M. Pavesi

Leakage current and reverse-bias luminescence in InGaN-based light-emitting diodes

This paper reports an electro-optical analysis of the correlation between reverse-bias leakage current and luminescence in light-emitting diodes based on InGaN. The results of the analysis suggest that (i) the main mechanism responsible for leakage current conduction is tunneling, (ii) leakage current is correlated with the presence of reverse-bias luminescence, (iii) leakage current flows through preferential paths, that can be identified by means of emission microscopy, and (iv) reverse-bias luminescence could be ascribed to the recombination of electron-hole pairs in the quantum well region.

Verification of electron distributions in silicon by means of hot carrier luminescence measurements

This paper investigates the use of hot carrier luminescence (HCL) measurements as a mean for the verification of carrier energy distribution functions in submicron silicon devices subject to high electric fields. To this purpose, physically-based two-dimensional (2-D) simulations of the spectral distribution of HCL are compared with extensive experimental data on special purpose n/sup +//n/n/sup +/ test structures that demonstrate lateral field profiles similar to real MOSFETs without the obscuring effects of a gate electrode. Good agreement between measured and simulated data is observed over wide channel length, bias, and temperature ranges, thus providing for the first time a direct verification of simulated electron energy distributions in a MOSFET-like environment.

Electron and hole-related luminescence processes in gate injection transistors

This paper reports an extensive investigation of the luminescence processes in GaN-based gate injection transistors (GITs). The results of the analysis indicate that: (i) GITs operating in on-state conditions can emit a weak luminescence signal; (ii) for moderate gate voltage levels, luminescence is originated at the edge of the gate toward the drain side, due to hot electrons accelerated by the high gate-drain electric field; (iii) for higher gate voltage levels, luminescence is originated in the region between the gate and the source of the devices, due to the recombination of the holes injected from the gate and the electrons present in the channel. Results obtained by means of electrical and optical measurements are compared throughout the paper to support the interpretation of the measurement results.

Effects of extreme dc-ageing and electron-beam irradiation in InGaN/AlGaN/GaN light-emitting diodes

Changes in the optical performances of blue light-emitting diodes based on InGaN/AlGaN/GaN heterostructures were studied after ageing stages consisting of electrical stress at a dc forward current of 100 mA and high-energy electron-beam irradiation. The structural modifications induced by ageing were investigated and a physical description of the nature of the damage was defined by means of complementary techniques such as electroluminescence, cathodoluminescence and current–voltage characterization at different temperatures. The onset of a broad optical band at about 3.08 eV and a consistent reduction of the quantum well optical efficiency revealed a substantial degradation of the heterostructure after ageing. The new band, ascribed to the formation of Mg-related complexes acting as shallow acceptors, occurred together with a dramatic change in the current–voltage curves. Its quenching after electron-beam irradiation and its recurrence after further electrical stress suggested the metastable nature of Mg-related complexes.

Cathode Hot Electrons and Anode Hot Holes in Tunneling MOS Capacitors

This paper presents simulations of electron and hole gate currents in thin oxide tunneling MOS capacitors, based on a newly developed Monte Carlo code for Si-SiO Si stacks. Fully bipolar simulations with state of the art Si and SiO transport models predict a previously neglected population of cathode hot electrons proportional to that of the anode hot holes, often regarded as responsible of oxide degradation. The bias dependence of this population is discussed in view of recently reported results on the role of hole injection and transport in device degradation.

Field dependence of the carrier injection mechanisms in InGaN Quantum wells: Its effect on the luminescence properties of blue light emitting diodes

Carrier injection and radiative recombination processes in InGaN∕GaN blue light emitting diodes are investigated by bias-dependent cathodoluminescence. The samples are designed with a single-quantum-well (SQW) light emitter and an adjacent multi-quantum-well (MQW) carrier injector of lower In content. In unbiased samples, the MQW emission dominates over the SQW at high temperatures (T>160K) or low beam currents (IB<5nA). This is ascribed to changes in the device energy band diagram, dependent on the field in the p-n junction and on the level of electron beam induced excitation of excess carriers. A field screening, resulting in a forward biasing of the devices, is highlighted. A maximum value of +2.65V is reached at T=100K for a carrier generation rate G0∼5.4×1013s−1. At a fixed electron beam power, the effects of an external applied field on the emitter efficiency are considered. The SQW emission is promoted in the forward-current regime, while in the reverse-current regime (−1⩽V<2.65 at T=100K) it under...

A comparative analysis of substrate current generation mechanisms in tunneling MOS capacitors

This paper presents a critical analysis of the origin of majority and minority carrier substrate currents in tunneling MOS capacitors. For this purpose, a novel, physically-based model, which is comprehensive in terms of impact ionization and hot carrier photon emission and re-absorption in the substrate, is presented. The model provides a better quantitative understanding of the relative importance of different physical mechanisms on the origin of substrate currents in tunneling MOS capacitors featuring different oxide thickness. The results indicate that for thick oxides, the majority carrier substrate current is dominated by anode, hole injection, while the minority carrier current is consistent with a photon emission-absorption mechanism, at least in the range of oxide voltage and oxide thickness covered by the considered experiments. These two currents appear to be strictly correlated because of the relatively flat ratio between impact ionization and photon emission scattering rates and because of the weak dependence of hole transmission probability on oxide thickness and gate bias. Simulations also suggest that, for thinner oxides and smaller oxide voltage drop, the photon emission mechanism might become dominant in the generation of substrate holes.

Evidence of substrate enhanced high-energy tails in the distribution function of deep submicron MOSFETs by light emission measurements

This letter reports direct experimental evidence that the high-energy tail of the hot carrier luminescence distribution of deep submicron silicon MOSFETs is essentially modified by the application of a substrate voltage. The bias and temperature dependence of the phenomenon are consistent with an enhancement of the high-energy tail of the energy distribution due to a second impact ionization event occurring at the drain to substrate junction.

Temperature dependence of the electrical activity of localized defects in InGaN-based light emitting diodes

Traps govern the temperature dependence of current in III-nitride quantum heterostructures, but frequently electrical measurements result unable to identify how many and what kind of defects take part in the conduction. The present work shows how a combined electrical and optical characterization in temperature can detect localized defects involved in injection mechanisms in InGaN∕AlGaN∕GaN blue light lmitting diodes. At least two different traps assisting the carrier injection by tunneling and playing an active role below and above 175K, respectively, are identified in or nearby the active layers.

Failure mechanisms of GaN-based LEDs related with instabilities in doping profile and deep levels

In this work we report on a reliability study carried out over InGaN/GaN LEDs. Accelerated life tests have been carried out at various biasing current levels and at various temperature. Failure modes have been identified and correlated with the instability of Magnesium as dopant in the p-layer and with the worsening of the efficiency of the quantum well in active layer.