Fabrication and Optical Characterization of Photodetector Based on Graphene/Semiconductor Heterojunction

Abstract

Recently, as the rising of the Internet of Things (IoT) era, the various sensors have been required. Among the IoT sensors, the infrared photodetectors are essential which helps to exchange information between humans and things. Infrared sensors have been applied in private as well as industry and military fields as bio-sensing, light detection, ranging (Lidar) system, motion detector, image sensor,and missile detection system. Besides, the portability of these devices has recently become important. Therefore, small size and high performance photodetectors are required. However, it is hard to make a small device with low price and high-performance infrared photodetector via conventional semiconductors; such as silicon (Si), germanium (Ge), and Indium gallium arsenide (InGaAs). In order to make a high-performance photodetector with small size and reasonable prices, two-dimensional materials have been intensively studied. Among the two-dimensional materials, graphene has attracted many researchers due to its unique properties such as broadband absorption, scalability, carrier multiplication, and ultrafast operation speed. In addiation, graphene photodetector has a lot of advantages in scalable size, low price, and easy fabrication process. However, when graphene is Ph.D/MS 20152077 ii utilized as the active channel of the photodetector, It shows limited performance by a high dark current, instability, and low photoresponsivity. Therefore, various approaches have been studied to break through the challenges and improve the performance of graphene photodetector. In this dissertation, the manner in which enhance the performance of photodetector will be discussed, especially using graphene photodetector and graphene/silicon junction photodetector. In chapter 1, the various types of graphene-based photodetector and graphene/semiconductor heterojunction photodetector will be introduced and discussed extensively. In chapter 2, asymmetric metal contact graphene photodetector will be discussed. In order to improve the performance of the graphene photodetector it was operated under 0V condition. Therefore, source and drain metal were chosen differently to make the potential gradient in the channel region to break the mirror symmetry of the band diagram in an equilibrium state. As a result, we demonstrated the graphene photodetector with asymmetric metal contact structure, which can be operated under 0V condition, and the dark current level was 70nA, and the photoresponsivity was 51mA/W. In chapter 3, decoration of abosrption materirals on the graphene photodetector will be introduced to improve the photoconductivity in broad wavelength range. The Bi2Te3 nanowires were decorated on the graphene surface by the drop-casting method. As a result, the photoconductive gain was improved by photoabsorption enhancement. The photoconductive gain was achieved upto 104, and the photoresponsivity under 2200nm wavelength was improved by nearly 200% compared to the only graphene channel photodetector. iii In chapter 4, the chemical doping applyied on the graphene/silicon photodetector was investigated. In graphene/silicon junction diode, the dark current is initially lower than the graphene photodetector. Nevertheless, in the case of graphene/p-silicon junction diode, the dark current is large compared to the graphene/n-silicon photodetector due to the lower Schottky barrier height. It needs to reduce the dark current in the graphene/p-silicon. In order to reduce the dark current of graphene/p-silicon photodetector, the chemical doping method was applied to the graphene surface to make n-type graphene and to modulate the barrier height between the graphene/p-silicon junction. We demonstrated the low dark current via chemical doping compared to the pristine graphene/p-silicon junction photodetector, and the photoresponsivity was 0.3 A/W and detectivity was 5.9 × 1010 Jones under 850 nm illumination. In chapter 5, we provide the method to understand the internal gain of the device. The internal photoemission(IPE) spectroscopy was investigated on the ZrO2 thin film. The IPE measurement was applied to the metal-insulator-metal (MIM) device. The IPE spectroscopy result was analyzed. The information about the origin of internal gain from the ZrO2 film was investigated. Also, the origin of internal gain, the leakage current, and AC characterization was combined to figure out the defect energy level change obtained by IPE comparing before and after the RTA process.