By means of the slow positron beam Doppler-broadening technique, the depth profile of microvoids across a p-i-n double junction solar cell has been resolved. VEPFIT fitting results indicate an approximately uniform density of the defects throughout the solar cell, but with an enhanced concentration at all of the interfaces possibly due to network mismatch. In order to evaluate the internal electric field, Variable Energy Positron Annihilation Spectroscopy (VEPAS) measurements have been performed on a single junction pin solar cell at different biases. The internal electric field effect on positrons has also been examined in terms of the bias dependence of positron drift in a-Si:H single junction pin solar cell.

The effects of light-soaking on either phosphorus- or boron-doped a-Si:H films were studied as functions of the doping level and the temperature. In the case of boron-doped films, the most important effect is the improvement of the conductivity during light-soaking, which is related to the activation of boron. On the contrary, phosphorus-doped films present a remarkable stability, although lightly phosphorus-doped ones show a decrease of their conductivity by five orders of magnitude when light-soaking is performed below 40 °C. This effect is attributed to the formation of P-H complexes which are stable at low temperature only. Our results suggest that in both types of doped a-Si:H films the creation of metastable defects is a second order effect with respect to the activation or passivation of dopants, which results from their interaction with hydrogen.

Current transport in metal-semiconductor-metal structures based on amorphous silicon alloys has been studied in relation to the density of dangling bond state defects. The density of defects was changed by varying alloy composition or by current stressing. We show that the change of current-voltage characteristics and activation energy with defect density and the onset of Poole-Frenkel conduction with composition require charged defects. It is found that there are more charged defects in amorphous silicon nitride (a-Si1−xNx:H) than in amorphous silicon carbide (a-Si1−xCx:H). In addition, an excess of negatively charged dangling bond defects compared to positively charged dangling bond defects is observed in a-Si1−xNx:H films. This is attributed to the presence of N4+ act as the donor states in silicon nitride. We find that the density of charged dangling bond defects can be higher than 1019cm−3.

A series of pure hydrogenated amorphous silicon (a-Si:H) samples as well as carbon (aSi:C:H) and sulfur alloys (a-Si:S:H) were investigated by means of the photoconductive time-of-flight technique. Drift moblility (µd) measurements reveal a fast decrease of the electron µd upon C or S addition, while the hole µd does not change significantly. Contrary to the electron transients, hole transient currents do not show the typical space-charge-limited (SCL) features, even for high light intensities where these features are normally seen. SCL features are observed to disappear for electron transients as well in the a-Si:H alloys with increasing alloy content. Above results are all explained starting from the a-Si:H density of states (DOS) model where the valence band (VB) tail is much broader than the conduction band (CB) tail. Introducing additional disorder by alloying broadens both the VB and the CB tails while the relative increase of the CB tail is much greater than that of the VB tail.

Molecular dynamics simulations find light-induced metastable defects to be silicon dangling bonds accompanied by (Si-H)2 defect complexes that have two Si-H bonds. These complexes are formed by pairs of hydrogen breaking a silicon bond. This supports the model of Branz. These defects are the analogue of the H2* defect in c-Si and their energy correlates with the bond-angle strain. Several features of annealing including E-field induced effects are well accounted for by the (Si-H)2 defect.

We present a systematic atomic-scale analysis of the interactions of SiH3radicals originating in silane containing discharges with Si(001)-(2×1) and H-terminated Si(001)-(2×1) surfaces. Through simulations, we show that the hydrogen coverage of the surface is the key factor that controls both the surface reaction mechanism and the reaction probability. The SiH3radical reacts readily with the pristine Si(001)-(2×1) surface during the initial stages of growth while its reactivity with the corresponding H-terminated surface is considerably lower. Deposition of a-Si:H from SiH3 radicals has also been simulated by repeatedly impinging SiH3 radicals onto Si(001)-(2×1) surfaces maintained at 500 °C. During deposition under these conditions, the dominant mechanism of hydrogen removal from the surface is through abstraction by SiH3 radicals, which subsequently return to the gas phase as silane. The important reaction processes that take place during film growth have been identified and their energetics has been analyzed.

The concentration dependence of hydrogen diffusion was studied in hydrogenated crystalline and amorphous silicon prepared by hydrogen implantation into crystalline Si wafers and into amorphous silicon of low hydrogen concentration. The results are compared with data for plasma-grown a-Si:H and µc-Si:H films. The increase of the diffusion coefficient with rising hydrogen concentration in a-Si:H is explained by an (equilibrium) energy band model of hydrogen diffusion whereas the decrease of the diffusion coefficient in c-Si:H is explained by a trapping model. The different behavior is attributed to a greater flexibility of the amorphous Si network compared to the crystalline Si lattice which is also visible in a difference in hydrogen-related microstructure formation.

Device-quality a-Si:H films were prepared by glow discharge CVD with pure or H-diluted silane as well as by hot-wire CVD. The hydrogen content was varied from ∼2 to 15 at. %. The Si-H bond absorption and its light-soaking-induced changes were studied by IR and differential IR absorption spectroscopes. The results indicate that the more stable sample exhibits an increase of the absorption at wave number ∼2000 cm−1, and the less stable one exhibits a decrease at ∼2040 cm−1and an increase at ∼1880 cm−1.

A new model for Staebler-Wronski effects is proposed. Defect pairs of (D°:D+) or (D°:D°) as a precursor of neutral dangling bonds are produced by electron-phonon interactions. Most of the defect-pairs immediately rebond after creation, but some of them separate resulting in wandering dangling bonds. The separation and the wandering take place through bond-switching. When wandering dangling bonds collide, most of them make covalent bonds via defect-pairs. The rate equations based on these processes are given and annealing effects on photo-generated dangling bonds are studied.

With the accumulation of experimental data, it has been recognized by many that the light-induced metastable change of a-Si:H, Staebler-Wronski effect (SWE), may be related to a structural instability of the whole a-Si:H network. However, direct evidence of such a structural change is still lacking. In the present paper, the efforts of our laboratory in this direction will be reviewed, including the light-induced changes of Si-H bond absorption, low frequency dielectric response, and an apparent photo-dilation effect.

A new microscopic and kinetic model of light-induced metastability in hydrogenated amorphous silicon (a-Si:H) is described. Recombination and trapping of photoinduced carriers excite hydrogen from deep Si-H bonds into a mobile configuration, leaving a dangling bond (DB) defect at the site of excitation. Normally, mobile H are recaptured at DB defects and no metastability or net DB production results. However, when two mobile H collide, they form a metastable two-hydrogen complex and leave two spatially-uncorrelated Staebler-Wronski DBs. Thermal and light-induced annealing occur when mobile H are excited from the metastable two-H complex; they diffuse and are recaptured to DBs. The microscopic model is entirely compatible with electron-spin-resonance results showing neither DB-DB nor DB-H spatial correlation of the light-induced DBs. The model leads to new differential equations describing the evolution of the mobile H and DB densities. These equation equations explain the observed room-temperature Ndb∼G2/3t1/3 dependence of DB creation upon the electron-hole pair creation rate (G) and time. The model also accounts for both t1/3-kinetics at 4.2K and t1/2-kinetics under laser-pulse soaking. Neither of these results can be explained within the prevailing electron-hole pair recombination model.

This paper proposes intrinsic reaction pathways for generation of metastable defects in hydrogenated undoped or intrinsic amorphous silicon (i-a-Si:H). Since these pathways involve only silicon (Si) and hydrogen (H) atoms, this approach is valid for device grade materials in which concentrations of oxygen (0) atoms, and nitrogen-hydrogen (N-H) groups are present at concentrations below about 1019 cm−3. Ab initio calculations demonstrate that the proposed generation pathway reactions are exothermic with relatively small reaction barriers (< 0.4 eV).

We have attempted to observe changes in the Raman scattering spectrum of a-Si:H due to light exposure. We do not find any detectable shift of the position or change in the width of the TO phonon peak. We find, however, other unexpected degradation phenomena in the Raman spectrum of a-Si:H samples which are attributed to light induced changes in the high energy tail of room temperature luminescence. Raman scattering has a relatively large (10-20% of TO peak height) broad background which extends to 2000cm−1 in the Stokes region and tails into the anti-Stokes region. Multiple phonon processes, geminate radiative recombination and surface oxide contributions are considered. Degradation phenomena and the broad background signal are observed in a-Si:H deposited on different substrates as well as in free-standing films. Degradation processes in bare fused quartz substrates are also investigated. Small structural changes surrounding the light-induced defects are proposed and discussed.

We have studied the degradation kinetics of undoped a-Si:H films which contain a significant fraction of silicon microcrystallites. The degradation rate is found to be exceptionally slow in the first stage of degradation, then the defect density follows the “normal” t1/3rate and finally saturates. We present a model which relates this abnormal kinetics to the microcrystallites which are embedded in the amorphous matrix.

Polarized electroabsorption method has been used to study photo-induced structural changes in hydrogenated amorphous silicon. The field-modulated absorption signal consists of two components, one of which is the true polarization-dependent electroabsorption serving as an indicator of the structural disorder, and the other is the thermoabsorption resulted from the temperature modulation due to Joule heating. The thermoabsorption component has removed from the observed field-modulated absorption signal to make an accurate and reliable evaluation of structural disorder by phase-separation procedure. As a result, about 15-25 % of the observed signal arises from the thermoabsorption effect for the Tauc gap region. Nevertheless, any essential alteration is not needed for our previous PEA results. The internal stress as well as density have been measured to provide another evidences for the photo-induced structural change. It is found that amorphous silicon film expands and the density tends to decrease upon light-exposure, the temporal behaviors of which coincide with that of the PEA ratio factor indicating disorderness of the amorphous network structure. The results permit us to conclude that a large scaled change in the amorphous network structure occurs under light-exposure, which might proceed the light-induced creation of metastable dangling bond defects.

The effects of light exposure on neutral defect density at the surface of nanocrystalline Si are investigated by electron-spin resonance (ESR) experiments. A decrease of the neutral dangling bond density by light soaking was observed in this nanostructure. The reduction rate of ESR signal intensity becomes large with increasing light exposure intensity, and the reduction occurs from the excitation energy higher than 2 eV in vacuum. The reduction of the defect density can be explained in terms of the conversion of neutral states to charged states by carrier trapping.

The temperature dependence (4.2 - 300 K) of the electron spin resonance (ESR) signal at g = 1.998 and the electronic conductivity in microcrystalline silicon are measured and related with each other for a series of n-type samples. In addition the hyperfine interaction with the phosphorus donor states is investigated. The conductivity at low temperatures can be interpreted as hopping in impurity states with activation energies of a few meV very similar to n-doped crystalline silicon. The ESR intensity follows a 1/T-Curie law dependence. The carrier densities determined from the dopant concentration, electrical transport and ESR, respectively, agree surprisingly well. Little hyperfine intensity is found which is explained by exchange interaction of electrons with more than one dopant nucleus or occupation of tail states at low doping levels. It is concluded that the excess carriers introduced through doping do for their major part contribute to both transport and ESR. The results are in accordance with a position of the electrons with g=1.998 in impurity or tail states of the crystalline grains at low temperatures. With increasing temperature carriers get excited into the conduction band - a transition which is also monitored in the conductivity - where the ESR line is subject to considerable line broadening.

Two-pulse-echo field-sweep spectra of boron-doped samples show in addition to dangling bonds (DB) a very broad feature of approximately 500 G width. This feature is the sum of several lines whose relative intensities change with doping and temperature. The total intensity increases with doping. The spin-lattice relaxation time TI of the resonance at g=1.998 referred to as conduction electrons (CE) has been studied for undoped and P-doped microcrystalline silicon. The discrepancy between T1 values previously reported has been resolved and the values for CE are at least two orders of magnitude smaller than those of the DB. In undoped samples cross-relaxation between the CE and DB spin systems might explain the similar T1 values obtained for CE and DB in inversion recovery measurements.

We have studied subgap absorption of intrinsic a-Si:H induced by below- and above-gap photo-excitation. We find very similar photoinduced subgap absorption spectra when excited with 2.4 eV or 1.2 eV light. Both spectra exhibit a power-law dependence on laser intensity ΔT ∼ 1a, where a is 0.5 and 0.7 for 2.4 and 1.2 eV excitation energy, respectively. This behavior indicates a change in the recombination mechanism as a function of excitation energy. The PA spectrum excited at 1.2 eV shows a strong dependence on bias illumination. Bias illumination bleaches the absorption according to a power-law as ΔT = c(Ebias)·Iβ, where β is approximately 0.85 and independent of probe energy and bias energy. The parameter c(Ebias) increases superlinearly with bias illumination energy for Ebias > 1.7 eV.

We have carried out a comprehensive study of the electronic properties of two series of optimized a-Si1−xGex:H alloys fabricated at United Solar Systems Corporation (“Uni-Solar”) and Harvard University, encompassing the composition range 0.2 ≤ × ≤ 1.0. Both series of samples exhibit deep defect densities that obey quite accurately the spontaneous bond-breaking model proposed by M. Stutzmann which, by considering how the defect formation energy varies with the position of Fermi energy, we have been able to extend to doped samples.

We have also extended our studies to include measurements of ambipolar diffusion lengths and the effects of light-induced degradation, and thus have been able to demonstrate a direct relation between these transport properties and the measured defect levels before and after degradation.