In this review article a comprehensive analysis of the developments in ultraviolet (UV) detector technology is described. At the beginning, the classification of UV detectors and general requirements imposed on these detectors are presented. Further considerations are restricted to modern semiconductor UV detectors, so the basic theory of photoconductive and photovoltaic detectors is presented in a uniform way convenient for various detector materials. Next, the current state of the art of different types of semiconductor UV detectors is presented. Hitherto, the semiconductor UV detectors have been mainly fabricated using Si. Industries such as the aerospace, automotive, petroleum, and others have continuously provided the impetus pushing the development of fringe technologies which are tolerant of increasingly high temperatures and hostile environments. As a result, the main efforts are currently directed to a new generation of UV detectors fabricated from wide band‐gap semiconductors the most promising ...

Semiconductor ultraviolet detectors

New concept ultraviolet photodetectors

Charge-coupled devices (CCDs) are presently the technology of choice for most imaging applications. In the 23 years since their invention in 1970, they have evolved to a sophisticated level of performance. However, as with all technologies, we can be certain that they will be supplanted someday. In this paper, the Active Pixel Sensor (APS) technology is explored as a possible successor to the CCD. An active pixel is defined as a detector array technology that has at least one active transistor within the pixel unit cell. The APS eliminates the need for nearly perfect charge transfer--the Achilles' heel of CCDs. This perfect charge transfer makes CCD's radiation 'soft,' difficult to use under low light conditions, difficult to manufacture in large array sizes, difficult to integrate with on-chip electronics, difficult to use at low temperatures, difficult to use at high frame rates, and difficult to manufacture in non-silicon materials that extend wavelength response. With the active pixel, the signal is driven from the pixel over metallic wires rather than being physically transported in the semiconductor. This paper makes a case for the development of APS technology. The state of the art is reviewed and the application of APS technology to future space-based scientific sensor systems is addressed.

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Active Pixel Sensors: Are CCD's Dinosaurs?

CMOS based imagers are beginning to compete against CCDs in many areas of the consumer market because of their system-on-a-chip capability. Sensitivity, however, is a main weakness of CMOS imagers and enhancements and deviations from standard CMOS processes are necessary to keep up sensitivity with downscaled process generations. In the introductory section several definitions for the sensitivity of image sensors are reviewed with regard to their potential to allow meaningful comparison of different detector structures. In the main section, the standard CMOS sensor architecture is compared to detector structures designed to improve the sensitivity, namely the photogate (PG), the pinned photodiode (PPD) and the thin film on ASIC (TFA) approach. The latter uses a vertical integration of the photodiode on top of the pixel transistors. A careful analysis of the relevant electrical, optical and technological parameters and many previously published experimental data for different imagers reveals that only the PPD and the TFA enhancements provide satisfactory sensitivity and withstand scaling down to 0.18 /spl mu/ processes. Due to the higher fill factor and the higher quantum efficiency TFA provides significantly better values than PPD. The radiometric sensitivity of a 5 /spl mu/m/spl times/5 /spl mu/m TFA pixel is found to amount to 11.9 V/(/spl mu//cm/sup 2/) for a 0.25 /spl mu/m process and 27.5 V/(/spl mu/J/cm/sup 2/) for a 0.18 /spl mu/m process.

Sensitivity of CMOS based imagers and scaling perspectives

In the last few decades, advances and breakthroughs of carbon materials have been witnessed in both scientific fundamentals and potential applications. The combination of carbon materials with traditional silicon semiconductors to fabricate solar cells has been a promising field of carbon science. The power conversion efficiency has reached 15-17% with an astonishing speed, and the diversity of systems stimulates interest in further research. Here, the historical development and state-of-the-art carbon/silicon heterojunction solar cells are covered. Firstly, the basic concept and mechanism of carbon/silicon solar cells are introduced with a specific focus on solar cells assembled with carbon nanotubes and graphene due to their unique structures and properties. Then, several key technologies with special electrical and optical designs are introduced to improve the cell performance, such as chemical doping, interface passivation, anti-reflection coatings, and textured surfaces. Finally, potential pathways and opportunities based on the carbon/silicon heterojunction are envisaged. The aspects discussed here may enable researchers to better understand the photovoltaic effect of carbon/silicon heterojunctions and to optimize the design of graphene-based photodevices for a wide range of applications.

Carbon/Silicon Heterojunction Solar Cells: State of the Art and Prospects.

The authors report the UV photoresponse of an a-SiC/a-Si heterojunction p-i-n diode with the structure of glass/TCO (transparent conducting oxide, SnO/sub 2/:F)/p-a-SiC:H/i-a-Si:H/n-a-Si:H/Al. The diode has been designed for a high-sensitivity and low-noise UV detector. The diode has its peak responsivity (0.254 A/W) and quantum efficiency (81.5%) at 385 nm. This structure possesses (1) the window effect by using the wide-bandgap a-SiC:H as the front layer (p-layer) and (2) the carrier confinement effect at the p-SiC:H/i-a-Si:H interface. Enhancements are proposed to raise UV response and suppress long-wave responsivity. The diode was designed to be operated under zero external bias to suppress the dark-current-induced noise. Results show a 200% higher UV sensitivity than a GaAsP Schottky photodiode in the 200-400-nm wavelength region. >

An amorphous SiC/Si heterojunction p-i-n diode for low-noise and high-sensitivity UV detector

The effects of material and structure parameters on an amorphous-silicon-germanium alloy Schottky barrier diode's responsivity have been investigated in detail. The effects contradict each other. A compromise is made in selecting parameters for construction of a Si/sub 1-x/Ge/sub x/:H Schottky barrier for an IR detector. The optimized amorphous-silicon-germanium alloy Schottky diode using x=0.43 alloy with a 900-nm-thick i-layer was found to have a peak at 850 nm with a responsivity of 0.6 A/W under -2-V bias. The diode also has a response time of 33 mu s and slight photodegradation. Thus, the diode becomes a candidate for manufacturing the infrared OEIC (optoelectronic integrated circuit) on glass substrate in applications which require size, reproducibility, and low cost more than sensitivity. >

A metal-amorphous silicon-germanium alloy Schottky barrier for infrared optoelectronic IC on glass substrate application

A high reverse breakdown voltage hydrogenated amorphous carbon (a-C:H)/crystalline silicon (c-Si) heterojunction diode has been successfully prepared by the photochemical vapour deposition (photo-CVD) method. This method has the advantage of being free from RF radiation. There is a lower density of fixed charges at the Al/a-C:H interface and in the bulk of a-C:H film. Hence the photo-CVD a-C:H/c-Si heterojunction diode shows smaller voltage offset in forward bias and breakdown voltage shrinkage in reverse bias compared to those deposited by plasma enhanced-CVD (PECVD).

Amorphous carbon/crystalline silicon high voltage heterojunction diode prepared by photochemical vapour deposition

An amorphous-crystalline silicon heterojunction switching device with the structure of a-Si( n+-i-p)/c-Si(n)/a-Si(p) has been developed and fabricated. This device exhibits good switching and gate control capabilities. By connecting the devices in antiparallel, a heterojunction device with bidirectional switching characteristics can be obtained. With either polarity of the voltage applied to the anode, the same switching behavior of this device is obtained, which is suitable for ac switch applications.

Novel Amorphous-Crystalline Silicon Heterojunction Switching Devices with Bidirectional Switching Characteristics

Amorphous SiN x :H thin films were prepared by plasma-enhanced CVD process. The effects of preparation conditions on the optical properties, compositions, and deposition rates of a-SiN x :H films were systematically studied by means of spectrophotometer, ellipsometer, and infrared spectroscopy measurements. It has been found that the refractive indices decrease with increasing the NH 3 gas flow rate and the rf power, and increase with increasing the electrode spacing. Conversely, the deposition rates increase with increasing the NH 3 gas flow rate and the rf power, and decrease with increasing the electrode spacing. There was no obvious effect of substrate temperatures ranging from 250 to 320°C on the optical properties and deposition rates of a-SiN x :H films. The decrease of the refractive index and the increase of the deposition rate were due to the increase of the nitrogen composition x occurring in the a-SiN x :H films. Refractive indices n and extinction coefficients k of a-SiN x :H films can be varied from n = 3.6 and k = 0.1 for x = 0 (pure a-Si:H) to n = 1.8 and k = 0 for x = 1.33 (pure a-Si 3 N 4 :H) at the wavelength of 633 nm.

L. C. Kuo

Papers

An amorphous SiC/Si heterojunction p-i-n diode for low-noise and high-sensitivity UV detector

A metal-amorphous silicon-germanium alloy Schottky barrier for infrared optoelectronic IC on glass substrate application

Amorphous carbon/crystalline silicon high voltage heterojunction diode prepared by photochemical vapour deposition

Novel Amorphous-Crystalline Silicon Heterojunction Switching Devices with Bidirectional Switching Characteristics

Optical Characterization of Amorphous SiNx:H Films Prepared by Plasma-Enhanced CVD