Physical properties of amorphous gallium arsenide

The physical properties of crystalline III-V compounds with zincblende structure are well known [1]. The situation of tetrahedral coordinated amorphous semiconductors is much more complicated. They can be easily investigated, but large structural variations can be found with the sample history. This makes the analysis of the experimental data difficult and comparison among data collected on different samples not simple. The deposition conditions and possible annealing processes influence physical properties of the material. The amorphous compounds contain defects, which alter the density of states. They can be dangling bonds or bonds between like atoms, i.e. wrong-bonds. When the atoms involved remain tetrahedral bonded, it is possible to consider two types of like-bonds generated by an arsenic atom on a gallium site or vice versa. If the deposition conditions lead to isolated like-bonds, we can consider each of them separately. Thus, arsenic like-bond act as deep donors and gallium like –bonds act as deep acceptor. The bonding in III-V is largely ionic and thus chemical disorder produces large changes through the coulomb interaction. The degree of tolerable chemical disorder depends on the compound ionicity, which determines the energy cost of a wrong-bond compared to the normal heteropolar bond. Both structural (dangling bond type) and chemical (wrong bond type) defects in amorphous III-V compounds introduce states in the mobility gap and/or at the band edges. Configurational disorder refers to variations in angle bond and bond lengths from values characteristic of the crystal. Additional problems come from the necessity to control the film composition. Two desiderated properties of the film are in contradiction: amorphicity and stoichiometry. At stoichiometry, a continuous random network (CRN) best describes III-V materials with even-membered rings only. The gap density of staes is very similar to that of the crystalline compound, with an addition of states between the upper two peaks of the valence band. All this is a result of the loss of long-range order and not of the presence of As-As bonds. Wrong-bonds seem not present and this can be due to the fact that Ga and As have almost equal atomic radii. The structure of the paper presents in Sec.1 the most general properties of material deposited with suitable growth parameters. Sec. 2 reviews composition, structural and morphological properties, considering also theoretical results, while Sec. 3 is devoted to theoretical and experimental aspects of the density of states. Sec. 4 and Sec. 5 present optical results and phonon spectra, respectively. Finally, Sec. 6 discusses the electrical transport properties and Sec. 7 applications and devices. We used a probably unusual structure of the paper: in fact, we chosen to present practically a “card” for each quoted paper, giving the main experimental data, as an immediate reference.