Amorphous chalcogenides, of which selenium is the simplest representative, exhibit a number of unique properties such as the ability to undergo various transformations under the action of the bandgap light. On illumination, the absorption edge shifts to lower energies, and subsequent annealing near the glass-transition temperature leads to a recovery of the initial parameters as demonstrated in Figure 1. Such a photo-induced change could not be observed either in amorphous group IV semiconductors or a-As or in crystalline chalcogenides. A good review of the initial stage of these studies was made by de Neufville.

Reversible photodarkening can also be observed in pure chalcogens, but this process can be achieved only at lower temperatures, which is understandable if one takes into account that the glass-transition temperature of selenium is just above room temperature. This result indicates that there is a correlation between the temperature at which photodarkening is annealed out and the flexibility of the glassy network.

Reversible changes in the optical absorption are accompanied by (reversible) changes in electrical and photoelectric properties, volume, microhardness, glass-transition temperature, and dissolution rate in various solvents, to name a few. The totality of these changes has led investigators to the conclusion that the photoinduced changes in optical absorption are caused by changes in structure.

The dynamic change of the dangling bond (db) intensity in hydrogenated amorphous silicon (a-Si:H) during H2 and Ar plasma treatments was observed using an in-situ electron-spinresonance (ESR) technique. The experimental results show that the time to reach the steady state between gas-phase H atoms and the a-Si:H surface is less than 1 sec, and Ar plasma treatments create a top-surface region with an extremely high db density.

We report new results on microstructures and electronic properties of nanocrystalline silicon (nc-Si:H) films prepared by a triode PECVD method with a hydrogen diluted silane plasma. The transition from a-Si:H to nc-Si:H was obtained by varying the ratio of H2 to SiH4 while the bias of the grid electrode was fixed at 100.V with respect to the substrate. The threshold value of [H2]/[H2 ]+[SiH4] is about 93.3%. With increasing value of [H2]/[H2]+[SiH4], the volume fraction of crystallinity in nc-Si:H films increased from 12% to 50%, while the average grain size remaind constant. This is quite different from the structures of samples prepared by the conventional diode PECVD with H2 dilution. The traveling-wave technique was applied to study the transport properties of nc-Si:H films, and experimental results confirmed percolation transport in nc-Si:H films.

The spin-lattice relaxation times (T1) of conduction electron (CE) and dangling bond (DB) centers in μc-Si:H have been directly measured using the 3-pulse inversion recovery method. For both CE and DB, the inversion recovery curve follows a stretched exponential form. T1 of DB is about twice the T1 of CE, however the temperature dependence of T1 seems to be the same for both CE and DB and can be approximated by T−4 While the DB echo decay is modulated by both 29Si and 1H nuclei, we found no modulation of the CE echo decay by the H nucleus, indicating that CE centers are located in H-depleted phases in μc-Si:H. The modulation results are direct evidence that CE centers are located in the crystalline grains and DB centers in the amorphous phases.

In situ measurements of optical absorption and x-ray diffraction for small supported silver particles were performed under defined atmospheres at temperatures below 723 K. We found, for the first time, reversible changes both in optical absorption spectrum and lattice parameter of silver particles against the cyclic exchange of ambient gases between 5% H2/He and 1% 02/He. The present data indicate that the adsorption of oxygen results in a reduction in surface stress of silver particles, leading to a change in the shape of particles.

A novel technique has been proposed for determining the density-of-state (DOS) distribution in the mobility gap of highly resistive amorphous semiconductors, using amorphous/crystalline heterojunction structures. This technique has been tested and applied on undoped a-Si:H and a-SiGe:H films, covering the optical gap (E0 ) range of 1.30 to 1.76 eV. For undoped a-Si:H with E0 =1.76 eV, the peak of the midgap DOS distribution has been locatedst 0.85 eV below the conduction band edge, EC , with a value of 5.6×1015 cm-3 eV-1 . For undoped a-SiGe:H (E0 =1.55 eV) the same has been obtained 0.71 eV below EC with a magnitude of 7.9×1016 cm-3 eV-1. Those midgap states have been found to be correlated with singly-occupied dangling bonds.

Firstly, on the basis of the classical model on the surface process of adsorbed species, a guiding principle is proposed for the preparation of high-quality a-SiGe:H alloys via plasma-CVD technique. Secondly, a- SiGe:H alloys were prepared by the rf-discharge technique with three intentional modifications; (A) from SiH4/GeH4 using a diode reactor (conventional method), (B) from SiH4/GeH4 using a triode reactor (triode method), (C) from SiH4/GeH4/H2 mixture using the diode reactor (H2-dilution method). Finally, growth mechanism is discussed in terms of the selection of long-lifetime radicals in a plasma and the H-coverage factor of the growing surface, providing us a clue to materials design.