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GaN‐based materials are the hottest research topic in UV photodetectors (PDs) because of their low operating voltage, small volume, long lifetime, high‐temperature resistance, and low energy consumption. However, there are still fundamental issues to be overcome, and the most important issue is to get a photoconductive gain. In this paper, the following new approaches are provided to innovatively improve the photoconductive gain of UV PDs in GaN‐based materials. First, the aspect ratio of the 1D GaN microwire (MW) structure is dramatically improved by analyzing the pulse growth mechanism using the metal‐organic vapor deposition system. Second, the comprehensive strain behavior in the MW epitaxial growth system is successfully analyzed. Third, the fabricated metal‐semiconductor‐metal‐based MW UV PD shows photoresponsivity and sensitivity of 28.365 A W−1 and 93.16%, respectively, at the −2 V bias, which significantly outperforms the conventional structures in the UV region. Finally, a trap‐assisted Poole–Frenkel effect‐based energy bandgap mechanism, that allows the defect level formed by lattice mismatch between the substrate and GaN to be used as an electron carrier path, is newly defined. This study will present the direction of future UV PDs by providing a new MW structure based on GaN materials, a third‐generation semiconductor. [ABSTRACT FROM AUTHOR]

Abstract: Self-assembled InAs quantum dots (QDs) covered by an In0.15Ga0.85As layer with two different thicknesses were, respectively, grown on GaAs(001) substrates by a solid-source molecular beam epitaxy, and were investigated by cross-sectional transmission electron microscopy (XTEM) and photoluminescence spectroscopy under the various bias conditions. From the XTEM images the shape, especially height, of InAs QDs can be significantly modified by changing the thickness of an In0.15Ga0.85As overgrowth layer, resulting in the modification of the optical properties. With an increase in the QD height leading to more isotropic shape of QDs, the relatively better optical properties such as larger energy-level spacing between the ground states and the first excited states, and enhanced stability to the external bias conditions were demonstrated. [Copyright &y& Elsevier]

Abstract: The characteristics of Si-doped and undoped GaN/Si(111) heteroepitaxy with composite buffer layer (CBL) and superlattice are compared and discussed. While as-grown Si-doped GaN/Si(111) heteroepitaxy shows lower quality compared to undoped GaN, crack-free n-type and undoped GaN with the thickness of 1200nm were obtained by metalorganic chemical vapor deposition (MOCVD). In order to achieve the crack-free GaN on Si(111), we have introduced the scheme of multiple buffer layers; composite buffer layer of Al0.2Ga0.8N/AlN and superlattice of Al0.2Ga0.8N/GaN on 2-in. Si(111) substrate, simultaneously. The FWHM values of the double-crystal X-ray diffractometry (DCXRD) rocking curves were 823arcsec and 745arcsec for n-GaN and undoped GaN/Si(111) heteroepitaxy, respectively. The average dislocation density on GaN surface was measured as 3.85×109 and 1.32×109 cm−2 for n-GaN and undoped GaN epitaxy by 2-D images of atomic force microscopy (AFM). Point analysis of photoluminescence (PL) spectra was performed for evaluating the optical properties of the GaN epitaxy. We also implemented PL mapping, which showed the distribution of edge emission peaks onto the 2 inch whole Si(111) wafers. The average FWHMs of the band edge emission peak was 367.1 and 367.0nm related with 3.377 and 3.378eV, respectively, using 325nm He-Cd laser as an excitation source under room temperature. [Copyright &y& Elsevier]

We have investigated photoluminescence (PL) and electron Hall mobility for unintentionally doped GaN epitaxial layers grown by low-pressure metalorganic chemical vapor deposition on c-plane Al2O3 substrates. Four GaN films having identical dislocation density but remarkably different electron Hall mobility were exploited. At low temperature (12 K), a PL line associated with a bound exciton was observed and strong correlations were found between the Hall mobility and the PL intensity of the exciton transition. That is, relative PL intensity of the bound exciton to a donor-bound exciton monotonously increased with decreasing the electron mobility of the GaN films. This correlation was interpreted in terms of electrical compensation. Efforts to find the chemical origin of the PL line led to the conclusion that the BE line originated neither from threading dislocations nor from extrinsic point defects. Intrinsic acceptors such as Ga vacancy and GaN anti-site were suspected as plausible origin. [Copyright &y& Elsevier]

Spatial variations of donor concentration and diffusion length/lifetime were studied for epitaxial laterally overgrown undoped n-GaN samples by electron beam induced current (EBIC) and microcathodoluminescence (MCL). The dependence of the EBIC signal collection efficiency on the probing beam accelerating voltage shows that the local electron concentration is three times lower and the local lifetime about twice as high in the laterally overgrown regions compared to the regions grown in the SiO2 mask windows. Band edge MCL profiling shows that the lifetime difference could be an order of magnitude higher. EBIC scans along the SiO2 stripes in the low dislocation density overgrown regions show long propagation distances of holes in the quasineutral part of the structure, explained by the existence of a potential profile forming a trough for transport of holes while spatially separating nonequilibrium carriers. [ABSTRACT FROM AUTHOR]

We investigated the emission characteristics of InGaN/GaN light-emitting diodes (LEDs) fabricated on lens-shaped (LS) patterned-sapphire substrates (PSS) by using time-resolved photoluminescence (TRPL) and confocal-scanning-electroluminescence microscopy (CSEM). The carrier lifetimes evaluated from the TRPL spectra for the LEDs on the LS-PSS (LS-LEDs) at 10K were relatively shorter than those of the LEDs on a conventional planar substrate (C-LED). However, the carrier lifetimes for the LS-LEDs were relatively long compared to that of the C-LED at room temperature. In the CSEM images of the LS-LEDs, the emission beam around the center region of the LS pattern was relatively weaker than that of the edge region. In addition, the beam profile for the LS-LEDs showed different shapes according to the pattern structures. The emission beam around the boundary region of the LS pattern showed periodic fluctuation with the peak-to-peak distance of 814nm. [ABSTRACT FROM AUTHOR]

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]

Abstract: We report the influences of growth parameters on the characteristics of GaAs quantum rings (QRs) and quantum dots (QDs) formed on AlGaAs/GaAs by the droplet epitaxy (DE) method. After forming Ga droplets on the AlGaAs/GaAs surface, varying amounts of arsenic (As) flux were introduced to fabricate the GaAs quantum structures. By decreasing the As flux from 8×10−5 to 3×10−5 Torr, the shape of the GaAs quantum structures was changed from QDs to elongated QRs. With further decreasing As flux, the shape of the elongated QRs became symmetric. The formation characteristics of the GaAs QRs from the QDs with the amount of As flux were discussed in terms of migration behaviors of the gallium (Ga) atoms on the GaAs(001)-c(4×4) surface. The effects of the amount of Ga supply and the growth temperature for the deposition of Ga droplets on the formation of the GaAs quantum structures were also considered. [ABSTRACT FROM AUTHOR]

Abstract: Monocrystalline ZnO thin films on p-GaN/sapphire (0001) substrate were grown by single step hydrothermal technique at 90 °C. ZnO thin films were grown in aqueous solution of zinc nitrate and ammonium hydroxide. The X-ray diffraction, scanning electron microscopy and room temperature photoluminescence analysis were carried out to characterize structural, morphological and optical properties of the films. The thin films revealed single crystalline nature and wurtzite symmetry. The films were c-axis oriented and have honeycomb like pitted surface morphology. The epitaxial relationship between ZnO film and p-GaN buffer layer was observed to be (0001)[112¯0]ZnO||(0001)[112¯0]GaN. Sharp luminescent peak centered at 376 nm due excitonic emission and broad deep level emission peak were obtained from room temperature photoluminescence measurement of the ZnO thin film. [Copyright &y& Elsevier]

Abstract: The growth of GaN on the patterned substances has proven favorable to achieve thick, crack-free GaN layers. Based on these methods, we specially designed periodically patterned Si substrate process, which is referred to as lateral epitaxy on patterned Si substrate (LEPS). High crystalline quality GaN are obtained by using this technique. In this paper, numerical modeling of transport and reaction of species is performed to estimate the growth rate of GaN from the reaction of trimethyl gallium (TMG) and ammonia. The effect of fabricated structure of feature scale model will be predicted by using the topography simulator, and deposition profile of the GaN on the pattern will be discussed. The effect of flow conditions and pattern shape and periodicity will also be addressed, which can be critical for the quality of crystal growth. The dependency of step coverage and conformality of patterned mask will also be discussed. [Copyright &y& Elsevier]

Self-assembled InAs quantum dots (QDs) with In0.15Ga0.85As were grown by a molecular beam epitaxy and their optical properties were investigated by photoluminescence (PL) spectroscopy. For InAs QDs inserted in an asymmetric In0.15Ga0.85As quantum well, the emission peak position of QDs is 1.30 μm (0.953 eV) with narrower PL linewidth and larger energy-level spacing between the ground states and the first excited states compared to those of QDs embedded in a GaAs matrix. While the room temperature PL yield for InAs QDs in a GaAs matrix was reduced by 1/99 from that measured at 18 K, the reduction in PL yield for InAs QDs, grown on a 1 nm In0.15Ga0.85As layer, with a 6 nm In0.15Ga0.85As overgrowth layer was only 1/27. Also, using the In0.15Ga0.85As overgrowth layer significantly reduced the temperature sensitivity of the peak energy for InAs QDs. The relatively better temperature PL characteristics of the QDs with In0.15Ga0.85As, as well as the ability to control the emission peak position and the energy-level spacing are interesting and important for device applications. [Copyright &y& Elsevier]

The characteristics of GaN/Si(1 1 1) epitaxial layers grown by metalorganic vapor phase epitaxy(MOVPE) with AlxGa1−xN/AlN composite buffer layer(CBL) where x varies from 0.07 to 0.15 in AlxGa1−xN were investigated. The x values of AlxGa1−xN grown with various trimethyl-gallium(TMG) flow rates while feeding trimethyl-aluminum(TMA) flow constant were determined by triple crystal X-ray diffractrometry(TCXRD). The surface images of GaN/Si(1 1 1) epitaxy observed by SEM show some cracks, which are due to thermal mismatch between GaN and Si. According to double crystal X-ray diffractometry (DCXRD) rocking curves of GaN(0002), the crystal quality of GaN/Si(1 1 1) grown on Al0.11Ga0.89N/AlN CBL exhibits the best result with FWHM of 886 arcsec. PL spectrum at RT of the GaN/Si(1 1 1) epitaxy grown on the CBL shows a sharp band edge emission peak at 365 nm with average FWHM of 45 meV, which is compared with the best result achieved so far for GaN/Si. Therefore, it can be concluded that the AlxGa1−xN/AlN CBL having suitable Al mole fraction of AlxGa1−xN layer plays an important role in improving the quality of GaN/Si(1 1 1) epitaxy by reducing the lattice and thermal mismatch between GaN and Si(1 1 1). [Copyright &y& Elsevier]

We have studied the effects of Al0.1Ga0.9N(150 nm)/AlN composite nucleation layers (CNLs) having different thicknesses of AlN ranging from 20 to 41 nm on the growth characteristics of GaN/Si(1 1 1) epitaxy. The surface morphology of the GaN epitaxial layers which were grown on Al0.1Ga0.9N(150 nm)/AlN CNLs showed that the number of thermal etch pits and cracks were abruptly decreased with the increase of AlN thickness from 20 to 35 nm. However, the morphology of GaN epitaxy which was grown on Al0.1Ga0.9N(150 nm)/AlN CNL having AlN of thickness 41 nm above 35 nm showed that the number of etch pits increased again. The GaN/Si(1 1 1) epitaxy grown by using Al0.1Ga0.9N(150 nm)/AlN(35 nm) CNL showed that the highest crystallinity having a FWHM of 1157 arcsec for the (0 0 0 2) diffraction. Photoluminescence (PL) spectrum at room temperature for GaN/Si(1 1 1) epitaxy grown using Al0.1Ga0.9N(150 nm)/AlN(35 nm) CNL showed sharp band edge emission at 364 nm, which do not have yellow luminescence related to various defects such as vacancy and dislocation. The PL spectra at room temperature for the GaN layers grown using other CNLs showed yellow luminescence at around 580 nm in addition to the band edge emission. Moreover, the FWHM of the main exitonic peak at 10 K for the GaN/Si(1 1 1) epitaxy, which was grown using Al0.1Ga0.9N(150 nm)/AlN(35 nm) CNL, has the lowest value of 12.81 meV. It is obvious that the Al0.1Ga0.9N(150 nm)/AlN CNL having suitable thickness of AlN plays an important role in improving the crystallinity and optical properties of GaN/Si(1 1 1) heteroepitaxy without showing any defects such as pits and cracks over the surface by reducing the mismatch of thermal expansion coefficient and lattice constant between GaN and Si(1 1 1) compared with other nucleation layer such as AlxGa1−xN or AlN alone. [Copyright &y& Elsevier]

We report the role of a thin AlN buffer layer introduced between AlxGa1−xN and GaN grown on sapphire (0 0 0 1) by MOVPE. The heterostructures have high x values ranging from 0.35 to 0.50. After growing the 20 nm thick AlN buffer layer on GaN/sapphire (0 0 0 1) epitaxy, the 1.0 μm thick AlxGa1−xN epitaxial layers were grown by increasing the flow rate of TMA. The measured Al mole fractions of AlxGa1−xN/GaN heteroepitaxy grown using the thin AlN buffer from each TCD rocking curve are 0.35, 0.37, 0.45 and 0.50, respectively. As the incorporation rate of Al in the AlxGa1−xN increases, the crystallinity is improving and the RMS values scanned by AFM of their surfaces become lower. The optoelectronic characteristics of these heteroepitaxial layers were evaluated by cathodoluminescence and found to improve with the increase of x. To the contrary these trends of improvement, it is usually known that the properties such as crystallinity, surface morphology and optical properties of the AlxGa1−xN/GaN heteroepitaxial layers grown without AlN buffer layer become worse with the increase of x above 0.2. The electrical resistivities of Al0.35Ga0.65N/GaN, Al0.37Ga0.63N/GaN, Al0.45Ga0.55N/GaN and Al0.5Ga0.5N/GaN heterostructure, which were grown with the thin AlN buffer, measured by four point probing method are 13.5, 18.1, 31.7 and 36.2 MΩ cm, respectively. The resisitivity increases with the raising of x. It may be caused by the increment of the intrinsic own-resistance. [Copyright &y& Elsevier]

Abstract: The possibilities of the Electron Beam Induced Current (EBIC) method for the characterization of epitaxial lateral overgrowth (ELOG) GaN structures are demonstrated. The diffusion length and local donor concentrations in the structures studied are obtained by fitting the dependence of collected current in the EBIC mode on accelerating voltage with calculated one. It is shown that the local donor concentrations in the regions of lateral overgrowth measured by the EBIC are about three times lower than that in the vertically grown regions. This difference is observed in the structures with the donor concentration varying in the range from 1015 to 1017 cm−3. These results could be ascribed to the anisotropy of Si incorporation efficiency during growth. A comparison of the EBIC results with the results of capacitance–voltage (C–V) profiling allows us to estimate the precision of EBIC dopant concentration measurements. [Copyright &y& Elsevier]