Mechanical milling and subsequent annealing in air at temperatures between 210 and 1200°C have been carried out on high purity ZnO powder to study the defect generation and recovery in the material. Lowering of average grain size (from 76±1to22±0.5nm) as a result of milling has been estimated from the broadening of x-ray lines. Substantial grain growth in the milled sample occurs above 425°C annealing temperature. Positron annihilation lifetime (PAL) analysis of the samples shows a distinct decrease of the average lifetime of positrons very near the same temperature zone. As indicated from both x-ray diffraction (XRD) and PAL results, high temperature (>700°C) annealed samples have a better crystallinity (or lower defect concentration) than even the nonmilled ZnO. In contrast, the measured optical band gap of the samples (from absorption spectroscopy) does not confirm lowering of defects with high temperature annealing. Thermally generated defects at oxygen sites cause significant modification of the optical absorption; however, they are not efficient traps for positrons. Different thermal stages of generation and recovery of cationic as well as anionic defects in granular ZnO are discussed in the light of XRD, PAL, and optical absorption studies.

Annealing effect on nano-ZnO powder studied from positron lifetime and optical absorption spectroscopy

Phosphorus ions were implanted in ZnO single crystals with energies of 50–380keV having total doses of 4.2×1013–4.2×1015cm−2. Positron annihilation measurements reveal the introduction of vacancy clusters after implantation. These vacancy clusters grow to a larger size after annealing at a temperature of 600°C. Upon further annealing up to a temperature of 1100°C, the vacancy clusters gradually disappear. Raman-scattering measurements reveal the enhancement of the phonon mode at approximately 575cm−1 after P+ implantation, which is induced by the production of oxygen vacancies (VO). These oxygen vacancies are annealed out up to a temperature of 700°C accompanying the agglomeration of vacancy clusters. The light emissions of ZnO are suppressed after implantation. This is due to the competing nonradiative recombination centers introduced by implantation. The recovery of the light emission occurs at temperatures above 600°C. The vacancy-type defects detected by positrons might be part of the nonradiative rec...

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Production and recovery of defects in phosphorus-implanted ZnO

We have applied positron annihilation spectroscopy to show that 2-MeV electron irradiation at 300 K creates primary Ga vacancies in GaN with an introduction rate of 1 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The Ga vacancies recover in long-range migration processes at 500--600 K with an estimated migration energy of 1.5 (2) eV. Since the native Ga vacancies in as-grown GaN survive up to much higher temperatures (1300--1500 K), we conclude that they are stabilized by forming complexes with oxygen impurities. The estimated binding energy of 2.2 (4) eV of such complexes is in good agreement with the results of theoretical calculations.

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Thermal stability of isolated and complexed Ga vacancies in GaN bulk crystals

Native point defects control many aspects of semiconductor behavior. Such defects can be electrically charged, both in the bulk and on the surface. This charging can affect numerous defect properties such as structure, thermal diffusion rates, trapping and recombination rates for electrons and holes, and luminescence quenching rates. Charging also introduces new phenomena such as nonthermally photostimulated diffusion, thereby offering distinctive mechanisms for defect engineering. The present work incorporates the first comprehensive account of semiconductor defect charging, identifying correspondences and contrasts between surfaces and the bulk as well as among semiconductor classes (group IV, groups III–V, and metal oxides). For example, small lattice parameters, close-packed unit cells, and basis atoms with large atomic radii all inhibit the formation of ionized interstitials and antisites. The charged defects that exist in III–V and oxide semiconductors can be predicted with surprising accuracy from the chemical potential and oxygen partial pressure of the ambient. The symmetry-lowering relaxations, formation energies, and diffusion mechanisms of bulk and surface defect structures often depend strongly on charge state with similar qualitative behavior, although for a given material surface defects do not typically take on the same configurations or range of stable charge states as their counterparts in the bulk.

Charged point defects in semiconductors

We apply positron annihilation spectroscopy to identify ${V}_{\mathrm{N}}\mathrm{\text{\ensuremath{-}}}{\mathrm{M}\mathrm{g}}_{\mathrm{G}\mathrm{a}}$ complexes as native defects in Mg-doped GaN. These defects dissociate in postgrowth annealings at $500--800\text{ }\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. We conclude that ${V}_{\mathrm{N}}\mathrm{\text{\ensuremath{-}}}{\mathrm{M}\mathrm{g}}_{\mathrm{G}\mathrm{a}}$ complexes contribute to the electrical compensation of Mg as well as the activation of $p$-type conductivity in the annealing. The observation of ${V}_{\mathrm{N}}\mathrm{\text{\ensuremath{-}}}{\mathrm{M}\mathrm{g}}_{\mathrm{G}\mathrm{a}}$ complexes confirms that vacancy defects in either the N or Ga sublattice are abundant in GaN at any position of the Fermi level during growth, as predicted previously by theoretical calculations.

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Vacancy defects as compensating centers in Mg-doped GaN.

1 Introduction.- 2 Experimental Techniques.- 3 Basics of Positron Annihilation in Semiconductors.- 4 Defect Characterization in Elemental Semiconductors.- 5 Defect Characterization in III-V Compounds.- 6 Defect Characterization in II-VI Compounds.- 7 Defect Characterization in Other Compounds.- 8 Applications of Positron Annihilation in Defect Engineering.- 9 Comparison of Positron Annihilation with Other Defect-Sensitive Techniques.- A1 Semiconductor Data.- A2 Trapping Model Equations.- References.

R. Krause-Rehberg

Papers

Positron annihilation in semiconductors : defect studies