Extended Exposure of Gallium Nitride Heterostructure Devices to a Simulated Venus Environment Savannah R. Eisner Hannah S. Alpert Caitlin A. Chapin Stanford University Stanford University Stanford University 496 Lomita Mall 496 Lomita Mall 496 Lomita Mall Stanford, CA 94305 Stanford, CA 94305 Stanford, CA 94305 [email protected] [email protected] [email protected] Ananth Saran Yalamarthy Peter F. Satterthwaite Ardalan Nasiri Stanford University Stanford University University of Arkansas 496 Lomita Mall 496 Lomita Mall 700 Research Center Blvd. Stanford, CA 94305 Stanford, CA 94305 Fayetteville, AR 72701 [email protected] [email protected] [email protected] Sara Port Simon Ang Debbie G. Senesky University of Arkansas University of Arkansas Stanford University 332 Arkansas Ave 700 Research Center Blvd. 496 Lomita Mall Fayetteville, AR 72701 Fayetteville, AR 72701 Stanford, CA 94305 [email protected] [email protected] [email protected] AlGaN/GaN heterostructure is suitable for robust, Venus- Abstract—Further development of harsh environment capable electronics. electronics capable of uncooled operation under Venus surface atmospheric conditions (~460°C, ~92 bar, corrosive) would enable future missions to the surface of Venus to operate for up to a year. Wide band-gap gallium nitride (GaN) heterostructure TABLE OF CONTENTS devices are attractive candidates for Venus lander missions due to their ability to withstand high-temperature exposure. Here, 1. INTRODUCTION ....................................................... 1 we present the first assessment of the electrical integrity of GaN- 2. DEVICE FABRICATION AND OPERATION ............... 1 based devices subject to Venus surface atmospheric conditions. Three unique device architectures were fabricated at the 3. EXPERIMENTAL METHODS ..................................... 4 Stanford Nanofabrication Facility and exposed in a Venus 4. RESULTS AND DISCUSSION ...................................... 4 simulation chamber for 244 hours at the University of Arkansas 5. CONCLUSIONS .......................................................... 8 Center for Space and Planetary Sciences. The three device architectures tested were InAlN/GaN high electron mobility ACKNOWLEDGEMENTS ....... ERROR! BOOKMARK NOT transistors (HEMTs), InAlN/GaN Hall-effect sensors, and DEFINED. AlGaN/GaN UV photodetectors, which all have potential REFERENCES ........ ERROR! BOOKMARK NOT DEFINED. applications in the collection and readout of sensor data from BIOGRAPHY ............................................................... 11 Venusian landers. After exposure, HEMT threshold voltage had shifted only ~1% and gate leakage current remained on the same order of magnitude, demonstrating stability of the IrOx gate under supercritical CO2 ambient. Fluctuations in drain 1. INTRODUCTION current after exposure are attributed to thermal detrapping and electrically-activated trapping processes. Measurements of the The longevity and scope of proposed missions to the surface InAlN/GaN 2DEG properties in virgin and exposed Hall-effect of Venus are currently limited by the challenge of developing sensors were comparable. Furthermore, the Hall-effect sensors electronics that can survive the ~460°C, ~92 bar corrosive exhibited a maximum change of only +11.4% in current-scaled sensitivity and -6.6% in voltage-scaled sensitivity post-exposure. environment [1], [2]. Despite the use of cooling measures, The UV photodetectors with 362 nm peak wavelength exhibited data transmission from previous Venus landers lasted only an average decrease in responsivity of 38% after exposure, two hours due to failure of the silicon electronics [2]. Long which is thought to be due to strain relaxation or ohmic contact term missions (beyond 2 months) to the surface of Venus are degradation. Similar performance of the InAlN/GaN HEMTs needed to make seismic observations, determine detailed and Hall-effect sensors before and after exposure highlights the mineralogy, and characterize atmosphere-surface viability of this material platform for development of Venus interactions over an extended duration. This data can surface electronics, while the decrease in AlGaN/GaN UV illuminate the origin and diversity of terrestrial bodies (e.g. photocurrent requires further analysis to assess whether the how Venus and Earth diverged in climate and geology), as 978-1-7281-7436-5/21/$31.00 ©2021 IEEE well as the factors that determine the evolution of life on atmospheric conditions has been reported. Here, we describe some planets and not others. Moreover, a surface study of the the first demonstration of the stability of GaN heterostructure runaway greenhouse gas effect on Venus can increase our devices after 244-hour (>10 days) exposure to a simulated understanding of the processes that control climate on Earth- Venus surface environment. like planets [1], [3], [4]. The three distinct GaN heterostructure device types Wide band-gap semiconductors such as silicon carbide (SiC) characterized in this exposure study were chosen due to their and gallium nitride (GaN) have recently emerged as potential sensing and telecommunication applications on promising material platforms for uncooled extreme board a Venus lander. GaN HEMT devices can be used for environment electronics due to their stability at high pressure, chemical, and IR sensing [19]–[23]. Additionally, temperatures, inherent radiation tolerance, and chemical HEMTs have applications in read-out and transmission of resistance [5]–[7]. Furthermore, GaN heterostructures have sensor data to orbiters. Hall-effect sensors are often used for the unique ability to form a polarization-induced two- detecting the position and speed of rotating components (e.g., dimensional electron gas (2DEG) at the interface between gears, motors) aboard a spacecraft, as well as monitoring the GaN and a III-nitride alloy (e.g. AlGaN or InAlN). Unlike reliability of other on-board electronics via non-invasive doped junctions, the 2DEG channel is largely unaffected by current sensing. Ultraviolet (UV) photodetectors have adverse dopant scattering and diffusion effects at elevated applications in radiative heating characterization during temperatures. Previous studies on the high-temperature atmospheric entry. This data can be used to improve thermal operation of GaN heterostructure devices in inert and air protection systems (TPS) on future Venus surface landers. ambient are promising [8]–[16]. InAlN/GaN high electron Furthermore, these three GaN heterostructure device mobility transistors (HEMTs) have been reported to operate architectures have compatible fabrication flows, which for 25 hours at 1000°C in vacuum [16]. would allow for monolithic integration of uncooled sensors with signal readout and transmission. Future maturation of Even with these recent breakthroughs, there have been few this technology could substantially lower mission weight and studies of the effects of long-term exposure to a Venus cost by reducing packaging and shielding requirements. surface environment, where the atmosphere is ~97% supercritical CO2, on wide band-gap electronics [1], [17]. In 2. DEVICE FABRICATION AND OPERATION the most rigorous experiment to date, the stable operation of SiC junction field effect transistor (JFET) integrated circuits The HEMT devices were fabricated at the Stanford (ICs) under Venus surface atmospheric conditions was Nanofabrication Facility on an indium aluminum nitride demonstrated for 60 days [18]. The devices were directly (In0.18Al0.82N)/GaN-on-silicon wafer grown via metal- exposed (e.g. no packaging or cooling) inside the NASA organic chemical vapor deposition (MOCVD) and supplied Glenn Extreme Environment Rig (GEER) and tested in situ by NTT Advanced Technology Corporation. A cross- over the duration. The ICs experienced a small increase in sectional diagram of the device and heterostructure is shown frequency, primarily over the first 20 days of exposure, which in Figure 1(a). Starting from the Si (111) wafer, the stack the authors attributed to burn-in. No comparable assessment consisted of a 300-nm-thick buffer layer, 1-μm-thick GaN of GaN electronics operating under Venus surface layer, 0.8-nm-thick AlN spacer, and a 10-nm-thick InAlN Figure 1. Cross-sectional schematic of the (a) InAlN/GaN HEMTs, (b) InAlN/GaN Hall-effect sensors, and (c) AlGaN/GaN UV photodetectors exposed to the Venus-simulant environment. Interfaces where the 2DEG is present are represented by white dashed line. 2 barrier layer. HEMT active regions were isolated through a readout. The sensitivity of a Hall-effect device with respect BCl3/Cl2 inductively coupled plasma (ICP) reactive ion etch to supply current is referred to as the current-scaled (RIE). Ti/Al/Mo/Au (10/200/40/80 nm) ohmic contacts were sensitivity (Si) and is inversely proportional to sheet carrier electron beam evaporated, patterned via lift-off process, and concentration, rapid thermal annealed (RTA) at 850°C for 35 seconds in N2. Iridium (15 nm) was evaporated and patterned as the = = (2) Schottky contact, followed by Ti/Au (20/200 nm) bond metal evaporation and patterning. The Ir gate metal was oxidized at 300°C for 5 minutes via RTA to form IrOx. The use of IrOx A reduction in the sheet carrier density of the 2DEG increases gates on GaN HEMTs has been reported to reduce leakage the sheet resistance of the device, leading to a drop in current currents compared to as-deposited Ir gates, and high- under constant voltage conditions, and thereby increasing