Epitaxy Of Gaas On Nano-patterned Si Substrates

As the age of Moore’s law is drawing to a close,continuing increase in computing performance is becoming increasingly hard-earned,while demand for bandwidth is insatiable.One way of dealing with this challenge is the integration of active photonic material with Si,allowing high-speed optical inter-and intra-chip connects on one hand,and the economies of scale of the CMOS industry in optical communications on the other.One of the most essential active photonic materials is GaAs,stemming from its capability in combination with its related materials to produce lasers.However,integrating GaAs with Si remains a challenging subject.Defects arise due to differences in lattice constants,differences in thermal expansion coefficients and.Approaches to counter these problems include the epitaxial growth on patterned,which involves growing GaAs on openings,then laterally from openings in a mask deposited on Si substrate,and finally realizing coalescence.This approach solves some of these problems by filtering out the dislocations（originating from GaAs/Si interface）by the bottom of the mask and the vertical sidewalls of the opennings.However,filtering may not be complete and the coalescence will result in rough surface and introduce new sources for formation of defects.This thesis is focused on the coalescence of GaAs on nano-patterned Si substrates by metal organic chemical vapor deposition（MOCVD）.The main achievements are listed below.1.Fabricated nano-patterned Si（001）substrates using SiO₂ as a mask and studied the cleaning method for patterned substrates.Trench-patterned substrates（opening wide:120 nm,period:240 nm）and round-hole patternd substrates（diameter:350 nm）were fabricated by nanoimprint lithography,laser holographic lithography,deep-uv lithography and reactive ion etching.We analyzed the types of pollutants on the surface of the patterned substrates,and put forward a cleaning treatment,finally obtained clean substrates without any pollutants for epitaxy.2.Studied the effects MOCVD growth parameters on the growth selectivity of GaAs on patterned Si substrates.Under the growth rate of 1A/s and pressure of 70 Torr,perfect growth selectivity had been achieved.We found that GaAs growth selectivity had no direct relationship with the temperature.3.Proposed a three-stage epitaxial lateral overgrowth（ELO）method.This method divided the epitaxy of GaAs on patterned substrates into three growth stage:the first selective growth stage,the second lateral overgrowth and coalescence stage,and the third planar growth stage.The growth rate,Ⅴ/Ⅲ ratio,temperature and growth time were changed ineach stage.By this method,the neighbouring GaAs epitaxial layers on the trench-patterned substrates had coalesced rapidly and uniformly,yielding a flat surface.This method could also reduce the dislocations induced by the coalescence.4.Studied the effects of trench opening orientations on the GaAs lateral overgrowth.We used four opening orientation,[110],[-110],[010]and[410],for the epitaxy of GaAs on trench-patterned Si substrates.The experiments showed that the lateral overgrowth rate of GaAs was best under the seed line orientation of[410];GaAs cross-sections were different under different seed line orientations.The growth thickness of GaAs epitaxial layer under the aforementioned four orientations were 542.7 nm,510.0 nm,511.8 nm and 396.3 nm,with the surface roughness（10×10μm2）of 16.5 nm,21.4 nm,10.5 nm and 6.29 nm,respectively.5.Analyzed and experimentally confirmed the formation mechanism of coalescence-induced dislocation during the coalescence of different growth regions,two-zipper coalescence growth mode.GaAs was selectively deposited and coalescence on trenched-patterned substrates by the three-stage ELO growth method.Measurement results showed that the coalescence of neighbouring GaAs epitaxial layer occurred from two starting points and then proceeds toward the central region.The last coalesced point would promote the formation of new dislocations.6.Conducted ’the the epitaxy of GaAs on Si（001）round-hole-patterned substrates.We conducted the GaAs epitaxy on round-hole-patterned Si substrates using the three-stage ELO growth method.Under the selective growth stage,GaAs had good growth selectivity.Growth fronts of GaAs pillars in the opening windows were faceted.Under the lateral overgrowth and coalescence stage,when the height of GaAs pillar exceeded the Si02 mask,the growth proceeded in a particular direction that had an angle with the growth direction（001）.The side walls of the GaAs pillars were surrounded by four crystal face.The coalescence of these GaAs pillars resulted in large surface roughness.