Surface and Defect Structure of Epitaxial Gallium Phosphide on Si(001)
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sec. The overall surface structure was revealed from a growth run with t=90 minutes. The deposition process was real-time monitored by PRS and compared to atomic force microscope (AFM) and high resolution TEM. For the PRS monitoring a p-polarized HeNe laser beam with a wavelength of 632.8nm, for which the growing film is transparent, was used. The beam impinging the surface in an adjustable angle between 70 deg to 75 deg, close to the Brewster angle of the Si substrate. The reflected beam is detected by a photo diode, processed through a phase sensitive detector and read in a computer. The out of the plane of incidence scattered light intensity is monitored by a high sensitive photo multiplier tube in order to correlate surface and interface roughness with the observed ex-situ AFM and TEM structures. RESULTS High resolution transmission electron microscopy (HRTEM) and atomic force microscope (AFM) support the mechanism of three dimensional nucleation and stacking faults of GaP on Si [3]. HRTEM results show stacking faults along the ( 111 ) plane in GaP films grown on ( 100) Si. Heteroepitaxial growth without filament assisted hydrogen provided AFM scans with surface morphology that represents a growth mechanism with nucleation facets along the ( 111 )planes seen in figure 1.
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Fig. 1. AFM scans with surface morphology that represents a growth mechanism with nucleation facets along the [111 )planes ...... 198
The initial clean silicon surface as measured by AFM yields a rms surface roughness of 0.5 nm. Figure 2 shows the AFM image and PRS signal for a sample with 20 sec of GaP growth resulting in three dimensional nucleation islands and an rms surface roughness of 1.9 nm. After 40 seconds the surface has a contiguous film with an rms surface roughness of imm as shown in figure 3. Prolonged growth of 90 minutes produces a film approximately 4455A as measured by PRS with an rms surface roughness of 2.26 nm.
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DISCUSSION During the initial nucleation stage, corrugations in the surface affect the PRS reflectance, which can be described by an effective dielectric function[6] r. = e(l + 2q) + ea(1- q)
(1)
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over a range for the corrugation parameter of 0.-q•1 as the film goes from ambient to contiguous film. Therefore, during partial coverage of the surface the effective dielectric function is smaller than that of the contiguous film. This may be seen in the increased reflectance during film nucleation as it deviates from two-dimensional growth as shown in the initial peak feature in figure 4 (a).
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