Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session A46: Focus Session: Wide Bandgap Semiconductors I |
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Sponsoring Units: DMP Chair: Mary Ellen Zvanut, University of Alabama Room: Baltimore Convention Center 349 |
Monday, March 13, 2006 8:00AM - 8:36AM |
A46.00001: Efficiency in Piezoelectric GaInN/GaN Green Light Emitting Diodes. Invited Speaker: The color of green is most closely related to human's perception of light, while red and blue have more of a signal character. Wide bandgap GaInN/GaN heterostruture devices successfully expanded the wavelength range of light emitting diodes (LEDs) from the red to blue and the UV. Performance in the green (500 -- 570 nm), however, substantially lags behind requirements for energy efficient solid state white lighting. In particular, it is a significant challenge to extent the wavelength range from the blue back to the lower emission energies of green. Problems to incorporate high fractions of In and large biaxial strain are compounded by strong polarization properties of the material system. From detailed bandstructure spectroscopy we reveal that and how the piezoelectric properties play the pivotal role in the light emission properties. On these premises, polarization optimized green LED dies have been developed with superior performance at 525 nm and the deep green to 560 nm. In an ongoing process, device performance and efficiencies are analyzed and compared to a large set of sample data and provide relevant optimization feedback. [Preview Abstract] |
Monday, March 13, 2006 8:36AM - 8:48AM |
A46.00002: Growth and Characterization of III-Nitride LEDs on Vicinal Sapphire Substrates N. Khan, J. Li, J. Y. Lin, and H. X. Jiang GaN and related III-N alloys have great potential applications in optoelectronic devices such as light emitting diodes (LEDs), laser diodes (LDs) and high power electronic devices such as transistors. For improved performance and greater versatility of these devices, it is essential to improve the quality of materials and devices. It has been reported that growth on vicinal or misaligned substrates compared to flat ones, lead to smoother and higher crystalline quality thin films. Improved brightness and quantum efficiency, and a reduction of the forward current at which maximum quantum efficiency occurs are among the improved optical properties resulting from the use of misaligned substrates and a thick base layer. We report on the characteristics of InGaN based blue/green LEDs grown by metal organic chemical vapor deposition (MOCVD) on specially fabricated dome shaped sapphire substrates. These dome shaped substrates provide varying degrees of off-cut within a single wafer. Surface morphologies, crystalline qualities and emission properties across the entire wafer surface have been mapped by various techniques, including optical and atomic force microscopy (AFM), x-ray diffraction and electroluminescence and photoluminescence spectroscopy. The results are compared with those of blue/green LEDs grown on conventional on-axis substrates. [Preview Abstract] |
Monday, March 13, 2006 8:48AM - 9:00AM |
A46.00003: Improvement of AlGaN-based deep-ultraviolet light emitting diodes performance with a nano-textured/roughened sapphire substrate Muhammad Khizar, Yasin Raja Performance improvement of the fabricated AlGaN-based deep-ultraviolet light emitting diode (DUV LEDs) on sapphire with nano-textured/roughned substrate is reported. The output optical powers as well as the current-voltage characteristics of the processed devices with and without textured/roughened substrate are compared. Lapping/polishing and dry-etching techniques were employed for nano-texturing/roughening of the substrate. Magnetron dry etching was performed using a mixed CF$_{2}$Cl$_{2}$/Ar feed gases followed by an inductively coupled plasma (ICP) etching in BCl$_{3}$/Cl$_{2}$ plasma chemistry. An etching rate of $\sim $145 nm/min for the sapphire was easily obtained. The result show that the optical emission power of the fabricated devices with textured/roughened sapphire substrate is higher as compared to similar un-textured/roughened devices. Based on the analysis, it is concluded that a textured/roughened substrate surface with an RMS $\sim $ 15 -- 25 nm shows improved light extraction efficiency, which is the result of the reduced internal reflection of the light caused by the nano-textured/roughened surface profile thereby causing deviation for critical angle. [Preview Abstract] |
Monday, March 13, 2006 9:00AM - 9:12AM |
A46.00004: Growth and Optical Properties of Al rich AlN/AlGaN Quantum Wells T.M. Al tahtamouni, N. Nepal, M.L. Nakarmi, J.Y. Lin, and H.X. Jiang Al rich AlGaN alloys are promising materials for the applications in the optoelectronic devices such as deep ultraviolet (UV) emitters and detectors in the spectral range down to 200 nm. AlGaN based UV emitters ($\lambda <$340nm) has applications in bio-chemical agent detection and medical research/ health care. To realize deep UV emission ($\lambda <$ 280 nm) Al rich AlGaN based quantum wells (QWs) are required. We report here the growth of AlN/Al$_{x}$Ga$_{1-x}$N$_{ }$QWs (x$>$0.65) on AlN/sapphire templates by metalorganic chemical vapor deposition (MOCVD). Deep UV photoluminescence (PL) was employed to study the optical properties of the QWs. Well width (Al composition) dependence was studied by varying the QW thickness (Al composition) with fixed x $\sim $ 0.65 (well width at 3 nm). Optical properties of these QWs such as the effects of alloy fluctuation, temperature, strain and piezoelectric field, carrier and exciton localizations on the quantum efficiency have been studied. Carrier and exciton dynamics were probed. Implications of our findings on the applications of Al rich AlN/AlGaN QWs for UV emitters and detectors will also be discussed. [Preview Abstract] |
Monday, March 13, 2006 9:12AM - 9:24AM |
A46.00005: Optical properties of Er doped III-nitride epilayers and quantum wells synthesized by MOCVD C. Ugolini, M.L. Nakarmi, N. Nepal, J.Y. Lin, and H.X. Jiang The wide bandgap semiconductor, GaN, is an excellent host for Er since the thermal quenching of radiative intra-4f \textit{Er}$^{3+}$ transitions is very low in Er implanted or Er doped GaN, and the solubility of Er in GaN is high. Due to these properties and the characteristic transition of 1.54 $\mu $m of Er$^{3+}$, Er doped GaN structures are promising for Er related emitters operating in the infrared region. In recent studies, Er doped III-Nitride epilayers were obtained by ion implantation or molecular beam epitaxy (MBE). But, in-situ Er doping of III-Nitride epilayers has not been achieved by metalorganic chemical vapor deposition (MOCVD), mostly due to the low vapor pressure and lack of suitable, metalorganic Er sources. However, due to the large output and high-crystalline quality of MOCVD grown III-nitride epilayers, use of this process to grow Er doped III-nitride epilayers would be very useful for commercial applications. We report on the synthesis of Er doped III-nitride epilayers and quantum wells (QW) grown by MOCVD, and their optical properties. Optical properties in the visible and infrared region for epilayers and QW's of different Er concentrations and growth conditions are discussed. The mechanisms of optical transitions involving different intra-4f Er$^{3+}$ energy levels are also discussed. Lastly, potential applications of Er doped III-nitride structures in the communication wavelength will be presented. [Preview Abstract] |
Monday, March 13, 2006 9:24AM - 9:36AM |
A46.00006: Structural and optical properties of GaN films grown on GaAs substrates by molecular beam epitaxy O. Maksimov, V. Heydemann, P. Fisher, H. Du, M. Skowronski, Y. Gong, I. Kuskovsky GaN emerged during the past decade as a wide band gap semiconductor with promising material properties for the development of short-wavelength optoelectronic, high frequency and high power electronic devices. Most GaN research has been focused on the growth of high quality GaN epitaxial layer on A12O3 and SiC substrates. The MBE growth of GaN on GaAs substrates is much less studied, although it provides several advantages, such as a closer thermal expansion coefficient matching and a possibility to stabilize cubic $\beta $-GaN. In this work we report on MBE growth of GaN films on the (100) GaAs substrates. Contrary to previous literature accounts, we do not employ an As beam, neither during oxide desorption nor during GaN nucleation and growth. We determine that direct growth on thermally desorbed GaAs results in a polycrystalline film that contains inclusions of both $\alpha $ and $\beta $ GaN. Low-temperature nitridation followed by annealing improves structural properties resulting in the growth of $\alpha $-GaN with 0002 orientation. [Preview Abstract] |
Monday, March 13, 2006 9:36AM - 9:48AM |
A46.00007: Characterization of stress in thin-film wurtzite GaN grown on c-plane sapphire by molecular beam epitaxy. Francisco Parada, Michael Marciniak, LaVern Starman, Jason Foley, Yung Kee Yeo The stress distribution in wurtzite gallium nitride (GaN) grown on c-plane sapphire substrates by molecular beam epitaxy is characterized. Micro ($\mu )$-Raman spectroscopy is particularly useful for stress characterization because of its ability to measure the spectral shifts in Raman peaks in a material, and correlate those shifts with stress and strain. The phonon deformation potential is determined by applying pressure to the material using a four-point strain fixture while simultaneously monitoring the applied pressure using a strain gauge and recording the Raman spectrum. The deformation potentials are then used to determine stress distribution; the spectral positions of the E$_{2}$ Raman mode ($\nu $ = 569 cm-1) in GaN and A$_{1g}$ Raman mode ($\nu $ = 418 cm$^{-1})$ in sapphire are recorded at each spatial position in a raster map. The $\mu $-Raman spectroscopy is performed using a Renishaw InVia Raman spectrometer with argon ion ($\lambda $ = 514.5 nm, h$\nu $ = 2.41 eV) and helium-neon ($\lambda $ = 633 nm, h$\nu $ = 1.96 eV) excitation sources, and the data is collected across the samples with 5- to 10-$\mu $m spatial resolution. Inherent stress and evidence of significant damage in the GaN layer due to MEMS processing will be discussed. [Preview Abstract] |
Monday, March 13, 2006 9:48AM - 10:00AM |
A46.00008: The Influence of Phonons and Phonon Decay on the Optical Properties of GaN D. Y. Song, M. Basavaraj, S. A. Nikishin, M. Holtz, V. Soukhoveev, A. Usikov, V. Dmitriev The temperature dependences of vibrational and optical properties of high-quality GaN are studied using Raman and photoluminescence (PL) spectroscopies in the range 20 to 325 K. The Raman-active A$_{1}$(LO) phonon has temperature dependence described well by combined two- and three-phonon decay. The temperature dependences of E$_{2}^{2 }$phonon are almost entirely dominated by the thermal expansion, and the contribution of three-phonon decay process is very small throughout interested temperature range. The shallow neutral donor-bound exciton (D$^{0}$,X) and two free excitons (X$_{A}$ and X$_{B})$ are observed at low temperatrue PL spectra. Also seen are two A$_{1}$(LO) phonon sidebands (PSBs), originating from the X$_{B}$ free exciton, with the characteristic asymmetry attributed to interactions between discrete and continuum states. Analysis of the band-edge excitons reveals that energy gap shrinkage and exciton linewidths are completely described based on electron-phonon interactions with phonon properties consistent with the Raman analysis. First and second PSBs have temperature dependence associated with the A$_{1}$(LO) phonon. The shift, broadening, and asymmetry of the PSBs are explained by Segall-Mahan theory adding the decay mechanism of A$_{1}$(LO) phonon and the exciton broadening from electron-phonon interactions. Work at Texas Tech University supported by National Science Foundation grant ECS-0323640. [Preview Abstract] |
Monday, March 13, 2006 10:00AM - 10:12AM |
A46.00009: Bandedge transitions in GaN and AlN at elevated temperatures N. Nepal, M. L. Nakarmi, J. Li, J. Y. Lin, and H. X. Jiang GaN and AlN are recognized as promising materials for high power high temperature optoelectronic and electronic devices. For device realization, understanding fundamental properties of these materials at elevated temperature is very important. Probing the optical properties such as the temperature dependence of the bandedge transitions reveals important information pertaining to the fundamental band structures, exciton and carrier recombination and activation processes. Deep-ultraviolet photoluminescence (PL) spectroscopy has been employed to study the bandedge transitions in metalorganic chemical vapor deposition grown GaN and AlN epilayers up to 800 K. The temperature dependence of the PL emission intensity revealed two different activation processes in both GaN and AlN epilayers. The first process occurring below T$_{t}$ = 325 K (T$_{t}$ = 500 K) for GaN (AlN) is due to the activation of free excitons to free carriers. Whereas the second activation process occurring above T$_{t}$ with an activation energy of 0.29 eV (0.3 eV) for GaN (AlN) is believed to be associated with the existence of a satellite valley ($\Gamma _{3})$ at about 0.3 eV above the conduction band minimum, $\Gamma _{1}$. Consequence of this satellite valley in the conduction band on the high temperature and high power device applications of GaN and AlN will be discussed. [Preview Abstract] |
Monday, March 13, 2006 10:12AM - 10:24AM |
A46.00010: Hot carrier dynamics in bulk GaN under short laser pulse excitation S. Rudin, E. Bellotti, G. A. Garrett, M. Wraback In this work, we apply the semiconductor Bloch equation formalism to the study of the electron-hole dynamics in bulk GaN under short laser pulses excitation. Both coherent and non-coherent processes contribute to the dynamics on short time scales. The numerical solution of the equations of motion for the electron-hole plasma and the polarization is obtained by using a generalized Monte Carlo algorithm. This includes a direct solution of the coherent process and a stochastic description of the dephasing mechanisms. Screened carrier-carrier interaction and carrier-phonon interaction are considered in the dephasing processes. The model also includes the description of the interaction of the ultra-short laser pulse with the semiconductor media. The non-parabolic two-band analytical approximation was used in this work. The interaction of LO-phonons with electrons in GaN is much stronger than in GaAs, and the LO-phonon-electron scattering rate in GaN is almost one order higher than that in GaAs. The computed time dependent carrier densities were used to obtain time dependent luminescence at different probe wave-lengths for different excitation powers. The results were compared with the experimental results obtained by the subpicosecond time-resolved spectroscopy of UV luminescence. [Preview Abstract] |
Monday, March 13, 2006 10:24AM - 10:36AM |
A46.00011: Nonequilibrium carrier dynamics in AlGaN/GaN surface quantum wells monitored by time-resolved photoluminescence spectroscopy Y. D. Glinka, J. V. Foreman, W. Davenport, H. O. Everitt, X. Zhang, I. P. Wellnius, J. F. Muth, J. Roberts, P. Rajagopal, J. Cook, E. Piner, K. Linthicum The photoluminescence (PL) from a GaN surface quantum well of an AlGaN/GaN heterostructure has been studied by using a wide range of ultrafast ($\sim $200 fs) laser excitations and temperatures (4.2 -- 300 K) for quantum wells of varying widths. We observe a broad ($\sim $100 meV) PL band whose peak shifts linearly from 3.5 to 3.8 eV with decreasing GaN quantum well thickness from 2.9 to 1.5 nm. The spectral shift of the band is accompanied by the corresponding temporal shift in the range $<$30 ps. The dynamics of nonequilibrium electrons in the GaN layer confined by the AlGaN barrier and the vacuum level is discussed, with particular emphasis on carrier interactions with both acoustic and optical phonons. [Preview Abstract] |
Monday, March 13, 2006 10:36AM - 10:48AM |
A46.00012: Time Resolved Photoluminescence Measurements in hexagonal Mg$_{x}$Zn$_{1-x}$O (x=0.13) alloys Shiva Hullavarad, R. Vispute, S. Dhar, Henry Everitt, John Foreman, I. Takeuchi In this work we present results on the optical characterization of Pulsed Laser Deposited epitaxially grown thin films of Mg$_{x}$Zn$_{1-x}$O on sapphire. The crystalline quality and composition of the alloys are characterized by X-Ray Diffraction and Rutherford Back Scattering (RBS) -- ion channeling techniques respectively. The composition of the films is found to have strong dependence on the growth parameters. The photo generated carrier decay times are analyzed by time-resolved photoluminescence (TRPL) measurements. Since most optical and electrical devices are operated at room temperature, understanding the fundamental excess carrier recombination dynamics at 300 K is required to evaluate the relevant radiative and non-radiative recombination mechanisms and thus to improve the performance of devices. We have observed from initial PL measurements evidence of varying amounts of non-radiative relaxation processes competing with radiative PL in the MgZnO samples. ~More importantly, we have observed a decay time of 125 ps for films from TR-PL measurements. The efficiency of the radiative recombination, and therefore the material quality, is strongly related to the decay time of the particular transition. The correlation of the results obtained from RBS and PL-TRPL measurements will be presented. [Preview Abstract] |
Monday, March 13, 2006 10:48AM - 11:00AM |
A46.00013: Optical and magnetic properties of Zn$_{1-x}$Co$_{x}$O and ZnCo$_{2}$O$_{4}$ thin films. K. Samanta, P. Bhattacharya, R.S. Katiyar, W. Iwamoto, R.R. Urbano, C.P. Pagliuso, C. Rettori The transition metal doped wide bandgap ZnO are considered possible candidates for room temperature ferromagnetic semiconductor for spintronic devices. Thin films of Co substituted ZnO and ZnCo$_{2}$O$_{4}$ were deposited using pulsed laser deposition on c-axis (0001) oriented Al$_{2}$O$_{3}$ substrates. The XRD results showed all the films were highly (002) oriented with a less intense peak of (311) for ZnCo$_{2}$O$_{4}$ thin film. Micro-Raman spectra of ceramic targets showed the modes related to wurtzite ZnO and spinel ZnCo$_{2}$O$_{4}$ structure. In thin films of Zn$_{1-x}$Co$_{x}$O no such modes related to ZnCo$_{2}$O$_{4}$ were detected. The intensity of E$_{1}$(LO) and multiphonon peaks at 540 and 584 cm$^{-1}$, respectively, increased with increase in the Co substitution. The optical absorption of the films showed that the band gap decreased with the increase of Co concentrations at room temperature along with the sub-bandgap absorption due to $d-d$ transitions of Co$^{2+}$. Similar sub-bandgap $d-d$ transition was also observed in absorption spectra ZnCo$_{2}$O$_{4}$ thin film. [Preview Abstract] |
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