III-nitride m-plane violet narrow ridge edge-emitting laser diodes with sidewall passivation using atomic layer deposition.

A novel deep-ridge laser structure with atomic-layer deposition (ALD) sidewall passivation was proposed that enhances the optical characteristics of 8-µm ridge width III-nitride violet lasers on freestanding m-plane GaN substrates. The internal loss was determined using the variable stripe length method, where the laser structure with ALD sidewall passivation showed lower internal loss compared to the conventional shallow-ridge laser design. ALD sidewall passivation plays a critical role in device improvements; compared to the lasers without ALD sidewall passivation, the lasers with ALD sidewall passivation yield improved optoelectrical performance and longer lifetime under continuous-wave operation at high current density. This work demonstrates the importance of ALD sidewall passivation to laser performance, which enables high energy efficiency.

Hybrid tunnel junction enabled independent junction control of cascaded InGaN blue/green micro-light-emitting diodes.

We demonstrate vertical integration of nitride-based blue/green micro-light-emitting diodes (µLEDs) stacks with independent junctions control using hybrid tunnel junction (TJ). The hybrid TJ was gown by metal organic chemical vapor deposition (p + GaN) and molecular-beam epitaxy (n + GaN). Uniform blue, green and blue/green emission can be generated from different junction diodes. The peak external quantum efficiency (EQE) of the TJ blue µLEDs and green µLEDs with indium tin oxide contact is 30% and 12%, respectively. The carrier transportation between different junction diodes was discussed. This work suggests a promising approach for vertical µLEDs integration to enhance the output power of single LEDs chip and monolithic µLEDs with different emission colors with independent junction control.

High external quantum efficiency (6.8%) UV-A LEDs on AlN templates with quantum barrier optimization.

AlGaN-based UV-A LEDs have wide applications in medical treatment and chemical sensing; however, their efficiencies are still far behind visible LEDs or even shorter wavelengths UV-C counterparts because of the large lattice mismatch between the low-Al-content active region and the AlN substrate. In this report, we investigated the composition and thickness of the quantum barrier in the active region in terms of LED performance. Due to the improved strain management and better carrier confinement, efficient UV-A LEDs (320 nm - 330 nm) with EQEs up to 6.8% were demonstrated, among the highest efficiencies at this wavelength range.

High external quantum efficiency (6.5%) InGaN V-defect LEDs at 600†nm on patterned sapphire substrates.

Highly efficient long-wavelength InGaN LEDs have been a research focus in nitride LEDs for their potential applications in displays and solid-state lighting. A key breakthrough has been the use of laterally injected quantum wells via naturally occurring V-defects which promote hole injection through semipolar sidewalls and help to overcome the barriers to carrier injection that plague long wavelength nitride LEDs. In this article, we study V-defect engineered LEDs on (0001) patterned sapphire substrates (PSS) and GaN on (111) Si. V-defects were formed using a 40-period InGaN/GaN superlattice and we report a packaged external quantum efficiency (EQE) of 6.5% for standard 0.1 mm2. LEDs on PSS at 600 nm. We attribute the high EQE in these LEDs to lateral injection via V-defects.

Localization Effect in Photoelectron Transport Induced by Alloy Disorder in Nitride Semiconductor Compounds.

Near-band-gap photoemission spectroscopy experiments were performed on p-GaN and p-InGaN/GaN photocathodes activated to negative electron affinity. The photoemission quantum yield of the InGaN samples with more than 5% of indium drops by more than 1 order of magnitude when the temperature is decreased while it remains constant for lower indium content. This drop is attributed to a freezing of photoelectron transport in p-InGaN due to electron localization in the fluctuating potential induced by the alloy disorder. This interpretation is supported by the disappearance at low temperature of the peak in the photoemission spectrum that corresponds to the contribution of the photoelectrons relaxed at the bottom of the InGaN conduction band.

Fully transparent metal organic chemical vapor deposition-grown cascaded InGaN micro-light-emitting diodes with independent junction control.

In this work, we present fully transparent metal organic chemical vapor deposition (MOCVD)-grown InGaN cascaded micro-light-emitting diodes (µLEDs) with independent junction control. The cascaded µLEDs consisted of a blue emitting diode, a tunnel junction (TJ), a green emitting diode, and a TJ, without using any conductive oxide layer. We can control the injection of carriers into blue, green, and blue/green junctions in the same device independently, which show high optical and electrical performance. The forward voltage (Vf) at 20 A/cm2 for the TJ blue µLEDs and TJ green µLEDs is 4.06 and 3.13 V, respectively. These results demonstrate the efficient TJs and fully activated p-type GaN in the cascaded µLEDs. Such demonstration shows the important application of TJs for the integration of µLEDs with multiple color emissions.

Dependence of carrier escape lifetimes on quantum barrier thickness in InGaN/GaN multiple quantum well photodetectors.

We reported significant improvements in device speed by reducing the quantum barrier (QB) thicknesses in the InGaN/GaN multiple quantum well (MQW) photodetectors (PDs). A 3-dB bandwidth of 700 MHz was achieved with a reverse bias of -6 V. Carrier escape lifetimes due to carrier trapping in the quantum wells (QWs) were obtained from both simulation and experimental fitting, identifying carrier trapping as the major speed limiting factor in the InGaN/GaN MQW PDs.

Size-independent low voltage of InGaN micro-light-emitting diodes with epitaxial tunnel junctions using selective area growth by metalorganic chemical vapor deposition.

High performance InGaN micro-size light-emitting diodes (µLEDs) with epitaxial tunnel junctions (TJs) were successfully demonstrated using selective area growth (SAG) by metalorganic chemical vapor deposition (MOCVD). Patterned n + GaN/n-GaN layers with small holes were grown on top of standard InGaN blue LEDs to form TJs using SAG. TJ µLEDs with squared mesa ranging from 10×10 to 100×100 µm2 were fabricated. The forward voltage (Vf) in the reference TJ µLEDs without SAG is very high and decreases linearly from 4.6 to 3.7 V at 20 A/cm2 with reduction in area from 10000 to 100 µm2, which is caused by the lateral out diffusion of hydrogen through sidewall. By contrast, the Vf at 20 A/cm2 in the TJ µLEDs utilizing SAG is significantly reduced to be 3.24 to 3.31 V. Moreover, the Vf in the SAG TJ µLEDs is independent on sizes, suggesting that the hydrogen is effectively removed through the holes on top of the p-GaN surface by SAG. The output power of SAG TJ µLEDs is ∼10% higher than the common µLEDs with indium tin oxide (ITO) contact.

560†nm InGaN micro-LEDs on low-defect-density and scalable (20-21) semipolar GaN on patterned sapphire substrates.

We demonstrate InGaN-based semipolar 560 nm micro-light-emitting diodes with 2.5% EQE on high-quality and low-defect-density (20-21) GaN templates grown on scalable and low-cost sapphire substrates. Through transmission electron microscopy observations, we discuss how the management of misfit dislocations and their confinement in areas away from the active light-emitting region is necessary for improving device performance. We also discuss how the patterning of semipolar GaN on sapphire influences material properties in terms of surface roughness and undesired faceting in addition to indium segregation at the proximity of defected areas.

Electrically driven, polarized, phosphor-free white semipolar (20-21) InGaN light-emitting diodes grown on semipolar bulk GaN substrate.

We demonstrate a simple method to fabricate efficient, electrically driven, polarized, and phosphor-free white semipolar (20-21) InGaN light-emitting diodes (LEDs) by adopting a top blue quantum well (QW) and a bottom yellow QW directly grown on (20-21) semipolar bulk GaN substrate. At an injection current of 20 mA, the fabricated 0.1 mm2 size regular LEDs show an output power of 0.9 mW tested on wafer without any backside roughing, a forward voltage of 3.1 V and two emission peaks located at 427 and 560 nm. A high polarization ratio of 0.40 was measured in the semipolar monolithic white LEDs, making them promising candidates for backlighting sources in liquid crystal displays (LCDs). Furthermore, a 3dB modulation bandwidth of 410 MHz in visible light communication (VLC) was obtained in the micro-size LEDs (µLEDs) with a size of 20×20 µm2 and 40×40 µm2, which could overcome the limitation of slow frequency response of yellow phosphor in commercial white LEDs combing blue LEDs and yellow phosphor.

Improved performance of AlGaInP red micro-light-emitting diodes with sidewall treatments.

The electrical and optical improvements of AlGaInP micro-light-emitting diodes (µLEDs) using atomic-layer deposition (ALD) sidewall passivation were demonstrated. Due to the high surface recombination velocity and minority carrier diffusion length of the AlGaInP material system, devices without sidewall passivation suffered from high leakage and severe drop in external quantum efficiency (EQE). By employing ALD sidewall treatments, the 20×20 µm2 µLEDs resulted in greater light output power, size-independent leakage current density, and lower ideality factor. The forward current-voltage characteristic was enhanced by using surface pretreatment. Furthermore, ALD sidewall treatments recovered the EQE of the 20×20 µm2 devices more than 150%. This indicated that AlGaInP µLEDs with ALD sidewall treatments can be used as the red emitter for full-color µLED display applications.

Efficient tunnel junction contacts for high-power semipolar III-nitride edge-emitting laser diodes.

We demonstrate high-power edge-emitting laser diodes (LDs) with tunnel junction contacts grown by molecular beam epitaxy (MBE). Under pulsed conditions, lower threshold current densities were observed from LDs with MBE-grown tunnel junctions than from similarly fabricated control LDs with ITO contacts. LDs with tunnel junction contacts grown by metal-organic chemical vapor deposition (MOCVD) were additionally demonstrated. These LDs were fabricated using a p-GaN activation scheme utilizing lateral diffusion of hydrogen through the LD ridge sidewalls. Secondary ion mass spectroscopy measurements of the [Si] and [Mg] profiles in the MBE-grown and MOCVD-grown tunnel junctions were conducted to further investigate the results.

Realization of thin-film m-plane InGaN laser diode fabricated by epitaxial lateral overgrowth and mechanical separation from a reusable growth substrate.

A nonpolar edge emitting thin film InGaN laser diode has been separated from its native substrate by mechanical tearing with adhesive tape, combining the benefits of Epitaxial Lateral Overgrowth (ELO) and cleavability of nonpolar GaN crystal. The essence of ELO is mainly to weakening strength between native substrate and the fabricated laser device on top of it. We report a 3 mm long laser bar removed from its native GaN substrate. We confirmed edge emitting lasing operation after cleaving facets on a separated thin bar. Threshold current density of the laser was measured to be as low as 2.15 kA/cm2.

Impact of roughening density on the light extraction efficiency of thin-film flip-chip ultraviolet LEDs grown on SiC.

Discovering ways to increase the LED light extraction efficiency (LEE) should help create the largest performance improvement in the power of UV AlGaN LEDs. Employing surface roughening to increase the LEE of typical AlGaN UV LEDs is challenging and not well understood, yet it can be achieved easily in AlGaN LEDs grown on SiC. We fabricate thin-film UV LEDs (~294-310 nm) grown on SiC-with reflective contacts and roughened emission surface-to study and optimize KOH roughening of N-face AlN on the LEE as a function of roughened AlN pyramid size and KOH solution temperature. The LEE increased the most (2X) when the average AlN pyramid base diagonals (d) were comparable to the electroluminescence (EL) wavelength in the AlN layer (d ~λEL; 42-52 pyramids/µm2), but the LEE enhancement diminished when d was much larger than λEL (d ~5.5λEL; 2-3 pyramids/µm2). The UV LEDs had a 10 nm p-GaN contact layer, and the forward voltage was ~6 V at ~8 A/cm2, with a voltage efficiency (VE) of ~70%. The VE of the LEDs did not change after KOH roughening. This work suggests important implications to increase the LEE of AlGaN LEDs.

Prospects for 100% wall-plug efficient III-nitride LEDs.

The possibility of a III-nitride LED with 100% or greater wall-plug efficiency is examined considering recent observations of the phenomenon for smaller bandgap mid-IR LEDs under extremely low-bias operation [Phys. Rev. Lett. 108, 1 (2012)]. Thermoelectric pumping of carriers by lattice heat enables ≥ 100% WPE, but this effect is relatively weaker for the wider band gap III-nitrides. This work assesses the electrical and optical performance of several state-of-the-art nitride devices and summarizes the requirements and prospects for approaching ≥ 100% WPE for III-nitride LEDs operating at technologically relevant current densities (> 1 A/cm2).

Zinc oxide clad limited area epitaxy semipolar III-nitride laser diodes.

We report continuous-wave (CW) blue semipolar (202¯1) III-nitride laser diodes (LDs) that incorporate limited area epitaxy (LAE) n-AlGaN bottom cladding with thin p-GaN and ZnO top cladding layers. LAE mitigates LD design limitations that arise from stress relaxation, while ZnO layers reduce epitaxial growth time and temperature. Numerical modeling indicates that ZnO reduces the internal loss and increases the differential efficiency of TCO clad LDs. Room temperature CW lasing was achieved at 445 nm for a ridge waveguide LD with a threshold current density of 10.4 kA/cm2, a threshold voltage of 5.8 V, and a differential resistance of 1.1 Ω.

Development of high performance green c-plane III-nitride light-emitting diodes.

The effect of employing an AlGaN cap layer in the active region of green c-plane light-emitting diodes (LEDs) was studied. Each quantum well (QW) and barrier in the active region consisted of an InGaN QW and a thin Al0.30Ga0.70N cap layer grown at a relatively low temperature and a GaN barrier grown at a higher temperature. A series of experiments and simulations were carried out to explore the effects of varying the Al0.30Ga0.70N cap layer thickness and GaN barrier growth temperature on LED efficiency and electrical performance. We determined that the Al0.30Ga0.70N cap layer should be around 2 nm and the growth temperature of the GaN barrier should be approximately 75° C higher than the growth temperature of the InGaN QW to maximize the LED efficiency, minimize the forward voltage, and maintain good morphology. Optimized Al0.30Ga0.70N cap growth conditions within the active region resulted in high efficiency green LEDs with a peak external quantum efficiency (EQE) of 40.7% at 3 A/cm2. At a normal operating condition of 20 A/cm2, output power, EQE, forward voltage, and emission wavelength were 13.8 mW, 29.5%, 3.5 V, and 529.3 nm, respectively.

Semipolar InGaN quantum-well laser diode with integrated amplifier for visible light communications.

GaN-based semiconductor optical amplifier (SOA) and its integration with laser diode (LD) is an essential building block yet to be demonstrated for III-nitride photonic integrated circuits (PICs) at visible wavelength. This paper presents the InGaN/GaN quantum well (QW) based dual-section LD consisting of integrated amplifier and laser gain regions fabricated on a semipolar GaN substrate. The threshold current in the laser gain region was favorably reduced from 229mA to 135mA at SOA driving voltages, VSOA, of 0V and 6.25V, respectively. The amplification effect was measured based on a large gain of 5.7 dB at VSOA = 6.25V from the increased optical output power of 8.2 mW to 30.5 mW. Such integrated amplifier can be modulated to achieve Gbps data communication using on-off keying technique. The monolithically integrated amplifier-LD paves the way towards the III-nitride on-chip photonic system, providing a compact, low-cost, and multi-functional solution for applications such as smart lighting and visible light communications.

Using tunnel junctions to grow monolithically integrated optically pumped semipolar III-nitride yellow quantum wells on top of electrically injected blue quantum wells.

We report a device that monolithically integrates optically pumped (20-21) III-nitride quantum wells (QWs) with 560 nm emission on top of electrically injected QWs with 450 nm emission. The higher temperature growth of the blue light-emitting diode (LED) was performed first, which prevented thermal damage to the higher indium content InGaN of the optically pumped QWs. A tunnel junction (TJ) was incorporated between the optically pumped and electrically injected QWs; this TJ enabled current spreading in the buried LED. Metalorganic chemical vapor deposition enabled the growth of InGaN QWs with high radiative efficiency, while molecular beam epitaxy was leveraged to achieve activated buried p-type GaN and the TJ. This initial device exhibited dichromatic optically polarized emission with a polarization ratio of 0.28. Future improvements in spectral distribution should enable phosphor-free polarized white light emission.

Gigabit-per-second white light-based visible light communication using near-ultraviolet laser diode and red-, green-, and blue-emitting phosphors.

Data communication based on white light generated using a near-ultraviolet (NUV) laser diode (LD) pumping red-, green-, and blue-emitting (RGB) phosphors was demonstrated for the first time. A III-nitride laser diode (LD) on a semipolar (2021¯)  substrate emitting at 410 nm was used for the transmitter. The measured modulation bandwidth of the LD was 1 GHz, which was limited by the avalanche photodetector. The emission from the NUV LD and the RGB phosphor combination measured a color rendering index (CRI) of 79 and correlated color temperature (CCT) of 4050 K, indicating promise of this approach for creating high quality white lighting. Using this configuration, data was successfully transmitted at a rate of more than 1 Gbps. This NUV laser-based system is expected to have lower background noise from sunlight at the LD emission wavelength than a system that uses a blue LD due to the rapid fall off in intensity of the solar spectrum in the NUV spectral region.