Back Illumination Mode Research Articles

A comparative study of reduced graphene oxide synthesized through different techniques for UV-C radiation detection application

In this work, reduced Graphene oxide (rGO) is synthesized through three different chemical routes of synthesis and they are used for the fabrication of UV radiation detectors working in the 100–280 nm range. The fabricated device is a Silicon based radiation detectors with these different synthesized samples of rGO as the sensing material. The photon detector has an architecture of a field effect transistor working in the back-gate mode of operation. The device response is studied in presence of UV rays for all the three types of devices in the back-illumination mode of operation. A comparative analysis of the performance of these three devices is done to analyze the effect of rate of reduction on the device performance and its correlation with the properties of rGO. It was observed that higher the rate of reduction, better is the performance of the device. The effect can be correlated with the improved electrical conductivity and reduced bandgap of rGO.

Highly ordered combined structure of anodic TiO2 nanotubes and TiO2 nanoparticles prepared by a novel route for dye-sensitized solar cells

Combined structure of anodic TiO2 nanotubes and TiO2 nanoparticles (TiNTs-TiNPs) has been synthesized by a facile combination of hydrothermal and chemical vapor deposition methods. Ordered TiO2 nanotubes with smooth walls were fabricated by two step anodization method in ethylene glycol containing NH4F at 50 V. This nanotubular array after annealing at 450 °C was subjected to the hydrothermally produced gaseous environment in an autoclave with diluted TiCl4 solution at its bottom. Vapors of TiCl4 were allowed to react chemically with water vapors for predefined time durations at 180 °C that resulted in the deposition of TiO2 nanoparticles on tubes’ surface and side walls. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) confirmed that for one hour reaction duration, nanoparticles were evenly coated on the walls of nanotubes, whereas, longer durations tend to deteriorate the tubular structure. Consequently, the ordered TiNTs-TiNPs array produced after one hour coating has shown better performance for dye-sensitized solar cell DSSC) in back illumination mode with 130% increase in efficiency as compared to the device based on bare TiO2 nanotubes. The same photoanode has higher reflective properties with higher scattering ability. The solar cell based on this photoanode exhibits higher external quantum efficiency and effective charge transport properties. This study shows that porous ordered 1D structures based on TiO2 are of crucial importance for the high performance of DSSCs.

Performance comparison of front- and back-illuminated modes of the AlGaN-based p-i-n solar-blind ultraviolet photodetectors

The authors report the comparison of front- and back-illuminated mode operations of Al0.4Ga0.6N positive-intrinsic-negative solar-blind photodetectors (PDs) grown on the double-side polished sapphire substrates by metalorganic chemical vapor deposition. It is shown that the responsivity in back-illumination mode of fabricated PDs can be almost three times as that in front-illumination mode under the same reverse bias. In addition, a wide spectral response between 300 nm and 370 nm is observed, which is not expected for solar-blind PDs in both illumination modes, while the PDs in back-illumination mode have a stronger ability to restrain the long-wavelength response, showing larger solar-blind/ultraviolet rejection ratio than front-illumination mode. The reasons for the performance differences are discussed.

Highly sensitive near IR detectors using n-type porous Si

Abstract Porous n-type Si has been used to fabricate PtSi Schottky detectors. The n-type Si substrates were made porous by anodic etching under ultraviolet radiation. The micrometer size pores were completely filled by Pt through electrodeposition. After annealing and subsequent etching of excess Pt a uniform PtSi layer was formed over the entire surface of the pores. The Schottky junction resulting from PtSi/porous Si surface was reverse biased and its response to 0.8–1.2 μm of radiation was determined in back illumination mode. Responsivities as large as 100 A/W at 0.8 μm, corresponding to a gain larger than 90 in efficiency, were obtained. This indicates that an internal gain mechanism such as avalanche multiplication is occurring in the porous samples, which is also corroborated by the general I–V characteristics. The use of these detectors as highly sensitive near IR detectors is discussed.

An Improved CCD Register Driven from Overflow Drain through Barrier.

Conventional CCD registers for full frame or frame transfer scheme image sensors have disadvantages such as low light sensitivity due to light absorption in electrodes, high dark current generated at Si-Si02 interfaces, and small charge handling capability that results from the surface pinning mode of operation. To solve these problems, we proposed CCD register driven through a barrier in February 2000. The register cell was an inverted version of the photo-diode with a vertical overflow drain that is used as the driving electrode. In simulating the register's characteristics in back illumination mode, we found that all the above-disadvantages disappeared, but the characteristics were rather sensitive to the height, width, and location of SiO2 film required to isolate electrodes. Therefore, some difficulty remains in fabrication processes. This paper proposes a new version of DTB-CCD where a specific-feature SiO2 layer covers a part of the barrier surface. This layer not only helps to minimize the structure sensitiveness of performance but also makes it possible to employ the conventional overlapping poly-Silicon electrode process. A thin SiO2 layer between electrodes and optimized width and thickness of the Si02 layer on the barrier minimizes potential barrier height in channels and thus improve transfer efficiency. The simulated charge handling capability of the improved DTB-CCD was 3-4 times greater than that of a two-phase CCD register driven by surface pinning mode.

Highly sensitive PtSi/porous Si Schottky detectors

Presents the first experimental results on PtSi/porous Si Schottky detectors. Si pores have been filled by Pt through electrodeposition. Under proper temperature treatment, Pt reacts with Si creating a PtSi layer that uniformly covers the walls of the pores. The excess unreacted Pt inside the pores is etched away leaving empty spaces behind. The spectral response of such a detector is very wide, covering from 0.9 up to at least 7 /spl mu/m of IR radiation in back illumination mode. Excellent responsivities, such as 60 A/W at 1 /spl mu/m and 0.96 A/W at 4 /spl mu/m of IR radiation is exhibited. Reverse bias current-voltage characteristics exhibit a breakdown type behavior with a breakdown voltage at about 10 V. The general shape of the reverse bias I-V curve, the wide spectral range, and high responsivity are explained through tunneling and avalanche multiplication. It is proposed that large fringing fields developed at sharp edges of the porous surface cause tunneling and avalanche multiplication.

Internal photoemission in CoGa/GaAs Schottky barriers, possible injection of electrons into the L valley

Internal photoemission in epitaxially grown films of CoGa/GaAs Schottky barriers by back illumination is used to study the injection of electrons into L and X valleys of the conduction bands of n-type GaAs. The general theory of Fowler predicts that the photocurrent per absorbed photon should vary as the square of the photon energy hν for [(hν−Φb/kT]≥0, where Φb is the difference between the Fermi level and Γ point in the GaAs conduction band. Unlike the case for Au/GaAs Schottky barriers, we observe two thresholds in the photocurrent measurements of uniform epitaxial CoGa/GaAs contacts. The second threshold formed at ≊0.3 eV higher than the first threshold (Γ point) is interpreted as the injection of carriers into the L valley of the GaAs from the metal side. A third threshold at ≊0.5 eV above the Γ point requires a photon energy greater than the energy gap of GaAs (1.4 eV) and hence cannot be observed in the back illumination mode.

Ultraviolet, visible, and infrared response of PtSi Schottky-barrier detectors operated in the front-illuminated mode

The quantum efficiency of PtSi Schottky-barrier detectors has been measured as a function of wavelength from 0.23 to 7 mu m. For front-illuminated PtSi/p-Si devices operated at low temperatures, quantum efficiencies of 40 to 70% are obtained in the ultraviolet (UV) and visible regions with little loss of the infrared (IR) photoresponse that is obtained for operation in the conventional back-illumination mode. For room-temperature operation of front-illumination PtSi/n-Si devices, the quantum efficiencies are approximately the same in the UV and visible regions, but the IR response decreases abruptly beyond the Si absorption edge. Room-temperature transmission and reflection measurements have been used to determine the values of the real and imaginary parts of the complex dielectric constant for PtSi at wavelengths from 0.2 to 3 mu m. A simple model, used with these values and published values of the dielectric constant for Si, yields calculated quantum efficiencies in the UV and visible regions that agree quite well with the measured efficiencies. The temporal response of front-illuminated PtSi/p-Si detectors in the visible and IR regions is found to be fast enough for operation at video frame rates. >

PtSi Schottky-barrier focal plane arrays for multispectral imaging in ultraviolet, visible, and infrared spectral bands

PtSi Schottky-barrier detectors, which are conventionally used in the back-illumination mode for thermal imaging in the 3-5- mu m infrared (IR) spectral band, are shown to exhibit excellent photoresponse in the near-ultraviolet and visible regions when operated in the front-illumination mode. For devices without antireflection coatings, external quantum efficiency in excess of 60% has been obtained for wavelengths between 400 and 800 nm. The efficiency decreases below 400 nm but is still about 35% at 290 nm. High-quality imaging has been demonstrated in both the visible and 3-5- mu m spectral bands for front-illuminated 160*244-element PtSi focal plane arrays integrated with monolithic CCD readout circuitry.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>