Proceedings Volume 6473

Gallium Nitride Materials and Devices II

Hadis Morkoc, Cole W. Litton
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Proceedings Volume 6473

Gallium Nitride Materials and Devices II

Hadis Morkoc, Cole W. Litton
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 7 February 2007
Contents: 14 Sessions, 50 Papers, 0 Presentations
Conference: Integrated Optoelectronic Devices 2007 2007
Volume Number: 6473

Table of Contents

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Table of Contents

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  • Front Matter: Volume 6473
  • Growth
  • Electrical and Optical Characterization
  • Special Topics
  • Point Defects
  • FETs I
  • LEDs II
  • Lasers I
  • Defects
  • FETs II
  • Lasers II
  • Extended Defects
  • FETs III
  • LEDs IV
Front Matter: Volume 6473
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Front Matter: Volume 6473
This PDF file contains the front matter associated with SPIE Proceedings Volume 6473, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.
Growth
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New possibility of MOVPE-growth in GaN and InN: polarization in GaN and nitrogen-incorporation in InN
In the application of nitride semiconductors for electronic and optical devices, spontaneous and piezoelectric polarizations have been discussed recently. On the contrary, in light emitting devices, polarization is expected to be absent. To suppress the polarization effect, GaN growth on A-plane and R-plane sapphire substrates has been attempted. A-plane sapphire has crystallographical symmetry different from GaN. R-plane sapphire has large lattice-mismatch from GaN. In this paper, GaN grown on an M-plane sapphire substrate which has been focused in 1990 is reviewed. M-plane sapphire has a lattice-mismatched to GaN by less than 3%. Single-phase GaN was grown on sapphire tilted 15 degrees from an M-plane and its inclination of c-axis to the nominal axis of a substrate was by 32 degree. This number is much attractive to suppress the polarization effect in light emitting devices. This paper also describes N-polar GaN grown by MOVPE. Differently from the reported data about N-polar GaN this N-polar GaN with a surface as smooth as Ga-polar one was obtained and the density of threading dislocations was in order of 1018/cm2. p-type doping was also possible. This N-polar is very suitable for the growth of InN, which has the high equilibrium-vapor-pressure of nitrogen, because N polarity has the advantage in the capture of nitrogen. The growth and the properties of N-polar InN on N-polar GaN templates are reviewed. Finally, the perspectives of InN in device applications are introduced.
Two-step epitaxial lateral overgrowth of a-plane GaN by MOCVD
We report on growth and characterization of epitaxial lateral overgrown (ELO) (112-0) a-plane GaN by metalorganic chemical vapor deposition (MOCVD). The ELO samples were grown using a SiO2 striped mask pattern consisting of 4 &mgr;m wide open windows and 10 &mgr;m or 20 &mgr;m wide SiO2 stripes. Different growth rates in Ga- and N-wings along with the wing tilt create a major obstacle for achieving a fully coalesced flat surface in ELO-GaN. To address this problem we have employed a two-step growth method that is able to provide a high height to width aspect ratio in the first growth step followed by enhanced lateral growth in the second step by controlling the growth temperature. Depending on the growth conditions, lateral growth rate of the wings with Ga-polarity were from 2 to 5 times larger than that of the N-polarity wings. We investigated the effects of growth parameters on wing tilt, which was observed to be ~0.25° from the Kikuchi lines using large angle convergent beam electron diffraction (LACBED) and accompanied by some twist (0.09°) between the two opposite wings. Transmission electron microscopy (TEM) results showed that the threading dislocation density in the resulting fully coalesced overgrown GaN was reduced from 4.2×1010 cm-2 in the window area to 1.0×108 cm-2 in the wing area, and that the wing areas contained relatively high density of basal stacking faults, 1.2×104 cm-1. The recombination of carriers/excitons localized at stacking faults was evident in far-field near bandedge photoluminescence (PL) measured at 10 K. Moreover, atomic force microscopy (AFM) measurements revealed two orders of magnitude higher density of surface pits in window than in wing regions, which could be decorating dislocation termination on surface. Time-resolved PL measurements for the a-plane ELO-GaN samples revealed biexponential decays. The recombination times were significantly increased ( &tgr;1 =80 ps and &tgr;2 =250 ps) compared to the standard a-plane epitaxial layers (<45 ps), and ratio of the slow decaying component magnitude to the fast decaying one was more than 1.5, showing considerable reduction of nonradiative centers by lateral overgrowth. In addition, room temperature near-field optical microscopy studies revealed the improved optical quality in the wing regions of the overgrown GaN. As revealed from far-field PL, the band edge luminescence at room temperature was more than two orders of magnitude weaker than the yellow luminescence. Therefore, the overall spectrally integrated near field PL was collected, and its intensity was noticeably stronger in the wing areas with both Ga and N polarity. The much weaker emission at the windows and meeting fronts of the two opposite wings were consistent with the observations of high density of dislocations in the window regions and new defects originating at the meeting boundaries from TEM.
Low dislocation density GaN grown by MOCVD with SiNx nano-network
GaN epitaxial layers grown on SiC and sapphire suffer from high density of line and point defects. To address this problem, new growth methods using in situ or ex situ nano-network masks as dislocation filters have been introduced recently. In this work, we report on metalorganic chemical vapor deposition (MOCVD) of GaN layers on 2-inch sapphire substrates using in situ SiNx nano-networks intended for defect reduction. SiNx interlayers with different deposition times were employed after ~2 &mgr;m GaN grown on sapphire, which was followed by ~3.5 &mgr;m GaN overgrowth. With increasing SiNx coverage, full width at hall maximum (FWHM) values of (0002) and (101-2) X-Ray diffraction (XRD) peaks monotonously decrease from 252 arc sec to 217 arc sec and from 405 ar csec to 211 arc sec, respectively for a 5.5 &mgr;m thick film. Similarly, transmission electron microscopy (TEM) revealed that screw and edge type dislocation densities as low as 4.4x107 cm-2 and 1.7x107 cm-2 were achieved. The use of SiNx nanonetwork also increases the radiative recombination lifetimes measured by time-resolved photoluminescence to 2.5 ns from less than 0.5 ns in control GaN. We have also fabricated Ni/Au Schottky diodes on the overgrown GaN layers and the diode performance was found to depend critically on SiNx coverage, consistent with TEM, XRD and TRPL results. A 1.13eV barrier height was achieved when SiNx layer was used compared to 0.78 eV without any SiNx nanonetwork. Furthermore, the breakdown voltage was improved from 76 V to 250 V with SiNx nanonetwork.
Growth and polarity control of GaN and AlN on carbon-face SiC by metalorganic vapor phase epitaxy
Growth and polarity control of GaN and AlN on carbon-face SiC (C-SiC) by metalorganic vapor phase epitaxy (MOVPE) are reported. The polarities of GaN and AlN layers were found to be strongly dependent on the pre-growth treatment of C-SiC substrates. A pre-flow of trimethyaluminum (TMAl) or a very low NH3/TMAl ratio resulted in Al(Ga)-polarity layers on C-SiC. Otherwise, N-polarity layers resulted. The polarities of AlN and GaN layers were conveniently determined by their etching rate in KOH or H3PO4, namely the etching rate on N-polarity is substantial larger, a method reported earlier. We suggest that the Al adatoms form several Al adlayers on C-SiC and change the incorporation sequence of Ga(Al) and N leading to a metal polarity surface. In addition, the hexagonal pyramids, typical on N-polarity GaN surface, are absent on N-polarity GaN grown on off-axis C-SiC owing to high density of terraces on the substrate surface. The properties of GaN layers grown on C-SiC have been studied by X-ray diffraction and are reported in this paper.
Electrical and Optical Characterization
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Enhanced luminescence from AlxGa1-xN/AlyGa1-yN quantum wells grown by gas source molecular beam epitaxy with ammonia
Sergey A. Nikishin, Boris A. Borisov, Gregory A Garrett, et al.
We report the structural and optical properties of AlxGa1-xN/AlyGa1-yN quantum wells (QWs) structures grown by gas source molecular beam epitaxy with ammonia on sapphire (0001) substrates. QWs structures consist of five pairs of AlyGa1-yN, 0.3xGa1-xN, 0.55
Narrow-width photoluminescence spectra of InGaN quantum wells grown on GaN (0001) substrates with misorientation toward [1100] direction
Koichi Tachibana, Hajime Nago, Shin-ya Nunoue
The optical properties of InGaN quantum wells on misoriented GaN (0001) substrates were investigated. The fluctuation of peak wavelength and full width at half maximum of micro-photoluminescence from InGaN quantum wells was large when the misorientation angle was 0.0o. The micro-photoluminescence showed narrow-width spectra, with full width at half maximum below 60 meV, of InGaN quantum wells grown on GaN (0001) substrates with a misorientation angle of around 0.28o toward [11-00] direction. These results indicate that InGaN quantum wells have high crystalline quality when InGaN quantum wells are grown with misorientation angle between 0.2o and 0.3o toward [11-00] direction.
AFM and CAFM studies of ELO GaN films
V. Kasliwal, J. C. Moore, X. Ni, et al.
The techniques of atomic force microscopy (AFM) and conductive AFM (CAFM) have been used to study the morphology and conduction properties of a-plane GaN films grown via epitaxial lateral overgrowth (ELO). Four GaN samples were prepared using metal organic chemical vapor deposition (MOCVD) with slightly different growth conditions. In AFM images, the coalesced ELO films show undulations, where the window regions appear as depressions with a higher defect density than surrounding areas. At reverse bias above 20 V, lower quality samples show localized leakage defect sites inside the window regions, whereas higher quality samples show no localized leakage. This behavior is consistent with previous observations on non-ELO samples where significantly enhanced localized leakage occurs at voltages above 15 V. Surface oxidation was also observed, where continuous scanning at reverse bias results in decreased conduction. This CAFM study confirms that ELO-grown GaN samples show enhanced reverse-bias leakage inside window regions where a higher defect density is present.
Magneto-transport properties of MOVPE-grown AlxGa1-xN/AlN/GaN heterostructures with high-mobility two-dimensional electron gas
N. Biyikli, H. Cheng, Ç. Kurdak, et al.
We study AlxGa1-xN/AlN/GaN heterostructures with a two-dimensional-electron-gas (2DEG) grown on different GaN templates using low-temperature magneto-transport measurements. Heterostructures with different Al compositions are grown by metal-organic vapor phase epitaxy (MOVPE) on three different templates; conventional undoped GaN (u- GaN), epitaxial lateral overgrown GaN (ELO-GaN), and in situ ELO-GaN using a SixNy nanomask layer (SiN-GaN). Field-dependent magneto-resistance and Hall measurements indicated that in addition to 2DEG, the overgrown heterostructures had a parallel conducting layer. The contact resistance for the parallel channels was large so that it introduced errors in the quantitative mobility spectrum analysis (QMSA) of the data. Notwithstanding complexities introduced by parallel conducting channels in mobility analysis in SiN-GaN and ELO-GaN samples, we were able to observe Shubnikov-de Haas (SdH) oscillations in all samples, which confirmed the existence of 2DEGs. To characterize the parallel channel, we repeated the transport measurements after the removal of the 2DEG by etching the heterostructure. The 2DEG carrier density values were extracted from the SdH data, whereas the zero-field 2DEG conductivity was determined by subtracting the parallel channel conductivity from the total conductivity. The resulting 2DEG mobility was significantly higher (about a factor of 2) in the ELO-GaN and SiN-GaN samples as compared to the standard control sample. The mobility enhancement is attributed to the threading dislocation reduction by both ELO techniques.
Investigation of current voltage characteristics of n-GaN/i-AlxGa1-xN/n-GaN structures
X. Ni, J. Xie, Y. Fu, et al.
Although standard GaN device structures used for FETs, light emitters, and detectors have been investigated reasonably extensively, the device structures relying on the particulars of current transport over barriers in this material system have not received as much attention, to a large extend due to the insufficient quality of the layers. Unless special measures are taken, the defects present in the barrier material induce current conduction paths that preclude any possibility of observing the fundamental current conduction mechanisms. To overcome this impediment, high quality GaN layers, followed by the vertical single barrier heterostructures, have been grown on sapphire substrates using epitaxial lateral epitaxy in a metal organic chemical vapor deposition system with the aid of an in-situ deposited SiNx nanonet. Structural and optical properties of the films indicate their superior nature. With these templates in hand, n-GaN/i-AlxGa1-xN/n- GaN structures with varying barrier width and height have been prepared and tested for their IV characteristics. The rectification observed is consistent with the barrier design. Because the band bending is affected by polarization charge, which is dependent on pressure, current vs. voltage measurements under pressure have also been recorded. In this presentation, the details of the measurements and analyses, as well as the pertinent aspects of growth related issues will be discussed.
Characterization of the carrier dynamics and interface-state charge fluctuations in quaternary AlInGaN multiple quantum well heterostructures
Chih-Chun Ke, Cheng-Wei Hung, Da-Chuan Kuo, et al.
Nitride-based light-emitting diodes (LEDs) have recently attracted to understand the emission mechanisms in novel multiple quantum well (MQW) heterosturctures. To understand substantially the unique spectral response, it is necessary to examine the carrier transport behavior. In this work, we studied the unique correlations between the carrier dynamics and optical characteristics of the quaternary AlInGaN MQW heterostructures with different barrier widths. It has been found that the photoluminescence peak energy of quaternary AlInGaN MQW blueshifts when decreasing the barrier width. This is attributed to the redistribution among the well and barrier of the strong electrostatic fields induced by polarization effect. It resulted in not only the diminutions of the charge density induced by piezoelectric field, but also the increments of the interface-state charge distribution from the collective influence of alloy disorder and interface roughness. We resort the Arrhenius plots to demonstrate the localized effect originated from indium fluctuation. Our results show the exciton-localization effect can be enhanced monotonically by increasing the barrier widths. On the other hand, we corroborated the surface charge density increased while increasing barrier widths between the epitaxial layers in this investigation.
Optical properties of Berthelot-type behaviors in quaternary AlInGaN multiple quantum well heterostructures
Cheng-Wei Hung, Chih-Chun Ke, Da-Chuan Kuo, et al.
The anomalous Berthelot-type optical properties of quaternary AlInGaN heterostructure with different quantum well pairs have been systematically investigated in this study. The Berthelot-type model refers to the temperature dependence of emission intensity with blue then red shift behavior in disorder material system. The photoluminescence of the AlInGaN heterostructures is also found to exhibit such unique luminescence features as S-shaped emission peak energy similar to Berthelot-type properties over temperature. We ascribed the phenomenon to the spinodal decompositions, which will lead to the appearance of the Berthelot-type behavior. The increase of quantum well pairs will cause the incorporation of indium and/or aluminum atoms in the AlInGaN nanostructures more obviously, resulting in augmentation of the degree of crystalline randomization. In other words, the higher degree of disorder in AlInGaN heterostructures is observed to manifest not only the extension of static microbarrier width, but also the enhancement of carrier localization effects.
Characterization of the carrier localization confinement for InGaN/GaN multiple quantum well heterostructures with hydrogen-flow treatments
Ta-Chuan Kuo, Wei-Jen Chen, Chih-Chun Ke, et al.
Major developments in group-III nitride semiconductors have led to the commercial production of InGaN-based blue/green multiple quantum well (MQW) laser diodes (LDs) and light-emitting diodes (LEDs) for use in varied applications. The main approaches have been adopted to meet the increasing demands for improved efficiency in modern optoelectronic devices; enhancing the light extraction and the quantum efficiency. In this work, the improvement of carrier localization confinement in InGaN/GaN multiple quantum well structures has been achieved by introducing hydrogen-flow treatment into the growth procedures. To characterize the radiative recombination mechanisms in the active layers, the temperature-dependent photoluminescence (PL) of InGaN/GaN MQW structures have been measured. It has been found the strong temperature-dependent blueshift of the emission peak energy for the conventional MQW sample due to band filling effect. As the temperature increased, for the MQW sample with hydrogen-flow treatment, it has been found the emission peak of PL spectra exhibited an obvious red-blue-red shift, i.e., S-shaped shift. By introduction of hydrogen flow during the growth procedures, it has been expected not only to encourage atom coherence motions tend to three-dimension cluster formations but also to provide a stronger localization confinement ability to enhance exciton radiative recombinations in the band tail of the density of states. From the Arrhenius plot of PL intensity, compared with the value of 120 meV achieved for the conventional MQW sample, the higher activation energy value of 300 meV for the MQW sample with hydrogen-flow treatment implies that there was better confinement ability for the excess charge carriers.
Special Topics
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Wide bandgap UV photodetectors: a short review of devices and applications
Franck Omnès, Eva Monroy, Elias Muñoz, et al.
Ultraviolet detectors are of a great interest to a wide range of industrial, military, environmental and even biological applications. This paper intends first to review some of the most relevant recent developments in the field of wide bandgap semiconductor UV detectors, and to give an overview of their applications. A special focus is given on III-nitride based devices, which more and more clearly represent to date one of the most promising and flexible technical solutions for UV detection.
Spontaneous polarizations, electrical properties, and phononic properties of GaN nanostructures and systems
Takayuki Yamanaka, Ke Sun, Yang Li, et al.
Spontaneous polarizations of GaN nanostructures and quantum dots are calculated for different surface terminations. In addition, dimensionally-confined phonons in GaN-based nanostructures are discussed. GaN-based nanostructures have applications in a variety of systems and concepts including: non-charge-transfer-based devices and single-photon detectors based on GaN-based double-barrier quantum-well injectors, conductive-polymer collectors, and colloidal quantum dot recombination regions. In this paper, application of photodetectors is considered along with the related application of using colloidal quantum-dot-based ensembles for solar cell applications.
Point Defects
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Summary of deep level defect characteristics in GaN and AlGaN
The characteristics of defects that are predicted to be dominant for n-GaN or p-GaN are reviewed, and compared to measurements. Measurements are discussed to extract the concentration, transition energy, charge state, and lattice relaxation, each of which can be predicted from theory. Additional considerations are discussed related to the defects that are expected to occur in highest concentration. All of the native defects with transitions in the band gap are expected to act as minority carrier traps, in spite of the predominance of characterization using majority carrier devices. Defects detected by deep level transient spectroscopy are also frequently cited as being associated with a dislocation based on the capture kinetics. Possibilities for other capture mechanisms exist and are modeled along with capture at a dislocation in order to provide a method to distinguish between the mechanisms. This work provides a framework for systematically progressing towards identifying the composition of defects.
Interplay of Ga vacancies, C impurities, and yellow luminescence in GaN
F. Reurings, F. Tuomisto
We have applied positron annihilation spectroscopy to study the role of gallium vacancies in the yellow luminescence of gallium nitride. We measured the Ga vacancy concentrations of a set of wurtzite GaN layers grown by metal-organic chemical-vapor deposition (MOCVD) on sapphire and containing different amounts of C and exhibiting different intensities in the yellow range of the photoluminescence spectrum. Interestingly, the relationship between the Ga vacancies, C impurities and yellow luminescence depends on the electrical conductivity of the material. In semi-insulating samples, we observe a correlation between the yellow luminescence and the carbon concentration, while there is anti-correlation between these two and the gallium vacancy concentration. In contrast, in the n-type samples both the yellow luminescence and the Ga vacancy concentration are independent of the carbon content. These results support the view that the gallium vacancy is not the only defect related to yellow luminescence observed in GaN, but that another, carbon-related mechanism is involved as well.
Point defect reduction in GaN layers grown with the aid of SiNx nanonet by metalorganic chemical vapor deposition
Reduction of deep centers in GaN layers grown employing nano-ELO SiNx porous nanonetworks has been studied by deep-level transient spectroscopy (DLTS). The obtained concentrations of deep traps in layers with SiNx nanonetworks were compared with an otherwise identical reference sample and with another sample grown by employing conventional ELO technique. Two traps, labeled A (0.54-0.58 eV) and B (0.20-0.23 eV), were delineated in all layers with trap A being dominant in the temperature range 80-400 K. The concentration of trap A in SiNx layers was found to be lower by 2-4 times compare to the reference sample. The minimum concentration 7.5x1014 cm-3 was obtained in the layer grown on SiO2 stripe pattern which is ~6 times lower compare to the reference sample. We have found the logarithmic capture mechanism up to ~20 ms for deep center A. Considering that the lateral growth mainly reduces the edge dislocations in our films it is tempting to suggest that structural defects that may have a direct and or indirect role in the creation of the dominant trap which we believe are located close to each other along the edge threading dislocation lines. In addition, a small blue shift, compare to a strain free layers, of the neutral-donor-bound-exciton line (D0XA) observed in the photoluminescence spectra of the samples grown with lateral overgrowth is indicative of partial strain relief.
FETs I
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Quantum 1/f noise in GaN FETs, HFETs, MODFETs, and their oscillators' phase noise
Peter H. Handel, Amanda M. Hall, Hadis Morkoç
GaN-based FETs, HFETs and MODFETs are ideally suited for use in high-power amplifiers and oscillators, due to their large band gap and high operating voltages. According to the quantum 1/f theory, the larger effective mass implied for the carriers also leads to lower fundamental 1/f noise and to lower resulting phase noise close to the carrier frequency. We have therefore studied the quantum 1/f noise sources in the channel and in the gate insulation. For the channel, a combination of conventional and coherent Quantum 1/f Effect (Q1/fE) is present, with the conventional Q1/fE dominant in the sub-threshold part of the channel toward the drain. It turns out that the quantum 1/f parameter "s" that determines the fraction of the two forms of Q1/fE, is no longer increasing proportionally to the width of the device w, when the latter exceeds the length of the channel. A logarithmic dependence on w is obtained for s instead. This is why an extremely large width w does not automatically lead to coherent Q1/fE in HFETs. Conventional Q1/fE applies for the gate insulation, with contributions of the much larger piezoelectric Q1/fE in spontaneously polarized AlGaN, if gate leakage is present. The noise figure is calculated, including all contributions. Finally, the minimal expected oscillator phase noise is calculated from the Q1/fE in the dissipative elements, even for perfectly linear amplifiers, by multiplication with the inverse fourth power of the quality factor, as was first done by us for quartz in 1979.
Accumulation of hot phonons in GaN and related structures
Arvydas Matulionis, Ilona Matulionien?
Interaction of hot electrons with longitudinal optical (LO) phonons and the resultant accumulation of the phonons is investigated in the GaN-based channels subjected to high electric fields. Physical background of microwave noise technique for experimental investigation of hot phonons is described. The technique is applied to the 2DEG and 3DEG channels located in AlGaN/GaN and AlGaN/AlN/GaN heterostructures and silicon-doped GaN. The noise technique has yielded the pioneering result on the hot-phonon lifetime in the standard AlGaN/GaN channel of interest for microwave power transistors; the result is confirmed later by independent time-resolved intersubband absorption technique. At the high electron density, typical for 2DEG channels, the hot-phonon lifetime is independent of the lattice temperature; the lifetime is essentially shorter as compared with the values for bulk GaN at a low electron density. The dependence of lifetime on electron density available from time-resolved Raman hot-phonon lifetime measurements is discussed in terms of plasmon-assisted disintegration of LO-phonon-like quasiparticles launched by high-energy electrons.
Subpicosecond time-resolved Raman studies of LO phonons in GaN
K. T. Tsen, Juliann G. Kiang, D. K. Ferry, et al.
Subpicosecond time-resolved Raman spectroscopy has been used to measure the lifetime of the LO phonon mode in GaN at T = 300K for photoexcited electron-hole pair density ranging from 1016cm-3 to 2x1019 cm-3. The lifetime has been found to decrease from 2.5 ps, at the lowest density to 0.35 ps, at the highest density. Our experimental findings should help resolve the recent controversy over the lifetime of LO phonon mode in GaN.
AlGaN/GaN MODFET regrown by rf-MBE on MOCVD templates
J. Xie, H. Morkoç, L. Zhou, et al.
AlGaN/GaN devices are typically grown on foreign substrates such as SiC and sapphire due to lack of commercial bulk GaN. Metalorganic chemical vapor deposition (MOCVD) is a widely used method for growth of GaN templates for these structures even for other growth methods. Because the growth temperature during molecular beam epitaxy (MBE) is low, dislocation motion is hindered leading to a high dislocation density, particularly pure edge type, when grown directly on foreign substrates. On the other hand, low background doping, sharp interface and well-controlled growth rate allow MBE to grow high performance modulation doped field effect transistor (MODFET) structures on MOCVD GaN templates. However, the regrowth interface in this case has been reported to act as a parallel channel unless Zn-doped GaN templates were used. [J. Appl. Phys. v92 p338] In this paper we report on the control of the regrowth interface of GaN/AlGaN MODFETs by rf-assisted MBE on GaN templates prepared by MOCVD. We have found that the defective parallel channel at the regrowth interface could be effectively eliminated by a proper growth procedure and pre-cleaning using KOH combined with high temperature (800oC) thermal annealing in vacuum. Reflection high energy electron diffraction (RHEED) was used to monitor the interface quality to a first order during the initial growth stages. Electrical and structural properties at the regrowth interface were analyzed by capacitance-voltage (CV) measurements and transmission electron microscopy (TEM). Al0.3Ga0.7N/GaN MODFET structures grown under the optimized conditions exhibited a maximum transconductance of 230 mA/mm for a 1&mgr;m gate length.
AlGaN/GaN MOS transistors using crystalline ZrO2 as gate dielectric
Xing Gu, Natalia Izyumskaya, Vitaly Avrutin, et al.
Epitaxial growth of ZrO2 has been achieved on MOCVD-grown GaN(0001) templates by oxides molecular beam epitaxy using reactive H2O2 for oxygen and organometallic source for Zr. Utilizing a low temperature buffer layer followed by high temperature insitu annealing and high-temperature growth, monoclinic (100)-oriented ZrO2 thin films were obtained. The full width at half maximum of ZrO2 (100) rocking curve was 0.4 arc degree for 30-nm-thick films and the rms roughness for a 5&mgr;m by 5 &mgr;m AFM scan was 4 Å. The employment of epitaxial ZrO2 layer in the AlGaN/GaN heterojunction field effect trasnsistor as a gate dielectric has resulted in the increase of the saturation-current density and pinch-off voltage as well as in near symmetrical gate-drain I-V behavior.
LEDs II
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InGaN/GaN nanocolumn LEDs emitting from blue to red
K. Kishino, A. Kikuchi, H. Sekiguchi, et al.
Self-assembled GaN nanocolumns were grown on sapphire and Si substrates by rf-plasma-assisted molecular-beam-epitaxy, clarifying the growth condition. The nanocolumn crystal showed a highly efficient photoluminescence (PL) emission at the room temperature, which intensity was 4 times stronger than that of a high-quality GaN substrate. InGaN/GaN quantum-disk nanocolumn LEDs were fabricated on n-type (111) Si substrates. For a macroscopic emission area of 500-&mgr;m-diameter, a broad electro-luminescence (EL) emission spectrum extending from the blue to the red region was observed. Microscopic EL measurement was performed for a 3-&mgr;m-diameter detection area, demonstrating a drastic spectral narrowing. In the microscopic EL spectrum, no blue shift of the emission wavelength was observed when the injection current increased. This suggests that the carrier localization or/and the piezo-electric field is minimized in nanocolumns. Selective growth of GaN nanocolumns was performed by use of patterned pre-deposited Al layers.
Nitride-based light-emitting diodes with p-AlInGaN surface layers prepared at various temperatures
C. W. Kuo, C. M. Chen, C. H. Kuo, et al.
We have prepared bulk p-AlInGaN layers and light emitting diodes (LEDs) with p-AlInGaN surface layers by metalorganic chemical vapor deposition (MOCVD). It was found that surfaces of the LEDs with p-AlInGaN layers were rough with high density of hexagonal pits. It was also found that pit width and pit density depend on the growth temperature of the p-AlInGaN layer. Furthermore, it was found that we can achieve 62% enhancement in output intensity from the LED with 820°C p-AlInGaN cap layer without increasing the LED operation voltage.
AlGaN-based deep ultraviolet light emitting diodes with reflection layer
We report on the effect of reflection layer on electrical and optical characteristics of AlGaN-based multiple-quantum well deep-UV flip chip light-emitting diodes. Relatively thick Al metal as a reflector layer was deposited on the p-contact of deep-UV LEDs by e-beam evaporator at a nominal chamber pressure of 2 x 10-6 mtorr. AlGaN-based flip chip deep-UV LEDs using Ni/Au/Al/Ti/Au composite reflection layers exhibited a significant improvement in their optical output power and extraction efficiencies. Efforts have also been involved to improve the reflection properties of the packaged devices without increase their forward voltages. Present results suggest that, for efficient deep-UV flip chip LEDs, the deposited Al-reflection layer should have an optimal thickness of ⩾ 200 nm.
Lasers I
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Recent achievements of AlInGaN based laser diodes in blue and green wavelength
AlInGaN based blue and blue-green LDs were investigated with regard to the characteristics of GaN semiconductor laser diodes. High power, single mode blue LDs with high COD level (~334mW under CW operation at 25°C, kink-free at 150mW) and long lifetime (~10000 hours under CW operation, 50mW 25°C) were achieved. No significant characteristic differences between blue LDs on LEO-GaN/sapphire and GaN substrate were observed. The blue-green LD which has the wavelength of 485 nm was successfully fabricated and demonstrated under CW operation 25°C, while it showed poor performances of LD characteristics compared to those of blue LDs. We believe that the poor performance of blue-green LDs were caused by the piezo-electric effect by lattice mismatch along C-axis of GaN, In fluctuation by lattice mismatch and In solubility limit in InGaN QWs and thermal annealing which was performed during the p-layer growth.
Long lifetime cw InGaN laser diodes by molecular beam epitaxy
M. Kauer, W. S. Tan, S. E. Hooper, et al.
The growth and fabrication of 405 nm InGaN laser diodes by molecular beam epitaxy (MBE) has made rapid progress over the last three years. In 2004, the authors reported the first MBE-grown nitride laser diodes. In mid-2005 the authors then demonstrated room-temperature continuous-wave (cw) operation. This was achieved by significantly reducing the threshold current density to 5.6 kA/cm2 for facet-coated LDs. The lifetime of these first MBE-grown cw lasers was up to 3 minutes, limited by power dissipation. In this paper we report on the progress we have made in reducing operating voltage and power dissipation, enabling a significant increase in laser lifetime. Uncoated 2x1000 &mgr;m ridge waveguide lasers fabricated on freestanding GaN substrates have a continuous-wave (cw) threshold current of 110 mA, corresponding to a threshold current density of 5.5 kA/cm2. For 2x600 &mgr;m laser diodes the minimum threshold current is 70 mA. Cw laser lifetime vs. power dissipation data is presented, with a maximum lifetime of 2.6 hours for the best laser. The lifetime versus power dissipation data shows that the MBE-grown lasers follow a similar trend as lasers grown by metalorganic chemical vapor deposition (MOCVD). We also report length dependence measurements of these long lifetime lasers, with a gain G0 of 2000-2200 cm-1 and an internal loss &agr;i=30-45 cm-1.
Degradation studies of InGaN/GaN heterostructure laser diodes using a Kelvin Force Microscope
We demonstrate the potential of Kelvin Probe Force Microscopy (KPFM) for analyzing degradation effects in GaN-based laser diodes (LDs). Thereby, the surface potential at the mirror facet was measured locally for both, unbiased LDs and LDs exposed to a well-defined current. In the unbiased case, our KPFM measurements demonstrate the impact of aging on the mirror facet, which we attribute to a photon enhanced facet oxidation. In case of an externally applied voltage, the local variation of the Kelvin voltage across the heterostructure layer sequence is analyzed. A clear correlation between macroscopic I-V-characteristics and the microscopic data obtained with the KPFM is found.
Defects
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The influence of alloy disorder and hydrostatic pressure on electrical and optical properties of In-rich InGaN compounds
T. Suski, G. Franssen, A. Kamińska, et al.
We discuss the influence of indium segregation-induced disorder effects in InxGa1-xN alloys. Changes of the transport mechanism between InN and InxGa1-xN with x=0.58 were demonstrated by means of temperature-dependent conductivity measurements. Furthermore, an increase of (i) full width at half maximum of photoluminescence (PL) and (ii) the Stokes shift between PL and absorption was seen for samples approaching an In content of 0.5, which can also be attributed to growing disorder. Hydrostatic pressure dependent PL measurements of In-rich InGaN alloys are diacussed. Due to the fact that PL in InGaN originates from regions with higher-than-average In content, the luminescence pressure coefficient dEE/dp should not be associated with the average In content, but with the In content which is in accordance with the energy of the photon emission. This correction leads to a reduction of the large bowing of dEE/dp (associated with the band gap) which was reported earlier. Furthermore, it is shown that the electron concentration in InN has a significant influence on the measured value of dEE/dp.
Defect studies in HVPE GaN by positron annihilation spectroscopy
We have used positron annihilation spectroscopy to study GaN grown by hydride vapor epitaxy. Our results imply that Ga vacancies in GaN are the dominant intrinsic acceptor defects in n-type nominally undoped pure material. Our experiments also show the universal role of the Ga vacancy as the most important compensating acceptor over four orders of magnitude of intentional oxygen doping. We have been able to identify the in-grown Ga vacancy-related defects as VGa-ON pairs and to determine both the binding energy of the pair as well as the formation energy of the isolated Ga vacancy. The experiments performed in polar homo-epitaxial GaN and non-polar hetero-epitaxial GaN give support for a growth-surface-dependent defect incorporation model. On the other hand, the incorporation of impurities in hetero-epitaxial Ga-polar GaN seems to be dominated by effects related to extended defects such as dislocations. The preliminary results obtained in electron irradiated GaN show that negatively charged isolated Ga vacancies are produced together with non-open volume negative ion-type defects when 2-MeV electrons are used, while neutral N vacancy related defects seem to trap positrons in the material irradiated with 0.45 MeV electrons.
Lanthanide impurity level location in GaN, AlN, and ZnO
A method that has proven succesful in locating the energy levels of divalent and trivalent lanthanide ions (Ce, Pr,..., Eu,...Yb, Lu) in wide band gap inorganic compounds like YPO4 and CaF2 is applied to locate lanthanide levels in the wideband semiconductors GaN, AlN, their solid solutions AlxGa1-xN, and ZnO. The proposed schemes provide a description of relevant optical and luminescence properties of these lanthanide doped semiconductors. Especially, the relation between thermal quenching of Tb3+ emission and the location of the energy levels is explained.
FETs II
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AlGaN/GaN field-plate FETs for microwave power applications
H. Miyamoto, Y. Ando, Y. Okamoto, et al.
This paper describes the performance of AlGaN/GaN Field-Plate FETs and amplifiers for microwave power applications. Recessed-gate FETs with a single field-modulating plate (FP) and advanced dual field-modulating plates (FP's) FP are developed for high-voltage microwave power operation. The developed single FP-FETs exhibited a 230-W CW output power at 2 GHz and a 100-W CW output power at 5 GHz. The developed dual FP-FET provides higher gain, increased linearity and stability since the second FP effectively reduces feedback capacitance. Under a 2.15-GHz W-CDMA modulation scheme, the dual-FP-FET achieved a-state-of-the-art combination of 160-W output power and a 17.5 dB linear gain. The developed amplifier using two device dice for W-CDMA base stations delivers a 370-W peak output power and the amplifier for other L/S band high power applications delivers a pulsed 750-W output power at 2.14 GHz.
Insulator engineering in GaN-based MIS HFETs
Narihiko Maeda, Masanobu Hiroki, Noriyuki Watanabe, et al.
Insulated-gate AlGaN/GaN heterostructure field-effect transistors (HFETs), i.e., GaN-based metal-insulator-semiconductor (MIS) HFETs, have been fabricated that exhibited excellent DC and RF characteristics together with the reduced gate leakage current (Ig). The Al2O3/ Si3N4 bilayer gate insulator was used to simultaneously utilize (i) the high-quality interface between Si3N4 and AlGaN, and (ii) the high resistivity and high dielectric constant of Al2O3. The Ig was less than 10-4 A/mm even at a gate voltage of +3 V. In a device with a gate length of 0.1 &mgr;m, the drain current was 1.30 A/mm, and the cut-off frequency (f&Tgr;) and maximum oscillation frequency (fmax) were 70 and 90 GHz, respectively. Moreover, the deposition effect of insulators on the electrical properties in AlGaN/GaN heterostructures has been examined and theoretically analyzed for Si- and Al-based insulators (Si3N4, SiO2, AlN, and Al2O3), because insulators are commonly used for surface passivation as well as the gate insulator, and hence, clarifying the insulator deposition effect is a fundamental issue in GaN-based HFETs. The increase in the two-dimensional electron gas (2DEG) was observed for all the insulators, and the effect was found to vary among insulators. One result is that Al2O3 was most effective to increase Ns. The results were explained in terms of the change in the potential profile. The band engineering including insulators is proposed to be indispensable for interpreting and designing the device performance, because, through the potential profile change, the essential device parameters are altered such as the source resistance, the channel resistance under the insulated-gate, and its threshold voltage.
Thermal analysis of AlGaN/GaN HFETs using electro-thermal simulation and micro-Raman spectroscopy
Tatsuya Fujishima, Kaoru Inoue, Kenichi Kosaka, et al.
We report on investigation of self-heating effects in AlGaN/GaN HFETs (heterostructure field effect transistors) using numerical simulations and micro-Raman spectroscopy. In the numerical simulations, we used a temperature-dependent thermal conductivity for each constituent material. To reduce the size effect of the device, we added wide thermal diffusion regions to active device region. Both AlGaN/GaN HFETs on sapphire and SiC substrates were studied using both electro-thermal 2D (two-dimensional) simulations and also analytical 3D thermal simulations. Good agreements between the simulated and measured surface temperature distributions have been obtained, which supports the validity of simulation models. The simulated temperature distribution for HFETs on SiC substrates was found to have a much sharper peak than that on sapphire substrates. In addition, the region around the gate edge on the drain side usually showed a maximum temperature for the devices operating at drain voltages less than about 40 V, but this region shifted toward the drain side when the drain voltage was increased up to 50-80 V. These results show that micro-Raman spectroscopy can be used for high-resolution temperature measurements.
Epitaxial growth and characterization of AlGaN/GaN HEMT devices on SiC substrates for RF applications
Ashok K. Sood, Yash R. Puri, Frederick W. Clarke, et al.
GaN / Al1-xGaxN-based hetero-structures have demonstrated a versatility in RF electronic applications which is practically unmatched by any other material system. There are many device structures under consideration for use in RF and Power amplifiers, suitable for both commercial and military applications. In this paper, we will discuss HEMT device design and growth of GaN/AlGaN layers on semi-insulating SiC substrates by MBE and MOCVD. Both of the growth techniques have shown high quality GaN /AlGaN epitaxial layers and have demonstrated very uniform epitaxial layers with high mobility. The MBE growth was carried out using RF Plasma Assisted MBE. The MOCVD growth was performed in a close-coupled showerhead reactor operating at low pressure. All HEMT structures were grown on 2-inch semi-insulating SiC substrates. Several of the HEMT wafers grown by these two growth techniques were characterized in detail using AFM measurements of the surface roughness, and non-destructive characterization via contact-less sheet resistance mapping, optical reflectance, and high-resolution X-ray diffraction. Several of the wafers were fabricated into HEMT devices, and the results on these devices are also presented.
Lasers II
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TM-mode lasing and anisotropic polarization properties of AlGaN multiple quantum well lasers in deep-ultraviolet spectral region
Hideo Kawanishi, Masanori Senuma, Takeaki Nukui
We investigated rolls of crystal-field split-off valence band (CH VB) in the deep-UV laser by measuring a transverse-magnetic-field (TM)-mode polarization of lasing and spontaneous surface and edge emissions above and below the lasing threshold from the AlGaN multiple-quantum-well (MQW) laser at 240.8 nm and the anisotropic optical properties of c-plane AlxGa1-xN multiple quantum wells and m-plane AlxGa1-xN single layer. These results indicate that CH VB plays an important role in (E//c) polarization and anisotropic emissions from AlxGa1-xN with x≧0.5.
Comparison of optical properties of InGaN/GaN/AlGaN laser structures grown by MOVPE and MBE
T. Swietlik, C. Skierbiszewski, R. Czernecki, et al.
Metalorganic vapor phase epitaxy (MOVPE) and plasma assisted molecular beam epitaxy (MBE) were used as alternative techniques to fabricate similar group-III-nitride laser structures. Utilization of high-pressure-grown GaN substrates resulted in reduction of threading dislocation density down to 105 cm-2. Light amplification features of the measured structures were evaluated by means of the variable stripe length method. Maximum peak modal gain values of 180 cm-1 for the MOVPE-grown sample and 315 cm-1 for the MBE-grown one were reached at corresponding pump power of 464 kWcm-2. Temperature-dependent photoluminescence measurements yielded activation energies of 41 meV nad 22 meV for MOVPE- and MBE-grown samples, respectively. Saturation lengths of 350 &mgr;m and 250 &mgr;m determined for MOVPE and MBE structures indicate reduced rate of nonradiative recombination compared to heteroepitaxy on foreign substrates. Differences in nonradiative recombination processes between the investigated structures lead to deviations in threshold for stimulated emission in favor of the MBE-grown sample.
Progress in etched facet technology for GaN and blue lasers
Alfred Schremer, Cristian Stagarescu, Jeff Hwang, et al.
We report recent progress in chemically assisted ion beam etching (CAIBE) of GaN/AlGaN materials leading to improved performance of 405nm blue lasers fabricated with etched mirrors. Using a proprietary Etched Facet Technology (EFT) designed for GaN, we have fabricated ridge lasers in conventional GaN/sapphire material. Typical 3&mgr;m ridge lasers with 600&mgr;m cavity lengths exhibit threshold currents of 150mA with high yield and cross wafer uniformity. This represents a factor of five reduction in threshold current over previous results. Additional processing (such as FIB) was not required to improve the mirror verticality and smoothness as in previous work. Continuing improvements in laser performance are anticipated with further optimization of facet smoothness, laser design, and improved epitaxial material. We are also investigating the benefits of shorter cavity lasers, made feasible by etching, to realize improvements in laser reliability and yield. The yield advantage is based on the concept that shorter cavity devices will intercept fewer defects per device. Combined with EFT advantages like low cost wafer-scale testing and monolithic integration, this is a promising approach for next generation blue lasers for optical storage applications.
High quality UV AlGaN/AlGaN distributed Bragg reflectors and microcavities
Oleg Mitrofanov, S. Schmult, M. J. Manfra, et al.
We demonstrate high-reflectivity crack-free Al0.18Ga0.82N/Al0.8Ga0.2N distributed Bragg reflectors (DBR) and monolithic microcavities grown by molecular beam epitaxy on thick c-axis GaN templates. The elastic strain energy in the epilayer is minimized by compensating the compressive and tensile stress in every period of the DBR structure. A 25 period DBR mirror provides a 26nm-wide stop band centered at 347 nm with the maximum reflectivity higher than 99%. The high-reflectivity DBRs can be used to form high Q-factor monolithic AlGaN/AlGaN microcavities.
High reflectivity ultraviolet distributed Bragg reflector based on AlGaN/AlGaN multilayer
AlGaN/AlGaN distributed Bragg reflectors (DBRs) designed for the ultraviolet spectral region have been attained. The crack-free structures were grown on c-plane sapphire by plasma assisted molecular beam epitaxy (MBE). To minimize the built-in strain in DBRs, a thin buffer layer was used directly on c-plane sapphire. A peak reflectivity of 95% at 381 nm with a 21 nm stop band width was obtained at room temperature (RT) using a 32.5 pairs Al0.7Ga0.3N/Al0.15Ga0.85N DBR. With a driving force for DBRs and emitting regions in wide band gap semiconductor microcavities, such as those based on GaN and ZnO, is the quest for cavity polariton which is the coupled mode between the exciton and photon modes. Moreover, the exploitation of cavity polaritons could be expected in the course of the development of extremely low-threshold optoelectronics devices.
Extended Defects
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Structural characterization of III-nitrides using electron microscopy
David J. Smith, Lin Zhou, Martha R. McCartney
Electron microscopy methods have been used in recent collaborative studies to investigate the defect microstructure of III-nitride materials and devices. An approach based on convergent beam diffraction allowed the elemental composition of pseudomorphic InGaN/GaN quantum well structures to be determined on the nanometer scale. Indium compositional fluctuations in InGaN quantum wells caused local electric field inhomogeneities that seemed to be more pronounced near the onset of InGaN layer growth, suggesting strain relaxation as a strong contributing factor. Relaxed InN quantum dots were invariably associated with threading dislocations in the underlying GaN buffer layer, and the interfacial misfit was accommodated by periodic dislocation arrays. Lateral phase separation in InAlN/GaN heterostructures possibly originating from misfit-strain relaxation at the heterointerface, resulted in the development of a vertical 'honeycomb' structure. The structural and electronic properties of AlGaN/GaN heterostructures grown by molecular beam epitaxy have been correlated with the Al/N flux ratio during nucleation layer growth. Electron microscopy played a central role in contributing to the development of ferromagnetic Cr-doped nitride semiconductors.
Nanoheteroepitaxy of GaN on columnar SiC substrates by metalorganic chemical vapor deposition
Preliminary results on nanoheteroepitaxy of GaN on silicon face (Si-face) and carbon face (C-face) nano-columnar SiC (CSC) by metalorganic chemical vapor deposition (MOCVD) are reported. The CSC substrates are fabricated from standard SiC wafers by photo-enhanced electrochemical etching, with typical diameter of pores around 20nm. Noticeable reduction of threading dislocations (TDs) in GaN is realized on the CSC substrates. On the C-face CSC, GaN nuclei have an inverted pyramidal shape which contains high density of stacking faults (SFs). These SFs block possible extension of TDs into upper portion of the layer. On the Si-face CSC, TDs are annihilated by forming nanoscale TD half-loops over the surface pores. These nanoscale TD loops confine the defective layer in GaN to within ~50 nm thickness from the GaN/CSC interface. High density (~5x108 cm-2) of remnant TDs still presents in GaN grown on CSC, chiefly because the surface damages on CSC were not properly removed before growth.
FETs III
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Characterization of transient behavior of AlGaN/GaN HEMTs
We have studied transient behavior of AlGaN/GaN high electron mobility transistors (HEMTs) using various kinds of measurement techniques such as frequency dispersion of the drain conductance, low-frequency noise, drain current DLTS, and Kelvin probe force microscopy. It has been shown that the frequency dispersion has a correlation with the low-frequency noise. In order to study the transient behavior of the device in more detail, the drain current DLTS was applied to various types of HEMTs. Even though electron-trap-related negative peaks were observed for all types of devices, surface-states-related positive peaks were observed only for devices without Si3N4 passivation film. It has been shown by KFM measurement that the surface potential of the device which was subjected to the gate bias stress increased with time due to the emission of electrons which were captured at the surface states. These phenomena are consistent with the current collapse model where the collapse is caused by the electrons which are injected from the gate electrode and captured at the surface states.
Charge trapping on defects in AlGaN/GaN field effect transistors
Oleg Mitrofanov, M. J. Manfra
The presence of electronic traps in GaN-based devices limits device performance and reliability. Crystallographic defects in the bulk and electronic states on the surface act as trapping centers. We review the trapping phenomena in GaN-based high electron mobility transistors and discuss a characterization method, current transient spectroscopy, applied for trap identification. Probing the charge trapping mechanisms allows us to extract the trap characteristics including the trapping potential, the binding energy of an electron on the trap, and the physical location of the active centers in the device.
Analytical model, simulation, and parameter extraction of AIGaN/GaN HEMT for microwave circuit applications
Hasina F. Huq, Syed K. Islam
In this paper an improved temperature model for AlGaN/GaN high electron mobility transistor (HEMT) is presented. The two-dimensional Gaussian Standing Wave (GSW) equation is used to include the dependence of electron drift velocity on the longitudinal electric field. The effects of channel conductance in the saturation region and the parasitic resistance due to the undoped GaN buffer layer have been included. The effect of both spontaneous and piezoelectric polarization induced charges at the AlGaN/GaN heterointerface has been incorporated. The proposed model is used to determine the output current-voltage characteristics and small-signal microwave parameters of HEMTs. The major tasks of this paper include the establishment of the compact model including the polarization effects and the validation of the analytical results and experimental data with numerical simulator. High fT (10-70 GHz) values and high current levels (~650 mA/mm) are achieved. The calculated critical parameters and the simulation results suggest that the performance of the proposed device degrades at elevated temperatures.
1/f noise in the dark current of GaN QWIPs
Amanda M. Hall, Peter H. Handel
The dark current of Quantum Well Inter-subband Photodetectors is affected by 1/f noise that limits the detectivity. This paper applies for the first time conventional quantum 1/f noise expressions to calculate the expected level of 1/f noise in QWIPs and applies the resulting engineering formulas to the case of GaInAs/InP and GaN/AlGaN QWIPs. Both the collisionless and collision-dominated cases are considered. The elementary process causing the dark current is the transfer of an electron from one well to the neighboring well. This happens under the influence of the applied electric field, and has in general both thermally activated and tunneling components. The larger the applied electric field, the larger is the squared velocity change of the carriers, and the larger is the obtained conventional quantum 1/f effect. The detectivity of the devices is calculated on this basis. Quantum well intersubband photodetectors (QWIPs) can be extended in principle from infrared into the THz region.
LEDs IV
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Light extraction analysis for GaN-based LEDs
Tung-Xian Lee, Kuo-Fong Go, Te-Yuan Chung, et al.
Optical analysis of the light extraction based on Monte-Carlo ray tracing in high-efficiency GaN-based LEDs with use of textured structure is presented. The simulation indicates that the introduction of micro pyramid array with slanted surfaces can effectively improve the light extraction efficiency. In addition, the light extraction of three types of LED with or without an epoxy lens is analyzed, where the patterned substrate with pyramid structure arrays is shown most effective way to increase light extraction efficiency in an encapsulated LED with an epoxy lens.
Convergence of optical spectroscopic system for characterization of InGaN/GaN multi-quantum well light-emitting diodes
We present a converged spectroscopic system design for performing photoreflectance (PR), electroreflectance (ER), electroluminescence (EL), photoluminescence (PL) and photovoltage (PV) measurements of semiconductors. The design of the experimental setup is described in detail. To test the performance of the system, measurements of a series of InxGa1-xN/GaN light emitting semiconductor with different indium composition of InGaN layer are carried out by use of this system. The experimental reflection and luminescence spectra are analyzed and discussed. The experimental results demonstrate the performance of this system. Optical and electrical properties of In0.15Ga0.85N/GaN multi-quantum well (MQW) light-emitting diodes (LEDs) with different quantum well (QW) thicknesses were investigated by electric-field dependent ER spectroscopy. From the ER measurements, we have observed the well-resolved transition peaks related to InGaN QW. Furthermore, the transitions related to yellow luminescence (YL) from Si-doped GaN and blue luminescence (BL) from Mg-doped GaN were observed in the ER spectra of In0.15Ga0.85N/GaN MQW LEDs. With increasing QW thickness, the additional transitions related to InGaN QW can be attributed to the recombination of excitons localized at the shallow potential states in InGaN QW, originating from the In-poor InGaN regions caused by indium phase separation in InGaN QW. By applying a reverse bias voltage, the ER features related to InGaN QW were shifted to higher energy, resulting from the reduction of quantum confined Stark effect in InGaN QW with increasing reverse bias voltage. On the other hand, the ER features from YL and BL band related to the deep and the shallow impurity state exhibit redshift and broaden with reverse bias voltage. These results can be attributed to the reduction of Coulomb interaction between donor and acceptor caused by the increase of depletion regions with increasing reverse bias voltage.
Confocal scanning electroluminescence spectro-microscope for multidimensional light-emitting property analysis
S. Hong, G. Onushkin, J. S. Park, et al.
We report new type of micro-EL instrument and its applications for light emitting devices. Our new micro-EL, so-called confocal scanning electroluminescence sprctro-microscope (CSESM) has not only fast image acquisition time but also high image resolution. The newly developed CSESM is combined with confocal laser scanning photoluminescence micsoscope, i.e. micro-PL. Therefore, micro-EL distribution can be directly matched with micro-PL and mechanical chip structure of LED. It is fruitful for providing a fast and non-destructive method to analyze the homogeneity of LEDs in its completely proceeded form. Using this apparatus, we study local intensity and wavelength distribution of electroluminescence for InGaN/GaN blue LED chip. Our results represent that local fluctuations of electroluminescence intensity and wavelength position are closely connected with the fluctuation of local current density, i.e. current spreading features on LED chips.