Raman spectroscopic investigation of evaporated PbO layers

Lead oxide (PbO) is one of the possible candidates to be used as a direct conversion material for X-ray imaging . It can be deposited by evaporation in layers of several 100 μm thickness, which is required for such X-ray detectors. Raman spectroscopy offers information about the phase composition of the deposited material and the micro-focus option of the micro-Raman set-up delivers this phase information spatially resolved on a microscopic scale. The lateral resolution of several microns enables cross sectional measurements, which provide information about the vertical homogeneity of the evaporated layers. The features of the Raman spectra of lead oxide and related materials are discussed in detail. Several differently prepared PbO layers have been analyzed and the Raman results are compared as a function of deposition process parameters. Typically PbO growth starts with a seeding layer containing large fractions of orthorhombic (yellow) PbO material. On top of this seeding layer, which has a thickness of below 2 μm, the layer growth is always dominated by the tetragonal (red) phase of PbO. The phase composition can be influenced by several deposition process parameters and its dependence on deposition rate is investigated in detail.