A combination of high electron saturation velocity, high electron mobility, large breakdown field, and high electron density can be simultaneously achieved in GaN high electron mobility transistors (HEMTs), making them attractive for high-power, high-frequency power amplifiers. N-polar GaN epi-structures offer scaling of the gate-to-channel distance without losing 2DEG density and a built-in back-barrier for better pinch-off performance. Additionally, N-polar GaN enables the implementation of a thick in-situ GaN-cap layer, which keeps surface states away from the channel and minimizes their effect on device performance. By using N-polar HEMT epi-structure with a GaN cap (deep recess HEMT) 8 W mm−1 output power with 27.8% power added efficiency (PAE) at 94 GHz has been demonstrated, which is more than twice the output power and efficiency of conventional Ga-polar AlGaN/GaN HEMTs [1]. Despite numerous benefits, there are challenges associated with N-polar deep recess HEMTs. Dry etching used in recess etching causes surface damage in the gate area and the trench sidewalls leading to high gate leakage and low breakdown voltage. These issues are currently addressed by inserting SiNx gate dielectric deposited via metal organic chemical vapor deposition (MOCVD), a high temperature deposition technique that could be a limiting factor in device fabrication and integration. Moreover, inserting a thick dielectric layer under the gate increases the gate-to-channel distance, which is not desirable in scaled devices. Therefore, there is a need for developing a gate insulator with a high dielectric constant with low bulk and interface trap density. Moreover, a low-temperature deposition technique such as ALD allows for more flexibility in the fabrication process and is more desirable for manufacturing. Recent studies in our group have shown the successful application of ALD- HfO2 as gate dielectric in N-polar planar GaN HEMTs. [2] In this work, for the first time, a thermal-ALD HfO2 was used as the gate dielectric in N-polar deep recess GaN HEMTs. DC, pulsed IV, and small signal measurements were conducted to evaluate the device performance. Results show low gate leakage current, high transconductance, high dielectric breakdown field, and low dispersion, which indicates a high quality dielectric and dielectric/GaN interface. This work shows the potential for HfO2 as a gate insulator in N-polar deep recess HEMTs with a scaled channel. MOCVD-grown N-polar deep recess GaN HEMT structure with a 12 nm thick GaN channel and a 47.5 nm thick GaN-Cap was used for this study. The devices presented here have a 100 μm gate width, and 1.75 μm source to drain distance. The etched region length for the gate recess is 200 nm and placed 650 nm away from the source. Ti/Au gate metal has 450 nm thickness, 900 nm length, and 300 nm away from the source. DC measurements showed 0.9 A/mm maximum drain current, 2 Ohm.mm on-resistance, and 2.2 V knee voltage at 1 V gate bias. The threshold voltage was -1.5 V. The maximum transconductance of 420 mS/mm was measured at VG of -0.75 V and VD of 3 V. Gate leakage current of 140 nA/mm, and a 6.2 MV/cm dielectric breakdown field. More importantly, near zero hysteresis was observed in DC measurements. Pulsed IV measurements revealed less than 5% dispersion. Unity current gain frequency (fT) of 30.5 GHz and 53 GHz maximum oscillation frequency (fMAX) were measured. These results show the potential of using HfO2 as the gate dielectric in the future ultra-scaled N-polar deep recess GaN HEMTs. [1] B. Romanczyk et al., "W-band N-polar GaN MISHEMTs with high power and record 27.8% efficiency at 94 GHz," 2016 IEEE International Electron Devices Meeting (IEDM), 2016, pp. 3.5.1-3.5.4, doi: 10.1109/IEDM.2016.7838339. [2] S. Mohanty, A. Jian, K. Khan, and E. Ahmadi, “Demonstration of N-polar GaN MIS-HEMT with high-k atomic layer deposited HfO 2 as gate dielectric,” Accepted to appear in Journal of Electronic Materials (JEMS).

N-Polar GaN Deep Recess HEMTs with ALD HfO2 as Gate Dielectric

Abstract Body: The (In,Ga)N material system is attractive for photovoltaic applications as its optical absorption covers the solar spectrum. Thick InGaN films (hundreds of nm thick) with InN mole fraction larger than 10% are required for this purpose. However, the growth of high In-content InGaN films remains challenging due to 10% lattice mismatch between InN and GaN and the large difference between the thermal stability of GaN and InN. While the optimum growth temperature of GaN by plasma-assisted molecular beam epitaxy (PAMBE) is ~700 °C-730 °C, the growth temperature of InGaN is typically below 600 °C. Therefore, lower growth temperatures are required to grow InGaN films within the miscibility gap. InGaN films can be grown pseudo-morphically on GaN below a critical thickness. As the thickness increases beyond the critical thickness, the InGaN film relaxes. This relaxation may occur through different mechanisms depending on the growth technique and growth conditions (e.g., growth temperature). V-defect formation from threading dislocations is a common way of strain relaxation, especially in InGaN films grown via metal organic chemical vapor deposition, and has been observed in molecular beam epitaxy (MBE) grown InGaN films as well [1]. Pyramidal slip through the Matthews-Blakeslee mechanism and basal plane slip of misfit dislocation are other mechanisms through which strain relaxation occurs in InGaN films. Critical thickness reduces as InN mole fraction in InGaN increases. Bazioti et al [2] studied relaxation mechanism in InGaN films grown by PAMBE. They showed that at lower indium contents, strain relaxation dominantly occurred via formation of V-defects, with concurrent formation of an indium-rich interfacial zone. As indium content increased, this mechanism was replaced by the introduction of a self-formed strained interfacial InGaN layer of lower indium content, as well as multiple intrinsic basal plane stacking faults and threading dislocations in the rest of the film. In this talk, we will present growth of 700 nm-thick InGaN with an average InN mole fraction of ~8% coherently strained to GaN confirmed by XRD reciprocal space map scans. This is several times thicker than the critical thickness of In0.08Ga0.92N. Surprisingly, we did not observe generation of new threading dislocations in the InGaN films or misfit dislocations at the interface. Instead, we show that the strain is managed by self-formation of a superlattice structure. The superlattice was confirmed via scanning transmission electron microscopy and atom probe tomography. Photoluminescence (PL) measurements at room temperature revealed peak wavelength of ~450 nm, corresponding to In0.18Ga0.82N with a full width half max (FWHM) of ~36 nm. Reference: 1. C. Bazioti et al. Structure and strain variation in InGaN interlayers grown by PAMBE at low substrate temperatures Phys. status solidi 252 1155–62 (2015). 2. C. Bazioti et al. Defects, strain relaxation, and composition grading in high indium content InGaN epilayers grown by molecular beam epitaxy J. Appl. Phy. 118,155301 (2015).

Demonstration of Self-Assembled InGaN/GaN Superlattice on GaN Template Grown by Plasma-Assisted Molecular Beam Epitaxy

Abstract Body: The (In,Ga)N alloy system is attractive for optoelectronic applications as it enables direct bandgap tuning within a large range (0.7 eV–3.4 eV). While highly efficient GaN-based blue light emitting diodes (LEDs) have been commercially available for a decade, GaN red LEDs suffer from low efficiency. Red LEDs require high In content InGaN (>35%) QWs. Growth of high In-content InGaN on the GaN substrate is challenging due to high vapor pressure of InN. InN starts decomposing at 500 °C which is well below optimum GaN growth temperature of ~700 °C in molecular beam epitaxy (MBE). There is a large lattice mismatch between InN and GaN (∼10%). Therefore, there is a great deal of interest in developing relaxed InGaN buffer as pseudo-substrate for optoelectronics applications. Various techniques have been employed in the past to obtain relaxed InGaN buffer layers including graded InGaN transition film and growth of InGaN on micrometer-size tiles of porousified GaN. Growth on oxide substrates, such as ScAlMgO4 and ZnO have also been explored as an alternative. ZnO has the same crystal structure as (In,Ga)N (wurtzite) and is in-plane lattice-matched with In0.2Ga0.8N [1]. Recently, we demonstrated growth of high-quality ∼300 nm-thick (In,Ga)N films with the In content varying from 11% to 23% on ZnO [2]. For this purpose, we developed low-temperature atomically smooth GaN thin films (~2 monolayers (ML)) to suppress the interfacial reaction between nitrides and the ZnO substrate at elevated temperatures using metal-enhanced epitaxy (MEE). In this talk, we will present successful growth of high-quality 1µm-thick InGaN with In content ranging from 19.5% to 30.5% on ZnO substrate by plasma-assisted molecular beam epitaxy. PL analysis showed a relatively uniform InGaN composition on samples grown at 600 °C and 580 °C. Lowering the growth temperature to 560 °C led to clusters with higher In content. The uniformity of low temperature InGaN growth was improved by periodic desorption of In from the surface. Although, STEM image showed a high threading dislocation density generated at the substrate interface, TDD reduced significantly through annihilation of dislocations as InGaN thickness was increased. A self-assembled superlattice structure of the InGaN on ZnO was confirmed by scanning transmission electron microscopy and atom probe tomography. We will show the relaxation of InGaN on ZnO by XRD reciprocal space map measurement. Reference: 1. A. Kobayashi et al. Pulsed sputtering epitaxial growth on m-plane InGaN lattice matched to ZnO, Sci. Rep. 7 1-6 (2017). 2. K Khan, Growth of high quality (In,Ga)N films on O-face ZnO substrates by plasma-assisted molecular beam epitaxy, AIP Adv. 10 075120 (2020)

Kamruzzaman Khan

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