Infrared Physics & Technology 53 (2010) 173–176

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Infrared Physics & Technology

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Distinction investigation of InGaAs photodetectors cutoff at 2.9 lm

Cheng Li a,b, Yonggang Zhang a,c,*, Kai Wang a,b, Yi Gu a, Haosibaiyin Li a, YaoYao Li a

a State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Science, Shanghai 200050, Chinab Graduate School of the Chinese Academy of Science, Beijing 100049, Chinac Key Laboratory of Infrared Imaging Materials and Detectors, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China

a r t i c l e i n f o

Article history:Received 25 August 2009Available online 1 December 2009

Keywords:InGaAs photodetectorsMolecular beam epitaxyShort wavelength infraredTrap assisted tunneling current

1350-4495/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.infrared.2009.11.002

* Corresponding author. Address: State Key LaboratoInformatics, Shanghai Institute of Microsystem anChinese Academy of Science, Shanghai 200050, Chfax: +86 21 62513510.

E-mail address: ygzhang@mail.sim.ac.cn (Y. Zhang

a b s t r a c t

Using double heterojunction structure with linearly graded InxAl1–xAs as buffer layer and In0.9Al0.1As ascap layer, wavelength extended In0.9Ga0.1 As detectors with cutoff wavelength of 2.88 lm at room tem-perature have been grown by using gas source molecular beam epitaxy, their characteristics have beeninvestigated in detail and compared with the detectors cutoff at 2.4 lm with similar structure as wellas commercial InAs detectors. Typical resistance area product R0A of the detectors reaches 3.2 X cm2

at 290 K. Measured peak detectivity reaches 6.6E9 cm Hz1/2/W at room temperature.� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Many efforts have been made to extend the cutoff wavelengthof the InxGa1–xAs photodetectors from about 1.7 lm (x = 0.53,lattice matched to InP substrate) to the longer wavelength sidefor the importance of the short wave infrared (SWIR) band of1–3 lm [1–11], which exhibiting particular spectroscopic featuresincluding strong water absorption band around 1.4, 1.9 and2.7 lm, as well as the high contrast for earth observation fromthe satellite [12]. However, to our knowledge the longest cutoffwavelength of the detectors at room temperature is still around2.6–2.7 lm [2,11], which corresponding to a lattice mismatch ofabout 2.2% for x � 0.85. Further extending of the cutoff wavelengtharises two questions. First, at even larger lattice mismatch, are theperformances of the detectors still suitable for many applications,or the performances degradation because of the epitaxial defectsstill moderate? Second, at certain operation wavelength (e.g.k � 3 lm), are the performances of the InxGa1–xAs detectors(x > 0.85) still superior to the lattice matched detectors with longercutoff wavelength (e.g. InAs, x = 1 with cutoff wavelength of3.6 lm) especially at room temperature [13], or competitive toexisting commercial devices [14]? To answer those questions, inthis paper further extending of the cutoff wavelength of InxGa1–xAsdetectors up to 2.9 lm is demonstrated, and their performancesare investigated in detail. A comprehensive comparison of the

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detector features have been made with our InxGa1–xAs detectorscutoff at 2.4 lm as well as the commercial Judson InAs detectorscutoff at 3.6 lm [14].

2. Experimental details

In this work the detector epi-wafers with structure similar to[9] were grown by using a VG Semicon V80H GSMBE system. Inthe growth of 2.9 lm detector (sample A), the pre-heat treatmentof the substrate was carried out in the phosphor pressure. After thedesorption of surface oxygen, the substrate temperature was de-creased to 460 �C (measured by thermal couple) to start the growthof a thin InP buffer (0.15 lm in thickness), then begun the linearlygraded N+ InxAl1–xAs buffer layer (2.4 lm in thickness) where theIndium composition x was graded from 0.52 (lattice matched toInP) to 0.90 at a mismatch grading rate of 1.1%/lm using a corre-lating temperature ramping procedure as described in [9]. Afterthat, a In0.90Ga0.10As absorption layer (2.0 lm in thickness) wasgrown, which was lightly doped with Si, then ended with Be dopedP+ In0.90Al0.10As cap layer (0.6 lm in thickness). As a comparison, a2.4 lm detector wafer (sample B) where the Indium compositionsof InxAl1–xAs and InxGa1–xAs are 0.78 was also grown subsequentlyusing similar structural and growth parameters. The grown sam-ples show shiny surface, but slight cross ripples can be clearly seenalong the [0 1 1] and [0 1 �1] directions, which are typical mor-phology related to the existence of mismatch dislocations [15],especially at large positive mismatched conditions.

After growth, the wafers were processed into mesa-type detec-tors. Mesas with different diameters were defined by photolithog-raphy and wet chemical etching using H3PO4/H2O2 solution, then

0.0 0.2 0.4 0.6 0.8 1.010-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

0 10 20 30 40 5010-5

10-4

10-3

10-2

10-1

V(mV)

Dar

k C

urre

nt D

ensi

ty (A

/cm

2 )

Reverse Bias (V)

sample AT=110~350KStep:20K

0.0 0.2 0.4 0.6 0.8 1.0Reverse Bias (V)

sample BT=170~350KStep:20K

Fig. 2. Measured reverse I–V characteristics of sample A in the temperature range

174 C. Li et al. / Infrared Physics & Technology 53 (2010) 173–176

passivated by Si3N4 using plasma enhanced chemical vapor depo-sition (PECVD), no antireflection (AR) coatings were applied. Afterthat, ohmic contacts were formed by electron beam evaporationof Ti/Pt/Au in combination of lift-off, then alloyed and diced intochips. Finally the detector chips were packaged into TO-can or dualin line (DIL) packages for further measurements.

The I–V properties of the detectors were evaluated by using aHP4156A precise semiconductor analyzer. A HP4280A C–V meterwas used for C–V characterization. In the I–V measurements, theDIL packaged chips were installed on the cold head of a closed cy-cle helium cryostat to control their temperatures. The responsespectra of the detectors were measured by using a Nicolet Magna760 Fourier transform infrared (FTIR) spectrometer at room tem-perature and zero bias with a CaF2 beam splitter and Ever-Glo IRsource, where the output signal from the detector was fed intothe preamplifier of the spectrometer.

from 110 K to 350 K and sample B in the temperature range from 170 K to 350 K instep of 20 K. Inset shows the enlarged characteristics of sample A at lower bias of 0–50 mV.

10-4

10-3

10-2

10-1

100

101

102001002003004

Bias At -10mV Sample A Sample B

Dar

k cu

rrent

(μA)

Ea=0.33eV

Ea=0.46eV

T(K)

3. Results and discussion

Fig. 1 shows the C–V characteristics of the detectors measured at300 K and 1 MHz, the diameters of the detector mesa are 500 lm.From Fig. 1 the capacitances at 0 V bias are 153 pF (78.1 nF/cm2)and 127 pF (64.5 nF/cm2) for samples A and B respectively. Theelectron concentrations educed from the C–V date are below1E16 cm�3 for sample A and around 3E16 cm�3 for sample B byusing N = (1/qe0erA

2) [C3/(dC/dV)] and Xd = e0erA/C, where A and Care the area and capacitance of the detectors. It can be seen thatat the same doping temperature sample A shows lower carrierconcentration in the InGaAs absorption layer. One possible reasonis due to the carrier depletion cause by the electrical inactivationof dopant and electron trap, for example, formed by misfit disloca-tions [16].

0.0 0.2 0.4 0.6 0.8 1.0

80

100

120

140

1600.18 0.20 0.22 0.24

1E15

1E16

1E17

0.14 0.16 0.18 0.20 0.22 0.24 0.26

1E15

1E16

1E17

Sample B

C(p

F)

VR(V)

VR(V)

XD(μm)

XD(μm)

ND(c

m-3)

ND(c

m-3)

0.00 0.05 0.10 0.15 0.20 0.25

80

100

120

140

160

C(p

F)

Sample A

Fig. 1. Measured C–V characteristics of the photodetectors and calculated carrierconcentration in the absorption layer of samples A and B.

3 4 5 6 7 8 9 1010-5

1000/T(K-1)

Fig. 3. Arrhenius plot of the dark current versus reciprocal temperature of samplesA and B at reverse bias voltage of 10 mV.

Typical reverse I–V characteristics of sample A measured in thetemperature from 110 K to 350 K with step of 20 K are shown inFig. 2. For comparison the reverse I–V characteristics of sample Bfrom 170 K to 350 K with the same step are also shown. The detec-tor diameters of samples A and B are 300 lm and 500 lm respec-tively. The purpose to use a larger diameter for sample B is that theminimum measurable current on our cryostat system is on the or-der of 10 pA limited by EM interference, therefore a lower temper-ature could be reached if a larger diameter is used. The currentfeatures of those two samples with different cutoff wavelengthsare quite different. For sample A at the reverse bias VR = 10 mV,the dark current decreases only about an order of magnitude from2.0 lA (2.8E�3 A/cm2) at 290 K to 0.14 lA (2.0E�4 A/cm2) at190 K, whereas for sample B the dark current decreases three or-ders of magnitude from 0.67 lA (3.4E�4 A/cm2) at 290 K to456 pA (2.3E�7 A/cm2) at 190 K, so the current transport mecha-nisms for those two samples should also be different. To see thedark current characteristics more clearly, Arrhenius plots of thedark current at �10 mV are made for both samples as shown inFig. 3. For sample A, a fixed slope can be only seen at higher tem-perature range from about 250 K to 350 K, whereas current dropsaturation can be clearly seen at lower temperatures. For sampleB, fixed slope extends in whole measurement temperature range,no current drop saturation can be seen until 190 K. This meansthat, for sample B with lower Indium composition of x = 0.78 andtherefore shorter cutoff wavelength, at reverse bias of VR = 10 mVthe thermally activated dark current, which is expressed as Id /

3 4 5 6 7 8 910-1

100

101

102

103

104

105

106400 300 200

Sample A Sample B Judson InAs

R0A(

Ωcm

2 )

1000/T(K-1)

T(K)

Fig. 4. Measured zero bias resistance area products R0A versus temperature ofsamples A and B. For comparison the data of Judson InAs photodetector is alsoshown, which is from Ref. [14].

1.0 1.5 2.0 2.5 3.0 3.5 4.00.01

0.1

1

R(a

.u.)

λ(μm)

Sample B

Sample A

Judson InAs

Fig. 5. Measured response spectra of the wavelength extended photodetectors ofsample A (In0.90Ga0.10As) and sample B (In0.78Ga0.22As) at room temperature, inwhich absorption features around 1.4, 1.9 and 2.7 lm caused by water vapor in theoptical path were clearly shown. For comparison the response spectrum of JudsonInAs photodetector is also shown, which is from Ref. [14].

Table 1Measured black body and peak detectivity of samples A and B at room temperatureusing a 900 K black body.

Sample (lm) Vs/Vn g-Factor D�bb (cm Hz1/2/W) D�kp (cm Hz1/2/W)

A (2.9) 1.4E3 7.0 9.4E8 6.6E9B (2.4) 3.3E3 14 2.2E9 3.1E10

1.0 1.5 2.0 2.5 3.0 3.5 4.0

1E9

1E10

Judson InAs

Sample A

D* (c

mH

z1/2 /W

)

Wavelength(μm)

Sample B

Fig. 6. Measured black body and peak detectivity versus wavelength of samples Aand B (the spectra are smoothed). For comparison the data of Judson InAsphotodetector is also shown, which is from Ref. [14].

C. Li et al. / Infrared Physics & Technology 53 (2010) 173–176 175

exp(�Ea/kT), is dominant in a wider temperature range. Activationenergy of 0.46 eV can be deduced. However, for sample A withhigher Indium composition of x = 0.9 and cutoff wavelength at2.9 lm, at reverse bias of VR = 10 mV the thermally activated darkcurrents only dominant at higher temperature range, where activa-tion energy of 0.33 eV can be deduced. The band gaps Eg of the InG-aAs layers at room temperature are about 0.43 eV and 0.52 eV forsample A and B respectively. From the results it can be inferredthat, at lower bias of �10 mV, the diffusion current may be themain current from above room temperature to lower than 190 Kfor sample B because its Ea is more close to Eg, while for sampleA from 250 K to 350 K, the G–R current and diffusion current bothhave important contributions. At temperature lower than about250 K, the dark current of sample A decreases slowly. This meansthe trap assisted tunneling current, which represents electronstunnel from the occupied trap states to the empty band states, be-gins to play an important role. From above results, the perfor-mances of 2.9 lm InGaAs detectors seem still quite competitiveat higher operation temperatures until about 250 K, but the effectsof further decreasing the operation temperature to improve theperformances of the detectors will be limited, mainly because ofthe trap states caused by the large lattice mismatch related defects.Despite the lower energy gap of the material, larger lattice mis-match accelerates the degradation of the detector performancesfurther.

The shunt resistances R0 of the detectors at 0 V bias were alsomeasured. Fig. 4 shows the typical resistance area product R0A ver-sus reciprocal temperature. At 290 K, R0A of 3.2 X cm2 (R0 = 4.5 kX)and 24 X cm2 (R0 = 12 kX) are measured for sample A and sampleB, respectively. When the detectors cooling down to 250 K, the R0Aincreases exponentially to 17 X cm2 (R0 = 24 kX) and 534 X cm2

(R0 = 272 kX) for sample A and sample B. The R0A of commercialJudson InAs detector is also shown in Fig. 4, with 0.15 X cm2

(R0 = 0.3 kX) at 295 K and 1.5 X cm2 (R0 = 3 kX) at 253 K respec-tively (detector diameter of 250 lm). It can be seen that the R0Aof 2.9 lm InGaAs detector is higher than lattice matched JudsonInAs detector until about 220 K.

Fig. 5 shows the measured response spectra of the detectors atroom temperature, for reference the response of Judson InAs detec-tor is also plotted [14]. For sample A, the 50% cutoff wavelength is2.88 lm. The response fluctuations around the response peak atabout 2.7 lm are due to the trace water vapor absorption alongthe optical path, which can also be found at the wavelengths around1.4 and 1.9 lm. For sample B, the peak response and 50% cutoffwavelength are around 2.24 lm and 2.38 lm, respectively. Theblack-body response measurements of the detectors were also per-formed at room temperature with a TB = 900 K black body. The mea-sured peak detectivity D�kp of 2.9 lm detector (sample A) is

6.6E9 cm Hz1/2/W, while for 2.4 lm detector (sample B) is3.1E10 cm Hz1/2/W, as listed in Table 1. Moreover, the wavelengthdependent detectivity of the detectors for sample A and sample Bat room temperature are shown in Fig. 6. In Fig. 6 the responsespectra are smoothed to eliminate the water absorption bands.The detectivity spectrum of Judson InAs detector at room tempera-ture is also shown in Fig. 6 for comparison, which has a value of3.7E9 cm Hz1/2/W at response peak about 3.45 lm [14]. Atwavelength of 2.65 lm, the detectivity of sample A reaches6.6E9 cm Hz1/2/W, twice that of Judson InAs detector of2.9E9 cm Hz1/2/W at the same wavelength. From Fig. 6 it could beinferred that, extending the cutoff wavelength of InGaAs detectorson InP substrate up to about 3 lm is still quite valuable. Thereforefor certain applications where the cutoff wavelength less than3 lm is enough, this detector is a good choice especially at highoperation temperatures. Furthermore, the wavelength extendingInGaAs detectors grown on InP substrate are very suitable for thearray integration illuminated from the substrate (back) side, wherethe detector arrays are often connected to the read-out circuitsusing flip-chip bounding. The S. I. InP substrate is transparent tothe detected light, whereas in this case the InAs substrate is opaque.However, for applications using discrete device at even longer

176 C. Li et al. / Infrared Physics & Technology 53 (2010) 173–176

wavelength, lattice matched InAs detector may be more suitableespecially at lower operation temperatures, further extending thecutoff wavelength of InGaAs detectors remains a challenge.

4. Conclusions

In conclusion, GSMBE grown wavelength extended 2.9 lmIn0.90Ga0.10As detectors with linearly graded InxAl1–xAs buffer layerand In0.90Al0.10As cap layer have been demonstrated. For detectorswith 300 lm mesa diameter, the typical dark current at VR = 10 mVis 2.0 lA at 290 K and 0.11 lA at 190 K respectively. Resistancearea product R0A of the detector reaches 3.2 X cm2 at 290 K. Mea-sured peak detectivity reaches 6.6E9 cm Hz1/2/W at room temper-ature. The performance of this detector is quite competitive forcertain applications, especially at higher temperatures. For thosewavelength extending InxGa1–xAs detectors with large mismatchto InP substrate, suppression of the trap states induced by misfitdislocations due to the larger lattice misfit remains a challenge.On the other hand, there is still much room for the optimizationof the detector performances.

Acknowledgements

This work was supported in part by Key Basic Research Programof China under Grant 2006CB604903, NSFC Project under Grant60876034 and open fund of Key Laboratory of Infrared ImagingMaterials and Detectors, Shanghai Institute of Technical Physics,Chinese Academy of Sciences.

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