for full-stack device. In annealed condition, the ratio is 17 for half-stack device and 1.1

for full-stack device. It can easily be seen that a higher 1550nm/980nm ratio is obtained

in half-stack devices.

4.3.1.1.3 Infrared/visible comparison

 Fig 4.4 shows the spectral distribution of the visible emission of ZnS:ErF3 half-

stack and full-stack devices. The three emission peaks in the spectrum agree with those

reported for erbium doped ZnS film [47]. The faint green light emitted by this system is

the result of the transitions at -525 nm (2H11/2 4115/2) and -550 nm (4S3/2 4115/2) [47].

In addition, there is another transition at -660 nm (4F9/2 4115/2). In the as-deposited

condition, half-stack devices have higher visible emission intensities than full-stack

devices. In the annealed condition, the visible 525 nm and 550nm signals from half-stack

devices are stronger, while at 660nm the visible intensity of full-stack is stronger.

 The peak height ratio of 1550nm peak to 550nm peak at 40V above threshold in

half-stack and full-stack devices is given in Fig 4.5. The data shown in Fig 4.5 displays

the same trend as the ratio of 1550nm/980nm reported previously. The ratio is higher for

half-stack devices compared to full-stack devices in both the as deposited and annealed

conditions; however the ratio is 9 before and 25 after annealing.

 The effect of annealing at 425oC on infrared intensity can also be seen from the

data in Table 4.4. Annealing enhances the infrared intensity in both half-stack devices

and full-stack devices. In addition, it can also be seen from Fig 4.3 and Fig 4.5 that

annealing at 4250C increased both the 1550nm/980nm ratio and the 1550nm/550nm ratio.

Other data suggest that annealing at higher temperatures may decrease the

1550nm/550nm intensity ratio [50].