Showing papers by "Toshio Kamiya published in 1999"

Optimization of Transparent Conductive Oxide for Improved Resistance to Reactive and/or High Temperature Optoelectronic Device Processing

Abstract: Research on improved amorphous silicon-based devices has focused on materials prepared at high temperatures and/or those grown under very reactive conditions. The use of these conditions for device applications requires the development of more robust transparent conductive oxide (TCO) substrates. A thin (<10 nm) ZnO coating on a SnOx-coated glass substrate could withstand RF (13.56 MHz) and very high frequency (VHF: 144 MHz) hydrogen plasma treatments; however, the TCO was strongly reduced by a higher density, higher energy electron cyclotron resonance (ECR) hydrogen plasma or a higher temperature. Ga-doped ZnO (GZO) TCO substrates exhibited greater resistance to hydrogen plasma induced reduction. RF magnetron sputter deposited crystalline GZO thin films were deposited and optimized at temperatures higher than 150°C on glass substrates. The electron mobility and the Ga doping efficiency were improved with increasing GZO deposition temperature. The performance of a-Si:H solar cells fabricated under standard conditions (~220°C) on these GZO substrates increased with increased GZO deposition temperature. The performance of a-Si:H solar cells prepared under more reactive and/or at higher deposition temperatures on 250°C deposited GZO was also examined. Both high temperature (280°C)-deposited narrow-bandgap a-Si:H(Ar) and ECR hydrogen plasma deposited a-Si:H(Cl) based solar cells were significantly improved using high temperature deposited GZO substrates.

Comparison of Microstructure and Crystal Structure of Polycrystalline Silicon Exhibiting Varied Textures Fabricated by Microwave and Very High Frequency Plasma Enhanced Chemical Vapor Deposition and Their Transport Properties

Abstract: Micro- and crystal structures of polycrystalline silicon (poly-Si) films fabricated by low temperature (≤360°C) plasma enhanced chemical vapor deposition (PECVD) were examined. Crystal orientation could be controlled by varying the source gas ratio SiF4/H2. (220) oriented films were obtained at low gas flow rate ratios while (400) preferentially oriented films were obtained at higher SiF4/H2 ratios either by a remote-type microwave PECVD or a capacitive coupled parallel electrode very high frequency (VHF) PECVD. It was found that micro- and crystal structures were a strong function of orientation; that is, the crystal lattice in the (220) oriented film was under tensile strain and the crystalline grain had strong anisotropy of grain size. In contrast, the crystal lattice in the (400) oriented film was under compressive strain and evident anisotropy in the grain size could not be found. Furthermore, it was confirmed that the deposition of SiHn and/or SiHnFm and etching by fluorinated species and their competition played an important role in the selective growth. Fluorine-related species were also effective in growing large crystalline grains. Hall mobility of electron for (400) oriented films showed a monotonic increase with carrier density and achieved a large mobility of ~10 cm2/Vs.

Role of Seed Crystal Layer in Two-Step-Growth Procedure for Low Temperature Growth of Polycrystalline Silicon Thin Film from SiF4 by a Remote-Type Microwave Plasma Enhanced Chemical Vapor Deposition

Abstract: Role of seed crystal layer which played in low temperature growth of polycrystalline silicon (poly-Si) thin films was investigated by two-step-growth (TSG) process. The TSG involves two different deposition processes, which are called as `seed process' and `growth process'. In order to satisfy the conflicting demands such as low temperature, high rate and high quality crystal growth, the deposition conditions in the seed and growth processes were examined. As a result, it is confirmed that the seeding of high quality crystal layer is effective to improve crystallinity of growing poly-Si film, especially for the case grown at lower temperature than 300°C. In order to fully promote crystallinity, some thick and high crystallinity seed layer is needed. In addition, epitaxial-like growth on the seed layers can be realized by optimizing deposition conditions both during the seed and growth processes. When H2/SiF4 flow ratios larger than those used during the seed process were used during the growth process, lower growth temperatures were possible with maintaining a smooth interface between the seed and the grown poly-Si layers. In essence, the hydrogen mixing ratio and deposition temperature are complementary parameters, thus it was possible to reduce deposition temperatures with maintaining large crystal fraction and oriented structure, if the hydrogen mixing ratio was increased to an appropriate amount. Furthermore, in situ ellipsometry analysis indicated that, under optimal conditions, the interface between the seed and the growing film can indeed be smooth and epitaxial-like. TSG is a promising technique by which to fabricate high quality poly-Si thin films on glass substrates at low temperatures.

High rates and very low temperature fabrication of polycrystalline silicon from fluorinated source gas and their transport properties

Abstract: Low temperature (50-300°C) growth of polycrystalline silicon (poly-Si) by very high frequency (100MHz) glow-discharge plasma enhanced CVD using SiF4 and H2mixtures was studied. The poly-Si microstructure was strongly affected by the SiF4/H2 gas flow ratio. For example, either (220) or (400) preferentially oriented films were prepared by appropriate SiF4/H2 ratio selection. The addition of small SiH4 flows to the SiF4/H2 mixtures could be used to increase the growth rate while the SiF4/H2 continued to control the film structures such as preferential orientation. Highly crystalline films were grown at a growth rate of 0.52nm/s using SiF4/H2/SiH4 flow rates of 30/90/2.Osccm (respectively). However, at higher SiH4 flows amorphous films were deposited. Under the small SiF4/H2 ratio condition, highly crystallized poly-Si was grown at temperatures as low as 50°C. N/i/Pt Schottky diode solar cells were prepared using these poly-Si for both the n- and the i-layers. These solar cells exhibited good performance; for example, open circuit voltages over 0.32V. N-i-p solar cell results are very promising with 6.2% of conversion efficiency being achieved in the initial trials.

Fabrication of Solar Cells Having SiH2Cl2 Based I-Layer Materials

Abstract: Intrinsic amorphous silicon films were fabricated using electron cyclotron resonance (ECR) assisted chemical vapor deposition and SiH2Cl2 source gas. Intrinsic layers were used for material characterization and also for the absorber layer of solar cells. The highly reducing atmosphere produced by the high energy ECR hydrogen plasma used to deposit these intrinsic films caused some degradation and/or etching of the previously deposited solar cell doped layers as well as the SnO2-coated glass substrates. The p-layer etching rates were greater than those of the n-layer when these layers were exposed to ECR hydrogen plasma. Optimum photovoltaic performance was achieved when an optimized n/i interfacial buffer layer was used for a solar cell deposited in the n-i-p sequence. Better solar cell performances were obtained when the solar cells were measured under n-side illumination. In part, the buffer layer optimization involved careful consideration of band gap matching to the relatively wide band gap (1.85 eV) intrinsic layers prepared from SiH2Cl2. Further performance gains were possible through transparent conductive oxide/substrate optimization. For example, the open circuit voltage (Voc) increased to ~0.89 V when gallium-doped zinc oxide/glass substrates were used compared to ~0.63 V when tin oxide/glass substrates were used. Interface recombination and minority carrier diffusion lengths were probed by n- and p-side illuminated quantum efficiency measurement and analysis. The electron and hole µτ products were estimated to be 4.4×10-8 cm2/V and 3.5×10-8 cm2/V, respectively. The stability of the solar cells was also examined.

Effect of Halogen on the Structure of Low Temperature Polycrystalline Silicon Thin Films Fabricated on Glass Substrates

Abstract: Polycrystalline silicon thin films were fabricated by VHF (100-144MHz) plasma enhanced chemical vapor deposition. Three different source materials were used to grow the films on glass substrates: (1) SiH2Cl2/H2, (2) SiF4/H2 and (3) SiH4/H2 mixing gases. It was found that the gas mixing ratio where crystal silicon grows strongly depends on the selection of source gas: i.e., crystal growth occurred at mixing ratios (SiF4/H2) smaller than 30/10 sccm while the crystal growth in SiH4/H2 system required much smaller mixing ratios, such as ≤1/50sccm. Microstructures of the films were also strongly influenced by the source material. (220) orientation structures were easily obtained when SiF4, SiH2Cl2 or B2H6 were used, compared to SiH4. In addition, (400) preferentially oriented film grew on glass when the film was grown at a gas mixing ratio of SiF4/H2=30/10sccm and a substrate temperature of 200°C. Chlorinated source gases including SiHnClm (n+m=4) are also expected to produce (400) oriented growth.

Amorphous Silicon Solar Cells Techniques for Reactive Conditions

Abstract: The preparation of amorphous silicon films and solar cells using SiH2Cl2 source gas and electron cyclotron resonance assisted chemical vapor deposition (ECR-CVD) was investigated. By using buffer layers to protect previously deposited layers improved a-Si:H(Cl) solar cells were prepared and studied. The high quality a-Si:H(Cl) films used in this study exhibited low defect densities (∼1015cm−3) and high stability under illumination even when the deposition rate was increased to ∼15A/s. The solar cells were deposited in the n-i-p sequence. These solar cells achieved VOC values of ∼ 0.89V and ∼ 3.9% efficiency on Ga doped ZnO (GZO) coated specular substrate. The a-Si:H(C1) electron and hole μτ products were ∼10−8cm2/V.

Amorphous Silicon Solar Cell Techniques for High Temperature and/or Reactive Deposition Conditions

Abstract: Several promising new methods for amorphous silicon solar cell preparation involve high substrate temperatures and/or very reactive atmospheres When incorporated into solar cells, the performance of these layers has often been less than expected due to enhanced diffusion and/or chemical reactionsThis poor performance results from the harsh deposition environments Deleterious effects include darken of TCO coated glass substrates due to hydrogen diffusion to and hydrogen reduction at the TCO interface when solar cells are prepared in the p-i-n deposition sequence Alternatively, the deposition of TCO layers onto amorphous layers also involves rather harsh oxidizing conditions that have a deleterious effect on the top most amorphous silicon-based p-layers Strategic use of blocking layers results in remarkably improved solar cell performance A thin Cr layer (probably becoming Cr{sub 2}O{sub 3}) shows ability to improve the performance of both n-i-p and p-i-n solar cells by inhibiting both O and H diffusion