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Growth of p-type ZnSe by metalorganic vapor phase epitaxy using ethylazide as a new nitrogen source
j. . . . . . . .
ELSEVIER
ClNYSTAI. GROWTH
Journal of Crystal Growth 159 (1996) 130-133
Growth of p-type ZnSe by metalorganic vapor phase epitaxy using ethylazide as a new nitrogen source K. Inoue *, K. Yanashima, T. Takahashi, J.S. Hwang, K. Hara, H. Munekata, H. Kukimoto Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226, Japan
Abstract The growth of nitrogen-doped ZnSe epilayers has been investigated by metalorganic vapor phase epitaxy (MOVPE) using ethylazide as a novel dopant source in which no N-H bonds are involved. Nitrogen was successfully incorporated in the ZnSe epilayers, whereas hydrogen was also found in the epilayers, primarily in the form of N-H bonds. By thermally annealing the samples, we have found that N-H bonds can be dissociated, yielding p-type ZnSe layers with hole concentrations of mid- 1017 cm- 3.
1. Introduction ZnSe-based materials have excellent characteristics for blue-light-emitting devices [1,2]. one of the key issues for realizing those devices is the achievement of low resistivity p-type layers, for which nitrogen is expected to be one of the most suitable p-type dopants. The control of p-type conductivity, however, is still a serious problem especially in ZnSe-based epilayers grown b y metalorganic vapor phase epitaxy (MOVPE). This is primarily due to the incorporation of hydrogen in the epilayers, which results in the formation of N - H bonds and thus the hydrogen passivation of nitrogen acceptors [3]. In this study we report the growth and characterization of N-doped ZnSe by MOVPE using ethylazide (C2HsN3) as a new p-type dopant source. Note that no N - H bond is involved in this source. We aim at studying the influence of dopant source on the amounts of N and H in ZnSe epilayers. The effect of
* Correspondingauthor. Fax: + 81 45 921 1492.
thermal annealing on electrical properties has also been investigated.
2. Experimental procedure The growth was carded out in an atmospheric pressure MOVPE system consisting of a cold wall horizontal reactor, with diethylzinc (DEZn) and diethylselenide (DESe) being primary sources, and ethylazide (EtN 3) being the p-type dopant source. Typical flow rates of DEZn, DESe and EtN 3 were 3.5, 9.0 and 0-100 /~mol/min, respectively. The V I / I I flow ratio is 2.6. Prior to the growth, a p-type GaAs: Zn(100) substrate was chemically etched by a mixture of a 1NH4OH : 1H202 : 10H20 solution for 1 min, followed by surface treatment with a (NH4)2S x water solution. The substrate was then thermally etched for 10 rain at 500°C in the reactor under a hydrogen atmosphere. It was then followed by the growth of N-doped ZnSe layers at substrate temperatures of 400-470°C. Both uniform and spike dopings of N were examined in this work. As for the
0022-0248/96/$15.00 © 1996ElsevierScienceB,V. All fights reserved SSDI 0022-0248(95)00760- 1
K. lnoue et aL /Journal of Crystal Growth 159 (1996) 130-133
spike doping, the influence of the source supply sequence on N and H incorporation has also been studied by varying systematically the sequence of both primary and dopant gas sources.
131
10 4 ZnSe:N
ZnSe ~aAI bufferf
A
~ 10" "i
,.Z
3. Results and discussion
0
We have found that the amount of N in the ZnSe epilayers can be controlled to a great extent by changing the supply rate of ethylazide. Fig. 1 shows the photoluminescence (PL) spectra at 25 K of four different ZnSe epilayers grown at 400°C with various EtN 3 supply rates. Samples were doped uniformly by continuous supply of EtN 3 together with primary sources. The PL spectrum of an undoped ZnSe layer is dominated by a sharp free exciton emission band. A sample grown with a small supply o f E t N 3 s h o w s a neutral-acceptor-bound exciton emission band (A°X) together with donor-acceptor pair (DAP) emission bands. With increasing the flow rate of ethylazide, the PL spectra tum out to be dominated by DAP emission bands, as also shown in the figure. These results indicate that ethylazide, compared with NH 3 [4] and t-BNH 2 [5,6], can be readily decomposed during the MOVPE growth at
4.9 runol/rnln
3.4
Z
"' k-
2.2 rtmoI/mln A D A P
a.
FE
I
I
I
2.6 2.7 PHOTON ENERGY
t
I
2.8
{eV)
Fig. 1. Photoluminescence (PL) spectra at 25 K for various ZnSe layers grown at 400°C using EtN 3 as a nitrogen source. The spectrum of an undol~l ZnSe layer is also shown for comparison. Samples were excited by the 325 nm line of a HeCd laser.
'.~,-., ..,. ,/......... ,.'!
t/~
'..f ":
:i 10 2
¢,n
1111 0
200
400
600
DEPTH (rim) Fig. 2. SIMS depth profile of a N-doped ZnSe layer grown on ZnSe/GaAs(100).
relatively low growth temperatures ( < 400°C) to yield nitrogen-doped ZnSe epilayers. The incorporation of N has been confirmed by secondary ion mass spectroscopy (SIMS) depth profiles. In Fig. 2, we show SIMS data of a heavily N-doped ZnSe sample grown at 400°C with 50 /zmol/min of EtN 3. Here, N is detected in the form of N + Se (total mass number 94). Also noticeable are relatively high hydrogen counts, being indicative of the incorporation of a large amount of hydrogen together with nitrogen. Epilayers exhibit high electrical resistance, suggesting hydrogen passivation of nitrogen acceptors. Growth at relatively high temperatures has also been studied to suppress hydrogen incorporation. We have found, however, that incorporation of nitrogen is also suppressed unexpectedly. For example, as shown in Fig. 3, the PL spectrum of the sample grown at 470°C with 100 /zmol/min of EtN 3 is dominated by the A°X emission band, which is usually a sign of the lightly doped situation. We infer that the suppression of N incorporation is attributed to the reduced sticking efficiency of EtN 3 on a ZnSe surface. To compensate this effect, we have actually increased the EtN 3 supply rate. However, it has turned out to promote the reaction between EtN 3 and primary sources (DEZn and DESe) in the vapor phase. In order to pursue the effective N incorpora-
132
K. Inoue et al. / Journal of Crystal Growth 159 (1996) 130-133
A°dX A
L 2.6 2.7 2.8 PHOTON ENERGY (eV)
Fig. 3. PL spectrum of N-doped ZnSe grown at 470°C with uniform doping of E t N 3. The supply rate of E t N 3 w a s 100 /~mol/min.
tion, we have studied the spike doping of EtN 3 under different growth conditions. Fig. 4 shows the PL spectrum of the ZnSe:N epilayer prepared by the spike-doping technique. The PL spectrum is clearly dominated by the DAP emission, being indicative of the incorporation of a relatively large amount of N acceptors. Studying the source supply sequence systematically, we have
25 K exc. 325
nm
DAP
i I
2.6
!
2.7 2.8 PHOTON ENERGY (eV)
Fig. 4. PL spectrum of N-doped ZnSe grown at 470°C with spike doping of EtN 3. The supply rate of EtN 3 was 40 p.mol/min. Process of spike doping is described in the text.
found that first supplying the DEZn is very importam to have nitrogen incorporated in the ZnSe epilayers. In other words, the Zn-stabilized surface enhances the incorporation of nitrogen. For example, the source supply sequence of the sample shown in the figure is as follows: (1) 3 s DEZn supply, (2) 10 s supply of EtN3, where a flow rate was 40 /zmol/min, together with DESe for the nitrogen spike doping, and (3) 22 s supply of both DEZn and DESe for ZnSe epitaxy. Comparing the PL spectrum shown in Fig. 4 with that of a typical MBE-grown ZnSe:N sample [7], we estimate that the nitrogen concentration of this sample is roughly over 1018 cm -3. We also notice, by comparing the PL data of Fig. 3 with Fig. 4, that nitrogen incorporation is effectively enhanced by the spike doping. According to the Raman scattering measurements, however, we again find the presence of hydrogen in the form of N - H bonds (vibrations of stretching and wagging modes), strongly suggesting the hydrogen passivation of nitrogen acceptors. The effective ionized acceptor concentration of the as-grown sample, obtained by C-V measurements, was mid-1015 cm -3 at RT. The measurements were performed for Au/ZnSe: N / p ÷ - GaAs (100) Schottky diodes at a frequency of 100 Hz. The effect of thermal annealing has also been investigated. The annealing was carried out in the MOVPE reactor under a N 2 atmosphere. Another ZnSe/GaAs (100) sample was placed face-to-face on the sample as a cap to prevent evaporation of Zn and Se. By thermally annealing the samples under a N 2 atmosphere, we have found that hydrogen in the layers can be reduced significantly without causing an appreciable degradation in crystal quality. Fig. 5 shows the plot of ionized-acceptor concentration as a function of annealing temperature. The annealing time is fixed at 30 min. Samples used in the experiment were grown in a similar fashion as the one for the sample of Fig. 4. The inset shows a C-2-V plot (100 Hz) for a sample annealed at 570°C, indicating a good linearity with a built-in voltage of 1.9 V. As seen in the figure, the acceptor concentration increases two orders of magnitude with annealing temperature, resulting in the maximum concentration of 5 )< 1017 cm -3 at the annealing temperature of 570°C. Beyond this temperature, we have found that both electrical and optical properties tend to degrade.
K. lnoue et al./Journal of Crystal Growth 159 (1996) 130-133 101e
133
4. Summary O
We have investigated the growth of N-doped ZnSe layers by MOVPE using ethylazide as a novel dopant source in which N - H bonds are not included. Incorporation of N in ZnSe epilayers has been successfully achieved, whereas a relatively large amount of hydrogen has also been detected in the layers. Thermal annealing of the layers has resulted in a reduction of hydrogen concentration, yielding p-type ZnSe layers with hole concentrations of mid-10 ]7 cm -3.
O E o
10" 2
|
=-
-2
101s 500
V O L T A G E (V)
I I 550 600 TEMPERATURE (°C)
650
Fig. 5. A plot of Na - N d value as a function of annealing temperature. The inset shows a C - 2 - V plot (100 Hz) for a sample annealed at 570°C.
Before annealing, N - H vibration modes are observed by Raman scattering, while, after annealing at 570°C for 30 min, signals due to these modes disappear. These results indicate that the N - H bonds are thermally dissociated by the annealing process, resulting in the activation of nitrogen acceptors. Annealed samples show PL spectra composed of a broad DAP emission band, being similar to the heavily doped MBE-grown ZnSe : N layers. The hole concentrations that we can currently obtain after the annealing process are mid-1017 cm -3.
References [1] H. Okuyama, Y. Morinaga and K. Akimoto, J. Crystal Growth 127 (1993) 335. [2] S. Itoh, N. Nakamura, S. Matsumoto, M. Nagai, K. Nakano, M. Ozawa, H. Okuyama, S. Tomita, T. Ohata, M. Ikeda, A. lshibashi and Y. Mori, Jpn. J. Appi. Phys. 33 (1994) 938. [3] J.A. Wolk, J.W. Ager III, K.J. Duxstad, E.E. Hailer, N.R. Taskar, D,R. Dorman and D.J. Olego, Appl. Phys. Lett. 63 (1993) 2756. [4] A. Ohki, N. Shibata and S. Zembutu, Jpn. J. Appl. Phys. 27 (1988) L909. [5] Sz. Fujita, T. Asano, K. Maehara and Sg. Fujita, Jpn. J. Appl. 32 (1993) L1153. [6] Sz. Fujita and Sg. Fujita, J. Crystal Growth 145 (1994) 552. [7] I.S. Hanksson, J. Simpson, S.Y. Wang, K.A. Prior and B.C. Cavenett, Appl. Phys. Lett. 61 (1992) 2208.
ELSEVIER
ClNYSTAI. GROWTH
Journal of Crystal Growth 159 (1996) 130-133
Growth of p-type ZnSe by metalorganic vapor phase epitaxy using ethylazide as a new nitrogen source K. Inoue *, K. Yanashima, T. Takahashi, J.S. Hwang, K. Hara, H. Munekata, H. Kukimoto Imaging Science and Engineering Laboratory, Tokyo Institute of Technology, 4259 Nagatsuda, Midori-ku, Yokohama 226, Japan
Abstract The growth of nitrogen-doped ZnSe epilayers has been investigated by metalorganic vapor phase epitaxy (MOVPE) using ethylazide as a novel dopant source in which no N-H bonds are involved. Nitrogen was successfully incorporated in the ZnSe epilayers, whereas hydrogen was also found in the epilayers, primarily in the form of N-H bonds. By thermally annealing the samples, we have found that N-H bonds can be dissociated, yielding p-type ZnSe layers with hole concentrations of mid- 1017 cm- 3.
1. Introduction ZnSe-based materials have excellent characteristics for blue-light-emitting devices [1,2]. one of the key issues for realizing those devices is the achievement of low resistivity p-type layers, for which nitrogen is expected to be one of the most suitable p-type dopants. The control of p-type conductivity, however, is still a serious problem especially in ZnSe-based epilayers grown b y metalorganic vapor phase epitaxy (MOVPE). This is primarily due to the incorporation of hydrogen in the epilayers, which results in the formation of N - H bonds and thus the hydrogen passivation of nitrogen acceptors [3]. In this study we report the growth and characterization of N-doped ZnSe by MOVPE using ethylazide (C2HsN3) as a new p-type dopant source. Note that no N - H bond is involved in this source. We aim at studying the influence of dopant source on the amounts of N and H in ZnSe epilayers. The effect of
* Correspondingauthor. Fax: + 81 45 921 1492.
thermal annealing on electrical properties has also been investigated.
2. Experimental procedure The growth was carded out in an atmospheric pressure MOVPE system consisting of a cold wall horizontal reactor, with diethylzinc (DEZn) and diethylselenide (DESe) being primary sources, and ethylazide (EtN 3) being the p-type dopant source. Typical flow rates of DEZn, DESe and EtN 3 were 3.5, 9.0 and 0-100 /~mol/min, respectively. The V I / I I flow ratio is 2.6. Prior to the growth, a p-type GaAs: Zn(100) substrate was chemically etched by a mixture of a 1NH4OH : 1H202 : 10H20 solution for 1 min, followed by surface treatment with a (NH4)2S x water solution. The substrate was then thermally etched for 10 rain at 500°C in the reactor under a hydrogen atmosphere. It was then followed by the growth of N-doped ZnSe layers at substrate temperatures of 400-470°C. Both uniform and spike dopings of N were examined in this work. As for the
0022-0248/96/$15.00 © 1996ElsevierScienceB,V. All fights reserved SSDI 0022-0248(95)00760- 1
K. lnoue et aL /Journal of Crystal Growth 159 (1996) 130-133
spike doping, the influence of the source supply sequence on N and H incorporation has also been studied by varying systematically the sequence of both primary and dopant gas sources.
131
10 4 ZnSe:N
ZnSe ~aAI bufferf
A
~ 10" "i
,.Z
3. Results and discussion
0
We have found that the amount of N in the ZnSe epilayers can be controlled to a great extent by changing the supply rate of ethylazide. Fig. 1 shows the photoluminescence (PL) spectra at 25 K of four different ZnSe epilayers grown at 400°C with various EtN 3 supply rates. Samples were doped uniformly by continuous supply of EtN 3 together with primary sources. The PL spectrum of an undoped ZnSe layer is dominated by a sharp free exciton emission band. A sample grown with a small supply o f E t N 3 s h o w s a neutral-acceptor-bound exciton emission band (A°X) together with donor-acceptor pair (DAP) emission bands. With increasing the flow rate of ethylazide, the PL spectra tum out to be dominated by DAP emission bands, as also shown in the figure. These results indicate that ethylazide, compared with NH 3 [4] and t-BNH 2 [5,6], can be readily decomposed during the MOVPE growth at
4.9 runol/rnln
3.4
Z
"' k-
2.2 rtmoI/mln A D A P
a.
FE
I
I
I
2.6 2.7 PHOTON ENERGY
t
I
2.8
{eV)
Fig. 1. Photoluminescence (PL) spectra at 25 K for various ZnSe layers grown at 400°C using EtN 3 as a nitrogen source. The spectrum of an undol~l ZnSe layer is also shown for comparison. Samples were excited by the 325 nm line of a HeCd laser.
'.~,-., ..,. ,/......... ,.'!
t/~
'..f ":
:i 10 2
¢,n
1111 0
200
400
600
DEPTH (rim) Fig. 2. SIMS depth profile of a N-doped ZnSe layer grown on ZnSe/GaAs(100).
relatively low growth temperatures ( < 400°C) to yield nitrogen-doped ZnSe epilayers. The incorporation of N has been confirmed by secondary ion mass spectroscopy (SIMS) depth profiles. In Fig. 2, we show SIMS data of a heavily N-doped ZnSe sample grown at 400°C with 50 /zmol/min of EtN 3. Here, N is detected in the form of N + Se (total mass number 94). Also noticeable are relatively high hydrogen counts, being indicative of the incorporation of a large amount of hydrogen together with nitrogen. Epilayers exhibit high electrical resistance, suggesting hydrogen passivation of nitrogen acceptors. Growth at relatively high temperatures has also been studied to suppress hydrogen incorporation. We have found, however, that incorporation of nitrogen is also suppressed unexpectedly. For example, as shown in Fig. 3, the PL spectrum of the sample grown at 470°C with 100 /zmol/min of EtN 3 is dominated by the A°X emission band, which is usually a sign of the lightly doped situation. We infer that the suppression of N incorporation is attributed to the reduced sticking efficiency of EtN 3 on a ZnSe surface. To compensate this effect, we have actually increased the EtN 3 supply rate. However, it has turned out to promote the reaction between EtN 3 and primary sources (DEZn and DESe) in the vapor phase. In order to pursue the effective N incorpora-
132
K. Inoue et al. / Journal of Crystal Growth 159 (1996) 130-133
A°dX A
L 2.6 2.7 2.8 PHOTON ENERGY (eV)
Fig. 3. PL spectrum of N-doped ZnSe grown at 470°C with uniform doping of E t N 3. The supply rate of E t N 3 w a s 100 /~mol/min.
tion, we have studied the spike doping of EtN 3 under different growth conditions. Fig. 4 shows the PL spectrum of the ZnSe:N epilayer prepared by the spike-doping technique. The PL spectrum is clearly dominated by the DAP emission, being indicative of the incorporation of a relatively large amount of N acceptors. Studying the source supply sequence systematically, we have
25 K exc. 325
nm
DAP
i I
2.6
!
2.7 2.8 PHOTON ENERGY (eV)
Fig. 4. PL spectrum of N-doped ZnSe grown at 470°C with spike doping of EtN 3. The supply rate of EtN 3 was 40 p.mol/min. Process of spike doping is described in the text.
found that first supplying the DEZn is very importam to have nitrogen incorporated in the ZnSe epilayers. In other words, the Zn-stabilized surface enhances the incorporation of nitrogen. For example, the source supply sequence of the sample shown in the figure is as follows: (1) 3 s DEZn supply, (2) 10 s supply of EtN3, where a flow rate was 40 /zmol/min, together with DESe for the nitrogen spike doping, and (3) 22 s supply of both DEZn and DESe for ZnSe epitaxy. Comparing the PL spectrum shown in Fig. 4 with that of a typical MBE-grown ZnSe:N sample [7], we estimate that the nitrogen concentration of this sample is roughly over 1018 cm -3. We also notice, by comparing the PL data of Fig. 3 with Fig. 4, that nitrogen incorporation is effectively enhanced by the spike doping. According to the Raman scattering measurements, however, we again find the presence of hydrogen in the form of N - H bonds (vibrations of stretching and wagging modes), strongly suggesting the hydrogen passivation of nitrogen acceptors. The effective ionized acceptor concentration of the as-grown sample, obtained by C-V measurements, was mid-1015 cm -3 at RT. The measurements were performed for Au/ZnSe: N / p ÷ - GaAs (100) Schottky diodes at a frequency of 100 Hz. The effect of thermal annealing has also been investigated. The annealing was carried out in the MOVPE reactor under a N 2 atmosphere. Another ZnSe/GaAs (100) sample was placed face-to-face on the sample as a cap to prevent evaporation of Zn and Se. By thermally annealing the samples under a N 2 atmosphere, we have found that hydrogen in the layers can be reduced significantly without causing an appreciable degradation in crystal quality. Fig. 5 shows the plot of ionized-acceptor concentration as a function of annealing temperature. The annealing time is fixed at 30 min. Samples used in the experiment were grown in a similar fashion as the one for the sample of Fig. 4. The inset shows a C-2-V plot (100 Hz) for a sample annealed at 570°C, indicating a good linearity with a built-in voltage of 1.9 V. As seen in the figure, the acceptor concentration increases two orders of magnitude with annealing temperature, resulting in the maximum concentration of 5 )< 1017 cm -3 at the annealing temperature of 570°C. Beyond this temperature, we have found that both electrical and optical properties tend to degrade.
K. lnoue et al./Journal of Crystal Growth 159 (1996) 130-133 101e
133
4. Summary O
We have investigated the growth of N-doped ZnSe layers by MOVPE using ethylazide as a novel dopant source in which N - H bonds are not included. Incorporation of N in ZnSe epilayers has been successfully achieved, whereas a relatively large amount of hydrogen has also been detected in the layers. Thermal annealing of the layers has resulted in a reduction of hydrogen concentration, yielding p-type ZnSe layers with hole concentrations of mid-10 ]7 cm -3.
O E o
10" 2
|
=-
-2
101s 500
V O L T A G E (V)
I I 550 600 TEMPERATURE (°C)
650
Fig. 5. A plot of Na - N d value as a function of annealing temperature. The inset shows a C - 2 - V plot (100 Hz) for a sample annealed at 570°C.
Before annealing, N - H vibration modes are observed by Raman scattering, while, after annealing at 570°C for 30 min, signals due to these modes disappear. These results indicate that the N - H bonds are thermally dissociated by the annealing process, resulting in the activation of nitrogen acceptors. Annealed samples show PL spectra composed of a broad DAP emission band, being similar to the heavily doped MBE-grown ZnSe : N layers. The hole concentrations that we can currently obtain after the annealing process are mid-1017 cm -3.
References [1] H. Okuyama, Y. Morinaga and K. Akimoto, J. Crystal Growth 127 (1993) 335. [2] S. Itoh, N. Nakamura, S. Matsumoto, M. Nagai, K. Nakano, M. Ozawa, H. Okuyama, S. Tomita, T. Ohata, M. Ikeda, A. lshibashi and Y. Mori, Jpn. J. Appi. Phys. 33 (1994) 938. [3] J.A. Wolk, J.W. Ager III, K.J. Duxstad, E.E. Hailer, N.R. Taskar, D,R. Dorman and D.J. Olego, Appl. Phys. Lett. 63 (1993) 2756. [4] A. Ohki, N. Shibata and S. Zembutu, Jpn. J. Appl. Phys. 27 (1988) L909. [5] Sz. Fujita, T. Asano, K. Maehara and Sg. Fujita, Jpn. J. Appl. 32 (1993) L1153. [6] Sz. Fujita and Sg. Fujita, J. Crystal Growth 145 (1994) 552. [7] I.S. Hanksson, J. Simpson, S.Y. Wang, K.A. Prior and B.C. Cavenett, Appl. Phys. Lett. 61 (1992) 2208.