2010 International Conference on Information, Networking and Automation (ICINA)

Design and Properties Analysis of a Single-chip White InGaN/GaN LED

Fu Xiansong*, Niu Pingjuan, Xu Wen cui Engineering Research Center of High Power Solid State

Lighting Application System, Ministry of Education, School of Information and Communication Engineering,

Tianjin Polytechnic University, Tianjin, P. R. China, 300160

e-mail: fuxiansong@tjpu.edu.cn

Abstract-LED can not only improve the quality and efficiency of light, but also save energy and reduce environmental pollution. Developing LED industry has great significance for the national economy and the people's livelihood. In the paper, main methods for fabricating white LED and recent progresses in white LED are studied. And then, a novel single-chip white InGaN/GaN LED is designed. Fanally, the I-V characteristics and optical characteristics of the LEDs are analyzed. Turn-on voltage of the single-chip white light LED is 2.7V and reverse leakage current is O.18mA at reverse-bias voltage of lSV.The single-chip white LED emits white light as the injection current is from SmA to 6OmA.

Keywords- single-chip white LED; metal-organic vapor phase epitaxy(MOCVD); quantum dots; quantum wells

1. INTRODUCTION

The emergency of white LED is an important milestone that LED is from identifying function to lumination function. Compared with a conventional flashlight that uses an incandescent lamp, the LED has several advantages, such as long lifetime, low power dissipation, light weight and small size, but it also has several shortages such as low luminous flux, high cost and so on. With the improvement of luminous efficiency and luminous flux, LED has been widely used in the field of scientific research and production. Especially in recent years, the successful development of high luminous efficiency and high brightness white LED make the application of LED possible. The white LED was recognized the promising new production in LED industry. It will replace incandescent, fluorescent and high-pressure gas discharge lamp and become the fourth generation of solid light source in 21 st century.

LED is a solid-state semiconductor devices, which can be directly converted electricity to light. LED is the heart of a semiconductor chip. Semiconductor chip is composed of two parts, one part is the P-type semiconductor, the hole­dominated inside, the other part is the N-type semiconductor, and here is mainly electron. At forward biased, the electron inject from N region to P region, the hole inject from P region to N region, the hole with electronic composite, and then to be issued in the form of photon energy, and this is the principle of LED luminescence. Assume that luminescence occur in the P region, the injected electrons and valence band holes compound directly, or the injected electrons are captured by luminescence center and then compound with

Tian Haitao, Chen Hong* Beijing National Laboratory of Condensed Matter, Institute of Physics, Chinese Academy of Sciences,

Beijing, P. R. China, 100080 e-mail: haitaot@yahoo.com.cn

the holes. Some electrons are captured by non-luminescence center and then compound with the holes. The released energy is not enough to form the visible light. The greater the ratio which the amount of recombination luminescence compare with the non-recombination luminescence is, the higher the photo quantum efficiency is. The luminescence principle of LED is shown in Fig.l.

.----------1 II 1--+------, Conduction band Luminescence

Electron injection center

• • • •

o 0 I I N region I I I

�

Hole injection

Luminescence �""""'--"'--10 0 0 0

I I Valence band I I P region

Junction region I

Figure 1. Luminescence principle of LED

II. IMPLEMENTA TION METHOD OF WHITE LED

In the early 90, the use of MOCVD epitaxial material and multiple quantum wells made GaN based blue and green LED achieved a historic breakthrough. And its brightness was caught up with the red LED. It forms red, green and blue trichromatic system completely. White LED was originally realized by closely packaging together a red, green, blue (R, G, B) triple of such LED chips based on AlGaAs or AlGaInP/GaAs, GaP:N, and InGaN, respectively. Red, green and blue LED is packed in one tube and the luminous efficiency can reach 30lm/W [1]. The luminous efficiency is high and the color rendering is well. But the shortages of this method are the complex driving circuit of the LED and the higher cost of the trichromatic three-chip-based. As temperature increase the optical parameters of red, green and

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2010 International Conference on Information, Networking and Automation (ICINA)

blue LED change in different, so the output and peak wavelength of this white LED is very sensitive to the variation of temperature, time and injection current and the white LED is unstable.

The most commonly used method to achieve white LEOs is to use GaN-based blue LED chip in combination with phosphor, embedded into epoxy resin as a light converter. The material in which the Ce emission has been used in commercialized applications is Y3Als012:Ce

3+ or short written as Y AG:Ce, a garnet structure. This phosphor has been used in discharge lamps to convert the 435-nm emission into yellow as a so-called color correction material [2]. This luminous method has high luminous efficiency, simple preparation, high-temperature stability and good color rendering. However, color changes with the angle and the coherence of light is poor. Obviously, according to the principle of colourometry, blue light exciting red or green phosphor can also produce white light.

Near-UV or violet LED exciting R, G, B phosphors can also produce white light. The R, G, B phosphor combination mostly used for the preparation of tricolor white blends contains an earth-alkali sulfide (R), an earth-alkali thiogallate (G) and BaMgAllO017(B). All these three materials are europium doped, the luminescent center being Eu2+ [3]. It has high color rendering and simple preparation method. But the luminous efficiency and temperature stability are low and the UV light is easily missed.

Light source classification of white LED is shown in Table 1 [4].

TABLE!. THE LIGHT SOURCE CLASSIFICATION OF WHITE LED

Method Ught Source Luminescent Material Illustration

Blue LED InGaNIY AG phosphor 15-201m/W

Blue LED lnGaNIYAG phosphor blue light exciting red, green

Single-chip and blue phosphor

Blue LED ZnSe epitaxial layer produce blue light,

exciting substrate produce yellow lig

UV LED InGaN/phosphor UV light exciting VAG phosphor

Blue LED+Ycl1ow/Grccn InGaN/GaP complementary color produce

Double-chip LED white light

Blue LED+Ycllow InGaN/AIGaln? complementary color produce

LED white light

Trip-chip Blue LED+Grccn

LED+Rcd LED InGaN/AIGalnP 301m!W

III. A NOVEL SINGLE-CHIP WHITE LIGHT-EMITTING

INGAN/GAN DIODE

The samples were grown on (0001 )-oriented sapphire substrates by using low-pressure metal-organic vapor phase epitaxy(MOCVD). The used indium, gallium, and nitrogen sources were trimethylindium (TMI), trimethylgallium, and highly purified ammonia (NH3), respectively. The sources for p-type and n-type dopings were bicyclopentadienyl magnesium (Cp2Mg) and silane (SiH4), respectively. After a 30nm thick GaN nucleation layer was deposited on the sapphire substrate at 520°C, a 311m thick Si-doped GaN layer was grown at 1040 °C with a doping density of 3xlQ

18 cm-2, followed by a 220nm thick InGaN UL grown at 845°C. Then an active region of four period InGaN(3nm)!GaN(14nm) multi-quantum wells (MQWs) was deposited at 714°C and

814°C, respectively, for InGaN and GaN layers. At last, a 0.211m thick Mg-doped p-GaN was grown at 920 °C. The nominal Indium content in the InGaN underlying layer(UL) and quantum wells are 0.05 and 0.17, respectively. The LED chips were fabricated by photolithography and etching, with sizes of 0.3xO.3 mm2. TiAI and NiAu alloy are used for n­and p-type Ohmic contacts, respectively [5-6]. The structural diagram of single-chip white InGaN/GaN LED is shown in Fig.2.

Mg-doped p-GaN

GaN isolated layer

4xlnGaN/GaN quantum-wells

InGaN unerlying layer UL

Si-doped n-GaN

GaN buffer layer

(0001 )-oriented sapphire substrate

Figure 2. The structural diagram of single-chip white InGaN/GaN LED

The single-chip white LED emits white light as the driving current is from 5mA to 60mA. The output power of the white LED is 24.4mcd at the driving current of 60mA [4].

IV. PROPERTIES ANALYSIS OF SINGLE-CHIP WHITE

INGAN/GAN LED

LED is not only the semiconductor device, but also the luminescent device. Consequently, electrical characteristic and optical characteristic of the single-chip white LED must be studied. And then the related characteristic parameters are measured.

A. Test and Analysis on Electrical Characteristic of Single-chip White LED

The relationship of the diode current I to the diode voltage V is the diode Volt-ampere characteristic, namely I­V characteristic. 1-V characteristic is the significant characteristic of the diode, and the core of LED is pn­junction. Therefore the I-V characteristic curve ofLEDs and 1-V characteristic curve of ordinary diodes are similar. I-V characteristic of the single-chip white LED is tested by the method of combining the probe station and the semiconductor graphic instrument. The results are shown in the Fig. 3.

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2010 International Conference on Information, Networking and Automation (ICINA)

Voltage (V)

t 2 3 � Vottage (V)

(a) I-V curve at forward-bias (b) I-V curve at reverse-bias

Figure 3. I-V characteristics curve of the single-chip white LED

As shown in the Fig.3(a), the forward-bias 1-V characteristic curve, tum-on voltage of the single-chip white light LED is 2.7V, and the operating current is 20mA at forward-bias voltage of 3.5V. As shown in the Fig. 3 (b), the reverse-bias I-V characteristic curve, reverse leakage current is 0.18mA at reverse-bias voltage of 15V. Tum-on voltage of the LED is small, and the LEDs work stably. It is concluded that the electrical parameters of the single-chip white LED is consistent with those of blue LED and green LED in the marked. The single-chip white LED need not change the existing LED the application standard.

B. Test and Analysis on Optical Characteristic of Single­chip White LED

Photoluminescence (PL) is the phenomenon that material can emit the light under the illumination stimulation, and it is commonly used methods to measure the properties of semiconductor material.

In the paper, the PMS50 ultraviolet - visible light - near­infrared spectral analysis system, which made by the Hangzhou EVERFINE photo-electrical instrument corporation, is used for LED device photometric and colorimetric measurement. The test system may provide to 200nm�800nm (from ultraviolet ray to visible light) spectral analysis function; Or it provides 380nm� 11 OOnm (from visible light to near-infrared light) the spectral analysis function. The test results conforms to the eIE standard completely. And it can be used to test the luminous powder spectrum, LED active photoelectric devices spectrum and so on.

Fig. 4 shows the spectral distribution graph of the single­chip white LED at injection currents of 1 rnA, 5mA, 20mA and 60mA respectively.

(a) At injection current of ImA

(b) At injection current of SmA

(c) At injection current of 20m A

(d) At injection current of 60mA

Figure 4. The spectral distribution graph of the single-chip white LED at injection currents of ImA, 5mA, 20mA and 60mA respectively

At very low injection current, such as ImA, a yellow emission peak about 575nm dominates spectral distribution graph. When the injection current increases to 5mA, blue light peaked about 430nm appears, and its intensity increases rapidly with the increasing of injection current. When the injection current is 20mA, the intensity of blue light is greater than that of yellow light. For injection currents from 20 to 60mA, the intensity ratio of the blue light to the yellow light is almost constant, which is important for solid state lighting because it overcomes the problem of color shift with increasing injection current, as commonly occurred in phosphor-converted white LED [5-6].

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2010 International Conference on Information, Networking and Automation (ICINA)

V. CONCLUDES

Base on the (0001 )-oriented sapphire substrates, a single­chip white LED designed in the paper. The 1-V characteristics and optical characteristics of the LEDs are analyzed. Tum-on voltage of the single-chip white light LED is 2.7V and reverse leakage current is 0.18mA at reverse­bias voltage of lSV.The single-chip white LED emits white light as the current is from SmA to 60mA.

ACKNOWLEDGMENT

This research is supported by Natural Science Foundation of Tianjin (08JCZDJC241 00).

REFERENCES

[1) Subramanian MutlJu, Sehuunnans F J, Pashley M D. "Red, green, and blue LED based white light generation: Issues and control," Industry Applications Conference, 2002. 37th lAS Annual Meeting, 2002, pp. 13-18.

[2) Regina M. M, Mueller G. 0, Krames M. R, et al. "High-power phosphor-converted light-emitting diodes based on ill -Nitrides," IEEE J Select Topics Quantum Electron, vol. 8, pp. 339-345, February, 2002.

[3) U. Kaufmann, M. Kunzer, K. Kohler. "Single Chip White LEOs," phys. stat sol. (a), vol. 192, pp. 246-253, February, 2002.

[4) Fu Xiansong, Xu Wencui, Niu Pingjuan, et al. "Design of a Single­chip White Light-emitting InGaN/GaN Diode," 2010 International Conference on Mechanical and Electrical Technology, September, 2010, pp. 761-764.

[5) X H. Wang, L. W Guo,H. Q Jia, Z. G. Xing, Y. Wang, X J Pei, J M. Zhou, and H. Chen, "Control performance of a single-chip white light emitting diode by adjusting strain in InGaN underlying layer," Applied Physics Letters, Volume 94, March, 2009.

[6) X H. Wang, H. Q Jia, L. W Guo, Z G. Xing, Y. Wang, X J Pei, J M. Zhou, and H. Chen, "White light-emitting diodes based on a single InGaN emission layer," Applied Physics Letters, Volume 91, October, 2009.

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