1 实验方法
1.1 铌酸钾钠薄膜的制备
用溶胶凝胶法制备K0.5Na0.5NbO3(KNN)和xMn-(K0.5Na0.5)NbO3前体液，其中x分别为0.05和0.10。将原料乙醇铌在90℃水浴加热1 h溶解在乙二醇甲醚中，得到含铌溶液。一份含铌溶液用来制备氧化铌种子层的前体液。向其他铌溶液按摩尔比加入乙酸钾、乙酸钠和乙酸锰，其中钾钠过量摩尔比10%，以弥补在薄膜制备过程中钾钠的挥发。水浴加热30 min后加入乙酰丙酮为稳定剂，再用醋酸调节溶胶溶度至0.4 mol/L。然后将溶胶静置48 h。为了准备薄膜，将溶胶旋涂在衬底和带单层氧化铌种子层的衬底上，然后在热板上热解，最后置于管式炉退火，重复6次得到薄膜。其中氧化铌种子层的制备工艺与KNN薄膜和KNNM薄膜制备工艺相同。
实验中使用Pt/Ti/SiO2/Si衬底，使用匀胶机旋涂薄膜，转速为3500 r/min，时间30 s。采用层层退火的退火方式，即旋涂一次就热解和退火一次，重复6次。其中单次的热解温度和退火时间分别为200℃和700℃，对应的热解时间和退火时间为5 min和10 min。
1.2 铌酸钾钠薄膜性能的表征
用Cu Kα 辐射(λ=0.15418 nm)的X-射线衍射(XRD; Bruker D8 Advance)分析薄膜的结晶相。用FESEM(SU8000，Hitachi)表征薄膜的形态。使用Precision Premier II铁电测试系统(辐射技术)分析薄膜的铁电性能和漏电流。使用阻抗分析仪(TH2829C，同惠)测量介电常数。所有测量均在室温下进行。在进行电学测量之前，使用直流溅射在KNN膜的表面上制备金顶电极(ϕ=200 μm)。
2 结果和讨论
2.1 铌酸钾钠薄膜的相结构和表面形貌
图1给出了KNN薄膜的XRD。可以看出，薄膜样品都具有典型的钙钛矿晶体结构，没有出现第二相。KNN、5%Mn-KNN、10%Mn-KNN、NbO/5%Mn-KNN和NbO/10%Mn-KNN薄膜的(100)结晶峰位置分别位于22.326°、22.469°、22.509°、22.162°和22.080°，与纯KNN薄膜相比，掺杂Mn元素的KNN薄膜(100)结晶峰位置向更高角度偏移。这表明，Mn离子的引入不影响薄膜的晶体结构，并且Mn能进入KNN的晶格中[15~17]。掺杂Mn的KNN薄膜晶格向高角度偏移，表明晶格尺寸减小。由于Mn2+半径(0.067 nm)小于K+(0.138 nm)和Na+(0.102 nm)[18]，而稍大于Nb5+半径(0.065 nm)，一部分Mn2+可能是A位掺杂。而带种子层的掺杂锰的KNN薄膜的结晶峰向更低角度偏移，表明KNN薄膜的晶格尺寸增大。薄膜受到的应力可用公式
图1
图2a~d分别给出了KNN、5%Mn-KNN和NbO/5%Mn-KNN薄膜的微观表面形态和KNN截面形态。可以看出，薄膜的形态有显著的差异，未掺杂的KNN薄膜其晶粒尺寸大小不一致，厚度约为250 nm，且在薄膜表面可见明显的裂纹和缺陷。一方面，表面形态缺陷可能是薄膜内部残余应力引起的，而内部残余应力是KNN薄膜与基底的热膨胀系数不匹配所致。另一方面，钾钠在热解和退火过程中挥发也使薄膜产生晶格缺陷。图2b给出了掺杂锰元素的KNN薄膜表面形貌，可见均匀分布的晶粒和一些纳米孔和裂纹。与未掺杂的KNN薄膜相比，薄膜表面缺陷较小，表明掺杂锰元素使薄膜的表面形态改善[15]。图2c给出了带氧化铌种子层的掺杂锰元素KNN薄膜，与未带种子层的掺Mn元素的KNN薄膜相比，带种子层的KNN薄膜表面的晶粒和裂纹更细密，没有出现明显的裂纹和孔洞。作为过渡层，氧化铌种子层抑制了薄膜内部残余应力引起的裂纹。
图2
图2
薄膜的SEM形貌
Fig.2
SEM images of the films (a) KNN films; (b) 5%Mn-KNN films; (c) NbO/5%Mn-KNN films; (d) KNN films section
2.2 铌酸钾钠薄膜的电学性能
图3给出了KNN系薄膜的介电频谱，其曲线在1 kHz至1 MHz频率范围内表现出共同的特征，即介电常数值随着频率的提高而略有减小，损耗正切值先减小然后略有增加(与频率无关)。同时，在高频区域中KNN薄膜的损耗角正切值低且稳定，表明KNN薄膜材料有微波应用的潜在价值。与纯KNN薄膜相比，掺杂锰元素的KNN薄膜及带种子层的KNN薄膜的介电常数值都有所增大。在1 kHz下，KNN介电常数最小为342.98，5%Mn-KNN和10% Mn-KNN次之，分别对应为424.92和353.63，NbO/5%Mn-KNN和NbO/10%Mn-KNN薄膜的介电常数较大分别对应为492.49和475.52。掺杂锰元素能提高介电常数，其原因是Mn元素补偿了因碱金属离子挥发引起的电荷缺陷并改善了KNM薄膜的微观结构。而带有种子层的锰掺杂的KNN薄膜介电常数值的增大，可能是其晶粒和表面形貌更细密所致。
图3
图4给出了KNN系薄膜的PE电滞回线和电场在200 kV/cm下薄膜的剩余极化强度和最大极化强度值。可以看出，纯KNN薄膜只显示了在较低电场下的PE电滞回线，且随着电场强度的增大而显示出不饱和的电滞回线。与其余KNN薄膜相比，纯KNN薄膜的剩余极化值最大。其原因是，纯KNN薄膜的剩余极化值中有漏电流的较大贡献。与之相对的，即使电场强度加大两倍，锰掺杂的KNN薄膜电滞回线也表现更加饱和，表明掺杂Mn元素可提高薄膜的铁电性能。值得注意的，在掺杂锰元素的KNN薄膜中引入种子层使薄膜的电滞回线更加饱和，并且带种子层锰掺杂摩尔浓度为10%的KNN薄膜其电滞回形更加细长，表明其较大的最大极化强度和较小的剩余极化强度值。具体表现是，在600 kV/cm的电场下薄膜的最大极化值和剩余极化值分别为20.33 µC/cm2和2.94 µC/cm2，其最大极化值与剩余极化值的差值为17.39 µC/cm2。这与Zhao等[10]用锰钽复合掺杂制备的KNN薄膜报道的值相当，表现出良好的储能材料的潜在价值。图4f给出了KNN薄膜在200 kV/cm电场下的最大极化值和剩余极化值。其中横坐标分(a)、(b)、(c)、(d)和(e)分别对应KNN、5%Mn-KNN、NbO/5%Mn-KNN、10%Mn-KNN和NbO/10%Mn-KNN薄膜。结果表明，掺杂锰元素和引入种子层，能有效提高薄膜铁电性能和降低薄膜的漏电流。
图4
图4
KNN、5%Mn-KNN、NbO/5%Mn-KNN、10% Mn-KNN、NbO/10%Mn-KNN薄膜的电滞回线以及薄膜在200 kV/cm电场中的最大极化值和剩余极化值
Fig.4
Hysteresis loop of KNN films (a), 5% Mn-KNN films (b), NbO/5% Mn-KNN films (c), 10% Mn-KNN films (d), NbO/10% Mn-KNN films (e) and Maximum polarization value and residual polarization value of the films in 200 kV/cm (f)
2.3 铌酸钾钠薄膜的漏电流与漏电机制
图5给出了KNN系薄膜的漏电流和电场的IE曲线。可以看出，纯KNN薄膜的漏电流最大，这是薄膜薄膜表面有较多的缺陷所致。而薄膜表面出现缺陷的原因，一方面是热解和退火过程中钾钠挥发，而另一方面是热应力失配。掺杂Mn元素可使薄膜表面缺陷减小，并且Mn元素能减少一部分钾钠挥发导致的薄膜晶格缺陷，这表现为图中比纯KNN薄膜有更小的漏电流。与其余KNN薄膜相比，带种子层且掺杂锰元素的KNN薄膜的漏电流最小，下降幅度也较大(下降一个数量级)。其原因是，一方面薄膜表面的缺陷更细小。另一方面，绝缘的氧化铌种子层进一步减小了薄膜的漏电流。Sun和Li等也观察到类似的结果[21,22]。为了进一步分析漏电流特性和传导机制，画出了KNN、5%Mn-KNN、NbO/5%Mn-KNN的漏电流与电场的关系曲线，以及lg(J)与lg(E)关系函数图，如图5b所示。施加电场达到83.3 kV/cm时，纯KNN薄膜的拟合线斜率发生改变。当电场小于83.3 kV/cm时拟合斜率接近2，表明导电机制为空间电荷传导[23]。电场大于83.3 kV/cm时拟合斜率接近1，为欧姆传导。掺杂锰元素的KNN薄膜，其导电机制与KNN薄膜类似。电场小于133.4 kV/cm时为空间电荷传导，电场大于133.4 kV/cm时为欧姆传导。带种子层且掺杂锰元素的KNN薄膜，其漏电流机制发生改变。电场小于158.5 kV/cm时，其导电机制为欧姆传导。电场大于158.5 kV/cm时其拟合斜率为4.96，表明其导电机制为肖特基发射机制[24,25]。这些结果表明，掺杂锰元素并不能改变薄膜的漏电流机制。而带种子层且掺杂锰元素的KNN薄膜，其漏电流改善的原因是漏电流机制发生改变。
图5
图5
KNN薄膜的电流密度与电场关系曲线以及KNN、5%Mn-KNN和NbO/5%Mn-KNN薄膜的In(J)与In(E)的关系曲线
Fig.5
Leakage current density (J) as a function of the applied electric field (E) for the KNN films (a) and ln(J)-ln(E) characteristics of KNN，5%Mn-KNN，NbO/5%Mn-KNN films (b)
3 结论
用溶胶凝胶法可制备K0.5Na0.5NbO3(KNN)和xMn-(K0.5Na0.5)NbO3薄膜。Mn掺杂使KNN薄膜的介电性能、铁电性能和漏电流提高，引入氧化铌种子层使薄膜的漏电流机制发生转变，使薄膜的最大极化值和更小的剩余极化值更大。
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