The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev. 142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys. 6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. ...

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A comprehensive review of zno materials and devices

X-ray photoelectron spectroscopy: Towards reliable binding energy referencing

Contemporary theories of defects and impurities in semiconductors rely to a large extent on supercell calculations within density-functional theory using the approximate local-density approximation (LDA) or generalized gradient approximation (GGA) functionals. Such calculations are, however, affected by considerable uncertainties associated with: (i) the ``band-gap problem,'' which occurs not only in the Kohn-Sham single-particle energies but also in the quasiparticle gap (LDA or GGA) calculated from total-energy differences, and (ii) supercell finite-size effects. In the case of the oxygen vacancy in ZnO, uncertainties (i) and (ii) have led to a large spread in the theoretical predictions, with some calculations suggesting negligible vacancy concentrations, even under Zn-rich conditions, and others predicting high concentrations. Here, we critically assess (i) the different methodologies to correct the band-gap problem. We discuss approaches based on the extrapolation of perturbations which open the band gap, and the self-consistent band-gap correction employing the $\text{LDA}+U$ method for $d$ and $s$ states simultaneously. From the comparison of the results of different gap-correction, including also recent results from other literature, we conclude that to date there is no universal scheme for band gap correction in general defect systems. Therefore, we turn instead to classification of different types of defect behavior to provide guidelines on how the physically correct situation in an LDA defect calculation can be recovered. (ii) Supercell finite-size effects: We performed test calculations in large supercells of up to 1728 atoms, resolving a long-standing debate pertaining to image charge corrections for charged defects. We show that once finite-size effects not related to electrostatic interactions are eliminated, the analytic form of the image charge correction as proposed by Makov and Payne leads to size-independent defect formation energies, thus allowing the calculation of well-converged energies in fairly small supercells. We find that the delocalized contribution to the defect charge (i.e., the defect-induced change of the charge distribution) is dominated by the dielectric screening response of the host, which leads to an unexpected effective $1/L$ scaling of the image charge energy, despite the nominal $1/{L}^{3}$ scaling of the third-order term. Based on this analysis, we suggest that a simple scaling of the first order term by a constant factor (approximately 2/3) yields a simple but accurate image-charge correction for common supercell geometries. Finally, we discuss the theoretical controversy pertaining to the formation energy of the O vacancy in ZnO in light of the assessment of different methodologies in the present work, and we review the present experimental situation on the topic.

Assessment of correction methods for the band-gap problem and for finite-size effects in supercell defect calculations: Case studies for ZnO and GaAs

Several hundred thousands of tons of ZnO are used by per year, e.g. as an additive to concrete or to rubber. In the field of optoelectronics, ZnO holds promises as a material for a blue/UV optoelectronics, alternatively to GaN, as a cheap, transparent, conducting oxide, as a material for electronic circuits, which are transparent in the visible or for semiconductor spintronics. The main problem is presently, however, a high, reproducible and stable p-doping. We review in this contribution partly critically the material growth, fundamental properties of ZnO and of ZnO-based nanostructures, doping as well as present and future applications, with emphasis on the electronic and optical properties including stimulated emission. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

ZnO : From basics towards applications

The CdTe nanoparticles were prepared in aqueous solution using different mercaptocarboxylic acids such as 3-mercaptopropionic acid (MPA) and thioglycolic acid (TGA) as stabilizing agents following the synthetic route described in ref 9. The pH-dependent photoluminescence (PL) of MPA- and TGA-stabilized CdTe nanoparticles was systematically investigated before and after addition of poly(acrylic acid) (PAA) into the CdTe solutions. Experimental results reveal that lowering the pH can increase the PL efficiency of both MPA- and TGA-stabilized CdTe. Moreover, the PL of the CdTe can further be increased in the presence of PAA in low pH range. X-ray photoelectron spectroscopy (XPS) was employed to investigate the interaction between the carboxyl groups from PAA and CdTe nanoparticles which were assembled in polymer matrix by a layer-by-layer self-assembly method to exclude interference from other species in CdTe solutions. XPS results demonstrate that the S/Te ratio of CdTe particles decreases after the additio...

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The Influence of Carboxyl Groups on the Photoluminescence of Mercaptocarboxylic Acid-Stabilized CdTe Nanoparticles

The effect of the spin-exchange interaction between electron and hole is investigated for the case of excitons originating from one of the $p$-like valence bands and an $s$-like conduction band, as is the case for IIb-VIb compounds. A general exciton matrix is constructed, starting from the work of Pikus. It includes spin-orbit, crystal-field, spin-exchange, and deformation-potential interactions. Use of this matrix then allows a theoretical fit to our experimental data which describes the shift of exciton levels under uniaxial pressure in ZnO, CdS, and CdSe. This fit results in the determination of six deformation potentials, two spin-orbit parameters, the crystal-field parameter, and the exchange parameter. The general theory, when adapted to the zinc-blende structure, allows us to fit our data on cubic ZnS and ZnSe, resulting in a determination of two deformation potentials and the spin-exchange parameter for each compound.

Spin Exchange in Excitons, the Quasicubic Model and Deformation Potentials in II-VI Compounds

X-ray induced electron-emission measurements were used to determine the energy levels of core electrons in ZnO, ZnS, ZnSe, ZnTe, CdO, CdS, CdSe, CdTe, HgS, HgSe, and HgTe. The investigated energy range extends from the bottom of the valence band to about 1200 eV below the Fermi level. Chemical shifts were determined by comparing our results with experimental values for the pure elements. These shifts are plotted as a function of the fractional ionicity values determined by Phillips and Van Vechten, Pauling, and Coulson. Spinorbit-splitting values were experimentally determined for the first time for several levels including the $\mathrm{Zn}3d$, $\mathrm{Cd}4d$, and $\mathrm{Hg}5d$ levels. Furthermore, our measured energy values for these levels are used to determine the absolute energy values of the initial and final states of transitions normally labeled ${d}_{2}$ in ultraviolet reflectivity and electron-energy-loss measurements. Our results for ZnSe and CdTe are compared with self-consistent relativistic orthogonalized-plane-wave calculations for the excitation energies of these compounds. Agreement with these theoretical calculations is best for the levels closest to the valence band and appears to be angular momentum dependent.

Electronic Core Levels of the II B − VI A Compounds

X-ray-induced photoemission studies were used to determine the energy levels of core electrons in ZnO, ZnS, ZnSe, and ZnTe. The investigated energy range extends from the Fermi level to about 1200 eV below the valence band. The observed Auger transitions in these compounds are in good agreement with the measured energy levels. Some of the uncertainties in using this technique for the study of electron energy levels in semiconducting or insulating crystals were investigated and are discussed. The measured energy levels are compared with published experimental data for the respective pure elements. Comparison is also made with energy values predicted by several different types of theoretical calculations. Significant conclusions are: (i) The location of the Zn $3d$ level in the zinc chalcogenides has been unambiguously determined and (ii) the discrepancy between our measured values and the theoretically determined energy values is angular momentum dependent.

Electronic Core Levels of Zinc Chalcogenides

Photoemission measurements have been made of the upper $d$ levels in ZnO, ZnSe, ZnTe, CdS, CdSe, CdTe, HgSe, and HgTe using 21.2-eV (584-\AA{}) and 16.8-eV (740-\AA{}) radiation. All the samples except for HgSe and HgTe were cleaved and measured in an oil-free ion-pumped vacuum system at a pressure in the ${10}^{\ensuremath{-}7}$-${10}^{\ensuremath{-}8}$ Torr range. The results agree extremely well with values obtained by x-ray-induced-electron-emission spectroscopy. The spectra obtained for HgSe and HgTe make it possible to positively verify the identification of certain peaks observed in photoemission spectra of the IIB-VIA compounds as being due to the upper $d$ levels. The photoemission results reported in this study are also compared with both reflectivity and energy-loss measurements to obtain information about the optical density of states in the conduction bands of these compounds.

uv photoemission measurements of the upper d levels in the IIB-VIA compounds

X-ray-induced electron-emission measurements were used to determine the energy levels of core electrons in GaP, GaAs, InAs, and InSb. The investigated energy range extends from the bottom of the valence band to about 1400 eV below the Fermi level. Samples were cleaned by using argon-ion bombardment, and the gold $4{f}_{\frac{7}{2}}$ electron level was used to provide an energy reference level. Chemical shifts were determined by comparing the results with previously published experimental values for the pure elements. Several spin-orbit-splitting values were experimentally determined. Two maxima in the conduction-band density of states were located with respect to the bottom of the conduction band by comparing the photoemission data for the outermost core $d$ and $p$ electrons with transition energies measured by uv absorption, uv reflectivity, and electron-energy-loss experiments. One maximum is located between 0.8 and 1.4 eV above the bottom of the conduction band depending on the compound and the other is located between 3 and 4 eV.

D. W. Langer

Papers

Spin Exchange in Excitons, the Quasicubic Model and Deformation Potentials in II-VI Compounds

Electronic Core Levels of the II B − VI A Compounds

Electronic Core Levels of Zinc Chalcogenides

uv photoemission measurements of the upper d levels in the IIB-VIA compounds

X-Ray Photoemission Measurements of GaP, GaAs, InAs, and InSb