Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session H15: Focus Session: Dilute Nitride Semiconductors: From Atoms to Devices I |
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Sponsoring Units: FIAP DCMP DMP Chair: Rachel S. Goldman, University of Michigan Room: LACC 405 |
Tuesday, March 22, 2005 8:00AM - 8:36AM |
H15.00001: Quasi-localized states, electron scattering and carrier mobility in GaNAs Invited Speaker: Mobility of n-type carriers is an important determining characteristic of the dilute nitride semiconductors. The introduction of small amounts of nitrogen, which is so effective in reducing the band-gap, also drastically reduces the mobility of GaAs. Solar power applications are limited by the low carrier mobility (typically $\sim$ 200 cm$^2$$(Vs)^{-1}$) and an understanding of the microscopic mechanisms governing transport could lead to substantial improvements in growth methods. The strong perturbation to the low-lying conduction bands by substitutional nitrogen makes a theoretical treatment of carrier transport difficult in these systems. Using a combination of parameterized tight-binding calculations of GaNAs, which allow us to accurately describe the energies and wavefunctions of electronic states in the low-lying conduction bands, we characterize the electronic states near the conduction band edge as a linear combination of isolated nitrogen states and GaAs conduction band states. Using the resulting spectrum of quasi-localized states coupled to itinerant states of the GaAs host, we calculate the effect of alloy scattering on the n-type carrier mobility in the regime of degenerate and non-degenerate doping within a resonant scattering approach and the relaxation time approximation to the Boltzmann transport equation. Excellent agreement is found with measured values of the mobility. By analysing scattering from different states in the spectrum of quasi-localized states, we identify scattering by random clusters of nitrogen atoms (i.e. nitrogen atoms which share Ga nearest -neighbours) as a major limiting factor on the mobility. [Preview Abstract] |
Tuesday, March 22, 2005 8:36AM - 8:48AM |
H15.00002: Influence of Nitrogen Incorporation Mechanisms on Optical and Electronic Properties of GaAsN Alloys M. Reason, H.A. McKay, W. Ye, D. Mao, R.S. Goldman, X. Bai, C. Kurdak, V. Rotberg We have examined the effects of N incorporation mechanisms on the properties of GaAs$_{1-x}$N$_{x}$ alloys. Nuclear reaction analysis and Rutherford backscattering spectrometry in channeling and non-channeling conditions reveal significant non-substitutional incorporation of N, presumably as either N-N or N-As split interstitials. Our optical absorption measurements reveal a substitutional nitrogen composition-dependent bandgap reduction, which is less significant than typical literature reports. However, when the data are corrected to account for the typical 20\% incorporation of non-substitutional nitrogen, all measurements reveal a bandgap reduction of $\sim$150 meV per 1\% N. Thus, GaAsN bandgap bowing is significantly influenced by substitutional nitrogen ad is smaller than previously reported. In addition, GaAsN films with minimal interstitials exhibit perhaps the highest room temperature electron mobilities reported to date. We have also developed an \textit{in-situ} approach to control N incorporation in heterostructures, using a separately-pumped plasma chamber. This approach enables significant improvements in superlattice interface abruptness and electron mobilities in GaAsN-based superlattices and modulation-doped heterostructures. [Preview Abstract] |
Tuesday, March 22, 2005 8:48AM - 9:00AM |
H15.00003: Two-Dimensional Electron Transport in Selectively Doped GaAsN/AlGaAs Heterostructures X. Bai, H.A. McKay, R.S. Goldman, C. Kurdak We have studied two-dimensional electron transport in high quality selectively doped GaAsN/AlGaAs heterostructures grown by molecular beam epitaxy. Four terminal magnetoresistance and Hall measurements are performed on Van der Pauw and gated Hall bar samples in a variable temperature cryostat at temperatures down to 1.6 K. In samples with an approximate nitrogen concentration of 0.02{\%}, we observe integer quantum Hall effect and a well-resolved Shubnikov-de Haas oscillations at magnetic fields down to 0.7 Tesla. We have also observed a sharp negative magnetoresistance feature arising from weak localization effect at low magnetic fields. The electron mobility is found to be around 15,000 cm$^2$/V $\cdot$ s and weakly dependent on carrier concentration for carrier concentrations above 2.2 $\times$ 10$^{11}$ cm$^{-2}$ and found to be dropping rapidly with decreasing carrier concentration for carrier concentrations below 1.6 $\times$ 10$^{11}$ cm$^{-2}$. For comparison, we have also studied nitrogen-free heterostuctures; the electron mobilities of these control samples were about an order of magnitude larger. We will discuss the electron scattering due to different types of nitrogen related defects in this material system. [Preview Abstract] |
Tuesday, March 22, 2005 9:00AM - 9:12AM |
H15.00004: Localization and exciton line-width broadening in the dilute nitride, GaNAs Dermot McPeake, Ivana Bosa, Andrew Lindsay, Stephen Fahy, Eoin P. O'Reilly The connection between carrier mobility and exciton line-width broadening in GaNAs is studied in a one-band, effective mass approach. Exciton energy levelsĀ are calculated numerically in a supercell geometry, using the Born-Oppenheimer approximation for the electron-hole wavefunction in the presence of the electron-hole Coulomb interaction and a random alloy potential acting on the electron. Under the assumption of completely random alloying, the variance (per unit volume) of the random potential acting on the electron is inversely proportional to the electron mobility. Results for the electron-hole recombination spectrum are compared to recent experiments. [Preview Abstract] |
Tuesday, March 22, 2005 9:12AM - 9:48AM |
H15.00005: The unusual band structure properties of dilute nitride GaAsN alloys Invited Speaker: The incorporation of low concentrations of N in GaAs leads to qualitatively new alloy phenomena and electronic properties, which have increased the power and scope of band-structure engineering as a tool for the design of a new generation of electronic devices. The large electronegativity of the N atoms combined with the stretching and compression of neighbouring bonds in GaAs strongly perturbs the band structure properties of the host crystal: N-impurities and N-clusters act to ``disrupt'' the extended Bloch states of GaAs at characteristic resonant energies in the conduction band [1-3]. This leads to a strongly modified energy-wavevector dispersion relation of the conduction electrons and to a large red-shift of the band gap. Here we use magneto-tunneling spectroscopy to probe directly the unusual band structure properties of GaAsN. Of particular interest is the strongly non-parabolic conduction band in which an inflection point occurs in the energy-wavevector dispersion relation at relatively modest wavevectors. We demonstrate that this property can be tailored by quantum confinement effects or by an applied hydrostatic pressure. Also it can be exploited to realize a new type of non-linear device in which electrons are accelerated by a large electric field up to and beyond the inflection point, thus leading to a large negative differential drift velocity effect of potential interest for novel emitters and detectors of high-frequency radiation. [1] W. Shan et al., Phys. Rev. Lett. 82, 1221 (1999). [2] J. Endicott et al., Phys. Rev. Lett. 91, 126802 (2003). [3] A. Lindsay and E .P. O'Reilly, Phys. Rev. Lett. 93, 196402 (2004). [Preview Abstract] |
Tuesday, March 22, 2005 9:48AM - 10:00AM |
H15.00006: Excited states of acceptor-like excitons bound to isolated nitrogen in GaAs D.J. Wolford, E.A. Stinaff, K.W Ver Steeg, Tyson D. Hoffmann We report on the first observation of the excited states of the hole for acceptor-like excitons bound to isolated nitrogen impurities (N $\cong $ 2 x 10$^{18 }$cm$^{-3})$ in GaAs under pressure. A large absorption-resonance in both transmission and photoluminescence excitation (PLE) spectroscopy leads to identification of the 2S$_{3/2}$ excited-hole-state associated with the ground-state nitrogen isoelectronic bound exciton, known as the N$_{X}$ state from alloy studies. Comparison to the established EM-theory of Baldereschi and Lipari for excited-state acceptors provides good agreement. We thus deduce the hole ground-state ionization energy, finding 19.2 meV -- virtually independent of pressure. These data then also provide direct information about the remaining electron binding energy at isolated N in GaAs, which we find to vary from 0 (level degeneracy with the $\Gamma _{1}$-edge) to some 135 meV, depending upon pressure. These data provide further understanding of the ``deep-level'' isoelectronic N-trap in GaAs and its alloys, including the here-to-fore elusive N$_{\Gamma }$state. [Preview Abstract] |
Tuesday, March 22, 2005 10:00AM - 10:12AM |
H15.00007: Planar modulation of the conduction band edge in GaNxAs1-x Kirstin Alberi, O.D. Dubon, A. Minor, W. Shan, K.M. Yu, W. Walukiewicz, S.J. Chung, D.E. Mars, F. Zavaliche A novel approach for inducing a planar modulation of the conduction band for the fabrication of quantum structures within a GaN$_{x}$As$_{1-x}$ thin film is presented. This dilute nitride alloy exhibits a giant bandgap bowing as a result of the downward shift of the conduction band edge upon the incorporation of N into GaAs. We demonstrate that amorphization of a GaN$_{x}$As$_{1-x}$ film followed by rapid thermal annealing results in the realease of N from the As sublattice and the formation of eptiaxial, single-crystalline GaAs. Selective amorphization of GaN$_{x}$As$_{1-x}$ has been achieved by Ga ion implantation using a focused ion beam (FIB). For typical FIB implantation conditions, an amorphous region approximately 20 nm deep and 50 nm wide is produced. Upon annealing the pattern of amorphized film regrows, yielding co-existing regions of GaAs and (unimplanted) GaN$_{x}$As$_{1-x}$ across the surface. Thus, dot and wire structures are fabricated through appropriate FIB implantation pattern design. [Preview Abstract] |
Tuesday, March 22, 2005 10:12AM - 10:24AM |
H15.00008: Electron Transport in Dilute Alloys of GaAs(1-x)N(x) Richard Ahrenkiel, Joseph Luther, Steven Johnston, Wyatt Metzger Alloys of GaAs(1-x)N(x) where x $<$ 0.03 are currently very popular because of the abnormally large bowing coefficient with these dilute nitrogen concentrations. Electron transport in p-type samples is problematic due to low carrier mobilites and diffusion lengths. Our time-resolved photoluminescence measurements indicate very short recombination lifetimes; these lifetimes are far shorter than would be predicted by radiative recombination theory. Our DLTS studies of the alloy have shown strong electron trapping with an emission energy of about 0.2 eV. This trap does not show filling behavior, even with very long (10 s) filling pulses. Also, the minority carrier DLTS peak occurs without injecting, forward-bias electron pulses, indicating pair generation in the junction. Our recent temperature-dependent current voltage measurements of n-GaAs/p- GaAs(1-x)N(x) heteojunctions indicate a reverse saturation current, J$_{0}$, that is decadesof magnitude larger than found in similar doped GaAs homojunctions. Furthermore, there is a large step in J$_{0}$ at temperature above about 200 K, which increases with nitrogen concentration, x. These data can be attributed to nitrogen complexes or clusters that began to form an impurity band very low x-values. Our model of electron transport, that is compatible with the above data, will be described. [Preview Abstract] |
Tuesday, March 22, 2005 10:24AM - 10:36AM |
H15.00009: Dilute GaAs-Nitride Alloys Grown with Bismuth by MBE D. A. Beaton, T. Tiedje, S. Tixier, S. E. Webster, N. R. Zangenberg, E. C. Young, P. Wei, F. Schiettekatte, S. Francoeur Under conventional MBE growth conditions for the dilute nitride GaN$_{x}$As$_{1-x}$, a Bi flux sufficient to saturate the growth surface yields negligible Bi incorporation due to its strong tendency to surface segregate. However the Bi increases the N incorporation efficiency by up to 50{\%} and improves the electronic quality of the material by reducing the density of gap states as measured by variable temperature photoluminescence. Under conditions of low As overpressure and low growth temperature ($\sim $370C) Bi can be incorporated into GaAs either by itself or in combination with N. Similar to the dilute nitride, the dilute bismide exhibits strong concentration dependent reductions in the bandgap. We observe strong room temperature photoluminescence at 1150 nm in a dilute quaternary GaN$_{x}$As$_{1-x-y}$Bi$_{y}$ alloy lattice matched to GaAs. The quaternary alloy shows a bandgap reduction that is equal to or larger than the reduction due to the sum of the effects of Bi and N individually, in contrast to GaInNAs where the combined effect of In and N is less than the sum of the effects of In and N individually. [Preview Abstract] |
Tuesday, March 22, 2005 10:36AM - 10:48AM |
H15.00010: Role of Nitrogen on Emission Wavelengths of InAs Quantum Dots: InAs/GaAs Interfaces and Strain-compensating GaAsN Burying Layers I. Suemune, S. Ganapathy, K. Uesugi, N. Matsumura, Y. Nabetani, T. Matsumoto Extension of emission wavelengths of InAs QDs longer than 1.3 $\mu $m has been difficult. Our group has demonstrated GaAsN strain- compensating layers (SCL) on InAs QDs and emission wavelength was extended to 1.55 $\mu $m. Two main issues of physical origin extending wavelengths will be discussed. One is role of N at InAs/GaAs interfaces. Intentional ``nitridation'' of InAs QDs surfaces just before burying with GaAs layers was studied. From modeling of relation between lowest energy peak and QD-states energy separations, main factor for red-shift was attributed to suppression of Ga inclusion in InAs QDs, consistent with VFF model calculation. The other is study of compressive strain in InAs QDs and tensile strain in GaAsN SCLs with channeling in RBS, showing lattice constants parallel to surface remained unchanged and those normal to surface were extended in InAs QDs and shrinked in GaAsN SCLs, demonstrating effect of GaAsN SCLs. [Preview Abstract] |
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