Himwas et al. Nanoscale Research Letters 2011, 6:496 http://www.nanoscalereslett.com/content/6/1/496 N A N O E X P R E S S Optical properties of as-grown and annealed InAs quantum dots on InGaAs cross-hatch patterns Chalermchai Himwas, Somsak Panyakeow and Songphol Kanjanachuchai* Abstract InAs quantum dots (QDs) grown on InGaAs cross-hatch pattern (CHP) by molecular beam epitaxy are characterized by photoluminescence (PL) at 20 K. In contrast to QDs grown on flat GaAs substrates, those grown on CHPs exhibit rich optical features which comprise as many as five ground-state emissions from [1-10]- and [110]-aligned QDs, two wetting layers (WLs), and the CHP. When subject to in situ annealing at 700°C, the PL signals rapidly degrades due to the deterioration of the CHP which sets the upper limit of overgrowth temperature. Ex situ hydrogen annealing at a much lower temperature of 350°C, however, results in an overall PL intensity increase with a significant narrowing and a small blueshift of the high-energy WL emission due to hydrogen bonding which neutralizes defects and relieves associated strains. Keywords: quantum dots, cross-hatch patterns, photoluminescence, annealing, InAs, InGaAs. Open Access Introduction Self-assembled InGaAs quantum dots (QDs) have been intensively investigated during the last decade due to their high crystalline quality [1]. InGaAs QDs conven- tionally grown on on-axis (100)-GaAs substrates are optically active and typically emit in the 1.0 to 1.3 eV range [2]. Those grown unconventionally - on high- index substrates [3], pre-patterned layers [4], or cross- hatch patterns [5-9] - exhibit similar optical characteris- tics with a possibility to obtain lateral QD alignment, further expanding the range of optoelectronic applica- tions which includes lasers [10] and detectors [11]. These QDs are usually embedded in a junction/mirror structure and have to be overgrown by GaAs or AlGaAs. The active (QD) and overgrown layers, however, have different growth temperature requirements: QDs growth temperature is low (approximately 470°C to 520°C) to prevent In desorption, but subsequent overlayer growth temperature is high (580°C and above), especially if the layer contains slow-diffusing species such as Al. The fundamental difference in growth temperature require- ments and its inevitability lead to extensive investigation of the properties of InGaAs QDs annealed in situ * Correspondence: songphol.k@chula.ac.th Semiconductor Device Research Laboratory (Nanotec Center of Excellence), Department of Electrical Engineering, Faculty of Engineering, Chulalongkorn University, Bangkok 10330, Thailand [12-15] and ex situ [15-21]. In terms of luminescence, it is well established that conventional InGaAs QDs that underwent annealing would: (1) exhibit a blueshift in their ground-state emission, (2) have narrower line- width, and, in some cases, (3) emit at an increased intensity due to interdiffusion and intermixing of cations and the reduction in non-radiative recombination cen- ters in the surrounding matrix [13-20]. Annealing stu- dies of unconventional InGaAs QDs such as those grown on InGaAs metamorphic or cross-hatch patterns (CHPs), however, have received much less attention, partly because of the perceived inferiority due to the presence of misfit dislocations (MDs) at the InGaAs/ GaAs heterointerface [22] and partly because the full explanation of the rich optical features of these types of QDs is still lacking. In this paper, InAs QDs on InGaAs CHPs, controlled InAs QDs, and controlled InGaAs CHPs are grown by molecular beam epitaxy (MBE) and subject to high-tem- perature in situ and low-temperature ex situ annealing. The optical properties of the samples - as-grown and annealed - as characterized by photoluminescence (PL) show that QDs on CHP have rich optical features and that high-temperature in situ annealing severely degrades them while low-temperature ex situ annealing improves them. The mechanisms responsible for the © 2011 Himwas 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.