Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session L40: Properties of Semiconductor Nitrides |
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Sponsoring Units: FIAP Chair: Kathleen Kash, Case Western Reserve University Room: Colorado Convention Center 503 |
Tuesday, March 6, 2007 2:30PM - 2:42PM |
L40.00001: Measurement of the Enthalpy and Free Energy of GaN Timothy J. Peshek, Kathleen Kash, John C. Angus Direct measurement of the thermochemical properties of GaN has proven difficult, and differences in reported values remain. We present a technique for direct measurement of the free energy of formation of GaN by finding the partial pressures of ammonia gas in hydrogen for spontaneous formation and elimination of a GaN film on a liquid Ga surface in the temperature range 750-1050 C. These data were used to calculate the enthalpy of formation of GaN. The measured values are compared to those obtained by other means, e.g. calorimetry or direct reaction of N$_{2}$ and molten Ga. [Preview Abstract] |
Tuesday, March 6, 2007 2:42PM - 2:54PM |
L40.00002: First-principles study of the thermodynamics of InGaN alloys C.K. Gan, D.J. Srolovitz We present the most rigorous density-functional study [1] to date of the thermodynamics of In$_x$Ga$_{1-x}$N alloys. These systems have attracted considerable theoretical and experimental attention due to its enormous potential for high-power, high-frequency, and high-temperature optoelectronic applications. Theoretical calculations of the pseudo-binary phase diagram require an accurate description of the interactions between the constituent atoms, a good representation of a random alloy, the inclusion of the effect of lattice vibrations, and a consideration of the configurational entropy. We have used accurate density-functional theory to calculate the heat of formation of the alloy (represented by special quasi-random structures), as well as the phonon spectra for the lattice vibrational contribution to the free energy. We find that the wurtzite structure is always more stable than the zinc-blende structure for all temperatures and compositions investigated, in agreement with experiment. We find that the lattice vibrations lead to a reduction of the critical temperature by more than 20\%, leading to a temperature of 1654 K and 1771 K for the wurtzite and zinc-blende structures, respectively. The lattice vibrations also change the shape of the binodal and spinodal curves. This result suggests that quaternary alloy additions may increase the vibrational contribution to the stability of the disordered phase. Our predicted phase diagrams have been used to interpret several key experiment measurements on MOCVD In$_x$Ga$_{1-x}$N films. The importance of In$_x$Ga$_{1-x}$N alloys has also prompted the study of the surface thermodynamics of InN [2]. New results will be presented. \newline [1] C. K. Gan, Y. P. Feng, and D. J. Srolovitz, Phys. Rev. B, {\bf 73} 235214 (2006). \newline [2] C. K. Gan and D. J. Srolovitz, Phys. Rev. B., {\bf 74} 115319 (2006). [Preview Abstract] |
Tuesday, March 6, 2007 2:54PM - 3:06PM |
L40.00003: Theoretical study of thermodynamic and electronic properties of the Zincblende In$_{x}$Ga$_{1-x}$N alloys Zhe Liu, Paulo Piquini, Giancarlo Trimarchi, Alex Zunger Semiconductor alloys often show distinct atomic-scale microstructures such as long-range, or short-range order, clustering and phase- separation. Such microstructures directly affect the electronic properties. To establish how the atomic microstructure in InGaN zinc-blende alloys affects the electronic structure we (1) Calculate the equilibrium alloy phase-diagram both for bulk (``free-floating'') alloy as well as for the epitaxial alloy using the mixed-basis cluster expansion (MBCE) approach. The MBCE Hamiltonians are evaluated by a number of total-energy inputs from First- principle LDA calculations. Given the Cluster-expansion, we calculate the miscibility gaps, and short range order through Monte Carlo simulations. (2) Calculate the electronic properties of the ensuing microstructure using a supercell approach with atoms placed where the thermodynamic calculation dictates, and the electronic properties are obtained from plane-wave empirical pseudopotential approach. [Preview Abstract] |
Tuesday, March 6, 2007 3:06PM - 3:18PM |
L40.00004: Thermoelectrical properties of InGaN Bed Pantha, Rajendra Dahal, Jing Li, Zhaoyang Fan, Jingyu Lin, Hongxing Jiang, G. Pomrenke III-nitride semiconductors offer tremendous scope for the enhancement of the thermoelectric figure of merit (\textit{ZT}) through the use of the bandgap engneering, alloying and nanostructure manipulation. Although III-nitride materials have been extensively studied for visible and ultraviolet light emitters and detectors and high power transistors during the past decade, very little work has been done with respect to their applications for thermopower technology. The \textit{ZT }value of a thermoelectrical material can be enhanced by decreasing the thermal conductivity. In this study, we have employed the 3-Omega method to characterize the thermal conductivities of III-nitride nano-structures. It was found that the incorporation of indium in GaN significantly reduced the thermal conductivity. A systematic study has been carried out on the dependence of the thermal conductivity of InGaN on the In-content. Various growth schemes for introducing scattering centers and InGaN/GaN supperlattices for phonon-blocking were carried out with the aim of further reducing the thermal conductivity and enhancing the \textit{ZT} value. The results of these studies will be reported in this talk. [Preview Abstract] |
Tuesday, March 6, 2007 3:18PM - 3:30PM |
L40.00005: ABSTRACT WITHDRAWN |
Tuesday, March 6, 2007 3:30PM - 3:42PM |
L40.00006: Local electronic and optical behavior of ELO $a$-plane GaN A.A. Baski, J.C. Moore, U. Ozgur, V. Kasliwal, X. Ni, H. Morkoc Conductive atomic force microscopy (CAFM) and near-field optical microscopy (NSOM) were used to study $a$-plane GaN films grown via epitaxial lateral overgrowth (ELO). The ELO films were prepared by metal organic chemical vapor deposition on a patterned SiO$_{2}$ layer with 4-$\mu $m wide windows, which was deposited on a GaN template grown on $r$-plane sapphire. The window regions of the coalesced ELO films appear as depressions with a high density of surface pits. At reverse bias below 12~V, very low uniform conduction (2~pA) is seen in the window regions. Above 20~V, a lower-quality sample shows localized sites inside the window regions with significant leakage, indicating a correlation between the presence of surface pits and leakage sites. Room temperature NSOM studies also suggest a greater density of surface terminated dislocations in the window regions, while wing regions explicitly show enhanced optical quality of the overgrown GaN. The combination of CAFM and NSOM data therefore indicates a correlation between the presence of surface pits, localized reverse-bias current leakage, and low PL intensity in the window regions. [Preview Abstract] |
Tuesday, March 6, 2007 3:42PM - 3:54PM |
L40.00007: Electrical properties of Si-implanted Al$_{x}$Ga$_{1-x}$N with high Al mole fraction Elizabeth Moore, Mee-Yi Ryu, Yung Kee Yeo, Robert Hengehold Ion implanted AlGaN has not been well investigated, in particular AlGaN with high Al mole fraction, compared to the research of ion-implanted GaN. Therefore, a systematic electrical activation study of Si-implanted AlGaN with Al mole fraction from 0.1 to 0.5 has been made as a function of ion dose and anneal temperature. The AlGaN wafers were grown on sapphire substrates by migration enhanced metal organic vapor deposition. Silicon ions were implanted at 200 keV with doses from 1x10$^{14}$ to 1x10$^{15}$ cm$^{-2}$ at room temperature. The samples were proximity cap annealed from 1100 to 1350 $^{o}$C with a 500 {\AA} AlN cap in a nitrogen environment. The results of Hall Effect measurements show that electrical activation efficiencies of 77{\%} and 53{\%} were obtained respectively for the Al$_{0.4}$Ga$_{0.6}$N and Al$_{0.5}$Ga$_{0.5}$N implanted with a dose of 1x10$^{15}$ cm$^{-2}$ and annealed at 1350 for 20 min. The Al$_{0.1}$Ga$_{0.9}$N samples exhibited a mobility of 100 cm$^{2}$/V\textbullet s, while the Al$_{0.5}$Ga$_{0.5}$N showed a mobility of half that value. It has been found that the samples with higher Al mole fraction generally require a higher anneal temperature and/or longer anneal time for a better electrical activation. Also, the mobility of the sample was found to decrease as the Al mole fraction increases. [Preview Abstract] |
Tuesday, March 6, 2007 3:54PM - 4:06PM |
L40.00008: Hydrostatic Pressure Study of GaN-Based FETs Y. Zhang, B. Nielsen, E.E. Mendez, J. Jimenez Because of their large breakdown voltage and high saturation velocity, GaN-based field effect transistors (FETs) are best suited for applications that require high power and high frequency, but they face serious reliability problems. In these FETs the charge in the channel results from spontaneous and piezoelectric polarization, so any stress produced during device operation can change the charge in the channel and affect the device's electrical performance. With the aim of better understanding the role of stress, we have studied the current-voltage characteristics of GaN FETs under hydrostatic pressure up to 10 kbar. N-channel FETs were fabricated from GaN-Al$_{0.28}$Ga$_{0.72}$N structures grown on SiC substrates. The source-drain current was measured for voltage up to 15V, at various gate voltages, from turn-on (typically -3V) up to 1V. We have observed that for a given source-drain voltage the current increased monotonically with pressure. At 10 kbar, the increase in the saturation current relative to 1 atm was found to be device-dependent, but typically between 5{\%} and 15{\%}. These results, along with the pressure effects on the devices' intrinsic short term degradation and on strongly degraded GaN FETs devices, allow us to speculate about the possible origin of device degradation. [Preview Abstract] |
Tuesday, March 6, 2007 4:06PM - 4:18PM |
L40.00009: Concentration dependence of the E$_{ +}$ transition in dilute GaAs$_{1-x}$N$_{x}$ alloy. B. Fluegel, A. Ptak, A. Mascarenhas, S. Tixier, E. C. Young, T. Tiedje We investigate dilute isoelectronic doping in molecular-beam epitaxially-grown GaAs using low-temperature micro-photoluminescence to measure the above-bandgap transition energies of $E_{+}$ and the spin-orbit transition, $E_{SO}$. In the case of dilute nitrides, GaAs$_{1-x}$N$_{x}$, we examine the functional shape of the concentration dependence of E$_{+}(x)$ in the low-$x$ limit, with $x$ as low as 0.04 {\%}. Comparison with the concentration dependence of the E$_{0}$ bandgap gives compelling evidence against the picture of bandgap reduction via repulsion of the conduction band minimum from the impurity level acting alone. [Preview Abstract] |
Tuesday, March 6, 2007 4:18PM - 4:30PM |
L40.00010: Symmetry of the N-H$_{2}$ complex in GaAsN S. Kleekajai, M. Stavola, K.R. Martin, W.B. Fowler, M. Capizzi, A. Polimeni The III-N-V alloys have attracted much recent attention because of a large reduction of the band-gap energy that occurs for N concentrations of a few percent. The hydrogenation of these alloys eliminates the effect of N [1]. A previous IR study of the vibrational properties of GaAsN:H showed that the dominant N- and H-containing defect has two weakly coupled N-H modes [2], a finding that is inconsistent with an H$_{2}$* structure proposed in earlier studies. Recent theory [3-5] suggests a defect with C$_{1h}$ symmetry. While uniaxial stress results do not provide vibrational splittings sufficiently large to reveal the symmetry of the N-H$_{2}$ center, new IR absorption experiments performed for the H-wagging modes of GaAsN:H yield results that are consistent with the vibrational properties predicted for the C$_{1h}$ structure [3]. This work is supported by NSF Grant DMR 0403641. \newline [1] A. Polimeni \textit{et al.}, Phys. Rev. B \textbf{63}, 201204(R) (2001). \newline [2] F. Jiang \textit{et al.}, Phys. Rev. B \textbf{69}, 041309(R) (2004). \newline [3] W. B. Fowler \textit{et al.}, Phys. Rev. B \textbf{72}, 035208 (2005). \newline [4] M.-H. Du \textit{et al.}, Phys. Rev. B \textbf{72}, 073202 (2005). \newline [5] G. Ciatto, Phys. Rev. B \textbf{71}, 201301 (2005). [Preview Abstract] |
Tuesday, March 6, 2007 4:30PM - 4:42PM |
L40.00011: Phonon spectra of the high-frequency modes in dilute nitride random GaN$_x$As$_{1-x}$ alloy Amanuel Teweldeberhan, Stephen Fahy The broadening of the highly localized isolated vibrational mode in GaNAs has been studied using the force constants between nearest-neighbor and second-nearest neighbor nitrogen pairs obtained from density functional theory supercell calculations. The phonon spectra of the high-frequency modes associated with substitutional nitrogen atoms in the random alloy are calculated for different nitrogen concentrations. [Preview Abstract] |
Tuesday, March 6, 2007 4:42PM - 4:54PM |
L40.00012: Simulation of time-resolved photoluminescence in the strongly disordered dilute nitride GaAsN Ivana Bosa, Dermot McPeake, Stephen Fahy We calculate the time-resolved photoluminescence spectra at finite temperatures in the dilute nitride GaAsN by direct numerical solution of the rate equation for the electron distribution. Electron energy levels and wave functions are calculated numerically in a supercell geometry with a strong random alloy potential acting on the electron. The rate equation includes phonon-assisted transitions between states, radiative and non-radiative recombination. Results are compared to experiments. [Preview Abstract] |
Tuesday, March 6, 2007 4:54PM - 5:06PM |
L40.00013: Photoluminescence blinking of InGaN Single Quantum Well: a study on time correlation Ruggero Micheletto, Sho Suzuki, Yoichi Kawakami We studied the photoluminescence of Indium Gallium Nitride (InGaN) single quantum well crystals and found that in particular conditions it becomes unstable (optical blinking). This peculiar optical phenomenon is confined to few hundreds nanometers domains and is presumably related to the presence of dislocations or impurities in the quantum well. Moreover, time evolution of these blinking domains is not regular, but characterized by a random-like behavior with a certain degree of auto-correlation. It was found that distant domains do not blink independently, but are partially time-correlated in couples or groups. This suggests the existence of preferred carriers paths within the crystal; these pathways are not detectable by other means, but reveal themselves only through blinking. We believe this phenomenon is of interest to better understand fundamental carrier recombination processes and may help in the effort to realize novel and more efficient optical devices. [Preview Abstract] |
Tuesday, March 6, 2007 5:06PM - 5:18PM |
L40.00014: Anomalous Beating Pattern in Wurtzite Al$_{x}$Ga$_{1-x}$N/GaN Heterostructures Wan-Tsang Wang, Ikai Lo, Ming-Hong Gau, Yen-Liang Chen, Jih-Chen Chiang We have confirmed the $k$-dependent spin splitting in wurtzite Al$_{x}$Ga$_{1-x}$N/GaN heterostructures. Anomalous beating pattern in Shubnikov-de Haas measurements arises from the interference of Rashba and Dresselhaus spin-orbit interactions. The dominant mechanism for the $k$-dependent spin splitting at high value k is attributed to Dresselhaus term which is enhanced by the $\Delta _{C1}-\Delta _{C3}$ coupling of wurtzite band folding effect. [Preview Abstract] |
Tuesday, March 6, 2007 5:18PM - 5:30PM |
L40.00015: Femtosecond time-resolved photoreflectance of InN thin films. Tsong-Ru Tsai, Chih-Fu Chang, S.-F. Gwo The kinetics of the nonequilibrium photoexcited carriers in high-quality InN is investigated using femtosecond time-resolved pump-probe reflectivity measurements at room temperature. We observed that both of the hot-carriers relaxation times and carrier recombination times decrease with increasing photoexcited carrier density. We attribute the hot-carriers relaxation times anomaly is caused by the impact-ionization effect. And the carrier density-dependent recombination times can be explained by the Auger recombination (AR). The AR rate was found to have a quadratic rather than a cubic dependence on carrier density. The experimental results allowed the coefficients for impact-ionization, AR and a defect capture time in InN to be estimated as 2.3x10$^{-9}$cm$^{3}$/s, 2.5x10$^{-10}$cm$^{3}$/s and 535 ps, respectively. [Preview Abstract] |
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