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...

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We report on a metal-insulator-semiconductor AlGaN/GaN heterostructure field-effect transistor (MIS-HFET) using Al 2 O 3 simultaneously for channel passivation layer and as a gate insulator which was deposited by plasma enhanced atomic layer deposition(PE-ALD). Capacitance-voltage measurements show successful surface passivation by the Al 2 O 3 dielectric layer. For a gate length 1.2 µm with 15 µm sourceto-drain spacing the maximum drain current was 1.22 A/mm, the maximum transconductance was 166 mS/mm and the gate leakage current was 4 nA/mm at V gs = -20 V which is at least three orders of magnitude lower than that of conventional AlGaN/GaN HFETs. IntroductionThe AlGaN/GaN heterostructure field effect transistor (HFET) has a great potential for high voltage, current, and power device applications due to its inherent wide bandgaps, high electron saturation velocity, and piezoelectric field-induced high 2-DEG the density at hetero-interface [1][2][3]. Many research works have been carried out to improve the power and high frequency characteristics of the nitride-based HFETs [4]. However, it has been observed that the Schottky gate of HFET tends to degrade with large gate leakage current and cause an occasional current collapse when the HFETs are operating under high power and high frequency conditions [5]. The degradation of the gate leads to premature breakdown and, hence, an adverse effect on device performances such as output power, rf efficiency, and noise figure. Such a premature breakdown is caused by the gate-drain diode breakdown as a result of the thermionic emission, crystal defects due to lattice mismatch, or the thermal effect as a consequence of the surface hopping conduction of the gate leakage current.Various gate insulation layers, such as SiO 2 , Si 3 N 4 , Ga 2 O 3 , and AlN have been employed as one of the possible solutions to solve the gate leakage problem by passivating the surface [6]. Other groups showed that a thin SiO 2 or Si 3 N 4 layer under the gate is effectively reducing the gate leakage current by several orders. Another advantage of using the insulating layer was thought that it reduces the electrical field in the underlying nitride semiconductor and, therefore, increase the gate breakdown voltage [7].

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