Session J14: Focus Session: Dopants and Defects in Zinc Oxide

Shallow acceptor complexes in p-type ZnO

ZnO films grown on sapphire substrates by organometallic vapor phase epitaxy exhibit p-type behavior when sufficient N is properly incorporated and followed by an appropriate annealing sequence. While substitutional N on the O sublattice is a deep acceptor, shallow acceptor complexes involving N, H and V$_{Zn}$ can provide useful 10$^{18}$ cm$^{-3}$ p-type films. Taking advantage of Raman, SIMS, and Hall-effect data, we establish a two-step growth scheme to form a metastable double donor complex, N$_{{\mathrm{Zn}}^-}$V$_{\mathrm{O}}$, then convert it to a single shallow acceptor complex, V$_{{\mathrm{Zn}}^-}$N$_{\mathrm{O}^-}$H$^{+}$ during \textit{in situ} annealing in N$_{2}$O. The V$_{{\mathrm{Zn}}^-}$N$_{\mathrm{O}^-}$H$^{+}$ complex accepts electrons at ionization energies of 134 meV, rendering it an efficient p-type dopant at room temperature.   

Hindered rotation of the 3326 cm$^{-1}$ OH center in ZnO?

Experiments on H in ZnO have found two OH vibrational lines at 3611 and 3326 cm$^{-1}$ [1,2]. The IR line at 3611 cm$^{-1}$ has been assigned to isolated H$^{+}$ in a bond centered configuration. The band at 3326 cm$^{-1}$ has been assigned to H$^{+}$ in an antibonding configuration in the vicinity of another defect, perhaps Ca [3]. The 3326 cm$^{-1}$ band has a distinctive dependence on temperature, consisting of several overlapping components whose intensities show thermally activated behavior over the temperature range 4K to 50K. This behavior is reminiscent of previous results for the hindered rotation of an off-axis OD-Li center in MgO [4]. Our results suggest that the off-axis motion of OH centers in oxides might be a common occurrence. \\[4pt] [1] E.V. Lavrov \textit{et al.}, Phys. Rev. B \textbf{79}, 165210 (2009).\\[0pt] [2] S.J. Jokela and M.D. McCluskey, Phys. Rev. B \textbf{72}, 113201 (2005).\\[0pt] [3] M.D. McCluskey and S.J. Jokela, Physica B \textbf{401-402}, 355 (2007).\\[0pt] [4] K. Martin \textit{et al.}, Phys. Rev. B \textbf{75}, 245211 (2007).   

Quantum Monte Carlo models of substitutional point defects in zinc oxide and zinc selenide

Introducing dopants into semiconductors allows manipulation of electrical and optical properties, useful for applications such as optoelectronics and photovoltaics. While first principles quantitative descriptions of the defects properties in semiconductors are critical to understanding and engineering dopants in semiconductors, obtaining accurate descriptions has proven challenging in the past. Here we demonstrate the use of quantum Monte Carlo (QMC) methods to describing the properties of point defects in zinc oxide and zinc selenide. Due to its direct treatment of electron correlation, the QMC method is capable of accurate calculation of band gaps and defect behaviors. We describe the energetics and potential barrier to forming gallium DX-center defects according to QMC in zinc selenide, and compare the description to those of conventional and hybrid DFT. We also use QMC to determine the defect transition levels for nitrogen defects in zinc oxide, and show that QMC obtains descriptions that are in good agreement with GW and beyond-DFT approaches. Our results demonstrate the importance of accurate descriptions of electron correlation in the calculation of defect properties of semiconductors.   

Structural Stability and Ionic Defects in ZnO from Quantum Monte Carlo

An accurate method capable of describing atomic, molecular and solid-state systems is required to take a full advantage of computer-aided materials discovery and design. We show that the many-body \textit{ab-initio} diffusion monte carlo (DMC) method is now a practical method for such tasks. We have studied the atomic, molecular and solid-state properties of the Zn-O system with DMC. In particular, the equation of state of bulk Zn and ZnO in the rock salt, zinc blend and wurtzite phases and the properties of ionic defects in wurtzite ZnO were studied. The first ionization potential of O and Zn, and the atomization energy of O$_{2}$, ZnO dimer, and wurtzite ZnO as well as the band gap of this material were evaluated with DMC, and the results agree with experimental measurements to within 0.2 eV. The DMC atomization energy of bulk Zn, 1.00(1) eV, is also in good agreement with the experimental value, 1.35 eV, considering the complexity of this metallic system. The DMC calculated properties of Zn and ZnO under pressure, and the formation energy for the oxygen vacancy, hydrogen impurities and Zn interstitial defects in ZnO will be discussed in comparison with results from experiments and density functional theory approximations.   

Intrinsic point defects and their interaction with impurities in mono-crystalline zinc oxide

Zinc oxide (ZnO) is a direct and wide band-gap semiconductor with several attractive features, like an exciton binding energy of $\sim$ 60 meV, for light emitting devices, photovoltaics and spintronics. In the past decade, ZnO has received tremendous attention by the semiconductor physics community and many challenging issues have been addressed, especially the ``native'' n-type conductivity, the role of intrinsic point defects, and the realization of reproducible p-type doping. The latter is, indeed, decisive for a true breakthrough of ZnO-based optoelectronics. In this contribution, recent progress in our understanding of the interaction between intrinsic point defects and impurities in ZnO will be discussed. Aluminum (Al) is often introduced intentionally to accomplish high n-type conductivity since Al on Zn-site (Al$_{Zn})$ acts as a shallow donor. However, Al$_{Zn}$ was recently found to react strongly with Zn vacancies (V$_{Zn})$ and the resulting complex (Al$_{Zn}$-V$_{Zn})$ is energetically favorable. The Al$_{Zn}$-V$_{Zn}$ complex is a deep acceptor limiting the n-type doping efficiency and this finding is expected to hold in general for complexes between V$_{Zn}$ and group-III elements. Further, implantation of self-ions (Zn and O) has been demonstrated to affect radically the balance of intrinsic point defects where an excess of Zn interstitials gives rise to a dramatic depletion of residual Li impurities on Zn-site (Li$_{Zn})$ whilst the opposite holds for an excess of O interstitials. In fact, this behavior appears to be of general validity and Li depletion occurs for a wide variety of implanted elements incorporated into the Zn sub-lattice while Li pile-up occurs for elements residing on O-site. Finally, the most prominent deep-level defect in ZnO, labelled E3, will be shown to involve hydrogen. E3 exists in most ZnO materials, irrespective of the growth method used, and evidence for a center formed by reaction between interstitial hydrogen and primary defects on the Zn sub-lattice will be given.   

Dynamical Interplay Between Intrinsic Defects and Impurity Ions in Very Dilute Fe-doped MgO Thin Films

The nature of intrinsic defects and impurities in the dielectric layer of a typical magneto tunneling junction is of great interest to understand tunneling of spin-polarized currents. In this connection, we have carried out studies of the electronic and magnetic properties of Mg$_{1-x}$Fe$_{x}$O. In particular, we have compared results from first principles calculations based on Density Functional Theory with highly accurate experiments. The measurements were performed with a Quantum Interference Device and by using soft x-ray absorption spectroscopy. Our study reveals basic defect units composed a Fe impurity coupled to one or two Mg vacancies. The trimer unit (i.e. Fe with two Mg vacancies) produces a magnetic net spin opposed to the dilute magnetism present in the oxide matrix. These findings could pave a way for engineering dielectric layers with high endurance and optimal tunneling properties by controlling the concentration of impurities and defects in the oxide matrix. Work supported by the US DOE.   

Exploring Cd-Zn-O-S alloys for optimal buffer layers in thin-film photovoltaics

The development of thin-film photovoltaics has largely focused on alternative absorber materials, while the choices for other layers in the solar cell stack have remained somewhat limited. In particular, cadmium sulfide (CdS) is widely used as the buffer layer in typical record devices utilizing absorbers like Cu(In,Ga)Se$_{\mathrm{2}}$ (CIGSe) or Cu$_{\mathrm{2}}$ZnSnS$_{\mathrm{4}}$ (CZTS) despite leading to a loss of solar photocurrent due to its band gap of 2.4 eV. While different buffers such as Zn(S,O,OH) are beginning to become competitive with CdS, the identification of additional wider-band gap alternatives with electrical properties comparable to or better than CdS is highly desirable. Here we use hybrid functional calculations to characterize Cd$_{x}$Zn$_{1-x}$O$_{y}$S$_{1-y}$ candidate buffer layers in the quaternary phase space composed by Cd, Zn, O, and S. We focus on the band gaps and band offsets of the alloys to assess strategies for improving absorption losses from conventional CdS buffers while maintaining similar conduction band offsets known to facilitate good device performance. We also consider additional criteria such as lattice matching to identify regions in the composition space that may provide improved epitaxy to CIGSe and CZTS absorbers. Lastly, we incorporate our calculated alloy properties into simulations of typical CIGSe devices to identify the Cd$_{x}$Zn$_{1-x}$O$_{y}$S$_{1-y}$ buffer compositions that lead to the best performance.   

Impact of Mg Content on (Mg,Zn)O Native Point Defects

The two most thermodynamically stable defects in ZnO are oxygen vacancies (V$_{O})$ and zinc vacancies (V$_{Zn})$. These native point defects are electrically charged and can contribute to free carrier densities. Experiment shows that Mg addition to ZnO significantly changes native defect densities. To better understand this dramatic decrease in V$_{Zn}$ and V$_{O}$-related defects with increasing Mg content up to x$=$0.44 and the subsequent increase, we performed density functional theory (DFT) calculations using PAW potentials within PBE using VASP. The results showed to be very sensitive to DFT method used and chemical-potential calculation. For the latter, the literature shows that one can assume that the oxygen chemical potential equals that of the atoms in the oxygen molecules at a given surrounding partial oxygen pressure. However, one can also postulate that the total defect concentrations conserve the stoichiometry, or limiting potentials from elemental equilibrium phases can be used. The experimentally observed dependence helped identify the correct way to reproduce the experimental dependence of formation energy on Mg concentration, which will be discussed in detail in this presentation.   

A hybrid functional study of oxygen interstitial defects in amorphous In-Ga-Zn-O semiconductors

Amorphous indium-gallium-zinc oxide (a-IGZO) semiconductors have attracted much attention because these materials are considered as the replacement of amorphous Si in high performance thin film transistors (TFTs). Although a-IGZO has been the subject of extensive studies due to its superior properties, problems such as threshold voltage shift main as obstacles for device applications. While O-vacancy defects were suggested to be responsible for the device instability under negative bias illumination stress (NBIS), there is no systematic study for the origin of instability under positive bias stress (PBS). In this work, we reveal the origin of PBS instability by performing hybrid density functional calculations for O interstitials in a-IGZO. The defect configuration of an O interstitial depends on its charge state. While an O-O dimer is stable in neutral state, it is easily broken by capturing eletrons, acting as an electron trap. Based on the results for the formation energy and transition level of an interstitial O, we propose that excess O atoms in a-IGZO are responsible for the PBS instability.   

Ionic liquid gated IGZO thin film field effect transistors

Ionic liquid gated field effect transistors have been extensively studied due to their low operation voltage, ease of processing and the realization of high electric fields at low bias voltages. Here, we report ionic liquid (IL) gated field effect transistor based on amorphous Indium Gallium Zinc Oxide (IGZO) thin film active layers. Conveniently, our device structure includes a conventional bottom gate SiO$_{2}$ insulator so the transfer characteristics of the IL could be directly compared to an equivalent 100 nm thick SiO$_{2}$ gate insulator. The transport measurement of the IL revealed the intrinsic n-channel property of the IGZO thin film with high ON/OFF ratio $\sim$ 10$^{5}$ and a large field effect electron mobility of 2.54 cm$^{2}$V$^{-1}$S$^{-1}$ at 300K and a threshold voltage of 0.1V. Comparable measurements on the bottom SiO$_{2}$ gate insulator revealed an ON/OFF ratio $\sim$ 10$^{9}$ and field effect electron mobility of 5.24 cm$^{2}$V$^{-1}$S$^{-1}$ and a threshold voltage of 4.0V. Interestingly, temperature dependent measurements revealed that the ionic liquid electric double layer can be ``frozen-in'' when dropped below the glass transition temperature which could lead to new switching and possibly non-volatile memory applications.   

ALD growth of non-polar ZnO-based heterojunctions for UV lighting: structural, electrical and optical properties

Non-polar, semi-polar, and polar ZnO films can be well controlled to epitaxially grow on different substrates such as p-Si (111), p-GaN, and sapphire substrates by atomic layer deposition at 200 $^o$C by introducing different interlayers of Al$_2$O$_3$, or InGaN or AlN or even none. The electroluminescence of the n-ZnO/Al$_2$O$_3$/p-GaN heterojunctions was dominated by a blue emission under forward biases, whereas it was violet emission under reverse biases. Under an ultralow driven current density, a blue emission could be observed from the nanocrystalline ZnO/GaN heterojunctions. Well defined Al nanoparticles (NPs) arrays with different shapes were fabricated on the surface of ZnO by electron-beam lithography. The theoretical analysis based on the finite-difference time-domain method was carried out to show the shape dependence of the localized surface plasmon resonance wavelength. By top excitation of the Al NP arrays coupled with ZnO, a 2.6-fold enhancement in peak photoluminescence intensity was measured. Furthermore, the enhancement strongly depends on the NP’s shape, revealing an important way of geometrical tuning of the UV-emission.