Study of cracking mechanism in GaN/α-Al2O3 structure

Abstract

Imperfection (cracking, stress) in (0001)GaN/(OOOIja-AI203 grown by using the metalorganic chemical. vapor deposition (MOCVD) or chemical vapor deposition (CVD) method is investigated. First, the epitaxial relation is confirmed by using the oscillating-crystal method. Subsequently, imperfections, especially cracks, are observed in detail by optical microscopy and x-ray diffraction topography. Cracking occurs at the layer thickness larger than 13!-lm in the crystals having epitaxial layers of good quality, while it does not appear even at the thickness of30 !-lm in crystals including inferior layers. Finally, stress in the J.ayer and in the substrate are determined by measuring the warp of the wafer. Compressive stress in the good epitaxial. layer is 1. 6 X 10 9 dyne cm -2, and in this case, cracking occurs at the maximum tensil.e stress in the substrate less than 3 X lOS dyne cm -2. However, cracking does not occur even at the compressive stress of 3 X 10 9 dyne cm -2 in the inferior inyer and at the tensile stress of l. X 10 9 dyne cm -2 in the substrate. From these facts, a possible mechanism of cracking is proposed. During the epitaxial growth, microcracks occur in the substrate at the interface owing to the lattice mismatching. During the cooling, macrocracks which can be observed are grown from the already existing microcracks owing to the thermal stress. In the latter course, the cracking in the substrate must be simultaneously propagated to the epitaxial layer.

Reactor and growth process optimization for growth of thick GaN layers on sapphire substrates by HVPE

“…Usually, it is described [6,7] moving the focus even after successful healing in much thicker GaN layers [8]. Although it is beyond the scope of this work to analyze the MOVPE nucleation procedure used here in detail, it is suspected that some still unknown property of the GaN/sapphire template can be responsible for the finding that cracks can be avoided on particular template structures.…”

“…on sapphire already after the growth of only a few micrometers of GaN. In early investigations evidence was found for the formation of microcracks in the sapphire substrate during the GaN growth, but the formation of macrocracks which extend to the GaN surface was assumed to originate from the thermal mismatch only [6]. Later when layer growth exceeded 100 mm it was found that effective relaxation processes exist [7].…”

Stress analysis of transferred thin-GaN LED by Au-Si wafer bonding

“…12 Due to the CTE mismatch between GaN and sapphire, a compressive stress level would be produced in the GaN epi-layer after MOCVD growth process. [13][14][15][16] The compressive stress level of MOCVD grown GaN epi-layer is in the range of 450~650 MPa at the thickness range from 50µm to 2µm. In the present study, the stress in the transferred GaN epi-layer would mainly result from the CTE mismatch between GaN and Si.…”

The Peculiarities of Strain Relaxation in GaN/AlN Superlattices Grown on Vicinal GaN (0001) Substrate: Comparative XRD and AFM Study

“…6c). It is known [29, 30] that this type of crack morphology is attributed to the combination of cleavage of -like planes in GaN and parting of -like planes in α-Al 2 O 3 , because cracking in the substrate and the epitaxial layer will occur simultaneously. With cracks generated at the (0001) GaN/(0001) α-Al 2 O 3 interface, they run dominantly along the GaN and α-Al 2 O 3 directions.…”