Stress engineering with AlN/GaN superlattices for epitaxial GaN on 200 mm silicon substrates using a single wafer rotati
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We are reporting on stress engineering utilizing AlN/GaN superlattices (SLs) for epitaxy of GaN layers on 200 mm silicon substrates carried out in Veeco’s Propel™ rotating disk, single wafer metal organic chemical vapor deposition (MOCVD) reactor. The TurbodiscÒ reactor is designed to have homogeneous alkyl/hydride flow distribution and uniform temperature profile, which translate into excellent uniformity and concentric symmetry in epilayer thickness and alloy composition. This feature results in uniform and controllable stress in epilayers across large-size substrates. Crack-free 2 lm GaN layers were grown on 200 mm Si using uniformly strained AlN/GaN SLs with periods of 3–5 and 10–30 nm, respectively. Compressive and tensile stress can be precisely adjusted by changing the thickness of the AlN and GaN layers in the SLs, resulting in controllable wafer curvature/bow after cool down. For a fixed period thickness structure, the effects of growth conditions, such as growth rate of GaN, AlN V/III ratio, and growth temperature, on wafer stress were investigated.
I. INTRODUCTION
Contributing Editor: Joan Redwing a) Address all correspondence to this author. e-mail: [email protected] This paper has been selected as an Invited Feature Paper. DOI: 10.1557/jmr.2015.194
thermal stress during cool-down resulting in concave bending of Si wafer and cracks in the GaN epilayers due to the large difference (;54%) in thermal expansion coefficient (CTE) between GaN (CTEGaN 5 5.59 106 K1) and Si (CTESi 5 2.59 106 K1). To avoid Ga meltback etching, other materials have to be used as nucleation or transition layers to isolate GaN from Si, such as AlN,12–18 SiC,19–21 HfN,22 ZnO,23 Al2O3,24 or rare earth oxides or nitrides.25 In the case of using AlN as a nucleation layer, Al pretreatment or preflow14–18 is often used to wet the Si surface without the introduction of ammonia (NH3) to prevent the formation of an amorphous SiNx layer.26,27 However, an amorphous thin layer of SiNx can still exist at the Si–AlN interface due to the reaction between N and the Si substrate. 28–31 The conditions for TMAl preflow (such as time, TMAl flow, and temperature)31–34 are critical in affecting the Si–AlN interface, which can determine the structural quality, surface morphology, and surface pits in the AlN layer and the subsequently grown GaN layers. The stress in uniformly strained epilayers is primarily caused by the lattice and thermal mismatch between epilayers and substrates. The stress in the nonrelaxed epilayers can bend the substrates either during epitaxy or after cool-down. For compressively strained epilayers, either due to lattice mismatch (aepi . asubstrate) or thermal mismatch (CTEepi , CTEsubstrate) during cool-down, the bending of the substrates will be convex, as seen in Fig. 2(a). For tensile-strained epilayers, either due to lattice mismatch (aepi , asubstrate) or thermal mismatch (CTEepi . CTEsubstrate) during cool-down, the bending
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Ó Materials Research Society 2015
AlGaN based high electron mobility transistors grown on larg
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