Mehmet Güneş

1/f Noise in doped and undoped amorphous silicon

Abstract We measured the spectrum of conductance fluctuations in n-type, p-type, and undoped hydrogenated amorphous silicon (a-Si:H) as a function of temperature. In general, the spectra can be fit to a power law, 1/ f α , although in the p-type and undoped samples deviations from a strict power law occur. For n-type and p-type samples, the noise magnitude increases with temperature by approximately a factor of 5 from 295 to 450 K. The slope parameter, α , also increases with temperature in the p-type samples from near unity to 1.4 but not in the n-type sample where it remains near 1.05 independent of temperature. The undoped sample could be measured only over a limited range of elevated temperatures, but α does trend larger. The undoped and lightly doped material have similar noise levels but larger p-type doping reduces the noise by two orders of magnitude. Correlation measurements indicate the 1/ f noise is Gaussian for all samples. However, intermittent random-telegraph noise is observed in n-type material.

1/f noise in amorphous silicon and silicon-germanium alloys

We report measurements of conductance noise of a-Si1-xGex:H in two different geometries; one where the current flow is transverse to the surface and the other longitudinal to it. Because of the large increase in sample resistance in going from transverse to longitudinal conduction, it was not possible to measure both geometries at the same temperature. However, the temperature trends are compatible with a common noise source. For both geometries, alloying with up to 40% Ge reduces the noise magnitude by a factor of 50 over that found in a-Si:H.

Photoconductivity spectroscopy in hydrogenated microcrystalline silicon thin films

Steady-state photoconductivity and sub-bandgap absorption measurements by the dual-beam photoconductivity (DBP) 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.

Conductance fluctuations in undoped hydrogenated amorphous silicon–germanium alloy thin films

Abstract We report coplanar conductance fluctuations of device quality, undoped hydrogenated amorphous silicon–germanium alloy thin films (a-SiGe:H) measured from 430 to 490 K. The a-SiGe:H alloys produce noise power spectra similar to coplanar undoped a-Si:H films in the same temperature range. The noise power spectrum S n does not fit a single 1/ f α power law but rather has two distinct regions, each accurately fitted by a power law, but with different slopes. The low frequency slope α 1 is similar to that observed in undoped a-Si:H films varying from 1.30 to 1.46 for different Ge concentrations and shows a slight temperature dependence. At higher frequencies, the slope α 2 is less than unity and temperature independent but depends on the Ge content of the film. α 2 decreases from 0.60 for no Ge (pure a-Si:H) to 0.15 for 40 at.% Ge. The noise power at lower frequencies increases and at higher frequencies decreases substantially as the temperature increases from 430 to 490 K. We infer that similar noise mechanisms are operating in undoped a-SiGe:H and a-Si:H films but that the Ge content is influencing the noise, particularly the slope at higher frequencies. In addition, the noise has the expected quadratic dependence on bias current and obeys Gaussian statistics.

Conductance fluctuations in undoped intrinsic hydrogenated amorphous silicon films prepared using several deposition techniques

Abstract Coplanar conductance fluctuations in a range of device quality undoped hydrogenated amorphous silicon (a-Si:H) films prepared using different deposition systems were measured in the temperature range of 440–505 K for frequencies from 2 Hz to 3 kHz. The 1/ f α type noise spectra had two different power law dependencies, one at lower frequencies with slope α 1 close to unity and a second region at higher frequencies with slope α 2 around 0.60. The noise power density decreases with increasing temperature in the high frequency region, but only increases much less with temperature at low frequencies. The results indicate that the noise in undoped intrinsic a-Si:H films is due to two independent noise mechanisms operating simultaneously.

1/F NOISE IN UNDOPED HYDROGENATED AMORPHOUS SILICON AND SILICON-GERMANIUM ALLOYS

; instead, there are two distinct regions. In Region 1 at lower frequencies, the slope parameter  is above unity and the noise power is sample dependent and shows only a slight temperature dependence. Region 2, at higher frequencies, has a different slope of about 0.60 that is similar for all samples, and the noise power decreases with temperature. Thus region 1 dominates the spectrum at higher temperatures and region 2 dominates at lower. For undoped a-SiGe:H films, the noise spectra show similar behavior except that the noise power varies with temperature in region 1. In both regions and for all the samples the noise power depends quadratically on dc bias current and obeys Gaussian noise statistics. The origin of this unusual noise spectrum is not yet understood.

Native and Light Induced Defect States in Wide Band Gap Hydrogenated Amorphous Silicon-Carbon(a-Si1-X Cx :H) Alloy Thin Films

In this study, wide band gap a-Si1-x Cx :H alloy thin films prepared with and without hydrogen dilution of (SiH4 +CH4) were characterized using optical absorption, dark conductivity, steady-state photoconductivity, sub-bandgap absorption obtained with both photothermal deflection spectroscopy (PDS) and dual beam photoconductivity (DBP), and electron spin resonance (ESR) techniques. Experimental results of steady-state photoconductivity and sub-bandgap absorption for different generation rates were analyzed using a detailed numerical model based on Simmons-Taylor statistics. The densities, energy location and nature of the native and light induced defect states in diluted and undiluted a-Si1-x Cx :H alloy thin films were derived from the best fits to the experimental data. The extracted parameters for defect states were compared with those of a-Si:H films both in the annealed and light degraded states.