Indirect exciton dispersion in III–V semiconductors: “Camel's back” in GaP

A new perturbation approach to exciton dispersion in indirect gap semiconductors is developed. For GaP and AlSb existence of the “camel's back” in exciton dispersion is confirmed, and a precise value of the “camel's back” parameter for X^c₁-minima in GaP is reported: E(X^c₁)?E_min(Δ^c₁)=3.5±0.3 meV. At the X-point the 21.44 and 19.48 meV exciton binding energies in GaP are obtained. The corresponding valley-anisotropy splitting is 1.96 meV.     

Shallow impurity states and the free exciton binding energy in gallium phosphide

A new set of donor and acceptor ionization energies in GaP is deduced from photoexcitation spectra. Energy spectrum of donor states confirms an existence of the “camel's back” with a conduction band minima displacement ≈ 0.08(2π/a) from the X, and the corresponding energy shift ≈ 3meV. The free exciton binding energy in GaP is correctly determined: E_ex = 21 ± 2 meV.     

Fine structure of indirect exciton in GaP

Wavelenght modulation spectra of the 1s indirect exciton in GaP are reported. Four lines are found which can be easily explained taking into account the “camel's back” structure of the conduction band. An exciton splitting of 0.9 meV^- ±0.2 has been measured.     

Theory of donor states in GaP: The role of the camel's back

We present a theoretical study of the influence of the camel's back structure on the low lying excited states of the donors in GaP. Based on comparison with the observed spectrum, we conclude that the appropriate values for the parameters describing the camel's back are $\text{Δ}\text{E}\text{= 3.2}\text{meV and k}{}_{\mathrm{o}}\text{= 0.083 (}\text{a}{}_{\mathrm{o}}\text{2π}\text{)}$.     

The location and shape of the conduction band minima in cubic silicon carbide

We have measured the inter-bound state excitation spectrum of the N_C donor in cubic β-SiC through the ‘two-electron’ transition satellites observed in the luminescent recombination of excitons bound to neutral N donors. Transitions are seen to p as well as s-like donor states although the transition oscillator strength is derived from interaction with the impurity core since parity is conserved through inter-valley scattering by p-like X phonons. The Zeeman splitting of a luminescence line involving the 2p± donor state yield the electron mass parameter m_t = 0.24 ± 0.01 m₀. This and the directly measured energy separations of the 2p₀ and 2p± states yields m_t/m₁ = 0.36 ± 0.01 with the static dielectric constant K = 9.92 ± 0.1. Mutually consistent central cell corrections of 1.1 and 8.4 meV are observed for the 2s(A₁) and 1s(A₁) donor states, the latter being in agreement with a recent estimate from electronic Raman scattering by Gaubis and Colwell. The ionization donor energy of the N_C donor, 53.6 ± 0.5 meV is consistent with earlier, less accurate estimates from donor-acceptor pair and free to bound luminescence. There is no evidence for a ‘camel's back’ conduction band structure in cubic SiC, unlike GaP. The two-phonon sidebands of the N_C donor exciton luminescence spectrum in SiC can be constructed by X and Г phonons only.     

A dense electron-hole-liquid in Ga0.08Al0.92As

An electron-hole-liquid in the polar ternary compound Ga_xAl_1?xAs is observed. Temperature, excitation intensity, and time dependent luminescence spectra for x = 0.08 reveal the high carrier density n = 1.6 × 10¹⁹ cm^-3, critical temperature T_c = 52⁺³_-2K and binding energy E_B = 16 meV.The experimental results agree with theoretical predictions presented here, only if the newly discovered camel's back in the conduction band at X-point is accounted for, thus demonstrating the importance of this band structure anomaly.     

Revised fine splitting of excitons in diamond

We study low-strain synthetic high pressure, high temperature diamonds by cathodoluminescence and observe novel fine structure in the free exciton and the boron-bound exciton emission. The basic spectral structure is a doublet with DeltaE approximately 11 meV common to both exciton spectra. This resolves the previously found inequivalence of free exciton ( approximately 7 meV) and bound exciton ( approximately 12 meV) fine splitting. It is argued that for a spin-orbit interaction Delta(0) much smaller than the excitonic binding ( E(X) approximately 80 meV) and the excitonic localization ( E(loc) approximately 51 meV) at the boron acceptor, the orbital momentum and the spin of the particles constituting the electron-hole pair are recoupled to form spin singlet and triplet exciton states as the elementary excitations.     

The absorption edges of GeS and Urbach's rule

The low-energy absorption tail of the E∥a exciton in GeS obeys Urbach's rule over at least three orders of magnitude in absorption coefficient for 4.2 ? T ? 240 K. The E∥b polarization (for which no exciton is observed) does not obey Urbach's rule. The E∥a Urbach edge is caused by interactions between the excitons and a 8.7 meV rigid-layer vibrational mode. Our results are consistent with Sumi and Toyazawa's theory of the Urbach edge, and inconsistent with the models of Dow, Redfield and Skettrup. The absorption edge of GeS is confirmed as a direct edge.     

Absorption measurements at high pressure (0–10kbar) on strained superlattices

Absorption data on strained GaAs_1?xP_x-GaAs superlattices (SL, 128-period, barrier size L_B≈75 Å, quantum-well size L_z≈75 Å, alloy composition x≈0.25) are presented in the range 0–10 kbars. The absorption curves obtained show no exciton show no exciton peaks such as seen in lattice- matched Al_xGa_1?xAs-GaAs SL's, and the pressure coefficient decreases from 11.5 meV/kbar to ≈ 10.5 meV/kbar in the wells and ≈6.5 meV/kbar at energies approaching and above the barrier energies. This behavior is attributed to the fluctuations in strain caused by the alloy disorder, and clustering, of the barriers.     

Identification of Cu and Ag acceptors in CdTe

We report here on the identification of the two dominant acceptor levels in high purity p type CdTe, with Cu and Ag on Cd site. This identification is based on back doping experiments coupled with electrical measurements and photoluminescence studies. Cu and Ag can form easily complex centers when a supersaturation is achieved. The way of obtained good doping without complexation, is explained. The principal bound exciton lines are at 1,5896 eV (Cu) and at 1,5885 eV (Ag). The precise hole binding energies obtained from optical data are E_A (Cu) = 146 meV and E_A (Ag) = 108 meV.     

The “Urbach” absorption edge in ALN

The shape of the intrinsic absorption edge of the AlN single crystals has been interpreted under assumption of the absorption of Wannier excitons in the electric field predominantly of charged impurities. The best fit of experimental data is obtained forE _G6·2–6·3 eV and exciton binding energyR70–80 meV.     

Fine structure splitting and biexciton binding energy in single self-assembled InAs/AlAs quantum dots

We report on photoluminescence investigations of individual InAs quantum dots embedded in an AlAs matrix which emit in the visible region, in contrast to the more traditional InAs/GaAs system. Biexciton binding energies, considerably larger than for InAs/GaAs dots, up to 9 meV are observed. The biexciton binding energy decreases with decreasing dot size, reflecting a possible crossover to an antibinding regime. Exciton and biexciton emission consists of linearly cross polarized doublets due to a large fine structure splitting up to 0.3 meV of the bright exciton state. With increasing exciton transition energy the fine structure splitting decreases down to zero at about 1.63 eV. Differences with InAs/GaAs QDs may be attributed to major dot shape anisotropy and/or larger confinement due to higher AlAs barriers.     

Wavelength modulated absorption in SiC

Wavelength modulated absorption spectra of the free excitons in 6H, 15R and 3C SiC have been measured. The spin-orbit splitting of the valence bands of the uniaxial and cubic polytypes are found to be 7 and 10 meV respectively. Using a new value for the exciton binding energy of 27 meV, an improved value of the fundamental gap of cubic SiC, E_g = 2.417 eV, is derived. Due to the small spin-orbit splitting, the valence bands are highly non-parabolic at low energies.     

The Xc1 camel's-back parameters for cubic III–V semiconductors

Band-parameter calculations for the X^c₁-minima were carried out. They confirm that the camel's-back structure is typical for cubic III–V semiconductors. We report a significant temperature dependence of the density-of-states effective mass and discuss the conditions which give a negative sign for two principal values of the conduction-band effective-mass tensor.     

Free exciton luminescence lineshape of Hg0.3Cd0.7Te

The photoluminescence spectrum of Hg_0.3Cd_0.7Te at 77K includes a narrow, high energy free exciton line. This experimental spectrum is in good agreement with the theoretical free exciton lineshape, the Gaussian broadening of this line is due to alloy inhomogeneity, and the binding energy of the bound exciton with respect to the free exciton at 77K is 13 ± 4 meV.     

Time-resolved photoluminescence study of GaInAs/AlGaInAs superlattices

We present results of time-resolved photoluminescence experiments performed at 77 K on a GaInAs/AlGaInAs superlattice grown by molecular beam epitaxy and lattice matched to an InP substrate. The superlattice is the intrinsic part of a p–i–n diode. Photoluminescence spectra, reconstructed at various delay times between 5 ps and 100 ps after the laser pulse, show lines associated to the 1s and 2s heavy-hole exciton states and to the free carrier recombination. This result provides a direct determination of the binding energy of the heavy-hole exciton which is shown to be equal to 15 meV. Such a large value of the Rydberg is due to the fluctuations of composition which cause the heavy-hole exciton to be localized within a single well. The spectra also exhibit a shoulder which corresponds to the electron-to-light-hole transition. The 2s heavy-hole-exciton transition is coupled to the latter by an LO phonon. Finally a transition 21 meV below the 1s heavy-hole-exciton energy is related to Be residual impurities.     

Exciton states in asymmetric GaInNAs/GaAs coupled quantum wells in an applied electric field

The electronic and optical properties of exciton states in GaInNAs/GaAs coupled quantum well (CQW) structure have been theoretically investigated by solving the Schrödinger equation in real space. The effect of well width on the exciton states has been also studied by varying the well width from 5?nm to 10?nm in asymmetric structures. The electron, hole and exciton states are calculated in the presence of an applied electric field. It is found that there are two direct (bright) exciton states with the largest oscillator strengths. Their energies weakly depend on the electric field due to the compensation between the blue shift and red shift of the electron–hole pair states. In addition, these two states are overlap in the case of symmetric CQWs and one of them is then shifted to higher energy in asymmetric CQWs. The ground state exciton has the binding energy of approximately 7.3?meV and decrease to around 3.0?meV showing the direct to indirect transition of the ground state. The direct–indirect crossover is observed at different electric field for different structure. It happens at the electric field when the e1–e2 electron anticrossing or h1–h2 hole anticrossings is observed, so that the crossover can be controlled by the well width of CQWs structure.     

Electronic structure and electron dynamics at Si(100)

The electronic structure and electron dynamics at a Si(100) surface is studied by two-photon photoemission (2PPE). At 90 K the occupied D_up dangling-bond state is located 150±50 meV below the valence-band maximum (VBM) at the center of the surface Brillouin zone ̄ and exhibits an effective hole mass of (0.5±0.15)m_e. The unoccupied D_down band has a local minimum at ̄ at 650±50 meV above the VBM and shows strong dispersion along the dimer rows of the c(4×2) reconstructed surface. At 300 K the D_down position shifts comparable to the Si conduction-band minimum by 40 meV to lower energies but the dispersion of the dangling-bond states is independent of temperature. The surface band bending for p-doped silicon is less than 30 meV, while acceptor-type defects cause significant and preparation-dependent band bending on n-doped samples. 2PPE spectra of Si(100) are dominated by interband transitions between the occupied and unoccupied surface states and emission out of transiently and permanently charged surface defects. Including electron–hole interaction in many-body calculations of the quasi-particle band structure leads us to assign a dangling-bond split-off state to a quasi-one-dimensional surface exciton with a binding energy of 130 meV. Electrons resonantly excited to the unoccupied D_down dangling-bond band with an excess energy of about 350 meV need 1.5±0.2 ps to scatter via phonon emission to the band bottom at ̄ and relax within 5 ps with an excited hole in the occupied surface band to form an exciton living for nanoseconds. PACS 73.20.At; 79.60.Bm; 79.60.Dp; 79.60.Ht     

Two-photon spectroscopy of SnO2 under hydrostatic pressure

The pressure shift of S excitons in the rutile-type semiconductor tin oxide (SnO₂) is measured by two-photon absorption. From these data the pressure coefficients of the band gap (62.0 meV/GPa) and of the exciton binding energy (0.87 meV/GPa) are determined.     

Camel's back induced stabilization of electron-hole liquids : GaP

A camel's back-like nonparabolicity of the longitudinal electron mass enhances the density of states and strongly stabilized an electron-hole-liquid. In GaP therefore the EHL density is doubled to 8.6 × 18¹⁸cm^?3 and the Fermi energy ratio E_F^h/E_F^e changes from 1.9 to 4.9. The theoretical binding energy agrees with the experimental E_B=17.5±3meV interpreting the luminescence at 2.30 eV as a superposition of liquid and plasma recombination radiation.