Session H24: Electrical and Thermal Properties of Semiconductors

Electrical characterization of SiGeSn grown on Ge substrate using ultra high vacuum chemical vapor deposition

There has been recently considerable interest in growing Si$_{y}$Ge$_{1-x-y}$Sn$_{x}$ alloys for the fabrication of photonic devices that could be integrated with Si technologies. We report temperature dependent Hall (TDH) measurements of the hole concentration and mobility from high quality p-type doped Si$_{0.08}$Ge$_{0.90}$Sn$_{0.02}$ layers grown on p-type doped Ge substrates using ultra high vacuum chemical vapor deposition. The TDH measurements show the hole sheet density remains constant at low temperatures before slightly decreasing and dipping at $\sim $ 125 K. It then exponentially increases with temperature due to the activation of shallow acceptors. At temperatures above $\sim $ 450 K, the hole sheet density increases sharply indicating the onset of intrinsic conduction in the SiGeSn and/or Ge layers. To extract the electrical properties of the SiGeSn layer alone, a parametric fit using a multi layer conducting model is applied to the measured hole concentration and mobility data. The analysis yields boron and gallium doping concentrations of 3x10$^{17}$ cm$^{-3}$ and 1x10$^{18}$ cm$^{-3}$ with activation energies of 10 meV and 11 meV for the SiGeSn layer and Ge substrate, respectively. Furthermore, a temperature independent hole sheet concentration of $\sim $5x10$^{15}$ cm$^{-2}$ with a mobility of $\sim $250 cm$^{2}$/Vs, which is believed to be due to an interfacial layer between the SiGeSn layer and the Ge substrate, is also determined.   

Informatic Strategies for Screening Electron Mobility of Candidate Semiconducting Materials

While carrier transport properties are critical to semiconductor efficiency, screens for potentially new materials based upon mobility measurements can be problematic. In the early stages of materials development, measured electron mobilities are often unreliable indicators of their eventual performance and serve as a poor basis to assess the longer term potential of candidate materials. In this paper, we describe an information-based approach for estimating an effective upper limit, using the specific case of the II-VI semiconductors. The optical (polaron) electron mobility has been developed as a screening property, supported by informatic estimates of dielectric properties. This mobility represents a practical screen, providing an estimate of the potential bounding value at room temperature. Using this basis, partial screening criteria based on compositional factors can also be constructed.   

Measurement of Electron Effective Mass in GaAs$_{1-x}$Bi$_{x}$

Magnetic field and temperature dependent resistivity measurements on n-type GaAs$_{1-x}$Bi$_{x}$ epitaxially grown films show clear Shubnikov de Haas oscillations in the range 0 $\le x \le $ 0.0088. An overall decrease in the electron effective mass is observed for this range of compositions. Accounting for the known giant bandgap bowing of GaAs$_{1-x}$Bi$_{x}$, the measured changes in the electron effective mass are in qualitative agreement with perturbation theory applied to the known bandgap reduction for this alloy, confirming that bismuth mainly perturbs the valence band. The stronger compositional dependence of the measured masses is attributed to effects from the bismuth isolated state.   

Piezo-resistance in $n$-type Si$_{1-x}$Ge$_x$ alloys as a function of alloy composition and strain

We use first-principles electronic structure methods[1,2] to predict the piezoresistance of $n$-type Si$_{1-x}$Ge$_x$ at various alloy compositions and strain configurations. The gauge factor, $G = d\rho/d\epsilon/\rho$, where $\rho$ is resistivity and $\epsilon$ is strain, varies strongly with both composition and strain. Nonlinear changes in resistivity with strain may arise due to changing occupancy of the higher-conductance $L$ valley relative to the lower-conductance $\Delta$ valley, coupled to a change in inter-valley alloy and phonon scattering. [1] F. Murphy-Armando and S. Fahy, J. Appl. Phys., accepted for publication (2011). [2] F. Murphy-Armando and S. Fahy, Phys. Rev. Lett., 97, 096606 (2006)   

Non-diffusive thermal conductivity in semiconductors at room temperature

The ``textbook'' value of phonon mean free path (MFP) in silicon at room temperature is $\sim $40 nm. However, a large contribution to thermal conductivity comes from low-frequency phonons with much longer MFPs. We find that heat transport in semiconductors such as Si and GaAs significantly deviates from the Fourier law at distances much longer than previously thought, $\ge $1 $\mu $m at room temperature and above. We use the laser-induced transient thermal grating technique in which absorption of crossed laser pulses in a sample sets up a sinusoidal temperature profile monitored via diffraction of a probe laser beam. By changing the period of the thermal grating we vary the thermal transport distance within the range $\sim $1-10 $\mu $m. In measurements performed on thin free-standing Si membranes and on bulk GaAs the thermal grating decay time deviates from the expected quadratic dependence on the grating period, thus providing model-independent evidence of non-diffusive transport. The simplicity of the experimental configuration permits analytical treatment of non-equilibrium phonon transport with the Boltzmann transport equation. Our analysis shows that at small grating periods the effective thermal conductivity is reduced due to diminishing contributions of ``ballistic'' low-frequency phonons with long MFPs.   

Thermal conductivity and isotope effect in wide bandgap semiconductors

We have calculated the lattice thermal conductivity, $k$, of wurtzite InN, GaN, and AlN using an exact numerical solution of the phonon Boltzmann transport equation and \textit{ab initio} calculations of interatomic force constants. We find good agreement with experiment for the thermal conductivities around and above room temperature. The large frequency gap between the acoustic and the optic phonon branches in these materials limits anharmonic phonon scattering leading to large enhancements to $k$ with isotopic enrichment. We comment on the roles of various phonon scattering mechanisms on $k$ in these wide bandgap semiconductors.   

Conductance beyond the Landauer limit and charge pumping in quantum wires driven by linearly polarized radiation

Periodically driven systems, which can be described by Floquet theory, have been proposed to show characteristic behavior that is distinct from static Hamiltonians. Floquet theory proposes to describe such periodically driven systems in terms of states that are indexed by a photon number in addition to the usual Hilbert space of the system. In this work, we propose a way to measure directly this additional state by the measurement of the conductance of a single channel quantum point contact. Specifically, we show that a single channel wire augemented with a grating structure when irradiated with microwave radiation can show a DC conductance above the limit of one conductance quantum set by the Landauer formula. Another interesting feature of the proposed system is that being non-adiabatic in character, it can be used to pump a photo-current even with linearly polarized light. This circumvents the topological restrictions of adiabatic pumping, which necessisate the use of circularly polarized light. J.S. thanks the Harvard Quantum Optics center for support.   

Positive and Negative Coulomb Drag in a 1D Quantum Circuit

We report Coulomb drag measurements between tunable vertically-coupled quantum wires. The wires are fabricated in a GaAs/AlGaAs double quantum well heterostructure with a 15 nm barrier separating the quantum wells. The Coulomb drag signal is mapped out versus the number of subbands occupied in each wire, and regions of both positive and negative drag are observed (D. Laroche \textit{et. al.} Nature Nanotechnology, doi:10.1038/nnano.2011.182). The observation of negative Coulomb drag at a high one-dimensional electronic density is not predicted by the usual momentum-transfer model for Coulomb drag and shows that the existing picture of the drag effect in one-dimension is incomplete. In order to clarify the origin of this negative signal, temperature dependencies of the Coulomb drag are presented both in the positive and in the negative drag regimes. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Phonon Mediated Optical Stark Effect for Organic-Semiconductor Heterostructures

The system of macroscopic equations of hybrid excitons, photons and phonon is investigated in order to model the optical Stark effect for the hybrid systems with combination of semiconductor and organic materials. The unique properties of the hybrid system will allow tuning the system to get the most preferable outcomes. One of the key points is to study the interaction of different components of the system with each other to get the condition to facilitate the observation of phonoriton and the phonon-mediated Stark effect.   

Electron phonon coupling and carrier lifetimes in III-nitride ternary semiconductors

III-nitride semiconductors have a large bandgap and find applications in high power devices, in which the thermal management of energy generated becomes a key issue. The transfer of energy from the energetic carriers to the lattice is determined by the electron-phonon coupling for the crystal. In this work, we present our results on the electron-phonon coupling in ternary and quaternary semiconductors. The full phonon dispersion and electron-phonon coupling is determined using ab-initio methods. We then evaluate the carrier lifetimes for the emission of LO phonons by including the full zone and not only the zone center phonons. We expect that the treatment of the electron-phonon coupling effects over the full Brillouin zone could be critical for III-nitride thermal management, and will be directly compared to the results where only the zone center phonons are considered.   

Vibrational structure of defect luminescence bands in GaN from electronic structure calculations

Optical methods are among the most powerful to characterize defects in materials. The study of optical signatures based on state-of-the-art electronic structure methods is therefore very important. In this work we investigate the vibrational structure of luminescence bands pertaining to deep defect levels in GaN. Since luminescence lineshapes depend sensitively on defect geometries and vibrational frequencies, these should be described accurately. The latter is achieved through the use of hybrid density functionals. Both quasi-localized and bulk phonons are included in our description. In the case of transitions accompanied by very large lattice relaxations, anharmonic effects become sizeable, and these are also accounted for. For the defects studied a very good agreement with available experimental data is achieved. For instance, in the case of wide luminescence bands the resulting line widths are within 0.05 eV of the experimental values. This work was supported by the Swiss NSF and by NSF.   

Ultrafast Carrier Dynamics in GaAs(110) Studied by Time- and Angle-Resolved Photoelectron Spectroscopy

Ultrafast carrier dynamics in GaAs is of particular importance to optoelectronic devices and solar cell technologies. We employ Time- and Angle-Resolved Photoelectron Spectroscopy to elucidate the dynamics of both the occupied and unoccupied states of GaAs(110) upon femtosecond infrared laser excitation. We observe in the conduction band an optically excited population and energy shift, which both decay in $\sim$10 ps. The valence band also exhibits energy shifting dynamics which encompasses multiple temporal regimes. More intriguingly, valence band dynamics are also observed for negative pump-probe delays. We explain these observations by a carrier-transport-induced electrostatic potential change within a Drude-like picture.   

Intraexcitonic Autler-Townes effect in terahertz-driven semiconductor quantum wells

When a two-level system is resonantly driven by intense light non-perturbative phenomena such as Rabi oscillations and their frequency equivalent, the AC Stark or Autler-Townes effect, can be observed. The latter one manifests itself in an absorption line splitting where the magnitude is linear in the light field strength and where the symmetry of the splitting is determined by the detuning from resonance. Known from molecular spectroscopy [1], the effect has also been observed in solid state structures with its much broader line widths, e.g. for intersubband transitions [2]. Here, we present the first unambigous evidence of this effect in undoped GaAs/AlGaAs quantum wells using the hydrogen atom like intraexcitonic 1s and 2p states of the heavy-hole exciton. These states with a transition energy of 9 meV are resonantly coupled by strong terahertz light from a free-electron laser. For low fields our findings are qualitatively explained by a simple two-level model whereas deviations occur in the 10 kV/cm field range where the rotating-wave approximation of the simplified model breaks down and exciton ionization occurs. Due to the small Rydberg energy we can easily reach a highly non-trivial regime where the Rabi energy and the transition energy become comparable to the photon energy.\\[4pt] [1] S. H. Autler and C. H. Townes, Phys. Rev. 100, 703 (1955). \\[0pt] [2] S. G. Carter et al., Science 310, 651 (2005). \\[0pt] [3] M. Wagner et al., Phys. Rev. Lett. 105, 167401 (2010).   

Raman Spectroscopy and Scanning Electron Microscopy of the Bismuth Sillenites Bi$_{25}$InO$_{39}$ and Bi$_{25}$FeO$_{39}$

The Raman spectrum of Bi$_{25}$InO$_{39}$, a new type of bismuth sillenite, is reported along with the spectra of Bi$_{25}$FeO$_{39}$. Their spectra are remarkably similar to each other and to other bismuth sillenites reported in the literature. The similarities show that the new samples were successfully grown in the sillenite structure. The parameters of each Raman mode were obtained by fitting the spectra to a Lorentzian oscillator model, and the modes were assigned to symmetry-allowed modes of the I23 space group. The assignments were made by comparison to other materials with the sillenite structure. Scanning Electron Microscope images of the samples are also presented.   

Two-dimensional Fourier-Transform spectra of PbS semiconductor quantum dots

Investigating the correlations of multiple excitons in semiconductor quantum dots is a challenging many-body problem that has drawn considerable experimental and theoretical attention over the last two decades. Nonlinear four-wave mixing experiments have long been known to provide direct probes for the many-body effects in the ultrafast dynamics of excitons in semiconductor nanostructures. With the advent of two-dimentional Fourier-transform (2DFT) spectroscopy many-body contributions can be isolated and identified. 2DFT spectra of colloidal quantum dots will be presented, providing new understanding of the role of many-body interactions in the excitonic decoherence of these nanomaterials.