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81 Mat. Res. Soc. Symp. Proc. Vol. 588 ©2000 Materials Research Society

EXPERIMENT Samples has been grown by MOVPE on (0001)-SiC subtrates. The heterostructures are formed by a buffer layer of AIN followed by a GaN layer of about 1 Pan. On top, 100 nm thick InGaN layers with different In contents have been deposited. Trimethylaluminium, triethylgallium, trimethylindium, and ammonia were used as precursors for Al, Ga, In, and N, respectively. The growth temperatures were 1 100*C, 1000*C and 780"C for the AIN layers, the GaN and the InGaN layers, respectively. Structural characterization has been carried out by means of a Park Scientific Instruments atomic force microscope (AFM) in contact mode, a Philips CM 20 transmission electron X-ray diffraction microscope (TEM) operated at 200 keV accelerating voltage and measurements (XRD) performed with a Phillips X'Pert MRD diffractometer equipped with a PW3050120 goniometer, a channel cut Ge[220] analyser and a 4-crystal monochromator in Ge[220] mode. Optical characterization has been carried out by means of reflectance spectrometry, photoluminescence (PL) and cathodoluminescence (CL). Reflectance measurements were performed using a Perkin Elmer Lambda 900 double beam spectrometer with a resolution between 1 and 5 nm in the range 0.6-4 eV. PL measurements have been performed under excitation with the 351.1 nm line of an Ar÷ laser, using an excitation power of 20 mW. A continuos flow He cryostat was used for temperature variation from 5 to 300 K. The luminescence was detected with a 0.8 m double monochromator and a cooled photomultiplier in the photon counting mode. CL measurements were carried out at 80 K in a Hitach S-2500 scanning electron microscope. The experimental setup for CL measurements consists of an optical lens that focused the light on a guide. A computer controlled Oriel 77200 monochromator and a Hamamatsu R928 photomultiplier were used. RESULTS Figure 1 shows an AFM image of a 2x2 gtm area of a typical sample. The black dots in the surface image correspond to uniformely distributed pinholes with a density about 10 9 cm-2 . The pinholes shown here are about 50 nm diameter. Flat surface areas are an indication of stepflow growth mode. With increasing In concentration in the layers the pinhole density increases, as I. The Indium shown in Table concentrations in the InGaN layers were A measured by XRD using the (0002) rocking 180 curves, and taking into account the

a

pseudomorphic growth of the layers. Table I. Pinhole densities and In concentrations in the films. INDIUM SAMPLE PINHOLE DENSITY CONCENTRATION (cm-2)

Figurel. AFM image of sample with 8% Indium. Surface area 2x2 g'm. Smooth areas are found indicating step-flow growth mode. 9 2 Pinholes in a density in the range of 10 cm are also found.

B C D

82

(%)

3.0

A

1.2x1

I.5x0 I.7xI0

6.6 8.1 13.0

Sample A, with 3% Indium concentration, has not been taken into account to compare the pinhole density with the In content because this sample presents pinholes with different diameter sizes that often g