Kamran Forghani

Strain and defects in Si-doped (Al)GaN epitaxial layers

Si is the most common dopant in (Al)GaN based devices acting as a donor. It has been observed that Si induces tensile strain in (Al)GaN films, which leads to an increasing tendency for cracking of such films with the increase of Si content and/or the increase of Al content. Based on x-ray investigations, the Si-doped films have a larger in-plane lattice constant than their undoped buffer layers, indicating involvement of a mechanism other than the change of lattice constants expected from an alloying effect. In this work, we present a model about Si dislocation interaction while debating other proposed models in the literature. According to our model, Si atoms are attracted to the strain dipole of edge-type dislocations in (Al)GaN films. It is expected that Si is more incorporated on that side of the dislocation, which is under compression leading to the formation of off-balanced dipoles with reduced compressive component. In response to such off-balanced dipoles—appearing as tensile dominant strain dipol...

AlGaN-Based 355 nm UV Light-Emitting Diodes with High Power Efficiency

High-efficiency AlGaN-based 355 nm UV light-emitting diodes (LEDs) grown on low-dislocation-density AlGaN/sapphire templates are reported. Flip-chip-mounted mesa LEDs exhibit an output power of 9.8 mW (22.7 mW) at a DC current of 40 mA (100 mA), corresponding to an external quantum efficiency of 7% at 40 mA. Due to the low forward voltage of only 3.8 V at 40 mA, this translates into a power efficiency of 6.5% at 40 mA. These performance data have been achieved under optimized growth conditions for the low-In-content (AlGaIn)N single quantum well and a reduced thickness of the absorbing p-GaN top layer.

Simulation supported analysis of the effect of SiNx interlayers in AlGaN on the dislocation density reduction

SiNx interlayers can act as anti-surfactants and drastically reduce the dislocation density in pure GaN layers. In our work we could observe a very efficient dislocation annihilation of the a-type threading dislocations (TD) at a fractional SiNx monolayer even in AlxGa1?xN layers with relatively high Al content of x=0.2, grown on c-plane sapphire by MOVPE. The investigations were focused on the effect of the SiNx interlayer on the dislocation density reduction of the a-type TDs. Weak-beam dark-field (WBDF) and high-resolution (HR) TEM analyses directly at the SiNx interface indicate that the most frequently occurring effect for the reduction of the a-type TDs is the conversion of an a-type TD into an a-type basal dislocation due to lateral overgrowth of SiNx by AlGaN. To confirm this effect, an appropriate dislocation model was developed for the a-type TD in AlGaN and its bending due to the SiNx nano-mask. Corresponding image calculations were performed and compared with the experiments.

Composition dependent valence band order in c-oriented wurtzite AlGaN layers

The valence band order of polar wurtzite aluminum gallium nitride (AlGaN) layers is analyzed for a dense series of samples, grown heteroepitaxially on sapphire substrates, covering the complete composition range. The excitonic transition energies, found by temperature dependent photoluminescence (PL) spectroscopy, were corrected to the unstrained state using input from X-ray diffraction. k⋅p theory yields a critical relative aluminum concentration xc=(0.09±0.05) for the crossing of the uppermost two valence bands for strain free material, shifting to higher values for compressively strained samples, as supported by polarization dependent PL. The analysis of the strain dependent valence band crossing reconciles the findings of other research groups, where sample strain was neglected. We found a bowing for the energy band gap to the valence band with Γ9 symmetry of bΓ9=0.85eV, and propose a possible bowing for the crystal field energy of bcf=−0.12eV. A comparison of the light extraction efficiency perpendic...

Study of threading dislocation density reduction in AlGaN epilayers by Monte Carlo simulation of high- resolution reciprocal-space maps of a two-layer system

High-resolution X-ray diffraction in coplanar and noncoplanar geometries has been used to investigate the influence of an SiNx nano-mask in the reduction of the threading dislocation (TD) density of high-quality AlGaN epitaxial layers grown on sapphire substrates. Our developed model, based on a Monte Carlo method, was applied to the simulation of the reciprocal-space maps of a two-layer system. Good agreement was found between the simulation and the experimental data, leading to an accurate determination of the dislocation densities as a function of the overgrowth layer thickness. The efficiency of the SiNx nano-mask was defined as the ratio of the TD densities in the AlGaN layers below and above the mask. A significant improvement in the AlGaN layer quality was achieved by increasing the overgrowth layer thickness, and a TD density reduction scaling law was established.

Cathodoluminescence and photoluminescence study on AlGaN layers grown with SiNx interlayers

In this study the optical properties of high quality c-plane AlGaN layers grown on c-plane sapphire by metal organic vapor phase epitaxy have been investigated. Submonolayers of SiNx have been deposited in situ to reduce the dislocation density. After subsequent AlGaN growth atomic force microscopy shows hexagonal hillocks. They consist of differently oriented facets, which contain different amounts of Al as we find in low temperature scanning electron microscope cathodoluminescence measurements. In macroscopic photoluminescence measurements, this leads to doublet emission bands. Further AlGaN overgrowth planarizes the surface, both emission bands coalesce, and the defect density is reduced.

Studies on Defect Reduction in AlGaN Heterostructures by Integrating an In-situ SiN Interlayer

We have decreased the dislocation density in AlxGa1-xN epitaxial layers grown on sapphire wafers by introducing an in-situ deposited SiN nano-mask layer. Taking together results obtained by transmission electron microscopy, photoluminescence, cathodoluminescence, and X-ray diffraction, we were able to derive a schematic model about the AlGaN growth on the SiN nanomask: On the open pores of the nano-mask, Ga-rich AlGaN hillocks develop, whereas on the SiN layer Al-rich AlGaN nucleates owing to the reduced selectivity of Al-containing material. Once the hillocks are formed, Ga-rich material is more efficiently incorporated on the inclined side-facets leading to an Al-rich coverage of the central c-plane part of the hillocks. We observed a bending of the dislocations towards the side-facets of the hillocks, which eventually leads to dislocation bundles with increased probability of dislocation annihilation, separated by fairly defect-free regions. Thus, we could achieve a significant reduction of the edge-type dislocation density in these epitaxial layers.

High Power Efficiency AlGaN-Based Ultraviolet Light-Emitting Diodes

High-efficiency AlGaN-based 355 nm UV light-emitting diodes (LEDs) grown on low-dislocation-density AlGaN/sapphire templates with an output power of 9.8 mW (22.7 mW) at a DC current of 40 mA (100 mA) are reported. The corresponding maximum external quantum efficiency and maximum power efficiency are 7.2 and 6.5%, respectively. Based on a rate equation model, a method is presented to derive the extraction as well as the injection and internal quantum efficiency as a function of the driving current. The thus obtained injection and internal quantum efficiencies amount to 51 and 47% at 40 mA, the extraction efficiency to 29%.

Efficient 350 nm LEDs on low edge threading dislocation density AlGaN buffer layers

Improving the crystal quality of AlGaN epitaxial layers is essential for the realization of efficient III-nitride-based light emitting diodes (LEDs) with emission wavelengths below 365 nm. Here, we report on two different approaches to improve the material quality of AlGaN buffer layers for such UV-LEDs, which are known to be effective for the MOVPE growth of GaN layers. Firstly, we grew AlGaN on thin GaN nucleation islands which exhibit a threedimensional facetted structure (3D GaN nucleation). Lateral overgrowth of these islands results in a lateral bending of dislocation lines at the growing facets. Secondly, in-situ deposited SiNx interlayers have been used as nano-masks reducing the dislocation density above the SiNx layers. Both approaches result in reduced asymmetric HRXRD ω-scan peak widths, indicating a reduced edge-type dislocation density. They can be applied to the growth of AlGaN layers with an Al concentration of at least 20%, thus suitable for LEDs emitting around 350 nm. On-wafer electroluminescence measurements at 20 mA show an increase in output power by a factor of 7 and 25 for LED structures grown on 3D GaN nucleation and SiNx interlayer, respectively, compared to structures grown on a purely 2D grown low Al-content AlGaN nucleation layer. Mesa-LEDs fabricated from the LED layer sequences grown on buffers with SiNx interlayer exhibit a low forward voltage of 3.8 V at 20 mA and a maximum continuous wave (cw) output power of 12.2 mW at 300 mA.