Sheng-Min Yu

TiO 2 -Based Metal–Semiconductor–Metal Ultraviolet Photodetectors Deposited by Ultrasonic Spray Pyrolysis Technique

This paper uses nonvacuum ultrasonic spray pyrolysis deposition method to grow TiO₂ for ultraviolet (UV) detection. The analyses of the materials like X-ray photoelectron spectroscopy, X-ray diffraction, and photoluminescence were investigated. The 600 °C annealing temperature is the optimum condition to obtain the anatase TiO₂. The metal-semiconductor- metal (MSM) photodetectors (PDs) with 3-, 5-, 7-, 10-μm finger spacing were fabricated and the performance was investigated. The PD with 10-μm finger spacing has the lowest dark current of 2.92 × 10^-11 A and the highest UV-to-visible rejection ratio (RUV/VIS) of 2.1 × 10⁵ at 5 V. The PD with 5-μm finger spacing has the lowest noise equivalent power of 2.57 × 10^-9 W and the highest detectivity (D*) of 5.46 × 108 cmHz^0.5W^-1. The rising time and the falling time of the PD are 5 and 12 s. In addition, the TiO₂-based MSM PD in this paper operated normally at 450 K; however, the performance is slightly degraded. The mechanisms causing degradation at high temperature were investigated.

Characterization of TiO 2 -Based MISIM Ultraviolet Photodetectors by Ultrasonic Spray Pyrolysis

In this letter, non-vacuum ultrasonic spray pyrolysis deposition was used to grow Al₂O₃/TiO₂ thin film as the metal-insulator-semiconductor-insulator-metal ultraviolet photodetector (MISIM UV PD). The anatase TiO₂ with 400 °C annealing was used as the active layer of the UV PD. X-ray diffraction and Raman spectra were used to characterize the crystal phase of the TiO₂. The Al₂O₃/TiO₂ MISIM UV PD has lower dark current (~57.2 pA), higher photoresponse (~24.48 A/W), and higher detectivity (~8.25 × 10¹³ Jones), all of which were better than the TiO₂ MSM UV PD. Similar device performance was obtained from the SiO₂/TiO₂ MISIM UV PD. The external quantum efficiency of the Al₂O₃/TiO₂ and SiO₂/TiO₂ MISIM UV PDs was 8204% and 6840%. Such high external quantum efficiency results from the internal photoconductive gain and the interfacial trap controlled charge injection.

An Experimental Study of Worst Case Scenarios of Nitric Acid Decomposition in a Toluene Nitration Process

Several worst case scenarios were carried out for safety evaluation of an existing nitration process. Heterogeneous nitric acid decomposition is observed to be the worst credible scenario in the toluene nitration process. Fire or explosion may subsequently develop due to temperature or pressure runaway from the undesired reaction. Gassy nitric acid decomposition can easily be triggered by process derivation from the desired reaction. In this study, a heat-flux calorimeter (C80–II) and an adiabatic calorimeter (VSP2) were used to measure the thermokinetics and determine the runaway characteristics for assessing worst credible scenarios. The heat of reaction of toluene mononitration is measured to be 190.9±6 3.6/(kJ mol −1 ). The calculated Arrhenius parameters are Ea (kJ mol −1 ) = 30.9±2.1 and A (mol −1.5 l 1.5 s −1 ) = (1.38 ± 0.07) × 10 −2 . The ‘worst case’ scenarios were chosen at the following conditions: loss of cooling, stirrer failure, batch-wise charging, and decomposition of reactants. The experimentally determined worst case was found to be a combination of a loss of cooling with batch-wise charging coupled with decomposition of nitric acid with full mixing. Adiabatic temperature rise, self-heat rate, final pressure and non-condensable gas quantity in the VSP2 experiment were used to assess thermal explosion in a nitration reactor. Unstable and critical onset temperature to runaway is determined as about 140°C for the process in this study.

Ultrafast Al(Si)-induced crystallisation process at low temperature

Aluminium-induced crystallisation process can be accelerated by a factor of about 50 by the doping of Si atoms into the initial Al layer. This process is known as Si-AIC (aluminium-induced crystallisation). The grain size and crystallographic orientation of the grown poly-Si thin film produced are modified due to the fact that the presence of excess Si in the initial Al layer alters the nucleation and growth behaviour of the Si grains as compared with the traditional AIC process. In the present study, the nucleation mechanism and growth rate of Si grains for Si-AIC are analysed and quantitatively compared with those for AIC using time-series transmission electron microscopy/energy-dispersive X-ray spectroscopy (TEM/EDS) images. It is found that the activation energy for grain growth was significantly reduced in the Si-AIC process, by 0.7 eV compared with the AIC process.

Epitaxial growth of heavily boron-doped Si by Al(B)-induced crystallisation at low temperature for back surface field manufacturing

P-type polycrystalline Si film on a foreign substrate can be fabricated at temperatures lower than 773 K by an aluminium-induced crystallisation process. However, the ultimate carrier concentration of the Si film is limited to approximately 3 × 1018 cm−3 because of the low solid solubility of Al in Si at temperatures below 773 K. In this study, a process called B-AIC is developed in which boron is co-doped with Al to increase the carrier concentration in Si films to ∼1019 cm−3 at temperatures as low as 673 K. The carrier concentration can be tuned by the initial thickness of a-Si layer in the B-AIC process. Beside the fabrication of polycrystalline Si film on glass, the epitaxial growth of this heavily doped p++-Si film can also be realized on a single crystalline Si wafer via a solid phase epitaxy mechanism.

Orientation selected epitaxy for grain enlargement of AIC poly-Si seed layers

A novel method for producing a highly-(100) orientated, large grain of poly-Si seed layer on glass by multi-round aluminum-induced crystallization (AIC) is developed. A flat 200nm poly-Si layer was first fabricated by regular AIC process to be a based layer. The second round AIC process was carried out immediately on the based layer to epitaxially grow up to a 400nm poly-Si layer by solid phase epitaxy (SPE) mechanism. The structure of enlarged epitaxial grain was examined by transmission electron microscopy (TEM), the orientation map as well as histogram of grain size from large area were performed by electron backscatter diffraction (EBSD), and the crystallinity of multi-round AIC was verified by Raman spectrometry. The lateral growth and grain suppression can be clearly observed in cross-sectional TEM analysis. The average grain size can be determined from analysis of histogram. The speed of epitaxial growth is strongly influenced by the orientation of the growth plane. We utilize this characteristic to promote the {100} proportion and reduce others to achieve a highly oriented seed-layer for followed thickening step. The population of {100} crystallographic plane is obtained statistically from orientation map and a pole figure analysis. The mechanism of the multi-round AIC will be discussed in detail in the conference.

Investigation of TiO 2 -Based MISIM Ultraviolet Photodetectors With Different Insulator Layer Thickness by Ultrasonic Spray Pyrolysis Deposition

This work uses ultrasonic spray pyrolysis deposition to grow titanium dioxide and aluminum oxide (Al₂O₃) films, respectively, as an active layer and an insulator layer of the metal-insulator-semiconductor-insulator-metal (MISIM) photodetector (PD). 10-, 20-, and 30-nm-thick Al₂O₃ films were deposited and the I-V characteristics in the dark and under illumination were measured and investigated. The dark current was suppressed to 11.6 pA for the MISIM PD with the 30-nm Al₂O₃. In addition, the carrier transportation mechanisms of the dark current are analyzed. The photoresponsivity of the MISIM PD with the 10-nm Al₂O₃ was 8.22 A/W (at 10 V), which is much higher than 0.84 A/W of the metal-semiconductor-metal PD. The noise equivalent power and detectivity of the MISIM PD with the 10-nm Al₂O₃ were 2.28 × 10^-10 W and 2.4 × 10⁹ Jones. The PDs showed a slight degradation when the ambient temperature was up to 450 K.

Spray pyrolysis coating Al 2 O 3 :Cl/TiO 2 bilayer for PERC

In this study we use low-cost spray pyrolysis method to produce a Al₂O₃:Cl/TiO₂ bilayer as rear passivation layer to replace the common known Al₂O₃/SiNx laminate in PERC structure silicon solar cells. The TiO₂ and TiO₂/Al₂O₃ mixed thin film acts a competent barrier from molten aluminum paste during the co-firing process and high refractive index of those thin films demonstrated excellent internal back reflectance (IBR). Very high negative fixed charge concentration is up to 5 × 10¹² cm^-2 in TiO₂ thin film being observed and the interface trap density (D_it) can be controlled via Ti/Al ratio and ultra-thin Al₂O₃:Cl interfacial layer to maintain good passivation ability and the lowest result of D_it is 8 × 10¹⁰ cm^-2eV^-1 with a 6nm Al₂O₃:Cl interfacial layer. Through the use of Al₂O₃:Cl interfacial layer, the carrier lifetime of bulk wafer increase from 60μs to 120μs. All of the results show that this concept is very promising for high efficiency silicon solar cell application.