STUDY OF INTERSUBBAND TRANSITIONS IN InGaAs/GaAs QUANTUM WELLS

Abstract

Infrared photodetectors are used to detect and image thermal heat patterns that all object emits. The detectors are classified into different types based upon the electron transitions. Some of the applications are military night vision, fiber communications and landing of aircrafts at night. The main focus of this project is to design and optimise the multiquantum step well infrared photodetector using InGaAs/GaAs/AlGaAs compound semiconductor for multicolour detection in the range of 3-12 µm. The main advantages of using the step quantum well are to cover the wide bandwidth, that is, to cover multiple wavelengths in the infrared spectmm and the use of mature compound semiconductors. The another intention to design such a detector is that the GaAs based devices have gained industrial maturity and the new functions may be obtained during the studies. Recent research has found that normal incidence is possible in quantum well structure using InGaAs well. In addition to that, the small mass of the InGaAs quantum well structure makes the device faster compare to GaAs or AlGaAs based well structures. This project also involves the design work for a square well to understand the basic operation and characteristics. Generally, the performance of the device can be evaluated by calculating the oscillator strength, dark current and detectivity of the device etc.,. Recent research has shown that the detectivity of device is maximum when the energy level of the structure is just above the barrier height, that is, at the continuum state. In our project, we designed the step well structure such that the third energy state is just above the barrier height (~ 28 meV), which makes the detector with higher detectivity. The Transfer Matrix Technique using Analytical Dispersion Relation has been followed to calculate the energy levels in the device structure. The calculated energy levels are 0.125 eV in the well, 0.273 eV in the step and 0.364 eV in the continuum state. It has been found that the Transfer Matrix Technique using Tunnelling Probability Approach depends on the barrier width, that is, the energy levels in the structure are not consistent. This effect has been presented clearly with proper illustrations. The wavefunction of the each energy state has been constructed by solving the Schrodinger equation at each boundary and the wavefunctions are also normalised in order to find the oscillator strength. After engineering the width of the well and the step many times, it has been found that the third energy state is just above the barrier height when the well width of 30 ° A and the step width of 48 °A. The barriers are taken to be greater than 1500 A to reduce the dark current to some extent. Finally, the oscillator strength of the device has been calculated for different widths of both well and step and this effect has been illustrated clearly. Obviously, the structure has been optimised with the well-spread oscillator strength for both 1 -> 2 and 1 -> 3 electron transitions. The optimised oscillator strength for 1 -> 2 is 0.55 and 1 -> 3 is 0.44 and the corresponding wavelength ranges are 8.1 µm and 5.1 µm, respectively.