventional a-Si:H film, by low-cost means without lowering a film-deposition rate.
The present invention will now be described based on an embodiment thereof where an a-Si:H film is deposited using a silane gas.
In a first embodiment of the present invention, a technique of increasing a gas flow rate in a plasma region, generating a thermophoretic force which acts on large clusters in gaseous phase, and capturing large clusters by an inner wall of a hole to remove the large clusters is used for preventing the incorporation of large clusters in an a-Si:H film to be deposited.
In an actual example using the apparatus 10 illustrated in
As described above, in this embodiment, the incorporation of large clusters in a deposited film on the substrate is prevented based on the high-speed gas flow and the thermophoretic force in the through-holes 16, and the large clusters are captured and removed by the inner walls of the through-holes 16.
In
The technique according to the first embodiment makes it possible to facilitate increasing a film-depositing area so as to achieve a high film-deposition rate of 1 nm/s or more.
In a second embodiment of the present invention, a cluster removal filter is used as one of large-cluster removal means.
As shown in
In an actual example using the apparatus 20 illustrated in
An a-Si:H thin film deposited in the above manner had characteristics equivalent to those in the first embodiment, as indicated by black circle marks in
In the second embodiment, a plurality of the cluster removal filters may be arranged in a superimposed manner so as to maximally reduce the incorporation of large clusters in an a-Si:H thin film to be deposited.
In a third embodiment of the present invention, a gas curtain (high-speed silane gas flow) is used as one of the large-cluster removal means, and employed in an amorphous silicon thin film deposition apparatus 30 (hereinafter referred to simply as “apparatus 30”) illustrated in
In this embodiment, the high-frequency power feeder circuit is designed to feed 2W of VHF power having a frequency of 60 MHz, to the high-frequency electrode 32, to create a plasma. Further, first and second silane gas inlet ports 35, 36 are provided in one of opposite lateral walls of the reaction chamber 31 in vertically space-apart relation to each other, and first and second vacuum pumps 37, 38 are provided in the other lateral wall at respective positions corresponding to the first and second silane gas inlet ports, in such a manner that a low-speed gas flow “a” is formed between the high-frequency electrode 32 and the earth electrode 33 and on the side of the high-frequency electrode 32, and a high-speed gas flow “b” is formed between the high-frequency electrode 32 and the earth electrode 33 and on the side of the earth electrode 33. Specifically, a silane gas is introduced from the silane gas inlet ports 35 while discharging the silane gas through the vacuum pump 37, so as to set a flow rate of the low-speed gas flow “a”, at about 1 to 10 cm/s. Further, a silane gas is introduced from the silane gas inlet ports 36 while discharging the silane gas through the vacuum pump 38, so as to set a flow rate of the high-speed gas flow “b” immediately above the substrate 34, at about 20 to 100 cm/s. More specifically, the flow rate of the high-speed gas flow “b” immediately above the substrate 34 is set at a value greater than an in-film diffusion rate (about 10 cm/s) of large clusters and less than a diffusion rate (about 200 cm/s) of SiH3 radicals as a film precursor. In conventional film depositing techniques, a set of a gas inlet port and a vacuum pump are provided, and a gas flow rate is typically set at 5 cm/s.
In this embodiment, a viscous force induced by the high-speed gas flow “b” immediately above the substrate 34 is exerted on large clusters so as to prevent the large clusters from being incorporated in a deposited thin film on the substrate 34. In other words, the high-speed gas flow “b” immediately above the substrate 34 acts as a large-cluster removing gas curtain so as to prevent large clusters from being incorporated in a deposited thin film on the substrate 34.
An a-Si:H thin film deposited in the above manner had characteristics equivalent to those in the first and second embodiments. In the technique according to the third embodiment, a plurality of elongated electrodes each having a size, for example, of 200 cm×10 cm, may be arranged to increase an film-depositing area and reduce a volume of gas to be used, so as to achieve a film-deposition rate of 0.3 nm/s.
The present invention makes it possible to deposit a hydrogenated amorphous silicon thin film free from a light-induced degradation, through a plasma CVD process. This thin film can be used as a power generation layer of a solar cell to achieve a high-efficiency solar cell free from a light-induced degradation.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-244333 | Aug 2004 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP05/15007 | 8/17/2005 | WO | 4/4/2007 |
