Study On The Strain Distribution Of The Barrier Layer And The Polarization In GaN-Based Electron Devices

GaN material has lots of excellent properties, such as wide band gap, high critical breakdown electric field, high saturated electron drift velocity, chemical stability, and superior transport properties of the two-dimensional electron gas (2DEG) caused by the polarization effect. As a result, GaN-based heterojunction field-effect transistors (HFETs) are very suitable for the electronic devices in radiation, high temperature, high voltage, high frequency, high power. GaN-based HFETs have a wide range of applications in the wireless communication base stations, car electronics, power transmission, satellite, radar, aerospace, nuclear industry, monolithic microwave integrated circuits (MMICs) and other fields of national economy and national defense construction.The research around the GaN-based electronic devices has last for more than 20 years. Great improvements have been made in the parameters of the GaN-based electronic devices, which have been penetrating into consumer markets. However, the reliability of the devices is still one of the bottlenecks for the large scale application. As a result, the study of reliability is one of the hottest topics in the researches of the GaN-based electronic devices nowadays. The group-Ⅲ-nitride materials used in the devices are polar materials, whose most important characteristic is the polarization effect. This makes the group-III-nitride materials are different from other semiconductor material systems. The total polarization is the sum of spontaneous and piezoelectric polarizations, which can cause some great impacts on the carrier concentration, distribution, and composite of the group-III-nitride materials. For the GaN-based heterostructure materials (including AlGaN/GaN, AIN/GaN and InAlN/GaN, etc.), the influences of the polarization effect will become more intense. For these heterojunction materials without any doping, the density of 2DEG can be up to an order of 1013cm-2 in the quantum well of heterogeneous interface only by the polarization stress. Studies show that each process of the device manufacturing can have some influences on the uniformity of the strain in the GaN-base HFETs barrier layer. Nonuniformity of the strain can lead to the nonuniformity of the polarization charges, which could scatter the 2DEG in the channel of the devices. This process is the polarization coulomb field scattering. Prior studies have suggested that the polarization coulomb field scattering is one of the most important scattering mechanisms in GaN-based HFETs, and it has a very important influence on the carrier mobility of the devices. Some other studies have shown that the change of the strain in the barrier layer plays an extremely important role in the degradation and failure of devices during the working process. Therefore, the study on the strain distribution of the barrier layer and the polariztion in GaN-based electron devices is very important to improve the device characteristics and stability.The work in this dissertation first presents a method to determine the strain of the barrier layer under the gate in GaN-based HFETs; then analyses the impacts on the performance of the AlGaN/AlN/GaN HFETs caused by the rapid thermal annealing of the gate, gate metals and gate structures; finally studies the effects of GaN cap layer thickness on AIN/GaN heterostructure and the effects of side-Ohmic contact processing in AIN/GaN HFETs. The main contents of the dissertation are listed below:1. A method to determine the strain of the barrier layer under the gate in GaN-based HFETs.It has been shown that polarization-induced charges play an important role in the electrical properties of GaN-based HFETs, and the polarization is closely related to the strain of the barrier layer. The strain of the barrier layer under the gate directly affects the 2DEG electron density of the channel. However, the normal methods to determine the strain can’t directly determine the strain of the AlGaN barrier layer due to its thickness and the resistance of the gate metal. Thus, it is necessary to propose a simple and universal approach to evaluate the strain of the barrier layer under the gate metal. Based on the forward current-voltage (I-V) characteristics and the capacitance-voltage (C-V) curves between the gate and source, a method to determine the strain of the barrier layer under the gate in GaN-based HFETs has been presented. With this proposed method, the strain of the AlGaN barrier layer for the prepared AlGaN/AlN/GaN HFETs with the normal-Ohmic contacts and the side-Ohmic contacts has been analyzed and determined. It was found that the normal-Ohmic contact processing greatly affected the strain of the AlGaN barrier layer and the tensile strain of the AlGaN barrier layer was gradually reduced from the middle to the Ohmic contacts for the AlGaN/AlN/GaN HFETs with the normal-Ohmic contacts, while the strain of the AlGaN barrier layer was weakly affected by the side-Ohmic contact processing. The polarization coulomb field scattering in the devices with side-Ohmic contacts was much weaker than that in the devices with normal-Ohmic contacts. The side-Ohmic contacts processing can make the electrical performances of GaN-based HFETs improved.2. Effects of rapid thermal annealing of the gate electrodes on the electrical properties of the AlGaN/AlN/GaN HFETsIn this study, we investigated the effects of rapid thermal annealing on the electrical properties of the AlGaN/AlN/GaN HFETs with Ti/Al/Ni/Au and Ni/Au gate electrodes. For the devices with Ti/Al/Ni/Au gate electrodes, using the measured electrical characteristics and micro-Raman spectroscopy, we found that the uneven distribution of the strain caused by the Schottky metals was a major factor that generates the polarization Coulomb field scattering in AlGaN/AlN/GaN HFETs, and appropriate rapid thermal annealing could make the strain distribution of the AlGaN barrier layer uniform, weaken the polarization Coulomb field scattering, improve the 2DEG electron mobility and the Schottky barrier height, and make the direct current characteristics of the HFETs better. Of course, the electrical performances mentioned above became deteriorated after excessive annealing. For the devices with Ni/Au gate electrodes, we found that the thermal stability of Ni/Au gate electrode is very good after rapid thermal annealing at 400℃ since the gate capacitance and the 2DEG density are basically unchanged. However, when the gate annealing temperature further rise, up to 600℃ and 700℃, we can see that the density of the 2DEG under the gate falls sharply, and the threshold voltage of the devices drifts to 0 V apparently at the same time. This may be due to the diffusion of the metal atoms into the AlGaN barrier layer after a rapid thermal annealing, which leads to the decrease of the dielectric constant of the AlGaN material. Through the strain under the gate, we found that high temperature can damage the lattice structure of the barrier layer, and the higher the annealing temperature, the stronger the damage.3. Influences of the gate metals and structures on the electrical properties of the AlGaN/AlN/GaN HFETsIn this study, we investigated the influences on the electrical properties of the AlGaN/AlN/GaN HFETs due to the thickness of the Schottky contact metals, the types of gate metals, the floating gate structures and the split-gate structures. For the thickness of the gate metal, Ni/Au Schottky contacts with three different thicknesses (50 nm/50 nm,50 nm/150 nm and 50 nm/300 nm) were deposited on strained AlGaN/AIN/GaN heterostructures. We found that the intensity of the polarization Coulomb field scattering was little affected by the thickness of the Schottky contact metals. For the types of gate metals, six different gate metals (Al/Au, Au, Cu/Au, Pt/Au, Ni/Au, and Fe/Au) were deposited on strained AlGaN/AlN/GaN heterostructures. We found that the intensity of the polarization Coulomb field scattering was greatly affected by the types of gate metals. For the devices with Al gate metal, the Al/AlGaN interface undergoes a Ga-Al exchange chemical reaction driven by the large heat of formation of AlN as compared to that of GaN at room temperature. The reaction leads to thinner AlGaN barrier layer, which causes the 2DEG sheet carrier density under the Schottky contact metals at zero gate bias lower. The reaction also leads to longer distance between the gate metal and the 2DEG channel, which causes the control ability of the Schottky contact to the 2DEG weaker and results in the much lower threshold voltage of the device with Al Schottky contact compared to other devices. For the metals mainly undergo a physical interaction with the AlGaN barrier layer, there is a negative correlation between the intensity of the polarization Coulomb field scattering and the Young’s modulus of the gate metal. For the the floating gate structures, we design the structures without floating gate, with unilateral floating gate and with bilateral floating gate structures. We found that the polarization Coulomb field scattering on the devices with the floating gate are stronger than the devices without the floating gate, and the larger the number of the floating gate is, the stronger the polarization Coulomb field scattering is. For the split-gate structures, we design the structures with the same gate area but different gate fingers. We found that the polarization Coulomb field scattering become stronger with the increase of the gate fingers. The change of the intensity of the polarization coulomb field scattering is mainly caused by the number of the uneven distributions of the polarization charges along the channel.4. Studies about the effects of GaN cap layer thickness on AIN/GaN heterostructure and the effects of side-Ohmic contact processing in AIN/GaN HFETsTo date, it has been reported that GaN cap layer can significantly reduce the leakage current, and is widely used as the cap layer in the GaN-based heterostructure. In addition, the thickness of the GaN cap layer affects the 2DEG density and the 2DEG electron mobility in AIN/GaN heterostructures. In this study, by analyzing the temperature-dependent Hall measurement results for the prepared AIN/GaN heterostructure materials with different GaN cap layer thicknesses, we found that very thin GaN cap layer can’t avoid oxidation, while increasing the thickness of GaN cap layer leads to an increase in the dislocation density and the interface roughness. The optimized thickness of GaN cap layer for the AIN/GaN heterostructures with 3 nm AlN barrier layer is around 4 nm. It is also found that the strained a-axis lattice constant of the AlN barrier layer is less than that of GaN for the AIN/GaN heterostructures with 3 nm AlN barrier layer.In addition, using the measured C-V and I-V characteristics of the rectangular AIN/GaN HFETs with the side-Ohmic contacts, it was found that the polarization Coulomb field scattering in the AIN/GaN HFETs was greatly weakened after the side-Ohmic contact processing, however, it still could not be ignored. It was also found that, with side-Ohmic contacts, the polarization Coulomb field scattering was much stronger in AIN/GaN HFETs than in AlGaN/AlN/GaN and In0.17Al0.83N/AlN/GaN HFETs, which was attributed to the extremely thinner barrier layer and the stronger polarization of the AIN/GaN heterostructure.