Devices

The fundamental conception behind this Basic Technology programme is that of producing high-density arrays of micron-sized light-emitting diode (LED) elements from AlInGaN quantum well inorganic semiconductor materials.

As has been noted elsewhere on this web-site, the AlInGaN alloy system allows high-performance LED epi-structures to be produced at, in principle, any specified wavelength across the range 245nm – 550nm. The range 370nm – 550nm (near-UV through blue to green) relies on relatively mature InGaN semiconductor growth technology and is in effect available “off the shelf” at both Sheffield and Strathclyde Universities, although there are special requirements (see below) for certain device formats. The range 245nm – 370nm, on the other hand (deep to near UV), which relies on alloys incorporating aluminium, is much more challenging and is the subject of intensive ongoing research worldwide. The project was conceived (in 2003) with an awareness that developments in deep-UV-emitting nitrides were imminent. Remarkable progress has been achieved in the meantime, led by groups in the United States (we have close links to the leading teams at South Carolina and Kansas State Universities) but recently joined by the Sheffield University partners in this project. Sheffield have already produced good broad-area (conventional format) test LED’s emitting at 340nm, and are currently working on devices emitting at 315nm and below.

Once the epi-wafer has been supplied, with emission characteristics appropriate to the target application, multi-step photolithographic patterning allows the micro-pixellated LED elements to be specified and fabricated. In principle, depending upon the pitch between elements and the electrical addressing scheme used, any form of device pixel shape can be produced, down to a minimum diameter of around 5µm. The pixel shapes we have explored to date include micro-disks, micro-rings and micro-stripes, as defined by the photolithography mask design and patterned into the semiconductor using inductively-coupled plasma (ICP) dry etching. A typical diameter of these pixels is ~20µm, with centre-to-centre spacing ~30μm. Thus we have been able to produce, for example, 4,096 micro-disk elements in an active area ~2mm x 2mm.

It is central to most of the applications in this project that these high-density pixels be capable of “individual” electronic control. This is being explored via two approaches, the first involving matrix-addressing and the second involving local control from a CMOS backplane. It is a feature of the AlInGaN epistructures, which are (usually) grown on sapphire substrates, that light emission can be obtained either from the top surface of the epistructure or through the substrate (with is an optical material in its own right, transparent into the deep ultraviolet). The matrix-addressing scheme involves a sequentially-driven row/column grid of electrical contacts to address the individual pixels, and we are currently exploring this in both epi-up and substrate-up geometries. The backplane approach, on the other hand, allows “true” local control of the individual elements, which can be operated continuously, phase-modulated, or pulsed down to a sub-nanosecond time regime.

Colour conversion in nitride LEDs is normally achieved using inorganic phosphors. However, the typical grain size in these phosphors is comparable to the size of our pixels, and the phosphor materials themselves lack flexibility and functionality. Instead, in this project and involving the contributions of the Imperial College Solid-State group and the Strathclyde University Department of Chemistry, we are pursuing hybrid devices made by integrating AlInGaN semiconductors with light-emitting polymers (LEP’s). LEP’s, conjugated molecules of central importance in molecular electronics, are the basis of many forms of organic light-emitting displays, and have also been shown to be suitable gain media for use in (optically-pumped) lasers and optical amplifiers. These polymers are solution processable, and can be patterned and integrated with the inorganic devices in a number of ways. Techniques we are exploring include “conventional” photolithography, ink-jet printing, laser-activated direct writing and LED-activated direct writing. We have shown that, using a particular family of LEP’s called polyfluorenes, we can achieve down-conversion of a UV micro-pixellated LED to blue, green and red wavelengths, respectively, and also produce white light emission by blending the polymers in suitable combination. Blending the LEP’s in a photo-curable polymer matrix facilitates “direct writing” and a special form of this is to use the LED pixels themselves to produce a self-aligned microstructures in the LEP blend. The excitation of the polymer can be achieved by a radiative process, which involves light-emission by the inorganic material, absorption by the polymer and subsequent re-emission, but it can also be accomplished in a non-radiative way by direct dipole-dipole coupling between the inorganic structure and the polymer – the so-called Förster resonant energy transfer (FRET) process.

Materials for sources

The micro-LED sources used in the project are fabricated from epitaxial (single crystal) thin film multilayer structures of III-nitride semiconductors, grown using a technique known as metal organic chemical vapour deposition. The group III metals aluminium, gallium and indium combine with nitrogen to form a continuous series of alloys of general chemical formula Al xGa yIn 1-x-yN. Direct bandgaps in this system range from 6.3 eV for binary AlN to 0.7 eV for binary InN, as shown in the figure. Practical LEDs use designs with one or more quantum well (QW) layers of the active light emitting material. These QWs are typically about 2 nanometres in thickness, and are grown in an undoped region of the device structure sandwiched between thicker n- and p-doped layers. The mechanism of light generation is the recombination of electrons and holes injected on opposite sides of the active region.

Ternary (In,Ga)N alloy QWs are satisfactory for LEDs emitting in the green to violet visible region, and broad-area LEDs using such active regions have been in commercial mass production for other a decade. (In,Ga)N QWs are also employed in the structures grown at Strathclyde for work on Förster resonant energy transfer into light-emitting polymers. However, the efficiency of (In,Ga)N-based LEDs falls off rapidly for wavelengths less than 400 nm, and the efficient generation of shorter UV wavelengths continues to pose major research challenges. The leading results worldwide on LEDs emitting at wavelengths below 300 nm have been obtained using all-( Al, Ga)N device structures with no indium content in the QWs. However, the optimum choice of active region for devices emitting at 300-380 nm is currently less clearcut. Approaches using quaternary alloy QWs (ie. containing some indium), binary GaN QWs with ( Al, Ga)N barrier layers, and active regions containing zero-dimensional GaN quantum dots, have all shown promise.

There are several other challenges in developing UV LED structures in addition to the choice of the active region. These include issues of tensile strain and cracking in aluminium-rich layers, efficient outcoupling of light emitted in the LED, and optimisation of contacting and current spreading for p-type layers. The Sheffield group has put particular effort into a novel growth technique which addresses the first two of these issues. This involves growing thick buffer layers of high-quality AlN directly on sapphire substrates, in place of the GaN buffer layers employed in traditional device structures. ( Al, Ga)N and quaternary alloy layers grown on these AlN templates are in a state of compressive strain, such that cracking issues are avoided. The AlN buffer layer and sapphire substrate are also highly transparent to UV light emitted from the device. Therefore this growth approach has excellent compatibility with the work on flip-chip bonding of micro-LED arrays, and integration of emitters with micro-optics, being pursued by other partners in the project consortium.