Thermal Stability of Si/Ge Hetero-Interface Grown by Atomic-Layer Epitaxy
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Table. I Experimental conditions (a) 2ML-Si adsorption on Ge(100) "2"!a step Substrate temperature Ts=300°C Substrate temperature T0cs=530°C SiH 4 pressure Ps= 17 mTorr DCS pressure Pocs= 17 mTorr SiH 4 exposure time ts= 1200 s DCS exposure time tocs=60s
".1st step
Substrate temperature DCS pressure DCS exposure time
(b) Si-ALE on 2ML-Si/Ge(100) TSi.ALE=530 0C H2 flow rate 1.0seem Pcs= 17mTorr Filament temperature 2000K tocs=20s Atomic H exposure time tH=15s
pressure for conventional gas-source MBE[6-8]. After the 2ML-thick Si layer growth and the removal of surface terminating C1 atoms by atomic H exposure, the clean Si surface is recovered and the successive Si-ALE (i.e., cyclic exposures of DCS and atomic H[1 2,13]) can be done without degradation of the abruptness of the interface. Experiments were carried out using a ultrahigh vacuum multi-chamber ALE system having an ALE growth chamber, an analysis-chamber with CAICISS and RHEED apparatus and a pretreatment-chamber with a load-lock transfer system. The Si coverage was confirmed by the intensity ratio between Si 2p and Ge3d core level spectrums obtained by ex-situ XPS. Epitaxial growth was also confirmed by RHEED. Details of sequence were described in our previous paper[ 10] , and experimental parameters are tabulated in Table I. RESULTS AND DISCUSSION TOF-spectrums of as-prepared samples TOF-spectrums are shown schematically in Fig. I for the Si and Ge substrates and also for the Si/Ge heterostructure. Signals from bulk Ge and Si are shown by bold curves in Figs. (a) and (b) respectively, and can be divided into two components; an elastic scattering component (heavily hatched area) caused by direct collision of He ions with surface atoms and (a) Ge substrate: an inelastic scattering component (lightly hatched area) caused by multiple collisions of He ions with atoms inside. The elastic scattering components from the surface Si S(b)3l -Si sulbstrate ° ~inelastic a set........................... and Ge atoms can appear around 4660ns and 4280ns, respectively, and inelastic components follow the elastic components as "1 C "T schematically shown in the figure. Signal M intensity for the Si substrate is weaker than S(c) Si/Ge heterostructure that for the Ge substrate. This is caused by the difference of a scattering cross-section between Si and Ge atoms. Namely, cross-section for a Si atom is nearly 20% of that for a Ge atom. a6 (d) STS For the Si/Ge heterostructure, the elastic component of the Ge signal from the 0 400a'c 5000 substrate surface reduces and the elastic component of the Si signal from the surface Time of flight (ns) Si may be added as schematically shown in Fig.(c). TOF-spectrum itself is not sensitive Fig.1. Schematic view of TOF spectrums and STS.
34
He+: 2 keV I
in S
Gei
Si:
a- 350 Azimuth [110]
(d)3ML
,Ge Si!
-.CC 0
CO
%I.- c2ML
CU C,-
4000 5000 Time of flight (ns)
(b)1ML
CU
6000
Fig.3.STS for IML-thick Si layer on Ge surface with respect to Ge substrate.
ý(a)Ge-sub. 4000 5000 6000 Time of flight (ns) Fig.2.TOF-spectrums o
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