Optical properties of serpentine superlattices

Abstract

Recent developments on ultrafine submonolayer epitaxy on vicinal substrates1 by molecular beam epitaxy (MBE)2 or metalorganic chemical vapor deposition (MQCVD)3,4 made it possible to fabricate arrays of quantum wires named tilted superlattices (TSL) and serpentine superlattices (SSL).2,5 Figure 1 (a) and (b) show the cross sectional profile of a TSL and a SSL. A TSL or a SSL is directly grown on an off axis or vicinal GaAs substrate by alternating deposition of two materials of different compositions by either MBE or MOCVD. The end result is an array of quantum well wires (QWW) shown in figure 1 with a period T, which is determined by the tilt angle, α of the substrate. Such QWW are considered to be superior to bulk and quantum well structures in laser applications due to a larger temperature coefficient, larger relaxation frequency, and smaller threshold current. Furthermore, these structures have several significant advantages in comparison with the arrays of QWW fabricated with the fine line lithography techniques. The lateral dimensions are in the low nanometer range which is suitable for obtaining sufficient quantum size effect, they are obtained processing free, hence is free from processing damage. There is one practical difficulty in the fabrication of a TSL, however. To keep the growth interface vertical one has to know the exact growth rates and keep them constant throughout the growth. Any deviation from the correct value would tilt the growth interface. This difficulty can be circumvented if one deliberately varies the growth rate from less than the correct value to greater than the correct value.5 Then one grows a superlattice with a curved growth interface and somewhere within the grown layer one obtains a vertical interface, hence two dimensional confinement as shown in figure 1 (b). In this paper a study of the optical properties of the SSL for quantum wire laser applications is undertaken. The most important criteria for this application is a large subband spacing, larger than the thermal energy and broadening of the energy levels due to unavoidable imperfections, and narrow subband widths to create well localized and sharp gain peaks.