Properties of Laser-Deposited Yttria Films on CdTe and Silicon Substrates

Abstract

Yttria (Y2O3) films have been used for a variety of applications including MIS diodes, capacitors, insulators in transistor gates and electro-luminescent devices, and as optical AR coatings. Several conventional Physical Vapor Deposition (PVD) techniques have been used to deposit Y2O3 films such as evaporationl,2,3,4, ion beam, RF and DC magnetron sputtering5,6,7,8. Here we report on the growth of Y2O3 films using excimer-based Pulsed-Laser Deposition (PLD). This study was motivated by the need to deposit high-quality yttria films onto the backsides of CdTe and CdZnTe substrates to act as an Infra-Red (IR) Anti-Reflective (AR) coating. AR coatings can theoretically increase the efficiency of back-side illuminated HgCdTe photo-voltaic IR detectors from about 65% to 80% over the 3 to 5 μm and 8 to 12 μm bands. For detector fabrication Y2O3 is deposited onto the substrate backside, after HgCdTe diodes have been formed. Low deposition temperatures (T< 75°C) are thus essential in order to reduce Hg diffusion and minimize the creation of vacancies in the HgCdTe lattice. Unfortunately, conventional PVD techniques inadvertently heat uncooled substrates to temperatures above 100°C, even at modest deposition rates (1 Å /sec). Effective cooling of the CdTe or CdZnTe is not practical since these substrates are very fragile and cannot be clamped with significant pressure to cooled backing plates. Furthermore, the fact that the HgCdTe diode array has already been formed on the front-side of the substrate precludes the use of thermal grease or epoxy to improve contact with a water cooled stage. PLD on the other hand, has the ability to deposit Y2O3 fims at relatively high deposition rates when compared to sputtering, with only a small rise in substrate temperature.