Processing of InP and GaAs Surfaces by Hydrogen and Oxygen Plasmas: In Situ Real Time Ellipsometric Monitoring
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study were commercially available SI (100) InP wafers and SI GaAs wafers with the three different orientations (100), (110) and (11 )A. The samples were introduced, through a load lock chamber, in the MOCVD reactor without any ex situ pre-treatment, and were positioned on a molybdenum susceptor whose temperature is monitored by a K-thermocouple. All the hydrogen treatments were performed at a r.f. power of 60 watt, a pressure of 1 torr, a H 2 flow rate of 800 sccm, and the substrate temperature was changed in the range 25-350'C. For r.f. plasma oxidation, the plasma was fed by 0 2 :Ar=10:2 sccm mixture, the r.f. power was 100 watt, the pressure was varied in the range 7-70 mtorr, the surface temperature was 130'C, and a dc bias between 0 and +70 V was applied to the substrates. A Hg lamp, with maximum emission at 250 nm, was used to irradiate the substrate surface in order to investigate the effect of UV light on the oxidation kinetics and oxide chemistry. Ellipsometric spectra were recorded in the range 1.5 - 5.5 eV at room temperature before and after plasma treatments and were modelled by using the Bruggemann Effective Medium Approximation (BEMA) [9]. As for the process kinetics, the previous plasma treatments were monitored in real time by single wavelenght ellipsometry (SWE). The wavelength corresponding to the E2 interband critical point (see fig. 1) was used to investigate the cleaning process of InP and GaAs, due to the high sensitivity to the surface state at E2. To study the GaAs oxidation kinetics, the wavelenght of 359 nm, where the ellipsometric measurement is insensitive to temperature variation, was used. A Perkin-Elmer 5300 spectrometer was used for the ex situ XPS analysis by using a standard Mg source (1253.6 eV). RESULTS AND DISCUSSION Hydrogen plasma treatment for native oxide removal Surface temperature and exposure time are crucial parameters in hydrogen plasma treatment of III-V surfaces. The effect of surface temperature on the cleaning kinetics, reported by us in a previous paper [10], has evidenced that a substrate temperature of 230'C is the optimum to selectively remove oxygen from InP surfaces. In addition, Gottscho reported [2] that H-atom cleaning at room temperature is able to reduce As-oxide but not Ga-oxide, whereas the complete GaAs native oxide reduction can be achieved at temperature of about 200'C. Figure 1 shows SE spectra for (100) InP and GaAs before and after hydrogen cleaning performed at T=230°C. To evaluate the quality of the surface resulting from the present hydrogen treatment, the reference SE spectra for c-InP and c-GaAs provided by Aspnes are also shown. The recorded spectra of the pseudodielectric function, (e) = (r)+ (ci), have been modeled using BEMA theory and, as demonstrated previously [10], the best-fit optical model for the hydrogen plasma cleaned III-V surfaces is the one-layer model also shown in fig. 1. From this model, it comes out that the hydrogen cleaning of III-V surfaces results in a less dense top layer, including voids, due to the selective oxyge
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