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In this work, we demonstrate the potential of a gallium nitride (GaN)-based visible-blind ultraviolet (UV) photodetector (PD) on a commercially viable 150-mm Si wafer. The influence of thermionic field emission (TFE) and Poole–Frenkel (PF) mechanisms on the current transport of the PD has been analyzed. Conduction due to the TFE mechanism dominates in the moderate electric fields (1.25 kV/cm ${ kV/cm), while the influence of PF is prominent at higher electric fields. A bulk trap energy level of 0.374 eV is obtained with PF conduction analysis. A high responsivity of 33.3 A/W at 15 V with a 362-nm incident wavelength has been achieved in the presence of an internal gain. The internal gain of the PD is also assisted by TFE and PF mechanisms. The PD exhibits a low dark current of 4.7 nA as well as high detectivity of $4.6\times 10^{12}$ Jones at the abovementioned bias. The demonstrated robustness and high performance show the promise of III–nitride PDs for commercial applications.

In this article, we report the temperature-dependent transistor characteristic of Epi-Gd2O3/AlGaN/GaN metal oxide semiconductor high electron mobility transistor (MOSHEMT) and compare its properties with that of AlGaN/GaN metal-Schottky high electron mobility transistor (HEMT) grown on 150 mm Si (111) substrate. Introducing an epitaxial single crystalline Gd2O3 between the metal gate and AlGaN barrier not only improves the gate leakage current significantly but also enhances its thermal stability. We observe that there is no significant change in the gate leakage current even at 473 K compared to that measured at room temperature (RT) (298 K), and this is also evident in the transistor’s subthreshold behavior at 473 K. We have determined the electric field within the Gd2O3 as well as AlGaN and investigated the leakage conduction mechanism through Gd2O3. The ${I}_{ \mathrm{ON}}/{I}_{ \mathrm{OFF}}$ of the transistor was measured as high as ~108 even at 473 K with the lowest ${V}_{\text {TH}}$ shift (91.4 mV) with temperature. Our measurements also confirm the presence of polar optical phonon scattering, which directly affects the 2-D electron gas (2DEG) mobility at high temperatures and thus the electrical characteristics of HEMT and MOSHEMT.

This letter reports the demonstration of N-polar Al0.8Ga0.2N/AlN continuously-graded-channel polarization-doped field-effect transistors (PolFETs) on SiC. A PolFET with a source to drain distance of $12~\mu \text{m}$ exhibited a maximum drain current of 62.8 mA/mm and an ON/OFF current ratio of $1.1\times 10^{{4}}$ . The maximum drain current was stable between 20 °C and 250 °C operating temperatures. With the addition of 30-nm-thick Al2O3 gate insulator the maximum drain current increased to 126 mA/mm.

Due to the functional limitations of SiO2 for SOI applications, alternative dielectric materials have been investigated. Alternative SOI materials in this work include, AlN and AlGaN. The dielectrics were deposited using MOCVD, and with the aid of PECVD deposited SiO2, and the SiO2 was directly bonded to a handle Si wafer. Tensile tests were performed on the samples to examine the fracture behavior and maximum tensile stresses, with results being comparable to a traditional SOI. Characterization was undertaken using TEM to understand the microstructural and interfacial properties of alternative SOI. High crystal quality Al(Ga)N was achieved on a Si(111) substrate that generally contained well defined chemical interfaces. Finally, synchrotron X-ray diffraction topography was used to understand the topographical strain profile of the device and handle wafers. Topography results showed different strain network properties between the device and handle wafer. This work has demonstrated preliminary feasibility of using alternative dielectrics for SOI applications.

In this work, atomic layer etching (ALE) of thin film Ga-polar GaN(0001) is reported in detail using sequential surface modification by Cl2 adsorption and removal of the modified surface layer by low energy Ar plasma exposure in a standard reactive ion etching system. The feasibility and reproducibility of the process are demonstrated by patterning GaN(0001) films by the ALE process using photoresist as an etch mask. The demonstrated ALE is deemed to be useful for the fabrication of nanoscale structures and high electron mobility transistors and expected to be adoptable for ALE of other materials.

In this letter, we report the impact of epitaxial Gd2O3 on the electrical properties of an AlGaN/GaN high electron mobility transistor (HEMT) grown on a 150 mm diameter Si (111) substrate. Incorporation of epitaxial Gd2O3 grown by the molecular beam epitaxy technique under a metal gate (metal/Gd2O3/AlGaN/GaN) causes six orders of magnitude reduction in gate leakage current compared to metal/AlGaN/GaN HEMT. We observe that epi-Gd2O3 undergoes complete structural changes from hexagonal to monoclinic as the thickness of the layer is increased from 2.8 nm to 15 nm. Such structural transformation is found to have a strong impact on electrical properties whereby the gate leakage current reaches its minimum value when the oxide thickness is 2.8 nm. We find a similar trend in the density of interface traps (Dit) having a minimum value of 2.98 × 1012 cm-2 eV-1 for the epioxide layer of thickness 2.8 nm. Our measurements also confirm a significant increase in the two dimensional electron gas (2DEG) density (∼40%) at AlGaN/GaN interface with epioxide grown on AlGaN, thus confirming the contribution of epitaxial lattice strain on 2DEG modulation.

We report on studies of electrically excited non-equilibrium 2D electrons and 2D plasmons in an AlGaN/GaN nanostructure. Optical access to 2D plasmons is provided by means of a metal grating fabricated at the nanostructure surface, while the properties of 2D electrons are examined in the samples without metal grating. The paper focuses on the creation of highly non-equilibrium conditions when the effective temperature of 2D electrons is much higher than the crystal lattice temperature. Such conditions are realized by applying short electrical pulses with a low repetition frequency. A method has been developed for independently determining the temperature of hot electrons and the temperature of the crystal lattice under an applied electric field. It has been shown that under highly non-equilibrium conditions the spectral density of terahertz electroluminescence of 2D plasmons can significantly exceed that of 2D electrons at a certain frequency.

Metalorganic chemical vapor deposited (MOCVD) aluminum nitride (AlN) on vertical sidewalls can be used to implement piezoelectric in-plane actuation and sensing in microelectromechanical system (MEMS) sensors. The AlN films should optimally cover conformally the sidewalls and have good crystal quality with c-axis oriented microstructure for optimal piezoelectric properties. Previous MOCVD AlN research has focused on using AlN as a buffer layer for other III-nitrides and so far, AlN growth has not been studied on large vertical surfaces. In this study, AlN thin films were grown using MOCVD on vertical sidewalls of fabricated templates and the conformality and crystal quality was characterized. The growth template fabrication was optimized with respect to surface roughness, the conformal coverage was analyzed by measuring the thickness profiles of the films, and the crystal quality was investigated using in-plane XRD and TEM. The AlN films have good crystal quality (FWHM 1.70°–3.44°) and c-axis orientation on vertical Si(1 1 1) sidewalls. However, the thicknesses of the films reduce approximately at a rate of 0.8–1.2 nm/ μ m down the sidewall. Lowering the reactor pressure improved the conformal coverage while changing the growth mode from columnar to step-flow, which also improved the film morphology.

We present the electrical characterization of N-polar AlN layers grown by metal-organic vapor phase epitaxy and the demonstration of N-polar AlN-channel metal-semiconductor field-effect transistors (MESFETs). A high concentration of silicon is unintentionally incorporated during the high-temperature growth of N-polar AlN, causing a high buffer leakage current. The silicon concentration decreases from 2 x 10(18) to 9 x 10(15) cm(-3) with decreasing growth temperature, reducing the buffer leakage current to 5.6 nA/mm at a 100 V bias. The N-polar AlN MESFET exhibits an off-state drain current of 0.27 nA/mm and a transistor on/off ratio of 4.6 x 10(4) owing to the low leakage of AlN buffer layers. (C) 2018 The Japan Society of Applied Physics

Almost all modern inorganic light-emitting diode (LED) designs are based on double heterojunctions (DHJs) whose structure and current injection principle have remained essentially unchanged for decades. Although highly efficient devices based on the DHJ design have been developed and commercialized for energy-efficient general lighting, the conventional DHJ design requires burying the active region (AR) inside a pn-junction. This has hindered the development of emitters utilizing nanostructured ARs located close to device surfaces such as nanowires or surface quantum wells. Modern DHJ III-N LEDs also exhibit resistive losses which arise from the DHJ device geometry. The recently introduced diffusion-driven charge transport (DDCT) emitter design offers a novel way to transport charge carriers to unconventionally placed ARs. In a DDCT device, the AR is located apart from the pn-junction and the charge carriers are injected into the AR by bipolar diffusion. This device design allows the integration of surface ARs to semiconductor LEDs and offers a promising method to reduce resistive losses in high power devices. In this work, we present a review of the recent progress in gallium nitride (GaN) based DDCT devices, and an outlook of potential DDCT has for opto- and microelectronics.

The optical and electrical properties of silicon-doped epitaxial gallium nitride layers grown on sapphire have been studied. The studies have been performed over a wide range of silicon concentrations on each side of the Mott transition. The critical concentrations of Si atoms corresponding to the formation of an impurity band in gallium nitride (∼2.5 × 1018 cm−3) and to the overlap of the impurity band with the conduction band (∼2 × 1019 cm−3) have been refined. The maximum of the photoluminescence spectrum shifts nonmonotonically with increasing doping level. This shift is determined by two factors: (1) an increase in the exchange interaction leading to a decrease in the energy gap width and (2) a change in the radiation mechanism as the donor concentration increases. The temperature dependence of the exciton luminescence with participating optical phonons has been studied. The energies of phonon-plasmon modes in GaN: Si layers with different silicon concentrations have been measured using Raman spectroscopy.

Devices based on nanostructures hold great potential to improve the performance of present light emitter technologies. In this paper, we examine a buried multiquantum well III-nitride diffusion injected light-emitting diode (DILED), where the active region is located outside the p-n junction and current injection to the active region takes place through bipolar diffusion. We study the current–voltage behavior and light emission characteristics of the DILED as a function of temperature experimentally and theoretically. We show that in contrast to conventional LEDs, the light output efficiency of the DILED increases when temperature is increased from 0 °C to 100 °C. This anomalous temperature behavior is shown to be linked to the strong temperature dependency of ionized acceptor density in the p-doped region, which increases the hole diffusion current into the active region. This highlights the fundamental difference in the operating principle of the DILED compared with conventional LEDs. In addition to optical and electrical characterization of a DILED, we also study the relation of the observed yellow-band luminescence to Shockley–Read–Hall recombination, compare the measurements to charge carrier transport simulations, and present an equivalent circuit model of the DILED structure for additional insight into the new current injection scheme.

White beam synchrotron radiation X-ray topography (SR-XRT) and X-ray diffraction (XRD) measurements were used to non-destructively study the defect structure of a bulk GaN wafer, grown by the ammonothermal method. SR-XRT topographs revealed high crystal quality with threading dislocation density 8.8×10 4 cm −2 and granular structure consisting of large, slightly misaligned grains. The threading dislocations within grains were identified as mixed and screw type, while no pure threading edge dislocations were observed.

High-power operation of conventional GaN-based light-emitting diodes (LEDs) is severely limited by current crowding, which increases the bias voltage of the LED, concentrates light emission close to the p-type contact edge, and aggravates the efficiency droop. Fabricating LEDs on thick n-GaN substrates alleviates current crowding but requires the use of expensive bulk GaN substrates and fairly large n-contacts, which take away a large part of the active region (AR). In this work, we demonstrate through comparative simulations how the recently introduced diffusion-driven charge transport (DDCT) concept can be used to realize lateral heterojunction (LHJ) structures, which eliminate most of the lateral current crowding. Specifically in this work, we analyze how using a single-side graded AR can both facilitate electron and hole diffusion in DDCT and increase the effective AR thickness. Our simulations show that the increased effective AR thickness allows a substantial reduction in the efficiency droop at large currents, and that unlike conventional 2D LEDs, the LHJ structure shows practically no added efficiency loss or differential resistance due to current crowding. Furthermore, as both electrons and holes enter the AR from the same side without any notable potential barriers in the LHJ structure, the LHJ structure shows an additional wall-plug efficiency gain over the conventional structures under comparison. This injection from the same side is expected to be even more interesting in multiple quantum well structures, where carriers typically need to surpass several potential barriers in conventional LEDs before recombining. In addition to simulations, we also demonstrate selective-area growth of a finger structure suitable for operation as an LHJ device with 2µm distance between n- and p-GaN regions.

In this paper we report on the MOCVD growth and characterization of GaN structures and InGaN single quantum wells grown on pillar patterned GaN/sapphire templates. During the regrowth a network of voids was intentionally formed at the interface of sapphire substrate and GaN epitaxial layer. The regrowth process was found to decrease the threading dislocation density of the overgrown layer. The quantum well sample grown on patterned template showed significantly higher optical output in photoluminescence measurements compared to the reference sample with identical internal quantum efficiency characteristics. We attribute the increase to enhanced light extraction efficiency caused by strong scattering and redirection of light from the scattering elements.

Carrier lifetimes of the recombination processes in light emitting diodes (LEDs) depend strongly on the operating point. We measure the carrier lifetimes of InGaN/GaN LEDs at low current densities by applying the differential carrier lifetime analysis. In the measurement setup the LED is driven with constant current modulated with small amplitude alternating current (AC) and the carrier lifetime is obtained from the phase difference between input AC and light output. In addition to the optical method, we also extract the lifetimes directly from the capacitance and differential resistance of the LED obtained using an impedance analyzer and compare the results to lifetimes obtained by the optical setup. The carrier lifetimes were found to increase from hundreds of nanoseconds to microseconds when the current density was decreased from tens of A/cm2 to tens of mA/cm2. Both measurement methods resulted in similar lifetimes (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We study theoretically and experimentally transverse and longitudinal electromagnetic waves in n-GaN epitaxial layers. The studies are carried out on the epitaxial layers with various doping levels. Simulation of the reflectivity and absorptivity spectra is performed in a wide frequency range. Additionally, the radiation emission spectra are simulated for various temperatures of the GaN epitaxial layer. It is shown that the resonance peculiarities of the optical spectra are located closely to the frequencies of the coupled plasmon phonon modes. Experimental studies of the reflectivity spectra have been performed in the spectral range 2-20 THz. The experimental spectra are well fitted by the simulated ones and can be used for contactless determination of the electron concentration and mobility in GaN epitaxial layers. The experimental and theoretical results of the present work provide insights for the development of GaN-based devices aimed to absorb/emit terahertz or mid infrared radiation selectively.

We present the results of experimental and theoretical studies of the surface plasmon polariton excitations in heavily doped GaN epitaxial layers. Reflection and emission of radiation in the frequency range of 2-20 THz including the Reststrahlen band were investigated for samples with grating etched on the sample surface, as well as for samples with flat surface. The reflectivity spectrum for p-polarized radiation measured for the sample with the surface-relief grating demonstrates a set of resonances associated with excitations of different surface plasmon polariton modes. Spectral peculiarities due to the diffraction effect have been also revealed. The characteristic features of the reflectivity spectrum, namely, frequencies, amplitudes, and widths of the resonance dips, are well described theoretically by a modified technique of rigorous coupled-wave analysis of Maxwell equations. The emissivity spectra of the samples were measured under epilayer temperature modulation by pulsed electric field. The emissivity spectrum of the sample with surface-relief grating shows emission peaks in the frequency ranges corresponding to the decay of the surface plasmon polariton modes. Theoretical analysis based on the blackbody-like radiation theory well describes the main peculiarities of the observed THz emission.

Owing to its great potential in optoelectronic devices, a lot of effort has recently been put into improving the crystal quality of heteroepitaxial GaN. In this study, GaN layers grown on patterned GaN/sapphire substrates are non-destructively investigated by X-ray diffraction techniques. The effect of the patterning is characterized by determining the strain and lattice tilt and twist in the overgrown GaN layers. High resolution X-ray diffraction (HRXRD) scans and reciprocal space maps (RSMs) were utilized for this purpose. Synchrotron radiation X-ray topography (SR-XRT) was used to examine the microstructure of the samples. The patterning was found to significantly improve the crystal quality of the GaN layers, especially by reducing the amount of edge dislocations. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present the results of GaN re-growth on hexagonally patterned GaN templates. Sapphire was used as the original substrate and the samples were grown by metalorganic vapor phase epitaxy (MOVPE). The re-growth on the patterned templates results in the formation of voids at the GaN/sapphire interface. Our extensive scanning electron microscopy (SEM)-based experimental investigations show that the void shape can be controlled from nearly vertical to fully inclined configurations. It was found that the initial hexagon hole diameter plays a key role in determining the final profile of the void sidewalls. X-ray diffraction analysis of the GaN layers indicates that the layers with inclined sidewall voids have an improved crystalline quality. Knowledge of the void configurations in the GaN layers and a possibility to control their shape can help in enhancing light extraction from the light emitting structures.

The commercial significance of superlattice structures is increasing due to greater demand of optoelectronic devices, such as the light emitting diode (LED). In order to optimize these devices, an accurate and reliable characterization method is needed. This paper describes in detail the characterization of superlattices with X-ray scattering techniques. The thicknesses of the individual layers are determined by X-ray reflectivity (XRR) measurements and the state of strain, the lattice constants and the compositions of ternary compounds by X-ray diffraction (XRD) measurements. The method is non-destructive, and yields unique results, unlike characterizations based on simulation of symmetric XRD scans. These simulations were used for verification of results. The method was tested on InGaN and AlGaN superlattice structures. The measured and simulated parameter values agreed very well. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the electrical characterization of Si-ion implanted AlN layers and the first demonstration of metal-semiconductor field-effect transistors (MESFETs) with an ion-implanted AlN channel. The ion-implanted AlN layers with Si dose of 5 × 1014 cm-2 exhibit n-type characteristics after thermal annealing at 1230°C. The ion-implanted AlN MESFETs provide good drain current saturation and stable pinch-off operation even at 250°C. The off-state breakdown voltage is 2370V for drain-to-gate spacing of 25μm. These results show the great potential of AlN-channel transistors for high-temperature and high-power applications.

GaN films were fabricated by metal organic vapor phase epitaxy (MOVPE) on patterned sapphire substrates (PSSs) with either direct or inverse type patterned structures. Both of these two types of PSSs had their own unique GaN growth process which depart from the standard growth on the planar c-plane. GaN films on PSSs showed decreased threading dislocation (TD) density. However, differences between the crystal quality of the GaN films grown on PSSs were observed. It was also found out with one of the pattern type that the TD density varied laterally and followed the periodicity of the pattern on the sapphire surface.

The effects following ion irradiation of GaN-based devices are still limited. Here we present data on the photoluminescence (PL) emitted from InGaN/GaN multiple quantum well (MQW) structures, which ...

This paper describes a study on the effect of InGaN underneath layer (UL) on InGaN/GaN LEDs operating in the spectral range from 425 to 460 nm. Samples were grown by metalorganic vapor phase epitaxy (MOVPE). The In content of the UL is studied in the range of 0–2.7%. Multiple quantum well (MQW) surface morphology and crystal structure were characterized by atomic force microscopy (AFM), high resolution X-ray diffraction (XRD) and secondary ion mass spectroscopy (SIMS). Electroluminescence (EL) intensity was found to increase almost 50% at 20 mA injection current by introducing an optimized UL under the MQW stack in the LED structure. Also a decrease in the full width of half maximum (FWHM) and the blue-shift of the EL spectrum compared to the reference sample were observed in the all UL consisting samples.

In this work, we have investigated the effect of AlGaN electron blocking layer (EBL) doping level and thickness on the optical properties of near-UV (405 nm) InGaN/GaN multiple quantum well (MQW) light emitting diodes (LEDs) grown by metal organic chemical vapor deposition (MOCVD). Photoluminescence (PL) and electroluminescence (EL) of LEDs with a 50-nm-thick undoped GaN spacer layer between the EBL and the MQW stack were systematically studied. We will present evidence showing that the optimal EBL doping and thickness has an important role in the optimization of near-UV LED output power. By employing the optimized EBL structure we were able to enhance the EL intensity by a factor of five compared to the similar structure without the EBL.

Various techniques for morphological evolution of InGaN/GaN multiple quantum well (MQW) structures grown by metalorganic chemical vapor deposition have been evaluated. Atomic force microscopy, photoluminescence (PL) and X-ray diffraction measurements have been used for characterization. It is shown that inclusions, that are generated into the V-defects in the InGaN quantum wells (QW), can be removed by introducing a small amount of hydrogen during the growth of GaN barriers. This hydrogen treatment results in partial loss of indium from the QWs, but smooth surface morphology of the MQW structure and improved optical quality of InGaN wells are obtained. The density of the V-defects could be reduced by reducing the dislocation density of the underlying GaN buffer.

We report on the effect of GaN buffer threading dislocation (TD) optimization and InGaN/GaN quantum well (QW) hydrogen ðH2Þ treatment on the efficiency of GaN light emitting diodes (LEDs) operating in the spectral range from 400 to 500 nm. A tenfold reduction of the TD density in the GaN buffer increased the efficiency of blue LEDs operating at high current density, while in green LEDs it had very little effect. The reduced TD density also increased the compressive strain in the InGaN QWs, and caused blue shift to the electroluminescence (EL) peak wavelength. The H2 treatment of the QWs increased strain inside the MQW stack. It was possible to apply the H2 treatment only to UV LEDs, as the increased strain in blue and green LEDs caused relaxation of the MQW stack. Although this resulted in smooth surface morphology of the MQW stack, it did not lead to any increase in the efficiency of the UV LEDs. r 2006 Published by Elsevier B.V. PACS: 73.21.Fg; 78.55.Cr; 78.60.Fi; 85.60.Jb

Although suitable for the reduction of the threading dislocation density in GaN layers the widely used two-step MOCVD method does not work as efficiently for AlGaN. This is due to slow surface diffusion of the Al species. In the present paper, the previously reported in situ multistep method for MOCVD growth of high-quality GaN films is adopted for the growth of Al 0.12 Ga 0.88 N films on c-plane sapphire. The developed method for AlGaN growth is virtually GaN free in the sense that no continuous film of GaN is needed near the substrate interface. Crack-free layers of Al 0.12 Ga 0.88 N with a thickness of about 2μm are grown by the method. A sparse distribution of 3D GaN nucleation islands and stimulation of threading dislocation reactions enable a reduction of the threading dislocation density down to 5 x 10 8 cm -2 in the Al 0.12 Ga 0.88 N films. The threading dislocation density is evaluated by etch-pit density measurements. High-resolution X-ray diffraction and transmission electron microscopy are used to study the crystallinity of the Al 0.12 Ga 0.88 N layers. Reflectometry is utilized to analyze film growth in situ. The surface morphology of GaN nucleation layers and Al 0.12 Ga 0.88 N epilayers is characterized by atomic force microscopy.

Charge injection to the prevailing and emerging light-emitting devices is almost exclusively based on the double heterojunction (DHJ) structures that have remained essentially unchanged for decades. In this letter, we report the excitation of a near surface indium gallium nitride (InGaN) quantum well (QW) by bipolar carrier diffusion from a nearby electrically excited pn-homojunction. The demonstrated near surface QW emitter is covered only by a 10nm GaN capping leaving the light-emitting mesa perfectly free of metals, other contact, or current spreading structures. The presented proof-of-principle structure, operating approximately with a quantum efficiency of one fifth of a conventional single QW reference structure, provides conclusive evidence of the feasibility of using diffusion injection to excite near surface light-emitting structures needed, e.g., for developing light emitters or photo-voltaic devices based on nanoplasmonics or free-standing nanowires. In contrast to the existing DHJ solutions or optical pumping, our approach allows exciting nanostructures without the need of forming a DHJ, absorbing layers or even electrical contacts on the device surface.

Due to their potential to improve the performance of light-emitting diodes (LEDs), novel device structures based on nanowires, surface plasmons, and large-area high-power devices have received increasing amount of interest. These structures are almost exclusively based on the double hetero junction (DHJ) structure, that has remained essentially unchanged for decades. In this work we study a III-nitride diffusion injected light-emitting diode (DILED), in which the active region is located outside the pn-junction and the excitation of the active region is based on bipolar diffusion of charge carriers. This unorthodox approach removes the need of placing the active region in the conventional current path and thus enabling carrier injection in device structures, which would be challenging to realize with the conventional DHJ design. The structure studied in this work is has 3 indium gallium nitride / gallium nitride (InGaN/GaN) quantum wells (QWs) under a GaN pn-junction. The QWs are grown at diferent growth temperatures for obtaining distinctive luminescence peaks. This allows to obtain knowledge on the carrier diffusion in the structure. When the device is biased, all QWs emit light indicating a significant diffusion current into the QW stack.

The heteroepitaxial growth of gallium nitride (GaN) on sapphire substrates by metal-organic chemical vapor deposition is most commonly carried out using the two-step growth process. This process involves the deposition of a thin GaN nucleation layer (NL) at a temperature of approximately 450–6001C. The morphology of this low-temperature film after annealing is known to have a crucial effect on the quality of GaN buffer layers. In this paper, we report on efficient control of the GaN NL morphology using a multistep technique. The technique is used to control the size and reduce the density of nucleation islands (NIs) on the NL in order to optimize the surface morphology for a subsequent higher temperature overgrowth step. Together with process parameter optimization a density as low as 1 � 10 7 cm � 2 for the NIs is obtained. The NL morphology is analyzed by atomic force microscopy. The dislocation density of GaN buffer layers grown on multistep NLs is evaluated by etch-pit density measurements and X-ray diffraction is used to support and elaborate the results. The crystalline quality of individual NIs is studied by transmission electron microscopy. Measurements indicate that the multistep technique is successfully used to significantly reduce the threading dislocation density in GaN films. A threading dislocation density of 1.0 � 10 8 cm � 2 is demonstrated with the method.

We present cathodoluminescence (CL) studies on low energy e-beam irradiated (LEEBI) metal-organic vapor phase epitaxy (MOVPE) grown GaN films. High intensity LEEBI has been reported to reduce the band-edge photoluminescence intensity of MOVPE grown GaN films. Here we observe similar reduction of band-edge CL intensity with increasing LEEBI dose. The irradiation damage is found to be concentrated in the LEEBI energy dissipation depth by CL depth profiling. We have previously attributed the LEEBI induced reduction of band-edge intensity to the activation of in-grown Ga-vacancies. Here we observe no increase in the relative intensity of defect related yellow or blue CL emission peaks in the LEEBI treated samples. This indicates that blue or yellow emission in undoped GaN is not related to in-grown Ga-vacancies.

In this study, homoepitaxial GaN grown on ammonothermal GaN substrates is non-destructively investigated by synchrotron radiation X-ray topography (SR-XRT) and X-ray diffraction (XRD). The homoepitaxial GaN layer was found to be of excellent crystal quality with individual dislocations clearly visible in the SR-XRT images. SR-XRT images and XRD rocking curves suggest the dislocations are mainly of mixed type. To the best of our knowledge the measured dislocation density is the lowest reported in a homoepitaxial GaN film (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report a study of the structural and optical properties of near-surface InGaN/GaN single quantum wells, grown by metalorganic chemical vapour deposition, as a function of underneath layer structure and GaN capping thickness. Special attention is paid to characterize properties which are important for non-radiative coupling applications, such as emission intensity at peak wavelength and surface morphology. We observe that utilization of indium containing underneath structures results in high optical quality while increasing surface roughness. Optical performance can be further improved with InGaN/GaN superlattice structures instead of a single InGaN underneath layer. Time-resolved photoluminescence measurements of samples with different GaN capping thicknesses show that room temperature photoluminescence decay time increases with decreasing GaN capping thickness until surface states related non-radiative recombination processes start to play a significant role at very small capping thicknesses (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present our studies on low energy electron beam induced damage to gallium nitride semiconductor material. We have observed a clear reduction in InGaN single quantum well and bulk GaN band-to-band emission intensity induced by low energy electron beam irradiation. The dose and energy of the e-beam were 0 - 500 μC/cm2 and of 5 - 20 keV, respectively. We attributed the damage mechanism to electron beam induced defect electromigration. The electromigration is caused by a localized electric field generated by high local current density. However, further studies are needed to confirm the damage mechanism. These results should be considered in electron beam related fabrication, characterization and usage of GaN-based materials (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Native bulk gallium nitride (GaN) has emerged as an alternative for sapphire and silicon as a substrate material for III-N devices. While quasi-bulk GaN substrates are currently commercially available, single crystal GaN substrates are considered essential for future high performance light emitters and power devices. The ammonothermal method is currently considered one of the most feasible methods to grow large truly bulk crystals of GaN at low cost and high structural quality. High crystalline quality GaN substrates sliced from ammonothermally grown crystals have been demonstrated and utilized in homoepitaxy for III-N devices. However, despite the high crystalline quality the properties of as-grown ammonothermal GaN crystals and substrates are affected by the presence of impurities and other defects that hinder their use for device applications. Here, the main developments of ammonothermal growth of GaN and the effects of impurities, native point defects, and dislocations on the material properties are summarized. Additionally, measurement techniques that enable the evaluation of point defect concentration and low dislocation density distribution over a large area on bulk GaN substrates are reviewed.

We demonstrate that higher crystalline quality, lower strain and improved electrical characteristics can be achieved in gallium nitride (GaN) epitaxy by using a silicon on insulator (SOI) substrate compared to a bulk Si substrate. GaN layers were grown by metal-organic vapor phase epitaxy on 6-inch silicon (Si) and SOI wafers using the standard step graded AlGaN and AlN approach. The GaN layers grown on SOI exhibited lower strain according to X-ray diffraction analysis. Defect selective etching measurements suggested that using a SOI substrate reduces the dislocation density approximately by a factor of two. Furthermore, growth on SOI substrate allows to use a significantly thinner AlGaN buffer compared to bulk Si. Synchrotron radiation X-ray topography analysis confirmed that the stress relief mechanism in GaN on SOI epitaxy is the formation of dislocation network to SOI device Si layer. In addition, the buried oxide layer significantly improves the vertical leakage characteristics as the onset of the breakdown is delayed by approximately 400 V. These results show that the GaN on SOI platform is promising for power electronics applications.

The crystal quality of bulk GaN crystals is continuously improving due to advances in GaN growth techniques. Defect characterization of the GaN substrates by conventional methods is impeded by the very low dislocation density and a large scale defect analysis method is needed. White beam synchrotron radiation x-ray topography (SR-XRT) is a rapid and non-destructive technique for dislocation analysis on a large scale. In this study, the defect structure of an ammonothermal c-plane GaN substrate was recorded using SR-XRT and the image contrast caused by the dislocation induced microstrain was simulated. The simulations and experimental observations agree excellently and the SR-XRT image contrasts of mixed and screw dislocations were determined. Apart from a few exceptions, defect selective etching measurements were shown to correspond one to one with the SR-XRT results.

We report on the growth and characterization of InGaN/ InxAlyGa1–x–yN multiple quantum well (MQW) structures emitting at 380 nm. The samples were grown by metalorganic vapour phase epitaxy (MOVPE) and were analyzed by using high resolution X-ray diffraction (HRXRD), room and low temperature photoluminescence (PL) measurements. The effect of changing the indium precursor flow during the barrier and capping layer growth on the structural and optical quality of multiple quantum wells (MQWs) is explored. The HRXRD results show that the MQWs grown under the set of mentioned conditions are of very good structural quality. The temperature dependent PL results from our samples show that increasing the In content in the barriers enhances to a certain limit the internal quantum efficiency of the MQW structures. We also report that changing the In content in the capping layer effects the peak emission energy and does not have any significant impact on the optical quality of the MQW structures. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A new multistep MOCVD method for growing GaN is used to suppress threading dislocations in GaN epilayers on c -plane sapphire. A nucleation island density of as low as 2.5 × 107 cm–2 is reported. Developed subsequent overgrowth prevents the formation of new islands and stimulates the inclination of threading dislocations inside nucleation islands before their coalescence. GaN epilayers with a threading dislocation density of 5.0 × 107 cm–2 are grown by the method. Nucleation island morphology and threading dislocation density are analyzed by atomic force microscopy. Transmission electron microscopy is used to support the results for the threading dislocation density and to evaluate the epitaxial relationship of the GaN films. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Although conventional III-V compound semiconductors are often considered not to exhibit an efficiency droop, a pronounced low temperature droop was recently measured in AlGaInP/GaAs multi-quantum well structures. In this work, we investigate the efficiency droop in simple optically pumped lattice matched InGaAs/InP single well heterostructures to exclude charge transport related effects from the measurements. The results show that droop is present in this very simplistic setup and, furthermore, starts approximately at the same carrier density as in typical III-N structures. Our results suggest that in its most fundamental form, droop can be explained by Auger-like processes.

High-resolution Raman mapping of the stress distribution in an etched GaN micro-pillar template and a 5 μm thick GaN layer grown on a micro-pillar patterned GaN template is investigated. Raman mapping of the E2 (high) phonon shows differences in stress between the coalescing boundary, the top surface of the pillar region and around the GaN micro-pillar. Increased compressive stress is observed at the coalescing boundary of two adjacent GaN micro-pillars, when compared to the laterally grown GaN regions. The electron channeling contrast image reveals the reduction of threading dislocation density in the GaN layer grown on the micro-pillar patterned GaN template.

International audience; We demonstrate that an extraction enhancement by a factor of 2.8 can be obtained for a GaN quantum well structure using metallic nanostructures, compared to a flat semiconductor. The InGaN/GaN quantum well is inserted into a dielectric waveguide, naturally formed in the structure, and a silver grating is deposited on the surface and covered with a polymer film. The polymer layer greatly improves the extraction compared to a single metallic grating. The comparison of the experiments with simulations gives strong indications on the key role of weakly guided modes in the polymer layer diffracted by the grating.

Variation of absorption of terahertz radiation in lateral electric field was investigated in GaN epitaxial layers. Different behaviour of the absorption modulation in electric field was observed for radiation polarized along electric field and perpendicular to it. Joint analysis of optical and transport measurements let us obtain field dependencies of mobility, electron concentration and absorption cross-section. For terahertz radiation polarized perpendicular to the electric field, results are in accordance with Drude model of free electron absorption. Another polarization demonstrates significant deviation that is yet to be studied more thoroughly.

We present results on optical degradation of gallium nitride based materials under low energy electron beam irradiation (LEEBI). GaN thin film and GaN/InGaN quantum well samples, grown by metal-organic vapor phase epitaxy (MOVPE), were exposed to a tightly focused (ø = 2 nm, J = 0-130 kA/cm2), rapidly scanning electron beam (e-beam) with energy of 5-20 keV and dose of 0-500 μC/cm2. The irradiation severely reduced the band-to-band photoluminescence of the exposed sample areas. Performing positron annihilation spectroscopy measurements on the irradiated films revealed an important increase of Ga-vacancy concentration as a function of the irradiation dose. Based on the measurements we propose that in-grown passive VGa-Hn complexes are present in MOVPE grown GaN (and its alloys), and are activated by LEEBI.

We report on the observation and experimental studies of impurity breakdown and terahertz luminescence in n-GaN epilayers under external electric field. The terahertz electroluminescence is observed in a wide range of doping levels (at noncompensated donor density from 4.5×10[sup 16] to 3.4×10[sup 18] cm[sup −3]). Spectra of terahertz luminescence and photoconductivity are studied by means of Fourier transform spectrometry. Distinctive features of the spectra can be assigned to intracenter electron transitions between excited and ground states of silicon and oxygen donors and to hot electron transitions to the donor states.

GaN layers grown by metal organic vapour phase epitaxy on sapphire were imaged by synchrotron radiation X-ray topography. The threading dislocations could not be resolved in the topographs due to t ...