Free-standing GaN, nearly equal in area to the original 2 inch wafer, was produced from 250–300 µm thick GaN films grown on sapphire by hydride vapor phase epitaxy (HVPE). The thick films were separated from the growth substrate by laser-induced liftoff, using a pulsed laser to thermally decompose a thin layer of GaN at the film-substrate interface. Sequentially scanned pulses were employed and the liftoff was performed at elevated temperature (>600°C) to relieve postgrowth bowing. After liftoff, the bow is only slight or absent in the resulting free GaN.

Films of GaN have been separated from a sapphire growth substrate by illuminating the interface with a pulsed ultraviolet laser that induces localized thermal decomposition of the GaN. Free-standing lms and devices can be produced in this way. This process is also an alternative to surface etching for patterning of lms and can be used for other nitride materials and more complex lm systems by c hoosing an appropriate illumination wavelength or by including a strategically placed sacri cial absorbing layer during the lm growth.This process exploits the thermally activated decomposition of GaN that begins to occur above about 800 C, resulting in the e usion of nitrogen gas. We h a ve recently shown that this decomposition can be induced with high spatial resolution by heating the material with a short laser pulse (less than 10 ns). 1] The rapid generation of heat allows a high localized temperature to be reached before the heat is conducted out of the illuminated region. In this way, surface patterning was achieved at illumination intensities above about 0.2 J/cm 2 with 355 nm wavelength, near the absorption threshold of GaN.In order to achieve lm lifto , we h a ve n o w activated this mechanism at an internal interface rather than the surface. This requires that the interface can be illuminated with a wavelength of light that is then absorbed there. This is satis ed, for example, by t h e i n terface between the nitride lm and a sapphire substrate, commonly used for epitaxial growth. In this important case, we could illuminate the interface through the sapphire substrate with 355 nm light that is absorbed by GaN with an extinction length near 100 nm. The decomposition of the GaN produces nitrogen gas at the interface that expands and separates the two i n terface sides. To de ne an interface for separation, a lm material with lower optical absorption threshold could be inserted. Films of InxGa1;xN w ould satisfy this in a GaN lm system.For our tests, we used the 355 nm third harmonic of a Q-switched Nd:YAG laser with a nominal pulse duration of 6 ns. The beam diameter is 7 mm, and 100-200 mJ pulses were required, depending on the incidence angle at the sample. The sapphire substrates were polished on both sides to permit illumination of the interface after growth. Films of GaN were grown by MOCVD. 2] The lifto was performed with a single pulse. Excess gallium remaining after decomposition was removed by holding the material brie y over fuming HCl.To i n vestigate patterning, a lm was illuminated with the striped interference pattern from two o verlapping beams. The result, for a sample that was sequentially structured with two crossed patterns of this type, is shown in Fig. 1. The GaN lm has been completely removed in the regions where the interface was su ciently heated, leaving a clean substrate surface. The trenches widen away f r o m t h e i n terface, leaving sloping sidewalls on the remaining mesas. This wedge-like form of the removed material is an indication of the explosive nature of the lifto . Further i...

Patterned etching of GaN films was achieved with laser-induced thermal decomposition. High-energy laser pulses are used to locally heat the film above 900 °C, causing rapid nitrogen effusion. Excess gallium is then removed by conventional etching. At exposures of 0.4 J/cm2 with 355 nm light, etch rates of 50–70 nm per pulse were obtained. Illumination with an interference grating was used to produce trenches as narrow as 100 nm.