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Hydrothermal growth and characterization of nitrogen-doped ZnO crystals
ARTICLE IN PRESS
Journal of Crystal Growth 287 (2006) 381–385 www.elsevier.com/locate/jcrysgro
Hydrothermal growth and characterization of nitrogen-doped ZnO crystals Buguo Wanga,, M.J. Callahanb, L.O. Bouthilletteb, Chunchuan Xuc, M.J. Suscavageb a
Solid State Scientific Corporation, 27-2 Wright Road, Hollis, NH 03049, USA Sensors Directorate, Air Force Research Laboratory, Hanscom AFB, MA 01731, USA c Department of Physics, West Virginia University, Morgantown, WV 26506, USA
b
Available online 4 January 2006
Abstract Hydrothermal growth of nitrogen-doped ZnO crystals was performed in alkaline solutions with the addition of 1 N LiNO3 at T dissoultion =T growth ¼ 365 1C=350 1C, and in a 20% NH4OH solution at 490 1C/475 1C, respectively. An average nitrogen concentration of approximately 1018 atom/cm3 was incorporated in the crystals. X-ray diffraction showed that the crystals grown from the 1 M LiNO3 solution are of high quality, exhibiting high resistivity. The spontaneously nucleated crystals were obtained from the NH4OH solution; a weak peak at 3.236 eV and a peak at 3.332 eV of photoluminescence were found at 18 K from these crystals after annealing at 600 1C for 2 h. The emission peaks correspond to the nitrogen-associated donor–acceptor pair (DAP) and electron-acceptor emissions in ZnO:N prepared by other techniques reported in the literature. r 2005 Elsevier B.V. All rights reserved. PACS: 81.10.Dn; 85.30.Fg; 78.55.m; 78.66.Fd; 61.72.Vv Keywords: A1. p-Type; A1. Nitrogen-doped ZnO; A2. Hydrothermal growth; B3. Hall measurements; B3. Photoluminescence
1. Introduction ZnO is attracting considerable attention for its possible applications to UV light emitters, spin functional devices, gas sensors, transparent electronics and surface acoustic wave devices [1]. However, the most significant impediment to the widespread exploitation of ZnO-related materials in electronic and photonic applications is the difficulty in carrier doping, particularly as it relates to achieving p-type material [2]. Most candidate p-type dopants in ZnO introduce deep acceptor levels [1]. Copper doping introduces an acceptor level with an energy 0.17 eV below the conduction band. Silver behaves as an acceptor with a deep level 0.23 eV below the conduction band. Lithium introduces a deep acceptor, and induces ferroelectric behavior. It appears Corresponding author. Solid State Scientific Corporation, c/o Air Force Research Laboratory SNHC/AFRL, 80 Scott Drive, Hanscom AFB, MA 01731, USA. Tel.: +1 781 377 5261; fax: +1 781 377 7812. E-mail address: buguo@solidstatescientific.com (B. Wang).
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.11.049
that the most promising dopants for p-type ZnO are the group V elements [3], and most efforts on p-type doping of ZnO have focused on nitrogen doping. There have been several reports suggesting acceptor doping levels with nitrogen substitution [4–7]. Many researchers are concentrating on producing highly crystalline ZnO epitaxial layers using a heterogeneous substrate such as sapphire. However, the speed in developing homoepitaxy will be accelerated if highly crystalline ZnO substrates become available. One possible reason for the difficulties in p-doping ZnO could be the presence of self-compensating doping mechanisms due to defects, which is seen in other II–VI materials. Therefore, unlike GaN-based devices, it may not be possible to produce efficient long-life time ZnO-based LEDs and laser diodes on highly defective ZnO films grown on heterogeneous substrates. The growth of ZnO single crystals has been carried out mainly by four methods—the chemical vapor transport method (CVT), the flux method, the high-pressure melt growth method and the hydrothermal method. The
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hydrothermal method has been known as a method to grow highly crystalline and large-size crystals for ZnO substrates under a relatively low temperature with low cost [8–12]. In this paper, we report the growth of nitrogendoped ZnO single crystals by the hydrothermal method and characterization of the grown crystals by chemical analysis, photoluminescence (PL) and Hall measurements. 2. Experimental procedure 2.1. Nitrogen-doped ZnO crystal growth Hydrothermal autoclaves made of high-strength steel were used for crystal growth. Because of the high normality of the mineralizing solution, a sealed platinum liner was used to isolate the crystal growth environment from the walls of the autoclave. The nutrient was prepared from 99.99% ZnO (Alfa Aesar) powder having particle size less than 3 mm, which was sintered for 12 h in air at 1150 1C in a platinum crucible. The seeds were (0 0 0 1) plates of ZnO from previous hydrothermal growth runs. Two different sets of conditions were employed to incorporate nitrogen into the crystals successfully: (1) using 3 N NaOH, 1 N KOH and 1 N LiNO3 as the growth solution at the growth temperature 350 1C and the dissolution zone at 365 1C, fill at 75% with pressure of 2.9 kpsi (200 bar); and (2) using 20 vol% NH4OH; 80% 5 N KOH as the growth solution and a higher temperature is used with the growth temperature at 475 1C and the dissolution zone at 490 1C, 70% fill with pressure of 20 kpsi (1.36 kbar). The experiments were conducted at AFRL, Hanscom AFB.
crystals. Fig. 1 is a photograph of a nitrogen-doped ZnO crystal grown from 1 N LiNO3 solution. Impurities were determined by GDMS from spontaneous crystallites obtained from 1 N LiNO3 as follows: N25.0, Li 10, Al 3.8, Si 6.4, Fe 4.0 [ppm wt]. SIMS measurements showed an average 1 1018 atom/cm3 of nitrogen incorporated in the crystals grown from 1 N LiNO3, as shown in Fig. 2. About 8 1018 atom/cm3 of nitrogen incorporation was measured by inert gas fusion analysis in the crystals grown from the NH4OH solution. High-resolution, four circle X-ray diffraction was performed on the p facets of the as-grown crystals from the 1 N LiNO3 solution. The full-width at half-maxima of the rocking curves (triple axis o2y scans) were 57 arcsec for the (0 0 0 2) face and 76 arcsec for the (1 0 1 1) face, indicating high structural quality. However, the seeded growth of crystals from the NH4OH solutions was affected by NH+ 4 in the solution; only spontaneously nucleated crystals were obtained. The skeletal growth can be seen when both LiNO3 and NH4OH
2.2. Characterization
3. Results and discussion 3.1. Nitrogen incorporation in ZnO crystals The nitrogen-doped ZnO crystals are slightly greenishyellow compared with the regular lithium-doped ZnO
1 cm Fig. 1. Nitrogen-doped ZnO crystal grown from 1 N LiNO3 solution.
1022 N, Si, H CONCENTRATION (atoms/cc)
The obtained crystals were analyzed by glow discharge mass spectroscopy (GDMS) and secondary ion mass spectroscopy (SIMS) to determine how much nitrogen was incorporated into the crystals. The crystals were also annealed in oxygen at 600 1C for 2 h. Surface properties were analyzed by double axis X-ray rocking curves with a high-resolution four circle X-ray diffractometer. PL at room temperature was measured with a 0.25 m grating monochrometer, CCD detector and a 275 nm, 5 ns pulsed laser source; PL at 18 K was measured with a 1.26 m grating spectrometer; an RCA C31034A photomultiplier and a HeCd laser were used in both measurements. The Hall measurements were performed at West Virginia University. The surface of the samples was polished before using an Hg/In solder to make electrical contact.
1021 1020 1019
H
1018
N Si
1017 1016
0
0.50
1.00
1.50 2.00 DEPTH (µm)
2.50
3.00
3.50
Fig. 2. SIMS profiles of nitrogen, hydrogen and silicon, of concentrations in a nitrogen-doped ZnO crystal grown from a 1 N LiNO3 solution.
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0.45
PL RELATIVE INTENSITY
0.40 0.35
18K EXCITATION 325nm 12mW CW SPEX SLIT 0.3mm PMT1KV
369nm 3.36eV
0.30
anneal 600°C, 2hrs pre-anneal
0.25 0.20
368.5nm 3.365eV
0.15 0.10
367.75nm 3.372eV 367.2nm
0.05
3.377eV
0.00 3.30 3.31 3.32 3.33 3.34 3.35 3.36 3.37 3.38 3.39 3.40 ENERGY (eV) 0.050 18K EXCITATION 325nm 12mW CW SPEX SLIT 0.3mm PMT1KV
0.045 PL RELATIVE INTENSITY
were used as mineralizers, as shown in Fig. 1. This is similar to the report by Demianets et al. [13,14], although our crystals were grown in an NH4OH solution at a much higher temperature. ZnO crystals tend to have a dendrite + growth in the presence of NH+ is 4 . The absence of Li another factor influencing the crystal growth of ZnO from NH4OH solution. Li+ is usually added to hydrothermal solvents to improve the growth quality of ZnO crystals. Since Zn3N2 crystals can be grown from supercritical ammonia solution at 450–500 1C [15], a higher temperature is necessary to grow nitrogen-doped ZnO crystals with less hydrogen incorporation. Since the N3 ionic radius is larger than that of O2, nitrogen doping might reduce the crystal quality; moreover, hydrogen incorporation may play a role in electrical charge compensation. In addition, using ammonia water as the solvent leads to a sharp increase in pressure, since the pressure–temperature curve is steeper than that of water [16]. A good pressure balance between liner and autoclave is required to prevent the liner from leaking. Typically, less than 10% of the nitrogen incorporated in the ZnO crystal was activated by annealing, which accounts for about 1017 atom/cm3 nitrogen acceptors. This acceptor concentration is lower than the concentrations of n-type impurities such as silicon and aluminum. Usually, hydrothermal ZnO crystals contain aluminum and silicon, each at 1017 atom/cm3. Therefore, our nitrogen-doped ZnO is not p-type, as discussed further in Section 3.3. More work on reducing impurities in the crystal is required.
383
0.040 0.035 0.030 0.025
anneal 600°C, 2hrs (upper) pre-anneal (lower)
0.020 3.236eV
0.015
3.332eV
0.010 0.005
3.2. PL measurements Room-temperature and low-temperature PL was measured on the as-grown surface C+ and C planes of the nitrogen-doped ZnO crystal grown from both LiNO3 and NH4OH solutions, before and after annealing at 600 1C for 2 h. PL obtained from the as-grown and annealed ZnO crystal grown in the LiNO3 solution showed little difference from the PL of regular Li-doped ZnO [17]. PL obtained from ZnO crystal in NH4OH solutions, before annealing, showed I6a (3.3604 eV), I4 (3.365 eV), I1 (3.372 eV) and AL (3.377 eV) peaks; however, after annealing, a very weak peak at 3.236 eV, which was attributed to nitrogen-associated DAP [4,18] and a peak at 3.332 eV due to nitrogen-related defects were seen in the PL spectra at 18 K [19], as shown in Fig. 3. 3.3. Electrical properties A two-point resistivity measurement of the nitrogendoped ZnO crystal grown from LiNO3 solution was performed. A room temperature resistivity of 3 105 O cm for the as-grown crystal was measured, increasing to 1 108 O cm after annealing at 600 1C in air. The temperature dependence of the resistivity and carrier concentration was measured at temperatures from 283 to 360 K; the resistivity became too high to be measured
0.000 3.10
3.15
3.20 3.25 ENERGY (eV)
3.30
3.35
Fig. 3. Low temperature photoluminescence obtained from the C+ surface of a nitrogen-doped ZnO crystal grown from NH4OH solutions before and after annealing. After annealing, two weak peaks of 3.236 and 3.332 eV were enhanced, which are attributed as the nitrogen-associated DAP and electron-acceptor emissions in N:ZnO crystals.
accurately at sample temperatures below 283 K. The electrical properties measured on a nitrogen-doped crystal grown from 1 N LiNO3 are listed in Table 1. The Hall coefficient is negative, so this is the Hall electron concentration. The Hall mobilities are low, which probably indicates that the samples are heavily compensated. Hall measurements performed on pieces from the zincterminated end of a no-nitrogen-but-lithium-doped boule showed that the Hall carrier concentration is typically 1015 cm3 at room temperature (about 1000 times higher than the result from the nitrogen-doped boule grown from the 1 N LiNO3 solution, see Table 1). For pieces from the oxygen-terminated end of the seed used for the nitrogendoped ZnO crystal growth, the resistivity was about 0.07 O cm, room-temperature Hall concentration was about 2 1018 cm3 and mobility was about 43 cm2/V s. The above properties are typical for hydrothermally grown lithium-doped ZnO crystals [12]. Fig. 4 is a plot of data of
ARTICLE IN PRESS
Temperature (K)
Hall carrier concentration ( 1012 cm3)
Hall mobility (cm2/V s)
Resistivity ( 104 O cm)
283 300 320 340 360
1.5 3.5 4.7 15 19
18.0 11.1 13.9 6.8 8.3
24 16 9.5 6.0 3.9
Hall concentration and Hall mobility measured from 87 to 360 K on the seed region of the nitrogen-doped ZnO crystal used for the data measurements in Table 1. From Fig. 4 one can see that there is no significant temperature dependence at this high level of donors. Although detailed electrical measurements of the nitrogen-doped ZnO crystal grown from NH4OH solution are under way, preliminary room temperature two-point resistivity measurements on the as-grown crystal are in the range of MO. High resistivity was also measured previously on hydrothermal ZnO crystals grown in ammonia water [20]. Thus, nitrogen-doped ZnO crystals obtained from our two different experiments are all highly insulating. Possible reasons for the increased resistivity of nitrogen-doped ZnO crystals are: (1) activation of nitrogen; (2) depletion of hydrogen in the crystal after annealing; and (3) improved stoichiometry. Since the concentrations of silicon and aluminum (each at 1017 cm3 level) are higher than activated nitrogen (at 1017 cm3 level) in the annealed samples, the nitrogen-doped ZnO crystals are not p-type, but instead are highly insulating due to compensation. Nitrogen-doped ZnO thin films have been prepared by thermally oxidizing Zn3N2 films [5]. NH3-doped ZnO thin films on sapphire substrates were epitaxially grown at 610 1C by metalorganic chemical vapor deposition [6]. Clearly, these films contain nitrogen. Minegishi et al. [7] reported the growth of p-type ZnO by the simultaneous addition of NH3 in hydrogen carrier gas with excess Zn. Photoinduced paramagnetic resonance studies of N-doped ZnO crystals indicate the presence of an acceptor state due to nitrogen substitution [4]. However, no definitive p-type behavior in these films was observed in Hall measurements, suggesting that compensating sites or complexes may have been incorporated in the growing films. With respect to p-type doping, ZnO displays significant resistance to the formation of shallow acceptor levels. There have been several explanations put forward in explaining doping difficulties in wide band gap semiconductors. First, there can be compensation by native point defects or dopant atoms that locate on interstitial sites. The defect compensates for the substitutional impurity level through the formation of a deep level trap. In some cases, strong lattice relaxations can drive the dopant energy level deeper into the gap. The incorporation of aluminum,
70 3x1018 60
50
40
1018 8x1017 6x1017
Hall Mobility (cm2/V-s)
Table 1 Electrical properties measured on a nitrogen-doped ZnO crystal grown from 1 M LiNO3 solution
Hall Carrier Concentration (cm-3)
B. Wang et al. / Journal of Crystal Growth 287 (2006) 381–385
384
30 50
100
150
200 250 300 TEMPERATURE (K)
350
400
Fig. 4. Plot of data of Hall concentration and Hall mobility taken from 87 to 360 K on the seed region of the same nitrogen-doped ZnO crystal used for data measurements in Table 1. The seed was cut from a Li-doped ZnO crystal grown from a pure alkali solution.
silicon, hydrogen and OH, as well as defects in the crystals, would play roles in compensation. So, nitrogendoped ZnO crystals grown from hydrothermal solutions may also have similar problems to other growth techniques. However, hydrothermal crystals have fewer defects because growth occurs at near-equilibrium conditions. If the concentration of unintentionally introduced impurities can be reduced, hydrothermal growth of p-type nitrogendoped ZnO crystals should be possible. 4. Summary Nitrogen-doped ZnO crystals have been successfully grown by the hydrothermal technique using (1) 1 N LiNO3, 1 N KOH and 3 N NaOH as the growth solution at T dissoultion =T growth ¼ 365 1C=350 1C, and (2) 20% NH4OH and 80% 5 N KOH as the growth solution at 490 1C/ 475 1C, respectively. An average of 1018 atom/cm3 (25200 ppm atomic concentration) of nitrogen was incorporated in the crystals. The crystals from 1 N LiNO3 solution are high quality and highly insulating. The growth of crystals from NH4OH solutions was dendritic, and the grown crystal shows 18 K photoluminescence peaks at 3.236 and 3.332 eV, which were assigned as nitrogenassociated DAP and electron-acceptor emissions in the literature, after the crystals were annealed at 600 1C for 2 h. However, due to impurity compensation, the nitrogendoped crystals grown under hydrothermal conditions are still not p-type. Nevertheless, the growth of p-type nitrogen-doped ZnO crystals by the hydrothermal technique should become possible once unintentional impurities in the crystal are reduced. Acknowledgements The work in AFRL and SSSC was funded by Air Force Office of Scientific Research (Dr. Gernot Pomrenke, program manager). We thank Capt. Eric Grant for help in cutting and polishing, and Dr. Sheng-Qi Wang for X-ray diffraction of samples.
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References [1] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Progr. Mater. Sci. 50 (2005) 293. [2] R. Triboulet, J. Perrie`re, Progr. Cryst. Growth Character. Mater. 47 (2003) 65. [3] D.C. Look, Semicond. Sci. Technol. 20 (2005) S55. [4] N.Y. Garces, L. Wang, N.C. Giles, L.E. Halliburton, G. Cantwell, D.B. Eason, J. Appl. Phys. 94 (2003) 519. [5] B.S. Li, Y.C. Liu, Z.Z. Zhi, D.Z. Shen, Y.M. Lu, J.Y. Zhang, X.W. Fan, R.X. Mu, D.O. Henderson, J. Mater. Res. 18 (1) (2003) 8. [6] J. Wang, G. Du, B. Zhao, et al., J. Crystal Growth 255 (2003) 293. [7] K. Minegishi, Y. Koiwai, Y. Kikuchi, K. Yano, M. Kasuga, A. Shimizu, Jpn. J. Appl. Phys. 36 (1997) L1453. [8] R.A. Laudise, E.D. Kolb, A.J. Caporaso, J. Am. Ceram. Soc. 47 (1964) 9. [9] K. Maeda, M. Sato, I. Niikura, T. Fukuda, Semicond. Sci. Technol. 20 (2005) S49. [10] M. Suscavage, M. Harris, D. Bliss, P. Yip, S.Q. Wang, D. Schwall, L. Bouthillette, J. Bailey, M. Callahan, D.C. Look, D.C. Reynolds,
[11]
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R.L. Jones, C.W. Litton, MRS Int. J. Nitride Semicond. Res. 4S1 (1999) G3.40. R.R. Monchamp, R.C. Puttbach, J.W. Nielson, Hydrothermal growth of zinc oxide crystals, Technical Report AFML-TR-67-144, Airtron Division of Litton Industries, Morris Plains, NJ, Contract No. AF33(657)-8795, November, 1964. D.F. Croxall, R.C.C. Ward, C.A. Wallace, R.C. Kell, J. Crystal Growth 22 (1974) 117. L.N. Demianets, D.V. Kostomarov, I.P. Kuzmina, S.V. Pushko, Crystallogr. Rep. 47 (Suppl. 1) (2002) S86. I.P. Kuzmina, V.F. Antonova, in: The Growth of Crystals, 4 (1964) 125–129. B. Fro¨hling, H. Jacobs, Z. Anorg. Allg. Chem. 623 (1997) 1103. B. Wang, M. Callahan, to be published. N.C. Giles, N.Y. Garces, L. Wang, L.E. Halliburton, Proc. SPIE 5359 (2004) 267. M. Watanabe, M. Sakai, H. Shibata, H. Tampo, P. Fons, et al., Appl. Phys. Lett. 86 (2005) 221907. J.F. Rommelue`re, L. Svob, J. Mimila-Arroyo, S. Hassani, F. Jomard, et al., Proc. SPIE 5359 (2004) 279. E.D. Kolb, R.A. Laudise, J. Am. Ceram. Soc. 49 (1966) 302.
Journal of Crystal Growth 287 (2006) 381–385 www.elsevier.com/locate/jcrysgro
Hydrothermal growth and characterization of nitrogen-doped ZnO crystals Buguo Wanga,, M.J. Callahanb, L.O. Bouthilletteb, Chunchuan Xuc, M.J. Suscavageb a
Solid State Scientific Corporation, 27-2 Wright Road, Hollis, NH 03049, USA Sensors Directorate, Air Force Research Laboratory, Hanscom AFB, MA 01731, USA c Department of Physics, West Virginia University, Morgantown, WV 26506, USA
b
Available online 4 January 2006
Abstract Hydrothermal growth of nitrogen-doped ZnO crystals was performed in alkaline solutions with the addition of 1 N LiNO3 at T dissoultion =T growth ¼ 365 1C=350 1C, and in a 20% NH4OH solution at 490 1C/475 1C, respectively. An average nitrogen concentration of approximately 1018 atom/cm3 was incorporated in the crystals. X-ray diffraction showed that the crystals grown from the 1 M LiNO3 solution are of high quality, exhibiting high resistivity. The spontaneously nucleated crystals were obtained from the NH4OH solution; a weak peak at 3.236 eV and a peak at 3.332 eV of photoluminescence were found at 18 K from these crystals after annealing at 600 1C for 2 h. The emission peaks correspond to the nitrogen-associated donor–acceptor pair (DAP) and electron-acceptor emissions in ZnO:N prepared by other techniques reported in the literature. r 2005 Elsevier B.V. All rights reserved. PACS: 81.10.Dn; 85.30.Fg; 78.55.m; 78.66.Fd; 61.72.Vv Keywords: A1. p-Type; A1. Nitrogen-doped ZnO; A2. Hydrothermal growth; B3. Hall measurements; B3. Photoluminescence
1. Introduction ZnO is attracting considerable attention for its possible applications to UV light emitters, spin functional devices, gas sensors, transparent electronics and surface acoustic wave devices [1]. However, the most significant impediment to the widespread exploitation of ZnO-related materials in electronic and photonic applications is the difficulty in carrier doping, particularly as it relates to achieving p-type material [2]. Most candidate p-type dopants in ZnO introduce deep acceptor levels [1]. Copper doping introduces an acceptor level with an energy 0.17 eV below the conduction band. Silver behaves as an acceptor with a deep level 0.23 eV below the conduction band. Lithium introduces a deep acceptor, and induces ferroelectric behavior. It appears Corresponding author. Solid State Scientific Corporation, c/o Air Force Research Laboratory SNHC/AFRL, 80 Scott Drive, Hanscom AFB, MA 01731, USA. Tel.: +1 781 377 5261; fax: +1 781 377 7812. E-mail address: buguo@solidstatescientific.com (B. Wang).
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.11.049
that the most promising dopants for p-type ZnO are the group V elements [3], and most efforts on p-type doping of ZnO have focused on nitrogen doping. There have been several reports suggesting acceptor doping levels with nitrogen substitution [4–7]. Many researchers are concentrating on producing highly crystalline ZnO epitaxial layers using a heterogeneous substrate such as sapphire. However, the speed in developing homoepitaxy will be accelerated if highly crystalline ZnO substrates become available. One possible reason for the difficulties in p-doping ZnO could be the presence of self-compensating doping mechanisms due to defects, which is seen in other II–VI materials. Therefore, unlike GaN-based devices, it may not be possible to produce efficient long-life time ZnO-based LEDs and laser diodes on highly defective ZnO films grown on heterogeneous substrates. The growth of ZnO single crystals has been carried out mainly by four methods—the chemical vapor transport method (CVT), the flux method, the high-pressure melt growth method and the hydrothermal method. The
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hydrothermal method has been known as a method to grow highly crystalline and large-size crystals for ZnO substrates under a relatively low temperature with low cost [8–12]. In this paper, we report the growth of nitrogendoped ZnO single crystals by the hydrothermal method and characterization of the grown crystals by chemical analysis, photoluminescence (PL) and Hall measurements. 2. Experimental procedure 2.1. Nitrogen-doped ZnO crystal growth Hydrothermal autoclaves made of high-strength steel were used for crystal growth. Because of the high normality of the mineralizing solution, a sealed platinum liner was used to isolate the crystal growth environment from the walls of the autoclave. The nutrient was prepared from 99.99% ZnO (Alfa Aesar) powder having particle size less than 3 mm, which was sintered for 12 h in air at 1150 1C in a platinum crucible. The seeds were (0 0 0 1) plates of ZnO from previous hydrothermal growth runs. Two different sets of conditions were employed to incorporate nitrogen into the crystals successfully: (1) using 3 N NaOH, 1 N KOH and 1 N LiNO3 as the growth solution at the growth temperature 350 1C and the dissolution zone at 365 1C, fill at 75% with pressure of 2.9 kpsi (200 bar); and (2) using 20 vol% NH4OH; 80% 5 N KOH as the growth solution and a higher temperature is used with the growth temperature at 475 1C and the dissolution zone at 490 1C, 70% fill with pressure of 20 kpsi (1.36 kbar). The experiments were conducted at AFRL, Hanscom AFB.
crystals. Fig. 1 is a photograph of a nitrogen-doped ZnO crystal grown from 1 N LiNO3 solution. Impurities were determined by GDMS from spontaneous crystallites obtained from 1 N LiNO3 as follows: N25.0, Li 10, Al 3.8, Si 6.4, Fe 4.0 [ppm wt]. SIMS measurements showed an average 1 1018 atom/cm3 of nitrogen incorporated in the crystals grown from 1 N LiNO3, as shown in Fig. 2. About 8 1018 atom/cm3 of nitrogen incorporation was measured by inert gas fusion analysis in the crystals grown from the NH4OH solution. High-resolution, four circle X-ray diffraction was performed on the p facets of the as-grown crystals from the 1 N LiNO3 solution. The full-width at half-maxima of the rocking curves (triple axis o2y scans) were 57 arcsec for the (0 0 0 2) face and 76 arcsec for the (1 0 1 1) face, indicating high structural quality. However, the seeded growth of crystals from the NH4OH solutions was affected by NH+ 4 in the solution; only spontaneously nucleated crystals were obtained. The skeletal growth can be seen when both LiNO3 and NH4OH
2.2. Characterization
3. Results and discussion 3.1. Nitrogen incorporation in ZnO crystals The nitrogen-doped ZnO crystals are slightly greenishyellow compared with the regular lithium-doped ZnO
1 cm Fig. 1. Nitrogen-doped ZnO crystal grown from 1 N LiNO3 solution.
1022 N, Si, H CONCENTRATION (atoms/cc)
The obtained crystals were analyzed by glow discharge mass spectroscopy (GDMS) and secondary ion mass spectroscopy (SIMS) to determine how much nitrogen was incorporated into the crystals. The crystals were also annealed in oxygen at 600 1C for 2 h. Surface properties were analyzed by double axis X-ray rocking curves with a high-resolution four circle X-ray diffractometer. PL at room temperature was measured with a 0.25 m grating monochrometer, CCD detector and a 275 nm, 5 ns pulsed laser source; PL at 18 K was measured with a 1.26 m grating spectrometer; an RCA C31034A photomultiplier and a HeCd laser were used in both measurements. The Hall measurements were performed at West Virginia University. The surface of the samples was polished before using an Hg/In solder to make electrical contact.
1021 1020 1019
H
1018
N Si
1017 1016
0
0.50
1.00
1.50 2.00 DEPTH (µm)
2.50
3.00
3.50
Fig. 2. SIMS profiles of nitrogen, hydrogen and silicon, of concentrations in a nitrogen-doped ZnO crystal grown from a 1 N LiNO3 solution.
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0.45
PL RELATIVE INTENSITY
0.40 0.35
18K EXCITATION 325nm 12mW CW SPEX SLIT 0.3mm PMT1KV
369nm 3.36eV
0.30
anneal 600°C, 2hrs pre-anneal
0.25 0.20
368.5nm 3.365eV
0.15 0.10
367.75nm 3.372eV 367.2nm
0.05
3.377eV
0.00 3.30 3.31 3.32 3.33 3.34 3.35 3.36 3.37 3.38 3.39 3.40 ENERGY (eV) 0.050 18K EXCITATION 325nm 12mW CW SPEX SLIT 0.3mm PMT1KV
0.045 PL RELATIVE INTENSITY
were used as mineralizers, as shown in Fig. 1. This is similar to the report by Demianets et al. [13,14], although our crystals were grown in an NH4OH solution at a much higher temperature. ZnO crystals tend to have a dendrite + growth in the presence of NH+ is 4 . The absence of Li another factor influencing the crystal growth of ZnO from NH4OH solution. Li+ is usually added to hydrothermal solvents to improve the growth quality of ZnO crystals. Since Zn3N2 crystals can be grown from supercritical ammonia solution at 450–500 1C [15], a higher temperature is necessary to grow nitrogen-doped ZnO crystals with less hydrogen incorporation. Since the N3 ionic radius is larger than that of O2, nitrogen doping might reduce the crystal quality; moreover, hydrogen incorporation may play a role in electrical charge compensation. In addition, using ammonia water as the solvent leads to a sharp increase in pressure, since the pressure–temperature curve is steeper than that of water [16]. A good pressure balance between liner and autoclave is required to prevent the liner from leaking. Typically, less than 10% of the nitrogen incorporated in the ZnO crystal was activated by annealing, which accounts for about 1017 atom/cm3 nitrogen acceptors. This acceptor concentration is lower than the concentrations of n-type impurities such as silicon and aluminum. Usually, hydrothermal ZnO crystals contain aluminum and silicon, each at 1017 atom/cm3. Therefore, our nitrogen-doped ZnO is not p-type, as discussed further in Section 3.3. More work on reducing impurities in the crystal is required.
383
0.040 0.035 0.030 0.025
anneal 600°C, 2hrs (upper) pre-anneal (lower)
0.020 3.236eV
0.015
3.332eV
0.010 0.005
3.2. PL measurements Room-temperature and low-temperature PL was measured on the as-grown surface C+ and C planes of the nitrogen-doped ZnO crystal grown from both LiNO3 and NH4OH solutions, before and after annealing at 600 1C for 2 h. PL obtained from the as-grown and annealed ZnO crystal grown in the LiNO3 solution showed little difference from the PL of regular Li-doped ZnO [17]. PL obtained from ZnO crystal in NH4OH solutions, before annealing, showed I6a (3.3604 eV), I4 (3.365 eV), I1 (3.372 eV) and AL (3.377 eV) peaks; however, after annealing, a very weak peak at 3.236 eV, which was attributed to nitrogen-associated DAP [4,18] and a peak at 3.332 eV due to nitrogen-related defects were seen in the PL spectra at 18 K [19], as shown in Fig. 3. 3.3. Electrical properties A two-point resistivity measurement of the nitrogendoped ZnO crystal grown from LiNO3 solution was performed. A room temperature resistivity of 3 105 O cm for the as-grown crystal was measured, increasing to 1 108 O cm after annealing at 600 1C in air. The temperature dependence of the resistivity and carrier concentration was measured at temperatures from 283 to 360 K; the resistivity became too high to be measured
0.000 3.10
3.15
3.20 3.25 ENERGY (eV)
3.30
3.35
Fig. 3. Low temperature photoluminescence obtained from the C+ surface of a nitrogen-doped ZnO crystal grown from NH4OH solutions before and after annealing. After annealing, two weak peaks of 3.236 and 3.332 eV were enhanced, which are attributed as the nitrogen-associated DAP and electron-acceptor emissions in N:ZnO crystals.
accurately at sample temperatures below 283 K. The electrical properties measured on a nitrogen-doped crystal grown from 1 N LiNO3 are listed in Table 1. The Hall coefficient is negative, so this is the Hall electron concentration. The Hall mobilities are low, which probably indicates that the samples are heavily compensated. Hall measurements performed on pieces from the zincterminated end of a no-nitrogen-but-lithium-doped boule showed that the Hall carrier concentration is typically 1015 cm3 at room temperature (about 1000 times higher than the result from the nitrogen-doped boule grown from the 1 N LiNO3 solution, see Table 1). For pieces from the oxygen-terminated end of the seed used for the nitrogendoped ZnO crystal growth, the resistivity was about 0.07 O cm, room-temperature Hall concentration was about 2 1018 cm3 and mobility was about 43 cm2/V s. The above properties are typical for hydrothermally grown lithium-doped ZnO crystals [12]. Fig. 4 is a plot of data of
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Temperature (K)
Hall carrier concentration ( 1012 cm3)
Hall mobility (cm2/V s)
Resistivity ( 104 O cm)
283 300 320 340 360
1.5 3.5 4.7 15 19
18.0 11.1 13.9 6.8 8.3
24 16 9.5 6.0 3.9
Hall concentration and Hall mobility measured from 87 to 360 K on the seed region of the nitrogen-doped ZnO crystal used for the data measurements in Table 1. From Fig. 4 one can see that there is no significant temperature dependence at this high level of donors. Although detailed electrical measurements of the nitrogen-doped ZnO crystal grown from NH4OH solution are under way, preliminary room temperature two-point resistivity measurements on the as-grown crystal are in the range of MO. High resistivity was also measured previously on hydrothermal ZnO crystals grown in ammonia water [20]. Thus, nitrogen-doped ZnO crystals obtained from our two different experiments are all highly insulating. Possible reasons for the increased resistivity of nitrogen-doped ZnO crystals are: (1) activation of nitrogen; (2) depletion of hydrogen in the crystal after annealing; and (3) improved stoichiometry. Since the concentrations of silicon and aluminum (each at 1017 cm3 level) are higher than activated nitrogen (at 1017 cm3 level) in the annealed samples, the nitrogen-doped ZnO crystals are not p-type, but instead are highly insulating due to compensation. Nitrogen-doped ZnO thin films have been prepared by thermally oxidizing Zn3N2 films [5]. NH3-doped ZnO thin films on sapphire substrates were epitaxially grown at 610 1C by metalorganic chemical vapor deposition [6]. Clearly, these films contain nitrogen. Minegishi et al. [7] reported the growth of p-type ZnO by the simultaneous addition of NH3 in hydrogen carrier gas with excess Zn. Photoinduced paramagnetic resonance studies of N-doped ZnO crystals indicate the presence of an acceptor state due to nitrogen substitution [4]. However, no definitive p-type behavior in these films was observed in Hall measurements, suggesting that compensating sites or complexes may have been incorporated in the growing films. With respect to p-type doping, ZnO displays significant resistance to the formation of shallow acceptor levels. There have been several explanations put forward in explaining doping difficulties in wide band gap semiconductors. First, there can be compensation by native point defects or dopant atoms that locate on interstitial sites. The defect compensates for the substitutional impurity level through the formation of a deep level trap. In some cases, strong lattice relaxations can drive the dopant energy level deeper into the gap. The incorporation of aluminum,
70 3x1018 60
50
40
1018 8x1017 6x1017
Hall Mobility (cm2/V-s)
Table 1 Electrical properties measured on a nitrogen-doped ZnO crystal grown from 1 M LiNO3 solution
Hall Carrier Concentration (cm-3)
B. Wang et al. / Journal of Crystal Growth 287 (2006) 381–385
384
30 50
100
150
200 250 300 TEMPERATURE (K)
350
400
Fig. 4. Plot of data of Hall concentration and Hall mobility taken from 87 to 360 K on the seed region of the same nitrogen-doped ZnO crystal used for data measurements in Table 1. The seed was cut from a Li-doped ZnO crystal grown from a pure alkali solution.
silicon, hydrogen and OH, as well as defects in the crystals, would play roles in compensation. So, nitrogendoped ZnO crystals grown from hydrothermal solutions may also have similar problems to other growth techniques. However, hydrothermal crystals have fewer defects because growth occurs at near-equilibrium conditions. If the concentration of unintentionally introduced impurities can be reduced, hydrothermal growth of p-type nitrogendoped ZnO crystals should be possible. 4. Summary Nitrogen-doped ZnO crystals have been successfully grown by the hydrothermal technique using (1) 1 N LiNO3, 1 N KOH and 3 N NaOH as the growth solution at T dissoultion =T growth ¼ 365 1C=350 1C, and (2) 20% NH4OH and 80% 5 N KOH as the growth solution at 490 1C/ 475 1C, respectively. An average of 1018 atom/cm3 (25200 ppm atomic concentration) of nitrogen was incorporated in the crystals. The crystals from 1 N LiNO3 solution are high quality and highly insulating. The growth of crystals from NH4OH solutions was dendritic, and the grown crystal shows 18 K photoluminescence peaks at 3.236 and 3.332 eV, which were assigned as nitrogenassociated DAP and electron-acceptor emissions in the literature, after the crystals were annealed at 600 1C for 2 h. However, due to impurity compensation, the nitrogendoped crystals grown under hydrothermal conditions are still not p-type. Nevertheless, the growth of p-type nitrogen-doped ZnO crystals by the hydrothermal technique should become possible once unintentional impurities in the crystal are reduced. Acknowledgements The work in AFRL and SSSC was funded by Air Force Office of Scientific Research (Dr. Gernot Pomrenke, program manager). We thank Capt. Eric Grant for help in cutting and polishing, and Dr. Sheng-Qi Wang for X-ray diffraction of samples.
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