The phase conjugation between the deformable mirror and the wavefront sensor in the aberration correction of a terawatt Ti:sapphire laser is studied experimentally and theoretically in this paper. At varying values of phase-conjugation precision, we focus the corresponding beams into spots of the same size of 5.1 μm × 5.3 μm with a f/4 parabola in the 32 TW/36 fs Ti:sapphire laser system. The results show that the precision of conjugation can induce an intensity modulation but does not significantly affect the wavefront correction.
High electron mobility transistors(HEMTs)show tremendous potential for high mobility, high breakdown voltage, low conduction, low power consumption, and occupy an important piece of the microelectronics field. High-resistivity-cap-layer high electron mobility transistor (HRCL-HEMT) is a novel device structure. Based on the hole compensation mechanism, the p-GaN is converted into high resistance semiconductor material by hydrogen plasma implantation. Thus, the surface of the p-GaN layer will have a serious bombardment damage under the hydrogen plasma implantation. In practical work, it is also very challenging in the accurately control of the hydrogen injection rate, injection depth and injection uniformity. To achieve the required depth of injection, the energy of hydrogen plasma is often injected in more than the required dose or multiple injections times. The energy of hydrogen plasma plays a huge influence on the surface of the p-GaN layer. leakage current will be generated on the device surface, which deteriorate the electrical performance of the device. In this work, to protect the surface of p-GaN layer, a 2 nm Al2O3 film was deposited on the surface of the p-GaN cap layer to reduce the implantation damage caused by hydrogen Plasma treatment. The research shows that after the device deposited Al2O3 film before hydrogen Plasma treatment, the gate reverse leakage current was reduced by an order of magnitude, the ratio of ION and IOFF was increased by about 3 times. Meanwhile, the OFF-state breakdown voltage was increased from 410 V to 780 V. In addition, when the bias voltage was 400 V, the dynamic RON of devices A and B were 1.49 and 1.45 respectively, the device B showed a more stable dynamic performance. To analyze the gate leakage mechanism, a temperature-dependent current IG-VG testing was carried out, and it was found that the dominant mechanism of gate leakage current was Two-dimensional variable range hopping (2D-VRH) at reverse gate voltage. The reason for reducing the gate reverse current was analyzed, the Al2O3 film increases the activation energy of trap level and changed the surface states of HR-GaN; Furthermore, Al2O3 film blocked the injection of too much H plasma, reduced the density of AlGaN barrier and channel trap states, and weakened the current collapse.
As reported by several market analysts, GaN-based power devices show great potential applications in the low and medium voltage range ( < 900 V). For high voltage ( > 1200 V), including ship transportation and power grid, the future applications of GaN highly depend on the development of vertical devices based on GaN substrates. Several vertical devices have been reported, such as current aperture vertical electron transistors (CAVETs), U-shape trench metal-oxide-semiconductor field-effect transistors (UMOSFETs), and fin power transistors. And the UMOSFETs show potential advantages due to greater simplicity in material epitaxy and fabrication process. In the fabrication of UMOSFETs, the U-shape trench dry etching is the most critical process. The GaN sidewalls after dry etching directly affect the interface state characteristics in the MOS structure and the channel electron transport. In this work, etching optimization including etching radio-frequency (RF) power and etching mask is investigated and process-dependent electrical characteristics of GaN UMOSFETs are also studied. The appropriate decrease of RF power ensuring the steep sidewalls can effectively improve the channel electron mobility from 35.7 cm<sup>2</sup>/(V·s) to 48.1 cm<sup>2</sup>/(V·s) and consequently increase the ON-state current and reduce the ON-state resistance. Larger etching damage to the p-GaN sidewall caused by higher RF power leads the scattering effects to increase and the mobility of the channel carriers to decrease. The interface state density at the channel can be extracted by the subthreshold swing. The interface state density decreases to 1.90 × 10<sup>12</sup> cm<sup>–</sup><sup>2</sup>·eV<sup>–1</sup> when the RF power is regulated to 50 W, which is only half of the interface state density when RF power is 135 W. Similar breakdown voltages (350-380 V) are measured for these devices with varying RF power, which are governed by gate early breakdown. Positive valence band offset is formed in the SiO<sub>2</sub>/GaN MOS structure and the early breakdown occurs due to the holes accumulating at the SiO<sub>2</sub>/GaN interface. The etching uniformity at the bottom of U-shape trench can be improved by using the SiO<sub>2</sub> hard masks instead of photoresist masks. Sub-trenches at both ends of the trench bottom are observed in the device with photoresist masks, leading the carrier scattering to increase and ON-state current to decrease. Besides, the interface state density decreases from 3.42 × 10<sup>12</sup> cm<sup>–2</sup>·eV<sup>–1</sup> to 2.46 × 10<sup>12</sup> cm<sup>–2</sup>·eV<sup>–1</sup> with a SiO<sub>2</sub> hard mask layer used. Compared with 1.6 μm photoresist mask, the thinner SiO<sub>2</sub> mask with a thickness of 500 nm has a small sidewall area, which weakens the high-energy ion reflection in the inductively coupled plasma system. Consequently, the over-etching at the bottom ends of the trench is improved significantly and therefore the fabricated GaN UMOSFET has higher channel mobility and a lower interface state density.
The Pt/Au Schottky contacts to InGaN samples with different background carrier concentrations are fabricated. The crystal qualities of InGaN samples are characterized by X-ray diffraction (XRD) and atomic force microscope (AFM), and the correlation between threading dislocation density of InGaN and growth temperature is further clarified. The full width at half maximum (FWHM) values of the InGaN (0002) XRD rocking curves show that the density of threading dislocations in InGaN, which can seriously deteriorate InGaN crystal quality and surface morphology, decreases rapidly with increasing growth temperature. The Hall measurements show that the background carrier concentration of InGaN increases by two orders of magnitude as growth temperature decreases from 750 to 700℃, which is due to a reduced ammonia decomposition efficiency leading to the presence of high-density donor-type nitrogen vacancy (VN) defects at lower temperature. Therefore, combining the studies of XRD, AFM and Hall, it can be concluded that the higher growth temperature is favorable for realizing the InGaN film with low density of VN defects and threading dislocations for fabricating high-quality Schottky contacts, and then the barrier characteristics and current transport mechanism of Pt/Au/n-InGaN Schottky contact are investigated by current-voltage measurements and theory analysis based on the thermionic emission (TE) model and thermionic field emission (TFE) model. The results show that Schottky characteristics for InGaN with different carrier concentrations manifest obvious differences. It is noted that the high carrier concentration leads to the Schottky barrier height and the ideality factor obtained by TE model are quite different from that by TFE model due to the presence of high density of VN defects. This discrepancy suggests that the VN defects lead to the formation of the tunneling current and further reduced Schottky barrier height. Consequently, the presence of tunneling current results in the increasing of total transport current, which means that the defects-assisted tunneling transport and TE constitute the current transport mechanism in the Schottky. However, the fitted results obtained by TE and TFE models are almost identical for the InGaN with lower carrier concentration, indicating that TE is the dominant current transport mechanism. The above studies prove that the Pt/Au/n-InGaN Schottky contact fabricated using low background carrier concentration shows better Schottky characteristics. Thus, the properly designed growth parameters can effectively suppress defects-assisted tunneling transport, which is crucial to fabricating high-quality Schottky devices.
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