The Chromium Impurity in ZnTe: Changes of the Charge State Detected by Optical and EPR Spectroscopy

In gasphase-grown p-type ZnTe crystals, the substitutional Cr impurity is detected in the neutral (Cr2+) and both ionized states Cr3+ and Cr+. While these three oxidation states are now identified by EPR, two of them emerge in the optical spectra as well. Optical irradiation of the samples at low temperatures influences the relative concentrations of these species. By monitoring the respective EPR or optical signals, thresholds for photoionization and photo-neutralization processes are derived. In addition to the strong 5E(D)→5T2(D) optical absorption of Cr2+(d4), the samples display a luminescence band near 4500 cm-1, structured by Jahn—Teller interaction. For the first time, a no-phonon line at 4989 cm-1 is resolved, accompanied by a TA(L) satellite and some local-mode structure. In EPR, the angular dependence of the ∆M = ±2 transition characteristic for Cr2+(d4) ions on lattice sites indicates the 5E(D) ground state subject to a static Jahn-Teller effect in the three (100) directions. Whereas an internal optical transition of Cr3+(d3) could not be identified, in EPR, an isotropie signal at g=3.3 discloses a strongly Jahn—Teller-disturbed 4T1(F) ground state. On irradiation with band-tail photons, the ionized acceptor state Cr+(d5) becomes detectable by EPR. Furthermore, an emission with an onset near 10,300 cm-1 is recorded for the first time. Its excitation and optical stimulation spectra are studied in wide spectral ranges; the exponential decay with ζ≅1 ms supports an assignment to a spin-forbidden transition. The acceptor ionization energy is determined as 10,500 cm-1 (1.3 eV)

Static and dynamic ionization levels of transition metal-doped zinc chalcogenides

Transition metal (TM) impurities in semiconductors have a considerable effect on the electronic properties and on the lattice vibrations. The unfilled d shell permits the impurity atoms to exist in a variety of charge states. In this work, the static donor and acceptor ionization energies of ZnX:M, with X = S, Se, Te and M:Sc, Ti, V, Fe, Co, Ni are obtained from first principles total energy calculations and compared with experimental results in the literature where they exist. From these results, many of the TM-doped zinc chalogenides have an amphoteric behavior. To analyze the rule of the deep gap levels in both the radiative and non-radiative processes, the dynamic ionization energies are obtained as a function of the inward and outward M–X displacements. In many cases, the changes in the mass and the force constants resulting from the substitution of an impurity center for a lattice atom are small. When the charge or the environment of the impurity changes, the electron population tend to remain compensated. As consequence, the changes in the lattice vibrational modes are small

Electron paramagnetic resonance related to optical charge-transfer processes in ZnSe:Ti

In ZnSe crystals grown by different techniques, titanium ions are incorporated as Ti2+(d2) and Ti3+(d1) centers on Zn sites. A strong Jahn-Teller (JT) effect acts on the doubly degenerate 2E ground state of Ti3+. The electron paramagnetic resonance (EPR) at T=3 K indicates a quasistatic JT effect of the ground state and a quasidynamic one of the first excited state for the strain-split vibronic 2E−2A2 manifold. These effects are distinguished by their angular variations and the g values. Ti2+ causes an isotropic EPR signal. Excitation and sensitization spectra of Ti2+ and Ti3+ luminescence transitions are explained within a one-electron model connecting internal (d-d) with charge-transfer transitions involving the valence and conduction bands. Both charge states are sensitive to illumination with near-infrared light. The ions can be mutually converted, as shown by photo-EPR and sensitization experiments. The Ti2+/Ti3+ donor level is situated approximately 8500 cm−1 below the edge of the conduction band

Identification of titanium dopants in CdS, CdSe and Cd(S,Se) crystals by luminescence and EPR methods

CdS, CdSe and Cd(S,Se) crystals were grown and doped by Ti in a sublimation process. By EPR, the ground states of Ti2+(d2) and Ti3+(d1) are detected, indicating a substitutional incorporation as TiCd. For the first time, the Jahn-Teller ion Ti3+ is observed by its 2T2(D) → 2E(D) transition in photoluminescence

Vapour growth, EPR and optical study of Znse:Ti single crystals

Znse:Ti crystals were grown by seeded chemical vapour transport (SCVT) in a hydrogen atmosphere. In situ doping with titanium was carried out from an additional TiSe source in the quartz ampoule at 1460 K. The Good homogeneity of the Ti distribution and the presence of only a few defects have been demonstrated by etch pit and X-ray topography studies. For the frist time, Ti3+ (3d1) centres in a II–VI semiconductor compund are detected. The concentration of localized centres of Ti2+ and Ti3+ in high-resistivity material determined by EPR is about 5 × 1016 cm−3. Fe3+, Ni2+ and traces of Mn2+ are additionally observed. Below 77K an anisotropic EPR spectrum of Ti3+ (d1) is recorded, indicating both a dynamic and a static Jahn–Teller effect of the 2E(D) ground state. At lower temperatures clear evidence of the static Jahn–Teller effect is observed. The frist emission and excitation spectra of Ti impurities in II–VI compounds are presented. An emission near 3400 cm−1 is assigned to the 3T2(F) → 3A2(F) transition of Ti2+ (d2). Its excitation structures coincide with the known absorption bands to 3T1(P) and 3T1(F). A further structured luminescence band at 4700 cm−1 is related to 2T2 → 2E(D) transition of Ti3+ (d1). Ti ions form a deep donor level Ti2+/Ti3+ situated approximately 14,100 cm−1 above the valence band. This photo-ionization threshold is derived from the excitation measurements in accordance with previous photoconductivity experiments

Optical and Magnetic Properties of Titanium Ions in CdTe and (Cd, Zn)Te

Bridgman-grown CdTe and (Cd, Zn)Te crystals doped with Ti are investigated by means of near-infrared optical spectroscopy and electron paramagnetic resonance (EPR). In high-resistivity CdTe material, Ti centres are detected by EPR with a concentration of abou 1016 cm-3. Below 60 K, the isotropic spectrum of the Ti2+(3d2) ground state 3A2(F) is recorded, showing strong saturation at lower temperatures. Below 8 K, a second spectrum appears due to transitions within the vibronic 2E ground state of Ti3+(3d1 caused by simultaneous action of a dynamic Jahn-Teller effect and random strains. Both these centres show isotropic hyperfine structure with 12 equivalent next-nearest Cd neighbours, proving that the Ti impurity substitutes for Cd at lattice sites of Td symmetry. The EPR signals are photosensitive in the visible and near-infrared regions. The luminescence spectra of Ti centres in Cd1-xZnxTe:Ti with x = 0.15 show the 3T2(F) 3A2(F) transition of Ti2+ centred at 2 600 cm-1 and the 2E(D) emission of Ti3+ around 3200 cm-1. The excitation spectrum of the Ti2+ luminescence shows three excitation bands, viz. to 3T1(F), 3T1(P), and near-gap levels

Optical and paramagnetic properties of titanium centres in ZnS

ZnS:Ti crystals grown by different methods are investigated at low temperatures (2 K $\leq T \leq 80$ K) by electron paramagnetic resonance (EPR) and optical spectroscopies, mainly with regard to their photoluminescence properties, including excitation and sensitisation spectra. The more familiar lattice-neutral Ti$^{2+}(d^2)$ ion on a cubic site (AN) presents an isotropic EPR signal with g=1.928 observable up to 80 K. Signals of axial PN and AS sites are detected in the same temperature range. This Ti2+ ion exhibits the $^3T_2(F) \rightarrow{}^3A_2(F)$ transition in emission, structured by no-phonon lines (NPL) of centres in various environments, with the AN site represented at 3613 cm-1, and the $^3T_1(F)$ and $^3T_1(P)$ bands in excitation spectra. Substitutional Ti$^{3+}(d^1)$ is identified by an anisotropic EPR spectrum at T = 3.5 K, indicating a quasistatic Jahn-Teller effect at the doubly degenerate $^2E(D)$ ground state, while in the first excited vibronic state a quasidynamic JT effect in the strain-split vibronic $^2E/^2A_2$ manifold is found. A new luminescence band centred at 4500 cm-1 with NPL structures near 5000 cm-1 represents AN, PN and AS sites in the $^2T_2(D) \rightarrow{}^2E(D)$ transition. Charge-transfer processes are described in a one-particle model, depicting the Ti3+/Ti2+ donor level at 12 900 cm-1 below the conduction-band edge