The polarization properties of the three kinds of bound excitons bounded at neutral donors(BED

0

) neutral acceptors(BEA

0

) and Donor-Acceptor pairs (D-A) in GaN crystals were studied.The polarizability of the emission of BED

0

and BEA

0

increases quaduatically with the increase of the excitation intensity (Iex) in the range of 10

4

W/cm

2

~10

6

W/cm

2

.And a saturation appears thereafter.The initial polarization of the photoluminescence from BED

0

and BEA

0

can be explained by the anisotropic dielectric constant of uniaxial crystal.And the change of polarization with the Iex is due to the different extent of the dipole-dipole interaction between bound excitons at different density.The GaN crystal layers were grown by the VPE on sapphire substrates oriented in the (0001) direction.The samples were put in the crystal ESR-900 and cooled down to 77K.And they were then excited by a light beam of wavelength of 308nm from an excimer laser EMG-102.With a set if neutral attenuators and a lens

Iex on samples could be easily changed from 10

4

W/cm

2

to 10

7

W/cm

2

.A polarizer was put before a double grating monochrometer to pick out bound exciton emissions polarized in parallel and vertical directions to the C-axis of GaN wafers respectively.After the monochrometer

the light signal was messured by a photomultiplier C-31034 with cooling system and recorded by a X-Y recorder.Fig.1 and Fig.2 show the emission spectra of different samples.Fig.3 shows the optical scattering plane in experimental setup.Fig.4 and Fig.5 show the P values under different Iex for different samples.Therelations of BED

0

and BEA

0

are shown in Fig.6 and Fig.7 respectively.From Fig.6 and Fig.7

we can see that the ralation P=(A+BIex)2(7)is valid in the Iex range of 10

3

W/cm

2

-10

5

W/cm

2

.When Iex is higher than 10

5

W/cm

2

P tends to a saturation point.It might be caused by the concentration limitation of neutral donors and neutral acceptors.