K. H. Jung

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.

Microstructural investigations of light-emitting porous Si layers

The structural and morphological characteristics of visible‐light‐emitting porous Si layers produced by anodic and stain etching of single‐crystal Si substrates are compared using transmission electron microscopy and atomic force microscopy (AFM). AFM of conventionally anodized, laterally anodized and stain‐etched Si layers show that the layers have a fractal‐type surface morphology. The anodized layers are rougher than the stain‐etched films. At higher magnification 10 nm sized hillocks are visible on the surface. Transmission electron diffraction patterns indicate an amorphous structure with no evidence for the presence of crystalline Si in the near‐surface regions of the porous Si layers.

Control of porous Si photoluminescence through dry oxidation

We demonstrate the applicability of thermal oxidation to control the photoluminescence (PL) from quantum‐sized structures in porous silicon. Uniform photoluminescence samples with intense visible light observed under ultraviolet light at room temperature were quickly obtained without a long time hydrofluoric acid (HF) immersion. Applying different oxidation times or temperatures provides a very practical technique to control the luminescence color. By this way, we have observed a shift in the luminescence peak from 7600 to 6200 A and a reduction in the spectral width from ∼1600 to ∼950 A.

Intense photoluminescence from laterally anodized porous Si

We have studied photoluminescence (PL) from porous Si anodized laterally along the length of the Si wafer. Broad PL peaks were observed with peak intensities at ∼640 to 720 nm. Strong PL intensity could be observed from 550 to 860 nm. Room‐temperature peak intensities were within an order of magnitude of peak intensities of AlGaAs/GaAs multi‐quantum wells taken at 4.2 K, and total intensities were comparable. A blue shift of peak intensities from ∼680 to 620 nm could be observed after thermal anneal at 500 °C in O2 and subsequent HF dip.

Effects of H and O passivation on photoluminescence from anodically oxidized porous Si

We have studied the mechanism of photoluminescence (PL) change in porous Si layers (PSLs) by gradually replacing the hydrogen‐terminated surface with an oxygen‐terminated surface by anodic oxidation at room temperature. The observed PL change did not follow the change in the silicon hydrides detected by transmission Fourier transform spectroscopy (FTIR). FTIR spectra show that the silicon hydrides decreased while the PL increased. The results of this study show that the polysilane species is not solely responsible for efficient luminescence from PSLs. In addition, an enhancement of PL intensities after laser exposure was observed from anodically oxidized PSLs.

Transmission electron microscopy study of chemically etched porous Si

We have developed a new, minimal damage approach for examination of luminescent porous Si (PS) layers by transmission electron microscopy (TEM). In this approach, chemically etched (CE) PS layers are fabricated after conventional plan‐view TEM sample preparation. Our TEM studies show that crystalline, polycrystalline, and amorphous phases exist in the same CE sample. The microstructure is believed to gradually change from crystalline to amorphous during chemical etching in a HF‐HNO3‐H2O solution. The microcrystallites in the polycrystalline region are estimated to be 15–100 A, while the pore size is on the order of 400 A.

Study of thermal oxidation and nitrogen annealing of luminescent porous silicon

The properties of thermally oxidized porous Si were studied by Fourier‐transform infrared spectroscopy and secondary ion mass spectroscopy. The results show that residual hydrogen exists in the 1000 °C/10 min thermally oxidized porous Si film in the form of SiOH bonds. The removal of these hydrogen atoms by annealing at 1000 °C in N2 reduces the photoluminescence.

Visible photoluminescence from porous Si formed by annealing and chemically etching amorphous Si

We have studied the visible photoluminescence (PL) and microstructure of amorphous Si (a‐Si) after annealing and etching. The a‐Si layers were grown on (100)Si substrates and partially crystallized by annealing between 550–1150 °C. Porous Si layers (PSLs) were then produced by etching in HF‐HNO3. While no visible PL was observed from unannealed and etched a‐Si, visible PL was detected after annealing and etching. The observation of visible PL after etching coincided with the observation of Si microcrystallites in the annealed layer. The results suggest that an initial crystalline structure is important for fabricating luminescent PSLs.

Photoinduced luminescence enhancement from anodically oxidized porous Si

We have investigated the phenomenon of photoluminescence (PL) increase in anodically oxidized porous Si with increasing laser illumination time by transmission Fourier transform infrared spectroscopy (FTIR), PL spectroscopy, and electron paramagnetic resonance. The adsorption of oxygen without hydrogen loss was observed during laser illumination by FTIR. The PL intensity increased linearly, while the dangling bond (DB) density decreased with increasing illumination time. By assuming that the decrease of DB density has a linear response to the illumination time, we identify that the change in DB density is mainly responsible for the observed PL increase after laser illumination.

Thin-film thermo-optic GexSi 1− x Mach–Zehnder interferometer

We report on the design and fabrication of Ge(x)Si(1-x) Mach-Zehnder interferometers based on the thermo-optic effect. The modulators exhibit 8 dB of extinction with a pi-phase-shift power requirement, P(pi), of 245 mW. The 3-dB bandwidth of these thermo-optic devices is 88 kHz. Finally, we present calculations indicating the effect of various parameters on the modulation of the Mach-Zehnder interferometer.