5.4.1 Surface Chemistry of Sulfidized GaAs Surface
Figure 5.4 displays As 2p3/2 and Ga 2p3/2 photoemission spectra of the as-deposited Al2O3/GaAs bilayer with and without (NH4)2S interfacial passivation; the thickness of the overlying Al2O3 thin film after 60 deposition cycles was ca. 63 (±3) Å. Thus, we employed the high surface sensitivity of this technique (maximum sampling depths of ca. 31 and ca. 55 Å for the As 2p3/2 and Ga 2p3/2 spectra, respectively) to characterize the content of
GaAs-related chemical species diffused into the high-k layer [22]. In accordance with the Ga 2p3/2, As 2p3/2, Al 2p, and O 1s core levels, we evaluated the average concentration of existed GaAs oxides and the stoichiometric ratio of O to Al, based on the mixture of Ga2Ox and As2Ox in the Al2Ox bulk layer. As indicated in Table 5.2, the concentrations of both the Ga and As oxide species were much less than 0.5%. So that, we infer that both kinds of native oxides, As2Ox and Ga2Ox, in the forms of their stoichiometric oxides and suboxides more likely formed close to oxide-substrate interface. But, we cannot rule out the possibility that a tiny amount of As oxides diffused into the Al2O3 high-k film during deposition at substrate temperature up to 300 oC. Besides, the formation of these oxides, in particular the gallium oxides, could be suppressed by forming Ga- and As-related sulfur bonds at the dielectric interface. Interestingly, it appears that, the stoichiometry of Al2Ox in the deposited film may correlate with the degree of GaAs oxides formed nearby the interface. Provided that Al2O3
was directly deposited on GaAs, the O/Al chemical ratio was as high as 1.75, i.e., an oxygen-enriched Al2Ox dielectric film; long-term sulfide immersion could return this value to a nearly ideal stoichiometric ratio of 1.57 as a result of the enhanced surface stability.
The As 3d and Ga 3d core level spectra in Fig. 5.5 allowed us to characterize the interfacial composition close to the substrate, because the inelastic mean free paths (IMFPs) were above 25 Å. We found that an amorphous As layer was present on the bulk GaAs;
moreover, its content could be decreased upon chemical treatment with (NH4)2S. We suggest that these interfacial As atoms could have arisen through two mechanisms: (a) the dilute HCl solution used to clean the GaAs might have led to the formation of elemental As atoms covering the GaAs surface [23]; (b) thermal transformation of the surface As oxides might have occurred through reactions with the GaAs substrate [24]. The As suboxides can desorb at ca. 150–200 °C, and the As2Ox species having higher oxidization states (x = 3 or 5) will sublime at temperatures of ca. 250–300 °C [24]-[26]. This ready thermal desorption indicates that mechanism (b) probably occurred during ALD deposition at 300 °C. We also calculated
the relative contributions of elemental As and the As2Ox and Ga2Ox components in the respective As 3d and Ga 3d spectra (Table 5.3). Evidently, the sulfur-terminated GaAs surface successfully decreased the content of the As-rich layer and its chemical oxides.
The O 2s photoelectron signal observed at 23.4 eV, which mostly originated from the overlying Al2O3 thin film, overlapped with the Ga 3d core level. The content of Ga–O oxide species existing near the Al2O3–GaAs interface decreased after sulfidization. According to the literature [27], [28], the Ga–S chemical bond, relative to the As–S bond, has the higher bond strength up to 400 °C, thereby effectively restraining the growth of Ga2Ox. These sulfur species are not readily identified in a S 2p spectrum because of partial overlap with the signal of bulk Ga 3s, but a small number of GaSx bonds were detectable at a peak position of ca.
162.2 eV (not shown here) [29]. Along with the high stabilization of the Ga2O3 stoichiometric oxide, these features taken together reasonably explain the absence of the Ga–O signal in the Ga 2p3/2 spectra after surface sulfidization. In view of the improved behavior, we believe that the sulfur-terminated GaAs substrate enhanced the deposition quality and dielectric stoichiometry of the ALD-Al2O3 thin films.
5.4.2 Capacitor and Gate Leakage Characteristics
Figure 5.6 displays the C–V and I–V characteristics of the various samples. The (NH4)2S-treated sample displayed a higher oxide capacitance accompanying a decreased C–V frequency dispersion, relative to those of the untreated sample; a reduction in hysteresis width and Jg were also achieved [Figs. 5.6(b) and 5.6(c)]. These results are indicative of the abatement of the Fermi level pinning effect in the capacitor properties; in other words, modulation of the surface potential and carrier manipulation was enhanced. As stated above, sulfide pretreatment suppressed the amounts of unstable As–As and As–O species, which are believed to be a source of high-density interfacial traps. The value of Dit close to the midgap
was estimated to be ca. 1 × 1013 cm–2 eV–1 for the untreated sample; subsequent sulfide passivation reduced this value to ca. 7 × 1012 cm–2 eV–1. Previous studies have found that sulfidized GaAs improved the MOS characteristics as a result of eliminating As-related surface defects [30], [31]. The subsequent PMA at 400 °C, however, not only caused a larger frequency dispersion and broadened hysteresis width at the depletion region but also increased the value of Jg by nearly four orders of magnitude. The underlying mechanisms responsible for the PMA-induced gate leakage degradation are discussed below.