Diamond for Image Intensifier and Photodetection Applications

An image intensifier is a device that intensifies low light-level images to light levels that can be seen with the human eye or can be detected by a camera. An image intensifier consists of a vacuum tube with several conversion and multiplication screens. An incident photon will hit a light sensitive photo-cathode screen. Photons are absorbed in the photocathode and give rise to emission of electrons into the vacuum. These electrons are accelerated by an electric field to increase their energy and focus them on the multi channel plate (MCP).

As a wide band gap (5.5eV) semiconductor diamond offers a range of properties that make its integration into image intensifier devices very promising in terms of enhanced device performance. For example, under appropriate conditions the surface of diamond can display a negative electron affinity (NEA), allowing for high secondary electron yields (SEY) to be achieved, with values greater than 100 being achieved. Diamond, grown by chemical vapour deposition (CVD) methods, can also support very high carrier mobilities and has a high electric field breakdown strength. Given that it takes 13eV to create electron-hole pairs in diamond when irradiated by electrons 'cascade gain' of an electron flux within diamond can be achieved; this can lead to an electron transmission gain of >10 if the transmitted electrons emerge from an NEA diamond surface. Diamond can also be doped p-type by the inclusion of boron.

Existing MCP technology leads to a secondary electron 'gain' of around 1.9 when incoming electrons impact the channel regions of the plate. Whilst cascading results in over-all gains of a few thousand at the exit of the MCP, these large values only arise for the electrons that are effective in the initial stages of secondary generation. As 1.9 is a statistical value some incoming electrons will not result in further cascade and are effectively 'lost' degrading the resultant image. It is thus desirable that gain levels are increased at the entrance to the MCP and for a short distance into the MCP.

Another limitation to the performance of current MCP-based image intensifiers involves the loss of 'focus' caused by the emergence of 'hot' electrons from the exit of the MCP. It is the consideration of these issues, along with the properties of diamond described above that allows for the development of several ideas for considerably enhancing the performance of existing image intensifiers, namely:

[1] A diamond pre-amplifier stage. A thin diamond membrane (displaying transmission electron gain), to pre-amplify the photo-generated electrons prior to their entry into the MCP for image intensifiers.
[2] Diamond coating the MCP for enhanced SEY (displaying reflective gain) for image intensifiers. A thin diamond layer displaying NEA to enhance the SEY for each electron collision within the MCP

In addition the replacement of the MCP within an image intensifier device with a 'stack' of diamond membranes may offer an alternative to Avalanche Photodiodes (APD) for ultra-low light fast photodetection. If each membrane offers a transmission gain of ~10, then a 4-layer stack may offer a gain of some 10,000. This would lead to a completely new generation of photodetectors.

[3] A diamond membrane stack for multi-stage electron amplification (transmission gain) within a low light fast photodetector

Low light level photo-detection and imaging is essential in many sectors, ranging from manufacturing, automotive and aerospace, medical and scientific instrumentation, space science and astronomy as well as defence related activities. In the latter case night vision is now an essential aspect of modern warfare, and continual improvements are sought to keep NATO forces a step-ahead within the volatile arenas around the world that they find themselves within. It is also the case that more local terror threats along with civil search and rescue tasks all require the UK to have sophisticated night vision capabilities. The proposed activity will have a very broadly based, very high impact on a number of sectors of high importance to the UK and the EU.

[1] Image Intensifiers for night vision

The manufacture of enhanced image intensfiers. The proposed activity offers the prospect of a generation of image intensifiers that will considerably outperform current devices. As a project partner, the EUs Photonis SAS is well placed to expolit such an acheivement and has indeed expressed a strong interest in expoiting diamond technology within its products. The UKs Photek Ltd also manufacture image intensifiers and can be expected to be able to exploit the outcomes of the project.

Access to the military markets is complex, and Photonis are ideally placed to achieve this, and the UKs Qioptiq Ltd provides night vision systems for the military using image intensifiers therefore also being well placed to exploit the developments achieved.

The UKs BAE Systems plc is the second largest defence sector manufacturer and is well placed to exploit enhanced night vision devices in many areas, including within its aerospace division.

It is anticipated that night vision within the automotive industry will emerge as a multi-billion dollar market (see case in support); 'high end' vehicles (luxury, performance) will be the entry point here and the UK is particularly strong here, with an important industry in parts development and supply. The UK also manufactures large volumes of mainstream cars for export, to which the technology can be expected to trickle-down. Low light level machine vision is also an increasing important use of image intensifiers within, for example, the food industry.

[2] Photodetection

The enhanced MCP-based technology and the completely new type of photodetector (based on stacked diamond membranes) have the potential to make an enormous impact on, again, a very broad range of applications sectors. In addition to the manufacturing capabilities of Photonis and Photek already noted the UK hosts a number of SMEs, with which Jackman already has past/present links, which are well placed to exploit the projects outcomes to develop and manufacture detectors. These include Centronic Ltd, E2V Ltd, Applied Scintillation Technologies Ltd and Micron Semiconductor Ltd; the latter already has an in-house programme developing diamond based radiation detectors.

In terms of application, the UK has a thriving space science and astronomy community for whom these devices would be of considerable interest. A major interest will be in the use of ultra-compact, fast, high gain new generation photodetectors that will emerge for laser range finding. Qioptiq Ltd in the UK will be ideally placed to liaise with Photonis or one of the other manufacturing companies to develop a new generation of, for example, weapons sights using such devices. High-speed low light level detection applications exist in the medical and scientific instrument fields as well as advanced manufacturing environments.