Optical characterisation of quantum well infrared photodetectors (QWIPs) for gas sensing applications
Although much work has been done on λ∼5-12 μm quantum well infrared photodetectors (QvWPs), the distinctive feature of this project is the use of strain compensated materials on InP substrates, AIAs (tensile) and InGaAs (compressive), to achieve shorter wavelengths and higher temperature operation. Stepped wells, high thin barriers, and strained layers have been used to achieve λ∼2-5 μm and also enhanced normal incidence absorption. These structures also give an additional degree of freedom to control the position of the excited states in the QWIPs conduction band. The strain-balancing allows the use of Indium (In) concentrations up to 84% and hence deep wells with a large band offset relative to the outer barrier (which predominantly controls the dark current). The conduction band offset (ΔEc) for these structures (with respect to the wide InAlAs barrier) is estimated to be ∼675 meV. In the course of this work, we have also been able to estimate the subband nonparabolicity (m* and α) from absorption spectra in highly doped quantum wells (QWs). In this thesis, the main results which I present are on a comparative study of the intersubband absorption in a series of double barrier QW (DBQW) structures grown on GaAs substrates (Chapter 5) and InP substrates (Chapter 6). The background and theory of QWs is given in Chapter 1. In Chapter 2, the experimental procedure is detailed, while the theoretical model to calculate the conduction band profile and energy levels and the comparison of this model with literature values are presented in Chapter 3. Early results are discussed in Chapter 4. Finally, a summary and future work are outlined in Chapter 7.