沉积功率对溅射制备镓镁共掺杂氧化锌薄膜光学和电学性能的影响.pdf

第 42 卷第 6 期2023年 11 月Vol.42 No.6Nov.2023中南民族大学学报（自然科学版）Journal of South-Central Minzu University（Natural Science Edition）Influence of deposition power on the optical and electrical performance of sputtered gallium-magnesium co-doped zinc oxide thin filmsZHONG Zhiyou1,2，WAN Xin1，GU Jinghua3*，LONG Hao1,2，YANG Chunyong1,2，CHEN Shoubu1（1 College of Electronic Information Engineering，South-Central Minzu University，Wuhan 430074，China；2 Hubei Key Laboratory of Intelligent Wireless Communications，South-Central Minzu University，Wuhan 430074，China；3 Experimental Teaching and Engineerring Training Center，South-CentralMinzu University，Wuhan 430074，China）Abstract The transparent conductor thin films of gallium-magnesium co-doped zinc oxide（ZnO：Ga-Mg）were deposited by magnetron-sputtering process.The deposition power dependence of structural，morphological，optical and electrical properties of the thin film was characterized by various techniques.The experimental results indicate that all the thin films have hexagonal wurtzite structure with highly c-axis preferred orientation along the（002）plane，and the deposition power strongly affects the properties of ZnO：Ga-Mg thin films.The thin film prepared at the deposition power of 150 W exhibits the best crystallinity quality and photoelectric properties，with the highest average visible transmittance of 92.2%，the lowest resistivity of 1.1810-3 cm，the maximum figure of merit of 1.04104-1cm-1，and the minimum lattice strain of 1.9510-3 and dislocation density of 1.171015 m-2.The optical constants of the thin films were obtained by the optical characterization methods.The optical dispersion behavior of the thin films was studied in terms of the single-oscillator Wemple-DiDomenico（WDD）model，and the oscillator parameters，non-linear optical constants and optical energy-gaps were achieved.The results demonstrate that the deposition power is one of the most important processing parameters to affect the structure，optical and electrical properties of ZnO：Ga-Mg thin films.Keywords ZnO；thin films；doping；photoelectric properties中图分类号 TM914；文献标志码 A 文章编号 1672-4321（2023）06-0809-10doi：10.20056/ki.ZNMDZK.20230612沉积功率对溅射制备镓镁共掺杂氧化锌薄膜光学和电学性能的影响钟志有1,2，万鑫1，顾锦华3*，龙浩1,2，杨春勇1,2,陈首部1（1 中南民族大学 电子信息工程学院，武汉 430074；2 中南民族大学 智能无线通信湖北省重点实验室，武汉 430074；3 中南民族大学 实验教学与工程训练中心，武汉 430074）摘要 采用控溅射工艺制备了镓镁掺杂氧化锌（ZnO：Ga-Mg）透明导电薄膜，通过多种表征技术研究了沉积功率对薄膜结构、形貌、光学和电学性能的影响.实验结果表明，所有薄膜均为六角纤锌矿结构并且具有c轴择优取向生长特点，并且沉积功率明显影响薄膜的性能.沉积功率为150 W时所制备的ZnO：Ga-Mg薄膜具有最好的结晶质量和光电性能，对应的平均可见光透过率为92.2%、电阻率为1.1810-3 cm、品质因数为1.04104-1cm-1、晶格应变为1.9510-3、位错密度为1.171015 m-2.另外，利用光学表征方法获得了ZnO：Ga-Mg薄膜的光学常数，同时根据单振子WDD模型研究了薄膜的光学色散性质，得到了薄膜的振子参数、非线性光学常数和光学能隙.研究结果表明沉积功率是影响ZnO：Ga-Mg薄膜结构和光电性能的最重要的工艺参数之一.关键词 氧化锌；薄膜；掺杂；光电性能收稿日期 2022-11-25 *通信作者 顾锦华，研究方向：光电子材料与器件，E-mail：作者简介 钟志有（1965-），男，教授，博士，研究方向：能源光电子、光电信息功能材料与器件，E-mail：基金项目 国家自然科学基金资助项目（12075322）；中南民族大学实验室研究资助项目（SYYJ2022008）第 42 卷中南民族大学学报（自然科学版）Gallium-doped zinc oxide（ZnO：Ga）is a promising transparent conductor material which has numerous applications in modern technologies such as organic light emitting diodes（OLEDs）1-2，flat panel displays（FPDs）3-4，thin film transistors（TFTs）5-6，photovoltaic cells（PVCs）7-8，gas sensitive devices9-10 and ultraviolet（UV）photodetectors11-13.Besides good electrical conductivity and high visible light transparency，the ZnO：Ga thin films have a variety of advantages，such as material abundance，non-toxicity，low manufacture cost，high exciton binding energy（about 60 meV at room temperature），broad direct energy-gap（about 3.3 eV at room temperature）and excellent chemical-stability under hydrogen plasma，as compared to the tin-doped indium oxide（In2O3：Sn）thin films14-17.In order to further improve the optical and electrical properties of ZnO：Ga thin films，the co-doping process with two elements have been used.Up to now，the titanium-gallium（Ti-Ga），aluminum-gallium（Al-Ga），gallium-zirconium（Ga-Zr），gallium-indium（Ga-In），boron-gallium（B-Ga），gallium-fluorine（Ga-F），magnesium-gallium（Mg-Ga）and nickel-gallium（Ni-Ga）co-doping cases have been reported18-27.However，few reports have been devoted to the non-linear optical properties and optical dispersion behavior of the co-doped ZnO thin films.In this work，the Ga-Mg co-doped ZnO（ZnO：Ga-Mg）thin films were prepared by radio-frequency（RF）magnetron-sputtering process under various deposition powers.The dependence of structure，morphology，photoelectric and non-linear optical properties of the thin films on power was investigated in detail.In addition，the optical constants of the thin films were obtained using the optical characterization methods，and the optical dispersion behavior was studied in terms of the single-oscillator Wemple-DiDomenico（WDD）model.1ExperimentalA quartz glass was employed as the transparent substrate，and a ceramic target（ZnO：95 wt%，Ga2O3：3 wt%，MgO：2 wt%，4N in purity）was used as the sputtering source material.The ZnO：Ga-Mg samples were prepared on the quartz glass substrates by RF magnetron-sputtering system（13.56 MHz）.The base pressure in deposition chamber was kept blow 2.2510-4 Pa and high purity argon gas（5N in purity）was used as the working gas.Prior to the ZnO：Ga-Mg samples deposition，the pre-sputtering for 20 min was conducted in order to clean contamination on the ceramic target surface.The ZnO：Ga-Mg samples were prepared under the following processing parameters：7.5 cm of target-substrate distance，300 of substrate temperature，3.5 Pa of gas pressure and 40 min of sputtering time.In order to investigate the effect of deposition power on the properties of ZnO：Ga-Mg thin films，the power was controlled from 110 to 170 W.The thickness of the ZnO：Ga-Mg thin films was measured by an Alpha-step 500 type surface profiler.The surface morphology of the thin films was observed by a JSM-6700F type scanning electron microscope（SEM）.The electrical properties of the thin films were investigated at room temperature using a RH-2035 type four-point probe measurement system.The X-ray diffraction（XRD）patterns of the thin films were characterized with a D8-Advanced type diffractometer using standard Cu K source（wavelength=0.15406 nm）.The optical transmittance（T）of the thin films were measured at room temperature by using a TU-1901 type double beam UV-visible spectrophotometer.The optical constants of the ZnO：Ga-Mg thin films were obtained from the measured transmittance data using the method of optical spectrum fitting28.All measurements were performed in ambient air.2Results and discussionFigure 1 shows the XRD patterns of the standard ZnO powder（PDF 036-1451）and the ZnO：Ga-Mg samples prepared at various powers.From the figure，we note that these XRD peaks of the investigated samples can be assigned to ZnO according to PDF 036-1451 card.All the investigated samples exhibit a dominant（002）peak with slight（101）and（004）peaks，indicating that the ZnO：Ga-Mg thin films have 810第 6 期钟志有，等：沉积功率对溅射制备镓镁共掺杂氧化锌薄膜光学和电学性能的影响hexagonal wurtzite structure of ZnO with preferred c-axis orientation along the（002）plane，regardless of power.Also，no diffraction peaks from other impurities can be detected from Figure 1，which indicates that all the ZnO：Ga-Mg thin films in this work do not have any phase segregation or secondary phase formation.Similar results have been reported by other researchers who investigated the structural properties of ZnO-based thin films18，21，29-30.The intensity of（002）diffraction peak（I(002)）for the ZnO：Ga-Mg samples as a function of power is shown in Figure 2a.As can be seen，the value of I(002)rises first and thereafter drops with the increment of power.The ZnO：Ga-Mg thin film deposited at the power of 150 W presents the highest intensity of（002）diffraction peak.The degree of preferred（002）orientation of the ZnO：Ga-Mg thin films was quantified by means of the orientation factor（P(002)）31：P(002)=I(002)NI(hkl)100%，（1）where I(002)is the intensity of（002）diffraction peak，the subscript h，k and l are Miller indices，I(hkl)denotes the diffraction intensity of the（hkl）plane，and N is the number of the diffraction peaks.Figure 2b shows the variation of P(002)with power for the ZnO：Ga-Mg thin films.It is found that with increasing power from 110 to 170 W，the P(002)value raises first and subsequently falls.When the power is 150 W，the maximum P(002)value can be obtained，indicating that the ZnO：Ga-Mg sample has the highest c-axis preferred orientation when the power is 150 W.The mean particle size（Dm）of the ZnO：Ga-Mg samples was evaluated using the Debye-Scherrer formula31-32：Dm=0.89cos，（2）where is the Bragg s diffraction angle，is the full-width at half-maximum（FWHM，in radians）of（002）diffraction peak，and denotes the wavelength of XRD measurement used（=0.15406 nm）.The lattice strain（0）and the dislocation density（0）can be obtained by means of the following relationships33-34：0=Dmsin-tan，（3）0=1D2m，（4）where Dm represents the mean particle size，is the value of FWHM in radians，denotes the Bragg s angle，and is the wavelength of X-ray used.Figure 3 shows the variation in the，Dm，0 and 0 values of the investigated thin films as a function of power.It can be noticed from Figure 3 that with rising power from 110 to 170 W，the values of，0 and 0 increase in advance and then fall，but the Dm takes on an opposite trend.When the power is 150 W，the ZnO：Ga-Mg thin film exhibits the optimum crystallinity quality and structural properties，with the narrowest （4.9110-3 rad），the largest Dm（29.3 nm），the lowest 0（1.9510-3）and the minimum 0（1.171015 m-2），respectively.This result demonstrates that the grain growth and structural properties of the ZnO：Ga-Mg thin films are subjected to the deposition power.图1 PDF 036-1451和不同功率时所制备薄膜的XRD图谱Fig.1XRD patterns of PDF 036-1451 and the thin films prepared at various powers图2 不同功率时所制备薄膜的I（002）和P（002）数值Fig.2The values of I（002）and P（002）for the thin films prepared at various powers811第 42 卷中南民族大学学报（自然科学版）Figure 4 presents the dependence of optical transmittance T on wavelength for the ZnO：Ga-Mg samples prepared at various powers.As can be seen，all the T-curves exhibit an interference pattern where the transmittance falls rapidly at the edge of the bands，which indicates excellent film crystallinity and low surface roughness.Also，the absorption edge is observed to blue shift firstly with the increasing power from 110 to 150 W and then red shift from 150 to 170 W，as shown in the inset of Figure 4.The power dependence of the mean transmittance（Tm）in the visible wavelength range for the ZnO：Ga-Mg samples is presented in Figure 5a.Note that the Tm value exceeds 85.7%for the investigated samples regardless of power，which indicates that all the ZnO：Ga-Mg thin films possess high transparency in the visible light region.The highest value（92.2%）of Tm for the ZnO：Ga-Mg sample can be achieved when the power is 150 W.图3 不同功率时所制备薄膜的，Dm，0和0数值Fig.3The values of，Dm，0 and 0 for the thin films prepared at various powers图4 不同功率时所制备薄膜的T-曲线Fig.4The curves of T-for the thin films prepared at various powers图5 不同功率时所制备薄膜的Tm，和FM数值Fig.5The values of Tm，and FM for the thin films prepared at various powers812第 6 期钟志有，等：沉积功率对溅射制备镓镁共掺杂氧化锌薄膜光学和电学性能的影响The enhancement of optical transmittance may be caused by the improvement of crystallinity quality and structural properties of the ZnO：Ga-Mg thin film.In order to quantify the photoelectric properties of the ZnO：Ga-Mg transparent conductor oxide(TCO)thin films，the figure of merit（FM）was introduced.The FM is defined by the following relation35-36：FM=1log()1 Tm，（5）where denotes the resistivity of the investigated sample，and Tm is the average transmittance in the visible range.The power dependence of and FM for the ZnO：Ga-Mg samples is shown in Figure 5.It is observed from Figure 5b that the falls firstly with the rising power from 110 to 150 W and then increases from 150 to 170 W.The minimum （1.1810-3 cm）of the ZnO：Ga-Mg sample can be obtained when the power is 150 W.The optimal value in this work is comparable to the results of previous studies.For the ZnO-based thin films deposited by RF-sputtering technique，the lowest values were reported to be ranging from 7.2310-4 to 1.5210-3 cm19，37-38.Note also from Figure 5c that the FM values are found to be 1.26103，2.49103，1.04104 and 1.67103-1cm-1 for the ZnO：Ga-Mg samples fabricated at the power of 110，130，150 and 170 W，respectively.Clearly，the FM raises first and subsequently falls with the increment of power，the ZnO：Ga-Mg thin film deposited at the power of 150 W exhibits the maximum FM value，indicating that the optimum deposition power is 150 W for preparing ZnO：Ga-Mg thin film in the present work.The direct optical energy-gap（Edg）of the ZnO：Ga-Mg thin films was evaluated by using the Tauc s relation in the region of high absorption39-40：(h)2=B(h-Edg)，（6）=ln()1 Ttf，（7）where h is Plancks constant，is the photon frequency，B is an energy-independent constant，tf is the film thickness，and is the absorption coefficient of the thin film41-42.Figure 6 shows the(h)2 vs.h plots of the ZnO：Ga-Mg samples prepared at various powers.As can be seen，a good straight line can be obtained in the band edge region for all the investigated thin films.The straight-line portion of the curve gives the direct optical energy-gap Edg when extrapolated to（a）110 W；（b）130 W；（c）150 W；（d）170 W图6 不同功率时所制备薄膜的（h）2-h曲线Fig.6The curves of（h）2-h for the thin films prepared at various powers813第 42 卷中南民族大学学报（自然科学版）zero（(h)2=0）.The values of Edg are evaluated to be ranging from 3.41 to 3.49 eV for the ZnO：Ga-Mg samples prepared at various powers.Obviously，the obtained Edg values of all the ZnO：Ga-Mg thin films are larger than that of standard ZnO sample（3.30 eV）18，43.The broadening in Edg of the ZnO：Ga-Mg thin films mainly be attributed to the Burstein-Moss（B-M）effect44-47.Similar results have been observed by many researchers who studied the optical properties of ZnO-based thin films47-50.Based on the measured transmittance，the optical constants including extinction coefficient（k）and refractive index（n）of the ZnO：Ga-Mg thin films were obtained by the method of optical spectrum fitting28，and the dependence of k and n on for all the samples is shown in Figure 7.It can be seen from Figure 7a that the k values of the investigated samples are very small at long wavelength region，which indicates that all the investigated thin films have high visible transparency.Similar to the k-curves，the n gradually falls with raising for all the ZnO：Ga-Mg samples.The result suggests that all the ZnO：Ga-Mg thin films exhibit the normal dispersion characteristics in the visible wavelength range31.For the ZnO：Ga-Mg samples deposited at the power of 110，130，150 and 170 W，the values of k and n are 1.1610-2，1.87；8.9310-3，1.96；3.5410-3，1.91；and 8.4510-3，1.94 at=450 nm，respectively.The result is in agreement with the previously reported works51-53.The refractive index dispersion behavior of the ZnO：Ga-Mg samples was studied according to the single-oscillator WDD model as follows54-55：(n2-1)-1=Ed-1(Eo-Eo-1E2)，（8）E=ch-1，（9）where is the wavelength of incident light，h denotes Pl