Electroreflectance and Photoreflectance Studies of Electric Fields in Pt/GaN Schottky Diodes and AlGaN/GaN Heterostructu
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Electroreflectance and Photoreflectance Studies of Electric Fields in Pt/GaN Schottky Diodes and AlGaN/GaN Heterostructures S. Shokhovets1, R. Goldhahn1, G. Gobsch1, O. Ambacher2, I.P. Smorchkova3, J.S. Speck3, U. Mishra3, A. Link4, M. Hermann4 and M. Eickhoff4 1 Institute of Physics and 2Center for Micro- and Nanotechnologies, Technical University Ilmenau, 98684 Ilmenau, Germany 3 Electrical and Computer Engineering Department and Materials Department, University of California, Santa Barbara, CA 93106, U.S.A. 4 Walter Schottky Institute, Technical University Munich, 85748 Garching, Germany
ABSTRACT We have performed electroreflectance and photoreflectance studies of Pt/GaN Schottky diodes with Ga- and N-face polarity as well as AlGaN/GaN based transistor heterostructures. The experimental data were analyzed using electric field-dependent dielectric functions of GaN and AlGaN. Inhomogeneities in the electric fields were taken into account by application of a multi-layer formalism. We observed an increase of the electric field strength underneath the Schottky contact and in the AlGaN barrier with increasing temperature. The results are explained in terms of temperature dependent densities of ionized impurities and surface charges.
INTRODUCTION A detailed investigation of electric fields caused by Schottky contacts as well as the piezoelectric and spontaneous polarization of wurtzite group III nitrides is crucial to understand the performance of GaN based devices. Among the methods suitable for a direct measurement of the electric field strength, electroreflectance (ER) and photoreflectance (PR) have proven their high sensitivity and usefulness [1]. These methods utilize the dependence of the dielectric function (DF) of a material on the electric field strength F. However, investigations of particular mechanisms of electro-optical response and systematic applications of ER and PR methods to nitrides are rare up to date. In most cases, Franz-Keldysh oscillations (FKO) are employed for optical determination of the electric field strength [2-4]. It should be noted that strong excitonic effects are characteristic of nitrides which are not included into the one-electron FKO picture. A more general approach must be used which takes into account the electric field-dependent contributions of discrete excitons and excitonic continuum to the DF [5-9]. Quenching of discrete excitons in high electric fields leads to the formation of an exciton dead layer (EDL). Recently [9] it has been shown that the resulting spatial changes of optical constants along a depletion region determine the shape of ER spectra for GaN Schottky diodes leading to so-called “rotation” ER spectra. In addition, the linear electro-optic (LEO) effect has been found to dominate below the band gap [10]. In this paper, we focus on the determination of electric fields in Pt/GaN Schottky diodes and in barriers of AlGaN/GaN heterostructures using the electric field-dependent DF.
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SAMPLES AND EXPERIMENTAL DETAILS The diodes were formed on 1.5 µm
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