Quantum-Dot Molecules for Potential Applications in Terahertz Devices
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Quantum-Dot Molecules for Potential Applications in Terahertz Devices Valeria Gabriela Stoleru1, Elias Towe2, Chaoying Ni1, and Debdas Pal2 1 Department of Materials Science and Engineering, University of Delaware, Newark, DE 19716, U.S.A. 2 Department of Electrical and Computer Engineering, Carnegie Mellon University, Pittsburgh, PA, 15213, U.S.A.
ABSTRACT The experimental and theoretical results of the electronic and optical properties of quantum dot artificial molecules (AMs), formed by pairs of electronically coupled quantum dots (QDs), are presented here in order to identify the necessary conditions for the development of new types of terahertz (THz) injection lasers based on intraband carrier transitions. We have performed analytical calculations to obtain the spatial strain distribution in vertically aligned (In,Ga)As QDs grown on (001) GaAs substrates by molecular beam epitaxy. Electronic coupling of the dots, mainly governed by the thickness of the separating barrier between the dot layers, is allowed due to the strain field-assisted self-organization of the dots. The calculated strain field reproduces our cross sectional high-resolution transmission electron microscopy observations very well. We further take into account the microscopic effects of the spatial strain distribution on carrier confinement potentials, and compute the electronic structure of the AM. Our calculations of the peak luminescence energies are in good agreement with our experimental results and those of others. The growth of quantum dot molecules represents a major step in tailoring the electronic and optical properties of the nanostructures. INTRODUCTION Since their first demonstration in 1994, quantum cascade lasers have seen constant innovation as tunable coherent sources in the mid-infrared range of the electromagnetic spectrum [1]. Research in this field is continuously expanding: new material systems are being explored, [2] ultrahigh-speed operation and mode locking have been demonstrated, [3] and new spectral ranges outside the mid-infrared (MIR) regime are under investigations [4, 5]. Over the years, quantum cascade lasers have shown tremendous performance improvements and technological progress. At present time they are the only semiconductor lasers operating above room temperature in the 5-12 µm wavelength range, [6] with peak output power exceeding 100 mW at 300 K [7]. Semiconductor emitters fall broadly into two categories: interband and intraband devices. The former are based on transitions from conduction to valence bands, whereas the latter use quantum confined states within the conduction band. The problems which limit laser operation by keeping efficiencies low and laser thresholds high are quite different in these two cases, but both stem from high competing transition rates due to nonradiative processes. In interband devices Auger scattering dominates, and in intraband devices, though Auger scattering is still fast, phonon emission is even faster. While the choice of either the interband or intraband structu
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