When applying the models discussed above to the experimental data, the desired output is Q, or the position dependent total collected charge. The inputs to the model are the total generated charge, which is discussed below, and theΩweighting function described in Section 3.3.3. Once the inputs are known, the model predictions can be calculated in seconds using any modern computer programming language.
Figure 3.7 shows the mean collected charge for the TPA measurements along with the model predictions for the data (red and blue lines). The results shown in Figure 3.7 are discussed in detail in Section 3.5. The purpose of this section is to describe how the data points shown in Figure 3.7 are related to the model predictions.
The model, as it is presented here, does not require the geometry and device doping as explicit inputs. The geometric aspects of the device that are relevant to the analysis are described by the Ω function. Appendix A shows that Ω can be calculated using (A.6);
however, it is also possible to estimate, rather than calculate, Ωusing data from experi- mental charge-collection measurements. The first step needed to estimateΩfrom data is to estimate the amount of generated charge when the laser spot is completely contained in the silicon portion of the device. While analytical methods are available for calculating
Figure 3.7: Collected charge as a function of focal plane position in the diode. Notice the better agreement for the ADC model as the laser pulse energy is increased enough to produce a plateau in charge collection. This plateau is a strong indication of regional partitioning effects occurring in the device.
the TPA-induced carrier density when only the carrier generation term is considered [39], these methods provide, at best, a rough approximation to the generated carrier density in the device. An accurate analytical calculation to determine the amount of charge deposited dur- ing the TPA process within a particular device is not presently available and is a difficult problem that would require considerable computational effort. This complex calculation can be avoided by assuming in the analysis that follows that the collected charge at the maximizing focal plane position is an estimate of the total generated charge, regardless of whether low-level or high-level conditions apply. Referring to the experimental data points in Figure 3.7, the estimates of generated charge are
Qgen=
0.08 pC @ 0.43 nJ2 0.81 pC @ 2.59 nJ2 2.7 pC @ 4.12 nJ2 7.8 pC @ 10.43 nJ2
(data). (3.4)
Having estimated the generated charge for each laser energy used, the next step is to esti- mateΩ.
Although a large amount of spatial resolution regarding laser intensity is needed to calculate (as opposed to measure) the generated charge, less spatial resolution regarding the initial generated charge is needed when using 3.1 or 3.2 to estimate the collected charge.
In particular, a sufficiently small laser spot can be approximated as a point source in the latter calculations. This approximation will be used to estimate Ω. Also, it is assumed here that the lowest laser intensity used in the measurements, which produced Figure 3.7a and will be called the “low-intensity measurements”, produces low-level conditions so that 3.1 applies. Let QEX P(Z)be the experimentally measured collected charge from the low- intensity measurements as a function of the focal plane depth Z. In other words,QEX P(Z) is the set of points in Figure 3.7a. Using the point-source approximation (i.e., qPI is the
generated charge multiplied by the Dirac delta function centered at depth Z) with 3.1 gives
QEX P(Z) =QgenΩ(Z)
or
Ω(Z) = QEX P(Z)
Qgen (low intensity). (3.5)
Note thatQEX P(Z)was measured only at the discrete values of Z indicated by the filled black circles in Figure 3.7a. Estimates ofΩbetween these points are obtained from linear interpolations. OnceΩwas determined using the data points in Figure 3.7a, it could be used along with 3.1-3.3 to develop the low-level (red) and ADC (blue) lines in Figures 3.7b-3.7d.
In other words, the only knowledge that the red and blue lines of Figure 3.7 have of the data points in Figure 3.7 is what is shown in 3.4 plus the data points in Figure 3.7a used to con- structΩ. Were it possible to accurately calculate the charge deposited by TPA in the device (Qgen) as opposed to using 3.4, the red and blue curves shown in Figure 3.7 would require no knowledge of the data points in Figures 3.7b-3.7d whatsoever. The predictions could be made using only the low-intensity TPA data (to define Ω) together with the calculated values forQgen.
The low-level curves in Figure 3.7 were calculated by combining 3.5 with 3.1. How- ever, this calculation does not use the point-source approximation. If it did, the prediction would exactly reproduce the points in Figure 3.7a because the points in Figure 3.7a were used to defineΩ. Instead, a smoother curve was produced by givingPI some spatial extent.
Specifically, it is treated as constant over a 12 µm range (6µm on each side of the focal plane depth). This is consistent with recent measurements of the TPA carrier generation region’s “effective length” [57]. While the effective length of the TPA carrier generation region does increase with increasing laser pulse energy, a 12 µm range is a reasonable approximation for the laser pulse energies used here.
Similar calculations are used to obtain the blue curves, except that 3.1 is replaced by
3.2 and 3.3. This calculation requires an estimate of the diffusion-coefficient ratioDm/DM, which is related to the mobility ratio through the Einstein relation [25]. Because of (A.9), accuracy is needed only in the ambipolar region portion of the quasi-neutral region, which is a weak-field region, so we use low-field mobilities. For a p-type substrate, this ratio is the low-field electron mobility divided by the low-field hole mobility. The ratio used here is 2.8, which is consistent with mobilities reported in the literature for the doping concentration used in the tested device [25].