Luminescence of bound exciton and bound multiexciton complexes in Si:Li

We report on the observation of a number of new lines in the photo- luminescence spectrum of Si:Li. Some of these lines are interpreted as luminescence from excited initial states of the bound exciton and bound multiexciton complexes. Thermodynamic measurements of the binding energy for one and two exciton complexes are reported. One of the bound exciton excited states is correctly positioned to be the final state for the observed decay of the two exciton complex.     

A high resolution investigation of the recombination radiation from Si containing the acceptors B,Al, Ga,In and Tl

A high resolution investigation has been made of the luminescence from Si doped with the acceptors B, Al, Ga, In and Tl in the spectral energy range 0.96–1.16 eV using 50 keV electron excitation. Luminescence from Tl-doped material has not previously been reported. Particular attention is directed to the splitting of the no-phonon and phonon-assisted bound exciton lines which has been detected for the first time in luminescence studies. Thermalizing doublet structure has been observed for Al, Ga and In-doped Si and a thermalizing triplet structure for tl-doped material, and three possible models for the fine structure are considered. Satellite lines on the low energy side of the bound exciton lines, which have previously only been observed in B, P and Li-doped Si and attributed to multiple bound exciton complexes, are reported for Al and Ga-doped material. Recent models for these lines are discussed together with their relationship with electron-hole droplet condensation. Structure associated with free electron to bound hole transitions are reported and discussed.     

Observation of bound multiple-excitons in germanium

A series of sharp emission lines was seen at 4.2 K and at high excitation levels in germanium doped with phosphorus or arsenic. The series starts with the bound exciton and converges towards the maximum of the luminescence band of the condensed electron-hole state. The lines are interpreted as the decay of bound multiple-exciton complexes.     

Disappearance of Be-related photoluminescence in Be-doped Ge under large uniaxial stresses

Increasing uniaxial stress applied to Ge doped with the double acceptor Be results in the disappearance of first the luminescence due to two-exciton complexes bound to Be, and at a higher stress the Be bound exciton itself, with a concomitant increase in free exciton intensity by a factor of 25. These results can be readily understood in terms of the shell model of excitonic complexes. Similar lines observed in Zn-doped Ge behave in a qualitatively different way under uniaxial stress, indicating that the existing model for the origin of the Zn lines must be modified.     

Systematics of bound excitons and bound multiexciton complexes for shallow donors in silicon

We report luminescence measurements of transitions involving bound multiple exciton complexes in Si:Sb and Si:As. The systematics of the donor systems Si:P, Si:As and Si:Sb are examined. We find that the splittings between the two observed series of lines are independent of complex number and donor type.     

Resonant exciton trapping at impurity states in silver bromide

Excitation spectra near the indirect exciton edge of AgBr at 1.8K are reported for several luminescence lines from weakly localized excitons. Excitation below the exciton absorption threshold reveals several excited bound exciton states the energetic positions of which are determined. For excitation above the threshold, strong energy dependent structure is observed. It is interpreted in terms of resonant trapping of free excitons in both ground and excited bound exciton states associated with emission of LO(Γ), long wavelength acoustic and intervalley TA(X) and LA(X) phonons as well as combinations and overtones of these. From measurements in doped crystals two bound exciton systems are found to be correlated with Cd²⁺ and Ca²⁺, respectively.     

Luminescence of exciton-neutral donor complex in CdTe

Zeeman splittings and energy shifts of luminescence lines due to the exciton-neutral donor complex have been studied under magnetic fields in CdTe. Fine structures of the two-electron transitions associated with this complex are identified. The doublet structure of the initial state of the exciton bound to the neutral donor is assigned to the electron-hole spin-exchange interaction.     

Bound exciton in CuGaS2

In CuGaS₂ crystals absorption and luminescence spectra at the temperature 9 K at excitation by different wavelengths of Ar laser are investigated. A series of lines available in luminescence and absorption spectra is found. Another series of lines is found only in absorption spectra. The found series of lines of absorption and luminescence are determined by excitons bound on neutral acceptor. A model of electron transitions between the energy levels of the exciton bound on neutral acceptor is proposed.     

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.     

The puzzle of double-doped Si(B,In): Sharp line series in near-band-edge photoluminescence

We observe a large number of new lines in the near-band-edge (1060 to 1125 meV) photoluminescence of double-doped Si(B, In). The indium bound exciton lines appear as expected, however no bound exciton or bound multiexciton lines associated with boron are seen. The intensity of the new lines decreases relative to the intensity of the indium bound exciton line with increasing exciton power and decreasing temperature in the range from 1.2 to 20 K studied.     

Indirect exciton luminescense and Raman scattering in CdI2

Intrinsic luminescence and Raman scattering in 4HCdI₂ have been investigated at 2 K. Weak emission bands observed near the absorption edge are attributed to the phonon-assistes indirect exciton luminescence. Several new Raman lines are observed under resonant excitation in addition to known lines. The symmetry of the phonon modes associated with the indirect transitions as well as with Raman scattering is discussed.     

Effect of heteroboundary spreading on the properties of exciton states in Zn(Cd)Se/ZnMgSSe quantum wells

Exciton states in Zn(Cd)Se/ZnMgSSe quantum wells with different diffusion spreading of interfaces are studied by optical spectroscopy methods. It is shown that the emission spectrum of quantum wells at low temperatures is determined by free excitons and bound excitons on neutral donors. The nonlinear dependence of the stationary photoluminescence intensity on the excitation power density and the biexponential luminescence decay are explained by the neutralization of charged defects upon photoexcitation of heterostructures. With the stationary illumination on, durable (about 40 min) reversible changes in the reflection coefficient near the exciton resonances of quantum wells are observed. It is shown that, along with the shift of exciton levels, the spreading of heteroboundaries leads to three effects: an increase in the excitonphonon interaction, an increase in the energy shift between the emission lines of free and bound excitons, and a decrease in the decay time of exciton luminescence in a broad temperature range. The main reasons for these effects are discussed.     

MBE生长的掺Be p-GaAs的光荧光谱

本文报道利用国产分子束外延(MBE)设备和Be元素生长的p-GaAs的低温光荧光谱,观察到FE,(D°,X),(A°,X),(D+,X)等束缚激子发光,判断外延层有较好的质量。在1.503～1.515eV之间观察8条与缺陷有关的(d,x)发光线。仔细分析了与Be,Ge受主有关的(BA)和(DA)发光现象。     

Studies of relaxation times of secondary radiations in ZnTe by circular polarization correlation

Population and spin relaxation rates concerned with exciton luminescence and multiple LO-phonon emission lines are studied in ZnTe at 77 K by means of optical circular polarization correlation between the exciting and emitted lights. The decay times of the LO lines are determined to be much less than 10^-11sec, which are definitely shorter than the obtained exciton lifetime of 10^-10-10^-9sec. It is concluded that the LO lines are not due to hot luminescence but due to resonance Raman scattering.     

Extrinsic two dimensional excitons and their high density effects in BiI3 single crystals

Sharp absorption lines near the fundamental edge caused by planar stacking faults in BiI₃ crystals have been investigated on absorption and luminescence spectra. Under high excitation-photon densities these lines show peak-shift and line broadening. The high density effects on the exciton bound at the defects shows a two-dimensional feature of the exciton motion.     

Double acceptor bound exciton in Ge

Six photoluminescence lines from excitons bound to neutral zinc acceptors in germanium are observed. The six lines consist of three replicas associated with zero, TA and LA phonons. Two lines from each replica correspond to the transitions from the split ground states of the bound exciton complex. The binding energy of the bound exciton is found to be 3.2 meV.     

Effects of <001> uniaxial stress on the photoluminescence spectrum of Ge doped with Zn

The effects of <001> uniaxial stresses on the photoluminescence spectrum of Ge doped with the double acceptor Zn are found to be considerably different from those previously reported for Ge: Be. The Ge: Zn results are, however, in complete agreement with the predictions of our model for the Ge: Zn spectrum, in which one of the most prominent lines is considered to result from the accidental superposition of a bound exciton and a bound multiexciton complex transition. As in the case of Ge: Be, sufficiently large stresses result in the disappearance of all luminescence associated with the double acceptor impurities, a result expected from a shell model analysis of these excitonic systems.     

The optical properties of Be,Mg and Zn-diffused gallium phosphide

Interest in the Ga-site acceptors Be and Mg was stimulated by the possibility that they might produce efficient luminescence on association with O, analogous to the well-known red Zn-O luminescence in GaP but at higher transition energy. Attention was directed to diffusion doping by Be and Mg of GaP O-doped during growth because the reactivity of Be and Mg with O renders double doping during crystal growth very difficult. Structured green donor-acceptor pair spectra were observed at 1.6°K from many Be-diffused crystals, yielding an accurate measure of (E_A)_BE, 50 ± 1 meV. Moderately efficient orange-red luminescence was also observed below ∼ 100 °K from these crystals, but the intensity of this luminescence decreased rapidly to negligible levels by ∼ 200°K. This luminescence also contains sharp structure at 1.6°K, of a form characteristic of the decay of excitons bound to complex centres. Many sharp phonon replicas occur, involving local modes as well as characteristic GaP modes. One set of no-phonon lines, at least, near 2.19 eV, shows zero-field splitting, luminescence decay times and behaviour in magnetic and external strain fields characteristic of exciton decay at a centre with <100>; or <111>-type symmetry axes, containing no extra electronic particles. The exciton state is split by 2.4 meV by J-J coupling, and the axial field of the centre splits the hole states by ∼ 1.0 meV. These bound excitons are specifically characteristics of diffused GaP and appear analogous to bound excitons observed below 2.12 eV in Zn-diffused GaP. It is probable that the relevant centres contain diffusion components such as Be or Zn interstitials and improbable that O_P is involved. By contrast, weak orange bound exciton luminescence observed in Mg-diffused GaP does involve O, presumably as O_P. No analysis of the magneto-optical behaviour of this Mg-related bound exciton was possible in our crystals, so its symmetry axis was not established. It is possible that this is the Mg_Ga-O_P bound exciton. If so, the two-fold reductions in the exciton localisation energy from ∼ 0.32 eV to ∼ 0.15 eV and in the mass of the Ga-site substituent has produced dramatic changes in the form of the phonon cooperation between the Zn-O and “Mg-O” excitons. The “Mg-O” exciton luminescence is not dominant in our crystals, even at low temperature. The exciton state is again split by a local crystal field as well as by J-J coupling, but here the former splitting is predominant; 2_∈0 = 3.9 meV, Δ = 0.60 meV.     

PHOTOLUMINESCENCE STUDY OF GaAs/AlGaAs QUANTUM WELL HETEROSTRUCTURE INTERFACES

Photoluminescence (PL) ia used to study the interface properties of GaAs/AlGaAs quan-tum well (QW) heterostructures prepared by molecular beam epitaxy with growth interrup-tion (GI). The discrete luminescence lines observed for the sample with GI are assigned to the splitting of the heavy-hole exciton associated with heterointerface islands with the lateral size greater than excjton diameter and mean height less than one monolayer, and the spectra have the Gaussian lineshapes. The results strongly support the microroughness model. We also study the temperature dependence of the exciton energies and find that excitons are localized at the interface roughness at low temperature even in the sample with GI, The lateral size of the microroughness of the GI sample is estimated to be less than 5nm from the exciton localization energy.     

Cross-shaped photoluminescence of excimers in perylene crystals

Cross-shaped excimer (self-trapped exciton) luminescence from α- and β-perylene single crystals of 50–100 μm was found when they were excited at the center of the crystals with a continuous-wave (cw) laser resonant with the exciton absorption. The cross shape is formed by the two lines which intersect at the excited position and are perpendicular to the sides of the crystals of parallelogram shape. Luminescence is emitted from the excited spot and 4 side edges in the cross shape. The most striking feature is that the luminescence intensity at the edges was as high as or higher than at the excited spot. The possibility of the exciton propagation or the waveguide effect is rejected both experimentally and theoretically. This phenomenon can be reasonably explained only when the radiative transition probability of excimers is significantly enhanced at the crystals side edges than at the center due to the lower symmetry.