Strained GaN quantum-well FETs on single crystal bulk AlN substrates

Abstract

We report the first realization of molecular beam epitaxy grown strained GaN quantum well field-effect transistors on single-crystal bulk AlN substrates. The fabricated double heterostructure FETs exhibit a twodimensional electron gas (2DEG) density in excess of 2×10 13 /cm 2 . Ohmic contacts to the 2DEG channel were formed by n + GaN MBE regrowth process, with a contact resistance of 0.13 Ω·mm. Raman spectroscopy using the quantum well as an optical marker reveals the strain in the quantum well, and strain relaxation in the regrown GaN contacts. A 65-nm-long rectangular-gate device showed a record high DC drain current drive of 2.0 A/mm and peak extrinsic transconductance of 250 mS/mm. Small-signal RF performance of the device achieved current gain cutoff frequency f T ∼ 120 GHz. The DC and RF performance demonstrate that bulk AlN substrates offer an attractive alternative platform for strained quantum well nitride transistors for future high-voltage and high-power microwave applications.State-of-art gallium nitride based electronic devices have demonstrated excellent performance in highfrequency and high-power applications 1-5 . These devices are on thick GaN buffer layers, most of which are on SiC substrates for efficient thermal dissipation. The heteroepitaxially grown GaN layers inherently incorporate high density of dislocations (typically ∼ 10 9 /cm 2 ), which give rise to reliability issues and degrade breakdown characteristics.In this letter we show that thin strained GaN quantum well double heterostructures on bulk AlN substrates offer an attractive alternative approach for high-performance nitride electronics. To meet the scaling requirements for high-speed high-power RF applications, tight electrostatic control and quantum confinement of charge carriers are highly desired. The wide direct band gap of AlN (∼6.2 eV) and its large band offset with GaN offers the maximal vertical confinement of carriers in nitride channels. The large polarization charge of AlN induces high density two-dimensional electron gases (2DEG) in the quantum well, which is desired for high current drive and lateral scaling of gate lengths. The thermal conductivity of AlN, estimated 6 to be as high as ∼340 W/m·K can be comparable 7 to that of SiC ∼370 W/m·K, and offers the potential benefit of reducing thermal boundary resistance. Thus AlN simultaneously satisfies the conflicting requirements of high thermal conductivity and high electrical resistivity for high-power microwave electronics. Importantly, the low dislocation density (∼ 10 4 /cm 2 ) of single-crystal bulk AlN substrates has the potential for defect-free channels and barriers in principle, which is a promising prospective for improving thermal robustness, reliability, breakdown, and noise characteristics of these devices 8 . The presence of a very thin GaN quantum well active region embedded in the AlN/GaN/AlN double heta) Electronic mail: djena@cornell.edu. erostructure also enables selective optical-marker based Raman metrology of strain 9 present in the active layer...

Ultrathin AlN‐Based HEMTs Grown on Silicon Substrate by NH3‐MBE

“…Contrary to the 2DEG density, the 2DEG mobility is not affected by the GaN channel thickness, at least in the investigated range. Figure shows a benchmark of the mobility as a function of the 2DEG density for different AlN‐based HEMTs grown on various substrates . The mobility values are almost constant, from 622 to 636 cm 2 V −1 s −1 , for the studied structures grown on Si.…”

“…It is usually hard to obtain such low contact resistance without using a regrown Ohmic contact step. For example, the regrowth of highly n‐type doped GaN allows to get 0.13 Ω mm on very similar AlN‐based HEMTs grown on AlN substrate . Concerning HEMTs with an AlN barrier on thick GaN buffer layer on Si, usually, an additional etching step of the cap is necessary, prior to the metal deposition and annealing at>850°C is needed to routinely achieve values around 0.35 Ω mm…”

“…Aluminum nitride‐based high electron mobility transistors (HEMTs) are original and promising structures for the next generation of power and RF electronics. This concept has been introduced by Jena et al, where a thin GaN channel is sandwiched in between an AlN barrier and an AlN buffer layer. The originality and the key of this structure is to compressively strain the GaN channel on AlN, in order to grow an almost strain free several nanometer thick AlN barrier.…”

“…It demonstrates the potential of using ultrathin AlN‐based epistacks on Si. To further explore this potential, we decide to implement the concept of AlN‐based HEMTs, recently proposed by Jena et al, on silicon substrate, where the thin GaN channel and the AlN barrier are directly grown on the AlN buffer layer. The total epitaxial thickness is mainly limited by the thickness of AlN that can be grown before cracking, which is typically below 350 nm.…”

Microstructure and mechanical property of high growth rate SiC via continuous hot‐wire CVD

“…In recent years, its outstanding irradiation resistance and biocompatibility have been widely recognized, which make it a promising candidate material for nuclear fusion reactors and biomedical applications . Besides, as one of the competitive third‐generation semiconductor materials, its wide bandgap and high saturated electron drift velocity enable SiC devices to be applied in high voltage modulus of electric vehicles and components in solar power system . Although a high growth rate is crucial to meet the rising market demand and reduce the cost in mass production, the associated growth defects would not only limit its electronic performance, also severely degrade the mechanical properties of SiC filament.…”