The effects of doping type on structural and electrical properties of silicon nanocrystals layers grown by plasma enhanced chemical vapor deposition

Abstract

The purpose of this paper is to compare the influence of boron and phosphor doping on the optical, structural and electrical properties of hydrogenated amorphous silicon (a-Si:H) thin films elaborated at low radiofrequency power and low hydrogen dilution. Structural properties of deposited films were investigated by scanning electron microscopy (SEM), grazing incidence X-ray diffraction (GI-XRD) and Raman spectroscopy. The SEM analysis shows that the best homogeneity is observed in the layer doped with low diborane flow rate. Reflectance, XRD and Raman measurements show that the doping of thin films induces the formation of nanocrystallites (nc-Si) embedded in the amorphous matrix with higher density in the phosphorus-doped layers compared to those of boron-doped one. The temperature dependent electrical properties of Au/a-Si:H Schottky diodes were investigated using current–voltage characteristics (I–V), admittance spectroscopy technique and capacitance–voltage characteristics [C(V)] in the temperature range of 100–400 K. From the I–V analyses based on thermionic emission (TE) theory we notice the heterogeneity of the barrier height of the Schottky diodes. From the electrical conductivity measurements, we plotted the evolution of the logarithm of the conductivity σ·T as a function of 1000/T. Activation energies EaLow and EaHigh of the samples in low and high temperature ranges respectively are estimated. The decrease of EaLow with increasing boron flow rate is attributed to an increase in the density of the traps in the band gap of silicon. On the other hand, we found that EaHighincreases when the flow rate of boron increases and decreases when the flow rate of phosphorus increases. We attributed these behaviors to the increase in the crystallinity of the n-type layers and to the presence of a deep defect in the p-type layers. C−2(V) plots show the presence of two linear regions. We found that the response of the nanocrystalline phase becomes more pronounced at high doping flow rate. Optical, structural and electrical measurements confirmed that the type and the density of doping are important parameters that influence the electrical properties of nc-Si:H Schottky diodes.