Low-noise, Low-dark-current GaN Diodes for UV Detectors
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Low-noise, Low-dark-current GaN Diodes for UV Detectors Peter W. Deelman and Robert N. Bicknell-Tassius Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109 Sergey Nikishin and Henryk Temkin Department of Electrical Engineering, Texas Tech University, Lubbock, TX 79409 ABSTRACT We report mesa-isolated Schottky barrier photodetectors fabricated on n-GaN. Singleelement detectors were constructed from nitride epilayers grown by gas source molecular beam epitaxy (GSMBE) on Si(111). Chlorine-based reactive ion etching was used to form two-level mesas. The detectors were front-illuminated through 100 Å Pd semitransparent Schottky contacts on the upper mesas; ohmic contact on the lower mesas was made using standard Ti/Al/Ti/Au metallurgy. Silicon dioxide grown by plasma-enhanced chemical vapor deposition provided both surface passivation and electrical isolation. The dark current of an 86 × 86 µm2 single-element detector is 2.10 × 10-8 A/cm2 at –2 V bias, and the zero-bias noise power density at 1Hz is as low as 9 × 10-29 A2/Hz. In addition, we present preliminary results for p-n diodes fabricated from epilayers grown on sapphire. The dark current of a 50 × 50 µm2 single-element detector is 1.6 × 10-6 A/cm2 at –2 V bias. INTRODUCTION Ultraviolet (UV) sensors for space astronomy have requirements distinct from those for industrial or military applications. First, they must be insensitive to light at optical wavelengths (commonly referred to as being solar blind), because, for example, astronomical objects often emit 104–108 visible photons for every UV photon[1]. These detectors are further required to generate as little noise and dark current as possible, since noise arising from the background often dominates in faint UV observations. Detector arrays must be addressed using low-noise readout techniques and they must be resistant to the effects of operation in space. With their wide bandgaps, high thermal conductivities, chemical inertness, and radiation hardness, UV detectors implemented in GaN, AlN, and their alloys offer significant potential for solar-blind UV detectors capable of operating at high temperatures and in hostile environments. Device quantum efficiencies of (Al)GaN-based detectors are potentially several times greater than those of competing UV detector technologies[2]. In the last several years, nitride-based solar-blind UV detectors have made the transition from theoretical interest to practical application. While the vast majority of these detectors were fabricated from epilayers grown on sapphire substrates by metalorganic chemical vapor deposition (MOCVD), only a few utilized Si as the substrate and only a few grew epilayers by molecular beam epitaxy (MBE)[3-6]. Nevertheless, the use of silicon offers many benefits: largearea, low-cost, highly perfect substrates are readily available, a very sophisticated backside process technology has been developed over many years, and thermal expansion mismatch between detector arrays and readout electronics would be mitigated. In this
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