No abstract

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 quantitative agreement between the model and the experimental data is obtained.

The optimization of an InAs0.91Sb0.09 based infrared detector has been performed. The importance of the interfaces between the active region and the contacts in generation recombination phenomena is demonstrated. The two sides of the active region are optimized independently through heterostructure band gap engineering. The use of an Al0.15In0.85As0.91Sb0.09 quaternary makes it possible reach a detectivity of 4.4×109cm√Hz∕W at 290 K and 1.4×1010cm√Hz∕W at 250 K at 3.39μm, offering the perspective of a noncryogenic infrared imaging in the 3–5μm band with quantum detectors.

BiFeO 3 is the prototypical multiferroic and one of the few with both (anti)ferroic ordering temperatures above 300 K. While its magnetic and ferroelectric properties and their coupling have been investigated intensely, offering opportunities in spintronics, little is known concerning its optical properties and their coupling to the ferroic orders. For applications in the microwave range, we report on the integration of BiFeO3 onto low permittivity substrates. Such integrated films show good ferroelectric and optical properties consistent with those of films grown on SrTiO3 substrates. Prospects for the use of BiFeO3 in optical applications are discussed.