Abstract
Influence of sapphire pretreatment conditions on crystalline quality of AlN epilayers has been investigated by metal organic chemical vapor deposition (MOCVD). Compared to alumination treatment, it is found that appropriate sapphire nitridation significantly straightens the surface atomic terraces and decreases the X-ray diffraction (0002) full width at half maximum (FWHM) to a minimum of 55 arcsec, indicating a great improvement of the tilting feature of the grain structures in the AlN epilayer. More importantly, there is no inversion domains (IDs) found in the AlN epilayers, which clarifies that optimal sapphire nitridation is promising in the growth of high quality AlN. It is deduced that the different interfacial atomic structures caused by various pretreatment conditions influence the orientation of the AlN nucleation layer grains, which eventually determines the tilting features of the AlN epilayers.
Highlights
It means that appropriate sapphire nitridation effectively reduces the planar tensions as well as the density of screw threading dislocations, since both of them are demonstrated to be responsible for the terrace meander[10,14]
It is observed that Sample F features a rough surface morphology delineated by hexagonal faceted nanocolumns similar to the observations in literatures[10,15], which suggests the possible formation of inversion domains (IDs) in AlN epilayers on the long-time nitrided sapphire
When prolonging the nitridation time to 600 s (Sample F), (0002) FWHM changes little, but (10-12) value increases to 922 arcsec, which is believed to result from the coalescence of 3D nanocolumns with different polarities as shown below
Summary
The samples (A-F) were grown on 2-in. 0.2° off-cut c-sapphire substrates by MOCVD, using an AIXTRON 3 × 2 in. Two-step growth procedure was adopted as follows: first, a 20 nm-thick AlN nucleation layer (NL) was deposited on sapphire at 950 °C, and the chamber temperature was raised to 1240 °C for the growth of 1 μm-thick high temperature AlN (HT-AlN) epilayers. V/III ratio for NL and epilayers was 7500 and 500, respectively. Prior to the NL growth, sapphire substrates were pretreated under different conditions, and the other growth parameters were kept the same for all samples. The surface morphology were characterized by a Bruker Dimension ICON-PT atomic force microscopy (AFM). The symmetric (0002) and asymmetric (10-12) -scan curves of all samples were measured by a Bruker AXS D8 Discover HRXRD
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