Environmental Engineering Reference
In-Depth Information
Figure 7.5
AtypicalIPCEcurvemeasuredfora3
×
3mm
2
solarcell.Note
theseverelossesattheblueendandclosetotheGaAscut-offwavelengthof
∼
855 nm.
thedepletionregionis
∼
1.50
μ
mandstartsfrom
∼
0.87
μ
mfromthe
front surface.
This experimental evidence indicates that although the desired
device design was as shown in Fig. 7.1, during the first MOVPE
growth of the materials, only a 1.50
μ
m thick depletion region has
beenformedwithinthedevice.Thethicknessof0.87
μ
mtothefront
and 0.83
μ
m to the back of the device remains at nearly flat-band
conditions due to high doping levels (see Fig. 7.7). The slight slopes
available are due to the bandgap grading in these two regions. The
ideal situation would be to have a
∼
3
μ
m thick depletion region to
coverthefullwidthofthedevice.Thisobservationinvokedthestudy
of doping concentration using secondary ion mass spectroscopy
(SIMS) profiling through the device structure.
7.3.5
SIMS Profiling
A sample of the same wafer has been used for depth profiling by
SIMS at Loughborough Surface Analytical Ltd. A typical profile is
showninFig.7.8forthisdevicestructure.TheCandSiprofileswere
quantified (left-hand scale), whereas the Al and As profiles were
