Optical characterization of InGaN heterostructures for blue light emitters and vertical cavity lasers: Efficiency and recombination dynamics

OPTICAL CHARACTERIZATION OF INGAN HETEROSTRUCTURES FOR BLUE LIGHT EMITTERS AND VERTICAL CAVITY LASERS: EFFICIENCY AND RECOMBINATION DYNAMICS By Serdal Okur, Ph.D. A thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at Virginia Commonwealth University. Virginia Commonwealth University, 2014. Major Director: Ümit Özgür, Associate Professor, Electrical and Computer Engineering This thesis explores radiative efficiencies and recombination dynamics in InGaN-based heterostructures and their applications as active regions in blue light emitters and particularly vertical cavities. The investigations focus on understanding the mechanism of efficiency loss at high injection as well as developing designs to mitigate it, exploring nonpolar and semipolar crystal orientations to improve radiative efficiency, integration of optimized active regions with high reflectivity dielectric mirrors in vertical cavity structures, and achieving strong exciton-photon coupling regime in these microcavities for potential polariton lasing. In regard to active regions, multiple double heterostructure (DH) designs with sufficiently thick staircase electron injection (SEI) layers, which act as electron coolers to reduce the overflow of hot electrons injected into the active region, were found to be more viable to achieve high efficiencies and to mitigate the efficiency loss at high injection. Such active regions were embedded in novel vertical cavity structure designs with full dielectric distributed Bragg reflectors (DBRs) through epitaxial lateral overgrowth (ELO), eliminating the problems associated with semiconductor bottom DBRs having narrow stopbands and the cumbersome substrate removal process. Moreover, the ELO technique allowed the injection of carriers only through the high quality regions with substantially reduced threading dislocation densities compared to regular GaN templates grown on sapphire. Reduced electron-hole wavefunction overlap in polar heterostructures was shown to hamper the efficiency of particularly thick active regions (thicker than 3 nm) possessing three-dimensional density of states needed for higher optical output. In addition, excitation density-dependent photoluminescence (PL) measurements showed superior optical quality of double heterostructure (3 nm InGaN wells) active regions compared to quantum wells (2 nm InGaN wells) suggesting a minimum limit for the active region thickness. Therefore, multiple relatively thin but still three dimensional InGaN active regions separated by thin and low barriers were found to be more efficient for InGaN light emitters. Investigations of electroluminescence from light emitting diodes (LEDs) incorporating multi DH InGaN active regions (e.g. quad 3 nm DH) and thick SEIs (two 20 nm-thick InGaN layers with step increase in In content) revealed higher emission intensities compared to LEDs with thinner or no SEI. This indicated that injected electrons were cooled sufficiently with thicker SEI layers and their overflow was greatly reduced resulting in efficient recombination in the active region. Among the structures considered to enhance the quantum efficiency, the multi-DH design with a sufficiently thick SEI layer constitutes a viable approach to achieve high efficiency also in blue lasers. Owing to its high exciton binding energy, GaN is one of the ideal candidates for microcavities exploiting the strong exciton-photon coupling to realize the mixed quasiparticles called polaritons and achieve ideally thresholdless polariton lasing at room temperature. Angle-resolved PL and cathodoluminescence measurements revealed large Rabi splitting values up to 75 meV indicative of the strong exciton-photon coupling regime in InGaN-based microcavities with bottom semiconductor AlN/GaN and a top dielectric SiO2/SiNxDBRs, which exhibited quality factors as high as 1300. Vertical cavity structures with all dielectric DBRs were also achieved by employing a novel ELO method that allowed integration of a high quality InGaN cavity active region with a dielectric bottom DBR without removal of the substrate while forming a current aperture through the ideally defect-free active region. The full-cavity structures formed as such were shown to exhibit clear cavity modes near 400 and 412 nm in the reflectivity spectrum and quality factors of 500. Although the polar c-plane orientation has been the main platform for the development of nitride optoelectronics, significant improvement of the electron and hole wavefunction overlap in nonpolar and semipolar InGaN heterostructures makes them highly promising candidates for light emitting devices provided that they can be produced with good crystal quality. To evaluate their true potential and shed light on the limitations put forth by the structural defects, optical processes in several nonpolar and semipolar orientations of GaN and InGaN heterostructures were investigated. Particularly, stacking faults were found to affect significantly the optical properties, substantially influencing the carrier dynamics in nonpolar (1-100), and semipolar (1-101) and (11-22)GaN layers. Carrier trapping/detrapping by stacking faults and carrier transfer between stacking faults and donors were revealed by monitoring the carrier recombination dynamics at different temperatures, while nonradiative recombination was the dominant process at room temperature. Although it is evident that nonpolar (1-100)GaN and semipolar (11-22)GaN require further improvement of material quality, steady-state and time-resolved PL measurements support that (1-101)-oriented GaN templates and InGaN active regions exhibit optical performance comparable to their highly optimized polar c-plane counterparts, and therefore, are promising for vertical cavities and light emitting device applications

Carrier dynamics in bulk GaN

Carrier dynamics in hydride vapor phase epitaxy grown bulk GaN with very low density of dislocations, 5–8 × 105 cm−2, have been investigated by time-resolved photoluminescence (PL), free carrier absorption, and light-induced transient grating techniques in the carrier density range of 1015 to ∼1019 cm−3 under single and two photon excitation. For two-photon carrier injection to the bulk (527 nm excitation), diffusivity dependence on the excess carrier density revealed a transfer from minority to ambipolar carrier transport with the ambipolar diffusion coefficient D a saturating at 1.6 cm2/s at room temperature. An extremely long lifetime value of 40 ns, corresponding to an ambipolar diffusion length of 2.5 μm, was measured at 300 K. A nearly linear increase of carrier lifetime with temperature in the 80–800 K range and gradual decrease of D pointed out a prevailing mechanism of diffusion-governed nonradiative recombination due to carrier diffusive flow to plausibly the grain boundaries. Under single photon excitation (266 and 351 nm), subnanosecond transients of PL decay and their numerical modeling revealed fast processes of vertical carrier diffusion, surface recombination, and reabsorption of emission, which mask access to pure radiative decay

Improved quantum efficiency in InGaN light emitting diodes with multi-double-heterostructure active regions

InGaN light emitting diodes(LEDs) with multiple thin double-heterostrucutre (DH) active regions separated by thin and low energy barriers were investigated to shed light on processes affecting the quantum efficiency and means to improve it. With increasing number of 3 nm-thick DH active layers up to four, the electroluminescence efficiency scaled nearly linearly with the active region thickness owing to reduced carrier overflow with increasing total thickness, showing almost no discernible efficiency degradation at high injection levels up to the measured current density of 500 A/cm2. Comparison of the resonant excitation dependent photoluminescence measurements at 10 K and room temperature also confirmed that further increasing the number of DH layers beyond six results in degradation of the material quality, and therefore, increasing nonradiative recombination. Using multiple DH active regions is shown to be a superior approach for quantum efficiency enhancement compared with simply increasing the single DH thickness or the number of quantum wells in LED structures due to better material quality and larger number of states available

Structural, compositional and mechanical characterization of plasma nitrided CoCrMo alloy

Thesis (Master)--Izmir Institute of Technology, Physics, Izmir, 2009Includes bibliographical references (leaves: 103-107)Text in English; Abstract: Turkish and Englishxv, 107 leavesPlasma nitriding techniques can be used to create wear and corrosion protective layers on the surface of CoCrMo alloys by modifying the near surface layers of these materials. In the present study, a medical grade CoCrMo alloy was nitrided in a low-pressure ( 60 mTorr) R. F. plasma at 400 Cfor 1, 2, 4, 6, and 20 hours under a gas mixture of 60% N2 . 40% H2. The structural as well as compositional characterization of the plasma nitrided layers were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and glow discharge optical emission spectroscopy (GDOES). The hardness and wear behaviour of the nitrided layers were performed by a microhardness tester and a pin-on-disk wear apparatus. The experimental analyses indicate that the expanded austenite phase, YN, with high N contents ( 30 at.%) is formed by the plasma nitriding process at 400 C. However, at longer nitriding times (6 and 20 h) there is decomposition into CrN in the YN matrix and a preferential (200) orientation of YN grains parallel to the surface develops. Based on the microscopy analyses of the electrochemically and Ar ion beam etched nitrided sample cross-sections and on the GDOES data, the YN layer thicknessesare found to be ranging from 2 to 10 microns. Based on the thickness data, an average N diffusion coefficient for the CoCrMo samples plasma nitrided at 400 C is estimated to be near 2x10-11 cm2/s. While significant improvements in hardness and wear volume reductions are observed for all the plasma nitrided alloys compared to the untreated alloy, the CoCrMo alloys with the YN structure only had the best combined wear-corrosion protection

Growth and characterization of α-, β-, and ϵ-phases of Ga2O3 using MOCVD and HVPE techniques

Heteroepitaxial films of Ga $_2$ O $_3$ were grown on c-plane sapphire (0001). The stable phase β-Ga $_2$ O $_3$ was grown using the metalorganic chemical vapor deposition technique, regardless of precursor flow rates, at temperatures between 500 $^\circ$ C and 850 $^\circ$ C. Metastable α- and ϵ-phases were grown when using the halide vapor phase epitaxy (HVPE) technique, at growth temperatures between 650 $^\circ$ C and 850 $^\circ$ C, both separately and in combination. XTEM revealed the better lattice-matched α-phase growing semi-coherently on the substrate, followed by ϵ-Ga $_2$ O $_3$ . The epitaxial relationship was determined to be [ $\bar {1}100$ ] ϵ-Ga $_2$ O $_3$ $\|$ [ $11\bar {2}0$ ] α-Ga $_2$ O $_3$ $\|$ [ $11\bar {2}0$ ] α-Al $_2$ O $_3$ . SIMS revealed that epilayers forming the ϵ-phase contain higher concentrations of Cl introduced during HVPE growth

Recombination dynamics of excitons with low non-radiative component in semi-polar (10-11)-oriented GaN/AlGaN multiple quantum wells

International audienceOptical properties of GaN/Al0.2Ga0.8N multiple quantum wells grown with semi-polar (10-11) orientation on patterned 7°-off Si (001) substrates have been investigated. Studies performed at 8 K reveal the in-plane anisotropic behavior of the QW photoluminescence (PL) intensity for this semi-polar orientation. The time resolved PL measurements were carried out in the temperature range from 8 to 295 K to deduce the effective recombination decay times, with respective radiative and non-radiative contributions. The non-radiative component remains relatively weak with increasing temperature, indicative of high crystalline quality. The radiative decay time is a consequence of contribution from both localized and free excitons. We report an effective density of interfacial defects of 2.3 × 1012 cm−2 and a radiative recombination time of τloc = 355 ps for the localized excitons. This latter value is significantly larger than those reported for the non-polar structures, which we attribute to the presence of a weak residual electric field in the semi-polar QW layers