Warren A. Turner

Studies on the structure of a-Ge: H using differential scanning calorimetry, gas evolution on heating and transmission electron microscopy

Abstract This paper contains a review of our recent studies on the medium-range order in glow-discharge-produced a-Ge: H using a combination of differential scanning calorimetry, isothermal calorimetry, gas evolution, and transmission and scanning electron microscopy. Taken together with evidence from Raman and infrared spectroscopy and X-ray diffraction, and evidence of changes in the structural parameters with carefully chosen annealing procedures, these data suggest correlations between deposition, structural and photoelectronic parameters and properties. The data show conclusively that the best material for fundamental and device studies has a high atomic density and a homogenous structure with minimal admixture of low-density tissue to high-density islands. In material prepared under less than optimum conditions, there occurs, to a greater or lesser degree, heterogeneity revealed directly by electron microscopy, and indirectly by structural relaxation at relatively low temperatures and even two stage...

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 ...

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.

Comparison of the structural properties of a-Si:H prepared from SiH4 and SiH4 + H2 plasmas, and correlation of the structure with the photoelectronic properties

Abstract The addition of hydrogen gas to silane gas during the deposition of high-quality a-Si: H has frequently been reported to improve the optical and electronic responses of the material. For our preparations of device-quality a-Si: H(: D) in an r.f. glow-discharge reactor from SiH4, SiH4 + H2, and SiH4 + D2, we find no significant differences between the optical and electronic properties of the as-deposited materials. However, structural examinations by gas evolution and high-resolution transmission electron microscopy (HRTEM) reveal marked dissimilarities, the gas evolution being an especially discriminating tool. Moreover, annealing at several temperatures, followed by a full complement of optical, electronic and HRTEM measurements, demonstrates differences in the optoelectronic properties that can be reasonably correlated with the structural differences.

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.

The Effect of Hydrogen Dilution of the Gas Plasma on Glow Discharge a-Ge:H

A series of films of amorphous hydrogenated germanium has been produced using the r.f. glow discharge technique by varying the H 2 /GeH 4 ratio in the gas plasma from 0 to 50 while keeping all other deposition parameters constant. Electronic, optical, and structural characterization has been performed. No significant changes in optical and electronic properties were observed for this range of dilution, in contrast to repeated reports on the hydrogenated silicon-germanium alloy system. However, small systematic changes in structure are observed using TEM techniques. We conclude that the observed differences are unimportant in determining the optical and electronic properties of this material.

Optimization of the Properties of Undoped a-Ge:H

High density, non-porous, highly photoconductive films of undoped a-Ge:H showing minimal microstructure have been prepared out of an r.f. glow discharge of GeH 4 on substrates mounted on the powered electrode of a diode reactor. These films, prepared under the systematic variation of substrate temperature, discharge power and dilution of the plasma by H 2 are markedly different from those produced on the unpowered electrode. An optimum level of applied power is found which does not differ significantly from that used to prepare state-of-the-art a-Si:H. For the reactor geometry used, dilution of the plasma with H 2 is found to be essential to the preparation of high quality a-Ge:H. These films contain similar amounts of bound hydrogen to that found in good a-Si:H, and also relatively large quantities of unbound hydrogen. Transport is activated with a Fermi level near mid-gap, the dangling bond spin density is 5 × 10 16 spins/cm 3 , and the samples exhibit photoluminescence. The electron μA has been measured using the TOF technique. The conditions for preparation of optimized material are quite different for those found to optimize the preparation of high quality a-Si:H in this reactor. This is asserted to be a principal cause for our finding, along with other laboratories, of inferior properties for a-SiGe:H alloys when compared with those of a-Si:H.

The effect of sample substrate on the structural properties of co-deposited a-Ge:H films☆

Abstract Gas evolution, differential scanning calorimetry, transmission electron microscopy, and Raman scattering have been used to give information about possible differences in the structure of films of a-Ge:H co-deposited on substrates of Corning 7059 glass, aluminum, crystalline Si, carbon coated mica, beryllium, and NaCl. The only appreciable differences in the above spectra were found for films deposited on aluminum foil, but these were very clear. For example, differential scanning calorimetry spectra yield a single exothermic peak near 520°C for films of a-Ge:H deposited on 7059 glass at 100 and 150°C, while co-deposited films on aluminum foil show two exothermic peaks at different temperatures. This, along with differences in the other spectra, indicate that the structure of the films on aluminum foil is different from that for the other films. For films deposited at higher temperatures the spectra are less substrate dependent, consistent with the occurrence of smaller structural/chemical variations on a nanometer scale.

Molecular HD and D2 in amorphous semiconductors

Deuteron magnetic resonance (DMR) spectroscopy resolves several distinct molecular HD and D2 species present in plasma-deposited deuterated a-Si and a-Ge films. Newly-discovered deuteron multiple spin echoes uniquely identify immobile HD and ortho-D2 molecules. Differentiable components of a 76 kHz doublet DMR line shape arise from para-D2 in the presence of lage static electric field gradients. Measurement of para to ortho conversion of D2 at 4.2 K provides further resolution of the microvoid D2 configurations. The microvoid HD and D2 population is inversely related to sample quality and ranges down to <0.02 at.% in high quality samples.

Deuteron magnetic resonance in amorphous germanium

Abstract Deuteron magnetic resonance (DMR) studies of plasma-deposited a-Ge:D,H have been made at 46 MHz. Resonance line shapes indicate that 62% of the contained 1.3 at.% deuterium is in the form of molecular D 2 and HD in microvoids. No germanyl rotors are observed, in spite of the large microvoid volume. Temperature variations of spin relaxation times are compared with those for similarly-prepared high quality a-Si:D,H and with an earlier a-Ge:D,H sample. Below 50 K a characteristic 76 kHz doublet identifies para-D 2 and some features of microvoid electric field gradients and geometry.