and this gives an indication of the maximum spatial resolu-

tion, which is less than 50 lm in our best devices. These

results are presented in Table 2.

6. Conclusions

In this workwe produced three different PSD structures

based on amorphous silicon, all of which showed promising

results.

l

Structures 1 and 2 appear to workas a pseudo n±i-SB

an n-layer, a-Si:H acting as the intrinsic layer and a Schott-

ky barrier being formed with the Pt and a-Si:H. Improved

contacts and the addition of resistive layers to the struc-

tures will improve device sensitivities. Structure 3 works

as a straightforward Schottky barrier device.

l

We tested all devices under red and white light. Even

allowing for different optical intensities, devices per-

formed better by a large margin in terms of linearity and

sensitivity, under white light than with the red laser light.

It is assumed that the optical response of a-Si:H to white

light is greatly enhanced due to the energy gap of the

absorbing region more closely matching that of the light

source.

l

Measurements taken along x- and y-directions were suffi-

ciently promising to indicate that there is potential for use-

ful two-dimensional PSDs.

l

Nonlinearities of the devices were usually less than 5 %

for increments down to 50 lm indicating useful devices

for high-resolution applications.

l

Future workwill focus on the optimization of our devices

to give linear results and high outputs in two dimensions.

We will attempt to increase the size of our devices and the

major challenges will be to maximize the active area while

achieving high spatial resolution. Further attempts will be

made to tailor material bandgaps to respond optimally to

particular light sources.

±

[1] W. Schottky, Phys. Z. 1930, 31, 913.

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[3] T. Takeda, in Amorphous and Microcrystalline Semiconductor Devices:

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[4] E. Fortunato, G. Lavareda, R. Martins, F. Soares, L. Fernandez, Sens.

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[5] L. Lindholm, private communication.

[6] R. Martins, E. Fortunato, Thin Solid Films 1999, 337, 158.

[7] S. Arimoto, H. Yamamoto, H. Ohno, H. Hasegawa, Electron. Lett. 1983,

19, 628.

[8] E. Fortunato, G. Lavareda, M. Vieira, R. Martins, Rev. Sci. Instrum. 1994,

65, 3784.

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[10] E. Fortunato, R. Martins, Solid State Phenom. 1995, 44±46, 883.

[11] J. Geist, in Sensor technology and Devices (Ed: L. Ristik), Artech House,

Norwood, MA 1994.

[12] D. E. Carlson, IEEE Trans. Electron Devices 1977, ED-24, 449.

[13] Y. Uchida, in Amorphous Semiconductors (Ed: Y. Hamakawa), North-

Holland, Amsterdam 1984.

[14] T. Moustakas, in Semiconductors and Semimetals (Ed: J. Pankove), Aca-

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1994, 144, K37.

4

Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 0935-9648/01/12±1307-04 $ 17.50+.50/0

Adv. Mater. 2001, 13, No. 12±13, July 4

J. Henry, J. Livingstone/Thin-Film Amorphous Silicon Position-Sensitive Detectors

Fig. 3. Device J25a (Structure 2) tested under focused white light in photovol-

taic mode resulting in a sensitivity of 350 lV per 500 lm increment.

Table 2. Calculated nonlinearities measured under focused white light for incre-

ments of 500 lm, 250 lm, 100 lm, and 50 lm.

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