Chemical mapping of indium rich quantum dots in InGaN/GaN quantum wells
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Chemical mapping of indium rich quantum dots in InGaN/GaN quantum wells N Sharma, H K Cho1, J Y Lee1 and C J Humphreys Department of Metallurgy and the Science of Materials, University of Cambridge, Pembroke Street, Cambridge, CB2 3QZ, U.K. 1 Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong,Yusong-gu, Taejon, 305-701, South Korea ABSTRACT Indium clustering in InGaN/GaN multiple quantum wells (MQWs) is believed to be responsible for the high luminescent efficiency of GaN based light emitting diodes. In this paper we show that substantial clustering can be induced by reducing to zero the interruption time between growth of the GaN barrier layer on the InGaN quantum well. Photoluminescence (PL) shows that this has the effect of increasing the luminescence intensity and decreasing the band gap energy (higher indium concentration). The clusters or quantum dots were examined and quantified by energy filtered transmission electron microscopy (EFTEM), which was used to form chemical distribution maps of indium, gallium and nitrogen. In this paper we will show that this technique can accurately calculate the indium concentration and distribution in the quantum wells. The calculations show that InxGa1-xN quantum dots (width = 1.3nm) exhibit an In concentration of up to x = 0.5, which are embedded in a quantum well matrix with x = 0.05.
INTRODUCTION It has been suggested that the high luminescent efficiency of InGaN/GaN MQWs is due to quantum dot type indium clusters in the InGaN layers [1,2]. This has been suggested be due to the low solubility of InN in GaN [3]. An important requirement for the determination of indium clusters is the accurate measurement of the indium concentration. The techniques which have been used to measure this are X-ray diffraction (XRD) [4], high resolution TEM (HRTEM) [5] and energy dispersive X-ray analysis (EDX) [6]. For most cases HRXRD and HRTEM are used to measure the (0002) c-spacing. As thin InGaN layers are grown pseudomorphically (aGaN = aInGaN) the c-spacing will be adjusted with the a-spacing by an amount determined by Poisson’s ratio, which will be different for GaN and InN. Also in the case of HRTEM, the thin samples deemed necessary change the projected lattice spacing through surface relaxation effects, making quantitative analysis difficult. Direct analysis using EDX in the STEM [6] is an excellent method for acquiring qualitative comparisons of local regions. However the N K peak is strongly absorbed by the detector window and hence cannot be used for calibration of indium concentrations. Chen et al. [7] used a combination of molecular beam epitaxial growth of thin InGaN layers and scanning tunneling microscopy to show that indium segregates to the growth surface to form a ’floating layer’ during growth due to weak In-N bonds compared to Ga-N bonds. They also observed N-vacancy nanopits, 5nm apart, which were claimed to relieve the quantum well strain. Total energy calculations showed a substantial energy reducti
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