Reaction kinetics and mechanisms of inorganic hydrides on germanium surfaces

The surface reactivity of germanium is of interest because of novel Si$\sb{x}$Ge$\sb{1-x}$ heterostructure applications and the insight into semiconductor surface chemistry attainable through comparative studies on silicon and germanium. The adsorption of the inorganic hydrides H$\sb2$S, H$\sb2$O, NH$\sb3$, and HX (X = Cl, Br) on Ge(100) was investigated by temperature-programmed desorption (TPD) for the first time. The initial sticking probability $S\sb0$ for H$\sb2$S exhibits at most only a minor temperature effect, remaining roughly constant at $\sim$0.23 within the temperature range 173 K $\le$ T $\le$ 373 K. Adsorbed H$\sb2$S decomposes upon heating into H$\sb2$ and GeS, which desorb at 570 K and 660 K, respectively. The initial sticking probability for H$\sb2$O depends strongly upon substrate temperature, dropping from 0.28 at 173 K to $\sim$0.02 at 273 K. Adsorbed water decomposes to yield H$\sb2$ and GeO, which desorb at peak temperatures of 570 K and 660 K, respectively. Both HCl and HBr adsorb on Ge(100), and desorb upon heating to 570 K as H$\sb2$ and to 580 K as HX in roughly equal proportions. The remaining X(a) desorbs as GeX$\sb2$ at 680 $\sim$ 760 K. The initial sticking probability for HBr is 0.77 at 273 K, falling slightly to 0.55 at 373 K, whereas S$\sb0$ for HCl falls from 0.30 at 273 K to 0.066 at 373 K. The peak shapes and independence of peak temperature with coverage for H$\sb2$ and HX for all adsorbates indicate approximately first-order desorption kinetics, but GeX$\sb2$ desorption follows second-order desorption kinetics. Adsorption of NH$\sb3$ was not observed, implying $S\sb0$ $\le$ 6 $\times$ 10$\sp{-5}$. The adsorption and desorption behavior of these molecules can be understood by regarding the Ge(100) dimer atoms as being linked by a strained double bond and examining analogous addition and elimination reactions of molecular germanium compounds