Problems of Real Scalar Klein-Gordon Field
  We examine the negative energy solution in Klein-Gordon equation in terms of the number of field components. A scalar field has only one component, and there is no freedom left for an anti-particle since the Klein-Gordon equation failed to take the negative energy solution into account. This is in contrast to the Dirac equation which has four components of fields. It is shown that the current density for a real scalar field is always zero if the field is classical, but infinite if the field is quantized. This suggests that the condition of a real field must be physically too strong. 
Sachiko Oshima, Seiji Kanemaki and Takehisa Fujita