Diagnóstico del proceso de gestión de Ventas de la unidad

Many different factors can affect the work function measured and throughout the literature there is a large variance in the expected value. The work function of copper for clean polycrystalline copper is given as around 4.5eV by multiple references [114, 115] but can vary from 4.4 to 4.8eV [116, 117] with the surface roughness and strain on the surface. The absorption of oxy- gen is also known to cause the work function to increase anywhere from 0.125eV to 0.675eV for single crystal faces [118] and Cu2O has been quoted to have a WF as high as 4.84eV [119]. As such it is difficult to know exactly what work function our copper surface will have and we have to rely on our KP measurements and calibration performed in section 2.2.4.

As mentioned previously by polishing the copper surface with Brasso, instead of the standard solvent cleaning method using in the fabrication of the ThGEMs, the photocurrent measured increases significantly. To investigate this effect further we will look at the CPD of the copper surfaces using the different cleaning methods.

4.2. COPPER

different cleaning processes: A 5 minute ultrasonic bath of 50:50 acetone and isopropanol so- lution is always done first to remove the photoresist from the PCB then the surfaces are either polished with Brasso or subsequently cleaned with a solution of 5% Micro-90 cleaning solution and distilled water in an ultrasonic bath for 5 minutes.

From the table it can be seen that the main source of error is the determination of the tip work

Cleaning Method CPD (mV) Error WF (eV) Error

Solvent -180 20 4.52 0.07

Brasso -260 30 4.44 0.08

M90(5%) -660 25 4.04 0.07

Water -50 10 4.65 0.07

Table 4.1: CPD measured for copper surfaces under different cleaning methods

function (0.07eV). With the error during measurement and between samples of similar condi- tions having a smaller error.

The solvent cleaned copper has a very similar WF to that found for clean polycrystalline copper in literature [114, 115], where Brasso cleaned copper showed slightly lower WF, but still within error.

Interestingly, the samples cleaned in the micro90 solution resulted in much lower measured WF, this could be due to the absorbed water on the surface, as it has been shown that this can reduce the WF significantly. However, Cu surfaces cleaned in standard solvent and following distilled water bath showed a higher WF in contrast to the literature [120, 121] where the work function is shown to decrease as much as 1eV for water absorbed onto clean copper surface.

As well as absorption of water, oxygen and other contaminants can cause significant changes to the work function measured, there is often a large variation across different patches on a surface as shown by figures 4.1a& 4.1bwhich show the CPD measured over several points on the same sample surface for solvent cleaned and Brasso polished Cu surface respectively. The time in between each measurement was small, so significant changes due to absorption of contaminants are unlikely.

The standard deviation across a single sample is about 30mV for both samples, however for some samples this deviation can be even larger especially in the case of vacuum grown samples that can sometimes have non-uniform or patchy films. The size of this variance across a sample limits the ability to detect small work function changes on a sample, without having multiple scans across the sample surface or measuring the exact same position on the sample.

Despite causing a large increase in the QY, the WF of the polished copper surfaces is similar to fresh copper samples cleaned in solution. Further proof of polishing not affecting the work function of the copper surface is that polishing already measured solvent cleaned copper surfaces showed no measurable change. This suggests that the improvement in response is not due to a shift in the work function as any change measured is too small to explain the amount of extra output signal achieved. Therefore, it is more likely that the quantum yield has been increased

4.2. COPPER

(a) (b)

Figure 4.1: CPD variations across a single copper sample for a) Solvent Cleaning & b) Brasso polished surface

instead, by a change to either the surface roughness or composition of the surface. Li et al. [116], showed a reduction in surface roughness causes the WF to increase. This change has not been noticed in the WF measurements between solvent cleaned and the polished Cu surfaces. Therefore the most likely candidate for increase in signal is a change in the composition of the surface.

Figure 4.2 shows the CPD measurements over a period of 20 days of copper surface both sol-

Figure 4.2: CPD measurements for a solvent cleaned ThGEM over a period of 20 days exposed to ambient air

4.2. COPPER

vent cleaned and polished with Brasso exposed to ambient air. It can be seen that there may be an initial rise in work function, but this happens over a period of around a few days, after that the work function appears to remain reasonably stable. The initial rise is more noticeable for Brasso polished samples, with a∆WF of about 0.1eV.