It can be shown from kinetic gas theory that the impingement rate (Φ) of a gas on a surface can be given by
Φ = √ P
2πmkBT
(cm−2s−1) (3.13)
Where P is the gas pressure, m is the mass of an impinging gas atom, kB is
Boltzmann’s constant, and T is the temperature of the gas. By assuming that every
impinging gas atom sticks to the surface and has a molecular diameter of 300 pm, the time to form a monolayer can be expressed as [18]
Monolayer formation time = 2.5×10−6
P(torr) s (3.14)
It can be seen that to keep a surface free from contaminants for a period of one hour, it must be in a vacuum better than 7×10−10Torr or 1×10−9mbar. Keeping a clean surface is vital when using XPS to measure the band alignment of a pair of materials; any surface adsorption can have a major effect on the band bending at the surface and will lead to an incorrect interpretation of band alignment results [19, 20]. Typical surface contaminants include surface oxidation (for metal, or metal terminated surfaces), adsorbed carbon compounds, and adsorbed hydroxide species.
To move samples from the MBE to the XPS system without breaking vacuum a portable UHV transfer system was developed. It consists of a magnetically coupled linear/rotary drive with a stroke of 609 mm, a 20 l s−1 Ion Pump, a gate valve and a port aligner. A gripping mechanism is attached to the end of the linear drive and is actuated by rotating the drive. The mechanism engages or unlocks when there is resistance during rotation, i.e., when the sample holder is being held in free space it will rotate with the linear drive, but if the sample holder is held against a surface the increased resistance will cause the drive rotation to lock/unlock the gripping mechanism. The sample holder is gripped by its ‘nipple’, which is shown
48mm
(a) (b)
14mm
(c) (d) (e)
Figure 3.12: Photographs of (a) the original MBE sample holder for 10x10mm
substrates, (b) the new, modified MBE sample holder (c) the original XPS sample holder, (d) the newly designed XPS sample holder, (e) the backside of the new XPS sample holder.
circled in Figure 3.12(c). All components are connected with ConFlat (CF) 35 mm inner-diameter flanges. The transfer system connects to the load lock on both the MBE and XPS. As the MBE and XPS are in the same room, the ion pump can remain powered during movement of samples from one system to the other. As the ion pump provides a pressure reading by measuring the current in the pump, a separate pressure gauge is not necessary. In future, a non-evaporable getter pump will be added to the system. This will aid the base pressure of the transfer system and will enable the transfer of samples between systems which are located in other rooms, such as the scanning tunneling microscope and magnetron sputtering tool. The transfer system has a base pressure better than 1×10−9mbar after bake-out. A sample spends approximately 1–2 h in the transfer system. While this includes the time to vent the loadlock of the MBE so that the transfer system can be removed, and the physical movement and attachment of the transfer system to the XPS, the majority of the time is spent waiting for the XPS loadlock to be pumped down to an acceptable pressure (<1×10−7mbar) to minimise the risk of contaminating the transferred sample.
are shown in Figures 3.12(a) and 3.12(c) respectively. The MBE sample holder shown is made of 1.5 mm thick molybdenum and is designed to hold 10x10mm square
substrates. The substrate sits in the recess of the sample holder and remains face down through all manipulation in the MBE. It is held by gravity, resting only by it’s corners on four metal clips. This shields as little of the substrate surface as possible from the deposition, and provides the radiant heater with a direct line of sight to the substrate backside.
In comparison, the XPS is designed for much smaller flag-shaped sample holders, as seen in Figure 3.12(c). The sample is usually secured to the sample holder with UHV-compatible conductive copper tape, or held in by spot welded tantalum brackets if the sample is to be heated. The sample must be held securely as it is rotated upside down as it moves from the load lock to the analysis chamber of the XPS.
A new XPS sample holder was designed which was compatible with both systems, and is shown in Figure 3.12(d) and 3.12(e). It is constructed out of molybdenum so as to be suitable with high temperatures. As in the MBE, the sample is secured only at its corners, but a 1 mm diameter molybdenum wire bent into a ‘U’- shape (not shown in Figure 3.12) is placed against the sample backside and held in by its natural springiness. This is used to prevent the sample from falling out of the holder and also forces the substrate into contact with the sample holder to provide electrical contact with the XPS. This is necessary to prevent substrate charging, which makes determination of binding energy shifts extremely difficult. Substrates used with this sample holder had ∼50 nm of gold deposited onto their corners to provide this electrical contact from film to XPS.
While this sample holder is naturally compatible with the internal mechanisms of the XPS, a piece had to be designed for the MBE to allow the sample holder to be moved between the load lock and main chamber. This is shown in Figure 3.12(b). It was originally a holder for 1" wafers; it has now been modified by removing a portion of the holder and adding two molybdenum supports that are attached to the sample holder with screws. The supports allow the new XPS sample holder to rest on its edges, with the sample facing down. The portion which has been cut out allows an XPS sample holder which is attached to the linear drive to be placed into this sample holder. A piece of bent tantalum foil acts as a spring to hold an XPS sample holder in place.
To demonstrate the effectiveness of the transfer system and the detrimental effect that surface contamination can have on XP spectra, XPS was performed on two samples of Mn2Au. One was moved from the MBE to the XPS using the vacuum transfer system, while the other was removed from the dry nitrogen-vented MBE loadlock, mounted on an XPS sample plate, and inserted in the XPS loadlock as quickly as possible. This resulted in an air exposure of approximately 15 minutes. A
7 0 0 6 5 0 6 0 0 5 5 0 5 0 0 4 5 0 4 0 0 3 5 0 3 0 0 2 5 0 2 0 0 1 5 0 1 0 0 5 0 0 5 3 4 5 3 1 5 2 8 5 2 5 In te n s it y ( A rb it ra ry U n it s ) B i n d i n g E n e r g y ( e V ) V a c u u m T r a n s f e r r e d A i r E x p o s e d
Figure 3.13: XP survey spectra of vacuum transferred and air exposed Mn2Au
samples. The intense peaks originating from Mn are shown in purple (—), those from Au are shown in gold (—), while those from contaminants are shown in blue (—). Inset: High resolution XPS of the O 1s peak showing the large amount of oxide
species that accumulate on an air-exposed sample.
comparison of the two XP survey spectra is shown in Figure 3.13. Typical spectral features due to the aforementioned contaminants appear as features in the O 1s peak at 530 eV and C 1s peak at 284 eV.
The material shown here (Mn2Au) is more suitable to highlight contamination than the p-type metal oxides which this thesis is based on, as the O 1s core level present in all oxides can obscure any oxygen-containing contaminants such as adsorbed hydroxide or carbon monoxide. For the vacuum transferred sample, less that 10% of the signal originates from surface contaminants, while for the air exposed sample, the signal is dominated by contaminants, with only 20% of the signal originating from the Mn2Au.
The Mn and Au core levels are seen to be sharper and more intense, and some features such as the Au 4p3/2 at 546 eV are highly obscured in the air-exposed sample. As can be seen in the inset of Figure 3.13, the oxygen content of the vacuum-transferred sample is minimal. There is also no detectable carbon on the surface of this sample.