A. Anopchenko

High power efficiency in Si-nc/SiO2 multilayer light emitting devices by bipolar direct tunneling

We demonstrate experimentally bipolar (electrons and holes) current injection into silicon nanocrystals in thin nanocrystalline-Si/SiO2 multilayers. These light emitting devices have power efficiency of 0.17% and turn-on voltage of 1.7 V. The high electroluminescence efficiency and low onset voltages are attributed to the radiative recombination of excitons formed by both electron and hole injection into silicon nanocrystals via the direct tunneling mechanism. To confirm the bipolar character, different devices were grown, with and without a thick silicon oxide barrier at the multilayer contact electrodes. A transition from bipolar tunneling to unipolar Fowler–Nordheim tunneling is thus observed.

Silicon Nanocrystals as an Enabling Material for Silicon Photonics

Silicon nanocrystals (Si-nc) is an enabling material for silicon photonics, which is no longer an emerging field of research but an available technology with the first commercial products available on the market. In this paper, properties and applications of Si-nc in silicon photonics are reviewed. After a brief history of silicon photonics, the limitations of silicon as a light emitter are discussed and the strategies to overcome them are briefly treated, with particular attention to the recent achievements. Emphasis is given to the visible optical gain properties of Si-nc and to its sensitization effect on Er ions to achieve infrared light amplification. The state of the art of Si-nc applied in a few photonic components is reviewed and discussed. The possibility to exploit Si-nc for solar cells is also presented. In addition, nonlinear optical effects, which enable fast all-optical switches, are described.

Low-voltage onset of electroluminescence in nanocrystalline-Si/SiO2 multilayers

Thin film metal-oxide-semiconductor light emitting devices (LEDs) based on nanocrystalline silicon multilayer structure were grown by plasma-enhanced chemical vapor deposition. Room temperature electroluminescence was studied under direct current and time-resolved pulsed-current injection schemes. Multilayer LEDs operating at voltages below 5 V and electroluminescence turn-on voltage of 1.4–1.7 V are demonstrated. The turn-on voltage is less than 3.2 V which corresponds to the barrier height at the silicon oxide interface for electrons. Electrical injection in the multilayer LED is controlled by direct tunneling of electrons and holes among silicon nanocrystals. This injection regime is different than the Fowler–Nordheim tunneling that controls the electron injection in single thick layer LED operating at high voltages. A comparison of the power efficiency for the multilayer based LED and a similar single thick layer LED shows larger power efficiency for the former than for the second. Our results open ne...

Electrical conduction and electroluminescence in nanocrystalline silicon-based light emitting devices

Electrical transport and light emission properties of plasma-enhanced chemical vapor deposition grown light emitting devices (LEDs) based on nanocrystalline silicon have been studied. Various active layer compositions have been used. Electroluminescence and current-voltage measurements have been performed on metal-oxide-semiconductor structures. We found that Poole–Frenkel emission and trap-assisted tunneling between traps located at the nanocrystalline silicon interfaces are consistent with the measurements. The interface trap density was estimated. Its dependence on the composition of the active layer is discussed. We propose an equivalent electrical circuit model for the LED based on complex impedance measurements. Nanocrystalline silicon electroluminescence in the near infrared region is explained by hot-electron injection and impact ionization mechanism. It is concluded that the trap-assisted tunneling and charge trapping limit the external power efficiency of this kind of devices.

Light emission properties and mechanism of low-temperature prepared amorphous SiNX films. II. Defect states electroluminescence

In this paper, we present a room-temperature electroluminescence (EL) study of amorphous nonstoichiometric silicon nitride (SiNX) films. The light-emitting device is formed by an ITO/SiNX/p-type silicon structure. EL shows a yellowish broad emission spectrum with a power efficiency of 10−6. The EL peak energy depends on the bias voltage rather than on the silicon content in SiNX. By fitting the current-voltage characteristic with existing models, we found that under high voltages the Poole–Frenkel hole conduction is the main carrier transport mechanism in these devices. Injected electrons are captured by silicon dangling bonds (K center) and recombine with holes, which are localized in valence band tail states. Unbalanced hole and electron injection and nonradiative recombination are the main constraints on the EL efficiency of SiNX.

Erbium emission in MOS light emitting devices: From energy transfer to direct impact excitation

The electroluminescence (EL) at 1.54 μm of metal–oxide–semiconductor (MOS) devices withEr3C ions embedded in the silicon-rich silicon oxide (SRSO) layer has been investigated under different polarization conditions and compared with that of erbium doped SiO2 layers. EL time-resolved measurements allowed us to distinguish between two different excitation mechanisms responsible for the Er3C emission under an alternate pulsed voltage signal (APV). Energy transfer from silicon nanoclusters (Si-ncs) to Er3C is clearly observed at low-field APV excitation. We demonstrate that sequential electron and hole injection at the edges of the pulses creates excited states in Si-ncs which upon recombination transfer their energy to Er3C ions. On the contrary, direct impact excitation of Er3C by hot injected carriers starts at the Fowler–Nordheim injection threshold (above 5 MV cm(-1)) and dominates for high-field APV excitation.

Graded-size Si quantum dot ensembles for efficient light-emitting diodes

We propose a simple way to engineer the energy band gap of an ensemble of silicon nanocrystal (Si-NC) embedded in SiO2 via thickness/composition profiling of Si-NC multilayers. By means of a complementary metal-oxide-semiconductor compatible process, light emitting diodes (LEDs) which incorporate graded energy gap Si-NC multilayers in the active region have been grown. Electrical and optical properties of these graded Si-NC LEDs demonstrate the ability of the proposed method to tailor the optoelectronic properties of Si-NC devices.

Superlinear photovoltaic effect in Si nanocrystals based metal-insulator-semiconductor devices

Superlinear-variation in short circuit photocurrent with increasing incident optical power has been observed in metal-insulator-semiconductor structures having a silicon rich oxinitride active layer containing silicon nanocrystals. A model has been elaborated where an internal gain mechanism explains the superlinear photovoltaic effect. The internal gain mechanism is due to secondary carrier generation (SCG) from sub-bandgap levels in the nanocrystal. SCG is caused by impact excitation from the photogenerated conduction band electrons. The sub-bandgap levels are associated to traps formed at the dielectric/Si-nanocrystals interface.

Modeling of silicon nanocrystals based down-shifter for enhanced silicon solar cell performance

A transfer matrix model of a luminescent down-shifter (LDS) layer, consisting of silicon nanocrystals (Si-NCs) embedded in a silicon oxide matrix, on a silicon solar cells is presented. To enhance the efficiency of the silicon solar cell, we propose using a SiO2/Si-NCs double layer stack, as an anti-reflection-coating (ARC) and as a LDS material. The optical characteristics of this stack have been simulated and optimized as a front surface coating. The cell performances have been simulated by means of a two-dimensional device simulator and compared with the performances of a reference silicon solar cell. We found a 6% relative enhancement of the energy conversion efficiency with respect to the reference cell. We demonstrate that this enhancement results from the lower reflectance and from the down-shifter effect of the Si-NCs activated coating stack.

Toward a 1.54

In this paper, we report on the first attempt to design, fabricate, and test an on-chip optical amplifier which works at 1540 nm and can be electrically driven. It is based on an asymmetric silicon slot waveguide which embeds the active material. This is based on erbium-doped silicon rich silicon oxide. We describe the horizontal asymmetric slot waveguide design which allows us to get a high field confinement in a nanometer thick active layer. In addition, we detail the complex process needed to fabricate the structure. The waveguides have been characterized both electrically as well as optically. Electroluminescence can be excited by hot carrier injection, due to impact excitation of the Er ions. Propagation losses have been measured and high values have been found due to processing defects. Pump and probe measurements show a voltage dependent strong attenuation of the probe signal due to free carrier accumulation and absorption in the slot waveguide region. At the maximum electrical pumping level, electroluminescence signal is in the range of tens of μW/cm 2 and the overall loss of the device is only -6 dB. Despite not demonstrating optical amplification, this study shines some light on the path to achieve an all-silicon electrically driven optical amplifier.