Hybrid gan led with capillary bonded ii–vi mqw color converting membrane for visible light communications
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Hybrid GaN LED with capillary-bonded II–VI MQW color-converting membrane for visible light
communications
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2015 Semicond. Sci. Technol. 30 035012
(http://iopscience.iop.org/0268-1242/30/3/035012)
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Semiconductor Science and Technology
Semicond. Sci. Technol. 30 (2015) 035012 (7pp) doi:10.1088/0268-1242/30/3/035012
Hybrid GaN LED with capillary-bonded II–VI
MQW color-converting membrane for visible
light communications
Joao M M Santos1, Brynmor E Jones1, Peter J Schlosser1, Scott Watson2,
Johannes Herrnsdorf1, Benoit Guilhabert1, Jonathan J D McKendry1,
Joel De Jesus3, Thor A Garcia4, Maria C Tamargo4, Anthony E Kelly2,
Jennifer E Hastie1, Nicolas Laurand1 and Martin D Dawson1
1
Institute of Photonics, SUPA, University of Strathclyde, 106 Rottenrow, Glasgow, G4 0NW, UK
2
School of Engineering, University of Glasgow, Glasgow, G12 8LT, UK
3
Department of Physics, The Graduate Center and The City College of New York, CUNY, New York, NY
10031, USA
4
Department of Chemistry, City College of New York, 138th Street and Convent Avenue, New York, NY
10031, USA
E-mail: [email protected]
Received 26 August 2014, revised 18 December 2014
Accepted for publication 5 January 2015
Published 27 January 2015
Abstract
The rapid emergence of gallium-nitride (GaN) light-emitting diodes (LEDs) for solid-state
lighting has created a timely opportunity for optical communications using visible light.
One important challenge to address this opportunity is to extend the wavelength coverage
of GaN LEDs without compromising their modulation properties. Here, a hybrid source for
emission at 540 nm consisting of a 450 nm GaN micro-sized LED (micro-LED) with a
micron-thick ZnCdSe/ZnCdMgSe multi-quantum-well color-converting membrane is
reported. The membrane is liquid-capillary-bonded directly onto the sapphire window of the
micro-LED for full hybridization. At an injection current of 100 mA, the color-converted
power was found to be 37 μW. At this same current, the −3 dB optical modulation
bandwidth of the bare GaN and hybrid micro-LEDs were 79 and 51 MHz, respectively. The
intrinsic bandwidth of the color-converting membrane was found to be power-density
independent over the range of the micro-LED operation at 145 MHz, which corresponds to
a mean carrier lifetime of 1.9 ns.
Keywords: photonics, GaN LED, semiconductor, color-converters, visible light communications
(Some figures may appear in colour only in the online journal)
1. Introduction converting phosphors. This approach is effective for illumi-
nation purposes but is not well suited to VLC because of the
Development of light-emitting diode (LED) technology is long (μs) excited-state lifetime of phosphors. The resulting
driven mainly by the need for efficient solid-state lighting, but sources have modulation bandwidths limited to ∼1 MHz,
it is also creating opportunities for new applications such as whereas unconverted blue InGaN-based LEDs have band-
visible light communications (VLCs) [1]. VLC requires light widths of 20 MHz up to 400 MHz depending on their formats,
sources that are not only efficient but also have high modula- with single-color direct-LED-emission data links at >1 Gbit s−1
tion bandwidth and are wavelength versatile. White-emitting rate having been demonstrated [2]. There is therefore a need to
LEDs, and color-converted LEDs in general, are typically explore color-converting materials with shorter excited-state
obtained by combining blue InGaN LEDs with down- lifetimes to complement rare-earth phosphors. Organic
0268-1242/15/035012+07$33.00 1 © 2015 IOP Publishing Ltd Printed in the UK
Semicond. Sci. Technol. 30 (2015) 035012 J M M Santos et al
semiconductors and colloidal quantum dots are potential can- growth buffer. It consists of nine ZnCdSe quantum wells with
didates but they necessitate advanced encapsulation schemes CdMgZnSe barriers and was designed to absorb the pump
for long-term environmental stability [3]. emission in-barriers. The quantum wells emit at 540 nm in a
Here we introduce an alternative approach based on resonant periodic gain configuration for potential alternative
using inorganic MQW semiconductor epi-layer membranes as use as a laser gain medium [9]. The InP substrate was
photo-pumped color converters for the underlying InGaN removed by a combination of mechanical polishing and wet
LEDs. This technology benefits from being based on all- etch process using a solution of HCl:H3PO4 in a ratio of 3:1,
inorganic semiconductors and therefore promises to be robust followed by the removal of the InGaAs layer with a solution
[4, 5]. It also leads to extremely compact sources, as the of H3PO4:H2O2:H2O, in a ratio of 1:1:6 for maximum etch
membrane can be integrated (as we show) by techniques such selectivity with the II–VI material [10]. The epi-side is fixed
as liquid capillary bonding. There are several options for onto a temporary glass substrate for this step, using a wax, for
wavelength coverage across the visible spectrum with avail- mechanical support during processing. After substrate
able semiconductor alloys for MQW membranes including removal, the II–VI layer was transferred from the glass and
III–V AlGaInP (yellow to red) and InGaN (green) materials capillary-bonded using deionized water onto the sapphire
and II–VI CdMgZnSe (green to orange) materials [4, 6]. As window of the LEDs to complete the hybrid device [11]. The
proof-of-principle of a generic approach, the hybrid LED resulting MQW membranes have a thickness of less than
demonstrator discussed here is obtained by capillary-bonding 2.5 μm, and a typical surface area of a few mm2. Images of
a 540 nm emitting, II–VI CdMgZnSe MQW membrane onto the hybrid device and structural membrane layout are shown
an array of 450 nm InGaN micro-size LEDs (micro-LEDs), in figure 1.
on the sapphire side, taking advantage of the high modulation
bandwidths associated with these small-sized LEDs [7].
2.2. Experimental setups
Micro-LEDs operate in a size regime where physical char-
acteristics that play a major role for VLC, e.g. current density, The device was characterized in terms of optical power,
differential carrier lifetime and junction capacitance, diverge emission spectrum and modulation bandwidth prior to and
from those of conventional format broad area LEDs. This size after membrane integration. For the power measurements, the
regime firmly applies to devices with dimensions of 100 μm device under test was fixed on top of an optical power meter
(as used in this work) and below, where devices can handle with a 30 mm cage plate in between to enable the optional
kA cm−2 of dc current density and possess significantly higher placement of an optical long-pass filter (500 nm cut-off
modulation bandwidths [7]. We chose II–VI materials for wavelength). Hereafter, in sections 3 and 4, we refer to the
converters in this initial study because (i) they are readily wet micro-LED with no membrane as the ‘bare LED’, the LED
etched into epitaxial membranes, (ii) they can be designed to with integrated membrane as the ‘hybrid micro-LED’, and the
offer coverage of the visible spectrum promising white-light micro-LED with membrane and filter in place at detection
generation, and (iii) they permit an alternative approach to simply as the ‘integrated MQW membrane’. Optical spectra
green emission at high-modulation-bandwidth, useful for e.g. were recorded with an Ocean Optics USB4000 Fiber Optic
introducing coarse wavelength division multiplexing (blue Spectrometer (2.5 nm resolution).
and green) into optical wireless and polymer optical fiber For the frequency response and modulation bandwidth
communications. measurements, a set of aspheric lenses with focal length of
In the following, the hybrid device fabrication and char- 32 mm and high numerical aperture (NA = 0.612) were used
acterization methods are described in section 2. Sections 3 and to collect and focus the micro-LED emission into a Femto
4 report and discuss, respectively, the continuous-wave (CW) HSA-X-S-1G4-SI fast photoreceiver (bandwidth of 1.4 GHz).
and dynamic characteristics of the hybrid source. The LED device was simultaneously driven with a dc bias
current and a frequency-swept modulated signal (0.250 Vpp)
that were combined with a wideband (0.1–6000 MHz) Bias-
2. Device fabrication and characterization Tee. An Agilent HP 8753ES network analyzer was used to
provide the modulation signal and to record the device fre-
2.1. Device design and fabrication
quency response as detected by the photoreceiver. The long-
pass filter was placed before the detection for measurements
The 450 nm wavelength micro-LEDs used to pump the MQW of the integrated MQW membrane characteristics.
membrane were fabricated using a commercial p-i-n GaN The intrinsic modulation bandwidth of the stand-alone
LED structure grown on c-plane sapphire, following the membrane was also characterized as a function of the exci-
procedure reported in [8]. Here, the fabricated chip is made of tation power density (section 4). For this, the membrane was
several 100 × 100 μm square micro-LEDs spaced 450 μm supported on a glass substrate and remotely pumped with a
apart, figure 1. After fabrication, the chip was placed onto a bare micro-LED. A pair of lenses was placed directly after the
printed circuit board and wire bonded to tracks connected to bare device to focus its light onto the stand-alone membrane.
SMA connectors, such that the micro-LEDs could be Further optics were used to collect the light emitted by the
addressed individually. membrane and focus it onto the photodetector. A Coherent®
The II–VI MQW membrane structure was grown by BeamMaster was used to measure the micro-LED beam size
molecular beam epitaxy on an InP substrate and InGaAs incident onto the membrane surface and hence to deduce the
2
Semicond. Sci. Technol. 30 (2015) 035012 J M M Santos et al
Figure 1. (a) Plan view micrograph of the hybrid device. The micro-LED pixels are the smaller elements as labeled, some of which are
bonded underneath the II–VI membrane. Between the membrane and the underlying LEDs is the sapphire substrate, (b) II–VI membrane
when pumped by a blue LED, (c) II–VI MQW structural design.
incident power density. The full width at half maximum membrane. The II–VI membrane is designed to absorb
(FWHM) spot-size was found to be 330 μm. The bandwidth 97–98% of 450 nm monochromatic light. Because of the
measurement of the stand-alone membrane was otherwise as bandwidth of the bare micro-LED spectrum (23 nm FWHM)
described above. the effective micro-LED light absorption is 85% ± 1%. The
band-edge of the membrane is at around 460 nm and therefore
the long-wavelength tail of the micro-LED emission is not
3. CW characteristics fully absorbed, explaining the ‘apparent’ 475 nm peak in the
spectrum of the integrated MQW membrane.
Normalized spectral measurements for the bare micro-LED, The power transfer functions (optical power versus bias
the hybrid micro-LED and the integrate MQW membrane (i.e. current) for the bare, hybrid micro-LEDs and integrated
the contribution of the converted light at 540 nm) are pre- MQW membrane are shown in figure 3. At 100 mA dc, which
sented in figure 2. The bare micro-LED has emission centered was found to be the maximum current before thermal rollover,
at 445 nm. The hybrid LED spectrum shows a peak centered the measured optical powers from the bare micro-LED and
at 475 nm and a secondary peak at 540 nm. The emission at the integrate MQW membrane (i.e. the color-converted light
540 nm comes from the light emitted by the II–VI MQW contribution) are 4 mW and 37 μW, respectively. The total
3
Semicond. Sci. Technol. 30 (2015) 035012 J M M Santos et al
Figure 2. Emission spectra from bare and hybrid pixels and the II–VI
band-edge normalized absorption. Figure 4. Frequency responses for 80 mA bias current of the bare
and hybrid micro-LED, as well as the intrinsic response of the color-
converting membrane (integrated MQW membrane), with the
respective bi-exponential fits.
4.1. Bandwidth measurements
The electrical frequency responses of the bare micro-LED and
integrated MQW membrane were measured at different levels
of bias current. The −3 dB optical bandwidth values (the
frequency at which the optical power is half the dc value, fco)
was obtained for each bias current by fitting the data, con-
sidering a possible multi-exponential decay of the lumines-
cence. The following expressions were used for the fits [12]:
a i ωτi2
∑i
N (ω) =
(1 + ω τ ) ,
2 2
i
(1)
Figure 3. CW optical powers of the bare, hybrid unfiltered micro- ∑i a i τi
LED and integrated MQW membrane.
a i τi
∑i
D (ω) =
(1 + ω τ ) ,
2 2
i
(2)
power of the hybrid micro-LED is 0.58 mW. The power ∑i a i τi
conversion efficiency—defined as the ratio between 540 nm
over incident 450 nm power—was found to be 1% ± 0.1% as 1/2
derived from the power transfer functions. The membrane (
M (ω) = N (ω)2 + D (ω)2 ) . (3)
structure used for this initial proof of principle was not pri-
marily designed for color-conversion and the high index In the expressions (1)–(3), the ω and τi parameters
contrast between the membrane material and air (2.5:1) represent the angular frequency of the modulation and the
results in a significant amount of waveguided light, which is time constant of the ith exponential decay process, respec-
then lost through reabsorption and edge emission. Improved tively. ai is a multiplicative factor indicating the contribution
epilayer design of the membrane and implementation of light strength of the ith decay process. N, D and M are the sine and
extraction schemes would improve this value significantly. cosine intensity decay transforms and the demodulation fac-
tors [12].
The frequency responses of the bare micro-LED and
integrated MQW membrane when driven at 80 mA bias cur-
4. Dynamic characteristics rent and their corresponding fitting curves are depicted in
figure 4. They are fitted considering a bi-exponential decay of
The modulation properties of the hybrid micro-LED are the luminescence, one component of the decay being domi-
important for VLC applications and represent the main focus nant. The resulting parameters for this particular case are,
of this study. This section looks into the dynamic character- a1 = 0.0037, a2 = 0.9963, τ1 = 0.2 μs, τ2 = 3 ns, fco = 69 MHz
istics of the bare micro-LED and the MQW membrane and and a1 = 0.0024, a2 = 0.9976, τ1 = 0.2 μs, τ2 = 4.6 ns and
what this implies for the hybrid LED modulation properties. fco = 47 MHz respectively.
4
Semicond. Sci. Technol. 30 (2015) 035012 J M M Santos et al
Figure 5. The −3 dB optical bandwidths of the bare μLED and of the
hybrid micro-LEDs—the intrinsic bandwidth values of the mem- Figure 6. Bandwidth dependence over different power densities.
brane (integrated MQW membrane) are also plotted.
4.2. Power density dependence of the intrinsic bandwidth of
The data for the intrinsic response of the color-converting the color-converting membrane
membrane is obtained by removing the frequency response
contribution of the bare micro-LED from the overall response The dependence of the color-converter bandwidth on the
of the hybrid device [3]. This is also plotted in figure 4 along incident micro-LED pump power density was further studied.
with its fit. This response is accounted for by a mono-expo- Since the micro-LED is in flip-chip configuration, i.e. the
nential decay of the MQW membrane photoluminescence and emission occurs through the sapphire substrate, the excitation
its bandwidth is determined by the following bandwidth- area at the membrane/sapphire interface is determined by the
lifetime relation, divergence of the micro-LED light and the propagation length
through the sapphire. Due to the small LED size, the exci-
3 tation spot incident on the membrane can in good approx-
fco = . (4)
2πτi imation be assumed to be circular with diameter equal to the
substrate thickness, i.e., 330 ± 20 μm in this case [15]. The
In equation (4), fco is the optical bandwidth and τi the intrinsic bandwidth data of the membrane as previously
photoluminescence lifetime, also the carrier lifetime. The determined is then plotted again in figure 6 as a function of
mono-exponential decay parameters found for this curve are, the incident excitation power density (‘integrated mem-
a1 = 1, τ1 = 2.0 ns and fco = 139 MHz. brane’). For pump power density between 1 and 1.75 W cm−2,
Figure 5 plots the optical bandwidth values versus the the average intrinsic bandwidth is 145 MHz.
InGaN LED bias current for the bare and hybrid micro-LED In order to observe with more confidence the bandwidth
as well as the intrinsic modulation bandwidth of the mem- behavior of the MQW membrane at excitation power density
brane. The data is also summarized in table 1 along with the below 1 W cm−2, it was independently characterized, i.e.
respective goodness of fit (GoF) values. The GoF is a sta- when separated from the micro-LED, as a stand-alone
tistical analysis that characterizes how well the fit describes membrane. In this case the membrane was held onto a glass
the measured data, i.e., is a parameter often used to describe substrate and remotely pumped with a 450 nm micro-LED as
the discrepancy between a set of raw data and a model. With explained in section 2. Results are plotted in figure 6 (black
GoF = 1 being a perfect fit and GoF < 1 indicating some dis- squares, ‘stand-alone membrane’).
crepancy between data. The vertical error bars in figure 6 represent the root mean
The micro-LED bandwidth is current dependent and square errors of the fits of the frequency responses. Overall,
reaches 79 MHz at 100 mA. This current dependency can be the presented bandwidth values are within the error bars of the
attributed to the reduced carrier lifetime in the InGaN micro- surrounding points so there is no significant power density
LED active region as the current, and hence the carrier den- dependence of the membrane intrinsic bandwidth. The aver-
sity, increases [13, 14]. The typical intrinsic response of the age bandwidth over the range of 450 nm light power density
membrane is 145 MHz, much faster than conventional phos- corresponding to the operation of the integrated MQW
phors, see also figure 6. The hybrid micro-LED behavior is membrane is around 145 MHz. This corresponds to an
the combination of the frequency responses of the color- effective carrier lifetime of 1.9 ns. This value can be corro-
converting membrane and the underlying micro-LED. The borated by direct time-correlated single photon counting,
result is a modulation bandwidth of 51 MHz limited by the TCSPC, measurement of the stand-alone membrane. Such
slower of the two components, i.e. the micro-LED response in TCSPC analysis was done using an Edinburgh Instruments
this case. Higher modulation bandwidths are expected by system using an EPL-450 Picosecond pulsed diode laser as
using blue micro-LEDs of even smaller dimensions [2]. the excitation source (450 nm). Differences in the TCSPC
5
Hybrid GaN LED with capillary-bonded II–VI MQW color-converting membrane for visible light
communications
This content has been downloaded from IOPscience. Please scroll down to see the full text.
2015 Semicond. Sci. Technol. 30 035012
(http://iopscience.iop.org/0268-1242/30/3/035012)
View the table of contents for this issue, or go to the journal homepage for more
Download details:
IP Address: 134.129.120.3
This content was downloaded on 16/05/2015 at 16:05
Please note that terms and conditions apply.
Semiconductor Science and Technology
Semicond. Sci. Technol. 30 (2015) 035012 (7pp) doi:10.1088/0268-1242/30/3/035012
Hybrid GaN LED with capillary-bonded II–VI
MQW color-converting membrane for visible
light communications
Joao M M Santos1, Brynmor E Jones1, Peter J Schlosser1, Scott Watson2,
Johannes Herrnsdorf1, Benoit Guilhabert1, Jonathan J D McKendry1,
Joel De Jesus3, Thor A Garcia4, Maria C Tamargo4, Anthony E Kelly2,
Jennifer E Hastie1, Nicolas Laurand1 and Martin D Dawson1
1
Institute of Photonics, SUPA, University of Strathclyde, 106 Rottenrow, Glasgow, G4 0NW, UK
2
School of Engineering, University of Glasgow, Glasgow, G12 8LT, UK
3
Department of Physics, The Graduate Center and The City College of New York, CUNY, New York, NY
10031, USA
4
Department of Chemistry, City College of New York, 138th Street and Convent Avenue, New York, NY
10031, USA
E-mail: [email protected]
Received 26 August 2014, revised 18 December 2014
Accepted for publication 5 January 2015
Published 27 January 2015
Abstract
The rapid emergence of gallium-nitride (GaN) light-emitting diodes (LEDs) for solid-state
lighting has created a timely opportunity for optical communications using visible light.
One important challenge to address this opportunity is to extend the wavelength coverage
of GaN LEDs without compromising their modulation properties. Here, a hybrid source for
emission at 540 nm consisting of a 450 nm GaN micro-sized LED (micro-LED) with a
micron-thick ZnCdSe/ZnCdMgSe multi-quantum-well color-converting membrane is
reported. The membrane is liquid-capillary-bonded directly onto the sapphire window of the
micro-LED for full hybridization. At an injection current of 100 mA, the color-converted
power was found to be 37 μW. At this same current, the −3 dB optical modulation
bandwidth of the bare GaN and hybrid micro-LEDs were 79 and 51 MHz, respectively. The
intrinsic bandwidth of the color-converting membrane was found to be power-density
independent over the range of the micro-LED operation at 145 MHz, which corresponds to
a mean carrier lifetime of 1.9 ns.
Keywords: photonics, GaN LED, semiconductor, color-converters, visible light communications
(Some figures may appear in colour only in the online journal)
1. Introduction converting phosphors. This approach is effective for illumi-
nation purposes but is not well suited to VLC because of the
Development of light-emitting diode (LED) technology is long (μs) excited-state lifetime of phosphors. The resulting
driven mainly by the need for efficient solid-state lighting, but sources have modulation bandwidths limited to ∼1 MHz,
it is also creating opportunities for new applications such as whereas unconverted blue InGaN-based LEDs have band-
visible light communications (VLCs) [1]. VLC requires light widths of 20 MHz up to 400 MHz depending on their formats,
sources that are not only efficient but also have high modula- with single-color direct-LED-emission data links at >1 Gbit s−1
tion bandwidth and are wavelength versatile. White-emitting rate having been demonstrated [2]. There is therefore a need to
LEDs, and color-converted LEDs in general, are typically explore color-converting materials with shorter excited-state
obtained by combining blue InGaN LEDs with down- lifetimes to complement rare-earth phosphors. Organic
0268-1242/15/035012+07$33.00 1 © 2015 IOP Publishing Ltd Printed in the UK
Semicond. Sci. Technol. 30 (2015) 035012 J M M Santos et al
semiconductors and colloidal quantum dots are potential can- growth buffer. It consists of nine ZnCdSe quantum wells with
didates but they necessitate advanced encapsulation schemes CdMgZnSe barriers and was designed to absorb the pump
for long-term environmental stability [3]. emission in-barriers. The quantum wells emit at 540 nm in a
Here we introduce an alternative approach based on resonant periodic gain configuration for potential alternative
using inorganic MQW semiconductor epi-layer membranes as use as a laser gain medium [9]. The InP substrate was
photo-pumped color converters for the underlying InGaN removed by a combination of mechanical polishing and wet
LEDs. This technology benefits from being based on all- etch process using a solution of HCl:H3PO4 in a ratio of 3:1,
inorganic semiconductors and therefore promises to be robust followed by the removal of the InGaAs layer with a solution
[4, 5]. It also leads to extremely compact sources, as the of H3PO4:H2O2:H2O, in a ratio of 1:1:6 for maximum etch
membrane can be integrated (as we show) by techniques such selectivity with the II–VI material [10]. The epi-side is fixed
as liquid capillary bonding. There are several options for onto a temporary glass substrate for this step, using a wax, for
wavelength coverage across the visible spectrum with avail- mechanical support during processing. After substrate
able semiconductor alloys for MQW membranes including removal, the II–VI layer was transferred from the glass and
III–V AlGaInP (yellow to red) and InGaN (green) materials capillary-bonded using deionized water onto the sapphire
and II–VI CdMgZnSe (green to orange) materials [4, 6]. As window of the LEDs to complete the hybrid device [11]. The
proof-of-principle of a generic approach, the hybrid LED resulting MQW membranes have a thickness of less than
demonstrator discussed here is obtained by capillary-bonding 2.5 μm, and a typical surface area of a few mm2. Images of
a 540 nm emitting, II–VI CdMgZnSe MQW membrane onto the hybrid device and structural membrane layout are shown
an array of 450 nm InGaN micro-size LEDs (micro-LEDs), in figure 1.
on the sapphire side, taking advantage of the high modulation
bandwidths associated with these small-sized LEDs [7].
2.2. Experimental setups
Micro-LEDs operate in a size regime where physical char-
acteristics that play a major role for VLC, e.g. current density, The device was characterized in terms of optical power,
differential carrier lifetime and junction capacitance, diverge emission spectrum and modulation bandwidth prior to and
from those of conventional format broad area LEDs. This size after membrane integration. For the power measurements, the
regime firmly applies to devices with dimensions of 100 μm device under test was fixed on top of an optical power meter
(as used in this work) and below, where devices can handle with a 30 mm cage plate in between to enable the optional
kA cm−2 of dc current density and possess significantly higher placement of an optical long-pass filter (500 nm cut-off
modulation bandwidths [7]. We chose II–VI materials for wavelength). Hereafter, in sections 3 and 4, we refer to the
converters in this initial study because (i) they are readily wet micro-LED with no membrane as the ‘bare LED’, the LED
etched into epitaxial membranes, (ii) they can be designed to with integrated membrane as the ‘hybrid micro-LED’, and the
offer coverage of the visible spectrum promising white-light micro-LED with membrane and filter in place at detection
generation, and (iii) they permit an alternative approach to simply as the ‘integrated MQW membrane’. Optical spectra
green emission at high-modulation-bandwidth, useful for e.g. were recorded with an Ocean Optics USB4000 Fiber Optic
introducing coarse wavelength division multiplexing (blue Spectrometer (2.5 nm resolution).
and green) into optical wireless and polymer optical fiber For the frequency response and modulation bandwidth
communications. measurements, a set of aspheric lenses with focal length of
In the following, the hybrid device fabrication and char- 32 mm and high numerical aperture (NA = 0.612) were used
acterization methods are described in section 2. Sections 3 and to collect and focus the micro-LED emission into a Femto
4 report and discuss, respectively, the continuous-wave (CW) HSA-X-S-1G4-SI fast photoreceiver (bandwidth of 1.4 GHz).
and dynamic characteristics of the hybrid source. The LED device was simultaneously driven with a dc bias
current and a frequency-swept modulated signal (0.250 Vpp)
that were combined with a wideband (0.1–6000 MHz) Bias-
2. Device fabrication and characterization Tee. An Agilent HP 8753ES network analyzer was used to
provide the modulation signal and to record the device fre-
2.1. Device design and fabrication
quency response as detected by the photoreceiver. The long-
pass filter was placed before the detection for measurements
The 450 nm wavelength micro-LEDs used to pump the MQW of the integrated MQW membrane characteristics.
membrane were fabricated using a commercial p-i-n GaN The intrinsic modulation bandwidth of the stand-alone
LED structure grown on c-plane sapphire, following the membrane was also characterized as a function of the exci-
procedure reported in [8]. Here, the fabricated chip is made of tation power density (section 4). For this, the membrane was
several 100 × 100 μm square micro-LEDs spaced 450 μm supported on a glass substrate and remotely pumped with a
apart, figure 1. After fabrication, the chip was placed onto a bare micro-LED. A pair of lenses was placed directly after the
printed circuit board and wire bonded to tracks connected to bare device to focus its light onto the stand-alone membrane.
SMA connectors, such that the micro-LEDs could be Further optics were used to collect the light emitted by the
addressed individually. membrane and focus it onto the photodetector. A Coherent®
The II–VI MQW membrane structure was grown by BeamMaster was used to measure the micro-LED beam size
molecular beam epitaxy on an InP substrate and InGaAs incident onto the membrane surface and hence to deduce the
2
Semicond. Sci. Technol. 30 (2015) 035012 J M M Santos et al
Figure 1. (a) Plan view micrograph of the hybrid device. The micro-LED pixels are the smaller elements as labeled, some of which are
bonded underneath the II–VI membrane. Between the membrane and the underlying LEDs is the sapphire substrate, (b) II–VI membrane
when pumped by a blue LED, (c) II–VI MQW structural design.
incident power density. The full width at half maximum membrane. The II–VI membrane is designed to absorb
(FWHM) spot-size was found to be 330 μm. The bandwidth 97–98% of 450 nm monochromatic light. Because of the
measurement of the stand-alone membrane was otherwise as bandwidth of the bare micro-LED spectrum (23 nm FWHM)
described above. the effective micro-LED light absorption is 85% ± 1%. The
band-edge of the membrane is at around 460 nm and therefore
the long-wavelength tail of the micro-LED emission is not
3. CW characteristics fully absorbed, explaining the ‘apparent’ 475 nm peak in the
spectrum of the integrated MQW membrane.
Normalized spectral measurements for the bare micro-LED, The power transfer functions (optical power versus bias
the hybrid micro-LED and the integrate MQW membrane (i.e. current) for the bare, hybrid micro-LEDs and integrated
the contribution of the converted light at 540 nm) are pre- MQW membrane are shown in figure 3. At 100 mA dc, which
sented in figure 2. The bare micro-LED has emission centered was found to be the maximum current before thermal rollover,
at 445 nm. The hybrid LED spectrum shows a peak centered the measured optical powers from the bare micro-LED and
at 475 nm and a secondary peak at 540 nm. The emission at the integrate MQW membrane (i.e. the color-converted light
540 nm comes from the light emitted by the II–VI MQW contribution) are 4 mW and 37 μW, respectively. The total
3
Semicond. Sci. Technol. 30 (2015) 035012 J M M Santos et al
Figure 2. Emission spectra from bare and hybrid pixels and the II–VI
band-edge normalized absorption. Figure 4. Frequency responses for 80 mA bias current of the bare
and hybrid micro-LED, as well as the intrinsic response of the color-
converting membrane (integrated MQW membrane), with the
respective bi-exponential fits.
4.1. Bandwidth measurements
The electrical frequency responses of the bare micro-LED and
integrated MQW membrane were measured at different levels
of bias current. The −3 dB optical bandwidth values (the
frequency at which the optical power is half the dc value, fco)
was obtained for each bias current by fitting the data, con-
sidering a possible multi-exponential decay of the lumines-
cence. The following expressions were used for the fits [12]:
a i ωτi2
∑i
N (ω) =
(1 + ω τ ) ,
2 2
i
(1)
Figure 3. CW optical powers of the bare, hybrid unfiltered micro- ∑i a i τi
LED and integrated MQW membrane.
a i τi
∑i
D (ω) =
(1 + ω τ ) ,
2 2
i
(2)
power of the hybrid micro-LED is 0.58 mW. The power ∑i a i τi
conversion efficiency—defined as the ratio between 540 nm
over incident 450 nm power—was found to be 1% ± 0.1% as 1/2
derived from the power transfer functions. The membrane (
M (ω) = N (ω)2 + D (ω)2 ) . (3)
structure used for this initial proof of principle was not pri-
marily designed for color-conversion and the high index In the expressions (1)–(3), the ω and τi parameters
contrast between the membrane material and air (2.5:1) represent the angular frequency of the modulation and the
results in a significant amount of waveguided light, which is time constant of the ith exponential decay process, respec-
then lost through reabsorption and edge emission. Improved tively. ai is a multiplicative factor indicating the contribution
epilayer design of the membrane and implementation of light strength of the ith decay process. N, D and M are the sine and
extraction schemes would improve this value significantly. cosine intensity decay transforms and the demodulation fac-
tors [12].
The frequency responses of the bare micro-LED and
integrated MQW membrane when driven at 80 mA bias cur-
4. Dynamic characteristics rent and their corresponding fitting curves are depicted in
figure 4. They are fitted considering a bi-exponential decay of
The modulation properties of the hybrid micro-LED are the luminescence, one component of the decay being domi-
important for VLC applications and represent the main focus nant. The resulting parameters for this particular case are,
of this study. This section looks into the dynamic character- a1 = 0.0037, a2 = 0.9963, τ1 = 0.2 μs, τ2 = 3 ns, fco = 69 MHz
istics of the bare micro-LED and the MQW membrane and and a1 = 0.0024, a2 = 0.9976, τ1 = 0.2 μs, τ2 = 4.6 ns and
what this implies for the hybrid LED modulation properties. fco = 47 MHz respectively.
4
Semicond. Sci. Technol. 30 (2015) 035012 J M M Santos et al
Figure 5. The −3 dB optical bandwidths of the bare μLED and of the
hybrid micro-LEDs—the intrinsic bandwidth values of the mem- Figure 6. Bandwidth dependence over different power densities.
brane (integrated MQW membrane) are also plotted.
4.2. Power density dependence of the intrinsic bandwidth of
The data for the intrinsic response of the color-converting the color-converting membrane
membrane is obtained by removing the frequency response
contribution of the bare micro-LED from the overall response The dependence of the color-converter bandwidth on the
of the hybrid device [3]. This is also plotted in figure 4 along incident micro-LED pump power density was further studied.
with its fit. This response is accounted for by a mono-expo- Since the micro-LED is in flip-chip configuration, i.e. the
nential decay of the MQW membrane photoluminescence and emission occurs through the sapphire substrate, the excitation
its bandwidth is determined by the following bandwidth- area at the membrane/sapphire interface is determined by the
lifetime relation, divergence of the micro-LED light and the propagation length
through the sapphire. Due to the small LED size, the exci-
3 tation spot incident on the membrane can in good approx-
fco = . (4)
2πτi imation be assumed to be circular with diameter equal to the
substrate thickness, i.e., 330 ± 20 μm in this case [15]. The
In equation (4), fco is the optical bandwidth and τi the intrinsic bandwidth data of the membrane as previously
photoluminescence lifetime, also the carrier lifetime. The determined is then plotted again in figure 6 as a function of
mono-exponential decay parameters found for this curve are, the incident excitation power density (‘integrated mem-
a1 = 1, τ1 = 2.0 ns and fco = 139 MHz. brane’). For pump power density between 1 and 1.75 W cm−2,
Figure 5 plots the optical bandwidth values versus the the average intrinsic bandwidth is 145 MHz.
InGaN LED bias current for the bare and hybrid micro-LED In order to observe with more confidence the bandwidth
as well as the intrinsic modulation bandwidth of the mem- behavior of the MQW membrane at excitation power density
brane. The data is also summarized in table 1 along with the below 1 W cm−2, it was independently characterized, i.e.
respective goodness of fit (GoF) values. The GoF is a sta- when separated from the micro-LED, as a stand-alone
tistical analysis that characterizes how well the fit describes membrane. In this case the membrane was held onto a glass
the measured data, i.e., is a parameter often used to describe substrate and remotely pumped with a 450 nm micro-LED as
the discrepancy between a set of raw data and a model. With explained in section 2. Results are plotted in figure 6 (black
GoF = 1 being a perfect fit and GoF < 1 indicating some dis- squares, ‘stand-alone membrane’).
crepancy between data. The vertical error bars in figure 6 represent the root mean
The micro-LED bandwidth is current dependent and square errors of the fits of the frequency responses. Overall,
reaches 79 MHz at 100 mA. This current dependency can be the presented bandwidth values are within the error bars of the
attributed to the reduced carrier lifetime in the InGaN micro- surrounding points so there is no significant power density
LED active region as the current, and hence the carrier den- dependence of the membrane intrinsic bandwidth. The aver-
sity, increases [13, 14]. The typical intrinsic response of the age bandwidth over the range of 450 nm light power density
membrane is 145 MHz, much faster than conventional phos- corresponding to the operation of the integrated MQW
phors, see also figure 6. The hybrid micro-LED behavior is membrane is around 145 MHz. This corresponds to an
the combination of the frequency responses of the color- effective carrier lifetime of 1.9 ns. This value can be corro-
converting membrane and the underlying micro-LED. The borated by direct time-correlated single photon counting,
result is a modulation bandwidth of 51 MHz limited by the TCSPC, measurement of the stand-alone membrane. Such
slower of the two components, i.e. the micro-LED response in TCSPC analysis was done using an Edinburgh Instruments
this case. Higher modulation bandwidths are expected by system using an EPL-450 Picosecond pulsed diode laser as
using blue micro-LEDs of even smaller dimensions [2]. the excitation source (450 nm). Differences in the TCSPC
5