A thermodynamic approach is presented to assess the extent of anion exchange reactions during the heteroepitaxy (molecular beam epitaxy) of dissimilar anion III-V compound semiconductor structures. It is shown that the extent of anion exchange can be predicted by the change in the Gibbs free energy. Bond strength changes can only be used as a guide in comparing the relative tendency for exchange, rather than as a criterion. The driving force for anion exchange strongly depends on the conditions during interface formation. A number of important factors, including bond strength, misfit strain energy, surface structure and energy, the equilibrium between dimers V 2 and tetramers V 4 , and segregation are discussed in terms of their contributions to the thermodynamics. r
P/As anion exchange is exploited to modify stacked InAs/GaAs quantum dot structures grown by molecular beam epitaxy (MBE). It is shown that the vertical alignment and size uniformity can be remarkably improved via P/As anion exchange. This, therefore, demonstrates a promising approach to tuning the quantum dot morphologies and structures, and hence, the electronic and optoelectronic properties. q Self-assemble by exploiting the Stranski-Krastanov (S-K) growth of highly lattice-mismatched semiconductors is widely recognized as a powerful technique for fabrication of semiconductor quantum dot structures for novel electronic and optoelectronic applications [1,2]. In the S-K growth mode, coherent islands are formed through a spontaneous nucleation and growth process, which is fundamentally based on surface morphological instability driven by relaxation of the strain energy in lattice-mismatched heterostructures. This in situ growth process offers advantages in comparison to ex situ techniques, such as lithography. However, the spontaneous nature of the S-K growth generally produces islands distributed randomly and with a distribution of island sizes. This size non-uniformity causes considerable broadening of optical transition energies and poses a serious limitation to device applications. In multilayer structures, or quantum dot superlattices, it has been shown that the buried dots influence the nucleation and growth of the islands in subsequent layers, thereby giving rise to vertical alignment in the growth direction with improved size uniformity [3-6]. However, practically the optimization of growth parameters to realize perfect vertical alignment and improved size uniformity of islands in stacked quantum dot structures is difficult. Factors, such as growth interruption, growth temperature, growth rate, and spacer layer thickness, can remarkably influence the vertical alignment and size uniformity [7-9]. It has been widely observed that the island size tends to increase in the upper dot layers, although the vertical alignment is well controlled [3,8,10,11]. We have been investigating the modification of quantum dot size, composition, strain, and optical properties via changes in growth conditions and the use of dissimilar anion anneal steps after formation of the dots. In our anion exchange approach, the InAs quantum dots are annealed under a P 2 flux. Previous work has shown that (1) P-for-As anion exchange, or intermixing of P and As in the top surface InAs layer, occurs during P 2 anneal; (2) the P 2 anneal of InAs quantum dots can change the quantum dot morphology remarkably depending on the annealing temperature [12-14]. In this communication, we report our anion exchange approach by improving vertical alignment and island size uniformity in molecular beam epitaxy (MBE)-grown InAs/GaAs dot superlattice structures. The MBE-grown samples consist of five-layers of two mono-layers (MLs) of InAs, separated by 10 nm GaAs 0038-1098/02/$-see front matter q 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 3...
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