An investigation of group IV alloys and their applications in bipolar transistors

This thesis investigates the use of carbon in group IV alloys and their potential uses in bipolar transistors. The first part of the thesis investigates the ability of carbon to suppress transient enhanced diffusion in SiGe heterojunction bipolar transistors, whilst the second part deals with the impact of carbon incorporation on the electrical properties of polycrystalline silicon and silicon-germanium films. A background doping concentration (10"2"0cm"-"3) of C has been introduced into the base of SiGe HBTs with the aim of studying the effects of C on TED of B from the base. An electrical method is used to extract the bandgap narrowing in the base of SiGe and SiGe:C HBTs through measurements of the temperature dependence of I_c at different C/B reverse biases. The method is very sensitive to small amounts of dopant out-diffusion from the base and hence is ideal for determining the effect of C on TED. Extracted BGN values of 115meV and 173meV were obtained for the SiGe and SiGe:C HBTs respectively, for a C/B reverse bias of 0V. Increasing the C/B reverse bias to 1V increased the extracted BGN of the SiGe HBT to 145meV, but left the SiGe:C value unchanged. This demonstrates that no parasitic energy barrier exists in the SiGe:C HBT and that TED has been suppressed. The effect of carbon position and concentration has been studied by introducing a peak C concentration of 10"2"0cm"-"3 in the collector and 1.1x10"1"9cm"-"3 or 1.5x10"1"9cm"-"3 C in the base. From these measurements it has been shown that TED is only suppressed in the device with 1.5x10"1"9cm"-"3 C in the base, indicating that a C concentration of 1.5x10"1"9cm"-"3 is needed to suppress TED and that the C needs to be co-located with the B profile. The effects of carbon on the electrical properties of polycrystalline Si and SiGe films have been investigated. The resistivity, Hall mobility (#mu#_H) and effective carrier concentration (N_E_F_F) of n- and p-type polySi_1_-_yC_y and polySi_0_._8_2_-_yGe_0_._1_8C_y layers have been measured for carbon contents between 0% and 8%. For the n-type polySi_1_-_yC_y and polySi_0_._8_2_-_yGe_0_._1_8C_y layers, the addition of small amounts of C (#<=# 0.9%) was found to severely increase the resistivity of the layers, caused by a drop in N_E_F_F and #mu#_H. In contrast, for the p-type polySi_1_-_yC_y and polySi_0_._8_2_-_yGe_0_._1_8C_y layers, the effect of C on the resistivity was much less dramatic for C concentrations up to 7.8%. Measurements of the grain boundary energy barriers for the n-type polySi_1_-_yC_y and polySi_0_._8_2_-_yGe_0_._1_8C_y layers, extracted from the temperature dependence of the resistivity, showed that there was a square law dependence on carbon content. This is consistent with carbon increasing the grain boundary trap density. In contrast, the grain boundary energy barriers in the p-type polySi_1_-_yC_y layers exhibited a linear dependence on carbon content. This behaviour of C in p-type layers has been attributed to a shift in the dominant trap energy level towards the valence band at high C concentrations. (author)SIGLEAvailable from British Library Document Supply Centre- DSC:DXN039397 / BLDSC - British Library Document Supply CentreGBUnited Kingdo