researchers from ibm have developed a technique called controlled spalling and used it to produce think layers from gallium nitride (gan) crystals, a common semiconductor material, without causing damage and demonstrated the ability to measure basic physical properties such as strain-induced optical effects and fracture toughness.

this image shows a thick bulk gallium nitride (gan) crystal wafer (2 inches in diameter)
with a gan film in the foreground fabricated by controlled spalling.
(stephen bedell/ibm research)

according to an article from the american institute of physics (aip), “single-crystal gan wafers are extremely expensive, where just one 2-inch wafer can cost thousands of dollars, so having more layers means getting more value out of each wafer. thinner layers also provide performance advantages for power electronics, since it offers lower electrical resistance and heat is easier to remove.”

the researchers first deposit a layer of nickel on the surface of the material being removed, roll a layer of tape onto the nickel, hold the substrate so it cannot move, and peel the tape off. the stressed nickel creates a crack in the substrate that runs parallel to the surface. this is done at room temperature.

this discovery is notable because it is the simplest layer transfer process and the is one of the only processes that is material agnostic, according to the article.

“their next discovery was how to make the crack initiation consistent and reliable,” the article continued. “while there are many ways to generate a crack -- laser, chemical etching, thermal, mechanical, etc. -- it turns out that the simplest way…is to terminate the thickness of the nickel layer very abruptly near the edge of the substrate.”

this method creates a stress discontinuity at the edge of the film, so that a small pull on the tape will create a crack. the researchers will be working to use controlled spalling to create high-voltage gan devices.

the research was recently published in journal of applied physics. the abstract stated:

“successful wafer-scale layer transfer from high-quality 2-in. diameter bulk gallium nitride substrates was demonstrated using the controlled spalling technique. the crystal quality of both the as-fractured bulk substrate and the spalled gan film was assessed using transmission electron microscopy analysis, and the defect density was below the detection limit (mid 10⁷ cm⁻²) for both samples.

“by using the experimentally determined critical conditions for tensile stress and thickness of the ni stressor layer, an effective fracture toughness k_icof 1.7 mpa m???√m could be calculated for [0001] fracture using the suo and hutchinson mechanical model.

“the resulting in-plane contraction of the gan film after spalling permitted a novel method for measuring film strain without knowledge of the elastic properties of the material.

“this was used to measure the raman e₂(high) peak shift coefficient of δω(cm⁻¹) = 1411ε which, when converted to a stress coefficient (2.95 cm⁻¹/gpa), was in agreement with only one other literature value.”