Growth of homoepitaxial diamond films on superpolished substrates in a pulsed microwave plasma

Abstract

Abstract We used (100)-cut and (111)-cut, synthetic Ib-type diamonds as substrates for our investigations. The diamonds were Ti/Pt/Aubonded to a tungsten holder and the (100)-cut substrates were prepolished in a conventional manner on a diamond scaife. Superpolishing was done with KNO 3 on a rotating iron plate. We achieved a roughness of about 80 pm rms on the (100)-cut as well as on the (111)-cut substrates, whereas the typical roughness of commercial diamond substrates is about 2 nm rms or more. To compare the properties of the substrate with those of the diamond layer, we applied a standard photolithographic method with a sputtered mask of SiO 2 to achieve selective diamond growth with sharp and straight edges. The homoepitaxial diamond films with thicknesses up to 10 μm were grown in a quartz-tube microwave set-up. The microwave power of 730 W was pulsed at repetition frequencies of about 10 Hz. The temperature was measured pyrometrically and with a specially developed gas thermometer. The dependencies of the growth rate on methane concentration and substrate temperature were investigated and the films were characterized by Talystep profilometry, Nomarski microscopy and reflection high energy electron diffraction (RHEED). The results showed that the roughness and the quality of the chemical vapour deposition-diamond films strongly depend on the growth conditions but also on the substrates and the substrate pretreatment, particularly if there is a damage layer caused by cutting or polishing. On superpolished (100)-cut substrates we achieved a relatively slow increase of roughness with film thickness. Homoepitaxial films up to 1 μm thick had a roughness still below 1 nm rms. RHEED patterns of these homoepitaxial films proved that their crystal quality was at least as high as those of the substrate. On superpolished (111)-cut substrates the roughness increased much faster.