Wei-Lin Wang

Growth of nonpolar (112¯0) ZnO films on LaAlO3 (001) substrates

Nonpolar (112¯0) ZnO films were grown on LaAlO3 (001) single crystal substrates at temperature from 300 to 750 °C by pulsed laser deposition method. The films were examined using x-ray diffraction, reflection high energy electron diffraction, and photoluminescence measurements for the crystallinity. The surface morphology of ZnO films from atomic force microscopy exhibits L-shaped domains. Cross-sectional transmission electron microscopy with selected area diffraction reveals two types of a-plane ZnO domains perpendicular to each other with in-plane orientation relationships of [0001]ZnO∥[11¯0]LAO and [11¯00]ZnO∥[11¯0]LAO.

Effect of graded-temperature arsenic prelayer on quality of GaAs on Ge/Si substrates by metalorganic vapor phase epitaxy

The growth of GaAs epitaxy on Ge/Si substrates with an arsenic prelayer grown with graded temperature ramped from 300 to 420 °C is investigated. It is demonstrated that the graded-temperature arsenic prelayer grown on a Ge/Si substrate annealed at 650 °C not only improves the surface morphology (roughness: 1.1 nm) but also reduces the anti-phase domains’ (APDs) density in GaAs epitaxy (dislocation density: ∼2 × 107 cm−2). Moreover, the unwanted interdiffusion between Ge and GaAs epitaxy is suppressed by using the graded-temperature arsenic prelayer due to the low energy of the Ge-As bond and the use of a low V/III ratio of 20.

Effect of substrate misorientation on the material properties of GaAs/Al0.3Ga0.7As tunnel diodes

The effect of substrate misorientation on the material quality of the N++–GaAs/P++–AlGaAs tunnel diodes (TDs) grown on these substrates is investigated. It is found that the misorientation influences both surface roughness and interface properties of the N++–GaAs/P++–AlGaAs TDs. Smooth surface (rms roughness: 1.54 A) and sharp interface for the GaAs/Al0.3Ga0.7As TDs were obtained when the (100) tilted 10° off toward [111] GaAs substrate was used. Besides, the oxygen content in N++–GaAs and P++–AlGaAs layers grown on the 10° off GaAs substrates was reduced due to the reduction of sticking coefficient and number of anisotropic sites.The effect of substrate misorientation on the material quality of the N++–GaAs/P++–AlGaAs tunnel diodes (TDs) grown on these substrates is investigated. It is found that the misorientation influences both surface roughness and interface properties of the N++–GaAs/P++–AlGaAs TDs. Smooth surface (rms roughness: 1.54 A) and sharp interface for the GaAs/Al0.3Ga0.7As TDs were obtained when the (100) tilted 10° off toward [111] GaAs substrate was used. Besides, the oxygen content in N++–GaAs and P++–AlGaAs layers grown on the 10° off GaAs substrates was reduced due to the reduction of sticking coefficient and number of anisotropic sites.

Defects in m-plane ZnO epitaxial films grown on (112) LaAlO3 substrate

The crystallographic orientations of m-plane ZnO on (112) LaAlO3 (LAO) substrate are [1¯21¯0]ZnO∥[111¯]LAO and [0001]ZnO∥[1¯10]LAO. The defects in m-plane ZnO have been systematically investigated using cross section and plan-view transmission electron microscopy (TEM). High-resolution TEM observations in cross section show misfit dislocations and basal stacking faults (BSFs) at the ZnO/LAO interface. In the films, threading dislocations (TDs) with 1/3⟨112¯0⟩ Burgers vectors are distributed on the basal plane, and BSFs have 1/6⟨202¯3⟩ displacement vector. The densities of dislocations and BSFs are estimated to be 5.1×1010 cm−2 and 4.3×105 cm−1, respectively. In addition to TDs and BSFs, plan-view TEM examination also reveals that stacking mismatch boundaries mainly lie along the m-planes and they connect with planar defect segments along the r-planes.

Crystal Quality of 3C-SiC Influenced by the Diffusion Step in the Modified Four-Step Method

The atomic arrangement and bonding characteristics of void-free 3C-SiC/Si(100) grown by the modified four-step method are presented. Without the diffusion step, Si-C bonds are partially formed in the as-carburized layer on Si(100). The ratio of C-C bonds to Si-C bonds is about 7:3, which can be lowered to about 1:9 after the diffusion step at 1350°C for 5 min or at 1300°C for 7 min according to C 1s core level spectra. The residual C-C bonds cannot be removed, which is associated with an irregular atomic arrangement (amorphous) located either at the 3C-SiC/Si(100) interface or at the intersection of twin boundaries in the 3C-SiC buffer layer based on the lattice image taken by transmission electron microscope. The diffusion step helps the formation of Si-C bonds more completely and results in a SiC buffer layer of high quality formed on Si(100) before the growth step. However, twins and stacking faults still appear in the 3C-SiC buffer layer after the diffusion step. The formation mechanism of the 3C-SiC buffer layer is proposed and discussed.

The cooling effect on structural, electrical, and optical properties of epitaxial a-plane ZnO:Al on r-plane sapphire grown by pulsed laser deposition

Here, the unambiguous effect of cooling rate on structural, electrical, and optical properties of a-plane ZnO:Al on r-plane sapphire grown by pulsed laser deposition at 700 °C is reported. A high cooling rate (∼100 °C/min) can result in stripe morphology along m-direction and significant deformation on the epitaxial films of a-plane ZnO:Al with deteriorated crystallinity and significantly lowered resistivity. Also, photoluminescence spectra exhibit high intensities of excess violet and green emissions with low intensity of near band edge luminescence. Comparison with pure a-plane ZnO films is also presented.

Enhancement of InGaN ∕ GaN Flip-Chip ITO LEDs with Incline Sidewalls Coated with TiO2 ∕ SiO2 Omnidirectional Reflector

The light extraction enhancement of GaN-based flip-chip indium-tin oxide light-emitting diodes (FC ITO LEDs) with an inclined sidewall coated with TiO 2 /SiO 2 omnidirectional reflectors (ODRs) is presented. At a driving current of 350 mA and a chip size of 1 × 1 mm, the light output power and the light extraction enhancement of the FC ITO LEDs coated TiO 2 /SiO 2 ODRs with inclined sidewall reached 183 mW and 15% when compared with the results from the same device, FC ITO LEDs coated TiO 2 /SiO 2 ODRs with vertical sidewall. Furthermore, by examining the radiation patterns of the FC ITO LEDs, the increased optical power within 150° cone contributed to the stronger enhancement around the vertical direction of an inclined sidewall ODR within blue regime. Our work offers promising potential for enhancing output powers of commercial light-emitting devices.

Study of InN epitaxial films and nanorods grown on GaN template by RF-MOMBE.

This paper reports on high-quality InN materials prepared on a GaN template using radio-frequency metalorganic molecular beam epitaxy. We also discuss the structural and electro-optical properties of InN nanorods/films. The X-ray diffraction peaks of InN(0002) and InN(0004) were identified from their spectra, indicating that the (0001)-oriented hexagonal InN was epitaxially grown on the GaN template. Scanning electron microscopic images of the surface morphology revealed a two-dimensional growth at a rate of approximately 0.85 μm/h. Cross-sectional transmission electron microscopy images identified a sharp InN/GaN interface and a clear epitaxial orientation relationship of [0001]InN // [0001]GaN and (2¯110)InN // (2¯110)GaN. The optical properties of wurtzite InN nanorods were determined according to the photoluminescence, revealing a band gap of 0.77 eV.

Effect of growth temperature on a-plane ZnO formation on r-plane sapphire

The effect of growth temperature on a-plane ZnO formation on r-plane sapphire has been systematically investigated by employing in situ high pressure reflection high-energy electron diffraction, atomic force microscopy, and high-resolution x-ray diffraction. For film growth above and below 600 °C, it is shown that there is a significant difference in growth rate and surface morphology due to the differences in the growth mode. Stripelike morphologies were observed on the surface of a-plane ZnO grown at low temperature (LT) because of differences in the growth rate along the c-axis and the growth rate normal to the c-axis. Furthermore, annealing of films grown at low temperature results in more pronounced stripe morphology and in improvement of crystallinity.

Diamond plates on dome-like particles: preparation, characterization and field emission properties

Thin diamond microplates have been grown on dome-like/hemispherical carbon particles on titanium carbide by a microwave plasma chemical vapour deposition (MPCVD) method using a gas mixture of methane and hydrogen. The diamond microplates have a thickness of about 200 nm. A thin (300 nm) film of titanium carbide was formed during carburization of sputtered titanium on an Si(100) substrate in MPCVD. The hemispherical carbon particles were covered with diamond microplates. The diamond microplates are isolated electron-emitting spherules and exhibit a low threshold (50 V µm−1) and high current density (0.92 mA cm−2) in their field emission properties. A possible mechanism for the formation of the diamond microplates and hemispherical carbon particles is presented.