Background Impurity Reduction and Iron Doping of Gallium Nitride Wafers
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Background Impurity Reduction and Iron Doping of Gallium Nitride Wafers Robert P. Vaudo, Xueping Xu, Allan D. Salant, Joseph A. Malcarne, Edward L. Hutchins, and George R. Brandes ATMI, Inc., Danbury, CT 06810, U.S.A. ABSTRACT Background impurities and the resulting electrical characteristics were studied for GaN wafers grown using hydride vapor phase epitaxy at various growth conditions. The electron concentration was found to decrease with increasing GaN thickness, by orders of magnitude in the first few microns of growth, but continuing gradually for thousands of microns. Physical removal of the backside degenerate layer enabled improved analysis of the electrical properties. Secondary ion mass spectroscopy was used to determine that the presence of oxygen and silicon accounted for the electron concentration for unintentionally n-type doped material. The concentration of oxygen was found to vary more than that of silicon and increased with decreasing growth temperature. The resistivity was measured to be as high as 1 ohm-cm, corresponding to a carrier concentration of 1016 cm-3. Iron was demonstrated to effectively compensate the residual donors and increased the resistivity to greater than 109 ohm-cm at room temperature and greater than 3x105 ohm-cm at 250 °C. An activation energy for the iron-doped GaN was determined by variable temperature resistivity measurements to be 0.51 eV.
INTRODUCTION A promising pathway for improving the reliability of GaN-based electronic devices is to form the device on low defect density GaN substrates, which can be produced by hydride vapor phase epitaxy (HVPE). In addition to low defect density [1,2], the HVPE approach is inherently scalable to very large wafer area. Early reports of GaN growth by HVPE [3,4] produced material that was heavily n-type with a carrier concentration in excess of 1018 cm-3. Since that time, the background impurities have been reduced. The current state-of-the-art HVPE growth of unintentionally doped GaN exhibits an electron concentration on the order of 1016 cm-3 [5-7]. Motoki [8] reports a background level of 5x1018 cm-3 due to auto-doping from the SiO2 mask used. While the background electrical concentration has been reduced by several orders of magnitude since the 1970s, the purity of the material must be further increased and/or a deep level compensating impurity must be incorporated to make the material suitable for high frequency electronic applications. Kuznetsov [9] reported 0.2-0.4 µm thick Zn-doped GaN layers by HVPE and calculated a specific resistivity as high as 1012 ohm-cm. Activation energies of 0.18 and 0.7 eV were found in low and high temperature regimes, respectively. In this work, we grew unintentionally doped GaN under a variety of growth conditions and studied how the background impurity concentration changed with process conditions. Iron doping was employed to compensate the residual donors to make thick, semi-insulating GaN with a room temperature resistivity in excess of 109 ohm-cm.
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