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Effects of the passivation layer deposition temperature on the electrical and optical properties of GaN-based light-emitting diodes
ARTICLE IN PRESS
Journal of Luminescence 127 (2007) 441–445 www.elsevier.com/locate/jlumin
Effects of the passivation layer deposition temperature on the electrical and optical properties of GaN-based light-emitting diodes Guangdi Shen, Xiaoli Da, Xia Guo, Yanxu Zhu, Nanhui Niu Institute of Electronic Information and Control Engineering, Beijing Optoelectronic Technology Laboratory, Beijing University of Technology, Beijing 100022, China Received 6 July 2006; received in revised form 3 January 2007; accepted 7 February 2007 Available online 20 February 2007
Abstract In this paper, silicon oxynitride is deposited through plasma-enhanced chemical vapor deposition (PECVD) to serve as an antireflection passivation layer. We have studied the effects of the deposition temperature (from 100 to 300 1C) on the electrical and optical performances of GaN-LEDs. It is found that the light output of GaN-LEDs improves greatly after the deposition of SiON antireflection passivation layer at 200 1C and is better than that of GaN-LEDs whose layers are deposited at 100 and 300 1C. The electrical properties of GaN-LED do not degrade at 100 and 200 1C, but degrade significantly at 300 1C. r 2007 Elsevier B.V. All rights reserved. Keywords: Temperature; PECVD; Passivation layer; GaN-LEDs
1. Introduction Light-emitting diodes (LEDs) based on GaN materials are widely studied for applications of large-scale color displays and signal lights. In particular, they have become a focus in developing white LEDs for general illumination, taking advantage of their high-brightness feature [1,2]. There are two principal approaches to improve LED efficiency. One is to increase the internal quantum efficiency determined by crystal quality and epitaxial layer structure; the other is to increase the light extraction efficiency. For blue GaN-LED, the high internal quantum efficiency has already reached 70% [3], but there is still a great potential to improve the external quantum efficiency [4–7], which is limited by the large difference in the refractive index between the semiconductor and the air. For a conventional GaN-based LED, the critical angle [yc ¼ sin1 ðnair =nGaN Þ] for the light generated in the active region is determined by the refractive indexes of GaN (nGaN ) and the air (nair ), which are 2.5 and 1, respectively. In this case, it leads to a Corresponding author. Tel.: +86 10 67391641x835; fax: +86 10 67391641x822. E-mail address: [email protected] (X. Da).
0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.02.002
critical angle of 231, and as a result, a very small amount of light is possible to escape from LEDs. Moreover, some of the light escaping from the LED will be reflected back into LED due to the high interfacial reflectivity between GaN materials and the air. Therefore, only a small portion of light can really escape from a GaN-LED. Neglecting the light emitted from sidewalls and backside, only about 4% of the internal light will be emitted from the surface [8]. The light that has not escaped from the surface is reflected back into the substrate and is then repeatedly reflected until it is finally reabsorbed by the active layer or the electrodes, unless it escapes through the sidewalls. In order to improve the external quantum efficiency, we designed an antireflection layer coated on the GaN-LEDs. The antireflection layer can decrease the interfacial reflectivity between the GaN and the air. Furthermore, the antireflection layer can also enlarge the critical angle for the light escaping. For example, after coating the antireflection layer with the refractive index of 1.54, the critical angle for the light escape from p-GaN surface can be enlarged to 381 and the critical angle for the light escaping from antireflection layer is 40.51, so that more light can escape from the surface of a LED after coating an antireflection layer on p-GaN.
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In this paper, the SiON antireflection passivation layers were deposited on GaN-LEDs through plasma-enhanced chemical vapor deposition (PECVD) at 100, 200 and 300 1C, respectively, to serve as antireflection layers. We have investigated the influence of the SiON deposition temperature on the electrical and optical performances of GaN-LEDs. The light output of GaN-LEDs improves greatly after the deposition of SiON antireflection passivation layer at 200 1C, and is better than that of GaN-LEDs whose layers are deposited at 100 and 300 1C. As for the electrical properties of GaN-LED, they do not degrade at 100 and 200 1C, but degrade significantly at 300 1C. 2. Experiments The GaN-LEDs epilayers are composed of a GaN nucleation layer, a Si-doped n-type GaN, a 20 nm InxGa1xN (x ¼ 0.08–0.1) strain relief layer, five periods of InxGa1xN/GaN multi-quantum well (MQW) layers, and a Mg-doped p-type GaN, grown in sequence on a sapphire (0 0 0 1) substrate through metal organic chemical vapor deposition (MOCVD) [9]. The carrier concentrations in the p-GaN and n-GaN layers are 5 1017 cm3 and 5 1018 cm3, respectively. In our experiments, the sample was annealed at 720 1C for 30 min in a nitrogen atmosphere to activate the Mg dopants in the p-GaN layer. To fabricate the LEDs, a mesa-type surface of the p-GaN layer had been etched by means of an induced coupled plasma (ICP) until the n-GaN layer was exposed for n-type ohmic contact formation. A Ni/Au (50 A˚/50 A˚) layer was deposited onto the p-GaN layer as a light-transmitting layer, and then was annealed at 500 1C for 1 min in a N2 and O2 mixture to achieve a good metal ohmic contact and a low absorption. A Ti/Al/Ti/Au (15 nm/100 nm/20 nm/200 nm) layer was deposited on the n-GaN layer to serve as an n-electrode. After the metal electrodes had been formed, the SiON passivation layer was deposited on these samples at 100, 200 and 300 1C, respectively. Lee et al. [10] have reported that the SiH4 and NH3 processes can passivate the Si doping in n-GaAs through the reaction: Siþ þ H ¼ ðSiHÞ0 , where most of the hydrogen in the SiH4/NH3 mixture originates from the ammonia [11]. Therefore, we designed deposition processes without ammonia. These processes used a mixture of silane diluted to 5% in nitrogen as the source of silicon, nitrous oxide as the source of oxygen, and highly purified nitrogen as the source of nitrogen. When the deposition temperature was 100 or 300 1C, the gas flows of SiH4, N2O and N2 were set to 200, 120 and 350 sccm, respectively. The working pressures of the two processes are 500 and 1000 mTorr. When the deposition temperature was 200 1C, the silicon oxynitride film was deposited at a working pressure of 500 mTorr, 200 sccm SiH4, 180 sccm N2O and 600 sccm N2. In order to get good uniformity and low stress of the film, we applied the dual frequency mode in these three deposition processes, where the high
frequency (HF) was 13.56 MHz with the power of 30 W and the low frequency (LF) was 100 kHz with the power of 40 W. The forward voltage and light output (LOP) were measured before and after the deposition of the passivation layer by the LED tester and each device was driven at 20 mA. The refractive index and thickness of the films were obtained by the variable angle spectroscopic ellipsometry. The current–voltage (I–V) curves of n–n electrode were measured using a precision semiconductor parameter analyzer (Agilent 4156). The tape pull test was conducted to evaluate the adhesion of the SiON films. The uniformity of GaN step profile capped with SiON film was represented by a SEM micrograph. The transmittance of the films was measured through the Hitachi spectrophotometer. 3. Results and discussion The refractive index and thickness of the films were obtained by the variable angle spectroscopic ellipsometry and are shown in Table 1. In order to evaluate the adhesion of the SiON films, the tape pull test was conducted by adhering white tape onto the SiON film side using FURUKAWA brand white tape. To this surface, one end of a strip of white single-coated tape was pressed firmly and then pulled away in the vertical direction using a smooth pull of 1 s duration. Each sample coated with SiON film was examined under 100 power OLYMPUS BX51M microscope to determine if any SiON had been removed. This method was taken as a practical measure of the adhesion strength. It is observed that none of the SiON films is peeled off from the samples. According to the results of the tape pull test, the SiON films on the three samples have shown good adhesion. Sapphire is a kind of insulating substrate, which is very difficult to lift off to expose the n-GaN for fabricating n-electron. We etched wafer from the p-GaN to the n-GaN layer through the method of ICP to form a mesa structure. Fig. 1 shows a SEM picture in which the sample is capped with a SiON layer. We can see that SiON film on the top step, bottom step and sidewall is very uniform and presents good step coverage properties. Tables 2–4 show the electrical and optical characteristics of GaN-LEDs before and after the deposition of SiON film at 100, 200 and 300 1C, respectively. The LED wafer was put on the probe station to obtain the characteristic distribution of the wafer before and after the deposition of SiON film. We drove 20 mA current in each device and measured the light output (relative light intensity) and Table 1 The refractive index and thickness of SiON film deposited at 100, 200 and 300 1C in PECVD Deposition temperature (1C)
Refractive index
Thickness (nm)
100 200 300
1.5385 1.5426 1.5926
229.1 218.4 189.5
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Table 4 The optical and electrical characteristics of a GaN-LED wafer before and after the deposition of the SiON layer at 300 1C
Fig. 1. A SEM picture of GaN step profile capped with SiON film.
Table 2 The optical and electrical characteristics of a GaN-LED wafer before and after the deposition of the SiON layer at 100 1C Measurement items
Value range
Before passivation (%)
After passivation (%)
Light output
0.20–0.25 0.25–0.30 0.3–0.35
5.89 88.84 0
0.42 6.93 87.39
Forward voltage (V)
3.1–3.2
85.37
92.33
3.2–3.3
9.16
2
Table 3 The optical and electrical characteristics of a GaN-LED wafer before and after the deposition of the SiON layer at 200 1C Measurement items
Value range
Before passivation (%)
After passivation (%)
Light output
0.18–0.20 0.20–0.22 0.26–0.28 0.28–0.30
73.06 15.58 0 0
0 0.39 75.39 12.93
Forward voltage (V)
3.1–3.2
93.71
23.76
3.2–3.3 3.3–3.4
5.45 0.08
68.96 5.57
forward voltage (Vf). The numbers in the three tables are the data of light output (relative light intensity) and forward voltage. From Table 2, we can see that the light output has been improved dramatically. After the
Measurement items
Value range
Before passivation (%)
After passivation (%)
Light output
0.1–0.12 0.12–0.14 0.14–0.16 0.16–0.18
6.71 38.43 48.14 0
0.95 0.96 28.73 50.63
Forward voltage (V)
3.5–3.6
41.74
3.6–3.7 3.7–3.8 4–5 5–6 6–7
25.72 16.94 0 0 0
0 0 0 48.63 15.3 19.62
deposition of a 229.1 nm SiON layer with a refractive index of 1.5385, the percentage of light output in the range 0.3–0.35 increased greatly up to 87.39%. For example, the light output of a chip has increased from 0.3 to 0.338, an increase of 12.6% after the passivation. From Table 3, we can see the optical characteristic has been improved dramatically after coating a 218.4 nm SiON layer with a refractive index of 1.5426. Moreover, the antireflection effect of SiON film is better than that of the SiON films deposited at 100 1C. Before passivation, the range of light output is 0.18–0.22, and it was enhanced to 0.26–0.30 after the deposition of SiON antireflection passivation layer at 200 1C. For example, the light output of a chip increased from 0.245 to 0.336, an increase of 37.1% after the passivation. From Table 4, we can see the antireflection effect is still obvious. After the deposition of a 189.5 nm SiON layer with a refractive index of 1.5926 at 300 1C, the percentage of light output in the range 0.16–0.18 increased greatly from 0 to 50.63%. For example, the light output of a chip increased from 0.168 to 0.193, an increase of 14.8% after the passivation. In order to further investigate the relationship between the deposition temperature and the light output, we measured the different etched rates of the SiON films, which are 960 nm/min at 100 1C, 344 nm/min at 200 1C and 600 nm/min at 300 1C, respectively. This result indicates that the SiON film deposited at 200 1C is more compact than those deposited at 100 and 300 1C. We measured the transmittance of the SiON deposited at 100 and 200 1C to verify the relationship between the densification and the transmittance. From Fig. 2, we can see the transmittance of the SiON film deposited at 100 and 200 1C was 91.5% and 91.7%, respectively. This result means that although the densifications of the two films were quite different, their transmittance rates for light were close. The ideal single antireflective layer is designed to ensure that the optical thickness of the film is odd times of a quarter of the wavelength and the refractive index is the extraction root of the refractive index of GaN. Thickness and the refractive
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Fig. 2. The transmittance of the glass substrate and the SiON film fabricated at 100 and 200 1C, respectively.
index are the factors influencing the effects of the antireflection layer. In our previous investigation [12], the SiON passivation layer with the refractive index of 1.54 had good antireflection effect for GaN-LEDs. This has clearly illustrated the results that the light output of GaNLED improves greatly after depositing a SiON passivation layer at 200 1C. Comparing the electrical properties given in these three tables, it is obvious that the electrical properties of the GaN-LED degrade, when the deposition temperature increases. As shown in Table 4, when the deposition temperature was enhanced to 300 1C, the electrical and optical characteristics of a GaN-LED wafer degraded significantly, where the range of forward voltage increased from 3.5–3.8 V to 4–7 V. However, the forward voltage of LED increased slightly after the 200 1C deposition process, where the percentage in the range 3.2–3.3 V increased from 5.45% to 68.96% (as shown in Table 3). After the passivation of the chip at 100 1C, the percentage of the forward voltage (Vf) in the range 3.2–3.3 V reduced obviously. The forward electrical characteristics of GaNLEDs improved after the 100 1C SiON deposition process. We observed the GaN-LED chips under the microscope (Olympus BX51 M). Three samples are shown in Fig. 3. Samples (a), (b) and (c) are treated by the 100, 200 and 300 1C deposition processes of SiON film, respectively. From Fig. 3, no obvious change was found in the morphology of the Ni/Au contact layer. However, comparing these three pictures in Fig. 3, we found that the morphology of the n-electrode (Ti/Al/Ti/Au) surface changed after the 300 1C deposition process. The surface of n-electrode became rough and the color changed. It may result from the degradation of Al. It has been reported that at high temperature, Al of the metal contact melts and tends to ball up, which, in turn, results in rough surfaces and increased ohmic contact resistances [13,14]. Fig. 4 shows current–voltage (I–V) characteristics of Ti/Al/Ti/Au contact on n-type GaN, with SiON films deposited at 100, 200 and 300 1C. After the 100 1C
Fig. 3. The microscope photograph of part of GaN-LED chip after the deposition of the SiON antireflection layer at (a) 100 1C, (b) 200 1C and (c) 300 1C.
Fig. 4. I–V characteristics of Ti/Al/Ti/Au contact on n-type GaN at 100, 200 and 300 1C.
deposition process, the Ti/Al/Ti/Au contact on n-GaN exhibited linear I–V characteristics, which indicates that the 100 1C deposition temperature did not degrade the electrical properties of GaN-LEDs. After the 200 1C deposition process, the Ti/Al/Ti/Au contact on n-GaN exhibited quasi-linear I–V characteristics, which illustrates that the 200 1C deposition temperature has little bad effects on the Ti/Al/Ti/Au contact. However, after the 300 1C deposition process, the metal contact showed nonlinear
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performances of GaN-LEDs. The light output of GaNLEDs improves greatly after the deposition of SiON antireflection passivation layer at 200 1C and is better than that of GaN-LEDs whose films are deposited at 100 and 300 1C. Moreover, the electrical properties of GaN-LED do not degrade at 100 and 200 1C, but degrade significantly at 300 1C. Acknowledgments
Fig. 5. The accelerated life test of two GaN-LEDs with SiON film deposited at 100 and 200 1C.
This work was sponsored by Beijing Municipal Science & Technology Commission (KZ200510005003), Funding Project for Academic Human Resources Development in Institutions of Higher Learning Under the Jurisdiction of Beijing Municipality (05002015200504) and National Basic Research Program of China (2006CB604902). References
I–V characteristics, which was probably due to the formation of rectifying Schottky contacts. Therefore, the results have verified that the Ti/Al/Ti/Au will deteriorate significantly after the deposition at 300 1C and the electrical properties of GaN-LEDs are strongly dependent on the deposition temperature. The accelerated life test was applied on two GaN-LEDs with SiON film deposited at 100 and 200 1C, respectively, and was performed at room temperature in the air. The driving current is 50 mA. These test conditions were held throughout the testing. In Fig. 5, the light output power was normalized. We can see that the characteristics shown by the GaN-LED with SiON film deposited at a temperature of 100 1C are similar to the one with SiON film deposited at 200 1C. 4. Conclusion In this paper, we have investigated the effects deposition temperature (from 100 to 300 1C) of oxynitride through plasma-enhanced chemical deposition (PECVD) on the electrical and
of the silicon vapor optical
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Journal of Luminescence 127 (2007) 441–445 www.elsevier.com/locate/jlumin
Effects of the passivation layer deposition temperature on the electrical and optical properties of GaN-based light-emitting diodes Guangdi Shen, Xiaoli Da, Xia Guo, Yanxu Zhu, Nanhui Niu Institute of Electronic Information and Control Engineering, Beijing Optoelectronic Technology Laboratory, Beijing University of Technology, Beijing 100022, China Received 6 July 2006; received in revised form 3 January 2007; accepted 7 February 2007 Available online 20 February 2007
Abstract In this paper, silicon oxynitride is deposited through plasma-enhanced chemical vapor deposition (PECVD) to serve as an antireflection passivation layer. We have studied the effects of the deposition temperature (from 100 to 300 1C) on the electrical and optical performances of GaN-LEDs. It is found that the light output of GaN-LEDs improves greatly after the deposition of SiON antireflection passivation layer at 200 1C and is better than that of GaN-LEDs whose layers are deposited at 100 and 300 1C. The electrical properties of GaN-LED do not degrade at 100 and 200 1C, but degrade significantly at 300 1C. r 2007 Elsevier B.V. All rights reserved. Keywords: Temperature; PECVD; Passivation layer; GaN-LEDs
1. Introduction Light-emitting diodes (LEDs) based on GaN materials are widely studied for applications of large-scale color displays and signal lights. In particular, they have become a focus in developing white LEDs for general illumination, taking advantage of their high-brightness feature [1,2]. There are two principal approaches to improve LED efficiency. One is to increase the internal quantum efficiency determined by crystal quality and epitaxial layer structure; the other is to increase the light extraction efficiency. For blue GaN-LED, the high internal quantum efficiency has already reached 70% [3], but there is still a great potential to improve the external quantum efficiency [4–7], which is limited by the large difference in the refractive index between the semiconductor and the air. For a conventional GaN-based LED, the critical angle [yc ¼ sin1 ðnair =nGaN Þ] for the light generated in the active region is determined by the refractive indexes of GaN (nGaN ) and the air (nair ), which are 2.5 and 1, respectively. In this case, it leads to a Corresponding author. Tel.: +86 10 67391641x835; fax: +86 10 67391641x822. E-mail address: [email protected] (X. Da).
0022-2313/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2007.02.002
critical angle of 231, and as a result, a very small amount of light is possible to escape from LEDs. Moreover, some of the light escaping from the LED will be reflected back into LED due to the high interfacial reflectivity between GaN materials and the air. Therefore, only a small portion of light can really escape from a GaN-LED. Neglecting the light emitted from sidewalls and backside, only about 4% of the internal light will be emitted from the surface [8]. The light that has not escaped from the surface is reflected back into the substrate and is then repeatedly reflected until it is finally reabsorbed by the active layer or the electrodes, unless it escapes through the sidewalls. In order to improve the external quantum efficiency, we designed an antireflection layer coated on the GaN-LEDs. The antireflection layer can decrease the interfacial reflectivity between the GaN and the air. Furthermore, the antireflection layer can also enlarge the critical angle for the light escaping. For example, after coating the antireflection layer with the refractive index of 1.54, the critical angle for the light escape from p-GaN surface can be enlarged to 381 and the critical angle for the light escaping from antireflection layer is 40.51, so that more light can escape from the surface of a LED after coating an antireflection layer on p-GaN.
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In this paper, the SiON antireflection passivation layers were deposited on GaN-LEDs through plasma-enhanced chemical vapor deposition (PECVD) at 100, 200 and 300 1C, respectively, to serve as antireflection layers. We have investigated the influence of the SiON deposition temperature on the electrical and optical performances of GaN-LEDs. The light output of GaN-LEDs improves greatly after the deposition of SiON antireflection passivation layer at 200 1C, and is better than that of GaN-LEDs whose layers are deposited at 100 and 300 1C. As for the electrical properties of GaN-LED, they do not degrade at 100 and 200 1C, but degrade significantly at 300 1C. 2. Experiments The GaN-LEDs epilayers are composed of a GaN nucleation layer, a Si-doped n-type GaN, a 20 nm InxGa1xN (x ¼ 0.08–0.1) strain relief layer, five periods of InxGa1xN/GaN multi-quantum well (MQW) layers, and a Mg-doped p-type GaN, grown in sequence on a sapphire (0 0 0 1) substrate through metal organic chemical vapor deposition (MOCVD) [9]. The carrier concentrations in the p-GaN and n-GaN layers are 5 1017 cm3 and 5 1018 cm3, respectively. In our experiments, the sample was annealed at 720 1C for 30 min in a nitrogen atmosphere to activate the Mg dopants in the p-GaN layer. To fabricate the LEDs, a mesa-type surface of the p-GaN layer had been etched by means of an induced coupled plasma (ICP) until the n-GaN layer was exposed for n-type ohmic contact formation. A Ni/Au (50 A˚/50 A˚) layer was deposited onto the p-GaN layer as a light-transmitting layer, and then was annealed at 500 1C for 1 min in a N2 and O2 mixture to achieve a good metal ohmic contact and a low absorption. A Ti/Al/Ti/Au (15 nm/100 nm/20 nm/200 nm) layer was deposited on the n-GaN layer to serve as an n-electrode. After the metal electrodes had been formed, the SiON passivation layer was deposited on these samples at 100, 200 and 300 1C, respectively. Lee et al. [10] have reported that the SiH4 and NH3 processes can passivate the Si doping in n-GaAs through the reaction: Siþ þ H ¼ ðSiHÞ0 , where most of the hydrogen in the SiH4/NH3 mixture originates from the ammonia [11]. Therefore, we designed deposition processes without ammonia. These processes used a mixture of silane diluted to 5% in nitrogen as the source of silicon, nitrous oxide as the source of oxygen, and highly purified nitrogen as the source of nitrogen. When the deposition temperature was 100 or 300 1C, the gas flows of SiH4, N2O and N2 were set to 200, 120 and 350 sccm, respectively. The working pressures of the two processes are 500 and 1000 mTorr. When the deposition temperature was 200 1C, the silicon oxynitride film was deposited at a working pressure of 500 mTorr, 200 sccm SiH4, 180 sccm N2O and 600 sccm N2. In order to get good uniformity and low stress of the film, we applied the dual frequency mode in these three deposition processes, where the high
frequency (HF) was 13.56 MHz with the power of 30 W and the low frequency (LF) was 100 kHz with the power of 40 W. The forward voltage and light output (LOP) were measured before and after the deposition of the passivation layer by the LED tester and each device was driven at 20 mA. The refractive index and thickness of the films were obtained by the variable angle spectroscopic ellipsometry. The current–voltage (I–V) curves of n–n electrode were measured using a precision semiconductor parameter analyzer (Agilent 4156). The tape pull test was conducted to evaluate the adhesion of the SiON films. The uniformity of GaN step profile capped with SiON film was represented by a SEM micrograph. The transmittance of the films was measured through the Hitachi spectrophotometer. 3. Results and discussion The refractive index and thickness of the films were obtained by the variable angle spectroscopic ellipsometry and are shown in Table 1. In order to evaluate the adhesion of the SiON films, the tape pull test was conducted by adhering white tape onto the SiON film side using FURUKAWA brand white tape. To this surface, one end of a strip of white single-coated tape was pressed firmly and then pulled away in the vertical direction using a smooth pull of 1 s duration. Each sample coated with SiON film was examined under 100 power OLYMPUS BX51M microscope to determine if any SiON had been removed. This method was taken as a practical measure of the adhesion strength. It is observed that none of the SiON films is peeled off from the samples. According to the results of the tape pull test, the SiON films on the three samples have shown good adhesion. Sapphire is a kind of insulating substrate, which is very difficult to lift off to expose the n-GaN for fabricating n-electron. We etched wafer from the p-GaN to the n-GaN layer through the method of ICP to form a mesa structure. Fig. 1 shows a SEM picture in which the sample is capped with a SiON layer. We can see that SiON film on the top step, bottom step and sidewall is very uniform and presents good step coverage properties. Tables 2–4 show the electrical and optical characteristics of GaN-LEDs before and after the deposition of SiON film at 100, 200 and 300 1C, respectively. The LED wafer was put on the probe station to obtain the characteristic distribution of the wafer before and after the deposition of SiON film. We drove 20 mA current in each device and measured the light output (relative light intensity) and Table 1 The refractive index and thickness of SiON film deposited at 100, 200 and 300 1C in PECVD Deposition temperature (1C)
Refractive index
Thickness (nm)
100 200 300
1.5385 1.5426 1.5926
229.1 218.4 189.5
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Table 4 The optical and electrical characteristics of a GaN-LED wafer before and after the deposition of the SiON layer at 300 1C
Fig. 1. A SEM picture of GaN step profile capped with SiON film.
Table 2 The optical and electrical characteristics of a GaN-LED wafer before and after the deposition of the SiON layer at 100 1C Measurement items
Value range
Before passivation (%)
After passivation (%)
Light output
0.20–0.25 0.25–0.30 0.3–0.35
5.89 88.84 0
0.42 6.93 87.39
Forward voltage (V)
3.1–3.2
85.37
92.33
3.2–3.3
9.16
2
Table 3 The optical and electrical characteristics of a GaN-LED wafer before and after the deposition of the SiON layer at 200 1C Measurement items
Value range
Before passivation (%)
After passivation (%)
Light output
0.18–0.20 0.20–0.22 0.26–0.28 0.28–0.30
73.06 15.58 0 0
0 0.39 75.39 12.93
Forward voltage (V)
3.1–3.2
93.71
23.76
3.2–3.3 3.3–3.4
5.45 0.08
68.96 5.57
forward voltage (Vf). The numbers in the three tables are the data of light output (relative light intensity) and forward voltage. From Table 2, we can see that the light output has been improved dramatically. After the
Measurement items
Value range
Before passivation (%)
After passivation (%)
Light output
0.1–0.12 0.12–0.14 0.14–0.16 0.16–0.18
6.71 38.43 48.14 0
0.95 0.96 28.73 50.63
Forward voltage (V)
3.5–3.6
41.74
3.6–3.7 3.7–3.8 4–5 5–6 6–7
25.72 16.94 0 0 0
0 0 0 48.63 15.3 19.62
deposition of a 229.1 nm SiON layer with a refractive index of 1.5385, the percentage of light output in the range 0.3–0.35 increased greatly up to 87.39%. For example, the light output of a chip has increased from 0.3 to 0.338, an increase of 12.6% after the passivation. From Table 3, we can see the optical characteristic has been improved dramatically after coating a 218.4 nm SiON layer with a refractive index of 1.5426. Moreover, the antireflection effect of SiON film is better than that of the SiON films deposited at 100 1C. Before passivation, the range of light output is 0.18–0.22, and it was enhanced to 0.26–0.30 after the deposition of SiON antireflection passivation layer at 200 1C. For example, the light output of a chip increased from 0.245 to 0.336, an increase of 37.1% after the passivation. From Table 4, we can see the antireflection effect is still obvious. After the deposition of a 189.5 nm SiON layer with a refractive index of 1.5926 at 300 1C, the percentage of light output in the range 0.16–0.18 increased greatly from 0 to 50.63%. For example, the light output of a chip increased from 0.168 to 0.193, an increase of 14.8% after the passivation. In order to further investigate the relationship between the deposition temperature and the light output, we measured the different etched rates of the SiON films, which are 960 nm/min at 100 1C, 344 nm/min at 200 1C and 600 nm/min at 300 1C, respectively. This result indicates that the SiON film deposited at 200 1C is more compact than those deposited at 100 and 300 1C. We measured the transmittance of the SiON deposited at 100 and 200 1C to verify the relationship between the densification and the transmittance. From Fig. 2, we can see the transmittance of the SiON film deposited at 100 and 200 1C was 91.5% and 91.7%, respectively. This result means that although the densifications of the two films were quite different, their transmittance rates for light were close. The ideal single antireflective layer is designed to ensure that the optical thickness of the film is odd times of a quarter of the wavelength and the refractive index is the extraction root of the refractive index of GaN. Thickness and the refractive
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Fig. 2. The transmittance of the glass substrate and the SiON film fabricated at 100 and 200 1C, respectively.
index are the factors influencing the effects of the antireflection layer. In our previous investigation [12], the SiON passivation layer with the refractive index of 1.54 had good antireflection effect for GaN-LEDs. This has clearly illustrated the results that the light output of GaNLED improves greatly after depositing a SiON passivation layer at 200 1C. Comparing the electrical properties given in these three tables, it is obvious that the electrical properties of the GaN-LED degrade, when the deposition temperature increases. As shown in Table 4, when the deposition temperature was enhanced to 300 1C, the electrical and optical characteristics of a GaN-LED wafer degraded significantly, where the range of forward voltage increased from 3.5–3.8 V to 4–7 V. However, the forward voltage of LED increased slightly after the 200 1C deposition process, where the percentage in the range 3.2–3.3 V increased from 5.45% to 68.96% (as shown in Table 3). After the passivation of the chip at 100 1C, the percentage of the forward voltage (Vf) in the range 3.2–3.3 V reduced obviously. The forward electrical characteristics of GaNLEDs improved after the 100 1C SiON deposition process. We observed the GaN-LED chips under the microscope (Olympus BX51 M). Three samples are shown in Fig. 3. Samples (a), (b) and (c) are treated by the 100, 200 and 300 1C deposition processes of SiON film, respectively. From Fig. 3, no obvious change was found in the morphology of the Ni/Au contact layer. However, comparing these three pictures in Fig. 3, we found that the morphology of the n-electrode (Ti/Al/Ti/Au) surface changed after the 300 1C deposition process. The surface of n-electrode became rough and the color changed. It may result from the degradation of Al. It has been reported that at high temperature, Al of the metal contact melts and tends to ball up, which, in turn, results in rough surfaces and increased ohmic contact resistances [13,14]. Fig. 4 shows current–voltage (I–V) characteristics of Ti/Al/Ti/Au contact on n-type GaN, with SiON films deposited at 100, 200 and 300 1C. After the 100 1C
Fig. 3. The microscope photograph of part of GaN-LED chip after the deposition of the SiON antireflection layer at (a) 100 1C, (b) 200 1C and (c) 300 1C.
Fig. 4. I–V characteristics of Ti/Al/Ti/Au contact on n-type GaN at 100, 200 and 300 1C.
deposition process, the Ti/Al/Ti/Au contact on n-GaN exhibited linear I–V characteristics, which indicates that the 100 1C deposition temperature did not degrade the electrical properties of GaN-LEDs. After the 200 1C deposition process, the Ti/Al/Ti/Au contact on n-GaN exhibited quasi-linear I–V characteristics, which illustrates that the 200 1C deposition temperature has little bad effects on the Ti/Al/Ti/Au contact. However, after the 300 1C deposition process, the metal contact showed nonlinear
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performances of GaN-LEDs. The light output of GaNLEDs improves greatly after the deposition of SiON antireflection passivation layer at 200 1C and is better than that of GaN-LEDs whose films are deposited at 100 and 300 1C. Moreover, the electrical properties of GaN-LED do not degrade at 100 and 200 1C, but degrade significantly at 300 1C. Acknowledgments
Fig. 5. The accelerated life test of two GaN-LEDs with SiON film deposited at 100 and 200 1C.
This work was sponsored by Beijing Municipal Science & Technology Commission (KZ200510005003), Funding Project for Academic Human Resources Development in Institutions of Higher Learning Under the Jurisdiction of Beijing Municipality (05002015200504) and National Basic Research Program of China (2006CB604902). References
I–V characteristics, which was probably due to the formation of rectifying Schottky contacts. Therefore, the results have verified that the Ti/Al/Ti/Au will deteriorate significantly after the deposition at 300 1C and the electrical properties of GaN-LEDs are strongly dependent on the deposition temperature. The accelerated life test was applied on two GaN-LEDs with SiON film deposited at 100 and 200 1C, respectively, and was performed at room temperature in the air. The driving current is 50 mA. These test conditions were held throughout the testing. In Fig. 5, the light output power was normalized. We can see that the characteristics shown by the GaN-LED with SiON film deposited at a temperature of 100 1C are similar to the one with SiON film deposited at 200 1C. 4. Conclusion In this paper, we have investigated the effects deposition temperature (from 100 to 300 1C) of oxynitride through plasma-enhanced chemical deposition (PECVD) on the electrical and
of the silicon vapor optical
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