Radio Frequency sputtered Si 1-x Ge x and Si 1-x Ge x O y thin films for uncooled infrared detectors Mukti M. Rana, Donald P. Butler ⁎ The University of Texas at Arlington, Department of Electrical Engineering, P.O. Box 19072, Arlington, Texas 76019, USA Received 16 December 2004; received in revised form 14 February 2006; accepted 23 February 2006 Available online 31 March 2006 Abstract Thin films of Si 1-x Ge x and Si 1-x Ge x O y were deposited by radio frequency (rf) magnetron sputtering at room temperature from a single target of Si 1-x Ge x in the Ar or Ar:O 2 environment. The silicon and oxygen concentrations were varied in a parametric investigation of the dependence of the electrical and optical characteristics of the thin films on composition. As Si concentration was increased in the Si 1-x Ge x films, the temperature coefficient of resistance (TCR) was decreased. For Si 1-x Ge x O y films, the addition of oxygen to the Si 1-x Ge x , increased the activation energy and TCR. The TCR was measured to vary from - 2.27% to - 8.69%/K. The optical bandgap was increased with the increasing concentration of oxygen in Si 1-x Ge x O y . A suitable atomic composition of Si 1-x Ge x O y for uncooled infrared detector applications was found to have a TCR of - 5.10%/K. © 2006 Elsevier B.V. All rights reserved. Keywords: Silicon germanium; Sensors; Bolometers; Sputtering 1. Introduction Infrared detectors are classified into two main categories, photon detectors in which the absorption of electromagnetic radiation creates charge carriers in a semiconductor and thermal detectors in which the absorption of infrared radiation heats the material thereby changing a measurable electrical property. Bolometers are thermal detectors whose resistance changes with temperature. It is important for a bolometer to have good in- frared (IR) absorption, a high temperature coefficient of resis- tance (TCR) and have low 1/f-noise to achieve a greater value of voltage responsivity (R v ) and detectivity (D*). The voltage responsivity is defined as the detector output voltage per unit of detector input power. R v ¼ gbRI b G th ð1 þ x 2 s 2 Þ 1=2 ð1Þ where R v is the voltage responsivity, η is the optical absorption coefficient of the thermister, β is the TCR, R is the DC resistance of the device, I b is the bias current, G th is the thermal con- ductance between the microbolometer and its surroundings, ω is the modular frequency of the incident IR radiation and τ is the thermal time constant of the bolometer. The detectivity is de- fined as follows: D * ¼ R v ffiffiffiffiffiffiffiffiffi AΔf p Δv n ð2Þ where Δv n is the total noise voltage observed in the electrical bandwidth Δf and A is the area of the detector. Moreover, in order to integrate the detector with the read out circuitry, it is preferred that the sensing material or thermister of bolometer have moderate resistivity and be a conventional semiconductor so its processing is fully compatible with conventional post- complementary metal oxide semiconductor (CMOS) silicon micromachining technology. Common materials used as the thermister in microbol- ometers include the semiconducting phase of Y–Ba–Cu–O [1], poly-Si [2], poly-SiGe alloys [3], VO x [4], metal resistors such as Ti [5], Nb [6], Pt [7], and amorphous Si and Ge [8]. Silicon and germanium alloys are a good choice for the thermometer because they are conventional semiconductors and their pro- cessing can be fully compatible with post-CMOS micromachin- ing technology. The semiconducting phase of Y–Ba–Cu–O has a high TCR of - 3.1%/K but this is not a conventional semi- conductor. Dobrzanski et al. [2] and Sedky et al. [3] used poly- Thin Solid Films 514 (2006) 355 – 360 www.elsevier.com/locate/tsf ⁎ Corresponding author. E-mail address: dbutler@uta.edu (D.P. Butler). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.02.088