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The dependence of dark conductivity and room temperature Raman spectra on boron and hydrogen incorporation in thin films of hydrogenated amorphous silicon (a-Si:H) prepared by plasma enhanced chemical vapor deposition was investigated. It was found that the dominant conductivity is Mott variable range hopping conduction. However, at lower temperatures, Efros-Shklosvkii hopping conduction is observed and contributes to the total conductivity. For structural characterization, transverse optical (TO) and transverse acoustic (TA) modes of the Raman spectra were studied to relate changes in short- and mid-range order to the effects of boron and hydrogen incorporation. With an increase of hydrogen incorporation and/or substrate temperature, both short and mid-range order improve, whereas the addition of boron results in the degradation of the short range order. The line width and frequency of the Raman TO Raman peak correlate with electrical measurements and suggest that this technique can be used for non-destructive characterization of a-Si:H.
Noise and electrical conductivity measurements were made at temperatures ranging from approximately 270°K to 320°K on devices fabricated on as grown Boron doped p-type a-Si:H films. The room temperature 1/f noise was found to be proportional to the bias voltage and inversely proportional to the square root of the device area. As a result, the 1/f noise can be described by Hooge’s empirical expression . The 1/f noise was found to be independent of temperature in the range investigated even though the device conductivity changed by a factor of approximately 4 over this range. Conductivity temperature measurements exhibit a T-0.25 dependence, indicative of conduction via localized states in the valence band tail [2,3]. In addition, multiple authors have analyzed hole mobility in a-Si:H and find that the hole mobility depends on the scattering of mobile holes by localized states in the valence band tail [4-7]. We conclude that the a-Si:H carrier concentration does not change appreciably with temperature, and thus, the resistance change in this temperature range is due to the temperature dependence of the hole mobility. Our results are applicable to a basic understanding of noise and conductivity requirements for a-Si:H materials used for microbolometer ambient temperature infrared detection.
In this study, we employed Multiple Internal Reflection Infrared Spectroscopy (MIR-IR) to characterize chemical bonding structures of boron doped hydrogenated amorphous silicon (a-Si:H(B)). This technique has been shown to provide over a hundred fold increase of detection sensitivity when compared with conventional FTIR. Our MIR-IR analyses reveal an interesting counter-balance relationship between boron-doping and hydrogen-dilution growth parameters in PECVD-grown a-Si:H. Specifically, an increase in the hydrogen dilution ratio (H2/SiH4) was found to cause the increase in the Si-H bonding and a decrease in the B-H and SiH2 bonding, as evidenced by the changes in corresponding IR absorption peaks. In addition, although a higher boron dopant gas concentration was seen to increase the BH and SiH2 bonding, it also resulted in the decrease of the most stable SiH bonding configuration. The new chemical bonding information of a-Si:H thin film was correlated with the various boron doping mechanisms proposed by theoretical calculations.
This paper presents the formation and the characterization of silicon germanium oxide (SixGeyO1-x-y) infrared sensitive material for uncooled microbolometers. RF magnetron sputtering was used to simultaneously deposit Si and Ge thin films in an Ar/O2 environment at room temperature. The effects of varying Si and O composition on the thin film's electrical properties which include temperature coefficient of resistance (TCR) and resistivity were investigated. The highest achieved TCR and the corresponding resistivity at room temperature were -5.41 %/K and 3.16×103 ohm cm using Si0.039Ge0.875O0.086 for films deposited at room temperature.
An increasingly important application of thin film hydrogenated amorphous silicon (α-Si:H) is in infrared detection for microbolometer thermal imaging arrays. Such arrays consist of thin α-Si:H films that are integrated into a floating thermally isolated membrane structure. Among the α-Si:H material properties affecting the design and performance of microbolometers is the microstructure. In this work, Raman spectroscopy is used to study changes in the microstructure of protocrystalline p-type α-Si:H films grown by PECVD as substrate temperature, dopant concentration, and hydrogen dilution are varied. The films exhibit the four Raman spectral peaks corresponding to the TO, LO, LA, and TA modes. It is found that the TO Raman peak becomes increasingly well defined (decreasing line width and increasing intensity), and shifts towards the crystalline TO energy as substrate temperature is increased, H dilution of the reactants is increased, or as dopant concentration is decreased.
Highly sensitive hydrogenated amorphous silicon (a-Si:H) microbolometer arrays have been developed that take advantage of the high temperature coefficient of resistance (TCR) of aSi:H and its relatively high optical absorption coefficient. TCR is an important design parameter and depends on material properties such as doping concentration. Ultra-thin (∼2000 Å) aSiNx:H/a-Si:H/ a-SiNx:H membranes with low thermal mass suspended over silicon readout integrated circuits are built using RF plasma enhanced chemical vapor deposition (PECVD) and surface micromachining techniques. The IR absorptance of the bolometer detectors is enhanced by using quarter-wave resonant cavity structures and thin-film metal absorber layers. To ensure high thermal isolation the microbolometer arrays are vacuum packaged using wafer level vacuum packaging. Imaging applications include a 120×160 a-Si:H bolometer pixel array IR camera operating at ambient temperature. Non-imaging applications are multi-channel detectors for gas sensing systems.
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