J. Sarathy

Thermal treatment studies of the photoluminescence intensity of porous silicon

Thermal annealing studies of the photoluminescence (PL) intensity and Fourier‐transform infrared spectroscopy have been performed concurrently on porous Si. A sharp reduction in the PL intensity is observed for annealing temperatures ≳300 °C and this coincides with desorption of hydrogen from the SiH2 surface species. A brief etch in HF can restore the luminescence of the samples annealed below 400 °C. We conclude that SiH2 is essential to the visible luminescence in porous Si.

Photoluminescence and formation mechanism of chemically etched silicon

Room‐temperature photoluminescence (PL) from Si chemically etched (CE) in HF‐HNO3‐based solution has been observed. Scanning electron microscopy reveals that the etched Si has a surface morphology similar to that of luminescent porous Si fabricated by conventional anodization. PL spectra show an order of magnitude smaller luminescent intensity and a shorter wavelength intensity peak for CE Si. A CE Si thickness limitation was observed. The formation of CE Si can be readily explained by a local anodization model.

Demonstration of photoluminescence in nonanodized silicon

The formation of photoluminescent porous Si in an etchant solution made from the HF‐HNO3‐CH3COOH system is reported. The porous Si is characterized on the basis of its photoluminescence (PL) spectra and the degradation of the PL during exposure to laser irradiation. The surface topography as characterized by atomic force microscopy (AFM) reveals features on the order of 400–600 A. The effect of annealing the porous Si in vacuum on the PL intensity is described and correlated to the breakdown of Si—H bonds on the porous Si surface.

Chemically induced shifts in the photoluminescence spectra of porous silicon

The observation of photoluminescence (PL) spectral shifts during anodization of porous Si and after immersion in different chemical solutions is reported. These shifts in the PL spectra are attributed to changes in the surface chemistry achieved by changing the composition of the electrolyte in which the samples are immersed. Using this approach the emission has been repeatedly cycled (≳100 times) between green and red.

Photoluminescence of porous silicon buried underneath epitaxial GaP

Recent observations of visible, room‐temperature photoluminescence in porous Si have stimulated research aimed at the realization of efficient, Si‐based electroluminescent devices. To achieve electroluminescence, it may be beneficial to generate carriers with sufficient energy to populate the states of the quantum‐confined Si structures. A viable method to accomplish this is to utilize a wide‐band‐gap heterojunction injector, such as GaP. Toward that end, we report the successful formation of porous Si buried underneath GaP islands, and we demonstrate that the buried porous Si layer exhibits strong photoluminescence (λ≊7000 A).

Study of the Structure and Chemical Nature of Porous Si and Siloxene by STM, AFM, XPS, and LIMA

In situ scanning tunneling microscopy (STM) and ex situ atomic force microscopy (AFM) were used to examine the surface morphology of anodized p-Si(100) electrodes in F - -containing solutions. In addition to the formation of a mainly pitted and rough surface, in situ STM observation of anisotropic etching of Si(100) in dilute (1%) HF showed the formation of well-defined features, such as peninsulas, a 27 nm wide V-groove, and many protruding 5 nm wide micropyramids. High-resolution in situ STM resolved atomic features at the V-groove limiting (111) facets. Although this slightly etched Si sample contained no quantum pillars, it luminesced orange under UV irradiation, in the same way as a porous Si layer prepared by anodization in a more concentrated HF (1:1 HF:EtOH) solution

Normal-incidence grating couplers in Ge-Si.

We report a normal-incidence grating coupler that is monolithically integrated with a Ge(x)Si(1-x) rib waveguide. The design, fabrication, and testing procedures are described for a 0.33-microm-period grating coupler integrated with a Ge(0.04)Si(0.96) waveguide. Also, a theoretical analysis based on coupler-mode theory has been carried out to predict the coupling efficiency and its dependence on the angle of incidence of light. For normal incidence the measured efficiency of 6.5% was found to be in good agreement with the theoretical prediction of 6.9%. The measured FWHM of the acceptance angle about the normal was 5 degrees , compared with the theoretical prediction of 3 degrees .

Photoluminescence and Structure of Chemically Etched Si

We demonstrate for the first time that chemical etching of Si in HF-HNO 3 -based solution without applying bias can produce a room temperature photoluminescent porous Si layer. Scanning electron microscope studies reveal a surface morphology similar to that of the conventionally anodized porous Si. The formation mechanism of the chemically etched (CE) film can be explained by a local anodization concept. X-ray diffraction studies on the luminescent CE porous Si show a broad amorphous peak.

The Role of Silicon Monohydride and Dihydride in the Photoluminescence of Porous Silicon and Photoluminescence of Porous Silicon Buried Underneath Epitaxial GaP

Thermal annealing studies of the photoluminescence (PL) intensity and Fourier-transform infrared (FTIR) spectroscopy have been performed concurrently on porous Si. A sharp reduction in the PL intensity is observed for annealing temperatures > 300 °C and this coincides with desorption of hydrogen from the SiH 2 surface species. The role of silicon hydride species on the photoluminescence intensity has been studied. The surfaces of luminescent porous Si samples were converted to a predominate SiH termination using a remote H-plasma. The as-passivated samples were then immersed in various concentrations of hydrofluouric solutions to regulate the recovery of SiH 2 termination on the surface. Photoluminescence measurements and transmission Fourier-transform infrared spectroscopy have shown that predominant silicon monohydride (SiH) termination results in weak photoluminescence. In contrast, it has been observed that the appearance of silicon dihydride (SiH 2 ) coincides with an increase in the photoluminescence intensity. To achieve electroluminescence it will be beneficial to generate carriers with sufficient energy to populate the states of the quantum-confined Si structures. A viable method to accomplish this is to utilize a wide-bandgap heterojunction injector such as GaP. Toward that end we report the successful formation of porous Si buried underneath GaP islands and we demonstrate that the buried porous Si layer exhibits strong photoluminescence.

Crystallographically limited submicrometer gratings in (100) and (211) silicon

We demonstrate the fabrication of submicrometer gratings in (100) and (211) Si with periodicities that are appropriate for normal-incidence coupling into Ge-Si waveguides. Gratings of periodicities in the 0.25-0.75microm range were fabricated by a standard holographic lithography technique. Crystallographically preferential wet etching that effectively terminates at the {111} family of planes facilitates the fabrication of symmetric gratings in (100) Si and of blazed gratings in (211) Si. The gratings used in this study are appropriate for the fabrication of integrated first- and second-order, normal-incidence grating couplers at a 1.30-microm wavelength. The blazed gratings show a preferential coupling of more than 80% of the total diffraction into the (+1) diffraction order at normal incidence. The maximum first-order diffraction efficiency obtained with these gratings was approximately 28% in (100) Si and 23% in (211) Si.