E. Rimini

A melting model for pulsing‐laser annealing of implanted semiconductors

The transition to single crystal of ion‐implanted amorphous Si and Ge layers is described in terms of a liquid‐phase epitaxy occurring during pulsing‐laser irradiation. A standard heat equations including laser light absorption was solved numerically to give the time evolution of temperature and melting as a function of the pulse energy density and its duration. The structure dependence of the absorption coefficient and the temperature dependence of the thermal conductivity were accounted for in the calculations. In this model the transition to single crystal occurs above a well‐defined threshold energy density at which the liquid layer wets the underlying single‐crystal substrate. Experiments were performed in ion‐implanted amorphous layers of thicknesses ranging between 500 and 9000 A. The energy densities of the Q‐switched ruby laser ranged between 0.2 and 3.5 J/cm2; time durations of 20 and 50 ns were used. The experimental data are in good agreement with the calculated values for the amorphous thickn...

Amorphous-to-crystal transition of nitrogen- and oxygen-doped Ge2Sb2Te5 films studied by in situ resistance measurements

The amorphous-to-crystal transition has been studied through in situ resistance measurements in Ge2Sb2Te5 thin films doped by ion implantation with nitrogen or oxygen. The dependence of the electrical resistivity and structure on the annealing temperature and time has been investigated in samples with different dopant concentrations. Enhancement of the thermal stability and increase of the mobility gap for conduction have been observed in O- and N-doped amorphous Ge2Sb2Te5. Larger effects have been found in the case of nitrogen doping.

Ion-beam-induced epitaxial crystallization and amorphization in silicon

Abstract The ion-beam-induced epitaxial crystallization (IBIEC) and planar amorphization of amorphous Si (a-Si) layers onto single-crystal Si substrates is reviewed. In particular, the dependence of the process on substrate temperature, on substrate orientation and on the energy deposited by the impinging ions into electronic and elastic collisions is treated in detail and discussed. Emphasis is also given to the influence of impurities on IBIEC, where a variety of different phenomena are observed. For instance, fast diffusers, such as Au, are seen to be swept by the moving c-a boundary and present intriguing segregation profiles. Slow diffusers such as As or O, on the other hand, have not enough mobility to move over long-range distances even in the presence of irradiation, but they can strongly modify the kinetics of IBIEC. Dopants such as B, P and As, for example, enhance the ion-induced growth rate by a factor of 2–3, while O retards it. It is also shown that by decreasing the substrate temperature (or by increasing the ion flux) IBIEC can be reversed resulting in a planar layer-by-layer amorphization. This phenomenon evidences the unique non-equilibrium features of ion-assisted phase transitions in silicon which are the result of a dynamic balance between defect production rate and defect annihilation rate. These data are discussed, mainly in comparison with the purely thermally activated growth of a-Si and a possible explanation of the observed phenomena is presented in terms of a simple model. Finally, new possible applications of the phenomenon, such as the ion-induced regrowth of deposited Si layers and of deposited GeSi heterostructures, are illustrated, demonstrating the high potentialities of ion-beam processing in producing epitaxial layers in a non-conventional manner.

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.

Arsenic diffusion in silicon melted by high‐power nanosecond laser pulsing

The time evolution of temperature and melting in amorphous silicon layers laser irradiated was calculated numerically. Experiments were performed in Si crystals implanted with 400‐keV As to a dose of 5×1015/cm2 and illuminated with 50‐ns‐duration Q‐switched ruby laser pulse in the energy range 1.0–3.0 J/cm2. Comparison between experimental and calculated results allows a quantitative understanding of the amorphous–to–single‐crystal transition. A good agreement was found between the experimental As profiles after laser irradiation and those calculated with a diffusion coefficient of 10−4 cm2/s for As in liquid silicon.

Screening Length and Quantum Capacitance in Graphene by Scanning Probe Microscopy

A nanoscale investigation on the capacitive behavior of graphene deposited on a SiO2/n(+) Si substrate (with SiO2 thickness of 300 or 100 nm) was carried out by scanning capacitance spectroscopy (SCS). A bias V(g) composed by an AC signal and a slow DC voltage ramp was applied to the macroscopic n(+) Si backgate of the graphene/SiO(2)/Si capacitor, while a nanoscale contact was obtained on graphene by the atomic force microscope tip. This study revealed that the capacitor effective area (A(eff)) responding to the AC bias is much smaller than the geometrical area of the graphene sheet. This area is related to the length scale on which the externally applied potential decays in graphene, that is, the screening length of the graphene 2DEG. The nonstationary charges (electrons/holes) induced by the AC potential spread within this area around the contact. A(eff) increases linearly with the bias and in a symmetric way for bias inversion. For each bias V(g), the value of A(eff) is related to the minimum area necessary to accommodate the not stationary charges, according to the graphene density of states (DOS) at V(g). Interestingly, by decreasing the SiO(2) thickness from 300 to 100 nm, the slope of the A(eff) versus bias curve strongly increases (by a factor of approximately 50). The local quantum capacitance C(q) in the contacted graphene region was calculated starting from the screening length, and the distribution of the values of C(q) for different tip positions was obtained. Finally the lateral variations of the DOS in graphene was determined.

How far will silicon nanocrystals push the scaling limits of NVMs technologies

For the first time, memory devices with optimized high density (2E12#/cm/sup 2/) LPCVD Si nanocrystals have been reproducibly achieved and studied on an extensive statistical basis (from single cell up to 1 Mb test-array) under different programming conditions. An original experimental and theoretical analysis of the threshold voltage shift distribution shows that Si nanocrystals have serious potential to push the scaling of NOR and NAND flash at least to the 35 nm and 65 nm nodes, respectively.

Dependence of trapping and segregation of indium in silicon on the velocity of the liquid‐solid interface

The segregation phenomena of In‐implanted Si have been observed following the melting and epitaxial regrowth of surface layers by pulsed ruby laser irradiation. The velocity of the liquid‐solid interface on recrystallization has been varied from 1.8 to 5.2 m/s in two independent ways. Indium is observed to be trapped on substitutional sites, in excess of solid solubilities, or segregated in a thin surface layer. Trapping increases and segregation decreases as the interfacial velocity is raised. The complete depth profiles can be fitted with unique interfacial segregation coefficients which are velocity dependent. The material that has been segregated to the surface shows cell structure of approximately 100 A diameter arising from lateral segregation due to constitutional supercooling. The cells are not present at velocities of 5 m/s. The critical dependence of In trapping and segregation on velocity in the range 2–5 m/s is interpreted in terms of interfacial residence times.

Gettering of metals by He induced voids in silicon

He bubbles were formed in Si substrates implanting He at doses ranging from 5 × 1015 / cm2 to 1 × 1017 / cm2 and energies in the range of 40–300 keV. Bubbles are formed only if a dose of 1 × 1016 /cm2 at 40 keV is exceeded, while at 300 keV a dose of 5 × 1016 /cm2 has to be reached. If bubbles are present in the as-implanted sample after annealing at 700°C voids are formed. The void evolution during subsequent thermal processing was studied in detail. The thermal stability is found to be excellent for long thermal treatments and an increase in the void diameter with increasing the annealing temperature was observed. The gettering reactivity of these voids is higher than for conventional gettering processes.

Solute trapping by moving interface in ion‐implanted silicon

Experiments are reported for Te and Ag implantation in silicon, as examples of slow and fast diffusers, after furnace or laser annealing. Slow diffusers are substitutionally located at concentrations in great excess of the maximum solid solubility after both processes. Fast diffusers inhibit the solid‐phase epitaxial regrowth or are rejected at the sample surface after laser irradiation. Although the epitaxial growth occurs with velocities which differ up to ten orders of magnitude after furnace or laser annealing, the supersaturation is interpreted as due to the same basic mechanism: solute trapping at the moving interface when the residence time is larger than the one monolayer regrowth time. This process is controlled by the diffusion coefficient in the two adjacent phases.