GaSb superlattices by exploring the optimum molecular beam epitaxy growth process

Infrared Physics & Technology 67 (2014) 8–13

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Infrared Physics & Technology journal homepage: www.elsevier.com/locate/infrared

High quality mid-wavelength infrared InAs/GaSb superlattices by exploring the optimum molecular beam epitaxy growth process Zhicheng Xu, Jianxin Chen ⇑, Fangfang Wang, Yi Zhou, Chuan Jin, Qingqing Xu, Li He Key Laboratory of Infrared Imaging Materials and Detectors, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 500 Yutian Road, Shanghai 200083, China

h i g h l i g h t s  A systematic study of InAs/GaSb superlattices (SLs) epitaxial process.  Demonstrated that As BEP has a lower limit to bring down lattice-mismatch.  A novel shutter sequence was designed to achieve lattice matched SLs materials.  A high quality P-I-N superlattice mid-infrared detector structure was realized.

a r t i c l e

i n f o

Article history: Received 8 April 2014 Available online 24 June 2014 Keywords: InAs/GaSb superlattices Melocular beam epitaxy Interfaces Dark current

a b s t r a c t In this paper we report on the growth of mid-wavelength infrared superlattice materials by molecular beam epitaxy. We focused on the effects of process parameters, such as arsenic beam equivalent pressure and shutter sequences, on the key material properties, such as the lattice mismatch and the surface morphology. Though a smaller As beam equivalent pressure helps to reduce the lattice mismatch between the superlattice and the GaSb substrate, the As beam equivalent pressure itself has a lower limit below which the material’s surface morphology will degrade. To achieve fully lattice-matched superlattice materials, a novel shutter sequence in the growth process was designed. With well-designed interface structures, a high quality P-I-N superlattice mid-infrared detector structure was realized. At 77 K the dark current density at 50 mV bias was 2.4  108 A/cm2 and the resistance-area product (RA) at maximum (50 mV bias) was 2.4  106 X cm2, and the peak detectivity was then calculated to be 9.0  1012 cm Hz1/2/W. The background limited infrared photodetector (BLIP) level can be achieved at a temperature of 113 K. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Type-II InAs/GaSb superlattices (SL) are attractive for infrared detection due to their excellent properties, such as high absorption coefficients comparable to HgCdTe, high material uniformity, and reduced tunneling currents, suppressed Auger recombination rates. [1,2]. Youngdale et al. reported Auger lifetimes in SL materials were approximately two orders of magnitude longer than those in HgCdTe with the same energy gap [3], which translated into an improved detectivity D. According to theoretically modeling a factor of 4.5 greater in detectivity for the best SL as compared to the best HgCdTe could be achieved [4]. In the past few years, this novel infrared technology has developed quickly and large format superlattice focal plane arrays have successfully been realized [5–8]. Despite the rapid progress, the ⇑ Corresponding author. Tel.: +86 21 25051488. E-mail address: [email protected] (J. Chen). http://dx.doi.org/10.1016/j.infrared.2014.05.020 1350-4495/Ó 2014 Elsevier B.V. All rights reserved.

performances of the SL infrared detectors have so far not reached their theoretical prediction and are still inferior to that of the HgCdTe detectors. One important issue is that high performance infrared photodetectors are very demanding on superlattice material qualities. The dark currents of photodetectors are closely associated with the material imperfections, such as defects and dislocations. Therefore, lattice-matched and dislocation-free epitaxial layers are the key to high performance devices. However, epitaxial growth of InAs/GaSb superlattice materials are challenging because the superlattice detectors usually consist of several hundred periods of alternating InAs and GaSb layers and the InAs/GaSb interfaces have complicated structures. Since InAs has 0.6% smaller lattice constant than GaSb, proper interface layers have to be inserted between InAs and GaSb layers for strain balance. As a result, the interface layers will play an important role in determining the SL quality [9–13]. It is essential to control the interface structures to achieve well lattice-matched superlattice materials and improve their crystalline and optical qualities. In this

Z. Xu et al. / Infrared Physics & Technology 67 (2014) 8–13

paper we explored the optimal growth conditions for mid-wavelength infrared (MWIR) SL material. The arsenic beam equivalent pressure was optimized at first. A novel shutter sequence was then designed in the growth process for strain compensation. With the well-designed interface structures, a high performance MWIR photodiode was fabricated and characterized. 2. Experiments InAs/GaSb superlattice structures were grown on n-type doped (1 0 0) GaSb substrates by solid-source molecular beam epitaxy (MBE). Valved cracking cells for arsenic and antimony sources were employed to produce dimer As2 and Sb2, respectively. The beam equivalent pressures (BEP) were measured by an in situ ion gauge. The gallium and antimony BEPs are 7.0  107 Torr and 3.0  106 Torr, respectively, corresponding to GaSb growth rate of 0.22 nm/s. The indium BEP was 8.0  107 Torr to obtain an InAs growth rate of 0.13 nm/s. Arsenic BEP was used as a parameter for process optimization. A few monolayers (ML) of nominal InSb were inserted at both the InAs-on-GaSb and the GaSb-on-InAs interfaces for strain balance. We used a two-step approach to tune the lattice mismatch between SL layers and the GaSb substrates. At the first step we changed the arsenic BEP in different growth runs. Four samples with 50 periods of 7 ML InAs/7 ML GaSb SLs were grown employing different As BEPs of 2.4  106 Torr (sample A), 3.0  106 Torr (B), 3.5  106 Torr (C) and 4.0  106 Torr (D), respectively, during InAs layer growth. The shutter sequences of the four growth runs are shown as in Fig. 1a. At the InAs-on-GaSb

Interface

Interface

(a)

Interface

Interface

9

interfaces, the gallium cell shutter was firstly closed, left the antimony cell shutter open for 2 s, and the indium cell was opened for 1.3 s before the arsenic shutter to resume InAs growth. At the GaSb-on-InAs interface, the arsenic shutter was firstly closed and left the indium shutter open for 1.3 s to have the growing surface covered by one layer of indium atoms, the indium shutter was closed and the antimony shutter was opened for 2 s Sb-soak time, the gallium shutter was then opened to switch to GaSb growth. All the nominal InSb interfaces in the four samples have the same layer thickness of 0.17 nm. Based on the first step, we then optimized the MBE shutter sequences. Two samples, E and F, also consisting of 50 periods of 7 ML InAs/7 ML GaSb SLs were grown back-to-back. The two growth runs employed the same arsenic BEP of 3.0  106 Torr, but different MBE shutter sequences. The sample E also used the shutter sequences as shown in Fig. 1a. The shutter sequences for the growth of the sample F were redesigned as shown in Fig. 1b. At the GaSb-on-InAs interfaces, the arsenic shutter and the indium shutter were simultaneously closed for one second at first, i.e., inserting a growth interruption. Then the antimony cell shutter was opened for 1.5 s before opening the indium shutter to have the growing surface covered by one layer of indium atoms. The indium shutter open times at all the interfaces were 1.6 s to obtain nominal InSb layer thicknesses of 0.21 nm for both samples E and F. Finally, a P-I-N detector structure was grown on a 2-in. diameter GaSb substrate. It consists of a 1-lm-thick P-type GaSb layer, 50 periods of Be doped 8 ML InAs/8 ML GaSb, a unintentionally doped 2.5-lm-thick 8 ML InAs/8 ML GaSb SLs absorber region, 50 periods of Si doped 8 ML InAs/8 ML GaSb, and a 30-nm-thick Si doped InAs layer. The bottom P-GaSb and the top N-InAs were served as electrical contact layers. The SL growths in the P-I-N structure employed the MBE shutter sequences as shown in Fig. 1b. The surface morphology of all samples was measured by a Bruker Digital 8 atomic force microscope (AFM). Photoluminescence (PL) measurements were conducted using a step-scan Fourier transform infrared (FTIR) spectrometer at a temperature of 77 K. Details about the FTIR-PL configuration and measurement techniques can be found elsewhere [14–16]. To compare the luminescence efficiencies, the samples were mounted side-by-side on the holder during the measurements. The superlattice structural properties, such as the periods and the lattice-mismatch between the SLs with the GaSb substrates, were measured by a high resolution X-ray diffractometer (HRXRD). The P-I-N detector structure was processed into square mesas by wet chemical etching. Ohmic contacts were made by electron beam deposition of TiPtAu metals. Current–voltage and optical response measurements were performed at liquid nitrogen temperatures. The detectors are designed to receive the irradiance from the front sides. The architecture of the single-pixel detectors can be found in our previous paper [17].

3. Results and discussion

(b) Fig. 1. (a) Shutter sequences for the growth runs of the sample A, B, C, D and E, (b) shutter sequences for the growth of the sample F.

The HRXRD x2h scanning curves around the GaSb (0 0 4) for the four SLs samples with different arsenic BEP were shown in Fig. 2. The 0th order SL satellite peak moved to a lower angle with a decreased arsenic BEP, showing that the combined lattice constant of the SL material was a function of the arsenic BEP. To pinpoint the mechanisms, the four measured SL HRXRD curves were simulated using a four-layer model [18]. Through the least square fitting, we found that there were certain amounts of arsenic in the InSb interface layers, especially at the GaSb-on-InAs interfaces, which means the interface layers are actually InAsSb ternary compounds. This phenomenon can be understood that the As flux does not immediately become zero upon the close of the As shutter due

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to its high background pressure. Therefore, a considerable amount of residual As atoms will still present at the growing surface during InSb interface layer growth, resulting in arsenic incorporation in the interface layers. The As fraction at the GaSb-on-InAs interfaces of the four SLs samples has been extracted as shown in Fig. 3, and the lattice mismatch (Da/a) and As fraction are also summarized in Table 1. The lattice mismatch was reduced from 4.2  103 to 9.0  104 as the arsenic BEP decreased from 4.0  106 Torr to 2.4  106 Torr, correspondingly the arsenic composition decreases from 0.99 to 0.42. Because the InAsSb lattice constant becomes bigger with lower As fraction, the combined lattice constant of the superlattices becomes bigger as well, and hence, the lattice mismatch between the SLs and the GaSb substrates becomes smaller, as revealed in Fig. 2. Therefore, it is a good practice to accomplish lattice-match between the SLs and the GaSb substrates by reducing the arsenic BEP as possible. However, in addition to the lattice mismatch, smooth surface morphology is also important for high quality SL materials. The

GaSb substrate

Intensity (a.u.)

(004)

30.3

30.4

30.5

30.6

Omega-2Theta ( o ) Fig. 2. The HRXRD x2h scanning curves around the GaSb (0 0 4) for the 50 periods of 7 ML InAs/7 ML GaSb SLs with different arsenic BEP varying from 2.4  106 Torr, 3.0  106 Torr, 3.5  106 Torr to 4.0  106 Torr (from the bottom to the top), and the peak of the GaSb substrate and the zero-order peak of SLs (SL-0) are only depicted.

As component (a.u.)

1.0

C

D

3.5

4.0

0.8

B 0.6

A 0.4 2.5

3.0

As BEP (10

-6

Torr )

Fig. 3. The dependence of the arsenic fraction on the arsenic beam equivalent pressure. The arsenic fraction was extracted from HRXRD curves by simulation.

Table 1 The dependence of the lattice mismatch (Da/a) and arsenic fraction of the GaSb-onInAs interface layers on the arsenic beam equivalent pressure. Sample

A

B

C

D

As BEP (Torr) Da/a (%) As fraction

2.4  106 0.09 0.42

3.0  106 0.24 0.70

3.5  106 0.35 0.90

4.0  106 0.42 0.99

AFM images with 10 lm  10 lm scanning areas of the four samples are shown in Fig. 4(a)–(d). The four samples surfaces exhibited the clear island structures, which were caused by the low growth temperature (400 °C) for the SLs. For a comparison, a GaSb sample grown at a higher temperature (560 °C) revealed smooth atom steps with no islands, as shown in Fig. 4(e). Besides, macro-point defects can be observed on the SL surface with arsenic BEP of 2.4  106 Torr. In contrary, the samples with arsenic BEPs of 3.0  106 Torr, 3.5  106 Torr and 4.0  106 Torr have similar and defect-free surface morphologies. The surface morphology degradation was caused by MBE growth drifting from the As-rich condition due to an insufficient arsenic flux. Our measurements show that an arsenic BEP of 3.0  106 Torr is necessary to maintain a As-rich growth mode. Such an arsenic BEP corresponds to As/In ratio of 3.7 in our cases. However, the lattice mismatch at the arsenic BEP of 3.0  106 Torr is 2.4  103 which corresponds to a critical thickness of only 3.1 lm. Such a critical thickness is not enough because an SL infrared photodetectors usually require a thicker absorption layer to achieve high quantum efficiency (QE) [19]. Though one can further reduce the indium and arsenic BEPs simultaneously to bring down the lattice mismatch while maintaining As-rich growth mode, this will decrease the growth rate as well. Considering a photodiode usually is several micro-meter thick, a too low growth rate will cause epitaxial time too long. Therefore, reducing arsenic BEP to bring down lattice mismatch has a limit. In our case, the optimal arsenic BEP is 3.0  106 Torr for InAs/GaSb superlattice growth. Based on the experiments and analysis in the first step, we optimized the shutter sequence in order to further eliminate As background atoms before the nominal InSb interface layer growth. Therefore a novel interface shutter sequence was designed as shown in Fig. 1b. Two samples, E and F, were grown back to back. The sample E used the shutter sequences in Fig. 1a while the sample F in Fig. 1b. The arsenic BEP were both 3.0  106 Torr for the growth of the two samples. The measured HRXRD x–2h curves showed that the 0-th peak from the SLs layers of the sample F almost coincided with that from the GaSb substrate. The calculated lattice mismatch was as small as 4.5  104. In contrast, the lattice mismatch between the SLs layers and the GaSb substrate of the sample E is 2.1  103. This result is consistent with that of the sample B which was also grown with an arsenic BEP of 3.0  106 Torr and have a lattice mismatch of 2.4  103. The slightly smaller lattice mismatch of the sample E is mostly resulted from a slightly thicker nominal InSb interface layer in the superlattice. The extracted As fraction in the InAsSb compounds at the GaSb-on-InAs interfaces for the samples E and F were about 0.70 and 0.50, respectively. The results demonstrated that it was very effective to tune lattice mismatch by the interface structure design. During the growth interruption, the amount of arsenic atoms at the growing surface was reduced due to arsenic desorption and lacks of arsenic flux supply. The growing surface was further covered by a layer of Sb atoms by the following Sb soaking, which increases the probability of the combination of indium and Sb atoms. The photoluminescence (PL) spectra of the two samples at 77 K were shown in Fig. 5. The PL intensity of the sample F is about 10 times higher than that of the sample E. High luminescence

Z. Xu et al. / Infrared Physics & Technology 67 (2014) 8–13

(a) RMS : 0.464 nm

(b) RMS : 0.289 nm

(c) RMS:0.321 nm

(d) RMS : 0.376 nm

11

(e) RMS: 0.180 nm Fig. 4. The AFM images with 10 lm  10 lm scanning areas of the samples (a) A, (b) B, (c) C, (d) D and (e) an GaSb sample.

efficiency in the sample F implied less non-radiative recombination defect centers in superlattice materials. It is well known that nonradiative defect centers are an important source of the dark currents of infrared photodiodes. A P-I-N SL detector structure was grown on a 2-in. diameter GaSb substrate to further evaluate the superlattice material quality with the optimized interface design. The photodiode has a superlattice structure of 8 ML InAs/8 ML GaSb in each period in order to extend the cutoff wavelength longer than 4.5 lm. To achieve well lattice-matched 8 ML InAs/8 ML GaSb SLs materials, the Sb soak time after the growth interruption at the GaSb-on-InAs interfaces was increased from 1.5 s to 2.5 s to further reduce the

As fraction in the interface layers. Simulation from a HRXRD measurement showed that the As fraction was about 0.41. To evaluate growth uniformity, seven HRXRD curves were scanned along the diameter of the wafer, 7 mm apart from each other. The lattice mismatches between the SL layer and the GaSb substrate at different locations and the full width at half maximum (FWHM) of the first-order peaks, extracted from HRXRD curves, have been summarized in Fig. 6. The Z value in Fig. 6 is the distance from per measured point to the center of the wafer. All the FWHMs are only about 0.007° and the lattice mismatches are around zero, which demonstrates that the detector structure has the excellent crystalline quality and uniformity. It is noted that the mismatch and

Z. Xu et al. / Infrared Physics & Technology 67 (2014) 8–13

10

0

6

10

10

-1

10

-2

10

-3

10

-4

10

-5

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10

-7

10

-8

10

10

-9

10

5

10

77 K

4

10

3

10

2

10

1

10

RA (Ω∗cm 2 )

FWHM did not show flat lines across the radius of the sample. The spikes up and down was most likely caused by the measurements, and did not reflect the sample itself. In our HRXRD measurements the setup parameters were fully optimized only for the first scan. The same parameters except the sample’s radius position were used for the following scans. Therefore the parameters may not be optimal for these measurements. The processed photodiodes had a 100% cutoff wavelength of 4.7 lm at 77 K, as shown in Fig. 7. The peak current responsivity was 0.8 A/W at 4.0 lm, corresponding to a quantum efficiency of 25%. The measured dark current density-bias voltage (J–V) curve

Current Density (A/cm 2 )

12

0 -1

-0.4

-0.2

0.0

0.2

0.4

Bias (V) 1.5

Fig. 8. The dark current density and dynamic resistance-area product as a function of the applied bias. The photodiode had an area of 500 lm  1000 lm.

Sample F

1.0 10 10

CO 2 absorption

0.5

Sample E

R0A (Ω∗cm2 )

0.0 3

4

5

6

wavelength ( μm) Fig. 5. Photoluminescence spectra of the samples E and F.

10 10 10 10 10

6 5

BLIP 2π FOV 300 K Background

3 2

12

10

11

10

1 10

10

0

-1

60 0.04

9

80

100

120

140

160

10 180

Temperature T (K) Mismatch FWHM

0.02

0.008

FWHM ( 0 )

0.00

-0.02 0.006

-0.04 -21

-14

-7

0

7

14

21

Z (mm)

1.0 0.8 0.6 0.4 0.2 0.0 3.0

3.5

4.0

Fig. 9. Temperature dependent R0A and peak detectivity at 4 lm (100% cutoff is at 4.7 mm at 77 K) with zero bias. BLIP performance can be achieved at 113 K.

and its associated dynamic resistance-area product (RA) – voltage (V) curve at 77 K were shown in Fig. 8. The dark current density at 50 mV bias was 2.4  108 A/cm2 and the RmaxA (at 50 mV bias) and the R0A (at zero-bias) were 2.4  106 X cm2 and 5.4  105 X cm2, respectively. Johnson noise is the major noise component at zero bias, and thus the specific detectivity D of the device with current responsivity Ri can be calculated from the equation:

D ¼ Ri

Fig. 6. The lattice mismatch between SLs layer and GaSb substrate and full width at half maximum (FWHM) of the first-order peaks extracted from the HRXRD curves. The Z value is the distance from the measured point to the center of the wafer.

Responsivity (A/W)

10

D*

4

10

Mismatch (%)

13

R 0A

D* (cm Hz1/2/W )

Intensity (a.u.)

77 K

4.5

5.0

wavelength ( μm) Fig. 7. The current responsivity spectrum of the 8 ML InAs/8 ML GaSb superlattice photodetector. The 100% cutoff wavelength was 4.7 lm.

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi R0 A=4kT

ð1Þ

where R0 is the zero-bias differential resistance of the device, A is the device area, k is the Boltzmann constant, and T is temperature. A peak detectivity D of 9.0  1012 cm Hz1/2/W is obtained at 4.0 lm. To get a rough idea about the dependence of the detector performances on operating temperature, the dark current density-bias voltage (J–V) curves at different temperatures were also measured, and the R0A for different temperatures was extracted, as shown in Fig. 9. The Johnson noise limited detectivity D, calculated according to Eq. (1) assuming the current responsivity unchanged with temperature, is shown as blue squares in Fig. 9. The background limited infrared photodetector (BLIP) level assuming a 2p field of view and 100% quantum efficiency for 300 K background are also shown as a dashed line. The BLIP operating temperature for the device is 113 K. 4. Conclusion In summary, we optimized the molecular beam epitaxy growth process of mid-infrared SL material. Our results indicated that the

Z. Xu et al. / Infrared Physics & Technology 67 (2014) 8–13

arsenic BEP for InAs layers had an important impact on the lattice constant and surface morphology of SL materials. To achieve lattice-matched and defect free SLs, a novel shutter sequence in the growth process was designed. Finally, a high quality P-I-N superlattice mid-infrared detector structure was demonstrated. The dark current density at 77 K at 50 mV bias was 2.4  108 A/cm2 and the resistance-area product (RA) at maximum (50 mV bias) was 2.4  106 X cm2, and the peak detectivity was then calculated to be 9.0  1012 cm Hz1/2/W. The background limited infrared photodetector (BLIP) level can be achieved at temperature 113 K. Conflict of interest There is no conflict of interest.

[6]

[7]

[8]

[9]

[10]

[11]

Acknowledgements [12]

The authors thank Mr. Xiren Chen and Dr. Jun Shao for their helps with infrared PL measurements. This work was supported in part by the National Natural Science Foundation of China (NSF) with Grant Nos. 61176082 and 61290302, and the National Basic Research Program of China (973 program) under Grant No. 2012CB619203.

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