Fabrication And Characterization Of InSe Field Effect Transistors And Photodetectors

With the advent of graphene,two-dimensional materials with atomic layer thickness have become a research hotspot due to their physical properties and application prospects.Graphene has a series of outstanding characteristics such as high mobility,high mechanical strength,high thermal conductivity,high electrical conductivity and low light absorption rate,showing great potential applications as transparent conductive films and super-capacitor electrode materials.However,its zero band gap has limited the application of this material in microelectronics.Indium selenide（InSe）is a typical III-VI semiconductor material and has been considered to have a significant impact on development of future electronics because of its high carrier mobility and adjustable band gap.On the other hand,InSe is quite sensitive to the environment conditions.For example,the electrical stability and performance of InSe-based devices can be affected by the exposure and interaction of the InSe surface to chemical species in air,such as water or oxygen,which limits the development in electronics.In this thesis,InSe field effect transistors（FET）with different channel thickness were fabricated by mechanical exfoliation,and the optical,electrical and interface properties were systematically studied.In addition,the electrical properties of InSe FET were improved using different interface treatment methods.A self-aligned process and an alumina（Al₂O₃）layer as a passivation layer were used to protect the surface of InSe from oxygen,water molecules,and photoresist.After Al₂O₃ encapsulation,the mobility and stability of the FET were improved.Furthermore,transistor based photodetector were fabricated on rigid and flexible substrates.The key experiments in this thesis as follows:1.Research on the fabrication of InSe FET by shadow maskEffect of contact metal on the mobility of InSe FET:Work function of the contact electrode metal and the Fermi level of the semiconductor material determined the contact type and contact resistance of the InSe FET,which directly affected the device channel voltage division and carrier transport efficiency.In this thesis,titanium,gold,aluminum,and palladium were used to be contact electrodes.By comparing the work function of the metal and the film formation quality of the electrode,titanium was finally selected as the contact metal for InSe FET.The mobility（μ）of the device is 200 cm²/Vs,contact resistance（Rc）is 9.7KΩ,on off ratio is 1.2× 106,and the hysteresis is 2.5 V with the gate voltage from-30 to 30V.Relationship between InSe thickness and field effect mobility:The InSe FET with different channel thickness were fabricated.In particular,the mobility increased as the layer thickness of InSe increases from 20 to 30 nm,and decreased as the InSe thickness increased from 30 to 65 nm.For thinner layered InSe,the interaction distance between charged impurities on the substrate and free carriers in InSe was smaller,which enhanced carrier scattering and reduces the mobility.For thicker InSe layers,because the source and drain electrodes were contacted only directly to the top InSe layer,electron transport into the bottom layers involved additional interlayer resistors,resulting in a lower mobility of the FET.Interface processing:In FET,due to the fracture of the lattice structure at the boundary,different defects existed at the interface between the semiconductor material and the dielectric layer,resulting in degradation of device performance.In order to reduce such degradation caused by trap effects,the dielectric layer（SiO₂）was hydrophobized by buffer oxide etching（BOE）solution and hexamethyldisilazane（HMDS）.Such hydrophobization of the substrate reduced the hydroxyl groups and hydrogen bonds formed between the hydroxyl groups and water molecules,which reduced the interface trap state density from 3.3×1012 to 1.1×1012 cm²eV-1.Meanwhile the mobility of the device increased from 146 to 548 cm²/Vs.In addition,3aminopropyltriethoxysilane（APTES）was used to protect the channel of InSe FET,and the ntype doping was performed while filling the Se vacancies,which increased the mobility of the device from 72 to 115 cm²/Vs.Improved performance of InSe FET by PMMA encapsulation:The electrical stability and performance of InSe-based devices can be affected by the exposure and interaction of the InSe surface to chemical species in air.Therefore,Polymethyl methacrylate（PMMA）was spinned onto the surface of InSe FET,and the electric performance was compared before and after PMMA encapsulation.After PMMA encapsulation,the mobility increased from 186 to 220 cm²/Vs,and a similar improvement was measured on other FET in which the InSe layer thickness ranges from 20 to 60 nm.In addition,the electrical stability of the FET was investigated by applying a constant gate bias stress before and after PMMA encapsulation.After PMMA encapsulation,InSe FET showed better electrical stability.Under negative bias,the threshold voltage decreased by 1.8 V,and the rate of change of mobility droped by 9.4%;under forward bias,the threshold voltage decreased by 0.2 V,and the rate of change of mobility decreased by 8.7%.2.Research on the fabrication of InSe FET by self-aligned lithographyA high performance InSe transistor was designed and fabricated by self-aligned process and by encapsulating the InSe flake with an alumina（Al₂O₃）layer immediately after transferring it onto the substrate.The encapsulated transistor shows high electronic performance with a mobility of 857 cm²/Vs,hysteresis of only 1.6 V with gate voltage ranged from-100 to 100 V,contact resistance of 180 Ω,and on-off ration of 1.0×108.To the best of our knowledge,these electrical parameters have reached the world’s leading level.Due to the screen effect of Al₂O₃,the mobility of InSe FET is hardly affected by the surface coulomb effect.After 50 days of storage in air,the transfer characteristic curve was basically not shifted.The mobility decreased from 710 to 621 cm²/Vs in the first 10 days,and almost did not change in the next 40 days.The hysteresis of the device kept at 1.6 ± 0.3 V within 50 days.We also prepared MoS2-based FET by using Al₂O₃ self-aligned lithography.The mobility was 344 cm²/Vs,which was higher than the MoS2-based high mobility FET reported in most literatures.The self-aligned lithography provides a universal method to improve the air stability and electronic performance of not only InSe but also other 2D material based devices.3.InSe photodetector based on rigid substrateHigh performance InSe was fabricated by Al₂O₃ self-aligned lithography.In order to test the photoresponse,various illumination lights（450,520,and 808nm）were used to illuminate the InSe channel in ambient environment.With the illumination intensity of 20 mW/cm²,the responsivity of InSe photodetector was 1.7×104,4.5×103,and 2.3×103 A/W,respectively.Furthermore,we studied the relationship between responsivity and incident light intensity at 450 nm.When the incident light intensity was 0.001 mW/cm²,the responsivity of the device was 2.3×107 A/W,and the specific detectivity（D*）was 7.5×1012 Jones.Such responsivity was higher than that of devices tested under vacuum,and it was better than most photodetectors based on other two-dimensional materials.Moreover,we used 30 nm Al₂O₃ as the dielectric layer to further improve the photoelectric performance of the device by optimizing the interface performance.The device’s responsivity was 2.1 ×108 A/W,the detectivity was 1.9×1013 Jones,the rising time is 179.2 p.s,and the decay time is 130.0 μs.4.InSe photodetector based on flexible substrateBased on Al₂O₃ encapsulation and self-aligned lithography,InSe FET photodetector was fabricated on a flexible polyethylene naphthalate（PEN）substrate,with a response rate of 4.5×106 A/W at a wavelength of 450 nm and a specific detectivity was 3.2×1011 Jones.The rising time was 249.1 μs,and the decay time was 261.2 μs.In addition,we conducted bending tests on flexible samples,and the electrical properties of the devices showed a relatively high stability under different degrees of bending.The transfer characteristic curve of the device returned to the original state after returning to the non-bending state,which showed InSe FET based on PEN substrates have good mechanical flexibility.