Luminescence of color centers formed in alkali-earth-doped yttrium orthoaluminate crystals

Alkali-earth-doped yttrium orthoaluminate crystals grown in a reducing atmosphere are found to show bright photoluminescence (PL) in visible wavelength regions under the excitation by UV light source. From the results of transmission, PL, PL excitation and time-resolved PL spectra for the samples with different types and concentrations of dopants and the comparison to the results for the samples grown under different conditions, the origin of principal emission is determined to be color centers stabilized by heterovalent ions. The observed fast lifetime and high quantum yield of the luminescence can be explained by dipole-allowed transition between the levels localized in a vacancy. Comparing the effects on the optical properties from several types of dopant ions and taking influence from the UV irradiation into account, a model for the structure of emission centers is proposed.     

Quantum dynamics of phonon assisted emission of carriers at deep level centers

A formalism is proposed to investigate quantum dynamics of localized states involving highly non-adiabatic time-evolution of electron-lattice systems. The effect of electron itinerancy is projected onto the dynamics of local variables through an integral kernel of Volterra's integral equation. The method is applied to the problem of thermal emission of carriers at deep level centers in semiconductors. It is shown that the real situation is in the adiabatic limit, and the probability of thermal emission of the trapped carriers is one per a single lattice oscillation, if the amplitude of the oscillation exceeds a critical value but zero if not.     

Charge compensation and the luminescence of Cr3+ in KMgF3

Studies of the photoluminescence spectra of Cr³⁺ ions in KMgF₃ crystals co-doped with Cr³⁺ and Ni²⁺ ions are reported. Several crystal field sites are identified by the different R-line spectra due to the ² E⁴ A ₂ transition and broadband luminescences associated with the ⁴ T ₂⁴ A ₂ transitions. Cr³⁺ ions substituting without local charge compensation on the octahedral Mg²⁺ site give rise to a low temperature R line in photoluminescence at =702.3 nm with a radiative decaytime of 3 ms at T=14 K. At T=300 K this isotropic centre gives rise to an unpolarized broadband ⁴ T ₂⁴ A ₂ emission, which results from the thermal occupancy of an excited ⁴ T ₂ state just above the ² E level which, at lower temperature, gives rise to emission in the R-line. Other crystal field sites are due to some Cr³⁺ ions having Mg²⁺ or K⁺ vacancies in nearest-neighbour positions, these vacancies being required to maintain charge neutrality in doped fluoride perovskites. The Cr³⁺–K⁺ vacancy complex results in the centre having trigonal symmetry, and low temperature, photoluminescence via R ₁ and R ₂ lines at 716.8 nm and 716.0 nm, respectively. Finally, Cr³⁺ ions having a nearest neighbour Mg²⁺ vacancy have tetragonal symmetry, experiencing weak crystal fields. In consequence, the ⁴ T ₂ level lies below ² E and the photoluminescence spectrum at low temperature takes the form of a polarized broad ⁴ T ₂⁴ A ₂ band with peak at 760 nm and radiative decaytime of 54 s.     

Band gap states of V and Cr in 6H-silicon carbide

Ti, V and Cr in n-type 6H-SiC were investigated by radiotracer deep level transient spectroscopy (DLTS). Doping with the radioactive isotopes ⁴⁸V and ⁵¹Cr was done by recoil implantation followed by annealing (1600 K). Repeated DLTS measurements during the elemental transmutation of these isotopes to ⁴⁸Ti and ⁵¹V respectively revealed the corresponding concentration changes of band gap states. Thus, three levels were identified in the band gap: a Cr level at 0.54 eV and two V levels at 0.71 and 0.75 eV below the conduction band edge. There are no deep levels of Ti in the upper part of the band gap. Received: 28 April 1997/Accepted: 16 May 1997     

Effect of Tunneling Current on Schottky Barrier Height in ZnO Varistors at Low Temperature

On the basis of the Schottky barrier and thermionic emission models, the temperature dependence of barrier height in ZnO varistors is investigated by the I - V characteristics in a wide temperature range from 93 K to 373 K. The obtained barrier height decreases with reducing temperature, which is ascribed to the contribution of tunneling current in measured current. From the proposed equivalent circuit, it is suggested that two current components coexist. One is thermionic emission current, which reflects the thermionic emission barrier height. The other is tunneling current, which appears even at low voltage, especially in low temperature ranges, and thus makes the barrier height obtained from measured current vary with temperature.     

Efficient VUV sensitization of Eu3+ emission by Tb3+ in potassium rare‐earth double phosphate

The luminescence properties of K₃Tb(PO₄)₂ activated by Eu³⁺ were studied at excitation over the 120–300 nm wavelength range. It is demonstrated that Tb³⁺ ions, exhibiting a strong absorption band in the vacuum‐ultraviolet (VUV), can provide efficient sensitisation of Eu³⁺ emission in this wave length range, giving rise to intense red luminescence at 150 nm excitation. A proof is given for the concept of VUV sensitisation enabling the engineering of luminescence materials with enhanced conversion efficiency of VUV radiation into visible light. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photoluminescence-spectra study of yttria-stabilized zirconia

Spectral analyses of photoluminescence of yttriastabilized zirconia single crystals are carried out in the temperature range from 90 K to 330 K. Over the whole temperature range the emission spectrum can be decomposed into two broad bands. The main band is centred in the yellow-orange region of the spectrum and the secondary in the green region. The temperature dependence of the emission spectrum is tentatively analyzed in terms of the first three moments of the two band shapes. In addition, thermal quenching energy is determined to be 0.10 eV for both the yellow-orange and green bands. Results are discussed on basis of the major defects, oxygen vacancies and complexes formed by associating with the Y³⁺ ions.     

Electronic structure of light emitting centers in Er doped Si

Ab-initio calculations are carried out for the Er-related electrically active centers in Si. Our proposed microscopic model is consistent with photoluminescence measurements on Si:Er and Si:Er:O samples. For isolated Er, the tetrahedral interstitial site is the stable configuration, being related to the photoluminescence lines in Si:Er. Several configurations containing oxygen and fluorine atoms, surrounding the Er impurities, are proposed to simulate the effects of co-implantation. The results suggest that six oxygen atoms around substitutional Er can stabilize the center, which can be related to the strong photoluminescence lines in Si:Er:O samples. On the other hand, no configuration containing fluorine atoms could explain the stronger photoluminescence lines resulting from fluorine co-implantation. Received: 9 September 2002 / Accepted: 12 September 2002 / Published online: 17 December 2002 RID="*" ID="*"Corresponding author. Fax: +55-11/3091-5585, E-mail: jjusto@lme.usp.br     

Hydrogen-related donor in silicon crystals grown in a hydrogen atmosphere

Neutron transmutation doped (NTD) silicon crystals grown in a hydrogen atmosphere have been investigated by infrared absorption spectroscopy at a low temperature (10 K). An effective-mass-like donor state HD^0/+ has been found at 110.8 meV below the conduction band bottom after rapid thermal annealing (RTA). The HD^0/+ formation mechanism after NTD and RTA is briefly discussed, and tentatively attributed to H atoms present in the vicinity of some residual irradiation defects, like a complex of a H atom and a H-saturated vacancy.     

Doping Defects in Two-Dimensional Holographic Photonic Crystals Using a Continuous-Wave Visible Laser

It is demonstrated that defects of any shape or size can be doped in holographic photonic crystals using a cw visible laser and spherical/cylindrical lens. Defects with different sizes at any depth in the material can be obtained by controlling the position of the foca/point of the lens and exposure value. We facilitate the implementation of sub-wavelength arbitrary point or line defects in large-size 2D holographic photonic crystals.     

Photoluminescence study of acceptor-carbon complexes in irradiated silicon: Aluminum-related defects

The paper reports photoluminescence studies on two defects, All at 0.836 eV and Al2 at 0.886 eV, created by aluminum complexing in irradiated silicon after thermal annealing at around 300 °C. The optical data include perturbation spectroscopy (uniaxial stress and magnetic field) revealing rhombic I (Al1) or monoclinic I (Al2) site symmetry of the defects. For Al a carbon isotope shift in the no-phonon line is detected. These and other data are discussed in conjunction with Ga- or B-related defects exhibiting similar optical features.     

Screening due to non-equilibrium carriers in presence of deep repulsive traps

The characteristics of screening length due to non-equilibrium electrons at low lattice temperatures are investigated theoretically, when the lifetime of the carriers is controlled by deep repulsive traps. The calculations have been made covering a wide range of the electric field, starting from a weakly heated carrier ensemble up to the onset of impurity breakdown. Apart from becoming field dependent, the screening length now shows a rather complicated dependence upon the lattice temperature. The numerical results for high-purity covalent semiconductors like Ge and Si show that the characteristics of the screening length are now significantly different from what one obtains for an ensemble of carriers that is in thermodynamic equilibrium with the lattice atoms.     

Influence of Mg doping on the behaviour of polaronic light‐induced absorption in LiNbO3

Transient light‐induced absorption changes α_li(t), caused by optically generated small polarons, are investigated in LiNbO₃:Mg below and above the optical‐damage‐resistance threshold (ODRT). The lifetime of α_li(t) is reduced by three orders of magnitude above the ODRT while a significantly enhanced amplitude is observed in the infrared. Our observations are in full accordance with the predictions of microscopic models for the ODRT, namely the removal of Nb_Li antisite defects upon incorporation of Mg ions, and an enhanced dark conductivity. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Strong quenching of praseodymium f-f luminescence induced by a surface of Y2SiO5:Pr nanocrystal

The direct comparison of the luminescence decay data obtained for nano- and bulk Y₂SiO₅:Pr³⁺ crystals has revealed that the concentration threshold of luminescence quenching is strikingly low for nanocrystals. Nanocrystal inhomogeneous stress field induced by a surface stimulates the segregation of the doped Pr³⁺ ions within the surface layer that provides the relaxation of elastic tension arising due to the difference of the ionic radii of Pr³⁺ and Y³⁺. The Pr³⁺ irregular distribution in the nanocrystal volume results in the Pr³⁺ local concentration increasing that facilitates the luminescence quenching.     

Electrical switching in germanium telluride glasses doped with Cu and Ag

Electrical switching in germanium telluride glasses containing metallic atoms (Cu and Ag) has been investigated. All these glasses are found to exhibit memory switching. The switching fields of these glasses are compared with the thermal parameters evaluated from DSC studies and the results are explained on the basis of the thermal model. The composition dependence of the switching field and the thermal parameters show interesting variations at the critical compositions which correspond to the rigidity percolation and the chemical thresholds of these glasses. Received: 11 January 1999 / Accepted: 14 April 1999 / Published online: 14 July 1999     

The role of hydrogen- and hydroxyl-doping in CsBr:Eu

Hydrogen- and hydroxyl-doped CsBr:Eu²⁺ storage phosphors were investigated by photostimulated luminescence (PSL) spectra, absorption spectra and PSL lifetime spectra. Hydrogen- and hydroxyl-doping plays an active role related to the trap centers in CsBr:Eu²⁺ storage phosphors. A sharp increase in the PSL yield was observed for non-hydroxyl-doped CsBr:Eu²⁺ annealed in 5%H₂+95%Ar atmosphere. For hydroxyl-doped CsBr:Eu²⁺, the PSL lifetime (361 ns) is obviously shorter than that of typical CsBr:Eu²⁺ storage phosphors.     

Temperature dependence of persistent photo-conductivity due to DX centers in AlxGa1−xAs:Si

Conductivity and Hall effect measurements were performed on molecular beam epitaxy grown Al_xGa_1–xAs:Si samples, which show a large persistent photoconductivity effect. We observe one, two, and three minima in the temperature-dependent carrier concentration during the heating process after having first illuminated the samples (x=0.25, 0.30, and 0.37, respectively) at low temperature. We interpret this structure in terms of the existence of different types of large lattice relaxation DX centers.     

Magnetism without magnetic ions in non-magnetic perovskites SrTiO3, SrZrO3 and SrSnO3

Using the full-potential linearized augmented plane-wave (FP-LAPW) method with the generalized gradient approximation (GGA) for the exchange-correlation potential, we studied spin polarization induced by replacement of oxygen atoms by non-magnetic 2p impurities (B, C and N) in non-magnetic cubic SrMO₃ perovskites, where M=Ti, Zr and Sn. The results show that the magnetization may appear because of the spin–split impurity bands inside the energy gap of the insulating SrMO₃ matrix. Large magnetic moments are found for the impurity centers. Smaller magnetic moments are induced on the oxygen atoms around impurities. It is shown that SrTiO₃:C and SrSnO₃:C should be magnetic semiconductors while other compounds in this series (SrTiO₃:B, SrTiO₃:N and SrZrO₃:C) are expected to exhibit magnetic half-metallic or pseudo-half-metallic properties.     

Si DX centers in GaAs at large hydrostatic pressures

A number of experimental and theoretical studies indicate that DX centers in GaAs, its alloys and other III–V semiconductors have negative U properties. Using far infrared localized vibrational mode (LVM) spectroscopy of Si donors in GaAs under large hydrostatic pressure in a diamond anvil cell we have discovered an LVM of the Si DX center. From the ratio of the LVM absorption lines of Si_Ga and Si_DX and the compensation in our GaAs samples, we show unambiguously that two electrons are trapped when the ionized shallow Si donors transform into negatively charged DX centers, in full agreement with the negative U model.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Trap centers in Au-implanted MOS structures

Trap centers in the Si-SiO₂ interface region of MOS structures doped by ion implantation of gold have been investigated using constant capacitance deep level transient spectroscopy (CC-DLTS). Gold doses of 10¹²–3 × 10¹³ cm^–2 were implanted into the back surface of the wafers and were then redistributed during a diffusion anneal for 30 min at 1100° or 900° C. Three Au-related trap levels have been observed in the interface region, which were attributed to the Au-donor (E _v +0.35 eV), the Au-acceptor (E _v +0.53 eV), and the Au-Fe complex (E _v +0.45 eV). The trap concentration profiles show that the Si-SiO₂ interface affects the Au concentration in a depth range of 1 m from the interface and that gettering of Au occurs at the interface. The interface state density is independent of the Au concentration at the interface even for concentrations of 10¹⁵ cm^–3.