Ching-Lien Hsiao

Self-assembled vertical GaN nanorods grown by molecular-beam epitaxy

Dislocation-free vertical GaN pillars in nanoscale were grown on Si (111) surface through self-assembly by molecular-beam epitaxy. No extra catalytic or nanostructural assistance has been employed. These nanorods have a lateral dimension from ≲10 nm to ∼800 nm and a height of ≲50 nm to ≳3 μm protruding above the film, depending on the growth parameters. The top view of the nanorods has a hexagonal shape from scanning electron microscopy. Transmission electron microscopy shows that the nanorods are hexagonal, single crystal GaN along the c-axis. An extra peak at 363 nm originated from nanorods was observed in photoluminescence spectra at 66 K, which is ascribed to the surface states according to the results of surface passivation. Micro-Raman spectroscopy on a single nanorod reveals E1 and E2 modes at 559.0 and 567.4 cm−1, respectively. Large strain was observed in both the transmission electron micrograph and the Raman shift. A possible growth mechanism is discussed.

Generation of electricity in GaN nanorods induced by piezoelectric effect

Conversion of mechanical energy into electric energy has been demonstrated in GaN nanorods. The measurement was achieved by deflecting GaN nanorods with a conductive atomic force microscope PtIr tip in contact. The mechanism relies on the coupling between piezoelectric and semiconducting properties in GaN nanorod, which creates a strain field and drives the charge flow across the nanorod. The result shown here opens up an opportunity for harvesting electricity from wasted mechanical energies in the ambient environment, which may lead to the realization of self-powered nanodevices.

Micro-Raman spectroscopy of a single freestanding GaN nanorod grown by molecular beam epitaxy

Micro-Raman spectra were measured on a single freestanding GaN nanorod, which was grown by molecular beam epitaxy. A sharp linewidth of E2(high) mode of 2.1cm−1 measured in the x(y,y)x¯ configuration indicates the high crystalline quality of the nanorod. The angle-dependent Raman spectroscopy shows that the integrated intensities of these first-order Raman modes follow the theoretical sinusoidal functions. The forbidden E1(LO) mode that appeared in the x(z,z)x¯ scattering configurations is assigned to the quasi-LO phonon mode. Power-dependent Raman spectroscopy shows redshift with increasing laser power density due to sample heating which is confirmed by Stokes and anti-Stokes measurements. The broadband centered at 708.5cm−1 is ascribed to the surface mode of the nanostructure.

Electronic-grade GaN(0001)/Al2O3(0001) grown by reactive DC-magnetron sputter epitaxy using a liquid Ga target

Electronic-grade GaN (0001) epilayers have been grown directly on Al2O3 (0001) substrates by reactive DC-magnetron sputter epitaxy (MSE) from a liquid Ga sputtering target in an Ar/N2 atmosphere. T ...

Photoluminescence spectroscopy of nearly defect-free InN microcrystals exhibiting nondegenerate semiconductor behaviors

Nearly defect-free InN microcrystals grown on Si(111) substrates have been realized by plasma-assisted molecular beam epitaxy. High-resolution transmission electron microscope images reveal that these microcrystals exhibit single-crystalline wurtzite structure. Low temperature photoluminescence (PL) shows a strong emission peak at 0.679eV with a very narrow linewidth of 17meV at excitation power density of 3.4W∕cm2. Temperature-dependent PL spectra follow the Varshni equation well, and peak energy blueshifts by ∼45meV from 300to15K. Power-density-dependent PL spectroscopy manifests direct near-band-edge transition. A low carrier density of 3×1017cm−3 has been estimated from PL empirical relation, which is close to the critical carrier density of the Mott transition of 2×1017cm−3.

Polycrystalline to Single-Crystalline InN Grown on Si(111) Substrates by Plasma-Assisted Molecular-Beam Epitaxy

The structural evolution of InN from microsized grains to nanocolumns, and to a two-dimensional epifilm grown on Si(111) substrates was realized by plasma-assisted molecular-beam epitaxy. Grainy InN was grown at a higher substrate temperature, and a higher NBEP/InBEP ratio, and on a low-temperature InN buffer layer. A high-quality InN epifilm was grown at a lower substrate temperature, and a lower NBEP/InBEP ratio, and on a high-temperature AlN buffer layer with a room-temperature Hall mobility and a carrier concentration of 860 cm2/(Vs) and 8.9×1018 cm-3, respectively. Photoluminescence spectroscopy showed a unique peak in the infrared region indicating that the energy gap of the InN is in the range of 0.64–0.66 eV.

Epitaxial GaN nanorods free from strain and luminescent defects

Raman spectroscopy, cathodoluminescence imaging, and electron backscatter diffraction have been used to characterize the GaN nanorods as compared to their supporting matrix. The nanorods are strain free, distinguished from the mechanically and thermally stressed matrix that bears the brunt of all lattice mismatch and thermal strain, strain relaxation, and the related defect generation. This thus allows the loosely attached nanorods to grow to measurable perfection in electronic and crystal structures. The nanorods are crystallographically aligned with the matrix as well as the substrate.

Hydrogen in InN: A ubiquitous phenomenon in molecular beam epitaxy grown material

We study the unintentional H impurities in relation to the free electron properties of state-of-the-art InN films grown by molecular beam epitaxy (MBE). Enhanced concentrations of H are revealed in the near surface regions of the films, indicating postgrowth surface contamination by H. The near surface hydrogen could not be removed upon thermal annealing and may have significant implications for the surface and bulk free electron properties of InN. The bulk free electron concentrations were found to scale with the bulk H concentrations while no distinct correlation with dislocation density could be inferred, indicating a major role of hydrogen for the unintentional conductivity in MBE InN.

Electron accumulation at nonpolar and semipolar surfaces of wurtzite InN from generalized infrared ellipsometry

The free electron properties of nonpolar (112¯0)-oriented and semipolar (101¯1)-oriented wurtzite InN films are studied by generalized infrared ellipsometry (GIRSE). We demonstrate the sensitivity of GIRSE to the surface charge accumulation layer and find a distinct surface electron accumulation to occur at all surfaces. The obtained surface electron sheet densities are found to vary from 0.9×1013 to 2.3×1014 cm−2 depending on the surface orientation and bulk electron concentration. The upper limits of the surface electron mobility parameters of 417–644 cm2/V s are determined and discussed in the light of electron confinement at the surface.

Energy relaxation of InN thin films

The energy relaxation of InN thin films has been studied by ultrafast time-resolved photoluminescence technique. The obtained carrier cooling curves can be explained by carriers releasing excessive energy through the carrier–LO-phonon interaction. The extracted effective phonon emission times decrease as the photoexcited carrier concentration reduces and come close to the theoretical prediction of 23fs at small carrier concentration. The reduction of energy loss rate at high photoexcited carrier density is attributed to the hot phonon effect.