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Size and density control of In droplets at near room temperatures
Lee, J H,Wang, Zh M,Kim, N Y,Salamo, G J IOP Pub 2009 Nanotechnology Vol.20 No.28
<P>We report on the ability to control the size and density of In droplets on GaAs(100) substrates at near room temperatures using solid source molecular beam epitaxy. We specifically demonstrate the height, diameter and density control of In droplets as functions of substrate temperature (<I>T</I><SUB>sub</SUB>) and monolayer (ML) coverage. For a range of density (∼10<SUP>9</SUP>–10<SUP>10</SUP> cm<SUP>−2</SUP>), the growth window is revealed to be between 20 and 70 °C. For a fixed ML coverage, the size and density of droplets can be controlled by controlling the <I>T</I><SUB>sub</SUB>. For a fixed <I>T</I><SUB>sub</SUB>, by controlling the ML coverage, droplet size and density can be controlled. Even at near room temperatures (20–70 °C), In atoms are extremely sensitive to surface diffusion and this enables the control of the size and density of droplets. This study provides an aid to understanding the formation of In droplets at near room temperatures and can find applications in the formation of quantum structures and/or nanostructures based on droplet epitaxy.</P>
Origin of nanohole formation by etching based on droplet epitaxy
Li, X.,Wu, J.,Wang, Z.,Liang, B.,Lee, J.,Kim, E. S.,Salamo, G. Royal Society of Chemistry 2014 Nanoscale Vol.6 No.5
Creating and manipulating materials at the nanoscale with controllable size, shape and nucleation site is an important task to meet the urgent demands for quantum structures with designed properties. In the last ten years, droplet epitaxy has been emerging as a versatile fabrication method for various complex nanostructures, such as quantum dots, quantum rings, double-rings, and so on. However, there is a lack of understanding of the deep nanohole formation based on droplet epitaxy at a high substrate temperature. Here we fabricate self-organized GaAs nanoholes by Ga droplet etching at high temperature based on droplet epitaxy, and they present good optoelectronic properties and have promising applications in fabrication of nanodevices due to their unique topology. A theoretical model is correspondingly proposed to explain the basic mechanism and simulate the time evolution of the nanohole structures. Our analysis shows that the morphology of the nanohole nanostructures can be well controlled through regulating experimental conditions.
Effects of rapid thermal annealing on the optical properties of strain-free quantum ring solar cells
Wu, Jiang,Wang, Zhiming M,Dorogan, Vitaliy G,Li, Shibin,Lee, Jihoon,Mazur, Yuriy I,Kim, Eun Soo,Salamo, Gregory J Springer 2013 Nanoscale research letters Vol.8 No.1
<P>Strain-free GaAs/Al<SUB>0.33</SUB>Ga<SUB>0.67</SUB>As quantum rings are fabricated by droplet epitaxy. Both photoresponse and photoluminescence spectra confirm optical transitions in quantum rings, suggesting that droplet epitaxial nanomaterials are applicable to intermediate band solar cells. The effects of post-growth annealing on the quantum ring solar cells are investigated, and the optical properties of the solar cells with and without thermal treatment are characterized by photoluminescence technique. Rapid thermal annealing treatment has resulted in the significant improvement of material quality, which can be served as a standard process for quantum structure solar cells grown by droplet epitaxy.</P>
State filling dependent luminescence in hybrid tunnel coupled dot-well structures.
Mazur, Yuriy I,Dorogan, Vitaliy G,Ware, Morgan E,Marega, Euclydes,Benamara, Mourad,Zhuchenko, Zoryana Ya,Tarasov, Georgiy G,Lienau, Christoph,Salamo, Gregory J RSC Pub 2012 Nanoscale Vol.4 No.23
<P>A strong dependence of quantum dot (QD)-quantum well (QW) tunnel coupling on the energy band alignment is established in hybrid InAs/GaAs-In(x)Ga(1-x)As/GaAs dot-well structures by changing the QW composition to shift the QW energy through the QD wetting layer (WL) energy. Due to this coupling a rapid carrier transfer from the QW to the QD excited states takes place. As a result, the QW photoluminescence (PL) completely quenches at low excitation intensities. The threshold intensities for the appearance of the QW PL strongly depend on the relative position of the QW excitonic energy with respect to the WL ground state and the QD ground state energies. These intensities decrease by orders of magnitude as the energy of the QW increases to approach that of the WL due to the increased efficiency for carrier tunneling into the WL states as compared to the less dense QD states below the QW energy.</P>
Low-Density Quantum Dot Molecules by Selective Etching Using in Droplet as a Mask
Jihoon Lee,Wang, Z M,Hirono, Y,Dorogan, V G,Mazur, Y I,Eun-Soo Kim,Sang-Mo Koo,Seunghyun Park,Sangmin Song,Salamo, G J IEEE 2011 IEEE TRANSACTIONS ON NANOTECHNOLOGY Vol.10 No.3
<P>We demonstrate low-density quantum dot molecules (QDMs) by selective etching using In droplets as a mask. Selective etching is performed with InGaAs QDMs buried underneath GaAs capping layer, on which In droplets are formed by droplet epitaxy using molecular beam epitaxy. During the chemical etching, the droplets act as a mask and QDMs underneath the droplets that only survive. Photoluminescence measurement from the selectively etched QDMs in mesa structures shows a much reduced intensity, which indicates low-density QDMs. This technique provides a simple and flexible method to attain low-density QDMs. The density can be easily modified by the control of the size and density of In droplets, which is suitable for single QDM spectroscopy and for their device applications.</P>
InGaAs quantum dot molecules during selective etching using an In droplet mask
Lee, Jihoon,Wang, Zhiming,Hirono, Yusuke,Kim, Eun-Soo,Koo, Sang-Mo,Dorogan, Vitaliy G,Mazur, Yuriy I,Song, Sangmin,Park, Gamyoung,Salamo, Gregory J Institute of Physics [etc.] 2011 Journal of Physics. D, Applied Physics Vol.44 No.2
<P>We investigated the optical transition of InGaAs quantum dot molecules (QDMs) during selective etching of GaAs using In droplets to demonstrate low-density QDMs. During the selective etching, In droplets act as nanoscale masks and only QDMs underneath the droplets survive, by which process low-density QDMs are fabricated. The thickness of selective GaAs etching is systematically varied and a gradual red-shift is observed with the increased etching thickness. The continuing red-shift can be explained by the strain relaxation due to GaAs etching. This technique to achieve low-density QDMs by selective etching using droplets as nanoscale mask is a simple and flexible approach. This study can find applications in single QDM spectroscopy and other spectroscopic techniques.</P>
Jihoon Lee,Wang, Z M,Eun-Soo Kim,Nam-Young Kim,Seung-Hyun Park,Salamo, G J IEEE 2011 IEEE TRANSACTIONS ON NANOTECHNOLOGY Vol.10 No.3
<P>The evolution of self-assembled InGaAs tandem nanostructures consisting a hole and pyramid is demonstrated using droplet epitaxy on various type-A high-index GaAs surfaces: (3 1 1)A (4 1 1)A, (5 1 1)A and (7 1 1)A. Under an identical fabrication condition depending on the index of surfaces, the resulting density and size of nanostructures are characteristic. The variation of density and size of nanostructures is explained with the relationship of the density of monolayer steps. An empirical model that describes the mechanism of self-assembled tandem nanostructures consisting a hole and pyramid is suggested as the concurrent occurrence of intermixing between droplets and substrate, dissolution of substrate and anisotropic surface diffusion.</P>
Lei Gao,Hirono, Y.,Ming-Yu Li,Jiang Wu,Sangmin Song,Sang-Mo Koo,Eun-Soo Kim,Wang, Z. M.,Jihoon Lee,Salamo, G. J. IEEE 2012 IEEE TRANSACTIONS ON NANOTECHNOLOGY Vol.11 No.5
<P>A sharp contrast on the density and size of Ga metal droplets is observed on various photolithographically patterned GaAs (100). As clearly evidenced by high-resolution scanning electron microscope, Ga metal droplet density and size surprisingly differ on etched and unetched surfaces under an identical growth condition. The apparent contrast on the density and size of droplets is clearly observed at the interface between etched and unetched areas. Ga droplets exhibit much higher density and the size is much smaller on etched surface; meanwhile, the density is an order of magnitude lower and the size is much larger on unetched surface. Along different directions, [011] and [01-1], due to anisotropic surface diffusion, the density is about twice higher along [011] for the same strip pattern.</P>