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The Definition of Multiple Bandgaps in Quantum-Dot Material by Intermixing
- A. Catrina Bryce, John H. Marsh, Dan A. Yanson, Olek P. Kowalski, Shin-Sung Kim
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- Journal:
- MRS Online Proceedings Library Archive / Volume 829 / 2004
- Published online by Cambridge University Press:
- 26 February 2011, B1.6
- Print publication:
- 2004
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We present the recent progress in the intermixing of InGaAs/GaAs quantum dot (QD) material. Quantum dot intermixing (QDI) allows the tuning of the energy bandgap in selected areas of the wafer or optoelectronic device, thus modifying its emission or absorption properties, in much the same way as in quantum well (QW) material. QDI has recently received increasing interest, as it combines bandgap engineering with the predicted advantages that quantum dots offer, such as low temperature-sensitivity of threshold current, high modulation frequency and low chirp.
We have applied the dielectric-cap-based techniques that were originally developed for QW structures, to the intermixing of InGaAs/GaAs/AlGaAs QD material with an emission wavelength of 1280 nm. Intermixing was achieved by sputtering a QDI-enhancing cap in some areas, and a QDI-suppressing cap in other areas, followed by a high-temperature anneal cycle. Extremely large bandgap blue-shifts of up to 280 nm have been obtained with an anneal temperature of 800 °C. The shifts were inferred from the photoluminescence (PL) spectra measured at 77 K under red-laser excitation.
To be of use in many applications, QDI must be able to provide a multiplicity of bandgaps on a single substrate. Multiple bandgaps can be created by varying the thickness of the QDI-enhancing cap, repeating the anneal cycle several times, or varying the coverage density of QDI-enhancing features over that of QDI-suppressing ones. The latter approach, termed selective intermixing in selected areas (SISA), involves the deposition of QDI-enhancing patterns of various area fill factors, which, upon annealing, will cause different degrees of intermixing in the underlying regions.
To demonstrate the SISA process in the QD material, we defined patterns containing lines and squares of various sizes (3 - 100 μm) and area fill factors (5% - 95%). The wafer was then annealed at 725 °C for 1 minute. As expected, the observed bandgap shifts were commensurate with the fill factor, with a 5% coverage providing a minimum shift (0 - 10 nm) and ∼40% a maximum one (∼ 120 nm). At fill factors above 40%, the shifts appeared to saturate and even decrease slightly. The effect of the feature size and shape was not very pronounced, with smaller features generating somewhat larger shifts. This may be due to the fabrication-related size bias that will have the strongest effect on the fill factor of smaller features. The PL spectra measured from patterns of large-size features (20 μm or more) often had a lopsided shape and broader peak width, which may be attributed to the limited spatial resolution of the measurement probe.
Quantum Well Intermixing Using Sputtered Silica for Photonic Integrated Circuits Operating Around 1550 nm
- John H. Marsh, A. Catrina Bryce, Olek P. Kowalski, Stewart D. McDougall, Maolong Ke, Bocang Qiu, Yahong Qian
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- Journal:
- MRS Online Proceedings Library Archive / Volume 607 / 1999
- Published online by Cambridge University Press:
- 10 February 2011, 479
- Print publication:
- 1999
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- Article
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A novel technique for quantum-well intermixing has been developed using sputtered silica. The technique relies on the generation of point defects via plasma induced damage during the deposition of the sputtered silica. The presence of the defects before annealing allows the intermixing to take place at lower annealing temperatures (>200 C) than those needed for conventional impurity free vacancy disordering. The low annealing temperature has allowed the technique to be applied to InP based quantum well systems such as InGaAs/InGaAsP and InGaAs/InGaAlAs, and provides a simple and reliable process for the fabrication of both wavelength tuned lasers and monolithically integrated devices operating around 1550 nm.
Wavelength tuned broad area oxide stripe lasers have been demonstrated in InGaAs-InAlGaAs and InGaAs-InGaAsP quantum wells. Oxide stripe lasers with integrated intermixed slab waveguides have enabled the production of a narrow (3 degrees), single lobed far field pattern in InGaAs-InAlGaAs devices. Extended cavity ridge waveguide lasers operating around 1550 nm have been demonstrated with low loss (alpha = 4.1/cm) waveguides, the loss beinglimited by free carrier absorption in the waveguide cladding layers. Ridge waveguide, deeply etched surface grating DBR lasers were fabricated both with and without intermixing the grating section. Measurements show that a significant improvement in performance is obtained from the DBR lasers when the grating section is intermixed.
The intermixing technique has been further developed in order to realise 3 bandgaps on a chip in a single annealing step for the integration of amplifiers, passive waveguides and bandgap tuned electro-absorption modulators. Modulation depths of 25 dB were measured from the modulators.
The results illustrate that the technique can routinely be used to fabricate low-loss optical interconnects and bandgap tuned devices, offering a very promising route toward photonic integration.