FABRICATION OF III-NITRIDE BASED ENERGY EFFICIENT ULTRAVIOLET PHOTODETECTOR

Abstract

III-Nitride semiconductor devices reserve their utmost place in the ubiquitous class of technology due to its applicability in a wide range of applications, which include solar cells, light-emitting diodes (LEDs), laser diodes (LDs), photodetectors (PDs), etc. Consequently, the extensive range of applications and projected gigantic market size generates continuous demand for more efficient, durable, and quality device products. The researchers have world-wide taken up this challenge of the never-ending requirement of technological improvisation and devoting their rigorous efforts to achieve better materials quality and device design. Among numerous domestic, commercial, and defence applications of III-nitride semiconductor-based devices, the detection of harmful ultraviolet (UV) radiation (<400 nm) found to be an extreme necessity. Universally, technologist finds potential needs to detect UV for plume detection, flame detection, chemical as well as biological analyses, secure space to space communications, environmental monitoring, astronomical research, and antimissile technology. Thus, the recommended ideal photodetector (PD) device should be adverse environment friendly with excellent sensitivity, high responsivity, high signal to noise ratio, and high spectral selectivity. Henceforth, as compared to conventional Sibased UV PD devices, III-nitride is proven to be a promising candidate due to their remarkable credentials such as wide-direct bandgap, high thermal conductivity, superb radiation hardness, which makes them operable in harsh environmental conditions. Besides, the detectors with Si-based technology severely loses signal to noise ratio as well as efficiency as a function of increasing dark current with elevated temperature. The solution to these problems has been provided by usage of the bulky cooling arrangement, expensive optical filters, and their operational instrumentation setups. Therefore, the movement of the semiconductor market from the Si-based to III-nitride based can be well understood. However, a key challenge is the presence of a large density of defects and dislocations due to lack of suitable lattice-matched substrates, and low resistance Ohmic contacts. Thus, particularly in the case of energy-efficient UV PD fabrication, the curiosity to grow good quality epitaxial III-nitride films has been mainly directed towards the study on GaN nanostructures (NSs). Thereby, the xiv growth of high surface to volume ratio based GaN NSs increases the incident photon absorption sites without sacrificing the device nano dimensionality. Besides, the possibility of integration of GaN with existing Si-based device technology leads us towards growth & fabrication on Si (111) substrates. Till date, numerous methodologies have been adopted to grow and optimize stress relaxed, higher aspect ratio, high crystalline quality as well as an excellent opto-electrical transport based GaN NSs by using plasma-assisted molecular beam epitaxy (PAMBE). Despite improvisation in crystalline quality, grown NSs as well as device geometry, recent advancement for enhancing the performance of PD devices emphasized on other exciting and potential approaches such as hybridization of III-nitride materials with other UV sensitive semiconductors, functionalization by novel metal nanoparticle’s nanoplasmonics and sensitization by highly conductive quantum dots. Thus, the thesis aims to explore and meticulously investigate the precise control epitaxial growth of high surface to volume ratio oriented GaN NSs on Si (111) substrate using the PAMBE system and their utilization as energy-efficient UV PDs. Besides, the effort made in this work excavates and execute the new prospects for emerging next-generation highly efficient photodetectors via. ZnO/GaN heterojunction hybridization, Au-NPs functionalization, and GQDs sensitization. The thesis consists of seven chapters, briefly described below: Chapter 1 gives a brief overview of the photodetection devices which are sensitive towards the UV range. Further, the significant contribution of III-nitrides in the fabrication of energy-efficient and durable UV PDs has been discussed. Moreover, the introduction of III-nitrides semiconductor’s inherent physical, chemical, optical properties with their bandgap engineering has been elaborated, which were found responsible for the fabrication of narrow to broadband PDs for harsh environment. Additionally, to enhance the performance of existing III-nitride UV PD technology, other potential approaches such as nitride’s surface functionalization, hybridization and sensitization have also been suggested. Chapter 2 describes the detailed mechanism of the technique used for the growth of III-nitride semiconductors along with various in-situ as well as ex-situ analytical tools xv and methods utilized for probing the structural, morphological, optical and electrical properties of the grown structures. This chapter illustrates a brief description of the steps involved in devices fabrication process and evaluation of their performance parameters as well. Chapter 3 elucidates the growth of nanoisland shaped, lower stress, and strain facilitated GaN-NS on Si (111) substrate via PAMBE and fabrication of GaN-NS based UV photodetection device even with NS’s tiny dimensionality. The threedimension (3D) growth of GaN-NS in real-time was observed by the in-situ RHEED technique, which displays transformation from streaky to the spotty pattern. A microRaman technique has been employed to elaborate on NS’s crystallinity and lower stress value, which is found to be in good agreement with related lower strain as evaluated by HR-XRD spectra. An observed sharp near band edge emission at 363.2 nm by room temperature photoluminescence measurement signifies the presence of GaN. After that, the as-grown ultra-thin GaN NSs were utilized to fabricate energyefficient self-powered UV PD, wherein non-homogeneous GaN nanoislands were perceived on the Si surface with a thickness of ~30 nm and an average distribution density of 2 ×1010 cm -2 . Despite nano dimensional GaN NSs film, the capability of UV detection of fabricated PD added novelty to this work, where performance parameters such as photosensitivity (~102 ), detectivity (~109 Jones), responsivity (1.76 mA/W) and NEP: noise equivalent power (3.5 × 10-11 WHz-1/2) under selfpowered mode were observed. The transient photo-response measurement revealed a rapid rise and decay time constants of ~18 ms and ~27 ms, respectively. Under varying optical power (1 mW to 13 mW), the GaN PD displayed significant enhancement in photocurrent with increasing optical power. The performance of the fabricated detector has also been analyzed under the photoconductive mode of operation, where it revealed significantly enhanced responsivity (23 fold) and detectivity (~1000 fold). Such nanostructured self-powered GaN-based UV PD paves the way towards the fabrication of energy-efficient optoelectronic devices. Chapter 4 presented the nanoplasmonic impact of chemically synthesized Au nanoparticles (NPs) on the performance of GaN NS based UV PD is analyzed. The devices with uniformly distributed Au NPs on GaN NSs (nanoislands) prominently xvi respond toward UV illumination (325 nm) in both self-powered as well as photoconductive modes of operation and have shown fast and stable time-correlated response with significant enhancement in the performance parameters. A comprehensive analysis of the device design, laser power, and bias-dependent responsivity and response time is presented. The fabricated Au NP/GaN nanoislandbased device yields the highest responsivity of ∼ 216 mA/W, detectivity of ∼ 109 jones, reduced NEP of ∼ 1.8 × 10−12 W Hz−1/2, External quantum efficiency (EQE) of ∼ 82%, and fast response/recovery time of ∼ 40 ms. Moreover, the study also illustrates the mechanism where light interacts with the chemically synthesized NPs guided by the surface plasmon to enhance the device performance effectively. Further, the decoration of low dimensional Au NPs on GaN NSs acts as a detection enhancer with fast recovery time. It paves the way toward the realization of energyefficient optoelectronic device applications. Chapter 5 elaborates on the fabrication of GaN-Nanotowers (GaN-NTs) based on highly efficient UV PD with distinct AlN buffer layer thickness and NTs lengths. The unique nanotower (tapered ended) morphology of an epitaxially grown hexagonal stacked nanocolumnar structure with truncated strain and high surface-to-volume ratio contributes to the significantly enhanced performance of the fabricated detector towards UV illumination. The fabricated GaN-NT UV PDs displays very low dark current (~12nA) & very high ILight /IDark ratio (>104 ) along with the highest responsivity of 485 A/W. The device exhibits very high EQE ~105 %, fast time-correlated transient response (~430 µs), very low NEP (~10-13 WHz-1/2), and excellent UV/Vis rejection ratio. Therefore, the utilization of such GaN-NT structures can be advantageous towards the fabrication of energy-efficient ultraviolet photodetector. Chapter 6 introduce the concept of performance augmentation of existing III-nitride (GaN) UV PDs technology employing hybridization using UV sensitive & compatible semiconductors (ZnO), surface functionalization by novel metal Gold (Au) NPs and sensitization by highly conductive graphene quantum dots (GQDs). Thereby, as grown (as discussed in chapter 5) vertically aligned longer GaN NTs with higher AlN thickness oriented highly responsive UV demonstrated fast response with excellent stability when functionalized with Au Nanoparticles, GQDs, and ZnO Nanorods. xvii Initially, GaN-NTs structure is hybridized by ZnO Nanorods (ZnO NRs), wherein, to capture maximum incident photons, a strategically formulated model of NSs over NSs has been proposed as an enhanced device active surface area. Consequently, the fabricated GaN-NTs based device with ZnO NRs hybridization, GQDs sensitization, and Au NPs functionalization significantly accelerate the performance of the device where a prominent three order reduction in dark current is observed along with gigantic R, lower NEP and enormously enhanced EQE of 7042 A/W, 1.84×10-14 W.Hz-1/2 and 2.7×106 % respectively. Mechanism elaborating the enhanced device performance with an appropriate energy band diagram has been discussed in detail. The fabricated highly sensitive device can lead a path towards future optoelectronic applications of integrated III-Nitride technology. Chapter 7 enlightens the major conclusions derived from the thesis work and the scope of future work.