Jan Linnros

Analysis of the stretched exponential photoluminescence decay from nanometer-sized silicon crystals in SiO2

Time resolved photoluminescence (PL) decays have been measured for Si nanocrystals embedded in silicon dioxide. The nanocrystals were formed by implanting 40 keV Si ions into a 1000 A thick film of thermally grown SiO2, followed by thermal annealing at 1000–1200 °C. The observed luminescence, peaking at 700–850 nm, is compared to similar measurements performed on porous Si emitting in the same wavelength range. The results show that the PL from the nanocrystals exhibits a stretched exponential decay with characteristic decay time τ in the range 10–150 μs and dispersion factor β in the range 0.7–0.8. Both parameters are, however, higher for the nanocrystals compared to those of porous Si indicating superior passivation of the nanocrystals in the SiO2 matrix. Evidence is also presented for a single exponential behavior at the decay end suggesting a remaining fraction of excitons in isolated nanocrystals. We attribute the highly nonlinear dose dependence of the PL yield to a nucleation process for the nanocr...

Silicon Nanoribbons for Electrical Detection of Biomolecules

Direct electrical detection of biomolecules at high sensitivity has recently been demonstrated using semiconductor nanowires. Here we demonstrate that semiconductor nanoribbons, in this case, a thin sheet of silicon on an oxidized silicon substrate, can approach the same sensitivity extending below the picomolar concentration regime in the biotin/streptavidin case. This corresponds to less than approximately 20 analyte molecules bound to receptors on the nanoribbon surface. The micrometer-size lateral dimensions of the nanoribbon enable optical lithography to be used, resulting in a simple and high-yield fabrication process. Electrical characterization of the nanoribbons is complemented by computer simulations showing enhanced sensitivity for thin ribbons. Finally, we demonstrate that the device can be operated both in inversion as well as in accumulation mode and the measured differences in detection sensitivity are explained in terms of the distance between the channel and the receptor coated surface with respect to the Debye screening length. The nanoribbon approach opens up for large scale CMOS fabrication of highly sensitive biomolecule sensor chips for potential use in medicine and biotechnology.

Photoluminescence spectroscopy of single silicon quantum dots

Photoluminescence (PL) from single silicon quantum dots have been recorded and spectrally resolved at room temperature. The Si nanocrystals (NCs) were fabricated using electron-beam lithography and ...

Recombination-enhanced extension of stacking faults in 4H-SiC p-i-n diodes under forward bias

The extension of stacking faults in a forward-biased 4H-SiC PiN diodes by the recombination-enhanced motion of leading partial dislocations has been investigated by the technique of optical emission microscopy. From the temperature dependence of the measured velocity of the partials, an activation energy of 0.27 eV is obtained. Based on this and analysis of the emission spectra, a radiative recombination level of 2.8 eV for the stacking fault, and two energy levels for the partial dislocation, a radiative one at 1.8 eV and a nonradiative at 2.2 eV, have been determined.

Carrier lifetime measurements using free carrier absorption transients. II. Lifetime mapping and effects of surface recombination

A novel noncontact technique for semiconductor wafer mapping of the charge carrier lifetime is reviewed. The principle is based upon measurements of free carrier absorption transients by an infrared probe beam following electron-hole pair excitation by a pulsed laser beam. The technique is demonstrated here for Si wafer lifetime mapping, both for homogenous wafers as well as for wafers having pn junctions, and the performance and limitations are addressed. In a companion article J. Appl. Phys. 84, 275 (1998), (part I), the injection dependence is treated in more detail. We demonstrate the broad range of accessible injections, allowing on-wafer lifetime mapping both of the minority carrier lifetime and of the high injection lifetime as well as of Auger recombination effects. Furthermore, problems of surface recombination for bare Si surfaces are elucidated while pn junctions or other barriers are shown to suppress diffusion of carriers to surfaces. Also, a scheme for low temperature surface passivation is ...

High quantum efficiency for a porous silicon light emitting diode under pulsed operation

A high quantum efficiency for a porous silicon light emitting diode (LED) is demonstrated under pulsed operation. The LED is fabricated from a p+nn+ structure by anodic/hydrofluoric etching followed by deposition of a transparent gold contact. The LED shows a rectifying behavior and emits orange‐red light under forward bias with a spectral width significantly narrower than the corresponding photoluminescence spectrum. By calibrated measurements, we here demonstrate electroluminescence external quantum efficiencies of ∼0.2% under pulsed operation corresponding to internal quantum efficiencies of a few percent.

Waveguiding effects in the measurement of optical gain in a layer of Si nanocrystals

We discuss applicability of the variable stripe length method to experimental investigation of optical gain in a luminescent layer that behaves like a planar waveguide. We show that an interplay between the output direction of guided light modes and the numerical aperture of the collection optics may lead to an artifact manifesting itself as an apparent but false gain. We propose a way to circumvent this inconvenience by using a “shifting excitation spot” complementary measurement. The method is demonstrated on a layer of Si nanocrystals embedded into a synthetic silica plate.

Carrier lifetime measurements using free carrier absorption transients. I. Principle and injection dependence

A contactless, all-optical technique for semiconductor charge carrier lifetime characterization is reviewed. The technique is based upon measurements of free carrier absorption transients by an infrared probe beam following electron-hole pair excitation by a pulsed laser beam. Main features are a direct probing of the excess carrier density coupled with a homogeneous carrier distribution within the sample, enabling precision studies of different recombination mechanisms. We show that the method is capable of measuring the lifetime over a broad range of injections (1013–1018 cm−3) probing both the minority carrier lifetime, the high injection lifetime and Auger recombination, as well as the transition between these ranges. Performance and limitations of the technique, such as lateral resolution, are addressed while application of the technique for lifetime mapping and effects of surface recombination is outlined in a companion article [J. Appl. Phys. 84, 284 (1998), part II]. Results from detailed studies ...

Auger recombination in 4H-SiC: Unusual temperature behavior

The band-to-band Auger recombination in 4H-SiC material is studied using a time-resolved photoinduced absorption technique. The Auger recombination coefficient is derived from the kinetics of electron-hole plasma in heavily doped n-type 4H-SiC and in low-doped 4H-SiC epitaxial layers in the temperature interval 300–565 K. Within this range, its value decreases from γ3=(7±1)×10−31 cm6 s−1 to γ3=(4±1)×10−32 cm6 s−1. The observed pronounced reduction of Auger recombination rate with temperature is correlated to temperature dependent threshold energy of Auger process.

Formation of wide and deep pores in silicon by electrochemical etching

Abstract Electrochemical etching of n-type silicon in hydrofluoric acid electrolyte is now well known as a technique for micro- or macroporous silicon formation. It is commonly admitted that the width of pores can extend over four orders of magnitude, from 2 nm to 20 μm. In this study the feasibility of using this technique to form larger pores is demonstrated. The use of a water–ethanol solvent mixture (1:1) is shown to modify the electrochemistry of silicon dissolution and pore formation. The formation of stable wide pores requires adjustment of the etching current during the pore formation as a function of the evolution of the current–voltage curve with etching time. An array of 42-μm wide pores with 2-μm wall thickness and 200-μm depth were etched using this method. The feasibility to etch pores up to 100 μm in width is also presented. The results enable to conclude that the electrochemical etching of n-type silicon could be used to form vertical structures, without restrictions concerning the wall spacing. This provides a useful tool for micro-machining.