Investigation of Chemical Adsorbate Effects on Blue and Red Emitting Porous Silicon Samples
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were removed from the teflon cell, rinsed with millipore water and allowed to dry in air. The porosity of the films was about 73%. Blue emitting samples were prepared electrochemically in a 50:.50 solution of EtOH:HF (50% commercially available) using a constant current density of 10 mA/cm 2 for 25 min. The p-type wafers had a resistivity of 5-15 ohm cm. The high temperature thermal oxidation took place in a oxygen purged oven for 3-10 min. at 1000-1050 *C. Room temperature steady-state PL spectra of the red samples were obtained using a SPEX fluorimeter. To avoid photobleaching, the excitation wavelength was moved from the UV to the visible (450 nm). The samples are mounted on a teflon holder in a cuvette such that the position remained fixed while various liquid solvents were introduced and extracted. All solvents were extracted from the cell before the PL spectrum was remeasured. Room temperature time-resolved measurements of the blue samples were made using the third harmonic of a picosecond mode-locked Nd-YAG laser with maximum power of 0.3 GW/cm 2 per pulse and a repetition rate of 5 Hz. The PL spectra were obtained using a gated optical multichannel analyzer consisting of a 0.5 m grating monochromator and a linear diode array. All light was collected within the first 200 nsec after excitation. Scattered laser light was removed using a UV cut off filter in front of the monochromator. The sample was mounted and liquid solvents were introduced and extracted as described above. The FTIR spectra were obtained using a Nicolet MTIR instrument model 20SXC with a liquid nitrogen cooled MCT detector. The contribution from the silicon substrate was removed from the FTIR data using a difference method. RESULTS Figure la illustrates the PL intensity of a red sample after 1 min. liquid methanol (MeOH) exposure. The PL, which is centered around 660 nm, is 100% quenched after MeOH exposure. Partial recovery is achieved after blowing N2 on the sample for 5 min. Benzene, millipore water and 10% HNO 3 also quenched the PL but not as efficiently. Because MeOH has a strong capillary function it has the ability to penetrate the pores more effectively than the other solvents. 17 The PL intensity was enhanced about 30% upon exposure to pH=1 HCI. This is attributed to the solvent's ability to passivate the surface thereby decreasing the number of dangling bonds which act as nonradiative recombination sites. All PL effects described above were reversible. Figure lb shows the Si-Hx stretch region of the electrochemically prepared sample described above after MeOH exposure. All three Si-H stretches that correspond to Si monohydride, dihydride and trihydride (at 2090, 2110 and 2140 cm- 1 respectively) are slightly quenched and the band is broadened significantly after MeOH exposure. These bands are resolvable again after all the MeOH has evaporated. Benzene and water had no effect on the FTIR spectrum whereas 10% HNO 3 and pH=1 HCI showed several effects. 10% HNO 3 broadened the Si-Hx stretch band and enhanced the Si-O stretch (1100 cm
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