Reflectivity and Photoreflectivity Spectra of Structures with Quantum Wells Based on ZnO
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Reflectivity and Photoreflectivity Spectra of Structures with Quantum Wells Based on ZnO A. M. Klyueva, N. G. Filosofovb, A. Yu. Serovb, V. F. Agekyanb, C. Morhainc, and V. P. Kochereshkoa, * a
Ioffe Institute, St. Petersburg, Russia St. Petersburg State University, St. Petersburg, Russia c Centre de Recherche sur l’Hétéro-Epitaxie et ses Applications CRHEA-CNRS, Sophia–Antipolis–Valbonne, France *e-mail: [email protected] b
Received June 13, 2020; revised June 13, 2020; accepted June 16, 2020
Abstract—Reflectivity and modulated photoreflectivity spectra of ZnO and Zn1 – xMgxO epitaxial layers as well as ZnO/Zn1 – xMgxO quantum wells have been studied at normal and oblique light incidence. Comparison of experimental and calculated spectra allowed refinement of certain parameters of excitons in these structures and the order of energy bands in ZnO. Keywords: heterostructures, quantum wells, excitons, spectroscopy DOI: 10.1134/S1063783420110165
1. INTRODUCTION Wide-bandgap semiconducting compounds such as ZnO, GaN, MgO, and AlN are extremely promising materials for optoelectronics. Due to wide band gap, these structures are convenient for the fabrication of semiconductor lasers operating in UV range and superhigh-voltage transistors. These structures due to piezoelectric effect frequently possess built-in electric field with strength up to several million volts per centimeter [1]. At the same time, these materials contain rather high concentration of impurities up to 1018 cm–3, which leads to the partial screening of this built-in field. As a result, the value of the built-in field varies depending on structure type and even on investigation conditions. Therefore, many properties of exciton and trion states are dependent on the built-in electric field and proved to be poorly controllable. Even interpretation of exciton and trion lines in the spectra of these structures is unambiguous. There is considerable discrepancy in the reported values of exciton parameters [2]. Moreover, since the 1960s [3, 4], the order of energy bands in zinc oxide and heterostructures based on ZnO is under discussion until now [5]. This fact substantiates the urgency of studies of these materials. In this work, we experimentally and theoretically studied reflectivity and photoreflectivity spectra at normal and oblique light incidence from heterostructures containing both ZnO and Zn1 – xMgxO layers and ZnO/Zn1 – xMgxO quantum wells at x = 0.22.
2. EXPERIMENT AND RESULTS A buffer ZnO layer of about 1 μm thick was grown on a sapphire substrate; next, Zn1 – xMgxO barrier layer of 200 nm thick was grown; after that, ZnO quantum well of 7.1 nm thick, another Zn1 – xMgxO barrier layer of 200 nm thick, ZnO quantum well of 2.7 nm thick, and Zn1 – xMgxO covering layer of 200 nm thick were grown. Reflectivity spectra were registered using a halogen lamp and an MDR-406 monochromator equipped with a photomultiplier in photon counting mode, sample temperature was 5 K in all experiments. Protoreflectivity spectra were record
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