Polarised hot electron luminescence in p-GaAs: Electric field effects

Electrons photo-excited to high-energy conduction band states of GaAs exhibit complex energy and momentum distributions determined by the anisotropic valence band structure and the optical matrix elements. In p-type GaAs a fraction of these hot electrons combine with localised acceptor states, producing a hot electron luminescence (HEL) spectrum with a cascade of peaks corresponding to discrete energy losses resulting from LO-phonon emission. The highest peak involves unscattered electrons, and their energy distribution is due to warping of the initial heavy-hole (HH) bands. We report measurements of the line shape of this 0-HH peak, and its polarisation profile which identifies emission from electrons along particular directions. An applied electric field of 1 kV cm⁻¹ distorts the hot electron momentum distribution, and this is reflected in the polarisation profile. These line shapes and profiles, with and without field, are calculated using a computer model incorporating a band structure and optical matrix elements, the effect of electric field being included using a k-broadening model. The data and model are in good quantitative agreement assuming an electron lifetime of 100 fs, and confirm the expected differences in the profiles for different excitation polarisation states and applied field directions.     

Effects of cation order on the energy bands of GaAs-AlAs heterostructures

Pseudopotential band structure calculations of (GaAs)_n ? (A$\text{l}$As)_n with n = 1, 2, 3 and of Ga_0.5A$\text{l}$_0.5As show that the effects of cation order are small due to the weakness of the Ga-A$\text{l}$ potential difference. The energy bands of the n = 1 heterostructure can be obtained by folding those of the alloy except for small energy shifts and splittings which can be handled in perturbation theory. The superlattice energy gap decreases with increasing n, the shrinkage being 4 meV for n = 2 and 10 meV for n = 3.     

Inter- and intra-subband relaxation of hot electrons in GaAs/AlGaAs quantum wells

In this work, we have studied the inter- and intra-subband scattering of hot electrons in quantum wells using the hot electron-neutral acceptor luminescence technique. We have observed direct evidence of the emission of confined optical phonons by hot electrons excited slightly above the n=2 subband in GaAs/Al_0.37Ga_0.63As quantum wells. Scattering rates of photoexcited electrons via inter- and intra-subband LO phonon emission were calculated based on the dielectric continuum model. We found that, for wide wells with the Al composition of our experiments, both the calculated and experimental results suggest that the scattering of the electrons is dominated by the confined LO phonon mode. In the calculations, scatterings among higher subbands are also dominated by the same type of phonon at well width of 10 nm.     

Intrinsic radiative recombination from quantum states in GaAs-AℓxGa1−xAs multi-quantum well structures

From absorption, emission, luminescence excitation and electron spin orientation studies of undoped GaAs-A?_xGa_1?xAs superlattices we demonstrate the $\text{intrinsic}$ nature of the radiative recombination process. This is in direct contrast to recombination observed in similar purity thick GaAs material. Moreover, our results do not support a recent suggestion that enhanced LO phonon-electron coupling should occur in such superlattice structures.     

Photoconductivity and photovoltaic excitation spectra and their wavelength derivative in Cu2O

Photoconductivity and photovoltaic excitation spectra, both direct and wavelength modulated, have been performed on monocrystalline Cu₂O from 6 to 300K. At low temperatures both types of direct spectra exhibit the excitonic structures already observed by other techniques such as absorption and reflectivity. The wavelength-modulated PC spectra reveal additional structures and in particular well-defined oscillations in the green fundamental continuum, related to photocarrier LO phonon interactions. The analysis of these results shows that the 149cm^?1 (18.5 meV) LO phonon is responsible for the observed periodic structures. The values found for the energy of the collector level (2.305 ±0.001 eV) and for the effective mass ratio $\text{(}\text{m}{}_{\mathrm{h}}\text{m}{}_{\mathrm{e}}\text{bsime; 12)}$, indicate that the oscillations are produced by the electrons excited from the heavy hole valence band Γ₈⁺ to the Γ₁⁺ conduction band. A study of the position and of the contrast of the phonon oscillations as functions of temperature illustrates the behaviour of the gap displacement with changes in temperature and throws some light onto the mechanisms responsible for the existence of such oscillations.     

Magnetophonon resonances of the two-dimensional electron gas in GaAs/AlGaAs heterostructures

We have studied magnetophonon resonances of the two-dimensional (2D) electron gas at the GaAs/AlGaAs interface in a single interface heterostructure and a superlattice. In magnetic fields up to 30T, one set of oscillations is observed, corresponding to the coupling of 2D electrons with LO phonons of GaAs. The effective mass obtained directly from the magnetic field position of the fundamental resonance, where the Landau splitting equals the bulk phonon energy, and of the next two harmonics is $\text{m}{}^1\text{= (0.071 ± 0.0015)m}{}_0$. A comparison with cyclotron resonance measurements on the same system and with bulk GaAs data gives an upper limit of about 2% for the mass corrections due to polaron effects and due to the confinement of the electron gas.     

Excited states of Ag and Cu acceptors in CdTe

We report on resonantly excited luminescence (REL) at 1.8 K and infrared absorption (IRA) between 6–12 K of CdTe doped with Ag and Cu impurities. The luminescence was excited with a tunable dye laser. Two hole transitions up to $\text{6}\text{S}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ state have been observed, the bound exciton lines have been identified. $\text{2}\text{P}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ states have been observed in IRA measurements. The binding energies as deduced from REL experiments are 107.5 meV for Ag and 146 meV for Cu. The fitted values of the valence band parameters are Ro = 27 ± 3 meV μ = 0.73 ± 0.03 and δ = 0.12 ± 0.01.     

Infrared cyclotron resonance in InSb,GaAs and Ge in very high magnetic fields

Infrared cyclotron resonance was observed in n-type InSb, GaAs and Ge in very high magnetic fields up to 1.3 MOe at room temperature using a CO₂ laser. A large shift of the cyclotron mass due to the non-parabolicity of the energy band was found in each material. The band edge masses of electrons at room temperature were evaluated to be $\text{m}{}^1\text{= 0.0127 m}$ for InSb, $\text{m}{}^1\text{= 0.065}$m for GaAs and $\text{m}{}^1{}_{\mathrm{t}}\text{= 0.086}$m for Ge. The linewidth was measured in GaAs and Ge in the high fields.     

Raman scattering by two LO-phonons near Γ in GaAs

Near resonance the second order phonon spectra of semiconductors exhibit peaks which correspond to 2 LO phonons with wave-vector $\text{q}\text{→}\text{?o}$ (2 LO(Γ)). The frequencies of these 2 LO(Γ)-peaks are studied for photon energies varying across the E_o + Δ_o gap of GaAs. These peaks are shown to be slightly shifted to lower frequencies compared with the exact 2 LO(Γ) frequency. The peak positions when plotted as a function of the incident photon energy display an oscillatory behaviour which corresponds to the fact that LO-phonons of different wave vectors are created in the Raman process as the photon energy is tuned across the E_o+Δ_o resonance.     

Photoluminescence of Mn doped epitaxial GaAs

We have measured the near band edge photoluminescence of Mn doped liquid phase epitaxially grown GaAs. The photoluminescence spectra at 2°K shows, at low excitation intensities, a structure of up to eight sharp peaks (widths .2 to 1.0 meV) between 1.517 and 1.512 eV, besides the lower energy bands near 1.41 eV due to the deep Mn acceptor level and the usual donor-acceptor bands around 1.47 eV. Attempts to relate the sharp lines to the Mn electronic states, introduced by doping, were unsuccessful. It is our belief that the presence of this particular impurity in our samples allows for whatever states are responsible for the sharp line structure, to reveal themselves in the emission spectrum. A most unespected result is that near band edge sharp line luminescence is observed for impurity concentration as high as 10¹⁸cm^-3.     

Photoelectron diffraction effects in XPS angular distributions from GaAs(110) and Ge(110) single crystals

Angular distribution measurements of XPS intensities have been made for various spectral lines from GaAs(110) and Ge(110) single-crystal surfaces. Observed angular distribution curves (ADC's) showed steep intensity variations and sharp peaks due to X-ray photoelectron diffraction (XPED) phenomena. The effects of the type of transition process (photoelectron or Auger), electron kinetic energy and crystal structure on the XPED patterns were examined. Considerably different ADC patterns were observed for high-energy photoelectrons and Auger electrons and for low-energy photoelectrons. ADC's for Ga 3d, As 3d and Ge 3d showed almost the same patterns for scans of the type [110] → [100] → [1$\text{1}$0], but they showed substantially different patterns for [110] → [11$\text{1}$] → [00$\text{1}$] scans. These features correspond well with the structural characteristics of GaAs and Ge crystals. A discussion of the applicability of XPS angular distribution measurements to the geometric analysis of crystal surfaces is presented.     

Shallow acceptor bound exciton and free exciton states in high-purity zinc telluride

We investigated excitons bound to shallow acceptors in high-purity ZnTe and measured excitation spectra of two-hole luminescence lines at 1.6 K using a tunable dye-laser. The electron-hole coupling in the bound exciton (BE) states appears to be very different for the various acceptors even for almost identical exciton localisation energies. Three different types of BE are reported. For the Li-acceptor BE we observe three sub-components separated by 0.22 and 0.17 meV and interpreted as J = $\text{1}\text{2}$, $\text{3}\text{2}$, $\text{5}\text{2}$ states. The Ag-acceptor BE exhibits a strong ground state and a weak excited state at 1.3 meV higher energy. For the as yet unidentified k-acceptor we observe a single BE level, degenerate with the Ag-acceptor BE ground state. Dips in the excitation spectra due to absorption into free exciton 1S, 2S, and 3S states yield an exciton Rydberg R₀ = 12.8±0.3 meV and a free exciton binding energy FE(1S) = 13.2±0.3 meV.     

Observation of stokes and anti-stokes vibrational sidebands in the optical emission spectrum of Eu atoms isolated in an argon matrix

The 4f⁶5d6s² → 4f⁷ 6s² emission spectrum of Eu atoms isolated in an argon matrix shows (a) zero-phonon lines and (b) Stokes vibrational sidebands due to resonance modes in the $\text{Ar}$: Eu phonon spectrum. In addition, unexpected anti-Stokes vibrational side bands are observed, which might be due to “hot luminescence”.     

Electronic structure of GaAs/AlGaAs quantum double rings in lateral electric field

A three-dimensional model of GaAs/A1GaAs quantum double rings in the lateral static electric field is investigated theoretically.The eigenvalue problem with the effective-mass approximation is solved by means of the finite-element method.The energy levels and wave functions of quantum-confined electrons and heavy holes are obtained and show an agreement with our previous theoretical and experimental studies.It is shown in the approximation of neglecting the Coulomb attraction between the electron and heavy hole that a relatively large Stark shift of exciton emission of 4 meV is attainable with an applied electric field of 0.7 kV/cm.     

Luminescence property studies of α-Al2O3 by means of nanosecond time-resolved VUV spectroscopy

The luminescence spectra of corundum monocrystals grown by different methods are investigated by means of a time-resolved spectroscopy method at temperatures 90 K and 300 K. The existence of fast and slow emission in the VUV luminescence spectra of irradiated and nonirradiated crystals was observed. We observed luminescence bands with a maximum at 326 nm produced by F ⁺ centers. A new type of fast luminescence at the band of 270 nm was found. This is known as cross-luminescence and is connected with the recombination of valence band electrons with the holes in the low ground band. It was shown that the band of 410 nm isn't due to to anionic centers (F-centers), but is determined by the short lifetime center of emission (F ^- -centers). Received 20 October 1998 and Received in final form 20 January 1999     

Terahertz emission from square wells in a longitudinal electric field

Terahertz emission accompanying heating of two-dimensional electrons by a strong electric field applied along size-quantized GaAs/AlGaAs layers is observed and investigated. The emission is due to indirect optical transitions of hot electrons in the bottom size-quantization band. The experimentally obtained emission spectra are compared with the spectra calculated taking into account scattering of electrons by optical phonons, impurities, and interfacial roughness and electron-electron scattering. Satisfactory agreement is obtained. The temperature of the hot electrons is determined from a comparison of the spectra. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 507–511 (10 April 1998)     

ZnCdSe/ZnSe多量子阱的生长和激子光学性质的研究

用分子束外延在GaAs衬底上生长了ZnCdSe/ZnSe多量子阱结构.利用X射线衍射(XRD)、变温度PL光谱和ps发光衰减等研究了ZnCdSe/ZnSe多量子阱结构和激子复合特性.由变温PL光谱讨论了随温度升高辐射线宽展宽和辐射复合效率降低的机理.     

Low temperature photoluminescence study of Ga As defect states

Low temperature(77 K)photoluminescence measurements have been performed on different GaAs substrates to evaluate the GaAs crystal quality.Several defect-related luminescence peaks have been observed,including 1.452 eV,1.476 eV,1.326 eV peaks deriving from 78 meV Ga_(As) antisite defects,and 1.372 eV,1.289 eV peaks resulting from As vacancy related defects.Changes in photoluminescence emission intensity and emission energy as a function of temperature and excitation power lead to the identification of the defect states.The luminescence mechanisms of the defect states were studied by photoluminescence spectroscopy and the growth quality of GaAs crystal was evaluated.     

The anisotropy of the hot-hole drift velocity in Ge

This paper presents a microscopical interpretation of the anisotropy of the hot-hole drift velocity in Ge at 77°K. The theory makes use of the Monte Carlo technique in solving the Boltzmann equation and is based on a single valence band model (the heavy mass one) warped and parabolic. The theoretical calculations carried out in a general form show that: (i) the anisotropy of the hot-hole drift velocity originates from the warping of the valence band structure; (ii) the strong interaction of holes with optical phonons via emission processes plays a fundamental role in the anisotropy supporting the reliability of the streaming motion model first suggested by Pinson and Bray.The comparison between theory and experiment at 77°K in Ge is satisfactory for the whole range of fields examined $\text{(50 ? E ? 10}{}^4\text{V}\text{cm}\text{)}$, confirming the basic validity of the present approach. The importance of optical and acoustic scattering mechanisms have been found comparable, the coupling strength ratio $\text{w}{}_{\mathrm{op}}\text{w}{}_{\mathrm{ac}}$ assuming the value 1.36.     

Adsorbate-induced surface resonances observed in photoemission from a (2 × 2)R45° sulphur layer on Pd(100)

Angular-resolved photoemission spectra from a $\text{(}\text{2}\text{×}\text{2}\text{)R45°}$ sulphur layer on Pd(100) reveal peaks independent of photon energy, which show strong dispersion in the electron energy range 3…9 eV relative to the vacuum level. The appearance of these levels can be correlated with absolute gaps in the projected bulk band structure of palladium. We interpret these features, which are also observed in secondary electron emission, as adsorbate-induced surface resonances above E_F. The sulphur 3p-derived levels below E_F show strong dispersion effects in the [110] but not in the [100] crystal azimuth. The projected band structure also shows that $\text{sd}$ hybridisation gives rise to an absolute gap at approximately this energy in the [110] $\text{(}\text{ΓX}\text{)}$ direction.