Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session S14: Focus Session: Dopants and Defects in III-V and II-VI Semiconductors |
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Sponsoring Units: DMP FIAP Chair: Kirstin Alberi, National Renewable Energy Laboratory Room: 008A |
Thursday, March 5, 2015 8:00AM - 8:12AM |
S14.00001: Carrier Decay and Diffusion Dynamics in Single-Crystalline CdTe as seen via Microphotoluminescence Angelo Mascarenhas, Brian Fluegel, Kirstin Alberi, Yong-Hang Zhang The ability to spatially resolve the degree to which extended defects impact carrier diffusion lengths and lifetimes is important for determining upper limits for defect densities in semiconductor devices. We show that a new spatially and temporally resolved photoluminescence (PL) imaging technique can be used to accurately extract carrier lifetimes in the immediate vicinity of dark-line defects in CdTe/MgCdTe double heterostructures. A series of PL images captured during the decay process show that extended defects with a density of 1.4x10$^{-5}$ cm$^{-2}$ deplete photogenerated charge carriers from the surrounding semiconductor material on a nanosecond time scale. The technique makes it possible to elucidate the interplay between nonradiative carrier recombination and carrier diffusion and reveals that they both combine to degrade the PL intensity over a fractional area that is much larger than the physical size of the defects. Carrier lifetimes are correctly determined from numerical simulations of the decay behavior by taking these two effects into account. Our study demonstrates that it is crucial to measure and account for the influence of local defects in the measurement of carrier lifetime and diffusion, which are key transport parameters for the design and modeling of advanced solar-cell and light-emitting devices. [Preview Abstract] |
Thursday, March 5, 2015 8:12AM - 8:24AM |
S14.00002: Carrier scattering mechanisms in p-type transparent copper-alloyed ZnS: Crystalline vs. amorphous Rachel Woods-Robinson, Alireza Faghaninia, Jason K. Cooper, Hieu H. Pham, Cynthia Lo, Lin-Wang Wang, Joel W. Ager Crystalline (wurtzite and sphalerite) and amorphous forms of copper-alloyed ZnS (Cu$_{\mathrm{x}}$Zn$_{\mathrm{1-x}}$S) are p-type conducting transparent thin film materials with near-record figures of merit for applications in photovoltaics and optoelectronics. Remarkably, the conductivity of amorphous Cu$_{\mathrm{x}}$Zn$_{\mathrm{1-x}}$S, 42 S/cm at x $=$ 0.30, is nearly as high as crystalline Cu$_{\mathrm{x}}$Zn$_{\mathrm{1-x}}$S (54 S/cm at x $=$ 0.21). This contrasts with typical observations of poorer carrier transport in amorphous materials. By combining experiment and computation, we investigate the defect physics underlying hole transport in amorphous and crystalline Cu$_{\mathrm{x}}$Zn$_{\mathrm{1-x}}$S. Structural probes (EXAFS, TEM and wide-angle XRD) are used to determine bonding characteristics and lattice order, and serve as inputs to ab initio hybrid functional HSE calculations of the electronic band structure. Hall effect, temperature dependent conductivity (15K to 500K), and XPS valence band measurements and ab initio calculations show that hole conduction occurs in a hybridized S-3p and Cu-3d valence band for amorphous and crystalline films. The hole scattering mechanisms which limit the conductivity will be discussed in the context of theoretical carrier transport model based on Boltzmann transport equation, ab initio calculated band structure, and phonon dispersion. [Preview Abstract] |
Thursday, March 5, 2015 8:24AM - 8:36AM |
S14.00003: Diffusion of Interstitial Defects in CdTe Su-Huai Wei, Ji-Hui Yang, Jie Ma, Joongoo Kang CdTe is one of the most promising candidates for thin-film photovoltaic applications and it is well known that Cu and Cl diffusions play critical roles in improving the CdTe solar cell efficiency. However, the diffusion behavior of these impurities as well as the host elements in CdTe has not been clearly understood. Using first-principles calculations, the diffusion behaviors of the cation atoms (Cd and Cu) and the anion atoms (Te and Cl) at different charged states are investigated. We find that, due to different electronic level occupations and level splittings, the diffusions of the cation atoms and anion atoms are very different. We explain why Cu can diffuse much faster than other elements and show that the diffusion speeds of the impurities can be controlled by tuning the Fermi level of CdTe. We have also developed a general diffusion coefficient theory for multi-barrier diffusion. Our calculated diffusivity of the interstitial impurities agrees well with available experimental measurements. [Preview Abstract] |
Thursday, March 5, 2015 8:36AM - 8:48AM |
S14.00004: ABSTRACT WITHDRAWN |
Thursday, March 5, 2015 8:48AM - 9:00AM |
S14.00005: First-principles study of low $\Sigma$ grain boundaries in CdTe Ji-Sang Park, Joongoo Kang, Ji-Hui Yang, Wyatt Metzger, Su-Huai Wei Grain boundaries (GBs) play critical roles in determining physical properties of polycrystalline materials. In this study, we investigate stability and electronic structure of GBs in CdTe through first-principles density functional calculations. We consider low $\Sigma$ symmetric tilt GBs including $\Sigma$3 (111), $\Sigma$3 (112), $\Sigma$5 (120), and $\Sigma$5 (130) GBs. We find that the $\Sigma$3 (111) GB is more stable than the other GBs considered in this study because it contains no dangling bonds and wrong bonds. The stability of the $\Sigma$3 GBs is independent on the chemical potential of Cd and Te whereas that of the $\Sigma$5 GBs depends on the chemical potentials. Unexpectedly, we find that the $\Sigma$5 (120) GBs are able to be more stable than the $\Sigma$3 (112) GBs, despite that the $\Sigma$3 (112) GBs have often been used as a model system to study GBs in polycrystalline thin-film photovoltaic materials. The $\Sigma$5 (120) GBs found in this study are not electrically harmful even though the GBs contain wrong bonds. [Preview Abstract] |
Thursday, March 5, 2015 9:00AM - 9:12AM |
S14.00006: Role of phonon-assisted processes in Auger recombination in InAs Jimmy-Xuan Shen, Daniel Steiauf, Anderson Janotti, Chris G. Van de Walle Auger recombination has been identified as an important loss mechanism in InAs and InAs-alloy based light emitters, yet the mechanisms are not fully understood. In particular, it is unclear whether direct or indirect processes dominate. The direct process involves only Coulomb interaction, while the indirect process is mediated by the absorption or emission of a phonon. We present results of first-principles calculations of the direct and indirect phonon-assisted Auger coefficients in InAs and related alloys. The direct process is usually assumed to dominate in small-band-gap semiconductors. However, we find that indirect phonon-assisted processes also contribute significantly to the Auger rate. We untangle the contributions of the electron-electron-hole and hole-hole-electron processes, and pinpoint the phonon modes that are most relevant to the indirect process. [Preview Abstract] |
Thursday, March 5, 2015 9:12AM - 9:24AM |
S14.00007: Electrical characterization of GaAsN and GaAsBi single-quantum-well diodes J. Occena, R.L. Field III, A.S. Teran, T. Jen, C. Kurdak, J.D. Phillips, R.S. Goldman The highly mismatched alloys GaAsN and GaAsBi have been identified as promising candidates for the active layer of optoelectronic devices because their bandgaps can be tuned with minimal change in lattice parameter. Since the conduction band minimum (CBM) of GaAsBi and the valence band maximum (VBM) of GaAsN are approximately aligned with the CBM and VBM of GaAs, with corresponding significant valence and conduction band offsets, GaAsN/GaAsBi is expected to exhibit a staggered Type II band-offset. The resulting spatial separation of charge and optical absorption at wavelengths smaller than the fundamental bandgap, on either side of the junction, is promising for both infrared detectors and solar cells. To date, few groups have grown GaAsNBi alloys and the GaAsN/GaAsBi heterostructure has yet to be realized. To explore the N- and Bi-related states near the CB and VB in GaAs(N)(Bi) alloys, Schottky diodes containing GaAsN(Bi) quantum wells (QWs) are grown by molecular-beam epitaxy at low temperature, using Si as either an n- or p-type dopant. For both GaAsN and GaAsBi QWs, rectifying current-voltage characteristics are observed, with reasonable diode ideality factors. We will discuss the influence of increasing N and Bi fraction on the formation of N- and Bi-related states. We will also discuss progress towards measurements of the VB and CB offsets using admittance spectroscopy. [Preview Abstract] |
Thursday, March 5, 2015 9:24AM - 9:36AM |
S14.00008: First-Principle Tight Binding-Like Parametrizations of GaAs$_{\mathrm{1-x}}$Bi$_{\mathrm{x}}$ and InAs$_{\mathrm{1-x}}$Bi$_{\mathrm{x}}$ Electronic Structures S.C. Badescu, M.E. Grupen, J. Hader, J.V. Moloney, S.W. Koch The anion substitution with Bi atoms in large concentration (1-5{\%}) has been proven to be an effective means for tuning the energy bandgap and the spin-orbit in materials like GaAs and InAs. In order to describe these materials in opto-electronic device simulations it is necessary to use simple and accurate parametrizations of their bandstructures. We describe here tight binding--like parametrizations of first-principle bandstructures to compare the better-know GaAs$_{\mathrm{1-x}}$Bi$_{\mathrm{x}}$ with the newer InAs$_{\mathrm{1-x}}$Bi$_{\mathrm{x}}$. Accurate bandgaps are included via hybrid density functionals, and spin-orbit split-offs of the valence bands as well as the d-orbitals for In are found to be crucial. Essential features such as the strong perturbation of the Luttinger Hamiltonian and the strong anticrossing between valence bands and impurity d-orbitals are captured in a zone-unfolding picture. [Preview Abstract] |
Thursday, March 5, 2015 9:36AM - 9:48AM |
S14.00009: Isoelectronic Traps in Gallium Phosphide Theresa Christian, Kirstin Alberi, Daniel Beaton, Brian Fluegel, Angelo Mascarenhas Isoelectronic substitutional dopants can result in strongly localized exciton traps within a host bandstructure such as gallium arsenide (GaAs) or gallium phosphide (GaP). These traps have received great attention for their role in the anomalous bandgap bowing of nitrogen or bismuth-doped GaAs, creating the dramatic bandgap tunability of these unusual dilute alloys. In the wider, indirect-bandgap host material GaP, these same isoelectronic dopants create bound states within the gap that can have very high radiative efficiency and a wealth of discrete spectral transitions illuminating the symmetry of the localized excitonic trap state. We will present a comparative study of nitrogen and bismuth isoelectronic traps in GaP. Research was supported by the U. S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division under contract DE-AC36-08GO28308 and by the Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF), made possible in part by the American Recovery and Reinvestment Act of 2009, administered by ORISE-ORAU under contract no. DE-AC05-06OR23100. [Preview Abstract] |
Thursday, March 5, 2015 9:48AM - 10:00AM |
S14.00010: Direct observation of the symmetry of core states of a single Fe impurity in GaAs Juanita Bocquel, Victoria Kortan, Richard Campion, Bryan Gallagher, Michael E. Flatt\'e, Paul Koenraad We report the observation of the two mid-gap core d-states of differing symmetry for a single Fe atom embedded in GaAs by scanning tunneling microscopy. By voltage control we can manipulate the charge state (Fe3$+$ or Fe2$+)$ of the embedded Fe atom. For Fe in the Fe3$+$ state two different deep core d-states with E and T2 symmetry are distinguished by the strength of their hybridization with the surrounding host electronic structure. The mid-gap state of Fe that does not hybridize via sigma-bonding is strongly localized to the Fe atom, whereas the mid-gap state that does hybridize via sigma-bonding is extended, and comparable in size to other acceptor states measured previously. Tight-binding calculations of these mid-gap states agree with the measurements, and illustrate that such measurements can determine the degree of hybridization via pi-bonding of impurity d-states. In addition to fundamental probing of mid-gap d-state electronic structure, measurements of such intrinsically spin-orbit-entangled, single-dopant states may be of use for high-speed electrical control of single spins. [Preview Abstract] |
Thursday, March 5, 2015 10:00AM - 10:12AM |
S14.00011: Excitons bound to stacking fault planes in GaAs: a novel 2D excitonic system Todd Karin, Xiayu Linpeng, Sarah Harvey, Arne Ludwig, Andreas Wieck, Kai-Mei Fu This work takes a first step in characterizing the unique optical properties of stacking fault defects in GaAs grown by molecular beam epitaxy. We observe narrow-band bright luminescence from carriers bound to the attractive strain potential formed by a stacking fault. The strong radiative emission is concentrated in two narrow bands ($\sim90~\mu$eV width) at either 828.65 or 830.40 nm depending on the stacking fault orientation. Stacking fault defects can be imaged using far-field confocal microscopy by collecting the the narrow band photoluminescence. Polarization-resolved photoluminescence and magnetic field measurements are consistent with a theory of light-hole excitons bound to the stacking fault plane with a quantization axis normal to the plane. Moreover, the wavelength shift of the excitonic emission provides a direct measure of the strain potential due to this fundamental growth defect. The narrow linewidth and high homogeneity across many defects suggests excitons are bound to a single atomically-thin stacking fault plane. This work opens the door to a novel, highly homogeneous, 2D light-hole excitonic system in the well-characterized material GaAs. [Preview Abstract] |
Thursday, March 5, 2015 10:12AM - 10:24AM |
S14.00012: Raman scattering and time-resolved photoluminescence characterization of defects in GaAs/AlGaAs double heterostructures Zehra Cevher, Patrick Folkes, Yuhang Ren GaAs/AlGaAs heterostructures remain to be one of the most promising materials for the fabrication of photodetectors, semiconductor lasers, and photovoltaic devices. We used Raman scattering and time-resolved photoluminescence spectroscopy to study the structural defects of GaAs/AlGaAs double heterostructures grown with various As/Ga flux ratios and substrate temperatures. The optimized structure was obtained with the As/Ga flux ratio equal to 15 and the substrate temperature close to 595$^{o}$C. The high residual impurity of the AlGaAs layers was activated and elevated as the ratio was increased from 15 to 40. As the growth temperature was decreased from 595 to 550 $^{o\, }$C, the layers became more defective. Moreover, we reveal that the defect levels were depressed by including a Bragg reflector layer. [Preview Abstract] |
Thursday, March 5, 2015 10:24AM - 10:36AM |
S14.00013: Profiling the local carrier concentration across a semiconductor quantum dot J.C. Walrath, Yen-Hsiang Lin, S. Huang, R.S. Goldman We profile the local carrier concentration, n, across epitaxial InAs/GaAs quantum dots (QDs) consisting of 3D islands on top of a 2D alloy layer. We use scanning thermoelectric microscopy to measure the temperature gradient-induced voltage, which is then converted to the local Seebeck coefficient, S. The S profile is then converted to a conduction band-edge profile and compared with Poisson-Schrodinger band-edge simulations. Our combined computational-experimental approach suggests a reduced carrier concentration in the QD center in comparison to that of the 2D alloy layer. The relative roles of free carrier trapping and/or dopant expulsion are discussed. [Preview Abstract] |
Thursday, March 5, 2015 10:36AM - 10:48AM |
S14.00014: Influence of arsenic species on the growth and properties of GaAsBi alloys R.L. Field III, J. Occena, T. Jen, M. Luengo-Kovac, B. Yarlagadda, V. Sih, C. Kurdak, R.S. Goldman Due to the significant bandgap reduction associated with bismuth incorporation, dilute bismuthide semiconductor alloys have been proposed for high-efficiency optoelectronic devices. To achieve significant incorporation of bismuth into GaAsBi, molecular beam epitaxy at low temperature is required. Furthermore, many groups use As$_{\mathrm{2}}$ for low-temperature growth of GaAsBi, presumably due to historical reports of improved photoluminescence for low-temperature growth of GaAs with As$_{\mathrm{2}}$ in comparison with As$_{\mathrm{4}}$. Here, we show that Bi incorporation into GaAs is favorable over a wider range of growth conditions with As$_{\mathrm{4}}$ in comparison with As$_{\mathrm{2}}$. The preference for Bi incorporation with As$_{\mathrm{4}}$ is associated with the differences in the likelihood for As$_{\mathrm{2}}$ vs. As$_{\mathrm{4}}$ to replace weakly bonded surface Bi$_{\mathrm{2}}$. For growth with As$_{\mathrm{4}}$, the electron mobility for GaAsBi:Si is as high as 2500 cm$^{\mathrm{2}}$/V-s for Si-doped (n $\approx $ 10$^{\mathrm{18}}$ cm$^{\mathrm{-3}})$ GaAsBi, higher than reported values for growth using As$_{\mathrm{2}}$. The hole mobility of Si-doped GaAsBi is essentially independent of $x$ up to 0.043, making Si a promising alternative to C or Be for $p$-type doping of GaAsBi and related bismuthide alloys. In addition, a comparison of the photoluminescence spectra of films grown with both As$_{\mathrm{2}}$ and As$_{\mathrm{4}}$ will be discussed. [Preview Abstract] |
Thursday, March 5, 2015 10:48AM - 11:00AM |
S14.00015: Radiative properties of multi-carrier bound excitons in GaAs Kai-Mei Fu, Todd Karin, Russell Barbour, Charles Santori, Yoshihisa Yamamoto, Yoshiro Hirayama Excitons in semiconductors can have multiple lifetimes due to spin dependent oscillator strengths and interference between different recombination pathways. In addition, strain and symmetry effects can further modify lifetimes via the removal of degeneracies. We present a convenient formalism for predicting the optical properties of k=0 excitons with an arbitrary number of charge carriers in different symmetry environments. Using this formalism, we investigate the radiative lifetime of the neutral acceptor-bound exciton (A$^0$X) in GaAs. We predict three distinct lifetimes for the 12-state three-carrier complex. We confirm this prediction through polarization dependent and time-resolved photoluminescence experiments. We find the acceptor bound-exciton lifetimes to be $(T_0,3 T_0,0.75 T_0)$ where $T_0=(0.61\pm0.12)$ ns. Furthermore, we provide an estimate of the intra-level and inter-level exciton spin-relaxation rates. The results are in contrast to the previously reported single 1.6$\pm$0.6 ns lifetime for this system and highlight the importance of a unified treatment of all recombination pathways when deriving the radiative properties of multi-carrier excitons. [Preview Abstract] |
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