Epitaxial Junctions Research Articles

Epitaxial Al/GaN and Au/GaN junctions on as‐grown GaN(0001)1 × 1 surfaces

AbstractGaN(0001) epilayers were fabricated by rf‐plasma enhanced molecular beam epitaxy on GaN templates. The templates were grown by metalorganic chemical vapor deposition on sapphire. The layers exhibited the 2 × 2 reconstruction of the Ga‐face during growth and the 1 × 1 reconstruction upon cooling. On such surfaces, Al/n‐GaN and Au/n‐GaN junctions were fabricated in‐situ by molecular beam epitaxy. X‐ray photoemission spectroscopy studies allowed us to determine n‐type Schottky barrier heights of 0.61 ± 0.06 and 0.98 ± 0.06 eV, respectively, for the two types of epitaxial junctions. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Defect formation in epitaxial GaP junctions under heat treatment

A theoretical and experimental investigation is reported into the influence of hydrogen heat treatment on the electrical and electroluminescent characteristics of GaP LEDs. It is shown that electrical transport in processed specimens is mainly by tunneling. The tunneling rate as a function of some properties is examined. A mechanism of electrical transport in the LEDs is proposed. The role of hopping conduction is discussed, and its parameters are determined. Electroluminescence spectra are measured and explained in terms of thermally induced structural changes in defects.

Hysteretic current–voltage characteristics and resistance switching at an epitaxial oxide Schottky junction SrRuO3∕SrTi0.99Nb0.01O3

Transport properties have been studied for a perovskite heterojunction consisting of SrRuO3 (SRO) film epitaxially grown on SrTi0.99Nb0.01O3 (Nb:STO) substrate. The SRO/Nb:STO interface exhibits rectifying current–voltage (I–V) characteristics agreeing with those of a Schottky junction composed of a deep work-function metal (SRO) and an n-type semiconductor (Nb:STO). A hysteresis appears in the I–V characteristics, where high resistance and low resistance states are induced by reverse and forward bias stresses, respectively. The resistance switching is also triggered by applying short voltage pulses of 1μs–10ms duration.

Magnetotransport in Exchange-Coupled Magnetite Junctions

Epitaxial magnetic junctions of theoretically half-metallic electrode material magnetite were fabricated with a cobalt chromite paramagnetic insulating tunnel barrier. This spinel structure barrier was chosen to be isostructural to the magnetite electrodes. Highly crystalline trilayers with low surface roughness were grown. Strong exchange coupling between the electrodes is observed across the barrier. Magnetoresistance measurements are reminiscent of a single ferromagnetic layer and not a junction.

Accuracy of scanning capacitance microscopy for the delineation of electrical junctions

In this work, we investigate the theoretical and the experimental accuracy limits of the delineation of pn junctions by scanning capacitance microscopy. We compare three different techniques basing both on experimental data and on one- and two-dimensional simulations of scanning capacitance measurements of epitaxial and shallow junctions.

Characterization of low frequency noise in epitaxial NbN/AlN/NbN tunnel junctions

We measured the low frequency noise characteristics and subharmonic gap structures of epitaxial NbN/AlN/NbN tunnel junctions with different current densities. For all of the junctions, the voltage noise power spectrum S/sub v/(f) showed a frequency dependence that is well described by 1/f behavior. Subharmonic gap structures were clearly observed in the dV/dI-V curves at voltage 2/spl Delta//ne, and the values of n increased with the current density J/sub c/ in the junctions. We estimated the 1/f noise parameter /spl eta/ using the empirical theory of Rogers and Buhrman for the S/sub v/(f), and investigated the correlation between the /spl eta/ and the inverse junction quality 1/Q. We found that the /spl eta/-1/Q relationship for our epitaxial tunnel junctions gave a different behavior for nonepitaxial tunnel junctions. The tunnel barrier properties were investigated by applying a pulse voltage to the junctions at 4.2 K, and measuring the applied voltage dependence of the I-V characteristics and low frequency noise amplitude.

Observation of inverse magnetoresistance in epitaxial magnetite/manganite junctions

We have fabricated and characterized epitaxial Fe3O4/CoCr2O4/La0.7Sr0.3MnO3 magnetic trilayer junctions (MTJs) grown on (110) and (100) SrTiO3 substrates. Large inverse junction magnetoresistance (JMR) as high as −25% has been observed at a field of 4 kOe in (110) oriented MTJs. The improvement of the JMR over that observed in previous epitaxial magnetite junctions is attributed to the choice of the spinel structure CoCr2O4 barrier, which minimizes structural disorder at the barrier/Fe3O4 interface. As a comparison, studies on (100) MTJs elucidate the effect of the magnetic domain state of the ferromagnetic electrodes on the JMR.

Negative spin polarization of Fe3O4 in magnetite/manganite-based junctions.

Epitaxial oxide trilayer junctions composed of magnetite (Fe3O4) and doped manganite (La0.7Sr0.3MnO3) exhibit inverse magnetoresistance as large as -25% in fields of 4 kOe. The inverse magnetoresistance confirms the theoretically predicted negative spin polarization of Fe3O4. Transport through the barrier can be understood in terms of hopping transport through localized states that preserve electron spin information. The junction magnetoresistance versus temperature curve exhibits a peak around 60 K that is explained in terms of the paramagnetic to ferrimagnetic transition of the CoCr2O4 barrier.

Highly crystallized as-grown smooth and superconducting MgB2 films by molecular-beam epitaxy

We have investigated the growth of superconductive thin films of magnesium diboride (MgB2) by molecular-beam epitaxy. A Si(111) substrate with a seed layer of MgO was used for the growth of these films by varying parameters such as the growth temperature, Mg:B flux ratio and deposition rate as well as the background pressure. It was found that highly crystallized films could already form at 250 °C; however, only in a narrow window of growth parameters. The highest critical temperature of 35.2 K with a sharp transition (ΔTC of 0.5 K) was observed for films grown at 300 °C. Using a capping layer of MgO proved to be highly beneficial for the preservation and the smoothness of these films. Together with the fact that MgO proved to be a good seed layer for thin films of MgB2 makes it an ideal candidate for growing all epitaxial MgB2 Josephson junctions.

7.4 kV, 330 A (pulsed), single chip, high temperature 4H-SiC pin rectifier

The design, fabrication process, and high temperature characteristics of a 4H-SiC rectifier with a 7.4 kV, 330 A (pulsed) capability is reported. A highly-doped p-type epitaxial anode layer and junction termination extension were used to obtain good on-state and stable reverse blocking characteristics. A forward voltage drop of 4.5 V was observed at 100 A/cm/sup 2/ (36 A). Measurements in the 25-200/spl deg/C range show little change in reverse leakage characteristics up to 4.5 kV. Reverse recovery measurements for a forward current density of 100 A/cm/sup 2/ show a modest 33% increase (from 18 to 24 A) in the peak reverse recovery current, and only a 113% increase (from 1.8 to 3.8 /spl mu/C) in Q/sub /spl pi// at 200/spl deg/C compared to 25/spl deg/C.

Large area, ultra-high voltage 4H-SiC p-i-n rectifiers

This paper reports the design, fabrication and high temperature characteristics of 1 mm/sup 2/, 4 mm/sup 2/ and 9 mm/sup 2/ 4H-SiC p-i-n rectifiers with 6 kV, 5 kV, and 10 kV blocking voltage, respectively. These results were obtained from two lots in an effort to increase the total power levels on such rectifiers. An innovative design utilizing a highly doped p-type epitaxial anode layer and junction termination extension (JTE) were used in order to realize good on-state as well as stable blocking characteristics. For the 1 mm/sup 2/ and 4 mm/sup 2/ rectifier, a forward voltage drop of less than 5 V was observed at 500 A/cm/sup 2/ and the peak reverse recovery current shows a modest 50% increase in the 25/spl deg/C to 225/spl deg/C temperature range. On the 10 kV, 9 mm/sup 2/ rectifier, a forward voltage drop of less than 4.8 V was observed at 100 A/cm/sup 2/ in the entire 25/spl deg/C to 200/spl deg/C temperature range. For this device, the reverse recovery characteristics show a modest 110% increase in the peak reverse recovery current from 25/spl deg/C to 200/spl deg/C. A dramatically low Q/sub rr/ of 3.8 /spl mu/C was obtained at a forward current density of 220 A/cm/sup 2/ at 200/spl deg/C for this ultra high voltage rectifier. These devices show that more than three orders of magnitude reduction in reverse recovery charge is obtained in 4H-SiC rectifiers as compared to comparable Si rectifiers.

Gallium Nitride Power Device Design Tradeoffs

carrier lifetime, termination, HFET, polarization charge Abstract. Design curves for GaN power devices are developed by a power-law fit to theoretically calculated ionization coefficients, including parallel-plane depletionwidth and breakdown voltage versus doping. An epitaxial junction termination extension structure is described for vertical GaN devices and the effect of AlGaN/GaN polarization charge on breakdown in HFETs is considered. A key design relationship for GaN unipolar devices, the specific on-resistance is estimated versus breakdown voltage and for bipolar devices, the carrier lifetime requirement versus breakdown voltage is calculated.

Effects of resonant interface states on tunneling magnetoresistance

Based on model and ab initio calculations we discuss the effect of resonant interface states on the conductance of epitaxial tunnel junctions. In particular we show that the ``hot spots'' found by several groups in ab initio calculations of symmetrical barriers of the ${\mathbf{k}}_{\ensuremath{\Vert}}$-resolved conductance can be explained by the formation of bonding and antibonding hybrids between the interface states on both sides of the barrier. If the resonance condition for these hybrid states is met, the electron tunnels through the barrier without attenuation. Even when both hybrid states move together and form a single resonance, strongly enhanced transmission is still observed. The effect explains why, for intermediate barrier thicknesses, the tunneling conductance can be dominated by interface states, although hot spots only occur in a tiny fraction of the surface Brillouin zone.

Transport properties and electronic structure of epitaxial tunnel junctions

We present ab initio calculations for the electronic ground-state and transport properties of epitaxial Fe/semiconductor/Fe (0 0 1) tunnel junctions. The ground state properties are determined by the ab initio Screened KKR Green's function method and the transport properties by a Green's function formulation of the Landauer–Büttiker formalism. We focus on tunnel junctions with a semiconducting ZnSe barrier and compare them to results for junctions with Si and GaAs barriers. We comment on the presence of metal-induced gap states (MIGS) in the semiconductor, the spin polarization of which strongly depends on the nature of the barrier. We investigate furthermore the influence of one atomic layer at the interface of a non-magnetic metal (Cu, Ag, Al) and of a magnetic 3d transition metal.

Large magnetoresistance in Fe/MgO/FeCo(001) epitaxial tunnel junctions on GaAs(001)

We present tunneling experiments on Fe(001)/MgO(20 Å)/FeCo(001) single-crystal epitaxial junctions of high quality grown by sputtering and laser ablation. Tunnel magnetoresistance measurements give 60% at 30 K, to be compared with 13% obtained recently on (001)-oriented Fe/amorphous-Al2O3/FeCo tunnel junctions. This difference demonstrates that the spin polarization of tunneling electrons is not directly related to the density of states of the free metal surface—Fe(001) in this case—but depends on the actual electronic structure of the entire electrode/barrier system.

1/f noise in high current density NbN/AlN/NbN tunnel junctions

Low frequency noise characteristics of epitaxial NbN/AlN/NbN tunnel junctions have been investigated. For all of our junctions with different current densities, the voltage noise power spectrum S/sub v/(f) showed a frequency dependence of 1/f type. The magnitude of the S/sub v/(f) exhibited two distinct types of dependency on the current density and AlN barrier thickness. We consider that this may result from a difference in the crystal structures of the AlN barrier by consulting with the barrier thickness dependence of tunnel barrier heights. We also estimated the 1/f noise parameter /spl eta/ using the Rogers and Buhrman's empirical theory for the S/sub v/(f), and investigated the relationship between the /spl eta/ and the current density J/sub c/ and the tunnel barrier thickness d/sub AlN/. The tunnel barrier was characterized by investigating the /spl eta/-d/sub AlN/ relation. It was found that the /spl eta/-J/sub c/ and /spl eta/-d/sub AlN/ relation in our high current density junctions, i.e, in the epitaxial tunnel junctions, are different from nonepitaxial tunnel junctions.

Evidence for charge gain mechanism in SI-GaAs detectors with epitaxial junction

We made a characterisation of GaAs detectors with an epitaxial p-type layer deposited on the 200 /spl mu/m semi-insulating substrate. The charge collection efficiency for 60-keV photons and 5.49 MeV alpha particle depends on the doping level of the p-layer. When completely depleted (reverse bias >200-300 V), the collected charge can be greater than 100%, implying the presence of some charge gain mechanism. At the same reverse bias and doping concentration, the collected charge depends also on the size of the contact pad. Moreover, the lower the p-doping, the lower the current density. The present findings confirm our previous work, obtaining gains up to 4:1.

Numerical simulation of Bragg-case section topographic images ofdislocations in silicon

Synchrotron white beam with a wavefront limited by a 5 µm slit was used for obtaining the Bragg-case section patterns in siliconsubstrates and epitaxial layers. The section images contained variousinterference fringes, such as the Uragami fringes and fringes caused bycrystal curvature. The system of fringes connected with individual defectswas also observed.The experimental images were compared with simulated theoretical imagesobtained by numerical integration of the Takagi-Taupin equations. Areasonably good correspondence was obtained for dislocations inclined tothe surface and misfit dislocations. The elements of the image wereanalysed using the visualization of |Dh|2 and |Do|2 intensities inthe plane of diffraction, where an additional amount of transmitted waveintensity indicated the decomposition of wavefields or the reflection ofthe redirected waves from the surfaces. Comparative studies of simulatedprecipitation images and modification of dislocation images caused bycurvature and by the diffusion of an epitaxial junction were alsoperformed.

Phosphorus and arsenic profile control for high performance epitaxial base bipolar junction devices

In this study, we report on the incorporation behavior of n-type species in undoped layers grown subsequent to doped layers at low temperature (700–750 °C) reduced pressure chemical vapor deposition of silicon/silicon germanium (Si/Si1−xGex) performed in a single wafer epitaxial deposition system. Significant amounts of these species are observed even in an undoped layer grown subsequent to the doped layer. The presence of these dopants in the subsequently grown undoped layer is attributed to segregation at the moving growth interface. The arsenic segregation is confirmed to be significantly higher than phosphorus. In addition, an increase in both the phosphorus and arsenic incorporation is observed in the presence of a highly doped boron layer.

Effects of surface defects on the performance of 4H– and 6H–SiC pn junction diodes

Effects of surface defects on performance of kV-class 4H– and 6H–SiC epitaxial mesa pn junction diodes were investigated. Mapping studies of surface morphological defects have revealed that triangular-shaped defects severely degrade high-blocking capability of the diodes whereas shallow round pits and scratch give no direct impact. The perimeter recombination and generation, instead of the bulk process, are responsible for forward recombination current and reverse leakage current of the diodes, respectively. Effective minority carrier lifetimes are mainly limited not by bulk recombination but by perimeter recombination.