• Isometsä, Joonas
• Liu, Hanchen
• Vähänissi, Ville
• Savin, Hele
• Leiviskä, Oskari
• Fung, Tsun Hang

Abstract

Atomic layer deposited (ALD) thin films have proven to be a highly effective method to reduce electronic recombination losses caused by defects present at the Si surfaces. Likewise, germanium (Ge) surfaces suffer from the same recombination problem and indeed, various ALD-based surface passivation schemes have been tried recently on them as well. The current methods utilize mainly so-called field effect passivation based on the negative fixed charge present in the film, such as aluminum oxide (Al2O3). The fixed charge induces an electric field to the vicinity of the substrate surface and thus prevents surface recombination by repelling electrons away from the surface. The negative charge can, however, be detrimental for certain applications. Therefore, there is a motivation to find a material which provides either a positive fixed charge or even better the ability to tailor the charge polarity.<br/>In this work we propose plasma-enhanced atomic layer deposited (PE-ALD) silicon oxide (SiO2) layers as a positive charge containing material for passivation of Ge surfaces and apply them as further charge tailoring interlayers for Ge/Al2O3 interfaces, as was demonstrated previously for Si/Al2O3 interfaces. First, we study 10 nm thick PE-ALD SiO2 films on n-type single-crystalline Ge wafers from which the charge polarity is determined. Next, the impact of PE-ALD SiO2 layers at Ge/Al2O3 interface is studied by varying the SiO2 interlayer thickness in the range of 1-20 nm. The passivation quality is monitored by measuring the minority carrier lifetime (τeff) and the thin film charge (Qtot) is determined from contactless capacitance-voltage (C-V) measurement.<br/>The results demonstrate that a bare PE-ALD SiO2 film provides lifetimes in a similar range (&gt; 1 ms) as previous state-of-the-art Ge surface passivation schemes. Surface recombination is seen to increase when depositing negative corona charge at the surface (i.e. effective neutralization of fixed charge) indicating the formation of positive charge on the Ge/SiO2 interface. Figure 1 presents both the τeff and the Qtot obtained with an SiO2 interlayer with varying nominal thickness at Ge/Al2O3 interface. C-V measurements show that ALD SiO2 interlayers at the Ge/Al2O3 interface allow us to tailor the effective charge polarity from negative to positive by gradually increasing the SiO2 layer thickness from ultrathin to thicker layers. Changes in the interlayer thickness at the Ge/Al2O3 interface caused a shift from negative effective charge to positive as the thickness of SiO2 increased. This also influences the τeff measured from these samples, implying an altering presence of field-effect passivation.<br/>

Topics

• impedance spectroscopy
• surface
• thin film
• aluminum oxide
• aluminium
• mass spectrometry
• Silicon
• defect
• Germanium