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1. Very High Density (>10 14 cm −2 ) Polarization‐Induced 2D Hole Gases Observed in Undoped Pseudomorphic InGaN/AlN Heterostructures

   https://doi.org/10.1002/aelm.202101120  

   Chaudhuri, Reet ; Zhang, Zexuan ; Xing, Huili Grace ; Jena, Debdeep ( January 2022 , Advanced Electronic Materials)

   High hole densities are desired in p‐channel field effect transistors to improve the speed and on‐currents. Building on the recently discovered undoped, polarization‐induced GaN/AlN 2D hole gas (2DHG), this work demonstrates the tuning of the piezoelectric polarization difference across the heterointerface by introducing indium in the GaN channel. Using careful design and epitaxial growths, these pseudomorphic (In)GaN/AlN heterostructures result in some of the highest carrier densities of >10¹⁴cm⁻²in a III‐nitride heterostructure—just an order below the intrinsic crystal limit of ≈10¹⁵cm⁻². These ultra‐high density InGaN/AlN 2DHGs show room temperature mobilities of 0.5–4 cm²V⁻¹s⁻¹and do not freeze out at low temperatures. A characteristic alloy fluctuation energy of 1.0 eV for hole scattering in InGaN alloy is proposed based on the experiments.

    

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2. In Situ Crystalline AlN Passivation for Reduced RF Dispersion in Strained‐Channel AlN/GaN/AlN High‐Electron‐Mobility Transistors

   https://doi.org/10.1002/pssa.202100452  

   Chaudhuri, Reet ; Hickman, Austin ; Singhal, Jashan ; Casamento, Joseph ; Xing, Huili Grace ; Jena, Debdeep ( October 2021 , physica status solidi (a))

   The recent demonstration of  W mm⁻¹output power at 94 GHz in AlN/GaN/AlN high‐electron‐mobility transistors (HEMTs) has established AlN as a promising platform for millimeter‐wave electronics. The current state‐of‐art AlN HEMTs using ex situ‐deposited silicon nitride (SiN) passivation layers suffer from soft gain compression due to trapping of carriers by surface states. Reducing surface state dispersion in these devices is thus desired to access higher output powers. Herein, a potential solution using a novel in situ crystalline AlN passivation layer is provided. A thick, 30+ nm‐top AlN passivation layer moves the as‐grown surface away from the 2D electron gas (2DEG) channel and reduces its effect on the device. Through a series of metal‐polar AlN/GaN/AlN heterostructure growths, it is found that pseudomorphically strained 15 nm thin GaN channels are crucial to be able to grow thick AlN barriers without cracking. The fabricated recessed‐gate HEMTs on an optimized heterostructure with 50 nm AlN barrier layer and 15 nm GaN channel layer show reduction in dispersion down to compared with in current state‐of‐art ex situ SiN‐passivated HEMTs. These results demonstrate the efficacy of this unique in situ crystalline AlN passivation technique and should unlock higher mm‐wave powers in next‐generation AlN HEMTs.

    

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3. Enhancement of 2DEG effective mass in AlN/Al 0.78 Ga 0.22 N high electron mobility transistor structure determined by THz optical Hall effect

   https://doi.org/10.1063/5.0087033  

   Kühne, Philipp ; Armakavicius, Nerijus ; Papamichail, Alexis ; Tran, Dat Q. ; Stanishev, Vallery ; Schubert, Mathias ; Paskov, Plamen P. ; Darakchieva, Vanya ( June 2022 , Applied Physics Letters)

   We report on the free charge carrier properties of a two-dimensional electron gas (2DEG) in an AlN/Al x Ga 1– x N high electron mobility transistor structure with a high aluminum content ( x =  0.78). The 2DEG sheet density [Formula: see text] cm −2 , sheet mobility [Formula: see text] cm 2 /(Vs), sheet resistance [Formula: see text] [Formula: see text], and effective mass [Formula: see text] at low temperatures [Formula: see text] are determined by terahertz (THz) optical Hall effect measurements. The experimental 2DEG mobility in the channel is found within the expected range, and the sheet carrier density is in good agreement with self-consistent Poisson–Schrödinger calculations. However, a significant increase in the effective mass of 2DEG electrons at low temperatures is found in comparison with the respective value in bulk Al 0.78 Ga 22 N ([Formula: see text]). Possible mechanisms for the enhanced 2DEG effective mass parameter are discussed and quantified using self-consistent Poisson–Schrödinger calculations. 

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4. High-density polarization-induced 2D electron gases in N-polar pseudomorphic undoped GaN/Al0.85Ga0.15N heterostructures on single-crystal AlN substrates

   https://doi.org/10.1063/5.0107159  

   Zhang, Zexuan ; Encomendero, Jimy ; Kim, Eungkyun ; Singhal, Jashan ; Cho, YongJin ; Nomoto, Kazuki ; Toita, Masato ; Xing, Huili Grace ; Jena, Debdeep ( August 2022 , Applied Physics Letters)

   The polarization difference and band offset between Al(Ga)N and GaN induce two-dimensional (2D) free carriers in Al(Ga)N/GaN heterojunctions without any chemical doping. A high-density 2D electron gas (2DEG), analogous to the recently discovered 2D hole gas in a metal-polar structure, is predicted in a N-polar pseudomorphic GaN/Al(Ga)N heterostructure on unstrained AlN. We report the observation of such 2DEGs in N-polar undoped pseudomorphic GaN/AlGaN heterostructures on single-crystal AlN substrates by molecular beam epitaxy. With a high electron density of ∼4.3 ×1013/cm2 that maintains down to cryogenic temperatures and a room temperature electron mobility of ∼450 cm2/V s, a sheet resistance as low as ∼320 Ω/◻ is achieved in a structure with an 8 nm GaN layer. These results indicate significant potential of AlN platform for future high-power RF electronics based on N-polar III-nitride high electron mobility transistors.

    

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5. Record >10 MV/cm mesa breakdown fields in Al 0.85 Ga 0.15 N/Al 0.6 Ga 0.4 N high electron mobility transistors on native AlN substrates

   https://doi.org/10.1063/5.0083966  

   Khachariya, Dolar ; Mita, Seiji ; Reddy, Pramod ; Dangi, Saroj ; Dycus, J. Houston ; Bagheri, Pegah ; Breckenridge, M. Hayden ; Sengupta, Rohan ; Rathkanthiwar, Shashwat ; Kirste, Ronny ; et al ( April 2022 , Applied Physics Letters)

   The ultra-wide bandgap of Al-rich AlGaN is expected to support a significantly larger breakdown field compared to GaN, but the reported performance thus far has been limited by the use of foreign substrates. In this Letter, the material and electrical properties of Al 0.85 Ga 0.15 N/Al 0.6 Ga 0.4 N high electron mobility transistors (HEMT) grown on a 2-in. single crystal AlN substrate are investigated, and it is demonstrated that native AlN substrates unlock the potential for Al-rich AlGaN to sustain large fields in such devices. We further study how Ohmic contacts made directly to a Si-doped channel layer reduce the knee voltage and increase the output current density. High-quality AlGaN growth is confirmed via scanning transmission electron microscopy, which also reveals the absence of metal penetration at the Ohmic contact interface and is in contrast to established GaN HEMT technology. Two-terminal mesa breakdown characteristics with 1.3  μm separation possess a record-high breakdown field strength of ∼11.5 MV/cm for an undoped Al 0.6 Ga 0.4 N-channel layer. The breakdown voltages for three-terminal devices measured with gate-drain distances of 4 and 9  μm are 850 and 1500 V, respectively. 

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