Alx Ga1 Research Articles

Tuning composition in graded AlGaN channel HEMTs toward improved linearity for low-noise radio-frequency amplifiers

Compositionally graded channel AlGaN/GaN high electron mobility transistors (HEMTs) offer a promising route to improve device linearity, which is necessary for low-noise radio-frequency amplifiers. In this work, we demonstrate different grading profiles of a 10-nm-thick AlxGa1−xN channel from x = 0 to x = 0.1 using hot-wall metal-organic chemical vapor deposition (MOCVD). The growth process is developed by optimizing the channel grading and the channel-to-barrier transition. For this purpose, the Al-profiles and the interface sharpness, as determined from scanning transmission electron microscopy combined with energy-dispersive x-ray spectroscopy, are correlated with specific MOCVD process parameters. The results are linked to the channel properties (electron density, electron mobility, and sheet resistance) obtained by contactless Hall and terahertz optical Hall effect measurements coupled with simulations from solving self-consistently Poisson and Schrödinger equations. The impact of incorporating a thin AlN interlayer between the graded channel and the barrier layer on the HEMT properties is investigated and discussed. The optimized graded channel HEMT structure is found to have similarly high electron density (∼ 9 × 10 12 cm−2) as the non-graded conventional structure, though the mobility drops from ∼ 2360 cm2/V s in the conventional to ∼ 960 cm2/V s in the graded structure. The transconductance gm of the linearly graded channel HEMTs is shown to be flatter with smaller g m ′ and g m ″ as compared to the conventional non-graded channel HEMT implying improved device linearity.

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During high-power optical picosecond pumping of the GaAs layer of the Alx Ga1−x As−GaAs−Alx Ga1−x As heterostruc- ture, stimulated picosecond emission arises in it. It has been experimentally revealed how, upon saturation of the emission gain, the maximum energy density of its spectral component and the time to reach this density depend on the photon energy, on the parameters of the amplification and relaxation of the component. It follows from these dependences that the indicated density and time are influenced by the slowing down of the transport of nonequilibrium carriers in the energy space. The slowdown is caused by the interaction of carriers with emission. It has been found that as the diameter of the active region approaches zero, the measured characteristic relaxation time of the component tends to the theoretical universal residual relaxation time prolonged due to the indicated transport slowdown.

Spectrum and normalized modes of acoustic phonons in multilayer nitride-based nanostructure

Using the model of an elastic continuum, precise analytical solutions of the equations of motion are established and a theory of the spectrum and components of the displacement field of acoustic phonons is developed for a two-well resonant-tunneling nanostructure with AlN – quantum barriers and GaN∕Alx Ga1−x N – quantum wells. For the experimentally realised nanostructure – the active zone of a quantum cascade detector – depending on its geometric parameters, the spectrum of acoustic phonons and normalized components of the displacement fields were calculated.

Ordered Al xGa1- xAs Nanopillar Arrays via Inverse Metal-Assisted Chemical Etching.

The ternary III-V semiconductor compound, Al xGa1 -xAs, is an important material that serves a central role within a variety of nanoelectronic, optoelectronic, and photovoltaic devices. With all of its uses, the material itself poses a host of fabrication difficulties stemming from conventional top-down processing, including standard wet-chemical etching and reactive-ion etching (RIE). Metal-assisted chemical etching (MacEtch) techniques provide low-cost and benchtop methods that combine many of the advantages of RIE and wet-chemical etching, without being hindered by many of their disadvantages. Here, inverse-progression MacEtch (I-MacEtch) of Au-patterned Al xGa1 -xAs is demonstrated for the first time and is exploited for the generation of vertical and ordered nanopillar arrays. The etching solution employed here consists of citric acid (C6H8O7) and hydrogen peroxide (H2O2). The I-MacEtch evolution is tracked in time for Al xGa1 -xAs samples with compositions defined by x = 0.55, x = 0.60, and x = 0.70. The vertical and lateral etch rates (VER and LER, respectively) are shown to be tunable with Al fraction and temperature of the etching solution, based on modification of catalytically injected hole distributions. Control over the VER/LER ratio is demonstrated by tailoring etch conditions for single-step fabrication of ordered AlGaAs nanopillar arrays with predefined aspect ratios. Maximum VER and LER values of ∼40 nm/min and ∼105 nm/min, respectively, are measured for Al0.55Ga0.45As at a process temperature of 65 °C. The I-MacEtch nanofabricationmethodology outlined in this study may be utilized for the processing of many devices, including high electron mobility transistors, distributed Bragg reflectors, lasers, light-emitting diodes, andmultijunction solar cells containing AlGaAs components.

Electronic properties of Si-doped AlxGa1−xN with aluminum mole fractions above 80%

The dependence of the activation energy as well as the energetic levels of the neutral charge state and the DX center of the Si donor in AlxGa1−xN:Si samples on aluminum content and SiH4/III ratio were investigated by electron paramagnetic resonance (EPR) measurements, Van-der-Pauw resistivity measurements, and Hall-effect measurements. It was found by EPR measurements that the energy distance of the neutral charge state of the Si donor from the conduction band increases with increasing aluminum content from 61 meV for x = 0.82 to 106 meV for x = 0.89. Additionally, the formation of a DX center below the neutral charge state which is deepening from 6 meV for x = 0.82 to 58 meV for x = 0.89 is observed. This results in a linearly increasing activation energy with increasing aluminum content from 67 meV for x = 0.82 to 164 meV for x = 0.89. This is consistent with the activation energies as determined by Hall-effect measurements showing a linear increase from 24 meV for x = 0.85 to 211 meV for x = 0.96, as well as the activation energies as determined by Van-der-Pauw resistivity measurements. By varying the SiH4/III ratio we observed a formation of a minimum resistivity in accordance with the room temperature charge carrier density. However, no clear dependence of the activation energy was observed. EPR measurements of samples with a high SiH4/III ratio hint to an increased incorporation probability of a deep secondary donor species which might explain the increase in resistivity.

Dielectric function in the spectral range (0.5–8.5)eV of an (Alx Ga1−x)2O3 thin film with continuous composition spread

We determined the dielectric function of the alloy system (Alx Ga1−x)2O3 by spectroscopic ellipsometry in the wide spectral range from 0.5 eV to 8.5 eV and for Al contents ranging from x = 0.11 to x = 0.55. For the composition range x < 0.4, we observe single phase material in the β-modification and for larger Al content also the occurrence of γ-(Al,Ga)2O3. We derived spectra of the refractive index and the absorption coefficient as well as energy parameters of electronic band-band transitions by model analysis of the dielectric function. The dependence of the dielectric functions lineshape and the energy parameters on x is highly continuous, reflecting theoretical expectations. The data presented here provide a basis for a deeper understanding of the electronic properties of this material system and may be useful for device engineering.

Heterojunction solar cell with 6% efficiency based on an n-type aluminum–gallium–oxide thin film and p-type sodium-doped Cu2O sheet

In this paper, we describe efforts to enhance the efficiency of Cu2O-based heterojunction solar cells fabricated with an aluminum–gallium–oxide (Al–Ga–O) thin film as the n-type layer and a p-type sodium (Na)-doped Cu2O (Cu2O:Na) sheet prepared by thermally oxidizing copper sheets. The optimal Al content [X; Al/(Ga + Al) atomic ratio] of an AlX–Ga1−X–O thin-film n-type layer was found to be approximately 2.5 at. %. The optimized resistivity was approximately 15 Ω cm for n-type AlX–Ga1−X–O/p-type Cu2O:Na heterojunction solar cells. A MgF2/AZO/Al0.025–Ga0.975–O/Cu2O:Na heterojunction solar cell with 6.1% efficiency was fabricated using a 60-nm-thick n-type oxide thin-film layer and a 0.2-mm-thick Cu2O:Na sheet with the optimized resistivity.

The Effect of Multivalley Bandstructure on Thermoelectric Properties of Al x Ga1−x As

We present the theoretical modeling of the thermoelectric properties of Alx Ga1−xAs. It was shown that, contrary to the known good thermoelectric materials, the optimum composition happens far from the point at which the multiple bands meet. This unexpected optimum material composition is related to the detrimental effect of multivalley bandstructure. A semi-empirical model was employed to predict the thermoelectric properties versus alloy composition, temperature, and doping concentration. The electrical conductivity, Seebeck coefficient and figure-of-merit (ZT) were calculated with consideration of the energy-dependent relaxation time and multivalley band structure for AlxGa1−xAs. The theoretical model was verified by comparison with different sets of experimental data on both electrical and thermal transport properties. It was shown that the multivalley bandstructure in AlxGa1−xAs affects the Seebeck coefficient in two counteracting processes; however, it always reduces the electrical conductivity and the electronic thermal conductivity. It was shown that the multivalley bandstructure also affects the lattice thermal conductivity. In contrast to several good thermoelectric materials in which their multivalley band structure enhances the ZT, in AlxGa1−xAs, the ZT reduces at the composition x at which the three bands of Γ, X, and L meet each other. Therefore, the maximum ZT happens far from this point. The optimum x also depends on temperature and reduces with temperature. Therefore, the Al concentration must decrease across the thermoelectric leg from the cold to the hot side. At the optimum composition, the ZT of AlxGa1−xAs is predicted to be comparable to that of good thermoelectric materials at high temperatures.

Polarization Induced High Al Composition AlGaN p–n Junction Grown on Silicon Substrates

Wide bandgap Alx Ga1−x N (x = 0.7−1) p–n junction is realized on a silicon substrate through polarization induced doping. Polarization induced positive charge field is produced by linearly grading from AlN to Al0.7 Ga0.3 N, and negative charge field is generated by an inverted grading from Al0.7 Ga0.3 N to AlN. The polarization charge field induced hole density is on the order of 1018 cm−3 in the graded Alx Ga1−x N:Be (x = 0.7−1) p–n junction. Polarization doping provides a feasible way to mass produce III-nitride devices on silicon substrates.

Structural and optical properties of Alx Ga1–xN nanowires

Abstractmagnified imageThe structural properties of Alx Ga1–xN nanowires with high Al content grown by plasma‐assisted molecular beam epitaxy have been studied by high‐resolution electron microscopy and X‐ray diffraction. Evidence for marked chemical composition fluctuations has been found that have been assigned to kinetical effects during growth. Concomitantly, microphotoluminescence and nanocathodoluminescence experiments on single nanowires have revealed the presence of sharp emission lines, as an evidence of carrier localization. This feature has been assigned to several spatial scales of composition fluctuations, in agreement with the kinetically‐driven growth model proposed for Alx Ga1–xN nanowires. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Characterization of MBE‐grown AlGaN layers heavily doped using silane

AbstractThe Alx Ga1–xN layers with a wide range of AlN molar fraction (0 &lt; x &lt; 0.6) were grown by ammonia MBE on (0001)‐oriented sapphire substrates using silane as the silicon dopant. The structural, electrical and optical properties of the layers were investigated by atomic force microscopy, X‐ray diffraction, Hall, Raman, photoluminescence and cathodoluminescence measurements. It was found that doping affects surface morphology, dislocation density and luminescence intensity of the Alx Ga1–xN layers. A sharp decrease of the luminescence intensity for the heavily doped GaN and Al0.3Ga0.7N layers was explained by enhancement of the nonradiative recombination rate in degenerate GaN layers and by an increase in the defect concentration in AlGaN layers. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Growth of GaAs/AlxGa1−xAs layers by LPE method and their characterization by SIMS

Growth of thin layers of compound semiconductors such as GaAs and Alx Ga1−x As was obtained by Liquid Phase Epitaxy (LPE) at 838–828 ° C in thickness range of 0.1–4.3 μm which was estimated by Scanning Electron Microscopy (SEM). By Secondary Ion Mass Spectroscopy (SIMS) measurements, type of impurity atoms and their density and uniformity with respect to thickness were measured. In this way we are sure that variation of impurity atoms such as Si, Te, Sn and Ge indicates that epilayers were formed uniformly and it demonstrated that the LPE growth was a suitable way to obtain a good quality of epitaxy layers. Amount of composition parameter x in the compound semiconductor AlxGa1−xAs was measured which varies from 0.13 to 0.4. In-depth profiles of SIMS indicated that Al atom can be replaced by Ga atom in the GaAs crystalline structure.

Mg‐doped Al0.85Ga0.15N layers grown by hot‐wall MOCVD with low resistivity at room temperature

AbstractWe report on the hot‐wall MOCVD growth of Mg‐doped Alx Ga1–xN layers with an Al content as high as x ∼ 0.85. After subjecting the layers to post‐growth in‐situ annealing in nitrogen in the growth reactor, a room temperature resistivity of 7 kΩ cm was obtained indicating an enhanced p‐type conductivity compared to published data for Alx Ga1–xN layers with a lower Al content of x ∼ 0.70 and a room temperature resistivity of about 10 kΩ cm. It is believed that the enhanced p‐type conductivity is a result of reduced compensation by native defects through growth conditions enabled by the distinct hot‐wall MOCVD system. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

MOVPE growth and characterization of a ‐plane AlGaN over the entire composition range

AbstractWe report the metal organic vapor phase epitaxy (MOVPE) growth and characterization of non‐polar (11$ \bar 2 $0) a ‐plane Alx Ga1–xN on (1$ \bar 1 $02) r ‐plane sapphire substrates over the entire composition range. Alx Ga1–xN samples with ∼0.8 μm thick layers and with x = 0, 0.18, 0.38, 0.46, 0.66, and 1.0 have been grown on r ‐plane sapphire substrates. The layer quality can be improved by using a 3‐stage AlN nucleation layer and appropriate V/III ratio switching following nucleation. All a ‐plane AlGaN epilayers show an anisotropic in‐plane mosaicity, strongly influenced by Al incorporation and growth conditions. Careful lattice parameter measurements show anisotropic in‐plane strain that results in an orthorhombic distortion of the hexagonal unit cell, making Al composition determination from X‐ray diffraction difficult. In general lower Al incorporation is seen in a ‐plane epilayers compared to c ‐plane samples grown under the same conditions. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

High‐field electron mobility in InGaAs quantum wells

AbstractA new approach to design the electron–optical phonon scattering rate in a quantum well (QW), based on the dependence of interface phonon frequency upon QW interface composition, is suggested. Possibilities to regulate a scattering rate of electrons by interface phonons in the InGaAs QW by selecting QW barrier materials are considered. It is shown that the electron–interface phonon scattering rate in the In0.2Ga0.8As QW can be changed by several times by choosing content x in the Alx Ga1–x As QW barriers and in the inserted Inx Al1–x As barrier in the QW centre. It is observed experimentally that the low‐field mobility decreases and the high‐field saturated drift velocity increases with the increase of the Iny Ga1–y As/ Inx Al1–x As interface phonon energy. It is shown that the electron drift velocity saturates at the electric field F &gt; 4 kV/cm and achieves values of 1.3–1.5 × 107 cm/s. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Large area mapping of the alloy composition of Alx Ga1–xN using infrared reflectivity

AbstractThe energy position of a dip observed in the IR‐reflectance spectra recorded from wurtzite c ‐plane Alx Ga1–x N epitaxial films grown on SiC substrate reflects the composition of the alloy. A calibration procedure is presented with the possibility of mapping for large area wafer. The technique is non‐destructive, scalable and fast. The limitations are discussed and comparisons with other techniques are made. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Surface phonon polariton modes of wurtzite structure Alx Ga1–xN thin film

AbstractThe polar phonon polariton modes in a wurtzite thin film structure are theoretically investigated in the present paper. It is confirmed that there are two types of phonon polariton modes, i.e. the surface phonon polariton (SPP) modes and the confined phonon polariton (CPP) modes in wurtzite thin film systems. The frequency ranges of the SPP and CPP modes are discussed in detail. The dispersive equation of the SPP modes is also deduced. Numerical calculations on Alx Ga1–x N thin film structures reveal that, for a given free wave number kt in the x ‐direction, there are two branches of SPP modes in general. The Al concentration x of Alx Ga1–x N materials and the thickness d of the wurtzite thin film have an important influence on the dispersive properties of the polariton modes, especially as the thickness d is small. Our calculated results are completely consistent with the recent experimental observation in Alx Ga1–x N thin film systems. We confirm that the SPP modes observed are the low‐frequency branches of SPP modes. Furthermore, the frequency values of the high‐frequency branches of SPP modes are explicitly given, and a possible method for observing these high‐frequency SPP modes is also brought forward. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Optical Hall effect studies on modulation‐doped AlxGa1–xAs:Si/GaAs quantum wells

AbstractWe study the temperature (10 K … 293 K) dependence of the optical Hall effect (OHE) in modulationdoped Alx Ga1–x As:Si / GaAs (x = 0.45) superlattice structures with different quantum well thickness (dGaAs = 16.9 nm and dGaAs = 3.7 nm) using generalized magnetooptic ellipsometry at far‐infrared wavelengths. Free electrons are identified within the wells, but not within the doped barriers. The observed OHE can be fully explained within the Drude model and thermionic rate equations. The quantum‐well free electron density (N = 1.3 × 1017 cm–3) increases five times upon sample cooling within the wells with dGaAs = 3.7 nm, and remains constant within the wells with dGaAs = 16.9 nm. We describe this behavior as a steady state of three quantum well electron condensation processes: Coulombactivation of electron states at the Alx Ga1–x As/GaAs interface, quantum‐well‐barrier reservoir interaction, and irreversible electron emission into the host crystal. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Quantum Hall effect in long and in mobility adjusted GaAs/Alx Ga1–xAs samples

AbstractWe study the behaviour of the magnetoresistance in the integer quantum Hall regime in dependence on the sample length and mobility. In Hall‐bars with macroscopic lengths of up to 96 cm we find that the longitudinal specific magnetoresistance (ρxx) decreases with the distances between voltage probes. The Hall resistance in these samples shows also peculiar features – we measure very deep dips between the quantised values of the plateaus. We give a qualitative explanation for these dependencies, supposing a fully equilibrated electron transport and high back‐scattering in the sample with very long dimensions. In order to prove these assumptions, we fabricated samples with shorter dimensions but with an enhanced inter‐edge channel scattering by focused ion implantation at the sample boundaries. The Hall resistance shows similarities to those of very long samples. Implanting Ga+ ions homogeneously into Alx Ga1–x As/GaAs heterostructures offers the possibility to modify the electron mobility while keeping a constant carrier density. In this way, we vary the electron mobility of the same heterostructure within an order of magnitude whereas the electron density remains constant. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Electrostatic electron piston pump with in‐plane gate transistors

AbstractWe have demonstrated an electron pump with in‐plane gate (IPG) transistors fabricated by focused ion beam (FIB) implantation on GaAs–Alx Ga1–x As modulation‐doped heterostructure with a two‐dimensional electron gas (2DEG). The maskless fabrication process does not require any alignment between the circuit elements including the sources, the drains and the gates of the IPG transistors. Although, the device is operating at low frequencies (tens of kHz), it is capable to produce a current more than one order of magnitude higher than other electron pumps based on Coulomb blockade and single electron tunneling effects. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)