p-GaN Thickness Research Articles

1.7-kV vertical GaN p-n diode with triple-zone graded junction termination extension formed by ion-implantation

Edge termination has emerged as an important area in the design and realization of vertical GaN power electronic devices. While the material properties of GaN are promising for high-performance devices, in practice the breakdown voltage can be compromised by inadequate edge termination (ET). While solutions in other materials (e.g. Si, SiC) are well-known, these are challenging to implement in GaN due to inherent difficulties in p-type doping GaN. In this work, we report a etch-free triple-zone graded junction termination extension (JTE) for vertical GaN diodes formed by Nitrogen ion implantation. The triple-zone design offers lower peak fields for effective field control. In addition, the proposed triple-zone JTE is beneficial for increasing the fabrication process window and allowing for more variability in epitaxial wafer growth in terms of p-GaN doping and thickness while maintaining high breakdown voltage. The fabricated GaN p-n diodes with triple-zone JTE obtain a maximum breakdown voltage of 1.73 kV with specific on-resistance Ron of 0.4 mΩcm2. Temperature-dependent reverse characteristics show that the devices have a positive temperature coefficient of the breakdown voltage indicating an avalanche breakdown mechanism. These results suggest that vertical GaN p–n diodes with N-implanted triple-zone JTE are promising for power applications.

Sidewall geometric effect on the performance of AlGaN-based deep-ultraviolet light-emitting diodes.

In this study, deep-ultraviolet light-emitting diodes (DUV LEDs) with different chip sidewall geometries (CSGs) are investigated. The structure had two types of chip sidewall designs that combined DUV LEDs with the same p-GaN thickness. By comparing the differences of the characteristics such as the external quantum efficiency droops, light output power, light extraction efficiency (LEE), and junction temperature of these DUV LEDs, the self-heated effect and light-tracing simulation results have been clearly demonstrated to explain the inclined sidewalls that provide more possibility pathway for photons escape to increase the LEE of LEDs; thus, the DUV LEDs with the CSG presented improved performance. These results demonstrate the potential of CSG for DUV LED applications.

Design Space of GaN Vertical Trench Junction Barrier Schottky Diodes: Comprehensive Study and Analytical Modeling

We report gallium nitride (GaN) vertical trench junction barrier Schottky (TJBS) diodes and systematically analyzed the effects of the key design parameters on the reverse and forward characteristics of the devices. By taking advantage of the shielding effects from both the trenches and pn junctions in the TJBS structure, the high electric field at the Schottky contact region can be effectively suppressed. We found that the doping concentration, thickness, and spacing of p-GaN, as well as the depth and angle of the trench sidewalls are closely associated with the electric field distribution and the reverse characteristics of the TJBS diodes. With an optimal set of design parameters, the local electric field crowding at either the corner of the trench or the edge of the p-GaN can also be alleviated, resulting in a boosted breakdown voltage of up to 1250 V in the TJBS diodes. In addition, an analytical model was developed to explore the physical mechanism behind the forward conduction behaviors. We believe that the results can provide a systematical design strategy for the development of low-loss, high-voltage, and high-power GaN power diodes towards an efficient power system.

Charge Control in Schottky-Type p-GaN Gate HEMTs With Partially and Fully Depleted p-GaN Conditions

The Schotty-type p-GaN gate high-electron-mobility transistors (HEMT) feature a unique gate structure. A comprehensive understanding of the charge control mechanism in the p-GaN gate region is a fundamental step for the optimization of this technology. In this work, a physics-based analytical model is presented which takes into consideration all the capacitive effects from gate metal deep into the GaN buffer. According to our analysis, the p-GaN layer can be either partially depleted by the metal/p-GaN Schottky junction or fully depleted, depending on the doping concentration and thickness of the p-GaN layer. Our model accurately captures the charge control properties under both conditions and is validated against TCAD numerical simulations. For a certain p-GaN thickness, a lightly doped p-GaN leads to a full-depletion condition, such that the acceptor concentration directly affects the band diagram at AlGaN/GaN interface. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {th}}$ </tex-math></inline-formula> of the HEMT increases quickly with acceptor concentration in p-GaN. With sufficiently high acceptor concentration in p-GaN, the device reaches the partial-depletion condition, the acceptor concentration loses its influence over the band diagram at the location of the AlGaN/GaN interface, since the Fermi-level at the AlGaN surface is pinned near the valence band of p-GaN. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {th}}$ </tex-math></inline-formula> starts to decrease with acceptor concentration, but at a relatively slow rate. The maximum <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {th}}$ </tex-math></inline-formula> is obtained near the boundary between partial-and full-depletion conditions. In consideration of the process margin, the device designed with a partially depleted p-GaN is preferable, since it ameliorated the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${V}_{\text {th}}$ </tex-math></inline-formula> sensibility against acceptor concentration.

GaN power IC normally-on and normally-off transistors technology and simulation

GaN technology has been waiting to be widely adopted because of its specific technical requirements. Integration of transistor and driver in a single die will enable to overcome problems with gate driving, high cost of circuit and low device reliability. This paper demonstrates technology of GaN-on-Si normally-on and normally-off transistor with different p-GaN cap-layer thickness as well as simulation of these devices. The simulation data confirm experimental results. P-GaN cap-layer thickness affects the current channel density: the more p-GaN thickness, the less channel density. The fabricated transistors have a maximum drain current in open state of about 800 mA/mm.

Factors Affecting Surface Plasmon Coupling of Quantum Wells in Nitride-Based LEDs: A Review of the Recent Advances.

Surface plasmon (SP)-enhanced quantum-well (QW) LEDs have proved their potential in replacing conventional lighting devices for their high-performance capabilities in ultraviolet (UV), blue and green spectral ranges. The SP-enhanced QW-LEDs have applications in light emission enhancement, light polarization, color conversion, and speed modulation. The electric field of the plasmonic mode of a metal couples with the exciton energy of QWs in resonance results in efficiency enhancement to several folds. The strength of the SP–QW coupling is mainly influenced by the type of metal used for SP enhancement, the metal nanostructure geometry, and the penetration depth of the SP fringing field in the p-GaN. The use of an appropriate dielectric interlayer between the metal and the p-GaN allows further control over SP resonance with QW emission wavelength. The penetration depth defines the p-GaN thickness and the QW period number for effective SP–QW coupling. The optimization of these parameters is key to achieve high efficiencies in SP-enhanced QW-LEDs for various applications. This review explains the SP enhancement mechanism and the key challenges facing the SP enhancement of QW-LEDs. The main factors that affect the SP–QW coupling have been explained in detail based on recent reports devoted to this field.

Numerical Investigation of Light Extraction Efficiency of GaN-based Vertical Blue Micron-Scale Light-Emitting Diode Structures.

The light extraction efficiency (LEE) of GaN-based vertical blue micron-scale light-emitting diode (μ-LED) structures was investigated numerically using three-dimensional finite-difference timedomain (FDTD) methods. The entire μ-LED chip was included in the FDTD computational domain to determine the LEE accurately. As the lateral dimensions of μ-LEDs increased from 5 to 30 μm, the LEE decreased gradually because of the increased portion of light trapped inside the LED chip and the increased light absorption in the GaN layers with increasing chip size. The LEE varied strongly with the p-GaN thickness for the μ-LED with a flattop surface, which could be explained by the strong dependence of the spatial distribution of the emission patterns on the p-GaN thickness. This dependence on the p-GaN thickness decreased when the surface of the μ-LED chip was patterned. A high LEE of >80% could be achieved in LEDs with properly chosen parameters. The FDTD simulation results presented in this study are expected to be employed advantageously in designing μ-LED structures with a high LEE.

Light Extraction Efficiency of GaN-Based Micro-Scale Light-Emitting Diodes Investigated Using Finite-Difference Time-Domain Simulation

We conducted a systematic investigation of the light extraction efficiency (LEE) of GaN-based vertical micro-scale light-emitting diode (μ-LED) structures using three-dimensional finite-difference time-domain (FDTD) simulations. The LEE of μ-LED structures was found to have a strong dependence on structural parameters such as the shape of the chip cross section, chip dimension, and p-GaN thickness. The LEE of a μ-LED with a circular cross section was lower than that of a μ-LED with a square cross section by 5∼10% owing to the coupling of light with high-quality-factor whispering gallery modes. The LEE of a μ-LED structure decreased as the chip dimension increased, which could be attributed to the increased portion of trapped light inside the LED chip and increased light absorption in the GaN with an increasing chip dimension. In addition, the LEE varied significantly with the thickness of the p-GaN layer, which could be explained by the strong dependence of the angular distribution of the emission pattern on the p-GaN thickness. The FDTD simulation method presented in this study is expected to be advantageously employed in designing μ-LED structures with high LEE.

Color conversion efficiency enhancement of colloidal quantum dot through its linkage with synthesized metal nanoparticle on a blue light-emitting diode.

Four surface-modified and, hence, positively charged metal nanoparticles (NPs) of different localized surface plasmon (LSP) resonance wavelengths are synthesized for linking with negatively charged, red-emitting colloidal CdZnSeS/ZnS quantum dots (QDs) on the top surface of a blue-emitting InGaN/GaN quantum well (QW) light-emitting diode (LED) through electro-static force. The metal NP-QD linkage leads to a short distance between them for producing their strong surface plasmon (SP) coupling, such that QD absorption and emission can be enhanced. Meanwhile, the small p-GaN thickness in the LED results in strong SP coupling between the LSP resonance of metal NP and the QWs of the LED, leading to enhanced QW emission and, hence, stronger QD excitation. All those factors together result in the increase of the color conversion efficiency of the QD.

A Novel Kilovolts GaN Vertical Superjunction MOSFET With Trench Gate: Approach for Device Design and Optimization

In this paper, a gallium nitride (GaN) vertical superjunction (SJ) MOSFET with trench gate is proposed and studied by the TCAD simulation. In order to achieve the typical electric field ( $E$ -field) modulation effect by the P+/N/N+ structure in the conventional Si SJ-MOS, the P-GaN/unintentional doped (UID)-GaN/N+-GaN stack is alternatively used by taking into account the unavailability of P+-GaN. In this manner, the P-GaN and N+-substrate serve as the field-stop (FS) layers in the proposed GaN SJ-MOS. In the forward bias, the enhancement-mode function of the device is enabled by the gated side-wall and aperture composite channel that leads to a 1.47-V threshold voltage of the device. In the reverse bias, the breakdown of the device is governed by two mechanisms: 1) the punchthrough (PT) in the top P-GaN/N-drift junction due to the possible under design of the P-GaN thickness; 2) the avalanche breakdown triggered by high $E$ -field due to the possible under design of the bottom UID-GaN thickness. Hence, from the uniform $E$ -field distribution point of view, a device optimization approach for GaN vertical SJ-MOS is proposed. The hole and electron densities of P-GaN and N-drift are optimized to achieve a uniform $E$ -field distribution in the device both in the lateral and vertical directions, which is found to be $6 \times 10^{16}$ cm $^{-3}$ . Moreover, the thickness ratio of P-GaN/UID-GaN is designed to obtain an identical intrinsic breakdown for the top P-GaN/N-drift junction and bottom UID/N-drift region, which enables the maximum Baliga’s figure-of-merit (BFOM) of the device. The concept of device design and optimization is of great interests for GaN vertical device for over kilovolts applications.

Evaluation of Light Extraction Efficiency of GaN-Based Nanorod Light-Emitting Diodes by Averaging over Source Positions and Polarizations

Light extraction efficiency (LEE) of GaN-based nanorod blue light-emitting diode (LED) structures is investigated using finite-difference time-domain (FDTD) simulations. When the LEE is calculated for different source positions inside the nanorod, the LEE is found to depend strongly on the source positions and the polarization directions for each source position, implying that the LEE of nanorod LED structures should be evaluated by averaging over source positions and polarization directions for determining the LEE accurately. The averaged LEE of nanorod LED structures is simulated as the radius, the p-GaN thickness, and the n-GaN thickness is varied, and the optimum structural parameters can be obtained. In addition, the far-field pattern is simulated when considering the averaging effects, and the circularly symmetric and uniform emission distribution is obtained.

Dependencies of surface plasmon coupling effects on the p-GaN thickness of a thin-p-type light-emitting diode.

The high performance of a light-emitting diode (LED) with the total p-type thickness as small as 38 nm is demonstrated. By increasing the Mg doping concentration in the p-AlGaN electron blocking layer through an Mg pre-flow process, the hole injection efficiency can be significantly enhanced. Based on this technique, the high LED performance can be maintained when the p-type layer thickness is significantly reduced. Then, the surface plasmon coupling effects, including the enhancement of internal quantum efficiency, increase in output intensity, reduction of efficiency droop, and increase of modulation bandwidth, among the thin p-type LED samples of different p-type thicknesses that are compared. These advantageous effects are stronger as the p-type layer becomes thinner. However, the dependencies of these effects on p-type layer thickness are different. With a circular mesa size of 10 μm in radius, through surface plasmon coupling, we achieve the record-high modulation bandwidth of 625.6 MHz among c-plane GaN-based LEDs.

Effects of p-type GaN thickness on optical properties of GaN-based light-emitting diodes

The influence of p-type GaN (pGaN) thickness on the light output power (LOP) and internal quantum efficiency (IQE) of light emitting diode (LED) was studied by experiments and simulations. The LOP of GaN-based LED increases as the thickness of pGaN layer decreases from 300 nm to 100 nm, and then decreases as the thickness decreases to 50 nm. The LOP of LED with 100-nm-thick pGaN increases by 30.9% compared with that of the conventional LED with 300-nm-thick pGaN. The variation trend of IQE is similar to that of LOP as the decrease of GaN thickness. The simulation results demonstrate that the higher light efficiency of LED with 100-nm-thick pGaN is ascribed to the improvements of the carrier concentrations and recombination rates.

Enhancing the light extraction efficiency of AlGaN deep ultraviolet light emitting diodes by using nanowire structures

The performance of conventional AlGaN deep ultraviolet light emitting diodes has been limited by the extremely low light extraction efficiency (&amp;lt;10%), due to the unique transverse magnetic (TM) polarized light emission. Here, we show that, by exploiting the lateral side emission, the extraction efficiency of TM polarized light can be significantly enhanced in AlGaN nanowire structures. Using the three-dimensional finite-difference time domain simulation, we demonstrate that the nanowire structures can be designed to inhibit the emission of guided modes and redirect trapped light into radiated modes. A light extraction efficiency of more than 70% can, in principle, be achieved by carefully optimizing the nanowire size, nanowire spacing, and p-GaN thickness.

Investigation of the Purcell effect in GaN-based vertical LED structures using FDTD simulation

The spontaneous emission (SE) rate in GaN-based vertical light-emitting diode (LED) structures can be modified significantly depending on the local density of states near the high-reflectance mirror. In this study, this Purcell effect in vertical LED structures is investigated numerically using finite-difference time-domain simulation. The Purcell factor of vertical LED structures is found to vary periodically with the p-GaN thickness, while it is almost independent of the n-GaN thickness, indicating strong influence of the p-type electrode reflector on the SE modification. The peak of the Purcell factor is obtained to be as high as 1.5 at a properly chosen p-GaN thickness, which leads to substantial increase in the internal quantum efficiency (IQE) of LEDs. Since the influence of the Purcell factor on IQE becomes more conspicuous as IQE decreases, the Purcell effect of vertical LED structures is expected to mitigate the green-gap and the efficiency droop problems of current LED technologies.

Improving the Vertical Light-Extraction Efficiency of GaN-Based Thin-Film Flip-Chip LEDs With p-Side Deep-Hole Photonic Crystals

This study systematically investigates the light-extraction efficiency (LEE) of GaN-based thin-film flip-chip light-emitting diodes (TFFC LEDs) with deeply etched photonic crystals (PhC). The optimal structure can be obtained by scanning the structural parameters using a three-dimensional finite-difference time-domain simulation. The concentration of the internal emission pattern within a limited extraction cone is significantly influenced by several structural parameters. The light-collection enhancement within an extraction cone explains the peak LEE caused by optimized structural parameters such as the p-GaN thickness, the air-hole depth, the lattice constant and the filling factor. The maximum extraction efficiency can reach 65% in the case of a p-GaN thickness of 140 nm, air-hole depth of 240 nm, lattice constant of 350 nm and filling factor of 0.40. Additionally, fabricating the optimal electrical contacts on the planar top surface of the n side can contribute to a higher potential enhancement.

Large enhancement of light extraction efficiency in AlGaN-based nanorod ultraviolet light-emitting diode structures

Light extraction efficiency (LEE) of AlGaN-based nanorod deep ultraviolet (UV) light-emitting diodes (LEDs) is numerically investigated using three-dimensional finite-difference time-domain simulations. LEE of deep UV LEDs is limited by strong light absorption in the p-GaN contact layer and total internal reflection. The nanorod structure is found to be quite effective in increasing LEE of deep UV LEDs especially for the transverse magnetic (TM) mode. In the nanorod LED, strong dependence of LEE on structural parameters such as the diameter of a nanorod and the p-GaN thickness is observed, which can be attributed to the formation of resonant modes inside the nanorod structure. Simulation results show that, when the structural parameters of the nanorod LED are optimized, LEE can be higher than 50% and 60% for the transverse electric (TE) and TM modes, respectively. The nanorod structure is expected to be a good candidate for the application to future high-efficiency deep UV LEDs.PACS41.20.Jb; 42.72.Bj; 85.60.Jb

Enhanced Electro-Static Discharge Endurance of GaN-Based Light-Emitting Diodes with Specially Designed Electron Blocking Layer

GaN-based light-emitting diodes (LEDs) with specially designed electron blocking layers (EBLs) between the multiple quantum wells (MQWs) and the top p-GaN layer have been developed. The EBLs consist of Mg-doped p-AlGaN/GaN superlattice (SL) with the layer thickness of p-AlGaN varied from 1 to 10 nm and the layer thickness of p-GaN fixed at 1 nm in this study. It was found that under a 2000 V reverse bias voltage condition, the electro-static discharge (ESD) yield increased from 61.98 to 99.51% as the thickness of p-AlGaN in the EBLs was increased from 1 to 10 nm. Since the ESD yield was 97.80%, and maximum value for LEDs' light output power (LOP) and minimum value for the forward voltage (Vf) were achieved when the thickness of p-AlGaN in the EBLs was 9 nm with a 20 mA injection current, it was concluded that the p-AlGaN/GaN SL EBLs with the combination of 9-nm-thick p-AlGaN and 1-nm-thick p-GaN would be beneficial to the fabrication of the GaN-based LEDs with high brightness, high ESD endurance, and low Vf.

Influence of Interference on Extraction Efficiency of Ultraviolet Vertical Light-Emitting Diodes

We report on enhanced efficiency of ultraviolet vertical light-emitting diodes (VLEDs) with interference between the reflective mirror and the multiple quantum well. The dimensions of the cavity are fixed at 30 nm for the p-AlGaN layer, while various thicknesses of p-GaN from 60 nm to 140 nm were used. The light output power of the VLED in constructive compared with destructive interference condition increased by 23.9% at 350 mA. These improvements could be attributed to the predominant constructive interference of vertical radiation due to an optical cavity with optimal p-GaN thickness.

A new method to measure the carrier concentration of p-GaN

A new method to measure the carrier concentration of p-GaN is proposed. The main idea is as follows: the difference between p-n+ structure GaN ultraviolet photodetector’s quantum efficiency at two different wavelengths varies remarkably with increasing reversed bias, and the most characteristic change occurs at a reversed voltage under which the p-GaN layer starts to be completely depleted, consequently the carrier concentration of p-GaN can be derived basing on this effect. The simulation results prove the validity of the idea even under the condition of high surface recombination velocity and bad ohmic contact. The thickness choice of p-GaN samples during the carrier concentration test experiment using this method is investigated. It is shown that the optimized thickness of p-GaN decreases with the increase of carrier concentration of p-GaN samples.