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In this study, we used chemical vapor transport to grow undoped and niobium (Nb)-doped tungsten diselenide (WSe2) thin sheets and then investigated the structural and photoelectronic characteristics of both samples. X-ray photoelectron spectroscopy confirmed the presence of Nb atoms in the Nb-doped WSe2 sample. Hall effect measurements of the transport behavior of the carriers in the undoped and Nb-doped WSe2 indicated that the Nb-doped WSe2 was of p-type and had a higher carrier concentration and lower mobility than the undoped WSe2. The current density–voltage characteristics indicated that doping with Nb led to a decrease in resistance. Photoconductivity measurements revealed that the responsivity of the Nb-doped WSe2 was an order of magnitude greater than that of the undoped WSe2. Thus, doping Nb atoms into WSe2 not only provides effective carriers but also enhances the photoresponsivity significantly. Accordingly, doping WSe2 with Nb atoms would appear to be useful for the fabrication of highly sensitive photodetectors.In this study, we used chemical vapor transport to grow undoped and niobium (Nb)-doped tungsten diselenide (WSe2) thin sheets and then investigated the structural and photoelectronic characteristics of both samples. X-ray photoelectron spectroscopy confirmed the presence of Nb atoms in the Nb-doped WSe2 sample. Hall effect measurements of the transport behavior of the carriers in the undoped and Nb-doped WSe2 indicated that the Nb-doped WSe2 was of p-type and had a higher carrier concentration and lower mobility than the undoped WSe2. The current density–voltage characteristics indicated that doping with Nb led to a decrease in resistance. Photoconductivity measurements revealed that the responsivity of the Nb-doped WSe2 was an order of magnitude greater than that of the undoped WSe2. Thus, doping Nb atoms into WSe2 not only provides effective carriers but also enhances the photoresponsivity significantly. Accordingly, doping WSe2 with Nb atoms would appear to be useful for the fabrication of highly sensi...

We reported optical properties of a-plane ZnO/ZnMgO multiple quantum wells (MQWs) structure grown by the pulse laser deposition system. The emission peak energy of a-plane ZnO/ZnMgO MQWs kept invariant in the power-dependent photoluminescence (PL) measurement, indicating the nonpolar characteristics due to the lack of built-in electric fields. Large exciton binding energy of 68 meV was deduced and no apparent S-curve appeared in temperature-dependent PL results, demonstrating less carrier localization effect in a-plane ZnO/ZnMgO MQWs. Large difference in electronic transition levels of 45 meV due to the valence band splitting was observed in the polarization dependent absorption spectrum. Furthermore, the high degree of polarization of 92% and 56% at 20 and 300 K in PL emission of a-plane ZnO/ZnMgO MQWs were obtained.

In this paper, we report a detailed study on the evolution of surface morphology and microstructure of nonpolar a-plane GaN (a-GaN) through controlled growth interruptions. Microscopy imaging shows that the two-step a-GaN growth went through a roughening-recovery process. The first-step growth (under high V/III and high pressure) produced a rough surface with tall mesas separated by voids. The second-step growth (under low V/III and low pressure) promoted the lateral growth and filled up the voids. Striations that formed during the island coalescence persisted throughout the second-step growth, but could be relieved by an additional third-step growth. The morphological evolution was explained according to the kinetic Wulff plots. The microstructure of the a-GaN films was investigated by transmission electron microscopy (TEM) and x-ray rocking curve analysis. Most of the extended defects observed in the plan-view TEM images were I1 type basal-plane stacking faults (BSFs) and their associated partial dislocations (PDs). It is found that the bending of PDs (at the inclined/vertical growth fronts) within the basal plane toward the m-axes was responsible for the substantial reduction in threading PDs and the increase in BSF dimension. Based on a careful correlation between the morphological evolution and the microstructure development, we proposed a model explaining the possible mechanisms for the great reduction in defect density during the two-step growth process.

The crystal quality of a-plane GaN films was improved by using epitaxial lateral overgrowth on a nanorod GaN template. The investigation of x-ray diffraction showed that the strain in a-plane GaN grown on r-plane sapphire could be mitigated. The average threading dislocation density estimated by transmission electron microscopy was reduced from 3×1010 to 3.5×108 cm−2. From the temperature-dependent photoluminescence, the quantum efficiency of the a-plane GaN was enhanced by the nanorod epitaxial lateral overgrowth (NRELOG). These results demonstrated the opportunity of achieving a-plane GaN films with low dislocation density and high crystal quality via NRELOG.

Pt/GaN Schottky diodes fabricated on m-plane (N-polar) layers grown on sapphire exhibit much larger responses to dilute concentrations (4% in N2) of hydrogen at room temperature than comparable Ga-polar devices. This is consistent with previous density functional theory indicating a very high affinity of hydrogen for the N-face surface of GaN. The rectifying current-voltage characteristics of N-face diodes make a transition to more Ohmic-like behavior after hydrogen exposure, leading to very large (∼106) maximum percentage changes in current relative to Ga-face (∼10%) or AlGaN/GaN heterostructure diodes (∼170%). The strong affinity of the N face of GaN for hydrogen also leads to a slower recovery of these diodes when hydrogen is removed from the ambient.

a-plane InGaN/GaN multiple quantum wells of different widths ranging from 3 to 12 nm grown on r-plane sapphire by metal-organic chemical vapor deposition were investigated. The peak emission intensity of the photoluminescence (PL) reveals a decreasing trend as the well width increases from 3 to 12 nm. Low temperature (9 K) time-resolved PL (TRPL) study shows that the sample with 3-nm-thick wells has the best optical property with a fastest exciton decay time of 0.57 ns. The results of cathodoluminescence and micro-PL scanning images for samples of different well widths further verify that the more uniform and stronger luminescence intensity distribution are observed for the samples of thinner quantum wells. In addition, more effective capturing of excitons due to larger localization energy Eloc and shorter radiative lifetime of localized excitons are observed in thinner well width samples in the temperature dependent TRPL.

We report the reduction in basal-plane stacking faults (BSFs) in m-plane GaN grown on m-plane SiC. The origin of BSFs is linked to heteronucleation of m-plane GaN and the presence of N-face basal-plane sidewalls of three-dimensional islands. Graded AlGaN layers help to alleviate mismatched nucleation and the generation of BSFs. Transmission electron microscopy shows that the density of BSFs is decreased to the low 105cm−1. Anisotropy in on-axis x-ray rocking curves, a salient feature in m-plane GaN heteroepitaxial layers, is greatly reduced. A possible mechanism of BSF generation, and the demonstration of improved InGaN∕GaN quantum well emission are presented.

Extended defect reduction in GaN can be achieved via direct growth on stripe patterned (11¯02) r-plane sapphire substrates by metal organic chemical vapor deposition. The striped mesa is along [112¯0] with two etched sides in {0001} and {11¯01} faces. GaN grown on both etched facets in epitaxy exhibit different crystallographic relationships with sapphire substrate which are (11¯02)sapphire‖(112¯0)GaN and [112¯0]sapphire‖[1¯100]GaN, and (0001)sapphire‖(0001)GaN and [112¯0]sapphire‖[1¯100]GaN, respectively. The dislocation densities can be significantly reduced through epitaxial growth on the inclined lateral faces of mesas. Dislocation density in the order of ∼107cm−2 can be achieved in the tilted GaN.

a-plane InGaN∕GaN multiple quantum wells of different widths ranging from 3to12nm were grown on r-plane sapphire by metal organic chemical vapor deposition for investigation. The peak emission intensity of the photoluminescence (PL) reveals a decreasing trend as the well width increases from 3to12nm. Low temperature (9K) time-resolved PL study shows that the sample with 3-nm-thick wells has a better optical property with a fast exciton decay time of 0.57ns. The results of cathodoluminescence and micro-PL scanning images for samples of different well widths further verify the more uniform and stronger luminescence intensity distribution observed for the samples of thinner quantum wells, indicating that the important growth parameters for a-plane InGaN∕GaN multiple quantum wells could be dominated by the In fluctuation and crystal quality during the epitaxial growth.

The authors have used metal organic chemical vapor deposition to grow InGaN∕GaN multiple quantum well (MQW) nanostripes on trapezoidally patterned c-plane sapphire substrates. Transmission electron microscopy (TEM) images clearly revealed that the MQWs grew not only on the top faces of the trapezoids but also on both lateral side facets along the [0001] direction defined by the selected area electron diffraction pattern. Meanwhile, dislocations that stretched from the interfaces between the GaN and the substrates did not pass through the MQWs in the TEM observation. Microphotoluminescence measurements verified that the luminescence efficiency from a single nanostripe was enhanced by up to fivefold relative to those of regular thin film MQW structures. Observation of the cathodoluminescence identified the areas of light emission and confirmed that enhanced emission occurred from the nanostripes.

The crystal quality of a-plane GaN films was improved by using epitaxial lateral overgrowth on trenched a-plane GaN buffer layers. Not only the threading dislocation density but also the difference of anisotropic in-plane strain between orthogonal crystal axes can be mitigated by using trenched epitaxial lateral overgrowth (TELOG). The low threading dislocation density investigated by the cross-sectional transmission electron microscopy was estimated to be 3×107cm−2 on the N-face GaN wing. On the other hand, the Ga-face GaN wing with a faster lateral overgrowth rate could be influenced by the thin GaN layer grown on the bottom of the trenches, resulting in higher dislocation density generated. As a result, the authors concluded that a narrower stripped GaN seeds and deeper stripped trenches etched into the surface of sapphire could derive a better quality a-plane GaN film. Finally, they demonstrated the fast coalescence process of TELOG GaN films below 10μm thick.