- Email: [email protected]
New type blue-light emitting diode using epitaxial ZnS films grown on GaAs by MOVPE
Physica B 185 (1993) North-Holland
PHYSICA El
500-504
New type blue-light emitting diode using epitaxial ZnS films grown on GaAs by MOVPE Shigeki Yamaga Department
of 2nd Development,
Engineering
Headquarters,
New Japan Radio Co. Ltd., Saitama. Japan
A new type of blue-light emitting diode (LED) has been fabricated by using epitaxial ZnS films on GaAs by low-pressure metalorganic vapor phase epitaxy. The emission mechanism has been investigated and the LED structure has been optimized. A bright blue emission was observed with a maximum quantum efficiency of 0.3%.
1. Introduction
ZnS is a promising material for blue-light emitting diodes (LEDs) because of its wide direct band gap. It is well known that the bluelight emission can be obtained in ZnS by doping with ‘coactivators’ (e.g. Al, Cl, I). However, the control of electrical properties is difficult due to the ‘self-compensation effect’. Bulk ZnS LEDs have been fabricated by using a metal-insulator-semiconductor (MIS) structure with relatively high quantum efficiency [1,2]. However, a high temperature is required and a long annealing time to get a low-resistivity n-type ZnS bulk crystal 121. Furthermore, the reproducibility of the device performance has been low because of the mismatch of the lattice constant and the thermal expansivity between the insulator and ZnS. Recently, low-resistivity n-type epitaxial ZnS films have been grown by metalorganic vapor phase epitaxy (MOVPE) using ‘coactivators’ as donor dopant sources [3-51. Especially we have achieved the control of carrier (electron) density of an n-ZnS film on GaAs by iodine doping [5]. However, it is very difficult to grow p-type ZnS. Correspondence to: S. Yamaga, Department of 2nd Development, Engineering Headquarters, New Japan Radio Co. Ltd., l-1 Fukuoka 2-chome, Kamifukuoka-city, Saitama 356, Japan. 0921.4526/93/$06.00
0
1993 - Elsevier
Science
Publishers
Therefore it is important to develop a new structure for the blue LED instead of a conventional pn junction. We have fabricated a new type of blue LED using epitaxial ZnS films on GaAs by low-pressure MOVPE for the first time [6]. This LED is based on the MIS structure with undoped and donor-doped epitaxial ZnS films as an insulator and an emission layer. A fairly bright blue electroluminescence (EL) has been observed at room temperature, but the external quantum efficiency was about 0.04%. In this work, the optimization of the LED structure has been investigated to improve the quantum efficiency. The thickness of the insulating layer and the carrier density of the n-ZnS layer have been investigated to achieve higher quantum efficiency.
2. Device fabrication A cross-sectional view of an epitaxial ZnS blue LED is shown in the inset of fig. 1. Si-doped n+-GaAs (n = 1018 cm-‘) was used as a conductive substrate. Low-resistivity n-ZnS films were grown by using methyliodide as a donor dopant source in low-pressure MOVPE. The carrier density can be controlled from 10” to 10’” cm-’ by a changing dopant flow rate [5]. Undoped ZnS films (intrinsic ZnS films:
B.V. All rights
reserved
S. Yamaga
‘-._
./’ 0
/ New type blue-LEDs
----________
__-
1 7000
5000~~“E~ENGTH
dooo
Fig. 1. Electroluminescence (EL) and photoluminescence (PL) spectra of a MiS LED at room temperature. The EL spectrum was measured under reverse-bias conditions and the PL spectrum was measured by using a He-Cd laser as an excitation source. The inset shows the cross-sectional view of the sample.
i-ZnS) were used as an insulating layer and grown by using the atomic layer epitaxy (ALE) technique [7]. This ALE technique is more suitable for obtaining ultra-thin epitaxial films with a uniform film thickness. The i-ZnS layer thickness was varied from 0 to 1OOOA. Furthermore, Au and In-Ga have been used as surface Schottky metal and back-side ohmic contact metal, respectively.
501
using epitaxial ZnS films
conditions, i.e. a negative voltage was applied to the Au contact [6]. Figure 2 shows the band diagram of the metal-intrinsic semiconductor (MiS) structure for both forward and reverse bias cases. For the forward-bias case, since the barrier height (&,,,) at the interface of Au and ZnS is lower than 0.5E,, the minority carrier (hole) is difficult to be generated by forward current flows [9], resulting in no EL emission. Meanwhile, for the reverse-bias case, when the bias voltage is increased, the current transport mechanism is changed from generationrecombination dominated current to tunneling dominated current. These tunneling electrons are accelerated in the i-ZnS layer by the electric field, resulting in the generation of electron-hole pairs through an impact ionization process. The impact ionization process needs a high field and the effective ionization threshold energy is about 1.5Ep [lo]. A high-resistivity i-ZnS layer is necessary to maintain a high electric field. But when the i-ZnS layer becomes thicker the tunneling-current density is reduced and the generated hole density is also reduced, resulting in a lower blue-light emission. To obtain high-intensi(a) Forword
bias
Lf%zL
3. Emission
mechanism
Figure 1 shows the electroluminescence (EL) and the photoluminescence (PL) spectrum of the same sample at room temperature (RT). Both spectra show a blue-light radiation. The higherenergy emission in the PL spectrum is originated from self-activated (SA) centers related to iodine donors [8]. It has been found that the concentration of SA centers increases with increasing the electron density [5]. That is, heavily doped samples show a higher-intensity blue-PL emission. The EL spectrum appears to be very similar to the PL spectrum as shown in fig. 1. The EL emission may originate from the SA centers in the iodine-doped ZnS. EL emission was obtained under reverse-bias
(b) Reverse
bias
Ehn
_____-_
Es*
i-ZnS Fig. 2. Energy band diagrams of the MiS LED forwardand (b) reverse-bias condition.
under
(4
502
S. Yamaga
i New type blue-LEDs
ty blue EL it is necessary to optimize the i-ZnS layer thickness. Holes generated by impact ionization in the i-ZnS layer are swept to the Schottky contact as shown in the figure (case 1). The holes generated in the n-ZnS depletion region also drift to the Schottky region. However, if they are trapped to the SA centers, because the SA center behaves as an acceptor level, they recombine with free electrons and can radiate blue EL (case 2). In addition to the above case, the accelerated electrons can excite holes directly to the SA centers, providing recombination-radiation centers for free electrons. These processes seem to be the main emission mechanisms of the blue EL in a reverse-biased MIS structure. Furthermore, a few hot electrons entering into the flat-band region of n-ZnS can generate effective holes (case 3). In order to obtain high-emission efficiency in MIS LEDs using an impact ionization process, it is important to increase both the emission center density (SA center density) and the number of hol’es generated in the n-ZnS depletion layer. The SA center density can be increased by increasing the donor, doping level. The number of holes are increased by increasing tl& ionization rate and the carrier’ traveling length [lo]. The carrier traveling length in case 2 (the width of the n-ZnS depletion layer) is widened by decreasing the doped donor density. The lower doping density, however, reduces the SA center resulting in a poorer emission efficiency. It is necessary to find the optimum donor doping density.
4. Results and discussion Figure 3 shows current-voltage (Z-V) characteristics for MIS LEDs under reverse-bias conditions when the i-ZnS layer thickness is changed from 0 to 1000 A. The carrier density of the n-ZnS layer was kept about 10” cmm3. In the case of Schottky type (without i-ZnS layer) no blue emission was observed. This shows that the i-ZnS layer is essential for obtaining blue EL and impact ionization is the main process to generate
using epitaxial ZnS films
I -21-6
Layer
Thickness
RT I U
5
15
RE”ERSE’“BIAS
(V1
20
Fig. 3. Current-voltage characteristics at RT of several MiS LEDs, of which the i-ZnS layer thickness was changed from 0 to 1000 A. The negative voltage was applied to the Au contact (reverse-bias condition).
minority carriers. In the case of MIS structures, the EL emission starts being observed near the inflection point of each Z-V curve. This point is called the ‘emission starting voltage’, which increases with i-ZnS layer thickness. The Z-V curves shift to higher voltages with increase of the i-ZnS layer thickness. It was found that the tunneling current is dominant above the emission starting voltage. Figure 4 shows the dependence of the emission starting voltage and the emission efficiency (quantum efficiency) in terms of i-ZnS layer thickness. The emission starting voltage increases as the i-ZnS layer thickness increases, as shown in fig. 3. This shows the electrons tunneling through the Schottky barrier, contributing to the impact ionization process. The emission efficiency was measured under constant-current conditions at RT. The highest efficiency is obtained when the i-ZnS layer thickness is about 350 A. The samples with a thinner i-ZnS layer show a lower emission efficiency because the tunneling electrons cannot be accelerated
S. Yamaga
I New type blue-LEDs
R T 0
5
KY3
,
0
I-ZnS
LA&?
THICKNESS
(A)
5. Conclusion
I
\
\
\
I
\ \ \
/I
\
New type blue LEDs have been fabricated by using epitaxial ZnS films on GaAs. The blue EL emission mechanism has been explained by an impact ionization process in a reverse-biased MiS structure. Furthermore, the i-ZnS layer thickness and the carrier density of the n-ZnS layer has been optimized to obtain high emission efficiency. As a result, the emission efficiency has been enhanced to a maximum of 0.3%.
Acknowledgements
The author wishes to thank Prof. A. Yoshikawa in Chiba University for continuous advice and encouragement of this work and Mr. W. Imajyuku for his helpful experimental assistance.
RT
s-.
about 350 A. When the carrier density is about 1018 crnm3, the emission efficiency becomes maximum to 0.3%. The carrier density above 10” cmm3 shows a lower emission efficiency because of the narrower n-ZnS depletion layer and the shorter carrier traveling length. Furthermore, when the n-ZnS carrier density is about 1017 cme3, the junction current is hardly flowing and the blue-light emission has diminished. Even when the junction current flows, the blue emission cannot be observed due to a very low density of emission centers.
1000
enough for the impact ionization. Meanwhile for the thicker i-ZnS layer the n-ZnS depletion layer thickness is reduced, resulting in a shorter traveling length. Figure 5 shows the dependence of the emission efficiency on the carrier density of the n-Z& layer, where the i-ZnS layer thickness was kept
‘0
503
k
Fig. 4. Dependence of the emission starting voltage and the emission efficiency of MIS LEDs in terms of the i-ZnS layer thickness. The carrier density of the n-ZnS film was kept about lOI cm-‘. The emission efficiency was measured under constant-junction-current conditions (50 mA).
I’
using epitaxial ZnS films
I
‘cl
o\
ci
i-ZnS
z
: 350
[II
A
s w
10'7
CARRIER
10'8
DENSITY
Fig. 5. Dependence of the emission on the carrier density of an n-ZnS layer thickness was kept at 350A.
References
10’9
(cm-31
efficiency of MIS LEDs layer, where the i-ZnS
H. Katayama, S. Oda and H. Kukimoto, Appl. Phys. Lett. 27 (1975) 697. 121K. Kurisu and T. Taguchi, in: Proceedings of the 4th International Workshop on Electroluminescence, Tottori, Japan (1988) p. 367. J. Cryst. Growth [31 T. Yasuda, K. Hara and H. Kukimoto, 77 (1986) 485. A. Yoshikawa and H. Kasai, J. Cryst. [41 S. Yamaga, Growth 86 (1988) 252. A. Yoshikawa and H. Kasai, J. Cryst. [51 S. Yamaga, Growth 106 (1990) 683. Jpn. J. Appl. Phys. 30 (1991) 104. 161 S. Yamaga,
504
S. Yamaga
I New type blue-LEDs
[7] S. Yamaga and A. Yoshikawa, in: Proceedings of the 5th International Conference on II-VI Compounds, to appear in J. Cryst. Growth. [8] T. Yokogawa, T. Taguchi, S. Fujita and M. Satoh, IEEE Trans. Electron Devices 30 (1983) 271.
using epitaxial ZnS films [9] N.B. H.M. (1977) [lo] S.M. 1985)
Lukyanchikova, G.S. Pekar, N.N. Tkachenko, Shin and M.K. Sjeinkman, Phys. Stat. Sol. 41 299. Sze, Semiconductor Devices (Wiley, New York, ch. 2.
PHYSICA El
500-504
New type blue-light emitting diode using epitaxial ZnS films grown on GaAs by MOVPE Shigeki Yamaga Department
of 2nd Development,
Engineering
Headquarters,
New Japan Radio Co. Ltd., Saitama. Japan
A new type of blue-light emitting diode (LED) has been fabricated by using epitaxial ZnS films on GaAs by low-pressure metalorganic vapor phase epitaxy. The emission mechanism has been investigated and the LED structure has been optimized. A bright blue emission was observed with a maximum quantum efficiency of 0.3%.
1. Introduction
ZnS is a promising material for blue-light emitting diodes (LEDs) because of its wide direct band gap. It is well known that the bluelight emission can be obtained in ZnS by doping with ‘coactivators’ (e.g. Al, Cl, I). However, the control of electrical properties is difficult due to the ‘self-compensation effect’. Bulk ZnS LEDs have been fabricated by using a metal-insulator-semiconductor (MIS) structure with relatively high quantum efficiency [1,2]. However, a high temperature is required and a long annealing time to get a low-resistivity n-type ZnS bulk crystal 121. Furthermore, the reproducibility of the device performance has been low because of the mismatch of the lattice constant and the thermal expansivity between the insulator and ZnS. Recently, low-resistivity n-type epitaxial ZnS films have been grown by metalorganic vapor phase epitaxy (MOVPE) using ‘coactivators’ as donor dopant sources [3-51. Especially we have achieved the control of carrier (electron) density of an n-ZnS film on GaAs by iodine doping [5]. However, it is very difficult to grow p-type ZnS. Correspondence to: S. Yamaga, Department of 2nd Development, Engineering Headquarters, New Japan Radio Co. Ltd., l-1 Fukuoka 2-chome, Kamifukuoka-city, Saitama 356, Japan. 0921.4526/93/$06.00
0
1993 - Elsevier
Science
Publishers
Therefore it is important to develop a new structure for the blue LED instead of a conventional pn junction. We have fabricated a new type of blue LED using epitaxial ZnS films on GaAs by low-pressure MOVPE for the first time [6]. This LED is based on the MIS structure with undoped and donor-doped epitaxial ZnS films as an insulator and an emission layer. A fairly bright blue electroluminescence (EL) has been observed at room temperature, but the external quantum efficiency was about 0.04%. In this work, the optimization of the LED structure has been investigated to improve the quantum efficiency. The thickness of the insulating layer and the carrier density of the n-ZnS layer have been investigated to achieve higher quantum efficiency.
2. Device fabrication A cross-sectional view of an epitaxial ZnS blue LED is shown in the inset of fig. 1. Si-doped n+-GaAs (n = 1018 cm-‘) was used as a conductive substrate. Low-resistivity n-ZnS films were grown by using methyliodide as a donor dopant source in low-pressure MOVPE. The carrier density can be controlled from 10” to 10’” cm-’ by a changing dopant flow rate [5]. Undoped ZnS films (intrinsic ZnS films:
B.V. All rights
reserved
S. Yamaga
‘-._
./’ 0
/ New type blue-LEDs
----________
__-
1 7000
5000~~“E~ENGTH
dooo
Fig. 1. Electroluminescence (EL) and photoluminescence (PL) spectra of a MiS LED at room temperature. The EL spectrum was measured under reverse-bias conditions and the PL spectrum was measured by using a He-Cd laser as an excitation source. The inset shows the cross-sectional view of the sample.
i-ZnS) were used as an insulating layer and grown by using the atomic layer epitaxy (ALE) technique [7]. This ALE technique is more suitable for obtaining ultra-thin epitaxial films with a uniform film thickness. The i-ZnS layer thickness was varied from 0 to 1OOOA. Furthermore, Au and In-Ga have been used as surface Schottky metal and back-side ohmic contact metal, respectively.
501
using epitaxial ZnS films
conditions, i.e. a negative voltage was applied to the Au contact [6]. Figure 2 shows the band diagram of the metal-intrinsic semiconductor (MiS) structure for both forward and reverse bias cases. For the forward-bias case, since the barrier height (&,,,) at the interface of Au and ZnS is lower than 0.5E,, the minority carrier (hole) is difficult to be generated by forward current flows [9], resulting in no EL emission. Meanwhile, for the reverse-bias case, when the bias voltage is increased, the current transport mechanism is changed from generationrecombination dominated current to tunneling dominated current. These tunneling electrons are accelerated in the i-ZnS layer by the electric field, resulting in the generation of electron-hole pairs through an impact ionization process. The impact ionization process needs a high field and the effective ionization threshold energy is about 1.5Ep [lo]. A high-resistivity i-ZnS layer is necessary to maintain a high electric field. But when the i-ZnS layer becomes thicker the tunneling-current density is reduced and the generated hole density is also reduced, resulting in a lower blue-light emission. To obtain high-intensi(a) Forword
bias
Lf%zL
3. Emission
mechanism
Figure 1 shows the electroluminescence (EL) and the photoluminescence (PL) spectrum of the same sample at room temperature (RT). Both spectra show a blue-light radiation. The higherenergy emission in the PL spectrum is originated from self-activated (SA) centers related to iodine donors [8]. It has been found that the concentration of SA centers increases with increasing the electron density [5]. That is, heavily doped samples show a higher-intensity blue-PL emission. The EL spectrum appears to be very similar to the PL spectrum as shown in fig. 1. The EL emission may originate from the SA centers in the iodine-doped ZnS. EL emission was obtained under reverse-bias
(b) Reverse
bias
Ehn
_____-_
Es*
i-ZnS Fig. 2. Energy band diagrams of the MiS LED forwardand (b) reverse-bias condition.
under
(4
502
S. Yamaga
i New type blue-LEDs
ty blue EL it is necessary to optimize the i-ZnS layer thickness. Holes generated by impact ionization in the i-ZnS layer are swept to the Schottky contact as shown in the figure (case 1). The holes generated in the n-ZnS depletion region also drift to the Schottky region. However, if they are trapped to the SA centers, because the SA center behaves as an acceptor level, they recombine with free electrons and can radiate blue EL (case 2). In addition to the above case, the accelerated electrons can excite holes directly to the SA centers, providing recombination-radiation centers for free electrons. These processes seem to be the main emission mechanisms of the blue EL in a reverse-biased MIS structure. Furthermore, a few hot electrons entering into the flat-band region of n-ZnS can generate effective holes (case 3). In order to obtain high-emission efficiency in MIS LEDs using an impact ionization process, it is important to increase both the emission center density (SA center density) and the number of hol’es generated in the n-ZnS depletion layer. The SA center density can be increased by increasing the donor, doping level. The number of holes are increased by increasing tl& ionization rate and the carrier’ traveling length [lo]. The carrier traveling length in case 2 (the width of the n-ZnS depletion layer) is widened by decreasing the doped donor density. The lower doping density, however, reduces the SA center resulting in a poorer emission efficiency. It is necessary to find the optimum donor doping density.
4. Results and discussion Figure 3 shows current-voltage (Z-V) characteristics for MIS LEDs under reverse-bias conditions when the i-ZnS layer thickness is changed from 0 to 1000 A. The carrier density of the n-ZnS layer was kept about 10” cmm3. In the case of Schottky type (without i-ZnS layer) no blue emission was observed. This shows that the i-ZnS layer is essential for obtaining blue EL and impact ionization is the main process to generate
using epitaxial ZnS films
I -21-6
Layer
Thickness
RT I U
5
15
RE”ERSE’“BIAS
(V1
20
Fig. 3. Current-voltage characteristics at RT of several MiS LEDs, of which the i-ZnS layer thickness was changed from 0 to 1000 A. The negative voltage was applied to the Au contact (reverse-bias condition).
minority carriers. In the case of MIS structures, the EL emission starts being observed near the inflection point of each Z-V curve. This point is called the ‘emission starting voltage’, which increases with i-ZnS layer thickness. The Z-V curves shift to higher voltages with increase of the i-ZnS layer thickness. It was found that the tunneling current is dominant above the emission starting voltage. Figure 4 shows the dependence of the emission starting voltage and the emission efficiency (quantum efficiency) in terms of i-ZnS layer thickness. The emission starting voltage increases as the i-ZnS layer thickness increases, as shown in fig. 3. This shows the electrons tunneling through the Schottky barrier, contributing to the impact ionization process. The emission efficiency was measured under constant-current conditions at RT. The highest efficiency is obtained when the i-ZnS layer thickness is about 350 A. The samples with a thinner i-ZnS layer show a lower emission efficiency because the tunneling electrons cannot be accelerated
S. Yamaga
I New type blue-LEDs
R T 0
5
KY3
,
0
I-ZnS
LA&?
THICKNESS
(A)
5. Conclusion
I
\
\
\
I
\ \ \
/I
\
New type blue LEDs have been fabricated by using epitaxial ZnS films on GaAs. The blue EL emission mechanism has been explained by an impact ionization process in a reverse-biased MiS structure. Furthermore, the i-ZnS layer thickness and the carrier density of the n-ZnS layer has been optimized to obtain high emission efficiency. As a result, the emission efficiency has been enhanced to a maximum of 0.3%.
Acknowledgements
The author wishes to thank Prof. A. Yoshikawa in Chiba University for continuous advice and encouragement of this work and Mr. W. Imajyuku for his helpful experimental assistance.
RT
s-.
about 350 A. When the carrier density is about 1018 crnm3, the emission efficiency becomes maximum to 0.3%. The carrier density above 10” cmm3 shows a lower emission efficiency because of the narrower n-ZnS depletion layer and the shorter carrier traveling length. Furthermore, when the n-ZnS carrier density is about 1017 cme3, the junction current is hardly flowing and the blue-light emission has diminished. Even when the junction current flows, the blue emission cannot be observed due to a very low density of emission centers.
1000
enough for the impact ionization. Meanwhile for the thicker i-ZnS layer the n-ZnS depletion layer thickness is reduced, resulting in a shorter traveling length. Figure 5 shows the dependence of the emission efficiency on the carrier density of the n-Z& layer, where the i-ZnS layer thickness was kept
‘0
503
k
Fig. 4. Dependence of the emission starting voltage and the emission efficiency of MIS LEDs in terms of the i-ZnS layer thickness. The carrier density of the n-ZnS film was kept about lOI cm-‘. The emission efficiency was measured under constant-junction-current conditions (50 mA).
I’
using epitaxial ZnS films
I
‘cl
o\
ci
i-ZnS
z
: 350
[II
A
s w
10'7
CARRIER
10'8
DENSITY
Fig. 5. Dependence of the emission on the carrier density of an n-ZnS layer thickness was kept at 350A.
References
10’9
(cm-31
efficiency of MIS LEDs layer, where the i-ZnS
H. Katayama, S. Oda and H. Kukimoto, Appl. Phys. Lett. 27 (1975) 697. 121K. Kurisu and T. Taguchi, in: Proceedings of the 4th International Workshop on Electroluminescence, Tottori, Japan (1988) p. 367. J. Cryst. Growth [31 T. Yasuda, K. Hara and H. Kukimoto, 77 (1986) 485. A. Yoshikawa and H. Kasai, J. Cryst. [41 S. Yamaga, Growth 86 (1988) 252. A. Yoshikawa and H. Kasai, J. Cryst. [51 S. Yamaga, Growth 106 (1990) 683. Jpn. J. Appl. Phys. 30 (1991) 104. 161 S. Yamaga,
504
S. Yamaga
I New type blue-LEDs
[7] S. Yamaga and A. Yoshikawa, in: Proceedings of the 5th International Conference on II-VI Compounds, to appear in J. Cryst. Growth. [8] T. Yokogawa, T. Taguchi, S. Fujita and M. Satoh, IEEE Trans. Electron Devices 30 (1983) 271.
using epitaxial ZnS films [9] N.B. H.M. (1977) [lo] S.M. 1985)
Lukyanchikova, G.S. Pekar, N.N. Tkachenko, Shin and M.K. Sjeinkman, Phys. Stat. Sol. 41 299. Sze, Semiconductor Devices (Wiley, New York, ch. 2.