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Phase separation in Ga-doped MgZnO layers grown by plasma-assisted molecular-beam epitaxy
ARTICLE IN PRESS
Materials Science in Semiconductor Processing 6 (2003) 539–541
Phase separation in Ga-doped MgZnO layers grown by plasma-assisted molecular-beam epitaxy Chihiro Harada*, Hang-Ju Ko, Hisao Makino, Takafumi Yao Center for Interdisciplinary Research, Tohoku University, Aramaki, Aobaku, Sendai 980-8578, Japan Received 10 March 2003; received in revised form 30 July 2003; accepted 10 August 2003
Abstract We report on the phase separation of Ga-doped MgZnO layers grown by plasma-assisted molecular-beam epitaxy. Based on X-ray diffraction, low-temperature (10 K) photoluminescence, and reflection high-energy electron diffraction observations, it is possible to classify the phase of Ga-doped MgZnO layers into three regions depending on the incorporated Ga concentration ([Ga]). Single-phase Mg0.1Zn0.9O layers are grown when [Ga] is less than 1 1018 cm 3. For [Ga] between 1 1018 cm 3 and 1 1020 cm 3, ZnO and Mg0.2Zn0.8O coexist, where electron transport is considered to be via two-channel conduction. When [Ga] exceeds 1 1020 cm 3, the Ga-doped MgZnO layers become polycrystalline, where carrier compensation takes place presumably due to grain boundaries. r 2003 Elsevier Ltd. All rights reserved. PACS: 81.05.Dz; 61.72.Ww; 64.75.+g Keywords: MgZnO; Ga doping; Phase separation; P-MBE
1. Introduction ZnO is a wide bandgap semiconductor with a direct bandgap energy of 3.37 eV at room temperature. The bandgap energy can be varied by making ternary compounds. MgO and CdO have bandgap energies of 7.8 eV [1] and 2.4 eV [2], respectively. The bandgap is increased by alloying with MgO, while decreased by alloying with CdO. The n-type conductivity of ZnO layers has been controlled up to 1020 cm 3 using Ga as a donor [3]. However, the conductivity of MgZnO layers has not been investigated yet. In this study, we have investigated the effects of Ga doping into MgZnO layers grown by plasma-assisted molecular-beam epitaxy (P-MBE) on crystallographic properties of MgZnO. The Mg–Zn– Ga–O quaternary system is complicated. MgO and ZnO
*Corresponding author. Tel.: +81-22-215-2074; fax: +8122-215-2073. E-mail address: [email protected] (C. Harada).
are of rocksalt and wurtzite structures, respectively. Therefore MgO cannot dissolve in ZnO completely. According to the phase diagram of the ZnO–MgO ternary system, the thermodynamic solubility limit of MgO in ZnO is less than 4 mol% [4]. It has been reported that MgZnO films can be grown with Mg compositions up to 33 mol% using pulsed laser deposition [5]. If Ga is added to the Mg–Zn–O ternary system, there is a possibility of the formation of additional phases [6]: corundum Ga2O3, spinel MgGa2O4, ZnGa2O4, and homologous Ga2O3[ZnO]m. We have found high incorporation of Ga induced phase separation in MgZnO.
2. Experimental Ga-doped MgZnO layers were grown on sapphire (0 0 0 1) substrates by P-MBE. Prior to the growth of Ga-doped MgZnO, MgO buffer (B3 nm) and LT–ZnO buffer (B30 nm) were grown on a sapphire substrate.
1369-8001/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2003.08.003
ARTICLE IN PRESS C. Harada et al. / Materials Science in Semiconductor Processing 6 (2003) 539–541
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After annealing the double buffer layers, we obtained a smooth and relaxed surface as evidenced by a streaky reflection high-energy electron diffraction (RHEED) pattern. Ga-doped Mg0.1Zn0.9O layers were grown on high-quality ZnO surfaces at 450 C. The compositions of Mg and Zn were determined by inductively coupled plasma (ICP) optical emission spectroscopy. The nominal Mg composition was fixed at 10%. It was confirmed that undoped Mg0.1Zn0.9O layers grown at 450 C were single crystalline. In order to control Ga doping concentration, we varied the Ga cell temperature from 320 C to 500 C. The growth process was monitored in situ by RHEED. The thickness of Ga-doped MgZnO layers was about 500 nm. Ga concentration ([Ga]) was measured by secondary ion mass spectroscopy (SIMS), and the phase separation in Mg0.1Zn0.9O:Ga layers was confirmed by X-ray diffraction (XRD) and lowtemperature (10 K) photoluminescence (PL). The electrical properties of Ga-doped MgZnO layers were characterized at room temperature by Hall measurements using the van der Pauw method.
3. Results and discussion Fig. 1 shows (0002) 2y–o XRD curves of (a) an undoped MgZnO layer and Ga-doped MgZnO layers with [Ga] of (b) 3.1 1019 cm 3 and (c) 1.8 1021 cm 3. The diffraction curve of the undoped Mg0.1Zn0.9O shows a sharp peak at 2y=34.514 with a broad shoulder extending towards the lower angle side. The presence of such shoulder implies the existence of a compressive strain in the layer. With the increase of [Ga] up to 3.1 1019 cm 3, the single XRD peak from the (0002)Mg0.1 Zn 0.9 O
MgZnO layer split into two peaks at 2y=34.449 and 34.581 as shown in Fig. 1(b). These two peaks correspond to (0 0 0 2)ZnO and (0 0 0 2)Mg0.2Zn0.8O, respectively, which indicates that excess Ga doping of more than [Ga] of 1 1018 cm 3 induces phase separation of Mg0.1Zn0.9O layers into Mg0.2Zn0.8O and ZnO. When the [Ga] is increased more than 1 1020 cm 3, the XRD peak of MgZnO disappears and that of ZnO . remains with prominent Pendellosung fringes on the tails. The layer was polycrystalline as observed by RHEED during growth. Fig. 2 shows low-temperature PL spectra of (a) the undoped MgZnO layer and Ga-doped MgZnO layers with [Ga] of (b) 3.1 1019 cm 3and (c) 1.8 1021 cm 3. The samples were excited by a He–Cd laser with the emission line at 325 nm for the most of the experiments, and by a frequency-converted Nd:YAG laser pulse at 266 nm for measurements in the higher-energy region. The emission peak of undoped Mg0.1Zn0.9O is observed at 3.621 eV. The peak position is lower in energy than that predicted by linear interpolation of the bandgap of ZnO (3.37 eV) and MgO (7.8 eV). However it is consistent with the reported PL peak position of Mg0.1Zn0.9O [5]. As shown in Fig. 2(b), the PL spectrum of Ga-doped MgZnO layers with a higher [Ga] of 3.1 1019 cm 3 consists of a broad and dominant peak at 3.781 eV and small peaks at lower energies. These small peaks at lower energy are clearly resolved when measured using the He–Cd laser, while only a broad peak is observed when measured using the Nd:YAG laser. The peak at 3.781 eV corresponds well to the bandgap of Mg0.2Zn0.8O [5], while the peaks at 3.362 and 3.326 eV can be attributed to bound-excitonemission in ZnO. A broad peak at 3.192 eV is also observed and can be attributed to impurity-related recombination in ZnO. Since the excitation wavelength
3.621 eV
(0002)ZnO
(a)
(b)
(a) PL intensity (a.u.)
XRD intensity (a.u.)
(0002)Mg0.2Zn 0.8 O
3.326 eV 3.19 eV
× 50
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× 30
3.32 e V
3.375 eV
excitedby He-Cd laser
excited by Nd:YAG laser
(c) 34.2
34.4 34.6 2- (deg.)
34.8
Fig. 1. (0 0 0 2) 2y–o XRD curves of (a) the undoped MgZnO layer and Ga-doped MgZnO layers with [Ga] of (b) 3.1 1019 cm 3 and (c) 1.8 1021 cm 3. The intensity is plotted on a logarithmic scale.
×5
3.36 eV
(c) 34.0
3.78 1 e V
3.362 eV
2.8
× 15
3.0
3.2
3.4 3.6 photon energy (eV)
3.8
4.0
Fig. 2. Low-temperature PL spectra of (a) the undoped MgZnO layer and Ga-doped MgZnO layers with [Ga] of (b) 3.1 1019 cm 3 and (c) 1.8 1021 cm 3.
ARTICLE IN PRESS C. Harada et al. / Materials Science in Semiconductor Processing 6 (2003) 539–541
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Mg0.2Zn0.8O in the Ga-doped layer in region II. Therefore, the electron transport in the Ga-doped layers in region II should be considered in terms of two channel conduction in ZnO and Mg0.2Zn0.8O. In region III, Gadoped MgZnO layers are polycrystalline, in which there exist many grain boundaries in the layers. It is well known that grain boundaries in ZnO polycrystals play a role as electron traps [7]. The decrease in carrier concentration can be attributed to electron compensation at grain boundaries.
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Ga concentration (cm ) Fig. 3. Carrier concentration (open squares) and resistivity (solid circles) of Ga-doped MgZnO layers measured at room temperature by the van der Pauw method. Regions I, II, and III are classified in terms of the phases of Ga-doped MgZnO layers.
by the frequency-converted Nd:YAG laser is much shorter than that the He–Cd laser, the PL emission comes from a deeper region from the surface when excited by the He–Cd laser compared with the Nd:YAG excitation. The difference observed in the ZnO emission region between the two excitations suggests that the feature of phase separation differs between the surface and bulk. For [Ga] more than 1 1020 cm 3, only the emission from ZnO is observed. These results are in good agreement with the XRD results and clearly suggest that phase separation occurs in Ga-doped MgZnO layers at high [Ga]. Fig. 3 shows the carrier concentration (open squares) and resistivity (solid circles) of Ga-doped MgZnO layers. Based on the PL, XRD, and RHEED results, it is possible to classify the phase of Ga-doped MgZnO layers as follows: single phase Mg0.1Zn0.9O in region I, two phases of ZnO and Mg0.2Zn0.8O in region II, and polycrystalline phase in region III. In spite of the increase of [Ga] up to 1018 cm 3 in region I, the carrier concentration only slightly increases to around 1 1017 cm 3. On the other hand, it increases sharply from 1 1017 to 1.4 1020 cm 3 with increasing [Ga] from 1 1018 to 1 1020 cm 3 in region II. The resistivity of Ga-doped MgZnO layers gradually decreases from 18 to 3.3 O cm in region I and rapidly decreases to 1.8 10 3 O cm with increasing [Ga] in region II. When the [Ga] is increased up to 1020 cm 3, the carrier concentration decreases and the resistivity again increases (region III in Fig. 3). The smaller carrier concentration compared with the [Ga] in region I indicates that Ga impurities do not work effectively as a donor in region I. However, Ga impurity works effectively as a donor in region II, since it increases with [Ga]. Note that there are two phases of ZnO and
4. Conclusions We investigated the effects of Ga doping in Mg0.1Zn0.9O layers grown by P-MBE at 450 C on crystalline properties. It is found that phase separation occurred in Ga-doped MgZnO at a high Ga concentration. Ga-doped MgZnO layers for [Ga] less than 1 1018 cm 3 are single-phase Mg0.1Zn0.9O layers. On the other hand, two phases of ZnO and Mg0.2Zn0.8O coexist in Ga-doped layers with [Ga] between 1 1018 cm 3 and 1 1020 cm 3, where Ga impurity works effectively as a donor. In this region the electron transport in Ga-doped layers is considered to be due to two conduction channels of ZnO and Mg0.2Zn0.8O phases. When [Ga] increases more than 1 1020 cm 3, the Ga-doped MgZnO layers become polycrystalline, in which the carrier concentration decreases due to electron compensation presumably at grain boundaries.
References [1] Bredow T, Gerson R. Effect of exchange and correlation on bulk properties of MgO, NiO, and CoO. Phys Rev B 2000;61:5194. [2] Yan M, Lane M, Kannewurf CR, Chang RPH. Highly conductive epitaxial CdO thin films prepared by pulsed laser deposition. Appl Phys Lett 2001;78:2342. [3] Ko HJ, Chen YF, Hong SK, Wenisch H, Yao T. Ga doped ZnO films grown on GaN templates by plasma-assisted molecular-beam epitaxy. Appl Phys Lett 2000;77:3761. [4] Sarver JF, Katnack FL, Hummel FA. Phase equilibria and manganese-activate fluorescence in the system Zn3(PO4)2– Mg3(PO4)2. J Electrochem Soc 1959;106:960. [5] Ohtomo A, Kawasaki M, Koida T, Masubuchi K, Koinuma H, Sakurai Y, Yoshida Y, Yasuda T, Segawa Y. MgxZn1 xO as a II–VI widegap semiconductor alloy. Appl Phys Lett 1998;72:2466. [6] Galasso FS. Structure and properties of inorganic solids. New York: Pergamon Press; 1970. [7] Winston RA, Cordaro JF. Grain-boundary interface electron traps in commercial zinc oxide varistors. J Appl Phys 1990;68:6495.
Materials Science in Semiconductor Processing 6 (2003) 539–541
Phase separation in Ga-doped MgZnO layers grown by plasma-assisted molecular-beam epitaxy Chihiro Harada*, Hang-Ju Ko, Hisao Makino, Takafumi Yao Center for Interdisciplinary Research, Tohoku University, Aramaki, Aobaku, Sendai 980-8578, Japan Received 10 March 2003; received in revised form 30 July 2003; accepted 10 August 2003
Abstract We report on the phase separation of Ga-doped MgZnO layers grown by plasma-assisted molecular-beam epitaxy. Based on X-ray diffraction, low-temperature (10 K) photoluminescence, and reflection high-energy electron diffraction observations, it is possible to classify the phase of Ga-doped MgZnO layers into three regions depending on the incorporated Ga concentration ([Ga]). Single-phase Mg0.1Zn0.9O layers are grown when [Ga] is less than 1 1018 cm 3. For [Ga] between 1 1018 cm 3 and 1 1020 cm 3, ZnO and Mg0.2Zn0.8O coexist, where electron transport is considered to be via two-channel conduction. When [Ga] exceeds 1 1020 cm 3, the Ga-doped MgZnO layers become polycrystalline, where carrier compensation takes place presumably due to grain boundaries. r 2003 Elsevier Ltd. All rights reserved. PACS: 81.05.Dz; 61.72.Ww; 64.75.+g Keywords: MgZnO; Ga doping; Phase separation; P-MBE
1. Introduction ZnO is a wide bandgap semiconductor with a direct bandgap energy of 3.37 eV at room temperature. The bandgap energy can be varied by making ternary compounds. MgO and CdO have bandgap energies of 7.8 eV [1] and 2.4 eV [2], respectively. The bandgap is increased by alloying with MgO, while decreased by alloying with CdO. The n-type conductivity of ZnO layers has been controlled up to 1020 cm 3 using Ga as a donor [3]. However, the conductivity of MgZnO layers has not been investigated yet. In this study, we have investigated the effects of Ga doping into MgZnO layers grown by plasma-assisted molecular-beam epitaxy (P-MBE) on crystallographic properties of MgZnO. The Mg–Zn– Ga–O quaternary system is complicated. MgO and ZnO
*Corresponding author. Tel.: +81-22-215-2074; fax: +8122-215-2073. E-mail address: [email protected] (C. Harada).
are of rocksalt and wurtzite structures, respectively. Therefore MgO cannot dissolve in ZnO completely. According to the phase diagram of the ZnO–MgO ternary system, the thermodynamic solubility limit of MgO in ZnO is less than 4 mol% [4]. It has been reported that MgZnO films can be grown with Mg compositions up to 33 mol% using pulsed laser deposition [5]. If Ga is added to the Mg–Zn–O ternary system, there is a possibility of the formation of additional phases [6]: corundum Ga2O3, spinel MgGa2O4, ZnGa2O4, and homologous Ga2O3[ZnO]m. We have found high incorporation of Ga induced phase separation in MgZnO.
2. Experimental Ga-doped MgZnO layers were grown on sapphire (0 0 0 1) substrates by P-MBE. Prior to the growth of Ga-doped MgZnO, MgO buffer (B3 nm) and LT–ZnO buffer (B30 nm) were grown on a sapphire substrate.
1369-8001/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2003.08.003
ARTICLE IN PRESS C. Harada et al. / Materials Science in Semiconductor Processing 6 (2003) 539–541
540
After annealing the double buffer layers, we obtained a smooth and relaxed surface as evidenced by a streaky reflection high-energy electron diffraction (RHEED) pattern. Ga-doped Mg0.1Zn0.9O layers were grown on high-quality ZnO surfaces at 450 C. The compositions of Mg and Zn were determined by inductively coupled plasma (ICP) optical emission spectroscopy. The nominal Mg composition was fixed at 10%. It was confirmed that undoped Mg0.1Zn0.9O layers grown at 450 C were single crystalline. In order to control Ga doping concentration, we varied the Ga cell temperature from 320 C to 500 C. The growth process was monitored in situ by RHEED. The thickness of Ga-doped MgZnO layers was about 500 nm. Ga concentration ([Ga]) was measured by secondary ion mass spectroscopy (SIMS), and the phase separation in Mg0.1Zn0.9O:Ga layers was confirmed by X-ray diffraction (XRD) and lowtemperature (10 K) photoluminescence (PL). The electrical properties of Ga-doped MgZnO layers were characterized at room temperature by Hall measurements using the van der Pauw method.
3. Results and discussion Fig. 1 shows (0002) 2y–o XRD curves of (a) an undoped MgZnO layer and Ga-doped MgZnO layers with [Ga] of (b) 3.1 1019 cm 3 and (c) 1.8 1021 cm 3. The diffraction curve of the undoped Mg0.1Zn0.9O shows a sharp peak at 2y=34.514 with a broad shoulder extending towards the lower angle side. The presence of such shoulder implies the existence of a compressive strain in the layer. With the increase of [Ga] up to 3.1 1019 cm 3, the single XRD peak from the (0002)Mg0.1 Zn 0.9 O
MgZnO layer split into two peaks at 2y=34.449 and 34.581 as shown in Fig. 1(b). These two peaks correspond to (0 0 0 2)ZnO and (0 0 0 2)Mg0.2Zn0.8O, respectively, which indicates that excess Ga doping of more than [Ga] of 1 1018 cm 3 induces phase separation of Mg0.1Zn0.9O layers into Mg0.2Zn0.8O and ZnO. When the [Ga] is increased more than 1 1020 cm 3, the XRD peak of MgZnO disappears and that of ZnO . remains with prominent Pendellosung fringes on the tails. The layer was polycrystalline as observed by RHEED during growth. Fig. 2 shows low-temperature PL spectra of (a) the undoped MgZnO layer and Ga-doped MgZnO layers with [Ga] of (b) 3.1 1019 cm 3and (c) 1.8 1021 cm 3. The samples were excited by a He–Cd laser with the emission line at 325 nm for the most of the experiments, and by a frequency-converted Nd:YAG laser pulse at 266 nm for measurements in the higher-energy region. The emission peak of undoped Mg0.1Zn0.9O is observed at 3.621 eV. The peak position is lower in energy than that predicted by linear interpolation of the bandgap of ZnO (3.37 eV) and MgO (7.8 eV). However it is consistent with the reported PL peak position of Mg0.1Zn0.9O [5]. As shown in Fig. 2(b), the PL spectrum of Ga-doped MgZnO layers with a higher [Ga] of 3.1 1019 cm 3 consists of a broad and dominant peak at 3.781 eV and small peaks at lower energies. These small peaks at lower energy are clearly resolved when measured using the He–Cd laser, while only a broad peak is observed when measured using the Nd:YAG laser. The peak at 3.781 eV corresponds well to the bandgap of Mg0.2Zn0.8O [5], while the peaks at 3.362 and 3.326 eV can be attributed to bound-excitonemission in ZnO. A broad peak at 3.192 eV is also observed and can be attributed to impurity-related recombination in ZnO. Since the excitation wavelength
3.621 eV
(0002)ZnO
(a)
(b)
(a) PL intensity (a.u.)
XRD intensity (a.u.)
(0002)Mg0.2Zn 0.8 O
3.326 eV 3.19 eV
× 50
(b)
× 30
3.32 e V
3.375 eV
excitedby He-Cd laser
excited by Nd:YAG laser
(c) 34.2
34.4 34.6 2- (deg.)
34.8
Fig. 1. (0 0 0 2) 2y–o XRD curves of (a) the undoped MgZnO layer and Ga-doped MgZnO layers with [Ga] of (b) 3.1 1019 cm 3 and (c) 1.8 1021 cm 3. The intensity is plotted on a logarithmic scale.
×5
3.36 eV
(c) 34.0
3.78 1 e V
3.362 eV
2.8
× 15
3.0
3.2
3.4 3.6 photon energy (eV)
3.8
4.0
Fig. 2. Low-temperature PL spectra of (a) the undoped MgZnO layer and Ga-doped MgZnO layers with [Ga] of (b) 3.1 1019 cm 3 and (c) 1.8 1021 cm 3.
ARTICLE IN PRESS C. Harada et al. / Materials Science in Semiconductor Processing 6 (2003) 539–541
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Mg0.2Zn0.8O in the Ga-doped layer in region II. Therefore, the electron transport in the Ga-doped layers in region II should be considered in terms of two channel conduction in ZnO and Mg0.2Zn0.8O. In region III, Gadoped MgZnO layers are polycrystalline, in which there exist many grain boundaries in the layers. It is well known that grain boundaries in ZnO polycrystals play a role as electron traps [7]. The decrease in carrier concentration can be attributed to electron compensation at grain boundaries.
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Ga concentration (cm ) Fig. 3. Carrier concentration (open squares) and resistivity (solid circles) of Ga-doped MgZnO layers measured at room temperature by the van der Pauw method. Regions I, II, and III are classified in terms of the phases of Ga-doped MgZnO layers.
by the frequency-converted Nd:YAG laser is much shorter than that the He–Cd laser, the PL emission comes from a deeper region from the surface when excited by the He–Cd laser compared with the Nd:YAG excitation. The difference observed in the ZnO emission region between the two excitations suggests that the feature of phase separation differs between the surface and bulk. For [Ga] more than 1 1020 cm 3, only the emission from ZnO is observed. These results are in good agreement with the XRD results and clearly suggest that phase separation occurs in Ga-doped MgZnO layers at high [Ga]. Fig. 3 shows the carrier concentration (open squares) and resistivity (solid circles) of Ga-doped MgZnO layers. Based on the PL, XRD, and RHEED results, it is possible to classify the phase of Ga-doped MgZnO layers as follows: single phase Mg0.1Zn0.9O in region I, two phases of ZnO and Mg0.2Zn0.8O in region II, and polycrystalline phase in region III. In spite of the increase of [Ga] up to 1018 cm 3 in region I, the carrier concentration only slightly increases to around 1 1017 cm 3. On the other hand, it increases sharply from 1 1017 to 1.4 1020 cm 3 with increasing [Ga] from 1 1018 to 1 1020 cm 3 in region II. The resistivity of Ga-doped MgZnO layers gradually decreases from 18 to 3.3 O cm in region I and rapidly decreases to 1.8 10 3 O cm with increasing [Ga] in region II. When the [Ga] is increased up to 1020 cm 3, the carrier concentration decreases and the resistivity again increases (region III in Fig. 3). The smaller carrier concentration compared with the [Ga] in region I indicates that Ga impurities do not work effectively as a donor in region I. However, Ga impurity works effectively as a donor in region II, since it increases with [Ga]. Note that there are two phases of ZnO and
4. Conclusions We investigated the effects of Ga doping in Mg0.1Zn0.9O layers grown by P-MBE at 450 C on crystalline properties. It is found that phase separation occurred in Ga-doped MgZnO at a high Ga concentration. Ga-doped MgZnO layers for [Ga] less than 1 1018 cm 3 are single-phase Mg0.1Zn0.9O layers. On the other hand, two phases of ZnO and Mg0.2Zn0.8O coexist in Ga-doped layers with [Ga] between 1 1018 cm 3 and 1 1020 cm 3, where Ga impurity works effectively as a donor. In this region the electron transport in Ga-doped layers is considered to be due to two conduction channels of ZnO and Mg0.2Zn0.8O phases. When [Ga] increases more than 1 1020 cm 3, the Ga-doped MgZnO layers become polycrystalline, in which the carrier concentration decreases due to electron compensation presumably at grain boundaries.
References [1] Bredow T, Gerson R. Effect of exchange and correlation on bulk properties of MgO, NiO, and CoO. Phys Rev B 2000;61:5194. [2] Yan M, Lane M, Kannewurf CR, Chang RPH. Highly conductive epitaxial CdO thin films prepared by pulsed laser deposition. Appl Phys Lett 2001;78:2342. [3] Ko HJ, Chen YF, Hong SK, Wenisch H, Yao T. Ga doped ZnO films grown on GaN templates by plasma-assisted molecular-beam epitaxy. Appl Phys Lett 2000;77:3761. [4] Sarver JF, Katnack FL, Hummel FA. Phase equilibria and manganese-activate fluorescence in the system Zn3(PO4)2– Mg3(PO4)2. J Electrochem Soc 1959;106:960. [5] Ohtomo A, Kawasaki M, Koida T, Masubuchi K, Koinuma H, Sakurai Y, Yoshida Y, Yasuda T, Segawa Y. MgxZn1 xO as a II–VI widegap semiconductor alloy. Appl Phys Lett 1998;72:2466. [6] Galasso FS. Structure and properties of inorganic solids. New York: Pergamon Press; 1970. [7] Winston RA, Cordaro JF. Grain-boundary interface electron traps in commercial zinc oxide varistors. J Appl Phys 1990;68:6495.