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Coalescence Promises Perfect GaN Boules

Dislocation-free crystals can be made by forming GaN points seeds by the sodium-flux method and joining them together via coalescence.

Makers of nitride LEDs, lasers and transistors all want to build their devices on large, dislocation-free substrates. Unfortunately, they don’t exist today, but they soon could thanks to the work of researchers at Osaka University, Japan.

Earlier this year, this team reported how they could make dislocation-free GaN crystals using a GaN point seed. Now they report how to join a pair of them together in a stainless steel tube that combines a melt of gallium, sodium and carbon with nitrogen gas held at a pressure of 3.6 MPa. After 200 hours of growth at 870 °C, crystals with dimensions of several millimetres have been formed that are free from dislocations associated with coalescence.

“We are now aiming to fabricate 8-inch GaN substrates by our newly developed coalescence process," says corresponding author Mamoru Imade.

There are no reports of any GaN crystals of that size today. The incumbent method for producing boules of this wide bandgap material, HVPE, has enabled the fabrication of 4-inch substrates with a dislocation density of about 106 cm-2. Lower values are possible with techniques such as ammonothermal growth and a sodium-flux technique, but they yield smaller crystals.

Polish GaN crystal developer Ammono has reported that its ammothermal growth can yield 1-inch GaN crystals with a dislocation density of 5 x 103 cm-2. However, according to Imade, this technique suffers from a low growth rate and high levels of impurity in the crystals. In comparison, in the last few years he and his co-workers have used the sodium-flux technique to produce 2-inch crystals with a dislocation density of 104 cm-2 on seeds with a dislocation density of 108 cm-2.

The sodium-flux method can yield GaN crystals grown on the point seed. 600 hours of growth results in the formation of this crystal,which is positioned against a backdrop of graph paper with a scale of 1 mm per division

However, in their most recent work, the Osaka university researchers begin by producing a pair of GaN point seeds. To do this, they mount a sapphire plate with two 1.2 mm holes, separated by 0.5 mm, on a 8 µm-thick c-plane GaN seed layer grown on sapphire. On this structure they deposit GaN by the sodium flux method. 

Point seeds form through the apertures, and when a pair of them is arranged along the a-direction, they coalesce without generating dislocations at this interface. In comparison, arranging the point seeds along the m-direction produced inferior results, with void appearing at the coalescence boundary.

To search for dislocations in the coalesced material, the team performed room-temperature cathodoluminescence imaging on cut and polished crystals with a Horiba Imaging-CL DF-100. Any dark spots found in the images would result from non-radiative carrier recombination at dislocations.

For bothtypes of crystal formed by coalescence, imaging revealed the absence of dark spots in the large areas apart from the coalescence boundary. However, near to this interface, dark spots were seen in material formed from seeds aligned along the m-direction. In contrast, no spots were visible in the crystal created from seeds in the a-direction.

Meanwhile, X-ray diffraction measurements of the full-width half maximum for GaN (0002) suggest that the crystalline quality of the material resulting from a-direction coalescence is as good as that distant from the coalescence boundary. This was not the case for m-direction coalescence.

Many researchers believe that substrates made from high quality crystals that are free from dislocations should lead to improved device performance, but this conjecture is yet to be fully tested.

“This is one of our current research topics," says Imade. “We are now investigating device performance on these substrates."

The sodium-flux method that has been pioneered by researchers at Osaka University, Japan, has produced material with a diameter of up to 4 inches.

N. Dharmarasu et al. Appl. Phys. Express 5091003 (2012)

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