Quantum Well Intermixing Using Sputtered Silica for Photonic Integrated Circuits Operating Around 1550 nm
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Ga vacancy concentration at the surface and an enhanced Ga-Al interdiffusion rate'. While certain caps enhance the out-diffusion, others suppress it and it is therefore possible to achieve spatially selective intermixing using photolithographic patterning of the appropriate capping layers. Both SrF22 and P:SiO23 have proved effective caps for suppressing the intermixing process, enabling the fabrication of extended cavity lasers with low passive section loss. The process has not been used successfully on InP based QW systems due to the high temperatures required ( 900 °C) in order to induce intermixing. Both the InGaAs/InGaAsP and InGaAs/InGaAIAs systems will intermix due to thermal interdiffusion at much lower temperatures, 650 *C 4. Ion implantation processes5 and laser processes 6 have been developed for intermixing these material systems and have been used in the fabrication of devices. At Glasgow, a technique has been developed which has been used to intermix a large range of III-V QW semiconductors: GaInP/AlGaInP, GaAs/AlGaAs, InGaAs/GaAs, InGaAs/InGaAsP and InGaAs/InGaAlAs. The technique uses sputtered SiO as the intermixing cap and processing takes place at relatively low annealing temperatures around 700 *C. The application of this technique for the fabrication of photonic integrated circuits operating at wavelengths around 1.55 pm will be discussed. THE QUANTUM WELL INTERMIXING TECHNIQUE The technique involves sputtering a layer of silica onto the surface of the sample and annealing at temperatures around 600 *C to 750 *C in a rapid thermal annealer (RTA) for times around 60 s. Intermixing studies have been carried out on laser structures in both the InGaAs/InGaAsP and the InGaAs/InGaAlAs quantum well systems. All samples used in the study were nominally lattice matched to InP and grown on InP substrates. The InGaAsP (P-Q) structure was grown by metal organic vapour phase epitaxy (MOVPE) and comprised a MQW active section with 5x65 A QWs separated by 120 A InGaAsP (X,=1.26 ,im) barriers within a 0.36 Pm thick stepped graded index waveguide, surrounded by Si doped n-type (lxl0 17 cm- 3 ) and Zn doped p-type (lxl0 17 cm- 3 ) InP cladding layers with respective thicknesses of 1.0 pm and 1.4 pm. The 19 cm- 3 ) InGaAs layer. The sample was capped by a 100 nm thick p+ Zn doped (lxl0 InAlGaAs (Al-Q) sample was grown by molecular beam epitaxy and 8 100S comprised a 0.5 pm Si doped n-type 7 0 80InAlAs cladding layer (5x101 cm3), a MQW active region containing 60U 0 6x70 A QWs separated by 80 0 InAlGaAs barriers centrally placed 40U within a 0.34 pm undoped C 03 rJ. 20waveguide core, a 2 pm Be doped (5x10 1 7 cm- 3 ) p-type InAlAs 0cladding layer and a p+ (Ix10 19 cm700 750 600 650 550 Anneal Temperature (-C) 3) InGaAs cap. Bandgap shifts were determined by measuring the Fig.1. Energy shift as a function of annealing photoluminescence (PL) of the temperature for P-Q samples annealed with sputtered samples at 77 K. Si0 2 caps (N), plasma deposited SiO2 caps (J) and The bandgap shifts were Al-Q samples annealed with
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