Session R18 - Semiconductors V: Optical Properties and Spectroscopy of Bulk Semiconductors.
MIXED session, Thursday morning, March 23
206AB, MCC

[R18.001] Resonance Raman scattering studies of GaAsN

We use resonance Raman scattering to study the nature of the recently discovered [Perkins et al., PRL 82, 3312 (1999).] nitrogen-induced E_+ level located in the conduction band of GaAs_1-xN_x (0.001\le x \le0.022). Our experimental data is consistent with the interpretation that the E_+ level is derived from the nitrogen-induced \Gamma-L mixing of the bulk GaAs states and it is not derived from the resonant nitrogen impurity level. We also observe a broadening of an LO phonon line which we interpret as an indication of the existence of another state that originates from the localized resonant isoelectronic nitrogen "deep impurity state".

[R18.002] Formation of an isoelectronic impurity band in heavily doped GaP:N and GaAs:N and the quantum confinement of electrons in the impurity band

It is well-known that the heavy doping of the isoelectronic impurity nitrogen in GaP, GaAs and related alloy systems results in a large band-gap reduction and other unusual effects. However, the origins for these observed effects are still not well understood. We perform a systematic study of the nitrogen related optical transitions in GaP:N and GaAs:N as a function of nitrogen doping level. Our results reveal how the impurity states evolve from discrete bound states to an impurity band with varying nitrogen doping level, and the band structure changes due to the impurity band formation. We also demonstrate the quantum confinement effects for the electrons in the impurity band. Other nitrogen doping induced abnormalities in the electronic properties, such as the deformation potential, will also be reported.

[R18.003] Stimulated Emission of Nitrogen Traps in GaAs:N

We report He-temperature experiments on narrow, <110% >-cleaved platelet cavities of N-doped GaAs loaded into diamond cells for intense photopumping. We find that strong direct-donor exciton PL predominates thru 20 kbar. At 22 kbar the electronic level of isolated N abruptly transforms from a conduction band resonance into the intensely emitting single-excitonic state known as N_X, which behaves as a multi-band ``deep trap'' level --- thus, rapidly plunging into the host gap as pressure is increased. At \sim 30 kbar, the bi-excitonic ``shallow'' state known as N_\Gamma separately distinguishes itself from N_X% and follows the \Gamma _1-edge. While we find evidence for only spontaneous emission for the N_X level, the N_\Gamma level is found from 30 to 40 kbar to support a broadened band comprising numerous longitudinal modes of the GaAs cavity, which upon further photo-pumping line-narrows into an intense laser mode centered upon the N_\Gamma exciton. Beyond \Gamma _1-X_1 crossing at 41.5 kbar, N_\Gamma lasing quenches due to level hybridization with the approaching X_1 band states, thus rapidly losing oscillation strength. These data suggest that stimulated emission involving N traps derives from bi-excitonic properties.

[R18.004] Recombination Lifetime and Spectral Response of GaAs_1-xN_x Double Heterostructures

The ternary semiconductor GaAs_1-x N_x with 0 < x < 0.3 can be grown epitaxially on GaAs. The alloy has a very large bowing coefficient and the bandgap can be reduced to about 1.0 eV with about a 3GaAs_1-x N_x is not intentionally doped but is typically p-type in the low 1 x 10 ^17 cm^3 range. Here, we measured the internal spectral response and recombination lifetime of a number of alloys using the ultra-high frequency photoconductive decay (UHFPCD) method. The data shows that the photoconductive excitation spectra of the GaAs_0.97N_0.03 alloy shows gradual increase in response through the absorption edge near Eg. This contrasts with most direct bandgap semiconductors that show a steep onset of photoresponse at the bandgap. The recombination lifetimes frequently are much longer than expected from radiative recombination and often exceeded 1.0 microseconds . These long lifetimes are cause by either charge separation or a weak radiative recombination coefficient or both. The data was analyzed in terms of a band model that includes large band tails or potential fluctuations in the conduction band. The latter are caused by the random distribution of nitrogen atoms in the alloy. Because of these potential fluctuations, the conduction band electrons are very immobile. By contrast, the valence band holes are quite mobile and produce strong photoconductivity signals.

[R18.005] Lifetimes in N-doped GaAs Under Pressure

Time-decays have been collected at 1.8 - 6 K for photoluminescent states in GaAs:N versus pressure (0 - 100 kbar) in diamond cells. We summarize this beginning at 70 - 80 kbar, wherein the N_X (A and B) exciton states of GaAs:N most clearly resemble their corresponding A and B states in GaP:N, with X_1-binding energies of \sim 32 meV. Here, single exponentials are found with deduced N_X lifetimes of >\sim 600 ns, just as reported for GaP:N(J.D. Cuthbert et al., Phys Rev. \textbf154), 763 (1967). As pressure is reduced the N_X level behaves as a multi-band ``deep trap,'' departing from the X_1 edge with binding energies increasing to a maximum of \sim 130 meV at the \Gamma _1-X_1 band crossing (41.5 kbar)(D.J. Wolford et al., Solid State Commun. \textbf53, )1069 (1985) --- for which N_X lifetime has precipitously dropped to \sim 45 ns. This may be compared with co-existing D_X^0 - excitons recombining at a fixed and intermediate lifetime of \sim 125 ns. Reducing pressure still further (direct-gap range) N_X lifetime smoothly tends toward an apparent limiting minimum of 10 - 15 ns as the N_x states approach the \Gamma _1 - degeneracy at \sim 21 kbar. Remarkably, this limiting estimate is confirmed by direct measure of \sim 10 ns for N_X-resonant-state lifetime at <\sim 20 kbar, by examining optical replicas within the gap of a silent (totally absorbed) above-gap N_X recombination.

[R18.006] N Traps Under Pressure in the Dilute GaAsN_x Limit

We report 1.8-6 K PL and PLE at pressures up to 100 kbar involving N impurities in high-purity n-type GaAs at concentrations (>\sim 10^19 cm ^-3) just short of host alloying. Samples were loaded into gasketed diamond cells containing a hydrostatic (>1 in 10^4) ^4 He-pressure-medium. From atmosphere we find \Gamma _1-donor-exciton ( \sim 6-meV deep) PL predominating and shifting with \Gamma _1 at the expected 10.74 meV/kbar(D.J. Wolford et al., Solid State Commun., \textbf53), 1069 (1985). Abruptly at 22 kbar both N_X and N_\Gamma states of isolated N --- first identified(D.J. Wolford et al., Phys. Rev. Lett. \textbf36, )1400 (1976) in GaAs_1-xP_x:N, respectively, as single-exciton and bi-excitonic states --- emerge from the \Gamma _1-band and establish themselves as sharp (<1-meV wide) excitonic lines separately and uniquely distinguishable in \textitboth PL and PLE. As in the alloy(D.J. Wolford et al., Phys. Rev. Lett. \textbf36, )1400 (1976), the N_X state shows (1) selective acoustic cooperation and strong LO ^\Gamma configuration-coordinate coupling (through some 6 orders), with mirror symmetry between PL and PLE and (2) an exciton-level following no single bandedge with pressure (especially \Gamma _1 from which it emerges) --- varying instead highly nonlinearly (e.g., from \sim 4 meV/kbar at 22 kbar to \sim 0 meV/kbar at \sim 70 kbar) as the multi-band ``deep-trap'' it has been variously modeled to be. Predominating for N_X we also find distinct and relatively large allowed-A-line and forbidden-B-line splitting (0.8 meV in GaP) which varies smoothly with pressure (e.g. \sim 1.5 meV at 22 kbar to \sim 6.5 meV near 70 kbar) --- a result suggestive of varying electron-hole localization and/or enhanced strain (and possibly reduced symmetry) surrounding N in GaAs.

[R18.007] Fundamental Band Gap in GaNP Alloys

Optical transitions in GaN_xP_1-x alloys (0.007>x>0.031) have been studied using optical absorption, photoluminescence, and photomodulation spectroscopies. The GaN_xP_1-x samples were grown by gas-source molecular beam epitaxy. An absorption edge appears in GaN_xP_1-x at the energy below the indirect \Gamma-X transition in GaP, and the absorption edge shifts to lower energy with increasing N content. Observation of strong photomodulation spectral signals associated with the absorption edge indicates that a direct interband transition is taking place in GaN_xP_1-x and suggests that the fundamental band gap has changed from indirect in GaP to direct in GaN_xP_1-x. This N-induced transformation of the fundamental band gap from indirect to direct is explained in terms of band anti-crossing that considers an interaction between highly localized nitrogen states and the extended states at the \Gamma minimum of the conduction band.

[R18.008] In-Situ Spectroscopic Ellipsometry of Low-Temperature-Grown GaAs

We present real-time, in-situ spectroscopic ellipsometry (SE) measurements of the pseudodielectric function \langle \epsilon(E) \rangle = \langle \epsilon_1(E) \rangle + i \langle \epsilon_2(E) \rangle of low-temperature-grown GaAs (LTG-GaAs) layers on GaAs (001) substrates at growth temperatures from T_g = (180 to 300)^oC, as a function of As_2:Ga flux ratio R and layer thickness. The peak in \langle \epsilon_2(E) \rangle near the GaAs interband critical point E_1 broadens and decreases in amplitude monotonically with decreasing T_g and/or increasing R, and hence, with increasing excess arsenic composition of the LTG-GaAs. The SE measurements during the growth also display a clear signature of exceeding the critical epitaxial thickness limit h_epi, above which the LTG-GaAs growth is polycrystalline for T_g = 250^oC, and amorphous for T_g = 180^oC. Our results suggest that the arsenic composition and structural properties of LTG-GaAs can be controlled in real time, using in-situ SE, by varying T_g and R.

[R18.009] The electronic structure of Bi_2Te_3 - an angle resolved photoemission study

We carried out angle resolved photoemission studies of Bi_2Te_3, the best thermoelectric material at room temperature to date. Valence and conduction band dispersions have been determined along major symmetry lines in the three-dimensional Brillouin zone. The results are compared with the predictions of band structure calculations.

[R18.010] The Soft X-ray Fluorescence Spectra of Phosphorus Compounds

Soft x-ray fluorescent spectra have been measured for the III-V compounds BP, GaP and InP, for the transition metal phosphides Mn_3P_2, Fe_2P and Ni_2P, and for Zn_3P_2 and Cd_3P_2. The results are compared with band structure calculations and published results. Hybridization of the Fe d-states with phosphorus states permits the location of metal d-bands within the valence band to be determined from P L_2,3 spectra. As the d-bands are filled form Mn to Fe and Ni, the d-band featues move to larger bindng energies. For the Zn and Cd compounds, they lie in a gap between the upper and lower valence bands, and in GaP lie below the valence bands. Two unusual features were observed. The normally suppressed P L_1 spectrum was observed in resonance with the B K spectrum, and strong resonant inelastic x-ray scattering was observed at the Ga M_2,3 edge associated with energy losss to electronic excitations from the Ga d-levels.

[R18.011] The Optical Properties of Silicon Revisited

Recent measurements^1 of the optical properties of silicon at ultraviolet and soft-x-ray energies have made it appropriate to conduct a comprehensive survey of the optical constants. A principal concern is the degree to which measured values satisfy theoretical constraints such as the f sum rule. To clarify this issue, we are developing a composite set of silicon optical constants covering the entire spectral range. Our approach employs a self-consistent Kramers-Kronig analysis that treats all the available measures of electromagnetic response on an equal footing, a procedure demonstrated in an earlier study of aluminum^2. Data considered include reflection, absorption, refractive-index, electron-energy-loss, and stopping-power measurements. An important issue is the extent to which surface roughness and oxide layers affect measured quantities. Preliminary results indicate that a composite set of bulk optical constants can be developed that is consistent with both the principal theoretical constraints and with recent published measurements. We are currently seeking input concerning any as-yet-unpublished measurements for inclusion in our data base.

^1R. Soufli and E. M. Gullikson, Applied Optics 36, 5499 (1997). ^2E. J. Shiles, T. Sasaki, M. Inokuti, and D. Y. Smith, Phys. Rev. B 57, 1612 (1980).

[R18.012] Photoreflectance Studies of Te-doped GaSb

We report a measurement of the photoreflectance (PR) responce of bulk and epitaxially grown Te-doped GaSb samples at E_0+\Delta_0 transition from 4 K to room temperature. The spectra are analyzed using the third derivative of the Lorentzian functional form of the dielectric function and the temperature dependence of the E_0+\Delta_0 transition energy has been obtained. We find that the value of the E_0+\Delta_0 transition energy in a bulk grown sample, with an electron concentration of about 1.6x10^17/cm^3, is 1.583 eV at 4K. For more heavily doped epitaxially grown samples with an electron concentration of 7.4x10^17/cm^3, the value of E_0+\Delta_0 is determined to be 1.613 eV. Similar results are also obtained at the E_0 transition. The contributions of many-body effects such us band filling and band-gap renormalization to these transitions will be discussed.

[R18.013] Exciton-Phonon droplets with Bose-Einstein condensate: transport and optical properties

We explore a nonlinear field model to describe the interplay between the ability of excitons to be Bose-condensed and their interaction with other modes of a crystal. We apply our consideration to the moving droplets of paraexcitons in 3D Cu_2O crystals.

Taking into account the exciton-phonon interaction, we argue that at T < T_c the moving packet of excitons and phonons can contain an exciton-phonon condensate, or a self-consistent exciton-phonon field.

Within the simple quasi-1D model, the stationary equations for the condensate can be solved exactly, and these equations support localized ballistic solutions. We discuss the conditions for the inhomogeneous condensate to appear in the crystal, and the characteristic width L_0 of ballistic condensates is estimated in the limit of T \rightarrow 0. As the real experiments are performed at T \simeq T_c, we discuss the dependency L_0(T) as well. The stability conditions of the moving condensate are analyzed by use of Landau arguments. It follows that the condensate can move through the crystal as a stable droplet.

To separate the coherent and non-coherent parts of the exciton packet, we suggest to turn off the phonon wind by changes in design of the 3D crystal and boundary conditions for the moving droplet. Finally, we speculate about the possibility to convert the coherent exciton field into the coherent photon one by colliding two moving exciton-phonon condensates.

[R18.014] Relaxation kinetics of excitons in cuprous oxide

Until recently, we believed cuprous oxide could host a Bose-Einstein condensate of excitons. Intense photoexcitation of this semiconductor produces an exciton gas in which the kinetic energy distribution matches that expected in a Bose gas near the critical density for Bose-Einstein condensation. The apparent gas temperature, however, was generally greater than that of the lattice, exciton annihilation limited attainable densities, and the system seemed very reluctant to form a condensate. Our measurements of the exciton density, based on the absolute brightness of photoluminescence, indicate that the density is not nearly high enough to exhibit quantum statistics. These experimental facts suggest that the interesting kinetic energy distributions result from competing excitation and relaxation processes in a highly non-equilibrium gas of excitons. A detailed but straightforward application of the Boltzmann equation predicts exciton distributions matching those observed, but with occupation numbers much less than one.

[R18.015] Quadrupole polariton effect in Cu_2O under two-photon resonant excitation

We studied the direct rediative emission of 1s orthoexcitons in Cu_2O, resulting from two-photon resonant excitation, as a function of the emission angle, time and lattice temperature. The data indicate that the emission is due to quadrupole polariton decay. At low temperature, the relationship between polariton decay and resonant second harmonic generation will be discussed. As temperature increases, the quadrupole polariton evolves into unddressed excitons as a result of phonon-exciton scattering.

Part R of program listing