Research Of Strain Engineered Ge/GeSi Multiple Quantum Wells Electro-absorption Modulators

With the rapid development of information technology and global internet economy,new technologies and applications keep emerging from now and then.With the market scale growth of big data,internet of things,cloud computing and other applications,the demond for wider bandwidth,longer distance,lower energy consumption,smaller device size of data transmission is becoming urgent.In the situation of intra-chip,inter-board and long-distance data transmission,the traditional data transmission technology using copper wire can no longer meet the demands,and is becoming a bottleneck for the development of semiconductor industry.Optical interconnect technology using integrated optical circuit has the advantages of low energy consumption,large band width,good anti-interference ability and has wide application prospects.On the chip scale,silicon photonic technology is a key to achieving optical interconnect because of its CMOS-compatible processing and can be large-scale manufactured with low cost.Silicon based photonic circuits can be integrated with CMOS electronic circuits.Silicon optical modulator is a very important device in silicon based integrated optical circuit.Germanium and silicon are both IV semiconductor materials and have similar features,Ge and Ge-rich Ge Simaterials can be processed within a CMOS foundry.The room-temperature direct bandgap of Ge corresponds to the communication wavelength and is very close to its indirect bandgap energy.Ge/SiGe quantum well can confine both the electron and hole states in the well enhancing the chance of direct bandgap transitions,achieving the direct bandgap absorption.Quantum-confined Stark effect(QCSE)can be utilized to achieve electro-absorption modulator with energy consumption lower than 100 f J/bit and modulation bandwidth higher than 100 Gb/s.Under the support of the National Natural Science Foundation of China,this thesis will focus on strain engineered Ge/SiGe multiple quantum wells electro-absorption modualtors,the design of Ge/SiGe quantum well,waveguide,coupling structure and suspended microbridge structures,both simulations and experiments.The main research achievements of these studies are summarized as follows:(1)We study the physical principle of quantum-confined Stark effect of Ge/SiGe multiple quantum wells,and establish the band structure of Ge/SiGe multiple quantum wells through the 8 band k?p method.We analyze the interaction of valence band light hole and heavy hole states of Ge/SiGe multiple quantum wells,and compare the difference between different conduction band electron states.We calculate the absorption coefficient of Ge/SiGe multiple quantum wells,taking in consideration of both direct bandgap transition absorption and indirect bandgap absorptions.(2)A low bias voltage Ge/SiGe multiple quantum wells electro-absorption modulator is proposed to operate at the wavelength of 1550 nm.Through the optimization of Ge/SiGe quantum well parameters,the compressive strain in the well is effectively reduced,shifting the absorption edge to longer wavelength.A large well width is adopted to increase the Stark shift of absorption edge under electric field.A small barrier width is adopted to increase the proportion of well area,increasing the effective absorption coefficient.The device couples with the passive waveguide through taper couplers,and has the advantages of compact design and high integration density.The taper has a segemented design and has a length of only 12 ?m,with a coupling coefficient higher than 90%.The modulator has a length of 104?m and has an extinction ratio of 20.3 d B under 0.5 V bias and 1 V driving voltage for 1550 nm wavelength,the corresponding insertion loss is-7.4 d B.(3)An uniaxially tensile strained Ge/SiGe multiple quantum wells waveguide integrated electro-absorption modulator is proposed and demonstrated.Through suspended microbridge structure,we can introduce uniaxial tensile strain into the Ge/SiGe multiple quantum wells and change the bandgap of the material,adjusting the operation wavelength of the device.The simulation shows that when the uniaxial tensile strain is 1.6%,the absorption edge of the material is shifted to 1550 nm wavelength and the modulator has an extinction ratio of 17.8 d B for 1550 nm wavelength under 0 V/ 2V operation.Experimentally,by comparing the absorption spectra of devices with and without suspended microbridges,we find that the abosrption edge is shifted to the 1530 nm wavelength and the operation wavelength control by strain is achieved.The Raman spectra test results show that 1.52%uniaxially tensile strain is introduced into the bridge.The modulator has a total length of 300?m using grating coupler,and photocurrent absorption contrast is 6.3 d B for 1558 nm and4.8 d B for 1550 nm wavelength,under 1 V/2 V operation.The 3 d B bandwidth is 11.3 GHz under 0 V bias voltage.(4)A waveguide integrated low polarization independence Ge/SiGe multiple quantum wells electro-absorption modulator utilizing biaxial tensile strain is proposed and demonstrated.Through a special design of suspended microbridge,we can introduce biaxial tensile strain into the Ge/SiGe multiple quantum wells.Through theoretical analysis,we find that biaxial tensile strain can be utilized to adjust the polarization independence of Ge/SiGe multiple quantum wells.The simulation shows that when the biaxial tensile strain is 0.78%,the modulator has a polarization independence extinction ratio of 7.6 d B for 1485 nm wavelength under 0 V/ 2V operation.Experimentally,we fabricate different devices using EBL and UV exposure,and test the extinction ratio,insertion loss,photocurrent response and high frequency response through butt coupling.The test results show that the device has a maximum extinction ratio of 9.3 d B under 0 V/4 V voltage condition.Under 0 V/2 V voltage condition,the device has a polarization independent extinction ratio of 4.6 d B for1479 nm wavelength and insertion loss of-37.5 d B.Under 2 V reverse bias condition,the device has a 3 d B bandwidth of 8.7 GHz.The fabrication process of this device is CMOS-compatible and can be manufactured through UV exposure,suitable for waveguide integrated polarization independent optical intensity modulation.