Carrier dynamics and coherent acoustic phonons in nitride heterostructures

We model generation and propagation of coherent acoustic phonons in piezoelectric InGaN/GaN multi-quantum wells embedded in a \textit{pin} diode structure and compute the time resolved reflectivity signal in simulated pump-probe experiments. Carriers are created in the InGaN wells by ultrafast pumping below the GaN band gap and the dynamics of the photoexcited carriers is treated in a Boltzmann equation framework. Coherent acoustic phonons are generated in the quantum well via both deformation potential electron-phonon and piezoelectric electron-phonon interaction with photogenerated carriers, with the latter mechanism being the dominant one. Coherent longitudinal acoustic phonons propagate into the structure at the sound speed modifying the optical properties and giving rise to a giant oscillatory differential reflectivity signal. We demonstrate that coherent optical control of the differential reflectivity can be achieved using a delayed control pulse.Comment: 14 pages, 11 figure

Ultrafast spectroscopy of propagating coherent acoustic phonons in GaN/InGaN heterostructures

We show that large amplitude, coherent acoustic phonon wavepackets can be generated and detected in In$_x$Ga$_{1-x}$N/GaN epilayers and heterostructures in femtosecond pump-probe differential reflectivity experiments. The amplitude of the coherent phonon increases with increasing Indium fraction $x$ and unlike other coherent phonon oscillations, both \textit{amplitude} and \textit{period} are strong functions of the laser probe energy. The amplitude of the oscillation is substantially and almost instantaneously reduced when the wavepacket reaches a GaN-sapphire interface below the surface indicating that the phonon wavepackets are useful for imaging below the surface. A theoretical model is proposed which fits the experiments well and helps to deduce the strength of the phonon wavepackets. Our model shows that localized coherent phonon wavepackets are generated by the femtosecond pump laser in the epilayer near the surface. The wavepackets then propagate through a GaN layer changing the local index of refraction, primarily through the Franz-Keldysh effect, and as a result, modulate the reflectivity of the probe beam. Our model correctly predicts the experimental dependence on probe-wavelength as well as epilayer thickness.Comment: 11 pages, 14 figure

Origin of the Broad Lifetime Distribution of Localized Excitons in InGaN/GaN Quantum Dots

We derive an energy-dependent decay-time distribution function from the multi-exponential decay of the ensemble photoluminescence (PL) of InGaN/GaN quantum dots (QDs), which agrees well with recently published single-QD time-resolved PL measurements. Using eight-band k.p modelling, we show that the built-in piezo- and pyroelectric fields within the QDs cause a sensitive dependence of the radiative lifetimes on the exact QD geometry and composition. Moreover, the radiative lifetimes also depend heavily on the composition of the direct surrounding of the QDs. A broad lifetime distribution occurs even for moderate variations of the QD structure. Thus, for unscreened fields a multi-exponential decay of the ensemble PL is generally expected in this material system.Comment: 5 pages, 4 figures. accepted at Physica Status Solid

Carrier-induced refractive index change and optical absorption in wurtzite InN and GaN: Fullband approach

Based on the full band electronic structure calculations, first we consider the effect of n-type doping on the optical absorption and the refractive index in wurtzite InN and GaN. We identify quite different dielectric response in either case; while InN shows a significant shift in the absorption edge due to n-type doping, this is masked for GaN due to efficient cancellation of the Burstein-Moss effect by the band gap renormalization. For high doping levels the intraband absorption becomes significant in InN. Furthermore, we observe that the free-carrier plasma contribution to refractive index change becomes more important than both band filling and the band gap renormalization for electron densities above 10$^{19}$~cm$^{-3}$ in GaN, and 10$^{20}$~cm$^{-3}$ in InN. As a result of the two different characteristics mentioned above, the overall change in the refractive index due to n-type doping is much higher in InN compared to GaN, which in the former exceeds 4\% for a doping of 10$^{19}$~cm$^{-3}$ at 1.55~$\mu$m wavelength. Finally, we consider intrinsic InN under strong photoexcitation which introduces equal density of electron and holes thermalized to their respective band edges. The change in the refractive index at 1.55~$\mu$m is observed to be similar to the n-doped case up to a carrier density of 10$^{20}$~cm$^{-3}$. However, in the photoexcited case this is now accompanied by a strong absorption in this wavelength region due to $\Gamma^v_5 \to \Gamma^v_6$ intravalence band transition. Our findings suggest that the alloy composition of In$_x$Ga$_{1-x}$N can be optimized in the indium-rich region so as to benefit from high carrier-induced refractive index change while operating in the transparency region to minimize the losses. These can have direct implications for InN-containing optical phase modulators and lasers.Comment: Revised with an appendix, two additional figures, and more discussions; 10 pages, 14 figures; published versio

Calibration of Polarization Fields and Electro-Optical Response of Group-III Nitride Based c-Plane Quantum-Well Heterostructures by Application of Electro-Modulation Techniques

The polarization fields and electro-optical response of PIN-diodes based on nearly lattice-matched InGaN/GaN and InAlN/GaN double heterostructure quantum wells grown on (0001) sapphire substrates by metalorganic vapor phase epitaxy were experimentally quantified. Dependent on the indium content and the applied voltage, an intense near ultra-violet emission was observed from GaN (with fundamental energy gap Eg = 3.4 eV) in the electroluminescence (EL) spectra of the InGaN/GaN and InAlN/GaN PIN-diodes. In addition, in the electroreflectance (ER) spectra of the GaN barrier structure of InAlN/GaN diodes, the three valence-split bands, Γ9, Γ7+, and Γ7−, could selectively be excited by varying the applied AC voltage, which opens new possibilities for the fine adjustment of UV emission components in deep well/shallow barrier DHS. The internal polarization field Epol = 5.4 ± 1.6 MV/cm extracted from the ER spectra of the In0.21Al0.79N/GaN DHS is in excellent agreement with the literature value of capacitance-voltage measurements (CVM) Epol = 5.1 ± 0.8 MV/cm. The strength and direction of the polarization field Epol = −2.3 ± 0.3 MV/cm of the (0001) In0.055Ga0.945N/GaN DHS determined, under flat-barrier conditions, from the Franz-Keldysh oscillations (FKOs) of the electro-optically modulated field are also in agreement with the CVM results Epol = −1.2 ± 0.4 MV/cm. The (absolute) field strength is accordingly significantly higher than the Epol strength quantified in published literature by FKOs on a semipolar (112¯2) oriented In0.12Ga0.88N quantum well

Surfactants and digital alloys for strain relief in III-nitride distributed Bragg reflectors and related heterostructures via metal organic vapor phase epitaxy

III-Nitride based semiconductors have emerged as one of the promising materials for electronic and opto-electronic devices, including but not limited to, solid state emitters, photodetectors, and transistors. Despite commercial success, several issues ranging from material growth to device fabrication remain unresolved and continue to hinder the efficiency of these devices. One such issue includes strain management in III-Nitride heterostructures. The binary alloys in the (Al,In,Ga)N family are characterized by a large lattice and thermal mismatch which leads to defect formation and cracking within heterostructures. These defects are detrimental to device fabrication and operation. This work investigates growth based techniques to manage strain in III-Nitride heterostructures and thereby reduce defect formation.;In particular, this work focuses on the development surfactant assisted growth and digital alloys as strain relieving techniques to minimize cracking in Aluminum Gallium Nitride (AlxGa1- xN) alloys and related heterostructures via Metal Organic Vapor Phase Epitaxy. Indium has been investigated as a surfactant in the growth of AlN/GaN Distributed Bragg Reflectors (DBRs) and has been shown to reduce the cracking by a factor of two. Using variable temperature x-ray diffraction studies, indium has been shown to influence the thermal expansion coefficients of the AlN layers. The digital growth technique has been investigated as a viable method for achieving high quality, crack free AlxGa 1-xN films. Alloys with an AlN mole fraction ranging from 0.1 to 0.9 have been grown by adjusting the periodicity of these short period superlattice structures. High resolution x-ray diffraction has been used to determine the superlattice period along with the a- and c-lattice parameter of the structure. High aluminum content digital AlxGa1-xN alloys have been employed in DBRs for high reflectivity, \u3e94%, crack-free structures. The characterization of these structures via scanning electron microscopy, atomic force microscopy, and x-ray diffraction is presented along with the results from the integration of the DBR with visible wavelength LEDs

The research on optical properties of InN and InGaN films

随着人类社会进入信息化社会，信息量飞速增长。为适应生活的需求以及时代的要求，以半导体为代表的材料和器件迅速发展，遍及人类生活的各个领域。InN与InGaN材料因其带隙随In组分x变化从0.7到3.4eV连续可调，其对应的吸收光谱的波长从紫外部分(365nm)可以一直延伸到近红外部分(1770nm)，几乎完整地覆盖了整个太阳光谱，这为设计新型太阳能电池、超高亮度发光二极管（LED）以及全彩显示提供了极大的可能，所以InN与InGaN材料近年来逐渐成为研究的热点。由于薄膜的光学常数（如折射率、吸收系数、色散关系等）是描述固体的独立光学常数，是确定和描述其他物理量的基础，并且高性能的光电子器件的设计...With the society entered the the information-oriented society, the amount of information the rapid growth. In order to meet the daily needs and requirements of the times, as the representative of semiconductor materials and devices rapid development throughout all areas of human life. Because of the band gap of the InN and InGaN material changes with In component continuously, which can adjust fro...学位：理学硕士院系专业：物理科学与技术学院_凝聚态物理学号：1982013115298

OPTICAL MODE PATTERN STUDY OF GAN BASED LEDS WITH AND WITHOUT NANOSCALE TOP GRATING

This study analyzes optical confinement factor and light emitting mode order for three different GaN LEDs: a conventional LED, thin Film LED, and thin Film LED with a photonic crystal (PhC) grating. For the first structure, we increase the thickness of AlxGa1-xN from 0 to 600nm, alter the x composition in AlxGa1-xN from 0.05 to 0.2 in steps of 0.05, and adjust the p-GaN and n-GaN thicknesses each from 0 to 200nm. For the second structure, we alter the n-GaN substrate thickness from 300-1000nm in steps of 100nm and 1000-4000nm in steps of 1000nm. These simulations show that increasing the substrate thickness causes the light emitting mode order to increase. The higher the mode, the more current is needed to make the device emit light. Higher current leads to shorter device lifetime. The last structure contains a photonic crystal grating with a period T = 100nm, 230nm, 460nm, 690nm, 920nm, 1500nm, 2000nm, 3000nm and 50% duty cycle. For each grating period, we display the effects on optical confinement factor and optical field intensity. The results show that changing the grating period does not affect the mode order, but does affect the optical field intensity. A larger grating period corresponds to lower optical field intensity. Maximizing optical field intensity increases the brightness of the device. The simulation method above can be used to improve the efficiency, brightness, and lifetime of GaN LEDs by reducing the effects of transverse mode coupling and maximizing the optical field intensity

Microstructure of BAlN and InGaN Epilayers for Optoelectronic Applications

abstract: In this dissertation, various characterization techniques have been used to investigate many aspects of the properties of III-nitride materials and devices for optoelectronic applications. The first part of this work is focused on the evolution of microstructures of BAlN thin films. The films were grown by flow-modulated epitaxy at 1010 oC, with B/(B+Al) gas-flow ratios ranging from 0.06 to 0.18. The boron content obtained from X-ray diffraction (XRD) patterns ranges from x = 0.02 to 0.09, while Rutherford backscattering spectrometry (RBS) measures x = 0.06 to 0.16. Transmission electron microscopy indicates the sole presence of the wurtzite crystal structure in the BAlN films, and a tendency towards twin formation and finer microstructure for B/(B+Al) gas-flow ratios greater than 0.15. The RBS data suggest that the incorporation of B is highly efficient, while the XRD data indicate that the epitaxial growth may be limited by a solubility limit in the crystal phase at about 9%. Electron energy loss spectroscopy has been used to profile spatial variations in the composition of the films. It has also located point defects in the films with nanometer resolution. The defects are identified as B and Al interstitials and N vacancies by comparison of the observed energy thresholds with results of density functional theory calculations. The second part of this work investigates dislocation clusters observed in thick InxGa1-xN films with 0.07 ≤ x ≤ 0.12. The clusters resemble baskets with a higher indium content at their interior. Threading dislocations at the basket boundaries are of the misfit edge type, and their separation is consistent with misfit strain relaxation due the difference in indium content between the baskets and the surrounding matrix. The base of the baskets exhibits no observable misfit dislocations connected to the threading dislocations, and often no net displacements like those due to stacking faults. It is argued that the origin of these threading dislocation arrays is associated with misfit dislocations at the basal plane that dissociate, forming stacking faults. When the stacking faults form simultaneously satisfying the crystal symmetry, the sum of their translation vectors does add up to zero, consistent with our experimental observations.Dissertation/ThesisDoctoral Dissertation Physics 201