MBE Growth of GaN Heterostructures for High Performance HEMTs

<p>GaN-based high electron mobility transistors (HEMTs) are promising for high speed and high power electronics applications. AlN and In_0.17Al_0.83N enable maximum possible vertical scaling of barriers in Ga-polar GaN-based HEMTs for high speed performance. High quality epitaxial growth of these barriers is a pre-requisite for superior device performance. In this work, plasma-assisted molecular beam epitaxy (MBE) was used to study the epitaxial growth of AlN/GaN and InAlN/GaN heterostructures. For a single AlN/GaN heterojunction (with sheet electron density of ~ 1.73x10¹³ cm^-2) we measured Hall-effect mobility of ~ 1834/11460 cm²V^-1s^-1 (at room temperature/10 K). This is among the highest values reported in literature for single AlN/GaN heterojunctions and indicates the superior quality of these MBE grown films. For double AlN/GaN heterojunctions, we measured (under single channel approximation) very high mobility ~ 1890/14430 cm²V^-1s^-1 (at room temperature/10 K) and sheet electron density of ~ 1.87x10¹³ cm^-2 at room temperature. This is the highest mobility reported to date for multiple AlN/GaN heterojunctions. Employing these films we demonstrated double-channel metal-oxide-semiconductor HEMTs (DC-MOSHEMT). This is the first report of AlN/GaN heterojunction based DC-MOSHEMT in literature to the best of our knowledge. By implementing top barrier recess and regrown contacts in future, these double barrier heterostructures may be employed to reduce DC - RF dispersion in III-nitride HEMTs. </p> <p>To explore another novel approach of solving this problem, we investigated low temperature (LT-) MBE growth of AlN for in-situ surface passivation of HEMT structures. We found a window below 250 °C to grow partially amorphous LT-AlN. For a LT-AlN capped AlN/GaN HEMT, we report a low DC-RF dispersion with gate lag and drain lag below 2%. Additionally, we investigated crystalline AlN growth at a low temperature of ~ 480 °C. This was motivated by novel MBE grown AlN barrier HEMT structures with high mobility InGaN channels for Terahertz applications. </p> <p>To fully exploit the potential of MBE grown high quality HEMT structures, low resistance (< 0.1 Ω.mm) regrown source/drain ohmic contacts are essential. From a systematic study using MBE grown bulk n⁺ GaN films, effect of Si doping concentration on contact resistance was analyzed.</p>