1 先進奈米科技暨 應用光電實驗室 Southern Taiwan University

Silicon nano-crystalline structures fabricated by a sequential plasma hydrogenation and annealing technique Student ： Jen-Chieh Cheng Professor ： Chih-Cheng Kao Y. Abdi a,1, P. Hashemi a,1, S. Mohajerzadeh a, *, M. Jamei a,1, M.D. Robertson b, M.J. Burns b, J.M. MacLachlan b 2 a Thin Film and Nano-Electronic Laboratory, Nano-Electronics Center of Excellence, University of Tehran, Tehran, Iran b Department of Physics, Acadia University, Wolfville, Nova Scotia, Canada B4P 2R6

OUTLINE Introduction Experiments Results and discussion – SEM – PL Spectrum – FTIR Spectrum – TEM – HREM Conclusions 3

Introduction Nano-crystalline,porous silicon(PS) films are promising materials in the areas of optoelectronics and microelectronics due to their visible luminescence characteristics at room temperature. 4

Experiments Step1:N-type(100)silicon substrates with a resistivity of 1- 5 Ω-cm were cleaned in standard RCA #1 solution (NH 4 OH/H 2 O 2 /H 2 O=1:1:5),rinesd with deionized water and blow dried in air. Step2:The samples were then coated with about 100nm of thermally grown silicon dioxide at a temperature of 1100 ℃. 5

Step3:A 100nm thick layer of amorphous silicon was then deposited using an E-beam evaporation system with the substrate temperature kept at 300 ℃ and a base pressure of 1.3×10 -4 Pa. Step4:The amorphous silicon-coated substrates were then placed in a direct-current plasma-enhanced-chemical- vapor-deposition (dc-PECVD) system to perform the hydrogenation–annealing sequence. 6

Step5:Specimens were prepared for plasma power densities ranging between 4.5 W/cm 2 and 6.5 W/cm 2 and substrate temperatures ranging between 350 °C and 450 °C for 30 min. Step6:The subsequent annealing step was conducted in- situ at a substrate temperature 70 °C higher than what was used for the hydrogenation step for a period of 35 min and three successive hydrogenation–annealing steps were applied to each of the samples. 7

Step7:During the hydrogenation step, the pressure of the reactor was maintained at 200 Pa and the hydrogen flow was set at 20 sccm. 8

A schematic drawing of the hydrogenation process in a dc-PECVDsystem nm dc-PECVD system

PECVD reactions system 10

Quantum confinement effects 11

SEM images of the surface of silicon hydrogenated at a power density of 6.5 W/cm 2 for 15 min and at temperatures of (left) 350 °C and (right) 400 °C. 12 Result of SEM

SEM images of the surface of silicon hydrogenated at temperature of 400 °C for 30 min and at plasma power densities of (left) 3.5 W/cm 2 and (right) 4.5 W/cm 2. Result of SEM 13

Collection of PL spectra from samples prepared at three temperatures of 350 °C, 375 °C and 400 °C and different plasma power densities during hydrogenation of 4.5 W/cm 2 and 6.5 W/cm 2. By raising the temperature a blue shift in the peak of the emitted light is observed while higher plasma powers result in a reduction in the light intensity. Result of PL spectrum 14

FTIR spectrum of a sample prepared on a silicon substrate without an interfacial oxide showing the presence of Si–O bonds. No clear evidence of Si–H is observed although a trace of Si–O–H bonds is visible in this image. Result of FTIR spectrum 15

(a) Dark-field TEM image of a sample prepared at 350 °C at a plasma power density of 6.5 W/cm 2 with the associated SADP inset. (b) Grain size distribution histogram as measured from the dark-field image. Result of TEM 16

(a) Dark-field TEM image of a sample prepared at 400 °C with a power density of 6.5 W/cm 2, and the associated (b) grain size distribution histogram. Result of TEM 17

(a) Bright-field and (b) dark-field cross-sectional TEM images of a sample prepared at 400 °C with a plasma power density of 4.5 W/cm 2. A selected area electron diffraction pattern of the sample was inserted in part (a)showing the ring pattern characteristic of a polycrystalline structure. The rectangle in part (b) represents the location of the high-resolution electron microscopy (HREM) image provided in part (c). (c) An HREM image showing Si {111} lattice fringes of the nano-sized silicon grains. Result of TEM and HREM 18

A plot of the PL peak emission wavelength against the peak in the average grain diameter distribution. The error bars are the standard deviations of the grain diameter distributions. 19

An optical image of the light-emitting behavior of patterned nanocrystalline porous Si under UV illumination showing the words “Thin Film”, as inserted in a box in the image. The patterning of this structure has been achieved by standard photolithography using positive resists. After patterning and developing the resist, the thin silicon film was removed by chemical etching. 20 Rusult of lithography

Conclusions In summary, we report a method for the fabrication of nanocrystalline porous silicon from a deposited amorphous silicon layer. The energetic hydrogen ions result in the formation of nano-sized grains which in turn leads to the creation of porous layers. 21

We believe that in the process of hydrogenation, hydrogen radicals replace the dangling bonds of the silicon atoms in the amorphous structure and when depassivating the previously hydrogenated bonds, energy is transferred to the silicon atom enhancing the chance for nucleation and growth of the nano- crystals. 22

Higher processing temperatures would lead to samples with a smaller average grain size, whereas lower plasma power densities resulted in more packed structures with a smaller surface structure size. 23

Thanks for your attention 24