Microstructure of non-polar GaN on LiGaO2 grown by plasma-assisted MBE
© Shih et al; licensee Springer. 2011
Received: 20 October 2010
Accepted: 15 June 2011
Published: 15 June 2011
We have investigated the structure of non-polar GaN, both on the M - and A-plane, grown on LiGaO2 by plasma-assisted molecular beam epitaxy. The epitaxial relationship and the microstructure of the GaN films are investigated by transmission electron microscopy (TEM). The already reported epi-taxial relationship and for M -plane GaN is confirmed. The main defects are threading dislocations and stacking faults in both samples. For the M -plane sample, the density of threading dislocations is around 1 × 1011 cm-2 and the stacking fault density amounts to approximately 2 × 105 cm-1. In the A-plane sample, a threading dislocation density in the same order was found, while the stacking fault density is much lower than in the M -plane sample.
Gallium nitride (GaN), as one of the most important wide band semiconductors today, has far-reaching applicability in electronic and optoelectronic devices. Its hexagonal crystal structure, however, exhibits a polar axis in the  direction along which a polarization is present. The resulting polarization fields lead to intrinsically existent internal electric fields which give rise to a strong quantum-confined Stark effect when group III-nitride heterostructures are grown along the  direction. As a consequence, electrons and holes are spatially separated in such structures, leading to a reduced wave function overlap and a decreased radiative transition energy.
One way to circumvent these unwanted effects is to use non-polar surfaces of the hexagonal nitride structure such as the M -plane and A-plane for epitaxial growth procedures. The lack of available substrates for homoepitaxy on non-polar crystal planes requires alternative substrates for heteroepitaxy. While various substrates have been considered for this purpose, LiGaO2 (LGO) presents the unique opportunity for growth of C -, M -, and A-plane-oriented GaNs on a very well lattice-matched substrate, depending on the substrate surface orientation used. C -plane GaN growth has been demonstrated on (001) LGO by a number of groups, e.g. . Recently, M - and A-plane GaN growth has been reported on (100) LGO  and (010) LGO , respectively.
In this article, we demonstrate a first analysis of M - and A-plane GaN films on LGO showing strong evidence for a high-phase purity of non-polar GaN. The TEM studies confirm the epitaxial relationship of M -plane GaN on (100) LGO and A-plane GaN on (010) LGO and give insight to their defect structure.
The two samples discussed in this report were grown by plasma-assisted molecular beam epitaxy (PAMBE). Details on the growth of the films as well as a first structural analysis including an investigation by X-ray diffraction can be found in our previous reports in [2, 3]. The M -plane GaN sample was grown on (100) LGO, and the A-plane GaN sample was grown on (010) LGO. A plan view TEM sample of the M -plane GaN film was prepared by mechanical polishing and subsequent Ar-ion milling. Two cross-sectional
TEM samples were fabricated for each of the GaN samples. The M -plane samples were cut by a focused ion beam (FIB), one looking onto the C -plane and one onto the A-plane. Mechanical polishing and Ar-ion milling were used in the preparation of the A-plane GaN TEM sample with the C -plane as the sample surface, while FIB cutting was used to produce the A-plane GaN TEM sample with the M -plane as the TEM sample surface. The samples were analyzed using a JEOL 3010 TEM as well as a FEI Tecnai F20 TEM, each operated with an electron acceleration voltage of 200 kV.
Results and conclusion
M - and A-plane GaN films grown on (100) and (010) LGO, respectively, were analyzed by transmission electron microscopy. We show that the epitaxial relationship of the film deduced is in agreement with previous reports. Threading dislocations and stacking faults are the main defects in the films. For the case of the M -plane GaN sample, a threading dislocation density of 1 × 1011 cm-2 and stacking fault density of about 2 × 105 cm-1 were found. The A-plane sample shows a threading dislocation density on the same order; however, a much lower stacking fault density is found in comparison to the M -plane sample.
focused ion beam
plasma-assisted molecular beam epitaxy
transmission electron microscopy.
The authors from Karlsruhe acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG) and the State of Baden-Württemberg through the DFG-Center for Functional Nanostructures (CFN) within the sub-project A 2.7. One of the authors (R. Schuber) is grateful for support from the Karlsruhe House of Young Scientists (KHYS) for providing a research scholarship which helped in making a contribution to conduct this study.
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