Technical Insight
III-Nitride Micro-Displays (Nitride News)
Semiconductor micro-displays, which require the integration of a dense array of micro-size LEDs on a single semiconductor chip, have not been successfully fabricated to date. So far, large flat-panel LED displays, which are used on large buildings and in sports stadiums, are made up of a massive number of discrete LEDs. Last year, the group led by Hongxing Jiang and Jingyu Lin at Kansas State University (Manhattan, KS) demonstrated boosted emission efficiencies in interconnected microdisk LEDs [see Compound Semiconductor 6(8), p.14 and Appl. Phys. Lett. 2000, 77, p.3236]. Subsequently the successful fabrication of a prototype blue micro-display based on InGaN/GaN quantum wells was reported at the 2000 Fall MRS meeting in Boston. KSU s prototype device measures 0.5 0.5 mm2 and consists of 10 10 pixels, each being 10 m in diameter (see Figures , ). Although micro-displays are tiny, they can provide a virtual image comparable to viewing a 21-inch diagonal TV/computer screen when put into a headset and viewed through a lens system. Micro-displays can be used in a variety of devices such as head-mounted displays, camcorders and viewfinders, and are suitable for such diverse applications as computing, entertainment, military, law enforcement, fire fighting, medicine etc. A New Display Technology Current micro-displays are based on liquid crystal display (LCD) technology or organic light emitting diodes (OLEDs). Unlike LCDs, which require an external light source, III-nitride blue micro-displays are self-luminescent, resulting in both space and power savings. They also allow viewing from any angle without color shift and degradation in contrast. On the other hand, the current generation of OLEDs are only reliable when driven at current densities many orders of magnitude lower than semiconductor LEDs, and hence are not yet suitable for high-intensity (outdoor) use. Additionally, III-nitrides are grown on sapphire substrates that are transparent to light and hence can serve as a natural surface for image display, reducing the steps for device packaging. Emission properties such as electroluminescence (EL) spectra, output power versus forward current (L-I) characteristic, viewing angle, uniformity, and speed of these III-nitride micro-displays have been measured. Due to the unique properties of III-nitride wide bandgap semiconductors, micro-displays fabricated from these materials can potentially provide unsurpassed performance. Properties include high brightness, resolution and contrast; high temperature and high power operation; high shock resistance and wide viewing angles; and long life and low power consumption. All these features enhance the present capabilities of miniature display systems. It was also shown that the speed of these devices was very fast (around 0.3 ns). Since the fabrication of micro-size LEDs is much less sophisticated than VCSELs, the group believes that III-nitride blue micro-size LED arrays are also very attractive for inexpensive optical links. For more information, contact Hongxing Jiang at jiang@phys.ksu.edu. The group s paper entitled "III-Nitride Blue Micro-displays" by H. X. Jiang et al. is to be published in the February 26, 2001 issue of Applied Physics Letters.
