AlGaN layers are very important constituent of practically any GaN-based electronic device such as light emitting diodes (LEDs), laser diodes (LDs) or high electron mobility transistors (HEMTs). The device epitaxy conditions should be optimized in a way to obtain homogenous layers of smooth morphology that will provide flat interfaces and uniform layer thickness. In plasma-assisted molecular beam epitaxy (PAMBE) on polar (0001) GaN substrates it is usually realized under metal-rich conditions. It means that all the growth long, 2-3 atomic layers of gallium are maintaned constantly on the surface which enable the growth of high quality AlGaN layers. However, in this case, the so called “growth window” is quite narrow in comparison to N-rich conditions. The recent advent of nonpolar and semipolar GaN substrates not only opened new possibilities related to the decreased piezoelectric field in such devices, but also posed a question about the growth mechanism on semipolar and nonpolar surfaces.

In this work we investigate the growth mechanisms on different crystal surfaces under N-rich conditions in PAMBE. We describe the multiquantum well (MQW) AlGaN/GaN structures fabricated simultaneously on semipolar (2021), m-plane (1010) and c-plane (0001) GaN substrates under nitrogen-rich (N-rich) conditions. The experimental details can be found in Refs. 1-2.

A smooth surface with atomic steps was found only for the semipolar (2021) structure as opposite to m-plane and c-plane surfaces which were rough and covered by islands (see Fig. 1). Transmission electron microscopy (TEM) studies of the semipolar MQW structure showed uniform AlGaN layers with sharp interfaces and no extended defect formation, which proves good structural quality (see Fig. 2a). At higher resolution, on the surface and at the interfaces of the semipolar MQWs, {1011}and {1010} nano-facets were found. In contrast to the semipolar case, the TEM studies of the m-plane structure showed no clear periodicity and very uneven distribution of Al (see Fig 2b). The AlN was formed on island top surfaces and AlGaN grew only on island slopes that were tilted from the m-plane. In parallel this result was also confirmed by X-ray diffractometry. The growth rate was slightly smaller for the semipolar MQW structure and much smaller for the m-plane than the c-plane one. This can be attributed to Ga losses from semipolar and m-plane surfaces, in contrast to c-plane where no Ga losses were observed. The mechanisms for the Ga losses under N-rich conditions are discussed.

Acknowledgements: This work has been partially supported by the National Science Centre Grant No. 02950, the National Centre for Research and Development Grant No. IT13426, INNOTECH 157829, the European Union within SINOPLE grant No. 230765 and European Union funds by the European Social Fund.

References:

[1] M. Sawicka, C. Chèze, H. Turski, J. Smalc-Koziorowska, M. Kryśko, S. Kret, T. Remmele, M. Albrecht, G. Cywiński, I. Grzegory, and C. Skierbiszewski, J. Cryst. Growth  in press (2013) doi: 10.1016/j.jcrysgro.2013.04.045.

[2] J. Smalc-Koziorowska, M. Sawicka, T. Remmele, C. Skierbiszewski, I. Grzegory, M. Albrecht, Appl. Phys. Lett., 99, 061901(2011)

^*Contact: [email protected]

Fig. 1. AFM images of the AlGaN/GaN structures grown on (a) semipolar (2021), (b) m-plane (1010), and (c) c-plane (0001) surface under N-rich conditions at 740°C. Inset to Fig. 1(a) shows a smaller scan with atomic steps marked with dashed lines.

Figure 2. (a) TEM image of the semipolar (2021) AlGaN/GaN structure taken along the [1120] axis. The contrast changes due to thickness gradient. The position of the interfaces is marked with dashed lines. (b) STEM image taken along [1120] axis of m-plane AlGaN/GaN structure grown under N-rich conditions showing no periodic structure. Al-rich areas appear dark because of the Z-number contrast. 

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