Properties Of Surface-modified AlGaN Solar-blind Ultraviolet Photodetectors

Benefiting from the breakthrough of AlN and AlGaN material growth technology,AlGaN-based ultraviolet optoelectronic devices have risen rapidly.AlGaN material has excellent electronic properties such as wide and continuously tunable direct band gap,high electron saturation drift rate,high breakdown field strength and high thermal conductivity,and has great development potential in the field of short-wavelength solar-blind ultraviolet optoelectronic devices.However,the crystal quality of AlGaN materials with high Al composition will decrease with the increase of Al content,resulting in internal dislocations,surfaceGadangling bonds and complex surface state defects during the preparation of AlGaN,which greatly affects the device performance.In addition,due to the strong self-compensation effect,low solubility,and high activation energy of AlGaN materials,it is difficult to realize high-efficiency p-type doped AlGaN materials.Therefore,the back-to-back Schottky contact metal-semiconductor-metal(MSM)structure without doping is beneficial to the fabrication of solar-blind UV photodetectors based on AlGaN-based solar-blind UV photodetectors with high Al composition.In this paper,we focus on the surface modification of low dark current MSM solar-blind UV photodetectors based on AlGaN substrates.The effects of three different surface modification methods on device performance were studied respectively.The main research results are as follows:1.First,we introduce the metal electrode modification process of AlGaN-based MSM structure photodetector,and study the effect of the modification process on the device performance.The hexadecanethiol(CH3(CH2)15-SH)organic molecule was chemisorbed on the electrode of the high Al composition Al0.6Ga0.4N MSM solar-blind UV photodetector.The CH3(CH2)15-SH electrode modification reduces the work function of the metal electrode and changes the height of the Schottky barrier in the metal-semiconductor rectifying contact.Compared with the unmodified reference photodetector,the AlGaN photodetector modified with CH3(CH2)15-SH electrode exhibits an order of magnitude higher photocurrent and a high peak photoresponse of 7.598 A/W at 10 V bias voltage.This metal electrode modification significantly increases the photocurrent and responsivity of the device.At the same time,the gain mechanism of CH3(CH2)15-SH organic molecules on device performance was discussed through the theoretical model of Schottky contact and the energy band diagram of metal-AlGaN contact.2.Although the electrode metal modification process can improve the photoelectric performance of the AlGaN-based MSM structure photodetector to a certain extent,it slightly increases the dark current of the device.In addition,the modification method did not reduce the influence of AlGaN material defects on device performance.Therefore,a solar-blind UV photodetector of Al0.6Ga0.4N MSM modified on the surface of AlGaN by Octadecanethiol(ODM)organic molecules was prepared.ODM organic molecules are chemically adsorbed on the surface of AlGaN material with high Al composition in solution phase,and their alkane chains are cleaved by high temperature annealing at 300 °C.The S atoms on the ODM organic molecules are chemically bonded with theGadangling bonds on the surface of the AlGaN material to form Ga-S bonds.The crystal quality and surface morphology of Al0.6Ga0.4N material were characterized by XRD,AFM and SEM,and the existence of ODM organic molecules was characterized by XPS.Compared with the unmodified reference device,the dark current of the Al0.6Ga0.4N MSM photodetector surface-modified with ODM organic molecules is significantly reduced,and the peak responsivity at 250 nm is increased by about 3 times.It is proved that the ODM organic molecular surface modification process can effectively reduce the adverse effects caused byGadangling bonds and surface states of AlGaN materials and optimize the optoelectronic properties of the device.3.Surface modification with organic molecular layers can provide devices with additional properties to enhance their functions,thereby effectively enhancing the performance of AlGaN-based optoelectronic devices.Therefore,the self-assembly of5,10,15,20-tetrakis(4-hydroxyphenyl)-porphyrin(5,10,15,20-T(4-OH P)P)organic molecules in solution phase was investigated.Chemically adsorbed on the surface hydroxylated AlGaN material to form an Al0.6Ga0.4N-based MSM solar-blind UV photodetector with a Ni/Mo L/AlGaN/Mo L/Ni structure.The chemisorption properties of porphyrin organic molecules were characterized by XPS,and 5,10,15,20-T(4-OH P)P organic molecules combined with hydroxyl groups on the surface of AlGaN to form a dense and ordered organic monolayer(Mo L).The high dipole moment of the organic molecule and the properties of its porphyrin molecule can reduce the interfacial potential caused by material defects in AlGaN and lead to an increase in the number of electrons in the semiconductor conduction band under illumination conditions,thereby increasing the photogenerated carriers of the device.Surface modification of 5,10,15,20-T(4-OH P)P organic molecules significantly improves the photocurrent and responsivity of the Al0.6Ga0.4N MSM photodetector compared with the unmodified reference photodetector,with a high peak responsivity of 5.49 A/W at 8 V,and the light-dark current ratio is significantly improved.Even at a high temperature of 425 K,the leakage dark current measured by the surface-modified AlGaN device did not increase significantly,indicating that the 5,10,15,20-T(4-OH P)P organic molecule modified Al0.6Ga0.The 4N MSM photodetector has the potential for high temperature solar-blind UV detection applications.In addition,the potential mechanism of this porphyrin organic molecule to enhance the performance of the AlGaN MSM solar-blind UV photodetector is explored through energy band diagrams.