Semiconducting AlN Electrical Devices

Aluminum nitride (AlN) is a material of great interest for high performance power electronics, extreme environment semiconductor devices, radio frequency devices, and deep ultraviolet (DUV) optoelectronics due to its excellent electrical and thermal properties. Compared to other commonly used semiconductors (i.e., Si, SiC, GaN, and β-Ga2O3), AlN has the highest critical electric field and theoretical breakdown voltage, which lead to the highest Baliga’s and Johnson’s figures-of-merit (BFOM & JFOM)3. AlN also has the second highest saturation velocity, and thermal conductivity among semiconductors with a commercially available substrate3. However, AlN has traditionally only been an insulator without the ability to be converted to a semiconductor via doping. We recently demonstrated substantial bulk doping (carrier concentrations above 1018 cm-3) for both p- and n-type AlN using Be and Si dopants, respectively. From these results, we also demonstrated rectification and a 6 V turn-on in AlN PN diodes1,2,3.<br/><br/>Electrical characterization of homojunction aluminum nitride (AlN) PIN diodes is presented along with an update on the prospects for an AlN FET. Recently, using the metal modulated epitaxy (MME) variation of molecular beam epitaxy that controls the surface chemistry to enable low temperature, high quality epitaxy, substantial bulk n- and p-doping in AlN as well as AlN homojunction PN diodes are demonstrated, overcoming the longstanding barrier to doping of AlN1,2. P-type (p=4.4x1018 cm-3, 0.04 ohm-cm ) and N-type (n=3x1018 cm-3, 0.02 ohm-cm) bulk conducting AlN is demonstrated with a narrow window for substrate temperatures for achieving semiconducting AlN. With proper surface cleaning procedures, MME grown AlN is indistinguishable from the HVPE AlN on sapphire template via TEM. Current-voltage-temperature (IVT) data from 25 °C to 300 °C is used to investigate these diodes. Diodes with poor substrate cleaning shows a kinked, temperature independent IV behavior in the low forward voltage range associated with defect assisted tunneling originating from stacking faults near the re-growth interface. No such kink, no stacking faults but still temperature independent IV is observed for properly cleaned substrates. These quasi-vertical devices consist of 1 μm highly n-type AlN:Si, a lightly Si-doped intrinsic AlN layer with varied thickness, and a 200 nm highly p-type AlN:Be.<br/><br/>IVT measurements from 25 °C to 300 °C in deep forward bias is presently limited by high series resistance originating from contacts to the plasma etched n-type AlN layer which has ~100x higher contact resistance than as-grown n-type AlN. However, the current increases by ~2 orders of magnitude as temperature increases with contact limited specific-resistance improving by &gt;250x with an activation energy of ~0.3 eV. Forward current below the turn on voltage appear to be dominated by tunneling as suggested by temperature independence. Transistor structures will be discussed and at present are, again, contact limited to ~0.1 A/mm and only modest current modulation. Contact metallization, surface preparation of the AlN templates, and various device geometry parameters as important areas for future optimization in order to allow AlN devices to eventually push the limits in the field power electronics.<br/><br/>References:<br/>1 H. Ahmad, J. Lindemuth, Z. Engel, C. Matthews, T. McCrone, W.A. Doolittle, Adv. Mater. 33, 2104497 (2021).<br/>2 H. Ahmad, Z. Engel, C.M. Matthews, S. Lee, and W.A Doolittle, J. Appl. Phys. 131, 175701 (2022).<br/>3 W. Alan Doolittle, et al, Accepted in Appl. Phys. Lett. (2023)