Profiling electric fields in GaN/InGaN/GaN single quantum wells by electron holography
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Profiling electric fields in GaN/InGaN/GaN single quantum wells by electron holography Juan Cai, M. R. McCartney and F. A. Ponce Department of Physics and Center for Solid State Science Arizona State University, Tempe, AZ 85287-1504 ABSTRACT We use electron holography to profile the local internal potential due to spontaneous polarization and piezoelectric effects in strained quantum well structures of wurtzitic group III nitrides. Profiles of the electrostatic potential across a GaN/InxGa1-xN/GaN quantum well structure show the existence of internal electric fields of about –2.2 ± 0.6 MV/cm, and a potential drop across the quantum well of 0.6 ± 0.16 V. The electric fields indicate an average indium composition of 15% in the quantum well, for a thickness of 2.7 nm. This indium composition compares well with measurements by energy-disperse spectroscopy of 18 ± 2 %. Screening effect is not observed under these experimental conditions. INTRODUCTION It is well known that large polarization fields exist in strained heterostructures of the wurtzitic group III nitrides [1]. These fields result from the polar nature of these materials in combination with the strain that is due to the large lattice mismatch within the InGaN ternary system. Reports in the literature use these fields and the resulting quantum-confined Stark effect (QCSE, as shown in Fig. 1) to explain the optical characteristics associated with InGaN quantum wells (QWs), such as peak broadening, red shifts, long carrier lifetimes, etc. [2]. Therefore, the measurement of these fields is critical for understanding the light emission of InGaN QWs. Theoretical calculations by Bernardini, on the spontaneous and piezoelectric polarization for group III nitride binaries using ab-initio methods [1], predict the existence of internal electric fields from 0 to –15MV/cm for isolated InxGa1-xN QWs with GaN barriers (Negative values indicate r that the polarization is in the - c direction). The value of the internal electric field in InGaN wells can be inferred from experimental methods, such as photoluminescence, photoreflectance [2,3], and electron holographic techniques [4]. A comparison between theoretical and experimental values, shown in Fig. 2, reveals large discrepancies, and indicates the need for a more accurate method to map the potential distribution across the InGaN QWs. Cherns et al. studied the internal fields in GaN/In0.52Ga0.48N/GaN quantum wells (QW) using electron holography (EH) and reported the observation of an internal field of the order of 4 MV/cm [4]. McCartney recently reported the use of EH to profile the potential across an AlGaN/InGaN/AlGaN diode with a 44 nm well and attributed the differences from the predicted energy profiles to the masking of the expected electric field by free charge carriers in this wide well structure [5]. In this paper, we use EH to analyze a thin InGaN QW structure, and report the observation of large electric fields in the quantum well. G11.20.1
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∆V=Ed E
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(a) h ω ≈ Eg
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h ω < Eg
Fig. 1. Band alignment
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