Somit Talwar

Crystallinity, strain, and thermal stability of heteroepitaxial Si1−xGex/Si (100) layers created using pulsed laser induced epitaxy

Heteroepitaxy of Si1−xGex/Si alloy layers on Si (100) substrates has been achieved using pulsed laser induced epitaxy (PLIE). The energy of 1 to 20 pulses from a spatially homogenized XeCl excimer laser beam is used to melt a structure consisting of electron‐beam evaporated Ge on Si (100) substrates. Alloy films with different Ge fractions are investigated and films with up to 21% Ge content are found to exhibit excellent crystallinity, as confirmed by MeV‐ion channeling along the 〈100〉 direction. MeV‐ion channeling is also used to determine the level of strain in the layers. This is done by comparing angular yield curves around the 〈110〉 direction for the substrate and alloy layer. The strain values obtained match with calculations for an ideally strained layer state. The strain is also measured for layers that have been subjected to different thermal cycles. A high level of strain is preserved in the alloy layer even after heating to 950u2009°C for 1 h. This unusual thermal stability is believed to be due t...

Resistless, area‐selective ultrashallow P+/N junction fabrication using projection gas immersion laser doping

The selective fabrication of ultrashallow p+/n junctions in silicon using projection gas immersion laser doping is reported. The method offers substantial improvement and simplification in junction formation to integrated circuit manufacturers, since several processing steps required for conventional doping techniques like ion implantation are eliminated. Spatially selective incorporation of boron into silicon without the use of any masking layer on the wafer surface is achieved. A pulsed excimer laser beam is patterned using a chromeless reticle and the pattern is transferred through a projection system onto a wafer that is kept in a BF3 dopant gas ambient. The depth of the fabricated junctions is 60 nm with a surface concentration of 5×1019 cm−3. The vertical and lateral distribution of boron in silicon after patterned laser processing is investigated using secondary ion mass spectroscopy (SIMS) and time‐of‐flight SIMS (ToF‐SIMS). Vertical and lateral dopant profiles are steep and clearly resolved.

Laser-induced lateral epitaxy in fully depleted silicon-on-insulator junctions

Junction formation by laser-induced lateral epitaxy was studied on a fully depleted silicon-on-insulator substrate (25–30 nm Si on 375 nm silicon dioxide). Selective laser melting of amorphous films with a 35 ns 308 nm XeCl laser pulse was characterized in situ using transient conduction and optical reflectance techniques. Lateral epitaxy from a channel edge was observed for 146 nm after a 300u2009mJ/cm2 irradiation. The initial 28 nm of epitaxy was nearly defect free, followed by an increasing density of twins and ultimately terminating in an amorphous quench. The microstructure is discussed as a lateral equivalent of laser-induced amorphization of bulk Si.

Formation of poly-Si1−xGex using excimer-laser processing

Abstract Polycrystalline silicon-germanium (poly-SiGe) films can be an attractive alternative to poly-Si in several technologies like TFTs where thermal budget allowances may be limited. In this work, we investigate the formation of poly-Si 1− x Ge x films by using a XeCl pulsed excimer laser to induce either alloying (PLIA) or crystallization (PLIC) processes. In the first case, the starting material consisted of an amorphous Ge layer electron-evaporated onto poly-Si grown on quartz substrates. Both the Ge mole fraction and the alloyed junction depth can be controlled by varying the laser energy fluence and/or the number of laser pulses. As an example, poly-Si 1− x Ge x layers with x = 0.3 can be easily obtained by this technique. In the second case, thin amorphous Si/Ge layers, prepared by successive evaporations of the two elements onto oxidized Si substrates, were submitted to the XeCl laser irradiation and the resulting films were investigated using the TEM and Raman spectroscopy techniques. They showed that the processed layers (120 nm thick) are polycrystalline with grains as large as 200 nm and an almost equal Si and Ge content.

Fabrication and doping of poly-SiGe using excimer-laser processing

Pulsed Laser-Induced Epitaxy (PLIE)/Gas Immersion Laser Doping (GILD) offers several advantages over alternative epitaxial processes, especially because the process can be made spatially selective. Here, a pulsed XeCl excimer laser is used to grow poly-Si1−xGex layers with Ge fractions up to 30% by intermixing a structure of electron beam-evaporated a-Ge on poly-Si deposited on quartz. Arsenic or boron dopant is incorporated during the melt process by using, respectively, an AsF5 or BF3 gas ambient. RBS and SIMS analysis reveal that the Ge metallurgical depth, the dopant junction depth and the incorporated dopant dose scale with the laser energy density and the number of laser pulses. The sheet resistance values reached after GILD process are low enough to be suitable for the fabrication of source and drain for poly-SiGe TFTs.

Fabrication and characterization of selectively grown Si1-xGex/Si p+/n heterojunctions using pulsed laser induced epitaxy and gas immersion laser doping

Abstract A pulsed XeCl excimer laser is used to grow ideally strained heteroepitaxial Si1-xGex/Si layers with Ge fractions up to 21% by intermixing a structure of electron beam evaporated a-Ge on Si(100). The rapid regrowth process induces an interfacial grading of the Ge fraction, which results in unusual stability of the layer strain upon heat treatment, as confirmed by MeV-ion channeling along and . Boron dopant is incorporated during the meltprocess by using a BF3 gas ambient. Hall/van der Pauw and SIMS analysis reveal that the incorporated dopant dose scales with the number of laser pulses. The junction depth is controlled by the incident laser fluence. The melt time is monitored in-situ utilizing the transient reflectance of the sample during the phase transformations. A patterned reflective aluminum mask is used to obtain spatially selective melting. In-plane Hall mobilities are found to be lower for the heteroepitaxial junctions than for Si homojunctions. We believe this is due to different transport behavior for holes in the observed doping regime of 1018–1020 cm-3. Quasiplanar p+/n heterojunction diodes are fabricated and exhibit near-ideal forward I-V characteristics. Heterojunction diodes exhibit lower turn-on voltages than equivalent Si homojunction diodes, indicative of a lowered bandgap. The turn-on voltages also depend on the B junction depth with respect to the Si1-xGex/Si interface. Both quantities are controlled independently by separating the epitaxy from the doping step.

Creating process margin in laser thermal processing: Application to formation of titanium silicide

Recently, we have demonstrated that, due to differential thermal budget, laser silicidation is an attractive alternative for deep submicron metal-oxide field effect transistors. In laser thermal processing, any spatial beam nonuniformities or pulse to pulse energy fluctuations lead to varying Si melt depth and hence variations in the silicide depth. In this letter, we report that amorphization is a possible solution for this problem. We demonstrate that stochiometric titanium disilicide can be fabricated using laser thermal processing. We also show that the depth of the silicide can be defined by amorphization and that process margin can be created in laser thermal processing.

Impurity distribution and electrical characteristics of boron-doped Si1?x Ge x /Si p +/N heterojunction diodes produced using pulsed UV-laser-induced epitaxy and Gas-immersion laser doping

A two-step pulsed UV-laser process which independently controls the metallurgical and electrical junction depth of a Si1−xGex/Si heterojunction diode has been implemented. Pulsed Laser-Induced Epitaxy (PLIE) combined with Gas-immersion Laser Doping (GILD) are used to fabricate boron-doped heteroepitaxial p+/N Si1−xGex/Si layers and diodes. Borontrifluoride is used as the gaseous dopant source in the GILD process step. Boron incorporation and activation are investigated as a function of laser energy fluence and the number of laser pulses using SIMS and Halleffect measurements. The dose of incorporated dopant is on the order of 1013 cm−2 per pulse. The B profiles obtained are flat except for a peak at the interface resulting from segregation effects. The B and Ge distributions are compared with shifts in the turn-on voltage of p+/N Si1−x/Si heterojunction diodes produced by the process. The GILD/PLIE process is spatially selective with the resulting diodes fabricated being quasiplanar. Hole mobilities in the heavily doped Si1−xGex films are found to be slightly lower than in comparable Si films.

Heteroepitaxial Si/Si1−xGex/Si structures produced using pulsed UV‐laser processing

High quality heteroepitaxial regrowth of arsenic‐implanted a‐Si on Si1−xGex layers with Ge fractions between 0 and 0.2 is accomplished using pulsed laser induced epitaxy (PLIE). The structures of boron‐doped Si1−xGex on Si(100) are created beforehand using a combination of (PLIE) and gas immersion laser doping. A small amount of Ge and B backdiffusion from the Si1−xGex film into the top Si layer is observed. During the laser pulse, the implanted arsenic diffuses up to the maximum melt depth so that melt depth and junction depth coincide. The a‐Si is sputter deposited to a thickness of 900 A, and the As is implanted to a dose of 5×1014 cm−2 at 40 keV. A single XeCl excimer laser pulse with an energy fluence to be selected between 0.6 and 0.8 J/cm2 is sufficient to heteroepitaxially regrow the a‐Si and activate the implanted arsenic.

Excimer Laser Induced Crystallization of Amorphous Silicon-Germanium Films

Application of excimer laser crystallization of Amorphous silicon (a-Si) has introduced a new, interesting potential technology for the fabrication of polycrystalline (poly-Si) thin film transistors. We are currently studying polycrystalline Si 1−x Ge x thin films in order to determine whether this material can lead to improved electrical properties or to better processing requirements when compared with polycrystalline Si films. In this work we analyze by RBS, TEM, Raman spectroscopy and surface reflectance, the structure of thin Amorphous Si 1−x Ge x films after irradiation with a XeCl excimer laser. The Amorphous SiGe films were prepared by evaporation of Si and Ge onto oxidized Si substrates using an electron gun in vaccum. The effects of laser energy fluence during irradiation are investigated. The Amorphous to crystalline transition is followed by in-situ measurement of time-resolved reflectivity.