Structural properties of 10 μm thick InN grown on sapphire (0001)

The structural properties of a 10 μm thick In-face InN film, grown on Al₂O₃ (0001) by radio-frequency plasma-assisted molecular beam epitaxy, were investigated by transmission electron microscopy and high resolution x-ray diffraction. Electron microscopy revealed the presence of threading dislocations of edge, screw and mixed type, and the absence of planar defects. The dislocation density near the InN/sapphire interface was 1.55×10¹⁰ cm⁻², 4.82×10⁸ cm⁻² and 1.69×10⁹ cm⁻² for the edge, screw and mixed dislocation types, respectively. Towards the free surface of InN, the density of edge and mixed type dislocations decreased to 4.35×10⁹ cm⁻² and 1.20×10⁹ cm⁻², respectively, while the density of screw dislocations remained constant. Using x-ray diffraction, dislocations with screw component were found to be 1.2×10⁹ cm⁻², in good agreement with the electron microscopy results. Comparing electron microscopy results with x-ray diffraction ones, it is suggested that pure edge dislocations are neither completely randomly distributed nor completely piled up in grain boundaries within the InN film.     

Dependences of structural and electrical properties on thickness of polycrystalline Ga-doped ZnO thin films prepared by reactive plasma deposition

Polycrystalline Ga-doped (Ga content: 4 wt%) ZnO (GZO) thin films were deposited on glass substrates at 200 C by a reactive plasma deposition with DC arc discharge technique. The dependences of structural and electrical properties of GZO films on thickness, ranging from 30 to 560 nm, were investigated. Carrier concentration, n, and Hall mobility, μ, increases with increasing film thickness below 100 nm, and then the n remains nearly constant and the μ gradually increases until the thickness reaches 560 nm. The resistivity obtained of the order of 10⁻⁴ Ω cm for these films decreases with increasing film thickness: The highest resistivity achieved is 4.4×10⁻⁴ Ω cm with n of 7.6×10²⁰ cm⁻³ and μ of 18.5 cm²/V s for GZO films with a thickness of 30 nm and the lowest one is 1.8×10⁻⁴ Ω cm with n of 1.1×10²¹ cm⁻³ and μ of 31.7 cm²/V s for the GZO film with a thickness of 560 nm. X-ray diffraction pattern for all the films shows a hexagonal wurtzite structure with its strongly preferred orientation along the c-axis. Full width at half maximum of the (002) preferred orientation diffraction peak of the films decreases with increasing film thickness below 100 nm.     

Carrier concentration and shallow electron states in In-doped hydrothermally grown ZnO

Single-crystal ZnO has been hydrothermally grown with additional In₂O₃ in the solution. Schottky barrier contacts have been deposited by electron beam evaporation of Pd onto the face. Capacitance–voltage measurements have been performed to reveal the carrier concentration as a function of the In₂O₃ content in the solution, and secondary-ion mass spectrometry was used to measure the resulting In concentration in the samples. For an In₂O₃ content of 2×10¹⁹ cm⁻³, the average free electron concentration increased to 5×10¹⁸ cm⁻³ compared to 4×10¹⁷ cm⁻³ for the non-doped material. An increase of the In₂O₃ content to 4×10¹⁹ cm⁻³ leads to a measured carrier concentration of approximately 1×10¹⁹ cm⁻³; however, only up to a quarter of the incorporated In became electrically active. From thermal admittance spectroscopy measurements two prominent electronic levels are found, and compared with to the non-doped material case, the freeze-out of the shallow doping in the In-doped samples takes place at lower temperatures (below 80 K).     

Low resistance nonalloyed Al-based ohmic contacts on n-ZnO:Al

We have investigated the electrical properties of nonalloyed Al, Al/Au, and Al/Pt ohmic contacts on n-type ZnO:Al (2×10¹⁸ cm⁻³). All Al-based nonalloyed ohmic contacts on the n-ZnO:Al reveal linear current–voltage behavior with low specific contact resistivity of 8.5×10⁻⁴ (Al), 8.0×10⁻⁵ (Al/Au) and 1.2×10⁻⁵ Ω cm² (Al/Pt), respectively. Using secondary ion mass spectroscopy (SIMS) and x-ray photoelectron spectroscopy (XPS) depth profiles, it was found that the O atoms in the ZnO:Al layer outdiffused to Al metal layer while the Al atoms indiffused to the surface region of ZnO:Al. This interdiffusion between Al and O atoms at room temperature results in an increase of doping concentration in the surface region of the ZnO:Al and reduces a specific contact resistivity of the Al-based ohmic contacts without thermal annealing process.     

In-plane gate transistors in AlxGa1−xN/GaN heterostructures written by focused ion beams

Focused ion beam implantation of gallium and dysprosium was used to locally insulate the near-surface two-dimensional electron gas of Al_xGa_1−xN/GaN heterostructures. The threshold dose for insulation was determined to be 2×10¹⁰ cm⁻¹ for 90 keV Ga⁺ and 1×10⁹ cm⁻¹ for 200 keV Dy²⁺ at 4.2 K. This offers a tool not only for inter-device insulation but also for direct device fabrication. Making use of “open-T” like insulating line patterns, in-plane gate transistors have been fabricated by focused ion beam implantation. An exemplar with a geometrical channel width of 1.5 μm shows a conductance of 32 μS at 0 V gate voltage and a transconductance of around 4 μS, which is only slightly dependent on the gate voltage.     

Correlation between morphological, electrical and optical properties of GaN at all stages of MOVPE Si/N treatment growth

GaN has been grown using Si/N treatment growth by MOVPE on sapphire (0001) in a home-made vertical reactor. The growth was monitored by in situ laser reflectometry. The morphological, electrical and optical properties of GaN are investigated at all the growth stages. To this aim, the growth was interrupted at different stages. The obtained samples are ex situ characterized by scanning electron microscopy (SEM), room temperature Van der Pauw–Hall electrical transport and low temperature (13 K) photoluminescence (PL) measurements. The SEM images show clearly the coalescence process. A smooth surface is obtained for a fully coalesced layer. During the coalescence process, the electron concentration (n) and mobility (μ) vary from 2×10¹⁹ cm⁻³ to 2×10¹⁷ cm⁻³ and 12 cm²/V s–440 cm²/V s, respectively. The PL maxima shift to higher energy and the FWHM decreases to about 4 meV. A correlation between PL spectra and Hall effect measurements is made. We show that the FWHM follows a n^2/3 power law for n above 10¹⁸ cm⁻³.     

Large area SiC substrates and epitaxial layers for high power semiconductor devices — An industrial perspective

We review the progress in the industrial production of SiC substrates and epitaxial layers for high power semiconductor devices. Optimization of SiC bulk growth by the sublimation method has resulted in the commercial release of 100 mm n-type 4H-SiC wafers and the demonstration of micropipe densities as low as 0.7 cm⁻² over a full 100 mm diameter. Modelling results link the formation of basal plane dislocations in SiC crystals to thermoelastic stress during growth. A warm-wall planetary SiC-VPE reactor has been optimized up to a 8×100 mm configuration for the growth of uniform 0.01–80-micron thick, specular, device-quality SiC epitaxial layers with low background doping concentrations of <1×10¹⁴ cm⁻³, and intentional p- and n-type doping from 1×10¹⁵ to >1×10¹⁹ cm⁻³. We address the observed degradation of the forward characteristics of bipolar SiC PiN diodes [H. Lendenmann, F. Dahlquist, J.P. Bergmann, H. Bleichner, C. Hallin, Mater. Sci. Forum 389–393 (2002) 1259], and discuss the underlying mechanism due to stacking fault formation in the epitaxial layers. A process for the growth of the epitaxial layers with a basal plane dislocation density <10 cm⁻² is demonstrated to eliminate the formation of these stacking faults during device operation [J.J. Sumakeris, M. Das, H.McD. Hobgood, S.G. Müller, M.J. Paisley, S. Ha, M. Skowronski, J.W. Palmour, C.H. Carter Jr., Mater. Sci. Forum 457–460 (2004) 1113].     

Ni–Al ohmic contact to p-type 4H-SiC

Investigations on Ni/Al alloys to form ohmic contacts to p-type 4H-SiC are presented in this paper. Different ratios of Ni/Al were examined. Rapid thermal annealing was performed in argon atmosphere at 400 C for 1 min, followed by an annealing at 1000 C for 2 min. In order to extract the specific contact resistance, TLM test structures were fabricated. A specific contact resistance of 3×10⁻⁵ Ω cm² was obtained reproducibly on Al²⁺ implanted p-type layers, having a doping concentration of 1×10¹⁹ cm⁻³. The lowest specific contact resistance value measured amounts to 8×10⁻⁶ Ω cm².     

Search for γγ→ηb in ee collisions at LEP 2

A search for the pseudoscalar meson η_b is performed in two-photon interactions at LEP 2 with an integrated luminosity of 699 pb⁻¹ collected at e⁺e⁻ centre-of-mass energies from 181 GeV to 209 GeV. One candidate event is found in the six-charged-particle final state and none in the four-charged-particle final state, in agreement with the total expected background of about one event. Upper limits of Γ_γγ(η_b)×BR(η_b→4 charged particles)<48 eV, Γ_γγ(η_b)×BR(η_b→6 charged particles)<132 eV are obtained at 95% confidence level, which correspond to upper limits of 9.0% and 25% on these branching ratios.     

Power factor enhancement in Pr-substituted alloys prepared by high-pressure sintering

Bi₈₅Sb_15−xPr_x (x=0,1,2,3) alloys with partial substitution of Pr for Sb were synthesized by mechanical alloying followed by high-pressure sintering. The crystal structure was characterized by X-ray diffraction. The electrical conductivity and Seebeck coefficient were measured in the temperature range of 80–300 K. The results show that the electrical conductivity and Seebeck coefficient of Pr-substituted samples are both larger than those of the reference sample, Bi₈₅Sb₁₅, in the whole measurement temperature range. The power factor of Bi₈₅Sb₁₃Pr₂ reaches a maximum value of 3.83×10⁻³ W K⁻² m⁻¹ at 235 K, which is about four times larger than that of the reference sample, Bi₈₅Sb₁₅, at the same temperature.     

Characterization of hybrid organic and inorganic functionalised membranes for proton conduction

Titanium zirconium phosphate and organic polymer hybrid (poly-vinyl alcohol, (3-glycidoxypropyl)-trimethoxysilane and ethylene glycol) based membranes were investigated for their potential application as proton conductors. The hybrid materials were characterized by XRD, FTIR, SEM, TGA and impedance spectroscopy analysis. It was found that embedding of functionalised inorganic particles (TiZrP) into composite polymer matrix allowed for some crystallinity formation, and cross-linking of hydroxyl groups during annealing or reactions within the organic and inorganic phases during synthesis. A complex structure was formed, as many FTIR peaks were masked by more defined peaks assigned to P–O–R bonds. The high concentration of phosphorus in the TiZrP (1:1:9 molar ratio) samples resulted in more hydrophilic particles. This was further reflected in the hybrid membranes as the water losses increased from 13 to 25 wt.% as a function of the TiZrP content changing from 10 to 50 wt.% in the final hybrid membrane, respectively. As a result, proton conductivity increased by two to three orders of magnitude from blank (organic phase only) membranes (2.61 × 10^− 5 S cm^− 1) to TiZrP hybrid membrane (2.41 × 10^− 2 S cm^− 1) at 20 °C. Proton conduction changed as a function of temperature and the Ti1Zr1P9 particles content, mainly attributed to the membrane ability to retain water, thus complying with the Grotthus mechanism.     

Structural and optical characterisation of InN layers grown by MOCVD

In the following, we report investigations of the dependencies of the structural, optical and electrical characteristics of InN thin films grown by MOCVD on the growth temperature. The layer thicknesses range from 70 to 400 nm. Their carrier concentrations range from 7×10¹⁸ to 4×10¹⁹ cm⁻³. Hall mobility values from 150 to 1300 cm²/V/s were determined in these films. The variation of the growth temperature and V/III ratio brought about different growth modes and rates. Using TEM, in addition to measuring layer thickness, we also determined the growth mode along with the structural quality of the InN layers. The surface roughness was obtained from AFM measurements. The layer crystalline quality was also investigated by means of X-ray diffraction in the rocking mode. Photoluminescence measurements performed at room temperature and at 7 K gave emission at around 0.7 eV.     

Growth and characterisation of GaN with reduced dislocation density

Two GaN MOVPE growth methods to reduce the threading dislocation (TD) density have been explored. The combined effects of (1) in situ SiN_x masking of the sapphire substrate and (2) starting the epitaxial growth at low V-to-III ratio on the GaN film quality were studied by atomic force microscopy, transmission electron microscopy and high-resolution X-ray diffraction. It was found that the annealing condition of the low-temperature nucleation layer after in situ SiN_x masking is critical in order to decrease the density of nucleation sites and hence increase the average grain size to about 5 μm. However, the coalescence of large grains with vertical side facets results in the formation of dense bundles of TDs at the grain boundaries combined with large numbers of basal-plane dislocation loops throughout the film. The formation of these dislocations can be prevented by starting the epilayer growth at low V-to-III ratio, resulting in the formation of grains with inclined side facets. The interaction of the TDs with the inclined side facets causes the dislocations to bend 90 as the grains grow in size and coalesce. GaN films with dislocation densities as low as 1×10⁸ cm⁻², giving full-width at half-maximum values of 180 and 220 arcsec for respectively (002) and (302) omega scans, were achieved by the combination of in situ masking and low V–III ratio epilayer growth. Hall carrier mobility values in excess of 900 cm² V ⁻¹ s⁻¹ were deduced for Si-doped layers.     

Atomic layer deposition and characterization of Ga-doped ZnO thin films

Low-resistivity n-type ZnO thin films were grown by atomic layer deposition (ALD) using diethylzinc (DEZ) and H₂O as Zn and O precursors. ZnO thin films were grown on c-plane sapphire (c- Al₂O₃) substrates at 300 C. For undoped ZnO thin films, it was found that the intensity of ZnO () reflection peak increased and the electron concentration increased from 6.8×10¹⁸ to 1.1×10²⁰ cm⁻³ with the increase of DEZ flow rate, which indicates the increase of O vacancies () and/or Zn interstitials (Zn_i). Ga-doping was performed under Zn-rich growth conditions using triethylgallium (TEG) as Ga precursor. The resistivity of 8.0×10⁻⁴ Ω cm was achieved at the TEG flow rate of 0.24 μmol/min.     

Oxide ion conductivity and relaxation in CaREZrNbO7 (RE = La, Nd, Sm, Gd, and Y) system

CaREZrNbO₇ (RE = La, Nd, Sm, Gd and Y) system changed from fluorite (F)-type to pyrochlore (P)-type structure when the ionic radius ratios, r(Ca²⁺–RE³⁺)_av/r(Zr⁴⁺–Nb⁵⁺)_av were larger than 1.34. Thus, the La, Nd, and Sm compounds have a cubic P-type structure and the Gd and Y ones have a defect F-type structure. The electrical conductivity was measured using complex-plane impedance analysis over a wide temperature (300–750 °C) and frequency (1 Hz–1 MHz) ranges. The conductivity relaxation phenomenon was observed in these compounds and the relaxation frequencies were found to show Arrhenius-type behavior and activation energies were in good agreement with those obtained from high temperature conductivity plots. These results support the idea that the relaxation process and the conductivity have the same origin. The ionic conductivity of CaREZrNbO₇ (RE = La, Nd, Sm, Gd and Y) system showed the maximum at the phase boundary between the F-type and P-type phases. On the other hand, the activation energy for the conduction decreased in the F-type phase and increased in the P-type phase with increasing ionic radius ratio. Among the prepared compounds, CaGdZrNbO₇ showed the highest ionic conductivity of 9.47 × 10^− 3 S/cm at 750 °C which was about twice as high as that observed in Gd₂Zr₂O₇ (4.2 × 10^− 3 S/cm at 800 °C). The grain morphology observation by scanning electron microscope (SEM) showed well-sintered grains. AC impedance measurements in various atmospheres further indicated that they are predominantly oxide ion conductors at elevated temperatures (> 700 °C).     

High-quality InAlN/GaN heterostructures grown by metal–organic vapor phase epitaxy

We fabricated high-quality InAlN/GaN heterostructures by metal–organic vapor phase epitaxy (MOVPE). X-ray diffraction measurements revealed that InAlN/GaN heterostructures grown under optimal conditions have flat surfaces and abrupt heterointerfaces. Electron mobility from 1200 to 2000 cm²/V s was obtained at room temperature. To our knowledge, this mobility is the highest ever reported for InAlN/GaN heterostructures. We also investigated the relationship between the Al composition and sheet electron density (N_s) for the first time. N_s increased from 1.0×10¹² to 2.7×10¹³ cm⁻² when the Al composition increased from 0.78 to 0.89.     

Preparation and characterization of ZnO thin films prepared by thermal oxidation of evaporated Zn thin films

In this paper, the experimental results regarding some structural, electrical and optical properties of ZnO thin films prepared by thermal oxidation of metallic Zn thin films are presented.Zn thin films (d=200–400 nm) were deposited by thermal evaporation under vacuum, onto unheated glass substrates, using the quasi-closed volume technique. In order to obtain ZnO films, zinc-coated glass substrates were isochronally heated in air in the 300–660 K temperature range, for thermal oxidation.X-ray diffraction (XRD) studies revealed that the ZnO films obtained present a randomly oriented hexagonal nanocrystalline structure. Depending on the heating temperature of the Zn films, the optical transmittance of the ZnO films in the visible wavelength range varied from 85% to 95%. The optical band gap of the ZnO films was found to be about 3.2 eV. By in situ studying of the temperature dependence of the electrical conductivity during the oxidation process, the value of about 2×10⁻² Ω⁻¹ m⁻¹ was found for the conductivity of completely oxidized ZnO films.     

AlGaN/GaN HEMTs grown on silicon (001) substrates by molecular beam epitaxy

We report the realization of an AlGaN/GaN HEMT on silicon (001) substrate with noticeably better transport and electrical characteristics than previously reported. The heterostructure has been grown by molecular beam epitaxy. The 2D electron gas formed at the AlGaN/GaN interface exhibits a sheet carrier density of 8×10¹² cm⁻² and a Hall mobility of 1800 cm²/V s at room temperature. High electron mobility transistors with a gate length of 4 μm have been processed and DC characteristics have been achieved. A maximum drain current of more than 500 mA/mm and a transconductance g_m of 120 mS/mm have been obtained. These results are promising and open the way for making efficient AlGaN/GaN HEMT devices on Si(001).     

Probe calibration of the scanning thermal microscope in the AC mode

Scanning thermal microscope based DC measurements of local thermal conductivity are relevant to insulating (<20 W m⁻¹ K⁻¹) materials only. We aim at using the 3ω method to enhance the sensitivity of the device to a larger range. In this paper we present both a thermal model and experimental results from the calibration procedure to study the thermal behaviour of stand-alone probes. The two approaches provide data in very good agreement on the full measured frequency domain. Several geometric and thermal parameters are deduced from the comparison. Those quantities are key inputs for future heat transfer modeling of the tip–sample contact.     

Electrical characterisation of phosphorus-doped ZnO thin films grown by pulsed laser deposition

Phosphorus-doped ZnO films were grown by pulsed laser deposition using a ZnO:P₂O₅-doped target as the phosphorus source with the aim of producing p-type ZnO material. ZnO:P layers (with phosphorus concentrations of between 0.01 to 1 wt%) were grown on a pure ZnO buffer layer. The electrical properties of the films were characterised from temperature dependent Hall-effect measurements. The samples typically showed weak n-type conduction in the dark, with a resistivity of 70 Ω cm, a Hall mobility of μ_n0.5 cm² V ⁻¹ s⁻¹ and a carrier concentration of n3×10¹⁷ cm⁻³ at room temperature. After exposure to an incandescent light source, the samples underwent a change in conduction from n- to p-type, with an increase in mobility and decrease in concentration for temperatures below 300 K.