Final Report for AOARD Grant FA2386-09-1-4104 “Study on Locally Confined Deposition of Si Nanocrystals in High-Aspect-Ratio Si Nano-Pillar Arrays for Nano-Electronic and Nano-Photonic Applications

We investigate the PL characteristics of the equivalent-size controlled Si-QDs by employing the nano-porous AAO membrane as the template for growing Si-rich SiOx nano-rods to achieve the spatially confined synthesis of Si-QD. The ultra-bright PL can be emitted from the SiOx nano-rod in nano-porous AAO membrane. In addition, we preliminarily demonstrate the in-situ high-temperature synthesis of Si nanocrystal based broadband optical waveguide amplifiers with gain coefficient of >100 cm and a power gain of >13 dB. Introduction: Implementation of Si/Ge nanocrystal based light-emitting diode, optical amplifier, laser diode made on silica and Si nanowire and Al2O3 nanoporous membrane is an intriguing topic, however, which has yet not been completely realized for versatile applications in all-Si-based lighting flexible photonic modules or optical interconnect systems in near future. The main bottleneck is attributed to the inappropriate control on the envorpmental and parametric factors set by the commercial chemical vapor deposition systems. Our previous researches reveal that the analomous gas fluence and flowing rate is mansatory for the synthesis of Si nanocrystals on different nanoporous substrates with a strictly tooling deposition system. In last year, we propose to establish an ultralow-plasma and high-temperature chemical vapor deposition (UPHT-CVD) system which is particularly designed for spatially confined synthesis of size and wavelength controllable Si nanocrystals with optimized quantum efficiency in Si nanowire substrate, SiC or Al2O3 nanoporous membrane. In this work, we investigate the PL characteristics of the equivalent-size controlled Si-QDs by employing the nano-porous AAO membrane as the template for growing Si-rich SiOx nano-rods to achieve the spatially confined synthesis of Si-QD. The ultra-bright PL can be emitted from the SiOx Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 08 DEC 2009 2. REPORT TYPE FInal 3. DATES COVERED 03-06-2009 to 02-12-2010 4. TITLE AND SUBTITLE Study on Locally Confined Deposition of Si Nanocrystals in High-Aspect-Ratio Si Nano-Pillar Arrays for Nano-Electronic 5a. CONTRACT NUMBER FA23860914104 5b. GRANT NUMBER 5c. PROGRAM ELEMENT NUMBER 6. AUTHOR(S) Hao-Chung Kuo 5d. PROJECT NUMBER 5e. TASK NUMBER 5f. WORK UNIT NUMBER 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) National Chiao-Tung University,1001 Ta-Hsieh Rd,Hsinchu 300,Taiwan,TW,300 8. PERFORMING ORGANIZATION REPORT NUMBER N/A 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) AOARD, UNIT 45002, APO, AP, 96337-5002 10. SPONSOR/MONITOR’S ACRONYM(S) AOARD 11. SPONSOR/MONITOR’S REPORT NUMBER(S) AOARD-094104 12. DISTRIBUTION/AVAILABILITY STATEMENT Approved for public release; distribution unlimited 13. SUPPLEMENTARY NOTES 14. ABSTRACT This is the report of an investigation of the photoluminescence characteristics of equivalent-size controlled silicon quantum dots by employing a nano-porous aluminum oxide membrane as the template for growing silicon-rich nano-rods to achieve the spatially-confined synthesis of silicon quantum dots. 15. SUBJECT TERMS Electro-optics, nano photonics, semiconductor 16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF ABSTRACT Same as Report (SAR) 18. NUMBER OF PAGES 5 19a. NAME OF RESPONSIBLE PERSON a. REPORT unclassified b. ABSTRACT unclassified c. THIS PAGE unclassified Standard Form 298 (Rev. 8-98) Prescribed by ANSI Std Z39-18 nano-rod in nano-porous AAO membrane. The synthesizing mechanism for such a spatially confined synthesized of Si-QD buried between 1D SiOx nano-rod and 2D thin-film is elucidated. High-resolution transmission electron microscopy (HRTEM) is performed to determine the spatially confined size distribution of the Si-QDs. Furthermore, to realize the concept of spatially confined synthesis between theory of Si atomic diffusion and crystal nucleation mechanism, the effective diffusion characteristics of Si in sub-stoichiometric SiOx is also to determine. Study on controllable Si-QD size uniformly for improving the standard deviation of Si-QD size dependent spectral linewidth is emphasized. The relationship between Si-QD size and annealing temperature for controlling the emission wavelength, and temperature effect on spatially confined synthesis of Si-QD are proposed to promote the light emission power. Experiment: The nano-porous AAO membrane with specified pore diameter of 100±5 nm (Fig.1 (a) and (b)) was used as a template to deposit the 1D Si-rich SiOx nano-rods through plasma-enhanced chemical vapor deposition (PECVD) at RF power, working pressure, working temperature, and N2O/SiH4 flowing ratio of 50 W, 0.5 Torr, 350oC and 5.5, respectively. All of the SiOx-on-AAO samples were deposited within 120 seconds to preserve the sample thickness. To precipitate Si-QD, the 1D Si-rich SiOx nano-rod embedded in nano-porous AAO specimens were annealed in quartz furnace with flowing nitrogen gas at temperature varying from 350 to 800oC. To confirm the results of spatially confined synthesis of Si-QDs, the SiOx film grown on (100)-oriented p-type Si substrate is also prepared for comparison. In particular, the Si-rich SiOx films were also annealed at 1100oC14, for precipitating the Si-QD in the host matrix without spatial confinement. Subsequently, the room-temperature PL of the samples pumped by He-Cd laser at wavelength and average power of 325 nm and 6.3 W/cm2, respectively, was analyzed using a fluorescence spectrophotometer (CVI, DK240) with a single-grating of 0.06 nm resolution and converted into electrical signal by a photomultiplier (Hamamatsu, model R928) with operational voltage of 1.0 volt. In addition, the working distance between the focusing lens and the sample was fine-tuned to maximize the PL intensity. The HRTEM (JEOL, JEM-2010) and TEM X-ray energy-dispersive spectrometer (TEM-XEDS, Link ISIS 300) were employed to characterize the Si-QD size and O/Si composition ratio within 1D SiOx nano-rods and 2D SiOx thin-film. In addition, the Si-QD distributions have also been obtained by processing HRTEM images, and the lattice spacing of Si-QD was confirmed by using the related software (Digital Micrograph 3.3.1 and Origin R7.0 SRO). Results and Discussion: First of all, the ultralow-plasma and high-temperature CVD method for spatially confined synthesis of size and wavelength controllable Si nanocrystals in Al2O3 nanoporous membrane has been demonstrated. We have focused on the nanocrystallite Si based colorful MOSLEDs with broadband wavelength tunability made by Si-rich SiOx/SiNx multi-layer with buried Si nanoclusters of precisely controlled size on roughened Si nano-pillars or Al2O3 nanoporous membrane Si substrate. Important roles of the interfacial diffusion effect induced dislocation and phase transitions in Si-rich SiOx/SiNx multilayered thin films system, the surface energy and solute strain energy effects in surface segregation, and strain induced grain boundary migration in SiOx-Al2O3 nanocomposite material, the impact of ripening effect on self-aggregated nano-clusters with reducing surface energy, the charge capacity within the hydrogen passivated Si nanocrystal will be realized to achieve non-stoichiometric and nanocrystal-enriched synthesis for enabling wavelength tunable luminescence. We have demonstrated the spatially confined synthesized of SiOx nano-rod in nano-porous AAO membrane with ultra-bright PL after lower annealing temperature at 500oC. A higher concentration with volume density of 5.51×10 cm and tiny size with diameter of only 2.0±0.1 nm of the excessive Si-QD can easier be obtained. HRTEM results indicate that the geometric form varied from 2D to 1D could also effectively reduce a standard deviation of Si-QD size from 25% to 12.5%. By using the diffusion controlled growth mechanism to estimate the diffusion characteristics, we obtain the activation energy of 6.284 kJ/mole and diffusion length of 1.74 nm for SiO1.5 nano-rod in nano-porous AAO membrane to corroborate the spatially confined Si-QD precipitation. These results also indicate that the varied composition ratio of SiOx host matrix will affect the value of activation energy. In contrast, the same SiOx film grown on smooth Si wafer only exhibits a PL intensity of 0.08 count/nm. The PL can further broaden and red-shift to 600 nm for 800C annealed sample, however, while the PL intensity greatly attenuates due to both the enlargement of Si-QD size and the reduction of the emission photon energy. In addition, raising annealing temperature beyond 800C, also leads to crack the nano-porous AAO membrane due to an extremely large bending strain. The impact of this work elucidates that the monotonically decreasing dimension of SiOx host matrix can concurrently contribute to the spatially confined precipitation of Si-QD and low temperature phase-separation between Si-QD and SiO2 matrices, thus facilitating super-luminescent PL with enhanced power, blue-shifted wavelength, and narrowing spectral linewidth. Fig. 1 The plane-view and cross-sectional FESEM micrograph of 1D Si-rich SiOx nano-rod buried nano-porous AAO membrane, respectively. Such Si nanocrystals synthesized in Si nanopillars or Al2O3 nanoporous membrane for white-light emitters with optimized quantum efficiency is characterized. Previously, the maximum electroluminescent power and external quantum efficiency up to 10 W and 1% are proposed. However, this relies on the optimization on the critical reactant-gas recipe and detuning fluence ratios of diluted Silane, nitrious oxide and ammonia during for the UPHT-CVD synthesis of Hydrogen passivated Si nanocrystal within these nanoporous membranes at extremely low plasma power. Besides, the study on phase transitions in multilayered thin films system, interfacial segregation phenomenon, ripening effect and tunable composition ratio with controllable Si nanocrystal size for improving the size dependent quantum efficiency and charge capacity must take into consideration. Moreover, the reduction on escaping rate of electrons from Si nanocrystals needs to be achieved for overcoming the by-passing effect of carriers and to promote the light emission efficiency. The spatially confined synthesis of Si quantum dots (Si-QDs) embedded in low-temperature (500C) annealed Si-rich SiOx nano-rod deposited in nano-porous anodic aluminum oxide (AAO) membrane using PECVD with ultra-bright photoluminescence spectra are characterized. In comparison with the same SiOx film grown on planar Si substrate, a blue-shifted PL by = 96 nm is clearly observed from such 1D Si-rich SiOx nano-rods in nano-porous AAO membrane. In particular, a narrower PL spectral linewidth (FWHM) shrinks from 210 to 140 nm with the PL intensity enhancing up to two orders of magnitude, corresponding to the reduction on both the average Si-QD size and its standard deviation from 2.60.2 to 2.00.1 nm and from 25% to 12.5%, respectively. The diffusion controlled growth mechanism is also employs to determine the activation energy of 6.284 kJ/mole and diffusion length of 1.74 nm for SiO1.5 nano-rod in nano-porous AAO membrane to corroborate the spatially confined Si-QD precipitation. HRTEM results verify that the variation on the geometric form of the host matrix from 2D to 1D could effectively constrain the SiOx nano-rod diameter for unifying the buried Si-QD size at even lower annealing temperature. Raising the annealing temperature beyond 800C not only red-shifts the PL wavelength, but also leads to a monotonic reduction on PL intensity and cracks the nano-porous AAO membrane due to an extremely large bending strain.