E.A. Davis

Shallow donor state of hydrogen in indium nitride

The nature of the electron states associated with hydrogen in InN has been inferred by studying the behavior of positive muons, which mimic protons when implanted into semiconductors. The muons capture electrons below 60 K, forming paramagnetic centers with a binding energy of about 12 meV. Together with an exceedingly small muon-electron hyperfine constant indicative of a highly delocalized electron wave function, the results confirm the recently predicted shallow-donor properties of hydrogen in InN.

Transient photodarkening in amorphous chalcogenides

Abstract Illumination of amorphous arsenic chalcogenides with band-gap light induces photodarkening and volume expansion. In addition to metastable changes, which persist after the light is switched off, transient effects occur during and after illumination. We present measurements of these metastable and transient effects in evaporated films of a-As 2 S 3 , a-As 2 Se 3 and a-Se. The results are discussed in terms of a model in which, on photoexcitation, layer- or chain-like clusters of atoms expand and slip relative to each other.

The a-SiOx:Hy thin film system I. Structural study by IR spectroscopy

Abstract A systematic study of the trends in the IR spectra of a-SiO x :H y films with both y and x is presented for the first time. The measurements carried out on four series of room temperature rf sputtered films reveal a dramatic decrease in the concentration of bonded hydrogen in the films as x increases beyond 0.7. The spectral features are compared with those of previously reported low-oxygen high-temperature glow-discharged films, and it is found that the x = 0.7 series of films has the monohydride HSi 3 O as the predominant structural unit, while the x = 1.1 and 1.5 series contain this units as well as isolated dihydride units. The fine structure in the SiH stretch peak generally supports the above conclusions and explicitly indicates the presence of HSi 3 O, H 2 Si 2 O, HSi 2 O 2 and HSiO 3 units in the x = 1.5 series of films.

Optical properties of amorphous SiNx(:H) films

The optical absorption edges and the dispersion behaviour of the refractive index have been determined as a function of composition x for thin films of a-SiNx(:H), prepared (i) by glow-discharge decomposition of a gaseous mixture of silane and ammonia and (ii) by radio-frequency sputtering of silicon in an argon-nitrogen(-hydrogen) atmosphere. The data for films with and without hydrogen have been used to evaluate certain characteristic energies and these are then related to other experimental results, in particular those from photo-emission measurements.

Crystallization of amorphous selenium and As0.005Se0.995

The effect of temperature on the crystallization kinetics of bulk amorphous selenium and the alloy As0.005Se0.995 has been studied using differential scanning calorimetry. A time-temperature-transformation (TTT) curve has been plotted from isothermal results over the temperature range 320 to 490 K, and shows the presence of two minima for both materials. Using the empirical Avrami expression for solid-state transformations, a number of kinetic parameters has been determined.

Hydrogen in silicon

Abstract Hydrogen incorporated intentionally or inadvertently in silicon is associated with important effects that can be beneficial or detrimental to material properties and device performance. In crystalline or polycrystalline silicon, its role in passivating surfaces and impurities is an important part of silicon technology. In amorphous silicon its presence satisfies dangling bonds and relieves strain in the network. However, hydrogen can also lead to defect creation and, in a-Si:H, its motion is involved in a variety of metastable phenomena. Theoretical calculations and experimental studies (the latter principally using muonium to simulate hydrogen) have given a fairly clear picture of the sites and energy levels associated with isolated hydrogen. The situation in a-Si:H, in which hydrogen exists in a variety of bonding configurations, is much more complex and the interplay between structural and electronic effects has not been clearly resolved. Nevertheless, a reasonably coherent description of many phenomena is beginning to emerge.

Shallow versus deep hydrogen states in ZnO and HgO

The muonium states mimicking interstitial hydrogen in ZnO and HgO are compared. Whereas in ZnO a theoretically predicted shallow donor state is confirmed, in HgO we find a considerably deeper state. The respective ionization temperatures are around 40 K and 150 K and the donor ionization energies are 19±1 and 136±3 meV, deduced from the temperature dependence of the µSR (muon spin-rotation) signal amplitudes. The µSR spectra provide a comprehensive characterization of the undissociated paramagnetic states: the hyperfine parameters, which measure the electron spin density on and near the muon, differ by a factor of ~30. These define a hydrogenic radius of 1.1 nm in ZnO but indicate a much more compact electronic wavefunction in HgO, more akin to those of Mu* and the AA9 centre in Si. These data should largely carry over to hydrogen as a guide to its electrical activity in these materials.

The effect of varying substrate temperature on the structural and optical properties of sputtered GaAs films

Stoichiometric GaAs films have been prepared by RF sputtering over an appreciable range of substrate temperature Ts. The structural and optical properties of these films have been investigated by means of a variety of experimental techniques. Transmission electron microscope (TEM) experiments reveal that for Ts>or= approximately 70 degrees C the structure consists of nanocrystals embedded in an amorphous network. EXAFS, XPS and infrared experiments are consistent with each other, indicating that configurational disorder in the amorphous network of the GaAs films decreases as Ts is raised. Most of the disorder relates to bond-angle disorder; the spread in bond angle around Ga atoms is found to be less than that around As atoms for a given substrate temperature. The changes observed in the fundamental absorption edge with increasing Ts are discussed in relation to various possible defects in amorphous GaAs.

Free-carrier behaviour in a-Si1−yNiy:H

Abstract The reflectivity and transmission of a-Si1−yNiy:H alloys, prepared by radio-frequency sputtering, have been measured for films with nickel concentrations on both sides of the semiconductor-metal transition. With increasing nickel content, the absorption edge broadens and the optical gap decreases, reaching zero at approximately y = 0.26. For larger values of y, the reflectivity falls with increasing photon energy, a variation which can be interpreted in terms of free-carrier Drude-like behaviour with a very short scattering time.

Structural investigation of the a-Ga1−xAsx system

a-Ga 1− x As x ( x >0.5) films have been prepared by rf sputtering. The local structure and bonding configurations in these films have been investigated by EXAFS measurements. The As-coordination is shown to change from fourfold at stoichiometry towards threefold for As-rich films, while the Ga-coordination remains at 4 throughout the composition range studied. The EXAFS data are correlated with optical (visible and infrared) absorption experiments undertaken on the same films to provide a consistent picture of the local structure.