No abstract

Planar GaP epilayers on Si(111) are considered as virtual substrates for III–V-related optoelectronic devices such as high-efficiency nanowire-based tandem absorber structures for solar energy conversion, next generation LEDs, and fast photodetectors. Rotational twin domains in such heteroepitaxial epilayers are found to strongly impede vertical nanowire growth. We investigate the twin-induced defect density and surface morphology of B-type GaP/Si(111) virtual substrates in dependence on the GaP nucleation process by metalorganic chemical vapor deposition. By employing quantitative high-resolution X-ray diffraction (HR-XRD)), scanning electron and atomic force microscopy (SEM and AFM), we reveal the significant influence of nucleation temperature and substrate miscut direction on the formation of rotational twin domains during a two-step GaP growth approach. The epilayer defect density is drastically decreased by low temperature GaP nucleation on Si(111) misoriented 3° toward [1̅1̅2], where rotational twin domains are suppressed below 5% and the layers exhibit a smooth surface morphology. We demonstrate that these virtual substrates are highly suitable for vertical GaP nanowire growth.

Metalorganic vapor phase epitaxy of III-V compounds commonly involves arsenic. We study the formation of atomically well-ordered, As-modified Si(100) surfaces and subsequent growth of GaP/Si(100) quasisubstrates in situ with reflection anisotropy spectroscopy. Surface symmetry and chemical composition are measured by low energy electron diffraction and X-ray photoelectron spectroscopy, respectively. A two-step annealing procedure of initially monohydride-terminated, (1 × 2) reconstructed Si(100) in As leads to a predominantly (1 × 2) reconstructed surface. GaP nucleation succeeds analogously to As-free systems and epilayers free of antiphase disorder may be grown subsequently. The GaP sublattice orientation, however, is inverted with respect to GaP growth on monohydride-terminated Si(100).

Stable InP (001) surfaces are characterized by fully occupied and empty surface states close to the bulk valence and conduction band edges, respectively. The present photoemission data show, however, a surface Fermi level pinning only slightly below the midgap energy which gives rise to an appreciable surface band bending. By means of density functional theory calculations, it is shown that this apparent discrepancy is due to surface defects that form at finite temperature. In particular, the desorption of hydrogen from metalorganic vapor phase epitaxy grown P-rich InP (001) surfaces exposes partially filled P dangling bonds that give rise to band gap states. These defects are investigated with respect to surface reactivity in contact with molecular water by lowtemperature water adsorption experiments using photoemission spectroscopy and are compared to our computational results. Interestingly, these hydrogen-related gap states are robust with respect to water adsorption, provided that water does not dissociate. Because significant water dissociation is expected to occur at steps rather than terraces, surface band bending of a flat InP (001) surface is not affected by water exposure.