Session W3: Advances in ZnO Physics and Applications

Hybrid functional studies of defects and impurities in ZnO

Zinc oxide is regarded as a highly promising material for light-emitting diodes and lasers. Its features include a direct band gap of 3.4 eV, a large exciton binding energy of 60 meV, and the availability of high-quality single-crystal substrates. Despite the rapid development, fundamental issues regarding $p$-type doping remain unresolved. The most significant barrier to realizing ZnO-based optoelectronic devices is the difficulty in producing reliable and reproducible $p$-type material. Among the possible acceptor impurities, N has been considered the most promising because it has an atomic size close to that of O. In addition, N has been conclusively shown to act as a shallow acceptor in other II-V semiconductors, such as ZnSe. In spite of many published reports on $p$-type conductivity in N-doped ZnO, reproducibility and stability are still major issues, and devices based on $p-n$ homojunctions have remained elusive. In this work, we study the properties of the nitrogen acceptor using advanced density functional techniques. Our first principles calculations are based on hybrid functionals, which include a portion of exact exchange and correct the band gap of semiconductors, allowing us to accurately predict defect and impurity transition levels. Contrary to the conventional wisdom, we find the N acceptor has an exceedingly high ionization energy of 1.3 eV above the valence band, meaning that N cannot lead to hole conductivity in ZnO [1]. We have also analyzed the optical transitions (absorption and luminescence) and charge distribution associated with the N impurity, which offer characteristic signatures that can be compared to experimental results. \\[4pt] [1] J. L. Lyons, A. Janotti, and C. G. Van de Walle, APL \textbf{95}, 252105 (2009).   

Limits of Conductivity in ZnO Thin Films: Experiment and Theory

Transparent conductive oxides (TCOs) have major (multi-{\$}B) roles in applications such as flat-panel displays, solar cells, and architectural glass. The present workhorse TCO is indium-tin-oxide (ITO), but the recent huge demand for ITO has made In very expensive; moreover, it is toxic. The most commonly suggested replacement for ITO is ZnO, doped with Al, Ga, or In, and indeed the ISI lists 628 papers on Group-III-doped ZnO in 2009. However, to our knowledge, none of these papers has included calculations of donor N$_{D}$ and acceptor N$_{A}$ concentrations, the fundamental components of conductivity in semiconductors. We have developed a simple model for the calculation of N$_{D}$ and N$_{A}$ from temperature-dependent measurements of carrier concentration n, mobility $\mu $, and film thickness d. With the inclusion of phonon scattering in the model, excellent fits of n and $\mu $ are obtained from 15 -- 300 K. Experimentally, we have shown that highly conductive ZnO films can be grown by pulsed laser deposition in a pure Ar ambient, rather than the usual O$_{2}$. In a 278-$\mu $m-thick film, we have achieved a room-temperature resistivity $\rho $ = 1.96 x 10$^{-4}$ $\Omega $-cm, carrier concentration n = 1.14 x 10$^{21}$ cm$^{-3}$, and mobility $\mu $ = 28.0 cm$^{2}$/V-s. From our model, we calculate N$_{D}$ = 1.60 x 10$^{21}$ and N$_{A}$ = 4.95 x 10$^{20}$ cm$^{-3}$; however, the model also predicts that a significant reduction of N$_{A}$ would give $\mu $ = 42.5 cm$^{2}$/V-s and $\rho $ = 7.01 x 10$^{-5} \quad \Omega $-cm, a world record. Such a reduction in N$_{A}$ may be possible by in-diffusion of Zn after growth, since there is evidence that one of the major acceptor species in these films is the Zn-vacancy/Ga$_{Zn}$ complex. We can also decrease the resistivity by annealing in forming gas, and have recently attained $\rho $ = 1.46 x 10$^{-4} \quad \Omega $-cm, n = 1.01 x 10$^{21}$ cm$^{-3}$, and $\mu $ = 42.2 cm$^{2}$/V-s, giving N$_{D}$ = 1.13 x 10$^{21}$ and N$_{A}$ = 1.09 x 10$^{20}$ cm$^{-3}$. In very thin films, quantum effects must be considered.   

The Electronic Properties of Native Point Defects at ZnO Surfaces and Interfaces

Despite nearly sixty years of research, several fundamental issues surrounding ZnO remain unresolved. Among the key roadblocks to ZnO optoelectronics have been the difficulty of p-type doping and the role of compensating native defects. Oxygen vacancies (V$_{O})$, Zn interstitials (Zn$_{I})$, and residual impurities such as H, Al, Ga, and In are reported to be donors in ZnO, while Zn vacancies (V$_{Zn})$ are considered to be acceptors. Electrically active complexes of V$_{O}$, Zn$_{I}$, and V$_{Zn}$ can also exist. Although their impact on free carrier compensation and recombination is recognized, the physical nature of the donors and acceptors dominating carrier densities in ZnO and their effects on carrier injection at contacts is unresolved. The impact of these electronic states on ZnO carriers at the nanoscale is only now being explored. We can now address these issues using a combination of depth-resolved and scanned probe techniques. Taken together, we clearly identify the optical transitions and energies of V$_{Zn}$ and V$_{Zn}$ clusters, effects of annealing on their spatial distributions in ion-implanted ZnO, and how V$_{Zn}$ and V$_{Zn}$ clusters modify the near- and sub-surface carrier densities. Indeed, these native point defects can directly impact the activation of extrinsic dopants. We have now discovered that nanostructures form spontaneously on ZnO polar surfaces and create sub-surface V$_{Zn}$ locally because of Zn diffusion that feeds the nanostructure growth. Overall, this work reveals the interplay between ZnO electronic defects, polarity, and surface nanostructure. \\[4pt] [1] Y. Dong, F. Tuomisto, B. G. Svensson, A. Yu. Kuznetsov, and L. J. Brillson, ``Vacancy defect and defect cluster energetics in ion-implanted ZnO,'' Phys. Rev. B \textbf{81}, 081201(R) (2010).\\[0pt] [2] D. Doutt, H. L. Mosbacker, G. Cantwell,J. Zhang, J. J. Song, and L. J. Brillson, ``Impact of near-surface defects and morphology on ZnO luminescence,'' Appl. Phys. Lett. \textbf{94}, 042111 (2009).   

Realization of high performance random laser diodes

For the past four decades, extensive studies have been concentrated on the understanding of the physics of random lasing phenomena in scattering media with optical gain. Although lasing modes can be excited from the mirrorless scattering media, the characteristics of high scattering loss, multiple-direction emission, as well as multiple-mode oscillation prohibited them to be used as practical laser cavities. Furthermore, due to the difficulty of achieving high optical gain under electrical excitation, electrical excitation of random lasing action was seldom reported. Hence, mirrorless random cavities have never been used to realize lasers for practical applications -- CD, DVD, pico-projector, etc. Nowadays, studies of random lasing are still limited to the scientific research. Recently, the difficulty of achieving `battery driven' random laser diodes has been overcome by using nano-structured ZnO as the random medium and the careful design of heterojunctions. This lead to the first demonstration of room-temperature electrically pumped random lasing action under continuity wave and pulsed operation. In this presentation, we proposed to realize an array of quasi-one dimensional ZnO random laser diodes. We can show that if the laser array can be manipulated in a way such that every individual random laser can be coupled laterally to and locked with a particular phase relationship to its adjacent neighbor, the laser array can obtain coherent addition of random modes. Hence, output power can be multiplied and one lasing mode will only be supported due to the repulsion characteristics of random modes.   

Nanogenerators and Piezotronics

Developing wireless nanodevices and nanosystems is of critical importance for sensing, medical science, environmental/infrastructure monitoring, defense technology and even personal electronics. It is highly desirable for wireless devices to be self-powered without using battery. This is a new initiative in today's energy research for mico/nano-systems in searching for sustainable self-sufficient power sources [1]. We have invented an innovative approach for converting nano-scale mechanical energy into electric energy by piezoelectric zinc oxide nanowire arrays [2]. As today, a gentle straining can output 1-3 V from an integrated nanogenerator, using which a self-powered nanosensor has been demonstrated. A commercial LED has been lid up [3-5]. Due to the polarization of ions in a crystal that has non-central symmetry, a piezoelectric potential\textit{ (piezopotential)} is created in the crystal by applying a stress. The effect of piezopotential to the transport behavior of charge carriers is significant due to their multiple functionalities of piezoelectricity, semiconductor and photon excitation. Electronics fabricated by using inner-crystal piezopotential as a ``gate'' voltage to tune/control the charge transport behavior is named \textit{piezotronics [6,7].Piezo-phototronic effect} is a result of three-way coupling among piezoelectricity, photonic excitation and semiconductor transport, which allows tuning and controlling of electro-optical processes by strain induced piezopotential [8]. \\[4pt] [1] Z.L. Wang, \textit{Scientific American}, 298 (2008) 82-87; \\[0pt] [2] Z.L. Wang and J.H. Song, \textit{Science}, 312 (2006) 242-246. \\[0pt] [3] R.S. Yang, Y. Qin, L.M. Dai and Z.L. Wang, \textit{Nature Nanotechnology}, 4 (2009) 34-39. \\[0pt] [4] S. Xu, Y. Qin, C. Xu, Y.G. Wei, R.S. Yang, Z.L. Wang, \textit{Nature Nanotechnology}, 5 (2010) 366. \\[0pt] [5] G. Zhu, R.S. Yang, S.H. Wang, and Z.L. Wang , Nano Letters, 10 (2010) 3151. \\[0pt] [6] Z.L. Wang, \textit{Adv. Mater}., 19 (2007) 889-992. \\[0pt] [7] W.Z. Wu, Y.G. Wei and Zhong Lin Wang , Adv. Materials, DOI: adma.201001925. \\[0pt] [8] Y.F. Hu, Y.L. Chang, P. Fei, R.L. Snyder and Z.L. Wang, \textit{ACS Nano}, $4$ (2010) 1234--1240. \\[0pt] [9] Research supported by DARPA, DOE, NSF, Airforce, NIH, Samsung. For details: http://www.nanoscience.gatech.edu/zlwang/.