H. F. Liu

Piezotronic Effect on the Transport Properties of GaN Nanobelts for Active Flexible Electronics

The transport properties of GaN nanobelts (NBs) are tuned using a piezotronic effect when a compressive/tensile strain is applied on the GaN NB. This is mainly due to a change in Schottky barrier height (SBH). A theoretical model is proposed to explain the observed phenomenon.

Effects of substrate on the structure and orientation of ZnO thin film grown by rf-magnetron sputtering

X-ray diffractions, Nomarski microscopy, scanning electron microscopy, and photoluminescence have been used to study the effects of substrate on the structure and orientation of ZnO thin films grown by rf-magnetron sputtering. GaAs(001), GaAs(111), Al2O3(0002) (c-plane), and Al2O3(11¯02) (r-plane) wafers have been selected as substrates in this study. X-ray diffractions reveal that the ZnO film grown on GaAs(001) substrate is purely textured with a high c-axis orientation while that grown on GaAs(111) substrate is a single ZnO(0002) crystal; a polycrystalline structure with a large-single-crystal area of ZnO(0002) is obtained on a c-plane Al2O3 substrate while a ZnO(112¯0) single crystal is formed on an r-plane Al2O3 substrate. There is absence of significant difference between the photoluminescence spectra collected from ZnO∕GaAs(001), ZnO∕GaAs(111), and ZnO∕Al2O3(0002), while the photoluminescence from ZnO∕Al2O3(11¯02) shows a reduced intensity together with an increased linewidth, which is, likely, due...

Influence of stress on structural properties of AlGaN/GaN high electron mobility transistor layers grown on 150 mm diameter Si (111) substrate

Effects of stress imposed by individual nitride layers on structural properties of an AlGaN/GaN high-electron-mobility-transistor (HEMT) structure, which was grown on a 150 mm diameter Si (111) substrate by metal-organic chemical vapor deposition employing high-temperature step-graded AlxGa1−xN/AlN buffer layers, were studied using transmission electron microscopy, visible micro-Raman spectroscopy, and high-resolution x-ray diffraction. It is revealed that all the nitride layers are more or less tensile strained on the Si (111) substrate; however, strain relaxations occurred at all the heterointerfaces except for the AlGaN/(AlN/)GaN two-dimensional electron gas interface, which is desired for achieving high performance HEMT. The wafer curvature, an important parameter for large area epitaxy of GaN-on-Si, is modeled on the basis of stress distribution within individual layers of the multilayered AlGaN/GaN HEMT structure via the close-form expression developed by Olsen and Ettenberg [J. Appl. Phys. 48, 2543...

Towards large area and continuous MoS2 atomic layers via vapor-phase growth: thermal vapor sulfurization.

We report on the effects of substrate, starting material, and temperature on the growth of MoS(2) atomic layers by thermal vapor sulfurization in a tube-furnace system. With Mo as the starting material, atomic layers of MoS(2) flakes are obtained on sapphire substrates while a bell-shaped MoS(2) layer, sandwiched by amorphous SiO(2), is obtained on native-SiO(2)/Si substrates under the same sulfurization conditions. An anomalous thickness-dependent Raman shift (A(1g)) of the MoS(2) atomic layers is observed in Mo-sulfurizations on sapphire substrates, which can be attributed to the competition between the effects of thickness and the surface/interface. Both effects vary with the sulfurizing temperatures for a certain initial Mo thickness. The anomalous frequency trend of A(1g) is missing when using MoO(3) instead of Mo as the starting material. In this case, the lateral growth of MoS(2) on sapphire is also largely improved. Furthermore, the area density of the resultant MoS(2) atomic layers is significantly increased by increasing the deposition temperature of the starting MoO(3) to 700 °C; the adjacent ultrathin MoS(2) grains coalesce in one or other direction, forming connected chains in wafer scale. The thickness of the so-obtained MoS(2) is generally controlled by the thickness of the starting material; however, the structural and morphological properties of MoS(2) grains, towards large area and continuous atomic layers, are strongly dependent on the temperature of the initial material deposition, and on the temperature of sulfurization, because of the competition between surface mobility and atom evaporation.

Enhanced performance of GaN nanobelt-based photodetectors by means of piezotronic effects

AbstractGaN ultraviolet (UV) photodetectors (PDs) have attracted tremendous attention due to their chemical stability in harsh environments. Although Schottky-contacted GaN-based UV PDs have been implemented with better performance than that of ohmic contacts, it remains unknown how the barrier height at local Schottky contacts controls the sensors’ performance. In this work, the piezotronic effect was employed to tune the Schottky barrier height (SBH) at local contacts and hence enhance the performances of Schottky-contacted metal-semiconductor-metal (MSM) structured GaN nanobelt (NB)-based PDs. In general, the response level of the PDs was obviously enhanced by the piezotronic effect when applying a strain on devices. The responsivity of the PD was increased by 18%, and the sensitivity was enhanced by from 22% to 31%, when illuminated by a 325 nm laser with light intensity ranging from 12 to 2 W/cm2. Carefully studying the mechanism using band structure diagrams reveals that the observed enhancement of the PD performance resulted from the change in SBH caused by external strain as well as light intensity. Using piezotronic effects thus provides a practical way to enhance the performance of PDs made not only of GaN, but also other wurtzite and zinc blende family materials.

Structural and optical properties of InGaN∕GaN multiple quantum wells grown on nano-air-bridged GaN template

Structural and optical properties of InGaN∕GaN multiple quantum wells (MQWs) grown on nano-air-bridged GaN template by metal organic chemical vapor deposition were investigated. The InGaN∕GaN MQWs on nano-air-bridged GaN demonstrate much better surface morphology, revealing low defect density ∼4×108cm−2 with step flow features measured by atomic force microscopy. The photoluminescence measurement shows one magnitude higher in intensity from less defective InGaN MQWs compared to that of the control InGaN MQWs. The improvement in photoluminescence of the InGaN MQWs is benefited from the reduction of threading dislocation density in the InGaN∕GaN active layers and GaN template, revealed from cross-sectional transmission electron microscopy. High resolution x-ray diffraction analysis results show higher indium mole fraction in the MQWs when grown on nano-air-bridged GaN template, due to the strain relaxation in the nano-air-bridged GaN template. This higher indium incorporation is consistent with the redshift...

Doping behavior of phosphorus in ZnO thin films: Effects of doping concentration and postgrowth thermal annealing

Effects of phosphorus concentration, [P], and postgrowth annealing on the x-ray diffraction and Raman scattering from ZnO:P thin films are presented. The ZnO (0002) diffraction peak exhibits a large monotonic angular shift with increasing [P] up to 5.1×1019 cm−3 while its shift upon annealing is dependent on [P]. No extrinsic phases were detected for the samples studied. Raman spectra reveal disorder-activated vibrational modes, around 276, 510, 582, and 643 cm−1, with increasing [P]. They also reveal local vibrational modes corresponding to PO (∼370 cm−1) and PZn (∼482 cm−1) upon annealing. The intensity evolution of the Raman features, together with the x-ray diffraction results, indicates that phosphorus substituting for zinc is favored at low annealing temperatures while increasing annealing temperature tends to convert phosphorus doping configurations from PZn to PO.

Annealing effects on electrical and optical properties of ZnO thin-film samples deposited by radio frequency-magnetron sputtering on GaAs (001) substrates

The effects of thermal annealing on Hall-effect measurement and photoluminescence (PL) from undoped n-type ZnO/GaAs thin-film samples have been studied. The evolutions of carrier concentration, electrical resistivity, and PL spectrum at various annealing conditions reveal that the dominant mechanism that affects the electrical and PL properties is dependent on the amount of thermal energy and the ambient pressure applied during the annealing process. At low annealing temperatures, annihilation of native defects is dominant in reducing the carrier concentration and weakening the low-energy tail of the main PL peak, while the GaAs substrate plays only a minor role in carrier compensations. For the higher temperatures, diffusion of Ga atoms from the GaAs substrate into ZnO film leads to a more n-type conduction of the sample. As a result, the PL exhibits a high-energy tail due to the high-level doping.

Synthesis and phase evolutions in layered structure of Ga2S3 semiconductor thin films on epiready GaAs (111) substrates.

We report on synthesis and properties of p-type Ga2S3 semiconductor thin films that were prepared by sulfurizing epiready n-type GaAs (111) surface at elevated temperatures. Comparisons of structural and optical properties among the thin films, peeling-off resulted microtubes, and the remains after peeling-off give a clear clue to the crystal growth and phase evolutions of Ga2S3. Three layers of Ga2S3 are clearly identified in the thin films. They are layer i, cubic Ga2S3 epitaxially grown on the GaAs (111) substrate; layer ii, polycrystalline cubic Ga2S3 on top of layer-i; and layer iii, monoclinic and/or hexagonal Ga2S3 on top of layer ii. The onset of peeling-off occurred in layer i and/or at the interface between layer i and ii. Both the phase evolutions and the location of peeling-off are associated with a Ga out diffusion growth mechanism. Absorption spectroscopy revealed a direct bandgap of 3.0 eV, whereas photoluminescence spectra showed defects (excited Ga vacancies) related red (1.62 eV) and green (2.24 eV) emissions of the Ga2S3 films; both are qualitatively consistent with those reported values obtained at lower sample temperatures from Ga2S3 single crystals. These results, together with a large on/off current ratio (i.e., ∼14 at a bias of 4.0 V) of the resultant hetero p-Ga2S3/n-GaAs junction under a blue laser (405 nm, 3.0 mW) illumination, shed light on consequent integrations of Ga2S3- and GaAs-based optoelectronic devices, e.g., high-power laser radiation sensors.

Effects of oxygen on low-temperature growth and band alignment of ZnO∕GaN heterostructures

Continuous ZnO thin films have been grown at low temperature (400°C) on GaN∕c-sapphire substrates in a radio-frequency magnetron-sputtering chamber employing a substoichiometric ZnO target with and without extra oxygen feeding. The effects of oxygen on the growth and band alignment of the ZnO∕GaN heterostructures were investigated by using scanning-electron microscopy, x-ray diffraction, photoluminescence and transmittance/absorbance, ultraviolet-resonant Raman scattering, and x-ray photoelectron spectroscopy. Very remarkable changes of the structural and optical properties resulted from the introduction of oxygen: the surface hexahedral facets were diminished; the size of the surface islands and, hence, the compressive strains were reduced; ultraviolet transparency of the ZnO film was enhanced, together with an increased band gap due to the reduced intrinsic shallow-donor defects; and hence, the free-electron concentration. The offset in valence bands of the ZnO∕GaN heterostructure was increased by ∼90me...