On Preparation And Photoelectric Modification Of Ge_1-xC_x Thin Films By DC Magnetron Sputtering And Doping Of Aluminum

The composition of Ge1-xCx thin film is simple and easy to obtain,and the optical band gap can be adjusted in a wide range depending on the Ge/C content and the Ge-C bonding ratio.It can be used as a new type of efficient and low-cost semiconductor thin film material with great potential in the field of infrared optical/photovoltaic materials.Unbalanced conditions are required to prepare Gei_xCx films.At present,Ge and C gas sources containing H combined with plasma-assisted vapor deposition are used to prepare thin films with high cost,difficult to control the proportion of Ge/C components,and poor thermal stability due to H in service.Therefore,this work attempts to explore the feasibility and major technical difficulties of preparing H-free Gei_xCx thin films directly based on low cost Ge/C(graphite)pure component target material by using multiple target alternating DC sputtering,an effective method to control Ge/C content in thin films based on sputtering yield model was studied,by changing sputtering conditions and orthogonal experimental optimization of sputtering process parameters related to unbalanced conditions,the key factors and laws affecting the proportion of Ge-C bond formation in films were revealed,experimental study on the effect and basic rule of Al doping on improving photon absorption and electrical properties of Ge1-xCx films.Research shows:1)The H-free Ge1-xCx film can be prepared by DC-MS technology combined with Ge and C pure component target alternate sputtering.2)Based on the sputtering yield model and parameter modification,the C content of ge-c binary film can be effectively regulated.The C content regulation error level is maintained between-7%and 4%under various sputtering conditions.3)When the C content is uniform at 31.5 at.%,the formation ratio of Ge1-xCx in Ge-C binary films prepared by changing the target power and target configuration varies from 2.5%to 8.2%,and the optical band gap of thin films varies from 0.91 ev to 1.11ev.Compared with low target power,high target power deposition(IGE?2.31 A,Ic=3.5 A)is not conducive to the formation of high Ge1-xCx ratio and obtaining wide optical band gap level.4)Based on the non-equilibrium condition related sputtering parameters such as C target current(ic),substrate bias(US)and substrate speed(?),the orthogonal experiment was designed to deposit Ge-C binary film.It was found that the sputtering parameters had an impact on the formation ratio of Ge1-xCx and the optical band gap of Ic>?>Us.With the change of sputtering parameters,the formation ratio of the film Ge1-xCx varies from 4.1%to 8.9%,and the corresponding optical band gap varies from 1.08 eV to 1.15 eV.The optical band gap is basically consistent with the variation trend of the formation ratio of Ge1-xCx.Among various parameter combinations,when Ic=0.96 A,U,=-35 V,and ?=7 r/min,the non-equilibrium conditions are most favorable and the highest level of Ge1-xCx ratio and optical band gap level is obtained.5)Although it is feasible to prepare H-free Ge-C binary films by alternating deposition of pure component targets,there are technical difficulties in achieving uniform mixing of Ge and C components.Especially when Ge-C binary film is deposited with high target power,Ge and C components tend to be layered.At this time,only the mixing zone near the interlayer interface is likely to form Gei-xCx.However,at low target power,even if the substrate bias,substrate rotation speed and other non-equilibrium conditions are favorable,the Ge clusters distributed at nanoscale on the amorphous C matrix will form an mosaic structure in the Ge-C binary film.At this time,only the Ge cluster peripheral mixing zone has the possibility of Gei-xCx formation.Although the Ge1-xCx formation ratio is improved(compared with the high power condition),it still leads to the unsatisfactory overall Ge1-xCx formation ratio and optical band gap level of the film.6)A simple sputtering yield model can be used to control the Al doping ratio with a certain accuracy(about 5 at.%).The error is mainly caused by the low sputtering threshold,which leads to the self-sputtering loss of Al components in the film forming process.NWhen the actual Al doping ratio is higher than 10 at.%,the conductivity of Ge1-xCx thin film can be improved by nearly two orders of magnitude,but its absorption property does not change much.However,Al doping also causes the optical band gap of thin film to drop gradually from 1.11 eV before doping to 0.80 eV(corresponding to the actual ratio of Al doping about 27.3 at.%).Therefore,A1 doping is only beneficial to improve the overall photoelectric performance of the film at a lower ratio.