Gap states in a-Si:H by photoconductivity and absorption

The spectral dependence of photoconductivity in a series of a-Si:H samples has been analyzed in order to derive the absorption coefficient α. It has been found that the exponent β which relates photoconductivity and generation rate varies with photon energy and chopping frequency. Its critical influence upon the value of α is established. The Urbach tail in the region 1.4–1.8 eV has been studied and found to be independent of extrinsic gap states and hydrogen content. Finally an inverse correlation has been determined between the photoconductivity lifetime and the extrinsic gap-state density, as measured from the corresponding integrated area of the absorption coefficient.     

Electronic states in a-Si:H/c-Si heterostructures

We have investigated PECVD-deposited ultrathin intrinsic a-Si:H layers on c-Si substrates using UV-excited photoemission spectroscopy (hν = 4–8 eV) and surface photovoltage measurements. For samples deposited at 230 °C, the Urbach energy is minimal, the Fermi level closest to midgap and the interface recombination velocity has a minimum. The a-Si:H/c-Si interface density of states is comparable to that of thermally oxidized silicon interfaces. However, the measured a-Si:H dangling bond densities are generally higher than in thick films and not correlated with the Urbach energy. This is ascribed to additional disorder induced by the proximity of the a-Si:H/c-Si interface and H-rich growth in the film/substrate interface region.     

Deep-level spectroscopy of reactively sputtered a-Si:H films

We have performed light-induced modulation of absorption spectroscopy, thermally stimulated current spectroscopy, capacitance measurements as a function of temperature and frequence, and electron spin resonance experiments on reactively sputtered a-Si:H films. We find evidence for seven different localized gap states in a range of ±0.4 eV around the midgap region. The possible origin of these states is discussed.     

Two characteristic photoluminescence states in a-Si:H and its alloys

Photoluminescence (PL) in a-Si:H and its alloys exhibits interesting characteristics related to the metastability of a disordered material. Two lifetime components, which are characterized with respective specific peak lifetimes of about 1 ms and 10 μs are commonly observed in a PL-lifetime distribution throughout the systems, irrespective of their difference in a localized tail-state distribution. With the aid of the modulated IR-absorption measurement that can detect structural instability in the vicinity of the Si–H bond, the characteristics of the two lifetime components are discussed based on the model that the PL corresponding to those lifetime components is caused by a localized PL center associated with some special structural unit.     

Transient photoconductivity studies of band tails states in compensated a-Si:H

The density of conduction band tail states has been determined for a series of compensated a-Si:H films using transient photoconductivity measurements. Relative to undoped material, the energy width of shallow tail states lying within 0.45 eV of the conduction band edge are found to be insensitive to the level of compensated doping for doping levels up to 1000 vppm. An observed reduction in photocurrent magnitude as the doping is increased is consistent with a corresponding reduction in the electron extended state mobility. The magnitude of the mobility reduction is found to be quantitatively consistent with potential fluctuations which arise from ionized dopants for compensated doping levels up to about 100 vppm.     

Optical absorption spectra of a-Si:H MIS structure measured by transient photocapacitance spectroscopy

The transient photocapacitance spectroscopy (TPC) was applied to measure the optical absorption spectra of hydrogenated amorphous silicon (a-Si:H) in metal-insulator–semiconductor (MIS) structure. The measured spectra show the features of optical transitions from both D⁰ and D⁻ defect bands to conduction band. The stability of measurement was checked against the change of probe frequency. The availability of TPC in detecting the defect density changes was also tested by checking the well-known defect density increase in a-Si:H after light illumination.     

Effect of the hydrogen content on optical band gap in a-Si:H:N

The effect of the hydrogen content on the optical band gap in glow discharge a-Si:H:N produced from SiH₄ and N₂ mixtures is demonstrated. The hydrogen content in the film can be well controlled by making a non-plasma-excited region above the substrate surface during deposition. When the concentration of hydrogen is high, a large amount of nitrogen atoms included in the film does not cause a widening of the optical band gap. The effects of the hydrogen content on the infrared absorption peak wavelength relevant to SiN lattice vibration and the optical band gap are discussed.     

Defects and microvoids in a-Si and a-Si:H

We report the first measurements of positron-annihilation spectra of samples of both pure and hydrogenated amorphous silicon. Comparison of these spectra with that of crystalline silicon indicates that the lowest-lifetime component can be identified as the contribution mainly from valence-band electrons. Both the pure (a-Si) and the hydrogenated (a-Si:H) samples exhibit a component with intermediate lifetime, which we attribute to small vacancies consisting of about 4 missing atoms. Finally, only a-Si:H shows a significant long-lived line (τ > 5 ns), which arises from large microvoids, with ～ 100 missing atoms. The existence of these microvoids in a-Si:H is consistent with recent reports of the presence of occluded H₂ gas under high pressure in such films.     

Xerographic spectroscopy of gap states in Se-rich amorphous semiconductors review

One of the most important parameters which determine the performance of many modern devices based on amorphous semiconductors is the drift mobility-lifetime product, μτ. There has been much interest in determination of charge-carrier ranges in amorphous semiconductors by various measurement techniques. Although the mobility, μ, can be measured by the conventional time-of-flight transient photoconductivity technique, the determination of the lifetime, τ, is often complicated by both experimental and theoretical limitations. The present article provides an overview of xerographic measurements as a tool for studying the electrical properties of amorphous semiconductors. First, details of the experimental set-up are discussed. Thereafter, the analysis and interpretation of dark discharge, the first cycle residual potential, cycled-up saturated residual potential are considered. It is shown that from such measurements the charge-carrier lifetime, τ, the range of the carriers, μτ, and the integrated concentration of deep traps in the mobility gap can be readily and accurately determined. Xerographic measurements on Se-rich amorphous photoconductors have indicated the presence of relatively narrow distribution of deep hole traps with integrated density of about 10¹³ cm^? 3. These states are located at ~ 0.85 eV from the valence band. A good correlation was observed between residual potential and the hole range, in agreement with the simple Warter expression. The capture radius is estimated to be r_c = 2–3 Å. Since r_c for pure a-Se and a-As_xSe_1 ? x is comparable to the Se–Se interatomic bond length in a-Se, it can be suggested that deep hole trapping centers in these chalcogenide semiconductors are neutral-looking defects, possibly of intimate valence-alternation pair (IVAP) in nature. The absence of any electron spin resonance signal (ESR) at room temperature seemed to be a strong argument in favor of this suggestion. Finally, photoinduced effects on xerographic parameters are discussed. It has been shown that photoexcitation of a-As_xSe_1 ? x amorphous films with band-gap light alters deep hole and electron states. During room-temperature annealing photosensitized states relax to equilibrium. Recovery process becomes slower with increasing As content. Qualitative explanation of the observed behavior may be based on associating the deep states with C₃⁺ and C₁^?intimate-valence-alternation–pair (IVAP) centers.     

Local bonding of hydrogen in a-Si:H,a-Ge:H and a-Si,Ge:H alloy films

This paper reviews the local bonding of hydrogen (and deuterium) in a-Si:H(D), a-Ge:H(D) and a-Si, Ge:H(D) alloy films. We specify the types of atomic environments that have been identified through vibrational spectroscopy, primarily infrared (IR) absorption. We emphasize local modes and discuss the atomic motions that are responsible for the various spectral features. We discuss correlations between the occurrence of specific local bonding groups, e.g., polysilane and polygermane configurations, and the deposition techniques and parameters, including the substrate temperature (T_s), the gas mixtures and the RF power into a glow discharge, etc. We include a discussion of the theoretical approaches that have been used to treat vibrational modes in these materials. We emphasize the approximations that are valid because of the relatively light mass of the hydrogen and deuterium atoms compared with those of the silicon and germanium atoms. Finally we highlight the effects of neighboring alloy or impurity atoms on the frequencies of hydrogen vibrations in a-Si:H, and point out the differences between oxygen atom incorporation in a-Si and a-Ge alloys.     

Investigation of the density of localized states in a-Si using the field effect technique

The electronic properties of amorphous solids are largely determined by the distribution of localized states $\text{N}$ (?) in the mobility gap. In this paper, the field effect technique is applied to the experimental study of $\text{N}$ (?) in specimens of a-Si prepared by the glow discharge method and by vacuum evaporation. The experimental approach and the analysis of the results are discussed in some detail. $\text{N}$ (?) curves, extending over an energy range of up to 0.5 eV have been obtained for a series of glow discharge specimens, deposited at substrate temperatures between 310 and 570 K. The results show structure in the gap states, a well-defined minimum almost in the centre of the mobility gap and a rapid rise in $\text{N}$ (?), 0.18 eV below ?_c, which is identified with the onset of band tail states. The field effect data confirm that the predominant conduction mechanism at room temperature changes from hole hopping to transport in extended electron states, as the Fermi level is moved through the minimum in $\text{N}$ (?) The effects of annealing on the state distribution have been investigated, showing that $\text{N}$ (?_f) can be reduced by one or two orders of magnitude. The nature of the gap states is discussed and the divacancy is suggested as a basic model for the electronic states involved.     

Radiation-induced defects in a-Si:H by 1.5 MeV He4 particles studied by photoconductivity and photothermal deflection spectroscopy

We report radiation effects on intrinsic a-Si:H thin films subjected to a 1.5 MeV He⁴ beam for particle fluences up to 10¹⁶ cm⁻². Photothermal deflection spectroscopy is used to obtain information on the sub-gap density of states. Photoconductivity detects changes in the μτ-product of the electrons. Steady-state photocarrier grating technique is used for measuring the ambipolar diffusion length and estimating the hole μτ-product. The 1.5 MeV He⁴ beam radiation results in pronounced changes in the a-Si:H absorption spectrum. Optical absorption due to deep defects increases with particle fluence by more than one order of magnitude. Electronic transport properties consistently degrade with increasing particle fluence and correlate with the density of radiation-induced defects.     

New approach to capacitance spectroscopy for interface characterization of a-Si:H/c-Si heterojunctions

A new technique for characterization of interface defects in a-Si:H/c-Si heterostructure solar cells from capacitance spectroscopy measurements under illumination at forward bias close to open-circuit voltage is described. The proposed method allows to significantly increase the sensitivity to interface defects compared to conventional capacitance measurements at zero or small negative bias. Results of numerical modelling as well as experimental data obtained on n-type a-Si:H/p-type c-Si heterojunctions are presented. The sensitivity of the proposed method to interface states and the influence of various parameters like band mismatch, density of interface defects, recombination velocity at the back contact are discussed.     

Recombination efficacy in a-Si:H p-i-n devices

The performance of hydrogenated amorphous silicon solar cells (a-Si:H) is limited due to the relatively high defect concentration in the intrinsic layer, leading to high recombination rates. The dark current-voltage (JV) characteristics of these cells are mainly determined by recombination processes. Several parameters influence these processes: the density-of-states distribution in the mobility gap, the spatial distribution of recombination centers, and the recombination efficacy. As a result the ideality factor is non-integer and varies with voltage. The temperature dependence of the dark JV curves reveals the bias dependent activation energy, a measure for the mobility gap, and the temperature dependence of the recombination efficacy.In this contribution we present results of accurate dark JV measurements at different temperatures and a comparison to computer simulations. By analyzing the temperature dependence we study the influence of the recombination efficacy on the recombination rate and determine where in the device recombination is most effective. Simulations show that the shape of the recombination efficacy curve depends on the cross-section ratio for electron and hole trapping. Whereas the recombination rate is mainly determined by the local defect density in the device and controls the JV characteristic, the recombination efficacy controls the temperature dependence.