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.     

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.     

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.     

Reevaluation of bandgap and free exciton binding energy of GaP

Very sensitive measurements on the spectral behaviour of the free-to-bound excitation σ_pl^°(hν) from the valence band to the deep 0 donor in GaP at low temperatures are presented. Evaluation of the threshold energy for the electronic transition, together with the known value of the 0 binding energy, provides a simple and accurate way to determine the indirect bandgap E_g of GaP. Our new value is E_g = 2.3525 ± 0.003 eV at 1.5 K, which gives an exciton binding energy E_x = 24 ± 3 meV, considerably larger than previously used values. These data also imply an upward revision of acceptor binding energies in GaP with 10 ± 2 meV.     

Valence band spin-orbit splitting in CdTe and Cdf−xMnxTe and giant zeeman effect in the Γ7 band of Cd1−xMnxTe

Optical transitions from spin-orbit split-off valence band Γ₇ to the conduction band Γ₆ were directly observed in transmission measurements on thin samples of CdTe and Cd_1?xMn_xTe. Precise values of spin-orbit splittings were determined. Giant Zeeman splitting of Γ₇ - Γ₆ exciton in Cd_1?xMn_xTe was observed and found coherent with corresponding splitting of Γ₈-Γ₆ exciton.     

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.     

Spectroscopic search for the free carrier screening of the excitonic binding

A spectroscopic determination of the energy gap E_g and the exciton energy E_x in highly excited Ge at T = 5–20K is presented. Within 0.05 meV we observe no shift of E_g and E_x up to electron-hole densities of 10¹⁴?10¹⁵ cm^?3. In this range all previous theories predict a sizeable band renormalization (ΔE_g ≈?0.3 meV to ? 2 meV).     

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.     

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.     

Size effect on different impurity levels in semiconductors

The different impurity levels in crystals are investigated basing on a deformation potential model which is related to the impurity size effect. The donor levels such as E_d(O) and E_d(Se) in lightly doped GaP are found to be equal to 898.55 and 103.00 meV, respectively, in perfect agreement with the experimental results of Vink et al.     

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 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 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.     

Photoluminescence spectra of Ga-doped and Al-doped 4H-SiC

Blue and violet photoluminescences of Ga-doped and Al-doped 4H-SiC single crystals grown from a Si melt have been studied at 2 ~ 200 K. Luminescence spectra under continuous excitation and their dependences on temperature and excitation intensity as well as temperature dependence of the luminescence intensity are measured. Time-resolved spectra and decay curves after pulsed excitation also are observed at various temperatures. The luminescences at 2 K are found to be due to pair recombination between the N donor introduced unintentionally and the Ga or Al acceptor. The spectra of the two samples resemble each other in shape, and each consists of a zero-phonon peak and its phonon replicas. At higher temperatures, another emission appears due to the recombination of free electrons with bound holes at the acceptors in place of the pair emission. From the energies of the zero-phonon peaks of these two kinds of emissions, the ionization energies of the Ga and the Al acceptors are determined to be 249 ± 3meV+E_x and 168 ± 3 meV + E_x, respectively, where E_x is the exciton binding energy of 4H-SiC, and that of the N donor is estimated to be 55 ± 7 me V using an appropriate approximation.     

Magneto-optical studies of excitons in AgGaS2

The reflection, absorption and luminescence spectra of the lowest excitons in AgGaS₂ are measured in magnetic fields. The results can be explained by the quasicubic model. The exchange splittings of the Γ₄, Γ₅ and Γ₃ exciton state are evaluated. The Zeeman splittings of the Γ₅ and Γ₅-Γ₃ states yield g-values of + 3.8 and 3.7, respectively. They are determined mainly by the contributions of the hole and conduction electron spins.     

Luminescence of CuInS2: II. Exciton and near edge emission

The near-edge (exciton) emission of CuInS₂ is investigated for various material-compositions as a function of temperature. From these investigations the exciton ionization energy (20 meV) and the temperature dependence of the energy gap were determined. For the first time, recombination of the free exciton belonging to the deeper lying Γ₇ valence bands has been observed. Moreover, six different bound exciton emission lines and a donor to valence band transition were detected. These emissions could be assigned in terms of the defect-chemical model presented in Part I.     

Study of the 1.65 eV pair recombination in cinnabar powders: Thermal quenching and total light decay

We have studied the luminescence of cinnabar powders excited by an electron beam. The shape of the emission spectrum is a broad band, whose bandwidth is about 320 meV and which is peaked at 1.65 eV at liquid helium temperature. The study of thermal quenching leads to a donor depth E_D ～ 170 meV; the Bohr radius of this level is approximately 4 Å.Time resolved spectroscopy curves, which have been published in a previous paper, are theoretically analyzed and we derive the maximum recombination probability W_max ～ 10⁷ s^-1, i.e. the theoretical recombination probability extrapolated to the limit of very close pairs.From the above results we compute the theoretical decay curves of the total light emission and we compare them to the experimental curves. A reasonable agreement is observed between theory and experiments.     

Photoluminescence of ZnMnTe quantum-well structures in a magnetic field

Photoluminescence spectra of strained structures Zn_{1 ? x} Mn _x Te/Zn_{1 ? y} Mg _y Te with magnetic quantum wells and nonmagnetic barriers are studied. The Zeeman splitting of the heavy exciton is found to follow an unusual behavior: both spin components shift down in energy. The heavy-exciton photoluminescence Zeeman components are observed to be inversely distributed in intensity, with the higher energy component being stronger than the lower energy component. The Zeeman splitting of the exciton in a magnetic field is calculated. The data obtained permit refinement of some parameters of the energy spectrum and magnetic properties of these structures.     

Donor-like excited states of exciton bound to neutral acceptor in ZnTe

Photoexcitation spectroscopy has been used to study the excited states of the neutral c-acceptor bound exciton complex A^c₁ in ZnTe. We have detected four excited states at ～ 11.2 meV above the bound exciton ground state. Zeeman effects on these excited states have also been studied. The results show that they correspond to excitations of the bound electron to donor-like 2p and 2s orbital states. This represents an unambiguous experimental evidence of the pseudo-donor model previously suggested by Rühle and Bimberg for acceptor bound exciton complexes when m_e ? m_h.