Beyond Spatial Correlation Effect in Micro-Raman Light Scattering: An Example of Zinc-blende GaN/GaAs Hetero-interface

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Adsorption and desorption kinetics of gallium atoms on 6H-SiC(0001) surfaces

Gallium (Ga) surface adsorption and desorption kinetics on 6H-SiC(0001) are investigated using reflection high-energy electron diffraction. It is found that for Ga adsorption, a wetting layer bonds strongly to the SiC(0001) surface. Additional Ga atoms form droplets on top of the wetting layer. The Ga droplets behave like a metallic liquid. The activation energies for desorption are determined to be 3.5 eV for Ga in the wetting layer and 2.5 eV for Ga in the droplets. It is further found that the desorption of Ga atoms from the wetting layer follows a zero-order kinetics, i.e., the desorption rate is independent of the number of adsorbed atoms. ©2000 The American Physical Society.published_or_final_versio

Electrically and magnetically controlled optical spanner based on the transfer of spin angular momentum of light in an optically active medium

An optical spanner is a light beam that can exert a torque on an object. It is demonstrated in this Rapid Communication that, with the aid of applied electric and magnetic fields, a light beam with initially linear polarization and initially zero total spin angular momentum can interact with an optically active medium, resulting in a change of the ratio of left-handed circularly polarized photons to right-handed ones. Thus the total spin angular momentum of the light is changed, which leads to a torque, creating an electrically and magnetically controlled optical spanner on the medium. For a linearly polarized 632.8 nm laser beam incident on a 100-mu m-long Ce:Bi12TiO20 whisker crystal with 5 mu m radius, if the magnetic field is fixed at -1.8 T, both the left- (right-)handed circularly polarized photon number and the total spin angular momentum vary with the applied electric field in a sinusoidal way, which means the torque exerted by the optical spanner on the crystal also varies sinusoidally with the electric field. It is found that at 50 (or-50) kV/cm, 56% right- (left-)handed circularly polarized photons are translated into left- (right-)handed ones, which corresponds to a transfer of 0.56h spin angular momentum contributed by each photon

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

GaN (0001) films grown by molecular beam epitaxy (MBE) were studied using scanning tunneling microscopy (STM). 'Ghost' islands were observed on surfaces grown under excess Ga conditions. These ghost islands were associated to a metastable, intermediate nucleation state of the surface.published_or_final_versio

Initial stage of GaN growth and its implication to defect formation in films

In situ scanning tunneling microscopy (STM) observations of initial growth processes of GaN by molecularbeam epitaxy reveal important differences between growth on vicinal versus flat SiC(0001) substrates. Based on stop-growth STM studies, we explain why there are orders of magnitude reductions in the density of threading screw dislocations in the vicinal films. It is shown that on vicinal surfaces, three-dimensional (3D) islands develop into a characteristic shape. The islands coalesce much sooner than on flat surfaces. Consequently, fewer defects are created at their boundaries.published_or_final_versio

Comparative study on the broadening of exciton luminescence linewidth due to phonon in zinc-blende and wurtzite GaN epilayers

The broadening of exciton luminescence linewidth due to phonon in zinc-blende and wurtzite GaN epilayers was discussed. The coupling parameters between exciton and acoustic and longitudinal optical phonons were obtained for both structures. The analysis showed that the coupling constants of both exciton-acoustic optial phonon coupling and exciton-longitudinal optical phonon coupling for zinc-blende GaN were almost twice as much as the corresponding values of wurtzite GaN.published_or_final_versio

Evidence for a Type-II band alignment between cubic and hexagonal phases of GaN

The study of photoluminescence spectra of a series of thin, undoped, hexagonal GaN films containing cubic GaN inclusions grown by molecular-beam epitaxy on 6H-SiC was presented. It was shown that an emission peak at ∼3.17 eV in thin, hexagonal GaN films exhibits behaviors typical of a spatially indirect transition. The values of the band offsets extracted from the data were in good agreement with theoretical predictions.published_or_final_versio

Anisotropic step-flow growth and island growth of GaN(0001) by molecular beam epitaxy

GaN(0001) thin films are grown using radio frequency plasma assisted molecular beam epitaxy. By changing the growth temperature, anisotropic growth rate behavior is observed in both the step-flow growth mode and the 2D island growth mode. Tunneling scanning microscopy reveals, in the step-flow growth mode, strong influences from the growth anisotropy on the shape of the terrace edges, resulting in striking differences between hexagonal and cubic films. In the 2D nucleation growth mode, triangularly shaped islands are formed. The significance of growth anisotropy to growing high quality GaN films is discussed.published_or_final_versio

Direct observation of a Ga adlayer on a GaN(0001) surface by LEED Patterson inversion

A low-energy electron diffraction (LEED) Patterson function (PF) with multiple incident angles is used to obtain three-dimensional interatomic information of hexagonal GaN(0001) grown on a 6H-SiC(0001)-√3 x √3 surface. A Ga-Ga atomic pair between the Ga adlayer and the terminating Ga layer is observed in the LEED PF. This provides direct experimental evidence to support the structural model proposed by first-principles calculations. The LEED PF also shows that the GaN film has a hexagonal structure and the surface has single-bilayer steps.published_or_final_versio

Step bunching of vicinal GaN(0001) surfaces during molecular beam epitaxy

Step bunching of vicinal GaN(0001) surface during epitaxial growth is observed by scanning tunneling microscopy. Large step stiffness and repulsive step-step interaction are suggested based on step morphology observations. The size of the bunch changes with time, depending on the direction in which the substrate is heated by a direct current. This observation provides evidence for the electromigration effect causing the step bunching, and from the field dependence we infer that adatoms, which are likely N, have effective positive charges. ©2000 The American Physical Society.published_or_final_versio