Space charge limited currents of holes in intrinsic hydrogenated amorphous silicon (a-Si:H) have been obtained using novel p+-intrinsic-p+ (p-i-p) structures. The presence of these hole space charge limited currents is verified from their temperature dependence and their dependence on a wide range of intrinsic layer thickness. The carrier transport and space charge limited currents in the p-i-p structures are compared with those of n-i-n structures and the results are discussed in terms of a self consistent density of states in the gap.

The thermalization of electrons in the medium temperature range (120K ≤ T ≤ 200K) is investigated by time-of-flight experiments on a-SiGe:H mim structures. The evaluation of the pre-transit current decay reveals a dual slope characteristic which reflects an initial hopping down (I ∼ t−1) followed by a thermally activated (I ∼ t−p 0 < p < 1) process. The transition time from hopping down to multiple trapping sensitively depends on temperature and alloy composition. The experimentally deduced features can be explained by the increased tail state density at the conduction band mobility edge of a-SiGe:H samples compared to a-Si:H.

The energy distribution and temperature dependence of the conduction and valence band tail density of states in a-Si:H and a-Si,Ge:H alloys is determined via total yield photoelectron spectroscopy. All films are observed to possess purely exponential conduction and valence band tail densities of states; however, the characteristic energy of the conduction band tail increases much more rapidly with temperature in the range of 300K to 550K than that of the valence band tail. This indicates that over that temperature range the conduction band tail is considerably more susceptible to thermal disorder than to structural disorder whereas the reverse holds for the valence band tail.

The disorder in a-Ge:H thin films produced by the plasma-enhanced chemical vapor deposition (PECVD) technique is strongly reduced when the GeH4 gas is diluted at 1% in H2 and the radiofrequency power density is increased to 0.1 W.cm−2. This improvement is attributed to a better surface passivation by the hydrogen atoms during the growth. However, the poor transport properties indicate a still high defect density. The midgap defect absorption and the Urbach energy, obtained from the photothermal deflection spectra calibrated with optical data, both decrease as a function of the film thickness. The optical defect density is calibrated with EPR spin measurements. For a-Ge:H films thicker than 2 μm, obtained at a deposition temperature Ts ranging from 150 to 250°C, the Urbach tail parameter E° is lower than 50 meV and not sensitive to Ts ; the dangling bond density is around 4.107 cm−3, which is higher by a factor of 100 than in a-Si:H. Preliminary transport measurements indicate that the Fermi level density of states is larger than 1018 cm−3.eV−1.

The ambipolar diffusion length (L*) and the effective lifetime (τ*) in undoped a-Si:H films have been measured by the Flying Spot Technique (FST). This technique consists in measuring the decay range of charges in Schottky barriers and the potential induced by a laser beam with a constant motion rate. The data show that the FST measurements are quite suitable to determine separately the effective diffusion length and the effective lifetime of the carriers, taking into account the surface effect. The results also indicate that the photosensitivity is not the best parameter to characterize film quality. Indeed, films with high photosensitivity can present a poor lifetime and diffusion length that make them unsuitable to be used in optoelectronic devices. The results obtained by FST have been correlated with the Density of States (DOS) inferred by the Space Charge Limited Current (SCLC) measurements performed in the temperature range of 300–430 K and by the Constant Photocurrent Method (CPM) in forward biased Schottky diodes at room temperature. The dependence of the DOS on temperature shows that the SCLC technique is clearly influenced by the thermally activated carriers.

Here the species detected by a quadropole mass analyser and resulting from the decomposition of SiH4 induced by a plasma [using a diode-like PECVD (Plasma Enhanced Chemical Vapour Deposition) system, in which has been varied the following experimental conditions: temperature of the substrate (Ts); deposition pressure (pd); power density (dp) and electrode geometries], have been correlated with the electro-optical properties and Density of States (DOS), inferred from Constant Photocurrent Measurements (CPM). The mass spectra analysis reveal the existence of atomic and molecular hydrogen as well as on the SiH3 and SiH2 species whose ratios are strongly dependent on electrode geometries used, that also roles the adequated deposition conditions for producing good quality materials. Thus, for the recessed electrode we observe an enhancement on plasma density at low self bias voltage (Vdc), leading to the production of high quality films at Ts = 180 °C, using moderate dp and pd in the range of 75–80 mtorr, with Vdc ≃ 8 V. On the other hand, for the grid electrode we observe an enhancement in molecular hydrogen at growth surface for high Ts, leading to the production of the best films for Ts in the range of 280–300 °C, using dp ≃ 25 mWcm−3, pd ≃ 100 mtorr and Vdc ≃ 30 V. For both electrodes we observe that high bombardment is important for getting microstructures, defect free and dense. The CPM data show that DOS increases with pd indicating that the enhancement on the number of collisions in the gas phase lead to the increase on chain polymerization formation. We also observe that the hydrogen coverage onto the growth surface is important for activating the growth mechanism, producing homogeneous material with a low DOS.

The room temperature cathodoluminescence (CL) of diamond-1ike materials produced by glow discharge is reported. The CL spectra of these samples (4 eV optical gap) display two bands located at 2.3 eV and 2.8 eV which are frequently found in natural and synthetic diamond. X-ray diffraction spectra (before electron bombardment) are identical to those obtained in natural powder diamond, confirming the existence of crystalline particles. The room temperature CL of phosphorous doped samples is also reported.

Structural changes in amorphous silicon carbide samples (3.0 eV optical gap) associated with electron bombardnent are presented and elucidated by IR studies.

Modulated primary photocurrent (MPC) studies on pin structures show spatial variations of the gap state distribution across the i-layer that can be correlated with Fermi level shifts by band bending towards interfaces. These results as well as reverse bias annealing effects are explained in terms of the defect pool model. It is demonstrated that MPC measurements are basically identical to TOF measurements with clear advantages in the post-transit time regime.

We employ total yield photoelectron spectroscopy to measure the density of occupied states at the clean a-SixGe1_x:H alloy surface. The near surface defect states are observed to lie at 0.57 eV above the valence band edge with a density of 4×l017 cm−3, independent of Ge content. The valence band tail characteristic energy is also measured to be independent of alloy composition with an average value of 54 meV. We demonstrate that thermodynamic equilibrium at the surface between weak bonds (forming the valence band tail) and the dangling bonds provides an excellent description of the experimental data and explains why the surface state density in a-Si:H cannot be lowered below the 1011 to 1012 cm−2 range.

The interface structure of a-Si:H films deposited by glow discharge decomposition of Si2H6 on SnO2 was characterized on an atomic scale with the use of in situ XPS. The method analyzes the variation of the intensity of photoelectron emission originating from the bottom SnO2 layer and transmitting through the upper a-Si:H layers of various thicknesses. It was evaluated that a-Si:H was partially oxidized as it grew to a thickness of about 15 A, while the SnO2 bottom layer was reduced to metallic Sn and SnOx (0 < x < 2) states as deep as 2 A and 12 A from the interface, respectively,

A method for the determination of surface and interface states is suggested, based on the observation of interference fringe pattern in photo-thermal spectra. Some experimental results are presented and interpreted by means of a theoretical approach. The method allows in most cases to get an evaluation of surface and interface density of states.

The performance and reliability of silicon-based hetero-structure devices depend critically on their interfacial characteristics. Here we present high-resolution TEM (HRTEM) results for the a-SiC:H/c-Si (100) interface, substrate doping, and interface electrical characteristics derived from an Al/undoped a-SiC:H/p-Si metal-insulator-semiconductor (MIS) structure. The HRTEM study reveals an interface with a maximum asperity of three atomic planes. The substrate dopant profile for depths less than an extrinsic Debye length from the interface is computed from an iterative fit to the C-V data. A density of interface traps (Dit) of 1011 eV−1cm−2 at midgap is obtained.

A detailed study of hydrogenated amorphous silicon (a-Si:H) surfaces during etching and subsequent low temperature (T≤200°C) oxidation was carried out using in-situ and spectroscopic ellipsometry (SE). The microstructural information from SE is correlated with the properties of the corresponding evaporated Pd Schottky barrier structures. Oxide layers thinner than ∼ 10 Å have little effect on the diode characteristics and electron surface recombination velocities. This is consistent with a porous structure for these oxides. Evidence is found from growth kinetics that higher density, compact oxide growth occurs for greater thicknesses. These oxides have a large effect on electron transport and surface recombination velocities consistent with the formation of metal-insulator-semiconductor structures.

We report high-resolution small-angle x-ray reflectivity measurements on plasma-deposited a-Ge:H/a-Si:H periodic amorphous multilayers. We show that reflectivity spectra are quite complex, involving components due to relatively smooth and rough layer interfaces at different depths in the sample respectively. Model computations show that interface roughness increases from 3 É at the substrate-sample interface to more than 15 É at the top surface of a 0.5 μm multilayer stack.

We present a new analysis approach which combines photoconductivity and photother-mal deflection spectroscopies in a-Si:H thin films of various thicknesses to profile electronic defects.

Amorphous multilayers with low interface defect density were deposited in an ultrahigh-vacuum RF-glow discharge reactor by computer-controlled fast gas switching. An extensive film characterization was done including determination of carbon content by PES (proton enhanced scattering), grazing-incidence X-ray diffraction, measurement of ambipolar diffusion length, FTIR spectroscopy and hydrogen effusion. From PDS (photothermal deflection spectroscopy) a defect density of below 1011 cm−2 per interface is found.

Many of the photoelectronic properties of a-Si:H, including photoconductivity and ambipolar diffusion length, were observed to deteriorate rapidly with decreasing film thickness. Calculations show that the thickness dependence can be attributed mainly to the effect of surface recombination. In fact, any surface recombination velocity above 100 cm/s will result in an equally good fit to the experimental data. We conclude that the intrinsic photoelectronic properties of bulk a-Si:H and other related materials can be measured only on thick samples (≥1μm) and by using uniformly absorbing light (at long wavelengths).

Amorphous silicon models have been computer-generated by melt-quenching and film deposition molecular dynamics simulations, employing classical interatomic Si-potentials. The structural, vibrational and electronic properties of these models is described. Dangling-bond gap states are well localized whereas, floating bonds gap states are considerably less localized with wavefunction amplitudes on the neighbors of the five-coordinated atom. In contrast to melt-quenched models, the a-Si films displayed voids, a 15–28% lower density than c-Si, and no five- coordinated atoms. A-Si:H models with 5 and 22% hydrogen, and both monohydride and dihydride species, have also been developed.

A new hydrogenated amorphous silicon (a-Si:H) density of states (d.O.s.) in+cluding the transition levels of both neutral (T3o) and charged (T3+ and T3−) dangling-bond defects is proposed. We derive closed-form and numerical solutions for the d.o.s. from a thermodynamic equilibrium theory of defect concentrations in which material inhomogeneity is assumed to give rise to ∼1020 cm−3 of electrostatic potential fluctuations. The connection between thermodynamic transition level energy and defect formation energy implicit in this and other “defect pool” models is included explicitly in the calculation. We calculate the d.o.s. for a range of parameters and for different values of Fermi energy. We apply the calculated d.o.s. to explain and unify various experimental results in a-Si:H. In particular, we reconcile recent depletion-width-modulated ESR data with the near-perfect Curie law T-dependence of the dangling-bond spin density observed by several groups. It is seen +that the depletion results in roughly equal numbers of T3−T3–>° and T3°–>T3+ transitions despite the positive value of effective correlation energy. We also discuss possible sources of the short-to-medium range potential fluctuations in amorphous silicon.

The existence of saturation (or steady state) in the density of light-induced defects in amorphous Si:H is shown to have major importance for the interpretation of the nature and origin of these defects. First, a number of characteristics of the steady-state and transient responses to light and temperature are described and contrasted. These lead to the conclusion that the saturation value is the only useful criterion of the number of defects in these materials. We then describe a new atomic model for defects, unifying both dopant-induced and light-induced defects. This model invokes foreign atoms in defects, and saturation reflects the limitation imposed by the numbers of such atoms. Many other observed properties of defects are explained by this model.