Study On TCO Films As Electrode And Al₂O₃ Films As Buffer Layer In Micro-Lasers

With the development of nano-optics and nano-optoelectronics,laser size decreases and device performance gets improved.But in the current-pump model,the placement of metal electrodes and the current injection are bottlenecks in micro-lasers design and application.TCO films are expected to use as bridge electrodes to transport current from metal electrodes which are placed far away to core layers of the lasers.Meanwhile,the big difference of refractive indexes between the TCO?about1.8².0?and the semiconductor?popularly-used InP and GaAs,3.2³.4?will enhance the confinement of the light,as cladding materials.Basic requirements for such TCO functional films are as followings: as electrode materials,they are supposed to have good electrical conductivity to reduce their electric consumption;as cladding materials,they are supposed to have low optical loss coefficient;above all,Ohmic contacts with low specific resistance and good thermal stability must be achieved between the interfaces of TCO film and semiconductor substrate.Thus,in this paper,by ion beam assisted deposition?IAD?,In₂O₃ films and ZnO and SnO₂equally-cosubstituted ZnO-In₂O₃-SnO₂?ZITO?films were deposited onto regular semiconductor substrates.Not only their optical and electrical properties but also the contact properties between the TCO films and the semiconductor substrates were studied systematically.In₂O₃ films were deposited onto n-InP and p-InP substrates by IAD.Their electrical and optical properties were tuned by controlling the O₂ flow rate of the assited ion beam resources.It turns out,with the increasing O₂ flow rate from 0 sccm to 7 sccm,the electrical conductivity and the optical loss coefficient at wavelength of1550 nm of the film both decrease,while the refractive index increases.When the O₂ flow rate was 3 sccm,conductivity and optical loss coefficient at wavelength of 1550 nm of the In₂O₃ film were 99.9 S/cm and 831.1 cm-1.Effects of plasma pretreatment on InP substrates and rapid contact annealing temperature on the interfacial contact properties between In₂O₃ films and InP substrates were discussed.H₂ plasma pretreatment on InP substrates can lower the contact barrier height and is conductive to form Ohmic contacts,but O₂ plasma pretreatment to the semiconductor substrates works on the contrary,so allOhmic contacts for In₂O₃/n-InP pretreated by H₂ plasma and all Schottky contacts forIn₂O₃/n-InP pretreated by O₂ plasma;contact annealing treatments at temperatures lower than 400 ? do not have great effects on the interfacial electrical property;noOhmic contacts were ahieved between In₂O₃ films and p-InP substrates.Though the conductivity of In₂O₃ film prepared with the O₂ flow rate of 3 sccm is not high,its optical loss coefficient is small.Meanwhile,goodOhmic contacts were achieved between In₂O₃ films and n-InP substrates pretreated by H₂ plasma.For as-deposited In₂O₃ films and n-InP substrates pretreated by H₂ plasma,the specific contact resistance was 1.37×10-6?.cm2.In the rapid contact annealing process,with the increasing annealing temperature,their contact resistance first increased slowly,but when the annealing temperature was improved to 450 ?,their specific contact resistance increased significantly and the interfacial contact property got worse.It indicates that its thermal stability is good in the post contact annealing process,when the annealing temperature is lower than 450 ?.So,In₂O₃ film prepared with the O₂ flow rate of 3 sccm suits for n-InP based micro-lasers and the rapid contact annealing temperature is ought to be no higher than 400 ?.Also by IAD,ZITO films were deposited onto n-InP and p-GaAs substrates.Variation laws of the electrical and optical properties of ZITO films in dependence with the O₂ flow rate during film deposition are the same as those of In₂O₃ films.When the O₂ flow rate was 0 sccm,the conductivity and the optical loss coefficient at1550 nm of ZITO film were 525.2 S/cm and 9775.3 cm-1and its refractive index was low as 1.32 which means big difference in refractive index will be obtained between TCO films and semiconductor substrates and the confinement of the light will be enhanced.Effects of plasma pretreatment of the substrates prior to ZITO film deposition and the rapid contact annealing treatments on the interfacial contact properties were studied.It turns out complicated variation laws were found in as-deposited ZITO film contacts to n-InP: the barrier height of contacts between ZITO films and solvent-cleaned substrates decreases with the increasing O₂ flow rate;for H₂-cleaned substrates,the contact barrier heights are all lower than those of the solvent-cleaned and the O₂-cleaned ones,resulting in allOhmic contacts between ZITO films and H₂-cleaned n-InP substrates,but the biggest interfacial contact resistance was observed at ZITO-5 film;while for O₂-cleaned substrates,Schottky contacts were found between n-InP substrates and both ZITO-3 film and ZITO-5 film with high contact barrier heights;whileOhmic contacts were obtained between n-InP and bothZITO-0 film and ZITO-7 film,originating from its multicomponent of ZITO by comparation with In₂O₃ films.When the O₂ flow rate was 0 sccm,though the optical loss of ZITO film is high,it possesses high conductivity meanwhile.And at this time,goodOhmic contact was achieved between ZITO films and n-InP substrates pretreated by H₂ plasma.For as-deposited samples,the specific contact resistance was 1.84×10-4 ?.cm2.In the rapid contact annealing process,with the increasing annealing temperature,their conact resistances first increased slowly,but after annealed at 450 ?,the specific contact resistance decreased and was 1.24×10-4 ?.cm2,meaning its thermal stability is good in the post rapid contact annealing process.So ZITO film prepared at the O₂ flow rate of0 sccm suits for n-InP based micro-lasers and the rapid contact annealing temperature can be high as 450 ?.Especially,Schottky contact was found between ZITO films as-deposited at 0sccm and p-GaAs substrates pretreated by H₂ plasma,but after annealed at 360 ?and 400 ?,Ohmic contact were achieved.Al₂O₃ films prepared by traditional r.f magnetron sputtering were studied in this paper and the deposition time was tuned in order to get compact,continuous and ultra-thin films to be used in micro-lasers as dielectric layers.When the deposition time is 30 min,the thickness of the Al₂O₃ film is about 30 nm and the film is compact and continuous,but its optical and electrical properties are not good enough as dielectric layers.