Nenad Lalic

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...

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.

Electroluminescence of single silicon nanocrystals

We report on measurements of room-temperature electroluminescence from single silicon nanocrystals. The electrically driven emission reveals typical characteristics of single-nanocrystal luminescence: the peak wavelength variations, narrowing of spectral bands, a high degree of linear polarization, and intensity fluctuations (blinking) observed on a scale of minutes. From the count rate statistics of individual nanocrystals, we conclude that the yield of radiative emission is as high as 19%. These findings may open a route to highly efficient all-silicon light emitters.

Characterization of a porous silicon diode with efficient and tunable electroluminescence

Electrical and optical properties of electroluminescent porous silicon pn diodes have been investigated. The measurements were performed under pulsed conditions to maintain high quantum efficiency (∼0.2%) operation. Diodes with different peak wavelengths, obtained by varying the etching parameters, were used. A high correlation between electroluminescence (EL) and photoluminescence (PL) of porous silicon (PSi) structures was observed: similar luminescence peak position, spectral width, quantum efficiencies, decay time constants, and luminescence quenching at increasing temperature, suggesting the same recombination mechanism for both EL and PL. The temperature dependence of the forward current and EL revealed a thermally activated conduction mechanism with an activation energy of ∼0.1–0.2 eV which is attributed to potential barriers in the undulating silicon wires. EL quenching and a redshift of the EL peak at an increased temperature were also observed. This is attributed to shorter nonradiative lifetime...

Electroluminescence microscopy and spectroscopy of silicon nanocrystals in thin SiO2 layers

Stable continuously operable electroluminescent diodes have been fabricated by Sii-ion implantation and annealing of thin SiO2 layers on a silicon substrate. The external quantum efficiency of the ...

A porous silicon light-emitting diode with a high quantum efficiency during pulsed operation

A high quantum efficiency for a porous silicon light-emitting diode (LED) is demonstrated. The LEDs, fabricated by anodic etching from a p+ nn+ structure, exhibit external quantum efficiencies of ∼0.2% under pulsed operation. Corresponding internal quantum efficiencies of a few percent approach those of the photoluminescence. The color of the emitted light can be tuned from orange to the infrared range by variation of etching parameters in agreement with the quantum confinement model. The spectral distribution is voltage independent, as well as the quantum efficiency for voltages above 20 V.

Instabilities in electroluminescent porous silicon diodes

The stability of high quantum efficiency, porous silicon (PS) electroluminescent (EL) diodes have been analyzed under pulsed operation. The results show that EL exhausting is due to a charging of the PS network which recovery is thermally activated (Er∼0.4 eV). This suggests a charge trapping mechanism at larger crystallites with low bandgap, inhibiting further injection of carriers. Also, forward current is found to be thermally activated, however, with a distinctly lower activation energy (Ej∼0.1–0.2 eV), possibly related to jumping of carriers over barriers between nearby crystallites. Finally, high current densities may lead to thermal runaway causing permanent damage to the structure.

Porous silicon diodes operated near the electroluminescence threshold

Abstract Transient behavior and the onset voltage of the electroluminescence (EL) has been measured for a porous silicon light-emitting diode (PSi LED). Diodes were fabricated by anodization of the pn diode structure with contacts realized by deposition of gold pads. Higher onset voltages of the EL have been found for samples with shorter peak wavelengths. This is attributed to higher injection barriers for those samples, in agreement with a smaller expected average crystallite size. A red-shift of the EL is observed at the voltages near the EL threshold and an EL mechanism based on exciton generation is suggested.

Relevance of the carrier transport change in porous silicon at the onset of one excited pair per crystallite

Abstract We report on the first experimental observation of transient free-carrier absorption-detected gratings in highly-luminescent porous silicon membranes. The sloping changes on 400 μs long decay scale are observed below the injection onset for one excited electron–hole pair per nanocrystallite which are related to a notable shortening in the microsecond recombination form. A preliminary microscopic model for the dispersive recombination is suggested including the photoinduced fraction of uncorrelated carriers, which recombine according to a power decay, in contrast to the excitons which recombine according to a stretched exponential. At low excitation, we extract the upper limit of exciton diffusion coefficient and diffusion length as D =3.4×10 −5 cm 2 s −1 and L =315 nm.

Short-term degradation of porous silicon light-emitting diodes

Abstract We report on two important features of light-emitting diodes based on a porous silicon p-n junction: Strong decrease of the electroluminescence intensity is observed for pulses longer than ~1 ms and the influence of ambient gases on the LED operation is discussed.