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Refining periodically orientated GaN

Non-linear optics could benefit from alternating layers of N-polar and Ga-polar GaN.

Transmission electron microscopy reveals the damage to the annealed Al2O3 near the interface with GaN

Researchers from the US Naval Research Laboratory are claiming to have developed a superior approach to making a structure with selectively switched GaN polarity.

“[Our work] is a breakthrough,” says team-member Jennifer Hite. She explains that the team have demonstrated a simple method for forming a thin sapphire-like layer on Ga-polar GaN. This process holds the key to polarity changes from Ga-polar to N-polar material.
Kite believes that this simple method is preferable to the previous best approach: That required a magnesium-based layer for inversion of GaN, and had unwanted side effects, such as uncontrolled formation of inversion domains and faceted interfaces.

“The biggest application space for our work is in non-linear optics, basically using the alternating polarity for frequency conversion through quasi-phase matching.” For a semiconductor structure to realise quasi-phase matching, its crystal orientation must be periodically altered to produce a large second order non-linearity.

Quasi-phase matching in GaN was reported in 2003 by a team from Bell Laboratories, New Jersey. However, in that case the researchers employed GaN heteroepitaxial thin films on sapphire, which add strain and limit the thickness of the structure. A thickness of a millimetre or more is needed, which accounts for the interest in GaN-on-GaN structures for high-power, non-linear optics.

The starting point for forming such structures is a 2 mm-thick film of c-plane GaN deposited on a-plane sapphire by MOCVD. Following chemical cleaning, this epiwafer is placed in an ALD tool and a 15 nm-thick film deposited. After patterning 16 µm-wide stripes with a positive photo-resist and photolithography technique, etching in hydrofluoric acid removes Al2O3 from designated regions of the wafer.

Stripping the photoresist leaves stripes of Al2O3 on Ga-polar GaN, before the wafer is loaded into an MOCVD chamber. Annealing in an ammonia atmosphere at 1100 °C  for 40 minutes converts amorphous Al2O3 to a crystalline form, before a GaN buffer is deposited at 670 °C, followed by GaN growth at 1100 °C.

Using two growth steps and an intermediate clean is the only way to make periodically orientated semiconductor structures. That may mean it’s viewed as a complexprocess, but this not a barrier to volume manufacturing, insists Kite: “Silicon technologies require multiple tools and growth steps – Intel has actually incorporated ALD to deposit high-kdielectrics.”

Researchers confirm the presence of N-polar and Ga-polar regions by etching samples for up to 40 minutes in potassium hydroxide. N-polar material is far more chemically reactive than its Ga-polar cousin, with etching creating hexagonal faceting, while the Ga-polar material maintains a smooth surface.

Studies of initial structures identified N-polar regions with ‘lips’ of material along the border and ‘troughs’ in the middle. Lowering the V/III ratio enables a coalesced, more uniform N-polar structure. However, further optimisation is needed, because there are Ga-polar inclusions in the N-polar areas.

Scrutinising structures with a transmission electron microscope reveals that all the GaN is c-orientated. The interface between the  Al2O3 and the upper GaN is sharp, but the bottom 10 nm of  Al2O3 is damaged, due to annealing.

Plans for the future begin with reducing Ga-polar inclusions by optimising the N-polar initiation on the ALD layer. “The following step will be to grow thick structures by HVPE, and then test the material for quasi-phase matching,” says Hite.
Periodically orientated GaN is formed with a process involving atomic layer deposition (ALD) of amorphous Al2O3, which takes on a more crystalline form after annealing

J. Hite et. al. 
Appl. Phys. Express 7 025502 (2014)
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