Elimination of AlGaN epilayer cracking by spatially patterned AlN mask

Abstract

The inherent problem in III-nitride technology is the cracking of AlGaN layers that results from lattice mismatch between AlGaN and GaN. In case of thin substrates (30–90μm), such as, bulk GaN grown by the high-pressure/high-temperature method, the bowing of AlGaN∕GaN strained structures becomes an additional problem. To eliminate cracking and bowing, AlGaN layers were grown on GaN substrates with an AlN mask patterned to form 3–15μm wide windows. In the 3μm window, the AlGaN layer was not cracked, although its thickness and Al composition exceeded critical values for growth on nonpatterned substrates. Dislocation density in the windows was of 5×106∕cm2.

High Al-content AlInGaN epilayers with different thicknesses grown on GaN/sapphire templates

“…It is worth noting that the relaxation of tensile strain can also contribute to the PL blue shift [14]. It is believed that, at a given high Al-content (>0.4) and low In-content (0.003-0.016), the relaxation of elastic energy in AlInGaN epilayer will occur by cracks when the critical thickness (h crit ) is reached, referring to the similar phenomenon reported in AlGaN epilayers grown on GaN templates [19,20]. We calculated the critical thickness based upon the fracture theory described in Ref.…”

“…On the other hand, the smaller one (indicated by hollow arrows) has a density of $2.5 Â 10 8 cm À2 , and is formed when the epilayer thickness is over that of S2. It is likely related to newly created screw or mixed screw-edge character TDs in AlInGaN epilayers which are born after the relaxation of tensile strain [20]. …”

Influence of the trench depths of grooved GaN templates on the characteristics of overgrown AlGaN films

“…The relaxation of misfit strain occurs through the generation of crack networks. The crack networks on the surfaces of an AlGaN film grown on a GaN template are observed when the thickness of the AlGaN layer exceeds its critical value which decreases on increasing the Al content [1][2][3]. The formation of cracks on an AlGaN surface may result in the deterioration of device performances.…”

“…A high TD density in AlGaN affects internal quantum efficiency of UV LEDs having AlGaN active layers more strongly than that of the blue ones with InGaN active layers. Epitaxial lateral overgrowth (ELOG) and selective-area epitaxy using stripepatterned mask materials such as SiO 2 or Si 3 widely employed to reduce TD density in GaN films grown on sapphire substrates. ELOG method was found to effectively reduce the TD density as low as 10 7 cm À2 , which was reported to tremendously enhance the lifetime of III-nitride LDs [7].…”

Nitride-based laser diodes and superluminescent diodes

“…Unfortunately, a high lattice mismatch between GaN and AlN (2.4%) generates tensile strain. In thick AlGaN layers it leads to wafer bowing and layer cracking [3]. Furthermore, the GaN substrate, which is transparent for traveling light, may form a parasitic waveguide resulting in increased optical losses and a reduced optical confinement factor [4][5].…”