Bimodal size distribution of self-assembledInxGa1−xAsquantum dots

Abstract

~Received 2 July 2001; revised manuscript received 9 April 2002; published 23 September 2002! We investigate quantization of energy levels in self-assembled In xGa12xAs quantum dots that are embedded in a GaAs matrix. We use capacitance and photoluminescence spectroscopies to analyze the evolution of the energy levels with varying amounts of deposited InxGa12xAs. These techniques suggest that the size distribution of the quantum dots contains two well-separated peaks. Transmission electron microscopy confirms a bimodal size distribution and further shows that the big and the small quantum dots have different shapes. In addition, we use an effective-mass based method to calculate the lowest energy states of quantum dots with the physical dimensions obtained by transmission electron and atomic force microscopies. Our results allow us to construct the energy-level diagrams of the two kinds of quantum dots. The strong quantization of the energy levels of selfassembled semiconductor quantum dots ~QDs!, a manifestation of their nanoscale size, can be utilized for optoelectronic applications such as QD lasers 1,2 and charge-storage devices. 3,4 From a fundamental point of view, charge carriers populating semiconductor QDs can be considered as model systems for quantum-mechanical interactions between confined electrons and holes. 5,6 The self-assembly process poses interesting questions regarding the growth dynamics of the QDs and the resulting distribution of sizes and shapes. 7‐10 For example, it has been found that certain growth conditions result in a bimodal or even multimodal 11,12 QD size distribution. In previous work, bimodal QD systems have been investigated with atomic force microscopy ~AFM! 13‐19 and photoluminescence ~PL! spectroscopy. 16,19,20 In this report, we combine PL and capacitance spectroscopies at liquid He temperature, atomic force microscopy ~AFM! and transmission electron microscopy ~TEM!, and effective-mass based calculations to obtain a complete picture of the evolution of the energy levels in the QDs with varying InxGa12xAs coverage. The PL and capacitance measurements show that two distinct sets of QDs with wellseparated energy levels coexist in the sample. The TEM and AFM not only measure the average sizes of the two kinds of QDs, but also show that the shapes of the big and small QDs are different. Combining these measurements with our calculations, we construct a consistent picture for the energy-level diagrams of both kinds of QDs and also speculate about the growth dynamics that give rise to the bimodal size distribution.