Structure and Stability of Si(114)-(2x1)

We describe a recently discovered stable planar surface of silicon, Si(114). This high-index surface, oriented 19.5 degrees away from (001) toward (111), undergoes a 2x1 reconstruction. We propose a complete model for the reconstructed surface based on scanning tunneling microscopy images and first-principles total-energy calculations. The structure and stability of Si(114)-(2x1) arises from a balance between surface dangling bond reduction and surface stress relief, and provides a key to understanding the morphology of a family of surfaces oriented between (001) and (114).Comment: REVTeX, 4 pages + 3 figures. A preprint with high-resolution figures is available at http://cst-www.nrl.navy.mil/papers/si114.ps . To be published in Phys. Rev. Let

Surface photovoltage in undoped n-type GaN

Steady-state and transient surface photovoltage (SPV) in undoped GaN is studied in vacuum and air ambient at room temperature and 400 K with a Kelvin probe. The results are explained within a phenomenological model accounting for the accumulation of photogenerated holes at the surface, capture of free electrons from the bulk over the near-surface potential barrier, and emission of electrons from surface states into the bulk. Simple analytical expressions are obtained and compared with experimental results. In particular, the proposed model explains the logarithmic decay of the SPV after stopping illumination. Internal and external mechanisms of the SPV are discussed in detail. It is shown that an internal mechanism dominates at low illumination intensity and/or small photon energies, while external mechanisms such as charging of a surface oxide layer and photoinduced processes play a significant role for above-bandgap illumination with sufficient intensity

Photoadsorption and photodesorption for GaN

The effect of an ambient environment on the surface photovoltage and photoluminescence observed for GaN is studied. In air ambient the upward band bending gradually increases under UVillumination and is explained by the photoinduced chemisorption of surface adsorbates. Specifically, the increase in negative surface charge is consistent with the transfer of electrons from surface states or bulk to oxygen species physisorbed at the GaNsurface. In contrast, the upward band bending gradually decreases in vacuum under UVillumination and can be explained by the photoinduced desorption of these species. The photoadsorption and photodesorption of negatively charged species cause the surface depletion region to increase and decrease, respectively. This change in depletion region width is consistent with the observed decrease in photoluminescence intensity in air ambient and its significant increase in vacuum for a sample with low free electron concentration

Temperature-dependent Kelvin probe measurements of band bending in p-type GaN

The band bending in a Mg-doped, p-type GaN film grown by hydride vapor phase epitaxy was studied at various temperatures. At 295 K, the band bending in dark was calculated to be approximately −1.5 eV. However, when the sample was heated to 600 K for 1 h in dark before performing a measurement at 295 K, the calculated value of band bending in dark became about −2.0 eV. These results are explained by the fact that increasing the sample temperature exponentially increases the rate at which the band bending restores and allows for a more accurate value of band bending to be measured

Para to Ortho transition of metallic dimers on Si(001)

Extensive electronic structure calculations are performed to obtain the stable geometries of metals like Al, Ga and In on the Si(001) surface at 0.5 ML and 1 ML coverages. Our results coupled with previous theoretical findings explain the recent experimental data in a comprehensive fashion. At low coverages, as shown by previous works, `Para' dimers give the lowest energy structure. With increasing coverage beyond 0.5 ML, `Ortho' dimers become part of low energy configurations leading toward a `Para' to `Ortho' transition at 1 ML coverage. For In mixed staggered dimers (`Ortho' and `Para') give the lowest energy configuration. For Ga, mixed dimers are non-staggered, while for Al `Para' to `Ortho' transition of dimers is complete. Thus at intermediate coverages between 0.5 and 1 ML, the `Ortho' and `Para' dimers may coexist on the surface. Consequently, this may be an explanation of the fact that the experimental observations can be successfully interpreted using either orientation. A supported zigzag structure at 0.5 ML, which resembles ${\rm (CH)_x}$, does not undergo a dimerization transition, and hence stays semi-metallic. Also, unlike ${\rm (CH)_x}$ the soliton formation is ruled out for this structure.Comment: 8 pages, 6 figure

Investigation of inversion domains in GaN by electric-force microscopy

Inversion domains in III-nitride semiconductors degrade the performance of devicesfabricated in them. Consequently, it is imperative that we understand their electrostatic manifestation, the growth conditions under which such domains form, and an effective means of their identification. In what is nominally referred to as Ga-polarity samples, N-polarity domains have a polarization that is reversed with respect to the remainder of the surface, and therefore, have a different potential under strain. We have used surface-potential electric-force microscopy (SP-EFM) to image the electrostaticsurface potential of GaNgrown on sapphire, which is strained due to the thermal mismatch between the substrate and GaN. Employing a control sample with side-by-side Ga- and N-polarity regions, we have established the EFM mode necessary to identify inversion domains on GaN samples grown by molecular-beam epitaxy. This method is not sensitive to topology and has a spatial resolution of under 100 nm. The measured surface potentials for Ga-face and N-face regions are +25±10 and −30±10 mV, respectively, with respect to the sapphire substrate, where the sign is consistent with Ga- and N-polarity GaN under compressive strain due to thermal mismatch with the sapphire substrate

Electronic behavior of the Zn- and O-polar ZnO surfaces studied using conductive atomic force microscopy

We have used conducting atomic force microscopy (CAFM) to study the morphology and electronic behavior of as-received and air-annealed (0001) Zn- and (0001¯) O-polar surfaces of bulk ZnO. Both polar surfaces exhibit relatively flat morphologies prior to annealing, which rearrange to form well-defined steps upon annealing in air at 1050 °C for 1 h. Long-term exposure to air results in surface layer pitting and the destruction of steps for both the as-received and air-annealed (0001¯)surfaces, indicating its enhanced reactivity relative to the (0001) surface. CAFM I-V spectra for polar surfaces are similar and indicate Ohmic to rectifying behavior that depends on the maximum applied ramp voltage, where higher voltages result in more conducting behavior. These data and force-displacement curves suggest the presence of a physisorbed H2O layer, which is removed at higher voltages and results in higher conduction

Effects of hydrogen on the morphology and electrical properties of GaN grown by plasma-assisted molecular-beam epitaxy

We study the effect of introducing hydrogen gas through the rf-plasma source during plasma-assisted molecular-beam epitaxy of GaN(0001). The well-known smooth-to-rough transition that occurs for this surface as a function of decreasing Ga flux in the absence of H is found to persist even with H present, although the critical Ga flux for this transition increases. Under Ga-rich conditions, the presence of hydrogen is found to induce step bunching (facetting) on the surface. Conductive atomic force microscopy reveals that leakage current through dislocation cores is significantly reduced when hydrogen is present during the growth

Nonpolar m-plane GaN on patterned Si(112) substrates by metalorganic chemical vapor deposition

The concept of nonpolar (11¯00) m-plane GaN on Si substrates has been demonstrated by initiating growth on the vertical (1¯1¯1) sidewalls of patterned Si(112) substrates using metalorganic chemical vapor deposition. The Si(112) substrates were wet-etched to expose {111} planes using stripe-patterned SiNx masks oriented along the [1¯10] direction. Only the vertical Si(1¯1¯1) sidewalls were allowed to participate in GaNgrowth by masking other Si{111} planes using SiO2, which led to m-plane GaNfilms.Growth initiating on the Si(1¯1¯1) planes normal to the surface was allowed to advance laterally and also vertically toward full coalescence. InGaN double heterostructure active layers grown on these m-GaN templates on Si exhibited two times higher internal quantum efficiencies as compared to their c-plane counterparts at comparable carrier densities. These results demonstrate a promising method to obtain high-quality nonpolar m-GaN films on large area, inexpensive Si substrates

Study of SiNx and SiO2 passivation of GaN surfaces

The optical properties of GaN films have been found to be sensitive to SiNx and SiO2 surface passivation. The main effect of such passivation on photoluminescence(PL) data is an increase of the PL intensity for near-band-edge emission. This effect is attributed to the removal of oxygen from the surface of GaN and the subsequent formation of a protective layer during passivation. The increase in PL intensity is more pronounced for samples passivated with SiO2, which demonstrate initially lower PL intensity and a lower equilibrium concentration of free electrons. A nearly constant band bending of approximately 1.0 eV at the surface has been observed for as-grown and passivated samples by scanning Kelvin probe microscopy (SKPM). This constant value is explained by pinning of the Fermi level at the surface. In addition, we have demonstrated that passivation of the GaN surface between the contacts of a Schottky diode leads to a reduction of the leakage current observed at reverse bias. It was found that the surface potential measured by SKPM increases as a function of distance from the Schottky contact much faster after SiNx passivation. We suggest that the passivation reduces the total density of surface states and therefore reduces surface recombination