Optical an photoelectric properties odf ZnSe:Ti crystals

The photoconductivity and photoluminescence spectra of ZnSe:Ti crystals in the visible and infrared spectral regions are studied. It is established that the high-temperature impurity-induced photoconductivity of ZnSe:Ti crystals is defined by the optical transitions of electrons from the ³ A ₂(F) ground state to highenergy excited states, with the subsequent thermal transitions of electrons to the conduction band. The efficient excitation of intracenter luminescence of ZnSe:Ti crystals is achieved by light from the region of the intrinsic absorption of Ti²⁺ ions.     

Study of the impurity photoconductivity and luminescence in ZnSe:Ni crystals in the visible spectral region

The photoconductivity and photoluminescence spectra of ZnSe:Ni crystals in the visible spectral region are studied. It is established that the high-temperature impurity photoconductivity of ZnSe:Ni crystals is controlled by the optical transitions of electrons from the ground state ³ T ₁(F) to high-energy excited states, with subsequent thermally activated transitions of electrons to the conduction band. A photoconductivity band associated with the photoionization of Ni impurities is revealed. The intracenter luminescence of ZnSe:Ni crystals is efficiently excited with light corresponding to the intrinsic absorption region of Ni²⁺ ions.     

硫化锌基底可见-红外多波段的光学薄膜

由于硫化锌(ZnS)晶体透光区域较宽,常被用于多波段可见与红外光学系统中。介绍了在ZnS基底上制备的多波段光学薄膜,在400~700nm的可见波段及8~12μm的中长波红外波段实现高透射率,在1064nm及1540nm两点实现高反射率,其入射角度为45°。选取了ZnS和YbF3作为高低折射率材料,设计并通过软件优化出合理的膜系,采用电子枪离子辅助沉积系统进行镀制。该薄膜能够承受雨淋、盐雾、高低温等环境测试,满足使用要求。这对于红外光学系统的应用具有极其重要的意义。     

Effect of iron impurities on the photoluminescence and photoconductivity of ZnSe crystals in the visible spectral region

The photoconductivity and photoluminescence of ZnSe:Fe crystals in the visible spectra region are studied. The scheme of optical transitions within Fe²⁺ impurity centers is established. It is shown that the high-temperature photoconductivity of ZnSe:Fe crystals is controlled by optical transitions of electrons from the ⁵ E(F) ground state to the higher levels of excited states of Fe²⁺ ions, with subsequent thermal activation of the electrons to the conduction band. Efficient excitation of intracenter luminescence of ZnSe:Fe crystals is attained with light corresponding to the region of intrinsic absorption in Fe²⁺ ions.     

The Effect of Thermal Treatment on the Morphology and Charge Carrier Dynamics in a Polythiophene–Fullerene Bulk Heterojunction

The influence of various thermal treatment steps on the morphology and the photoconductive properties of a non‐contacted, 50 nm thick blend (50:50 wt.‐%) of [6,6]‐phenyl C₆₁‐butyric acid methyl ester (PCBM) and poly(3‐hexyl thiophene) (P3HT) spin‐coated from chloroform has been studied using transmission electron microscopy (TEM) and the electrodeless time‐resolved microwave conductivity technique. After annealing the film for 5 min at 80 °C, TEM images show the formation of crystalline fibrils of P3HT due to a more ordered packing of the polymer chains. The thermal treatment results in a large increase of the photoconductivity, due to an enhancement of the hole mobility in these crystalline P3HT domains from 0.0056 cm² V^–1 s ^–1 for the non‐annealed sample to 0.044 cm² V^–1 s ^–1 for the sample annealed at 80 °C. In contrast, the temporal shape of the photoconductivity, with typical decay half‐times, τ_1/2, of 1 μs for the lowest excitation intensities, is unaffected by the temperature treatment. Further annealing of the sample at 130 °C results in the formation of three different substructures within the heterojunction: a PCBM:P3HT blend with PCBM‐rich clusters, a region depleted of PCBM, and large PCBM single crystals. Only a minor increase in the amplitude, but a tenfold rise of the decay time of the photoconductivity, is observed. This is explained by the formation of PCBM‐rich clusters and large PCBM single crystals, resulting in an increased diffusional escape probability for mobile charge carriers and hence reduced recombination.     

Structural studies of ZnS:Cu (5 at %) nanocomposites in porous Al<Subscript>2</Subscript>O<Subscript>3</Subscript> of different thicknesses

We present EXAFS, XANES, and X-ray diffraction data on nanoscale ZnS:Cu (5 at %) structures fabricated by the thermal deposition of a ZnS and Cu powder mixture in porous anodic alumina matrices with a pore diameter of 80 nm and thicknesses of 1, 3, and 5 μm. The results obtained are compared with data on ZnS:Cu films deposited onto a polycor surface. According to X-ray diffraction data, the samples contain copper and zinc compounds with sulfur (Cu₂S and ZnS, respectively); the ZnS compound is in the cubic (sphalerite) and hexagonal (wurtzite) modifications. EXAFS and XANES studies at the K absorption edges of zinc and copper showed that, in samples deposited onto polycor and alumina with thicknesses of 3 and 5 μm, most copper atoms form the Cu₂S compound, while, in the sample deposited onto a 1-μm-thick alumina layer, copper atoms form metallic particles on the sample surface. Copper crystals affect the Zn–S interatomic distance in the sample with a 1-μm-thick porous Al₂O₃ layer; this distance is smaller than in the other samples.     

Photoconductivity and luminescence in GaSe crystals at high levels of optical excitation

The photoconductivity and luminescence of GaSe layered crystals at high levels of optical excitation are studied experimentally. The specific features observed in the photoconductivity and photoluminescence spectra are controlled by the nonlinear optical absorption in the region of excitonic resonance.     

Photoluminescence in CuGaSe2

The photoluminescence spectra of CuGaSe₂ have been investigated at 76 K in the range 0.65 < λ < 1.1μ. The crystals were grown by iodine transport, and had p-type conductivity. The photoexcitation was provided by the single-line outputs of a CW Ar⁺ laser, and the luminescence spectra were analyzed using a m grating monochromator.The luminescence observed consisted of a rather sharp structure near the energy gap, plus two broad peaks at considerably smaller energies. The broad peaks, centered around 1.52 and 1.30 eV respectively, are attributed to transitions involving deep localized states. The sharper structure near the energy gap is resolved into two peaks, one at 1.72 eV and another at 1.68 eV. This structure is explained by radiative recombination of free excitons (peak at 1.72 eV), and by recombination of free electrons with bound holes (peak at 1.68 eV).A thermal treatment done on the CuGaSe₂ crystals changed both their electrical properties and their luminescence spectra. These changes produced by sample treatment, plus measurements of the excitation power dependence and the polarization of the luminescent radiation, are in agreement with the interpretation presented.     

Energy states of a Cr2+ ion in ZnSe crystals

ZnSe:Cr single crystals fabricated by diffusion doping are studied. The optical density spectra in the energy range 0.4–3 eV are investigated. The chromium concentration in the investigated crystals is determined by the absorption-edge shift. The energy states of the Cr²⁺ ion in ZnSe crystals are calculated. The nature of optical transitions determining the optical properties of ZnSe:Cr single crystals in the visible and IR spectral regions is established. It is shown that effective excitation of the IR luminescence of ZnSe:Cr crystals is implemented by light from the fundamental absorption region of Cr²⁺ ions.     

InGaAs/GaAs quantum dot heterostructures for 3–5 μm IR detectors

Specific features in the formation of InAs quantum dots (QD) by MOCVD were studied in relation to the growing time or equivalent thickness of the InAs layer. TEM and photoluminescence studies have shown that, as the growing time of QDs in a GaAs matrix becomes longer, both the size and shape of the QDs are modified; namely, the aspect ratio increases. Selectively doped multilayer InGaAs/GaAs QD structures were fabricated, and photoconductivity in the IR range was studied for lateral and vertical electron transport. Under a normal incidence of light, intraband photoconductivity in the mid-IR range, 2.5–5 μm, was observed at temperatures of up to 110 K.     

Optical switching in thin film electroluminescent devices with inherent memory characteristics

Quantitative data is presented concerning the optical process of writing and erasing of memory electroluminescent (EL) devices. The switching process is dependent on the energy of the exciting photons and the energy required varies from approximately 10 µJ/cm²at 325 nm (associated with bandgap photoconductivity)to about 75 mJ/cm²at 515 nm (associated with impurity photoconductivity). Experimental evidence is presented to show that necessary and sufficient conditions for EL cell erasure are the photoconductive discharging of the insulator-ZnS charge and the decay of the bulk excitation of the ZnS:Mn film. The technique of photoinduced detrapping of the stored space charge is shown to lead to the determination of midgap as the approximate location in energy of the deep traps in one of our sample films.     

Interband emission of cadmium thiogallate

The edge cathodoluminescence of CdGa₂S₄ single crystals was studied at a temperature of 80 K. The true emission spectrum was obtained with allowance made for self-absorption. New intense lines related to band-to-band transitions and recombination via a deep acceptor level were observed. The results obtained are interpreted using the data on photoconductivity and fundamental absorption of this compound.     

Analytical Expression of Relaxation Luminescence in ZnS：Er^3＋ Thin Films

The relaxation luminescence of ZnS:Er^3 thin films is studied with luminescence dynamics model.The excitation and emission processes of Er^3 in ZnS host are described through the resonant energy transfer method.Taking the energy storing effect of the traps into account,an expression is obtained by using the convolution formula,which may describe luminescence decay process containing the multiple relaxation luminescence peaks.The experimental results confirm that the relaxation characteristics of the electroluminescence are related to the carriers captured by the bulk traps.     

Increase in quantum efficiency of IR emission in elastically strained narrow-gap semiconductors

The rate of radiative band-to-band recombination in elastically strained direct-band narrow-gap semiconductors increases owing to the valence band transformation. At the same time, the rate of nonradiative band-to-band transitions (Auger recombination) decreases dramatically. As a result, the quantum efficiency of IR emission in the range of band-to-band transitions can be essentially raised and, as calculation shows, tends to limiting values close to unity. Experimental data were obtained for InSb crystals under strong excitation.     

Absorption, luminescence excitation, and infrared transmittance spectra of ZnS(O)-ZnSe(O) crystals in the context of the band anticrossing theory

The paper reports the results of optical studies of the absorption, luminescence excitation, and infrared transmittance spectra for the set of ZnS(O)-ZnSe(O) alloys with highly inconsistent properties of anions. It is shown that the band anticrossing theory applied to the alloys provides a general interpretation of their specific optical properties not properly understood previously. A band model of transitions with absorption in a complex multiband formed due to oxygen is presented. The specific features of the absorption and luminescence excitation spectra are interpreted to gain an insight into the distribution and states of oxygen in the crystals. The effect of oxygen on the transmittance band of the ZnS-ZnSe alloys in the near-infrared spectral region is considered. A new approach to the interpretation of the infrared absorption bands associated with oxygen is developed. The calculation algorithms that provide a means for determining the spectral position of the bands in relation to the dissolved oxygen content are suggested.     

Spectral properties and energy transfer in Er3+/Yb3+ co-doped LiYF4 crystal

Laser crystals of LiYF4 (LYF) singly doped with Er3+ in 2.0% and co-doped with Er3+/Yb3+ in about 2.0%/1.0% molar fraction in the raw composition are grown by a vertical Bridgman method. X-ray diffraction (XRD), absorption spectra, fluorescence spectra and decay curves are measured to investigate the structural and luminescent properties of the crystals. Compared with the Er3+ singly doped sample, obviously enhanced emission at 1.5 μm wavelength and green and red up-conversion emissions from Er3+/Yb3+ co-doped crystal are observed under the excitation of 980 nm laser diode. Meanwhile, the emission at 2.7 μm wavelength from Er3+ singly doped crystal is reduced. The fluorescence decay time ranging from 18.60 ms for Er3+ singly doped crystal to 23.01 ms for Er3+/Yb3+ co-doped crystal depends on the ionic concentration. The luminescent mechanisms for the Er3+/Yb3+ co-doped crystals are analyzed, and the possible energy transfer processes from Yb3+ to Er3+ are proposed.     

Blue electroluminescence in reverse-biased ZnS(Zn,Al) diodes

Diodes fabricated from ZnS crystals annealed in molten zinc containing 2% aluminium have been found to yield bright blue electroluminescence under reverse-bias at room temperature. Study of the electroluminescence and cathodoluminescence spectra for these diodes together with electron microprobe analysis and measurements of their current-voltage characteristics has led to an explanation of the blue electroluminescence excitation and emission mechanisms. The surface (～ 800 μm) of the ZnS(An,Al) crystal was found to be aluminium-rich. The aluminium provided additional donors to give a low-resistivity region. This region contained ZnAl₂S₄ phases in additional to the V_ZnAl complex defects that were thought to be responsible for the blue electroluminescence emission.     

Long-term photoconductivity decay in n-InGaAs/GaAs heterostructures with coupled quantum wells under band-to-band excitation

The photoconductivity kinetics in n-In _x Ga_{1 ? x} As/GaAs heterostructures with double-coupled quantum wells and δ doping of one of them has been experimentally studied under interband excitation. It is shown that the long-term photoconductivity decay observed in the temperature range from 10 to 70 K is due to the chaotic potential associated with composition fluctuations in quantum-well layers.     

Absorption and photoionization of the donor level in CdF2 semiconductor crystals

A model of strong vibronic interaction is proposed to interpret the specific features of infrared absorption and photoionization in CdF₂ semiconductor crystals. The model takes into account the polaronic nature of the conductivity in these crystals and the profound configuration shift of the free and bound polaron states. It is shown that the intense infrared absorption band in the crystals is not due to the transitions of charge carriers from hydrogen-like donor levels to the conduction band, but is caused by the phonon replicas of intracenter transitions. The low-temperature photoconductivity (in the temperature range 0–70 K) is a result of tunneling transitions between the phonon states of bound and free polarons, since these states are separated by rather high potential barriers. Overcoming the barriers in both directions is responsible for equilibration in the polaron subsystem upon the photoexcitation of charge carriers. The tunneling character of this process is responsible for the slight variation in the equilibration time in the above-indicated temperature range.