Investigation of positively charged acceptor states in Si and Ge under uniaxial stress by phonon induced conduction

We use superconducting Al-tunnel junctions as tunable phonon generators in the meV-range to determine ground state splitting at zero stress of positively charged states associated with single acceptors in Si and double acceptors in Ge. From the stress and energy dependence of the conductivity induced by high frequency phonon irradiation of the corresponding two- and three-hole states we find that the splitting is below 0.1 meV for the ground states of Si:B⁺ and Ge:Be⁺, and 1 meV and 1.2 meV for Si:Ga⁺ and Si:Al⁺, respectively. These level separations are comparable to those found previously for the ground states of the corresponding acceptor bound excitons A⁰X. For the deeper acceptors Si:In⁺, Si:Tl⁺, and Ge:Zn⁺ the results are qualitatively different.     

Site-dependent donor and acceptor levels in 6 H-SiC

The photoluminescence of N- and Al- or Ga-doped 6 H-SiC has been studied. Radiative transitions between free electrons and holes bound to acceptors in cubic-like and hexagonal-like sites are assigned. Donor-acceptor pair transitions between donor and acceptor impurities in cubic-like and hexagonal-like sites are analyzed. From studies on these transitions, the ionization energies of impurities in cubic-like and hexagonal-like sites in 6 H-SiC are estimated to be 248.5 and 239 meV for Al acceptors, 333 and 317 meV for Ga acceptors, and 155 and 100 meV for N donors, respectively. The ratios of the ionization energies of acceptors in the cubic-like sites to the hexagonal-like sites are almost constant, ≈1.04-1.05. The origin of the difference of the ionization energies of impurities in the different sites is discussed on the basis of the quantum defect model. For acceptors this is mainly caused by differences in the local dielectric constants.     

Resonant states of shallow acceptors in uniaxially deformed germanium

The states of shallow acceptors in uniaxially deformed germanium are studied theoretically. A non-variational numerical computational method is developed for determining the energy and wave functions of localized states of holes in the acceptor field as well as the states of the continuous spectrum (including resonant impurity states). The dependence of the energy of the lower resonant state on strain is studied. It is found that this state is formed from the excited 4Γ ₈ ⁺ state with a binding energy of 1.3 meV (in the absence of deformation) and not from the ground state. The results presented in this work may be useful in the study of the conditions for the generation of far IR radiation in deformed p-Ge, which involves optical transitions between resonant and localized acceptor states.     

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.     

Edge emissions of ion-implanted CdS

Edge emissions of CdS crystals implanted with Li⁺, N⁺, Ne⁺ and P⁺ are studied. Li and P are found to form acceptors and give rise to I₁ lines in the blue and donor-acceptor pair recombination emissions in the green. N gives rise to two kinds of I₂ lines (excitons bound to neutral donors) and donor-acceptor pair recombination emissions. An acceptor due to a defect is found in N⁺ and Ne⁺ implanted CdS. The temperature range of thermal treatment where impurity centers are formed is also investigated. The relationship among impurity centers produced by N⁺ implantation is, moreover, discussed based upon the results obtained here and previous works.     

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.     

Copper,the dominant acceptor in refined,undoped zinc telluride

Comparative measurements have been made of optical absorption and photoluminescence of refined undoped and Cu in-diffused ZnTe single crystals. Strong increases in a bound exciton BE line near 2.375 eV previously identified with the electrically dominant point defect acceptor ‘a’, with binding energy E_A ? 149 meV, suggests that this acceptor is substitutional Cu_Zn. Similarly strong increases in a relatively broad band at slightly higher energy suggests the simultaneous incorporation of shallow donors, possibly interstital Cu_I. These findings indicate that intrinsic defects such as V_Zn neither control the Fermi level in refined ZnTe nor produce shallow acceptors with E_A ? 250 meV, contrary to much previous speculation.     

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.     

Photoluminescence and capacitance transients in highly Mg-doped GaN

Behaviors of the photoluminescence blue-band and near-bandgap peak and the relevant thermal ionization energies of the shallow and deep Mg-related acceptors have been studied, respectively. The 2.989 eV blue-band is attributed to the deep donor–acceptor-pair transitions involving a deep Mg-related acceptor at E_v+0.427 eV. The blueshift with increasing excitation power is explained by variation in the contribution of close and distant donor–acceptor-pairs to the luminescence. The redshift with increasing temperature results from thermal release of carriers from close donor–acceptor-pairs. The 3.26 eV near-bandgap peak is attributed to the shallow donor–acceptor-pair transitions involving a shallow Mg-related acceptor at E_v+0.223 eV. The relevant thermal ionization energies of the shallow and deep Mg-related acceptors, being about E_v+0.16 and E_v+0.50 eV, are determined from deep-level transient Fourier spectroscopy measurements. Received: 11 July 2001 / Accepted: 9 August 2001 / Published online: 2 May 2002     

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.     

Shallow acceptor levels in Ge/GeSi heterostructures with quantum wells in a magnetic field

Shallow acceptors in Ge/GeSi heterostructures with quantum wells are studied theoretically and experimentally in the presence of a magnetic field. It is shown that, in addition to the cyclotron resonance lines, magnetoabsorption spectra reveal transitions from the acceptor ground state to excited states related to Landau levels from the first and second confinement subbands, as well as the resonances caused by ionization of A⁺ centers.     

Energy band structure of TiO2 and it's dependence on uniaxial stress

The electronic band structure of titanium dioxyde is calculated. The evolution of the levels and of the optical transitions under an uniaxial stress along the c axis has also been calculated.We confirm the existence, of a first order forbidden direct optical gap, and of a maximum along the Δ direction; the theoretical pressure coefficients ?E/?P for the Γ⁺₇ → Γ⁺₆ and Δ₅ (max) → Γ⁺₆ transitions are respectively 0.600 and 0.950 meV kbar^-1 while the corresponding experimental values are 0.625 and 0.985 meV kbar^-1.     

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.     

Low-temperature photoluminescence in CuIn<Subscript>5</Subscript>S<Subscript>8</Subscript> single crystals

Photoluminescence (PL) spectra of CuIn₅S₈ single crystals grown by Bridgman method have been studied in the wavelength region of 720–1020 nm and in the temperature range of 10–34 K. A broad PL band centred at 861 nm (1.44 eV) was observed at T = 10 K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.5– 60.2 mW cm^?2 range. Radiative transitions from shallow donor level located at 17 meV below the bottom of the conduction band to the acceptor level located at 193 meV above the top of the valence band were suggested to be responsible for the observed PL band. An energy level diagram showing transitions in the band gap of the crystal has been presented.     

Low energy electron spectroscopy of CO in the 10.600–14.400 eV energy loss range

Excitation spectra of CO have been obtained at low electron impact energy in the 10.600–13.400 eV energy loss range for scattering angles from 10 to 120°, with a 35 meV experimental resolution. The angular behaviour of the observed peaks is used to discriminate singlet-singlet and singlet-triplet transitions. Previously calculated Rydberg states are observed, in particular the triplet analogue of the F¹Σ⁺ state. A new high energy valence triplet state is identified; the first observed vibrational level is at 11.595 eV and the vibrational spacing is 90 meV. Upper levels are strongly affected by predissociation.     

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.     

Luminescence and lattice defects in Cu In S2

We have studied the cathodoluminescence of single crystals of Cu In S₂ obtained by iodine chemical transport. As grown crystals and crystals annealed in In, S, (In + S), or in vacuum, were used. Two types of spectrum were observed, one for n type crystals due to S vacancies, and the other for p type or compensated crystals, which is interpreted as donor-acceptor pair transitions (determined by Time Resolved Speetroscopy) between the S vacancy (donor ~90 meV) and the Cu vacancy (acceptor ~45 meV), and by the corresponding free-to-bound transitions.     

Excitation spectroscopy on the P,Q, R isoelectronic lines in indium doped silicon

Excitation spectroscopy by a tunable NaF: (F⁺₂)¹ color center laser was successfully applied to the isoelectronic center responsible for the P, Q, R lines in indium doped silicon. This was facilitated by the extremely high P line luminescence intensities achieved after the special heat treatment which was recently reported. We determined three highly excited levels at energies of 11.4 meV, 19.4 meV, and 36.4 meV above the P line upper state.     

X射线吸收谱测算锂氮共掺杂氧化锌薄膜局域电子结构

为了实现对Li—N共掺杂p型ZnO薄膜的形成机制以及其稳定p型导电原因的揭示,利用X射线光电子谱及基于同步辐射光源的X射线吸收精细结构谱测试对薄膜的局域电子结构进行了测算分析.获得了Li—N成键及Li—N复合型受主形成的信号,利用光致发光测量计算其受主能级为122 mV.证实了薄膜中Li—N复合型受主的形成,而Li—N...     

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.