Density Of V-defects Research Articles

EFFECT OF THE DENSITY OF SURFACE V-DEFECTS ON LASER PROPERTIES OF InGaN/GaN HETEROSCTRUCTURES WITH MULTIPLE QUANTUM WELLS GROWN ON SILICON SUBSTRATES

We investigated the radiative properties of InGaN/GaN heterostructures with multiple quantum wells (MQWs) grown on silicon substrates with different thicknesses of quantum wells at optical excitation. The correlation of laser and photoluminescent properties with the surface morphology of the gallium nitride
 coating layers and the density of V-defects has been established. It is shown that, with a growth in the density of V-defects, the threshold power density of the excitation of the generation of InGaN/GaN heterostructures with MQWs increases.

Role of V-defect density on the performance of III-nitride green LEDs on sapphire substrates

In this study, we experimentally investigated the role of V-defect density on the performance of green III-nitride LEDs grown on sapphire substrates by metalorganic chemical vapor deposition. We systematically varied the threading dislocation (TD) density from 4 × 108 to 1 × 109 cm−2 by changing the V/III ratio during initial high temperature GaN growth. A 30-period InGaN/GaN superlattice promoted V-defect formation and growth at TDs, where the density of V-defects was correlated to the TD density. By interrupting the LED growth and examining the surface of the active region, we quantified the average size and density of V-defects. In a series of LEDs, we measured a systematic decrease in forward voltage (VF) with V-defect density. At a V-defect density of 5.0 × 108 cm−2 and TD density of 1 × 109 cm−2, green LED devices were demonstrated with λ = 523 nm and VF = 2.94 V at 20 A cm−2. These results highlight the potential of using V-defect engineering to achieve low VF long wavelength LEDs on sapphire substrates, where opening of remaining threading dislocations into V-defects presents an opportunity for further VF reduction.

Electrical Properties of Graphene Contacts to AlGaN/GaN Heterostructures

A nanoscale electrical characterization of graphene (Gr) contacts to AlxGa1-xN/GaN heterostructures has been carried out using conductive atomic force microscopy. The impact of the AlGaN microstructure on the current transport at Gr/AlGaN interface was evaluated considering two Al0.25Ga0.75N/GaN heterostructures with very different quality in terms of surface roughness and defectivity, i.e. a uniform and defect-free sample and a sample with a high density of V-defects, that locally cause a reduction of the AlGaN thickness. Rectifying contacts were found on the bare (Gr-free) AlGaN surfaces of both samples, but with a more inhomogeneous and lower Schottky barrier height (ΦB≈0.6 eV) in the presence of V-defects with respect to the case of the uniform AlGaN (ΦB≈0.9 eV). Very different electrical behaviour was observed for Gr on the two AlGaN samples, i.e. a low barrier height Schottky contact (ΦB≈0.4 eV) for the uniform AlGaN and an Ohmic contact for the defective AlGaN. Both Schottky and ohmic Gr/AlGaN contacts exhibit an excellent lateral uniformity, that can be ascribed to an averaging effect of the Gr electrode over the AlGaN interfacial inhomogeneities.

Effect of intentional p-GaN surface roughening on the performance of InGaN/GaN solar cells

The effect of intentional p-GaN surface roughening on the performance of c-plane InGaN/GaN solar cells was investigated. Surface roughness was introduced by growing the p-GaN at a relatively high rate and low temperature which resulted in a faceted surface with a high density of V-defects. Increasing the surface roughness led to a 69.4% increase in short circuit current density. Similar surface roughening techniques should also be applicable for increasing the extraction efficiency of InGaN/GaN light-emitting diodes.

Influence of growth conditions on the V-defects in InGaN/GaN MQWs

The influence of the growth temperature, TMIn/TEGa and V/III ratio on the V-defects of InGaN/GaN multi-quantum wells (MQWs) has been investigated and discussed. When the TMIn flow increases from 180 to 200 sccm, the density of V-defects increases from 2.72 × 1018 to 5.24 × 1018 cm−2, and the V-defect width and depth increase too. The density also increases with the growth temperature. The densities are 2.05 × 108, 2.72 × 1018 and 4.23 × 108 cm−2, corresponding to a growth temperature of 748, 753 and 758 °C respectively. When the NH3 flows are 5000, 6600 and 8000 sccm, the densities of the V-defects of these samples are 6.34 × 1018, 2.72 × 1018 and 4.13 × 1018 cm−2, respectively. A proper V/III ratio is needed to achieve step flow growth mode. We get the best quality of InGaN/GaN MQWs at a growth temperature of 753 °C TMIn flow at 180 sccm, NH3 flow at 6600 sccm, a flatter surface and less V-defects density. The depths of these V-defects are from 10 to 30 nm, and the widths are from 100 to 200 nm. In order to suppress the influence of V-defects on reverse current and electro-static discharge of LEDs, it is essential to grow thicker p-GaN to fill the V-defects.

Effect of indium surfactant on stress relaxation by V-defect formation in GaN epilayers grown by metalorganic chemical vapor deposition

The effect of indium surfactant on the stress in GaN films grown on SiC at 950 °C by metalorganic chemical vapor deposition was investigated using a combination of in situ wafer curvature measurements and ex situ high resolution x-ray diffraction (HRXRD). As the molar flow rate of trimethylindium was varied from 0 to 4.5 μmol/min during growth, the real-time stress measurements showed that the mean compressive stress of the GaN films decreased from −0.60 to −0.30 GPa. The lattice constants of the GaN epilayers determined by HRXRD confirmed the stress relaxation promoted by the presence of indium while the rocking curve measurements showed that the threading dislocation (TD) density of GaN films remains nearly unchanged. Atomic force microscopy measurements showed that the indium improved step-flow growth, but simultaneously it drove V-defect formation on the GaN surface, which plays a critical role in stress relaxation of GaN films. Cross-sectional transmission electron microscopy revealed the minor contribution of plastic dislocation motion to stress relaxation by localized TD bending toward V-defects. A nucleation and growth model for the V-defect formation was developed to explain that V-defects are energetically favorable to form at TDs under indium-rich conditions. This model shows that the energy barrier for V-defect formation is significantly reduced when indium is present, which leads to the relaxation of misfit strain energy by increasing the size and density of V-defects. Initiation of V-defects and the role of TDs in V-defect formation are discussed based on the presented model.

Effect of barrier growth temperature on morphological evolution of green InGaN/GaN multi-quantum well heterostructures

Surface morphology of green InGaN/GaN multi-quantum wells (MQWs) on a sapphire substrate with various high temperature grown GaN barriers has been evaluated. Keeping the InGaN well growth temperature constant at 740 °C, a series of MQWs were grown with GaN barrier temperatures varied up to 910 °C. GaN barriers grown below 800 °C lead to the generation of a high density of V-defects and inclusions embedded within V-defects as observed by atomic force microscopy. Scanning electron microscopy and cathodoluminescence (CL) studies revealed that the embedded inclusions are of two kinds: one of them appears as bright spots in CL mapping while the other appears as the surrounding region. Temperature ramping and subsequent interruption for GaN barrier growth suppresses both kinds of inclusion defects and also significantly reduces the V-defect density. An inclusion-free smooth surface is obtained for green emitting InGaN/GaN MQWs with the GaN barrier grown at 910 °C.

A study of dislocations in InGaN/GaN multiple-quantum-well structure grown on ( [formula omitted]) sapphire substrate

Transmission electron microscope (TEM) and X-ray diffraction (XRD) measurements performed on an InGaN/GaN multiple-quantum-well (MQW) structure deposited on ( 1 1 2 ̄ 0 ) and (0 0 0 1) sapphire substrates have been investigated. The grown MQW deposited on ( 1 1 2 ̄ 0 ) sapphire substrate are still oriented along the (0 0 0 1) direction due to an unusual growth mechanism, which is confirmed by XRD measurement. TEM measurements of the initial stage of the high-temperature GaN growth after the low-temperature buffer GaN layer on ( 1 1 2 ̄ 0 ) sapphire are different from the TEM results obtained for GaN on (0 0 0 1) sapphire substrate, unlike the XRD patterns which were identical. In addition, a further TEM study indicates that the sample on ( 1 1 2 ̄ 0 ) sapphire substrate shows higher threading dislocation density than that on (0 0 0 1) sapphire substrate, a fact that is related to the initial stage of the high-temperature GaN layer. Moreover, the sample deposited on ( 1 1 2 ̄ 0 ) sapphire substrate shows a higher density of V-defects incorporating a threading dislocation. The plane-view images of scanning electron microscopy also show that there is a higher density of small dark spots appearing in the paired form compared with that on (0 0 0 1) sapphire substrate, which generally correspond to screw dislocations. It is generally accepted that V-defects and screw dislocations result in phase separation in InGaN around these defects and finally give rise to a strong exciton-localization effect. This result can explain why an enhanced exciton-localization effect was recently observed in the InGaN/GaN MQW deposited on ( 1 1 2 ̄ 0 ) sapphire substrate.