Surface structure and polarization effects in GaN Thin Films as studied by Electric Force Microscopy
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Surface structure and polarization effects in GaN Thin Films as studied by Electric Force Microscopy K. M. Jones, P. Visconti 1, F. Yun, M. A. Reshchikov, A. A. Baski, and H. Morkoç Virginia Commonwealth University, Dept. of Electrical Engineering, Richmond, VA, 23284 1
Also with: Istituto per lo Studio di Nuovi Materiali per l’Elettronica, CNR, 73100, Lecce, ITALY
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ABSTRACT Stress induced piezoelectric fields in III-nitride semiconductors induce charge at the surface that is comparable to the free carrier concentration intended by doping. These fields, coupled with mixed Ga-polarity and N-polarity within the film, are detrimental to device performance due to an additional scattering caused by a reversal of the electric field normal to the surface. Consequently, investigation of the polarization effects is important and timely as nitride semiconductor devices are being contemplated for system applications. By imaging the surface potential by electric force microscopy (EFM), both a qualitative and a quantitative measure of polarization induced charge distribution across the surface could be obtained. In this vein, we undertook an EFM study in a series of samples. EFM images of as-grown and progressively etched samples not only led to the identification and imaging of inversion domains, but also showed the evolution of different polarities. The polarization of the surface was calculated and was found to agree reasonably well with the values found experimentally. The role of polarization was addressed by extraction of the second harmonic term from EFM signal. INTRODUCTION The increased use of III-N semiconductors in commercial electronic devices has led to a need for a better understanding of material defects that can hinder device performance [1, 2, 3, 4]. One such type of defect is the inversion domains, or regions of mixed polarity in a III-N film. These regions of different polarity cause electric fields normal to the surface that cause scattering of carriers in devices [5]. By imaging the potential at the surface by surface-contact-potential electric force microscopy (SCP-EFM), both a qualitative and a quantitative measure of polarization induced charge distribution across the surface can be obtained. We prepared and imaged with SCP-EFM a standard reference sample containing regions of Ga and N-polarity surfaces, and sapphire all on the same sample. Having established the particular mode of SCPEFM operation best suited to measure III-V systems, we studied GaNs film grown by molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) with the intent to find inversion domains. There are various configurations of an EFM that can be used to find the surface contact potential, but all use an alternating current applied to the tip or the sample. The resulting oscillating voltage interacts with the electric field assicated with the sample to produce a force on the tip [6, 7]
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V 2 ∂C F =− 2 ∂z 1 ∂C 1 2 ∂C 1 ∂C 2 2 =− (Vdc − Vs )Vac sin (ωt ) + Vac cos(2ωt ) (Vdc −
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