C-plane Light-emitting Diodes Research Articles

Low-efficiency-droop c-plane InGaN/GaN light-emitting diodes through the use of thick single quantum wells and doped barriers

Efficiency droop at high current densities is a problem for InGaN-based light-emitting diodes (LEDs), especially for conventional c-plane devices. The large internal electric fields in c-plane quantum wells (QWs) lead to an increase in the active region carrier density (n), causing the electrical efficiency droop onset to occur at low current densities. Here, we present an approach to reduce the internal electric fields (Eint) in c-plane QWs by placing doped p-type and n-type GaN barriers close to the QW. The reduced Eint also allows a thick QW active region design, which helps to lower n to further reduce the droop. The concept of using doped barriers to control Eint is explained using theory and device simulations. Following that, multiple series of thick single QW (SQW) LEDs were grown and characterized. Key parameters in the epitaxial design such as the doping levels and the relative position of the doped barriers were systematically studied and optimized. By using optimized doped barriers and a thick SQW, c-plane LEDs with a low-efficiency droop of 14% at 300 A/cm2 [with respect to the peak external quantum efficiency (EQE)] and a high peak EQE of 49% were demonstrated.

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.

1.3 GHz E-O bandwidth GaN-based micro-LED for multi-gigabit visible light communication

The data rate of a visible light communication (VLC) system is basically determined by the electrical-to-optical (E-O) bandwidth of its light-emitting diode (LED) source. In order to break through the intrinsic limitation of the carrier recombination rate on E-O bandwidth in conventional c-plane LEDs based on InGaN quantum wells, a blue micro-LED with an active region of nano-structured InGaN wetting layer is designed, fabricated, and packaged to realize a high-speed VLC system. The E-O bandwidth of the micro-LED can reach up to 1.3 GHz. Based on this high-speed micro-LED, we demonstrated a data rate of 2 Gbps with a bit error rate (BER) of 1.2 × 10 − 3 with simple on-off keying signal for a 3-m real-time VLC. In addition, a 4-Gbps VLC system using quadrature phase shift keying-orthogonal frequency-division multiplexing with a BER of 3.2 × 10 − 3 is also achieved for the same scenario. Among all the point-to-point VLC systems based on a single-pixel LED, this work has the highest distance-bandwidth product of 3 GHz·m and the highest distance-rate product of 12 Gbps·m.

A 3D simulation comparison of carrier transport in green and blue c-plane multi-quantum well nitride light emitting diodes

Until recently, the electrical efficiency of green nitride light-emitting diodes (LEDs) was considerably lower than that of blue LEDs. This is particularly surprising as one would expect a reduced forward voltage with increasing emission wavelength. In this paper, we theoretically investigated the impact of the number of quantum wells on the forward voltage of III-nitride LEDs with x = 0.15 (blue) and x = 0.24 (green) InxGa1–xN QWs. The simulated dependence of current density (J) on applied diode bias (V) shows a significant increase of 1.9 V in the forward voltage between one and five quantum well (QW) c-plane green LED structures. Artificially turning off the polarization fields in the simulation does not entirely suppress this effect. Due to the large band offsets in the green LED multiple QW stack, simulations indicate a sequential band filling of the QW sequence. This mechanism should not be limited to c-plane LEDs and could also be present in nonpolar or semipolar devices.

High-Bandwidth Green Semipolar (20–21) InGaN/GaN Micro Light-Emitting Diodes for Visible Light Communication

Light-emitting diodes (LEDs) have been regarded as promising candidates for visible light communication (VLC); however, strong internal polarization fields in common c-plane LEDs, especially green ...

Coaxial semipolar InGaN/GaN microwire array LED with substantially suppressed efficiency droop

GaN nano/micro-structure based light-emitting diodes (LEDs) have drawn much attention owing to their potential applications in display and optoelectronics integration. Here, we have fabricated coaxial semipolar InGaN/GaN multiple quantum wells (MQWs) microwire array LED with superior performances in suppressing efficiency droop. The results show that our as-synthesized microwire has two semipolar planes (1‾101) and (11‾01), and the InGaN/GaN MQWs have superior crystal quality. In addition, the efficiency droop ratio of our device is about 9.7% as the injected current increases from 3 to 100 A/cm2, which is largely declined by 47% compared with that of the conventional polar c-plane LEDs. Meanwhile, the microwire array LED reveals a small wavelength shift (3 nm) as the injected current increases from 3 to 23 A/cm2. The effective advances in the device should be attributed to the weaker quantum-confined stark effect of InGaN/GaN MQWs in semipolar plane. This work proposes a high repeatability method to fabricate microwire array LED for future optoelectronic integrated systems.

Improving modulation bandwidth of c-plane GaN-based light-emitting diodes by an ultra-thin quantum wells design.

The GaN-based light emitting diodes (LEDs) have a great potential for visible light communication (VLC) due to their ubiquitous application in general lighting, but the modulation bandwidth of conventional c-plane LEDs is limited by carrier recombination rate in InGaN quantum wells (QWs) due to the polarization-field-induced quantum confined Stark effect (QCSE). Furthermore, the high modulation bandwidth on c-plane sapphire substrates can only be achieved at high current densities. Here, blue LEDs with ultra-thin InGaN QWs (1nm) and GaN barriers (3nm) are grown on c-plane sapphire substrate to suppress QCSE and extend the cut-off frequency from 214 MHz for conventional LEDs to 536 MHz at a current density of 2.5 kA/cm2, which is comparable to devices grown on semi-polar substrates.

(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

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.

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.

Large Stokes-like shift in N-polar InGaN/GaN multiple-quantum-well light-emitting diodes

N-polar (−c-plane) InGaN/GaN light-emitting diodes (LEDs) were grown by metalorganic vapor phase epitaxy, and their optoelectronic properties were evaluated by electroreflectance (ER) and electroluminescence (EL) measurements. In −c-plane LEDs, the emission energy was much lower than that in c-plane LEDs. By comparing EL and ER results, we found that the emission energy was also much lower than the transition energy. The transition energy is in good agreement with X-ray diffraction analysis results. These results indicate that −c-plane LEDs exhibit a larger Stokes-like shift than do c-plane LEDs. This Stokes-like shift is due to the strong potential fluctuation, which is possibly caused by the specific growth patterns of −c-plane III–nitrides. The dominant emission centers of the −c-plane LEDs were suggested to be the localized states of InGaN islands.

(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.

Lighting for the 21st century with laser diodes based on non-basal plane orientations of GaN

Abstract

Suppression of metastable-phase inclusion in N-polar (0001¯) InGaN/GaN multiple quantum wells grown by metalorganic vapor phase epitaxy

The metastable zincblende (ZB) phase in N-polar (0001¯) (−c-plane) InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic vapor phase epitaxy is elucidated by the electron backscatter diffraction measurements. From the comparison between the −c-plane and Ga-polar (0001) (+c-plane), the −c-plane MQWs were found to be suffered from the severe ZB-phase inclusion, while ZB-inclusion is negligible in the +c-plane MQWs grown under the same growth conditions. The ZB-phase inclusion is a hurdle for fabricating the −c-plane light-emitting diodes because the islands with a triangular shape appeared on a surface in the ZB-phase domains. To improve the purity of stable wurtzite (WZ)-phase, the optimum conditions were investigated. The ZB-phase is dramatically eliminated with decreasing the V/III ratio and increasing the growth temperature. To obtain much-higher-quality MQWs, the thinner InGaN wells and the hydrogen introduction during GaN barriers growth were tried. Consequently, MQWs with almost pure WZ phase and with atomically smooth surface have been demonstrated.

Regularly patterned non-polar InGaN/GaN quantum-well nanorod light-emitting diode array.

The growth and process of a regularly patterned nanorod (NR)- light-emitting diode (LED) array with its emission from sidewall non-polar quantum wells (QWs) are demonstrated. A pyramidal un-doped GaN structure is intentionally formed at the NR top for minimizing the current flow through this portion of the NR such that the injection current can be effectively guided to the sidewall m-plane InGaN/GaN QWs for emission excitation by a conformal transparent conductor (GaZnO). The injected current density at a given applied voltage of the NR LED device is similar to that of a planar c-plane or m-plane LED. The blue-shift trend of NR LED output spectrum with increasing injection current is caused by the non-uniform distributions of QW width and indium content along the height on a sidewall. The photoluminescence spectral shift under reversed bias confirms that the emission of the fabricated NR LED comes from non-polar QWs.

Semipolar (202̄1) Single-Quantum-Well Red Light-Emitting Diodes with a Low Forward Voltage

We have demonstrated the InGaN/GaN single-quantum-well (SQW) red light-emitting diodes (LEDs) grown on the free-standing GaN (202̄1) substrate with a forward voltage as low as 2.8 V at 20 mA. A low p-GaN growth temperature is required to prevent the structure deterioration during the p-GaN growth. The reduction of the forward voltage was observed as the emission wavelength increased in the (202̄1) SQW LEDs, which is attributed to its reversed polarization-related electric field compared to the conventional c-plane LEDs.

Analyzing the physical properties of InGaN multiple quantum well light emitting diodes from nano scale structure

We report on the influence of nanoscale indium fluctuations on physical properties for multiple quantum well (QW) light emitting diodes (LEDs). A commercial grade c-plane LED was analyzed by atom probe tomography, and the indium composition distribution was extracted. The influence of the degree of fluctuation and number of quantum wells were analyzed by a two-dimensional Poisson and drift-diffusion solver with very fine mesh and compared to the experimental result and a simple normal quantum well model. The studies show that the indium fluctuation will significantly impact the device’s internal quantum efficiency, droop behavior, and current-voltage curves. Including the influence of indium-fluctuation gives a better prediction of the device performance.

Influence of polarity on carrier transport in semipolar (2021¯) and (202¯1) multiple-quantum-well light-emitting diodes

We investigate the influence of polarity on carrier transport in single-quantum-well and multiple-quantum-well (MQW) light-emitting diodes (LEDs) grown on the semipolar (202¯1) and (2021¯) orientations of free-standing GaN. For semipolar MQW LEDs with the opposite polarity to conventional Ga-polar c-plane LEDs, the polarization-related electric field in the QWs results in an additional energy barrier for carriers to escape the QWs. We show that semipolar (2021¯) MQW LEDs with the same polarity to Ga-polar c-plane LEDs have a more uniform carrier distribution and lower forward voltage than (202¯1) MQW LEDs.

Submicro-electroluminescence spectroscopy of dodecagonal micropit-arranged blue light-emitting diodes

We investigate the local electroluminescence (EL) characteristics of c-plane blue InGaN/GaN light-emitting diodes (LEDs) with periodically arranged dodecagonal micropits (DMs) which have {1 0 −1 −1} and {1 1 −2 −2} facets using confocal scanning electroluminescence microscopy (CSEM). By increasing the injection current from 1 to 20 mA, the EL peak for c-plane LEDs is shifted to a shorter wavelength (by up to 5.5 nm), but the EL peaks for semi-polar {1 0 −1 −1} and {1 1 −2 −2} facets are nearly unchanged because of the substantially reduced polarization-related electric fields in the quantum wells. CSEM images show various emitting distributions due to the different emission origins as a function of peak position in the EL spectrum. The present observations can help one to improve the understanding of the spatial origin of increased external quantum efficiency in InGaN LEDs with DM arrays.

Improved light-emitting diode performance by conformal overgrowth of multiple quantum wells and fully coalesced p-type GaN on GaN nanowires

We demonstrate a light-emitting diode (LED) structure with multiple quantum wells (MQWs) conformally grown on semipolar and nonpolar plane facets of n-GaN nanowires (NWs), followed by deposition of fully coalesced p-GaN on these nanowires. Overgrowth on the nanowires’ tips results in inclusion of high density voids, about one micron in height, in the GaN film. The light output intensity of NWs LEDs is more than three times higher than corresponding c-plane LEDs grown simultaneously. We believe this results from a reduced defect density, increased effective area of conformally grown MQWs, absence of polar plane orientation, and improved light extraction.