R. Brüggemann

Urbach energy dependence of the stability in amorphous silicon thin-film transistors

We investigate the relationship between the stability of amorphous silicon thin-film transistors (a-Si:H TFTs) and the bulk properties of a-Si:H films. Threshold voltage shifts in a-Si:H TFTs are characterized by the thermalization energy Eth for different times and temperatures and fitted by {1+exp[(Eth−Ea)/kT0]}−2. We find that kT0 exhibits a clear correlation to the Urbach energy, but the more significant parameter Ea seems to depend only on the deposition-induced microstructure and not on the Urbach energy, the hydrogen content, or the hydrogen diffusion coefficient.

Electronic and optical properties of hot-wire-deposited microcrystalline silicon

Abstract We conducted a study on undoped and doped hydrogenated microcrystalline silicon ( μ c-Si) samples deposited by hot-wire chemical vapor deposition for determination of the structural, electronic and optical properties. Light scattering, investigated by total and specular transmission and reflection, as well as angular resolved measurements, results mainly from the surface but with an intrinsic contribution from the interior. The optical properties resemble that of monocrystalline Si: the refractive index in the visible, the reflectance peaks in the near ultraviolet which may only appear after surface polishing, and the absorption coefficient which is larger than in monocrystalline silicon and varies with sample thicknesses. Depending on the doping level, the dark conductivity prefactor and activation energy exhibit either normal or anti-Meyer–Neldel rule behavior. The mobility-lifetime product from steady state photoconductivity strongly depends on the position of the Fermi energy with a minimum for low p-type doping, suggesting the importance of information on the Fermi energy if the mobility-lifetime product is given as an indicator for material quality of microcrystalline Si.

Modulated and transient photoconductivity in a-As2 Se3 *

Modulated photocurrent phase shift measurements (MPC) have been used to probe the valence band tail density of states (DOS) in a-As2 Se3. In the absence of obvious structure, use of optical bias to define a ‘thermalisation energy limit’ allows MPC to be used to determine the attempt-to-escape frequency. Computer modelling of steady state and transient photoconductivity reveals inconsistencies in a charged defect interpretation.

Colour separation in a-Si:H based p-i-i-n sensors: temperature and intensity dependence

Abstract We report investigations of temperature and intensity dependencies of two p-i-i-n colour sensor devices of hydrogenated amorphous silicon and silicon carbide. They differ only in their band gaps of the front two layers causing an expected change in the detectable spectral range. Current/voltage measurements between 253 and 373 K and different photon fluxes (10 13 to 10 15 photons cm −2 s −1 ) for illumination in the red, green and blue spectral range show a dependence on both parameters. The photocurrent response for the red spectral range is more affected than for shorter wavelengths. Colour detection remains possible over the whole parameter range. Additionally, numerical modelling complements the study and gave insight into the internal physical processes.

Paramagnetic Defects in Undoped Microcrystalline Silicon Deposited by the Hot-Wire Technique

We report on a study of ESR and conductivity on a series of hot-wire CVD microcrystalline silicon samples prepared with different hydrogen dilution of silane. We observe two different types of dangling bond defects in ESR in different microscopic environments. One type of defect is located at outer surfaces accessible to oxygen and/or chemicals, the other is located at inner boundaries presumably at columnar structures. We correlate changes of the defect density induced by either annealing, exposure to air or wet-chemical treatment with the morphology and electronic properties of the films. We find that annealing at 200 °C induces irreversible changes in donor concentration as monitored by an ESR signal at g = 1.9981±3.

Time and frequency domain studies of photoconductivity in amorphous semiconductors

Abstract We present a general spectroscopic technique for the computation of the distribution of gap-states (DOS) in amorphous semiconductors from transient photocurrent decay (TPC). The technique assumes trap-limited and is otherwise model-independent. It is valid whether the TPC exhibits anomalous or conventional dispersion, and also works without modification for pre- and post-recombination regions of the decay. A numerical Fourier integral procedure is used to convert the TPC i ( t ) data to frequency domain spectra I ( ω ). The DOS is then computed using a procedure developed by the authors [1] for analysis of modulated photocurrent (MPC) data. The method avoids distortions and computational difficulties associated with other TPC analytical techniques. We report on the application of the method to experimental data on a-Si:H, demonstrating the wide energy range of states accessed, and highlighting the observation that the observed long-time power-law TPC decay, normally associated with a featureless exponential state distribution is consistent with structure in the DOS.

The operational principle of a new amorphous silicon based p-i-i-n color detector

The operational principle of a new type p-i-i-n color sensor is described with the aid of numerical modeling. The modeling results account for the color detection mechanism recently presented that this kind of structure exhibits [T. Neidlinger, M. B. Schubert, G. Schmid, and H. Brummack, in Amorphous Silicon Technology—1996, edited by E. A. Schiff et al. (Materials Research Society, Pittsburgh, 1996), p. 147]. By band gap engineering the experimental red response is maximized at larger reverse bias voltage whereas the green response has its maximum at low reverse bias voltage. The numerical modeling qualitatively reproduces the characteristic shape of the steady-state current-voltage curves at different illumination wavelengths. At low and at high reverse bias voltages the influence of the internal variables and parameters is identified and leads to the experimentally observed response. The potential profile of the p-i-i-n structure is of crucial importance to the color detection mechanism. At larger wave...

Characterization of high electronic quality a-SiC:H films by μτ products for electrons and holes

Intrinsic a-SiC:H films with optical bandgaps E g ≤ 2.0 eV have been optimized with respect to minimum PDS Urbach energy E 0 . We demonstrate that the increasing density of midgap defects induced by C incorporation controls AM1 photoconductivity at room temperature whereas the low temperature photoconductivity is governed by additionally generated conduction band tail states.

Electronic Properties of Hot-Wire Deposited Nanocrystalline Silicon

We compare the electronic properties of nanocrystalline silicon from hot-wire chemical vapor deposition in a high-vacuum and an ultra-high-vacuum deposition system, employing W and Ta as filament material. From the constant photocurrent method we identify a band gap around 1.15 eV while, in contrast, a Tauc plot from optical transmission data guides to a wide band gap above 1.9 eV. The sudden change-over from nanocrystalline to amorphous structure in a hydrogen dilution series is also find in the dark and photoconductivity measurements. The samples show a metastability effect in the dark conductivity upon annealing in vacuum with an increase in the dark conductivity, with the large dark conductivity decreasing slowly after the annealing cycle when the cryostat is flushed with air. We identify larger values for the mobility-lifetime products, which corresponds to the smaller defect density shoulder in constant photocur- rent spectra, for the ultra-high-vacuum deposited material compared to the high-vacuun counterpart.

Investigation of collection efficiencies much larger than unity in a-Si:H p-i-n structures

An apparent quantum efficiency much larger than unity is observed under reverse bias voltage conditions, in hydrogenated amorphous silicon p-i-n structures. High collection efficiencies are measured for low-level probe illumination of the device n side with red light, with simultaneous bias illumination from the device p side with strongly absorbed blue light. The photogating effect responsible varies experimentally with reverse bias voltage, and collection efficiencies for the probe excitation of up to 50 are obtained. Detailed computer simulations corroborate such high values of quantum efficiency and the underlying mechanisms for the effect are revealed. We present the influence on quantum efficiency of bias light wavelength and photon flux, probe light photon flux, applied voltage, and defect density.