Research On The Electrically Driven Germanium Light Source

The appetite of emerging data-intensive fields such as next-generation wireless and optical communications,cloud storage and servers,high-performance computing,and data centers have surpassed the transmission capabilities of traditional electrical interconnections.Because optical interconnection has the characteristics of low loss,high speed and large bandwidth,it is considered to be an alternative to achieve high-performance data transmission.A light source plays an indispensable role in photonic integrated circuits,optical interconnection,optical communication and numerous other optical applications scenarios.With large optical gain originating from their direct bandgap transitions,?-? materials have made great progress in widely tunable,low noise and narrow linewidth lasers through heterogeneous integration on SOI platform.However,an electrically driven group ? laser is still a long-term pursuit in the silicon photonics field for its better CMOS compatibility and financial advantages,which is conducive to mass production.The stumbling block to a silicon laser is the indirect band structure,hindering the optical transitions.Ge,a group ?material with near direct band structure configuration,whose ? valley is only 140 meV higher than the L valley,has been proved a good candidate as its directness of the band structure can be improved through bandgap engineering.With the funding from National Natural Science Foundation of China(NSFC),a series of theoretical and experimental studies have been carried out on electrically driven Ge light sources in this paper.The innovative research results obtained can be summarized as follows:(1)The direct bandgap light emission of the strained Ge microbridge and its temperature dependence are theoretically analyzed.The 8-band k·p method was used to study the improvement of strain on the band directness of band structure and luminescence characteristics of Ge.Based on the finite element method,the variation of strain with temperature in the Ge microbridge is analyzed,and combined with the thermal broadening of the Fermi-Dirac distribution,the variation of optical properties with temperature in different microbridge structures is classified and discussed.Due to the temperaturesensitivity of the Ge microbridge structure,the above-mentioned research will play a guiding role in the design of Ge microbridge lasers.(2)A monolithic integrated emission detection configuration is designed and fabricated based on microbridges,including lateral p-i-n junctions,waveguides and gratings.The operating wavelength range of the emitting bridge and the detecting bridge are matched through the designed same dimensions of the two microbridges,as well as the strain.Strainenhanced spontaneous emission and the effect of spectra red-shifting on low-loss transmission of on-chip light are discussed.Temperature dependence experiments reveal that in devices with highly strain-enhanced structure(?x/?0=12.4),the strain variation can offset the effect of electron thermalization,so that the performance of the device remains stable when temperature changes around room temperature.For the case of ?x/?0=7.6,the effect of strain is reduced,and the thermal broadening of the Fermi-Dirac distribution makes the spontaneous radiation increase as the temperature rises.(3)A germanium(Ge)waveguide laser under external phonon injection is designed to reduce laser threshold.To take the phonon injection and the acousto-optic overlap into consideration,the theory of the photon-phonon laser action is further developed.The phononic crystal waveguide is introduced in the laser structure to intensify acousto-optic interaction and characteristics of photon-phonon laser action in Ge waveguide are analyzed.With the external phonon injection,the two-quantum transition can be facilitated and the photon-phonon laser action is able to be established.The impacts of phononic crystal waveguide parameters,overlap of optical and acoustic fields and phonon injection on the laser behavior are discussed.Optimal waveguide structural parameters are obtained to enhance acousto-optic interaction through the enlargement of the overlap of optical and acoustic fields.The results indicate that for a Ge waveguide with the length of 200 ?m,the threshold current is reduced to 0.2 ?A and the slope efficiency reaches 0.7 W/A when the average phonon injection concentration is about 2.5×1021 cm-3.The proposed scheme offers an effective approach to achieve laser oscillation in integrated Ge waveguide.