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What is the monolithic integrated tri-color LED developed by this university in the United States?
- Sep 19, 2017 -

Based on gallium nitride technology and existing manufacturing facilities, strain engineering can provide a feasible method for micro-display.

Based on the strain engineering of indium gallium nitride (InGaN) Multiple quantum wells, the University of Michigan has developed a monolithic integrated amber-green-blue LED (Fig 1). The strain engineering is achieved by etching different diameters of nano-columns. 

Fig. 1. The various diameters of the nano-column led array from the Top-down manufacturing schematic

The researchers hope to produce a red-green-blue led in the future with a 635nm luminous quantum well, providing a viable method for a micro-display based on this pixel led. Other potential applications include illumination, biosensors and optical genetics.

In addition to support from the National Science Foundation (NSF), Samsung supports manufacturing and equipment design. Researchers hope to develop a chip-level multicolor LED platform based on existing manufacturing infrastructure.

Epitaxial materials are grown on 2-inch no-patterned sapphires by means of metal-organic chemical vapor deposition (MOCVD). The luminous active region consists of 5 2 5nm InGaN traps separated by a 12nm gan gate. The electronic barrier layer and the P-contact layer are composed of 20nm gallium nitride (P-al0.2ga0.8N) and 150nm P-gan respectively.

The Nano-column is formed by using electron beam lithography, and the nickel mask is used for the mixed wet and dry etching process. Most of the etching is dry inductively coupled plasma, and the wet etching phase is used to achieve the final diameter and to remove damage from the dry etching step. The etching depth is about 300nm. During the entire manufacturing process, the etching mask is protected to protect the P-gan surface.

After the plasma-enhanced chemical vapor deposition (PECVD) of 50nm silicon nitride was performed, the structure was formed by using a rotational-coated glass to isolate N and P-gan parts.

Dry-type corrosion of the flat structure to expose the tip of the column. Remove the nickel mask material with nitric acid solution. P-Contact Nickel/gold metallization is thermally annealed in the air.

The electrical performance of the device shows a low leakage of about 3x10-7a per pixel at 5V reverse bias. The low leakage is attributed to two factors-the flattened quantum well provides a low current crowding effect, and the restriction of the strain-initiated carrier to the center of the nano-column. The risk of a reduced effect due to greater current density in a narrower column can be improved by reducing the strain, thus reducing the quantum limit "stark effect" of the electric field caused by the charge polarization of the chemical bonds in the-nitride.

The pixels consist of columns with different diameters and different colors (Fig 2). As the diameter increases, the wavelength becomes longer and the variation is greater. The researchers attributed the change to quantum well thickness changes on the wafer.

QQ screenshot 20170916103202. png

Fig. 2. (a) room temperature electroluminescent spectra of Blue (487nm), Green (512nm), Orange (575nm) and Amber (600nm) light obtained from 50nm, 100nm and 800nm diameter nano columns and thin film led pixels.

(b) The wavelength of light obtained by one-dimensional stress relaxation theory.

(c) The position of the main peak under various biased voltages.

With the increase of voltage and current injection, more and more loose narrow nanotubes also show less wavelength blue shift. 800nm diameter nano column pixel blue shift between 2.8V and 4V is 40nm. This is due to the research team sifting through the strain-dependent voltage field in the trap.

The team fixed the bias voltage and changed the intensity through pulse frequency modulation, thus stabilizing the output wavelength of the pixel. Through this experiment, it is shown that all pixel types give stable wavelength and relative electroluminescence intensity, and the duty ratio of the pulse signal is changed almost linearly. The pulse width is 400μs. The pulse frequency varies between 200Hz and 2000Hz.