bluuebunny/crossref_metadata_2025 huggingface dataset:
High-Gain, High-Speed ZnO MSM Ultraviolet Photodetectors
Authors: Shen H. , Wraback M. , Gorla C. R. , Liang S. , Emanetoglu N. , Liu Y. , Lu Y.
Published: September 2012

High quality zinc oxide (ZnO) films were epitaxially grown on R-plane sapphire substrates by metalorganic chemical vapor deposition at temperatures in the range 350-600°C. In-situ nitrogen compensation doping was performed using NH3. The metalsemiconductor-metal ultraviolet-sensitive photodetectors were fabricated on nitrogencompensation-doped epitaxial ZnO films. The photoresponsivity of these devices exhibits a linear dependence upon bias voltage up to 10 V, with a photoresponsivity of 400 A/W at 5 V. The rise and fall times are 1 and 1.5 μs, respectively.
Electron Beam Pumping in Nitride Vertical Cavities with GaN/ Al0.25 Ga0.75 N Bragg Reflectors
Authors: Klausing H. , Aderhold J. , Fedler F. , Mistele D. , Stemmer J. , Semchinova O. , Graul J. , Dänhardt J. , Panzer S.
Published: September 2012

Electron beam pumped surface emitting lasers are of great interest for a variety of applications, such as Laser Cathode Ray Tubes (LCRT) in projection display technology or high power UV light sources for photolithography.

Two distributed Bragg reflector (DBR) samples were grown by plasma assisted molecular beam epitaxy (PAMBE). The active regions of the samples are a GaN:Si bulk layer and a multihetero (MH) structure, respectively.

Also, a separately grown single DBR stack was studied to find optical transmission and reflection properties which were compared to transfer matrix simulations.

Scanning electron beam pumping at 80 K with an excitation energy of 40 keV at varying beam currents revealed luminescence emission maxima located at about 3.45 eV for the sample with the MH structure active region. Optical modes appeared for excitation powers greater than 0.85 MW/ cm2. Further increasing the excitation power density the number of modes increased and a broadening and redshift of the luminescence spectrum could be observed.

Based on our experimental results, we discuss the dependence of optical parameters of the nitride vertical cavity and sample surface reactions on primary electron beam power.
Microstructure-Based Lasing in GaN/AlGaN Separate Confinement Heterostructures
Authors: Bidnyk S. , Lam J. B. , Little B. D. , Gainer G. H. , Kwon Y. H. , Song J. J. , Bulman G. E. , Kong H. S.
Published: September 2012

We report on an experimental study of microstructure-based lasing in an optically pumped GaN/AlGaN separate confinement heterostructure (SCH). We achieved low-threshold ultra-violet lasing in optically pumped GaN/AlGaN separate confinement heterostructures over a wide temperature range. The spacing, directionality, and far-field patterns of the lasing modes are shown to be the result of microcavities that were naturally formed in the structures due to strain relaxation. The temperature sensitivity of the lasing wavelength was found to be twice as low as that of bulk-like GaN films. Based on these results, we discuss possibilities for the development of ultra-violet laser diodes with increased temperature stability of the emission wavelength.
Sign of the Piezoelectric Field in Asymmetric GaInN/AlGaN/GaN Single and Double Quantum Wells on SiC
Authors: Im Jin Seo , Hangleiter A. , Off J. , Scholz F.
Published: September 2012

We study both GaInN/GaN/AlGaN quantum wells with an asymmetric barrier structure grown on SiC substrate and GaN/AlGaN asymmetric double quantum well (ADQW) structures. In the first case, a time-resolved study reveals an enhanced oscillator strength when the AlGaN barrier is on top of the GaInN quantum well. In comparison to our previous study of the same structure grown on sapphire, we find that the sign of the field is the same in both cases: the field points towards the substrate. In the case of ADQW, we observed not only intrawell transitions of both a 4 nm and a 2 nm QW separated by a 2.5 nm AlGaN barrier but also an interwell transition between the two QWs in the photoluminescence. The lifetimes and emission energies of the transitions can be well explained by the existence of the piezoelectric field built in the QWs.
Pulsed-Laser-Deposited AlN Films for High-Temperature SiC MIS Devices
Authors: Vispute R. D. , Patel A. , Baynes K. , Ming B. , Sharma R. P. , Venkatesan T. , Scozzie C. J. , Lelis A. , Zheleva T. , Jones K. A.
Published: September 2012

We report on the fabrication of device-quality AlN heterostructures grown on SiC for high-temperature electronic devices. The AlN films were grown by pulsed laser deposition (PLD) at substrate temperatures ranging from 25 °C (room temperature) to 1000 °C. The as-grown films were investigated using x-ray diffraction, Rutherford backscattering specttroscopy, ion channeling, atomic force microscopy, and transmission electron microscopy. The AlN films grown above 700 °C were highly c-axis oriented with rocking curve FWHM of 5 to 6 arc-min. The ion channeling minimum yields near the surface region for the AlN films were ∼2 to 4%, indicating their high degree of crystallinity. TEM studies indicated that AlN films were epitaxial and single crystalline in nature with a large number of stacking faults as a results of lattice mismatch and growth induced defects. The surface roughness for the films was about 0.5 nm, which is close to the unit cell height of the AlN. Epitaxial TiN ohmic contacts were also developed on SiC, GaN, and AlN by in-situ PLD. Epitaxial TiN/AlN/SiC MIS capacitors with gate areas of 4 * 10-4 cm2 were fabricated, and high-temperature current-voltage (I-V) characteristics were studied up to 450 °C. We have measured leakage current densities of low 10-8 A/cm2 at room temperature, and have mid 10-3 A/cm2 at 450°C under a field of 2 MV/cm.
The Formation of In-Rich Regions at the Perphery of the Inverted Hexahonal Pits of Ingan Thin-Films Grown by Metalorganic Vapor Phase Epitaxy
Authors: Li P. , Chua S. J. , Hao M. , Wang W. , Zhang X. , Sugahara T. , Sakai S.
Published: September 2012

InGaN thin films were grown by low-pressure metalorganic-vapor-phase-epitaxy (MOVPE) and characterized by cathodoluminescence (CL) and scanning electron microscopy (SEM). SEM images showed that InGaN samples have inverted hexagonal pits which are formed by the In segregation on the (1011) surfaces. Room temperature CL at the wavelengths corresponding to the GaN band edge, the In-poor and In-rich regions showed that the In–rich regions formed at the periphery of the hexagonal pits.
Emission Enhancement of GaN/AlGaN Single-Quantum-Wells Due to Screening of Piezoelectric Field
Authors: Kinoshita A. , Hirayama H. , Riblet P. , Ainoya M. , Hirata A. , Aoyagi Y.
Published: September 2012

Photoluminescence (PL) enhancement due to the screening of piezoelectric field induced by Si-doping is systematically studied in GaN/AlGaN quantum wells (QWs) fabricated by metal organic vapor-phase-epitaxy (MOVPE). The PL enhancement ratio of QWs for Si-doped directly into the wells was much larger than that for doped only into the barrier layers. This result shows that the crystal quality of the quantum well is not so damaged by heavy Si-doping, which is different from the cases of GaAs or InP material systems. The PL intensity enhancement ratio was especially large for thick wells. The typical value of the enhancement ratio was 30 times for a 5 nm-thick single QW. The optimum Si-doping concentration was approximately 4×1018 cm-3. From the well width dependence of the PL enhancement ratio and PL peak shift under high excitation conditions, we determined that the dominant effect inducing the PL enhancement is screening of piezoelectric field in the QWs. These results indicate that Si-doping is very effective for the application of GaN/AlGaN QWs to optical devices.
Correlation between Structural Properties and Optical Amplification in InGaN/GaN Heterostructures Grown by Molecular Beam Epitaxy
Authors: Kaschner A. , Holst J. , von Gfug U. , Hoffmann A. , Bertram F. , Riemann T. , Rudloff D. , Fischer P. , Christen J. , Averbeck R. , Riechert H.
Published: September 2012

We comprehensively studied InGaN/GaN heterostructures grown by molecular beam epitaxy (MBE) using a variety of methods of optical spectroscopy, such as cathodoluminescence microscopy (CL), time-integrated and time-resolved photoluminescence. To correlate the fluctuations in emission wavelength with values for the optical amplification we performed gain measurements in edge-stripe geometry. The lateral homogeneity can be drastically improved using a template of GaN grown on the sapphire substrate by metal-organic vapor phase epitaxy (MOVPE). Gain values up to 62 cm-1 were found in samples with low indium fluctuations, which is comparable to values for high-quality InGaN/GaN heterostructures grown by MOVPE.
Optical Properties of AlGaN Quantum Well Structures
Authors: Hirayama Hideki , Enomoto Yasushi , Kinoshita Atsuhiro , Hirata Akira , Aoyagi Yoshinobu
Published: September 2012

We demonstrate 230-250 nm efficient ultraviolet (UV) photoluminescence (PL) from AlN(AlGaN)/AlGaN multi-quantum-wells (MQWs) fabricated by metalorganic vapor-phase-epitaxy (MOVPE). Firstly, we show the PL properties of high Al content AlGaN bulk (Al content: 85-95%) emitting from near band-edge. We systematically investigated the PL properties of AlGaN-MQWs consisting of wide bandgap AlGaN (Al content: 53-100%) barrier. We obtained efficient PL emission of 234 and 245 nm from AlN/Al0.18Ga0.82N and Al0.8Ga0.2N/Al0.18Ga0.82N MQWs, respectively, at 77 K. The optimum value of well thickness was approximately 1.5 nm. The emission from the AlGaN MQWs were several tens of times stronger than that of bulk AlGaN. We found that the most efficient PL is obtained at around 240 nm from AlGaN MQWs with Al0.8Ga0.2N barriers. Also, we found that the PL from AlGaN MQW is as efficient as that of InGaN QWs at 77 K.
Characterization of InGaN Quantum Wells Grown by Molecular Beam Epitaxy (MBE) Using Ammonia as the Nitrogen Source
Authors: Semendy F. , Li L.K. , Jurkovic M.J. , Wang W.I.
Published: September 2012

Single quantum well InGaN was grown by molecular beam epitaxy with ammonia as the nitrogen source. The samples were grown on (0001) sapphire substrates. The photoluminescence (PL) intensity of InGaN quantum wells showed band-edge emissions at 2.71eV at low temperature (10 K). PL was investigated as a function of excitation intensity and temperature. The relationship between PL intensity and excitation intensity, as well as the relationship between PL intensity and lattice temperature was studied. Also studied was the combined effect of temperature and intensity variation. Detailed results are reported here.
Comparative Study of Structural Properties and Photoluminescence in InGaN Layers with a High in Content
Authors: Vantomme A. , Wu M.F. , Hogg S. , Langouche G. , Jacobs K. , Moerman I. , White M.E. , O'Donnell K.P. , Nistor L. , Landuyt J. Van , Bender H.
Published: September 2012

Rutherford backscattering and channeling spectrometry (RBS), photoluminescence (PL) spectroscopy and transmission electron microscopy (TEM) have been used to investigate macroscopic and microscopic segregation in MOCVD grown InGaN layers. The PL peak energy and In content (measured by RBS) were mapped at a large numberof distinct points on the samples. An indium concentration of 40%, the highest measured in this work, corresponds to a PL peak of 710 nm, strongly suggesting that the lightemitting regions of the sample are very indium-rich compared to the average measured by RBS. Cross-sectional TEM observations show distinctive layering of the InGaN films. The TEM study further reveals that these layers consist of amorphous pyramidal contrast features with sizes of order 10 nm. The composition of these specific contrast features is shown to be In-rich compared to the nitride matrix.
Optical Spectroscopy of Ingan Epilayers in the Low Indium Composition Regime
Authors: Crawford M. H. , Han J. , Banas M. A. , Myers S. M. , Petersen G. A. , Figiel J. J.
Published: September 2012

Photoluminescence (PL) spectroscopy was carried out on a series of Si-doped bulk InGaN films in the low indium (In) composition regime. Room temperature PL showed a factor of 25 increase in integrated intensity as the In composition was increased from 0 to 0.07. Temperature dependent PL data was fit to an Arrhenius equation to reveal an increasing activation energy for thermal quenching of the PL intensity as the In composition is increased. Time resolved PL measurements revealed that only the sample with highest In ( x=0.07) showed a strong spectral variation in decay time across the T=4K PL resonance, indicative of recombination from localized states at low temperatures. The decay times at room temperature were non-radiatively dominated for all films, and the room temperature (non-radiative) decay times increased with increasing In, from 50-230 psec for x=0-0.07. Our data demonstrate that non-radiative recombination is less effective with increasing In composition.
The Use of Micro-Raman Spectroscopy to Monitor High-Pressure High-Temperature Annealing of Ion-Implanted GaN Films
Authors: Kuball M. , Hayes J.M. , Suski T. , Jun J. , Tan H.H. , Williams J.S. , Jagadish C.
Published: September 2012

We have investigated the high-pressure high-temperature annealing of Mg/P-implanted GaN films using visible and ultraviolet (UV) micro-Raman spectroscopy. The results illustrate the use of Raman spectroscopy to monitor processing of GaN where fast feedback is required. The structural quality and the stress in ion-implanted GaN films was monitored in a 40nm-thin surface layer of the sample as well as averaged over the sample layer thickness. We find the nearly full recovery of the crystalline quality of ion-implanted GaN films after annealing at 1400-1500°C under nitrogen overpressures of 1.5GPa. No significant degradation effects occurred in the GaN surface layer during the annealing. The high nitrogen overpressures proved very effective in preventing the nitrogen out-diffusion from the GaN surface. Stress introduced during the annealing was monitored. Raman spectra of ion-implanted GaN films were investigated at different temperatures and excitation wavelengths to study the GaN phonon density of states.
A Study of Annealed GaN Grown by Molecular Beam Epitaxy Using Photoluminescence Spectroscopy.
Authors: Bell Abigail , Harrison Ian , Korakakis Dimitris , Larkins Eric C. , Hayes J. M. , Kuball M.
Published: September 2012

Photoluminescence (PL) spectroscopy has been used to investigate the effect annealing has on molecular beam epitaxially grown GaN in different ambients. By observing the changes in the PL spectra as a function of ambient temperature and atmosphere used, important information concerning the origin of defects within GaN has been found. Samples were annealed in different atmospheres, (including oxygen, oxygen and water vapour, nitrogen and argon), different temperatures. In the 2.0eV-2.8eV region of the PL spectra, two peaks appeared at approximately 2.3eV and 2.6eV, somewhat higher than the usual yellow luminescence peak. We find that the 2.6eV peak is dominant for high annealing temperatures and the 2.3eV peak dominates at low annealing temperatures for the samples annealed in oxygen. When annealed in argon and nitrogen the 2.6eV peak dominates at all annealing temperatures. Changes in the PL spectra between anneals were also seen in the 3.42eV region. The 3.42eV peak is often assigned to excitons bound to stacking faults. Power resolved measurements indicate that in our sample the cause is a donor acceptor pair transition.
Nonlinear Optical Characterization of GaN Layers Grown by MOCVD on Sapphire
Authors: Tiginyanu Ivan M. , Kravetsky Igor V. , Pavlidis Dimitris , Eisenbach Andreas , Hildebrandt Ralf , Marowsky Gerd , Hartnagel Hans L.
Published: September 2012

Optical second and third harmonic generation measurements were carried out on GaN layers grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates. The measured d33 is 33 times the d11 of quartz. The angular dependence of second-harmonic intensity as well as the measured ratios d33/d15= -2.02 and d33/d31 = -2.03 confirm the wurzite structure of the studied GaN layers with the optical c-axis oriented perpendicular to the sample surface. Fine oscillations were observed in the measured second and third harmonic angular dependencies. A simple model based on the interference of the fundamental beam in the sample was used to explain these oscillations.
Picosecond Photoinduced Reflectivity Studies of GaN Prepared by Lateral Epitaxial Overgrowth
Authors: Wraback M. , Shen H. , Eiting C.J. , Carrano J.C. , Dupuis R.D.
Published: September 2012

The pump-probe technique has been used to perform room temperature studies of the photoinduced changes in the reflectivity ΔR associated with exciton and carrier dynamics in GaN prepared by lateral epitaxial overgrowth. For resonant excitation of cold excitons, the ΔR decay possesses a 720 ps component attributed to the free exciton lifetime in this high quality material. For electrons with small excess energy (< 50 meV), the strong increase in the ΔR decay rate with decreasing excitation density suggests that screening of the Coulomb interaction may play an important role in the processes of carrier relaxation and exciton formation. The faster decay times at a given carrier density observed for hot (> 100 meV) electron relaxation are attributed to electron-hole scattering in conjunction with the screened electron-LO phonon interaction.
Photoluminescence Enhancement and Morphological Properties of Carbon Codoped GaN:Er
Authors: Overberg M.E , Abernathy C.R. , Pearton S. J. , Wilson R. G. , Zavada J. M.
Published: September 2012

The surface morphology and the room temperature 1.54 µm photoluminescence (PL) intensity from GaN:Er grown by gas source molecular beam epitaxy have been investigated as a function of C concentration as introduced by CBr4. Similar to previous results with increasing Er level, increasing the C concentration initially improved the surface smoothness as measured by atomic force microscopy (AFM) and scanning electron microscopy (SEM), with RMS roughness improving by a factor of seven over undoped GaN. The PL also improved dramatically. However, the highest amounts of C investigated produced a decrease in the PL as well as a roughening of the film surface. These effects indicate that the GaN:Er had reached its C solubility limit, producing an increased amount of defect induced nonradiative recombination.
Comparison of the Optical Properties of Er3+ Doped Gallium Nitride Prepared by Metalorganic Molecular Beam Epitaxy (MOMBE) and Solid Source Molecular Beam Epitaxy (SSMBE)
Authors: Hömmerich U. , Seo J. T. , MacKenzie J. D. , Abernathy C. R. , Birkhahn R. , Steckl A. J. , Zavada J. M.
Published: September 2012

We report on the luminescence properties of Er doped GaN grown prepared by metalorganic molecular beam epitaxy (MOMBE) and solid-source molecular beam epitaxy (SSMBE) on Si substrates. Both types of samples emitted characteristic 1.54 µm PL resulting from the intra-4f Er3+ transition 4I13/2→4I15/2. Under below-gap excitation the samples exhibited very similar 1.54 µm PL intensities. On the contrary, under above-gap excitation GaN: Er (SSMBE) showed ∼80 times more intense 1.54 µm PL than GaN: Er (MOMBE). In addition, GaN: Er (SSMBE) also emitted intense green luminescence at 537 nm and 558 nm, which was not observed from GaN: Er (MOMBE). The average lifetime of the green PL was determined to be 10.8 µs at 15 K and 5.5 µs at room temperature. A preliminary lifetime analysis suggests that the decrease in lifetime is mainly due to the strong thermalization between the 2H11/2 and 4S3/2 excited states. Nonradiative decay processes are expected to only weakly affect the green luminescence.
Comparison of Implant Isolation Species for GaN Field-Effect Transistor Structures
Authors: Dang G. , Cao X. A. , Ren F. , Pearton S. J. , Han J. , Baca A. G. , Shul R. J. , Wilson R. G.
Published: September 2012

Different ions (Ti+, O+, Fe+, Cr+) were implanted at multiple energies into GaN field effect transistor structures (n and p-type). The implantation was found to create deep states with energy levels in the range EC –0.20 to 0.49 eV in n-GaN and at EV +0.44 eV in p-GaN after annealing at 450-650 oC. The sheet resistance of the GaN was at a maximum after annealing at these temperatures, reaching values of ∼4×1012 Ω/⊏ in n-GaN and ∼1010 Ω/⊏ in p-GaN. The mechanism for the implant isolation was damage-related trap formation for all of the ions investigated, and there was no evidence of chemically induced isolation.
A Thermodynamic Approach to Ohmic Contact Formation to p-GaN
Authors: Liu Bo , Ahonen Mikko H. , Holloway Paul H.
Published: September 2012

A new ohmic contact scheme for gallium nitride is presented. The use of Nitride-forming metal Over Gallide-forming metal, “NOG”, can modify the thermodynamic activity of N and Ga near the interface. This in turn can modify the near-surface point defect concentrations, particularly the vacancies of Ga and N. The principle of this contact scheme was shown to be consistent with results from Ni/Au, Ni/Zn-Au, Ta/Ti, and Ni/Mg/Ni/Si contacts. In the present study, the “NOG” scheme was used to design Ni/Ti/Au and Ni/Al/Au metallization, and addition of Ti and Al nitride-forming metals to the Ni gallide-forming metal led to lower but still high contact resistance. Ti was shown to be better than Al as the nitride-forming metal based on the decrease of resistance in as deposited contacts. Compared to Ni/Au, four times more current was measured in Ni/Ti/Au contacts to p-GaN after anneal at 300°C for 5min. However the addition of the Ti nitride-forming metal led to lower stability at 500°C.
Growth and Characterization of Piezoelectrically Enhanced Acceptor-Type AlGaN/GaN Heterostructures
Authors: Michel A. , Hanser D. , Davis R.F. , Qiao D. , Lau S.S. , Yu L.S. , Sun W. , Asbeck P.
Published: September 2012

Acceptor (Mg)-doped AlGaN/GaN heterostructures were grown via MOVPE and compared to similarly doped GaN standard films grown in the same reactor. Chemical analysis of the films, via secondary ion mass spectrometry (SIMS), revealed comparable Mg concentrations of ∼2×1019 atoms/cm3 in all films. The Mg-doped GaN standard sample had a sheet conductance of 7-μS compared to a sheet conductance of 20-μS for an AlGaN/GaN heterostructure. The sheet conductance of the AlGaN/GaN heterostructures was higher due to piezoelectric acceptor doping and modulation doping effects in addition to conventional Mg acceptor doping.
Deep Levels in n-Type Schottky and p+-n Homojunction GaN Diodes
Authors: Hierro A. , Kwon D. , Ringel S. A. , Hansen M. , Mishra U. K. , Denbaars S. P. , Speck J. S.
Published: September 2012

The deep level spectra in both p+-n homojunction and n-type Schottky GaN diodes are studied by deep level transient spectroscopy (DLTS) in order to compare the role of the junction configuration on the defects found within the n-GaN layer. Both majority and minority carrier DLTS measurements are performed on the diodes allowing the observation of both electron and hole traps in n-GaN. An electron level at Ec-Et=0.58 and 0.62 V is observed in the p+-n and Schottky diodes, respectively, with a concentration of ∼3-4×1014cm−3 and a capture cross section of ∼1-5×10−15cm2. The similar Arrhenius behavior indicates that both emissions are related to the same defect. The shift in activation energy is correlated to the electric field enhanced-emission in the p+-n diode, where the junction barrier is much larger. The p+-n diode configuration allows the observation of a hole trap at Et-Ev=0.87 eV in the n-GaN which is very likely related to the yellow luminescence band.
Fermi Level Pinning at GaN-Interfaces: Correlation of Electrical Admittance and Transient Spectroscopy
Authors: Witte H. , Krtschil A. , Lisker M. , Rudloff D. , Christen J. , Krost A. , Stutzmann M. , Scholz F.
Published: September 2012

In GaN layers grown by molecular beam epitaxy as well as metal organic vapor phase epitaxy significant differences were found in the appearance of deep defects de-tected by thermal admittance spectroscopy as compared for deep level transient spectros-copy measurements. While, thermal admittance spectroscopy measurements which were made under zero bias conditions only show thermal emissions at activation energies between 130 and 170 meV, further deep levels existing in these GaN layers were evidenced by transient spectrocopy. This discrepancy is explained by a pinning effect of the Fermi level at the metal / GaN interface induced by high a concentration of the deep levels showing up in thermal admittance spectroscopy. We compare our results with a GaAs:Te Schottky- diode as a refernec sample. Here, both spectroscopic methods give exactly the same deep level emissions.
Low-Frequency Noise in SiO2/AlGaN/GaN Heterostructures on SiC and Sapphire Substrates
Authors: Pala N. , Gaska R. , Shur M. , Yang J. W , Khan M. Asif
Published: September 2012

The low-frequency noise in GaN-based Metal-Oxide-Semiconductor Heterostructure Field Effect Transistors (MOS-HFETs) and HFETs on sapphire and n-SiC substrates were studied. Hooge parameter at zero gate bias was calculated about 8 × 10−4 for both types of the devices. The AlGaN/GaN MOS-HFETs exhibited extremely low gate leakage current and much lower noise at both positive and negative gate biases. These features demonstrate the high quality of the SiO2/AlGaN heterointerface and feasibility of this technology for high-power microwave transmitter and high-power, high-temperature switches.
Influence of Internal Electric Fields on the Ground Level Emission of GaN/AlGaN Multi-Quantum Wells
Authors: Bonfiglio A. , Lomascolo M. , Traetta G. , Cingolani R. , Carlo A. Di , Sala F. Della , Lugli P. , Botchkarev A. , Morkoc H.
Published: September 2012

The spectroscopic investigation of GaN/AlGaN quantum wells reveals that the emission energy of such structures is determined by four parameters, namely composition, well-width, strain and charge density. The experimental data obtained by varying these parameters are quantitatively explained by an analytic model based on the envelope function formalism which accounts for screening and built-in field, and by a full self-consistent tight-binding model.
Identification of As, Ge and Se Photoluminescence in GaN Using Radioactive Isotopes
Authors: Stötzler A. , Weissenborn R. , Deicher M. , Collaboration Isolde
Published: September 2012

We report on experiments which unequivocal identify the chemical nature of optical transitions related to As (2.58 eV), Ge (3.398 eV) and Se (1.49 eV) found in the photoluminescence (PL) spectra of GaN. For this purpose epitaxial GaN layers were doped by ion implantation (60 keV, 3×1012 cm−2) with the radioactive isotopes 71As and 72Se. The isotope 71As (half-life 64.28 h) decays first into 71Ge (11.43 d), which finally transmutes into stable 71Ga. The isotope 72Se decays via 72As (26 h) into stable 72Ge. These chemical transmutations were monitored with photoluminescence spectroscopy (PL). The half-lives resulting from exponential fits on our PL data are in excellent agreement with the half-lives of the isotopes. Our experiments clearly show that in each case the luminescence center involves exactly one As, Ge or Se atom. In addition to this, the results imply that no optically active GaN antisite exists.
Dislocation Arrangement in a Thick LEO GaN Film on Sapphire
Authors: Dunn Kathleen A. , Babcock Susan E. , Stone Donald S. , Matyi Richard J. , Zhang Ling , Kuech Thomas F.
Published: September 2012

Diffraction-contrast TEM, focused probe electron diffraction, and high-resolution X-ray diffraction were used to characterize the dislocation arrangements in a 16[.proportional]m thick coalesced GaN film grown by MOVPE LEO. As is commonly observed, the threading dislocations that are duplicated from the template above the window bend toward (0001). At the coalescence plane they bend back to lie along [0001] and thread to the surface. In addition, three other sets of dislocations were observed. The first set consists of a wall of parallel dislocations lying in the coalescence plane and nearly parallel to the substrate, with Burgers vector (b) in the (0001) plane. The second set is comprised of rectangular loops with b = 1/3 [11 20] (perpendicular to the coalescence boundary) which originate in the coalescence boundary and extend laterally into the film on the (1 100). The third set of dislocations threads laterally through the film along the [1 100] bar axis with 1/3<11 20>-type Burgers vectors These sets result in a dislocation density of ∼109 cm−2. High resolution X-ray reciprocal space maps indicate wing tilt of ∼0.5°.
Dislocation Mechanisms in the GaN Lateral Overgrowth by Hydride Vapor Phase Epitaxy
Authors: Kuan T. S. , Inoki C. K. , Hsu Y. , Harris D. L. , Zhang R. , Gu S. , Kuech T. F.
Published: September 2012

We have carried out a series of lateral epitaxial overgrowths (LEO) of GaN through thin oxide windows by the hydride vapor phase epitaxy (HVPE) technique at different growth temperatures. High lateral growth rate at 1100°C allows coalescing of neighboring islands into a continuous and flat film, while the lower lateral growth rate at 1050°C produces triangular-shaped ridges over the growth windows. In either case, threading dislocations bend into laterally grown regions to relax the shear stress developed in the film during growth. In regions close to the mask edge, where the shear stress is highest, dislocations interact and multiply into arrays of edge dislocations lying parallel to the growth window. This multiplication and pileup of dislocations cause a large-angle tilting of the laterally grown regions. The tilt angle is high (∼8 degrees) when the growth is at 1050°C and becomes smaller (3-5 degrees) at 1100°C. At the coalescence of growth facets, a tilt-type grain boundary is formed. During the high-temperature lateral growth, the tensile stress in the GaN seed layer and the thermal stress from the mask layer both contribute to a high shear stress at the growth facets. Finite element stress simulations suggest that this shear stress may be sufficient to cause the observed excessive dislocation activities and tilting of LEO regions at high growth temperatures.
A Study of the Effect of V/III Flux Ratio and Substrate Temperature on the in Incorporation Efficiency in inxGa1−x/GaN Heterostructures Grown by Rf Plasma-Assisted Molecular Beam Epitaxy
Authors: O'Steen M. L. , Fedler F. , Hauenstein R. J.
Published: September 2012

Laterally resolved high resolution X-ray diffraction (HRXRD) and photoluminescence spectroscopy (PL) have been used to assess In incorporation efficiency in InxGa1−xN/GaN heterostructures grown through rf-plasma-assisted molecular beam epitaxy. Average alloy composition over a set of InxGa1−xN/GaN superlattices has been found to depend systematically upon both substrate temperature (Tsub) and V/III flux ratio during growth. A pronounced thermally activated In loss (with more than an order-of-magnitude decrease in average alloy composition) is observed over a narrow temperature range (590–670oC), with V/III flux ratio fixed. Additionally, the V/III flux ratio is observed to further strongly affect In incorporation efficiency for samples grown at high Tsub, with up to an order-of-magnitude enhancement in In content despite only a minor increase in V/III flux ratio. PL spectra reveal redshifts as In content is increased and luminescence efficiency which degrades rapidly with decreasing Tsub. Results are consistent with In loss arising from thermally activated surface segregation + surface desorption processes during growth.
Polarity Determination for MOCVD Growth of GaN on Si(111) by Convergent Beam Electron Diffraction
Authors: Zhao L. , Marchand H. , Fini P. , Denbaars S. P. , Mishra U. K. , Speck J. S.
Published: September 2012

The polarity of laterally epitaxially overgrown (LEO) GaN on Si(111) with an AlN buffer layer grown by MOCVD has been studied by convergent beam electron diffraction (CBED). The LEO GaN was studied by cross-section and plan-view transmission electron microscopy (TEM). The threading dislocation density is less than 108cm−2 and no inversion domains were observed. CBED patterns were obtained at 200 kV for the <1100> zone. Simulation was done by many-beam solution with 33 zero-order beams. The comparison of experimental CBED patterns and simulated patterns indicates that the polarity of GaN on Si(111) is Ga face.
Evidence from EELS of Oxygen in the Nucleation Layer of a MBE grown III-N HEMT
Authors: Eustis Tyler J. , Silcox John , Murphy Michael J. , Schaff William J.
Published: September 2012

The presence of oxygen throughout the nominally AlN nucleation layer of a RF assisted MBE grown III-N HEMT was revealed upon examination by Electron Energy Loss Spectroscopy (EELS) in a Scanning Transmission Electron Microscope (STEM). The nucleation layer generates the correct polarity (gallium face) required for producing a piezoelectric induced high mobility two dimensional electron gas at the AlGaN/GaN heterojunction. Only AlN or AlGaN nucleation layers have provided gallium face polarity in RF assisted MBE grown III-N's on sapphire. The sample was grown at Cornell University in a Varian GenII MBE using an EPI Uni-Bulb nitrogen plasma source. The nucleation layer was examined in the Cornell University STEM using Annular Dark Field (ADF) imaging and Parallel Electron Energy Loss Spectroscopy (PEELS). Bright Field TEM reveals a relatively crystallographically sharp interface, while the PEELS reveal a chemically diffuse interface. PEELS of the nitrogen and oxygen K-edges at approximately 5-Angstrom steps across the GaN/AlN/sapphire interfaces reveals the presence of oxygen in the AlN nucleation layer. The gradient suggests that the oxygen has diffused into the nucleation region from the sapphire substrate forming this oxygen containing AlN layer. Based on energy loss near edge structure (ELNES), oxygen is in octahedral interstitial sites in the AlN and Al is both tetrahedrally and octahedrally coordinated in the oxygen rich region of the AlN.
Formation of BN and AlBN During Nitridation of Sapphire Using RF Plasma Sources
Authors: Ptak A.J. , Ziemer K.S. , Holbert L.J. , Stinespring C.D. , Myers T.H.
Published: September 2012

Evidence is presented that nitrogen plasma sources utilizing a pyrolytic boron nitride liner may be a significant source of B contamination during growth and processing. Auger electron spectroscopy analysis performed during nitridation of sapphire indicate the resulting layers contain a significant amount of BN. The formation of Al1−xBxN would explain the observation of a lattice constant several percent smaller than AlN as measured by reflection high-energy electron diffraction. The presence of cubic inclusions in layers grown on such a surface may be related to the segregation of BN during the nitridation into its cubic phase.
The Effect of Al in Plasma-Assisted MBE-Grown GaN
Authors: Zsebök Otto , Thordson Jan V. , Zhao Qingxiang , Södervall Ulf , Ilver Lars , Andersson Thorvald G.
Published: September 2012

We have grown GaN, with addition of a 0.10 to 0.33 % Al, on sapphire(0001) substrates by solid-source RF-plasma assisted MBE. The Al-concentration was determined by secondary ion-mass spectrometry and Auger-electron spectroscopy, while the layer quality was assessed by photoluminescence and high-resolution scanning electron microscopy. Microscopy revealed a meandering pattern and a surface roughness varying with Al-content. The smallest surface roughness was obtained at 0.10 % Al. Photoluminescence revealed two main peaks attributed to the neutral donor-bound exciton. Its energy increased slightly with Al-concentration, which established a correlation between the Al-concentration and the band gap.
Electrical Properties of Cubic InN and GaN Epitaxial Layers as a Function of Temperature
Authors: Fernandez J.R.L. , Chitta V.A. , Abramof E. , Silva A. Ferreira da , Leite J.R. , Tabata A. , As D.J. , Frey T. , Schikora D. , Lischka K.
Published: September 2012

Carrier concentration and mobility were measured for intrinsic cubic InN and GaN, and for Si-doped cubic GaN as a function of temperature. Metallic n-type conductivity was found for the InN, while background p-type conductivity was observed for the intrinsic GaN layer. Doping the cubic GaN with Si two regimes were observed. For low Si-doping concentrations, the samples remain p-type. Increasing the Si-doping level, the background acceptors are compensated and the samples became highly degenerated n-type. From the carrier concentration dependence on temperature, the activation energy of the donor and acceptor levels was determined. Attempts were made to determine the scattering mechanisms responsible for the behavior of the mobility as a function of temperature.
Metal Organic Vapor Phase Epitaxy of GaAsN/GaAs Quantum Wells Using Tertiarybutylhydrazine
Authors: Schmidtling T. , Klein M. , Pohl U.W. , Richter W.
Published: September 2012

GaAsN epilayers and quantum wells with a good structural quality and surface morphology were grown by low pressure metal organic vapor phase epitaxy using tertiarybutylhydrazine as a novel nitrogen source. The dependence of nitrogen incorporation on growth temperature was studied for epitaxy with arsine and tertiarybutylarsine precursors. A nitrogen content of 6.7 % was achieved using tertiarybutylhydrazine and tertiarybutylarsine at a low growth temperature of 530 °C. The observed room temperature luminescence shows an increasing redshift with increasing nitrogen contents of the wells.
TEM Study of the Morphology Of GaN/SiC (0001) Grown at Various Temperatures by MBE
Authors: Sarney W.L. , Salamanca-Riba L. , Ramachandran V. , Feenstra R.M , Greve D.W.
Published: September 2012

GaN films grown on SiC (0001) by MBE at various substrate temperatures (600° - 750° C) were characterized by RHEED, STM, x-ray diffraction, AFM and TEM. This work focuses on the TEM analysis of the films' features, such as stacking faults and dislocations, which are related to the substrate temperature. There are several basal plane stacking faults in the form of cubic inclusions for samples grown at low temperatures compared to those grown at high temperatures. The dislocation density is greatest for the film grown at 600°C, and it steadily decreases with increasing growth temperatures. Despite the presence of various defects, x-ray analysis shows that the GaN films are of high quality. The double crystal rocking curve full width at half maximum (FWHM) for the GaN (0002) peak is less than 2 arc-minutes for all of the films we measured and it decreases with increasing growth temperature.
Threading Dislocation Density Reduction in GaN/Sapphire Heterostructures.
Authors: Kvit A. , Sharma A. K. , Narayan J.
Published: September 2012

Large lattice mismatch between GaN and α-Al2O3 (15%) leads to the possibility of high threading dislocation densities in the nitride layers grown on sapphire. This investigation focused on defect reduction in GaN epitaxial thin layer was investigated as a function of processing variables. The microstructure changes from threading dislocations normal to the basal plane to stacking faults in the basal plane. The plan-view TEM and the corresponding selected-area diffraction patterns show that the film is single crystal and is aligned with a fixed epitaxial orientation to the substrate. The epitaxial relationship was found to be (0001)GaN∥(0001)Sap and [01-10]GaN∥[-12-10]Sap. This is equivalent to a 30° rotation in the basal (0001) plane. The film is found to contain a high density of stacking faults with average spacing 15 nm terminated by partial dislocations. The density of partial dislocations was estimated from plan-view TEM image to be 7×109 cm−2. The cross-section image of GaN film shows the density of stacking faults is highest in the vicinity of the interface and decreases markedly near the top of the layer. Inverted domain boundaries, which are almost perpendicular to the film surface, are also visible. The concentration of threading dislocation is relatively low (∼;2×108 cm−2), compared to misfit dislocations. The average distance between misfit dislocations was found to be 22 Å. Contrast modulations due to the strain near misfit dislocations are seen in high-resolution cross-sectional TCM micrograph of GaN/α-Al2O3 interface. This interface is sharp and does not contain any transitional layer. The interfacial region has a high density of Shockley and Frank partial dislocations. Mechanism of accommodation of tensile, sequence and tilt disorder through partial dislocation generation is discussed. In order to achieve low concentration of threading dislocations we need to establish favorable conditions for some stacking disorder in thin layers above the film-substrate interface region.
Simulations of Defect-Interface Interactions in GaN
Authors: Chisholm J. A. , Bristowe P. D.
Published: September 2012

We report on the interaction of native point defects with commonly observed planar defects in GaN. Using a pair potential model we find a positive binding energy for all native defects to the three boundary structures investigated indicating a preference for native defects to form in these interfaces. The binding energy is highest for the Ga interstitial and lowest for vacancies. Interstitials, which are not thought to occur in significant concentrations in bulk GaN, should form in the (11 20) IDB and the (10 10) SMB and consequently alter the electronic structure of these boundaries.
Optical and Electrical Properties of MBE Grown Cubic GaN/GaAs Epilayers Doped by Si
Authors: As D.J. , Richter A. , Busch J. , Schöttker B. , Lübbers M. , Mimkes J. , Schikora D. , Lischka K. , Kriegseis W. , Burkhardt W. , Meyer B.K.
Published: September 2012

Si-doping of cubic GaN epilayers grown by an rf plasma-assisted molecular beam epitaxy on semi-insulating GaAs (001) substrates is investigated by secondary ion mass spectroscopy (SIMS), photoluminescence (PL) and by Hall-effect measurements. SIMS measurements show a homogeneous incorporation of Si in cubic GaN epilayers up to concentrations of 5*1019 cm−3. PL shows a clear shift of the donor-acceptor emission to higher energies with increasing Si-doping. Above a Si-flux of 1*1011cm−2s−1 the near band edge lines merge to one broad band due to band gap renormalization and conduction band filling effects. The influence of the high dislocation density (≈1011cm−2) in c-GaN:Si on the electrical properties is reflected in the dependence of the electron mobility on the free carrier concentration. We find that dislocations in cubic GaN act as acceptors and are electrically active.
Efficient Acceptor Activation in AlxGa1−xN/GaN Doped Superlattices
Authors: Goepfert I. D. , Schubert E. F. , Osinsky A. , Norris P. E.
Published: September 2012

Mg-doped superlattices consisting of uniformly doped AlxGa1−xN and GaN layers are analyzed by Hall-effect measurements. Acceptor activation energies of 70 meV and 58 meV are obtained for superlattice structures with an Al mole fraction of x = 0.10 and 0.20 in the barrier layers, respectively. These energies are significantly lower than the activation energy measured for Mg-doped GaN thin films. At room temperature, the doped superlattices have free hole concentrations of 2 × 1018 cm−3 and 4 × 1018 cm−3 for x = 0.10 and 0.20, respectively. The increase in hole concentration with Al content of the superlattice is consistent with theory. The room temperature conductivity measured for the superlattice structures are 0.27 S/cm and 0.64 S/cm for an Al mole fraction of x = 0.10 and 0.20, respectively.
Structural and Optical Property Investigations on Mg-Alloying in Epitaxial Zinc Oxide Films on Sapphire
Authors: Sharma A.K. , Jin C. , Kvit A. , Narayan J. , Muth J.F. , Teng C.W. , Kolbas R.M. , Holland O.W.
Published: September 2012

We have synthesized single-crystal epitaxial MgZnO films by pulsed-laser deposition. High-resolution transmission electron microscopy, X-ray diffraction and Rutherford backscattering spectroscopy/ion channeling were used to characterize the microstructure, defect content, composition and epitaxial single-crystal quality of the films. In these films with up to ∼ 34 atomic percent Mg incorporation, an intense ultraviolet band edge photoluminescence at room temperature and 77 K was observed. The highly efficient photoluminescence is indicative of the excitonic nature of the material. Transmission spectroscopy revealed that the excitonic structure of the alloys was clearly visible at room temperature. Post-deposition annealing in oxygen reduced the number of defects and improved the optical properties of the films. The potential applications of MgZnO alloys in a variety of optoelectronic devices are discussed.
AlGaN/GaN High Electron Mobility Transistor Structure Design and Effects on Electrical Properties
Authors: Piner E.L. , Keogh D.M. , Flynn J.S. , Redwing J.M
Published: September 2012

We report on the effect of strain induced polarization fields in AlGaN/GaN heterostructures due to the incorporation of Si dopant ions in the lattice. By Si-doping (Al)GaN, a contraction of the wurtzite unit cell can occur leading to strain in doped AlGaN/GaN heterostructures such as high electron mobility transistors (HEMTs). In a typical modulation doped AlGaN/GaN HEMT structure, the Si-doped AlGaN supply layer is separated from the two-dimensional electron gas channel by an undoped AlGaN spacer layer. This dopant-induced strain, which is tensile, can create an additional source of charge at the AlGaN:Si/AlGaN interface. The magnitude of this strain increases as the Si doping concentration increases and the AlN mole fraction in the AlGaN decreases. Consideration of this strain should be given in AlGaN/GaN HEMT structure design.
Negative Differential Conductivity in AlGaN/GaN HEMTs: Real Space Charge Transfer from 2D to 3D GaN States?
Authors: Deng J. , Gaska R. , Shur M. S. , Khan M. A. , Yang J. W.
Published: September 2012

We report on non-thermal negative differential conductivity (NDC) in AlGaN/GaN HEMTs grown on sapphire substrates by low-pressure MOCVD. The sheet electron density was on the order of few times 1012cm−2 and the Hall mobility was 1,000 cm2/V.s. The HEMTs had threshold voltage close to zero and could operate at high positive gate bias up to 3 to 3.5 Volts, with a very low gate leakage current. NDC was observed at the gate bias larger than 1.5V and at the drain biases between approximately 0.5Vg and Vg. We excluded the possibility of self-heating as the cause, since the NDC occurs at relatively small power levels where self-heating effects are negligible.

An explanation we provided for the NDC effect is the new mechanism of real space charge transfer from 2D to 3D GaN states, which leads to a decrease in the channel mobility at large 2D electron gas densities. The observed low leakage can be explained by an enhanced molar fraction of aluminum at the heterointerface that results in a larger conduction band discontinuity. Our model that accounts for the piezoelectric and pyroelectric effects is consistent with the observed NDC effect. The Hall mobility dependence on the gate bias and sheet carrier concentration [1] is consistent with the real space transfer mechanism.

This NDC effect in GaN/AlGaN HEMTs may find applications in high-performance digital circuits at elevated temperatures.