This thesis describes the fabrication and characterization of GaN homojunction visible-blind ultraviolet (UV) p-i-n avalanche photodiodes (APDs) grown by metalorganic chemical vapor deposition (MOCVD) on free-standing bulk GaN substrates. The objective of this research is to develop GaN UV p-i-n APDs with high linear-mode avalanche gains and the Geiger-mode operation for single photon detection. Low noise, high responsivity, and high detectivity are also required for fabricated APDs used as photodiodes in the photovoltaic mode (zero bias) and the photoconductive mode (low reverse bias). High material defect density and immature fabrication technology have hampered the development of III-nitride APDs in the past. In this thesis, sidewall leakage reduction methods have been developed to achieve significant improvement in dark current density, noise performance, and photo detection performance. A record linear-mode avalanche gain > 10⁵ for GaN APDs was demonstrated at λ = 360 nm. The first Geiger-mode deep UV (DUV) APD using front-illuminated homojunction p-i-n diode structure on a free-standing bulk GaN substrate was also measured with single photo detection efficiency (SPDE) of 1.0 % and dark count probability (DCP) of 0.03 at 265 nm. The performance of fabricated homojunction GaN p-i-n photodiodes was also evaluated in the photoconductive mode as well as the photovoltaic mode. For an 80-µm-diameter device biased at - 20 V (in the photoconductive mode) the dark current density is lower than 40 pA/cm² which is the lowest value achieved for any III-nitride photodiode so far. Its responsivity is 0.140 A/W at 360 nm with an ultraviolet-visible rejection ratio of 8×10³. The room-temperature noise equivalent power is 4.27×10 ⁻¹⁷ W-Hz-[superscript 0.5] and the detectivity D* is 1.66×10¹⁴ cm-Hz[superscript 0.5]-W ⁻¹ at - 20 V. The minimum detectable optical power is as low as 100 fW. They are among the best values reported for reverse-biased GaN p-i-n photodiodes to date.

Quantum dot infrared photodetectors (QDIPs) have emerged as a promising technology in the mid- and far-infrared (3-25 μm) for medical and environmental sensing that have a lot of advantages over current technologies based on Mercury Cadmium Telluride (MCT) and quantum well (QW) infrared photodetectors (QWIPs). In addition to the uniform and stable surface growth of III/V semiconductors suitable for large area focal plane applications and thermal imaging, the three dimension confinement in QDs allow sensitivity to normal incidence, high responsivity, low darkcurrent and high operating temperature. The growth, processing and characterizations of these detectors are costly and take a lot of time. So, developing theoretical models based on the physical operating principals will be so useful in characterizing and optimizing the device performance. Theoretical models based on non-equilibrium Green's functions have been developed to electrically and optically characterize different structures of QDIPs. The advantage of the model over the previous developed classical and semiclassical models is that it fairly describes quantum transport phenomenon playing a significant role in the performance of such nano-devices and considers the microscopic device structure including the shape and size of QDs, heterostructure device structure and doping density. The model calculates the density of states from which all possible energy transitions can be obtained and hence obtains the operating wavelengths for intersubband transitions. The responsivity due to intersubband transitions is calculated and the effect of having different sizes and different height-to-diameter ratio QDs can be obtained for optimization. The dark and photocurrent are calculated from the quantum transport equation provided by the model and their characteristics at different design parameter are studied. All the model results show good agreement with the available experimental results. The detectivity has been calculated from the dark and photocurrent characteristics at different design parameters. The results shows a trade off between the responsivity and detectivity and what determines the best performance is how much the rate of increase of the photocurrent and dark current is affected by changing the design parameters. Theoretical modeling developed in the thesis give good description to the QDIP different characteristics that will help in getting good estimation to their physical performance and hence allow for successful device design with optimized performance and creating new devices, thus saving both time and money. / Thesis / Doctor of Philosophy (PhD)

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Approved for public release; distribution in unlimited. / Traditionally, digital imaging systems rely on the use of dedicated photodetectors to capture specific wavelengths in the visible spectrum. These photodetectors, which are commonly made of silicon, are arranged as arrays to capture the red, green and blue wavelengths. The signal captured by the individual photodetectors must then be interpolated and integrated to obtain the closest color match and the finest possible resolution with reference to the actual object. The use of spatially separated detectors to sense primary colors reduces the resolution by a factor of three compared to black and white imaging. The FOVEON detector technology greatly improves the color and resolution of the image through its vertically arranged, triple well photodetector. This is achieved by exploiting the variation of absorption coefficient of silicon with wavelength in the visible spectrum. Hence, in a silicon detector, the shorter wavelength (e.g. blue) would be mainly absorbed at a shallow depth. A longer wavelength (e.g. red) would penetrate the material deeper than the shorter wavelengths and be primarily absorbed at a greater depth. By producing a layered silicon detector, all three primary colour wavelengths of red, green and blue can be captured simultaneously. This thesis aims to study the FOVEON camera's ability to image light from the near Infrared (NIR) to the Ultra-Violet (UV) range of the electromagnetic spectrum. The imaged obtained using a set of bandpass filters show that the camera has response both in the UV as well as NIR regions. / Major, Singapore Armed Forces

In this work, homojunction interfacial workfunction internal photoemission (HIWIP) detectors based on GaAs, and heterojunction interfacial workfunction internal photoemission (HEIWIP) detectors based mainly on the Gallium Arsenide/Aluminum Gallium Arsenide material system are presented. Design principles of HIWIP and HEIWIP detectors, such as free carrier absorption, photocarrier generation, photoemission, and responsivity, are discussed in detail. Results of p-type HIWIPs based on GaAs material are presented. Homojunction detectors based on p-type GaAs were found to limit their operating wavelength range. This is mainly due to band depletion arising through carrier transitions from the heavy/light hole bands to the split off band. Designing n-type GaAs HIWIP detectors is difficult as it is strenuous to control their workfunction. Heterojunction detectors based on Gallium Arsenide/Aluminum Gallium Arsenide material system will allow tuning their threshold wavelength by adjusting the alloy composition of the Aluminum Gallium Arsenide/Gallium Arsenide barrier, while keeping a fixed doping density in the emitter. The detectors covered in this work operate from 1 to 128 micron (300 to 2.3 THz). Enhancement of detector response using resonance cavity architecture is demonstrated. Threshold wavelength extension of HEIWIPs by varying the Al composition of the barrier was investigated. The threshold limit of approximately 3.3 THz (92 micron), due to a practical Al fraction limit of approximately 0.005, can be overcome by replacing GaAs emitters in Gallium Arsenide/Aluminum Gallium Arsenide HEIWIPs with Aluminum Gallium Arsenide/Gallium Arsenide emitters. As the initial step, terahertz absorption for 1 micron-thick Be-doped Aluminum Gallium Arsenide epilayers (with different Al fraction and doping density) grown on GaAs substrates was measured. The absorption probability of the epilayers was derived from these absorption measurements. Based on the terahertz absorption results, an Aluminum Gallium Arsenide/Gallium Arsenide HEIWIP detector was designed and the extension of threshold frequency (f0) to 2.3 THz was successfully demonstrated. In a different study, switching in Gallium Arsenide/Aluminum Gallium Arsenide heterostructures from a tunneling dominated low conductance branch to a thermal emission dominated high conductance branch was investigated. This bistability leads to neuron-like voltage pulses observed in some heterostructure devices. The bias field that initiates the switching was determined from an iterative method that uses feedback information, such as carrier drift velocity and electron temperature, from hot carrier transport. The bias voltage needed to switch the device was found to decrease with the increasing device temperature.

Highly responsive organic photodetectors allow a plethora of applications in fields like imaging, health, security monitoring, etc. Photomultiplication-type organic photodetectors (PM-OPDs) are a desirable option due to their internal amplification mechanism. However, for such devices, significant gain and low dark currents are often mutually excluded since large operation voltages often induce high shot noise. Here, a fully vacuum-processed PM-OPD is demonstrated using trap-assisted electron injection in BDP-OMe:C60 material system. By applying only −1 V, compared with the self-powered working condition, the responsivity is increased by one order of magnitude, resulting in an outstanding specific detectivity of ≈1013 Jones. Remarkably, the superior detectivity in the near-infrared region is stable and almost voltage-independent up to −10 V. Compared with two photovoltaic-type photodetectors, these PM-OPDs exhibit the great potential to be easily integrated with state-of-the-art readout electronics in terms of their high responsivity, fast response speed, and bias-independent specific detectivity. The employed vacuum fabrication process and the easy-to-adapt PM-OPD concept enable seamless upscaling of production, paving the way to a commercially relevant photodetector technology.