Emitter profiles

In order to get accurate data on the amount of inactive P after diffusion it is worth comparing the atomic phosphorus and the electron emitter profiles. The profiles have been analyzed by Secondary Ion Mass Spectroscopy (SIMS) and Stripping Hall Profiling (SHP) yielding the atomic P and the electron profile concentration, respectively. The profiles were measured on polished substrates. When the influence of a post RTD high-temperature step such as RTO was investigated, measurements were carried out on two different wafers. One wafer was only diffused and the other one was additionally subjected to the second high-temperature step after removal of the PSG.

SOD P509

Fig. 3.5 shows the profile of a 100 /sq emitter diffused in 30 s at 900 C (thermal budget III of Table 3.3) using the P509 SOD. For any depth, the P concentration is higher than the electron concentration. The P surface concentration is at 10²¹cm^-3, whereas the electron surface concentration is well below at 3 to 4 10²⁰ cm^-3. The latter value perfectly fits to the solid solubility of P in Si at the applied diffusion temperature (see Tab. 3.1). However, this means that there is a huge proportion of electrically inactive P serving as an internal source during a second high-temperature treatment subsequent to removal of the PSG. According to Fig. 3.5, during RTO at 950 C, the atomic P diffuses deeper into the silicon and becomes activated.

After RTO, the P and the electron profile show a good agreement, both exhibiting a surface

0 50 100 150 200 10¹⁶

10¹⁷ 10¹⁸ 10¹⁹ 10²⁰ 10²¹

after RTO (950 °C, 30 s)

Base doping RTD parameters

860 °C, 120 s 900 °C, 30 s 950 °C, 5 s

E le ct ro n co nc en tra tio n [c m

]

Depth [nm]

Fig. 3.6: Emitter electron profiles measured by Stripping Hall after different RTD runs using the P509 SOD and a subsequent RTO step for 30 s at 950 C . If appropriate parameters are chosen for the initial RTD, the corresponding emitter profiles are almost identical after RTO.

concentration of 3 to 4 10²⁰ cm^-3 and a junction depth of about 200 nm. Because of the activation of P, a decrease in from 100 /sq to around 65 /sq occurs. The same effect has been observed for the thermal budgets I and V of diffusion as given in Table 3.3.

decreases during the oxidation and, as shown in Fig. 3.6, the corresponding electron profiles nearly coincide, exhibiting negligible dependence on the initial diffusion parameters.

It is worth noting, that all profiles follow the characteristic kink and tail behavior which is typical for the diffusion of P in Si under extrinsic conditions (i.e. [178]). Because of the decrease in , the RTD parameters have to be adjusted carefully in the case of solar cell fabrication in order to obtain the desired 80-90 /sq emitter after RTO emitter surface passivation.

SOD P507

Using the SOD P507, Fig. 3.7 reports the profiles of a 105 /sq emitter diffused at 970 C for 40 s and of a 31 /sq emitter diffused at 1060 C for 30 s, respectively. Obviously, the profiles of the P (SIMS) and the electron (SHP) concentration coincide for both emitters, indicating that all the diffused P is electrically active and that no SiP precipitates are present. Small deviations between SIMS and SHP in the surface near concentration of the 31 /sq emitter should not be overestimated, since they are within the accuracy of the measurements (e.g. 10 % for SIMS [147]). Hence, in contrast to the P509 and the P508, the P507 yields no inactive P.

0 100 200 300 400 10¹⁶

10¹⁷ 10¹⁸ 10¹⁹ 10²⁰ 10²¹

1060 °C, 30 s R_sheet=31 Ω/sq

970 °C, 40 s R_sheet= 105 Ω/sq

SHP SIMS

C on ce nt ra tio n [c m

]

Depth [nm]

Fig. 3.7: Comparison of the P (SIMS) and the electron (SHP) emitter profiles for the SOD P507. The 105 /sq emitter was diffused at 970 C whereas the 31 /sq emitter was diffused at 1060 C.

This corresponds well to the fact that does not decrease during RTO as has been shown in Fig. 3.4. If there is a change in at all, it is a slight increase caused by lowering of the surface concentration due to deeper diffusing P. This holds especially for lightly doped and hence surface doping dominated emitters.

Remarkably, the P507 apparently yields surface near concentrations slightly below the solubility of P in Si at the utilized diffusion temperatures. For example, at 1060 C, is approximately 5.5 10²⁰ cm^-3 [122], whereas, according to the profile measurements, is only at 3.6 10²⁰ cm^-3. The same observations were made by Noël et al. who also studied RTD using the P507 SOD [125]. There has to be a fundamental difference in the chemical composition of the different SODs.

Emitters for high-efficiency solar cells with evaporated contacts

For the fabrication of high-efficiency solar cells with photolithographically defined and evaporated front contacts, an emitter with in the range of 80 to 90 /sq is the optimal choice. For the SODs P509 and P507 the RTD and RTO processes were worked out yielding the desired . In the case of the P509, RTD was conducted at 875 C for 30 s whereas in the case of the P507, RTD was carried out at 975 C for 40 s. The left graph in Fig. 3.8 shows the respective P emitter profiles after RTO as measured by SIMS. It has been shown already that after RTO the electron and the P profile, i.e. the SIMS profile, coincide for both type of P sources. The highly concentrated P509 yields a surface concentration of 3 10²⁰cm^-3whereas

0 100 200 300 10¹⁶

10¹⁷ 10¹⁸ 10¹⁹ 10²⁰ 10²¹

0 100 200 300 400 500 10¹⁶

10¹⁷ 10¹⁸ 10¹⁹ 10²⁰ 10²¹

P c on ce nt ra tio n [c m

]

Depth [nm]

P507 after RTD + RTO

P509

Depth [nm]

CFD standard emitter POCl₃

820 °C, 1 h

Fig. 3.8: Left) Comparison of the SIMS profiles after RTO of 90 /sq RTD emitters, developed for high-efficiency solar cells, using the SODs P509 and P507, respectively. Right) Profile of the standard 90 /sq CFD emitter diffused at 820 C for 1 h using POCl₃.

the lower concentrated P507 yields a surface concentration of only 1.5 10²⁰cm^-3despite the much higher diffusion temperature. The higher diffusion temperature necessary for the P507 reflects in an increased junction depth compared to the junction depth obtained with the P509.

The standard CFD emitter

The right graph in Fig. 3.8 shows the P profile of the 90 /sq emitter diffused with the standard CFD process at 820 C in 1 h in a conventional quartz tube furnace. The standard process uses POCl3 as a P source. In this work it has been used as a reference process in the fabrication of RTP solar cells from RGS (section 4.5) and EFG (section 4.6) silicon sheets. Obviously, its junction depth is roughly twice as deep as that of RTD emitter of comparable sheet resistance.

Though the active electron profile is not available, we suppose that it certainly coincides with the P profile in the kink region and for sure in the tail region. In the surface near region, however, it should be in the range of the solid solubility of P in Si at the diffusion temperature, which is 2.5 10²⁰ cm^-3 at 820 C [122]. Since the P concentration is at approximately 5 10²⁰ cm^-3 this means that the CFD emitter features inactive P in its surface near region.