The Properties of a-Ge/a-SiO Superlattices

Abstract

We have fabricated a-Ge/a-SiOx superlattices with a-Ge layer thicknesses between 8Å to 192Å and a-SiOx layer thicknesses between 8Å to 275Å by ultra high vacuum vapour deposition. A new rotating substrate assembly facilitated the production of the superlattice films. This assembly enables superlattices with three different periods to be produced at once. The properties of as deposited and vacuum annealed (thirty minutes at temperatures of 200°C and every 40°C from 400°C to 760°C) a-Ge/a-SiOx superlattices have been investigated using transmission electron microscopy, Auger spectroscopy, a new photographic small angle x-ray diffraction technique, far infrared spectroscopy, Raman spectroscopy, dc and ac electrical conductivity measurements, thermopower measurements and small and large angle x-ray diffraction measurements. The as deposited superlattices are periodic and the a-Ge layers are continuous for layers equal or thicker than l3Å, while the a-SiOx layers are continuous for layer thicknesses equal to or greater than 19Å. We model the vibrational spectra using a four layer model for each period involving a pure a-Ge layer, a pure a-SiOx layer and two interface layers. From this model we deduce that the mixed interface region is ~7Å wide and the bond angle and bond length mismatch at the interfaces does not induce a strained a-Ge region near the interfaces. The as deposited a-SiOx layers have an x of about one for a-SiOx layers equal to or thicker than 26Å, indicating that phase separation into a-Si and a-SiO2 islands has not occurred. We use a novel technique involving the temperature dependence of the 2D variable range hopping conductivity in the a-Ge layers to deduce that the density of electronic defect states at each interface due to the localised defect state band about the Fermi level is ~4x1010cm-2. Furthermore, we find that the bond angle and bond length mismatch at the interfaces has not resulted in an excess of dangling bond defects when compared to the a-Ge layers. From the thermopower measurements we find that the superlattices with thin a-Ge layers are slightly more disordered than our thick a-Ge film. For conduction perpendicular to the layers we observe field activated variable hopping which is dominated by the a-SiOx layers for superlattices with a-SiOx layers l2Å or thicker. The ac data is successfully fitted with a simple model involving a Schottky interface region and a bulk region with ac variable range hopping in both regions. From isochronal annealing studies we observed that the Ge layer crystallisation temperature is larger in the superlattices with thin Ge layers. This has been successfully modelled in terms of Ge microcrystallites that nucleate away from the a-Ge/a-SiOx interfaces and rapidly grow up to the interfaces after which growth ceases. From this model we find that nucleation is retarded within ~12Å of the a-Ge/a-SiOx interfaces. After Ge layer crystallisation and upon further annealing we deduced that Si slowly diffuses from the a-SiOx layers into the crystallised Ge layers and once in the crysteltised Ge layers the Si rapidly diffuses leading to Si being uniformly distributed. throughout the crystallised Ge layers. From x-ray measurements after the final thirty minute anneal at 760°C, we found that the superlattices were still periodic and the x-ray spectra could be modelled with superlattice periods that contained a uniform a-SiOx layer and a uniform a-GeySi1-y layer where the rough or mixed interfacial regions were still no more than 8Å in extent.