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Characteristics of Mg-doped GaN epilayers grown with the variation of Mg incorporation
Journal of Crystal Growth 193 (1998) 300—304
Characteristics of Mg-doped GaN epilayers grown with the variation of Mg incorporation Cheul-Ro Lee!,*, Jae-Young Leem!, Sam-Kyu Noh!, Seong-Eun Park!, Ju-In Lee!, Chang-Soo Kim!, Sung-Jin Son", Ki-Young Leem" ! Epitaxial Semiconductor Group, Material Evaluation Center, KRISS, Daeduck Science Town, Yusung, Taejeon 305-600, South Korea " SPRC, Jeonbuk National University, Jeonju 561-756, Jeonbuk, South Korea Received 10 March 1998
Abstract We have studied the growth characteristics of Mg-doped GaN epilayers grown by MOCVD with the variation of Cp Mg flow rate. To optimize the p-type conductivity. We investigated the dependence of acceptor concentration on the 2 dopant source (Cp Mg) flow rate. The van der Pauw technique, double-crystal X-ray diffractometry (DCXRD) and 2 photoluminescence (PL) were used to characterize their crystallographic, electrical and optical properties. As the incorporation of Mg in GaN epitaxy increases, the surface morphology and crystallinity of the layers become rough and worse because of the increase of lattice distortion due to the large difference of the atomic size between Ga and Mg. With the increase of Mg incorporation, the resistivity of the epilayers increases abruptly without discontinuity because of the increase of much Mg—H complex not cracked. So, it is possible to know that only the partial amount of Mg—H complex in the layers are unbound by annealing at a certain condition. In spite of the continuous increase of Mg incorporation, the hole concentration of the epilayers first increases and then decreases from a certain Mg flow rate. As well as the results of hole concentration, the blue emission intensity of the layers in PL spectra at room temperature first increases and then decreases from a certain Mg flow rate. Therefore, it can be concluded that there is a limitation in Mg activation and the hole concentration decreases from this limit though the incorporation of Mg increases. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: GaN : Mg
1. Introduction Wide band gap GaN and related III—V nitrides are promising materials for applications in optoelectronics and in high-temperature electronic
* Corresponding author.
devices. Although some researchers succeeded in developing blue LED and LDs using these materials [1—4], there are still serious problems to be solved such as the high resistivity and low hole concentration of p-type GaN : Mg [5,6] for fabricating a device with high efficiency, operating at low voltage and for decreasing the contact resistance between the metal/p-type semiconductor. It is
0022-0248/98/$ — see front matter ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 4 3 6 - 9
C.-R. Lee et al. / Journal of Crystal Growth 193 (1998) 300–304
known that the Mg—H complex [7—9] synthesized from decomposed NH and Cp Mg during growth 3 2 plays an important role in high resistivity and low hole concentration of the Mg doped p-type GaN epilayer. So, the thermal annealing or low-energy electron beam irradiation (LEEBI) treatment of it after growth is absolutely necessary for unbinding the complex [10,11]. In general, the increase in the hole concentration of Mg doped GaN epilayer is dominantly dependent on the effective condition of activation heat treatment. Therefore, a more detailed study on the Mg-doped GaN including growth technique and activation heat treatment is needed to optimize the device efficiency which are fabricated by III—V nitrides. In this paper, we report on the characteristics of Mg doped GaN epilayers grown by MOCVD with the variation of Cp Mg flow rate. To optimize the 2 p-type conductivity, we investigate the dependence of acceptor concentration on the dopant source (Cp Mg) flow rate. The van der Pauw technique, 2 double crystal X-ray diffractometry (DCXRD) and photoluminescence (PL) were used to characterize their crystallographic, electrical and optical properties.
301
170 cm2/V s, respectively. After growth, all the Mg doped GaN epilayers was annealed in N gas at2 mosphere for 30 min at 850°C in order to unbind the Mg—H complex and activate Mg as acceptors. Each sample annealed was evaluated by an optical microscope, DCXRD, Hall measurement and PL.
3. Results Fig. 1 shows the surface morphologies of Mgdoped GaN epilayers A, B, C, D and E grown with various flow rates of Cp Mg observed by the op2 tical microscope. The mirror-like surface without any defect such as hillocks can be seen in the undoped GaN epilayer which was grown without any intentional flow of Cp Mg. But, the surfaces of 2 the GaN : Mg layers become rough as the flow rate
2. Experimental procedure The Mg-doped GaN epilayers were grown in a horizontal MOCVD reactor at a reduced pressure of 300 Torr. NH and MO-sources flow were 3 separated to reduce undesirable parastic reaction [12]. Trimethylgallium (TMG), bis-magnesium (Cp Mg) and ammonia (NH ) were used as Ga, 2 3 Mg, and N sources, respectively. The Mg doped GaN epilayers of about 1.7 lm thick were grown on sapphire(0 0 0 1) for 40 min at 1070°C after growing the GaN nucleation layers of about 30 nm thick at 520°C. During growth of Mg doped GaN epilayers, the flow rates of NH , TMG and H were 3 2 2000 sccm, 42 lmol/min and 4000 sccm, respectively. The flow rates of Cp Mg during growth are 2 0 (sample A), 8 (sample B), 18 (sample C), 21 (sample D) and 23 sccm (sample E). The electron concentration and carrier mobility of the undoped GaN epilayer grown under the upper condition without Cp Mg flow are 4]1017/cm3 and 2
Fig. 1. Surface morphologies of Mg-doped GaN epilayers grown with the flow rate of Cp Mg (A)"0, (B)"8, (C)"18, 2 (D)"21, (E)"23 sccm.
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of Cp Mg increases during growth. The rough sur2 face means that the epilayer is grown also three dimensionally except in two-dimensional lateral growth. It is considered that the rough surface resulted from the large difference of the atomic size between Ga and Mg [5,13]. In general, the atomic size of the Mg atom which is the substitutional element for Ga in GaN, is much smaller than that of Ga. This difference make the lattice distort and, the lattice distortion generates dislocations and causes the layer to grow three dimensionally [14]. So, it can be concluded that the lattice distortion and dislocation density of GaN epilayers increase with the increase of Mg incorporation. Fig. 2 shows the FWHMs of the DCXRD for (0 0 0 2) diffraction from the Mg doped GaN epilayers A, B, C, D and E. The FWHMs of the sample A, B, C, D and E are 250, 330, 460, 450 and 440 arcmin, respectively. As the incorporation rate of Mg in GaN increases, the FWHM decreases. So, it is possible to conclude that the dislocation density, which is resulted from the difference of atomic size between Ga and Mg, of the Mg doped GaN epilayers increases with the increase of Mg incorporation. Fig. 3 shows the resistivities of Mg-doped GaN epilayers A, B, C, D and E. The Ni/Au bilayer 200 A_ /1500 A_ thick used as a metal contact was deposited on each Mg doped GaN epilayer for electrical measurements using the van der Pauw
Fig. 2. Crystallinities of Mg-doped GaN epilayers grown with the variation of Mg incorporation.
Fig. 3. Resistivities of Mg-doped GaN epilayers grown with the variation of Mg incorporation.
technique. The epilayers were also thermally annealed rapidly in an RTA system to decrease the contact resistance of the interface between metal/semiconductor. The measured resistivities of the samples A, B, C, D and E are 0.5, 1.0, 5.0, 13 and 30 ) cm, respectively. As the incorporation rate of Mg in GaN increases, the resistivity increases abruptly in spite of the same annealing conditions. As mentioned, the Mg—H complex plays an important role in the high resistivity of Mg doped GaN epilayer. So, it can be known that a mount of Mg—H complex not cracked in GaN : Mg increases with the increase of Cp Mg flow rate. Judging from 2 Fig. 3, it can be seen that all the Mg—H complexes in the layers are not cracked and there are limitations in unbinding them at certain annealing conditions. Carrier concentrations and mobility were measured at room temperature by Hall measurements using the Ni/Au electrode [15]. Fig. 4 shows the
C.-R. Lee et al. / Journal of Crystal Growth 193 (1998) 300–304
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results of Hall measurements on the Mg doped GaN epilayers A, B, C, D and E. The GaN : Mg epilayer A free from the intentional incorporation of Mg showed n-type conduction with an electron concentration of 2]1017/cm3. Though the sample B with 8 sccm of Cp Mg also showed n-type con2 duction, the electron concentration of it decreased abruptly to 4]1016/cm3 compared with that of sample A. It is considered to have resulted from the compensation of activated acceptor Mg on the donor. The epilayer C with 18 sccm of Cp Mg showed 2 p-type conduction with a hole concentration of 2.1]1017/cm3. So, it can be thought that the type of Mg-doped GaN changes in the flow rate between 8 and 18 sccm of Cp Mg. The epilayer D with 2 21 sccm of Cp Mg also showed p-type conduction 2 having a hole concentration of 2.0]1017/cm3, which is very similar with that of sample C. Though the sample E with 23 sccm of Cp Mg also showed 2 p-type conduction with hole concentration of 7]1016/cm3, which is lower than those of samples
C and D. Therefore, the N —N begins to fall when ! $ the flow rate of Cp Mg is over 23 sccm and event2 ually the film turns out to be almost semi-insulating. Judging from Fig. 4, it can be concluded that the activation of Mg does not increase proportionally with the increase of Mg incorporation and there is a saturation point in the activation of Mg at a certain incorporation level. Fig. 5 shows the room-temperature (RT) photoluminescence (PL) spectra of the Mg doped GaN epitaxial layers at different levels of Mg incorporation. For the undoped GaN sample A, the PL spectrum is dominated by band edge emission at 358 nm, while the donor—acceptor (D—A) pair emission at 445 nm and deep level (DL) emission around 600 nm are weak. From the PL spectrum of sample A without intentional doping of Mg, it can be seen that Mg is slightly doped in this layer [16], which probably resulted from the memory effect of the residual Mg in the reactor. When the Mg is introduced into GaN films (sample B), band edge
Fig. 4. Hole concentrations of Mg-doped GaN epilayers grown with the variation of Mg incorporation.
Fig. 5. PL spectra at room temperature of Mg-doped GaN epilayers grown with the variation of Mg incorporation.
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emission peak disappears and the intensity of D—A pair, blue, emission peak begins to increase. The intensity of blue emission peak first increases until 21 sccm of Cp Mg and then decreases from 2 23 sccm of Cp Mg. Judging from these PL spectra 2 in Fig. 5 and the results of carrier concentration in Fig. 4, it is obvious that the activation of Mg does not increase proportionally with the increase of Mg incorporation and there is a saturation point. The reason why the intensity of the blue emission peak begins to decrease from 23 sccm of Cp Mg is prob2 ably due to the increased Mg—H complexes not cracked in the layer.
4. Summary As the incorporation of Mg in GaN epitaxy increases, the surface morphology and crystallinity of the layers become rough and worse because of the increase of lattice distortion due to the large difference of the atomic size between Ga and Mg. With the increase of Mg incorporation, the resistivity of the epilayers increases abruptly without discontinuity because of the much increased amount of Mg—H complexes not cracked. So, it is possible to conclude that only a partial amount of Mg—H complex in the layers are unbound by annealing at a certain condition. In spite of the continuous increase of Mg incorporation, the hole concentration of the epilayers first increases and then decreases from a certain Mg flow rate. As well as the results of hole concentration, the blue emission intensity of the layers in PL spectra at room temperature first increases and then decreases from a certain Mg flow level. Therefore, it can be concluded that there is a limitation in Mg activation and the hole con-
centration decreases from this limit though the incorporation of Mg increases. Acknowledgements This work was supported by Korean Ministry of Science and Technology. References [1] S. Nakamura, J. Crystal Growth 145 (1994) 911. [2] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. [3] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, Jpn. J. Appl. Phys. 34 (1995) L797. [4] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, H. Kiyoko, Y. Sugimoto, Jpn. J. Appl. Phys. 35 (1996) L74. [5] Yasuo Ohba, Ako Hatano, Jpn. J. Appl. Phys. 33 (1994) PL1367. [6] S. Nakamura, T. Mukai, M. Senoh, N. Iwasa, Jpn. J. Appl. Phys. 31 (1992) pL1258. [7] S. Fischer, C. Wetzel, E.E. Haller, APL 67 (9) (1995) 1298. [8] S.J. Pearton, J.W. Lee, C. Yuan, APL 68 (19) (1996) 2690. [9] D.J. Chadi, APL 71 (20) (1997) 2970. [10] S. Nakamura, T. Mukai, M. Senoh, N. Iwasa, Jpn. J. Appl. Phys. 31 (1992) pL139. [11] H. Amano, M. Kito, K. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. 28 (1992) L2112. [12] Cheul-Ro Lee, Sung-jin Son, In-Hwan Lee, Jae-Young Leem, Sam-kyu Noh, J. Crystal Growth 182 (1997) 11. [13] W. Gotz, N.M. Johnson, J. Walker, D.P. Bour, R.A. Street, APL 68 (5) (1996) 667. [14] A. Cros, R. Dimitrov, H. Angerer, O. Ambacher, M. Stutzmann, S. Christiansen, M. Albrecht, H.P. Strunk, J. Crystal Growth 181 (1997) 197. [15] J.T. Trexler, S.J. Miller, P.H. Holloway, M.A. Khan, Matter. Res. Soc. Symp. Proc. 395 (1997) 819. [16] Y. Li, Y. Lu, H. Shen, M. Wraback, C.Y. Hwang, M. Schurman, W. Mayo, T. Salagaj, Mater. Res. Soc. Symp. Proc. 395 (1997) 369.
Characteristics of Mg-doped GaN epilayers grown with the variation of Mg incorporation Cheul-Ro Lee!,*, Jae-Young Leem!, Sam-Kyu Noh!, Seong-Eun Park!, Ju-In Lee!, Chang-Soo Kim!, Sung-Jin Son", Ki-Young Leem" ! Epitaxial Semiconductor Group, Material Evaluation Center, KRISS, Daeduck Science Town, Yusung, Taejeon 305-600, South Korea " SPRC, Jeonbuk National University, Jeonju 561-756, Jeonbuk, South Korea Received 10 March 1998
Abstract We have studied the growth characteristics of Mg-doped GaN epilayers grown by MOCVD with the variation of Cp Mg flow rate. To optimize the p-type conductivity. We investigated the dependence of acceptor concentration on the 2 dopant source (Cp Mg) flow rate. The van der Pauw technique, double-crystal X-ray diffractometry (DCXRD) and 2 photoluminescence (PL) were used to characterize their crystallographic, electrical and optical properties. As the incorporation of Mg in GaN epitaxy increases, the surface morphology and crystallinity of the layers become rough and worse because of the increase of lattice distortion due to the large difference of the atomic size between Ga and Mg. With the increase of Mg incorporation, the resistivity of the epilayers increases abruptly without discontinuity because of the increase of much Mg—H complex not cracked. So, it is possible to know that only the partial amount of Mg—H complex in the layers are unbound by annealing at a certain condition. In spite of the continuous increase of Mg incorporation, the hole concentration of the epilayers first increases and then decreases from a certain Mg flow rate. As well as the results of hole concentration, the blue emission intensity of the layers in PL spectra at room temperature first increases and then decreases from a certain Mg flow rate. Therefore, it can be concluded that there is a limitation in Mg activation and the hole concentration decreases from this limit though the incorporation of Mg increases. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: GaN : Mg
1. Introduction Wide band gap GaN and related III—V nitrides are promising materials for applications in optoelectronics and in high-temperature electronic
* Corresponding author.
devices. Although some researchers succeeded in developing blue LED and LDs using these materials [1—4], there are still serious problems to be solved such as the high resistivity and low hole concentration of p-type GaN : Mg [5,6] for fabricating a device with high efficiency, operating at low voltage and for decreasing the contact resistance between the metal/p-type semiconductor. It is
0022-0248/98/$ — see front matter ( 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 4 3 6 - 9
C.-R. Lee et al. / Journal of Crystal Growth 193 (1998) 300–304
known that the Mg—H complex [7—9] synthesized from decomposed NH and Cp Mg during growth 3 2 plays an important role in high resistivity and low hole concentration of the Mg doped p-type GaN epilayer. So, the thermal annealing or low-energy electron beam irradiation (LEEBI) treatment of it after growth is absolutely necessary for unbinding the complex [10,11]. In general, the increase in the hole concentration of Mg doped GaN epilayer is dominantly dependent on the effective condition of activation heat treatment. Therefore, a more detailed study on the Mg-doped GaN including growth technique and activation heat treatment is needed to optimize the device efficiency which are fabricated by III—V nitrides. In this paper, we report on the characteristics of Mg doped GaN epilayers grown by MOCVD with the variation of Cp Mg flow rate. To optimize the 2 p-type conductivity, we investigate the dependence of acceptor concentration on the dopant source (Cp Mg) flow rate. The van der Pauw technique, 2 double crystal X-ray diffractometry (DCXRD) and photoluminescence (PL) were used to characterize their crystallographic, electrical and optical properties.
301
170 cm2/V s, respectively. After growth, all the Mg doped GaN epilayers was annealed in N gas at2 mosphere for 30 min at 850°C in order to unbind the Mg—H complex and activate Mg as acceptors. Each sample annealed was evaluated by an optical microscope, DCXRD, Hall measurement and PL.
3. Results Fig. 1 shows the surface morphologies of Mgdoped GaN epilayers A, B, C, D and E grown with various flow rates of Cp Mg observed by the op2 tical microscope. The mirror-like surface without any defect such as hillocks can be seen in the undoped GaN epilayer which was grown without any intentional flow of Cp Mg. But, the surfaces of 2 the GaN : Mg layers become rough as the flow rate
2. Experimental procedure The Mg-doped GaN epilayers were grown in a horizontal MOCVD reactor at a reduced pressure of 300 Torr. NH and MO-sources flow were 3 separated to reduce undesirable parastic reaction [12]. Trimethylgallium (TMG), bis-magnesium (Cp Mg) and ammonia (NH ) were used as Ga, 2 3 Mg, and N sources, respectively. The Mg doped GaN epilayers of about 1.7 lm thick were grown on sapphire(0 0 0 1) for 40 min at 1070°C after growing the GaN nucleation layers of about 30 nm thick at 520°C. During growth of Mg doped GaN epilayers, the flow rates of NH , TMG and H were 3 2 2000 sccm, 42 lmol/min and 4000 sccm, respectively. The flow rates of Cp Mg during growth are 2 0 (sample A), 8 (sample B), 18 (sample C), 21 (sample D) and 23 sccm (sample E). The electron concentration and carrier mobility of the undoped GaN epilayer grown under the upper condition without Cp Mg flow are 4]1017/cm3 and 2
Fig. 1. Surface morphologies of Mg-doped GaN epilayers grown with the flow rate of Cp Mg (A)"0, (B)"8, (C)"18, 2 (D)"21, (E)"23 sccm.
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of Cp Mg increases during growth. The rough sur2 face means that the epilayer is grown also three dimensionally except in two-dimensional lateral growth. It is considered that the rough surface resulted from the large difference of the atomic size between Ga and Mg [5,13]. In general, the atomic size of the Mg atom which is the substitutional element for Ga in GaN, is much smaller than that of Ga. This difference make the lattice distort and, the lattice distortion generates dislocations and causes the layer to grow three dimensionally [14]. So, it can be concluded that the lattice distortion and dislocation density of GaN epilayers increase with the increase of Mg incorporation. Fig. 2 shows the FWHMs of the DCXRD for (0 0 0 2) diffraction from the Mg doped GaN epilayers A, B, C, D and E. The FWHMs of the sample A, B, C, D and E are 250, 330, 460, 450 and 440 arcmin, respectively. As the incorporation rate of Mg in GaN increases, the FWHM decreases. So, it is possible to conclude that the dislocation density, which is resulted from the difference of atomic size between Ga and Mg, of the Mg doped GaN epilayers increases with the increase of Mg incorporation. Fig. 3 shows the resistivities of Mg-doped GaN epilayers A, B, C, D and E. The Ni/Au bilayer 200 A_ /1500 A_ thick used as a metal contact was deposited on each Mg doped GaN epilayer for electrical measurements using the van der Pauw
Fig. 2. Crystallinities of Mg-doped GaN epilayers grown with the variation of Mg incorporation.
Fig. 3. Resistivities of Mg-doped GaN epilayers grown with the variation of Mg incorporation.
technique. The epilayers were also thermally annealed rapidly in an RTA system to decrease the contact resistance of the interface between metal/semiconductor. The measured resistivities of the samples A, B, C, D and E are 0.5, 1.0, 5.0, 13 and 30 ) cm, respectively. As the incorporation rate of Mg in GaN increases, the resistivity increases abruptly in spite of the same annealing conditions. As mentioned, the Mg—H complex plays an important role in the high resistivity of Mg doped GaN epilayer. So, it can be known that a mount of Mg—H complex not cracked in GaN : Mg increases with the increase of Cp Mg flow rate. Judging from 2 Fig. 3, it can be seen that all the Mg—H complexes in the layers are not cracked and there are limitations in unbinding them at certain annealing conditions. Carrier concentrations and mobility were measured at room temperature by Hall measurements using the Ni/Au electrode [15]. Fig. 4 shows the
C.-R. Lee et al. / Journal of Crystal Growth 193 (1998) 300–304
303
results of Hall measurements on the Mg doped GaN epilayers A, B, C, D and E. The GaN : Mg epilayer A free from the intentional incorporation of Mg showed n-type conduction with an electron concentration of 2]1017/cm3. Though the sample B with 8 sccm of Cp Mg also showed n-type con2 duction, the electron concentration of it decreased abruptly to 4]1016/cm3 compared with that of sample A. It is considered to have resulted from the compensation of activated acceptor Mg on the donor. The epilayer C with 18 sccm of Cp Mg showed 2 p-type conduction with a hole concentration of 2.1]1017/cm3. So, it can be thought that the type of Mg-doped GaN changes in the flow rate between 8 and 18 sccm of Cp Mg. The epilayer D with 2 21 sccm of Cp Mg also showed p-type conduction 2 having a hole concentration of 2.0]1017/cm3, which is very similar with that of sample C. Though the sample E with 23 sccm of Cp Mg also showed 2 p-type conduction with hole concentration of 7]1016/cm3, which is lower than those of samples
C and D. Therefore, the N —N begins to fall when ! $ the flow rate of Cp Mg is over 23 sccm and event2 ually the film turns out to be almost semi-insulating. Judging from Fig. 4, it can be concluded that the activation of Mg does not increase proportionally with the increase of Mg incorporation and there is a saturation point in the activation of Mg at a certain incorporation level. Fig. 5 shows the room-temperature (RT) photoluminescence (PL) spectra of the Mg doped GaN epitaxial layers at different levels of Mg incorporation. For the undoped GaN sample A, the PL spectrum is dominated by band edge emission at 358 nm, while the donor—acceptor (D—A) pair emission at 445 nm and deep level (DL) emission around 600 nm are weak. From the PL spectrum of sample A without intentional doping of Mg, it can be seen that Mg is slightly doped in this layer [16], which probably resulted from the memory effect of the residual Mg in the reactor. When the Mg is introduced into GaN films (sample B), band edge
Fig. 4. Hole concentrations of Mg-doped GaN epilayers grown with the variation of Mg incorporation.
Fig. 5. PL spectra at room temperature of Mg-doped GaN epilayers grown with the variation of Mg incorporation.
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emission peak disappears and the intensity of D—A pair, blue, emission peak begins to increase. The intensity of blue emission peak first increases until 21 sccm of Cp Mg and then decreases from 2 23 sccm of Cp Mg. Judging from these PL spectra 2 in Fig. 5 and the results of carrier concentration in Fig. 4, it is obvious that the activation of Mg does not increase proportionally with the increase of Mg incorporation and there is a saturation point. The reason why the intensity of the blue emission peak begins to decrease from 23 sccm of Cp Mg is prob2 ably due to the increased Mg—H complexes not cracked in the layer.
4. Summary As the incorporation of Mg in GaN epitaxy increases, the surface morphology and crystallinity of the layers become rough and worse because of the increase of lattice distortion due to the large difference of the atomic size between Ga and Mg. With the increase of Mg incorporation, the resistivity of the epilayers increases abruptly without discontinuity because of the much increased amount of Mg—H complexes not cracked. So, it is possible to conclude that only a partial amount of Mg—H complex in the layers are unbound by annealing at a certain condition. In spite of the continuous increase of Mg incorporation, the hole concentration of the epilayers first increases and then decreases from a certain Mg flow rate. As well as the results of hole concentration, the blue emission intensity of the layers in PL spectra at room temperature first increases and then decreases from a certain Mg flow level. Therefore, it can be concluded that there is a limitation in Mg activation and the hole con-
centration decreases from this limit though the incorporation of Mg increases. Acknowledgements This work was supported by Korean Ministry of Science and Technology. References [1] S. Nakamura, J. Crystal Growth 145 (1994) 911. [2] S. Nakamura, T. Mukai, M. Senoh, Appl. Phys. Lett. 64 (1994) 1687. [3] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, Jpn. J. Appl. Phys. 34 (1995) L797. [4] S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, H. Kiyoko, Y. Sugimoto, Jpn. J. Appl. Phys. 35 (1996) L74. [5] Yasuo Ohba, Ako Hatano, Jpn. J. Appl. Phys. 33 (1994) PL1367. [6] S. Nakamura, T. Mukai, M. Senoh, N. Iwasa, Jpn. J. Appl. Phys. 31 (1992) pL1258. [7] S. Fischer, C. Wetzel, E.E. Haller, APL 67 (9) (1995) 1298. [8] S.J. Pearton, J.W. Lee, C. Yuan, APL 68 (19) (1996) 2690. [9] D.J. Chadi, APL 71 (20) (1997) 2970. [10] S. Nakamura, T. Mukai, M. Senoh, N. Iwasa, Jpn. J. Appl. Phys. 31 (1992) pL139. [11] H. Amano, M. Kito, K. Hiramatsu, I. Akasaki, Jpn. J. Appl. Phys. 28 (1992) L2112. [12] Cheul-Ro Lee, Sung-jin Son, In-Hwan Lee, Jae-Young Leem, Sam-kyu Noh, J. Crystal Growth 182 (1997) 11. [13] W. Gotz, N.M. Johnson, J. Walker, D.P. Bour, R.A. Street, APL 68 (5) (1996) 667. [14] A. Cros, R. Dimitrov, H. Angerer, O. Ambacher, M. Stutzmann, S. Christiansen, M. Albrecht, H.P. Strunk, J. Crystal Growth 181 (1997) 197. [15] J.T. Trexler, S.J. Miller, P.H. Holloway, M.A. Khan, Matter. Res. Soc. Symp. Proc. 395 (1997) 819. [16] Y. Li, Y. Lu, H. Shen, M. Wraback, C.Y. Hwang, M. Schurman, W. Mayo, T. Salagaj, Mater. Res. Soc. Symp. Proc. 395 (1997) 369.