Shengrui Xu

Yellow luminescence of polar and nonpolar GaN nanowires on r-plane sapphire by metal organic chemical vapor deposition.

We have grown horizontal oriented, high growth rate, well-aligned polar (0001) single crystalline GaN nanowires and high-density and highly aligned GaN nonpolar (11-20) nanowires on r-plane substrates by metal organic chemical vapor deposition. It can be found that the polar nanowires showed a strong yellow luminescence (YL) intensity compared with the nonpolar nanowires. The different trends of the incorporation of carbon in the polar, nonpolar, and semipolar GaN associated with the atom bonding structure were discussed and proved by energy-dispersive X-ray spectroscopy, suggesting that C-involved defects are the origin responsible for the YL in GaN nanowires.

Performance Comparison of Conventional and Inverted Organic Bulk Heterojunction Solar Cells From Optical and Electrical Aspects

The conventional and inverted organic solar cells (OSC and IOSC) based on the bulk heterojunction structure are investigated from both optical and electrical aspects. When the optical aspect is considered only, with the increase of the active layer thickness, the number of photons absorbed in the active layer and the external quantum efficiency tend to increase with the obvious interference behavior for both OSC and IOSC. However, compared to OSC, IOSC shows a better performance except for the thicknesses around which the interference maxima of OSC are obtained. When the electrical aspect is also considered, an effective area in the active layer will be induced by the charge drift length (L), and only the photons absorbed in this effective area have contribution to the photocurrent. By considering optical and electrical aspects together, OSC and IOSC show different behaviors. Compared to IOSC, OSC performs better for relatively thick active layers. Simultaneously, the optical modulation effect is also investigated by introducing an optical spacer layer. It is found that the optical spacer layer can notably enhance the performance of OSC with thin and thick active layers, while it could only degrade the performance of IOSC with relatively thick active layers.

Threading dislocation reduction in transit region of GaN terahertz Gunn diodes

An effect of the position of notch-doping layer in 1-μm GaN Gunn diode on threading dislocations (TDs) distribution is investigated by transmission electron microscopy. Compared with the top-notching-layer (TNL) structure, the bottom-notching-layer (BNL) structure can efficiently reduce the TDs density and improve the crystal quality in the transit region of GaN Gunn diode because it exhibits twice-transition of growth mode from atomic step flow to layer-by-layer nucleation and leads to a significant annihilation of TDs before penetrating into the transit region. X-ray diffraction and Raman spectroscopy reveal that the BNL structure has less compressive stress than the TNL structure.

Influence of vicinal sapphire substrate on the properties of N-polar GaN films grown by metal-organic chemical vapor deposition

The influence of vicinal sapphire substrates on the growth of N-polar GaN films by metal-organic chemical vapor deposition is investigated. Smooth GaN films without hexagonal surface feature are obtained on vicinal substrate. Transmission electron microscope results reveal that basal-plane stacking faults are formed in GaN on vicinal substrate, leading to a reduction in threading dislocation density. Furthermore, it has been found that there is a weaker yellow luminescence in GaN on vicinal substrate than that on (0001) substrate, which might be explained by the different trends of the carbon impurity incorporation.

Asymmetric quantum-well structures for AlGaN/GaN/AlGaN resonant tunneling diodes

Asymmetric quantum-well (QW) structures including the asymmetric potential-barrier and the asymmetric potential-well are proposed for AlGaN/GaN/AlGaN resonant tunneling diodes (RTDs). Theoretical investigation gives that an appropriate decrease in Al composition and thickness for emitter barrier as well as an appropriate increase of both for collector barrier can evidently improve the negative-differential-resistance characteristic of RTD. Numerical simulation shows that RTD with a 1.5-nm-thick GaN well sandwiched by a 1.3-nm-thick Al0.15Ga0.85N emitter barrier and a 1.7-nm-thick Al0.25Ga0.75N collector barrier can yield the I-V characteristic having the peak current (Ip) and the peak-to-valley current ratio (PVCR) of 0.39 A and 3.6, respectively, about double that of RTD with a 1.5-nm-thick Al0.2Ga0.8N for both barriers. It is also found that an introduction of InGaN sub-QW into the diode can change the tunneling mode and achieve higher transmission coefficient of electron. The simulation demonstrates th...

Spatial distribution of crystalline quality in N-type GaN grown on patterned sapphire substrate

The epitaxial layers of n-type GaN were grown on both planar and patterned sapphire substrate (PSS) by metal organic chemical vapor deposition. By comparing the epitaxial layers grown on planar substrate, GaN grown on PSS exhibited many improvements both on surface morphology and crystalline quality according to the characterization of atoms force microscopy, and high resolution X-ray diffraction. Spatially resolved micro-Raman scattering results were performed for mapping the spatial variations in crystalline quality of the n-type GaN grown on PSS. According to the variations on the intensity and the full width at half maximum of GaN E2 (high) peaks, the crystalline quality improvement occurred in the lateral growth regions which correspond to center region of the pyramid patterns. We proposed that the bending of dislocations during the lateral growth plays an important role in the spatial variations of GaN crystalline quality. Cross sectional transmission electron microscope and spatial cathodoluminescence mapping results further supported the explanation of the dislocation inhibition during the growth process of GaN grown on PSS.

Effects of interlayer growth condition on the transport properties of heterostructures with InGaN channel grown on sapphire by metal organic chemical vapor deposition

The effects of AlN interlayer growth condition on the properties of InAlN/InGaN heterostructures are investigated in detail. Since the properties of InGaN channel are different from the traditional GaN channel, two-step AlN interlayer is proposed, which is proven to be more suitable for the InGaN channel heterostructures than the interlayers grown at constant temperature. Test results show that two-step AlN interlayer can not only significantly improve the interface morphology between the InGaN channel and barrier layers but also make an effective protection of the high-quality InGaN channel. The electron mobility of the InAlN/InGaN heterostructure with two-step AlN interlayer achieves 890 cm2/V s with a high two-dimensional-electron-gas density of 1.78 × 1013 cm−2. The gratifying results indicate that the InGaN channel heterostructure with two-step interlayer is a promising candidate for microwave power devices.

Effects of growth temperature on the properties of InGaN channel heterostructures grown by pulsed metal organic chemical vapor deposition

Pulsed metal organic chemical vapor deposition (P-MOCVD) is introduced into the growth of high quality InGaN channel heterostructures. The effects of InGaN channel growth temperature on the structural and transport properties of the heterostructures are investigated in detail. High resolution x-ray diffraction (HRXRD) and Photoluminescence (PL) spectra indicate that the quality of InGaN channel strongly depends on the growth temperature. Meanwhile, the atomic force microscopy (AFM) results show that the interface morphology between the InGaN channel and the barrier layer also relies on the growth temperature. Since the variation of material properties of InGaN channel has a significant influence on the electrical properties of InAlN/InGaN heterostructures, the optimal transport properties can be achieved by adjusting the growth temperature. A very high two dimension electron gas(2DEG) density of 1.92 × 1013 cm−2 and Hall electron mobility of 1025 cm2/(V⋅s) at room temperature are obtained at the optimal growth temperature around 740 °C. The excellent transport properties in our work indicate that the heterostructure with InGaN channel is a promising candidate for the microwave power devices, and the results in this paper will be instructive for further study of the InGaN channel heterostructures.

The performance enhancement of an InGaN/GaN multiple-quantum-well solar cell by superlattice structure

An enhanced InGaN/GaN multiple-quantum-well (MQW) solar cell was produced and characterized through the superlattice structure (SLS) insertion. The experiments demonstrated that the conversion efficiency of the fabricated device increased from 0.61 to 1.61%, compared to the device without SLS. The promising result was considered to originate from the SLS insertion. From Raman analysis and theoretical calculation of the electron transmissivity, it was demonstrated that the plane strain of GaN was effectively released when the SLS was inserted and the electron tunneling effect was enhanced. Consequently, the collection of photo-generated electrons was strengthened, which thereby led to the conversion efficiency increase.

Improved carrier mobility of InAlN/GaN heterostructure grown on cone-shaped patterned sapphire substrate

InAlN/GaN heterostructures were grown on both patterned and planar sapphire substrates by metal–organic chemical vapor deposition. Hall measurements showed that both samples possessed similar two-dimensional electron gas densities, whereas the electron mobility of the heterostructure grown on a patterned substrate was evidently superior. The enhanced mobility was attributed to the reduced threading dislocation density and improved channel-barrier interface morphology. Furthermore, Raman measurement results showed that there was less residual compressive strain in the GaN grown on the pattered sapphire substrate, which could be the reason for the morphology improvement. Consequently, the patterned sapphire substrate is very promising for further improving the performance of high-electron-mobility transistors.