Diamond with an ultra-wide bandgap shows intrinsic performance that is extraordinarily superior to those of the currently available wide-bandgap semiconductors for deep-ultraviolet (DUV) photoelectronics and microelectromechanical systems (MEMS). The wide-bandgap energy of diamond offers the intrinsic advantage for solar-blind detection of DUV light. The recent progress in high-quality single-crystal diamond growth, doping, and devices design have led to the development of solar-blind DUV detectors satisfying the requirement of high Sensitivity, high Signal-to-Noise ratio, high spectral Selectivity, high Speed, and high Stability. On the other hand, the outstanding mechanical hardness, chemical inertness, and intrinsic low mechanical loss of diamond enable the development of MEMS sensors with boosted sensitivity and robustness. The micromachining technologies for diamond developed in these years have opened the avenue for the fabrication of high-quality single-crystal diamond mechanical resonators. In this review, we report on the recent progress in diamond DUV detectors and MEMS sensors, which includes the device principles, design, fabrication, micromachining of diamond, and devices physics. The potential applications of these sensors and a perspective are also described.
Diamond is an element semiconductor material owning numerous extraordinary properties such as the highest mechanical hardness in nature and the highest thermal conductivity among all the known semiconductors. By virtue of the extreme hardness, diamonds are widely used as abrasive materials in industry, such as grinding tools, blades and for cutting, drilling, and polishing. In addition to the outstanding mechanical properties, diamond also has the highest figure-of-merits for semiconducting devices due to its extraordinary properties such as the ultra-wide bandgap (UWBG) energy, the highest thermal conductivity (22W/mm K), high carriers mobilities, large breakdown electric field (exceeding 10MV/cm), high chemical inertness, and thermal stability.
In contrast to diamond electronics, diamond as DUV or radiation detectors are more developed since shallow dopants are not prerequisite and simple device structure can be adopted. The utilization of diamond as DUV detector has the intrinsic merit of solar blindness (wavelength <280nm) due to the large bandgap energy of 5.5 eV. Therefore, an ideal diamond DUV detector shows no or weak response to the light with wavelength longer than 280nm on the earth. Traditional UV-enhanced Si photodetector has intrinsic limitations in UV detection owing to its narrow bandgap energy of 1.1 eV. For other wide bandgap semiconductors such as GaN and SiC, the bandgaps are not high enough to reach the solar blindness. When using these semiconductors for DUV detection, filters are needed to reduce the background noise from the solar light. The large bandgap energy of diamond also offers the advantage of extremely low dark current. Compared to other UWBG semiconductors such as Ga2O3, diamond is expected to show much stronger radiation hardness upon high-power DUV illumination due to the single element nature and strong chemical carbon-carbon bonds.
Fig1
The PPC effect can be avoided by using the photovoltaic mode of the CSPD [72], although the responsivity was as low as 0.5mA/W at 220nm light. Note that at zero bias, the responsivity varied little as the light intensity, as shown in Figure 3. A high response time was also observed.
Photodiodes based on p(i)n-junction is the most desirable structure for photovoltaic operation due to the fast speed and high responsivity. Koizumi et al obtained the first n-type diamond epilayer grown on the {111}-oriented diamond substrate by phosphorus doping and demonstrated the DUV LED diode. The p-i-n photodiode was used to DUV or extreme UV detection. The DUV sensitivity at 200nm was around 27.2mA/W , much higher than that of the diamond SPD at zero bias. The photocurrent ratio between 210nm and 310nm was more than 104 of the p-i-n photodiode at zero bias.
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