Detailed structural investigations have been performed on a-Si1−xGex:H in 0 ≤ x ≤ l namely Raman, X-ray Photoelectron, and Photothermal Deflection Spectroscopies, which reveal the silicon sub-matrix to be considerably more rigid than the germanium one. In Si-Ge dominated networks distinct deviations from an overall random distribution of Si and Ge atoms can be deduced from a semi-quantitative analysis of the Raman data.

Utilizing the high optical absorption of amorphous silicon in conjunction with advanced signal processing capabilities of modern ASICs offers the possibility of simply realizing smart color sensors with random pixel access and without any losses in active area due to the signal processing circuitry. Thin-film photodiodes have to be incorporated into a layer sequence of ASIC/insulator/metal/a-Si:H based multilayer/TCO for this so-called Thin Film on ASIC (TFA) concept[l]. Amorphous silicon and silicon carbide photodiodes have been studied, and discrimination of the fundamental colors (red, green and blue) has been obtained from 2-terminal n-i-p-i-n devices at small bias voltages within ±2 V. Changing of the bias voltage applied to the two back-to-back p-i-n junctions allows for adjusting the location of predominant photo-carrier collection, and therefore enables color readout according to the energy dependent absorption profile of the incident light. Film thickness and optical bandgap of the individual layers have been optimized to achieve optimum color sensitivity. Switching experiments in the voltage mode give some information about conceivable frame rates these TFA sensors could be operated at. Light-induced degradation does not hamper the performance of the sensors thanks to low light intensity, reverse bias conditions, and small i-layer thickness.

We present an amorphous silicon-based n-i-p-i-n three-color sensor with a layer sequence of substrate/metal/n-i-p-i-n/transparent contact. The color sensitivity (red, green and blue) is realized by applying small bias voltages within /spl plusmn/2 V. For the first time, this structure offers the possibility to integrate a color sensor on top of an ASIC chip, where pixel-based signal processing can be performed. Film thickness and optical band gap of the individual layers have been optimized to achieve maximum color separation. The surface of the metal-back contact is found to be critical for the performance of the sensor. A rough surface of the metal is responsible for short circuits at the common p-contact of the back-to-back p-i-n junction diodes. The steady-state I-V characteristics of the dark and photo currents have been studied. The dynamic range of the sensor is already suitable for optoelectronic applications. Sensor performance is not affected by metastable effects after prolonged light soaking. Speed limitations have been evaluated from switching experiments in the voltage mode. >

Fast transient photocurrent measurements have been performed on a-Si:H based pin diodes with thicknesses down to 0.5 μm. On a thick sample drift mobilities derived in either the small-signal or the space-charge limited current mode are compared. On real solar cell devices in SCL voltage (integral) Mode the electron lifetimes τ are determined from the extraction time of the photo-generated charge in dependence on the degradation state of the sample. A clear and reproducible correlation between lifetime and degradation state has experimentally been established. Lifetimes τ, for which an intensity dependence has been observed in the SCL voltage Mode, are compared to the μτ-product from a small-signal charge collection Measurement. The influence of the real absorption profile on the measurements is discussed.

Reversible changes of σd and σph in a-Ge:H after light degradation and thermal quenching have been investigated. Quantitatively these two methods of defect generation cause very different changes of σ: Light degradation effects are dependent on light intensity. High intensity decreases σph by about 10%. Fast thermal quenching lowers σd and σph by a factor of 2–3. Qualitatively the two different ways of degradation yield similar defect kinetics which can be described by a stretched exponential. This implies common microsocopic defect reactions. The equilibrium temperature as well as the relaxation time is strongly dependent on hydrogen content. Combining IR spectroscopy, which gives information about hydrogen bonding configuration, and thermal quenching experiments indicates that mainly H on inner surfaces influence metastable effects in a-Ge:H.

Infrared (ir) spectroscopy is used to investigate the structural properties of a-SiC:H in a wide compositional range and as a function of film thickness. Hydrogen content NH increases considerably with increasing carbon fraction. For low carbon alloys this is mainly due to an increase of hydrogen bonded to silicon, incorporated in a mono- or dihydride form. Above Eg=2.3eV the proportion of hydrogen incorporated in C-H bonds increases considerably. Oxidation of high C alloys is observed. Converting experimental transmission exactly into absorption data yields thickness independent NH values. It is shown that the previously reported discrepancy between the hydrogen content calculated from ir and nuclear reaction techniques is an artifact of the ir analysis.

a-Ge:H films were prepared by conventional glow discharge, remote plasma chemical vapour deposition, a novel Ar- or H2-jet technique and photo-CVD using Xe-excimer radiation. Structural properties were characterized by optical absorption (far-IR to vis.), Raman scattering, and photothermal deflection spectroscopy. Optoelectronic properties were investigated by measuring dark conductivity σd(T) and the mobility lifetime product ημτ was calculated from photoconductivity measurements. The results are discussed comparatively in terms of the influence of deposition parameters on film properties.

A thorough investigation of plasma-CVD amorphous hydrogenated boron (a-B:H) has been conducted with the main emphasis on how in this amorphous semiconductor the variation of the coordination number in comparison to a-Si:H influences the structural disorder and density of states in the band tails. a-B:H was deposited from different concentrations of B2H6 diluted in H2 by both DC- and RF-plasma-CVD. The influence of the change of substrate temperature, pressure, flow and deposition power on the structural, optical and electronic properties of the material was examined. Raman-scattering and IR-absorption reveal the clear non-crystallinity of the deposited films. Almost all samples show some photoconductivity with a σphoto/σdark ratio from 10−1 to 3x101. Although a strong influence of some of the deposition parameters on bandgap and refractive index, hydrogen content, dark and photoconductivity was observed, the density of states in the band tails as measured by PDS was relatively high, showing always a rather flat Urbach slope of about 180 - 220 meV. An explanation for this unexpected almost uniform huge Urbach slope might be that even in the amorphous state the behaviour of boron is still dominated by its electron deficiency character which leads to a certain amount of three centre bonds (as seen in IR-absorption) and results in comparison to amorphous hydrogenated silicon in even stronger constraints of the amorphous network and obviously in more potential fluctuations.

The effusion of hydrogen from a-Si:H, a-SiGe:H and a-Ge:H was studied in-situ by infrared spectroscopy. This allows monitoring of Si-H (Ge-H) bond breaking in different configurations. Additional information is obtained from changes in refractive index and from the wavelength shift of absorption bands upon tempering. Already before the onset on H-effusion changes in hydrogen binding are observed. During effusion film density increases as a result of crosslinking in voids. Information on bond angle distribution and crystallisation is obtained from Raman measurements.

Reversible changes of the electrical properties in undoped and doped a-Ge:H by thermal quenching and subsequent annealing have been investigated. For undoped a-Ge:H the dark conductivity sigma(d) shows two regimes, separated by the equilibration temperature T(E) of about 170-degrees-C. Above this temperature sigma(d) becomes independent of thermal history, indicating that the structure is in thermal equilibrium and a unique function of T. Below 170-degrees-C sigma(d) exhibits a pronounced downward kink in the Arrhenius plot and becomes dependent on the cooling rate. Relaxation behaviour can be described with a stretched exponential and the time constant for equilibration tau(E) follows a thermal activated form with an activation energy of 1.1eV. Heavily boron-doped films show the opposite effect, namely an increased conductivity after fast quenching.

A series of boron doped a-Si:H films have been characterized by PDS, FTIR, Raman, and SIMS in order to evaluate the effects of boron incorporation on structural properties and hydrogen bonding. Doping by B2H6 or B(CH3)3 does not significantly enhance the overall disorder of the silicon network showing up in the TO-like Raman halfwidth whereas remarkable changes in local, defect related structures are evident from PDS. An analysis of the data suggests two bands of defects in the pseudogap at low boron concentration and only one band for higher concentration. To account for Fermi level positions, shifts of the hole transport path well into the valence band tail upon doping must be invoked.

Amorphous hydrogenated germanium films have been deposited in a conventional glow discharge reactor (diode as well as triode configuration) from GeH4 with and without additional H2 dilution. In this study special emphasis was put on samples deposited at different substrate temperatures, different neutral gas densities and dilution ratios. The deposition rate has been varied from 0.2 to 2.5 Å/s. Optical, structural and electronic properties have been studied by VIS/NIR thin film spectroscopy, PDS, Raman backscattering, FTIR, as well as temperature dependent dark and photoconductivity. Hydrogen content was monitored by quantitative SIMS analysis and FTIR spectroscopy. p]By ’soft’ deposition, i.e. high pressure and H2 dilution ratio, Fermi level positions near midgap (Eact =0.48eV at Eg=1.0ev) are achieved. Nevertheless the optimum ratio σph/σd is restricted to 0.6 at room temperature (λ=632.8nm). To point out the influence of Fermi level position for transport properties, we investigated two samples with different Fermi level positions in detail. σd(T) is strongly dependent on Fermi level positions, as well as the absolute values of σph(T), whereas the temperature dependence of σph(T) does not differ greatly. This is an indication, that Fermi level position and recombination centers of a-Ge:H can be effectively influenced by changing deposition parameters.

In order to evaluate structural implications of different deposition techniques (namely standard diode vs. triode+H2-dilution) for a-SiGe:H growth and performance we report on Raman studies, optical and PDS data. Ge based structural features turn out to be unaffected by ‘soft’ deposition whereas significant relaxation of the silicon matrix could be observed.