Yen-Teng Ho

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.

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.

Performance Improvements of AlGaN/GaN HEMTs by Strain Modification and Unintentional Carbon Incorporation

An advanced AlGaN/GaN HEMT structure, grown on a sapphire substrate by MOCVD utilizing a high temperature (HT) AlN interlayer (IL) and a multilayer high-low-high temperature (HLH) AlN buffer layer, demonstrates a superior performance both in breakdown voltage (>200 V) and maximum drain current (IDSS = 667 mA/mm). The HT AlN IL produces an additional compressive strain into the above GaN layer. Accordingly, an AlGaN barrier, grown on the more compressive GaN, introduces less tensile strain leading to an improvement in surface morphology (RMS = 0.19 nm in 2 × 2 μm2), a remarkable increase in 2DEG mobility by 46% (μs = 1900 cm2/Vs) and a decrease in densities of defects acting as paths for the leakage current through the AlGaN barrier. A high semi-insulating buffer is achieved by eliminating leakage paths both through the buffer layer and the buffer-substrate interfacial layer. These result from an increase in unintentional carbon introduced by AlN layers, especially by a low temperature AlN layer; which are grown under low pressure (50 Torr). Lastly, the decrease in AlGaN barrier tensile strain and low leakage current in the advanced HEMTs structure using an HT AlN IL and an HLH AlN buffer are promising for an improvement in AlGaN/GaN HEMTs’ reliability.

Defects in semipolar

The microstructure of semipolar [Formula: see text] ZnO deposited on (112) LaAlO3/(La,Sr)(Al,Ta)O3 was investigated by transmission electron microscopy. The ZnO shows an in-plane epitaxial relationship of [Formula: see text] with oxygen-face sense polarity. The misfit strain along [Formula: see text] and [Formula: see text] is relieved through the formation of misfit dislocations with the Burgers vectors [Formula: see text] and [Formula: see text], respectively. The line defects in the semipolar ZnO are predominantly perfect dislocations, and the dislocation density decreases with increasing ZnO thickness as a result of dislocation reactions. Planar defects were observed to lie in the M-plane and extend along 〈0001〉, whereas basal stacking faults were rarely found.

(1 1\bar {2}\bar {2})

The microstructure of semipolar [Formula: see text] ZnO deposited on (112) LaAlO3/(La,Sr)(Al,Ta)O3 was investigated by transmission electron microscopy. The ZnO shows an in-plane epitaxial relationship of [Formula: see text] with oxygen-face sense polarity. The misfit strain along [Formula: see text] and [Formula: see text] is relieved through the formation of misfit dislocations with the Burgers vectors [Formula: see text] and [Formula: see text], respectively. The line defects in the semipolar ZnO are predominantly perfect dislocations, and the dislocation density decreases with increasing ZnO thickness as a result of dislocation reactions. Planar defects were observed to lie in the M-plane and extend along 〈0001〉, whereas basal stacking faults were rarely found.

ZnO grown on (112) LaAlO3/(La,Sr)(Al,Ta)O3 substrate by pulsed laser deposition

This work describes optical and electrical characteristics of InGaP/GaAs/Ge triple-junction (T-J) solar cells with CdS quantum dots (QDs) fabricated by a novel chemical solution. With the anti-reflective feature at long wavelength and down-conversion at UV regime, the CdS quantum dot effectively enhance the overall power conversion efficiency more than that of a traditional GaAs-based device. Experimental results indicate that CdS quantum dot can enhance the short-circuit current by 0.33 mA/cm2, which is observed for the triple-junction solar cells with CdS QDs of about 3.5 nm in diameter. Moreover, the solar cell conversion efficiency is improved from 28.3% to 29.0% under one-sun AM 1.5 global illumination I–V measurement.

Defects in semipolar (11(2)over-bar(2)over-bar) ZnO grown on (112) LaAlO3/(La, Sr) (Al, Ta)O-3 substrate by pulsed laser deposition

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.

The effect of CdS QDs structure on the InGaP/GaAs/Ge triple junction solar cell efficiency

The electrical and material properties of WSe₂ doped by W:Ta co-sputtering process were examined. High acceptor doping concentration (6×10¹³cm^-2) and good effective hole mobility (16.5cm²/Vs) were realized. As a result, low sheet resistance (17kΩ/sq) and contact resistance with Pd metal (11.4kΩ-μm) were measured using TLM structures.

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

The effect of resistance (Rc) reduction of Ti/Au contact on MoS2 by ICP Ar plasma pretreatment is presented. The lowest Rc of 1.72 ohm-cm was achieved with the ICP treatment condition of 50W (ICP) /2W (chuck) for 10 sec. Compared with the contact without treatment (19.6 ohm-cm), the Rc improves over 10 times, demonstrating the advantage of Ar plasma pre-treatment on MoS2 contact.

Reliable doping technique for WSe 2 by W:Ta co-sputtering process

The effects of surface pre-treatments and the role of an AlN buffer layer for 2H-SiC growth on c-plane sapphire substrates by thermal CVD are investigated. While the crystallinity of SiC directly grown on sapphire substrate always degrades with a hydrogen pre-treatment but improves by optimizing carbonization, the crystallinity of SiC grown on sapphire substrate using an AlN buffer grown by MOCVD improves with sufficient time of exposure to the H pre-treatment but always deteriorates with carbonization. Detailed microstructural analysis by phi-scan x-ray diffraction reveals that SiC film grown on sapphire substrate consists of crystalline domains with two different crystallographic orientations which are rotated relative to each other along the [111] axis by 60°. A highly oriented hexagonal 2H-SiC film is obtained on low-cost c-plane sapphire substrate by using an AlN buffer. 2H-SiC is unambiguously determined not only by phi-scan x-ray diffraction but also by high-resolution transmission electron microscopy. The growth relationship between 2HSiC and 2H-AlN are coherent due to the favorable bonding of C and Al between SiC and AlN.