Session W23: Semiconductors: Theory & Spectra II

Line width resonance of the longitudinal optical phonon in GaAs:N

We extend resonant Raman scattering studies of Mascarenhas et al. [PRB68, 233201 (2003)] of GaAs$_{1-x}$N$_x$ to the ultra-dilute nitrogen doping concentrations, whereby we unambiguously resolve the line width resonances of the LO phonon. A discontinuity is observed in the LO phonon line width resonance energy as a function of concentration. With decreasing nitrogen concentration the $E_W$ line width resonance energy reduces by ca. 40 meV at $x=0.4\%$. This value corresponds to the concentration, at which the localized to delocalized transition manifests itself in the electro-reflectance signature line widths.   

Micro-Raman study of InAs/GaSb superlattices from front and cleaved edge

The InAs/GaSb superlattice (SL) has a ``broken-gap'' type II band alignment, with its effective bandgap being able to be tuned by changing the thickness of individual layers. Therefore, it is of great interest for mid- and far-IR detection. Because the SL does not have common cation or anion at the interface, there are two types of interfacial layers: InSb and GaAs, that impact the device performance. We investigate the SLs grown on either GaSb or InAs substrate and with difference interfacial treatment using confocal micro-Raman spectroscopy on both front surface and cleaved edge of the epilayer with polarization.   

Raman Investigation of p-type Amorphous Silicon Thin Films

Thin film layers of p-type a-Si:H of differing doping concentration and hydrogen dilution were investigated by Raman spectroscopy to determine their effect on short- and mid-range order. In this study, the TA and TO peaks were used to study the microstructure of the thin films. Our analyses reveal an interesting counter-balance relationship between the boron-doping and hydrogen-dilution growth parameters. Specifically, an increase in the hydrogen dilution ratio (H$_{\mathrm{2}}$/SiH$_{\mathrm{4}})$ was found to cause the increase in the short-range order, as evidenced by an increase in the TO frequency and a decrease in the FWHM of the spectral peak. However, an increase in the doping concentration resulted in a decrease in the short-range order, as evidenced by a decrease in the TO frequency and an increase in the FWHM of the spectral peak. These results will be correlated with Multiple Internal Reflection Infrared Spectroscopy, electrical transport and noise in a-Si:H thin films to determine the effects of doping and hydrogen on the transport mechanisms in a-Si:H.   

Scanning tunneling microscope study of La- and Sb-doped BaSnO$_3$ thin films

The La-doped BaSnO$_3$(BLSO) system was found to exhibit high electron mobility and high oxygen stability along with its transparency in visible spectrum. Additionally, we recently observed a significant difference in electron mobility values between BLSO and Sb-doped BSO (BSSO) epitaxial thin films. In order to elucidate the origin of the different mobility in BLSO and BSSO thin films, we have investigated a density of states (DOS) of BLSO and BSSO by scanning tunneling microscopy and spectroscopy. Our measurements were performed at 77 K in ultra-high vacuum of 2x10$^{-10}$ Torr. We will compare the DOS of the conduction band of BLSO with that of BSSO. Only in the conduction band of BSSO, we found a specific peak that can be identified as due to the localized Sb impurity states. Our results provide strong evidence for the strong influence of localized Sb impurity states on the electron mobility. We will explain our data by anisotropy of scattering on the Fermi surface by resorting to band structure calculations of BLSO and BSSO.   

Origin of Charge Separation in III-Nitride Nanowires under Strain

The structural and electronic properties of BN, AlN and GaN nanowires (NWs) under different strain condition are investigated using first-principles calculations. We found an anomaly of band gap change with respect to the applied external uniaxial strain. We show that this is due to the band crossing caused by the crystal field splitting at the top of the valance band. Due to the difference of the atomic relaxation at the core and surface regions of the NW, we show that electron and hole separation can be achieved when the compressive uniaxial strain exceeds the critical value $|\epsilon_c|$.   

Band offsets at GaN/ZnGeN$_2$ interfaces

The interfaces of GaN/ZnGeN$_2$ are of interest because of their close lattice match and hence suitability of GaN as substrate for ZnGeN$_2$ film growth. In the present work, the band offsets for various polar and non-polar interfaces between GaN and ZnGeN$_2$ are determined from full potential linear muffin-tin orbital (FP-LMTO) within the local density approximation (LDA). We determine the dipole potential formed at the interface from self-consistent supercell calculations and then add the difference between the bulk band-edges energy levels. Quasiparticle self-consistent GW corrections of the bulk band edges relative to the LDA edges are added. The strain state of the ZnGeN$_2$ is determined by assuming an unstrained GaN substrate with the ZnGeN$_2$ in-plane lattice constants matched to the substrate and the perpendicular lattice constant determined by minimizing the elastic strain energy using the known elastic constants. We find that the offset is type II, meaning staggered instead of straddled alignment, which is of interest for photovoltaic applications as holes and electrons would separate in different regions. The band offset depends slightly on interface direction. The orientation averaged valence band maximum of GaN is 0.86 eV lower than ZnGeN$_2$'s. The charge neutrality point alignment model is tested and found to give a significantly smaller band offset.   

Pentacene Derivatives: Electronic Structure and Spectra

The variation in composition and structure of the substituent groups of pentacene compounds promises a broad range of electronic structures and behaviors and provides a vast and alluring field of inquiry with avenues of exploration. These include the development of synthetic schema, the process of design for novel derivatives and, in order to identify those hypothesized compounds which demonstrate the desired behavior, the identification and refinement of computational tools that make accurate predictions about the electronic behavior of theoretical compounds. Two computational techniques and six pentacene derivatives are here examined. One technique was used to predict the vibrational spectra of the compounds, in order to both acquire data about the optical conductivity of the compounds and to establish a pool of theoretical data against which experimental data will be compared. The molecular orbital energy level diagram of the same six compounds was derived using a second approach, with the same goals of discerning between valid and invalid predictive schema by comparison with pending experimental data and between hypothesized compounds which show promise and those which present little potential for use in organic semiconductor technology.   

Many-body physics of intersubband polaritons

Intersubband polaritons are light-matter excitations originating from the strong coupling between an intersubband quantum well electronic transition and a microcavity photon mode. Up to now intersubband polaritons have been observed in a wide range of the electromagnetic spectrum from the mid-infrared to the THz regime. Due to their composite bosonic nature, the matter part of these excitations is responsible for a non-trivial dynamics of cavity polaritons. We studied how the Coulomb electron-electron interaction and the Pauli saturation of the electronic transitions affect the many-body physics of intersubband polaritons [1]. As a first application we calculated the efficiency of intersubband polariton-polariton scattering, paving the way to promising quantum non-linear optics especially in the THz regime.\\[4pt] [1] L. Nguyen-Th\^e, S. De Liberato, M. Bamba, C. Ciuti, submitted.   

Band-gap variations in polytypes of SiC: misleading parameter ``hexagonality''and proposal of new parameter

Silicon carbide (SiC) has been discovered in various polytypes. Each polytype is characterized by its stacking of atomic planes. The band gap varies substantially in each polytype from 2.40 eV to 3.33 eV in spite that the local atomic structures are identical to each other. Recently, we have clarified the mechanism of this intriguing property based on the density functional theory [1]. We have found that the Kohn-Sham orbital at the conduction-band bottom extends broadly in the internal space called channels, and thus floating in the matter. Therefore, important parameter describing the band-gap variations is the channel length, not ``hexagonality,'' which is thought to be important for the band-gap variations. \\[4pt] [1] Y.-I. Matsushita, S. Furuya, A. Oshiyama, PRL, 108, 246404 (2012).   

Band gap formation in tetrahedrally bonded I$_3$-V-VI$_4$ semiconductors-the role of V lone pairs

An interesting class of tetrahedrally coordinated ternary compounds have attracted considerable interest because of their potential as good thermoelectrics. These compounds, denoted as I$_3$-V-VI$_4$, contain three monovalent-I (Cu, Ag), one pentavalent-V (P, As, Sb, Bi), and four hexavalent-VI (S, Se, Te) atoms; and can be visualized as ternary derivatives of the II-VI zincblende or wurtzite semiconductors, obtained by starting from four unit cells of (II-VI) and replacing four type II atoms by three type I and one type V atoms. In trying to understand their electronic structures and transport properties, some fundamental questions arise: whether V atoms are indeed pentavalent and if not how do these compounds become semiconductors, what is the role of V lone pair electrons in the origin of band gaps, and what are the general characteristics of states near valence band maxima and conduction band minima. We will answer some of these questions using ab initio electronic structure calculations (density functional methods with both local and nonlocal exchange-correlation potentials). Some part of this work has been published in Dat et al, J. Phys.: Condens. Matter 24, 415502 (2012).   

NMR spectroscopy around filling factor three

We probe the spin signatures of a two-dimensional electron system, confined to a GaAs quantum well, around filling factor three ($\nu \sim 3$) using resistively detected nuclear magnetic resonance (RDNMR) spectroscopy at milliKelvin temperatures. Whereas the existence of spin textures, known as skyrmions, around filling factor one is well established, an understanding of the spin degrees of freedom for odd-integer states in higher Landau levels remains elusive. It is believed that for skyrmions to exist at $\nu \sim 3$, the Zeeman energy needs to be smaller than in the case of $\nu \sim 1$ [1]. We measured the spin-lattice relaxation time, $T_1$, which is sensitive to these spin textures as they trigger a rapid nuclear spin relaxation. Our $T_1$ measurements around $\nu = 3$ at 5 T find a small spin-lattice relaxation rate, suggesting the absence of skyrmions. In addition, our Knight shift measurements corroborate this interpretation. Furthermore, we report striking anomalies in the RDNMR spectral line shape and discuss their origin in conjunction with our findings. \\* $[1]$N. R. Cooper, Phys. Rev. B 55, R1934 (1997).   

Phonon-Assisted Auger Recombination in Gallium Arsenide and Gallium Nitride from First Principles

GaN and GaAs and their alloys are technologically important materials for solid-state optoelectronic devices such as light emitting diodes. The internal quantum efficiency of these devices, defined as the fraction of electron-hole pairs converted to photons, is limited by non-radiative loss mechanisms. Auger recombination is such a mechanism which decreases the efficiency at high current densities. In this process, the energy and momentum of an electron-hole pair is transferred to a third carrier. Numerically it is found that this process does not lead to relevant loss rates. However, if a phonon is emitted or absorbed at the same time, Auger loss rates increase by several orders of magnitude. We calculate the Auger recombination rate coefficients from first principles using density functional theory. Treating also the phonons from first principles allows us to analyze which modes and wave vectors contribute predominantly to Auger recombination and the non-radiative loss in these materials.   

Core level shift and charge transfer of Sr templates on Si(001) for epitaxial oxide growth: theoretical and experimental study

Sub-monolayer Sr templates are used as a transition layer in the epitaxial growth of perovskite oxides on semiconductors. However, a detailed understanding of how the template enables oxide growth on Si(001) is still lacking. Sr on Si(001) shows different structural and electronic properties as a function of Sr coverage. Using a combination of \textit{in situ }reflection high energy electron diffraction (RHEED) and \textit{in situ} x-ray photoelectron spectroscopy (XPS), we observed both the Si 2p and Sr 3d core levels shift toward higher binding energy as Sr coverage increases up to one half monolayer. In addition, increase of Sr coverage leads to unbuckling of the Si dimer atoms as evidenced by the merging of the up and down dimer core level components as Sr donates charge to the dimer atoms. The work function of Si also shifts with Sr coverage as observed using ultraviolet photoelectron spectroscopy (UPS).   

Inelastic electron and light scattering from the elementary electronic excitations in quantum wells

The most fundamental approach to an understanding of electronic, optical, and transport phenomena which the condensed matter physics offers is generally founded on two experiments: the inelastic electron scattering and the inelastic light scattering. This work embarks on providing a systematic framework for the theory of inelastic electron scattering and of inelastic light scattering from the electronic excitations in quantum wells. To this end, we start with the Kubo's correlation function to derive the generalized dielectric function, the inverse dielectric function, and the Dyson equation for the screened potential within the framework of Bohm-Pines' random-phase approximation. This is followed by a thorough development of the theory of inelastic electron scattering and of inelastic light scattering. After trying and testing the eigenfunctions, we compute the density of states, the Fermi energy, the full excitation spectrum made up of single-particle and collective (plasmon) excitations, the loss functions for the inelastic electron scattering, and the Raman intensity for the inelastic light scattering. It is found that HREELS can be a potential alternative of the overused Raman scattering for investigating collective excitations in such nanostructures.