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Ü. Özgür

Virginia Commonwealth University

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H. Morkoç̌

Virginia Commonwealth University

109

V. Avrutin

Virginia Commonwealth University

X. Ni

University of Chinese Academy of Sciences

H. Morkoç

Virginia Commonwealth University

Jacob H. Leach

Kyma Technologies (United States)

Henry O. Everitt

Duke University

X. Li

Dalian University of Technology

Serdal Okur

Princeton Optronics (United States)

Kai Ding

Shanghai University

Y. Fu

Ankang University

Cooperative Institutions

Virginia Commonwealth University

161

Kyma Technologies (United States)

Vilnius University

Arizona State University

North Carolina State University

Centre National de la Recherche Scientifique

Center for Physical Sciences and Technology

University of South Carolina

Beijing University of Technology

Chinese Academy of Sciences

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Terahertz oscillations in gallium nitride quantum-well channels predicted by hot-electron noise temperature behavior at microwave frequency

Journal of Applied Physics (2020)

M. Ramonas J. Liberis Artūr Šimukovič E. Šermukšnis A. Matulionis

The almost periodic streaming motion of accelerated electrons under moderate electric fields coupled with almost periodic emission of longitudinal optical (LO) phonons is studied in a gallium nitride quantum-well—a promising pathway for terahertz (THz) oscillations. The optimal conditions for the LO-phonon-terminated streaming depend, among others, on the density of the electron gas, the low-field electron mobility, the lattice temperature, and the electric field in a very specific way. The present manuscript exploited the electron noise temperature measured at an X band frequency as a marker for the oscillations at THz frequencies. The idea was tested on a deterministic model for a GaN two-dimensional electron gas (2DEG) through calculation of the electron noise temperature spectra in the Langevin approach for the frequency range from 1 GHz to 10 THz. The noise temperature at 10 GHz was found to be in a strong anticorrelation with the THz peaks in the noise temperature spectrum. In particular, a weaker dependence on the applied electric field at 10 GHz implies stronger THz oscillations. In an experiment, the microwave hot-electron noise measurements were carried out for AlGaN/AlN/GaN heterostructures with the 2DEG channel at 10 GHz under pulsed electric field conditions in order to mitigate the effect of Joule heating of the channel. The plateau-like behavior of the noise temperature, in its dependence on the electric field, was obtained for the 2DEG channels with rather low electron densities (2.5×1012 cm−2) in a good agreement with the model. The aforementioned plateau in the electron noise temperature observed at 10 GHz can be used as an indicator for the THz oscillations.

10.1063/5.0015257

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Molecular beam epitaxy of Zn-polar ZnO/BeMgZnO heterostructure field effect transistors on GaN and c-sapphire: a comparative study

Kai Ding V. Avrutin N. Izyumskaya Ü. Özgür H. Morkoç̌

Previously we have conducted researches on the molecular beam epitaxy (MBE) of Zn-polar BeMgZnO/ZnO heterostructure field effect transistor (HFET) structures on GaN templates, which exhibit both high two-dimensional gas (2DEG) density and high electron mobility with relatively low Mg contents via the strain modulation of Be and Mg co-incorporation. In this contribution, we report on the growth of the HFET structures directly on sapphire substrate by employing a buffer consisting of a rock-salt structure MgO layer, a low-temperature (LT) ZnO layer and a high-temperature (HT) ZnO layer. Compared with growth of O-polar on sapphire, in which a thin and wurtzite MgO buffer is deposited at high temperatures above 700 °C, the MgO buffer for Zn-polar growth has to be reduced to 450 °C, in order to obtain smooth interface and surface for the BeMgZnO/ZnO HFETs. The residual electron sheet concentration in Zn-polar ZnO layers is ~2×10¹² cm^-2 on GaN while semi-insulating Zn-polar ZnO layers on sapphire have been obtained via controlling the buffer growth conditions, which is vital to the realization of HFET device structures.

Wurtzite crystal structure

Electron Mobility

Wide-bandgap semiconductor

10.1117/12.2550534

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Electron mobility in InGaN channel heterostructure field effect transistor structures with different barriers

Applied Physics Letters (2007)

Jun Xie Jacob H. Leach X. Ni M. Wu Rintaro Shimada

InGaN possesses higher electron mobility and velocity than GaN, and therefore is expected to lead to relatively better performances for heterostructure field effect transistors (HFETs). However, the reported mobilities for AlGaN∕InGaN HFETs are lower than GaN channel HFETs. To address this issue, we studied the effect of different barriers on the Hall mobility for InGaN channel HFETs grown by metal organic chemical vapor deposition. Unlike the conventional AlGaN barrier, the AlInN barrier can be grown at the same temperature as the InGaN channel layer, alleviating some of the technological roadblocks. Specifically, this avoids possible degradation of the thin InGaN channel during AlGaN growth at high temperatures; and paves the way for better interfaces. An undoped In0.18Al0.82N∕AlN∕In0.04Ga0.96N HFET structure exhibited a μH=820cm2∕Vs, with a ns=2.12×1013cm−2 at room temperature. Moreover, with an In-doped AlGaN barrier, namely, Al0.24In0.01Ga0.75N, grown at 900°C, the μH increased to 1230cm2∕Vs with a ns of 1.09×1013cm−2 for a similar InGaN channel. Furthermore, when the barrier was replaced by Al0.25Ga0.75N grown at 1030°C, μH dropped to 870cm2∕Vs with ns of 1.26×1013cm−2 at room temperature. Our results suggest that to fully realize the potential of the InGaN channel HFETs, AlInN or AlInGaN should be used as the barrier instead of the conventional AlGaN barrier.

Barrier layer

Electron Mobility

Wide-bandgap semiconductor

10.1063/1.2824461

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Citations (45)

Impact of ballistic electron transport on efficiency of InGaN based LEDs

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE (2011)

Fan Zhang X. Li S. Liu Serdal Okur V. Avrutin

InGaN light emitting diodes (LEDs), which have become key components of the lighting technology owing to their improved power conversion efficiencies and brightness, still suffer from efficiency degradation at high injection levels. Experiments showing sizeable impact of the barrier height provided by an electron blocking layer (EBL) or the electron cooling layer prior to electron injection into the active region strongly suggest that the electron overflow resulting from ballistic and quasi-ballistic transport is the major cause of efficiency loss with increasing injection. Our previous report using a first order simple overflow model based on hot electrons and constant LO phonon scattering rates describes well the experimental observations of electron spillover and the associated efficiency degradation in both nonpolar m-plane and polar c-plane LEDs with different barrier height EBLs and electron injection layers. LEDs without EBLs show three to five times lower efficiencies than those with Al_0.15Ga_0.85N EBLs due to significant electron overflow to the p-type region in the former. For effective means of thermalization in the active region within their residence time and possibly longitudinal optical phonon lifetime, the electrons were cooled prior to their injection via a staircase electron injector, i.e. an InGaN staircase structure with step-wise increased In composition. The investigated m-plane and c-plane LEDs with incorporation of staircase electron injector show comparable electroluminescence performance regardless of the status of EBL. This paper discusses hot electron effects on efficiency loss, means to cool the electrons prior to injection.

Indium gallium nitride

10.1117/12.875806

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Citations (3)

Effectiveness of TiN porous templates on the reduction of threading dislocations in GaN overgrowth by organometallic vapor-phase epitaxy

Applied Physics Letters (2005)

Y. Fu Y. T. Moon Feng Yun Ü. Özgür Jun Xie

Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Twitter Facebook Reddit LinkedIn Tools Icon Tools Reprints and Permissions Cite Icon Cite Search Site Citation Y. Fu, Y. T. Moon, F. Yun, Ü. Özgür, J. Q. Xie, S. Doğan, H. Morkoç, C. K. Inoki, T. S. Kuan, Lin Zhou, David J. Smith; Effectiveness of TiN porous templates on the reduction of threading dislocations in GaN overgrowth by organometallic vapor-phase epitaxy. Appl. Phys. Lett. 24 January 2005; 86 (4): 043108. https://doi.org/10.1063/1.1849833 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentAIP Publishing PortfolioApplied Physics Letters Search Advanced Search |Citation Search

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10.1063/1.1849833

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Citations (57)

Excitonic fine structure and recombination dynamics in single-crystallineZnO

Physical Review B (2004)

Ali Teke Ü. Özgür S. Doğan Xing Gu H. Morkoç

The optical properties of a high quality bulk $\mathrm{ZnO}$, thermally post treated in a forming gas environment are investigated by temperature dependent continuous wave and time-resolved photoluminescence (PL) measurements. Several bound and free exciton transitions along with their first excited states have been observed at low temperatures, with the main neutral-donor-bound exciton peak at $3.3605\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ having a linewidth of $0.7\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ and dominating the PL spectrum at $10\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. This bound exciton transition was visible only below $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, whereas the A-free exciton transition at $3.3771\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ persisted up to room temperature. A-free exciton binding energy of $60\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ is obtained from the position of the excited states of the free excitons. Additional intrinsic and extrinsic fine structures such as polariton, two-electron satellites, donor-acceptor pair transitions, and longitudinal optical-phonon replicas have also been observed and investigated in detail. Time-resolved PL measurements at room temperature reveal a biexponential decay behavior with typical decay constants of $\ensuremath{\sim}170$ and $\ensuremath{\sim}864\phantom{\rule{0.3em}{0ex}}\mathrm{ps}$ for the as-grown sample. Thermal treatment is observed to increase the carrier lifetimes when performed in a forming gas environment.

10.1103/physrevb.70.195207

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Citations (690)

Epitaxial lateral overgrowth of (11¯00) m -plane GaN on m -plane 6H-SiC by metalorganic chemical vapor deposition

Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE (2008)

X. Ni Ü. Özgür Serguei Chevtchenko Jia-Cai Nie H. Morkoç̌

Planar m-plane GaN was grown on (11¯00) m-plane 6H-SiC substrates using high-temperature AlN nucleation layers by metalorganic chemical vapor deposition. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images showed striated features on the sample surface aligned along the GaN [11¯20] direction, which are perpendicular to those associated with a-plane (11¯20) GaN. The epitaxial relationship between the m-GaN and 6H-SiC was analyzed using high-resolution x-ray diffraction (XRD). In order to reduce the defect density, epitaxial lateral overgrowth (ELO) was carried out on an m-GaN template with mask stripes along the GaN [11¯20] direction, which makes the lateral growth fronts advance along the GaN c-axis. On-axis XRD rocking curves show that the full width at half maximum (FWHM) values for the ELO samples were reduced by nearly half when compared to those of the m-plane template without ELO. Clear atomic steps were observed in the wing regions by AFM. The absence of the striated features that are associated with the template could be indicative of the reduction of basal stacking faults in the ELO wings. Lowtemperature photoluminescence (PL) spectra showed an excitonic emission at 3.47eV, a basal stacking fault (BSF)- related emission at 3.41 eV, and other defect-related emissions at 3.29 eV and 3.34 eV. For exact notation please see manuscript

Stacking fault

Wide-bandgap semiconductor

10.1117/12.763848

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Citations (1)

Control of Coherent Acoustic Phonons in InGaN Multiple Quantum Wells

arXiv (Cornell University) (2000)

Ü. Özgür Chang‐Won Lee Henry O. Everitt

Using sub-picosecond optical pump-probe techniques, coherent zone-folded longitudinal acoustic phonons (ZFLAPs) were generated and controlled in an InGaN multiple quantum well structure. A one-pump, one-probe differential transmission technique revealed that carriers injected near the barrier band edge were quickly captured into the quantum wells and generated strong coherent ZFLAP oscillations. Two-pump differential transmission was used to generate and control coherent ZFLAP oscillations through the relative timing and amplitude of the two pump pulses. Enhancement and suppression of ZFLAP oscillations were demonstrated, including complete cancellation of generated acoustic phonons for the first time in any material system. Coherent control was used to demonstrate that ZFLAPs are generated differently in InGaN multiple quantum wells than in GaAs/AlAs superlattices.

Coherent control

Picosecond

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Impact of active layer design on InGaN radiative recombination coefficient and LED performance

Journal of Applied Physics (2012)

X. Li Serdal Okur Fan Zhang V. Avrutin Ü. Özgür

The relative roles of radiative and nonradiative processes and the polarization field on the light emission from blue (∼425 nm) InGaN light emitting diodes (LEDs) have been studied. Single and multiple double heterostructure (DH) designs have been investigated with multiple DH structures showing improved efficiencies. Experimental results supported by numerical simulations of injection dependent electron and hole wavefunction overlap and the corresponding radiative recombination coefficients suggest that increasing the effective active region thickness by employing multiple InGaN DH structures separated by thin and low barriers is promising for LEDs with high efficiency retention at high injection. The use of thin and low barriers is crucial to enhance carrier transport across the active region. Although increasing the single DH thickness from 3 to 6 nm improves the peak external quantum efficiency (EQE) by nearly 3.6 times due to increased density of states and increased emitting volume, the internal quantum efficiency (IQE) suffers a loss of nearly 30%. A further increase in the DH thickness to 9 and 11 nm results in a significantly slower rate of increase of EQE with current injection and lower peak EQE values presumably due to degradation of the InGaN material quality and reduced electron-hole spatial overlap. Increasing the number of 3 nm DH active regions separated by thin (3 nm) In0.06Ga0.94N barriers improves EQE, while maintaining high IQE (above 95% at a carrier concentration of 1018 cm−3) and showing negligible EQE degradation up to 550 A/cm2 in 400 × 400 μm2 devices due to increased emitting volume and high radiative recombination coefficients and high IQE. Time-resolved photoluminescence measurements revealed higher radiative recombination rates with increasing excitation due to screening of the internal field and enhanced electron and hole overlap at higher injection levels. To shed light on the experimental observations, the effect of free-carrier screening on the polarization field at different injection levels and the resulting impact on the quantum efficiency were investigated by numerical simulations.

Quantum Efficiency

Active layer

Wide-bandgap semiconductor

10.1063/1.3699199

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Citations (32)

Index

Prof. Dr. Hadis Morkoç Ü. Özgür

3d-states 281 configuration 281 4s-states 281 configuration 281 a ab initio total energy calculations 312, 314 ab initio treatments 309 absorption coefficient 135, 145, 149-151, 161 absorption/emission processes 135 absorption rate 149 acceptor binding energy 254 acceptor concentration 255 acoustic/optical phonon modes 38 acoustic phonon modes 34 acoustic phonon scattering 64 acoustic phonon wave equations 36 acoustic velocity 449 acoustic waves 37 devices 447 longitudinal 37 propagation 36 transverse 37 A-free exciton 176 temperature-dependent peak positions 176 Ag Schottky contacts 407 Al-doped ZnO (AZO) 421, 443 aluminum-titanium oxide (ATO) 443 angle-of-minimum deviation methods 190 angle-resolved photoemission spectroscopy (ARPES) 14, 15, 17, 20 angular dependent magneto-optical spectroscopy 156 angular dispersive X-ray diffraction 29 angular momentum 280 anisotropic exchange 305 anomalous Hall coefficient 302 anticrossing 426 antiferromagnetic 285 antiferromagnetism 287 antisites 251 a-plane sapphire 107 atom positions for basal ZnO grown arsenic-doped BeZnO 417 Au Schottky contacts 407 average energy per mode 140 average lifetime 151 AX center 252 b backscattering geometry 41 bandgap 351 engineering 351 pressure dependence 22 temperature dependence 22 band structure 20, 25 band-to-band model 300 band-to-band transitions 151 barrier height (f B ) 388 basal stacking faults 84 Becke three parameter Lee-Yang-Parr (B3LYP) hybrid density functional method 316, 317 Be x Zn 1Àx O alloy 359 biexciton 202 binding energies 202 emission 202 spectral shape 202 biexponential decay 209 bimolecular process 151 bio sensors 453, 454 Bi 1.5 Zn 1.0 Nb 1.5 O 7 (BZN) 445 black body radiation 140 Blombergen-Rowland (BR) interactions mechanism 305-307 Bohr magneton(s) 291 Bohr radius 157 Boltzmann transport 65 bond angles 3 j469

Commonwealth

DEPT

10.1002/9783527623945.index

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Citations (1)