Study On Novel Structure Of Gan-based Terahertz Device And Related Material Growth

Compared to other conventional semiconductor material, GaN terahertz device hasmore electron effective mass, higher longitudinal phonon energy, faster electronscattering between sub-band, greater negative resistance current peaks valley, highertwo dimension electron gas density and so on, showing that GaN has evident advantagesin the field of terahertz application. However, the growth of GaN material technologycompared with traditional semiconductor materials is still not mature enough to obtainhigh crystal quality, the high defect density in GaN materials is still high, therefore, itcan directly result in GaN terahertz device performance attenuation, and more evenbring in breakdown and burning. In order to promote the application of terahertz GaNdevice, the related devices material quality problem must be first taken into account.The traditional improvement of crystal quality can be achieved byEpitaxial-lateral-overgrowth (ELOG) or interlayer technologies. However, ELOGtechnology requires multi-step regrowth and mask technology, which is cumbersomeand costly to be achieved. Interlayer technology can filter dislocation defect, but it canintroduce large lattice-mismatch with GaN layer therefore lead to generation of misfitdislocations at the interface, which could not effectively reduce the dislocation density.To solve these problems, this paper takes the special structure of GaN terahertz devicesinto consideration, and then carry out the adjustment and optimization for growthprocess, thus effectively filter the dislocation density, significantly improved the crystalquality in the transit region of devices. Some encouraging results have been obtainedthrough a series of experimental tests, the main research conclusions are as follows:1, We report the GaN based THz Gunn diode epitaxial growth through MOCVD,including the bottom notch-doping accelerating region structure (BNL) and the topnotch-doping accelerating region structure (TNL). Subsequently, the relatedmeasurement and analysis are carried out, and independent establishment of the growthdynamics model of the epitaxial defect is proposed to explain the annihilation ofdislocation in the active region.2, We propose an improved small-signal equivalent model to eliminate frequencydispersion phenomenon in capacitance-voltage (C-V) measurement, and introduce anew mathematic method to calculate the amount of bulk defect existing in the GaNbuffer layer. As well known, the existence of bulk defect (especially deep-level pointdefect) can degrade high-field transport characteristic during further device operation.To have a further investigation, we make use of PL, improved C-V, SIMS, HRXRD, and TEM measurements to take a deep insight into the bulk defect in the device. In addition,the experiment indicates that the annealing treatment reduces the point defect density ofsamples by around30%, which can weaken the charge-trapping effect introduced bydeep-level crystal defects in GaN and encourage the improvement of radio frequencypower and conversion efficiency in terahertz device operation.3, We report the design and experiment of the sub-micron transit regionAlGaN/GaN heterostructure THz Gunn diode with a tri-graded AlxGa1-xN layer. Wecarry out a detailed investigation about the effects of AlGaN hot electron injecting layer(HEI) on blocking the dislocation in GaN transit region and improving the crystalquality. In the following, the related experiment demonstrates that the bending andannihilation of the screw and edge dislocations occur at the AlGaN/GaN heterointerface.We propose that the bottom AlGaN-HEI structure is a promising candidate for thefabrication of GaN Gunn diodes in terahertz regime. At the same time, we report on thedislocation annihilation in N-polar GaN with a pseudomorphicly growngraded-Al-fraction AlGaN interlayer. The improvement for the crystal quality ofN-polar GaN can be introduced in optimizing the N-polar related THz deviceperformance.4, We report the crystal quality optimization of the InAlN/GaN heterointerface.PMOCVD was employed to grow InAlN interlayer. Subsequently, a low temperature(LT)-GaN protective layer was deposited on the InAlN interlayer, to prevent the risk ofindium cluster formation and indium redistribution during the following hightemperature top GaN growth. According to the TEM measurement, it demonstrates thatthe InAlN/GaN heterointerface can not only filter the dislocations with great efficiency,but also avoid the generation of misfit dislocation. Therefore, the InAlN layer grown byPMOCVD and the LT-GaN layer can significantly bring in high quality InAlN/GaNheterointerface, thus improve the crystal quality of the whole material. In brief, ourwork provides a valuable reference to the related terahertz devices application.