The Photocurrent Phenomenon Of Conductive Surface Layer Of Strontium Titanate Single Crystal

SrTiO₃ crystal is an insulator of perovskite.Early,the study of SrTiO₃ focused on the SrTiO₃ conductive layer obtained by doping metal ions such as Nd,Cr,La etc.Ten years ago,it is found that two-dimensional electron gas（2DEG）with high mobility in the interface of SrTiO₃/La AlO₃（STO/LAO）,thus the researchers paid more attention on SrTiO₃ and found other properties in the interface of STO/LAO,such as high mobility（10000cm2/v·s,at 4K）,superconductivity and persistent photocurrent.And later the researchers found that Ar⁺ bombardment also caused conductive layer with high mobility in the surface of SrTiO₃ and the properties of this conducting layer are similar to STO/LAO interface.Therefore,we can use an easier way to transform SrTiO₃ from insulator to metal,instead of STO/LAO,and even in this way it could enlarge its utilization potentiality.Photocurrent phenomenon has been being a key topic of scientific research since it was found in selenium conductor.It found that optical excitation phenomenon is an effective way to explore the trap distribution of semiconductor.Especially,sub-bandgap photocurrent is been thought by the contribution of band-in trap energy levels,and its decay curve can extract trap characteristics of materials.Moreover,nowadays the photoelectric device is the indispensable component of social productivity.Thus the study on the photoelectric response of materials makes great difference.Photoelectric response phenomenon is one of properties of Ar⁺ bombarded SrTiO₃ conductive layer.As is sated above,in this paper,it studies photocurrent properties of the surface conductive layer of SrTiO₃ crystal by Ar⁺ bombardment.Its main idea is as follows:Firstly,using Ar⁺ beam energy of Hall ion source is 200 eV at a vacuum of 8 x 10-4 Pa and under the condition of 0.8A electron beam to bombard SrTiO₃ to get stable SrTiO₃ surface conductive layer.The sheet resistance of SrTiO₃ conductive layer dependence of the bombardment time with logRs^-t.In a certain period of time,as the bombardment increases,the mobility increases rapidly and saturated soon.While the carrier concentration will continue to increase and the surface carrier concentration is on the order of 1014.In this paper,the mobility of the metal sample at 77 K is about 160 cm2/v·s.The field effect test of SrTiO₃ surface conductive layer shows that the semiconductor sample has a field effect phenomenon and is turned on at VGS> 0 when the back gate voltage VGS <0 is off.Secondly,when we use a laser with a wavelength of 520 nm（2.38 eV）to excite the SrTiO₃ conductive layer,only the samples with a semi-state and a weak metal state had a photocurrent response.Their photocurrent decay characteristics are different from those of the reported metal-state samples at 405 nm and UV light excitation.Their photocurrent decay faster at low temperatures.The analysis of the photocurrent decay curve shows there is a broad shallow-level trap state near the conduction band of the SrTiO₃ conductive layer in the semiconducting state and the central level of the trap state is below the conduction band at 82.8 meV.At high temperatures,more deep-level traps contribute to photocurrent,so decay is slower.For the metallic conductive layer,the decay of the photocurrent is independent of the temperature,and the thermal barrier of the nonequilibrium carrier in the trap trapping conduction band is only about 5.0 meV.This very low potential barrier and photonic current decay rate is not dependent on the typical characteristics of the temperature indicates that the metal conductive layer is mainly multi-phonon auxiliary tunneling effect caused by the composite mechanism.Thirdly,by analyzing the formation mechanism of photonic current in the sub-bandgap,the study builds the relationship between the photon-borne unbalanced carriers and the position of the hole trap and extracts the hole trap distribution of SrTiO₃ conductive layered.The results show that the hole trap at the high temperature（T> 250K）is far from the valence band to contribute to the photocurrent.Finally,the back gate electric field can not modulate the surface resistivity of the metallic SrTiO₃ conductive layer.However,it can significantly affect the photogenerated unbalanced carrier: when the back gate electric field is negative,the decay of the photocurrent becomes faster and the back gate is positive becomes slower.These results show that the decay of SrTiO₃ surface photocurrent is also affected by the surface barrier.The role of surface barrier facilitates the separation of photogenerated carriers（electronhole pairs）in space and reduces their recombination rates,leading to the occurrence of persistent photocurrent phenomena.