Study Of The Polymer Surface Modification Effect On UV Detector Performance

This paper studied from the aspects of barrier height, which produced by the contactof metal and semiconductor. The polyethyleneimine (PEI) as an interfacial layer wasintroduced to modify the substrate material of TiO2and Zr0.2Ti0.8O2. The mechanism ofimproving the device’s performance was studied. However，the effect of TiO2surfacemodified PMMA on humidity was studied, a method to improve the device stability wasfound. By studying the effects of the polymer surface modification with PEI and PMMA onthe performance of the device, the meaningful results have been achieved as follows:The TiO2thin film was prepared by using the sol-gel method. The surface of TiO2filmis smooth and no crack, which is suitable for fabricating the detector. XRD results indicatedthat the prepared TiO2is the structure of anatase and grain size is about23.4nm. At3V bias,the photocurrent and dark current is2.92μA and3.17nA, respectively. The UV/visiblerejection ratio is about three orders of magnitude, indicating a higher sensitivity. It has laida good material basis for the subsequent surface modification.PEI as interface modification was introduced into the UV detector for the first time.The dipole layer was formed at the interface by electrostatic self-assembly. Since thedirection of the dipole layer was contrary to the direction of the built-in electric fieldgenerated by the contact of the metal and the semiconductor, the height of contact barrierwas reduced and the width of the depletion layer was narrowed, which is benefit for thecarrier transmission. Thus the performance of the device was effectively improved. Themain contents include using two different hydroxylation treatment method on the surface ofTiO2film, studying the effect of different PEI mass fraction, different spin coating speed onthe performance of the devices. For hydroxylation by KOH solution treatment:1) At thesame speed of4000rpm spin-coated PEI solution of different mass fraction, found that thephotocurrent is the largest after using0.2wt%of the PEI solution to modify the devices,the photocurrent of devices reaches maximum value of92.68μA, and the responsivityreaches a maximum value1594A/W at270nm;2) under the same mass fraction, thephotocurrent of the devices and responsivity are presented first increased and thendecreased with increasing the speed of spin coating, and the best performance of devices was obtained at4000rpm. The curve of response time display that the rise time and falltime of the devices are17.8ms and17.2s, respectively. For hydroxylation by UV/O3irradiation treatment:1) at4000rpm, the device performance of spin coated with0.2wt%PEI is the best one. The responsivity of the devices reaches the maximum value of706A/Wat270nm.2) spin-coated PEI solution with different rpm, the performance of the device isthe best one at4000rpm. The response time curve shows that the rise time of device is103.6ms, and fall time is about11.86s.The analysis of UPS and energy distribution shows: compared with pure TiO2, Ecutoffwas decreased0.96eV and0.46eV, respectively. The secondary electrons cut-off edge ofPEI/K-TiO2and PEI/U-TiO2devices was shifted, which led to that the barrier height wassignificantly decreased after the device modification by PEI. Due to the lowered of thebarrier height and narrowed the width of the depletion layer, the photocurrent and rise timewere obviously improved. Compared with TiO2device, the photocurrent and responsivityof PEI/K-TiO2were increased about32and33times, respectively, and the rise time wasshortened about60times, but the fall time was prolonged2times. The photocurrent andresponsivity of PEI/U-TiO2were improved about14and15times, respectively. The risetime was shortened about10times, but the fall time was prolonged to1times. Photocurrentversus time curve showed that the device performance fell slightly after modification, butstill higher than that of before modification, proved that the device stability after PEImodification is good. The results show that the overall performance of the devices has beenobviously improved by PEI modification.Zr0.2Ti0.8O2thin film as the base material was modified in order to verify the improvedperformance of UV detector by PEI is a universal method. The effect of the differenthydroxylation process, different hydroxylation time on device performance was studied.The results show that the performance of device by KOH treated24h is better than that oftreatment12h and36h, the performance of device by UV/O3treated20min is better thanthat of treatment10min and30min. The photocurrent of the device after KOH treated24hwas larger than that of the devices without modification. The photocurrent was increased22times. The maximum of the responsivity is1228A/W. For the device with UV/O3treated 20min, the maximum photocurrent value was41.22μA, and the responsivity was709A/W.Compared to the UPS curve of Zr0.2Ti0.8O2, the barrier height of UV/O3treated20min andKOH treated24h was reduced0.43and0.7eV, respectively. Since the barrier height of thedevice by KOH treated24h was reduced even more, therefore the performance of thedevice was even better. In addition, the rise time of the modified device was obviouslyshortened than that the unmodified device, but the fall time was prolonged. The mainreason was that Au Fermi level formed after modification was close to the conduction bandof Zr0.2Ti0.8O2after PEI modification. Therefore, Au was not easy to capture the hole,which caused the reducing of the carrier recombination rate. Due to barrier height wasreduced after PEI modification, the depletion layer became thinner, so that the capacitanceof the depletion region and the RC time constant is increased, resulting in a slow fall time.In order to reduce the effect of humidity on device performance, the PMMA wasmodified on the detector surface, and studied the changes of device performance before andafter PMMA modification. The discussion about the influence of humidity was performed.Before PMMA modification, the dark current of the device increased with the increase ofRH humidity. After PMMA modification, dark current of the device was obviously reduced,the reason was that the modification of PMMA can be effectively isolated from oxygen,water molecules adsorbed on the film surface, so the ionic conductivity generated byhydrolysis was reduced, leading to the dark current decreased. After proper increasing thethickness of PMMA, the dark current of the device was basically stable. Dark currentversus time test results showed that the dark current maintained at around0.7nA aftermodification, which did not vary with the change of humidity, and improved the long-termstability of the device after PMMA modification.In this paper, the effect of polymer surface modification on the device performancewas studied. The mechanisms to improve the performance of photocurrent, responsivityand response time are explained. It provides a new way to improve the performance of theUV detector.