Structure in Amorphous Semiconductors: Extrinsic and Intrinsic

Abstract

A detailed study of the atomic-scale structure of amorphous semiconductors utilizing Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy is reported. Samples were examined in both extrinsic (preparation specific) and intrinsic (minimum energy) forms. The amorphous elemental semiconductors exhibit structural disorder in the form of both bond-length and bond-angle distortions. As formed, amorphous Ge displays a fabrication-dependent non-Gaussian inter-atomic distance distribution with implantation-induced defects in the amorphous phase accommodated as three- and five-fold coordinated atoms. Thermal annealing yields a reduction in both bond-length and bond-angle distortion, as measured by EXAFS and Raman spectroscopies, respectively, where the total relaxation enthalpy is consistent with differential scanning calorimetry measurements. In a fully-relaxed state, amorphous Ge retains four-fold coordination and the inter-atomic distance distribution is Gaussian and independent of the implanted ion dose. The amorphous compound semiconductors contain chemical disorder in addition to structural disorder. As formed, amorphous compound semiconductors including the Ga and In phosphides and arsenides all exhibit chemical disorder manifested as homopolar bonding. Though low-temperature thermal annealing lessens the Debye-Waller factor and the homopolar bonding fraction, the latter is not eliminated. Point-defect annealing in the form of homopolar bond annihilation is thus operative during structural relaxation of the amorphous phase. Residual chemical disorder necessitates the presence of odd-membered rings and thus demonstrates the elastic energy required to produce a continuous-random-network without homopolar bonding (or equivalently with only even-membered rings) must exceed the Coulomb energy inherent with anion-anion or cation-cation repulsion.