formation of uniform high density and small size ge si quantum dots by scanning pulsed laser annealing of pre deposited ge si film

Formation of uniform high-density and small-size Ge/Si quantum dots by scanning pulsed laser annealing of pre-deposited Ge/Si film , Hamza Qayyum, Chieh-Hsun Lu, Ying-Hung Chuang, Jiunn-Yuan Lin, and Szu-yuan Chen Citation: AIP Advances 6, 055323 (2016); doi: 10.1063/1.4953057 View online: http://dx.doi.org/10.1063/1.4953057 View Table of Contents: http://aip.scitation.org/toc/adv/6/5 Published by the American Institute of Physics AIP ADVANCES 6, 055323 (2016) Formation of uniform high-density and small-size Ge/Si quantum dots by scanning pulsed laser annealing of pre-deposited Ge/Si film Hamza Qayyum,1,2,3 Chieh-Hsun Lu,1,2 Ying-Hung Chuang,1,4 Jiunn-Yuan Lin,4 and Szu-yuan Chen1,2,3,a Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan Department of Physics, National Central University, Zhongli, Taoyuan 320, Taiwan Molecular Science and Technology Program, Taiwan International Graduate Program, Academia Sinica, Taipei 115, Taiwan Department of Physics, National Chung Cheng University, Chiayi 621, Taiwan (Received 11 March 2016; accepted 19 May 2016; published online 25 May 2016) The capability to fabricate Ge/Si quantum dots with small dot size and high dot density uniformly over a large area is crucial for many applications In this work, we demonstrate that this can be achieved by scanning a pre-deposited Ge thin layer on Si substrate with a line-focused pulsed laser beam to induce formation of quantum dots With suitable setting, Ge/Si quantum dots with a mean height of 2.9 nm, a mean diameter of 25 nm, and a dot density of 6×1010 cm−2 could be formed over an area larger than mm2 The average size of the laser-induced quantum dots is smaller while their density is higher than that of quantum dots grown by using Stranski-Krastanov growth mode Based on the dependence of the characteristics of quantum dots on the laser parameters, a model consisting of laser-induced strain, surface diffusion, and Ostwald ripening is proposed for the mechanism underlying the formation of the Ge/Si quantum dots The technique demonstrated could be applicable to other materials besides Ge/Si C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/) [http://dx.doi.org/10.1063/1.4953057] I INTRODUCTION In recent years, the growth of quantum dots (QDs) embedded in a solid-state device has become an active research field because of their novel electrical and optical properties Among various materials, Ge/Si quantum dots is of particular interest due to its small band gap, non-toxicity, and compatibility with the well-developed Si technology Many applications of Ge/Si QDs have been explored, such as optoelectronic devices,1 solar cells,2 and thermoelectricity.3 Conventionally, Ge/Si QDs are grown by depositing Ge atoms on a Si substrate under the condition for Stranski-Krastanow (SK) growth mode The 4.2% lattice mismatch between the Si and Ge layers is the driving force for the dot formation In the beginning of Ge deposition, strain builds up but still accommodated Once the Ge layer thickness exceeds the critical thickness, the strain relaxes by forming Ge dots.4 The unassisted SK growth exhibits poor control over the evolution of quantum dots, since the dot nucleation is fluctuation-driven in nature This results in unsatisfactory and uncontrollable QD size and density, inadequate for fabricating optoelectronic devices.5 Therefore, it is critical to have a way for a high degree of control over formation of QDs To date, the most promising approach for control of the size and density of QDs appears to be assisted SK growth by templating the underlying Si layer using various methods such as nano-indentation and lithography with electron or ion beams.6–8 In these methods, lithographically or mechanically produced pits serve as preferred nucleation sites for deposited atoms However, the a E-mail: sychen@ltl.iams.sinica.edu.tw 2158-3226/2016/6(5)/055323/11 6, 055323-1 © Author(s) 2016 055323-2 Qayyum et al AIP Advances 6, 055323 (2016) QD size and density are limited by the resolution of the lithographic instruments, and the process is time-consuming It has also been reported that QD size can be reduced and QD density increased by surfactant-mediated growth of QDs using sub-monolayer Sb or C,9–11 or ultra-thin SiO2 layer.12 Nevertheless, such methods inevitably introduce interfacial species which may modify the electronic structure of Ge/Si QDs On the side of top-down approaches, Dais et al has recently reported the growth of densely packed QD arrays with a density of 8.16 × 1010 cm−2 by using extreme ultraviolet interference lithography.13 However, this method relies on large synchrotron radiation facility and thus are not time- and cost-effective The close proximity between adjacent QDs also results in electronic coupling between QDs, rendering this method unsuitable for many applications Pulsed laser annealing is an emerging material fabrication method based on localized photochemical and photothermal reaction Unlike conventional thermal annealing, which heats up the whole device, the irradiation with a pulsed laser exerts thermal annealing only in the region requiring the treatment without causing a detrimental effect in other regions of the device The selective heating of the surface by pulsed laser enables processing of the targeted layer/region on any kind of substrate Motivated by these advantages, pulsed laser annealing has been used for producing nanodots or nanocones on a semiconductor surface.14–18 However, in all of the previous works reporting on pulsed laser annealing of semiconductors, the fabrication of nanostructures is limited to a small area Uniformity over a large area is hard to achieve in these methods because it is hard to get a large laser beam profile with the required smoothness Therefore, these methods lead to bad uniformity of the nanostructures on chip-size scale and thus are not ideal for fabricating optoelectronic devices In this paper, we report a method termed scanning pulsed laser annealing (SPLA) for fabricating Ge/Si QDs with small dot size and high dot density uniformly over a large area It is achieved by scanning a pre-deposited Ge thin layer on Si substrate with a line-focused pulsed laser beam to induce formation of Ge/Si QDs With suitable setting, Ge/Si quantum dots with a mean height of 2.9 nm, a mean diameter of 25 nm, and a dot density of 6×1010 cm−2 could be formed over an area larger than mm2 The average size of the laser-induced QDs is smaller while their density is higher than that of QDs grown by using Stranski-Krastanov growth mode Based on the dependence of the characteristics of QDs on the laser parameters, a model consisting of laser-induced strain, surface diffusion, and Ostwald ripening is proposed for the mechanism underlying the formation of the Ge/Si QDs II EXPERIMENTAL The Ge/Si QD samples were made by first depositing thin Ge layers on Si substrates and then subjected the Ge/Si films to scanning pulsed laser annealing of various conditions to induce formation of Ge dots on the Si substrates The two processes were carried out in the same vacuum chamber, which was evacuated to < × 10−4 torr for both processes Pulsed laser deposition (PLD) was used to deposit the Ge layers A Ge disk (purity >99.99%) of 10-mm diameter and 5-mm thickness was used as the PLD target The Si (100) substrates were cleaned by the sequence of dipping in a 0.56 M HF solution for min, dipping in a 0.28 M HF solution for min, flushing with deionized water, rinsing with isopropyl alcohol, and drying with nitrogen gas, right before being loaded into the vacuum chamber The Ge target was mounted on a motorized holder, which was controlled by a computer program to automatically rotate or translate the target after each laser shot This provided a different location for the next ablation pulse in order to avoid formation of large craters and thus change of on-target laser alation parameters A third-harmonic Q-switched Nd:YAG laser of 355-nm wavelength, 8-ns pulse width, and 10-Hz repetition rate was used as the ablation laser The p-polarized ablation beam was focused with a spherical lens onto the Ge target at 45◦ incidence angle The on-target beam size was set to 300 m (vertical)ì420 µ m (horizontal) in clear aperture, resulting in a peak laser fluence of 120 J/cm2 The 1-cm×1-cm substrates were mounted on a motorized carousel-type holder which contained six substrate mounts, allowing for switching of the substrates under vacuum A mask was installed in front of the substrate holder to shield the substrates except for the one being coated The PLD substrate temperature was set at 400 ◦C for all the samples used in this experiment The substrates were positioned in the normal direction of the target, and the target-to-substrate distance was set at cm 055323-3 Qayyum et al AIP Advances 6, 055323 (2016) The deposition rate was 0.03 nm/s, which was measured with a quartz microbalance (SQM-160, Sigma Instruments) calibrated by using a surface profiler for film thickness measurement SPLA were carried out by using a second-harmonic Q-switched Nd:YAG laser of 532-nm wavelength, 10-ns pulse width, and 10-Hz repetition rate The p-polarized annealing beam was expanded with a plano-concave cylindrical lens in the vertical direction and shrunk with a planoconvex cylindrical lens in the horizontal direction to produce a line-shaped beam of 20 mm (vertical)×0.5 mm (horizontal) in full width at half maximum (FWHM) on the surface of the PLD-produced Ge/Si films with an incidence angle of 45◦ The Ge/Si films were mounted on a motorized holder that could move in the horizontal direction to allow scanning of the Ge/Si films by the annealing laser beam at a variable scan speed The Ge/Si films were kept at room temperature during pulsed laser annealing Atomic force microscopy (AFM) (NanoWizard II, JPK Instruments) with a cantilever of