Research On Interface Characterization Of GaN-based Metal-Oxide-Semiconductor Diode

Due to its superior physical properties such as high breakdown field, large saturation electron velocity, high electron mobility and great thermal stability, the wide bandgap semiconductor gallium nitride（GaN） becomes an ideal material for fabricating high electron mobility transistors（HEMTs） working under high-power and high-frequency conditions. Over the last decades, benefiting from the development of material growth and micro-processing technologies, the electrical properties of GaN-based HEMT have been gradually improved. However, limited by the capture effect of surface defect electron in the barrier layer, the device often exhibits significant current collapse phenomenon. Experiments show that depositing a passivation layer on the semiconductor surface can effectively suppress the current collapse of GaN-based HEMTs, reducing the gate leakage current and realizing the enhanced mode of the device. However, the large amount of Al₂O₃/n-GaN interface traps reduce the passivation effect. Thus, evaluating the deep level interface states correctly, improving the passivation effect and increasing the interface quality have important scientific and practical values. For wide bandgap semiconductors, however, limited by the extremly long time constant of electron emission and low generation rate of minority carriers, the traditional capacitance and conductance methods are unable to effectively characterize the deep level interface states distribution of Al₂O₃/GaN. Therefore, developing a new characterization method is important for investigating the deep level interface states in wide bandgap semiconductor structures.Firstly, the basic material featrures of GaN and the physics problems encountered in explaining the properties of GaN-based HEMTs are introduced briefly. The research status of interface states characterizing methods for insulating layer/semiconductor structures is analyzed. Then, the Ni/Au/Al₂O₃/n-GaN metal-oxide-semiconductor（MOS） structure is prepared by the atomic layer deposition（ALD） techinique. Finally, based on the different physical methodology to traditional methods, the deep level interface states of Al₂O₃/n-GaN are characterized according to experimental data. The major research work of this thesis can be summarized as follows.In the first part, the transparent electrode Ni/Au/Al₂O₃/n-GaN MOS structure is fabricated by using the ALD technique. Based on the fact that deep-level interface states can be recombined by photo-generated holes, we use the variable-wavelength light assisted capacitance-voltage（C-V） method to characterize the average distribution of Al₂O₃/n-GaN deep level interface states, and analyze the frequency dispersion phenomena in the accumulation zone. Experimental results show that, the room-temperature capacitance curve shows a deep depletion behavior because the minority carrier generation rate in GaN is very low, unable to keep up with the DC bias scanning speed and the changing of semiconductor surface potential, and then continuing to enter the deep depeletion zone. Interface states can exchange carriers with the semiconductor, causing the capacitance curve to stretch along the voltage direction. Neglecting the oxide layer interface states, the calculated average interface states density of Al₂O₃/n-GaN is in the order of 1012 cm-2eV-1. The frequency dispersion appearing in the accumulation zone is tentatively explained by two possible reasons. On one hand, defects at the oxide layer border can exchange carriers with the semiconductor; on the other hand, the interface states can exchange carriers with the metal, providing that the oxide layer is thin enough.In the second part, we use the backside ultra-violet（UV） photo-assisted high-frequency C-V method to study the capacitance characteristics of Al₂O₃/n-GaN on a mercury probe station, and analyze the spectral response characteristics of samples. Experimental results show that, the capacitance curve after illumination exhibits shift and shape distortion, indicating that the system has introduced a number of positive charges and the electrical state of deep-level interface states changes. The calculated oxide border trap density is about 3.95×1011 cm-2, and the decaying distribution of deep-level interface state density in the Al₂O₃/n-GaN is characterized. It shows that the maximum density distribution value near the conduction band is about 2.5×1012 cm-2eV-1, then it quickly decrease to about 9×1010 cm-2eV-1 at the energy of 0.8 eV below conductance band. The capacitance changing before and after the illumination can be understood as follows: the interface states are recombined with the photo-generated hole, causing the changes of surface potential and the space charge region width, and hence affecting the change of capacitance.