Browsing by Author "Talvitie, Heli"
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- Effect of Contamination Level and Gettering Parameters on Solar Cell Performance in Iron Contaminated Silicon
A4 Artikkeli konferenssijulkaisussa(2011) Vähänissi, Ville; Haarahiltunen, Antti; Talvitie, Heli; Yli-Koski, Marko; Savin, Hele - Transitiometallien vaikutus hapen erkautumiseen piissä
Helsinki University of Technology | Master's thesis(2007) Talvitie, HeliPuolijohdekomponenttien dimensioiden pienentyessä puolijohdemateriaalin virheettömyys tulee yhä merkittävämmäksi. Yleisesti käytetyssä Czochralski-menetelmällä kasvatetussa piissä on epäpuhtautena happea, joka erkautuu lämpökäsittelyissä. Kiekon sisällä happierkaumat ja hapen erkautumisen aiheuttamat kidevirheet getteroivat prosessoinnin aikana piihin tulevia metallisia epäpuhtauksia ja tekevät pinnan IC-alueesta puhtaamman. MEMS-komponenteissa hapen erkautumisen aiheuttamat kidevirheet kuitenkin heikentävät märkäetsattujen pintojen laatua. Tässä työssä tutkittiin raudan ja kuparin vaikutusta hapen erkautumiseen, erkautuneen hapen määrään sekä kokonaisvirhetiheyteen ja pinousvikatiheyteen. Raudalla kontaminoiduille ja kontaminoimattomille kiekoille tehtiin hapen erkauttamiseksi kaksivaiheisia Iämpökäsittelyjä. Ensimmäisen, erkaumien ydintämiseksi tehdyn lämpökäsittelyn aikaa ja lämpötilaa muutettiin, toinen, erkaumien kasvattamiseksi tehty Iämpökäsittely pidettiin samana. Kuparikontaminoiduissa kiekoissa erkautettiin ensin kupari, minkä jälkeen niille tehtiin vastaavia hapenerkautuskäsittelyjä kuin rautakontaminaatiokokeissa. Kiekoista mitattiin metallikonsentraatio elinaikamittauksilla ja erkautuneen hapen määrä FTIR-spektroskopiamittauksilla. Lisäksi valikoiduista kiekoista määritettiin kokonaisvirhetiheys ja pinousvikatiheys tekemällä virhesyövytys Wright-etsillä. Erkautuneen hapen määrä ei rautakontaminoiduissa kiekoissa ollut suurempi verrattuna kontaminoimattomiin, mutta saman kaksivaiheisen lämpökäsittelyn kokeneisiin kiekkoihin, toisin kuin useimpien kirjallisuusviitteiden perusteella oletettiin. Virhesyövytystulosten mukaan rautakontaminaatio ei myöskään lisää kokonaisvirhetiheyttä tai pinousvikatiheyttä. Kuparierkaumienkaan ei havaittu vaikuttavan erkautuneen hapen määrään tai virhetiheyksiin. - Effect of transition metals on oxygen precipitation in silicon
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2008) Talvitie, Heli; Haarahiltunen, Antti; Yli-Koski, Marko; Savin, Hele; Sinkkonen, JuhaEffects 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. - Effective Passivation of Black Silicon Surfaces by Atomic Layer Deposition
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2013) Repo, Päivikki; Haarahiltunen, Antti; Sainiemi, Lauri; Yli-Koski, Marko; Talvitie, Heli; Schubert, Martin C.; Savin, HeleThe poor charge-carrier transport properties attributed to nanostructured surfaces have been so far more detrimental for final device operation than the gain obtained from the reduced reflectance. Here, we demonstrate results that simultaneously show a huge improvement in the light absorption and in the surface passivation by applying atomic layer coating on highly absorbing silicon nanostructures. The results advance the development of photovoltaic applications, including high-efficiency solar cells or any devices, that require high-sensitivity light response. - Impact of phosphorus gettering parameters and initial iron level on silicon solar cell properties
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2012) Vähänissi, Ville; Haarahiltunen, Antti; Talvitie, Heli; Yli-Koski, Marko; Savin, HeleWe 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. - Iron gettering in silicon using doped layers and bulk defects
Sähkötekniikan korkeakoulu | Doctoral dissertation (article-based)(2011) Talvitie, HeliThe removal of iron impurities to desired regions in silicon wafers has been studied using phosphorus and boron doped layers and bulk defects as gettering sites. Techniques to remove metal impurities, so-called gettering techniques, are needed for improving the performance of both the microelectronic and photovoltaic silicon devices, although the desired location of impurities may be different in various applications. In this work, both separate and simultaneous influences of the doped layers and bulk defects on the gettering behaviour of iron, e.g. the gettering efficiency and gettering mechanisms, were investigated. The phosphorus diffusion gettering studies at low temperatures enabled the determination of a more accurate segregation coefficient for iron between a phosphorus diffused layer and bulk silicon. Comparison between the phosphorus diffusion gettering experiments and similar experiments with boron showed that boron diffusion gettering can in some cases be nearly as effective as the phosphorus diffusion gettering. The gettering studies with implanted boron layers revealed that the gettering occurs also by precipitation, not only by segregation. Competitive gettering between an implanted boron layer and bulk defects was investigated using specially designed gettering anneals. It was found that depending on the desired location of iron in silicon wafers in different applications, iron can be collected either to the doped layers or the bulk defects. The gettering anneals were also applied to a microelectronic device process and their effect on the electronic device parameters was evaluated. These results contribute to the understanding of iron behaviour in silicon. Thus, they can help when designing the gettering anneals both for microelectronic and photovoltaic fabrication processes. - Modeling boron diffusion gettering of iron in silicon solar cells
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2008) Savin, Hele; Talvitie, Heli; Yli-Koski, Marko; Haarahiltunen, Antti; Asghar, M.I; Sinkkonen, JuhaIn this paper, a model is presented for boron diffusion gettering of iron in silicon during thermal processing. In the model, both the segregation of iron due to high boron doping concentration and heterogeneous precipitation of iron to the surface of the wafer are taken into account. It is shown, by comparing simulated results with experimental ones, that this model can be used to estimate boron diffusion gettering efficiency of iron under a variety of processing conditions. Finally, the application of the model to phosphorus diffusion gettering is discussed. - Modeling phosphorus diffusion gettering of iron in single crystal silicon
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2009) Haarahiltunen, Antti; Savin, Hele; Yli-Koski, Marko; Talvitie, Heli; Sinkkonen, JuhaWe propose a quantitative model for phosphorus diffusion gettering (PDG) of iron in silicon, which is based on a special fitting procedure to experimental data. We discuss the possibilities of the underlying physics of the segregation coefficient. Finally, we show that the proposed PDG model allows quantitative analysis of gettering efficiency of iron at various processing conditions. - Phosphorus and boron diffusion gettering of iron in monocrystalline silicon
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2011) Talvitie, Heli; Vähänissi, Ville; Haarahiltunen, Antti; Yli-Koski, Marko; Savin, HeleWe 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. - Significant minority carrier lifetime improvement in red edge zone in n-type multicrystalline silicon
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2013) Vähänissi, Ville; Yli-Koski, Marko; Haarahiltunen, Antti; Talvitie, Heli; Bao, Yameng; Savin, HeleWe have carried out experiments on both boron diffusion gettering (BDG) and phosphorus diffusion gettering (PDG) in n-type multicrystalline silicon. We have focused our research on the highly contaminated edge areas of the silicon ingot often referred to as the red zone. Due to poor carrier lifetime attributed to these areas, they induce a significant material loss in solar cell manufacturing. In our experiments, the red zone was found to disappear after a specific BDG treatment and a lifetime improvement from 5 μs up to 670 μs was achieved. Outside the red zone, lifetimes even up to 850 μs were measured after gettering. Against the common hypothesis, we found higher dopant in-diffusion temperature beneficial both for the red zone and the good grains making BDG more efficient than PDG. To explain the results we suggest that high temperature leads to more complete dissolution of metal precipitates, which enhances the diffusion gettering to the emitter. - Silicon Surface Passivation by Al2O3: Effect of ALD Reactants
School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä(2011) Repo, Päivikki; Talvitie, Heli; Li, Shuo; Skarp, Jarmo; Savin, HeleWe have studied the surface passivation of p- and n-type silicon by thermal atomic layer deposited (ALD) Al2O3. The main emphasis is on different ALD reactant combinations and especially on using ozone as an oxidant. Thermal stability of Al2O3 will also be briefly addressed. Our results show that in p-type CZ-Si Al2O3 leads to much higher passivation than thermal oxidation, independent of the reactants. The best minority carrier lifetimes are measured when a combination of Al2O3 and TiO2 is used. In n-type CZ-Si similar results are obtained except the choice of reactants seems to be more crucial. However, the combination of Al2O3 and TiO2 results again in the best passivation with measured lifetimes well above 10 ms corresponding surface recombination velocities of ∼2 cm/s. Finally, we demonstrate that Al2O3 passivation is also applicable in high resistivity n-type FZ-Si and in ∼1 Ωcm p-type multicrystalline Si.