Properties of Iodine-Doped CdTe Layers on (211) Si Grown at High Substrate Temperatures by MOVPE
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https://doi.org/10.1007/s11664-020-08420-3 Ó 2020 The Minerals, Metals & Materials Society
TOPICAL COLLECTION: U.S. WORKSHOP ON PHYSICS AND CHEMISTRY OF II-VI MATERIALS 2019
Properties of Iodine-Doped CdTe Layers on (211) Si Grown at High Substrate Temperatures by MOVPE M. NIRAULA ,1,2 K. YASUDA,1 R. TORII,1 Y. HIGASHIRA,1 R. TAMURA,1 B.S. CHAUDHARI,1 T. KOBAYASHI,1 H. GOTO,1 S. FUJII,1 and Y. AGATA1 1.—Graduate School of Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 4668555, Japan. 2.—e-mail: [email protected]
The properties of iodine-doped (211) CdTe layers grown on (211) Si substrates by metalorganic vapor-phase epitaxy at high substrate temperatures from 325°C to 450°C were studied. The growth rate of the doped layer increased with increasing substrate temperature before reaching a maximum value of 2.6 lm/h at 425°C, after which it decreased slightly. On the other hand, the room-temperature electron density showed a strong dependence on the Te/Cd precursor flow-rate ratios, where the electron density increased with a decreasing Te/Cd ratio. The highest electron density of 2.591018 cm3 was obtained by growing the epilayer at a substrate temperature of 400°C and Te/ Cd ratio of 0.05. This was considered to be due to decreased donor compensation at a small Te/Cd ratio. Good correspondence was observed between the results obtained from Hall measurements and photoluminescence measurements. Key words: CdTe epilayers, iodine doping, high substrate temperature, Si substrate, doping mechanism
INTRODUCTION Epitaxial growth of single-crystal CdTe on largearea readily available substrates such as GaAs or Si is a promising way to obtain large-area crystals that are required for x-ray, gamma-ray, or infrared (IR) device technology. Actual device applications require highly doped materials with controllable electrical properties. For example, x-ray and gamma-ray detectors that are fabricated in a pCdTe/n-CdTe/n+-Si heterojunction diode structure require a thick and highly doped n-CdTe layer in order to reduce detector dark current.1,2 The nCdTe/n+-Si heterojunction contains a large number of dislocations resulting from the large lattice mismatch and difference in thermal expansion coefficients between the two materials. Dislocations are well known as potential sources of increased
(Received February 2, 2020; accepted August 14, 2020)
diode dark currents.3,4 The dark current can be reduced by suppressing the spread of the depletion layer towards the n-CdTe/n+-Si junction, which can be achieved by making n-CdTe thicker and with high carrier concentrations. There are several reports on n-type doping of CdTe epilayers using iodine or indium as dopant, with room-temperature electron density exceeding 1018 cm3.5–7 Most of those studies, however, were performed at a low substrate temperature (< 230°C) on either CdTe or CdZnTe substrates in molecular beam epitaxy (MBE) growth. The low-temperature growth was performed to suppress the formation of native defects and hence to enhance dopant activity.5–7
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