p-AlGaN Layer Research Articles

MOVPE-grown AlGaN-based tunnel heterojunctions enabling fully transparent UVC LEDs

We report on AlGaN-based tunnel heterojunctions grown by metalorganic vapor phase epitaxy enabling fully transparent UVC LEDs by eliminating the absorbing p-AlGaN and p-GaN layers. Furthermore, the electrical characteristics can be improved by exploiting the higher conductivity of n-AlGaN layers as well as a lower resistance of n-contacts. UVC LEDs with AlGaN:Mg/AlGaN:Si tunnel junctions exhibiting single peak emission at 268 nm have been realized, demonstrating effective carrier injection into the AlGaN multiple quantum well active region. The incorporation of a low band gap interlayer enables effective tunneling and strong voltage reduction. Therefore, the interlayer thickness is systematically varied. Tunnel heterojunction LEDs with an 8 nm thick GaN interlayer exhibit continuous-wave emission powers >3 mW near thermal rollover. External quantum efficiencies of 1.4% at a DC current of 5 mA and operating voltages of 20 V are measured on-wafer. Laterally homogeneous emission is demonstrated by UV-sensitive electroluminescence microscopy images. The complete UVC LED heterostructure is grown in a single epitaxy process including in situ activation of the magnesium acceptors.

Improve the electrical and optical performance of deep ultraviolet light-emitting diodes with a w-shaped p-AlGaN layer

We theoretically investigate the effects of three independent p-type contact layers on the photoelectric properties of deep ultraviolet light-emitting diodes (DUV-LEDs). From the simulation results, the DUV-LED using a w-shaped p-AlGaN layer has a better photoelectric performance. The internal quantum efficiency and output power in the new structure are significantly increased compared to the two other structures. Besides, the novel structure reveals lower contact resistance and higher electroluminescence intensity, and the efficiency droop is very small at 200 mA injection current. These improvements are attributed to not only decreased electron leakage but also increased hole injection efficiency, thereby enhancing radiative recombination rates. The specially designed p-type layer provides an attractive solution for improving the photoelectric performance of DUV-LEDs with the Al-rich p-AlGaN contact layers.

Reflectance of a reflective photonic crystal p-contact layer for improving the light-extraction efficiency of AlGaN-based deep-ultraviolet light-emitting diodes

We utilized the finite-difference time-domain method (FDTD) to investigate the reflectance of an air void photonic crystal (PhC) on the p-electrode of an AlGaN-based deep ultraviolet (DUV) light-emitting diode (LED). Firstly, a transparent p-AlGaN layer with cylindrical air voids on a Ni(1nm)/Al(140nm) metal electrode was optimized to maximize the reflectance at normal incidence. It was shown that by having the optimum AlGaN PhC on the metal electrode, the reflectance as a function of the angle of incidence was advantageously redistributed to increase LEE. The calculated angle-dependent reflectances were converted to average reflectance considering the power distribution of dipole sources in the TE and TM modes. The average reflectances of a reference structure and the PhC structure for the TM mode at wavelengths around 283 nm were 77.1 and 85.2%, respectively. Thus, an incremental increase of 8% in average reflectance for the TM mode was obtained by adopting the optimized PhC. Secondly, we investigated a PhC with air voids in two different layers, a p-GaN layer and a p-AlGaN layer on the Ni/Al. The calculated average reflectances at 283 nm were 38% and 42% for the TE and TM modes, respectively, when the thickness of the p-GaN layer was 70 nm. The average reflectances for the TE and TM modes with a uniform 70nm thick p-GaN layer without a PhC were 4.2 and 3.6%, respectively. This clearly shows that the optimized PhC can reduce light absorption in both the p-GaN layer and the metal electrode.

On the polarization effect of the p-EBL/p-AlGaN/p-GaN structure for AlGaN-based deep-ultraviolet light-emitting diodes

The external quantum efficiency (EQE) and light output power (LOP) for III-nitride based deep ultraviolet light-emitting diodes (DUV LEDs) are strongly influenced by the hole injection efficiency. This work shows that the hole concentration in the active region is strongly subject to the polarization level of the p-region that consists of the p-type electron blocking layer (p-EBL), the p-AlGaN and the p-GaN layers. The hole injection becomes poor once the p-EBL/p-AlGaN and p-AlGaN/p-GaN interfaces are of the [000–1] polarity. On the contrary, the improved hole injection can be obtained once the polarization level that is of the [0001] polarity for the p-EBL/p-AlGaN and p-AlGaN/p-GaN interfaces increases. The increased polarization level of the [0001] polarity can not only reduce the hole depletion within the p-AlGaN layer, increase the energy for the holes, but also reduce the valence band barrier height of the p-EBL for holes, which in turn facilitates the hole injection capability and enhances the EQE for DUV LEDs.

Performance improvements in AlGaN-based ultraviolet light-emitting diodes due to electrical doping effects

We report a new doping method for the fabrication of wide-bandgap (WB) semiconductors such as p-AlGaN using electric fields and the application of this method to AlGaN-based UV light-emitting diodes (LEDs) to evaluate its effect at the device level. We prepared four LED samples with different work function (WF) energies using Pt, Ni, Ti, or Mg as contact metals and applied electric fields between these metals and the p-AlGaN surface across indium-doped tin oxide (ITO)/AlN thin films to facilitate diffusion of the metal atoms into the p-AlGaN layer. Compared to the samples with reference ITO electrodes (10 or 100 nm), ohmic behavior on the p-AlGaN surface was improved in the samples doped with Pt, Ni (high WF), and Mg (low WF but shallow dopant), but not for the sample doped with Ti (low WF). Furthermore, Mg-doped samples exhibited the lowest contact resistance with reasonably high transmittance among the four samples; accordingly, the lowest forward voltage and highest light-output power were achieved with UV LEDs using ITO/AlN/Mg electrodes. This electrical doping method could be useful for WB semiconductors fabricated with materials such as p-AlGaN and p-ZnO, which are difficult to dope using either thermal or optical doping method.

Polarization-induced hole doping in N-polar III-nitride LED grown by metalorganic chemical vapor deposition

Polarization-induced doping has been shown to be effective for wide-bandgap III-nitrides. In this work, we demonstrated a significantly enhanced hole concentration via linearly grading an N-polar AlxGa1-xN (x = 0–0.3) layer grown by metal-organic chemical vapor deposition. The hole concentration increased by ∼17 times compared to that of N-polar p-GaN at 300 K. The fitting results of temperature-dependent hole concentration indicated that the holes in the graded p-AlGaN layer comprised both polarization-induced and thermally activated ones. By optimizing the growth conditions, the hole concentration was further increased to 9.0 × 1017 cm−3 in the graded AlGaN layer. The N-polar blue-violet light-emitting device with the graded p-AlGaN shows stronger electroluminescence than the one with the conventional p-GaN. The study indicates the potential of the polarization doping technique in high-performance N-polar light-emitting devices.

Improved performance of AlGaN-based deep ultraviolet light-emitting diode using modulated-taper design for p-AlGaN layer

AlGaN-based deep ultraviolet light-emitting diodes adopting modulated-taper design for p-AlGaN layer have been investigated. It is found that remarkable enhancements both in light output power (LOP) and internal quantum efficiency (IQE) can be realized using the modulated-taper design compared to the conventional single-Al-component AlGaN ones. It is verified that the upward energy band of the last quantum barrier, especially symmetric ones, can increase effective potential barrier height for electrons and at the same time, can decrease the barrier height for holes, which is beneficial for high hole concentration distribution within the quantum wells, therefore accounting for the improved IQE as well as LOP.

Influence of hydrogen impurity on the resistivity of low temperature grown p-AlxGa1-xN layer (0.08 ≤ x ≤ 0.104)

Low temperature grown p-AlGaN layer with a small resistivity is crucial to improve the performance of the GaN-based laser diodes (LDs). In this study, growth temperature of the p-AlxGa1-xN (0.08 ≤ x ≤ 0.104) layers are controlled to be relatively low, and the influence of hydrogen impurity on the resistivity is investigated in detail. According to the dependence of hole concentration and resistivity on hydrogen impurity concentration, it is found that when Mg doping concentration is unchanged, reducing hydrogen impurity concentration in p-AlxGa1-xN (0.08 ≤ x ≤ 0.104) layers could reduce the resistivity effectively, which is attributed to weaken the compensation or passivation effect of hydrogen impurities on Mg acceptors. A p-Al0.09Ga0.91N layer with a low resistivity of 4.6 Ω cm has been achieved.

III-Nitride Phototransistors Fabricated on a Light-Emitting-Diode Epitaxial Wafer

Instead of using a wafer with a bipolar junction transistor n-p-n epitaxial structure, we successfully fabricated AlGaInN-based near-ultraviolet (UV) heterojunction phototransistors (HPTs) on a commercial wafer with a light-emitting-diode (LED) epitaxial structure by employing silicon diffusion to convert a part of the p-AlGaN layer into an n-AlGaN layer. High responsivity of more than 77 A/W in the near-UV spectral range, including a peak responsivity of 120 A/W at 382 nm, was obtained at a bias voltage of only 3 V. The corresponding photocurrent gain was approximately 790. The response speed and photocurrent versus incident light intensity were also characterized. The results indicate that the LEDs can be monolithically integrated with high-responsivity HPTs.

(Invited) AlGaN-Based PN Diodes for Power Electronics

As the performance of GaN- and SiC-based power devices becomes limited by fundamental material properties, semiconductors with larger bandgaps are appealing due to their superior material properties. Most notable is that the critical electrical field scales as the bandgap to the 2.0-2.5 power. Thus, diodes employing wide bandgap semiconductors are expected to operate at higher breakdown voltages with thinner and more heavily-doped drift regions, leading to lower resistive losses. AlGaN alloys with their much larger bandgap compared to SiC and GaN are thus appealing for high voltage pn diodes and transistors. Additionally, the formation of AlGaN-based heterojunctions and the utilization of polarization fields offer design options not possible for devices based on materials such as SiC and diamond. Here we report on the growth and performance of pn diodes fabricated from Al0.30Ga0.70N (Eg=4.1eV) and Al0.74Ga0.26N (Eg=5.3eV) alloys, illustrating the potential for AlGaN alloys as a viable material for next-generation power diodes. Quasi-vertical pn diodes consisting of Al0.30Ga0.70N epilayers were grown by metalorganic vapor phase epitaxy on 1.3 mm thick sapphire substrates. The added stiffness of such thick sapphire substrates (3x thicker than typical sapphire) reduces wafer bow and eliminates cracking in epitaxial structures up to 19 μm thick. Thick epitaxial structures are needed to achieve the > 4 microns thick drift layers necessary for multi-kilovolt breakdown voltages. A series of pn diode structures were grown where the thickness of the lightly doped drift layer (No = 2-5x1016 cm-3) was varied from 4.3 to 11 μm. All structures were completed with a 0.35 μm thick Mg-doped Al0.30Ga0.70N p-layer followed by a compositional grade to a p-GaN contact layer. Diodes employed a junction edge termination structure around the p-contact to reduce the peak electric field at the contact metal to avoid premature breakdown. Breakdown voltages from 1600 to 3000V were achieved in reverse bias and a forward turn-on voltage of 6 to 8 volts was recorded. To our knowledge, this is the first vertical AlGaN-based pn diode exhibiting a breakdown voltage in excess of 1 kilovolt. The differential specific on-resistance was higher than expected based on consideration of the drift region only (typically 16-20 mΩ-cm2), which is tentatively attributed to the resistance due to lateral current spreading in these front-contacted, quasi-vertical diodes. An effective critical electric field of 5.9 MV/cm was determined from the reverse current-voltage characteristics based on a punch-through, as dictated by the doping and thickness of the drift region. We have also investigated quasi-vertical pn diodes consisting of Al0.74Ga0.26N epilayers to exploit the expected higher critical electric field in higher aluminum mole fraction alloys. These diodes consisted of 4.5 μm thick AlN epilayers grown on sapphire followed by a heavily doped n-type contact layer. Next a lightly doped (No = 2-4x1016 cm-3) 5 or 8 μm thick drift layer was grown. The combination of a thick AlN buffer and Al0.74Ga0.26N layers resulted in x-ray diffraction linewidths of 164-197 (237–284) arc seconds for the (0002) and (10-12) reflections for the drift layer, respectively. By optimizing the growth conditions used for the drift layer, the density of electron compensating defects can be reduced to ~ 1x1016 cm-3 — a level reported for GaN pn diodes. Unlike GaN diodes where drift layer doping at mid to low 1015 cm-3 is necessary for breakdown voltages of several kilovolts, the free electron concentration in these Al0.74Ga0.26N pn diodes is significantly higher than the level of compensation, making doping much easier to control and illustrating the benefit of higher critical electric field. The drift layer was followed by p-type AlGaN and finally a compositional grade to a p-GaN contact layer. It is the p-AlGaN layer that is a significant challenge in Al-rich pn diodes. Unlike pn diodes based on Al0.30Ga0.70N epilayers, simple homojunction pn diodes are not practical in Al-rich AlGaN alloys due to the low hole concentration owing to the very deep (Ea > 400 meV) acceptor level. Consequently, other approaches must be considered for hole transport to the pn junction. We will report on the current-voltage characteristics of pn heterojunction diodes where a Mg-doped p-type Al0.30Ga0.70N layer is grown on the n-type Al0.74Ga0.26N drift layer. Additionally, the current-voltage characteristics of pn diodes where holes are generated in the p-region by field ionization of Mg acceptors will also be presented. Finally, practical pn diodes require specific on-resistance lower than what is inherent for front-contacted quasi-vertical device architectures grown on insulating substrates. We will report an approach that has realized vertically conducting AlGaN pn diodes employing AlGaN overgrowth on conducting GaN substrates.

Design considerations for AlGaN-based UV LEDs emitting near 235 nm with uniform emission pattern

The uniformity of emission from deep ultraviolet light emitting diodes (UV LEDs) is investigated. The AlGaN-based heterostructures of the UV LEDs emitting around 235 nm were grown by metalorganic vapor phase epitaxy on epitaxially laterally overgrown AlN/sapphire substrates. The impact of different device designs on the spatial distribution of the electroluminescence for a series of UV LEDs is studied. Due to the relatively high resistivities of n-AlGaN and p-AlGaN layers, ranging from 10 to 0.1 Ω cm as well as specific contact resistances approaching 1 Ω cm2, the emission patterns revealed heavy current crowding at the mesa edges causing a drop of power in the center of the emitting area and an asymmetry towards the side of the bonding pad of the n-contact. Simple analytical models considering the transfer and the current spreading length could only qualitatively explain the observed emission pattern. Using a 3D electro-thermal simulation of the current spreading in the LEDs the experimentally observed emission pattern could also be quantitatively reproduced. Based on these findings the 3D electro-thermal simulation was employed to optimize the contact geometry of the deep UV LEDs in order to achieve a more uniform power distribution.

Investigation of the light-extraction efficiency in 280 nm AlGaN-based light-emitting diodes having a highly transparent p-AlGaN layer

The light-extraction efficiency (LEE) of 280 nm AlGaN-based ultraviolet light-emitting diodes (UV-LEDs) with a highly transparent p-AlGaN layer was investigated using a three-dimensional (3D) finite-difference time-domain method. This investigation clearly demonstrated that two major problems need to be solved to obtain an LED structure that guarantees the high LEE for both TE and TM polarized light emissions. The first problem was shown to be light confined in quantum wells (QW) and the n-AlGaN layer. By analyzing a conventional LED structure with a highly transparent p-AlGaN layer and an AlN/sapphire template, the major cause of the light confinement was confirmed as the total internal reflection (TIR) at the interfaces between the EBL, QW, n-AlGaN, and AlN layers. Especially, light confined in the QWs is severe due to TIR at the interfaces adjacent to them. This light diminishes with time due to absorption. By removing the AlN/sapphire layer beneath the n-AlGaN layer, adopting an Al electrode, introducing cone-shaped pillars on the n-AlGaN layer and having cylindrical holes through the Al electrode, LEEs of 54.3% and 49.3% were attained for the TE and TM polarized emissions, respectively. This result demonstrates that just by offering proper light escape angles directly to the light confining layers, attainment of LEEs of more than 50% is possible. The second problem is severe light absorption by the metal electrode. LEE vs. the reflectivity of the metal electrode was analyzed for several LED structures. In the extracted results, an exponential increase in LEE as the metal reflectivity increases was shown, which means reflection at the interface between the p-AlGaN layer and the metal electrode is repeated several times until the light escapes from the LED. Also, it was shown that once the reflectivity of the metal electrode is more than 90%, any additional small increase in reflectivity increases LEE significantly. These results emphasize that without high reflectivity of the metal electrode and without releasing the light confined in several of the epitaxial layers, attaining LEEs of over 70% is difficult for AlGaN-based LEDs operating at 280 nm.

Degradation Characteristics of Normally-Off p-AlGaN Gate AlGaN/GaN HEMTs With 5 MeV Proton Irradiation

Proton irradiation at 5 MeV was performed on normally-off p-AlGaN gate AlGaN/GaN high electron mobility transistors (HEMTs). The increase of on-resistance and the degradation of subthreshold characteristics were observed after irradiation. The reduction of on-current was induced by displacement damage which also affected ungated devices with no p-AlGaN gate layer. Thermal annealing partially recovered this reduction. The subthreshold degradation with the threshold voltage (Vth) shift was proportional to the irradiation dose. TCAD simulation indicated that the decrease of hole concentration in p-AlGaN layer resulted in the negative shift of Vth and the subthreshold degradation.

Design of p-type cladding layers for tunnel-injected UV-A light emitting diodes

We discuss the engineering of p-AlGaN cladding layers for achieving efficient tunnel-injected III-Nitride ultraviolet light emitting diodes (UV LEDs) in the UV-A spectral range. We show that the capacitance-voltage measurements can be used to estimate the compensation and doping in the p-AlGaN layers located between the multi-quantum well region and the tunnel junction layer. By increasing the p-type doping concentration to overcome the background compensation, on-wafer external quantum efficiency and wall-plug efficiency of 3.37% and 1.62%, respectively, were achieved for the tunnel-injected UV LEDs emitting at 325 nm. We also show that interband tunneling hole injection can be used to realize UV LEDs without any acceptor doping. The work discussed here provides new understanding of hole doping and transport in AlGaN-based UV LEDs and demonstrates the excellent performance of tunnel-injected LEDs for the UV-A wavelength range.

Tailoring of Energy Band in Electron-Blocking Structure Enhancing the Efficiency of AlGaN-Based Deep Ultraviolet Light-Emitting Diodes

Electron-leakage is a deep-rooted problem for nitride-based light-emitting diodes (LEDs), particularly for AlGaN-based deep-ultraviolet (DUV) LEDs. In this paper, a specific design for the electron-blocking structure in AlGaN DUV LEDs, increasing the Al composition of the last quantum barrier to the same value as that of the electron-blocking layer accompanied with composition-graded p-AlGaN layer, is proposed to reduce electron leakage. Simulation results demonstrate that this design can effectively reduce electron leakage and hence increase internal quantum efficiency. Furthermore, fabricated devices with at an emitting wavelength of 292 nm verify remarkably enhanced light-output power. At 20-mA injection current, the proposed structure achieved a light-output increment as high as 98%, compared with the conventional structure.

Improved efficiency of near-ultraviolet LEDs using a novel p-type AlGaN hole injection layer

We investigate a novel near-ultraviolet light-emitting diode (NUV-LED) with a p-type AlGaN (pAlGaN) hole injection layer to replace the conventional p-type GaN layer. The optical properties are studied numerically with simulations. Our calculated results indicate that a pAlGaN layer can significantly improve both light output power and internal quantum efficiency of a NUV-LED. The light power of NUV-LED with constant and gradually increasing Al content of the pAlGaN layer increases by 215% and 266% compared to a conventional LED. We also find that the elimination of the interface barrier and suppression of the polarization field are the key factors that lead to the improved NUV-LED performance.

GaN-on-Si blue/white LEDs: epitaxy, chip, and package**Project supported financially by the National Natural Science Foundation of China (Nos. 61522407, 61534007, 61404156), the National High Technology Research and Development Program of China (No. 2015AA03A102), the Science & Technology Program of Jiangsu Province (Nos. BA2015099, BE2012063), the Suzhou Science & Technology Program (No. ZXG2013042), and the Recruitment Program of Global Experts (1000

The dream of epitaxially integrating III-nitride semiconductors on large diameter silicon is being fulfilled through the joint R&D efforts of academia and industry, which is driven by the great potential of GaN-on-silicon technology in improving the efficiency yet at a much reduced manufacturing cost for solid state lighting and power electronics. It is very challenging to grow high quality GaN on Si substrates because of the huge mismatch in the coefficient of thermal expansion (CTE) and the large mismatch in lattice constant between GaN and silicon, often causing a micro-crack network and a high density of threading dislocations (TDs) in the GaN film. Al-composition graded AlGaN/AlN buffer layers have been utilized to not only build up a compressive strain during the high temperature growth for compensating the tensile stress generated during the cool down, but also filter out the TDs to achieve crack-free high-quality n-GaN film on Si substrates, with an X-ray rocking curve linewidth below 300 arcsec for both (0002) and (101̄2) diffractions. Upon the GaN-on-Si templates, prior to the deposition of p-AlGaN and p-GaN layers, high quality InGaN/GaN multiple quantum wells (MQWs) are overgrown with well-engineered V-defects intentionally incorporated to shield the TDs as non-radiative recombination centers and to enhance the hole injection into the MQWs through the via-like structures. The as-grown GaN-on-Si LED wafers are processed into vertical structure thin film LED chips with a reflective p-electrode and the N-face surface roughened after the removal of the epitaxial Si(111) substrates, to enhance the light extraction efficiency. We have commercialized GaN-on-Si LEDs with an average efficacy of 150–160 lm/W for 1mm2 LED chips at an injection current of 350 mA, which have passed the 10000-h LM80 reliability test. The as-produced GaN-on-Si LEDs featured with a single-side uniform emission and a nearly Lambertian distribution can adopt the wafer-level phosphor coating procedure, and are suitable for directional lighting, camera flash, streetlighting, automotive headlamps, and otherlighting applications.

Enhanced light extraction in 260 nm light-emitting diode with a highly transparent p-AlGaN layer

AlGaN LEDs have been studied as efficient light sources in DUV. One of the central issues in DUV LEDs is their low light extraction efficiency owing to the absorption in a p-contacting layer and a metal electrode. We report the fabrication of a 260 nm LED containing a transparent p-AlGaN layer. The LED showed a relatively good current injection with an increase in forward voltage compared with a p-GaN LED. Its efficiency reached 2%, almost equivalent to that of the p-GaN LED. In addition, a nearly zero absorption in the p-contacting layer increased the light extraction efficiency by using a reflective metal electrode.

Surface plasmon coupling for suppressing p-GaN absorption and TM-polarized emission in a deep-UV light-emitting diode.

The radiated power enhancement (suppression) of an in- (out-of-) plane-oriented radiating dipole at a desired emission wavelength in the deep-ultraviolet (UV) range when it is coupled with a surface plasmon (SP) resonance mode induced on a nearby Al nanoparticle (NP) is demonstrated. Also, it is found that the enhanced radiated power propagates mainly in the direction from the Al NP toward the dipole. Such SP coupling behaviors can be used for suppressing the transverse-magnetic (TM)-polarized emission, enhancing the transverse-electric-polarized emission, and reducing the UV absorption of the p-GaN layer in an AlGaN-based deep-UV light-emitting diode by embedding a sphere-like Al NP in its p-AlGaN layer.

Novel high-current density GaN-based normally off transistor with tensile-strained quaternary InAlGaN barrier

A GaN-based normally off heterostructure field effect transistor (HFET) with high current density and with a quaternary InAlGaN barrier instead of an AlGaN barrier is investigated. It is difficult to obtain both high-current operation and normally off operation in the GaN-based HFET because of the need to control the polarization-induced charge. In order to obtain a normally off operation using quaternary InAlGaN barrier, the InAlGaN barrier is selectively removed and the p-AlGaN layer is formed as the gate electrode. The obtained threshold voltage of the InAlGaN-based HFET is +1.1 V. The maximum drain current reaches as high as 0.73 A/mm, which is almost twice that of a conventional AlGaN-based normally off gate injection transistor (GIT).