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.     

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.     

Valence band contributions to photoluminescence excitation spectra of tightly bound holes in zincblende semiconductors

Photoluminescence excitation (PLE) spectra of deep acceptor states in ZnSe, for example the Cu-related luminescence band at ≈1.95 eV, contain a prominent excitation band at ≈3.25 eV. This band lies above the structure marking the lowest direct E_O band gap E_g by the spin-orbit splitting energy Δ of the valence bands at Γ. The higher energy feature is either absent or greatly de-emphasised in the PLE spectra of shallow acceptor states in ZnSe and of the oxygen iso-electronic trap in ZnTe, where the electron rather than the hole is tightly bound. However, a significant PLE component at E_g + Δ is observed for deep acceptor-like states in ZnTe, where Δ is ≈0.95 eV. Efficient PLE at E + Δ for luminescence from deep acceptor-like states is shown to be consistent with the extended wave-vector contributions to the bound state wave-functions of holes of binding energies ≈Δ.     

Evidence for a deep acceptor level in p-InSb from the variation of hole density with uniaxial stress

The hole density of Cd doped p-InSb has been studied as a function of uniaxial compressive stress along [001] at T = 77 K. Analysis implies the presence of a deep acceptor level whose activation energy decreases with compressive stress, dE_A/dχ = ?3.9 meV/kbar.     

Evidence for a donor-acceptor pair band in CuInSe2

The broad-band emission, observed in p-type crystals, was studied as a function of excitation intensity and temperature. The band peak is usually in the energy range of 0.93–0.95 eV at 4.2°K. The band shifts to higher energy as the excitation intensity of temperature increases. This behavior is consistent with a donor-acceptor pair-band mechanism. The acceptor energy E_A is 85 ± 2 meV. The acceptors and donors involved in the pair band appear to be Cu and Se vacancies.     

Deep level transient spectroscopy of hole traps in Zn-annealed ZnTe

Hole traps in Schottky barrier diodes of p-ZnTe that had previously been annealed in liquid Zn have been investigated using Deep Level Transient Spectroscopy (DLTS). Three traps have been investigated in 3etail and have activation energies of between E_v = 0.28 eV and E_v + 0.59 eV. All traps are assigned to acceptor defects because of their large capture cross sections for holes and one of these is tentatively assigned to a V_Zn native acceptor. From capacitance-temperature plots we deduce that Cu_Zn is the dominant shallow acceptor in as-grown material but after annealing the Cu_Zn concentration is reduced and a yet shallower acceptor, probably Li_Zn, predominates.     

Impurity centers in ZnSe:Na crystals

The influence of sodium impurity on photoluminescence (PL) spectra of ZnSe crystals doped in a growth process from a Se+Na melt is investigated. It is shown that the introduction of the impurity results in emergence of emission bands in the PL spectra due to the recombination of exciton impurity complexes associated with both donors and hydrogen-like acceptors. Apart from that, four bands generated by donor-acceptor pairs recombination and a band produced by electronic transitions from the conduction band to a shallow acceptor are discussed. As a result of the analysis it is concluded that Na impurity forms in ZnSe lattice Na_Zn hydrogen-like acceptors with activation energy of 105±3 meV, Na_i donor centers with activation energy of 18±3 meV, as well as Na_ZnV_Se and Na_iNa_Zn associative donors with activation energy of 35±3 and 52±9 meV, respectively.     

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.     

Identification of Acceptor States in Li-N Dual-Doped p-Type ZnO Thin Films

Li-N dual-doped p-type ZnO （ZnO：（Li, N）） thin films are prepared by pulsed laser deposition. The optical properties are studied using temperature-dependent photoluminescence. The Lizn-No complex acceptor with an energy 1evel of 138 me V is identified from the free-to-neutral-acceptor （e, A0 ） emission. The Haynes factor is about 0.087 for the Lizn-No complex acceptor, with the acceptor bound-exciton binding energy of 12meV. Another deeper acceptor state located at 248 meV, also identified from the （e, A0） emission, is attributed to zinc vacancy acceptor. The two acceptor states might both contribute to the observed p-type conductivity in ZnO：（Li,N）.     

Fabrication and photoluminescence of P doped ZnO nanobelts by thermal evaporation method

Uniform and flat single crystal ZnO:P nanobelts (NBs) were fabricated on Si (1 0 0) substrates by the thermal evaporation method. The growth process, free-catalyst self-assembly vapor-solid (V-S) mechanism, was described and investigated deeply in terms of thermodynamics and kinetics. Then, the photoluminescence (PL) properties of ZnO NBs were studied in a temperature range from 10 to 270 K. At 10 K the recombination of acceptor-bound exciton (A⁰X) was predominant in the PL spectrum, and was attributed to the transition of P_Zn−2V_Zn complex bound exciton. The active energy of A⁰X and acceptor binding energy were calculated to be 17.2 and 172 meV, respectively. The calculated acceptor binding energy of P doped ZnO nanostructure is in good agreement with that of P doped ZnO film.     

Impurity–defect emission from undoped Cd<Subscript>1–<Emphasis Type="Italic">x</Emphasis> </Subscript>Zn<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>Te single crystals near the fundamental absorption edge

Shallow impurity–defect states in undoped Cd_1–xZn_xTe (x ～ 3–6%) single crystals have been studied using low-temperature photoluminescence measurements. It has been found that the effect exerted by zinc is mainly reduced to a rigid shift of all the specific features associated with the exciton radiation, which made it possible, with a high (～0.3 meV) accuracy, to measure the band gap and the zinc concentration in solid solutions. Hydrogen-like donors with the ground-state energy of ～14 meV and four types of acceptors with average activation energies of 59.3 ± 0.6 meV, 69.6 ± 1.5 meV, 155.8 ± 2.0 meV, and 52.3 ± 0.6 meV have been identified in all the crystals studied. Based on a comparison with the results of the analysis of the impurity background and the data available in the literature on impurity–defect emission in undoped CdTe, the first three acceptors can be assigned to the substitutional impurities Na_Cd, P_Te, and Cu_Cd, respectively. The most shallow acceptor (52.3 ± 0.6 meV) is a complex defect in which there is a nonstandard excited level separated by only 7 meV from the ground level. This level is formed apparently due to the removal of degeneracy, which is characteristic of T_D acceptors, by the low-symmetry potential of the complex defect.     

Dependence of the interband transition and the activation energy on the ZnTe spacer thickness in CdTe/ZnTe double quantum dots

Photoluminescence (PL) measurements were carried out to investigate the interband transition and the activation energy in CdTe/ZnTe double quantum dots (QDs). While the excitonic peaks corresponding to the interband transition from the ground electronic subband to the ground heavy-hole (E₁-HH₁) in the CdTe/ZnTe double QDs shifted to higher energy with decreasing ZnTe spacer thickness from 30 to 10 nm due to transformation from CdTe QDs to Cd_xZn_1−xTe QDs, the peaks of the (E₁-HH₁) transitions shifted to lower energy with decreasing spacer thickness from 10 to 3 nm due to the tunneling effects of the electrons between CdTe double QDs. The decrease in the activation energy with decreasing ZnTe spacer thickness might originate from an increase in the number of defects in the ZnTe spacer. The present results can help improve the understanding of the interband transition and the activation energy in CdTe/ZnTe double QDs.     

Study of acceptor states in CdTe by donor-acceptor pair excitation luminescence

Excitation of donor-acceptor pair luminescence has been studied in CdTe doped with lithium or chlorine. The excitation spectrum of the lithium acceptor is determined and fitted with the effective mass theory of Baldereschi and Lipari. Revised values of the valence band parameters are deduced: μ = 0.8, δ = 0.054, Ry = 24 meV. The analysis of the 1.45 eV luminescence band in compensated Cl-doped crystals shows the existence of donor-acceptor pair transitions. Three acceptor centers are identified: E_A = 89, 111 and 119 meV, and the contribution of a deep donor (E_D > 40 meV) is demonstrated. Besides intracentre type excitation transitions of the 1.45 eV band have been observed in non-compensated chlorine-doped crystals. Thus several recombination channels and distinct acceptor states contribute to the composite 1.45 eV luminescence band.     

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.     

Photoluminescence study of manganese in AlSb

We have investigated Czochralski-grown manganese-doped AlSb by low-temperature photoluminescence spectroscopy. Discrete donor-acceptor pair lines are resolved, which involve the Mn acceptor. A Mn acceptor binding energy (E _A =92±5 meV) is deduced from the donor-acceptor pair luminescence.     

A PTIS study of quenched-in acceptors in germanium

The shallow acceptors produced in germanium crystals by quenching from 820–925 C have been studied for the first time using Photo-Thermal Ionization Spectroscopy (PTIS). We have found two acceptor-continua, which correspond to the E_v + 8.4 meV and E_v + 12 meV levels observed in earlier Hall measurements. With the lower energy continuum there are associated two previously unobserved hydrogenic acceptors. They are shallower than any known acceptor in germanium: their ionization energies are 8.69 ±0.01 meV and 9.48 ±0.01 meV. We attribute the acceptors to two different defects because of differences in their creation and annealing behaviour. No discrete lines were found to be associated with the higher energy continuum. We estimate the acceptor ionization energy to be about 14 meV. Finally, we have observed a number of as yet unexplained negative lines superimposed on both acceptor-continua.     

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.     

On directly measuring relative Fermi energies of noble metals and their alloys

We present the first evidence of direct measurement of relative Fermi energies in alloys and between pure metals. From applying the “atomic” concept of core hole final state screening, the Auger energy shift of noble metal A equals E_F^A?E_F(x). High resolution Auger shifts in P_1?x^tCu_x,Au_xCu_1?x and Au_xAg_1?x demonstrate experimental verification of this simple relation. We find E_F^CuE_F^Au ? ? 0.2 eV, and E_F^Pt ? E_F^Cu and E_F^Ag ? E_F^Au.     

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.     

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).