Spin-Orbit Coupling and Zero-Field Electron Spin Splitting in AlGaN/AlN/GaN Heterostructures with a Polarization Induced
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Spin-Orbit Coupling and Zero-Field Electron Spin Splitting in AlGaN/AlN/GaN Heterostructures with a Polarization Induced Two-Dimensional Electron Gas Ç. Kurdak1,2, N. Biyikli2, H. Cheng1, U. Ozgur2, H. Morkoç2, and V. I. Litvinov3 1 Physics Department, University of Michigan, Ann Arbor, MI, 48109 2 Department of Electrical Engineering, Virginia Commonwealth University, Richmond, VA, 23284 3 WaveBand/Sierra Nevada Corporation, 15245 Alton Parkway, Suite 100, Irvine, CA, 92618
ABSTRACT We studied spin-orbit coupling in wurtzite AlxGa1-xN/AlN/GaN heterostructures with different Al concentrations using weak antilocalization measurements at 1.6 K. Using the persistent photoconductivity effect we cover a carrier density range extending from 0.8 ×1012 cm-2 to 10.6 ×1012 cm-2 . The zero-field electron spin-splitting energies extracted from the weak antilocalization measurements scaled with the Fermi wavevector kF as 2(αk F + γk F 3 ) with effective linear and cubic spin-orbit parameters of α = 5.13 ×10−13 eV ⋅ m and γ = 1.2 ×10−31 eV ⋅ m3 , respectively. INTRODUCTION Following the prediction of above room temperature ferromagnetism in GaN based dilute magnetic semiconductors [1], there has been growing interest in exploring the use of AlGaN/GaN heterostructures for spintronic applications. In this context, it is essential to understand the spin-orbit coupling and the associated zero-field electron spin-splitting in GaN based two-dimensional electron systems. It is well known that the spin-splitting for a twodimensional electron system originates from both the structural inversion asymmetry (SIA) of the quantum well [2] as well as the bulk inversion asymmetry (BIA) of the crystal [3]. The spinsplitting arising from the SIA, also known as the Rashba effect, scales linearly with the Fermi wavevector kF whereas the BIA for a wurtzite structure has contributions that are both linear and cubic in kF [4-6]. The contributions of these terms have not been measured independently for the GaN/AlGaN heterostructures. Furthermore, there has been conflicting reports on the size of spinsplitting energies based on beat patterns of the Shubnikov-de Haas oscillations [7.8]. To resolve these issues, we and two other groups have recently used weak antilocalization (WAL) measurements to extract spin-splitting energies, instead [9-11]. WAL measurements, which are reliable for studying the spin-orbit coupling, have been consistent with each other and revealed that the spin splitting energies scale linearly with kF . In this work we extended the WAL measurements to higher carrier densities and report the contribution of the cubic term for the first time. EXPERIMENT We used five AlxGa1-xN/AlN/GaN heterostructures with different Al concentrations in this work. The heterostructures were all grown by metalorganic vapor phase epitaxy on c-plane sapphire substrates and consist of the following layers: a 3 μm GaN buffer layer, a 1 nm AlN
interfacial layer, a 25 nm AlxGa1-xN layer and a 3 nm GaN cap layer where the Al composition x=0.1, 0.1
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