Engineering MBE grown InAs/InGaAs Sub-monolayer Quantum Dot Infrared Photodetectors

 This thesis presents the experimental perusal of InAs/InxGa1–xAs quantum- dot infrared photodetectors (QDIPs) grown by molecular- beam epitaxy (MBE) technique in sub- monolayer (SML) configuration. Existing technologies on quantum- well (QW) and superlattices (SLs) have thrived well and demonstrated a varying degree of advancements and bottlenecks by a number of research communities. The main hurdles amid the
counterparts are achieving the required carrier confinement, higher absorption efficiency (photoconductive gain), high-temperature operability (lower dark current), and faster response to the normal- incidence operation. Intrinsic three- dimensional (3D) carrier confinement property of QDs within delta (δ)- function like density- of- states (DOS) serves for this purpose, providing a lot of controllability on the confined energy levels (dis
creteness). Moreover, the phonon bottleneck effect in QDs impedes the carrier- phonon scattering thereby facilitiating the carrier lifetime and intersubband (ISB) optical emission. The growth of self- assembled QDs occur as a result of lattice mismatch between the two epilayers whose magnitude alters the strain distribution in the heterostructure tremendously.
Epitaxial growth process of SML QDs involves the cyclic deposition of InAs QDs and InGaAs matrix on GaAs substrate. Compared to conventional Stranski- Krastanow (SK) mode, SML QDs owing to its smaller size distribution and absence of 2D wetting layer (WL) render the following: (i) higher QD density (∼ 5×1010cm–2), (ii) tunable geometry, (iii) lesser strain, (iv) size inhomogeneity, and (v) more in-plane absorption (s/p polarized ratio). Findings indicate that the QD size, shape and composition can be i
maneuvered by varying the growth parameters, affecting the position of discrete energy levels inside the QDs (confinement). A similar effect can be anticipated by the proper choice of capping layer (CL) material (a.k.a matrix) such as InGaAs, capable of reducing the lattice- induced misfit strain, enhancing QD dimensions for long photoluminescence (PL) emission at room temperature (300 K). Hence, the manifestation of SML QDs can be regarded as the potential choice for the enablement of QDIPs. Nevertheless, the QD size inhomogeneity and In- Ga intermixing are inescapable though, limiting the PL redshift below 1.3- 1.55 μm and for these reasons strict growth parameters has to be deployed. Detailed optimization studies were carried out to investigate the growth parameters on the performance of QDIPs along with fundamental studies to determine the optical and structural properties. Various approaches to lower the size dispersion, strain, defects and dark current were figured out which include: (i) controlling the extent of strain- field coupling and relaxation through InGaAs CL thickness, (ii) multiple stacking of QD and CL to improve the strain and photoconductive gain (g), (iii) deep carrier confinement inside QDs through ML coverage (& multiple stacks); furthermore In% content and defect density estimation in QDs, and (iv) subsiding In- Ga intermixing and defects via thermal annealing that drive towards higher detectivity and long-wave IR (LWIR) spectral response. Besides, the last part of thesis was focused on the use of GaAsN (ternary) and
quaternary cappings (Q-CL) involved with Sb/N/SbN that unlock the door to 1.3 μm and beyond PL emission at 300 K. This is the first-ever report discussing the inclusion of Q CL in SML dots for mitigating (balancing) the compressive strain and dark current (high temperature operability:uncooled detector) essential for the next- generation focal-plane arrays (FPAs) design, performed using Nextnano++ simulation tool. Another tech nique to control the dark current in lasers, via the use of wide bandgap AlxInyGa1–x–yAs
barriers were illustrated to achieve temperature- insensitive lasing operation at 300 K. Thus, all the above variability studies have helped in engineering the following: (i) QD dimensions and density, (ii) morphology preservation, (iii) control of strain and band edge potentials (carrier confinement), (iv) In- Ga intermixing, and (v) size homogenization, and (vi) optical gain through ternary and quaternary cappings. Finally, the observation of a strong bias dependent spectral photoresponse demonstrated the capability of post
growth spectral tunability around the mid (3- 5 μm)- long wavelength (8- 10 μm) IR atmospheric window useful for most commercial applications and next generation FPAs