Metallic thin film on sige si substrates 7

Chapter 7 Conclusions 7.1 Conclusions We have studied the desorption of Ge from Si0.8Ge0.2 virtual substrate upon annealing and the behavior of Ni atoms deposited on both clean and H-ter

Chapter 7 Conclusions

7.1 Conclusions

We have studied the desorption of Ge from Si0.8Ge0.2 virtual substrate upon

annealing and the behavior of Ni atoms deposited on both clean and H-terminated

Si0.8Ge0.2(001) substrates by performing in-situ XPS and ex-situ AFM measurements

The behaviors of Ni on H-Si(001) and H-Ge(001) were also similarly studied for a

systematic comparison The results obtained are summarized as following:

(1) We have identified two temperature regions The Si0.8Ge0.2 substrate

remains stable in composition and unchanged in surface morphology between RT and

500oC (region-I) Above 500oC (region-II), Ge at surface region desorbed while Ge in

the bulk diffused to surface region, which resulted in a decrease in Ge surface

concentration and formation of three-types of holes Our model has suggested that the

Ge behavior in region-II can be successfully described by the desorption and diffusion

mechanism To avoid the degradation of underlying Si1-xGex substrate, temperatures

used for metal deposition and subsequent processing steps should be in region I, i.e.,

not higher than 500oC

(2) Ni reacted strongly with the Si, Ge and Si0.8Ge0.2 substrates to form thin,

smooth and continuous NiSi-like, NiGe-like and NiSi0.8Ge0.2-like layers at room

temperature on both clean and hydrogen terminated Si, Ge and Si0.8Ge0.2 surfaces,

respectively Terminating the surface with hydrogen lead to a smoother morphology,

but it did not suppress the reaction of Ni with Si, Ge and Si0.8Ge0.2 surfaces at RT

(3) Ni growth on H-terminated Si, Ge, Si0.8Ge0.2 and clean Ge surfaces

proceeded via a pseudo-layer-by-layer mode, while it changed to small close-packed

island growth mode on the clean Si and Si0.8Ge0.2 surfaces XPS however can not

distinguish between these two kinds of growth modes By controlling the growth time,

we are able to grow smooth and continuous Ni thin films varying from NiSi-like layer

to pure metallic Ni layer

(4) At low Ni coverage (≤33%), Ni was protected from oxidation even after

more than one year’s exposure to air This can be attributed to the formation of

NiSi-like and NiSi0.8Ge0.2-like layers, substantiating the existence of bonding between Ni

and the Si, Ge & Si0.8Ge0.2 substrates even at the presence of hydrogen At high Ni

coverage (≥41%), both Ni and the substrates (Si/Ge) were oxidized on all Si, Ge and

Si0.8Ge0.2 substrates A power law can be used to fit the evolution of SiO2/Si and

(GeO2+GeO)/Ge ratio as a function of oxidation time

(5) Different phases were formed when the ultra-thin Ni film (~2-6Å) grown on

hydrogen-terminated Si, Ge and Si0.8Ge0.2 surfaces were annealed from RT to 620oC

Above 300oC, two time regions can be identified Region-I (annealing less than 30

minutes) is characterized by a sharp decrease in Ni%, which is attributed to Ni inward

diffusion Region-II (annealing longer than 30 minutes) is represented by a steady state

Ni/Si, Ni/Ge and Ni/Si0.8Ge0.2 intensity ratio at different temperatures The steady state

value of Ni/Si, Ni/Ge and Ni/Si0.8Ge0.2 can be attributed to the formation of respective

silicide and germanosilicide phase structures as well as clustering of formed 3D islands

during annealing

(6) Both rectangular and square islands were formed on Si(001), Ge(001) and

Si0.8Ge0.2(001) substrates above 400oC These islands were elongated along the two

perpendicular [110 ] and [110] directions They grew bigger and taller but decreased in

density with temperature The formation of such islands may be explained by the

diffusion anisotropy along and across the dimer row Although the crystallinity of

silicide, germanide and germanosilicide improved after annealing, the morphology of

these films has degraded from being smooth and continuous to one that is decorated

with these 3D islands

7.2 Future work

(1) We have monitored the Si, Ge and Si0.8Ge0.2 oxide intensity change as a

function of time with/without the presence of Ni thin films in Chapter 5 A

cross-section TEM experiments would be helpful in identifying the structure of the

Ni/H-Si(Ge, Si0.8Ge0.2) after oxidation and have a comparison with those observed by XPS.

The oxide/substrate ratio seemed to increase linearly with oxidation time in log

scales, irregardless of Ni coverages Therefore, it would be interested if a model can be

proposed in order to describe the power law dependence of the oxide/substrate ratio

with time in order to substantiate the oxidation process we propose

In addition, the two-gradient behaviour in Ge oxidation process can be further

explored in order to provide evidence for the claimed hydrogen-termination

break-down mechanism A temperature-dependence mass spectroscopy experiment would be

able to identify the hydrogen desorption temperature (and hence a bond strength of

H-Ge) for a series of H-Ge(001) samples with various Ni coverages

(2) In Chapter 6, the appearance of both rectangular and square islands with flat

tops is unexpected because such structures do not have the lowest surface energy

Diffusion anisotropy along and across the dimer row is proposed to account for the

formation of these islands However, there is no direct evidence to support this claim

because all the morphology studies were done ex-situ after growth Therefore,

experiments using in-situ STM should be employed to investigate and monitor the

formation process of both rectangular and square islands as a function of annealing

time and temperature at various Ni coverages Only then can we possibly address how

a flat surface becomes island like in terms of diffusion of the species or inherent strain

associated

The 3D islands grew bigger and taller but decreased in density with

temperature Therefore, a quantitative correlation between island density, annealing

time and temperature through in-situ STM and some computation is clearly warranted

as a future work in order to grow the 3D islands with not only the controlled density

but also the desired size/height Such ability could be very promising for the future

nano-device application

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