Robert D. McKeag

Polycrystalline diamond photoconductive device with high UV‐visible discrimination

Planar metal–diamond–metal photoconductive devices have been fabricated from free standing large grain (20–30 μm) polycrystalline thin film diamond. An interdigitated electrode design with spacings of 20 μm was used to produce effective UV photodetecting devices at bias values in the range 0.1–10 V. A methane‐air treatment has been used to modify the structures such that unprecedented performance characteristics have been recorded (106 higher response to 200 nm than visible wavelengths, <0.1 nA dark currents); spectral features similar to those observed in natural diamond crystals have been observed indicating that the treatment used led to near ideal electronic characteristics from polycrystalline material.

Diamond UV photodetectors: Sensitivity and speed for visible blind applications

Abstract UV photodetectors displaying visible blind characteristics (> 10 6 discrimination between 200 nm light and wavelengths longer than 230 nm), low dark currents and high sensitivity have been fabricated. Interdigitated planar devices have been formed on CVD diamond films with differing grain sizes; the highest yet reported photoconductive gain levels (10 6 ) have been measured, although these devices are moderately slow to turn off. Post-fabrication gas treatments have enabled a good sensitivity (gain ∼ 50) to be achieved alongside switching speeds that are fast enough to be used with conventional display technology enabling imaging applications to be considered.

Photoconductive properties of thin film diamond

Abstract The origin of photoconductivity within diamond is briefly reviewed. Several applications for thin film diamond formed by chemical vapour deposition (CVD) could be realised if the extrinsic photoconductive response could be controlled, including the fabrication of deep UV photodetectors which are “blind” to visible light. It is possible to reduce the extrinsic photoconductive response otherwise apparent in CVD films by a combination of careful device design and well chosen post fabrication gaseous treatments; the nature of the treatments used and how they may be affecting defects within the diamond are discussed. Highly sensitive deep UV photodetectors can be produced in this manner, exhibiting external quantum efficiencies of greater than 500%.

Thin film diamond UV photodetectors: Photodiodes compared with photoconductive devices for highly selective wavelength response

Abstract Silicon-supported and free-standing thin films of diamond have been used to fabricate photoconductive and photodiode structures for the detection of UV light. On free-standing (80 μm thick) material, a planar interdigitated design with 20-μm electrode spacings is found to offer unprecedented wavelength discrimination between deep UV and visible light, with dark currents

Diamond photodetectors for next generation 157-nm deep-UV photolithography tools

Abstract Next generation photolithography stepper tools will operate at 157 nm and require robust solid state photodetectors to ensure efficient operation and facilitate direct beam monitoring for photoresist exposure dosimetry. There is currently no commercial detector system able to fully meet all the demanding requirements of this application. Diamond, which is intrinsically visible blind and radiation hard, is an obvious candidate for consideration. In this paper we report the results of the first study to assess the viability of thin film polycrystalline diamond photodetectors for use in 157 nm F 2 –He based laser lithography tools. Co-planar inter-digitated gold photoconductor structures were fabricated on free standing thin film diamond and exposed to pulses from an industrial F 2 –He laser in the fluence range 0–1.4 mJ cm −2 . The electrical and optical characteristics of the devices have been measured and are compared to the response of a standard vacuum photodiode. The suitability of the diamond devices for use at 157 nm is discussed.

High collection efficiency CVD diamond alpha detectors

Advances in Chemical Vapour Deposited (CVD) diamond have enabled the routine use of this material for sensor device fabrication, allowing exploitation of its unique combination of physical properties (low temperature susceptibility (>500/spl deg/C), high resistance to radiation damage (>100 Mrad) and to corrosive media). A consequence of CVD diamond growth on silicon is the formation of polycrystalline films which has a profound influence on the physical and electronic properties with respect to those measured on monocrystalline diamond. We report the optimisation of physical and geometrical device parameters for radiation detection in the counting mode. Sandwich and co-planar electrode geometries are tested and their performances evaluated with regard to the nature of the field profile and drift distances inherent in such devices. The carrier drift length before trapping was measured under alpha particles and values as high as 40% of the overall film thickness are reported. Further, by optimising the device geometry, we show that a gain in collection efficiency, defined as the induced charge divided by the deposited charge within the material, can be achieved even though lower bias values are used.

Diamond photoconductors: operational lifetime and radiation hardness under deep-UV excimer laser irradiation

Abstract The first study of long term pulse exposure and fluence level on the performance of CVD diamond photodetectors subjected to 193 nm excimer laser radiation has been performed. Whilst diamond is considered ‘radiation hard’ it is shown that damage to detector performance can be provoked at laser fluence levels considerably below that required for graphitisation or ablation. However, the application of defect passivation treatments prior to device use acts to considerably reduce the damaging effect of the radiation, such that devices suitable for stable laser monioring applications can be realised.

Thin film diamond alpha detectors for dosimetry applications

Abstract Diamond is a resilient material with rather extreme electronic properties. As such it is an interesting candidate for the fabrication of high performance solid state particle detectors. However, the commercially accessible form of diamond, grown by chemical vapour deposition (CVD) methods, is polycrystalline in nature and often displays rather poor electrical characteristics. This paper considers the way that this material may be used to form alpha particle dosimeters with useful performance levels. One approach adopted has been to reduce the impurity levels within the feed-stock gases that are used to grow the diamond films. This has enabled significant improvements to be achieved in the mean carrier drift distance within the films leading alpha detectors with up to 40% collection efficiencies. An alternative approach explored is the use of planar device geometry whereby charge collection is limited to the top surface of the diamond which comprises higher quality material than the bulk of the film. This has lead to collection efficiencies of greater than 70%, the highest yet reported for polycrystalline CVD material based detectors. Techniques for improving the characteristics of these devices further are discussed.

Imaging deep UV light with diamond-based systems

Diamond grown by chemical vapour deposition (CVD) techniques has shown great promise for the fabrication of high sensitivity, low dark current, fast and visible-blind deep UV photodetectors. In addition to the careful choice of substrate material, defect passivation treatments applied to the diamond after growth have been found to considerably enhance the detector characteristics achieved. In this paper, we report on the first purposefully designed one-dimensional CVD diamond imaging array for the detection of nanosecond 193-nm excimer laser pulses using this approach. It is shown to perform extremely well, giving less than 2% pixel-to-pixel variation in signal response, and is fast enough to avoid any sign of charge build-up during prolonged operation.

Diamond deep UV photodetectors: reducing charge decay times for 1-kHz operation

Abstract Diamond grown by chemical vapour deposition (CVD) methods is thought to be ideal for the fabrication of visible blind, fast deep UV photodetectors. However, careful device design and selection of high-quality CVD thin film diamond is, in itself, insufficient for the realisation of high performance devices. Post-growth device treatments are capable of transforming the optoelectronic properties of the material such that commercially interesting devices result. In the present study we have shown that sequentially applied methane–air treatments continue to modify both the gain level and speed of the device. Three such treatments give an optimal gain level, whilst more treatments than this lead to an improved turn-off speed. For the first time we have demonstrated the successful operation of a CVD diamond photoconductive device at at least 1 kHz at 193 nm, a frequency that is required for state-of-the-art excimer laser applications at this wavelength.