The Chemical Modification Of The Si(111)-7×7 Surface By XH_n (X=N, P, O, And S) Molecules

Chemical modification of silicon surface has been an active field in surface chemistry study because of its potential applications in integrated circuits and semiconductor device manufacture.In order to satisfy the neeed for microelectronic industry evolution,various chemical modification techniques,which makes silicon material possess more excellent properties,such as light,electricity,mechanics and so on,have been achieved after numerous studies.However,during the course of exploration for endowing silicon material with extra functions,industrial production processes are usually what people first master,and little is known about the corresponding intermediate processes and reaction mechanisms,which may be helpful in designing and developing new silicon materials.In this paper,based on our density functional cluster model calculations,we report the dissociative adsorption mechanisms of NH₃,PH₃,H₂O,and H₂S on the Si(111)-7×7 surface,whereas these four simple molecules are the very common gas resources that are used to modify silicon.The relevant results are as follows:(1) The dissociation of NH₃ to NH₂(a) and H(a) easily occurs across an adatom-rest atom pair,and the rest atom(Si_r) is more reactive than the adatom(Si_a) toward NH₃.Further N-H bond dissociation can be facilitated by the elevated temperature.It starts by an NH_x(x=2,1) insertion.H₂ desorptions can be achieved at high temperature to form a surface Si=N or a subsurface Si₃N unit,both of which are the building blocks in silicon nitride.(2) The intial dissociation of PH₃ on the Si(111)-7×7 surface exhibits the same site selectivity as NH₃.Further P-H bond dissociation can be accomplished by two pathways.On the one hand,PH_x(x=2,1) can insert into the Si-Si backbond at elevated temperature,and a surface Si=P or a subsurface Si₃P unit was formed via subsequent H₂ liberations.On the other hand,the PH₂ species adsorbed on the Si_r site may completely dissociate across the other two unsaturated adatoms,and P adsorbed on the Si_r site may desorb in P₂ gas form at high temperature. (3) The dissociation of water across an adatom-rest atom pair is not the only reaction mechanism.With increasing exposures to water,oxygen atoms are gradually accumulated around the Si_a site,up to the formation of Si_a⁴⁺ oxidation state,and the thermal stability of these oxidation species gets higher and higher.For the oxidation channel from OH group migration to O atom insertion,the activation energy barriers for OH migration are 58.4,65.3, and 79.2 kcal/mol,respectively,whereas those for O atom insertions beginning with directly destroying Si_a-Si_s backbonds by a foreign H₂O molecule are much lower,the respective values are 19.0,16.7,and 24.8 kcal/mol.Hence,kinetically,the latter is more favorable than the fomer.(4) Being distinct from NH₃,PH₃,and H₂O,the initial dissociation of H₂S on Si(111)-7×7 is a very fast reaction not via molecular precursor.It should be attributed to the fact that H₂S possesses the weakest basicity among these molecules.(5) Similar to the oxidation process of Si(111)-7×7 by water,the gradual insertion of S atom into Si_a-Si_s backbond is possible to occur,and the insertion pathway,which begins with directly destroying Si_a-Si_s backbonds by a foreign H₂S molecule,remains more favorable.However,when H₂S attacks at the Si_a site to split Si_a-Si_s backbond with its S or H atom,there is no distinct thermodynamical and kinetical difference between these two attacking behaviors,whereas Si_a-Si_s backbond is easilier broken when H₂O attacks at the Si_a site with its O atom.This mainly lies in the fact that the electronegativity of oxygen is larger than that of sulfur.