M. Vaněček

Density of the gap states in undoped and doped glow discharge a-Si:H

Abstract The density of the gap states in undoped and phosphorus and antimony doped a-Si:H has been determined from the absorption coefficient measurement. A constant photocurrent method was used to determine the value of optical absorption in the low absorption region. A quantitative model has been suggested to explain all experimental data. The model is based on a Gaussian shaped maximum, connected with the dangling bond, within the exponential band tails just below the middle of the gap. Moderate and strong coping rises the concentration of dangling bonds.

Direct measurement of the gap states and band tail absorption by constant photocurrent method in amorphous silicon

A direct way to obtain the spectra dependence of the optical-absorption coefficient in the low-absorption region (10−1−103cm−1) on thin amorphous silicon films is presented. The constant photocurrent method (CPM) is described compared with normalized photoconductivity spectra measurement and the results on a-Si:H are presented. Implications of the CPM for the gap states spectroscopy are suggested.

Determination of the mobility gap in amorphous silicon from a low temperature photoconductivity measurement

Abstract Measurement of the spectral dependence of photoconductivity and quantum efficiency at liquid helium temperature is presented. The mobility gap value was found to be quite close to the Tauc optical gap value for undoped good quality material.

Photoelectrical and optical study of p-type a-Si:H used as a photoreceptor

Abstract Deconvolution of the optical absorption coefficient α(E) with the help of the photothermopower measurement is used to derive density of states in boron doped p-type a-Si:H. Deep gap level 1 eV above the valence band with halfwidth 0.2 eV is detected.

Steady-state transport under non-equilibrium conditions in undoped and phosphorus doped a-Si:H at very low temperature

Photocurrent spectra normalized per number of photons absorbed have been measured at liquid helium temperature on samples of amorphous hydrogenated silicon (a-Si:H) with different level of phosphorus doping. Results have been compared with the measurement on undoped samples. The measurements show that for a device quality a-Si:H the transport path does not change with phosphorus doping except for a high doping level.

Temperature modulated-SCLC, used for the study of the light induced, metastable density of states of a-Si:H

Abstract The density of states has been measured for undoped a-Si:H by means of a temperature modulated space-charge-limited currents (TM-SCLC) method in an annealed state A and after prolonged illumination - state B. With increasing illumination time the peak in the DOS at E-E C ∻ 0.61 eV grows and we attribute this to the D− Si dangling bond level. From the difference in the position of this D− level and the stable Fermi level in state B ( E F − E C ∻ 0.79 eV ) we deduce the effective correlation energy of Si dangling bond in undoped a-Si:H to be Ueff = +0.36 ± 0.03 eV.

Photocarrier collection efficiency in amorphous silicon down to liquid helium temperature

Abstract Carrier collection efficiency has been measured from the room temperature down to 15 K as a function of applied electric field (up to 2.5 × 10 5 V cm −1 ) at different excitation wavelengths (450 to 720 nm). More than 50% of photogenerated carriers can be collected at an electric field of approximately 1 × 10 5 V cm −1 even at very low temperature (15 K).

Studies of the Gap States Density in Undoped and Doped Amorphous Hydrogenated Silicon

Density of the gap states in undoped and phosphorus and antimony doped n-type a-Si:H has been determined from the absorption coefficient measurement. A constant photocurrent method was used to determine the value of optical absorption in a low absorption region. A quantitative model has been suggested to determine the density of states (DOS) within a gap below the Fermi level from the spectral dependence of the absorption coefficient. The model is based on a Gaussian shaped maximum, connected with the dangling bonds, between the exponential valence and conduction band tails. The doping rises this maximum in the DOS.

Initial growth region of the glow discharge a-Si: H

Abstract The time dependence of the optical emission in SiH 414 nm band and of the temperature of silane gas were measured after the switching on of the r.f. glow discharge. The results emphasize the influence of plasma stabilization on the initial growth region of amorphous silicon. This 60–300 A thick layer has a higher concentration of gap states.

Photoconductivity detected S‐W related defect levels in a‐Si:H

A constant photocurrent method was used to study the Staebler‐Wronski effect on a‐Si:H samples with a coplanar configuration of electrodes on Schottky diodes. Results on the coplanar samples are partly influenced by surface (interface) band bending but both samples show a higher value of optical absorption coefficient α in an absorption shoulder after prolonged illumination. Some experimental details of the method are discussed. A model for the density of states in p‐type a‐Si