On the dynamics of InGaN dot formation by RF-MBE growth
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On the dynamics of InGaN dot formation by RF-MBE growth Tomohiro Yamaguchi, Sven Einfeldt, Stephan Figge, Carsten Kruse, Claudia Roder and Detlef Hommel Institute of Solid State Physics, University of Bremen, P.O. Box 330440, 28334 Bremen, Germany.
ABSTRACT The growth of InGaN on GaN (0001) by plasma-assisted molecular beam epitaxy was investigated with special focus on the dynamics of the formation of the dots. A metastable 2D growth regime, where the surface changes from smooth to rough by thermal treatment during growth interruption, existed previous to the 2D-3D transition. Both small regular-shaped dots and large irregular-shaped islands were observed. The large islands were suppressed by choosing correct growth conditions. The critical thickness for the transition from 2D to 3D growth also depended on the growth conditions. The growth of GaN capping layer to cover InGaN dotstructure was also attempted.
INTRODUCTION Group-III nitride semiconductors have been investigated as their materials with a wide range of band gaps which makes them suitable for various optoelectronic devices.1,2 In particular, InGaN quantum wells (QWs) have been used as an active layer of optical devices, and have led to the commercialization of light emitting diodes (LEDs) and laser diodes (LDs). The radiative recombination in these structures has been well known to result from localized carriers which were attributed to indium-rich clusters in the InGaN exhibiting a quantum dot (QD)-like behavior.3-5 Such QDs have been predicted to improve the performance of light emitting devices such as LEDs and LDs.6 At the moment, the self-assembled growth of InGaN QDs on GaN with indium mole fractions of around 0.2 to 0.4 by plasma-assisted7,8 and ammonia-source9,10 molecular beam epitaxy (MBE) as well as metalorganic vapor phase epitaxy (MOVPE)11,12 has been reported. However, still there are many open fundamental questions concerning the growth mechanisms, especially for InGaN with a high indium mole fraction. There has been actually few report for high indium mole fraction.13 A deeper understanding of the growth mechanism of InGaN QDs will help to optimize their growth conditions and to exploit their potential in devices. In this paper, the epitaxial growth of relatively indium-rich InxGa1-xN dots (x = 0.4∼0.7) on GaN (0001) templates has been investigated with special focus on the dynamics of the island formation. In-situ measurements by reflection high-energy electron diffraction (RHEED) enabled us to propose a metastable 2D growth regime of InGaN on GaN. The growth condition dependences, especially on a growth temperature and a growth rate, to form dot structure were also studied.
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EXPERIMENTAL DETAILS InGaN was grown in a conventional MBE system (EPI 930) by supplying gallium and indium from hot-lip effusion cells and supplying nitrogen through a radio frequency (RF) plasma source (EPI Unibulb). 2-µm thick GaN layers previously deposited by MOVPE on sapphire (0001) substrates served as templates for the MBE growth. Their th
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