Ferromagnetic (Ga,Mn)As Layers and ... - OPUS Würzburg

28 3. MBE Growth of Ferromagnetic (Ga,Mn)As Layers
cell shutter port
RHEED fluorescent screen
manipulator in
growth position
liquid nitrogen
lead-through
cell ports
cell watercooling
pyrometer port
sample holder
pressure gauge
liquid nitrogen
cooling shroud
main shutter
port
240W lamp for bakeout
turbopump V70
cryopump UHV CT8
Figure 3.1: Schematic cross section of the UHV MBE chamber. The cells are mounted
on the left side, aligned towards the sample holder sitting on the manipulator (shaded area)
occupying the central cavity.
on thermal activation at a temperature T , is given by
p = b e −E/kT . (3.1)
E is the activation energy, k the Boltzmann factor and b a temperature dependent
material parameter. For the small range of temperatures employed during growth
in an effusion cell, the dependence is approximately exponential. Therefore, assuming
favorable growth conditions, the amount of incorporated Mn (determined by HRXRD,
see Section 3.5.1) also increases exponentially with rising cell temperature, as shown
in Fig. 3.2 for a number of layers grown over a range of suitable Mn cell temperatures.
As indicated by the trendline, an increase of T Mn by 20 ◦ C leads to a doubling of the
Mn content.
3.2.2 The Growth Temperature
The solubility of Mn in GaAs at thermal equilibrium is very low. (Ga,Mn)As is a
metastable compound which can only be grown under conditions far from the equilib-