Electroreflectance Spectra of InGaN/AlGaN/GaN p-n-Heterostructures
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Electroreflectance Spectra of InGaN/AlGaN/GaN p-n-Heterostructures Alexander E. Yunovich1, Lev Avakyants1, Mansur Badgutdinov1, Pavel Bokov1, Anatoly Chervyakov1, Stanislav Shirokov1, Elena Vasileva2, Anatoly Feopentov2, Fedor Snegov2, Dmitry Bauman2, and Boris Yavich2 1 Dept. of Physics, M.V.Lomonosov Moscow State University, Leninskie gory, Moscow, 119992, Russian Federation 2 “Svetlana-Optoelectroniks” JSC, Saint-Petersburg, 194156, Russian Federation ABSTRACT
Electroreflectance (ER) spectra of InGaN/AlGaN/GaN p-n- heterostructures with multiple quantum wells (MQW) are studied. Structures with MQW InGaN/GaN were grown for blue LEDs by MOCVD technology and “flip-chip” mounted. The ER spectral maxima correspond to the high energy side of electroluminescence spectral line. The ER spectra caused by FranzKeldysh effect are approximated by Aspnes theory. The ER spectra in a range 400 ÷ 800 nm have interference bands caused by the change of refraction index in the structure.
INTRODUCTION
InGaN/AlGaN/GaN structures grown for light-emitting diodes (LEDs) give a possibility to study p-n- heterostructures with quantum wells by the methods of modulation spectroscopy. If they are flip-chip mounted by p- side to the heat sink ([1, 2]) it is possible to light up the structure through the transparent substrate and n- layer and observe reflectance spectra from the structure as a whole. It is possible to change electric field in the active layer of the junction by applying reverse voltage to the contacts. The electric field changes energy spectra in quantum wells, changing reflectance spectra. Methods of photoreflectance studies of AIIIBV heterostructures with quantum wells were previously described elsewhere [4]. The photoreflectance and contact free electroreflectance (ER) were used for study InGaN/GaN/AlGaN heterostructures with quantum wells [4, 5]. The ER method is advanced here for optical studies of LED’s InGaN/AlGaN/GaN p-n- heterostructures described in [2]. EXPERIMENT
InGaN/AlGaN/GaN p-n- heterostructures for blue LEDs were grown by MOCVD on sapphire substrates and flip-chip mounted by p- side to the heat sink. The structure had following layers: the buffer i-GaN, the width < 0.5 µm; n-GaN - 3.5 µm; 5 quantum wells/barriers InGaN/GaN (active MQW region) - ≅0.03/0.12 µm; bloking barrier i-GaN (samples #4 and #80) or p-AlGaN (samples #119 and #120) - ≅0.02 µm and a cap p-GaN of ≅0.11 µm. A reflective mirror was made on the p-side of the LED. The area of the crystal was ≅0.4 мм2. The structures were lighted through the transparent sapphire substrate. The optical reflection was modulated by periodic (370 Hz) rectangular pulses; bias voltage was changed in the range -7 V ÷ +2 V. Electroluminescence spectra were studied at room temperature by the methods described in [6]. Fig 1 shows electroreflectance spectra of four samples measured at reverse bias +1.8 V and amplitudes of pulses -7.8 V. Spectrum of the sample #80 has only interference bands in the range 1.4 ÷ 2.0 eV. Spectra of the samples #4, #119, #120 ha
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