Two-Photon Raman scattering and high excitation luminescence in ZnTe

Two-Photon-Raman Scattering (TPRS) and the luminescence of ZnTe are investigated when the samples are highly excited with a tunable narrow-band dye-laser. In luminescence, one observes emission bands due to the well-known inelastic exciton-exciton scattering at intermediate excitation intensities, and the recombination radiation of an electronhole plasma (EHP) at the highest excitation levels. For the first time, TPRS is reported in ZnTe. From the change in the TPRS lines in magnetic fields up to 10T we deduce a diamagnetic shift of 1.2°10^–2 meV/T² of the free longitudinal exciton. This value is in good agreement with results obtained by other authors from reflection spectroscopy.     

Studies of relaxation times of secondary radiations in ZnTe by circular polarization correlation

Population and spin relaxation rates concerned with exciton luminescence and multiple LO-phonon emission lines are studied in ZnTe at 77 K by means of optical circular polarization correlation between the exciting and emitted lights. The decay times of the LO lines are determined to be much less than 10^-11sec, which are definitely shorter than the obtained exciton lifetime of 10^-10-10^-9sec. It is concluded that the LO lines are not due to hot luminescence but due to resonance Raman scattering.     

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.     

Spectral dependent kinetics of the EHD luminescence in As-doped Ge

Following pulsed laser excitation of As-doped Ge with impurity concentrations between 10¹⁵-10¹⁷cm^-3, we observe the electron-hole drop (EHD) and excitonic luminescence decay. The spectrum resolved no-phonon (NP) EHD luminescence kinetics are found to depend on its spectral position. “Plateaus” on the kinetic curves for the high energy side of NP-spectrum are observed at high excitation. The data suggest this luminescence is due to the states which are in dynamical equilibrium during some time after excitation pulse.     

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.     

Channels of radiative recombination and phase transitions in a system of nonequilibrium carriers in a Si0.93Ge0.07/Si thin quantum well

The “exciton gas-plasma” transition (the Mott transition) in a Si_0.93Ge_0.07/Si thin quantum well is investigated using low-temperature photoluminescence. It is demonstrated that this transition is smooth and occurs in the concentration range from approximately 6 × 10¹⁰ to 1.2 × 10¹² cm^?2. At a temperature of 23 K and excitation densities of higher than 10 W/cm², the shape and location of the luminescence line associated with the electron-hole plasma remain unchanged with an increase in the pump density. This can indicate the occurrence of an “electron-hole gas-liquid” transition. It is shown that, in the spectrum of the quantum well, the luminescence of boron-bound excitons dominates at liquid-helium temperatures and low excitation densities, whereas the free-exciton luminescence dominates at temperatures above 10 K. The influence of the homogeneous and inhomogeneous broadening on the electron-hole plasma and exciton luminescence is discussed.     

Electron-hole drops in A2B6 compounds with blende type structure

Evidence of electron-hole drops (EHD) in ZnTe, ZnSe and ZnS single crystalsof blende type structure based on studies of luminescence and optical gain measurements under laser excitation is presented. It is established that at T =4.2 K the luminescence and the inverse population of the most pure crystals near the self-absorption edge is due to the radiative recombination of EHD. The main parameters of the drops in these materials are obtained by comparison of experimental and theoretical luminescence spectra.     

Resonant Raman scattering from CdTe/ZnTe self-assembled quantum dot structures

We report resonant Raman scattering results of CdTe/ZnTe self-assembled quantum dot (QD) structures. Photoluminescence spectra reveal that the band gap energies of the CdTe QDs decrease with the increase of CdTe thickness from 2.0 to 3.5 monolayers, which indicates that the size of the QDs increases. When the CdTe/ZnTe QD structures are excited by non-resonant excitation, a longitudinal optical (LO) phonon response from the ZnTe barrier material is observed at 206 cm⁻¹. In contrast, when the CdTe/ZnTe QD structures are resonantly excited near the band gap energy of the QDs, additional phonon modes emerge at 167 and 200 cm⁻¹, while the ZnTe LO phonon response completely disappears. The 167 cm⁻¹ mode corresponds to the LO phonon of the CdTe QDs. A spatially resolved Raman scattering from the cleaved edge of the QD sample reveals that the 200 cm⁻¹ mode is strongly localized at the interface between the CdTe QDs and ZnTe cap layer. This phonon mode is attributed to the interface optical (IO) phonon. The analytically calculated value of the IO phonon energy using a dielectric continuum approach, assuming a spherical dot boundary, agrees well with the experimental value.     

Ti掺杂ZnTe体材料的优化光致发光光谱

针对光致发光光谱法研究ZnTe:Ti的困难,对包括激发能量、激发功率与激发光斑大小、水汽干扰和测量温度等实验条件进行了细致的优化.发现: (1)低于ZnTe禁带宽度的激发能量能给出相对强的光致发光光谱; (2) 水汽干扰既影响谱线的相对强度又增大谱线能量的测定误差; (3)相对强的激光聚焦有利于获得较好的光致发光光谱.可靠地观察到位于3903.5和3905.9cm^-1能量位置处的零声子光致发光谱线. 根据两谱线的能量间距和相对强度随 温度的变化关系,并借助于晶体场理论对四面体晶体场中 关键词： Ti掺杂 ZnTe 光致发光     

The luminescence of II–VI compounds with blende type structure under high excitation

The luminescence of ZnSe is investigated as a function of excitation intensity for temperatures between 5 K and 300 K. At low excitation we observe emission due to free and bound excitons and due to donor-acceptor pair recombination. At higher excitation, the emission is dominated by inelastic exciton-exciton and exciton-free carrier scattering at lower and higher temperatures, respectively. A “M-band” observed in ZnSe and ZnTe is tentatively ascribed to a biexciton decay. The biexciton binding energies are 2±1 meV for ZnSe and 1,5±1 meV for ZnTe.     

Photostimulated formation of sensitized anti-Stokes luminescence centers in AgCl(I) microcrystals

The low-temperature photostimulated activation of sensitized anti-Stokes luminescence in heterogeneous systems based on AgCl(I) microcrystals with adsorbed organic dye molecules and their aggregates is investigated. It is shown that the observed considerable (by more than an order of magnitude) enhancement of the intensity of this luminescence is caused by the formation of silver atoms and few-atom clusters on the surface of AgCl(I) microcrystals, which increase the efficiency of a two-quantum excitation of sensitized anti-Stokes luminescence by optical radiation in the range 630–730 nm with the flux density 10¹³–10¹⁵ quantum cm^?2s^?1. Analysis of all the experimental results indicates that the excitation mechanism of anti-Stokes luminescence is based on successive electron transfer or electron-excitation energy transfer from a dye molecule to an atomic-molecular dispersive silver center.     

Anomalous phonon wind effect on the lateral exciton migration in ultrathin quantum well CdTe/ZnTe

The effect of nonequilibrium acoustic phonon flux on the photoluminescence of an ultrathin quantum well CdTe/ZnTe upon its quasi-resonant excitation by a He-Ne laser was studied. It is found that the phonon flux generated by an external source affects the quantum well luminescence bandshape even at small lasing power and large (up to 1 cm) distance between the phonon generation zone and the quasi-resonant luminescence excitation zone. It is assumed that the phonon flux stimulates exciton in-plane (lateral) migration in the quantum well through the tunneling between the local potential minima accompanied by induced phonon emission.     

Thermal quenching of luminescence in erbium doped semiconductors

The nature of the temperature dependence of luminescence intensity from Er⁺ ions in GaInAsP, Si, InP, GaAs, AlGaAs, ZnTe, as observed by Favennecet al [1] has been examined in terms of a double exponential model. The smaller activation energy is found to be 58–100 meV, characteristic of a localized energy barrier at the Er⁺ centre while the higher activation energy is approximately 0.8E _g attributed to an Auger non-radiative process of carrier excitation into bands. This model has been found to describe the observed temperature dependences with reasonably good agreement.     

Time-resolved luminescence of ZnIn2S4 at low temperature

Summary A detailed study of the time-resolved luminescence of ZnIn₂S₄ after pulsed-laser excitation at 30 K is presented. Results are given on the dependence of the emitted PL intensity at different emission wavelengths on the excitation intensity, on the time-resolved spectra in the range (0÷10⁻¹) s of delay time with respect to excitation and on the time decay of the luminescence intensity for different emission wavelengths. The results are discussed in the framework of a simple model for the recombination processes at low temperatures yielding a good overall agreement with the experimental data. To speed up publication, the authors of this paper have agreed to not receive the proofs for correction.     

Effect of excitation level on the photoluminescence of barium thiogallate activated with europium and cerium ions

The photoluminescence of barium thiogallates doped with Eu²⁺, Ce³⁺, and Eu²⁺ + Ce³⁺ ions is studied over a wide range of excitation levels (10^–3–10⁶ W/cm²). Introduction of 3 at.% Eu and 3 at.% Ce instead of 5 at.% Eu into a BaGa₂S₄ matrix doubles the luminescence quantum yield of the phosphor. Doped BaGa₂S₄ exhibits a high linearity in its luminescence intensity as a function of excitation level (a constant efficiency) up to 2·10⁴ W/cm² for excitation pulse durations of 8 ns, which corresponds to cw pumping at a power density of about 5·10² W/cm² in terms of the concentration of excited ions. It is shown that using BaGa₂S₄:Eu,Ce along with the “yellow” phosphor of a Nichia NS6L083 LED may increase its color rendering index from 0.64 to 0.80 with no reduction in its luminous efficiency.     

Manifestation of outgoing resonance in stokes and anti-stokes spectra of ZnTe and ZnMgTe quantum wires

The spectra of resonant light scattering by ZnTe quantum wires have been measured at excitation energies of 2.18–2.72 eV. The quantum wires have been grown on Si(100) and GaAs(100) substrates by molecular beam epitaxy. The effect of outgoing resonance with the electron transition energy E ₀ on the intensity of phonon lines of the Stokes spectrum and on the intensity ratio of the Stokes and anti-Stokes spectral lines has been studied. The energy E ₀ has been determined in ZnTe and ZnMgTe quantum wires from the edge luminescence spectra.     

Photoluminescence associated with impurity clusters in phosphorous doped silicon

Photoluminescence measurements as a function of excitation level and temperature are presented for Si(P) containing 7.5×10¹⁷ P atomd/cm³. The luminescence in Si(P) for intermediate concentrations in the range from 1×10¹⁶ to 2×10¹⁸ cm^?3 is interpreted in terms of recombination of trapped carriers at clusters of impurities.     

Anti-stokes luminescence of solid AgCl<Subscript>0.95</Subscript>I<Subscript>0.05</Subscript> solutions

An anti-Stokes luminescence band with λ_max = 515 nm of microcrystals of solid AgCl_0.95I_0.05 solutions excited by a radiation flux of density 10¹³–10¹⁵ quanta/cm²·sec in the range 600–800 nm at 77 K was detected. It is shown that the intensity of this luminescence and the frequency of its excitation depend on the prior UV-irradiation of samples. Analysis of the stimulated-photoluminescence spectra and the anti-Stokes luminescence excitation spectra of the indicated microcrystals has shown that to the centers of anti-Stokes luminescence excitation correspond local levels in the forbidden band of the crystals. These states are apparently due to the atomic and molecular disperse silver particles that can be inherent in character or formed as a result of a low-temperature photochemical process. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 72, No. 6, pp. 738–742, November–December, 2005.     

The excitation mechanism of rare-earth ions in silicon nanocrystals

A detailed investigation on the excitation mechanisms of rare-earth (RE) ions introduced in Si nanocrystals (nc) is reported. Silicon nanocrystals were produced by high-dose 80-keV Si implantation in thermally grown SiO₂ followed by 1100 °C annealing for 1 h. Subsequently some of the samples were implanted by 300-keV Er, Yb, Nd, or Tm at doses in the range 2×10¹²–3×10¹⁵ /cm². The energy was chosen in such a way to locate the RE ions at the same depth where nanocrystals are. Finally an annealing at 900 °C for 5 min was performed in order to eliminate the implantation damage. These samples show intense room-temperature luminescence due to internal 4f shell transitions within the RE ions. For instance, luminescence at 1.54 μm and 0.98 μm is observed in Er-doped nc, at 0.98 μm in Yb-doped nc, at 0.92 μm in nc and two lines at 0.78 μm and 1.65 μm in Tm-doped nc. Furthermore, these signals are much more intense than those observed when RE ions are introduced in pure SiO₂ in the absence of nanocrystals, demonstrating the important role of nanocrystals in efficiently exciting the REs. It is shown that the intense nc-related luminescence at around 0.85 μm decreases with increasing RE concentration and the energy is preferentially transferred from excitons in the nc to the RE ions which, subsequently, emit radiatively. The exact mechanism of energy transfer has been studied in detail by excitation spectroscopy measurements and time-resolved photoluminescence. On the basis of the obtained results a plausible phenomenological model for the energy transfer mechanism emerges. The pumping laser generates excitons within the Si nanocrystals. Excitons confined in the nc can either give their energy to an intrinsic luminescent center emitting at around 0.85 μm nor pass this energy to the RE 4f shell, thus exciting the ion. The shape of the luminescence spectra suggests that excited rare-earth ions are not incorporated within the nanocrystals and the energy is transferred at a distance while they are embedded within SiO₂. Rare-earth excitation can quantitatively be described by an effective cross section σ_eff taking into account all the intermediate steps leading to excitation. We have directly measured σ_eff for Er in Si nc obtaining a value of ≈2×10⁻¹⁷ cm². This value is much higher than the cross section for excitation through direct photon absorption (8×10⁻²¹ cm²) demonstrating that this process is extremely efficient. Furthermore, the non-radiative decay processes typically limiting rare-earth luminescence in Si (namely back-transfer and Auger) are demonstrated to be absent in Si nc further improving the overall efficiency of the process. These data are reported and their implications. Received: 9 April 1999 / Accepted: 10 April 1999 / Published online: 2 June 1999     

Red Mn2+-luminescence in hexagonal aluminates

On UV excitation, a red luminescence band peaking at 690 nm is observed in Eu, Mn doped aluminate lattices containing large trivalent cations and high Mn²⁺-concentrations. The green Mn²⁺ luminescence band occuring at lower Mn²⁺ contents is quenched by the appearance of the red band. The excitation spectra of the red luminescence show that energy transfer takes place from the green Mn²⁺ centres which are located at tetrahedrally surrounded sites to the red emitting Mn²⁺ centres. The last-named are most probably Mn²⁺ ions surrounded octahedrally by oxygen ions. One of these oxygen ions is located at the crystallographic position normally occupied by the large trivalent cation. The dependence of the intensity of the red luminescence on the Mn²⁺ concentration in La-aluminate samples can be explained if it is assumed that the Mn²⁺ ions in tetrahedral sites (the green emitting ones) are all located near the oxygen ions at the large cation sites.