Advances in the LED Materials and Architectures fro Energy-Saving Solid State Lighting towards Lighting Revolution

Cataloged from PDF version of article.In this paper, we review the recent developments (in years 2010–2011) of energysaving solid-state lighting. The industry of white light-emitting diodes (LEDs) has made significant progress, and today, white LED market is increasing (mostly with increasing LED screen and LED TV sales). The so-called Blighting revolution[ has not yet really happened on a wide scale because of the lighting efficiency at a given ownership cost. Nevertheless, the rapid development of the white LEDs is expected to soon trigger and expand the revolution

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

Influence of silicon doping on vacancies and optical properties of AlxGa1-xN thin films

The authors have used positron annihilation spectroscopy and photoluminescence measurements to study the influence of silicon doping on vacancy formation in AlGaN:Si structures. The results show a correlation between the Doppler broadening measurements and the intensity from 510nm photoluminescence transition. The reduction in the W parameter when the [Si]∕[Al+Ga] fraction in the gas phase is above 3×10exp−4 indicates that the positrons annihilate in an environment where less Ga 3d electrons are present, i.e., they are trapped in group-III vacancies. The observation of vacancies at these silicon concentrations coincides with the onset of the photoluminescence transition at 510 nm.Peer reviewe

Characteristics Of InGaN Based Red Led Epiwafer

This paper describes a preliminary work of developing InGaN based red LED using two-flow MOCVD reactor, focussing on characteristics of the LED epiwafer from the aspects of optics, surface and crystalline structure. The active region multiquantum wells (MQWs) of the LED consists of In0.3Ga0.7N quantum well and GaN quantum barrier, which were alternatively grown for a period of 4 with Al0.28Ga0.72N as a cap layer between the quantum well and the quantum barrier. From EL measurement, the LED showed a broad emission peak at 615 nm. The broad emission can be related to the presence of hexagonal pits on the LED’s surface, as witnessed through SEM and AFM measurements. Despite of that, XRD measurement implied the LED has a relatively good crystalline structure with FWHM of ~234 arcsec in (002) and (102) scans. XRD-RSM measurement suggests that the MQWs are under strain. On the basis of this work, further effort on reducing the hexagonal pits is required to improve the characteristics of the LED epiwafer so that it is useful to be processed into functional devices

Thermally enhanced blue light-emitting diode

We investigate thermoelectric pumping in wide-bandgap GaN based light-emitting diodes (LEDs) to take advantage of high junction temperature rather than avoiding the problem of temperature-induced efficiency droop through external cooling. We experimentally demonstrate a thermally enhanced 450 nm GaN LED, in which nearly fourfold light output power is achieved at 615 K (compared to 295 K room temperature operation), with nearly no reduction in the wall-plug efficiency (i.e., electrical-optical energy conversion efficiency) at bias V< ℏ ω/q. The LED is shown to work in a mode similar to a thermodynamic heat engine operating with charged carriers pumped into the active region by a combination of electrical work and Peltier heat (phonons) drawn from the lattice. In this optimal operating regime at 615 K, the LED injection current (3.26 A/cm[superscript 2]) is of similar magnitude to the operating point of common high power GaN based LEDs (5–35 A/cm[superscript 2]). This result suggests the possibility of removing bulky heat sinks in current high power LED products thus realizing a significant cost reduction for solid-state lighting.Bose (Firm)Singapore. Agency for Science, Technology and Researc

Quantitative observation and discrimination of AlGaN- and GaN-related deep levels in AlGaN∕GaN heterostructures using capacitance deep level optical spectroscopy

Deep levels were observed using capacitance deep level optical spectroscopy (DLOS) in an AlGaN/GaN heterostructure equivalent to that of a heterojunction field effect transistor. Band gap states were assigned to either the AlGaN or GaN regions by comparing the DLOS spectra in accumulation and pinch-off modes, where the former reflects both AlGaN- and GaN-related defects, and the latter emphasizes defects residing in the GaN. A band gap state at E-c-3.85 eV was unambiguously identified with the AlGaN region, and deep levels at E-c-2.64 eV and E-c-3.30 eV were associated with the GaN layers. Both the AlGaN and GaN layers exhibited additional deep levels with large lattice relaxation. The influence of deep levels on the two-dimensional electron gas sheet charge was estimated using a lighted capacitance-voltage method. (c) 2006 American Institute of Physics

Si doping effect on strain reduction in compressively strained Al0.49Ga0.51N thin films

Evaluation of the structural properties of 200-nm-thick Si-doped Al0.49Ga0.51N films, grown on nominally relaxed 1-mum-thick Al0.62Ga0.38N buffer layers on sapphire, revealed that increased Si doping promoted the relaxation of the compressively strained layers. The degree of strain relaxation R of the Al0.49Ga0.51N films, as determined by x-ray diffraction (XRD), increased from R=0.55 to R=0.94 with an increase in disilane injection from 1.25 nmol/min to 8.57 nmol/min. Transmission electron microscopy analysis showed that the edge threading dislocations (TDs) in the Al0.49Ga0.51N layers were inclined, such that the redirected TD lines had a misfit dislocation component. The calculated strain relaxation due to the inclined TDs was in close agreement with the values determined from XRD. We propose that the TD line redirection was promoted by the Si-induced surface roughness. (C) 2003 American Institute of Physics

Effect of threading dislocation density on Ni∕n-GaN Schottky diode I-V characteristics

The impact of threading dislocation density on Ni/n-GaN Schottky barrier diode characteristics is investigated using forward biased current-voltage-temperature (I-V-T) and internal photoemission (IPE) measurements. Nominally, identical metal-organic chemical vapor deposition grown GaN layers were grown on two types of GaN templates on sapphire substrates to controllably vary threading dislocation density (TDD) from 3x10(7) to 7x10(8) cm(-2). I-V-T measurements revealed thermionic emission to be the dominant transport mechanism with ideality factors near 1.01 at room temperature for both sample types. The Schottky barrier heights showed a similar invariance with TDD, with measured values of 1.12-1.13 eV obtained from fitting the I-V-T results to a thermionic emission-diffusion model. The I-V-T results were verified by IPE measurements made on the same diodes, confirming that the Ni/n-GaN barrier heights do not show a measurable TDD dependence for the TDD range measured here. In apparent contrast to this result is that the measured forward bias I-V characteristics indicate a shift in the observed forward bias turn-on voltage such that at the higher TDD value investigated here, a larger turn-on voltage (lower current) is observed. This difference is attributed to localized current blocking by high potential barrier regions surrounding threading dislocations that intersect the Ni/GaN interface. A simple model is presented that reconciles both the observed voltage shift and variations in the extracted Richardson constant as a function of threading dislocation density. With this model, an average local barrier surrounding dislocation of similar to 0.2 V is obtained, which diverts current flow across the forward biased Schottky interface to nondislocated regions. (c) 2006 American Institute of Physics

Electron mobility in graded AlGaN alloys

Polarization gradients in graded AlGaN alloys induce bulk electron distributions without the use of impurity doping. Since the alloy composition is not constant in these structures, the electron scattering rates vary across the structure. Capacitance and conductivity measurements on field effect transistors were used to find mobility as a function of depth. The effective electron mobility at different depths calculated from theory closely matched the measured mobility. Local bulk mobility values for different AlGaN compositions were found, and the electron mobility in AlGaN as a function of alloy composition was deduced. These were found to match with theoretical calculations