  We present the results of our ultra-deep Keck/DEIMOS spectroscopy of
z-dropout galaxies in the SDF and GOODS-N. For 3 out of 11 objects, we detect
an emission line at ~ 1um with a signal-to-noise ratio of ~ 10. The lines show
asymmetric profiles with high weighted skewness values, consistent with being
Lya, yielding redshifts of z=7.213, 6.965, and 6.844. Specifically, we confirm
the z=7.213 object in two independent DEIMOS runs with different spectroscopic
configurations. The z=6.965 object is a known Lya emitter, IOK-1, for which our
improved spectrum at a higher resolution yields a robust skewness measurement.
The three z-dropouts have Lya fluxes of 3 x 10^-17 erg s^-1 cm^-2 and
rest-frame equivalent widths EW_0^Lya = 33-43A. Based on the largest
spectroscopic sample of 43 z-dropouts that is the combination of our and
previous data, we find that the fraction of Lya-emitting galaxies (EW_0^Lya >
25A) is low at z ~ 7; 17 +- 10% and 24 +- 12% for bright (Muv ~= -21) and faint
(Muv ~= -19.5) galaxies, respectively. The fractions of Lya-emitting galaxies
drop from z ~ 6 to 7 and the amplitude of the drop is larger for faint galaxies
than for bright galaxies. These two pieces of evidence would indicate that the
neutral hydrogen fraction of the IGM increases from z ~ 6 to 7, and that the
reionization proceeds from high- to low-density environments, as suggested by
an inside-out reionization model.
