First-principle study of extrinsic defects in CuScO2 and CuYO2

Using first-principles methods, we studied the extrinsic defects doping in transparent conducting oxides CuMO₂ (MSc, Y). We chose Be, Mg, Ca, Si, Ge, Sn as extrinsic defects to substitute for M and Cu atoms. By systematically calculating the impurity formation energy and transition energy level, we find that BeCu is the most prominent extrinsic donor and CaM is the prominent extrinsic acceptor. In addition, we find that Mg atom substituting for Sc is the most prominent extrinsic acceptor in CuScO₂. Our calculation results are expected to be a guide for preparing n-type and p-type materials through extrinsic doping in CuMO₂ (MSc,Y).     

Vacancies and impurities in InP studied using positron lifetime and an improved Doppler-broadening spectrometer

In ^- in Zn-doped p-InP. The line-shape parameter measured by an improved Doppler-broadening spectrometer shows a significant difference between n-type and Fe-doped SI-InP materials: the S-parameter varies from 0.5203 in n-InP to 0.5184 in Fe-doped SI-InP by four times the standard deviation. A possible reason for this difference is discussed. Received: 7 January 1997/Accepted: 8 September 1997     

Anisotropy of the electron momentum distribution in Bi2Sr2CaCu2O8+δ superconductor studied by positron annihilation

The temperature dependent (30-300 K) Doppler broadening of the positron annihilated γ-radiation measurement has been investigated on single crystalline Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) high T_c superconducting samples along two different crystallographic orientations. It has been observed that throughout the temperature range the electron momentum distribution has a larger value along the crystallographic c-axis than in the a-b plane. The temperature dependent Doppler broadened positron annihilation γ-radiation lineshape analysis shows a step like increase of S-parameter at the temperature region 92-116 K.     

Thermally stimulated current study of electron-irradiation induced defects in semi-insulating InP obtained by multiple-step wafer annealing

Electron-irradiation induced defects in semi-insulating (SI) InP wafers with Fe concentration ranging from 1.5×10¹⁵ to 2.5×10¹⁵ cm⁻³, which have been obtained by multiple-step wafer annealing (MWA) under phosphorus vapor pressure, were studied using a thermally stimulated current (TSC) method. New traps, e₁, e₂, e₃, e₄ and e₅, with activation energies of 0.22, 0.28, 0.37, 0.44 and 0.46 eV, respectively, were observed. Based upon the annealing behavior of traps and the calculated defect levels, traps e₁ and e₅ produced by the irradiation with electron doses above 1×10¹⁵ cm⁻² were linked to In_P and P_In antisite defects, respectively, that probably form complexes. Traps e₃ and e₄ produced by the irradiation with doses above 1×10¹⁴ cm⁻² were associated with In and P vacancy related defects, respectively.     

On the sensitivity limit of positron annihilation: detection of vacancies in as-grown silicon

14 cm^-3 in FZ-Si was obtained. Received: 2 June 1998 / Accepted: 20 November 1998 / Published online: 24 February 1999     

Slow positron annihilation studies of vacancy-type defects in the near-surface region of Cu and Nb before and after wear

Received: 22 June 1998 / Accepted: 9 November 1998     

Positron annihilation in diamond,silicon and silicon carbide

Recent positron lifetime and doppler broadening results on silicon, diamond and silicon carbide are presented in this contribution. In as-grown Czochralski Si ingols vacancies are found to be retained after growth at concentrations typically around 3×10¹⁶/cm³. 10 MeV eleciron irradiation of variously doped Si wafers shows that only high doping concentrations well in excess of the interstitial oxygen concentration causes an increase in the amount of monovacancies retained.In porous silicon very long-lived positronium lifetimes in the range 40–90 ns are found. Polycrystalline diamond films contain various types of vacancy agglomerates but these are found to be inhomogeneously distributed from crystallite to crystallite. Electron irradiation of silicon carbide results in two vacancy-related lifetimes which are interpreted as resulting from carbon and silicon vacancies.Paper presented at the 132nd WE-Heraeus-Seminar on Positron Studies of Semiconductor Defects, Halle, Germany, 29 August to 2 September 1994     

First-principles study on the electronic structures and absorption spectra for the PbWO4 crystal with lead vacancy

The electronic structures and absorption spectra for the perfect PbWO₄ (PWO) crystal and the crystal containing lead vacancy have been calculated using density functional theory code CASTEP with the lattice structure optimized. The calculated absorption spectra of the PWO crystal containing exhibit seven absorption bands peaking at 1.72 eV (720 nm), 2.16 eV (570 nm), 2.81 eV (440 nm), 3.01 eV (410 nm), 3.36 eV (365 nm), 3.70 eV (335 nm) and 4.0 eV (310 nm), which are very close to the experimental values. It predicts that the 330, 360, 420, 500-750 nm absorption bands are related to the existence of in the PWO crystal.     

Positron annihilation in vacancies at grain boundaries in metals

The behavior of vacancies in selected coherent grain boundaries (GBs) in Fe and Ni is studied by means of molecular dynamics simulations. Corresponding positron lifetimes are calculated using the atomic superposition method. There is a difference between the vacancy behavior in Fe and Ni in dependence on temperature. In Ni, vacancies at GBs appear to diminish substantially their free volume (and lifetime) with the increasing temperature, which can be attributed to ‘vacancy delocalization’. Contrary, GB vacancies remain stable up to apparently higher temperatures in Fe.     

Mechanism of Th, Zr doping in PbWO4 crystals

To check the doping mechanism of trivalent ion doping in PbWO₄ (PWO) and verify the formation of dipoles [2M_Pb³⁺-V_Pb], the researches on an investigation of tetravalent ion (Th⁴⁺, Zr⁴⁺) doped PWO was conducted by dielectric loss spectroscopy, thermoluminescence and optical absorption spectroscopy. The doping mechanism of tetravalent ion doping is similar to that of trivalent ion doping, while the dipole defects should be formed more easily due to the convenience for the two defects V_Pb and M_Pb⁴⁺ to get close to each other. However, in the case of Zr⁴⁺ doping, Zr was found to enter interstitial sites because of its small ionic radius, with the introduced charges compensated by V_Pb far away and hence does not exert an obvious influence on the performance of this material. Thus, no dielectric relaxation was found in the DS (dielectric spectra) experiment.     

First-Principles Study of Defects in CuGaO2

Using the first-principles methods, we study the electronic structure, intrinsic and extrinsic defects doping in transparent conducting oxides CuGaO₂. Intrinsic defects, acceptor-type and donor-type extrinsic defects in their relevant charge state are considered. The calculation result show that copper vacancy and oxygen interstitial are the relevant defects in CuGaO₂. In addition, copper vacancy is the most efficient acceptor. Substituting Be for Ga is the prominent acceptor, and substituting Ca for Cu is the prominent donors in CuGaO₂. Our calculation results are expected to be a guide for preparing n-type and p-type materials in CuGaO₂.     

Useful vacancies: Positron beam interrogation of fluorine-vacancy complexes in semiconductor device structures

The formation, migration and agglomeration in silicon of fluorine-vacancy complexes have been monitored by single-detector Doppler broadening spectroscopy. After electronics engineers found that fluorine ion implantation effectively eliminated the transient-enhanced diffusion of dopants in the creation of ultra-shallow junctions, a vital step in the further miniaturization of device structures, positron beams have played a pivotal role in providing an insight into the mechanisms underlying this phenomenon, being able to detect FV complexes in implanted and annealed samples. Secondary Ion Mass Spectrometry has provided complementary information on fluorine concentrations so that the nature of the F_mV_n complexes can be further assessed. New results on Si and SiGe structures are presented.     

Studies on the origins of the absorption spectra in PbWO4 crystal

The electronic structures and absorption spectra for both the perfect PbWO₄ (PWO) crystal and the three types of PWO crystals, containing V_Pb²⁻, V_O²⁺ and a pair of V_Pb²⁻-V_O²⁺, respectively, have been calculated using CASTEP codes with the lattice structure optimized. The calculated absorption spectra indicate that the perfect PWO crystal does not occur absorption band in the visible and near-ultraviolet region. The absorption spectra of the PWO crystal containing V_Pb²⁻ exhibit seven peaks located at 1.72 eV (720 nm), 2.16 eV (570 nm), 2.81 eV (440 nm), 3.01 eV (410 nm), 3.36 eV (365 nm), 3.70 eV (335 nm) and 4.0 eV (310 nm), respectively. The absorption spectra of the PWO crystal containing V_O²⁺ occur two peaks located at 370 nm and 420 nm. The PWO crystal containing a pair of V_Pb²⁻-V_O²⁺ does not occur absorption band in the visible and near-ultraviolet region. This leads to the conclusions that the 370 and 420 nm absorption bands are related to the existence of both V_Pb²⁻ and V_O²⁺ in the PWO crystal and the other absorption bands are related to the existence of the V_Pb²⁻ in the PWO crystal. The existence of the pair of V_Pb²⁻-V_O²⁺ has no visible effects on the optical properties. The calculated polarized optical properties are well consistent with the experimental results.     

Ordered zinc-vacancy induced Zn0.75Ox nanophase structure

Using transmission electron microscopy, a new nano-phase structure of Zn_0.75O_x induced by Zn-vacancy has been discovered to grow on wurtzite ZnO nanobelts. The superstructure grows epitaxial from the surface of the wurtzite ZnO nanobelts and can be fitted as an orthorhombic structure, with lattice parameters a′=2a, and c′=c, where a and c are the lattice parameters of ZnO. The superstructured phase is resulted from high-density Zn vacancies orderly distributed in the ZnO matrix. This study provides direct observation about the existence of Zn-vacancies in ZnO.     

Nonmagnetic impurities induced magnetism in SnO2

A density functional study is performed to investigate the magnetism induced by the nonmagnetic impurity substitution for the cation in SnO₂. The calculated results show that the K impurity substitution leads to a robust magnetism in SnO₂, and the induced magnetic moments are mainly attributed to the first shell of oxygen atoms surrounding the impurity atom. Meanwhile, no magnetism is observed in SnO₂ doped with Ca which implies a decreasing tendency of induced magnetic moments for Sn substituted by vacancy, K, and Ca. It is also demonstrated that the magnetic coupling constant oscillates as a function of K-K separation distance, and the Curie temperature above room temperature can be obtained in K-doped SnO₂.     

Computer simulation of intrinsic defects in YAlO3 single crystal

Computer simulation techniques were used to investigate intrinsic defects in YAlO₃ single crystal. A set of short-range potential parameters were derived using a relaxed fitting procedure incorporating with the known crystal properties. These parameters were then applied within the framework of the shell model. The simulation results reveal that oxygen Frenkel disorder and the antisite defect of Al ion substituting the Y ion dominate the intrinsic defects in YAlO₃. An analysis of redox reactions corroborate that the oxidation is most likely to occur via forming interstitial oxygen, while the oxidation via filling oxygen vacancies and reduction reaction may predominate at high temperature. The activation energy of oxygen vacancy migration on conduction was also studied.     

Positron annihilation lifetime studies on La0.5Pb0.5Mn1−yCryO3

Concentration dependent positron annihilation lifetime (PAL) measurements on Cr doped La_0.5Pb_0.5Mn_1−yCr_yO₃ (y=0.075, 0.15, 0.3, 0.35, 0.45) samples showing metal-insulator transitions (MIT) between 178 and 276 K (depending on y) reveal anomalous variation of average and bulk PAL, τ_av and τ_B, respectively, around y=0.35. Such anomaly has not, however, been observed from the corresponding magnetic susceptibility and resistivity data. Interestingly, the model parameters (polaron radius, number of ion sites per unit volume) obtained from fitting the high temperature (above MIT, T_p) resistivity data with small polaron hopping model show the signature of the said anomaly around the same concentration. Nonlinear variations of τ_av and τ_B support the existence of small polarons (T>T_p) which act as defect centers in such rare-earth manganites. Analysis of X-ray diffraction pattern confirms a change of lattice parameters indicating a structural transformation from rhombohedral (for y≤0.30) to orthorhombic (for y≥0.35) in the present system.     

Neon ion irradiation studies on MgB2 superconductor

160 MeV of neon ion irradiation has been carried out on MgB₂ polycrystalline pellets at various doses. There has not been any significant change in T_c except at the highest dose of 1×10¹⁵ ions/cm². Increase in resistivity has been noticed. Resistivity data have been fitted with Bloch-Grüneisen function to extract the values of Debye temperature, residual resistivity and temperature coefficient of resistivity for irradiated as well as unirradiated samples. There has not been any significant effect on electron-phonon coupling due to irradiation as evident from Debye temperature and the electron-phonon coupling constant.     

High Fe2+/3+ trap concentration in heavily compensated implanted InP

High Fe concentrations (up to 2×10¹⁹ cm^-3) have been implanted in n-doped InP to compensate the substrate donors. The resulting semi-insulating layers have been investigated by current–voltage (I-V) measurements and photo-induced current transient spectroscopy (PICTS) analyses to characterise the Fe activation process and to study the Fe related deep levels. The activation of the Fe^2+/3+ trap has been assessed by the identification of the deep level located at E_C-0.64 eV. The outcomes of the PICTS measurements have been correlated with the electrically active Fe concentration calculated from a numerical simulation of the I-V characteristics. We observe an increasing linear relation between the electrically active Fe concentration and the substrate doping density, with a maximum active Fe concentration as high as 2×10¹⁸ cm^-3, i.e. more than an order of magnitude above the equilibrium Fe solid solubility. These data are presented and their implications discussed. Received: 4 September 2000 / Accepted: 7 February 2001 / Published online: 23 May 2001     

Anisotropic defect reduction in non-polar a-plane GaN grown by hydride vapor phase epitaxy on maskless patterned templates

Several non-polar a-plane GaN films had been grown by hydride vapor phase epitaxy (HVPE) on different designed metal organic chemical deposition (MOCVD) GaN templates, which exhibited various ridge-like sidewall facets surface morphologies. The templates induced a lateral growth at the early stage of the HVPE growth, and resulted in a kind of maskless epitaxy lateral overgrown (ELO) process. It is found that the dislocation reduced differently along [1 0 0 0] and [] directions in these HVPE a-plane GaN layers. In [0 0 0 1] direction, the dislocation reduction resulted from the optimal surface roughness value of the template. In [] direction, the inclined facet might be a main factor for the dislocation reduction in HVPE-GaN films. The maskless ELO process had a significant influence on decreasing the dislocation density.