Tal Margalith

Indium tin oxide contacts to gallium nitride optoelectronic devices

We have fabricated GaN-based light-emitting diodes using transparent indium tin oxide (ITO) p contacts. ITO-contacted devices required an additional 2 V to drive 10 mA, as compared to similar devices with metal contacts. However, ITO has lower optical absorption at 420 nm (α=664 cm−1) than commonly used thin metal films (α=3×105 cm−1). Uniform luminescence was observed in ITO-contacted devices, indicating effective hole injection and current spreading.

4 Gbps direct modulation of 450 nm GaN laser for high-speed visible light communication

We demonstrate high-speed data transmission with a commercial high power GaN laser diode at 450 nm. 2.6 GHz bandwidth was achieved at an injection current of 500 mA using a high-speed visible light communication setup. Record high 4 Gbps free-space data transmission rate was achieved at room temperature.

Demonstration of a III-nitride vertical-cavity surface-emitting laser with a III-nitride tunnel junction intracavity contact

We report on a III-nitride vertical-cavity surface-emitting laser (VCSEL) with a III-nitride tunnel junction (TJ) intracavity contact. The violet nonpolar VCSEL employing the TJ is compared to an equivalent VCSEL with a tin-doped indium oxide (ITO) intracavity contact. The TJ VCSEL shows a threshold current density (Jth) of ∼3.5 kA/cm2, compared to the ITO VCSEL Jth of 8 kA/cm2. The differential efficiency of the TJ VCSEL is also observed to be significantly higher than that of the ITO VCSEL, reaching a peak power of ∼550 μW, compared to ∼80 μW for the ITO VCSEL. Both VCSELs display filamentary lasing in the current aperture, which we believe to be predominantly a result of local variations in contact resistance, which may induce local variations in refractive index and free carrier absorption. Beyond the analyses of the lasing characteristics, we discuss the molecular-beam epitaxy (MBE) regrowth of the TJ, as well as its unexpected performance based on band-diagram simulations. Furthermore, we investigat...

Development of selective lateral photoelectrochemical etching of InGaN/GaN for lift-off applications

The authors have developed a wet band gap-selective photoelectrochemical etching process to produce deep undercuts (∼500 μm) into InGaN/GaN heterostructures. These undercuts were used in a lift-off process which successfully transferred device-scale (100 μm diameter, 5 μm thick) disks from their underlying sapphire substrates to another substrate. Experiments were conducted using a lamp-and-filter arrangement, employing n-type and p-type GaN pieces as filters. Polishing was conducted to smooth the resulting substrate-transferred GaN disks.

Nonpolar III-nitride vertical-cavity surface-emitting lasers incorporating an ion implanted aperture

We report on our recent progress in improving the performance of nonpolar III-nitride vertical-cavity surface-emitting lasers (VCSELs) by using an Al ion implanted aperture and employing a multi-layer electron-beam evaporated ITO intracavity contact. The use of an ion implanted aperture improves the lateral confinement over SiNx apertures by enabling a planar ITO design, while the multi-layer ITO contact minimizes scattering losses due to its epitaxially smooth morphology. The reported VCSEL has 10 QWs, with a 3 nm quantum well width, 1 nm barriers, a 5 nm electron-blocking layer, and a 6.95- λ total cavity thickness. These advances yield a single longitudinal mode 406 nm nonpolar VCSEL with a low threshold current density (∼16 kA/cm2), a peak output power of ∼12 μW, and a 100% polarization ratio. The lasing in the current aperture is observed to be spatially non-uniform, which is likely a result of filamentation caused by non-uniform current spreading, lateral optical confinement, contact resistance, and...

Growth and fabrication of short-wavelength UV LEDs

Ultra-violet light emitting diodes with a peak wavelength of 293 nm were grown by MOCVD on AlN on sapphire. The maximum output power was 15 μW at 100 mA DC current injection for on wafer, room temperature testing. We have shown that by forming an interdigitated multi-fingered n-contact compared to a square geometry LED, the series resistance is reduced by ~ 8 - 15 Ω at 100 mA. This results in a 2 - 4 V reduction in drive voltage at 100 mA. The quantum wells exhibit a sharp electroluminescence peak at 293 nm with a 9 nm full-width at half maximum, but deep level related emission was observed at 2.56, 2.80, 3.52, and 3.82 eV. The high energy peaks, 3.52 and 3.82 eV, saturate with increasing drive current while the low energy peaks, 2.56 and 2.80 eV, increase with drive current proportional to the quantum well emission. This indicates the recombination mechanism for the low energy and high energy peaks is fundamentally different. We have also shown that forward bias leakage current in these devices is another factor limiting the quantum efficiency.

Catastrophic optical damage in GaInN multiple quantum wells

We have observed progressive damage due to reabsorption of stimulated emission in optically pumped laser-quality GaInN–GaN multiple quantum wells. The degradation occurred on a time scale consistent with the lifetime of electrically pumped lasers incorporating the same active region, suggesting that the failure mechanism was in part catastrophic optical damage, and not just heating in the p contact and p cladding as is often assumed.

Enhanced light extraction from free-standing InGaN/GaN light emitters using bio-inspired backside surface structuring

Light extraction from InGaN/GaN-based multiple-quantum-well (MQW) light emitters is enhanced using a simple, scalable, and reproducible method to create hexagonally close-packed conical nano- and micro-scale features on the backside outcoupling surface. Colloidal lithography via Langmuir-Blodgett dip-coating using silica masks (d = 170-2530 nm) and Cl2/N2-based plasma etching produced features with aspect ratios of 3:1 on devices grown on semipolar GaN substrates. InGaN/GaN MQW structures were optically pumped at 266 nm and light extraction enhancement was quantified using angle-resolved photoluminescence. A 4.8-fold overall enhancement in light extraction (9-fold at normal incidence) relative to a flat outcoupling surface was achieved using a feature pitch of 2530 nm. This performance is on par with current photoelectrochemical (PEC) nitrogen-face roughening methods, which positions the technique as a strong alternative for backside structuring of c-plane devices. Also, because colloidal lithography functions independently of GaN crystal orientation, it is applicable to semipolar and nonpolar GaN devices, for which PEC roughening is ineffective.

2.6 GHz high-speed visible light communication of 450 nm GaN laser diode by direct modulation

Gallium Nitride (GaN) based light emitting diodes (LEDs) has been considered as a next generation lighting source due to its high efficiency, long lifetime, and high brightness. In addition to the lighting purpose, visible light communication (VLC) on LEDs has been studied since it is easily available in the existing light infrastructure. Exponentially increasing wireless data traffic in radio frequency (RF) also motivated the need for VLC. However, the modulation bandwidth of LEDs with a long carrier recombination lifetime (~ns) is limited up to ~ 400 MHz even though orthogonal frequency division multiplexing (OFDM) rather than direct modulation could improve data rate up to 3 Gbit/s [1]. It can be seen that laser based VLC is necessary because the modulation bandwidth of laser diodes is limited by photon lifetime (~ps). In addition, since Denault et al. reported 76 lm/W efficacy for 442 nm laser based white lighting, the blue laser is promising lighting source readily available for white lighting communication, which is also called light fidelity (Li-Fi) [2]. Recent progress on high-speed VLC using a 422 nm laser diode showed 1.4 GHz bandwidth and 2.5 Gbps. Even if this is about three times larger bandwidth than blue LED VLC, the system was limited by the bandwidth of the photo detector (PD) due to the absence of commercially available high speed PD covering in the blue region with enough responsivity [3]. Our study demonstrated the first novel VLC system limited by the bandwidth of 450 nm laser diode with a high-speed UV-extended PD giving 2.6 GHz modulation bandwidth and 4 Gbit/s data transmission rate.

Nitride-based lasers: advances in cavity design

Unique constraints encountered in the III-N semiconductor system, such as a lack of cleaving planes and resistance to wet etchants, make conventional approaches to the fabrication of laser diodes difficult to implement, and result in the need for novel cavity designs. Improvements in in-plane cavities include etching facets using a focused ion beam and the incorporation of gratings to decrease losses associated with poor mirrors. Towards the fabrication of an electrically pumped VCSEL, the issue of the bottom mirror can be addressed by the use of dielectric mirrors, and either the removal of the sapphire substrate or its incorporation into the cavity by using a curved backside mirror. This paper will review recent developments in these areas at UC Santa Barbara.