Light-emitting Silicon Research Articles

Nanocrystalline Silicon for Optoelectronic Applications

Light emission in silicon has been intensively investigated since the 1950s when crystalline silicon (c-Si) was recognized as the dominant material in microelectronics. Silicon is an indirect-bandgap semiconductor and momentum conservation requires phonon assistance in radiative electron-hole recombination (Figure 1a, top left). Because phonons carry a momentum and an energy, the typical signature of phonon-assisted recombination is several peaks in the photoluminescence (PL) spectra at low temperature. These PL peaks are called “phonon replicas.” High-purity c-Si PL is caused by free-exciton self-annihilation with the exciton binding energy of ~11 meV. The TO-phonon contribution in conservation processes is most significant, and the main PL peak (~1.1 eV) is shifted from the bandgap value (~1.17 eV) by ~70 meV—that is, the exciton binding energy plus TO-phonon energy (Figure 1a).

Photoluminescent Siloxenes in Nanoporous Aluminum Oxide

Light-emitting silicon systems are of interest both scientifically and technologically. Luminescence can arise either from silicon structures or from siloxene-like systems. The formation of photoluminescent siloxenes by spontaneous decomposition of silanes in the channels of aluminum oxide membranes is reported, which opens up simple novel routes to stable light-emitting systems. The advantages of using nanoporous aluminum oxide membranes are discussed, the presence of siloxene layers on the pore surfaces is proved, and the photoluminescence data are presented.

Integration of Electrically Isolated Porous Silicon Leds for Applications in CMOS Technology

AbstractPreviously we reported on integrated porous silicon (PSi) based LEDs with local bipolar drivers that would have possible applications in an active-matrix configuration for display or optical signal transmission [1]. We now report further progress in device engineering and integration that has enabled the fabrication of improved light-emitting silicon devices based on oxide-passivated nanocrystalline silicon (OPNSi) that can be integrated with standard CMOS technology. Design of experiments methodologv was used to direct device engineering experiments, providing a better understanding of how process parameters influence the resulting device performance. It was discovered that the formation of stacking fault defects in the LED region prior to anodization has a predominant effect on both carrier transport and electroluminescence (EL) capabilities of the devices. Process development work has resulted in the fabrication of OPNSi LEDs that offer many of the required attributes of a useful silicon light-emitter. The devices exhibit EL at a bias < 5V, diode-like rectifying I-V characteristics, good stability under DC bias, and uniform emission over the LED contact area. These LEDs have been combined with a new fabrication technique which enables the formation of electrically isolated integrated LEDs in the single-crystal substrate. Each diode structure is formed in its own individual well, enabling junction-isolated devices without a common substrate electrode. Such a process is required to realize integrated LEDs which can be individually addressed in an X-Y array configuration using full-rail voltage modulation. Details of the LED characteristics and device integration will be discussed.

Light-emitting nanocrystalline silicon by low-pressure chemical-vapor deposition of disilane

Porous silicon is an attractive material for silicon optoelectronics. The great advantage of porous silicon lies on the simple way of production which makes silicon nanostructures easily available. After several papers have been published on this topic, we are able to identify some disadvantages connected to the porous nature of the material and to the method of fabrication. Other dry processes can be used to produce Si nanostructures. In this paper we present a method fully compatible with the standard semiconductor technology. The optical and structural properties of the nanocrystalline films so far obtained are presented, together with some promising results indicating good electrical properties.

Light emitting silicon, recent progress

Recent results concerning selected optical and mainly transport properties of light-emitting silicon are critically reviewed. A short discussion is devoted to the question of the nature of the optical gap in porous silicon (direct/indirect). Special attention is given to the transport mechanism in strongly luminescent nanoporous silicon. It is concluded that the existence of tail states is crucial for the transport and a three-layer model of silicon nanoparticles is presented. Prospects for electroluminescent devices based on light-emitting silicon are evaluated from the point of view of their efficiency and stability.

Multiple Source Quantum Well Model of Porous Silicon Light Emission

Based on photoluminescence, Fourier transform infrared spectroscopy, and atomic force microscopy results, a new light emitting model for porous silicon (multiple source quantum well model) is proposed.

The influence of preparation conditions on the photoluminescence spectra of light-emitting Si prepared by laser pulse irradiation of a-Si:H

We report on room-temperature photoluminescence (PL) properties of light-emitting silicon produced by excimer (XeCl, ArF) laser crystallization of undoped hydrogenated amorphous silicon (a-Si:H). PL intensity increases linearly with the number of applied laser pulses. We annealed the recrystallized a-Si:H samples to modify the PL intensity and the spectral position. When compared with the XeCl laser processing, ArF laser crystallization produces more intense PL, shifted towards the blue region. Time-resolved reflectivity spectra and nucleation theory are used to study the difference between the ArF and XeCl laser crystallizations.

A Comparison of Porous Silicon and Silicon Nanocrystallite Photoluminescence Quenching with Amines

We report here a comparison of the ability of the monodentate Lewis base n-propylamine and the bidentate molecule ethylenediamine to quench the photoluminescence of light-emitting silicon in three different structural environments. These include porous silicon fabricated from p-type substrates; porous silicon from n-type substrates, and Si nanocrystallites derived from porous silicon. Both types of porous Si substrates were characterized by conventional transmission electron microscopy, while the Si nanocrystallites were characterized by high-resolution transmission electron microscopy. The fractional changes in photoluminescence as a consequence of amine addition were measured and fitted to a static equilibrium model from which adduct formation constants were calculated. It is found that the Si lumophores embedded in the porous oxides on n- and p-type substrates interact similarly with n-propylamine and ethylenediamine; however, ultrasonic extraction of the nanocrystalline Si fragments from the porous oxide layers and subsequent exposure to these amines reveals a near order of magnitude difference between the equilibrium constants for n-propylamine (log K ∼ 4.9) and ethylenediamine (log K ∼ 5.8) addition.

Polarization analysis of hot-carrier light emission in silicon

In this paper a theoretical-evaluation is given of the absolute intensity and polarization of light emission from silicon devices due to conduction-conduction (c-c) and valence-valence (v-v) direct transitions. The matrix elements of the momentum operator between Bloch states have been obtained from a full band-structure calculation performed with the pseudopotential method. Results have been obtained by using both analytical model distribution functions and realistic hot-carrier distributions obtained from Monte Carlo (MC) simulations based on the same band model. They show a polarization degree of a few per cent, which should be observable for these transitions.

Effect of substrate roughness on porous silicon light emission

Porous silicon layers fabricated on substrates of different roughness are studied. The photoluminescence characteristics of these samples are reported at room and liquid nitrogen temperature. The initial roughness of the substrates regulates the porous silicon formation mechanism. The results are explained in terms of the quantum confinement model. The rougher the starting substrate, the narrower is the size distribution of the emitting structures.

Visible light emission from Si materials

The photoluminescence of electrochemically etched silicon under excitation in the wavelength range from 325 to 514 nm is described by a single energy gap (of order of 1.9 eV). The Raman spectra can be explained by a Gaussian phonon confinement to a geometrical structure size of about 3 nm. In addition, the novel method of spark erosion for a non-liquid preparation of light-emitting silicon has been used, and it is shown that the light emission spectrum, apart from its overall intensity, is independent of environment gas species (N 2, O 2 or H 2). In areas of higher electric field strength, presumably leading to a smaller effective size, a blue shift is observed in all these cases. This appears to rule out any siloxene-derivate models and suggests a quantum confinement as the basic phenomenon.

Optocoupler based on the avalanche light emission in silicon

The photon emission in silicon light-emitting diodes is very inefficient, due to the indirect bandgap. Moreover the photon energy of 1.1 eV, which equals the bandgap, is outside the visible part of the spectrum and is not sufficient to generate electron-hole pairs in a silicon photodetector. When used in avalanche mode, the emission spectrum covers the visible range and the range where silicon photodetectors show high sensitivity. Therefore, silicon integrated displays and silicon integrated optical devices, such as optocouplers can be realized. When used in an accurate analog optocoupler circuit, the low efficiency of the current transfer ratio of the optocouplers makes the design of the electronic circuit more difficult. However, a circuit of an input stage of an isolated amplifier is given using an optocoupler based on the avalanche light emission in silicon.