xN Epitaxial Layers Research Articles

Electron paramagnetic resonance of VN–VGa complex in BGaN

A metastable photoinduced electron paramagnetic resonance (EPR) signal at low temperatures is reported for GaN alloyed with boron (i.e., BxGa1−xN) epitaxial layers grown at temperatures ranging from 840 to 1090° C. An isotropic EPR line with g = 2.004 is observed with an intensity depending on the growth temperature for all the samples with boron content between 0.73% and 2.51%. The temperature dependence of EPR intensities is compared with the results of high-resolution photoinduced transient spectroscopy. This enables a link between particular traps and the EPR signal. The activation energies of these traps are consistent with the theoretical position of the VN–VGa complex. Thermal annihilation of the EPR signal with 30 meV activation energy corresponds to shallow donor ionization. A model explaining the light-induced EPR signal that involves redistribution of electrons between deep and shallow donors, mediated by photoionization to the conduction band, is proposed.

Low Al-content n-type Al[formula omitted]Ga[formula omitted]N layers with a high-electron-mobility grown by hot-wall metalorganic chemical vapor deposition

In this work, we demonstrate the capability of the hot-wall metalorganic chemical vapor deposition to deliver high-quality n-AlxGa1−xN (x = 0–0.12, [Si] = 1×1017 cm−3) epitaxial layers on 4H-SiC(0001). All layers are crack-free, with a very small root mean square roughness (0.13–0.25 nm), homogeneous distribution of Al over film thickness and a very low unintentional incorporation of oxygen at the detection limit of 5×1015 cm−3 and carbon of 2×1016 cm−3. Edge type dislocations in the layers gradually increase with increasing Al content while screw dislocations only raise for x above 0.077. The room temperature electron mobility of the n-AlxGa1−xN remain in the range of 400–470 cm2/(V.s) for Al contents between 0.05 and 0.077 resulting in comparable or higher Baliga figure of merit with respect to GaN, and hence demonstrating their suitability for implementation as drift layers in power device applications. Further increase in Al content is found to result in significant deterioration of the electrical properties.

MBE growth and donor doping of coherent ultrawide bandgap AlGaN alloy layers on single-crystal AlN substrates

Single-crystal Aluminum Nitride (AlN) crystals enable the epitaxial growth of ultrawide bandgap Al(Ga)N alloys with drastically lower extended defect densities. Here, we report the plasma-MBE growth conditions for high Al-composition AlGaN alloys on single-crystal AlN substrates. An AlGaN growth guideline map is developed, leading to pseudomorphic AlxGa1−xN epitaxial layers with x ∼0.6–1.0 Al contents at a growth rate of ∼0.3 μm/h. These epitaxial layers exhibit atomic steps, indicating step flow epitaxial growth, and room-temperature band edge emission from ∼4.5 to 5.9 eV. Growth conditions are identified in which the background impurity concentrations of O, C, Si, and H in the MBE layers are found to be very near or below detection limits. An interesting Si segregation and gettering behavior is observed at the epitaxial AlGaN/AlN heterojunction with significant implications for the formation and transport of 2D electron or hole gases. Well-controlled intentional Si doping ranging from ∼2 × 1017 to 3 × 1019 atoms/cm3 is obtained, with sharp dopant density transition profiles. In Si-doped Al0.6Ga0.4N epilayers, a room-temperature free electron concentration of ∼3 × 1019/cm3, an electron mobility of ∼27 cm2/V s, and an n-type resistivity of ∼7.5 m Ω cm are obtained. The implications of these findings on electronic and photonic devices on single-crystal AlN substrates are discussed.

Strain-stress study of AlxGa1\u2212xN/AlN heterostructures on c-plane sapphire and related optical properties

This work presents a systematic study of stress and strain of AlxGa1−xN/AlN with composition ranging from GaN to AlN, grown on a c-plane sapphire by metal-organic chemical vapor deposition, using synchrotron radiation high-resolution X-ray diffraction and reciprocal space mapping. The c-plane of the AlxGa1−xN epitaxial layers exhibits compressive strain, while the a-plane exhibits tensile strain. The biaxial stress and strain are found to increase with increasing Al composition, although the lattice mismatch between the AlxGa1−xN and the buffer layer AlN gets smaller. A reduction in the lateral coherence lengths and an increase in the edge and screw dislocations are seen as the AlxGa1−xN composition is varied from GaN to AlN, exhibiting a clear dependence of the crystal properties of AlxGa1−xN on the Al content. The bandgap of the epitaxial layers is slightly lower than predicted value due to a larger tensile strain effect on the a-axis compared to the compressive strain on the c-axis. Raman characteristics of the AlxGa1−xN samples exhibit a shift in the phonon peaks with the Al composition. The effect of strain on the optical phonon energies of the epitaxial layers is also discussed.

Electron effective mass in In0.33Ga0.67N determined by mid-infrared optical Hall effect

Mid-infrared optical Hall effect measurements are used to determine the free charge carrier parameters of an unintentionally doped wurtzite-structure c-plane oriented In0.33Ga0.67N epitaxial layer. Room temperature electron effective mass parameters of m⊥*=(0.205±0.013) m0 and m∥*=(0.204±0.016) m0 for polarization perpendicular and parallel to the c-axis, respectively, were determined. The free electron concentration was obtained as (1.7 ± 0.2) × 1019 cm−3. Within our uncertainty limits, we detect no anisotropy for the electron effective mass parameter and we estimate the upper limit of the possible effective mass anisotropy as 7%. We discuss the influence of conduction band nonparabolicity on the electron effective mass parameter as a function of In content. The effective mass parameter is consistent with a linear interpolation scheme between the conduction band mass parameters in GaN and InN when the strong nonparabolicity in InN is included. The In0.33Ga0.67N electron mobility parameter was found to be anisotropic, supporting previous experimental findings for wurtzite-structure GaN, InN, and AlxGa1−xN epitaxial layers with c-plane growth orientation.

Dependence of Al incorporation on growth temperature during laser molecular beam epitaxy of AlxGa1−xN epitaxial layers on sapphire (0001)

We report the successful growth of AlxGa1−xN (0 ≤ x ≤ 0.25) epitaxial films on c-plane sapphire substrate by using laser molecular beam epitaxy technique. The role of growth temperature on the Al incorporation and the structural, electronic and optical properties of the AlxGa1−xN epitaxial layers grown in the temperature range 500–700 °C have been systematically studied. The atomic force microscopy analysis shows that the grain size of AlxGa1−xN increases with increase in growth temperature and flat surface epilayers are obtained at ≥ 600 °C. The Al incorporation is confirmed with high resolution x-ray diffraction, x-ray photo electron microscopy and photoluminescence studies. It is observed that the growth temperature plays a critical role in determining the Al composition, which increases with increasing growth temperature. AlxGa1−xN layer with about 23% of Al composition is obtained on sapphire (0001) substrate at a growth temperature of 700 °C, which is about 100–150 °C lower than the conventional molecular beam epitaxy growth.

Strongly nonparabolic variation of the band gap in InxAl1−xN with low indium content

80–120 nm thick InxAl1−xN epitaxial layers with 0 < x < 0.224 were grown by metalorganic vapour phase epitaxy on AlN/Al2O3-templates. The composition was varied through control of the growth temperature. The composition dependence of the band gap was estimated from the photoluminescence excitation absorption edge for 0 < x < 0.11 as the material with higher In content showed no luminescence under low excitation. A very rapid decrease in band gap was observed in this range, dropping down below 5.2 eV at x = 0.05, confirming previous theoretical work that used a band-anticrossing model to describe the strongly x-dependent bowing parameter, which in this case exceeds 25 eV in the x → 0 limit. A double absorption edge observed for InAlN with x < 0.01 was attributed to crystal-field splitting of the highest valence band states. Our results indicate also that the ordering of the valence bands is changed at much lower In contents than one would expect from linear interpolation of the valence band parameters. These findings on band gap bowing and valence band ordering are of direct relevance for the design of InAlN-containing optoelectronic devices.

Surface properties of AlN and Al x Ga1−x N epitaxial layers characterized by angle resolved X-ray photoelectron spectroscopy

The AlN and AlxGa1−xN (x = 0.35, 0.73) epitaxial layers were grown on c-plane sapphire substrates by metal–organic chemical vapor deposition. The surface properties of these layers were quantitatively characterized by angle resolved X-ray photoelectron spectroscopy. The characterization results revealed that the air-exposed surfaces of the Al-containing AlN and AlGaN films were covered by a thin Al-oxide layer due to the large chemical affinity of aluminum to oxygen. The as-deposited AlN film was proved to be a nitrogen-deficient film. On the other hand, the Ga Auger effect for the as-deposited AlGaN film was found to be greatly suppressed by increasing Al composition. Moreover, the Al composition in the as-deposited AlGaN films was demonstrated to be non-uniform since Al atom is more easily oxidized than Ga atom, especially for the AlGaN film with relatively high Al content. Our work will be beneficial to understanding and controlling of the surface properties for Al-containing III-nitrides.

Strain Distributions in Non-Polar a-Plane InxGa1−xN Epitaxial Layers on r-Plane Sapphire Extracted from X-Ray Diffraction

By using x-ray diffraction analysis, we investigate the major structural parameters such as strain state and crystal quality of non-polar a-plane InxGa1−xN thin films grown on r-sapphire substrates by metalorganic chemical vapour deposition. The results of the inplane grazing incidence diffraction technique are analyzed and compared with a complementary out-of-plane high resolution x-ray diffraction technique. When the indium composition is low, the a-plane InxGa1−xN layer is tensile strain in the growth direction (a-axis) and compressive strain in the two in-plane directions (m-axis and c-axis). The strain status becomes contrary when the indium composition is high. The stress in the m-axis direction σyy is larger than that in the c-axis direction σzz. Furthermore, strain in the two in-plane directions decrease and the crystal quality becomes better with the growing of the InxGa1−xN film.

Evolution of the mosaic structure in InGaN layer grown on a thick GaN template and sapphire substrate

The InxGa1−xN epitaxial layers, with indium (x) concentration changes between 0.16 and 1.00 (InN), were grown on GaN template/(0001) Al2O3 substrate by metal organic chemical vapour deposition. The indium content (x), lattice parameters and strain values in the InGaN layers were calculated from the reciprocal lattice mapping around symmetric (0002) and asymmetric (10–15) reflection of the GaN and InGaN layers. The characteristics of mosaic structures, such as lateral and vertical coherence lengths, tilt and twist angle and heterogeneous strain and dislocation densities (edge and screw dislocations) of the InGaN epilayers and GaN template layers were investigated by using high-resolution X-ray diffraction (HR-XRD) measurements. With a combination of Williamson–Hall (W-H) measurements and the fitting of twist angles, it was found that the indium content in the InGaN epilayers did not strongly effect the mosaic structures’ parameters, lateral and vertical coherence lengths, tilt and twist angle, or heterogeneous strain of the InGaN epilayers.

Luminescence studies in InxGa1−xN epitaxial layers with different indium contents

The optical properties of InxGa1−xN epitaxial layers (x=0.02, 0.04, 0.11, 0.15, 0.30 and 0.33) grown by metalorganic chemical vapor deposition (MOCVD) have been investigated by temperature-dependent photoluminescence (PL) measurement. The surface morphologies of InGaN samples are studied by scanning electron microscopy (SEM) images. The PL feature at 12K has shown an increase in full-width at half-maximum (FWHM) with increasing In content. An anomalous S-shaped temperature dependence of the PL peak energy exhibited by InGaN films with higher In content enabled the evaluation of the exciton localization energy. The broadened FWHM and S-shaped emission shift are attributed to larger compositional fluctuation due to compositional inhomogeneity of In. Additionally, the luminescence mechanism relating to the phase separation has to be considered for the much larger FWHM value and the pronounced S-shaped behavior for the InGaN samples with In content of 0.30 and 0.33.

Carrier dynamics in epilayers and nanocolumns of ternary AlGaN with a tunable bandgap

We apply several optical time-resolved techniques to investigate the dynamics of excess carriers in AlxGa1−xN nanocolumns grown on (111) silicon substrates and in thick AlxGa1−xN epitaxial layers deposited on sapphire. We demonstrate that carrier lifetime drops in nanocolumns by several times if compared to epilayers with similar Al content; in addition, recombination rate displays a strong nonlinearity on excitation. On the other hand, carrier localization effects observed in the epilayers completely disappear in nanocolumns.

SIMS‐EDX system for the quantitative analysis of solids

We incorporated a Si drift X‐ray detector into a Cameca ims‐6f ion microprobe to perform a nondestructive energy‐dispersive X‐ray spectrometric analysis inside the SIMS instrument. Using thick AlxGa1−xN epitaxial layers and thin SixGe1−x films as examples, we demonstrate that integration of the EDX method (device) with the SIMS technique (instrument) can be extremely efficient for the quantitative analysis of complex materials. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Photoluminescence and Raman scattering in spatially inhomogeneous heteroepitaxial InGaN layers

Spatial inhomogeneities of the indium distribution in In x Ga1–x N epitaxial layers grown on sapphire substrate with a GaN buffer layer were investigated using photoluminescence (PL) in addition to confocal scanning Raman spectroscopy (RS) and PL. Broad emission bands from In-enriched InGaN nanoclusters (700–900 nm) and from the volume outside the clusters (about 460 nm) were observed in PL spectra of an epitaxial InGaN layer with an average In content of 25.7%. It was established that larger micro-PL intensities corresponded to energetically shallower clusters. The observed broadly asymmetric A1(LO) RS band of InGaN confirmed that the In concentration in the layer was highly variable. Modeling the LO phonon band by two Lorentzian curves gave an average In concentration of 21% in the volume outside the clusters and 37% in the nanoclusters, which was considerably higher than the average concentration in the layer and agreed well with their PL band positions.

Silicon concentration dependence of optical polarization in AlGaN epitaxial layers

The optical polarization of Si-doped AlxGa1−xN epitaxial layers (x=0.37–0.95) has been studied by means of photoluminescence (PL) spectroscopy. The predominant polarization component of the band-edge PL switched from E⊥c to E∥c at an Al composition between 0.68 and 0.81. This critical Al composition was much higher than in previous reports for AlGaN epitaxial layers. In addition, the predominant polarization in Al0.55Ga0.45N epitaxial layers switched from E⊥c to E∥c with increasing Si concentration. Therefore, the topmost valence band changed from the heavy-hole band to the crystal-field split-off-hole band with decreasing in-plane compressive strain induced by Si doping.

The effect of temperature and ammonia flux on the surface morphology and composition of In xAl 1−xN epitaxial layers

An interesting recent development in the Group III nitrides is the growth of InAlN lattice matched to GaN, with applications in distributed Bragg reflectors (DBRs), high electron mobility transistors (HEMTs) and as etch-layers. This work presents a systematic study of the effects of changing the key growth conditions of ammonia flux and growth temperature in InAlN growth by metal-organic vapour phase epitaxy (MOPVE) and describes our current optimised parameter set. We also particularly concentrate on the details of surface morphology assessed by atomic force microscopy (AFM). The nanoscale surfaces are characterised by low hillocks and dislocation pits, while at a larger scale microscopic indium droplets are also present. However, these droplets are eliminated when the layers are capped with GaN. Other trends observed are that increasing the growth temperature will lower the indium incorporation approximately linearly at a rate of approximately 0.25% per °C, and that increasing the ammonia flux from 44.6 to 178.6 mmol min −1 increased the indium incorporation, but further increases to 446 mmol min −1 did not result in any further increase.

The structural quality of AlxGa1−xN epitaxial layers grown by digitally alloyed modulated precursor epitaxy determined by transmission electron microscopy

Al x Ga 1 − x N layers of varying composition (0.5&amp;lt;xAl&amp;lt;1.0) grown in the digitally alloyed modulated precursor epitaxial regime employing AlN and GaN binary sublayers by metal organic chemical vapor deposition on AlN templates were characterized by transmission electron microscopy techniques. Fine lamellae were observed in bright field images that indicate a possible variation in composition due to the modulated nature of growth. In higher Ga content samples (xAl&amp;lt;0.75), a compositional inhomogeneity associated with thicker island regions was observed, which is determined to be due to large Ga-rich areas formed at the base of the layer. Possible causes for the separation of Ga-rich material are discussed in the context of the growth regime used.

Stokes Shift and Band Gap Bowing in InxGa1-xN (0.060 ≤ x ≤ 0.105) Grown by Metalorganic Vapour Phase Epitaxy

We presented the results of electrical and optical studies of the properties of InxGa1−xN epitaxial layers (0.060 ≤ x ≤ 0.105) grown by metalorganic vapour phase epitaxy. Resistivity and Hall effect measurements of the samples were carried out at room temperature. Optical properties of the samples were characterized by photoluminescence and optical absorption spectroscopy. The comparison between the photoluminescence and the optical absorption measurements gives the Stokes shift. We explained the observed Stokes shift in terms of Burstein–Moss effect. The band gap versus composition plot for InxGa1−xN alloys is well fitted with a bowing parameter of ≈ 3.6 eV. PACS numbers: 73.61.−r, 78.20.−e, 78.40.Fy, 78.55.−m

Generation of misfit dislocations by basal-plane slip in InGaN∕GaN heterostructures

The authors have observed that for InxGa1−xN epitaxial layers grown on bulk GaN substrates exhibit slip on the basal plane, when in the presence of free surfaces that intercept the heterointerface and for indium compositions x⩾0.07. This leads to almost complete relaxation of the local misfit strain by generation of radial-shape dislocation half loops. For x⩾0.17, generation of straight misfit dislocations by glide on the secondary ⟨112¯3⟩ {112¯2} slip system is observed, in addition to the radial-shape half loops at surface pits. These two mechanisms act independently with no observed interaction between them, leading to the conclusion that slip on the basal plane occurs first during the growth process. The secondary slip system is activated later and involves a significantly higher critical stress energy.

Guided-wave acousto-optic diffraction in AlxGa1−xN epitaxial layers

The acousto-optic (AO) diffraction of guided optical waves from surface acoustic waves in AlxGa1−xN layers grown on sapphire substrates by Migration Enhanced Metalorganic Chemical Vapor Deposition (MEMGCVD™) technique was studied at the optical wavelengths of 442 and 633nm and acoustic wavelength of 16μm. In the near-to-Bragg diffraction regime, the diffraction efficiency from 90% to 95% was attained at SAW powers of 0.28 and 0.72W for the blue and red light, respectively. The simulation using photo-elastic and electro-optic constants reported in literature revealed the prevailing contribution of the photo-elastic effect to the AO diffraction. The calculated SAW power required to attain the diffraction maximum was about seven times larger than the measured values. This discrepancy implies that the photo-elastic constants of AlN and GaN available from literature are underestimated. The increase in the diffraction efficiency with the decreasing optical wavelength is in a good agreement with the theoretical prediction. This feature makes AlGaN very promising for AO applications in the deep UV region.