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.     

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.     

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.     

Defect-state engineering in a-Si:H: An effective tool for studying processes during light-induced degradation

Charge deep-level transient spectroscopy (Q-DLTS) has proved to be a powerful tool for investigating the defect-state distribution in hydrogenated amorphous silicon (a-Si:H) in both annealed and light-soaked states. In this article, we report on Q-DLTS experiments that were designed to further investigate our recently published model for the Staebler–Wronki effect (SWE). By subjecting an a-Si:H based metal/oxide/semiconductor structure to negative (positive) bias voltage during the annealing above the equilibration temperature a programmed p-type (n-type) defect-state distribution can be established in the undoped a-Si:H layer. In this way defect-state engineering in a-Si:H is achieved that enables us to study the role of different components of the defect-state distribution in the light-induced degradation process. Additional possibility to control the role of a particular defect-state component in the degradation process is to apply a bias voltage also during light soaking. The observed changes in the programmed defect-state distributions due to light soaking under various conditions give additional information on the SWE and can be interpreted with our model for the SWE.     

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.     

Investigation of gap states in phosphorous-doped ultra-thin a-Si:H by near-UV photoelectron spectroscopy

A variant of photoelectron spectroscopy with near-UV light excitation was established and applied to an n-type doping series of ultra-thin a-Si:H layers (layer thickness ～10 nm). Using this technique, the position of the surface Fermi level E_Fs is obtained and the density of recombination active defect states in the a-Si:H band gap down to ～10¹⁵ states/cm³ can be detected. Defect densities are generally about one order of magnitude higher than in the bulk of thicker (several 100 nm) layers, and the minimum achievable distance of E_Fs from the conduction band is ～360 mV for doping with 10⁴ ppm PH₃. The optimum doping for the fabrication of solar cells is almost one order of magnitude lower. This discrepancy may be explained by enhanced recombination at the a-Si:H/c-Si interface at high doping levels, and in addition by an efficient recombination pathway where charge carriers tunnel from c-Si via a-Si:H band tail states into the a-Si:H and subsequently recombine at dangling bond states.     

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.     

On the role of hydrogen in a-Si

Samples of a-Si sputtered in a hydrogen enriched argon atmosphere were prepared under various hydrogen partial pressures and deposition temperatures and successfully doped with lithium. A decrease of the electrical conductivity and a shift of the absorption edge to the higher energy were observed and ascribed to the diminishing number of the localized states in the band gap due to the presence of hydrogen.     

The effect of crystallization on doping efficiency in a-Si:H films

The structure of A-Si:H films can be changed by substitutional doping of boron and phosphours dopants. Proper regulation of technological parameters, Ts and r. f. power, can result in a mode transformation from amorphous morphology into microcrystalline or polysilicon like one. The crystallization of amorphous silicon film leads to a further reduction in its room-temperature resistivity by about three order of magnitude and a reduction in its activation energy of electrical conductivity.     

Thermal stability of light-induced defects in hydrogenated amorphous silicon: Effect on defect creation kinetics and role of network microstructure

The kinetics of light-induced defect creation in a-Si:H is studied in early-time limit and as function of pre-existing defects of different thermal stability by electron spin resonance and optical spectroscopy techniques. Both for cw and for laser pulse exposures, the early-time kinetics follows sublinear t^β time dependences, similar to the long-time limit. In addition, the overall defect creation rate is not a single function of the total defect number. Instead, it depends on the thermal stability, or annealing energy distribution, of the defects present in the film. Furthermore, creation of the thermally less stable defects is unaffected by the presence of a large number of stable defects introduced by pre-exposure at a higher temperature. These findings question the existing defect creation models. Thermal stability of the light-induced defects depends on the network microstructure, the less stable defects being created in a-Si:H deposited near microcrystalline transition.     

Preparation and analysis of reactively evaporated a-Si:H

a-Si:H formed by the method of reactive evaporation of silicon with supplying hydrogen ion has much the same properties as films prepared by silane grow discharge (GD) decomposition. Growth kinetics in this method is described. Hydrogen ion suppresses the oxidation of silicon and it is incorporated as mono-hydride, whose content is proportional to hydrogen ion current. Adsorption energy of hydrogen ion is some 130 meV. Dark conductivity of a-Si:H depends mainly on energy gap associated with hydrogen content. Spin density is determined by the collision phenomenon between Si-H and spin in depositing.     

Characterization and luminescence of a-Si:H:Cl films

a-Si:H:Cl films have been deposited by glow-discharge and characterized by infrared transmission, optical absorption and photoluminescence. The influence of growth parameters on the H and Cl content has been investigated. The luminescence spectra show that three different radiative transitions can occur, at 0.75, 0.95 and ～1.3 eV. These bands have been interpreted respectively in terms of the following recombinations: defect to defect, defect to band tail, band tail to band tail.     

Initial stage of growth of the GD a-Si:H

The time dependences of the optical emission intensities of SiH,H and He lines and also of the gas temperature have been studied. It is demonstrated that 10–100 sec are necessary for full stabilization of plasma. This can explain the different properties of the initial 30–300 Å layer of a-Si:H.