Absorption and luminescence spectra of YAG:Ce 3+ crystals have been measured at various temperatures between 4.2 ~ and 300~ The high resolution, low temperature spectra show re~olved fine structure. Zero-phonon transitions have been assigned and the phonon replicas correlated with lattice phonon energies. The characteristic absorption band of YAG:Ce 8+ near 460 nm decreases in ]ntensity, whereat that near 340 nm increases~ with increasing temperature. This effect is explained in terms of a tetragonal crystal field splitting of both the 4f 1 ground state and 5d 1 excited states of Ce 8+ in YAG. When allowance is made for changes in the absorption of pump light with temperature the photoluminescence quantum efficiency of Ce z+ in YAG is constant up to 300~ The significance of these crystal field effects for the use of YAG: Ce as a high temperature lamp phosphor is discussed.Ce3+-activated yttrium aluminum garnet (Y3A15012, referred to as YAG) has primarily been developed as a CRT phosphor for flying-spot scanner applications (1, 2). The characteristic Ce 3+ broad-band luminescence in the visible region is now well known, and the phosphor is designated P46 in the JEDEC screen classi-

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The detailed structure of porous Si (PS) layers formed in p-type wafers with resistivities 0.01-25 Omega cm has been investigated using reflectance, transmission, ellipsometry and photoluminescence techniques. Marked differences were observed in the optical properties of PS formed in degenerate or non-degenerate Si and these results are correlated with the results of other techniques. The optical techniques together with effective medium modelling have been shown to be useful non-destructive methods for either assessment of PS density or detection of unsuspected phases. The degenerate PS layers consistently showed good retention of the single-crystal characteristics of the starting wafer, only c-Si and voids being detected. For these samples, good agreement was obtained between optical and gravimetric densities. However, the non-degenerate PS had much greater variability, with greater loss of crystallinity and significant incorporation of oxygen, due to partial oxidation having occurred on or immediately after anodisation. Oxide fractions have been determined both optically and gravimetrically, with up to 50% oxide being detected in some samples. Non-degenerate PS samples with high oxygen concentrations appeared to be in the form of a chemical mixture, SiOx, from interpretation of the optical constants. Photoluminescence measurements together with the other techniques indicated a complex mixture of phases in the latter samples-voids, alpha -Si:O (and/or alpha -Si:H), an unknown amorphous phase and silicon oxide. This complex structure probably contributes to the observed instability of thick non-degenerate PS layers when heated in UHV as part of the cleaning procedure prior to epitaxial growth, all degenerate samples being able to withstand heat treatment.

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