An electrically driven quantum dot-in-nanowire visible single photon source operatingup to 150KSaniya Deshpande and Pallab Bhattacharya

Citation: Applied Physics Letters 103, 241117 (2013); doi: 10.1063/1.4848195 View online: http://dx.doi.org/10.1063/1.4848195 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/24?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Germanium doping of self-assembled GaN nanowires grown by plasma-assisted molecular beam epitaxy J. Appl. Phys. 114, 103505 (2013); 10.1063/1.4820264 Blue single photon emission up to 200K from an InGaN quantum dot in AlGaN nanowire Appl. Phys. Lett. 102, 161114 (2013); 10.1063/1.4803441 Single photon emission from InGaN/GaN quantum dots up to 50K Appl. Phys. Lett. 100, 061115 (2012); 10.1063/1.3683521 Position controlled nanowires for infrared single photon emission Appl. Phys. Lett. 97, 171106 (2010); 10.1063/1.3506499 Growth of single quantum dots on preprocessed structures: Single photon emitters on a tip Appl. Phys. Lett. 86, 091911 (2005); 10.1063/1.1869544

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An electrically driven quantum dot-in-nanowire visible single photonsource operating up to 150 K

Saniya Deshpande and Pallab Bhattacharyaa)

Center for Photonics and Multiscale Nanomaterials, Department of Electrical Engineering and ComputerScience, University of Michigan, Ann Arbor, Michigan 48109-2122, USA

(Received 27 September 2013; accepted 29 November 2013; published online 13 December 2013)

We demonstrate electrically pumped single photon emission up to 150 K from a single InGaN

quantum dot embedded in a GaN nanowire junction diode. The InGaN dot-in-nanowire p-n junctions

were grown on silicon by molecular beam epitaxy. The exciton electroluminescence from individual

dot-in-nanowires is in the green spectral range (k� 520 nm) and is detectable up to 150 K. Second

order autocorrelation measurements performed at the exciton energy at an ambient temperature of

125 K show a background corrected g(2)(0) equal to 0.35, indicating dominant single photon

emission. The steady state nanowire temperature under these conditions is estimated to be 150 K due

to Joule heating induced by the large nanowire series resistance. Time resolved photoluminescence

measurements yield an exciton radiative lifetime of 1.1 ns. VC 2013 AIP Publishing LLC.

[http://dx.doi.org/10.1063/1.4848195]

Developing electrically driven single photon sources

(SPSs) is a crucial step towards realizing practical quantum

information processing and quantum cryptography schemes.

While a variety of systems such as atoms, dye molecules, di-

amond color centers, semiconductor impurities, and quantum

dots have been used to demonstrate single photon emission,

most of these schemes rely on optical pumping by an exter-

nal laser.1–5 Semiconductor quantum dots are perhaps the

most promising candidates for practical SPSs because they

can be easily incorporated into a diode heterostructure or

microcavity, in addition to their thermal stability, fast recom-

bination lifetimes, narrow spectral linewidths, control over

emission wavelength, and polarization of emission in a pre-

ferred direction. There have been few reports of electrically

pumped single photon emission from semiconductor quan-

tum dots inserted in p-i-n diodes.6–9

A major drawback with commonly used III-V quantum

dots (QDs), such as InAs QDs, for single photon devices is the

need for operation at cryogenic temperatures. This difficulty

can potentially be overcome by using III-nitrides and II-VI

semiconductors with large bandgap offsets and improved zero-

dimensional confinement of carriers. Room temperature single

photon emission has been observed in CdSe/ZnSe nanowire-

QDs with optical pumping but only in nanowires grown on

high quality ZnSe pseudosubstrates.10 II-VI nanocrystals

which also offer high temperature operation suffer from photo-

bleaching and fluorescence intermittency.11 Group III-nitrides

have emerged as attractive materials for ultraviolet-blue single

photon emission. Epitaxial self-organized GaN and InGaN

QDs have been used to demonstrate optical single photon

emission up to 200 K.12–15 However, self-organized QDs ex-

hibit several limitations for electrically pumped single photon

sources: poor light extraction efficiency due to QDs embedded

in a planar matrix, leakage of carriers into the two dimensional

wetting layer at higher temperatures, a large dislocation den-

sity due to heteroepitaxy, and a large built-in electric field of

the order of a few MV/cm which has a detrimental effect on

the radiative recombination process. On the other hand, GaN

and InGaN nanowire-QDs have several distinct advantages.

GaN nanowires grown epitaxially on silicon substrates are

almost free of extended defects.16–19 They are relatively strain-

free due to radial strain relaxation during epitaxy, which leads

to significantly smaller piezoelectric polarization fields and

larger exciton oscillator strength in the dot-in-nanowire hetero-

structure. The surface recombination velocity in nitride nano-

wires is small and of the order of �103 cm/s.20 Due to these

favorable attributes, electrically injected single photon emis-

sion in the blue spectral range has been recently demonstrated

with a single InGaN/GaN dot-in-nanowire, albeit at cryogenic

temperatures.21

Here, we report electrically driven green (k� 520 nm)

single photon emission from an In0.37Ga0.63N/GaN dot-in-na-

nowire. This is an important spectral range for single photon

sources because silicon based single photon avalanche diodes

have their quantum efficiency maxima in the green spectral

range (�550 nm). It is also suitable for free space quantum

cryptography applications where the short emission wave-

length would be economical by allowing for the reduction in

emitter and receiver telescope sizes. Additionally, plastic and

polymer optical fibers have transmission windows in this

wavelength range. At the operation temperature of 150 K,

multi-stage thermoelectric coolers can substitute liquid he-

lium or nitrogen cooling. The InGaN/GaN dot-in-nanowire

system can potentially emit single photons over the entire

visible spectrum. The quantum confinement energies in the

In0.37Ga0.63N/GaN system are large (�1 eV), which prevent

leakage of carriers out of the QD at higher temperatures.

This, in combination with the use of a p-doped Al0.15Ga0.85N

electron blocking layer and reduced device resistance, plays a

key role in enabling higher operation temperatures for the

InGaN/GaN dot-in-nanowire single photon source.

The dot-in-nanowire p-n junctions were grown on

Si (111) by plasma assisted molecular beam epitaxy (PA-

MBE) in a nitrogen rich growth environment. The nanowirea)Email: pkb@eecs.umich.edu

0003-6951/2013/103(24)/241117/4/$30.00 VC 2013 AIP Publishing LLC103, 241117-1

APPLIED PHYSICS LETTERS 103, 241117 (2013)

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heterostructure, shown schematically in Fig. 1(a), consists of

a single In0.37Ga0.63N dot at the center of a GaN p-n junction,

where Mg and Si were used as the respective dopants. The

growth is initiated without Ga droplet nucleation and the re-

sultant nanowires have a mean diameter of �30 nm. The n

and p doping levels are estimated to be 3 � 1018 cm�3 and 5

� 1017 cm�3 based on Hall measurements performed on sim-

ilarly doped planar layers. During the growth of the quantum

dot, the substrate temperature was lowered to 520 �C for effi-

cient incorporation of In and an In flux of 8� 10�8 Torr was

used. The Indium composition in the In0.37Ga0.63N quantum

dot was determined using EDX (energy dispersive X-ray)

spectroscopy performed in a scanning transmission electron

microscope. A Mg-doped Al0.15Ga0.85N electron blocking

layer was grown on top of the GaN capping layer to ensure

confinement of carriers in the QD. The Aluminum fraction in

the electron blocking layer was confirmed by SEM-EDX and

X-ray diffraction characterization of AlGaN nanowires,

grown under identical growth conditions. Other growth pa-

rameters are similar to those described previously.21 The op-

tical properties of the nanowire ensemble were investigated

by temperature dependent photoluminescence measurements

and the data are shown in Fig. 1(b). The dot-in-nanowire

emission has a photoluminescence peak at �520 nm and rel-

atively narrow emission linewidth of �40 nm at room tem-

perature. No defect-related yellow band was observed in the

PL spectra, which confirms good crystalline quality of the

nanowires. An internal quantum efficiency (IQE) of 36%

was measured in this sample, assuming that the IQE at 10 K

is 100%.

The oblique view scanning electron microscope (SEM)

image of n-GaN/ In0.37Ga0.63N /p-GaN nanowires in Fig. 2(a)

depicts a dense forest of vertically aligned nanowires. The

nanowire density is �1010/cm2. The inset shows a high

resolution transmission electron microscope (TEM) image

of the In0.37Ga0.63N quantum dot, confirming a defect-free

InGaN/GaN interface and dot height of �3 nm. Nanowires

were first ultrasonicated in a solution of isopropyl alcohol

(IPA) for 30 min and then dispersed on a (001) silicon wafer

coated with 100 nm of thermal oxide, pre-patterned with

alignment markers. Single nanowires were located using SEM

imaging and ohmic contacts were defined on the ends of sin-

gle nanowires by electron beam lithography and electron

beam evaporation. The metallization used for making contact

to n-GaN was Ti/Au (5 nm/50 nm) and that to p-GaN was

Pd/Au (5 nm/ 50 nm). The nanowire surface was treated with

dilute buffered hydrofluoric acid prior to p-contact metal depo-

sition to ensure perfect contact metallurgy. The contacts

showed excellent ohmic behavior after annealing at 200 �C in

air. The room temperature current-voltage characteristics of the

single nanowire p-n junction are presented in Fig. 2(b). The se-

ries resistance at a bias of 17 V is �25 MX. A SEM image of a

fabricated device is seen in Fig. 2(c). After wirebonding the

fabricated sample on to a chip carrier, the micro electrolumi-

nescence (micro-EL) was collected by a microscope objective

lens (numerical aperture of 0.5), dispersed by a high resolution

monochromator and detected by a photomultiplier tube using

phase sensitive lock-in amplification. Electroluminescence

measurements were performed under cw electrical excitation in

the temperature range of 10–150 K in a continuous flow

Helium cryostat. The micro-EL spectrum at 10 K shows two

predominant sharp resonances around �2.39 eV. As seen in

Fig. 2(d), the integrated intensity of these transitions has a

nearly linear and quadratic dependence on injection current,

hence these resonances can be ascribed to the exciton and biex-

citon transitions, respectively. The strength of the exciton

FIG. 1. (a) Schematic of a single dot-in-nanowire diode heterostructure; (b)

Temperature dependent photoluminescence spectra of InGaN/GaN dot-in-

nanowire ensemble shown in Fig. 2(a).

FIG. 2. (a) Scanning electron micrograph of an as-grown nanowire ensemble

imaged at a tilt of 45�. A high resolution image of an In0.37Ga0.63N/GaN QD

is seen in the inset; (b) I-V characteristics of the single nanowire diode; (c)

SEM image of a single InGaN/GaN nanowire diode with contacts fabricated

on an oxide coated Si wafer; (d) Electroluminescence intensity as a function

of injection current for exciton and biexciton transitions at 10 K.

241117-2 S. Deshpande and P. Bhattacharya Appl. Phys. Lett. 103, 241117 (2013)

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transition drops for injection currents beyond 10 nA due to

competition from the biexciton state. The biexciton lumines-

cence also saturates at the high injection currents, which sug-

gests triexcitons and higher order excitons cannot be excited

in these dots.

Micro-EL spectra for progressively higher device tem-

peratures are shown in Fig. 3(a). The normalized integrated

intensity of exciton and biexciton luminescence is plotted as

Arrhenius plots in Fig. 3(b). The plots show that the radiative

efficiency of the exciton luminescence is almost 75% at

150 K which indicates that the dominant mode of carrier

recombination up to this temperature is through radiative

channels. This has been confirmed through independent tem-

perature dependent time resolved photoluminescence

(TRPL) measurements where a reduction in the recombina-

tion lifetime due to dominant non-radiative recombinations

is observed at high temperature. The biexciton EL quenches

rapidly with temperature and shows a radiative efficiency of

�25% at 100 K, which is the highest temperature at which

the biexciton luminescence can be distinguished. Since the

exciton transition exhibits better temperature stability, we

investigated it as a high temperature single photon emitter

and studied its temperature dependent EL characteristics.

The low temperature exciton emission linewidth is 2 meV.

The variation of the exciton emission energy as a function of

temperature is shown in Fig. 3(c). The emission energy fol-

lows the variation predicted by the Varshni equation:

E ¼ E0 �aT2

bþ T; (1)

with a¼ 4.5 � 10�4 eV/K and b� 629 K. The absence of a

S-shaped dependence, which is commonly observed in

InGaN quantum wells,22 can be attributed to a reduced polar-

ization field and the absence of local In clusters in the quan-

tum dot.

Single photon emission from the single InGaN dot in

GaN nanowire was studied in a Hanbury–Brown and Twiss

(HBT) measurement system with Si-based avalanche photo-

diodes (APDs). The second-order autocorrelation from the

exciton luminescence at 10 K is seen in Fig. 4(a). The pro-

nounced dip in the correlation signal with respect to the

Poisson-normalized level verifies suppression of multiphoton

emission events. The correlation data can be analyzed with

the function

g2 sð Þ ¼ 1� A� expð�s=srÞ; (2)

where (1-A) is equal to the g(2)(0), and sr is the recombination

lifetime. Using this function, the value of g(2)(0) derived for

the exciton transition at 10 K is 0.25 and the recombination

lifetime sr is �1.4 ns. The inset shows the transient decay of

the exciton transition which was independently measured at

10 K by TRPL using a frequency tripled Ti:Sapphire laser

(k¼ 267 nm, repetition rate¼ 80 MHz, pulse width¼ 130 fs)

excitation source. The data can be analyzed with a stretched

FIG. 3. (a) Temperature dependent micro-electroluminescence spectra from

the single In0.37Ga0.63N QD. Variation of (b) integrated intensity of exciton

and biexciton emission, (c) exciton emission peak energy.

FIG. 4. (a) Second order correlation at exciton energy at 10 K, under injection

current density of 265 A/cm2. Time-resolved photoluminescence intensity

measured at the exciton peak at 10 K is plotted in the inset; (b) Second order

correlation at the exciton energy at 150 K. Single photon emission with back-

ground corrected g(2)(0) equal to 0.35 is observed. Inset shows a pulsed single

photon stream originating from single QD electroluminescence.

241117-3 S. Deshpande and P. Bhattacharya Appl. Phys. Lett. 103, 241117 (2013)

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exponential model: I¼ Io exp[�(t/s)b], which yields values of

the stretching parameter b¼ 0.90 and recombination lifetime

s¼ 1.1 ns. The latter is in good agreement with the lifetime

obtained from the analysis of second order correlation data.

Assuming that all non-radiative recombination centers are

frozen at 10 K, this lifetime approximates the radiative life-

time of exciton emission. The b value of 0.9 indicates the ab-

sence of significant clustering effects in the quantum dot.

Joule heating due to the large nanowire series resistance

raises the junction temperature above the ambient tempera-

ture. We have modeled the heat transfer problem by solving

the three dimensional Laplace equation. The simulation

reveals an actual device temperature of 150 K at an ambient

temperature of 125 K, when the current density through the

nanowire is 354 A/cm2. Figure 4(b) shows the second order

correlation measured under these conditions, from which a

g(2)(0) value of 0.44 is derived, confirming single photon

emission from the InGaN dot. At 150 K, the exciton linewidth

broadens to �14 meV and partially merges with the biexciton

transition, which causes a degradation in the g(2)(0) value.

The finite value of the second order correlation at zero time

delay may be attributed to the time resolution of the measure-

ment equipment and to the background signal. The jitter asso-

ciated with the photon counting avalanche photodiodes limits

the time resolution of the HBT system. Since the InGaN dot

recombination lifetime is short (�1 ns) the linewidth of the

correlation trace at zero delay becomes comparable to the jit-

ter, causing broadening of the dip accompanied by a signifi-

cant reduction of its amplitude. Background noise includes

the dark counts in the detectors, emission from the biexciton

line and any stray light that enters the detection system. To

correct for the effect of background counts, we apply a cor-

rection using: gb(2)(0)¼ 1þ (g(2)

meas(0)�1)/q2, where q is the

ratio of signal-to-total counts, including dark counts.23 The

gb(2)(0) values derived after background correction for the

exciton emission at 10 K and 125 K are 0.11 and 0.35, respec-

tively. The inset shows a periodic stream of single photons

recorded by a single photon detector. The start trigger to the

counter was provided by the 80 MHz Ti:Sapphire laser and

stop signal is received from the single QD electrolumines-

cence. A single photon emission event is recorded within 1 ns

of each start trigger, which implies the single photon source

can be driven at frequencies in the GHz range.

In conclusion, single nanowire diodes were fabricated

from MBE grown GaN nanowire p-n junctions containing a

single InGaN dot. The electroluminescence spectrum from

the single InGaN dot shows resonances due to exciton and

biexciton transitions. The exciton resonance is observable up

to 150 K. Second order autocorrelation measurements per-

formed at the exciton energy demonstrates photon anti-

bunching up to 150 K with g(2)(0)¼ 0.35 at this temperature.

The exciton has a fast radiative decay time of 1 ns, making

the device attractive as a high frequency electrically injected

single photon source. From previous studies we also know

that the single photons are linearly polarized along the nano-

wire c-axis.21

The work was supported by the National Science

Foundation (MRSEC program) under Grant No. DMR-

1120923.

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