High-performance GE/GESN photodetectors in near- and mid-infrared range

Demand for Near- (NIR) and Mid-infrared (MIR) range detection has increased drastically for practical applications in optical sensing, imaging, and communications. Silicon (Si)-based photonic integrated circuits (PICs) have drawn attraction for such applications with cost-effectiveness, low power consumption, ultra-compact device footprint, and complementary metal-oxide-semiconductor (COMS) compatibility. However, Si is an indirect bandgap material with a bandgap of 1.12 eV, which makes it challenging to cover the wavelength beyond 1,100 nm. Although numerous efforts have been put into extending its cut-off wavelength, the large Si bandgap even restricts the whole NIR range photodetection. Germanium (Ge), a Group IV element, is CMOS compatible with an indirect bandgap of 0.66 eV. Since the direct conduction band of Ge bulk is located only 140 meV higher than the indirect band, a Ge photodetector is able to do NIR photodetection. Germanium-tin (GeSn) photodetectors have been introduced for MIR range photodetection, taking advantage of its bandgap tunability and excellent optical and electrical properties. An increase in Sn content shrinks GeSn bandgap and improves carrier mobility. In addition, GeSn lasers have been reported with direct bandgap properties, accelerating the development of the monolithic integration of Group IV-based optoelectronic integrated citcuits in the MIR range. As such, Ge and GeSn optical components are primarily attractive to NIR and MIR range applications, respectively. Nevertheless, III-V compound semiconductor photodetectors, e.g., indium gallium arsenide (InGaAs) photodetectors, dominate the industry despite their CMOS incompatibility, high complexity, and high cost. In order to utilize Ge/GeSn material systems for low-cost and CMOS compatible photodetectors, a low dark current and a high photon collection efficiency are required. During the epitaxial Ge/GeSn growth on a Si substrate, high density defects/dislocations are generated due to the large differences in the thermal expansion coefficient and lattice constants. The dislocations/defects act as trap states that degrade optical and electrical properties in Ge/GeSn material systems. Therefore, mitigation of defects in Ge/GeSn material systems should be implemented to improve optical performances. Direct wafer bonding (DWB) and layer transfer techniques enable a transfer of Ge/GeSn epitaxial layers directly on insulator platforms. The insulator platform improves not only optical confinement but also provides electrical isolation. In addition, defective Si/Ge/GeSn interfaces can be readily eliminated with the techniques, leading to enhanced photodetection in terms of dark currents and optical responsivities. This thesis explores high-performance Ge/GeSn photodetectors, which are fabricated on insulator platforms. In addition, furnace annealing in oxygen (O2) ambient and germanium-oxide (GeOx) formation via ozone (O3) oxidation are conducted to achieve a sub-mA/cm2 dark current for a Ge photodetector. Also, optical responsivity for the Ge photodetector is improved by gourd-shaped hole array structures. With these advanced techniques, improved detectivity is obtained, comparable to commercial Ge bulk and extended III-V photodiodes. GeSn alloys are investigated for MIR range photodetection. It is revealed that trap states near a Ge/GeSn interface degrade electrical and optical properties in GeSn material systems. In order to mitigate the trap-related carrier dynamics, waveguide GeSn photodetectors are demonstrated on an insulator platform. DWB and layer techniques enable the realization of a GeSn-on-insulator (GeSnOI) platform without the GeSn/Ge interface layer. A demonstrated GeSn photodetector on the GeSnOI platform provides the photodetection beyond 2,000 nm. The proposed Ge/GeSn photodetectors would be widely applicable to Si-based PICs for high-efficiency NIR and MIR range photodetection.Doctor of Philosoph

Ultrafast light emission at telecom wavelengths from a wafer-scale monolayer graphene enabled by Fabry-Perot interferences

Ultrafast light emission from monolayer graphene shows attractive potential for developing integrated light sources for next-generation graphene-based electronic-photonic integrated circuits. In particular, graphene light sources operating at the telecom wavelengths are highly desired for the implementation of graphene-based ultrahigh-speed optical communication. Currently, most of the studies on ultrafast light emission from graphene have been performed in the visible spectrum, while studies on ultrafast emission at the telecom wavelengths remain scarce. Here, we present experimental observations of strong ultrafast thermal emission at telecom wavelengths from wafer-scale monolayer graphene. Our results show that the emission spectra can be strongly modified by the presence of the cavity effect to produce an enhanced emission at telecom wavelengths. We corroborate our experimental results with simulations and show that by designing a suitable cavity thickness, one can easily tune the emission profile from visible to telecom wavelength regardless of the pump power. In addition, we demonstrate that the insertion of a monolayer of hexagonal boron nitride between graphene and the substrate helps improve the thermal stability of graphene, thereby providing more than five times enhancement of the ultrafast thermal emission. Our results provide a potential solution for stable on-chip nanoscale light sources with ultrahigh speed modulation.Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)National Research Foundation (NRF)Agency for Science, Technology and Research (AME IRG (A2083c0053)); National Research Foundation Singapore (NRF2018-NRF-ANR009 TIGER, NRF-CRP19-2017-01); Ministry of Education - Singapore (AcRF TIER 1 (RG 115/21), AcRF TIER 2 (MOE2018-T2-2-011 (S)))

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))

Dark current analysis of germanium-on-insulator vertical p-i-n photodetectors with varying threading dislocation density

Dark current characteristics of germanium (Ge) vertical p-i-n photodetectors were studied. Ge photodetectors were demonstrated on the germanium-on-insulator (GOI) platforms realized via direct wafer bonding and layer transfer. GOI platforms with two different threading dislocation densities (TDDs) of 3.2 × 106 cm−2 (low TDD) and 5.2 × 108 cm−2 (high TDD) were varied via furnace annealing in oxygen ambient. An ultra-low dark current density of 1.12 mA/cm2 for epi-Ge photodetectors was obtained for a low TDD Ge photodetector. This is reduced by a factor of 53 in comparison with a high TDD Ge photodetector. A dominant leakage contribution component shifts from bulk leakage to surface leakage as TDD decreases to 3.2 × 106 cm−2, suggesting that advanced surface passivation is required to further reduce the leakage current. Through an activation energy study, it is revealed that a primary bulk leakage mechanism shifts from Shockley–Read–Hall (SRH) leakage to diffusion leakage in a temperature range of 323–353 K. The surface leakage performed with plasma enhanced chemical vapor deposition-deposited SiO2 is governed by SRH and trap-assisted tunneling leakage processes. Two orders of magnitude enhancement in the effective carrier lifetime is observed with the reduction in TDD. This work suggests that bulk leakage current density and effective lifetime analysis provide a better understanding of TDD-dependent dark leakage current study.NRF (Natl Research Foundation, S’pore)Published versio

Grating and hole-array enhanced germanium lateral p-i-n photodetectors on an insulator platform

Germanium (Ge) lateral p-i-n photodetectors with grating and hole-array structures were fabricated on a Ge-on-insulator (GOI) platform. Owing to the low threading dislocation density (TDD) in the transferred Ge layer, a low dark current of 0.279 μA was achieved at -1 V. The grating structure enhances the optical absorption by guiding the lateral propagation of normal incident light, contributing to a 3× improved responsivity at 1,550 nm. Compared with the grating structure, the hole-array structure not only guides the lateral modes but also benefits the vertical resonance modes. A 4.5× higher responsivity of 0.188 A/W at 1,550 nm was achieved on the 260 nm Ge absorptive layer. In addition, both the grating and the hole-array structure attribute to a 2× and a 1.6× enhanced 3dB bandwidth at -5 V due to significantly reduced capacitance. The planar configuration of -- photodiodes is favorable for large-scale monolithic integration. The incorporated surface structures offer promising approaches to reinforce the responsivity and bandwidth simultaneously, paving the way for the development of high-performance Ge photodetectors on silicon substrate.Ministry of Education (MOE)National Research Foundation (NRF)Published versionNational Research Foundation Singapore (NRF–CRP19–2017–01), Ministry of Education AcRF Tier 2 (T2EP50121-0001), Ministry of Education AcRF Tier 1 (2021-T1-002- 031 (RG112/21))

High speed and ultra-low dark current Ge vertical p-i-n photodetectors on an oxygen-annealed Ge-on-insulator platform with GeOx surface passivation

Germanium (Ge) vertical p-i-n photodetectors were demonstrated with an ultra-low dark current of 0.57 mA/cm2 at -1 V. A germanium-on-insulator (GOI) platform with a 200-mm wafer scale was realized for photodetector fabrication via direct wafer bonding and layer transfer techniques, followed by oxygen annealing in finance. A thin germanium-oxide (GeOx) layer was formed on the sidewall of photodetectors by ozone oxidation to suppress surface leakage current. The responsivity of the vertical p-i-n annealed GOI photodetectors was revealed to be 0.42 and 0.28 A/W at 1,500 and 1,550 nm at -1 V, respectively. The photodetector characteristics are investigated in comparison with photodetectors with SiO2 surface passivation. The surface leakage current is reduced by a factor of 10 for photodetectors via ozone oxidation. The 3dB bandwidth of 1.72 GHz at -1 V for GeOx surface-passivated photodetectors is enhanced by approximately 2 times compared to the one for SiO2 surface-passivated photodetectors. The 3dB bandwidth is theoretically expected to further enhance to ∼70 GHz with a 5 µm mesa diameter.National Research Foundation (NRF)Published versionNational Research Foundation Singapore Competitive Research Programme (NRF-CRP19-2017- 01)

Insights into the Origins of Guided Microtrenches and Microholes/rings from Sn Segregation in GermaniumTin Epilayers

We demonstrate the self-assembly synthesis of millimetre-long guided trenches and micro-holes/rings in the supersaturated GeSn epilayers through two approaches: epitaxial growth engineering and thermal annealing treatment. It reveals that the ordered trenches originate from a central nucleation point which typically accompanied by micro-hole/ring formation. These trenches are caused by the migration of Sn droplets on the film surface with the orientation dominantly along or axis, determined by the Sn-content of the epilayers and formation temperature. The holes/rings are postulated to be caused by the local droplet etching due to the development of Ge-Sn eutectic. The morphological and compositional evolution of the Sn-segregation is characterized by the combination of optical and electronic microscopy, spectroscopy, and atomic force microscope measurements. This work provides a comprehensive understanding of the mechanism for the Sn segregation in GeSn and suggests the new degree of freedom to the growth and engineering of droplet-assisted micro-structures.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 and Ministry of Education Tier-1 Project under Grant 2019-T1-002-040

Effects of high-temperature thermal annealing on GeSn thin-film material and photodetector operating at 2 µm

Here, we explore the thermal stability of GeSn epilayers with varying Sn contents (3-10%) at an annealing temperature ranging from 300 to 750°C. It is found that ordered nanopatterns are formed on the surface of GeSn with Sn content of 8% without excessive Sn precipitation after thermal annealing at 700°C. Despite being annealed at high temperatures, the GeSn maintains its crystal structure, which is confirmed by the X-ray Diffraction (XRD), Raman spectrum, and secondary-ion mass spectrometry (SIMS). The corresponding photocurrents of the photodetectors at the wavelength of 2 µm also indicate the crystal quality of the GeSn alloys does not deteriorate significantly after high-temperature annealing (675-700°C). Meanwhile, the decrease of dark current with the enhancement of Ip/Id ratio (on-off ratio) indicates the improvement of detectivity of the photodetector due to the annealing process. Furthermore, the annealing temperature is optimized to 550°C to achieve 200% enhancement of photocurrents of the GeSn photodetectors operated at 2 µm.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 and Ministry of Education Tier-1 Project under Grant 2019-T1-002-040

Monolithic Germanium-tin pedestal waveguide for mid-infrared applications

Germanium-tin (GeSn) is a CMOS-compatible group-IV material. Its growth, however, is plagued by the tendency of Sn segregation and the generation of defects within the GeSn layer when it is grown on the lattice-mismatched substrate. Thus far, thin GeSn has been reported for use in a direct-band gap for near-mid infrared light source and photodetector. In this communication, we report the growth of high quality single-crystalline GeSn (∼ 1 μm) with low compressive stress (−0.3%) and low defects (3 × 10 7 /cm 2 ) on Ge buffer on Si substrate. The as-grown GeSn is then fabricated into pedestal waveguide of width 1.25 μm. An estimated propagation loss of 1.81 dB/cm and bending loss of 0.19 dB/ bend are measured at 3.74 μm. In the absence of Ge-O absorption peaks at 820 and 550 cm −1 , under optimal fabrication and measurement condition, the proposed GeSn waveguide might possibly support light propagation for wavelength beyond 25 μm.National Research Foundation (NRF)Published versionThis research project is supported by the National Research Foundation, Singapore, under its Competitive Research Program (CRP Award NRF-CRP19-2017-01)

A highly ordered and damage-free Ge inverted pyramid array structure for broadband antireflection in the mid-infrared

With increasing demand for infrared (IR) photonics and optoelectronics, germanium (Ge) has recently regained attention due to its outstanding optical properties in the near infrared (NIR) and mid infrared (MIR) ranges. Here we present a highly ordered and damage-free microscale Ge inverted pyramid array fabricated by HF-free metal-assisted chemical etching. The surface quality of the inverted pyramid is systematically investigated, demonstrating the good preservation of single crystallinity with a minimized amount of defects at etched surfaces. In addition, an outstanding antireflection performance of the Ge inverted pyramid is realized in a broadband MIR wavelength range up to 15 μm. The damage-free Ge inverted pyramid array, together with its strong antireflection capability in the MIR range, provides an outstanding platform for future MIR photonic and optoelectronic applications.Agency for Science, Technology and Research (A*STAR)Ministry of Education (MOE)National Research Foundation (NRF)Submitted/Accepted versionThis work was supported by A*STAR, Singapore, Advanced Manufacturing and Engineering (AME) Young Individual Research Grant (YIRG) under Project A2084c0066 and Ministry of Education (MOE), Singapore, under grant AcRF TIER 1-2018- T1-002-115 (RG 173/18). The simulation part was supported by the National Research Foundation, Singapore, under the Competitive Research Program (NRF-CRP19-2017-01) and MOE, Singapore, under AcRF Tier 1 2019-T1-002-040 (RG147/19 (S))