Electron emission processes are central to many effects at surfaces and interfaces, and to many techniques for examining the near-surface region. Most obviously we have emission from the solid into the vacuum, the electron overcoming the work function barrier in the process. This happens in both thermal emission, as described in sect. a) below, and in photoemission and Auger electron spectroscopy, described in section 3.3. In a high electric field, the barrier height can be substantially reduced, resulting in cold or thermally assisted field emission, as discussed here in sect. b). Finally an incoming beam can result in secondary electron emission, as described in sect. c), and hot electrons can penetrate internal barriers by ballistic emission, as described in connection with the microscopy of semiconductors in Section B. All of these effects are connected with electron sources for various types of microscopy. Consequently, you can think of this lecture as providing underpinning for the lectures in section 3 on (electron) microscope techniques.
Thermionic emitters in the form of pointed wires or rods are used as electron sources in many electron optical devices such as oscilloscopes, TV and terminal displays, and both scanning and transmission varieties of electron microscopes. A good thermionic emitter has to have a combination of a low work function and a high operating temperature. However, as can be seen from tabulations such as Table A10, higher melting point metals typically have higher work function. Thus the search is on for metals with a moderate work function which are sufficiently strong, or creep-resistant, near to their sublimation temperature, which in many cases is a long way below the melting T. Note that an additional possibility is to take a high melting point material and to coat it with a thin low work function layer. This is done for high current applications (TV and terminals) in sealed vacuum systems: we return to this later.
The standard material for comparison is a polycrstalline tungsten ‘hairpin’ filament with work function around 4.5 V, made of drawn wire a few tenths mm in diameter, bent, and situated in a triode structure, using a gate electrode called a Wehnelt. The competition is between the brightness of the source and its lifetime, which decreases markedly as the operating T is increased. For example, standard W-filaments used as electron microscope sources may have a lifetime of around 15 hours when operated at 2800 K, but this extends to maybe 50 hours when T is dropped to 2700 K (Orloff).
In instruments such as analytical SEM, TEM and STEM, we need to force as much current into a small spot as possible, in order to extract a high spatial resolution signal which has a sufficient signal to noise ratio (SNR), as discussed in section 3.5. This means that there has been an intensive search for materials with better performance as thermionic emitters than LaB6. It is clear that the desired material must be very stable at high T, and moreover must have a stable surface. Borides, carbides and nitrides are natural candidates, which have strong (largely ionic) bonds and can be, or can be made, adequately conducting.
Futamoto et al. (Surface Sci 100 (1980) 470) investigated mixed rare earth borides (LaxM1-xB6), where several metals M were tried out. They found that these additions made the emission go down rather than up, but that after some use, they improved somewhat, but never exceeded the performance of pure LaB6. Using a microprobe AES setup, they investigated the surface composition of the tips, and found that the other metallic elements evaporate faster, leaving a surface layer, a few nm thick enriched with La; emission properties thus remained remarkably similar across the series. Swanson et al. (Surface Sci 107 (1981) 263) changed the surface plane away from (100), measuring the lifetimes for a given emission current: no luck, (100) was the best!
Electron Microscopy conferences typically have a few papers on carbides and nitrides; some of these have promising properties, but they are not stable enough to be used routinely. Thus (100) LaB6 stays! The competition has come from field emission as described below.
Field emission requires a very good vacuum, and often, even in UHV, emission is not due to the clean surface. A typical field emitter needs to be ‘flashed’ to clean it, usually by passing a current through a loop on which it is mounted. After flashing the emission current is high, but rather unstable; the current decays with time, and becomes more stable as it does so. This is due to contamination of the tip, either from the vacuum, or more often from diffusion of adsorbed surface species to the tip. Thus the real nature of a field emission tip during use, and indeed of an STM tip, is somewhat shrouded in mystery.
These adsorbate and diffusion effects can be turned on their head, and put to good use scientifically, in Field Emission Spectroscopy and F.E. Microscopy (FEM). This field was pioneered by Gomer, whose 1961 book contains many of the important features of the methods: his review article (R.Gomer, Rep. Prog. Phys. 53 (1990) 917) should be consulted for more recent results on diffusion using the current fluctuation method. Three types of example are given here.
FEM images the tip itself, are obtained using with a plate anode which my be coated with phosphor to detect the intensity of emission from different crystal planes; in more recent experiments a channel plate would be used as an intermediate amplifier. An image from Gomer’s book is shown in diagram A18. This also shows (see the original to be convinced) that the shapes/ shadow of adsorbed molecules can also be seen on the emission pattern; however, the main feature is to show the variation of emission with crystallographic orientation. It is on this basis that faces such as W(310) were subsequently chosen as field emission tips for electron optical instruments.
In-situ deposition of individual metal atoms on the tip has been shown to caused jumps in the emitted current, as illustrated in diagram A19. Todd and Rhodin (Surface Sci. 42 (1974) 109) showed that they could distinguish 1,2 and 3 W-atoms arriving, and their subsequent desorption when the field remained on. Then they investigated the response of individual W (hkl) faces to adsorption of different alkali adatoms (Na,K,Cs), which all increase emission via lowering the work function.
The subtelty can be increased further: using a slot in different orientations allows one to explore diffusion anisotropy, since the measurement is dominated by diffusion parallel to the short axis of the slot. An example of O/W(110) is shown in diagram A21 from the work of Tringides and Gomer (Surface Sci 155 (1985) 254); in this case the work function of the oxygen covered surface is greater than the clean metal, so the adsorbate reduces emission. Note that O-diffusion was found to be anisotropic in the ratio about 2:1.
Adsorption is useful for an electron source if the adsorbate increases emission. The stringent vacuum requirements of field emission can be reduced somewhat if one both increases the operating T, and also uses an adsorbed layer which reduces the work function. This is Thermal Field Emission (TFE). Typical thin layers, which have long been used in TV and other sealed tube applications, are refractory (Ba, Sr) oxides, pasted onto, or indirectly heated by, the W filament; however, these coatings degrade badly if let up to atmsophere.
However, there are cases where the secondary electrons can be seen to convey more specific surface information. Under clean surface conditions, a change of surface reconstruction, or an adsorbed layer, will change the work function, the surface state occupation, and may also, in a semiconductor, change the extent of band bending in the surface region. The technique developed from this effect is biassed-secondary electron imaging, (b-SEI), since biassing the sample negatively to ~ -500V causes the low energy electrons to escape the patch fields at the surface. This signal is much more sensitive as regards imaging, than the corresponding Auger microscopy, as discussed in section 3.5. It has been shown that this technique is sufficient to detect sub-ML deposits with good SNR, as illustrated in diagram A23 for Ag/Si(111), and in diagram 119 (section 3.5) for Ag/W(110).
Once again, the corresponding spectroscopy is useful in determining the origin of the contrast, as shown in diagram A24 (Futamoto et al, Surface Sci. 150 (1985) 430). There it was noted that the case of 2ML Ag deposited onto W(110) causes an increase in the secondary yield at the lowest energies, which can be readily explained by a decrease in the work function. However, the sub-ML Ag/Si(111) case shown in A24(b) gives an increase in yield at higher energies too, and it is probable that a change in band-bending is involved. In the latest work on sub-ML Cs/Si(100), the detection sensitivity of b-SEI was pushed to below 1% of a ML, reflecting the strongly ionic bonding of Cs, which causes a large surface dipole, at low coverage (Milne et al, Phys. Rev. Lett. 73 (1994) 1396, Surface Sci 336 (1995) 63). This form of surface microscopy has been exploited to measure diffusion of sub-ML and multilayer deposits over large distances (many micrometers), as described in section E.
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