Creating AlGaAs Photodetectors

O. Rabinovich, S. Didenko, S. Legotin and M. Basalevskiy

National University of Science and Technology “MISiS”, P.O. Box 409, 119313, Russian Federatation

Keywords: AlGaAs, Photodiode, Photodetector, Dark Current, Scintillators.

Abstract: AlGaAs/GaAs photodetectors operate at room temperature in the visible spectrum. Distinctive features of

the photodetectors are: high absolute spectral sensitivity up to 0.112 A / W at λ

max

= 530-570 nm;

photodiodes showed the low dark current of 4.7 nA and 530 nA, accordantly, at 5 V reverse bias. The shift

of the spectral characteristics which is associated with an increase in the band gap was detected.

1 INTRODUCTION

Photodetectors for the visible spectral range can be

used as photodetectors in scintillation counters in

experiments with space rays in neutrino physics, in

experiments with particle accelerators, etc. Getting

information about the particles that came to Earth

from space, is an important research task. For

example, the fastest accelerator allows getting the

particles energy about 10

eV, while the energy of

the particles arrived from outer space, can reach

-10

eV. Also due to the sensitivity lack in the

infrared range and high sensitivity in the visible

region, such photodetectors may be used, for

example to control the liquid crystal display (LCDs)

backlight.

Scintillation detector consists of a scintillator

which emits photons when struck by ionizing

radiation and a photodetector that converts light

from the scintillator into an electrical signal. In

present-time detectors optical fibers are used to

improve light focusing from the scintillator.

Typically, the scintillators used for experiments in

high energy physics, emit at a maximum wavelength

max

from 375 to 430 nm. For detection of it light

photomultiplier tubes (PMT) are used because their

maximum spectral sensitivity is almost perfectly

matches the maximum of emission spectrum of

scintillator. But PMT has some disadvantages, that is

why, nowadays, researchers and engineers are

working on finding an alternative to replace the

PMT. The most common alternative is silicon

p-i-n diode (Ryzhikov and Kozin D et al., 2003) or

silicon PMT (SiPM) (Patent RF, 2005; Herbert and

D’Ascenzo et al, 2006; Bloser and Legere et al.

2014).

AlGaAs solid state solutions are high promising

to create photodetectors for this spectral range. Si

has maximum sensitivity about 900 nm, GaAs –

about 800 nm, but by adding Al it can be achieved

high sensitivity in the visible region (400-500 nm)

due to increased bandgap. During the photodetectors

creation it has been assumed that scintillation plate

type SC-301 with maximum in the emission

spectrum about 420 nm is used. To improve the light

focusing optical fibers are used. The wavelength of

transferred light to photodetector is approximately

476 nm.

In this paper photodetectors AlGaAs / GaAs for

detecting light from the scintillator are presented. On

the basis of heterostructures using the standard

contact photolithography photodiodes were

produced (tabl. 1).

Details of the construction and growth method of

these structures are described in (Bloser and Legere.

2014; Murashev and Legotin et al. 2014; Bazalevsky

and Didenko et al. 2014; Legotin and Murashov et

al. 2014; Baryshnikov and Didenko et al. 2012;

Koltsov and Didenko, et al. 2012; Legotin and

Rabinovich et al. 2014). Photodiodes circular Mesa,

the chip size of 4 x 4 mm and a photosensitive

window diameter of 1.5 mm have been mounted in

the housing type TO-39.

Rabinovich O., Didenko S., Legotin S. and Basalevskiy M..

Creating AlGaAs Photodetectors.

DOI: 10.5220/0005334100870089

In Proceedings of the 3rd International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS-2015), pages 87-89

ISBN: 978-989-758-093-2

 2015 SCITEPRESS (Science and Technology Publications, Lda.)

Table 1: Basic structure for photodiode.

Layer’s title

Concentration,

(cm

-3

)

Layer’s

width, (nm)

Contact layer, p

GaAs

(Be)

2 x 10

“Window” p

AlAs (Be)

2 x 10

pAl

0.35

0.65

As(Be)

2 x 10

500

nAl

0.35

0.65

As(Si)

2 x 10

500

Buffer n

GaAs (Si)

1.5 x 10

200

Substrate n

GaAs(Si)

1.5 x 10

400 μm

2 RESULTS AND DISCUSSION

Schematic cross-sections of finished devices are

shown in figure 1.

Figure 1: Cross section for photodiode.

Dark current and breakdown voltage were measured

by Agilent B1500A semiconductor device analyzer.

Reverse current–voltage characteristic (I-V) of

photodiode is presented in Figure 2. The best

samples demonstrated the dark current I

= 3.38 nA

at a reverse bias U

rev

= 5 V. I-V control was carried

out at each stage of devices production, from cutting

wafers into chips till the packaging process (Fig. 3).

The measurements showed that after all

production steps the dark current increased and the

breakdown voltage decreased insignificantly

(Fig. 3).

Calculations of spectral response were carried

out for a p-i-n GaAs structure. It consists of a 0.82

microns thick top p-GaAs layer doped to 5·10

; 45 microns thick i-GaAs region with electron

density 1·10

-3

and heavily doped n

-substrate

from the back side of the structure. Surface

recombination rate was taken as 2·10

cm/s. Figure 4

shows a spectral response of described structure. For

clarity contribution of different regions is shown.

Figure 2: Reverse photodiode I-V characteristic.

Figure 3: Photodiode I-V at wafer and after packaging.

Figure 4: Calculated spectral response of p-i-n GaAs

structure.

Figure 5 shows that the photosensitivity of the

structure from the short-wave edge is determined by

electrons in p-region, and from the long-wavelength

edge by charge carriers generated in the depletion

region and holes in the lightly doped i-region. Thus

blue-shift of spectral response can be achieved by

reducing surface recombination rate and by

increasing mobility and lifetime of electrons in p-

region. Also modeling of p-i-n heterostructure with

PHOTOPTICS2015-InternationalConferenceonPhotonics,OpticsandLaserTechnology

p Al

1-x

As layer was performed.

Figure 5: Photodiodes spectral characteristics.

Calculated spectral characteristics are shifted to the

short-wave range and overall sensitivity increases.

At the same time it could be seen that the greatest

increase in sensitivity at wavelengths less than 0.4

microns takes place not for the widest bandgap

material. Someone can find it strange, but the fact is

that in the more wide bandgap-materials the electron

mobility decreases sharply (from 3.1·10

/V·s for

0.25

0.75

As to 5.4·10

/V·s for

0.419

0.581

As) for the same surface recombination

rate and lifetime of charge carriers.

Spectral characteristics were measured using a

monochromator MDP and apparatus for measuring

the spectral sensitivity of photodetectors

TTM3.435.088. Figure 5 shows the relative spectral

characteristics of the photodiodes.

It’s seen that, at changing the Al mole fraction by

0.05, maximum of spectral sensitivity was shifted by

40 nm to shorter wavelengths. At the same time the

sensitivity at a wavelength of 475 nm was increased

almost twice by choosing the optimum composition

and thickness of the active p-layer. To achieve

greater sensitivity in shorter wavelengths for

photodiodes antireflection coating on “window”

layer will be used.

Photodetectors created for detecting light from

the scintillator based on AlGaAs/GaAs

heterostructures were produced. Photodetectors

measurements showed the low dark current at the

level of 0.5 nA for phototransistors and 10 nA for

photodiodes, for at the same time high breakdown

voltage was about 600 V and 20 V, accordantly.

Experimentally determined values of

composition and thickness for the Al

1-x

As solid

solution allowed to increase the sensitivity almost

twice from 40 to 75 % at a wavelength 475 nm.

Absolute spectral sensitivity of photodiodes reached

0.13 A/W at λ = 570 nm.

ACKNOWLEDGEMENTS

This study was supported by the Federal Targeted

Program “The development of electronic component

base and radio electronics” for 2008 – 2015, state

contract № 14.430.12.0003.

The work was financially supported Basic part of

State the Federal Targeted Program by the Ministry

of Education and Science of the Russian Federation

within the framework of the federal target program

of Russia in 2014 - 2016 years

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CreatingAlGaAsPhotodetectors