P. Wickboldt

Structural, optical, and electrical characterization of improved amorphous hydrogenated germanium

High‐quality amorphous hydrogenated germanium has been deposited using the diode rf glow discharge method out of a gas plasma of GeH4 and H2. The optical, electrical, and structural properties of this material have been extensively characterized. The optical and electrical properties are all consistent with material containing a low density of defect related states in the energy gap. In particular, this material has an ημτ=3.2×10−7 cm2/V, ratio of photocurrent to dark current of 1.3×10−1, and flux dependence of the photocurrent with γ=0.79 at 1.25 eV measured using photoconductivity, a μτ=4×10−8 cm2/V measured using time of flight, an Urbach energy of 51 meV and α at 0.7 eV of 8.3 cm−1 measured using photothermal deflection spectroscopy, a dangling bond spin density of 5×1016 cm−3 measured using electron spin resonance, photoluminescence with a peak energy position of 0.81 eV and full width at half maximum of 0.19 eV, an activation energy of 0.52 eV and σ0 of 6.1×103 (Ω cm)−1 measured using dark conductiv...

HIGH PERFORMANCE GLOW DISCHARGE A-SI1-XGEX:H OF LARGE X

Radio frequency glow discharge chemical vapor deposition has been used to deposit thin films of a-Si1−xGex:H which possess optoelectronic properties that are greatly improved over any yet reported in the range of x⩾0.6. These films were deposited on the cathode (cathodic deposition) of an rf discharge. Their properties are assessed using a large variety of measurements and by comparison to the properties of alloys conventionally prepared on the anode (anodic deposition). Steady state photoconductivity measurements yield a quantum-efficiency-mobility-lifetime product, ημτ, of (1–3)×10−7 cm2 V−1 for 1.00⩾x⩾0.75 and (6–10)×10−8 cm2 V−1 for 0.75⩾x⩾0.50, and photocarrier grating measurements yield ambipolar diffusion lengths several times greater than previously obtained for alloys of large x. It is confirmed that the improvements in phototransport are not due to a shift in the Fermi level. In fact, results of recent measurements on lightly doped samples strongly suggest that for these cathodic alloys neither ...

Stress and thermomechanical properties of amorphous hydrogenated germanium thin films deposited by glow discharge

Stress measurements of a-Ge:H thin films deposited by rf glow discharge using a large variety of deposition conditions are reported. It was observed that the stress of the films is strongly related to their structure. Tensile films are usually porous or have many defects, while compressive films are usually homogeneous. High quality films are always compressive. A strong correlation of the stress in the films with the unbonded hydrogen concentration was observed, which may explain the origin of the compressive stress. There was no systematic or consistent link between the stress and the bonded hydrogen content or the deposition rate. The thermal expansion coefficient and the elastic constant were determined for high quality films.

A study of the properties of hydrogenated amorphous germanium produced by r.f. glow discharge as the electrode gap is varied the link between microstructure and optoelectronic properties

Abstract Hydrogenated amorphous germanium films were deposited by r.f. glow discharge on the powered electrode of a diode-type deposition system. By varying the electrode gap D and keeping all other deposition parameters constant, a continuous monotonic change in both the optoelectronc and the structural properties of the films was observed. As D was decreased from 3.2 to 1.2 cm the product nμτ of the quantum efficiency n, mobility μ and lifetime τ increased from 2.1 × 10−9 to 1.7 × 10−7cm2 V−1, and the photoluminescence intensity increased. Other measurements taken indicate that changes in the electronic density of states cannot account for the observed improvement in the phototransport. Rather, we implicitly link this improvement to the observed changes in film structure. As D was decreased, the structure varied continuously from being heterogeneous of the order of 200 A and porous, to being homogeneous and non-porous. This change in structure was inferred from measurements of film stress, transmission ...

The mott lecture. Structural and electronic properties of amorphous SiGe:H alloys

It is universally found that the photoelectronic properties of a-Ge:H and a-Si 1-x G x :H alloys are much poorer than those of a-Si:H. For example, the efficiency-mobility-lifetime products from steady-state photoconductivity measurements and the mobility-lifetime products from time-of-flight data decrease by about a factor of 100 between a-Si:H and a 50% Ge alloy. Estimates of the gap density-of-states from spin resonance or optical absorption, however, indicate increases of only an order of magnitude, and measurements of the Urbach tail parameter show essentially no change throughout the alloy series. There would appear to be a growing consensus that these results are reconciled by a model of a rapidly decreasing electron mobility with alloying

Structural properties of amorphous hydrogenated germanium

The medium-range order on a scale of 100 A is an important determinant of the properties of amorphous semiconductor films such as a-Ge:H: if the material has a high-density, closed structure, it is likely to have good phototransport properties and to be environmentally stable; if, on the other hand, it has an open network structure, with many voids and wide grain boundaries, it is very likely to be a poor photoelectronic material and to have properties changing with time because of atmospheric contamination. Two types of film with these extreme, contrasting structural attributes, prepared under different conditions in an rf PECVD reactor with parallel-plate electrodes of different area, have been compared. The measurements reported are primarily structural: infrared vibrational absorption, gas evolution versus temperature, transmission and scanning electron microscopy, differential scanning calorimetry, deuteron magnetic resonance and forward recoil spectroscopy. An encompassing, self-consistent explanation of the results of these measurements in terms of the different constitution of the films is proposed. Several specific problems are addressed: the identification of the features in the infrared spectra, the influence of O on the spectra and on gas evolution, and the presence of H 2 molecules in the material.

Room‐temperature 1.5 μm luminescence of co‐deposited erbium and germanium

Nanocrystalline germanium films containing erbium were deposited by thermal evaporation under 0.6 Torr of argon onto crystalline silicon wafer substrates. Weak broad photoluminescence (PL) around 1.5 μm was observed at room temperature. Annealing under 10−7 Torr of vacuum for 3 h at 500 °C produced no change in the PL spectrum. After 1 h oxidation in air at 500 °C the PL intensity increased by an order of magnitude with reduction of the spectral linewidth and appearance of distinct structures, a portion of which is similar to that observed for Er‐implanted Si:O. Subsequent increase in oxidation time reduced the PL intensity slightly with no change in the spectral shape. The PL intensity exhibits a sublinear increase with pump power and approaches saturation at 200 mW. Raman spectra before and after anneal are also presented. Annealing increased the average grain size from 5 to 10 nm. The PL spectrum of erbium metal after oxidation in air at 500 °C is quite different from that of these oxidized Ge:Er films.

Visible room-temperature photoluminescence from oxidized germanium

Abstract Strong visible photoluminescence (PL) has been observed at room temperature from oxidized amorphous hydrogenated germanium. Excitation by the 488 nm line from an argon ion laser yields a broad PL peak at 2.2 eV with a full-width-half-maximum (FWHM) of 0.5 eV. The luminescence intensity depends on the morphology of the untreated a-Ge:H film produced by plasma enhanced chemical vapor deposition (PECVD). Films ranging from ones with coarse columnar microstructure and poor photoelectronic properties to ones with microstructure similar to that of device-quality a-Si:H were intentionally chosen. After oxidation in air at 500°C for one hour, the films with coarse microstructure yield the highest luminescence intensity, comparable to that from porous silicon, while the best films with minimum microstructure yield orders of magnitude lower intensity. Removing the germanium oxide from the film reduces the luminescence intensity. Powder-like Ge was also produced by thermal evaporation in 0.3 Torr of argon. After annealing in air at 500°C for two hours strong PL is observed with peak position and spectral width similar to those from oxidized poor PECVD a-Ge:H and commercial GeO2.

Stress measurement of glow discharge produced a-Ge:H thin films and its relation to electronic and structural properties

Stress measurements are reported for thin films of a-Ge:H deposited under a large variety of conditions using glow discharge CVD. The stress is nonuniform with thickness, and, for films exhibiting high tensile stress, changes reversibly due to atmospheric contamination. Strong correlations are observed for stress with hydrogen content, structure, and photoresponse. In particular, higher photoresponse is observed for films with a higher compressive stress.

Plasma deposited non-stoichiometric hydrogenated germanium sulfide a-Ge1−xSx:H (x < 0.3)

Abstract Hydrogenated amorphous germanium-sulfur alloys were prepared by PCVD from H 2 , GeH 4 and SF 6 . Addition of sulfur reduces the hydrogen concentration in the films by a factor of two and decreases the activation energy of the conductivity from 0.5 eV for a-Ge:H to 0.14 eV and is related to n-type doping. Sulfur incorporation significantly improves the quality of films prepared at the unpowered electrode: the ημτ-product increases by three orders of magnitude to 10 −6 cm 2 V −1 , the tensile stress is reduced and there is no significant post-oxidation under ambient pressure. The IR-absorption band around 360 cm −1 related to GeS bond stretching vibrations, increases proportional to sulfur concentration. Radial distribution functions show a continuous reconstruction of the Ge network with sulfur.