Optical and Electrical Properties of GaN-Based Light Emitting Diodes Grown on Micro- and Nano-Scale Patterned Si Substrate

Optical and Electrical Properties of GaN-Based

Light Emitting Diodes Grown on Micro- and

Nano-Scale Patterned Si Substrate

Ching-Hsueh Chiu, Chien-Chung Lin, Dong-Mei Deng, Da-Wei Lin, Jin-Chai Li,

Zhen-Yu Li, Gia-Wei Shu, Tien-Chang Lu, Ji-Lin Shen,

Hao-Chung Kuo, Senior Member, IEEE, and Kei-May Lau, Fellow, IEEE

Abstract— We investigate the optical and electrical

character-istics of the GaN-based light emitting diodes (LEDs) grown on

micro- and nano-scale patterned silicon substrate (MPLEDs and

NPLEDs). The transmission electron microscopy images reveal

the suppression of threading dislocation density in InGaN/GaN

structure on nano-pattern substrate due to nano-scale epitaxial

lateral overgrowth. The plan-view and cross-section cathodo

luminescence mappings show less defective and more

homoge-neous active quantum-well region growth on nano-porous

sub-strates. From temperature-dependent photoluminescence (PL)

and low temperature time-resolved PL measurement, NPLEDs

have better carrier confinement and higher radiative

recom-bination rate than MPLEDs. In terms of device performance,

NPLEDs exhibit smaller electroluminescence peak wavelength

blue shift, lower reverse leakage current and decrease in

effi-ciency droop when compared with the MPLEDs. These results

suggest the feasibility of using NPSi for the growth of high quality

and power LEDs on Si substrates.

Index Terms— Light emitting diodes, metal-organic chemical

vapor deposition, nano-scale epitaxial lateral overgrowth, silicon

substrate.

Manuscript received December 15, 2010; revised February 1, 2011; accepted February 7, 2011. Date of current version May 13, 2011. This work was supported in part by the National Science Council of Taiwan, under Grant NSC 98-3114-E-009-002-CC2.

C.-H. Chiu, D.-W. Lin, J.-C. Li, Z.-Y. Li, T.-C. Lu, and H.-C. Kuo are with the Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan (e-mail: [email protected]; [email protected]; jinchaili@ xmu.edu.cn; [email protected]; [email protected]; hckuo@ faculty.nctu.edu.tw).

C. C. Lin is with the Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan. He is also with the Institute of Photonic Systems, College of Photonics, National Chiao Tung University, Tainan 71150, Taiwan (e-mail: [email protected]).

D.-M. Deng is with the Department of Photonics and Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan. He is also with the Photonics Technology Center, Department of Electrical and Electronic Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong (e-mail: [email protected]).

K.-M. Lau is with the Photonics Technology Center, Department of Elec-trical and Electronic Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong (e-mail: [email protected]).

G.-W. Shu and J.-L. Shen are with the Department of Physics, Chung Yuan Christian University, Chung-Li 32023, Taiwan (e-mail: [email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JQE.2011.2114640

I. I

T

HE wide band gap GaN-based semiconductors have

received enormous attention for various applications,

such as short-haul optical communication, traffic and

sig-nal lights, back lights for liquid-crystal displays, and

indoor/outdoor lightings. Typically, GaN-based light emitting

diodes (LEDs) were grown on sapphire or SiC substrate

by heteroepitaxial techniques in a metal-organic chemical

vapor deposition (MOCVD) system [1]–[3]. However, the low

thermal and electrical conductivities make sapphire less perfect

as a substrate for the GaN epilayers, meanwhile the high price

and mechanical defects hinder SiC substrate’s acceptability in

the LED market. Silicon has been considered as an alternative

substrate material due to its low manufacturing cost,

avail-ability of large size wafers, and good thermal and electrical

conductivities. Thus, many efforts have been dedicated to the

realization of GaN based LEDs on Si substrates [4]–[8]. Even

though good progress has been made, there are still several

problems when using Si substrate for GaN epitaxial layers.

The large lattice mismatch between GaN and Si (almost 17%)

leads to high threading dislocation densities (TDDs) (around

10

_{− 10}

_cm

_{) in the subsequent GaN epilayers. The other}

major problem is the thermal expansion coefficient difference

(56%) between two materials, which induces a high tensile

stress during the thermal cycling in MOCVD and often results

in cracks and damages of epilayers [9]. To reduce the density

of cracks and threading dislocations of GaN grown on Si,

a number of approaches have been reported, such as using

AlN multilayer combined with graded AlGaN layer as buffer

[10], epitaxial lateral overgrowth of GaN on micro-patterned

Si [11], and nano heteroepitaxial (NHE) lateral overgrowth

of GaN on nanopore array Si [12], etc.. These methods

effectively reduce the tensile stress and thus the crystal

qual-ity of GaN was greatly improved. Recently, our co-workers

reported fabrication of GaN-based device structure on a

nano-scale patterned silicon substrate [13] that shows significant

improvement on reduction of TDDs, surface morphology and

light emission. In the mean time, the optical and electrical

properties of InGaN/GaN MQWs grown on these patterned

silicon substrates have not been fully studied yet. In this paper,

we examine various optical and electrical characteristics of

GaN based LEDs grown on micro and nano-scale patterned

Aluminium Al₂O₃ Al3+_↑ Al3+ O2−_/OH−_↓ Electrolyte 3. 4. E → E → E → ↑O↓2

Fig. 1. Schematic diagram of pore formation at the beginning of the anodization.

Si substrates (MPLEDs and NPLEDs), and the experimental

results can lead us to believe that NPLED is in general superior

to its micro-scale counterpart.

II. E

The micro-scale pattern Si (MPSi) substrate was prepared

into 340

μm × 340μm square islands on a 2-inch silicon

substrate. These islands are separated by 3

μm deep and

20

μm wide trenches, in the 110 and 112 directions, and

were patterned by an STS inductively coupled plasma-reactive

ion etching (ICP-RIE) system on 2-inch Si substrate. The

self-ordered anodized aluminum oxide (AAO) procedures are

depicted in Fig. 1. Firstly, a 500 Å thick SiO

film acting

as the isolation layer was formed by thermal oxidation on a

2-inch Si (111) substrate. After that, 500 Å Ti and 3500 Å

Al were deposited on it one by one using an AST

electron-beam evaporator. The Ti improved the adhesion of the Al

layer and promoted the uniformity of the porous alumina in

the anodization step. The procedure of anodization can be

summarized in the following four steps [13], [14]: first we

deposit a non-conductive oxide layer and submerge the wafer

into the electrolyte. Second, due to the inherent roughness,

the electric field will locally concentrate at the high curvature

points (Step 2). This local high field leads to a field-enhanced

or/and temperature-enhanced dissolution of formed oxide, and

thus, pores grow with the gradually dissolved alumina (Step 3).

When the formation and dissolving of alumina reach an

equilibrium state, a stable growth of pores can be realized,

as shown in step 4.

Finally after the alumina nano-particles were formed, the

oxide and then the underneath semiconductor layer can be

removed by generic etching process (as shown in Fig. 2).

In general, there are some parameters influencing the

self-ordered anodized aluminum oxide (AAO), such as the anodic

voltage, type and concentration of electrolyte, temperature, etc.

[15] Among these factors, the anodic voltage is one of the

AAO AAO AAO Si (111) substrate Si (111) substrate Ti SiO₂ Si (111) substrate Ti SiO2 Si (111) substrate Ti SiO2

Fig. 2. Schematic diagram for preparing the porous Si substrate. TABLE I

SUBSTRATESUSED FORGROWINGGaN LAYERS

Substrate Diameter (nm) Spacing (nm) Depth (μm)

A-1 200 100 1

A-2 120 150 1

A-3 120 150 0.25

most important factors for adjusting inter-pore distance. It is

reported that the inter-pore distance was proportional to the

anodic voltage, and could get the following relation [14],

2

.5(nm/V) U ≤ Dint ≤ 2.8(nm/V) U

(1)

[16], [17] where Dint is the inter-pore distance, and U is the

applied voltage.

The aforementioned equation (1) can served as a baseline

for the process. However, in different material system, there

should be one or more optimal conditions for the subsequent

material quality. In this work, several designs were carried

out to find out the optimized processes, and we summarize the

physical characteristics in Table 1. Their outcomes of epitaxial

layer quality can be visually distinguished from Fig. 3(a)

to 3(c). When the size of the pore is too large, the coalescence

of GaN layer can not be fully developed due to large pore

diameter to mesa width ratio. If the depth of the pore is too

large, the surface morphology will also be affected badly.

We choose the condition in Fig. 3(c) as the final template

for NPLED because it can deliver the best quality of the

material. In addition to the anodic voltage and timing control,

the common condition of anodization electrolyte was at 6 °C

in 0.3M phosphoric acid for 30min. After anodization,

self-assembled AAO nano arrays were uniformly distributed on the

Si surface. By ICP etching, the AAO pattern was transferred

to the Si substrate. The AAO mask was then removed by wet

etching.

When all AAO steps were carried out successfully,

nano-pore arrays were uniformly distributed on the entire 2-inch Si

substrate with an average nano-pore diameter of 150 nm,

inter-pore distance of 120 nm, and an etched depth of 250 nm. In the

next step, LED structures with In

Ga

N/GaN MQWs

Fig. 3. Opticalmicroscope of GaN layer grown on substrate (a), (b), and (c). P-GaN InGaN/GaN MQWsx5 n-GaN u-GaN AlGaN AlN MPSi (a) (b) NPSi AlN/AlGaN

Fig. 4. Schematic of GaN-based LED structures grown on (a) MPSi and (b) NPSi.

an Aixtron 2000HT system. The epitaxial structure of the

GaN-based LED overgrowth on MPSi and NPSi substrate is

depicted in Fig. 4. Detailed substrate preparation and growth

procedure for LED on MPSi and NPSi substrate were reported

elsewhere [18], [19].

After the InGaN/GaN structures were grown, we performed

standard LED lithographic process, metallization, and etch

procedure in order to define device mesa and make p/n

contacts of the LED layers. Once the device fabrication is

finished, we engaged four different types of measurements:

cathodo luminescence (CL), photoluminescence (PL),

time-resolved photoluminescence (TRPL) and electroluminescence

(EL). The spatially resolved CL imaging was obtained by

scanning electron microscope (JEOL-7000F SEM system)

with a fixed viewing scale. The temperature dependent PL

measurements were done by a 325 nm He-Cd laser at 35 mW

excitation power. Low temperature TRPL measurements were

performed at 10 K using time-correlated single-photon

count-ing and a pulsed GaN diode laser operatcount-ing at a wavelength

of 396 nm as the excitation source. In the EL measurement

system, the current source is Kiethley 238, and the best

mea-surement resolution at 1 nA injection could reach 10 fA with a

accuracy of 0.3%. We can perform a serious of current-voltage

measurement and data storage by Lab View human-machine

control interface. Finally a generic device LIV measurement

by the standard probe station and Kiethley current source will

demonstrate superior power output and efficient droop in our

NPLED device.

III. R

D

First step to compare these two material growth methods is

to check their material quality. In order to analyze the detailed

epitaxial layer quality, we used TEM to compare the cross

section between two types of devices in Fig. 5. A comparison

of Fig. 5(a) and 5(b) shows that the dislocation density in the

NPSi sample is reduced much more than that of MPSi’s. The

TDDs for MPSi is estimated to be 2.5

× 10

cm

at the

bottom of the n-GaN layer, and it decreases to 4.6

× 10

cm

at the top of the n-GaN layer and 6.2

× 10

_cm

_{in the p-GaN}

Fig. 5. TEM images of LEDs grown on (a) MPSi and (b) NPSi; (c) and (d) region of between AlGaN layer and Si substrate for NPSi using g= (0002).

1 µm 1 µm

Fig. 6. Top view CL images on samples of energy for (a) micro-scale and (b) nano-scale pattered Si substrate.

region. On the other hand, for the epilayer grown on NPSi,

fewer dislocations are observable within the range of view. As

shown in Fig. 5(b), the TDDs at the bottom of the n-GaN layer

is about 1.1

× 10

cm

; however, the TDDs at the top of

the n-GaN layer drop down to 5.7

× 10

cm

, and it is only

8.8

× 10

cm

in the p-GaN region. The reduction of TDDs

NPSi over MPSi is about 10 times. Fig. 5(c) and 5(d) are

TEM images are taken at the interface of epilayer/NPSi. As

can be seen in Fig. 5(c), there are many dislocations bent and

terminated in AlGaN layer or near the epilayer/NPSi interface.

As a result, the density of TDDs in the subsequent quantum

well region was much lower.

Next, we will examine our results by CL. CL is a very

important technique when we need non-invasive assessment

of crystal quality. Fig. 6(a) and 6(b) display the plan-view

CL emission images with a 10 kV accelerating voltage at

room temperature. At first glance, MPLED showed more “dead

zone” or black spots than NPLED. These dark areas in the CL

images are regions where minority carriers get consumed by

dislocations due to high non radiative recombination velocity

[20]. The other feature we would like to point out is that the

emission intensity of MPLED is less uniform than NPLED’s.

This was mainly due to indium composition fluctuation and the

(a) 4000 3500 3000 2500 2000 A v era g e Intensit y (a. u .) 1500 1000 500 0.0 0.2 0.4 Interpore distance (µm) 0.6 (b) 0.8 1.0 Experimental Fit JEOL SEI 10.0 kV×23.000 1 µm WD 12.2 mm

Fig. 7. (a) Cross section CL intensity at nano-scale patterned Si/GaN inter-face. (b) Average intensity between silicon holes against interpore distance.

phase separation [21], [22]. These CL images suggest that the

pitch between the etched silicon holes might play an important

role since the nano-patterned sample looks much better. To

further investigate how the pitch of nano-patterns affects the

photon emission efficiency, we cleaved through nano-porous

wafers and performed the cross section CL measurement.

The upper half of Fig. 7(a) shows the cross section CL

intensity of NPLEDs’ quantum well region, and it is taken

at the same horizontal location aligned to the nano-scale

pat-terned Si/GaN substrate underneath (bottom half of Fig. 7(a)).

We noticed that CL intensity is much stronger when etched

silicon holes are closer. To quantitatively evaluate this

obser-vation, we plot the average intensity between silicon holes

against interpore distances in Fig. 7(b). When the interpore

distance reduced to 0

.2μm (200nm) or less, the integrated

luminescence intensity grows sharply. From previous research

by Sugahara, et. al. [23], the CL efficiency (

η) can be related

to sample recombination behavior given by:

η = 1 −



2r

L



−

8

L



r exp



−

r

− r

L



dr

(2)

where L

is the mean dislocation distance, L is the diffusion

length in InGaN, and r

is the radius in which non-radiative

recombination consumes all carriers (the dark spot). If other

material characteristics is the same and assume uniform

exci-tation, the only factor that can affect the luminescent intensity

is L

, the mean dislocation distance. So when material has

fewer defects, the efficiency is higher. From the trend of data,

we can reasonably conclude that the higher density of

nano-0 10 P L Intensit P L Intensit 20 b b 30 1000/T (K−1) (a) (b) 1000/T (K−1) 40 50 0 10 20 30 40 50

Fig. 8. (Color online.) PL intensity for (a) micro-scale and (b) nano-scale pattered Si substrate plotted as a function of 1000/T.

size interpore area bears less dislocation and thus tends to have

strong light emission.

Just like CL can reveal the crystal quality, PL can let us

find out the possible radiative recombination mechanism in

the quantum well region. It has been shown that thermal

quenching of PL intensity can be explained by carriers’

thermal emission out of a confining potential with an activation

energy correlated with the depth of the confining potential

[24]. Therefore, it is expected that the deeper localization

with better confinement should have larger activation energy.

Fig. 8(a) and 8(b) display the temperature dependence of PL

intensity fitted by Arrhenius equation as following [25]

:

I

(T ) =

I

1

+ A exp



−



+ B exp





(3)

where I(T) is the temperature-dependent PL intensity, I

is the

PL intensity at 20 K, k

is Boltzmann’s constant, A and B are

the rate constants, and E

and E

are the activation energies

for two different nonradiative channels, which correspond to

the low temperature and high temperature regions [26]. For

high temperature region, thermal quenching can be fitted with

activation energy (E

) 59 and 87 meV for MPLEDs and

NPLEDs, respectively. In particular, the activation energy for

NPLEDs is 47.4 % higher than that for MPLEDs, leading to

a minor overflow of carriers outside the InGaN MQW active

region. The discrepancy should rise from either anisotropic

distribution in the active region or mixture of thermionic

emission from potential minimum to barrier. Based on above

result, the PL-intensity improvement in the NPLEDs can be

attributed to the stronger localization effects and better carrier

confinement in In

Ga

N/GaN MQW active region [27].

Potential variation affects how easy the carrier can be

confined, and the combining rate can be regarded as how fast

the carriers can recombined. The information about carrier

recombination rate can be obtained from decaying behavior

of photoluminescence. The low temperature TRPL decay for

both samples was shown in Fig. 9. Because the measurement

was carried out at 10K, the influence of the nonradiative

recombination process could be excluded [28]. The TRPL

results can be fitted by a bi exponential decaying function: [29]

I

(t) = I

(0) exp



−

t

τ



+ I

(0) exp



−

t

τ



(4)

where I(t) is the PL intensity at time t;

τ

and

τ

represent

0 5 10 15 Life Time (ns) Normalized Intensit y (a. u .) 20 MPLEDs NPLEDs 25 30 35

Fig. 9. Comparison of low-temperature TRPL between MPLEDs and NPLEDs. 100 80 60 40 20 0 0 2 0 Reverse voltage (V) Re v erse c u rrent (mA) −5 −4 −3 −2 −1 0 −2 −4 −6 −8 −10 4 MPLEDs NPLEDs MPLEDs NPLEDs Forward voltage (V) For w ard c u rrent (mA) 6 8

Fig. 10. Forward I–V characteristics of all fabricated LEDs, and the inset is reverse I–V characteristics of all fabricated LEDs.

constant

(τ

) usually represents the radiative recombination

of excitons and the relaxation of QW excitons from free or

extended states toward localized states [29], [30]. Our fitting

shows

τ

= 3.2 and 1 ns for MPLEDs and NPLEDs,

respec-tively. The slow decay time

(τ

) accounts for communication

between localized states and localized excitons [29], [30]. The

fitting shows

τ

= 9.4 and 3.2 ns for MPLEDs and NPLEDs,

respectively. In both fast and slow constants, NPLEDs’

life-time is generally shorter than MPLEDs’ at low temperature.

S. Chichibu, et. al. reported the electron-hole pairs in the

potential minima of QWs can be referred to as localized

excitons, and the emission efficiency can still be enhanced

even though the wave function overlap is weakened [31]. In

the case of MPLEDs and NPLEDs, much higher radiative

recombination rate observed in TRPL can be interpreted as

direct evidence of stronger localized confinement in NPLEDs

than MPLEDs, and also an indication of more efficient

light-emitter.

The final trial of this nano-size template is to test the light

emitting efficiency from the real device. LED devices with a

chip size of 350

× 350 μm

were fabricated on both MPLEDs

and NPLEDs. Fig. 10 shows the forward I-V characteristics

of both samples. At 20 mA forward current, both samples

exhibited diode voltages around 4.7 V. In addition, at the

200 MPLEDs NPLEDs 160 120 80 40 0 200 160 120 80 40 0 400 420 427 nm 429 nm 425 nm 440 EL Intensit y (a. u .) EL Intensit y (a. u .) Wavelength (nm) (a) (b) 460 5 mA 10 mA 20 mA 40 mA 60 mA 80 mA 100 mA 5 mA 10 mA 20 mA 40 mA 60 mA 80 mA 100 mA 480 400 420 440 Wavelength (nm) 460 480

Fig. 11. EL spectra of (a) MPLEDs and (b) NPLEDs at different drive currents. 4 100 80 60 Normalized EQE (%) 40 20 0 100 80 60 Normalized EQE (%) 40 20 0 2 Li g ht Intensit y (a. u .) 0 4 2 Li g ht Intensit y (a. u .) 0 0 20 40 60

Current density (A/cm2₎

(a) (b)

80

MPLEDs NPLEDs

100 0 20 40 60

Current density (A/cm2₎ 80 100

Fig. 12. Integrated EL intensity and normalized EQE as a function of forward current density for (a) MPLEDs and (b) NPLEDs, respectively.

reverse bias (shown in the insert plot of Fig. 10), the leakage

current of the NPLEDs is smaller than MPLEDs. Several

types of dislocations can contribute to the reverse-bias leakage

current [32], and one of the most dominant type is the screw

dislocation [32], [33]. The reduction of screw type dislocations

can certainly help to reduce the reverse-bias current, and our

measurement indicates a better crystal quality of LEDs grown

on NPSi substrate, which confirms with TEM results.

Fig. 11 shows EL spectrums as a function of injection

current for MPLEDs and NPLEDs. The emission peak

wave-length of NPLEDs is slightly shorter than MPLEDs’. In our

previous study, we performed Raman backscattering

measure-ment at room temperature. The regular Raman shift of E

(High) in stress-free GaN layer is around 567.2 cm

. In this

paper, the E

(High) shift is 565.4 cm

and 564.5 cm

, for

samples on NPSi and on MPSi, respectively. The deviation of

the E

(High) peaks from the intrinsic position is proportional

to the residual tensile stress. For GaN, the E

(High) mode

shifts linearly with stress in 2.9 cm

/GPa for biaxial stress.

We can thus estimate the tensile stress in NPSi and MPSi are

0.62 GPa and 0.93 GPa, respectively. This indicates that the

LEDs grown on NPSi exhibited lower strain than on MPSi.

Therefore, the LEDs grown on NPSi possess a reduced QCSE.

The related MPSi and NPSi Raman measurement results have

been previously published by Dongmei Deng et. al. [18].

Moreover, we can see EL emission peak wavelength of

MPLEDs exhibits blue shift from 429 nm to 427 nm with

increasing drive current as shown in Fig. 11(a). However, we

obtained almost unshifted EL peak with increasing injection

current. This result indicates that the quantum confined stark

effect (QCSE) does become weaker due to the strain relaxation

in epitaxial layer overgrown on NPSi template [34].

Finally, Fig. 12 shows the light output intensity and

nor-malized external quantum efficiency (EQE) as a function of

forward current density for both samples. The light

output-lower in MPLED. The data from NPLED, however,

demon-strates good material quality with a reduced efficiency droop

and much less soft increase in EQE.

However, it rolls over beyond 20mA/cm

with a reduced

EQE. The EQE is decreased to 62% of its maximum value

when the current density at 100mA/cm

. In contrast, the

NPLEDs exhibits 20% efficiency droop with increasing the

injection current density to 100mA/cm

. It can be attributed to

reduced polarization field which also echoes to weaker QCSE

under the circumstance of reduced strain in overgrown layers

on NPSi template [36].

IV. C

In conclusion, the optical and electrical properties of LEDs

grown on micro and nano-scale patterned Si substrate were

investigated. We demonstrated a more homogeneous growth

of InGaN/GaN active layers under this nano-scale template

by plan-view and cross-section CL mapping. From

tempera-ture dependent PL and low temperatempera-ture TRPL measurement,

NPLEDs has better carrier confinement and higher radiative

recombination rate than MPLEDs. On the actual device

perfor-mance, NPLEDs exhibits smaller peak wavelength blue shift,

lower reverse leakage current and decreases efficiency droop

compared with the MPLEDs. The results suggest a weaker

QCSE due to relaxation of strain in the epitaxial layers on

nano-scale patterned substrate, which can be really useful for

the next generation of large area, Si-based heteroepitaxy of

GaN related optoelectronic devices.

A

The authors would like to thank S. C. Wang of National

Chiao-Tung University, Hsinchu, Taiwan, for useful

discus-sion.

R

[1] E. F. Schubert, Light Emitting Diodes, 2nd ed. Cambridge, U.K.: Cambridge Univ. Press, 2003, pp. 21–22.

[2] J. Han, M. H. Crawford, R. J. Shul, J. J. Figiel, M. Banas, L. Zhang, Y. K. Song, H. Zhou, and A. V. Nurmikko, “AlGaN/GaN quantum well ultraviolet light emitting diodes,” Appl. Phys. Lett., vol. 73, no. 12, pp. 1688–1690, Sep. 1998.

[3] S. Nakamura, S. Pearton, and G. Fasol, “The blue laser diode,” in A Complete Story, 2nd ed. Berlin, Germany: Springer-Verlag, 2000, p. 48. [4] S. Guha and N. A. Bojarczuk, “Ultraviolet and violet GaN light emitting diodes on silicon,” Appl. Phys. Lett., vol. 72, no. 4, pp. 415–417, Jan. 1998.

[5] C. A. Tran, A. Osinski, R. F. Karlicek, and I. Berishev, “Growth of InGaN/GaN multiple-quantum-well blue light-emitting diodes on silicon by metalorganic vapor phase epitaxy,” Appl. Phys. Lett., vol. 75, no. 11, pp. 1494–1496, Sep. 1999.

[6] S. Dalmasso, E. Feltin, P. de Mierry, B. Beaumont, P. Gibart, and M. Leroux, “Green electroluminescent (Ga, In, Al)N LEDs grown on Si (111),” Electron. Lett., vol. 36, no. 20, pp. 1728–1730, Sep. 2000. [7] M. A. Sánchez-García, F. B. Naranjo, J. L. Pau, A. Jiménez, E. Calleja,

and E. Muñoz, “Ultraviolet electroluminescence in GaN/AlGaN single-heterojunction light-emitting diodes grown on Si(111),” J. Appl. Phys., vol. 87, no. 3, pp. 1569–1571, 2000.

strates,” Mater. Sci. Eng. B, vol. 93, nos. 1–3, pp. 77–84, May 2002. [10] K. Cheng, M. Leys, S. Degroote, M. Germain, and G. Borghs, “High

quality GaN grown on silicon(111) using a SixNyinterlayer by metal-organic vapor phase epitaxy,” Appl. Phys. Lett., vol. 92, no. 19, pp. 192111-1–192111-3, 2008.

[11] S.-J. Lee, G. H. Bak, S.-R. Jeon, S. H. Lee, S.-M. Kim, S. H. Jung, C.-R. Lee, I.-H. Lee, S.-J. Leem, and J. H. Baek, “Epitaxial growth of crack-free GaN on patterned Si(111) substrate,” Jpn. J. Appl. Phys., vol. 47, pp. 3070–3073, Apr. 2008.

[12] J. Liang, S.-K. Hong, N. Kouklin, R. Beresford, and J. M. Xu, “Nanoheteroepitaxy of GaN on a nanopore array Si surface,” Appl. Phys. Lett., vol. 83, no. 9, pp. 1752–1754, Sep. 2003.

[13] B. Van der Linden, H. Terryn, and J. Vereecken, “Investigation of anodic aluminium oxide layers by electrochemical impedance spectroscopy,” J. Appl. Electrochem., vol. 20, no. 5, pp. 798–803, 1990.

[14] D. Deng, “InGaN based light-emitting diodes grown by metal organic chemical vapor deposition,” Ph.D. thesis, Dept. Electr. Electron. Eng., Hong Kong Univ. Sci. Technol., Clear Water Bay, Hong Kong, 2010. [15] J. Choi, “Fabrication of monodomain porous alumina using nanoimprint

and its applications,” Ph.D. thesis, Inst. Microstruct. Phys., Martin-Luther-Universität Halle-Wittenberg, Halle, Germany, 2003.

[16] K. Ebihara, H. Takahashi, and M. Nagayama, “Structure and density of anodic oxide films formed on aluminium in oxalic acid solutions,” J. Met. Finish. Soc. Jpn., vol. 34, no. 11, pp. 548–553, 1983.

[17] H. de L. Lira and R. Paterson, “New and modified anodic alumina membranes: Part III. Preparation and characterisation by gas diffusion of 5 nm pore size anodic alumina membranes,” J. Membr. Sci., vol. 206, nos. 1–2, pp. 375–387, Aug. 2002.

[18] D. Deng, N. Yu, Y. Wang, X. Zou, H.-C. Kuo, and K. M. Lau, “InGaN-based light-emitting diodes grown and fabricated on nanopatterned Si substrates,” Appl. Phys. Lett., vol. 96, no. 20, pp. 201106-1–201106-3, May 2010.

[19] B. Zhang, H. Liang, Y. Wang, Z. Feng, K. W. Ng, and K. M. Lau, “High-performance III-nitride blue LEDs grown and fabricated on patterned Si substrates,” J. Cryst. Growth, vol. 298, pp. 725–730, Jan. 2007. [20] S. J. Rosner, E. C. Carr, M. J. Ludowise, G. Girolami, and H. I. Erikson,

“Correlation of cathodoluminescence inhomogeneity with microstruc-tural defects in epitaxial GaN grown by metalorganic chemical-vapor deposition,” Appl. Phys. Lett., vol. 70, no. 4, pp. 420–422, Jan. 1997. [21] H. J. Chang, C. H. Chen, Y. F. Chen, T. Y. Lin, L. C. Chen, K. H. Chen,

and Z. H. Lan, “Direct evidence of nanocluster-induced luminescence in InGaN epifilms,” Appl. Phys. Lett., vol. 86, no. 2, pp. 021911-1– 021911-3, Jan. 2005.

[22] T. Sugahara, M. Hao, T. Wang, D. Nakagawa, Y. Naoi, K. Nishino, and S. Sakai, “Role of dislocation in InGaN phase separation,” Jpn. J. Appl. Phys., vol. 37, no. 108, pp. L1195–L1198, 1998.

[23] T. Sugahara, H. Sato, M. Hao, Y. Naoi, S. Kurai, S. Tottori, K. Yamashita, K. Nishino, L. T. Romano, and S. Sakai, “Direct evidence that dislocations are non-radiative recombination centers in GaN,” Jpn. J. Appl. Phys., vol. 37, no. 4A, pp. L398–L400, 1998.

[24] Y. Sun, Y.-H. Cho, H.-M. Kim, and T. W. Kang, “High efficiency and brightness of blue light emission from dislocation-free InGaN/GaN quantum well nanorod arrays,” Appl. Phys. Lett., vol. 87, no. 9, pp. 093115-1–093115-3, Aug. 2005.

[25] A. Yasan, R. McClintock, K. Mayes, D. H. Kim, P. Kung, and M. Razeghi, “Photoluminescence study of AlGaN-based 280 nm ultraviolet light-emitting diodes,” Appl. Phys. Lett., vol. 83, no. 20, pp. 4083–4085, Nov. 2003.

[26] M. Leroux, N. Grandjean, B. Beaumont, G. Nataf, F. Semond, J. Massies, and P. Gibart, “Temperature quenching of photoluminescence intensities in undoped and doped GaN,” J. Appl. Phys., vol. 86, no. 7, pp. 3721–3728, 1999.

[27] S. Nakamura and S. F. Chichibu, Introduction to Nitride Semiconductor Blue Lasers and Light Emitting Diodes. New York: Taylor & Francis, 2000, pp. 242–243.

[28] T. Onuma, A. Chakraborty, B. A. Haskell, S. Keller, S. P. DenBaars, J. S. Speck, S. Nakamura, U. K. Mishra, T. Sota, and S. F. Chichibu, “Localized exciton dynamics in nonpolar (1120) InxGa1−xN multiple

quantum wells grown on GaN templates prepared by lateral epitaxial overgrowth,” Appl. Phys. Lett., vol. 86, no. 15, pp. 151918-1–151918-3, Apr. 2005.

[29] W. Z. Lee, G. W. Shu, J. S. Wang, J. L. Shen, C. A. Lin, W. H. Chang, R. C. Ruaan, W. C. Chou, C. H. Lu, and Y. C. Lee, “Recombination dynamics of luminescence in colloidal CdSe/ZnS quantum dots,” Nan-otechnology, vol. 16, no. 9, pp. 1517–1521, 2005.

[30] T. S. Ko, T. C. Lu, T. C. Wang, J. R. Chen, R. C. Gao, M. H. Lo, H. C. Kuo, S. C. Wang, and J. L. Shen, “Optical study of a-plane InGaN/GaN multiple quantum wells with different well widths grown by metal-organic chemical vapor deposition,” J. Appl. Phys., vol. 104, no. 9, pp. 093106-1–093106-8, Nov. 2008.

[31] S. Chichibu, T. Sota, K. Wada, and S. Nakamura, “Exciton localization in InGaN quantum well devices,” J. Vac. Sci. Technol. B.: Microelectron. Nanometer Struct., vol. 16, no. 4, pp. 2204–2214, Jul. 1998.

[32] E. J. Miller, E. T. Yu, P. Waltereit, and J. S. Speck, “Analysis of reverse-bias leakage current mechanisms in GaN grown by molecular-beam epitaxy,” Appl. Phys. Lett., vol. 84, no. 4, pp. 535–537, Jan. 2004. [33] E. G. Brazel, M. A. Chin, and V. Narayanamurti, “Direct observation

of localized high current densities in GaN films,” Appl. Phys. Lett., vol. 74, no. 16, pp. 2367–2369, Apr. 1999.

[34] C.-H. Chiu, P.-M. Tu, C.-C. Lin, D.-W. Lin, Z.-Y. Li, K.-L. Chuang, J.-R. Chang, T.-C. Lu, H.-W. Zan, C.-Y. Chen, H.-C. Kuo, S.-C. Wang, C.-Y. Chang, C.-H. Chiu, P.-M. Tu, D.-W. Lin, Z.-Y. Li, T.-C. Lu, H.-W. Zan, H.-C. Kuo, and S.-C. Wang, “Highly efficient and bright LEDs overgrown on GaN nanopillar substrates,” IEEE J. Sel. Topics Quantum Electron., vol. PP, no. 99, pp. 1–8, 2010.

[35] M. F. Schubert, S. Chhajed, J. K. Kim, and E. F. Schubert, D. D. Koleske, M. H. Crawford, S. R. Lee, A. J. Fischer, G. Thaler, and M. A. Banas, “Effect of dislocation density on efficiency droop in GaInN/GaN light-emitting diodes,” Appl. Phys. Lett., vol. 91, no. 23, pp. 231114-1– 231114-3, Dec. 2007.

[36] Y. B. Tao, Z. Z. Chen, F. F. Zhang, C. Y. Jia, S. L. Qi, T. J. Yu, X. N. Kang, Z. J. Yang, L. P. You, D. P. Yu, and G. Y. Zhang, “Polarization modification in InGaN/GaN multiple quantum wells by symmetrical thin low temperature-GaN layers,” J. Appl. Phys., vol. 107, no. 10, pp. 103529-1–103529-5, May 2010.

Ching-Hsueh Chiu was born in Hsinchu, Taiwan, in 1983. He received the B.S. degree in physics from the Chung Yuan Christian University, Chung-Li, Tai-wan, in 2006, and the M.S. degree in electrophysics from the National Chiao Tung University (NCTU), Hsinchu, in 2008. He is currently pursuing the Ph.D. degree in the Department of Photonics, NCTU.

He joined the Semiconductor Laser Technol-ogy Laboratory, NCTU, in July 2008. His current research interests include III–V compound semicon-ductor materials growth by metal organic chemical vapor deposition and characteristic study under the instruction Prof. H.-C. Kuo and T.-C. Lu.

Chien-Chung Lin was born in Taipei, Taiwan, in 1970. He received the B.S. degree in electrical engineering from the National Taiwan University, Taipei, in 1993, and the M.S. and Ph.D. degrees in electrical engineering from Stanford University, Stanford, CA, in 1997 and 2002, respectively. His thesis work focused on design, modeling and fab-rication of micromachined tunable optoelectronic devices.

He has been with the National Chiao-Tung Uni-versity (NCTU), Tainan, Taiwan, since 2009, where he holds a position as an Assistant Professor. The major research efforts in his group are in design and fabrication of semiconductor optoelectronic devices, including light emitting diodes, solar cells and lasers. Before joining NCTU, he was with different start-ups in the United States. After graduating from Stanford in 2002, he joined E2O Communications, Inc., Calabasas, CA, as a Senior Optoelectronic Engineer. In 2004, he joined Santur Corporation, Fremont, CA, where he initially worked as a Member of Technical Staff then became a Manager of laser chip engineering. He had worked on various projects such as monolithic multi-wavelength distributed feedback (DFB) laser arrays for data and telecommunications applications and yield and reliability

analysis of DFB laser arrays. He has more than 30 journal and conference publications. His current research interests include optically and electrically pumped long-wavelength vertical cavity surface emitting lasers.

Dr. Lin is a member of the IEEE Photonic Society and the Electron Devices Society.

Dong-Mei Deng received the Graduate degree with the Doctor degree in electronic and computer engi-neering at the Hong Kong University of Science and Technology, Kowloon, Hong Kong, in 2010, with a research background in metalorganic chemical vapor deposition growth.

She is currently with the Hong Kong Applied Science and Technology Research Institute, Shatin, Hong Kong.

Da-Wei Lin received the B.S. degree in photonics and the M.S. degree in electro-optical engineering from the National Chiao Tung University, Hsinchu, Taiwan, in 2009 and 2010, respectively. He is cur-rently pursuing the Ph.D. degree in the Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu.

His current research interests include optical mea-surement and analysis and nano-structure analysis for GaN-based light emitting diodes.

Jin-Chai Li received the B.S. degree in physics and the Ph.D. degree in microelectronics and solid state electronics from Xiamen University, Xiamen, China, in 2002 and 2008, respectively.

She joined Prof. S.C. Wang’s Group in the Department of Photonics, National Chiao Tung University, Hsinchu, Taiwan, in 2009, as a Post-Doctoral Research Fellow. Her current research interests include first principle simulation and epi-taxial growth of III–V materials and optoelectronic devices.

Zhen-Yu Li was born in Chiayi, Taiwan, on October 2, 1980. He received the B.S. degree from the Department of Electronic Engineering, Chung Yuan Christian University, Chung-Li, Taiwan, in 2003, and the Ph.D. degree in engineering from Chung Yuan Christian University in 2007.

He joined Prof. S.C. Wang’s Group in the Institute of Electro-Optical Engineering, National Chiao Tung University, Hsinchu, Taiwan, in October 2007, as a Post-Doctoral Research Fellow. His current research interests include metal organic chemical vapor depo-sition heteroepitaxial growth, process and characterization of optoelectronic devices, such as vertical cavity surface emitting lasers, resonant cavity light emitting diodes and solar cells.

He is currently a Post-Doctoral Researcher at Chung Yuan Christina University. His current research inter-ests include bandgap engineering of semiconductors and time-resolved phenomena in nanomaterials.

Tien-Chang Lu received the B.S. degree in electri-cal engineering from the National Taiwan University, Taipei, Taiwan, in 1995, the M.S. degree in electrical engineering from the University of Southern Califor-nia, Los Angeles, in 1998, and the Ph.D. degree in electrical engineering and computer science from the National Chiao Tung University, Hsinchu, Taiwan, in 2004.

He joined the Department of Photonics, National Chiao Tung University, as a faculty member in August 2005. He has authored or co-authored more than 100 international journal papers. His current research interests include design epitaxial growth, process and characterization of optoelectronic devices, such as Fabry-Perot type semiconductor lasers, vertical cavity surface emitting lasers, resonant cavity light emitting diodes (LEDs), microcavities, photonic crystal surface emitting lasers, wafer-fused flip-chip LEDs and solar cells.

Prof. Lu is a recipient of the Exploration Research Award of Pan Wen Yuan Foundation in 2007 and the Excellent Young Electronic Engineer Award in 2008.

Ji-Lin Shen received the Ph.D. degree in physics from the National Taiwan University, Taipei, Taiwan, in 1994.

He completed post-doctoral research in the Elec-trical Engineering Department, University of Cal-ifornia, Los Angeles, then was an Assistant Pro-fessor at Chung Yuan Christian University, Chung-Li, Taiwan, in 1998. He is currently a Professor in the Physics Department, Chung Yuan Christian University. His current research interests include optical characterization of semiconductor materials and nano-materials.

Hao-Chung Kuo (M’98–SM’06) received the B.S. degree in physics from the National Taiwan Univer-sity, Taipei, Taiwan, the M.S. degree in electrical and computer engineering from Rutgers University, New Brunswick, NJ, in 1995, and the Ph.D. degree from the Electrical and Computer Engineering Depart-ment, University of Illinois at Urbana-Champaign, Urbana, in 1998.

He has an extensive professional career both in research and industrial research institutions that includes Research Assistant in Lucent Technologies, Bell Laboratories, Murray Hill, NJ, from 1993 to 1995, and a Senior Research and Development Engineer in the Fiber-Optics Division, Agilent Technologies, Santa Clara, CA, from 1999 to 2001, and LuxNet Corporation, Fremont,

He has authored or co-authored 200 internal journal papers, two invited book chapters, six granted and 10 pending patents. His current research interests include semiconductor lasers, vertical cavity surface-emitting lasers, blue and ultraviolet light-emitting diode lasers, quantum-confined optoelectronic structures, optoelectronic materials, and solar cells.

Prof. Kuo is an Associate Editor of the IEEE/Optical Society of Amer-ica (OSA) JOURNAL OF LIGHTWAVE TECHNOLOGY and JOURNAL OF

SELECTEDTOPICS IN QUANTUMELECTRONICS—Special Issue on solid state lighting in 2009. He received the Ta-You Wu Young Scholar Award from the National Science Council Taiwan in 2007 and the Young Photonics Researcher Award from the OSA/Society of Photo-Optical Instrumentation Engineers Taipei Chapter in 2007.

Kei May Lau (S’78–M’80–SM’92–F’01) received the B.S. and M.S. degrees in physics from the University of Minnesota, Minneapolis, and the Ph.D. degree in electrical engineering from Rice Univer-sity, Houston, Texas.

She started as a Senior Engineer at M/A-COM Gallium Arsenide Products, Inc., Lowell, MA, where she worked on epitaxial growth of GaAs for microwave devices, development of high-efficiency and mm-wave impact ionization avalanche transit-time diodes, and multi-wafer epitaxy by the chloride transport process. After two years in industry, she joined the faculty of the Electrical and Computer Engineering Department, University of Massa-chusetts (UMass), Amherst, where she became a Professor in 1993. She initiated metalorganic chemical vapor deposition, compound semiconductor materials and devices programs at UMass. Her research group has performed studies on heterostructures, quantum wells, strained-layers, III–V selective epi-taxy, high-frequency and photonic devices. She spent her first sabbatical leave at Lincoln Laboratory, Massachusetts Institute of Technology, Lexington, and was with the Electro-Optical Devices Group. She developed acoustic sensors at the DuPont Central Research and Development Laboratory, Wilmington, DE, during her second sabbatical leave. She was a Visiting Professor at Hong Kong University of Science and Technology (HKUST), Kowloon, Hong Kong, in 1998. She has been a Chair Professor/Professor in the Electronic and Computer Engineering Department at HKUST since 2000. Her current research interests include metamorphic growth of III–V devices on silicon substrates.

Prof. Lau is a recipient of the National Science Foundation Faculty Awards for Women Scientists and Engineers in 1995 and Croucher Senior Research Fellowship in 2008. She served on the IEEE Electron Devices Society Administrative Committee and was an Editor of the IEEE TRANSACTIONS ONELECTRONDEVICESfrom 1996 to 2002. She also served on the Electronic Materials Committee of the Minerals, Metals and Materials Society of American Institute of Materials Engineers, and was an Editor of the Journal of Crystal Growth.