TECHNOLOGY GaN SUBSTRATES
can be fulfilled if this material is grown by the ammonothermal method, which involves forming GaN from a solution in supercritical ammonia.
Fig. 2: HVPE-GaN crystals, thicker than 1 mm, crystallized on 1-inch Ammono-GaN wafers in a few hours (courtesy of T. Sochacki, IHPP PAS).
2010. However, the hetero-epitaxial growth causes the substrates to be riddled with a high density of defects, which typically number 106
- 107 cm-2 .
What’s more, the growth on a foreign substrate causes a large lattice bow – for example, when GaN is grown on sapphire the bowing radius of the (0001) crystallographic planes is always below 10 m. This is not good enough for epitaxy and device processing on 2-inch wafers, which require bowing radii exceeding 30 m.
Lattice bow also plays havoc with the growth of thick HVPE-GaN boules, the fabrication of large, freestanding substrates, and the use of freestanding HVPE-GaN crystals as seeds for crystal
multiplication. Note that there is no point in crystallizing more material on a seed with a bent lattice, since crystalline quality only deteriorates further with growth.
For the growth of epi-structures and the processing of devices, the wafer surface must be offcut uniformly with an accuracy of one to two tenths of a degree in a specific crystallographic direction, which produces a specific step structure on the wafer’s surface. This low offcut promotes bilayer step flow, controls the composition of ternary alloys, and enables uniform incorporation of dopants. Lattice bow precludes all this. If substrates have a significant lattice bow, it is impossible to form homogeneous device layers across the wafer, and low production yield results. For a 2-inch wafer, for example, if the tolerance for the deviation of the offcut across the wafer is below 0.1°, which is a typical value, then the bow radius must be greater than 30 m to be able to meet this tolerance (see table 1 to understand the interplay between the deviation of the surface offcut and the bowing radius for wafers of various sizes).
A better foundation
Fig. 3: A single growth center observed on the as-grown, 1-inch crystal surface (courtesy of T. Sochacki, IHPP PAS).
The problem of lattice bow is not intrinsic to the HVPE process, but is rather a result of hetero-epitaxy. So, what is needed is to begin with a structurally perfect GaN seed – a requirement that
Fig. 4: a) A polished freestranding HVPE-GaN wafer sliced from the Ammono-GaN seed (courtesy of T. Sochacki, IHPP PAS); b) Wafer surface after the defect selective etching; the etch pit density was 5x104 Weyher, IHPP PAS).
cm-2
The best partner in this approach, which is based on HVPE deposition on ultra- high-quality GaN seeds, is the world’s leading grower of ammonothermal GaN, Ammono S.A. This Polish firm uses an approach that is analogous to the hydrothermal crystallization of quartz − but with supercritical ammonia replacing water – to produce GaN with many attractive attributes: exceptional lattice flatness, demonstrated by bowing radii of the (0001) crystallographic planes of around 100 m; dislocation densities of typically just 5 x 104
cm-2 cm-3 ; and a
free carrier, n-type concentration that may be varied from 5 x 1017 2 x 1019
cm-3 GaN substrate). to (see Figure 1a for a 2-inch
(courtesy of J.L.
June 2014
www.compoundsemiconductor.net 49
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