Low Excitation Densities Research Articles

Photoluminescence on high-quality AlxGa1−xAs grown by metalorganic vapor-phase epitaxy using alane bis(dimethylethylamine)

Photoluminescence (PL) spectra are reported of initial results of AlxGa1−xAs grown by metalorganic vapor-phase epitaxy (MOVPE), using a new precursor, alane bis(dimethylethylamine), as the aluminum source. The advantage of this new precursor over other alane precursors used previously is that it is liquid at room temperature. Using this new precursor instead of trimethylaluminum (TMAl), we found a reduction by a factor 6 in carbon incorporation when it was used together with trimethylgallium (TMGa), whereas a reduction by a factor 50 was found when it was used in combination with triethylgallium (TEGa). At low excitation density the linewidth of the separate donor bound exciton (D0,X) was 2.6 meV at an Al fraction of 0.31. This is comparable with the smallest values ever reported in literature for MOVPE-grown AlxGa1−xAs with an Al fraction higher than 20%. This narrow linewidth indicates a very uniform aluminum composition.

PHOTOLUMINESCENCE OF ZnSe-ZnS STRAINED-LAYER SUPERLATTICES

The photoluminescence (PL) emission spectra of ZnSe-ZnS strained-layer surerlattices (SLS) grown by atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD) have been studied in this paper. With the high density excitation (N2-Laser 337.1nm), the PL of the SLS consist in only one emission peak due to the n = 1 heavy hole excitons recombination, and with the low density excitation (high pressure Hg-Lamp 365.nm) the PL consist of both the band-edge emission and the deep-centre emission. One gap-edge emission peak without the deep-centre emission have been observed in the low-excitation PL, and the sample was thought to have good crystaline quality. The subbands of ZnSe-ZnS SLS were calculated using the theoies of the stress-induced baud structures and the Kronig-Penney band model. The two emission peaks, the energy of which are greater than the energy-gap of ZnSe single crystal films in the absence of strain, were observed in the PL spectra of ZnSe-ZnS SLS for the first time. According to the subband calculation, the two peaks were thought to be caused by the transitions between the n= 1 electronic subband and the n= 1 heavy, light holes' subbands, respectively.

Carrier Injection and Space Charge Effects in a-Si:H / Crystaiine Si Heterojunctions

ABSTRACTA study of excess carrier kinetics in a-Si:H / crystalline n-Si heterojunctions by transient photoconductivity measurements as a function of the excitation density is presented. It is observed that excess carriers can be injected from the a-Si:H in the c-Si part of the junctions, where the time associated to this injection process can be clearly resolved. The number of injected carriers increases with the number of excess carriers induced in the a-Si:H at low excitation density reaching a limit in the moderate excitation range. The driving force of the injection process is tentatively ascribed to the space charge field at the interface. The injection efficiency is estimated to be about 10%.

Linear-to-quadratic transition in electronically stimulated sputtering of solid N2 and O2.

Electronic excitations produced by MeV protons and helium ions lead to ejection of molecules from solid N{sub 2} and O{sub 2}. The yield is shown to be determined by the near-surface excitation density, rather than the primary ionization cross section. It is linear in the excitation density at low excitation densities and quadratic at high excitation densities. The linear yield can be described by discrete, nonradiative transfers of electronic energy into kinetic energy ( spikes'') that utilize {approx lt}12% of the total electronic energy deposited for N{sub 2} and {approx lt}37% for O{sub 2}. A statistical model is used to calculate the transition from low to high excitation density due to the overlap of either Maxwellian or non-Maxwellian spikes. It is found that the separate N{sub 2} and O{sub 2} data sets are consistent in this model, that excitations do not diffuse very far from where they are created, that the linear-to-quadratic transition is described better by the non-Maxwellian spikes, and that the energy derived in the linear regime is adequate to account for the nonlinear yields.

Influence of electron-hole scattering on the plasma thermalization in doped GaAs.

The cooling of an electron-hole plasma in bulk n- and p-type doped GaAs is calculated without assuming complete thermalization between electrons and holes. Electron-hole scattering and nonequilibrium optical phonons are included in the simulation. In n-doped GaAs, we show that at low excitation density (i.e., when the photogenerated plasma density is small compared to the donor density), the electrons are at first colder than the holes but heat up quickly and subsequently stay close to the hole temperature. At low density in p-doped GaAs, however, the holes stay cold and the electron cooling is mainly controlled by electron-hole scattering.

Localization effects, energy relaxation, and electron and hole dispersion in selectively doped n-type AlyGa1-yAs/InxGa1-xAs/GaAs quantum wells.

The nonequilibrium electron-hole pair (e-h) system in a strained ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum well (QW) has been studied. The heterostructures were selectively doped n-type ${\mathrm{Al}}_{\mathit{y}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$As/ ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs (x=0.1--0.2) having an electron concentration ${\mathit{n}}_{2\mathrm{D}}$=(0.4--1.0)\ifmmode\times\else\texttimes\fi{}${10}^{12}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$. Simultaneous measurements of the quantum Hall effect and photoluminescence in a magnetic field were found to be crucial to obtain a reliable understanding of the system. The main features of the emission spectra from a weakly excited QW (the concentration of photoexcited e-h pairs is less than ${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$) were found to be determined by hole localization due to fluctuations in the ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As composition (\ensuremath{\Delta}x/x1%), QW thickness (\ensuremath{\Delta}${\mathit{L}}_{\mathit{z}}$/${\mathit{L}}_{\mathit{z}}$)5%, and the density of charged impurities. The hole relaxation time was found to depend on the excited e-h pair density and QW quality. At the low excitation density of ${10}^{\mathrm{\ensuremath{-}}3}$ W/${\mathrm{cm}}^{2}$ the hole temperature reaches 20 K at a lattice temperature of 2 K. Finally, the E-k dispersion of electrons and holes were measured from the emission spectra under magnetic field. The electron effective mass was determined to be 0.072${\mathit{m}}_{0}$ and 0.067${\mathit{m}}_{0}$ for QW's with x=0.13 and 0.18, respectively. The valence band is strongly nonparabolic with the hole mass 0.105${\mathit{m}}_{0}$ at the band edge.

Unusual properties of photoluminescence from partially ordered Ga0.5In0.5P

Ga0.5In0.5P grown lattice matched to GaAs by metalorganic chemical vapor deposition (MOCVD) exhibited domains of varying degrees of column III sublattice ordering. Continuous-wave photoluminescence spectra were single peaked and relatively narrow, but the peak wavelengths from samples grown at low (630–670 °C) temperatures varied strongly with excitation density at low measurement temperatures, while peak wavelength did not vary for high (775 °C) temperature growth. The half width was 6.5 meV in the latter case, the narrowest reported from MOCVD-grown Ga0.5In0.5P. Time-resolved photoluminescence of partially ordered GaInP at liquid-helium temperatures is reported for the first time. For samples grown at low temperatures, the spectral peak displayed a slow (τ≳1 μs) decay at low excitation density. The decay was more rapid (τ=1.8 ns) at higher excitations and at higher photon emission energies. Possible explanations discussed include spatial separation of carriers and trapping.<squeeze;1.6p>

Polar optic phonon and Γ→L intervalley scattering times in GaAs from steady-state hot-electron luminescence spectroscopy

We deduce the Γ→L intervalley and polar optic phonon scattering times of hot electrons in bulk GaAs from cw hot (e,A0) luminescence spectra at low excitation densities and their dependence on electron kinetic energy. We obtain the lifetime broadening due to these two processes from comparison with line shape calculations using a 16×16 k⋅p Hamiltonian, a full integration over k space, and a dipole model for the optical matrix elements. We find for the LO-phonon emission time τLO=(132±10)fs. The threshold for Γ→L scattering is determined as 330±10 meV, above which a distinct decrease in total lifetime is observed. Γ→L scattering times of 150–200 fs are deduced, and we discuss a corresponding deformation potential.

High‐Density Spectroscopy of ZnSe/GaAs Epilayers in the Near‐Band‐Edge Region

AbstractZnSe epilayers grown by MBE on (100) GaAs substrates are optically investigated in comparison with bulk ZnSe crystals using photoluminescence and excitation spectroscopy under high excitation densities. High‐density luminescence bands appear in both materials for relatively low excitation densities (about 30 kW/cm2), but their energetic positions do not correspond. The intensity‐dependent shape, and the behaviour of the dominant band in bulk ZnSe in excitation spectroscopy are similar to that of the M band in other II VI semiconductors. The main bands growing up in the ZnSe/GaAs films are much more corresponding to typical P‐band emission of exciton‐exciton collision. Further, the transmission of ZnSe epilayers released from the GaAs substrate is studied in dependence on the laser intensity.

Nonlinear Luminescence of Heavily Doped CdS: In Crystals

AbstractLuminescence data of heavily doped CdS:In crystals at various exciation densities are presented. The spectra are compared to theoretical calculations, which are based upon the assumption of indirect band‐to‐band transitions. At low excitation density some highly self‐compensated samples exhibit an enhanced luminescence at energies around 2.45 cV. This is due to conduction band acceptor transitions, which are easily saturable. Samples of intermediate In concentration show a narrow‐band highexcitation line, which is attributed to an EHP recombination. The temperature‐dependent behavior of this line indicates the occurrence of optical gain in these samples.

Time-resolved measurements of tunneling between double quantum wells in In0.53Ga0.47As/InP

Tunneling of electrons between double quantum wells is investigated by time-resolved photoluminescence in the picosecond regime. The samples contain two In0.53Ga0.47As quantum wells with different widths, separated by various InP barriers. At low excitation density, the luminescence decay time of the narrower quantum well depends strongly on the thickness of the barrier, revealing the lifetimes to be tunneling controlled. A semiclassical model explains the observed nonresonant tunneling escape times. With increasing density, the luminescence decay time of the narrower quantum well strongly increases and finally saturates due to effective mass filtering, which leads to a lineup of the electron levels in both wells and resonant tunneling.

Picosecond absorption studies of intermolecular electronic energy transfer in micellar systems. II. Polarization-dependent studies of energy migration and energy trapping at low donor excitation densities

A picosecond excite-and-probe beam investigation on intermolecular electronic energy transfer (EET) between dye molecules in one-component systems (electronic energy migration - EM) and two-component systems (electronic energy trapping - ET) is presented and compared with existing theories for EET in restricted volumes. The experiments were carried out on a transient absorption spectrometer based on synchronously pumped dye laser (pulse duration 5 ps). Donor excitation density was less than 1%. In the case of ET good agreement has been obtained with the Förster theory, in which in addition an inhomogeneous spatial distribution of the dye molecules has been considered (part I of this study). In the case of EM direct evidence for the theory of Ediger and Fayer has been obtained by measuring the polarization anisotropy at early times after excitation.

Picosecond time evolution of photoexcited hot-electron mobility in GaAs and the speed of photoresponse

The transient mobility of hot electrons photoexcited in undoped GaAs by subpicosecond laser pulses is calculated. For this, we solve the time-dependent Boltzmann transport equation in the presence of a low-frequency, weak electric field. The attention is focused mainly on the role of intracentral Γ valley scattering in determining the delay in the mobility rise on the picosecond time scale, and the hot-electron energies are assumed to be below the thresholds for possible side-valley transfers (Γ→L,X). We consider the mobility response under two separate conditions of excited carrier density, namely, (1) low-density excitations for which the electron–longitudinal phonon (LO) Fröhlich interaction initially dominates in the carrier relaxation and (2) high-density excitations for which the electron-electron interaction is faster than all other collisions. The mobility of hot electrons is very small (<1000 cm2/V s) just after photogeneration. It rises to its maximum value with a time constant decided by the various scattering processes which are influenced by the values of the carrier density and the lattice temperature. We find that the mobility rise can be quite slow up to the scale of several picoseconds, even when the possible delay due to side-valley scattering is absent.

Cooling of hot carriers in three- and two-dimensional Ga0.47In0.53As.

Carrier cooling is investigated by means of time-resolved photoluminescence spectroscopy in an undoped ${\mathrm{Al}}_{0.48}$${\mathrm{In}}_{0.52}$As/${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As/InP heterostructure and in undoped and modulation-doped ${\mathrm{Ga}}_{0.47}$${\mathrm{In}}_{0.53}$As/${\mathrm{Al}}_{0.48}$${\mathrm{In}}_{0.52}$As multiple-quantum-well structures. The cooling dynamics of electrons and holes is analyzed by a theoretical model based on Fermi-Dirac statistics and taking into account polar-optical scattering and acoustic-deformation-potential scattering as energy-loss and Auger processes as heating mechanisms. The energy loss of holes by polar-optical scattering is close to theoretical expectations in the low-excitation regime, but is reduced by 2 orders of magnitude for high-excitation densities. This reduction is attributed to a hot phonon effect. For electrons the polar-optical energy-loss rate is strongly reduced even at low excitation densities as compared to theoretical expectations. We assume that screening of the Coulomb interaction between carriers and lattice might play a role beside the hot phonon overpopulation to explain this phenomenon.

Dynamics of carrier capture in an InGaAs/GaAs quantum well trap

We report on a time-resolved luminescence study of the dynamics of carrier capture in a strained quantum well of InGaAs embedded in GaAs confining layers. Efficient carrier capture takes place at low temperature (T<70 K) and low excitation density (∼250 nJ/cm2 per pulse), while at room temperature and higher excitation density the rate of carrier collection decreases as the carriers fill all the available states in the quantum well allowed by the Fermi–Dirac statistics.

One-dimensional energy transfer in GdCl3

We report the observation of the intrinsic fluorescence of anydrous gadolinium trichloride GdCl3 when laser light is absorbed into the first excited state 6P7/2. With the aid of a frequency doubled pulsed dye laser, we are able to analyze carefully the excitation and emission spectra as well as the decay times over a range of temperatures including that at which the material undergoes a ferromagnetic transition at 2.2 K. At 4.4 K and above, experiments at low excitation density (N0<1014 excited ions/cm3) show that the intrinsic and the impurity induced trap fluorescence dynamics are well described in terms of a fast diffusion and trapping model. When the excitation density is increased anti-Stokes fluorescences appear efficiently; they are assigned to the 6D9/2, 6I7/2→8S7/2 transitions. High excitation density effect on the intrinsic decay as well as the decay of the anti-Stokes fluorescence are nicely reproduced by a theoretical model involving mainly an exciton–exciton annihilation process. Moreover, an excited state absorption mechanism contributes notably to the anti-Stokes fluorescence intensity at t=0 with increasing the excitation density. In the ferromagnetic phase of GdCl3, at 1.5 K, the one-dimensional nature of the exciton diffusion (8S7/2↔6P7/2) is confirmed by the ‘‘incoherent’’ exciton decay analysis using an appropriate model recently developed by Cibert et al. Finally a diffusion length spreading over 2500 visited Gd3+ normal sites is evaluated, knowing the hopping time as a function of the nearest neighbor interactions (nn) in the crude approximation of an ideal two level system in a linear chain of atoms.

Hot-carrier energy-loss rates in GaAs/AlxGa1−xAs quantum wells

We present a systematic study of the cooling of hot carriers in undoped, n-type doped, and p-type doped GaAs/${\mathrm{Al}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As quantum wells of different well widths by time-resolved luminescence spectroscopy. The energy loss of the carriers due to interaction with optical phonons is independent of dimensionality and well width. The energy loss of electrons is highly reduced at all excitation densities compared with a simple theory of the interactions; for holes, the energy loss comes close to theory at low excitation density. The reduction of the energy-loss rate by optical-phonon scattering cannot be explained by screening or degeneracy but rather is consistent with a hot-phonon effect. The energy-loss rate due to deformation-potential scattering with acoustical phonons increases with decreasing well width.

Conductivity response of nonthermal hot carriers photoexcited by subpicosecond pulses in GaAs.

A simplified formulation of the problem of calculating transient electrical conductivity \ensuremath{\sigma}(t) for nonequilibrium carriers excited by a subpicosecond laser pulse in semiconductors such as GaAs is presented. The Boltzmann equations for the distribution function of various types of hot carriers in the valence and conduction bands are set up in the presence of an electric field, and solved in the weak-field approximation. For assumed low-density excitations, the dynamics of the excited carriers is dominated by the LO-phonon emission via the strong Fr\ohlich interaction. The energy relaxation due to this interaction is treated within the model of ``cascade emission'' of LO phonons. The other slower relevant scattering processes are approximated by an effective relaxation time, for simplicity. The initial photoexcitation of carriers, by a laser beam of a short-time duration T, is taken to occur in a sufficiently narrow range of energy 2\ensuremath{\Delta}. The time dependence of \ensuremath{\sigma}(t) arising from the conduction electrons and the holes in the two valence bands is calculated explicitly as a function of excitation energies ${\ensuremath{\varepsilon}}_{0}$ of these carriers (measured from the extrema of the appropriate bands), the initial energy spread 2\ensuremath{\Delta}, generating laser pulse width T, the polarization state of the laser beam, and the effective quasielastic relaxation time ${\ensuremath{\tau}}_{1}$.It is found that at sufficiently short times, \ensuremath{\sigma}(t) is a strong function of the initial carrier excitation energies. The conductivity rapidly decreases and becomes negative for initial conduction-electron excitation energy \ensuremath{\varepsilon}${\ifmmode \tilde{}\else \~{}\fi{}}_{0}$, measured in units of the long-wavelength LO-phonon energy \ensuremath{\Elzxh}${\ensuremath{\omega}}_{\mathrm{LO}}$, close to an integer m. For such special values of \ensuremath{\varepsilon}${\ifmmode \tilde{}\else \~{}\fi{}}_{0}$, \ensuremath{\sigma}(t) shows a pronounced negative dip initially, before saturating to the usual positive value determined by the relaxation time ${\ensuremath{\tau}}_{1}$. For other \ensuremath{\varepsilon}${\ifmmode \tilde{}\else \~{}\fi{}}_{0}$, not close to an integer, the conductivity steadily rises in time to saturate again at the value determined by ${\ensuremath{\tau}}_{1}$. The contribution of electrons excited from the light-hole band to \ensuremath{\sigma}(t) is found to be appreciable but insufficient to offset the -\ensuremath{\sigma}(t) effect. The hot holes have only a small contribution. The polarization of the laser beam is found to have a noticeable quantitative effect on the size of the negative dip in \ensuremath{\sigma}(t).

Energy relaxation in p- and n-GaAs quantum wells: Confinement effects

We have measured the energy relaxation of carriers in p- and n-type GaAs quantum wells using time-resolved photoluminescence. At low excitation densities (low carrier temperatures) the energy loss rate for holes is greater than for electrons, but it is not observed to depend on well width for values greater than 60Å. At high excitation densities the rate is found to increase significantly for narrow wells (25Å).

Energy transfer and inversion saturation in Tm, Ho: YAG

We report on the spectroscopic properties and the fundamental aspects of the Tm-Ho energy transfer in YAG under low and high excitation densities. Strong Tm-Ho interaction causes efficient down conversion of 781.5 nm pump light and saturation of the 2000 nm transition of Ho 3+ in Tm,Ho:YAG laser crystals at 300 K.