InAlAs/InGaAs strain compensated quantum cascade structures for short wavelength infra-red emission

The inherently shorter transition times for the emission process of quantum cascade lasers, as compared to conventional band-to-band semiconductor diode devices (picoseconds compared to nanoseconds), has been utilised here to examine the feasibility of short infra-red wavelength devices working near the telecom band of 1.55 μm. To achieve this wavelength in intersubband structures requires extremely large conduction band discontinuities. In this paper a material system, using highly strain compensated InAlAs/InGaAs quantum wells (QWs) and barriers grown by MBE on InP substrates, is discussed for realising these discontinuities to obtain a quantum cascade device (initially with potential emission at 2.7 μm with the aim of then developing 1.55 μm). Preliminary characterisation of this material system, using Quantum Well Infrared Photodetector (QWIP) structures, is also presented, showing agreement with design models and that quality growth can be achieved for In_0.8Ga_0.2As quantum wells/ In _0.35Al_0.65As barriers grown by MBE. Comparative studies of lattice matched, compressively strained and strain compensated structures are also presented demonstrating excellent structural and optical behaviour of the strain compensated structure