Ivan Gueorguiev Ivanov

Vibrational properties and structure of undoped and Al-doped ZnO films deposited by RF magnetron sputtering

Highly conductive and transparent in the visible range Al-doped ZnO (ZnO:Al) and undoped ZnO films have been deposited by RF magnetron sputtering. Reflection high-energy electron diffraction observations characterized them as textured. The habitus of the microcrystallites forming the texture depends on the Al doping. The layer texture of undoped ZnO films has texture axis parallel to the substrate. The ZnO:Al films, instead, show a columnar texture with texture axis perpendicular to the substrate. The Raman spectra of the films obtained by non-resonant excitation are completely different from those of the target material which is polycrystalline ZnO. For the interpretation of the different bands in the Raman spectra the existence of a depletion region near the grain boundaries has been assumed. The most intensive band in the Raman spectra at approximately 570 cm−1 has been assigned to electric field-induced Raman scattering on longitudinal optical phonons. The built-in electric field in the depletion region induces the Raman activity of the B2 modes and a band at 276 cm−1 appears in the spectra. Phonon modes highly localized near the grain boundaries have been detected at 516 cm−1 and 468 cm−1 which are well pronounced in the Raman spectra for the doped samples. Localized modes were observed also in the infrared reflection spectra of the doped films. Surface enhanced Raman scattering has been applied and the band in the range 830–920 cm−1 has been interpreted as due to adsorbates from the ambient air. It has been shown that the non-resonant Raman scattering can be used for qualitative study of some details of the microstructure of the zinc oxide films like the built-in electric field and the adsorbates in the films.

REACTIVE MAGNETRON SPUTTER DEPOSITED CNX : EFFECTS OF N2 PRESSURE AND GROWTH TEMPERATURE ON FILM COMPOSITION, BONDING, AND MICROSTRUCTURE

The effects of growth processes on the chemical bond structure, microstructure, and mechanical properties of carbon–nitride (CNx) thin films, deposited by reactive magnetron sputtering in a pure N2 discharge, are reported. The film deposition rate RD increases with increasing N2 pressure PN2 while N/C ratios remain constant. The maximum N concentration was ∼35 at. %. RD was found to be dependent upon the film growth temperature Ts. For a given PN2, RD decreased slightly as Ts was increased from 100 to 600 °C. The variations in RD with both PN2 and Ts can be explained by ion‐induced desorption of cyano radicals CNx from both the target and growth surfaces during deposition. X‐ray photoelectron spectroscopy and Fourier transform infrared spectroscopy (FTIR) analyses showed that N atoms in films grown at Ts≳350 °C with low nitrogen partial pressures PN2, ∼2.5 mTorr, were bound to C atoms through hybridized sp2 and sp3 configurations. For low Ts=100 °C and higher PN2, 10 mTorr, triple‐bonded C≡N was detected ...

Reactive magnetron sputter deposition of CNX films on Si(001) substrates : film growth, microstructure and mechanical properties

There is currently considerable interest in producing new materials with extreme combinations of mechanical properties such as high hardnesses and moduli. One example of such a material is crystalline C3N4, which has been predicted to have a bulk modulus higher than that of diamond. In this paper we report on experiments carried out to synthesize CNx thin films. The films were grown in an unbalanced magnetron-sputtering system by reactive sputtering of C in N2 discharges. Si(001) substrates with the native oxide removed by thermal desorption and then kept at temperatures ranging from 150 to 600°C and substrate bias voltages Vs between 7.5 and -200 V were used. The films were analysed using X-ray diffraction, transmission electron microscopy (TEM). Auger electron spectroscopy, Rutherford backscattering and nano-indentation tests. Typically the films were grown at rates of 5 nm s−1 to total thicknesses of 300 nm. Owing to an extensive re-sputtering, only low negative bias voltages (−80<Vs<7.5V) could be used while still retaining reasonable growth rates. Films with a maximum N concentration of ∼28 at.% were obtained under the growth conditions used. Films analysed using TEM proved to be crystalline with a turbostratic structure in both high resolution imaging and electron diffraction. Nano-indentation tests showed load vs. deflection curves with an extremely large elastic recovery (compared with the Si substrate alone) of between 82% and 74% of the total indentation displacement for loads of 1, 5 and 10 mN. The “load-on” (i.e. with no allowance for recovery of the substantial elastic component of the displacement) hardness values calculated from the measurements were between 7 and 9 GPa.

Annealing effects on optical properties of low temperature grown ZnO nanorod arrays

Vertically well-aligned ZnO nanorods on Si substrates were prepared by a two-step chemical bath deposition method. The structure and optical properties of the grown ZnO nanorods were investigated b ...

Growth of SiC by "Hot-Wall" CVD and HTCVD

A reactor concept for the growth of high-quality epitaxial SiC films has been investigated. The reactor concept is based on a hot-wall type susceptor which, due to the unique design, is very power efficient. Four different susceptors are discussed in terms of quality and uniformity of the grown material. The films are grown using the silane–propane–hydrogen system on off-axis (0001) 6H- and 4H-SiC substrates. Layers with doping levels in the low 1014 cm—3 showing strong free exciton emission in the photoluminescence spectra may readily be grown reproducibly in this system. The quality of the grown layers is also confirmed by the room temperature minority carrier lifetimes in the microsecond range and the optically detected cyclotron resonance data which give mobilities in excess of 100000 cm2/Vs at 6 K. Finally, a brief description will be given of the HTCVD technique which shows promising results in terms of high quality material grown at high growth rates.

High temperature chemical vapor deposition of SiC

A growth process has been investigated for the epitaxial growth of silicon carbide. The technique can simply be described as chemical vapor deposition (CVD) at high temperatures, hence the name high temperature CVD (HTCVD). The growth process however, differs greatly from that of the CVD process due to the significant sublimation and etch rates at the extreme growth temperatures (1800–2300°C). The grown rates obtained with the HTCVD are in the order of several tens of μm/h to 0.5 mm/h. The purity and crystallinity of the growth layers are outstanding showing strong free exciton related photoluminescence.

Nitrogen doping concentration as determined by photoluminescence in 4H– and 6H–SiC

Low‐temperature photoluminescence (PL) spectroscopy is used for determination of the nitrogen doping concentration in noncompensated 4H– and 6H–SiC by comparing the intensity of nitrogen‐bound exciton (BE) lines to that of the free exciton (FE), the latter being used as an internal reference. The results are compared with a previous work performed for the case of 6H–SiC only. A line‐fitting procedure with the proper line shapes is used to determine the contribution of the BE and FE lines in the PL spectrum. The ratio of the BE zero‐phonon lines (R0 and S0 in 6H, Q0 in 4H) to the FE most intensive phonon replica around 77 meV exhibits very well a direct proportional dependence on the doping as determined by capacitance–voltage (C–V) measurements for both polytypes. The use of fitting procedure which takes into account the real line shapes, the influence of the spectrometer transfer function, and the structure of the PL spectrum in the vicinity of the FE replica allows us determination of the N‐doping conce...

Properties of molecular-beam epitaxy-grown GaNAs from optical spectroscopy

GaNxAs1−x layers with different nitrogen concentrations x grown on (001)GaAs substrates by molecular-beam epitaxy have been studied by photoluminescence, optical absorption, and Raman spectroscopy. The content of nitrogen in the layers was determined by x-ray diffraction and secondary-ion-mass spectrometry. The samples can be classified in three categories with respect to the concentration of N: with doping nitrogen concentration, with average content of N less than 0.3, and with x close to 1. From optical measurements and from analysis of x-ray diffraction spectra, different phases are observed in the GaNxAs1−x layers: GaAs, GaN, and the solid ternary solution GaNxAs1−x. In Raman spectra both GaAs-like and GaN-like optical phonons are observed. We have estimated the fundamental band-gap energy in the GaNxAs1−x alloy with low nitrogen concentration up to x=0.04 from absorption measurements, and in GaNxAs1−x with high nitrogen concentration x>0.96 from photoluminescence spectra. Fitting of the experimental...

Liquid phase epitaxial growth of SiC

The characteristics of 4H and 6H-SiC epitaxial growth from the liquid phase by using a sandwich configuration are presented. The preparation procedure of the two-component solvent and the growth technique are described. Growth rates exceeding 300 lm/h have been obtained. The transport of solute is a⁄ected by formation of complexes within the liquid zone which decrease the growth rate. The growth rate depends mainly on the temperature gradient but is also influenced by the solvent composition. Important growth parameters such as temperature gradient and substrate o⁄-orientation have a profound influence on the morphological stability. It is shown that if these are not properly chosen, constitutional supercooling may appear. The polytype of the substrate is reproduced in the grown material and the structural quality is good. Common defects are discussed. ( 1999 Elsevier Science B.V. All rights reserved.

GROWTH OF EPITAXIAL ALN(0001) ON SI(111) BY REACTIVE MAGNETRON SPUTTER-DEPOSITION

2H–AlN(0001) layers have been grown on Si(111) by reactive magnetron sputtering from an Al target in Ar+N2 gas mixtures at temperatures Ts=400–900 °C. Variations in reactive gas consumption, target voltage, and current–voltage characteristics versus nitrogen partial pressure were used to determine deposition parameters required to yield stoichiometric AlN with growth rates ≥2 μm h−1. High‐resolution cross‐sectional transmission electron microscopy (XTEM) analyses of films grown at 900 °C showed that the initial 6–8 monolayers were (111)‐oriented cubic 3C before transforming to the (0001)‐oriented 2H polytype. The epitaxial relationship was found by XTEM and x‐ray diffraction (XRD) to be 2H–AlN(0001)//3C–AlN(111)//Si(111) with 2H–AlN[1210]//3C–AlN[110]//Si[110]. High‐resolution XRD ω−2Θ and ω rocking curve widths for films grown at Ts=900 °C were 70 and 500 arc sec, respectively, the lowest values yet reported.2H–AlN(0001) layers have been grown on Si(111) by reactive magnetron sputtering from an Al target in Ar+N2 gas mixtures at temperatures Ts=400–900 °C. Variations in reactive gas consumption, target voltage, and current–voltage characteristics versus nitrogen partial pressure were used to determine deposition parameters required to yield stoichiometric AlN with growth rates ≥2 μm h−1. High‐resolution cross‐sectional transmission electron microscopy (XTEM) analyses of films grown at 900 °C showed that the initial 6–8 monolayers were (111)‐oriented cubic 3C before transforming to the (0001)‐oriented 2H polytype. The epitaxial relationship was found by XTEM and x‐ray diffraction (XRD) to be 2H–AlN(0001)//3C–AlN(111)//Si(111) with 2H–AlN[1210]//3C–AlN[110]//Si[110]. High‐resolution XRD ω−2Θ and ω rocking curve widths for films grown at Ts=900 °C were 70 and 500 arc sec, respectively, the lowest values yet reported.