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Limiting buffer layer current leakage is essential for the realization of high breakdown fields in GaN-on-Si high electron mobility transistors (HEMTs). In this report, we demonstrate the importance of controlling the surface morphology of the AlN nucleation layer (NL) in limiting this leakage. Testing on a self-consistent series of samples grown under two different AlN NL conditions revealed the presence of leakage paths within the epilayers grown using a single temperature AlN NL owing to the presence of surface pits. The introduction of a higher temperature AlN in the NL drastically reduced the pit density and led to a large reduction (>103) in the lateral and vertical buffer leakage in HEMT structures. Using conductive atomic force microscopy, secondary ion mass spectroscopy, and temperature-dependent carrier transport measurements, we confirm that these pits—which originate in the AlN NL, thread vertically, and propagate into the device structures—are associated with leakage paths, thus reducing the field that can be dropped across the epilayers. This is explained by invoking preferential oxygen segregation at their side-facets. It is shown that when a pit-free surface is maintained, a vertical field of 1.6 MV/cm can be achieved for HEMTs. This study is expected to benefit the development of high-performance GaN HEMTs in moving toward the theoretical breakdown field of III-nitrides. [ABSTRACT FROM AUTHOR]

The bending and interaction of threading dislocations are essential to reduce their density for applications involving III-nitrides. Bending of dislocation lines also relaxes the compressive growth stress that is essential to prevent cracking on cooling down due to tensile thermal expansion mismatch stress while growing on Si substrates. It is shown in this work that surface roughness plays a key role in dislocation bending. Dislocations only bend and relax compressive stresses when the lines intersect a smooth surface. These films then crack. In rough films, dislocation lines which terminate at the bottom of the valleys remain straight. Compressive stresses are not relaxed and the films are relatively crack-free. The reasons for this difference are discussed in this work along with the implications on simultaneously meeting the requirements of films being smooth, crack free and having low defect density for device applications. [ABSTRACT FROM AUTHOR]

An efficient buffer layer scheme has been designed to address the issue of curvature management during metalorganic chemical vapour deposition growth of GaN on Si (111) substrate. This is necessary to prevent cracking of the grown layer during post-growth cooling down from growth temperature to room temperature and to achieve an allowable bow (<40 μm) in the wafer for carrying out lithographic processes. To meet both these ends simultaneously, the stress evolution in the buffer layers was observed carefully. The reduction in precursor flow during the buffer layer growth provided better control over curvature evolution in the growing buffer layers. This has enabled the growth of a suitable high electron mobility transistor (HEMT) stack on 2" Si (111) substrate of 300 μm thickness with a bow as low as 11.4 μm, having a two-dimensional electron gas (2DEG) of mobility, carrier concentration, and sheet resistance values 1510 cm2/V–s, 0.96 × 1013/cm2, and 444 Ω/□, respectively. Another variation of similar technique resulted in a bow of 23.4 μm with 2DEG mobility, carrier concentration, and sheet resistance values 1960 cm2/V–s, 0.98 × 1013/cm2, and 325 Ω/□, respectively. [ABSTRACT FROM AUTHOR]