Atomic Self-Ordering in Heteroepitaxially Grown Semiconductor Quantum Dots Due to Relaxation of External Lattice Mismatc
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Atomic Self-ordering in Heteroepitaxially Grown Semiconductor Quantum Dots due to Relaxation of External Lattice Mismatch Strains Peter Möck1*, Teya Topuria1, Nigel D. Browning1, Robin J. Nicholas2, and Roger G. Booker3 1
Department of Physics (MC 273), University of Illinois at Chicago, 845 W. Taylor Street, Chicago, IL 606077059, U.S.A; *[email protected] 2 Department of Physics, Clarendon Laboratory, University of Oxford, Parks Road, Oxford, OX1 3PU, U.K. 3 Department of Materials, University of Oxford, Parks Road, Oxford, OX1 3PH, U.K.
ABSTRACT Thermodynamic arguments are presented for the formation of atomic order in heteroepitaxially grown semiconductor quantum dots. From thermodynamics several significant properties of these systems can be derived, such as an enhanced critical temperature of the disorder-order transition, the possible co-existence of differently ordered domains of varying size and orientation, the possible existence of structures that have not been observed before in semiconductors, the occurrence of atomic order over time, and the occurrence of short range order when the growth proceeds at low temperatures. Transmission electron microscopy results support these predictions. Finally, we speculate on the cause for the observed increase in life time of (In,Ga)As/GaAs quantum dot lasers [H-Y. Liu et al., Appl. Phys. Lett. 79, 2868 (2001)]. INTRODUCTION The growth of self-assembled semiconductor quantum dots (QDs) by the heteroepitaxial StranskiKrastanow mode and its variants has developed rapidly over recent years [1]. Regardless of the deposition parameters, alloying of the elements or compounds (e.g. the formation of (Ge,Si) [2], (In,Ga)As [3], (Cd,Zn)Se [4], ect) that constitute the QD system is known to take place during the growth. This alloying is even considered to be crucial for the Stranski-Krastanow growth mode to operate in semiconductors [5]. Since the deposition and alloying are random events, these QDs will have the structure of the prototype of the constituents of the alloy, i.e. the atoms will (initially) be randomly distributed. A tacit assumption throughout the quantum dot community is that Stranski-Krastanow grown random alloy QDs are structurally stable. Recent experimental evidence, however, indicates that this is for several III-V and II-VI compound semiconductor systems not the case [6-8]. In these systems it appears that it is atomically ordered or phase separated QDs that are structurally stable, i.e. the initial random alloy transforms to a partially ordered superlattice structure. In hind sight, it is surprising that structural transformations by means of atomic ordering and phase separation within Stranski-Krastanow grown QDs have so far not been taken into account, as there is a large body of experimental and theoretical literature which describes such transformations in heteroepitaxial semiconductor alloy layers, e.g. [10], [11], and review [12]. As these effects lead to both a reduction of the stored lattice mismatch energy and a lower band gap, such ordering should
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