Nanocolumn Light-emitting Diodes Research Articles

Fabrication and optical properties of regularly arranged GaN-based nanocolumns on Si substrate

Nanocolumn light-emitting diodes (LEDs) are expected to achieve the monolithic integration of the three primary-color micro-LEDs for micro-LED displays. From the viewpoints of low cost and large-area substrates, a technology for the regular arrangement of nanocolumns on Si substrates is required. The improvement of GaN nanocolumns on Si would be an important advance for the preparation of high efficiency optical devices. In this paper, the effects of column diameter and height on the photoluminescence (PL) and cathodoluminescence properties of GaN nanocolumn arrays were investigated. The PL intensity of the 700-nm high (tall) nanocolumn was three times stronger than that of the 350-nm high (short) nanocolumn. Although the PL intensity decreased dramatically with an increasing diameter for the shorter nanocolumns, it retained its high value (up to 220 nm) for the taller GaN nanocolumns. For the latter specimens, a decrease in the number of emitting nanocolumns, which would reduce emission efficiency, was suppressed by the dislocation filtering effect. Moreover, yellow luminescence was suppressed for taller nanocolumns. In the low-temperature-PL spectra, the peak observed at 3.41 eV, related to a stacking fault, increased with diameter regardless of height. These results indicate that the appropriate design of column height and diameter is of considerable importance for obtaining high efficiency emissions. Finally, InGaN/GaN quantum wells were fabricated on the regularly arranged GaN nanocolumn platform. Blue, green, and red (RGB) emission colors with no significant change in emission intensity were observed. These results constitute an important step toward the monolithic integration of RGB micro-LEDs.

Self-Organized Eu-Doped GaN Nanocolumn Light-Emitting Diode Grown by RF-Molecular-Beam Epitaxy

High-concentration doping of Eu ions working as luminescent centers in GaN is required to improve light output of optical devices. GaN nanocolumns can be used to achieve high crystallinity in high-concentration Eu-doped GaN (GaN:Eu). In this study, optical properties of a GaN:Eu nanocolumn grown by RF-plasma-assisted molecular-beam epitaxy (RF-MBE) are investigated and GaN:Eu nanocolumn light-emitting diodes (LEDs) are demonstrated. Suppression of yellow luminescence and concentration quenching of PL intensity are observed at RT for GaN:Eu nanocolumns. When the effect of Eu concentration on optical properties is analyzed using rate equation, an increase in the Eu concentration is observed to affect 5D0 lifetime corresponding to back transfer, and not the effective cross-section corresponding to transfer efficiency. Based on these results, GaN:Eu nanocolumn LEDs are fabricated on an n-type (111) Si substrate. Clear rectification characteristics and sharp red luminescence are observed. The light output increases with increasing Eu concentration up to a concentration of 6 × 1020 cm−3. These results suggested that GaN:Eu nanocolumns provide feasible crystal to realize novel optical devices.

Selective area growth of InGaN-based nanocolumn LED crystals on AlN/Si substrates useful for integrated μ-LED fabrication

Triangular-lattice nanopillar-array templates with a lattice constant of 280 nm and with AlN disks on top of the underlying Si pillars were prepared on 2-in. AlN/Si substrates through nanoimprint lithography and dry etching. Regularly arranged GaN nanocolumn arrays with a 220-nm diameter were grown on the templates to fabricate wide-area emission InGaN/GaN nanocolumn light emitting diodes (LEDs). An LED chip with an Indium Tin Oxide (ITO) electrode with an area of 3 × 4 mm2 operated at a current of 100 mA emitted blue-green light (504 nm in wavelength) from the entire surface of the large emission area.

Thermally Engineered Flip-Chip InGaN/GaN Well-Ordered Nanocolumn Array LEDs

Thermally engineered flip-chip (FC) InGaN/GaN well-ordered nanocolumn array light-emitting diodes (LEDs) are demonstrated for the first time. A directional photoluminescence beam profile was observed from the n-side bottom of the GaN nanocolumns, which maintained an optically damage-free periodic arrangement of nanocolumns through the FC process involving Si substrate removal by wet etching. The developed LED achieved single-peak emission at $\sim 570$ nm in the middle of the visible range. The thermal resistance of the nanocolumn LED was significantly reduced using the FC configuration.

Selective-area growth of GaN nanocolumns on Si(111) substrates for application to nanocolumn emitters with systematic analysis of dislocation filtering effect of nanocolumns

The growth of highly uniform arrays of GaN nanocolumns with diameters from 122 to 430 nm on Si (111) substrates was demonstrated. The employment of GaN film templates with flat surfaces (root mean square surface roughness of 0.84 nm), which were obtained using an AlN/GaN superlattice (SL) buffer on Si, contributed to the high-quality selective-area growth of nanocolumns using a thin Ti mask of 5 nm thickness by rf-plasma-assisted molecular beam epitaxy. Although the GaN template included a large number of dislocations (dislocation density ∼1011 cm−2), the dislocation filtering effect of nanocolumns was enhanced with decreasing nanocolumn diameters (D). Systematic transmission electron microscopy (TEM) observation enabled us to explain the dependence of the dislocation propagation behavior in nanocolumns on the nanocolumn diameter for the first time. Plan-view TEM analysis was performed for nanocolumns with D = 120–324 nm by slicing the nanocolumns horizontally at a height of ∼300 nm above their bottoms and dislocation propagation through the nanocolumns was analyzed by the cross-sectional TEM observation of nanocolumns with D ∼ 200 nm. It was clarified that dislocations were effectively filtered in the bottom 300 nm region of the nanocolumns, the dislocation density of the nanocolumns decreased with decreasing D, and for narrow nanocolumns with D < 200 nm, dislocation-free crystals were obtained in the upper part of the nanocolumns. The dramatic improvement in the emission properties of GaN nanocolumns observed with decreasing diameter is discussed in relation to the decreased dislocation density. The laser action of InGaN/GaN-based nanocolumn arrays with a nanocolumn diameter of 170 nm and a period of 200 nm on Si under optical excitation was obtained with an emission wavelength of 407 nm. We also fabricated red-emitting InGaN-based nanocolumn light-emitting diodes on Si that operated at a wavelength of 652 nm, demonstrating vertical conduction through the AlN/GaN SL buffer to the Si substrate.

Monolithic integration of four‐colour InGaN‐based nanocolumn LEDs

The monolithic integration of four-colour indium gallium nitride (InGaN)-based nanocolumn light-emitting diodes (LEDs) is demonstrated. In the integrated nanocolumn LED unit, blue-, sky-blue-, green- and yellow-emitting micro-LEDs (LEDs 1–4) with a 65 μm diameter circular indium tin oxide emission window were arrayed in a 2 × 2 square lattice with a lattice constant of 190 μm. LEDs 1–4 consisted of nanocolumn arrays arranged in a triangular lattice with a lattice constant of 300 nm and their nanocolumn diameters at the position of the InGaN/gallium nitride (GaN) multiple quantum wells (MQWs) were 119, 145, 188 and 231 nm, respectively. The increase in nanocolumn diameter from LED 1 to LED 4 resulted in increasing emission peak wavelengths, which were 465, 489, 510 and 570 nm for LEDs 1–4, respectively. On the same substrate, a red-emitting micro-LED was prepared, in which the nanocolumn diameter was increased to 260 nm by using a 350 nm-lattice-constant nanocolumn array. A combination of different lattice constants in an integrated LED unit is expected to contribute to the achievement of red–green–blue–yellow (RGBY)-colour-integrated nanocolumn LEDs.

Green-Light Nanocolumn Light Emitting Diodes With Triangular-Lattice Uniform Arrays of InGaN-Based Nanocolumns

Green-light nanocolumn light emitting diodes (LEDs) consisting of triangular-lattice uniform arrays of InGaN-based nanocolumns with lattice constants of 245-350 nm were fabricated with TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mask selective-area growth by RF-plasma-assisted molecular beam epitaxy. The built-in core/shell structure of InGaN/GaN multiple quantum wells was self-assembled, confining carriers in the core of the nanocolumns. The characteristics of the nanocolumn LEDs were evaluated at room temperature under dc current injection in the range from 4.5 to 450 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The emission wavelengths were 515 to 550 nm, and small current-induced spectral blueshifts of 2-11 nm were observed. The linewidth narrowing at a low current density was very small for the nanocolumn LEDs, in which nanocolumns with the same size were homogeneously arranged. The sidewalls of the nanocolumns were passivated by the deposition of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , contributing to the elimination of current leakage paths. The external quantum efficiency was improved with the passivation. Radiation beam angular profiles of the nanocolumn LEDs were evaluated and directional beam radiation was observed at specific wavelengths, which was attributed to the photonic band edge of the periodic nanocolumn arrangement.

Effect of Be‐doping on InGaN/GaN nanocolumn light‐emitting diode structures by rf‐plasma‐assisted molecular‐beam epitaxy

AbstractGaN nanocolumns are one‐dimensional high‐quality dislocation‐free nano‐crystals. The effect of Be‐doping on GaN nanocolumns was investigated using InGaN/GaN nanocolumn LED structure grown by RF‐plasma‐assisted molecular‐beam epitaxy. Be was doped into the upper cladding region of the nanocolumn light‐emitting diodes (LEDs). Be is expected to be a p‐type dopant of GaN with a shallow accepter level, but p‐type conductivity was not obtained in these experiments. However, we observed a remarkable enhancement of the lateral growth of the GaN nanocolumn by Be doping; that is, the surface of the Be‐doped region became smooth and continuous with increasing Be doping concentration. The root mean square (RMS) surface roughness was reduced from 35 to 10 nm with increasing Be cell temperature from 710 to 975 °C. Cross‐sectional TEM observation revealed that no threading dislocations were generated in the observed area at the conjunction boundary between neighboring nanocolumns during lateral growth. This suggests that a high‐quality overgrowth layer with a low threading dislocation density can be realized on Be‐doped GaN nanocolumns. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)