Nuclear Magnetic Resonance (NMR) of Porous Silicon
- PDF / 500,090 Bytes
- 6 Pages / 412.92 x 646.2 pts Page_size
- 36 Downloads / 273 Views
443
Mat. Res. Soc. Symp. Proc. Vol. 452 0 1997 Materials Research Society
EXPERIMENTAL In preparation for the CP experiments, 'H and 19F direct polarization spectra were acquired on several freshly prepared PS samples. By using rectangular sections of PS layers intact on their silicon wafer substrate, background signals were minimized.[ 11] For 2Si NMR, porous Si was made from 4" diameter (100) p-type Si wafers with resisitivities of 1-10 0 cm, a current density of 20 mA/cm 2, and an anodization time of 90 minutes. The 30 wt% HF electrolyte was prepared by diluting 48 wt% aqueous HF with ethanol. A few seconds of increased current density at the end of the anodization produced delamination [12] of the PS layer which was collected by filtration for MAS. After anodization, the PS was washed with distilled water. About two months elapsed between the growth and the initial characterization. Upon completion of these measurements, the as-prepared PS sample was divided into two parts. The first one was oxidized in air at - 200* C for 4 hours. After NMR and FTIR measurements were taken, this first part was further oxidized for about 60 hours at the same temperature, and the corresponding changes in NMR and FTIR were compared. The second part of the sample was reduced by soaking it in HF for 10 minutes, followed by drying in air without being washed by water. NMR measurements were taken immediately after the HF treatment in order to minimize any signal originating from the oxidation of the sample in air. FTIR of the same sample was taken after the NMR characterization. All NMR spectra were obtained using a home-built solid-state NMR spectrometer with a 29 Si resonance frequency of 53.64 MHz. For CP-MAS NMR, the PS samples were spun at a frequency - 2900 Hz. The 900 pulse time was 4 ts, the cross polarization 3 ms and the dipolar decoupling time was 10 ms. Also, since the recycle delay (40 s) between signal averages is determined by the recovery of proton magnetization, the shorter T, constants of the proton resonance allow for more signal averages in a given time, also leading to improved sensitivity. All 29Si NMR spectra are plotted in ppm from the tetramethylsilane (TMS) reference. RESULTS The as-prepared PS layer used for 29 Si for was 110 gtm thick and had a Brunauer-EmmettTeller (BET) surface area of 893 m2/g. The Barret-Joyner-Halenda (BJH) desorption method revealed a monodisperse pore size distribution with an average diameter 42 A. The photoluminescence spectrum showed a single feature centered at -700 nm, having a FWHM of approximately 150 nm. Proton NMR of freshly prepared samples measured H concentrations ranging from 3x 0'4 to 3x10' 5 per cm 2 of PS surface area. The density of surface atoms on the different crystallographic faces of silicon is between 6x1014 and lxlO15 per cm 2 of surface area. [13] Thus, the hydrogen concentration range is equivalent to 0.5 to 3 monolayers and is sufficient for cross-polarizing nearby 29 Si nuclei. In contrast, the F content of all samples was found to be below our NMR det
Data Loading...
