Session ZC13 - Spectroscopy of Semiconductors.
ORAL session, Friday afternoon, March 26
Room 267W, GWCC

[ZC13.01] Optical Absorption in the High-Density One Dimensional Electron-Hole Plasma

We use bosonization and diagrammatic methods to give a detailed treatment of the phase diagram and the optical absorption spectrum of the high density limit of the one dimensional electron-hole plasma. We allow for different electron and hole velocities and determine the optical signatures of the different phases and of the phase transition points. Extension of the method to treat recombination at impurities and nonequilibrium effects is briefly outlined.

[ZC13.02] Coherent Control of Absorption and Propagation in Semiconductors

We present the first demonstration of coherent control of absorption and propagation in semiconductors in both spectrum and time domain. The phase-locked two collinear pulses which is separated from Ti:sapphire laser have passed through the sample. The laser pulses out of sample were upconverted in order to probe their temporal evolution and spectrum. The investigated sample was high quality GaAs multiple quantum well, so that there is pronounced pulse distortion with quantum beat, which shows free induction decay of excitons excited by incident laser pulse. The absorption caused by second pulse is strongly dependent on the phase of the second pulse, which is more evident from the fact that pulse distortion is controlled by the relative phase of two incident pusles. In addition, various specta will be shown for fixed time dely between two laser pulses at each time delay of upconverting pulse.

[ZC13.03] Studies of Random Telegraph Noise in the photoluminescence of single InP quantum dots

We have investigated random telegraph noise in the emission spectra of single InP quantum dots. The switching frequency increases with excitation power density and temperature. Spectra taken in the on-state and the off-state show no evidence of a Stark shift. A simple model based on defects is used to explain the results.

[ZC13.04] Optical second harmonic spectra of Si(001) with H and Ge ad-atoms: first principles theory and experiment

A theory of SHG from the Si(001) surface is developed based on a first principles description of eigenvalues and eigenvectors using pseudopotential and/or full potential linearized augmented plane-wave methods. We demonstrate for the first time that optical transitions from surface atom and/ or ad-atom states contribute much more strongly to the SHG than to the linear optical response of the surface. Calculated results are compared with measured SHG spectra of the Si(001) surface with and without hydrogen [1]. The observed ten-fold decrease of SHG intensity with mono-hydride termination, which was not explained by previous microscopic calculations [2], is now quantitatively explained in terms of redistribution of the ad-atom related electronic states on the surface. New experimental [3] and theoretical spectroscopic SHG results for Ge-covered Si(001) surface, which show strikingly different dependence on ad-atom coverage, will also be presented. This work was supported by NSF grant CHE-8920210.

1. J. I. Dadap et al., Phys. Rev. B 56, 13367 (1997). 2. B. I. Mendoza et al., Phys. Rev. Lett. 81, 3781 (1998). 3. P. Parkinson et al., submitted to Appl. Phys. B (1998).

[ZC13.05] Second Order Raman Scattering of Hexagonal GaN Films

In contrast to first order Raman scattering where signals come from only the center of Brillouin zone, second order Raman measurements provide information on the vibrational states from the entire zone. In this study, second order polarized Raman scattering of hexagonal GaN single crystal films deposited on sapphire substrates using low pressure metal organic chemical deposition (MOCVD) was carried out at variable temperature and at different angles between the laser polarization and the c axis of the film. The intensity of the Raman bands involving acoustic phonons shows more remarkable increase with temperature than those involving only optical phonons, while the line widths of the former remain largely constant. These temperature dependence results are interpreted using the anharmonic crystal theory and they agree with our results from the high temperature first order Raman data. Some new Raman bands, observed by performing angle-rotation measurements, are assigned and discussed.

[ZC13.06] Electron Energy-Loss for anisotropic systems: Application to GaN (10øverline10)

We derive expressions for the theory of electron energy-loss (EEL) spectroscopy in the general case of an optically anisotropic surface on top of an optically anisotropic substrate (R. Esquivel-Sirvent and C. Noguez, Phys. Rev B 58), 7367 (1998).. As an example of the applications for anisotropic media, we present the calculated electron energy-loss for the unreconstructed GaN (10øverline10) wurzite surface (C. Noguez, Phys. Rev B 58), 12641 (1998).. An EEL differential spectroscopy technique is proposed in order to determine the atomic reconstruction and anisotropy of this important surface.

This work has been supported in part by Grants No. CONACyT-3035PE and No. UNAM-DGAPA-IN104297.

[ZC13.07] Spectroscopic signature of a symmetry breaking surface phase transition: Ag/Si(111) Root3xRoot3 \rightarrow 3x1

One of the most investigated but challenging metal/semiconductor prototypical system is Ag/Si(111). Its Root3xRoot3 R(30^o) reconstruction, completed at one monolayer (ML), was solved only after decades of efforts [1]. Yet, despite the small unit cell, the atomic geometry of the symmetry breaking (3x1) structure, completed at 1/3 ML and prepared at high temperature, has just been determined experimentally [2] and theoretically [3]. We have investigated by high-resolution photoelectron spectroscopy the Si2p and Ag3d core-levels at the Italian third generation synchrotron facility, ELLETRA in Trieste. We have studied the electronic changes associated to the Root3xRoot3 R(30^o) \rightarrow 3x1 phase transition. Spectroscopic signatures of the dimensionality reduction are identified and related to the new bonding configurations.

[1] see e.g.: G. Le Lay et al., Surf. Sci., 307-309 (1994) 280 [2] C. Collazo-Davila et al., Phys. Rev. Lett., 80,1678 (1998) [3] S. C. Erwin and H. H. Weitering, Phys. Rev. Lett., 81,(1998) 2296

[ZC13.08] First-Principles Calculation of Photoreflectance Spectrum from a Strained In_0.18Ga_0.82As/GaAs Single Quantum Well

Pump-probe modulation spectroscopy techniques, such as photoreflectance spectroscopy, allow contactless, room temperature, full wafer characterization of multilayer structures for electronic and optoelectronic devices. Empirical lineshape fits can be used to obtain optical transition energies, but provide little information on the nature of the transitions. In this work we have modeled the photoreflectance spectrum for a 100 ÅIn_0.18Ga_0.82As/GaAs single quantum well from a first-principles calculation. This calculation includes results from a 6x6 k dot p model for strained quantum wells which provides matrix elements, transition energies, and wavefunctions used to calculate the excitonic binding energies and overlap integrals as part of the absorption spectrum calculation. Differential reflectivity is calculated from changes in the quantum well absorption spectrum resulting from photoexcitation of the sample by a chopped pump beam. The results of our calculation are in excellent agreement with the experimental data for reasonable values of sample parameters and experimental conditions.

[ZC13.09] Determination of Two-dimensional Electron Gas Density of a Double-Side Delta Doped Pseudomorphic High Electron Mobility Transistor Structure from Room Temperature Photoluminescence and Contactless Electroreflectance

Using room temperature photoluminescence and contactless elctroreflectance(CER) we have determined the 2-D electron gas density of a double-side doped Ga_0.8Al_0.2As/In_0.2Ga_0.8As PHEMT structure fabricated by metal-organic chemical vapor deposition. From a detailed line shape fit to the CER signal, the 2-D electron gas density is obtained. The fit consists of the first derivative of a broadened step-like joint density of states (screened exciton) multiplied by a Fermi level filling factor. The 2-D density is also evaluated from a self-consistent Poisson's-Schrodinger(SCPS) theoretical calculation, which evaluates mC-nH (transition between m^thelectron and n^th heavy-hole valence subbands) InGaAs channel intersubband energies and E_f(Fermi level) energy for intended material parameters. There is good agreement for the 2-D gas density between the lineshape fit and the SCPS calculation.

[ZC13.10] Magnetic Field Dependence of the Optical Conductivity of a-Si1-xRE_x Alloys

We have studied the magnetic field (0-15 Tesla) dependence of the conductivity (30-4000 cm^-1) in a-Si1-xRE_x (RE=Y,Gd) in the vicinity of the metal-insulator transition. For non-magnetic Y doping, there is no measurable magnetic field dependence to the conductivity. In contrast, Gd-doped films reveal a large field-induced enhancement of the optical conductivity starting from ømega=0 and extending up to 4000 cm^-1. Sum-rule analysis shows that the effective spectral weight associated with the response of free carriers is strongly temperature- and field-dependent. We also discuss the possible effects of photo-induced conductivity on the measurement.

Part Z of program listing