Molecular beam epitaxy of single-phase wurtzite N-polar ScxAl1−xN (x ∼ 0.11–0.38) has been demonstrated on sapphire substrates by locking its lattice-polarity to the underlying N-polar GaN buffer. Coherent growth of lattice-matched Sc0.18Al0.82N on GaN has been confirmed by x-ray diffraction reciprocal space mapping of the asymmetric (105) plane, whereas lattice-mismatched, fully relaxed Sc0.11Al0.89N and Sc0.30Al0.70N epilayers exhibit a crack-free surface. The on-axis N-polar crystallographic orientation is unambiguously confirmed by wet chemical etching. High electron mobility transistors using N-polar Sc0.18Al0.82N as a barrier have been grown on sapphire substrates, which present the existence of well confined two-dimensional electron gas. A Hall mobility of ∼564 cm2/V s is measured for a 15-nm-thick Sc0.18Al0.82N barrier sample with a sheet electron concentration of 4.1 × 1013 cm−2, and the corresponding sheet resistance is as low as 271 Ω/sq. The polarity-controlled epitaxy of ScxAl1−xN provides promising opportunities for applications in high-frequency and high-power electronic and ferroelectric devices.
GaN-based high electron mobility transistors (HEMTs) have demonstrated high frequency power amplification with considerably larger output power densities than that available from amplifiers based on other material systems such as GaAs or InP. To further increase the operating frequency while maintaining the high output power in HEMTs, the gate-to-channel distance needs to be reduced significantly. This leads to a reduced two-dimensional electron gas (2DEG) density (ns) and mobility (μ) in Ga polar HEMT structures resulting in a larger sheet resistance. This work demonstrates that by proper design of the back-barrier in N-polar GaN-based scaled-channel HEMT structures, a high 2DEG density can be maintained while scaling the channel thickness. Scaled-channel GaN-based HEMT structures with an AlN/GaN (0.5 nm/1.5 nm) digital alloy as the back-barrier were grown on an on-axis N-polar GaN substrate via plasma-assisted molecular beam epitaxy. A record high electron mobility of 2050 cm2/vs was achieved on an N-polar HEMT structure with a 10 nm-thick channel, while maintaining 8 × 1012 cm−2 2DEG density. By modifying the barrier structure, we demonstrated a combination of 2DEG density and a mobility of 1.7 × 1013 cm−2 and 1420 cm2/V s, respectively, leading to a record low sheet resistance of ∼258 Ω/□ on 7 nm-thick channel N-polar HEMT structures.
Electron transport in N-polar GaN-based high-electron-mobility transistor (HEMT) structures with a combination of In0.18Al0.82N-AlN as the barrier was studied via temperature-dependent van der Pauw Hall and Shubnikov de Haas measurements. In contrast to Ga-polar HEMT structures, no persistent photoconductivity could be detected. In a sample with 10 nm thick InAlN, only one oscillation frequency was observed, demonstrating that a single sublevel is present. From the oscillations, a two-dimensional electron gas carrier density of 8.54 × 1012 cm−2 and a mobility of 4970 cm2/V s were extracted at 1.7 K. This sample was further investigated using ionic liquid gating. The charge density was varied from 7.5 × 1012 cm−2 to 9.6 × 1012 cm−2. The electron mobility significantly declined with decreasing charge density. This is in contrast to Ga-polar HEMT structures, where the electron mobility typically increases slightly as the charge density decreases.
It has been previously shown that hole/donor traps at the (Al, Ga) N/GaN interface cause DC-RF dispersion and output conductance in N-polar GaN high electron mobility transistors (HEMTs). In this work, we systematically studied the impact of hole trap energy and density on two-dimensional electron gas (2DEG) density using Silvaco Atlas. Simulation results revealed that the exclusion of the hole traps in the model results in an underestimation of 2DEG density compared to experimentally obtained 2DEG density. By comparing simulations with the experimental results, a hole trap with a density of 3 × 1013 cm−2 at 280 meV above the valence band at the AlN/GaN negative polarization interface was estimated. Three different silicon doping schemes were then examined to suppress the effect of traps. Delta doping (15 nm) at the GaN buffer and barrier interface (doping Scheme-A) is effective in compensating traps present at that interface but insufficient to compensate traps near the GaN channel. Similarly, doping the back-barrier (doping scheme-B) is sufficient to neutralize traps in the middle of the back-barrier and close to the channel but inadequate to neutralize traps at the buffer–barrier interface. Series-C doping employs a combination of doping schemes A and B that effectively neutralizes traps present at all interfaces while simultaneously modulating the 2DEG charge density. An ultra-scaled 5-nm-thick GaN HEMT epitaxial structure was also designed by band engineering that can maintain high 2DEG density in the channel (2 × 013 cm−2) with less than 5% parasitic charge and trap ionization over a wide range of doping from 6 × 1018 cm−3 to 1 × 1019 cm−3.
In this paper, we report on the observation of self-assembled InGaN/(In)GaN superlattice (SL) structure in a nominal “InGaN” film grown on N-polar GaN substrate. 350 nm thick InGaN films were grown at different temperatures ranging from 600 to 690 °C. Structural characterization was conducted via atomic force microscopy, scanning transmission electron microscopy, high-resolution x-ray diffraction, and XRD reciprocal space map. A SL structure was unexpectedly observed on all samples. However, the In content in each layer varied depending on growth temperature. By increasing the substrate temperature to 670 °C, a periodic structure composed of 3 nm In0.26Ga0.74N and 3 nm of GaN with a surface roughness of ∼0.7 nm was achieved. This work establishes a method for the growth of InGaN films with high structural quality on N-polar GaN and opens a new pathway for the design and fabrication of various electronic and optoelectronic devices with enhanced performance.
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