AlGaN nanowire light-emitting diodes grown by molecular beam epitaxy

We report on the achievement of high ef ﬁciency green, yellow, and red InGaN/AlGaN dot-in-a-wire nanowire light-emitting diodes grown on Si(111) by molecular beam epitaxy.. The peak emis[r]

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Original Article

light-emitting diodes grown by molecular beam epitaxy

M.R Philipa, D.D Choudharya, M Djavida, K.Q Leb,c, J Piaod, H.P.T Nguyena,e,*

a Department of Electrical and Computer Engineering, New Jersey Institute of Technology, University Heights, Newark, NJ 07102, USA

b Faculty of Science and Technology, Hoa Sen University, Ho Chi Minh City, Viet Nam

c Center for Mesoscopic Sciences, Institute for Molecular Science, Myodaiji, Okazaki, Aichi 444-8585, Japan

d Epitaxial Laboratory Inc., Tiana Place, Dix Hills, NY 11746, USA

e Electronic Imaging Center, New Jersey Institute of Technology, University Heights, Newark, NJ 07102, USA

a r t i c l e i n f o

Article history:

Received 12 May 2017

Received in revised form

19 May 2017

Accepted 19 May 2017

Available online 31 May 2017

Keywords:

Light-emitting diodes

Molecular beam epitaxy

Nanowire

Coreeshell

III-Nitride

a b s t r a c t

We report on the achievement of high efﬁciency green, yellow, and red InGaN/AlGaN dot-in-a-wire nanowire light-emitting diodes grown on Si(111) by molecular beam epitaxy The peak emission wavelengths were altered by varying the growth conditions, including the substrate temperature, and In/

Gaflux ratio The devices demonstrate relatively high (>40%) internal quantum efficiency at room temperature, relative to that measured at 5 K Moreover, negligible blue-shift in peak emission spectrum associated with no efficiency droop was measured when injection current was driven up to 556 A/cm2 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open

access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)

1 Introduction

Phosphor-free white light-emitting diodes (LEDs) have been

intensively studied and identiﬁed as an emerging platform for

future solid-state lighting and displays To realize high

perfor-mance, low cost phosphor-free white LEDs, it is critically important

to develop high efﬁciency LEDs with emission wavelengths in the

deep green to red spectral range [1,2] The achievement of such

devices using conventional InGaN/GaN quantum well

hetero-structures has been difﬁcult, due to the presence of large densities

of dislocations and polarizationﬁelds [3] Moreover, they suffer

from efﬁciency droop, which has been explained by the Auger

recombination[4], poor hole transport[5,6], and carrier

delocal-ization[7]in InGaN/GaN heterostructures In this regard,

InGaN-based nanowire heterostructures have been intensively

investi-gated, which provides a relatively defect-, strain- and

polarization-free platform for realizing high performance LEDs and other nanoscale devices [8e11] Tunable emission has been demon-strated using InGaN/GaN nanowire heterostructures[12e14] In spite of the progress, however, the achievement of high efﬁciency nanowire LEDs has remained elusive

Due to the large surface-to-volume ratios, the surface plays a key role in the operation and electrical and optical properties of III-nitride nanowire LEDs[15,16] Depending on the energy levels of the surface states, as well as the surface stoichiometry, Fermi-level pinning has been theoretically predicted and experimentally observed on the (11-00) plane[17,18], i.e the lateral surfaces of commonly reported GaN nanowire LEDs The resulting lateral electric field, as well as the associated surface nonradiative recombination is highly detrimental to the performance of GaN-based nanowire LEDs In this context, we have recently developed InGaN/AlGaN dot-in-a-wire nanoscale heterostructures, which can provide enhanced carrier confinement and carrier injection, thereby leading to high emission efficiency[19] Moreover, by en-gineering color emission of such InGaN/AlGaN nanowire hetero-structures, we demonstrated full-color emission across the visible spectrum, leading to achievement of phosphor-free white LEDs [19] In this paper, we focus on developing high efficiency green/ yellow, and red LEDs for their applications in high power

phosphor-* Corresponding author Department of Electrical and Computer Engineering,

New Jersey Institute of Technology, University Heights, Newark, NJ 07102, USA.

E-mail addresses: khaidotle@ims.ac.jp (K.Q Le), hieu.p.nguyen@njit.edu

(H.P.T Nguyen).

Peer review under responsibility of Vietnam National University, Hanoi.

Contents lists available atScienceDirect Journal of Science: Advanced Materials and Devices

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / j s a m d

http://dx.doi.org/10.1016/j.jsamd.2017.05.009

2468-2179/Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license

Journal of Science: Advanced Materials and Devices 2 (2017) 150e155

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Fig 1 (a) Schematic illustration of an InGaN/AlGaN dot-in-a-wire LED heterostructure; (b) A 45tilted scanning electron microscopy image showing the morphology of the InGaN/ AlGaN dot-in-a-wire heterostructures grown on a Si(111) substrate by molecular beam epitaxy.

ﬂow of InGaN/AlGaN nanowire LEDs.

M.R Philip et al / Journal of Science: Advanced Materials and Devices 2 (2017) 150e155 151

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free white LEDs The fabrication and characterization of InGaN/

AlGaN dot-in-a-wire nanowire LEDs on Si(111) substrates were

performed We observe that the emission wavelengths can be

tuned from green to red spectral range by varying the sizes and/or

compositions of the dots Moreover, relatively high (>40%) internal

quantum efﬁciencies were measured for GaN-based nanowire yellow LEDs No efﬁciency droop was observed for injection current

as high as 556 A/cm2at room temperature

2 Experimental The InGaN/AlGaN dot-in-a-wire LED heterostructures, illus-trated inFig 1(a), were spontaneously formed on n-type Si(111) substrates under nitrogen rich conditions by a Veeco Gen II mo-lecular beam epitaxial (MBE) system equipped with a radio-frequency plasma-assisted nitrogen source GaN nanowires were doped n- and p-type using Si and Mg, respectively The growth conditions for GaN include a growth temperature of ~750e800C,

nitrogenﬂow rate of 1 sccm, and forward plasma power of ~350 W The device active region consists of ten vertically aligned InGaN dots, separated by 3 nm AlGaN barrier layers The InGaN/AlGaN quantum dot heterostructures were grown at relatively low tem-peratures (560e620C) to enhance the incorporation of In The

nanowire diameter and density can be controlled by the substrate temperature and/or In/Gaﬂux ratios, while the nanowire length can be adjusted by the growth duration[20,21] Using such nano-wire structures, have demonstrated that, by varying the growth conditions, high brightness green, yellow and red emissions from the InGaN/AlGaN dot-in-a-wire heterostructures can be achieved Surface morphology of the dot-in-a-wire LED heterostructures was studied by scanning electron microscopy (SEM) A 45 degree-titled SEM image is illustrated inFig 1(b) The wire areal density

is ~1 1010cm2 Moreover, during the growth of AlGaN barrier layer, an AlGaN shell layer is spontaneously grown due to the diffusion-controlled growth process that fully covers the InGaN dot active region[22] Such coreeshell heterostructures exhibit dras-tically reduced nonradiative surface recombination, and enhanced carrier injection efﬁciency More detailed information on the MBE growth and structural properties of this InGaN/AlGaN coreeshell nanowire structures can be found elsewhere[19,22]

Fig 2presents the device fabrication process It involves the use

of a polyimide resist for surface planarization, standard photoli-thography and dry etching processes The nanowire LED sample (Fig 2(a)) wasﬁrst fully covered with polyimide resist, followed by oxygen plasma dry etching to expose the top of nanowires for metallization, shown inFig 2(b) Top p-contact with Ni(5 nm)/Au (5 nm)/ITO (200 nm) was then deposited on the nanowire surface, illustrated in Fig 2(c) Subsequently, thick layers of Ni(10 nm)/ (100 nm) and Ti (10 nm)/Au (100 nm) were deposited on top of ITO transparent contact to sever grid-metal contact to facilitate the carrier injection and backside metal contact, respectively, shown in Fig 2(d) The LED devices with areal size of 300 300mm2were chosen for characterization

3 Results and discussion Performance characteristics of the dot-in-a-wire LEDs were measured under pulsed bias conditions with 1% duty cycle to minimize junction heating effect Strong green, yellow, and red emissions were measured from the InGaN/AlGaN dot-in-a-wire LEDs at room temperature, shown inFig 3 The electrolumines-cence (EL) spectra of the green, yellow, and red nanowire LEDs under different injection current varied from 30 mA to 400 mA were shown inFig 3(a)e(c), respectively At injection current of

400 mA, the peak emission wavelengths at ~535, 585, and 645 nm for green, yellow, and red LED devices, respectively It may also be noticed that the spectral linewidths increases progressively with emission wavelengths, varying from 75, 105, to 125 nm for the green, yellow, and red-emitting devices, respectively This is a direct consequence of the enhanced In phase separation with

Fig 3 Room temperature electroluminescence spectra under different injection

cur-rents for (a) green, (b) yellow, (c) red nanowire LEDs The inset of each ﬁgure shows the

corresponding light emissions from green, yellow and red nanowire LEDs.

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increasing In compositions, that leads to the formation of In-rich

nanoclusters in the dots as well as in the barrier layers The

emis-sion properties of LEDs are depended on the compositions, the sizes

of the dots, and the diameter of the nanowires as well These

pa-rameters can be controlled by adjusting substrate temperature,

growth duration, and In/Gaﬂux ratio The achievement of strong

emission in long wavelength from green to red region attributed to

the successful usage of coreeshell nanowire heterostructure

asso-ciated with the embedded quantum dots

We have further investigated variations of the peak emission

wavelengths with injection currents for the dot-in-a-wire LEDs

Illustrated inFig 3(a) and (b), peak wavelengths of the green and

yellow-emitting LEDs are virtually invariant with increasing

cur-rent, suggesting the presence of a negligible quantum-conﬁned

Stark effect [12,23] A very small blue shift (~3e5 nm), on the

other hand, was observed for the red-emitting devices, illustrated

inFig 3(c) The highly stable emission characteristics are further

illustrated inFig 4, which shows locations of the light emission

from the various LEDs on the 1931 Commission International de

l'Eclairage (CIE) chromaticity diagram under injection currents

from 100 to 400 mA As expected, the green (triangles) and yellow

(circles) LEDs exhibit nearly invariant CIE chromaticity coordinates

of (x¼ 0.22, y ¼ 0.55) and (x ¼ 0.44, y ¼ 0.50) with increasing

current, respectively The red (stars) devices show very small

var-iations, with the derived CIE chromaticity coordinates being

(xz 0.54e0.55, y z 0.36e0.37)

We have further studied the currentevoltage and light-current

characteristics of the yellow LEDs The defect density, polarization

field, and internal electric field induces quantum-confined Stark

effect were minimized also results in perfect diode performances

with very low leakage current of ~0.5 mA at6 V, as presented in

Fig 5(a) We have further conﬁrmed that the dot-in-a-wire yellow

LEDs exhibit virtually no efﬁciency droop at room temperature

Illustrated in Fig 5(b), the output power increases linearly with current for the entire measurement range (up to ~ 556 A/cm2) This observation is consistent with recent studies that Auger recombi-nation is signiﬁcantly reduced in InGaN/GaN nanowire hetero-structures, due to the reduced defect densities[16] Additionally, hole transport problem may also be minimized with the use of p-type modulation doping in the device active region[14]and the use

of self-distributed AlGaN multi-shell electron blocking layer [22,24]

Finally, we have investigated the internal quantum efﬁciencies (IQEs) of the dot-in-a-wire LEDs by comparing the integrated electroluminescence intensity measured at 300 K to that measured

at 5 K under the same injection current Shown inFig 5(c), the IQEs increase with injection currents and reach maximum values of 44.7% for injection currents at ~300 mA (~333 A/cm2) for the yellow-emitting LEDs, suggesting a small, or negligible efficiency droop under relatively high current injection conditions The peak IQE was measured at high injection current (~333 A/cm2) which is significantly higher than that of conventional InGaN/GaN thin-film LEDs which is normally at 10e20 A/cm2 [14,25] The very slow rising trend of the IQE has been commonly reported in nanowire LEDs[14,26,27]and is attributed to the presence of large surface states and defects on nanowire surfaces More detailed information was presented in our previous study[16,28] The negligible ef fi-ciency droop is attributed to the strong carrier confinement pro-vided by the quantum dot heterostructures, the enhanced carrier injection, the reduced electron overflow and other higher order effects on the device quantum efficiency [29,30] Compared to previously reported InGaN/GaN nanowires[31]or well/disk-in-a-wire LED heterostructures[32,33]in the same wavelength range, the unique dot-in-a-wire heterostructures exhibit significantly higher IQE which can be explained due to the higher effective carrier confinement along the wire radial direction, thereby

Fig 4 1931 Commission International de l'Eclairage (CIE) chromaticity diagrams of the light emission of green (triangles), yellow (circles), and red (stars) LEDs.

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minimizing the nonradiative carrier recombination on the lateral

surfaces Moreover, the signiﬁcantly improved IQEs measured in

the dot-in-a-wire LEDs is attributed to the signiﬁcantly enhanced

carrier conﬁnement and reduced nonradiative carrier

recombina-tion associated with the presence of surface states, as well as the

enhanced carrier injection to the device active region

For practical lighting applications, the performances of such nanowire LEDs should be further improved for the enhanced light extraction efﬁciency We had previously reported that nanowire LEDs with large diameters exhibit higher carrier injection efﬁciency due to the reduced surface nonradiative recombination, compared

to that of a smaller diameter wires However, large diameter nanowires can suffer from large dislocation density in the nanowire structures, resulted in the reduced quantum efficiency Therefore, the optimized nanowire density, height, and diameter play critical roles and lead to further enhanced light extraction efficiency Moreover, the Ni/Au/ITO top metal contact should be further optimized or replaced by a better transparent contact design for higher light extraction efficiency

4 Conclusion

In summary, we have demonstrated that, with the use of self-organized InGaN/AlGaN dot-in-a-wire heterostructures, GaN-based nanowire LEDs can exhibit relatively high internal quan-tum efﬁciency in the deep green to red wavelength range under electrical injection Moreover, the devices show highly stable emission characteristics with increasing current and virtually no

efﬁciency droop at relatively high injection conditions (up to 556 A/

cm2) These results provide a significant progress for future solid-state lighting applications wherein the usage of low cost, high ef-ficiency, smart LEDs are realized Such nanowire LEDs are perfectly suited for wearableflexible electronics as well as high speed LEDs for visible light communications

Acknowledgments This work is supported by New Jersey Institute of Technology grant 210124 and the National Science Foundation grant EEC-1560131

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