Semipolar Light-emitting Diodes Research Articles

Study of the Luminescence Decay of a Semipolar Green Light-Emitting Diode for Visible Light Communications by Time-Resolved Electroluminescence.

Time-resolved photoluminescence (TRPL) is often used to study the excitonic dynamics of semiconductor optoelectronics such as the carrier recombination lifetime of III-nitride light-emitting diodes (LEDs). However, for any real-world application that requires LEDs under electrical injection, TRPL suffers an intrinsic limitation due to the absence of taking carrier transport effects into account. This becomes a severe issue for III-nitride LEDs used for visible light communication (VLC) since the modulation bandwidth for VLC is determined by the overall carrier lifetime of an LED, not just its carrier recombination lifetime. Time-resolved electroluminescence (TREL), which can characterize the luminescence decay of an LED under electrical injection to simulate real-world conditions when used in practical applications, is required. Both TRPL and TREL have been carried out on a semipolar LED and a standard c-plane LED (i.e., polar LED) both in the green spectral region for a comparison study. The (11-22) green semipolar LED exhibits much faster differential carrier lifetimes than the c-plane LED. In addition to a fast exponential component and a slow exponential component of 0.40 and 1.2 ns, respectively, which are similar to those obtained by TRPL, a third lifetime of 8.3 ns due to transport-related effects has been obtained from TREL, which has been confirmed by capacitance measurements. It has been found that the overall carrier lifetime of a c-plane LED is mainly limited by RC effects due to a junction capacitance, while it is not the case for a semipolar LED due to intrinsically reduced polarization, demonstrating the major advantages of using a semipolar LED for VLC.

A Systematic Exploration of InGaN/GaN Quantum Well-Based Light Emitting Diodes on Semipolar Orientations -=SUP=-*-=/SUP=-

Light-emitting diodes (LEDs) based on group III-nitride semiconductors (GaN, AlN, and InN) are crucial elements for solid-state lighting and visible light communication applications. The most widely used growth plane for group III-nitride LEDs is the polar plane (c-plane), which is characterized by the presence of a polarization-induced internal electric field in heterostructures. It is possible to address long-standing problems in group III-nitride LEDs, by using semipolar and nonpolar orientations of GaN. In addition to the reduction in the polarization-induced internal electric field, semipolar orientations potentially offer the possibility of higher indium incorporation, which is necessary for the emission of light in the visible range. This is the preferred growth orientation for green/yellow LEDs and lasers. The important properties such as high output power, narrow emission linewidth, robust temperature dependence, large optical polarization ratio, and low-efficiency droop are demonstrated with semipolar LEDs. To harness the advantages of semipolar orientations, comprehensive studies are required. This review presents the recent progress on the development of semipolar InGaN/GaN quantum well LEDs. Semipolar InGaN LED structures on bulk GaN substrates, sapphire substrates, free-standing GaN templates, and on Silicon substrates are discussed including the bright prospects of group III-nitrides. Keywords: Group III-nitride semiconductor, semipolar, light-emitting diodes, InGaN/GaN quantum well.

Characterization of semi-polar (20overline{2}1) InGaN microLEDs

In this paper, semi-polar (20overline{2}1) InGaN blue light-emitting diodes (LEDs) were fabricated and compared the performance with those of LEDs grown on c-plane sapphire substrate. LEDs with different chip sizes of 100 μm × 100 μm, 75 μm × 75 μm, 25 μm × 25 μm, and 10 μm × 10 μm were used to study the influence of chip size on the device performance. It was found that the contact behavior between the n electrode and the n-GaN layer for the semi-polar (20overline{2}1) LEDs was different from that for the LEDs grown on the c-plane device. Concerning the device performance, the smaller LEDs provided a larger current density under the same voltage and presented a smaller forward voltage. However, the sidewall’s larger surface to volume ratio could affect the IQE. Therefore, the output power density reached the maximum with the 25 μm × 25 μm chip case. In addition, the low blue-shift phenomenon of semi-polar (20overline{2}1) LEDs was obtained. The larger devices exhibited the maximum IQE at a lower current density than the smaller devices, and the IQE had a larger droop as the current density increased for the LEDs grown on c-plane sapphire substrate.

Influence of an InGaN superlattice pre-layer on the performance of semi-polar (11\u201322) green LEDs grown on silicon

It is well-known that it is crucial to insert either a single InGaN underlayer or an InGaN superlattice (SLS) structure (both with low InN content) as a pre-layer prior to the growth of InGaN/GaN multiple quantum wells (MQWs) served as an active region for a light-emitting diode (LED). So far, this growth scheme has achieved a great success in the growth of III-nitride LEDs on c-plane substrates, but has not yet been applied in the growth of any other orientated III-nitride LEDs. In this paper, we have applied this growth scheme in the growth of semi-polar (11–22) green LEDs, and have investigated the impact of the SLS pre-layer on the optical performance of semi-polar (11–22) green LEDs grown on patterned (113) silicon substrates. Our results demonstrate that the semi-polar LEDs with the SLS pre-layer exhibit an improvement in both internal quantum efficiency and light output, which is similar to their c-plane counterparts. However, the performance improvement is not so significant as in the c-plane case. This is because the SLS pre-layer also introduces extra misfit dislocations for the semi-polar, but not the c-plane case, which act as non-radiative recombination centres.

Lift-off of semipolar blue and green III-nitride LEDs grown on free-standing GaN

Light emitting diodes (LEDs), with active blue and green emitting and sacrificial multi-quantum well layers, were epitaxially grown using metal organic chemical vapor deposition on free-standing semipolar (202¯1) GaN substrates. NanoLEDs were then fabricated and released into solution using an approach based on forming a mm-scale mesa, Au–Au thermocompression bonding to a submount, large-area photoelectrochemical etching, and colloidal lithography. Photo- and cathodoluminescence (CL) measurements demonstrated that nanoLEDs were optically active after fabrication and released into the solution. Monte Carlo simulations of the electron trajectory through GaN/InGaN were performed to understand the patterns shown in CL images. The fabrication process developed herein could provide a viable route to highly efficient, nanoscale blue and green light emitters for applications in next-generation display technologies.

Effect of a patterned sapphire substrate on indium localization in semipolar (11-22) GaN-based light-emitting diodes

We have demonstrated the control of arrowhead-like surface structures in a semipolar (11-22) GaN film grown on a patterned sapphire substrate (PSS). The PSS can reduce the anisotropic crystallographic tilts of the semipolar (11-22) GaN film compared with a normal sapphire substrate (NSS). Semipolar (11-22) GaN-based light-emitting diodes (LEDs) grown on the PSS generated higher optical output power than the NSS. In addition, two emission peaks were observed for the semipolar (11-22) GaN-based LEDs grown on both substrates. However, as the injection current increased, the longer-wavelength emission peak of the LED grown on the NSS remained strong; meanwhile, the shorter-wavelength emission peak of the LED grown on the PSS was dominant over the longer-wavelength emission. Based on micro-electroluminescence images, we found that a longer-wavelength emission could be initiated at the front edge of the arrowhead-like surface structure; this emission expanded to the side edge regions of these structures as the injection current increased. Based on these results, we propose that using a PSS could decrease the number of arrowhead-like surface structures of semipolar (11-22) InGaN quantum wells; this further leads to a reduction in the strong indium localization produced at the arrowhead-like surface structure.

Suggestions on Efficiency Droop of GaN-based LEDs

InGaN/GaN-based light-emitting diodes (LEDs) are widely used in modern society and industry among different areas. However, InGaN/GaN LEDs suffer from an efficiency droop issue: The internal efficiency decreases during high current injection. The efficiency droop significantly affects the development of GaN-based LEDs devices in efficiency and light-output areas. Therefore, the improvement of the droop phenomenon has become a significant topic. This paper introduces several possible mechanisms of droop phenomenon based on different hypotheses including Auger Recombination, Carrier Delocalization and Electron Leakage. Furthermore, some proposals to mitigate efficiency droop, including semipolar LEDs, electron blocking layer(EBL), quaternary alloy and chip design will be discussed and analyzed. Also, it will provide some suggestions for the further optimization of droop phenomenon in each proposal.

Polarized monolithic white semipolar (20–21) InGaN light-emitting diodes grown on high quality (20–21) GaN/sapphire templates and its application to visible light communication

We demonstrate efficient, polarized and monolithic white semipolar (20–21) InGaN light-emitting diodes (LEDs) grown on high crystal quality 4-inch (20–21) GaN/sapphire template. Materials growth by metal-organic chemical vapor deposition (MOCVD) and characterization by atom probe tomography (APT) were carried out. The fabricated regular 0.1 mm2 size LEDs show a high electrical performance with an output power of 3.9 mW at 100 mA, an emission spectrum with two peaks located at 445 nm and 565 nm, a CIE point of (0.37, 0.42) and a polarization ratio of 0.30, which make them promising candidates for backlighting in liquid crystal displays (LCDs) application. Moreover, the fabricated square phosphor-free white μLED with size ranging from 20 to 60 μm, exhibit a high 3 dB modulation bandwidth of 660 MHz in the visible light communication (VLC) system, which benefits from the shorter carrier lifetime grown on the semipolar (20–21) plane. To our best knowledge, this is the first demonstration of monolithic white semipolar μLEDs in the VLC application, which can overcome the limitation of the slow frequency response of yellow phosphors converted commercial white LEDs. These results demonstrate the huge potentials to produce high efficiency monolithic white semipolar InGaN LEDs on cost-effective large area sapphire substrates.

Confocal photoluminescence investigation to identify basal stacking fault\u2019s role in the optical properties of semi-polar InGaN/GaN lighting emitting diodes

High spatial-resolution confocal photoluminescence (PL) measurements have been performed on a series of semi-polar (11–22) InGaN light emitting diodes (LEDs) with emission wavelengths up to yellow. These LED samples have been grown on our high crystal quality semi-polar GaN templates which feature periodically distributed basal stacking faults (BSFs), which facilitates the study of the influence of BSFs on their optical performance. Scanning confocal PL measurements have been performed across BSFs regions and BSF-free regions. For the blue LED, both the emission intensity and the emission wavelength exhibit a periodic behavior, matching the periodic distribution of BSFs. Furthermore, the BSF regions show a longer emission wavelength and a reduced emission intensity compared with the BSF-free regions. However, with increasing indium content, this periodic behavior in both emission intensity and emission wavelength becomes weaker and weaker. When the indium content (and correspondingly, wavelength) increases up to achieve yellow emission, only random fluctuations have been observed. It is worth highlighting that the influence of BSFs on the optical properties of semi-polar InGaN LEDs is different from the role of dislocations which normally act as non-radiative recombination centers.

(Invited) High-Speed GaN-Based Micro-Scale Light-Emitting Diodes for Visible-Light Communication

The increasing demand for wireless data communication and popularity of solid-state lighting has prompted research into visible-light communication (VLC) systems using GaN-based light-emitting diodes (LEDs). VLC is a promising candidate for next-generation (5G and beyond) light-fidelity (Li-Fi) network systems. To support multi-Gb/s data rates, VLC systems will require efficient LEDs with large modulation bandwidths. Conventional lighting-class LEDs cannot achieve high-speed operation due to their large chip size, large active region volume, and phosphor-converted output. Conversely, micro-scale LEDs (micro-LEDs) offer a viable path to high-speed operation due to their lower parasitic capacitance and ability for operation at high current densities where the carrier lifetime is significantly reduced. Furthermore, conventional c-plane LEDs suffer from polarization-related electric fields, which reduce the overlap between the electron and hole wave functions and lower the carrier recombination rate. Since modulation bandwidth is proportional to the carrier recombination rate (inversely proportional to the carrier lifetime), the overlap between the wave functions should be maximized for high-speed operation. Nonpolar and semipolar orientations have significantly reduced polarization effects and wave function overlaps approaching unity. These orientations can enable high-efficiency LEDs with simultaneously large modulation bandwidths, especially at relatively low current densities. In this work, we introduce VLC and discuss progress on the growth, fabrication, and characterization of high-speed micro-LEDs. Polar (0001), nonpolar (1010), and semipolar (2021) InGaN/GaN micro-LEDs on free-standing GaN substrates are investigated for their small-signal modulation characteristics as a function of current density, temperature, device area, and active region design. We demonstrate that the modulation bandwidths are well correlated with the wave function overlap in the active region by examining the orientation-dependence of the bandwidth. Record modulation bandwidths for GaN-based LEDs above 1 GHz are achieved for the nonpolar and semipolar orientations. We also show the modulation characteristics of an electrically injected nanowire-based LED, which achieves a modulation bandwidth of 1.2 GHz. Finally, we present a small-signal modeling method for determining the RC characteristics, differential carrier lifetime, carrier escape lifetime, and injection efficiency of the LEDs under electrical injection and discuss the results in the context of efficiency droop. Figure 1: Bandwidth vs. current density for the nonpolar LEDs (open black triangles), semipolar LED (open blue circles), and polar c-plane LED (open green squares) compared to other reported polar c-plane LEDs on sapphire or with a flip-chip design and compared to semipolar (1122) LEDs. The bandwidth characteristics for a nanowire-based LED are also included. Also shown are the crystal orientations used in the orientation-dependent bandwidth study. Figure 1

AlxGa1−xN-based semipolar deep ultraviolet light-emitting diodes

Deep ultraviolet (UV) emission from AlxGa1−xN-based light-emitting diodes (LEDs) fabricated on semipolar () (r-plane) AlN substrates is presented. The growth conditions are optimized. A high NH3 flow rate during metalorganic vapor phase epitaxy yields atomically flat AlyGa1−yN (y > x) on which AlxGa1−xN/AlyGa1−yN multiple quantum wells with abrupt interfaces and good periodicity are fabricated. The fabricated r-AlxGa1−xN-based LED emits at 270 nm, which is in the germicidal wavelength range. Additionally, the emission line width is narrow, and the peak wavelength is stable against the injection current, so the semipolar LED shows promise as a UV emitter.

Size-Dependent Bandwidth of Semipolar ( $11\overline {2}2$ ) Light-Emitting-Diodes

The limited modulation bandwidth of commercial light-emitting diodes (LEDs) is one of the critical bottlenecks for visible light communications. Possible approaches to increase the bandwidth include the use of micron sized LEDs, which can withstand higher current densities, as well as the use of LED structures that are grown on different crystal planes to the conventional polar c-plane. We compare c-plane InGaN/GaN LEDs with semipolar ( $11\overline {2}2$ ) LEDs containing a 4- and 8-nm single quantum well. The modulation bandwidth of semipolar LEDs with active areas varying from $200\times 200$ to $30\times 30\,\,\mu \text{m}^{2}$ is shown to be governed by both current density and size. A small signal bandwidth of over 800 MHz for a relatively low applied current density of 385 A/cm2 is reported for $30\times 30 \,\,\mu \text{m}^{2}$ LEDs with 8-nm thick quantum well. An optical link using an easy non-return-to-zero ON–OFF keying modulation scheme with a data rate of 1.5 Gb/s is demonstrated.

Toward ultimate efficiency: progress and prospects on planar and 3D nanostructured nonpolar and semipolar InGaN light-emitting diodes

Nonpolar and semipolar III-nitride-based blue and green light-emitting diodes (LEDs) have been extensively investigated as potential replacements for current polar c-plane LEDs. High-power and low-efficiency-droop blue LEDs have been demonstrated on nonpolar and semipolar planes III-nitride due to the advantages of eliminated or reduced polarization-related electric field and homoepitaxial growth. Semipolar (202¯1) and (202¯1¯) LEDs have contributed to bridging “green gap” (low efficiency in green spectral region) by incorporating high indium compositions, reducing polarization effects, and suppressing defects. Other properties, such as low thermal droop, narrow spectral linewidth, small wavelength shift, and polarized emission, have also been reported for nonpolar and semipolar LEDs. In this paper we review the theoretical background, device performance, material properties, and physical mechanisms for nonpolar and semipolar III-nitride semiconductors and associated blue and green LEDs. The latest progress on topics including efficiency droop, thermal droop, green-gap, and three-dimensional nanostructures is detailed. Future challenges, potential solutions, and applications will also be covered.

Impact of crystal orientation on the modulation bandwidth of InGaN/GaN light-emitting diodes

High-speed InGaN/GaN blue light-emitting diodes (LEDs) are needed for future gigabit-per-second visible-light communication systems. Large LED modulation bandwidths are typically achieved at high current densities, with reports close to 1 GHz bandwidth at current densities ranging from 5 to 10 kA/cm2. However, the internal quantum efficiency (IQE) of InGaN/GaN LEDs is quite low at high current densities due to the well-known efficiency droop phenomenon. Here, we show experimentally that nonpolar and semipolar orientations of GaN enable higher modulation bandwidths at low current densities where the IQE is expected to be higher and power dissipation is lower. We experimentally compare the modulation bandwidth vs. current density for LEDs on nonpolar (101¯0), semipolar (202¯1¯), and polar 0001 orientations. In agreement with wavefunction overlap considerations, the experimental results indicate a higher modulation bandwidth for the nonpolar and semipolar LEDs, especially at relatively low current densities. At 500 A/cm2, the nonpolar LED has a 3 dB bandwidth of ∼1 GHz, while the semipolar and polar LEDs exhibit bandwidths of 260 MHz and 75 MHz, respectively. A lower carrier density for a given current density is extracted from the RF measurements for the nonpolar and semipolar LEDs, consistent with the higher wavefunction overlaps in these orientations. At large current densities, the bandwidth of the polar LED approaches that of the nonpolar and semipolar LEDs due to coulomb screening of the polarization field. The results support using nonpolar and semipolar orientations to achieve high-speed LEDs at low current densities.

Efficient Semipolar (11-22) 550 nm Yellow/Green InGaN Light-Emitting Diodes on Low Defect Density (11-22) GaN/Sapphire Templates.

We demonstrate efficient semipolar (11-22) 550 nm yellow/green InGaN light-emitting diodes (LEDs) with In0.03Ga0.97N barriers on low defect density (11-22) GaN/patterned sapphire templates. The In0.03Ga0.97N barriers were clearly identified, and no InGaN clusters were observed by atom probe tomography measurements. The semipolar (11-22) 550 nm InGaN LEDs (0.1 mm2 size) show an output power of 2.4 mW at 100 mA and a peak external quantum efficiency of 1.3% with a low efficiency drop. In addition, the LEDs exhibit a small blue-shift of only 11 nm as injection current increases from 5 to 100 mA. These results suggest the potential to produce high efficiency semipolar InGaN LEDs with long emission wavelength on large-area sapphire substrates with economical feasibility.

Study of Low-Efficiency Droop in Semipolar ( <formula formulatype="inline"> <tex Notation="TeX">$20\bar{2}\bar{1}$</tex> </formula>) InGaN Light-Emitting Diodes by Time-Resolved Photoluminescence

The superior low-efficiency droop performance of semipolar ( $20\bar{2}\bar{1}$ ) InGaN light-emitting diodes (LEDs) makes it a hot candidate for efficient solid-state lighting and full-color displays. To unveil the mystery of this low droop and high efficiency, the emission dynamics of semipolar ( $20\bar{2}\bar{1}$ ) LEDs is investigated by time-resolved and steady-state photoluminescence (PL) measurements. Much smaller carrier lifetimes (radiative and nonradiative lifetime) were obtained from semipolar ( $20\bar{2}\bar{1}$ ) InGaN QWs compared with those on the $c$ -plane samples, possibly due to the reduced quantum-confined Stark effects and smaller indium fluctuation on semipolar InGaN samples. The experimental findings indicate a much reduced excess carrier density in semipolar ( $20\bar{2}\bar{1}$ ) InGaN LEDs, which will impact the device performance. Based on this, a modified ABC equation with weak phase-space-filling (PSF) effect was used to model the droop characteristics of semipolar ( $20\bar{2}\bar{1}$ ) LEDs.

Analysis of low efficiency droop of semipolar InGaN quantum well light-emitting diodes by modified rate equation with weak phase-space filling effect

We study the low efficiency droop characteristics of semipolar InGaN light-emitting diodes (LEDs) using modified rate equation incoporating the phase-space filling (PSF) effect where the results on c-plane LEDs are also obtained and compared. Internal quantum efficiency (IQE) of LEDs was simulated using a modified ABC model with different PSF filling (n0), Shockley-Read-Hall (A), radiative (B), Auger (C) coefficients and different active layer thickness (d), where the PSF effect showed a strong impact on the simulated LED efficiency results. A weaker PSF effect was found for low-droop semipolar LEDs possibly due to small quantum confined Stark effect, short carrier lifetime, and small average carrier density. A very good agreement between experimental data and the theoretical modeling was obtained for low-droop semipolar LEDs with weak PSF effect. These results suggest the low droop performance may be explained by different mechanisms for semipolar LEDs.

Growth and characterization of nonpolar (10-10) ZnO transparent conductive oxide on semipolar (11–22) GaN-based light-emitting diodes

We have grown thin films of nonpolar m-plane (10-10) ZnO on a semipolar (11–22) GaN template by atomic layer deposition (ALD) at low growth temperatures (<200 °C). The surface morphology of the ZnO film is found to be an arrowhead-like structure, which is a typical surface structure of the semipolar (11–22) GaN films. On increasing the growth temperature of the ZnO films, the concentration and mobility of the charge carriers in the ZnO film are increased. However, the optical transmittance decreases with an increase in the growth temperature. Based on these results, we have fabricated semipolar (11–22) GaN-based light-emitting diodes (LEDs) with nonpolar m-plane ZnO film as a transparent conductive oxide (TCO) to improve the light extraction efficiency. In spite of a decrease in the optical transmittance, the operation voltage of semipolar (11–22) GaN-based LEDs is found to decrease with an increase in the growth temperature, which might be due to the improvements in the electrical properties and current spreading effect, resulting in an increase in the optical output power.

(11-22) semipolar InGaN emitters from green to amber on overgrown GaN on micro-rod templates

We demonstrate semipolar InGaN single-quantum-well light emitting diodes (LEDs) in the green, yellow-green, yellow and amber spectral region. The LEDs are grown on our overgrown semipolar (11-22) GaN on micro-rod array templates, which are fabricated on (11-22) GaN grown on m-plane sapphire. Electroluminescence measurements on the (11-22) green LED show a reduced blue-shift in the emission wavelength with increasing driving current, compared to a reference commercial c-plane LED. The blue-shifts for the yellow-green and yellow LEDs are also significantly reduced. All these suggest an effective suppression in quantum confined Stark effect in our (11-22) LEDs. On-wafer measurements yield a linear increase in the light output with the current, and external quantum efficiency demonstrates a significant improvement in the efficiency-droop compared to a commercial c-plane LED. Electro-luminescence polarization measurements show a polarization ratio of about 25% in our semipolar LEDs.

Internal quantum efficiency and carrier injection efficiency of c‐plane, and InGaN/GaN‐based light‐emitting diodes

The electroluminescence (EL) output power of c‐plane InGaN/GaN‐based light‐emitting diodes (LEDs) is much higher than that of semipolar and LEDs at the same operation current. In order to elucidate the reasons for this behavior, we have fitted the pulsed EL data by the well‐known ABC model to extract the internal quantum efficiency (IQE) and the carrier injection efficiency (CIE) to clarify which parameter weighs more for the poor EL output power of the semipolar LEDs. The CIE shows large differences, 78%, 4%, and 4% for the c‐plane, and LEDs, respectively, whereas the IQE values are fairly the same for all three structures. The fit of resonant photoluminescence (PL) data at room temperature confirms the similar IQE values for all three structures. The CIE was increased from 4% to 10% for the planar LED with better electrical conductivity of the p‐GaN layer.