Mario Saggio

Voids in Silicon by He Implantation: From Basic to Applications

The mechanism of bubble formation when He is implanted into silicon is described. Many experiments are reviewed and several techniques are considered. During implantation and subsequent annealing, complex He n –V m clusters are formed, trapping vacancies, while Si self-interstitials recombine directly at the surface. By increasing temperature He atoms out-diffuse, and the entire process produces a supersaturation of vacancies (void formation). Their evolution is studied during isothermal and isochronal annealing, describing the mechanisms involved; that is, direct coalescence or Ostwald ripening. The internal surface is an efficient trap for self-interstitials and for metals. The gettering mechanism is governed by a surface adsorption at low impurity concentration while at high value a silicide phase is observed. The high getter capability is ensured by the large number of traps introduced (10 17 –10 19 cm −3 ). Finally, voids introduce mid gap energy levels that act as minority carrier recombination centers, providing a powerful method to control lifetime locally in silicon devices. The reviewed results demonstrate that the trap levels are due to the dangling bonds present on the void surface. This property can be used in many applications.

Materials issues and device performances for light emitting Er-implanted Si

Abstract The mechanisms of photo and electroluminescence from Er-implanted crystalline Si have been investigated and the crucial issues for the achievement of higher efficiency have been identified. Photoluminescence experiments show that Er-related levels are the gateway for the energy transfer from the electronic system of the semiconductor to the internal 4f shell of the Er ions. Er excitation is in fact thought to occur by the recombination of an electron-hole pair bound to an Er-related level. Higher yield and reduced temperature quenching of the luminescence can be obtained by engineering of the properties of these levels by codoping with O or other impurities. Room temperature electroluminescence has been achieved from Er doped crystalline Si diodes under both forward and reverse bias. Under forward bias the same mechanism identified from photoluminescence experiments is operative and therefore similar requirements have to be met in order to improve efficiency. On the other hand a higher room temperature electroluminescence yield is obtained under reverse bias. In this case the energy transfer occurs by impact excitation of the Er ions by hot carriers. Crucial issues for excitation mechanisms are the proper design of the diode structure in order to optimize the hot carrier distribution and the increase of the fraction of incorporated ions which are efficiently excited.

Thermal stability of the current transport mechanisms in Ni-based Ohmic contacts on n- and p-implanted 4H-SiC

Studying the temperature dependence of the electrical properties of Ohmic contacts formed on ion-implanted SiC layers is fundamental to understand and to predict the behaviour of practical devices. This paper reports the electrical characterization, as a function of temperature, of Ni-based Ohmic contacts, simultaneously formed on both n- or p-type implanted 4H-SiC. A structural analysis showed the formation of the Ni2Si phase after an annealing leading to Ohmic behaviour. The temperature-dependence of the specific contact resistance indicated that a thermionic field emission mechanism (TFE) dominates the current transport for contacts formed on p-type material, while a field emission (FE) is likely occurring in the contacts formed on n-type implanted SiC. The values of the barrier height were 0.75 eV on p-type material and 0.45 eV on n-type material. The thermal stability of the current transport mechanisms and related physical parameters has been demonstrated upon a long-term (up to 95 h) cycling in the temperature range 200–400 °C.

Fowler-Nordheim tunneling at SiO2/4H-SiC interfaces in metal-oxide-semiconductor field effect transistors

The conduction mechanisms and trapping effects at SiO2/4H-SiC interfaces in metal-oxide-semiconductor field effect transistors (MOSFETs) were studied by Fowler-Nordheim (FN) tunnelling and frequency dependent conductance measurements. In particular, the analysis of both MOS capacitors and MOSFETs fabricated on the same wafer revealed an anomalous FN behavior on p-type implanted SiC/SiO2 interfaces. The observed FN instability upon subsequent voltage sweeps was correlated to the charge-discharge of hole trap states close the valence band edge of 4H-SiC. The charge-discharge of these traps also explained the recoverable threshold voltage instability observed in lateral MOSFETs.

Radiation damage and implanted He atom interaction during void formation in silicon

He was implanted in silicon wafers to several doses (5×1015–4×1016 cm−2) at different temperatures (from −196 up to 400 °C). Void formation and evolution was observed by cross-sectional and plan view transmission electron microscopy analyses. We observed that void density and morphology are strictly related to substrate temperature during He implantation. Experiments show that for substrate temperature between 10 and 90 °C or higher than 150 °C, void formation is inhibited; when voids are observed, a few degrees of difference significantly change their density. The results can be interpreted by considering the interaction between He and the radiation damage produced during He implantation that forms stable bubbles.

Radiation damage–He interaction in He implanted Si during bubble formation and their evolution in voids

Abstract He atoms were implanted in crystalline and pre-amorphized silicon wafers at doses in the 2×10 16 1×10 17 cm −2 range. Using transmission electron microscopy (TEM) we monitored the evolution of He bubbles into voids upon thermal annealing. Bubbles are formed in both crystalline and amorphous silicon. However, in amorphous material bubble interaction with the moving crystalline–amorphous interface during the epitaxial regrowth prevents their evolution into voids. By implanting He at different target temperatures in crystalline Si, thus by changing the structure of radiation damage, we found that the interaction between point defects and He atoms is essential for the generation of He bubbles and for their subsequent evolution into voids.

Two-dimensional profiling by selective etching: the role of implant induced secondary defects

We have investigated the selective chemical etching of B-doped n-type silicon in a HF:HNO3 mixture either under illumination with ultraviolet light or using an electrolytic cell. The etching profiles are observed by transmission electron microscopy after a double cross section sample preparation. They extend up to the junction depth with a regular shape if no crystallographic defects are present in the sample. Extended defects affect the etching profiles and steps appear. The experimental results are interpreted by taking into account the role of the free carriers generated by light or by an external current source. This information is of fundamental importance to develop a two-dimensional junction profiling technique for micrometer size features by selective etching and transmission electron microscopy observations.

Two‐Dimensional Aluminum Diffusion in Silicon: Experimental Results and Simulations

Two-dimensional diffusion of aluminum implanted through a mask in silicon has been measured after furnace or rapid thermal annealings. Chemical staining on cross-sectioned samples was used to determine both the vertical and the lateral function depths. The spreading resistance standard procedure was adopted to measure the vertical profiles while a new procedure was developed for the determination of the lateral diffusion profiles. The difference in the measured profile by staining and spreading resistance is related to the spilling phenomenon that distorts the carrier concentration profile on beveled samples. Moreover, experimentally aluminum diffusion parameters and segregation coefficient at the Si/SiO 2 interface have been introduced in advanced one-dimensional (SUPREM III) and two-dimensional (SUPREM IV) process simulators. The good agreement between the simulated and the experimental data, for both the lateral and the vertical directions, indicates that these process simulators can reproduce with accuracy the aluminum diffusion

Nanoscale electro-structural characterisation of ohmic contacts formed on p-type implanted 4H-SiC

This work reports a nanoscale electro-structural characterisation of Ti/Al ohmic contacts formed on p-type Al-implanted silicon carbide (4H-SiC). The morphological and the electrical properties of the Al-implanted layer, annealed at 1700°C with or without a protective capping layer, and of the ohmic contacts were studied using atomic force microscopy [AFM], transmission line model measurements and local current measurements performed with conductive AFM.The characteristics of the contacts were significantly affected by the roughness of the underlying SiC. In particular, the surface roughness of the Al-implanted SiC regions annealed at 1700°C could be strongly reduced using a protective carbon capping layer during annealing. This latter resulted in an improved surface morphology and specific contact resistance of the Ti/Al ohmic contacts formed on these regions. The microstructure of the contacts was monitored by X-ray diffraction analysis and a cross-sectional transmission electron microscopy, and correlated with the electrical results.

Performance Analysis of Merged p-i-n–Schottky Diodes With Doping Compensation of the Drift Region

In this paper, standard-cell Schottky rectifiers along with silicon-based merged p-i-n-Schottky (MPS) and p-i-n diodes, which are realized using a super junction technology, have been analyzed and compared by conducting extensive device and mixed-mode simulations through a 2-D finite-element grid. The main issues that concern these devices, such as the forward voltage drop, the leakage characteristic, and the reverse recovery, are treated, and the superior performances exhibited by the MPS rectifier with respect to the p-i-n diodes are experimentally validated. First, the basics on the used technology are reported by focusing on the high voltage capability of the new devices along with the low forward voltage drop during the on -state conduction. The reverse-recovery behavior belonging to the MPS diode has been analyzed by exploring through several simulations the internal plasma dynamics. 2-D simulations of the turn-on behavior relative to the Schottky, p-i-n, and MPS rectifiers have been carried out in order to analyze the effects of the voltage overshoot phenomenon eventually occurring in the three diode structures