金属/SiC半导体接触的SiC表面等离子体改性研究
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摘要
SiC半导体由于具有禁带宽度大、临界击穿电场和热导率高等特点,在高温、高压、大功率器件领域具有广阔的应用前景。金属/SiC半导体接触包括SiC欧姆接触和肖特基接触,是构成SiC器件最基本也是最重要的结构,其质量直接影响SiC器件的效率、增益和开关速度等性能指标。制备良好的SiC欧姆接触和肖特基接触是提高SiC器件性能和可靠性的基础。金属半导体接触特性不仅与金属功函数和半导体掺杂浓度有关,还受半导体表面态的影响。相对于Si、GaN等其它半导体而言,SiC半导体表面存在更加复杂的表面态,如用传统的RCA湿法清洗获得的SiC表面仍然存在C、O等污染物,使得金属/SiC接触特性受SiC表面特性的影响非常大。因此,开发新的SiC表面处理技术并利用表面态调控改善金属/SiC欧姆接触和肖特基接触特性显得尤为重要。
本文提出并开发了低温低损伤的SiC半导体表面电子回旋共振(ECR)微波氢等离子体处理新技术。利用该方法对SiC表面态进行调控,研究了金属/SiC接触特性的改善效果,并对相应的改善机理以及相关的理论做了深入的研究。主要研究内容及结果如下：
1、SiC表面ECR微波氢等离子体处理研究。利用反射式高能电子衍射(RHEED)、原子力显微镜(AFM)以及X射线光电子能谱(XPS)分析了氢等离子体处理对SiC表面结构、形貌、化学及电学性质等的影响。RHEED分析结果表明,经氢等离子体处理后,SiC表面原子排列规则,单晶取向性好,且表面未发生重构;AFM分析发现,SiC表面经处理后变得非常平整,表面均方根粗糙度降低至0.268nm; XPS分析结果显示,经氢等离子体处理后,SiC表面O含量明显降低,C污染物全部被去除,并且表面具有较好的抗氧化性；经计算发现,处理后的SiC表面态密度降低至1010cm-2eV-1量级。表面污染物的去除和表面态密度的显著降低为SiC器件后续工艺,提高器件性能及可靠性提供了有利的保障。
2、ECR氢等离子体预处理对金属/n型4H-SiC接触欧姆特性的改善效果及机理研究。对比分析了处理前后Ti/4H-SiC接触的I-V特性和比接触电阻率,发现经处理后,Ti与较高掺杂浓度(1×1018cm-3)的4H-SiC接触后无需退火就形成了低比接触电阻率的欧姆接触。为了明晰欧姆接触的形成机理,在对欧姆接触理论充分研究的基础上,对不同功函数的金属与不同掺杂浓度的SiC接触经氢等离子体处理前后的电学特性进行了系统的对比研究。研究结果表明,Ti/4H-SiC欧姆接触形成的机理在于在接触界面处形成了低的肖特基势垒。氢等离子体处理降低了SiC表面态密度,使得费米能级钉扎效应被消除,势垒高度不再受制于高的表面态密度。并且在镜像力降低、隧穿效应、能带变窄等势垒降低机制的共同作用下,经氢处理后,Ti/4H-SiC接触势垒高度降至0.41eV,无需退火就形成了低比接触电阻率的欧姆接触。避免采用高温退火以及重掺杂SiC衬底制备良好的欧姆接触将有利于提高器件的可靠性,降低SiC器件制备成本。
3、ECR氢等离子体预处理对金属/n型4H-SiC肖特基接触特性的改善效果及机理研究。对比分析了处理前后Ni、Pt/4H-SiC接触的I-V特性和势垒高度值,发现经氢处理后,Ni、Pt接触的势垒高度增大,说明整流特性得到改善,并且其整流特性在经过低温400℃退火后得到优化。为了研究其改善机理,利用XPS对表面费米能级以及表面态密度的变化进行了分析。结果显示,经氢等离子体处理后,SiC表面费米能级向导带底方向移动,表面态密度降低。400℃退火后,基本出现了平带的情况,表面态密度达到最低。通过结合低功函数金属Ti/4H-SiC接触的实验结果,对金属/4H-SiC接触的势垒高度与金属功函数和表面态密度的关系进行了讨论,发现实验现象与Cowley和Sze的势垒理论相吻合。SiC肖特基接触特性的改善将有利于促进SiC肖特基接触相关器件的发展。
4、金属/n型4H-SiC接触势垒不均匀性分布问题研究。通过利用ECR氢等离子体处理对SiC表面特性进行有效控制,采用I-V-T和C-V-T方法对Pt与不同表面特性的4H-SiC接触的电学特性进行了研究。根据实验结果,分析讨论了金属/4H-SiC接触特性与表面特性之间的关系。结果表明,Pt/4H-SiC接触势垒高度以及势垒不均匀性分布与SiC表面特性存在强烈的依赖关系。其有效势垒高度随着表面费米能级钉扎程度的降低而升高。在费米能级完全被钉扎或者完全解钉扎的情况下,界面势垒分布均匀并分别满足Bardeen和Schottky-Mott模型。然而当表面费米能级被部分钉扎时,金属/SiC接触界面势垒呈Gaussian分布,并且其不均匀性分布程度随钉扎程度的降低而降低。分析产生这些结果的原因可能是,经不同的表面处理后,表面态密度的大小和不均匀性分布程度发生了变化,并由此构建了金属/SiC接触势垒高度及势垒不均匀性分布与表面态密度以及表面费米能级位置的关系模型,明晰了金属/SiC接触势垒不均匀性分布的形成机理。为准确控制金属/SiC接触的势垒高度,进而准确控制金属/SiC接触特性以及SiC器件性能提供了实验和理论依据。
Silicon carbide (SiC) is a promising candidate for applications in high temperature, high voltage, high power electronic devices because of its outstanding properties such as wide band gap, high critical electric field and high thermal conductivity. Metal/SiC contacts, serving as Ohmic contacts or rectifying contacts form an intrinsic and vital part of SiC devices. The efficiency, gain and switching speed of these devices are strongly depended on the quality of Metal/SiC contacts. Therefore, the formation of good Ohmic contacts and Schottky contacts on SiC is a key factor in improving the electrical performance and reliability of SiC devices. The electrical properties of metal-semiconductor contact not only depend on the work function of metal and the doping concentration of semiconductor, but also on the surface states of semiconductor. Compared to other semiconductors like Si and GaN, the surface states on SiC surfaces are much more complex. For example, SiC surfaces are still contaminated by C and O impurity ions after the traditional RCA cleaning. Consequently, the electrical properties of metal/SiC contacts are strongly influenced by the surface properties of SiC. Therefore, it is of great importance to improve the properties of SiC Ohmic and Schottky contacts through developing new surface treatment technology to improve the surface properties of SiC.
In this paper, low-temperature low-damage electronic cyclotron resonance (ECR) microwave hydrogen plasma treatment (HPT) was employed to improve the properties of SiC surface. With the HPT, the improvements in electrical characteristics of metal/SiC contacts were studied. Also, the corresponding mechanisms and related theories were further investigated. The main research contents and results are as follows:
1. The improvements in the properties of4H-SiC surfaces with the HPT have been studied. The effects of the HPT on the structure, morphology, chemical and electronic properties of surfaces were characterized by in-situ reflection high energy electron diffraction (RHEED), Atomic Force Microscope (AFM) and X-ray photoelectron spectroscopy (XPS). With the HPT, RHEED results indicate that smooth, atomically ordered, unreconstructed SiC surfaces are achieved. AFM results display that RMS is reduced to be as low as0.268nm. The XPS results show that the surface oxygen is greatly reduced and the carbon contaminations are completely removed from the4H-SiC surfaces. The hydrogenated SiC surfaces exhibit an unprecedented stability against oxidation in the air with the density of surfaces states as low as1010cm-2eV-1scale. The removal of surface contaminations and the reduction of surface state density would be significant for improving the properties and reliabilities of SiC devices.
2. The improvements in the Ohmic properties of Ti/n-type4H-SiC contact with the HPT and the corresponding mechanism have been studied. It is found that Ti Ohmic contacts to relatively highly doped (1×1018cm-3) n-type4H-SiC with low resistivity have been produced without high temperature annealing. To elucidate the Ohmic formation mechanism of Ti contact to4H-SiC with the HPT, the electrical properties of metals with different work functions contact to SiC with different doping concentrations before and after the HPT are systematically investigated. The experimental results show that the Ohmic behavior of Ti contact is mainly attributed to low barrier height at Ti/4H-SiC contact interface. The HPT releases the Fermi level from pinning by reducing surface state density. Consequently, barrier height is significantly decreased to as low as0.41eV by the low surface state density together with the effects of band-gap narrowing, image-force and thermionic-field emission at relatively high doping. The formation of SiC Ohmic contact without high temperature annealing and high doping concentration would be helpful for improving the stability of SiC electronic devices and reduce the production cost.
3. The improvements in the rectifying properties of Ni, Pt/n-type4H-SiC contacts with the HPT and the corresponding mechanism have been studied. It is found the rectifying behaviors of Ni, Pt/4H-SiC contacts have been remarkably enhanced, and are optimized after annealing at400℃. In order to reveal the mechanism involved, XPS was employed to investigate the changes in the position of surface Fermi level and surface state density. XPS measurements show that the surface Fermi level moves toward the conduction band edge by the HPT. It almost attains the bulk Femi level position after annealing at400℃with the lowest density of surface states. Combining the observation of Ti/4H-SiC contact, the the correlation between the barrier height of metal/4H-SiC contact, the metal work function and the surface state density is discussed. It is found the experimental results obey the barrier height theory proposed by Cowley and Sze. The improvement of rectifying properties of metal/4H-SiC contact may help promote the development of Schottky contact related SiC electronic devices.
4. The barrier inhomigeneities at metal/n-type4H-SiC contact has been studied. Through controlling the surface properties of4H-SiC by the HPT for different periods and annealing process, the Schottky barrier properties of Pt/4H-SiC contacts are investigated by current-voltage-temperature (I-V-T) and capacitance-voltage (C-V-T) measurements. Using the experimental results, the dependence of the barrier height and barrier inhomogeneities on the SiC surface properties is discussed and analyzed. It is found that the barrier height and barrier inhomogeneities of Pt contacts to4H-SiC appear to be strongly influenced by the surface properties of4H-SiC. The effective barrier height increases with decreasing the degree of Fermi level pinning. Electrically homogeneous contacts with barrier heights close to Bardeen limit and ideal Schottky limit are observed for the samples with "pinning" and "pinning free" of Fermi level, respectively. However, when the Fermi level is partially pinned, Gaussian distribution of inhomogeneous barrier height is found and the inhomogeneity decreases with reducing the degree of Fermi level pinning. An analysis model, by considering changes in the magnitude and spatial distribution of surface state density after different pretreatments, is proposed to clarify the correlation between the barrier height, the barrier inhomogeneties of metal/SiC, and the surface state density and the position of surface Fermi level of SiC. This study provides a clear insight into physical information on the barrier inhomogeneties of metal/SiC, which is of significance in controling the Schottky barrier of metal/SiC contact precisely and further improving the performance and reliability of SiC electronic devices.
本文提出并开发了低温低损伤的SiC半导体表面电子回旋共振(ECR)微波氢等离子体处理新技术。利用该方法对SiC表面态进行调控,研究了金属/SiC接触特性的改善效果,并对相应的改善机理以及相关的理论做了深入的研究。主要研究内容及结果如下：
1、SiC表面ECR微波氢等离子体处理研究。利用反射式高能电子衍射(RHEED)、原子力显微镜(AFM)以及X射线光电子能谱(XPS)分析了氢等离子体处理对SiC表面结构、形貌、化学及电学性质等的影响。RHEED分析结果表明,经氢等离子体处理后,SiC表面原子排列规则,单晶取向性好,且表面未发生重构;AFM分析发现,SiC表面经处理后变得非常平整,表面均方根粗糙度降低至0.268nm; XPS分析结果显示,经氢等离子体处理后,SiC表面O含量明显降低,C污染物全部被去除,并且表面具有较好的抗氧化性；经计算发现,处理后的SiC表面态密度降低至1010cm-2eV-1量级。表面污染物的去除和表面态密度的显著降低为SiC器件后续工艺,提高器件性能及可靠性提供了有利的保障。
2、ECR氢等离子体预处理对金属/n型4H-SiC接触欧姆特性的改善效果及机理研究。对比分析了处理前后Ti/4H-SiC接触的I-V特性和比接触电阻率,发现经处理后,Ti与较高掺杂浓度(1×1018cm-3)的4H-SiC接触后无需退火就形成了低比接触电阻率的欧姆接触。为了明晰欧姆接触的形成机理,在对欧姆接触理论充分研究的基础上,对不同功函数的金属与不同掺杂浓度的SiC接触经氢等离子体处理前后的电学特性进行了系统的对比研究。研究结果表明,Ti/4H-SiC欧姆接触形成的机理在于在接触界面处形成了低的肖特基势垒。氢等离子体处理降低了SiC表面态密度,使得费米能级钉扎效应被消除,势垒高度不再受制于高的表面态密度。并且在镜像力降低、隧穿效应、能带变窄等势垒降低机制的共同作用下,经氢处理后,Ti/4H-SiC接触势垒高度降至0.41eV,无需退火就形成了低比接触电阻率的欧姆接触。避免采用高温退火以及重掺杂SiC衬底制备良好的欧姆接触将有利于提高器件的可靠性,降低SiC器件制备成本。
3、ECR氢等离子体预处理对金属/n型4H-SiC肖特基接触特性的改善效果及机理研究。对比分析了处理前后Ni、Pt/4H-SiC接触的I-V特性和势垒高度值,发现经氢处理后,Ni、Pt接触的势垒高度增大,说明整流特性得到改善,并且其整流特性在经过低温400℃退火后得到优化。为了研究其改善机理,利用XPS对表面费米能级以及表面态密度的变化进行了分析。结果显示,经氢等离子体处理后,SiC表面费米能级向导带底方向移动,表面态密度降低。400℃退火后,基本出现了平带的情况,表面态密度达到最低。通过结合低功函数金属Ti/4H-SiC接触的实验结果,对金属/4H-SiC接触的势垒高度与金属功函数和表面态密度的关系进行了讨论,发现实验现象与Cowley和Sze的势垒理论相吻合。SiC肖特基接触特性的改善将有利于促进SiC肖特基接触相关器件的发展。
4、金属/n型4H-SiC接触势垒不均匀性分布问题研究。通过利用ECR氢等离子体处理对SiC表面特性进行有效控制,采用I-V-T和C-V-T方法对Pt与不同表面特性的4H-SiC接触的电学特性进行了研究。根据实验结果,分析讨论了金属/4H-SiC接触特性与表面特性之间的关系。结果表明,Pt/4H-SiC接触势垒高度以及势垒不均匀性分布与SiC表面特性存在强烈的依赖关系。其有效势垒高度随着表面费米能级钉扎程度的降低而升高。在费米能级完全被钉扎或者完全解钉扎的情况下,界面势垒分布均匀并分别满足Bardeen和Schottky-Mott模型。然而当表面费米能级被部分钉扎时,金属/SiC接触界面势垒呈Gaussian分布,并且其不均匀性分布程度随钉扎程度的降低而降低。分析产生这些结果的原因可能是,经不同的表面处理后,表面态密度的大小和不均匀性分布程度发生了变化,并由此构建了金属/SiC接触势垒高度及势垒不均匀性分布与表面态密度以及表面费米能级位置的关系模型,明晰了金属/SiC接触势垒不均匀性分布的形成机理。为准确控制金属/SiC接触的势垒高度,进而准确控制金属/SiC接触特性以及SiC器件性能提供了实验和理论依据。
Silicon carbide (SiC) is a promising candidate for applications in high temperature, high voltage, high power electronic devices because of its outstanding properties such as wide band gap, high critical electric field and high thermal conductivity. Metal/SiC contacts, serving as Ohmic contacts or rectifying contacts form an intrinsic and vital part of SiC devices. The efficiency, gain and switching speed of these devices are strongly depended on the quality of Metal/SiC contacts. Therefore, the formation of good Ohmic contacts and Schottky contacts on SiC is a key factor in improving the electrical performance and reliability of SiC devices. The electrical properties of metal-semiconductor contact not only depend on the work function of metal and the doping concentration of semiconductor, but also on the surface states of semiconductor. Compared to other semiconductors like Si and GaN, the surface states on SiC surfaces are much more complex. For example, SiC surfaces are still contaminated by C and O impurity ions after the traditional RCA cleaning. Consequently, the electrical properties of metal/SiC contacts are strongly influenced by the surface properties of SiC. Therefore, it is of great importance to improve the properties of SiC Ohmic and Schottky contacts through developing new surface treatment technology to improve the surface properties of SiC.
In this paper, low-temperature low-damage electronic cyclotron resonance (ECR) microwave hydrogen plasma treatment (HPT) was employed to improve the properties of SiC surface. With the HPT, the improvements in electrical characteristics of metal/SiC contacts were studied. Also, the corresponding mechanisms and related theories were further investigated. The main research contents and results are as follows:
1. The improvements in the properties of4H-SiC surfaces with the HPT have been studied. The effects of the HPT on the structure, morphology, chemical and electronic properties of surfaces were characterized by in-situ reflection high energy electron diffraction (RHEED), Atomic Force Microscope (AFM) and X-ray photoelectron spectroscopy (XPS). With the HPT, RHEED results indicate that smooth, atomically ordered, unreconstructed SiC surfaces are achieved. AFM results display that RMS is reduced to be as low as0.268nm. The XPS results show that the surface oxygen is greatly reduced and the carbon contaminations are completely removed from the4H-SiC surfaces. The hydrogenated SiC surfaces exhibit an unprecedented stability against oxidation in the air with the density of surfaces states as low as1010cm-2eV-1scale. The removal of surface contaminations and the reduction of surface state density would be significant for improving the properties and reliabilities of SiC devices.
2. The improvements in the Ohmic properties of Ti/n-type4H-SiC contact with the HPT and the corresponding mechanism have been studied. It is found that Ti Ohmic contacts to relatively highly doped (1×1018cm-3) n-type4H-SiC with low resistivity have been produced without high temperature annealing. To elucidate the Ohmic formation mechanism of Ti contact to4H-SiC with the HPT, the electrical properties of metals with different work functions contact to SiC with different doping concentrations before and after the HPT are systematically investigated. The experimental results show that the Ohmic behavior of Ti contact is mainly attributed to low barrier height at Ti/4H-SiC contact interface. The HPT releases the Fermi level from pinning by reducing surface state density. Consequently, barrier height is significantly decreased to as low as0.41eV by the low surface state density together with the effects of band-gap narrowing, image-force and thermionic-field emission at relatively high doping. The formation of SiC Ohmic contact without high temperature annealing and high doping concentration would be helpful for improving the stability of SiC electronic devices and reduce the production cost.
3. The improvements in the rectifying properties of Ni, Pt/n-type4H-SiC contacts with the HPT and the corresponding mechanism have been studied. It is found the rectifying behaviors of Ni, Pt/4H-SiC contacts have been remarkably enhanced, and are optimized after annealing at400℃. In order to reveal the mechanism involved, XPS was employed to investigate the changes in the position of surface Fermi level and surface state density. XPS measurements show that the surface Fermi level moves toward the conduction band edge by the HPT. It almost attains the bulk Femi level position after annealing at400℃with the lowest density of surface states. Combining the observation of Ti/4H-SiC contact, the the correlation between the barrier height of metal/4H-SiC contact, the metal work function and the surface state density is discussed. It is found the experimental results obey the barrier height theory proposed by Cowley and Sze. The improvement of rectifying properties of metal/4H-SiC contact may help promote the development of Schottky contact related SiC electronic devices.
4. The barrier inhomigeneities at metal/n-type4H-SiC contact has been studied. Through controlling the surface properties of4H-SiC by the HPT for different periods and annealing process, the Schottky barrier properties of Pt/4H-SiC contacts are investigated by current-voltage-temperature (I-V-T) and capacitance-voltage (C-V-T) measurements. Using the experimental results, the dependence of the barrier height and barrier inhomogeneities on the SiC surface properties is discussed and analyzed. It is found that the barrier height and barrier inhomogeneities of Pt contacts to4H-SiC appear to be strongly influenced by the surface properties of4H-SiC. The effective barrier height increases with decreasing the degree of Fermi level pinning. Electrically homogeneous contacts with barrier heights close to Bardeen limit and ideal Schottky limit are observed for the samples with "pinning" and "pinning free" of Fermi level, respectively. However, when the Fermi level is partially pinned, Gaussian distribution of inhomogeneous barrier height is found and the inhomogeneity decreases with reducing the degree of Fermi level pinning. An analysis model, by considering changes in the magnitude and spatial distribution of surface state density after different pretreatments, is proposed to clarify the correlation between the barrier height, the barrier inhomogeneties of metal/SiC, and the surface state density and the position of surface Fermi level of SiC. This study provides a clear insight into physical information on the barrier inhomogeneties of metal/SiC, which is of significance in controling the Schottky barrier of metal/SiC contact precisely and further improving the performance and reliability of SiC electronic devices.
引文
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