Thin-Film Amorphous Silicon

Position-Sensitive Detectors

By Jasmine Henry* and John Livingstone

1. Introduction

Position-sensitive detectors, or PSDs, comprise an impor-

tant class of optical sensor, producing an electrical output,

either voltage or current, which utilizes the lateral photo-

voltaic effect to give a linear relation between the output and

the location of a spot of light impinging on a semiconductor

surface. This phenomenon was first described by Schottky in

PSDs are used for a variety of optical applications, such as

machine tool alignment, medical instrumentation, remote

optical alignment, robotic vision, and other applications

requiring precision measurements. Other interesting applica-

ing, angle measurement, rotation monitoring, guidance sys-

tems, and roles where precise automated control is necessary.

PSDs are different to photodiode and other device arrays,

e.g., those formed using charge coupled devices (CCDs), in

that they can provide continuous information with no internal

are that PSDs have better sampling frequencies (10 MHz to

10 kHz compared to 2 kHz) and they are cheaper. CCDs have

the advantage that they are more effective at eliminating the

The wavelength sensitivity of these devices is, like all semi-

conductor optical devices, dependent upon the optical energy

gap of the absorbing material so that a typical silicon-based

single-crystal device has a maximum response in the region of

1000 nm. Our a-Si devices appear to have an optimal response

to white light (peakwavelength 690 nm), which corresponds

to an effective energy gap in the region of 1.8 eV. Light inten-

sity does not appear to affect the linearity of device output,

however, the magnitude of the response is directly related to

the input power while operating below optical saturation. The

devices, however, appear to lose linearity close to the contacts

and this is attributed to edge effects related to electric field

Typical light saturation for silicon-based PSDs is around

tude such that the voltage drop across the sheet resistance is

so large that it equals reasonable values of reverse bias across

the device in photodiode mode. At this point the p±n junction

will be forward biased and hence the PSD no longer functions

in this mode. Our devices have shown the best linearities in

photovoltaic mode, with no advantage being gained in photo-

diode mode implying that we are approaching saturation with

some of our optical sources. In a photodiode, saturation

means that the production of photocurrent is saturated and

can no longer increase with increasing light intensity. This

leads to an accumulation of charge in the diode which slows it

down. To remove these charges after the light is turned off, a

2. Mechanisms of Position-Sensitive Detector

Operation

The simplest model of a PSD is that of a crystal-based

device with a highly conducting top layer on a lower conduc-

tivity substrate, with appropriately placed contacts. They can

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

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

1

Optical position-sensitive detectors are a useful class of sensor with a wide range of

applications in machine control systems, industrial alignment and robotic vision. They

have distinct advantages over most arrayed discrete optical devices in that they can

produce continuous optical signals, and versions based on thin-film amorphous sili-

con are not restricted by crystal growth limits and so have the potential to be fabri-

cated in large area format. Sputter-deposited hydrogenated a-Si also has features such

as excellent adhesion to glass substrates, precise film thickness, and hydrogen content control, which are of some

interest in device design and fabrication.

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[*] Dr. J. Henry, Dr. J. Livingstone

Department of Electrical and Electronic Engineering

University of Western Australia

35 Stirling Highway, Crawley, W.A. 6009 (Australia)

E-mail: jasmine@ee.uwa.edu.au