The properties of nitrogen and oxygen in silicon

A novel dislocation locking technique is used to study the behaviour of nitrogen and oxygen in silicon. Specimens containing well-defined arrays of dislocation half-loops are subjected to isothermal anneals of controlled duration, during which nitrogen or oxygen diffuses to the dislocations. The stress required to move the dislocations away from the impurities is then measured. Measurement of this unlocking stress as a function of annealing time and temperature allows information on the transport of nitrogen and oxygen to be deduced. Despite being present in a concentration of just 3E14cm-3 in some specimens, nitrogen is found to provide substantial benefits to the mechanical properties of float-zone silicon (FZ-Si). The segregation of nitrogen at dislocations is stable to at least 1200 degrees centigrade and the unlocking stress measured at 550 degrees centigrade is of similar magnitude to that found previously for oxygen in Czochralski silicon (Cz-Si). The unlocking stress initially rises linearly with annealing time, before it takes a constant value. The rate of the initial rise is dependent on temperature and the 1.5eV activation energy found agrees with that found previously. The rate of the initial rise also depends on nitrogen concentration. In the 500 to 700 degrees centigrade temperature range, the unlocking stress is found to decrease linearly as the temperature at which the unlocking process takes place increases. The results of a pre-annealing experiment confirm that oxygen monomers and dimers in Cz-Si exist in thermodynamic equilibrium at 550 degrees centigrade. Numerical simulation of oxygen diffusion to dislocations allows values of the effective diffusivity of oxygen in Cz-Si with four different oxygen concentrations to be deduced. At 500 degrees centigrade, the effective diffusivity depends upon oxygen concentration in a way which is consistent with oxygen dimers being responsible for transport. The transport of oxygen in Cz-Si at 550 to 600 degrees centigrade is found to be unaffected by nitrogen doping at a level of 2.1E15cm-3. The dislocation locking technique has also been used to study the effect of high concentrations of shallow dopants on oxygen transport in Cz-Si in the 350 to 550 degrees centigrade temperature range. Oxygen transport has been found to be unaffected by a high antimony concentration ~3E18cm-3, but is found to be enhanced by, on average, a factor of approximately 44 in Cz-Si with a high boron concentration ~5E18cm-3. Furthermore, deep-level transient spectroscopy (DLTS) and high-resolution DLTS (HR-DLTS) are used to study the electrical activity of defects in silicon. A deep-level with an enthalpy of 0.50eV and a concentration of order 10E11cm-3 is found in n-type nitrogen-doped FZ-Si and n-type nitrogen-doped neutron transmutation doped FZ-Si. No additional deep-levels are found in either material, for which the detection limit is 6E10cm-3. No deep-levels are found in p-type nitrogen-doped Cz-Si, for which the detection limit is approximately 10E12cm-3. DLTS and HR-DLTS are also used to investigate the electrical activity of oxygen-decorated dislocations in Cz-Si and states associated with oxygen at dislocation cores have been identified.

Keywords: deep-level
silicon
DLTS
nitrogen
oxygen
boron
electrical activity
float-zone
dislocation
locking
doping
transport
Czochralski
antimony
diffusion
Advisor: Wilshaw, P