Growth of GaN Single Crystals by Li3N Flux with Mn as Addition H. Li, H. Q. Bao, G. Wang, B. Song, W. J. Wang, and X. L. Chen* Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100190, P. R. China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 8 2775–2779
ReceiVed October 12, 2007; ReVised Manuscript ReceiVed May 4, 2008
ABSTRACT: Metal Mn was introduced into Li3N flux to grow GaN single crystals from Ga melt. Colorless, transparent GaN single crystals with an average size of 2-3 mm were obtained at 800 °C under N2 pressure of about 2 atm. The effects of Mn on the quality, size distribution, morphologies, and formation mechanism of GaN crystals were investigated. It was found that the proper addition of Mn favored GaN growth, impeded the homogeneous nucleation to some extent, and resulted in a more uniform size distribution and high quality of GaN crystals as compared with that of GaN grown using Li3N as flux only. The formation mechanism was enhanced by Mn addition. These results suggested a promising new route for the growth of large size and high quality GaN single crystals from a Li-Ga-N system in the future by optimizing the molar ratio of Mn to Li3N. Introduction GaN, a wide direct band gap semiconductor (3.44 eV at 300 K), has attracted considerable attention as a material for fabrication of both optoelectronic devices like blue/green light emitting diodes (LEDs), violet laser diodes (LDs) and hightemperature, high-power, high-frequency transistors.1–4 Besides, the GaN, AlN, and InN with wurtzite structure can form solid solutions, making it possible to utilize the wide range of band gaps from 0.7 to 6.2 eV.5–7 However, GaN has usually been deposited by vapor phase heteroepitaxially on R-Al2O3, MgAl2O4 and other substrates like SiC, MgO, LiGaO2 due to the lack of bulk GaN crystals substrates.8 A drawback of heteroepitaxy is that a high density of defects, due particularly to lattice mismatch and different thermal expansion coefficient between epitaxial layer and substrate, will greatly deteriorate the performance and lifetime of GaN based devices. To overcome these problems, GaN single crystals are highly demanded as substrates materials in the process of homoepitaxial growth. However, it is a notorious challenge to prepare bulk GaN single crystals either by the Czochralski method or Bridgman method, due to its extremely high decomposition pressures9 and high theoretical melting temperature.10 Up to now, several techniques have been developed to grow large size and high quality bulk GaN single crystals including the hydride vapor phase epitaxy (HVPE) method,11,12 high pressure solution growth (HPSG) method,9,13–16 ammonothermal method,17 and flux method.18–25 Currently, using the HVPE method, 2 in. freestanding GaN wafers have been successfully obtained. Nevertheless, the growth process is quite difficult to control, resulting in a high cost of the wafers. The HPSG method has been developed to grow bulk GaN single crystals since 1980s9,13 and GaN single crystals of up to 10 mm have been synthesized.14–16 Nevertheless, this method needs high N2 pressure and high temperature (usually 1-2 GPa, 1400-1500 °C) due to its extremely high decomposition pressure9 and high theoretical melting temperature.10 As a relatively moderate technique, the ammonothermal method has attracted comprehensive attention since the 1990s. GaN single crystals with sizes about 10 × 10 mm and 10 × 13 mm have been achieved by this method in recent years.17,18 In this route, GaN single crystals are obtained * To whom correspondence should be addressed. Tel.: +86-10-8264-9039; fax: +86-10-8264-9646; e-mail:
[email protected].
in supercritical ammonia with a temperature of 500 °C and a pressure of 400-500 MPa. However, the ammonia involved in this technique is corrosive and the manipulation process is inconvenient. The flux method has been used to grow bulk GaN single crystals using various fluxes such as NaN3,19 Li3N,20–23 Na-Ca,24,25 and Ca3N2.26 Bulk GaN single crystals with the largest size of 4 mm were prepared using Li3N as flux at nitrogen pressures as low as 1-2 atm at about 780 °C.20–23 The obtained GaN single crystals were colorless and transparent, suggesting that this pathway may be a promising method to grow high quality and large size GaN single crystals. The limitation of this method is that homogeneous nucleation occurs in random orientation and in a wide temperature range during the growth process, inhibiting a further increase in the size for single crystal grain. A feasible solution is to increase the critical supersaturation at which the GaN begins to nucleate. The critical supersaturation was enhanced to a narrow temperature range through the use of a Ca-Li3N composite flux, leading to a more uniform size distribution of GaN.27 But the obtained GaN crystals were nitrogen deficient and the value of the full-width at half-maximum (fwhm) from the (100) reflection in the X-ray rocking curve was 237.6 arcsec.27 Metallic manganese is capable of dissolving nitrogen according to the Mn-N binary phase diagram. Moreover, Mn has been used to increase the nitrogen solubility in the AlN growth process.7 It is reasonable to speculate that the addition of Mn may be helpful to increase the nitrogen solubility in the Ga-based melts and inhibit homogeneous nucleation of GaN to some extent. In this work, we present the results of the growth of GaN single crystals by Li3N flux with Mn addition. Colorless, transparent, and high quality GaN single crystals with an average size of about 2-3 mm were obtained at about 800 °C and under N2 pressure of about 2 atm by Li3N flux with Mn and Ga molar ratios of 3 to 97, 10 to 90, and 30 to 70. It was found that the addition of Mn at 3% and 10% molar ratios to the Ga into Li3N flux improves the quality of the obtained GaN single crystals, increases the nitrogen content in the GaN single crystals, and results in a more uniform size distribution of GaN as well. Experimental Section The starting materials used for the growth of bulk GaN single crystals were Ga (99.999%), metal Mn (99.9%) powder, and bulk Li3N (synthesized using Li metal (99.9%) and N2 (>99.999%) as reactants at about 400 °C). In a typical process, the Mn and Ga at a molar ratio
10.1021/cg700984r CCC: $40.75 2008 American Chemical Society Published on Web 07/17/2008
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Figure 1. (a) PXRD pattern of sample “A” after milling; the inset shows PXRD pattern of sample “B” after milling. (b) XRD pattern of sample “A”; the inset shows XRD pattern of sample “B”. of 3:97 were first put into a tungsten crucible with a 50 mm inner diameter and 60 mm depth. Then the tungsten crucible was placed inside an induction heating furnace. The schematic diagram for the furnace can be found elsewhere.22,23,30 After being evacuated to a vacuum of about 2.0 × 10-3 Pa, the system was charged with high purity Ar (>99.999%) until the pressure increased to about 2 atm. After that, the crucible was heated to 800 °C and kept at this temperature for 6 h, and then cooled down to room temperature naturally. In succession, lamella Li3N weighted at a certain molar ratio to Ga (the molar ratio of Li3N to Ga was 1:3) was put into the tungsten crucible containing the as-synthesized Ga-Mn alloy and put into the induction heating furnace. The schematic diagram for the furnace can be found in ref 23. Then the system was charged with high pure N2 up to about 2 atm after being evacuated to a vacuum of about 1.4 × 10-3 Pa. The crucible was heated to 800 °C and kept at this temperature for 12 h, and then was slowly cooled at a rate of 3 °C/day over two days, and then was heated to 800 °C at a rate of 3 °C/h. The process was repeated for three cycles. Finally, the crucible was cooled down to room temperature by shutting off the power of the furnace. The whole growth process lasted for about 180 h. For comparison, GaN was also synthesized under the same conditions without addition of Mn. Plenty of shiny crystal platelets were obtained after soaking the reaction products in hot HCl and HNO3 aqueous solution respectively and drying. The shining platelets were characterized by X-ray diffraction (XRD) on a MAC-M18XHF diffractometer with Cu KR radiation at 50 kV and 200 mA for structural analysis. X-ray rocking curve data were collected on a high resolution X-ray diffraction (X′Pert PRO) equipped with a Ge (220) monochromator. The morphologies of the crystals were observed by an optical microscope and a scanning electron microscope (SEM, FEI, XL-30). The element compositions of the crystals were characterized by energy dispersive X-ray spectroscope (EDS) attached to SEM and inductively coupled plasma-atomic emission spectrometry (ICP-AES). Raman spectra of the crystals were collected at room temperature by a multichannel modular triple Raman system (JY-64000) using the 532 nm line of a solid-state laser as the excitation source.
Results Figure 1a shows the powder XRD (PXRD) pattern of the obtained shining platelets obtained with Mn as addition (labeled as sample “A”) after milling. All diffraction peaks could be well indexed based on a hexagonal cell with a ) 3.189 Å and c ) 5.185 Å (ICDD-PDF No: 50-0792), indicating the obtained platelets were GaN crystals. No impurity phases are found under the resolution of the X-ray diffractometer. It is worth noting that the (002) reflection is obviously preferred although the sample has been milled before XRD measurement. Figure 1b presents the XRD pattern of the obtained shining platelets. Only (002) and (004) reflections appear, implying the basal face of the obtained crystals was the c face. For the GaN single crystals obtained without Mn as addition (labeled as sample “B”), the
Figure 2. X-ray rocking curve of sample “A”; the inset shows X-ray rocking curve of sample “B”.
basal face was also the c face, determined from the PXRD and XRD patterns of the sample (the PXRD and XRD pattern of the sample is respectively shown in the inset of Figure 1a,b). Figure 2 displays the X-ray rocking curve from (002) reflection of sample “A”. The fwhm value of the peak is 43.2 arcsec, much smaller than that of GaN obtained by Ca-Li3N composite flux,27 indicating well-crystallized GaN platelets were obtained; for sample “B”, the fwhm value of the (002) reflection peak in the X-ray rocking curve is 151.2 arcsec (as shown in the inset of Figure 2), much larger than that of sample “A”, indicating the addition of Mn can improve the quality of the obtained GaN single crystals. The size of sample “A” was relatively uniform. The size of the most platelets was in the range of 1-3 mm. Figure 3a shows the optical photograph of sample “A”. Figure 3b illustrates the size distribution of the obtained GaN shown in the Figure 3a. As shown in Figure 3b, more than 40% of the crystals had sizes of 1-1.5 mm, and about 50% of the crystals had sizes of 1.5-2.5 mm, while about 8% of the crystals had sizes above 2.5 mm. The size distribution of the crystals was much more uniform and larger than that of crystals obtained by Ca-Li3N composite flux,27 while for sample “B”, most of the crystals are 1-2 mm in size (as shown in the inset of Figure 3b); thus much smaller crystals were obtained in the absence of Mn. Thus it can be seen that the addition of Mn into the Li3N flux aids in the growth of larger and more uniformly distributed GaN single crystals. It also can be seen from Figure 3a that colorless and transparent platelet GaN crystals were obtained. However, some researchers have reported that the synthesized GaN single crystals were slightly yellowish,28 green, and black amber.19 According to their analysis, the color was due to nitrogen deficiency or impurity. Therefore, the obtained colorless and
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Figure 3. (a) Typical optical photograph of sample “A”. (b) Size distribution of sample “A”, the inset shows the size distribution of sample “B”.
Figure 4. SEM images and EDS spectrum of sample “A”. (a) Typical SEM image of sample “A”. (b) Typical layered structure of sample “A”. (c) EDS spectrum of sample “A”.
transparent GaN crystals should lack these problems. The EDS spectrum of the crystal (as shown in Figure 4c) reveals that the crystal only consisted of N and Ga elements. The atomic ratio of N to Ga was about 46.62:53.38, higher than the reported values characterized by EDS and that of GaN ontained without Mn as addition (The atomic ratio of N to Ga was about 33.51: 66.49). No Mn element was found in the obtained GaN crystals characterized by both of EDS and ICP-AES. The crystal morphology of sample “A” can be very clearly observed by SEM. As can be seen from the low magnification SEM image (Figure 4a), the contour of the obtained GaN was irregular. Figure 4b shows the cross-section of the crystal. The thickness of the crystal was about 30 µm, similar to that of the crystal grown by Li3N flux (20-300 µm).20 The crystal has a layered structure distinctly seen from Figure 4b, suggesting a layered growth mechanism. Figure 5 displays a typical room temperature Raman scattering spectrum of sample “A”. Three phonon modes are observed at 143, 567, and 734 cm-1 in the spectrum, which are in good agreement with the reported E2 (low), E2 (high),
Figure 5. Room temperature Raman scattering spectrum of sample “A”.
and A1 (LO) phonon modes of GaN.29 According to c-plane back scattering modes, only three phonon modes observed in
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Figure 6. PXRD patterns of the as-prepared products, (a) obtained in the bottom layers of the Li3N lamella, and (b) obtained in the upper layers of the Li3N lamella.
Figure 6 would be appear in the spectrum.29 The result further indicates that the basal face of the obtained GaN crystals was the c face. No forbidden phonon modes are found in the spectrum, revealing a good quality of the obtained GaN single crystals. Additionally, the peak of the E2 (high) phonon mode is very sharp and intense, implying the obtained crystals had high quality. The size distribution and quality of GaN single crystals was also studied as a function of the molar ratio of Mn to Li3N. It was found that, compared with Mn in Li3N flux at a 3% molar ratio of Mn to Ga, the size distribution and quality of GaN was slightly affected with 10% Mn, while the quality was greatly deteriorated with 30% Mn. Discussion The formation mechanism for GaN by Li3N with Mn addition can be explained according to the formation enthalpy values for Li3GaN2, Mn3GaN, and GaxMny alloy. The value of formation enthalpy for Li3GaN2 at 1073 K is -298 kJ · mol-1,23 whereas the value of formation enthalpy for the most stable phase in the Mn-Ga-N ternary system at 1073 K, Mn3GaN, is -242.3 kJ · mol-1, much lower than that of GaxMny alloy.31 It can be deduced that the most stable phase is Li3GaN2 from the values of formation enthalpies for Li3GaN2, Mn3GaN, and GaxMny alloy. Thus, Li3GaN2, not Mn3GaN, formed when Li3N lamella was added into the as-prepared GaxMny alloy, leading to the formation of the Li-Ga-N ternary system. This can also be proven by the PXRD results of the various parts of the Li3N lamella after reaction (Figure 6). Figure 6, a and b, shows the PXRD pattern of the milled upper and lower sections, respectively, of the Li3N lamella immerged in the Ga-Mn melt after reaction. From the PXRD patterns, it can be seen that the bottom part of the Li3N lamella consisted mainly of GaN (ICDD-PDF No: 50-0792) and unreacted metal Ga (ICDD-PDF No: 050601), while the upper part was mainly composed of Li3GaN2 (ICDD-PDF No: 65-3190). No Mn3GaN was found by XRD measurements. Thus, no Mn-Ga-N ternary system existed in the melt, and the Li-Ga-N ternary system was undisturbed by Mn addition. However, the EDS results suggest that Mn addition increases the N content in the obtained GaN crystals compared to that of crystals synthesized by Li3N flux and Ca-Li3N composite flux.27 This suggests that Mn increase gaseous nitrogen solubility
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in the ternary system, for Mn has the ability to dissolve N. A higher concentration of nitrogen in the Li-Ga-N ternary system resulted in a higher nitrogen transportation in the ternary system, which is very critical to the growth of GaN. Furthermore, Mn is considered to dilute the concentration of Ga in the total system to some extent, leading to dissolving a higher concentration of Ga in the Li-Ga-N ternary melt. The higher concentration of Ga in the melt somewhat inhibits the evaporation of Li from the melt due to the formation of Li-Ga solution.30 Thus, the higher the concentration of Ga, the higher is the concentration of Li. The higher concentration of Li is helpful to dissolve Li3N and Li3GaN2,32 leading to easier formation of the Li-Ga-N system. Moreover, in the Ga-rich Li-Ga-N ternary solution, the higher concentration of Li in the melt further increases the content of N in the melt.32 As mentioned above, a higher concentration of nitrogen in the Li-Ga-N ternary system resulted in a higher nitrogen transportation in the ternary system, which is very critical to the growth of GaN. Thus, it can be seen that the addition of Mn increases the N concentration in the solution in the Li-Ga-N ternary system. The higher concentration of nitrogen in the Li-Ga-N ternary system seems to increase the critical supersaturation of GaN.27 The higher the critical supersaturation, the narrower temperature range of the nucleation occurred. The size distribution is considered to be greatly related to the nucleation process. The nucleation may already occur prior to cooling and the nucleation continues in the subsequent cooling at a wide temperature range in the process by Li3N, resulting in a wide size distribution of GaN crystals. Thus, without Mn, the nucleation occurred spontaneously over a wide temperature range, which made it impossible to grow larger GaN crystals by modulating the temperature. However, adding a small amount of Mn to the Ga melt decreased the temperature range for nucleation, which made it possible to grow larger sized GaN crystals by modulating the temperature. As mentioned above, Mn is considered to increase the concentration of N in the molten solution. Therefore, GaN single crystals with a larger size, higher quality, more uniform size distribution, and higher N content are obtained by Li3N flux with Mn addition. However, further investigation is still needed to further optimize the Li3N flux with the addition of Mn. Conclusions In conclusion, colorless and transparent GaN single crystals with an average size of 1-3 mm and high quality were grown by Mn addition into Li3N. The basal face of the obtained GaN crystals was the c face. The formation mechanism was enhanced by the addition of metal Mn. Mn favors GaN growth both in size enlargement and more uniform size distribution compared with Li3N as flux only. These results suggested a promising new route for the growth of large size and high quality GaN crystals from a Li-Ga-N system in the future by optimizing the molar ratio of Mn to Li3N. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 50472075, 59972040, and 59925206) and by “863” program (Grant No. 2006AA03A107).
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