Tomonori Nishimoto

Effect of higher-silane formation on electron temperature in a silane glow-discharge plasma

Electron temperature measured by an optical-emission spectroscopy shows a strong substrate temperature dependence in a silane glow-discharge plasma. The electron temperature increases with time after turning on the plasma at a low substrate temperature of 150 °C, while it stays constant at a high substrate temperature of 400 °C. The electron temperature is drastically reduced when the source gas silane is diluted with hydrogen at low substrate temperatures. These results suggest that the electron temperature in silane plasma is strongly affected by an electron-attachment process to higher-order silane molecules whose formation reactions show negative activation energies with gas temperature and are also suppressed by the presence of hydrogen molecules.

Amorphous silicon solar cells deposited at high growth rate

Abstract A series of investigations was performed to improve stabilized efficiency of hydrogenated amorphous silicon (a-Si:H) solar cells deposited at a high growth rate of 15–20 A/s by the plasma-enhanced chemical vapor deposition method. The deterioration of film stability accompanied by an increase of deposition rate was found to be closely correlated with the increase of Si–H 2 bond hydrogen content in the deposited film. Plasma diagnosis results by quadrupole mass spectrometry and optical emission spectroscopy showed that reduction of electron temperature of plasma can effectively suppress the formation of higher-order silane-related species in the plasma and can improve film stability. According to the guiding principles deduced from the plasma diagnosis, we successfully improved the stability of cell performance and obtained a considerably improved stabilized efficiency of 8.2% at a high rate of 20 A/s. Key issues for improving stabilized efficiency of high growth-rate a-Si:H solar cells, by making the best use of plasma diagnostic techniques, are presented and discussed.

Guiding principles for obtaining stabilized amorphous silicon at larger growth rates

Abstract Guiding principles are proposed for preparing stabilized hydrogenated amorphous silicon thin films (a-Si:H) from silane glow-discharge plasmas. Higher order silane related chemical species (HSRS) produced in a silane plasma are suggested as species responsible for modifying the network structure in a resulting film, leading to photo-induced degradation. Formation kinetics of HSRS in the plasma and its contribution ratio to film growth have been studied using rate equations for the successive reactions of SiH2 forming HSRS. As a consequence, excitation frequency to reduce the electron temperature in the plasma and hydrogen dilution ratio to scavenge SiH2 have been indicated as the most important controllable parameters to obtain stabilized a-Si:H at a constant substrate temperature.

Anomalous behavior of electron temperature in silane glow discharge plasmas

Abstract The electron temperature measured by an optical emission intensity ratio of Si* to SiH* in a silane (SiH 4 ) glow-discharge plasma shows an anomalous behavior against film preparation conditions such as gas pressure and substrate temperature. When increasing the gas pressure, the electron temperature decreases first, takes a minimum value at a certain pressure range and then it increases. The electron temperature decreases with increasing substrate temperature, which is quite the opposite trend to a conventional non-reactive hydrogen plasma. These anomalous behaviors of electron temperature in silane plasmas have been explained in terms of feed-back phenomenon in the plasma, starting from an electron-attachment event to higher silane molecules produced in the plasma, causing an increase of electron temperature due to an increase of electron-loss rate, followed by an enhanced production of higher silane molecules. It has also been suggested that the responsible higher silane molecules for the above mentioned feedback phenomenon is penta-silane, Si 5 H 12 .

Chemical-reaction dependence of plasma parameter in reactive silane plasma

Abstract Electron temperature in a silane glow-discharge plasma, being an important plasma parameter for determining photo-induced instability in the resulting hydrogenated amorphous silicon (a-Si:H), has been studied under various film-preparation conditions. We have used an optical-emission-intensity ratio of Si* to SiH* (Isi*/IsiH*) which corresponds to the high-energy-tail slope of the electron-energy-distribution function in the plasma as a measure of electron temperature in a reactive silane glow-discharge plasma. We have found quite differently from the conventional non-reactive glow-discharge plasma such as hydrogen plasma that the electron temperature in the silane plasma is strongly modified by the substrate temperature (gas temperature) especially under high silane-gas partial-pressure condition. This anomalous behavior of the electron temperature in the silane plasma has been explained by means of gas-phase-polymerization reaction and electron-attachment process to the polymers in the plasma. The electron temperature has been remarkably reduced when a hydrogen-dilution method and a cathode-heating method are used which are considered to control polymer-formation reactions in the silane plasma together with utilization of conventional electron-temperature-controlling methods such as a very high plasma-excitation frequency and an application of magnetic field for electron-confinement. As a consequence of the reduction of electron temperature in the silane plasma, highly stabilized a-Si:H has been successfully obtained even under high growth rate conditions of 1.5 nm s-1.

Effect of halogen additives on the stability of a-Si:H films deposited at a high-growth rate

We deposited a-Si: H,F films at a high-growth rate (∼ 15 A/s) using a SiH 4 and SiF 4 gas mixture to examine the effect of halogen additives on the film stability against light exposure. Fluorinated a-Si: H films show a high conductivity over 5 × 10 - 5 S/cm and the Schottky cells made with fluorinated films exhibit an improved fill factor after light-soaking. SIMS measurements show an increased oxygen incorporation into the film at a SiF 4 flow of 5 sccm or larger, while virtually no increase is seen when a small SiF 4 flow rate of 1 sccm is used. This is presumably an indication that a small amount of SiF 4 can actually help improve the stability of a-Si:H films against light exposure.

Excitation frequency dependence of the optical emission intensity vs. deposition rate relationship in silane plasmas

Abstract The relationship between the optical emission intensity (Isi*) and the deposition rate (DR) of hydrogenated amorphous silicon thin films has been investigated in silane plasmas under various plasma conditions. We have indicated that the integrated value of Isi* should be used by taking into account the distance of the Isi*-measuring positions from the substrate surface for studying the optical emission intensity–DR relationship. One-to-one correspondence between the integrated value of Isi* and the DR has been obtained under various plasma conditions when keeping the electron temperature in the plasma constant. We have proposed a prediction method in the variation of the slope of the integrated value of Isi*–DR relationship by measuring the emission intensity ratio of Isi*/IsiH* for plasmas showing different electron temperatures.

Electrode distance dependence of photo-induced degradation in hydrogenated amorphous silicon

Electrode distance between cathode and anode is one of the important parameters for fabricating hydrogenated amorphous silicon (a-Si:H) using plasma-enhanced chemical-vapor-deposition system with parallel plate electrodes. In this work, we have investigated the relationship between electrode distance and stability of a resulting a-Si:H. The stability is improved with decreasing electrode distance. At shorter electrode distance, formation of higher silane-related-reactive species is suppressed by heating effect of gas molecules near the cathode due to a proximity to the heated anode. Using cathode heating method, the stability of a-Si:H is improved even at long electrode distances.

Gas Phase Diagnosis of Disilane/Hydrogen RF Glow Discharge Plasma and Its Application to High Rate Growth of High Quality Amorphous Silicon

Gas phase diagnosis of disilane/hydrogen plasma was carried out using mass spectrometry. At high growth rate (20 A/s) conditions using pure disilane as a source gas, the partial pressure of disilane molecules measured by mass spectrometry was more than one order of magnitude higher than in the case when mono-silane was used as a source gas. The stability of amorphous silicon films prepared from disilane was improved by the hydrogen dilution technique, although the disilane partial pressure in this condition was much higher than in the case when mono-silane was used as a source gas for device quality films. The relation between the gas phase species and the stability of the resulting films is studied. It was found that increase in disilane related signal intensity do not decrease film stability directly.

Relationship between the photo-induced degradation characteristics and film structure of a-Si:H films prepared under various conditions

We have investigated the photo-induced degradation characteristics of high growth-rate (/spl sim/20 /spl Aring//s) a-Si:H films prepared under various conditions and find that the stability of the film is closely related to the Si-H/sub 2/ bond density in the film. The results obtained by the Schottky cell measurements indicate that highly stable films with low Si-H/sub 2/ bond density are obtained at moderately high temperatures of around 350/spl deg/C. As our first trial, we made a small number of nip solar cells at high growth-rate (/spl sim/20 A/s) and confirmed the consistency between the result of the Schottky cell behavior and the solar cell performance. The best performance cell we have obtained so far exhibits a 9.3% initial efficiency, with a stabilized efficiency of 7.4% after a 500 h, AM1.5, 100 mW/cm/sup 2/ light exposure.