Shih et al. Nanoscale Research Letters 2011, 6:409 http://www.nanoscalereslett.com/content/6/1/409 N A N O E X P R E S S Open Access Effects of crossed states on photoluminescence excitation spectroscopy of InAs quantum dots Ching-I Shih1, Chien-Hung Lin2, Shin-Chin Lin1, Ta-Chun Lin2, Kien Wen Sun1, Oleksandr (Alex) Voskoboynikov2, Chien-Ping Lee2 and Yuen-Wuu Suen3* Abstract In this report, the influence of the intrinsic transitions between bound-to-delocalized states (crossed states or quasicontinuous density of electron-hole states) on photoluminescence excitation (PLE) spectra of InAs quantum dots (QDs) was investigated. The InAs QDs were different in size, shape, and number of bound states. Results from the PLE spectroscopy at low temperature and under a high magnetic field (up to 14 T) were compared. Our findings show that the profile of the PLE resonances associated with the bound transitions disintegrated and broadened. This was attributed to the coupling of the localized QD excited states to the crossed states and scattering of longitudinal acoustical (LA) phonons. The degree of spectral linewidth broadening was larger for the excited state in smaller QDs because of the higher crossed joint density of states and scattering rate. Introduction Self-assembled semiconductor nanostructures with three-dimensional carrier confinement provide the ulti- mate quantum system with discrete energy levels that can be tailored and controlled to tune the electrical and optical properties of these nanostructures. In particular, InAs on GaAs (001) self-assembled quantum structures is one of the most-studied systems. Quantum dots (QDs) provide another approach in making lasers, photodetectors, and memory devices as well as finding applications in quantum computing. Quantum dot lasers are predicted to have a high efficiency, low threshold current densities, and low temperature dependence of the threshold current [1-3]. The use of QDs may offer possibilities for low-power nonlinear devices. Therefore, the understanding of optical properties in these nanos- tructures is of extreme relevance for device applications to be a realistic prospect. Theoretically, carriers confined in QDs show an atomic-like energy spectrum, characterized by discrete low-lying confined states, followed by spatially deloca- lized states associated to the InAs wetting layer and to the GaAs cap layer. The current perspective and analysis of the controversies regarding the phonon bottleneck in * Correspondence: ysuen@phys.nchu.edu.tw 3Institute of Nanoscience, National Chung Hsing University, 250 Kuo Kuang Rd., Taichung 402, Taiwan Full list of author information is available at the end of the article semiconductor QDs have been discussed by Prezhdo [4]. However, this simple picture fails when the actual dots are probed using advanced local probe techniques that provide excellent spatial and spectral resolutions. These techniques enable us to study only a few dots or even one dot. Near-field photoluminescence excitation (PLE) spectra of single quantum dots display 2D-like conti- nuum states and a number of sharp lines between a large zero-absorption region and the 2D wetting layer edge [5]. The carriers were also found to relax easily within continuum states, and make transitions to the excitonic ground state by resonant emission of localized phonons. Limitations of the isolated artificial atom pic- ture of an InAs QD were investigated in Ref. [6]. This study showed that the continuum background in the up-converted photoluminescence signal is possibly related to the wetting layer. Microphotoluminescence excitation spectra for neutral excitons revealed a conti- nuum-like tail and a number of sharp resonances above the detection energy [7]. Temperature-dependent PL studies of an ensemble of self-assembled (In, Ga)As QDs provide insight into the nature of the continuous states between the wetting layer and QDs [8]. However, in contrast to other findings, PLE results of single InGaAs dot experiments by Hawrylak and co-workers showed spectra free of any continuum background and sharp emission lines [9]. © 2011 Shih et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.