Sandro Solmi

Precipitation as the phenomenon responsible for the electrically inactive phosphorus in silicon

The physical nature of the electrically inactive phosphorus in silicon was investigated by annealing experiments performed on laser annealed specimens doped by ion implantation up to 5×1021 at/cm3. The hypothesis of point defects, which compensate or make the excess dopant electrically inactive, is contradicted by the experimental results. It was verified that phosphorus solubility corresponds to the electrically active concentration in equilibrium with the inactive dopant, and that the latter is precipitated phase. This was confirmed by transmission electron microscopy (TEM) examinations with the weak beam technique, which detected a high density of very small coherent precipitates. This method allowed us to observe particles of the same kind even on specimens thermally predeposited in conditions typical of device technology. In both cases the amount of precipitates was consistent with the inactive dopant concentration. In addition these experiments show that precipitation is associated with a high enhan...

Transient enhanced diffusion of arsenic in silicon

The transient enhanced diffusion (TED) of As in silicon samples implanted at 35 keV with dose 5×1015 cm−2 has been investigated in the temperature range between 750 and 1030 °C by comparing experimental and simulated profiles. For temperatures higher than 900 °C the phenomenon is of modest entity and vanishes after a few seconds, whereas at lower temperatures diffusivity enhancements of some order of magnitude have been observed. The anomalous shift of the junction depth, evaluated at 2×1018 cm−3, is about 12 nm at 900 °C and increases up to 45 nm at 750 °C. It has been verified that the two are the contributions, that generate the interstitial excess responsible for the TED: (i) the implantation damage and (ii) the aggregation in clusters of the As atoms. From an experiment that allows us to separate the two contributions, we estimate that about one third of the TED observed in the first 20 min of annealing at 800 °C is due to the defects produced by clustering. The influence of clustering on the shape o...

On the growth of stacking faults and dislocations induced in silicon by phosphorus predeposition

The growth of extrinsic stacking faults associated with SiP platelets induced by phosphorus predeposition in thin silicon foils has been followed by transmission electron microscopy. The role of the precipitates and of the phosphorus diffusion on the climb of the Frank partials is discussed. Unfaulting reactions, observed to annihilate isolated extrinsic Frank loops, are assumed to take place also in the case of the stacking faults associated with precipitates, resulting in the generation of 60° perfect dislocations, which have been revealed by x‐ray topography. Different climb rates for the 60° dislocations have been observed after predeposition experiments performed by varying the oxidation and the phosphorus‐doping kinetics; the relationship between these two parameters and the vapor partial pressures of the gas flowing in the furnace has been obtained as a result of careful experimental work. From the results of all our experiments, it has been deduced that the ingoing phosphorus, rather than the asso...

SiP precipitation within the doped silicon lattice, concomitant with phosphorus predeposition

From experiments of thermal predeposition of phosphorus in silicon, performed at 920 °C and followed by neutron activation analysis and TEM observations, it is concluded that SiP precipitation takes place in the furnace during the isothermal process. In fact, the surface concentration of phosphorus increases with predeposition time attaining values well above the solid solubility; in addition the flat rodlike SiP precipitates grow with the time of predeposition, according to a parabolic law. Finally, considerations of the size of the precipitates indicate that they are not formed during the cooling stage subsequent to predeposition.

Nitrogen Implantation to Improve Electron Channel Mobility in 4H-SiC MOSFET

Normally off 4H-SiC MOSFET devices have been fabricated on a p-type semiconductor and electrically characterized at different temperatures. A gate oxide obtained by nitrogen ion implantation performed before the thermal oxidation of SiC has been implemented in n-channel MOSFET technology. Two samples with a nitrogen concentration at the SiO2/SiC interface of 5 X 1018 and 1.5 X 1019 cm-3 and one unimplanted sample have been manufactured. The sample with the highest N concentration at the interface presents the highest channel mobility and the lowest threshold voltage. For increasing temperature, in all the samples, the threshold voltage decreases, and the electron channel mobility increases. The latter case attains a maximum value of about 40 cm2/V ldr s at 200degC for the sample with the highest N concentration. These trends are explained by the reduction of interface electron traps in the upper half of the band gap toward the conduction band edge. These results demonstrate that N implantation can be effectively used to improve the electrical performances of an n-type surface channel 4H-SiC MOSFET.

Effects of heating ramp rates on the characteristics of Al implanted 4H–SiC junctions

The role of the heating rate in the postimplantation annealing process of SiC was investigated. Structural and electrical properties of an Al+ implanted junction in n-type 4H–SiC were studied for a 30min annealing at 1600°C in argon atmosphere and changing the heating rate between 7 and 40°C∕s. With the increasing of the heating rate the surface roughness increases, but the electrical performance of the devices improves. In particular, the faster ramp-up rate reduces the sheet resistance of the implanted layer of about 40% with respect to the slowest process and significantly reduces the leakage current of the diodes.

Arsenic uphill diffusion during shallow junction formation

The behavior during annealing of low-energy As-implanted Si have been investigated by comparing secondary ion mass spectrometry (SIMS) and simulated profiles. Z-contrast scanning transmission electron microscopy (STEM) imaging has also been used to determine the As local distribution in proximity of the sample surface. The implants have been performed with energies between 1 and 10keV both through a thermally grown 11nm thick oxide and without any oxide mask. SIMS and STEM profiles show, after short annealing at 800–1000°C, an As pileup in the first nanometers of the Si matrix in proximity of the SiO2∕Si interface. We demonstrate that this phenomenon can be explained with a “Fickian” standard diffusion by assuming the presence of unspecified “dopant traps” near the SiO2∕Si interface that cause a drastic reduction of the dopant able to diffuse inside the bulk. We have also verified that removing before annealing the superficial 4nm of Si does not eliminate the As pileup. Different mechanisms proposed in li...

Reverse annealing, clustering, and electron mobility in arsenic doped silicon

The reverse annealing phenomena and the nature of the As clusters were studied on silicon on insulator samples uniformly doped with As at concentrations up to 7.6×1020 cm−3. Carrier concentration and electron mobility were determined by Hall and resistivity measurements after annealing at temperatures in the range 550–800 °C. The amplitude of transient reactivation, which can involve up to 20% of the clustered As, depends on dopant concentration and on the annealing temperatures and times. The results of a detailed study of the influence of these parameters are reported. The occurrence of reverse annealing confirms the existence of different types of As clusters, whose distribution and features depend on the experimental conditions. The effect of clustered As on the electron mobility was measured and a quantitative relationship for this phenomenon is reported. The amplitude of this effect is in agreement with the view that at room temperature As clusters are electrically neutral.

Analysis of electron traps at the 4H–SiC/SiO2 interface; influence by nitrogen implantation prior to wet oxidation

Electron states at the SiO2/4H–SiC interface have been investigated using capacitor structures and especially, the influence of excess nitrogen, introduced by ion implantation, at the interface is studied in detail. Implanted and nonimplanted n-type samples with an interfacial concentration of nitrogen of ∼1019 cm−3 and 1016 cm−3, respectively, were analyzed by capacitance-voltage (C-V) measurements, performed at different temperatures and probe frequencies, and thermal dielectric relaxation current (TDRC) measurements performed in the temperature range of 35–295 K. Three main categories of electron states are disclosed, true interface states (Dit), fast near interface states (NIToxfast) and slow near interface states (NIToxslow). The density versus energy distributions of Dit and NIToxfast have been deduced from the TDRC data and they are shown to give a close quantitative agreement with the shape and frequency dependence of the C-V curves. Further, the amount of NIToxslow extracted from TDRC is demonstr...

Clustering equilibrium and deactivation kinetics in arsenic doped silicon

The equilibrium between clusters and dopant in solution was studied on silicon on insulator specimens uniformly doped with As at concentrations CAs from 1 to 7.6×1020 cm−3. The values of the carrier density n* after equilibration at 700, 800, and 900 °C are reported. With increasing dopant concentration n* rapidly saturates to the limiting value of the carrier density ne, thus simulating a precipitation process. It is shown that the values of n* at different temperatures and dopant concentrations can be calculated by an equation derived in the Appendix by using a simple cluster model. The deactivation was analyzed by isothermal annealing of the specimens at temperatures in the range 550–800 °C. At high temperature the kinetics accurately complies with the rate equation −dn/dt=A{exp[−(E−αn)/kT]−(n0−n)/(n0−n*)exp[−(E−αn*)/kT]} which is the one reported in [D. Nobili, S. Solmi, M. Merli, and J. Shao, J. Electrochem. Soc. 146, 4246 (1999)] complemented by the second term on the right to account for the declus...