Playing with Polarity

Abstract

We review the influence of GaN crystal polarity on various properties of epitaxial films and electronic devices. GaN films grown on sapphire by MOCVD or HVPE usually exhibit Ga-face polarity. N-face polarity is obtained either on the backside of such layers after removal from the substrate, or by turning the crystal polarity in MBE growth via a thin AlN buffer layer. In addition to rather obvious differences in their structural and morphological features, Ga-and N-face samples differ also in their electronic properties. Thus, different Schottky barrier heights are observed for both polarities, the position and detailed properties of spontaneously formed twodimensional electron gases vary with polarity, and the adsorption of gases and ions also show an influence of the two different surfaces. A particular interesting possibility is the growth of lateral polarity heterostructures with predetermined macroscopic domains of different polarity separated by inversion domain boundaries. These structures make use of the crystal polarity as a new degree of freedom for the investigation of electronic properties of III-nitrides and for novel devices. Introduction The fact that the III-nitrides InN, GaN, and AlN are essentially ionic solids with a strong charge transfer between the very electronegative nitrogen atoms and the less electronegative metal atoms (In, Ga, or Al) has important consequences for many physical properties of this material system.The thermodynamically stable wurtzite structure of III-nitrides has a polar axis parallel to the c-direction of the crystal lattice. Deviations of the real atomic charge distributions from the point charge model of the ideal wurtzite lattice give rise to a macroscopic spontaneous polarization P of the III-nitrides, which can reach values up to 0.1 C/m 2 . Electrostatically, such macroscopic lattice polarizations are equivalent to twodimensional fixed lattice charge densities s ¼ P with values between 10 13 and 10 14 e/cm 2 located at the two surfaces of a sample (about 1-10% of a monolayer) [1,2]. A direct consequence of this large macroscopic polarization and the corresponding fixed surface charges is the appearence of large internal electric fields,

Photoelectron emission microscopy observation of inversion domain boundaries of GaN-based lateral polarity heterostructures

“…1,2 A gradient in the polarization induces bound surface charges and creates strong internal electric fields in the film. The sign of the bound charges at the surface and the direction of the field depend on the orientation of the polarization, which is determined by film polarity; Ga or N face.…”

“…4 Recently, intentionally grown GaN-based lateral polarity heterostructures ͑LPHs͒, in which Ga-and N-face regions were grown laterally on the same surface and separated by inversion domain boundaries ͑IDBs͒, have become of interest for potential application in optoelectric devices. 1,2 In particular, it has been proposed that an IDB can be employed as a tunnel junction barrier between regions with twodimensional electron gases ͑2DEGs͒ in a lateral GaN/ AlGaN/GaN heterostructure. 2 The spontaneous and piezoelectric polarization of the GaN leads to a polarization bound charge at the surface and interface.…”

“…1,2 In particular, it has been proposed that an IDB can be employed as a tunnel junction barrier between regions with twodimensional electron gases ͑2DEGs͒ in a lateral GaN/ AlGaN/GaN heterostructure. 2 The spontaneous and piezoelectric polarization of the GaN leads to a polarization bound charge at the surface and interface. The Ga-face surface exhibits a negative bound charge, and the N-face surface exhibits a positive bound charge.…”

Hydrogen-surfactant-assisted coherent growth of GaN on ZnO substrate

“…Moreover, In the recent years, the topic of growing N-polar GaN with comparable high crystal quality to Ga-polar GaN has attracted much attention [98][99][100], due to the special advantages of N-polar thin films over Ga-polar ones [43,[101][102][103][104]. Mg, In and Si are used to assist to get rid of the disadvantages of the N-polar film growth on traditional sapphire and SiC substrates [99,105].…”

Raman characterization of damaged layers of 4H-SiC induced by scratching

“…However, the removal of only a single atomic layer of Si or C is restricted by the surface polarity of SiC. 12 Accordingly, SiC layers rather than a Si or C atomic layer would amorphize at the initial stage of amorphization during scratching.…”