Evaluating the size dependent quantum ef

Evolution of QE of the 57 lm gated photodiode with gate bias and diode operation bias reveals different surface recombination mechanisms.. Recently, gating technique demonstrated its cap

Evaluating the size-dependent quantum efficiency loss in a SiO2-Y2O3 hybrid gated

type-II InAs/GaSb long-infrared photodetector array

G Chen, A M Hoang, and M Razeghi

Citation: Applied Physics Letters 104, 103509 (2014); doi: 10.1063/1.4868486

View online: http://dx.doi.org/10.1063/1.4868486

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Evaluating the size-dependent quantum efficiency loss in a SiO2-Y2O3hybrid gated type-II InAs/GaSb long-infrared photodetector array

G Chen, A M Hoang, and M Razeghia)

Center for Quantum Devices, Department of Electrical Engineering and Computer Science, Northwestern

University, Evanston, Illinois 60208, USA

(Received 5 January 2014; accepted 25 February 2014; published online 14 March 2014)

Growing Y2O3 on 20 nm SiO2 to passivate a 11 lm 50% cut-off wavelength long-wavelength

infrared type-II superlattice gated photodetector array reduces its saturated gate bias (VG,sat) to7

V Size-dependent quantum efficiency (QE) losses are evaluated from 400 lm to 57 lm size gated

photodiode Evolution of QE of the 57 lm gated photodiode with gate bias and diode operation

bias reveals different surface recombination mechanisms At 77 K and VG,sat, the 57 lm gated

photodiode exhibits QE enhancement from 53% to 63%, and it has 1.2 105A/cm2dark current

density at200 mV, and a specific detectivity of 2.3  1012Jones.V C 2014 AIP Publishing LLC

[http://dx.doi.org/10.1063/1.4868486]

InAs/GaSb type-II superlattice (T2SL) has been making

rapid improvements,1 6but its performance has not reached

its theoretical limits7 because of surface leakage current,

which is more severe in the long-wavelength-infrared-regime

(LWIR) Although different passivation techniques have

been attempted,812 surface leakage still limits its

perform-ance Recently, gating technique demonstrated its capability

of eliminating surface leakage current,13,14 restoring

quan-tum efficiency (QE), and focal plane array (FPA)

applica-tion.15 To suppress the high saturated gate bias (VG,sat),

reducing dielectric layer thickness and applying high-k

dielectric, yttrium sesquioxide (Y2O3), were both attempted

but ultrathin dielectric layer is impractical for FPA

applica-tion Therefore, high-k dielectric becomes the only opapplica-tion

Thanks to its high dielectric constant, 600 nm thick Y2O3

passivated 100 lm pitch gated array was realized with 3

times lower VG,sat than the one passivated with SiO2.15

However, thinning Y2O3below 600 nm still resulted in high

gate leakage current and reduced the fabrication yield, which

made realization of small pitch, lowVG,satgated array

diffi-cult Therefore, improving the quality of Y2O3will be

im-portant for further suppressing the VG,sat and scaling down

pixel size Most importantly, the QE loss with different mesa

sizes has not been evaluated and the evolution of the QE

with gate bias (VG) and diode operation bias (VOP) can reveal

different surface recombination mechanisms, which are very

important information for further reducing pixel size for

higher resolution FPA In this Letter, we reported a method

of improving Y2O3quality to increase the fabrication yield,

evaluated the mesa size-dependent QE loss from LWIR

T2SL Pþ-p-M-Nþgated photodetector and revealed the

evo-lution of surface recombination withVGandVOP

Same structure material as Ref 15 was processed into

one die containing unpassivated diode (UPD) for reference,

17 dies of single gated photodetectors and one die of gated

photodetector arrays The 17 single gated photodetector

sam-ples contained both gated diode (GD) and ungated diode

(UGD), and differed from each other by passivation methods

and passivation layer thicknesses Sample S1 to S6 were pas-sivated with SiO2by plasma-enhanced chemical vapor depo-sition (PECVD), sample Y1 to Y6 were passivated with

Y2O3 by ion-beam sputtering deposition (IBD), and their thickness were reported in TableII Sample H1 to H5 used SiO2-Y2O3hybrid passivation by first depositing 20 nm thick SiO2 using PECVD and then depositing additional 50 nm,

100 nm, 200 nm, 300 nm, and 600 nm of Y2O3 using IBD The gated photodetector arrays contained square detector arrays with pixel size of 400 lm, 320 lm, 250 lm, 150 lm,

120 lm, 57 lm, and 37 lm 220 nm thick SiO2-Y2O3hybrid passivation layer was used for array fabrication The process-ing detail was reported in Ref.15

As shown in Figure1(a) and TableI, the dark current densities of UGD with SiO2, Y2O3, and SiO2-Y2O3hybrid passivation are similar and all better than UPDs The VG,sat

of each sample is plotted in Figure 1(b) and reported in Table II Samples passivated with Y2O3 (Y1 to Y6) show

2.7 times lower VG,sat than those using SiO2 (S1 to S6) Since the dielectric constant of SiO2is3.9 and the inter-face charge density between the SiO2/T2SL and Y2O3/T2SL are similar,14 the dielectric constant of Y2O3 grown on etched T2SL sidewall surface is 11, which is around the lowest value reported.16The capacitance of the sample with SiO2-Y2O3hybrid passivation can be expressed by Eq.(1), where eSiO2 and eHybridY

2 O3 are dielectric constants of SiO2 and

Y2O3grown on thin SiO2, dSiO2and dY2O3are the thicknesses

of SiO2and Y2O3dielectric layers, and A is the sidewall sur-face area Since samples with SiO2-Y2O3hybrid passivation and the SiO2passivation have the same SiO2/T2SL interface, their interface charge densities are the same and the dielec-tric constant of Y2O3grown on thin SiO2(eHybridY

2 O 3 ) can be cal-culated from Eq (2), where VSiO2

G;sat and VHybridG;sat are the saturated gate biases of SiO2and SiO2-Y2O3hybrid passiva-tion gated sample eHybridY

2 O 3 is estimated to be 20.8–22.6, which

is around the highest value reported16

Hybrid

Y 2 O 3 eSiO2

eSiO2dY2O3þ eHybridY2O3 dSiO2

a)

Email: razeghi@eecs.northwestern.edu

0003-6951/2014/104(10)/103509/4/$30.00 104, 103509-1 V C 2014 AIP Publishing LLC

2 O3 ¼

e 2 SiO2dY2O3

dSiO2þdY2O3

VSiO2G;sat

VHybridG;sat

eSiO2 eSiO2dY2O3

dSiO2þdY2O3

VSiO2G;sat

VHybridG;sat

The fabrication yield of 220 nm thick SiO2-Y2O3hybrid

passivated gated photodetector reaches 90% and is higher

than those with SiO2and Y2O3passivation because of Y2O3

quality improvement (Figure 1(c)) Therefore, SiO2-Y2O3

hybrid gated array with 220 nm thick passivation layer with

small pitch can be realized withVG,sat¼ 7 V The average

I–V characteristics of the 37 lm GDs at VG¼ 0 and 7 V

are shown in Figure 2 At 200mV operation bias, where

the QE is saturated, the GDs exhibit four orders of

magni-tude lower dark current density than the one at zero gate bias

and reach 1.2 105A/cm2 The differential resistance area

product at200 mV (RA200 mV) is 2.8 104X cm2 The

dark current density of the SiO2-Y2O3hybrid gated array at

VG,satis similar as the ones with SiO2and Y2O3passivated

single gated photodetector (TableI)

Since the optical response of 37 lm pixel was close to

the measurement system noise level, 57 lm pixel is used to

represent the optical property of the gating array The

spec-tral QE curves of the 57 lm GD array atVOP¼ 200 mV

and atVG¼ 0 and 7 V are shown in Figure 3 At VG¼ 0

V, the QE at peak responsivity (QEpeak200mV, k¼ 8.6 lm) is

53% AtVG¼ 7 V, the QEpeak200mVincreases to 63%, which

is 18.9% increment This increment is due to the

suppres-sion of surface recombination.15 As predicted, the surface

assisted QE loss should have mesa size-dependent

behav-ior, which is illustrated in Figure 4 The QE at VG¼ 0

decreases linearly with P/A, where A and P are area and

pe-rimeter of the pixel, respectively At VG¼ 7 V, due to

minimizing of surface recombination, diodes with different

sizes exhibit the same level of QE, QEpeak200mV¼ 63% For

the large size mesa, the loss of QE is negligible The

QEpeak200mVof 400 lm pixel increases from 61.7% at VG¼ 0

to 63% atVG¼ 7 V

As shown in Eq.(3), the measured photocurrent density (Jphoton) consists of one or more of the following four com-ponents: photocurrent density generated in the bulk (Jbulk), the recombination current density within the depletion region

of the p-n junction (Jp-n), the recombination current density within the surface depletion region (JSDR), and the surface recombination current density (JS) (Jbulk  Jp-n) reaches maximum value atVOP¼ 200 mV because the material has condition band misalignment between the M-barrier barrier and p-region Since Jbulk and Jp-n are independent of the diode size, the following analysis will be focused on JSDR andJS

When the sidewall surface is accumulated or in flat band condition,JSDRandJSdo not contribute to the recom-bination of the photocurrent.17When the sidewall surface is depleted, both JSDRandJScontribute to the recombination

of the photocurrent The magnitude ofJSDRdepends onVG because the surface depletion width changes with VG, and the magnitude of JS is strongly related to the SiO2/T2SL interface trap density When the surface is inverted, JSdo not contribute to the recombination of the photocurrent any-more because all SiO2/T2SL interface traps are filled, which means they are not activated and can not acted as recombination-generation centers In this case, the recombi-nation in the surface depletion region (JSDR) becomes maxi-mum because the surface depletion width is in maximaxi-mum, xdmax, whose value depends on theVOP.17

The expressions of each component are shown in Eqs (3)–(7),17 where ni is the intrinsic carrier concentration, which is around 5.9  1014cm3 as reported in Ref 5, W and xdmax are the bulk depletion width and the maximum surface depletion width, s0 pn and s0 SDR are the carrier life time in the bulk and surface depletion region, d is the mesa depth, which is 5.5 lm, s0is surface recombination velocity,

KSis the superlattice dielectric constant, which is 15.4,5e0is the vacuum permittivity, q is the electron charge, NAis the acceptor concentration, which is1016cm3, and /Fpis the quasi-Fermi potential, which is 23.3 mV

FIG 1 (a) Comparison of dark current density between UPD and UGD differ-ent passivations (b) Correlation between saturated gate bias and passi-vation layer thickness of samples with different passivations (c) The yield of single gated photodetector with differ-ent passivations.

TABLE I Dark current density at 200 mV (J 200 mV ) of UPD, UGD, and GD at V G,sat

Passivation N/A SiO 2 Y 2 O 3 Hybrid SiO 2 Y 2 O 3 Hybrid

103509-2 Chen, Hoang, and Razeghi Appl Phys Lett 104, 103509 (2014)

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Jphoton¼ Jbulk Jpn  JSDR JS; (3)

Jpn ¼ 1

2q

ni

JSDR ¼1

2q

nid

s0 SDR

xdmax

P A

 

(5)

JS¼1

2qnis0d

P A

 

xdmax ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2Kse0VOPþ 2/Fp

qNA

s

As shown in Figure5, the QE is evaluated with bothVG

andVOP First, we fix theVGat 0 V and7 V, and gradually

change VOP from 0 to 500 mV At VG¼ 0 V (magenta

curve), the sidewall surface is inverted,14 and JS does not

contribute to the recombination of the photocurrent JSDRis maximum and its maximum value increases with VOP increases Therefore, the QE reaches its maximum value of 53% at VOP¼ 200 mV, where the (Jbulk  Jp-n) is maxi-mum, and then the QE starts decreasing At VG¼ 0 V and VOP¼ 200 mV, xdmax is estimated to be around 0.2 lm; and in this case,JSDRis the only term causes the P/A depend-ent QE behavior thus s0 SDRcan be estimated from the slope

of the fitting line in Figure 4and is estimated to be around 5.3 ps AtVG¼ 7 V (blue curve), the surface is under accu-mulation, the surface depletion region vanishes and both JSDRandJSdo not contribute to the photocurrent Therefore, the QE reaches maximum value of 63% at VOP¼ 200 mV and stays in the same level up toVOP¼ 500 mV

Second, we fix theVOPat 0 V and200 mV, and gradu-ally change VGfrom 0 to12 V The evolutions of the QE curves of VOP¼ 0 (red curve) and 200 mV (green curve) are similar except that the QE at VOP¼ 200 mV is higher than VOP¼ 0 mV because of the bias-dependent QE behav-ior When 0⬉ VG⬉ 1 V, the QE does not change with VG because the sidewall surface is still inverted, the contribution

of JSDR does not change, and there is no contribution from

JS When1⬉ VG⬉ 3 V, the sidewall surface is depleted, and the SiO2/T2SL interface traps become active, which means bothJSandJSDRcause the reduction of the QE When

VG⬉ 3 V, the JSDRkeeps decreasing because the surface depletion width decreases, resulting in the increasing of the

QE At VG¼ 7 V, the surface is accumulated, therefore, and the QE reaches maximum value of 63% and stays the same asVGincreases Therefore, there are different surface recombination mechanisms associated with the QE loss and their influences depend on the surface condition

TABLE II Saturated gate bias of different types of passivation and passivation layer thicknesses.

Sample S1/Y1 S2/Y2/H1 S3/Y3/H2 S4/Y4/H3 S5/Y5/H4 S6/Y6/H5

FIG 2 Comparison of the dark current density and differential

resistance-area product of 37 lm pixel at V G ¼ 0 and 7 V.

FIG 3 Saturated spectral QE of the 57 lm SiO 2 -Y 2 O 3 hybrid passivated

GDs at V op ¼ 200 mV and at V G ¼ 0 and 7 V.

FIG 4 QE at peak responsivity (k ¼ 8.6 lm) and at V op ¼ 200 mV of dif-ferent sizes pixels at V G ¼ 0 and V G ¼ 7 V.

Figure 6 exhibits the evolution of specific detectivity

(D*) of the 57 lm pixel with VG and VOP The high D*

region (D* > 1012Jones) mainly falls in the highVGand low

VOP region That is because the increment of VG can

sup-press surface leakage current, resulting in lower dark current

density and higher value of differential RA product; and the

increment ofVOP, on the contrary, results in higher dark

cur-rent and reduction of the RA value Moreover, as VG

increases, the high D* region can extend to higher VOP,

which means the high performance operation region of the

gated pixel increases That is because by eliminating the

sur-face leakage current, the dark current density of the pixel

become less sensitive withVOP The maximum D* value is

2.3 1012Jones atVOP¼ 100 mV and VG¼ 7 V

In summary, we showed that the SiO2-Y2O3 hybrid passivation improved the quality of Y2O3, resulting in much higher dielectric constant and improvement of fabri-cation yield, which realized the 37 lm pitch gated array and further suppressed the VG,sat to 7 V More importantly, the evolution of QE with different pixel sizes,VG, andVOP revealed different surface recombination mechanisms and carrier life time in the surface depletion region Thanks to the gating technique to suppress the surface recombination,

at 77 K, the QE of the 57 lm size detectors was improved

by 18.9% At VG,sat (VG¼ 7 V), the 57 lm gated diode array exhibits J200 mVof 1.2 105A/cm2, RA200 mVof 2.8 104X cm2, a 63% quantum efficiency, and a detectiv-ity of 2.3 1012Jones

The authors acknowledge the support, interest, and sci-entific discussion of Dr Fenner Milton, Dr Meimei Tidrow,

Dr Joseph Pellegrino, and Dr Sumith Bandara from the U.S Army Night Vision Laboratory; Dr William Clark, Dr Priyalal Wijewarnasuriya, and Dr Eric DeCuir, Jr from U.S Army Research Laboratory; Dr Nibir Dhar from DARPA; and Dr Murzy Jhabvala from NASA Goddard Space Flight Center

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FIG 5 QE mapping of 57 lm pixel in the SiO 2 -Y 2 O 3 hybrid gating array

with V G and V OP Top inset: The evolution of QE with V OP at V G ¼ 0 and

7 V Right inset: The evolution of QE with V G at V OP ¼ 0 and 200 mV.

FIG 6 Specific detectivity mapping of 57 lm pixel in the SiO 2 -Y 2 O 3 hybrid

gating array with V G and V OP

103509-4 Chen, Hoang, and Razeghi Appl Phys Lett 104, 103509 (2014)

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