R. Beresford

The effect of the III/V ratio and substrate temperature on the morphology and properties of GaN- and AlN-layers grown by molecular beam epitaxy on Si(1 1 1)

Abstract We have studied the effect of the III/V ratio and substrate temperature on the growth of GaN and A1N films on Si(1 1 1) substrates by molecular beam epitaxy, where active nitrogen was generated by a radio frequency plasma source. In the case of GaN, two distinct regimes of growth (Ga-rich and N-rich conditions) lead to different crystal morphologies and luminescence properties. Scanning electron micrographs of the cleaved edges of films grown under highly N-rich conditions reveal columnar features, while growth under Ga-excess results in compact layers. The lowtemperature photoluminescence associated with the N-rich films is dominated by intense and narrow exciton lines, with peaks having full-width at half-maximum of less than 2 meV, whereas the Ga-rich films exhibit weaker and broader emissions. For increasing substrate temperatures above 700°C, stoichiometry is reached at higher Ga/N ratios, pointing to an enhancement of Ga desorption characterized by an activation energy of 2.5 eV. A similar study of A1N films shows that the desorption of A1 in terms of growth rate is not relevant for the substrate temperature range studied (850–920°C). III/V ratios close to the stoichiometric value and substrate temperatures above 900°C lead to high-quality A1N layers on Si(l 1 1) substrates. Complete relaxation is reached, for both GaN and A1N, in films with thicknesses well below 1 μm.

A study of low temperature crystallization of amorphous thin film indium–tin–oxide

Deposition of tin-doped–indium-oxide (ITO) on unheated substrates via low energy processes such as electron-beam deposition can result in the formation of amorphous films. The amorphous-to-crystalline transformation was studied in this system using in situ resistivity, time resolved reflectivity, glancing incidence angle x-ray diffraction, and transmission electron microscopy. The resistivity of 180 nm thick In2O3(9.9 wt. %SnO2) was monitored during isothermal anneals at 125, 135, 145, and 165 °C. The dependence of the resistance on the volume fraction of crystalline phase was established using glancing incidence angle x-ray diffraction and a general two phase resistivity model for this system was developed. These studies show that, upon annealing, as-deposited amorphous ITO undergoes both a structural relaxation and crystallization. Structural relaxation of the amorphous material includes local ordering that increases the ionized vacancy concentration which, in turn, increases the carrier density in the ...

High-mobility amorphous In2O3–10wt%ZnO thin film transistors

The authors report on the fabrication and characterization of thin film transistors that use sputter deposited amorphous indium zinc oxide both for the channel and source-drain metallizations in a gate-down configuration. The channel and source-drain layers were deposited from a single In2O3–10wt%ZnO ceramic target using dc magnetron sputtering onto an unheated substrate. The carrier densities in the channel (2.1×1017∕cm3) and source/drain regions (3.3×1020∕cm3) were adjusted by changing the reactive oxygen content in the sputter chamber during deposition. The resulting transistors operate as depletion mode n-channel field effect devices with saturation mobility of 20cm2∕Vs and on/off current ratio of 108.

Interband tunneling in polytype GaSb/AlSb/InAs heterostructures

Polytype heterostructures of GaSb/AlSb/InAs show interband tunneling due to the 0.1 eV overlap of the InAs conduction band and the GaSb valence band. This broken‐gap configuration results in a novel mechanism for negative differential resistance that has potential applications in high‐speed devices. We have demonstrated for the first time interband tunneling in single‐barrier and double‐barrier polytype heterostructures. Single‐barrier structures show negative differential resistance due to the change in interband tunneling with applied bias. A peak‐to‐valley ratio of 2.7:1 at 77 K was observed in this case. Double‐barrier structures using an InAs quantum well exhibit resonant interband tunneling with a peak‐to‐valley current ratio of more than 60:1 at 77 K. This structure is promising for applications to three‐terminal devices because of the very wide quantum well that can be achieved.

Microstructure and photoluminescence of GaN grown on Si(111) by plasma‐assisted molecular beam epitaxy

Wurtzite GaN films on AlN buffer layers were grown on Si(111) by plasma‐assisted molecular beam epitaxy. High resolution x‐ray diffraction and scanning electron microscope studies indicate that the mosaic disorder decreases with increasing film thickness and increasing growth temperature. The grain size increases with the growth temperature. The best (0002) diffraction peak full width at half‐maximum is 22 arcmin for a film 1.7 μm thick. Prominent low‐temperature exciton luminescence is observed at 3.46 eV. The plasma I‐V characteristics are measured with a Langmuir probe near the growth position and analyzed to extract the nitrogen ion density and energy for the growth conditions used.

Resonant tunneling in AlSb/InAs/AlSb double-barrier heterostructures

We report the first observations of resonant tunneling in the AlSb/InAs material system, with a maximum peak‐to‐valley current ratio of 1.8:1 at room temperature and 9:1 at 77 K. The large AlSb/InAs barrier height of 1.8 eV for electrons and high‐mobility InAs will be advantageous in device applications. In particular, the small electron effective mass in InAs makes it possible to demonstrate quantum effects in a 24 nm well, the longest coherence distance reported for double‐barrier tunneling structures. We estimate that an AlSb/InAs resonant tunneling transistor can significantly outperform similar devices based on AlGaAs/GaAs.

Microstructure of AlN on Si (111) grown by plasma‐assisted molecular beam epitaxy

We have grown monocrystalline AlN on Si (111) substrates over the temperature range 550–900 °C using electron cyclotron resonance plasma assisted molecular beam epitaxy. The best (0002) peak omega rocking curve full width at half‐maximum value obtained was 26 min for a film deposited at 900 °C. All films nominally displayed the AlN[0001]∥Si[111] orientation. The exact angle between AlN[0001] and Si[111] decreased from 2.1° to 1.1° and the (0002) peak widths improved with increasing substrate temperature. Mosaic‐type disorder was shown by high resolution x‐ray diffraction to be the dominant cause of the ω rocking curve peak full widths.

Nanoheteroepitaxy of GaN on a nanopore array Si surface

We report the growth by molecular beam epitaxy and the optical characterization of GaN films nucleated on a Si(111) surface that has been patterned by dry etching an ordered array of nanometer-scale pores prior to the growth. The etching is performed using an anodized aluminum oxide membrane as a mask. The nanopore array surface with the pore diameter of 60 nm and periodicity of 110 nm exhibits significant effects on emissivity and the optical properties of the resulting film. Room-temperature photoluminescence intensity increases by a factor of 5 for GaN grown on nanoporous Si. Peak shifts in photoluminescence and Raman spectroscopy suggest that the material grown on nanopores may be more relaxed than films grown on flat substrates. The effects of nanopore topography on the nucleation of GaN films offer a potential path to significant improvement of III-nitride heteroepitaxy for device applications.

Competition between tensile and compressive stress mechanisms during volmer-weber growth of aluminum nitride films

Stress evolution during molecular-beam epitaxy of AIN films was monitored with in situ curvature measurements. Changes in the growth rate produced large stress variations, with more tensile stress observed at higher growth rates. For example, at a growth temperature of 750°C the instantaneous steady-state stress in films with similar grain sizes varied from −0.15GPa at a growth rate of 90nm∕h, to approximately 1.0GPa at a growth rate of 300nm∕h. To explain these results, we develop a kinetic model of stress evolution that describes both tensile and compressive mechanisms. The tensile component is based on a mechanism which is proposed here as an inherent feature of grain-boundary formation. The compressive component is based on our recent model of atom insertion, driven by the excess chemical potential of surface adatoms that is created by the growth flux. The combined model predicts that the stress is largely governed by the competition between tensile and compressive mechanisms, which can be convenientl...

DEMONSTRATION OF A SILICON FIELD-EFFECT TRANSISTOR USING ALN AS THE GATE DIELECTRIC

As a precursor to cointegration of blue and ultraviolet light emitting nitrides with silicon microelectronics, an AlN on Si(111) metal‐insulator‐semiconductor field effect transistor has been fabricated and characterized. Current–voltage data show the threshold voltage and transconductance to be −0.8 V and 6 ms/mm, respectively, for a 50 nm insulator thickness. The ideal threshold voltage is 0.09 V. From the transconductance, a channel mobility of 45 cm2/V s is inferred. Capacitance versus voltage (C–V) measurements on a 210 nm film showed a flatband voltage of −4.2 V, corresponding to 8×1011 cm−2 trapped charges at the interface. Comparison of the experimental high‐frequency (1 MHz) C–V plot to the theoretical plot indicates the presence of fast interface states. High‐resolution transmission electron microscopy studies showed the AlN/Si(111) interface to be surprisingly well ordered in places, with the large lattice mismatch accommodated via a quasiperiodic array of misfit dislocations.