Fabrication technology for high light-extraction ultraviolet thin-film flip-chip (UV TFFC) LEDs grown on SiC

The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to their poor light extraction efficiency (LEE). To improve the LEE of AlGaN LEDs, we developed a fabrication technology to process AlGaN LEDs grown on SiC into thin-film flip-chip LEDs (TFFC LEDs) with high LEE. This process transfers the AlGaN LED epi onto a new substrate by wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a highly selective SF6 plasma etch that stops at the AlN buffer layer. We optimized the inductively coupled plasma (ICP) SF6 etch parameters to develop a substrate-removal process with high reliability and precise epitaxial control, without creating micromasking defects or degrading the health of the plasma etching system. The SiC etch rate by SF6 plasma was ~46 \mu m/hr at a high RF bias (400 W), and ~7 \mu m/hr at a low RF bias (49 W) with very high etch selectivity between SiC and AlN. The high SF6 etch selectivity between SiC and AlN was essential for removing the SiC substrate and exposing a pristine, smooth AlN surface. We demonstrated the epi-transfer process by fabricating high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further enhance the light extraction, the exposed N-face AlN was anisotropically etched in dilute KOH. The LEE of the AlGaN LED improved by ~3X after KOH roughening at room temperature. This AlGaN TFFC LED process establishes a viable path to high external quantum efficiency (EQE) and power conversion efficiency (PCE) UV-C LEDs.Comment: 22 pages, 6 figures. (accepted in Semiconductor Science and Technology, SST-105156.R1 2018

Full real-space analysis of a dodecagonal quasicrystal

The atomically resolved real-space structure of a long-range-ordered dodecagonal quasicrystal is determined based on scanning tunnelling microscopy. For the BaTiO3-derived oxide quasicrystal which spontaneously forms on a Pt(111) surface, 8100 atomic positions have been determined and are compared with an ideal Niizeki–Gähler tiling. Although the Niizeki–Gähler tiling has a complex three-element structure, the abundance of the triangle, square and rhomb tiling elements in the experimental data closely resembles the ideal frequencies. Similarly, the frequencies of all possible next-neighbour tiling combinations are, within the experimental uncertainty, identical to the ideal tiling. The angular and orientational distributions of all individual tiling elements show the characteristics of the dodecagonal quasicrystal. In contrast, the analysis of the orientation of characteristic and more complex tiling combinations indicates the partial decomposition of the quasicrystal into small patches with locally reduced symmetry. These, however, preserve the long-range quasicrystal coherence. The symmetry reduction from dodecagonal to sixfold is assigned to local interaction with the threefold substrate. It leads to atomic flips which preserve the number of quasicrystal tiling elements

Two-dimensional wetting layer structures of reduced ternary oxides on Ru(0001) and Pt(111)

Long-range ordered structures of reduced oxide films with monolayer thickness derived from BaTiO3 and SrTiO3 on Ru(0001) and Pt(111) are investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Upon ultrahigh vacuum annealing at 1100 K, a hexagonal phase is observed for BaTiO3 on Ru(0001), which forms similarly from SrTiO3 on Pt(111). At higher temperatures, a triangle–square tiling called σ-phase develops in the BaTiO3/Ru(0001) system, with a unit cell rotation of 15° against the Ru(0001) substrate. Furthermore, it is shown that this 15° rotated σ-phase also forms in the BaTiO3/Pt(111) system in addition to the already known 8° rotated σ-phase. The results emphasize a strong flexibility in the structural parameters of the reduced oxide wetting layers in response to the substrate interaction strength

Exceptionally Slow Rise in Differential Reflectivity Spectra of Excitons in GaN: Effect of Excitation-induced Dephasing

Femtosecond pump-probe (PP) differential reflectivity spectroscopy (DRS) and four-wave mixing (FWM) experiments were performed simultaneously to study the initial temporal dynamics of the exciton line-shapes in GaN epilayers. Beats between the A-B excitons were found \textit{only for positive time delay} in both PP and FWM experiments. The rise time at negative time delay for the differential reflection spectra was much slower than the FWM signal or PP differential transmission spectroscopy (DTS) at the exciton resonance. A numerical solution of a six band semiconductor Bloch equation model including nonlinearities at the Hartree-Fock level shows that this slow rise in the DRS results from excitation induced dephasing (EID), that is, the strong density dependence of the dephasing time which changes with the laser excitation energy.Comment: 8 figure

Optical constants, band gap, and infrared-active phonons of (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.35 (LSAT) from spectroscopic ellipsometry

Using spectroscopic ellipsometry, the authors determined the optical constants (complex dielectric function) for (LaAlO₃)0.3(Sr₂AlTaO₆) 0.35 (LSAT) from 0.01 to 6.5 eV. Above 0.5 eV, the data were described with a sum of two Tauc-Lorentz oscillators and two poles. A direct gap of 5.8 ± 0.1 eV was found. An Urbach tail extends to even lower photon energies and makes the crystal opaque above 4.8 eV. Using Fourier-transform infrared ellipsometry, the lattice dynamics was studied. Nine pairs of transverse/longitudinal phonons were found and attributed to disorder in the La/Sr sublattice, ordering in the Al/Ta sublattice, and two- phonon absorptio

Femtosecond Time-Resolved Reflectivity of Ge

We have measured the transient reflectivity changes of bulk Ge after excitation with 140 fs laser pulses at 1.5 eV. The electron and hole carrier dynamics arc calculated using an ensemble Monte Carlo method. The observed reflectivity changes are due to three mechanisms: Diffusion, band gap renormalization, and carrier dynamics, particularly scattering of light holes to the heavy hole band via optical phonons

Growth of highly conductive Al-rich AlGaN:Si with low group-III vacancy concentration

Publisher Copyright: © 2021 Author(s).The impact of AlGaN growth conditions on AlGaN:Si resistivity and surface morphology has been investigated using metalorganic chemical vapor deposition. Growth parameters including growth temperature, growth rate, and trimethylindium (TMI) flow have been systematically studied to minimize the resistivity of AlGaN:Si. We observed a strong anticorrelation between AlGaN:Si conductivity and growth temperature, suggesting increased silicon donor compensation at elevated temperatures. Secondary ion mass spectrometry and positron annihilation spectroscopy ruled out compensation by common impurities or group-III monovacancies as a reason for the observed phenomenon, in contrast to theoretical predictions. The underlying reason for AlGaN:Si resistivity dependence on growth temperature is discussed based on the possibility of silicon acting as a DX center in Al0.65Ga0.35N at high growth temperatures. We also show remarkable enhancement of AlGaN:Si conductivity by introducing TMI flow during growth. A minimum resistivity of 7.5 m? cm was obtained for n-type Al0.65Ga0.35N, which is among the lowest reported resistivity for this composition.& nbsp;(c)& nbsp;2021 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/).& nbsp;Peer reviewe

Effect Of Nucleation Time With Tmal Preflow Assistance On Reducing Dislocation Density Of Aln Layer For AlGaN-Based UVC LED

AlGaN-based UVC LEDs have now received numerous attentions due to their ability to eliminate coronaviruses which cause COVID-19 disease. It is therefore essential to improve the efficiency of the LEDs to make them compatible for large scale applications. One of the major challenges to improve the efficiency is to reduce the dislocation density in AlN layer; the base layer for the LEDs, to be below 109 cm-2 . Thus far, many works have been proposed to reduce the dislocation in the AlN layer. However, by properly adjusting the AlN nucleation time in the growth of the AlN layer, the dislocation can be reduced. The effect might be more significant with TMAl preflow assistance, which is applied after the growth of the nucleation. In this work, we will present the effect of the nucleation time with the assistance of TMAl preflow on reducing the dislocation density in the overgrown AlN layer. With 60 seconds of nucleation, the density of the dislocation in the AlN layer can be as low as 9.0 x 108 cm-2 . In addition. the role of the TMAl preflow assistance will be justified. The AlN layer was subsequently used to grow a 255 nm UVC LED. The diode characteristic and CL emission of the LED will be discussed towards the end of the presentation

Effect Of Nucleation Time With TMAl Preflow Assistance On Reducing Dislocation Density Of Aln Layer For

AlGaN-based UVC LEDs have now received numerous attentions due to their ability to eliminate coronaviruses which cause COVID-19 disease. It is therefore essential to improve the efficiency of the LEDs to make them compatible for large scale applications. One of the major challenges to improve the efficiency is to reduce the dislocation density in AlN layer; the base layer for the LEDs, to be below 109 cm-2. Thus far, many works have been proposed to reduce the dislocation in the AlN layer. However, by properly adjusting the AlN nucleation time in the growth of the AlN layer, the dislocation can be reduced. The effect might be more significant with TMAl preflow assistance, which is applied after the growth of the nucleation. In this work, we will present the effect of the nucleation time with the assistance of TMAl preflow on reducing the dislocation density in the overgrown AlN layer. With 60 seconds of nucleation, the density of the dislocation in the AlN layer can be as low as 9.0 x 108 cm-2. In addition. the role of the TMAl preflow assistance will be justified. The AlN layer was subsequently used to grow a 255 nm UVC LED. The diode characteristic and CL emission of the LED will be discussed towards the endof the presentation