Ranges of Deposition Temperatures Applicable for Metalorganic Vapor Phase Epitaxy of GaN Films Via the Technique of Pend
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of pendeo-epitaxy (PE) is anticipated to yield the same result--a continuous layer of low defect density GaN--with only one lithography step and a single growth of GaN beyond the seed layer. As its name implies, pendeo-epitaxy is the epitaxial growth of crystalline material that hangs from freestanding forms and is suspended above the substrate. In the particular PE approach used in this research, etched columnar GaN forms are capped with a silicon nitride mask layer. As such, pendeo-epitaxial growth of the GaN films originates only from the sidewalls of these columns. The material grows laterally and vertically until it coalesces between and over the silicon nitride masks located atop the columns in the manner employed in the conventional LEO approach. A continuous layer of low defect density GaN is thus created. This process route is capable of producing continuous layers over large areas; it is limited only by the size of the substrate. EXPERIMENTAL PROCEDURE Pendeo-epitaxial growth of GaN films was performed in the manner shown schematically in Figure 1.
SiNx mask
Sseed
GaN
AIN
6H-SiCr
(a)
(b) SPE m sGaN
•,• •
S•NX GaNj iseed mask AIN
(c)
6H-SiC
(d)
Figure 1. Schematic diagram showing the process steps for growth of pendeo-epitaxial GaN: (a) GaN seed layer, (b) etched GaN columns prior to PE growth, (c) partial growth of PE GaN showing growth only from the sidewalls, (d) coalesced growth of PE GaN. Each substrate was prepared via growth of a I gtm thick GaN seed layer at 1000IC on an AIN buffer layer previously grown at 1 100'C on a 6H-SiC(0001) substrate in a cold-wall, vertical, pancake-style, RF inductively heated metalorganic vapor phase epitaxy (MOVPE) system. Additional details of the growth experiments have been previously reported [4]. The consecutive deposition of a growth mask layer of silicon nitride (SiNx) and an etch mask layer of nickel were achieved on the GaN seed layer using plasma enhanced chemical vapor deposition
(PECVD) and e-beam evaporation, respectively. The latter mask layer was patterned using standard photolithography techniques in parallel 2 gim wide stripes spaced 3 gim edge-to-edge and oriented along the [jioo] direction of the GaN film. Long columns containing the GaN seed material were produced via inductively coupled plasma (ICP) etching through the SiNx, GaN and AIN layers and into the 6H-SiC substrate. Detailed procedures for the ICP etching have been previously reported [5]. The nickel etch mask was removed by dipping in HNO 3 for approximately five minutes. The samples were subsequently cleaned by consecutive dips in trichloroethylene, acetone, methanol, and HCl for five minutes each and blown dry with nitrogen. Pendeo-epitaxial growth of GaN from the (i 120) sidewalls of the columns was performed at 45 torr and temperatures ranging from 1000°C to II 000 C via MOVPE. Reactants consisting of 26 jimol/min triethylgallium (TEG) and 1500 sccm ammonia were delivered into the growth chamber and entrained in 3000 sccm of hydrogen diluent. The morphological microstruc
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