Design And Fabrication Of High Efficient Si-based Ge Light Emitters For Si-based Optoelectronic Integrated Circuits

The realization of high efficient Si-based light emitting devices is one of the greatest challenges for Si-based optoelctronic integrated circuits.For its quasi-direct band structure,high carrier mobilities,large absorption coefficient at telecom wavelength?-1.55?m?and compatibility with Si architecture,Ge is considered as one of the most promising candidate materials for Si-based light source.Via enhancement of tensile strain level and n-type doping concentration in Ge,the light emitting efficiency of Ge can be effectively improved.This thesis focuses on preparation of Si-based Ge materials,structure design and fabrication of high efficient Si-based Ge light emitting devices.The work summary and innovation of this thesis are listed below:1.Based on Fermi-Dirac model for carrier distribution and Van de Walle's deformation theory,the influence of tensile strain,temperature and doping on the spontaneous emission?SE?properties of degenerated Ge was systematically investigated.It was found that tensile strain is beneficial to improve occupation levels of carriers.The SE of Ge increases effectively under tensile strain and becomes less sensitive to temperature.As elevation of sample temperature,the energy difference between ' and L valley shrinks rendering increase of both peak intensity and integrated intensity of Ge SE.Compared to p-type doping,n-type doping is more favorable to improve the light emitting efficiency of Ge.2.Via varying width of SiN_x-capped Ge and Ge-on-insulator?GOI?stripes,the tensile strain in Ge layers was modulated.The tensile strain in Ge becomes larger as broadening of stripe width and becomes constant above certain width for Ge stripes,while diminishes with broadening of stripe width for GOI stripes.By combination with phosphorus ion implantation and excimer laser annealing,a n+/p junction with a peak donor concentration of?5×10¹⁹ cm^-3 and a junction depth of 44 nm was achieved.Due to n-type heavy doping,electroluminescence?EL?from Ge direct band recombination at around 1600 nm and defect related luminescence at around 1660 nm was observed simultaneously.3.Light emitting diodes?LEDs?with tensile-strain Si_0.13Ge_0.87/Ge multiple quantum wells on Si substrate were fabricated.Electroluminescence from recombination of carriers between ?1 and HH1 of tensile-strain Ge wells was observed under continuous injection currents at room temperature.Due to Joule heating effect of the injection current,the peak energy of EL decreases superquadratically with increase of injection current density.Under constant injection currents,the EL intensity from Ge wells becomes stronger as elevation of sample temperature.4.Lateral p-Si_0.05Ge_0.95/i-Ge/n-Si_0.05Ge_0.95 heterojunction LEDs on SOI substrate were proposed and fabricated.The light extraction efficiency of the device was improved with the lateral p-i-n structure;the SiGe/Ge/SiGe double heterojunctions led to improvement of carrier injection ratio by-137 folds;due to resonant effect between 'Si/BOX' interface and the surface of device,the EL intensity of the diode was effectively enhanced;through annealing of the material at 800?,tensile strain of?0.30%was induced into Ge.With the above designs,the EL peak intensity of the lateral heterojunctions was enhanced by?4 folds compared to vertical Ge p-i-n homojunctions under a same injection level.5.Vertical resonant cavity LEDs?RCLEDs?with dual active regions consisting of highly n-doped SiGe/Ge MQWs and Ge epilayer with tensile strain on SOI substrate were proposed.The SE from Ge MQWs,whose peak energy is slightly larger than the direct bandgap of Ge epilayer,serves as pumping light for the Ge epilayer,rendering extra non-equilibrium electrons in ? valley.By combination with vertical microcavity,the optical path is greatly elongated and the luminescence of Ge MQWs escaped from the dual active regions can be recycled to optically pump the Ge epilayer.With the unique design,optical gain from Ge epilayer was observed in the range of wavelength from 1625 to 1700 nm under continuous injection currents.Based on the result,a vertical cavity surface emitting laser?VCSEL?was fabricated paving the way to further optimization of device structure.6.A modified Ge condensation method was proposed to fabricated high quality SiGe-on-insulator?SGOI?substrates.As enrichment of Ge content in SiGe layer,the condensation temperature was gradually decreased from 1150? to 900?.With the modified recipe,uniform SGOI and GOI substrates were obtained with high crystal quality and low surface roughness?RMS<1.0 nm?.The modified Ge condensation method can be applied to fabricate Ge-rich SGOI?Ge content>0.70?substrates with high compressive strain?up to-1.23%?.7.A three dimentional Ge condensation model was setup.Based on the model,SiGe/Ge heterojunction nanowires on insulator with tunable Ge fraction in SiGe was designed and fabricated.A MSM photodiode with Si0.09Ge9.91/Ge heterojunction nanowires on insulator was further fabricated with a reverse saturation dark current of 2.7 nA,which was significantly lower than that of general Ge photodiodes by more than 3 orders of magnitude.Due to existence of tensile strain and the Schottky barrier height of metal/Si_0.09Ge_0.91 contact in the nanowires,the cutoff detection wavelength was extended to?2400 nm.