Speaker
Prof.
Eduard Hulicius
(Institute of Physics, AS CR, v.v.i.)
Description
Although the self-assembled InAs/GaAs quantum dots (QDs) are intensively studied during more than the last twenty years, the mass production of QD lasers started only in 2010 and the production of telecommunication QD lasers was announced even last year [1]. The main problem is the complexity of QD preparation process which includes not only the epitaxial layer growth but also Stranski-Krastanow formation, subsequent dissolution of QDs during capping, diversity of QD size and shape or the surfactant behavior of In atoms. The situation is even more complicated when QDs are covered by InGaAs or GaAsSb strain reducing layer (SRL) to shift the QD luminescence toward telecom wavelength. The GaAsSb SRL seems to be more suitable for MOVPE grown QD structures [2]. However, in these structures the surfacting behavior of In and Sb atoms is enhanced which complicates growth of multiple QD (MQD) structures for semiconductor laser application.
In this talk I will present our results obtained on QD and MQD structures with InAs/GaAs QDs covered by GaAsSb SRL. The growth of these structures was studied in situ by reflectance anisotropy spectroscopy (RAS), which offer direct observation of processes during the structure growth such as quantum dot formation, and dissolution or surfacting of In and Sb atoms. The surfacting of In atoms in structures where QDs are covered only by GaAs or by GaAsSb SRL will be compared. Enhanced In surfacting was observed for structures with GaAsSb SRL. Possible ways how to suppress surfacting of both types of atoms and how to prevent their transport on epitaxial surface to subsequent QD layer in MQD structures will be suggested. Different interruption and growth rates of the separation layer growth under varying temperature or composition gradient of GaAsSb SRL will be discussed with respect to the suppression of undesired surfacting of In and Sb atoms. The conclusions derived from RAS measurements will be supported by TEM, AFM and photoluminescence (PL) results.
Properties of InAs/GaAs QDs prepared by the MOVPE technology covered by GaAsSb SRL with extremely long emission wavelength at 1.8 μm will be presented. The prolongation of the emission wavelength was achieved by the introduction of GaAsSb SRL with Sb content of about 30% in the solid phase. The high Sb concentration in the SRL causes the preservation of QD size, which is about 15 nm wide at the base and 5 nm high. Increased QD size prolongs the PL wavelength. Furthermore, high content of antimony leads to a creation of type II heterostructure for which a red shift of the PL wavelength and decrease of the PL intensity is typical. Low PL intensity may complicate light emitting applications; however, fast separation of carriers in the type II structure is an advantage for detector or solar cell application, especially with the long wavelength. With respect to the perspective application of this structure, the photocurrent (PC) measurement was chosen as the complementary characterization method. A depression of PC for quantum well wavelength region (900-1200 nm) was observed for positive bias, while the PC from QDs (over 1200 nm) is not sensitive to the electric field orientation at all. Explanation of this unexpected phenomenon will be suggested.
[1] Semiconductor lasers: Fundamentals and Applications, edited by A Baranov, E Tournie, Elsevier 2013, ISBN 0857096400.
[2] A. Hospodková et al, J. Cryst. Growth 370 (2013) 303. doi: 10.1016/j.jcrysgro.2012.08.007
Are you currently a postgraduate student? (Yes/No)
No
Primary author
Prof.
Eduard Hulicius
(Institute of Physics, AS CR, v.v.i.)
