非彈性電子穿隧能譜（Inelastic Electron Tunneling Spectroscopy，IETS）在穿隧元件上的運用十分廣泛，從早期1960年開始在氧化物上吸附化學有機物質的研究，到近十幾年來對MOS（金屬-氧化物-半導體）穿隧元件中的二氧化矽及高介電常數氧化物的研究，都在材料的能譜研究領域上引領出重大的進程。針對MOS元件的研究中，舉凡相關上電極金屬、底電極矽半導體、閘極氧化物及存在於氧化物中的其他化學鍵結之聲子能量，甚而是MOS元件中常出現的缺陷等，都能透過IETS精確地量測出來。MOS元件對於來自相反電子穿隧方向所量測到不同能譜的獨有特性，可物理性地解析出所偵測到的聲子是位於較為靠近上電極金屬與氧化物之間的界面，抑或氧化物與矽半導體之間的界面。本實驗研究所量測的氧化物涵蓋SiO2、HfO2、Y2O3、疊層式Y2O3/HfO2，以及疊層式HfO2/Y2O3。IETS在高介電常數氧化物HfO2上的研究結果，與過去在實驗與理論上發表有關光學反應下所誘發的聲子能譜，兩者之間有高度的一致性。甚或相較於近幾年在傅立葉轉換紅外線能譜（Fourier Transform Infrared Spectroscopy，FTIR）及其他IETS的研究成果，展現出更為高靈敏度的能譜量測現象。另外，IETS在疊層式Y2O3/HfO2 MOS穿隧元件的研究，也能精確的揭示出造成此元件在半導體載子速度（carrier mobility）的提升中，其所對應的重要物理原因。

Inelastic electron tunneling spectroscopy (IETS) in silicon metal-oxide-semiconductor systems with SiO2, pure HfO2, pure Y2O3, stacked Y2O3/HfO2, and stacked HfO2/Y2O3 as gate dielectrics have all been studied in this work. Information of electrode phonons, dielectric phonons, chemical bonding, and trap-related states in MOS structures has been able to be revealed by IETS. The bias polarity dependence of IETS has enabled the distinguishability to analyze microstructures either near the metal gate interface or near the silicon substrate interface. IETS results from HfO2 MOS tunnel junctions are consistent with published experimental and theoretical works relevant to Raman- and Infrared- active HfO2 phonon modes in monoclinic and tetragonal phases. IETS results of stacked Y2O3/HfO2 gate dielectrics, compared to pure HfO2, show less formation of hafnium silicate and less HfO2 and Y2O3 phonon peaks that can be detected with energies close to silicon phonons. Both phenomena observed are probably associated with less charge trapping and less ionic polarization of HfO2 and Y2O3 or intrinsically smaller ionic polarization of Y2O3 which may contribute to the improvement of channel carrier mobility due to suppressed coulomb scattering and reduced remote phonon scattering, respectively.

Contents

1. Introduction to High- MOS Technology
1.1 MOS Device Shrinking & SiO2 Scaling Limits-----------------------------------1
1.2 Alternative High- Gate Dielectrics----------------------------------------------2
1.3 Candidates for High- Gate Dielectrics------------------------------------------3
2. Tunneling Spectroscopy
2.1 Progress Making for Tunneling Spectroscopy-------------------------------------6
2.2 Fundamental Principles of IETS-----------------------------------------------------6
2.3 Energy Loss Mechanisms & Trap-Related Figures Revealed in IETS---------7
2.4 Bias Polarity Dependence of IETS--------------------------------------------------9
3. Experimental Details
3.1 Sample Preparation------------------------------------------------------------------13
3.2 UHV Molecular Beam Epitaxy Multi-chamber System------------------------18
3.3 Measurement Methods--------------------------------------------------------------20
3.3.1 AC Voltage Modulation Technique----------------------------------------20
3.3.2 Voltage-Bias Modulation Approach----------------------------------------21
3.4 Experimental Setups-----------------------------------------------------------------22
3.4.1 Electronic Instrumentation---------------------------------------------------22
3.4.2 Noise Consideration----------------------------------------------------------25
3.5 Measurement Operation-------------------------------------------------------------28
3.5.1 Lock-in Amplifier Settings------------------ --------------------------------28
3.5.2 Spectral Resolution-----------------------------------------------------------32
3.5.3 Preliminary Works before Measurement-----------------------------------33
3.6 Data Analysis-------------------------------------------------------------------------34
3.6.1 Inspection for Data Accuracy------------------------------------------------34
3.6.2 Dual Temperature Technique------------------------------------------------36
3.7 Summary------------------------------------------------------------------------------38
4. Experimental Results
4.1 Silicon Dioxide-----------------------------------------------------------------------45
4.1.1 Electrodes of the MOS system----------------------------------------------45
Phonons of the Aluminum Gate---------------------------------------------45
Phonons of the Silicon Substrate--------------------------------------------46
4.1.2 Oxides of the MOS system---------------------------------------------------47
Phonons of Aluminum oxide------------------------------------------------47
Phonons of Silicon Dioxide--------------------------------------------------47
4.1.3 IETS of Al-SiO2-Si Tunnel Junction---------------------------------------48
4.2 Hafnium Oxide-----------------------------------------------------------------------54
4.2.1 Hafnium Oxide Vibrational Modes-----------------------------------------54
4.2.2 IETS of Hafnium Oxide------------------------------------------------------55
4.3 Stacked Y2O3/HfO2 & HfO2/Y2O3 Gate Dielectrics-----------------------------68
4.3.1 IETS of Stacked Y2O3/HfO2 Gate Dielectric------------------------------68
4.3.2 IETS of Yttrium Oxide-------------------------------------------------------69
4.3.3 IETS of Stacked HfO2/Y2O3 Gate Dielectric------------------------------70
4.4 IETS Comparison between HfO2 and Stacked Y2O3/HfO2 Gate Dielectric--77
4.5 Summary------------------------------------------------------------------------------77
5. Conclusion------------------------------------------------------------------------------81
6. Bibliography---------------------------------------------------------------------------82

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