Electrical Properties of Midwave and Longwave InAs/GaSb Superlattices Grown on GaAs Substrates by Molecular Beam Epitaxy
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In the present work, we report on the in-plane electrical transport properties of midwave (MWIR) and longwave infrared (LWIR) InAs/GaSb type-II superlattices (T2SLs) grown by molecular beam epitaxy (MBE) system on GaAs (001) substrate. The huge lattice mismatch between the T2SL and GaAs substrate is reduced by the growth of GaSb buffer layer based on interfacial misfit array (IMF) technique. In order to compensate the strain in the InAs/GaSb T2SL, we utilized a special shutters sequence to get InSb-like and GaAs-like interfaces. It is found that the MWIR InAs/GaSb T2SL exhibits a p- and n-type conduction at low and high temperatures, respectively. Interestingly, the conduction change temperature is observed to be dependent on the growth temperature. On the other hand, LWIR T2SL conduction is dominated only by electrons. It is important to note that the dominant scattering mechanism in LWIR T2SL at low temperatures is the interface roughness scattering mechanism.
KeywordsMolecular beam epitaxy Type-II superlattices Hall effect High-resolution X-ray diffraction
Focal plane arrays
Full width at half maximum
High operation temperature
High-resolution X-ray diffraction
Interfacial misfit array
Interface roughness scattering
Molecular beam epitaxy
Reflection high-energy electron diffraction
Reciprocal space map
Since InAs/GaSb T2SL has been conceptualized by Sai-Halasz et al.  in 1977, great attentions have been paid in the investigation of this semiconductor material. Photodetectors based on this T2SL present theoretically higher potential over mercury cadmium telluride (HgCdTe) and the state-of-the-art infrared material systems for the next generation of infrared (IR) applications [2, 3]. Interestingly, InAs/GaSb T2SL exhibits an unusual type-II broken gap band lineup where the InAs conduction band minimum is located 140 meV lower than the GaSb valence band top . Consequently, the fundamental transition between the heavy-hole subbands and the conduction band bottom depends on the thickness of the InAs or GaSb layer . However, the main advantage of this alignment is the reduction of Auger recombination rate thanks to the suppression of some non-radiative pathways in the valence band . In addition, the band-to-band tunneling is decreased significantly due to the large effective masses (≈ 0.04 m0) of electrons and holes . These two latter features permit the reduction of the dark current, which leads to the high operation temperature (HOT) of the photodetector.
InAs/GaSb T2SL is traditionally grown on lattice-matched GaSb substrate. However, this latter is expensive and available in small sizes less than 3 in., which impede the realization of large-format focal plane arrays (FPAs). Moreover, GaSb substrates are not “epi-ready” and their growth surfaces contain many macroscopic defects . Furthermore, the absorption coefficient is relatively high in GaSb substrate (≈ 100 cm−1) for IR radiation above 5 μm . Due to its numerous advantages, GaAs has been proposed as a viable candidate for the growth of InAs/GaSb T2SL [9, 10, 11, 12]. Indeed, they are “epi-ready,” cost-efficient, and available in large sizes up to 6 in. Besides, GaAs has an absorption coefficient two orders of magnitude lower than that of GaSb. Unfortunately, a huge lattice mismatch (~ 7.5%) exists between GaAs and InAs/GaSb T2SL that results in high misfit dislocation density (109 cm−2) . Therefore, it is compulsory to concept new growth techniques to relieve the strain and reduce the dislocation density. Among these techniques are low-temperature nucleation  and IMF technique [15, 16].
In order to improve the performances of photodetectors based on InAs/GaSb T2SL, a better understanding of fundamental parameters is needed. One of these parameters is the background carrier concentration which is associated with the minority carrier lifetime and diffusion lengths. It is worth noting that InAs and GaSb bulk materials have opposite polarity of carriers’ concentration. Indeed, InAs and GaSb materials grown using molecular beam epitaxy (MBE) are residually n- and p-type, respectively [17, 18]. Consequently, the conduction of the InAs/GaSb T2SL is predicted to be dependent on the thickness of each constituent.
In this paper, we investigate the in-plane transport properties of 10 ML InAs/10 ML GaSb and 24 ML InAs/7 ML GaSb T2SLs dedicated for the detection in MWIR and LWIR regions, respectively, grown on semi-insulating GaAs (001) substrates. This study is achieved by performing a temperature-dependent Hall effect measurement using the Van der Pauw method. Besides, the influence of the growth temperature on the conduction of the InAs/GaSb T2SL is presented.
InAs/GaSb T2SL samples have been grown on semi-insulating GaAs (001) substrates in a RIBER Compact 21-DZ solid source MBE system. This latter is equipped with standard effusion cells for group III elements (indium (In) and gallium (Ga)) and valved cracked cells for group V materials (arsenic (As) and antimony (Sb)). The cracker temperatures were kept at 900 °C for both As and Sb to produce As2 and Sb2, respectively. The manipulator thermocouple (TC) and BandiT (BT) are utilized to monitor the growth temperature. This latter has been calibrated from the GaAs oxide desorption temperature. Following the deoxidization of GaAs substrates at 610 °C (measured by BT), a 250-nm-thick GaAs layer was deposited at 585 °C (BT) to get a smooth starting surface. Subsequently, a 1-μm-thick GaSb buffer layer has been grown using IMF technique at a BT temperature of 440 °C [16, 19]. This technique consists on the formation of a periodic array of 90° misfit dislocation at the GaAs/GaSb interface leading to a low dislocation density (≈ 106 cm−2) . After the growth of GaSb buffer layer, the BT cannot be used anymore due to the emissivity changes, surface roughening, and extra radiative absorption mechanisms . Thus, the growth temperature of the InAs/GaSb T2SL is controlled only by the TC. MWIR 10 ML InAs/10 ML GaSb T2SLs are grown at different substrate temperatures, 330, 390, and 400 °C (TC) to investigate the influence of the growth temperature on the transport properties. On the other hand, LWIR 24 ML InAs/7 ML GaSb T2SL has been deposited at only 390 °C. In order to compensate the strain between InAs and GaSb, special shutters sequence, which was reported to lead to a better structural quality [22, 23], was used as follows: growth of InAs was followed by Sb soak of 8 s to form InSb-like bonds, whereas GaSb growth was followed by 2 s of As soak to grow GaAs-like interface. The V/III flux ratio is 8.3 and 4.6 for InAs and GaSb, respectively. Besides, the growth rate is 0.5 ML/s for both InAs and GaSb. The growth was monitored in situ by reflection high-energy electron diffraction (RHEED) system.
The grown samples have been assessed by high-resolution X-ray diffraction (HRXRD) of PANalytical X’Pert to investigate the structural properties. The Cu Kα1 radiation (λ ≈ 1.5406 Å) originating from a line focus and a four bounce Ge (004) monochromator have been utilized. The transport properties were evaluated by Hall effect measurements using the Van der Pauw method in an ECOPIA system, with a temperature range of 80–300 K. Measurements were performed on square samples of 6 × 6 mm2; contact was made by indium dots in each corner. A magnetic field of 0.4 T was applied normal to the samples.
Results and Discussion
In summary, InAs/GaSb T2SLs have been grown on GaAs substrate using GaSb buffer layer based on IMF technique. Moreover, these T2SLs have been demonstrated for MWIR and LWIR detection regions. It has been found that MWIR T2SL exhibits a change in the conduction type, form p- to n-type as the temperature increases. Furthermore, the temperature at which the change occurs increases as the growth temperature of the T2SLs increases. This conduction type change is attributed to the existence of two impurity levels with two different activation energies. On the other hand, the LWIR InAs/GaSb T2SL conduction is demonstrated to be n-type for the whole range of temperature. In addition to the conventional scattering mechanisms, the IRS mechanism is proved to be the dominant scattering mechanism at low temperatures. These results allow a better understanding of the physical properties of the InAs/GaSb T2SL, which leads to the improvement of IR photodetector performances based on this material.
This paper has been completed with the financial support of the Polish National Science Centre: grant no.: OPUS/UMO-2015/19/B/ST7/02200.
The source of funding of this paper is the grant no.: OPUS/UMO-2015/19/B/ST7/02200.
Availability of Data and Materials
The conclusions made in this manuscript are based on the data (main text and figures) presented and shown in this paper.
DB grew the samples, made the HRXRD and Hall effect characterization, and wrote the manuscript. ŁK, KM, JB, and AK contributed on the design and MBE growth of the grown samples. PM, JP, and AR helped in the theoretical analysis and the interpretation of the results. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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