Physics

Despite the fact that ZnO has been a widely and efficiently employed for centuries as a material of choice in many areas, its new promising applications became apparent only recently, with the advent of novel growth techniques producing bulk ZnO crystals of a very high quality. This translated into a high potential for optoelectronics, spintronics, and high-temperature, high-power microelectronics. 
For many of these applications, and especially for the nanoscale-based, the condition of the surface and the subsurface region is a key performance-defining factor. Because of the large surface/volume ratio in ZnO nanostructures, device performance is determined essentially by the surface and near-surface properties. Current understanding of the relationship between the morphology of the ZnO nanostructures and their defect properties is still largely incomplete. The nature of the surface and sub-surface defect states is still ambiguous and only in a small number of studies in the past few years attempts were made to correlate properties of these states with the morphology of the nanocrystals themselves on the one hand and on the other hand to modify these states in a controllable fashion. In our studies we investigate a number of different ZnO systems, for which studies of surface/interface defect properties of ZnO may yield a significant outcome. In particular, a distinct class of surface/interface processes of interest for us is the influence of defects on the optolelectronic phenomena and structural properties in various ZnO nanostructures. We aim to extend our understanding of the fundamental mechanisms in such systems and their influence on the valuable applied properties. 

Due to its potential optoelectronic applications (lasers and light emitters, planar waveguide devices, flat panel displays, etc.), the rare earth (Er, Tm, Tb, Dy) doping of semiconductors, and ZnO in particular, is an important field of study. In such materials, the shielded 4f levels of the rare earth (RE) ions produce narrow optical transitions. Despite the fact that the rear earth ions are excellent candidates for luminescent centers, their matrix environment often limits the luminescence efficiency. It is important to elucidate the optimum conditions for high-efficiency luminescence of those materials, and ZnO is one of the most important representatives. In addition, the rare earth ion dopant can serve as a sensitive probe of the chemistry and structure of its host. We are interested in addressing numerous problems and unresolved issues in this area such as difficulties with incorporating the RE ions into the host ZnO lattice, unknown mechanisms of energy transfer from the matrix to the RE species, almost unidentified relationship between their structural and optical properties. There is no clear model of the upconversion transitions in the RE-doped ZnO systems. 

Experimental Facilities 
Experimental instrumentation available at our lab includes: photoluminescence (PL) spectroscopy, Raman spectroscopy, and a high-vacuum (HV) analysis/processing multi-chamber setup. One of the main advantages of this multifunctional HV system is a versatile combination of in situ processing and characterization tools. It includes remote plasma treatment simultaneous with resistive annealing and ability to accommodate a number of subsurface-sensitive and surface-specific spectroscopic probes such as surface photovoltage (SPV) spectroscopy and Auger electron spectroscopy (AES). 

We employ remote plasma as a tool for tailoring surface and sub-surface properties. Remote plasma processing refers to the arrangement, in which the surface-plasma contact occurs outside the plasma-generating region. The main advantage of using remote plasma follows from the fact that the chemically driven changes at the surface occur without significant temperature variations. This allows a separate control of the temperature at the surface of a specimen. In our research we demonstrated that well-defined remote-plasma treatment procedures allow control and qualitative improvement of key performance parameters of the studied surfaces.
 
SPV is known for its ability to detect surface states and distinguish their charge sates and donor- vs. acceptor-like nature. It is based on a vibrating Kevlin probe positioned near the surface of the sample of interest. Light of a variable frequency generates transitions from/to the gap states, modifies the population of the surface states, and induces changes in the surface barrier heights. This translates into the variation of the surface potential detected by the Kelvin probe. The SPV technique offers notable advantages: identification of conduction vs. valence band nature of the deep level transitions and the deep level positions within the band gap; ability to measure surface defect densities less than 1010 cm-2 as well as their cross sections. Additional information can be deduced from the SPV transient measurements. The adjacent AES probe provides surface-specific information about surface stiochiometry and contamination. 

Complementary to the in vacuo spectroscopic probes, we have in our lab a multi-purpose optical bench setup allowing PL and Raman studies in a wide range of temperatures (6K – 320K), polarizations, and geometries, as well as several laser beams: ultraviolet and visible continuous wave HeCd and ultraviolet pulsed nitrogen. This facility provides information about optoelectronic, structural and chemical properties of the studied systems. 

The research in the group is focused on the development and application of quantum theoretical techniques for the study of atomic and molecular systems. Projects range from the study of the structure and properties of molecular clusters to the development of new theoretical and computational techniques as well as more fundamental questions regarding the interpretation of quantum theory. 

Molecular Clusters 
Pure and mixed molecular clusters of carbon, silicon, and germanium atoms provide an interesting and challenging group of molecular systems to a theorist. Sufficiently high-level theoretical methods have to be employed to provide accurate data to be used in conjunction with the analysis of experimental data. 

In these theoretical studies we employ state-of-the-art computational techniques to solve the quantum mechanical many-body electron problem in the Born-Oppenheimer approximation. These techniques includes the so called coupled cluster methods where the electronic wave-function is essentially expressed in an infinite sum with certain constraints that lead to a finite computational scheme. Relatively recently, an alternative approach, the density functional method (DFT), has been developed for the description of electronic ground states. Here, instead of attempting to describe the electronic wave function, the focus is on calculating the electron density. Such DFT techniques can provide very accurate information at a very modest computational cost and enable us to study and describe large molecular clusters more accurately. 

Although advanced software is available for electronic structure calculations an additional challenge is to provide results to experimentalists that are meaningful in that they come with some type of “error bars” to facilitate in the comparison with real experimental results. One of the major goals in the group is to develop and employ techniques in a way to facilitate the resolution of experimental spectra. As a result a number of new theoretical techniques that serve as interfaces between theory and experiment have been developed. 

Electronic Structure Methods 
Quantum theoretical and computational methods are developed and refined in order to perform efficient calculations of the electronic structure of atoms and molecules. Our main interest has been in coupled cluster methods and the closely related many-body perturbation theoretical techniques. 

Fundamental Quantum Theory 
Quantum theory is a well-established theory which provides a highly accurate description of microcosmic phenomena. Although developed in the early part of the 20th century several problems concerning the interpretation of quantum theory still remain. Ongoing projects involve the study of the structure of the theory in the complex energy plane using complex scaling techniques as well as investigations of new and alternative descriptions of the quantum theory measurement.