Relaxation of the metastable defect density in undoped amorphous silicon is observed after keV electron irradiation. The time constant for relaxation has an activation energy close to 1 eV, similar to that for light-induced defects. Relaxation appears to follow two or more stages. A large initial density relaxes rapidly, followed by slower relaxation more characteristic of light-induced defects. Separation of these components allows for a better comparison of e-beam and light-induced saturation defect density.

The power spectrum of coplanar current fluctuations in n-type doped hydrogenated amorphous silicon (a-Si:H) has been measured before and after metastable defect creation by light soaking. The average magnitude and spectral slope of the 1/f noise are not affected by illumination, however significant changes in the higher order statistics are observed. Associated with the decrease of conductivity upon light soaking (the Staebler-Wronski effect), there is a decrease in the correlation of the noise power which characterize the non-Gaussian noise in the annealed state. These changes in the noise statistics are reversible by annealing. The light-induced changes in the non-Gaussian statistics provide experimental support for models of light induced defect creation which involve long-ranged and many body interactions.

Bandtail defect carrier trapping dynamics in high quality photo-CVD grown a-Si1-xGex:H (x=0.3 and 0.62) and PECVD deposited a-Si:H films were studied using photo-modulated electron spin resonance (PMESR). Comparison of the dark spin densities with junction capacitance (drive-level profiling) methods indicate that a majority of the defects are neutral in the a-Si:H and 30% Ge samples, but the majority of the defects in the 62% Ge sample are charged. Lineshape deconvolution of the PMESR signals in the in-phase and quadrature phase as a function of photo-modulation frequency, excitation energy and excitation intensity for both intrinsic (state A) and light soaked (state B) states revealed three significant insights: in state A all samples behave similarly, state B behavior of the a-Si:H is dramatically different from the state B alloyed samples, and strong evidence for 'safe hole traps' (SHT) is observed in the state B a-Si:H material.

We have grown a-Si1-x,Cx:H films by reactive magnetron sputtering, varying the H2 partial pressure from 0 to 6 mTorr and maintaining the substrate temperature at 275°C and argon and methane partial pressures at 1.70 and 0.10 mTorr, respectively.

We investigate the correlation between electronic properties and the fraction of H bonded in "microstructure," defined by the ratio of SiHx stretching mode absorptions in IR and the low temperature H2release in thermal evolution. Our results on sputtered films disprove the monotonic decrease in carrier transport with increasing microstructure fraction which is commonly observed for a-Si1-xCx:H grown by glow discharge of SiH4 and CH4. We find that the electronic properties and microstructure depend on film composition and growth technique, and that the electronic properties are not uniquely determined by the microstructure of hydrogen bonding.

Amorphous silicon carbide films have been deposited by PECVD in SiH4+CH4+H2 mixtures at different hydrogen dilutions. The optoelecuonic properties of the films have been measured by transmittance-reflectance spectroscopy, photothermal deflection spectroscopy and photo and dark electrical conductivity. Structural properties have been obtained by FTIR spectroscopy. It was found that high hydrogen dilution leads to materials of improved quality, p-i-n device structures have been deposited with intrinsic layers at different hydrogen dilution levels.

Electronic properties of C-rich a-Si1 -xCx:H alloys are investigated through the characterization of MIM-type structures, by means of current-voltage measurements carried out at different temperatures. These measurements provide some new insight in both the dielectric behavior of the material and the physics of the metal/a-Si1-xCx:H junction.

Electrophotographic dark decay measurements have been used to determine the surface density of states (SDOS) of a-Si:C:H. Injection of trapped charge from these deep states into the conduction band governs the dark discharge of a photoconductor, provided bulk generation and bulk space charge are negligible. It is found that the SDOS profiles peak around 0.60 eV below the conduction band for materials with different carbon concentration. This observation implies that the energy position of these states is fixed with respect to the conduction band edge, even though the optical band gap of these materials increases with increasing carbon concentration. The nature of these states may be ascribed to D− states, whose density is strongly enhanced by filling D° states when the material is charged negatively. Furthermore, we observed that the SDOS around 0.60 eV below the conduction band edge is approximately the same for materials with up to 8 at.% carbon. From temperature dependent measurements a value of 2·108 s−1 was obtained for the attempt-to-escape frequency.

This paper deals with the electrical properties in the parallel direction of compositionally modulated amorphous silicon/amorphous silicon-carbon multilayers. Conductivity of three series of samples with varying well and barrier thicknesses is studied. The results show that dark conductivity decreases when reducing a-Si:H layer thickness. This is interpreted as an alloy effect at interfaces. The role of the a-Si1-xCx:H layers in the photoconductivity decrease observed in series with variable mean composition and constant well thickness is discussed

Heterostructure Schottky diode samples each composed of a sub-micron thick layer of intrinsic hydrogenated amorphous silicon-carbon (a-Si1−xCx:H) deposited on an n-type crystalline silicon (c-Si) substrate are used to measure the a-Si1−xCx:H/c-Si band offsets via junction capacitance techniques. The samples range in carbon concentration from x=0.0−0.3. First, a thermally activated capacitance step due to the response of defects at the amorphous/crystalline interface is evident in capacitance vs. temperature spectra taken on all these samples. The bias-dependence of this step’s activation energy provides a direct measure of the a-Si1−xCx:H/c-Si interface potential as a function of c-Si depletion width in each sample. By application of Poisson’s equation, we find that the a-Si1−xCx:H/c-Si conduction band offset ΔEc. increases from 0.00 to 0.10 eV as x increases from 0.00 to 0.26. Second, while under reverse-bias at low temperature, we optically pulsed each sample with c-Si band-gap light to create trapped holes at the a-Si1−xCx:H/c-Si valence band offset ΔEV. By noting the threshold for the subsequent optical release of these trapped holes by sub-band gap light, we found that ΔEV increases from 0.67 to ≥0.83 eV as x increases from 0.00 to 0.26.

The microstructure of undoped and phosphorus doped Si:H and Si,C:H films was analyzed by selected-area diffraction, conical dark-field imaging, energy-dispersive x-ray spectroscopy and electron energy-loss spectroscopy in transmission electron microscopes. Thin films were synthesized by remote plasma-enhanced chemical vapor deposition and characterized in terms of degree of crystallinity. The distribution of phosphorus in the Si:H and Si,C:H films, and carbon in the Si,C:H films was evaluated. The results indicate that i) the microstructure of a film may be two phase, consisting of silicon microcrystallites in an amorphous matrix, ii) phosphorus doping as well as the presence of carbon influences the degree of crystallinity by reducing the average size and volume fraction of microcrystallites, iii) the presence of carbon and phosphorus doping completely suppresses the crystalline phase, iv) phosphorus is distributed at approximately the same concentration in both the crystalline and amorphous phases of diphasic films, and v) carbon is detected in the amorphous phase of the Si,C:H films.

In this paper we present a detailed study of doping and alloying effects in a-Ge:H due to the incorporation of phosphorus, nitrogen, oxygen, boron, carbon and silicon. The film characterization was performed by means of quantitative analysis , optical absorption in the band and subband range and conductivity measurements. N and O were found to form donor states resulting in an increase of the dark conductivity up to five orders of magnitude. The maximum value of 0.3 (Ωcm)-1 has been obtained by N-doping of about 1 at% solid phase concentration, which exceeds the effect of P doping. B doping causes a decrease of the dark conductivity to a minimum of 10-6 (Ωcm)-1 at 150 ppm solid phase concentration. Addition of Si and C up to concentrations of about 5 at % does not influence the subgap absorption, where best values of E0<50 meV and α(0.6 eV)<10 cm-1 were obtained for the Urbach energy and midgap absorption, respectively. A Si content of 1 at % decreases the dark conductivity, whereas the normalized photoconductivity remains at a high level of ŋμτ=10-5 cm2V-1.

The activated conductivity of a-Ge:H between room temperature and 460K was investigated using heating and cooling rates in the range between .001 and 0.1 K/s. A splitting of the cooling curves obtained at different rates, which defines the so called equilibrium temperature TE, is observed mainly between 420 and 430K. Taking into consideration that TE depends on the maximum cooling rate, the present results are in good agreement with those reported by Eberhardt et al. The higher cooling rate always leads to the lower conductivity at any temperature below TE. These effects can be rationalized in terms of a reversible shift of the Fermi level towards midgap at higher temperature. Though reversible changes of the mobility cannot be excluded, they cannot account for our set of experimental data. Rather, changes in the density of electronic states within the mobility gap can explain the effects observed.

Modulated photocurrent (MPC) spectroscopy has been used to investigate the energy distribution of deep defect states in photo-CVD grown a-Si1 -xGex:H alloys. We observe a distinct electron trapping feature at a thermal energy of 0.55eV below Ec for higher Ge concentration (x = 0.62) alloys. For those samples with Ge concentration between 30-50 at. %, an anomalously large phase shift was observed within a temperature window between 150K to 240K. The modulated photocurrent exhibits strong quenching at the edges of this window. We suggest that these MPC spectra are controlled by deep defect relaxation processes in this temperature region, which causes electron thermal emission processes to be greatly suppressed. This effect can be reduced by increasing the reverse bias. Under such conditions, the energy position of these defect states can be roughly estimated to lie near Ec - 0.33eV. This result supports the recent result obtained from transient photocapacitance and photocurrent measurement.

Proton and deuteron double resonance methods have been used to study the structure of high quality plasma-deposited a-Si:H,D and a-Ge:H,D films. The micro-structure of the a-Si is dominated by nanovoids with dimension <3 Å while the a-Ge shows many larger microvoids. We have observed new proton HD multiple echoes in the films by using a three pulse sequence double resonance method. The new echoes imply that HD molecules in a-Ge principally are located in large microvoids, which is not the case in high quality a-Si.

Thin amorphous Si1−xBx films were co-evaporated onto pre-oxidized (100) Si wafers and quartz substrates, by using a high vacuum system with 2 electron guns. Films were deposited in a composition range from x=0 to x=0.5. In order to study how the structural and optical properties depended on concentration and annealing temperature, heat treatments were carried out at temperatures from 400 up to 1000°C. The films were characterized by means of transmission electron microscopy (TEM), Auger electron spectroscopy (AES), and light absorption spectrophotometry. It is shown that the amorphous Si1−xBx alloy is stable up to 1000°C at certain compositions and that the optical band gap of the alloy increases gradually with increasing annealing temperature up to 700 - 900°C, and then increases rapidly when annealed at a higher temperature by about 0.5 eV. These changes can be associated with microstructural alterations. The relationship between the microstructure and the band gap of the films is discussed.

Thin films of non stochiometric hydrogenated amorphous silicon oxide (a-SiOxHy.) with x=0.09...1.14, y=0.06...0.20 were prepared by dc-magnetron sputtering from a crystalline silicon target within an argon-hydrogen-water vapor atmosphere. The film properties were investigated by infrared and optical spectroscopy, Rutherford backscattering spectroscopy (RBS) and elastic recoil detection (ERD), luminescence and activated conductivity. Absorption bands related to either silicon-hydrogen or silicon-oxygen vibration modes were observed at 650, 800, 870 and 1000cm-1. Above a water pressure of 0.06Pa, the oxygen content saturates at about 50 at %. The static refractive index n ∞ the optical Tauc gap ETand the logarithmic dark conductivity at room temperature show a linear dependence on the films' oxygen content. Films prepared under certain conditions show visible photoluminescence.

We have investigated a-Si,N:H alloys as an alternative wide band-gap, photo-active material. The entire alloy range between a-Si:H and a-Si3N4:H can be formed by a remote plasma-enhanced chemical-vapor deposition (PECVD) process. Other studies have demonstrated that a-Si,N:H alloys could be doped to form window materials for p-i-n devices. This paper focuses on alloy materials with E04 bandgaps to about 2.2 eV. We have prepared these a-Si,N:H alloys, characterized their microstructure, and studied their photoconductivity, sensitivity to light-soaking and transport properties. For example, with increased alloying we show that i) the white-light photoconductivity and ii) the kinetics and magnitude of the decay of photoconducitivity under intense illumination (the Staebler-Wronski effect), are about the same as for PV-grade a-Si:H.

We have studied the depositions of amorphous silicon, silicon carbon alloy, doped microcrystalline silicon in order to apply these films as the component materials for the p-i-n and double stacked solar cells. We have obtained low band gap a-Si:H by decreasing the deposition rate under the proper preparation conditions and highly conductive, thin microcrystalline Si and SiC layers. We have developed a stable a-Si/a-Si double stacked solar cell with a conversion efficiency of ∼ % using narrow band gap a-Si:H as a i-layer of bottom cell.The performance of this cell does not degrade until 100 hrs illumination under 350 mW/cm2.

A total-area conversion efficiency of 12.0% (100cm2) has been achieved for a single-junction a-Si solar cell. The film deposition rate (Rd) plays an essential role in controlling the optical and electrical properties of "device-quality" a-Si:H for high-efficiency solar cells. The properties of conventional "device-quality" a-Si:H films deposited from 100% SiH4 are primarily determined by the balance between Ts and Rd. A lower or higher deposition rate results in a-Si:H with a narrower or wider bandgap, respectively. This enables the properties of a-Si:H to be controlled independent of Ts. The controllable range of a-Si:H properties can be widened by effectively utilizing factors such as ion bombardment. For example, a high dilution of SiH4 with H2 ( H2 SiH4 = 10 or more) or an H2 plasma treatment after deposition results in a-Si:H with a very wide bandgap and low defect density, which cannot be achieved by using 100% SiH4- Controlling the properties of a-Si:H by applying vibrational / rotational energy is also investigated.

The combination of a carefully-designed deposition system, process optimization, and a proprietary processing technique, has resulted in reduced structural defects and photo-stable, high quality intrinsic a-Si films. The improvement in material properties is further evidenced by successful incorporation of the material into single junction devices with a record stabilized device efficiency of 7.5%.