Z. M. Ren

The effects of thermal annealing on ZnO thin films grown by pulsed laser deposition

ZnO thin films were grown on silicon (100) by pulsed laser deposition. Highly textured crystalline ZnO thin films can be grown at 600 °C. The films were then annealed at 600 °C in oxygen. The effects of annealing on chemical composition of the ZnO films were investigated by x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The XPS spectra indicate that water has been adsorbed and then dissociated into H and OH groups. The surface properties of ZnO were studied both by scanning tunneling microscopy and scanning tunneling spectroscopy (STS). A narrow potential well has been formed on the surface of the ZnO thin films due to high density of surface states and negatively biasing the ZnO thin films during STS measurement. The discrete energy levels can be measured by STS.

Studies of carbon nitride thin films synthesized by KrF excimer laser ablation of graphite in a nitrogen atmosphere

Carbon nitride thin films were deposited on silicon wafers by pulsed KrF excimer laser (wavelength 248 nm, duration 23 ns) ablation of graphite in a nitrogen atmosphere. Different excimer laser fluences and pressures of the nitrogen atmosphere were used in order to achieve a nitrogen content as high as possible in the deposited thin films. Fourier transform infrared and x-ray photoelectron spectroscopies were used to identify the binding structure and the content of the nitrogen species in the deposited thin films. The N/C ratio 0.42 was obtained at an excimer laser fluence of 0.8 J cm−2 at a repetition rate of 10 Hz under a nitrogen pressure of PN=100 mTorr. A high content of C=N double bond instead of C≡N triple band was indicated in the deposited thin films. Ellipsometry was used to analyze the optical properties of the deposited thin films. The carbon nitride thin films have amorphous-semiconductorlike characteristics with an optical band gap Eopt of 0.42 eV.

Ion-assisted pulsed laser deposition of aluminum nitride thin films

Aluminum nitride (AlN) thin films were deposited by nitrogen-ion-assisted pulsed laser ablation of an AlN target. A KrF excimer laser with a pulse duration of 23 ns and a wavelength of 248 nm was used as a light source for the ablation. A nitrogen ion beam with energies in a range of 200–800 eV is used to assist the deposition. The nitrogen ion implantation can compensate the possible loss of nitrogen species in the ablated plasma and can effectively assist the deposition by providing energetic nitrogen ions. Raman and Fourier transform infrared spectroscopy measurements were used to characterize the deposited thin films. The influences of the substrate temperature and the ion energy on the electronic and structural properties of the deposited thin films were studied.

Investigation of Li-doped ferroelectric and piezoelectric ZnO films by electric force microscopy and Raman spectroscopy

We have grown Li-doped ZnO films on silicon (100) using the rf planar magnetron sputtering method. The surface charges induced piezoelectrically by defect and by polarization can be observed by electric force microscopy. The Li-doped ZnO films have been proven to be ferroelectric. The Raman spectra of ZnO and Li-doped ZnO films have been measured.

Room temperature synthesis of c-AlN thin films by nitrogen-ion-assisted pulsed laser deposition

Cubic aluminum nitride (c-AlN) thin films have been deposited at room temperature on silicon substrates by nitrogen-ion-assisted pulsed laser ablation of a hexagonal AlN target. The deposited thin films exhibit good crystal properties with sharp x-ray diffraction peaks. The influences of the nitrogen ion energy on the morphological, compositional, and electronic properties of the AlN thin films have been studied. The nitrogen ions can effectively promote the formation of Al–N bonds and improve the crystal properties of the deposited thin films. A nitrogen ion energy of 400 eV is proposed to deposit high quality c-AlN thin films.

Electronic and optical properties of carbon nitride thin films synthesized by laser ablation under ion beam bombardment

Carbon nitride thin films were deposited by 532 nm Nd:YAG laser ablation of graphite assisted by a nitrogen ion beam bombardment on silicon substrates. Different nitrogen ion beam energies (200, 400, and 600 eV) were used while the laser parameters remained fixed. X-ray photoelectron spectroscopy (XPS) and ellipsometry measurements were carried out to analyze the electronic and optical properties. The XPS C 1s spectrum for the C–N binding is at 286.5 eV while the N 1s spectrum has a corresponding peak of C–N binding at 396.9 eV. The optical gap Eopt is on the order of magnitude of 10−1 eV and increases with the N/C ratio in the deposited film. Linear dependence of the refractive index n and the extinction coefficient k on photon energy E in the range of 1.5–3.5 eV are established.

Influence of ion-beam energy and substrate temperature on the synthesis of carbon nitride thin films by nitrogen-ion-assisted pulsed laser deposition

Carbon nitride thin films were deposited on silicon wafers by pulsed KrF excimer laser (wavelength 248 nm, duration 23 ns) ablation of graphite with assistance of nitrogen ion beam bombardment. X-ray photoelectron spectroscopy, Raman spectroscopy, and ellipsometry were used to identify the binding structure, nitrogen content, and optical properties of the deposited thin films. The influence of the nitrogen ion beam energy on the compositional, electronic, and optical properties of the deposited thin films was investigated. The thin films deposited with nitrogen ion bombardment had an N/C ratio of 0.43. Raman spectroscopy measurements indicated the formation of CN triple bonds in the deposited thin films. The optical band gap Eopt was observed to decrease with the nitrogen ion energy. A nitrogen ion energy between 50 and 100 eV was deduced to be the optimal condition for depositing the carbon nitride thin films.

Quasiparticle band structures of wurtzite and rock-salt ZnO

Band structures of both wurtzite and rock-salt ZnO were investigated using the ab initio pseudopotential method with both local density approximation (LDA) and GW approximation. The error in approximating 3d electrons as core electrons was investigated for both LDA and quasiparticle calculations. The differences between the band structures obtained by the GW approximation and LDA were explained. The quasiparticle band structures were compared with experimental results. The spin–orbit splitting was calculated for both wurtzite and rock-salt ZnO with the LDA. The density of states was investigated with the GW approximation.

Raman Spectroscopy Studies of the Influence of Substrate Temperature and Ion Beam Energy on CNx Thin Films Deposited by Nitrogen-Ion-Assisted Pulsed Laser Deposition

Carbon nitride thin films were deposited by nitrogen-ion-assisted pulsed laser ablation of graphite. A KrF excimer laser with pulse duration of 23 ns and wavelength of 248 nm was used as the laser source for the ablation. Raman spectroscopy measurements were used to characterise the deposited thin films. The influences of substrate temperature and nitrogen ion beam energy on the electronic properties of the deposited thin films were studied. The suitable parameters of substrate temperature and ion energy were suggested given our deposition conditions and setup in order to obtain large graphite-like crystallite structures or to realize a high content of amorphous CNx. X-ray photoelectron spectroscopy (XPS) was also adopted to assist the characterisation and evaluation of the deposited CNx thin films.

Carbon nitride thin film synthesized on iron buffer layers

Carbon nitride thin films were deposited on iron buffer layers by pulsed laser deposition assisted with ion implantation. Two types of samples (A) and (B) were prepared with and without iron layers. Several techniques were used to study the properties of the samples. Scanning tunneling microscopy (STM) was used to observe the surface structures of the samples. The difference in their surface morphologies was studied. The STM measurements also provided the relation between tunneling current and bias voltage to study the local density of states of the sample surface by calculating (dI/dV)/(I/V). Three band edges were observed from the calculated curve. Measurements by Raman and Fourier transform infrared (FTIR) spectra were carried out to study the electronic properties of the samples. The Raman spectra showed the presence of triply bonded carbon nitride bonds (C≡N) in sample (A), while only single bonds were observed in sample (B) by FTIR spectra. The mechanical properties were studied by nanoindentation. ...