Sung-Sik Chang

Novel technique for preparing porous silicon

We have performed photoluminescence studies on porous, p‐type as well as n‐type silicon wafers which have been prepared in air or in a dry nitrogen atmosphere, utilizing a spark‐erosion technique. This sample preparation, which does not involve aqueous solutions or fluorine contaminants, yields similar photoluminescence spectra as those obtained by anodic etching in HF or unbiased etching in various HF‐containing reagents. The wavelength of the photoluminescence peaks are somewhat shifted into the blue region compared to porous silicon obtained by anodic etching. We have also taken photoluminescence spectra on amorphous silicon, SiO2, and oxidized, annealed porous silicon. Our results are interpreted in the light of the presently suggested theories.

Luminescence properties of Zn nanowires prepared by electrochemical etching

We have prepared samples of Zn wire structures by electrochemical anodization and measured their photoluminescence spectra at various temperatures. Scanning electron micrographs (SEM) were also taken on these samples. It was found that the dimension of the Zn wires has a definite influence on the photoluminescence (PL) spectra. Specifically, large wire dimensions exhibit a blue/violet luminescence. In addition to this blue/violet luminescence, nanowires of Zn about 20 nm in diameter exhibit a PL peak at 379 nm, which is the characteristic of ZnO. Low-temperature PL measurements on these Zn nanowires reveal three distinct excitonic emission peaks similar to high-quality ZnO. These results strongly suggest that high-quality ZnO nanowires can be formed by electrochemical etching of Zn.

On the origin of photoluminescence in spark‐eroded (porous) silicon

Photoluminescence measurements and high‐resolution transmission electron microscopy studies on spark‐treated (porous) silicon have been performed. Contrary to suggestions put forward by others, it has been found that spark erosion does not yield structures comparable to those obtained for irradiated, that is, damaged silica. Instead, evidence is given that spark treatment of single crystalline silicon wafers produces randomly oriented nanometer‐sized silicon crystallites surrounded by a SiO2 matrix. This configuration is believed to be responsible for the observed room temperature visible photoluminescence.

BRIGHT VISIBLE PHOTOLUMINESCENCE OF SPARK-PROCESSED GE, GAAS, AND SI

High‐frequency spark discharges were applied to single‐crystalline wafers of Ge, GaAs, and Si. The spark‐processed (sp‐) samples were characterized by photoluminescence (PL) and Raman measurements. Strong and stable luminescence with wavelengths centered at 416 and 525 nm was observed in sp‐Ge and sp‐Si layers, respectively, when excited with a 325 nm laser beam. A considerable blue shift of the PL (compared to the unsparked specimen) was also detected for sp‐GaAs with an average peak wavelength around 500 nm. The Raman shifts of the spark‐processed materials indicate that nanocrystals were formed, having diameters of 3.5–4 nm for Si and about 6 nm for Ge. A correlation between the PL wavelengths, the nanocrystal sizes, and the different semiconductor materials has been established based on the effective‐mass approximation. Making use of this model the nanocrystallite sizes have been found to range between ∼3 nm for Si and ∼5 nm for Ge. The related wavelengths for optical transitions confirm the PL result...

Novel Sn powder preparation by spark processing and luminescence properties

Abstract We have prepared nanosize Sn powders by spark processing. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to study the morphologies and sizes of these powders. Furthermore, X-ray diffraction (XRD) analyses are performed on spark processed Sn (sp-Sn) powders. Spark processed Sn powders are spherically shaped particles with diameters in the range of 3–5 nm. The photoluminescence characteristics of nanosized Sn powders are examined at various temperatures. In addition to the luminescence peak of 2.06 eV originating from SnO 2 , the photoluminescence studies of sp-Sn at room temperature and low temperature reveal an additional high-energy emission band in the violet range (2.95 eV). The violet PL band exhibits a continuous blue shift and shows an increase of PL intensities with decreasing temperature. These experimental results indicate that the violet PL band of sp-Sn powders may have originated from quantum size effects.

Luminescence properties of ambient air aged and thermally oxidized porous silicon

The photoluminescence (PL) properties of ambient air aged porous silicon (PS) and thermally oxidized PS have been studied at various temperatures. Furthermore, the decay dynamics, and stability characteristics (as a function of laser irradiation time) have been measured, and X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) have been carried out. Both ambient air aged and thermally oxidized PS show blue and green luminescence, but the luminescent properties of these samples are quite different from each other. Specifically, the constant PL peak position and abnormal PL intensity variations with decreasing temperatures are detected for ambient air aged PS. However, a substantial continuous blue shift and an increase of PL intensity with decreasing temperatures are observed for thermally oxidized PS. These different observations are attributed to differences in the structural configurations of the samples.

Comparison of photoluminescence behavior of porous germanium and spark-processed Ge

The photoluminescence (PL) characteristics of spark-processed Ge (sp-Ge) and of anodically etched, porous Ge have been studied at various temperatures. Further, scanning electron micrographs and XPS profiles have been taken. The optical properties of these samples have been found to be quite different from each other. Specifically, two new PL peaks emerge in sp-Ge at low temperatures which are seen at room temperature only as shoulders. Concomitantly, the room temperature PL peak vanishes at low temperatures. A similar observation is not made for porous Ge. Moreover, the PL peak wavelengths in sp-Ge remain constant during cooling whereas a substantial shift of these peak wavelengths occur during cooling for porous Ge. These different observations are attributed to the differences in the structural configurations of the samples.

Time-resolved photoluminescence measurements in spark-processed blue and green emitting silicon

Abstract Time-resolved photoluminescence (PL) measurements on spark-processed Si ( sp -Si) are compared with those on dry-oxidized porous Si ( p -Si). Both types of substances yield non-exponential decay times in the nanosecond region which are essentially independent of the detection wavelength. However, subtle differences between photoluminescing sp -Si and oxidized p -Si exist. Specifically, blue/violet emitting sp -Si has a peak wavelength near 410 nm (3eV) under steady state conditions whereas oxidized p -Si luminesces with a maximum centred around 460–480 nm (2.7 – 2.58eV). Further differences include the peak structures in the PL spectra, the decay dynamics, and certain features in the lifetime distribution. It is concluded from the data that sp -Si and p -Si derive their PL from somewhat different mechanisms. Moreover, differences in decay times between SiO 2 and sp -Si suggest that silica does not seem to be the major cause for PL in sp -Si.

Comparison of anodically etched porous silicon with spark-processed silicon

Abstract Anodically etched porous Si (AEPS) and spark-processed silicon (sp-Si) show interesting differences in some of their luminescence-related properties. Among these differences are: (1) the photoluminescence (PL) spectra of sp-Si have maxima between 550 and 410 nm (depending on the processing conditions) when sp-Si is excited at 325 nm, whereas PL spectra of conventional (unannealed) AEPS generally peak between 640 and 730 nm (depending on the degree of porosity); (2) the PL intensity of sp-Si remains relatively stable under UV illumination, whereas the PL intensity of conventional AEPS decreases substantially under these conditions; (3) the PL intensities of sp-Si and AEPS, when excited at 325 nm in the lower W cm−2 range, are initially comparable; (4) the pumping wavelength has a definite influence on the PL spectra of sp-Si (in contrast, the peak wavelength of conventional AEPS stays essentially constant under varying excitation wavelengths); (5) spark-processed Si displays a stable and visible cathodoluminescence (when exposed to electrons having an energy of about 0.8 keV) whereas conventional AEPS visibly cathodoluminesces under the same conditions for only a fraction of a second; and (6) The vibrational spectra for sp-Si as measured with Fourier transform infrared spectroscopy (FTIR) favour those modes which involve silicon-oxygen bonds. FTIR spectra of conventional AEPS additionally show some hydrogen-related vibrational modes.

Strong, blue, room-temperature photoluminescence of spark-processed silicon

Abstract This paper reports on photoluminescence (PL) and Raman-shift measurements of spark-processed silicon which has been prepared under various conditions. It is observed that spark-processing conducted in a continuous air stream leads to PL spectra whose maxima are shifted substantially into the blue region (i.e., 410 nm) compared to spectra of similar samples prepared in stagnant air or nitrogen. The PL intensity of the blue-luminescing samples was found to be comparable with that of the most efficient, anodically-etched (red-luminescing) porous Si. Possible mechanisms causing the observed PL in spark-processed Si are critically discussed.