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In this work, we report on the successfully growing Hg 1− x Mn x Se bulk crystals using a mixed, travelling heater method and Bridgman method, two-step procedure. Firstly, and with the aim of reducing Hg high pressure related to the high temperature synthesis reaction between the components in elemental form, HgSe crystals were synthesized and grown by the cold travelling heater method. Secondly, previously sublimated Mn and Se were incorporated to complete the desired composition. Then, the Bridgman growth was carried out by heating the alloy at a temperature of about 880 °C and lowering it at rate of 1 mm/h through a gradient of 25 °C/cm. The Hg 1− x Mn x Se crystals were characterized by scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffractometry, Fourier transform infrared spectroscopy and magnetic susceptibility measurements. The summary of the experimental results allows us to be optimistic with the potential of Hg 1− x Mn x Se as regards using Hg 1− x Mn x Te and Hg 1− x Cd x Te for infrared photodetection. Keywords Crystal growth Semimagnetic semiconductors HgMnSe Infrared 1 Introduction Stimulated by recent advances in spintronics, the interest on diluted magnetic semiconductors (DMS) has been relaunched in the last years [1] . Dilute magnetic semiconductors, also called semimagnetic semiconductors, are semiconductor compounds in which some of the ions have been replaced by magnetic atoms. The properties of these materials are related to the presence of exchange interactions between localized spins of half-filled outer shells of the magnetic ions and the delocalized conduction and valence band charge carriers. We can name Faraday rotation and giant magnetoresistance (GMR) as the most representative [2] . These features have been proved to be useful in technological applications such as optical filters, lasers and photodiodes [3,4] . Quantitatively speaking, both magnetic and nonmagnetic properties of the DMSs can be tuned by varying the concentration of the added magnetic ions. Nevertheless, the amounts of magnetic species that can be incorporated into the semiconductor host lattice is limited by their solubility. So, the higher the solubility of the magnetic ions into the semiconductor host, the wider the application range we get. In this sense, Mn is, for long, the widest used magnetic specie in II–VI semiconductor hosts. In the field of infrared and far infrared detection, Hg 1− x Mn x Te has traditionally attracted industrial interest due to its potential as a substitute of the well-known Hg 1− x Cd x Te material [3,4] . In fact, the fabrication, characterization and optimization of Hg 1− x Cd x Te and Hg 1− x Mn x Te based photodetectors is a matter of interest [5,6] . As shown in Fig. 1 , the range of solubility of Mn in HgTe (about 37%) allows a wavelength application window running from 1.5 to 12 μm, covering the full far infrared spectrum (for comparison, the Cd concentration dependence of Hg 1− x Cd x Te band gap is also included in Fig. 1 ). A successful proposal in order to open the gap is to make use of the solid solution Hg 1− x − y Cd x Mn y Te [7] . However, the complexity of this quaternary material makes finding alternative materials necessary. A natural substitute to Hg 1− x Mn x Te is the not so well-known Hg 1− x Mn x Se. As also shown in Fig. 1 , the range of solubility of Mn in HgSe (about 38%) allows a wavelength application window running from 1.0 to 12 μm, covering both the near and far infrared spectrum. Consequently, Hg 1− x Mn x Se appears to be a promising alternative to Hg 1− x Mn x Te and Hg 1− x Mn x Te as photodetective material in the infrared region. On the other hand and, as can be easily accepted, in order to fabricate HgMnSe-based devices, high-quality material, with good transport and optical properties at device level, is needed. The first synthesis and characterization of the solid solution HgTe–MnTe is traditionally attributed to Delves and Lewis [8] . Since then, several methods have been applied to grow Hg 1− x Mn x Te bulk crystals. Although the Bridgman method is the most extensively used, other techniques have also been employed, e.g., solid-state recrystallization [9] , travelling heater method [10,11] and a modified two-phase-mixture method [12] . On the other hand, and up to our knowledge, only the Bridgman method has been reported for the bulk growth of Hg 1− x Mn x Se crystals [13] . It is noticeable that the Se-rich phase diagram of Hg 1− x Mn x Se has not yet been stated, and the development of the bulk crystal growth methods for Hg 1− x Mn x Se is a matter of interest. An important trouble associated with the bulk growth of mercury-based DMSs materials in general, and with Hg 1− x Mn x Se, in particular, is the high pressure of Hg vapor during the alloy synthesis from the constituent in elemental form. Very thick wall ampoules have to be used to ensure the safety, and the Hg vapor convective streams generate inhomogeneities in Hg 1− x Mn x Se crystals, such as local off-stoichiometric regions and clusters of Hg vacancies. These inhomogeneities can seriously deteriorate the electronic characteristics of Hg 1− x Mn x Se. In some cases, they can hardly be corrected by means of very long (several weeks) post-growth annealing processes [2] . In this paper, in order to overcome the difficulties caused by Hg vapor during the growth of bulk Hg 1− x Mn x Se crystals, we propose the use of low-temperature pre-synthesized HgSe, instead of elemental mercury and selenium, as starting material in the growth process. So synthesized HgSe, together with appropriate amounts of elemental Mn and Se, will allow to achieve the desired composition. This method has been successfully applied to the crystal growth of Hg 1− x Mn x Te, from HgTe with Mn and Te [14] . 2 Crystal growth procedure As shortly explained before, Hg 1− x Mn x Se bulk crystals were produced by a two-step procedure including (I) the alloy synthesis using HgSe crystals grown by the cold travelling heater method (CTHM) [15] and elemental Mn and Se to complete the desired composition followed by (II) the Bridgman growth. Silica ampoules (10 mm I.D. and 13 mm O.D.) were used for the CTHM and, subsequently synthesis and Bridgman processes. The ampoules were submitted to a standard cleaning with organic solvents and aqua regia, shortly etched with a 20% aqueous solution of HF, repeatedly washed with deionized H 2 O, backed out under vacuum for 24 h at 1100 °C and graphitized by cracking of methane vapors. The HgSe starting material was synthesized and grown by the CTHM technique. The detailed process is described in [16] . The solvent zone consisted of a certain amount of selenium, depending on the Hg–Se phase diagram, growth temperature and thermal characteristics of furnace. The materials for the CTHM process were stoichiometric quantities of 9 N Hg and 6 N Se, which were charged into the ampoule that was, finally, sealed to a pressure of about 10 −6 Torr and placed into a home-made THM furnace, which was slowly heated to 670 °C. After stabilizing the temperature, the ampoule was lowered at a rate of 2.5 mm/day trough the furnace temperature profile. About the manganese, we should note that the higher temperatures achieved during the growth process usually leads the manganese to attack the quartz walls. This effect can be important in the HgMnSe case, with temperatures over 100 °C higher than in the HgMnTe case. For this reason, a sublimation process was applied to the commercially available Mn. A small amount of commercial manganese ( γ -Mn) was placed in an evacuated ampoule. The temperature was held at about 1000 °C during 12 h, in a dynamic vacuum. After this time, the manganese (now α -Mn, more resistant to the oxidation) was extracted. About the selenium, no cleaning process was necessary. The as-grown HgSe crystal was then loaded together with appropriated amounts of sublimated Mn and elemental Se into another ampoule which was sealed under vacuum (10 −6 Torr) and placed in a four zones vertical Bridgman furnace. A very slow temperature raising program was utilized for preventing damages due to exothermic reactions during synthesis. After the controlled raise of temperature, a plane zone at 880 °C, high enough to melt the charge, was established. This temperature was held for 48 h to allow a good homogenization of the charge. Finally, the ampoule was lowered at a rate of 1.5 mm/h through a 25 °C/cm temperature gradient to grow Hg 1− x Mn x Se. This way, ingots of 6–8 cm length were obtained. Several growths with different nominal compositions were grown and analyzed, as mentioned above. The Hg 1− x Mn x Se crystals were characterized by scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDAX), X-ray diffractometry (XRD), Fourier transformed infrared (FTIR) spectroscopy and magnetic-susceptibility measurements. 3 Characterization results and discussion In order to evaluate the morphological quality of the material, SEM studies were performed over a significant number of regions, taken from different ingots. So, the Hg 1− x Mn x Se ingots were axially cut with a wire saw, then mechanically slandered with SiC powder and mechanic-chemically polished in a 5% bromine-in-methanol solution. Over the cleaned surface, both secondary and backscattered modes were used. No inclusions or defects were observed at a magnification which ranged from ×150 to ×15000. For the EDAX axial compositional characterization, Hg and Mn percentage was determined by detecting the Hg L α and Mn K α radiation lines. Typical profiles of Mn concentration throughout the Hg 1− x Mn x Se ingot length, for nominal composition of x =0.09, are shown in Fig. 2 . With this composition, the band gap is expected to coincide with that of Hg 1− x Mn x Te for x =0.11, used in far-infrared specific applications. Due to the intrinsic segregation of the Bridgman method, there is a compositional gradient near the first frozen end zone end of the ingot. From local fitting calculations using the static Pfann model [2] , the Mn segregation coefficient was found to be about 2–2.5 at this starting region. These values are slightly less than those reported for the Hg 1− x Mn x Te growth with comparable composition and growth conditions [14] , and leads to more homogeneous material. After this initial region, the Mn concentration is roughly constant for the remainder ingot, except the end part of the ingot (i.e., the last to freeze zone of about 20% ingot length, which is not shown in Fig. 2 ). The radial composition uniformity was also checked by cutting several Hg 1− x Mn x Se crystals, perpendicular to the growth axis, in slices of 1 mm thickness. These samples were then mechanically and chemically polished, and further EDAX measurements were carried out. The areas closer to the axis of the ingot were found to have a very good homogeneity. The edges showed a slightly upper Mn concentration (1–3 at%) probably related to radial temperature gradients into the ingot and the interaction between the material and the ampoule during growth. The same behavior has also been reported for the growth of Hg 1− x Mn x Te crystals [14] . In order to, firstly, check the single phase morphology of crystals and, secondly, to determine the lattice parameter as a function of Mn concentration, homogeneous zones, which had been characterized by EDAX measurements, were powdered and used to perform standard θ –2 θ X-ray powder diffraction scans. Measurements were carried out in a Seifert XRD-3000TT two axis diffractometer. About the former studies, no other phases but the cubic one corresponding to the HgSe host was detected. Consequently, there is no need of post-annealing processes to force phase transitions. On the other hand, the dependence of the lattice parameter with the composition in the range x =0–0.20 is shown in Fig. 3 . As with the other semimagnetic semiconductors, we found a linear Vergard relationship in the cubic phase region [2] . We include in Fig. 3 the linear fit from the experimental data. Specifically, Hg 1− x Mn x Se lattice parameter could be used as a substrate in the range 6.085–5.937 Å. For the analysis of the evolution of band gap energy with Mn composition, far infrared transmittance measurements were performed on samples from different regions of the ingot. So, room temperature transmittance measurements were performed with the help of a Pelkin Elmer FTIR system. The studied samples were carefully slandered to a thickness between 100 and 150 μm while maintaining a mirror-like surface. Typical results of the FTIR measurements, from which absorption coefficient was calculated, are shown in Fig. 4 . As it is known, Hg 1− x Mn x Se has a direct band gap that can be derived, in a first approximation, from the squared absorption coefficient at room temperature. The band gap value was found to depend on the Mn concentration E g [ eV , 300 K ] = 4.17 x - 0.06 in good accord with the literature data obtained by the different methods [2] , and optimal to cover the full infrared region. With alternative applications in mind, magnetic measurements have also been performed. For magnetic measurements, several samples of about 1 mm 3 were prepared. The samples were selected from different zones of the ingot having Mn concentration of x =0.07, x =0.13 and x =0.18. After cleaning by etching in a bromine-in-methanol solution, the samples were grounded. Then, they were placed into a nonmagnetic capsule, suitable for the SQUID type susceptometer. The model was a MPMS-XL-5 of Quantum Design. Most of the II–Mn–VI semimagnetic semiconductors display the same magnetic behavior [2] . At the solubility limit, the magnetization shows a typical Brillouin-function response. On the other hand, at high temperature, magnetic susceptibility follows a Curie–Weiss law, representative of a Mn–Mn antiferromagnetic interaction. Finally, for higher manganese concentrations, a knee can be found in the susceptibility function at low temperature, indicative of a spin-glass transition [17] . Galazka and co-workers have explained this sort of behavior by including higher order terms in the calculation of the susceptibility. These new terms take account of the contribution to the susceptibility of clusters of doublets and triplets of manganese ions, distributed homogeneically [2,17] . Fig. 5 displays the results of the measured magnetic susceptibility of our samples compared with values obtained from [11] . The good agreement of both data indicates that our material has a homogeneous distribution of the manganese ions. So, the use of HgSe as a starting material does not obstruct the incorporation of the manganese ions in the cubic lattice. On the other hand, the Weiss temperature can be calculated from the extrapolation of the high temperature lineal response of magnetic susceptibility. Because we have found no previously reported values in the literature, we compare them to those calculated for Hg 1− x Mn x Te. As Fig. 6 shows, and as we could suspect, both compounds display a qualitatively analogous response. 4 Conclusions A technique for growing Hg 1− x Mn x Se bulk crystals by the Bridgman method using CTHM-grown HgSe as starting material instead of elemental Hg, with adequate amounts of Mn and Se, has been reported. This technique has proved to overcome the troubles related to elevated Hg vapor pressure that can produce off-stoichiometric regions and high concentration of Hg vacancies. The summary of the characterization results, mainly structural and optical, suggests that the parameters of Hg 1− x Mn x Se crystals grown by the proposed technique are good enough to make possible the matching of these crystals to specific applications, such as photoconductive detector in the infrared region. References [1] S. Von Molnar D. Read New materials for semiconductor spin-electronics Proc. IEEE 91 2003 715 726 [2] J.K. Furdyna J. Kossut Diluted Magnetic Semiconductors 1988 Academic San Diego, CA [3] A. Rogalski Hg 1− x Mn x Te as a new detector material Infrared Phys. 31 1991 117 166 [4] J. Piotrovski A. Rogalski New generation of infrared photodetectors Sensors Actuators A 67 1998 146 152 [5] L.A. Kosyachenko S.E. Ostapov A.V. Markov I.M. Rarenko Electronic transport properties of HgMnTe n(+)-p junctions Infrared Phys. Technol. 44 2003 1 10 [6] M.H. Rais C.A. Musca J.M. Dell J. Antoszewski B.D. Nener L. Faraone HgCdTe photovoltaic detectors fabricated using a new junction formation technology Microelectron. J. 31 2000 545 551 [7] O.A. Bodnaruk A.V. Markov S.E. Ostapov I.M. Rarenko A.F. Slonetskii Bandgap and intrinsic carrier concentration in HgCdMnTe and HgCdZnTe Semiconductors 34 2000 415 417 [8] R.T. Delves B. Lewis Zinc blende type HgTe–MnTe solid solutions (I) J. Phys. Chem. Solids 24 1963 549 556 [9] R.G. Mani T. McNair C.R. Lu R. Grober Crystal growth of Hg 1− x Mn x Te by solid state recrystallization J. Cryst. Growth 97 1989 617 621 [10] P. Gille U. Össner N. Puhlmann H. Niebsch T.T. Piotrowski Growth of Hg 1− x Mn x Te crystals by the traveling heater method Semicond. Sci. Technol. 10 1995 353 357 [11] C. Reig C.J. Gómez-García V. Muñoz-Sanjosé A new approach to the crystal growth of Hg 1− x Mn x Te by the cold travelling heater method J. Cryst. Growth 223 2001 357 362 [12] S. Takeyama S. Narita New techniques for growing highly-homogeneous quaternary semimagnetic semiconducting alloy Hg 1− x−y Cd x Mn y Te Jpn. J. Appl. Phys. 24 1985 1270 1273 [13] A. Pajaczkowska Physicochemical properties and crystal growth of AIIBVI–MnBVI systems Prog. Cryst. Growth Charact. 1 1978 289 326 [14] C. Reig N.V. Sochinskii V. Muñoz Low pressure synthesis and Bridgman growth of Hg 1− x Mn x Te J. Cryst. Growth 197 1999 688 693 [15] R. Triboulet P.V. Khoan G. Didier Cold Traveling Heater Method, a novel technique of synthesis, purification and growth of CdTe and ZnTe J. Cryst. Growth 101 1990 216 220 [16] C. Reig Y.S. Paranchych V. Muñoz-Sanjosé Crystal growth of HgSe by the cold travelling heater method Cryst. Growth Des. 2 2002 91 92 [17] G.D. Khattak C.D. Amarasekara S. Nagata R.R. Galazka P.H. Keesom Specific-heat, magnetic-susceptibility and the spin-glass transition in Hg 1− x Mn x Se Phys. Rev. B 23 1981 3353 3354 |
Year | DOI | Venue |
|---|---|---|
2007 | 10.1016/j.mejo.2007.01.016 | Microelectronics Journal |
Keywords | Field | DocType |
cold travelling heater method,hg high pressure,bridgman method,energy dispersive x-ray analysis,bridgman growth,crystal growth,x-ray diffractometry,xse bulk crystal,travelling heater method,high temperature synthesis reaction,xse crystal,infrared detection,infrared,magnetic susceptibility,scanning electron microscopy,high pressure,fourier transform infrared spectroscopy | Magnetic susceptibility,Analytical chemistry,Crystal growth,Scanning electron microscope,Fourier transform spectroscopy,Optics,Crystal,Engineering,Photodetection,Infrared,Condensed matter physics,Fourier transform infrared spectroscopy | Journal |
Volume | Issue | ISSN |
38 | 3 | Microelectronics Journal |
Citations | PageRank | References |
1 | 0.81 | 1 |
Authors | ||
3 | ||
Name | Order | Citations | PageRank |
|---|---|---|---|
| C. Reig | 1 | 1 | 1.15 |
| C. J. Gómez-García | 2 | 1 | 0.81 |
| V. Muñoz-Sanjosé | 3 | 1 | 0.81 |