The lack of a suitable, lattice matched substrate for the growth of the group III nitrides typically restricts GaN film growth to substrates such as sapphire or SiC, despite the large lattice and thermal mismatch. With the use of AlN or GaN nucleation layers (NL), GaN films of sufficient quality have been produced for blue LEDs. However, for laser and large-area microwave device applications, the large number of dislocations (> 10 8 cm −2 ) limit device performance, and techniques are desired to reduce dislocation density during the growth process. Here, we demonstrate how low temperature AlN interlayers (IL) sandwiched between high temperature (HT) GaN layers can be used to improve the electrical, optical, and structural properties of Si doped GaN films. A nearly two-fold increase in mobility is observed in Si doped GaN grown using 5 AlN IL compared to GaN grown on a single AlN NL. For GaN films grown on multiple AlN IL, cross-sectional transmission electron microscopy images reveal a significant reduction in the screw dislocation density and photoluminescence spectra reveal a reduction in yellow band intensity. An analysis of the electrical data based on a single donor/single acceptor model suggests that the improved electron mobility is the result of a reduced acceptor concentration in the top GaN film. The reduction in the calculated acceptor concentration may be associated with the reduction of the screw dislocation density.

The effect of rapid thermal annealing on strain reduction in 1.15 MeV S-implanted GaAs wafers irradiated to a dose of 5 × 1014/cm2 has been studied by double-crystal x-ray diffraction technique. X-ray rocking curves exhibit characteristic thin film fringes between the peak of unstrained GaAs and the major peak of the strained region. The maximum strain, i.e., the separation between the two peaks, as well as the number of minor fringes decreases with increasing RTA temperature, while the relative spacing between the fringes remains constant. At temperatures above 900°C, the main peaks begin to overlap; however, a residual positive strain can be measured for temperatures as high as 1100°C.

X-ray diffraction (XRD) rocking curves were mapped across 4H-SiC, 3-inch, 8° off-cut substrates prior to and after epitaxial growth, where a pattern of slightly higher defectivity region was clearly seen. This same pattern was apparent in both cross-polarization images of the epiwafers and microwave photoconductivity decay (μ-PCD) lifetime maps of the epilayers, where the latter shows the lifetime in the high defectivity regions had drastically decreased. Within the short lifetime regions, electron trap concentrations were similar to that as in the long lifetime regions as determined by deep level transient spectroscopy; however, the extended defect density was significantly higher. Consequently, high spatial resolution XRD can be a valuable tool in preselecting substrates for epitaxial growth to produce low defect density material with long injected carrier lifetimes.

A multi-component solvent has been developed to dissolve solid gallium nitride (GaN) source material and grow single GaN crystals from the solution on GaN seeds. A thermal gradient is used to maintain GaN dissolution at the hot end of the crucible and to grow GaN crystals on a seed at the colder end. Crystals were grown at nitrogen pressure of 0.23–0.25MPa and temperature of 800°C. Optical microscopy, X-ray diffraction, micro Raman scattering and photoluminescence spectroscopy confirmed the high structural and optical quality of the GaN crystals.

The authors demonstrate here that GaN films with good surface morphology and structural, optical, and electronic properties can be grown on metallic titanium carbide substrates. X-ray rocking curve and Raman scattering measurements confirmed the high crystalline quality of the wurtzite structure film. Variable temperature photoluminescence measurements of sharp and intense emission lines provided insights into the nature of the recombination processes, the carrier background type, and the carrier concentration. The high quality of the interface and substrate Ohmic contacts was verified. The ability to grow high-quality films on metallic substrates provides the means for advanced vertical and high-power and/or high-temperature device fabrication.

High-reflectance group III-nitride distributed Bragg reflectors (DBRs) were deposited by metal-organic chemical-vapor deposition on Si(111) substrates. A reflectance greater than 96% was demonstrated for an AlN∕GaN DBR with a stop band centered in the blue-green range of the visible spectrum. Crack-free GaN cap layers were grown on the DBR structures to demonstrate the opportunity to build III-nitride optoelectronic devices in this material. The DBR structure was under significant strain due to growth on a mismatched substrate, although the GaN cap layer was shown to be strain-free.

Bulk growth of 4H-SiC is challenging due to the required high growth temperatures and gradients used in sublimation physical vapor transport that are difficult to control, particularly over large diameter boules. We used x-ray diffraction mapping to show concave crystal plane curvature in substrates from five commercial suppliers with two suppliers producing wafers with ⩽2° curvature. The extent of curvature varied little for substrates from any particular supplier. Maps of peak position and full width at half maximum from symmetric and asymmetric reflections were used to identify defective regions in the crystal. Closer examination of the rocking curves in the defective regions found a low density of low angle grain boundaries only in substrates from one supplier.

Details of the recently developed 'zone technique' for the absolute measurement of lattice parameter and strain in single-crystal solids and thin films are presented. The method is based on measuring X-ray rocking curves from a few equatorial planes within a suitable zone and correcting their peak positions at once with a single zero offset. In contrast to the comparative method, which usually requires use of two opposite azimuthal directions, those in the zone technique can often be completed in only one azimuthal setting. A typical strained layer in the cubic system can be fully and rapidly characterized with only three rocking curves. The technique is suitable for routine applications under typical laboratory conditions, and for high-precision measurements of nearly perfect crystals in a controlled environment, with a potential parts in 10 million accuracy. This degree of accuracy is a direct consequence of the zero offset correction procedure, which effectively cancels a large portion of the misalignment errors in the diffractometer. The use of the (n, -n) geometry substantially reduces the errors of eccentricity compared to the Bond technique, and its stronger reflections enable the measurement of small samples about 0.05 mm in length with relative ease. The technique is illustrated with examples, and its extension to the triple-axis (omega-2theta) instruments is discussed.

Single crystal 3C-SiC epitaxial layers have been grown on SOI substrates using low-pressure chemical vapour deposition (LPCVD). The SOI substrates consist of nominally 150 Å Si layers bonded to 100 mm poly 3C-SiC substrates using direct wafer bonding and SOI film transfer techniques. Miscut Si(100) films were incorporated into the wafer bonding process for the first time in an effort to further reduce anti-phase domain formation in the 3C-SiC films. The Si films were transferred from Si(100) wafers miscut 4° toward the (110) direction. For growth of 3C-SiC layers, a two-step process is needed. First the Si is carbonized using propane mixed in a hydrogen carrier gas to convert the surface to SiC at atmospheric pressure. Next SiC growth is conducted by the addition of silane into the gas mix and a reduction of the process to 150 Torr. Previous characterization of these films via SEM and XRD indicated that the films were single crystal and oriented with respect to the starting Si bonded film. In this work we present results of TEM analysis of these films which show details of the atomic structure at the 3C-SiC/poly-crystalline SiC interface where a thin (6 nm thick) amorphous region has been detected, likely due to SiO2 used to wafer bond the Si film to the poly substrate. A detailed study of this material system is presented with the aim of providing feedback for possible improvements in the 3C-SiC growth process.

Single crystal 3C-SiC epitaxial layers have been grown on SOI substrates using low-pressure chemical vapor deposition (LPCVD). The SOI substrates consist of nominally 100 A Si layers bonded to 100 mm poly 3C-SiC substrates using direct wafer bonding and SOI film transfer techniques. Miscut Si(100) films were incorporated into the wafer bonding process for the first time in an effort to further reduce anti-phase domain formation in the 3C-SiC films. The Si films were transferred from Si(100) wafers miscut 4degrees toward the [211] direction. The 3C-SiC layers were grown using a two-step process. First the Si was carbonized using propane mixed in a hydrogen carrier gas to convert the surface to SiC at atmospheric pressure. Next SiC growth was conducted by the addition of silane into the gas mix and a reduction of the process pressure to 150 Torr. SEM and XRD analysis were performed on films grown for 30 and 60 min at 1380degreesC (and an additional hour at 1550degreesC) indicate that the film is single crystal and oriented in the [001] direction of the starting Si bonded film.

Using plan-view transmission electron microscopy (TEM), we have identified stacking faults (SFs) in 4H-SiC PiN diodes subjected to both light and heavy electrical bias. Our observations suggest that the widely expanded SFs seen after heavy bias are faulted dislocation loops that have expanded in response to strain of the 4H-SiC film, while faulted screw or 60° threading dislocations do not give rise to widely expanded SFs. Theoretical calculations show that the expansion of SFs depends on the Peach-Koehler (PK) forces on the partial dislocations bounding the SFs, indicating that strain plays a critical role in SF expansion.

Using site-specific plan-view transmission electron microscopy (TEM) and light-emission imaging (LEI), we have identified SFs formed during forward biasing of 4H-SiC PiN diodes. These SFs are bounded by Shockley partial dislocations and are formed by shear strain rather than by condensation of vacancies or interstitials. Detailed analysis using TEM diffraction contrast experiments reveal SFs with leading carbon-core Shockley partial dislocations as well as with the silicon-core partial dislocations observed in plastic deformation of 4H-SiC at elevated temperatures. The leading Shockley partials are seen to relieve both tensile and compressive strain during PiN diode operation, suggesting the presence of a complex and inhomogeneous strain field in the 4H-SiC layer.

The relationship of GaN resistivity to film microstructure and impurity compensation are investigated using transmission electron microscopy, secondary ion mass spectroscopy, X-ray diffraction, and resistance measurements. Unintentionally doped GaN films grown by MOCVD at varying pressures exhibit increased grain size, reduced carbon and oxygen impurity incorporation, reduction in the density of threading dislocations (TDs) with an edge component, and reduced resistivity with increasing growth pressure. Variation in resistivity over eight orders of magnitude is observed as a result of varying the MOCVD growth pressure in a controlled experiment. Our results suggest that disclocations play an important role in the resistivity of GaN. Evidence is presented of impurities segregating at TDs having an edge component, and acting as compensating centers. The control of such compensation as a function of MOCVD growth conditions is outlined.

Self-nucleated bulk AlN crystals were grown by thermodecomposition of AlCl3.NH3 vaporized in the low-temperature zone of a two-zone furnace. X-ray diffraction of the AlN crystals show single lines with a small linewidth indicating high single-crystalline quality. Polarized Raman scattering experiments of these samples confirm the x-ray results based on the detection of a small linewidth for all allowed optical phonons. Low-temperature cathodoluminescence spectra show very sharp emission bands close to the optical band gap, which have been assigned to free-excitons A and B, and exciton-bound to shallow neutral impurity. The latter has a full width at half maximum smaller than 1.0 meV. (C) 2003 American Institute of Physics.

Using site-specific plan-view transmission electron microscopy (TEM) and light emission imaging, we have identified stacking faults formed during forward biasing of 4H-SiC p-i-n diodes. These stacking faults (SFs) are bounded by Shockley partial dislocations and are formed by shear strain rather than by the condensation of vacancies or interstitials. Detailed analysis using TEM diffraction contrast experiments reveal SFs with leading carbon-core Shockley partial dislocations as well as with the silicon-core partial dislocations observed in plastic deformation of 4H-SiC at elevated temperatures. The leading Shockley partials are seen to relieve both tensile and compressive strain during p-i-n diode operation, suggesting the presence of a complex inhomogeneous strain field in the 4H-SiC layer. (C) 2003 American Institute of Physics.

Irradiation of sapphire with fast neutrons (0.8–10 MeV) at a fluence of 1022/m2 increased the c-axis compressive strength and the c-plane biaxial flexure strength at 600 °C by a factor of ∼2.5. Both effects are attributed to inhibition of r-plane twin propagation by damage clusters resulting from neutron impact. The a-plane biaxial flexure strength and four-point flexure strength in the c- and m-directions decreased by 10–23% at 600 °C after neutron irradiation. Neutron irradiation had little or no effect on thermal conductivity, infrared absorption, elastic constants, hardness, and fracture toughness. A featureless electron paramagnetic resonance signal at g=2.02 was correlated with the strength increase: This signal grew in amplitude with increasing neutron irradiation, which also increased the compressive strength. Annealing conditions that reversed the strengthening also annihilated the g=2.02 signal. A signal associated with a paramagnetic center containing two Al nuclei was not correlated with strength. Ultraviolet and visible color centers also were not correlated with strength in that they could be removed by annealing at temperatures that were too low to reverse the compressive strengthening effect of neutron irradiation.