The Faraday rotation (FR) in Mn-doped InSb (Mn-InSb) depends on extremely low doping of Mn into the InSb lattice. The polycrystalline Mn-InSb was prepared by direct alloying of InSb and Mn2Sb, followed by slow cooling of the melt. X-ray diffraction confirms the incorporation of Mn into the In sites, whereas the non-uniform precipitation of MnSb micro-nanoprecipitates was observed via SEM images. The FR of Mn-InSb was measured at room temperature in the 8 to 12 µm spectral range and compared to commercially-available undoped InSb as well as Te-doped InSb (Te-InSb) references and found to be about 1.27 - 1.35 times stronger than that of Te-InSb measured under the same conditions.
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A Faraday isolator is a nonreciprocal optical device which uses the Faraday effect to allow the propagation of light in one direction but prevents back-reflections towards the source. These devices are important to safeguard proper functioning of a laser source, especially when fast switching, large spectral bandwidth, or high laser power is needed. The development of Faraday isolators that meet these demands has been contingent on the fabrication of novel materials with high optical quality and the challenges associated with their integration. Due to the emergence of Si photonics and higher power laser sources, the near infrared (NIR) (0.75–3.0 µm) and mid-wave infrared (MWIR) (3.0–8.0 µm) regions have been the prime focus in the development of Faraday optical devices [1–5]. However, only a small number of material systems have been explored for the long-wave infrared (LWIR) (8.0–15 µm) . With the emergence of CO2 laser applications in the field of additive manufacturing , microfluidics , photo-acoustic spectroscopy , and surgery [10,11] recent activities have expanded in the exploration of new materials as efficient Faraday isolators in the LWIR. Some of the promising candidate materials for LWIR Faraday isolators include semiconductors [6,12,13], intrinsic Ge [14,15], dilute magnetic semiconductors (DMSs) , and magnetic sulfospinel [17–20]. Most of these materials have limitations ranging from expensive growth and optical fabrication processes, high absorption coefficient, low temperature sensitivity, and low Verdet constant.
DMSs consist of non-magnetic semiconductors doped with a small percentage of magnetic transition elements. The spin-spin exchange between the magnetic ions and the conduction and valence band electrons affects the energy band and electronic structure of the semiconductors, resulting in new physical properties such as giant FR in the presence of an applied magnetic field. The II-VI DMSs such as CdTe doped with Mn, Hg and/or Fe have shown high FR at room temperature due to large Zeeman splitting of the exciton levels [16,21]. The Mn-doped III-V DMSs (GaAs, GaN, AlN, and InN) have attracted interest in the field of spintronics, due to observation of room temperature ferromagnetism [22–26]. Although the above-mentioned DMSs show high FR, most of the work to assess their potential in isolator systems has been done in the NIR. Similarly, InSb is also a III-V semiconductor with a narrow band gap (0.18 eV), high carrier mobility (∼ 7.8 m2V−1s−1 at 300 K), and a large FR at room temperature, making it a good candidate for use in LWIR Faraday isolators . Undoped n-type InSb (wafer form) has been explored as a Faraday isolator in the 10 µm regime due to its free carrier transitions [6,27], inter-band effect [28,29], or both . The effects of doping InSb with Te, S, Se, Cd, or Zn on the electrical and structural properties of flash-evaporated InSb thin-films has also been studied  and has shown that the electrical properties of doped thin films are close to those of undoped bulk InSb with similar donor and acceptor concentrations. However, only n-type Te-InSb DMS has been explored and developed further for LWIR Faraday isolator applications [13,32,33].
In Mn-InSb the substitution of Mn2+ for In3+ forms a p-type Mn-InSb DMS . The high carrier mobility in the Mn-InSb is accompanied by strong p-d hybridization of lattice orbitals with Mn d-electrons, which makes Mn-InSb DMS an ideal candidate material for spintronics . Mn-InSb was first synthesized in thin-film form by low temperature molecular beam epitaxy . The films exhibited low Curie temperatures (Tc = 8.5 K to 20 K), and the ferromagnetic transition was explained by the influence of Mn ions on the electronic transport in the films . However, ferromagnetic transitions above room temperature have been observed in films of Mn-InSb grown by pulsed laser deposition  and liquid phase epitaxy , as well as in the bulk polycrystalline samples synthesized by a controlled ambient annealing technique . It has been shown that the room temperature ferromagnetism observed in heavily doped Mn-InSb systems is due to the formation of MnSb precipitates [38,40]. These results are supported by kinematic exchange theory, whereby the magnetization of Mn-InSb DMSs depends on the hole concentration, whereas the ferromagnetism is accompanied by MnSb atomic clusters. The influence of Mn concentration on the magneto-transport as well as other magnetic properties including magnetic susceptibility, Hall effect and resistivity of Mn-InSb has also been extensively studied [41–43]. Despite these studies, to our knowledge, the examination of Mn-InSb as a viable bulk candidate for LWIR Faraday isolator applications has, to date, not been explored. The examination of key attributes needed for consideration of Mn-InSb for LWIR application has been the focus of this effort.
In this work, we evaluate bulk Mn-InSb by comparing the physical properties that result from alloying InSb with a Mn source to those of undoped InSb and commercially-available Te-InSb. We discuss the compound’s formation, optical quality, and magneto-optical performance in the 8 to 12 µm spectral window in the context of LWIR Faraday isolator applications.
The key parameters for a suitable Faraday isolator include a low absorption coefficient (α) and a high Verdet constant (V) at the working wavelength, with figure-of-merit (FOM) often expressed as V/α. At room temperature, the LWIR transmission in InSb is dominated by the free-carrier transition band gap (Eg=0.117 eV), as compared to the liquid helium temperature transition band gap (Eg=0.236 eV) . The FR due to free-carrier transitions is given by Eq. (1):2): 27]. The inter-band FR is always opposite that originating from free-carrier transitions. Free carrier FR varies inversely to the effective mass of electrons. Since the effective mass of electrons in the conduction band is lower than that of valence band electrons, the free-carrier FR due to free electrons dominates the Faraday effect observed in the n-InSb at room temperature. However, the presence of a high concentration of free electrons will also result in an increase in free carrier absorption. As shown in Eq. (3), α depends directly on both Ne and λ2, leading to poor transmission in the LWIR. For low doping concentrations (Ne<1024 m−3), one expects a Drude-like free-carrier absorption:
In our study, Mn-InSb was prepared by direct melting of the starting materials, InSb and Mn2Sb, followed by slow cooling. The resulting polycrystalline Mn-InSb was characterized via x-ray diffraction (XRD) and scanning electron microscopy (SEM) to assess the resulting phases and material microstructure. Optical and magneto-optic properties of the processed material were evaluated on polished coupons, including IR transmission and room temperature FR measurements between 8 and 12 µm.
Bulk polycrystalline In(1-x)MnxSb ingots (10 g batches) were prepared using high purity 0.05 at% manganese (II) antimonide (99.5%), and electronic grade indium antimonide (99.99%) from Alfa Aesar. The compounds were weighed with an accuracy of ± 10−3 g and batched in a 10 mm-diameter quartz ampoule which had been previously etched with buffered oxide etch (BOE), rinsed with deionized water, and dried at 400 °C to ensure the removal of surface impurities. The quartz ampoule was sealed under vacuum (10 mTorr) using a methane-oxygen torch. The elements were mixed and melted in a rocking furnace for 24 h at 900 °C to promote the dissolution of Mn in InSb, followed by a gradual cool down to room temperature at a rate of −3 °C/min. The gradual cool down was important for realizing large chunks of the Mn-InSb pieces upon removal from the quartz ampoule. The prepared polycrystalline boule was cut into 1.5 mm-thin slices using a slow speed saw. The slices were polished on both sides using 2400 grit SiC paper with a final 0.05 µm Al2O3 slurry, polishing down to a final thickness of ∼ 0.95 mm. Polycrystalline undoped InSb was prepared by melting electronic grade InSb (99.99%) pieces (∼ 0.5 mm thick before melting) in a 10 mm-diameter quartz tube at 600°C for 6 hrs, followed by a gradual cool down to room temperature at −3 °C/min. Te-InSb wafers were purchased from WaferTech  and used as a reference material for FR measurements. These reference samples were an undoped (carrier concentration = 1014 cm−3) InSb wafer purchased from Galaxy Compound Semiconductor, Inc. and a Te-InSb wafer with carrier concentration of 2.02×1017 cm−3 purchased from WaferTech.
The FR of the reference wafers and the samples made in this work were measured employing the experimental setup shown in Fig. 1. A quantum cascade laser (QCL) from Block Engineering operating at 7.7–13.0 µm was used as a source, and the laser beam was passed through a GMW 5405 electromagnet with 76 mm pole face diameter and adjustable pole separation, connected to a chiller. The sample was placed between the two EM coils. The beam was passed through a Thorlabs linear polarizer and a neutral density (ND) filter and detected by a photodetector (Block Engineering External IR Detector Module). The photodetector signal was amplified using a Stanford Research Systems SR844 lock-in amplifier (LIA). The system was controlled by, and data was collected with, custom LabVIEW software. Infrared transmission measurements in the range of 5 to 15 μm were performed using ThermoFisher Scientific Nicolet iS5 Fourier Transform Infrared spectroscopy (FTIR) on polished Mn-InSb slabs. The resolution of the FTIR system was on the order of ± 1% in transmission. The structural and compositional analyses of the Mn-InSb samples were carried out using XRD and SEM/EDS, respectively. XRD patterns were obtained on a PAN analytical Empyrean XRD system fitted with a copper anode (λKα = 0.154 nm) and scans were performed in the 2θ range between 10 and 90°. A Zeiss Ultra 55 FEG SEM/EDS system was used to measure the grain structure as well as the presence of MnSb precipitates in the polycrystalline samples, and to assess the concentration of Mn, as summarized in Table 1.
3. Results and discussion
The FR (θp) measured in the 8 to 12 µm range at room temperature and 1 T magnetic field is used to calculate V using Eq. (5) (Fig. 2). The V of undoped InSb is negligible compared to its doped counterparts (V ≈ 0) at all wavelengths. Therefore, it is not shown in Fig. 2. In literature the V of n-type undoped InSb (L=0.52 mm) at wavelength 9.166 µm was 97.8 deg/T/mm when measured using a QCL source . The measured V of the Mn-InSb samples is about 1.3 times greater than the Te-InSb reference. Although both Mn-InSb samples had different doping (0.05 and 0.1 at%, respectively), the increase in V with respect to the wavelength is comparable. This similarity between the two Mn-InSb samples most likely indicates that both samples have reached the solubility limit of Mn2+ in InSb. Table 1 shows a summary of the Mn-InSb and Te-InSb samples used in this study. From Eq. (1), one would expect that:
Hence, the ratio of Ne of Te-InSb and Nh of Mn-InSb should only be accounted for by the difference in doping and effective masses, if MnSb precipitates do not add magnetic contribution to the FR as shown in Eq. (6). This is the case, shown in the resulting θp data shown in Table 1.
To further understand and compare the Te-InSb and the Mn-InSb samples, the absorption coefficient (α, cm−1) of these coupons is measured across the 5 to 20 µm spectral window as shown in Fig. 3. The α is estimated using Eq. (7):3) can be attributed to absorption from the free-carriers, scattering from MnSb precipitates, and poor surface quality due to polishing and/or the presence of micro-inclusions and MnSb nanoprecipitates [43,47]. All samples (except the Te-InSb) were hand polished with a nominal inspection polish (with rms roughness of 0.056 (± 0.015) µm), suggesting that the scatter loss between the as-fabricated experimental samples is comparable. Hence, Te-InSb exhibits a higher FOM despite having smaller V than its Mn-InSb counterparts.
The XRD patterns of the Mn-InSb samples are compared to that of the undoped InSb Fig. 4 (left). The InSb peak corresponding to the (311) plane at 2θ = 46.4817° is detected predominantly in the doped samples. The lattice constant a varies from 6.4747 Å in the undoped InSb to 6.4719 Å and 6.4727 Å in the 0.05 and 0.1 at% Mn-InSb samples, respectively. The reduction in a can be explained by the formation of an In(1-x)MnxSb solid solution of InSb and Mn, because the atomic radii of Mn and In are 1.27 Å and 1.67 Å respectively . Although the doped Mn-InSb samples show predominantly InSb peaks, small traces of MnSb peaks were observed. This is most evident in the 0.1 at% doped Mn-InSb sample, with a peak at 2θ = 32.3201° that corresponds directly to the peak in MnSb, as shown with a star in Fig. 4 (left). The presence of this peak indicates the precipitation of MnSb during the fabrication process. The formation of MnSb micro- and nanoprecipitates in the Mn-InSb samples is supported via kinematic exchange theory in the literature [38,48,49]. Figure 4 (right) shows the SEM images of the Mn-InSb samples; note that as discussed above for MnSb, micro-precipitates are observed in both the Mn-InSb samples. Further analysis of the size of the micro-precipitates was done by using ImageJ software. The micro-precipitates’ size varies from 0.185 (± 0.111) µm to 0.623 (± 0.314) µm in the 0.05 at% Mn-InSb and 0.1 at% Mn-InSb samples, respectively. The larger precipitates observed in the 0.1 at% Mn-InSb supports our hypothesis of MnSb precipitation during synthesis. Although the doping level of the Mn is below the detection limit of EDS (± 2.0 at%), further analysis of the Mn-InSb samples via EDS has been performed. Here, an undoped InSb has been used as a standard, and the Mn concentration in the Mn-InSb samples were measured as 3.5 and 5.0 at% respectively. The higher Mn concentration in the Mn-InSb again confirms the presence of MnSb micro-precipitates. The room temperature ferromagnetic behavior of MnSb micro-precipitates in Mn-InSb samples has been assessed earlier , but the impact of these phases on the FR has not been studied yet.
In summary, we have prepared and characterized melt-derived Mn-InSb alloys synthesized using Mn2Sb and InSb, followed by structural and magnetic characterization. XRD of the Mn-InSb samples illustrates the change in the lattice constant associated with incorporation of Mn in the In sites. The XRD of the 0.1 at% Mn-InSb also confirms the presence of MnSb micro-precipitates. The presence of MnSb micro-precipitates was also confirmed via SEM images, showing a nominal size of 0.185 (± 0.111) µm to 0.623 (± 0.314) µm in the 0.05 and 0.1 at% Mn-InSb, respectively. V of the Mn-InSb alloys has been measured and compared to an industry reference standard Te-InSb at λ = 10 µm, 1.0 T, and room temperature. The resulting V and FOM of the Mn-InSb samples were 1.27 and 1.35 times higher and 0.19 and 0.16 times lower than the Te-InSb standard, respectively. The similarity between the V of the two Mn-InSb samples, despite having a big difference in the size and percentage of the MnSb micro-precipitates, leads us to conclude that other than imparting scattering loss, the presence of MnSb has no impact on the FR of the Mn-InSb samples. However, further analysis of the role MnSb micro-precipitates play in the FR of the Mn-InSb is needed to fully understand this material’s potential as a LWIR magneto-optic candidate.
Charles Stark Draper Laboratory (SC001-0000001238).
We (RS) acknowledge the partial support of the UCF Pre-eminent Postdoctoral Fellowship (P3) Program.
All authors contributed equally to this work.
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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