The properties of a-(Si, Ge):H, F alloys prepared by glow discharge deposition from SiH4 and GeF4 are described. The measured IR absorption spectra, sub-gap absorption spectra, dark conduction activation energies, carrier drift mobilities and deep trapping lifetimes of these alloys are similar to those of alloys prepared from SiF4, GeF4, and H2. However, they have over an order of magnitude lower photoconductivity over most of the composition range. Infrared absorption measurements indicate that these alloys have a fluorine content less than 1.5 at.% and a Si-H2 content that increases with germanium concentration.

We report precise measurements of the fluorine content cF in amorphous silicongermanium alloy films (a-Si, Ge:H, F). The films were prepared by r.f. or d.c. glow discharge deposition. The fluorine concentration was measured by elastic scattering (NES) of 12 MeV protons and by IR absorption spectroscopy. Films measured by NES serve as calibration standards for other measurement techniques. Using the NES measurements, we update the published calibration coefficient for IR measurements for cF ranging from 0 to 10 at.%. The annealing properties up to 700°C of a-Si, Ge:H, F were studied by the gas evolution technique and by IR spectroscopy. Above 300°C the Si-F absorption peak decreases while the peak due to SiF4 molecules increases. No significant loss of F from the alloy films seems to occur up to 700°C.

Deuterated and fluorinated amorphous silicon-germanium alloys, a-Si, Ge:D, F, were studied by Fourier transform infrared (IR) spectroscopy. No Ge.-F modes are observed. The intensity of the Si-F and Si-F2 modes increases with Ge concentration. So does thae intensity of SiF4 which is trapped as isolated molecules. No DF (IR) or F2 (Raman) is observed. The IR spectra of alloys annealed at 300, 400, 500 and 600° C show that the fluorine in the Si-F and Si-F2 groups and in the SiF4 molecules is in thermochemical equilibrium.

We report measurements of internal photoemission from Ni, Au, and Pd contacts into a-Si, Ge:H, F alloys. The alloys were prepared by d.c. glow discharge decomposition of either SiF4 or SiH4 and GeF4, and H2. The sharp exponential drop in subgap absorption in these alloys, measured by the Constant Photocurrent Method (CPM), allows the determination of barrier heights using internal photoemission thresholds. The barrier heights of Ni, Au and Pd contacts are presented as a function of alloy composition. We find Ni has the lowest barrier heights while Au shows the highest barrier heights over the entire range of Eopt. We also find that for the Ni and Au contacts, ΦB varies as 1/2 the optical gap. In the case of Pd, ΦB shows a dependence of 1/3 the optical gap. We observed an increase in ΦB for Pd contacts when etched with a diluted HF solution prior to metallization. A similar increase in ΦD was not observed for the Au and Ni contacts.

Microcrystalline inclusions in hydrogenated and fluorinated amorphous silicongermanium alloys, α-Si, Ge:H, F, were studied. Microcrystals grown during RF or DC glow discharge deposition from SiF4, GeF4 and H2 consist of either pure Si or Ge. Microcrystals produced by thermal annealing of initially amorphous alloys are either microcrystalline Ge or microcrystalline Si-Ge alloys depending on the annealing temperature. Values for the grain size were calculated from X-ray diffraction (XRD) and Raman spectra. The grain size of the “grown” microcrystal ranges from 8 to 60 nm. These grown grains exhibit preferential orientation of the (220) planes parallel to the substrate surface. Microcrystals produced by high-temperature anneal are randomly oriented. Scanning electron micrographs of as-grown samples show protruding platelets several 100 nm long and several 10 nm wide. Fractured cross sections exhibit columnar structure.

Deuteron magnetic resonance line shapes and spin lattice relaxation times are presented for a-Si:D, F and a-SiGe:D, F. These parameters differ from those for typical a-Si:D, H samples, but in some respects are similar to those for an annealed a-Si:D, H sample. The a-SiGe:D, F spectra display an unusually large broad central weakly bound D resonance component and a barely-resolved Ge-D quadrupolar doublet. Comparisons indicate substantial differences in void morphology between the a-Si:D, F and a-SiGe:D, F.

Properties of a-Si1-xGex:H films prepared by magnetron sputtering (MG) and glow discharge decomposition (GD) were compared by means of NMR, ESR, IR and hydrogen-evolution measurements. For MG films, the content of dispersed H is roughly independent of x while the content of clustered H decreases with x. For GD films, both the contents of dispersed and clustered H decrease with x. ESR results reveal that most defects in the films are Ge dangling bonds and that the number of dangling bonds per Ge atom is roughly independent of x for MG films whereas it increases largely with x for GD films. Therefore the content of dispersed H has a good correlation with the number of Ge dangling bonds per Ge atom in a-Si1-xGex:H films. The ratio of the intensity of the IR peak at 2100 cm-1 to that at 2000 cm-1 decreases and increases with x, respectively, for MG and GD films, and the ratio of the intensity of the low temperature H evolution peak to that of the high temperature H evolution peak decreases and increases with x, respectively for MG and GD films.

Hydrogenated amorphous silicon-carbon alloy films were plasma-deposited from metnane and silane, varying gas ratio, R.F. power and substrate temperature. Carbon addition increases the optical gap, but also raises the dangling bond density while decreasing conductivity. Low C alloys can be gas-phase doped both p and n type. In the IR spectra the various Si-C stretching modes observed between 650 and 780 cm-1 are explained by back bonding variations. A tentative method of assigning this shift to back bonding of C to the Si is given. A distribution of modes is observed for all alloys, with each mode appearing even at 2% C. The distribution is sensitive to substrate temperature, but is stable after vacuum annealing to 400°C.

Amorphous hydrogenated silicon carbide alloy films (a-SiC:H) were produced by the decomposition of methane and silane in a glow discharge deposition system. A deposition rate of 13 Å/sec was achieved for good quality films. Amorphous SiC:H films of p+ type and n+ type of a band gap of 1.86 eV and 1.8 eV and the activation energy of 0.4 eV and 0.23 eV, respectively were obtained. Results show that p+ type and n+ type a-SiC:H films can be good window layers and good diffusion barriers for indium in polyimide/metal/n-i-p(p-i-n)/ITO amorphous silicon solar cells.

Doping of a-Si:H and a-SiC:H has been performed by plasma enhanced boron migration from thin (5–30 nm) Si0.2B0.8 layers on top of the substrate into the growing i-film. Transport coefficients for this plasma assisted diffusion (PAD) evaluated from SIMS profiles (D* = 10−15 … 10 cm2/s for a-Si:H, 10−15 cm2/s for a:SiC:H) by far exceed those for thermal diffusion after film deposition (10−21 cm2/s). Boron profiles are strongly governed by deposition parameters like substrate temperature, types and energies of radicals growing the film. Boron migration can be modelled assuming particle extraction from the network to the surface by bombardment of ions and atoms and high surface mobility of BHx radicals as a result from interaction with the plasma. Temperature dependent dark conductivity measurements show doping efficiencies comparable to B2H6 gas phase doping, but leaving plasma parameters optimized for i-film Leposition unchanged. Thin a-SiC:H layers are effectively p+-doped without affecting transmission, as no layer is optically detectable after PAD.

We present the results of a spectroscopic study of amorphous hydrogenated silicon nitride. The nitride was prepared by plasma enhanced chemical vapor deposition and was studied with infrared absorption (IR) and solid-state nuclear magnetic resonance (NMR) spectroscopies. This nitride film shows a particularly interesting distribution of hydrogen: hydrogen atoms are nearly exclusively bound to nitrogen.

The presence of selenium (Se) or Telurium (Te) in amorphous silicon films seems to improve the quality of these films by reducing the number of unsatisfied or dangling bonds and changing the optical gap and thermal activation energy. The thermal activation energies of a-Si:Sex and a-Si:Tex were always larger than that of a-Si films. The measured optical gap can be tailored to some value between 0.8 eV and 1.8 eV depending on the use of Se or Te and on the relative concentrations. The presence of Se or Te in a-Si films seems to have the same effect as that of hydrogen in a-Si films prepared by glow discharge of silane and used in solar cells.

The structural, compositional and electronic properties of the a-Si:H/a-SiNx:H interface are reported. High resolution TEM and light/dark ESR studies conclude that the interface has a finite width of the order of 10 Å. In addition, there is a high density of charges residing near the interface. Band bendings occur in both a-Si:H and a-SiNx:H, resulting in few neutral dangling bonds in the a-Si:H/a-SiNx:H multilayer structure. The depletion width in the nitride is of the order of 100 Å. The slow decay of the LESR in multilayers with thin sublayers is attributed to charges trapped in the slow states in the nitride.

Using in situ ellipsometry with a 3.4 eV probe, we have characterized the initial nucleation and interface structure of a-Si:H and μc-Si deposited on oxide/c-Si substrates. Multilayered a-Si:H/a-SiNx:H structures have also been studied and non-uniformities have been modeled. In none of the reported data could we model thin film growth with thickness independent optical functions. Once a film becomes opaque, ellipsometry is very sensitive to changes in the thickness of surface roughness layers. As a result, roughness on single and multilayered a-Si:H films has been studied as well as the renucleation that results from stopping and then restarting the a-Si:H growth plasma.

Transient photoconductivity is used to investigate the origin of slow states near the interface of a-Si:H and silicon nitride. A graded composition of the nitride layer near the interface greatly increases the density of slow states. We deduce that slow states are bulk nitride traps and that the magnitude of charge storage is largely determined by the composition dependence of the localization radius of electrons within these traps. The kinetics of charge storage and release are found to be very different and are interpreted in terms of an activation step at the interface.

The influence of the a-SiNx:H composition on the near-interface properties of the a-SiNx:H/a-Si:H layered structure was studied by photoluminescence measurements and the results were compared with a-Si TFT characteristics. It was found that the cw photoluminescence spectra of a-Si:H near the interface tend to shift to the low-energy side with an increase in nitrogen content of the a-SiNx:H layer. Time-resolved photoluminescence measurement revealed that the low-energy shift of the cw photoluminescence band is attributed to an enhancement of an additional broad band, which is caused by deep states, appearing in the low-energy side of the band arising from the transition between band tail electrons and holes. A decrease in field effect mobility in TFTs with an a-SiNx:H gate-insulator was observed when the nitrogen content of the a-SiNx:H increases, and correlated well with the low-energy shift of the cw photoluminescence band. These results suggest that the deep states in a-Si:H near the interface tend to increase with the nitrogen content of the a-SiNx:H. The origin of the states is believed to be a lattice strain induced in the a-Si:H layer.

An extensive study of contacts properties to undoped and doped hydrogenated amorphous silicon, undertaken in our laboratory, has shown that ohmicity and contact quality are very dependent on the reactivity of the metal and the quality of the metal / a-Si:H interface. For example, metals such as Sc, Mg or Ti form exceptionally good ohmic (very low barrier height) contacts, while others like Al, Cu, Mo or V form very poor quasi-ohmic contacts (average barrier height) to undoped films. In addition, metals such as Y, Ho, Hf or Er create fair quasi-ohmic (low barrier height) contacts to undoped films, at room temperature. The barrier height and the magnitude of current density can be adjusted to some degree not only by the proper choice of metal work function but also by changing material bulk resistivity or/and interface quality. Consequently, specific attention is devoted to these parameters which not only determine the quality of ohmic contact but also the dominant conduction mechanism across the barrier.

Silicide formation by reaction of palladium metal (Pd0) with hydrogenated amorphous silicon (a-Si:H) substrates was studied with Rutherford backscattering spectrometry (RBS), forward recoil spectrometry, x-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Upon low-temperature (200° C) annealing, RBS and TEM show a single-phase Pd2Si. This phase grows with the square root of time, and the activation energy is identical to that of the corresponding metal on single-crystal silicon substrates. The growth is slightly faster for hydrogenated amorphous silicon, which is attributed to its amorphous structure. During silicide formation, the hydrogen is released from silicides and presumably outdiffuses into a vacuum without interfacial accumulation. Thus, barrier formation does not occur, and the presence of hydrogen in the substrates has no effect on silicide growth.

The silicide electronic structure (core level binding energies, lineshapes, and d-band filling) of Pd2 Si on a-Si:H is identical to that of Pd2 Si formed on cr stalline silicon. Binding energy and peak shape analysis show the Pd2Si/Pd0 interface to be composed of one additional phase, Pd4Si, which has a well-defined binding energy (335.8 eV) and a narrow (FWHM = 1.1 eV), symmetric line shape. It has long been postulated that interface phases may be important in determining the phase sequence in silicide growth and the dominant diffusing species. This Pd4 Si interface phase may be important in understanding palladium silicide growth.

The transport of excess carriers in glow-discharge deposited a-Si:H/insulator heterostructures has been studied by time-of-flight and xerographic discharge techniques. Efficient injection of photocarriers from a-Si:H into, and transport through, relatively thick SiOx:N:H has been achieved. A mobility-lifetime product approaching 18−6 cm2V−1 is found for electrons in SiOx:N:H, and time resolved measurements indicate a room temperature mobility of 5×10−6 cm2v−1s−1 at a field of 2×104 Vcm−1, suggesting an electron lifetime of the order of 8.2 seconds. The results are contrasted with transport measurements on thermally grown SiO2 on Si and a transport model involving hopping through defect states within the gap of SiOx:N:H is discussed.

We have investigated the photosensitivity of the a-Si:H/P+Pc-Si heterojunction. It has been found that the capacitance of a-Si:H/P+Pc-Si heterojunction have an excellent photosensitivity, and potential application as a photosensitive capacitors.