UV detection and imaging
More sensitive detectors have increasing applications.
susan.savage@acreo.se
During the past decade there has been considerable interest in imaging systems able to record very low light levels in the ultraviolet range. Of especial interest for some applications has been detection in the so-called “solar-blind” region. Radiation from the sun in the wavelength region 240nm to 280nm is absorbed by the ozone layer in the earth’s atmosphere and so does not reach the ground. This causes the background level in this wavelength region to be greatly reduced, thus permitting detection of very low levels of earth-generated radiation.
Applications
There are many applications for UV detection and imaging, including
- Combustion/flame detection
- Air quality monitoring (see LIDAR below)
- Fluorescence (eg. biological agent detection, crime-scene analysis)
- Medical (eg. detection of repairs, cracks and damage to teeth; detection of sun damage, bite marks and bruises on skin)
- Imaging of surface texture not apparent to visible-light imaging
- Detection of changes in painted or coated surfaces
- Astronomy
There is considerable R&D ongoing to develop instruments suitable for detecting and monitoring pollution in the atmosphere. LIDAR (Light Detection and Ranging) systems are one such popular R+D area. These systems, which are based on UV detection, are suitable for the detection of such chemicals as ozone, SO2 and benzene, and also for the detection of some biological agents. Having a detector that is “solar-blind” can be greatly beneficial for these applications where detection of very low levels can be desirable.
UV detection
UV detection has traditionally been accomplished by the use of Photomultiplier tubes (PMTs), or narrow-band semiconductor (typically silicon) photodiodes and charge-coupled devices (CCDs). PMTs exhibit high gain and low noise and can be fairly visible-blind, but they are fragile and bulky instruments and require a high power supply. Photodiodes and CCDs offer the advantage of compact solid-state devices which require only moderate bias, and are compatible with the formation of large and efficient imaging arrays.
However, silicon devices have the disadvantage that their maximum responsivity occurs around 900nm and then falls off towards lower wavelengths. So, filters are required to block out longer wavelengths, which results in a significant effective loss of effective area of the instrument, and adds to the bulk and complexity of the measurement equipment.
Gallium Nitride detectors
There has been much interest during the past ten years in the development of UV photodetectors based on the III-N material system, (ie Gallium Nitride and its associated compounds). This material system can surmount many of the difficulties listed above. One advantage is that they have sensitive and fast detection in the wavelength band below 400nm. Also the fact that they can be tuned to detect only in the wavelength range of interest, thus eliminating the requirement for external filters. Of especial interest is the fact that they can be tuned to detect only in the solar-blind region.
The maximum wavelength detectable by III-N alloys, ie the cut-off point, can be adjusted at will by varying the material composition within practical limitations concerned with material quality. Cut-off points between 330nm and 420nm can be achieved relatively easily with today’s techniques. Development of techniques to achieve cut-off points of <300nm (visible-blind) and <280nm (solar-blind) are today on-going world-wide. The lowest cut-off point demonstrated to date is 250nm.
In addition, a design has been demonstrated where the detector also does not respond to lower wavelengths, leaving a narrow wavelength band within which a response is obtained. In this particular case reported, the response band occurred between 320 and 370nm, with the maximum response occurring between 340nm and 370nm.
It has been demonstrated that the contrast ratio at the cut-off edge can be more than three orders of magnitude, and the internal quantum efficiency about 82%. Speed of response from 90ps up to 31ns has been reported, and frequency response up to almost 1GHz. Noise measurements made on these device types showed that at small biases, the noise was below that of the measurement set-up.
Single-pixel III-N detectors can be purchased commercially from several companies today, for example UV monitors in the form of a wrist watch or clip-on watch that indicates the current UV exposure level. This consists of a III-N detector connected to a numerical and graphic display.
There are no UV cameras commercially available today that use III-N technology. This means that they detect up into the long wavelength region also, unless a filter is included in the housing.
Only a few isolated attempts have been made at the development of a multi-pixel III-N detector array. Most recently, a group consisting of North Carolina State University, Honeywell and the Army Research Laboratories have demonstrated arrays containing up to 128x128-pixels, that detect in the wavelength range 320 – 365nm. And a group at Northwestern Univerisity; Illinois, USA, have reported a back-illuminated III-N-based array operating at 280nm.
Acreo has combined its multi-year experience in III-N material technology and detector technology to demonstrate a III-N single pixel detector that detects at wavelengths up to 365nm. It is planned that this will be extended into the development of a UV camera within the near future.