Enrico Zanoni

Correlation between thermal resistance, channel temperature, infrared thermal maps and failure mechanisms in low power MESFET devices☆

Abstract Channel temperature T ch of GaAs MESFETs, determined by means of electrical measurements ( ΔV gs ), has been compared and correlated with thermal maps obtained by high resolution infrared microscopy. Results show that in low power devices with a small number of gates, the value of T ch derived from ΔV gs measurements is close to the maximum values of temperature measured on the hottest junctions on the chip. Local thermal inhomogeneities, in particular those observed in devices characterized by high values of R th , can explain and confirm results of failure analyses of MESFETs submitted to accelerated life tests in operating conditions where degradation phenomena occur in localized areas of the device.

Correlation between fabrication processes and thermal distribution in medium power MESFETs

Abstract Correlation between fabrication process and thermal distribution was evaluated by means of high resolution IR thermography in two 0.5 W MESFET structures, having the same layout but differing in the presence of via hole connections. The via hole structure allows a great improvement in device thermal behaviour, with decrease in Rtk and more uniform thermal distribution. The effect of process variations, such as gate misalignments, on temperature distribution within the device active area was also evaluated.

Techniques for latch-up analysis in CMOS IC's based on scanning electron microscopy☆

Abstract Scanning electron microscopy techniques for the characterization of latch-up phenomena in CMOS integrated circuits are reported, critically rewieved and compared with experimental techniques described in the literature, based on infra-red and scanning laser microscopy. Several examples of application are presented to demonstrate the capability of a SEM system for practical layout debugging. Described SEM techniques comprise: (a) sensitivity analysis by means of ebeam induced current, with digital control of the beam position and beam blanking to avoid MOS damaging; (b) identification of latch-up sites in steady-state by Digital Differential Voltage Contrast; (c) study of transient and triggering phenomena and localization of latch-up firing points by means of stroboscopic voltage contrast.

Microscopic, electrical and optical studies on InGaN/GaN quantum wells based LED devices

We report here on the micro structural, electronic and optical properties of a GaN-based InGaN/GaN MQW LED grown by the MOVPE method. The present study shows that the threading dislocations present in these LED structures are terminated as V pits at the surface and have an impact on the electrical and optical activity of these devices. It has been pointed that these dislocations were of edge, screw and mixed types. EBIC maps suggest that the electrically active defects are screw and mixed dislocations and behave as nonradiative recombinant centres.

Defect-related degradation of III-V/Silicon 1.55 µm DBR laser diodes

This paper reports on an extensive investigation on the degradation mechanisms that may limit the long term reliability of heterogeneous III-V/Silicon DBR laser diodes for integrated telecommunication applications in the 1.55 μm window. The devices under test, aged for up to 500 hours under different bias conditions, showed a gradual variation of both optical (L-I) and electrical (I-V, C-V) characteristics. In particular, the laser diodes exhibited an increase in the threshold current, a decrease of the turn-on voltage and an increase in the apparent charge density within the space-charge region, which was extrapolated from C-V measurements. For longer stress times, these two latter processes were found to be well correlated with the worsening of the optical parameters, which suggests that degradation occurred due to an increase in the density of defects within the active region, with consequent decrease in the non-radiative (SRH) lifetime. This conclusion is also supported by the fact that during stress the apparent charge profiles indicated a re-distribution of charge within the junction. A preliminary investigation on the physical origin of the defects responsible for degradation was carried out by DLTS measurements, which revealed the presence of five different deep levels, with a main trap located around 0.43 eV above the valence band energy. This trap was found to be compatible with an interface defect located between the In0.53AlxGa0.47-xAs SCH region and the InP layer.

White-light sources based on GaN laser diodes: analysis and application study

We report on the design and study of solid state laser sources for lighting applications. While LEDs are affected by droop, limiting efficacy at higher currents, a possible solution is represented by solid state laser lighting, where a blue (450nm) laser is exciting a luminescent material thus achieving white light. With this work we designed and tested several LARP (Laser-Activated-Remote-Phosphors) test structures, both diffused lighting and focused applications will be discussed. Results indicates that good efficiency are achievable, without any sensible droop also at high injection currents. Phosphors have also been subjected to thermal stability tests up to 550°C.

Degradation processes and origin in InGaN-based high-power photodetectors

GaN-based multi-quantum well devices are promising candidates as photodetectors in the UV to visible spectral range. Their complex structure and the extreme input optical power density still poses problems of reliability. In the devices under test, degradation takes place when the optical power density reaches values higher than 44 W/cm2 , and consists in a reduction in the efficiency of the device and in its output current. This degradation process is not sudden and is caused by a gradual increase in the defect concentration, detected by means of photocurrent spectroscopy experiments, that suggest the role of gallium vacancies and/or their complexes as the physical origin. A secondary effect is the reduction in open circuit voltage, likely originating from an improvement in dopant and/or contact quality.

Changes in the Mg profile and in dislocations induced by high temperature annealing of blue LEDsGallium Nitride Materials and Devices VIII

The efficiency of the injection and recombination processes in InGaN/GaN LEDs is governed by the properties of the active region of the devices, which strongly depend on the conditions used for the growth of the epitaxial material. To improve device quality, it is very important to understand how the high temperatures used during the growth process can modify the quality of the epitaxial material. With this paper we present a study of the modifications in the properties of InGaN/GaN LED structures induced by high temperature annealing: thermal stress tests were carried out at 900 °C, in nitrogen atmosphere, on selected samples. The efficiency and the recombination dynamics were evaluated by photoluminescence measurements (both integrated and time-resolved), while the properties of the epitaxial material were studied by Secondary Ion Mass Spectroscopy (SIMS) and Rutherford Backscattering (RBS) channeling measurements. Results indicate that exposure to high temperatures may lead to: (i) a significant increase in the photoluminescence efficiency of the devices; (ii) a decrease in the parasitic emission bands located between 380 nm and 400 nm; (iii) an increase in carrier lifetime, as detected by time-resolved photoluminescence measurements. The increase in device efficiency is tentatively ascribed to an improvement in the crystallographic quality of the samples.

Study of short-term instabilities in InGaN/GaN light-emitting diodes by means of capacitance voltage measurements and deep-level transient spectroscopy

In this paper we present a combined current-voltage, capacitance-voltage, Deep Level Transient Spectroscopy and electroluminescence study of short-term instabilities of InGaN/GaN LEDs submitted to forward current aging tests at room temperature. In the early stages of the aging tests at low forward current levels (15-20 mA), LEDs present a decrease in optical power, which stabilizes within the first 50 hours and never exceeds 10% (measured at 20 mA). The spectral distribution of the electroluminescence intensity does not change with stress, while C-V profiles detect changes consisting in apparent doping and/or charge concentration increase within quantum wells. This increase is correlated with the decrease in optical power. Capacitance Deep Level Transient Spectroscopy has been carried out to clarify the DC aging induced generation/modification of the energy levels present in the devices. Remarkable changes occur after the stress, which can be related to the doping/charge variation and thus to the efficiency loss.

Hot-electron-induced light emission and impact ionization in GaAs-based devices

In this paper, we characterize impact ionization effects in GaAs MESFETs, in AlGaAs/GaAs High Electron Mobility Transistors, HEMTs, in AlGaAs/InGaAs Pseudo-Morphic High Electron Mobility Transistors, PM-HEMTs, and AlGaAs/GaAs Heterojunction Bipolar Transistors, HBTs. Then, we correlate impact ionization and electroluminescence in each type of device, providing a deeper insight into the mechanisms responsible for the emission of photons in the high electric field regime. Finally, conclusions follow in Section 4.