Session S33: Semiconductors and Applications II

Examining the effect of Tl content on the optical, electrical, and structural properties of TlGaAs films

Tl containing III-V semiconductor alloy materials are of interest for use in new infrared detector structures and other applications requiring low bandgap material. Additions of relatively small amounts of Tl can significantly reduce the bandgap without large changes in lattice constant. However, the properties of the Tl containing III-Vs have not been extensively studied because it is difficult to produce material with high Tl content. In this work we examine the properties of Tl_xGa_1-xAs films grown at very low (<220°C) temperatures by solid-source molecular beam epitaxy (MBE). The optical, electrical, and structural properties of TlGaAs films have been characterized by a variety of techniques, including spectroscopic ellipsometry, high resolution x-ray diffraction, Rutherford back-scattering spectrometry, atomic force microscopy, Hall effect measurement, and transmission electron microscopy. We will discuss the properties of these TlGaAs films as a function of both MBE growth conditions and resulting film Tl composition, with a particular emphasis on the change in film optical and electrical properties with changing Tl content.   

Role of surface reconstruction and passivation mechanisms on the structural and electronic properties of diamond surfaces

Diamond is an insulating material in the bulk form which can exhibit two-dimensional p-type conducting characteristics when its surface is passivated with hydrogen. It is postulated that the formation of a conducting channel at the hydrogenated surface is mainly due to charge transfer between the surface and atmospheric adsorbates. A fundamental understanding of the source of p-type conductivity at the hydrogenated surface is key to the realization of diamond surface field effect transistor (FET). Though there have been many experimental studies in this direction, theoretical studies correlating surface termination and reconstruction to structural and electronic properties have been limited. To address these fundamental questions, we have utilized ab initio density functional theory (DFT) and performed systematic studies of surface reconstruction phenomena in (100) and (110) diamond surfaces. Based on the observed structural and electronic properties, we have made an attempt to establish a correlation between the surface reconstruction and passivation with the electronic properties of the diamond surfaces, such as surface energy, electron affinity, and effective mass.   

Examining the effects of changing film composition and temperature on the optical properties of GaAsBi epitaxial films

GaAs_1-xBi_x semiconductor alloys are being considered for a range of new infrared optoelectronic applications. This is because small changes in Bi content can produce large decreases in film bandgap, opening up new lattice parameter and bandgap combinations for devices. However, the optical properties of GaAsBi as a function of changing Bi content have not been thoroughly documented and seldom tested at temperatures relevant for infrared detectors or thermophotovoltaics. In this work we study the optical properties of GaAsBi films of varying Bi composition grown by solid source molecular beam epitaxy (MBE). Optical properties are measured by variable angle spectroscopic ellipsometry over a wavelength range from 300 nm to 32 μm and at temperatures ranging from 77K to 600K. The optical properties of the system are modeled so that the GaAsBi optical constants can be separated from those of the underlying substrate. Characterization of GaAsBi optical properties over a range of Bi compositions and temperatures will provide the necessary data for design of future devices and a better fundamental understanding of the material system.
  

A Computational study of Magnetic Field-controlled Acoustic Coherent Phonon Generation and Propagation in Ferromagnetic GaMnAs

We present a theoretical study of magnetic field-controlled generation and propagation of coherent longitudinal acoustic phonons (CLAP) generated by ultrafast laser pulses in a Ferromagnetic Ga_1-xMn_xAs (x=0.082) thin film grown on an intrinsic GaAs substrate subjected to an in-plane static magnetic field varying between 2-10T. Electron-phonon deformation potential coupling is considered for the generation of the CLAP at the surface as well as at the interface for a pump energy of 3.1 eV with their propagation being governed by a semi-classical wave equation. The field-induced variation in the bandstructure, dielectric functions, and, hence, the CLAP amplitude are calculated using an 8-band Pidgeon-Brown model including the effects of magnetic, Mn impurities. The calculated differential reflectivity at the probe energy 1.55 eV agrees well with experimental time-resolved differential reflectivity measurements and suggests that a substantial contribution to the signal comes from the interface. The magnetic field dependence of the reflectivity amplitude arises from the Seraphin coefficients.   

Two-photon absorption spectroscopy and polarization dependence of Gallium phosphide

The two-photon absorption (2PA) coefficient of Gallium phosphide (GaP) was measured by open-aperture z-scan in the spectral range 650 – 1200 nm. Anisotropy in the polarization dependence of 2PA is then used to determine the two independent components of the imaginary part of the third-order nonlinear susceptibility tensor corresponding to the zinc blende crystal structure of GaP. These results are then compared to first-principles calculations using length gauge theory. The strong 2PA in GaP qualifies this material to be used as a reference when extending this experimental and theoretic technique to other crystal and nanocrystal materials.   

Electroreflectance studies of GaAs/AlGaAs multiple quantum well based resonant Bragg structure at excited states

Electroreflectance (ER) Spectroscopy was employed to study the optical properties of GaAs/AlGaAs multiple quantum well (MQW) based resonant Bragg structure (RBS). The sample used in this experiment consists of 60 periods of quantum well structures with GaAs well and AlGaAs barrier layers grown by molecular beam expitaxy on a semi-insulating GaAs substrate. The sample structure was designed to achieve a double resonance condition to coincide the Bragg resonance peak with the exciton transitions at the second quantum state. Bragg peak can significantly be tuned by changing the angle of incidence of the light. Exciton energies can be tuned by changing the temperature and external electric field. The exciton energies are very sensitive to the thickness of the quantum wells. We performed low temperature ER measurements of the RBS samples of different thicknesses by tuning the angle of incidence of the light for double resonance and observed ER features related to the exciton transitions at excited states such x(e2-hh2), x(e2-hh1), x(e2-hh3) x(e2-lh1) and x(e1-hh3) exciton transitions along with the sharp features of x(e1-hh1) and x(e1-lh1) ground state exciton transitions. Details about the origin of the ER features related to the transitions will be presented.   

Analysis of the Critical Point Parameters of E0 and E0+Δ0 of Bulk Ge

Germanium is an indirect bandgap semiconductor having its onset of absorption at 0.8 eV at room temperature. Knowledge of the behavior of critical points (CPs) of Ge and other semiconductors is valuable for the further development of electronic and optoelectronic devices.
The dielectric function of Ge has been measured between 0.5 eV and 1.3 eV using spectroscopic ellipsometry at various temperatures between 10 K and 740 K. The interband CPs E₀ and E₀+Δ₀, where E₀ is the direct band gap of Ge and Δ₀ is the spin-orbit splitting occurring at the center of the Brillouin zone, lie in this energy range and are subject of our investigations. Applying an analysis in reciprocal space by performing a discrete Fourier transform of the data points and fitting the resulting Fourier coefficients, the parameters describing the line shape of E₀ are found as a function of temperature. Like for the CPs at higher energies, the authors find a red shift of the E₀ and E₀+Δ₀ energies which can be described by a Bose-Einstein factor taking into account electron-phonon interactions. The results of the reciprocal-space analysis are compared to the parameters determined by a parametric semiconductor fit.   

Quantitative Evaluation of Exciton-Exciton Interaction Energy in Bulk GaAs

The exciton-exciton(ex-ex) interaction has long been investigated from the viewpoint of excitonic nonlinear optics in a variety of semiconductors [1]. Recently, the problem has gained a renewed interest partly triggered by the progress of researches in the field of polariton Bose-Einstein condensation in a quantum well system embedded in a microcavity. In contrast to intensive theoretical studies devoted over decades, however, the quantitative evaluation of ex-ex interaction in bulk system has been lacked. Here we revisited this long-standing issue with near-infrared optical pump optical-probe/terahertz probe spectroscopy technique. In bulk GaAs at the lattice temperature 5K, we observed a blueshift of exciton absorption peak caused by the ex-ex interaction when the pump photon energy tuned to 1s-exciton resonance. Combined with the terahertz spectroscopy, the density dependence of the excitonic blueshift is evaluated, which allows to determine the ex-ex interaction strength quantitatively. The value coincides well with that predicted from mean-field theory. The dynamics of the excitonic blueshift will also be discussed with respect to the thermalization of cold exciton gas.
[1] S. Schmitt-Rink, D. S. Chemla and D. A. B. Miller, Phys. Rev. B, 32, 6601 (1985).   

Moments analysis of the dielectric function

For many purposes, the random-phase approximation (RPA) is an adequate approximation for dielectric screening when the latter is required within a calculation. Therefore, an efficient approximation to the RPA is similarly useful in a wide range of situations. In this talk, we will describe a model for the same based on knowledge of several approximate frequency moments of the irreducible polarizability function. These models rely, in part, on the Fourier transform of the two-point, one-electron density matrix. We test our model calculations against numerical density-function theory (DFT) calculations in isolated atoms, the homogeneous electron gas, and a wide variety of metals, semiconductors, and insulators.
  

Electronic Transport Studies of Boron-Doped Diamond-on-Graphene Heterostructures

Diamond-on-graphene heterostructures are a promising route for operating novel electronic devices at high temperatures due to their excellent thermal properties. For this purpose, boron-doped diamond thin films were grown in a microwave plasma-assisted chemical vapor deposition (CVD) system on a tantalum substrate which allows us to peel off diamond films [1]. Successive mechanical transfer of the diamond films onto CVD graphene on either Si/SiO₂ or quartz results in our heterostructure devices. We will present current-voltage characteristics as a function of temperature and their tunable gate dependence at specific temperatures. Varying the doping level of boron in the diamond films leads to variations in device behavior. Our latest results will be discussed.

[1] R. Bogdanowicz, M. Ficek, M. Sobaszek, A. Nosek , L. Golunski , J. Karczewski , A.Jaramillo-Botero, W.A. Goddard III , M. Bockrath and T. Ossowski, Growth and isolation of large area boron-doped nanocrystalline diamond sheets: A route towards diamond-graphene heterojunction, Advanced Functional Materials (submitted)   

Thermal transport in two-dimensional disordered electron systems at intermediate temperatures

Theoretical studies of transport properties of two-dimensional electron gases often focus on two limiting cases: (i) The diffusive regime that occurs at low temperatures where inelastic electron-electron scattering is substantially weaker than scattering by disorder. (ii) The hydrodynamic limit characterized by strong electron-electron scattering which requires relatively high temperatures. The latter has been widely studied in the context of strongly correlated electron systems and graphene. Here, we report a study of electron transport in an intermediate regime where both electron-electron scattering and scattering on impurities are equally important. To address this regime, we first analyzed a simplified kinetic equation which we supplemented by a more detailed analysis of the elastic and inelastic scattering terms. In particular, we derived the thermal conductivity of a degenerate electron system at intermediate temperatures.   

Comprehensive Characterization of Alkaline Earth Metal Halides using First Principles Calculations

Alkaline earth metal halides are among the scarce insulators with dielectric constants and band gaps larger than those of SiO₂. Here, we present a study of this class of high K/large band gap materials using first principles calculations with different levels of sophistication. In particular we focus on changes in their electronic and thermal properties as a function of composition and structure. Our results show that the most stable phases of these materials are those with larger band gaps. Determination of their band gaps requires hybrid approaches to overcome well-known limitations of density functional theory. Phonon properties and dielectric response are compared using different exchange-correlation functionals: (i) Localized Density Approximation (LDA), (ii) Generalized Gradient Approximation (GGA) and (iii) Generalized Gradient Approximation including van der Waals forces (GGA+vdW). We find that the dispersive forces correct the mode softening observed in low-frequency phonon modes in the GGA functionals yielding results similar to LDA. The different trends found in these materials class relative to their composition and structure are discussed.   

In-plane field dependence of spin resolved magnetic focussing in 2D hole systems

The spin orbit interaction allows for all-electrical control of spin. Transverse magnetic focussing provides a method for spatially separating spins so that spin dynamics can be studied. We perform transverse magnetic focussing experiments of heavy holes in highly symmetric (100) GaAs/AlGaAs heterostructures, and observe two clear spin resolved focussing peaks. It is tempting to use the area of the focussing peaks as a measure of the spin polarisation, however we find that the peak area is extremely sensitive to in-plane magnetic fields - as small as 0.1T. Additionally, we find that the peak area is asymmetric when the in-plane field direction is reversed, and this asymmetry is conserved under Onsager time reversal. This suggests that there is an additional non-spin polarisation effect at play, and that care must be taken when using focussing techniques to measure spin.   

Epitaxial growth of large grain size polycrystalline silicon on SiO2 by reduced pressure chemical vapor deposition

Epitaxial growth of polycrystalline silicon (poly-Si) on SiO₂ was successfully carried out by reduced pressure chemical vapor deposition (RP-CVD) using SiH₄/H₂ mixtures. The growth rate and grain size of ploy-Si were significantly improved by increasing the epitaxial temperature. The film deposition rate was 3.1nm/s with the epitaxial temperature at 820°C. The grain size of the poly-Si film could reach as large as 700nm on annealed seed layer. Good quality poly-Si film with large and uniform grain was obtained by direct deposition on SiO₂ substrate. The electrical properties of the doped poly-Si film were tested by Hall Effect and four-probe method.   

Chemistry Perspective to Design Novel Magnetic Semiconductors/Semimetals

Magnetic semiconductors combine the complementary functions of magnetic and semiconducting materials, providing tremendous potential for the development of new devices for today’s information technology. If employed in devices, these materials could lead to a new pathway to control information: by charging carriers not only in traditional electronics but also in spin dimensions. The Zener model was modified by Dietl et al. to explain the origin of ferromagnetism in Mn-doped GaAs and predicted the room-temperature ferromagnetism in other semiconductors and oxides. This remarkable work opened up the era of searching for room-temperature ferromagnetism in semiconductors. It still remains a lot of work to identify plausible targets for new magnetic materials. Previous studies mainly focused on hybrid structures consisting of ferromagnetic and semiconducting multilayers and Mn-doped binary semiconductors. However, only a small handful of intrinsic magnetic semiconductors have thus far been reported. Thus, the design of specifically targeted by synthetic scientists is highly demanded. In this talk, I will describe some empirical design rules and examples that have yielded new magnetic semiconductors/semimetals.