A Study of Electrical and Optical Properties of Boron-Doped Amorphous Silicon Deposited by RF-PECVD with Different B2H6/H2 Flow Rates
Authors: Dushaq Ghada , El-Atab Nazek , Rasras Mahmoud , Nayfeh Ammar 
Published: February 2020

Hydrogenated amorphous silicon (a-Si: H) with its inherently disordered nature has proved to be the material of choice for a large area of electronic and photonic devices. The high absorption coefficient of a-Si:H compared with c-Si make it an essential material and an important photovoltaic research topic. Furthermore, low cost, low temperature (200ºC-300ºC) and highly uniform a-Si:H films grown using PECVD allow for an effective channel material in MOSFET and many CMOS compatible devices. However the low hole mobility (approximately five orders of magnitude less than c-Si) and poor conductivity of a-Si:H due to the short range order of the amorphous material call for deeper  investigation of B/P-doped a-Si:H in order to achieve high efficiency and performance a-Si based devices. E. Johlin, et. all. (Phys. Rev. Lett, 2013)  discussed the major causes of low hole mobility based on analyzing the nature of the defects in the structure. It was shown that there are a wide range of structure defects which can act as a hole trapping states, for instance, dangling bond (under- coordinated silicon atoms), midgap states, bandtails and floating bonds (over-coordinated silicon atoms). However studies indicate that floating bonds are the strongest contributor to hole traps.

In this work the electrical and optical properties of a-Si:H by changing the flow rates of the diborane gas during the film deposition using RF-PECVD system is studied. In the experiment, three thick films with different standard cubic centimeter of B2H6 (1 sccm, 1.4 sccm and 2 sccm) are grown on quartz substrate at 250º C and 1000 mtorr. In all depositions 25 sccm of SiH4 is used as the silicon source. High-Resolution Scanning Electron Microscopy (HRSEM) cross section images of the samples show a ~ 1.9 µm thin film of a-Si:H. Furthermore, Elementary Dispersive Spectroscopy (EDS) data confirms the films constituents where a peak of boron is observed in the spectrum of the three films. The absorption spectrum and the index of refraction data of the three samples are extracted from UV/VIS/NIR spectrophotometer and variable angle ellipsometery, respectively. The absorption spectrum of the 1.4 sccm doped sample shows a maximum value of absorption around 1.5eV which is less than the typically reported band gap of a-Si: H, this can be interpreted in terms of hydrogen content of the sample. Hydrogen passivates dangling bonds from the energy gap but also widens the gap (D.A.Papaconstantopoulos,et.all, Phys.Rev B, 1980), as a result the incorporation of hydrogen can be used to tune the band gap of the fabricated films. Hall measurement is carried out to study the resistivity of the 1 sccm, 1.4 sccm, and 2 sccm B-doped a-Si:H and the results show 58.3 Ω.cm, 6.3 Ω.cm, and 82 Ω.cm, respectively. As can be seen from the resistivity values the 1.4 sccm doped sample gave the highest conductivity, however by increasing the flow rate to 2 sccm the electrical properties of the film degrade. In crystalline structure substitutional doping is used to introduce dopant in the structure.  On the other hand doping in amorphous structure can take place through dopant-defect pair. From the above results a flow rate of B2H6above 1.4 sccm can introduce more defect densities in the structure, which leads to low mobility values.

Using the electrical and optical data of the 1.4 sccm B-doped a-Si:H sample a bottom gate MOSFET transistor is fabricated (the process flow of the structure is attached). A highly doped n-type silicon substrate is cleaned with HF and ~ 400nm of SiO2is deposited for isolation purposes, after this SiN is deposited as a gate oxide.  PECVD is used to deposit 95nm of 1.4 sccm B-doped a-Si:H as the transistor channel. Finally, 200 nm of Al is deposited using e-beam evaporator to define the sourse and drain of the MOSFET. The quality of the interfaces and the effectiveness of the a-Si:H channel is studied.

This work was supported by Masdar Institute of Science and Technology. N. El-Atab acknowledges financial support provided by L’Oréal-UNESCO For Women in Science Middle East Fellowship.

Figure 1