Arsenic redistribution at the SiO 2 /Si interface during oxidation of implanted silicon Fabio Iacona, Vito Raineri, and Francesco La Via CNR-IMETEM, Stradale Primosole 50, 95121 Catania, Italy Antonio Terrasi and Emanuele Rimini INFM and Dipartimento di Fisica dell’Universita`, Corso Italia 57, 95129 Catania, Italy ~Received 13 April 1998! The behavior of ion-implanted As in ~100! silicon wafers, following thermal oxidation, has been investigated by Rutherford backscattering spectroscopy, atomic force microscopy, transmission electron microscopy, and extended x-ray-absorption fine structure. The adopted fluences ~3 310 15 and 3 310 16 cm 22 ! and oxidation conditions ~wet 920 °C, dry 1100 °C! span quite a broad range of phenomena, giving rise to As diffusion in the bulk, and/or segregation and precipitation at the SiO 2 /Si interface. The surface roughness is correlated to that measured at the interface, although the oxide presence strongly reduces the value with respect to that present at the interface. Rough interfaces and surfaces are formed when the arsenic concentration exceeds the solid solubility and precipitation occurs. The SiAs precipitates are characterized by a monoclinic structure with low surface energy for the ~100! facet, as determined by the Wulff plot. Residual roughness is left at the oxide surface even if precipitates initially formed dissolve during subsequent oxidation. The depth profile of the dopant has been quantitatively computed by the analytical solution of the diffusion equation, taking into account the interface movement, the As redistribution at the interface between oxide and bulk silicon, the formation, growth, and dissolution of precipitates, and, of course, the drive-in process. The dependence of the diffusion coefficient on the dopant concentration has been also considered and determined as a converging parameter, considering iteratively the differential equation solution. In all the investigated cases the agreement between experimental data and calculations has been found to be good. @S0163-1829~98!08539-7# I. INTRODUCTION The behavior of the most common dopants during thermal oxidation of silicon has been extensively studied, due to its tight correlation with the doped silicon electrical properties. 1–3 Indeed, during the SiO 2 /Si interface movement toward bulk silicon, impurities can be embedded in the SiO 2 layer or can pile up at the interface. These phenomena are quantitatively described by the segregation coefficient, de- fined as the equilibrium ratio between impurity concentration in the silicon and in the oxide. 1 Among the most common dopants, B has a segregation coefficient lower than 1, i.e., it is preferentially embedded in the oxide, while As, Sb, and P have a segregation coefficient higher than 1, i.e., they segre- gate at the interface during the oxidation process. From the electrical point of view, both situations cause a loss of dop- ant, because the atoms segregating at the interface form pre- cipitates when their concentration exceeds the solid solubil- ity. More recently, the relationship between impurity segrega- tion phenomena and morphology of the SiO 2 layers formed by thermal oxidation has been investigated in detail. The formation of precipitates at the SiO 2 /Si interface has been observed for silicon implanted with Pb, Ag, and other metals, 4 As, 5,6 and Ge. 6,7 Moreover, the formation of SiO 2 layers having a rough surface has been related to the SiO 2 /Si interface morphology. 6 In the case of Ge-implanted silicon, the formation of precipitates has been found to depend on the implanted dose. Oxidation of samples implanted with 3 310 15 Ge/cm 2 does not lead to precipitate formation, while the phenomenon is clearly visible when samples implanted with higher doses ~1 310 16 and 3 310 16 cm 22 ! are oxidized. The low-dose behavior has been explained through the capa- bility of the segregating Ge atoms to form flat and well- ordered Ge x Si 1 2x layers; when the concentration of segregat- ing atoms becomes too high, ordered layers cannot anymore be formed, and precipitation occurs. 6 In the case of As-implanted silicon, precipitation occurs already at 3 310 15 cm 22 , due to the relatively low As solu- bility. The phenomenon has been found to be strongly tem- perature dependent; 5,6 indeed, low-temperature oxidation ~920 °C! produces SiO 2 layers having a roughness higher than 0.5 nm ~i.e., about five times higher than that detected on oxides grown under the same conditions on unimplanted silicon!. On the other hand, by increasing the oxidation tem- perature up to 1100 °C, the surface roughness becomes com- parable to that of oxides grown on virgin silicon. Arsenic behavior has been qualitatively explained in terms of the competition between As diffusion rate, prevailing at 1100 °C, and the silicon oxidation rate, prevailing at 920 °C. 6 In this paper we discuss the As precipitation at the SiO 2 /Si interface during thermal oxidation of As-implanted silicon. In particular, we have first investigated the influence of different oxidation conditions and implanted doses on the As redistribution during silicon thermal oxidation; the related formation of As-containing precipitates has been discussed on the basis of Rutherford backscattering spectrometry ~RBS!, atomic force microscopy ~AFM!, transmission elec- tron microscopy ~TEM!, and extended x-ray-absorption fine- structure ~EXAFS! measurements. The observed interface PHYSICAL REVIEW B 15 OCTOBER 1998-II VOLUME 58, NUMBER 16 PRB 58 0163-1829/98/58~16!/10990~10!/$15.00 10 990 © 1998 The American Physical Society