Supplemental Material
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Tunnel Junction Enhanced Nanowire Ultraviolet Light Emitting Diodes
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ATM Golam Sarwar, Brelon J May, Julia I Deitz, Tyler J Grassman, David W McComb, and
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Roberto C Myers.
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Growth optimization of nanowire tunnel junction for LED integration:
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In this study, n-type (Si doped) GaN nanowires are nucleated on n-Si wafer at 730C for 5 minutes. After
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the nucleation step, the growth temperature is increased to 790 C and 100 nm of GaN is deposited. A 15
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nm n++ GaN layer is grown at a reduced growth rate using shutter pulsing method. The substrate
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temperature is then reduced to 600 C and a nominally 4 nm In0.25Ga0.75N quantum well is deposited
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followed by a 2.5 nm p++ GaN layer. After that the substrate temperature is increased to 790 C following
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a temperature protocol (as described in Table S1) and a doubly graded polarization doped UV LED
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heterostructure is grown.
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Table S1: Substrate temperature protocol for UV LED heterostructure
Sample
Description
TJ_A
Following the p++ GaN, graded AlxGa1-xN is grown from x=0 to x=0.4 at 600C. After
that the substrate temperature is increased to 790C with a ramp rate of 25C/min.
TJ_B
Following the p++ GaN, substrate temperature is increased immediately with 25C/min.
The substrate temperature reached the target temperature when 20% (~20 nm) of the ptype graded layer is completed.
TJ_C
Following the p++ GaN, substrate temperature is increased immediately with 50C/min.
The substrate temperature reached the target temperature when 10% (~10 nm) of the ptype graded layer is completed.
TJ_D
Following the p++ GaN, we stop the growth and increase the substrate temperature with
a ramp of 50C/min. When the substrate temperature reaches the set point of 790C, we
start the growth of p-type graded layer.
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Fig. S1: Integrated EL (a) and relative EQE (b) as a function of input current for TJ LED devices with
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different method described in Table S1. The results for the LEDs without TJ is also shown for
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comparison.
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The integrated EL vs current from all the devices are shown in Fig. S1(a). It is clearly evident that the
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temperature increase method has a dramatic effect on the performance of TJ integrated nanowire LEDs.
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The growth of low temperature (600C) graded AlGaN layer (TJ_A) shows order of magnitude less EL.
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This could be due to two factors. 1) AlGaN material quality degradation at low growth temperatures and
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large Mg incorporation at low temperature. 2) Low temperature growth increases the diameter of the
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nanowires due to suppressed adatom mobility. Which can cause less strain accommodation in the
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nanowire and can lead to the formation of non-radiative recombination centers in the form of strain
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induced defects. We see substantial increase in EL from the TJ LEDs when the temperature of the graded
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layer is increased immediately after the p++ GaN layer and observe best performance when the complete
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graded AlGaN layer is grown at high temperature.
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Fig S1(b) shows the relative EQE vs current in these devices. The TJ_A device shows no efficiency droop
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in the measured rage, i.e the peak EQE shifts toward very high currents and poses extremely low value.
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This happens when Shockley–Read–Hall (SRH) recombination increases and can be attributed to the
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strain induced defects as discussed earlier. For the TJ_B, TJ_C, and TJ_D devices we observe an increase
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in the relative EQE peak value and the peak EQE occurs at low currents. This indicates lower SRH
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recombination in these LEDs and attributed to the effective strain accommodation in these nanowire
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LEDs as well as lower Mg incorporation in the AlGaN region at elevated growth temperature. The peak
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efficiency of TJ_B and TJ_C LEDs are equal or higher compared to LEDs without TJ. The efficiency
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droop in these LEDs are 14% and 18% (green and blue curves), respectively at 18 mA (180 A/cm2) and
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much lower than the 66% (black curve) efficiency droop in LEDs without TJ.
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Structural characterization of TJ integrated nanowire UVLEDs:
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Fig. S2 shows the high angle annular dark field scanning transmission electron microscopy (HAADF-
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STEM) image and energy dispersive X-ray spectroscopy (EDXS) chemical composition (In, Ga, and Al)
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maps. We observe only Ga signal from the base of the nanowires, which corresponds to the n-type GaN
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base grown before the TJ. Localized In signal in the form of a disk is observed after the GaN base which
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confirms the existence of InGaN TJ in our nanowire samples. Following the InGaN disk, the Ga signal
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decreases, reaches a minimum and then increases while the Al signal increases, reaches maximum and
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then decreases. This confirms the back and forth composition grading in the polarization doped AlGaN
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regions. Three AlGaN disks separated by AlN barriers are also confirmed in the EDXS composition
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maps. STEM work was performed using an aberration (image) corrected FEI Titan 60-300 operated at
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300kV. The probe size, convergence angle and HAADF inner collection angle were 0.134nm, 13.2mrad
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and 45mrad, respectively. The instrument is fitted with a quadrant (SuperX) EDX detector with a solid
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angle of 0.9 srad. EDX acquisitions, approximately five minutes per spectrum image, were quantified
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using the Bruker Esprit software employing calculated k-factors.
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Fig. S2: HAADF STEM image and EDXS chemical composition maps of TJ integrated nanowire
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UVLEDs. Scale bar is 50 nm.
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Possible EQE limits in nanowire LEDs
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Surface recombination: The increased surface area in nanowires can act as a non-radiative
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recombination site. The carriers from the n- and p- side can be injected to the nanowire sidewalls
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instead of the active region and non-radiatively recombine. The surface recombination is
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minimized, to some extent, due to the spontaneous formation of core-shell structure. However,
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the surface recombination might still be a dominant factor in these nanowire LED devices. It was
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previously reported in nanowire visible LEDs that a thicker shell layer can decrease the surface
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recombination rate and dramatically increase the performance [1].
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Auger recombination: Previous studies in visible nanowire LEDs have shown that Auger
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recombination coefficients in nanowires are very small [2], [3] compared to its planar
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counterpart. However, an equivalent study in nanowire UV LEDs is not yet reported. Usually
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Auger recombination in higher bandgap (UV) quantum wells is expected to be more dominant
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than in lower bandgap (visible) quantum wells [4].
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Nanowire size deviation: The deviation in nanowire diameter can also affect the overall
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efficiency. Due to variation in the diameter and coalescence of several nanowires, some
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nanowire sites might possess a lower resistive path compared to the majority of the nanowires. In
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the ensemble measurements reported here, many nanowire LEDs are operated in parallel. It is
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possible that only a small number of nanowires are conducting a large portion of the current,
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operate at very high current densities, and emit a substantial amount of light; while most of the
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nanowires with relatively higher resistive paths conduct a smaller amount of current individually
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but large current as a whole due to their dominant number. The later kind of nanowires operate at
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extremely low current densities and emit no light. In effect these nanowires act as a conduction
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path but do not contribute toward the emission of ensemble LED devices thus causing low EQE
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values.
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Leakage due to shell region: The thin Al rich spontaneous shell may also cause a parallel leakage
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path. The injected carriers from n- and p-side can bypass the active region and conduct through
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the high composition AlGaN shell as the injection regions of both n- and p-side are made of AlN.
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Thus these bypassed carriers, although contributing to the conduction current, do not contribute
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to the emission.
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References:
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