Near-Field Scanning Optical Microscopy of Phase Separation Effects in Dilute Nitride Alloys.
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Near-Field Scanning Optical Microscopy of Phase Separation Effects in Dilute Nitride Alloys. Alexander Mintairov1, Thomas Kosel1, Kai Sun 1, Victor Ustinov2, and James Merz1. 1 Department of Electrical Engineering, University of Notre Dame, Notre Dame, IN 46556, USA 2 Ioffe Physico-Technical Institute, RAS, St. Petersburg 194021, Russia ABSTRACT The effect of nitrogen composition on structural parameters of intrinsic quantum dots (QDs) has been studied in GaAs1-yNy and InxGa1-xAs1-yNy alloys (y~0.015-0.03) using low-temperature near-field scanning optical microscopy (NSOM) combined with magneto-photoluminescence spectroscopy. We used measurements of the diamagnetic shift (magnetic field strength 0-10T), temperature dependent spectra (temperature range 5-300K) and near-field monochromatic images for the estimation of the size, nitrogen excess and density of QDs. The obtained values (size ~10-30 nm, nitrogen excess ~0.005 and density ~100 µm-3) suggest spontaneous formation (phase separation) of QDs. Strong lateral inhomogeniety of the QD distribution on a micron length scale was observed. INTRODUCTION Dilute nitride alloys (GaAs1-yNy and InxGa1-xAs1-yNy, y~0.01-0.03) have unusual electronic property - a ”giant bowing” parameter (b~20eV) which arises from the large electronegativity and small size of the nitrogen1,2. The resulting large energy scale for statistical composition fluctuation makes it possible to form quantum dots (QDs) in these alloys which appear in high spatial resolution low-temperature photoluminescence spectra as a series of sharp emission lines3. Here we use scanning near-field magneto-photoluminescence spectroscopy of these lines to study the effect of nitrogen composition on the structural properties of these QDs. We found that for nitrogen composition 0.01-0.03 the size, nitrogen excess and density of such intrinsic QDs exceed random statistics predictions, which implies that they originate from phase separation effects. EXPERIMENTAL DETAILS We studied GaAs1-yNy and InxGa1-xAs1-yNy (x~0.03-0.08 y~0.01- 0.03) epi-layers with thickness 0.1-1 µm grown on (001) semi-insulating GaAs substrates by solid source molecular beam epitaxy at temperature 450-520o C. The structural parameters of the samples are presented in Table 1. Samples 4 and 6 are InxGa1-xAs1-yN alloys , which were grown as GaAs0.983Ny0.017 /In0.365Ga0.635As0.981N0.019 and GaAs0.966Ny0.034 /InAs short-period-superlattices (SPSLs) with average N(In) compositions 0.0175(0.07) and 0.03(0.08), respectively, and consist of alternating Table 1 Composition and thickness of diluted nitride samples under study Sample No 1 2 3 4 5 6
Material GaAs1-yNy InxGa1-xAs1-yNy InxGa1-xAs1-yNy InxGa1-xAs1-yNy GaAs1-yNy InxGa1-xAs1-yNy
y,% 1.2 1.2 1.5 1.75 3.5 3
x ,% 0 3 5 7 0 8
Thickness, µm 0.1 1 0.2 0.3 0.12 0.1
Growth layer layer layer SPSL layer SPSL
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layers with thicknesses of individual layers 2.82/0.68 nm and 2.82/0.25 nm, respectively. In samples 4-6 the dilute nitride layers were sandwiched between AlGaAs layers of thickness 60
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