Comparative study on stress in AlGaN/GaN HEMT structures grown on 6H-SiC, Si and on composite substrates of the 6H-SiC/poly-SiC and Si/poly-SiC
Guziewicz , Marek1; Kaminska , Eliana1; Poisson, Maria, A.2; Lahreche, Hacene2; Langer, Robert2; Domagala, Jaroslaw1; Bove, Philippe2; Piotrowska, Anna1
1Poland;
2France

GaN-based devices for high power/high frequency/high temperature operation require substrates with high thermal conductivity and good matching of lattice and thermal expansion coefficients. Sapphire and silicon used as substrate material give high lattice and thermal mismatch and these substrates result in high film stress in grown epitaxial heterostructures. Special properties of silicon carbide are well compatible with GaN leading SiC to be excellent substrate for these devices, but the SiC wafer is still very expensive. A novel approach for engineering of suitable composite substrates has been recently proposed by the EC-funded HYPHEN Project [1]. The substrate is fabricated using the Smart CutTM concept and technology. The aim of our study is to compare stress in AlGaN/GaN HEMT structures grown on (i) mono-SiC 6H, (ii) 6H-SiC/mono-Si by MBE and on (iii) composite substrates such as mono-Si/poly-SiC and (iv) mono-SiC/poly-SiC substrate by MOCVD. The stress was determined using two methods. The first one relies on the measurements of wafer-bending technique, where the ADE Mapper Wafer Metrology System and the Tencor FLX-2320 Stress Measurement System are used. The substrate shape is optically scanned before and after transferring the monocrystalline layer on poly-SiC substrate as wall as after epitaxial growth of the AlGaN/GaN structure.The radius of wafer curvature is determined from curvature fitting to the wafer profile, and stress is calculated from Stoney’s equation. The second method relies high-resolution X-ray diffractrometry (HRXRD). In this method PHILIPS X'Pert MRD diffractometer is applied to measure film strain. Residual stress is calculated based on stress-strain relations regarding to biaxial stress in film plane. We show, that residual stress in AlGaN/GaN structure MBE grown with the AlGaN/AlN buffer on the mono-SiC is in compression state, but grown with the GaN/AlN buffer on 6H-SiC/mono-Si is in tension state. In contrast, HEMT structures grown on composite substrates of the mono-SiC/poly-SiC substrate or mono-Si/poly-SiC substrate are low stressed in the range of -200 MPa - +200 MPa.
[1] http://www.hyphen-eu.com/
Part of the research was supported by the grant from the EC HYPHEN Contract Number: FP6-027455
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