Study on the Structure and Properties of Hydrogen-free Germanium Carbide Films Prepared by Magnetron Co-Sputtering
|School||Harbin Institute of Technology|
|Keywords||Non-hydrogenated germanium carbide Co-magntron sputtering Bonding structure Mechanical and optoelectronic properites Uniform film oflarge area|
Great attention has been given to the development of Group Ⅳ materials inthe field of material science. The amorphous germanium carbide （a-Ge1xCx:H）,which have received considerable attention and experimental study, reflecting thehigh interest in these materials because of their exciting structural, optical, andelectrical properties. In particular, the refractive index values of a-Ge1xCx:H maybe adjusted by x in a wide range, this excellent performance makes the a-Ge1-xCx:Hfilms applicable for design and preparation of multilayer anti-reflection andprotection coatings of IR windows. In addition, the band gap of a-Ge1-xCx:H filmscan be changed with x in a very wide range, this makes a-Ge1-xCx:H films goodcandidates in the design of electronic devices and photovoltaic cells. Therefore, a-Ge1-xCx:H is a new semiconducting material with great potential application.However, a great deal of hydrogen content from the precursors has been remainedin the films. If the service temperature is raised up to350oC, the properties of a-Ge1-xCx:H films will gradually deteriorate because of the broken C–H and Ge–Hbonds in the films. Therefore, the amorphous non-hydrogenated germanium carbide(a-Ge1-xCx) films with a better stability are urgently demanded for the harshapplications. However, so far, there are very few reports on a-Ge1-xCxfilms. In thispaper, the a-Ge1xCxfilms were prepared using magnetron co-sputtering technique.The structural, mechanical, optical and electrical properties were focused on anddiscussed.The germanium target sputtering power and substrate temperature areimportant parameters affectting the film microstructure. As the germanium targetpower increases, the deposition rate and Ge content increase. The deposition rateand germanium content scarcely depond on the substrate temperature. XRD resultsshowed that the films are amorphous, and AFM results demonstrate that the largersurface roughness of the film with high germanium target sputtering power and lowsubstrate temperature. FTIR spectra show that Ge-C bond vibration peak positionsare at about610cm-1, and Ge target power and substrate temperature graduallyincrease, resulting in the Ge-C peak position shifted to lower wave number. Ramanspectroscopy results show that germanium clusters and carbon clusters are in thefilms. As the germanium target power increases from40W to160W, the ratio ofRaman I（D）/I（G） reduces from1.17to0.75, the G peak position reduces from1572cm-1to1563cm-1, which implies that the sp2carbon content gradually reduced in the film. As the substrate temperature increases from room temperature to600°C,the ratio of Raman I（D）/I（G） rises up from0.80to1.31, the G peak positionincreases from1561cm-1to1576cm-1, which implies that the sp2carbon contentgradually increases in the film. XPS results show that the carbon atoms form sp3C-C, sp2C-C and Ge-C in the film. As the germanium target power increases from40W to160W, the relative content of sp3C-C and Ge-C increases, while the relativecontent of sp2C-C significantly reduces. As the substrate temperature rises up fromroom temperature to600°C, the relative content of the Ge-C reduces slightly, therelative content of sp3C-C decreased rapidly, while the relative content of sp2C-Csignificantly increases, suggesting that a progressive inecrease in the size ofgraphitic clusters.The density, hardness,Young’s modulus and compressive stress of the a-Ge1-xCxfilms were respectively measured using X-ray reflectivity, surface profilemeter andnano-indenter. It was identified that the density increases from3.67g/cm3to4.65g/cm3, the hardness increases from5.6GPa to8.0GPa and Young’s modulusincreases from100GPa to123Gpa, as the germanium target power increases from40W to160W; the density increases from4.39g/cm3to4.47g/cm3, the hardnessincreases from7.5GPa to9.2GPa and Young’s modulus increases from121GPa to141Gpa, as the substrate temperature rises up from room temperature to600°C. Thecompressive stress decreases gradually from70MPa to5Mpa, as the germanium targetpower decreases. The compressive stress inecreases from50MPa to330Mpa, as thesubstrate temperature inecreases room temperature to600°C, hinting that the defectsreduce and coordination number of germanium atoms increases.In order to strudy the properties of thermal stability, the a-Ge1-xCxfilms depositedat140W and200°C were respectively annealed at400900°C for1h under vacuumconditions. The structural variations of the as-annealed a-Ge1-xCxfilms were studied byXPS and Raman spectroscopy. Raman analysis shows that the Ge-TO peak position risesup300cm-1, the I（D）/I（G） ratio rises up from1.10to1.20, the G peak positionincreases from1559cm-1to1569cm-1, as the annealing temperature rises up from400to700°C. Furthermore, the relative content of sp3C-C, sp3C-C and Ge-C almostremain below700°C, indicating very good thermal stabilities below this temperature.The hardness of the as-annealed a-Ge1-xCxfilms were disscussed. The results showthat the hardness increases from5.6GPa to8.0Gpa. It was identified that the hardnessincreases gradually from8.3GPa to11.8GPa with the annealed temperature from400to700°C, and then reduces sharply to4.9GPa at800°CThe optical and electrical properties of the a-Ge1-xCxfilms were repectively studied by ellipsometry, FTIR, and temperature dependence resistance measuringsystem. As the germanium target power increases, the refractive index increasesfrom3.0to4.5and the extinction coefficient increases from0.12to1.15at623.8nm, the refractive index increases from2.8to4.1at9μm, while the opticalgap reduces from1.55to1.05eV, and the average transmittance of a-Ge1-xCx/ZnSsysterm reduces from49.5%to42.5%in range of812μm. The refractive index,the extinction coefficient, the optical gap and the average transmittance of the a-Ge1-xCxfilms scarcely change as the substrate temperature increases. Thetemperature dependence of conductivity indicates the thermally activatedconduction of carriers in extended states and hopping conduction in localized bandtail states near conduction band edges at above and below400K, respectively. Asgermanium target power and substrate temperature increase, the room-temperatureconductivity increases and activation energy decreases.The absorption coefficient and response range of Ge1-xCxfilms are larger thanthat of a-Si:H films, and the especially Ge1-xCxfilm with low carbon content havemore advantage in the field of pv-tech application. According to the optical principle,a double-layer antireflection and protection coating was deposited on the single sidepolished polycrystalline ZnS. The value of the IR transmittance of ZnS wafer withthe double-layer coating is increased by8%at9.6μm, and the hardness is up to13.1GPa, which implies that a-Ge1-xCxfilms can be used as an effective antireflectionand protection coating for the ZnS IR window. The excellent optical transmission foran antireflection a-Ge1-xCxdouble-layer film on ZnS substrate is still maintainedafter annealing at400oC.To grow uniform films on a large substrate by magnetron sputtering with a smalltarget, a new theoretical model has been proposed, implemented and confirmed. Themodel was based on a magnetron sputtering system containing a rotation substrateholder and a step-moving target. A magnetron sputtering system containing a rotationsubstrate holder and an Φ50mm step-moving target has been established by us. Whentarget stay time was proportional to the target scanning area at the rotating substrateand target moving step was5mm, the relative deviation of film thickness distributionwas less than5%within a diameter of Φ300mm. The numerical results agreed wellwith measurements, which demonstrated that our model was applicable to depositinglarge uniform film.