Optical Spectroscopy of Dry-Etched GaN-Nanostructures
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techniques are more suitable, because etch rates of some 100 nm/min can be achieved. Recent studies have shown the influence of the etching parameters on the etch rate, the chemical composition and morphology of the etched surfaces [6-9]. To our knowledge no studies on the fabrication and optical characterization of GaN-nanostructures have been published up to now. EXPERIMENT For the purpose of the lateral patterning, GaN layers were grown using ECR plasma assisted molecular beam epitaxy (Eiko-I00-MBE, Astex CECR) on (0001) sapphire substrates. Using a modified ECR-aperture with a reduced aperture area (6 holes with diameter of 1 mm) we have obtained very flat surfaces suitable for high resolution patterning. During the growth the ECR source was operated at a microwave power of 65 W. The growth started with the deposition of an AIN buffer layer of 30 nm at a substrate temperature of 550'C. Then the GaN epilayer with thicknesses between 100 nm and 600 nm were grown at substrate temperatures between 640 0 C - 900 0 C. The layers were characterized by high resolution x-ray diffiactometry. They were all single crystalline and the rocking curves showed values of AO in the range of 910 arcmin which are very good values for such thin layers.
169 Mat. Res. Soc. Symp. Proc. Vol. 423 0 1996 Materials Research Society
For the fabrication of sub 100 nm GaN structures the samples were patterned with 30 nm thick Cr masks. The masks were defined by high resolution electron beam lithography in a 100 um thick layer of spin coated polymethylmethacrylate (PMMA 950K) and a lift off process. The mask patterns were transfered into the GaN by electron cyclotron resonance (ECR) enhanced reactive ion etching (RIE) using C1/Ar (1:9) as etching gas. The etch conditions including plasma self-bias voltage, chamber pressure, microwave input power, and gas mixture were optimized regarding the etch rates and the surface morphology. Figure 1 shows the dependence of the etch rate on the Cl 2-content (left) and the self-bias voltage (right). As the Cl-content is increased, the etch rate increases rapidly up to a Cl 2content of 10% and thereafter the etch rate saturates. From this it can be inferred that that surface of the Gan layer is essentially covered by Cl2 atoms at a Cl2-content of 10%. During the etching process the Ar+ ions cause two effects. They remove Ga and N from the sample surface due to physical sputtering and provide energy which promotes reactions between Ga and Cl2 to form GaCk products, which are volatile. 70
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Fig. 1: Etch rate of GaN dependent on the Cl-content (left) and self-bias voltage (right). By increasing the bins voltage from
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