GaN-based Ultraviolet Light-emitting Diodes Research Articles

Enhancing current spreading performance of GaN-based ultraviolet light-emitting diode chips through periodic doping composite barrier structures

In response to the issue of current crowding in GaN-based ultraviolet (UV) light-emitting diode (LED) chips, a periodic doping composite barrier structure is proposed. The results of the optical intensity distribution demonstrate that the device with the periodic doping composite barrier structure exhibits a more uniform light distribution compared to the device with traditional structures. Moreover, when the injected current is 20 mA, the light output power (LOP) and wall-plug efficiency (WPE) are improved by 11.99% and 13.5%, respectively. Experimental and simulation results indicate that the periodic doping composite barrier structure enhances the current spreading performance through two mechanisms: (1) the introduction of an electron barrier layer enhances lateral current transport; (2) increase the ratio of longitudinal to transverse resistivity of the material to further enhances current spreading performance.

Research on the Performance Improvement of GaN-Based Ultraviolet Light-Emitting Diode Chips with Magnetron-Sputtered AlN Nucleation Layers

In this paper, the influence of magnetron-sputtered AlN nucleation layers with different thicknesses on ultraviolet (UV) light-emitting diode (LED) chips is studied. First, magnetron-sputtered AlN layers with thicknesses of 8, 25, and 42 nm were prepared on a patterned sapphire substrate (PSS). Subsequently, a 2.8 μm-thick GaN layer was epitaxially grown. We found that when the magnetron-sputtered AlN was 25 nm thick, GaN had the best crystal quality and morphology. Therefore, we grew an 800 nm GaN layer on PSS to analyze the initial growth mechanism of the material. The scanning electron microscopy (SEM) images showed that when the thickness of the sputtered AlN layer was 25 nm, the GaN grains on the sidewall of the PSS cone were the least abundant. In addition, according to the selected area electron diffraction (SAED) test, the (011) crystal plane of GaN appeared on the sidewall of the PSS cone. The (011) crystal plane is the semipolar plane of GaN, which is not conducive to the growth of polar GaN materials. Finally, on this basis, we fabricated UV-LED chips. Among them, the UV-LED chips with the 25 nm magnetron-sputtered AlN layer exhibited the best electrical and optical performance. Overall, this work provides a pathway to improve the performance of UV-LED chips.

Evolution of Frenkel defect pairs in β-Ga2O3 thin film with the introduction of oxygen and its application in GaN-based ultraviolet light-emitting diode

In this work, β-Ga2O3 thin films were deposited by radio frequency (RF) magnetron sputtering technique. The effect of oxygen-argon flow ratio (OAFR) on the formation of Frenkel defect pairs was systematically analyzed. The Frenkel defect pairs would become abundant when the OAFR was 25:25 sccm, resulting in a severe damage in crystalline quality and an increase in the density of disorder. With the further increase of OAFR, Frenkel defect pairs would dramatically decrease or even vanish due to the significant reduction of oxygen vacancies (V0). Immediately after that, on the basis of avoiding the influence of Frenkel defect pairs inside the β-Ga2O3 films to the devices, the p-NiO/i-Ga2O3/n-GaN diode was fabricated from the as-prepared high-quality β-Ga2O3 film after post-deposition annealing as the electron-blocking layer. Subsequently, its electrical characteristics were investigated in detail which exhibited excellent rectification characteristics under all test temperatures. Correspondingly, the device achieved electroluminescence (EL) with a dominant sharp emission peak in the ultraviolet range (∼375 nm). In the end, the energy band diagram was used for in-depth analysis of the EL mechanism.

Enhanced ultraviolet electroluminescence performance from p-NiO/n-GaN heterojunctions by using i-Ga2O3 as electron blocking layer

In recent years, GaN materials have been applied to the field of optoelectronic devices due to their excellent photoelectric properties, especially in the field of light-emitting diodes, which has attracted considerable attention. In this article, we use structural engineering methods to further improve the performance of GaN-based ultraviolet light-emitting diodes. The magnetron sputtering and annealing treatment were used to prepare ultraviolet light-emitting diodes based on p-NiO/i-Ga2O3/n-GaN and p-NiO/n-GaN heterojunction structure. X-ray diffractometer, field emission scanning electron microscope and ultraviolet-visible spectrophotometer were used to study the crystal quality, surface morphology, and optical properties of Ga2O3 film. Compared with the p-NiO/n-GaN heterojunction diode, the p-NiO/i-Ga2O3/n-GaN heterojunction diode exhibits better rectification characteristics and more excellent electroluminescence characteristics. Under the injection voltage of 35 V, the EL spectrum intensity of p-NiO/i-Ga2O3/n-GaN heterojunction diode is nearly 37 times higher than that of p-NiO/n-GaN heterojunction diode. Finally, the working mechanism of the electroluminescence is analyzed in detail through the energy band diagram of the two diodes.

Development of highly efficient ultraviolet LEDs on hybrid patterned sapphire substrates.

A hybrid patterned sapphire substrate (HPSS) aiming to achieve high-quality Al(Ga)N epilayers for the development of GaN-based ultraviolet light-emitting diodes (UV LEDs) has been prepared. The high-resolution X-ray diffraction measurements reveal that the Al(Ga)N epilayers grown on a HPSS and conventional patterned sapphire substrate (CPSS) have similar structural quality. More importantly, benefiting from the larger refractive index contrast between the patterned silica array and sapphire, the photons can escape from the hybrid substrate with an improved transmittance in the UV band. As a result, in comparison with the UV LEDs grown on the CPSS, the LEDs grown on the HPSS exhibit a significantly enhanced light output power by 14.5% and more than 22.9% higher peak external quantum efficiency, owing to the boost of the light extraction efficiency from the adoption of the HPSS which can be used as a promising substrate to realize high-efficiency and high-power UV LEDs of the future.

Improving the Current Spreading by Fe Doping in n-GaN Layer for GaN-Based Ultraviolet Light-Emitting Diodes

GaN-based light-emitting diodes (LEDs) grown on the insulating sapphire substrate suffer from a severe current crowding effect. To alleviate the current crowding effect, we demonstrate a current spreading structure for 365-nm ultraviolet LED using Fe doping in n-GaN layer. The surface morphology was studied by atomic force microscope, and the crystal quality was evaluated by high-resolution X-ray diffraction, which shows that a thin insertion layer doped with Fe has less influence on the crystalline quality. Additionally, inserting a Fe-doped layer with appropriate doping concentration enables to induce a barrier in the n-type layer, which contributes to the uniform current distribution. As a result, compared with conventional LED, the light output power obtains a 170.6% enhancement at 100 mA. Furthermore, the uniform optical emission distribution is also achieved by inserting a Fe-doped layer. Finally, the conduction band structures and horizontal hole concentration distributions are calculated by Advanced Physical Models of Semiconductor Devices, which illustrates the mechanism of current spreading.

Micropyramid Vertical Ultraviolet GaN/AlGaN Multiple Quantum Wells LEDs on Si(111)

Micropyramid vertical GaN-based ultraviolet (UV) light-emitting diodes (LEDs) on Si(111) substrate have been fabricated by selective area growth to reduce threading dislocations and the polarization effects. There is no-light emission at the bottom and six planes of the pyramid at lower current due to the leakage current and nonradiative recombination of the dislocation at the bottom and the 90° threading dislocations (TDs) at six planes of the pyramid, and the top of the pyramid is the high-brightness region. The micropyramid UV LED has a high optical output intensity under a small current injection, and the series resistance of unit area is only a quarter of the conventional vertical LEDs, so the micropyramid UV LED would have a high output power under the drive circuit. The reverse leakage current of a single micropyramid UV LED is 2 nA at −10 V.

Increasing the Carrier Injection Efficiency of GaN-Based Ultraviolet Light-Emitting Diodes by Double Al Composition Gradient Last Quantum Barrier and p-Type Hole Supply Layer

A 365 nm AlxGa1-xN-based ultraviolet light-emitting diodes (LEDs) with double Al composition gradient last quantum barrier and hole supply layer structure has been studied. Experimental results show that the proposed structure enhances the carrier injection efficiency and suppresses the overflow of electrons. The introduction of three-dimensional hole gas further enhances the hole injection efficiency. As a result, the wall plug efficiency and electroluminescence intensity are significantly improved. In addition, the carrier concentration and the radiative recombination rate in the active region of the quantum well increases. Looking at the energy band diagram, one sees that the double Al composition gradient structure leads to higher electron blocking and alleviates the hole blocking effect. Overall, we conclude that the double Al composition gradient structure provides a pathway to achieve high-efficiency ultraviolet LEDs.

Improved Performance of GaN-Based Ultraviolet LEDs With Electron Blocking Layers Composed of Double-Peak p-Type Al x Ga1−x N/GaN Superlattice Layers

Past studies have demonstrated the positive impact of step-graded $p$ -type Al x Ga1− x N/GaN superlattice (SL) electron blocking layer (EBL) structures on the efficiency performance of ultraviolet (UV) GaN-based light-emitting diodes (LEDs). However, the optimal Al-grading structure of these SL EBLs remains unclear owing to a lack of systematic investigation. The present work addresses this issue by applying EBL composed of alternating half-peak, single-peak, and double-peak $p$ -type Al x Ga1− x N/GaN SL structures with varying values of $x$ ranging from 0.05 to 0.15 in 0.05 step increments. Simulation analysis is employed to obtain the internal quantum efficiency (IQE), energy band diagrams, polarization compensation factor, hole concentration, and electron concentration of GaN-based UV LEDs with three different SL-EBL structures. The results obtained at an injection current of 200 mA demonstrate that UV LEDs with double-peak SL-EBL structures provide the maximum IQE, which is ~38% greater than that of devices employing the conventional EBL in simulation experiment. This SL-EBL is demonstrated to improve the hole injection and electron overflow performance of GaN-based UV LEDs owing to the polarization charge and lattice mismatch at the p- Al x Ga1− x N/GaN interfaces. The reduced Al composition on the p- GaN side reduces the potential barrier of hole injection. Moreover, the predicted increase in the IQE of GaN-based UV LEDs with the optimal SL-EBL is verified experimentally.

Ultraviolet light-emitting diode arrays using Ga-doped ZnO as current spreading layer

In this article, the authors investigate the fabrication and characterization of the 4 × 4 GaN-based ultraviolet (UV) light-emitting diode (LED) array with a pixel size of 200 × 200 μm2 and a pitch of 200 μm. For comparison, the single broad-area LED with a size of 400 × 400 μm2 is also fabricated from the same epitaxial wafer as a reference. The Ga-doped ZnO (GZO) film is used as a current spreading layer onto the ultraviolet LEDs to enhance the light output power. The GZO film with a thickness of 100 nm was deposited by atomic layer deposition (ALD) and has an electron concentration of ∼8 × 1020 cm−3, a low resistivity of ∼4 × 10−4 Ω cm, and a high transmittance of ∼84% in the near UV wavelength range. The ALD-GZO film was also used as the internal wires to connect the pixels of an LED array. The LEDs have a peak wavelength of 370 nm with a slight redshift at higher injection currents. The 4 × 4 LED array has a better performance in the maximum light output power density of 7.25 W/cm2 at 240 mA and a highly linear light output power to injection current, while the broad-area LED only exhibits the maximum light output power density of 1.34 W/cm2 at 70 mA. The single pixel of a 4 × 4 UV LED array reveals a maximum light output power density of 7.25 W/cm2 at 240 mA and a wider divergence angle of 137°, which are better than the broad-area LED of 1.34 W/cm2 at 70 mA and 128°, respectively. Finally, the authors show the demonstration of photos of an 8 × 8 LED array.

Revealing the Role of Sidewall Orientation in Wet Chemical Etching of GaN-Based Ultraviolet Light-Emitting Diodes.

We demonstrated that the tetramethylammonium hydroxide (TMAH) solution possesses different etching abilities to the chip sidewalls with different orientations because the orientation of chip sidewall determines the exposed crystallographic plane of gallium nitride (GaN) and these crystallographic planes are with different chemical stability to the TMAH solution. After TMAH etching treatment, trigonal prisms were observed on sidewalls where m-plane GaN was exposed. For the investigated two types of light-emitting diodes (LEDs) with orthogonal arrangements, the LEDs with their larger sidewalls orientated along the [11–20] direction exhibited an additional 10% improvement in light output power after TMAH etching treatment compared to the LEDs with larger sidewalls orientated along the [1–100] direction.

Improvement in Light Output of Ultraviolet Light-Emitting Diodes with Patterned Double-Layer ITO by Laser Direct Writing.

A patterned double-layer indium-tin oxide (ITO), including the first unpatterned ITO layer serving as current spreading and the second patterned ITO layer serving as light extracting, was applied to obtain uniform current spreading and high light extraction efficiency (LEE) of GaN-based ultraviolet (UV) light-emitting diodes (LEDs). Periodic pinhole patterns were formed on the second ITO layer by laser direct writing to increase the LEE of UV LED. Effects of interval of pinhole patterns on optical and electrical properties of UV LED with patterned double-layer ITO were studied by numerical simulations and experimental investigations. Due to scattering out of waveguided light trapped inside the GaN film, LEE of UV LED with patterned double-layer ITO was improved as compared to UV LED with planar double-layer ITO. As interval of pinhole patterns decreased, the light output power (LOP) of UV LED with patterned double-layer ITO increased. In addition, UV LED with patterned double-layer ITO exhibited a slight degradation of current spreading as compared to the UV LED with a planar double-layer ITO. The forward voltage of UV LED with patterned double-layer ITO increased as the interval of pinhole patterns decreased.

Improved Light Extraction Efficiency of GaN-Based Ultraviolet Light-Emitting Diodes by Self-Assembled MgO Nanorod Arrays

This paper presents using ultrasonic spray pyrolysis deposition method to grow MgO nanorod arrays on the top surface and the sidewall of the GaN-based ultraviolet light-emitting diodes. It is found that when the thickness of the MgO film is over 60 nm, the self-assembled MgO nanorod arrays are obtained. Transmission electron microscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, refractive index, extinction coefficient, and photoluminescence spectrum are used to analyze the structural, chemical, and optical characteristics of the MgO nanorod arrays. The LED chip with the MgO nanorod arrays shows slightly degraded electrical characteristics for it requires a higher forward voltage to provide 0.35-A forward current and higher series resistance. However, the LED chip with the MgO nanorod arrays has great improvement in electroluminescence and efficiency. Compared to the LED chip with a standard SiO2 passivation layer, the LED chip with the MgO nanorod arrays shows improvement of 14.1% in light output power (LOP), 12% in external quantum efficiency and wall plug efficiency. The uniformity of the LED performance like forward voltage, LOP, and wall-plug efficiency is investigated in this paper.

Effects of GaN/AlGaN/Sputtered AlN nucleation layers on performance of GaN-based ultraviolet light-emitting diodes

We report on the demonstration of GaN-based ultraviolet light-emitting diodes (UV LEDs) emitting at 375 nm grown on patterned sapphire substrate (PSS) with in-situ low temperature GaN/AlGaN nucleation layers (NLs) and ex-situ sputtered AlN NL. The threading dislocation (TD) densities in GaN-based UV LEDs with GaN/AlGaN/sputtered AlN NLs were determined by high-resolution X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM), which revealed that the TD density in UV LED with AlGaN NL was the highest, whereas that in UV LED with sputtered AlN NL was the lowest. The light output power (LOP) of UV LED with AlGaN NL was 18.2% higher than that of UV LED with GaN NL owing to a decrease in the absorption of 375 nm UV light in the AlGaN NL with a larger bandgap. Using a sputtered AlN NL instead of the AlGaN NL, the LOP of UV LED was further enhanced by 11.3%, which is attributed to reduced TD density in InGaN/AlInGaN active region. In the sputtered AlN thickness range of 10–25 nm, the LOP of UV LED with 15-nm-thick sputtered AlN NL was the highest, revealing that optimum thickness of the sputtered AlN NL is around 15 nm.

Design of a tandem distributed Bragg reflectors specialized for enhancing the efficiency of GaN-based ultraviolet light-emitting diodes

We design a type of “tandem distributed Bragg reflector (DBR)” that shows advantages over normal DBR in ultraviolet region. We apply a tandem DBR centered at 390nm to a resonant cavity light emitting diodes model, consequently enhancing the extraction efficiency without detuning the cavity while narrowing the spectral width of the emitted light. The extraction efficiency into a numerical aperture exhibits a 36% increase compared to that of the normal structure. We further demonstrate the potential that the tandem DBRs (centered at 343nm and 246nm respectively) have for enhancing the efficiency in GaN-based deep UV device.

Highly polarized emission from a GaN-based ultraviolet light-emitting diode using a Si-subwavelength grating on a SiO2 underlayer

The polarization characteristics of a 370nm GaN-based ultraviolet light-emitting diode (UV-LED) were controlled by a subwavelength grating (SWG) on a low-refractive-index SiO2 underlayer inserted between the SWG and LED surface. Highly polarized UV emission was demonstrated by utilizing the Bloch eigenmode resonance in the SWG structure for the two orthogonal polarization states. The polarization ratio of the emission reached 16:1, which is the highest reported to date for direct emission from a GaN-based UV-LED. The decrease in UV emission was also prevented by suppressing the diffracted plane wave and by increasing the amplitude of the wave incident onto the SWG structure; this increase was achieved by taking advantage of the low refractive index of SiO2.

GaN-based ultraviolet light-emitting diodes with AlN/GaN/InGaN multiple quantum wells.

We demonstrate indium gallium nitride/gallium nitride/aluminum nitride (AlN/GaN/InGaN) multi-quantum-well (MQW) ultraviolet (UV) light-emitting diodes (LEDs) to improve light output power. Similar to conventional UV LEDs with AlGaN/InGaN MQWs, UV LEDs with AlN/GaN/InGaN MQWs have forward voltages (V(f)'s) ranging from 3.21 V to 3.29 V at 350 mA. Each emission peak wavelength of AlN/GaN/InGaN MQW UV LEDs presents 350 mA output power greater than that of the corresponding emission peak wavelength of AlGaN/InGaN MQW UV LEDs. The light output power at 350mA of AlN/GaN/InGaN MQWs UV LEDs with 375 nm emission wavelength can reach around 26.7% light output power enhancement in magnitude compared to the AlGaN/InGaN MQWs UV LEDs with same emission wavelength. But 350mA light output power of AlN/GaN/InGaN MQWs UV LEDs with emission wavelength of 395nm could only have light output power enhancement of 2.43% in magnitude compared with the same emission wavelength AlGaN/InGaN MQWs UV LEDs. Moreover, AlN/GaN/InGaN MQWs present better InGaN thickness uniformity, well/barrier interface quality and less large size pits than AlGaN/InGaN MQWs, causing AlN/GaN/InGaN MQW UV LEDs to have less reverse leakage currents at -20 V. Furthermore, AlN/GaN/InGaN MQW UV LEDs have the 2-kV human body mode (HBM) electrostatic discharge (ESD) pass yield of 85%, which is 15% more than the 2-kV HBM ESD pass yield of AlGaN/InGaN MQW UV LEDs of 70%.

Improved output power of GaN-based ultraviolet light-emitting diodes with sputtered AlN nucleation layer

In this work, the ultraviolet light-emitting diodes (UV-LEDs) at 380nm were grown on patterned sapphire substrate (PSS) by atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD). A sputtered AlN nucleation layer was utilized on the PSS to enhance the quality of the epitaxial layer. By using high-resolution X-ray diffraction, the full-width at half-maximum of the rocking curve shows that the UV-LEDs with sputtered AlN nucleation layer had better crystalline quality when compared to conventional GaN nucleation samples. From the scanning electron microscope (SEM) and transmission electron microscopy (TEM) images, it can be observed that the tip and sidewall portion of the pattern was smooth using the sputtered AlN nucleation layer. The threading dislocation densities (TDDs) are reduced from 6×107cm−2 to 2.5×107cm−2 at the interface between the u-GaN layers for conventional and AlN PSS devices, respectively. As a result, a much higher light output power was achieved. The light output power at an injection current of 20mA was enhanced by 30%. Further photoluminescence (PL) measurement and numerical simulation confirm that this increase of output power can be attributed to the improvement of material quality and light extraction.

Efficiency improvement of GaN-based ultraviolet light-emitting diodes with reactive plasma deposited AlN nucleation layer on patterned sapphire substrate.

The flip chip ultraviolet light-emitting diodes (FC UV-LEDs) with a wavelength of 365 nm are developed with the ex situ reactive plasma deposited (RPD) AlN nucleation layer on patterned sapphire substrate (PSS) by an atmospheric pressure metal-organic chemical vapor deposition (AP MOCVD). The ex situ RPD AlN nucleation layer can significantly reduce dislocation density and thus improve the crystal quality of the GaN epitaxial layers. Utilizing high-resolution X-ray diffraction, the full width at half maximum of the rocking curve shows that the crystalline quality of the epitaxial layer with the (RPD) AlN nucleation layer is better than that with the low-temperature GaN (LT-GaN) nucleation layer. The threading dislocation density (TDD) is estimated by transmission electron microscopy (TEM), which shows the reduction from 6.8 × 107 cm−2 to 2.6 × 107 cm−2. Furthermore, the light output power (LOP) of the LEDs with the RPD AlN nucleation layer has been improved up to 30 % at a forward current of 350 mA compared to that of the LEDs grown on PSS with conventional LT-GaN nucleation layer.

Performance enhancement of GaN-based flip-chip ultraviolet light-emitting diodes with a RPD AlN nucleation layer on patterned sapphire substrate

In this work, flip-chip ultraviolet light-emitting diodes (FCUV-LEDs) on patterned sapphire substrate (PSS) at 375 nm were grown by an atmospheric pressure metal organic chemical vapor deposition (AP-MOCVD). A specialized reactive plasma deposited (RPD) AlN nucleation layer was utilized on the PSS to enhance the quality of the epitaxial layer. By using high-resolution X-ray diffraction, the full-width at half-maximum of the rocking curve shows that the FCUV-LEDs with RPD AlN nucleation layer had better crystalline quality when compared to conventional GaN nucleation samples. From the transmission electron microscopy (TEM) image, it can be observed that the tip and incline portion of the pattern was smooth using the RPD AlN nucleation layer. The threading dislocation densities (TDDs) are reduced from 7 × 107 cm−2 to 2.5 × 107 cm−2 at the interface between the u-GaN layers for conventional and AlN PSS devices, respectively. As a result, a much higher light output power was achieved. The improvement of light output power at an injection current of 20 mA was enhanced by 30%. Further photoluminescence measurement and numerical simulation confirm such increase of output power can be attributed to the improvement of material quality and light extraction.