Comparative Growth of AlN on Singular and Off-Axis 6H and 4H-SiC by MOCVD
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Abstract A comparison study of the growth of aluminum nitride (AlN) single crystal epitaxy on 6H-SiC and 4H-SiC substrates has been performed. The material has been characterized using atomic force microscopy (AFM) and reflective high energy electron diffraction (RHEED). AlN crystals were deposited on the following 6H-SiC substrates: singular with and without an initial SiC epilayer, and 3.5ο off-axis with and without an initial SiC epilayer. AlN crystals were deposited on 8.0o off-axis 4H-SiC with and without initial SiC epilayers. AFM shows that the deposition of AlN on 6H-SiC and 4HSiC with an initial SiC epilayer displays high quality quasi-two dimensional growth as atomically flat or step flow epitaxy.
Introduction AlN is a wide band-gap (Eg = 6.2ev at RT) semiconductor material that is suitable for acoustic and visible to deep ultraviolet opto-electronic semiconductor devices [1][2]. One of the biggest drawbacks for growing nitride materials is the availability of suitable substrates [3] [2]. At this time there are no commercial nitride substrates, but researchers have learned to grow heteroepitaxially on semiconductors such as silicon carbide (SiC) and sapphire (Al2O3) to produce quality single crystal AlN material [4][5][3]. In this paper, the surface morphology of single crystal AlN deposited on 6H-SiC and 4H-SiC substrates with and without initial SiC epilayers are compared to determine which substrates promote superior growth.
Experimental Procedure The AlN experiments were performed in a low pressure MOCVD reactor. This reactor possesses a vertical pancake configuration and is heated resistively. The AlN
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depositions were performed between 1160oC and 1190oC at 10torr with a total H2 carrier flow of 5L. The precursors used were trimethylaluminum (TMA) and 5% ammonia (NH4) balanced in hydrogen (H2). The substrate materials for the experiment were 6H and 4H single crystal SiC. All of the substrates were supplied by Cree Research with bulk dopings on the order of 1018cm-3. The epilayers were grown on the SiC substrates by vapor phase or liqiud phase epitaxy. All the epilayers were approximately 4µm with dopings ranging from the high 1016cm-3 to low 1017cm-3. In total, there were 7 different silicon-faced substrates. The substrates were as follows:
Table 1. SiC Substrate Material Poly Type
Orientation
Epilayer*
6H
singular
n-type
6H
3.5o off-axis
p-type
6H
3.5o off-axis
p+-type
6H
singular
none
6H
3.5o off-axis
none
4H
8o off-axis
p-type
4H
8o off-axis
none
* On-axis by LPE and off-axis by VPE
Several surface cleaning experiments were performed to determine the best method for sample preparation before growth. Three different cleaning procedures were performed for both 6H and 4H-SiC substrates without epilayers. After being cleaned, the growth
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