Equilibrium surface segregation enthalpy of Ge in concentrated amorphous SiGe alloys

doi:10.1016/S0169-4332(03)00097-7

Applied Surface Science

Volumes 212–213, 15 May 2003, Pages 244-248

https://doi.org/10.1016/S0169-4332(03)00097-7 Get rights and content

Abstract

Auger electron spectroscopy technique was used to study surface segregation of Ge in concentrated amorphous Si_1-xGe_x thin film alloys. The alloys (with Ge bulk concentrations in the range of 18–58 at.%) were prepared by dc magnetron sputtering and annealed in a UHV chamber in the temperature range of 653–673 K. The surface equilibrium data (X_s(Ge) versus X_b(Ge)) were determined from segregation kinetics. We show that (i) the experimental data can be interpreted using McLean–Langmuir isotherms with a segregation enthalpy ΔH_(exp) equals to 5.3±0.5 kJ mol⁻¹and (ii) the estimated segregation enthalpy is lower than the theoretical one calculated in crystalline alloys neglecting the alloying and the size effects.

Introduction

Numerous works reported that, during the growth of epitaxial Si/Ge layers, the control of Ge concentration is difficult due to the preferential surface segregation of Ge [1], [2], [3]. The driving force of germanium surface segregation in Si–Ge system is due to the difference in the surface tension of the elements [4], [5] and the difference in size between Ge and Si atoms favours also the phenomenon (the atomic radius of these elements are the following r_Ge=0.137 nm and r_Si=0.132 nm). However, the changes of surface composition with temperature have been studied only in few works [6], [7] and to our knowledge only one experimental work is devoted to the study of equilibrium segregation of Ge in SiGe alloys [8], which can be described in terms of phase transformation and characterized by a segregation enthalpy.

In this paper, we present an AES study of Ge surface segregation kinetics in a series of concentrated homogeneous amorphous Si_1−xGe_x thin films in the temperature range of 653–673 K (amorphous Ge crystallizes at about 693 K [9]). The main difficulty of this study was to determine equilibrium data (X_s(Ge) versus X_b(Ge)) from AES kinetics results (i.e. the variations of the peak-to-peak heights of Si and Ge Auger signals versus time). Firstly, since the Ge (1147 eV) peak is not sensitive to the composition of the topmost layer the Si (92 eV) was used to follow the variation of composition in the surface layer. Secondly, because of the high bulk concentration of the segregating specie (Ge) in the alloys, the estimation of the atomic fraction of Ge in the first layer needs a modelisation. Thirdly, long annealing is necessary to restore homogeneity of the alloy after segregation (i.e. to estimate the bulk Ge concentration near the segregated layer).

Section snippets

Materials and techniques

In order to study the composition dependence of segregation, amorphous Si_1−xGe_x thin film alloys with different concentrations of Ge (in the range of 18–58 at.%) were prepared. The general experimental set-up as described in previous work [8] is the following: the samples (thickness of 20 nm) were prepared by dc magnetron sputtering onto a SiO₂ substrate (size of $8 mm ×8$ mm) at room temperature using a Si target covered by Ge pieces (base pressure in the equipment: 5×10⁻⁷ Pa, argon pressure during

AES results

Annealing of the samples are performed at 653 K (X_b(Ge): 0.18, 0.37, 0.55) and 673 K (X_b(Ge): 0.51, 0.58) to avoid crystallisation of the alloys. Typical variations of Si, Ge, and C Auger signals (peak-to-peak height) versus time during annealing are the following:

1.
the decrease of the Si (92 eV) peak-to-peak height is important in all experiments while the variation of the Ge (1147 eV) peak-to-peak height is not significant;
2.
at the end of the kinetics (for annealing time long enough) constant values

Determination of surface equilibrium data at 653–673 K

To estimate equilibrium data, we assumed the existence of a local equilibrium during the kinetics process [12]. It means that the surface and the bulk (near the surface) concentrations follows the equilibrium isotherm during Ge segregation.

A depletion (due to the local exchange between Si superficial atoms and Ge bulk atoms from the underlying layers) was expected in the bulk layers near the surface at the beginning of the process. Assuming that the annealing time was long enough to restore the

Comparison between experimental and theoretical Ge surface segregation isotherms

The obtained experimental equilibrium data X_s(Ge) versus X_b(Ge) at 653 and 673 K are reported in Table 1. and plotted in Fig. 2. As the Si–Ge system is almost ideal, we have fitted our data obtained for different bulk concentrations using a McLean–Langmuir type isotherm: $X_{s(Ge)} = X_{b(Ge)} K(T) 1+(K(T)−1)X_{b(Ge)} .$ From Eq. (3), we have determined the experimental segregation coefficient (taking it as a fitting parameter): $K^{exp} (653 K)=2.6±0.3$ . Then, supposing Arrhenius-type temperature dependence for the

Conclusion

We have studied by AES the segregation of germanium in series of amorphous Si_1−xGe_x thin film alloys at 653 and 673 K and the main results of this work are the following:

1.
The variation of Auger signals during annealing indicates a local exchange between Ge and Si atoms in the uppermost layers: Ge atoms replace the Si surface atoms to minimize the surface free energy as observed in crystalline alloys. For annealing time long enough, equilibrium data have been determined and an experimental

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Equilibrium surface segregation enthalpy of Ge in concentrated amorphous SiGe alloys

Abstract

Introduction

Section snippets

Materials and techniques

AES results

Determination of surface equilibrium data at 653–673 K

Comparison between experimental and theoretical Ge surface segregation isotherms

Conclusion

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Appl. Phys. Lett.