李晓伟等[8]计算了掺杂不同种类和含量的金属时非晶碳的电子结构，发现掺入过渡金属可降低这个体系的总畸变能量从而降低残余应力，掺杂Cu，可使其具有Cu-C反键态特征。Yaremchuk等[9]用磁控溅射技术沉积了不同Cu含量的a-C: H: Cu薄膜，发现掺杂Cu的非晶碳具有等离子体共振效应，共振吸收峰的位置向短波方向移动。Beauty等[10]用电化学沉积制备了不同Ni含量的非晶碳薄膜，随着Ni含量的降低，这种薄膜的光学带隙逐渐下降。Meskinis等[11]用反应磁控溅射制备Ni掺杂非晶碳新型敏感材料，研究了Ni掺杂对其电学和压阻性能的影响。结果表明，改变掺入Ni的含量可大范围调控非晶碳的电阻率，Ni含量低于4%(原子分数)的样品，其压阻系数大于3000，远比纯非晶碳和常用Si材料的高。
与晶体半导体不同，非晶半导体中既有扩展态又有定域态，而定域态电子只能通过隧穿或者热激活方式传导[12]。非晶碳中的碳原子主要以sp2和sp3杂化的形式存在，sp2杂化碳既有σ键又有π键。位于费米能级附近的π电子决定非晶碳的电学特性[13]，而π键又与原子间距和配位数密切相关。非晶碳复杂的键态结构，使其载流子输运机理的研究进展缓慢。Tripathi等[14]指出，纯非晶碳的载流子输运方式为Mott型变程跳跃传导。Abdolghaderi等[15]调控非晶碳中Ag的掺入量使载流子输运方式变为渗流特性。万蔡华等[16]发现，在非晶碳中掺入Fe使载流子的输运方式从低温到高温依次从Efros-Shklovskii(ES)型变为跳跃传导(<60 K)、Mott型三维变程跳跃传导(60~200 K)或热激活传导(>200 K)。掺杂多种金属与碳成键的特征差异较大[17,18]，金属能以固溶或以金属和碳化物纳米晶的形式存在于非晶碳基质中[7]。同时，随着金属含量的变化非晶碳的结构和载流子输运行为极为复杂。载流子的输运机理不明确，限制了碳基薄膜光电器件的设计和应用。
在非晶碳中掺入Cu使C 2p轨道与Cu 3d轨道杂化，电子占据能量较高的反键态，即Cu和C成反键特征[8]。这表明，a-C: Cu体系的电子结构不稳定，在非晶碳中掺杂Cu可使薄膜具有优异的光电特性。但是，掺杂过量的金属会降低非晶碳优异的机械性能和半导体特性。
与直流磁控溅射技术相比，高功率脉冲磁控溅射技术(HiPIMS)具有更高的离化率，可在较宽的范围调控离子能量，有利于调节薄膜的sp2-C/sp3-C比例和Cu含量从而制备出高致密涂层[19]。鉴于此，本文采用HiPIMS技术制备Cu含量(原子分数)为2.77%~7.28%的a-C: Cu薄膜，系统研究Cu含量对a-C: Cu薄膜组分结构的影响并分析薄膜的电学性能和载流子输运机理。
1 实验方法
1.1 样品的制备
用高功率脉冲磁控溅射制备不同Cu掺杂含量非晶碳薄膜，其设备(图1)包括一个离子束源和一个磁控溅射源。磁控溅射用的靶材为Cu(99.99%, 20 mm × 100 mm)与石墨(99.99%, 380 mm × 100 mm)拼接靶，使用HiPIMS电源，工作气体为Ar气。衬底为二氧化硅、石英玻璃以及单面抛光单晶硅片。将用无水乙醇清洗干净的衬底固定在四面体机架，然后置于真空沉积腔室中。如图1所示，将距离机架底部4、8、12、16 cm的样品分别命名为S1、S2、S3、S4。待腔室真空度达到2.7×10-3 Pa后向腔室通入40 mL/min的Ar气，离子束工作电压设定为1200 V，对衬底进行刻蚀清洗20 min。调节Ar气流量使腔室的气压达1.3×10-2 Pa。用HiPIMS电源沉积Cu掺杂非晶碳薄膜，溅射功率为500 W，频率为455 Hz，脉宽为200 μs。在机架上施加-200 V的偏压，并使其以120 Hz的频率自传。为了调控Cu含量，调整样品在机架上的位置以改变Cu靶与样品的靶基距。由于Cu的溅射产额大于C，因此越靠近Cu靶位置薄膜样品沉积速率越高。为了排除薄膜厚度对样品性能的影响，S1、S2、S3号样品的沉积时间为120 min，S4号样品的沉积时间为95 min，以使所有薄膜的厚度为205±20 nm。
图1
1.2 结构和性能表征
用热场发射扫描电子显微镜(SEM, Verios G4 UC)观察薄膜截面的形貌并测量其厚度。使用原子力显微镜(AFM)轻敲模式表征薄膜表面的形貌和粗糙度。使用X射线光电子能谱仪(XPS, Axis Ultra DLD)表征a-C: Cu薄膜的元素含量和化学键。测试前用4 kV的Ar+蚀刻样品4 min以去除表面污染物。用高斯和洛伦兹函数对C 1s峰进行分峰拟合，并计算sp2-C/sp3-C比例。用拉曼光谱(LAbRAMHR Evolution, 532 nm)分析a-C: Cu薄膜中碳原子键合结构。用X射线衍射仪(XRD, D8 ADVANCE DAVINCI)表征薄膜的相结构。用透射电子显微镜(TEM, Tecnai F20)表征薄膜的微观结构，根据电子能量损失谱(EELS)分析碳键结构。用紫外光电子能谱(UPS, Axis Ultra DLD)测量薄膜价带谱和二次电子截止边谱，其光源为能量21.22 eV的He I共振线。使用综合物理性能测试系统(PPMS, PPMS-EverCool)测量a-C: Cu薄膜在150~350 K的I-V特性和电阻率。使用紫外可见近红外分光光度计(Lambda 950)测量石英衬底上的薄膜在200~1600 nm范围的透过率并计算其光学带隙，测试间隔为1 nm，扫描速度为280 nm/min。
2 结果和讨论
2.1 薄膜的组分和结构
调控沉积时间，使a-C: Cu薄膜的厚度在186~221 nm范围变化。S1、S2、S3、S4样品的沉积速率分别为1.55、1.73、1.84和2.23 nm/min。随着样品靠近Cu靶，薄膜的沉积速率逐渐提高。图2给出了硅片上a-C: Cu薄膜截面的形貌。在Cu的含量较低时，Cu的掺入对薄膜的结构没有明显的影响，所有的薄膜均具有良好的致密性。同时，薄膜与Si衬底界面处也没有出现裂纹或剥落，表明a-C: Cu薄膜与衬底界面的结合良好。
图2
图2
Cu掺杂非晶碳薄膜截面的SEM形貌
Fig.2
SEM cross-sectional morphology of a-C:Cu films (a) S1, (b) S2, (c) S3, (d) S4
为了进一步研究Cu掺杂对非晶碳表面形貌的影响，用AFM表征了表面的形貌和粗糙度。图3给出了S1~S4样品的3D-AFM图像。可以看出，所有a-C: Cu薄膜的表面都连续和光滑，从S1到S4的粗糙度Ra分别为0.28、0.32、0.26、0.24 nm。Cu的掺入对非晶碳表面的粗糙度没有明显的影响，但是Cu含量不同的样品其表面形貌不同，S1、S2样品的表面凸起状结构较多，而S3、S4样品表面呈现更多的颗粒状结构。
图3
图3
Cu掺杂非晶碳薄膜的三维AFM表面形貌
Fig.3
3D-AFM images of a-C: Cu films (a) S1, (b) S2, (c) S3, (d) S4
图4给出了S1~S4样品的XPS光谱和拟合结果。图4a给出了全谱，可见所有a-C: Cu薄膜由C、Cu和O三种元素组成，其中O元素可能来自沉积过程中真空腔室中残余的空气[20,21]。如图4b所示，随着样品到机架底部的距离从4 cm逐渐增大到16 cm，Cu含量(原子分数)从2.77%提高到7.28%，表明实现了对Cu含量的调控。图4c给出了C 1s精细谱，为了得到sp2-C和sp3-C的相对含量，用高斯(80%)和洛伦兹(20%)函数将C 1s峰分为三个峰，分别对应结合能为284.6、285.4和286.6 eV的sp2-C、sp3-C和C-O/C=O[22]。如图4d所示，随着Cu含量的提高sp2-C的含量随之提高。Cu含量(原子分数，下同)为7.28%的sp2-C其含量最高(54%)，原因可能是Cu元素的引入促进了sp2-C的形成[23, 24]。图4e给出了Cu 2p的精细谱，位于933和953 eV附近的峰对应Cu 2p3/2和Cu 2p1/2，在933.0和953.2 eV之间没有出现卫星峰，表明a-C: Cu薄膜中没有Cu2+[25, 26]。为了进一步判断薄膜中Cu的存在形式，测试了Cu的俄歇谱(图4f)。位于568.3 eV的峰表明，a-C: Cu薄膜中的Cu主要以单质的形式存在[27]。
图4
图4
Cu掺杂量不同的a-C: Cu薄膜的XPS全谱、各元素的含量、C 1s精细谱及各杂化碳含量、Cu 2p的精细谱以及Cu的俄歇谱
Fig.4
XPS spectra (a), the contents of C, O, Cu (b), C 1s spectra (c), sp2, sp3, and C-O/C=O contents (d), Cu 2p spectra (e) and Cu LMM Auger spectra (f) of a-C: Cu films with different Cu contents
图5a给出了不同Cu含量a-C: Cu薄膜的拉曼光谱。在波数为800~2000 cm-1样品S1~S4的谱中都出现了典型的非晶碳非对称拉曼峰。用高斯拟合将非晶碳拉曼峰分为位于1360 cm-1附近的D峰和1550 cm-1的G峰[28]，其中D峰对应环状结构中sp2-C原子呼吸振动，G峰则对应环和链状结构中sp2-C原子的伸缩振动。根据G峰的半高宽(G-FWHM)、D峰与G峰的面积比(ID/IG)以及G峰位置(G-peak position)，可定性分析sp2-C团簇的尺寸和含量。如图5b所示，随着Cu含量从2.7%提高到7.28%，ID/IG从2.2增大到2.6，表明Cu的掺入有利于增大sp2-C团簇的尺寸。G峰位也随着掺入Cu含量提高而增大，Cu含量为7.28%时G峰位置达到最大值1549.5 cm-1。G-FWHM先增大后减小，表明Cu含量的提高使a-C: Cu薄膜结构的无序度先提高后降低。
图5
图5
Cu掺杂量不同的a-C: Cu薄膜的拉曼光谱及其拟合结果
Fig.5
Raman spectra (a), the fitting result (b) of a-C: Cu films with different Cu contents
图6给出了无衍射硅衬底上随Cu含量变化的a-C: Cu薄膜的XRD谱。可以看出，在所有样品的谱中均未出现明显的衍射峰，表明Cu含量为2.77%~7.28%的薄膜其主体仍为非晶态。
图6
图6
Cu掺杂量不同的a-C:Cu薄膜的XRD谱
Fig.6
XRD spectra of a-C:Cu films with different Cu contents
为了进一步探究薄膜的微观结构和掺杂Cu的存在形式，对Cu含量为2.77%和7.28%的样品进行了TEM表征。如图7a所示，Cu含量为2.77%的薄膜均匀致密，在HRTEM图中未出现明显的晶格条纹，表明非晶碳中的Cu以固溶的形式存在。值得指出的是，在图7b对应SAED的图中，衍射环(R=4.43 nm-1)对应的是Pt(111)晶面(d=0.229 nm)。在Cu含量为7.28%的非晶碳基质中出现了尺寸约为5 nm的纳米晶(图7c)，其SAED衍射环(R=4.76 nm-1)对应Cu的(111)晶面(d=0.209 nm)。这表明，随着Cu含量从2.77%提高到7.28%固溶在非晶碳中的Cu原子逐渐析出形成纳米晶，并均匀地分散镶嵌在非晶碳基质中。
图7
图7
Cu含量为2.77%和7.28%样品的高分辨电子图像和选区电子衍射图
Fig.7
HRTEM image and corresponding SAED of a-C: Cu film with 2.77% (a, b) and 7.28% Cu (c, d)
图8
图8
Cu掺杂量为2.77%和7.28%样品的电子能量损失谱
Fig.8
EELS spectrum of a-C: Cu film with 2.77% and 7.28% Cu
2.2 电学性能
图9
图9
Cu掺杂量不同的a-C: Cu薄膜300 K时的 I-V特性曲线和电阻率的变化
Fig.9
I-V characteristic plots (a) and electrical resistiv-ity (b) at 300 K of the a-C: Cu films with differ-ent Cu contents
图10给出了S1~S4样品150~350 K的I-V曲线。在整体上，所有样品在各温度下都表现出优异的线性I-V特性。测试电流一定时每个样品对应的电压都随着测试温度的升高而降低，表明a-C: Cu薄膜均具有典型半导体特性。同时，在相同温度下，测试电流一定时Cu掺杂量小的样品对应的电压值高。例如S1样品对应的电压值在所有样品中最大，与S1样品的薄膜电阻率R更大有关。
图10
图10
Cu掺杂量不同的a-C: Cu薄膜在150~350 K的I-V曲线
Fig.10
I-V plots of the a-C: Cu films with different Cu contents from 150 to 350 K
图11
图11
Cu掺杂量不同的a-C: Cu薄膜在150~350 K的R-T曲线
Fig.11
R-T behaviors of a-C: Cu films with different Cu contents from 150 to 350 K
其中R为电阻率，T为测试温度，R0和T0皆为常数，n取值1、1/4和1/2，其载流子输运行为分别对应热激活，Mott型三维变程跳跃传导以及计入Coulomb作用的变程跳跃传导。
图12给出了S1~S4样品150~350 K的拟合结果。对于掺入Cu含量较高的S3(5.4% Cu，原子分数)和S4(7.28% Cu，原子分数)样品，在150~350 K其ln(R)与T-1/4线性关系较为显著，对应的载流子传输方式为Mott型三维变程跳跃传导，如图12e、f所示。而Cu掺杂含量较低的S1(2.77% Cu，原子分数)和S2(3.88% Cu，原子分数)样品，在整个测试温度区间没有显示出单一的传输机制，在不同温度范围载流子的输运特性不同。如图12a、c所示，在150~250 K样品S1、S2的ln(R)与T-1/4呈现线性关系；而在250~350 K，ln(R)与T-1则显示出线性关系，如图12b、d所示。因此，Cu含量为2.77%和3.88%样品在250~350 K其热激活主要为输运机制；而在150~250 K，起主要作用的是Mott型三维变程跳跃传导。
图12
图12
Cu掺杂量不同的a-C: Cu薄膜在不同温度范围的ln(R)与T-1/4和T-1的关系曲线
Fig.12
Relationship between ln(R) and T-1/4 (a, c, e, f) and T-1 (b, d) at different temperature ranges for the samples with different Cu contents. The red lines are fitting results
2.3 能带结构
图13
图13
Cu掺杂量不同的a-C: Cu薄膜的透过率曲线、(αhν)2-hν曲线以及光学带隙
Fig.13
Transmittance (a), Tauc plots (b) and optical bandgap (c) of a-C: Cu films with different Cu contents
计算其光学带隙。其中α为光吸收系数，hν为入射光子能量，A为常数，Eg为材料光学带隙，指数n(可取1/2、3/2、2和3)的值与半导体材料以及跃迁类型相关。绘制(αhν)1/n 与hν曲线并将其线性部分延长至α=0，则对应的入射光子能量即为光学带隙。光吸收系数α可用朗伯比尔定律[33]
计算，其中d为薄膜厚度，单位为nm；T为透过率。
电学测试结果表明，掺入原子分数为2.77%至7.28% Cu的a-C: Cu薄膜仍具有典型的半导体特性，因此可将其作为半导体处理。Pandey和Majeed指出，a-C为直接带隙半导体[10,32]，n取1/2。图13b中S1~S4样品的(αhν)2-hν曲线，给出了Cu含量对a-C: Cu薄膜光学带隙的影响。如图13c所示，随着Cu含量的提高样品S1-S4的光学带隙从3.87 eV下降到2.93 eV，可能是sp2-C/sp3-C比例与结构的变化所致[34,35]。sp3-C只有σ键，sp2-C既有σ键又有π键。而与σ态相比，π电子更接近费米能级，其带隙主要由π电子控制[36]。非晶碳薄膜的光学带隙，可用于间接评估sp2-C的含量和团簇尺寸。其原因是，Eg随着sp2-C的含量的提高和团簇尺寸的增大而降低[23]。
根据能带理论，电子的传输与样品的功函数密切相关，用UPS可测得S1~S4样品的功函数。Ecut为样品二次电子截止边的切线与x坐标轴交点，如图14a所示，S1~S4样品的Ecut约为16.61 eV。其对应的功函数WF可从He I(21.22 eV)中减去Ecut得到[37]，约为4.61 eV。Cu的掺入，对a-C: Cu薄膜的功函数没有明显的影响。图14b给出了S1~S4样品价带顶(Ev)相对于费米能级(EF)的位置。可以看出，随着Cu含量的提高Ev相对EF的距离逐渐从1.62 eV降低到1.33 eV。以真空能级0 eV为参考，结合用紫外可见近红外透过测试测得的光学带隙，S1~S4的能带结构如图14c所示。对于本征半导体，其费米能级位于禁带中间。但是随着掺入Cu含量从2.77%提高到7.28%，S1~S4样品的费米能级向价带偏移了0.29 eV。
图14
图14
不同Cu掺杂量a-C: Cu薄膜 的二次电子截至边、价带相对费米能级的位置以及 (c) 能带结构
Fig.14
Secondary electron cutoff (a), the distance from valence band (Ev) to the Fermi level (EF) (b) and band structure (c) of a-C: Cu films with different contents
3 结论
(1) 用HiPIMS溅射Cu-C拼接靶，可制备Cu含量为2.77%~7.28%的a-C: Cu薄膜。掺杂的Cu以单质态存在于非晶碳结构中。随着Cu含量从2.77%提高到7.28%固溶Cu原子析出生成纳米晶，sp2-C含量和团簇尺寸增大。
(2) 随着非晶碳中Cu掺杂量的提高a-C: Cu薄膜的导电性增强，薄膜的电学性能仍为典型的半导体特性，在150~350 K薄膜的电阻率均随着温度的升高而降低。Cu含量为5.4%和7.28%的样品在150~350 K其载流子的传导方式均为Mott型三维变程跳跃传导，而Cu含量为2.77%和3.88%的样品在250~350 K为热激活，在150~250 K则为Mott型三维变程跳跃传导。
(3) 随着薄膜中Cu掺杂量的提高a-C: Cu薄膜的光学带隙减小，费米能级则向价带偏移。
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