Sep 6, 2011
InGaN/GaN heterostructures: building blocks for solid-state lighting
Nanowire-based LEDs are macroscopic devices that feature a great number of axial nanoLEDs connected in parallel. A single nanoLED has an average diameter of around 50 nm, a length approaching 1 µm and consists of an InGaN active region sandwiched between a n-type GaN base and a p-type GaN top section.
The team, which is based in France and includes researchers from CEA-Leti and INAC (CEA-Grenoble), has investigated several active region configurations, from thin InGaN/GaN multiple quantum wells to thicker single InGaN layers. And the study has led to the fabrication of several devices emitting in green, yellow and red spectral ranges with output powers up to 30 µW.
The fabrication of high-efficiency GaN-based LEDs with wavelengths longer than 550 nm is limited in a conventional planar (2D) layout by several physical aspects. The main challenge is related to strain considerations. Indeed, increasing the In content in InGaN alloy drives up the lattice parameter mismatch with GaN and causes internal mechanical strain. The presence of strain within In-rich quantum wells favours the formation of extended defects and enhances piezoelectric fields that promote a separation between electrons and holes. Both effects affect the quantum efficiency of the device.
Due to their one-dimensional (1D) geometry, InGaN/GaN nanowire heterostructures can efficiently reduce mechanical strain through free sidewalls. Consequently, an improvement in quantum efficiency can be expected for wire-based LEDs.
This is actually the case, at the microscopic scale at least. Indeed, thanks to electro-optical measurements performed with a confocal microscope, the scientists have been able to detect and analyse the emission coming from single active nanoLEDs.
Very promising results have been obtained in terms of output power – the team has measured more than 50 nW emitted by a single nanoLED. Unfortunately, the researchers also discovered that the main source of losses in the wire-based devices is due to electrical injection failure – less than 1% of the nanoLED is active under electrical injection.
Building better devices
The submicrometer resolved study revealed that there is still a lot of room for improvement and strengthened opinion on the potential of 1D heterostructures for light emission. The group believes that improving the nanowire growth homogeneity is the route to follow to increase the operability rate of the nanoLEDs and consequently enhance the performance of wire-based devices at the macroscopic scale.
Future developments will tell us whether this new wire-based technology can pave the way to more breakthroughs in solid-state lighting applications.
More information can be found in the journal Nanotechnology.
About the author
I am Anne-Laure Bavencove, a PhD student in the Optronics department at CEA-Leti (Laboratoire d'Electronique et de Technologie de l'Information) in Grenoble, France. My project is supervised by Prof. Le Si Dang from the Institut Néel and Philippe Gilet, a research engineer at CEA-Leti. The aim of my research is to fabricate integrated LEDs with GaN-based wires. I have investigated several wire configurations, from axial heterostructures grown by MBE to radial designs obtained by MOCVD. To explore the potential of these structures, I perform electro-optical characterization at different resolution scales. The work presented here is the result of a collaboration with the group "Nanophysique et Semiconducteur" from INAC (CEA-Grenoble), which has a strong expertise in MBE growth, optical and structural characterization of III-N nanowires. In particular, Gabriel Tourbot supervised by Bruno Daudin, is the PhD student in charge of the MBE growth of the samples.