  We review aspects of the thermodynamics of black holes and in particular take
into account the fact that the quantum entanglement between the degrees of
freedom of a scalar field, traced inside the event horizon, can be the origin
of black hole entropy. The main reason behind such a plausibility is that the
well-known Bekenstein-Hawking entropy-area proportionality -- the so-called
`area law' of black hole physics -- holds for entanglement entropy as well,
provided the scalar field is in its ground state, or in other minimum
uncertainty states, such as a generic coherent state or squeezed state.
However, when the field is either in an excited state or in a state which is a
superposition of ground and excited states, a power-law correction to the area
law is shown to exist. Such a correction term falls off with increasing area,
so that eventually the area law is recovered for large enough horizon area. On
ascertaining the location of the microscopic degrees of freedom that lead to
the entanglement entropy of black holes, it is found that although the degrees
of freedom close to the horizon contribute most to the total entropy, the
contributions from those that are far from the horizon are more significant for
excited/superposed states than for the ground state. Thus, the deviations from
the area law for excited/superposed states may, in a way, be attributed to the
far-away degrees of freedom. Finally, taking the scalar field (which is traced
over) to be massive, we explore the changes on the area law due to the mass.
Although most of our computations are done in flat space-time with a
hypothetical spherical region, considered to be the analogue of the horizon, we
show that our results hold as well in curved space-times representing static
asymptotically flat spherical black holes with single horizon.
