Semiconductor device having a planar III-N semiconductor layer and fabrication method
A semiconductor device having a planar III-N semiconductor layer includes a substrate including a wafer and a buffer layer of a buffer material different from a material of the wafer, the buffer layer having a growth surface, an array of nanostructures epitaxially grown from the growth surface, a continuous planar layer formed by coalescence of upper parts of the nanostructures at an elevated temperature T, where the number of lattice cells spanning a center distance between adjacent nanostructures are different at the growth surface and at the coalesced planar layer, and a growth layer epitaxially grown on the planar layer.
1. A method of fabricating a semiconductor device having a planar III-N semiconductor layer, comprising:
providing a substrate comprising a wafer, a buffer layer of a different material than the wafer, having a growth surface with a first lattice constant at the growth surface, and a mask layer on the growth surface, having an array of nanosized apertures;
epitaxially growing III-N material in the nanosized apertures to form nanostructures, the nanostructures comprising at least one III-N layer with a second lattice constant,
coalescing the nanostructures at an elevated temperature (T) to form a continuous layer, wherein the step of coalescing includes:
releasing column III material from upper ends of the nanostructures; and
filling out a spacing between the nanostructures by forming semiconductor material from the released column III material; and
epitaxially growing a III-N growth layer on said continuous layer;
wherein the III-N growth layer is configured such that thermal expansion between room temperature (RT) and T of the growth layer is different from the wafer; and
wherein a number of lattice cells spanning a distance between adjacent apertures is different at the growth surface of the buffer layer and at the continuous layer.
2. The method of claim 1 wherein:
the wafer is a Si or SiC wafer;
the second lattice constant is smaller than the first lattice constant; and
the number of lattice cells spanning the distance between adjacent apertures is lower at the growth surface of the buffer layer than at the coalesced layer.
3. The method of claim 1 wherein:
the wafer is a sapphire wafer;
the second lattice constant is larger than the first lattice constant; and
the number of lattice cells spanning the distance between adjacent apertures is higher at the growth surface of the buffer layer than at the coalesced layer.
4. The method of claim 1 , wherein the coalesced planar layer has a mean lattice spacing at the elevated temperature (T) which corresponds to its relaxed nominal lattice spacing at same T.