detect position in either a one-dimensional or a two-dimen-

electron±hole pairs are generated on both sides of the junc-

tion and carriers move down the potential gradient, with

minority electrons moving from the p-side to the n-side and

minority holes moving from the n-side to the p-side. This sets

up a forward bias and a transverse photovoltage is set upÐ

this is the basic mechanism of a solar cell output voltage,

when the whole receiver surface is illuminated with light. If

now a spot of light is shone on a junction that has one layer

that is much more conductive than the other, a lateral photo-

voltage is also set up and we can observe a voltage parallel to

the plane of the junction. As before, electrons and holes are

generated on both sides of the junction, and the minority

carriers are swept across the junction. The photogenerated

carriers are swept down the built-in potential to the highly

conducting layer, and spread rapidly in this plane, forming an

equipotential layer. These carriers from the equipotential

layer then re-inject into the less conductive layer. In this low-

conductivity layer, slow-moving excess carriers are still

located in the region of the light spot. It is the tendency of

these bunched carriers to recombine with the re-injected car-

riers that constitute the lateral photovoltage. This photovol-

tage is a measure of the location of the light beam. Further-

more, the linear behavior of the incremental change in the

voltage with distance along the receiver surface indicates the

voltaic mode and Figure 1 shows a simplified crystalline

example of a PSD in this mode. If however, the device is

reverse biased, re-injection is inhibited by the increased

potential barrier, recombination is reduced, and the same con-

tacts can be used to collect currents flowing in the base layer.

In this case, current can be used to measure the location of

the beam. This is the photodiode mode.

Amorphous silicon devices, such as those based on p±i±n

and similar structures, follow the same general principles but

the precise carrier flow mechanisms are more complex. As

with p±i±n solar cells, electron±hole pair generation occurs in

the intrinsic layer and carriers are then separated by the junc-

tion potentials so that electrons move to the n-layer and holes

to the p-layer. These p±i±n structures can be configured as

one- or two-dimensional devices and are widely researched.

Mechanisms and properties of these devices are extensively

Our workis focused on the optimization of the actual thin-

film PSD structure so that we have concentrated mainly on

one-dimensional devices with some two-dimensional devices

constructed. We have designed a novel configuration compris-

ing an indium tin oxide (ITO) layer, followed by an a-Si film

and a platinum layer, and a typical configuration, which forms

a Schottky barrier device, is shown in Figure 2.

3. a-Si-based Position-Sensitive Detectors

PSDs based on crystalline structure have been widely

limited area and high relative costs. These comparatively

small devices require complex and expensive optical systems

to ensure maximum utilization of their areas and to minimize

with pathlengths in the region of 20 mm, while commercial

devices range from 2.5 mm to about 30 mm. Our devices, both

crystalline and amorphous silicon, range from 16 mm to

20 mm by 10 mm.

Thin-film amorphous silicon is a well-established technol-

as a transparent film, which makes it possible to utilize it for

tages, including low-temperature processing capabilities, high

main drawbackis the high defect state density, which can be

greatly reduced using hydrogenation. Amorphous silicon can

be prepared using many techniques, among the more common

ones are radiofrequency (RF) and direct current (DC) glow

discharge, RF sputtering, chemical vapor deposition (CVD),

plasma-enhanced chemical vapor deposition (PECVD), and

ion discharge. Most of the latest workis on a-Si:H PSDs fabri-

tures produce excellent linearity between voltage output and

position. Of the plasma techniques, glow discharge and sput-

tering, the former has received the most attention probably

due to its ability to produce films with superior optical and

the advantage of permitting good control of the hydrogen

tent of a-Si:H has a major effect on the optical and electrical

2

Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2001 0935-9648/01/12±1307-02 $ 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. 1. A crystalline PSD configured for measurement in photovoltaic mode.

Fig. 2. A typical thin-film Schottky barrier device structure.