AlN/GaN/AlN quantum-well (QW) HEMTs have emerged as a promising device technology for high-power applications such as next-generation communication networks and military defense systems [1-2]. Thanks to their strong carrier confinement, high thermal conductivity, and high breakdown electric field of the AlN back barrier, QW-HEMTs are promising for highly scaled high-speed transistors capable of power amplification at high temperatures. Moreover, HEMTs grown on single-crystal AlN substrates would provide higher reliability thanks to their low dislocation densities (~104 /cm2) and the absence of thermal boundary resistance with the substrate in contrast to AlN on SiC, sapphire or other substrates. However, in thin GaN QWs along a crystal polar direction, a strong internal electric field can arise due to large polarization effects. The important consequences of the vertical electric field and compressive strain in the electronic transport of the thin GaN channel is not yet fully understood. Therefore, in this report we present a combined experimental and theoretical study concerning homoepitaxial AlN/GaN/AlGaN QW-HEMTs, in which both the internal electric field as well as the strain in the GaN channel are systematically varied simultaneously. A series of AlN/GaN/AlGaN QW HEMTs with different QW thickness, ranging from 20 to 200 nm, are grown by molecular beam epitaxy on single-crystal AlN substrates [See Fig. 1(a)]. As the QW thickness increases, the vertical electric field in the GaN channel decreases, resulting in a re-distribution of the electronic wavefunction. These effects can be clearly seen from the self-consistent Schrödinger–Poisson simulations displayed in Fig. 1(b). Structural analysis was performed using atomic force microscopy (AFM) and high-resolution X-ray diffraction (HRXRD). The symmetric ω/2θ diffraction pattern, shown in Fig. 1(c), and the reciprocal space map (RSM) around the asymmetric (-105) reflection, displayed in Fig. 1(d), indicate the relaxation process of GaN layer as the channel thickness increases. The AFM scans, displayed in Fig. 2, reveal smooth surface morphologies with sub-nanometer root-mean-square roughness for all the samples. While clear atomic steps are observed on the HEMT with a 20-nm-thick GaN well, spiral hillocks start to appear on QW HEMTs with thicker GaN layers; this can stem from screw dislocations formed due to GaN relaxation. For both samples featuring a 50-nm-thick GaN well and a 200-nm-thick GaN well, the formation of visible cracks is observed. To examine the transport properties of the 2DEGs, hall-effect measurements were carried out at room temperature and 77 K. As shown in Fig. 3(a), the electron density increases and saturates with increasing GaN thickness, reaching 4.7x1013 /cm2. Meanwhile, a room-temperature sheet resistance of Rsh ~ 340 Ω/sqr was achieved in QW HEMTs with both thin and thick channels. To put these results in context, Fig. 3(b) compares the room temperature transport properties of all the reported 2DEGs in Al(Ga)N/GaN/Al(Ga)N heterostructures. A clear trend relating GaN thickness and mobility is observed. The high mobility HEMTs are achieved in thick and relaxed GaN layers with middle to low carrier densities. Since our samples are located within the high 2DEG density region, there exists room for further mobility improvement by lowering the carrier density in the channel. [1] Hickman et al. Semicond. Sci. Technol. 36 044001 (2021) [2] Yaita et al. IEEE Electron Device Lett., vol. 42, no. 11, pp. 1592–1595, Nov. 2021

High Electron Density AlN/GaN/AlGaN Quantum-Well HEMTs on Al-Polar Single-Crystal AlN Substrates

In this work, epitaxial wurtzite phase BAlN films were grown by nitrogen plasma molecular beam epitaxy on different wurtzite phase substrates (e. g. nitrided sapphire, β-Nb2N film, GaN template). Effect of different growth parameters on composition of BAlN films was explored. B incorporation was found to increase with decreasing substrate temperature and with increasing B flux. In-situ electron diffraction and ex-situ x-ray diffraction measurements confirm the growth of wurtzite BAlN films at 300 in plane orientation with sapphire substrate and at 00 orientation with bottom Nb2N films. With increasing boron content in BAlN films, the c-parameter decreases while the a-parameter shows a non-monotonic trend. Ferroelectricity (with remnant polarization of ~16 μC.cm–2 and coercive field of ~1.45 MV.cm-1) and high dielectric constant (of ~18 at 500 kHz) were also observed in these BAlN films. A slight reduction in bandgap is seen with increasing boron content in BAlN films (from ~6.12eV for AlN films to ~5.95eV for B0.15Al0.85N film). This work also reports the first demonstration of polarization-induced two-dimensional electron gas (2DEG) in the epitaxial BAlN/GaN heterojunctions. Observation of ferroelectricity with low coercive field, remnant polarization (lower than the sputtered BAlN [1] and comparable to MBE grown AlScN films [2]) and high-k, makes BAlN an attractive barrier in GaN high electron mobility transistors (HEMTs). Therefore, BAlN/GaN heterostructures were grown on 2” and 3” wafers; showing polarization-induced 2DEG with good sheet resistance of ~650 ohm/sq, ~400-500 cm2/V.s mobility (400-500 cm2/V.s), and 1.4-1.8 e13 cm-2 charge. The results presented in this paper demonstrates MBE growth of BAlN films and heterostructures with interesting properties that would enable new electronic devices. These films could be useful for combining memory and logic functionalities in digital electronics and to mingle epitaxial ferroelectric and high-k barrier technology into RF and mm-wave power electronics. For instance, a new class of ferroelectric HEMTs devices can be formed by integrating epitaxial ferroelectric, high-k nitride barrier with III-V nitride semiconductors [3].

MBE Growth, Characterization of BAlN Films and 2D Electron Gas in Epitaxial BAlN/GaN Heterojunction

A: Group III–Nitrides—Devices

technical paper

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EMC 2023

The Electronic Materials Conference (EMC) is the premier annual forum on the preparation, characterization and use of electronic materials. 

Yu-Hsin Chen

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High Electron Density AlN/GaN/AlGaN Quantum-Well HEMTs on Al-Polar Single-Crystal AlN Substrates

MBE Growth, Characterization of BAlN Films and 2D Electron Gas in Epitaxial BAlN/GaN Heterojunction

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