An innovative new approach has been developed for modeling the expansion of laser-generated plumes into low-pressure gases where initially the mean free path may be long enough for interpenetration of the plume and background. The model is based on a combination of multiple elastic scattering and hydrodynamic formulations. Although relatively simple in structure, it gives excellent fits to new experimental data for Si in He and Ar, and provides for the first time a detailed, coherent explanation of the observed splitting of the plume into a fast and slow component. [S0031-9007(97)03820-9] PACS numbers: 79.20.Ds, 52.50.JmPulsed laser deposition (PLD) has become an important technique for depositing a variety of materials [1,2], most notably thin films [3] and superlattices [4] of high-T c superconductors. As a consequence, a very active field of research into laser ablation phenomena underlying PLD has developed [5]. But this is not a new field since it dates back to the earliest days of the laser era when many materials were irradiated with high-powered laser pulses [6][7][8]. Thus, the work reported here has applications far beyond the PLD process itself. More recent work has provided a wealth of new diagnostics with which to study the laser ablation process [9]. It is of crucial importance to know the constitution and dynamical behavior of the plume of ablated material in order to understand how film growth can be optimized by varying the laser parameters, the targetsubstrate distance, and ambient gases introduced into the deposition chamber. In particular, the often observed but little understood phenomenon of "plume splitting" [10] into fast (ϳ vacuum speed) and background-slowed components is of great interest because the fast component may damage the growing film or otherwise affect its microstructure. Also, clustering of film constituents in the gas phase or on the surface may cause problems, but may also provide a technologically important method of producing nanostructures [5,11,12].In this paper, a new modeling approach, combining multiple scattering and hydrodynamical elements, is described and applied to recently obtained experimental data on Si ablated into He and Ar gases. The resulting model is remarkably successful in describing quantitatively the data and resolves long-standing uncertainties about the interpretation of many previous experimental observations.Silicon was selected for the target in the experiments because it is well characterized, is readily obtained as single crystals, and has been thoroughly studied in the laser-annealing regime [13]. Background gases of He and Ar were chosen because their ionization energies are high (25 and 16 eV, respectively), hence avoiding ionization, and because one is lighter and one heavier than Si. KrF laser pulses of 3.0 J͞cm 2 provided a good supply of singly ionized Si for the ion-probe detector while avoiding higher ionization states. Measurements revealed only neutral and singly ionized Si in the plume and only neutral atoms in the background. A de...

No abstract

A comparative study of ArF-and KrF-laser generated carbon plasmas has been performed under PLD conditions of amorphous diamond-like carbon (DLC) films. Gated-ICCD speciesresolved imaging, luminescence spectroscopy and ion probe diagnostics have revealed distinct differences between the carbon plumes generated by ArF-and KrF-lasers. KrF-laser (6.7 Jlcm2) irradiation produces a less energetic carbon plasma containing larger amounts of luminescent C2 compared with ArF-laser ablation at the same energy fluence. Spectroscopic ellipsometry and EELS analysis of the DLC films deposited on Si and NaCl substrates were utilized to characterize the high quality ArF-and KrF-laser deposited films (up to 84% of sp3 bonded carbon for 7 Jkm2 -ArF-laser DLC film). The more energetic and highly-atomized ArF-laser carbon plasma appears to be responsible for the better diamond-like properties.

Sequential ion implantation followed by thermal annealing has been used to form encapsulated CdS, ZnS, and PbS nanocrystals in SiO2 and Al2O3 matrices. In SiO2, nanoparticles are nearly spherical and randomly oriented, and ZnS and PbS nanocrystals exhibit bimodal size distributions. In Al2O3, nanoparticles are facetted and oriented with respect to the matrix. Initial photoluminescence (PL) results are presented.

is DISCLAIMERPortions of this document may be illegible in electronic image products. Images are produced from the best available original document. GROWTH OF HIGHLY DOPED i-TYPE ZnTe FILMS BY PULSED LASER ABLATION IN MOLECULAR NITROGEN ABSTRACTHighly p-doped ZnTe films have been grown on semi-insulating GaAs (001) substrates by pulsed-laser ablation (PLA) of a stoichiometric ZnTe target in a high-purity N2 ambient without the use o any assisting (DC or AC) plasma source. Free hole concentrations in the mid-1019 cm-3 to >IO& cm-3 range were obtained for a range of nitrogen pressures The maximum hole concentration equals the highest hole doping reported to date for any wide band gap II-VI compound. The highest hole mobilities were attained for nitrogen pressures of 50-100 mTorr (-6.5-13 Pa). Unlike recent experiments in which atomic nitrogen beams, extracted from RF and DC plasma sources, were used to produce p-type doping during molecular beam epitaxy deposition, spectroscopic measurements carried out during PLA of ZnTe in N2 do not reveal the presence of atomic nitrogen. This suggests that the high hole concentrations in laser ablated ZnTe are produced by a new and different mechanism, possibly energetic beam-induced reactions with excited molecular nitrogen adsorbed on the growing film surface, or transient formation of Zn-N complexes in the energetic ablation plume. This appears to be thefirst time that any wide bund gap (Eg > 2 eV) II-VI compound (or other) semiconductor has been impurity-dopedfrorn the gas phase by laser ablation. In combination with the recent discovery that epitaxial ZnSel-xSx films and heterostructures with continuously variable composition can be grown by ablation from a single target of fixed composition, these results appear to open the way to explore PLA growth and doping of compound semiconductors as a possible alternative to molecular beam epitaxy.