CHAPTER 2

Surface Analytical Techniques

YING WEI CAI AND STEVEN L. BERNASEK

2.1 INTRODUCTION

A number of different surface analytical methods are used in the research described in this book. These methods provide structural, compositional, and molecular identity and reactivity information about the semiconductor surface before, during, and after functionalization. This chapter briefly summarizes the most commonly used of these techniques, providing a description of the operation of the method and an indication of the sort of information provided by the technique. There are a number of review papers and monographs [1] that describe these analytical tools in much more detail. This chapter is not meant to replace these references, but to provide a close at hand introduction to the techniques for researchers interested in the functionalization of semiconductor surfaces.

Compared to wet methods conducted in ambient environments, the dry methods for semiconductor surface modification and functionalization are performed in ultrahigh vacuum (UHV) that accommodates a wide spectrum of surface analytical techniques. With these techniques and under UHV conditions, the surface reaction process can be well controlled and characterized to study and explore the fundamentals of “in situ” modification and functionalization of semiconductor surfaces.

The development of UHV systems that can attain pressures from 10⁻⁷ to 10⁻¹¹ Torr not only enables the preparation and maintenance of a clean surface for a reasonable period of time but also makes more accurate and surface-sensitive analytical techniques feasible. With the reduction of pressure, gas molecules behave independently and collide only with the chamber wall, as the mean free path of the molecules increases to a magnitude that is thousands of times larger than the dimension of the UHV chamber. Also, when the density of gas molecules is as low as 3×10¹² m⁻³ at 10⁻¹⁰ Torr, the rate of collision of gas molecules on the surface drops to about 10¹⁰cm⁻²s⁻¹, keeping the sample surface uncontaminated for several hours. In addition, under this condition, ions, electrons, and photons can also travel freely without interaction with residual gas molecules, which is a critical condition for surface analytical techniques using these probes.

To achieve and maintain the UHV condition, a variety of pumping, sealing, and measuring technologies have been developed and implemented [2]. Vacuum pumps with different mechanisms work in different pressure ranges. Starting from atmospheric pressure, rotary mechanical pumps, sorption pumps, or turbomolecular pumps can attain pressure as low as 10⁻³, 10⁻⁵, or 10⁻⁷ Torr, respectively. Once the pressure is reduced to about 10⁻⁷ Torr, gas desorption, especially water vapor desorption, from the stainless steel chamber wall contributes significantly to the background pressure. At this stage, the chamber is baked at 150°C or higher to accelerate the desorption and pump away the gas. After cooling down, the background pressure can drop to about 10⁻¹⁰ Torr if pumped by turbomolecular pump, diffusion pump, or ion sputtering pump. The pressure is accurately measured by thermocouple gauge or ionization gauge for their respective pressure ranges. Thanks to the development of durable and high-temperature-tolerant sealing materials with extremely low vapor pressure, the UHV chamber can be baked and tightly sealed while the sample is transported, manipulated, and processed.

The above-mentioned critical vacuum components allow UHV chambers to accommodate a variety of scientific instruments. A mass spectrometer (MS) and an ion gun are generally included. In the MS, the gas molecule is ionized and fragmented, followed by examination of the fragment ions' mass-to-charge ratio. The distribution pattern of the charged fragments provides information about the gas composition in the UHV chamber and its working condition. Also, the MS is used as a detector for many analytical techniques. An ion gun ionizes and energizes inert gas molecules to bombard sample surfaces to remove contaminants for sample preparation.

Besides the MS and ion gun, a number of scientific instruments have been developed to characterize semiconductor surfaces, before and after modification. Although the principles and applications of these techniques vary widely, all share the characteristic of extreme surface sensitivity. A basic understanding of the frequently used surface analytical techniques will put the research described in this book into a clearer background and lead to an in-depth consideration of the development of research in related areas.

Based on the major applications of surface analytical technologies, the discussion in this chapter is organized to include surface structure, surface composition, electronic structure and vibrational properties, and kinetic and energetic probes.

2.2 SURFACE STRUCTURE

The structure of the semiconductor surface is the basis for further modification and functionalization, as it defines the properties, distribution, and spatial dimensions of reactive sites, which are the key to the understanding of the surface reaction mechanisms and processes. A number of technologies have been developed to detect and study the surface structure, among which low-energy electron diffraction (LEED), ion scattering methods (MEIS, ISS), and scanning tunneling microscopy (STM) are the most commonly used.

2.2.1 Low-Energy Electron Diffraction

The wave–particle duality suggests electrons diffract like water waves and photons. The diffraction of electrons was demonstrated experimentally by Davisson and Germer in 1927 [3]. However, only after its combination with UHV technologies did electron diffraction become a powerful tool to detect and study the structure of ordered surfaces.

The de Broglie relationship relates the electron's wavelength to its momentum, λ=h/p, where h is Planck's constant and p is the momentum. When the electron's energy is in the range of 20–200 eV, the range used in LEED, the wavelength of the electron varies from 0.866 to 2.74 Å. This wavelength is comparable to the dimension of the surface lattice, and the elastically backscattered electrons diffract and display unique patterns, revealing the structure of the surface. Electrons have a strong interaction with solid matter; thus, the mean free path of low-energy electrons is very short in solids (around 1 nm), making LEED highly surface sensitive.

In the experimental setup [4], a cathode filament held at a negative potential is electrically heated to emit electrons, which are then focused by electrostatic lenses, and accelerated onto the grounded metallic or semiconducting sample. If the electron-irradiated sample surface area is ordered, the elastically scattered electrons form a distinct diffraction pattern, which is displayed on a phosphor-coated screen. The inelastically scattered electrons are prevented from striking the phosphor display screen by a set of suppressor grids.

The LEED patterns and diffraction spot intensities provide information about the symmetry and atomic arrangements of superstructures, domains, and unit cells of the surface. Although the crystal structure of semiconductors and their surface periodicity are well understood today, LEED was the main technique decades ago used to investigate the semiconductor surface structures and atomic arrangements, based on the kinematic theory (single scattering), dynamical calculations, and crystallographic knowledge. LEED patterns and intensities can be calculated, and databases of observed LEED patterns have made the interpretation of LEED data more straightforward.

For organic modification and functionalization of semiconductor surfaces, LEED is normally used to verify and monitor the surface symmetry and structure during the sample preparation and modification, since the semiconductor crystal structure and surface reconstruction have been well studied and demonstrated. Moreover, the adlayers on modified semiconductor surfaces could also be characterized using this technique to determine the overlayer symmetry and order.

Even after the Si(111)-(7×7) reconstruction structure was well explained by the DAS (dimer–adatom–stacking fault) model [6], LEED still helped to experimentally improve the detailed knowledge about dimensions of the unit cell. For instance, Webb and coworkers [7] studied the reconstructed surface with LEED and dynamical calculations based on the DAS model. By adjusting the cluster parameters, they simulated intensity variation in the diffraction features with changing incident electron energy, and thus further refined the dimensions of the surface unit cell.

2.2.2 Ion Scattering Methods

Ion scattering uses ions impinging on surfaces to study surface elemental composition, thin layer thickness, and atomic arrangement by analyzing the scattered ions. Compared to the scattering of electrons, the ions have comparable mass with surface atoms. Therefore, the elastically scattered ions will lose a significant amount of energy to surface atoms based on the conservation of energy and momentum. In addition, the de Broglie wavelength of noble gas ions is much less than the distance between surface atoms, avoiding obvious diffraction of scattered ions.

Also accommodated in the UHV chamber, ion scattering spectrometry consists of several main components, including an ion source, beam manipulators, sample manipulators, and detectors. The noble gas, such as helium or argon, is bombarded by electrons to produce positively charged ions in the ion source. The ions are then accelerated, focused, and directed to the sample surface, while the position and orientation of the sample with respect to the ion beam is adjusted by a sample manipulator. The scattered ions are gathered by the detectors, which convert the number of ions with specific energy into electrical signals [8].

When scattered by surface atoms with unique masses, the ions' kinetic energy carries identifying information about the scattering surface atoms. The energy and intensity of the scattered ions are analyzed by an electrostatic analyzer or are determined by the time of flight of the ion, while gradually changing the beam's incident angle. The elemental composition of the surface can be deduced from the mass and energy of the incident and elastically scattered ions. In addition, the intensity of scattered ions with a particular energy is proportional to the abundance of related surface atoms. Moreover, when the energy of scattered ions from a known element deviates from the expected value, the ion may have passed through a thin layer of matter and lost part of its energy. In this case, based on the relationship between the energy loss and the ion path length through the particular material, the depth of the scattering atoms and thickness of the layer on top of them can be directly calculated. In addition, if the ion–nucleus repulsion is taken into consideration, surface atoms will shadow the incident ions from colliding with some atoms along the incident direction. Thus, tracking the intensity variation of scattered ions as a function of incident angle reveals the atomic arrangement and lattice dimensions of the surface.

Before the Si(111)-(7×7) reconstruction structure was confirmed by scanning tunneling microscopy, the importance and complexity of this surface attracted intensive investigations, including ion scattering methods (ISS). Culbertson et al. [9] carefully determined the surface atomic displacement of about 0.4 Å by studying the variations in scattered ion peak intensity as a function of incident ion energy and sample orientation.

2.2.3 Scanning Tunneling Microscopy and Atomic Force Microscopy

Although the surface symmetry, composition, and even atomic arrangement can be carefully determined by experimental results from LEED and ISS, as discussed above, it is no doubt that the visualization of surfaces at atomic resolution provides more direct and accurate information. The invention of scanning tunneling microscopy enabled scientists to “see” surface atoms and earned the inventors Gerd Binnig and Heinrich Rohrer the Nobel Prize [10].

According to quantum mechanics, electrons have some probability to be present in classically forbidden energy regions. These tunneling effects are the basis of STM. When a metal tip is brought close to a surface, the vacuum gap between them results in an energy barrier, with electrons from the two materials transmitted from the one with the higher Fermi level to the other without reaching the vacuum energy level. Scanning tunneling microscopy works on this principle, measuring and controlling the tunneling current between the sample and the tip. The tunneling current is extremely sensitive to the local charge densities and the distance between the tip and the surface. After the tip scans around the surface like a finger touching and feeling it, the topography and the charge density distribution of the surface can be depicted using the information on the tip position and the measured tunneling current.

Atomic force microscopy (AFM) follows the concept of STM, but it is “feeling” the force applied to the tip by surfaces, rather than the tunneling current between them [11]. Thus, it can be applied on both conductors and insulators. The AFM probe is typically a silicon or silicon nitride cantilever with a sharp tip at the suspended end, and the tip's radius of curvature is on a nanometer scale. When the probe is scanned across surface in the “contact” mode, its deflection is monitored and recorded to map the surface at the atomic scale. Furthermore, in addition to the mechanical contact force and other short-range forces working in the contact mode, the tip is also influenced by some “long-range” forces, such as van der Waals forces, when it is away from the surface in “noncontact” mode. Therefore, when the cantilever vibrates around its resonance frequency close above the sample surface, the vibration is modified by this long-range force, and thus provides information about topography of the surface. In order to detect both the long- and the short-range force without trapping the tip by the surface, the contact and noncontact modes have been combined in the “tapping” mode. In the tapping mode, the cantilever vibrates around its resonance frequency with an amplitude ranging from 100 to 200 nm; thus, the tip contacts the surface intermittently.

The STM and AFM tips are all precisely controlled and manipulated by piezoelectric elements that operate at the atomic scale. The piezoelectric elements are, in turn, driven by stable high-voltage power supplies, which are monitored and controlled by computer. The computer also generates the surface image based on position and current information from the control circuit. Although the AFM image interpretation appears to be relatively straightforward, the STM image is a convolution of topography and electronic structure, and must be carefully considered and interpreted.

The imaging of the Si(111)-(7×7) surface stands among the most important contributions of the STM. Shortly after the invention of the STM, the Si(111)-(7×7) surface was imaged at atomic resolution [12], and the atomic arrangement and charge distribution among adatoms and rest atoms were demonstrated. This work then quickly led to the proposal of the well-accepted DAS model [13]. The atomic image of the Si(111)-(7×7) surface was also obtained with AFM, and the image resolution was further improved by finely controlling the amplitude of the cantilever vibration to enhance the short-range force sensitivity of the AFM [14].

2.3 SURFACE COMPOSITION, ELECTRONIC STRUCTURE, AND VIBRATIONAL PROPERTIES

The understanding of surface structures of semiconductors provides fundamental knowledge for their modification and functionalization, which in turn requires the analysis of surface composition, electronic structure, and vibrational properties. A variety of surface-sensitive analytical techniques are used to study the physical, chemical, and structural properties of modified surfaces and adlayers. Among these techniques, Auger electron spectroscopy (AES), photoelectron spectroscopy (PES; XPS, UPS), high-resolution electron energy loss spectroscopy (HREELS), and some synchrotron-based methods are the most widely used.

2.3.1 Auger Electron Spectroscopy

The Auger effect was first discovered in the 1920s by Lise Meitner and Pierre Auger, describing the electron emitting relaxation process that occurs after the formation of a core hole in an atom. The ejected electron is called an Auger electron. The Auger electron's kinetic energy approximately equals the energy difference between the initially excited state and the relaxed state, which are characteristics of the emitting element. The most prevalent Auger peaks observed in spectra are normally KLL or LMM types, where the letters represent the initial states of the core hole and the involved electrons in the relaxation.

AES uses an electron gun (generating electrons with an energy of several keV), an X-ray source, or ion gun incident on the surface to eject core level electrons, creating the core hole state. After core holes are created Auger electrons are emitted from the surface, and their kinetic energy is analyzed, typically by a concentric hemispherical or a cylindrical mirror electron energy analyzer. The electrons are amplified with an electron multiplier, and their kinetic energy spectrum collected and recorded.

If an electron beam is used for core hole excitation, the Auger peaks in AES spectra are superposed on a strong background of inelastically scattered electrons, appearing as sharp peaks on a rapidly varying slope. Thus derivative spectra are often used to eliminate the influence of this secondary electron background [16]. The characteristic AES peaks can identify the elemental composition of sample surfaces, and their values, compared to standard spectra, suggest the chemical states of the respective elements. In addition, with information about electron collision cross section and Auger relaxation probability, the abundance of the elements in the surface region can be determined. More commonly, similar information can be obtained by referring to external standards. Furthermore, when an AES study is conducted while the sample is sputtered by an ion beam at a known rate, the depth profile of elemental composition can be acquired.

The AES is normally used to check the semiconductor surface cleanliness, as the clean silicon surfaces show only the Si LVV peak around 92.6 eV without contaminant peaks caused by carbon (KLL at about 263.6 eV) or oxygen (KLL at 508.6 eV). Upon surface modification, the Auger peaks shift to indicate a change in chemical environment. More recently, AES was also proposed to characterize the thickness of graphene films based on AES peak intensities [17].

2.3.2 Photoelectron Spectroscopy

In addition to the Auger electrons, photoelectrons emitted from atoms in the surface region also carry information about their identity, abundance, and bonding status. Core level and valence level electrons can be excited to become photoelectrons by irradiation with X-ray and ultraviolet sources, respectively.

In X-ray photoelectron spectroscopy (XPS), the X-ray photons are normally emitted by electron-bombarded Al or Mg targets. The X-ray photons are then directed to the sample surface, ejecting photoelectrons from the surface atoms. The incident X-ray photons can be passed through a monochromator, improving the energy resolution of the photoelectron peaks. Since electrons interact with the matter strongly, the photoelectrons have a short mean free path [18] of several nanometers, making XPS surface sensitive. The photoelectrons ejected directly from the excited atoms travel trough vacuum to reach the analyzer. The analyzer is normally a concentric hemispherical analyzer with fine small apertures, followed by amplifiers and electron signal converters. The signal is monitored and recorded, indicating the core level electron binding energy (peak position) and intensity (peak area). Binding energy is obtained from a knowledge of the photon source energy and the measured electron kinetic energy (B_E=hv−K_E).

The core level binding energy, referred to as the Fermi level, not only identifies the elemental composition of the surface but also suggests the electronic environment because the electron density surrounding the nuclei partially screens the core level electrons. However, the experiments should be conducted with care to avoid any charging effect, which also causes the binding energy to shift. Owing to the energy width of the X-ray source, the XPS spectra normally require deconvolution to reveal the intrinsic peaks based on the knowledge of peak properties and composition information. There are several software packages available to fit and deconvolute XPS peaks [19]. After fitting and deconvolution, the ratio of peak area can be used to determine the composition and chemical evolution of sample surfaces. Furthermore, varying the photoelectron detection angle changes the origin depth of the detected electrons and makes XPS capable of providing depth profile information [20].

Ultraviolet photoelectron spectroscopy (UPS) is similar to XPS, but the photons from a helium discharge lamp of 21.2 (HeI) or 40.8 (HeII) eV are typically used instead of X-ray photons. Thus, the valence level electrons are excited to form photoelectrons, leaving the core levels unaffected. The UPS peaks for semiconductor surfaces represent surface states and molecular orbitals of the surface and adsorbed layers, which are directly involved in surface reactions. Hence, the spectral character and changes upon surface reactions and modifications directly indicate the evolution of involved surface sites and relevant molecular orbitals. In addition, the onset of the UPS spectra can be used to determine the work function of sample surfaces, which has important application for organic–semiconductor interface fabrication.

XPS and UPS are the most intensively employed methods to characterize modified and functionalized semiconductor surfaces. An example is given by the investigation of adsorption of acetonitrile on the Si(111)-(7×7) surface [21]. In this work, Xu and coworkers extensively applied XPS, UPS, HREELS (discussed in the following sections), and STM to experimentally study the molecular binding state of acetonitrile. In this study, XPS was used to observe the chemical shift of the core level electron binding energy to determine the respective atom bonding environment. The N atom is involved in the binding as its 1s binding energy shifts from 400.1 eV for physisorbed molecules down to 398.8 eV for chemisorption. This peak is attributed to the Si–N species. UPS monitors changes in surface states and the involvement of acetonitrile molecular orbitals upon adsorption and shows that the surface state at 0.3 and 0.7 eV below the Fermi level attenuates with the increase in acetonitrile coverage.

2.3.3 Inverse Photoemission Spectroscopy

Inverse photoemission spectroscopy (IPES) is a surface-sensitive spectroscopy that investigates the unoccupied surface states between the Fermi level and the vacuum level, which cannot be probed by PES. It employs an inverted photoelectric process. If energetic electrons are coupled with an unoccupied surface state or molecular orbital, relaxation to a lower unoccupied state can occur, emitting photons of characteristic energy. When the incident electron energy is scanned over a certain range, the energy level of the unoccupied state can be derived from the known incident electron energy and the emitted photon energy.

Besides an electron gun, a Geiger–Muller tube, which converts ionizing photons to current pulses, or a diffraction grating together with a two-dimensional position-sensitive detector, is used to quantify the energy and number of emitted photons. The assignment of these states requires careful experimental design and support from theoretical work [22].

After the common acceptance of the DAS model, much effort was put into the detailed experimental investigation of the distribution of charge and states on the Si(111)-(7×7) surface. Thus, IPES was employed to study the unoccupied states. In the study done by Nicholls and Reihl [23], the empty surface state, which is 0.5 eV above the Fermi level, can be identified with the IPES technique.

2.3.4 Vibrational Spectroscopy

For the organic–semiconductor interface, vibrational spectroscopy is widely used to study molecular structure and even orientation of adsorbates because it is versatile under many experimental conditions and conclusive for structural determination. Among the vibrational spectroscopic methods, infrared (IR) spectroscopy and HREELS are most frequently used. However, these two techniques work on different principles with distinct selection rules and characters.

2.3.4.1 Infrared Spectroscopy

IR spectroscopy is a conventional technique to excite and probe the characteristic molecular vibrations. Infrared photon energy is comparable to the molecular vibrational energy scale; thus, some of the IR photons with particular energy can be absorbed by molecules, making them vibrationally excited. The vibrational energy, which is determined by bond strength and the reduced mass of the vibrating atoms, provides the chemical bond's intrinsic character. Thus, IR spectroscopy is able to identify the chemical bonds and molecular structure of the adsorbed species. Especially, when polarized IR is used to selectively excite dipole moments in adsorbed molecules, their orientations can be deduced.

The continuous IR light is normally emitted from electrically heated inert solids. The continuous IR light is then dispersed in a monochromator, and respective absorption intensities for the adsorbed molecule are measured. More commonly, Fourier transform IR (FTIR) [24] is used instead of a dispersive spectrometer. In the FT-IR, broadband light from a Michelson interferometer is reflected from the surface and adsorbed layer. Upon interaction with the sample, some of the photons across the IR region of the incident spectrum are absorbed by the molecular overlayer, and photons reflected from the sample are detected with sensitive IR detectors. The data from the detector (intensity versus interferometer mirror displacement) is then converted to absorption spectra (intensity versus frequency) through Fourier transformation. The FTIR has the advantage of high speed, high resolution, and high throughput, resulting in high sensitivity.

The FTIR is a powerful tool to measure the molecular vibrations of adsorbates and determine their structures on semiconductor surfaces, especially when high resolution is required. For instance, the C–H stretching frequencies are substantially influenced by the rehybridization of the C atom, and can be monitored by IR. When the C atom is sp³ rehybridized, the C–H stretching is mostly seen below 3000 cm⁻¹, and the C–H stretching with sp² rehybridization appears in the range from 3020 to 3100 cm⁻¹, while the sp rehybridized C–H bond can promote the vibration to around 3300 cm⁻¹ [25].

2.3.4.2 High-Resolution Electron Energy Loss Spectroscopy

HREELS uses a monochromatic low-energy electron beam of about 5 eV to probe adsorbed species on surfaces. When electrons impinge on solid surfaces, some of them are elastically scattered, as with low-energy electron diffraction, and some of the electrons lose energy to vibrationally excited surface species via impact, dipole, and intermediate negative ion resonance scattering mechanisms [26]. Similar to IR spectroscopy, the discrete energy losses correspond to the vibrational energy levels of adspecies, which in turn can be used to analyze their molecular and binding structures. Although the resolution is lower for HREELS (about 30 cm⁻¹) compared to FT-IR (as low as 2 cm⁻¹), HREELS is able to directly identify the vibrational features of substrate–degadspecies bonds, which normally lie in the extremely low-energy range. In addition, careful comparison of the HREELS spectra collected in specular and off-specular directions, respectively, can shed light on the orientation of adspecies. Peak intensity contributed from the dipole scattering mechanism is sensitive to both adsorbate orientations and reflection angle, while the impact scattered electrons are not.

Several generations of HREELS instruments have been developed with increasing resolution and improved control systems. To achieve high resolution, two 127° sector electron energy monochromators are used in sequence to well define the electron beam emitted from a filament. After scattering from a sample surface, the electrons are analyzed also by two consecutive 127° cylindrical sector analyzers, and then counted by detectors [27].

The elastically scattered electrons create a sharp intense peak as an energy reference in HREELS spectra. The inelastic peaks appear on the sloping background of the elastic peak. A resolution of about 30 cm⁻¹ or better can be routinely acquired. The interpretation and assignment of the HREELS peaks collected on modified semiconductor surfaces normally require assistance from theoretical work and well-studied IR or Raman spectra of related species.

The HREELS method has been extensively used in the study of organic modification of semiconductor surfaces. The HREELS technique can be used to detect the vibrations of substrate–adsorbate bonds (e.g., Si–O, Si–N, and Si–C) and to determine the structure and binding modes of adspecies. In addition, HREELS obtained in off-specular mode provides information about the geometry of the adsorbed molecules, as was found for adsorbed styrene molecules that adopt an orientation parallel to the surface on Si(111)-(7×7) [28].

2.3.5 Synchrotron-Based Methods

Synchrotron radiation, a tunable and highly monochromatic light source with high intensity and polarity, is key to a number of surface analytical techniques. Near-edge X-ray absorption fine structure spectroscopy (NEXAFS), energy scanned PES, and glancing incidence X-ray diffraction (GIXD) are frequently used to characterize the modified and functionalized semiconductor surface.

2.3.5.1 Near-Edge X-Ray Absorption Fine Structure Spectroscopy

When sample surfaces are irradiated with a tunable monochromatic X-ray beam from a synchrotron across a particular photon energy range, the sample absorbs photons of a specific energy, corresponding to core level electron excitation to the Fermi level, to unoccupied states or molecular orbitals of the surface–adsorbate complex. This excitation is followed by the Auger emission process. The electric current compensating the emitted Auger electrons from the grounded sample can be monitored to track the X-ray absorption [29]. Similar to XPS, NEXAFS presents peaks corresponding to core level electron binding energies. In addition to the main absorption peaks, X-ray absorption features near the transition arise from the excitation of core level electrons to unoccupied molecular orbitals, which are sensitive to the surrounding electronic and chemical environment of the surface adsorbed layer. These features serve as fingerprints for molecular and electronic structures. In addition, polarized X-rays from a synchrotron source can selectively excite electrons to certain unoccupied orbitals under particular selection rules, revealing their shape and orientation at the surface.

2.3.5.2 Energy Scanned PES

This technique takes advantage of the ready tunability of light from a synchrotron source to enhance the information obtainable from photoelectron spectroscopy carried out with a fixed frequency photon source. A typical example is the use of variable photon energy to scan the kinetic energy of ejected photoelectrons originating from a specific binding energy transition. In this way, a vertical profile of the composition of a composite or layered sample can be obtained [30].

The ability to readily scan the photon energy enables a number of photoelectron-based methods that provide useful electronic structure information about the modified semiconductor surface. Some example methods [31] are as follows:

1. Photoemission yield spectroscopy, where all the photoelectrons emitted at a particular incident photon energy are collected as a function of photon energy.
2. Constant initial state spectroscopy, where the difference in energy between the incident photons and the photoelectron energy analyzer is kept fixed while scanning the incident photon energy. This scanning of the final state energies provides information about the unoccupied state density of states.
3. Constant final state spectroscopy, where the electron energy analyzer is kept at a fixed energy as the incident photon energy is scanned. This provides information about the occupied density of states of the sample.

2.3.5.3 Glancing Incidence X-Ray Diffraction

X-ray Diffraction is a traditional crystal structure analytical methodology, which has contributed significantly to the determination of three-dimensional crystal and molecular structures. However, due to the ability of X-rays to penetrate deep into matter, conventional XRD technology is not sensitive to surfaces. To discriminate surface structural information from the bulk, a highly collimated intense X-ray beam from a synchrotron source irradiates the sample at a glancing incidence angle, which is typically smaller than the critical angle to enhance its surface sensitivity. While the incident beam and sample are fixed to maintain the incident angle, the scattered diffraction photons are detected at a continuously changing angle. The photon intensity versus detecting angle spectra provide information about crystal structures on surfaces and their atomic arrangements [32].

2.4 KINETIC AND ENERGETIC PROBES

Besides the compositional, electronic, and vibrational properties of the clean and modified semiconductor surfaces, the kinetic and energetic aspects of semiconductor surface reactions have attracted significant interest. The information related to reaction kinetics and energies provides fundamental scientific understanding and guides further modification and functionalization of semiconductor surfaces. Furthermore, after the modification and functionalization, the semiconductor surfaces are subject to a broad range of applications, which could require different surface properties such as stability on the one hand or reactivity on the other. Thermal programmed desorption (TPD) and molecular beam methods can help probe the reactions on surfaces from a kinetic and energetic aspect and provide information about the physical and chemical properties of the modified surface.

2.4.1 Thermal Programmed Desorption

Adsorbed molecules are bound in energy potential wells on the surface. Thermal energy can activate the adspecies in this potential well and cause it to desorb. The desorption temperature is indicative of the desorption energy, which is determined by the adsorption site, the binding configuration, and the interaction between adsorbates. By heating the sample while monitoring the desorbed fragments by mass spectroscopy, TPD can be used to study the surface coverage, thermal stability, and adsorption–desorption kinetics.

In the TPD experiment, the sample temperature is increased with a constant rate under computer control, which monitors the surface temperature and controls the heating power supply. Simultaneously, the desorbed species are tracked by mass spectrometry and the signal intensity of particular mass fragments is recorded versus temperature. Normally, the spectra acquired after different exposures to the sample are collected and compared.

The most straightforward information from the TPD spectra is the thermal stability of the adsorbed layers and the desorption temperature, which is directly linked to desorption and adsorption energies. The desorbed species also suggest the adsorption and desorption process and mechanisms. More importantly, the desorption peaks indicate different adsorption states with their corresponding desorption energies, differentiating physisorbed and chemisorbed species at different surface sites. The desorption peak area relates to the coverage of the species. With increased exposure, desorption peaks may change in position and shape, as well as peak area, providing information about desorption reaction order and other kinetic parameters [33].

For example, in the case of adsorption of acetonitrile on the Si(100) surface [34], the TPD spectra show first-order physisorption and two distinct chemisorption states, which desorb at 400 and 460 K, respectively. The desorption activation energies were calculated to be 24.6 and 29.8 kcal/mol, accordingly. On the basis of the spectral change with increasing surface exposure, the evolution of the adsorption states and a mechanism for the process was proposed. Combined with experimental results from other techniques, detailed adsorption structures and mechanisms can be concluded.

2.4.2 Molecular Beam Sources

A collimated molecular beam can be generated either by effusion of molecules from an oven or by high-pressure expansion through a skimmed nozzle source. These beam sources can be used as directed molecular dosers in the study of modified surfaces. One important application of these molecular beam sources is in the measurement of sticking coefficients of small molecules on the well-characterized surface. Because the adsorbed molecules do not contribute to the background pressure, the sticking coefficient can be determined on the basis of pressure variation in the experimental chamber when the sample surface is exposed to the collimated molecular beam. This method of King and Wells [35] is a widely used and accurate method for determining the probability of adsorption on a well-characterized modified surface. Measuring the sticking coefficient under a variety of conditions, such as differing coverages, temperatures, and preparation methods, leads to detailed information about adsorption kinetics and mechanisms.

2.5 CONCLUSIONS

A full spectrum of surface analytical techniques has been applied to study the modification and functionalization of semiconductor surfaces, ranging from structural analysis to reaction mechanism deduction. The accumulated knowledge of semiconductor surfaces and their modifications is based on these techniques. Also, the constant need for more accurate and more detailed understanding of the organic–semiconductor interface and its related applications motivates the development of surface analytical techniques. In order to obtain a complete picture of surface reactions, different surface analytical techniques are normally combined to conduct the analysis. The combination is determined by the properties of related surface reactions and the strengths of respective detection techniques. Although surface analytical techniques can provide direct evidence for properties and mechanisms of surface reactions and modification processes, theoretical work is typically needed in order to interpret the experimental data and make predictions on the basis of the results.

In the following chapters, we will constantly see the power and importance of these analytical techniques in their application to semiconductor surface modification and functionalization.

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