GaN-based Light-emitting Diode Structures Research Articles

Van der Waals epitaxy of nearly single-crystalline nitride films on amorphous graphene-glass wafer.

Van der Waals epitaxy provides a fertile playground for the monolithic integration of various materials for advanced electronics and optoelectronics. Here, a previously unidentified nanorod-assisted van der Waals epitaxy is developed and nearly single-crystalline GaN films are first grown on amorphous silica glass substrates using a graphene interfacial layer. The epitaxial GaN-based light-emitting diode structures, with a record internal quantum efficiency, can be readily lifted off, becoming large-size flexible devices. Without the effects of the potential field from a single-crystalline substrate, we expect this approach to be equally applicable for high-quality growth of nitrides on arbitrary substrates. Our work provides a revolutionary technology for the growth of high-quality semiconductors, thus enabling the hetero-integration of highly mismatched material systems.

Stripping GaN/InGaN epitaxial films and fabricating vertical GaN-based light-emitting diodes

The growth technology of GaN-based light-emitting-diodes (LEDs) greatly restricts their preparation of vertical structure. Now we can get GaN-based LEDs structure on silicon (Si) substrate, which makes us a big step forward in the preparation of vertical GaN-based LEDs. Herein, we report the fabrication of vertical GaN-based LEDs through a chemical etching process to removing Si substrate combing with inductively coupled plasma reactive ion etching (ICP-RIE) to remove the un-doped buffer followed by copper (Cu) electroplating technology. By collecting photoluminescence (PL) and Raman scattering spectra of the GaN-based epitaxial structure, it is confirmed that the residual strain between the epitaxial structure and growth substrate is effectively released. The corresponding electroluminescence (EL) measurement of the vertical GaN-based LEDs was conducted by using a probing test with an optical power meter, showing a good electrical property and optical property. The vertical GaN-based LEDs have better output power and better heat dissipation capacity. This research proposes a low-cost method to fabricate freestanding and high performance vertical GaN-based LEDs.

GaN-based light emitting diodes on nano-hole patterned sapphire substrate prepared by three-beam laser interference lithography

Nano-hole patterned sapphire substrates (NHPSSs) were successfully prepared using a low-cost and high-efficiency approach, which is the laser interference lithography (LIL) combined with reactive ion etching (RIE) and inductively coupled plasma (ICP) techniques. Gallium nitride (GaN)-based light emitting diode (LED) structure was grown on NHPSS by metal organic chemical vapor deposition (MOCVD). Photoluminescence (PL) measurement was conducted to compare the luminescence efficiency of the GaN-based LED structure grown on NHPSS (NHPSS-LED) and that on unpatterned sapphire substrates (UPSS-LED). Electroluminescence (EL) measurement shows that the output power of NHPSS-LED is 2.3 times as high as that of UPSS-LED with an injection current of 150 mA. Both PL and EL results imply that NHPSS has an advantage in improving the crystalline quality of GaN epilayer and light extraction efficiency of LEDs at the same time.

Local nanotip arrays sculptured by atomic force microscopy to enhance the light-output efficiency of GaN-based light-emitting diode structures

In this work, local nanotip arrays on GaN-based light-emitting (LED) structures were fabricated through nano-oxidation using an atomic force microscope (AFM). The photoluminescence (PL) intensity of the InGaN/GaN multiple quantum wells (MQWs) active layer and the light extraction efficiency of the LED structure were enhanced by forming this nanotips structure to serve as a graded-refractive index layer, which is further validated by the finite-difference time-domain analysis. The PL emission peak of the MQWs active layer has a blue-shift phenomenon that is caused by a partial reduction of the strain on the InGaN well. It is expected that our approach opens a promising route for simultaneously enhancing both the internal quantum efficiency and the light extraction efficiency of GaN-based LEDs. The proposed AFM-based method will be of importance for local patterning the light emitting components for optoelectronic applications.

Depth-resolved confocal micro-Raman spectroscopy for characterizing GaN-based light emitting diode structures

In this work, we demonstrate that depth-resolved confocal micro-Raman spectroscopy can be used to characterize the active layer of GaN-based LEDs. By taking the depth compression effect due to refraction index mismatch into account, the axial profiles of Raman peak intensities from the GaN capping layer toward the sapphire substrate can correctly match the LED structural dimension and allow the identification of unique Raman feature originated from the 0.3 μm thick active layer of the studied LED. The strain variation in different sample depths can also be quantified by measuring the Raman shift of GaN A1(LO) and E2(high) phonon peaks. The capability of identifying the phonon structure of buried LED active layer and depth-resolving the strain distribution of LED structure makes this technique a potential optical and remote tool for in operando investigation of the electronic and structural properties of nitride-based LEDs.

Numerical Study of ZnO-Based LEDs

2-D numerical simulation is employed to assess a number of possible design approaches aimed at optimizing the internal quantum efficiency (IQE) of ZnO-based light-emitting diodes (LEDs) grown along the c-axis. First, the relative performance of similar ZnO-based and GaN-based LED structures is compared and discussed. Second, the effects on IQE of thickness, doping, and alloy composition of the MgZnO electron blocking layer (EBL) is studied in order to maximize the carrier confinement in the active region. The optimum number of quantum wells is also addressed, and different strategies for barrier doping are considered, showing that, if the EBL is doped p-type, a similar doping in the barriers is not required to compensate for the spontaneous and piezoelectric interface charges and to enhance hole transport. Different choices of the geometrical and doping parameters of the n-type access region are considered, and the impact of different values of the electron mobility is determined. Finally, the analysis of a ZnO/BeZnO LED structure suggests that the incorporation of BeZnO layers does not provide significant advantages.

Oblique-angle sputtered AlN nanocolumnar layer as a buffer layer in GaN-based LED

This work presents an aluminum nitride (AlN) nanocolumnar layer sputtered at various oblique angles and its application as a buffer layer for GaN-based light-emitting diodes (LEDs) that are fabricated on sapphire substrates. The OA-AlN nanocolumnar layer has a diameter of about 30–60 nm. The GaN-based LED structure is perpendicularly extended from the OA-AlN nanocolumnar layer. Then, the nanocolumnar structure is merged into p-GaN layer to form a mesa structure with a diameter of about 200–600 nm on the surface of the GaN-based LED. Moreover, optical characteristics of the LED were studied using photoluminescence, along with the blue-shifts observed as well.

Vertical GaN-Based Light-Emitting Diodes Structure on Si(111) Substrate with Through-Holes

A novel vertical GaN-based light-emitting diode (LED) structure on a Si(111) substrate with through-holes was reported in this letter. The through-holes were formed by dry etching from the n-GaN layer to the Si substrate. Metals connecting the n-GaN layer and Si substrate were used to fill the holes. The series resistances induced by the AlN buffer layer and other interlayers were shorted by the metals filling the holes. Compared with those of the conventional LED structure, the series resistance and operating voltage at 20 mA were reduced from 26 to 22.5 Ω, and from 4.4 to 4.0 V, respectively. Light output intensity shows an increase of 29%.

Enhancement of light extraction in GaN based LED structures using TiO2 nano-structures

TiO2 nano-patterns were formed on the indium-tin-oxide (ITO) layer of GaN based light emitting diodes (LEDs) without a residual layer using a sol-imprinting process. A polydimethylsiloxane mold replicated from a Si master was used as the imprint stamp for the sol-imprinting process. The light extraction efficiency of LEDs was enhanced by the TiO2 nano-patterns formed on the ITO layer because the TiO2 nano-patterns locally modulated the refractive index of the ITO layer and enhanced the scattering of light at the ITO layer. Consequently, directly fabricated TiO2 nano-patterns on the ITO layer can esoln hance the light extraction efficiency of LEDs without plasma-induced damage.

Light extraction from GaN-based light emitting diode structures with a noninvasive two-dimensional photonic crystal

A noninvasive fabrication process involving soft nanoimprint lithography is used to pattern a photonic crystal (PhC) in titania film for enhanced light extraction from a GaN light emitting diode (LED). This technique avoids damaging the LED structure by the etching process, while photoluminescence measurements show extracted modes emitted from the quantum wells which agree well with modeling. A light extraction improvement of 1.8 times is measured using this noninvasive PhC.

GaN-based light-emitting diode structure with monolithically integrated sidewall deflectors for enhanced surface emission

To improve the overall surface emission efficiency, the structure of a standard GaN light-emitting diode (LED) was modified; the mesa sidewalls were etched at an angle, and deep enough to reach the sapphire substrate. Photoexcitation experiments, including photoluminescence and near- and far-field emission patterns, were performed on LED-like test devices, and results indicated that the angled sidewalls efficiently deflect photons that are initially guided laterally within the GaN epilayer in the off-surface direction. For a sidewall angle of 30deg, the total surface emission strength was improved by a factor exceeding three. Computer simulations produced results consistent with the experimental observations

Metalorganic vapor phase epitaxy grown InGaN∕GaN light-emitting diodes on Si(001) substrate

We present GaN-based light emitting diode structures on a Si(001) substrate. The 2.3μm thick, crack-free layers were grown by metalorganic vapor phase epitaxy using a high-temperature AlN seed layer and 4° off-oriented substrates. This allows us to grow a flat, fully coalesced, and single crystalline GaN layer on Si(001). For preventing crack formation, four AlN interlayers were inserted in the buffer structure. The optically active layers consist of five-fold InGaN∕GaN multiple quantum wells showing a bright electroluminescence at 490nm at room temperature. The crystallographic structure was analyzed by x-ray diffraction measurements and the optical properties were determined by photo- and electroluminescence.

Thick, crack-free blue light-emitting diodes on Si(111) using low-temperature AlN interlayers and in situ SixNy masking

Thick, entirely crack-free GaN-based light-emitting diode structures on 2 in. Si(111) substrates were grown by metalorganic chemical-vapor deposition. The ∼2.8-μm-thick diode structure was grown using a low-temperature AlN:Si seed layer and two low-temperature AlN:Si interlayers for stress reduction. In current–voltage measurements, low turn-on voltages and a series resistance of 55 Ω were observed for a vertically contacted diode. By in situ insertion of a SixNy mask, the luminescence intensity is significantly enhanced. A light output power of 152 μW at a current of 20 mA and a wavelength of 455 nm is achieved.