Ga

1-x

Mn

x

As thin films with a thickness of 200 nm and with various Mn mole concentrations(

x

=0

2.6%

4.2% and 6.3%) were grown on epi-ready semi-insulating GaAs(001) substrates by low temperature molecular beam epitaxy.The Curie temperature of samples

in the range of 30~50 K

was determined using a superconducting quantum interference device.For photo-modulated reflectance(PR) spectra measurements at room temperature

a halogen lamp dispersed by a monochromator was used as the light source.The He-Ne laser(632.8 nm) with a power of 1 mW was used as the modulation light and the chopper frequency was at 184 Hz.A Si detector was employed to collect the modulated signal.Franz-Keldysh oscillations(FKO) from epitaxial layers of the diluted magnetic semiconductor(Ga

Mn)As have been studied by PR at room temperature.The PR line shape is broadened with increasing Mn concentration but the critical points of

E

0

and

E

0

+Δ

0

are not shifted obviously.Based on FKO oscillation data

the calculated electric field of Ga

1-x

Mn

x

As is 21

45

77 and 160 kV/cm for

x

=0

2.6%

4.2% and 6.3%

respectively.It suggests that the surface electric field increases with the increase of Mn concentration up to 6.3%.In addition

Mn-related impurity state is observed

which is about 100 meV above the top of the valence band of the host GaAs.Based on depletion layer approximation

we solve the equations of semiconductor statistics together with Poisson’s equation for the relationship between the potential and charge concentration.The calculated hole concentration is about 10

17

cm

-3

which is two orders of magnitude less than the reported value due to the PR signal coming from the(Ga

Mn)As layer with a part of depletion of the delocalized holes

or a lower Curie temperature.Further work is needed to deplete all holes within the(Ga

Mn)As layer by applied external electric field.