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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 13 (2010) 180–184

1369-80

doi:10.1

n Corr

E-m

hskim7

journal homepage: www.elsevier.com/locate/mssp

Electrical and optical characterization of GaN-based light-emittingdiodes fabricated with top-emission and flip-chip structures

Hyunsoo Kim a,n, Sung-Nam Lee b, Jaehee Cho c

a School of Semiconductor and Chemical Engineering, Semiconductor Physics Research Center, Chonbuk National University, Jeonju 561-756, Republic of Koreab Department of Nano-Optical Engineering, Korea Polytechnic University, Siheung 429-793, Republic of Koreac School Department of Physics, Applied Physics and Astronomy, Department of Electrical, Computer, and System Engineering, and Future Chips Constellation,

Rensselaer Polytechnic Institute, Troy, NY 12180, USA

a r t i c l e i n f o

Available online 18 November 2010

Keywords:

GaN

Light emitting diode

Flip-chip

Top-emission

Forward voltage

Light extraction

01/$ - see front matter & 2010 Elsevier Ltd. A

016/j.mssp.2010.10.008

esponding author.

ail addresses: hskim7@jbnu.ac.kr,

@chonbuk.ac.kr (H. Kim).

a b s t r a c t

We investigated the electrical and optical characteristics of GaN-based light-emitting

diodes (LEDs) fabricated with top-emission and flip-chip structures. Compared with top-

emission LEDs, flip-chip LEDs exhibited a 0.25 V smaller forward voltage and an 8.7 Olower diode resistance. The light output power of the flip-chip LED was also larger than

that of the top-emission LED by factors of 1.72 and 2.0 when measured before and after

packaging, respectively. The improved electrical and optical output performances of flip-

chip LEDs were quantitatively analyzed in terms of device resistance and ray optics,

respectively.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

GaN-based light-emitting diodes (LEDs) have beenextensively investigated due to their applicability to var-ious applications including full-color indicators, full-colordisplays, back-light units (BLUs) for liquid crystal displays(LCDs), and next-generation solid-state lighting [1–11].Further, the recent market for LCD BLUs has attracted agreat deal of attention with respect to potential LEDapplications. For applications involving LCD BLUs, mostLED chips currently use top-emission structures (TELEDs),in which the top transparent-electrode is deposited onto ap-layer acting as a current-spreader as well as a light-transmitting p-contact [12]. However, as LED applicationsextend to solid-state lighting, it has become essential toadopt high-power chip structures such as the flip-chipconfiguration (FCLED), in which a thick reflective metallayer is deposited on the p-layer and the chip is inverted bymeans of the flip-chip bonding to a Si submount [5–7]. It is

ll rights reserved.

generally acknowledged that FCLED has several advantagesover TELED. Specifically, the level of current injection inFCLED can be increased by the use of a thick p-electrodewith negligible sheet resistance and efficient heat dissipa-tion through the Si submount. Second, light extraction canbe more enhanced in the FCLED configuration, since theemitted light can be extracted through a sapphire substrateacting as a graded refractive index medium [13].

In order to properly develop both TELED and FCLEDstructures for specific applications, it is important toaccurately understand the characteristics of both of theseLED structures. In this study, we systematically investi-gated the electrical and optical output performances ofTELED and FCLED structures. The relationship betweenforward voltage and diode series resistance was analyzedbased on the measured forward current versus voltagecurves. The light output power of both LED types wasobtained in the on-wafer probing configuration and wasanalyzed and compared with data obtained from a ray-tracing simulation. To understand the effect of lightextraction with respect to chip structure after packaging,the output powers of both LEDs were evaluated after a5 mm-indicator-lamp-packaging. We found that the elec-trical and optical output characteristics of FCLEDs were

H. Kim et al. / Materials Science in Semiconductor Processing 13 (2010) 180–184 181

highly superior to those of TELEDs. Specifically, FCLED andTELED had forward voltages of 3.26 and 3.51 V, dioderesistances of 6.6 and 15.3 O, and output powers of 4.6 and9.2 mW, respectively.

2. Device fabrication and characterization

Epilayer structures of the LED wafers evaluated in thisstudy consisted of a 3.4 mm-thick n-GaN layer, a GaN/InGaNmultiple quantum well with five periods, a p-AlGaN electronblocking layer and a 0.20 mm-thick p-GaN layer. For devicefabrication, the p-layer was first partially dry-etched by 0.8 mmto expose the n-layer using an inductively coupled plasma(ICP) etching system. Using an e-beam evaporator, a Ti/Al/Ti(30 nm/200 nm/30 nm) scheme was deposited onto the newlyexposed n-layer and then thermally annealed at 600 1C for 30 sin N2 ambient to form an ohmic contact. Next, Pd/Au (2 nm/3 nm) and Pd/Ag (2 nm/100 nm) schemes were evaporatedonto the p-layer to form a p-electrode for the TELED and FCLED,respectively. In addition, 100 nm-thick Pd and 100 nm-Aglayers were also deposited onto the p-layer as referencep-electrodes for the FCLED. To evaluate the effect of thep-electrode on LED performance, the specific contact resis-tances and reflectivities of all p-electrodes were evaluated

10-2100 nm

Pd/Au10-4

10-3

2 nm/100 nm100 nm2 nm/3 nm

Spe

cific

con

tact

resi

stnc

e (Ω

cm2 )

p-electrode scheme

Wavelength (nm)

0.6

0.8

1.0

Pd/Ag (2 nm/100 nm)

Ag (100 nm)

4000.0

0.2

0.4

Pd/Au (2 nm/3 nm)

Pd (100 nm)

Ref

lect

iity

Pd Pd/Ag Ag

450 500 550 600

Fig. 1. (a) Specific contact resistances and (b) reflectivity of the

p-electrode in different layer schemes.

using transfer length model (TLM) [14] patterns and a UV/VISspectrometer, respectively (Fig. 1(a) and (b)). It is noted that,for the measurement specific contact resistances, the totalresistance of adjacent TLM pads was determined using thecurrent measured at a low voltage of 0.1 V. Lastly, a Cr/Au(30 nm/500 nm) was deposited onto both n- and p-electrodesas a bonding pad. The electrical and optical characteristics ofthe LEDs were measured using an on-wafer testing config-uration, which consisted of a parameter analyzer (HP4155A)and a Si photodiode mounted on the bottom sides of LEDwafers (inset of Fig. 3(a) and (b)). The chip dimension was300�300 mm2 and the peak wavelength of the LEDs mea-sured at an injection current of 20 mA was around 392 nm. Toevaluate the actual output power, both TELED and FCLED LEDswere packaged with a 5 mm-indicator-lamp, in which the LEDchip flip-bonded to a Si submount with AuSn eutectic bondingwas die-bonded for FCLED packaging, while the LED chip wasdirectly die-bonded for TELED packaging. It is noted thatthe lead frame used in this package is coated with Ag,exhibiting a high reflectivity over 90% at the wavelength of400 nm. It is also noted that, to use high reflectivity of leadframe, a transparent UV-epoxy was used for die-bonding. After

0.03

0.04

0.05

TELED

10

0.00

0.01

0.02I (A

)

FCLED (Pd/Ag) FCLED (Pd) FCLED (Ag)

0.5

0.6

0.7

0.8

TELEDslope=15.3 Ω

V (V)

0.0

0.1

0.2

0.3

0.4 FCLED (Pd/Ag)slope=6.6 Ω

experimental fitted

I(dV

/dI)

0.00I (A)

0.02 0.04 0.06 0.08 0.10

2 3 4

Fig. 2. (a) Forward current versus voltage (I�V) characteristics of FCLEDs

fabricated with Pd, Pd/Ag, and Ag p-electrodes along with TELED with

Pd/Au and (b) replotted I(dV/dI)–I curves of FCLED (Pd/Ag) and TELED.

H. Kim et al. / Materials Science in Semiconductor Processing 13 (2010) 180–184182

die-bonding, a wire-bonding process was performed to estab-lish the electrical connection, onto which the UV-epoxy wasencapsulated.

3. Results and discussion

Fig. 1(a) shows the specific contact resistances of thep-electrode with different layer schemes. As shown inthe figure, once the Pd layer was used as the first layer,the ohmic contact could be obtained. For example, the

3

4

5

Photodiode

quartz

LED chip

( −) (+)Parameter analyzer

0.000

1

2

Nor

mal

ized

out

put p

ower

u

Current (

10

12

14

16

FCLED

0

4

6

8

Cal

cula

ted

extra

ctio

n ef

ficie

ncy

(%)

TELED

FCLE

p-electrode refle

0.01 0.02

20 40

TELED

Fig. 3. (a) Light outputs of FCLED and TELED as a function of injection current a

efficiency (solid line) of LEDs as a function of p-electrode reflectivity. The insets of

the plan views of electroluminescent FCLEDs and TELEDs measured at an injectio

branched structure, which was optimized in terms of uniform current spreadin

specific contact resistance of the Pd-based schemes was ina range of 1.9�2.9�10�4 O cm2, which was attributed tothe nature of the Pd layer having a work function as high as5.12 eV [15]. In addition, despite having a very thin Pdthickness of as low as 2 nm in the Pd/Au scheme, the lowspecific contact resistance revealed that the deposited Pdformed a continuous film. However, Ag schemes producepoor contact resistance, as high as 1.5�10�2 O cm2 [16].Fig. 1(b) shows reflectivities of p-electrodes with differentlayer schemes (deposited onto both-sides polished

A)

TELED FCLED (Pd) FCLED (Pd/Ag) FCLED (Ag)

2.0

2.5

FCLED (Ag)

calculated experimental

1.0

1.5FCLED (Pd/Ag)

D (Pd)

Nor

mal

ized

ligh

t out

put a

t 20

mA

ctivity (%)

0.03 0.04 0.05

60 80 100

nd (b) experimental output power (solid dots) and calculated extraction

Fig. 3(a) and (b) shows schematics for the on-wafer probing condition and

n current of 1 mA, respectively. Note that the p-bond pad for TELED has a

g.

H. Kim et al. / Materials Science in Semiconductor Processing 13 (2010) 180–184 183

sapphire). The reflectivity strongly depended on the layerscheme, particularly when the Pd layer was used. Thereflectivities measured at 392 nm were 36.1%, 43.7%, 60.5%,and 84.0% for Pd/Au, Pd, Pd/Ag, and Ag schemes, respec-tively, indicating that while Pd is essential for makingohmic contact, it significantly degrades reflectivity. Thelowest reflectivity of Pd/Au was attributed to its thinnessallowing light transmission through the Pd/Au layer.

Fig. 2(a) shows the forward current versus voltage (I�V)characteristics of FCLEDs fabricated with Pd, Pd/Ag, and Ag p-electrodes along with TELED with Pd/Au. The I�V curve ofFCLED fabricated with Ag was significantly degraded, which,as discussed above, can be attributed to the high p-contactresistance of the Ag electrode. In addition, the I�V curves ofFCLED fabricated with Pd and Pd/Ag were almost the sameand were much steeper than that of TELED. Specifically, theforward voltages measured at an injection current of 20 mA(Vf) were 3.26 and 3.51 V for FCLED and TELED, respectively.Neglecting the voltage drop caused by non-adiabatic injec-tion of carriers into the active region, the total voltage dropacross a forward-biased LED at 20 mA (Vf) can be written asfollows [4]

Vf ¼ Vdþ IRs, ð1Þ

where Vd represents the diode voltage, I the injected current(20 mA), and Rs the diode series resistance. Considering thatthe emission wavelength was 392 nm, the diode voltage canbe calculated as VdEEg/e=3.17 V, where Eg is the bandgapenergy and e the elementary charge. Under the assumptionthat the diode had high parallel resistance, namely, the diodehas no shunt path, the diode series resistance (Rs) can beobtained using the following equation [4]

IðdV=dIÞ ¼ IRsþnkT=e ð2Þ

where n is the ideality factor, k the Boltzmann constant, and T

the absolute temperature.

1.8

2.0

2.2

nGaN (2.54)

nsapp (1.78)

nencap (1.50)

nair (1.00)

TELED (Pd/Au)

1.0

1.2

1.4

1.6FCLED

T

LED typ

Nor

mal

ized

ligh

t out

put,

on-w

afer

pro

bing

FCLED (

Fig. 4. Light output powers of FCLEDs and TELEDs plotted before (on-wafer pro

FCLEDs and TELEDs within lamp-packages.

Fig. 2(b) shows the replotted experimental I(dV/dI)–I

curves of FCLED and TELED along with a linear fit result.For an accurate extraction of device parameters, currents lessthan 1 mA were neglected. Note that the linear fits were in agood agreement with the experimental data, indicating thatthe extracted parameters were reliable. The extracted Rs werefound to be 6.6 and 15.3 O for FCLED and TELED, respectively.The forward voltages (Vf) calculated using Eq. (1) were 3.30and 3.48 V for FCLED and TELED, respectively, which was inagreement with the experimental data. Considering that theepitaxial structure and specific p-contact resistances for bothLEDs are the same, such a large difference in diode seriesresistance (Rs) of 8.7 O could be solely attributed to thedifference in sheet resistances of the Pd/Ag and Pd/Au layerscaused by their different thicknesses. Indeed, the sheetresistances measured using a four-point probe system were�0 and 26 O/sq for Pd/Ag and Pd/Au, respectively. It isnoteworthy that the sheet resistance of the p-electrodedetermines the current spreading length and hence theeffective device area [12,17].

Fig. 3(a) shows the light outputs of FCLEDs and TELEDsas a function of injection current, where the output powerwas measured from the photodiode located at the bottomsides of the wafers as shown in the inset of Fig. 3(a). Thelight output power of the LEDs was significantly dependenton the p-electrode scheme. For example, the normalizedoutput powers measured at an injection current of 20 mAwere 1.00, 1.34, 1.72, and 2.28 for TELED, FCLED (Pd), FCLED(Pd/Ag), and FCLED (Ag), respectively. To explain the drasticchange with respect to p-electrode, measured outputpower was also replotted as a function of the p-electrodereflectivity, as shown in Fig. 3(b).

Fig. 3(b) shows the experimental output powers (soliddots) and calculated extraction efficiencies (solid line) ofLEDs as a function of the p-electrode reflectivity. Thecalculation of extraction efficiency was performed using

8

9

10

4

5

6

7

Act

ual o

utpu

t pow

er, p

acka

ged

(mW

)

nGaN (2.54)

nsapp (1.78)

nencap (1.50)

nair (1.00)

ELED

ePd) FCLED (Pd/Ag)

bing) and after packaging. The inset shows schematic chip geometries of

H. Kim et al. / Materials Science in Semiconductor Processing 13 (2010) 180–184184

a commercially available optical ray-tracing simulation[18]. The measured output powers were in excellentagreement with the calculated results, with the exceptionof TELED. This result suggested that for FCLED, a drasticchange of output power was caused by differentp-electrode reflectivities while TELED did not follow thecalculated curve. This was attributed to the nature of theTELED emitting light in both upward and downwarddirections, whereas the FCLED emitted light only down-ward, as shown in the inset of Fig. 3(b). To compare theoutput characteristics of both FCLED and TELED accurately,both LEDs were evaluated after a 5 mm-indicator-lamppackaging as shown in Fig. 4.

Fig. 4 shows the light output powers of FCLED (Pd/Ag)and TELED plotted before on-wafer probing and afterpackaging, along with the reference FCLED (Pd). Thepackaged FCLED (Pd/Ag) had significantly improved outputpower, as high as twice that of the packaged TELED.Specifically, the output powers were 9.15 and 4.59 mWfor FCLED and TELED, respectively. It is worth noting thatthe enhancement factor obtained after packaging (2.0) wasmuch larger than that obtained before packaging (1.72),which can be attributed to different LED chip geometriesbefore and after packaging. Before packaging (on-waferprobing conditions), the output powers of both TELED andFCLED were measured from the bottom side of a sapphiresubstrate as shown in the inset of Fig. 3(a). In this case, lightis able to escape with ease. According to Snell’s law [4], thecritical angle for light extraction from GaN to sapphire is aslarge as yc,sapp=sin�1(nsapp/nGaN)=44.51, where nsapp andnGaN are the refractive indices of sapphire (1.78) and GaN(2.54), respectively. After packaging; however, FCLEDshould emit light through sapphire substrate since itwas flip-bonded, the TELED should emit light directlytowards the encapsulant as shown in the inset of Fig. 4.This indicates that, for TELED, the critical angle couldbe reduced, i.e., yc,encap=sin�1(nencap/nGaN)=36.21, wherenencap=1.5. Therefore, the light extraction enhancements inFCLED before and after packaging can be different fromthose of TELED because of their different chip structures,namely, different refractive index sequences.

For the FCLED fabricated with Pd/Ag, it was interestingthat the enhancement factor obtained after packaging (1.12)was much smaller than that obtained before packaging(1.28). This result indicated that the effect of p-electrodereflectivity on light output became weaker after packaging.This was also related to the different refractive indexsequences before and after packaging. For example, theconditions were GaN(2.54)/sapphire(1.78)/air(1.0) beforepackaging and GaN(2.54)/sapphire(1.78)/encapsulant(1.5)/air(1.0) after packaging. Due to the enhanced effect of thegraded refractive index after packaging [13], light extraction

was more easily carried out than before packaging. Inaddition, since the LED was diced, the light could be extractedthrough the sapphire sidewall as well as the sapphire bottomsurface, meaning that the probability of internal reflectionevents of the wave-guided mode could be decreased afterpackaging [4], thus reducing the effect of p-electrode reflec-tivity on the light output.

4. Conclusion

The electrical and optical characteristics of GaN-basedFCLEDs and TELEDs were investigated. We found thatFCLED had a lower forward voltage than TELED by 0.25 Vand a smaller diode series resistance by 8.7 O, which wasdue to the reduced sheet resistance of the p-electrode. Thelight output power of FCLED was shown to be larger thanthat of TELED by a factor of 2.0. This could be attributed tothe high reflectivity of the p-electrode and the effect ofgraded refractive index layer sequences.

Acknowledgements

This paper was supported by research funding fromChonbuk National University in 2009.

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