Deep Levels Associated with Oxygen Precipitation in CZ Silicon and Correlation with Minority Carrier Lifetimes

Abstract

A dominant electron trap in boron-doped CZ silicon at Ec −0.41 ± 0.02 eV with a electron capture cross section >10−14 cm2 has been observed with DLTS in 2-step annealed (16 hrs at 800°C + 16 hrs at 1050°C) seed end wafers where oxygen precipitation is pronounced. A strong correlation between the generation lifetime τg (as calculated from junction reverse generation currents) and the density of this trap is also observed. Electron microscopy showed the dominant precipitates to be {100} plate type containing only Si and 0, with dislocations and punched-out loops in close proximity. Wafers from the center- or tang-sections of the ingot or those given a 1-step anneal (conditions which result in much less oxygen precipitation) contain a low density (<1.2 × 1012 cm−3) of a hole trap at Ev +0.25 eV not correlated with Tg. These wafers contain precipitates with a different morphology with much rower or minimal dislocation densities. Possible origins of the lifetime controlling electron trap is discussed.

Minority Carrier Lifetime in Czochralski Silicon Containing Oxide Precipitates

“…The results show that very good minority carrier lifetimes can be achieved in spite of the thermal treatments these samples have undergone. The highest lifetimes reported here are substantially higher than those in other studies of recombination at oxide precipitates [9][10][11][12][13].…”

“…Oxygen can also improve the mechanical strength of the Cz-Si by precipitation in the bulk [3] and by atomic decoration of dislocations [4,5], however excess precipitation can reduce mechanical stability. Oxygen-related defects can act as recombination centres, including thermal donor defects formed upon annealing at low temperatures [6], boronoxygen complexes formed upon illumination [7,8] and oxide precipitates [9][10][11][12][13].…”

“…In this current work, the recombination properties of carriers are studied in a wide spectrum of Cz-Si samples, containing almost all unstrained oxide precipitates at one end, a mixture of unstrained and strained oxide precipitates in the middle and almost all strained oxide precipitates at the other end. Previous studies of carrier lifetime in Cz-Si containing oxide precipitates [9][10][11][12][13] did not take the morphological transformation into account. Additionally, in this present work, other mechanisms contributing to lifetime (band-to-band recombination, Coulomb-enhanced Auger recombination and Shockley-Read-Hall recombination at iron and boron-oxygen related defects) have been accounted for, enabling the lifetime contribution due to the precipitates and associated defects (dislocations and stacking faults) to be isolated more accurately.…”

Parameterisation of injection-dependent lifetime measurements in semiconductors in terms of Shockley-Read-Hall statistics: An application to oxide precipitates in silicon

“…13 Oxide precipitates can also affect the mechanical stability of wafers at elevated temperatures 14,15 and can alter the mechanism of room temperature wafer fracture. 16 Oxide precipitates and surrounding defects act as recombination centres [17][18][19][20][21][22][23][24][25] and this can cause a reduction in solar cell efficiencies. 10,26 It is important to understand the mechanism by which recombination occurs at the precipitates and to be able to quantify their effect on minority carrier lifetime.…”

On the mechanism of recombination at oxide precipitates in silicon

“…[1][2][3][4][5][6] Oxide precipitates typically form unintentionally in vacancy-rich regions in rapidly pulled Cz-Si 5 and at crystallographic defects in mc-Si. 2,3 Precipitation of oxygen is generally undesirable for photovoltaics as it increases recombination, [7][8][9][10][11][12][13][14][15][16][17][18][19][20] but it is often desirable in silicon for microelectronics as precipitates and surrounding defects (dislocations and stacking faults) can confine device-ruining metallic contaminants away from active regions in internal gettering processes. 21,22 Although it has long been known that the precipitation of oxygen introduces recombination centres, [7][8][9][10][11][12][13][14][15][16][17][18][19][20] after at least 35 years of research, the physical origin of these centres is not fully understood.…”