Improved performance of InAs0.07Sb0.93

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... based on the epilayers. Ge immersion lenses were set on the ... Liquid nitrogen cooled HgCdTe detectors are dominant in 8–12 m (LWIR) wavelength region.
Optik 142 (2017) 68–72

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Improved performance of InAs0.07 Sb0.93 photoconductors operating at room temperature Yu Zhu Gao a,∗ , Xiu Ying Gong a , Ji Jun Li b , Yan Bin Feng b , Takamitsu Makino c , Hirofumi Kan c , Tadanobu Koyama d , Yasuhiro Hayakawa d a b c d

College of Electronics and Information Engineering, Tongji University, Shanghai 201804, China Huaxing Infrared Device Company, Xian 712099, China Hamamatsu Photonics K. K., 5000 Hirakuchi, Hamakita, Shizuoka 434-8601, Japan Research Institute of Electronics, Shizuoka University, Johoku 3-5-1, Hamamatsu, Shizuoka 432-8011, Japan

a r t i c l e

i n f o

Article history: Received 20 February 2017 Accepted 17 May 2017 Keywords: InAsSb Room temperature photodetector Melt epitaxy Spectral photoresponse

a b s t r a c t In this study, the improvement of detectivities D* at the wavelength of 8 and 9 ␮m of InAs0.07 Sb0.93 photoconductors are provided. InAs0.07 Sb0.93 , InAs0.05 Sb0.95 and InAs0.03 Sb0.97 thick epilayers were grown on InAs substrates by melt epitaxy (ME). The photoconductors were fabricated based on the epilayers. Ge immersion lenses were set on the devices. At room temperature, the photoresponse wavelength range was 2–10 ␮m. The peak detectivities Dp * (6.5 ␮m, 800) were larger than 1.0 × 109 cm Hz1/2 W−1 . The detectivities D* at 8 and 9 ␮m of InAs0.07 Sb0.93 detectors were raised to 5.01 × 108 cm Hz1/2 W−1 and 2.92 × 108 cm Hz1/2 W−1 respectively, which are higher than that of InAs0.05 Sb0.95 and InAs0.03 Sb0.97 detectors. It benefits from arsenic composition increasing in the epilayers. © 2017 The Author(s). Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction Room temperature infrared (IR) photon detectors are very useful for temperature measurements, gas sensors, laser detections and monitoring systems. They have some important advantages, such as very fast responding speed, small volume and good reliability, since they need not coolers. In 3–5 ␮m (MWIR) wavelength range, uncooled InAs/GaSb type-II superlattice photodetectors have been reported recently [1–5]. Liquid nitrogen cooled HgCdTe detectors are dominant in 8–12 ␮m (LWIR) wavelength region. However, it is racking of uncooled III–V photodetectors with cutoff wavelengths longer than 8 ␮m. In our previous papers, InAsSb thick epilayers with long wavelength were grown on InAs substrates by melt epitaxy (ME) [6–10]. The thickness of the epilayers reached 100 ␮m. This thickness effectively suppressed the affection of the lattice mismatch, and resulted in a low dislocation density (the order of 104 cm−2 ) and good crystal quality of the epilayers. However, the cutoff wavelength of InAs0.05 Sb0.95 photoconductors grown by ME was 8.3 ␮m[9,10]. In this paper, the detectivities of InAs0.07 Sb0.93 detectors at the wavelength of 8 and 9 ␮m were evidently improved comparing with that of InAs0.05 Sb0.95 and InAs0.03 Sb0.97 detectors. It is induced by increasing arsenic composition in the epilayers. At room temperature, the peak detectivity Dp * at the wavelength of 6.5 ␮m for InAs0.07 Sb0.93 detectors is 4.27 × 109 cm Hz1/2 W−1 , the detectivity D* at 8 ␮m is 5.01 × 108 cm Hz1/2 W−1 , and D* at 9 ␮m is 2.92 × 108 cm Hz1/2 W−1 .

∗ Corresponding author. E-mail address: [email protected] (Y.Z. Gao). http://dx.doi.org/10.1016/j.ijleo.2017.05.058 0030-4026/© 2017 The Author(s). Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

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2. Experimental procedure InAsSb epilayers were grown on (100)-oriented n-InAs substrates in a horizontal liquid phase epitaxy (LPE) growth system in high purity hydrogen ambient. The source materials were 7N Sb, In and non-doped InAs single crystals. The detailed growth process of melt epitaxy (ME) has been published in our previous paper [7]. The key point is as follows: at the growth temperature (about 500 ◦ C), the melt is pushed to contact with substrate, and the excess growth melt is immediately removed away from the substrate by pushing the melt holder. It is important that some melt is remained on the surface of the substrate at the growth temperature. Then the substrate was pushed under the flat part (the block) of the melt holder and cooled with a cooling rate of 0.5 ◦ C/min to obtain an epilayer. The size of InAsSb samples is 9 × 11 mm2 . The surface of the samples has been polished to mirror smooth by Al2 O3 powders. Room temperature photoconductors were fabricated based on InAsSb thick epilayers grown by ME. InAs substrates were entirely removed away during the device process. Thus the affection of the lattice mismatch between the epilayers and substrates were eliminated. Ge immersion lenses were set on the detectors without any antireflective coatings. The area of the sensitive elements is 0.05 × 0.05 cm2 . Indium is employed as the electrode. At room temperature, the spectral photoresponse of InAsSb photoconductors were measured by a Fourier transform infrared (FTIR) spectrometer, and the absolute responsivity was calibrated by a standard blackbody source at a temperature of 500 K and a modulation frequency of 800 Hz. The bias current applied on the detectors was 10 mA. 3. Results and discussion 3.1. Distributions of Sb, In and As The cross section of InAsSb samples was observed by a scanning electron microscopy (SEM, HITACHI S-4800). Fig. 1(a) shows a SEM cross-sectional image of an InAs/InAsSb sample. The boundary between the epilayer and substrate is basically flat and straight indicating the good crystal quality of the epilayer. It possibly benefits from the special growth process of ME and the slow cooling rate of 0.5 ◦ C/min during the epitaxial growth. The cracks in the epilayer are induced by the cleavage of sample. As is shown in Fig. 1(a), the thickness of InAsSb epilayer grown by ME reaches 120 ␮m. This thickness practically reduces the affection of the lattice mismatch. The composition distribution in the cross sections of InAs/InAsSb samples was measured by an energy dispersive x-ray analysis (EDAX) system attached SEM. Fig. 1(b) shows EDAX line-scan profiles of Sb, In and As distributions in the cross section of the sample along the epitaxial growth direction respectively. In Fig. 1(b), Sb is mainly distributed in InAsSb epilayer, and Sb in the epilayer is fairly homogeneous. The distribution tendency of In in the sample has not obvious varying due to containing of In both in the epilayer and substrate. As in InAs substrate is more than that in InAsSb epilayer, since the atomic fraction of As in the epilayer is only 7%. The abrupt change of Sb and As distributions at the boundary between the epilayer and substrate are observed in Fig. 1(b). 3.2. X-ray diffraction spectra X-ray diffraction (XRD) spectra were measured by an x-ray diffractometer (BRUKER D8 ADVANCE) at a voltage of 40 kV and a current of 40 mA. Fig. 2 shows XRD curves of an InAs/InAs0.07 Sb0.93 sample. As shown in Fig. 2, Cuk␣1 and Cuk␣2 diffraction peaks of InAsSb epilayers are clearly observed, and no other crystal structures appear. The growth direction of the epilayers is in accordance with the surface direction of InAs substrates, i.e. the (100) orientation. This demonstrates that InAsSb epilayers are indeed single crystals. The sharpness and the full-width at half-maximum (FWHM) of 280 of InAsSb (400) Cuk␣1 diffraction peak indicate the high quality of the epilayers. According to the Bragg diffraction equation, the lattice constant of InAsx Sb1-x samples is estimated to be 6.4492 Å. Based on Vegard Law [11], the arsenic mole fraction in the epilayers is estimated by x = (aInAsSb − aInSb )/(aInAs − aInSb )

(1)

where x is the arsenic mole fraction in the epilayers, and a is the lattice constant. The arsenic mole fraction is estimated to be 0.07 for the samples in Fig. 2. The arsenic atomic fraction in the epilayer measured by EDAX is 7.2%, which is in agreement with that obtained by XRD measurements. The lattice mismatch between InAs0.07 Sb0.93 epilayers and InAs substrates is 6.45%. Therefore, InAsSb single crystals with high quality were obtained by melt epiatxy technology under the rather large lattice mismatch. 3.3. Spectral photoresponse of room temperature InAs0.07 Sb0.93 photoconductors Fig. 3 shows a picture of InAsSb photoconductors. As shown in Fig. 3, there are Ge immersion lenses on the detectors. Fig. 4 shows spectral photoresponse of room temperature InAs0.07 Sb0.93 , InAs0.05 Sb0.95 and InAs0.03 Sb0.97 photoconductors labeled by sample a, b and c. At room temperature, the peak responsivity Rp is 394.1, 145.6 and 158.4 V/W at the

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Fig. 1. SEM image of an InAs/InAsSb sample (a), EDAX line-scan profiles of Sb, In and As distributions in the cross section of the sample (b).

Fig. 2. XRD curves of an InAs/InAs0.07 Sb0.93 sample. The full-width at half-maximum of InAsSb (400) Cuk␣1 diffraction peak is 280 .

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Fig. 3. InAsSb photoconductors with Ge immersion lenses.

Fig. 4. Spectral photoresponse of room temperature InAs0.07 Sb0.93 photoconductor labeled by a, InAs0.05 Sb0.95 photoconductor labeled by b and InAs0.03 Sb0.97 photoconductor labeled by c.

wavelength of 6.5 ␮m, and the peak detectivity Dp * (6.5 ␮m, 800 Hz) is 4.27 × 109 , 3.87 × 109 and 5.30 × 109 cm Hz1/2 W−1 for sample a, b and c respectively. It indicates the high sensitivity of room temperature InAsSb photodetectors. At the wavelength of 8 ␮m, the detectivity D* is 5.01 × 108 , 8.18 × 107 and 1.45 × 108 cm Hz1/2 W−1 for sample a, b and c respectively. At the wavelength of 9 ␮m, the detectivity D* is 2.92 × 108 , 6.14 × 107 and 1.32 × 107 cm Hz1/2 W−1 for sample a, b and c respectively. Therefore, the detectivity at 8 and 9 ␮m of InAs0.07 Sb0.93 detectors is higher than that of InAs0.05 Sb0.95 and InAs0.03 Sb0.97 detectors. This is attributed to arsenic composition increasing in InAs0.07 Sb0.93 epilayers. Ge immersion lenses were set on the detectors. The incident IR radiation was focused by Ge lenses, thus the radiation energy density on the photosensitive surfaces was increased. The immersion lens can raise the signal-to-noise ratio and detectivity of approximate one order of magnitude. InAsSb epilayers with the thickness larger than 100 ␮m grown by ME have the properties of bulk single crystals. The intrinsic semiconductors are more suitable for room temperature photodetectors. The high density of states in the valence and conduction bands of them gives rise to the strong absorption of IR radiation [12]. Moreover, the response time of InAsSb photodetectors is the order of 10−1 ␮s, which is at least three orders of magnitude faster than that of heat IR detectors. 4. Conclusions In summary, InAs0.07 Sb0.93 photoconductors were obtained using the thick epilayers with long wavelength. Spectral photoresponse of the detectors showed that the wavelength range is 2–10 ␮m. At room temperature, the peak detectivity Dp * at the wavelength of 6.5 ␮m was 4.27 × 109 cm Hz1/2 W−1 , the detectivity D* at 8 ␮m was 5.01 × 108 cm Hz1/2 W−1 , and D* at 9 ␮m was 2.92 × 108 cm Hz1/2 W−1 . The detectivities at 8 and 9 ␮m of InAs0.07 Sb0.93 detectors are higher than that of InAs0.05 Sb0.95 and InAs0.03 Sb0.97 detectors. It is caused by arsenic composition increasing in the epilayers. Acknowledgments The authors acknowledge the financial assistance provided by the Research Funds for National Defence in China.

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