Study On Distribution Of Phosphorous And Electrical Property In N-type Czochralski Polycrystalline Silicon

Polycrystalline silicon is one of the important basic materials for photovoltaic solar cells.The microstructure and electrical properties of polycrystalline silicon sheets determine the performance of the cells directly.In order to regulate the resistivity,the phosphorus was doped in n-type Czochralski polycrystalline silicon.As there are evaporation at gas-liquid interface and segregation at solid-liquid interface of phosphorus during solidification process,which cause the concentration and volatilization of phosphorous and results in the uneven distribution of the phosphorous element.The uneven distribution of P elements in silicon rods will affect the stability of the resistivity of silicon wafers and the electrical properties directly.However,there are few studies on the evaporation and electrical properties in n-type Czochralski polycrystalline silicon.Therefore,the distribution and change process of P elements in the n-type Czochralski polysilicon was studied in this paper.The distribution and change of phosphorous in the process was analyzed from the angles of evaporation and segregation.The condensation formula and mass transfer coefficient are derived.The resistivity and minority carrier lifetime characteristics of n-type Czochralski polysilicon rod were studied at the same time.The law of the influence on electric properties from phosphorous element content was studied and the main conclusions of this paper are as follows:?1?The content of phosphorous increases with the increase of solidification fraction in the n-type polycrystalline silicon.The concentration of phosphorous is 0.87 ppmw at the 7.7%solidification fraction and the value is higher than 2 ppmw at the last stage of solidification.The distribution of phosphorous in the cross section was uniform with small fluctuation along the radial.?2?The overall mass transfer coefficient of phosphorous volatilization process is 4.6×10^-7m/s,and its value increases with the increase of temperature.Considering the segregation and evaporation of phosphorous,the effective segregation coefficient of phosphorous is 0.53.The loss rate of phosphorous is 62.5%during the solidification process.?3?With the increase of the solidification fraction,the large size single crystal in the center of the polycrystalline silicon rod gradually stops growing.And the tiny multicrystal grain begins to form and the proportion of fine grain increases gradually.The size of grain decreases as the solidification time,and at the later stage of solidification the size of grain is maintained at1³ mm.?4?The dopant phosphorous content increases with the solidification fraction,and the resistivity decreases from 0.28?·cm at the beginning of solidification to 0.16?·cm.The area of short minority carrier lifetime increases with the increase of solidification fraction.A large number of grain boundaries leads to the decrease of the minority carrier lifetime,and the value of the minority carrier lifetime is reduced from 5.12 us to 0.85 us.