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Analysis of InGaN-delta-InN quantum wells for light-emitting diodesHongping Zhao, Guangyu Liu, and Nelson TansuCitation:Appl. Phys. Lett. 97, 131114 (2010); doi: 10.1063/1.3493188View online: http://dx.doi.org/10.1063/1.3493188View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v97/i13Published by theAmerican Institute of Physics.Related ArticlesSub-250 nm room-temperature optical gain from AlGaN/AlN multiple quantum wells with strong band-structurepotential fluctuations

Appl. Phys. Lett. 100, 061111 (2012)Deep traps and enhanced photoluminescence efficiency in nonpolar a-GaN/InGaN quantum well structuresJ. Appl. Phys. 111, 033103 (2012)Temperature dependence of the intraexcitonic AC Stark effect in semiconductor quantum wells

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Appl. Phys. Lett. 100, 043502 (2012)Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/Journal Information: http://apl.aip.org/about/about_the_journalTop downloads: http://apl.aip.org/features/most_downloadedInformation for Authors: http://apl.aip.org/authors

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Analysis of InGaN-delta-InN quantum wells for light-emitting diodes

Hongping Zhao,a Guangyu Liu, and Nelson Tansub

Department of Electrical and Computer Engineering, Center for Optical Technologies, Lehigh University,Bethlehem, Pennsylvania 18015, USA

Received 5 July 2010; accepted 5 September 2010; published online 1 October 2010

The design of InGaN-delta-InN quantum wells QWs leads to significant redshift for nitride active

region with large electron-hole wave function overlap

e_hh and spontaneous emission rate. Theanalysis was carried out by using self-consistent six-band kp band formalism. The design of activeregion consisting of 30 In0.25Ga0.75N QW with InN delta-layer leads to large e_hh of50% withemission wavelength in the yellow and red spectral regimes, which is applicable for nitride-basedlight-emitting diodes. 2010 American Institute of Physics. doi:10.1063/1.3493188

High-efficiency InGaN quantum wells QWs light-emitting diodes LEDs play an important role in solid-statelighting and display applications.13 Due to the polarizationfields in InGaN QW, the electron and hole wave functionsare confined in opposite directions leading to reduction of theelectron-hole wave function overlap e_hh . The detrimental

effect from charge separation becomes more severe, as theemission wavelength of the InGaN QWs is extended intogreen and red spectral regimes. Several approaches havebeen proposed to address the charge separation in InGaNQW as follow: nonpolar InGaN QW,4 InGaN QW withAlGaN -layer,5,6 staggered InGaN QW,715 type-II InGaN-based QW,1618 and strain-compensated InGaN QW.19

Recently, the use of staggered InGaN QWs with en-hanced matrix element results in increase in radiative recom-bination rate and radiative efficiency for nitride LEDs.713

Despite its larger overlap e_hh over conventional QW e_hh=37% for 420 nm; e_hh=17% for 500 nm ,the optimized e_hh for staggered InGaN QW decreases

e_hh=68% for 420 nm; e_hh= 32% for500 nm 8,9,12 as its emission wavelength extends tolonger spectral regime.

In this work, we present a nitride-based active regionwith high e_hh by employing InGaN-delta-InN QW. The in-sertion of ultrathin layer 36 of narrow-band gap InNalloy Eg =0.69 eV in InGaN QW leads to significant en-hancement ofe_hh. In contrast to the existing approaches toenhance the overlap,519 the optimized e_hh for the InGaN-delta-InN QW increases as its emission wavelength extendsto longer spectral regimes. The optical properties of InGaN-delta-InN QW are compared to those of the conventional

InGaN QW for LEDs.The calculation of the band structures and wave func-tions for InGaN-based QWs with GaN barriers is based on aself-consistent six-band kp formalism.1923 The details ofthe numerical model were presented in Ref. 19, and all thecomputation parameters were obtained from Refs. 24 and 25.In this calculation, all possible transitions between the con-fined states of the conduction bands and valence bands aretaken into account.

To illustrate the challenging issue to redshift the emis-sion wavelength in InGaN QWs, Fig. 1 plots the e_hh left

and peak emission wavelength right as a function of theindium In content for InGaN QW with QW thicknesses

dQW of dQW=2 nm and dQW=3 nm. As the In-content in-creases from 5% to 45%, the e_hh decreases from 60%

47% to 27.8% 6.8% while the emission wavelength in-creases from 372 nm 377 nm to 640 nm 871 nm for the

InGaN QW with dQW=2 nm dQW=3 nm . Similarly, whenthe dQW increases, the emission wavelength extends longerwhile the e_hh decreases significantly. Thus, it is importantto employ QW designs with enhanced e-hh, as the emissionwavelength is redshifted.

Figures 2 a and 2 b show the energy band lineup pro-files and the wave functions of the first conduction subband

C1 and the first valence subband HH1 at zone center for a conventional 30 In0.25Ga0.75N QW and b 30 In0.25Ga0.75N QW with inserted 3 InN delta-layer. Due tothe existence of the internal electric filed, the charge separa-tion leads to a significant reduction in e_hh of 14.8% forconventional 30 In0.25Ga0.75N QW with emission wave-

length of 492 nm. By inserting 3 InN delta-layer in thecenter of the 30 In0.25Ga0.75N QW Fig. 2 b , the largeband offset between InGaN and InN leads to the strong lo-calization of electron and hole wave functions toward thecenter for the QW resulting in enhanced e_hh of 29%. Theintroduction of the InN delta-layer in the InGaN QW leads tostrong modification of the electron and hole wave functionsresulting in significant increase in the e_hh, as well as ac-companied by a strong redshift of the transition wavelengthup to 590 nm. Thus, the InGaN-delta-InN QW is conceptu-

a Electronic mail: [email protected]

Electronic mail: [email protected].

350

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0.6

.

0 0.1 0.2 0.3 0.4 0.5

IndiumContent

e_

hh

Wavelength(nm)

dQW= 3nm

dQW= 2nm

2nm - InGaN QW

3nm - InGaN QW

FIG. 1. Color online Electron-hole wave function overlap e_hh left andpeak emission wavelength right vs indium content with InGaN QW thick-

ness dQW of dQW=2 nm and dQW=3 nm.

APPLIED PHYSICS LETTERS 97, 131114 2010

0003-6951/2010/97 13 /131114/3/$30.00 2010 American Institute of Physics97, 131114-1

http://dx.doi.org/10.1063/1.3493188http://dx.doi.org/10.1063/1.3493188http://dx.doi.org/10.1063/1.3493188http://dx.doi.org/10.1063/1.3493188

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ally very distinct from the three-layer staggered InGaNQW,912 where the latter overlap e_hh decreases as the emis-sion wavelength extends to the longer spectral regime.

Recently, Che et al.26 have proposed the use of asym-metric GaN/InN/InGaN/GaN QWs structure with InN mono-layer thickness, which leads to increase in e_hh from theintroduction of ultrathin InN layer resulting in strong local-ization of electron and hole wave functions in the InN/InGaN interface. In contrast to the asymmetric QWs,

26

our approachfocused on the insertion of InN delta-layer in the center ofInGaN QW, which leads to strong localization of electronand hole wave functions toward the center for the QW.

Figure 3 a shows the interband transition wavelengthversus the thickness d of the InN layer for 30 In0.25Ga0.75N /d InN QW structure. The interband transi-tion wavelength increases as the thickness of the InN layerincreases. Figure 3 b shows the overlap e_hh versus thethickness d of the InN layer for 30 In0.25Ga0.75N /d InN QW structure. When the InN layer thickness is 6

as delta layer , the e_hh increases as InN layer thicknessincreases due to the shift of the electron and hole wave func-

tions toward the center of the InGaN QW. Note that thedesign of InGaN QW with delta-InN layer 6 allowsone to redshift the emission wavelength significantly accom-panied by enhancement in the e_hh. Thus, the role of the InNlayer in the InGaN-delta-InN QW structure is very differentfrom that in the InN single quantum well SQW Fig. 3 b .When the InN layer thickness is 6 , the e_hh reduces asInN layer thickness increases, which indicates that the InNlayer behaves as SQW.

Figure 4 a shows the spontaneous emission spectra forconventional 30 In0.25Ga0.75N QW dash line , 3 0 In0.25Ga0.75N /3 InN QW dashed-dotted line , 3 0 In0.25Ga0.75N /4 InN QW dotted-dotted line and 30

In0.25Ga0.75N /6 InN QW solid line at carrier densities n of n =51018101018 cm3 at T=300 K. From theinsertion of InN delta-layer, the transition wavelength shows

0

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Wavefunctionsoverlap(e_

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(b)30- In0.25Ga0.75N / d- InN QW

InGaN-delta-InN QW

InN SQW-like

FIG. 3.

Color online a

Interband transition wavelength and

b

electron-hole wave function overlap e_hh vs InN layer thickness for 30 In0.25Ga0.75N /d InN QW for both cases of InGaN-delta-InN QW

d6 and InN SQW-like d6 .

FIG. 2. Color online Energy band lineups of a conventional 30 In0.25Ga0.75N QW, and b 30 In0.25Ga0.75N /3 InN QW with electronwave function of EC1 and hole wave function of HH1.

0

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0 5 10 15 20Carrier Density (x 1018cm-3)

30-In0.25Ga0.75N / 4- InN QW( ~ 632 nm)

Rsp

(x1027s-1cm-3)

(b)

30-In0.25Ga0.75N QW( ~ 492 nm)

30-In0.25Ga0.75N / 3- InN QW( ~ 590 nm)

30-In0.25Ga0.75N / 6- InN QW( ~ 747 nm)

1.6 1.8 2 2.2 2.4 2.60

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neousemissionspectrum

(x1027s-1cm-3eV-1)

30- In0.25Ga0.75N / 4-InN QW (~632 nm)

n = 5x1018 cm-3

n = 8x1018

cm-3

n = 10x1018 cm-3

(a)

30- In0.25Ga0.75N / 6-InN QW (~747 nm)

30- In0.25Ga0.75N QW( ~ 492 nm)

30- In0.25Ga0.75N / 3-InN QW (~590 nm)

ConventionalInGaNQW

InGaN-delta-InNQW

FIG. 4. Color online a Spontaneous emission spectra for conventional3 0 I n0.25Ga0.75N QW, 30 In0.25Ga0.75N /3 InN QW, 30 In0.25Ga0.75N /4 InN QW and 30 In0.25Ga0.75N /6 InN QW with car-rier density n=51018101018 cm3, and b spontaneous emission ra-diative recombination rate Rsp as a function of carrier density up to 201018 cm3 for the three QW structures at T=300 K.

131114-2 Zhao, Liu, and Tansu Appl. Phys. Lett. 97, 131114 2010


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significant redshift as compared to that of the conventionalInGaN QW. The 30 In0.25Ga0.75N /3 InN QW shows aredshift from 492 nm conventional InGaN QW to 590 nmwith ~3.4 times higher of the peak spontaneous emissionspectra 1.341027 s1 cm3 eV1 than that of the conven-tional one 3.91026 s1 cm3 eV1 at n=101018 cm3.The In0.25Ga0.75N QW with 6 InN delta-layer shows aredshift from 492 nm conventional InGaN QW to 747 nm

with ~7 times higher of the peak spontaneous emission spec-tra 2.731027 s1 cm3 eV1 than that of the conventionalone at n=101018 cm3.

Figure 4 b illustrates the spontaneous emission radiativerecombination rate per unit volume Rsp for conventional 30 In0.25Ga0.75N QW, 30 In0.25Ga0.75N /3 InN QW, 30 In0.25Ga0.75N /4 InN QW, and 30 In0.25Ga0.75N /6 InN QW as a function of carrier density. For the 30 In0.25Ga0.75N /3 InN QW structure, the Rsp is enhanced by44.6 times at each carrier density as compared to that forconventional 30 In0.25Ga0.75N QW. The enhancement ofthe Rsp for the 30 In0.25Ga0.75N /4 6 InN QW rangesbetween 5.28.3 6.412 times.

Note that high quality InN alloy have been reported formaterial grown by metal organic chemical vapor deposition

MOCVD Refs. 2733 and molecular beam epitaxy MBE .26,3337 The growths of InN material by MBE haveresulted in high electron mobility in the order of2370 cm2 / V s .34 The use of pulsed MOCVD has also re-sulted in high optical quality InN alloy.27,28 Recent experi-mental studies by MBE have also indicated the capabilitiesto grow InN with monolayer precision.26,37

The key idea from this work is to illustrate the advantagearises from the insertion of narrow-band gap delta-layer inInGaN QW, which enables the extension of emission wave-length while resulting in QW with large matrix element and

large radiative recombination rate. Experimental studies arerequired to clarify on the optimized design for realisticInGaN-delta-InN QW LEDs, which require the need to takeinto account the large lattice mismatch and phase separationissues in InN/InGaN heterostructure, as well as current injec-tion efficiency in nitride LEDs.38

Our analysis assumes that the InN delta-layer inserted inInGaN QW as uniform ultrathin layer, thus the active regionis treated as two-dimensional QW structure. However, theInN nonuniformity or clustering may lead to quantum-dotlike characteristics, which will require further investigation.However, it is important to point out that the challenges inthe high precision control in the epitaxy of very thin InN

layer may present potential advantage, in particular for real-izing broadband white emission from the potential thicknessvariation or InN clustering from the InN delta-layer.

In summary, the design of delta-InN layer inserted in theInGaN QW results in the ability to extend its emission wave-length into yellow and red spectral regime, with significantlyenhanced matrix element and spontaneous emission rate. Thedesign of 30 InGaN QW with 3 6 InN delta-layershows redshift in the emission wavelength by 100 nm

250 nm with enhancement of the spontaneous emissionrate of 44.6 times 6.412 times as compared to thatof the conventional InGaN QW. The InGaN-delta-InN QWhas potential for achieving high-efficiency nitride LEDs andlasers emitting in the yellow and red spectral regime.

This work is supported by Department of Energy DE-FC26-08NT01581 , National Science Foundation ECCS0701421 , and Class of 1961 Professorship Fund.

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131114-3 Zhao, Liu, and Tansu Appl. Phys. Lett. 97, 131114 2010

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