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Effects of rotating magnetic field on the microstructure and properties of a GaInSb crystal
Vacuum 174 (2020) 109177
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Effects of rotating magnetic field on the microstructure and properties of a GaInSb crystal Qiang Liu a, Jinwei Wang a, Guofang He b, Donghai Yang c, Weicai Zhang c, Juncheng Liu a, b, * a
School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin, 300387, China College of Chemistry and Chemical Engineering, Taishan University, Taian, 271000, China c The 46th Research Institute, China Electronics Technology Group Corporation, Tianjin, 300220, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: GaInSb Crystal growth Segregation Dislocation density Rotating magnetic field
High-quality Ga0.86In0.14Sb ingots (Φ25 � 100) were prepared with vertical Bridgman method (VB) applied a rotating magnetic field (RMF). The microstructure and properties of the ingots were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), metallographic microscopy, and a Hall Effect testing system. RMF improved the crystallinity of the samples, reducing the full width at half maximum (FWHM) of the XRD characteristic peak of the GaInSb ingot from 21400 to 74". RMF significantly also reduced both of the radial and axial segregation of indium in the GaInSb crystal. The radial segregation in a 25–75 mm axial section decreased to 0.085 mol% from 0.247 mol% per mm, and the axial segregation to 0.047 mol% from 0.140 mol% per mm. RMF decreased the dislocation density (or the etch pit density; EPD) from 1.047 � 105 cm 2 to 6.988 � 103 cm 2. It also improved the electrical properties of the crystal, with the average value of the axial carrier mobility increasing from 1.437 � 103 cm2/(V⋅s) to 1.618 � 103 cm2/(V⋅s) and the average value of the axial resistivity reducing from 1.352 � 10 3 Ω⋅cm to 1.162 � 10 3 Ω⋅cm. Finally, the uniformity of the axial carrier mobility and resistivity also improved significantly.
1. Introduction Ga1-xInxSb is a typical Group III–V ternary semiconductor whose lattice constant can be adjusted by changing the value of x to match the lattice constant of the epitaxial layers of a substrate material. Modifying x can also adjust the electrical properties of the material such as its band gap width, to maximize device performance [1–3]. Given these advan tages, GaInSb crystals have been used in a variety of applications, including infrared detectors, semiconductor lasers, and in optoelec tronic communications [4–6]. However, many difficulties remain in the preparation of large high-quality single GaInSb crystals. First, indium has a high segregation tendency. GaInSb can be regarded as a pseudo-binary solid solution composed of GaSb and InSb. A pseudo-binary phase diagram of GaSb–InSb [7] shows that the two species do not share compatible reactive regions, directly causing inhomogeneous component distribution during crystal growth. The equilibrium segregation coefficient of indium in GaInSb is only 0.2 [8], making it difficult to precisely control the chemical composition ratio during crystal growth. At the same time, inhomogeneous solute distri bution at the solid-liquid interface due to component segregation also
leads to “constitutional supercooling” [9]. Second, since preparation processes are immature, the shape of the solid-liquid interface during crystal growth is not always flat or slightly convex, resulting in large thermal stresses in crystals resulting in increased dislocation density and increasing the number of defects such as twins, microcracks, and in clusions [10,11]. Several studies have focused on preparing high-quality GaInSb crystals by the effective suppression of indium segregation while reducing dislocation density and the number of defects. The first approach sought to improve crystal growth methods. Kumagawa et al. [12] adopted the Czochralski (Cz) method to grow GaInSb ingots in an ultrasonic environment. The presence of the ultrasonic waves allowed raw materials to better adhere to seed crystal during growth, improving the shape of the solid-liquid interface and the uniformity of indium distribution, while reducing the crystal dislocation density. Dutta et al. [13] adopted the accelerated crucible rotation technique (ACRT) with the vertical Bridgman (VB) method, which effectively restrained the axial and radial segregation of indium and reduced crystal impurities. Arivanandhan et al. [14] grew GaInSb ingots under microgravity with GaSb seeds to maintain the stability of the growth interface, control
* Corresponding author. School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin, 300387, China. E-mail address: [email protected] (J. Liu). https://doi.org/10.1016/j.vacuum.2020.109177 Received 24 July 2019; Received in revised form 31 December 2019; Accepted 7 January 2020 Available online 9 January 2020 0042-207X/© 2020 Elsevier Ltd. All rights reserved.
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Fig. 2. (a) The OT3 as-grown GaInSb ingot; (b) The GaInSb wafer.
length 200 mm. The polycrystalline ingots were synthesized in a swing furnace at 850 � C for 12 h. In order to minimize the O and N contami nation from the residual gas, after the starting materials were put into the quartz crucible, first filled the quartz crucible with high-purity Ar2, then vacuumed to 1x10 3Pa, then filled the high-purity Ar2 to a pressure higher than one standard atmospheric pressure, then vacuumed to 1x10 3Pa again, and then sealed the quartz crucible with oxyacetylene flame. Noriaki Murakami etc. [22] prepared InGaSb crystal, they sealed the crucible at a vacuum degree lower than 1x10 4pa, and obtained high quality crystal. When Gadkari etc. [23] prepared GaSb crystals, they also adopted a similar process of vacuuming the crucible, filling Ar2 and vacuuming again to reduce the contamination of O and N from the re sidual gas in the crucible, and got higher quality crystals. The crystal ingots grew in a five-temperature-zone RMF-VB furnace (Fig. 1) at a growth rate of 2.0 mm/h and a temperature gradient of 10 � C/cm. The RMF intensity was constant at 30 mT, its frequency was set to 0 Hz, 25 Hz and 50 Hz, and the prepared ingots were labeled as OT1, OT2, and OT3, respectively. The OT3 as-grown GaInSb ingot is shown in Fig. 2(a). The ingots were then cut with a diamond slicer along the axis of the ingot, 3-mm-thick wafers were cut out from specific locations to prepare samples for test and characterization, see Fig. 2(a). For Hall Effect measurement, the sample was taken from the center of each wafer, a circular area with a diameter of about 10 mm, and its size was 5 mm � 5 mm � 3 mm. In order to compare the influence of varying RMF fre quency on the microstructure and properties of the ingots, three wafers were selected from the same axial position of each ingot. The OT1 wafers were labeled as a1, a2, and a3, the OT2 wafers b1, b2, and b3, and the OT3 wafers c1, c2, and c3. The c2 wafer is shown in Fig. 2(b). All the wafers were mechanically polished with Al2O3 suspensions to obtain mirror-like surfaces and etched with an etching solution (HNO3:HF: CH3COOH:H2O ¼ 1:1:2:5) to further remove the surface damage. The chemical composition was measured at five different points from the same origin on each wafer, as shown in Fig. 2(b). Point 3 was the center of the wafer, Points 1 and 5 were located 10 mm radially from the center, and Points 2 and 4 were located 5 mm radially from the center. These five test points were selected on two mutually perpendicular radii of a wafer surface, and the radii determined by points 1, 2 and 3 on each wafer were located on the same radial cross section as far as possible, so were points 3, 4 and 5. The measured value of point 3, the average value of points 1 and 5, and the average value of points 2 and 4, these three values were used to reflect the radial distribution of In. The etch pit densities (EPDs) of the wafers were measured by the international general 69-point counting [24], and calculated according to the following formula:
Fig. 1. Schematic diagram of RMF-VB crystal growth furnace.
crystal composition, and reduce component segregation. Houchens et al. [15] grew GaInSb ingots with the horizontal traveling heater method (HTHM) to control the component ratio and reduce the number of structural defects. These studies showed that sample components were uniform and the VGa (Ga vacancy) and dislocation density decreased. Gadkari et al. [16] grew GaInSb ingots with a modified vertical direc tional solidification (VDS) technique, creating samples with excellent electrical properties. The variation of either axial or radial indium concentration was less than 10%, and the dislocation density reduced to 1 � 103 cm 2. The second approach was to improve crystal quality via doping. Streicher et al. [17] used the VB method to grow a Ga0.8In0.2Sb ingot containing a trace amount of aluminum, which successfully inhibited indium segregation. The components of the ingot were uni form and its electrical properties were excellent. The third technique improved crystal quality by optimizing the parameters used during crystal growth. Mitric et al. [18] attempted to improve the quality of GaInSb ingots grown using the VB method by adjusting the intensity of an alternating magnetic field. However, this alternating magnetic field did not significantly reduce the radial segregation in the ingots. Barat et al. [19] studied the influence of a temperature gradient and velocity of a moving crucible on indium segregation in GaInSb during growth with the VB method, again observing that axial and radial segregation were not effectively suppressed. Stelian et al. [20] studied the VB growth of GaInSb by numerical simulation, and showed that higher solute con centrations increased the interfacial curvature during crystal growth, but decreased the stability of the solid-liquid interface, and increased the €tzold et al. [21] applied a axial and radial segregation in the ingot. Pa rotating magnetic field (RMF) during the growth of a single Ga-doped Ge crystal using the vertical gradient freezing (VGF) method, and studied the effect of the RMF frequency and intensity on the microstructure and properties of the Ge crystal Their results indicated that using a suitable magnetic field frequency and intensity can significantly improve the shape of the crystal solid-liquid interface, effectively reduce Ga segre gation in the crystal, and improve the electrical properties of the crystal. In this work, an RMF was used during GaInSb crystal growth with the VB method to improve component segregation. The effects of the RMF on the microstructure and properties of GaInSb crystals were investigated. 2. Experimental methods High-purity Sb (99.9999%), Ga (99.9999%), and In (99.999%) pel lets were used to grow Ga0.86In0.14Sb. Pellets were sealed in a highpurity quartz vacuum tube with an inner diameter of 25 mm and
EPD ¼ where 2
Σ ns P
(1) is the total number of etch pits, n ¼ 69 is the number of
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doping did not change the zinc sphalerite crystal structure of GaSb crystal, but changed the carrier type from p-type to n-type, reduced the number of GaSb acceptor states, improved the crystallinity, decreased the FWHM; and the dislocation density decreased to 1.275 � 103 cm 2, the carrier mobility increased to 2.512 � 103 cm2/(V�s), the resistivity decreased to 0.521 � 10 3 Ω cm. However, the In segregation in ingot was serious, the radial and axial segregation reached 2.32% and 4.84% per millimeter respectively. Herein, In segregation was decreased with the application of RMF. With the application of RMF and its increase of rotating frequency, In element was promoted to be the donor to replace the Ga position in the GaSb lattice, or fill the vacancy of Ga at the re sidual acceptor state and combine with it, or as the newly doped donor impurity to directly capture the electrons released by Ga vacancy and release the energy equivalent to the gap band of GaSb, so as to reduce the content of the GaSb acceptor state. Compared with OT1, the diffraction peaks of OT2 and OT3 are sharper and their FWHMs are smaller, indicating that the composition and structure of ingots are more uniform and the crystallinity is significantly improved.
Fig. 3. Effect of RMF on crystal XRD patterns (using a2, b2, c2 wafers).
3.2. Effect of RMF on crystal compositional distribution
counting points, and s is the area under the metallographic microscope. The ingot crystallization phase and crystallinity were characterized with X-ray diffraction (XRD; Bruker D8 Advance; Cu-Kα1, λ ¼ 0.15405 nm, 2θ ¼ 10–80� ), and ingot morphology was analyzed with scanning electron microscopy (SEM; Hitachi S-4800). The chemical compositions of the wafers were measured with energy dispersive spectroscopy (EDS; Hitachi S-4800). The dislocation densities of the wafers were measured with metallographic microscopy (Bimu 4XBE). The carrier mobility and resistivity of the wafers were measured with a Hall Effect testing system (Ecopia HMS-3000).
In order to analyze the segregation of indium in GaInSb ingots, the indium content at five individual points were measured with EDS using the GaInSb wafers, as shown in Fig. 1(b) and Table 1. The radial segregation of indium in an axial section 25–75 mm of the OT1, OT2, and OT3 ingots were 0.247 mol%, 0.144 mol%, and 0.085 mol% per mm, respectively, and axial segregation was 0.140 mol%, 0.076 mol%, and 0.047 mol% per mm, respectively. The experimental results of the Arivanandhan group [14] showed that indium axial segregation was 0.139 mol% per mm. The experimental results of the Gadkari group [16] showed that the axial and radial segregation of indium were closer to 0.200 mol% per mm. Compared to the above studies, the indium segregation obtained in this study was lower, while its uniformity was higher. The radial and axial segregation of indium in the three ingots are shown in Fig. 4. Ingot OT3 had the smallest radial and axial segregation, and the best quality. This is because the applied RMF strengthened the convection and heat and mass transfer in the melt. This caused the temperature gradient and diffusion layer thickness at the front of the solid-liquid interface to decrease, the probability of indium content deviating from its theoret ical value to decrease, and the effective segregation coefficient to in crease. Thus, the radial segregation of indium was significantly reduced, and axial segregation was inhibited effectively. Increasing the rotation frequency further strengthened the convection and heat and mass transfer in the melt, further decreasing the segregation of indium. However, if the frequency was too large or small, two melt streams at different temperatures would meet at the solid-liquid interface, leading to an asymmetric temperature distribution or uneven diffusion layer thickness, increasing indium segregation. Therefore, the segregation of indium can be effectively reduced only by using the proper rotation frequency [26,27].
3. Results and discussion 3.1. Effect of RMF on the structures of the crystal The XRD patterns of the ingots OT1, OT2, and OT3 are shown in Fig. 3. The main diffraction peaks of the three ingots are similar, indi cating that the application of the RMF and the changing rotation fre quency did not affect the zinc blende structure of GaInSb. The diffraction data of the (111) face of ingot OT1 were 2θ ¼ 25.325� , d ¼ 3.5139 Å, and FWHM ¼ 21400 , while those of the (111) face of ingot OT2 were 2θ ¼ 25.219� , d ¼ 3.5284 Å, and FWHM ¼ 10300 , and those of the (111) face of ingot OT3 were 2θ ¼ 25.139� , d ¼ 3.5395 Å, and FWHM ¼ 74". Comparing these data, when the RMF was applied and the rotation frequency increased, the crystal diffraction angle decreased and the overall trend shifted to the left. An increased lattice constant resulted in a decreased of the crystal FWHM. The analysis of X-ray diffraction (XRD) is mainly based on the Bragg diffraction equation, in which the interplanar spacing is a basic parameter, which is affected significantly by the kinds of ions in the lattice, including the ion radius and its charge number, dislocation, the residual stress and other structural defects. The residual stress, vacancy, dislocation and other defects in the crystal will reduce the intensity of the diffraction peak and increase the width of the diffraction peak. Wang et al. [25] prepared GaSb crystal and GaInSb crystals with vertical Bridgman method, and studied the influence of In doping on the struc ture and physical properties of GaSb crystal. The results showed that In
3.3. Effect of RMF on EPDs of the crystal The etch pits of the (100) faces of ingots OT1, OT2, and OT3 are shown in Fig. 5. The calculated EPDs of the faces were 1.047 � 105 cm 2, 7.264 � 103 cm 2, and 6.988 � 103 cm 2, respectively. In
Table 1 Indium concentration in GaInSb wafers (unit: at%). 1(5) 2(4) 3
a1
a2
a3
b1
b2
b3
c1
c2
c3
4.05* 3.16* 3.04
6.96* 5.64* 4.49
10.02* 8.95* 8.23
4.18* 3.55* 3.31
6.59* 5.47* 5.15
10.13* 9.73* 7.12
4.06* 3.56* 3.68
6.44* 5.72* 5.59
9.87* 8.59* 6.03
*: Value acquired from the average value of two points (1 and 5 or 2 and 4). 3
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Fig. 4. Effect of RMF on indium segregation. (a) Indium concentration as a function of radial position of GaInSb ingots, using a2, b2, and c2. (b) Indium concen tration as a function of axial position in GaInSb ingots, using Point 3 as an example.
Fig. 5. Effect of RMF on crystal EPDs, using a2, b2, and c2 as examples. EPDs of the (100) faces of (a) OT1, (b) OT2, and (c) OT3.
comparison, the Udayashankar group [10] showed that the EPD of the crystal was 7.008 � 105 cm 2, while the Tanaka group [11] showed that the EPD of the crystal was 5.624 � 104 cm 2. Compared to the above studies, herein the ingot EPDs were lower and the quality of the ingots was better. Further comparison of the EPDs of the three ingots in this experiment shows that when the RMF was applied, the EPD of the ingots significantly decreased and their distri bution gradually became uniform. The thermal stress is often the main cause of dislocations, which can move and proliferate under stress. The critical resolved shear stress (CRSS) model had been used to study the dislocation movement and proliferation in GaAs single crystals [28,29] and CdZnTe single crystals [30]. The CRSS to produce dislocations in the region near the solid-liquid interface is rather low, and the curvature of the solid-liquid interface plays an important role on the thermal stress generated in the crystal. A flat interface can reduce the thermal stress caused by a temperature gradient [31]. After applying an RMF, the convection and heat and mass transfer in the melt were strengthened.
The radial temperature gradient and maximum temperature difference of the solid-liquid interface were reduced, and the crystal should have a flatter solid-liquid interface. Thus, the thermal stress within the crystal and the EPD decreased, and the latter by one order of magnitude. However, when the rotation frequency increased from 25 Hz to 50 Hz, the EPD of the ingot did not decrease significantly, indicating that the modification of rotating frequency did not affect the ingot EPD. 3.4. Effect of RMF on the electrical properties of a crystal The carrier mobility and resistivity are important electrical proper ties of semiconductor materials, and directly determine the application of a material and its performance in semiconductor devices. Three ingots were tested at room temperature using a Hall Effect testing system, and the carrier mobility (μ) and resistivity (ρ) were calculated according to the following formula [32]:
4
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Fig. 6. Effect of RMF on crystal electrical properties, shown through distributions along the direction of crystal growth of the (a) mobility and (b) resistivity.
μ¼
jRH j
ρ
; ρ¼
V σ ad Il
carrier mobility increased from 1.437 � 103 cm2/(V⋅s) to 1.618 � 103 cm2/(V⋅s), and the average value of the axial resistivity decreased from 1.352 � 10 3 Ω⋅cm to 1.162 � 10 3 Ω⋅cm. Meanwhile, the uniformity of the axial carrier mobility and re sistivity also improved significantly.
(2)
where RH is the Hall coefficient, Vσ is the applied potential difference, I is the test current, a is the sample thickness, d is the sample width, and l is the sample length. The carrier mobility and resistivity distributions along the crystal growth axis for the three ingots are shown in Fig. 6. The carrier mobility increased (to 1.618 � 103 cm2/(V⋅s)) and the resistivity decreased (to 1.162 � 10 3 Ω⋅cm) by applying the RMF, and both the carrier mobility and resistivity were more uniform along the growth direction. These changes were even more pronounced when the rotation frequency increased. Carrier mobility reflects the average motion rate of carriers per unit electric field intensity and the conductivity of semiconductors. Carrier mobility is mainly affected by the scatterings in crystal, such as lattice vibration scattering, ionized impurity scattering, dislocation scattering, and other scatterings, including equivalent inter-valley scattering, neutral impurity scattering etc. [33] As mentioned above, RMF promoted the solid solution of In GaSb lattice, reduced the va cancies of Ga and the residual acceptor states, as well as In segregation, all of which could decrease the ionized impurity scattering and equiv alent inter-valley scattering significantly. Furthermore, RMF also reduced the dislocation density of the crystal, thus decreased the dislocation scattering. And RMF improved the crystallinity of the crys tal, i.e., the order of atomic lattice arrangement was improved, so the lattice vibration scattering got suppressed to some extent. And increasing the rotation frequency further strengthened this effect of RMF.
Declaration of interest statement No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 50972085), Advanced Research of Equipment Sharing Technology (No. 41422030103), and Key Technology R&D Program of Shandong Province (No. 2017GGX20104). References [1] B. Krishan, P.B. Barman, G.S. Mudahar, N.P. Singh, Growth of GaxIn1 xSb bulk crystals for infrared device applications by vertical Bridgman technique, Mater. Lett. 58 (2004) 1441–1445. [2] S. Abroug, F. Saadallah, N. Yacoubi, S. Abroug, Photothermal investigations of doping effects on opto-thermal properties of bulk GaSb, J. Alloy. Comp. 484 (2009) 772–776. [3] D.B. Gadkari, P. Shashidharan, K.B. Lal, N.A. Gokhale, A.P. Shah, B.M. Arora, Growth morphology and compositional analysis of InxGa1 xSb crystals-grown by vertical directional solidification technique, Indian J. Pure Appl. Phys. 37 (1999) 652–656. [4] I. Vurgaftman, G. Belenky, Y. Lin, D. Donetsky, L. Shterengas, G. Kipshidze, W. L. Sarney, S.P. Svensson, Interband absorption strength in long-wave infrared typeII superlattices with small and large superlattice periods compared to bulk materials, Appl. Phys. Lett. 108 (2016) 187–190. [5] D. Z. Ting, S. D. Gunapala, A. Soibel, J. Nguyen, A. Khoshakhlagh, Single-band and Dual-Band Infrared Detectors, U.S. Patent No. 8928029. 6 Jan. 2015. [6] R. Peng, S. Jiao, H. Li, L. Zhao, Dark current mechanisms investigation of surface passivation InAs/GaSb photodiodes at low temperatures, J. Alloy. Comp. 632 (2015) 575–579. [7] G.B. Stringfellow, Calculation of ternary phase diagrams of III–V systems, J. Phys. Chem. Solids 33 (1972) 665–677. [8] G.N. Kozhemyakin, Indium inhomogeneity in InxGa1 xSb ternary crystals grown by floating crucible Czochralski method, J. Cryst. Growth 220 (2000) 39–45. [9] J. Vincent, E. Di� eguez, Microstructure and solidification behavior of cast GaInSb alloys, J. Cryst. Growth 295 (2006) 108–113. [10] N.K. Udayashankar, K.G. Naik, H.L. Bhat, The influence of temperature gradient and lowering speed on the melt–solid interface shape of GaxIn1 xSb alloy crystals grown by vertical Bridgman technique, J. Cryst. Growth 203 (1999) 333–339. [11] A. Tanaka, J. Shintani, M. Kimura, T. sukegawa, Multi-step pulling of GaInSb bulk crystal from ternary solution, J. Cryst. Growth 209 (2000) 625–629. [12] M. Kumagawa, T. Tsuruta, N. Nishida, Y. Hayakawa, On voids in InxGa1 xSb crystals grown by an ultrasonic-vibration-introduced Czochralski method, Cryst. Res. Technol. 29 (1994) 1037–1044.
4. Conclusions (1) A RMF was applied to the VB process of Ga0.86In0.14Sb crystal growth, and high-quality GaInSb ingots (Φ25 mm � 100 mm) were prepared. (2) The RMF improved the crystallinity. A 50 Hz rotating frequency decreased the FWHM of the GaInSb ingot to 7400 , which was much lower than the 21400 of an ingot not exposed to the RMF. (3) The RMF significantly reduced both the radial and axial segre gation of indium in the GaInSb crystal. It reduced the radial segregation of indium in a 25–75 mm axial section of OT3 ingot to 0.085 mol% per mm, from the 0.247 mol% per mm for OT1 ingot not exposed to RMF, and the axial segregation to 0.047 mol% per mm from the 0.140 mol% per mm for OT1 ingot. (4) The applied RMF decreased the dislocation density of the crystal from 1.047 � 105 cm 2 to 6.988 � 103 cm 2, and improved the electrical properties of the crystal. The average value of the axial 5
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Vacuum journal homepage: http://www.elsevier.com/locate/vacuum
Effects of rotating magnetic field on the microstructure and properties of a GaInSb crystal Qiang Liu a, Jinwei Wang a, Guofang He b, Donghai Yang c, Weicai Zhang c, Juncheng Liu a, b, * a
School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin, 300387, China College of Chemistry and Chemical Engineering, Taishan University, Taian, 271000, China c The 46th Research Institute, China Electronics Technology Group Corporation, Tianjin, 300220, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: GaInSb Crystal growth Segregation Dislocation density Rotating magnetic field
High-quality Ga0.86In0.14Sb ingots (Φ25 � 100) were prepared with vertical Bridgman method (VB) applied a rotating magnetic field (RMF). The microstructure and properties of the ingots were characterized with X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), metallographic microscopy, and a Hall Effect testing system. RMF improved the crystallinity of the samples, reducing the full width at half maximum (FWHM) of the XRD characteristic peak of the GaInSb ingot from 21400 to 74". RMF significantly also reduced both of the radial and axial segregation of indium in the GaInSb crystal. The radial segregation in a 25–75 mm axial section decreased to 0.085 mol% from 0.247 mol% per mm, and the axial segregation to 0.047 mol% from 0.140 mol% per mm. RMF decreased the dislocation density (or the etch pit density; EPD) from 1.047 � 105 cm 2 to 6.988 � 103 cm 2. It also improved the electrical properties of the crystal, with the average value of the axial carrier mobility increasing from 1.437 � 103 cm2/(V⋅s) to 1.618 � 103 cm2/(V⋅s) and the average value of the axial resistivity reducing from 1.352 � 10 3 Ω⋅cm to 1.162 � 10 3 Ω⋅cm. Finally, the uniformity of the axial carrier mobility and resistivity also improved significantly.
1. Introduction Ga1-xInxSb is a typical Group III–V ternary semiconductor whose lattice constant can be adjusted by changing the value of x to match the lattice constant of the epitaxial layers of a substrate material. Modifying x can also adjust the electrical properties of the material such as its band gap width, to maximize device performance [1–3]. Given these advan tages, GaInSb crystals have been used in a variety of applications, including infrared detectors, semiconductor lasers, and in optoelec tronic communications [4–6]. However, many difficulties remain in the preparation of large high-quality single GaInSb crystals. First, indium has a high segregation tendency. GaInSb can be regarded as a pseudo-binary solid solution composed of GaSb and InSb. A pseudo-binary phase diagram of GaSb–InSb [7] shows that the two species do not share compatible reactive regions, directly causing inhomogeneous component distribution during crystal growth. The equilibrium segregation coefficient of indium in GaInSb is only 0.2 [8], making it difficult to precisely control the chemical composition ratio during crystal growth. At the same time, inhomogeneous solute distri bution at the solid-liquid interface due to component segregation also
leads to “constitutional supercooling” [9]. Second, since preparation processes are immature, the shape of the solid-liquid interface during crystal growth is not always flat or slightly convex, resulting in large thermal stresses in crystals resulting in increased dislocation density and increasing the number of defects such as twins, microcracks, and in clusions [10,11]. Several studies have focused on preparing high-quality GaInSb crystals by the effective suppression of indium segregation while reducing dislocation density and the number of defects. The first approach sought to improve crystal growth methods. Kumagawa et al. [12] adopted the Czochralski (Cz) method to grow GaInSb ingots in an ultrasonic environment. The presence of the ultrasonic waves allowed raw materials to better adhere to seed crystal during growth, improving the shape of the solid-liquid interface and the uniformity of indium distribution, while reducing the crystal dislocation density. Dutta et al. [13] adopted the accelerated crucible rotation technique (ACRT) with the vertical Bridgman (VB) method, which effectively restrained the axial and radial segregation of indium and reduced crystal impurities. Arivanandhan et al. [14] grew GaInSb ingots under microgravity with GaSb seeds to maintain the stability of the growth interface, control
* Corresponding author. School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin, 300387, China. E-mail address: [email protected] (J. Liu). https://doi.org/10.1016/j.vacuum.2020.109177 Received 24 July 2019; Received in revised form 31 December 2019; Accepted 7 January 2020 Available online 9 January 2020 0042-207X/© 2020 Elsevier Ltd. All rights reserved.
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Fig. 2. (a) The OT3 as-grown GaInSb ingot; (b) The GaInSb wafer.
length 200 mm. The polycrystalline ingots were synthesized in a swing furnace at 850 � C for 12 h. In order to minimize the O and N contami nation from the residual gas, after the starting materials were put into the quartz crucible, first filled the quartz crucible with high-purity Ar2, then vacuumed to 1x10 3Pa, then filled the high-purity Ar2 to a pressure higher than one standard atmospheric pressure, then vacuumed to 1x10 3Pa again, and then sealed the quartz crucible with oxyacetylene flame. Noriaki Murakami etc. [22] prepared InGaSb crystal, they sealed the crucible at a vacuum degree lower than 1x10 4pa, and obtained high quality crystal. When Gadkari etc. [23] prepared GaSb crystals, they also adopted a similar process of vacuuming the crucible, filling Ar2 and vacuuming again to reduce the contamination of O and N from the re sidual gas in the crucible, and got higher quality crystals. The crystal ingots grew in a five-temperature-zone RMF-VB furnace (Fig. 1) at a growth rate of 2.0 mm/h and a temperature gradient of 10 � C/cm. The RMF intensity was constant at 30 mT, its frequency was set to 0 Hz, 25 Hz and 50 Hz, and the prepared ingots were labeled as OT1, OT2, and OT3, respectively. The OT3 as-grown GaInSb ingot is shown in Fig. 2(a). The ingots were then cut with a diamond slicer along the axis of the ingot, 3-mm-thick wafers were cut out from specific locations to prepare samples for test and characterization, see Fig. 2(a). For Hall Effect measurement, the sample was taken from the center of each wafer, a circular area with a diameter of about 10 mm, and its size was 5 mm � 5 mm � 3 mm. In order to compare the influence of varying RMF fre quency on the microstructure and properties of the ingots, three wafers were selected from the same axial position of each ingot. The OT1 wafers were labeled as a1, a2, and a3, the OT2 wafers b1, b2, and b3, and the OT3 wafers c1, c2, and c3. The c2 wafer is shown in Fig. 2(b). All the wafers were mechanically polished with Al2O3 suspensions to obtain mirror-like surfaces and etched with an etching solution (HNO3:HF: CH3COOH:H2O ¼ 1:1:2:5) to further remove the surface damage. The chemical composition was measured at five different points from the same origin on each wafer, as shown in Fig. 2(b). Point 3 was the center of the wafer, Points 1 and 5 were located 10 mm radially from the center, and Points 2 and 4 were located 5 mm radially from the center. These five test points were selected on two mutually perpendicular radii of a wafer surface, and the radii determined by points 1, 2 and 3 on each wafer were located on the same radial cross section as far as possible, so were points 3, 4 and 5. The measured value of point 3, the average value of points 1 and 5, and the average value of points 2 and 4, these three values were used to reflect the radial distribution of In. The etch pit densities (EPDs) of the wafers were measured by the international general 69-point counting [24], and calculated according to the following formula:
Fig. 1. Schematic diagram of RMF-VB crystal growth furnace.
crystal composition, and reduce component segregation. Houchens et al. [15] grew GaInSb ingots with the horizontal traveling heater method (HTHM) to control the component ratio and reduce the number of structural defects. These studies showed that sample components were uniform and the VGa (Ga vacancy) and dislocation density decreased. Gadkari et al. [16] grew GaInSb ingots with a modified vertical direc tional solidification (VDS) technique, creating samples with excellent electrical properties. The variation of either axial or radial indium concentration was less than 10%, and the dislocation density reduced to 1 � 103 cm 2. The second approach was to improve crystal quality via doping. Streicher et al. [17] used the VB method to grow a Ga0.8In0.2Sb ingot containing a trace amount of aluminum, which successfully inhibited indium segregation. The components of the ingot were uni form and its electrical properties were excellent. The third technique improved crystal quality by optimizing the parameters used during crystal growth. Mitric et al. [18] attempted to improve the quality of GaInSb ingots grown using the VB method by adjusting the intensity of an alternating magnetic field. However, this alternating magnetic field did not significantly reduce the radial segregation in the ingots. Barat et al. [19] studied the influence of a temperature gradient and velocity of a moving crucible on indium segregation in GaInSb during growth with the VB method, again observing that axial and radial segregation were not effectively suppressed. Stelian et al. [20] studied the VB growth of GaInSb by numerical simulation, and showed that higher solute con centrations increased the interfacial curvature during crystal growth, but decreased the stability of the solid-liquid interface, and increased the €tzold et al. [21] applied a axial and radial segregation in the ingot. Pa rotating magnetic field (RMF) during the growth of a single Ga-doped Ge crystal using the vertical gradient freezing (VGF) method, and studied the effect of the RMF frequency and intensity on the microstructure and properties of the Ge crystal Their results indicated that using a suitable magnetic field frequency and intensity can significantly improve the shape of the crystal solid-liquid interface, effectively reduce Ga segre gation in the crystal, and improve the electrical properties of the crystal. In this work, an RMF was used during GaInSb crystal growth with the VB method to improve component segregation. The effects of the RMF on the microstructure and properties of GaInSb crystals were investigated. 2. Experimental methods High-purity Sb (99.9999%), Ga (99.9999%), and In (99.999%) pel lets were used to grow Ga0.86In0.14Sb. Pellets were sealed in a highpurity quartz vacuum tube with an inner diameter of 25 mm and
EPD ¼ where 2
Σ ns P
(1) is the total number of etch pits, n ¼ 69 is the number of
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doping did not change the zinc sphalerite crystal structure of GaSb crystal, but changed the carrier type from p-type to n-type, reduced the number of GaSb acceptor states, improved the crystallinity, decreased the FWHM; and the dislocation density decreased to 1.275 � 103 cm 2, the carrier mobility increased to 2.512 � 103 cm2/(V�s), the resistivity decreased to 0.521 � 10 3 Ω cm. However, the In segregation in ingot was serious, the radial and axial segregation reached 2.32% and 4.84% per millimeter respectively. Herein, In segregation was decreased with the application of RMF. With the application of RMF and its increase of rotating frequency, In element was promoted to be the donor to replace the Ga position in the GaSb lattice, or fill the vacancy of Ga at the re sidual acceptor state and combine with it, or as the newly doped donor impurity to directly capture the electrons released by Ga vacancy and release the energy equivalent to the gap band of GaSb, so as to reduce the content of the GaSb acceptor state. Compared with OT1, the diffraction peaks of OT2 and OT3 are sharper and their FWHMs are smaller, indicating that the composition and structure of ingots are more uniform and the crystallinity is significantly improved.
Fig. 3. Effect of RMF on crystal XRD patterns (using a2, b2, c2 wafers).
3.2. Effect of RMF on crystal compositional distribution
counting points, and s is the area under the metallographic microscope. The ingot crystallization phase and crystallinity were characterized with X-ray diffraction (XRD; Bruker D8 Advance; Cu-Kα1, λ ¼ 0.15405 nm, 2θ ¼ 10–80� ), and ingot morphology was analyzed with scanning electron microscopy (SEM; Hitachi S-4800). The chemical compositions of the wafers were measured with energy dispersive spectroscopy (EDS; Hitachi S-4800). The dislocation densities of the wafers were measured with metallographic microscopy (Bimu 4XBE). The carrier mobility and resistivity of the wafers were measured with a Hall Effect testing system (Ecopia HMS-3000).
In order to analyze the segregation of indium in GaInSb ingots, the indium content at five individual points were measured with EDS using the GaInSb wafers, as shown in Fig. 1(b) and Table 1. The radial segregation of indium in an axial section 25–75 mm of the OT1, OT2, and OT3 ingots were 0.247 mol%, 0.144 mol%, and 0.085 mol% per mm, respectively, and axial segregation was 0.140 mol%, 0.076 mol%, and 0.047 mol% per mm, respectively. The experimental results of the Arivanandhan group [14] showed that indium axial segregation was 0.139 mol% per mm. The experimental results of the Gadkari group [16] showed that the axial and radial segregation of indium were closer to 0.200 mol% per mm. Compared to the above studies, the indium segregation obtained in this study was lower, while its uniformity was higher. The radial and axial segregation of indium in the three ingots are shown in Fig. 4. Ingot OT3 had the smallest radial and axial segregation, and the best quality. This is because the applied RMF strengthened the convection and heat and mass transfer in the melt. This caused the temperature gradient and diffusion layer thickness at the front of the solid-liquid interface to decrease, the probability of indium content deviating from its theoret ical value to decrease, and the effective segregation coefficient to in crease. Thus, the radial segregation of indium was significantly reduced, and axial segregation was inhibited effectively. Increasing the rotation frequency further strengthened the convection and heat and mass transfer in the melt, further decreasing the segregation of indium. However, if the frequency was too large or small, two melt streams at different temperatures would meet at the solid-liquid interface, leading to an asymmetric temperature distribution or uneven diffusion layer thickness, increasing indium segregation. Therefore, the segregation of indium can be effectively reduced only by using the proper rotation frequency [26,27].
3. Results and discussion 3.1. Effect of RMF on the structures of the crystal The XRD patterns of the ingots OT1, OT2, and OT3 are shown in Fig. 3. The main diffraction peaks of the three ingots are similar, indi cating that the application of the RMF and the changing rotation fre quency did not affect the zinc blende structure of GaInSb. The diffraction data of the (111) face of ingot OT1 were 2θ ¼ 25.325� , d ¼ 3.5139 Å, and FWHM ¼ 21400 , while those of the (111) face of ingot OT2 were 2θ ¼ 25.219� , d ¼ 3.5284 Å, and FWHM ¼ 10300 , and those of the (111) face of ingot OT3 were 2θ ¼ 25.139� , d ¼ 3.5395 Å, and FWHM ¼ 74". Comparing these data, when the RMF was applied and the rotation frequency increased, the crystal diffraction angle decreased and the overall trend shifted to the left. An increased lattice constant resulted in a decreased of the crystal FWHM. The analysis of X-ray diffraction (XRD) is mainly based on the Bragg diffraction equation, in which the interplanar spacing is a basic parameter, which is affected significantly by the kinds of ions in the lattice, including the ion radius and its charge number, dislocation, the residual stress and other structural defects. The residual stress, vacancy, dislocation and other defects in the crystal will reduce the intensity of the diffraction peak and increase the width of the diffraction peak. Wang et al. [25] prepared GaSb crystal and GaInSb crystals with vertical Bridgman method, and studied the influence of In doping on the struc ture and physical properties of GaSb crystal. The results showed that In
3.3. Effect of RMF on EPDs of the crystal The etch pits of the (100) faces of ingots OT1, OT2, and OT3 are shown in Fig. 5. The calculated EPDs of the faces were 1.047 � 105 cm 2, 7.264 � 103 cm 2, and 6.988 � 103 cm 2, respectively. In
Table 1 Indium concentration in GaInSb wafers (unit: at%). 1(5) 2(4) 3
a1
a2
a3
b1
b2
b3
c1
c2
c3
4.05* 3.16* 3.04
6.96* 5.64* 4.49
10.02* 8.95* 8.23
4.18* 3.55* 3.31
6.59* 5.47* 5.15
10.13* 9.73* 7.12
4.06* 3.56* 3.68
6.44* 5.72* 5.59
9.87* 8.59* 6.03
*: Value acquired from the average value of two points (1 and 5 or 2 and 4). 3
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Fig. 4. Effect of RMF on indium segregation. (a) Indium concentration as a function of radial position of GaInSb ingots, using a2, b2, and c2. (b) Indium concen tration as a function of axial position in GaInSb ingots, using Point 3 as an example.
Fig. 5. Effect of RMF on crystal EPDs, using a2, b2, and c2 as examples. EPDs of the (100) faces of (a) OT1, (b) OT2, and (c) OT3.
comparison, the Udayashankar group [10] showed that the EPD of the crystal was 7.008 � 105 cm 2, while the Tanaka group [11] showed that the EPD of the crystal was 5.624 � 104 cm 2. Compared to the above studies, herein the ingot EPDs were lower and the quality of the ingots was better. Further comparison of the EPDs of the three ingots in this experiment shows that when the RMF was applied, the EPD of the ingots significantly decreased and their distri bution gradually became uniform. The thermal stress is often the main cause of dislocations, which can move and proliferate under stress. The critical resolved shear stress (CRSS) model had been used to study the dislocation movement and proliferation in GaAs single crystals [28,29] and CdZnTe single crystals [30]. The CRSS to produce dislocations in the region near the solid-liquid interface is rather low, and the curvature of the solid-liquid interface plays an important role on the thermal stress generated in the crystal. A flat interface can reduce the thermal stress caused by a temperature gradient [31]. After applying an RMF, the convection and heat and mass transfer in the melt were strengthened.
The radial temperature gradient and maximum temperature difference of the solid-liquid interface were reduced, and the crystal should have a flatter solid-liquid interface. Thus, the thermal stress within the crystal and the EPD decreased, and the latter by one order of magnitude. However, when the rotation frequency increased from 25 Hz to 50 Hz, the EPD of the ingot did not decrease significantly, indicating that the modification of rotating frequency did not affect the ingot EPD. 3.4. Effect of RMF on the electrical properties of a crystal The carrier mobility and resistivity are important electrical proper ties of semiconductor materials, and directly determine the application of a material and its performance in semiconductor devices. Three ingots were tested at room temperature using a Hall Effect testing system, and the carrier mobility (μ) and resistivity (ρ) were calculated according to the following formula [32]:
4
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Fig. 6. Effect of RMF on crystal electrical properties, shown through distributions along the direction of crystal growth of the (a) mobility and (b) resistivity.
μ¼
jRH j
ρ
; ρ¼
V σ ad Il
carrier mobility increased from 1.437 � 103 cm2/(V⋅s) to 1.618 � 103 cm2/(V⋅s), and the average value of the axial resistivity decreased from 1.352 � 10 3 Ω⋅cm to 1.162 � 10 3 Ω⋅cm. Meanwhile, the uniformity of the axial carrier mobility and re sistivity also improved significantly.
(2)
where RH is the Hall coefficient, Vσ is the applied potential difference, I is the test current, a is the sample thickness, d is the sample width, and l is the sample length. The carrier mobility and resistivity distributions along the crystal growth axis for the three ingots are shown in Fig. 6. The carrier mobility increased (to 1.618 � 103 cm2/(V⋅s)) and the resistivity decreased (to 1.162 � 10 3 Ω⋅cm) by applying the RMF, and both the carrier mobility and resistivity were more uniform along the growth direction. These changes were even more pronounced when the rotation frequency increased. Carrier mobility reflects the average motion rate of carriers per unit electric field intensity and the conductivity of semiconductors. Carrier mobility is mainly affected by the scatterings in crystal, such as lattice vibration scattering, ionized impurity scattering, dislocation scattering, and other scatterings, including equivalent inter-valley scattering, neutral impurity scattering etc. [33] As mentioned above, RMF promoted the solid solution of In GaSb lattice, reduced the va cancies of Ga and the residual acceptor states, as well as In segregation, all of which could decrease the ionized impurity scattering and equiv alent inter-valley scattering significantly. Furthermore, RMF also reduced the dislocation density of the crystal, thus decreased the dislocation scattering. And RMF improved the crystallinity of the crys tal, i.e., the order of atomic lattice arrangement was improved, so the lattice vibration scattering got suppressed to some extent. And increasing the rotation frequency further strengthened this effect of RMF.
Declaration of interest statement No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. Acknowledgement This work was supported by the National Natural Science Foundation of China (No. 50972085), Advanced Research of Equipment Sharing Technology (No. 41422030103), and Key Technology R&D Program of Shandong Province (No. 2017GGX20104). References [1] B. Krishan, P.B. Barman, G.S. Mudahar, N.P. Singh, Growth of GaxIn1 xSb bulk crystals for infrared device applications by vertical Bridgman technique, Mater. Lett. 58 (2004) 1441–1445. [2] S. Abroug, F. Saadallah, N. Yacoubi, S. Abroug, Photothermal investigations of doping effects on opto-thermal properties of bulk GaSb, J. Alloy. Comp. 484 (2009) 772–776. [3] D.B. Gadkari, P. Shashidharan, K.B. Lal, N.A. Gokhale, A.P. Shah, B.M. Arora, Growth morphology and compositional analysis of InxGa1 xSb crystals-grown by vertical directional solidification technique, Indian J. Pure Appl. Phys. 37 (1999) 652–656. [4] I. Vurgaftman, G. Belenky, Y. Lin, D. Donetsky, L. Shterengas, G. Kipshidze, W. L. Sarney, S.P. Svensson, Interband absorption strength in long-wave infrared typeII superlattices with small and large superlattice periods compared to bulk materials, Appl. Phys. Lett. 108 (2016) 187–190. [5] D. Z. Ting, S. D. Gunapala, A. Soibel, J. Nguyen, A. Khoshakhlagh, Single-band and Dual-Band Infrared Detectors, U.S. Patent No. 8928029. 6 Jan. 2015. [6] R. Peng, S. Jiao, H. Li, L. Zhao, Dark current mechanisms investigation of surface passivation InAs/GaSb photodiodes at low temperatures, J. Alloy. Comp. 632 (2015) 575–579. [7] G.B. Stringfellow, Calculation of ternary phase diagrams of III–V systems, J. Phys. Chem. Solids 33 (1972) 665–677. [8] G.N. Kozhemyakin, Indium inhomogeneity in InxGa1 xSb ternary crystals grown by floating crucible Czochralski method, J. Cryst. Growth 220 (2000) 39–45. [9] J. Vincent, E. Di� eguez, Microstructure and solidification behavior of cast GaInSb alloys, J. Cryst. Growth 295 (2006) 108–113. [10] N.K. Udayashankar, K.G. Naik, H.L. Bhat, The influence of temperature gradient and lowering speed on the melt–solid interface shape of GaxIn1 xSb alloy crystals grown by vertical Bridgman technique, J. Cryst. Growth 203 (1999) 333–339. [11] A. Tanaka, J. Shintani, M. Kimura, T. sukegawa, Multi-step pulling of GaInSb bulk crystal from ternary solution, J. Cryst. Growth 209 (2000) 625–629. [12] M. Kumagawa, T. Tsuruta, N. Nishida, Y. Hayakawa, On voids in InxGa1 xSb crystals grown by an ultrasonic-vibration-introduced Czochralski method, Cryst. Res. Technol. 29 (1994) 1037–1044.
4. Conclusions (1) A RMF was applied to the VB process of Ga0.86In0.14Sb crystal growth, and high-quality GaInSb ingots (Φ25 mm � 100 mm) were prepared. (2) The RMF improved the crystallinity. A 50 Hz rotating frequency decreased the FWHM of the GaInSb ingot to 7400 , which was much lower than the 21400 of an ingot not exposed to the RMF. (3) The RMF significantly reduced both the radial and axial segre gation of indium in the GaInSb crystal. It reduced the radial segregation of indium in a 25–75 mm axial section of OT3 ingot to 0.085 mol% per mm, from the 0.247 mol% per mm for OT1 ingot not exposed to RMF, and the axial segregation to 0.047 mol% per mm from the 0.140 mol% per mm for OT1 ingot. (4) The applied RMF decreased the dislocation density of the crystal from 1.047 � 105 cm 2 to 6.988 � 103 cm 2, and improved the electrical properties of the crystal. The average value of the axial 5
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