Room‐Temperature Interfacial Reactive‐Template Epitaxy of NiO/4H‐SiC for Dual‐Mode UV Photodetection

Nanoscale UV photodetectors based on ZnO and alloyed ZnO

This thesis presents a performance investigation of newly-developed ultra-shallow junction photodiodes (PureB-diodes) for ultraviolet (UV) radiation detection. The photodiodes are fabricated by pure boron chemical vapor deposition (PureB CVD) technology, which can provide nanometer-thin boron capping layer and nanometer-deep defect-free p+n junction. Due to its unique features, the PureB-diode technology is considered to be a promising solution for developing advanced UV photodetectors. Chapter 1 presents the motivation and the objectives of the research work. In view of one of the most demanding UV-related industrial applications: deep-UV (DUV) and extreme-UV (EUV) lithography; the major challenges for designing high-performance UV photodetectors are defined. In Chapter 2, the results of a comparative study of the existing ultraviolet (UV) photodetectors are presented, based on the performance requirements defined in Chapter 1. The subsequent analyses are restricted to silicon-based devices, due to their superior characteristics: high sensitivity, simplicity, low costs, and IC-compatibility. The optical performance of state-of-the-art Si-based UV photodiode is reviewed, and their advantages and drawbacks are summarized in comparison with a proposed ideal structure of Si-based UV photodiode. Owing to the nanometer-deep junction and the nanometer-thin ?-boron capping layer, PureB-diodes have an ultra-thin front absorption window which can greatly reduce the unwanted photon loss, consequently achieve a near-theoretical responsivity at 13.5- nm wavelength in the EUV spectral range. This unique feature indicates that PureB-diodes can potentially provide better sensitivity than any other commercially available Si-based photodetector over the full UV spectral range. Therefore, photodiodes fabricated by the PureB CVD technology was chosen as the main candidate to design high-sensitivity, high-stability photodiodes for UV radiation detection. In Chapter 3, for proper interpretation and analysis of the reported optical experimental data in this thesis, the fabrication process and the device structure of PureB-diodes are introduced. Due to the limited attenuation length (penetration depth) of DUV/VUV photons in silicon, the surface conditions of Si-based photodiodes play an important role in their optical performance. The surface structure and properties of PureB-diodes are discussed in detail. In Chapter 4, a superior DUV/VUV optical performance of PureB-diodes has been presented. A doping-profile-induced surface charge-collection effect is validated as an additional important factor for achieving the high DUV/VUV sensitivity, especially when the junction depth is significantly increased by extra thermal treatment. It has been demonstrated that, along with the ultra-shallow junction, an optimized surface-layer doping profile plus a damage-free doping technique could be an alternative way to reduce the equivalent thickness of the pre-absorption layer on top of the effective charge-collection zone of the photodiode. The observed doping-profile-enhanced effect of surface charge-collection offers significant opportunities for designing a high-sensitivity photodiode for the case that an “ultra-shallow junction” is difficult to apply; for instance, when a low surface sheet resistance is the primary goal. At the same time, this doping-profile-enhanced effect of surface charge-collection also enhances the compatibility of PureB CVD technology with standard Si processing, and becomes one of the inherent advantages of these silicon-based photodiodes when it comes to fabricating fully-integrated UV smart-sensor systems. In Chapter 5, two potential negative effects related to a high surface sheet resistance of PureB-diodes have been studied: unpredictable response times and high series resistance. With respect to the response time, it has been demonstrated experimentally that, when PureB-diodes have a high surface sheet resistance, as well as a large active area (in the order of mm2), their response time is only slightly affected by variations of the size and location of the incident light spot. In most of the practical applications, this influence is negligible. Yet, for achieving a shorter response time, it makes sense to reduce the series resistance. This can be achieved, for instance, by improving the structure of the PureB-diodes either by creating an extended anode with the help of an aluminum grid, or by extra thermal annealing. Although PureB-diodes with an aluminum grid have a higher responsivity for the same series resistance value, the local non-uniformity of the sensitivity limits the applications of this method. Alternatively, a low series resistance can also be achieved with a deeper junction of the PureB-diodes. Due to the high efficiency of collecting charge at the surface, PureB-diodes with a deeper junction still maintain a good VUV/EUV responsivity and an excellent uniformity. In Chapter 6, the performance stability of PureB-diodes is evaluated. The experimental results prove that the p+n junction formed by PureB CVD technology is stable under DUV/VUV and EUV radiation: after prolonged exposures, no performance degradation caused by the damage to the junction itself has been observed. Nevertheless, a surface oxidation, which can be positively charged by the photoelectron emission effect under UV radiation, was found to be the main reason for the small responsivity degradation that was measured after receiving some VUV/DUV and EUV radiation. This charging-induced responsivity loss can be avoided by forming a thicker boron layer or adding a conductive capping layer. The former can eliminate the harmful surface oxidation on PureB-diodes; the latter is helpful in minimizing the charge-accumulation on the diode surface. Moreover, PureB-diodes have also demonstrated good electrical performance stability under intensive EUV radiation when an extra nitride passivation layer is applied to protect the Si-SiO2 interface along the diode perimeter. The still observed small dark current increase can be further scaled down by reducing the reverse bias voltage to the mV range in future applications. In Chapter 7, the durability of PureB-diodes in detrimental environments (under aggressive cleaning) is evaluated. Experimental results prove that PureB-diodes have excellent robustness to hydrogen radical (H*) cleaning treatment: the nanometer-thin ?-boron layer and the underlying boron-doped ultra-shallow p+n junction do not degrade during cleaning with aggressive H*. In particular, no responsivity drop has been observed after continuous H* exposure. However, H*-induced defects created at the Si-SiO2 interface with only SiO2 surface passivation outside the photodiode sensitive area, nearby the diode perimeter, increase the dark current at certain biasing conditions. This effect is almost eliminated by either a low-temperature bake (~ 200 oC) or by including a thin Si3N4 layer in the passivation layer stack to prevent the hydrogen from reaching the Si-SiO2 interface. In Chapter 8, a short overview of the UV photodiode design strategies is offered and a summary of the major contributions of this research is presented. Next, the ongoing work and potential extensions are shortly discussed.

Self-powered photoelectrochemical (PEC) photodetectors hold great promise for underwater optical applications, yet suffer from sluggish carrier dynamics and limited stability. Herein, high-performance, self-powered, and stable PEC ultraviolet (UV) photodetectors were fabricated using metal-organic-framework derived ZnO nanocages (NCs). These topography-engineered ZnO NCs synchronously enhance UV absorption, facilitate photogeneration carrier separation, and promote charge transfer at the ZnO/electrolyte interface, thus optimizing the overall photoresponse performance. The ZnO NCs-based PEC device achieves an ultrahigh responsivity of 300.6 mA/W under 365 nm UV light irradiation, a fast response time of 10/20 ms, outstanding spectra selectivity (UV/visible rejection ratio of 2000), and excellent cycling stability (10,000 cycles), which is one of the best reported PEC UV photodetectors. Furthermore, the self-powered ZnO-based PEC PDs have good underwater optical imaging capability. This work provides a new idea for designing high-performance UV photodetectors for application in underwater environments.

High performance UV photodetector based on MoS2 layers grown by pulsed laser deposition technique

We report on a new approach to quickly synthesize high-quality single crystalline wide band gap silicon carbide (SiC) films for development of high-performance deep ultraviolet (UV) photodetectors. The fabricated SiC based UV photodetectors exhibited high response while maintaining cost-effectiveness and size miniaturization. Focus of the experiments was on studies of electrical and electronic properties, as well as responsivity, response and recovery times, and repeatability of the deep UV photodetectors. Raman scattering spectroscopy and scanning electron microscope (SEM) were used to characterize the SiC materials. Analyses of the SEM data indicated that highly flat SiC thin films have been obtained. Based on the synthesized SiC, deep UV detectors are designed, fabricated, and tested with various UV wavelength lights at different radiation intensities. Temperature effect and bias effect on the photocurrent strength and signal-to-noise ratio, humidity effect on the response time and recovery time of the fabricated detectors have been carefully characterized and discussed. The detectors appear to have a very stable baseline and repeatability. The obtained responsivity is more than 40% higher compared to commercial detectors. The good performance of the photodetectors at operating temperature up to 300 °C remains nearly unchanged.

In this paper the temperature dependence of responsivity and response time for 6H-SiC ultraviolet (UV) photodetector is simulated based on numerical model in the range from 300 K to 900 K. The simulation results show that the responsivity and the response time of device are less sensitive to temperature and this kind of UV photodetector has excellent temperature stability. Also the effects of device structure and bias voltage on the responsivity and the response time are presented. The thicker the drift region is, the higher the responsivity and the longer the response time are. So the thickness of drift region has to be carefully designed to make trade-off between responsivity and response time.

A photoelectrochemical type self-powered ultraviolet photodetector based on GaN porous films

Ultrafast, high‐sensitivity, visible‐blind ultraviolet (UV) photodetectors are crucial for practical applications, including: optical communication, environmental ozone hole monitoring, and combustion and chemical detection. However, commercial visible‐blind UV photodetectors based on traditional inorganic semiconductor materials involve expensive production processes, while the currently solution‐processed detectors still suffer from issues of low sensitivity or slow response speed. Herein, high‐performance visible‐blind UV photodetectors with simultaneously ultrafast response speed and high detectivity are achieved from solution‐processed micrometer‐thick methylammonium lead trichloride perovskite thin single crystals, grown in high quality with smooth surfaces by judiciously designing the substrate's wettability. The as‐fabricated UV photodetectors exhibit a low noise of 6.5 fA Hz−1/2, a low detection limit of 8.5 pW cm−2, and high specific detectivity of ≈6 × 1012 Jones. A fast response time of 15 ns and a large bandwidth of 25 MHz are obtained when decreasing the crystal thickness to 1 µm. The nanosecond response speed is several orders of magnitude faster than the best ever reported solution‐processed visible‐blind UV photodetectors and is comparable to reported UV photodetectors based on traditional (high‐temperature processed) inorganic semiconductors. The high performance combined with low‐cost fabrication makes these visible‐blind UV photodetectors highly attractive for applications in optical communication and ultrafast detection.

Enhanced ultraviolet (UV) photodetectors (PDs) with high responsivity comparable to that of visible and infrared photodetectors are needed for commercial applications. n-Type ZnO nanotubes (NTs) with high-quality optical, structural, and electrical properties on a p-type Si(100) substrate are successfully fabricated by pulsed laser deposition (PLD) to produce a UV PD with high responsivity, for the first time. We measure the current-voltage characteristics of the device under dark and illuminated conditions and demonstrated the high stability and responsivity (that reaches ∼101.2 A W-1) of the fabricated UV PD. Time-resolved spectroscopy is employed to identify exciton confinement, indicating that the high PD performance is due to optical confinement, the high surface-to-volume ratio, the high structural quality of the NTs, and the high photoinduced carrier density. The superior detectivity and responsivity of our NT-based PD clearly demonstrate that fabrication of high-performance UV detection devices for commercial applications is possible.

We have investigated the conductivity and photoconductivity response of undoped and Li-doped ZnGa2O4 epitaxial films grown using pulsed-laser deposition. A significant enhancement of the ultraviolet (UV) photoresponse is observed with Li doping that also correlates with an enhanced luminescent intensity. The wavelength dependence observed for creation of free carriers under UV excitation suggests that the transition is either band-to-band or involves a defect level near the band edge. Moderate n-type dark conductivity is observed for undoped films processed under reducing conditions. With Li doping, dark conductivity is reduced, suggesting that lithium ions in the zinc gallate lattice serve as deep acceptors. In addition, Li doping effectively eliminates persistent photoconductivity that is commonly observed in undoped films, suggesting the possible use of Li-doped ZnGa2O4 as a visible wavelength blind UV photodetector.

Morphology changes of In-doped ZnO nanosheets via ZnCl2 and InCl3 vapor formation during thermal dissipation annealing process and improved UV photoresponse properties

Temperature dependence of the pyro-phototronic effect in self-powered p-Si/n-ZnO nanowires heterojuncted ultraviolet sensors

BeCaZnO quaternary alloy: thin films and ultraviolet photodetectors

High performance self-powered ultraviolet photodetectors based on electrospun gallium nitride nanowires

A solution-processed ZnO quantum dots ultraviolet photodetector with high performance driven by low operating voltage