High Al Composition Research Articles

Comparative study of intersubband absorption in AlGaN/GaN and AlInN/GaN superlattices: Impact of material inhomogeneities

We report a systematic and quantitative study of near-infrared intersubband absorption in strained AlGaN/GaN and lattice-matched AlInN/GaN superlattices grown by plasma-assisted molecular-beam epitaxy as a function of Si-doping profile with and without \ensuremath{\delta} doping. For AlGaN/GaN, we obtained good theoretical agreement with experimental measurements of transition energy, integrated absorbance and linewidth by considering many-body effects, interface roughness, and calculations of the transition lifetime that include dephasing. For the AlInN/GaN system, experimental measurements of the integrated absorbance due to the superlattice transitions produced values more than one order of magnitude lower than AlGaN/GaN heterostructures at similar doping levels. Furthermore, observed transition energies were roughly 150 meV higher than expected. The weak absorption and high transition energies measured in these structures is attributed to columnar alloy inhomogeneity in the AlInN barriers observed in high-angle annular dark-field scanning transmission electron microscopy. We simulated the effect of these inhomogeneities using three-dimensional band-structure calculations. The inhomogeneities were modeled as AlInN nanorods with radially varying In composition embedded in the barrier material of the superlattice. We show that inclusion of the nanorods leads to the depletion of the quantum wells (QWs) due to localization of charge carriers in high-In-containing regions. The higher energy of the intersubband transitions was attributed to the relatively uniform regions of the QWs surrounded by high Al (95$%$) composition barriers. The calculated transition energy assuming Al${}_{0.95}$In${}_{0.05}$N barriers was in good agreement with experimental results.

Improvement of operation temperature in GaAs/AlGaAs THz‐QCLs by utilizing high Al composition barrier

AbstractTerahertz quantum cascade lasers (THz‐QCLs) are compact THz laser source with narrow emission linewidth and high output power. Currently, a maximum operation temperature (Tmax) of 199.5 K is reported by using a state‐of‐the‐art longitudinal optical (LO) phonon depopulation scheme AlGaAs/GaAs structure. We propose an effective method to obtain higher operating temperature for AlGaAs/GaAs THz‐QCL by increasing LO‐phonon scattering energy by utilizing high Al composition AlGaAs barrier. We have successfully fabricated an LO phonon depopulation scheme AlGaAs/GaAs THz‐QCL with double metal waveguide, and obtained 3.7 THz lasing. We obtained the Tmax of 150 K. We observed an increase of Tmax by about 7 K by increasing the Al composition of AlGaAs barrier from 15% to 30%. The characteristic temperature (T0) was also improved from 44.7 K to 69.9 K by changing the Al composition of AlGaAs barriers. The increase of Al composition in AlGaAs‐based THz‐QCL was shown to be promising way for obtaining higher operating temperature of THz‐QCL. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Normally-Off AlGaN/GaN High Electron Mobility Transistors with Thin and High Al Composition Barrier Layers

A GaN-based enhancement-mode high electron mobility transistor (HEMT) with a 1.5 nm GaN/9 nm Al0.65Ga0.35N thin barrier was reported. Without any treatment on barrier layer under the gate, the as-grown HEMTs exhibited a threshold voltage of 0.3 V, a maximum drain current density of 441 mA/mm at VGS = 3 V and a peak extrinsic transconductance of 204 mS/mm at VGS = 1.1 V. At the same time, both a low Schottky leakage current and an insignificant surface defects induced current dispersion were observed. Moreover, drain induction barrier lower (DIBL) effect was determined to be merely 3.28 mV/V at 1 mA/mm for a gate length of 0.5 µm. Additionally, post-gate annealing experiment at step temperatures up to 450 °C was implemented, only causing a minor shift in threshold voltage. These results demonstrated the substantial potential of thin and high Al composition barrier layers for high-voltage and highly reliable enhancement mode operation.

Optical investigation of strong exciton localization in high Al composition AlxGa1-xN alloys

The exciton localization in wurtzite AlxGa₁-xN alloys with x varying from 0.41 to 0.63 has been studied by deep-ultraviolet photoluminescence (PL) spectroscopy and picosecond time-resolved PL spectroscopy. Obvious S-shape temperature dependence was observed indicating that the strong exciton localization can be formed in high Al composition AlxGa₁-xN alloys. It was also found that the Al composition dependence of exciton localization energy of AlGaN alloys is inconsistent with that of the excitonic linewidth. We contribute the inconsistency to the strong zero-dimensional exciton localization.

Growth and characterization of a high-quality Al0.45Ga0.55N layer with AlGaN/AlN superlattices

We report the growth and characterization of a Si-doped, n-type AlGaN layer with 45% Al composition. For the application of n-type AlGaN layers with high Al compositions in ultraviolet emitters, we fabricated an n-Al0.45Ga0.55N layer with high crystalline quality and high electrical conductivity by inserting Al0.85Ga0.15N/AlN superlattices (SLs) to prevent cracks prior to growing the n-type AlGaN layer. The dislocation density in the n-AlGaN layer with a 45% Al composition and SLs was less than 2.4 × 1010 cm−2, which was lower than the dislocation density of 5.3 × 1010 cm−2 for the n-AlGaN layer without SLs. The resistivity, mobility, and free-electron concentration in the n-type Al0.45Ga0.55N layer with SLs were 2.2 × 10−2 Ω·cm, 55.0 cm2/V-s, and 5.0 × 1018 cm−3 at room temperature, respectively.

Inductively coupled plasma–reactive ion etching of c- and a-plane AlGaN over the entire Al composition range: Effect of BCl3 pretreatment in Cl2/Ar plasma chemistry

Inductively coupled plasma (ICP)–reactive ion etching (RIE) patterning is a standard processing step for UV and optical photonic devices based on III-nitride materials. There is little research on ICP-RIE of high Al-content AlGaN alloys and for nonpolar nitride orientations. The authors present a comprehensive study of the ICP-RIE of c- and a-plane AlGaN in Cl2/Ar plasma over the entire Al composition range. The authors find that the etch rate decreases in general with increasing Al content, with different behavior for c- and a-plane AlGaN. They also study the effect of BCl3 deoxidizing plasma pretreatment. An ICP deoxidizing BCl3 plasma with the addition of argon is more efficient in removal of surface oxides from AlxGa1−xN than RIE alone. These experiments show that AlxGa1−xN etching is affected by the higher binding energy of AlN and the higher affinity of oxygen to aluminum compared to gallium, with oxides on a-plane AlGaN more difficult to etch as compared to oxides on c-plane AlGaN, specifically for high Al composition materials. The authors achieve reasonably high etch rate (∼350 nm/min) for high Al-content materials with a smooth surface morphology at a low DC bias of ∼−45 VDC.

Aluminum-Induced Photoluminescence Red Shifts in Core–Shell GaAs/AlxGa1–xAs Nanowires

We report a new phenomenon related to Al-induced carrier confinement at the interface in core-shell GaAs/Al(x)Ga(1-x)As nanowires grown using metal-organic vapor phase epitaxy with Au as catalyst. All Al(x)Ga(1-x)As shells strongly passivated the GaAs nanowires, but surprisingly the peak photoluminescence (PL) position and the intensity from the core were found to be a strong function of Al composition in the shell at low temperatures. Large and systematic red shifts of up to ~66 nm and broadening in the PL emission from the GaAs core were observed when the Al composition in the shell exceeded 3%. On the contrary, the phenomenon was observed to be considerably weaker at the room temperature. Cross-sectional transmission electron microscopy reveals Al segregation in the shell along six Al-rich radial bands displaying a 3-fold symmetry. Time-resolved PL measurements suggest the presence of indirect electron-hole transitions at the interface at higher Al composition. We discuss all possibilities including a simple shell-core-shell model using simulations where the density of interface traps increases with the Al content, thus creating a strong local electron confinement. The carrier confinement at the interface is most likely related to Al inhomogeneity and/or Al-induced traps. Our results suggest that a low Al composition in the shell is desirable in order to achieve ideal passivation in GaAs nanowires.

Magnetotransport properties of high equivalent Al composition AlGaN/GaN heterostructures using AlN/GaN superlattice as a barrier

Magnetotransport properties of the two-dimensional electron gas (2DEG) in high equivalent Al composition AlGaN/GaN heterostructures using AlN/GaN superlattice as a barrier have been studied at low temperatures and high magnetic fields. Well resolved Shubnikov-de Haas oscillations were observed, indicating excellent quality of the quasi-AlGaN/GaN heterostructures. It is measured that the energy separation between the two subbands in the GaN triangular quantum well can be as large as 180.5 meV, depicting strong quantum confinement at the heterointerface. The strong quantum confinement results in a high 2DEG density of 2 × 1013 cm−2. The persistent photoconductivity investigation also indicates that the superlattice barrier layer has a low density of impurities/defects. It is believed that the AlN/GaN superlattice, instead of high Al composition alloy AlGaN layer, could greatly improve the device performance.

High-Temperature Characteristics in Recessed-Gate AlGaN/GaN Enhancement-Mode Heterostructure Field Effect Transistors with Enhanced-Barrier Structures

Recessed-gate AlGaN/GaN enhancement-mode (E-mode) heterostructure field-effect transistors (HFETs) with enhanced-barrier structures were fabricated, and their high-temperature characteristics were examined. Owing to enhanced-barrier structures, where a thin AlGaN layer with a higher Al composition is inserted into the AlGaN barrier, excellent E-mode operation with threshold voltage (Vth) higher than +3 V and drain current density (Id) higher than 600 mA/mm was obtained at room temperature. Interestingly, Id did not decrease much at high temperatures, i.e., Id decreased from 610 to 590 mA upon a temperature raise from RT to 300 °C. Open-gate devices with and without recessed-gate structures were fabricated and their two-terminal characteristics were comparatively examined, revealing that the observed peculiar characteristics are ascribed to recessed-gate structures. A model for explaining the observed high-temperature characteristics is proposed.

Polarization induced hole doping in graded AlxGa1−xN (x = 0.7 ∼ 1) layer grown by molecular beam epitaxy

Polarization induced hole doping on the order of ∼1018 cm−3 is achieved in linearly graded AlxGa1−xN (x = 0.7 ∼ 1) layer grown by molecular beam epitaxy. Graded AlxGa1−xN and conventional Al0.7Ga0.3N layers grown on AlN are beryllium (Be) doped via epitaxial growth. The hole concentration in graded AlxGa1−xN:Be (x = 0.7 ∼ 1) layers demonstrates that polarization generates hole charges from Be dopant. The Al0.7Ga0.3N layer is not conductive owing to the absence of carriers generated from the Be dopant without the inducement of polarization. Polarization doping provides an approach to high efficiency p-type doping in high Al composition AlGaN.

Al组分对MOCVD制备的AlxGa1-xN/AlN/GaN HEMT电学和结构性质的影响

采用金属有机化合物化学气相沉积(MOCVD)方法制备了不同Al组分(x=0.19,0.22,0.25,0.32)的Al x Ga1-x N/AlN/GaN结构的高电子迁移率晶体管(HEMT)材料。研究了Al x Ga1-x N势垒层中Al组分对HEMT材料电学性质和结构性质的影响。研究结果表明,在一定的Al组分范围内,二维电子气(2DEG)浓度和迁移率随着Al组分的升高而增大。然而,过高的Al组分导致HEMT材料表面粗糙度增大,2DEG迁移率降低,该实验现象在另一方面得到了原子力显微镜测试结果的验证。在最佳Al组分(25%)范围内,获得的HEMT材料的2DEG浓度和室温迁移率分别达到1.2×1013cm-2和1 680 cm2/(V·s),方块电阻低至310Ω/□。

Al_xGa_1−xN–based deep-ultraviolet 320×256 focal plane array

We report the synthesis, fabrication, and testing of a 320×256 focal plane array (FPA) of back-illuminated, solar-blind, p-i-n, Al(x)Ga(1-x)N-based detectors, fully realized within our research laboratory. We implemented a pulse atomic layer deposition technique for the metalorganic chemical vapor deposition growth of thick, high-quality, crack-free, high Al composition Al(x)Ga(1-x)N layers. The FPA is hybridized to a matching ISC 9809 readout integrated circuit and operated in a SE-IR camera system. Solar-blind operation is observed throughout the array with peak detection occurring at wavelengths of 256 nm and lower, and falling off three orders of magnitude by ~285 nm. By developing an opaque masking technology, the visible response of the ROIC is significantly reduced; thus the need for external filtering to achieve solar- and visible-blind operation is eliminated. This allows the FPA to achieve high external quantum efficiency (EQE); at 254 nm, average pixels showed unbiased peak responsivity of 75 mA/W, which corresponds to an EQE of ~37%. Finally, the uniformity of the FPA and imaging properties are investigated.

Strain-driven light-polarization switching in deep ultraviolet nitride emitters

Residual strain plays a critical role in determining the crystalline quality of nitride epitaxial layers and in modifying their band structure; this often leads to several interesting physical phenomena. It is found, for example, that compressive strain in AlxGa1-xN layers grown on AlyGa1-yN (x<y) templates results in an anti-crossing of the valence bands at considerably much higher Al composition than expected. This happens even in the presence of large and negative crystal field splitting energy for AlxGa1-xN layers. A judicious magnitude of the compressive strain can support vertical light emission (out of the c-plane) from AlxGa1-xN quantum wells up to x\approx 0.80, which is desirable for the development of deep ultraviolet light-emitting diodes designed to operate below 250nm with transverse electric polarization characteristics.

High mobility two-dimensional electron gases in nitride heterostructures with high Al composition AlGaN alloy barriers

We report high-electron mobility nitride heterostructures with &amp;gt;70% Al composition AlGaN alloy barriers grown by molecular beam epitaxy. Direct growth of such AlGaN layers on GaN resulted in hexagonal trenches and a low mobility polarization-induced charge. By applying growth interruption at the heterojunction, the surface morphology improved dramatically and the room temperature two-dimensional electron gas (2DEG) mobility increased by an order of magnitude, exceeding 1300 cm2/V s. The 2DEG density was tunable at 0.4–3.7×1013/cm2 by varying the total barrier thickness (t). Surface barrier heights of the heterostructures were extracted and exhibited dependence on t.

Characterization of Ti 1−xAl xN coatings with selective IR reflectivity

Ti 1− x Al x N thin films were deposited by reactive magnetron sputtering. The obtained different stoichiometries give rise to different optical properties as the films change from metallic to dielectric. In this work the IR reflectivity of these coatings is investigated taking into account different application fields for IR selective Ti 1− x Al x N thin films. Low Al content coatings present high reflectivity, high absorptance and low thermal emittance. High Al compositions give raise to coatings with high absorptance and high thermal emittance. The composition of the coatings was evaluated combining electron energy loss spectroscopy (EELS) and energy dispersive spectroscopy. Scanning electron microscopy (SEM) revealed a columnar structure. Reflectance spectra for the visible and infrared spectral ranges were used to obtain the solar absorptance and thermal emittance values, used to calculate the equilibrium temperature of the coatings. The thermal stability in air from 300 to 600 °C was also evaluated.

Growth and characterization of a-plane Al xGa 1− xN alloys by metalorganic chemical vapor deposition

The non-polar a-plane Al x Ga 1− x N alloys on GaN epitaxial layer with different Al compositions (0≤ x≤0.2) were grown on r-plane (1 1¯ 0 2) sapphire substrates by using low-pressure metalorganic chemical vapor deposition (MOCVD) and the Al composition x were estimated from the X-ray diffraction measurements. According to the result of asymmetric X-ray reciprocal space mapping, AlGaN layer was coherently strained to the underlying GaN template. The a-plane AlGaN alloy with relatively lower Al composition showed a flat surface with reduction of pits. The best mean roughness of the surface morphology was 1.18 nm. The photoluminescence (PL) result revealed that the PL peak position shifted from 3.42 to 3.87 eV with 0≤ x≤0.2. Apart from the shifted peak position with increasing Al content, the PL emission intensity and surface morphology of the a-plane AlGaN alloy with relatively low Al content show slightly better characteristics than that of the a-plane GaN and AlGaN with higher Al composition.

Growth and character stics of AlGaN/GaN HEMT structures with AlN/GaN superlattices as barrier layers

We report the growth and characterization of AlGaN/GaN heterostructures with AlN/GaN superlattices as the barrier layer.It is found that the surface morphology of the heterostructure is greatly improved compared with those using the conventional alloy AlGaN barrier layer.Meanwhile, electric properties of samples with high Al composition (≥40%) are superior to the conventional alloy sanples.Low sheet resistance (251 Ω/□) is obtained for our samples with 40% Al content.

Electrochemical behaviour and corrosion performance of Mg–Li–Al–Zn anodes with high Al composition

This study investigated the electrochemical and corrosion performance of Mg–Li–Al–Zn anodes with Al compositions of ∼3 wt.% and ∼9 wt.%. Mg–Li–Al–Zn alloy with ∼9 wt.% Al had a relatively negative open-circuit potential and a high discharge voltage in MgCl 2 electrolyte, owing to the distribution of numerous AlLi particles in the matrix of the alloy. AlLi particles were believed to transform to Al particles during the corrosion of the Mg–Li–Al–Zn anode. The high-Al anode material exhibited good corrosion performance since a dense and continuous Mg(OH) 2/Al composite layer covered the surface of the high-Al anode. Experimentally, increasing the Li + concentration in the electrolyte improved the corrosion performance of the Mg anode.

Structure and geochemistry of the adakitic Horoz granitoid, Bolkar Mountains, south-central Turkey, and its tectonomagmatic evolution

High-Al granitic and granodioritic rocks of the 55 Ma Horoz pluton in the Bolkar Mountains of southern Turkey provide important new constraints on the early Cenozoic evolution of the eastern Mediterranean region. The ENE–WSW-trending, sill-like pluton is intrusive into late Palaeozoic–early Mesozoic metamorphic rocks of the Central Tauride block, and is unconformably overlain by Plio-Pleistocene alluvial deposits. The metaluminous to peraluminous granitoids have high-K calc-alkaline to high-K shoshonitic compositions, and show enrichment in large ion lithophile and depletion in high-field strength elements relative to ocean ridge granite. Their high Al2O3 contents (15.9–20.06 wt%) and low SiO2, MgO, and Mg numbers are consistent with adakitic compositions. These geochemical features, coupled with low Sr/Y and La/Yb ratios and trace-element patterns, suggest that the Horoz magmas were produced in part by partial melting of a subduction-metasomatized mantle. The high-Al adakitic and calc-alkaline compositions are consistent with partial melting of a hydrated lithospheric mantle and an amphibolitic–eclogitic mafic lower crust that was triggered by delamination-induced asthenospheric upwelling. We propose that, following Palaeocene continental collision between the Tauride and Central Anatolian Crystalline Complex, the inferred lithospheric delamination was a result of foundering of the overthickened orogenic root. Asthenospheric upwelling beneath the young orogenic belt thermally weakened the crust, and caused uplift and tectonic extension leading to core complex formation (Nigde massif), development of an extensional volcanic province (Cappadocia), and tectonic collapse of the Central Tauride block (Bolkar Mountains). The shallow-level Horoz pluton was unroofed by ∼23 Ma as a combined result of crustal uplift and erosion throughout the Palaeogene.

A 1.33 µm InAs/GaAs quantum dot laser with a 46 cm−1 modal gain

We report on 1.33 µm quantum dot (QD) lasers grown on GaAs substrates that show a modal gain of 45 cm−1, low threshold current density of 150 A cm−2 and room-temperature continuous wave output power of 2.5 W. The active region is based on ten InAs/InGaAs/GaAs quantum dot layers formed by activated phase separation. High structural quality of the active region is achieved, owing to minimization of the total amount of strained material per QD layer. The optical confinement factor is increased by exploiting high Al composition (80%) in the cladding layers. A modal gain over 20 cm−1 in the 1315–1345 nm wavelength range is revealed by the Hakki–Paoli technique at a low current density of 500 A cm−2.