Optical properties of the Ge donor and acceptor in GaP

The observation of discrete pair line emission and the analysis of the spectra from Ge donor - C acceptor and S donor - Ge acceptor pairs in GaP is reported. The Ge donor binding energy is 0.200 ± 0.002 eV and the Ge acceptor binding energy is 0.258 ± 0.002 eV. The bound exciton recombination at neutral Ge donors was observed at 2.265 eV corresponding to an exciton binding energy of 0.063 eV. This is the most tightly donor bound exciton observed in GaP.     

Excitons and excitonic complexes in GaAs/AlGaAs quantum wells with a low-density quasi-two-dimensional electron and hole channel

The recombination spectra of excitons and excitonic complexes in un-doped GaAs/AlGaAs single quantum wells are investigated. It is shown on the basis of a study of the magnetic-field dependence of the emission spectra and the degree of optical orientation in zero magnetic field and on the basis of electrooptic measurements that not only the density but also the sign of the charge carriers in a well depend strongly on the photoexcitation energy. It is shown on the basis of a comparative analysis of the spin splitting of the recombination lines of free and bound excitons that the recombination line which was attributed earlier to a positively charged exciton corresponds to the recombination of an exciton bound on a neutral acceptor. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 707–713 (10 May 1998)     

Observation of a donor-acceptor pair recombination in the edge emission of ZnSe crystal by electro-luminescence

An injection type electro-luminescence in ZnSe crystal has been studied by using ZnSe-SnO₂ hetero-junction at 20°K. In the emission peak observed around 2·70 eV, a clear energy shift toward the higher energy side with increasing injection current density has been found at low temperatures, which could be associated with the D-A pair (donor-acceptor pair) recombination process. The energy sum of the donor and acceptor activation is estimated to be larger than 137 meV. In the higher temperature region, this emission line turns out due to the ‘free-to-bound’ recombination, and the related acceptor ionization energy is considered to be ～120 meV. By taking into consideration the energies of bound exciton emission, the exciton localization energies and the related donor and acceptor ionization energies are evaluated.     

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.     

GaN激子跃迁的时间分辨光谱学研究

用时间分辨光谱学方法研究低压有机金属化学汽相沉积生长的ＧａＮ中自由，束缚激子（ＢＸ）的跃迁，讨论了这些跃迁的光致发光谱，复合寿命及其与温度的关系，给出了中性施主束缚激子和自由激子（ＦＸ）的辐射复合寿命分别为０．１２ｎｓ和０．４ｎｓ。     

Bound multiple exciton complex in CdTe

A new line corresponding to the radiational decay of a bound multiple exciton complex (BMEC) consisting of two excitons bound to a neutral acceptor is found in luminescence spectra of CdTe. Zeeman splitting of an exciton bound to a neutral acceptor (BE) and of BMEC is studied.     

Bound excitons in p-doped GaAs quantum wells

Photoluminescence attributed to excitons bound to neutral impurities has been observed from GaAs quantum wells in Al_xGa_1?xAs-GaAs heterostructures grown by molecular beam epitaxy. The quantum wells were either doped with [Be] ≈ 10¹⁷ cm^-3 or Zn-diffused. At low temperatures both single and multiple quantum wells exhibited this extrinsic luminescence which is ascribed to the radiative recombination of the n=1 ground state heavy hole exciton E_1h bound to a neutral acceptor A^o. The dissociation energy E_D of the A^o-E_1h complex is obtained directly from the measured separation of this extrinsic peak from the intrinsic E_1h free exciton peak. For 46Å wide GaAs wells, E_D=6.5meV and E_D decreases with increasing well width.     

镓铟磷三元化合物中的自由激子和杂质N发光中心的结构位形

我们研究了GaxIn1-xP:N(x=0.99,0.98,0.96)的发光光谱,用Ar+离子激光器的458nm线激发。N的浓度为5×1017cm-3。图1示出了在6K下Ga0.99In0.01P:N的发光光谱。Mariette和Chevallier[1,2]以及Nelson和Holonyak,Jr[3]。认为Nx0带起源于束缚在孤立N中心上激子的辐射复合。     

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。     

Donor binding energy and radiative life time of exciton in a strained InGaN/GaN quantum wire

Donor binding energies of positively and negatively charged impurities in a strained InGaN/GaN cylindrical quantum wire are investigated. The interband optical transition with and without the exciton is computed as a function of wire radius. The exciton oscillator strength and the exciton lifetime for radiative recombination as a function of wire radius have been computed.     

Excitation and temperature dependence of the excitonic emissions in undoped and Mg-doped VPE InP

The photoluminescence of vapor phase epitaxial indium phosphide, both unimplanted and magnesium implanted, is investigated as a function of excitation intensity and temperature to gain a more complete understanding of the free exciton, exciton-bound donor, exciton-bound acceptor (residual and Mg), the 1.4155 eV emission identified with both the recombination of free holes with neutral donors and with bound excitons to ionized donors, and the recently identified 1.4122 eV emission (unique to Mg implanted layers). Emission peak broadening, quenching, and saturation is examined as the irradiance is varied from 0.002 to 33.3 W/cm² and the temperature from 2.6 to 50 K. The source of the 1.4122 eV emission is speculated to be a donor-acceptor recombination involving a donor center bond to an off-axis band and the Mg acceptor, or more likely, excitons bound to point defects which are created by the acceptor implant.     

Optical properties of p-type ZnO doped by lithium and nitrogen

A lithium and nitrogen doped p-type ZnO (denoted as ZnO: (Li, N)) film was prepared by RF-magnetron sputtering and post annealing techniques with c-Al₂O₃ as substrate. Its transmittance was measured to be above 95%. Three dominant emission bands were observed at 3.311, 3.219 and 3.346 eV, respectively, in the 80 K photoluminescence (PL) spectrum of the p-type ZnO:(Li, N), and are attributed to radiative electron transition from conduction band to a Li_Zn-N complex acceptor level (eFA), radiative recombination of a donor-acceptor pair and recombination of the Li_Zn-N complex acceptor bound exciton, respectively, based on temperature-dependent and excitation intensity-dependent PL measurement results. The Li_Zn-N complex acceptor level was estimated to be about 126 meV above the valence band by fitting the eFA data obtained in the temperature-dependent PL spectra.     

Spin Physics of Excitons in Colloidal Nanocrystals

We present a review of spin-dependent properties of excitons in semiconductor colloidal nanocrystals. The photoluminescences (PL) properties of neutral and charged excitons (trions) are compared. The mechanisms and the polarization of radiative recombination of a “dark” (spin-forbidden) exciton that determines the low-temperature PL of colloidal nanocrystals are discussed in detail. The radiative recombination of a dark exciton becomes possible as a result of simultaneous flips of the surface spin and electron spin in a dark exciton that leads to admixture of bright exciton states. This recombination mechanism is effective in the case of a disordered state of the spin system and is suppressed if the polaron ferromagnetic state forms. The conditions and various mechanisms of formation of the spin polaron state and possibilities of its experimental detection are discussed. The experimental and theoretical studies of magnetic field-induced circular polarization of PL in ensembles of colloidal nanocrystals are reviewed.     

Recombination dynamics of free and bound excitons in bulk GaN

We report new data on the transient photoluminescence behaviour of free and donor bound excitons in high quality bulk GaN material grown by HVPE. With 266 nm photoexcitation the no-phonon free exciton has a short decay time, about 100 ps at 2 K, assigned to nonradiative surface recombination. The LO replicas of the free exciton have a much longer decay at 2 K, about 1.4 ns, believed to be a lower bound for the bulk radiative lifetimes of the free excitons at 2 K. The donor bound exciton no-phonon lines exhibit a rather short (about 300 ps) nonexponential decay at 2 K, which appears to be dominated by a scattering process. The corresponding LO replicas and the two-electron transitions have a much longer decay. From the latter, the lower bound of the radiative lifetime of the O- and Si-bound excitons are 1800 ps and 1100 ps, respectively.     

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.     

Effect of binding energy on the kinetics of radiative and nonradiative recombination of bound excitons

Radiative and nonradiative lifetimes of bound excitons in GaSe with various binding energies are measured by the magnetooptical method using time-resolution spectroscopy. It is shown that both the radiative and nonradiative lifetimes of an exciton bound at an ionized center increase with the binding energy of an exciton with a defect. Possible reasons for this dependence are considered. The results of calculations obtained in the framework of the proposed model are in good agreement with experimental data.     

Magneto-Luminescence of excitons bound to neutral acceptor in cadmium telluride

The radiative recombination of excitons bound to neutral acceptor (A⁰, X) in high-purity CdTe has been investigated in magnetic fields up to 100 KG. A doublet structure observed in (A⁰, X) emission line is explained by considering the j-j coupling between two holes and an electron. Zeeman splittings in emission lines originate from the transition between J = 1/2 and 3/2 states in bound-exciton complex, and the acceptor-ground state. It is supposed that the acceptor which binds excitons is due to certain complex having C_3υ symmetry which is lower than the host lattice.     

Non-radiative exciton recombination through excitation energy transfer in quantum dot clusters

Excitation energy transfer (EET) processes in CdSe/CdZnS quantum dot (QD) clusters have been investigated in this study by measuring their time-resolved and spectrally resolved fluorescence intensities. The contributions of radiative and non-radiative exciton recombination through EET are evaluated, where the latter is expected to occur in a large class of QD ensembles because of the presence of nonluminescent QDs. It appears that the fluorescence decay in larger QDs serving as acceptor does not show an initial rise, in addition the lifetime of the acceptor QD is independent of the excitation wavelength, suggesting that an EET is followed mostly by non-radiative recombination.     

Nongeminate recombination in organic bulk heterojunction solar cells: Contribution of exciton–polaron interaction

A new model to explain nongeminate recombination in organic bulk heterojunction solar cells is presented. We suggest that the annihilation of excitons on charge carriers at the interface between donor and acceptor phases competes with the bimolecular recombination of Coulombically bound electron–hole pairs. The exciton–polaron interaction gives visible contribution to the reduction of Langevin recombination. An analytical formula, which describes the reduction prefactor, has been derived. We demonstrate that exciton–charge carrier interactions cause an increase of the recombination order. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.