Broad-area InAs=GaAs quantum dot lasers incorporating Intermixed passive waveguide N.Yu. Gordeev, W.K. Tan, A.C. Bryce, I.I. Novikov, N.V. Kryzhanovskaya, S.M. Kuznetsov, A.G. Gladyshev, M.V. Maximov, S.S. Mikhrin and J.H. Marsh InAs=GaAs quantum dot lasers incorporating passive waveguide created by post-growth intermixing processing have been studied. Emission wavelength of the passive section shows relative blueshift as high as 135 nm with respect to the emission wavelength of the active section. Intrinsic losses in the section formed by the intermixing are very small. Broad-area lasers with 100 mm stripe width incorporating intermixed section have demonstrated improvements in far-field pattern under both pulsed and continuous wave pumping current. Introduction: Semiconductor quantum dot medium is of great interest owing to a number of applications such as lasers, optical amplifiers and photodetectors. For future applications, device miniaturisation and integration into photonic integrated circuits (PICs) are required. A special post-growth bandgap engineering technique known as inter- mixing has been shown as a powerful tool to create quantum-well based PICs [1]. It allows tuning of the energy bandgap in selected areas of the semiconductor wafer. Similar approaches are being developed for quantum dots [2, 3]. In this Letter, we present our results on broad-area extended cavity lasers (ECL) fabricated by the quantum dot intermixing (QDI) process known as sputter induced disordering (SID). An ECL comprises an active section and mono- lithically integrated intra-cavity spatial mode filter, which suppress the lasing in higher order modes [4]. The SID technique [5] involves the generation of point defects during the deposition of a sputtered SiO 2 film (QDI-enhancing cap), followed by a high-temperature annealing stage to enable the diffusion of point defects through the structure. An SiO 2 film (QDI-suppressing cap) is deposited on the active sections by plasma-enhanced chemical vapour deposition (PECVD) to suppress the intermixing process. 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150 1200 1250 1300 as grown Yag : Nd CW RT peak wavelength, nm annealing temperature, °C sputtered silica PEC VD silica Fig. 1 Annealing temperature dependence of photoluminescence spectra maximum 900 1000 1100 1200 1300 1400 EL RT T anneal = 650∞C intensity, a.u. l, nm as grown PECVD sputtered Fig. 2 EL spectra of as-grown sample, sample with sputtered SiO 2 annealed at 650  C and sample with PECVD SiO 2 annealed at 650  C Experiment: The QD structure used in this study was grown by molecular beam epitaxy on an Si-doped GaAs substrate. The active region contained five layers of InAs quantum dots separated by 40 nm-thick GaAs spacers and was placed in the centre of a 0.4 mm-thick AlGaAs waveguide layer. This core layer was sand- wiched between AlGaAs 1.5 mm-thick cladding layers. The wafer composition and growth conditions are similar to ones described earlier [6]. The emission wavelength of the as-grown wafer was around 1280 nm. Initial properties of the laser wafer have been checked by fabrication of broad-area lasers with stripe width of 100 mm. The processed devices have demonstrated threshold current density of 150 A=cm 2 , internal quantum efficiency of 90% and internal losses of 3 cm 1 . Depending on the cavity length the lasers emitted in the wavelength range 1280–1286 nm. Two sets of samples have been prepared. Samples from the first set were capped with a 500 nm-thick SiO 2 cap deposited by plasma enhanced chemical vapour deposition (PECVD). Samples from the second set were capped with sputtered silica. Then test pairs combining samples from each set were annealed at various temperatures (600– 850  C) for 1min by rapid thermal annealing (RTA). Following the intermixing process, photoluminescence measurements (PL) were carried out to reveal the influence of anneal temperature on bandgap shift (see Fig. 1). Samples capped with sputtered silica showed stronger blueshift. Clear differential wavelength shift is seen in the whole annealing temperature range. The largest differential shift of 135 nm has been observed for an annealing temperature of 700  C. However higher annealing temperature results in a strong bandgap blueshift in samples capped with PECVD SiO 2 as well. Fig. 2 shows the electro- luminescence (EL) spectra measured at room temperature (RT) of an as- grown sample, a sample with sputtered SiO 2 annealed at 650  C and a sample with PECVD SiO 2 annealed at 650  C. A differential shift of 120 nm between PL spectra from samples capped with two types of SiO 2 is achieved at this annealing temperature. Overlap between the PL spectra is very small, which ensures that there will be no considerable absorption in the passive section. The QDI practically does not affect emission wavelength of the active region as the total bandgap blueshift for sample with PECVD SiO 2 is only 20 nm. Hence, annealing at 650  C is sufficient for the integration of the passive waveguide in QD lasers. –10 –5 0 5 10 1210 1220 1230 3 A, 1.5 deg 4.5 A, 1.8 deg 170 mW L = 3 + 0.15 mm I th ~ 2.9 A angle, deg l, nm Fig. 3 Far-field pattern and spectra for ECL with 3 mm-long active section and 150 mm-long passive section Extended cavity oxide stripe lasers have been fabricated from a specially prepared part of the wafer. It was partly capped with sputtered SiO 2 and PECVD SiO 2 . An intermixed region (capped with sputtered SiO 2 ) has been used as a passive section, while a nonintermixed region (capped with PECVD SiO 2 ) has been used as an active section. Devices with stripe width of 100 mm were fabricated. The samples were mounted p-side down on a copper heatsink covered with indium. To eliminate any heating effects the devices were tested under pulsed pump current of 300 ns duration at 1 kHz repetition frequency. Results and discussion: Fig. 3 shows the pump current dependence of the far-field pattern for a ECL with a 3 mm-long active section and a 150 mm-long passive section at one facet. Single-lobed narrow far- field distribution with a full-width at half-maximum (FWHM) of 2  is ELECTRONICS LETTERS 4th January 2007 Vol. 43 No. 1