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JOURNAL OF MATERIALS SCIENCE: MATERIALS IN ELECTRONICS 13 (2002) 295±298 Formation of ZnSe 1 x S x quantum dots under Volmer±Weber mode C. X. SHAN, X. W. FAN*, J. Y. ZHANG, Z. Z. ZHANG, Y. M. LU, Y. C. LIU, D. Z. SHEN, X. G. KONG Open Laboratory of Excited State Processes, Chinese Academy of Sciences, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, No. 1 Yan An Road, Changchun, 130021, People's Republic of China E-mail: [email protected] ZnSe 1 x S x quantum dots (QDs) grown directly onto GaAs substrates in Volmer±Weber (V±W) mode by low-pressure metallorganic chemical vapor deposition (MOCVD) are investigated. The value of x is determined by the photoluminescence (PL) spectra. The formation of QDs was con®rmed by atomic force microscopy (AFM) images. When the growth duration is short, the surface of the substrate is full of pyramid-like dots; while increasing the duration, another kind of dome-like dots with larger size appear. Meanwhile, the density of the dots decreases with increasing growth duration. The above facts are explained systematically taking into account the surface free energy. The PL characteristics of the dots are also assessed. With increasing growth duration, the emission peak of the dots shows an obvious red-shift, which is due to a decrease in quantum con®nement effect. # 2002 Kluwer Academic Publishers 1. Introduction Numerous attempts have been made to grow semicon- ductor nanostructures, such as quantum well and quantum dots (QDs). Quantum dots are especially highlighted because they can provide three-dimensional (3D) con®nement to electrons and excitons, thus providing lower threshold current density higher gains and higher quantum ef®ciency compared to 2D quantum wells and 1D quantum wires [1]. Several state-of-the-art methods have been applied to fabricate QD structures, high-resolution lithography combined with dry or wet etching [2, 3] and growth on masked substrates [4] are two cases in point. Unfortunately, such processes inevitably deteriorate the quality of the crystal. Recent reports on the growth under Stranski±Krastanow (S±K) mode offer possibilities for growing high quality QD structures [5±8]. However, growth under S±K mode needs precise control over the growth rate and the ®lm thickness, which is arduous for some growth systems such as vapor phase epitaxy (VPE), hot wall epitaxy (HWE), metallorganic chemical vapor deposition (MOCVD) and so on. Furthermore, growth under S±K mode requires a large lattice mismatch between epilayer and substrate. Therefore, it cannot be applied to grow low sulfur content ZnSeS QDs on GaAs substrates (lattice mismatch between ZnSe 0:88 S 0:12 and GaAs 5 0.3%). In the present paper, a relatively simple method, Volmer± Weber mode, in which a rough layer was introduced ®rst, and then the QDs were formed on the rough surface, was carried out to fabricate ZnSe 1 x S x QDs by MOCVD. ZnTe and ZnSe QDs structure have been grown successfully under this growth mode [9±11]. However, the formation process was not available at all. In this paper, the formation process of the QDs was also studied preliminarily from the surface morphologies and photo- luminescence characteristics. 2. Experimental The samples were prepared on GaAs (100) substrates in a horizontal low-pressure MOCVD system at 350 C, and the growth pressure was ®xed at 210 mm Hg. The precursors were dimethylzinc (DMZn) hydrogen sul®de (H 2 S) and hydrogen selenide (H 2 Se), respectively. The growth rate determined by the scanning electron microscopy (SEM) was 13 A Ê /s for the growth of the ZnSeS layers. The GaAs substrates were chemically polished in H 2 SO 4 : H 2 O 2 : H 2 O 3 : 1 : 1 at 40 C, and then boiled in hydrochloric acid. After being loaded into the reaction chamber, the substrates were annealed at 600 C in hydrogen ambience for 10 min to avoid the gallium, and arsenic oxide layer. The surface morphol- ogies of our samples were characterized by atomic force microscopy (AFM). The photoluminescence (PL) spectra were excited by the 325 nm line of a He±Cd laser at room temperature and the signals were measured by a Jobin± Yvon-630 micro-Raman spectrograph. *Corresponding author. 0957±4522 # 2002 Kluwer Academic Publishers 295

Formation of ZnSe1−x Sx quantum dots under Volmer–Weber mode

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