Towards a LED based on a photonic crystal nanocavity for single photon sources at telecom wavelength
ABSTRACT A fundamental step towards achieving an "on demand" single photon source would be the possibility of electrical pumping for a single QD and thus the integration of such a device in an opto-electronic circuit. In this work we describe the fabrication process and preliminary results of a Light Emitting Diode (LED) to be integrated with a PhC nanocavity at telecom wavelength. We demonstrate the possibility of an effective electric pumping of the QDs embedded into the membrane by contacting the n-doped and p-doped layers of the thin membrane, which allows the fabrication of a PhC nanocavity on it. (C) 2007 Elsevier B.V. All rights reserved.
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ABSTRACT: The progress in nanofabrication has made possible the realization of optic nanodevices able to handle single photons and to exploit the quantum nature of single-photon states. In particular, quantum cryptography (or more precisely quantum key distribution, QKD) allows unconditionally secure exchange of cryptographic keys by the transmission of optical pulses each containing no more than one photon. Additionally, the coherent control of excitonic and photonic qubits is a major step forward in the field of solid-state cavity quantum electrodynamics, with potential applications in quantum computing. Here, we describe devices for realization of single photon generation and detection based on high resolution technologies and their physical properties. Particular attention will be devoted to the description of single-quantum dot sources based on photonic crystal microcavites optically and electrically driven: the electrically driven devices is an important result towards the realization of single photon source “on demand”. A new class of single photon detectors, based on superconducting nanowires, the superconducting single-photon detectors (SSPDs) are also introduced: the fabrication techniques and the design proposed to obtain large area coverage and photon number-resolving capability are described. Keywordsphotonic crystal microcavities-quantum dots-single photon sources and detectors-superconducting single photon detectors-photon number-resolving detectorsOpto-Electronics Review 18(4):352-365. · 0.92 Impact Factor
Towards a LED based on a photonic crystal nanocavity for
single photon sources at telecom wavelength
M. Francardia, A. Gerardinoa,*, L. Baletb, N. Chauvinb, D. Bitauldb, C. Zinonib,
L.H. Lib, B. Alloingb, N. Le Thomasb, R. Houdre ´b, A. Fioreb
aInstitute for Photonics and Nanotechnologies-CNR, via Cineto Romano 42, 00156 Roma, Italy
bEcole Polytechnique Fe ´de ´rale de Lausanne (EPFL), Institute of Photonics and Quantum Electronics, CH-1015 Lausanne, Switzerland
Received 4 October 2007; received in revised form 16 December 2007; accepted 27 December 2007
Available online 12 January 2008
A fundamental step towards achieving an ‘‘on demand” single photon source would be the possibility of electrical pumping for a sin-
gle QD and thus the integration of such a device in an opto-electronic circuit. In this work we describe the fabrication process and pre-
liminary results of a Light Emitting Diode (LED) to be integrated with a PhC nanocavity at telecom wavelength. We demonstrate the
possibility of an effective electric pumping of the QDs embedded into the membrane by contacting the n-doped and p-doped layers of the
thin membrane, which allows the fabrication of a PhC nanocavity on it.
? 2007 Elsevier B.V. All rights reserved.
Keywords: Photonic crystals; Optical nanocavities; LED; Single photon sources
The technology of real single-photon devices is still in
its infancy, while the physical basis for single-photon
emission from single QDs seems well-established. The
required telecom wavelength (1300 or 1550 nm) poses sig-
nificant challenges both in the epitaxial growth of the
quantum dots (QDs) and in the measurement (InGaAs
or Ge avalanche photodiodes must be used, with lower
quantum efficiency and much higher noise). For these rea-
sons, the few demonstrations of single-photon emission in
the telecom bands [1–4] do not yet match the application
requirements. An approach to the fabrication of efficient
single-QD LEDs still has to be demonstrated , and a
systematic investigation of the temperature limitations is
missing. The enhancement of the spontaneous emission
rate of an emitter on resonance with a mode of an optical
cavity (Purcell effect ), can be used to increase the effi-
ciency of the source. Recently, using a modified L3 defect
nanocavity (3 in-line missing holes) in a photonic crystal
(PhC) on a GaAs membrane with a single layer of low
density (5–7 dot/lm2) QDs in its center, we obtained
quality factors Q as high as 16500 and measured, in res-
onance conditions, a Purcell factor of 8 at 1300 nm for
the first time [7,8]. On the basis of these achievements,
we have developed an original design to integrate an
LED device at 1300 nm with a PhC nanocavity. The fab-
rication process is very challenging and we demonstrate
an effective electrical pumping of the QDs into the mem-
brane both in case of high and low areal density QDs.
2. Fabrication process
The light source at k = 1300 nm consists of InAs QDs
grown by molecular beam epitaxy (MBE) [9,10]. The
LEDs have been fabricated on two heterostructures
grown on a GaAs substrate: the first one has 3 layers of
high QDs density (?300 dots/lm2) emitting at 1300 nm
0167-9317/$ - see front matter ? 2007 Elsevier B.V. All rights reserved.
*Corresponding author. Tel.: +39 0641522242; fax: +39 0641522220.
E-mail addresses: email@example.com (M. Francardi), gerardi-
firstname.lastname@example.org (A. Gerardino).
Available online at www.sciencedirect.com
Microelectronic Engineering 85 (2008) 1162–1165
at room temperature (RT); the second one has a single
layer of low QDs density (5–7 dots/lm2) emitting at
1300 nm at 5 K. The QDs were grown at the center of a
320 nm-thick GaAs membrane on top of a 1500 nm-thick
Al0.7Ga0.3As sacrificial layer and the doped layers are
contained in the membrane. The main idea is to inject
electrons from a top annular contact and holes from the
sides of the mesa, using highly-doped GaAs contact layers
to spread the current throughout the mesa to the center of
the cavity. Recombination will occur on the entire surface
of the mesa but the emission from the cavity center is
enhanced and can be isolated with a combination of spa-
tial and spectral filtering.
The fabrication process is based on e-beam lithography
(EBL) (Vistec, EPBG 5HR working at 100 kV) and thin-
film techniques. The first step consists in patterning of the
top, ring-shaped n-contact by lift-off of a multilayer of
Ni/Ge/Au/Ni/Au. The pattern is transferred by EBL on a
1 lm thick UVIII resist. The large thickness of this resist
layer allows us to lift-off the metal contact, deposited by
gun-evaporation (Ni/Ge) and thermal evaporation (Au)
for a total thickness of 155 nm. The contact is annealed at
400 ?C for 30 min with a thermal ramp of 5 ?C/min.
Together with the contacts, markers to be used for the
alignment of the successive exposures are also deposited.
Then, we expose mesa patterns with different diameters
Fig. 1. (a) Top view (SEM image) of a LED at the end of the fabrication process, s-pad: signal pad, g-pad: ground pad; (b) same device type in section
(after cleavage); (c) particular of the device with a PhC at the end of the process.
M. Francardi et al./Microelectronic Engineering 85 (2008) 1162–1165
(32–26–20–15–10 lm) on a 300 nm-thick HSQ (hydrogen
H3PO4:H2O2:H2O = 3:1:30 solution to obtain a height
between 290 nm and 330 nm. This wet etching process is
the most crucial step of our fabrication process: we need
to stop the wet etching exactly on the p-region of our mem-
brane. The membrane is composed by layers of GaAs (n
and p doped) with AlGaAs layers in between, so the etching
rate is not constant: to reach reproducible results we have to
optimise this step accurately. The HSQ layer is then
removed with a HF based solution. The p-contact is also
shaped in a ring, in order to obtain the best hole injection
around the mesa and it consists of 110 nm Ti/Au evapo-
rated on a patterned UVIII layer and subsequent lift-off
in acetone. A 200 nm-thick Si3N4layer is deposited by PEC-
VD to create an insulating layer between the n and p-con-
tacts. The Si3N4is then removed by selective reactive ion
etching (RIE) (50 sccm CHF3, 10 sccm O2, pressure:
55 mTorr, power: 200 W) from the n-contact on the top
of the mesa and from part of the p-contact. By lift-off of a
Cr/Au (thickness: 110 nm) layer we can connect the n-con-
tact on the top of the mesa with the ground pads on the
Si3N4surface by two bridge contacts. In the same process
step we also connect the p-contact with the signal pad on
the Si3N4layer. In order to carry out a continuous film over
the mesa lateral edge we performed two tilted Cr/Au evap-
orations. The lift-off step ends the fabrication of the LED.
Fig. 1a shows a SEM image of a LED (mesa diameter:
32 lm) at this stage of the process. In Fig. 1b a cleaved sec-
tion of a device of the same type is shown. It is possible to
see the n-contact onto the mesa and the bridge that brings
the n-contact down on the sample. The anular p-contact
under the Si3N4layer should also be noted. To integrate
the PhC nanocavity on the LED a 150 nm-thick SiO2layer
has been deposited by ECR-PECVD on the top of the sub-
strate. The PhC pattern is transferred by EBL on a 200 nm-
thick PMMA resist and the usual process to fabricate PhC
nanocavities has been performed . In Fig. 1c is shown a
particular of the mesa with a PhC cavity transferred on its
surface at the end of the process.
The electro-optical characterization of the LED has
been performed in a cryogenic elettro-probe station cou-
pled with a microelectroluminescence (lEL) setup. The
sample is mounted on a cold-finger cooled by a holder
dipped in liquid He (about 3 K). The optical emission from
the top of devices can escape from the cryogenic set up
through a window and is collected by a microscope objec-
tive (numerical aperture 0.3). A mirror reflects the Infra-
Red (IR) radiation (which is sent to an IR camera or
focused into an optical-fiber and sent to the spectrometer)
and transmits the visible radiation (that allows to observe
the samples surface by a CCD camera). The electrolumi-
nescence (EL) is dispersed into a 1 m focal length mono-
chromator equipped with a cooled InGaAs photodiode
array detector; the spectral resolution of the setup is better
than 30 leV (?0.04 nm).
The first tests have been carried out on high density
InAs QDs samples emitting at 1300 nm at RT. The I-V
curves measured on these devices (mesa area: ?1400 lm2)
show typical rectifying p–n junction characteristics with a
voltage threshold in direct polarization of ?1.5 V. At low
temperatures, a lower forward-bias current is measured,
presumably due to reduced thermal activation through
contact and heterostructure barriers. Nevertheless, the cur-
rent injected through the junction is in the range of mA, for
an applied voltage P4 V, sufficient to excite the InAs QDs
in the active region. In fact EPL signal has been measured
(Fig. 2a). From the IR camera image of a device under elec-
trical injection we observe (Fig. 2b) a large emission from
the edges of the mesa as well as from the center, attributed
to scattering of light propagating in a waveguide mode
within the mesa. So, we have processed the sample with
low density InAs QDs, implementing a gold cover around
the mesa border to reflect the scattered light to the sub-
Fig. 2. (a) EPL emission (RT) from a high density QD based LED for
different pumping currents. (b) Picture from an IR camera of a high
density QD LED under electrical injection. Note the light emitted from the
M. Francardi et al./Microelectronic Engineering 85 (2008) 1162–1165
strate side. Fig. 3a shows the IR camera picture of a work-
ing device: the emission comes from the proper region and
also the measured EPL spectum (Fig. 3b) is in agreement
with the PL signal of the same sample. The measurements
on the low density samples have been performed at 4 K.
We have presented a LED structure working at
1300 nm, designed to be integrated with a PhC nanocavity.
First steps in this last direction have been already per-
formed, optimizing the exposure process of the PhC nano-
cavities on the LED top and their transfer into the
membrane. Our next goals will be to demonstrate the cou-
pling of InAs QDs at 1300 nm to a PhC cavity mode, also
in electrical pumping conditions.
Swiss National Science Foundation, Italian MIUR-
FIRB program, the FP6 NoE ‘‘ePIXnet’’, EU project
‘‘QAP”, SER-COST program.
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Fig. 3. (a) EL emission from the same LED at 4 K. (b) Picture from an IR
camera of a low density QD LED under electrical injection.
M. Francardi et al./Microelectronic Engineering 85 (2008) 1162–1165