xN Layers Research Articles

P–n junction diodes with polarization induced p-type graded InxGa1–xN layer

In this study, p–n junction diodes with polarization induced p-type layer are demonstrated on Ga polar (0001) bulk GaN substrates. A quasi-p-type region is obtained by linearly grading the indium composition in un-doped InxGa1–xN layers from 0% to 5%, taking advantage of the piezoelectric and spontaneous polarization fields which exist in group III-nitride heterostructures grown in the typical (0001) or c-direction. The un-doped graded InxGa1–xN layers needed to be capped with a thin Mg-doped InxGa1–xN layer to make good ohmic contacts and to reduce the on-resistance of the p–n diodes. The Pol-p–n junction diodes exhibited similar characteristics compared to reference samples with traditional p-GaN:Mg layers. A rise in breakdown voltage from 30 to 110 V was observed when the thickness of the graded InGaN layer was increased from 100 to 600 nm at the same grade composition.

Influence of the heterostructure design on the optical properties of GaN and Al0.1Ga0.9N quantum dots for ultraviolet emission

The optical properties of AlyGa1-yN quantum dots (QDs), with y = 0 or y = 0.1, in an AlxGa1−xN matrix are studied. The influence of the QD layer design is investigated pointing out the correlations between the QD structural and optical properties. In a first part, the role of the epitaxial strain in the dot self-assembling process is studied by fabricating GaN QD layers on different AlxGa1−xN layers with 0.5 ≤ x ≤ 0.7. Photoluminescence (PL) measurements show the main influence of the increase of the internal electric field (Fint) on the QD optical response inducing a strong red shift in the emission energy as x increases. Time resolved combined with temperature dependent PL measurements enabled the estimation of the QD internal quantum efficiencies at low temperature showing values around 50%. In addition, a PL integrated intensity ratio up to 74% is shown, between 300 and 9 K. In the second part, the design of Al0.1Ga0.9N QDs was investigated, by varying the Al0.1Ga0.9N amount deposited. An increase of the transition energy (from 3.65 eV up to 3.83 eV) is obtained while decreasing the deposited amount. Calculations of the ground state transition energies as a function of the Al0.1Ga0.9N dot height give a value of Fint around 2.0 ± 0.5 MV/cm. Therefore, the propensity of Al0.1Ga0.9N dots to emit at much higher energies than GaN dots (a PL shift of ∼1 eV using a low excitation power) is seen as the consequence of the reduced Fint together with their smaller sizes.

Properties of GaN nanowires with ScxGa1−xN insertion

We report the first GaN nanowire structure incorporating a GaN/ScxGa1–xN axial heterostructure. A 20 nm ScxGa1−xN layer was inserted into GaN nanowires grown by a catalyst‐free self‐assembled method using plasma‐assisted molecular beam epitaxy (PA‐MBE). The insertion was characterised by energy dispersive X‐ray spectroscopy in the scanning transmission electron microscope (STEM‐EDX). High resolution electron microscopy revealed the ScxGa1−xN to have a wurtzite crystal structure as expected at this composition. Cathodoluminescence (CL) imaging in the scanning electron microscope (SEM) at 77 K revealed an emission peak at 377 nm which is not present in samples without the ScxGa1−xN insertion. A correlative study using STEM‐EDX and low temperature SEM‐CL indicates a direct link between the 377 nm emission and the ScxGa1−xN insertion. This is tentatively attributed to a type‐II band offset between the GaN and the ScxGa1−xN insertion.

Self-organization of palladium nanoislands on GaN and AlxGa1−xN/GaN heterostructures

We report on systematic study on the dewetting process of thin Pd layer and self-organized Pd nano-islands on SiO2, GaN and AlxGa1−xN/GaN heterostructures with various Al content. The influence of factors such as the thickness of metal layer, type of top layer of AlGaN/GaN heterostructures, temperature and time of annealing process on the dimensions, shapes and density of Pd islands was analyzed. Comparing the behavior of self-organization of Pd islands on Al0.25Ga0.75N/GaN and SiO2 we can conclude that solid-state dewetting process on SiO2 occures much faster than on Al0.25Ga0.75N. For substrates with SiO2 this process requires less energy and can arise for thicker layer. On the Al0.25Ga0.75N surface the islands take more crystalline shape which is probably due to surface reconstruction of Pd-Ga alloy thin layer on interface. For thin metal layer the coalescence of islands into larger islands similar to Ostwald ripening mechanism was observed. Greater surface roughness of AlxGa1−xN/GaN heterostructures with higher Al content causes an increase of surface density of islands and the reduction of their sizes which improves the roundness. In case of GaN and AlxGa1−xN layers with Al content lower than 20%, the surface degradation caused by annealing process was observed. Probably, this is due to the decomposition of layers with gallium droplet formation on catalytic metal islands.

Anisotropically biaxial strain in non-polar (112\u20130) plane InxGa1\u2212xN/GaN layers investigated by X-ray reciprocal space mapping

In this study, the indium composition x as well as the anisotropically biaxial strain in non-polar a-plane InxGa1−xN on GaN is studied by X-ray diffraction (XRD) analysis. In accordance with XRD reciprocal lattice space mapping, with increasing indium composition, the maximum of the InxGa1−xN reciprocal lattice points progressively shifts from a fully compressive strained to a fully relaxed position, then to reversed tensile strained. To fully understand the strain in the ternary alloy layers, it is helpful to grow high-quality device structures using a-plane nitrides. As the layer thickness increases, the strain of InxGa1−xN layer releases through surface roughening and the 3D growth-mode.

Infrared dielectric functions, phonon modes, and free-charge carrier properties of high-Al-content AlxGa1−xN alloys determined by mid infrared spectroscopic ellipsometry and optical Hall effect

We report on the analysis of a combined mid-infrared spectroscopic ellipsometry and mid-infrared optical Hall effect investigation of wurtzite structure c-plane oriented, crack-free, single crystalline, and high-Al-content AlxGa1−xN layers on 4H-SiC. For high-Al-content AlxGa1−xN, a two mode behavior is observed for both transverse and longitudinal branches of the infrared-active modes with E1 symmetry, while a single mode behavior is found for the longitudinal modes with A1(LO) symmetry. We report their mode dependencies on the Al content. We determine and discuss static and high frequency dielectric constants depending on x. From the analysis of the optical Hall effect data, we determine the effective mass parameter in high-Al-content AlxGa1−xN alloys and its composition dependence. Within the experimental uncertainty limits, the effective mass parameters are found isotropic, which depend linearly on the Al content. The combination of all data permits the quantification of the free electron density N and mobility parameters μ.

Exciton emission of quasi-2D InGaN in GaN matrix grown by molecular beam epitaxy

We investigate the emission from confined excitons in the structure of a single-monolayer-thick quasi-two-dimensional (quasi-2D) InxGa1−xN layer inserted in GaN matrix. This quasi-2D InGaN layer was successfully achieved by molecular beam epitaxy (MBE), and an excellent in-plane uniformity in this layer was confirmed by cathodoluminescence mapping study. The carrier dynamics have also been investigated by time-resolved and excitation-power-dependent photoluminescence, proving that the recombination occurs via confined excitons within the ultrathin quasi-2D InGaN layer even at high temperature up to ~220 K due to the enhanced exciton binding energy. This work indicates that such structure affords an interesting opportunity for developing high-performance photonic devices.

Compositionally graded InGaN layers grown on vicinal N-face GaN substrates by plasma-assisted molecular beam epitaxy

This work reports on compositionally graded (0001¯) N-polar InxGa1−xN layers. The InGaN grades with different final In compositions xf up to 0.25 were grown by plasma-assisted molecular beam epitaxy on vicinal GaN base layers with a miscut angle of 4° towards the m-direction. When increasing xf the surface morphology evolved from an interlacing finger structure, attributed to the Ehrlich-Schwöbel effect, towards fully strain-relaxed columnar features. Regardless of the crystal morphology and the strain state each graded sample exhibited a bright photoluminescence signal at room temperature spanning the whole visible range. Cross-sectional nanoscale cathodoluminescence evidenced a red-shift of the luminesced signal from 420 to 580nm along the grade and also showed strong lateral emission inhomogeneities.

Effect of growth conditions on the Al composition and optical properties of AlxGa1 −xN layers grown by atmospheric-pressure metal organic vapor phase epitaxy

The effect of growth conditions on the Al composition and optical properties of AlxGa1−xN layers grown by atmospheric-pressure metal organic vapor phase epitaxy is investigated. The Al content of the samples is varied between 3.0% and 9.3% by changing the gas flow rate of either trimethylaluminum (TMA) or trimethylgallium (TMG) while other growth parameters are kept constant. The optical properties of the AlxGa1−xN layers are studied by photoreflectance and time-resolved photoluminescence (TR-PL) spectroscopies. A degeneration in the material quality of the samples is revealed when the Al content is increased by increasing the TMA flow rate. When the TMG flow rate is decreased with a fixed TMA flow rate, the Al content of the AlxGa1−xN layers is increased and, furthermore, an improvement in the optical properties corresponding with an increase in the PL decay time is observed.

In-rich AlxIn1−xN grown by RF-sputtering on sapphire: from closely-packed columnar to high-surface quality compact layers

The structural, morphological, electrical and optical properties of In-rich AlxIn1−xN (0 < x < 0.39) layers grown by reactive radio-frequency (RF) sputtering on sapphire are investigated as a function of the deposition parameters. The RF power applied to the aluminum target (0 W–150 W) and substrate temperature (300 °C–550 °C) are varied. X-ray diffraction measurements reveal that all samples have a wurtzite crystallographic structure oriented with the c-axis along the growth direction. The aluminum composition is tuned by changing the power applied to the aluminum target while keeping the power applied to the indium target fixed at 40 W. When increasing the Al content from 0 to 0.39, the room-temperature optical band gap is observed to blue-shift from 1.76 eV to 2.0 eV, strongly influenced by the Burstein–Moss effect. Increasing the substrate temperature, results in an evolution of the morphology from closely-packed columnar to compact. For a substrate temperature of 500 °C and RF power for Al of 150 W, compact Al0.39In0.61N films with a smooth surface (root-mean-square surface roughness below 1 nm) are produced.

Structural and optical properties of AlxGa1−xN (0.33 ≤ x ≤ 0.79) layers on high-temperature AlN interlayer grown by metal organic chemical vapor deposition

High-Al-content AlxGa1−xN films with x varying from 0.33 to 0.79 were grown on GaN templates with the high temperature AlN (HT-AlN) interlayer by metal organic chemical vapor deposition (MOCVD). The best crystalline quality, among these AlxGa1−xN alloys, can be obtained for an AlN mole fraction x = 0.55, where the full-width at half-maximum of the Al0.55Ga0.45N (0002) diffraction peak was measured to be 259 arcsec by high resolution X-ray diffraction (HRXRD). The screw threading dislocation (TDs) density was 2 × 108 cm−2 evaluated by transmission electron microscope (TEM), which agreed with the calculations from Williamson-Hall plots. Moreover, cross-sectional TEM indicated that the HT-AlN interlayer could sufficiently reduce the threading dislocations (TDs) through generation of V trenches in the HT-AlN interlayer, since the TDs propagated along the V trenches, then bent into basal planes and annihilated with other dislocations. The study of optical properties indicated that obvious S-shape of temperature dependence on emission energy was observed for Al0.55Ga0.45N layers, which was attributed to exciton localization with energy (Eloc) ∼14.95 meV at 10 K resulting from potential fluctuation and band tail states. The time-resolved photoluminescence (TRPL) curves showed a bi-exponential decay at low temperature. The fast decay time implied the presence of the localized excitons enhancing radiative recombination, while the quite slow one was due to the dominance of trapping mechanisms originating from cation vacancy complexes and the VIII-related complexes.

Buffer effects on the mosaic structure of the HR-GaN grown on 6H-SiC substrate by MOCVD

High-resistive GaN (>108 Ω cm) layers have been grown with different buffer structures on 6H-SiC substrate using metalorganic chemical vapor deposition reactor. Different combination of the GaN/AlN super lattice, low temperature AlN, high temperature AlN and AlxGa1−xN (x ≈ 0.67) layers were used in the buffer structures. The growth parameters of the buffer layers were optimized for obtaining a high-resistive GaN epilayer. The mosaic structure parameters, such as lateral and vertical coherence lengths, tilt and twist angle (and heterogeneous strain), and dislocation densities (edge and screw dislocations) of the high-resistive GaN epilayers have been investigated using x-ray diffraction measurements. In addition, the residual stress behaviors in the high-resistive GaN epilayers were determined using both x-ray diffraction and Raman measurements. It was found that the buffer structures between the HR-GaN and SiC substrate have been found to have significant effect on the surface morphology and the mosaic structures parameters. On the other hand, both XRD and Raman results confirmed that there is low residual stress in the high-resistive GaN epilayers grown on different buffer structures.

Surface photovoltage studies of p-type AlGaN layers after reactive-ion etching

The surface photovoltage (SPV) technique was used to study the surface and electrical properties of Mg-doped, p-type AlxGa1−xN (0.06 &amp;lt; x &amp;lt; 0.17) layers. SPV measurements reveal significant deviation from previous SPV studies on p-GaN:Mg thin films and from the predictions of a thermionic model for the SPV behavior. In particular, the SPV of the p-AlGaN:Mg layers exhibited slower-than-expected transients under ultraviolet illumination and delayed restoration to the initial dark value. The slow transients and delayed restorations can be attributed to a defective surface region which interferes with normal thermionic processes. The top 45 nm of the p-AlGaN:Mg layer was etched using a reactive-ion etch which caused the SPV behavior to be substantially different. From this study, it can be concluded that a defective, near-surface region is inhibiting the change in positive surface charge by allowing tunneling or hopping conductivity of holes from the bulk to the surface, or by the trapping of electrons traveling to the surface by a high concentration of defects in the near-surface region. Etching removes the defective layer and reveals a region of presumably higher quality, as evidenced by substantial changes in the SPV behavior.

Growth of free-standing bulk wurtzite AlxGa1−xN layers by molecular beam epitaxy using a highly efficient RF plasma source

The recent development of group III nitrides allows researchers world-wide to consider AlGaN based light emitting diodes as a possible new alternative deep ultra–violet light source for surface decontamination and water purification. In this paper we will describe our recent results on plasma-assisted molecular beam epitaxy (PA-MBE) growth of free-standing wurtzite AlxGa1−xN bulk crystals using the latest model of Riber's highly efficient nitrogen RF plasma source. We have achieved AlGaN growth rates up to 3µm/h. Wurtzite AlxGa1−xN layers with thicknesses up to 100μm were successfully grown by PA-MBE on 2-inch and 3-inch GaAs (111)B substrates. After growth the GaAs was subsequently removed using a chemical etch to achieve free-standing AlxGa1−xN wafers. Free-standing bulk AlxGa1−xN wafers with thicknesses in the range 30–100μm may be used as substrates for further growth of AlxGa1−xN-based structures and devices. High Resolution Scanning Transmission Electron Microscopy (HR-STEM) and Convergent Beam Electron Diffraction (CBED) were employed for detailed structural analysis of AlGaN/GaAs (111)B interface and allowed us to determine the N-polarity of AlGaN layers grown on GaAs (111)B substrates. The novel, high efficiency RF plasma source allowed us to achieve free-standing AlxGa1−xN layers in a single day's growth, making this a commercially viable process.

Polarization induced three-dimensional hole gas in compositionally graded InxGa1−xN layer

A polarization-induced three-dimensional hole gas (3DHG) was demonstrated in undoped and compositionally graded InxGa1−xN layers. All samples were grown on Ga-face bulk GaN substrates by metal organic chemical vapor deposition. A high hole concentration of 2.8 × 1018 cm−3 was obtained in a 100-nm-thick InxGa1−xN layer where the indium composition was graded from x = 0 to x = 0.2. 3DHG density control by varying the indium composition and thickness of a compositionally graded InxGa1−xN layer was also demonstrated.

Analysis of the Tb3+ recombination in ion implanted AlxGa1−xN (0≤x≤1) layers

AlxGa1−xN layers with different AlN molar fractions, 0≤x≤1, implanted with Tb3+ ions were characterized using structural and optical techniques. After adequate thermal annealing treatments, all the layers evidence the 5D4→7FJ intra-4f8 transitions. The green emission from the 5D4 emitting level can be identified up to room temperature in samples with x≥0.53. The preferential population paths of the Tb3+ luminescence were assessed by photoluminescence excitation at RT where two main broad subgap excitation bands peaked at ~270nm and ~300nm were found for the layers with higher aluminium content. The analysis of the temperature-dependent Tb3+ integrated luminescence reveals distinct activation energies for different compositions, with AlN:Tb showing the highest thermal stability for the intraionic luminescence. The ion excitation paths are discussed on a basis of excitonic features and compared with the trend observed for other rare earth ions in nitrides, as well as theoretical predictions. A single exponential decay for the 5D4→7F6 transition was measured at RT and the lifetime of the transition was found to increase with increasing composition revealing the sensitivity of the ions to the surrounding medium.

Deep level defects in N-rich and In-rich InxGa1−XN: in composition dependence

This study focuses on the presence and electronic properties of deep levels in In- and N-rich InxGa1−xN materials with different growth temperature, causing the different In composition, grown by nitrogen plasma-assisted molecular beam epitaxy (PAMBE). Semi-transparent Ag/Au Schottky contacts were formed on the InxGa1−xN layers and the device performances has been investigated with varying In contents. Increase in the leakage current density with the increase of In content has been observed in both In- and N-rich samples. Deep level optical spectroscopy has shown three deeper levels in both In-rich and N-rich InxGa1−xN layers. 1.4–1.6 eV level below the conduction band has been observed in all the samples, while the energy values of the other deep levels varies by sample to sample. The deepest levels are found to be the most prominent with the concentration in the range of 1016 cm−3 for high In content InxGa1−xN for both In-rich and N-rich growth conditions. Almost 4–5 times higher total trap concentrations have been observed for higher In InxGa1−xN sample.

Kinetics of AlGaN metal–organic vapor phase epitaxy for deep-UV applications

AlxGa1−xN layers with high aluminum content of x ∼ 0.68–0.73 were grown in an 11 × 2-in. AIX 2400 G3 HT planetary reactor by metal–organic vapor phase epitaxy. Growth trends are analyzed by reaction-transport modeling in a wide range of growth conditions. Gas-phase nucleation resulting in both Al and Ga consumption into nanoparticles is a major mechanism affecting the growth efficiencies of AlN and GaN. Process windows suitable to grow multiple quantum wells (MQWs) for deep UV applications are found for a range of pressures, temperatures, and V/III ratios.

On the hole accelerator for III-nitride light-emitting diodes

In this work, we systematically conduct parametric studies revealing the sensitivity of the hole injection on the hole accelerator (a hole accelerator is made of the polarization mismatched p-electron blocking layer (EBL)/p-GaN/p-AlxGa1−xN heterojunction) with different designs, including the AlN composition in the p-AlxGa1−xN layer, and the thickness for the p-GaN layer and the p-AlxGa1−xN layer. According to our findings, the energy that the holes obtain does not monotonically increase as the AlN incorporation in the p-AlxGa1−xN layer increases. Meanwhile, with p-GaN layer or p-AlxGa1−xN layer thickening, the energy that the holes gain increases and then reaches a saturation level. Thus, the hole injection efficiency and the device efficiency are very sensitive to the p-EBL/p-GaN/p-AlxGa1−xN design, and the hole accelerator can effectively increase the hole injection if properly designed.

Quantification of matrix and impurity elements in AlxGa1−xN compounds by secondary ion mass spectrometry

The authors describe a comprehensive secondary ion mass spectrometry (SIMS) calibration procedure for the quantification of matrix and impurity elements of epitaxially grown AlxGa1−xN layers over the full compositional range of 0 ≤ x ≤ 1. For that a set of eight samples was grown by metalorganic vapor phase epitaxy, characterized with respect to AlN mole fraction and implanted with impurity and dopant elements (H, C, O, and Si). The compositional analysis using various techniques yielded consistent Al contents x with an accuracy of ±1%. For the quantitative characterization of impurities by SIMS, calibration curves were generated using a 14.5 keV Cs+ primary beam at an angle of incidence of 25°. Measured sputter rates decrease with a nearly linear slope as a function of Al content in the range of 0 ≤ x &amp;lt; 0.48. At higher Al concentrations the sputter rates show only a weak dependence on AlN mole fraction. Matrix ion intensity ratios of AlCs+/GaCs+ change linearly with direct and inverse proportionality as a function of x/(1−x). The absolute sensitivity factors for H, C, and Si follow an exponential reduction with increasing AlN mole fraction only for lower Al concentrations (0 ≤ x &amp;lt; 0.48). The calculated relative sensitivity factors are determined by the respective reference intensities depending on the AlN mole fraction.