Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Conference Object Citation - WoS: 6Citation - Scopus: 9Sub-Bandgap Optical Absorption Spectroscopy of Hydrogenated Microcrystalline Silicon Thin Films Prepared Using Hot-Wire Cvd (cat-Cvd) Process(Elsevier Ltd., 2006) Göktaş, Oktay; Işık, Nebile; Okur, Salih; Güneş, Mehmet; Carius, Reinhard; Klomfaß, Josef; Finger, FriedhelmHydrogenated microcrystalline silicon (μc-Si:H) thin films with different silane concentration (SC) have been prepared using the HW-CVD technique. Dual beam photoconductivity (DBP), photothermal deflection spectroscopy (PDS), and transmission measurements have been used to investigate the optical properties of the μc-Si:H films. Two different sub-bandgap absorption, α(hν), methods have been applied and analyzed to obtain a better insight into the electronic states involved. A good agreement has been obtained in the absorption spectrum obtained from the PDS and DBP measurements at energies above the bandgap. Differences between PDS and DBP spectra exist below the bandgap energy where DBP spectra always give lower α(hν) values and show a dependence on the SC. For some films, differences exist in the α(hν) spectra when the DBP measurements are carried out through the film and substrate side. In addition, for some films, there remains fringe pattern left on the spectrum after the calculation of the fringe-free absorption spectrum, which indicates structural inhomogeneities present throughout the film.Conference Object Citation - WoS: 8Citation - Scopus: 9Diffusion Length Measurements of Microcrystalline Silicon Thin Films Prepared by Hot-wire/Catalytic Chemical Vapor Deposition (hwcvd)(Elsevier Ltd., 2006) Okur, Salih; Güneş, Mehmet; Finger, Friedhelm; Carius, ReinhardHydrogenated microcrystalline silicon (μc-Si:H) films prepared by using the hot-wire/catalytic chemical vapor deposition (HWCVD) technique at low substrate temperatures between 185 °C and 220 °C with different silane concentrations (SC) were investigated using steady-state photocarrier grating (SSPG) and the steady-state photoconductivity methods (SSPC). Crystalline volume fractions (IC RS) obtained from Raman spectroscopy change from 0.22 to 0.77. The diffusion length (LD) is measured at generation rates between G = 1019 and 1021 cm- 3 s- 1. LD changes from 27 nm to 270 nm, with maximum values around SC = 5%. The dependence of LD on SC is similar to that observed for similar quality microcrystalline silicon films prepared using the VHF-PECVD technique. The grating quality factor, γ0, drops from about 0.9 to 0.5 after transition to the microcrystalline regime as indication of scattering from surface patterns.Conference Object Citation - WoS: 7Citation - Scopus: 10Sub-Bandgap Absorption Spectroscopy and Minority Carrier Transport Properties of Hydrogenated Microcrystalline Silicon Thin Films(National Institute of Optoelectronics, 2005) Güneş, Mehmet; Göktaş, Oktay; Okur, Salih; Işık, Nebile; Carius, Reinhard; Klomfaß, Josef; Finger, FriedhelmHydrogenated microcrystalline silicon thin films have been prepared using HW-CVD and VHF-PECVD techniques with different silane concentrations. The steady-state photoconductivity, dual beam photoconductivity, photothermal deflection spectroscopy and steady-state photocarrier grating (SSPG) methods have been used to investigate the optical and electronic properties of the films. Two different sub-bandgap absorption methods have been applied and analyzed to obtain a better insight into the electronic states involved. For some films, differences existed in the optical absorption spectra when the measurements were carried out through the film side and through the substrate side. In addition, for some films, fringe patterns remained on the spectrum after the calculation of the fringe free absorption spectrum, which indicates that structural inhomogeneities were present throughout the film. Finally, minority carrier diffusion lengths deduced from the SSPG measurements were investigated as a function of the crystalline volume fraction (I c RS) obtained from Raman spectroscopy. The longest diffusion lengths and lowest sub-bandgap absorption coefficients were obtained for films deposited in the region of the transition to the amorphous growth.Article Citation - WoS: 13Citation - Scopus: 14Electronic Transport Properties of Microcrystalline Silicon Thin Films Prepared by Vhf-Pecvd(Springer Verlag, 2004) Okur, Salih; Güneş, Mehmet; Göktaş, Oktay; Finger, Friedhelm; Carius, ReinhardSteady-state photocarrier grating (SSPG) and steady-state photoconductivity, σph, experiments have been carried out to investigate the electronic transport properties of undoped hydrogenated microcrystalline silicon (μc-Si: H) films prepared with very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD). Material with different crystalline volume fractions was obtained by variation of the silane concentration (SC) in the process gas mixture. Pure amorphous silicon material was investigated for comparison. The ambipolar diffusion length, L amb, which is dominated by the minority carrier properties, is obtained both from the best fit to the experimental photocurrents ratio, β, versus grating period (Λ), and from the "Balberg plot" for the generation rates between 1019 and 1021 cm -3 s-1. Lamb increases from 86 nm with increasing SC and peaks around 200 nm for the SC = 5.6% and decreases again for higher SCs. Lamb values obtained from the intercept of the Balberg plot result in a small difference of around 5% for most of the samples. Minority carrier mobility-lifetime (μτ)-products are much lower than those of majority carriers, however, both majority and minority carrier μτ-products in microcrystalline silicon are higher than those of undoped hydrogenated amorphous silicon. The grating quality factor (γ 0) changes from 0.70 to 1.0 indicating almost negligible surface roughness present in the samples.Conference Object Citation - WoS: 76Citation - Scopus: 80Stability of Microcrystalline Silicon for Thin Film Solar Cell Applications(Institute of Electrical Engineers, 2003) Finger, Friedhelm; Carius, Reinhard; Dylla, Thorsten; Klein, Stefan; Okur, Salih; Güneş, MehmetThe development of microcrystalline silicon (μc-Si:H) for solar cells has made good progress with efficiencies better than those of amorphous silicon (a-Si:H) devices. Of particular interest is the absence of light-induced degradation in highly crystalline μc-Si:H. However, the highest efficiencies are obtained with material which may still include a-Si:H regions and light-induced changes may be expected in such material. On the other hand, material of high crystallinity is susceptible to in-diffusion of atmospheric gases which, through adsorption or oxidation, affect the electronic transport. Investigations are presented of such effects concerning the stability of μc-Si:H films and solar cells prepared by plasma-enhanced chemical vapour deposition and hot wire chemical vapour deposition.Conference Object Citation - WoS: 1Citation - Scopus: 1Conductance Fluctuations in Vhf-Pecvd Grown Hydrogenated Microcrystalline Silicon Thin Films(Springer Verlag, 2003) Güneş, Mehmet; Johanson, Robert E.; Kasap, Safa O.; Finger, Friedhelm; Lambertz, AndreasCoplanar conductance fluctuations or excess noise of undoped hydrogenated microcrystal-line silicon (μc-Si : H) thin films grown by VHF-PECVD from silane-hydrogen mixtures with silane concentrations from 2% to 6% have been studied between room temperature and 470 K. We report that undoped μc-Si : H thin films show similar noise-power spectra to those of undoped a-Si : H films in a coplanar sample geometry. At lower temperatures, the noise with the slope α = 0.60 ± 0.07 and at higher temperatures, the noise with the slope α close to unity dominate the spectrum. The noise magnitude decreases with decreasing silane concentration and becomes strongly temperature dependent with increased crystal unity.Conference Object Citation - WoS: 5Citation - Scopus: 5Photoconductivity Spectroscopy in Hydrogenated Microcrystalline Silicon Thin Films(Springer Verlag, 2003) Güneş, Mehmet; Akdaş, Deniz; Göktaş, Oktay; Carius, Reinhard; Klomfaß, Josef; Finger, FriedhelmSteady-state photoconductivity and sub-bandgap absorption measurements by the dual-beam photoconductivity (DBF) method were carried out on undoped hydrogenated microcrystalline silicon thin films prepared by VHF-PECVD and hot-wire chemical vapor deposition. The results are compared with those of the constant-photocurrent method (CPM) and photothermal deflection spectroscopy (PDS). It is found that DBP, CPM, and PDS provide complementary data on the optoelectronic processes in microcrystalline silicon.Article Citation - WoS: 15Citation - Scopus: 141/F-noise Study of Undoped Intrinsic Hydrogenated Amorphous Silicon Thin Films(American Physical Society, 1999) Güneş, Mehmet; Johanson, Robert E.; Kasap, Safa O.Conductance fluctuations in four samples of undoped intrinsic hydrogenated amorphous silicon (a-Si:H) were measured in the temperature range of 450 K to 500 K and for frequencies from 2 Hz to 3 kHz. The noise spectra divide into two regions that each fit a 1/fα power law but with different slope parameters α and different temperature dependences. At low frequencies, α is greater than unity and increases with temperature. At high frequencies, α is near 0.6 and temperature independent, but the noise magnitude decreases rapidly with temperature. We infer from the different dependences on temperature that the noise is generated by two independent mechanisms operating simultaneously in a-Si:H. We also observe that the 1/f noise exhibits a quadratic dependence on bias current and Gaussian statistics.Article Citation - WoS: 37Citation - Scopus: 41Differences in the Densities of Charged Defect States and Kinetics of Staebler-Wronski Effect in Undoped (nonintrinsic) Hydrogenated Amorphous Silicon Thin Films(American Institute of Physics, 1997) Güneş, Mehmet; Wronski, Christopher R.A variety of undoped (nonintrinsic) hydrogenated amorphous silicon (a-Si:H) thin films was studied in greater detail using steady-state photoconductivity, σph, subband-gap absorption, α(hν), steady-state photocarrier grating (SSPG), and electron-spin-resonance (ESR) techniques both in the annealed and stabilized light soaked states. The experimental results were self-consisiently modeled using a detailed numerical analysis. It was found that large differences in the optoelectronic properties of device quality a-Si:H thin films can only be explained using a gap slate distribution which consists of positively charged D+ defect states above the Fermi level, the neutral D0 defect states, and the negatively charged D- defect states below the Fermi level. There are large differences both in the densities of neutral and charged defect states and R ratios in different a-Si:H films in the annealed state. The densities of both neutral and charged defect states increased, however, R ratios decreased in the stabilized light soaked state. Very good agreement was obtained between the densities of neutral defect states measured by ESR and those derived from the numerical analysis in the stabilized light soaked state. The kinetics of the Staebler-Wronski effect was also investigated. There was no direct correlation between the decrease of steady-state photoconductivity and increase of subband-gap absorption. The self-consistent fits to wide range of experimental results obtained with the three Gaussian distributions of charged defect states imply that this model is much better representation of the bulk defect states in undoped hydrogenated amorphous silicon thin films.