Reverse breakdown voltages larger than 1 kV have been reported for both unterminated Ga2O3 vertical rectifiers (1000- 1600 V) and field-plated Schottky diodes (1076-2300 V) with an epi thickness of 8-20 μm. If the doping is in the 1016 cm-3 range, the breakdown is usually in the 500-800V regime. Furthermore, the switching characteristics of discrete Ga2O3 vertical Schottky rectifiers exhibited reverse recovery times in the range of 20 to 30 ns. Large area (up to 0.2 cm2 ) Ga2O3 rectifiers were fabricated on a Si-doped n-Ga2O3 drift layer grown by halide vapor phase epitaxy on a Sn-doped n+ Ga2O3 (001) substrate. A forward current of 2.2 A was achieved in single-sweep voltage mode, a record for Ga2O3 rectifiers. The on-state resistance was 0.26 Ω·cm2 for these largest diodes, decreasing to 5.9 × 10-4 Ω·cm2 for 40x40 μm2 devices. We detail the design and fabrication of these devices. In addition, an inductive load test circuit was used to measure the switching performance of field-plated, edge-terminated Schottky rectifiers with a reverse breakdown voltage of 760 V (0.1 cm diameter, 7.85x10-3 cm2 area) and an absolute forward current of 1 A on 8 Μm thick epitaxial β-Ga2O3 drift layers. These devices were switched from 0.225 A to -700 V with trr of 82 ns, and from 1 A to -300 V with trr of 64 ns and no significant temperature dependence up to 125°C. There was no significant temperature dependence of trr up to 150°C.
We investigated the crystal structure, growth kinetics and electrical properties of BeMgZnO/ZnO heterostructures grown by Molecular Beam Epitaxy (MBE). Transmission Electron Microscopy (TEM) studies revealed that incorporation of Mg into the BeZnO solid solution eliminates the high angle grain boundaries that are the major structural defects in ternary BeZnO. The significant improvement of x-ray diffraction intensity from quaternary BeMgZnO alloy compared to ternary BeZnO was attributed to the reduction of lattice strain, which is present in the latter due to the large difference of covalent radii between Be and Zn (1.22 Å for Zn, 0.96 Å for Be). Incorporation of Mg, which has a larger covalent radius of 1.41Å, reduced the strain in BeMgZnO thin films and also enhanced Be incorporation on lattice sites in the wurtzite lattice. The Zn/(Be + Mg) ratio necessary to obtain single-crystal O-polar BeMgZnO on (0001) GaN/sapphire templates was found to increase with increasing substrate temperature:3.9, 6.2, and 8.3 at substrate temperatures of 450°C, 475°C, and 500°C, respectively. Based on analysis of photoluminescence spectra from Be0.03MgyZn0.97-yO and evolution of reflection high-energy electron diffraction patterns observed in situ during the MBE growth, it has been deduced that more negative formation enthalpy of MgO compared to ZnO and the increased surface mobility of Mg adatoms at elevated substrate temperatures give rise to the nucleation of a MgO-rich wurtzite phase at relatively low Zn/(Be + Mg) ratios. We have demonstrated both theoretically and experimentally that the incorporation of Be into the barrier in Zn-polar BeMgZnO/ZnO and O-polar ZnO/BeMgZnO polarization doped heterostructures allows the alignment of piezoelectric polarization vector with that of spontaneous polarization due to the change of strain sign, thus increasing the amount of net polarization. This made it possible to achieve Zn-polar BeMgZnO/ZnO heterostructures grown on GaN/sapphire templates with two-dimensional electron gas densities substantially exceeding those in Zn-polar MgZnO/ZnO and O-polar ZnO/MgZnO heterostructures with similar Mg content.
Optical and structural properties of InAs/InAsSb type-II superlattices (T2SL) and their feasibility for mid- and longwavelength
infrared (MWIR and LWIR) photodetector applications are investigated. The InAs/InAsSb T2SL structures
with a broad bandgap range covering 4 μm to 12 μm are grown by molecular beam epitaxy and characterized by highresolution
x-ray diffraction and photoluminescence (PL) spectroscopy. All of the samples have excellent structural
properties and strong PL signal intensities of the same order of magnitude, indicating that non-radiative recombination is
not dominant and the material system is promising for high performance MWIR and LWIR detectors and multiband
FPAs.
Semipolar (1-101) GaN layers were grown by metal-organic chemical vapor deposition on patterned (001) Si substrates.
The effects of reactor pressure and substrate temperature on optical properties of (1-101) GaN were studied by steadystate
and time-resolved photoluminescence. The optical measurements revealed that the optical quality of (1-101)-
oriented GaN is comparable to that of c-plane GaN film grown on sapphire. Slow decay time constants, representative of
the radiative recombination, for semipolar (1-101)GaN grown at 200 Torr are found to be very long (~1.8 ns), comparable
to those for the state-of-art c-plane GaN templates grown using in situ epitaxial lateral overgrowth through silicon nitride
nano-network. Defect distribution in the GaN stripes was studied by spatially resolved cathodeluminescence
measurements. The c+-wing regions of the GaN stripes were found to be dominated by a (D0,X) emission. Only a thin
slice of emission around 3.42 eV related to basal stacking faults was revealed in c--wing regions.
This paper describes structural properties of strain-balanced InAs/InAs1-xSbx type-II superlattices (SLs) with random and
modulated InAs/InAs1-xSbx alloy layers as grown on GaSb(001) substrates either by molecular beam epitaxy (MBE) or
metalorganic chemical vapor deposition. The SL periods and the average Sb compositions of the InAs/InAs1-xSbx alloys are
determined by comparison of simulations with (004) high-resolution X-ray diffraction (XRD) measurements. The most
intense SL peaks no longer correspond to the zero-order peak because of the large SL periods, and XRD studies of thick
individual InAs/InAs1-xSbx and InAs layers show envelope modulations of the SL peaks on either side of the substrate peak,
causing some satellite peaks to be more intense than the zero-order SL peak. From the substrate - zero-order SL peak
separations, the average SL strain in the growth direction is revealed to be less than ~0.2%. Calculated bandgap energies
agree closely with photoluminescence peaks for mid-wavelength and long-wavelength infrared samples. Cross-sectional
electron micrographs reveal the entire structure including the GaSb substrate and buffer layer, the SL periods, and the
GaSb cap layer. Growth defects are occasionally visible, some originating at the substrate/buffer interface, some starting
in the middle of the buffer layer, and some located only just within the SL. Higher magnification images of the SLs
grown by MBE reveal that interfaces for InAs/InAs1-xSbx deposited on InAs are considerably more abrupt than those of InAs
deposited on InAs/InAs1-xSbx with the most likely reason being segregation of the Sb surfactant during layer growth.
Non-polar (1-100 ) and semipolar (1-101)GaN layers were grown on (112) and (001) Si substrates, respectively, by metalorganic
chemical vapor deposition. In both cases, grooves aligned parallel to the <110> Si direction were formed by
anisotropic wet etching to expose vertical {111}Si facets for growth initiation. The effect of growth conditions (substrate
temperature, chamber pressure, ammonia and trimethylgallium flow rates) on the growth habits of GaN was studied. It
was found that low pressure and low ammonia flow rate are beneficial for m-facet formation, while high ammonia flow
rate promotes formation of (1-101) facets. Steady-state and time-resolved photoluminescence measurements revealed that
the optical quality of (1-101) oriented GaN is comparable to that of c-plane GaN film grown on sapphire. The nonpolar
(1-100 ) GaN shows only weak emission and fast non-radiative recombination rate. The poor optical quality of the mplane
GaN can be explained by carbon incorporation during the growth under low pressure. Although further
optimization of the growth conditions for better optical quality is required, preliminary results obtained for semipolar
(1-101) -oriented GaN are encouraging.
The electron microscope provides a wide range of techniques that are very well suited for structural characterization of
nanophotonic materials and devices. High-resolution electron microscopy (defect identification and strain field analysis),
Z-contrast imaging in the scanning transmission electron microscope (cation distribution), convergent-beam electron
diffraction (local lattice parameter and strain), and off-axis electron holography (internal electrostatic fields), represent
powerful complementary approaches for distinguishing between the often-competing effects of growth conditions and
compositional differences. These various TEM techniques have been used separately or in tandem in our recent
collaborative studies of III-nitride heterostructures and nanostructures, where lattice mismatch, compositional
inhomogeneities and phase separation were all important considerations that can possibly impair the structural quality of
the final material and/or device. Representative applications that illustrate the prospects and some of the problems
include the following: i) relaxed InN quantum dots; ii) deep-UV-emitting AlGaN quantum wells; iii) near-UV light-emitting
diodes based on InN/GaN quantum wells; and iv) blue-green LEDs based on GaN quantum-dot superlattices.
Due to their large conduction-band offsets, GaN/AlGaN quantum wells can accommodate intersubband transitions at
record short wavelengths throughout the mid-infrared and into the near-infrared spectral regions. As a result, they are
currently the subject of extensive research efforts aimed at extending the spectral reach and functionality of intersubband
optoelectronic devices. Here we review our recent work in this area, based on GaN/AlGaN quantum-well samples
grown by molecular beam epitaxy on sapphire substrates. In particular, we have investigated the intersubband
absorption properties of a wide range of structures, including isolated and coupled quantum wells. Furthermore, we
have developed a new class of ultrafast all-optical switching devices, based on intersubband cross-absorption saturation
in GaN/AlGaN quantum-well waveguides operating at fiber-optic communication wavelengths. Strong self-phase
modulation of ultrafast optical pulses has also been measured in these waveguides, revealing a large refractive-index
nonlinearity which is related to the same intersubband carrier dynamics. Finally, we have demonstrated optically
pumped intersubband light emission from GaN/AlN quantum wells resonantly excited with a pulsed OPO. The
measured room-temperature output spectra are peaked near 2 μm, which represents a new record for the shortest
intersubband emission wavelength from any quantum-well materials system.
Nitride semiconductor quantum structures feature some unique properties for intersubband device development,
including a record large conduction-band offset that allows extending the operating wavelength to the near-infrared
spectral region, and large optical phonon energies that are advantageous for the development of THz devices. In this
paper we review our recent work aimed at the demonstration of novel intersubband device functionalities using these
materials. In particular, we have developed ultrafast all-optical switching devices operating at fiber-optic
communication wavelengths, based on intersubband cross-absorption saturation in GaN/AlN quantum-well waveguides.
Strong self-phase modulation of ultrafast optical pulses has also been measured in these waveguides, revealing a large
intersubband refractive-index nonlinearity which is also promising for all-optical switching applications. Furthermore,
we have demonstrated optically pumped intersubband light emission from GaN/AlN quantum wells at the record short
wavelength of about 2 μm. Finally, we have used a rigorous Monte Carlo model to show that GaN/AlGaN quantum
wells are promising for the development of THz quantum cascade lasers capable in principle of operation without
cryogenic cooling.
Near lattice matched Al0.81In0.19N/GaN heterojunction structures are compared with conventional Al0.3Ga0.7N/GaN heterojunctions in terms of the sheet density and mobility and their dependence on barrier and spacer layer parameters. With the insertion of an AlN spacer, the mobility of both structures is improved dramatically. Self-consistent solution of Poisson-Schrödinger equations was developed in order to determine the band structure and carrier distribution in these GaN based heterostructures in an effort to gain insight into the experimental observations. Surface donor states were included to account for the origin of electrons in 2DEG, which is treated as charge neutralization conditions in the simulation. Also the change in the piezoelectric polarization due to the electromechanical coupling effect, and shift of band gap caused by uniaxial strain were both included in the calculations. The calculated sheet density is close to the measured values, especially for the AlGaN samples investigated, but a notable difference was noted in the AlInN cases. The discrepancy is confirmed to be caused by the existence of a Ga-rich layer on the top of AlN spacer during the growth interruption, which can split the 2DEG into two channels with different mobilities and lower the overall sheet density. When the modifications made necessary by this GaN layer are taken into account in our model for the AlInN barrier case, the calculations match with the experimental data. When the spacer thickness increase from 0.3 to 3 nm, the total sheet density was found to slightly increase experimentally, which agreed with the theoretical prediction.
AlGaN/GaN devices are typically grown on foreign substrates such as SiC and sapphire due to lack of commercial
bulk GaN. Metalorganic chemical vapor deposition (MOCVD) is a widely used method for growth of GaN templates
for these structures even for other growth methods. Because the growth temperature during molecular beam epitaxy
(MBE) is low, dislocation motion is hindered leading to a high dislocation density, particularly pure edge type, when
grown directly on foreign substrates. On the other hand, low background doping, sharp interface and well-controlled
growth rate allow MBE to grow high performance modulation doped field effect transistor (MODFET) structures on
MOCVD GaN templates. However, the regrowth interface in this case has been reported to act as a parallel channel
unless Zn-doped GaN templates were used. [J. Appl. Phys.v92p338] In this paper we report on the control of the
regrowth interface of GaN/AlGaN MODFETs by rf-assisted MBE on GaN templates prepared by MOCVD. We have
found that the defective parallel channel at the regrowth interface could be effectively eliminated by a proper growth
procedure and pre-cleaning using KOH combined with high temperature (800oC) thermal annealing in vacuum.
Reflection high energy electron diffraction (RHEED) was used to monitor the interface quality to a first order during
the initial growth stages. Electrical and structural properties at the regrowth interface were analyzed by capacitance-voltage
(CV) measurements and transmission electron microscopy (TEM). Al0.3Ga0.7N/GaN MODFET structures
grown under the optimized conditions exhibited a maximum transconductance of 230 mA/mm for a 1&mgr;m gate length.
Electron microscopy methods have been used in recent collaborative studies to investigate the defect microstructure of
III-nitride materials and devices. An approach based on convergent beam diffraction allowed the elemental composition
of pseudomorphic InGaN/GaN quantum well structures to be determined on the nanometer scale. Indium compositional
fluctuations in InGaN quantum wells caused local electric field inhomogeneities that seemed to be more pronounced
near the onset of InGaN layer growth, suggesting strain relaxation as a strong contributing factor. Relaxed InN quantum
dots were invariably associated with threading dislocations in the underlying GaN buffer layer, and the interfacial misfit
was accommodated by periodic dislocation arrays. Lateral phase separation in InAlN/GaN heterostructures possibly
originating from misfit-strain relaxation at the heterointerface, resulted in the development of a vertical 'honeycomb'
structure. The structural and electronic properties of AlGaN/GaN heterostructures grown by molecular beam epitaxy
have been correlated with the Al/N flux ratio during nucleation layer growth. Electron microscopy played a central role
in contributing to the development of ferromagnetic Cr-doped nitride semiconductors.
We report on the structural, electrical, and optical characterization of GaN epitaxial layers grown by metalorganic chemical vapor deposition (MOCVD) on SiNx and TiNx porous templates in order to reduce the density of extended defects. Observations by transmission electron microscopy (TEM) indicate an order of magnitude reduction in the dislocation density in GaN layers grown on TiNx and SiNx networks (down to ~108 cm-2) compared with the control GaN layers. Both SiNx and TiNx porous network structures are found to be effective in blocking the threading dislocation from penetrating into the upper layer. Supporting these findings are the results from X-Ray diffraction and low temperature photoluminescence (PL) measurements. The linewidth of the asymmetric X-Ray diffraction (XRD) (1012) peak decreases considerably for the layers grown with the use of SiNx and TiNx layers, which generally suggests the reduction of edge and mixed threading dislocations. In general, further improvement is observed with the addition of a second SiNx layer. The room temperature decay times obtained from biexponential fits to time-resolved photoluminescence (TRPL) data are increased with the inclusion of SiNx and TiNx layers. TRPL results suggest that primarily point-defect and impurity-related nonradiative centers are responsible for reducing the lifetime. The carrier lifetime of 1.86 ns measured for a TiNx network sample is slightly longer than that for a 200 μm-thick high quality freestanding GaN. Results on samples grown by a new technique called crack-assisted lateral overgrowth, which combines in situ deposition of SiNx mask and conventional lateral overgrowth, are also reported.
A series of six WC(x)/B4C multilayers was produced by dc magnetron sputtering in an atmosphere of argon with methane additions (from 0 to 15 percent). The microstructure and chemistry of these multilayers was studied using transmission/HREM, XRD, XPS, electron probe microanalysis, and ion beam analysis with MeV helium beams. The multilayers were shown to be completely amorphous. In addition to carbon incorporation, a significant amount of hydrogen was incorporated. The amounts of hydrogen and carbon present increased with the percentage of methane (up to the 12 percent sample), but the atom percent of argon in the multilayers was constant, regardless of the methane concentration. It was found that reflectivity values for Mg K-alpha radiation improved as the methane concentration increased, with the sample produced in a 12 percent methane atmosphere showing the highest reflectivity. Annealing of a representative sample caused a significant loss of hydrogen, and a decrease of the bilayer spacing.
State of the art diode rf-sputtering is used to fabricated high quality polarizing monochromators for neutrons. Optimization of the deposition parameters is achieved using in- situ kinetic ellipsometry and a great number of ex-situ characterization techniques such as grazing x-ray reflection, x-ray diffusion, alternating field gradient magnetometry. Mossbauer spectrometry, and electron microscopy. A precise picture of the structural characteristics of the system is deduced and related to the neutron performance as measured by polarized neutron reflectometry. We show that the structural behavior is controlled by crystallization of the iron layers and by the occurrence of amorphous interdiffused layers at each interface. As a consequence, the polarizing efficiency of these mirrors depends directly on the amount of iron involved in the interdiffusion. Using optimized deposition conditions, the flipping ratio is found to be around 40 for Fe/Ge mirrors with a medium period value of 120 angstroms. Positive spin-state reflectivity at the first Bragg peak is close to 100% when 150 bilayers are included and a controlled graded layer thickness allows the angular acceptance to be enhanced up to 39%.
Pierre Boher, Philippe Houdy, Louis Hennet, Zhigang Li, Abhijit Modak, David Smith, M. Idir, T. Moreno, Robert Barchewitz, Mikhael Kuehne, Peter Mueller, Jean-Pierre Delaboudiniere
Using a diode rf-sputtering technique, different magnesium silicide based multilayer systems have been deposited in very thin films for optical applications in the soft x-ray range. A detailed structural analysis of the different multilayers has been made using in-situ kinetic ellipsometry, ex-situ grazing x-ray reflection at the copper K-(alpha) line and transmission electron microscopy. The multilayer performances have been measured by synchrotron radiation at the magnesium K-(alpha) and L-(alpha) lines and related to the structural characteristics. For short wavelength, the W/Mg2Si system shows characteristics very similar to those of the more common W/Si system. Non-negligible interdiffusion and limited interface roughness allow the layer thicknesses to be reduced to very low values. Well-defined Bragg peaks are observed even when the double period is as low as 44 angstrom. First Bragg peak reflectivity as high as 31 has been measured at 9.89 angstrom for a multilayer with a double period equal to 84 angstrom and a limited number of periods. This preliminary result is very promising for future applications in the field of x-ray fluorescence analysis. W/Mg2Si and Si3N4/Mg2Si multilayers have also been fabricated for use at higher wavelengths around the Mg L-(alpha) line (286 angstrom). In the case of the W/Mg2Si multilayers have also been fabricated for use at higher wavelengths around the Mg L-(alpha) line (286 angstrom). In the case of the W/Mg2Si system, the tungsten layers are crystallized due to their higher thickness and consequently the interface roughness is slightly higher. In spite of this, more than 20 reflectivity at the first Bragg peak has been measured at normal incidence on different W/Mg2Si samples, with a selectivity two times better than conventional Mo/Si mirrors ((lambda) /(delta) (lambda) approximately equals 20). When we replace tungsten by a thin silicon nitride layer deposited by reactive sputtering, we increase the selectivity up to (lambda) /(delta) (lambda) approximately equals 30, and the thermal stability is drastically improved ( 800 degree(s)C).
W/BN multilayers are theoretically efficient x-ray mirrors at the nitrogen and boron K-(alpha) lines (31.3 angstroms and 67.6 angstroms, respectively). Their most attractive potential application is detection of light elements by x-ray fluorescence spectrometry. The performances of W/BN mirrors depend not only on the structural quality of the multilayers but also on the stoichiometry of the boron nitride layers, especially in the water window (20 - 40 angstroms). In order to get stoichiometric BN layers with low surface roughness, the deposition of thick boron nitride films has been studied in detail. In-situ kinetic ellipsometry, x-ray photoemission, grazing x-ray reflection and scanning electron microscopy show that quasi-stoichiometric BN films with low surface roughness are obtained only with a low total deposition pressure and an additional nitrogen partial pressure. This result is related to the chemical and structural properties of the BN films. W/BN multilayers with medium period value (2d approximately equals 120 angstroms) show about 80 of the maximum reflectivity at the W M4-5 line. When the period is reduced, the performances are reduced, but good quality W/BN multilayers with very low period values (2d approximately equals 50 angstroms) and a great number of periods ( 100) have been fabricated. The best structural quality is obtained when a low nitrogen partial pressure is introduced during the deposition of the BN layers. The optical indice contrast is improved and the tungsten-boron interdiffusion is reduced.
Si/SiO2 and Si/Si3N4 multilayers have been fabricated using a locally made reactive diode ri-sputtering system. The layer alternation is obtained by modulating a partial pressure of oxygen or nitrogen near the sample using a silicon target with argon as sputtering gas. O2 and N2 partial pressure conditions were optimized to deposit stoichiometric SiO2 and Si3N4 films without significant reaction with the silicon target. In situ kinetic ellipsometry was used to monitor both thick film and multilayer deposition. The different interfaces appear very sharp with a little contamination of the silicon layers especially using oxygen. The multilayers were characterized by grazing x-ray reflection (Cu-K α line), and the reflectivity was measured in the soft x-ray range (120-350 Å) by synchrotron radiation. Both Si/SiO2 and Si/Si3N4 multilayers exhibit well-defined Bragg peaks with very narrow bandpasses (two to three times lower than the conventional Mo/Si multilayer), and high absolute reflectivities (up to ≅22% at 130 Å). Finally, thermal stability of Si/Si3N4 multilayers was evaluated. We did not find any degradation after annealing up to 800°C, which is extremely high compared to conventional Mo/Si multilayers, which are generally destroyed above 500°C.
Si/Si02 and Si/Si3N4 multilayers have been fabricated using a locally made reactive diode if-sputtering systern.
'Ihe layer alternation is obtained by modulating a partial pressure of oxygen or nitrogen near the sample using
a silicon target and argon as sputtering gas. 02 and N2 partial pressure conditions were optimized to deposit
stoechiornetric Si02 and Si3TV4 films without significant reaction with the silicon target. In-situ kinetic ellipsometry
was used to monitore both thick film and multilayer deposition. 'I'he different interfaces appear very sharp with
a little contamination of the silicon layers especially using oxygen. The multilayers were characterized by gazing
X-ray reflection ( Cu - K line ), and the reflectivity was measured in the soft X-ray range (120 - 350 A ) by
synchrolroii radiation. BotI1 Si/Si02 and SiIS13N4 multilayers exhibit well defmed Bragg peaks with very narrow
bandpasses ( two to three times lower than the conventional M'/Si multilayer ),and high absolute reflectivities (up
to 22% at 1 30 A ). The soft X-ray performances of these mirrors are explained using the physical characteristics
deduced from kinetic ellipsometry, grazing X-ray reflection, infrared absorption and transmission electron
microscopy measurements.
Tungsten, iron, and rhodium materials have been deposited alternatively with carbon and boron carbide by diode RF-sputtering. The multilayer performances have been measured at the carbon or boron K-alpha lines depending on the layer spacing. It is found that tungsten and iron provide multilayers with good optical quality and optimized layer densities. This is related to the amorphous character of the tungsten and iron layers which results in low intrinsic roughness and limited interdiffusion. The experimental reflectivity is a factor of 2/3 lower than the theoretical value for W/C multilayers at 44.7 A. Rhodium layers alternated with carbon are crystallized, which induces significant interface roughness and poor soft X-ray performances. It is concluded that boron-carbide-based multilayers always exhibit lower interface roughness than carbon-based ones.
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