Evolution of femtosecond laser-induced damage in doped GaN thin films

Mg- and Si-doped GaN layers deposited by metalorganic chemical vapor deposition method were irradiated with femtosecond pulse duration laser of three different wavelengths 1,030, 515 and 343 nm. Both single and multiple shot laser induced damage thresholds of doped GaN layers were evaluated and discussed. The scanning electron microscopy employed with electron beam induced current and energy dispersive X-ray techniques were used to study laser damage morphology. It was observed that ablated area and laser-induced damage increased with irradiation fluence. The mechanism of damage generation by Gaussian beam profile laser was considered.     

Growth mechanism and characterisation of chemically grown Sb doped Bi2Se3 thin films

The synthesis of combinatorial Bi_2−xSb_xSe₃ thin films by arrested precipitation technique (APT) using triethanolamine-bismuth, triethanolamine-antimony and sodium selenosulphite as sources of Bi³⁺, Sb³⁺ and Se²⁻, respectively is investigated on commercial glass substrates. The growth mechanism of film formation, composition and surface morphology of the as deposited films were studied as a function of preparative parameters and bath composition. The films were monophasic, polycrystalline and covered the surface of the substrate completely. Energy dispersive X-ray analysis gave coherent elemental composition indicating single phase BiSbSe₃ was made. The good results obtained for Bi_2−xSb_xSe₃ thin films revealed that arrested precipitation technique is best suited for the deposition of large area thin films on conducting/nonconducting substrates to produce materials for device applications.     

掺Er/Er+O的GaN薄膜光学性质的研究

利用Raman散射谱研究了GaN注Er以及Er+O共注样品的振动模，并讨论了共注入O对Er离子发光的影响. 在Raman散射谱中，对于注Er的GaN样品出现了300 cm^-1和670 cm^-1两个新的Raman峰，而对于Er+O共注样品，除了上述两个峰外，在360 cm^-1处出现了另外一个新的峰，其中300 cm^-1峰可以用disorder-activated Raman scattering (DARS)来解释，670 cm^-1峰是由于与N空位相关的缺陷引起的，而360 cm^-1峰是由O注入引起的缺陷络合物产生的. 由于360 cm^-1模的缺陷出现，从而导致Er+O共注入GaN薄膜红外光致发光(PL)强度的下降. 关键词： GaN Er Raman散射 光致发光     

Growth and characterisation of GaN with reduced dislocation density

Two GaN MOVPE growth methods to reduce the threading dislocation (TD) density have been explored. The combined effects of (1) in situ SiN_x masking of the sapphire substrate and (2) starting the epitaxial growth at low V-to-III ratio on the GaN film quality were studied by atomic force microscopy, transmission electron microscopy and high-resolution X-ray diffraction. It was found that the annealing condition of the low-temperature nucleation layer after in situ SiN_x masking is critical in order to decrease the density of nucleation sites and hence increase the average grain size to about 5 μm. However, the coalescence of large grains with vertical side facets results in the formation of dense bundles of TDs at the grain boundaries combined with large numbers of basal-plane dislocation loops throughout the film. The formation of these dislocations can be prevented by starting the epilayer growth at low V-to-III ratio, resulting in the formation of grains with inclined side facets. The interaction of the TDs with the inclined side facets causes the dislocations to bend 90 as the grains grow in size and coalesce. GaN films with dislocation densities as low as 1×10⁸ cm⁻², giving full-width at half-maximum values of 180 and 220 arcsec for respectively (002) and (302) omega scans, were achieved by the combination of in situ masking and low V–III ratio epilayer growth. Hall carrier mobility values in excess of 900 cm² V ⁻¹ s⁻¹ were deduced for Si-doped layers.     

Surface modification of doped ZnO thin films

Effects of photo-assisted electrodeless and ion RF-sputter etching on the structural and optical properties of sputtered ZnO:Al thin films were investigated. Photo-assisted electrodeless etching was appropriate for getting “smooth” surfaces and ion RF-sputter etching by high power has significantly modified the surface roughness with an increase of the light diffuse transmittance.     

Structural and optoelectronic properties of amorphous GaN thin films

Amorphous gallium nitride (a-GaN) thin films were deposited on glass substrate by electron beam evaporation technique at room temperature and high vacuum using N ₂ as carrier gas. The structural properties of the films was studied by X-ray diffraction (XRD) and scanning electron microscope (SEM). It was clear from XRD spectra and SEM study that the GaN thin films were amorphous. The absorbance, transmittance and reflectance spectra of these films were measured in the wavelength range of 300–2200 nm. The absorption coefficient spectral analysis in the sharp absorption region revealed a direct band gap of E _g = 3:1 eV. The data analysis allowed the determination of the dispersive optical parameters by calculating the refractive index. The oscillator energy E ₀ and the dispersion energy E _d, which is a measure of the average strength of inter-band optical transition or the oscillator strength, were determined. Electrical conductivity of a-GaN was measured in a different range of temperatures. Then, activation energy of a-GaN thin films was calculated which equalled E _a = 0:434 eV.     

Synthesis and characterisation of co-evaporated tin sulphide thin films

Tin sulphide films were grown at different substrate temperatures by a thermal co-evaporation technique. The crystallinity of the films was evaluated from X-ray diffraction studies. Single-phase SnS films showed a strong (040) orientation with an orthorhombic crystal structure and a grain size of 0.12 μm. The films showed an electrical resistivity of 6.1 Ω cm with an activation energy of 0.26 eV. These films exhibited an optical band gap of 1.37 eV and had a high optical absorption coefficient (>10⁴ cm^-1) above the band-gap energy. The results obtained were analysed to evaluate the potentiality of the co-evaporated SnS films as an absorber layer in solar photovoltaic devices. PACS 78.40.Fy; 68.60.-p; 61.10.Nz; 68.55.-a; 78.66.-w     

Berkovich nanoindentation and deformation mechanisms in GaN thin films

The deformation mechanisms of GaN thin films obtained by metal-organic chemical vapor deposition (MOCVD) method were studied using nanoindentation with a Berkovich diamond indenter, micro-Raman spectroscopy and the cross-sectional transmission electron microscopy (XTEM) techniques. Due to the sharpness of the tip of Berkovich indenter, the nanoindentation-induced deformation behaviors can be investigated at relatively lower load and, hence, may cover wider range of deformation-related phenomena over the same loading range. The load-displacement curves show the multiple “pop-ins” during nanoindentation loading. No evidence of nanoindentation-induced phase transformation and cracking patterns were found up to the maximum load of 300 mN, as revealed from the micro-Raman spectra and the scanning electron microscopy (SEM) observations within the mechanically deformed regions. In addition, XTEM observation performed near the cross-section of the indented area revealed that the primary deformation mechanism in GaN thin film is via propagation of dislocations on both basal and pyramidal planes. The continuous stiffness measurement (CSM) technique was used to determine the hardness and Young's modulus of GaN thin films. In addition, analysis of the load-displacement data reveals that the values of hardness and Young's modulus of GaN thin films are 19 ± 1 and 286 ± 25 GPa, respectively.     

Magnetoelectrical properties of W doped ZnO thin films

ZnO thin films were deposited on the Si(100) substrate by rf sputtering using a 99.999% pure commercially bought and a home made target under 100 W power. The home made ZnO target, including 1–2% tungsten, was synthesized via solid state reaction. Thin films were deposited under a flow of 70% argon and 30% O₂ gas mixture followed by post-deposition annealing under 1 Torr oxygen atmosphere. Both deposition and post-deposition annealing were done at 420±1 °C. The structural analyses show that the films were in the [0002] preferred direction and that W atoms are bound to the oxygen atoms by replacing the Zn host atoms. Although no specific change was observed in the magnetic properties as a result of W doping, significant changes in the electrical properties were observed, as determined by the longitudinal and transversal magneto-electrical measurements. It was found that the W impurities induce better insulating properties due to lower carrier concentration and higher resistivity values. On the other hand, the enhanced positive magnetoresistivity and the existence of polarized spin currents, which were not specific for pure ZnO thin films, were observed in W doped ZnO films below 10 K.     

Pulsed laser deposition of doped skutterudite thin films

Ca_xCo₄Sb₁₂ skutterudite thin films have been prepared by pulsed laser deposition using a Nd:YAG laser working at 532 or 266 nm of wavelength. Characterization has been carried out by X-ray diffraction, atomic force microscopy and scanning electron microscopy. Emphasis has been put on the difficulty to obtain the skutterudite phase. Influence of the deposition temperature, the way of sticking the substrate, the laser fluence, the base pressure prior to deposition and the laser wavelength has been studied. All parameters revealed to have a drastic effect, and the skutterudite could only be achieved in a very narrow range of temperature and laser fluence, for a given wavelength, showing the importance on how these parameters are measured to ensure reproducible results.     

Growth of transparent conducting nano-structured In doped ZnO thin films by pulsed DC magnetron sputtering

Transparent conducting nano-structured In doped zinc oxide (IZO) thin films are deposited on corning 7059 glass substrates by bipolar pulsed DC magnetron sputtering with variation of pulsed frequency and substrate temperature. Highly c-axis oriented IZO thin films were grown in perpendicular to the substrate on the 30 kHz and 500 °C. The IZO films exhibited surface roughness of 3.6 nm similar to the commercial ITO and n-type semiconducting properties with electrical resistivity (carrier mobility) of about 5 × 10⁻³ Ω cm (14 cm²/V s). The optical characterization showed high transmittance of over 85% in the UV-vis region and exhibited the absorption edge of near 350 nm. In micro-Raman spectra, the origin of two additional modes is attributed to the host lattice defect due to the addition of In dopant. These results suggest that the IZO film can possibly be applied to make transparent conducting electrodes for flat panel displays.     

Fabrication and electrical characterisation of Zr-substituted BaTiO3 thin films

Thin films of zirconium-substituted barium titanate were deposited by chemical solution deposition on platinum-coated silicon substrates at a temperature of 700 °C. The films showed a polycrystalline perovskite structure. The grain size was found to decrease with increase of Zr substitution. The effect of Zr substitution on the dielectric constant and the leakage was studied. It was found that with increasing Zr content the phase transition becomes diffuse and relaxor-like. The dielectric constant was also found to decrease with increasing amounts of Zr after an initial increase. The hysteresis loops became thinner and the remanent polarisation was found to decrease. The leakage behaviour was explained by Schottky theory. The barrier heights of the different films were calculated and found to vary between 1.12 eV and 1.19 eV. PACS 68.55.-a; 81.20.Fw; 77.84.Dy     

XRD and XPS characterisation of transition metal silicide thin films

Binary transition metal silicides based on the systems Ti–Si, Fe–Si, Ni–Si and Cr–Si were fabricated on Si wafers by means of ion-beam co-sputter deposition and subsequent annealing. The crystalline structures of the phases formed were identified from the characteristic patterns acquired by means of X-ray diffraction (XRD) measurements. The phase formation sequences were described by means of the Pretorius' effective heat of formation (EHF) model. For the Ti–Si, Fe–Si and Ni–Si systems, single phase thin films of TiSi₂, β-FeSi₂ and NiSi₂ were generated as the model predicts, while a mixture of CrSi + CrSi₂ phases was obtained for the Cr–Si system. The surface chemical condition of individual specimens was analysed by using X-ray photoelectron spectroscopy (XPS). The chemical shifts of transition metal 2p_3/2 peaks from their metallic to silicide states were depicted by means of the Auger parameters and the Wagner plots. The positive chemical shift of 2.0 eV for Ni 2p_3/2 peak of NiSi₂ is mainly governed by the initial-state effects. For the other silicide specimens, the initial-state and final-state effects may oppose one another with similar impact. Consequently, smaller binding energy shifts of both negative and positive character are noted; a positive binding energy shift of 0.3 eV for the Fe 2p_3/2 level was shown for β-FeSi₂ and negative binding energy shifts of 0.1 and 0.3 eV were determined for CrSi + CrSi₂ and TiSi₂, respectively.     

Stoichiometric and structural alterations in GaN thin films during annealling

Annealling experiments were performed on GaN layers, grown on sapphire, over a wide range of temperatures (500–1100 °C). Rutherford Backscattering Spectrometry (RBS) was performed in random and 〈0001〉 channelling geometries using 2 MeV protons and helium ions to determine the stoichiometric and structural alterations produced during annealling. We present here, for the first time, a comprehensive and quantitative analysis of the depth distribution of both stoichiometric and structural changes in the near-surface region (∼750 nm) with a resolution of 50 nm for stoichiometric and 20 nm for structural changes. No decomposition was measured for temperatures up to 800 °C. Decomposition in the near-surface region increased rapidly with further increases in temperature, resulting in a near-amorphous region (500 nm) for annealling at 1100 °C. We describe the range of annealling conditions under which negligible stoichiometric and structural changes are observed. Our nanoscale resolution results are useful for the fabrication and operation of conventional and nanoscale optoelectronic and high-temperature devices. Received: 23 December 2002 / Accepted: 16 January 2003 / Published online: 28 March 2003 RID="*" ID="*"Corresponding author. Fax: +65-6777/6126, E-mail: scip0229@nus.edu.sg     

Nitrogen vacancy effects on the ferromagnetism of Mn doped GaN films

The amorphous gallium nitride thin films doped with Mn were deposited by Laser assisted Molecular Beam Epitaxy (LMBE). After annealing at different NH₃ flow rates, the high-quality GaMnN crystalline films with different concentration of nitrogen vacancies (V _N) were obtained, which were confirmed by the X-ray diffraction spectroscopy and Raman measurements. The magnetic behaviors of these films were also obtained to investigate the effects of nitrogen vacancies. It indicates that V _N play a significant role in the origin of ferromagnetism. The stronger room-temperature ferromagnetism is given with the higher V _N concentration when it is not beyond a critical concentration. Moreover, from our M(T) curves and Raman analysis, the films present high resistivity. The magnetism of films with high resistivity varies with concentration of nitrogen vacancies, which can be explained by the bound magnetic polarons (BMP) theory.     

Mn掺杂ZnO薄膜的结构及光学性能研究

通过脉冲激光沉积(PLD)法在SiO₂基片上制备了不同含量的Mn掺杂ZnO薄膜.X射线衍射、X射线能谱、原子力显微镜与紫外-可见分光光度计测试结果表明：少量的Mn离子的掺杂并没有改变薄膜的结构，薄膜具有(103)面的择优取向；PLD法制备的ZnO薄膜的成分与靶材基本一致，实现了薄膜的同组分沉积；薄膜表面比较平坦，起伏度小于80nm，颗粒尺寸主要集中在25nm附近；但是Mn离子的掺杂改变了ZnO薄膜的禁带宽度，随Mn掺杂含量的增加，ZnO薄膜的禁带宽度增加；当薄膜中Mn含量从6%增加到 关键词： PLD ZnO薄膜 Mn掺杂 吸收谱     

Emission properties of thin films of electroactive doped polymers

We have studied the effect of the intensity of the exciting radiation and the temperature on the emission properties of two kinds of thin-film samples based on blends of two types of organic electroactive materials: polyfluorene + iridium triphenylpyridinate and polyepoxypropylcarbazole + zero-th order PAMAM dendrimer with eosin. We have shown that an increase in the excitation intensity leads to an increase in the intensity of the luminescence of the polymer matrices and the iridium complex up to a power density of 300 kW/cm², and the emission of the dendrimer is rapidly saturated and does not return to the initial value when the excitation level decreases. Heating up to 170°C followed by cooling causes an increase in the intensity for all the components except the dendrimer. The data obtained show that annealing is an important method for improving the emission efficiency of the proposed thin-film structures, due to a change in the packing of the activator molecules in the polymer matrix leading to more efficient transfer of the excitation energy. Molecules of the studied dendrimer are not stable when exposed to optical radiation and temperature. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 6, pp. 820–825, November–December, 2007.     

Low-temperature activation of Mg-doped GaN with thin Co and Pt films

The activation of metallorganic chemical vapor deposition-grown Mg-doped GaN by N₂ annealing with thin Co and Pt films has been investigated. The Hall effect measurements revealed that both the Co and Pt films enhance activation of Mg acceptors as catalysts at low temperatures. A maximum hole concentration of p-type GaN was achieved at annealing temperature of 600 °C for the samples activated with both the Co and Pt films. It was also revealed that the activation of the acceptors is strongly affected by the thickness of the Co film.