Phosphorous and Boron Incorporation and Its Effect on Electronic and Optical Properties of Ge:H Films Deposited by LF Plasma

Abstract

In a previous work [1], the deposition conditions that provided low optical absorption related to both band tail and deep localized states have been found for both materials Ge: H and Si1-YGeY: H. In this work phosphorous and boron doping of Ge: H films have been systematically studied. These films were deposited by low frequency (LF) plasma under the conditions for low density of localized states whit optimal hydrogen dilution. The deposition parameters were as follow: substrate temperature Ts= 300 °C, discharge frequency f= 110 kHz, pressure P= 0.6 Torr, power W= 300 W, and the flow gas for the films of Ge: H was, germane flow QGeH4= 50 sccm, hydrogen flow QH2=2500 sccm, the phosphine flow was varied in the range of QPH3 = 20 to 100 sccm providing phosphorous concentration in gas phase in the range of XP= 4 to 20 %. For boron we used the same conditions as before, but the B2H6 flow was varied in the range of QB2H6 = 3 to 20 sccm providing concentration in gas phase in the range of XB = 0.3 to 4 %. SIMS profiling was used for determining the composition of the doped films. The hydrogen bonding was studied by FTIR. The temperature dependence of conductivity measured in DC regime was performed in a vacuum thermostat in order to study carrier transport. Optical measurements provided optical gap, absorption and refraction index. The Phosphorous incorporation to the solid film demonstrated that for the doping conditions used in this work, we obtained a constant P concentratio. But for boron incorporation, the concentration of it in the solid films increases linearly with its concentration in the gas phase. The influence of the P and B doping on the hydrogen concentration, activation energy and conductivity of the films is also studied and presented.