Bidirectional light-emitting diode as a visible light source driven by alternating current.

Gallium nitride-based light-emitting diodes have revolutionized the lighting market by becoming the most energy-efficient light sources. However, the power grid, in example electricity delivery system, is built based on alternating current, which raises problems for directly driving light emitting diodes that require direct current to operate effectively. In this paper, we demonstrate a proof-of-concept device that addresses this fundamental issue - a gallium nitride-based bidirectional light-emitting diode. Its structure is symmetrical with respect to the active region, which, depending on the positive or negative bias, allows for the injection of either electrons or holes from each side. It is composed of two tunnel junctions that surround the active region. In this work, the optical and electrical properties of bidirectional light emitting diodes are investigated under direct and alternating current conditions. We find that the light is emitted in both directions of the supplied current, contrary to conventional light emitting diodes; hence, bidirectional light-emitting diodes can be considered a semiconductor light source powered directly with alternating current. In addition, we show that bidirectional light-emitting diodes can be stacked vertically to multiply the optical power achieved from a single device.

Nanostars in Highly Si-Doped GaN.

Understanding the relation between surface morphology during epitaxy of GaN:Si and its electrical properties is important from both the fundamental and application perspectives. This work evidences the formation of nanostars in highly doped GaN:Si layers with doping level ranging from 5 × 1019 to 1 × 1020 cm-3 grown by plasma-assisted molecular beam epitaxy (PAMBE). Nanostars are 50-nm-wide platelets arranged in six-fold symmetry around the [0001] axis and have different electrical properties from the surrounding layer. Nanostars are formed in highly doped GaN:Si layers due to the enhanced growth rate along the a-direction ⟨112̅0⟩. Then, the hexagonal-shaped growth spirals, typically observed in GaN grown on GaN/sapphire templates, develop distinct arms that extend in the a-direction ⟨112̅0⟩. The nanostar surface morphology is reflected in the inhomogeneity of electrical properties at the nanoscale as evidenced in this work. Complementary techniques such as electrochemical etching (ECE), atomic force microscopy (AFM), and scanning spreading resistance microscopy (SSRM) are used to link the morphology and conductivity variations across the surface. Additionally, transmission electron microscopy (TEM) studies with high spatial resolution composition mapping by energy-dispersive X-ray spectroscopy (EDX) confirmed about 10% lower incorporation of Si in the hillock arms than in the layer. However, the lower Si content in the nanostars cannot solely be responsible for the fact that they are not etched in ECE. The compensation mechanism in the nanostars observed in GaN:Si is discussed to be an additional contribution to the local decrease in conductivity at the nanoscale.

Polarization Doping in a GaN-InN System-Ab Initio Simulation.

Polarization doping in a GaN-InN system with a graded composition layer was studied using ab initio simulations. The electric charge volume density in the graded concentration part was determined by spatial potential dependence. The emerging graded polarization charge was determined to show that it could be obtained from a polarization difference and the concentration slope. It was shown that the GaN-InN polarization difference is changed by piezoelectric effects. The polarization difference is in agreement with the earlier obtained data despite the relatively narrow bandgap for the simulated system. The hole generation may be applied in the design of blue and green laser and light-emitting diodes.

Role of Metallic Adlayer in Limiting Ge Incorporation into GaN.

Atomically thin metal adlayers are used as surfactants in semiconductor crystal growth. The role of the adlayer in the incorporation of dopants in GaN is completely unexplored, probably because n-type doping of GaN with Si is relatively straightforward and can be scaled up with available Si atomic flux in a wide range of dopant concentrations. However, a surprisingly different behavior of the Ge dopant is observed, and the presence of atomically thin gallium or an indium layer dramatically affects Ge incorporation, hindering the fabrication of GaN:Ge structures with abrupt doping profiles. Here, we show an experimental study presenting a striking improvement in sharpness of the Ge doping profile obtained for indium as compared to the gallium surfactant layer during GaN-plasma-assisted molecular beam epitaxy. We show that the atomically thin indium surfactant layer promotes the incorporation of Ge in contrast to the gallium surfactant layer, which promotes segregation of Ge to the surface and Ge crystallite formation. Understanding the role of the surfactant is essential to control GaN doping and to obtain extremely high n-type doped III-nitride layers using Ge, because doping levels >1020 cm−3 are not easily available with Si.

Electrically pumped blue laser diodes with nanoporous bottom cladding.

We demonstrate electrically pumped III-nitride edge-emitting laser diodes (LDs) with nanoporous bottom cladding grown by plasma-assisted molecular beam epitaxy on c-plane (0001) GaN. After the epitaxy of the LD structure, highly doped 350 nm thick GaN:Si cladding layer with Si concentration of 6·1019 cm-3 was electrochemically etched to obtain porosity of 15 ± 3% with pore size of 20 ± 9 nm. The devices with nanoporous bottom cladding are compared to the reference structures. The pulse mode operation was obtained at 448.7 nm with a slope efficiency (SE) of 0.2 W/A while the reference device without etched cladding layer was lasing at 457 nm with SE of 0.56 W/A. The design of the LDs with porous bottom cladding was modelled theoretically. Performed calculations allowed to choose the optimum porosity and thickness of the cladding needed for the desired optical mode confinement and reduced the risk of light leakage to the substrate and to the top-metal contact. This demonstration opens new possibilities for the fabrication of III-nitride LDs.

Dependence of InGaN Quantum Well Thickness on the Nature of Optical Transitions in LEDs.

The design of the active region is one of the most crucial problems to address in light emitting devices (LEDs) based on III-nitride, due to the spatial separation of carriers by the built-in polarization. Here, we studied radiative transitions in InGaN-based LEDs with various quantum well (QW) thicknesses-2.6, 6.5, 7.8, 12, and 15 nm. In the case of the thinnest QW, we observed a typical effect of screening of the built-in field manifested with a blue shift of the electroluminescence spectrum at high current densities, whereas the LEDs with 6.5 and 7.8 nm QWs exhibited extremely high blue shift at low current densities accompanied by complex spectrum with multiple optical transitions. On the other hand, LEDs with the thickest QWs showed a stable, single-peak emission throughout the whole current density range. In order to obtain insight into the physical mechanisms behind this complex behavior, we performed self-consistent Schrodinger-Poisson simulations. We show that variation in the emission spectra between the samples is related to changes in the carrier density and differences in the magnitude of screening of the built-in field inside QWs. Moreover, we show that the excited states play a major role in carrier recombination for all QWs, apart from the thinnest one.

InGaN blue light emitting micro-diodes with current path defined by tunnel junction.

We have fabricated tunnel-junction InGaN micro-LEDs using plasma-assisted molecular beam epitaxy technology, with top-down processing on GaN substrates. Devices have diameters between 5 µm and 100 µm. All of the devices emit light at 450 nm at a driving current density of about 10Acm-2. We demonstrate that within micro-LEDs ranging in size from 100 µm down to 5 µm, the properties of these devices, both electrical and optical, are fully scalable. That means we can reproduce all electro-optical characteristics using a single set of parameters. Most notably, we do not observe any enhancement of non-radiative recombination for the smallest devices. We assign this result to a modification of the fabrication process, i.e., replacement of deep dry etching by a tunnel junction for the current confinement. These devices show excellent thermal stability of their light emission characteristics, enabling operation at current densities up to 1kAcm-2.

Revealing inhomogeneous Si incorporation into GaN at the nanometer scale by electrochemical etching.

Typical methods of doping quantification are based on spectroscopy or conductivity measurements. The spatial dopant distribution assessment with nanometer-scale precision is limited usually to one or two dimensions. Here we demonstrate an approach to detect three-dimensional dopant homogeneity in GaN:Si layers using electrochemical etching (ECE). GaN:Si layers are grown by plasma-assisted molecular beam epitaxy. Dopant incorporation is uniform when the growth front morphology is atomically flat. Non-uniform Si incorporation into GaN is observed when step-bunches are present on the surface during epitaxy. In this study we show that local Si concentration in the area of step-bunch is about three times higher than in the area between step-bunches. ECE spatial resolution in our experiment is estimated to be about 50 nm. This makes ECE a simple and quantitative probing tool for local three-dimensional conductivity homogeneity assessment. Our study proves that ECE could be important both for fundamental studies of crystal growth physics and impurity incorporation and for ion-implanted structures and post-processing device control.

Anomalous photocurrent in wide InGaN quantum wells.

We show that in a wide In0.17Ga0.83N quantum well, placed within an undoped region of the pin diode, a photocurrent in the forward direction is observed. The photocurrent switches to reverse direction when the light intensity is increased and/or photon energy is above the bandgap of the quantum barrier. We propose a model showing that the anomalous photocurrent is due to the fact that when the carriers are pumped into the wide quantum well they cannot recombine until the built-in field is screened. For low-intensity light it takes a long time (milliseconds) for the screening to occur and during that time we observe current flowing in the forward direction. This current originates from the reorganization of carriers forming the depletion regions, rather than directly from the photogenerated carriers. The observed effects lead to the dependence of PC spectra on chopper frequency and on light power. They may also affect the operation of laser diodes and solar cells with wide InGaN quantum wells.