Thermal Stability of InGaAs Quantum Dots Under Large Temperature Transients
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Thermal Stability of InGaAs Quantum Dots Under Large Temperature Transients R. Rangarajan, V. C. Elarde, and J. J. Coleman Department of Electrical and Computer Engineering, University of Illinois, Urbana, IL 61801 ABSTRACT We report here studies of the thermal stability of InGaAs quantum dots that have been subjected to various thermal treatments. Atomic force microscopy and photoluminescence spectroscopy are used to analyze the effects of the thermal treatments. In this paper we present data that demonstrates a remarkable improvement in the thermal stability of quantum dots that were rapidly cooled down to room temperature following the growth of a GaAs capping layer. The observed thermal behavior is attributed to metastable states formed during post growth thermal cycle.
INTRODUCTION Photonic devices designed around quantum dots (QDs), zero dimensional systems, offer exciting advantages and potential over quantum well devices. Most applications of QDs, including photodetectors [1-4], high power lasers [5], and modulators [6], rely on further growth or processing steps involving high temperatures. However, the growth of quantum dots in InAs/GaAs materials system is complicated by thermal instability, which makes it difficult to incorporate them into devices. Thermal treatment is known to reduce photoluminescence efficiencies [7], introduce a blue shift in the emission wavelength [8], and better InGaAs/GaAs dots [9]. As a result thermal stability is highly desirable. Knowledge of thermal effects on QDs also attracts attention for application in adjusting QD characteristics [8,10,11]. In this paper we report studies of the thermal stability of InGaAs quantum dots. We confirm that post growth thermal treatments at temperatures significantly higher than the QD growth temperature can result in spectral blue shifts [8] and show that it eventually leads to the dissolution of the QDs. We present data confirming that introducing a post growth rapid cool down can significantly enhance the QD thermal stability. AFM and PL data indicate that the structural and optical properties of these treated dots are similar to QDs prior to any thermal treatment. The results are ascribed to the relaxation of QDs into a metastable state during the rapid cool down.
EXPERIMENT The samples in this work were grown in an atmospheric pressure metal-organic chemical vapor deposition (MOCVD) reactor. Common to all samples is a base structure, designed to be the lower half of a separate confinement heterostructure (SCH), and grown on an n-type, (100) oriented, GaAs substrate. The base structure, shown in Fig .1, is a GaAs buffer layer with a 1 µm Al0.74Ga0.26As cladding layer, both grown at 800°C, and a 65 nm GaAs inner barrier layer grown at 625°C. The temperature was ramped down to 500°C for the deposition of the self-assembled
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dot layer (~1.74 monolayers of In0.70Ga0.30As). The InGaAs quantum dots are capped with a 15 nm GaAs layer grown at 600°C. The control for this experiment is the base structure as described above cooled
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