Electrical measurements and numerical simulations of Ion Implanted 4H-SiC PiN diodes

In this thesis work 4H–SiC ion implanted PiN diodes are studied and characterised in detail. Electrical measurements (current–voltage curves and lifetime measurements) and Numerical simulations are performed with the aim to better understand physical phenomena which arises from the periphery of these diodes. This analysis is relevant as it is well known that perimeter currents affect performances of SiC devices

The Role of Predation in the Evolution of Sexual Dimorphism in Gasterosteus aculeatus

Sexual dimorphism —morphological differences between the sexes, contributes to morphological variation within a species and has been shown to occur in a variety of taxa. One mechanism hypothesized to generate dimorphism is differential niche partitioning between the sexes, which leads to selection acting differently on each sex. Differential predation is one factor that has been suggested to accompany sex-specific niche divergence. However, most studies investigating predation and sexual dimorphism have been conducted in the field, which has prohibited a direct test of the contribution of differential predation. To directly test for the contribution of differential predation to the evolution of sexual dimorphism, I utilized data from a manipulative predation experiment. In the experiment, a highly variable population of hybrid benthic-limnetic stickleback were either exposed to cutthroat trout predation or left in a trout-free control treatment for one generation. Morphological trait distributions were then compared between the two generations to compare the pattern of evolution between the sexes and estimate the strength of selection on sexual dimorphism. When looking at overall dimorphism we did not find a significant effect of predation. However, when looking at specific traits we found a trend towards a significantly greater amount of dimorphism in predation ponds for two traits, anal fin length and caudal peduncle depth. We also found evidence of differential patterns of selection between males and females when exposed to predators, with males experiencing more selection due to predation. These results together suggest that predation might play some role in the evolution of sexual dimorphism in stickleback

The role of defects on forward current in 4h-sic p-i-n diodes

We investigated the impact of defect states on the measured forward current-voltage (I -V) curves of ion-implanted planar 4H-SiC p-i-n diodes of a different anode dimension bymeans of a fine-tuned numericalmodel. Cross sections and activation energies of defects related to the carbon vacancy (EH6/7 and Z1/2) and Titanium (Ti) impurity used in our model were experimentally identified in the diodes of the same batch. We analyzed the effect of each individual defect on the I-V curves and estimated the unknown hole capture cross sections by ensuring the optimal match between simulated and measured currents. Small discrepancies between measured and simulated forward current-voltage curves of diodes of equal shape but a different perimeter-to-area ratio has been accounted for by considering, in the simulations, the presence of a fixed positive charge at the diode surface. By using this procedure, diodes of every dimension have been simulated without the use of adjustment parameters. These results are valuable in understanding the role of defects in the I-V curves of the ion-implanted SiC diode

Al+ Ion Implanted 4H-SiC Vertical p+-i-n Diodes: Processing Dependence of Leakage Currents and OCVD Carrier Lifetimes

The reverse and forward currents of Al+ ion implanted 4H-SiC p+-i-n diodes have been compared for identically processed devices except for the implanted Al concentration in the emitter, 6×1019 cm-3 against 2×1020 cm-3, and the post implantation annealing treatment, 1600°C/30 min and 1650°C/25 min against 1950°C/5min. The diodes’ ambipolar carrier lifetime, as obtained by open circuit voltage decay measurements, has been compared too. The devices with lower annealing temperature show lower leakage currents and higher ambipolar carrier lifetime; they also show lower current in ohmic conduction

Al+ implanted vertical 4H-SiC p-i-n diodes: experimental and simulated forward current-voltage characteristics

The temperature dependence of the forward and reverse current voltage characteristics of circular Al+ implanted 4H-SiC p-i-n vertical diodes of various diameters, post implantation annealed at 1950 °C/5 min, have been used to obtain the thermal activation energies of the defects responsible of the generation and the recombination currents, as well as the area and the periphery current component of the current voltage characteristics. The former have values compatible with those of the traps associated to the carbon vacancy defect in 4H-SiC. The hypothesis that only these traps may justify the trend of the current voltage characteristics of the studied diodes has been tested by simulations in a Synopsys Sentaurus TCAD suite

OCVD Lifetime Measurements on 4H-SiC Bipolar Planar Diodes: Dependences on Carrier Injection and Diode Area

In this paper, open-circuit voltage decay measurements of carrier lifetimes on 4H silicon carbide ion implanted planar p–i–n diodes of circular shape and different diameters are performed at increasing bias currents. The measured ambipolar carrier lifetimes have shown to be dependent on the carrier injection levels, quickly increasing at low-bias current up to reaching a saturation value, τHL, when the value of the average carrier density within the base exceedsthe intrinsic-regiondoping. The τHLmeasured in diodes of different area also demonstrate a marked dependence on the diode dimension, with longer lifetimes being typical of larger-diameter diodes, suggesting that a great contribution of recombination comes from the diode periphery. A bulk ambipolar lifetime τHL_Vol = 320 ns has been extracted fromthe area-dependent measured lifetimes

Perimeter and Area Components in the I-V Curves of 4H-SiC Vertical p⁺-i-n Diode With Al⁺ Ion-Implanted Emitters

This paper describes the separation between the area and the perimeter current density components of 4H-SiC vertical p⁺-i-n diode with a circular Al⁺ implanted emitter of different diameters in the range 150-1000 μm and for temperatures of measurement in the range 30-290 °C. It is shown that before the diode series resistances become dominant, the forward current is given by the sum of an area plus a perimeter component, both of exponential trend with ideality factor 2; while toward high voltages, an area component with exponential trend and ideality factor 1 adds to the previous components. Moreover, this paper shows that forward area and perimeter current density components can be used for a straightforward identification of the parameters controlling the current transport, provided that they can be fit by the p-n junction equations in the frame of the abrupt junction approximation. Finally, this paper shows that the area reverse current density can be used for the identification of the electrically active defects in the drift layer

Thermal stability of 1x1020 cm-3 Al+ implanted 4H-SiC after electrical activation at temperature >= 1850 °C

The effect of a 1500 °C treatment on 1x1020cm-3 Al+ ion implanted in 4H-SiC samples that have been previously annealed at 1850-1950 °C is studied in this work. Up to 240 min annealing time at 1500 °C, the Al electrical activation reached at 1850-1950 °C is preserved

Ni-Al-Ti ohmic contacts on 1 x1020 cm-3 Al+ ion implanted 4H-SiC

Ni/Al/Ti ohmic contacts with preserved form factor during alloying at 1000 °C, almost constant specific resistance value in the low 10-4 Ohmxcm2 decade, and a very weak temperature dependence in the range 25 - 290 °C, have been obtained on 1x 1020 cm-3 Al+implanted p-type 4H-SiC of different resistivity in the range 6 x10-2- 1 Ohmxcm2. A qualitative data analysis for understanding the hole transport mechanism through the formed metal/semiconductor interface is also shown