The influence of temperature on the photoluminescence properties of single InAs quantum dots grown on patterned GaAs
© Tommila et al.; licensee Springer. 2012
Received: 4 May 2012
Accepted: 6 June 2012
Published: 19 June 2012
We report the temperature-dependent photoluminescence of single site-controlled and self-assembled InAs quantum dots. We have used nanoimprint lithography for patterning GaAs(100) templates and molecular beam epitaxy for quantum dot deposition. We show that the influence of the temperature on the photoluminescence properties is similar for quantum dots on etched nanopatterns and randomly positioned quantum dots on planar surfaces. The photoluminescence properties indicate that the prepatterning does not degrade the radiative recombination rate for the site-controlled quantum dots.
KeywordsIII-V semiconductors InAs Quantum dots Site-controlled quantum dots Molecular beam epitaxy Nanoimprint lithography 78, optical properties, condensed-matter spectroscopy and other interactions of radiation and particles with condensed matter 78.67.-n, optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures 78.67.Hc, quantum dots
Single semiconductor quantum dots (QDs) are the building blocks for future information processing platforms, such as quantum cryptography and quantum computing [1–3]. QDs have been exploited as single [4, 5] and entangled photon sources [6, 7]. In order to enable such applications, the QDs should be fabricated at well-defined positions, rendering impractical the standard epitaxial processes that would result in randomly positioned QDs. The control of QD position can be achieved by creating preferential nucleation sites via patterning the deposited surface. Site-controlled quantum dots (SCQDs) have been fabricated utilizing various patterning techniques [8–12] combined with molecular beam epitaxy (MBE) or metalorganic vapor phase epitaxy. The optical properties of single QDs are easily deteriorated by defects, which are induced during the patterning and subsequent overgrowth. The defects cause nonradiative recombination channels, which degrade the QD photoluminescence (PL). Measuring the optical properties of a QD as a function of temperature provides a method to assess the quality of the QDs.
In this paper, we study the temperature-dependent PL of single SCQDs fabricated on a nanoimprint lithography patterned GaAs(100) surface. We compare their PL properties to the ones of single self-assembled quantum dots (SAQDs) grown on unpatterned surface.
The InAs QD sample was fabricated on a GaAs(100) substrate by combination of MBE and soft ultraviolet nanoimprint lithography (UV-NIL). First, a GaAs buffer layer, an AlGaAs barrier layer and a GaAs layer were grown by MBE. Second, the sample was patterned ex situ by UV-NIL using mr-UVCur06 (Micro Resist Technology GmbH, Berlin, Germany) as an etch mask and an EVG-620 (EV Group, St. Florian am Inn, Austria) mask aligner. The patterned area consisted of holes with a diameter of 100 nm arranged in a square lattice with the period of 1.5 μm. After patterning, the sample was chemically cleaned, and the native oxides were removed using IPA-, HCl-, and NH4OH-based solutions. The process is described in more detail in . After chemical treatment, the sample was loaded into the MBE reactor, and a 30-nm GaAs buffer layer was grown at 470°C. Growth-interrupted MBE [13, 14] at 540°C was used to form single SCQDs in the patterned holes and low density SAQDs randomly positioned outside the patterned area. Finally, the QDs were capped by 20 and 50 nm of GaAs grown at 540°C and at 590°C, respectively, and 50-nm AlGaAs grown at 590°C. The structure was finished by a 10-nm GaAs layer.
Micro-PL (μPL) measurements were performed at various temperatures up to 70 K using a continuous-wave laser emitting at 532 nm for excitation and a microscope objective (numerical aperture = 0.8) for diffraction-limited laser beam focusing and PL light collection. The PL signal was dispersed by a 50-cm spectrometer containing a 1,200 lines/mm grating and detected by a cooled Si CCD camera. The spectral resolution of the setup was 66 μeV.
Results and discussion
We have studied the influence of temperature on the PL properties of single site-controlled InAs QDs fabricated by a combination of soft UV-NIL and MBE. We have shown that the QDs in etched holes emitting at 945 nm have temperature-dependent PL properties similar to the self-assembled QDs grown on planar surface. The PL properties indicate that the defects induced by the patterning do not degrade significantly the emission of the site-controlled QDs. Thus, soft UV-NIL-positioned QDs are considerable candidates for fabricating large-scale optoelectronic devices.
Molecular beam epitaxy
Self-assembled quantum dot
Site-controlled quantum dot
Ultraviolet nanoimprint lithography
The research was carried out within the Academy of Finland project DAUNTLESS (decision number 123951). JT acknowledges the National Doctoral Programme in Nanoscience (NGS-NANO), the Vilho, Yrjö and Kalle Väisälä Foundation and the Industrial Research Fund at Tampere University of Technology (Tuula and Yrjö Neuvo fund) for the financial support. AS acknowledges funding from the Academy of Finland within the project 138940. TVH acknowledges financial support from the Finnish National Graduate School in Materials Physics, Jenny and Antti Wihuri Foundation, and Finnish Foundation for Technology Promotion. CS and TK acknowledge funding from the Deutsche Forschungsgemeinschaft via project KI 1257/1.
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