Digital Growth of Thick N-polar InGaN Films on Relaxed InGaN Pseudosubstrates
Smooth N-polar InGaN films and quantum wells (QWs) were grown by MOCVD on relaxed InGaN pseudo-substrates (PSs). All InGaN layers grown on PSs exhibited at least 50% higher In compositions compared to those grown on GaN base layers, confirming higher In uptake with reduced lattice mismatch. The N-polar InGaN PSs were grown by MBE on GaN-on-sapphire templates with a misorientation of 4˚ towards the m-GaN direction as reported previously by Hestroffer et al. The surfaces of the graded InGaN layers exhibited interlacing finger structures caused by the Ehrlich-Schwöbel effect. The MOCVD growth on the InGaN PSs was initiated with 3 nm GaN, restoring the original step structure of the MOCVD-grown GaN templates. 200 nm thick InGaN films were then deposited using a novel digital approach, alternating the growth of 2 nm InGaN with and without H2 present to avoid the formation of hexagonal defects typically seen on thick N-polar InGaN films. XRD reciprocal space maps confirmed that the regrown InGaN layers assumed the in-plane lattice spacing of the relaxed PS surfaces, larger than that of GaN. In addition the PL at 490 nm from the InGaN films grown on PSs was twice as intense compared to that from reference samples grown on GaN which emitted at 430 nm. Finally, 3 period QW structures composed of 2 nm thick InGaN wells and 9 nm thick GaN barriers were investigated. Details of the growth, structural and optical properties evaluated by AFM, XRD and PL will be discussed at the conference.