High density Si nanodots-fabrication and properties

Invited PaperHigh density Si nanodots- fabrication and propertiesJ. Xu*, J. Zhou, X. Li, Z. Cen, D. Chen, W. Li, L. Xu, Z. Ma, K. ChenNational Laboratory of Solid State Microstructures and Department of Physics, Nanjing University,Nanjing, China 210093ABSTRACTWe propose an approach to achieve single layer of Si nanodots arrays on insulating layer by using KrF pulsed excimer laser irradiation on ultra-thin hydrogenated amorphous silicon films followed by thermal annealing. Under the suitable fabrication conditions, the area density of formed Si nanostructures can be higher than 1011cm-2 as revealed by AFM images. The size of formed Si nanodots is 3-4 nm for sample with initial a-Si:H film thickness of 4 nm. Room temperature visible light emission can be observed from laser irradiated a-Si:H film after thermal annealing. The results on electron field emission properties were also presented in this paper.Keywords: Si nanodots, laser crystallization, luminescence, field emission1.INTRODUCTIONRecently, fabrication and properties of low-dimensional Si structures, such as Si quantum well and Si nanodots, have attracted much attention because of their potential applications in many kinds of electric and optical devices.1-2 So far, many approaches have been proposed to fabricate semiconductor nanostructures, especially Si nanodots.3-5 However, it is still an open question to develop an approach to obtain the dense and uniform Si nanodots with controllable size and position. More important, the used techniques must be compatible with the mature Si technology.Laser crystallization technique is widely used to crystallize hydrogenated amorphous silicon (a-Si:H) films with submicron thickness in order to achieve micro-crystalline Si materials due to its low temperature process.6 In our previous work, laser induced crystallization of amorphous Si films technique was proposed to prepare Si nanodots in hydrogenated amorphous SiN(a-SiN:H)/a-Si:H multilayers and an intense photo- and electro-luminescence was observed at room temperature.7 Since the optical band gap of a-SiN:H layer is much higher than that of a-Si:H films, the photon energy of irradiation laser was mainly absorbed by a-Si:H layers, which results in the phase transition from amorphous to crystalline state and the size can be well confined by the a-Si:H thickness.In this paper, we report our work on the formation of a single layer Si nanodots with high density on insulating layer by using KrF excimer laser annealing on ultrathin a-Si:H films. It is shown that Si nanodots array with size less than 10nm can be achieved and the area density can be higher than 1011/cm2. Visible light emission can be observed even from a single layer of Si nanodots at room temperature. It is also demonstrated that the prepared films containing Si nanodots show a good field emission characteristics which can be developed as a cold cathodes materials.2.EXPERIMENTFigure 1 is the schematic illustration of sample preparation process. Ultra-thin a-SiN:H and a-Si:H films were successively fabricated in conventional plasma enhanced chemical vapor deposition system. The film thickness of a-SiN:H layer is about 30-40 nm and a-Si:H film thickness is controlled by considering the deposition rate (0.1nm/s). The detailed deposition parameters can be found elsewhere.8 As-deposited a-Si:H films were then crystallized by using KrF excimer laser with wavelength of 248nm and the pulse duration of 30ns. The laser beam was focused and passed through a rectangle quartz mask to a-Si:H films. The irradiated area on the film surface is about 3x5 mm2. A single pulse was*E-mail: junxu@; Fax: 86-25-83595535; Tel: 86-25-83594836Sixth International Conference on Thin Film Physics and Applications, edited by Wenzhong Shen, Junhao Chu,Proceedings of SPIE Vol. 6984, 69840E, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.792268used to irradiate the samples and the laser fluence was change from 0 to 1.4J/cm 2. Subsequently, the irradiated samples were thermally annealed at 900o C for 30min in pure N 2 ambient. Crystalline Si wafers and quartz glass plates were used as substrates for various measurements.Atomic force microscopy (AFM, Digital Instruments Nano Scope III) was used to characterize the surface morphology of a-Si:H films before and after laser irradiation. Raman spectra were used to monitor the crystallization of a-Si:H films. The formation of Si nanodots was revealed by using planar and cross-sectional transmission elelctron microscopy (TEM). Photoluminescence measurements were carried out at room temperature by using Ar + laser as an excitation source operating at a wavelength of 488nm. Electron field emission properties for irradiated samples were measured in a vacuum chamber (<10-6 Torr). The samples acted as cathode electrode and the gap distance between the sample and Cu anode is in a range of 100-400µm.Fig.1. The schematic illustration of Si nanodots sample preparation process.3. RESULTS AND DISCUSSIONLaser induced crystallization of ultrathin a-Si:H films can be verified by using Raman scattering technique. Figure 2 is the Raman spectra for 4nm-thick a-Si:H film after laser irradiation at various fluence and subsequent thermal annealing. Two apparent bands can be identified which are TO modes related to amorphous Si-Si (~ 475cm -1) and crystallized Si-Si (~516cm -1) phases, respectively. It is found that the crystallization is occurred when the laser fluence exceeds 0.6J/cm 2. With increasing the laser fluence, the intensity of band located around 516cm -1 is gradually increased which suggests the increase of the crystallization ratio in amorphous Si layer.Fig.2. Raman spectra of 4nm-thick a-Si:H film after irradiation at different laser fluence. Inset is highresolution TEM image of irradiated sample. Fig.3. AFM top image for laser irradiated a-Si:H film.The film surface morphology is changed after laser irradiation as shown in figure 3, which is AFM top image of irradiated a-Si:H film with thickness of 4 nm. The scanning area is 2x2 µm2 and the vertical scale is 5nm/div. It is found that the smooth film surface of as-deposited sample becomes extensively fragmented after laser irradiation. High-density, well-defined outgrowths with nanometers size can be identified on the surface. The average lateral size of the outgrowth is about 10nm and the vertical size is around 3-4 nm with area density higher than 1011cm-2. The size distribution of Si nanostructures can be estimated from AFM images. It is found that the lateral size distribution can be well fitted by a Guass function and the size deviation is less than 18%, which means the uniform and dense Si nanostructures are formed on a-SiN:H layer by combining the laser crystallization and thermal annealing techniques.The formation of Si nanodots is directly revealed by planar and cross-sectional TEM images. It is found that the surface outgrowths contain at least one Si grain. Inset of figure 2 is the high resolution TEM image for irradiated samples with the initial a-Si:H film thickness of 4 nm. The formed Si grain with spherical shape can be well identified and the size is about 3.2 nm which is almost equal to the initial a-Si:H film thickness. It is suggested that the size of formed Si nanodots can be controlled by the a-Si:H film thickness due to the constrained recrystallization effect in ultrathin a-Si:H layer. Nano-second pulsed laser irradiation induces the melt of amorphous Si thin films when the laser fluence exceeds the threshold value. The subsequent super-freezing process results in the surface roughening of irradiated samples and the formation of Si nanodots. If the initial a-Si:H film thickness is thin enough and the size of formed Si nanodots can be well defined in the suitable laser irradiation fluence range. In our previous work, we demonstrated that the critical a-Si:H film thickness is around 15nm. It is also suggested that there exists a narrow window for laser irradiation fluence to obtain high-density Si nanostructures.9samples. A strong visible light is emitted from samples after laser irradiation and the subsequently thermal annealing as shown in figure 4. The PL spectra show a broad band entered at 660 nm with full width at half maximum (FWHM) of 180nm. The PL peak energy is almost the same for samples irradiated under the different laser fluence. It is found that the PL becomes weak and finally quenched in samples irradiated at high laser fluence. It looks like that the presence of Si nanodots plays an important role in the visible light emission. However, it is difficult to ascribe the luminescence to the quantum size effect of Si nanodots although the size-dependent luminescence spectra was observed.9 It is found that the luminescence peak energy keeps its position with changing the measurement temperature from 85K-300K. The temperature-dependent PL results indicate that the visible light emission from a single layer of Si nanodots in our casemay due to the radiative recombination via the interface states or luminescent centers existing in the Si nanodots surface rather than the band-to-band recombination in Si nanodots.The irradiated a-Si:H films also show a good electron field emission characteristics. Figure 5 gives the emission current from laser irradiated sample with initial a-Si:H thickness of 15 nm as a function of applied electric field. The emission current (I) was measured by applying voltage from 0 to 5000V. The macroscopic electric field (E) is simply deduced by dividing the applied voltage by the gap distance between anode and cathode electrodes. The sample was deposited on heavy-doped n-type Si (n +-Si) wafer covered by 30 nm a-SiN:H layer. The used laser irradiation fluence is about 1.0J/cm 2. As a reference, the field emission result from as-deposited a-Si:H film is also presented in figure 5. It is obviously that the electron field emission characteristics are improved after the laser irradiation. The threshold electric field defined as the emission current at 1µA, is 9.8V/µm for irradiated sample which is significantly reduced compared with that of the reference sample (~15 V/µm). The maximum emission current can reach as high as 0.1mA when the applied electric field exceeds 15V/µm.It is found that the field emission process from laser irradiated a-Si:H films obeys the Fowler-Nordheim (F-N) tunneling model described as J=A(βE)2exp(-B φ-3/2/βE), in which A and B are constant and β is the field enhancement factor.10 From the field emission results, one can deduce the field enhancement factor β by choosing Si work function of 3.59eV. It is found that the β-value for laser irradiated sample is increased by over 3-folds compared with that for as-deposited sample which means the effective tunneling barrier is significantly reduced after the laser irradiation. It has been reported that the main tunneling barrier of amorphous semiconductors is the front surface of the films. The electrons are injected from the substrate into the semiconductor films under the applied electric field, they are then pass through the film to the film front surface, finally, they are emitted by tunneling the surface barrier. Therefore, the surface morphology and the film microstructures are very critical for controlling the field emission behavior.The enhancement of electron field emission behavior after laser irradiation in our case can be attributed to the following two factors: one is the surface roughening as revealed by AFM measurements and another one is the formation of Si nano-dots in the films. As mentioned before, laser irradiation under the suitable fluence results in the appearance of dense and uniform outgrowths on the film surface. The surface roughening after laser irradiation can improve the field enhancement factor geometrically. Previously, It has been reported the fabrication of two-dimensional Si tip array by electrochemical etching technique. It is very interesting to find that the field emission properties can be improved byFig.5. Electron field emission characteristics of laser irradiated and as-deposited a-Si:H sample. The initial a-Si:H film thickness is 15 nm.forming the patterned structures.11 Since the area density in our case is obviously high, it is reasonable that the threshold electric field obtained here is significantly improved. It is also suggested that the formation of Si nanodots in the films can induce the internal field enhancement process which may increase the effective local electric field significantly and the field emission properties can be improved as a consequence. 124.CONCLUSIONSIn conclusion, KrF pulsed excimer laser irradiation on ultrathin a-Si:H films followed by thermal annealing technique was used to prepare a single layer of Si nanodots with high density and uniformity. The area density of formed Si nanostructures can be higher than 1011cm-2 and the size can be well controlled by the film thickness as revealed by the TEM images. Our approach can avoid the high temperature treatments (>1000o C) and therefore, the cheap glass substrates can be used. An intense light emission in visible light range was observed from the laser irradiated sample after subsequently thermal annealing which can be ascribed to the recombination of photo-excited electron-hole pairs via the surface luminescence centers of Si nanodots. The enhanced electron field emission behavior was also observed and the improvements can be attributed to the local electric field enhancement due to the surface roughening and the existence of Si nanodots in the films.ACKNOWLEDGEMENTSThe work was supported by NSF of China (No. 60425414, 60508009 and 10574069), NSF of Jiangsu Province (BK2006715) and the State Key Program for Basic Research of China (2007CB613401).REFERENCES1. A. Irrera, G. Franzo, F. Iacana, A. Canino, G. Di Stefano, D. Sanfilippo, A. Piana, P. Fallica and F. Priolo, "Light emiiting devices based on silicon nanostructures," Physica E 38, 181-187 (2007).2.L. Yu, K. Chen, L. Wu, J. Xu, W. Li and X. 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