Antimonide-based Superlattice Infrared Barrier Photodetectors

U. Zavala-Moran

, R. Alchaar

, J. P. Perez

, J. B. Rodriguez

, M. Bouschet

, V. H. Compean

F. de Anda

and P. Christol

IICO, UASLP, San Luis Potosí, Mexico

IES, Univ. Montpellier, CNRS, Montpellier, France

Keywords: Infrared Quantum Detector, Barrier Structure, Sb-based Heterostructure, Type-II Superlattice.

Abstract: Barrier structures are now the design of high performance antimonide-based (Sb-based) cooled infrared (IR)

quantum detectors. In this communication, we report on electrical and electro-optical characterizations of Ga-

free and Ga-containing type-II superlattices (T2SL) photodetectors structures grown by Molecular Beam

Epitaxy (MBE). Experimental measurements on Ga-free XBn and Ga-containing XBp samples were made by

photo-response measurements and dark current density-voltage (J-V) characteristics performed on detectors

as a function of temperature. Identification of bias voltage characteristics were extracted from measurements

and resulting dark current values were compared to the state of the art of infrared technology.

1 INTRODUCTION

In order to reduce the dark current, and therefore the

noise of the device, the first infrared (IR) barrier

detector was proposed by A. White in 1983 (White,

1983). He proposed an architecture device composed

of isotype (n-type) heterostructures where a high band

gap energy barrier layer was placed between the

absorbing and contact layers of smaller gap materials.

As the objective of the barrier layer is to block the

majority carriers (in this case electrons) while

allowing the photogenerated minority carriers (holes)

to reach the contact layer, he called this device "high

impedance photoconductor". However, at that time,

obtaining a very low or zero valence band offset was

difficult to achieve with the main IR detection

materials based on InSb and HgCdTe (MCT).

Everything changed in the mid-2000s with the

demonstration of the InAs-based barrier structure by

Maimon and Wicks (Maimon, 2006) and then

accelerated with the used of antimonide-based (Sb-

based) materials, such as InAsSb (Klipstein, 2011)

and type II superlattice (T2SL) structures lattice

matched to GaSb substrate (Klipstein, 2015), to build

IR barrier photodetectors.

Barrier structures are now the standard for the

design of Sb-based cooled IR photodetectors. Such

high-performance detector structure, which replaced

the usual pin photodiode, is made of n-type or p-type

photon absorbing layer, a barrier layer B and a contact

layer X that can be made from either the same, or a

different material, to that used for the photon

absorbing layer. Consequently, these quantum

detectors are named XBn or XBp structures, also

called bariodes.

Specialist of IR photonic Sb-based devices

fabricated by Molecular Beam Epitaxy (MBE) on

GaSb substrate, the Institute of Electronic and

Systems (IES) of the University of Montpellier

(France) has studied T2SL quantum detectors

dedicated to IR spectral domain since several years

(Rodriguez, 2005, Jaworowicz, 2010, Delmas, 2014).

This paper reports electro-optical and electrical

experimental measurements performed on Ga-free

InAs/InAsSb and Ga-containing InAs/GaSb T2SL

structures, grown by MBE, in XBn and XBp

photodetector configurations, respectively.

2 T2SL XBN AND T2SL XBP

BARRIER STRUCTURES

2.1 Advantages of Barrier Structure

To understand the advantages of the barrier detector,

is necessary to compare it with the pn junction

photodiode.

In the pn junction structure with reverse bias

applied, the presence of the space charge zone on

either side of the p/n interface blocks the transport of

Zavala-Moran, U., Alchaar, R., Perez, J., Rodriguez, J., Bouschet, M., Compean, V., de Anda, F. and Christol, P.

Antimonide-based Superlattice Infrared Barrier Photodetectors.

DOI: 10.5220/0009004900450051

In Proceedings of the 8th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2020), pages 45-51

ISBN: 978-989-758-401-5; ISSN: 2184-4364

the majority carriers and allows the transfer of the

minority ones. However, this zone with electric field

generates Shockley-Read-Hall (SRH) recombination

darkness current, thus generation-recombination

(GR) current, which may be important in the IR

spectral domain. Dark current of IR photodiodes,

such as InSb, is dominated by SRH current at its

operating temperature (~80K).

The main objective of the XBn or XBp barrier

structure is to reduce the contribution of the SRH

current to the dark current of the detector. For that, it

is required to control the electric field zone by

confining it in the high band gap barrier material

instead of in the absorbing zone structure. Then, SRH

processes occur in the high band gap material, not in

the IR absorbing layer. In addition, the barrier layer

plays the same role as the space charge zone in the pn

structure as blocking the majority carriers and

allowing the flow of the minority ones.

Consequently, when the barrier detector structure

is correctly designed, GR dark current is strongly

reduced, even eliminated, and the performances of the

photodetector are improved (Klipstein, 2015).

Figure 1 shows the possible XBn and XBp

designs.

Figure 1: Schematic view of XBn (a) and XBp (b)

structures. In each case, the contact layer (X) is on the left

and the IR absorbing layer (n or p) is on the right (according

to (Klipstein, 2015)).

2.2 T2SL Structures

The type-II superlattice (T2SL) was proposed at the

end of the 80's as a material for IR detection (Smith,

1987).

A superlattice is a stack of periodic layers whose

thickness is of the order of a few nanometers. It is an

artificial quantum structure composed of coupled

multi-quantum wells. Carriers (electrons and holes)

are then confined into energy minibands and optical

absorption occurs between two of these minibands.

In the case of Sb-based materials, such quantum

structure presents particular type-II band alignment

where the carriers are confined in the adjacent

materials (Figure 2) and, depending on the thickness

layers, the fundamental valence to conduction (V1-

C1) interminiband absorption (detector cut-off

wavelength λ

) can address all the thermal IR, from 3

to 30μm (Wei, 2004).

Figure 2 shows a Ga-containing InAs/GaSb T2SL

fundamental absorption spectrum at T= 300K.

Figure 2: T2SL absorption coefficient extracted from room

temperature absorption spectrum. In the inset, band

diagram and probability densities of fundamental C1 and

V1 levels of first electron and heavy hole (hh) minibands

are reported.

3 T2SL XBN AND T2SL XBP

DETECTORS: FABRICATION

AND CHARACTERIZATION

Lattice-matched to the GaSb substrate, the absorbing

zone is composed of strain compensated Ga-free

InAs/InAsSb or Ga-containing InAs/GaSb T2SL,

operating in the MWIR (midwave infrared 3-5µm) to

LWIR (longwave infrared 8-12µm) spectral domains,

respectively.

0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60

100

1000

Wavelength (µm)

I<f

= 60%

GaSb

InAs

First electron

miniband

First hole

miniband

absorption coefficient (cm

-1

)

Energie (eV)

T = 300K

Absorption

T2SL structure

6.5 6 5.5 5 4.5 4 3.5 3 2.5

PHOTOPTICS 2020 - 8th International Conference on Photonics, Optics and Laser Technology

3.1 Ga-free T2SL XBn Photodetector

The XBn detector operating in the MWIR domain is

made of 1µm-thick non-intentionally doped (nid) n-

type Ga-free InAs/InAs

0.69

0.31

T2SL absorbing layer,

80nm-thick p-doped AlAs

0.09

0.91

barrier layer and

100nm-thick n

-doped InAs/InAs

0.69

0.31

T2SL

contact layer (Figure 3). Equivalent Ga-free structure

has recently been fabricated on Si substrate (Durlin,

2019)

Figure 3: Schematic cross-section of the Ga-free T2SL XBn

photodetector on GaSb substrate.

Figure 4 displays the high-resolution x-ray

diffraction (HRXRD) spectrum of the T2SL XBn

structure grown by MBE on 2” (100) GaSb substrate.

The presence of numerous and intense satellite peaks

is a signature of the crystallographic structure’s quality

and their positions allow to calculate the InAs/InAsSb

period thickness of the T2SL stacking equal to 4.40

nm.

26 27 28 29 30 31 32 33 34 35

Intensity (cts/s)

Ω/2Θ(°)

T2SL period = 4.40 nm

Figure 4: High-resolution X-ray diffraction patterns of Ga-

free T2SL InAs (3.30nm)/InAs

0.69

0.31

(1.10nm) structure

on GaSb substrate.

From epitaxial SL material, circular mesa

photodiodes were fabricated using standard

photolithography with a mask set containing diodes

and photodiodes with several diameters, from 60µm up

to 310µm (figure 5). Mesa photodiodes were realized

by wet etching using citric acid solution and

polymerized photoresist was used to protect the mesa

surface from ambient air. Metallization were ensured

on the n-GaSb substrate and on the n-type T2SL cap

layer. Next, the devices are mounted in a cryogenic

probe station to perform dark current density-voltage

(J-V) measurements.

Figure 5: Top view of a processed sample, blind diodes (C)

and photodiodes (P) with several diameters from 60µm up to

310µm.

Figure 6 shows the J-V curves at temperatures

ranging from 77 to 250K, performed on a detector with

a 210 µm-diameter. At low temperature, until 110K,

the dark current is limited by the photonic current due

to the experimental set-up (probe station).

The shapes of the dark J(V) characteristics are in

accordance with the those obtained with XBn Ga-free

MWIR diodes, reported elsewhere (Baril, 2016 ; Ting,

2018).

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

-10

-9

-8

-7

-6

-5

-4

-3

Density current (A/cm

)

Bias (V)

77K

90K

110K

120K

130K

135K

140K

145K

150K

155K

160K

165K

170K

180K

190K

200K

220K

250K

Ga-free T2SL XBn MWIR detector

residual photonic current

Figure 6: Dark current density-voltage (J-V) characteristics

at different temperatures, from 77 to 250K, of a Ga-free

T2SL XBn MWIR detector.

Antimonide-based Superlattice Infrared Barrier Photodetectors

In the forward positive bias, the low dark current

is due to the thin n-type top contact layer compared to

the absorbing layer (100 nm vs 1µm). A slight

negative bias applied to the top contact layer

(typically - 200mV) is necessary for the device

operation.

At temperature lower than 180K, the slope of the

dark currents plots between -0.1V and -0.4V shows

the presence of GR current, indicating that the device

design must be optimised. At higher temperature, one

can see that the slope is reduced as diffusion current

becomes dominant in the dark current.

At the expected operation temperature equal to

150K, dark current density as low as 2x10

-6

A/cm

, at

typical bias operation equal to -200mV, is extracted.

Lower dark current could be obtained by a better

control of targeted doping values of MBE grown

layers.

The samples were then wire bounded onto a 68

pin LCC and placed in a liquid nitrogen cryostat in

order to perform non-calibrated photoresponse (PR)

measurements by using a Nicolet-870 Nexus Fourier

transform infrared (FTIR) spectrometer with non-

calibrated IR source. Figure 7 shows the front side

illuminated uncalibrated PR spectra obtained at

different biases and at T = 150K. One can observe a

50% cut-off wavelength at 4.2µm and that the

collected signal begins to saturate at bias applied

equal to -220mV.

2.5 3.0 3.5 4.0 4.5 5.0

Ga-free T2SL XBn MWIR detector

Uncalibrated photoresponse spectra (a.u)

Wavelength(µm)

-1.5V

-1V

-800mV

-500mV

-400mV

-350mV

-300mV

-280mV

-260mV

-240mV

-220mV

-200mV

-180mV

-160mV

-140mV

-120mV

-100mV

T = 150K

bias applied

Figure 7: Uncalibrated PR spectra recorded at T= 150K, for

different biases applied from -100 to 1500mV, of a Ga-free

T2SL XBn MWIR detector.

To explain the behaviour of the PR spectrum as a

function of bias applied, band diagrams of the XBn

device are calculated at T= 150K and reported in

0.00.51.01.5

-1

Energy (eV)

Position (µm)

T = 150K

bias

= 0V

Ga-free T2SL XBn MWIR detector

Barrier

Layer

Contact Layer

Absorbing Layer

0.0 0.5 1.0 1.5

-1

Contact Layer

Energy (eV)

Position (µm)

Ga-free T2SL XBn MWIR detector

T = 150K

bias

= -100mV

Barrier

Layer

Absorbing Layer

0.0 0.5 1.0 1.5

-1

Energie (eV)

Position (µm)

Ga-free T2SL XBn MWIR detector

T = 150K

bias

= -500mV

Absorbing Layer

Barrier

Layer

Contact

Layer

0.0 0.5 1.0 1.5

-1

Energie (eV)

Position (µm)

Ga-free T2SL XBn MWIR detector

T = 150K

bias

= -800mV

Absorbing Layer

Barrier

Layer

Contact

Layer

Figure 8: Calculated band diagrams using the ATLAS

software from SILVACO (Abautret, 2013) of the Ga-free

XBn barrier structure at 150K and 0V (a), -100mV (b), -

500mV (c), and -800mV (d).

PHOTOPTICS 2020 - 8th International Conference on Photonics, Optics and Laser Technology

Figure 8 for four typical bias voltages (V

bias

= 0V,

bias

= -100mV, V

bias

= -500mV, V

bias

= -800mV).

At V

bias

= 0V, the band diagram highlights the

presence of a potential barrier blocking the minority

heavy hole carriers. Even, at bias operation equal to -

100mV, this potential barrier remains, penalizing the

quantum efficiency (Figure 7).

At V

bias

= -500mV, the bias is high enough to

suppress the potential barrier allowing the transport

of hole minority carrier to the top contact layer. The

maximum of PR is collected.

3.2 Ga-containing T2SL XBp

photodetector

In this part, another kind of barrier structure is

considered. It is a Ga-containing InAs/GaSb T2SL

XBp detector operating in the LWIR domain.

To maximize the quantum efﬁciency, the Ga-

containing T2SL is p-type doping (Giard, 2014).

Consequently, the electrons are the minority carriers

and the barrier, made of InAs/AlSb T2SL (Delmas,

2017), is tailored to have a large offset in the valence

band and minimal offset in the conduction band,

therefore blocking majority holes and collecting

photo-generated electrons.

The XBp detector structure under consideration is

made of lightly p-type doped 3.2µm thick InAs/GaSb

T2SL absorbing layer, a heavily n-type doped wide

bandgap InAs/GaSb T2SL and a lightly n-type doped

InAs/AlSb barrier layer. Figure 9 shows the

calculated band diagram at 80K and under no bias.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Ga-containing T2SL XBp LWIR detector

InAs/GaSb T2SL

Absorbing Layer

InAs/AlSb T2SL

Barrier Layer

Energy (eV)

Position (µm)

Conduction band edge

Valence band edge

T= 80K

bias

= 0 V

InAs/GaSbT2SL

Contact Layer

Figure 9: Calculated band diagram of the Ga-containing

XBp barrier structure at 80K and 0V.

In addition of the photoluminescence (PL)

spectrum, figure 9 displays the non-calibrated photo-

response (PR) spectrum at V

bias

= -100mV for a

detector device having a 90 µm-diameter. The 50%

cut-off wavelength equal to 10.5µm for the spectral

PR corresponds to the PL peak, in agreement with the

targeted value suitable for LWIR applications. The

inset shows the PR measurements at different biases

and the PR starts to saturate after -50mV bias applied,

enough to reduce the possible conduction potential

barrier (Figure 9). Measured elsewhere (Höglund,

2018), the detector quantum efficiency (without anti-

reflection coating) reaches 35% at 80K. Figure 11a

reports normalized PR values extracted from the PR

measurements (inset Figure 11) at different biases and

at 10.5µm.

Dark current density-voltage measurement (J-V)

is also performed at 80K. J

dark

result is shown in

Figure 11b and the associated differential resistance

area product R

A is reported in Figure 11c (R

calculated from the derivative of the voltage over the

current and A is the device area).

7 8 9 1011121314151617

7 8 9 10 11 12 13 14

Photoresponse (a.u)

Wavelength (µm)

-25 mV

-50 mV

-75 mV

-100 mV

-150 mV

-200 mV

-300 mV

T=80K

Wavelength (µm)

PL intensity (a.u)

10.5 µm

Photoresponse (a.u)

T = 80K

Figure 10: PL and PR (at V

bias

= -100mV) measurements at

80K of a Ga-containing XBp T2SL detector device. The

inset shows the PR at different biases.

The shape of the curves displayed in Figure 11,

help us to identify the two main dark current regimes

of the LWIR detector. First, the operating bias V

.= -

100mV. At this bias, the device is fully turned on

(where R

A is at a maximum value), the diffusion

plateau is reached, and the PR value saturates. The

depletion region is still conﬁned within the barrier.

Next, the V

= -300 mV. At this bias, GR current

starts to dominate the dark current of the diode, the

barrier is fully depleted, and the depletion region

reaches the absorbing region.

From this analysis, it is possible to identify the

operating bias of the device, which is -100mV. At this

voltage, dark current density value as low as 8x10

-4

A/cm

and R

A product as high as 1160 Ω.cm

are

extracted at T = 80K.

Antimonide-based Superlattice Infrared Barrier Photodetectors

-3

-1

-5

-3

-1

0.0

0.2

0.4

0.6

0.8

1.0

1.2

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2

-0.2-0.4

-0.6

-0.8

-1.0

0.0

=-100 mVV

=-300 mV

0.2

Voltage (V)

A (Ω.cm²)

T=80K

dark

(A/cm²)

Normalized PR

T=80K

Figure 11: Experimental characterizations of a Ga-

containing XBp T2SL detector device. From top to bottom :

(a) normalized PR, (b) dark current density J

dark

, (c)

differential resistance area product (R

A) as a function of

the voltage at T = 80K. Two particular bias are identified :

The operating bias V

and the V

bias for which the G-R

current begins to dominate the dark current of the diode.

4 CONCLUSIONS

InAs/InAsSb Ga-free XBn T2SL MWIR and

InAs/GaSb Ga-containing XBp T2SL LWIR

quantum detectors have been fabricated and

characterized.

These photodetectors showed cut-off wavelength

around 4.2µm at 150K and 10.5µm at 80K,

respectively. Dark current density values, extracted

from J(V) measurements, are equal to 2x10

-6

A/cm

at 150K for the Ga-free device and 8x10

-4

A/cm

for

the Ga-containing one.

Compared to the state of the art (i.e; the Rule 07

(Tennant, 2008)), these values are from two to one

decades higher, respectively. Concerning the Ga-free

T2SL, such result is due to the presence of GR dark

current at operating bias. Improvement in the design

and on the control of doping layers during the MBE

growth are necessary to suppress this behaviour and

will be the subject of forthcoming studies. For the Ga-

containing T2SL, the origin of high dark current is

different. The current of the device is dominated by

diffusion current at operating bias but the

performance is penalized by the well-known poor

lifetime value of minority carriers in Ga-containing

T2SL (Svensson, 2011). A solution would be the

development of Ga-free T2SL quantum detector

structure dedicated for the LWIR spectral domain.

ACKNOWLEDGEMENTS

This work was partially funded by the French

“Investment for the Future” program (EquipEx

EXTRA, ANR 11-EQPX-0016), by the ESA contract

number 4000116260/16/NL/BJ and by the French

ANR under project HOT-MWIR (N° ANR-18-CE24-

0019-01).

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Antimonide-based Superlattice Infrared Barrier Photodetectors