Semiconductor quantum well lasers with nanoscale resonant periodic active layers

Abstract

Quantum dot semiconductor lasers have been the subject of significant investigation for many years. In part this is for potential applications based on desirable properties when compared with traditional quantum well-based lasers such as ultra-low threshold current density and reduced temperature sensitivity [1]. The most common technique for quantum dot growth is the self-assembly technique, in which dots form as a result of the large strain which exists between the dot material and substrate material. Unfortunately self-assembly results in dots with random positions and a broad size distribution. This leads to inhomogeneous broadening, decreased peak gain, and increased threshold current density. The lack of complete control over individual quantum dot properties inherent with the self-assembly technique has been the predominant obstacle to realizing their full potential for use in photonic devices.We have successfully applied selective area epitaxy using a patterned oxide mask to the growth of quantum dot laser [2] active regions. The growth of the dots within nanoscale patterns can highlight different growth rates on different crystal planes which leads to pyramidal growth. This is problematic for the growth of some structures, such as multiple active layers within a quantum dot. As an alternative, ordered nanopatterned quantum dots can be obtained by wet chemical etching [3] of previously grown multi-quantum well layers which have been incorporated into diode laser active regions. Unfortunatley, none of these approaches provide significant relief for the inhomogeneous broadening observed in lasers made with this kind of active region.We have recently developed [4] a novel type of nanostructure -called an inverted quantum dot or nanopore structure -which consists of an InGaAs quantum well which has been periodically (resonantly) perforated and then filled with the higher bandgap GaAs barrier material (Figs. 1, 2). This structure exhibits a similar quantized energy structure to that of quantum dots, with fundamental differences. Stronger coupling between unit cells results in the formation of allowed and forbidden energy bands instead of highly localized, fully discrete states. In addition, the periodicity of the lattice affects the nature of higher order states and can result in large increases in carrier lifetimes [5] in higher order states. A comparison of typical photoluminescence spectra for a nanopore and an otherwise identical quantum well structure is shown in Fig. 3.Here we describe in more detail the growth, processing and characteristics of semiconductor quantum well lasers with nanoscale resonant periodic active layer structures. We describe some of the interesting and potentially important effects arising from the introduction of nanoscale features (<50 nm) in the active medium. Methods for forming these structures, the fabrication process, and spectral results are described. 17 MA4.1 (Invited) 15:30 -16:00 978-1-4673-5060-0/$31.00 ©2013 IEEE