(530a) Physical Vapor Transport of Aluminum Nitride On Silicon Carbide Substrates: Parameters Affecting Nucleation

Employing native substrates in group III nitride semiconductor devices will reduce or eliminate strain in epitaxial layers caused by the mismatch of lattice constants and coefficients of thermal expansion.  Lowering strain reduces threading dislocation densities in epitaxial films.  These defects severely degrade the semiconductor’s optical and electrical properties, and thus the performance of devices employing these layers.  Thus, single crystal aluminum nitride is an attractive substrate for improving the performance of deep ultraviolet light emitting diodes and laser diodes with AlN and Al_xGa_1-xN epitaxial layers.  The quality of these semiconductors layers are particularly sensitive to defects.

In this study, the parameters affecting the nucleation of AlN crystals grown by physical vapor transport on silicon carbide single crystals wafers is reported.  Employing 4H-SiC and 6H-SiC seed crystals offers a means of rapidly scaling bulk AlN crystals to large diameters.  In addition, the choice of the SiC crystal plane determines the crystal plane of the AlN formed.  Nucleation is critically important, as this step defines the ultimate defect densities in the AlN crystals. 

Aluminum nitride crystals were grown from solid AlN sources at temperatures between 1400 °C to 1650 °C in a nitrogen ambient. Silicon carbide substrates with different planes and tilts were employed as seed crystals.  A comparison was made of the properties and growth rates of aluminum nitride crystals grown from oxygen-rich (AlN powder) and oxygen-depleted (dense, sintered polycrystalline) sources. 

Regardless of the SiC substrate plane (c- and m-planes),  misorientation tilt direction (toward [1-100] and [1-210] directions), or tilt magnitude (0, 7°, and 8° off-axis), the fastest growth direction was perpendicular to the c-axis.  This produced AlN platelet crystals with high a-axis to c-axis aspect ratios.  The AlN crystals nucleated as separate, individual islands on the SiC substrates, and grew parallel to the substrate’s c-plane (ie the (0001) plane).  This caused AlN to laterally overgrow the SiC substrate (separate from the substrate), resulting in the formation of voids at the SiC/AlN interface.

At high (1 wt%) oxygen concentrations in the source, severe anisotropic etching of the SiC substrate occurred simultaneous with AlN deposition. Deep etch pits were formed in the SiC substrate, presumably at dislocations.  In addition, AlN-SiC alloy hillocks formed, on which  AlN platelets would subsequently grow.  Consequently, the AlN platelets were attached to the substrate at discrete points corresponding to the top of the AlN-SiC hillocks.  Furthermore, a high oxygen concentration enhanced the growth rate of the AlN crystals.  For growth from a low oxygen concentration source, no etching of the SiC substrate was observed.

Thus, by careful selection of the process conditions, silicon carbide can be used as a seed crystal for bulk AlN crystal growth.  The AlN can be grown in a manner to reduce the degree it is strained by the SiC, and by long term continuous growth, free-standing AlN wafers can be produced.