Extension Functionality on Escalab 250 Xi J. F. Zhao Fermi Instruments (Shanghai) Co., Ltd XPS User Seminar, Nov. 7 th 2014
Outline Introduction to Standard XPS System Hardware Setup Basic Principal The extension functionality on Standard XPS System Why do we need to extend the Standard System What extension we could do on the ANAL Chamber: ISS (Ion Scattering Spectroscopy)-one hidden treasure on Escalab UPS to measure the work function of material IPES ( Inversed Photoemission Spectroscopy)-a step into the empty state Small modification on the sample holder What extension we could do on sample preparation: Quartz Gas Cell Small MBE System Other sample transfer techniques Summary 2
Introduction to XPS System X-ray Photoelectrons X-ray photon of energy hn hits the atom, and excites an electron (i.e. gives it more energy) If this energy is greater than the Binding Energy (BE) (plus a bit) it is ejected from the atom with a Kinetic Energy (KE) equal to the photon energy (hn) minus the binding energy (BE) minus a work function (y). This is the Photoelectron effect 3
Hardware Setup UHV system Ultra-high vacuum keeps surfaces clean Allows longer photoelectron path length X-ray source Typically Al Kα radiation Monochromated using quartz crystal Microfocus type for variable X-ray spot Electron analyser High transmission lens Multichannel detector Low energy electron flood gun Low energy e - (and Ar +, ideally) Analysis of insulating samples Optical system Good sample viewing and lighting Ease of sample navigation Ion gun Typically noble gas ions Sample cleaning Sputter depth profiling Vertical view camera ARXPS capability Electron transfer lens 4 Light Light Height setting camera Flood gun for charge compensation By sample tilting or by parallel ARXPS Non-destructive depth profiling *Based on Thermo Fisher K-Alpha System Hemispherical analyser 128 channel 2D Detector Depth profiling ion gun Toroidal quartz crystal Microfocused X-ray Monochromator source
Why need the extension?
Answer? Cost Saving What we could share with the XPS System: UHV environment Mu-metal Chamber Ion Source Electron Source Spherical Analyzer Contamination Free Cross-technique proof Clean Sample Preparation 6
ISS (Ion Scattering Spectroscopy)
Ion Scattering Spectroscopy Introduction Ion Scattering Spectroscopy (ISS) involves firing a beam of ions at a surface, and measuring the kinetic energies of the ions that are scattered back. A beam of monoenergetic ions is used, with ions lighter than the atoms in the sample so that they can be backscattered efficiently. ISS is an Energy Loss technique. The measured ions will lose some energy in the collision with the surface. The amount of energy lost depends on the particular atom that each ion collides with. ISS is highly surface-sensitive. Detected ions have mostly only interacted with the topmost layer of atoms. 8
Ion Scattering An ion impinging on a surface may collide with an atom. Where the collision is elastic, that is energy and momentum are conserved, the ion rebounds with less kinetic energy than it started with. Some energy is transferred to the surface atom, which recoils slightly. The amount of recoil, and thus the amount of energy lost by the ion, depends only on the mass of the atom. It does not (usually) depend on the environment of that atom in the solid. Incident Ion Mass = M 1 KE = E 0 Scattered Ion Mass = M 1 KE = E s θ Scattering Angle Scattering Atom Mass = M 2 9
Two-body collision model Energy Conservation Momentum Conservation 10
Experimental Setup The ISS experiment is fairly straightforward, requiring: an ion source a sample holder an ion kinetic energy analyser UHV environtment E/M Shielding The Escalab and Sigma/Theta Probe systems are all capable of performing ion scattering spectroscopy. One significant difference between systems is in the scattering angle, θ: Escalab Sigma/Theta θ = 130 o θ = 122 o Ions will be scattered in all directions, but only those scattered through a range of angles around θ will be detected by the analyzer. This angular acceptance also varies between systems. Ion Source Sample Analyser 11
ISS Calculations The relationship between the initial kinetic energy (E o ) and the final kinetic energy (E s ) depends on the masses of the scattered ion (M 1 ) and scattering atom (M 2 ), and on the angle through which the ion is scattered (θ): Equation 1 The primary beam energy (E o ) is usually determined by measuring the scattered peak energy (E s ) from a sample of known composition (such as gold). Then the known values of E o and M 1, together with the measured values of E s, can be used to calculate the unknown M 2 and so determine the composition of the surface of a sample. 12
Surface Sensitivity Any ions that penetrate below the topmost layer of atoms may: Experience multiple collisions, losing more energy Be neutralised by an electron Become embedded in the surface In these cases, the ion cannot be detected. There is a very low probability of an ion reaching a buried atom, backscattering elastically from it and leaving the surface to be detected. In particular, the noble gas ions have high neutralisation probabilities, and so are unlikely to survive as ions beneath the outermost atom layer. There are also shadowing effects where surface atoms obscure lower layers, increasing surface specificity even further. This means that ISS analyses almost exclusively the top layer of atoms on a sample. This makes ISS one of the most surface-sensitive techniques. For this reason, ISS is also highly sensitive to surface contaminants such as carbon - a very clean sample is needed for good results. 13
Non-Destructive Analysis ISS is usually considered to be non-destructive, especially when low-energy He + ions are used. The light, low-energy ions may become embedded in the surface but will not remove many atoms or change the surface chemistry significantly. By increasing the energy or mass of the ions, and the ion current, however, some sample etching will occur. This makes dynamic ISS possible, where low-energy ISS acquisitions are interspersed with high-energy etching steps. 14
Cross comparison 15
Intensity / counts per second ISS Calculator ISS Spectrum Plotter for Avantage Enter the Experiment Parameters in the boxes below. Paste an ISS dataspace into the spreadsheet, starting at M26, and then choose the X-axis scale. Parameters Eo 1000 ev Primary beam energy M1 4 amu Scattered ion mass Theta 130 degrees Scattering angle 20000 ISS Spectrum X-Axis Scale Options 18000 Kinetic Energy Original Ion KE Scale 16000 KE Ratio Ratio of Scattered Ion to Primary Ion Energy 14000 Mass Scattering Atom Mass 12000 Mass Ratio Ratio of Scattering Atom to Scattered Ion Mass Cutoff for Mass Scales 92 % 10000 8000 PASTE SPECTRUM INTO YELLOW BOX BELOW Axis Energy Elements= 476 6000 4000 2000 Kinetic Energy (E) ev Counts / s 50 83003.3 52 139766 54 172283 56 164517 58 149998 60 140009 62 127620 64 120517 66 111727 68 105898 70 97370.6 72 90461.9 74 84009.1 76 79970 78 75238.2 80 69751.4 82 64819.8 84 60335.3 86 56601.1 88 54385.9 90 49939.8 92 46007.6 94 43029.2 96 42066.7 98 39474.7 100 36597.2 102 34527.7 104 31674.6 106 30661.8 108 27653.6 110 26246.3 112 24787 114 22714.3 116 21692.4 0 50 250 450 650 850 Kinetic Energy / ev 16
Counts / s ISS Spectra : Gold An ISS survey spectrum from a pure gold sample is shown here. The incident ions were He +, with a primary beam energy of about 1000 ev. As is usual at the start of an experiment, the ion energy was not known precisely. The gold standard sample may be used to determine this energy. 60000 50000 40000 Au The main peak at 926 ev is from He + ions backscattered by Au atoms. 30000 20000 Using Equation 1, with M 1 = 4, M 2 = 197, E s = 926, and θ =130 o we can calculate the primary beam energy as E o = 990 ev. This primary beam energy can then be used to analyse ISS data from another sample, using the same ion beam conditions. 10000 0 100 200 300 400 500 600 700 800 900 1000 Kinetic Energy (ev) 17
Counts / s ISS Spectra : SiO 2 This ISS spectrum was obtained from a sample of SiO 2. From a gold standard, the primary beam energy E o was determined as ~910 ev. 4000 3000 A B There are two main peaks. The identities of the scattering atoms may be determined using Equation 1: 2000 1000 Peak A : E s = 393 ev M 2 = 16 Peak B : E s = 568 ev M 2 = 28 0 200 300 400 500 600 700 Kinetic Energy (ev) Peaks A and B correspond to O and Si respectively. 18
ISS Spectrum Scale Linearity Spectra are acquired on a KE scale, but the conversion to scattering atom mass is non-linear. This leads to an expansion of the mass scale with increasing mass. The same spectrum is shown below in KE (left) and Mass (right) scales. Note the changes in peak widths as the scale linearity changes. ISS Spectrum ISS Spectrum 3000 3000 2500 2500 Intensity / counts per second 2000 1500 1000 Intensity / counts per second 2000 1500 1000 500 500 0 200 300 400 500 600 700 800 900 1000 Kinetic Energy / ev 0 0 20 40 60 80 100 120 140 160 Mass / a.m.u 19
Other application 16 O/ 18 O as an example 16 O mass=16, Peak=431.2 ev 18 O mass=18, Peak=475.2 ev It could be quite obvious with the 12eV resolution 20
Mass Resolution The mass resolution for scattering atoms depends mainly on the energy resolution of the ion beam, and the angular and energy resolutions of the analyser. For typical pass energies (e.g. 50 ev on an E250) the analyser broadening is of the order of 1 ev. This is small compared to the other effects. For a small angular acceptance: (where A = M 2 /M 1 ) Best mass resolution is achieved where M 2 is similar to M 1, but this is not usually the case with He +, where M 1 =4. Better mass resolution can be obtained for heavy elements by using a heavier primary ion, such as Ne + or Ar +. Large scattering angles also improve mass resolution. The θ term means that the analyser angular acceptance is also an important factor. A wide angular acceptance will cause a loss of mass resolution. On the Escalab the upper iris may be used to reduce the angular acceptance. On SigmaProbe and ThetaProbe systems this facility is not available, so high mass resolution is not available on these systems. 21
ISS Test Rang Limit For Escalab (130º), we would estimate the lower limit of ISS is around mass=5 22
UPS on work function measurement
UV Source working principal 24
UPS Spectrum 25
Work Function measurement 26
Experimental Setup Clean Sample Surface Normally biased the sample to shift the 2 nd electron edge from the spectrometer Take the UPS spectrum with photon energy hʋ Measure the Fermi Energy, E Fermi Measure the 2 nd electron cut off edge E cut-off Work function = hʋ+e cut-off - E Fermi 27
Charge the sample while taking spectrum 28
IPES ( Inversed Photoemission Spectroscopy)- A step into the empty state
Photoemission Spectroscopy Albert Einstein 1921 Nobel Prize in Physics Core Level Valence Electron High Energy Excitation- XPS Low Energy Excitation- UPS 30
Inversed Photoemission Spectroscopy UPS IPES Spin Resolved If the electron source is spin orientated, the spin property of the unoccupied state could be resolved 31
Working Mode 可调光能模式 (TPE): 固定电子能量, 扫描不同能量的光子 1. 光栅 + 光电倍增管 : 光谱分光模式 (Spectrograph Mode) 固定入射电子能量, 对出射光子进行光谱分光 扫描末态 单色模式 (BIS): 扫描电子能量, 接收固定能量的光子 CaF2 窗 + 盖格管 : 单色测量模式 (Isochromatic Mode) 扫描入射电子能量, 固定探测光子的能量 扫描初态 接收角度小, 价格仪器贵 ; 能量分辨率高 接收角度大, 装置简单 ; 能量分辨率略低 32
Experimental Setup IPES is complete compatible with PES setup Other Requirements: 1. UHV 2. Electronic/Magnetic Shield 3. Sample Manipulator 4. Sputter for sample preparation Photon Detector Electron Source 33
Field of Application Solar Cell Spintronics Dilute Magnetic Semiconductor Storage Device OLED Topological Insulator 34
IPES _Application Case 1 Study on Solar Cell 35
IPES _Application Case 2 Surface State of absorbent Oxygen-induced enhancement of the spin-dependent effects in electron spectroscopies of Fe(001) PHYSICAL REVIEW B 59, 4027 36
IPES _Application Case 3 Band Structure Mapping Unoccupied band observed with IPES Ud-d=3.3ev PHYSICAL REVIEW B 75, 205124 2007 37
IPES _Application Case 4 PES+IPS study organic semiconductor HOMO IE LUMO EA 38
Band Structure PTCD A DiMe-PTCDA PTCDI 39
Quartz Gas Cell Attached to XPS System
Standard Gas Cell on Escalab 41
Limits of the HPGC on Escalab Temperature (600C) lower than expected. SS Chamber not suitable for corrosive gas reaction. High Cost of ownership Sealing Gasket consumption Not easy to clean/replace the gas cell SS will introduce Hydrogen into the chamber Possibility to contaminate the Prep/FEAL chamber Not only the sample is in the reaction atomsphere 42
Quartz Gas Cell Quartz is chemically inactive. Stable in high temperature and pressure. Quartz is a bad thermo conductor. Quartz is transparent to infrared. Easy and cheap to replace. Isolated with the UHV chamber. 43
Design Principal of Quartz Gas Cell Tube Furnace Design Only Quartz and Sample in the heating region Uniform Heating UHV Sealing Easy to service and low cost 44
真空系统及指标 技术协议要求 反应室系统拥有独立的机械泵 + 分子泵系统 并且经测试各项指标均已满足 : 极限真空 : 烘烤后最佳真空能达到 3.4E-4Pa, 即 (3.4E-6mbar) 系统抽速 : 从大气开始, 半个小时达 5.3E-3Pa 系统漏率 : 关机测试, 晚上 16:40 关机, 到第二天早上 8 点维持真空在 8Pa 20130929 测试极限真空如图所示 : 45
真空系统及指标 原系统 250Xi 为两室结构, 为配合定制反应室而升级为三级结构, 其中真空指标优于 5E-5mbar 为第三室真空指标, 现真空指标优于 1E-7mbar 46
技术协议要求 系统按要求配置 4 路反应气和一路吹扫氮气, 并配置了相应的气体流量计, 通过计算机对流量进行精确控制 47
技术协议要求 经测试, 已实现最快 30 度 /min 的加温测试, 以及在流通 1bar 气氛下的还原反应测试 进行了高温加热测试 : 1. 于 2013 年 11 月 8 日对系统进行了加热测试, 当时系统处于真空状态, 加热到最高加热温度 930 度 ( 已达到技术文件 1100K 指标 ), 保持时间 2 个半小时, 工作正常, 无漏气现象 ( 真空最后稳定在 5E-3Pa), 冷却水工作正常 升温速率, 从 20 度到 350 度升温时间 10 分钟,20 分钟从 350 度到 600 度,30 分钟从 600 度到 930 度, 升温速率满足技术文件最高 30 度每分钟要求 2. 最高温度时, 水冷工作情况以及设备各部件温度如右图所示 48
冷却测试 于 2013 年 11 月 12 日对样品冷却台进行了冷却测试, 最低温度可达 - 110 摄氏度以下, 满足技术文件 -100 摄氏度的要求 49
冷却速率测试 技术协议要求 对样品台的冷却速率进行了标定, 在样品冷却台的冷却块通过通冷氮气一个小时 ( 达到温度 -120 摄氏度 ) 之后, 将样品托插入冷却台, 温度于 4 分钟之内到达 -60 摄氏度, 接下去降温缓慢,10 分钟左右到达 -80 摄氏度,15 分钟左右达到 -90 度,30 分钟左右到达 -100 度, 满足指标要求 50
石英管内测温 应用户需要, 内置热电偶丝, 以毛细石英管铠装, 但仍需要注意以下问题 1. 内置热偶也不能精确反映样品真实温度, 只能一定程度上能测量内部的变化趋势 2. 热偶寿命问题, 可能不能长期工作 3. 内置热电偶可能会对真空造成影响 实物图 51
无缝传样交接 XPS 测试 可实现从快速进央室到反应室反应, 以及反应之后再反应室快速冷却, 再从快速进样室进入 XPS 分析腔进行分析测试的无缝传接, 并取得真实原位反应的结果. 反应过程使用 5%H2+95% 的 N2 对 Ag/Mg 基底上的 CuO 进行了原位反应测试 测试反应气压力为 1bar, 以 30ccm 的气流进行还原反应 原位反应 +XPS 分析测试内容包括 : 反应气路清洗 反应前 150 摄氏度通高纯氮去水汽 冷却至室温通混合气, 在加热原位反应 冷却至室温用高纯氮吹扫反应气, 在抽真空传入快速进样室 进行原位 XPS 测试 52
还原反应前 还原反应后 原位反应的结果如下, 可见 CuO 完全被还原为单质 Cu 形态 53
2 nd version Quartz Gas Cell in DICP 1. Upgrade the Tube Furnace to Split Open. 2. Upgrade the Tube Furnace to Lindberg Blue Mini with better temperature stability. 3. Upgrade the tube sealing for better UHV 4. Add in one ASG (Active Stain Gauge) to measure the pressure of different atmosphere 5. Upgrade the turbo pump so the pumping down is easier. 6. Upgrade the sample transfer mechanism. 54
Gas Cell with Escalab 250 XI 55
Details of the Gas Cell 56
Transfer Video 57
3 rd version HPGS on Escalab Further Reduce the size of the heating element and make the design more compact. 58
Small MBE System attached to XPS
Compact MBE system setup Maximum 7 K-cells Max Temp 1200C Working Temp 200-1800C 60
The growth of superconductor film FeSe/SrTiO 3 Thin Films 61
Experimental Detail Nb-SrTiO 3 substrate preparation Vacuum degas: 550 for 3H Se flux: 950 for 30min FeSe thin film MBE growth Fe Evaporation rate: 0.02Å/s Se Evaporation rate: 1.00Å/s Growth temperature: 490 Anneal temperature: 600 in situ ARPES measurement Photon energy: 21.2eV(Helium lamp); 7eV(Laser) Temperature:>25K 62
Electronic Structure Evolution 63
Thickness Dependence of the Lattice Constant 64
Phase Diagram 65
Other techniques for sample transfer without contamination
Gloves Box 67
UHV Suitcase The use of the combined Non-Evaporable Getter (NEG)/Ion-pump technology made it possible to design a truly portable and at the same time fully featured, ultra compact, light and slim UHV system. Ultra compact UHV-chamber NEXTorr D-100-5 combination pump Gate valve DN40CF (VAT) 3x viewport DN40CF Side/baseplates and carrying handle 68
Radical Distribution Chamber 69
Summary Several Things to Notice Only add components, NO modification on components. Be cautious what to add into the system: UHV compatible No magnetic parts into the ANAL Chamber No evaporation/contamination in the ANAL Chamber Do NOT modify the electronics/control unit of original system Always leave the possibility to recover Double Check the Safety Regulation (Electronic/Radiation/Mechanic) Procedure for extension Make sure it is a necessary for your research Make up the required specification Validate the specification with professionals Validate the specification with Thermo Fisher technical team Start 70
Thank You 71