Electrical And Photoelectric Characteristics Of GaN Schottky Junction With Fluorine Ions Implantation

Owing to the superior material properties of gallium nitride?GaN?,the GaN-based high electron mobility transistors?HEMTs?have witnessed rapid development during the last two decades in various applications such as high-frequency power amplifiers,high-efficiency power switches and integrated circuits capable of working at relatively high temperatures.In circuit applications,the enhancement-mode?normally-off?HEMTs are highly desirable because of simplified circuit configurations,reduced circuit complexity and intrinsic failure protection advantage.The fluorine?F?ion implantation approach was discovered when a large positive shift in V_th was observed in AlGaN/GaN HEMTs as F ions were incorporated in the barrier layer after the carbon tetrafluoride?CF₄?plasma treatment.Due to the strong electronegativity of the element F,the implanted F⁺ions can completely deplete the two-dimensional electron gas?2DEG?at the interface of AlGaN/GaN heterojunction and turn off the channel,converting depletion-mode HEMTs to enhancement-mode ones.Nevertheless,no p-type doping effect was observed in such experiments,mainly because the band gap of the AlGaN barrier is too large,the adjustment of the valence band by the F⁺ion is limited,and thus no hole quantum well can be formed.In addition,the thermal stability of F atoms in the barrier layer is not clear yet due to their small size and mass and large diffusion ability.Therefore,systematically studying the impact of F ion implantation on the electrical and photoelectrical properties of GaN-based devices helps to utilize this technique in fabricating various devices,having both scientific and practical significance.Compared with relatively complicated GaN-based HEMTs,the metal-semiconductor contact without piezoelectric polarization effect has simpler and clearer physical image,and thus is more suitable for analyzing the the influence of F ion implantation on GaN-based devices.The main content of this thesis is summarized as follows.1.F-implanted Ni/Au/n-GaN Schottky junctions are fabricated on self-supporting GaN substrates by using the standard lithography and lift-off techniques,and the metal-stack Ti/Al/Ni/Au is deposited by electron beam evaporation as the ohmic contact electrode.Meanwhile,un-implanted devices with the same structure are fabricated using the same process for subsequent comparisons.Compared with other substrates such as the patterned sapphire substrates and silicon substrates,the self-supporting GaN substrate has lower defect density(¹0⁶ cm^-2)according to the cathodoluminescence?CL?measurement results,which is favorable for preparing ideal Schottky contacts.Before the device preparation,the ion implantation process parameters are simulated by using the software SRIM,providing the distribution profile and injection damages of F ions in the GaN,which is comfirmed by the secondary ion mass spectrometry?SIMS?technique.Then,the room-temperature I-V and C-V characteristics of the F-implanted and unimplanted devices are measured,respectively.The results show that the leakage current of the F-implanted devices is reduced by 7 orders of magnitude compared with that of the un-implanted ones,indicating that the F ion implantation technique can greatly suppress the leakage current.The built-in potential barrier increases from 1.30 V to 3.22 V,thus the energy difference between the Fermi-level and the top of valence band in the equilibrium state is only a few kTs,suggesting that high-density free holes have been induced.2.The electrical characteristics of F-implanted GaN Schottky junctions are studied.The I-V curves of the device are measured in the temperature range of 300-480 K.The ideal factor n is extracted by using the classical thermal emission?TE?model for preliminarily judging the current transport mechanism,and then the temperature-variable I-V curves are fitted using different transport mechanisms.The results show that at low bias,the forward current is mainly determined by the trap-assisted tunneling?TAT?mechanism,while at higher bias,the TE current gradually replaces the recombination current with increasing temperature.Under reverse bias,current exhibits a strong temperature-dependence.The fitting results indicate that the tunneling probability exhibits a positive correlation and linear dependence with temperature.Besides,as the frequency increases from low?<1 MHz?to high?4 MHz?values,the depletion region capacitance increases rapidly with increasing frequency,showing obvious strong scattering of capacitance.The 1/C²-V relationship of F-implanted device at different frequencies also indicates that the highest position of the valence band of GaN in the equilibrium state has exceeded the Fermi-level,hence forming a hole quantum well structure.3.The photoelectrical and low-frequency noise characteristics of F-implanted GaN Schottky junctions are also investigated.Comparisons demonstrate the UV light response and continuous photoconductive effect of F-implanted devices.At the bias of-5 V,the peak responsivity is⁰.045 A/W,corresponding to a maximum external quantum efficiency of¹5.5%,and the UV to visible rejection ratio is¹0³.The average decay time constant?is about35 ms in the temperature range of 294-340 K.Electroluminescence?EL?can be observed in dark when the forward current injects the F-implanted device.In addition to the visible light,the EL spectrum also shows a significant luminescence peak at 375 nm,once again verifying that free holes are generated at a position where the surface Fermi-level is lower than the highest valence band.The rapid thermal annealing after ion implantation repairs a large number of lattice damages,so the internal defects and surface states are effectively suppressed.As a result,the noise current density is not significantly increased,and the impact of F ion implantation on GaN is acceptable in a controllable level.Particularly at the bias of-2 V,the F-implanted device exhibits good noise characteristics.The noise equivalent power is 9.72×10^-1111 W,and the detectivity is determined to be 1.42×10¹⁰ cm·Hz^1/2,exhibiting potential UV detection capability.