New Ga-Free InAs/InAsSb Superlattice Infrared Photodetector

J-P. Perez

1,2

, Q. Durlin

1,2

, C. Cervera

and P. Christol

1,2

Univ. Montpellier, IES, UMR 5214, F- 34000, Montpellier, France

CNRS, IES, UMR 5214, F- 34000, Montpellier, France

CEA-LETI, MINATEC Campus, 17 rue des Martyrs, F- 38054, Grenoble, France

Keywords: Infrared Photodetector, Type II Superlattice, Molecular Beam Epitaxy.

Abstract: We studied Ga-free InAs/InAsSb type-II superlattice (T2SL) in terms of period, thickness and antimony

composition as a photon absorbing active layer (AL) of a suitable XBn structure for full mid-wavelength

infrared domain (MWIR, 3-5µm) detection. The SL photodetector structures were fabricated by molecular

beam epitaxy (MBE) on n-type GaSb substrate and exhibited cut-off wavelength between 5µm and 5.5µm

at 150K. Electro-optical and electrical results of the device are reported and compared to the usual InSb

MWIR photodiode.

1 INTRODUCTION

Recently, InAsSb-based XB

n photovoltaic devices

(called bariodes) (P. Klipstein et al., 2011) and

lattice-matched to GaSb substrate have reached

impressively low dark current allowing temperature

operation as high as 150K

and cut-off wavelength

around 4.2µm. In this notation, "X" stands for the n-

or p-type contact layer, "B", for the n-type, wide

bandgap, barrier layer, and "n", for the n-type,

narrow bandgap, active layer. Such IR

photodetectors called HOT (High Operating

Temperature) detectors have been developed to

answer new needs like the compactness and the

reduction of cryopower which are key features for

the SWaP (Size Weight and Power) requirements

(A. Manissadjian et al., 2012). Nevertheless, only

the [3-4.2µm] part of the MWIR [3-5µm] domain is

addressed in that case.

Nevertheless, according to Planck's law and

considering a blackbody at 300K without any IR

system or transparency windows considerations, the

power emitted per unit area at the surface of the

blackbody in the [3-4.2µm] range represents only

18% of the total power in the [3-5 µm] range.

Therefore, taking into account the full MWIR

transparency window would significantly improve the

IR signal to noise ratio and finally the IR imaging

performances (Y. Reibel et al., 2015). Consequently,

there is an obvious need to extend the operational

wavelength of the XBn InAsSb HOT detector.

In that way, one can consider a type-II

InAs/GaSb superlattice (T2SL) on GaSb substrate

(R. Taalat et al., 2014). Unfortunately, such T2SL

devices are penalized by a low minority carrier

lifetime (around 100 ns in the MWIR) due to the

presence of Ga-related native defects in the SL

period (S.P. Svensson et al., 2011) leading typically

to a temperature operation lower than 110K for a

5μm cut-off (G. Chen et al., 2015). An extended cut-

off was achieved recently by using an InAsSb bulk

absorber material with a antimony content higher

than the one lattice-matched to GaSb, leading to a

cut-off wavelength higher than 5µm. This was

possible using a 1.5µm thick AlSb buffer layer (N.

Baril et al., 2017). An alternative to the previously

mentioned InAs/GaSb T2SL could be the Ga-free

InAs/InAsSb T2SL highlighting carrier lifetime

value as long as 9µs at 80K in the MWIR (B.V.

Olson et al., 2012). Moreover, results on first Ga-

free T2SL MWIR detectors have recently been

reported by US research groups (A. Haddadi et al.,

2015; D. R. Rhiger et al., 2016). Therefore, the

purpose of our work is to combine the XBn design

with a Ga-free InAs/InAsSb SL absorbing layer.

In this paper, InAs/InAsSb SL grown by

molecular beam epitaxy (MBE) is first studied.

Choices of superlattice period and antimony

composition (x

) of the InAsSb ternary alloy to

obtain high absorption in the full MWIR domain are

presented. MBE growth conditions to achieve strain-

balanced InAs/InAsSb SL structure on GaSb

232

Perez, J-P., Durlin, Q., Cervera, C. and Christol, P.

New Ga-Free InAs/InAsSb Superlattice Infrared Photodetector .

DOI: 10.5220/0006634002320237

In Proceedings of the 6th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2018), pages 232-237

ISBN: 978-989-758-286-8

substrate are then detailed. Next, optical

characterizations such as photoluminescence and

minority carrier lifetime lifetime performed on

dedicated structures, are reported. Finally, we report

electro-optical and electrical measurements of such

Ga-free SL structure in XBn

configuration.

2 CHOICE OF THE GA-FREE SL

PERIOD

InAs/InAs

1-x

SL can be strained balanced on

GaSb by setting the average lattice parameter of one

period of the SL equal to the lattice parameter of

GaSb. It follows that InAsSb and InAs layer

thicknesses (t

InAsSb

and t

InAs

) as function of the

antimony composition (x

) and SL period (P) can

be calculated by using Eqs(1) and (2):

0.09

GaSb InAs

InAsSb

InSb InAs Sb Sb

a a x x



   



(1)

InAs InAsSb

t t P

(2)

where a

GaSb

= 6.0954 Å ; a

InAs

= 6.0584 Å and a

InSb

=6.4794 Å are the lattice parameters of the binary

compounds.

Figure 1: Band structure simulation of an InAs QW in

InAs

0.65

0.35

strain balanced on GaSb substrate with

periodic boundary conditions at 150 K. The dotted lines

are the 1st quantized energy levels of the electron confined

in the InAs layer and the heavy hole in InAsSb. Ψ

is also

plotted so that the zero of probability coincides with the

corresponding quantized energy level.

Assuming a type II-b InAs/InAsSb band offset (D.

Lackner et al., 2012) with electrons confined in the

InAs layer and holes confined in the InAsSb one

(Fig. 1), the quantized miniband energies of the

strain balanced InAs/InAs

1-x

T2SL were

calculated with nextnano

commercial software (See

http://www.nextnano.de/nextnano3/).

At T=150K, for x

varying from 0.25 to 0.4 and

P varying from 4 nm to 8 nm, cut-off wavelength

(λ

or λ

VH1-C1

) corresponding to ground heavy hole

VH1 to conduction C1 interminiband energetic

transition, is plotted on Fig. 2. For each case, wave

functions overlap |<Ψe

|Ψhh

values calculated

for each fundamental VH1-C1 transition are also

specified.

Figure 2: Strain balanced InAs/InAs

1-x

T2SL on GaSb

substrate: Calculated cut-off wavelength at T = 150K and

associated wave functions overlap of the ground VH1-C1

miniband transition as a function of Sb concentration (x

)

and for different period thicknesses (P).

Consequently, to reach full MWIR detection, an

InAs/InAs

1-x

SL structure with x

= 0.35 and 5 ≤

P(nm) ≤ 6 could be of interest since numerical

simulations predict a cut-off wavelength between

4.8μm and 5.2μm and a wave functions overlap

between 55% and 66% (square with dashed lines

Fig. 2) at 150K.

3 MBE GROWTH

Ga-free InAs/InAsSb strain balanced SL lattice

matched to GaSb have been grown on n-type GaSb

Te-doped (100) substrates by solid source MBE

equipped with valved crackers set up to produce As

and Sb

species. Following the thermal oxide

desorption, a 400 nm-thick Te-doped GaSb buffer

layer was grown before the structure made of a 3μm

thick superlattice region composed of alternating

InAs(4.1 nm)/InAsSb(1.4 nm) non intentionally

doped (nid) layers and finally caped by a 150 nm-

thick n-doped Gasb layer (Fig. 3). Due to the well-

known competition for incorporation between

Arsenic (As) and Antimony (Sb) in the InAsSb

New Ga-Free InAs/InAsSb Superlattice Infrared Photodetector

233

layer, several test samples have been grown to find

accurate speed growth of these species leading to

lattice-matched structure on GaSb with respect to x

= 0.35 in InAsSb layers.

Figure 3: InAs/InAs

1-x

T2SL sample cross-section.

The corresponding high-resolution X-ray diffraction

(XRD) spectrum shown in Fig. 4 exhibits many

intense satellite peaks with a full-width at half-

Figure 4: High-resolution XRD spectrum (004) reflection

of InAs/InAs

0.65

0.35

T2SL sample (P = 5.5nm) lattice-

matched to the GaSb substrate.

maximum (FWHM) of the ﬁrst-order peak (SL0)

equals to 44 arcsec, attesting the good crystalline

quality of the layers. The targeted SL period value

(P = 5.5nm with layer thicknesses tInAs = 4.1 nm and

tInAsSb = 1.4nm calculated from Eqs (1) and (2)) is

confirmed by the simulated curve that matches the

experimental one.

In addition, the lattice mismatch calculated using

(T. Schuler-Sandy et al., 2015):

sin

sin ( )

substrate











(3)

where θ

substrate

is the angle in degrees of the substrate

peak measured by XRD and Δθ is the angular

difference between the substrate peak and the

epitaxial peak (the 0

order SL peak in this case) in

degrees is less than 300ppm.

The surface morphology has also been observed by

atomic force microscopy (AFM) on a 5x5μm

scan

area highlighting well-defined atomic steps (Fig. 5),

and measured root-mean-square (RMS) surface

roughness only equals to 0.15nm (that is, less than

one monolayer in the case of Sb-based materials).

Figure 5: AFM scan of InAs/In As

0.65

0.35

T2SL sample.

Clear monolayer steps can be observed.

4 OPTICAL

CHARACTERIZATIONS

The Ga-free InAs/InAsSb SL structure fabricated

could be the active zone of a XBn MWIR

photodetector. Such a device is designed to be

diffusion-current

limited. According to Eq. 4, the

diffusion-current is inversely proportional to the

minority carrier lifetime  and the carrier

concentration Nd:

d diff





(4)

where q is the electrical charge, ni the intrinsic

carrier concentration and Ldiff the minority carrier

diffusion length. As a consequence, determination of

 will give a trend on the expected dark current.

On top of this, it’s necessary to ensure that the SL

structure is suitable for the full MWIR domain. For

this purpose, photoluminescence (PL) measurements

allow to reach the bandgap energy of the structure,

which will correspond to the 50% cut-off energy of

the photodetector spectral response (see section 5.2).

4.1 Photoluminescence

Samples are placed in a cryostat allowing accurate

control of the temperature from 10K to 300K and are

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

234

optically excited with a 50W/cm

power density

from a 784nm laser diode modulated at 133kHz

through a CaF

window. The luminescence signal is

analyzed with a Nexus 870 FT-IR system equipped

with a MCT detector (12µm cut-off wavelength).

Figure 6: PL spectra at T = 80K of a P = 6nm

InAs/InAs

0.65

0.35

SL sample.

Fig. 6 shows the normalized PL signal at 80K of a

InAs/InAs

0.65

0.35

SL structure with a 6 nm period.

The PL peak position, observed at wavelength equal

to 4.7µm at 80K, is in agreement with the calculated

fundamental valence to conduction interminiband

transition (Fig. 1). This result confirms the choice of

a T2SL period close to 6nm.

4.2 Photoluminescence Decay

To reach the minority carrier lifetime ,

photoluminescence decay (PLD) measurements have

been performed on structure presented in Fig. 3.

Among contactless techniques, this method remains

one of the simplest and most straightforward. The

experimental set-up used for PLD measurements is

the one described by Delacourt et al. (B. Delacourt

et al., 2017). Since excess carriers are generated by a

pulsed Erbium-doped fiber laser at a wavelength of

1545 nm, it’s important to note that, at this

wavelength, the light is absorbed by the active layer

while the heavily doped buffer and cap layers are

transparents. The PLD signals measured at 80K and

150K, for the same level of injection, are reported in

Fig. 7.

The time-resolved signal is then fitted by a least

squares Levenberg–Marquardt method

(B. Delacourt

et al., 2017) to estimate the contributions from

Auger (

Auger

), SRH (

SRH

) and radiative (

rad

)

recombinations to the total carrier lifetime, since:

1 1 1 1

()

Auger SRH rad

   

   

  

(5)

Figure 7: PLD signals of Ga-free sample at T=80K and

T=150K.

Values of 565ns and 605ns have been

respectively extracted at T=150K and T=80K,

highlighting a minority carrier lifetime for a Ga-free

InAs/InAs

0.65

0.35

SL around five times higher than

the InAs/GaSb SL’s one. It is to note here that

such values may certainly be improved since no

attempts to reach optimal growth conditions have

been performed. Moreover, investigation of the

different mechanisms (not presented in this work)

showed that the lifetime was clearly SRH limited.

5 GA-FREE SL IN XBN

CONFIGURATION

The XBn structure is composed of a n-type

absorbing layer (AL), an unipolar barrier layer (BL)

and a contact layer (CL). The objective of BL is to

block majority carrier (electrons) while allowing

collection of minority carrier (holes). When properly

designed, the use of a wide bandgap BL enables

device operation limited by diffusion current

5.1 XBn Architecture

The MWIR XBn detector is made of 3µm-thick Ga-

free InAs/InAs

0.65

0.35

AL, 80nm-thick n-doped

AlAs

0.09

0.91

BL and a 150nm-thick n+-doped

InAs

0.91

0.09

CL (Fig. 8).

XRD scan of the complete detector structure is

shown in Fig. 9. No angular difference Δθ~0 (Eq.3)

was detected between the substrate peak and the 0

order AL, indicating lattice-matching of this layer.

In contrast, the BL is in compressive strain with a

lattice mismatch around 2200ppm and a 94.5% Sb

composition in the AlAsSb ternary alloy. Anyway,

with a thickness of only 80nm, no relaxation occurs

in this layer. In addition, from satellite peak

New Ga-Free InAs/InAsSb Superlattice Infrared Photodetector

235

positions (SL-3, SL-2,.., SL+3) we can estimate the

period of the AL : P=5.3nm in that case.

Figure 8: Schematic cross-section of the XBn

photodetector

Figure 9: XRD spectrum (004) reflection of the complete

XBn structure (P = 5.3nm).

5.2 Experimental Results and Analysis

Standard optical photolithography was used to

define detector mesas varying in size from 60 μm

down to 310 μm in diameter. The mesas were etched

down to the GaSb layer with a citric acid/H2O2 based

etch solution (O. Dier et al., 2004). The diodes were

passivated by a thin SiO2 dielectric layer deposited

by plasma enhanced chemical vapor deposition

(PECVD). Dark current measurements (Fig. 10) as a

function of bias and temperature were then

conducted under vacuum within a liquid nitrogen

cooled Dewar.

The Ga-free SL XBn detectors operate under

negative bias voltages (negative voltage on the top

contact). The bias operation is extracted at -2.5V.

This too high value, necessary to allow the transport

of holes (minority carriers) from the AL to CL

through the BL clearly indicates that the valence

band alignment between the BL and AL is not

Figure 10: Dark current density vs bias at temperatures

between 106K and 199K for a 310μm diameter device.

Figure 11: Normalized spectral response measured at

different temperatures for a 310μm diameter device at

bias

=-2.5V.

optimized. Normalized spectral response

characteristics measured at different temperatures

for a 310μm diameter device at bias operation V

2.5V are shown in Fig. 11.

From these measurements, we extract the cutoff

wavelength λ

= 4.6µm. This value, a bit lower than

the expected one, may probably be explained

because of a too short period of the SL (5.3nm

obtained vs 5.5 nm targeted).

6 PERFORMANCE

COMPARISON

From data presented in Fig. 10, we have plotted on a

Arrhenius graph (Fig. 12) the corresponding values

of the dark current density at V

bias

=-2.5V (red

PHOTOPTICS 2018 - 6th International Conference on Photonics, Optics and Laser Technology

236

circles) as a function of inverse temperature. In

addition, to compare the performances of our Ga-

free XBn device, we have also plotted in Fig. 12 the

data at bias operation of three different types of

InSb-based photodetectors (J. P. Perez et al., 2015)

(MWIR broadband detectors):

- InSb pn junction fabricated by standard planar

process,

- InSb pin junction fabricated by MBE,

- InSb nBn structure fabricated by MBE (A.

Evirgen et al., 2014).

The red line in Fig. 12 corresponds to the

diffusion regime of XBn structure. Consequently it

clearly appears that below 180K, our device is not

diffusion limited. Probably, due to its valence band

offset with the AL, AlAsSb is not the most

appropriate material for the BL. This is evidenced by

the large value bias operation we mentioned

previously in section 5.2. As a consequence, the

strong bias to be applied on our device to allow

holes (minority carriers) collection certainly leads to

tunnelling current contribution.

In Fig. 12, the horizontal dashed line indicates

the typical photonic current produced in the 3-5μm

band for f/3 optics by a III-V detector system with a

quantum efficiency = 80 % (M. A. Kinch et al.,

2010). On top of this, taking into account the criteria

according to which a high performance MWIR

detector must have a dark current density two

decades lower than its photocurrent (P. C. Klipstein

et al., 2004)), we also have reported in Fig. 12

the corresponding value (horizontal solid line):

6 10

-7

A/cm

Figure 12: Comparative Arrhenius plot of Ga-free SL with

three different types of MWIR broadband InSb-based

photodetectors.

Consequently, an optimized Ga-free SL structure in

XBn configuration should be able to operate at

temperature around 135K-140K, that is, a higher

temperature than InSb photodiode’s one operating in

the full MWIR spectral range.

ACKNOWLEDGEMENTS

Part of this work was supported by the French

“Investment for the future” program (Equipex

EXTRA ANR11-EQPX-0016).

REFERENCES

P. Klipstein et al., Opt Engin. 50, 061002 (2011).

A. Manissadjian et al., Proc. SPIE 8353, 835334 (2012).

Y. Reibel et al., Proc. SPIE 9451, 945110 (2015).

R. Taalat et al., J. Phys. D, Appl. Phys. 47 015101 (2014).

S. P. Svensson et al., J. Cryst. Gr., 334,103 (2011).

G. Chen et al., Opt. Lett. 40, 45 (2015).

N. Baril et al., Proc. of SPIE Vol. 10177 101771L-1,

(2017).

B. V. Olson et al., Appl. Phy. Lett. 101, 092109 (2012).

A. Haddadi et al., Appl. Phys. Lett. 106, 011104 (2015).

D. R. Rhiger et al., J. Electron. Mater. 45, 4646 (2016).

D. Lackner et al., J. Appl. Phy. 111, 034507 (2012).

See http://www.nextnano.de/nextnano3/ for more

information about the nextnano

software.

T. Schuler-Sandy et al., J. Cryst. Gr., 425, 29 (2015).

B. Delacourt et al., J. Elec.Mat. (2017) https://doi.org/

10.1007/s11664-017-5728-x.

O. Dier et al., Semicond. Sci. Technol., 19, 1250-1253,

(2004).

J. P. Perez et al., Proc. SPIE 9370 93700N-93700N-7

(2015)

A. Evirgen et al., Elec. Lett., 50, 20, 1472-1473, (2014)

M. A. Kinch et al., Proc. of SPIE, 7660, 76602V-1,

(2010).

P. C. Klipstein et al., Proc. of SPIE, 5406, 222-229 (2004).

New Ga-Free InAs/InAsSb Superlattice Infrared Photodetector

237