Role of the Substitutional Oxygen Donor in the Residual N-Type Conductivity in GaN
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INTRODUCTION Oxygen is one of the most common impurities which is frequently present during growth and processing steps as a contaminant, and can be unintentionally incorporated into many semiconductors including IlI-nitrides. This uncontrolled introduction of the oxygen impurity may affect and sometimes degrade the performance of semiconductor devices. The understanding of the role of oxygen in altering the material properties, and eventually a control of its introduction, are therefore of crucial importance for a reliable device operation. The substitutional oxygen donor has recently been suggested to be responsible for a long-standing materials problem - the residual n-type conductivity in oxygen-containing GaN. This conclusion was based both on theoretical calculations of the formation energy and the electronic structure of the oxygen donor in GaN [1,2] and on experimental observation of a strong correlation between the oxygen content in the material and the free electron concentration [3]. Though there is so far no direct experimental evidence available as to the exact form of the oxygen center and its geometric structure, the suggestion seemed to be reasonable and consistent until recently, when a new photoluminescence emission (denoted as the 0.88-eV PL below) of a deep center has provided microscopic information on local vibrations of the defect. In this paper, we shall demonstrate that these local vibrations resemble those reported earlier for the substitutional Op donor in GaP. This striking similarity, together with the electronic structure revealed from a detailed PL study of the deep center, has led us to suggest that the substitutional ON donor in GaN could be a deep donor, and thus to call for caution in assessing its role in the n-type conductivity of the material.
G 5.4 Mat. Res. Soc. Proc. Vol. 537 © 1999 Materials Research Society
EXPERIMENTAL The samples studied in this work were a variety of wurtzite GaN layers, typically 1- 200 ýtm thick, grown by hydride vapor phase epitaxy (HVPE) or metal organic chemical vapor deposition (MOCVD) on (0001) sapphire or 6H SiC substrates. The conductivity of the samples before electron irradiation varies over a wide range from highly n-type ([n] - higher 1018 cm-3), highly compensated ([n] - 106 - lower 1017 cm-3) to highly p-type ([p] - higher 1017 cm-3). These samples were irradiated by 2.5-MeV electrons at room temperature with a dose of Ix 10174x10 18 cm- 2, to enhance the 0.88-eV PL intensity. In PL experiments, the samples were excited by the 334 or 351 nm UV lines of an Argon ion laser. The resulting PL was spectrally dispersed by a 0.85-m double grating monochromator, and monitored by a cooled Ge detector. In temperature dependent PL studies, the sample temperature was varied between 1.5 K and room temperature. A magnetic field up to 14 T was applied in Zeeman studies of the PL spectra, with the aid of a superconducting magnet.
RESULTS AND DISCUSSION Defect Models A typical low-temperature 0.88-eV PL spectrum in GaN is shown as the upper curve in F
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