bluuebunny/crossref_metadata_2025 huggingface dataset:
Polarity control of InN grown by MOVPE on sapphire (0001)
Authors: Wang W. J. , Sugita K. , Nagai Y. , Houchin Y. , Hashimoto A. , Yamamoto A.
Published: June 2007

Polarity control of MOVPE InN on sapphire (0001) was investigated. Two kinds of polarity InN films on sapphire substrate by MOVPE have been prepared by controlling the substrate nitridation process and/or the LT‐GaN buffer annealing process. Surface of the nitrided sapphire is a crucial factor in determining the polarity of InN film. N‐polarity InN film was grown on the 1000 °C‐nitrided substrate without LT‐GaN buffer; while In‐polarity InN film was grown on the 900 °C‐nitrided substrate. In the case of InN growth with a LT‐GaN buffer, buffer annealing is a crucial issue in determining the polarity of InN film. N‐ and In‐polarity InN films were obtained on the long‐time‐annealed buffer and the short‐time‐annealed buffer, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Temperature dependence of the quantum efficiency in green light emitting diode dies
Authors: Li Y. , Zhao W. , Xia Y. , Zhu M. , Senawiratne J. , Detchprohm T. , Schubert E. F. , Wetzel C.
Published: June 2007

The electroluminescence of GaInN/GaN multiple quantum well light‐emitting diode dies is analyzed at variable low temperatures. We compare the external quantum efficiency of three dies of nominally identical structure but strongly different RT performance at 520 nm. For all dies, the external quantum efficiency increases as the temperature is lowered. A maximum is reached for all near 158 K while for lower temperatures as low as 7.7 K, the efficiency continuously drops. The low‐temperature efficiency is the lowest for the highest performing die at RT. The peak energy exhibits a blue shift from RT to 158 K followed by a red shift for lower temperatures. In the same low‐temperature range, a secondary emission peak appears near 390 nm that resembles a donor‐acceptor pair transition. The pronounced efficiency maximum is tentatively assigned to competition of carrier transport properties and the non‐radiative lifetime in the active region. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Platelets and needles: two habits of pressure grown GaN crystals
Authors: Bockowski M. , Grzegory I. , Łucznik B. , Kamler G. , Krukowski S. , Wróblewski M. , Kwiatkowski P. , Jasik K. , Porowski S.
Published: June 2007

The growth of the GaN needles by high pressure solution method is shown in details and compared to the platelets crystallization. The growth mechanism and main factors determining the crystal habit (needle and platelet) in high pressure solution method are presented and discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Inhomogeneity of InGaN quantum wells in GaN‐based blue laser diodes
Authors: Lee Sung‐Nam , Ryu H. Y. , Paek H. S. , Son J. K. , Sakong T. , Jang T. , Sung Y. J. , Kim K. S. , Ha K. H. , Nam O. H. , Park Y.
Published: June 2007

We investigated the inhomogeniety of InGaN quantum wells (QWs) with high In content in the GaN‐based blue laser diodes (LDs). The 2QWs LD structure show the twice higher photoluminescence intensity and the slightly higher electroluminescence intensity than single quantum well (SQW) LD. However, we can obtain the high power blue InGaN SQW LDs with the low threshold current density of 2.19 kA/cm2 and the high sloped efficiency of 0.8 W/A at the cw condition by reducing the number of QWs. From carrier distribution by simulation, blue InGaN 2QWs LDs represented the non‐uniformed hole distribution of n‐side 1st and p‐side 2nd well in InGaN/InGaN 2QWs region due to the low hole concentration and mobility of p‐type nitrides. From time‐resolved photoluminescence (TRPL), we obtained that 2QWs LD shows the some spatial localization states due to the multiple component PL decay process. Therefore, we can guess that the inhomogeniety of InGaN QWs would be quite restricted in deep InGaN QWs of GaN based blue LDs which results in significantly in homogeneities of QWs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Evidence of quantum dot‐like nano‐objects in InGaN quantum wells provided by narrow photoluminescence spectra from localized exciton
Authors: Gotoh H. , Akasaka T. , Tawara T. , Kobayashi Y. , Makimoto T. , Nakano H.
Published: June 2007

We report on narrow photoluminescence (PL) spectra obtained from spatially localized excitons in InGaN quantum wells (QW). These PL lines (less than 1 meV wide) are clearly detected in QWs on several buffer structures and substrates with the micro‐PL technique in low temperature regions. A narrow PL spectrum is one of the characteristics of an exciton confined in a quantum dot (QD). Our results directly confirm that QD‐like nano‐objects exist in InGaN QWs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
AlGaN deep ultraviolet LEDs on bulk AlN substrates
Authors: Ren Zaiyuan , Sun Q. , Kwon S.‐Y. , Han J. , Davitt K. , Song Y. K. , Nurmikko A. V. , Liu W. , Smart J. , Schowalter L.
Published: June 2007

We report the growth of sub‐300 nm ultraviolet light emitting diodes (UV LEDs) on bulk AlN substrates. Heteroepitaxial evolution study through interrupted growth experiments revealed that AlxGa1‐xN (x > 0.5) epilayers can be grown pseudomorphically with well‐defined step‐flow growth mode below a certain critical thickness. The build‐up of compressive strain energy eventually induces a morphological roughening followed by the admission of misfit dislocations. LEDs grown on bulk AlN substrates exhibit noticable improvement over those on sapphire in device impedance, efficiency and thermal characteristics under high‐level injection, pointing to a promising substrate platform for high performance III‐nitride ultraviolet optoelectronics. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Implantation damage recovery and carrier activation studies of Si‐implanted Al0.18Ga0.82N by temperature dependent Hall‐effect measurements
Authors: Ryu Mee‐Yi , Yeo Y. K. , Hengehold Robert L.
Published: June 2007

N‐type activation studies of Si‐implanted Al0.18Ga0.82N have been made as a function of anneal temperature and ion dose to obtain maximum possible activation efficiency. Nearly 100 and 95% electrical activation efficiencies were obtained for Si‐implanted Al0.18Ga0.82N with doses of 5x1014 and 1x1015 cm–2 and annealing at 1250 and 1200 oC for 25 min, respectively. The room temperature sheet resistivity decreases from 528 to 196 Ω/square with increasing anneal temperature from 1150 to 1250 oC for a dose of 5x1014 cm–2. Both sheet carrier concentration (activation efficiency) and mobility increase with anneal temperature, indicating an improved implantation damage recovery with anneal temperature. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A new system for growing thick InN layers by hydride vapor phase epitaxy
Authors: Kikuchi J. , Nishizawa Y. , Murakami H. , Kumagai Y. , Koukitu A.
Published: June 2007

A new system for hydride vapor phase epitaxy (HVPE) of thick InN layers using Cl2 was investigated. Thermodynamic analysis was performed for the reactions at the source and the growth zones. It was found that InCl3 can be generated preferentially at the second source zone by adding Cl2 to InCl from the first source zone. The analysis for the growth zone showed that a significantly large driving force for InN deposition is expected. It was shown that the growth of thick InN layers is possible by employing the new HVPE system. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Monte Carlo study of high‐field electron transport characteristics in AlGaN/GaN heterostructure considering dislocation scattering
Authors: Tomita Yoshihiro , Ikegami Hirofumi , Fujishiro Hiroki I.
Published: June 2007

The effect of the threading dislocation on the high‐field electron transport characteristics in the AlGaN/GaN heterostructure is studied using the Monte Carlo (MC) simulation technique. The three‐dimensional (3D) relaxation time of the dislocation scattering, where the threading dislocation is modeled as a line of charge, is applied to the simulation. Our study shows that the threading dislocation has a small effect on the electron velocity in the AlGaN/GaN heterostructure as compared with that in the bulk n‐GaN, which is owing to the strong screening effect by the highly concentrated carriers in the two‐dimensional electron gas (2DEG). Consequently, the AlGaN/GaN‐HEMT is insensitive to the existence of the threading dislocation piercing through the channel. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Lateral diffusion of photogenerated carriers in InGaN/GaN heterostructures observed by PL measurements
Authors: Vierheilig C. , Braun H. , Schwarz U. T. , Wegscheider W. , Baur E. , Strauß U. , Härle V.
Published: June 2007

In this paper we present a direct observation of the lateral diffusion of photogenerated carriers in InGaN/GaN quantum wells. The spatial distribution of the photoluminescence around a small exciting laser spot is analyzed by scanning in the focal plane of our confocal microscope. The results of a blue and a green emitting sample at varying excitation densities are discussed in the context of the theoretical model of the enhanced lateral diffusion. A lateral electric field which causes an additional radial drift motion was determined by analyzing the PL spectra and by solving a drift–diffusion equation for the lateral motion in the quantum well.
Growth and field emission of GaN nanowires grown by metal organic chemical vapour deposition
Authors: Inoue Y. , Tajima A. , Takeda S. , Ishida A. , Mimura H. , Sakakibara S.
Published: June 2007

Gallium nitride nanowires were grown on Si by Ni‐catalyst‐assisted metalorganic chemical vapor deposition via a vapor‐liquid‐solid growth mechanism. The GaN nanowires have a very high growth rate of >100 μm/h and a very high aspect ratio of 1000; ∼20 nm in diameter and ∼20 μm in length. In the field emission measurement, the GaN nanowires showed a low turn‐on field of 6 V/μm. The good result is attributed to the near‐vertical growth and the very high aspect ratio, as well as a low electron affinity. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Si+ implanted AlGaN/GaN HEMTs with reduced on‐resistance
Authors: Nomoto Kazuki , Mishima Tomoyoshi , Satoh Masataka , Nakamura Tohru
Published: June 2007

Incorporation of ion implantation to AlGaN/GaN high‐electron mobility transistor (HEMT) processing has been demonstrated. The source/drain regions formed using Si ion implantation into undoped AlGaN/GaN on sapphire substrate. Devices with gate length of 1 µm and source/drain distance of 5 µm were fabricated. In comparison with the conventional devices without ion‐implanted source/drain structures, the ion‐implanted devices showed excellent device performance. On‐resistance reduced from 10.2 to 4.0 Ω·mm. Saturation drain current and maximum transconductance increase from 600 to 1100 mA/mm and from 100 to 180 mS/mm, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Growth of thick a‐plane GaN on r‐plane sapphire by direct synthesis method
Authors: Nishino K. , Sakamoto A. , Sakai S.
Published: June 2007

Thick film growth of nonpolar a‐plane GaN was carried out on r‐plane sapphire by direct synthesis method for a substrate of homoepitaxy of a‐plane GaN. A 30 μm thick a‐plane GaN film was obtained at growth temperature at 1050 °C with a low‐temperature buffer layer deposited at 700 °C. The surface was smooth but has some pits. Without a buffer layer, 20 μm thick a‐plane GaN with much fewer pits was obtained. The best FWHM of XRD peak in ω‐mode obtained was less than 700 arcsec. This film can be used as a substrate for a‐plane III‐nitrides growth.
Electroluminescence uniformity in green LEDs and dual color blue/green stacking LEDs
Authors: Onushkin Grigory , Lee Jinhyun , Yang Jung‐Ja , Lee Seong‐Suk , Lee In‐Chul , Kim Min‐Ho , Koike Masayoshi
Published: June 2007

The uniformity of Electroluminescence (EL) of green and blue/green stacking InGaN Light Emitting Diodes (LED) has been studied and analyzed by various techniques. Comparative measurements of Photo‐Luminescence (PL), Cathode‐Luminescence (CL) and EL allow us to conclude that non‐uniform distribution of EL in studied LEDs is caused by non‐uniformity of acceptors distribution over the p‐GaN area. It was found that there are some defective areas in p‐GaN having locally low level of acceptor concentration. These areas correspond to the surface hexagonal shaped pyramids on p‐GaN surface. The locally low doping level changes the field distribution, injection properties and the EL emission properties for these defective points. Optimization of p‐GaN growth conditions will improve the EL uniformity and increase the efficiency of green and blue/green LEDs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Impact of Si+ implantation on reduction of contact resistance in Ti/Al contact to GaN
Authors: Satoh M. , Itoh N. , Nomoto K. , Nakamura T. , Mishima T.
Published: June 2007

For the Ti/Al contact to n‐type GaN, the impact of Si+ implantation on the reduction of contact resistance is investigated. The undoped GaN, grown on the sapphire substrate, is implanted by Si ions with the energy of 150 keV at the dose ranging from 5 x 1013 to 5 x 1014/cm2 and subsequently annealed at 1200 °C for 1 min in N2 gas flow. For the Ti(50 nm)/Al(200 nm) contacts annealed at 550 and 600 °C, as Si ion dose is increased, the specific contact resistance is decreased with the Schottky barrier height of 0.27 eV. The Ti/Al contact annealed at 600 °C reveals the exteremly low contact resistance as low as 1.2 x 10–7 Ωcm2 for the Si ion dose of 5 x 1014/cm2. Ti/Al contact annealed above 700 °C shows Rc in the order of 10–6 Ωcm2 even though Si ion dose increases. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Photo‐enhanced reactivation of Si donors deactivated by plasma‐induced defects in n‐type GaN
Authors: Nakamura Seiji , Ikadai Yuki , Suda Masayuki , Suhara Michihiko , Okumura Tsugunori
Published: June 2007

The behavior of plasma‐induced defects deactivating Si donors in GaN has been studied by using Schottky diodes with the UV illumination. It is interesting that the UV irradiating for the deactivated GaN leads to an enhancement of reactivation of the donors even at room temperature. It is found that the donor‐deactivated region in n‐GaN shifts toward the bulk region with increasing the illumination time. The effect of the UV illumination is thought to be the direct photodissociation of the donor‐defect complexes or the injection of holes as a minority carrier. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Boron addition effects on aluminum nitride fabricated by radio‐frequency plasma‐assisted molecular beam epitaxy
Authors: Yamashita Masashi , Yoshiya Masato , Ishikawa Yukari , Ohsato Hitoshi , Shibata Noriyoshi
Published: June 2007

Boron (B) added aluminum nitride (AlN) thin films have been fabricated by radio‐frequency plasma‐assisted molecular beam epitaxy and analyzed the effects of B addition to the AlN as a wide‐gap material. From first principles pseudo‐potential calculations, energy gap (Eg) of Al1‐xBxN at x = 0.25 obtained by a GGA exchange‐correlation term was estimated as 3.85 eV which is smaller than that of pure AlN. Refractive indices of AlBN (x = 0.29) measured by ellipsometry were smaller than those of pure AlN. Extinction coefficient of the Al1‐xBxN (x = 0.29) at 200 ‐ 500 nm increased by the B addition. The variation in the extinction coefficient can be explained by the decrease in calculated Eg. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Alloy composition fluctuation and band edge energy structure of In‐rich InxGa1–xN layers investigated by systematic spectroscopy
Authors: Ishitani Yoshihiro , Fujiwara Masayuki , Shinada Takuro , Wang Xinqiang , Che Son‐Bek , Yoshikawa Akihiko
Published: June 2007

After the finding of the bandgap energy of InN as less than 1 eV, the re‐examination of the bandgap of InGaN alloys and its bowing parameter is an issue of interest. We study the absorption spectra of In‐rich InGaN layers by taking account of the Burstein‐Moss effect and bandgap renormalization with the analysis of photoluminescence and infrared reflectance spectra. We find the far larger broadening of optical transition and lattice vibration energies than those expected from the alloy composition fluctuation based on the x‐ray diffraction patterns. We estimate that local potential fluctuation by the alloy composition fluctuation give rise to the strong localization of carriers and relaxation of the momentum conservation rule in optical transition. The average bandgap energies of the films show the bowing parameter less than 1.4 eV; the value down to 0.7 eV is possible. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Characterization of AlGaN/InGaN/AlGaN heterostructure with selective area growth of Te‐doped AlGaN cladding layer grown by mixed‐source HVPE
Authors: Jang K. S. , Kim K. H. , Hwang S. L. , Jeon H. S. , Yang M. , Ahn H. S. , Kim S. W. , Honda Y. , Yamaguchi M. , Sawaki N. , Jang S. H. , Lee S. M. , Koike M.
Published: June 2007

The selective area growth (SAG) of AlGaN/InGaN/AlGaN light‐emitting diodes (LEDs) is performed by mixed‐source hydride vapor phase epitaxy (HVPE). The structure is grown on a n‐GaN templated (0001) sapphire substrate. The SAG‐double heterostructure (DH) is consisted of a Te‐doped AlGaN cladding layer, an InGaN active layer, a Mg‐doped AlGaN cladding layer, and a Mg‐doped GaN capping layer. All of the epitaxial layers of LED structure are grown consecutively with a multi‐sliding boat system. Room‐temperature electroluminescence (EL) characteristics show an emission peak wavelength of 400 nm with a full width at half‐maximum (FWHM) of approximately 0.38 eV (at 20 mA). We find that the mixed‐source HVPE method with multi‐sliding boat system is possible to be one of the growth methods of III‐nitride LEDs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Effects of proton irradiation on electrical and optical properties of n‐InN
Authors: Emtsev V. V. , Davydov V. Yu. , Klochikhin A. A. , Sakharov A. V. , Smirnov A. N. , Kozlovskii V. V. , Wu C.‐L. , Shen C.‐H. , Gwo S.
Published: June 2007

Effects of proton irradiation on the optical and electrical properties of n‐InN with charge carrier concentrations of 2 – 5 × 1018 cm–3 have been investigated. Strong changes in photoluminescence spectra and electrical parameters of irradiated n‐InN are discussed. Proton irradiation of n‐InN results in the appearance of shallow donors in large concentrations. Comparison with the effects observed on heavily degenerate n‐InN after proton irradiation leads to the conclusion that these irradiation‐produced donors are native defects, most likely vacancies on the nitrogen sublattice.
Dependence of In mole fraction in InGaN on GaN facets
Authors: Nakao K. , Li D. B. , Liu Y. H. , Miyake H. , Hiramatsu K.
Published: June 2007

InGaN/GaN stripe structures and hexagonal pyramid structures were grown on sapphire (0001) by low‐pressure metalorganic vapor phase epitaxy (LP‐MOVPE). It was observed that the indium composition at (0001) was greater than that at {11$ \bar 2 $2}. For InGaN/GaN stripe and pyramid structures with {10$ \bar 1 $1}, the thickness, CL peak wavelength and CL intensity gradually increased from the bottom to the top of the facet. It is considered that this behavior on the facet is due to the indium diffusion mechanism or indium composition pulling effect. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Sensing dynamics and mechanism of a Pd/AlGaN/GaN Schottky diode type hydrogen sensor
Authors: Akazawa Masamichi , Hasegawa Hideki
Published: June 2007

Hydrogen sensing characteristics of surface‐controlled Pd/AlGaN/GaN Schottky diodes were investigated in air. Sensors showed high detection sensitivities and fast turn‐on and turn‐off characteristics. Sensing dynamics and mechanism were explained in terms of Schottky barrier height reduction due to hydrogen‐induced interface dipole. Here, the dipole formation is controlled by the Langmuir isotherm including an oxygen reaction, and current transport is due to thermionic field emission. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Growth of high‐quality 1‐inch diameter AlN single crystal by sublimation method
Authors: Mizuhara N. , Miyanaga M. , Fujiwara S. , Nakahata H. , Kawase T.
Published: June 2007

1‐inch diameter AlN single crystals with the thickness of 0.025–2 mm were grown on SiC substrates by the sublimation method. Crystalline qualities were evaluated by X‐ray diffraction and EPD measurements. The FWHM of X‐ray rocking curve for (0002) reflection was 281 arcsec with 1.2‐mm thick crystal and dislocation density measured by EPD was 1.6×106/cm2 with 0.8‐mm thick crystal. These results were well consistent with the dependence of those properties on the thickness of the crystal, which was found in our previous work on 10‐mm diameter crystals. According to this dependence, growing the thick AlN crystal on SiC substrates by the sublimation method is expected to lead to high‐quality free‐standing 1‐inch AlN substrates. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Bright electron emission from Si‐doped AlN thin films
Authors: Kishimoto A. , Inou Y. , Kita T. , Wada O.
Published: June 2007

We report the emission properties of the low temperature grown Si‐doped AlN thin films by reactive radio‐frequency magnetron sputtering technique. With increasing Si‐doping concentration, the emission characteristics have been improved. The low threshold electric field of 4.4 V/μm, and the saturation current density of 0.74 mA/cm2 has been obtained. According to analysis of the I‐V characteristics, it is found that the low effective work function causes the high emission performance. The luminance by using blue phosphor exceeds 700 cd/m2. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Nonpolar a‐ and m‐plane bulk GaN sliced from boules: structural and optical characteristics
Authors: Paskova T. , Kroeger R. , Hommel D. , Paskov P. P. , Monemar B. , Preble E. , Hanser A. , Williams N. M. , Tutor M.
Published: June 2007

Bulk GaN substrates grown by HVPE in the [0001] direction and sliced along the (11‐20) a‐plane and the (1‐100) m‐plane have been studied by SEM, TEM, HRXRD, CL and PL. Smooth morphology, low defect density, and narrow linewidths of the main XRD reflections and PL peaks are revealed and thus all the results demonstrate superior structural and optical quality of both a‐ and m‐plane bulk GaN material achieved by HVPE growth of boules in the conventional [0001] direction and subsequently sliced along nonpolar planes, implying the approach as the most promising to produce nonpolar GaN substrates. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Bending in HVPE grown GaN films: origin and reduction possibilities
Authors: Paskova T. , Becker L. , Böttcher T. , Hommel D. , Paskov P. P. , Monemar B.
Published: June 2007

We have studied the effects of film and substrate thicknesses on the bending and strain of thick GaN films grown by HVPE. Both experimental and simulation data, being in a very good agreement, show the highest bending at a critical film thickness slightly smaller than the substrate thickness, while the strain remains decreasing with only a small increase at much higher film thicknesses. Temperature dependent measurements allows a separation of the thermally induced component and an extrapolation of the bending to the growth temperature. The latter was found to be dependent on both the film and substrate thickness and to have a strong effect on the remaining bending of freestanding films. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Carrier transport studies of dichromatic InGaN‐based LEDs with spacer bandgap dependence
Authors: Feng Shih‐Wei , Pan C. C. , Chyi Jen‐Inn , Kuo Chien‐Nan , Chen Kuei‐Hsien
Published: June 2007

Carrier transport of dichromatic InGaN‐based LEDs with AlGaN spacer bandgap dependence has been studied. TREL measurements show that carrier dynamics could be well explained by the combined effects of carrier effective mass, carrier mobility, quantum confinement, and device structures. The experimental results provide important information for device designs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Characterization of AlGaN/GaN MOSHFETs with Al2O3 as gate oxide
Authors: Gregušová D. , Stoklas R. , Čičo K. , Heidelberger G. , Marso M. , Novák J. , Kordoš P.
Published: June 2007

We report on AlGaN/GaN metal‐oxide‐semiconductor heterostructure field‐effect transistors (MOSHFETs) with MOCVD deposited Al2O3 as a gate oxide. Properties of MOSHFETs with 9 nm and 14 nm thick Al2O3 are compared with conventional HFETs prepared simultaneously on the same layer structure. Lower gate leakage current (∼10–5 A/mm at –10 V) and higher saturated drain current (up to 40%) are obtained for MOSHFETs than those for HFETs. In contradiction to previously reported data, the extrinsic transconductance for MOSHFETs is also higher (up to 37% of peak values) than that for HFET. This indicates on semi‐conductive rather than insulating properties of Al2O3 gate oxide. Pulsed I–V measurements (pulse width 1 µs) yielded lower but still measurable current collapse in MOSHFETs compared to HFETs. Nevertheless, obtained results show that Al2O3 gate oxide, after optimising its microstructure and thickness, can be preferable for the preparation of AlGaN/GaN MOSHFETs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Intersubband transitions in InGaAsN/AlGaAs quantum wells with a high confinement energy
Authors: Duboz Jean‐Yves
Published: June 2007

When N is introduced in bulk InGaAs, the N level pushes the conduction band towards lower energies. In InGaAsN quantum wells however this repulsion leads to more complicated situations, in particular when confined levels are close to the N level. We calculate the energy of quantified levels in InGaAsN/AlGaAs quantum wells by using an analytical model based on the band anticrossing model and the repulsion by the nitrogen level. When the confinement becomes large enough to support subbands above the localized nitrogen level, subbands are split and have a dispersion strongly affected by the repulsion by the N level. In this case, intersubband transitions are deeply modified, with a splitting of the transitions and important spectral shifts of the lines, allowing to reach the 3‐4 µm spectral range. Due to the admixing of nitrogen wavefunction in the electron wavefunction, the amplitude of the intersubband transitions decreases. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Growth of highly resistive Ga‐polar GaN by LP‐MOVPE
Authors: Mita Seiji , Collazo Ramón , Dalmau Rafael , Sitar Zlatko
Published: June 2007

Electrical characteristics of 1.3 µm thick GaN films were investigated with respect to the polar type. Unintentionally doped N‐polar GaN grown on nitrided sapphire substrates had a hexagonal facetted morphology and consistently exhibited a high n‐type conductivity with a sheet resistance of ∼10 Ω/sq, corresponding to a carrier concentration on the order of 1019 cm–3. In contrast, Ga‐polar GaN films grown on low temperature AlN buffer layers showed a high sheet resistance of ∼109 Ω/sq. GaN films of mixed polarity exhibited sheet resistance in‐between the two extreme values obtained for polarity pure films. The results of this work show that the existence of N‐polar domains (even on the nano‐scale) in an otherwise Ga‐polar matrix significantly decreases the insulating properties of unintentionally doped GaN films. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Origin and elimination method of parasitic gate leakage current for AlGaN/GaN heterostructure field effect transistor
Authors: Baksht Tamara , Knafo Yaron , Kaplun Joseph , Bunin Gregory
Published: June 2007

The Schottky gate leakage current of AlGaN/GaN HFET was systematically investigated. Experimental results were acquired from gates of HFET and from round diodes on the same wafer. The performed analysis clearly shows that HFET gate current behaves according to additional physical mechanism. Hypothesis, that the source of parasitic leakage is the contact point between gate metal and two dimensional electron gas on the mesa edges, was theoretically and experimentally proved. Cost effective technological solution with SiO2 “patch” layer is presented. Transistors, manufactured with “patch” layer, have higher breakdown, two orders of magnitude lower gate leakage current, excellent performance and should be more reliable. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Electrical and optical characterization of M‐plane GaN films grown on LiAlO2 substrates
Authors: Rivera C. , Misra P. , Pau J. L. , Muñoz E. , Brandt O. , Grahn H. T. , Ploog K. H.
Published: June 2007

We report on a study of the electrical and optical properties of strained M‐plane GaN films grown on LiAlO2 substrates. We have electrically characterized Schottky barrier devices fabricated on strained M‐plane GaN films. The current leakage found in these devices is probably related to a surface effect. The influence of the current direction for in‐plane transport in M‐plane GaN was also studied. Polarized photoluminescence spectroscopy shows the effect of residual biaxial strain on the optical selection rules, allowing to identify intrinsic transitions involving holes from the two uppermost valence bands. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Inductively coupled plasma etching of GaN using SiCl4/Cl2/Ar for submicron‐sized features fabrication
Authors: Dylewicz R. , Hogg R. A. , Fry P. W. , Parbrook P. J. , Airey R. , Tahraoui A. , Patela S.
Published: June 2007

In this paper we report the optimization of the fabrication process of a grating coupler, which is fully integrated with a GaN planar waveguide. To our knowledge, this is the first report of a grating‐assisted optical coupler in gallium nitride. ICP dry etching of n‐doped GaN layers was investigated, where SiCl4/Cl2/Ar gas mixture was used under different etching conditions. We report n‐GaN ICP etching ratio of 520‐2680 Å min–1 as well as etching selectivity of GaN over SiO2 from 3 to 8, in the most cases. Grating ridge and grating groove width as well as the sidewalls slope were evaluated by SEM microscope. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Structural and optical properties of Si‐doped AlGaN/AlN multiple quantum wells grown by MOVPE
Authors: Li Da‐Bing , Katsuno Takuya , Aoki Masakazu , Miyake Hideto , Hiramatsu Kazumasa , Shibata Tomohiko
Published: June 2007

Undoped and Si‐doped AlxGa1–xN/AlN (x > 0.5) multiple quantum wells (MQWs) were grown on AlN/sapphire templates by metalorganic phase vapor epitaxy. The influence of Si doping on the structural and optical properties was investigated by means of high resolution X‐ray diffraction (HRXRD) and cathodoluminescence (CL) measurement. Comparing with unintentionally doped MQWs, the interface quality of the MQWs wasn't reduced by Si doping, even a little improved. CL measurement demonstrated that the MQWs behaved a deep ultraviolet emission. For MQWs with Si doped barrier layer, the deep UV emission peak characterizes blueshift with increasing the Si dopant concentration and the Si doping has no obvious effect on the CL intensity in the case of the dopant content lower than 1×1019 cm–3, however, with increasing the dopant content to 1020 cm–3, the efficiency of deep UV emission was drastically enhanced. The mechanism of enhancement was discussed in detail. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Free standing GaN layers with GaN nanorod buffer layer
Authors: Lee H. J. , Lee S. W. , Goto H. , Lee Hyo‐Jong , Ha J. S. , Cho M. W. , Yao T.
Published: June 2007

Thick GaN films were grown by HVPE (Hydride Vapor Phase Epitaxy) on c‐sapphire substrates with GaN nano‐rod buffer layers. Lateral epitaxial growth mode was adapted to grow GaN thick films on nano‐structure buffer. Thick GaN films were self‐separated during cooling down by thermal stress caused by the difference of thermal expansion coefficient (TEC) between GaN and sapphire. Since the nano‐rod buffer consists of nano‐rods and voids, it is mechanically weaker than planar GaN layers and contributes to the self‐separation of GaN thick films. 200 μm‐thick free‐standing GaN substrates show smooth surface morphology without any microcracks. The full width at half maximum (FWHM) of (0002) X‐ray rocking curve is 619 [arcsec]. Donor‐bound exciton emission is observed at 3.4718 eV in low‐temperature photoluminescence spectra, which is nearly the same peak position as bulk GaN. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Near‐UV to violet LEDs – Wavelength dependence of efficiency limiting processes
Authors: Kunzer M. , Kaufmann U. , Köhler K. , Leancu C. C. , Liu S. , Wagner J.
Published: June 2007

Two wavelength series (373‐435 nm) of GaInN/GaN triple quantum well LED‐structures, one on sapphire and one on GaN‐templates, as well as a quantum well thickness series were grown by MOVPE. In order to study luminescence efficiency limiting effects the LED‐structures were characterized by electroluminescence‐ and by temperature and excitation power dependent photoluminescence spectroscopy. The electroluminescence output and the internal quantum efficiency are peaked near 400 nm in both wavelength series, but output and efficiency are significantly enhanced for growth on templates. The photoluminescence data indicate that below 400 nm the efficiency, apart from insufficient vertical carrier confinement, is limited by poor localization within the quantum film plane while above 400 nm piezo‐electric fields appear to be an important efficiency limiting effect. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Nonlinear piezoelectric properties of GaN quantum dots nucleated at the edge of threading dislocations
Authors: Dłużewski Paweł , Young Toby D. , Jurczak Gregorz , Majewski Jacek A.
Published: June 2007

It was observed experimentally by Rouviere et al. that GaN/AlN Quantum Dots (QDs) nucleate at the edge of threading dislocations (Appl. Phys. Lett. 75, 2632 (1999) [1]). The preferred nucleation of QDs in this way is generally assumed to be due to the influence of the stress/strain field around the dislocation core, which in turn, gives the chemical and geometric conditions for nucleation of the QDs. We solve the finite element problem for QDs situated at the edge of threading dislocations where different lattice parameters, piezoelectric and spontaneous polarisation coefficients are assumed for the QD and its matrix. By solving the elastic and electric equilibrium problems we obtain both the residual stress and electric fields. The computational scheme employed here was obtained by linking two previous finite element algorithms described inreferences (P. Dłużewski et al., Comput. Mater. Sci. 29, 379 (2004) [2]) and (G. Jurczak et al., phys. stat. sol. (c) 2, 972 (2005) and S.P. Łepkowski et al., Phys. Rev. B 73, 245201 (2005) [3, 4], respectively). This approach allows us to get a deeper physical insight into the mechanics and electrical properties of QDs and ultimately determine the efficiency of light emission from these objects.
Continuous order‐disorder phase transition (2×2)→(1×1) on the (0001)AlN surface
Authors: Mansurov V. G. , Galitsyn Yu. G. , Nikitin A. Yu. , Kolosovsky E. A. , Zhuravlev K. S. , Osvath Z. , Dobos L. , Horvath Z. E. , Pecz B.
Published: June 2007

Smooth (0001)AlN surface on a (0001) sapphire substrate was prepared by ammonia Molecular Beam Epitaxy. A continuous (2×2)→(1×1) reconstruction phase transition on the (0001)AlN surface under ammonia flux was experimentally investigated by RHEED. The intensity of a fractional (0 1/2) streak was studied as a function of the sample temperature under different ammonia fluxes. The temperature of the transition increases with increasing of the ammonia flux (an effective activation energy is about 1.3 eV). The phase transition is described in the frame of the Bethe‐Peierls approximation. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Localized plasmons at pores and clusters within inhomogeneous indium nitride films
Authors: Shubina T. V. , Vasson A. , Leymarie J. , Gippius N. A. , Jmerik V. N. , Monemar B. , Ivanov S. V.
Published: June 2007

We report on reproducibly observed enhancement of luminescence within inhomogeneous InN films near both In clusters and pores. Low‐temperature absorption in these films deviates from that expected for a conventional semiconductor. The enhancement of emission is discussed in terms of coupling of excited radiative states with localized particle plasmons. It implies concentration of an electric field near the pores and clusters, which are considered, respectively, as metallic and dielectric ellipsoids within a semiconductor matrix. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
FE simulation of InGaN QD formation at the edge of threading dislocation in GaN
Authors: Dłużewski Paweł , Belkadi Amina , Chen Jun , Ruterana Pierre , Nouet Gerard
Published: June 2007

The stress induced diffusion process of In‐Ga segregation in InxGa1–xN layer deposited on GaN is simulated step by step by using a 3D nonlinear FE method. From the thermodynamical point of view this process is governed by the driving force induced by the gradient of residual stresses operating in an anisotropic nonlinear elastic structure. The source of stresses we consider is the set of threading dislocations examined in the plane view HRTEM investigation of GaN layer deposited on sapphire.
Impact of polarity control and related defects on the electrical properties of GaN grown by MOVPE
Authors: Collazo R. , Mita S. , Dalmau R. , Sitar Z.
Published: June 2007

By controlling the LT‐AlN nucleation layer, we have demonstrated growth of Ga‐ and N‐polar GaN thin films with identical growth rates on c‐plane sapphire via MOVPE. The Ga‐polar films were highly resistive, while N‐polar films exhibited n‐type conductivity. Under certain nucleation layer processing conditions, the GaN films were mixed‐polar. Using specular X‐ray reflectivity to calculate the density depth profile of the nucleation layers, we identified one mechanism during the processing evolution of the nucleation layer that could give rise to inversion domains within the film, giving raise to mixed‐polar GaN films. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Mechanisms of metalorganic vapor phase epitaxy of InGaN quantum wells on GaN microfacet structures
Authors: Ueda M. , Hayashi K. , Kondou T. , Funato M. , Kawakami Y. , Narukawa Y. , Mukai T.
Published: June 2007

The growth mechanisms during metalorganic vapor phase epitaxy of InGaN quantum wells (QWs) on three‐dimensional GaN microfacet structures composed of (0001) and {11‐22} planes were investigated. The cross section of the microfacet structures formed by regrowth was changed from triangle to trapezoid, depending on the mask geometries for the regrowth. The emission peak wavelength of the (0001) microfacet QWs was always longer than that of the {11‐22} microfacet QWs, and the difference is smaller than in planar (0001) and {11‐22} QWs. To clarify this observation, the local thicknesses of constituent InN and GaN on the GaN microfacets were estimated. Using a formula derived from the diffusion equation, the distribution of the local thickness is fitted to estimate the apparent diffusion lengths, λ. For Ga, λ was fitted to be 5.2 μm on (0001) and 3.1 μm on {11‐22}, while for In, λ was 2.7 μm on (0001) and 1.6 μm on {11‐22}. This suggests that the degree of migration of adatoms depends on both facets and atoms, which may cause the facet‐dependent In compositions and lead to multiwavelength emission from microfacet InGaN QWs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Mg‐doped high‐quality AlxGa1–xN (x=0‐1) grown by high‐temperature metal‐organic vapor phase epitaxy
Authors: Imura M. , Kato N. , Okada N. , Balakrishnan K. , Iwaya M. , Kamiyama S. , Amano H. , Akasaki I. , Noro T. , Takagi T. , Bandoh A.
Published: June 2007

We report on the growth of Mg‐doped AlxGa1–xN alloys and characterization of their optical properties. Under the low Mg concentration in the range 2×1017 to 1×1018 cm–3 of AlxGa1–xN, the activation energy of Mg acceptor became higher and higher with increase in the Al content of AlxGa1–xN. The experimental results for activation energies of Mg acceptor fitted quite well with the hydrogen atom like model. By the Haynes' rule, relationship between bound‐excition energy and activation energy of Mg acceptor was investigated, which coincided with our experimental results. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Effects of junction temperature and its variations on the performance of GaN‐based high power flip‐chip LEDs
Authors: Lin L. , Chen Z. Z. , Pan H. P. , Qi S. L. , Liu P. , Qin Z. X. , Yu T. J. , Zhang B. , Tong Y. Z. , Zhang G. Y.
Published: June 2007

In this paper, high power flip‐chip GaN‐based LEDs were fabricated, and then the effects of junction temperature and its variations were investigated. It was found that the optical output decreases seriously with rising temperature. The variations of junction temperature can be achieved to 9 °C at 350 mA in most regions of flip‐chip LED except for some hot regions around some electrodes. The combination of screening of quantum confined Stark effect (QCSE) and thermal effect surprisingly makes the peak wavelength constant from 300 mA to 800 mA in electroluminescence spectra. The full‐width at half‐maximum (FWHM) of emission peak was narrowed by thermal effect in constant current (CC) injection. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Crystallographic deterioration of MOVPE InN during the growth
Authors: Sugita K. , Nagai Y. , Houchin Y. , Hashimoto A. , Yamamoto A.
Published: June 2007

This paper reports the crystallographic degradation of MOVPE InN during the growth. Using FWHMs of X‐ray rocking curve, tilt ((0002)) and twist ((10‐10)) angle distributions are evaluated and effects of the major growth parameters, such as growth temperature, growth time and with/without GaN buffer in the degradation, are revealed. With increasing either thickness of grown InN or growth temperature up to 600 °C, the tilt angle distribution is markedly increased, indicating the crystallographic degradation of grown films. The use of a GaN buffer reduces such degradation. Since the twist angle distribution is scarcely changed by such growth parameters, the destruction of InN crystals during growth and annealing is concluded to be anisotropic. The trends of the crystallographic degradation revealed here are in good agreement with those for the electrical and optical degradation previously reported. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Metal catalyst enhanced growth of high quality and density GaN dots on Si(111) by implant source growth
Authors: Buckmaster Ryan , Goto Takenari , Hanada Takashi , Fujii Katsushi , Kato Takashi , Yao Takafumi
Published: June 2007

Using ion implantated material as a growth source, implant source growth, is an alternative growth method where ion implanted atoms diffuse to the surface and can be used to form nanostructures. By introducing a small amount of a steel based contaminate we were able to grow high quality and high density GaN nanodots on Si(111) which showed strong excitonic UV luminescence. Furthermore by varying the implanted Ga dose and annealing conditions, the size of the nanodots can be controlled. This method for producing GaN nanostructures may be useful for optical or electronic device applications. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Time‐resolved photoluminescence spectroscopy in a GaN/AlGaN SQW structure grown on a (111) Si substrate
Authors: Kim Eunhee , Narita Tetsuo , Honda Yoshio , Yamaguchi Masahito , Sawaki Nobuhiko
Published: June 2007

Time‐resolved photoluminescence (PL) spectroscopy was performed at 77 K for the near band edge emission band in a GaN/AlGaN single quantum well structure grown on a (111) Si substrate. The near band edge emission band was decomposed into several peaks characteristic to structural defects/dislocations in the sample. By using PL intensity correlation method, the decay time constant of the peak intensity was investigated. The radiative recombination time for the neutral‐donor‐bound exciton (DoX) peak was on the order of 800 ps, while the time constants for the defect/dislocation related peaks depended on the wavelength or the associated energy level ranging between 550 ps and 1,000 ps. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Unusual photoluminescence properties of vertically aligned InN nanorods grown by plasma‐assisted molecular‐beam epitaxy
Authors: Shen C.‐H. , Chen H.‐Y. , Lin H.‐W. , Wu C.‐Y. , Gwo S. , Klochikhin A. A. , Davydov V. Yu.
Published: June 2007

We report the unusual photoluminescence (PL) properties of vertically aligned InN nanorod arrays grown on Si(111) with a Si3N4 buffer layer. The optimum growth conditions of InN nanorods are obtained by controlling the III/V ratio and the growth temperature. Structural characterization by X‐ray diffraction and scanning electron microscopy indicates that individual nanorods are wurtzite InN single crystals with the growth direction along the c‐axis. Near‐infrared PL from InN nanorods is clearly observed at room temperature. However, in comparison to the PL from InN epitaxial films, the PL from InN nanorods is significantly lower in efficiency and exhibit anomalous temperature dependence. We propose that these unusual PL properties are results of considerable structural disorder (especially for the low‐temperature grown InN nanorods) and strong surface electron accumulation effect. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Two‐electron transition spectroscopy of shallow donors in bulk GaN
Authors: Paskov P. P. , Monemar B. , Toropov A. , Bergman J. P. , Usui A.
Published: June 2007

We report a detailed study of the recombination processes of the excitons bound to O and Si shallow donors in bulk GaN. Photoluminescence (PL) and time‐resolved PL measurement at different polarization and temperatures were carried out on a high quality 1‐mm thick sample grown by Hydride Vapor Phase Epitaxy. Together with the principal donor‐bound exciton (DBE) emission lines a large number of two‐electron transitions (TETs) related to the ground and excited states of both DBE were observed allowing an accurate determination of the donor binding energies. Polarization properties and optical selection rules as well as the recombination dynamics of different PL lines were also studied and discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Series resistance in a GaN/AlGaN/n‐Si structure grown by MOVPE
Authors: Honda Yoshio , Kato Satoshi , Yamaguchi Masahito , Sawaki Nobuhiko
Published: June 2007

GaN film was grown on a (111)Si substrate using AlGaN intermediate layer by selective metalorganic vapor phase epitaxy. The series resistance of the GaN/AlGaN/Si diode was evaluated as a function of the thickness and the alloy composition of the AlGaN intermediate layer. The resistance was decreased by reducing the thickness of the intermediate layer but showed a minimum value. The minimum resistance was evaluated as a function of the alloy composition. Assuming an exponential increase of the minimum resistance as a function of the conduction band offset in the AlGaN/Si interface, the resistance for a GaN/Si hetero interface was estimated to be 0.0018 Ω cm2. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Microstructure of A‐plane InN grown on R‐plane sapphire by ECR‐MBE
Authors: Watanabe S. , Kumagai Y. , Tsuyuguchi A. , Na H. , Naoi H. , Araki T. , Nanishi Y.
Published: June 2007

We used electron cyclotron resonance plasma‐excited molecular beam epitaxy (ECR‐MBE) to grow A‐plane (11$ \bar 2 $0) InN on nitridated R‐plane (1$ \bar 1 $02) sapphire and then measured the structural properties using transmission electron microscopy (TEM). We determined the epitaxial relationship between A‐plane InN and R‐plane sapphire to be (11$ \bar 2 $0)InN // (1$ \bar 1 $02)sapphire and [1$ \bar 1 $00]InN // [11$ \bar 2 $0]sapphire. Moreover, the results indicated that the nitridation of the sapphire produced a (001) cubic AlN layer, and this layer caused the subsequent InN to have its a‐axis normal to the interface. Also, by using two diffraction vector orientations in the TEM measurement, we found that dislocations with a screw component had a density of about 5 × 1010 cm–2, which is about ten times higher than that with an edge component. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)