- Email: [email protected]
Effects of thermal annealing of ZnO layers grown by MBE
Journal of Crystal Growth 214/215 (2000) 312}315
E!ects of thermal annealing of ZnO layers grown by MBE K. Ogata *, K. Sakurai, Sz. Fujita, Sg. Fujita, K. Matsushige Kyoto University Venture Business Laboratory, Kyoto University, Kyoto, 606-8501, Japan Department of Electronic Science and Engineering, Kyoto University, Kyoto, 606-8501, Japan
Abstract Thermal annealing of ZnO layers was done in N or O atmosphere and their e!ects were studied. Electron carrier density increases according to reevaporation of O from ZnO if annealed in N atmosphere. On the contrary, it decreases from the order of 10 to 10 cm\, and also optical properties are improved when annealed in O atmosphere at lower temperature. In that case, the number of interstitial Zn (Zn ) and O vacancies (V ) decrease, probably because the e!ective incorporation of O atom diminishes those donor levels. The crystallinity also improves with the annealing. The mobilities of the layer increase up to 51 cm/V s as annealing temperature increases, but are lower at lower temperatures. Solving this problem, annealing may become a promising technique for the fabrication of p-type ZnO layers. 2000 Elsevier Science B.V. All rights reserved. PACS: 61.72.Cc; 72.20.Dp; 81.15.Kk Keywords: ZnO; MBE; Thermal annealing; Electron carrier density; Mobility
1. Introduction For the short wavelength light-emitting devices, wide bandgap semiconductors have been widely studied in this decade. In particular, the technologies of both ZnSe- and GaN-based materials have made much progress [1,2]. Recently, another II}VI wide gap semiconductor ZnO (E "3.4 eV) [3] has been paid much attention to [5,6]. The exciton binding energy of ZnO is about 60 meV [3], roughly three times larger than that of ZnSe or GaN. In addition, the biexciton formation energy of ZnO is
* Corresponding author. Tel.: #81-75-753-7577; fax: #8175-753-7579. E-mail address: [email protected] (K. Ogata).
about 15 meV [4], also much larger. Therefore, ZnO is considered to be a promising material for novel exciton-related devices, such as low threshold lasers [7] and nonlinear optical devices [8], which are based on physics of excitons. However, at present, growth techniques of ZnO are still being developed, since the time when growth of epitaxial ZnO layers started as bu!er layers for GaN growth [9,10] several years ago. So, even in case of unintentionally doped ZnO layers, the layers show n-type conductivity with n-concentration of the order of 10 cm\, and control of electronic properties by impurity doping seems to be di$cult, especially for p-type layers, as had been quite di$cult in ZnSe and GaN. The origin of the donor levels is still unknown, but the interstitial Zn atom (Zn ) and O vacancy (V ) are thought to form donor levels [11,12]. To fabricate current injection
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 0 9 9 - 3
K. Ogata et al. / Journal of Crystal Growth 214/215 (2000) 312}315
313
pn junction devices, the realization of p-type conductivity is indispensable. Therefore, it is quite important to reduce these donor levels and to obtain high-quality layers. At the moment, attempts of nitrogen doping have resulted in no clear indication of acceptor formation yet [13]. Therefore, the suppression of native donor defects must be improved to achieve p-type doping. In this paper, we report the variation of electronic and optical properties of ZnO layers in terms of the thermal annealing conditions and e!ects on the material properties.
2. Experiments The ZnO layers were grown on (0 0 0 1) sapphire substrates by molecular beam epitaxy (MBE) at 7803C using RF oxygen plasma. Typical Zn pressure, O #ow rate and RF excitation power are 2.0;10\ Torr, 0.3 sccm and 300 W, respectively. The thickness, electron carrier density and mobility of a typical as-grown layer employed in this work are about 2500 As , 10 cm\ and 20 cm/V s, respectively. Thermal annealing after the growth was done in O or N atmosphere for 30 min. The annealing temperatures were varied between 500 and 10003C. Electrical properties were investigated by Hall measurement with the van der Pauw method, and optical properties by photoluminescence (PL) with a He}Cd laser (325 nm, 10 mW) at room temperature.
3. Results and discussion At "rst, it is quite important to investigate the e!ects of annealing atmosphere for the improvement of the layers. PL spectra taken after the annealing in N or O atmosphere at 10003C are shown in Fig. 1. As can be seen, the intensities of the excitonic emissions around 390 nm increase after the thermal annealing, but it is much more predominant when annealed in O atmosphere. Fur ther, it can be seen that the intensities of deep emissions are not a!ected by the thermal annealing, implying that it does not make deep centers in ZnO crystals.
Fig. 1. PL spectra of as-grown layer, annealed in N and in O atmosphere.
Fig. 2. Electron carrier density as a function of annealing temperatures.
Fig. 2 shows the variation of the electron carrier density in the ZnO layers as a function of the annealing temperature, where in this sample it is 2.4;10 cm\ in the as-grown state. It increases as high as 2.0;10 cm\ if annealed at 10003C. The increase in the carrier density can be attributed to the reevaporation of oxygen due to the hightemperature annealing. The following reactions may occur at high temperatures: (1) ZnOPZn #V #O !, 8 - ZnOPZn #O !. (2) As a result, both Zn and V act as donors. In addition to reevaporation of O atoms, formation of the spinel layers at the interface between the
314
K. Ogata et al. / Journal of Crystal Growth 214/215 (2000) 312}315
sapphire substrate and the ZnO epitaxial layer by higher temperature annealing [14] may also be the cause of degradation of the ZnO layers. On the other hand, we should notice that lower temperature annealing at 500 and 7003C decreases electron carrier density to 2.5;10 and 6.3; 10 cm\, respectively. In these cases, it is suggested that the reactions (1) and (2) are suppressed at lower temperatures and the O atmosphere enhan ces the reverse reactions of Eqs. (1) and (2). On the other hand, in N atmosphere, the electron carrier density increases to 2.6;10 cm\ when annealed at 5003C, as shown by the open circle in Fig. 2. This is probably because the N atmosphere cannot suppress su$ciently the reevaporation of oxygen during the annealing, i.e., the reverse reactions of Eqs. (1) and (2) are not enhanced compared to that in the O atmosphere. From these results, in order to obtain high-quality and lower donor level ZnO layers, thermal annealing in O atmosphere seems to be suitable. As for the mobility of the layers annealed in O atmosphere, interestingly it slightly decreases and then increases when the annealing temperature is lower or higher than the growth tempearture, as shown in Fig. 3. The maximum values after the annealing was 51 cm/V s, but still lower than that of high-quality bulk ZnO (205 cm/V s) [15]. The increase of the mobilities with the annealing temperature agrees with the results of the X-ray di!raction measurement on ZnO(0 0 0 2) shown in Fig. 4, which shows the di!raction intensities get stronger when annealed at higher temperature. However, at low temperature, Fig. 4 also suggests that the crystal quality becomes better, in spite of the slight decrease of the mobility. The FWHM of the X-ray di!raction, where it is 0.683 in the asgrown sample, decreases to 0.233 and the integrated intensity increases by more than 10 times after the annealing at 8503C due to the increase of the grain size of ZnO. This tendency of mobility is unusual, but agrees with that of polycrystalline silicon [16]. By paying attention to the grain size and carrier densities of the layers, it could be interpreted as follows. The mobility increases due to the increase of the grain size by the thermal annealing at higher temperature. In addition to that, those measured mobilities have higher values, since the electrons
Fig. 3. Electron mobility as a function of annealing temperatures.
Fig. 4. X-ray di!raction intensities of annealed ZnO layers.
can move beyond the barrier created at the grain boundary. On the contrary, after low-temperature annealing, the electron cannot pass the barrier, which reduces the mobilities since the Fermi level falls due to the decrease of electron density. Or it might be possible for the following reactions to occur during thermal annealing: ZnO#O QZn #V #O , (3) 8 ZnO#O QZn #O . (4) Then, the generation of interstitial oxygen (O ), which acts as deep acceptors [17] might make the mobility low. However, the papers on O are quite scarce, so it should be studied in the future. In Fig. 5, the PL spectra of ZnO layers annealed at various temperatures in O atmosphere are shown. At 5003C annealing, the excitonic emission gets stronger than that of higher temperature annealed samples, so the generation of nonradiative
K. Ogata et al. / Journal of Crystal Growth 214/215 (2000) 312}315
315
and the mobility increases to, e.g., 51 cm/V s. For the annealing in O at temperatures lower than the growth temperature, the electron carrier density decreases, e.g., from 2.4;10 to 2.5;10 cm\ by the 5003C annealing, and the crystallinity is also improved. However, the problem of the decrease in mobility remains. At the moment, thermal annealing in O at a low temperature seems to be a suit able choice from the standpoint of decreasing the concentration of residual donors.
References
Fig. 5. PL spectra of ZnO layers annealed at various temperatures.
recombination centers is greatly prevented. Therefore, it can be concluded that the annealing at higher temperatures not only makes the crystal quality better, but also generates many donor levels and nonradiative centers. On the other hand, at the moment, the lower temperature annealing in O atmosphere is more promising for obtaining ZnO layers suitable for p-type doping and for optical applications, although the problem of the decrease in the mobility must be solved.
4. Summary The e!ects of thermal annealing of ZnO layers grown by MBE were investigated. The annealing in O atmosphere is necessary compared to N atmo sphere in order to suppress the generation of V and Zn . If the annealing is done in O at temperatures higher than the growth temperature, the electron concentration increases, because the reevaporation of oxygen from ZnO cannot be suppressed. However, the crystallinity becomes better
[1] Y. Sanaka, H. Okuyama, S. Kijima, E. Kato, H. Noguchi, A. Ishibashi, Electron. Lett. 34 (1998) 1891. [2] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Matsushita, T. Mukai, Proceedings of Second International Symposium on Blue Laser and Light Emitting Diodes, 1998, p. 371. [3] A. Mang, K. Reimann, St. RuK benacke, Solid State Commun. 94 (1995) 251. [4] J.M. Hall, G. Brattner, M. Reuscher, C. Klingshirn, Phys. Stat. Sol. (b) 118 (1983) 179. [5] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, M.Y. Shen, T. Goto, Appl. Phys. Lett. 73 (1998) 1038. [6] A. Yamamoto, T. Kido, T. Goto, Y. Chen, T. Yao, A. Kasuya, Appl. Phys. Lett. 75 (1999) 469. [7] Y. Masumoto, T. Kawamura, K. Era, Appl. Phys. Lett. 62 (1993) 225. [8] Y. Yamada, T. Mishina, Y. Masumoto, Y. Kawakami, J. Suda, Sz. Fujita, Sg. Fujita, Phys. Rev. Lett. 52 (1995) R2289. [9] T. Detchprohm, K. Hiramatsu, H. Amano, I. Akasaki, Appl. Phys. Lett. 61 (1992) 2688. [10] M.A.L. Johnson, Sz. Fujita, W.H. Rowland Jr., W.C. Huges, J.W. Cook Jr., J.F. Schetina, J. Electron. Matter 25 (1996) 855. [11] V. Gavryushin, G. Rac\ iukaitis, D. Juodz\ bails, A. Kazlauskas, V. Kubertavic\ ius, J. Crystal Growth 138 (1994) 924. [12] D.C. Look, J.W. Hemsky, J.R. Sizelove, Phys. Rev. Lett. 82 (1999) 2552. [13] K. Iwata, P. Fons, A. Yamada, K. Matsubara, S. Niki, J. Crystal Growth 209 (2000) 526. [14] S. Liang, C.R. Gorla, N. Emanetoglu, Y. Liu, W.E. Mayo, Y. Lu, J. Electron. Matter 27 (1998) L72. [15] D.C. Look, D.C. Reynolds, J.R. Sizelove, R.L. Jones, C.W. Litton, G. Cantwell, W.C. Harsch, Solid State Commun. 105 (1998) 399. [16] K. Sagara, E. Murakami, Appl. Phys. Lett. 54 (1989) 2003. [17] P. PoK ppl, G. VoK lkel, Phys. Stat. Sol. (a) 125 (1991) 571.
E!ects of thermal annealing of ZnO layers grown by MBE K. Ogata *, K. Sakurai, Sz. Fujita, Sg. Fujita, K. Matsushige Kyoto University Venture Business Laboratory, Kyoto University, Kyoto, 606-8501, Japan Department of Electronic Science and Engineering, Kyoto University, Kyoto, 606-8501, Japan
Abstract Thermal annealing of ZnO layers was done in N or O atmosphere and their e!ects were studied. Electron carrier density increases according to reevaporation of O from ZnO if annealed in N atmosphere. On the contrary, it decreases from the order of 10 to 10 cm\, and also optical properties are improved when annealed in O atmosphere at lower temperature. In that case, the number of interstitial Zn (Zn ) and O vacancies (V ) decrease, probably because the e!ective incorporation of O atom diminishes those donor levels. The crystallinity also improves with the annealing. The mobilities of the layer increase up to 51 cm/V s as annealing temperature increases, but are lower at lower temperatures. Solving this problem, annealing may become a promising technique for the fabrication of p-type ZnO layers. 2000 Elsevier Science B.V. All rights reserved. PACS: 61.72.Cc; 72.20.Dp; 81.15.Kk Keywords: ZnO; MBE; Thermal annealing; Electron carrier density; Mobility
1. Introduction For the short wavelength light-emitting devices, wide bandgap semiconductors have been widely studied in this decade. In particular, the technologies of both ZnSe- and GaN-based materials have made much progress [1,2]. Recently, another II}VI wide gap semiconductor ZnO (E "3.4 eV) [3] has been paid much attention to [5,6]. The exciton binding energy of ZnO is about 60 meV [3], roughly three times larger than that of ZnSe or GaN. In addition, the biexciton formation energy of ZnO is
* Corresponding author. Tel.: #81-75-753-7577; fax: #8175-753-7579. E-mail address: [email protected] (K. Ogata).
about 15 meV [4], also much larger. Therefore, ZnO is considered to be a promising material for novel exciton-related devices, such as low threshold lasers [7] and nonlinear optical devices [8], which are based on physics of excitons. However, at present, growth techniques of ZnO are still being developed, since the time when growth of epitaxial ZnO layers started as bu!er layers for GaN growth [9,10] several years ago. So, even in case of unintentionally doped ZnO layers, the layers show n-type conductivity with n-concentration of the order of 10 cm\, and control of electronic properties by impurity doping seems to be di$cult, especially for p-type layers, as had been quite di$cult in ZnSe and GaN. The origin of the donor levels is still unknown, but the interstitial Zn atom (Zn ) and O vacancy (V ) are thought to form donor levels [11,12]. To fabricate current injection
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 0 9 9 - 3
K. Ogata et al. / Journal of Crystal Growth 214/215 (2000) 312}315
313
pn junction devices, the realization of p-type conductivity is indispensable. Therefore, it is quite important to reduce these donor levels and to obtain high-quality layers. At the moment, attempts of nitrogen doping have resulted in no clear indication of acceptor formation yet [13]. Therefore, the suppression of native donor defects must be improved to achieve p-type doping. In this paper, we report the variation of electronic and optical properties of ZnO layers in terms of the thermal annealing conditions and e!ects on the material properties.
2. Experiments The ZnO layers were grown on (0 0 0 1) sapphire substrates by molecular beam epitaxy (MBE) at 7803C using RF oxygen plasma. Typical Zn pressure, O #ow rate and RF excitation power are 2.0;10\ Torr, 0.3 sccm and 300 W, respectively. The thickness, electron carrier density and mobility of a typical as-grown layer employed in this work are about 2500 As , 10 cm\ and 20 cm/V s, respectively. Thermal annealing after the growth was done in O or N atmosphere for 30 min. The annealing temperatures were varied between 500 and 10003C. Electrical properties were investigated by Hall measurement with the van der Pauw method, and optical properties by photoluminescence (PL) with a He}Cd laser (325 nm, 10 mW) at room temperature.
3. Results and discussion At "rst, it is quite important to investigate the e!ects of annealing atmosphere for the improvement of the layers. PL spectra taken after the annealing in N or O atmosphere at 10003C are shown in Fig. 1. As can be seen, the intensities of the excitonic emissions around 390 nm increase after the thermal annealing, but it is much more predominant when annealed in O atmosphere. Fur ther, it can be seen that the intensities of deep emissions are not a!ected by the thermal annealing, implying that it does not make deep centers in ZnO crystals.
Fig. 1. PL spectra of as-grown layer, annealed in N and in O atmosphere.
Fig. 2. Electron carrier density as a function of annealing temperatures.
Fig. 2 shows the variation of the electron carrier density in the ZnO layers as a function of the annealing temperature, where in this sample it is 2.4;10 cm\ in the as-grown state. It increases as high as 2.0;10 cm\ if annealed at 10003C. The increase in the carrier density can be attributed to the reevaporation of oxygen due to the hightemperature annealing. The following reactions may occur at high temperatures: (1) ZnOPZn #V #O !, 8 - ZnOPZn #O !. (2) As a result, both Zn and V act as donors. In addition to reevaporation of O atoms, formation of the spinel layers at the interface between the
314
K. Ogata et al. / Journal of Crystal Growth 214/215 (2000) 312}315
sapphire substrate and the ZnO epitaxial layer by higher temperature annealing [14] may also be the cause of degradation of the ZnO layers. On the other hand, we should notice that lower temperature annealing at 500 and 7003C decreases electron carrier density to 2.5;10 and 6.3; 10 cm\, respectively. In these cases, it is suggested that the reactions (1) and (2) are suppressed at lower temperatures and the O atmosphere enhan ces the reverse reactions of Eqs. (1) and (2). On the other hand, in N atmosphere, the electron carrier density increases to 2.6;10 cm\ when annealed at 5003C, as shown by the open circle in Fig. 2. This is probably because the N atmosphere cannot suppress su$ciently the reevaporation of oxygen during the annealing, i.e., the reverse reactions of Eqs. (1) and (2) are not enhanced compared to that in the O atmosphere. From these results, in order to obtain high-quality and lower donor level ZnO layers, thermal annealing in O atmosphere seems to be suitable. As for the mobility of the layers annealed in O atmosphere, interestingly it slightly decreases and then increases when the annealing temperature is lower or higher than the growth tempearture, as shown in Fig. 3. The maximum values after the annealing was 51 cm/V s, but still lower than that of high-quality bulk ZnO (205 cm/V s) [15]. The increase of the mobilities with the annealing temperature agrees with the results of the X-ray di!raction measurement on ZnO(0 0 0 2) shown in Fig. 4, which shows the di!raction intensities get stronger when annealed at higher temperature. However, at low temperature, Fig. 4 also suggests that the crystal quality becomes better, in spite of the slight decrease of the mobility. The FWHM of the X-ray di!raction, where it is 0.683 in the asgrown sample, decreases to 0.233 and the integrated intensity increases by more than 10 times after the annealing at 8503C due to the increase of the grain size of ZnO. This tendency of mobility is unusual, but agrees with that of polycrystalline silicon [16]. By paying attention to the grain size and carrier densities of the layers, it could be interpreted as follows. The mobility increases due to the increase of the grain size by the thermal annealing at higher temperature. In addition to that, those measured mobilities have higher values, since the electrons
Fig. 3. Electron mobility as a function of annealing temperatures.
Fig. 4. X-ray di!raction intensities of annealed ZnO layers.
can move beyond the barrier created at the grain boundary. On the contrary, after low-temperature annealing, the electron cannot pass the barrier, which reduces the mobilities since the Fermi level falls due to the decrease of electron density. Or it might be possible for the following reactions to occur during thermal annealing: ZnO#O QZn #V #O , (3) 8 ZnO#O QZn #O . (4) Then, the generation of interstitial oxygen (O ), which acts as deep acceptors [17] might make the mobility low. However, the papers on O are quite scarce, so it should be studied in the future. In Fig. 5, the PL spectra of ZnO layers annealed at various temperatures in O atmosphere are shown. At 5003C annealing, the excitonic emission gets stronger than that of higher temperature annealed samples, so the generation of nonradiative
K. Ogata et al. / Journal of Crystal Growth 214/215 (2000) 312}315
315
and the mobility increases to, e.g., 51 cm/V s. For the annealing in O at temperatures lower than the growth temperature, the electron carrier density decreases, e.g., from 2.4;10 to 2.5;10 cm\ by the 5003C annealing, and the crystallinity is also improved. However, the problem of the decrease in mobility remains. At the moment, thermal annealing in O at a low temperature seems to be a suit able choice from the standpoint of decreasing the concentration of residual donors.
References
Fig. 5. PL spectra of ZnO layers annealed at various temperatures.
recombination centers is greatly prevented. Therefore, it can be concluded that the annealing at higher temperatures not only makes the crystal quality better, but also generates many donor levels and nonradiative centers. On the other hand, at the moment, the lower temperature annealing in O atmosphere is more promising for obtaining ZnO layers suitable for p-type doping and for optical applications, although the problem of the decrease in the mobility must be solved.
4. Summary The e!ects of thermal annealing of ZnO layers grown by MBE were investigated. The annealing in O atmosphere is necessary compared to N atmo sphere in order to suppress the generation of V and Zn . If the annealing is done in O at temperatures higher than the growth temperature, the electron concentration increases, because the reevaporation of oxygen from ZnO cannot be suppressed. However, the crystallinity becomes better
[1] Y. Sanaka, H. Okuyama, S. Kijima, E. Kato, H. Noguchi, A. Ishibashi, Electron. Lett. 34 (1998) 1891. [2] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Matsushita, T. Mukai, Proceedings of Second International Symposium on Blue Laser and Light Emitting Diodes, 1998, p. 371. [3] A. Mang, K. Reimann, St. RuK benacke, Solid State Commun. 94 (1995) 251. [4] J.M. Hall, G. Brattner, M. Reuscher, C. Klingshirn, Phys. Stat. Sol. (b) 118 (1983) 179. [5] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, M.Y. Shen, T. Goto, Appl. Phys. Lett. 73 (1998) 1038. [6] A. Yamamoto, T. Kido, T. Goto, Y. Chen, T. Yao, A. Kasuya, Appl. Phys. Lett. 75 (1999) 469. [7] Y. Masumoto, T. Kawamura, K. Era, Appl. Phys. Lett. 62 (1993) 225. [8] Y. Yamada, T. Mishina, Y. Masumoto, Y. Kawakami, J. Suda, Sz. Fujita, Sg. Fujita, Phys. Rev. Lett. 52 (1995) R2289. [9] T. Detchprohm, K. Hiramatsu, H. Amano, I. Akasaki, Appl. Phys. Lett. 61 (1992) 2688. [10] M.A.L. Johnson, Sz. Fujita, W.H. Rowland Jr., W.C. Huges, J.W. Cook Jr., J.F. Schetina, J. Electron. Matter 25 (1996) 855. [11] V. Gavryushin, G. Rac\ iukaitis, D. Juodz\ bails, A. Kazlauskas, V. Kubertavic\ ius, J. Crystal Growth 138 (1994) 924. [12] D.C. Look, J.W. Hemsky, J.R. Sizelove, Phys. Rev. Lett. 82 (1999) 2552. [13] K. Iwata, P. Fons, A. Yamada, K. Matsubara, S. Niki, J. Crystal Growth 209 (2000) 526. [14] S. Liang, C.R. Gorla, N. Emanetoglu, Y. Liu, W.E. Mayo, Y. Lu, J. Electron. Matter 27 (1998) L72. [15] D.C. Look, D.C. Reynolds, J.R. Sizelove, R.L. Jones, C.W. Litton, G. Cantwell, W.C. Harsch, Solid State Commun. 105 (1998) 399. [16] K. Sagara, E. Murakami, Appl. Phys. Lett. 54 (1989) 2003. [17] P. PoK ppl, G. VoK lkel, Phys. Stat. Sol. (a) 125 (1991) 571.