N. Xiang

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...

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.

Raman scattering probe of anharmonic effects due to temperature and compositional disorder in GaNxAs1−x

Using micro-Raman spectroscopy, we have investigated the vibrational properties of coherently strained GaNxAs1−x alloys grown on GaAs (001) substrates by molecular beam epitaxy. The effect of compositional disorder in GaNxAs1−x alloys has been studied by analyzing the broadening, asymmetry, and line shift of the first-order LO mode and nitrogen-induced localized vibration mode (NLVM). It is found that the line shape and peak shift of GaAs-like LO mode in GaNxAs1−x can be well described by the spatial correlation model. We have also analyzed Raman spectra of GaNxAs1−x alloys in the temperature range of 80–500K. The intensity of GaAs-like LO phonon and NLVM decreases with temperature and the linewidth of both these modes shows substantial broadening at higher temperature. The temperature dependence of phonon linewidth and peak frequency of LO modes are analyzed in terms of anharmonic damping effect induced by thermal and compositional disorders. We have observed that the anharmonicity in GaNxAs1−x is higher...

Anneal-induced interdiffusion in 1.3-μmGaInNAs∕GaAs quantum well structures grown by molecular-beam epitaxy

High-resolution x-ray diffraction (HRXRD) and photoluminescence (PL) have been used to study the diffusion of atoms in 8-nm Ga0.628In0.372N0.015As0.985∕GaAs quantum well, with and without dielectric encapsulants. These samples were repeatedly annealed in the temperature range of 680–800 °C over times of up to 16 000 s. HRXRD simulations, by using dynamic scattering theory and Fick’s diffusion model with a constant diffusion coefficient, demonstrate that the diffusion lengths are shorter than 2 nm under the annealing conditions studied. In this range of diffusion lengths, the transition energy Ee1−Hh1, numerically calculated from the Schrodinger equation using a potential derived from the diffusion equation, increases linearly as the square of the diffusion length. The steady-state PL blueshifts, after a fast initial blueshift due to the rearrangement of local nitrogen bonding configurations N–GamIn4−m(0⩽m⩽4), otherwise known as short-range order, as a function of annealing time are well fitted by using th...

Influence of N incorporation on In content in GaInNAs/GaNAs quantum wells grown by plasma-assisted molecular beam epitaxy

The authors investigated the synthesis of GaIn(N)As∕Ga(N)As multiple quantum wells by molecular beam epitaxy. Introducing N into the GaInAs appears to suppress the incorporation of In as indicated by reflective high-energy electron diffraction (RHEED). This effect is mainly due to the N-induced enhancement of In surface segregation at the growth front and is evidenced by the increasing damping rate of RHEED oscillations with N incorporation. The N-induced enhancement of In segregation in the GaInNAs quantum wells is confirmed by secondary-ion-mass spectroscopy and high-resolution x-ray diffractions, and its origin is discussed.

Radio-frequency magnetron sputtering and wet thermal oxidation of ZnO thin film

The authors studied the growth and wet thermal oxidation (WTO) of ZnO thin films using a radio-frequency magnetron sputtering technique. X-ray diffraction reveals a preferred orientation of [101¯0]ZnO(0002)∕∕[112¯0]Al2O3(0002) coexisted with a small amount of ZnO (101¯1) and ZnO (101¯3) crystals on the Al2O3 (0001) substrate. The ZnO (101¯1) and ZnO (101¯3) crystals, as well as the in-plane preferred orientation, are absent from the growth of ZnO on the GaAs(001) substrate. WTO at 550 °C improves the crystalline and the photoluminescence more significantly than annealing in air, N2 and O2 ambient; it also tends to convert the crystal from ZnO (101¯1) and ZnO (101¯3) to ZnO (0002). The evolution of the photoluminescence upon WTO and annealing reveals that the green and orange emissions, centered at 520 and 650 nm, are likely originated from oxygen vacancies and oxygen interstitials, respectively; while the 420 nm emission, which is very sensitive to the postgrowth thermal processing regardless of the subst...

Investigation of the V-pit related morphological and optical properties of InGaN∕GaN multiple quantum wells

In this work, the effects of large V-pits on the morphological and optical properties of InGaN∕GaN multiple quantum wells (MQWs) were studied using scanning electron microscopy, transmission electron microscopy, and photoluminescence. InGaN∕GaN MQWs with high-density large V-pits were grown by metal organic chemical vapor deposition. In addition to the regular c-plane MQWs, the MQWs grown on the {101¯1} faceted sidewalls of the V-pits were also observed, which gave much higher emission energies than those of the c-plane MQWs. Furthermore, when the low-temperature GaN buffer was very thin, the {112¯m} (m⩾2) faceted sidewalls of the V-pits were observed. It was then found that MQWs grown on such sidewalls had emission energies between those of the c-plane MQWs and those of the {101¯1} faceted sidewall MQWs.

Growth-temperature-and thermal-anneal-induced crystalline reorientation of aluminum on GaAs (100) grown by molecular beam epitaxy

The authors investigated the growth of Al thin films on GaAs (100) substrates by molecular beam epitaxy. It is found that the growth at 550°C results in a texture that consists of (100)Al[010]‖(100)GaAs[011] and (100)Al[010]‖(100)GaAs[010] rotated 45° with respect to each other, while the growth at 300°C leads to a mixture phase of (100)Al[010]‖(100)GaAs[011] and (110)Al[001]‖(100)GaAs[01¯1¯]. In situ annealing of the Al film grown at 300°C causes a reorientation of the crystalline from (100)Al[010]‖(100)GaAs[011] to (110)Al[001]‖(100)GaAs[01¯1¯]. The grain sizes of the Al film are increased by the increased growth temperature and in situ annealing; the ratio of the exposed to the covered surface is not changed significantly by changing the growth temperature but decreased by annealing; and the small islands in between the large ones are removed by annealing. These observations are explained based on island migration and coalescence.

Influence of GaNAs strain-compensation layers on the optical properties of GaIn(N)As∕GaAs quantum wells upon annealing

GaIn(N)As∕GaAs and GaIn(N)As∕GaNAs∕GaAs quantum well (QW) samples, with and without GaNAs strain-compensating layers (SCLs), were grown on GaAs (001) substrates by molecular beam epitaxy. Photoluminescence (PL) was used to study the effects of the GaNAs SCL on the properties of the Ga(In)NAs QWs upon annealing. We observed that the insertion of GaNAs SCL produced a distinct increase in the PL blueshift as a function of annealing time. X-ray diffraction from the strain-compensated GaIn(N)As QWs before and after annealing showed no N atom diffusion, but exhibited Ga–In atom interdiffusion across the QW interfaces. We compared the effects of the GaNAs SCL on the PL blueshift with those of the SiO2 encapsulant upon annealing. The increased PL blueshift caused by the GaNAs SCL for tann⩽40s is attributed to the further GamIn4−m–N (0⩽m⩽4) changes due to greater local strain caused by GaNAs (SCL) quantum barriers as compared with GaAs barriers. For tann>40s, the nonsaturable blueshift caused by GaNAs SCL is attri...

Growth of InAs on micro- and nano-scale patterned GaAs(001) substrates by molecular beam epitaxy

Molecular beam epitaxy of InAs on micro-?and nano-scale patterned GaAs(001) substrates was studied. An InAs epilayer grown on the micro-scale patterned substrate exhibits islands with {1?1?3}-type facets, and is similar to that grown on the flat (unpatterned) substrate. In contrast, the preferred growth of InAs on the nano-scale patterned substrate is in the direction and exhibits islands with {1?1?0}-type facets. The thickness of the dense dislocation networks at the interface due to strain relaxation is reduced by the micro-scale pattern in comparison with the flat substrate, while for growth on the nano-scale patterned substrate, the strain relaxes via the formation of stacking faults more than dislocations. X-ray diffraction reveals that the strains in the 300?nm InAs epilayers are nearly fully relaxed, and the patterns tend to decrease the lattice constants of the epilayer, implying mass transport of Ga atoms into the epilayer from the GaAs substrates.