Electronics on Single-Crystal, Bulk AlN Substrates

Abstract

AbstractThe quality of the substrate wafer on which a semiconductor device is fabricated plays a critical role in determining the final device performance and cost. Due to lack of availability of cheap, native substrates, the field of III-nitride semiconductor electronic devices has heavily relied on foreign substrates such as silicon carbide (SiC), sapphire and silicon (Si) for GaN-based RF and power transistors. Recently, the commercial opportunity offered by AlN-based optoelectronics devices such as deep-UV (DUV) LEDs and LASERs have renewed interest in development of single-crystal, bulk AlN substrates. It is well understood how these AlN-based optoelectronic devices will perform better on single-crystal AlN substrates due to orders of magnitude fewer defects and dislocations. The projected higher performance outweighs the high initial costs, thereby driving the research and development of single crystal AlN substrates. The integrated RF electronics platform on AlN is also expected to benefit from this development effort. This chapter quantitatively summarizes the advantages that a native, single-crystal AlN substrate will bring to the AlN RF electronics platform. Since the AlN platform utilizes a thick AlN buffer layer, epitaxial growth on bulk AlN substrates is the structurally purest realization of the GaN/AlN 2DHG and AlN/GaN/AlN 2DEG-2DHG bilayer heterostructures. This is because the epitaxial AlN grown on the current state-of-art single-crystal AlN substrates have 6 orders of magnitude fewer threading dislocations than AlN grown on SiC or Sapphire. For an RF transistor, this theoretically translates to higher breakdown voltages, higher on currents during high-frequency operation and better thermal performance. The absence of any thermal boundary between the buffer layer and substrate enables effective thermal management and therefore more energy efficient operation of the transistor. Initial experimental efforts have yielded enhanced 2D electron and hole mobilities at low temperatures in AlN-based heterostructures grown on single-crystal AlN substrates. These point to successful homoepitaxial AlN growth and reduced dislocation scattering of the carriers, which are promising steps towards an high-performance RF device. With AlN-based RF transistors on foreign substrates already demonstrating record high performance, native single-crystal AlN substrates will push the performance even higher and truly unleash the power of the AlN platform.KeywordsBulk substratesAlNHEMTp-FET2DEG2DHGGaNHeat sinkPhononsThreading dislocationsCrystal qualityXRDCurrent collapseTrapsGate leakageMobilities