Complete Report - University of New South Wales

Surprisingly high sensitivity of PL lifetime measurements
Using PL as a sensitive technique to determine small excess carrier concentrations in silicon
may at fi rst sight appear diffi cult given silicon’s indirect band structure and the resulting low
radiative recombination coeffi cient. However, the automated signal averaging, the geometry
of our PL setup and the use of a home built, low noise, high gain preamplifi er allowed effective
lifetimes to be determined down to injection levels < 10 9 cm -3 in 1 Ωcm p-type wafers (see
Fig. 4.6.6.5) with data acquisition times of only a few seconds [4.6.1], which is orders of
magnitude below the detection limit of a QSS-PC set-up used at our Centre. Using that QSS-
PC set-up, lifetime measurements become extremely noisy and inaccurate at injection levels
< 10 13 cm -3 . Due to this somewhat unexpected sensitivity, PL lifetime measurements can
also be used to determine very low effective lifetimes in quasi steady state mode and are by
no means restricted to high quality fl oat zone silicon.
The development of a computer controlled solid state light source for lifetime measurements
has also improved the sensitivity and accuracy of PC measurements, because the same
averaging techniques as used for PL can also be applied to PC measurements. While these
improvements render PC measurements at low injection levels less noisy they cannot avoid
some more fundamental limitations of PC measurements to be discussed now.
Depletion region modulation effect
In QSS-PC measurements at low to moderate injection conditions, the so-called depletion
region modulation (DRM) effect leads to a sharp artifi cial increase of the apparent minority
carrier lifetime on samples with a diffused or induced junction. Similar effects are expected
in any other experimental technique in which the measured signal is determined by the sum
of the minority and majority carrier concentrations. In contrast, theoretical analysis shows
that QSS-PL measurements should be largely unaffected by the DRM effect [4.6.2]. This was
proven experimentally by investigation of two 1Ωcm p-type wafers, one with Phosphorous
diffusions the other one with Boron diffusions on both sides [4.6.1]. Theoretically the artefacts
due to the DRM effect are expected only for the Phosphorus-diffused sample. Fig.4.6.6.4
shows the lifetimes of the Boron diffused sample determined from PC and from PL. Very good
agreement is observed between PL and PC with no indication in both measurements of an
infl uence of the DRM effect. In contrast, the PC lifetime from the Phosphorus doped sample
shown in Fig.4.6.6.5 increases steeply at injection levels < 5x10 12 cm -3 (up to an apparent
lifetime of 13ms, not shown in Fig.4.6.6.5). This increase is consistent with the theoretically
predicted effects resulting from the DRM effect. As theoretically predicted, the PL lifetime is
unaffected by these effects.
Figure 4.6.6.4: Effective lifetime from
QSS-PL (red) and from QSS-PC (black)
for a 1Ωcm p-type wafer with Boron
diffusions on both sides. Very good
agreement is observed between the
lifetimes from PL and from PC over a
wide injection level range.
111