Comparative study of metamorphic InAs layers grown on GaAs and Si for mid-infrared photodetectors
Introduction
Mid-infrared (mid-IR) spectral region is important for various applications such as molecular spectroscopy, gas detection, military counter measures and medicine since the majority of chemical elements possess strong vibrational absorption bands [1], [2]. Different types of photodetectors have been developed in the wavelength regime using diverse material systems [3], [4]. Cryogenic-cooled HgCdTe (MCT) detectors have long been the main choice for industry applications since its invention. However, MCT detectors require use of liquid nitrogen for its sufficient performance, which makes MCT very bulky and expensive [5].
GaAs-based quantum well and quantum dot infrared photodetectors (QWIP and QDIP) [6] and InAs(Sb) type-II-based superlattice detectors [7] have been extensively studied as an alternative material choice over MCT. They have achieved significant strides in increasing operation temperature and specific detectivity [8]. Recently, direct epitaxy of III-V materials on Si for mid-IR photodetectors has drawn much attention due to their potential for high throughput manufacturing, low-cost, and easy integration of III-V opto-electronics to the silicon photonics platform. Jiang Wu et al. have demonstrated epitaxially integrated GaAs-based QD infrared photodetectors on Si for 5 – 8 μm detection [9] and Sengupta et al. have reported QWIPs monolithically integrated on Si for 6 – 12 μm detection [10]. While InAs is a suitable material for 2 – 3.5 μm spectral wavelength, direct epitaxy of InAs on Si wafer and its material characterizations for mid-IR photodetectors has relatively been unexplored [11], [12].
Major challenges of InAs epitaxy on Si arise from the high lattice mismatch (11.6%) and polar (III-V)/non-polar (IV) growth. The polarity disparity which induces electrically active anti-phase domains (APDs) can be resolved by growing III-V materials on 4–6 degree offcut Si substrates or by growing APD-free GaP/Si wafers [13], [14]. The large lattice mismatch has yet been completely solved for high performance mid-IR InAs photodetectors on Si [15]. Especially, threading dislocations (TDs) in the metamorphic InAs layer result in high dark current density and low responsivity for the photodetectors [16], [17]. Therefore, it is crucial to quantitatively investigate the density of TDs in the InAs metamorphic layer monolithically grown on Si and correlate that to the InAs photodetector performance.
Here, we compare the crystalline quality of InAs buffer layers epitaxially grown on GaAs over Si and analyze the p-i-n photodetectors grown on the templates. Low growth temperature for InAlAs linearly graded buffers enabled smooth InAs layers on the substrates. Atomic force microscopy and high-resolution x-ray diffraction showed that the InAs layer grown on Si possesses similar structural quality compared to the one grown on GaAs. Fabricated InAs p-i-n photodetectors grown on GaAs showed a slightly higher peak responsivity of 0.092 A/W than the one grown on Si whose responsivity was 0.084 A/W at room temperature in photovoltaic mode. Transmission electron microscopy and electron channeling contrast imaging revealed consistent results that the threading dislocation density for InAs on GaAs is ~7.5 × 108 cm−2 while InAs on Si is ~1.1 × 109 cm−2. We conclude that reduction of threading dislocation density is imperative to improve the performance of InAs photodetectors monolithically integrated on Si.
Section snippets
Experimental details
All samples were grown by Veeco Gen930 solid-source molecular-beam epitaxy (MBE) machine equipped with an As-valved cracker cell. Metamorphic InAs buffer layers were grown via 700 nm-thick linearly graded InAlAs buffers on a semi-insulating GaAs wafer (Sample A) and 3 μm thick GaAs buffer on Si template (Sample B) as shown in Fig. 1. The GaAs/Si template used in this study has a threading dislocation density (TDD) of ~7 × 106 cm−2. More details about the growth of GaAs buffer on Si can be found
Results and discussion
Fig. 4 (a) displays the 2Theta-Omega (0 0 4) reflection symmetric scan results from both samples. It should be noted that the linearly graded InAlAs buffers maintain relatively high XRD intensities throughout the entire structure. The red curve of Sample B also exhibits strong satellite fringes around the GaAs peak because of the InGaAs/GaAs strained superlattices used in the GaAs/Si template growth. The omega scans (rocking-curves) on both samples are shown in Fig. 4 inset, and the FWHM values
Conclusion
In summary, we have systematically compared metamorphic InAs layers epitaxially grown on GaAs and Si. Smooth surface morphology was obtained from both samples, while the threading dislocation density was relatively high, 7.5 × 108 cm−2 and 1.1 × 109 cm−2 for InAs layers grown on GaAs and Si, respectively. InAs mid-infrared p-i-n photodetectors were demonstrated on the two templates, and the slightly lower responsivity for the InAs/Si device is attributed to the higher threading dislocation
CRediT authorship contribution statement
Geunhwan Ryu: Conceptualization, Methodology, Investigation, Writing - original draft. Soo Seok Kang: Investigation, Writing - original draft. Jae-Hoon Han: Investigation, Resources, Supervision. Rafael Jumar Chu: Investigation, Resources. Daehwan Jung: Conceptualization, Methodology, Investigation, Writing - review & editing, Supervision, Project administration. Won Jun Choi: Supervision, Project administration, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
Acknowledgements
This work was supported by National Research Foundation of Korea NO. NRF-2017M1A2A2048904, KIST Flagship Project (2E30100), and KIST Incubating program (2V08130). The authors are thankful to Dr. Do Kyung Hwang and Dr. Gyu Weon Hwang for their support in device measurement. We are also grateful to Cheol-Hwee Shim for TEM work.
Daehwan Jung (M’16) received the Ph.D. degree in Electrical Engineering from Yale University in 2016. He is currently a research staff at the Korea Institute of Science and Technology (KIST), Center for Opto-electronic Materials and Devices and an assistant professor at KIST School of University of Science and Technology in South Korea. His primary research interest is to study novel III-V materials growth for high performance optoelectronic and photonic devices. His current research project is
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Daehwan Jung (M’16) received the Ph.D. degree in Electrical Engineering from Yale University in 2016. He is currently a research staff at the Korea Institute of Science and Technology (KIST), Center for Opto-electronic Materials and Devices and an assistant professor at KIST School of University of Science and Technology in South Korea. His primary research interest is to study novel III-V materials growth for high performance optoelectronic and photonic devices. His current research project is to develop III-V optoelectronic devices epitaxially grown on silicon.
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Equally contributed.