Compound Semiconductor April May 2012

technology  LEDs

Figure 2: (a) Schematic illustration of InGaN/GaN dot-in-a-wire LEDs fabricated on an n-silicon (111) substrate.A single dot-in-a-wire LED structure with the incorporation of p-type modulation doping and an AlGaN electron-blocking layer (EBL) is shown in the inset.The flat energy band diagram is also illustrated. (b) Current-voltage characteristic of an InGaN/GaN dot-in-a-wire LED and the optical microscopy image of the device (inset).The series resistance is in the range of 20 - 50 Ω

Commercial promise

The processes that we use to create highly uniform, densely packed InGaN/GaN dot-in-a-wire arrays on silicon substrates are well suited to the fabrication of large-area LEDs (see Figure 2 a). Fabrication involves ‘planarization’ of the nanowire arrays using a polyimide resist, before a p-type Ni/Au/indium-tin-oxide contact is deposited on the top of the nanowire surface and an

n-metal Ti/Au contact is attached to the backside of the silicon substrate. This set of processes yields devices with excellent diode characteristics and negligible leakage current (see Figure 2 b).

By carefully selecting the height of the dots and their composition, we are able to fabricate InGaN/GaN dot-in- a-wire LEDs on silicon substrates with strong green,

Figure 3 (a)(Top) Optical images of the green,yellow,orange,and red-emitting dot-in-a-wire LEDs.(b) (left) Room-temperature relative external quantum efficiency (EQE) of the InGaN/GaN dot-in-a-wire LED device with the use of p-type modulation doping and an AlGaN electron-blocking layer.The simulated internal quantum efficiency (IQE) using the ABC model is shown for comparison.The optical image of the white LED is also shown in the inset.(c) (right) The Commission Internationale de l’Eclairage chromaticity diagram showing highly stable emission characteristics,with x and y in the ranges of ~ 0.33 – 0.35,and 0.36 – 0.38,respectively

36 www.compoundsemiconductor.net April / May 2012

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