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Wavelength-dispersive X-ray (WDX) spectroscopy was used to measure silicon atom concentrations in the range 35–100 ppm [corresponding to (3–9) × 1018 cm−3] in doped AlxGa1–xN films using an electron probe microanalyser also equipped with a cathodoluminescence (CL) spectrometer. Doping with Si is the usual way to produce the n-type conducting layers that are critical in GaN- and AlxGa1–xN-based devices such as LEDs and laser diodes. Previously, we have shown excellent agreement for Mg dopant concentrations in p-GaN measured by WDX with values from the more widely used technique of secondary ion mass spectrometry (SIMS). However, a discrepancy between these methods has been reported when quantifying the n-type dopant, silicon. We identify the cause of discrepancy as inherent sample contamination and propose a way to correct this using a calibration relation. This new approach, using a method combining data derived from SIMS measurements on both GaN and AlxGa1–xN samples, provides the means to measure the Si content in these samples with account taken of variations in the ZAF corrections. This method presents a cost-effective and time-saving way to measure the Si doping and can also benefit from simultaneously measuring other signals, such as CL and electron channeling contrast imaging.

Wavelength-dispersive X-ray (WDX) spectroscopy was used to measure silicon atom concentrations in the range 35-100 ppm [corresponding to (3-9) × 1018 cm-3] in doped AlxGa1-xN films using an electron probe microanalyser also equipped with a cathodoluminescence (CL) spectrometer. Doping with Si is the usual way to produce the n-type conducting layers that are critical in GaN- and AlxGa1-xN-based devices such as LEDs and laser diodes. Previously, we have shown excellent agreement for Mg dopant concentrations in p-GaN measured by WDX with values from the more widely used technique of secondary ion mass spectrometry (SIMS). However, a discrepancy between these methods has been reported when quantifying the n-type dopant, silicon. We identify the cause of discrepancy as inherent sample contamination and propose a way to correct this using a calibration relation. This new approach, using a method combining data derived from SIMS measurements on both GaN and AlxGa1-xN samples, provides the means to measure the Si content in these samples with account taken of variations in the ZAF corrections. This method presents a cost-effective and time-saving way to measure the Si doping and can also benefit from simultaneously measuring other signals, such as CL and electron channeling contrast imaging.

In this work, the growth and conductivity of semipolar AlxGa1−xN:Si with (11-22) orientation are investigated. AlxGa1−xN:Si (x = 0.60 ± 0.03 and x = 0.80 ± 0.02) layers were grown with different SiH4 partial pressures, and the electrical properties were determined using Hall measurements at room temperature. The aluminum mole fraction was measured by wavelength dispersive x-ray spectroscopy and x-ray diffraction, and the Si-concentration was measured by wavelength dispersive x-ray spectroscopy and secondary ion mass spectroscopy. Layer resistivities as low as 0.024 Ω cm for x = 0.6 and 0.042 Ω cm for x = 0.8 were achieved. For both aluminum mole fractions, the resistivity exhibits a minimum with the increasing Si concentration, which can be explained by compensation due to the formation of cation vacancy complexes at high doping levels. The onset of self-compensation occurs at larger estimated Si concentrations for larger Al contents.

In this work, reduced threading dislocation density AlN on (0 0 0 1) 6H-SiC was realized through the use of reduced temperature AlN interlayers in the metalorganic chemical vapor deposition growth. We explored the dependence of the interlayer growth temperature on the AlN crystal quality, defect density, and surface morphology. The crystal quality was characterized using omega rocking curve scans and the threading dislocation density was determined by plan view transmission electron microscopy. The growth resulted in a threading dislocation density of 7 × 108 cm−2 indicating a significant reduction in the defect density of AlN in comparison to direct growth of AlN on SiC (∼1010 cm−2). Atomic force microscopy images demonstrated a clear step-terrace morphology that is consistent with step flow growth at high temperature. Reducing the interlayer growth temperature increased the TD inclination and thus enhanced TD-TD interactions. The TDD was decreased via fusion and annihilation reactions.

Funding Information: This work was funded by the KACST-KAUST-UCSB Technology transfer program and the Solid State Lighting and Energy Electronics Center (SSLEEC) at UC Santa Barbara; a part of this work was carried out in the California NanoSystems Institute at UCSB. The research reported here made use of shared facilities of the UCSB MRSEC (Grant No. NSF DMR 1720256). A portion of this research was conducted in the UCSB nanofabrication facility, NSF NNIN network (Grant No. ECS-0335765). This work was partially funded by the Academy of Finland, Project No. 315082. The authors would like to gratefully thank Dr. Tom Mates for SIMS measurements and Dr. Stacia Keller and Dr. Mohammed Abo Alreesh for their insightful inputs. In addition, the authors would like to thank Dr. Youli Li from MRL and the cleanroom staff at the UCSB nanofabrication facility for the technical support provided. Publisher Copyright: © 2021 Author(s). The impact of AlGaN growth conditions on AlGaN:Si resistivity and surface morphology has been investigated using metalorganic chemical vapor deposition. Growth parameters including growth temperature, growth rate, and trimethylindium (TMI) flow have been systematically studied to minimize the resistivity of AlGaN:Si. We observed a strong anticorrelation between AlGaN:Si conductivity and growth temperature, suggesting increased silicon donor compensation at elevated temperatures. Secondary ion mass spectrometry and positron annihilation spectroscopy ruled out compensation by common impurities or group-III monovacancies as a reason for the observed phenomenon, in contrast to theoretical predictions. The underlying reason for AlGaN:Si resistivity dependence on growth temperature is discussed based on the possibility of silicon acting as a DX center in Al0.65Ga0.35N at high growth temperatures. We also show remarkable enhancement of AlGaN:Si conductivity by introducing TMI flow during growth. A minimum resistivity of 7.5 mΩ cm was obtained for n-type Al0.65Ga0.35N, which is among the lowest reported resistivity for this composition.

Coherent In x Al 1−x N (x = 0.15 to x = 0.28) films were grown by metalorganic chemical vapor deposition on GaN templates to investigate if the films obey Vegard’s Law by comparing the film stress-thickness product from wafer curvature before and after In x Al 1−x N deposition. The In composition and film thickness were verified using atom probe tomography and high resolution X-ray diffraction, respectively. Ex-situ curvature measurements were performed to analyze the curvature before and after the In x Al 1−x N deposition. At ∼In 0.18 Al 0.82 N, no change in curvature was observed following InAlN deposition; confirming that films of this composition are latticed matched to GaN, obeying Vegard’s law. The relaxed a 0 - and c 0 - lattice parameters of In x Al 1−x N were experimentally determined and in agreement with lattice parameters predicted by Vegard’s law.

AlGaN-based UV-C LEDs (260-300 nm) remain inefficient compared to InGaN visible LEDs due to optically absorptive layers limiting light extraction, optical polarization, and poor material quality. Sapphire, the most popular substrate material, is transparent and inexpensive but has many disadvantages in material quality and device performance. In contrast, SiC has small lattice mismatch with AlN (~1%), similar crystal structure, more chemically stable and contains no oxygen, which degrades the IQE and compensates holes. We report low threading dislocations density (TDD) AlN on SiC (TDD < 7x108cm-2) by metalorganic chemical vapor deposition (MOCVD). We demonstrate innovative thin-film flipchip (TFFC) LEDs with 7.8 mW at 95 mA at 278.5 nm grown on AlN/SiC with TDD~1x109 cm-2. (Respectively, EQE and WPE are 1.8% and 0.6%.) We also demonstrate that KOH roughening does not impact the IV voltage of TFFC LED. KOH roughening enhanced the light extraction efficiency (LEE) by 100% and ~180% for UV LEDs with 10 nm p-GaN and 5 nm p-GaN, respectively.

The energy difference between the lowest conduction band valleys is a fundamental semiconductor parameter affecting performance of electronic devices via intervalley electron scattering. Surprisingly, the intervalley energy (IVE) value in GaN is still disputed. Recent photoemission experiments showed that IVE is 0.90 – 0.95 eV, which is considerably smaller than the >2 eV values obtained by ab initio calculations. One of the suitable techniques to measure IVE is time-resolved spectroscopy. Excitation wavelength dependent photoluminescence and pump-probe transients allow pinpointing the onset of the intervalley scattering by increase of the electron relaxation time towards the bottom of the conduction band. In this work, we apply this approach by performing differential transmission (DT) and reflection (DR) measurements on n-GaN crystal. In DR, ultraviolet (UV) pump creates electrons in the Γ valley at energies around the scattering threshold, and the onset energy is determined by the change of the electron relaxation time towards the bottom of the conduction band. However, IVE evaluated using this technique is affected by the poor knowledge of the valence band dispersion at large k values. This problem is circumvented in the DT measurements, in which only conduction band states are involved. The DT decay time spectrum provided the IVE value of 0.97 ± 0.02 eV, close to the photoemission data. Comparison of DT and DR intervalley scattering onsets allowed estimating the hole mass as 1.4m0. Modelling of the DR transients with rate equations produced intra-and intervalley electron - LO phonon scattering times of 30 and 15 fs, respectively.

Discovering ways to increase the LED light extraction efficiency (LEE) should help create the largest performance improvement in the power of UV AlGaN LEDs. Employing surface roughening to increase the LEE of typical AlGaN UV LEDs is challenging and not well understood, yet it can be achieved easily in AlGaN LEDs grown on SiC. We fabricate thin-film UV LEDs (~294-310 nm) grown on SiC-with reflective contacts and roughened emission surface-to study and optimize KOH roughening of N-face AlN on the LEE as a function of roughened AlN pyramid size and KOH solution temperature. The LEE increased the most (2X) when the average AlN pyramid base diagonals (d) were comparable to the electroluminescence (EL) wavelength in the AlN layer (d ~λEL; 42-52 pyramids/µm2), but the LEE enhancement diminished when d was much larger than λEL (d ~5.5λEL; 2-3 pyramids/µm2). The UV LEDs had a 10 nm p-GaN contact layer, and the forward voltage was ~6 V at ~8 A/cm2, with a voltage efficiency (VE) of ~70%. The VE of the LEDs did not change after KOH roughening. This work suggests important implications to increase the LEE of AlGaN LEDs.

The growth mechanisms during metalorganic vapor phase epitaxy (11-22) oriented Al x Ga1-x N with x ∼ 0.80 on m-plane sapphire are studied with the intention of mitigating the expansion of misoriented grains, composed of the (1-10-3) crystal orientation and achieving a flat surface with only the (11-22) orientation. An increase in reactor pressure, metalorganic supply, and V/III ratio led to a decrease in the grain density from 1.0 × 109 cm−2 to 1.5 × 107 cm−2. By comparing different growth regimes, we found that the main factor suppressing the growth of the (1-10-3) orientation and decreasing the grain density in the AlGaN layers is the growth rate, which decreased with increasing reactor pressure, MO supply, and V/III ratio due to increasing pre-reactions in the gas phase. To mitigate pre-reactions even with lower growth rates, growth conditions with higher total flow and lower TMAl flow were chosen, yielding low growth rates of 0.13 μm h−1 and a grain density of 3.0 × 107 cm−2 at an aluminum mole fraction of 84%. To allow the growth of thick LED heterostructures we demonstrated that such a buffer can be overgrown with higher growth rate AlGaN, yielding a low grain density of 1.0 × 106 cm−2 and a smooth morphology with a rms roughness of 2.5 nm by avoiding misoriented crystal propagation during nucleation.

The loss of piezoelectricity in ferroelectric sodium bismuth titanate (NBT) in solid solution with barium titanate ((1− x )NBT− x BT) is studied by means of thermal depoling and Raman spectroscopy. In the range 0≤ x ≤13%, the highest thermal stability of the piezoelectric coefficients is found in x =13%, for which a longitudinal piezoelectric coefficient of 118 pC/N was nearly temperature-independent up to a maximum temperature of 165 °C. The composition nearest the morphotropic phase boundary, x =6%, exhibited the lowest thermal depoling temperature of 107 °C. Depoling of this composition is suggested to arise from a second order phase transition evidenced in the temperature-dependent Raman spectra.

Time-resolved transmission and reflection measurements were performed for bulk GaN at room temperature to evaluate the energy of the first conduction band satellite valley. The measurements showed ...

We investigate the bow of free standing (0001) oriented HVPE grown GaN wafers and demonstrate that their curvature is consistent with a compressive to tensile stress gradient (bottom to top) present in the wafers. The origin of the stress gradient and the curvature is attributed to the inclination of edge threading dislocations (TDs) with respect to original [0001] GaN wafer or boule growth direction. A model is proposed and a relation is derived for bulk GaN wafer curvature dependence on the inclination angle and the density of TDs. The model is used to analyze the curvature for commercially available GaN substrates, and is supported by X-ray diffraction and cathodoluminescence measurements. The results show a close correlation between the experimentally determined wafer curvature and TD density and those predicted from the theoretical model for varying inclination angles.

Author(s): SaifAddin, Burhan K; Almogbel, Abdullah; Zollner, Christian J; Foronda, Humberto; Alyamani, Ahmed; Albadri, Abdulrahman; Iza, Michael; Nakamura, Shuji; DenBaars, Steven P; Speck, James S | Abstract: The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes (LEDs) is limited due to their poor light extraction efficiency (LEE). To improve the LEE of AlGaN LEDs, we developed a fabrication technology to process AlGaN LEDs grown on SiC into thin-film flip-chip LEDs (TFFC LEDs) with high LEE. This process transfers the AlGaN LED epi onto a new substrate by wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a highly selective SF6 plasma etch that stops at the AlN buffer layer. We optimized the inductively coupled plasma (ICP) SF6 etch parameters to develop a substrate-removal process with high reliability and precise epitaxial control, without creating micromasking defects or degrading the health of the plasma etching system. The SiC etch rate by SF6 plasma was ~46 \mu m/hr at a high RF bias (400 W), and ~7 \mu m/hr at a low RF bias (49 W) with very high etch selectivity between SiC and AlN. The high SF6 etch selectivity between SiC and AlN was essential for removing the SiC substrate and exposing a pristine, smooth AlN surface. We demonstrated the epi-transfer process by fabricating high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further enhance the light extraction, the exposed N-face AlN was anisotropically etched in dilute KOH. The LEE of the AlGaN LED improved by ~3X after KOH roughening at room temperature. This AlGaN TFFC LED process establishes a viable path to high external quantum efficiency (EQE) and power conversion efficiency (PCE) UV-C LEDs.

Deep level transient spectroscopy (DLTS) and deep level optical spectroscopy (DLOS) were used to characterize defect states throughout the bandgap of unintentionally-doped InxAl1−xN grown by metal organic chemical vapor deposition for x = 0.18 (nominally lattice-matched) and x = 0.15 compositions. DLTS revealed broad peaks with energy levels of EC − 0.23 eV and 0.38 eV for In0.18Al0.82N and In0.15Al0.85N, respectively, tracking the difference in their conduction band minima [S. Schulz et al., Appl. Phys. Express 6, 121001 (2013)]. Capture kinetics studies revealed logarithmic filling behavior, which with the broad peaks, implies that an extended defect source is likely, consistent with threading dislocation densities (TDD) of ∼1 × 109 cm−2 measured for both structures. However, the trap concentration did not track the detailed TDD variation but instead followed the background oxygen content, which varied between 1.2 × 1018 cm−3 and 1.8 × 1018 cm−3 for the samples. Taken together with the logarithmic capture kinetics, this implies that dislocation-oxygen complexes could be the source for this trap. In spite of the high oxygen content in the samples, this state did not reveal DX-like behavior, supporting the assertion of an oxygen-dislocation complex as its likely source. DLOS also revealed additional states at EC − 1.63 eV, 2.09 eV, and 3.59 eV for In0.18Al0.82N and analogous states at EC − 1.70 eV, 2.70 eV, and 3.90 eV within In0.15Al0.85N. Lighted capacitance-voltage measurements indicated that the near mid-gap (EC − 2.09 eV and 2.70 eV) and near valence band (EC − 3.59 eV and 3.90 eV) states are their primary sources for carrier compensation.

Although a great deal of work has been done to understand defect chemistry in "soft" and "hard" PZT-based materials, there is little understanding of how defect chemistry influences the properties of lead-free piezoelectric materials. This paper reports a systematic investigation of doping on the ferroelectric and piezoelectric properties in Na 0.5 Bi 0.5 TiO 3 (NBT)-based ceramics. NBT-based ceramics have been synthesized by traditional solid state routes using several different dopants including lanthanum, manganese, and iron in 1 mol%. The addition of iron and manganese lead to an increase in the coercive field (E c ), a decrease in the piezoelectric coefficient (d33), and an increase in the thermal depoling temperature (T depole ), similar to the behavior of "hard" PZT. Lanthanum, on the other hand, leads to a decrease in the E c , an increase in d33, and a decrease in T depole similar to that seen in "soft" PZT.

We investigate the bow of free standing (0001) oriented hydride vapor phase epitaxy grown GaN substrates and demonstrate that their curvature is consistent with a compressive to tensile stress gradient (bottom to top) present in the substrates. The origin of the stress gradient and the curvature is attributed to the correlated inclination of edge threading dislocation (TD) lines away from the [0001] direction. A model is proposed and a relation is derived for bulk GaN substrate curvature dependence on the inclination angle and the density of TDs. The model is used to analyze the curvature for commercially available GaN substrates as determined by high resolution x-ray diffraction. The results show a close correlation between the experimentally determined parameters and those predicted from theoretical model.

Parasitic pre-reactions are known to play a role in the growth of aluminum nitride (AlN) via metal organic chemical vapor deposition, where they can deplete precursor molecules before reaching the substrate, leading to poor growth efficiency. Studies have shown that reducing the growth pressure and growth temperature results in improved growth efficiency of AlN; however, superior crystal quality and reduced impurity incorporation are generally best obtained when growing at high temperatures. This study shows that, with proper alkyl source dilution, parasitic pre-reactions can be suppressed while maintaining high growth temperatures. The results show an 18× increase in growth rate and efficiency of AlN films: from 0.04 μm h−1 to 0.73 μm h−1, and 26 μm mol−1 to 502 μm mol−1, respectively; under constant TMAl flow and a small change in total gas flow. This results in 6.8% of Al atoms from the injected TMAl being utilized for AlN layer growth for this reactor configuration. This is better than the standard GaN growth, where 6.0% of the Ga atoms injected from TMGa are utilized for GaN growth.

The crystal structure and property changes of sodium bismuth titanate (Na0.5Bi0.5TiO3, NBT) piezoelectric ceramics are reported as a function of La modification (0.5-2.0 at. %) and increasing temperature using high resolution x-ray diffraction, permittivity, depolarization, and polarization and strain hysteresis measurements. La substitution is found to decrease the depolarization temperature of NBT (e.g., 1.5 at. % La substitution lowers the depolarization temperature by 60 degrees C relative to the unmodified composition) with little impact on the room temperature polarization and strain hysteresis. The room temperature structures of the various NBT compositions were modeled using a mixture of the monoclinic Cc space group and the cubic Pm (3) over barm phase, where the Pm (3) over barm phase is used to model local regions in the material which do not obey the long range Cc space group. With increasing La substitution, the lattice parameter distortions associated with the Cc phase approached that of the prototypical cubic unit cell and the fraction of the Pm (3) over barm phase increased. The relationship between these crystallographic changes and the depolarization behavior of La-modified NBT is discussed. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4751357]