Research On The Technology Of Tensile Strained Germanium Light Source On Silicon Platform

With the rapid development of information technology and the proposal of‘Big data'and‘Internet of everything',there has been an explosive growth of the information volume in our society.The emergence of enormous information requires high-performance processing,transmission and storage of the information,more urgently than ever before.In the past decades,it is the continuous progress of the integrated circuit?IC?technology that contributes to the flourish of the information industry.However,with today's semiconductor fabrication technology gradually approaching its physical limit,the performance improvement of IC has been slowed down.On the other hand,the traditional electrical interconnect in IC device has also shown its bottleneck in the regime of high communication speed and high integration density.In order to seek out the solution for processing and transmitting information with ultra-high speed and low power dissipation,scientists have set their sights on photonics technology.Silicon photonics is widely regarded as a promising technology.Firstly,the bottleneck problem of information processing and transmission in the electrical domain can be overcome,taking advantage of the merits of photonics technology in terms of high speed,high parallelism and low power dissipation.Secondly,silicon photonics inherits the mature fabrication technology of silicon from the IC industry,i.e.the complementary metal-oxide-semiconductor?CMOS?technology,which accumulates tremendous investment and wisdom in the last decades.As a result,it is compatible with the current CMOS technology and thereby remarkably reduces the cost.Furthermore,thanks to the highly integrated feature,silicon photonics can revolutionize the conventional optical technology in areas of spectroscopy and sensing.The last two decades have witnessed a tremendous development in silicon photonics.However,there is still no ideal solution for a monolithically integrated light source which is the key device for silicon photonics.This hinders the application and development of silicon photonics.Supported by the National Basic Research Program of China,this thesis will focus on the theoretical analyses,device design and experimental investigation of silicon-based tensile stressed germanium light source.The main innovative achievements of these studies are described as follows:?1?We develop a theoretical model that can analyze the optical gain characteristics of tensile stressed bulk germanium and Ge/SiGe quantum well.This model includes the calculations of band structures near the?-point and L-points,direct gap optical gain and free carrier absorption.Based on the model,we can analyze the impacts of strain and n-type doping on the net optical gain spectra of the materials.Therefore,it can serve as a theoretical framework for the design of tensile stressed bulk germanium and Ge/SiGe quantum well light sources.?2?We calculate the net optical gain spectra of the uniaxially tensile stressed,n-doped Ge/SiGe quantum well and analyze the effects of strain value and doping concentration on the net peak gain and transparency carrier density.Theoretical results predict that the optical gain of TE-polarized light is larger than that of TM-polarized light in uniaxially tensile stressed Ge/SiGe quantum well.Meanwhile,with the increase of strain value,the net optical gain increases and the transparency carrier density decreases,dramatically.A net optical gain of2061 cm^-1 can be achieved with reasonable strain value of 4%and doping concentration of 1×10¹⁹ cm^-3.?3?We propose an electrically driven distributed Bragg reflector?DBR?laser based on uniaxially tensile stressed bulk germanium.The microbridge structure is utilized to introduce tensile strain,a longitudinal pn junction in the horizontal direction is designed for the injection of carriers and the optical resonator is constructed by the ridge waveguide and the Bragg gratings on the sidewall of the waveguide.Employing mechanical,optical and electrical transport simulation,the impacts of strain value and doping concentration on the threshold current density and the internal quantum efficiency of the laser are analyzed.Simulation results show that the threshold current density of the DBR laser with the current material quality is 80 kA/cm².This value can be further reduced to 29 kA/cm² by optimizing the material quality.In order to compare with the conventional vertically injected lasers,the threshold current density of our device is corrected using an equivalent method.After the correction,the threshold current density is around 4.8 kA/cm²,which is on the same order of magnitude as the?-?laser with double heterojunction.?4?We compare the threshold characteristics of the uniaxially tensile stressed bulk germanium laser and Ge/SiGe quantum well laser.A DBR resonator based on a broadband circular grating is proposed,which improves the fabrication tolerance significantly.Simulation results indicate that the uniaxially tensile stressed Ge/SiGe quantum well laser has a much higher threshold current density compared with its bulk germanium counterpart.Through the theoretical calculations and analysis of simulation results,it is revealed that the higher threshold current density of the Ge/SiGe quantum well laser is due to the larger number of states between the?-point and L-points and the lower carrier injection efficiency.?5?We introduce uniaxial tensile stress in Ge/SiGe quantum well using the microbridge structure.The shift of Raman peak and red-shift of photoluminescence?PL?peak are observed.The PL peak is shifted from 1450 nm to 1800 nm,which implies a strain value of2.1%in the Ge-well region.In addition,a significant enhancement of PL intensity induced by the uniaxial stress is observed.?6?We propose and experimentally demonstrate a uniaxially tensile stressed germanium light emitting diode?LED?based on a longitudinal p-i-n junction.Raman mapping measurement shows that a uniaxial stress with a strain value of 1.76%and uniform strain distribution is introduced in the active region of the LED.The devices exhibit decent electrical performance with a current on-off ratio of 10⁵ and an ideality factor of 1.92.In the electroluminescence?EL?measurement,we observe that the EL peak shifts from 1580 nm to1840 nm,which is consistent with the theoretical prediction.Furthermore,the strain-induced enhancement of EL in germanium is confirmed experimentally with an enhancement factor of16 for the integrated EL intensity.The proposed approach offers a pathway toward an electrically driven strained germanium laser with low threshold current.