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We report on the dependence of internal crystal structures on the electrical properties of a catalyst-free and undoped InAs nanowire (NW) formed on a Si(111) substrate by metal-organic chemical vapor deposition. Cross-sectional transmission electron microscopy images, obtained from four different positions of a single InAs NW, indicated that the wurtzite (WZ) structure with stacking faults was observed mostly in the bottom region of the NW. Vertically along the InAs NW, the amount of stacking faults decreased and a zinc-blende (ZB) structure was observed. At the top of the NW, the ZB structure was prominently observed. The resistance and resistivity of the top region of the undoped InAs NW with the ZB structure were measured to be 121.5 kΩ and 0.19 Ω cm, respectively, which are smaller than those of the bottom region with the WZ structure, i.e., 251.8 kΩ and 0.39 Ω cm, respectively. The reduction in the resistance of the top region of the NW is attributed to the improvement in the crystal quality and the change in the ZB crystal structure. For a field effect transistor with an undoped InAs NW channel, the drain current versus drain-source voltage characteristic curves under various negative gate-source voltages were successfully observed at room temperature. [ABSTRACT FROM AUTHOR]

A novel 3-step growth method is developed for growing n-GaN:Si nanowires (NWs) on a Si (111) substrate using the MOCVD process and this method is being reported for the first time. The aim of this study was to produce GaN NWs of high density and aspect ratio so that advanced GaN NW-based devices of larger effective surface area and high efficiency could be realized further. The results presented in this study succeeded to demonstrate n-GaN:Si NWs of ultrahigh density and aspect ratio. In the case of the 3-step growth recipe, the introduction of an intermediate growth step at a comparatively low temperature of 850 °C gave rise to longer n-GaN:Si NWs of density in the order of 108 cm−2 which is rare for a cost-effective technique like MOCVD. The increase in the NW density is attributed to a thin boundary layer produced during non-pulsed growth at the intermediate temperature. This paper also established the effects of the flow rate of a group-III precursor on the geometry and material properties of n-GaN:Si NWs. For the 3-step growth method, the optimal growth conditions were achieved with the 0.4 sccm TMGa flow at the final step. Comparative studies were done in detail between the 2-step and the 3-step growth methods and how the NW density and height were controlled. A five-fold increment in the photocurrent was noticed for n-GaN:Si NWs grown with the 3-step growth method when compared with the 2-step growth method. Judging from these results, it can be thought that the 3-step method is highly useful in the fabrication of optoelectronic devices using group III-nitride NWs such as photoconductive devices and LEDs. [ABSTRACT FROM AUTHOR]

InxGa1-xN quantum dots (QDs) were grown on GaN epitaxy using nitridation of nano-alloyed droplet (NNAD) method by metal-organic chemical vapor deposition (MOCVD) system. Before the InxGa1-xN QDs formation, In + Ga droplets were initially formed by the flow of TMI and TMG, which acts as a nucleation seed for the QDs growth. Density of the alloy droplets was increased with the increasing flow rate; however, droplet size was scarcely changed about 100–200 nm by flow rate. And InxGa1-xN QDs size can be easily changed by controlling the nitridation time or various factors. Also, the influence of GaN capping layer on the properties of InxGa1-xN QDs was discussed. [ABSTRACT FROM AUTHOR]

We have investigated the p-type doping characteristics such as crystallinity, electrical resistivity and optical properties of Al0.15Ga0.85N:Mg epilayers grown by MOCVD with various Mg incorporations. While the mirror-like surface without any defects such as cracks and hillocks can be seen in the undoped Al0.15Ga0.85N epilayer which was grown without any intentional doping of Mg, many hexagonal pyramids and small islands are observed in the p-Al0.15Ga0.85N:Mg which was grown with Mg incorporation rate of 50 nmol/min. Mg-doped Al0.15Ga0.85N layers exhibit little increase in the electrical resistivity with Mg incorporation until 40 nmol/min and then shows the abrupt increase above 50 nmol/min. The cathodeluminescene spectra of every Mg-doped layer show that the donor-to-acceptor (D–A) pair transition is dominated at 362 nm, which is red shifted by 19 nm against band edge luminescence emission of the undoped Al0.15Ga0.85N layer. By calculating the relative intensity between D–A pair transition of Mg-doped Al0.15Ga0.85N and band edge emission of the undoped, the change of optical characteristics with different Mg doping level was evaluated. This result suggested that the residual Mg–H complexes in highly Mg-doped Al0.15Ga0.85N are not cracked at a certain activation process and are responsible for the abrupt decrease of optical luminescence characteristics. [Copyright &y& Elsevier]

We have grown and characterized GaN/Si(1 1 1) epitaxial layers having different total thicknesses and periods of Al0.3Ga0.7N/GaN superlattices using metalorganic chemical vapor depositon. When the total thickness of Al0.3Ga0.7N/GaN superlattice is 60 nm, GaN/Si(1 1 1) epitaxy shows the best surface and crystal quality. Under the same total thickness of Al0.3Ga0.7N/GaN superlattice of 60 nm, as the number of periods of Al0.3Ga0.7N/GaN superlattices increases, the quality of GaN/Si(1 1 1) epitaxy is also improved. GaN/Si(1 1 1) epitaxy grown with optimized Al0.3Ga0.7N/GaN superlattice of total thickness of 60 nm and 20 periods, shows crack distance of about 120 nm between cracks, FWHM of double crystal X-ray diffractometry rocking curve for GaN(0 0 0 2) of 690 arcsec and FWHM of photoluminescence at RT of 34.49 MeV, which is compared with the best results reported for GaN/Si(1 1 1), so far. Therefore, we obtained a high-quality and nearly crack-free GaN/Si(1 1 1) epitaxy by optimizing Al0.3Ga0.7N/GaN superlattice, possibly for the applications of LED or other photon devices, and, we found that the optimized total thickness and periods of Al0.3Ga0.7N/GaN superlattice play a very important role in the improvement of quality and reducing cracks in the growth of GaN/Si(1 1 1) epitaxy. [Copyright &y& Elsevier]

The superior photoconductive behavior of a simple, cost-effective n-GaN nanorod (NR)-graphene hybrid device structure is demonstrated for the first time. The proposed hybrid structure was synthesized on a Si (111) substrate using the high-quality graphene transfer method and the relatively low-temperature metal-organic chemical vapor deposition (MOCVD) process with a high V/III ratio to protect the graphene layer from thermal damage during the growth of n-GaN nanorods. Defect-free n-GaN NRs were grown on a highly ordered graphene monolayer on Si without forming any metal-catalyst or droplet seeds. The prominent existence of the undamaged monolayer graphene even after the growth of highly dense n-GaN NRs, as determined using Raman spectroscopy and high-resolution transmission electron microscopy (HR-TEM), facilitated the excellent transport of the generated charge carriers through the photoconductive channel. The highly matched n-GaN NR-graphene hybrid structure exhibited enhancement in the photocurrent along with increased sensitivity and photoresponsivity, which were attributed to the extremely low carrier trap density in the photoconductive channel. [ABSTRACT FROM AUTHOR]

Abstract: We report on the size effects of nano-patterned Si(111) substrates on the selective growth of GaN nanowires (NWs) using metal organic chemical vapor deposition. The nano-patterns on Si(111) substrates were fabricated by etching process of Au nano-droplets. The size of nano-patterns fabricated on Si(111) substrates were corresponding to size of Au nano-droplets, and the diameter of GaN NWs grown on nano-patterns was similar to the size of nano-pattern. Dense and well-oriented GaN NWs were grown on Si(111) substrates corresponding to the nano-patterns with an average diameter of about 50nm. However, only a few GaN bulk grains, and mixed phase of a few NWs and bulk crystal of GaN were grown on the nano-patterned Si(111) having too small and large diameter, respectively, compare to the nano-patterns with diameter of 50nm. Our results suggested that the selective growth of GaN NWs is strongly affected by the size of nano-patterns and its related mechanism. [Copyright &y& Elsevier]

We have studied the effects of thermal convection during the growth of GaN epitaxial layers by horizontal MOCVD reactor. The epilayers were grown with the variation of reaction pressures in order to change the total gas velocity in the horizontal MOCVD reactor. The van der Pauw technique, SEM, double crystal X-ray diffractometry and photoluminescence were used in the characterization of their crystallographic, electrical and optical properties for investigating thermal convection effect of NH3. The GaN epilayer which was grown under thermal convection of NH3 related to low gas velocity shows rough surface having large islands. However, the quasi-mirror-like surface without any defect such as hillocks and islands can be seen in the sample which was grown free from thermal convection of NH3 due to suitable gas velocity. The growth rate is also changed, which results from different thermal convections of NH3 related to various gas velocities in the reactor during their growth. Judging from various data, it is possible to conclude that thermal convection of NH3 is taking place when the epilayer is grown in the reactor at a gas velocity below 0.32 m/s. And, the generated thermal convection of NH3 seriously affects the quality of GaN epilayers such as crystallinity, electrical characteristics and optoelectronic properties. [Copyright &y& Elsevier]

[Display omitted] • Novel InGaN quantum particles synthesized on high aspect ratio GaN nanowire structure. • External catalyst-free 1.36 eV InGaN nanowire electrode for broadband solar absorption. • 11.5 mmol cm−2 hydrogen generation with a photon-to-current efficiency of 13.75 % at −0.8 V versus RHE. GaN is a well-known material whose energy band edges can straddle the redox potentials deep in the visible and infrared wavelengths, thereby promising a drastically improved photon-to-current efficiency under applied bias. However, the material is still limited by the half-reactions of water splitting due to its high defect density, low light absorption, small reaction area, and large energy band bending. Here, our study provides a turn-key solution to all these issues. The synergistic effect of InGaN/GaN quantum pyramids on nanowires (QPs-NWs) directly addresses the performance degradation of the photocathodes (PCs). New InGaN QP structures on non-polar GaN nanowire show a unique tunable energy band (E g : ∼2 eV to ∼1.36 eV) by quantum-sliding interface recombination effect. Without the use of external catalysts, the photoelectrochemical water splitting (PEC-WS) of QPs-NWs PC demonstrated enhanced performance with a current density of 34.36 mA cm−2 and a photon-to-current efficiency of 13.75 % under the −0.8 to 0 V applied biasing condition, which is much higher than in previous reports. The current density and the H 2 production were measured to be ∼61.81 mA cm−2 and 11.5 mmol cm−2 for 10 h. The external catalyst-free electrode and the metal organic chemical vapor deposition (MOCVD) process will open a new platform for the commercialization of III-nitride based water splitting hydrogen technology. [ABSTRACT FROM AUTHOR]

The effects of different growth temperatures of n-GaN nanorods (NRs) on the material and electrical properties of n-GaN NRs–graphene hybrid device structures are being demonstrated for the first time. A high quality graphene transfer method was applied for transferring the graphene layer on Si (1 1 1) substrate and n-GaN NRs were synthesized on the graphene layer on Si using a metal organic chemical vapor deposition (MOCVD) process of high V/III ratio. No metal-catalyst or droplet seeds were formed when growing n-GaN NRs. The growth temperature of the n-GaN NRs was varied from 860 °C to 900 °C. Raman spectroscopy confirmed the prominent existence of an undamaged graphene layer in all of the highly-matched hybrid device structures under study. Improvement in the structural, crystalline and material properties was established from FE-SEM, XRD and PL studies for the hybrid structure where n-GaN NRs were grown at 890 °C. The same hybrid structure also showed a ten-fold enhancement in photocurrent along with increased sensitivity and photoresponsivity. Therefore, it can be concluded that a suitable growth temperature of n-GaN NRs is the most important factor for the fabrication of high quality n-GaN NRs–graphene hybrid structures. [ABSTRACT FROM AUTHOR]

Wereport the controlled synthesis of InGaN/GaN multiple quantumwell (MQW) uniaxial (c-plane) and coaxial (m-plane) nanowire (NW) heterostructures by metalorganicchemical vapor deposition. Two kinds of heterostructure NW light-emittingdiodes (LEDs) have been fabricated: (1) 10 pairs of InGaN/GaN MQWlayers in the c-plane on the top of n-GaN NWs whereMg-doped p-GaN NW is axially grown (2) p-GaN/10 pairs of InGaN/GaNshell structure were surrounded by n-GaN core. Here, we discuss acomparative analysis based on the m-plane and the c-plane oriented InGaN/GaN MQW NW arrays. High-resolutiontransmission electron microscopy studies revealed that the barrierand the well structures of MQW were observed to be substantially clearwith regular intervals while the interface regions were extremelysharp. The c-plane and m-plane orientedMQW single NW was utilized for the parallel assembly fabrication ofthe LEDs via a focused ion beam. The polarization induced effectson the c-plane and m-plane orientedMQW NWs were precisely compared via power dependence electroluminescence.The electrical properties of m-plane NWs exhibitedsuperior characteristics than that of c-plane NWsowing to the absence of piezoelectric polarization fields. Accordingto this study, high-quality m-plane coaxial NWs canbe utilized for the realization of high-brightness LEDs. [ABSTRACT FROM AUTHOR]

Abstract: We report on the growth of coaxial InxGa1−xN/GaN nanowires (NWs) on Si(111) substrates by using pulsed flow metalorganic chemical vapor deposition. The coaxial InxGa1−xN/GaN NWs were grown by a two step process in which the core (GaN) structure was grown at a higher temperature followed by the shell (InxGa1−xN) structure at a lower temperature. Dense and well-oriented coaxial InxGa1−xN/GaN NWs were grown with an average diameter and length of about 300±50nm and 1.5–2.0μm, respectively. The coaxial InxGa1−xN/GaN NW was confirmed by cathodoluminescence mapping and high-resolution transmission electron microscopy. It is proposed that the critical dissociation of precursors at an elevated growth temperature can lead to a clear formation of an outer-shell in coaxial InxGa1−xN/GaN NWs. [Copyright &y& Elsevier]

Abstract: We report on the selective area growth of GaN nanowires (NWs) on nano-patterned Si(111) substrates by metalorganic chemical vapor deposition. The nano-patterns were fabricated by the oxidation of Si followed by the etching process of Au nano-droplets. The size of formed nano-pattern on Si(111) substrate was corresponding to the size of Au nano-droplet, and the diameter of GaN NWs grown was similar to the diameter of fabricated nano-pattern. The interesting phenomenon of using the nano-patterned Si(111) substrates is the formation of very clear substrate surface even after the growth of GaN NWs. However, in the case of GaN NWs grown using Au nano-droplets, there was several nanoparticles including GaN bulk grains on the Si(111) substrates. The smooth surface morphology of nano-patterned Si(111) substrates was attributed to the presence of SiO2 layer which prevents the formation of unnecessary GaN particles during the GaN NW growth. Therefore, we believe that nano-patterning method of Si(111) which was obtained by the oxidation of Si(111) substrate and subsequent Au etching process can be utilized to grow high-quality GaN NWs and its related nano-device applications. [Copyright &y& Elsevier]

Abstract: We report on the growth of self-assembled GaN nanowires (NWs) on Si(111) substrates via pulsed metal-organic chemical vapor deposition (MOCVD) technique. Au colloid nanoparticles dispersed on Si(111) substrate were used as catalyst to conduct the GaN NWs growth. The size of the GaN NWs can be tailored by an easy and simple method of optimizing the Au colloids catalyst using Ga pre-deposition. The pulsed MOCVD growth mode was used to grow GaN NWs in which the group III and group V precursors were introduced alternately in the sequence. The surface morphology and optical characterization of the grown GaN NWs were studied by field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), photoluminescence (PL), and cathodoluminescence (CL). The XRD pattern confirmed the hexagonal structure of GaN NWs. The experimental results indicated that the GaN NW diameter depended on the size of Au+Ga droplets. The pulsed method combined with growth temperature considerably affected the shape and density of the GaN NWs. [Copyright &y& Elsevier]

Abstract: Gallium nitride (GaN) nanowire (NW) arrays were grown on a gold-coated Si(111) substrate by metalorganic chemical vapor deposition (MOCVD). Here we adopted two different experimental procedures to grow GaN NWs namely, with and without Ga predeposition on Au-coated Si(111) substrate before annealing. Comparative studies based on the morphology and growth behavior of GaN NWs were performed by using Au catalyst and Au + Ga solid solution nano-droplets. The grown GaN NWs were characterized by field-emission scanning electron microscopy (FE-SEM), energy dispersive X-ray (EDX) spectroscopy, photoluminescence (PL), cathodoluminescence (CL), high-resolution transmission electron microscopy (HR-TEM) and Raman spectroscopy. The nucleation of GaN NWs was explained in terms of Ga predeposition. As a result, we showed that the advantages of using the Au + Ga solid solution nano-droplets for growing GaN NWs. [ABSTRACT FROM AUTHOR]

Abstract: One-dimensional GaN nanostructures are promising materials for nano-device applications. In the present work, we demonstrated the successful growth of GaN NWs on platinum coated Si(111) substrates. The GaN NWs density and degree of alignment were found to be highly sensitive to the growth temperatures. The GaN NWs were characterized by field-emission scanning electron microscopy (FE-SEM), photoluminescence (PL), cathodoluminescence (CL) and high-resolution transmission electron microscopy (HR-TEM). The density of GaN NWs was increased monotonically with increase in growth temperature. Relatively, high density of GaN NWs was observed for the growth temperature of 950°C. It was interestingly observed that the peculiar bicrystalline structure of GaN NWs both contain identical crystal structure with a finite interface. We concluded that the Pt acts as a catalyst for GaN NWs growth, and that it provides control over density and position of GaN NWs growth, which can be ultimately utilized for nano-device fabrication. [Copyright &y& Elsevier]

Abstract: We have performed a detailed investigation of the metal-organic chemical vapor deposition (MOCVD) growth and characterization of InN nanowires formed on Si(111) substrates under nitrogen rich conditions. The growth of InN nanowires has been demonstrated by using an ion beam sputtered (∼10nm) Au seeding layer prior to the initiation of growth. We tried to vary the growth temperature and pressure in order to obtain an optimum growth condition for InN nanowires. The InN nanowires were grown on the Au+In solid solution droplets caused by annealing in a nitrogen ambient at 700°C. By applying this technique, we have achieved the formation of InN nanowires that are relatively free of dislocations and stacking faults. Scanning electron microscopy (SEM) showed wires with diameters of 90–200nm and lengths varying between 3 and 5μm. Hexagonal and cubic structure is verified by high resolution X-ray diffraction (HR-XRD) spectrum. Raman measurements show that these wurtzite InN nanowires have sharp peaks E 2 (high) at 491cm−1 and A1 (LO) at 591cm−1. [Copyright &y& Elsevier]

Abstract: Blue light-emitting diodes (LEDs) with an InGaN multi-quantum-well (MQW) structure were fabricated on a lens patterned sapphire substrate (LPSS) using a single growth process of metalorganic chemical vapor deposition (MOCVD). In the present study, the PSS with lens shape along the 〈0001〉sapphire direction was fabricated by inductively coupled plasma (ICP) dry etching method. When the blue LED device was operated at a forward-bias current of 20mA at room temperature, the output power and the luminous intensity were estimated to be 69.3μW and 159.2 mcd respectively. It was found that the luminous intensity of LPSS–LED was 2.5 times higher than that of the conventional sapphire substrate LED at an injection current of 20mA. At 20mA injection current the emission angle of LPSS–LED was 161.46° which is 1.17 times higher than that of CSS–LED. More importantly the LED on LPSS demonstrates a higher output power and an enhancement of the emission efficiency compared to that of LED on conventional sapphire substrate (CSS). In addition, the reduction of full width at half maximum in the HR-XRD curves of GaN on PSS suggested improved crystal quality. [Copyright &y& Elsevier]

Abstract: Gallium nitride (GaN) nanowire (NW) arrays were grown on a gold-coated Si(111) substrate by metal-organic chemical vapor deposition (MOCVD). We grew GaN NWs at various working pressures in order to examine its effect on the growth of NWs. The synthesized GaN NWs were characterized by field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX) spectroscopy, photoluminescence (PL), cathodoluminescence (CL), high-resolution transmission electron microscopy (HR-TEM) and Raman spectroscopy. The GaN NWs were highly single crystalline and possess uniform smooth surfaces. The HR-TEM images and selected area electron diffraction (SAED) patterns demonstrated that the GaN NWs were single-crystal wurtzite structure. The surfaces of the GaN NWs were clean, atomically sharp and without any other phases. The PL spectra revealed sharp peaks at 366nm with a full width at half maximum (FWHM) of 88meV, clearly indicating that the grown GaN NWs were highly crystalline. The strong E2 (high) phonon line appeared at 567cm−1 in the Raman spectrum reflects that the characteristics of wurtzite structure of GaN NWs. [Copyright &y& Elsevier]

Abstract: Blue light-emitting diodes (LEDs) with an InGaN multi-quantum well (MQW) structure were fabricated on a patterned sapphire substrate (PSS) using a single growth process of metal organic chemical vapor deposition (MOCVD). The electrical and optical properties of these LEDs were investigated. The crystal quality of epitaxial GaN film was improved by using the PSS structure. At 40mA injection current, the peak wavelength and the full-width at half-maximum of the electroluminescence spectra of PSS were 461 and 24nm, respectively. The electroluminescence intensity (EL) of LEDs grown on patterned substrate was 2.44 times greater than that of unpatterened sapphire substrate (UPSS). The operating voltage was measured about 3.1V for the LED with a PSS structure. This significant increase resulted from the improvement of the epitaxial quality of the InGaN/GaN epilayers and the improvement of the light extraction efficiency through patterned sapphire substrates. [Copyright &y& Elsevier]

Abstract: In x Ga1− x N quantum dots (QDs) were grown on GaN/sapphire (0001) substrates by employing nitridation of nano-alloyed droplet (NNAD) method using metal-organic chemical vapor deposition (MOCVD). In+Ga alloy droplets were initially formed by flowing the precursors TMIn and TMGa. Density of the In+Ga alloy droplets was increased with increasing precursors flow rate; however, the droplet size was scarcely changed in the range of about 100–200nm. Two cases of In x Ga1− x N QDs growth were investigated by varying the nitridation time and the growth temperature. It was observed that the In x Ga1− x N QDs size can be easily changed by controlling the nitridation process at the temperature between 680 and 700°C for the time of 5–30min. Self-assembled In x Ga1− x N QDs were successfully grown by employing NNAD method. [Copyright &y& Elsevier]