应用于太阳能电池的氢化非晶亚氧化硅的退火优化

Vol. 32, No. 5 Journal of Semiconductors May 2011Annealing optimization of hydrogenated amorphous silicon suboxide film for solarcell applicationJia Guangzhi( 贾广智 ), Liu Honggang( 刘洪刚 ) , and Chang Hudong( 常虎东 )Institute of Microelectronics, Chinese Academy of Sciences,Beijing 100029, ChinaAbstract: We investigate a passivation scheme using hydrogenated amorphous silicon suboxide (a-SiO x :H) filmfor industrial solar cell application. The a-SiOx :H films were deposited using plasma-enhanced chemical vapordeposition (PECVD) by decomposing nitrous oxide, helium and silane at a substrate temperature of around 250 ?C.An extensive study has been carried out on the effect of thermal annealing on carrier lifetime and surface recombi-nation velocity, which affect the final output of the solar cell. Minority carrier lifetimes for the deposited a-SiOx :Hfilms without and with the thermal annealing on 4 cm p-type float-zone silicon wafers are 270 s and 670 s,respectively, correlating to surface recombination velocities of 70 cm/s and 30 cm/s. Optical analysis has revealeda distinct decreaseof blue light absorption in the a-SiOx :H films compared to the commonly used intrinsic amor-phous silicon passivation used in solar cells. This paper also reports that the low cost and high quality passivationfabrication sequencesemployed in this study are suitable for industrial processes.Key words: a-SiO x :H; thermal annealing; PECVDDOI: 10.1088/1674-4926/32/5/052002 EEACC: 25201. IntroductionHigh-efficiency silicon solar cells feature low surface andbulk recombination rates, which can limit the open circuit volt-age and the fill factor of solar cell ?1 . Applying an effectivesurface passivation scheme in order to reduce surface recom-bination is a precondition for obtaining high efficiency solarcells. This is particularly true for heterojunction solar cells:these cells havean abrupt discontinuity at the interface betweenthe crystalline silicon andthe amorphous silicon (a-Si:H) emit-ter, which leads to a high density of dangling bonds and re-sults in a large density of defects in the bandgap?2 . Passivationschemes commonly used in photovoltaic applications are sili-con dioxide (SiO 2 /?1 , silicon nitride (SiN x /?3; 4 , hydrogenatedintrinsic amorphous silicon (a-Si(i):H) ?5 and amorphous sili-con carbide?6 . The highest efficiencies reported in the litera-ture have been achieved with SiO2 surface passivation, how-ever, the SiO2 is grown at around 1000 ?C and not readily appli-cable to industrial processes due to the issue of high cost. Sur-face passivation with intrinsic, hydrogenated amorphous sili-con (a-Si:H) deposited by plasma-enhanced chemical vapourdeposition (PECVD) at around 225 ?C, results in the samelow effective surface recombination velocity as thermal oxi-dation ?7 . However, Fujiwara et al. ?8 statedthat the growth ofa-Si:H at temperatures > 130 ?C often leads to anepitaxial layerformation on the c-Si, reducing solar cell performance. Also,due to the inherent strong blue light absorption, only ultrathina-Si(i):H films can be allowed to prevent losses.Recently, using hydrogenated amorphous silicon subox-ides (a-SiOx :H) as the passivation film was reported ?9 and ex-cellent passivation results were demonstrated. However, thehigh frequency (70 MHz) RF power and the hydrogen gassource used in this study are not suitable for industrial solarcell fabrication.A large amount of hydrogen atoms are produced in theplasma during the deposition of SiOx :H, which is believed toprovide additional bulk defect passivation in the silicon ma-terial and, consequently, improves the efficiency of the solarcells. Is not only the plasma that contains a high concentra-tion of hydrogen, so doesthe amorphous silicon suboxide filmitself ?10 . Therefore, the SiOx :H film also acts as a source ofhydrogen in subsequent post-deposition anneals and providesadditional bulk defect passivation ?11 . In our report, we usePECVD by decomposition of nitrous oxide, helium and silaneat a substrate temperature of around 250 ?C to deposit hydro-genated amorphous silicon suboxides (a-SiOx :H) and obtainan effective lifetime of 270 s on 4 cm p-type float-zonesilicon, and up to 670 s after annealing under a nitrogen at-mosphere. Additionally, the optical analysis shows much lesslight absorption compared with a-Si:H(i).2. ExperimentalTo verify the passivation effectiveness, we have fabricatedseveral samplesfrom two-sided polished (100)-oriented boron-doped float-zone (FZ) wafers with a thickness of 380 m and aresistivity of 4 cm. Firstly, we used the standard RCA clean-ing method to clean the entire wafer. After removing any poten-tial native oxides by a 30 s dip in a 5% HF solution, we deposithydrogenated amorphous silicon suboxides (a-SiO x :H) filmswith a thickness of about 100 nm on each side the wafer in a13.56 MHz direct PECVD reactor. The following conditionswere used: the SiH 4 gas flow is 10 sccm, the gas flow ratio ofSiH 4 : N2O D 1, the excitation power is 100 W and the depo-sition temperature Tdep is 250 ?C. The workflow followed canbe found in Fig. 2 and the cell design is schematically shownin Fig. 1.After the deposition, three of the samples were annealedCorresponding author. Email: liuhonggang@ime.ac.cnReceived 27 September2010, revised manuscript received 29 December 2010 c 2011 Chinese Institute of Electronics052002-1J. Semicond. 2011, 32(5) Jia Guangzhi et al.Fig. 1. Workflow of the lifetime investigation.Fig. 2. Structure of the design.under a nitrogen atmosphere at 300, 350 and 400 ?C, re-spectively. Microwave-detected photoconductance decay tech-niques (MW-PCD) were used to determine the effective car-rier lifetime of the samples. The optical characteristics of thehydrogenated amorphous silicon suboxides (a-SiO x :H) filmsprepared by PECVD are characterized by spectroscopic ellip-sometry (SE).3. Results and discussionTo determine the passivation quality of the a-SiO x :H films,the overall recombination of free carriers is evaluated via effec-tive lifetime measurementsusing the MW-PCD technique. Theeffective lifetime can be expressedas follows for simplify:1= eff D 1= bulk C 1= surface; (1)where eff is the measured effective lifetime, bulk is the bulklifetime (which combines the Auger, radiative and Shockley-Read-Hall recombinations) and surface is the characteristic sur-face recombination lifetime component related to surface re-combination through defects and emitter recombination, whichis determined by the wafer thickness W and surface recombi-nation Seff . From Eq. (1), it can be concluded that a high effindicates a good passivation.Since the mechanism of passivation imposed by the hy-drogenated amorphous silicon suboxides (a-SiOx :H) layer isstrongly related to the hydrogen bonds and ions within the filmnetwork, metastability effects are expected when high energyphotons irradiate this material or when thermal annealing pro-cessesare superimposed after PECVD. As reported in Fig. 1the measured effective lifetime is only 270.14 s just after de-position, but it increased quickly when the annealing tempera-Fig. 3. Measured effective lifetime asa function of the annealing tem-perature for a-SiOx :H passivated wafers and the sample without an-nealing for comparison.Fig. 4. Calculated surface recombination velocity asa function of theannealing temperature for annealeda-SiOx :H passivatedsamples.ture is higher than 350 ?C and reaches670.63 s at 400 ?C. Aneven higher effective lifetime is expected when the annealingtemperature is higher than 400 ?C.We have designed the wafer to be two-sided symmetricalpassivated and we can assumethat both surfaces provide a suf-ficiently low recombination velocity and have the same value(Seff D Sfront D Sback/ . The effective surface recombination ve-locity (Seff / can be expressed bySeff D W=2 .1= eff 1= bulk /; (2)where W is the thickness of the wafer. As the bulk lifetimebulk of float-zone crystal is so large, we use surface to expresseff approximately and then Seff can be expressed bySeff D W=2 surface: (3)However, the value of Seff that depends on the value ofthe bulk effective lifetime and the bulk lifetime bulk of float-zone crystal is so large, that we calculate the upper limit of thesurface recombination velocity by omitting the recombinationsthat occur in the bulk. It implies that the effective surface re-combination velocity Seff is always well below 100 cm/s andwould get down to less than 30 cm/s after annealing at 400 ?C(Fig. 4). However, when the annealing temperature is equal toor less than 350 ?C, there is no obvious improvement. The sub-sequentannealing of the samples at about 400 ?C drastically in-052002-2J. Semicond. 2011, 32(5) Jia Guangzhi et al.Fig. 5. Refractive index (n/ of the a-SiOx :H layer as afunction of theannealing temperature and the sample without annealing for compar-ison.Fig. 6. Absorption coefficient of a-SiOx :H deduced from SE data fit-ting compared with a-Si:H film.creases the effective carrier lifetime (Fig. 3). We attribute thisincrease to the diffusion of hydrogen in the amorphous siliconsuboxide layer to the interface and effective passivation of dan-gling bond states?12 . Figure 5 shows the effect of the annealingtemperature on the refractive index of the film. No significantchange in the value of refractive index and the thickness of thelayer were observed for the samples fired at different temper-atures.In order to learn more about the a-SiOx :H film, we haveused spectroscopic ellipsometry (SE) measurements to obtainthe optical properties of the a-SiO x :H film. We have the refrac-tive index n and the extinction coefficient k , and the absorptioncoefficient ? is calculated by? D 4k : (4)Figure 6 shows ? as a function of the photon energy for a-SiO x :H film compared to that of a-Si:H. The absorption in thea-SiO x :H film in the light region above 3.0 eV is significantlylower than that of a-Si(i):H. Therefore, for the same thicknessof the film, the fraction of light transferred to the wafer will in-crease drastically. The passivation quality depends strongly onthe thickness of the passivation film ?8; 9 , so the thickness couldbe increased to improve quality with decreasing absorption inthe passivation layer.4. ConclusionIn conclusion, we have demonstrated that the a-SiOx :Hfilm has the potential to provide excellent surface passivationand that the manufacture process is suitable for industrial use.We have achieved an effective lifetime of about 270 s on a4 cm p-type float-zone wafer before annealing, and obtainedan effective lifetime of up to around 670 s after annealingat 400 ?C. The corresponding up limit of effective surface re-combination velocity is less than 30 cm/s. However, when theannealing temperature is equal or less than 350 ?C, there is noobvious improvement. Thermal annealing causeslittle changeto the optic properties of a-SiOx :H films. In addition, the a-SiOx :H films exhibit a very low light absorption when the pho-ton energy is higher than 3 eV compared to a-Si(i):H, which isable to have a thicker passivation layer than standard a-Si(i):Hand therefore increases passivation quality.AcknowledgmentsWe would like to acknowledge Professors Xu Fei fromShanghai University and Zhang Zhiyong from Peking Univer-sity for measurements.References[1] Kerr M J, CuevasA. Very low bulk andsurface recombination inoxidized silicon wafers. Semicond Sci Technol, 2002, 17: 35[2] Kerr M J, Cuevas A. Recombination at the interface betweensilicon and stoichiometric plasma silicon nitride. Semicond SciTechnol, 2002, 17: 166[3] Lauinger T, Schmidt J, Aberle A G, et al. 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