M. J. Thompson

Effects of doping on transport and deep trapping in hydrogenated amorphous silicon

We report a quantitative comparison of the trapping rates of carriers at charged and neutral dangling bonds in hydrogenated amorphous silicon (a‐Si:H). The data are obtained from time‐of‐flight photoconductivity studies of doped and undoped samples. The temperature dependence of the trapping rates, as well as the effect of boron doping on the hole drift mobility, is also reported.

Electron drift mobility in amorphous Si: H

Abstract The ‘time of flight’ technique has been employed to study the transient photoconductivity due to excess electron carriers in amorphous Si:H, over a wide range of temperature and electric field. The data are analysed in terms of the temperature/field dependence of the carrier transit time, and also in terms of the variation in pulse shape with temperature. In both cases, the behaviour suggests interactions with a tail of states extending about 0·15 eV from the band edge and showing an almost linear variation of trap density with energy over at least the range 0·085 to 0·145 eV. The capture cross-section of the centres is estimated to be of order 10−17–10−l6cm2. Interaction with states beyond the tail is comparatively weak, and is dominated by deep trapping into centres believed to be dangling bonds.

Interference enhanced Raman scattering study of the interfacial reaction of Pd on a‐Si:H

The application of interference enhanced Raman scattering to probe the structure of metal/semiconductor interfaces is described. This technique is used to study the interface structures of Pd on hydrogenated amorphous Si (a‐Si:H) with particular attention to the effects of a native oxide layer on the a‐Si:H. It is found that for deposition on lightly oxidized a‐Si:H, ∠20 A of Pd is consumed to form a crystalline silicide interfacial structure with composition near Pd2Si. Annealing causes growth of this phase and further consumption of Pd until a second phase of Pd2Si is observed. If the deposition is repeated on more heavily oxidized a‐Si:H, no silicide formation is observed even at annealing temperatures of 400 °C.

Electronic states at the hydrogenated amorphous silicon/silcon nitride interface

Electronic states at the hydrogenated amorphous silicon (a‐Si:H)/silicon nitride interface are observed to depend strongly on the order of deposition. When the nitride is on top of the a‐Si:H, there is an interface charge of ∼5×1011 electrons/cm2 and the interface Fermi energy is 0.25 eV from the conduction band. The bottom nitride is also in electron accumulation but with a much smaller space charge. Our results are in marked disagreement with recent data.

Silicide formation in Pd‐a‐Si:H Schottky barriers

This letter gives the first report of a direct correlation of Schottky barrier characteristics to silicide growth on a‐Si:H. Changes in diode ideality factor (from 1.2 to 1.05) produced by annealing can now be directly attributed to growth of Pd2Si as demonstrated by Raman spectroscopy. Unannealed samples show long‐term changes in characteristics at room temperature due to slow silicide growth. However, Pd Schottky barriers possess ideal stable characteristics once silicide growth is complete.

Amorphous Si prepared in a UHV plasma deposition system

Abstract Although glow discharge decomposition of SiH 4 has been demonstrated to yield a-Si:H films with potential for large area device applications, the a-Si:H films often have high impurity level (∼ 10 19 t - 10 20 /cc). Moreover, cross contamination of dopants at interfaces between differently doped layers is a problem. Since both issues may be the key to improving material quality, a UHV plasma deposition systems has been built to address them. By controlling all sources of contaminants such as chamber walls, gas lines and gas bottles, the impurity levels have been significantly lowered to ∼10 17 −10 18 /cc. In addition, sharp interfaces between differently doped layers are obtained, which allow the preparation of multilayer structures.

Schottky barrier amorphous-crystalline interface formation

Abstract The electrical properties of metal-a-Si:H contacts will be discussed. The relationship between the Schottky barrier contacts and the structural properties of the interface is reviewed. Polycrystalline silicides form at the interface of Pd and Pt on a-Si:H; the formation of the silicides is accompanied by only small changes in the Schottky barrier height. Au and Al cause crystallization of a-Si:H when annealed to 250°C. The crystallization with Au contacts creates very little change in the electrical properties of the interface; however, the Al Schottky barriers become near ohmic when crystalline Si is formed.

The electrical characterization of surfaces, interfaces and contacts to a-Si:H

Abstract The present work shows how transient photoconductivity can be used to characterize surfaces, interfaces, and contacts to hydrogenated amorphous silicon (a-Si:H). At a silicon nitride/a-Si:H interface an electron accumulation layer is found, whereas the native oxide induces a depletion layer. The accumulation is much greater when the nitride is deposited on top of the a-Si:H, rather than vice versa. The injection properties of n+ contacts are also investigated, and a study is made of the conditions under which these are partially or completely depleted, thereby affecting the carrier injection.

Interfacial reactions between Au and hydrogenated amorphous Si

Solid state reactions at the interface between thin films of Au and hydrogenated amorphous Si are probed by interference enhanced Raman scattering, TEM, scanning Auger microprobe, and Schottky electrical measurements. It is found that crystalline Si islands with a dimension of ∠1 μm grow dendritically after annealing to temperatures above 150 °C. The islands apparently grow laterally due to diffusion of Si through Au. This formation of Si crystallites occurs at a temperature much lower than the normal crystallization temperature of ≳650 °C for the hydrogenated amorphous Si. Through this process the Schottky barrier is not destroyed. A model is proposed to describe the formation process, which includes the low‐temperature Si diffusion into Au to form a Au–Si intermixed phase, and the agglomeration of the Au‐rich regions.

Hole carrier drift-mobility measurements in a-Si: H, and the shape of the valence-band tail

Abstract A detailed study has been performed concerning the temperature and electric-field dependence of hole carrier time-of-flight pulses in amorphous silicon. Various aspects of the experimental behaviour are employed in the estimation of the energy distribution and other characteristics of localized states in the films. Over the depth range 0·2 to 0·45eV, the trap concentration is found to vary more rapidly than the exponential form assumed in earlier studies, and a Gaussian tail is in better agreement with the data. Localized-state capture cross-sections are calculated as 1–3 × 10−16 cm2, and are essentially independent of depth over the range studied. Taking the present data in conjunction with the energy distribution of states as determined by other techniques allows an estimation of the mobility of free holes. The value of about 10cm2V−1s−1 thus obtained is an order of magnitude larger than the previous estimate from time-of-flight measurements, but is comparable to the figures suggested by other ...