A Tem study of GaN Grown by ELO on (0001) 6H-SiC
- PDF / 1,925,487 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 58 Downloads / 169 Views
CTION The epitaxial lateral overgrowth (ELO) technique has proven to be a relevant process to decrease the dislocation densities from 1010 to less than 107 cm-2 in GaN layers grown on sapphire. It has been implemented in both Metalorganic Vapor Phase Epitaxy (MOVPE) [1-3] and Hybrid Vapor Phase Epitaxy (HVPE) [4, 5], and considerable attention has been focussed on it, since a blue laser diode grown on an ELO GaN substrate, resulted in a lifetime of more than 10000 hours [6]. The conventional ELO method can be
F99W2.5
described as follows: first, a GaN layer of a few micrometers thick is grown, then a dielectric (SiO2 or SixNy) mask is deposited. Using standard photolithographic techniques, stripes are opened in the mask. During regrowth, either by MOVPE or HVPE, growth only occurs in the openings i.e., selective area epitaxy is achieved. A lateral growth over the mask leads to full coalescence. A smooth surface suitable for device fabrication is obtained if the ratio of the lateral to vertical growth rates is high enough. The standard way to realize ELO is to induce the lateral growth from the very beginning of the regrowth process[7]. Whereas the misfit between GaN and 6H-SiC is 3.5% instead of 16% on sapphire. The epitaxial layers have similar defect densities on both substrates. This effect has until now been attributed to the mosaïc growth of GaN on these substrates, with slightly misoriented islands bounded by threading dislocations. The lattice mismatch between AlN and 6H-SiC is about 1% which makes it most adequate as a buffer layer for the growth of GaN. In the following, we investigate the possibility of using HVPE AlN and 2S-ELO GaN in order to improve the quality of the active layers.
EXPERIMENTAL Prior to epitaxy, the SiC was chemically cleaned and then H2 plasma treated, then a 100nm Al buffer layer was deposited by HVPE at 1100°C. The growth of GaN was next performed in a homemade MOVPE vertical reactor operating at atmospheric pressure. The carrier gas used in this study is a mixture of H2:N2. Trimethylgallium (TMGa) and NH3 are used as precursors. A HeNe laser reflectometry set-up is used to monitor in-situ the growth process. First a 2µm-thick GaN layer is deposited directly on sapphire. In order to have large areas with electron transparency, it was necessary to prepare TEM samples by the tripod method. In this technique, two specimens are glued face to face using epoxy for cross section preparation. Then the first face is polished using a series of diamond coated plastic discs down to 0.5nm granulometry followed by silica colloïdal 0.05nm solution. Then 0.5 to 1nm slab is cut and glued on the tripod in order to grind the second face down to electron transparency. During this step, the thickness is monitored by use of silicon as one of the slabs. Therefore, when the sample thickness comes down to 10 µm, silicon becomes transparent and the polishing is stopped when the thickness fringes cross the interface and extend into the area of interest. In this process, the silicon is used to protect the s
Data Loading...
