Kazuyoshi Ro

Role of grains in protocrystalline silicon layers grown at very low substrate temperatures and studied by atomic force microscopy

Abstract We have investigated the role of the sample thickness and silane dilution on the structure and electronic properties of protocrystalline silicon thin films deposited at very low substrate temperatures (∼80 ° C ) . Coincidence of the maxima in surface roughness and ambipolar diffusion length ( ≳100 nm ) with formation of the network of interconnected crystalline grain aggregates was observed. While the presence of the isolated grain aggregates improves the photoconductive properties before the percolation threshold is reached, further increase in crystallinity may have opposite effect due to detrimental role of increasing concentration of the defective grain boundaries.

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

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.

Fabrication of Polycrystalline Silicon Films from SiF4/H2/SiH4 Gas Mixture Using Very High Frequency Plasma Enhanced Chemical Vapor Deposition with In Situ Plasma Diagnostics and Their Structural Properties.

Polycrystalline silicon (poly-Si) films were fabricated by very high frequency (VHF) plasma enhanced (PE) chemical vapor deposition (CVD) from SiF4 and H2 gas mixture with small amounts of SiH4. Reactions and growth of poly-Si in the SiF4/H2/SiH4 system were discussed together with the results obtained from in situ plasma diagnostics, and compared with those obtained by microwave PECVD (MW CVD). As a result, similar relationships among growth temperature, SiF4/H2 gas flow ratio and film structure to those obtained with MW CVD were obtained with VHF CVD. For example, growth temperature could be reduced to 100°C while keeping a high crystal fraction (>80%) when small SiF4/H2 gas flow ratios were used. In contrast, under large SiF4/H2 gas flow ratios, crystal fraction rapidly decreased with decreasing temperature. The role of fluorine-related species in the growth of poly-Si was examined in relation to film microstructure and the results obtained from plasma diagnostics. Finally, guiding principles to achieve high rate and/or low-temperature growth of poly-Si by VHF CVD using SiF4/H2 gas mixtures were discussed.

Microstructure and photovoltaic properties of low temperature polycrystalline silicon solar cells fabricated by VHF-GD CVD using fluorinated gas

Abstract Polycrystalline silicon (poly-Si) photovoltaic devices were fabricated from SiF 4 , H 2 and SiH 4 gas mixtures using very high frequency (100 MHz) chemical vapor deposition (VHF CVD). The gas flow rate and the SiF 4 /H 2 /SiH 4 ratio were optimized for low temperature, device quality poly-Si growth. The n-layers used for the n/i/Pt Schottky diodes and n/i/p solar cells were deposited at 300°C and i-layers at 100–300°C to provide a seed layer for subsequent i-layer growth. The open circuit voltage increased with i-layer growth temperature in the n/i/Pt Schottky diodes. However, the optimal fill factor (FF) occurred at a ∼200°C i-layer growth temperature. The FF decreased at temperatures >250°C. The best solar cells had short circuit current of 24 mA/cm 2 and energy conversion efficiency of 6.22%. Also, it was possible to prepare high efficiency solar cells at relatively high growth rates, 0.5 nm/s, by optimizing the SiH 4 flow rate.

Structure and Properties of Silicon Thin Films Deposited at Low Substrate Temperatures

Silicon thin films were grown near the microcrystalline/amorphous boundary at substrate temperatures TS = 35–200°C and dilutions [H2]/[SiH4] = 25–167. The conductivity percolation threshold occurred at crystallinity ~60%, above the random composite threshold 33.3%, due to amorphous tissue coating crystalline grains and limiting the electronic transport. Higher content of hydrogen at lower TS facilitates formation of ordered silicon phase even close to room temperature (the sample grown at 35°C had 30 at% of hydrogen and crystallinity ~60%), providing a technological window for deposition of silicon thin films on cheap polymer substrates with electronic properties suitable for solar cells.

High Rates and Very Low Temperature Fabrication of Polycrystalline Silicon From Fluorinated Source GAS and Their Transport Properties

Low temperature (50--300 C) growth of polycrystalline silicon (poly-Si) by very high frequency (100 MHz) glow-discharge plasma enhanced CVD using SiF{sub 4} and H{sub 2} mixtures was studied. The poly-Si microstructure was strongly affected by the SiF{sub 4}/H{sub 2} gas flow ratio. For example, either (220) or (400) preferentially oriented films were prepared by appropriate SiF{sub 4}/H{sub 2} ratio selection. The addition of small SiH{sub 4} flows to the SiF{sub 4}/H{sub 2} mixtures could be used to increase the growth rate while the SiF{sub 4}/H{sub 2} continued to control the film structures such as preferential orientation. Highly crystalline films were grown at a growth rate of 0.52 nm/s using SiF{sub 4}/H{sub 2}/SiH{sub 4} flow rates of 30/90/2.0sccm (respectively). However, at higher SiH{sub 4} flows amorphous films were deposited. Under the small SiF{sub 4}/H{sub 2} 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 0.32 V. N-i-p solar cell results are very promising with 6.2% of conversion efficiency being achieved in the initial trials.

Silicon thin films deposited at very low substrate temperatures

Abstract A series of investigations were performed to study the influence of the substrate temperature on the structure and properties of silicon thin films. Substrate temperature was varied in the wide range of 35–200 °C. It has been shown that the films grown below 60 °C exhibit an unusual structural behavior. A sharp TO phonon peak at 520 cm −1 was detected in Raman spectra, which is associated with the crystalline structure. In contrast to these results, the same samples do not show any crystallite-related peak by X-ray diffraction and their optoelectronic properties (dark conductivity, activation energy and subgap absorption spectra) show amorphous features. A similar discrepancy was observed for a hydrogen dilution ratio ( r H =([SiH 4 ]+[H 2 ])/[SiH 4 ]) series of samples deposited at 60 °C. Hydrogen dilution ratio was varied from 25 to 170. It seems that at low substrate temperature a parameter window exists where the silicon thin films can be grown with the properties combining both crystalline and amorphous behavior.

Structure and transport properties of p-type polycrystalline silicon fabricated from fluorinated source gas

P-type polycrystalline silicon (poly-Si) thin films are fabricated from SiF4, H2 and SiH4 gas mixtures by plasma-enhanced chemical vapor deposition using in situ doping of B2H6 or BF3. Relationships between deposition condition, film structure and transport properties are focused on. In addition, ionization efficiency is discussed in connection with deposition condition and with B and F concentrations. When poly-Si is doped with B, crystal fractions are lower for 300°C-grown films than for 200°C-grown films. High B doping results in (220) preferential orientation even when no fluorinated gas is used in the gas source. These trends are very similar among films prepared using different doping sources, i.e. B2H6 and BF3. When SiF4 is used, the B ionization efficiency is very low, ∼10%. It is improved to ∼50% by removing SiF4 from the gas source. This low ionization efficiency shows good correlation with atomic concentration ratio of B/F in the poly-Si films, suggesting that some high incorporation of F compared to B inhibits the ionization of B. Using SiH4, H2 and BF3 without SiF4, highly (220) oriented, large grain poly-Si is obtained and it exhibits large mobility of 3.7 cm2/Vs.