Impurities In ZnSe Research Articles

Determining the Parameters of the Jahn–Teller Effect in Impurity Centers from Ultrasonic Experiments: Application to the ZnSe : Ni2+ Crystal

The earlier developed methodology of ultrasonic investigation for determining the parameters of the Jahn–Teller effect (JTE) in impurity centers in crystals is further developed in application to Ni2+ impurities in ZnSe : Ni2+. For all normal modes propagating along the [110] axis, the temperature dependences of the attenuation of ultrasound are measured in the frequency range 25–165 MHz. At a temperature of about 13 K, for all these modes, attenuation peaks are observed, which indicate the presence of local distortions of both trigonal and tetragonal types. Following the previously developed method, it is shown that these anomalies of attenuation are caused by the relaxation transitions between equivalent local distortions induced by the JTE at the Ni2+ centers. In the tetrahedral environment of selenium, this ion is in the threefold degenerate ground electron state 3T1, in which three types of the JTE can be realized. The data of ultrasonic experiments indicate the realization of the T ⊗ (e + t2) problem, in which the potential energy surface has four equivalent minima of the trigonal type, slightly deformed toward tetragonal saddle points. The trigonal vibronic constants, the stabilization energies of the trigonal minima, and the activation energy of the barriers between them are ev-aluated.

Определение параметров эффекта Яна-Теллера в примесных центрах из ультразвуковых экспериментов: приложение к кристаллу ZnSe : Ni-=SUP=-2+-=/SUP=-

AbstractThe earlier developed methodology of ultrasonic investigation for determining the parameters of the Jahn–Teller effect (JTE) in impurity centers in crystals is further developed in application to Ni^2+ impurities in ZnSe : Ni^2+. For all normal modes propagating along the [110] axis, the temperature dependences of the attenuation of ultrasound are measured in the frequency range 25–165 MHz. At a temperature of about 13 K, for all these modes, attenuation peaks are observed, which indicate the presence of local distortions of both trigonal and tetragonal types. Following the previously developed method, it is shown that these anomalies of attenuation are caused by the relaxation transitions between equivalent local distortions induced by the JTE at the Ni^2+ centers. In the tetrahedral environment of selenium, this ion is in the threefold degenerate ground electron state ^3 T _1, in which three types of the JTE can be realized. The data of ultrasonic experiments indicate the realization of the T ⊗ ( e + t _2) problem, in which the potential energy surface has four equivalent minima of the trigonal type, slightly deformed toward tetragonal saddle points. The trigonal vibronic constants, the stabilization energies of the trigonal minima, and the activation energy of the barriers between them are ev-aluated.

High-Fidelity and Ultrafast Initialization of a Hole Spin Bound to a Te Isoelectronic Center in ZnSe.

We demonstrate the optical initialization of a hole-spin qubit bound to an isoelectronic center (IC) formed by a pair of Te impurities in ZnSe, an impurity-host system providing high optical homogeneity, large electric dipole moments, and potentially advantageous coherence times. The initialization scheme is based on the spin-preserving tunneling of a resonantly excited donor-bound exciton to a positively charged Te IC, thus forming a positive trion. The radiative decay of the trion within less than 50ps leaves a heavy hole in a well-defined polarization-controlled spin state. The initialization fidelity exceeds 98.5% for an initialization time of less than 150ps.

Defects Energetics, Electronic Structure and Optical Properties of Cu-Doping and Zn Vacancy Impurities in ZnSe

Based on the first-principles within the density functional theory, the geometric structures of perfect zinc blend ZnSe, that with Zn vacancies (Zn0.875Se) and Cu-doped ZnSe (Zn0.875Cu0.125Se) were optimized using the plane-wave ultrasoft pseudopotential method. The defect formation energy, band structure, density of states, Mulliken charges, and optical spectra were calculated and discussed in detail. The results demonstrated that in Zn0.875Se and Zn0.875Cu0.125Se systems, because of the introduction of the vacancy acceptor level or acceptor impurity level, the band gap is reduced, and the absorption peaks show a remarkable redshift. Cu doping into the ZnSe system was found to be relatively stable, while the monovacancy system was not.

Photon antibunching and magnetospectroscopy of a single fluorine donor in ZnSe

We report on the optical investigation of single electron spins bound to fluorine donor impurities in ZnSe. Measurements of photon antibunching confirm the presence of single, isolated optical emitters, and magneto-optical studies are consistent with the presence of an exciton bound to the spin-impurity complex. The isolation of this single-donor-bound-exciton complex and its potential homogeneity offer promising prospects for a scalable semiconductor qubit with an optical interface.

Impurity vibration modes in II–VI and III–V mixed semiconductors

AbstractThe analysis of vibration frequencies in dilute semiconducting alloys is addressed, as part of a more vast paradigm for the description of lattice dynamics in semiconductor alloys within the “1 bond – 2 mode” concept. Specific cases considered are those of Be impurities in ZnSe (system with high elastic contrast; aspects of impurity clustering) and of (Ga,In)As alloys, exhibiting much smaller elastic contrast and almost vanishing value of the mode splitting parameter, Δ. First‐principles calculations of frequencies are done by Siesta method, using 64‐atom supercells. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Self-ordering of Mg and O isoelectronic impurities in ZnSe

Self-ordering of isoelectronic magnesium and oxygen impurities in ZnSe is described. The formation of 1O4 Mg tetrahedral cells in ZnSe-enriched MgxZn1−x OySe1−y alloy (x ≥ 4y) in the ultradilute limit (1 × 10−8 ≤ y ≤ 1 × 10−3) of oxygen concentrations is predicted. Under certain conditions that are established for the temperatures of growth (300°C) and annealing (500°C), all of the oxygen impurity atoms should be surrounded by Mg atoms only. The origin of this phenomenon is the thermodynamic preference for Mg-O and Zn-Se bond formation compared to Mg-S and Zn-O bonds and a decrease in the elastic-stress energy after self-ordering of the alloy.

Investigation of excitons bound to fluorine donors in ZnSe

The electrical and optical properties of fluorine impurities in ZnSe have been studied. Bulk and delta-doped ZnSe:F layers were grown by molecular beam epitaxy. Electrical measurements reveal evidence that a majority of fluorine impurities act as shallow donors. In delta-doped samples, we find strong photoluminescence due to the radiative recombination of fluorine donor bound excitons. We measured a fluorescence lifetime of less than 250 ps and determined the inhomogeneous line width to be below 2 meV at liquid helium temperature.

Jahn-Teller effect in the emission and absorption spectra ofZnS:Cr2+andZnSe:Cr2+

The Jahn-Teller effect is invoked to explain the fine structure (isolated zero-phonon lines) observed in both the infrared emission and absorption spectra of substitutional ${\mathrm{Cr}}^{2+}$ impurities in ZnSe and ZnS. The ground $^{5}D_{2}$ term of ${\mathrm{Cr}}^{2+}$ is split by crystal field into a $^{5}T_{2}$ ground multiplet and an excited $^{5}E$ multiplet. We look at transitions among levels belonging to these two multiplets, which happen to be in the near infrared region. Spin-orbit and spin-spin interactions are taken into account. The Jahn-Teller coupling is introduced as a linear coupling considering both $ϵ$ and ${\ensuremath{\tau}}_{2}$ phonons. The Lanczos-recursion procedure with a proper choice of the initial state is used to calculate the vibronic functions and energies. It is found that $ϵ$ modes only lead to intensities that do not agree well with those of the zero-phonon doublet observed both in emission and absorption in the cases of ZnS and ZnSe, while ${\ensuremath{\tau}}_{2}$ modes give a good explanation of transition energies and transitions strengths in the same cases. A discussion of the relatively high strength of the vibronic coupling for Cr in comparison with other impurities in the same compounds is also included.

Comparative Mössbauer spectroscopic study of iron impurities in ZnSe

An anomalous spectral doublet attributed to Fe2+ impurity pairs is observed in Mössbauer absorption spectra of 57Fe0.01Zn0.99Se below room temperature. In Mössbauer emission experiments, ZnSe:57Co polycrystal and powder sources exhibit below 45K the nucleogenic Fe1+ valence state born as an after effect of the electron-capture decay of 57Co. Above 8K, the proportion of the nucleogenic Fe1+ decreases under the effect of the thermally activated Fe1+→Fe2+ relaxation. The activation energy is estimated to 0.004eV. While nucleogenic Fe1+ is absent in ZnSe:57Co single crystal spectra, increasing proportions are detected in samples with an enhanced specific surface. The formation model of the nucleogenic Fe1+ is extended to other zinc chalcogenides and a tentative model is developed, which shows the role of the surface electric field in limiting the formation of nucleogenic Fe1+ in well crystallised source samples.

Study of site change of Li impurities in ZnSe by co-doping with iodine

Co-doping of lithium (Li) and iodine (I) in ZnSe was studied with metalorganic vapor-phase epitaxy. Since Li has the well-known metastability of substitutional acceptor sites and interstitial donor sites, the examination of the stability of Li sites will be effective to study the co-doping effect. Although the ZnSe films doped with Li showed photomuninescence (PL) spectra dominated by neutral-donor-bound excitons at 13 K, the PL spectra showed the systematic shift to the neutral-acceptor-bound excitons by co-doping with I doping. The corresponding turn-over of the electrical properties from n- to p-type was also observed.

Hyperspherical adiabatic approach for excitons bound to ionized donors in semiconductors

Binding energies for excitons trapped by impurities in ZnSe and CdS are calculated within the hyperspherical adiabatic approach. Nonadiabatic couplings are included in the radial equations, leading to energies lower than the variational values available in the literature. Resonant states similar to autoionizing lines in atoms are predicted to lie above the first electron-impurity ionization threshold.

First-principles study of phosphorus and nitrogen impurities in ZnSe.

Phosphorus and nitrogen defects in ZnSe have been studied by pseudopotential total-energy calculations. Various charge states of the phosphorus and nitrogen impurities ocupying zinc sites (antisite defects) and interstitial sites (interstitial defects) were considered: their structural and electronic properties were investigated, and formation energies were calculated as a function of Se and electronic chemical potentials. The calculated electronic properties and formation energies were used in the investigation of the role of impurities in the p-type doping problem of ZnSe. Our study suggests that the difficulty with p-type doping of ZnSe by phosphorus results from the compensation of the shallow acceptors by the antisite defects ${\mathrm{P}}_{\mathrm{Zn}}$ which act as triple donors. In the case of nitrogen, the concentrations of the antisite and interstitial defects are too low to cause such compensation, which is consistent with the successful p-type doping of ZnSe with nitrogen.

Pressure dependence of deep centers in II–VI semiconductors: Theory

In contrast to shallow acceptors or donors levels, the binding energies of deep impurity levels are sensitive to pressure. Transitions between shallow and deep levels can often be induced by pressure and useful information on energy levels and structural metastabilities can be derived. It is not generally well known that pressure has an opposite effect on deep donor states than on deep acceptor states. In both III–VI and II–VI semiconductors, pressure increases the binding energy of deep donors with respect to the Γ lc conduction band and, at sufficiently high pressures, can make the effective-mass state unstable relative to a deep state. In contrast, the results of recent experiments by Weinstein and co-authors [ High-Pressure Science and Technology, Vol. 1, p. 141, Plenum, NY (1979); J. Crystal Growth 138, 318 (1994); Phys. Rev. B (submitted)] show that the binding energies of deep acceptor states in ZnSe decrease with pressure. In this paper we present the results of ab initio pseudopotential calculations on the pressure dependence of the shallow and deep levels of Al, Ga and In donor impurities in CdTe, and of As and P acceptor impurities in ZnSe. The experimental results are proposed to arise from an upward shift in the energies of both the valence and conduction band edges with pressure. Deep states have a large degree of atomic-like character and tend to have a smaller pressure dependence than the band edges. The upward shift of the band edges, therefore, increases the binding energies of deep donors and decreases those of deep acceptors.

Iron and chromium impurities in ZnSe as centers of nonradiative recombination.

A detailed light-induced electron-spin-resonance study of iron and chromium impurities in ZnSe is presented. The position of the Fe 2+/3+ energy level in the band gap of ZnSe is determined. The role of iron and chromium impurities in nonradiative recombination processes in ZnSe is discussed.

Luminescence of deep phosphorous and arsenic impurities in ZnSe at high pressure.

High-pressure photoluminescence (PL) is reported for bulk and thin-film ZnSe containing ${10}^{18}$--${10}^{19}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ P and As acceptors using a cryogenic (4--300 K) diamond-anvil cell to 55 kbar. We focus on the deep PL bands in the red and green regions of the visible spectrum; the red emission bands (1-atm peaks at \ensuremath{\sim}6000 A in ZnSe:P and \ensuremath{\sim}7000 A in ZnSe:As) are thought to stem from ${\mathit{C}}_{3\mathit{v}}$-distorted acceptor sites. With increasing pressure in the range studied, the peak energies of all the deep bands approach but do not cross the ZnSe band gap. The average deformation potential of the valence edge relative to the P or As deep levels attributed to ${\mathit{C}}_{3\mathit{v}}$ sites is -1.2\ifmmode\pm\else\textpm\fi{}0.3 eV. The negative sign is confirmed by thermal quenching data at different pressures in ZnSe:P. These results imply that with pressure the valence edge shifts to higher energy, causing deep acceptor states to become less stable. We also observe under pressure strong transfer of oscillator strength from red to green emission, and several deep bands. However, since the shallow acceptor-bound exciton PL is not enhanced, we conclude that a deep-to-shallow transition has not occurred within the pressure range studied.

First-principles study of antisite and interstitial phosphorus impurities in ZnSe.

First-principles study of antisite and interstitial phosphorus impurities in ZnSe.

Acceptor and donor states of impurities in wide band gap II–VI semiconductors

The stability of substitutional versus displaced atomic configurations for N, Al, Ga, Cl, and isoelectronic O impurities in ZnSe, ZnTe, MgTe wide band gap semiconductors were examined via first-principles pseudopotential calculations. Negatively charged DX centers with very large lattice relaxations are found to be the stable states of Al and Ga donor impurities in ZnTe and MgTe but to be high energy metastable states in ZnSe and MgSe. The acceptor state of O in ZnSe is identified with a strongly bonded configuration in which a Zn-O bond replaces a Zn-Se bond. The idea of a beneficial defect-mediated doping mechanism is proposed and applied to an explanation of the properties of N impurities in ZnSe.

Donor-acceptor pair decay and evaluation of the recombination constant and the Bohr radii of shallow impurities in ZnSe

The recombination constant (W0) and the Bohr radii of Na interstitial donors (aD) and substitutional Na acceptors (aA) in Na doped ZnSe have been determined from the time decay of the close donor-acceptor pairs. The results are W0 = 6 x 108 s-1, aD = 37A˚, and aA = 5.6A˚. The measured Bohr radii are in excellent agreement with the v alues derived from scaled effective mass theory.

Column-V acceptors in ZnSe.

The structural and electronic properties of substitutional phosphorus and nitrogen impurities in ZnSe are studied using pseudopotential total-energy calculations. Substitutional phosphorus and nitrogen, in their neutral states, form shallow acceptors. In the case of phosphorus, the symmetry is lowered from ${\mathit{T}}_{\mathit{d}}$ to ${\mathit{C}}_{3\mathit{v}}$ by a Jahn-Teller distortion, and the hole is more localized on the phosphorus site. For nitrogen, however, the Jahn-Teller distortion is so small that the symmetry is approximately ${\mathit{T}}_{\mathit{d}}$, and the hole density is spread around nearby selenium-atom sites. For nitrogen in the positive charge state, two minimal-energy configurations are predicted: a stable state with approximately ${\mathit{T}}_{\mathit{d}}$ symmetry and a metastable state with ${\mathit{C}}_{3\mathit{v}}$ symmetry, which is 0.3 eV higher in energy than the stable one.