San Kang

Different characteristics of InGaN/GaN multiple quantum well heterostructures grown on m- and r-planes of a single n-GaN nanowire using metalorganic chemical vapor deposition

The growth behavior of coaxial nonpolar (m-plane) and semipolar (r-plane) oriented multiple quantum well (MQW) heterostructures simultaneously deposited under the same experimental conditions on core n-GaN nanowires (NWs) were investigated in this study. The core–shell-type InGaN/GaN MQW/n-GaN heterostructure NWs were grown on Si(111) substrates via the metalorganic chemical vapor deposition technique. We characterized these MQW NW heterostructures and discussed the comparative analyses of the formation of m-plane and r-plane oriented NW structures. Temperature-dependent photoluminescence (PL) measurements showed a distinct InGaN well structure emission wavelength at the m- and r-plane regions owing to the variation of the In composition. Further, the polarization-induced effects of the MQW NW structure were investigated by using power-dependent cathodoluminescence (CL) measurements. The non-radiative recombination located at the defective interface between the m- and r-planes was correlated with the CL mapping image profile and was also evidenced by scanning transmission electron microscopy (STEM) observations. This investigation addresses the understanding of the formation of nonpolar and semipolar oriented MQW NWs and offers insightful details of the structural and optoelectronic characterization of these core–shell NWs.

Hydrogen Generation using non-polar coaxial InGaN/GaN Multiple Quantum Well Structure Formed on Hollow n-GaN Nanowires

This article demonstrates for the first time to the best of our knowledge, the merits of InGaN/GaN multiple quantum wells (MQWs) grown on hollow n-GaN nanowires (NWs) as a plausible alternative for stable photoelectrochemical water splitting and efficient hydrogen generation. These hollow nanowires are achieved by a growth method rather not by conventional etching process. Therefore this approach becomes simplistic yet most effective. We believe relatively low Ga flux during the selective area growth (SAG) aids the hollow nanowire to grow. To compare the optoelectronic properties, simultaneously solid nanowires are also studied. In this present communication, we exhibit that lower thermal conductivity of hollow n-GaN NWs affects the material quality of InGaN/GaN MQWs by limiting In diffusion. As a result of this improvement in material quality and structural properties, photocurrent and photosensitivity are enhanced compared to the structures grown on solid n-GaN NWs. An incident photon-to-current efficiency (IPCE) of around ~33.3% is recorded at 365 nm wavelength for hollow NWs. We believe that multiple reflections of incident light inside the hollow n-GaN NWs assists in producing a larger amount of electron hole pairs in the active region. As a result the rate of hydrogen generation is also increased.

Fabrication of InxGa1−xN/GaN QDs with InAlGaN capping layer by coaxial growth on non-(semi-) polar n-GaN NWs using metal organic chemical vapor deposition for blue emission

This article demonstrates for the first time the merits of an immediate InAlGaN capping layer over self-assembled InxGa1−xN/GaN quantum dots (QDs) coaxially grown on the m-plane and r-plane of n-GaN nanowires on Si (111) substrate using metal organic chemical vapor deposition. For comparative analysis, we prepared InGaN/GaN QD samples both with and without quaternary capping. InAlGaN capping layer acted as a strain-driven phase separation alloy. Inhomogeneous surface strain over the dots helped this quaternary alloy in forming an indium concentration gradient over InxGa1−xN QDs and thus, indium out-diffusion from the dots was reduced. Quaternary alloy capped samples exhibited vertically stacked, highly dense, pyramidal InxGa1−xN/GaN QDs of improved carrier confinement grown as the active region on n-GaN NWs. In contrast, the nonexistence of InAlGaN capping over InGaN/GaN QDs caused deformation of the dots due to In–Ga inter-diffusion between the dots and the GaN barrier layer. Three kinds of InxGa1−xN/GaN QDs of different x with an InAlGaN capping layer were fabricated coaxially on n-GaN nanowire, whose emission wavelength were 380 nm, 450 nm and 510 nm respectively. These coaxially fabricated InxGa1−xN/GaN QDs on defect free n-GaN nanowires have various excellent characteristics and can be widely applicable to new optoelectronics semiconductor devices.

Synthesis of hybrid nanowires comprising uniaxial and coaxial InGaN/GaN MQWs with a nano-cap

We propose a novel hybrid nanostructure which comprises both uniaxial and coaxial multi-quantum wells (MQWs) on nanowires topped with an InGaN nano-cap. The growth process included both top-down and bottom-up approaches followed by the intentional growth of an InGaN nano-cap to offer larger active area. The In composition was optimized to absorb light at green and blue wavelengths by the uniaxial and coaxial quantum wells respectively. Extensive structural and optical characterizations were carried out. Field emission scanning electron microscopy (FE-SEM) revealed a high density of nanowires. High resolution transmission electron microscopy (HR-TEM) images displayed 5 pairs of uniaxial multi-quantum wells, 6 pairs of coaxial multi-quantum wells and the existence of the intended nano-caps. A photoluminescence (PL) spectrum was recorded for the grown structure at room temperature. The resultant emission spectrum comprised two distinct peaks resulting from each of the multi-quantum well assemblies and emission from the nano-cap. Cathodoluminescence (CL) mapping data revealed discrete bright field images of the InGaN nano-caps and uniaxial multi-quantum well structures. Emission peaks for the nano-caps and both of the multi-quantum well structures were observed in the CL point spectrum which in turn corroborated the PL measurement. During an energy-dispersive X-ray (EDX) study, a high composition of In was found in the nano-cap area along with the distinct presence of both types of multi-quantum wells. In addition, to investigate the opto-electronic device applicability of the grown MQW structure, the photocurrent was measured at various light intensities. The photocurrent density was observed to increase linearly with the increasing light power density. Also the photocurrent density was found to be higher for the hybrid structure than a single uniaxial or coaxial assembly.

Synthesis of n-AlGaN nanoflowers by MOCVD for high-performance ultraviolet-C photodetectors

AlxGa1−xN nanostructure based ultraviolet-C photodetectors have attracted nascent research attention owing to their low cost, tunable band gap, simple operation, and smaller and lightweight systems. However, the synthesis of high quality AlxGa1−xN nanostructures by MOCVD has been limited by their complex multi-component phase diagram and inhomogeneous composition. Here, we report the synthesis of Si-doped n-type compositionally uniform Al0.45Ga0.55N alloy with flower-like morphology (nanoflowers) by MOCVD. Quasi vertically aligned and preferentially c-axis oriented n-AlGaN nanoflowers consist of a large number of self-assembled one-dimensional nanowires that tend to grow radially from the center. The n-AlGaN nanowires are single crystalline in nature and grow along the (0002) direction. The low temperature (88 K) cathodoluminescence spectra of the AlGaN nanoflowers displayed a strong band edge emission at ∼280 nm, which shifted to ∼292 nm at room temperature. The photoresponsivity and sensitivity of the ultraviolet-C photodetectors fabricated with n-AlGaN nanoflowers are ∼0.72 A W−1 and ∼40%, respectively, at 2 V. The nanoflower based device exhibited remarkably superior photoresponse characteristics to the device fabricated with AlGaN nanorods or nanowires, which were synthesized under different MOCVD growth conditions. A large surface-to-volume ratio and a higher density of the nanoflowers enhance photon absorption, resulting in photocurrent gain, a substantially high quantum efficiency and hence improved photoresponse characteristics.

Characteristics of an oxide/metal/oxide transparent conducting electrode fabricated with an intermediate Cu–Mo metal composite layer for application in efficient CIGS solar cell

In order to improve the performance of Al-doped ZnO (AZO) based transparent conducting oxide (TCO) films, a metal Cu–Mo composite layer has been introduced between two AZO films to form an oxide/metal/oxide structure. The AZO/Cu–Mo/AZO (ACMA) multilayer film was prepared at room temperature by direct current magnetron sputtering equipped with multi-targets. To evaluate the performance of ACMA film as transparent conductors, the optical and electrical properties have been studied. These studies have shown the introduction of Cu–Mo composite layer significantly improves the performance of transparent conducting ACMA film compared to the AZO film as well as AZO/Cu/AZO (ACA) and AZO/Mo/AZO (AMA) multilayer films. The opto-electrical properties of the ACMA film critically depend on the thickness of the Cu–Mo composite layer. The highest figure of merit ∼7.96 × 10−4 Ω−1 is obtained for the ACMA film with a 10 nm thick Cu–Mo composite layer, indicating optimum conditions for the fabrication of transparent conducting electrodes. The CIGS solar cell fabricated with the ACMA electrode exhibits substantially higher efficiency (11.59%), compared to ACA or AMA electrodes. The gain in the cell efficiency is attributed to the improved opto-electrical characteristics of the ACMA electrode, resulting from the optimum optical transmittance and charge carrier collections.

A III-nitride nanowire solar cell fabricated using a hybrid coaxial and uniaxial InGaN/GaN multi quantum well nanostructure

III-Nitride nanowires are currently considered as next generation photovoltaic materials due to their excellent physical properties together with reduced dislocation densities, increased surface area and thus enhanced light absorption and direct path for carrier transport. Here, we investigate the photovoltaic characteristics of a solar cell fabricated from a novel hybrid nanostructure comprising uniaxial and coaxial InGaN/GaN multi-quantum wells (MQWs) along with an InGaN nano-cap layer. Various characterization methods were employed to study the optical and structural properties of the hybrid nanostructure. Transmission electron microscopy images revealed the hybrid nanostructure consists of distinct uniaxial and coaxial InGaN/GaN MQWs along with the InGaN nano-cap layer. The InGaN/GaN MQW architectures have a significant effect on the performance of the photovoltaic device. The solar cell fabricated with the hybrid nanostructure exhibits superior photovoltaic performance compared to the uniaxial as well as the coaxial InGaN/GaN nanowire MQW structures. The improved photovoltaic characteristic is primarily attributed to the considerably larger InGaN active area grown in the hybrid nanostructure. A conversion efficiency of 1.16% along with a fill factor of 70% was obtained for the device fabricated with the hybrid nanostructure. This study provides an experimental demonstration of the improvement of III-nitride nanowire based solar cells incorporating uniaxial and coaxial InGaN/GaN MQWs.

The synthesis of hybrid nanostructure comprising star-shaped GaN nanowires and Si nanoworms

Herein, we proposed a novel hybrid nanostructure comprising unique star-shaped GaN nanowires with Si nanoworms having drifting Au nanoparticles inside. Both nanostructures were simultaneously grown using a MOCVD chamber. Intentional stain was induced while growing the star-shaped GaN nanowires to achieve this new nanostructure. A planned experimental environment was deliberated to facilitate the simultaneous growth of the Si nanoworms along with the star-shaped GaN nanowires. After the growth process, various characterization techniques were applied to study the crystallinity and structural and optical properties of the hybrid structure. The growth mechanisms for both structures were also systematically investigated. It was found that a high concentration of the n-dopant source drives the growth of the star-shaped GaN nanowires by suppressing the lateral growth at the high dopant-concentrated vertices of the conventional hexagonal-shaped GaN nanowires. On the other hand, a surplus amount of the same dopant source assists the Si nanoworms to grow. Si nanoworms follow the base-growth mechanism, in which after a while, the Au nanoparticles start to drift inside the Si nanoworms. Finally, the photocurrent of the hybrid structure was measured and compared with that of the star-shaped GaN nanowires only. It is concluded from the photocurrent measurements that the grown hybrid structure is a better candidate for future optoelectronic devices.