Current-induced migration of surface adatoms during GaN growth by molecular beam epitaxy

The influence of electric fields on surface migration of Gallium (Ga) and Nitrogen (N) adatoms is studied during GaN growth by molecular beam epitaxy (MBE). When a direct current (DC) is used to heat the sample, long distance migration of Ga adatoms and diffusion asymmetry of N adatoms at steps are observed. On the other hand, if an alternating current (AC) is used, no such preferential adatom migration is found. This effect is attributed to the effective positive charges of surface adatoms. representing an effect of electro-migration. The implications of such current-induced surface migration to GaN epitaxy are subsequently investigated. It is seen to firstly change the distribution of Ga adatoms on a growing surface, and thus make the growth to be Ga-limited at one side of the sample but N-limited at the other side. This leads to different optical qualities of the film and different morphologies of the surface. (C) 2001 Elsevier Science B.V. All rights reserved.

Observation of "ghost" islands and surfactant effect of surface gallium atoms during GaN growth by molecular beam epitaxy

We observe "ghost" islands formed on terraces during homoepitaxial nucleation of GaN. We attribute the ghost islands to intermediate nucleation states, which can be driven into "normal" islands by scanning tunneling microscopy. The formation of ghost islands is related to excess Ga atoms on the surface. The excess Ga also affect island number density: by increasing Ga coverage, the island density first decreases, reaching a minimum at about 1 monolayer (ML) Ga and then increases rapidly for coverages above 1 ML. This nonmonotonic behavior points to a surfactant effect of the Ga atoms.

Role of surface defect states in visible luminescence from oxidized hydrogenated amorphous Si hydrogenated amorphous Ge multilayers

Nanocrystalline Ge embedded in amorphous silicon dioxide matrix was fabricated by oxidizing hydrogenated amorphous Si/hydrogenated amorphous Ge (a-Si:H/a-Ge:H) multilayers. The structures before and after oxidation were systematically investigated. The orange-green light emission was observed at room temperature and the luminescence peak was located at 2.2 eV. The size dependence in the photoluminescence peak energy was not observed and the luminescence intensity was increased gradually with oxidation time. The origin for this visible light emission is discussed. In contrast to the simple quantum effect model, the surface defect states of nanocrystalline Ge are believed to play an important role in radiative recombination process. (C) 1999 American Institute of Physics. [S0003-6951(99)02425-0].

Suppression of self-sustained field domain oscillation in GaAs/AlAs superlattice by hydrostatic pressure at room temperature

The behavior of room temperature self-sustained current oscillations resulting from sequential resonance tunneling in a doped weakly-coupled GaAs/AlAs superlattice (SL) is investigated under hydrostatic pressure. From atmosphere pressure to 6.5 kbar, oscillations exist in the whole plateau of the I-V curve and oscillating characteristics are affected by the pressure. When hydrostatic pressure is higher than 6.5 kbar, the current oscillations are completely suppressed although a current plateau still can be seen in the I-V curve. The plateau disappears when the pressure is close to 13.5 kbar. As the main effect of hydrostatic pressure is to lower the X point valley with respect to Gamma point valley, the disappearance of oscillation and the plateau shrinkage before Gamma - X resonance takes place are attributed to the increases of thermoionic emission and nonresonant tunneling components determined by the lowest Gamma - X barrier height in GaAs/AlAs SL structure.

Nano-Ag on vanadium dioxide. II. Thermal tuning of surface plasmon resonance

Thermal tuning of the localized surface plasmon resonance (LSPR) of Ag nanoparticles on a thermochromic thin film of VO2 was studied experimentally. The tuning is strongly temperature dependent and thermally reversible. The LSPR wavelength lambda(SPR) shifts to the blue with increasing temperature from 30 to 80 degrees C, and shifts back to the red as temperature decreases. A smart tuning is achievable on condition that the temperature is controlled in a stepwise manner. The tunable wavelength range depends on the particle size or the mass thickness of the metal nanoparticle film. Further, the tunability was found to be enhanced significantly when a layer of TiO2 was introduced to overcoat the Ag nanoparticles, yielding a marked sensitivity factor Delta lambda(SPR)/Delta n, of as large as 480 nm per refractive index unit (n) at the semiconductor phase of VO2.

DETERMINATION OF SURFACE-ROUGHNESS OF INP (001) WAFERS BY X-RAY-SCATTERING

The surface roughness of polished InP (001) wafers were examined by x-ray reflectivity and crystal truncation rod (CTR) measurements. The root-mean-square roughness and the lateral correlation scale were obtained by both methods. The scattering intensities in the scans transverse to the specular reflection rod were found to contain two components. A simple surface model of surface faceting is proposed to explain the experimental data. The sensitivities of the two methods to the surface structure and the role of the resolution functions in the CTR measurements are discussed.

EFFECT OF SURFACE-STRUCTURES UPON ULTRATHIN-FILM INTERFERENCE-FRINGES

Effect of surface structures upon ultrathin film interference fringes generated from extremely thin films or epitaxial layers grown on semiconductor wafers has been studied. Since dark regions of fringes correspond to the places where the thin films are destroyed or absent, the fringes are investigated to detect uneven surfaces with undesired structures. Therefore, surface microstructures can be detected and characterized effectively by the modification of the fringes.

TEMPERATURE-DEPENDENCE OF THE LOW-FIELD MOBILITY OF MINIBAND CONDUCTION IN SUPERLATTICES

The existing interpretation of the T-1 temperature dependence of the low-field miniband conduction is derived from certain concepts of conventional band theory for band structures resulting from spatial periodicities commensurable with the dimensionalities of the system. It is pointed out that such concepts do not apply to the case of miniband conduction, where we are dealing with band structures resulting from a one-dimensional periodicity in a three-dimensional system. It is shown that in the case of miniband conduction, the current carriers are distributed continuously over all energies in a sub-band, but only those with energies within the width of the miniband contribute to the current. The T-1 temperature dependence of the low-field mobility is due to the depletion of these current-carrying carriers with the rise of temperature.

Temperature dependence of the low-field mobility of a very narrow miniband in the degenerate case

Perpendicular transport in a specially designed, doped and weakly coupled GaAs/AlAs superlattice is investigated. A linear current-voltage at a bias within +/-10 mV and a negative differential velocity effect at a bias of about +/-40 mV are observed at low temperatures. The miniband conductance near zero electric field is studied as a function of temperature. It is found that the measured conductance increases slightly as the temperature increases to about 30 K, decreases as the temperature rises from 30 K to 70 K, and then increases strongly above 70 K, indicating the existence of a mobility gap.

Effect of Surface Potential Barrier on the Electron Energy Distribution of NEA Photocathodes

By calculating the energy distribution of electrons reaching the photocathode surface and solving the Schrodinger equation that describes the behavior of an electron tunneling through the surface potential barrier,we obtain an equation to calculate the emitted electron energy distribution of transmission-mode NEA GaAs photocathodes. Accord- ing to the equation,we study the effect of cathode surface potential barrier on the electron energy distribution and find a significant effect of the barrier-Ⅰ thickness or end height,especially the thickness,on the quantum efficiency of the cath- ode. Barrier Ⅱ has an effect on the electron energy spread, and an increase in the vacuum level will lead to a narrower electron energy spread while sacrificing a certain amount of cathode quantum efficiency. The equation is also used to fit the measured electron energy distribution curve of the transmission-mode cathode and the parameters of the surface barri- er are obtained from the fitting. The theoretical curve is in good agreement with the experimental curve.

Current-induced migration of surface adatoms during GaN growth by molecular beam epitaxy

The influence of electric fields on surface migration of Gallium (Ga) and Nitrogen (N) adatoms is studied during GaN growth by molecular beam epitaxy (MBE). When a direct current (DC) is used to heat the sample, long distance migration of Ga adatoms and diffusion asymmetry of N adatoms at steps are observed. On the other hand, if an alternating current (AC) is used, no such preferential adatom migration is found. This effect is attributed to the effective positive charges of surface adatoms. representing an effect of electro-migration. The implications of such current-induced surface migration to GaN epitaxy are subsequently investigated. It is seen to firstly change the distribution of Ga adatoms on a growing surface, and thus make the growth to be Ga-limited at one side of the sample but N-limited at the other side. This leads to different optical qualities of the film and different morphologies of the surface. (C) 2001 Elsevier Science B.V. All rights reserved.