AlN Insulator Research Articles

Photoelectric properties of MIS structures on high-resistivity p-type silicon with aluminium nitride tunnelling insulator

Photoelectric properties of MIS tunnel diodes produced on high-resistive p-type silicon wafers with thin aluminium nitride AlN insulator layer and Pd or Al metal gate electrodes were investigated. It was found that synthesized AlN films possess a fixed positive charge, which leads to the creation of near-surface inversion layer in silicon substrate. The ratio of the photocurrent to the dark current K = Iph / Idark (on / off ratio) was found to depend on the gate electrode material, illumination intensity and the applied reverse bias. For studied MIS structures K ratio varied from 10 4 to 10 5 and was two orders of magnitude higher than that for the control MS structure without the insulator layer. High on / off ratio of such MIS structures with AlN tunnelling insulator promotes their application as an effective photodetectors in optoelectronics.

Low-threshold voltage ultraviolet light-emitting diodes based on (Al,Ga)N metal–insulator–semiconductor structures

Al-rich III–nitride-based deep-ultraviolet (UV) (275–320 nm) light-emitting diodes are plagued with a low emission efficiency and high turn-on voltages. We report Al-rich (Al,Ga)N metal–insulator–semiconductor UV light-emitting Schottky diodes with low turn-on voltages of <3 V, which are about half those of typical (Al,Ga)N p–i–n diodes. Our devices use a thin AlN film as the insulator and an n-type Al0.58Ga0.42N film as the semiconductor. To improve the efficiency, we inserted a GaN quantum-well structure between the AlN insulator and the n-type AlxGa1−xN semiconductor. The benefits of the quantum-well structure include the potential to tune the emission wavelength and the capability to confine carriers for more efficient radiative recombination.

Fabrication and Analysis of AlN/GaAs(001) and AlN/Ge/GaAs(001) Metal–Insulator–Semiconductor Structures

Using sputtering-deposited AlN insulator films, we fabricated and analyzed AlN/GaAs(001) and AlN/Ge/GaAs(001) metal–insulator–semiconductor (MIS) structures; the former is obtained by the direct deposition of AlN on GaAs, while the latter includes a Ge interlayer between AlN and GaAs. By current–voltage (I–V) measurements, we obtained similar good insulating properties for both MIS structures. On the other hand, we observed rather different frequency dispersions in the capacitance–voltage (C–V) characteristics of the MIS structures; the AlN/Ge/GaAs(001) MIS structure exhibits a significantly smaller dispersion than the AlN/GaAs(001) MIS structure, attributed to a smaller interface state density, as confirmed by analysis using the conductance method. From the X-ray photoelectron spectroscopy analysis results of the MIS interfaces, we found that, in comparison with the AlN/GaAs(001) interface, the AlN/Ge/GaAs(001) interface exhibits suppressed As deficiency and Ga–O bonding, with an indication of their relation to interface states.

Properties of ${\hbox{MgB}}_{2}$ Thin Films and SIS Junctions Fabricated Under Various Fabrication Conditions

We investigated the optimal conditions to fabricate MgB2 superconductor-insulator-superconductor junctions. By investigating thin film qualities obtained under various deposition conditions, we found that the critical temperature depended strongly on the Mg sputtering power. Although a higher Mg sputtering power increased the MgB2 thin film's critical temperature, the dependence of the zero resistance transition width suggested that the higher Mg deposition power produced Mg-rich thin Alms. To evaluate the gap voltage of MgB2 thin films on c-plane sapphire substrates, we fabricated superconductor- insulator-normal metal junctions and determined that the gap voltage of MgB2 was 2.0 mV. Using these MgB2 thin Alms, we produced an MgB2/SiC/MgB2 junction and an MgB2/AlN/MgB2 junction. For both of these SIS junctions, we obtained clear gap structures, but at this stage, the AlN insulator appeared to be better than the SiC insulator based on current-voltage characteristics. We found that the current-voltage characteristics depend on the MgB2 thin film quality when using an AlN insulator. Our TEM measurements revealed epitaxial growth of the lower MgB2 thin films and amorphous-like growth of the upper MgB2 thin films. We obtained their depth profile with Auger electron spectroscopy, and the profile indicated that the trilayer has a nitrided and oxidized region at the insulator portion.

Fabrication and characterization of C 60 FET using highly c-axis oriented poly-crystalline AlN insulator film

Most important problem in C 60 field effect carrier injection technique is how to form an excellent interface between insulator and solid C 60 layers. In this paper, we have investigated the solid C 60 growth on the poly-crystalline AlN (0 0 0 1) layers with the highly and/or the randomly c-axis-oriented surfaces by molecular beam epitaxial (MBE) technique. Moreover, we have compared with the conductive characteristic of the C 60/AlN FET fabricated on the highly and/or the randomly c-axis-oriented poly-crystalline AlN layers. The results strongly indicate that the c-axis orientation is an important factor to improve the performance of the C 60/poly-crystalline AlN FET.

Characteristics of all-MgB2 superconductor–insulator–superconductor junctions obtained with as-grownMgB2 thin films

All-MgB2 SIS junctionswere fabricated on c-plane sapphire substrates using AlN film for the insulator layer. The critical temperature of the lowerMgB2 layer after the SIS junction fabrication process was 29 K, which remained the same as that of bareMgB2 films. On the other hand, the critical temperature of the upperMgB2 layer was depressed to 17 K. The current–voltage characteristics showed typical SIS properties withvery clear tunnelling currents and gap voltages, demonstrating an excellent insulator covering of thelower MgB2 films. The Josephson current decreased exponentially withincreasing AlN insulator layer thickness, and a Josephson current of115 A cm−2 was obtained for the junction with a 0.14 nm thick AlN insulator. However, the Josephson currents ofMgB2/AlN/MgB2 junctions werelower than that of MgB2/AlN/NbN junctions with the same AlN thickness; this indicated the existence of other insulatorlayers. The ideal modulation of Josephson current was observed by applying anexternal magnetic field; this indicated a uniform tunnelling current flow in thejunctions. The gap voltages were investigated on the basis of the current–voltagecharacteristics.

MgB2 thin films and MgB2/AlN/MgB2 SIS junctions

As-grown MgB2 thin films were fabricated by a sputtering method and a co-evaporation method. We found that the critical temperatures strongly depended on the deposition rate. The MgB2 thin films showed critical temperatures of 29K for the sputtering method and 35K for the co-evaporation method with relatively low substrate temperatures. For the fabrication of all MgB2 SIS junctions, we selected AlN as the material for the barrier layer, which has hexagonal crystal structure. In fabricating MgB2/AlN/NbN junctions, we found that the AlN deposition with a higher substrate temperature formed insulator layers other than AlN. We also prepared MgB2/AlN/MgB2 trilayers for SIS junctions using low AlN deposition temperature condition without breaking the vacuum. The MgB2/AlN/MgB2 junctions showed a clear Josephson current and gap structures. The critical current density was 120A/cm2 and the ratio of sub-gap resistance and the normal resistance was 3.3 when the AlN insulator thickness was 0.14nm. We showed current–voltage characteristics of the MgB2/AlN/MgB2 junctions with varying AlN layer thicknesses.

Insulator layer formation in MgB2 SIS junctions

The dependence of current–voltage characteristics on thin film deposition conditions was investigated using MgB2/AlN/NbN SIS junctions. By increasing the substrate temperature in AlN insulator deposition, the current density decreased and the normal resistance increased. The results indicated that an additional insulator layer between the MgB2 and AlN formed, either before or during the AlN deposition. The thickness of the additional insulator layer was increased with an increase in the AlN deposition temperature. From the dependence of current density on the thickness of AlN in low temperature depositions, the thickness of the additional insulator layer was estimated to be 1–1.5nm when the AlN insulator was deposited from 0.14 to 0.7nm. Moreover, with the current density of MgB2/AlN/MgB2 SIS junctions, further insulator layer formation was confirmed.

Solid C 60 layer growth on AlN (0001) surface for C 60 FET structure by MBE

One of the most important problems is how to form an excellent interface between the insulator and the C60 layers in the C60 field-effect doping technique. In this paper, we propose a new structure of the C60 field-effect transistor which consists of an epitaxial AlN layer as the insulator and a C60 epitaxial layer grown on an AlN (0001) surface. We have mainly investigated solid C60 growth on the AlN (0001) surface by the MBE technique in the present work. C60 grains with face centred cubic structure have been obtained on a smooth AlN surface. Four types of the C60 grains were obtained, that is, the 0° orientation and its twin grains and the 30° orientation and its twin grains. We have found that the 0° orientation grains only can be obtained by controlling the growth temperature. The results strongly indicate that an excellent interface between the AlN insulator and the C60 layer can be formed for device applications. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An AlN/GaN insulated gate heterostructure field effect transistor with regrown n+GaN source and drain contact

A nobel insulated gate heterostructure field effect transistor (IG-HFET) was demonstrated. The layer structure under the gate consisted of a thick AlGaN barrier, an n+GaN channel 15nm thick, and an AlN insulator 4nm thick, grown sequentially by MOCVD. The device operated with gate voltage up to +3V. The pinch-off voltage was about 0V, which resulted from the very high Schottky barrier height and the very thin AlN layer. The measured transconductance gm was 235mS/mm for Lg=1.4μm, which is the highest so far reported.

AlN/GaN insulated gate heterostructure FET withregrown n + GaN ohmic contact

An AlN/GaN insulated gate heterostructure FET is presented. An n+GaN channel is placed on a thick Al0.15Ga0.85N layer and is covered by a 4 nm thick AlN insulator layer. The FET has a threshold voltage of near 0 V. A Gm of 220 mS/mm for a 1.4 µm gate length was obtained.

The influence of the strain-induced electric field on the charge distribution in GaN-AlN-GaN structure

We show that strongly pronounced piezoelectric properties play a key role in GaN-AlN-GaN semiconductor-insulator-semiconductor (SIS) and related structures. In sufficiently thin AlN layers, the lattice constant mismatch is accommodated by internal strains rather than by the formation of misfit dislocations. These lattice-mismatch-induced strains generate polarization fields. We demonstrate that, in a GaN-AlN-GaN SIS structure with the growth axis along a (0001) crystallographic direction, the strain-induced electric fields can shift the flat band voltage and produce an accumulation region on one side and a depletion region on the other side of the AlN insulator. On which side of the insulator the accumulation region is produced depends on the type of atomic plane at the heterointerface (Ga or N). The surface charge density caused by the piezoeffect is on the order of 1012 cm−2. As a consequence of the asymmetry in the space charge distribution, the capacitance-voltage (C-V) characteristics of the SIS structure become asymmetrical. The asymmetrical shift of the C-V characteristics with respect to the origin is on the order of 1.5 V for a 30 Å AlN film. This asymmetry should vanish in a relaxed film. Hence, capacitance-voltage measurements of GaN-AlN-GaN SIS structures can be used for quantitative characterization as to the degree of AlN film relaxation, depending on the film thickness. This and related techniques should become important tools for the characterization of piezoelectric layered semiconductor films.