Advances of AlGaN-based High-Efficiency deep-UV LEDs

Hideki Hirayama

RIKEN, 2-1, Hirosawa, Wako, Saitama, 351-0198, Japan

Abstract We demonstrated AlGaN-based multi-quantum-well (MQW) deep-ultraviolet (UV) light-emitting diodes (LEDs) with wavelengths in the range of 222-351 nm, fabricated on low threading dislocation density (TDD) AlN template on sapphire. A high internal quantum efficiency (IQE) of 50-80% was achieved from Al-GaN or quaternary InAlGaN MQWs by fabricating them on low TDD AlN templates. Also, an electron injec-tion efficiency (EIE) was markedly improved by using multi-quantum barrier (MQB). Over 20 mW cw output power was obtained for 256-275 nm LEDs, which will be useful for sterilization applications. The maximum external quantum efficiencies (EQEs) were 1.8 and 2.75% for 247 and 270 nm AlGaN-LEDs, respectively.

Introduction Because of their wide direct transition energy

range in UV, which is between 6.2 eV (AlN) and 3.4 eV (GaN), AlGaN and quaternary InAlGaN are at-tracting considerable attention as candidate materi-als for the realization of deep ultraviolet (DUV) light-emitting diodes (LEDs) or laser diodes (LDs)[1]. DUV LEDs and LDs with emission wavelengths in the range of 220-350 nm have a lot of potential ap-plications, such as, sterilization, water purification, medicine and biochemistry, light sources for high density optical recording, white light illumination, fluorescence analytical systems and related informa-tion sensing fields, air purification equipment, and zero-emission automobiles.

In this report, I will mention about basic tech-niques for obtaining high-efficiency AlGaN DUV-LEDs and demonstrate advances of DUV-LEDs fabricated on sapphire substrates [1-6].

Low-TDD AlN on Sapphire

The samples were grown on sapphire (0001) substrates by low-pressure metal-organic chemical vapor deposition (LP-MOCVD). In order to obtain low threading dislocation density (TDD), crack free AlN template with atomically flat surface, we used NH3 pulse-flow multi-layer (ML) growth method.

Figure 1 shows gas flow sequence and sche-matic growth control image used for NH3 pulse-flow ML-AlN growth technique. The edge- and screw-type dislocation densities of ML-AlN were lower than 5×108 and 4×107 cm-2, respectively, as observed from the cross-sectional transmission electron mi-croscope (TEM) image.

We observed remarkable enhancement of DUV emission of AlGaN-QWs by fabricating them on the high-quality AlN template [2]. The PL intensity was increased by more than 100 times by reducing the XRC (10-12) FWHM from 1400 to 300 arcsec. The internal quantum efficiency (IQE) at room tempera-ture were estimated to be 30-50% and over 80% for 280 nm-band emission AlGaN and InAlGaN MQW, respectively, from the temperature dependence of integrated PL intensity [4].

Fig. 1 Gas flow sequence and schematic growth control image used for NH3 pulse-flow multilayer (ML)-AlN growth technique. 222-351 nm AlGaN-based DUV LEDs fab-

ricated on low TDD AlN Templates Figure 2 shows a schematic of the sample struc-

ture and emission image of an AlGaN MQW DUV LED fabricated on a ML-AlN template. We used a thin (1.3-1.7 nm-thick) QW in order to obtain a high IQE by suppressing the effects of the polarization field in the well. Ni/Au electrodes were used for both n-type and p-type electrodes. The size of the p-type electrode was 300×300 µm.

Figure 3 shows electroluminescence (EL) spectra of the fabricated AlGaN and InAlGaN-MQW LEDs. Single-peaked operations were obtained for every sample.

The efficiency of the AlGaN DUV-LED is not yet so high in comparison with that of blue LEDs, due to low electron injection efficiency (EIE) into the QW (10-30％) by the electron leakage caused by low hole concentrations in the p-type AlGaN layers, as well as by inferior light extraction efficiencies (lower than 8%) due to strong UV absorption in the p-side electrodes. We have succeeded in increasing EIE from approximately 20 to over 80 % by introducing multi-quantum barrier (MQB) electron-blocking

1. Growth of nucleation AlN layer (NH3 Pulse-flow)

2. Burying growth with lateral enhancement growth mode (NH3 pulse-flow)

NH3 se-flow growth ●Migration Enhance Epitaxy ●Al rich condition= stable Al (+c) polarity

3. Reduction of surface roughness with high-speed growth (continuous flow)

pul

TMAlNH3

Reduction of threading dislocation density (TDD)

4. Repeat 2 and 3

Crack-free thick AlN buffer with atomically flat surface

5s 3s 5s 3s 5s

TMAlNH3

5s 3s 5s 3s 5s

Sapphire Sapphire Sapphire Sapphire

AlN

AlN

AlNAlN

641978-1-4244-7113-3/10/$26.00 ©2010 IEEE

200 300 400Wavelength (nm)

0 50 1000

10

20

0

1

2

3

Current I (mA)

Out

put P

ower

(mW

)

EQE

(%)

layer (EBL). The output power was increased from 2.2 to 15 mW for 250 nm AlGaN LED by in-troducing MQB-EBL.

Fig. 2 Schematic sample structure and emission images of an AlGaN DUV LED. Fig. 3 EL spectra of AlGaN and InAlGaN-MQW LEDs.

Figure 4 shows I-L and EQE characteristics

for a 270 nm AlGaN-QW LED, measured under room temperature (RT) cw operation. The maxi-mum EQE obtained was 1.8 and 2.75 % for 247 and 270 nm AlGaN LEDs, respectively. We also obtained the maximum output power of over 20 mW for 256-275 nm LED under RT cw operation.

Max. EQE 2.75%

Al0.87Ga0.13N;Mg

ML-AlN Buffer(NH3 Pulse-Flow Method)

n-Al0.87Ga0.13N;Si

Ni/Au Electrode

GaN;Mg

Sapphire Sub.

Ni/Au

UV Output

Al0.79Ga0.21N/Al0.87Ga0.13N 3-layer MQW

Al0.98Ga0.02N;MgE-Blocking Layer

λ=270nm

AlGaN-LED

with MQB

Fig. 4 I-L and I-EQE characteristics for a 270 nm Al-GaN-QW DUV LED.

Figure 5 summarizes maximum output power of AlGaN and InAlGaN based DUV-LEDs fabricated on low TDD ML-AlN template achieved by our group. The output power of the 220-280 nm-band Al-GaN-based LEDs was dramatically increased in the past 3 years, by reducing the TDD of AlN template on sapphire, and by introducing MQB EBLs. We ob-tained 15-22 mW cw power LEDs with wavelength between 250-270 nm, which is directly useful for sterilization application.

200 220 240 260 280 30010-5

10-4

10-3

10-2

10-1

1

10

102

Max

. Out

put P

ower

(mW

)

Wavelength (nm)

NTT210nm0.02μW

Fig. 5 Output power of AlGaN-based DUV-LEDs for the various development stages obtained in our group.

CONCLUSIONS

We demonstrated AlGaN-based MQW DUV-LEDs with wavelengths in the range of 222-351 nm. A high-IQE of 50-80% was achieved from AlGaN or quaternary InAlGaN MQWs grown on low TDD AlN templates. Also, an EIE was markedly improved by using MQB-EBLs. Over 20 mW cw output power was obtained for 256-275 nm LEDs. The maximum EQEs were 1.8 and 2.75% for 247 and 270 nm Al-GaN-LEDs, respectively.

REFERENCES

[1] H. Hirayama, J. Appl. Phys. 97, 091101 1-19 (2005). [2] H. Hirayama, T. Yatabe, N. Noguchi, T. Ohashi, and

N. Kamata: Appl. Phys. Lett. 91, 071901 (2007). [3] H. Hirayama, N. Noguchi, T. Yatabe, and N. Kamata,

Appl. Phys. Express, 1, 051101 (2008). [4] H. Hirayama, S. Fujikawa, N. Noguchi, J. Norimatsu,,

T. Takano, K. Tsubaki, and N. Kamata, Phys. Stat. Sol. (a) 206, 1176 (2009).

[5] H. Hirayama, N. Noguchi, and N. Kamata, Appl. Phys. Express, 3, 032102 (2010).

[6] H. Hirayama, Y. Tsukada, N. Maeda, and N. Kamata, Appl. Phys. Express, 3, 031002 (2010).

200 250 300 350 400 450Wavelength (nm)

Nor

mal

ized

Inte

nsity

AlGaN-QWDUV LEDs

Measured at RT

222nm Pulsed227nm Pulsed234nm CW240nm CW248nm CW255nm CW261nm CW

InAlGaN-QWDUV LED282nm CW342nm CW351nm CW

TDD: 3～7×108cm-2

Using High EBL

Using MQB

Sterilization Wavelength

TDD:3×109cm-2

TDD>2×1010cm-2

Emission is weak、no single peak.

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