Single-Crystal Growth of ZnO:Ga by the Traveling-Solvent Floating

Jan 26, 2017 - have been grown by the traveling-solvent floating-zone technique using the ... The growth rate was 0.3−0.5 mm/h, which was far faster...
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Single-Crystal Growth of ZnO:Ga by the Traveling-Solvent FloatingZone Method Yunfeng Ma,*,†,‡ Yong Zeng,† Didier Perrodin,‡ Edith Bourret,‡ and Yijian Jiang† †

Institute of Laser Engineering, Beijing University of Technology, Beijing 100124, PR China Lawrence Berkeley National Laboratory, University of California, Berkeley, United States

‡

ABSTRACT: Transparent and blue ZnO:Ga (GZO) single crystals have been grown by the traveling-solvent floating-zone technique using the solvent B2O3 + MoO3 + Nb2O5. The crystals were typically 9−14 mm in diameter and 46−120 mm in length. The largest one was Φ12 mm × 120 mm in size. All GZO crystals were grown along the direction. The growth rate was 0.3−0.5 mm/h, which was far faster than 0.1 mm/day of that grown by the hydrothermal method. The crystalline quality has been characterized by single crystal X-ray diffraction and X-ray rocking curve measurements. Ga substituted on Zn-site and was saturated at about 0.5 wt % of Ga2O3 addition. The GZO crystal doped with 0.5 wt % Ga2O3 has the lowest electrical resistivity of 1.083 × 10−3 Ω·cm and the highest carrier concentration of 1.78 × 1020 cm−3.

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INTRODUCTION

direction and necessitated a long time for the production of GZO crystals. In this paper, the traveling-solvent floating-zone (TSFZ) method, combining the advantages of the floating zone technique and the high-temperature flux method, was used for growing GZO crystals. This growth feature is characterized by real-time monitoring on melting and crystallizing, and the composition and volume of the flux molten zone are almost not changed when crystal growth occurred. The high-temperature gradient near the solid−liquid interface increases significantly the crystallization driving force and Ga2O3 doping amount. We selected a new oxide mixture 67.7 atom % (ZnO) + 9.3 atom % (B2O3) + 16.3 atom % (MoO3) + 6.7 atom % (Nb2O5) as a solvent based on the analysis of phase diagrams of ZnO-B2O3,31 ZnO-MoO332 and ZnO-Nb2O533,34 systems, which reduced the growth temperature to below 1300 °C in which ZnO or GZO started to evaporate, suppressed the characterization of strong polarity crystallization based on ZnO materials, and successfully grew a series of ZnO: x wt % Ga2O3 (x = 0−1) (hereafter GZO-x wt %) single crystals with high quality and of centimeter size. The actual Ga2O3 doping amount reached 0.7 wt %, which is much higher than 0.053 wt % of GZO crystals grown by the hydrothermal method.10 That the grown GZO crystals are of high Ga2O3 doping amount and large doping range is important to systematical research on the electrical properties of GZO crystals as a function of Ga2O3 composition and finding optimal composition.

ZnO-based materials, doped with IIIA elements, have demonstrated some advantages over other transparent conducting oxides (TCOs) materials.1−3 It is generally believed that group IIIA elements occupy Zn sites, can act as a shallow energy level donor in ZnO and thus generate high-density free electrons in the TCOs.4,5 Among these elements, Ga is regarded as the best dopant for obtaining high-quality n-type ZnO materials due to its similar ionic radius to Zn2+,6−8 that prevents lattice distortion. Moreover, its high solubility in ZnO with good chemical and thermal stability makes it an ideal candidate.9,10 ZnO:Ga (GZO) material is a multifunctional semiconductor combining the characteristics of transparent conduction,11 ultrafast scintillation,12−14 and ultraviolet emission,15 etc. ZnO crystal has been grown using the following growth techniques: hydrothermal,16−21 flux,22−27 and vapor deposition method.28,29 But up to now, only the hydrothermal method was reported to grow GZO single crystal.10,30 The main drawback of this method is that a small amount of Ga2O3 in the solution caused a large extent influence on the growth behavior. It was necessary to restrict the Ga2O3 doping amount to about 0.1 mol %,30 which is far below the limit of Ga3+ substitution for Zn2+. The properties of GZO single crystals as a function of doped Ga2O3 amount were not studied. Up to now, the flux reported for growing pure ZnO crystal contains PbF2,22,24,26,27 P2O5 + V2O5,23 V2O5 + B2O3, MoO3 and V2O5 + MoO3.25 However, there was no report concerning GZO crystals grown by a high-temperature flux method. The conventional hydrothermal method or slow-cooling method failed to produce a large crystal in a well-defined growth © XXXX American Chemical Society

Received: August 20, 2016 Revised: January 21, 2017 Published: January 26, 2017 A

DOI: 10.1021/acs.cgd.6b01232 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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In order to characterize the composition distribution of as-grown GZO crystals, quantitative analyses were performed at intervals of 1.2 cm along growth direction of GZO-0.05 wt % crystal rod and at intervals of 1.1 mm across diameter of GZO-0.05 wt % wafer cut from the middle part of crystal rod using electron probe microanalysis (EPMA; JXA-8100) with the EDX mode. The samples used for the Hall-effect measurement were polished GZO crystal wafers cut perpendicular to the growth direction with ∼1 mm in thickness. The Ohmic contacts were prepared by soldering InGa dots on the four corners of each sample.

EXPERIMENTAL PROCEDURES

ZnO (Alfa Aesar, 99.99%) and Ga2O3 (Alfa Aesar, 99.99%) powders were weighed according to ZnO: x wt % Ga2O3, where x = 0−1. 0, ball-milled, oven-dried, and then sieved by 200 meshes. The prepared powder was loaded into a long rubber balloon, then vacuumed, sealed, and isostatic pressed under pressure of 70 M Pa to be a compact and uniform rod. The obtained rods were sintered at 1250−1300 °C for 24−48 h by a vertical furnace to form compact and uniform polycrystalline rods. B2O3 (Alfa Aesar, 99.999%), MoO3 (Alfa Aesar, 99.9995%), Nb2O5 (Alfa Aesar, 99.9995%), and ZnO (Alfa Aesar, 99.99%) powders were weighed according to the mole ratio 9.3:16.3:6.7:67.7, ball-milled, oven-dried, and sieved by 200 meshes. The prepared powder was loaded into a long rubber balloon, vacuumed, sealed, and isostatic pressed under pressure of 70 M Pa to be a compact and uniform flux rod. The obtained flux rods were sintered at 1100 °C for 24 h by a vertical furnace to be compact and uniform polycrystalline rods. The prepared flux rod was cut into crosssection wafers with 3−5 mm in thickness. The special thickness of wafer was selected as a flux depending on the diameters of the feed and seed rods. Normally, the diameter of the flux wafer should be less than that of feed and seed rods. The growth apparatus was an optical floating zone furnace (Crystal Systems Co., 10000H -HR-I-VPO-PC) equipped with four 500 W tungsten halide lamps focused by four polished elliptical mirrors. A part of GZO ceramic rod was fixed as a seed crystal to a bracket that was located on the lower rotary shaft. Its cross-section together with a flux wafer lie upon was adjusted to be horizontal, and the middle feed rod also named the upper rod was adjusted to be the middle too. Thereafter the quartz tube was loaded. The tip of the feed rod should be close to the flux wafer and removed to the center of the focus area of the halogen lamps together. Then the outpower of the halogen lamps were increased to 1028 W at a rate of 60 W/min, which makes the upper surface of flux wafer melt mildly. As shown in Figure 1, the

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RESULTS AND DISCUSSION Initial attempts to grow the ZnO crystal by the optical floating zone method directly at oxygen pressures ranging from atmospheric to 7 bar failed. It was difficult to melt the ZnO rod, because ZnO began to evaporate at about 1300 °C, which is far below its melting point of 1975 °C. Some whiskers were deposited on the tip of feed rod as shown in Figure 2.

Figure 1. Formation of molten zone established by the TSFZ technique. Figure 2. ZnO spherical whiskers around the tip of ZnO feed rod. upper rod was falling down slowly and adhering to the flux wafer and then pulled up together with the flux wafer quickly. The outpower was increased to 1100 W at a slower rate of 20 W/min to make the feed rod’s tip melt completely together with flux wafer for 0.5 h. Then upper and lower rods were connected to each other by molten flux and counter-rotated for 1 h at the upper and lower rates of 25−35 rpm and 25−35 rpm, respectively. The growth process then proceeded at a growth rate of 0.3−0.5 mm/h when molten zone became stable. The outpower was adjusted to 1100−1360 W depending on the Ga2O3 component. The crystals were grown in flowing air from a compressor that was attached to the floating-zone machine. Cooling time of grown crystals was set for 4.5−10 h when growth process finished. High-resolution X-ray diffraction (HRXRD) was performed with Ni-filtered Cu Kα radiation (Bruker D8 Discover) using a step size of 0.02° within the range of 20−80°. X-ray single crystal diffraction data were gathered on a CCD diffractometer (Bruker Smart APEX II) with graphite monochromatic MoKα radiation (λ = 0.71073 Å) at room temperature. The density of GZO crystals was measured by a buoyancy method using a METTLER TOLEDO Excellence XS Analytical Balances. High-resolution X-ray rocking curves of as-grown GZO-0.05 wt % and GZO-0.1 wt % crystals were recorded to check the crystalline quality.

Condensation of ZnO was also observed on the inner wall of quartz tube shielding light that was used for heating. Considering these observations, we attempted to grow GZO crystal employing a solvent that would form a stable liquid phase and suppress volatilization. The use of solvent in the optical floating zone method was well documented for a growth example of superconductors. However, the flux must be chosen carefully so as not to contaminate to the resulting crystal, especially to the semiconductor materials that were affected largely by foreign ions. We described the growth process in Experimental Procedures. For the flux selection, we considered the following solvents: V2O5−ZnO,35 B2O3−ZnO,31 P2O5−ZnO,36 MoO3− ZnO,32 WO3−ZnO,37 and Nb2O5−ZnO.33,34 Because they are oxides that lower the melting point below 1300 °C. Growth attempts using V2O5-, WO3-, and P2O5-based solvents failed due to severe penetration and solvent volatility. ZnO-B2O3, ZnO-MoO3, and ZnO-B2O3-MoO3 had low surface tension, which cannot sustain a stable molten zone between the two rod B

DOI: 10.1021/acs.cgd.6b01232 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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was stable. The solvent did not penetrate deeply into the feed or seed rod by capillary action along grain boundaries, thereby causing a steady growth state and a clear and smooth solid− liquid interface. As shown in Figure 4, we have grown pure ZnO and GZO crystals with 11 kinds of doping contents by the TSFZ method. The crystals were typically 9−14 mm in diameter and 46−120 mm in length. The largest one was Φ12 mm × 120 mm in size. They were light or dark blue in color depending on the Ga2O3 doping amount. The color of the polished cross-section wafers became lighter after annealing in air at 1100 °C for 24 h. As shown in Figure 5, annealed and polished cross-section wafers

tips during the crystal growth. ZnO-Nb2O5 provided high enough surface tension to sustain a stable molten zone but a high growth temperature 1300 °C which resulted in volatilizing of ZnO. At last, we found a new oxide combination of 9.3 atom % (B2O3) + 16.3 atom % (MoO3) + 6.7 atom % (Nb2O5) + 67.7 atom % (ZnO) as flux, which provided a low growth temperature and high enough surface tension. As shown in Figure 3, the molten zone of the GZO-0.3 wt % crystal growth

Figure 5. Annealed and polished cross section disks cut from middle region of GZO crystal rods.

typically 1 mm in thickness cut from the middle region of GZO crystal rods were much more transparent than the unannealed wafers. The crystal wafers with a low Ga2O3 doping amount are clearer than those with a high Ga2O3 doping amount. When the Ga2O3 concentration increased to 0.5 wt % and above, Ga2O3 segregation is observed and clearly marked by a ring layer on the outside of the GZO-0.9 wt % wafer in Figure 5. The quality of GZO crystals grown by the TSFZ method will be improved largely by some means, for example, optimizing the seed crystal, growth atmosphere, decreasing rod’s diameter, growth rate, and adjusting annealing condition, etc. The actual Ga2O3 dopant concentration and impurity concentration measured by EPMA are shown in Table 1. The maximum of actual doped Ga2O3 can reach is 0.701 wt %, which is far above 0.053 wt % of that grown by the hydrothermal method.10 The actual Ga2O3 doping amount at

Figure 3. Molten zone image of GZO-0.3 wt % crystal grown by the TSFZ technique.

Figure 4. GZO crystals grown by the TSFZ technique. C

DOI: 10.1021/acs.cgd.6b01232 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Table 1. Ga2O3 Dopant Concentration and Flux Impurity Content, Crystal Cell Parameters, and Actual Density of Grown GZO Crystals Measured by EPMA, Powder XRD, Buoyancy Method, Respectively crystal cell parameters sample no.

Ga2O3 dopant concentration/wt %

Nb2O5 impurity concentration/wt %

a/Å

c/Å

V/Å3

actual density/g/cm3

pure ZnO GZO-0.05 wt % GZO-0.1 wt % GZO-0.2 wt % GZO-0.4 wt % GZO-0.5 wt % GZO-0.6 wt % GZO-0.7 wt % GZO-0.8 wt % GZO-0.9 wt %

0 0.089 0.110 0.294 0.404 0.482 0.559 0.647 0.560 0.701

0.081 0.030 0 0.040 0.060 0 0.121 0.132 0.111 0.141

3.2490 3.2496 3.2492 3.2495 3.2494 3.2492 3.2494 3.2495 3.2498 3.2494

5.2057 5.2067 5.2062 5.2056 5.2062 5.2061 5.2058 5.2062 5.2068 5.2067

47.5898 47.6160 47.5991 47.6017 47.6049 47.5994 47.6004 47.6081 47.6208 47.6100

5.58403 5.76245 5.72082 5.63502 5.63595 5.65227 5.65078 5.67318 5.63966 5.64546

a range of 0−0.7 wt % was different from the Ga2O3 composition of GZO feed rod at a range of 0−0.9 wt %. The flux Nb2O5 impurity concentration and the saturation of substitution of Ga on Zn-sites resulted in a deviation in the Ga2O3 doping amount. For example, in the case of the GZO0.1 wt % and GZO-0.5 wt % crystal, the actual Ga2O3 compositions were 0.110 and 0.482 wt %, respectively. The deviation was nearly ignored when the Nb2O5 impurity content approximately equaled zero. The actual composition of GZO0.05% was 0.089 wt %, which is a little more than the expected composition. It is because the Nb2O5 impurity content of 0.030 wt % in the grown crystal influenced to a certain degree the stability of the solid−liquid surface and affected the doping amount. We can see some white or light-yellow material on the surface of the GZO-0.7, 0.8, 0.9, and 1.0 wt % crystal rod. The material was confirmed by EPMA to be a Ga2O3 rich compound. Thus, the substitution of Ga on the Zn-site was saturated. The educts influenced the stability of the solid− liquid surface of crystal growth, resulting in high Nb2O5 impurity concentration covering 0.111−0.141 wt %, which is larger than 0−0.081 wt % of samples with a low Ga2O3 doing amount. Back scattered electron (BSE) analysis provides an easy way to study the basic microstructure characters of a transparent crystal by using electron microscopy. BSE images with 500 times magnification of GZO-0.05 wt % cross-section wafers cut from different parts of crystal rod were captured and then shown in Figure 6 and Figure 7. As shown in Figure 6a−g, the BSE images of measured points across the diameter from side to side at intervals of 1.1 mm showed that a small quantity of bright flocculent stripes with the relative minor areas appeared and were unchanged on the quantity and relative area. As shown in Figure 7, the typical BSE images of GZO-0.05 wt % cross-section wafers cut along the length of grown rod at intervals of 1.2 cm were captured and showed bright flocculent stripes with small quantity and minor areas. We found that the color distribution is very uniform except for some stripes and spots. As shown in Table 2, the composition distribution of gray parts was measured by EPMA across the diameter of a GZO0.05 wt % cross-section wafer and along the growth direction of the GZO-0.05 wt % crystal rod. The impurity Nb 2 O 5 composition was very small and less than 0.08 wt %. MoO3 or B2O3 was not detected by EPMA based on its analysis resolution. The Ga2O3 composition measured along the axial and radial directions ranged from 0.02 to 0.14 wt %. Although the average Ga2O3 composition of GZO-0.05 wt % crystal

Figure 6. Back scattered electron (BSE) images of the cross-section wafer cut from the middle part of the GZO-0.05 wt % crystal rod. (a− g) BSE images of measured points across the diameter from side to side at an interval of 1.1 mm. (h−i) Two BSE images with a large impurity zone appeared on the wafer.

Figure 7. Typical back scattered electron (BSE) images of eight GZO0.05 wt % cross-section wafers cut particularly to the growth direction at intervals of 1.2 cm.

measured by EPMA was not equal to the expected value, it was neither systematically high nor systematically low across the diameter or along the growth direction. The measurement areas shown in Figure 6h,i were selected to check the composition of bright parts of BSE images. They were 48.58 wt % ZnO + 51.42 wt % Nb2O5 and 48.32 wt % ZnO + 51.58 wt % Nb2O5 + 0.1 D

DOI: 10.1021/acs.cgd.6b01232 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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significantly and systematically. This is because the Ga3+ is of similar ionic radius with Zn2+ and the flux impurity in the GZO crystal resulted in more stress and defects, causing a large influence on the cell parameter, density, and the composition variation. Figure 9a−d shows the rapid scanning diffraction photographs of as-grown ZnO, GZO-0.1 wt %, GZO-0.5 wt %, and

Table 2. Composition Distribution of Gray Parts Measured Across the Diameter of a GZO-0.05 wt % Cross-Section Wafer and along the Growth Direction of the GZO-0.05 wt % Crystal rod measuring position distance along the growth direction of crystal rod/cm 0 1.2 2.2 3.3

4.6 6.0 7.3 8.4

distance from center of wafer/mm

−3.3 −2.2 −1.1 0 1.1 2.2 3.3

corresponding BSE image Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure Figure

6a 6b 6c 5a 5b 5c 5d/6d 5e 5f 5g 6e 6f 6g 6h

Ga2O3 dopant concentration/ wt %

Nb2O5 impurity content/ wt %

0.04 0.07 0.02 0.02 0.10 0.05 0.14 0.14 0.15 0.02 0.08 0.08 0.17 0.05

0 0 0.05 0.07 0 0 0.08 0 0 0.06 0 0 0.07 0.03

Figure 9. Single crystal X-ray diffraction photographs for rapid scanning on grown (a) ZnO, (b) GZO-0.1 wt %, (c) GZO-0.5 wt %, and (d) GZO-0.05 wt % crystals; (e, f) lattice structure projection maps along the [001] and [100] crystal orientations of GZO-0.05 wt % crystal.

wt % Ga2O3, respectively. We did not detect the MoO3 and B2O3 impurities by EPMA. This is because MoO3 or B2O3 is of higher fluidity in the solid−liquid interface in the process of crystal growing due to their lower melt points compared to Nb2O5. The composition uniformity of the grown GZO crystals was not bad and should be improved further by adjusting the flux composition proportion to obtain a more stable molten zone for crystal growth. As shown in Figure 8, the powder X-ray diffraction (PXRD) patterns of the grown GZO crystals, within the resolution of measurement, were found to have a single phase. There was no obvious angle migration in every peak positions. The relative intensity of (100) peak was found to increase to a maximum value with increasing mass percentage of doped Ga2O3 up to 0.5. After this, it decreased with a further increase of the Ga concentration. This indicates that the Gallium atoms were incorporated into the ZnO lattice with tetrahedral coordination by substituting for Zn atoms in the case of 0−0.5 wt % doped amount crystals. The lattice substitution reached its limit at 0.5 wt %. As shown in Table 1, the lattice parameters, cell volume (a, c, V), and actual density of GZO crystals did not change

GZO-0.05 wt % crystals. They indicate that all diffraction spots are independent with high axial symmetry, initially considering them as single crystals. Figure 9e,f shows the lattice structure projection maps along the [001] and [100] crystal orientations of the GZO-0.05 wt % crystal. It can be seen that single crystal GZO-0.05 wt % has a hexagonal structure. There were no miscellaneous points in the projection map of lattice structure, which indicates that a high-quality single crystal GZO-0.05 wt % was obtained. In order to determine growth direction of GZO crystals, the cross-section wafers of GZO crystal rods were polished to be optical quality, measured by high-resolution XRD. As shown in Figure 10, the (002) and (004) peaks appeared on the XRD patterns of GZO crystals. That indicates that the GZO crystals were grown along the direction. As shown in Figure 11, the X-ray rocking curves from 002 reflection of as-grown GZO-0.05 wt %, GZO-0.1 wt % crystals showed that the diffraction peak is symmetrical and not split.

Figure 8. Powder X-ray diffraction pattern of GZO crystals. E

DOI: 10.1021/acs.cgd.6b01232 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 10. X-ray diffraction patterns of cross-section wafers of GZO crystals perpendicular to the growth orientation.

Figure 11. X-ray rocking curves from 002 reflection of GZO-0.05 wt %, GZO-0.1 wt % single crystals.

Ω·cm with increasing mass percentage of doped Ga2O3 up to 0.5. After this, it increased with further increasing of Ga concentration. The minimum resistivity achieved in the present work is about a seventh of 7.12 × 10−3 Ω·cm ever reported in the case of a GZO-0.053 wt % single crystal grown by the hydrothermal method.10 The composition 0.053 wt % was the highest doping amount that can reached by the hydrothermal method for crystal growing GZO crystals.10 A small amount of Ga2O3 in the solution caused a large influence on the growth behavior, which was accompanied by spontaneously nucleated crystallites. Sometimes the seeded crystals also cracked after growth.30 It can be seen that the Hall mobility (μ) of the GZO crystal decreased from 95.5 cm2/V·s (in the case of pure ZnO) to 32.3 cm2/V·s (in the case of GZO-0.5 wt %) and to 18.4 cm2/V·s (in the case of GZO-1.0 wt %) monotonically with increasing Ga2O3 concentration. Because as shown in Figure 5, lower crystalline quality and increased impurity defects with increasing Ga2O3 doping amount, the carrier concentration of GZO crystals initially increased to a value of 6.3 × 1019cm−3 for 0.5 wt % Ga2O3 concentration. Above this, carrier concentration decreased gradually and monotonically from the value of 6.3 × 1019 cm−3 for 1 wt % concentration of Ga2O3. The initial rapid increase/decrease of carrier concentration/resistively up to the Ga2O3 concentration of 0.5 wt % clearly indicates that most of the Ga was substituting Zn as the donor dopant in the ZnO lattice and hence contributing free electron to the ZnO lattice. However, a high dopant concentration of Ga in the ZnO

The full width at half-maximum (fwhm) of (002) reflections were 327.6 and 504.0 arc sec, respectively, indicating a good crystalline integrity of the GZO single crystal grown by the TSFZ method. Figure 12 shows resistivity, carrier concentration, and Hall mobility of GZO single crystals as a function of GZO-x wt % composition, respectively. All the GZO crystals were found to be n-type semiconductor. The resistivity of GZO crystals was found to decrease rapidly to a minimum value of 1.083 × 10−3

Figure 12. Resistivity, carrier concentration, and Hall mobility of GZO single crystals as a function of GZO-x wt % composition, respectively. F

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Table 3. Room Temperature Carrier Concentrations, Hall Mobilities and Resistivities of Pure ZnO, GZO-0.05 wt %, and GZO0.5 wt % Crystals Grown by the TSFZ Technique Compared with Pure ZnO, GZO-0.053 wt % Crystal Grown by the Hydrothermal Method and GZO (0.75 atom %) Thin Film, Respectively sample

carrier concentration n/cm−3

Hall mobility μ/cm2/V·s

pure ZnO ZnO-0.05 wt % Ga2O3 ZnO-0.5 wt % Ga2O3 pure ZnO (hydrothermal method)10 ZnO-0.053 wt % Ga2O3 (hydrothermal method)10 GZO (0.75 atom %) thin film41

× × × × × ×

95.5 84.6 32.3 239 81.5 41.3

4.02 6.87 1.78 4.09 1.16 2.9

lattice might result in the enhancement of Oi or VZn acceptor defects38,39 to keep the charge balance in the system due to compensation of free carriers. This may result in the gradual reduction of carrier concentration at higher Ga2O3 concentrations. This is perhaps a reason for the observed decrease/ increase of carrier concentration/resistivity when the Ga2O3 concentration was raised beyond 0.5 wt %. A similar variation of resistivity and carrier concentration has already been reported by others in the case of Ga-doped ZnO thin films.40 As shown in Table 3, the resistivity and carrier concentration of ZnO achieved in this study were about a fourth and 100 times of that of ZnO crystal grown by the hydrothermal method.10 The reason is that the actual composition contained 0.081 wt % Nb2O5 impurity. The Nb5+ occupies Zn2+ sites and acts as a multielectron donor in ZnO,41 and thus, a high quantity of free electrons can be generated in the ZnO crystal. The value of carrier concentration/resistivity/mobility in the case of the GZO-0.05 wt % crystal is a little less/more/less than that of the GZO-0.053 wt % crystal grown by the hydrothermal method.10 The result is abnormal, because the actual composition of the GZO-0.05 wt % crystal grown by this method was 0.089 wt % Ga2O3 + 0.030 wt % Nb2O5, which was more than 0.053 wt % Ga2O3 enough on n-type doping amount. The reason may be that the Oi or VZn acceptor defects of the TSFZ grown GZO-0.05 wt % crystal are more than that of hydrothermal-grown GZO-0.053 wt % crystal. The optimal Ga2O3 composition of thin film GZO (0.75 atom %) was 0.863 wt %, which is more than 0.048 wt % of GZO-0.5 wt % crystal, corresponding to better electrical properties, as shown in Table 3. This is because the crystalline quality decreases with further increasing of the Ga2O3 concentration from 0.5 wt % (as shown in Figure 5) and the Nb2O5 impurity amount started to increase to ∼0.11 wt % (as shown in Table 1), which results in vast acceptor defects. In the process of preparation, the GZO thin films were not affected by the flux impurity and having better composition uniformity and crystalline quality at the same heavy Ga2O3 doping amount. However, up to now, TSFZ is only the method for successfully growing heavy doped GZO crystals. So we should optimize the crystal quality and composition uniformity by continually modulating the flux ratio and growth parameters.

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10 1018 1020 1016 1019 1021

resistivity ρ/Ω·cm 1.63 1.07 1.08 6.39 7.15 5.1

× × × × × ×

10−2 10−2 10−3 10−2 10−3 10−5

amount. The crystal size was 9−14 mm in diameter and 46− 120 mm in length. The growth rate was typically 0.3−0.5 mm/ h, which is much faster than that of the hydrothermal method. On the basis of Hall effect measurements of GZO crystals, GZO-0.5 wt % is found to have the best electrical properties. It was noted that the TSFZ technique is a new method for growing cm-sized ZnO-based crystals along an expected crystallographic orientation. The solvent selected by us can sustain a steady molten zone for a long time. In the future, we will optimize seed quality or employ a lower growth rate to improve the crystal quality and change the composition ratio of the solvent to decrease the Nb2O5 impurity in the crystal. The good reproducibility, economy, and efficiency of this method made it possible for clarifying the intrinsic properties of ZnObased crystals as a function of doped elements or their doping amount.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yunfeng Ma: 0000-0002-6786-5696 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 11374031), China Scholarship Council under No. 201406540016 and China Postdoctoral Science Foundation (No. 2014M560863). The authors would like to express their thanks to Professor Yue Wang and Dr. Hong Xu for many valuable suggestions.

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REFERENCES

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CONCLUSIONS GZO (ZnO: x wt % Ga2O3; x = 0, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0) single crystals were grown by the TSFZ technique using oxides of 9.3 atom % (B2O3) + 16.3 atom % (MoO3) + 6.7 atom % (Nb2O5) + 67.7 at% (ZnO) as a flux. The actual mass percentages of Ga2O3 in GZO crystals were 0, 0.089, 0.110, 0.294, 0.404, 0.482, 0.559, 0.647, 0.560, and 0.701, respectively. These GZO crystals are transparent with light or dark blue color depending on the doped Ga2O3 G

DOI: 10.1021/acs.cgd.6b01232 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

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DOI: 10.1021/acs.cgd.6b01232 Cryst. Growth Des. XXXX, XXX, XXX−XXX