The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

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A comprehensive review of zno materials and devices

Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The p...

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Luminescence properties of defects in GaN

Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

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Light-emitting diodes

The isolation of a growing number of two-dimensional (2D) materials has inspired worldwide efforts to integrate distinct 2D materials into van der Waals (vdW) heterostructures. Given that any passivated, dangling-bond-free surface will interact with another through vdW forces, the vdW heterostructure concept can be extended to include the integration of 2D materials with non-2D materials that adhere primarily through non-covalent interactions. We present a succinct and critical survey of emerging mixed-dimensional (2D + nD, where n is 0, 1 or 3) heterostructure devices. By comparing and contrasting with all-2D vdW heterostructures as well as with competing conventional technologies, we highlight the challenges and opportunities for mixed-dimensional vdW heterostructures.

Mixed-dimensional van der Waals heterostructures

In this review, the structural, mechanical, thermal, and chemical properties of substrates used for gallium nitride (GaN) epitaxy are compiled, and the properties of GaN films deposited on these substrates are reviewed. Among semiconductors, GaN is unique; most of its applications uses thin GaN films deposited on foreign substrates (materials other than GaN); that is, heteroepitaxial thin films. As a consequence of heteroepitaxy, the quality of the GaN films is very dependent on the properties of the substrate—both the inherent properties such as lattice constants and thermal expansion coefficients, and process induced properties such as surface roughness, step height and terrace width, and wetting behavior. The consequences of heteroepitaxy are discussed, including the crystallographic orientation and polarity, surface morphology, and inherent and thermally induced stress in the GaN films. Defects such as threading dislocations, inversion domains, and the unintentional incorporation of impurities into the epitaxial GaN layer resulting from heteroepitaxy are presented along with their effect on device processing and performance. A summary of the structure and lattice constants for many semiconductors, metals, metal nitrides, and oxides used or considered for GaN epitaxy is presented. The properties, synthesis, advantages and disadvantages of the six most commonly employed substrates (sapphire, 6H-SiC, Si, GaAs, LiGaO 2 , and AlN) are presented. Useful substrate properties such as lattice constants, defect densities, elastic moduli, thermal expansion coefficients, thermal conductivities, etching characteristics, and reactivities under deposition conditions are presented. Efforts to reduce the defect densities and to optimize the electrical and optical properties of the GaN epitaxial film by substrate etching, nitridation, and slight misorientation from the (0 0 0 1) crystal plane are reviewed. The requirements, the obstacles, and the results to date to produce zincblende GaN on 3C-SiC/Si(0 0 1) and GaAs are discussed. Tables summarizing measures of the GaN quality such as XRD rocking curve FWHM, photoluminescence peak position and FWHM, and electron mobilities for GaN epitaxial layers produced by MOCVD, MBE, and HVPE for each substrate are given. The initial results using GaN substrates, prepared as bulk crystals and as free-standing epitaxial films, are reviewed. Finally, the promise and the directions of research on new potential substrates, such as compliant and porous substrates are described.

Substrates for gallium nitride epitaxy

The future of solid-state lighting relies on how the performance parameters will be improved further for developing high-brightness light-emitting diodes. Eventually, heat removal is becoming a crucial issue because the requirement of high brightness necessitates high-operating current densities that would trigger more joule heating. Here we demonstrate that the embedded graphene oxide in a gallium nitride light-emitting diode alleviates the self-heating issues by virtue of its heat-spreading ability and reducing the thermal boundary resistance. The fabrication process involves the generation of scalable graphene oxide microscale patterns on a sapphire substrate, followed by its thermal reduction and epitaxial lateral overgrowth of gallium nitride in a metal-organic chemical vapour deposition system under one-step process. The device with embedded graphene oxide outperforms its conventional counterpart by emitting bright light with relatively low-junction temperature and thermal resistance. This facile strategy may enable integration of large-scale graphene into practical devices for effective heat removal.

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Improved heat dissipation in gallium nitride light-emitting diodes with embedded graphene oxide pattern

In n-type GaN films grown on sapphire substrates by metal-organic chemical vapor deposition such as unintentionally GaN and intentionally Si-doped GaN and In-doped GaN, the electron capture behaviors were investigated by deep level transient spectroscopy with various filling pulse durations. Two distinct deep levels E1 and E2 were typically observed in unintentionally doped n-type GaN. After optimized growth of undoped GaN, deep level E1 disappears. With increasing Si doping, the trap concentration of deep level E2 is increased. However, In doping in n-type GaN growth was found to suppress the formation of deep level E2. The electrons captured at the traps E1 and E2 were found to depend logarithmically on the duration time of the filling pulse. From an analysis of a model involving barrier-limited capture rate, it can be concluded that deep level E1 is associated with linear line defects along dislocation cores while deep level E2 is related to point defects.

Electron capture behaviors of deep level traps in unintentionally doped and intentionally doped n-type GaN

GaN epitaxial layers on sapphire substrates were grown by the rotating disk metal organic chemical vapor deposition technique. Excitonic transitions from conduction band to spin-orbit split valence bands were observed. At 12 K we observed donor bound exciton and a very weak acceptor bound exciton. The temperature dependence of luminescence peak positions of free-excitons A and B were fitted to the Varshni’s equation to study the variation of the band gap with temperature. The linewidth of the free exciton (A) was studied as a function of temperature and was explained by theoretical model considering the scattering of excitons with acoustic phonons and longitudinal optical phonons. In the 12 K spectrum we also observed phonon-assisted excitonic transitions. The activation energy of the free exciton (A) was found to be 26 meV, while that of the donor bound exciton was 7 meV. The binding energy of the donor was estimated as 35 meV and that of the acceptor as 250 meV. The band gap of GaN was found to be 3.505 eV at 12 K and 3.437 at room temperature. All the parameters obtained in the present investigation are compared with those reported in the literature.

Photoluminescence studies of excitonic transitions in GaN epitaxial layers

Magnesium doped GaN epitaxial layers were grown by metal-organic chemical vapor deposition on sapphire substrate. Energy levels of these acceptors were investigated by systematic photoluminescence measurements in the temperature range of 12–300 K. Magnesium concentration was varied from <1×1019 to higher than 5×1019 cm−3. Photoluminescence measurements were made on the as-grown and annealed samples. We have observed various transitions related to donor to acceptor and their phonon replicas, conduction band to acceptors, and free excitons. Their dependence on temperature, concentration of the magnesium impurity and annealing conditions was discussed. In our study, two important observations were made. First, very deep level luminescence was not observed even in the highly magnesium doped as-grown samples. Second, free exciton transitions including valence band splittings were observed for the first time in the Mg-doped materials, demonstrating the high quality of the samples.

Magnesium acceptor levels in GaN studied by photoluminescence

Carrier localization phenomena in indium-rich InGaN/GaN multiple quantum wells (MQWs) grown on sapphire and GaN substrates were investigated. Temperature-dependent photoluminescence (PL) spectroscopy, ultraviolet near-field scanning optical microscopy (NSOM), and confocal time-resolved PL (TRPL) spectroscopy were employed to verify the correlation between carrier localization and crystal quality. From the spatially resolved PL measurements, we observed that the distribution and shape of luminescent clusters, which were known as an outcome of the carrier localization, are strongly affected by the crystalline quality. Spectroscopic analysis of the NSOM signal shows that carrier localization of MQWs with low crystalline quality is different from that of MQWs with high crystalline quality. This interrelation between carrier localization and crystal quality is well supported by confocal TRPL results.

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Chang-Hee Hong

Papers

Improved heat dissipation in gallium nitride light-emitting diodes with embedded graphene oxide pattern

Electron capture behaviors of deep level traps in unintentionally doped and intentionally doped n-type GaN

Photoluminescence studies of excitonic transitions in GaN epitaxial layers

Magnesium acceptor levels in GaN studied by photoluminescence

Carrier localization in In-rich InGaN/GaN multiple quantum wells for green light-emitting diodes