THE TECHNOLOGY

The technology being developed here is based on the light-emitting (electroluminescence) capabilities of gallium nitride semiconductor materials. These materials, forming the quaternary alloy family (Al,Ga,In)N, allow the fabrication of light-emitting diodes (LED’s) in the short-wavelength-visible and ultraviolet regions of the electromagnetic spectrum, at wavelengths in the approximate range of 250-550nm. These LED’s currently underpin a multi-billion dollar worldwide industry for illumination, displays and solid-state lighting.

Our motivation in this project is to customise gallium nitride LED’s for instrumentation and scientific use. In particular, we are focussing on micro-pixellated LED’s (so-called ‘micro-LEDs’) where the emitting area consists of several hundred to several thousand micro-sized light-emitting elements – each entire array being about the size of a match head! The generic characteristics of these sources are as follows:

1. The emission wavelength is determined by the structure designer and semiconductor crystal grower. The (AlGaIn)N alloy family presently allows wafers to be readily grown for efficient light emission in the wavelength range 370nm – 550nm, covering the near-ultraviolet, violet, blue and green. Extension deeper into the ultraviolet down to ~250nm has recently been demonstrated, covering the UVA and UVB regions of the spectrum and beginning to enter the UV-C region. The electroluminescence from these materials is generally single-peaked with a relatively narrow line-width (full width at half maximum ~10-15nm), allowing spectrally-selective excitation.
   
   
2. The size and shape of the individual emitting pixels is determined by the device designer and is controlled by the lithographic processing. Disk-like pixels of size in the range 4-20 μ m have been made so far, and other shapes have been produced including micro-rings and micro-stripes:
   
   

   64 x 64 micro-LED array	micro-stripes	micro-rings

   
   
3. With a suitable device layout and addressing scheme, the emission from each of the pixels in the array can be controlled separately. In other words, the devices can function as micro-displays which are pattern-programmable via a computer interface. Suitable addressing schemes which are being explored include matrix-addressing from an electrode grid and direct addressing of individual pixels via a CMOS-based silicon backplane.
   
   

   micro-LED driver	64 x 64 micro-LED array

   
   
4. The emission characteristics of these sources are suitable for optical projection. Thus a pattern programmed onto the arrays can be imaged to a separate ‘applications plane’. This may be immediately above the emitting display or at some distance, depending upon the optical relay system used to project the image.
   
   
   
5. The wavelength range and projected image power densities from these arrays are suitable for inducing physico-chemical changes in a wide range of materials. In particular, they are suitable for inducing interactions in organic materials as varied as photo-resists, light-emitting polymers, biopolymers, organic dyes and fluorescent biomarkers, and either stained or auto-fluorescing biological cells and tissues. The interactions include F ö rster resonant energy transfer (FRET), luminescence or fluorescence down-conversion, photo-polymerisation via either photolabile protecting groups or photoacid generation, control of local fields in photoconductive electrode systems, etc.
   
   
6. Depending upon the mode of addressing, there is great flexibility in the temporal mode of operation of the devices. Individual elements can be operated continuously, phase modulated or pulsed. In the pulsed regime, output of ~1ns (and potentially below) is achievable. This is short-enough compared to the fluorescence/luminescence decay times of organic dyes and polymers that applications such as highly-parallel time-resolved fluorimetry and high-throughput screening become possible, or array-based fluorescence lifetime imaging microscopy (FLIM). An exemplar time-correlated single-photon counting measurement of the impulse response of one pulsed pixel and the fluorescence its emission has induced in a Rhodamine dye sample, is shown below.

Flourescence decay of rhodamine 123 using blue micro-LED excitation

APPLICATIONS

The applications of this technology are potentially very broad indeed. In particular, it facilitates ‘lab-on-a-chip’ integration with control electronics, imaging optics, photodetectors and such applications platforms as cell cultures, microfluidics, microarrrays and photoconductive electrodes. Some major areas are as follows:

Hybrid organic/inorganic devices

• colour conversion
• energy transfer
• lasing

Direct-write patterning and synthesis of materials

• mask-free photolithography
• self-aligned photo-polymerisation
• custom synthesis of biopolymer arrays and ‘gene chips’

Novel forms of optical microscopy

• fluorescence lifetime imaging (FLIM)
• structured illumination microscopy
• confocal linescan imaging

Integrated light-patterned electrode structures

• massively-parallel manipulation of cells, microparticles, charged molecules
• electrohydrodynamic patterning of polymers