Laser Assisted Molecular Beam Deposition Research Articles

Analysis and Applications of ZnO Semiconductor Films Deposited by Laser and Sputtering Techniques

ABSTRACTIn this paper, ZnO thin films deposited by two methods have been studied. Specifically, the films were grown using i) Laser Assisted Molecular Beam Deposition (LAMBD) and ii) RF sputtering. Subsequent to film deposition, a subset of samples deposited using LAMBD were laser annealed. An additional set of samples (from LAMBD and RF sputtering) were annealed with N2 or forming gas at 600°C for 30mins.After deposition, optical and electrical properties of ZnO thin films have been studied. The application of ZnO to optical devices, including Metal-Semiconductor-Metal Photodetectors (MSM-PD) and solar cells, has been made. Several deposition experiments recently demonstrated that the thin films of RF-ZnO and LAMBD-ZnO have near ZnO parameters including refractive index close to 2, 1:1 stoichiometry ZnO, and 3.3 eV ZnO bandgap. Mixtures of single crystal and polycrystal grains were observed by Transmission Electron Microscopy (TEM) from LAMBD ZnO thin films. MSM current-voltage data show symmetrical photo current behavior. High ratio of photocurrent to dark current, good responsivity and fast pulse response of LAMBD-ZnO MSM were observed. ZnO/Si heterojunction solar cell result has been demonstrated and improvement in the ultraviolet light spectrum of spectral response has been shown in this paper.

Laser Assisted Molecular Beam Deposition of High Mobility Zinc Oxide

ABSTRACTWe have utilized laser assisted molecular beam deposition (LAMBD) to deposite zinc oxide (ZnO) at room temperature. The production of ZnO films via LAMBD utilizes an excimer laser induced ablation plume of zinc in combination with pulsed oxygen gases to create a molecular beam of ZnO clusters. The deposited films have been characterized structually and electrically in an as deposited state as well as after post deposition excimer laser annealing. The films undergo a clear structural change from a nanoparticle like film to either a microgranular film or a smooth continuous film depending upon the laser annealing power.Al ohmic contacts were made to both as-deposited and laser-annealed ZnO thin films. Thickness and refractive index, chemical composition, and surface morphology of the thin films were analyzed by using ellipsometry, electron spectroscopy for chemical analysic (ESCA), and feild emission scanning electron microscopy (FESEM), respectively. Dark current-voltage (I-V), DC photo I-V, Hall-effect, and photoluminescence measurements were employed for testing the device performance.In some cases, the ZnO thin film was changed from n-type into p-type after laser annealing, and photoconductive behavior was clearly seen on the laser-annealed samples, with values of 2.8 × 10−4 Ω−1. Also, both samples with and without laser annealing show near-band emission at ∼3.3 eV. Furthermore, the full width at half maximum (FWHW) values of the laser-annealed LAMB ZnO film was reduced from 20.02 nm or 21.68 nm to 18.2 nm when compared with that of non-laser annealed samples. This indicates that the laser annealing provided an improved stochiometric quality and crystallinity to the ZnO thin films.

Laser Assisted Molecular Beam Deposition of Thin Films for Gate Dielectrics Applications

AbstractHigh dielectric constant (k), the thermal stability and the chemical stability with respect to reaction with silicon of hafnium oxide (HfO2), and zirconium oxide (ZrO2) places them among the leading candidates for an alternative gate dielectric material. High dielectric constant HfO2 and ZrO2 thin films have successfully been deposited on silicon substrates at a temperature of 27 °C by Laser Assisted Molecular Beam Deposition (LAMBD). The LAMBD process is related to conventional Pulsed Laser Deposition (PLD). In the PLD process, the ablation plume impinges directly upon the substrate to deposit the thin film, whereas in the LAMBD process, the ablation material is expanded within a concurrently pulsed stream of a reactive gas. The gas pulse serves both to create the desired material and to transport the material to the substrate for deposition of the thin film. One advantage of the LAMBD process is that a chemically reactive carrier gas can be selected to produce the desired chemical products. Depositions yielded 35 nm to 135 nm thick HfO2, and ZrO2 films.Structural and chemical characterization of the films were performed by Auger electron spectroscopy (AES), Rutherford back-scattering (RBS), scanning electron microscopy (SEM), and x-ray diffraction (XRD). Film surface was investigated by atomic force microscopy (AFM) while optical characterization was also performed by means of spectroscopic ellipsometry (SE). Within the process window investigated, the film Hf/O and Zr/O ratios was found to be in the range 0.6 to 1.2. The as deposited films were amorphous with refraction index (RI) at 623 nm wavelength films in the range of 1.22 to 1.27 for the HfO2 and in the range of 1.23 to 1.19 for the ZrO2 films.

Up-converted emission observed from a hybrid inorganic:organic composite thin film

Hybrid inorganic:organic composites of TiO2:APSS were fabricated by mixing the constituents in the gas phase using the technique of laser-assisted molecular beam deposition. Ultraviolet-visible and infrared spectra of the composite thin films indicate that the organic monomeric chromophore is physically embedded within the titanium dioxide matrix without any thermal degradation. Two-photon pumped up-converted fluorescence emission is observed in the films, such that irradiation of the film at 800 nm results in a highly visible green emission.

Progress in thin film formation by laser assisted molecular beam deposition (LAMBD)

Laser assisted molecular beam deposition (LAMBD) is a pulsed laser ablation technique which utilizes a train of gas pulses to precisely control the chemistry and transport of species to be deposited on a substrate. The LAMBD technique has been used to grow films on a variety of substrates and to ablate a variety of target materials including: metals, metal oxides, metal halides, Si, C and SiC. In addition, a variety of more complex composite films such as Cu embedded polymers, YBa 2Cu 3O 7− x and organic doped TiO 2 and Al 2O 3 have also been produced. These films have been analyzed by Raman scattering, SEM, EDX, FTIR, ESCA, NMR, mass spectrometry and surface profile measurements to determine film structure and composition. Recent progress on source characterization by emission spectroscopy and a new source design will also be presented. These results demonstrate the potential of using LAMBD sources as a means of depositing a variety of high quality single and multiple component films.

On the Nature of Thin Films Generated during the Laser-Assisted Molecular Beam Deposition of Metal Plasma and Organic Vapors

Micrometer thick polymeric films containing embedded metallic nanoparticles have been fabricated via the technique of laser-assisted molecular beam deposition. A gaseous mixture of acetylene/acetone is expanded supersonically into the plasma plume of laser-ablated metallic (copper, titanium) targets and then expanded toward a substrate. The formation of these polymeric films is found to be not simply due to thermal effects of the plasma but also due to catalytic activity of the metal clusters. In the case of copper metal it is found to be physically embedded as metallic nanoparticles. In the case of the titanium, the metal is found to be bonded with oxygen.

Spectroscopic Characterization of Inorganic Host:Organic Dopant Films Fabricated via Laser-Assisted Molecular Beam Deposition

Thin films of hybrid inorganic:organic composites [TiO2:copper phthalocyanine, Al2O3:6-propionyl-2-(dimethylamino)naphthalene, and Al2O3:rhodamine 6G] were fabricated by mixing the constituents in the gas phase via the technique of laser-assisted molecular beam deposition. These films were characterized by UV−visible, infrared, and steady-state fluorescence spectroscopic techniques. UV−visible and infrared spectra of TiO2:copper phthalocyanine films indicate that the organometallic compound, copper phthalocyanine, is trapped within the host TiO2 matrix without degradation. The local microenvironment of the organic components in these composite films were studied by steady-state fluorescence spectroscopy. Emission spectra indicate that the organic dye molecule, rhodamine 6G, is trapped within the Al2O3 matrix as the monomer without aggregation. The spectral red shift of the emission peak and the full width at the half maximum suggest the environmentally sensitive probe, 6-propionyl-2-(dimethylamino)naphthalene, molecules are embedded within the Al2O3 matrix as opposed to being adsorbed on the Al2O3 surface.

The nature of copper in thin films of copper iodide grown by laser-assisted molecular beam deposition: comparative ESCA and EDXS studies

Thin molecular films of copper iodide have been grown by direct chemical reaction within a molecular beam using laser-assisted molecular beam deposition. Molecular iodine vapour entrained in a helium carrier gas stream was supersonically expanded into the laser-ablated plasma plume of vaporized copper, producing copper iodide. Films of this material were grown on substrates situated in the path of this molecular beam. The oxidation state of copper and the chemical composition of these films were studied by electron spectroscopy for chemical analysis and energy-dispersive X-ray spectroscopy, and, the results were compared with those of a high-purity Cul powder. The binding energy of the Cu 2p3/2 photoelectron line and the kinetic energy of the X-ray-induced Cu L3M4.5M4.5 Auger line are determined to be the same for both samples. The surface morphology and extent of surface inhomogeneities in copper iodide films were found to be largely dependent on the fluence of the incident laser and the expansion conditions. Secondary electron images of these films indicate clustering of copper iodide on the surface of the films during film growth. The copper in these films is found to be in the Cu+ ionization state.

Characterization of copper iodide thin films fabricated via laser-assisted molecular-beam deposition

The technique of laser-assisted molecular-beam deposition (LAMBD) has been used to fabricate a molecular film by reaction in a molecular beam. Molecular iodine vapor entrained into a stream of helium carrier gas was introduced via a supersonic expansion into the plasma plume of laser-evaporated copper to produce copper iodide. Films were deposited on substrates that were situated about 3 cm downstream on the path of the molecular beam. The surface morphology of films depends greatly on the fluence of the incident laser beam and expansion conditions. Films grown at low laser powers show small surface inhomogeneities in their electron micrographs compared to the films that were grown at higher laser powers. Copper in the LAMBD films is found to be mainly in the Cu+1 state as characterized by electron spectroscopy for chemical analysis. A comparison of the surface and structural properties of LAMBD films with that of a vacuum-evaporated film suggests that both films have similar lattice structures and compositions. It seems that LAMBD can be a powerful technique to deposit novel molecular and composite films for electronics and photonics.