technology detectors
completely insensitive to solar radiation reaching the Earth’s surface, because the ozone layer in our atmosphere absorbs all radiation below 280 nm, the cut- off wavelength for Al0.4
Ga0.6 N. Thanks to this complete
absence of sensitivity above a certain UV wavelength, imagers and detectors can be built with far fewer filters that offer superior detectivity, due to a reduction in background signal.
Degradation under high doses of UV radiation is also diminished by switching from silicon to AlGaN. This equips the detecting systems with greater robustness and better long-term stability. Additionally, AlGaN-based imagers do not require cooling, facilitating the instrument design.
Fig. 1. Cut-off wavelength (maximum detectable wavelength) of a material vs. its energy bandgap. Gallium nitride (365 nm) is sensitive only in the ultraviolet range (“visible blind”). Increasing the aluminum concentration in the compound above 40 percent (Al0.4
Ga0.6
below 280 nm, which is the lowest wavelength produced by the Sun to reach the surface of Earth (sea level) through the atmosphere
this means that additional filters are needed for removing unnecessary visible and infrared wavelengths. Adding filters brings major, unwanted consequences. In the EUV, all materials are highly absorbing, so any layer between the radiation source and the detector is highly undesirable because it considerably limits incoming flux. What’s more, if these filters degrade – for example, contamination and/or pinhole formation – this can degrade instrument’s performance. Rectifying this in a silicon foundry may not be a major issue, but it certainly is in a telescope operating in the EUV that is attached to a satellite or on- board a space station. Compounding all these issues, non-standard processing procedures are needed to make ultraviolet silicon-based imagers.
III-Ns strengths Turning to a wide bandgap semiconductor based on nitride alloys promises to improve the design and performance of EUV detectors. The binary compound GaN has already been used to make flame detectors operating in furnaces that can detect signals in the presence of hot backgrounds with no saturation, contrary to devices based on other technologies. These detectors are commonly referred to as ‘visible blind’, and have a cut- off wavelength – the upper limit of absorption – of 365 nm.
Far shorter cut-off wavelengths are possible by increasing the aluminum content in AlGaN. Take this to the extreme, AlN, and the bandgap hits 6.2 eV, translating to the highest detectable wavelength of only 200 nm, which is in the vacuum ultraviolet range. If an aluminum composition of at least 40 percent is applied, it’s possible to fabricate what is known as a ‘solar blind’ detector. This device is
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www.compoundsemiconductor.net January / February 2011
Fig. 2. Exploded schematic of the hybrid imager, with AlGaN-on-silicon detector chip (top) integrated by flip-chip bonding with the CMOS readout (bottom) using 10 µm pixel-to-pixel indium solder bumps. Image not to scale
N) provides “solar blindness”: cut-off
Efforts to develop GaN photodetectors for various applications have been going on for a couple of decades, with single-pixel photodetectors receiving the most attention. Some work has also been directed at the development of two-dimensional imagers, which are challenging to make, because the imager has to be integrated with the readout circuit. Uniting these two is not easy because the nitride active layers come with unwanted baggage – the substrate that provides a platform for their growth.
‘Face-up’ or ‘face-down’?
One option for integrating nitride layers and the read-out circuit is a ‘face-up’ approach, which requires the fabrication of through-wafer-vias to contact the readout through the substrate. This is possible for relatively large pixel-to-pixel pitches and thin substrates, but at the cost of a decreasing fill-factor - a part of the active pixel has to be used for interconnection. Complicating matters, fabricating vias with a high aspect ratio is tough in silicon, and would be even more challenging with the sapphire and SiC substrates that are commonly used for AlGaN heteroepitaxy.
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