Real-time Optical Monitoring of Epitaxial Growth Processes by p-Polarized Reflectance Spectroscopy
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with submonolayer resolution[4-9]. PRS is based on a light beam that is parallel-polarized to the plane of incidence (p-polarized light beam) impinges the surface at the Brewster angle (pB of the substrate, as schematically illustrated in fig. 1. On the silicon/vacuum interface the p-polarized reflectance component in the weak absorbent region ( X> 500 nm) is in the order of 10-4. Therefore, the reflected intensity is a sensitive function of any changes in the dielectric function of silicon surface, which may be due to either temperature-induced changes in the dielectric function of the substrate, a surface coverage/roughening, chemical surface modifications or overgrowth by a thin film having a dielectric function that differs from that of the substrate. The former effect provides for real-time monitoring of temperature changes at the substrate surface and the latter effects provide for the real-time monitoring of surface adsorption and overgrowth processes. A simultaneous application of laser light scattering (LLS) measurement provides additional insight in the nucleation process and into the evolution of the surface morphology during the deposition
process. In this paper we illustrate the capability of PRS on results obtained during homoepitaxial GaP growth and GaP heteroepitaxy on Si. EXPERIMENT To demonstrate the capability of PRS, we applied single-wavelength PRS during GaP
heteroepitaxy on Si and GaP homoepitaxy under pulsed chemical beam epitaxy conditions. During the deposition process, the surface is sequential exposed to the precursors TEG [Ga(C 2H 5) 3], and TBP [(C 4 H 9 )PH 2 ] with an continuos flow of hydrogen in the background. A ppolarized light beam generated by a HeNe laser (X=6328A) and a Glan-Thompson prism 341
Mat. Res. Soc. Symp. Proc. Vol. 406 ©1996 Materials Research Society
impinges on the substrate at an angle of incidence (p=72 ', which can be adjusted in the range of 70°-75°and set with an accuracy of 0.01'. The reflected beam is detected by a Si photodiode and the intensity of the scattered radiation (LLS) is simultaneously monitored by a photo multiplier tube (PMT) located 450 from the plane of incidence. The switching of the sources is synchronized with the data acquisition of the PRS and LLS.
SubstrateI rc
film
Figure 1.Schematic representation of the principle of PRS. RESULTS Figure 2 shows the time-evolution of the PRS and LLS signals during the growth of GaP on Si(001) at 350'C with a precursor cycle sequence time of 3 sec. Due to interference phenomena, minima and maxima are observed in the time evolution of the reflectivity as the film grows. Superimposed on the interference oscillations of the reflected intensity is a fine structure (see insert in fig. 2) that is strongly correlated to the timed sequence of the supply of precursors during the steady-state growth of GaP. Each peak in the fine structure represents a complete precursor cycle, consisting of a pulse of TBP, followed by a first delay period, a triethylgallium pulse, and a second delay period. In addition, a
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