The optical properties of the chromium doped spinel type semiconductor α-ZnAl2S4 have been examined over the temperature range 2–540 K. The intrinsic photoluminescence (PL) showed an intense ultraviolet peak at 381 nm at 296 K. From extrinsic absorption and emission spectra the transitions between the ground state A2g4 and the excited levels Eg2, T1g2, T2g2, T2g4, and T1g4 of Cr3+ ions were observed. A fit of the measured temperature dependence of the PL decay, using a four level model, yielded the lifetimes of the Eg2, T1g2, and T2g4 levels and the energy gaps between the Eg2–T1g2 and Eg2–T2g4 states which were found to be very close to the values obtained from steady-state measurements. A configurational coordinate diagram for Cr3+ ions in a α-ZnAl2S4:Cr spinel host has been constructed. Optical gain measurements at 198, 298, and 380 K displayed two separate spectral regions associated to the Cr3+ ions and possibly Cr4+ impurity centers. Gain values of up to 25 cm−1 at 198 K were obtained.
The advantages of very low excitation density photoluminescence for the quantitative determination of shallow impurity concentrations in ultrapure Si are demonstrated using the weakly-absorbed 1047 nm output of a Nd:YLF laser. Compared to the usual high excitation method, which uses an argon ion laser, the weak bound exciton luminescence is significantly enhanced relative to that of the free exciton, and competing luminescence from multiexcitonic species, which complicates the accurate determination of the bound exciton intensities, is eliminated. Also, the presence of the laser Raman line in the same spectral region as the luminescence allows for the accurate determination of relative luminescence quantum efficiencies and can reveal the presence of deep traps. Finally, we report calibrations for boron, phosphorus, and arsenic in the concentration range 1011 to 1015 cm−3.
A nondestructive technique is presented for the determination of trace levels of interstitial iron contamination in ultrapure silicon. This approach is based on the well-known ability of iron to undergo a reversible pairing reaction with boron near room temperature. A variety of float-zoned silicon samples with low concentrations of boron (∼1011 cm−3) were subjected to thermal annealing treatments to study changes in the apparent boron concentration as determined by the standard method of comparing the photoluminescence intensity of the boron bound exciton to that of the free exciton. Changes in the apparent boron concentration were attributed to the formation or dissociation of iron–boron pairs, allowing us to estimate the interstitial iron concentration in these samples. Remarkably, relatively mild thermal treatments can change the apparent boron concentration in some of these samples by up to a factor of ten.
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