Session J5: Electricity to Light Conversion: Solid State Lighting

Accurate treatment of spontaneous polarization in III-nitrides

The III-nitride compounds assume the wurtzite crystal structure in the ground state and therefore exhibit spontaneous and piezoelectric dipole moments in the c direction. Discontinuities in these moments at heterostructure interfaces result in electric fields in the layers, which can be detrimental because they separate electrons and holes in quantum wells. Accurate values for polarization differences are critical for understanding and engineering III-nitride heterostructures. Direct experimental measurement of spontaneous polarization has not been possible to date, and calculations are complicated by the necessity to choose a reference structure. The universal choice of reference structure for wurtzite has been zincblende; we demonstrate that this choice does not allow consistent determination of the differences of spontaneous polarizations between materials, which determine their physical manifestation. Using first-principles techniques based on hybrid density functional theory, we have determined polarization discontinuities using a consistent reference based on the hexagonal layered structure of these materials. We will discuss the results in light of available experimental data, and outline consequences for device simulations.   

Electronic and optical properties of AlN/GaN superlattices from first-principles calculations

Group-III-nitrides are important materials for efficient light emitters in the ultraviolet and visible range. Superlattices of AlN/GaN quantum wells are especially promising for ultraviolet light emission. We use first-principles calculations to investigate the electronic and optical properties of AlN/GaN quantum well superlattices. Density functional theory with quasiparticle corrections from the GW method provides accurate electronic band structures. We then solve the Bethe-Salpeter equation to predict exciton binding energies and fundamental optical emission energies from first principles, yielding good agreement with available experimental measurements. Ultimately, we elucidate the relationship between optical emission energy and well/barrier thickness, as well as demonstrate mitigation of the quantum confined Stark effect in ultra-narrow wells for enhanced radiative recombination efficiency. This research was supported by the J. Robert Beyster Computational Innovation Graduate Fellowship and in part by CSTEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science. Computational resources were provided by the DOE NERSC facility.   

Correlation between optical properties and strain relaxation in thick InGaN epitaxial films.

The alloy compositions, strain distributions and emission properties of thick In$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$N layers with x ranging from 0.13 to 0.38 are investigated. High resolution x-ray diffractions (XRD) and reciprocal space mapping (RSM) along an asymmetric axis reveal that the In composition inhomogeneity is accompanied by strain relaxations during the growth of thick InGaN layers. Photoluminescence (PL) results together with RSMs indicate that the observed double PL peaks are associated with the strained and relaxed phase in the InGaN films. It is further indicated that the relaxed phase in InGaN films exhibits better emission efficiency than the strained phase from the temperature-dependent PL measurements. Recombination dynamics from time-resolved PL measurements reveal that the carrier localization effect is more pronounced in the relaxed phase. According to the optical properties, the emission efficiency is strongly correlated with the localization effect in thick InGaN films.   

Characterization of $M$-plane InGaN/GaN Quantum Wells Grown on Misoriented $\gamma $-LiAlO$_{2}$ by Plasma-Assisted Molecular Beam Epitaxy

We have grown a series of InGaN/GaN quantum wells on misoriented LiAlO$_{2}$ (LAO) substrate with different growth parameters by plasma-assisted molecular beam epitaxy (PAMBE). The sample structure consists of \textbf{\textit{M}}-plane InGaN/GaN quantum wells(25nm GaN barrier/5nm InGaN well) with a $\sim$ 225nm GaN buffer layer between the quantum wells and LAO substrate. The mis-cut angle of LAO substrate was tilted 11$^{\circ}$ off [011] direction to match the atoms of GaN lattice. From the X-ray diffraction (XRD) measurement, we found two peaks of 34.6$^{\circ}$ and 32.2$^{\circ}$, indicating the LAO(100) substrate and [1\underline {1}00] oriented (\textbf{\textit{M}}-plane) for the GaN thin films, respectively. The surface morphology, optical property and microstructure of the samples were investigated by scanning electron microscope (SEM), photoluminescence (PL) and transmission electron microscope (TEM) measurements, as well.   

Growth of high-quality InN thin films on InGaN buffer layer by plasma-assisted molecular beam epitaxy

Four samples were grown on 2 inch \textbf{\textit{c}}-plane (0001) sapphire substrates with 4$\mu $m-thick GaN template. The InN thin films were grown on InGaN buffer layer by low-temperature plasma-assisted molecular beam epitaxy (PAMBE) system. These samples were grown under a varied temperature of InGaN buffer layers: 500$^{\circ}$C, 540$^{\circ}$C, 570$^{\circ}$C, and 600$^{\circ}$C. The structure properties of these samples were analyzed by X-ray diffraction (XRD). The interference fringes of InN grown on the sample 1 (the growth temperature of InGaN buffer layer at 500$^{\circ}$C) exhibit prominent oscillations, which indicates that the sample has a high quality and layer by layer epitaxial structure. The surface morphology and microstructure of samples were investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). We confirmed the smooth surface and high quality crystalline for the sample.   

Microstructure of Er optical centers in the large-bandgap semiconductor GaN

Photoluminescence properties at $\lambda =$1.54 $\mu $m from Er optical centers in GaN epilayers grown by metal-organic chemical vapor deposition are investigated in magnetic fields up to 17 T and high-resolution time-resolved PL spectroscopy. The magnetic field induced splitting is observed for all the main lines of the Er-related photoluminescence spectrum. For the most intense emission line, angular dependence of the splitting is measured in the (1120) crystallographic plane of the sample. The effective g-tensor, corresponding to the difference between individual g-tensors of the lowest multiplets of the ground and the first excited states, is experimentally determined. The magneto-optical measurements, the time-resolved photoluminescence and the temperature dependence of the photoluminescence spectroscopy show that the samples have two main optical centers and they can be excited selectively under band-to-band and resonance excitations.   

Effect of Dopant Activation on Device Characteristics of InGaN-based Light Emitting Diodes

Achieving high uniformity in growths and device characteristics of InGaN-based light-emitting diodes (LEDs) is important for large scale manufacturing. Dopant activation and maintaining control of variables affecting dopant activation are critical steps in the InGaN-based light emitting diodes (LEDs) fabrication process. In the epitaxy of large scale production LEDs, in-situ post-growth annealing is used for activating the Mg acceptor dopant in the p-AlGaN and p-GaN of the LEDs. However, the annealing temperature varies with respect to position in the reactor chamber, leading to severe uniform dopant activation issue across the devices. Thus, it is important to understand how the temperature gradient and the resulting variance in Mg acceptor activation will alter the device properties. In this work, we examine the effect of varying p-type doping levels in the p-GaN layers and AlGaN electron blocking layer of the GaN LEDs on the optoelectronic properties including the band profile, carrier concentration, current density, output power and quantum efficiency. By understanding the variations and its effect, the identification of the most critical p-type doping layer strategies to address this variation will be clarified.   

Electrical contact of wurtzite GaN mircrodisks on \textit{p}-type GaN template

We developed a back processing to fabricate a secure electrical contact of wurtzite GaN microdisk on a transparent p-type GaN template with the orientation, [10-10]$_{\mathrm{disk}}$ // [10-10]$_{\mathrm{template}}$. GaN microdisks were grown on LiAlO$_{2}$ substrate by using plasma-assisted molecular beam epitaxy [1]. In the further study, we analyzed the TEM specimen of a sample with annealed GaN microdisk/p-typed GaN template by selection area diffraction (SAD) to confirm the alignment of the microdisks with the template at the interface. From the I-V measurements performed on the samples, we obtained a threshold voltage of $\sim$ 5.9 V for the current passing through the GaN microdisks with a resistance of $\sim$ 45 K$\Omega $. The electrical contact can be applied to the nanometer-scaled GaN light-emitting diode. \\[4pt] [1] I. Lo, et at., Appl. Phys. Lett, \textbf{105}, 082101 (2014).   

Spatially resolved visible broad band multi-photon luminescence in GaN micro-pyramids

Multi-photon induced visible broad band luminescence was observed in position controlled m-polar GaN hexagonal micro-pyramids using a tunable near-infrared femtosecond laser source at room temperature. The GaN micro-pyramids were grown by metal-organic chemical vapor deposition using the epitaxial lateral overgrowth method. Excitation wavelengths corresponding to near band gap and below band gap energies were used. Spatially resolved optical characterization from a single micro-structure was achieved using multi-photon microscopy and spectroscopy. We observed broad band luminescence in the range of 400 nm to 600 nm for both near band gap and below band gap excitations. Power dependent excitation dependence confirms the origin of broad band visible emission as multi-photon induced luminescence (MPL). Band width of broadband visible light emission due to MPL can be controlled by selective spatial excitation of the GaN micro-pyramid.   

High Efficiency Alternating Current Driven Organic Light Emitting Devices Employing Active Semiconducting Gate Layers

In this work, we describe the role of semiconductor-polymer interfaces in alternating current (AC) driven organic electroluminescent (EL) devices. We implement inorganic semiconducting materials between the external contact and the active layers in organic light EL devices. Precise control of capacitance and charge injection is required to realize bright and efficient large area AC driven devices. We show how this architecture results in active gating to the polymer layers, resulting in the novel ability to control the capacitance and charge injection characteristics. We propose a model based on band bending at the semiconductor-polymer interface. Furthermore, we elucidate the influence of the semiconductor-polymer interface on the internally coupled magnetic field generated in an alternating current driven organic light emitting device configuration. Magnetic fields can alter the ratios of singlet and triplet populations, and we show that internal generation of a magnetic field can dramatically alter the efficiency of light emission in organic EL devices.   

Elasto-optic effect in semiconductors: a first principle approach using Maximally Localized Wannier Functions

Strain-induced changes of optical properties are of use in the design and functioning of devices that couple photons and phonons. The elasto-optic (or photo-elastic) effect describes a general materials property where strain induces a change in the dielectric tensor. Despite a number of experimental and computational works on this area, it is fair to say that a basic physical understanding of the effect and its materials dependence is lacking: for example, we know of no materials design rules for enhancing (or suppressing) elasto-optic response. Here, we begin by computing the elasto-optic tensor of silicon using Density Functional Theory (DFT). By~analyzing the longitudinal dielectric response to uniaxial strain, we find that a promising avenue to physical understanding of the basis of the effect is to work in a real space representation and to employ Maximally Localized Wannier Functions (MLWFs). We describe our results based on this approach. This work is supported by the National Science Foundation through grant NSF DMR-0808665.   

Directional Control of Plasmon-Exciton interaction with Plexcitonic Crystals

Plexcitons [1] are strongly coupled plasmon excitons modes. In this work, we developed a platform, consisting of one and two dimensional corrugated surface patterns coated with a thin metal film and a dye solution. This system shows a controlled coupling action based on the excitation direction of SPP modes [2]. Our scheme is based on the control of wavelengths of the forbidden SPP modes. Three kinds of patterns have been tested; a one dimensional uniform, a triangular, and a square lattice type crystals. For all three cases, lowest wavelength of the band gap is observed in $\Gamma $ to M direction. For triangular and square lattice cases, band gap center oscillates between two finite values for every 60$^{\circ}$ and 90$^{\circ}$s, respectively. We utilized this behavior to control SPP and J-aggregate coupling. We observe directional dependence of Rabi splitting energy varying between 0 meV and 60 meV . Square lattice gives the ability to tune a larger band gap, whereas triangular lattice gives higher number of symmetry points. Simulations show that, an 80 nm deep triangular lattice with 280 nm periodicity can result in omnidirectional decoupling of plexcitons. \\[4pt] [1] N. T. Fofang, T.-H. Park, O. Neumann, N. A. Mirin, P. Nordlander, and N. J. Halas, Nano Lett. 8, 3481 (2008).\\[0pt] [2] E. Karademir, S. Balci, C. Kocabas, and A. Aydinli, Opt. Express 22, 21912 (2014).   

Thermochemical, structural and electronic properties of amorphous oxides, nitrides and sulfides

Amorphous thin films materials become increasingly important components of many functional devices such as thin film displays, photovoltaic cells or thin film transistors. Due to lack of grain boundaries, they have superior uniformity and smoothest, flexibility and corrosion resistance. Amorphous thin films are typically prepared using physical vapor deposition (PVD) techniques at temperatures well below the melting point of deposited material (\textless 0.2Tm). Computational models of amorphous structures, however, are almost elusively constructed from a high temperature equilibrated crystal melt using simulated annealing (SA) protocol. To account for low temperature growth conditions of amorphous thin films we recently developed a new simulation technique [1]. The method, kinetically limited minimization (KLM), starts from a randomly initialized structure and minimizes the total energy in a number of local structural perturbation-relaxation events. We apply KLM to model amorphous structures of 20 binary oxides, nitrides and sulfides and compare their thermochemical, structural and electronic properties. \\[4pt] [1] Zawadzki P., Perkins J., Lany S.; Modeling amorphous thin films: Kinetically limited minimization. Physical Review B~\textbf{90} 094203 (2014)   

Colossal photoconductivity in transparent perovskite semiconductor BaSnO$_{3}$

We compare photoconductivities of BaSnO$_{3}$ (BSO) and SrTiO$_{3}$ (STO). Photoconductivity of STO has been intensively studied for its high potential for UV detector and optical devices. On the other hand, BSO has recently started to draw a large attention for its high electron mobility and thermal stability. BSO and STO have the same perovskite structure and similar band gap. Epitaxial thin films of BSO and STO were grown by pulsed laser ablation. The spectral responses measured by the monochromatic light showed peaks around the band gap of each film, an evidence that the electron-hole pair generation is the main mechanism for the photoconductivity in both materials. We have found the photoconductivity of BSO to be several orders of magnitude higher than that of STO. In addition, there exists a larger ``persistent'' photoconductivity in BSO. The high mobility of BSO, which is two orders of magnitude larger than that of STO at room temperature, should be partially responsible for the higher photoconductivity. The small difference between the direct gap and indirect gap of BSO may make band to band transition easier. We are also investigating the effect of dislocations on the ``persistent'' photoconductivity.   

$m$-Plane GaN Growth on ``Double Miscut'' Bulk Substrates for Blue Laser Diode Applications

Although nearly 100{\%} of today's commercial GaN devices are grown on the $c$-plane, the nonpolar $m$-plane is an alternative orientation that is free from polarization-induced electric fields, which separate carrier wavefunctions in $c$-plane InGaN quantum wells (QWs) and reduce radiative recombination rates compared to $m$-plane. Performance of $m$-plane blue laser diodes is currently limited by low In uptake and broad linewidth in the blue spectrum compared to $c$-plane. This work examines the impact of $m$-plane surface miscut on these performance aspects. The morphology was examined by atomic force microscopy for homoepitaxy on co-loaded substrates: on-axis, -1$^{\circ}$ $c-$miscut, 1$^{\circ}$ $a-$miscut. All three films showed regions of diagonal $a+c$ step direction where pure $a$- or $c$-direction steps were expected. These $a+c$ regions also emitted longer wavelength in fluorescence and cathodoluminescence than other step directions. ``Double miscut'' substrates in the combined $a$- and $c$-directions take advantage of the observed stable $a+c$ step direction and redshift. Multi-QWs on double miscut substrates exhibited \textless 30 nm linewidth in the blue spectrum and higher In uptake than ever achieved for standard miscut $m-$plane.