Hydrogenated amorphous silicon, amorphous germanium, and their alloys are of interest for uncooled infrared sensing microbolometer devices. In these applications, the properties of interest include controllable resistivity, high temperature coefficient of resistance TCR values, and low 1/f noise. Amorphous silicon (a-Si) and germanium (a-Ge) films exhibit relatively high TCR, may be deposited as uniform layers, and are prepared using more mature deposition processes than the other commonly used material in these types of devices, vanadium oxide. Recently, plasma enhanced chemical vapor deposited boron doped amorphous silicon germanium (a-Si<SUB>1-x</SUB>Ge<SUB>x</SUB>:H:B) and magnetron sputtered silicon germanium oxide (SiGe<SUB>x</SUB>O<SUB>y</SUB>) films have been reported with relatively high TCR values of -2.7 %/K and -4.86 %/K for films with resistivities of 100 Ωcm and 245 Ωcm, respectively [1,2]. Optimization of the a-Si<SUB>1-x</SUB>Ge<SUB>x</SUB>:H system for use in devices requires a better understanding of the relationship between TCR and 1/f noise in films with resistivity relevant to the current demands of the read out circuitry.
To assess how variations in composition and structure affect these electrical characteristics, a-Ge films have been deposited using DC magnetron sputtering in mixed atmospheres of argon, hydrogen and nitrogen. After sputtering, titanium top electrodes are prepared in a transfer length method (TLM) pattern for measurement of resistivity and TCR, while resistors with three different volumes are made for volume normalization of the 1/f noise measurement. The TCR is typically measured from room temperature to 55C. Spectroscopic ellipsometry over a range from 1.2 to 3.35 eV is used to extract the a-Ge film thickness and approximate the band gap obtained by fitting the complex dielectric function spectra using a Tauc-Lorentz oscillator . The TCR and equivalent activation energy can be thought of as a function of the tail states in the band structure, so that different slopes of the tail state densities influence how many carriers proportionally occupy these localized states with respect to the extended states in the conduction or valence band as temperature increases. Nitrogen dopant in germanium has been reported to have an activation energy of 0.23 eV by photoluminescence measurements with a corresponding TCR of -3.1 %/K [4,5], which are in agreement with our TCR measurements. Thus, it is desirable to draw correlations between the deposition conditions and the 1/f and TCR characteristics in terms of the tail states. In this work, a-Ge films have been prepared under variable deposition temperatures, sputtering atmospheres containing hydrogen and nitrogen, and with boron co-sputtering to assess how variations in the electrical properties (resistivity, TCR, 1/f noise) correlate to film composition and microstructure.