GaN/AlGaN 2DEGs in the quantum regime: Magneto-transport and photoluminescence to 60 tesla S. A. Crooker 1* , M. Lee 2 , R. D. McDonald 1 , J. L. Doorn 1 , I. Zimmermann 1 , Y. Lai 1 , L. E. Winter 1 , Y. Ren 3 , Y.-J. Cho 2 , B. J. Ramshaw 4 , H. G. Xing 2,3 , D. Jena 2,3* 1 National High Magnetic Field Laboratory, Los Alamos National Laboratory, Los Alamos, NM 87545, USA 2 School of Electrical and Computer Engineering, Cornell University, Ithaca, NY 14853, USA 3 Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, USA 4 Laboratory of Atomic and Solid State Physics, Cornell University, Ithaca, NY 14853, USA and * Authors to whom correspondence should be addressed: crooker@lanl.gov, djena@cornell.edu Using high magnetic fields up to 60 T, we report magneto-transport and photoluminescence (PL) studies of a two-dimensional electron gas (2DEG) in a GaN/AlGaN heterojunction grown by molecular-beam epitaxy. Transport measurements demonstrate that the quantum limit can be exceeded (Landau level filling factor ν< 1), and show evidence for the ν =2/3 fractional quantum Hall state. Simultaneous optical and transport measurements reveal synchronous quantum oscillations of both the PL intensity and longitudinal resistivity in the integer quantum Hall regime. PL spectra directly reveal the dispersion of occupied Landau levels in the 2DEG and therefore the electron mass. These results demonstrate the utility of high (pulsed) magnetic fields for detailed measurements of quantum phenomena in high-density 2DEGs. The wide-bandgap semiconductor GaN is a foun- dational material for solid-state lighting applications and high-power electronics. Furthermore, the two- dimensional electron gas (2DEG) that forms naturally at GaN/AlGaN heterointerfaces [1–3] is of considerable interest for high-electron mobility transistors. 2DEG structures grown by molecular-beam epitaxy (MBE) have exhibited low-temperature electron mobilities exceed- ing 10 5 cm 2 /Vs [4, 5], galvanizing interest in quan- tum phenomena and novel electron correlations in GaN- based materials. Indeed, transport measurements have shown a robust integer quantum Hall effect (IQHE) in GaN/AlGaN heterojunctions [2–9], and an indication of a fractional quantum Hall state (Landau level filling factor ν=5/3) was reported by Manfra et al. nearly two decades ago [10]. In comparison with the more widely studied GaAs-based 2DEGs, electrons in GaN-based 2DEGs have significantly heavier effective masses (≈0.24 m 0 versus ≈0.07 m 0 in GaAs, where m 0 is the bare electron mass), and the dielectric constant is smaller (ǫ≈9.5 in GaN ver- sus ≈13 in GaAs), so that enhanced electron-electron interactions are expected. In this regard, 2DEGs in GaN more closely resemble those found in other wide-bandgap semiconductors such as ZnO, where significant progress has recently been made [11]. However, peak mobilities in GaN-based 2DEGs are, to date, typically achieved at relatively large electron densi- ties n e ∼ 10 12 /cm 2 [12, 13], so that high magnetic fields B > ∼ 40 T are required to reach the so-called “quantum limit” wherein all the electrons reside in the lowest spin- polarized Landau level (i.e., ν ≤ 1). Such large B are (just) within reach of modern superconducting-resistive hybrid magnet technologies, but are routinely exceeded by pulsed magnets [14]. Pulsed fields can therefore en- able detailed studies of high-density 2DEGs, including not only transport but also optical measurements that probe the response of the 2DEG to a photogenerated hole, which have historically proven to be a very power- ful tool to measure screening and many-body effects in GaAs- and ZnO-based systems [15–24]. To this end, we report both transport and optical stud- ies of a high-mobility 2DEG in a GaN/AlGaN hetero- junction in pulsed magnetic fields to 60 T. We demon- strate that beyond the quantum limit, transport mea- surements show clear evidence for the ν =2/3 frac- tional quantum Hall effect (FQHE) state. Moreover, si- multaneous optical and transport studies reveal nearly- synchronous quantum oscillations of both the photolumi- nescence (PL) intensity and the longitudinal resistivity; however, the optical illumination required to perform PL significantly (and persistently) increases n e to the point where only ν ≥ 3 can be reached in 60 T in the same heterostructure. The GaN/AlGaN structure (see inset, Fig. 1a) was grown by MBE on a semi-insulating single-crystal GaN substrate with low dislocation density (∼ 5 × 10 4 /cm 2 ), following Ref. [13]. After the initial growth of a 300 nm GaN buffer layer, a thin 21 nm Al 0.07 Ga 0.93 N barrier layer was grown. A high-mobility 2DEG formed natu- rally at the interface due to the spontaneous polarization discontinuity across the junction [1]. The structure was capped by a final 3 nm GaN layer. For transport stud- ies, Ti/Au contacts were deposited and annealed at the corners of 3 mm × 3 mm squares in a van der Pauw geometry. The sample was mounted in a 3 He cryostat in a 60 T capacitor-driven pulsed magnet. The mag- net pulse pulse has a 9 ms rise time and total duration of ≈90 ms. Resistivity was measured using dc current, which avoided measurement-phase and RC time constant issues associated with high-frequency lock-in detection of high-resistance samples. Appropriate combinations of current and magnetic field direction were used to sym- metrize the data and subtract off any induced voltages due to the rapid-changing field, so that both the lon- This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0033047