E. Šípek

A-Si:H drift mobility - study of isotropy

Abstract The systematic difference has been observed between (μτ)cc and (μτ)ss - the mobility-lifetime product found in a-Si:H from TOF charge collection or steady state photoconductivity. Its recently suggested explanation (μτ-anisotropy) is briefly discussed. The new TOF measurements made under clear experimental conditions on unique a-Si:H samples in both “perpendicular” and “planar” directions convincingly show that a-Si:H is isotropic material with μ⊥ = μ∥. Even (μτ)⊥t (μτ)∥ when the same method (TOF change collection) is used.

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.

Initial stage of growth of the GD a-Si:H

Abstract The time dependences of the optical emission intensities of SiH,H and He lines and also of the gas temperature have been studied. It is demonstrated that 10–100 sec are necessary for full stabilization of plasma. This can explain the different properties of the initial 30–300 A layer of a-Si:H.

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).

Impact of AFM-induced nano-pits in a-Si:H films on silicon crystal growth

Conductive tips in atomic force microscopy (AFM) can be used to localize field-enhanced metal-induced solid-phase crystallization (FE-MISPC) of amorphous silicon (a-Si:H) at room temperature down to nanoscale dimensions. In this article, the authors show that such local modifications can be used to selectively induce further localized growth of silicon nanocrystals. First, a-Si:H films by plasma-enhanced chemical vapor deposition on nickel/glass substrates are prepared. After the FE-MISPC process, yielding both conductive and non-conductive nano-pits in the films, the second silicon layer at the boundary condition of amorphous and microcrystalline growth is deposited. Comparing AFM morphology and current-sensing AFM data on the first and second layers, it is observed that the second deposition changes the morphology and increases the local conductivity of FE-MISPC-induced pits by up to an order of magnitude irrespective of their prior conductivity. This is attributed to the silicon nanocrystals (<100 nm) that tend to nucleate and grow inside the pits. This is also supported by micro-Raman spectroscopy.

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.

Phototransport in a-Si:H at high electric fields

Time-of-flight (TOF) measurements on a-Si:H samples with different thicknesses (1.7–18 μm) have revealed that the drift mobility, μD, strongly increases and its activation energy, Eμ, decreases for electric fields F > 1 × 105 V/cm. This is partially related with a new transport mechanism and it requires a modification of standard equations used for finding of density of states (DOS). The means of spearating the field dependencies of the quantum efficiency, η, and the μτ product by measurements on an 18 μm self-supporting a-Si:H sample have been outlined. At 40 K, almost full charge collection was reached and η ≈ 1 was found. At an extremely high electric field (up to 4.5 × 105 V/cm), the μD under space-charge limited current (SCLC) conditions and also in a ‘small-signal’ case were deduced. Both values are in reasonable agreement. The observed drift mobility is independent of light intensity and continuously decreases with decreasing temperature; no increase of μD at T

Amorphous silicon vidicon target

The properties of glow discharge a-Si:H vidicon targets were investigated as a function of doping of the photoconductive layer and composition of the blocking layers. For 1 +5 ppm B2H6 doping rapid photoresponse (photoconductive lag less then 3% after 50 ms) and 100% quantum efficiency through the visible region were achieved. The decay lag of vidicon tube was mainly caused by properties of an electron beam scanning system (total lag ~ 5% after 50 ms).

Spectroscopic Investigation of Deep States in Amorphous Silicon - Changes with Doping and Applied Stress

Staebler-Wronski effect (SWE) is investigated under external strain, applied by bending thin glass substrate. Strong enhancement of increase of subgap optical absorption below 1.4 eV is observed when light soaking is done under high external strain. No general correlation in change of subgap absorption and change in mobility-lifetime product is found. Results are interpreted in terms of two mechanisms of SWE: rise in dangling bond density and change in a charge state of defects. This second mechanism is explained by breaking of impurity-dangling bond intimate pairs.