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Illumination & Displays

On display

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

From oemagazine July 2001
30 July 2001, SPIE Newsroom. DOI: 10.1117/2.5200107.0006

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 (see table). 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.

µ-LED displays

Figure 1. III-nitride µ-LEDs consist of an InGaN/GaN quantum well structure.

At Kansas State University, we have developed a III-nitride µ-LED technology suitable for microdisplays. To fabricate the wafers, we use sapphire substrates to grow 30-nm GaN buffer layers. The QW active layers consist of a 3.5-µm silicon-doped GaN bottom layer, a 0.1-µm silicon-doped superlattice composed of alternating 50-Å layers of aluminum-gallium-nitride (AlGaN) and GaN, a 50-Å layer of silicon-doped GaN, a 20-Å undoped indium-gallium-nitride (InGaN) active layer, a 0.14-µm magnesium- (Mg) doped superlattice consisting of alternating 50-Å layers of AlGaN and GaN, and a 0.5-µm Mg-doped GaN top layer (see figure 1). After deposition, we place the device in a rapid thermal anneal at 950°C for 5 s in nitrogen.

We have achieved p-layer concentrations of 5 X 1017 (hole mobility 12 cm2/Vs) and n-layer concentrations of 1.6 X 1018 (electron mobility 310 cm2/Vs) in these devices. By incorporating the AlGaN/GaN superlattice structure into our design, we enhanced the p-type concentration from 2 X 1017 to 5 X 1017 cm-3.

Figure 2. A scanning electron micrograph shows the µ-LEDs that form the independently addressable pixels that create changeable images (inset). (KANSAS STATE UNIVERSITY)

Our research group has fabricated and patented a novel prototype III-nitride semiconductor blue/purple microdisplay based on these structures (see figure 2). A typical device consists of a 10 X 10 array of pixels (individual µ-disk LEDs) with a total size of 0.5 X 0.5 mm2. Using photolithographic patterning and inductively coupled plasma dry-etching, we fabricated microdisplays with individual pixel sizes varying from 5 to 20 µm. The p- and n-type ohmic contacts consist of bilayers of nickel (20 nm)/gold (200 nm) and aluminum (300 nm)/titanium (20 nm), respectively. The p-type and n-contacts contacts were thermally annealed in nitrogen ambient for 5 minutes at 500°C and 650°C, respectively.

Figure 3. An optical microscope image shows the bonding scheme for III-nitride blue microdisplay that allows us to address each pixel individually to create images.

To obtain a working device with independently addressable pixels, we isolated the p-type contacts from the n-type layer by depositing a dielectric layer above the etch-exposed bottom n-type GaN (see figure 3). Conducting wires connect the n-type ohmic contacts with the contact pads that are used for current injection into those contacts. Other conducting wires connect the individual pixels through the top p-type ohmic contacts and the pixel control pads that are used for current injection into p-type ohmic contacts. Each pixel has its own control pad.

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 in figure 2, 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 τoff 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. oe

References

1. S. X. Jin, J. Li, J. Y. Lin, and H. X. Jiang, Appl. Phys. Lett. 76, 631 (2000).

2. H. X. Jiang, S. X. Jin, et al., Appl. Phys. Lett. 78, 1303 (2001).


Hongxing Jiang, Jingyu Lin

Hongxing Jiang is a professor and Jingyu Lin is an associate professor with Kansas State University, Department of Physics, Manhattan, Kansas.