Y.P. Zeng

Hillocks and hexagonal pits in a thick film grown by HVPE

A GaN film with a thickness of 250@mm was grown on a GaN/sapphire template in a vertical hydride vapor phase epitaxy (HVPE) reactor. The full-width at half-maximum (FWHM) values of the film were 141 and 498arcsec for the (002) and (102) reflections, respectively. A sharp band-edge emission with a FWHM of 20meV at 50K was observed, which corresponded to good crystalline quality of the film. Some almost circular-shaped hillocks located in the spiral growth center were found on the film surface with dimensions of 100@mm, whose origin was related to screw dislocations and micropipes. Meanwhile, large hexagonal pits also appeared on the film surface, which had six triangular {101@?1} facets. The strong emission in the pits was dominated by an impurity-related emission at 377nm, which could have been a high-concentration oxygen impurity.

Raman study on self-assembled InAs/InAlAs/InP(001) quantum wires

The phonons of self-assembled InAs/InAlAs/InP quantum wires (QWRs) have been studied by Raman scattering. The QWR LO phonons show an unusual frequency shift with the increase of the InAs deposited thickness due to dislocations. The QWR LO phonons are found to follow the selection rule of the LO phonons in bulk zinc-blende semiconductors. Because of the intermixing of In/Al atoms and the multiplication of dislocations, the post-growth thermal annealing treatment leads to a shift of the QWR LO phonons to lower frequency.

Hydride Vapor Phase Epitaxy Growth of Semipolar (1013)GaN on Patterned m-Plane Sapphire

We have investigated the hydride vapor-phase epitaxy growth of (1013)-oriented GaN thick films on patterned sapphire substrates (PSSs) ( 1010). From characterization by atomic force microscopy, scanning electron microscopy, double-crystal X-ray diffraction, and photoluminescence (PL), it is determined that the crystalline and optical qualities of (1013) GaN epilayers grown on the cylindrical PSS are better than those on the flat sapphire. However, two main crystalline orientations ( 1013) and (1122) dominate the GaN epilayers grown on the pyramidal PSS, demonstrating poor quality. After etching in the mixed acids, these ( 1013) GaN films are dotted with oblique pyramids, concurrently lining along the (3032) direction, indicative of a typical N-polarity characteristic. Defect-related optical transitions of the (1013) GaN epilayers are identified and detailedly discussed in virtue of the temperature-dependent PL. In particular, an anomalous blueshift-redshift transition appears with an increase in temperature for the broad blue luminescence due to the thermal activation of the shallow level.

Effect of microscope parameter and specimen thickness of spatial resolution of transmission electron backscatter diffraction

The spatial resolution of transmission electron backscatter diffraction (t‐EBSD) with a standard conventional EBSD detector was evaluated quantitatively based on the calculation of the correlation coefficient of transmission patterns which were acquired across a twin boundary in the sample of austenitic steel. The results showed that the resolution of t‐EBSD improved from tens of nanometres to below 10 nm with increasing accelerating voltage and thinning of specimen thickness. High voltage could enhance the penetration depth and reduce the scattering angle. And the thinning of specimen thickness would result in decreasing of the scattering events according to the theory of thermal diffuse scattering (TDS). In addition, the transmission patterns were found to be weak and noisy if the specimen was too thin, because of the decreasing intensity detected by the screen. Consequently, in this work, the best spatial resolution of 7 nm was achieved at 30 kV and 41 nm thickness. Moreover, the specimen thickness range was also discussed using Monte‐Carlo simulation. This approach was helpful to account for the differences of measured spatial resolutions, by t‐EBSD, of lamellas with different thickness.

Laser-driven fast electron ionization wave propagation in a dielectric target

Ultrafast shadowgraphy with sub-picosecond resolution is applied to investigate the propagation of laser-driven fast electron beams inside a dielectric target. Time-resolved expansion of an ionization sphere caused by hot electron transportation in the target is observed. An abnormal absorption area with a width of approximately 10 μm is observed near the ionization front and is consistent with the one-dimensional electron transport model. The observed distortions of the edge diffraction fringes near the surface of the target are analyzed to qualitatively retrieve the phase shift caused by the ionization front. A simplified three-dimensional model is proposed to analyze the underlying physics and indicates that the valley in the ionization front appears due to the unstable propagation of hot electrons in a self-induced fountain electric field, which is induced by the collective effect of the free electrons and the ions.

Effects of scattering on two-dimensional electron gases in InGaAs/InAlAs quantum wells

The effects of different scattering on two-dimensional electron gases (2DEGs) in lattice matched In0.53Ga0.47As/In0.52Al0.48As quantum wells with silicon δ-doped in one barrier layer have been investigated by means of magneto-transport measurements. For the studied samples, the 2DEGs have occupied two subbands. It is found that the dominant scattering mechanism is ionized impurities scattering for the two subbands’ electrons. Besides the dominant scattering mechanism of ionized impurities scattering, Coulomb scattering also plays a role in scattering mechanism for both subbands. Both the transport scattering time and the quantum scattering time of the second subband are larger than those of the first subband. It is because that the electrons of the first subband are, on average, closer to the ionized impurities in the doped layer, they will be scattered more strongly than those in the second subband. Due to the electron wave functions for the second subband spread more widely in the quantum wells, the all...

Spin dependence of electron effective masses in InGaAs/InAlAs quantum well

The effective masses for spin-up and spin-down electrons of a partially spin-polarized Fermi liquid are theoretically expected to be different. We extract the spin-up and spin-down effective masses from magnetotransport measurements at different temperatures for a two-dimensional electron gas in an In0.65Ga0.35As/In0.52Al0.48As quantum well exhibiting zero-field spin splitting. We apply two analytical methods, one involving the simultaneous fitting of fast Fourier transform (FFT) spectra and the other involving inverse FFT analysis. Both methods confirm that the effective masses for spin-up and spin-down are different, consistent with theoretical expectations.