Research Of Ge/SiGe Asymmetric Coupled Quantum Well Modulator Based On Silicon Substrate

After several years of development,numerous major breakthroughs have been made for silicon photonics integration technology,but silicon-based active devices are still the key to silicon photonics integration.The silicon germanium material system is currently the only material platform that can realize all silicon integrated active devices and is compatible with the mature complementary metal oxide semiconductor（CMOS）process,and has huge application prospects.The Ge/SiGe quantum well modulators based on the quantum confinement Stark effect have the advantages of low bias,high speed and low power consumption,thus have attracted extensive attention in recent years,but the performance can be further improved to meet the requirements of integration with CMOS circuits.With the support of the key project of the National Natural Science Foundation of China,this thesis proposes an asymmetrical Ge/SiGe coupled quantum well structure,and conducts theoretical calculations and analysis of its physical properties,and demonstrates an asymmetrical Ge/SiGe coupled quantum well waveguide modulator,and demonstrates a high efficiency grating coupler for GeSi integration.The major achievements of these researches can be summarized as follows:（1）Based on the 8 band k×p method,a theoretical model for calculating the band structure of Ge/SiGe quantum wells is constructed.The process of solving the energy eigenvalues and wave functions of the Ge/SiGe quantum wells is deduced in detail,and the total Hamiltonian,strain matrix element and momentum matrix element are given.At the last,the absorption spectrum expression is respectively given for the direct band gap absorption and the direct band gap absorption of the Ge/SiGe quantum wells.（2）An asymmetric Ge/SiGe coupled quantum well structure is proposed and its optoelectronic properties are calculated and analyzed theoretically.Compared with common Ge/SiGe multiple quantum wells,the red shift of the absorption band edge of the asymmetric Ge/SiGe coupled quantum well is more significant,and the electro-refractive index variation is also more obvious.When the applied electric field is 30 k V/cm,the variation of the refractive index takes a local maximum value of about 9×10^-3 at the wavelength of 1461nm,and the product of the corresponding half-wave voltage and the length of the phase shift region V_πL_πis about 0.01V×cm,which has a significant competitive advantage over other silicon based phase modulators.Besides,the effects of uniaxial tensile strain and biaxial tensile strain on the direct bandgap absorption spectrum of the Ge/SiGe asymmetric coupled quantum well are further calculated and analyzed.The uniaxial tensile strain can help to enhance the TE polarization of the Ge/SiGe asymmetric coupled quantum well and suppresses the TM polarization state,while the biaxial tensile strain helps to achieve low polarization dependent applications.（3）A silicon-based waveguide modulator based on the Ge/SiGe asymmetric coupled quantum well structure is demonstrated.The electrical test results show that the pn junction characteristics of the device are obvious and the dark current is small.The photocurrent response test implies the obvious quantum confinement Stark effect of Ge/SiGe asymmetric coupled quantum wells.Under 1 V and 2 V reverse bias voltages,the device can achieve modulation extinction ratios of up to 5 dB and 7.8 dB respectively.Under 1 V/2 V operation swing voltage,it can achieve an extinction ratio of about 5.7 dB.For phase modulation,under reverse bias voltages of 1 V and 2 V,the device can achieve the electro-refractive index variation of 1.4×10^-3 and 3.2×10^-3 at the wavelength of 1530 nm,and the product of the corresponding half-wave voltage and the length of the phase shift region V_πL_πis 0.055V×cmand 0.024V×cmrespectively,which is significantly smaller than other silicon-based phase modulators.Under the reverse bias of 0 V,1 V and 2 V,the 3 dB response bandwidth of the device is respectively about 24 GHz,27 GHz and 32 GHz,which is significantly higher than other Ge/SiGe multiple quantum well modulators that have been reported so far.The device can achieve both intensity modulation and phase modulation,and hold promise for realizing advanced modulation format in a more compact and simple system.（4）A high-efficiency grating coupler for GeSi integration is demonstrated.Due to the large loss from the butt-coupling scheme of GeSi devices,a GeSi grating coupler with a bottom metal layer as the reflecting mirror is designed to enhance the coupling efficiency between the on-chip GeSi devices and the optical fibers.The simulation results show that the coupling loss of the grating is decreased by 9.4 dB compared with the case without a metal layer and its directionality is also significantly improved.The minimum coupling loss of the GeSi grating obtained by simulation optimization is-1.34 dB and the calculation results show that the GeSi grating has a high tolerance for process deviations.In experiments,a back-side alignment process is developed to make the GeSi grating coupler and a double-faced alignment lithography machine is used to make the window of the deep etch region on the back of the substrate.The test results show that the minimum measured coupling loss of the GeSi grating coupler is about-2.7 dB the 3 dB bandwidth is more than 40 nm.