Unlike silicon and traditional III-V semiconductors, the III-nitrides exhibit high spontaneous and piezoelectric polarization charges at epitaxial polar heterojunctions. In the process of investigating scaling properties of gate-stacks consisting atomic-layer deposited Al 2 O 3 /III-Nitride heterojunctions, we find interface charges that appear closely linked to the polarization charges of the underlying nitride substrate. Through capacitance-voltage measurement on a series of samples of varying dielectric thicknesses, we find the presence and propose an origin of benign donor-type interface charges (Q it $6 Â 10 13 cm À2 ) at the AlN/Al 2 O 3 junction. This interface charge is almost equal to the net polarization charge in AlN. The polarization-related dielectric/AlN interface charge and the role of oxygen in the dielectric as a possible modulation dopant potentially offer opportunities for various device applications.GaN high electron mobility transistors (HEMTs) outperform Si devices for high voltage switching by virtue of their large bandgap and additionally possess the potential for very high speed switching. This requires highly scaled low sheetresistance HEMT structures with very thin barriers. However, ultrathin epitaxial barriers (such as AlN or InAlN) result in substantial leakage currents preventing the capability to block high drain voltages, and dielectrics can substantially mitigate this problem. Thus, dielectrics such as SiO (Ref. 4) are being investigated intensively both for composite gate stacks as well as for the suppression of current collapse 5 by passivating surface states in these devices. Atomic layer deposited (ALD) Al 2 O 3 has drawn the attention of the community due to its large bandgap and outstanding dielectric 6 and passivation 7 properties. The superior quality (in terms of uniformity) of ALD over sputtering and electron-beam deposition, coupled with high band gap ($6.5 eV), 8 high dielectric constant ($9.1), high break down field ($10MV/cm), high thermal (amorphous $1000 C), and chemical stability of ALD-grown Al 2 O 3 makes it a natural choice as a gate insulator for AlN/ GaN HEMTs (Ref. 9) and its variants. The study of the ALD Al 2 O 3 /III-nitride interface is of prime importance for device characteristics of AlN/GaN HEMTs. In this work, we present a comprehensive characterization of AlN/GaN MOS-HEMT gate stacks with ALD Al 2 O 3 of various thicknesses. Through capacitance-voltage (C-V) measurement, we find the presence and propose an origin of benign donor-type positive interface charge (Q it ) at the AlN/Al 2 O 3 junction and relate its presence to the polarization charges in AlN. The presence of Q it explains the trend of pinch-off voltage and twodimensional electron gas (2DEG) density with ALD thicknesses both qualitatively and quantitatively. Recent report 10 (appeared after this submission) on ALD/GaN structure also invokes positive interface charge to explain the trend of pinch-off voltage with ALD thicknesses. AlN/GaN HEMT structures were grown in a Veeco Gen 930 molec...

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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...

Double heterostructures of strained GaN quantum wells (QWs) sandwiched between relaxed AlN layers provide a platform to investigate the quantum-confined electronic and optical properties of the wells. The growth of AlN/GaN/AlN heterostructures with varying GaN quantum well thicknesses on AlN by plasma molecular beam epitaxy (MBE) is reported. Photoluminescence spectra provide the optical signature of the thin GaN QWs. Reciprocal space mapping in X-ray diffraction shows that a GaN layer as thick as $28 nm is compressively strained to the AlN layer underneath. The density of the polarization-induced two-dimensional electron gas (2DEG) in the undoped heterostructures increases with the GaN QW thickness, reaching $2.5 Â 10 13 /cm 2. This provides a way to tune the 2DEG channel density without changing the thickness of the top barrier layer. Electron mobilities less than $400 cm 2 /Vs are observed, leaving ample room for improvement. Nevertheless, owing to the high 2DEG density, strained GaN QW field-effect transistors with MBE regrown ohmic contacts exhibit an on-current density $1.4 A/mm, a transconductance $280 mS/mm, and a cut off frequency f T $ 104 GHz for a 100-nm-gate-length device. These observations indicate high potential for high-speed radio frequency and high voltage applications that stand to benefit from the extreme-bandgap and high thermal conductivity of AlN. V

The role of spontaneous and piezoelectric polarization in III-V nitride heterostructure devices is discussed. Problems as well as opportunities in incorporating polarization in abrupt and graded heterojunctions composed of binary, ternary, and quaternary nitrides are outlined.