The public’s declining trust in government in Korea

노트 : This article was presented at the Sixth Sino–US Public Administration International Symposium (“Rebuilding Trust after Global Financial Crisis”), June 5–6, 2012. Renmin University of China, Beijing

Low-threshold optically pumped lasing in highly strained Ge nanowires

The integration of efficient, miniaturized group IV lasers into CMOS architecture holds the key to the realization of fully functional photonic-integrated circuits. Despite several years of progress, however, all group IV lasers reported to date exhibit impractically high thresholds owing to their unfavorable bandstructures. Highly strained germanium with its fundamentally altered bandstructure has emerged as a potential low-threshold gain medium, but there has yet to be any successful demonstration of lasing from this seemingly promising material system. Here, we demonstrate a low-threshold, compact group IV laser that employs germanium nanowire under a 1.6% uniaxial tensile strain as the gain medium. The amplified material gain in strained germanium can sufficiently surmount optical losses at 83 K, thus allowing the first observation of multimode lasing with an optical pumping threshold density of ~3.0 kW cm^-^2. Our demonstration opens up a new horizon of group IV lasers for photonic-integrated circuits.Comment: 31 pages, 9 figure

Investigation of germanium and germanium-tin laser technologies towards electronic-photonic integrated circuits

Since the first electrical integrated circuits (ICs), the level of integration has been increasing with the scaling down of transistors. Although it is possible to integrate a number of transistors in a single chip, however, the ICs suffers from a performance bottleneck. To eliminate the performance bottleneck, an on-chip optical interconnect system has been in the limelight. However, developments in on-chip interconnect system have been made slow progress due to the absence of efficient on-chip light sources. Germanium (Ge) is the promising material for the on-chip optical interconnect system. However, Ge is not suitable for light sources because of its poor light emission efficiency. Thus, an efficient light source is the only missing key for the on-chip optical interconnect system. Throughout this thesis, we discuss our achievements about the efficient Ge based light sources, which can pave the way towards the on-chip optical interconnect system.Doctor of Philosoph

Strained germanium nanowire optoelectronic devices for photonic-integrated circuits

Strained germanium nanowires have recently become an important material of choice for silicon-compatible optoelectronic devices. While the indirect bandgap nature of germanium had long been problematic both in light absorption and emission, recent successful demonstrations of bandstructure engineering by elastic strain have opened up the possibility of achieving direct bandgap in germanium, paving the way towards the realization of various high-performance optical devices integrated on a silicon platform. In particular, the latest demonstration of a low-threshold optically pumped laser in a highly strained germanium nanowire is expected to vitalize the field of silicon photonics further. Here, we review recent advances and challenges in strained germanium nanowires for optoelectronic applications such as photodetectors and lasers. We firstly introduce the theoretical foundation behind strained germanium nanowire optoelectronics. And several practical approaches that have been proposed to apply tensile strain in germanium nanowires are further discussed. Then we address the latest progress in the developments of strained germanium nanowire optoelectronic devices. Finally, we discuss the implications of these experimental achievements and the future outlook in this promising research field.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio

Systematic study on photoexcited carrier dynamics related to defects in GeSn Films with low Sn content at room temperature

Germanium-Tin (GeSn) alloys have received much attention thanks to their optical/electrical properties and their operation in the mid-infrared range. However, dislocations/defects in GeSn films serve as trap states, limiting radiative recombination/generation via band-edges. In this work, the impact of the trap states in GeSn with varying Sn contents is investigated. The systematic study reveals that the defects/dislocations in GeSn contribute to the carrier dynamics, mainly originated from the trap states near GeSn/Ge interface. Through photoluminescence (PL) study, the broad PL peak of the trap state for GeSn exists at ~0.57 eV. The increase in Sn content mitigates the trap-related carrier dynamics. Besides, the increase in GeSn thickness effectively suppresses the interface-related carrier dynamic. By increasing thickness from 180 to 1,000 nm, the external quantum efficiency is enhanced by ~10×. This study provides a comprehensive understanding of trap-related carrier dynamics in a GeSn material system at room temperature.Ministry of Education (MOE)National Research Foundation (NRF)Accepted versionThis work was support by the National Research Foundation Singapore Competitive Research Programme under Grant NRF-CRP19-2017-01, Ministry of Education nAcRF Tier 1 2019-T1-002-040 (RG 147/19 (S)), Ministry of Education under grant AcRF Tier 2 (MOE2018-T2-2-011 (S))

Strain-relaxed GeSn-on-insulator (GeSnOI) microdisks

GeSn alloys offer a promising route towards a CMOS compatible light source and the realization of electronic-photonic integrated circuits. One tactic to improve the lasing performance of GeSn lasers is to use a high Sn content, which improves the directness. Another popular approach is to use a low to moderate Sn content with either compressive strain relaxation or tensile strain engineering, but these strain engineering techniques generally require optical cavities to be suspended in air, which leads to poor thermal management. In this work, we develop a novel dual insulator GeSn-on-insulator (GeSnOI) material platform that is used to produce strain-relaxed GeSn microdisks stuck on a substrate. By undercutting only one insulating layer (i.e., Al2O3), we fabricate microdisks sitting on SiO2, which attain three key properties for a high-performance GeSn laser: removal of harmful compressive strain, decent thermal management, and excellent optical confinement. We believe that an increase in the Sn content of GeSn layers on our platform can allow us to achieve improved lasing performance.Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)National Research Foundation (NRF)Published versionThe research of the project was in part supported by Ministry of Education, Singapore, under grant AcRF TIER 1 2019-T1-002-050 (RG 148/19 (S)). The research of the project was also supported by Ministry of Education, Singapore, under grant AcRF TIER 2 (MOE2018-T2-2-011 (S)). This work is also supported by National Research Foundation of Singapore through the Competitive Research Program (NRF- RP19-2017-01). This work is also supported by National Research Foundation of Singapore through the NRF-ANR Joint Grant (NRF2018-NRF-ANR009 TIGER). This work is also supported by the iGrant of Singapore A*STAR AME IRG (A2083c0053). The authors would like to acknowledge and thank the Nanyang NanoFabrication Centre (N2FC)

Biaxially strained germanium crossbeam with a high-quality optical cavity for on-chip laser applications

The creation of CMOS compatible light sources is an important step for the realization of electronic-photonic integrated circuits. An efficient CMOS-compatible light source is considered the final missing component towards achieving this goal. In this work, we present a novel crossbeam structure with an embedded optical cavity that allows both a relatively high and fairly uniform biaxial strain of ~0.9% in addition to a high-quality factor of >4,000 simultaneously. The induced biaxial strain in the crossbeam structure can be conveniently tuned by varying geometrical factors that can be defined by conventional lithography. Comprehensive photoluminescence measurements and analyses confirmed that optical gain can be significantly improved via the combined effect of low temperature and high strain, which is supported by a three-fold reduction of the full width at half maximum of a cavity resonance at ~1,940 nm. Our demonstration opens up the possibility of further improving the performance of germanium lasers by harnessing geometrically amplified biaxial strain.Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)National Research Foundation (NRF)Published versio

Optically pumped low-threshold microdisk lasers on a GeSn-on-insulator substrate with reduced defect density

Despite the recent success of GeSn infrared lasers, the high lasing threshold currently limits their integration into practical applications. While structural defects in epitaxial GeSn layers have been identified as one of the major bottlenecks towards low-threshold GeSn lasers, the effect of defects on the lasing threshold has not been well studied yet. Herein, we experimentally demonstrate that the reduced defect density in a GeSn-on-insulator substrate improves the lasing threshold significantly. We first present a method of obtaining high-quality GeSn-on-insulator layers using low-temperature direct bonding and chemical–mechanical polishing. Low-temperature photoluminescence measurements reveal that the reduced defect density in GeSn-on-insulator leads to enhanced spontaneous emission and a reduced lasing threshold by ∼10 times and ∼6 times, respectively. Our result presents a new path towards pushing the performance of GeSn lasers to the limit.Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)National Research Foundation (NRF)Mitacs; Innovation for Defence Excellence and Security, IDEaS; PRIMA Québec; Canada Foundation for Innovation; Canada Research Chairs; Natural Sciences and Engineering Research Council of Canada; iGrant of Singapore A*STAR (AME IRG (A2083c0053)); National Research Foundation Singapore (Competitive Research Program (NRF-CRP19-2017-01), NRF-ANR Joint Grant (NRF2018-NRF-ANR009 TIGER)); Ministry of Education - Singapore (AcRF TIER 1 2019-T1-002-050 (RG 148/19 (S)), AcRF TIER 2 (MOE2018-T2-2-011 (S)), AcRF Tier 2 (T2EP50121-0001 (MOE-000180-01))

GeSn/Ge multiquantum-well vertical-cavity surface-emitting p-i-n structures and diode emitters on a 200 mm Ge-on-insulator platform

An efficient monolithically integrated light source with complementary metal-oxide semiconductor (CMOS) compatibility remains the missing component to enable Si photonics for various applications. In particular, vertical-cavity-surface-emitting (VCSE) light sources, such as resonant cavity light-emitting diodes (RCLEDs) and vertical cavity surface-emitting lasers (VCSELs), are strong contenders due to their compact size, circular emission profile with low beam divergence, wafer-scale fabrication compatibility, high bandwidth and high coupling efficiency to fiber optic cables. We report the first demonstration of 8-inch wafer-scale GeSn/Ge multiple-quantum-well VCSE p-i-n structures and diodes for laser or light-emitting diode (LED) applications in Si photonics by wafer bonding and layer transfer techniques, which are challenging for all-epitaxy routes. Alternative dielectric layers (SiO2/SiN/SiO2), introduced by wafer bonding, under the emitting structure serve as the bottom mirror for the vertical cavity. The Ge0.92Sn0.08/Ge MQW layer is utilized to improve the material quality and to confine injected carries. As a result, more than 8× enhancement of light emission due to the vertical cavity resonance was demonstrated by photoluminescence spectroscopy. Besides, the spectral purity is enhanced by the single-mode cavity. The intensity of light emission is insensitive to the temperature range from 4 to 300 K and even becomes stronger at higher temperatures. The vertical cavity effect on the light emission is further verified by reflectivity spectroscopy and optical simulations. A positive gain can be achieved as indicated by optical gain calculations and an excellent carrier injection efficiency of the MQW VCSE diode is observed, showing its potential for electrically injected RCLEDs and VCSELs.Ministry of Education (MOE)National Research Foundation (NRF)Submitted/Accepted versionThis work was supported by National Research Foundation Singapore (NRF–CRP19–2017–01), Ministry of Education AcRF Tier 2 (T2EP50121-0001 (MOE-000180-01)) and Ministry of Education AcRF Tier 1 (2021-T1-002-031 (RG112/21))