23rd European Photovoltaic Solar Energy Conference, 1-5 September 2008, Valencia, Spain
Effect of Diffusions on Si surface passivation H. Jin, W. Jellett, C. Zhang, and K.J. Weber Centre for Sustainable Energy Systems, Faculty of Engineering and Information Technology, The Australian National University, Canberra ACT 0200, Australia email:
[email protected], ph: 61 2 6125 9741 ABSTRACT: Besides of forming pn junction, both Boron and Phosphorus diffusions have the effect of increasing the Si surface defect density. Electronic Spin Resonance measurements on (111) surfaces show that the density of the chief interface defect, the Pb center, increases as a result of a diffusion. Lifetime-Voltage measurements indicate that both B and P diffused Si surfaces have greater recombination velocities under accumulation conditions, than undiffused samples. Hydrogenated and dehydrogenated interface for (111) and (100) Si are investigated. Keywords: Diffusion, Defect, Pb, lifetime 1
INTRODUCTION
the change in interface defect properties resulting from the presence of a diffusion. Both SiO2/Si and Si3N4/SiO2/Si structures are investigated. In addition, Electron Spin Resonance (ESR) measurements are used to determine the Si-SiO2 interface surface paramagnetic defect density for B diffused and undiffused samples.
The formation of an n+ emitter using a phosphorus diffusion is one of the critical steps in the fabrication of conventional solar cells on p type material. Industrial screen printed solar cell processes involve a heavy (5070Ω/□) P diffusion step. The design of advanced solar cells requires a selective emitter structure, with a heavy P diffusion under the metal contact regions and a light P diffusion under the non-metal regions. Silicon solar cells based on n type, P doped material are also of great interest for photovoltaic applications. This interest stems from the fact that n type Cz wafers display a higher and stable lifetime than their B doped counterparts, due to the absence of boron-oxygen pairs which act as effective recombination centers. It is well known that diffusion can improve surface passivation by generating an electrical field hence reducing the surface concentration of holes or electrons and thus lowering the overall carrier recombination rate. However, diffusions can also lead to a decrease in passivation through additional recombination in the emitter bulk. This is particularly significant for heavily doped emitters. These cells usually require a B diffusion in order to form a p-n junction.
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EXPERIMENTAL DETAILS
Samples used for lifetime-voltage measurements were p type, float-zoned, (100), >100Ω-cm, 500µm thick and n type, float-zoned, (111), ~75Ω-cm, 500µm thick. After acid etching and a standard RCA clean, selected samples received a light phosphorus or boron diffusion. An 50 nm thick oxide was thermally grown at 1000oC in dry oxygen on both sides of all samples with an in-situ anneal in nitrogen, followed by annealing in forming gas (FGA, 5% H2 in 95% Ar) at 400oC for 30 min in order to passivate the Si-SiO2 interface. A ~50nm thick layer of LPCVD Si3N4 was deposited at 775oC and 0.5 torr on both sides of selected oxidized samples. Some Si/SiO2 samples received rapid thermal anneals (RTA) at 500oC for 1 min to partially dehydrogenate the interface. Importantly, diffused and undiffused samples were processed together except for the diffusion step in order to minimize variability due to processing. The profile (measured on mirror polished sample with background doping of 1.5e14cm-3) after all thermal steps is shown in figure 1 (a) and (b).
However, there is little information on the influence of dopant diffusions on the Si surface defect density, chiefly because this parameter is difficult to measure on heavily doped surfaces. Snel1 concluded that the Si-SiO2 interface defect density will be increased by both P and B diffusions beyond a certain threshold surface dopant concentration. However, his results are not universally accepted.
Using a shadow mask aluminium was evaporated over a circular area onto both sides of each quarter to a thickness of ~5nm to create a symmetric MOS structure. The diameter of the metallised area was at least 5mm larger than the sensing area of the inductive coil for lifetime measurements in order to minimise edge effects. Contact to the Al layers was made using In/Ga eutectic, while contact to the silicon bulk was made by removing a small amount of oxide in one of the corners of the sample and applying silver paste. The sample was positioned so that the metallised region was centered over the inductive coil. The metal thickness was carefully chosen to ensure minimal interference with the lifetime measurement. The effective lifetime τeff was measured using the transient photoconductivity method3.
In this paper, we discuss the results of measurements on both diffused and undiffused surfaces. Lifetime-voltage measurements2 are used to study of the influence of diffusions on surface recombination. In this measurement, the effective lifetime τeff and emitter saturation current density Joe are measured in deep inversion and accumulation, where the surface concentration of minority carriers becomes independent of surface doping, and almost independent of applied bias. By performing measurements with the wafer surface in accumulation, it is possible to directly compare diffused and undiffused surfaces, and to draw some qualitative conclusions about
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23rd European Photovoltaic Solar Energy Conference, 1-5 September 2008, Valencia, Spain
and comparison with a standard solution signal, obtained under similar conditions.
Boron Phosphorus
3
18
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RESULTS AND DISCUSSION
Figure 2 shows the modeled effect of surface charge on the surface minority carrier concentration in thermal equilibrium for two different substrate doping levels – one, a lightly doped substrate (p type, 1014cm-3), the other, a heavily doped substrate (p type, 1018cm-3). Bandgap narrowing was included in the calculation of the minority carrier concentration. In both cases, the minority carrier concentration displays a sharp peak for a given value of surface charge density and then rapidly decreases. On the left side of the peak, minority carriers are electrons (accumulation) while on the right side they are holes (inversion). As expected, the peak for the heavily doped sample occurs at a significant positive surface charge density which acts to repel the holes just beneath the insulator, while for a lowly doped sample the peak occurs for a low surface charge density. For large values of surface charge density, the surface minority carrier concentration saturates and becomes only weakly dependent on surface charge density, and nearly independent of substrate doping (particularly in accumulation). This implies that measurement of the emitter saturation current density Joe at high surface charge densities, particularly in accumulation, can be used to detect any differences in interface defect properties, provided that differences in recombination in the emitter itself can be accounted for.
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0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 depth (microns)
1E19 -3
carrier concentration (cm )
1E18
1E17
1E16
1E15
heavily doped sample lightly doped sample
-3
surface minority carrier concentration (cm )
-3
carrier concentration (cm )
20
10
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1E14 0.0
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1.0 depth 1.5 (microns) 2.0 2.5
Figure 1. Profile of Boron (top) and Phosphorus (bottom) diffusion Samples used for EPR measurements were n-type, ~4000Ω-cm, (111) Cz silicon wafers. Samples were cut with a diamond saw into 25mm×2.5mm pieces. They were subsequently etched to remove saw damage from the surfaces. After diffusion (on selected samples) and oxidation steps, a rapid thermal anneal was carried out at 700 °C for 3 min in a high flow of nitrogen gas to dehydrogenate the Pb centers at the Si–SiO2 interface. EPR measurements were undertaken using a Bruker 300E spectrometer operating at X Band (9.44GHz), fitted with an Oxford ES-9 liquid helium cryostat with temperature control via an Oxford ITC-4 controller. Measurements were done using a modulation frequency of 100KHz, modulation amplitude of 1G and a microwave power of 20µW at a temperature of 7K. 20µW was observed to be non-saturating. Samples were placed in 3mm ID quartz EPR tubes, which were flushed with pure argon to remove oxygen. The sample tubes were sealed with rubber septa and the sample end frozen to 77K. The angle between the sample surface and the magnetic field is within an error of 3o. The Pb centre concentration was calculated by double integration of the original signal
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-2 0 2 4 6 12 -2 surface charge density (×10 cm )
Figure 2: Surface minority carrier concentration in thermal equilibrium as a function of surface charge density for a lightly doped (dash line) and heavily doped (solid line) substrate Figure 3 compares the EPR spectra for the undiffused (a) and B diffused (b) samples. The EPR signal of the B diffused sample is characteristic of the Pb centre, but the Pb centre density is substantially greater (2.75 times) than that of the undiffused sample. Table I lists all expeirmenal values gained from EPR. 1343
23rd European Photovoltaic Solar Energy Conference, 1-5 September 2008, Valencia, Spain
contribution is much smaller (~2-3fA/cm2) than the observed increase in Joe.
EPR signal
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It can also be observed that the values of Joe for the undiffused sample under deep inversion and accumulation conditions are similar. This implies that the (111) Si–SiO2 interface with heavy n type or p type surface doping can, in principle, be passivated equally well, if it could be ensured that the heavy doping does not alter the interface. Similar results have also been obtained for (100) surfaces.
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applied magnetic field (G)
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no diff B P
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Joe (fA/cm )
EPR signal
a -3000 3360 3364 3368 3372 3376 3380 3384
-1000 -2000 b -3000 3360 3364 3368 applied 3372 3376 3380field 3384 magnetic (G)
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Figure 3. EPR signals for undiffused sample (a) and boron diffused sample (b). The magnetic field is set parallel to the (111) direction.
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TABLE I. Comparison of the g value, peak to peak linewidth (∆Bpp) and paramagnetic defect concentration of undiffused and boron diffused samples. ‘//’indicates magnetic field parallel to (111) direction, ‘⊥’indicates magnetic field perpendicular to (111) direction. The error in the g value is 4×10-5.
g//
Undiffused sample 2.00136
Diffused sample 2.00140
∆Bpp// (G)
1.9
1.9
g⊥
2.00870
2.00863
∆Bpp⊥ (G)
3.7
3.8
1.2
3.3
[Pb] (1013cm-2)
-20
-10
0
10 20 30 applied voltage (V)
Figure 4. Joe with applied bias for undiffused and B/P diffused samples Table I shows the saturated Joe values (under strong inversion conditions) for (100) p type Si samples. It also displays the lower limit Joe values contributed from surface for group B and C. Group A are as oxidized and FGAed samples. Group B samples received a subsequent RTA at 500oC for 1 min in pure N2. Group C received a 50 nm LPCVD nitride deposition. Table I: saturated Joe values for samples from group A, B and C
Diffused P Undiffused Difference Lower limit Joe from surface
C. Lifetime-Voltage measurements results Figure 4 shows the Joe values for (111) Si/SiO2 structures. The rectangle symbols represent the results from undiffused samples, the circle symbols represent boron diffused and triangle symbols represent phosphorus diffused results. Note that Joe measurements on undiffused samples are only meaningful when the surface remains in low level injection during the measurement – a condition that is met at sufficiently high applied (positive or negative) biases. The diffused samples show a higher Joe than the undiffused samples. Some of the increase in Joe is due to an additional component to Joe from the diffused region itself. However, this
Joe for group A (fA/cm2) 17 13 4 -
Joe for group B (fA/cm2) 49 30 19 15
Joe for group C (fA/cm2) 23 15 8 4
The RTA process has the effect of dehydrogenating the Si-SiO2 interface by removing H from active electronic surface defects. LPCVD nitride deposition can also depassivate the interface and create additional Si surface defects4. The higher Si-SiO2 interface defect density results in the more obvious differences between diffused and undiffused sample.
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23rd European Photovoltaic Solar Energy Conference, 1-5 September 2008, Valencia, Spain
For boron diffused samples, it is important to note that while Joe measurements were carried out on passivated (hydrogenated) samples, EPR measurements were done on de-hydrogenated samples in order to obtain the largest possible Pb signal and hence maximize measurement accuracy. The relative increase in Pb density (by a factor of 2.75) would be expected to translate to the same relative increase in (unpassivated) Pb centre density following hydrogenation only if the properties of the centres are completely unchanged. Nevertheless, the relative increases in Joe (factor of 2.33) and Pb centre density are equal within the error of the measurements, and we tentatively conclude that the increase in interface recombination on hydrogenated samples is likely to be chiefly due to an increase in un-passivated Pb centre density. Conclusion In conclusion, we have shown that a combination of lifetime-voltage and EPR measurements can be used to obtain information on the change in interface defect properties resulting from diffusion. The methods provide a new way to determine the effect of diffusions on surfaces, which can be of great value in optimizing diffusions and surface passivation schemes. References 1 J. Snel, Solid-State Electronics 24, 135-139 (1981). 2 W. E. Jellett and K. J. Weber, Applied Physics Letters 90, 042104 (2007). 3 R. A. Sinton and A. Cuevas, Applied Physics Letters 69, 2510-2512 (1996). 4 H. Jin, K. J. Weber, and P. J. Smith, Applied Physics Letters 89, - (2006).
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