Session J25: Focus Session: Electric-to-Light Conversion and Optics in Semiconductors II

Promises and challenges in solid-state lighting

Lighting technologies based on semiconductor light-emitting diodes (LEDs) offer unprecedented promises that include three major benefits: (i) Gigantic energy savings enabled by efficient conversion of electrical energy to optical energy; (ii) Substantial positive contributions to sustainability through reduced emissions of global-warming gases, acid-rain gases, and toxic substances such as mercury; and (iii) The creation of new paradigms in lighting driven by the unique controllability of solid-state lighting sources. Due to the powerful nature of these benefits, the transition from conventional lighting sources to solid-state lighting is virtually assured. This presentation will illustrate the new world of lighting and illustrate the pervasive changes to be expected in lighting, displays, communications, and biotechnology. The presentation will also address the formidable challenges that must be addressed to continue the further advancement of solid-state lighting technology. These challenges offer opportunities for research and innovation. Specific challenges include light management, carrier transport, and optical design. We will present some innovative approaches in order to solve known technical challenges faced by solid-state lighting. These approaches include the demonstration and use of new optical thin-film materials with a continuously tunable refractive index. These approaches also include the use of polarization-matched structures that reduce the polarization fields in GaInN LEDs and the hotly debated efficiency droop, that is, the decreasing LED efficiency at high currents.   

Spectroscopic Properties of Zinc Oxide Nanostructures Grown by Chemical Vapor Deposition

Zinc oxide (ZnO), with its large bandgap energy of 3.37 eV and exciton binding energy, is a promising semiconductor material for a number of optoelectronic devices and applications ranging from solid-state lighting to photovoltaics, and can potentially serve as a solid matrix for nanoscale sensors. Nanostructures of ZnO have recently attracted a lot of interest because of their good crystalline quality and unique optical characteristics. Here, we focus on the study physical properties of ZnO nanowires grown by chemical vapor deposition, including absorption, photoluminescence, Raman, and terahertz time-domain spectroscopy.   

Size effect of diverse ZnO nanostructures

ZnO nanostructures including nanowires, tetrapods and nanocrystals are investigated by low temperature photoluminescence spectroscopy at 20K and 90K. Three peaks at 3.360, 3.308 and 3.235 eV are attributed to recombinations of bound excitons (BX), the first and second order longitudinal optical replicas of free excitons respectively. At 20 K, the 3.360eV BX peak is dominant for all these ZnO nanostructures, and B$_{F}$, defined as the intensity ratio of BX line to the sum total of the three lines above, is close to unity. At 90K, we find the size dependence of B$_{F}$. Among the nanowires and tetrapods, B$_{F}$ changes monotonically from 0.7 for radius of 100nm, to less than 0.2 for r=2 $\mu$m. For nanocrystals, B$_{F} $ varies from 0.4 at r=100nm to less than 0.17 at r=600nm. A simple model is presented, in which nanowires and tetrapod legs are approximated by cylindrical geometry and nanocrystals by spheres, suggesting the above size dependency is due to the inhomogenous density distribution of the defects as binding sites for BX. These defects might also be responsible for the so-called ``anomalous blueshift'' of the NBE peak at RT.   

Size-dependent photoelastic effect in ZnO nanorods

Manipulation of internal strain by an external light beam called photoelastic effect has been firmly established in ZnO nanorods. The underlying mechanism of this interesting phenomenon arises from the combination of the screening of internal electric field and converse piezoelectric effect. We demonstrate that the photoelastic effect is more pronounced in thinner nanorods due to a larger surface to volume ratio. In addition to giving a good evidence for the existence of photoelastic effect in semiconductor nanorods, our finding also provides an excellent possibility for the development of nanoscale optical modulators.   

Spectroscopic evidence for limited carrier hopping interaction in amorphous ZnO thin film

The electronic structure of amorphous ZnO film (a-ZnO) was examined by O K-and Zn L3-edge x-ray absorption spectroscopy and valence band photoemission spectroscopy. Comparative studies of a-ZnO and a wurtzite ZnO (w-ZnO) revealed a decrease in Zn 4s-O 2p hybridization strength and the localization of Zn 4s band as a consequence of local structural disorder, indicating limited electron hopping interactions in a-ZnO. The 0.1 eV higher Fermi-level of a-ZnO compared to w-ZnO suggests that the electrical properties of a-ZnO are different from those in w-ZnO due to structural disorder, even in the absence of impurities or grain boundaries.   

Exciton Localization in ZnSe$_{1-x}$O$_{x}$

We have studied the relative magnitude of the localized density of states in ZnSe$_{1-x}$O$_{x}$ (0 $<$ x $<$ 0.063) through modeling of the temperature dependence of the photoluminescence (PL) and time-resolved PL at 15K. Alloying a small amount of ZnO with ZnSe results in a large decrease of the band gap from a downward shift of the conduction band. The random distribution of oxygen in the lattice results in local fluctuations of the conduction band edge, which form tails of localized states that localized excitons. This results in an anomalous blue shift of the PL emission with increasing temperature and a strong emission energy dependence to the PL decay time.   

Effective strain-induced band gap narrowing of anatase TiO2: A soft crystal direction

TiO2 is one of the promising materials for photoelectrochemical hydrogen production through water splitting. However, due to its large band gap (3.2. eV), it cannot absorb sun light effectively. To reduce its band gap, various approaches have been attempted, including applying strain. Using first-principles band structure method, we have studied the electronic and elastic properties of TiO2 with anatase and rutile phases. We calculated the band gap deformation potentials of both phases under hydrostatic, epitaxial and uniaxial strains. We find that the hydrostatic deformation potential of TiO2 is small. However, unlike the rutile phase, there is a soft direction in anatase phase. By applying strain in this soft direction, we show that the band gap of anatase TiO2 could be narrowed effectively, thus, offering an opportunity to reduce the band gap of TiO2. We demonstrate that this approach of tuning the band gap by applying strain along soft direction of a layered semiconductor is general and should be applicable to other anisotropic energy-related materials.   

Electronic structure of N doped TiO$_{2}$: Hard x-ray photoemission study

We have determined the electronic structure of N doped TiO$_{2}$ using a combination of synchrotron based hard x-ray photoemission spectroscopy (HAXPES) and first principle density functional theory calculations. Comparison with calculations and experiment reveals strong evidence of hybridization between different orbital components. Our results reveal that N doping of TiO$_2$ leads to the formation of oxygen vacancies and the combination of both N impurity and oxygen vacancies accounts for the observed visible light catalytic behavior of Ndoped TiO$_2$.   

Correlation between optical and structural properties of single II-VI compound semiconductor nanowires

Semiconductor nanowires have recently found tremendous interest in research community due to their prospective applications in areas such as electronic devices, photo-voltaics, and biological and chemical sensing. Among them, wide band-gap II-VI semiconductor nanowires have been the subject of intense research due to their potential applications in optical and opto-electronic devices in the UV and visible range. The suitability of these nanowires depends largely on, among other factors, their crystal structure, and type and density of structural defects. We present our work on the study of optical response of individual II-VI nanowires, and their correlation with the structural properties of the nanowires. We have employed spatially-resolved confocal micro-photoluminescence spectroscopy to study the optical properties and transmission electron microscopy to study the structural properties of the single nanowires.