Jean-Luc Reverchon

Wide bandgap UV photodetectors: a short review of devices and applications

Ultraviolet detectors are of a great interest to a wide range of industrial, military, environmental and even biological applications. This paper intends first to review some of the most relevant recent developments in the field of wide bandgap semiconductor UV detectors, and to give an overview of their applications. A special focus is given on III-nitride based devices, which more and more clearly represent to date one of the most promising and flexible technical solutions for UV detection.

Submicron metal–semiconductor–metal ultraviolet detectors based on AlGaN grown on silicon: Results and simulation

Solar blind metal–semiconductor–metal detectors have been fabricated based on AlGaN grown on Si by molecular-beam epitaxy. Submicron finger spacings were obtained by electron-beam lithography, and allowed us to demonstrate a significant improvement of the responsivity and the spectral selectivity. These results were explained by numerical two-dimensional calculations of the electric-field distribution. The simulation also explained the dependence of the response on applied bias.

GaN for x-ray detection

The potential of GaN based materials for x-ray detection is investigated. The absorption coefficient in GaN is measured as a function of photon energy between 6 and 40keV. Metal-semiconductor-metal photodetectors are fabricated and characterized. The response dependence on bias, the temporal dynamics, and the response dependence on detector geometry all together point toward a mixing of photovoltaic and photoconductive effects. Thanks to a large photoconductive gain, the detector has a decent responsivity at the expense of a large response time.

Effects of the buffer layers on the performance of (Al,Ga)N ultraviolet photodetectors

The fabrication of (Al,Ga)N-based metal–semiconductor–metal (MSM) photovoltaic detectors requires the growth of high-quality (Al,Ga)N films. Inserting a low-temperature deposited buffer layer enables the growth of an epitaxial layer with a reduced density of defects. Two structures using GaN and AlN buffer layers have been deposited by low-pressure metalorganic chemical vapor deposition and used to fabricate MSM interdigitated detectors. The devices have been characterized to investigate the effects of the buffer layers on the detector performances.

AlGaN-Based Linear Array for UV Solar-Blind Imaging From 240 to 280 nm

The realization of a linear array of 300 pixels, with a 26-mum pitch, based on AlxGa1-xN metal-semiconductor-metal photodetectors, is described. The composition of the active layer is chosen in order to optimize the solar-blind operation: A sharp cutoff at 280 nm is observed, with discrimination between far and medium ultraviolet (UV) of three orders of magnitude. The detector shows a peak responsivity of 12 mA/W and a dark current smaller than 1 fA at the typical polarization of 4 V. The maximum resolution is analyzed in terms of modulation transfer function (MTF): The best result is obtained for a front-side illumination, i.e., when an MTF of 0.45 is measured at the half Nyquist frequency (19.2 lp/mm). Some UV images, which are obtained in a pushbroom model, are reported. The visible rejection is proven by directly imaging the arc of a Xenon lamp: It is shown that only the relatively weak far-UV component contributes to the signal

Optoelectronic characterization of blue InGaN/GaN LEDS grown by MBE

Light emitting diodes were grown by molecular beam epitaxy using NH, as nitrogen precursor. The active layer is composed by a single plane of undoped InGaN layer with about 15% of In. The structure was buried by 2700 Angstrom of Mg-doped GaN (p = 1 x 10(17) cm(-3)). The turn on voltage is at 4.5 V and the operating voltage is 6.1 V at 20 mA. Temperature dependent I(V) characteristics reveal the predominance of tunneling injection current. We measure room temperature electroluminescence in the blue from 440 to 490 nm with a narrow full width at half maximum

Deep ultraviolet detection dynamics of AlGaN based devices

The photoconductive response of AlGaN based UV detectors to 193nm excimer laser radiation is presented. Two devices have been tested: a metal-semiconductor-metal (MSM) planar structure and a Schottky diode. The transient response of the MSM device closely follows the laser pulses, with a photoconductive decay time constant shorter than 3ns. Conversely, the Schottky diode shows a slower photoconductive rise and decay kinetics due to the material series resistance coupled with the junction capacitance. Moreover, a longer time constant tail is also evident in this case with a characteristic time of about 40ns, due to the presence of trap states localized at 0.2–0.3eV from the band edge. The detection dynamics has been evaluated by changing the beam energy density between 2×10−5 and 0.2mJ∕mm2. The signal increases linearly in the case of the MSM device up to 0.001mJ∕mm2, whereas, for a further intensity rise, the response shows a sublinear behavior. On the contrary, the Schottky diode showed a linear trend in...

High Performance Solar Blind Detectors Based on AlGaN Grown by MBE on Si

Solar blind metal-semiconductor-metal detectors have been fabricated based on AlGaN grown on Si by molecular beam epitaxy. Some devices were fabricated by electron beam lithography, We present the device characterization and performance, that includes a noise equivalent power as low as 30 fW at 280 nm.

Anomalous photoresponse of GaN x-ray Schottky detectors

GaN based materials are believed to be very stable materials, in particular, under irradiation by high energy photons such as x rays. We have studied x-ray detectors based on GaN Schottky diodes. Vertical Schottky diodes were fabricated based on a 20μm thick undoped GaN layer grown on a conductive GaN substrate. Their photoresponse to near UV light and to x rays was measured. While the response to near UV light was fast and linear as expected, anomalous behaviors were observed under x-ray illumination. The photocurrent increases as the third power of the incident x-ray flux. The photocurrent transient when the x rays is turned on are long and nonexponential (S shape) and strongly differs from the off transient which is fast and exponential. Also, a very strong quenching of the x-ray photoresponse is observed when the detector is simultaneously illuminated with visible light. All of these anomalous behaviors are explained in the frame of a complete model involving traps and tunnel currents. A reasonable qu...

Internal photoemission in solar blind AlGaN Schottky barrier photodiodes

We have analyzed the photoresponse of solar blind AlGaN Schottky barrier photodiodes below the alloy band gap energy, in the 3.5–4.5eV range, and we show that it is dominated by internal photoemission. The n-type Schottky barrier height is shown to increase linearly with the band gap energy of the AlGaN alloy. The amplitude of the internal photoemission signal is about 20 times smaller than the value given by the Fowler theory based on a free electron model. We explain this result by taking into account the interband transitions and the ballistic transport of photoexcited electrons in the metal. This low value of internal photoemission allows us to achieve a spectral rejection ratio between 280 and 320nm of more than 3 decades.