al$_{\bf 0.52}$in$_{\bf 0.48}$p sam-apd as a blue-green detector

$: Al$_{\bf 0.52}$In$_{\bf 0.48}$P SAM-APD as a Blue-Green Detector$
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Abstract— We demonstrate an Al0.52In0.48P homo-junction

Separate Absorption Multiplication Avalanche Photodiode

(SAM-APD) as a detector with narrow spectral response in the

blue-green part of the optical spectrum. Due to its wide band-gap,

this device has a dark current density of < 8 nA cm-2 at 99.9% of

the breakdown voltage at room temperature. This device has a

peak responsivity at 483 nm of 0.15 A/W when punched-through

and is capable of an avalanche gain higher than 100.

Index Terms—Optical communication, narrow band detector,

blue detector, avalanche photodiode

I. INTRODUCTION

HERE is an increasing interest in detectors for optical

communication systems recently for underwater

applications as they offer higher data bandwidth. High speed

data transmission of 1 Gbps over 50 m can be achieved

underwater by using a 532 nm laser [1] whereas conventional

acoustic systems can only give ~10 kbps [2] over a similar

distance. Unlike in fiber based communication systems, these

detectors may have to operate in the presence of extraneous

light sources such as sunlight and consequently may suffer

from a high background noise. To reduce such interference, the

detector should ideally only respond to the wavelength of

interest by having a narrow spectral response in the blue-green

part of the optical spectrum around 480 nm where seawater has

the least optical attenuation varying from < -0.1 dB/m to > -3.5

dB/m depending on its turbidity [2]. To maximize the range of

operation, a high sensitivity detector is required. Silicon is

usually the best choice for photodiode in this wavelength range

but its wide spectral response range makes it very susceptible to

any background illumination, necessitating the use of filters.

Although optical band-pass filters are available, they add cost

and complexity to the optical receiver. Furthermore, the filter

central wavelength varies with the incident beam angle and

therefore attenuates the signal power.

Al0.52In0.48P (AlInP thereafter) is the widest bandgap material

that can be grown lattice-matched on a GaAs substrate. Work

by Zhang et al. showed that a GaInP/AlInP PIN structure can

respond to ~ 480 nm with a narrow spectral response and a

quantum efficiency of 43 % in addition to negligible reverse

dark current density of < 42 nA∙cm-2

[3]. By utilizing the band

Manuscript submitted Januray 20, 2014. This work was supported by the

U.K. Ministry of Defense S&T Program via Defense Science and Technology

Program under Grant CDE 24907. J. S. Cheong, J. S. Ng, A. B. Krysa and J. P. R. David are with the

Department of Electronic and Electrical Engineering, University of Sheffield,

Mappin Street, Sheffield S1 3JD, U.K. (e-mail: [email protected],

[email protected], [email protected], [email protected]).

J. S. L. Ong is with the Microelectronic Engineering Department, University

of Malaya Perlis, Pauh Putra Campus, 02600 Arau, Perlis, Malaysia. (email: [email protected]).

discontinuities at the Ga0.52In0.48P-Al0.52In0.48P interface [4],

photocurrent due to short wavelength light is suppressed. The

hetero-junction capping layer behaves similarly to that of a

long-pass optical filter, resulting in a detector with a relatively

narrow spectral full-width-half-maximum (FWHM) of 45 nm.

An alternative way of removing the carriers created by short

wavelength light is to ensure that they recombine before they

reach the depletion region. This requires materials with short

minority carriers’ diffusion lengths. Potential materials are

aluminum-based alloy semiconductors such as AlGaAs and

AlGaInP, which high-quality wafer growth can be challenging

due to the strong affinity between aluminum and oxygen.

Residual oxygen during growth introduces deep-level traps and

reduces the minority carrier lifetime in aluminum containing

alloys [5]–[6]. This reduces the diffusion length significantly

and thereby potentially allows a material to have a narrow

spectral FWHM intrinsically without having a hetero-junction

interface. Unfortunately, the narrow spectral response also

results in a decrease in the peak responsivity. Avalanche

photodiodes (APDs) utilize the impact ionization process to

provide a gain mechanism and can provide an increase in

sensitivity, compensating for any reduction in responsivity.

Ong et al. [7] showed that homo-junction AlInP PINs retain a

low dark current density of < 6 nA∙cm-2

even at high reverse

bias up to 95% of breakdown voltage and therefore the material

may be used in an APD to improve the signal-to-noise ratio of a

photodiode. Since the electron ionization coefficient (α) is

larger than the hole ionization coefficient (β), electrons should

initiate the multiplication process for optimum noise

performance.

In this work we demonstrate a homo-junction AlInP based

APD that provides an inherently narrow FWHM centered at

around 480 nm and is capable of high gain.

II. DEVICE DESIGN, GROWTH & FABRICATION

To simulate the spectral response and FWHM of the AlInP

SAM-APD, the electron diffusion length (Le) and absorption

coefficient at 300 K in AlInP is required. To obtain these

parameters, homo-junction AlInP 1.0 µm PIN with 1.0 µm p+

cladding thickness was grown on a 10 degree off-axis GaAs

substrate by Metal Organic Vapour Phase Epitaxy (MOVPE)

and were fabricated using standard lithography process. The p+

cladding was chemically dry etched to thicknesses of 0.92, 0.43

and 0.26 µm with Inductive Coupled Plasma (ICP) and

subsequently the samples were wet-etched to form mesa diodes

of 25 – 200 µm radii.

Photocurrent measurements as a function of reverse bias

were performed on these devices using 442 nm illumination.

The increment of primary photocurrent was then simulated

using the analytical solutions [8] based on the current

continuity equation. Using an absorption coefficient (σ) at 442

Al0.52In0.48P SAM-APD as a Blue-Green Detector

J. S. Cheong, J. S. L. Ong, J. S. Ng, Member, IEEE, A. B. Krysa and J. P. R. David, Fellow, IEEE

T

mailto:[email protected]








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nm of 1 × 105 cm

-1 [9] and electron mobility (µe) of 100 cm

2/V.s

[10], Le was determined to be 0.14 µm, assuming a surface

recombination velocity (Se) of 1 × 105 cm/s similar to that of

GaAs [11]. Due to the presence of aluminum, the Se of AlInP

should be significantly higher than GaAs but the simulated

primary photocurrent showed insignificant change when Se was

increased from this nominal value, suggesting that Le is

relatively insensitive to this parameter.

To obtain the absorption coefficient, the spectral response of

the PIN diodes was then measured using a white light source

and a grating monochromator. As most of the absorption

process occurs in the p+ cladding, the absorption coefficient

was assumed to be similar regardless of the dopant type. The

absorption coefficient over the 400 – 600 nm wavelength range

was then deduced by modeling the spectral response which is

shown in Fig. 1a. The energy gap at Γ (EΓ) and X (EX) points of

the Brillouin zone of AlInP were estimated from the absorption

coefficients [12, 13], to give 2.31 eV and 2.60 eV respectively.

These values are consistent with those suggested by several

groups with values of EΓ and EX of 2.33 eV and 2.45- 2.63 eV

respectively at 300 K [14, 15, 16, 17]. The absorption

coefficients for wavelengths shorter than 470 nm increase more

rapidly since photon-absorption processes occur at the Γ point

of the Brillouin zone.

A simple AlInP PIN diode with a thick absorption region will

make a poor APD as besides having a large operating voltage, it

may suffer from poor noise performance due to both electrons

and holes initiating the multiplication process. To overcome

these problems, we propose an AlInP homo-junction

Separate-Absorption-Multiplication Avalanche Photodiode

(SAM-APD) in which the device electric field profile can be

tailored. A SAM-APD typically has a thin multiplication region

at high field and a thick absorption region at low field, ensuring

a relatively low operating voltage and high quantum efficiency

respectively. A 0.2 µm thick avalanche region AlInP PIN diode

has been reported with negligible tunneling current up to

breakdown voltage [7] so this was chosen as the multiplication

region thickness. A 1.0 µm thick un-doped absorption region

provides reasonable quantum efficiency at the desired peak

response wavelength of 480 nm and the electric fields in these

two regions are bridged by a 0.175 µm thick charge sheet layer

with doping density of 3 × 1017

cm-3

such that the electric field

in the multiplication region does not breakdown before

punch-through while ensuring a low electric field of ~ 100

kV/cm across the absorption region. Assuming that the charge

sheet is sufficiently thin such that there is no multiplication

occurring in that region, the breakdown field of this SAM-APD

is similar to that of a 0.2 µm PIN [7] and therefore such a

structure should give a breakdown voltage of ~ -40 V.

The modeling result in Fig. 1b shows that a narrow spectral

FWHM of 21 nm can be achieved from a homo-junction AlInP

SAM-APD with an active region thickness of 1.4 µm (the total

thickness of absorption region, charge sheet and multiplication

region) by increasing the p+ cladding thickness albeit at the

expense of device responsivity. The peak response wavelength

Fig. 1a. Absorption coefficients deduced by AlInP PINs.

Fig. 1b. Simulated spectral responses of fully depleted AlInP SAM-APD with

p+ cladding thicknesses of 0.2, 0.5, 1.0, 1.5 and 2.0 µm. Symbols shows experimental (circle) and simulation (rectangle) results of a 1.0 µm PIN.

Fig. 3. Structure detail and electric field profile of a fully depleted

(punch-through) AlInP SAM-APD.

Fig. 2. The responsivities at 480 nm and spectral FWHM of AlInP SAM-APD

(solid lines) with increasing p+ cladding thickness. Also shown the

multiplication required to achieve responsivity of 10 A/W (symbol with line).




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also shifts slightly to longer wavelength with increasing

cladding thickness since more carriers created at shorter

wavelengths recombine and are unable to contribute to the

photocurrent. Fig. 1b shows the responsivity roll-off at shorter

wavelengths improves with p+ cladding thickness and the

FWHM saturates at 21 nm for a 1 µm thickness as illustrated in

Fig. 2. However this also results in reduced peak responsivity

and consequently a higher multiplication gain is therefore

required to achieve a given responsivity, arbitrarily chosen to

be 10 A/W in Fig. 2. To have the narrowest FWHM of 21 nm,

1.0 µm p+ cladding was determined as the optimum thickness

with peak response wavelength at 480 nm in addition of

ensuring > 97 % of electrons generated in the p+ and absorption

region at such wavelength. The designed SAM-APD has the

structure as shown in Fig. 3. A highly doped, thin GaAs cap

was deposited on top of the p+-cladding to ensure a good ohmic

contact.

The SAM-APD was grown and fabricated using the same

growth and fabrication technique as that of the PIN diode. To

ensure the incident light is only absorbed by the AlInP p+

cladding layer, the thin GaAs cap is etched off in the central

window region as shown in Fig. 3 before forming mesa diodes.

III. EXPERIMENTAL RESULTS & ANALYSIS

Capacitance-voltage (C-V) measurements were performed to

obtain the doping densities and thicknesses of the absorption

region, charge sheet and multiplication region. This

information was subsequently corroborated by Secondary Ion

Mass Spectroscopy (SIMS) measurements. Using the doping

profile obtained from C-V and SIMS, good agreement was

obtained between experimental C-V and modeling results of the

structure from the SIMS measurement as shown in Fig. 4a. Due

to the dopant diffusion from the charge sheet, the absorption

region width is reduced to 0.5 µm with a 1.0 µm almost

uniformly doped charge sheet and multiplication region of 6 ×

1016

cm-3

which results in a higher punch-through and

breakdown voltage. The simulated electric field profiles of the

diode at breakdown voltage (-66.5 V) illustrated in Fig. 4b

using the extracted parameters is similar to that of a one-sided

p-n junction. The departure of the doping profile from the

intended design due to dopant diffusion however, should not

alter the primary spectral response since the active regions

thicknesses are similar.

I-V measurements were performed on the devices in dark

condition. The reverse I-V in Fig. 5a shows an abrupt

breakdown voltage of -66.5 V without the presence of

tunneling current despite a peak breakdown electric field

exceeding 1000 kV/cm.

It is important to estimate the primary photocurrent to

calculate the quantum efficiency at unity gain and also the gain

of the APD. Plimmer et al. [18] showed the local model of

impact ionization is able to predict the gain of a rapidly varying

electric field and using the reported ionization coefficients [7],

the gain (M) of the SAM-APD is simulated in Fig. 5b. The

predicted breakdown voltage obtained by extrapolating 1/M(V)

to zero agrees well with the experimental breakdown voltage

value.

Fig. 5b shows the responsivity of the same device with 480

nm illumination as a function of reverse bias. Using the gain

predicted by the local model, the modeled responsivity-voltage

(R-V) curve shows good agreement with the experimental

results and a gain of 167 is obtained at -65.9 V. The dark

current density of 5 nA cm-2

in our devices at a gain of 100 was

limited by the measurement system noise floor and compares

very favorably with the performance of previously grown

AlInP PIN [3] at low bias, an AlInP PIN (95% of breakdown

voltage) [7], a commercial gallium phosphide photodiode (< 70

nA cm-2

at 95% of breakdown voltage) [19] and a good

commercial silicon APD (28 nA cm-2

at a gain of 100) [20].

Fig. 6 shows that the spectral response of the SAM-APD

obtained in the same manner as for the PIN diodes is

independent of applied voltage up to near breakdown, giving a

peak response wavelength at 482 nm with a FWHM of 22 nm,

which is narrower than that of previous work [3] and a

Hamamatsu S5973-02 silicon photodiode [21]. Using the

doping densities and thicknesses extracted from the earlier C-V

measurements, the simulated spectral response has a good

agreement with experimental data at 0V. Excellent agreement

at higher biases up to -65.9 V can also be obtained by including

the effect of avalanche multiplication as shown in Fig. 5b. The

measured responsivity and external quantum efficiency at 483

nm is 0.15 A/W and 38 % respectively at the device

Fig. 4b. Simulated electric field profiles of design (solid line) and grown

(dashed line) structures at breakdown voltage.

Fig. 4a. Modelled C-V of designed (solid line) and grown (dashed line) 200

µm devices. Symbols are experimental obtained data, assuming uniform

doping densities in all regions.




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punch-through voltage of -45 V. This relatively low value of

responsivity is partly due to the lack of an anti-reflection

coating but primarily due to the poor diffusion length of

minority carriers in the p+ AlInP layer. A peak responsivity of

18.0 A/W is achieved at -65.9 V without affecting the FWHM

and it is two orders of magnitude higher than the previous work

[3] due to the presence of avalanche gain.

IV. CONCLUSION

We have demonstrated the successful growth of a highly

sensitive homo-junction AlInP SAM-APD with extremely low

dark current, narrow spectral response FWHM and peak

response near to the desired wavelength of 480 nm. Although

the charge sheet thickness widens due to the dopant diffusion,

the responsivity and dark current of the devices are not affected.

Using the absorption coefficients and ionization coefficients

derived from the PINs, the spectral response and multiplication

of the APD is accurately predicted. This material system

therefore may form the basis of detectors for underwater

communication systems.

ACKNOWLEDGMENT

The author would like to thank UK MOD S&T Programme

for funding this work through the Futures and Innovation

Domain of the Defence Science and Technology Laboratory

(DSTL).

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Fig. 5a. Dark current of an AlInP SAM-APD device and the measured photocurrent illuminated with 480 nm of light.

Fig. 5b. R-V (solid line) of an AlInP SAM-APD illuminated with 480 nm of light together with simulated responsivity (circle) using the gain obtained

from local model (dashed line).

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pdf.

Jeng Shiuh Cheong received the B.Eng degree in electrical

engineering and electronics from University of Tenaga

Nasional, Kajang, Malaysia, in 2011. He is currently

working toward the Ph.D. degree in electronic and electrical

engineering at the University of Sheffield, Sheffield, U.K.

His research is focused on experimental and theoretical

investigation of impact ionization in avalanche photodiodes

for underwater application.

Siok Lan Ong received her Ph.D degree from the University

of Sheffield, Sheffield, U.K. in 2013 in Electronic and

Electrical Engineering on the subject of impact ionization on

AlInP. She is presently a senior lecturer at University of

Malaysia Perlis, Arau, Malaysia.

Jo Shien Ng (M’99) received the B.Eng and Ph.D degrees

from University of Sheffield, Sheffield, U.K. in 1999 and

2003, respectively, in electronic and electrical engineering.

From 2003 and 2006, she was with the University of Sheffield

and was responsible for characterization within the National

Centre for III-V Technologies.

She became a Royal Society University Research Fellow in October 2006,

and her research interests include X-ray detectors, avalanche photodiodes,

Geiger-mode avalanche photodiodes, and material characterization.

Andrey B. Krysa graduated from the Moscow Engineering Physics Institute, Moscow, Russia, in 1990. He received the

Ph.D. degree in solid state physics from the Lebedev Physical

Institute of Russian Academy of Sciences, Moscow, in 1997. He joined the EPSRC National Centre for III–V

Technologies, University of Sheffield, Sheffield, U.K., in 2001.

Since that, he has been engaged in the MOVPE of the Group III phosphides and arsenides.

John P. R. David (SM’96–F’12) received the B.Eng. and

Ph.D. degrees in Electronic Engineering from The University of Sheffield, Sheffield, U.K.

In 1985, he joined the Central Facility for III-V

Semiconductors, Sheffield, where he was responsible for the characterization activity. From 2001-2002, he was with

Marconi Optical Components (now Bookham

Technologies). He is currently a Professor in the Department of Electronic and Electrical Engineering, The University of Sheffield and was

the Head of Department from 2009-2013. Prof. David was an IEEE Lasers and

Electro Optics Society Distinguished Lecturer from 2002 to 2004. His current research interests include III-V semiconductor characterisation and impact

ionization in analog and single photon avalanche photodiodes.


http://scitation.aip.org/content/aapt/journal/ajp/61/7/10.1119/1.17173


http://prb.aps.org/abstract/PRB/v51/i24/p17660_1



http://iopscience.iop.org/0268-1242/15/7/307

http://www.hamamatsu.com/resources/pdf/ssd/s5971_etc_kpin1025e07.pdf

http://www.hamamatsu.com/resources/pdf/ssd/s5971_etc_kpin1025e07.pdf

al$_{\bf 0.52}$in$_{\bf 0.48}$p sam-apd as a blue-green detector

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