Pio Capezzuto

Role of sapphire nitridation temperature on GaN growth by plasma assisted molecular beam epitaxy: Part I. Impact of the nitridation chemistry on material characteristics

The impact of the nitridation temperature on sapphire/GaN interface modifications and the structural, chemical, and optical properties of GaN epitaxial thin films with N plasma radicals is investigated. Based on ex situ spectroscopic ellipsometry and x-ray photoelectron spectroscopy analysis, it is found that the sapphire nitridation chemistry, specifically AlN versus oxynitride (NO) production, depends on the surface temperature. Nitridation at 200 °C produces a very thin AlN layer with 90% coverage, while high temperature nitridation leads to a 70% coverage of AlN layer containing NO. These initial stages of growth significantly impact the characteristics of the layers following the nitridation step, specifically the low temperature buffer, annealed buffer, and the GaN epitaxial layer. The annealed buffer on a 200 °C nitridation provides a homogeneous GaN thin layer covering most of the sapphire surface. This homogeneous GaN layer after annealing produces a superior template for subsequent growth, resul...

Plasma cleaning and nitridation of sapphire (α-Al2O3) surfaces: New evidence from in situ real time ellipsometry

The interaction of α-Al2O3 (0001) surfaces with H atoms and N atoms from remote rf plasmas used for the cleaning and nitridation processes, respectively, is investigated at temperatures in the range of 200–600 °C. The chemistry and kinetics of the above processes are monitored in real time by in situ spectroscopic ellipsometry. Also, the chemistry of the nitrided sapphire surfaces is verified by x-ray photoelectron spectroscopy (XPS) analysis. Specifically, H atoms treatments are effective in removing carbon contaminants from the sapphire surface at temperatures of 200–400 °C. Real time ellipsometry is suitable to detect the cleaning end point and to verify the onset of the H-atom diffusion into the sapphire substrate. Remote N2 plasma nitridation at 200 °C is found to yield homogeneous and smooth AlN layers of about 5 A, after approximately 25 min of nitridation, whereas high nitridation temperatures result in a damaged sapphire surface with AlN protrusions. Both ellipsometric and XPS data show that the ...

Dielectric function of nanocrystalline silicon with few nanometers (<3 nm) grain size

The dielectric function of nanocrystalline silicon (nc-Si) with crystallite size in the range of 1 to 3 nm has been determined by spectroscopic ellipsometry in the range of 1.5 to 5.5 eV. A Tauc–Lorentz parameterization is used to model the nc-Si optical properties. The nc-Si dielectric function can be used to analyze nondestructively nc-Si thin films where nanocrystallites cannot be detected by x-ray diffraction and Raman spectroscopy.

Rf glow discharge of SiF4-H2 mixtures : diagnostics and modeling of the a-Si plasma deposition process

Hydrogenated and fluorinated amorphous silicon films (a‐Si:H,F) have been deposited in a parallel‐plate rf plasma reactor fed with SiF4‐H2 mixtures. The plasma phase characterization has been performed by optical emission spectroscopy for the analysis of the emitting species (SiF*3,SiF*2,SiF*,H*, and H*2), mass spectrometry for the analysis of stable species, and Langmuir electrical probes for the evaluation of electron density (ne) and temperature (kTe). The deposition rate (rD) has been monitored by laser interferometer. The effect of the rf power, gas composition, total pressure, and dopant (B2H6, PH3) addition on the plasma phase composition and on the film growth rate has been studied. The data have been discussed on the basis of a chemical model where the chemisorption of SiF4 and SiF2 and the subsequent interaction with H atoms are the determinant steps also from the kinetic point of view. It has been found that the rate equation rD ∝ [H][SiF2] is able to fit the experimental results.

R.f. plasma deposition of amorphous silicon films from SiCl4-H2

Abstract Preliminary results on amorphous silicon films obtained by reduction of SiCl4 under hydrogen r.f. discharges at pressures between 1 and 5 Torr are presented. X- ray diffraction, IR and electron spin resonance spectroscopic techniques, optical absorption, electron spectroscopy for chemical analysis and room temperature electrical measurements were used for chemical and physical characterization of the deposited films. Chemical characterization of the films showed the presence of chlorine and hydrogen as SiH2, SiH2Cl and/or SiHCl2. Electron spin resonance measurements of a sample deposited at 430 K yielded a dangling bond density of 1016cm-3.

Role of sapphire nitridation temperature on GaN growth by plasma assisted molecular beam epitaxy: Part II. Interplay between chemistry and structure of layers

The effect of sapphire nitridation temperature on the chemistry and microstructure of the sapphire substrate/GaN interface, nucleation layer, and of the GaN epilayers grown by rf plasma assisted molecular beam epitaxy is investigated. It is found that a sapphire nitridation temperature as low as 200 °C improves the structural and optical quality of GaN epilayers. This result can be explained by the chemistry of the sapphire nitridation process, which is discussed in the framework of a model considering the competitive formation of AlN and oxynitride (NO). In particular, at 200 °C, NO desorbs from the sapphire surface, yielding an homogeneous 6 A AlN layer upon N2 plasma nitridation. This low temperature AlN template favors the nucleation of hexagonal GaN nuclei which coalesce completely resulting in a hexagonal GaN buffer layer that homogeneously covers the sapphire substrate. This condition promotes the growth of a high quality GaN epilayer. In contrast, high nitridation temperatures result in a mixed Al...

N2–H2 remote plasma nitridation for GaAs surface passivation

A remote N2–H2 (a mixture of 97% N2–3% H2) rf plasma nitridation procedure has been developed to form a very thin (∼5A) GaN layer successful in the electronic and chemical passivation of GaAs (100) surfaces. The interaction of the plasma with the GaAs surface has been controlled in situ and in real time by spectroscopic ellipsometry. The stability of the chemical and electronic passivation is demonstrated by the nonoxidation and by the nondecaying behavior of the photoluminescence efficiency of the GaAs passivated surface over months of air exposure.

Interrelation between nanostructure and optical properties of oxide thin films by spectroscopic ellipsometry

Abstract Nanostructured thin oxide films of SnO 2 , V 2 O 5 and indium tin oxide (ITO) were deposited by conventional and plasma-assisted chemical vapor deposition (CVD, PECVD) on different substrates and at different temperatures. Optical properties of the films were determined by spectroscopic ellipsometry in the energy range 1.5–5.5 eV. A parameterized analysis, based on Lorentzian oscillators combined with the Drude model, along with Bruggeman effective-medium approximation (BEMA) modeling, was used to determine the optical constants of thin films independently from the reference dielectric functions of the bulk materials. In this way, correlation between the optical properties and nanostructure of thin films could be established. In particular, in order to discuss the dependence of optical constants on grain size, SnO 2 nanostructured films have been considered, using a parameterization based on a double Lorentzian oscillator. Nanocrystalline V 2 O 5 thin films have demonstrated the correlation between optical constants (described by four Lorentzian oscillators) and crystalline/amorphous volume fractions. Finally, ITO thin film optical properties are described by a combination of a double Lorentzian oscillator with the Drude model; by this analysis, gradients in the structural and optical properties of ITO are demonstrated.

Anatomy of μc-Si thin films by plasma enhanced chemical vapor deposition: An investigation by spectroscopic ellipsometry

A detailed analysis of the anatomy of microcrystalline (μc-Si) films deposited by plasma enhanced chemical vapor deposition from both SiF4–H2 and SiH4–H2 mixtures is performed by spectroscopic ellipsometry (SE). Specifically, the μc-Si film anatomy consists of an interface layer at the substrate/μc-Si bulk layer, a bulk μc-Si layer, and a surface porous layer. All these layers have their own microstructures, which need to be highlighted, since it is this overall anatomy which determines the optical properties of μc-Si films. The ability of SE to discriminate the complex microstructure of μc-Si thin films is emphasized also by the comparison with the x-ray diffraction data which cannot provide unambiguous information regarding the distribution of the crystalline and the amorphous phases along the μc-Si film thickness. Through the description of the μc-Si film anatomy, information on the effect of the growth precursors (SiF4 or SiH4) and of the substrate (c-Si or Corning glass) on the growth dynamics can be...

Fundamental reactions controlling anion exchange during mixed anion heterojunction formation: Chemistry of As-for-Sb and Sb-for-As exchange reactions

As-for-Sb and Sb-for-As anion exchange reactions have been investigated by the exposure of GaSb surfaces to As2 and As4 species and by the exposure of GaAs to Sb2, respectively. The effect of surface temperature, anion soak time, and anion species (either As2 or As4) on the chemistry governing the anion exchange reactions during GaAsySb1−y∕GaSb and GaSbyAs1−y∕GaAs heterostructure formation by molecular beam epitaxy is examined. It is found that when GaSb surfaces are exposed to arsenic, the anion exchange reaction competes with the formation of isoelectronic compounds, AsSby, which form clusters precipitating in the GaAsySb1−y∕GaSb heterostructures. The relative amount of GaAs and AsSby depends on the surface temperature, the As soak time, and on the As species, i.e., As2 or As4. We observe specific process conditions that minimize AsSby formation, yielding more abrupt heterojunction interfaces. In the case of the Sb2∕GaAs system, the Sb-for-As anion exchange does not occur to a significant degree, but su...