Electrical properties, photoconductivity, and photoluminescence of coarse-grained p-ZnTe

Electrical and photoelectric properties and also low-temperature photoluminescence of coarsegrained p-ZnTe with a resistivity of 3.3 × 10¹⁰Ω cm at 77 K have been studied. The coarse-grained ZnTe sample in the shape of a pipe was synthesized at 730°C from vapor phase in cocurrent flows of components. X-ray structure analysis showed that there are single-crystalline grains in both the [111] and [100] directions. In spite of nonequilibrium conditions of rapid crystallization, X-ray diffractometry measurements showed that the ZnTe crystallinity is good. The results of low-temperature photoluminescence confirm that there is an ordered distribution of impurities in the lattice. The set of impurities is limited to Cu, Ag, Li, and O. The photoconductivity and photoluminescence spectra indicate that there are poorly studied centers with levels deep m the band gap. In the regions with a low density of twins, there is observed a high quantum yield of photoluminescence, the absence of transitions related to complex defects, the presence of intrinsic emission with pronounced polaritonic structure, and manifestation of an ample structure of two-hole transitions that are absent in the crystals obtained under conditions of slow growth.     

Photoconductivity and luminescence of CuInSe2 single crystals at a high level of optical excitation

The luminance-current and spectral characteristics of photoluminescence of the CuInSe₂ single crystals are studied. The superlinear portion of the excitation-intensity dependence of photoconductivity at low excitation intensities in compensated p-CuInSe₂ crystals is explained on the basis of a recombination model. The emission band that peaked at 0.98 eV in the n-CuInSe₂ photoluminescence spectrum corresponds to radiative recombination of electrons at the donor level with a depth of 0.04 eV. The maximum in the band intensity corresponds to the energy gap between the trap level and the valence band.     

Optical properties of photochromatic sulfur-doped chlorosodalite

Synthetic.photochromic sulfo-chlorosodalite, 6(NaAlSiO₄) ·2 NaCl(S), has been thoroughly investigated by measurements of optical absorption, photo-luminescence and cathodoluminescence. Depending on the sulfur ion form and concentration, the doped sodalite exhibits either sensitive tene-brescence or photoluminescence with long wavelength UV excitation. The photo-induced color absorption peaks at 5260A at 300°K with absorption coefficient, Δα_max >200 cm⁻¹ . This is by far the highest photo-induced absorption observed for synthetic chlorosodalite. At 80°K, the peak position of the absorption does not show significant shift within instrumental accuracy. In photoluminescence, the emission spectra as well as the excitation spectra are studied at both 300 and 78°K. Four characteristic spectral bands (IR, blue, red, and a band with oscillation in wavelength) are observed. The oscillatory S₂ ^- ion emission band starting about 2.35 eV and extending to lower energy and the IR band peaked at 1.4 eV are most efficiently excited by 3660A (3.4 eV), whereas the blue luminescence peaked at 2.7 eV has an excitation threshold of 3.9 eV. The red band is often masked by the oscillatory band and can be observed by higher energy excitation. The red and blue bands are also observable in the cathodoluminescence measurements of the sulfur-doped samples but not the undoped samples. Correlating the absorption, luminescence, and excitation spectral results, a quantitative model is derived to interpret the nature and the role of sulfur ions in the photochromic chlorosodalite material.     

New photoluminescence lines in Vanadium doped GaAs grown by MOVPE

We examined the electrical and optical properties of vanadium-doped GaAs grown by metalorganic vapour phase epitaxy using vanadium tetrachloride (VCl₄) as a novel dopant source. Samples with various vanadium incorporations were investigated. All samples were n type. The electron concentration dependence on the VCl₄ flow rate was established. At 15 K, by comparison with undoped layers grown in the same conditions, photoluminescence spectra of V-doped exhibited three new emission bands: at 1.41, 1 and 0.72 eV. The 1 and 0.72 eV band emissions were attributed to V²⁺ and V³⁺ intracenter emission, while the 1.41 eV band was suggested to be a donor-bound transition. The identity of the donor is tentatively attributed to a donor complex that associates vanadium to an arsenic vacancy. From Hall effect as function of temperature, the donor ionisation energy was estimated to be about 102±5 meV.     

Luminescence properties of MgxZn1-xSe prepared by Mg diffusion

The photoluminescence of ZnSe doped with the isoelectronic substituent Mg with a simple diffusion procedure has been studied. It was found that Mg easily enters substitutionally on Zn-site to form Mg_xZn_1-xSe. The diffused samples therefore show a graded bandgap due to a composition gradient in the surface region. In contrast to ZnSe the photoluminescence spectrum of Mg_xZn_1-xSe is dominated by a near bandgap emission at all temperatures between 1.5 and 300 K. The bandgap shift compared with “pure” ZnSe is estimated from photoluminescence excitation spectra. The luminous efficiency of Mg_xZn_1-xSe makes the material promising for future applications as light emitting diodes with a well defined narrow emission band at a wavelength determined by the Mg content x.     

Photoluminescence Studies of CuInS2-CuInSe2 Alloy Crystals

Photoluminescence spectra are presented for single crystals of CuInS₂, CuInSe₂ and CuInS_2ySe_2-2y alloys. The PL spectrum of stoichiometric CuInSe₂ is dominated by free exciton emission of 9 meV FWHM at 1.03 eV, but structure at 090, 0.94 and 0.97 eV is observed due to transitions involving residual native defect states. For pure CuInS₂ broad deep luminescence bands are obtained that involve several deep native defect states, e.g. donors at 45 and 160 meV below the CBE and acceptors at 85 meV above the VBE. These defects persist in S-rich CuInS₂/CuInSe₂ alloys, but excitonic emission is observed, in addition to the deep luminescence. Surprisingly CuInS_2ySe_2-2y crystals at y ≳ l are totally dominated by free exciton emission. This result shows that an increase of at least 20% over the bandgap of CuInSe₂ and excellent crystal quality can be achieved by partial substitution of Se by S.     

Behaviour of erbium implanted in InP

Erbium impurities were implanted in indium phosphide. The 2 K, 77 K and 300 K photoluminescence spectra show, after annealing at high temperature, the main erbium emission centered at 1.536μm. The variation of this Er-peak luminescence is studied as a function of the implanted dose, the annealing temperature and the annealing duration.     

Effect of nonstoichiometry and doping on the photoconductivity spectra of GeSe layered crystals

The polarization photoconductivity spectra of Bi-doped nonstoichiometric GeSe layered crystals grown by static sublimation were investigated. Two strongly polarized maxima at the photon energies hν_max = 1.35 eV (E ∥ a) and 1.44 eV (E∥ b) due to the V ₁ ^V → V ₁ ^c and Δ ₂ ^v → Δ ₁ ^c optical transitions, respectively, were found in the spectra of nominally undoped GeSe crystals near the intrinsic absorption edge at 293 K. In the low-temperature region, an exciton photoconductivity band peaked at hν_max=1.32 eV, which is due to exciton dissociation at the cation vacancies, was revealed. With an increase in excess Se in crystals, a sharp increase in the intensity of the exciton peak in the photoconductivity spectra was observed. It is shown that doping of GeSe crystals with donor Bi impurity is an effective tool of the control of their electrical and photoelectric properties. Although introduction of Bi into germanium monoselenide does not lead to the conductivity conversion from the p to n type, a sharp increase in the resistivity is observed, the crystals become photosensitive, and a strong impurity band peaked at 1.11 eV arises in the photoconductivity spectra.     

Optical spectroscopy of excitonic states in CuInSe2

Optical properties of structurally perfect CuInSe₂ single crystals were studied in the temperature range of 4.2–300 K with the use of photoluminescence, optical absorption, optical reflection, and wavelength-modulated optical reflection (WMOR). The intense lines of free excitons A (～1.0414 eV) and B (～1.0449 eV) with a half-width of ～0.7 meV at 4.2 K are found to be related to two extrema of valence band split by a crystal field. The excitons emission line C (～1.2779 eV) in WMOR spectra are related to a lower valence band split-off by spin-orbit interaction. Within the context of the quasi-cubic Hopfield model, the parameters of valence band splitting Δ_CF=5.2 meV and Δ_SO=234.7 meV defined by the crystal and spin-orbit interaction, respectively, are calculated. In the region of the fundamental absorption edge, the lines of bound excitons are found with a half-width ～0.3 meV that is indicative of a high quality of grown CuInSe₂ crystals.     

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.     

氢化对ZnO发光性质的影响

通过低温光致发光(PL)谱研究氢化对ZnO发光性质的影响.氢通过一个直流等离子体发生装置引入到ZnO晶体.研究发现氢的引入影响了束缚激子的相对发光强度,特别是氢化以后I4峰(3.363eV)的强度增加.与未氢化样品相比较,氢化样品显示出不同的温度依赖PL谱.在4.2K温度下,测量了氢化样品表面以下不同深度处的PL谱.研究发现I4峰的强度和I4峰与I8峰强度比随深度变化而变化,说明了在引入的氢和浅施主之间的直接联系.一般而言,氢化会增强带边发射并钝化绿光发射.     

UV and IR emission intensity in ZnO films,nanorods, and bulk single crystals doped with Er and additionally introduced impurities

For ZnO films, nanorods, and bulk single crystals doped with Er⁺ ions, it is shown that the effect of codopants introduced into the cation and ion sublattices and the observation of a high-intensity emission band at the wavelength λ_max = 1535 nm are defined by the local environment of the Er⁺ ion. Doping of the films and single crystals with Er⁺ ions by diffusion brings about an infrared (IR) emission band with a low intensity because of an inadequate concentration of impurity ions. The emission intensity of this band can be raised by introducing additional Ag, Au, or N⁺ impurities into the ZnO films. The UV-emission intensity of the Er-doped films and single crystals at λ_max = 368–372 nm is identical to that of the undoped films. ZnO nanorods doped with Er only or together with Al or Ga codopants exhibit only one IR band (at λ_max = 1535 nm), whose intensity decreases upon the introduction of codopants. Doping of the nanorods with the N⁺ gaseous impurity during growth (930 < T < 960°C) and then with the Er⁺ impurity by diffusion does not yield a substantial increase in the IR-emission intensity compared to the that of the corresponding band for nanorods not doped with the N⁺ impurity. In the Er-doped nanorods, whose photoluminescence spectra exhibit a high-intensity band at λ_max = 1535 nm, the UV emission band at λ_max = 372 nm is practically lacking.     

Photoluminescence of anodized layers of CdSiAs2

CdSiAs₂ belongs to the II-IV-V₂ group of chalcopyrite semiconductors with a direct band gap of 1.51 eV at T=300 K. In this paper we investigate the spectral dependence of the steady-state photoluminescence of CdSiAs₂ anodized layers. These layers were fabricated by electrochemical anodization of unoriented p-type CdSiAs₂ wafers in an solution of HF in ethanol. It is found that a broad photoluminescence band with a maximum at the photon energy ℏω=1.82 eV at 300 K arises. This band lies deep in the fundamental absorption region of CdSiAs₂ crystals. The dependence of the parameters of the photoluminescence spectra of anodized Si, GaAs, and CdSiAs₂ layers is discussed. Fiz. Tekh. Poluprovodn. 31, 313–314 (February 1997)     

Electroluminescent AuGaP diodes

Electroluminescence spectra under reverse bias conditions of Au-(n)GaP junctions are reported. Light emission is mainly due to temperature independent brehmsstrahlung radiation of hot electrons in the field of ionized impurities. Emission bands associated with carrier recombination via Si and O impurities are also present. The analysis of the conduction mechanism shows that the breakdown is caused by avalanche multiplication associated with formation of low resistance microplasma filaments, which give negative resistance effect at 77°K.     

Growth and properties of VPE GaP for green LEDs

The PH₃-HCl-Ga-H₂ technique for VPE growth of GaP is described. The influence of various growth parameters, including substrate temperature, orientation, and PH₃ flow rate on morphology and growth rate are described. For both VPE and LPE nitrogen doping is known to be a major factor in obtaining high green luminescence efficiency. The major emphasis of this paper is an examination of the effect of nitrogen concentration in the range less than 10¹⁹ cm⁻³ (using the Lightowlers correction factor) on the growth process and materials properties, such as defect structure, photoluminescence spectra (at 300 and 77K) and photoluminescence intensity and lifetime. The LED device performance (B/J and efficiency) is used as the final test of material quality. Nitrogen is found to be incorporated far in excess of the solubility limit, and the solid gas distribution coefficient for nitrogen is found to increase rapidly with decreasing temperature below 840°C . The optimum nitrogen concentration for high diode efficacy, photoluminescence intensity, and lifetime is found to be approximately 5 × 10¹⁸ cm⁻³, where diodes fabricated by Zn diffusion into the VPE GaP have efficiencies at a current density of 10 A/cm² of 0.1%, comparable to the state-of-the-art in the more widely used grown p-n junctions using LPE.     

The effect of composition on the properties and defect structure of the CdS-Ga2S3 solid solution

The effect of excess CdS in gallium thiogallate (CdGa₂S₄) on the spectrum of defect states in its band gap is investigated. Comparative investigations of Raman, photoluminescence, and cathodoluminescence spectra, as well as of the kinetics of photoconductivity in starting crystals (type A) and crystals obtained in conditions with excess CdS (type B), were carried out. It was found that the main types of defects in type A crystals are Cd and S vacancies, antisite Ga_Cd donor defects, as well as I_S defects, which are caused by the incorporation of carrier gas. In type B crystals, along with I_S, the main defects are antisite acceptor defects Cd_Ga as well as interstitial atoms Cd_i and S_i. It is demonstrated that the emission characteristics of the crystals investigated are determined by associations of the above defects. A luminescence band peaked at hν_m=0.971 eV was observed for the first time. This band is related to intracenter transitions in the d shell of Cd of the Cd_Ga defect, which is split due to the effects of the crystalline field and spin-orbit interaction, in the presence of S_i.     

Molecular beam epitaxy of CdTe and Hg1-xCdxTe ON GaAs (100)

Using the molecular beam epitaxial (MBE) technique, CdTe and Hg_1-xCd_xTe have been grown on Cr-doped GaAs (100) sub-strates. A single effusion cell charged with polycrystal-line CdTe is used for the growth of CdTe films. The CdTe films grown at 200 °C with a growth rate of ~ 2 μm/hr show both streaked and “Kikuchi” patterns, indicating single crystalline CdTe films are smoothly grown on the GaAs sub-strates. A sharp emission peak is observed at near band-edge (7865 Å, 1.577 eV) in the photoluminescence spectrum at 77 K. For the growth of Hg_1-xCd_xTe films, separate sources of HgTe, Cd and Te are used. Hg_0.6Cd_0.4Te films are grown at 50 °C with a growth rate of 1.7 μm/hr. The surfaces are mirror-smooth and the interfaces between the films and the substrates are very flat and smooth. As-grown Hg_0.6Cd_0.4Te films are p-type and converted into n-type by annealing in Hg pressure. Carrier concentration and Hall mobility of an annealed Hg_0.6Cd_0.4Te film are 1 × 10¹⁷ cm^?3 and 1000 cm²/V-sec at 77 K, respectively.     

Light emission from crystalline silicon and amorphous silicon oxide (SiOx) nanoparticles

Bright orange-red light emission was observed from single crystal silicon nanoparticles and silicon oxide (SiO_x) nanoparticles. The emission peak was recorded at about 1.5 eV both at room temperature and 77K. Varying the mean silicon particle size, we observed no effect of particle diameter on the emission wavelength. Amorphous silicon oxide (SiO_x) nanoparticles also showed essentially the same emission spectrum as the crystalline particles. The absence of change in the photoluminescence (PL) spectrum with variations in particle size and crystallinity indicates that quantum confinement is not the controlling PL mechanism. An examination of the hydrogen content with relation to the PL intensity showed no direct correlation; however, all samples did contain some hydrogen, so its effect on PL cannot be ruled out. To test for the presence of photoluminescent siloxene on the surface of the particles, nitric acid was applied; a violent reaction occurred with the silicon particles, while the SiO_x particles showed no reaction. Taken in conjunction with the emission data, these experiments demonstrate that the PL of the SiO_x is also not dependant on siloxene. Evidence points to an amorphous coating as the source of photoluminescence.     

Luminescence and electrical properties of MgxZn1-xTe alloys

The luminescence and transport properties of high-quality undoped and phosphorus-doped Zn1-xMgxTe alloys (x ≤ 0.50) have been investigated. At 4.2 and 300 K, the photoluminescence of unintentionally doped crystals is dominated by near-band-edge recombination mechanisms. In phosphorus-doped samples, the luminescence spectra exhibit free-bound transitions involving shallow acceptor centers (phosphorus in tellurium sites). Whenxand/orTincreases, a broad luminescence band also appears at lower energy (1.7-1.9 eV) which decreases the near-band-edge luminescence efficiency. This low-energy band could be due to phosphorus atoms occupying other sites in the lattice (metal sites, for example) and then acting like deep recombination centers. Such a behavior could also explain the electrical properties of phosphorus-doped crystals. The introduction of phosphorus leads to an increase of the free hole concentration p as compared to undoped crystals but all phosphorus atoms do not behave as shallow acceptors; an increasing fraction of these atoms would act like donors in sites other than the tellurium sites as x increases. In undoped materials, p decreases drastically when we add more magnesium and the hole mobility remains approximately constant. We think that this effect is due to compensation by residual donor impurities. On these undoped samples, light-emitting diodes (LED's) have been successfully obtained for the first time. Their quantum efficiency is reasonable if we take into account the low carrier concentration of the material. The emission peak position is 5390 Å for Zn0.9Mg0.1Te instead of 5550 Å for undoped ZnTe.     

Quantum-confinement effect in silicon-on-insulator films implanted with high doses of hydrogen ions

280-nm-thick silicon-on-insulator films are implanted with high doses of hydrogen with the energy 24 keV and the dose 5 × 10¹⁷ cm^?2. Peaks corresponding to optical phonons localized in the silicon nanocrystals 1.9?C2.5 nm in size are observed in the Raman spectra. The fraction of the nanocrystal phase is ??10%. A photoluminescence band with a peak at about 1.62 eV is detected. The intensity of the 1.62 eV band nonmonotonically depends on the measurement temperature in the range from 88 to 300 K. An increase in the radiative recombination intensity at temperatures <150 K is interpreted in the context of a two-level model for the energy of strongly localized electrons and holes. The activation energy of photoluminescence enhancement is 12.4 meV and corresponds to the energy of splitting of the excited state of charge carriers localized in the silicon nanocrystals.