Study On Electrical Characteristics Of Ohmic Contact On Lattice-matched InAlN/GaN Heterostructures

Due to the excellent physical properties, such as high breakdown field, high electron drift velocity, and good thermal stability, GaN-based high electron mobility transistors(HEMTs) have emerged as the most promising candidates for high power and high frequency applications. However, for conventional AlGaN/GaN devices, significant inverse piezoelectric effect in the barrier layer severely limits the reliability of devices working in the large signal mode for long time. Currently, one of the most effective solutions is to replace the traditional AlGaN/GaN heterojunction with the lattice matched In0.17Al0.83N/GaN one. On the other hand, in order to obtain as large output current as possible, the source and drain electrodes in HEMTs usually use the metal ohmic contacts, whose quality can directly affect the overall electrical performance of the device. Therefore, it is of great practical significance to study the electrical characteristics and current transport mechanism of ohmic contact on the lattice-matched InAlN/GaN heterojunction.In this thesis, the research background of lattice-matched InAlN/GaN heterostructure and the research status of ohmic contacts on the GaN-based heterostructures are firstly introduced. The fundamental theory of ohmic contact and the transmission line model(TLM) test method are explained in emphasis. Then, the lattice-matched In0.17Al0.83N/GaN heterostructures grown on Si and sapphire substrates are designed and fabricated, respectively, on which the TLM measurement structures of Ti/Al/Ni/Au ohmic contacts are fabricated. Finally, the temperature-variable current-voltage characteristics of TLM structures are measured by using the low- and high-temperature electrical testing systems, and the temperature dependence of specific contact resistivity(ρsc) and sheet resistance(Rsh) is particularly studied in the low(100-300 K) and high(300-523 K) temperature ranges, respectively. The transport mechanism of ohmic contact is also analyzed. The main conclusions of the thesis are drawn as follows.1. At low temperatures, ρsc increases with increasing temperature. The metal-like transport behavior of ohmic contact is proposed by analyzing the spike electrical contact mechanism, and the experimental data can be well fitted with the metal-like model. Rsh decreases gradually with decreasing temperature and reaches a certain value finally. By introducing the low-field mobility model of two-dimensional electron gas of GaN-based heterojunctions, such temperature dependence of Rsh can be explained by considering both the interface roughness scattering and polar optical phonon scattering at low temperatures.2. At high temperatures, ρsc increases first and then decreases with increasing temperature. Two transport mechanisms are proposed to explain the observed tendency. At raltively lower temperatures, the metal-like mechanism still dominates, while at relatively higher temperatures, the thermal field emmison(TFE) mechanism dominates. Therefore, we establish a parallel model based on the above two mechanisms. Experimental data can be perfectly fit by using this model. Then, the important physical parameters obtained by fitting are analyzed. The exponentially increasing tendancy of Rsh with increasing temperature can be related to the exponentially enhanced polar optical phonon scattering with increasing temperature at high temperatures.