Silicon grain arrays prepared using a pattern crystallization technique of pulsed-excimer laser irradiation

Silicon grain arrays were prepared using a pattern crystallization technique of pulsed KrF excimer laser irradiation. The precursor material was hydrogenated amorphous silicon (a-Si:H) thin films deposited on single crystal Si wafers by plasma-enhanced chemical vapor deposition. It was shown that Si grains with a uniform size and a well-defined periodicity embedded in the a-Si:H matrix were obtained by this simple technique. The grain size was less than 2 μm. Relativly strong photo-luminescence with two peaks at 720 and 750 nm was observed at room temperature. We expect to reduce Si grain sizes by optimizing the growth conditions of a-Si:H thin films and controlling the temperature distribution in the film during laser irradiation. Received: 21 November 2000 / Accepted: 12 December 2000 / Published online: 9 February 2001     

Effect of the laser fluence on structural and optical characterization of thin CdS films synthesized by femtosecond pulsed laser deposition

CdS thin films have been grown on Si(1 1 1) and quartz substrates using femtosecond pulsed laser deposition. X-ray diffraction, atomic force microscopy, photoluminescence measurement, and optical transmission spectroscopy were used to characterize the structure and optical properties of the deposited CdS thin films. The influence of the laser fluence (laser incident energy in the range 0.5–1.5 mJ/pulse) on the structural and optical characterizations of CdS thin films has been studied. The results indicate that the structure and optical properties of the CdS thin films can be improved as increasing the per pulse output energy of the femtosecond laser to 1.2 mJ. But when the per pulse output energy of the femtosecond laser is further increased to 1.5 mJ, which leads to the degradation of the structure and optical properties of the CdS thin films.     

Ultra-large lateral grain growth by double laser recrystallization of a-Si films

A new double laser recrystallization technique that can produce ultra-large direction- and location-controlled lateral grains is presented. An excimer laser is combined with a pulse-modulated Ar⁺ laser to yield grains of tens of micrometers in size. The effect of different parameters on lateral grain growth is investigated. These parameters include the time delay between the two lasers, the excimer-laser fluence, the Ar⁺-laser power and the pulse duration. The process has a wide process window and is insensitive to both the excimer-laser fluence and the Ar⁺-laser power fluctuations. Preheating of the a-Si film with the Ar⁺ laser before firing the excimer laser is necessary for inducing lateral grain growth. The transient excimer-laser irradiation is believed to generate nucleation sites for initiating the subsequent lateral grain growth. The surface roughness of the recrystallized poly-Si is measured by atomic force microscopy. Received: 14 September 2000 / Accepted: 24 February 2001 / Published online: 27 June 2001     

氧压对飞秒激光沉积ZnO/Si(100)薄膜光学性能的影响

利用飞秒脉冲激光沉积法在n-Si（100）单晶衬底上制备了ZnO薄膜, 分析了衬底温度、激光能量、氧压及退火处理对薄膜结构和光学性能的影响. X射线衍射结果表明, 当激光能量为15?mJ、氧压为10?mPa时, 80?℃生长的薄膜取向性最好. 场扫描电子显微镜结果显示薄膜的晶粒尺寸随激光能量的增加而减小、随衬底温度的升高而增大且退火后明显变大. 紫外-可见透射光谱显示薄膜具有90%以上的可见光透过率.光致发光谱表明当氧压为10 mPa时,除了ZnO的紫外本征峰外, 还有一波长为410 nm的强紫光峰, 当氧压增至20 mPa以上, 所有缺陷峰均消失, 只有376 nm处的紫外本征峰. 与纳秒激光法所制备的薄膜特性进行了比较, 结果表明, 虽然纳秒激光沉积所制备的薄膜具有更高的c轴取向度, 但飞秒激光沉积制备的薄膜具有更好的发光性能. 关键词： 氧化锌 飞秒脉冲激光沉积 透过率 光致发光     

Structural characterization and optoelectronic properties of GaN thin films on Si(111) substrates using pulsed laser deposition assisted by gas discharge

GaN films have been grown on Si(111) substrates with a thin AlN buffer layer using pulsed laser deposition (PLD) assisted by gas discharge. The crystalline quality, surface morphology and optoelectronic properties of the deposited films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL) spectroscopy, and room-temperature Van der Pauw–Hall measurements. The influence of the deposition temperature in the range 637–1037 K on the crystallinity of GaN films, the laser incident energy in the range 150–250 mJ/pulse on the surface morphology and the optoelectronic properties were systematically studied. The XRD analysis shows that the crystalline quality of the GaN films improves with increasing deposition temperature to 937 K, but further increase of the deposition temperature to 1037 K leads to the degradation of the crystalline quality. AFM results show that the surface roughness of the GaN films can be decreased with increasing laser incident energy to 220 mJ/pulse. Further increase of the laser incident energy to 250 mJ/pulse leads to an increase in the surface roughness. The optoelectronic properties of GaN films were also improved by increasing the laser incident energy to 220 mJ/pulse. GaN films which have a n-type carrier concentration of 1.26×10¹⁷ cm^-3 and a mobility of 158.1 cm²/Vs can be deposited at a substrate temperature of 937 K, a deposition pressure of 20 Pa and a laser incident energy of 220 mJ/pulse. Their room-temperature PL spectra exhibit a strong band-edge emission at 365 nm. PACS 81.15.Fg; 81.05.Ea; 78.20.-e; 73.61.Ey; 78.66.Fd     

高脉冲功率能量PLD法制备MgZnO薄膜中的沉积机理

用PLD法成功制备了一系列高质量的MgZnO薄膜。实验中发现高脉冲能量沉积薄膜的结构和发光特性随基片温度的变化规律与低脉冲能量下的结果不一样:基片在室温时高脉冲能量制备薄膜的XRD峰的半峰全宽比高基片温度时的结果相对更小;AFM显示其颗粒变大,柱状生长突出;PL谱紫峰与绿峰强度比最大,结晶质量反而提高。另一方面,与低脉冲能量时相反,增大氧气压强后高脉冲能量沉积的薄膜XRD半峰全宽变窄。结合实验现象和表征,合理解释了高脉冲能量沉积的机理。室温制备高质量MgZnO薄膜的PLD沉积机理对于以后在柔性衬底上沉积薄膜的研究有重要的参考价值。     

Reaction between amorphous Si and crystalline Al in Al/Si and Si/Al bilayers: microstructural and thermodynamic analysis of layer exchange

Aluminium-induced crystallization of amorphous silicon (a-Si) in Al/Si and Si/Al bilayers was studied upon annealing at 250 °C by X-ray diffraction and Auger electron spectroscopy. The Al/a-Si bilayers and a-Si/Al bilayers were prepared by sputter deposition on single-crystal silicon wafers with a silicon-oxide film on top. During the isothermal annealing a layer-exchange process occurred in both types of bilayers. A continuous polycrystalline silicon (poly-Si) film was formed within, and thereby gradually replacing, the initial Al metal layer. The sublayer sequence in the original bilayer influenced the speed of the poly-Si formation and the layer-exchange process. After annealing, the Al fiber texture in the as-deposited bilayers had become stronger, the Al crystallites had grown laterally, and the macrostress in the Al layer had been released. The amorphous Si layer had crystallized into an aggregate of nanocrystals with {111} planes parallel to the surface, with a crystallite size of about 15–25 nm. An extensive analysis of the Gibbs energy change due to annealing showed that the layer exchange may be promoted by the release of elastic energy and grain growth for the Al phase. PACS 05.70.Jk; 61.43.Dq; 68.35.Rh; 61.72.Cc; 68.55.Jk     

Write-Once Medium with Fe-Doped BiOx Thin Films for Blue Laser Recording

Fe：BiOx films are fabricated on K9 glass substrates by rf-magnetron sputtering of a BiFeO target under argon atmosphere with increasing sputtering power from 80 to 200 W at room temperature. It is found that the thin films grown at the sputtering power of 160 W can be formed at an appropriate deposition rate and have an improved surface morphology. The XPS result reveals that the films investigated are comprised of Bi, Fe and O elements. A typical XRD pattern shows that no phase transition occurs in the films up to 400℃. The results of the blue laser recording test demonstrate that the Fe：BiOx films have good writing sensitivity for blue laser beam （406.7nm） and good stability after reading 10000 times. The recording marks of 200nm or less are obtained. These results indicate that the introduction of Fe into BiOx films can reduce the mark size and improve the stability of the films.     

Numerical analysis of Excimer laser assisted processing of multi-layers for the tailored dehydrogenation of amorphous and nano-crystalline silicon films

The application of the striking electrical and optical properties of amorphous and nano-crystalline silicon in photovoltaic, photonic and nano-electronic devices is attracting increasing attention. In particular, its use both on polymeric substrates and in Integrated Circuit technology for the development of enhanced new devices has shown that processing techniques to produce amorphous hydrogenated and nano-crystalline silicon films avoiding high substrate temperatures are of great importance. A promising strategy to achieve this purpose is the combination of Hot-Wire Chemical Vapor Deposition at 150 °C with Excimer Laser Annealing, thus maintaining the substrate at relatively low temperature during the complete process.In this work we present a numerical analysis of Excimer Laser Annealing, performed at room temperature, of a multilayer structure of thin alternating a-Si:H and nc-Si films deposited on glass and grown by Hot-Wire Chemical Vapor Deposition. A set of two different layer thicknesses a-Si:H (25 nm)/nc-Si (100 nm) and a-Si:H (30 nm)/nc-Si (60 nm) were analysed for a total structure dimension of 900 nm. The aim is to determine the probable temperature profile to achieve controlled localized in depth dehydrogenation.Temperature distribution has been calculated inside the multilayer during the irradiation by a 193 nm Excimer laser, 20 ns pulse length, with energy densities ranging from 50 to 300 mJ/cm². Calculations allowed us to estimate the dehydrogenation effect in the different layers as well as the structural modifications of the same layers as a function of the applied laser energy.The numerical results have been compared to the experimental ones obtained in similar multilayer structures that have been analysed through Raman spectroscopy and TOF-SIMS in depth profiling mode.     

Improvement in electrical and photovoltaic properties of a-Si/c-Si heterojunction with slanted nano-columnar amorphous silicon thin films for photovoltaic applications

The flat a-Si and slanted nanocolumnar (S-nC) a-Si thin films were prepared on c-Si and corning glass substrates by e-beam physical vapor deposition (EB-PVD) technique. The structural properties of all the grown thin films were determined by X-Ray Diffraction (XRD) analysis and Raman spectroscopy. Surface and cross-sectional morphology of a-Si/c-Si and S-nC a-Si/c-Si heterojunctions were investigated by Field Emission Scanning Electron Microscopy (FE-SEM). Sculptured thin films demonstrate potential for significant nanoscale applications in the area of thin film technology. The electrical and photovoltaic properties of these heterojunctions have been investigated by means of dc current–voltage (I–V) measurements at room temperature in dark and light conditions. The S-nC STFs' performance has been found to be improvable on changing the morphology of the thin film. We have found that, the porous morphology of this structure improves the photosensitivity features in photovoltaic devices and solar cell technology. We gained a high open voltage value, such as 900 mV in S-nC a-Si/c-Si thin film, without any doping process.     

Fabrication and Investigation of Photovoltaic Converters Based on Polycrystalline Silicon Grown on Borosilicate Glass

The microcrystalline Si layers with grain sizes of up to several tens of micrometers were grown. The physical vapor deposition (PVD), amorphous–liquid–crystalline (ALC) transition technique and a steady-state liquid phase epitaxy (SSLPE) are used for the fabrication of three different samples. The first sample under consideration was prepared first by deposition of a-Si onto glass substrates by PVD at room temperature, followed by heating from the front side to ~300°C and deposition of an indium metallic solvent. At the preparation of the second sample, an additional silicon layer with the thickness of 400 nm was deposited. A sample, when after that a c-Si was grown on the seed layer by SSLPE from indium solution is referred as a third sample. The resulting samples have a strong absorption edge in the mid-infrared region around 1960 cm^?1. Six well-resolved oscillations with an average period of δB = 0.1214 T are revealed on the third sample’s magnetoresistance curve at gradually increasing of the magnetic field from zero up to 1.6 T. It is assumed that either Aharonov–Bohm effect or kinetic phenomena taking place in the grains boundaries at lateral current flow are responsible for those oscillations. Quantitative evaluations show that due to the strong absorption in mid-infrared region, enlargement of the photoresponse spectrum will occur and the efficiency of solar and other thermal energy conversion should be around ~10–15% higher than that of traditional PV cells based on silicon on glass structures.     

Microstructure and lateral conductivity control of hydrogenated nanocrystalline silicon oxide and its application in a-Si:H/a-SiGe:H tandem solar cells

Phosphorous-doped hydrogenated nanocrystalline silicon oxide(n-nc-SiO_x:H) films are prepared via radio frequency plasma enhanced chemical vapor deposition(RF-PECVD). Increasing deposition power during n-nc-SiO_x:H film growth process can enhance the formation of nanocrystalline and obtain a uniform microstructure of n-nc-SiO_x:H film. In addition,in 20 s interval before increasing the deposition power, high density small grains are formed in amorphous SiO_x matrix with higher crystalline volume fraction(I_c) and have a lower lateral conductivity. This uniform microstructure indicates that the higher Ic can leads to better vertical conductivity, lower refractive index, wider optical band-gap. It improves the back reflection in a-Si:H/a-SiGe:H tandem solar cells acting as an n-nc-SiO_x:H back reflector prepared by the gradient power during deposition. Compared with the sample with SiO_x back reflector, with a constant power used in deposition process,the sample with gradient power SiO_x back reflector can enhance the total short-circuit current density(Jsc) and the initial efficiency of a-Si:H/a-SiGe:H tandem solar cells by 8.3% and 15.5%, respectively.     

Nanocrystalline silicon films prepared by laser—induced crystallization

The excimer laser-induced crystallization technique has been used to investigate the preparation of nanocrystalline silicon (nc-Si) from amorphous silicon ($\al$-Si) thin films on silicon or glass substrates. The $\al$-Si films without hydrogen grown by pulsed-laser deposition are chosen as precursor to avoid the problem of hydrogen effluence during annealing. Analyses have been performed by scanning electron microscopy, atomic force microscopy, Raman scattering spectroscopy and high-resolution transmission--electron microscopy. Experimental results show that silicon nanocrystals can be formed through laser annealing. The growth characters of nc-Si are strongly dependent on the laser energy density. It is shown that the volume of the molten silicon predominates essentially the grain size of nc-Si, and the surface tension of the crystallized silicon is responsible for the mechanism of nc-Si growth.     

Growth of oriented Pb(ZrxTi1-x)O3 thin films on glass substrates by pulsed laser deposition

Oriented crystalline Pb(Zr_xTi_1-x)O₃ (x=0.53) (PZT) thin films were deposited on metallized glass substrates by pulsed laser deposition (1060-nm wavelength Nd:YAG laser light, 10-ns pulse duration, 10-Hz repetition rate, 0.35-J/pulse and 25-J/cm² laser fluence), from a commercial target at substrate temperatures in the range 380-400 °C. Thin films of 1-3 7m were grown on Au(111)/ Pt/NiCr/glass substrates with a rate of about 1 Å/pulse on an area of 1 cm². The deposited PZT films with perovskite structure were oriented along the (111) direction, as was revealed from X-ray diffraction spectra. Fourier transform infrared spectroscopy (FTIR) was performed on different PZT films so that their vibrational modes could be determined. Piezoelectric d₃₃ coefficients up to 30 pC/N were obtained on as-deposited films. Ferroelectric hysteresis loops at 100 Hz revealed a remanent polarization of 20 7C/cm² and a coercive field of 100 kV/cm.     

Effects of crystallinity on laser-induced voltage effect from Zn0.9Co0.1O thin film

Co-doped ZnO epilayer films were grown by pulsed laser deposition (PLD) on vicinal cut silicon and sapphire substrates. Changes in deposition time were observed as a moderate effect on the quality of the films, and the influence of the thickness on thermoelectric signals from Zn_0.9Co_0.1O thin films were discussed. The effect of one of the main deposition parameters, the deposition time, on the crystallinity and electron mobility properties of the Zn_0.9Co_0.1O thin films grown on sapphire was investigated by means of X-ray diffraction (XRD) and laser-induced voltage (LIV) effect. It shown that the XRD rocking curve full-width half-maximun (FWHM) decreased as time increasing, and the LIV signals were observed along the tilting angle of the substrate orientation when the pulsed KrF excimer laser of 248 nm were irradiated on the films. When the films illuminated in pulse lasers, the highest signals occurred in the films with best crystalline quality, and the signals were higher in the films grown on sapphire than those on silicon substrates. It suggested that the electrical resistivity and electron mobility have close relations with not only the crystallinity but also with the interface of the thin films.     

Subpicosecond excimer laser ablation of thick gold films of ultra-fine particles generated by a gas deposition technique

The results of UV laser ablation of gold nanoparticle films on glass substrates using femtosecond pulses are presented. Films of ultra-fine gold particles were prepared by an inert gas evaporation and deposition technique, resulting in a well-defined log-normal particle size distribution of (7ǃ) nm. The pulse length of the laser was 500 fs at a wavelength of 248 nm. Ablation thresholds, ablation rates at different fluxes, and the morphology of the ablated structures are presented. For the nanoparticle films studied an ablation rate five times higher than that of gold films prepared by the conventional evaporation technique was found. The ablation thresholds and rates are supposed to depend on the particle size and also on the evaporation pressure. These results are explained by taking into account the energy transport properties of nanocrystalline and conventionally evaporated gold films.     

沉积压力对氢化非晶硅薄膜特性的影响

采用等离子增强化学气相沉积(PECVD)系统,以乙硅烷和氢气为气源,普通钠钙玻璃为衬底制备了氢化非晶硅(a-Si∶H)薄膜,研究了沉积压力对非晶硅薄膜的沉积速率、光学带隙以及结构因子的影响。采用台阶仪、紫外可见分光光度计、傅里叶变换红外光谱仪和扫描电子显微镜等手段分别表征了a-Si∶H薄膜的沉积速率,光学带隙、结构因子和表面形貌。结果表明： 随着沉积压力的增加,沉积速率呈现先上升后下降的趋势,光学带隙不断下降。当沉积压力小于210 Pa时,以SiH键存在的H原子较多,而以SiH₂或SiH₃等形式存在的H较少; 当沉积压力大于210 Pa时,以SiH₂,(SiH₂)_n或SiH₃等形式存在的H较多。通过结构因子的计算,发现沉积压力在110～210 Pa的范围内沉积的薄膜质量较好。     

Grain size effect on twin density in as-deposited nanocrystalline Cu film

Here, we report the formation of twins and grain size dependence of twin density in nanocrystalline (NC) copper films fabricated by pulsed laser deposition. It is found that the percentage of grains containing twins decreases with decreasing grain size in the grain size range of 2–10?nm. Surprisingly, although the twins were formed during the deposition process without mechanical deformation, our analysis suggests that they are most likely deformation twins formed under high internal stress existing in the NC Cu films. This phenomenon may also happen in other NC metallic thin films where internal stresses are high.     

氧分压对PLD制备ZnO薄膜结构和发光性质的影响

采用脉冲激光沉积(PLD)技术,在Si(100)衬底上制备出高度c轴取向的ZnO薄膜。通过X射线衍射(XRD)谱,扫描电镜(SEM)和室温光致发光(PL)光谱的测量,研究了生长气氛压强的改变对薄膜结构和光致发光的影响。实验结果表明,当氧压从10Pa升高到100Pa时ZnO(002)衍射峰的半峰全宽(FWHM)增大。可以认为这是由于较高的氧压下,到达衬底表面的离子动能减小。这样部分离子没有足够的能量迁移到生长较快的(002)面,c轴取向变差,导致(002)衍射峰的强度降低,半峰全宽增大。随着氧压增大,紫外发光强度增强。这可能是氧压变大,薄膜的化学配比升高,说明化学配比对UV发光的影响要大于薄膜微结构的影响。改变氧气压强对薄膜的表面形貌也有较大的影响。     

The comparison between CdS thin films grown on Si(111) substrate and quartz substrate by femtosecond pulsed laser deposition

Two kinds of cadmium sulfate (CdS) thin films have been grown at 600 °C onto Si(111) and quartz substrates using femtosecond pulsed laser deposition (PLD). The influence of substrates on the structural and optical properties of the CdS thin films grown by femtosecond pulsed laser deposition have been studied. The CdS thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), photoluminescence (PL) and Raman spectroscopy. Although CdS thin films deposited both on Si(111) and quartz substrates were polycrystalline and hexagonal as shown by the XRD , SEM and AFM results, the crystalline quality and optical properties were found to be different. The size of the grains for the CdS thin film grown on Si(111) substrate were observed to be larger than that of the CdS thin film grown on quartz substrate, and there is more microcrystalline perpendicularity of c-axis for the film deposited on the quartz substrate than that for the films deposited on the Si substrate. In addition, in the PL spectra, the excitonic peak is more intense and resolved for CdS film deposited on quartz than that for the CdS film deposited on Si(111) substrate. The LO and TO Raman peaks in the CdS films grown on Si(111) substrate and quartz substrate are different, which is due to higher stress and bigger grain size in the CdS film grown on Si(111) substrate, than that of the CdS film grown on the amorphous quartz substrate. All this suggests that the substrates have a significant effect on the structural and optical properties of thin CdS films. PACS 81.15.Fg; 81.05.Ea; 78.20.-e; 78.67.-n; 42.62.-b