bluuebunny/crossref_metadata_2025 huggingface dataset:
In‐situ and real‐time monitoring of MOCVD growth of III‐nitrides by simultaneous multi‐wavelength‐ellipsometry and X‐ray‐diffraction
Authors: Simbrunner C. , Schmidegg K. , Bonanni A. , Kharchenko A. , Bethke J. , Woitok J. , Lischka K. , Sitter H.
Published: May 2006

Metal Organic Chemical Vapor Deposition (MOCVD) is nowadays the most frequently used industrial method for growing III‐nitrides. The possible spectrum of in‐situ diagnostic tools is quite narrow because the MOCVD growth process excludes all techniques based on ultra high vacuum conditions. Therefore, only optical methods like spectroscopic ellipsometry and X‐ray diffraction give the possibility to observe and analyse the growing surface in‐situ. We attached simultaneously a multi wavelength spectroscopic ellipsometer and a X‐ray diffraction system to our reactor and could determine parameters like growth rate and layer composition from both techniques. On the one hand the optical measurement gives surface sensitive information about the sample. On the other hand the measurement of the diffraction peak of hexagonal GaN and related alloys during growth gives additional information about the entire structure and the crystal quality. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Large signal analysis of AlGaN/GaN‐HEMT amplifier by coupled physical device‐circuit simulation
Authors: Fujishiro H. I. , Narita S. , Tomita Y.
Published: May 2006

The large signal time domain simulation of the 40 GHz one‐stage amplifier with the AlGaN/GaN‐HEMT is performed by means of the coupled physical device‐circuit simulation, in which the Monte Carlo (MC) device simulator is incorporated into the embedding circuit as a realistic physics‐based device model. The forward and the backward travelling waves are extracted from the current and the voltage signal waveforms at the input and the output terminals, and then the power performances of the amplifier are evaluated using them. The amplifier exhibits the linear power gain (G a) of 12.9 dB and the saturation output power (P o,sat) of 25.7 dBm at 40 GHz. The influence of the impact ionization on the power performances is also investigated. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
RF and DC characteristics in Al2O3/Si3N4 insulated‐gate AlGaN/GaN heterostructure field‐effect transistors with regrown ohmic structure
Authors: Maeda Narihiko , Makimura Takashi , Maruyama Takashi , Wang Chengxin , Hiroki Masanobu , Yokoyama Haruki , Makimoto Toshiki , Kobayashi Takashi , Enoki Takatomo
Published: May 2006

Al2O3/Si3N4 insulated‐gate AlGaN/GaN heterostructure field‐effect transistors (HFETs) have been fabricated, where the regrown ohmic structure was incorporated to reduce the contact resistance. Excellent DC and RF characteristics have been obtained together with the low gate leakage current as the result of employing the metal–insulator–semiconductor (MIS) structure. An HFET with a gate length (L g) of 0.1 μm has exhibited a drain current density (I d) and a transconductance (g m) of 1.30 A/mm and 293 mS/mm, respectively, with a reduced contact resistance of 0.3 Ω mm. The gate leakage current (I g) was as low as 1 × 10–8 A/mm in the reverse vias region, and only 4 × 10–5 A/mm even at a forward bias voltage of +3 V. In this device, the cutoff frequency (f T) and maximum oscillation frequency (f max) were estimated to be 70 and 90 GHz, respectively. In the HFETs with longer L g of 0.7 and 1.0 μm, f T and f max were 20 and 48 GHz (L g = 0.7 μm), respectively; and 14 and 35 GHz (L g = 1.0 μm), respectively. Thus, the Al2O3/Si3N4 MIS HFETs have proved to also exhibit excellent RF characteristics. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Electrical characterization of Si‐ion implanted AlxGa1–xN annealed at lower temperatures
Authors: Ryu Mee‐Yi , Yeo Y. K. , Zens T. W. , Marciniak M. A. , Hengehold R. L. , Steiner T.
Published: May 2006

Electrical activation studies of Si‐implanted Alx Ga1–x N (x = 0.1 and 0.18) grown on sapphire substrate have been made as a function of anneal time, anneal temperature, and ion dose. Silicon implantation was done at room temperature with a dose ranging from 5 × 1013 to 5 × 1015 cm–2 at 200 keV. The samples were proximity cap annealed from 1100 to 1250 °C for 5–40 min with a 500 Å‐thick AlN cap in a nitrogen environment. The electrical activation efficiencies of 84% and 75% were obtained for the Al0.1Ga0.9N after annealing at 1200 °C for 40 min for a dose of 5 × 1013 and 1 × 1014 cm–2, respectively. For the Al0.18Ga0.82N, nearly 100% activation for a dose of 5 × 1014 cm–2 and 94% for a dose of 1 × 1015 cm–2 were achieved after annealing at 1250 °C and 1200 °C for 20 min, respectively. Both the activation efficiency and mobility increase with anneal time and anneal temperature, indicating an improved implantation damage recovery. The highest mobility obtained at room temperature is 89 cm2/Vs for the Al0.1Ga0.9N samples having doses of 5 × 1013 and 1 × 1014 cm–2 and annealed at either 1200 °C for 40 min or 1250 °C for 20 min. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Analysis of TMGa/NH3/H2 reaction system in GaN‐MOVPE growth by computational simulation
Authors: Hirako A. , Koiso S. , Ohkawa K.
Published: May 2006

We report the major growth pathways of GaN metalorganic vapor‐phase epitaxial growth from a TMGa/NH3/H2 system studied by computational simulation. It has been found that the most major growth pathway is stepwise elimination of CH4 from TMGa:NH3 adducts (TMGa:NH3 → DMGaNH2 → MMGaNH → Ga–N → Growth), and the second major growth pathway is due to GaNH2 molecules sublimated from GaN layer. [Ga–N]n was generated by polymerization of Ga–N molecules around the susceptor heated. GaN layer was deposited on not only the substrate but also the heated reactor walls. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Deep‐UV LED controlled AlGaN‐based SAW oscillator
Authors: Ciplys D. , Shur M. S. , Sereika A. , Rimeika R. , Gaska R. , Fareed Q. , Zhang J. , Hu X. , Lunev A. , Bilenko Yu.
Published: May 2006

We have demonstrated the possibility of controlling the frequency of an AlGaN‐based SAW delay‐line oscillator by LEDs emitting in deep UV range. The energy bandgap of AlGaN corresponded to the cutoff wavelength of 270 nm. The deep UV LEDs used in these experiments were supplied by Sensor Electronic Technology, Inc. and had the photon energies 265 nm and 305 nm. We also used 375 nm LED from Nichia Corporation. The SAW delay‐line oscillator had the SAW delay line between the output and input of an RF wide‐band amplifier. Illumination of the SAW propagation path by a UV LED resulted in the downshift of the oscillator frequency. We attribute this shift to the interaction of the SAW with photoexcited carriers in the AlGaN layer. The responsivity of the SAW oscillator significantly varied with the LED wavelength and was slightly dependent on the incident optical power. At optical powers within 15–40 μW the responsivities were 0.5 kHz/μW at 265 nm, 0.18 kHz/μW at 305 nm, and 0.03 kHz/μW at 375 nm. No response was observed from the green (560 nm) LED with a similar optical power. The comparatively strong response at 305 nm wavelength (below the energy bandgap of AlGaN) indicated the importance of trapping effects. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Misfit dislocations in In‐rich InGaN/GaN quantum well structures
Authors: Costa P. M. F. J. , Datta R. , Kappers M. J. , Vickers M. E. , Humphreys C. J. , Graham D. M. , Dawson P. , Godfrey M. J. , Thrush E. J. , Mullins J. T.
Published: May 2006

Strain relaxation has been studied for the 10‐period quantum well (QW) heterostructures grown by MOVPE, In0.16Ga0.84N/GaN and In0.2Ga0.8N/GaN, both emitting with high efficiency in the blue and green regions, respectively. Additionally, a set of high‐In content InGaN/GaN QW structures, where growth parameters were kept constant but QW numbers varied between 1 and 10, were also analysed by TEM, XRD and PL. For the highly‐efficient green and the set of variable QW samples, TEM structural studies identified large well‐width fluctuations and misfit dislocations that thread into the sample surface and are generated in the quantum well stack. This contrasts with similar observations of blue‐emitting MQWs where misfit dislocations are not seen. PL measurements have been carried out for the In‐rich QW series and did not reveal a greater degradation of the optical properties with increasing numbers of quantum wells in the stack. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Group III‐nitride and SiC based micro‐ and nanoelectromechanical resonators for sensor applications
Authors: Förster Ch. , Cimalla V. , Lebedev V. , Pezoldt J. , Brueckner K. , Stephan R. , Hein M. , Aperathitis E. , Ambacher O.
Published: May 2006

Micro‐ and nanomechanical AlN and 3C‐SiC resonator beams with resonant frequencies between 20 kHz and 2 MHz, depending on the resonator geometry, have been realized and characterized at ambient conditions. Up to 200 nm thin epitaxial group III‐nitrides and SiC layers were grown on silicon (111) and (100) oriented substrates, respectively. The beams were dry‐etched by an electron cyclotron resonance (ECR) plasma and an inductive coupled plasma (ICP) technique. The freestanding resonator bars have dimensions in the sub‐µm to nm‐range. The operation principle based on the known magneto motive actuation and a thin conductive metal layer on top of the resonator realizes the detection. The main fabrication steps of the resonator beams are presented. The resonant frequencies, the quality factors of the MEMS and NEMS are investigated in dependence on the geometry and the residual strain in the epitaxial layers. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Investigation of buffer structures for the growth of a high quality AlGaN/GaN hetero‐structure with a high power operation FET on Si substrate using MOCVD
Authors: Yoshida Seikoh , Katoh Sadahiro , Takehara Hironari , Satoh Yoshihiro , Li Jiang , Ikeda Nariaki , Hataya Kohji , Sasaki Hitoshi
Published: May 2006

In order to obtain a high quality thick GaN layer on a 2‐inch Si substrate without any crack, we investigated three kinds of buffer layer (AlGaN graded structure, AlN/GaN super lattice (SL) structure, and AlN/thick GaN/AlN structure) using a metal‐organic chemical vapor deposition (MOCVD). As a result, we obtained a crack‐free AlGaN/GaN hetero‐structure with a smooth surface using three kinds of buffer structure. We also observed the cross‐sectional TEM image of three kinds of buffer structures. In the case of using an AlGaN graded buffer on the AlN buffer, the number of surface defect with the size of 10000 nm was comparatively larger compared with the other buffer structures. In the case of using an AlN (18 nm)/GaN (5 nm) super‐lattice (SL) buffer, a 1000 nm‐thick GaN was grown without crack. In the case of using three AlN (50 nm)/two thick GaN (200 nm) buffer structure, the threading dislocation using this buffer was 1.5 × 109 cm–2 and the value was smaller compared with the other buffer structures. This reduction effect of dislocation was larger than the other buffer structures. We also fabricated a HFET using the AlGaN/GaN heterostructure using three AlN/two thick GaN buffer. It was confirmed that the breakdown voltage of FET was over 400 V. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Over 1000 mW single mode operation of planar inner stripe blue‐violet laser diodes
Authors: Sasaoka C. , Fukuda K. , Ohya M. , Shiba K. , Sumino M. , Kohmoto S. , Naniwae K. , Matsudate M. , Mizuki E. , Masumoto I. , Kobayashi R. , Kudo K. , Sasaki T. , Nishi K.
Published: May 2006

We report the 1000 mW single mode operation of a planar blue‐violet laser diode with inner stripe waveguide. Kink‐free output surpassed 400 mW (CW) and 600 mW (50 ns pulsed, duty 50%) at temperatures up to 90 °C. Precise optical field design for narrow stripe waveguide as well as damage‐free stripe formation was a key to achieving watt class output. By utilizing buried AlN as optical and current confinement layers, a damage‐free inner stripe as narrow as 1.0 μm wide was successfully fabricated. Planar inner stripe structure also has the advantage of a small increase in the electrical resistance for such a narrow stripe, which is favorable for high power operations. The results here indicate the potential of this planar blue‐violet laser diode for high power applications as the next generation of optical disc systems. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
High‐nitrogen‐content InGaAsN films on GaAs grown by metalorganic vapor phase epitaxy with TBAs and DMHy
Authors: Sanorpim S. , Nakajima F. , Ono W. , Katayama R. , Onabe K.
Published: May 2006

We investigate the parameters used in the MOVPE growth of high‐N‐content Inx Ga1–x As1–y Ny films on GaAs and their structural and optical properties. The N incorporation control was found to mainly depend on the 1,1‐dimethylhydrazine (DMHy) molar flow rate and the growth temperature. With increasing the DMHy molar flow rate, the high‐resolution X‐ray diffraction measurements exhibited a clear diffraction peak shift, representing the increase in N incorporation. On the other hand, the N content was increased from y ∼ 0.5% to 3.6% with decreasing the growth temperature from 650 °C down to 500 °C. The red‐shift in both the photoluminescence (PL) peak and the photoreflactance (PR) signal (E 0 transition) with increasing N content was clearly observed. Finally, the lattice‐matched In0.06Ga0.94As0.979N0.021/GaAs film corresponding to the room‐temperature E 0 transition energy of 1.11 eV was achieved. It is noticeable that the structural and optical properties of the InGaAsN films grown using TBAs strongly depends on the N content, as in the case of corresponding films grown using AsH3. Besides, the PR spectral feature demonstrated the growth of the InGaAsN film whose bandgap is lower than 1.0 eV by using TBAs in place of AsH3. The present results show that the DMHy flow rate and the growth temperature play a critical role in the growth of high‐quality and single‐phase dilute‐nitride‐arsenide alloy semiconductors. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Waveguide mode dynamics of blue laser diodes
Authors: Pindl Markus , Schwarz Ulrich T.
Published: May 2006

We use a scanning near‐field microscope (SNOM) in combination with a time resolved detection scheme to measure the evolution of the near‐field and far‐field of InGaN laser diode (LD) waveguide modes. We observe lateral mode competition, filamentation, and beam steering. Here we compare the lateral mode dynamics for ridge waveguide LDs and oxide stripe LDs which are predominantly index and gain guided, respectively. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Comparative study on unpassivated and passivated AlGaN/GaN HFETs and MOSHFETs
Authors: Heidelberger G. , Bernát J. , Fox A. , Marso M. , Lüth H. , Gregušová D. , Kordoš P.
Published: May 2006

In this comparative study we investigate AlGaN/GaN‐based unpassivated and passivated HFETs and MOSHFETs with regards to DC‐, RF‐, and power‐performance. For optimal comparability, all devices emanate from the same wafer consisting of a SiC‐substrate, a 3 µm GaN‐ and a 30 nm Al0.28Ga0.72N‐layer. Devices are processed simultaneously to a large extend. Passivated devices are coated with a 10 nm thick SiO2‐layer between the electrodes, MOSHFETs contain a 10 nm thick SiO2‐layer serving as gate‐insulator underneath the gate and as conventional passivation‐layer between the electrodes. Unpassivated devices serve as reference. We present empirical evidence that MOSHFETs outperform both the conventional and the passivated HFETs with respect to DC‐, RF‐, and power‐performance, and we point out the different mechanisms responsible for the behaviour of the devices. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A comparative study of near‐UV emitting InGaN quantum wells with AlGaN and AlInGaN barriers
Authors: Zhu D. , Kappers M. J. , Costa P. M. F. J. , McAleese C. , Rayment F. D. G. , Chabrol G. R. , Graham D. M. , Dawson P. , Thrush E. J. , Mullins J. T. , Humphreys C. J.
Published: May 2006

InGaN multiple quantum well (MQW) structures with AlGaN and AlInGaN barriers were grown on sapphire by metalorganic vapour phase epitaxy (MOVPE). The high resolution X‐ray diffraction (HRXRD) and transmission electron microscopy (TEM) characterisation results show substantial improvements in the material quality by introducing a small amount of indium into the AlGaN barriers, resulting in improved optical properties, as indicated by low‐temperature (6 K) photoluminescence (LT‐PL) spectroscopy. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
X‐ray microdiffraction imaging investigations of wing tilt in epitaxially overgrown GaN
Authors: Lübbert D. , Mikulík Petr , Pernot Petra , Helfen Lukas , Craven Michael D. , Keller Stacia , DenBaars Steven , Baumbach Tilo
Published: May 2006

The crystalline quality in epitaxially laterally overgrown (ELO) GaN and the amount of wing tilt is characterized on a local basis, with high spatial and angular resolution. A method of full‐field X‐ray microdiffraction imaging, termed rocking curve imaging, is used to record simultaneously a large set of local X‐ray diffraction profiles originating from sample surface areas of micrometer size. x –ω maps of diffracted intensity allow to quantify the amount of wing tilt in individual lateral ELO periods as well as to monitor the fluctuations of tilt between adjacent periods. Automated shape analysis of the full set of local rocking curves provides a means to quantitatively characterize the local crystalline perfection of GaN. The ELO window and wing regions can be clearly separated; comparison indicates an average improvement of crystal quality by a factor 3–4 due to the lateral overgrowth process. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Morphology and microstructure of a ‐plane GaN layers grown by MOVPE and by low pressure solution growth (LPSG)
Authors: Hussy S. , Meissner E. , Birkmann B. , Brauer I. , Off J. , Scholz F. , Strunk H. P. , Müller G.
Published: May 2006

In this paper a ‐plane GaN‐layers, grown by two different techniques are compared, mainly focusing on the surface morphology and microstructure of the samples: Layers grown by metal organic vapor phase epitaxy (MOVPE) exhibit a flat surface interrupted by a certain number of pits with triangular shape. In contrast, specimens processed by LPSG developed a rough surface with a roof like morphology. The morphology is discussed in view to the facetting behaviour of the GaN. The observed thickness of the layers implies, that growth in this direction is slightly faster compared to the regular c ‐plane growth. TEM investigations showed that the microstructure of the a ‐plane samples is dominated by a high density of dislocations and stacking faults. In photoluminescence measurements a slightly different characteristic was seen for the materials grown by the two techniques. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Deep ultraviolet light‐emitting diodes
Authors: Hu X. , Deng J. , Zhang J. P. , Lunev A. , Bilenko Y. , Katona T. , Shur M. S. , Gaska R. , Shatalov M. , Khan A.
Published: May 2006

We report on the development of AlGaN‐based deep UV light emitting diodes (LEDs) with emission wavelengths from 254 to 340 nm, focusing on the improvement of 280 nm LEDs efficiency. Under optimal device structure the UV LEDs efficiency was found to strongly depend on the AlGaN material quality. Milliwatt‐power level LEDs were demonstrated for the 254–340 nm spectral range, and for 280 nm LEDs powers reaching 2.5 mW was achieved at 20 mA DC. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Strain‐relaxation in NH3‐source molecular beam epitaxy of AlN epilayers on GaN epitaxial templates
Authors: Koyama T. , Sugawara M. , Uchinuma Y. , Kaeding J. F. , Sharma R. , Onuma T. , Nakamura S. , Chichibu S. F.
Published: May 2006

Temporal evolution of surface morphology in AlN epilayers grown by NH3‐source molecular beam epitaxy on the GaN/(0001) Al2O3 epitaxial templates was correlated with changes in the degree of the residual strain and the layer thickness. They began to crack for the thickness as thin as 10 nm. However, atomic‐layer step‐and‐terrace surface structures were maintained for the thickness up to 32 nm. Tensile biaxial stress decreased with further increase in the thickness due to the lattice relaxation, which caused surface roughening. An 1580‐nm‐thick, nearly strain‐compensated AlN epilayer, of which threading dislocation density was reduced down to 6 × 109 cm–2, exhibited excitonic photoluminescence peaks at 6.002 and 6.023 eV at 9 K and a near‐band‐edge peak at 5.872 eV at 293 K. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Comparison of degradation mechanisms of blue‐violet laser diodes grown on SiC and GaN substrates
Authors: Furitsch M. , Avramescu A. , Eichler C. , Engl K. , Leber A. , Miler A. , Rumbolz C. , Brüderl G. , Strauß U. , Lell A. , Härle V.
Published: May 2006

In this work we present degradation studies of GaN based blue‐violet laser diodes grown on different substrates. By replacing the SiC substrate by GaN substrate, we change from hetero to homo epitaxy, while the threading dislocation density (TDD) is reduced by 3 orders of magnitude. A detailed analysis of small signal I –V curves shows an increase of non radiative (NR) recombination centers during aging for laser diodes on SiC substrate. This was not observed for lasers on GaN substrate due to the reduced TDD and therefore reduced number of diffusion channels for Mg‐atoms, acting as NR recombination centers in the active region. With an improved epitaxial structure on GaN substrate, we increased the lifetime of our lasers by a factor of 10. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Towards identification of degradation mechanisms in InGaN laser diodes grown on bulk GaN crystals
Authors: Marona L. , Riemann T. , Christen J. , Świetlik T. , Franssen G. , Wiśniewski P. , Leszczyński M. , Prystawko P. , Grzegory I. , Suski T. , Porowski S. , Czernecki R. , Perlin P.
Published: May 2006

In this work we present a reliability study of low dislocation density InGaN laser diodes fabricated on high‐pressure grown GaN monocrystaline substrates. The aging process was performed under pulse current conditions. Degradation of these devices manifests primarily in the increase of the threshold current. Interestingly, the differential efficiency of lasers remains stable at all times. The aging time dependence of the increase of the threshold current precisely follows a square root dependence. These observations suggest that the degradation results from the enhancement of the nonradiative recombination within the device active layers and is related to point defect diffusion. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Development of AlInGaN based blue–violet lasers on GaN and SiC substrates
Authors: Rumbolz C. , Brüderl G. , Leber A. , Eichler C. , Furitsch M. , Avramescu A. , Miler A. , Lell A. , Strauß U. , Härle V.
Published: May 2006

We present a process for high quality InGaN laser diodes on free standing GaN substrates with high reproducibility. We demonstrate a self‐aligned process to contact lasers with ridge width <2 μm. The stability of the process allows us to optimize the epitaxial structure on GaN substrates and we achieve cw‐threshold current densities of 2.9 kA/cm2 for 10 μm wide ridges and 4.7 kA/cm2 for 1.5 μm wide ridges. Cw‐slope efficiencies of 1.0 W/A are achieved for both widths. Beside this we notice a reduction of the ideality factor and the forward bias linked to an improvement of the epitaxial interfaces. For maximum optical output we could achieve 3.4 W from one output facet at pulsed operation and a catastrophic optical mirror damage (COMD) level of 67 MW/cm2. Furthermore we observe a dependency of the COMD level from the pulse width corresponding to filament formation. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optical study of the AlGaN/GaN high electron mobility transistor structures
Authors: Lin D. Y. , Lin W. C. , Shiu J. J.
Published: May 2006

We present an optical study on three Alx Ga1–x N/GaN high electron mobility transistor structures, which were grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates, with different Al compositions using contactless electroreflectance (CER), piezoreflectance (PzR) and surface photovoltage spectroscopy (SPS). The Al composition x can be determined from the band gap energy of Alx Ga1–x N layer observed in CER and PzR spectra. The determination of the built‐in electric field strength in the barrier layer (F) has been done by the analysis of Franz–Keldysh oscillations (FKOs) above the Alx Ga 1–x N band gap in CER spectra. A broad feature appeared at energies above the band gap of GaN was observed in each CER, PzR and SPS spectrum. We believe that it is related to the two‐dimensional electron gas (2DEG). The energy of the broad 2DEG minimum roughly corresponds to the Fermi energy plus the band gap. The Hall‐effect measurements have also been carried out in van der Pauw geometry at room temperature to reveal the sheet carrier density (n s). For the sample with lowest alloy composition (x = 0.07) the analysis of the FKOs yields F = 316 kV/cm at room temperature. When the temperature was cooled down to 50 K the electric field strength decreases to 188 kV/cm. This result can be explained by the change of piezoelectric polarization. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Strain and microstructure in Fe‐doped GaN layers grown by low pressure metalorganic vapour phase epitaxy
Authors: Azize M. , Leroux M. , Laugt M. , Gibart P. , Bougrioua Z.
Published: May 2006

In order to get semi‐insulating GaN layers with a low dislocation density on sapphire, two kinds of Fe doping were explored by low pressure metalorganic vapour phase epitaxy (LP‐MOVPE): the modulation doping (MD) and the continuous doping (CD). The high crystalline quality and the semi‐insulating character are obtained in the case of the Fe‐MD layers. The effect of Fe doping on the strain was investigated in both kind of layers (MD and CD) by X‐ray diffraction, photoluminescence and reflectivity. The lattice parameters are functions of the iron doping level, whatever the doping mode. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Selective‐area growth and fabrication of recessed‐gate GaN MESFET using plasma‐assisted molecular beam epitaxy
Authors: Hong Seung Jae , Chapman Patrick , Krein Philip T. , Kim Kyekyoon (Kevin)
Published: May 2006

For the first time, selective‐area growth (SAG) technique has been developed using plasma‐assisted molecular beam epitaxy (PAMBE), enabling fabrication of a recessed‐gate structure for the metal‐semiconductor field‐effect transistor (MESFET) without etching. On patterned SiO2 samples, polycrystalline GaN and single crystal n+‐GaN were observed to grow in the masked and unmasked regions, respectively. The regrown layers were analyzed using AFM. Ohmic contact formed on the n+‐GaN exhibited a vastly improved contact resistivity of 1.8 × 10–8 Ω cm2, giving rise to excellent device characteristics including a peak drain current of 360 mA/mm and a maximum transconductance of 46 mS/mm. The advantages of SAG were further investigated by comparing the dc characteristics of recessed‐gate and unrecessed MESFET. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optimization of AlGaN/GaN HEMTs for high frequency operation
Authors: Palacios T. , Dora Y. , Chakraborty A. , Sanabria C. , Keller S. , DenBaars S. P. , Mishra U. K.
Published: May 2006

In this paper, the structure and processing of AlGaN/GaN high electron mobility transistors have been optimized for maximum small signal gain at high frequencies. The effect of the gate resistance, gate‐to‐drain capacitance and output conductance on the power gain cut‐off frequency, f max, of the devices has been experimentally studied. The reduction of the gate width allowed a 4‐fold decrease in gate resistance which resulted in almost a 100% increase in f max. To minimize C gd, Γ‐shape gates have been processed instead of the conventional T‐shape submicron gates. Finally, to reduce the output conductance, the confinement of the 2‐dimensional electron gas was increased by using an ultra‐thin InGaN layer below the GaN channel. This InGaN back‐barrier caused a 2‐fold improvement in output conductance which allowed a 20% increase in f max. By optimizing all these parameters, AlGaN/GaN transistors with a record f max of 230 GHz were obtained. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Crystal quality and growth evolution of aluminum nitride on silicon carbide
Authors: Moe Craig G. , Wu Yuan , Keller Stacia , Speck James S. , DenBaars Steven P. , Emerson David
Published: May 2006

High quality base layer aluminum nitride films have become increasingly desirable with the advent of deep ultraviolet (<280 nm) emitters for water purification, solid state lighting, and biochemical detection applications. In this study, the influence of the MOCVD growth conditions on the structural properties of aluminum nitride grown on silicon carbide, both as‐delivered and chemomechanically polished, was studied. Pre‐deposition nitridation of the SiC substrate, growth temperature, growth rate, ammonia flow (V/III ratio), and gallium surfactants were explored. Films were analyzed with transmission electron microscopy (TEM), atomic force microscopy (AFM), and X‐ray diffraction. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Epitaxial lateral overgrowth of AlN layers on patterned sapphire substrates
Authors: Nakano K. , Imura M. , Narita G. , Kitano T. , Hirose Y. , Fujimoto N. , Okada N. , Kawashima T. , Iida K. , Balakrishnan K. , Tsuda M. , Iwaya M. , Kamiyama S. , Amano H. , Akasaki I.
Published: May 2006

Epitaxial lateral overgrowth (ELO) of low‐dislocation‐density AlN layers on trench‐patterned sapphire substrates is demonstrated for the first time. Sapphire (0001) substrates with trench patterns formed along two different directions, 〈10$ 10 \bar 10 $0〉 and 〈11$ 11 \bar 20 $0〉, were used. We can obtain fully coalesced AlN only on the sapphire substrate having 〈11$ 11 \bar 20 $0〉 trenches. The dislocation density of ELO‐AlN is as low as 6.7 × 108 cm–2. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
MOVPE growth and optical characterization of GaAsN films with higher nitrogen concentrations
Authors: Nakajima F. , Sanorpim S. , Ono W. , Katayama R. , Onabe K.
Published: May 2006

We have successfully grown high‐N‐content GaAsN films up to 5.1% on GaAs(001) substrates using tertiarybutylarsine (TBAs) as the As precursor by metalorganic vapour phase epitaxy (MOVPE). The narrow X‐ray diffraction (XRD) peaks and clear Pendellosung fringes indicate that the GaAsN/GaAs interface is fairly flat and the GaAsN layers are uniform. By the photoluminescence (PL) measurement at 10 K, the clear PL peaks related to the near‐band‐edge transition could be detected and the bandgap energy was red‐shifted to 1.16 eV in 1.9%‐N GaAsN film. But, in higher N‐content films no peak could be detected. So, post growth annealing in the reactor was applied to 4.7% and 5.1%‐N films, and resulted in an enhancement of the PL peak intensity, and the bandgap energy of 5.1%‐N film was consequently determined to be 0.95 eV at room temperature. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Self‐regulating mechanism of InN growth on GaN(0001) by molecular beam epitaxy; from nanostructures to films
Authors: Dimakis E. , Iliopoulos E. , Tsagaraki K. , Georgakilas A.
Published: May 2006

The surface kinetics of InN growth on GaN(0001) by plasma assisted molecular beam epitaxy was investigated. The surface coverage of the GaN(0001) surface by InN islands and the corresponding islands' growth rate along the [0001] direction, were studied as a function of substrate temperature. Both quantities were found to behave non‐monotonically with substrate temperature due to the interplay between In‐adatoms' surface migration and InN decomposition rate. In the range of temperatures were In‐adatoms' mobility is sufficient, a self‐regulating mechanism of InN islands' shape takes place. This mechanism insures stoichiometric ratio of In and N adatoms on the top (0001) surface of the islands. These findings are summarized in a physical model that describes the growth of InN on GaN(0001) surfaces. In the context of the model, four distinct growth regimes are defined as a function of substrate temperature for a given In/N flux ratio. The exploitation of the self‐regulating mechanism led to successful growth of InN films with good structural, morphological and electrical properties. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Gallium nitride based micro‐cavity light emitting diodes emitting at 498 nm
Authors: Pattison P. Morgan , Sharma Rajat , David Aurelian , Waki Ichitaro , Weisbuch Claude , Nakamura Shuji
Published: May 2006

A gallium nitride based micro‐cavity light emitting diode emitting at a peak wavelength of 498 nm has been fabricated. The epitaxial structure was grown by metalorganic chemical vapor deposition, and the device was fabricated using a laser lift‐off process. Cavity thinning was carried out using reactive ion etching until a cavity length of roughly 830 nm (corresponding to a cavity order of ∼8 for λ = 498 nm in GaN) was achieved. Electroluminescence measurements of the micro‐cavity device show a narrowed spectral width compared to the un‐thinned device, and angularly resolved spectral measurements yielded a spectral shift, evincive of micro‐cavity effects. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
InN growth on sapphire using different nitridation procedures
Authors: Drago M. , Werner C. , Pristovsek M. , Pohl U. W. , Richter W.
Published: May 2006

We studied optimal sapphire nitridation conditions with ammonia for the metal‐organic vapor phase epitaxy of InN using in‐situ spectroscopic ellipsometry (SE). SE indicates the formation of a complete AlN‐containing layer on top of the sapphire surface during the first 45 s of nitridation. Further nitridation leads to a growth of the AlN‐like layer by nitrogen diffusion and to roughening. Successively, a set of high quality InN films were grown onto sapphire substrates after different nitridation duration. Electronic characterisation clearly indicates optimum InN quality for 45 s of sapphire nitridation, i.e. when only the sapphire surface is nitridated, as determined by in‐situ SE. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Crack free monolithic nitride vertical‐cavity surface‐emitting laser structures and pillar microcavities
Authors: Lohmeyer Henning , Sebald Kathrin , Kruse Carsten , Kröger Roland , Gutowski Jürgen , Hommel Detlef , Wiersig Jan , Jahnke Frank
Published: May 2006

We report on the successful fabrication and optical investigation of monolithicly grown GaN based vertical‐cavity surface‐emitting laser structures featuring quality factors of 250. Short‐period AlN/InGaN superlattices are used for the low‐index layers of the distributed Bragg reflectors. Pillar microcavities are realized by focused‐ion‐beam etching. Micro‐photoluminescence measurements reveal the longitudinal and transversal mode spectra of the cavities in good agreement with theoretical calculations based on a vectorial transfer matrix method. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Novel devices based on the combination of nitride and II–VI materials
Authors: Murai A. , Kruse C. , McCarthy L. , Mishra U. K. , DenBaars S. P. , Hommel D.
Published: May 2006

We demonstrate the direct wafer bonding of GaN and ZnSe‐based materials. We report on the structural, optical, and electrical characteristics of InGaN/GaN light‐emitting diodes (LEDs) and ZnSe‐based II–VI materials combined by direct wafer bonding. Reflectivity, transmission electron microscopy (TEM), and current–voltage (I –V ) measurements were performed. The bonded sample exhibited a higher reflectivity at the wavelength of 510 nm than a control LED with an Al cap. Cross‐sectional TEM revealed a uniform wafer‐bonded interface with no voids or cavities for the low temperature sample, while the sample bonded at a higher temperature was observed to have lens‐shaped cavities at semiconductor interfaces. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Atomic structure of pyramidal defects in GaN:Mg: Influence of annealing
Authors: Liliental‐Weber Z. , Tomaszewicz T. , Zakharov D. , O'Keefe M. , Hautakangas S. , Saarinen K. , Freitas J. A. , Henry R. L.
Published: May 2006

The atomic structure of the characteristic defects (Mg‐rich hexagonal pyramids) in p‐doped bulk and MOCVD GaN:Mg thin films grown with Ga polarity was determined at atomic resolution by direct reconstruction of the scattered electron wave in a transmission electron microscope. Small cavities were present inside the defects, confirmed also with positron annihilation. The inside walls of the cavities were covered by GaN of reverse polarity compared to the matrix. Annealing of the MOCVD layers lead to slight increase of the defect size and an increase of the room temperature photoluminescence intensity. Positron annihilation confirms presence of vacancy clusters of different sizes triggered by the Mg doping in as‐grown samples and decrease of their concentration upon annealing at 900 and 1000 °C. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Optimization of AlGaN/GaN HEMTs for high frequency operation
Authors: Palacios T. , Dora Y. , Chakraborty A. , Sanabria C. , Keller S. , DenBaars S. P. , Mishra U. K.
Published: May 2006

In the article [1] featured at Editor's Choice, the structure and processing of AlGaN/GaN high electron mobility transistors (HEMTs) have been optimized for maximum small signal gain at high frequencies. The cover picture combines the sample structure – shown schematically and in a scanning electron microscopy image – with the band diagram of the sample with an InGaN back‐barrier used to increase the electron confinement in comparison to a standard HEMT.

The first author, Tomás Palacios, is currently a Project Scientist at UCSB. His research interest focuses on the search of novel GaN‐based transistors for mm‐wave applications and biological sensors. He is one of the winners of the physica status solidi Young Researcher Awards for his outstanding presentation at the 6th International Conference on Nitride Semiconductors held in Bremen, Germany, in 2005. Further articles from ICNS‐6 will also be published in phys. stat. sol. (b) 243, No. 7 (2006) and phys. stat. sol. (c) 3, No. 6 (2006).

The present issue of phys. stat. sol. (a) as well as phys. stat. sol. (c) 3, No. 5 (2006) also contain papers presented at the International Conference on Nanoscale Magnetism (ICNM‐2005) in Gebze, Turkey.
InN: Fermi level stabilization by low‐energy ion bombardment
Authors: Piper L. F. J. , Veal T. D. , McConville C. F. , Lu H. , Schaff W. J.
Published: May 2006

The near‐surface electronic properties of InN have been investigated with high‐resolution electron‐energyloss spectroscopy. Low‐energy (∼400 eV) nitrogen ion bombardment followed by low temperature annealing (<300 °C) was found to dramatically increase the n ‐type conductivity of InN, close to the surface. This is explained in terms of the formation of amphoteric defects from the ion bombardment and annealing combined with the band structure of InN. Low‐energy ion bombardment and annealing is shown to result in a damage‐induced, donor‐like defect‐profile instead of the expected electron accumulation for InN. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Analysis of buffer‐trapping effects on current collapse of GaN FETs
Authors: Horio K. , Takayanagi H. , Nakano H.
Published: May 2006

Two‐dimensional transient simulation of GaN MESFETs is performed in which a deep donor and a deep acceptor in a semi‐insulating buffer layer are considered. Quasi‐pulsed I‐V curves are derived from the transient characteristics, and are compared with the steady‐state I‐V curves. It is shown that the current collapse is more pronounced when the deep‐acceptor density in the buffer layer is higher and when an off‐state drain voltage is higher, because the trapping effects become more significant. It is suggested that to minimize current collapse in GaN‐based FETs, an acceptor density in a semi‐insulating GaN layer should be made low, although the current cutoff behaviour may be degraded. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Electronic states in nitride semiconductor quantum dots: A tight‐binding approach
Authors: Schulz S. , Czycholl G.
Published: May 2006

We investigate the electronic properties of a lens‐like InN/GaN QD with a wurtzite structure within a tightbinding (TB) model. The TB approach provides a theoretical description of these structures on a microscopic level. An sp 3‐TB model is used which well‐reproduces the band structure of the bulk material. The resulting parameters are used as an input to model the low dimensional heterostructure. Wurtzite nitrides exhibit a (spontaneous) polarization P which leads to the appearance of an electrostatic potential Vp . We incorporate the potential Vp in our TB model to examine the influence on the energy levels and the single particle wave functions for the investigated QD structure. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
An AlGaN/GaN two‐color photodetector based on an AlGaN/GaN/SiC HEMT layer structure
Authors: Marso M. , Fox A. , Heidelberger G. , Bernát J. , Lüth H.
Published: May 2006

The optoelectronic properties of MSM diodes that are based on an AlGaN/GaN HEMT layer system are investigated as function of the applied bias. For low voltages, the two‐dimensional electron gas acts as a barrier for the depletion layer. Therefore, only the upper AlGaN barrier layer contributes to the photocurrent. In the high voltage regime the depletion region penetrates the GaN buffer that adds the spectral responsivity of GaN to the pure AlGaN behaviour in the low voltage regime. The ratio of the responsivities at 350 nm and 300 nm wavelength can be switched from 0.01 at 2.5 V to 0.8 at 4 V bias. This property makes the MSM‐2DEG a candidate for use as two‐color photodetector. Device fabrication uses standard HEMT processing steps, allowing integration in HEMT circuits without the need of sophisticated growth or etching techniques. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Ion implantation doping for AlGaN/GaN HEMTs
Authors: Suita Muneyoshi , Nanjo Takuma , Oishi Toshiyuki , Abe Yuji , Tokuda Yasunori
Published: May 2006

Selective doping using ion implantation has been developed for the source/drain contacts of AlGaN/GaN high electron mobility transistors (HEMTs). As the annealing temperature of the electrical activation increased, the gate leakage current increased, although the specific contact resistance decreased. The characteristics of ion implanted HEMTs were improved using the optimized annealing temperature of 1150 °C. A saturation drain current of 0.68 A/mm and a maximum transconductance of 0.17 S/mm were obtained without degrading the off‐state breakdown voltage. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Extremely smooth surface morphology of GaN‐based layers on misoriented GaN substrates for high‐power blue‐violet lasers
Authors: Tachibana Koichi , Nago Hajime , Nunoue Shin‐ya
Published: May 2006

We have successfully realized a very smooth surface morphology of GaN‐based epitaxial layers on n‐GaN (0001) substrates with the misorientation angle of approximately 0.2° only toward [1¹100] direction, which is confirmed using Nomarski microscopy and atomic force microscopy. In x‐ray diffraction spectra, the narrow width of the satellite peak of superlattices also indicated the smooth surface morphology and abrupt interfaces of the epitaxial layers on the GaN (0001) substrate with the misorientation angle only toward [1$ \bar 1 $00] direction. The realization of smooth surface morphology of the layers grown on GaN (0001) substrates misoriented only towards [1$ \bar 1 $00] direction is considered to be closely related to the anisotropic inplane growth rate. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
High temperature characteristics of doped channel AlGaN/GaN MIS‐HFETs with thin AlGaN barrier layer
Authors: Wang C. X. , Maeda N. , Hiroki M. , Yokoyama Y. , Makimoto T. , Kobayashi T. , Enoki T.
Published: May 2006

The device performance of AlGaN/GaN‐based metal‐insulator‐semiconductor heterostructure field effect transistors (MIS‐HFETs) with a very thin AlGaN barrier and a heavily doped GaN channel for high 2DEG density has been investigated at elevated temperatures up to 250 °C. The devices exhibited ultra‐low gate current leakages under the reverse gate bias with very reasonable transconductance characteristics at both RT and high temperatures. MIS‐HFETs with doped channel showed much higher saturation drain current and weaker current collapse than the conventional devices at RT and high temperatures. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Towards a better understanding of nano‐islands formed during atmospheric pressure MOVPE
Authors: van der Laak N. K. , Oliver R. A. , Barnard J. S. , Cherns P. D. , Kappers M. J. , Humphreys C. J.
Published: May 2006

We report on transmission electron microscopy (TEM) characterisation of nano‐islands formed during atmospheric pressure growth of InGaN on GaN by metal‐organic vapour phase epitaxy (MOVPE). Although initial results seemed to indicate that three‐dimensional InGaN nano‐islands had formed, detailed TEM characterisation revealed that the islands do not have a hexagonal crystal structure, and electron energy loss spectroscopy suggests that the observed structures are metallic indium droplets formed during growth, which may be partially nitrided, rather than the expected strain‐induced epitaxial InGaN nano‐islands. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Hydrogen generation from aqueous water using n‐GaN by photoassisted electrolysis
Authors: Fujii Katsushi , Ohkawa Kazuhiro
Published: May 2006

We propose a new GaN application for water splitting using photoelectrolysis. Hydrogen gas generation from a counterelectrode was clearly observed using light‐illuminated n‐type GaN as a working photoelectrode in an electrolyte. The hydrogen generation in KOH was the highest. Corrosion of GaN in NaCl and HCl was, however, quite less than that in KOH during photoelectrolysis. A combination of GaN and HCl is promising in stable photo‐electrolysis for H2 generation. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
High quality tin zinc oxide/Ag ohmic contacts for UV flip‐chip light‐emitting diodes
Authors: Hong Hyun‐Gi , Ban K.‐Y. , Song June‐O , Cho J. , Park Y. , Kwak J. S. , Ferguson Ian T. , Seong Tae‐Yeon
Published: May 2006

High quality SnZnO (TZO)/Ag ohmic contacts to p ‐GaN (Na = 4×1017/cm3) for UV flip‐chip light‐emitting diodes (LEDs) were investigated. The TZO (2.5 nm)/Ag (250 nm) contacts produce contact resistivity of 1.58 × 10‐4 Ωcm2 and reflectance of 76% at 405 nm when annealed at a temperature of 530 °C for 1min in air. Near‐UV LEDs made with the annealed TZO (2.5 nm)/Ag (250 nm) contacts give reverse leakage current of ∼10‐8 A at reverse voltage of –10 V. Based on X‐ray photoelectron spectroscopy, possible ohmic formation mechanisms are discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
A comparative study on MOVPE InN grown on Ga‐ and N‐polarity bulk GaN
Authors: Wang W. J. , Miwa H. , Hashimoto A. , Yamamoto A.
Published: May 2006

The influence of substrate polarity on the growth of InN film by MOVPE was investigated using bulk GaN as a substrate. Single‐crystalline In‐ and N‐polarity InN films were obtained on Ga‐ and N‐polarity GaN substrate, respectively. Significant difference of the morphologies between the In‐ and N‐polarity InN films was found. For the In‐polarity InN film, the morphology was similar to that grown on sapphire substrate. The film surface was consisted of grains with small facets. In contrast, for the N‐polarity InN film, the surface was consisted of large hexagonal shape crystal grains with flat surface. The grain size was about 2 μm in diameter on the average, and two‐dimensional growth was enhanced obviously for each crystal grain. The influence of the growth temperature on the morphology, polarity, and optical property was also investigated. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)