Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates at Room Temperature: Interfacial Reaction and Growth Mode 4.1 Introduction In this chapter, we will study the growth dynamics of Ni on Si(001), Ge(001) Si0.8Ge0.2(001) surfaces using in-situ XPS and ex-situ AFM In particular, we will compare the growth mode, interfacial reaction and surface morphology of Ni thin films grown on hydrogen-terminated surfaces with those on clean surfaces 4.2 XPS and AFM results of Ni growth on Si substrates 4.2.1 Ni deposition on H-terminated and clean Si surfaces at RT Figure 4.1(a) and (c) show the typical Ni 2p3/2 spectra versus Ni coverage on both hydrogen terminated and clean Si(001) surface at RT using Mg Kα X-ray source, respectively At 0.1% Ni deposition on H-Si(001) surfaces, the Ni 2p3/2 peak was observed at a binding energy (B.E.) of 854.0 ± 0.1eV (Fig 4.1(a)), which is about 1.2eV shift away from that of pure metallic Ni 2p3/2 peak (852.8 ± 0.1 eV) This value is very close to that of NiSi (854.0 ± 0.1eV) With increasing Ni coverage, the binding energy of Ni 2p3/2 shifted gradually towards the lower values Eventually, it reached and stayed at 852.8 ± 0.1 eV, a value as expected for metallic Ni thin film The B.E of Si 2p from the substrate as shown in Fig 4.1(b) and 4.1(d) stayed consistently around 105 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode 99.2 ± 0.1 eV throughout and did not show any change in shape or position Identical XPS spectra evolution can be also observed for Ni deposition on both clean and hydrogen terminated Si(111) surfaces (not shown here) Ni 2p3/2 Si 2p 89%Ni 70%Ni 53%Ni x1.0 x1.9 x1.8 x1.7 40%Ni x1.8 29%Ni Intensity(a.u.) Intensity(a.u.) Pure Ni x102 89%Ni 70%Ni x19 x6.3 x3.6 40%Ni 53%Ni 13%Ni 5%Ni x2.2 x1.4 5%Ni x1.1 0.1%Ni x4.0 29%Ni 13%Ni x2.1 x1.0 x1.0 x9.4 x200 0.1%Ni Pure Si 858 856 854 852 850 Binding Energy(eV) 103 102 101 100 99 98 97 96 Binding Energy(eV) (b) (a) Ni 2p3/2 Si 2p 61% Ni 54% Ni x1.0 x1.5 x1.5 45% Ni x1.6 35% Ni x1.8 27% Ni Intensity(a.u.) Intensity(a.u.) Pure Ni 61% Ni 45% Ni 35% Ni 27% Ni x2.1 858 856 854 852 Binding Energy(eV) x1.9 9% Ni x6.0 9%Ni x2.7 16% Ni x3.6 16% Ni x8.4 x6.0 x3.8 54% Ni Pure Si 850 x1.5 x1.3 x1.0 103 102 101 100 99 98 97 96 Binding Energy(eV) (d) (c) Fig 4.1 Ni 2p3/2 and Si 2p spectra obtained from Ni deposited on (a-b) hydrogen terminated and (c-d) clean Si(001) surface at RT with increasing Ni coverage In comparison, Ni 2p3/2 spectra from bulk Ni and Si 2p spectra from pure Si(001) substrate were included At low metal coverages, a shift in BE of overlayers to higher values has previously been observed in the growth of Ir, Pd and Au on carbon180, Al and Cu on 106 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode graphite as well as TiO2 surfaces181-183 In these systems, the shifts in BE has been attributed to the result of incomplete screening of the final electronic (hole) state This occurs if small clusters are formed on the surfaces (termed the cluster effect)184-185 In the present work, the shift of Ni 2p3/2 to a higher binding energy on both hydrogenterminated Si(001) and Si(111) at low Ni coverage is unlikely to be attributed to cluster effects This is because the surface morphology after deposition of 10% or more Ni of H-Si(001) and H-Si(111) (Fig 4.2(a)&(b)) are relatively smooth and the RMS remained as ~2.1±0.5 Å throughout out the experiment Moreover, no clusters were observed to form on the Si surface (b) (a) Fig 4.2 represent àm ì àm AFM images after 10% Ni deposited on H-terminated (a) Si(001) and (b) Si(111) surfaces at RT, respectively Experimentally, we observed that the B.E of Ni 2p3/2 (854.0 ± 0.1eV) even at Ni coverage of 0.1% is very close to the value associated with bulk NiSi (854.0 ± 0.1 eV) Therefore, we attribute this to the formation of a NiSi phase and this occurs on both clean and hydrogen terminated Si(001) and Si(111) surfaces The formation of Ni-Si bond on clean Si surfaces is not unexpected since there are unsatisfied Si dangling bonds present at the surface186-187 However, it is more difficult to rationalise the observation of a Ni/H-Si reaction and formation of a Ni-Si bond on H-terminated 107 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode Si surfaces since all the dangling bonds are now passivated In this case, there are no available reactive Si dangling bonds to facilitate direct bond formation between Ni and surface Si atoms For this reaction to occur it would imply a reaction between Ni and the H-Si bond or with the Si back bonding at the H-Si-Si surface The interstitial model188 proposed for growth on clean Si surfaces may explain how Ni silicide reaction can take place at RT on both clean and H-terminated surfaces Results from experiments by employing strain-sensitive X-ray diffraction59 and first principle calculations189-190 have suggested that at the initial deposition stage, Ni atoms can diffuse into Si crystal by either preferably occupying the interstitial sites of the Si host or diffusing to the off-center bridge site (B site) in second layer between the dimer rows189-190 The consequence of Ni occupying interstitial sites or B sites in Si is the weakening of the Si-Si covalent bond Electrons in the neighboring Si-Si covalent bonds will no longer remain in their localized states and will have to share between the interstitial and Si atoms, leading to substantial Si-Ni p-d hybridization191-192 This can result in the formation of a Ni-Si covalent bond192 at the growth front of Ni/Si or Ni/HSi surfaces Given the limited mobility of adatoms at room temperature, the NiSi phase formed at RT would likely be different from the stable crystalline orthorhombic MnP structure191, and it has been widely agreed in literature that thick Ni silicide layers grown at RT are amorphous46,48,50 We therefore described this phase as a “NiSi-like” film The observation of spontaneous silicide reaction also contradicts the assumption that there is a threshold thickness for silicide reaction to be feasible193 Our results showed that silicide reaction can occur spontaneously at RT and at a low Ni coverage of ~0.1% Ni with respect to Si Therefore, the presence of hydrogen on Si surfaces does not suppress NiSi formation at RT as previously reported49,58-59,67,189-190 108 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode Nevertheless, NiSi2 is not observed on either clean or hydrogen-terminated Si surface, although it was reported to form when Ni is deposited on clean Si surfaces46,48,50 More interestingly, with increasing Ni coverage, the binding energy of Ni 2p3/2 progressively shifted from 854.0eV to 853.8eV, indicating a change from NiSi phase to a Ni-rich silicide (Ni2Si)5 Eventually it reached and stay at the signature value of a metallic film of 852.8eV The silicide formation sequence at RT is illustrated as following: Ni + Si NiSi Ni2Si Ni The implication is that this silicidation reaction only occurs at the initial Ni/Si or Ni/HSi-Si interface and thereafter more metallic silicide phases were produced until a pure metallic Ni layer was grown above it It is known that hydrogen desorption from the Si surface is only significant above 300oC, and that the Si-H bond is particularly strong with a bond energy of 3.9 eV compared to the Si-Si bond energy of 3.2 eV157 Therefore, the silicide phase formed is most likely also H-terminated, which is consistent with Hirose’s observation that the hydrogen atoms terminating the original Si surface are still present after the Ni deposition58 Unlike clean surfaces, further deposition of Ni/H-Si at RT will therefore occur above a H-terminated silicide layer instead Irrespectively, we therefore have growth of metallic nickel above initial silicide layers on both clean and H-terminated surfaces as Ni coverage increases The existence of the hydrogen atoms at Ni/H-NiSi interface may subsequently have an effect on the diffusion and nucleation behavior of following deposited Ni adatoms This in turn may lead to a different surface morphology compared to the clean surface, which will be discussed in the next section It is interesting to note that during Ni deposition, the B.E of Si 2p as shown in Fig 4.1 (b) and (d) remains at 99.3 ± 0.1 eV regardless of Ni coverages This is similar 109 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode to the binding energy derived from analysis of bulk Ni2Si, NiSi and NiSi2 samples of 99.39 ± 0.12 eV191, and also similar to clean H-Si(001) substrates (99.2 ± 0.1 eV) The Si 2p spectra from Ni silicides also not display any noticeable energy shift during annealing194 For the Si signal detected at low Ni coverage range, the main contribution would come primarily from the Si substrate (Si 2p photoelectron escape depth ~30 Å); hence a shift in Si 2p binding energy due to formation of Ni-Si bonds may not be obvious However, as seen in Fig 4.1(b & d), this remains true even in the Ni coverage of 89% We rationalize this observation as follows As shown by both semiempirical linear combination of atomic orbital (LCAO) extended Hückel scheme and linear muffin-tin orbital (LMTO) scheme, substantial Si-Ni p-d hybridization takes place for NiSi191 Therefore, Si forms covalent bonds with Ni even after forming Ni silicide192, which exercises little effect on Si’s electronic structure and hence the binding energy of Si 2p However, the bonding effects on Ni are much more significant since there is a change in bond type from a metallic bonding geometry to a covalent bonding structure when metal silicides are formed A shift in Ni 2p3/2 BE to a higher value is therefore expected since there is a charge transfer from Ni to Si192 4.2.2 The growth mode of Ni on H-terminated Si surfaces A standard method for the determination of growth mode using XPS is to measure the adsorbate and the substrate signals changes while increasing the exposure of overlayer metal The S-t plots (photoelectron intensity signal versus evaporation time) should have a characteristic shape depending on the growth modes of adsorbate metal: layer by layer growth (FM growth), three-dimensional growth mode (VolmerWeber growth, VW), the Stranski-Krastanov (SK) growth mode 110 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode In this experiment, Ni was evaporated from the e-beam evaporator onto the Si substrates while maintaining a flux of 20nA through out the experiments The growth was paused after every 10minutes for transferring the sample into XPS analysis chamber for scan without exposing the samples to air Mg Kα X-ray with Large Area (LA) lens mode was adopted to the scans due to its relatively stable intensity over time Fig 4.3 shows the evolution of the atomic fraction of Ni (Ni 2p3/2) and Si (Si 2p) as a function of Ni deposition time on both H-terminated Si(001) and Si(111) surface at room temperature The plots obtained on the H-terminated silicon substrates appear to be exponential-like, which implies that we may have layer by layer growth mode 90 80 80 70 70 60 50 Ni% Si% 40 30 Atomic Ratio (%) 100 90 Atomic Ratio(%) 100 60 Ni% Si% 50 40 30 20 20 10 10 0 20 40 60 80 100 120 140 160 180 Growth Time(mins) 0 10 20 30 40 50 60 70 80 90 Growth Time (mins) (b) (a) Fig 4.3 Evolution of the atomic fraction of Ni and Si during Ni evaporation on (a) HSi(001) and (b) H-Si(111) surfaces There is however no distinctive series of linear breaks as would be expected from an “ideal” layer by layer growth mode There are several possible reasons for this deviation from the ideal case (a) It may be related to the silicide reaction which occurs upon deposition of Ni on H-terminated Si surfaces and the intermixing of two components resulting in silicide formation This may cause the XPS signals to be different from the values calculated according to an ideal sharp unreacted interface 111 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode model; (b) Another possible deviation from the ideal case could be attributed to a growth whereby the second layer starts to grow before the first layer is completed This mode of growth is termed pseudo-layer-by-layer or "simultaneous multilayer" growth mode, and has been similarly observed in the cases of Ni growth on clean Ge(111)72 and on Ag(111) surfaces195 (c) Alternatively, we could also be observing StranskiKrastanov growth mode instead It is therefore not easy to distinguish the growth mode further without either resorting to some simple fitting or the use of other experimental techniques to probe the surface morphology evolution We will attempt to resolve this observation in two ways: (i) curve fitting the XPS data obtained through the used of known equation that describes pseudo-layer-by-layer growth39 and (ii) examining the surface morphology at various stages of growth using the AFM For pseudo layer by layer growth mode, the decay of XPS signal from substrate ( I sn ), i.e area under of Si 2p peak, as a function of material thickness (d) is given by: I sn = I s0 exp( − nd a λs ) (4.1) where I s0 is the XPS intensity from the pure substrate, n is the number of monolayers, and λs is the substrates’ photoelectron inelastic mean free path (IMFP) in the substrate n Similarly, the intensity of the adsorbate signal ( I a ), i.e area of Ni 2p3/2 peak, will increase as: n ∞ I a = I a [1 − exp(− nd a λa )] (4.2) 112 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode ∞ where I a is the signal intensity associated with a clean bulk-like film of the adsorbate, λa is the adsorbate’ photoelectron IMFP in the adsorbate Equation 4.1 and 4.2 can also be expressed as a function of time, where the product (n×da) can be written as a product of growth rate (G, in Å/min) and time (t, in minutes) The atomic fractions of CSi and CNi can thus be expressed as a function of time For atomic fractions of Si, we have, ISi (t) 2p CSi (t) = Si SSi ⋅ T2p 2p ISi (t) 2p Si SSi ⋅ T2p 2p + Ni I 2p3 (t) Ni Ni S2p3 ⋅ T2p3 100% ⋅ ⋅100% =  G⋅t ISi ⋅ exp  −  Si Si S2p T2p  λSi    G⋅t  G ⋅ t  ISi I∞ exp  − + Ni Ni Ni 1 − exp  −   Si Si S2p ⋅ T2p  λSi  S2p3 ⋅ T2p3   λ Ni    G⋅t  100% ⋅ exp  −   λSi  CSi (t) = Si SSi ⋅ T2p    G ⋅ t   I∞  G ⋅ t  2p Ni exp  −  +  I0 ⋅ SNi ⋅ T Ni  ⋅ 1 − exp  − λ   Ni    λSi   Si 2p3 2p3      (4.3) Ni Ni Si where S2p3 , T2p3 , SSi and T2p are the sensitivity factors and instrumental transmission 2p factors associated with detecting Ni 2p3/2 and Si 2p XPS peaks respectively For VG ESCALAB 220i-XL system using Mg Kα X-ray and large area lens mode, we have Ni Ni Si o S2p3 T2p3 = 2.28x104 and SSi T2p = 853196 Intensity ISi (Si substrate before 2p deposition) and I∞ (clean Ni foil) are determined on the day of the experiments Ni Ni Ni Si o Hence, the values of ISi , I∞ , S2p3 , T2p3 , SSi and T2p are thus constant By letting 2p Ni  I∞ SSi ⋅ TSi  K =  Ni ⋅ Ni 2p 2p  , the atomic faction of Si is thus given by: Ni  ISi S2p3 ⋅ T2p3    113 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode  G⋅t 100% ⋅ exp  −   λSi  CSi (t) =   G⋅t  G ⋅ t  exp  −  + K ⋅ 1 − exp  −   λSi   λ Ni    (4.4) and the atomic fraction of Ni is given by: Ni I 2p3 (t) C Ni (t) = Ni 2p3 S Si 2p Si 2p I (t) S ⋅T Si 2p + ⋅T Ni 2p3 Ni 2p3 Ni Ni 2p3 2p3 I S (t) ⋅T   G ⋅ t  1 − exp  −   λ Ni    ⋅100% =   G⋅t  G ⋅ t  ISi I∞ exp  − + Ni Ni Ni 1 − exp  −  S  Si Si S2p ⋅ T2p 2p3 ⋅ T2p3   λSi   λ Ni   100% ⋅ I∞ Ni Ni Ni S2p3 ⋅ T2p3   G ⋅ t  100% ⋅ 1 − exp −   λ Ni    C Ni (t ) =  G ⋅ t   G ⋅t   exp −   + 1 − exp − K  λ Ni    λSi   (4.5) In order to determinate the nature of the growth mode, equations 4.4 and 4.5 were used to fit the experimental data as given in Fig 4.3 The electron inelastic mean free path (IMFP), λSi and λNi, is a constant for a given material and kinetic energy197 As for the choice of λSi and λNi, we will use the empirical equation previously developed by M.P Seah and W.A Dench197 Their equation relates the IMFP (λ, in units of nm) to the corresponding photoelectron kinetic energy (Ekin, in units of eV) and the type of material through which the photoelectrons traverses and is given below by equation 4.6 λa = 538a + 0.41a E kin E kin (4.6) where a is the monolayer thickness of the film in units of nm, is given by 114 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode As explained earlier in Section 4.2 and 4.3, substantial p-d hybridization in both Si-Ni and Ge-Ni take place when Ni was deposited on Si and Ge substrates and reacted with the substrates Therefore, covalent bonds are formed between Si0.8Ge0.2 with Ni after forming Ni germanosilicide, which exercises little effect on the B.E of Si 2p and Ge3d since their bonding still remain as covalent bonds However, bond type in Ni has changed from a metallic bonding geometry to a covalent bonding structure when metal germanosilicide is formed, which exercises a significant effect on Ni and is displayed by a shift in Ni 2p3/2 BE to a higher value 4.4.2 The growth mode of Ni on H-terminated Si0.8Ge0.2(001) surfaces XPS is again employed as the technique to determinate the growth mode by measuring the adsorbate and the substrate signals changes while increasing the exposure of the overlayer metal In this experiment, Ni was evaporated from the ebeam evaporator onto the H-Si0.8Ge0.2(001) substrate while maintaining a flux of 20nA throughout the experiments The growth was paused after every 10minutes for transferring the sample into XPS analysis chamber for scan without exposing the samples to air Mg Kα X-ray with Large Area (LA) lens mode was adopted to the scans due to its relatively stable intensity over time Fig 4.21 shows the evolution of the atomic fraction of Ni (Ni 2p3/2), Si (Si 2p) and Ge (Ge 3d) as a function of Ni deposition time on H-terminated Si0.8Ge0.2(001) surface at room temperature The plots obtained on the H-Si0.8Ge0.2 substrates appear to be exponential-like, similar to what we obtained earlier when Ni was grown on H-Si & H-Ge surfaces This implies that we may have layer-by-layer growth mode 149 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode 100 90 Atomic Ratio(%) 80 70 60 Ni% Si% Ge% 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 100 110 120 Growth Time(mins) Fig 4.21 Evolution of the atomic fraction of Ni, Si and Ge during Ni evaporation on H-Si0.8Ge0.2 (001) surface There is however no distinctive series of linear breaks as would be expected from an “ideal” layer by layer growth mode The possible reasons for this deviation from ideal case have been explained earlier in Ni’s growth mode on Si surface It is not easy to distinguish the growth mode further without either resorting to some simple fitting or the use of other experimental techniques to probe the surface morphology We will attempt to resolve this observation in two ways; (i) curve fitting the XPS data obtained through the used of known equation that describes pseudo-layer-by-layer growth39 and (ii) examining the surface morphology at various stages of growth using the AFM Similar to equation 4.4 and equation 4.5, the atomic factions of Ni (CNi (t)), Si (CSi (t)) and Ge (CGe (t)) as a function of Ni growth time (t, in minutes) are given by equations 4.11, 4.12 and 4.13   G ⋅ t    λ Ni    C Ni (t) =   G ⋅ t   G⋅t  G⋅t 1 − exp  −   + K1 exp  − λ  + K exp  − λ   λ Ni     Ge  Si   100% ⋅ 1 − exp  − (4.12) 150 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode  G⋅t 100% ⋅ exp  −   λSi  CSi (t) =   G ⋅ t   G⋅t  G⋅t K1 1 − exp  −   + exp  − λ  + K exp  − λ  Si   λ Ni     Ge   (4.13)  G⋅t  100% ⋅ exp  −   λ Ge  CGe (t) =   G ⋅ t   G⋅t  G⋅t  K 1 − exp  −   + K exp  − λ  + exp  − λ  Si   λ Ni     Ge   (4.14) where G is growth rate in unit of Å/min λSi, λGe and λNi are the electron inelastic mean free paths (IMFP) of Si 2p, Ge 3d and Ni 2p3/2 photoelectrons, which are calculated to be 15, 15 and Å, respectively according to the empirical equation 4.6 developed by 197 M.P Seah and W.A Dench Si  I∞ SSi T2p  2p Ni K1, K2 and K3 are equal to  Ni  , Ni  ISi S2p3 T2p3     I∞  ISi SGe T Ge  SGe T Ge  Ni Ni Si Si Ge Ge Ni  Ni 3d 3d  , and  3d 3d  , where S2p3 , T2p3 , S2p , T2p , S3d and T3d are Si Ni IGe S2p3 T2p3  IGe SSi T2p    2p     the sensitivity factors and instrumental transmission factors associated with detecting Ni 2p3/2, Si 2p, and Ge 3d XPS peaks, respectively For VG ESCALAB 220i-XL Ni Ni system using Mg Kα X-ray and large area lens mode, we have S2p3 • T2p3 = 2.28x104 Si Ge o SSi • T2p = 853, and SGe • T3d = 1481196 Intensities Io and ISi (Si0.8Ge0.2 substrate before 2p 3d Ge deposition) and I∞ (clean Ni foil) were determined on the day of the experiments Ni Hence, K1, K2 and K3 are constants for each independent experiments at different days Ni Si Ni Ge Ge o since the values of ISi , I∞ , Io , SSi , S2p3 , S3d , T2p , T2p3 and T3d are constants 2p Ni Ge Figure 4.22 shows the experimental data for Ni% , Si% and Ge% evolution on H-Si0.8Ge0.2(001) surface re-plotted together with the fits (dashed lines) given by equations 4.11, 4.12 and 4.13 A reasonable fit using a single fitting parameter G can 151 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode be obtained for the atomic fraction of Ni, Si and Ge at various deposition times A value of G~0.39 ± 0.01 Å/min is found for the Ni growth on H-Si0.8Ge0.2(001) surface The implication of the above results is that Ni appears to follow a pseudo-layer-bylayer growth mode on these H-terminated Si0.8Ge0.2 surface The resulting morphology of the film following this growth mode will likely be smooth Consequently, the morphology of pure H-Si0.8Ge0.2(001) and the surfaces at various stages of growth were also examined by the AFM and were shown in Fig 4.23 The surface morphology after Ni deposition appears to be again decorated with continuous shallow close-packed Ni 2-D domes across the whole coverage range The domes typically have a size of ranging from 9±1 to 12±1nm and a height ranging from 1.0±0.2 to 2.0±0.2 Å Hence, the height to size aspect ratio is ~1:70, which suggests a significant wetting of the H-Si0.8Ge0.2 surfaces by Ni The surface appears to be smooth and flat with a RMS roughness of only ~ 2.4±0.2 Å throughout all coverage, which is consistence with the XPS simulation result that the growth of Ni on the H-terminated surface follows a pseudo layer by layer growth mode 152 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode 90 90 80 80 70 70 Atomic Ratio(%) 100 60 50 40 30 60 50 40 30 20 20 10 10 0 10 20 30 40 50 60 70 80 90 100 110 120 10 20 30 40 50 60 70 80 90 100 110 120 Growth Time(mins) Growth Time(mins) (a) (b) 100 90 80 70 Atomic Ratio(%) Atomic Ratio(%) 100 60 50 40 30 20 10 0 10 20 30 40 50 60 70 80 90 100 110 120 Growth Time(mins) (c) Fig 4.22 Diagrams (a), (b) and (c) represent Ni, Si and Ge atomic fractions for Ni/HSi0.8Ge0.2 (001) surface as a function of Ni deposition time, respectively The dashed lines represent the simulated pseudo-layer-by-layer growth mode calculated according to equation 4.11, 4.12 and 4.13 with λSi = 15 Å, λGe = 15 Å and λNi = Å 153 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode (a) (b) (c) (d) (e) (f) Fig 4.23 1àm ì 1àm AFM images of (a) H-Si0.8Ge0.2(001) and various Ni coverages equivalent to atomic fractions of (b) ~10%, (c) ~20%, (d) ~30%, (e) ~42% and (f) ~84% Ni on H-terminated Si0.8Ge0.2(001) surfaces at room temperature 4.4.3 Comparison with growth mode of Ni on clean Si0.8Ge0.2 (001) surface Since hydrogen desorption peak temperature from the Si & Ge surface was ~300oC157,158, a clean Si0.8Ge0.2 (001) surface was prepared by slowly heating a hydrogen-terminated Si0.8Ge0.2 (001) to 500oC and keeping it at 500oC for minutes After the Si0.8Ge0.2 substrate was cooled down to RT, similar Ni deposition conditions, i.e., a flux of 20nA and minutes growth followed by in-situ scanning, were adopted to probe the growth mode of Ni on clean Si0.8Ge0.2 (001) surface Figure 4.24 shows the experimental data for Ni%, Si% and Ge% evolution on clean Si0.8Ge0.2(001) surface as a function of Ni growth time plotted together with the fits (dashed lines) given by equations 4.11, 4.12 and 4.13 A reasonable fit through the experimental data 154 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode points can clearly be obtained for the atomic fraction of Ni, Si and Ge at various deposition times A value of G~0.38 ± 0.01 Å/min is found for the Ni growth on clean Si0.8Ge0.2(001) surface The implication of the above results is that Ni appears to follow a pseudo-layer-by-layer growth mode on this clean Si0.8Ge0.2 surface The resulting morphology of the film following this growth mode will likely be smooth Consequently, Fig 4.25 shows the m × m AFM scans taken of the surfaces of clean Si0.8Ge0.2 (001) and those with Ni coverages equivalent to atomic fractions of 13%, 21%, 41%, 58% and 80% on clean Si0.8Ge0.2(001) surface While the bare hydrogen-terminated Si0.8Ge0.2 (Fig 4.23(a)) was smooth, the clean Si0.8Ge0.2 surface in Fig 4.25(a) appeared to be rougher while decorating with larger domes in typical size ranging from 10±1 to 13±1nm with a height ranging from ~2.0±0.2 to 4.2±0.2 Å, which can be extracted from the line profile in Fig 4.25(b) The RMS of m×1 m AFM images for clean Si0.8Ge0.2(001) surface was 3.7Å, compared to 2.4Å for H-Si0.8Ge0.2 (001) surface 155 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode 90 90 80 80 70 70 Atomic Ratio(%) 100 60 50 40 30 60 50 40 30 20 20 10 10 0 10 20 30 40 50 60 70 10 20 30 40 50 60 70 Growth Time(mins) Growth Time(mins) (a) (b) 100 90 80 70 Atomic Ratio(%) Atomic Ratio(%) 100 60 50 40 30 20 10 0 10 20 30 40 50 60 70 Growth Time(mins) (c) Fig 4.24 Diagrams (a), (b), (c) represent Ni, Si and Ge atomic fractions as a function of Ni deposition time on clean Si0.8Ge0.2 (001) surface, respectively The dashed lines represent the simulated pseudo-layer-by-layer growth mode calculated according to equation 4.11, 4.12 and 4.13 with λSi = 15 Å, λGe = 15 Å and λNi = Å 156 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode Z[Å] (a) 0 10 20 30 40 50 60 70 80 90 X[nm] Z[nm] 0.8 (b) 0.6 0.4 0.2 0 20 40 60 80 100 120 140 160 X[nm] 1.2 Z[nm] 0.8 (c) 0.6 0.4 0.2 0 20 40 60 80 100 120 140 160 X[nm] Z[nm] 0.8 (d) 0.6 0.4 0.2 0 20 40 60 80 100 120 140 160 X[nm] 1.4 1.2 Z[nm] (e) 0.8 0.6 0.4 0.2 0 20 40 60 80 100 120 140 160 X[nm] (I) (II) Fig 4.25 1àm ì 1àm AFM images (column I) and line profiles (column II) of (a) clean Si0.8Ge0.2 (001) and various Ni coverages equivalent to atomic fractions of (b) ~13%, (c) ~21%, (d) ~41%, and (e) ~58% on clean Si0.8Ge0.2 (001) surface at room temperature 157 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode At 13% Ni coverage (see Fig 4.25(b)), the surface is decorated with Ni islands with size ranging from 10±1 to 15±1nm and the height between 0.2 and 0.8nm (the height measurement for all coverages is measured from the maximum to the height position marked by the island boundary) Compared to Ni domes on hydrogenterminated Si0.8Ge0.2(001) surface which have a size of ranging from 9±1 to 12±1nm and height ranging from 1.0±0.2 to 2.0±0.2nm, Ni domes on clean Si0.8Ge0.2 (001) surface slightly expanded in lateral dimension but grew significantly in height The islands thus have a height to size aspect ratio equivalent to 1:22, which is considerably smaller than the 1:70 observed for Ni growth on hydrogen-terminated surface Beside Ni islands, two types of dark voids started to appear on the surface, type-A (marked with circle box) and type-B (marked with square box) The type-A voids were normally wider than 40nm and deeper than 1.5nm The width of type-B voids were generally ~10-30nm and their depth were ~0.8-1.5nm Both types of voids were attributed to areas not covered by the deposited Ni, namely incomplete growth of the Ni layer However, since the depths of the type-A voids are higher than twice the height of the Ni domes while the type-B voids are close to or less than twice the height of the Ni domes, we assign type-A and type-B voids to area not covered by Ni domes for more than two consecutive layers and less than two consecutive layers, respectively The incomplete coverage of Ni on the surface has led to an increase of RMS from 3.7Å for clean Si0.8Ge0.2 (001) to 6.2Å for 13%Ni/clean Si0.8Ge0.2 (001) At a Ni coverage of 21%Ni (Fig 4.25(c)), line scan analysis again reveals that the Ni domes sizes slightly increased to ~12-20nm and the height still remained ~0.20.8nm Thus there was no significant increase in height or size even though the coverage had increased The further deposited Ni seemed to wet the previous layer, 158 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode filling in both the type-A and type-B voids and then growing on top of it, which were evidenced by the reduced density of both types of voids and a decrease in RMS to 5.1Å from an earlier value of 6.2Å in 13%Ni At a Ni% of 41% (Fig 4.25(d)), line scan analysis revealed that the surface seemed to have two layers of islands: the lower layer appeared to be around 0.2-0.4nm high and the higher one was taller by another 0.2-0.4nm, adding to an overall height of ~0.4-0.8nm The top layer was stacking above the proceeding layer formed earlier By increasing the Ni coverage equivalent to 58% (Fig 4.25(e)), we again find that the surface was still decorated by the two-layer structure as seen previously at 41% (see Fig 4.25(d)) The density of islands having a height between 0.4 and 0.8nm has increased while the density of lower islands with height of ~0.2-0.4nm has significantly decreased However, no islands having a height between 0.8-2.4nm are spotted The implication of these fixed size islands at various stages of deposition was that the growth mode of Ni on clean Si0.8Ge0.2(001) is close to “layer by layer” growth; i.e the 2nd stack of islands starts to grow when the 1st stack of islands has almost completely packed the initial surface Similarly the 3rd stack of islands is seen only when the 2nd layer is nearly covered by islands with dimension of ~10-20nm and height of 0.5-0.7nm In doing so, there is no significant 3D growth and film is also flat, although it will continue to leave voids on the previous layers and lead to a less dense film We therefore describe the growth of Ni on clean Si0.8Ge0.2 (001) as compactisland layer growth, similar to that of Ni growth on clean Si surfaces described earlier An aspect ratio of 1:22 would also imply some wetting of the Si0.8Ge0.2 surface by Ni during deposition, although it is much smaller than the 1:70 observed when Ni was grown on H-Si0.8Ge0.2(001) These factors coupled with the compact island growth thus make the distinction of growth mode by XPS between pseudo layer by layer and 159 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode compact islands growth difficult Without further analysis of the surface by AFM, we would not have been able to realise there is a difference in the growth dynamics and resulting films’ quality observed between H-terminated and clean Si0.8Ge0.2(001) surface 4.4.4 Role of surfactants on growth mode Similar to that on a Si substrate, the driving force for a Ni film to grow in a 2D layer by layer mode on Si0.8Ge0.2 surfaces requires the sum of surface free energy of Ni and interfacial energy of Ni/Si0.8Ge0.2 to be less than the surface free energy of Si0.8Ge0.2 substrate; i.e γNi/v + γNi/Si0.8Ge0.2 < γSi0.8Ge0.2/v (4.15) where γNi/v is the surface energy of Ni-vacuum interface, γNi/Si0.8Ge0.2 is the interfacial energy between Ni and Si0.8Ge0.2, and γSi0.8Ge0.2/v is the surface energy of Si0.8Ge0.2vacuum interface This driving force may again be rationalized qualitatively from consideration of bond enthalpies It has been reported that Ni-Ni, Ni-Ge, Ni-Si and SiGe bond enthalpies are 204 kJ mol-1, 290 ± 11 kJ mol-1, 318 ± 17 kJ mol-1 and 297 kJ mol-1, respectively179 Based on the above values, Ni-Ni bond appears to be significantly weaker than both Ni-Ge and Ni-Si bonds, which implies a preferred bond formation between Ni and Si or Ge in the substrate, rather than forming Ni-Ni bonds Consequently, the Ni atoms landing on the surface will spontaneously react with Si0.8Ge0.2 For Ni atoms that land on existing Ni, some may have sufficient energy to diffuse to the edge of the incomplete 2D Ni layer and then react with the remaining exposed Si0.8Ge0.2, while others may simply adsorb and diffuse on top of the Ni layer 160 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode In this way, pseudo 2D layer of Ni will form on the surface Once the Si0.8Ge0.2 surface is fully covered by Ni, additional Ni atoms can only react with and grow on the underlying Ni layer Bond enthalpy considerations suggest the presence of a driving force for the 2D growth mode of Ni at room temperature The present XPS experiment results also suggest that there is spontaneous bond formation between Ni and Si0.8Ge0.2 prior to any thermal treatment on both H-terminated and clean Si0.8Ge0.2 surfaces Thus wetting of Ni on both clean and H-terminated surface is energetically favorable The morphology obtained on H-Si0.8Ge0.2 (001) (Fig 4.23) and clean Si0.8Ge0.2 (001) (Fig 4.25) is rather different as shown by AFM examinations Smooth and continuous layer growth was seen on H-terminated surfaces while compact island growth was observed on clean Si surfaces It should be noted that while the Ni layers grown on either clean or on H-terminated surface oxidize when the samples are exposed to air, it will occur similarly on both types of surfaces with the same Ni coverage Furthermore, the surface morphology observed also does not change in size or form with increasing exposure time to air It is therefore unlikely that the oxidation processes modified the morphology of Ni grown on H-terminated and clean surfaces to be the same The difference in morphology observed on H-Si0.8Ge0.2(001) (Fig 4.23) and clean Si0.8Ge0.2(001) (Fig 4.25) can be rationalized as follows The passivation of Si0.8Ge0.2 dangling bond by hydrogen on the surface appears to promote diffusion of adatoms on the surface in such a way that arriving Ni adatoms have sufficient energy and hence longer diffusion length to either diffuse along the substrate surface so as to join existing 2D islands laterally, or diffuse to the edge of such islands and get incorporated there if they first land on top of these existing 2D islands In doing so, layer-by-layer growth mode is facilitated and the resulting surface of the film is thus 161 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode smooth and flat As the formation of Ni-Si and Ni-Ge bonding occurs on both bare and H-passivated Si0.8Ge0.2 surfaces, the above results can only be reconciled if only H “floats” to the surface above the initial deposited Ni layer When this happens, continuously wetting is allowed and the morphology thus obtained will be flat and smooth On a clean Si0.8Ge0.2 surface however, the Si and Ge dangling bonds can easily react with the deposited Ni adatoms upon deposition, which leads to a shorter diffusion length of Ni adatoms and therefore restricts the metal adatoms migration on the Si0.8Ge0.2 surfaces As a consequence Ni adatoms can neither diffuse long enough along the substrate surface to join existing 2D islands laterally, nor to diffuse to the edge of such islands and get incorporated there if they first land on top of these existing 2D islands The formation of Ni domes will occur at growth front of the Si0.8Ge0.2 surface, which will tend to grow upwards than sideward As a result, constant presence of the dark voids/trenches is expected since Ni adatoms are difficult to diffuse and fill in those voids due to a reduced mobility Thus, the surface obtained is rougher 4.5 Summary The surface and interfacial reaction, growth mode and surface morphology of Ni deposited on Si(001), Ge (001) and Si0.8Ge0.2 (001) substrates exhibit very similar trends It is found that Ni reacted immediately with the Si, Ge and Si0.8Ge0.2 substrates to form a thin, amorphous NiSi-like, NiGe-like, and NiSi0.8Ge0.2-like layer, respectively, at room temperature There reactions occurred on both clean and hydrogen-terminated surfaces Unlike other metals on H-Si(111) system, i.e., Al64, where hydrogen suppressed the formation of interfacial silicides, the presence of 162 Chapter Ni Growth on Si, Ge & Si0.8Ge0.2 Substrates At Room Temperature: Interfacial Reaction and Growth Mode hydrogen on Si, Ge and Si0.8Ge0.2 surfaces did not suppress the reaction between Ni adatoms and the underlying substrates during the initial stage of Ni deposition With further Ni deposition, the initial interfacial layers grew and became corresponding Nirich silicide, germanide and germanosilicide Eventually metallic Ni films grew on the top surfaces From XPS analysis of Ni and substrates’ signal evolution as a function of growth time, it seemed to suggest that Ni grew via a pseudo-layer-by-layer mode on all hydrogen-terminated and clean Si(001), Ge(001), and Si0.8Ge0.2(001) surfaces However, AFM images revealed that smoother surfaces were only achieved for Ni deposition on the substrates with hydrogen-terminated surfaces as well as on clean Ge(001) surface Rougher surface decorated with compact islands were observed when Ni was grown on clean Si(001) and Si0.8Ge0.2(001) substrates Therefore, Htermination appears to play a beneficial role for a smooth surface morphology during Ni growth on Si0.8Ge0.2 surface This effect again has a kinetic rather than energetic origin and appears to be a consequence of an increased surface diffusion rate of Ni adatoms The XPS curve method was unable to distinguish between very closedpacked small island growth mode from pseudo layer-by-layer growth mode 163 ... grows on H -Si( 001) (Fig 4. 5) and clean Si( 001) (Fig 4. 11) can be rationalized as follows The passivation of Si dangling bond by hydrogen on the surface appears to promote diffusion of adatoms on. .. surface morphology of Ni thin film grown on Ge substrates, although the effect is not as significant as on Si substrates 144 Chapter Ni Growth on Si, Ge & Si0 .8Ge0.2 Substrates At Room Temperature:... observation of a Ni/H -Si0 .8Ge0.2 reaction and formation of Ni -Si- Si, Ni -Si- Ge, Ni-Ge-Ge and Ni-Ge -Si bonds on H-terminated Si0 .8Ge0.2 surfaces since all the dangling bonds are now passivated In