Demonstration of Rashba spin splitting in GaN-based heterostructures

Abstract

The circular photogalvanic effect, induced by infrared radiation, has been observed in ͑0001͒-oriented n-GaN low dimensional structures. The photocurrent changes sign upon reversing the radiation helicity demonstrating the existence of spin splitting of the conduction band in k space in this type of materials. The observation suggests the presence of a sizeable Rashba type of spin splitting, caused by the built-in asymmetry at the AlGaN / GaN interface. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2158024͔ Gallium nitride is a potentially interesting material system for spintronics since it is expected to become ferromagnetic with a Curie temperature above room temperature if doped with manganese.1 Long spin relaxation times observed in this materials are another promising property for possible applications.2 Little is known so far about spin orbit interaction in GaN based heterojunctions like existence of Rashba spin splitting in the band structure which would provide a potential handle for spin manipulation.3 Strong spin-orbit effects are expected to be in narrow-gap materials only. 4 A large piezoelectric effect which causes a strong electric field at the Al x Ga 1−x N / GaN interface and the strong polarization induced doping effect, on the other hand, may result in a sizeable Rashba contribution to spin splitting of the band due to spin-orbit interaction ͑ϳ1 meV͒.5 Indeed a spin splitting of 9 meV was extracted from beatings of Shubnikov-deHaas oscillations in Al 0.25 Ga 0.75 N / GaN heterostructures. 6 However, such beatings were ascribed to magnetointersubband scattering by others.7 On the other hand, the observation of short spin relaxation times was attributed to D'yakonov-Perel mechanism which requires Rashba spin splitting. 8 To investigate the presence of a sizable spin splitting in this material class we investigate the circular photogalvanic effect ͑CPGE͒. 9-11The CPGE as well as k-linear spin splitting of the band structure are only permitted in gyrotropic media. In such materials a linear relation between polar vectors ͑electric current j, quasimomentum q, etc.͒ and axial vectors ͑photon angular momentum, spin, etc.͒ is allowed by symmetry. Both GaN / AlGaN low dimensional structures and bulk GaN belong to the family of wurtzite-type semiconductors which are gyrotropic. As were pointed out in Refs. 12 and 13 in these media the spin-orbit part of the Hamiltonian has the formHere the z axis is directed along the hexagonal c axis and is the vector of Pauli matrices. In bulk wurtzite materials the constant ␣ in the Hamiltonian ͑1͒ is solely due to bulk inversion asymmetry ͑BIA͒. In heterostructures, an additional source of k-linear spin splitting, induced by structure inversion asymmetry ͑SIA͒, exists. It occurs in asymmetric semiconductor heterostructures of any material. 3 If both, bulk and structure asymmetries, are present the resulting coupling constant ␣ is equal to the sum of BIA and SIA contributions to the spin-orbit part of the Hamiltonian. Note that in bulk III-V semiconductors wh...

Quantum ratchet effects induced by terahertz radiation in GaN-based two-dimensional structures

“…It has been observed in GaN-based structures demonstrating a substantial structural inversion asymmetry ͑SIA͒ in this wide band gap material. 7,[9][10][11] We have shown that SIA in GaN-based heterostructures results in a spin splitting of subbands in k space, which was later confirmed by magnetotransport measurements. [18][19][20][21] The Rashba spin splitting due to SIA, which is not expected in wide band semiconductors, in GaN/AlGaN heterostructures is caused by a large piezoelectric effect, 22 which yields a strong electric field at the GaN/AlGaN interface and a strong polarization induced doping effect.…”

“…Such experiments are of particular interest because such radiation, in partially circularly polarized light, has previously been used for investigation of the circular photogalvanic effect ͑CPGE͒, 4,7,14 which coexist with the LPGE yielding beatings in p dependence. The CPGE is characterized by a photocurrent whose direction is changed upon reversal of the radiation helicity.…”

“…In GaN-based structures first observation of LPGE has been reported quite recently. [7][8][9][10][11][12] Here, we present results of detailed experimental and theoretical investigation of LPGE in heterostructures based on GaN and its alloys with AlN. The commercial fabrication of blue and green light-emitting diodes have led to well established technological procedures of epitaxial preparation of these heterostructures and initiated a great activity on investigations of their properties.…”

Theory of electric dipole spin resonance in a parabolic quantum well

“…6,8 Currently, active experimental work on SO properties of microstructures including the wurtzite modifications of InAs, 44 InN, 45 GaN, [46][47][48] etc., is under way, and large SO splittings up to 9 meV have been reported. 47 Following the initial calculations of the band structure of wurtzite-type compounds, 49 more general models have been developed recently, 50,51 and it looks like the band folding in the hexagonal direction ͑resulting from the different size of the elementary cells in the zinc blende and wurtzite lattices͒ plays a role in developing large spin splittings.…”

Subnanosecond ellipticity detector for laser radiation

“…[4][5][6][7][8] We also note that the CPGE and LPGE are also observed at valence-toconduction band transitions, [9][10][11] at direct intersubband transitions, 1 and in wide band GaN semiconductor heterojunctions. 12 Thus, the applicability of the CPGE/LPGE detection scheme may well be extended into the visible, the near-infrared, and in the midinfrared spectral ranges.…”