| AVS 57th International Symposium & Exhibition | |
| Electronic Materials and Processing | Thursday Sessions |
| Session EM+SS-ThM |
| Session: | Nitride Surfaces and Interfaces |
| Presenter: | J.K. Hite, Naval Research Laboratory |
| Authors: | J.K. Hite, Naval Research Laboratory M.E. Twigg, Naval Research Laboratory M.A. Mastro, Naval Research Laboratory F.J. Kub, Naval Research Laboratory C.R. Eddy, Jr., Naval Research Laboratory |
| Correspondent: | Click to Email |
Gallium nitride (GaN), a highly advantageous material for both optical and electronic devices, can be grown in the (+/-) c-direction of its lattice with two different polar faces, nitrogen- (N-) or gallium- (Ga-) polar. The face or polar orientation of the material establishes many quite varied properties of the material, from chemical reactivity to dopant incorporation to spontaneous and piezoelectric-induced electric field directions in the crystal. Control of the polarization fields and, thus, polarization induced doping is the basis of Ga-polar and N-polar GaN-based high electron mobility transistor operation. On heterogeneous substrates, such as silicon carbide (SiC) and sapphire (Al2O3), the growth conditions, doping levels, and buffer or nucleation layer properties are used to control the polarity of resulting GaN epilayers. Further, in the case of heavily doped p-type layers, spontaneous polarity inversion has been demonstrated even on GaN epilayers, switching from Ga-polar to N-polar in the doped layer.1 However, this approach leads to uncontrolled inversion domain boundaries and often results in dopant clustering2 in the film, impacting film quality and resultant device performance.
In contrast, this new approach enables the controlled formation of Ga-polar GaN from a N-polar crystal. Instead of using concentrated doping, the polarity switch in this case hinges on both surface treatment and the addition of an optimized polarity inversion layer. Initial material characterization verified that the film was without N-polar inclusions or inversion domain boundaries. Chemical etching of the material in 4M KOH under slightly elevated temperatures (40°C) for 10-40 minutes as well as convergent beam electron diffraction3 are employed to verify the polarity of the films. The structural quality of the films is ascertained with transmission electron microscopy and x-ray diffraction. In addition, the dislocation density and grain size are determined through the use of electron channeling contrast imaging.4 While lateral polarity heterostructures have been of interest due to their unique electrical and structural properties,5 this method offers the promise of engineering both lateral and vertical polarity heterostructures and the potential of novel variable polarity-based devices.
1V. Ramachandran et al., Appl. Phys. Lett. 75, 808, 1999.
2M. Hansen et al., Appl. Phys. Lett. 80, 2469, 2002.
3F.A. Ponce et al., Appl. Phys. Lett. 69, 337, 1996.
4Y.N. Picard et al., Appl. Phys. Lett. 91, 094106, 2007.
5 M. Stutzmann et al., Phys. Status Solidi b 288, 505, 2001.