On Display

Microdisplays based on III-nitride wide band-gap semiconductors put the future in our hands.

Microdisplays are tiny, but when put into an eyeglasses headset and viewed through a lens system, they can provide a virtual image comparable to viewing a 21-in. diagonal TV or computer screen or larger. Microdisplays can satisfy demands for hands-free and highly mobile applications in areas such as computing, entertainment, military, law enforcement, fire fighting, and medicine.

During military action, for example, a head-mounted microdisplay would not only link the pilot to vital information about aircraft systems and the environment; it also would provide hands-free capability, which is vital when making split-second decisions and actions that can determine the success or failure of a mission. In a few years, microdisplays may allow people to use computers and watch television without a real monitor, offering mobility, privacy, and fun.

Current microdisplays are based on liquid-crystal-display (LCD) or organic-light-emitting-diode (OLED) technology. Semiconductor microdisplays, which require the integration of a dense array of micro-size LEDs on a single semiconductor chip offer a number of advantages over more conventional approaches. They have yet to be successfully fabricated, however, because color conversion for full-color displays cannot be achieved in conventional III-V or silicon semiconductors.

The unique properties of III-nitride wide band-gap semiconductors may bring a solution to the problem of semiconductor microdisplays by potentially offering performance superior to that of LCD and OLED displays. Unlike LCDs, which normally require an external light source, III-nitride blue microdisplays are self-luminescent, use less space and power, and allow viewing from any angle without color shift and degradation in contrast. Although OLEDs are also emissive devices, they must be driven at much lower current densities than semiconductor LEDs, limiting output intensity. Depending on the alloy composition, III-nitride devices achieve band gaps ranging from 1.9 eV indium nitride to 3.4 eV gallium nitride (GaN) to 6.2 eV aluminum nitride. The incorporation of indium yields extremely high emission efficiency, and the robust devices offer high power and high temperature operation as well as simple down-conversion of output color from UV/blue/green to red or yellow. In addition, 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.

Performance

The emission wavelengths of our µ-LEDs vary from violet to green (390 to 520 nm) as a function of the indium content in the InGaN active layers. Based on tests of the individual µ-LEDs in the display, plots of power output versus forward current indicate good uniformity of light emission between individual devices in the array. Despite the fact that we did not use lateral epitaxial overgrowth techniques to minimize threading dislocations in the GaN layers, the devices appear to be quite efficient. The escape cone for isotropic spontaneous emission from these µ-LEDs through sapphire substrate is about 100°, which demonstrates that µ-LED displays can provide a very wide viewing angle.

Operating speed is always a concern for displays. The turn-on response for our display is on the order of the system response (approximately 30 ps) and thus cannot be measured. The turn-off process, however, is in the form of a single exponential. We found that turn-off time toff decreases as a function of µ-LED size, dropping from 0.21 ns for a 15-µm device to 0.15 ns for an 8-µm device. This may be because the effects of surface recombination are enhanced in smaller µ-LEDs. Another possible explanation is an enhanced radiative recombination rate in µ-LEDs caused by the microcavity effect. With this fast speed and other advantages such as long operation lifetime, III-nitride µ-LED arrays may be used to replace lasers as inexpensive short-distance optical links, such as between computer boards, operating at frequencies as high as 10 GHz.

III-nitride microdisplays offer a number of advantages over conventional display technologies, including self-luminescence; high brightness, resolution, and contrast; operation at high temperature, power, and speed; wide field-of-view; full-color spectrum capability; reliability; robustness; long life; and low power consumption. Likewise, the ability of 2-D array integration with advantages of high speed, high resolution, low temperature sensitivity, and applicability under versatile conditions make III-nitride µ-LEDs a potential candidate for light sources in short- distance optical communications.

(By Hongxing Jiang and Jingyu Lin, Kansas State University,   OEmagazine,  July, 2001)

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