Study On Novel Heterostructures And Devices Based On High Al-content Nitride

Due to its superior material properties and great potential in high-temperature, high-frequency and high-power applications, III-nitride semiconductor has become the hotspot in microelectronics area. With improvements in material epitaxy technology, device fabrication process and continued innovation in device design, nitride-based electronic devices have made an extraordinary progress in terms of both performance and commercialization. In order to break the limit set by the conventional Al Ga N/Ga N heterostructure, novel high Al-content heterostructures spring up as the recent focus. The advantages provided by their bigger bandgap and stronger polarization effects have enabled them to demonstrate the capacity of outperforming well-estabilised Ga N-based HEMT. On the basis of these emerging heterostructures, the dissertation carries out a series of device design and optimization process, in an attempt to explore promising device structures applicable in high-frequency and high-voltage applications. The main work and achievements are summaried as follows:1. Study of high power gain field plate and advanced gate structures. Impact of the length of source field plate(FP) and dielectric thickness on power gain of conventional dual-FP devices is investigated by combining the theoretical simulation with related experiments. Results indicate that source FP can effectively shield gate-to-drain feedback capacitance, thus enhancing device’s power gain and operation stability;The effect presents a saturation trend as the FP length increases, whereas it rises continuously with a decrease in dielectric thickness under the FP. Furthermore, we proposed a novel dual-FP device incorporated with newly designed gate and source FPs. It features the same photolithography mask as gate electrode and exhibits no noticeable difference in breakdown characteristics with traditional T-gate HEMT but significantly higher power gain resulting from the source FP. Besides, the distance between the two FPs is identified to be the key factor in the electric field and gain modulation. And we realize the kind of devices with a distance of 0.4mm through process optimization, showing a 2.4d B higher power gain. Next, we successfully introduce the dual gate architecture to nitride seconductor system, and the power gain turn it out to be 5.0d B higher along with a wider stability region, which suggests this may be an ideal device for millimeter-wave opertion. Finally,advanced slant gate devices are fabricated, and both the gate leakage and current dispersion are dramatically suppressed owing to the inclusion of the linearly graded field plate effect which mitigates the electric field crowding at the gate edge effectively.2. Performance of the typical high Al-content Ga N/Al0.65Ga0.35N/Al N/Ga N HEMT is investigated in detail. Because of a thin barrier layer, the device behaves as an enhancement mode, exhibitng decent DC output characteristics, an ultralow Drain Induced Barrier Lowering, a low gate leakage current, a good current dispersion and breakdown performance with f T and fmax of 11.6GHz and 27.2GHz at a gate length of 0.5μm, respectively. These results reveal the great superiority of high-Al content heterostructures over conventional heterostructures in view of short channel effect and thereby high-frequency area.3. The systematic investigation of the band alignment properties between high K dielectric, Al2O3, La2O3, La2-x Alx O3 and Ga N is creatively performed. From La2O3 to Al2O3, energy band gap, valence and conduction band offset at the dielectric/Ga N interface all show a continual increase. In addition, band bending in the dielectric clearly indicates La2O3 introduces extra positive charges at the interface just opposite to the case of Al2O3, while La2-x Alx O3 with x=1 almost do not produce any changes. Based on the findings above, high-peformance MOS-HEMT with 15 nm Al2O3/La2-x Alx O3/Al2O3 stack gate is demonstrated, which attains a drain current level of 1367 m A/mm, nearly twice as large as that of HEMT. Morever, the substantial suppression of the gate leakage and smaller degradation of power gain than Si3N4 are achieved, effectively mitigating the leakage-gain tradeoff characteristics.4. Enhancement-mode(E-mode) devices based on high-Al content and thin barrier layer is fabricated by the use of newly developed O2 plasma implantation technology. The device exhibit a threshold voltage(VTH) of 1.02 V, a high peak transconductance of ~230m S/mm and especially a low subthreshold swing of 70 m V/Dec as well as a better VTH controllability. The major mechanism responsible for VTH modulation is determined to be partially oxidizing the barrier layer by several measurement techniques, and variation in strain inside barrier layer and thinning due to etching effect also contribute to a lesser extent. Furthermore, we develop a high-performance E-mode MOS-HEMT device using the stack gate. The device obtains a larger gate operation votage and excellent DC characteristics. The speci?c on-resistance is calculated to be 0.88mΩ·cm2 and the destructive breakdown voltage exceeds 480 V.5. To reduce the induced damage from implanted ions on channel, we propose a technique of fluorinated gate dielectrics instead of introducing ?uorine ions into barrier to achieve high-performance E-mode devices. Due to the electrons depletion effect of strong electronegativity of ?uorine ions in the surface layer,a positive VTH of 0.50 V is obtained with IDS,max=916m A/mm, Gm,max=342m S/mm, comparable to that of a conventional depletion-mode(D-mode) HEMT, an encouraging result among the best reports to date. As can be seen, the technique shows a great promise in millimeter-wave normally-off operation and E-mode/D-mode intergrated circuits.6. The demonstration of Al0.83In0.17N/Al N/Al0.05Ga0.95 N based E-mode devices is reported. Two methods, a composite one, ?uorine ions implantation combined with gate recess etching and O2 plasma treatment are exploited to positively shift the VTH. An improved ohmic contact process offers a low contact resistance of 0.12Ω·mm, overcoming the obstacle to ohmic fabrication in high Al-content heterostructures. Both devices show VTH of about 0.50 V. Thanks to the lager bandgap of Al Ga N buffer layer, the fabricated devices have very good subthreshold characteristics, for example, the O2 plasma-based device shows a subthreshold swing of merely 82 m V/Dec. Morever, we succeed in realizing an Al Ga N channel incorporated MOS-HEMT device. The VTH increases to 1.55 V originating from the inclusion of gate dielectric, with a maximum current of 254 m A/mm as well as an excellent off-state leakage and breakdown performance. It should be noted that results presented here related to E-mode Al0.83In0.17N-based devices is the first demonstration in the world.These reports here essentially indicates that advanced heterostructure designs, fabrication process and device structures jointly promote the continuous improvement of nitride-based devices.