Optical activation of copper in silicon studied by carrier lifetime measurements

In this work copper in silicon is studied by means of the microwave photoconductive decay technique. The motivation of this work is to develop a solution for the quantitative measurement of trace copper contamination in silicon using charge carrier lifetime measurements. The goal of this work is to study the influence of corona charge, oxygen defects and light intensity on the recombination activity of copper and the formation of copper precipitates at room temperature. The results of this thesis indicate that the optical activation of copper takes place under high excess charge carrier conditions when boron-doped silicon contains interstitial copper atoms. Due to the positive charge state of interstitial copper atoms, a positive corona charge on the silicon dioxide layer prevents interstitial copper diffusion from p-type bulk silicon to the silicon surface. Interstitial copper is shown to exist in boron-doped bulk silicon after long storage periods. It is suggested that the main reason for the optical activation of copper is the decrease in the precipitation barrier of copper due to the change in the charge state of copper precipitates. The recombination activity of copper after a long optical activation time is independent of light intensity. The density of growing copper precipitates increases when the light intensity, the density of oxygen defects or the density of interstitial copper is increased. The quantitative detection limit of copper is shown to be about 3×1012 cm−3 in boron-doped silicon without oxygen defects. However, oxygen defects increase the recombination activity of copper. It is estimated that a copper concentration of 5×1010 cm−3 is quantitatively measurable, if the bulk silicon contains small oxygen defects at high density.reviewe

Role of copper in light induced minority-carrier lifetime degradation of silicon

We investigate the impact of copper on the light induced minority-carrier lifetime degradation in various crystalline silicon materials. We demonstrate here that the presence of neither boron nor oxygen is necessary for the degradation effect. In addition, our experiments reveal that copper contamination alone can cause the light induced minority-carrier lifetime degradation.Peer reviewe

Room-temperature method for minimizing light-induced degradation in crystalline silicon

Although light-induced degradation (LID) in crystalline silicon is attributed to the formation of boron-oxygen recombination centers, copper contamination of silicon has recently been observed to result in similar degradation. As positively charged interstitialcopper stays mobile at room temperature in silicon, we show that the bulk copper concentration can be reduced by depositing a large negative charge onto the wafer surface. Consequently, light-induced degradation is reduced significantly in both low- and high-resistivity boron-doped Czochralski-grown silicon.Peer reviewe

Gettering of Iron in Silicon Solar Cells With Implanted Emitters

We present here experimental results on the gettering of iron in Czochralski-grown silicon by phosphorus implantation. The gettering efficiency and the gettering mechanisms in a high resistivity implanted emitter are determined as a function of both initial iron level and gettering anneal. The results show that gettering in implanted emitters can be efficient if precipitation at the emitter is activated. This requires low gettering temperatures and/or high initial contamination level. The fastest method to getter iron from the bulk is to rapidly nucleate iron precipitates before the gettering anneal. Here, this was achieved by a fast ramp to the room temperature in between the implantation anneal and the gettering anneal.Peer reviewe

Reduction of Light-induced Degradation of Boron-doped Solar-grade Czochralski Silicon by Corona Charging

This study aims at the reduction of light-induced degradation of boron-doped solar-grade Czochralski silicon wafers by corona charging. The method consists of deposition of negative charges on both surface sides of wafer and keeping the wafer in dark for 24 hours to allow the diffusion of positively-charged interstitial copper towards the surfaces. This method proves to be useful to reduce or eliminate light-induced degradation caused by copper. The degradation was significantly reduced in both intentionally (copper-contaminated of the negative charge was found to be proportional to the reduction strengthPeer reviewe

Effect of transition metals on oxygen precipitation in silicon

Effects of iron and copper impurities on the amount of precipitated oxygen and the oxide precipitate and stacking fault densities in Czochralski-grown silicon have been studied under varying thermal anneals. Silicon wafers were intentionally contaminated with iron or copper and subsequently subjected to different two-step heat treatments to induce oxygen precipitation. The iron contamination level was 2 × 10exp13cm-3 and copper contamination level 6 × 10exp13cm-3. Experiments did not show that iron contamination would have any effect on the amount of precipitated oxygen or the defect densities. Copper contamination tests showed some indication of enhanced oxygen precipitation.Peer reviewe

Phosphorus and boron diffusion gettering of iron in monocrystalline silicon

We have studied experimentally the phosphorus diffusion gettering (PDG) of iron in monocrystalline silicon at the temperature range of 650–800 °C. Our results fill the lack of data at low temperatures so that we can obtain a reliable segregation coefficient for iron between a phosphorus diffused layer and bulk silicon. The improved segregation coefficient is verified by time dependent PDG simulations. Comparison of the PDG to boron diffusion gettering (BDG) in the same temperature range shows PDG to be only slightly more effective than BDG. In general, we found that BDG requires more carefully designed processing conditions than PDG to reach a high gettering efficiency.Peer reviewe

Impact of phosphorus gettering parameters and initial iron level on silicon solar cell properties

We have studied experimentally the effect of different initial iron contamination levels on the electrical device properties of p-type Czochralski-silicon solar cells. By systematically varying phosphorus diffusion gettering (PDG) parameters, we demonstrate a strong correlation between the open-circuit voltage (Voc) and the gettering efficiency. Similar correlation is also obtained for the short-circuit current (Jsc), but phosphorus dependency somewhat complicates the interpretation: the higher the phosphorus content not only the better the gettering efficiency but also the stronger the emitter recombination. With initial bulk iron concentration as high as 2 × 1014 cm−3, conversion efficiencies comparable with non-contaminated cells were obtained, which demonstrates the enormous potential of PDG. The results also clearly reveal the importance of well-designed PDG: to achieve best results, the gettering parameters used for high purity silicon should be chosen differently as compared with for a material with high impurity content. Finally we discuss the possibility of achieving efficient gettering without deteriorating the emitter performance by combining a selective emitter with a PDG treatment.Peer reviewe

Light-induced degradation in copper-contaminated gallium-doped silicon

To date, gallium-doped Czochralski (Cz) silicon has constituted a solar cell bulk material free of light-induced degradation. However, we measure light-induced degradation in gallium-doped Cz silicon in the presence of copper impurities. The measured degradation depends on the copper concentration and the material resistivity. Gallium-doped Cz silicon is found to be less sensitive to copper impurities than borondoped Cz silicon, emphasizing the role of boron in the formation of copper-related light-induced degradation.Peer reviewe

Experimental evidence on removing copper and light-induced degradation from silicon by negative charge

In addition to boron and oxygen, copper is also known to cause light-induced degradation (LID) in silicon. We have demonstrated previously that LID can be prevented by depositing negative corona charge onto the wafer surfaces. Positively charged interstitial copper ions are proposed to diffuse to the negatively charged surface and consequently empty the bulk of copper. In this study, copper out-diffusion was confirmed by chemical analysis of the near surface region of negatively/positively charged silicon wafer. Furthermore, LID was permanently removed by etching the copper-rich surface layer after negative charge deposition. These results demonstrate that (i) copper can be effectively removed from the bulk by negative charge, (ii) under illumination copper forms a recombination active defect in the bulk of the wafer causing severe light induced degradation.Peer reviewe