Halide vapor phase epitaxy of thick GaN films on ScAlMgO₄substrates and their self-separation for fabricating freestanding wafers

Abstract

Halide vapor phase epitaxy of thick GaN films was demonstrated on ScAlMgO4 (SCAM) substrates, and their self-separation was achieved. The 320-µm-thick GaN film was self-separated from the SCAM substrate during the cooling process after the growth. This separation phenomenon occurred because of both the c-plane cleavability of SCAM and the difference in the thermal-expansion coefficients between GaN and SCAM. The dark-spot densities for the GaN films on the SCAM substrates were approximately 30% lower than those on sapphire substrates. These results indicate that SCAM substrates are promising for fabricating a high-quality freestanding GaN wafer at a low cost.

Direct Growth of Patterned Vertical Graphene Using Thermal Stress Mismatch between Barrier Layer and Substrate

“…In the fields of materials science and microelectronics, thermal stress is commonly a problem that cannot be ignored, because it can damage the original structure of devices or materials and thus degrade the electrical properties [ 34 , 35 , 36 ]. Only in a few cases, does thermal stress play a positive role, e.g., Ohnishi has reported the use of thermal stress at heterogeneous interfaces to achieve self-separation of GaN films from substrates, and pointed out two factors that drive this self-separation: two shear stresses in the opposite directions generated within the film and the substrate during cooling, and cracking caused by the maximum shear stress appearing at the sidewalls near the interface [ 37 ]. This self-separation mechanism is also applied in this work.…”

“…Below this value, the VG will not be able to provide enough stress to the Cr film during cooling, and in turn, the VG/Cr will not smoothly separate from the substrate. For this problem, some optimization schemes can be adopted in the future, such as appropriately increasing the thickness of the Cr film [ 37 , 41 ], or selecting materials with a suitable CTE as barrier layers.…”

Femtosecond Laser Lift‐Off with Sub‐Bandgap Excitation for Production of Free‐Standing GaN Light‐Emitting Diode Chips

“…This process includes the removal of the sapphire substrate from the thin GaN layer. Up to now, various techniques have been demonstrated for this purpose, including laser lift-off (LLO), [12] grinding and dry-etching of the sapphire, [13] the use of selectively etchable [14,15] or weakly bonded intermediate layers, [16,17] natural stress-induced separation after growth, [18][19][20][21] and controlled spalling. [22] Among these approaches, LLO has proven to be fast, efficient, and reliable.…”

Free-standing 2-inch bulk GaN crystal fabrication by HVPE using a carbon buffer layer

“…The cracking of a GaN layer can be suppressed by growing a bulk GaN layer with a thickness of several millimeters, or by reducing the bonding energy between a substrate and a GaN layer. Different methods were proposed to reduce the bonding energy between a substrate and a GaN layer, including porous layers created by dry etching [4], wet etching [5], electrochemical etching [6] or GaN decomposition induced by a TiN mask [7]; epitaxial lateral overgrowth over dielectric masks [8][9][10][11], employing substrates with a cleavage plane parallel to the c-axis of GaN [12,13], and weakly bonded buffer layers [14,15]. All these methods either require using non-standard substrates [12,13] or special ex situ pre-growth processing of the substrate like etching, buffer layer deposition, dielectric or metal mask fabrication, or GaN template structure growth.…”