GaInN Nanowire Array Light-Emitting Diode on

Mar 10, 2015 - The high speed on–off performance of GaN-based light-emitting diodes (LEDs) grown in c-plane direction is limited by long carrier lif...
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High-Speed GaN/GaInN Nanowire Array Light-Emitting Diode on Silicon(111) Robert Koester,† Daniel Sager,‡ Wolf-Alexander Quitsch,‡ Oliver Pfingsten,‡ Artur Poloczek,*,† Sarah Blumenthal,† Gregor Keller,† Werner Prost,† Gerd Bacher,*,‡ and Franz-Josef Tegude† †

Solid-State Electronics Department, ‡Werkstoffe der Elektrotechnik, Faculty of Engineering, and CENIDE, University Duisburg-Essen, D-47048 Duisburg, Germany S Supporting Information *

ABSTRACT: The high speed on−off performance of GaN-based light-emitting diodes (LEDs) grown in c-plane direction is limited by long carrier lifetimes caused by spontaneous and piezoelectric polarization. This work demonstrates that this limitation can be overcome by m-planar core−shell InGaN/GaN nanowire LEDs grown on Si(111). Time-resolved electroluminescence studies exhibit 90−10% rise- and fall-times of about 220 ps under GHz electrical excitation. The data underline the potential of these devices for optical data communication in polymer fibers and free space. KEYWORDS: GaInN/GaN, nanowire, LED, high frequency operation, photoluminescence, electroluminescence

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the quantum barriers4,9 or due to strong carrier injection, which recently results in a new speed record of 463 MHz modulation bandwidth in planar GaN based LEDs.10 A more stringent approach to cancel the quantum confined Stark-effect is based on growing the active quantum well region on a polarizationfree crystal plane. That is, nonpolar a-plane QW structures have been achieved using lateral overgrowth of side facets,11 whereas the m-plane growth direction is available by using different substrates such as SiC,12 LiGaO,13 or LiAlO2.14,15 In addition, bulk GaN substrates obtained from sliced hydride vapor phase epitaxy (HVPE) grown boules offer nonpolar crystal directions on which high power emission LEDs have been demonstrated.16−19 Despite the considerable efforts taken the realization of nonpolar luminescent layer structures remains difficult.19 An exciting development is the use of c-plane grown GaN nanowires20,21 as template for subsequent growth of radial junction orthogonal to the c-direction that is polarization free. GaN nanowires crystallize usually in the wurtzite c-plane [0001] on silicon (111) and c-plane sapphire substrates21 and

igh-speed short-range optical data communication in free space or in polymer optical fibers requires cheap and easy to handle light sources. Because of safety and cost restrictions, the current focus lies on light-emitting diodes, though lasers may offer a higher modulation speed. Whereas compact semiconductor light sources for the red spectral window around 650 nm are already established, an extension of the operation wavelength to the blue-green spectral range promise a significant reduction of losses in polymer optical fibers.1 Semiconductor light sources based on c-plane, wurtzite type InGaN/GaN quantum well light-emitting diodes (LEDs)2 cover the blue-green spectral regime and are thus of principal interest for communication via the polymer optical fiber.3 These devices offer huge luminous fluxes, but suffer from long electron−hole lifetimes in the InGaN/GaN quantum wells4 requiring advanced modulation techniques for high-speed operation.5 This is a consequence of strong internal electrical fields due to spontaneous and piezoelectric polarization,6−8 which are expected to fundamentally limit the switching speed of such devices. Hence, eliminating the internal electric fields is strongly required to push the high-speed on−off LED performance. Several approaches have been developed to tackle this issue. This includes partial screening of the internal field by doping of © XXXX American Chemical Society

Received: November 19, 2014 Revised: February 27, 2015

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DOI: 10.1021/nl504447j Nano Lett. XXXX, XXX, XXX−XXX

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Figure 1. Core−shell nanowire LED grown on Si(111): (a) SEM micrograph of the as-grown p- GaN/InGaN MQW(×5)/n-GaN core−shell, (b) array section covered with spin-on-glas and Ni/Au contacts, and (c) sketch of the nanowire array LED device.

In a first experiment, the absence of the polarization fields required for high modulation rates was proven by power- and time-dependent photoluminescence (PL) experiments on single nanowires. The as-grown nanowires were detached from the growth substrate by sonification in isopropyl alcohol and subsequently transferred onto a Si-substrate with a SiO2 insulation layer. In order to suppress thermally activated nonradiative processes, the ambient temperature was reduced to 7 K. Figure 2a shows PL-spectra obtained in the vicinity of the nanowire tip for different excitation densities ranging from 0.13 to 130 kWcm−2 at an excitation wavelength of 405 nm.

offer easy access to the polarization-free m-plane (11̅00) side facets suitable for InGaN/GaN quantum well deposition. The growth direction in the initial works on coaxial GaN/InGaN nanowires was (1120̅ )22 while the sapphire substrate-based works provided c-axis oriented deposition forming m-plane InGaN/GaN quantum well core−shell junctions using sapphire23−26 and Si(111) substrates. 27−31 Indeed, the suppression of the quantum confined Stark-effect in a mplane oriented layer14 and nanowire8,29 LED is shown to result in a reduced carrier recombination lifetime, which in principle should pave the way to high-speed electrically stimulated device operation. However, high-speed device performance of a GaN LED is not reported up to now. In this work, we present core−shell GaN/InGaN nanowire LEDs with high on/off switching rates. In our approach, we used a self-assembled and catalyst-free MOVPE (metal−organic vapor phase epitaxy) process to fabricate InGaN/GaN multiple quantum well core−shell nanowire LEDs with an active layer grown in m-plane direction. The growth is performed on conductive n-Si(111) substrates for the ease of n-contact formation. The suppression of the internal field is proved by time-resolved optical spectroscopy enabling us to demonstrate high-speed electroluminescence in the gigahertz regime. In Figure 1, scanning electron microscopy (SEM) micrographs of as grown nanowires (a) and a nanowire array covered with spin-on-glass and Ni/Au contacts are given. The nanowires were grown on highly n-conductive Si(111) substrate in order to enable a later array device fabrication using the conductive substrate as the LED cathode. A qualitative device sketch is presented in Figure 1c. The growth is carried out in a commercial Aixtron 3 × 2 in. showerhead MOVPE. After in situ native SiO2 removal under H2 and the deposition of a several nanometer thick AlN layer,32 the nanowire growth starts with a n-GaN core nanowire, followed by a InGaN/GaN multiple quantum well shell with a p-GaN contact layer, all grown in m-plane direction (for details see Supporting Information). The nanowire cores exhibit an excessive n-doping (≥1020 cm−3).33,34 The diameters are in the range of 400 nm to 1.5 μm and an overall length of 15 μm with a density of about 110 wires per (100 μm)2 is routinely obtained. The shell growth starts from about 2 μm above the substrate and exhibits a hexagonal shape with smooth m-plane facets and a diameter up to 2 μm. In a planar p-GaN reference layer grown on c-plane sapphire substrate at 910 °C, a hole density of 1017 cm−3 is found, while the I−V data discussed below suggest a much lower dopant density in the p-GaN shell grown in m-direction.

Figure 2. (a) Normalized time-integrated photoluminescence spectra for different excitation densities varying between 0.13 and 130 kWcm−2 recorded at a temperature of 7 K. (b) Peak intensities of the MQW- and the YL-emission, respectively, versus laser power. The red dashed line represents a linear fit.

For all excitation densities, the PL-spectra in Figure 2a show two characteristic emissions in the blue and the yellow spectral region, which are centered at around 448 and 570 nm, respectively. The broad emission peak around 570 nm is frequently seen in GaN and denoted as yellow luminescence (YL). This emission is often dedicated to recombination of charge carriers via acceptor states inside the bandgap, which are formed by nonintentionally introduced gallium vacancies.35 The blue emission around 448 nm is attributed to the recombination of charge carriers inside the radial MQW. The individual peaks and the slight broadening of the MQWemission to longer wavelengths at low excitation densities indicate radiative recombination via different localized states, B

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Figure 3. (a) Normalized spectrally integrated PL-decay characteristics of the MQW emission for different excitation densities varying between 0.13 and 130 kWcm−2, recorded at a temperature of 7 K. (b) PL-decay of the MQW-emission for different temperatures between 7 K and room temperature at a constant excitation density of 13 kWcm−2. The inset exhibits the charge carrier lifetime versus temperature extracted from a stretched exponential fit.

Figure 4. Electrical properties of the nanowire based LED: (a) measured/modeled I−V characteristics and (b) device model with parameters describing the whole array.

the excitation density over about 3 orders of magnitude does not result in a noticeable change of the carrier lifetime. This excludes a strong impact of nonradiative losses at low temperatures, which are expected to be saturated with increasing excitation density. It is obvious that for all excitation densities a nonexponential decay is observed. Such a nonexponential decay is reported for other InGaN/GaNMQW structures29,36,37 and is attributed to a local variation of the In-content and/or the well thickness inside the MQW, leading to localized exciton emission. The resulting nonexponential decay of the PL intensity after pulsed laser excitation can be described by a stretched exponential function36

which become saturated for high excitation densities. In case of pronounced internal electric fields inside the quantum well, a distinct blue shift is expected for increasing excitation density, caused by the well-known screening of the internal field by the optically induced charge carriers.7 The observed absence of a significant blue shift of the PL emission with excitation density over 3 orders of magnitude strongly indicates the absence of a polarization field. In Figure 2b, the PL-intensity of the MQW-emission as well as the YL is plotted versus excitation density. An almost linear increase of the PL-intensity is observed for both the MQW emission and the YL, respectively. We attribute the observation that the YL and the MQW emission are apparently independent, that is, no saturation effects occur, to the fact that the defects responsible for the YL are located outside the MQW, that is, within the p- and/or n-doped GaN layers of the nanowire. Figure 3a shows the spectrally integrated (from 435 to 455 nm) decay characteristics of the MQW-emission for different excitation densities at T = 7 K. First of all, the recombination lifetimes are in the ∼100 ps regime, at least 1 to 2 orders of magnitude shorter than usually obtained for c-plane InGaN/ GaN quantum wells,8 indicating the virtual absence of an internal electric field in our core−shell nanowires. A change of

⎡ ⎛ t ⎞β⎤ I(t ) = I0 exp⎢ −⎜ ⎟ ⎥ ⎣ ⎝τ⎠ ⎦

According to the formula the exponential decay is stretched by a parameter β, which in principal can take values between 0 and 1. Using β = 0.64 ± 0.014, the decay curve at, for example, P = 13 kWcm−2 can accurately be fitted with a charge carrier lifetime τ = 110 ± 10 ps. In order to get access to the carrier dynamics under ambient conditions, time-resolved PL studies were performed from 7 K C

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junctions often suffer from a high ideality factor indicating technological junction imperfection. Different effects like deeplevel-assisted tunneling, moderately doped heterojunctions or metal−semiconductor interfaces, in particular the metal-to-pGaN shell contact (Au/Ni-p-GaN), can be treated as a series of junctions where the effective ideality factor is the sum of all ideality factors involved with neff = ∑nk=1nk.40 The parallel resistance Rp represents the low-bias conductivity. We attribute this conductivity to the surface conductivity of each nanowire from the p-core to the n-stem. No contribution from the spinon-glas conductivity was found. In addition, the high reverse current is in line with a previous study,26 where it was reported that the interfaces of Ga-polar and N-polar sections within the nanowire might cause a high reverse current due to a significant minority carrier concentration. This effect gives rise to high current densities at low biases and has a negative effect on the efficiency of the nanowire LEDs. The fabricated nanowire array LED was used for electroluminescence (EL) measurements at room temperature. Figure 5a shows a typical EL-spectrum of the device probed for a

up to room temperature. The corresponding decay curves are plotted in Figure 3b. It is obvious that the recombination lifetime decreases with increasing temperature above about 20 K, a hint for thermally activated nonradiative losses. By fitting the data with a stretched exponential function, a lifetime can be extracted, which gradually decreases from 110 ps at 7 K to 20 ps at room temperature (see inset of Figure 3b). As both, PL intensity and PL lifetime tend to saturate for temperatures below 20 K, we conclude that the low-temperature decay time mainly reflects the radiative lifetime in our m-plane InGaN/ GaN quantum well nanowires. The stretch parameter β slightly changes from 0.633 at 7 K to 0.495 at 300 K. The reduction of the lifetime by a factor of 5−6 between 7 K and room temperature is in agreement with the observed loss in PLintensity by about a factor of 4−5 at room temperature with respect to 7 K. This indicates a PL efficiency of about 15−20% at room temperature as compared to 7 K. The charge carrier lifetime of 20 ps at room temperature is related to the absence of the internal electric field and thus far below the values of 1.1237 and 1.62 ns,38 reported for axial (i.e., c-plane) GaN/InGaN-nanowire-heterostructures at room temperature. Similar lifetimes between 21−35 ps are found for coaxial, polarization-free InGaN/GaN MQW nanowire structures at room temperature.29 The performance of the nanowire LED was studied under dc- and pulsed electrical excitation. For this purpose, an array of nanowires was top-contacted as schematically shown in Figure 1c. A highly transparent spin-on-glass was used enabling a high layer thickness of 10 μm to isolate the nanowire bottom. For the top p-contacts Ni/Au was selected that was evaporated under tilted angle in order to realize openings for the light emission. Within an area of A = 1 mm2, we found typically 50 illuminating nanowires properly connected to the top contact pattern. The low number of illuminating nanowires is attributed to both variations in nanowire height and caused by inhomogeneous contact resistances and thus different wire currents (filament effect),39 respectively. The measured I−V characteristic shows nonlinear pn-diode type behavior (Figure 4a). The resistivity of the core is negligible due to the excessive n-doping (≥1020 cm−3), while the observed current limitation (3.5 mA at +5 V) is attributed to the low conductivity of the pshell. Therefore, the electrical active pn-junction area is assumed to be mainly restricted to the top metal contact area. Because of the shadow evaporation the top contact metal area AP covers half of the nanowire surface above the isolation layer (Ap = htopπrshell). The measured Iext−Vext characteristic is modeled with the Shockley equation for a pn-junction diode with the extension of both, a series resistance RS and a parallel resistance RP as depicted in Figure 4b. The fit to the measured I−V characteristics gives a good agreement if the parallel resistance is set to RP = 120 kΩ and a series resistance of RS = 450 Ω is assumed. The ideality factor at low bias (Vext ≤ 0.1 V) is in the expected regime (n ≈ 2.5) but steadily increases with forward bias. In order to describe the measured curve in the whole forward bias regime the ideality factor is set to n = 15. With these parameters and a reverse current of I0 = 2.5 μA, the I−V characteristic is fitted to the measured data (Figure 4a). A remarkably good agreement is obtained in forward direction while the reverse current is somewhat underestimated. Both, the ideality factor and the reverse current, respectively, are far beyond the usual model of minority carrier transport across a pn-junction. Independent of the fabrication method (MBE,38 MOVPE30) the I−V characteristics of GaN nanowire pn-

Figure 5. Electroluminescence (EL) measurements of a GaInN/GaNMQW nanowire array LED: (a) dc EL intensity versus wavelength. The spectral window indicated by the shaded area was used for the time-resolved EL measurements, (b) high-speed measurement setup, and (c) time-resolved electroluminescence signal recorded for VDC = 8 V and VPulse = 6 V square pulses for a pulse width of 500 ps and a frequency of 1.1 GHz and (d) for a pulse width of 200 ps at a frequency of 1.3 GHz. The red points indicate the 90−10% rise- and fall-times.

surface area of a few square micrometers. No significant variation of the peak energy is found across the LED nanowire array. The peak energy is slightly shifted with respect to the PL spectrum presented for similar nanowires in Figure 3, which is a result of a slightly higher growth temperature and thus a lower In concentration of this device. For time-resolved electroluminescence measurements the device was connected via a bias-tee to a SubMiniature VersionA (SMA) based RF-sample holder (cf. Figure 5b) for measurements under ambient conditions. The DC-offset is provided by a Keithley source measurement unit (SMU) while the RF input was connected to an Agilent 81133A pulse pattern generator (PPG) with a source resistance of 50 Ω, capable to provide square pulses up to a frequency f = 3.35 GHz with rise- and fall D

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tMQW = 54 nm is deduced from TEM measurements. Taking these values and the epitaxy parameters, we calculated a weighted average dielectric constant of εr,MQW = 10.05 using εr,GaN = 8.9, εr,InN = 15.344 and a well composition of In0.3Ga0.7N. With these data the single wire junction capacitance Cpn can be roughly estimated to

times in the range of 100 ps. For device operation, a dc-offset is set to VDC = 8 V and rectangular voltage pulses with a height of VPulse = 6 V are added at a repetition frequency of f = 1.1 GHz. For the time-resolved EL measurements, a spectral detection window with a central wavelength of λ = 407 nm and a width of 22 nm was chosen (see Figure 5a). This regime is attributed to the luminescence of the InGaN/GaN multiple quantum wells and excludes the possible contribution from yellow defect luminescence. The data plotted in Figure 5c clearly demonstrate the high-speed performance of the LED device. A pronounced on/off modulation is achieved at 1.1 GHz with an on/off ratio of ∼5. From the measurements, 10−90% riseand fall-times of τfall = 210 ps and τrise = 220 ps, respectively, are extracted. The modulation time constants in the regime of 200 ps prove that the low carrier lifetime of m-plane luminescence allows for superior high-speed electroluminescence. For testing the current high frequency limit of the device and the setup, we reduced the nominal pulse width down to 200 ps by slightly raising the repetition frequency to f = 1.3 GHz. Indeed, the resulting data plotted in Figure 5d show that the modulation depth strongly decreases down to about 1.6. The shape of the measured EL signal becomes slightly asymmetric with rise- and fall-times of τrise = 170 ps and τfall = 310 ps, respectively. The speed data obtained deserve intense discussion as they are substantially superior to those reported earlier on any GaNbased LED.5,10,41,42 The extracted rise- and fall-times in the order of 200 ps of the room-temperature EL signal still exceeds the recombination lifetimes obtained from PL measurements by a factor of 10. Therefore, limitations of the measurement setup and the fabricated device are likely. The parasitic arraycapacitance CSOG is given by the parallel-plate capacitor defined by the conducting substrate and the top metal. Because of the large height of the spin-on glass (htop = 10 μm, εSOG = 2.8), the capacitance is CSOG = 2.5 pF for a device area A = 1 mm2. CSOG, together with the 50 ohms source resistance of the pattern generator, results in a cutoff frequency f 3db = 8 GHz, which is equivalent to a delay time of τRC = 124 ps. The intrinsic RC low-pass behavior is defined by the series resistance and the junction capacitance of the nanowire array pn-diode. According to the fundamentals of the LED frequency response, the dynamic LED behavior is limited by a series resistance and the geometrical junction capacitance.15,43 The series resistance of the array has been extracted from the forward I−V characteristics to RS = 450 Ω and is most probably dominated by the low conductivity of the p-shell and its contact resistance. The pn-junction capacitance Cpn of a nanowire is represented by a cylinder capacitor. Because of the low conductivity of the p-shell we assume that the outer cylinder electrode is restricted to the metal covered area. Because of the shadow evaporation, half of the cylinder area is covered with metal. An average contact height of htop = 3 μm is deduced from SEM data. We assume that only the part of the p-shell covered by the metal contact contributes to the current through the device, that is, the in-plane conductivity within the p-layer is negligible. Using highly spatially resolved PL measurements (not shown), we found distinct quantum well emission only within the upper 2−3 μm of the nanowires, which is in agreement with ref 24. A slight redshift of the luminescence (up to 13 nm) occurs toward to the wire tip, which can be attributed to a small gradient of the quantum well thickness and/or variations of the indium concentrations. An average quantum well thickness of

Cpn = πε0εr,MQW

htop

(

ln

R core + t MQW R core

)

= 8.2 fF

and sums up for an array of 50 wires to Carray = 0.41 pF. The resulting RC low pass frequency of f = 0.86 GHz is equivalent to a delay time constant τRC = 185 ps. These estimates support that the measured delays in Figure 5c,d are affected by both the measurement setup and the series resistance of the p-GaN contact, respectively. Thus, after careful optimization of the device design parameters even higher modulation speed may be possible. In summary, a GaN-based nanowire LED array grown by MOVPE on Si(111) substrate is fabricated, where m-plane InGaN/GaN multiple quantum well active layers are prepared around c-plane GaN nanowire cores. A radiative lifetime of about τ = 110 ps at T = 7 K is extracted from photoluminescence studies proving the absence of an intrinsic electric field and hence of spontaneous polarization inside the active quantum well area. Time-resolved electroluminescence studies exhibit 90−10% rise- and fall-times of 220 and 210 ps, respectively, after gigahertz excitation at room temperature, potentially allowing an on−off keying device operation at about 1 Gb/s. The measured data underline the high-speed potential of m-plane InGaN/GaN-nanowire based LEDs that might be used for short-range optical data communication in free space or via polymer optical fibers.

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ASSOCIATED CONTENT

S Supporting Information *

The growth procedure, the processing of the LED array device, and the high-speed optical measurement setup are described in detail. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel. +49 203 379 3878. *E-mail: [email protected]. Tel. +49 203 379 3406. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported by the EU Ziel2 project “NaSoL” with Grant 280334522 and by the German Research Foundation, Research Group FOR 1616 Dynamics and Interaction of Semiconductor Nanowires for Optoelectronics.

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REFERENCES

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DOI: 10.1021/nl504447j Nano Lett. XXXX, XXX, XXX−XXX