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The design of boron crucibles for MBE
Vacuum/volume Printed in Great
38lnumber Britain
Workshop
3lpages
188 to 190/l
0042-207X/88$3.00 Pergamon
988
+ .OO Press plc
note
The design of boron crucibles for MBE
In designing and constructing effusion sources for the source assembly of molecular beam epitaxy (MBE) growth chambers, a number of factors have to be considered, including rapid thermal response, uniformity of heating, selection of the evaporation materials and longer term stability. The effusion cells are ideally designed to approximate the Knudsen effusion cel11-4. In practice, however, the cell aperture is too large to be considered appropriate for a true Knudsen design. The apertures are therefore kept large to provide a useful rate of mass transfer3. Other problems associated with the construction of these cells are the difficulties in operating these cells at high temperatures (i.e. in excess of 1OOOC). This is due to the fact that at higher temperatures the chemical reaction of hot metal vapour which corrodes the cell depends on the cell materia12. Contamination is also a problem and is attributed mainly to the source materia15. In this work we report a new design of an effusion cell which is used successfully in the growth ofGaAs layers by MBE. The cell is
c---l675rnm
made from pure boron nitride (BN) (purity 99.999%). The dimensions of these crucibles are 50 mm long x 10 mm dia x 2 mm width (Figure 1). They are resistively heated using MO
0
x
T/c at the top of the oven T/c at the bottom of the oven
-
2mm hole may have to be rncreased to \ stop cloggl”g
t
4mm BN cop wth no gop between MO foil to reduce outgosing of wndlngs
+
Wall thickness 2mm
4
50mm
-
mm thick base
3 mm deep hole for_--thermocouple
(“Cl
Figure 2. Power input vs oven temperature calibration curves obtained for two thermocouples placed at the top and bottom position of the oven.
3 layers of MO foil for thermal shredding
2 wrndings of Ta wrre rnsulated by Alumrno (At 0,) tubing
loo0
500
Oven temperature
eater )nnections a
IOmm-
Figure 1. Schematic diagram of the effusion cell.
wire of 99.999% purity. The windings of the MO wire are very closely spaced and cover the outer surface of the cell up to the top to ensure uniform heating and to prevent crust formation near the opening. Three layers of radiation shields of highly pure (99.999%) and polished MO foils 0.1 mm thick are used to provide thermal insulation and independent temperature control. Temperature measurements of the cell are carried out using a platinumrhodium (Pt-13%Rh/Pt) thermocouple embedded in the bottom of the cells. For As, cells a thermocouple-controlled negative feedback arrangement (Eurothern Controller) is used to provide a temperature regulation of + 2°C. Calibration of the temperature versus power is made for both top and bottom positions of the cells. Calibration curves which are shown in Figure 2 demonstrate the temperature uniformity along the cell axis. Some temperature variations were observed between charged cells during normal operation and empty cells. The cells have been calibrated and used successfully in a uhv MBE chamber to grow 30 GaAs thin layers for different thicknesses ranging from 0.5-3 pm. Although boron nitride is a common material for GaAs effusion sources, it does have 189
Workshop
note
the discussion of these problems and the selection of another material (graphite) for effusion cells will be the subject of a future communication.
disadvantages;
References Acknowledgement
My thanks are due to Dr D Mukherjee for his useful suggestions and discussion, to I Rafferty for his technical assistance, and also to Dr N Ahmed for reading this letter.
190
’ A Y Cho and J R Aruther. Pro;/ Solicl Sr Chrw 10, 157 (1975). L K G Wagner, Circuutn. 34, 7 43 (1983). ’ D Mukherjee, Molecular beam growth and electrical analysis of PhTe layers on fused silica, PhD Thesis. The Hatfield Polytechnic (1980). J R A A Kubiak and E H C Parker, Appl Pl~r‘s .4, 35, 75 (1984). ’ P E Lusher and D M Collins, Proq Crust Growth Chumrt, 2, IS (1975).
38lnumber Britain
Workshop
3lpages
188 to 190/l
0042-207X/88$3.00 Pergamon
988
+ .OO Press plc
note
The design of boron crucibles for MBE
In designing and constructing effusion sources for the source assembly of molecular beam epitaxy (MBE) growth chambers, a number of factors have to be considered, including rapid thermal response, uniformity of heating, selection of the evaporation materials and longer term stability. The effusion cells are ideally designed to approximate the Knudsen effusion cel11-4. In practice, however, the cell aperture is too large to be considered appropriate for a true Knudsen design. The apertures are therefore kept large to provide a useful rate of mass transfer3. Other problems associated with the construction of these cells are the difficulties in operating these cells at high temperatures (i.e. in excess of 1OOOC). This is due to the fact that at higher temperatures the chemical reaction of hot metal vapour which corrodes the cell depends on the cell materia12. Contamination is also a problem and is attributed mainly to the source materia15. In this work we report a new design of an effusion cell which is used successfully in the growth ofGaAs layers by MBE. The cell is
c---l675rnm
made from pure boron nitride (BN) (purity 99.999%). The dimensions of these crucibles are 50 mm long x 10 mm dia x 2 mm width (Figure 1). They are resistively heated using MO
0
x
T/c at the top of the oven T/c at the bottom of the oven
-
2mm hole may have to be rncreased to \ stop cloggl”g
t
4mm BN cop wth no gop between MO foil to reduce outgosing of wndlngs
+
Wall thickness 2mm
4
50mm
-
mm thick base
3 mm deep hole for_--thermocouple
(“Cl
Figure 2. Power input vs oven temperature calibration curves obtained for two thermocouples placed at the top and bottom position of the oven.
3 layers of MO foil for thermal shredding
2 wrndings of Ta wrre rnsulated by Alumrno (At 0,) tubing
loo0
500
Oven temperature
eater )nnections a
IOmm-
Figure 1. Schematic diagram of the effusion cell.
wire of 99.999% purity. The windings of the MO wire are very closely spaced and cover the outer surface of the cell up to the top to ensure uniform heating and to prevent crust formation near the opening. Three layers of radiation shields of highly pure (99.999%) and polished MO foils 0.1 mm thick are used to provide thermal insulation and independent temperature control. Temperature measurements of the cell are carried out using a platinumrhodium (Pt-13%Rh/Pt) thermocouple embedded in the bottom of the cells. For As, cells a thermocouple-controlled negative feedback arrangement (Eurothern Controller) is used to provide a temperature regulation of + 2°C. Calibration of the temperature versus power is made for both top and bottom positions of the cells. Calibration curves which are shown in Figure 2 demonstrate the temperature uniformity along the cell axis. Some temperature variations were observed between charged cells during normal operation and empty cells. The cells have been calibrated and used successfully in a uhv MBE chamber to grow 30 GaAs thin layers for different thicknesses ranging from 0.5-3 pm. Although boron nitride is a common material for GaAs effusion sources, it does have 189
Workshop
note
the discussion of these problems and the selection of another material (graphite) for effusion cells will be the subject of a future communication.
disadvantages;
References Acknowledgement
My thanks are due to Dr D Mukherjee for his useful suggestions and discussion, to I Rafferty for his technical assistance, and also to Dr N Ahmed for reading this letter.
190
’ A Y Cho and J R Aruther. Pro;/ Solicl Sr Chrw 10, 157 (1975). L K G Wagner, Circuutn. 34, 7 43 (1983). ’ D Mukherjee, Molecular beam growth and electrical analysis of PhTe layers on fused silica, PhD Thesis. The Hatfield Polytechnic (1980). J R A A Kubiak and E H C Parker, Appl Pl~r‘s .4, 35, 75 (1984). ’ P E Lusher and D M Collins, Proq Crust Growth Chumrt, 2, IS (1975).