Hot Carrier Luminescence Research Articles

Spatial Non-Uniform Hot Carrier Luminescence From 4H-SiC p-i-n Avalanche Photodiodes

4H-SiC avalanche photodiodes (APDs) working in Geiger mode are promising devices for ultra-weak ultraviolet light detection. In this letter, the degree of avalanche uniformity across device mesa of 4H-SiC p-i-n APDs is studied by imaging hot carrier luminescence pattern of the devices in avalanche regime. It is found that the luminescence emission from the APD under avalanche always first appears in the [ $\overline 1 \overline 1 20$ ] side of the top contact electrode. As over-bias increases, the luminescence pattern gradually extends toward the [ $11\overline 2 0$ ] direction and finally covers the whole device mesa. A physical model is proposed to explain the non-uniform avalanche phenomenon, in which the substrate off-orientation related carrier lateral drift would cause asymmetric hole accumulation and screening of junction electrical field along the [ $11\overline 2 0$ ] direction.

Picosecond imaging of signal propagation in integrated circuits

Abstract: Optical analysis of integrated circuits (IC) is a powerful tool for analyzing security functions that are implemented in an IC. We present a photon emission microscope for picosecond imaging of hot carrier luminescence in ICs in the near-infrared spectral range from 900 to 1700 nm. It allows for a semi-invasive signal tracking in fully operational ICs on the gate or transistor level with a timing precision of approximately 6 ps. The capabilities of the microscope are demonstrated by imaging the operation of two ICs made by 180 and 60 nm process technology.

Synthesis, Morphological, and Electro-optical Characterizations of Metal/Semiconductor Nanowire Heterostructures.

In this letter, we demonstrate the formation of unique Ga/GaAs/Si nanowire heterostructures, which were successfully implemented in nanoscale light-emitting devices with visible room temperature electroluminescence. Based on our recent approach for the integration of InAs/Si heterostructures into Si nanowires by ion implantation and flash lamp annealing, we developed a routine that has proven to be suitable for the monolithic integration of GaAs nanocrystallite segments into the core of silicon nanowires. The formation of a Ga segment adjacent to longer GaAs nanocrystallites resulted in Schottky-diode-like I/V characteristics with distinct electroluminescence originating from the GaAs nanocrystallite for the nanowire device operated in the reverse breakdown regime. The observed electroluminescence was ascribed to radiative band-to-band recombinations resulting in distinct emission peaks and a low contribution due to intraband transition, which were also observed under forward bias. Simulations of the obtained nanowire heterostructure confirmed the proposed impact ionization process responsible for hot carrier luminescence. This approach may enable a new route for on-chip photonic devices used for light emission or detection purposes.

Enhanced Hot-Carrier Luminescence in Multilayer Reduced Graphene Oxide Nanospheres

We report a method to promote photoluminescence emission in graphene materials by enhancing carrier scattering instead of directly modifying band structure in multilayer reduced graphene oxide (rGO) nanospheres. We intentionally curl graphene layers to form nanospheres by reducing graphene oxide with spherical polymer templates to manipulate the carrier scattering. These nanospheres produce hot-carrier luminescence with more than ten-fold improvement of emission efficiency as compared to planar nanosheets. With increasing excitation power, hot-carrier luminescence from nanospheres exhibits abnormal spectral redshift with dynamic feature associated to the strengthened electron-phonon coupling. These experimental results can be well understood by considering the screened Coulomb interactions. With increasing carrier density, the reduced screening effect promotes carrier scattering which enhances hot-carrier emission from such multilayer rGO nanospheres. This carrier-scattering scenario is further confirmed by pump-probe measurements.

Avalanche Current Measurements in SPADs by Means of Hot-Carrier Luminescence

A growing number of applications require arrays of single-photon avalanche diode (SPAD) detectors with low timing jitter. In order to improve jitter without compromising other performance parameters, a clear understanding of avalanche dynamics and statistics is necessary. In this work, a noninvasive electro-luminescence technique has been employed to investigate the current growing in a SPAD device. The obtained results let us assess, for the first time experimentally, the avalanche spreading speeds and also confirmed our assumption that the current growth pace and its statistics critically depend on the space charge effects.

Extensive analysis of the luminescence properties of AlGaN/GaN high electron mobility transistors

This paper reports on an extensive analysis of the electroluminescence spectra of GaN-based high-electron mobility transistors (HEMT) submitted to different bias regimes. The results described within this paper indicate that: (i) under ON-state bias conditions, HEMT can emit a weak luminescence signal, localized at the edge of the gate toward the drain side; (ii) for low drain voltage levels, the electroluminescence spectrum has a Maxwellian shape, which is typical for hot carrier luminescence; (iii) for high drain voltage levels, parasitic emission bands are generated, possibly due to the recombination of hot electrons through defect-related sites. Electroluminescence data are compared with results of cathodoluminescence measurements, to provide an interpretation for the experimental results.

Comparison of oxide breakdown progression in ultra-thin oxide silicon-on-insulator and bulk metal-oxide-semiconductor field effect transistors

Enhanced oxide breakdown progression in ultra-thin oxide silicon-on-insulator p-type metal-oxide-semiconductor field-effect transistors is observed, as compared to bulk devices. The enhanced progression is attributed to the increase of hole stress current resulting from breakdown induced channel carrier heating in a floating-body configuration. Numerical analysis of hole tunneling current and hot carrier luminescence measurement are performed to support our proposed theory. This phenomenon is particularly significant to the reliability of devices with ultra-thin oxides and low operation gate voltage.

Characterization of breakdown in ultrathin oxides by hot carrier emission

Hot carrier luminescence excited at low voltages across ultrathin gate oxides is used to describe the localized transport during progressive oxide breakdown. The emission identifies the locations of initial breakdown spots in field effect transistors. The transitions from stress-induced leakage currents to reversible breakdown to final breakdown are accompanied by changes in the efficiency of the emission showing an evolution from inelastic to elastic transport.

Analysis of the hot carrier luminescence in AlxGa1?xAs/GaAs power HFET's

The processes related to the presence of hot carriers in power AlGaAs/GaAs heterostructure field effect transistors (HFET) have been studied by means of optical (electroluminescence) and electrical techniques. Electroluminescence spectra evidence the presence of three emission bands, superimposed on the maxwellian behavior typical for the hot electron emission. Two of the previous bands are related to the bandgap transitions of cold carriers in the conduction channel and in the barrier layer, while the third one is probably related to DX centers present in the Al x Ga 1-x As layers. We show that the optical measurements give more detailed information than the electrical ones; in particular the gate current seems to be a less indicative monitor of the impact ionization mechanisms than the light emitted in the conductive channel by cold carrier recombinations. No correlation between integrated light and currents is evidenced, except at low energies were an intra-band mechanism, related only to the drain current is observed. Spatial maps of the emitted light evidence that part of the cold holes produced by impact ionization move toward the source electrode and recombine with cold electrons in this region.

Substrate bias dependence of breakdown progression in ultrathin oxide pMOSFETs

Negative substrate bias-enhanced oxide breakdown (BD) progression in ultrathin oxide (1.4 nm) pMOS is observed. The enhanced progression is attributed to the increase of hole-stress current resulting from BD-induced, channel-carrier heating. The carrier temperature extracted from the spectral distribution of hot-carrier luminescence is around 1300 K. The substrate bias dependence of post-BD hole-tunneling current is confirmed from measurement and calculation. The observed phenomenon is particularly significant to ultrathin gate oxide reliability in floating substrate (SOI) and forward-biased substrate devices.

Photoluminescence characterization of a counterpropagating twin photons structure: hot carrier effects in AlGaAs

Counterpropagating signal and idler twin photons can be produced via parametric fluorescence by a pump beam incident at a certain angle on top of a III–V semiconductor waveguide, as long as the refractive indices of the waveguide material for the two photons––determined by the thicknesses and alloy compositions of the layers––satisfy the necessary in plane phase matching condition. A specially designed AlGaAs/AlAs counterpropagating twin photons structure is characterized by temperature and pumping power density dependence photoluminescence spectroscopy. Under the present conditions of photoexcitation the spectra show all the typical characteristics of hot carrier luminescence. The relaxation of photoexcited carriers and formation of hot electron gas are analysed and discussed. From the dependence on pumping power density of the carrier temperature extracted from the slope of the high energy tail of the spectra, it is shown that the main relaxation path of the hot electron gas is via emission of LO-phonons at low temperatures.

Hot carrier luminescence during porous etching of GaP under high electric field conditions

Electroluminescence is observed during porous etching of n-type GaP single crystals at strongly positive potential. The emission spectra, which include a supra-bandgap contribution, are markedly different from the spectra observed under optical excitation or minority carrier injection. The current density and electroluminescence intensity show a strong potential dependence and a similar hysteresis. The spectral characteristics of the luminescence suggest that both thermalised and hot charge carriers, generated by impact ionisation, are involved in light emission.

Spontaneous hot-carrier photon emission rates in silicon: Improved modeling and applications to metal oxide semiconductor devices

The recent publication of controversial experimental evidence on the origin of hot-carrier currents in 4\char21{}10 nm tunnel metal oxide semiconductor capacitors renewed the interest in improving hot-carrier luminescence models for silicon devices. This work presents several such improvements, aimed at making possible a physically based analysis of the hot-carrier luminescence effects taking place during tunneling experiments in relatively thick ${\mathrm{SiO}}_{2}$ layers. To this purpose, silicon band structure and scattering rate calculations have been extended well above 10 eV by considering eight conduction bands, instead of the usual four, so as to allow for a detailed description of the high-energy carriers injected from silicon into silicon dioxide during tunneling experiments. The absolute contributions of the direct and phonon-assisted, interband and intraband transitions of electrons and holes to the total photon emission rate are analyzed, so the results can be directly compared with the experimental data. To the best of our knowledge, it is for the first time that results for valence-to-valence band transitions of holes are presented and compared with those of conduction-to-conduction band transitions of electrons. Results can be directly compared with experimental data. Template results obtained with a variety of carrier distributions (Maxwellian, Gaussian, and Dirac's delta-like) are shown and implications for device analysis are discussed.

Tools for contactless testing and simulation of CMOS circuits

Short channel effects in MOSFETs are responsible for time-dependent hot-carrier luminescence, synchronous with the switching transitions in CMOS circuits. We propose an optical non-invasive inspection technique for high-speed signals, based on a high sensitivity solid-state photodetector with sharp time resolution. This tool is able to probe the fast electrical waveforms propagating through ULSI circuits without electrically loading the circuit under test. The measured time resolution of 50 ps allows an equivalent analog bandwidth of about 20 GHz. From the experimental results and the luminescence characterization of single transistors, we propose a SPICE model able to foresee the photoemission in complex ULSI circuits, down to transistor level. The optical testing equipment and the SPICE modeling are valuable tools for simulation, characterization and testing of fast ULSI circuits.

Time-resolved optical characterization of electrical activity in integrated circuits

If the rate of improvement in the performance of advanced silicon integrated circuits is to be sustained, new techniques for the measurement of electrical waveforms in operating circuits are needed. Critical factors dictating this requirement include the increased speed and complexity of circuits, the growing importance of faults that appear only during high-speed operation, and the use of flip-chip packaging technologies. Two recently developed all-optical methods for measuring the switching activity from the backside of a chip are described and compared. One is a passive approach based on the measurement of hot carrier luminescence emitted from the channel of a CMOS field-effect transistor (FET) during switching. The second uses a laser probe to sense the switching induced modulation of the silicon optical constants near an FET's source and drain.

Influence of hot-carrier luminescence from avalanche photodiodes on time-correlated photon detection

We present the results of our time-resolved measurements of hot-carrier luminescence from passively quenched Geiger-mode avalanche photodiodes. In time-correlated photon-counting (TCPC) experiments, hot-carrier luminescence interferes overwhelmingly with the coincidence spectrum, which results in artifacts. This potential problem should be taken into account in setting up TCPC experiments.

Watching chips work: picosecond hot electron light emission from integrated circuits

The picosecond pulses of hot carrier luminescence that are observed from individual submicron FETs in CMOS circuits can be used to describe the internal operation of integrated circuits. To effectively use the weak emission pulses, we have developed a method called picosecond integrated circuit analysis (PICA) which simultaneously images and time resolves the emission. PICA has been used to characterize the operation of integrated circuits from simple ring oscillators to a full microprocessors. Examples of circuit characterization and fault diagnosis are presented.

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.

Hot-carrier luminescence in sub-quartermicrometer high-speed GaAs MESFET's

Hot-carrier luminescence in high-speed GaAs MESFET's with sub-quartermicrometer gate length was investigated at drain voltages high enough to permit breakdown. The spectral distribution of emitted radiation was analyzed in the energy range of 1.4-2.5 eV. GaAs MESFET's with a gate length of 0.18 /spl mu/m yielded a prominent peak from the direct recombination across a GaAs bandgap of 1.43 eV. At energies above 1.65 eV, a broad continuous spectra with two peaks and a shoulder were detected. The two peaks coincide with the indirect recombination energies between holes in the /spl Gamma/ valley and electrons in the L or X valley. These peaks, however, were diminished at drain voltages as high as 7.5 V. It is suggested that the luminescence at energies above the bandgap mainly arises from the recombination of hot carriers, and the luminescence resulting from a phonon-assisted conduction to conduction-band transition is superimposed on it. The luminescence from the gate Schottky diode at reversed bias was also examined. There were no peaks from the direct recombination across the bandgap in the spectra. The light emission at the bandgap energy under the FET operation probably originates from the recombination of cold channel electrons and hot holes, which are generated by impact ionization and swept toward the source.

Monitoring hot-carrier degradation in SOI MOSFETs by hot-carrier luminescence techniques

This paper addresses the problem of hot-carrier degradation and lifetime monitoring in SOI MOSFETs by means of hot-carrier-induced luminescence measurements. The peculiar emission behavior of SOI devices is clarified over a broad range of bias conditions by means of comparison with that of BULK MOSFETs. It is shown that detailed analysis of hot-carrier luminescence measurements at different photon energies provides a noninvasive monitoring tool for various aspects of degradation, such as worst case bias conditions, threshold voltage shift, and variations of the electric field and hot-carrier population in the damaged region. The measured light intensity represents also a sensitive acceleration factor for the extrapolation of lifetimes to real operating conditions.