The gallium-site donors germanium and silicon in gallium phosphide

The node in the Bloch part of the electron wave function expected for a Ga-site donor in GaP removes the usual valley-orbit splitting and associated chemical shift. However, the T₂ ground state can still show a small spin-valley splitting into Γ₈ and Γ₇ states, as previously verified for the Sn donor. We find that the optical properties of the Ge and Si donors deviate appreciably from this “normal” behaviour. The Ge donor is anomalously deep, E_D ～ 202 meV, yet binds an exciton by ～63 meV consistent with the Haynes rule for neutral donors in GaP. We find that this exciton possesses the large oscillator strength, f～3.5 × 10^-3, Zeeman and piezo-optical splittings characteristic of a Γ₆, 1s(A₁) ground state, like a P rather than Ga-site donor. However, f and the exciton localization energy are consistent with expectation for E_D ～ 200 meV, as measured from the lowest set of X conduction band minima, if we assume a symmetric A₁-like wave function. A possible explanation for this unexpected result is advanced. The much shallower Si donor, E_D～82 meV, binds an exciton by only ～ 14 meV, also consistent with the Haynes rule. By contrast, we find this Ga-site donor to be normal except that our Zeeman and piezo-optical results indicate an inverted spin-valley splitting, about 25% of that for the still shallower Sn donor. We also discuss the numerous low-lying excited states, some anomalous phonon replicas in the Ge and Si donor bound exciton spectra and the magneto-optical properties of a sharp line near 2.24 eV, attributed to the decay of excitons bound to (S)_p-(Ge)_p donor-acceptor associates.     

The location of the lowest conduction band minima in gallium phosphide from bound exciton luminescence

Two weak satellite series with acceptor-independent displacement energies of 4.2₃ meV and 7.8₃ meV have been observed in the luminescence of excitons bound to neutral acceptors in GaP. These satellites contain broad background luminescence. Rather well defined superimposed peaks show relative strength which increases dramatically with decrease in exciton binding energy. The most plausible mechanism for these satellites involves bound exciton recombination with emission of one phonon to conserve momentum in the indirect transition and one or two further phonons to conserve momentum in g-type inter-valley scattering processes. This model is consistent with all known properties of the satellites and is strongly supported by a quantum-mechanical line width calculation. The narrow components arise from a diffuse tail on the bound exciton wave-function which is enhanced by the electron-hole correlation. The existence of this g-scattering process shows that the conduction band minima in GaP lie at 0.953 K^max_〈100〉, not exactly at K^max_〈100〉 as believed hitherto. This revision has important consequences for several properties of n-type GaP.     

Optical properties of excitons bound to neutral SiGa-donors in GaP and the degeneracy of the SiGa-donor ground state

Low temperature near band-edge absorption and luminescence spectra of Si-doped n-type GaP are reported. Electrical and optical evidence is presented that these spectra are due to the creation and decay of excitons bound to neutral Si donors on Ga sites. Several zero-phonon transitions, each with strong replications by momentum conserving phonons, were observed in both absorption and luminescence. From these measurements it is concluded that the ground state of the Si_Ga donor is split into two sublevels 0.6 meV apart. Crystals containing a sufficiently low concentration of S to be suitable for a reliable analysis of temperature-dependent Hall-effect measurements were selected by combining 300°K Hall-effect data with low temperature S-exciton absorption data. The electrical measurements support the identification of Si as the main shallow donor. Moreover, it is concluded from these measurements that the degeneracy of the ground state of a Ga-site donor is 3 times that of a P-site donor, in agreement with theoretical expectations.     

Interaction of excitons in indirect solid solutions of In1−xGaxP at high levels of excitation

The cathode luminescence spectra of GaP and solid solutions of In_1–xGa_xP (with an indirect energy band structure) obtained by liquid-phase epitaxy are investigated at T=77 and 9°K. It is established that the recombination of excitons on neutral donors and the pure exciton recombination with the participation of TA (=13±1 meV), LA (=31±1 meV), and TO (=43±2 meV) phonons. dominate in the low-temperature luminescence spectra of GaP and In_1–xGa_xP with a small concentration of photon impurities. The complex luminescence band that arises for sufficiently high levels of excitation and at T = 9°K is attributed to the luminescence of an electron-hole drop (EHD). The transformation of spectral form of the EHD band with the transition from GaP to In_1–xGa_xP is attributed to a simultaneous reduction in the contribution of the TA and TO replicas and to the appearance of a phononless component in the luminescence of the EHD.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 9, pp. 37–40, September, 1985.     

GaAs中N等电子陷阱的研究

对离子注N的GaAs样品作了77K的静压光致荧光研究。观察到了N陷阱中心元胞势束缚激子N_x的发光光谱及畸变势束缚激子N_T的发光峰。测量N_X能级的压力系数为2.8meV/kbar,常压下N的共振态高于导带边179meV。讨论了N等电子陷阱的电声子耦合强度及有效束缚激子半径随压力的变化关系。 关键词：     

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.     

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.     

The nature of excitonic complexes in ZnTe

Uniaxial stress experiments were used to investigate the nature of the luminescence lines observed at low temperatures in ZnTe in the vicinity of the absorption edge. The single crystals used in this experiment were grown from solution of ZnTe in tellurium. Both “as-grown” crystals and crystals annealed in Zn vapour were investigated. The most intense line in “as-grown” crystals is attributed to an exciton bound to a neutral acceptor. The binding energy of the exciton in this center is 6 meV. After annealing a new center appears in the same spectral region. Stress experiments as well as the temperature dependence of the intensity of the luminescence indicate that this center is a complex consisting of an exciton and an ionized donor. Splitting of J = 1 (Γ₅) and J = 2 (Γ₃ + Γ₄) levels was found to be 1.2 meV.     

Luminescence of exciton in GeS

The luminescence of one free and four bound excitons has been investigated in the layer compound GeS. Up to six phonon replicas of the bound excitons are observable. The energies of these phonons are in reasonable agreement with Raman- and IR reflectivity measurements. The luminescence is highly anisotropic and is only seen in the polarization E ∥ a which confirms that the transitions E ∥ b are forbidden. Measurements with E ∥ c were inconclusive because of the difficulties in the sample preparation. Time resolved measurements revealed decay times between 20 nsec and 1.5 μsec for the different bound excitons. One exciton has been identified as bound to a Si impurity. This is the first observation of intentional doping of GeS. Thermal dissociation energies for the bound excitons lie between 6.5 and 20 meV.     

Band-edge emission in CuI single crystals

The photoluminescence spectra of CuI single crystals have been studied at T = 4.2 K and at various excitation levels. The emission band of donor-acceptor pairs (DAP) with a maximum at about 4200 Å has been shown to possess a complex structure. Theoretical analyses and exciton spectroscopy data make it possible to calculate the ionization energies for the donors and acceptors participating in the formation of DAP, which are equal to E_D = = 0.045?0.065 eV and E_A = 0.155?0.170 eV, respectively. The fine structure of emission due to the annihilation of excitons bound on acceptor pairs (band maximum 4075 Å) has been detected and calculated. The energy of the longitudinal optical phonon participating in the exciton-phonon interaction (LO ? 18.7 meV) has been determined.     

Bose condensation of interwell excitons in double quantum wells

The luminescence of interwell excitons in double quantum wells GaAs/AlGaAs (n-i-n heterostructures) with large-scale fluctuations of random potential in the heteroboundary planes was studied. The properties of excitons whose photoexcited electron and hole are spatially separated in the neighboring quantum wells were studied as functions of density and temperature within the domains on the scale less than one micron. For this purpose, the surfaces of the samples were coated with a metallic mask containing specially prepared holes (windows) of a micron size an less for the photoexcitation and observation of luminescence. For weak pumping (less than 50 μW), the interwell excitons are strongly localized because of small-scale fluctuations of a random potential, and the corresponding photoluminescence line is inhomogeneously broadened (up to 2.5 meV). As the resonant excitation power increases, the line due to the delocalized excitons arises in a thresholdlike manner, after which its intensity linearly increases with increasing pump power, narrows (the smallest width is 350 μeV), and undergoes a shift (of about 0.5 μeV) to lower energies, in accordance with the filling of the lowest state in the domain. With a rise in temperature, this line disappears from the spectrum (T _c ≤ 3.4 K). The observed phenomenon is attributed to Bose-Einstein condensation in a quasi-two-dimensional system of interwell excitons. In the temperature range studied (1.5–3.4 K), the critical exciton density and temperature increase almost linearly with temperature.     

The nature of the predominant acceptors in high quality zinc telluride

We report very sharp bound exciton luminescence spectra in high quality melt-grown very lightly compensated ZnTe, p-type with N_A-N_D in the low 10⁺¹⁵ cm^-3. Bound exciton localisation energies at seven shallow neutral acceptors with E_A between ~55 and ~150 meV are very insensitive to E_A. Optical absorption and dye laser luminescence excitation spectroscopy were necessary to obtain a full separation of the transitions due to different acceptors, together with a study of certain ‘two-hole’ luminescence satellites in which the acceptor is left in a series of orbital states after bound exciton decay. Two shallow acceptors are P_Te and As_Te, a third possibly Li_Zn while a fourth, relatively prominent in our best undoped crystals, may be a complex. A deeper, 150 meV acceptor, frequently reported in the ZnTe literature and electrically dominant in most of our undoped crystals has the Zeeman character of a point defect. We present clear evidence from our spectra that this energy does not represent the binding of a single hole at a doubly ionized cation vacancy, a popular attribution since 1963. This acceptor may be covered by another impurity, possibly Cu_Zn. We also report bound phonon effects, lifetime broadening of excited bound exciton states and observe a single unidentified donor with E_D ~18.5 meV. This energy is determined using selective dye laser excitation at the weak neutral donor bound exciton line and from the onset of valence band to ionized donor photo-absorption.     

Intrinsic recombination radiation from GaP

Intrinsic recombination radiation has been detected in GaP crystals grown by vapour transport on GaAs and GaP substrates and in the form of needles, using 50 KeV electron beam excitation in the temperature range 25°K to 80°K. The three major components of the radiation are associated with the decay of free excitons with the emission of the transverse acoustic, longitudinal acoustic and transverse optic phonons which conserve momentum for transitions across the indirect energy gap. Structure has also been observed associated with the emission of two phonons, and for T > 77 >K a phonon absorption component can also be detected. In crystals grown on GaAs substrates, a no-phonon component has been observed with a threshold at the intrinsic exciton energy gap. This is thought to be associated with the presence of arsenic and there is a corresponding shift in the band gap to lower energy. close agreement is observed between the measured shapes of the emission components and those computed by the principle of detailed balance from the intrinsic edge absorption spectrum, when the broadening associated with the crystal imperfections and anomalous structure in the absorption spectrum are taken into account. The phonon energies derived from the emission spectra are in good agreement with the values determined from intrinsic edge absorption measurements.     

Excitons in highly optically excited gallium phosphide

A study has been made of luminescence in weakly (10¹⁵-10¹⁶ cm^-3) and heavily (10¹⁸-10¹⁹ cm^-3) N-doped GaP crystals induced by 1.78, 2.34 and 3.56 eV photons from Q-switched ruby or neodymium lasers with a KDP crystal for second harmonic generation. The results which were obtained at excitations up to 10²⁰ cm^-3 electron-hole pairs are interpreted as the transitions: single bound excitons, bound excitonic molecules, free excitons in weakly-doped GaP, as well as Auger processes and the formation of a new excitonic state similar to a solid metal with high density of single excitons and excitonic molecules bound to isoelectronic traps in heavily-doped GaP.     

K+3P-core excitons in potassium halides studied at low temperatures with high resolution

Reflection spectra of KF, KCl, KBr and KI single crystals have been measured at photon energies of approximately 20 eV with a bandwidth of 10 meV using synchrotron radiation in order to study fine structure and temperature dependence of the excitonic transitions associated with the K⁺3p core level. The crystals were cleaved under ultrahigh vacuum and cooled down to 20 K. Information on energy positions, halfwidths and line shapes for the K⁺3p-core excitons and their temperature dependence has been obtained. A new exciton predicted by the ligand field model was observed. For KI we evaluate an electron-hole exchange energy of only 30 (± 7) meV.     

Temperature dependent photoluminescence processes in ZnO thin films grown on sapphire by pulsed laser deposition

Temperature dependence of the photoluminescence (PL) transitions in the range of 10–300 K was studied for ZnO thin films grown on sapphire by pulsed laser deposition. The low temperature PL spectra were dominated by recombination of donor bound excitons (B_X) and their phonon replicas. With increasing temperature, free exciton (F_X) PL and the associated LO phonon replicas increased in intensity at the expense of their bound counterparts. The B_X peak with line width of ∼6 meV at 10 K exhibited thermal activation energy of ∼17 meV, consistent with the exciton-defect binding energy. The separation between the F_X and B_X peak positions was found to reduce with increasing temperature, which was attributed to the transformation of B_X into the shallower donor bound exciton complexes at consecutive lower energy states with increasing temperature, which are possible in ZnO. The energy separation between F_X peak and its corresponding 1-LO phonon replica showed stronger dependence on temperature than that of 2-LO phonon replica. However, their bound counterparts did not exhibit this behavior. The observed temperature dependence of the energy separation between the free exciton and it is LO phonon replicas are explained by considering the kinetic energy of free exciton. The observed PL transitions and their temperature dependence are consistent with observations made with bulk ZnO crystals implying high crystalline and optical quality of the grown films.     

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.     

Origins of green band emission in high-temperature annealed N-doped ZnO

Nitrogen-doped ZnO sample has been annealed in O₂ ambient at high temperature (1000 °C) to improve its photoluminescence property. Low-temperature photoluminescence spectra of the sample are dominated by three near-band-edge emissions at 3.377, 3.362, and 3.332 eV, which are ascribed to free exciton emission (FX_A), and neutral donor-bound exciton (D⁰X), and its two-electron satellite (TES), respectively. With increasing temperature in low temperature region, the intensity of FX_A increases and the green band (GB) shows a negative thermal quenching effect resulting from thermal dissociation of D⁰X with more free excitons and neutral donors formed. The doublet structure with energy space ∼30 meV and repeated separation of longitudinal-optical phonon energy of 72 meV are observed in GB at low temperatures. The temperature independent energy position of GB indicates a typical recombination characteristic within strongly localized complexes. The doublet structures are considered to originate from the ground and exited states of shallow donors recombining with deep acceptors such as zinc vacancies.     

Intrinsic excitons in 12CaO·7Al2O3

Unusual crystal structure of 12CaO·7Al₂O₃ is composed by a framework of positively charged nanocages, which enable accommodation of various negative ions (and even electrons) inside these cages. Different filling of cages leads to significant changes in electronic structure and as the result in luminescence properties, as well. Luminescence was studied using time-resolved spectroscopy in VUV in the temperature range from 6 to 300 K. Electron loaded samples exhibit UV luminescence band peaked at ～5 eV. The excitation spectrum of this emission has the onset at the energy gap value of 6.8 eV, and its decay is well described with the sum of two exponential functions with life-times of τ₁ = 3.7 ns and τ₂ = 29 ns, respectively. Its thermal quenching is well approximated by the sum of two Mott-Seitz type curves with the activation energies of 34 meV and 70 meV. Experimental results indicate that this luminescence is possibly due to radiative decay of two singlet self-trapped exciton states, which hole components are localized on two non-equivalent framework oxygens.     

Optical spectra and energy band structure of AgAsS2 crystals

Ground and excited states of three exciton series are observed in the region of fundamental absorption edge of AgAsS₂ crystals. The contours of exciton reflection spectra are calculated and the main parameters of excitons and energy bands are determined in the center of Brillouin zone. The optical reflection spectra are investigated at 30 K in E∥c and E⊥c polarizations in AgAsS₂ crystals in the region of 2-6 eV. The optical functions are calculated from the reflection spectra and a scheme of electronic transitions responsible for peculiarities of reflection spectra deep into the absorption band is proposed.