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K-cell Calibration

Calibration of the K-cells was attempted by two means;
 

1) Mn was deposited on a substrate of GaAs (001). Using XPS the surface concentration of Mn and substrate atoms can be calculated, however this method failed due to issues with the K-cells and was not pursued any further.

 

2) A quartz crystal microbalance (QCM) was attached to the chamber in place of the manipulator during one of the upgrade sessions. As material from the k-cells deposits on the crystal, the resonant frequency changes. From this frequency change one can calculate the mass deposited. Results for a range of cell temperatures are shown in the table below.

Deposition

Film

Z

g cm-3

Temp (°C)

Time (mins)

Å

Rate

Å min-1

1

Sb

0.768

6.6

490

20

2.07

0.1

2

Sb

0.768

6.6

470

35

2.26

0.065

3

Sb

0.768

6.6

450

35

1.6

0.046

4

Mn

0.377

7.2

720

74

-ve

-

5

6

Mn

Mn

0.377

0.377

7.2

7.2

750

800

48

20

-ve

0.4

-

0.02

 

The relationship between rate of film thickness growth and cell temperature is exponential in form [42]. Results for the Sb K-cell, suggest an exponential dependence, even though an accurate exponential fit cannot be made to the three available data points (time limitations only allowed for 3 experiments per K-cell). Results for the Mn K-cell are somewhat unexpected. At lower temperatures (deposition 4 and 5) an increase in QCM frequency is seen during deposition, suggesting the film on the QCM is in fact losing material. An increase in material is only seen at high temperature (deposition 6). This is likely due to desorption of Sb by heat radiated from the Mn K-cell. Furthermore, fluctuations in the water cooling temperature may have had an effect on the resonant frequency of the QCM. In future, Mn should be deposited onto a clean quartz crystal, and increase the deposition time to reduce the effect of water supply temperature fluctuations (which should be monitored throughout deposition). From the QCM results, a Sb and Mn K-cell operating temperature of 480°C and 800°C respectively was chosen.

 

The calibration of K-cells on Madge was performed by Chris Burrows using a Beam flux gauge to calculate beam equivalent pressures and a QCM to calculate rates of deposition. Similar issues were encountered calibrating the QCM as to those detailed above for Magnusson. For the Sb cell at 330°C the rate of arrival was found to be approximately 0.63 Å min-1, whilst for the Mn cell (717°C), the arrival rate was 0.34 Å min-1.