Electronic Properties and Gap State Defect Passivation of Si/SiO2 Nanostructures

A main loss factor in conventional solar cells is the mismatch of solar spectrum and bandgap energy of the absorber material. Thus, photons with energies above the bandgap generate hot carriers which lose their excess energy via thermalization. A concept in 3rd generation photovoltaics that aims to circumvent thermalization losses is to realize appropriate Si based nanound quantum structures, such as stacked Si/SiO2 multi quantum wells, that provide bandgap tunability and thus adaptation to the solar spectra via utilization of quantum size effects [1]. However, due to increased interface-tovolume ratios at reduced dimensions, charge carrier recombination at the SiO2/Si interfaces is facilitated, which lowers the solar cell efficiency. To overcome this drawback, experimental methods are required, that are capable of preparing well-defined Si and SiO2 layers and of minimizing interface losses. Moreover, adequate analytical tools have to be developed to reveal information about the chemical, structural, and photoelectrical properties at atomic level. These are the main issues that are addressed in this paper. The focus is on the study of a model system consisting of an ultrathin SiO2 layer upon a Si(111) wafer representing one of the interfaces as a constituting building block of such a Si/SiO2 quantum well device. For highest possible purity and control, complete preparation as well as full interface characterization was performed under ultrahigh vacuum (UHV) conditions. Under such conditions plasma oxidation using neutral, thermalized oxygen atoms is found to be superior to conventional thermal oxidation. The transmission electron microscopy image (TEM) in Fig. 1a shows such a homogeneous SiO2 layer on Si(111) with uniform thickness of about 2 nm and proves the successful formation of an abrupt interface structure. Analysis of the chemical shift by x-ray photoelectron spectroscopy (XPS) revealed dominant contributions of the oxidation states Si and Si, corresponding to Si and SiO2, respectively, and extremely low amounts of the intermediate oxidation states Si, Si, and Si (Fig. 1b) [2]. Thus, high quality ultrathin SiO2/Si interfaces with compositionally abrupt transitions were obtained by UHV plasma oxidation.