Session A28: Focus Session: Dopants and Defects in Semiconductors - ZnO

The trends of oxygen vacancy levels in metal oxides

Most of the $d$ or $d^{10}$ oxides such as ZnO, SnO$_{2}$, In$_{2}$O$_{3}$, and TiO$_{2 }$are wide-bandgap $n$-type semiconductors even though they are not intentionally doped. For quite a long time, it was commonly believed that oxygen vacancies (V$_{O})$ in metal oxides are the electron donors because the formation energy of V$_{O}$ in metal oxides is low and the electrical conductivity of the $n$-type oxides is closely linked to the formation of V$_{O}$ However, recent theoretical and experimental studies have put this point of view in question especially with different calculation methods involved. We present a detailed analysis of the wavefunction characters of oxygen vacancy in conventional metal oxides and unveil the chemical trend of oxygen vacancy transition energy levels with respect to the conduction-band minimum (CBM). We show that in the type-$s$ and type-$p$ metal oxides, where the character of an oxygen vacancy level is similar to that of the CBM, the oxygen vacancy levels are generally deep and become deeper when the cation size decreases. In type-$d$ metal oxides, the oxygen vacancy levels are generally shallow and can sometimes even be above the CBM. Our analysis is confirmed by the calculated trends of oxygen vacancy levels in representative metal oxides using hybrid density functional analysis. It also provides guidelines to search a metal oxide that has shallow V$_{O}$ donor levels, such as the one found in BiVO$_{4}$   

Highly Efficient Defect Emission from ZnO and ZnO:S

Bulk Zinc Oxide (ZnO) is a wide, direct band gap semiconductor with an energy of 3.4 eV that contains two emission bands: the UV band-edge emission and the green defect emission band. We have shown that the external quantum efficiency of the green band can exceed 50{\%}. To investigate the mechanism of efficient defect emission, vacuum annealed (ZnO:Zn) and sulfur-doped (ZnO:S) ZnO were investigated because of their strong defect emission and suppressed UV band edge emission. Continuous wave low temperature photoluminescence (PL) and PL excitation (PLE) spectra were measured for the two compounds. It was found that bound excitons, not free photo-excited carriers, mediate the defect emission in ZnO:Zn, while the defect emission from ZnO:S seems to originate from a Zn-S complex formed in the crystal lattice. Temperature-dependent PLE spectra for the defect and band edge emission were measured to estimate trapping and activation energies of the bound excitons. XPS and X-Ray diffraction studies were also performed to ascertain the concentration and nature of sulfur doping in the ZnO lattice. The results presented here offer hope that engineering defects in ZnO materials may significantly improve the quantum efficiency for white light phosphor applications.   

Acceptors in bulk and nanoscale ZnO

Zinc oxide (ZnO) is a semiconductor that emits bright UV light, with little wasted heat. This intrinsic feature makes it a promising material for energy-efficient white lighting, nano-lasers, and other optical applications. For devices to be competitive, however, it is necessary to develop reliable p-type doping. Although substitutional nitrogen has been considered as a potential p-type dopant for ZnO, theoretical and experimental work indicates that nitrogen is a deep acceptor and will not lead to p-type conductivity. This talk will highlight recent experiments on ZnO:N at low temperatures. A red/near-IR photoluminescence (PL) band is correlated with the presence of deep nitrogen acceptors. PL excitation (PLE) measurements show an absorption threshold of 2.26 eV, in good agreement with theory. Magnetic resonance experiments provide further evidence for this assignment. The results of these studies seem to rule out group-V elements as shallow acceptors in ZnO, contradicting numerous reports in the literature. If these acceptors do not work as advertised, is there a viable alternative? Optical studies on ZnO nanocrystals show some intriguing leads. At liquid-helium temperatures, a series of sharp IR absorption peaks arise from an unknown acceptor impurity. The data are consistent with a hydrogenic acceptor 0.46 eV above the valence band edge. While this binding energy is still too deep for many practical applications, it represents a significant improvement over the $\sim$ 1.3 eV binding energy for nitrogen acceptors. Nanocrystals present another twist. Due to their high surface-to-volume ratio, surface states are especially important. Specifically, electron-hole recombination at the surface give rises to a red luminescence band. From our PL and IR experiments, we have developed a ``unified'' model that attempts to explain acceptor and surface states in ZnO nanocrystals. This model could provide a useful framework for designing future nanoscale ZnO devices.   

Origin of the ``Red'' Luminescence Band in Bulk N-doped ZnO

Optically detected magnetic resonance (ODMR) at 24 GHz was performed on bulk ZnO crystals doped with nitrogen impurities (of high interest for p-type conductivity) to provide more details on the origin of a recently reported red/near-IR photoluminescence (PL) band.\footnote{M.C. Tarun et al., AIP Advances \textbf{1}, 022105 (2011).} PL at 7K revealed strong bandedge excitonic recombination at 3.364 eV, a broad ``green'' emission band at 2.45 eV, and a broad ``red'' PL band near 1.7 eV. Two luminescence-increasing ODMR signals were found on this ``red'' emission. The first was a sharp feature with g-value of 1.957 and FWHM of 1 mT and is attributed to shallow donors based on electron spin resonance (ESR) of n-type ZnO. The second feature exhibited a g-value near 2 and a broad, asymmetric lineshape with FWHM of $\sim $ 10 mT. A simulation of the spectrum showed that the broad resonance could be fit as the sum of three equally spaced lines with magnetic field splitting value and relative intensities in close agreement to those observed for deep nitrogen acceptors as identified from previous ESR studies. Thus, the ODMR results strongly suggest that the ``red'' PL is due to radiative recombination involving residual shallow donors and deep nitrogen acceptor centers.   

Elevated Temperature Dependent Transport Properties of As- and P-doped Zinc Oxide

Achieving highly conductive p-type zinc oxide (ZnO) is desired for the development of ZnO based optoelectronic devices. Understanding electrical properties of ZnO, doped with p-type dopants, is necessary for improving p-type conductivity. We employed temperature dependent Hall effect measurement to study the electrical transport properties of As- and P-doped ZnO epilayers. The samples were grown on sapphire substrates by magnetron sputtering technique. From the Hall effect measurements performed at elevated temperatures ranging from 20 to 750 K, we observed double activation processes in both As- and P-doped ZnO epilayers. We will compare the results of uniform doped and delta-doped samples. Correlation between electrical properties from these Hall effect measurements and optical properties from low temperature photoluminescence measurements will also be discussed.   

Impurity complexes and conductivity of Ga-doped ZnO

Using hybrid functional theory compared with experimental measurements, we investigate the in?uence of gallium impurities and their complexes on electrical properties of ZnO. In contrast to the behavior of isolated Ga impurities and native defects, the calculated formation energies of Ga complexes are consistent with experimental data. We show that for high levels of Ga doping the acceptor behavior of (Ga$_{Zn}-V_{Zn}$) and (Ga$_{Zn}-O_i$) complexes explains the conductivity measurements and compensation levels in ZnO. The computed binding energies of these complexes are in agreement with the binding energies obtained from the measurements of the temperature dependence of carrier mobility. The binding energy dependence on the Fermi level, as well as the computed barrier heights for the formation of complexes are also consistent with the latest experiments on annealing of Ga doped ZnO samples. Our results show that the formation of defect complexes is essential for capturing the physics Ga defects in ZnO.   

Cu-Doping of ZnO by Nuclear Transmutation and Electrical and Optical Characterization of Cu Acceptors

Cu doping is known to have a large effect on the electrical and optical properties of ZnO, its role is different from other dopants, and a fundamental understanding of this role is lacking. One problem of ZnO doping is arising from the difficulty in controlling dopant locations in conventional doping methods. In this work we doped Zn single crystals with copper acceptors by means of the nuclear transmutation doping (NTD) method, which gives highly uniform dopant distributions and has a much higher probability of controlling the dopant locations in the lattice. The Cu doping was confirmed by the infrared absorption signature of Cu$^{2+}$ at 5780 cm$^{-1}$. Hall-effect measurements indicated that the Cu acceptor level lies 0.160 eV below the conduction band minimum. With respect to optical properties, an interesting low-temperature thermal stimulated luminescence has been observed in as-grown and doped ZnO single crystals. This low-temperature luminescence can reveal the density of donors and acceptors in ZnO and their location in the band gap.   

The G0W0 band gap of ZnO: effects of plasmon-pole models

Carefully converged calculations are performed for the band gap of ZnO within the G0W0 approximation. The results obtained using four different well-established plasmon-pole models are compared with those of explicit calculations without such models (the contour-deformation approach). We evaluate the difference between plasmon-pole models that enforce the f-sum rule and those that are fitted to reproduce the low energy response. In the case of ZnO, plasmon-pole models enforcing the f -sum rule underestimate the low-frequency region of the dielectric function probably because of the presence of semi-core states in these calculations. Our results confirm that the band gap of ZnO is underestimated in the G0W0 approach as compared to experiment.   

Evidenece for surface states in ZnO nanostructures using non-linear optical spectroscopy

An unexpected presence of ferromagnetic (FM) ordering in nanostructured ZnO has been reported previously. Recently, from our detailed magnetization studies and \textit{ab initio} calculations, we attributed this FM ordering in nanostructured ZnO to the presence of surface states, and a direct correlation between the magnetic properties and crystallinity of ZnO was observed. Such defect induced surface states appear as green/yellow emission in the photoluminescence spectrum of ZnO nanostructures. In this study, through a systematic sample preparation of both pristine and Co-doped ZnO nanostructures, and detailed PL and nonlinear optical measurements, we confirm that the observed FM ordering is due to the presence of surface states.   

Terahertz properties of Tm (Tm=Cu, Ag) doped ZnO thin films

Optical properties of Zn$_{0.95}$Tm$_{0.05}$O (Tm=Cu, Ag) thin films are studied by terahertz time-domain spectroscopy (THz-TDS) at different temperatures (10K -- 300K) in the frequency range extending from 0.22 -- 3 THz. The measured complex dielectric response and conductivity are well fitted by a Drude-based model. Comparing with undoped ZnO thin films, the doping effect of Ag and Cu is investigated.   

The growth of non-polar ZnO and ZnO/Mg$_{0.25}$Zn$_{0.75}$O epi-films by radio-frequency magnetron sputtering

High quality non-polar ZnO and ZnO/Mg$_{x}$Zn$_{1-x}$O (x = 0.15, 0.25) epi-films have been successfully grown on m-plane and r-plane sapphire by using RF magnetron sputtering. The structural properties, including crystalline quality, strain state, and defect structures, of the ZnO and ZnO/Mg$_{x}$Zn$_{1-x}$O layers are thoroughly examined by synchrotron x-ray scattering, transmission electron microscopy and atomic force microscopy. We found the surface morphology of the a-plane oriented ZnO epi-films is smoother than that of m-plane oriented one. Moreover, the surface of Mg$_{0.25}$Zn$_{0.75}$O epi-layer is significantly better than the ZnO epi-film and exhibits the same orientation dependence. The optical properties of these samples are also investigated by temperature, polarization and power dependent photoluminescence, and polarization dependent Raman spectroscopy. The results reveal the a-plane ZnO epi-film grown on r-sapphire with a Mg$_{0.25}$Zn$_{0.75}$O buffer layer is promising for bright UV emission application.   

Excitonic Energy Shifts in Isotopically Controlled $I-III-VI_2$ Chalcopyrites: $CuGaS_2$ and $AgGaS_2$

$CuGaS_2$ and $AgGaS_2$ tetrahedrally co-ordinated chalcopyrites are ``genealogically related" to $II-VI$ semiconductors like ZnS. We have investigated the shifts in their excitonic signatures by controlling the isotopic mass of the $I$, $III$ or $VI_2$ constituent in the crystals grown by physical vapor deposition. The excitonic signatures are observed in wavelength modulated reflectivity employing a high S/N, LED based technique.\footnote{J. S. Bhosale, Rev. Sci. Instrum. 82, 093103 (2011)} For example it reveals a 3.9 meV shift for the A exciton in $Ag^{71}GaS_2$ with respect to that of natural $AgGaS_2$; a smaller increase occurs in ZnS\footnote{M. Cardona and M.L.W. Thewalt, Rev. Mod. Phys. 77, 575 (2002)}. These effects have been related to electron-phonon interaction caused by the zero-point vibrations. Similar effects, but with an opposite sign, have been observed for Cu-isotopes in $CuGaS_2$ as well as in the Cu-monohalides CuCl, CuBr, and CuI\footnote{D.Olguin et al., Solid State Commun. 122, 575 (2002)}; their origin is receiving considerable attention at present though not yet understood. In this context the excitonic temperature dependence\footnote{Cardona, op. cit}$^,$\footnote{H.Alawadhi et al., Phys. Rev.B 75, 205207 (2007)} will be discussed.   

Hybrid Hartree-Fock density functional study of transition-metal doped ZnO

Dilute magnetic semiconductors (DMS) obtained by doping semiconductors with transition metals (TM) hold much promise for spintronics. Transition metal doped ZnO (ZnO:TM) has been investigated for a possible room-temperature DMS. Density functional theory gives incorrect prediction for the band gap of ZnO which leads to diverging interpretations for the magnetic behavior of ZnO:TM [1,2]. Here we report Heyd-Scuseria-Ernzerhof (HSE) hybrid functional study of the electronic structure of ZnO:TM (TM=Cu, Ni, Co, Fe, Mn). The hybrid functional corrects for both the bandgap problem on the host and the lack of correlation in the impurity, without the use of \textit{ad-hoc }intra-atomic potentials. Our results show although the HSE opens the band gap of the host, the Hubbard splitting of the impurity levels makes the empty impurity levels reside in the host conduction band. This leaves open the possibility for spin polarized carriers. We discuss the validity of the results and explore their implications for the magnetic behavior of ZnO:TM. [1] H. Raebiger, S. Lany, and A. Zunger, Physical Review B 79, 165202 (2009). [2] P. Gopal and N. A. Spaldin, Phys.l Review B 74, 094418 (2006).