A Study of the Effect of Growth Rate and Annealing on GaN Buffer Layers on Sapphire
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heated, low pressure MOCVD reactor. The sources used were trimethylgallium (TMGa), high purity ammonia, and UHP hydrogen as a carrier gas. The reactants were introduced into the reaction chamber by a custom radial injection manifold which allows for separate injection of the group III and group V sources. Prior to loading, the substrates were chemically cleaned for 1 minute in a solution of 3:1 HCI:HNO 3 , followed by 5 minutes in trichloroethylene, then rinses in acetone and methanol. The substrates were then dried under a nitrogen flow and loaded into the reactor. Prior to growth, the sapphire substrates were heated in H2 at 1100 'C for 10 minutes. Experimental conditions for the reactor during the depositions were as follows: pressure 100 Torr, ammonia flow = 3.0 slm, growth temperature = 480 °C, and total gas flow of 4.5 slm. Growth rates from 30 to 440 A/min were studied for the GaN buffer layers. The layers studied were all nominally 300 A thick. The annealed samples were ramped up to a reactor temperature of 1025 °C in the same ammonia and hydrogen flow as was present during the buffer deposition. X-ray diffraction (XRD) measurements were made on the buffer lavers using a Philips Materials Research Diffractometer equipped with a 4-bounce germanium (220) monochromator and an open detector. Measurements were made in the rocking mode with the Bragg condition set for the [0002] reflection. The AFM work was done on a Digital Instruments Nanoscope III Atomic Force Microscope, using an e-beam grown tip with a 200 A radius. For each sample, the measurements were performed in scanning mode over an area of 0.4pin x 0.4ýtm. RESULTS The Effect of Growth Rate on Buffer Laver Characteristics AFM measurements revealed that the buffer laver consisted of a dense array of growth islands, as observed previously [1,3,6]. All GaN buffer layers showed XRD peaks indicating that thev were crystalline, however, the rocking curve peaks were typically very broad (about I to 2 degrees). This behavior is typical of polvcrystalline material and indicates that there is a large variation in the direction among the buffer "islands." For buffers grown on c-plane sapphire, the XRD peaks also showed a narrow, higher intensity peak at the center of this broad shoulder (see inset in figure 1). The presence of this higher intensity pealk indicates a preferred orientation among the polycrystals. On both c and a-plane sapphire samples, only c-oriented GaN could be detected; that is, only the (0002) and (0004) diffraction peaks of GaN were observed in the x-ray diffraction spectra. Figure 1 shows that the integrated (0002) GaN XRD pealk intensity for buffers grown on c-plane sapphire varies with the growth rate of the film. At a low deposition rate, the buffer is in crystalline form, while for a higher growth rate, the area under the full XRD peak decreases, indicating that more of the deposited material remains amorphous at the higher growth rate. GaN buffers grown on a-plane
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sapphire show no significant variation in crystallinity with the
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