Raman Characterization of Protocrystalline Silicon Films
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1153-A16-04
Raman Characterization of Protocrystalline Silicon Films A. J. Syllaios1, S. K. Ajmera1, G. S. Tyber1, C. Littler2, R. E. Hollingsworth3 1
L-3 Communications Infrared Products, 13532 N.Central Expressway, MS37, Dallas, TX 75243 Physics Dept., University of North Texas, 1155 Union Circle #311427, Denton, TX 76203 3 ITN Energy Systems, 8130 Shaffer Parkway, Littleton, CO 80127 2
ABSTRACT An increasingly important application of thin film hydrogenated amorphous silicon (αSi: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.
INTRODUCTION Thin amorphous silicon and silicon alloy films such as silicon germanium (Si1-xGex) are widely used in several applications including solar cells and thin film transistors (TFTs) [1] as well as uncooled infrared detector arrays [2,3,4]. Such films exhibit a wide range of electrical and structural properties [5]. One of the key material properties that affect an array of electrical parameters is the degree of structural order in the film. This structural order manifests physically in terms of crystallinity or embedded crystallinity in the amorphous matrix. In addition to crystallinity, strained bonds, dangling bonds, bond energy distribution, voids, and grain structure can all influence the effective bandgap and hence the electrical transport properties of the material. Deposited film morphology can be controlled by the selection of specific deposition conditions such as temperature, reactant and dopant concentrations, the use of nucleation or seed layers, and film thickness. Hydrogen dilution in the deposition plasma affects the nucleation of crystallites and results in a phase transition from amorphous to protocrystalline. In some applications such as solar cells, enhanced performance is observed for films that are deposited near the amorphous to crystalline phase transition [6]. For uncooled infrared detectors, film morphology directly influences key properties such as resistivity, temperature coefficient of resistance (TCR) and noise characteristics which impact detector performance. Microbolometer arrays consist of a focal plane of pixels that are suspended above an integrated circuit and thermally isolated through the use of extreme
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