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Graphene interlayer for current spreading enhancement by engineering of barrier height in GaN-based light-emitting diodes

Jung-Hong Min, Myungwoo Son, Si-Young Bae, Jun-Yeob Lee, Joosun Yun, Min-Jae Maeng, Dae-Gyeon Kwon, Yongsup Park, Jong-In Shim, Moon-Ho Ham, and Dong-Seon Lee

Open Access Open Access

Abstract

Pristine graphene and a graphene interlayer inserted between indium tin oxide (ITO) and p-GaN have been analyzed and compared with ITO, which is a typical current spreading layer in lateral GaN LEDs. Beyond a certain current injection, the pristine graphene current spreading layer (CSL) malfunctioned due to Joule heat that originated from the high sheet resistance and low work function of the CSL. However, by combining the graphene and the ITO to improve the sheet resistance, it was found to be possible to solve the malfunctioning phenomenon. Moreover, the light output power of an LED with a graphene interlayer was stronger than that of an LED using ITO or graphene CSL. We were able to identify that the improvement originated from the enhanced current spreading by inspecting the contact and conducting the simulation.

© 2014 Optical Society of America

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Figures (7)
Fig. 1
Fig. 1 Fabrication process of the three types of LEDs with different current spreading layers (CSLs). Based on a mesa structure, the LEDs were manufactured with a graphene current spreading layer (‘GR’), 150-nm thick ITO CSL (‘ITO’), and ITO on graphene interlayer (‘ITO on GR’).

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Fig. 2
Fig. 2 Results of Raman spectroscopy. ‘1’ through ‘6’ show the Raman spectra with respect to sapphire, graphene transferred onto sapphire, GaN-based LED, graphene transferred on GaN-based LED, ‘ITO’ on GaN-based LED, and graphene interlayer between ITO and GaN-based LED, respectively. G and 2D peaks from graphene can be clearly observed in all graphene-containing samples.

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Fig. 3
Fig. 3 (a) Transmittance values of ‘GR’, ‘ITO’, and ‘ITO on GR’. (b) Sheet resistance of each material obtained by Hall measurements. (c) Work functions of ‘GR’ and ‘ITO’ measured by ultraviolet photoelectron spectroscopy [38]. (d) Current-voltage characteristics of ‘GR’, ‘ITO’, and ‘ITO on GR’.

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Fig. 4
Fig. 4 (a) Light output power versus input current for ‘GR’, ‘ITO’, and ‘ITO on GR’ LEDs indicated by solid, dashed, and dotted black lines, respectively. Light output power versus input power for ‘ITO’ and ‘ITO on GR’ LEDs are also shown as dashed and dotted red lines, respectively. ‘ITO on GR’ showed a higher light output power than that of ‘ITO’. (b) Electroluminescence spectra of ‘ITO’ and ‘ITO on GR’ LEDs at 100 mA; the spectra were taken at 10 mA for the ‘GR’ LED because of a malfunction of the LED at low current injection.

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Fig. 5
Fig. 5 (a) The electroluminescence (EL) image of the ‘GR’. (b) An optical micro scope image of a probe-tip used in EL measurement. (c) An optical microscope image of the p-type metal pad region of the malfunctioning LED after removing the metal layers. Red-colored mesh represents the point of Raman mapping. Folding layers and a void are clearly observed. (d) and (e) show the results of Raman mapping using the intensity of G (1594 cm−1) and 2D (2700 cm−1), respectively.

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Fig. 6
Fig. 6 (a), (b), and (c) show the band diagram of the CSLs and the p-type GaN: (a) ITO/p-type GaN, (b) graphene/p-type GaN, (c) ITO/graphene/p-type GaN. (d) and (e) show a schematic of the current flow from the p-type metal pad to the active layer through ‘ITO’ or ‘ITO on GR’ CSLs, respectively.

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Fig. 7
Fig. 7 (a) and (b) are optical microscope images of luminescence for ‘ITO’ and ‘ITO on GR’ LEDs at 100 mA. (c) Luminous intensity between the p-type and n-type metal pads, marked by black and red arrows in (a) and (b). Distance at x-axis signifies the length from the p-type metal pad to n-type metal pad. (d) and (e) are simulation results for current injection into the active layer through the ‘ITO’ or ‘ITO on GR’ CSLs.

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Equations (2)

Equations on this page are rendered with MathJax. Learn more.

L s = ( ρ c + ρ p t p ) t n ρ n
B= E g −( ϕ m − χ s )
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