J. Washburn

Microstructure of Ti/Al and Ti/Al/Ni/Au Ohmic contacts for n-GaN

Transmission electron microscopy has been applied to characterize the structure of Ti/Al and Ti/Al/Ni/Au Ohmic contacts on n‐type GaN (∼1017 cm−3) epitaxial layers. The metals were deposited either by conventional electron‐beam or thermal evaporation techniques, and then thermally annealed at 900 °C for 30 s in a N2 atmosphere. Before metal deposition, the GaN surface was treated by reactive ion etching. A thin polycrystalline cubic TiN layer epitaxially matched to the (0001) GaN surface was detected at the interface with the GaN substrate. This layer was studied in detail by electron diffraction and high resolution electron microscopy. The orientation relationship between the cubic TiN and the GaN was found to be: {111}TiN//{00.1}GaN, [110]TiN//[11.0]GaN, [112]TiN//[10.0]GaN. The formation of this cubic TiN layer results in an excess of N vacancies in the GaN close to the interface which is considered to be the reason for the low resistance of the contact.

Some observations on the amorphous to crystalline transformation in silicon

An atomistic model for the transformation of amorphous (α) to crystalline silicon films while in contact with a crystalline substrate is presented. The atomic structure of the {100}, {110}, and {111} surfaces is examined and related to the observed interface migration rates. The assumption that for an atom to attach successfully to the crystal it must complete at least two undistorted bonds, leads to the prediction that the {100} amorphous/crystalline interface should advance fastest and the {111} slowest. The origin of crystal defects is discussed in terms of the atomistic recrystallization mechanism. Microtwins are found to be a logical consequence of crystallization on the {111} surfaces but are not expected to form on any other interface. Once microtwins are formed they can increase the recrystallization rate of a {111} surface. This phenomenon is both described in the model and experimentally observed.

Growth and properties of GaAs/AlGaAs on nonpolar substrates using molecular beam epitaxy

Using molecular beam epitaxy, we have successfully grown device quality GaAs/AlGaAs on (100)‐oriented Ge and Si substrates. Modulation doped field effect transistors have been fabricated from these layers which exhibit room‐temperature transconductances as high as 160 and 170 mS/mm for layers on Ge and Si, respectively, and showed no looping in either case. At 77 K, these values rose to 345 and 275 mS/mm for Ge and Si, respectively. Analysis by transmission electron microscopy of layers grown on Ge showed that the antiphase disorder was contained within the 250‐A‐thick initial layer which was grown at a 0.1‐μ/h growth rate at a substrate temperature of 500 °C. For both the layers grown on Si and Ge specular surface morphologies were obtained. The photoluminescence of GaAs/AlGaAs quantum wells grown on Si and Ge was similar in intensity to the same quantum well structures grown on GaAs. In quantum wells grown on Ge, the luminescence was dominated by a donor‐acceptor recombination at 1.479 eV, which appears...

Ni, Pd, and Pt on GaAs: A comparative study of interfacial structures, compositions, and reacted film morphologies

The reactions between (100) GaAs and the near-noble metals Ni, Pd, and Pt have been investigated by application of high-resolution transmission electron microscopy (TEM), energy-dispersive analysis of x rays in the scanning TEM and Rutherford backscattering spectrometry. Emphasis is placed on the evolution of the phase distributions, film compositions, and interface morphologies during annealing at temperatures up to 480 /sup 0/C. The first phase in the Ni/GaAs reaction is shown to have the nominal composition Ni/sub 3/GaAs. Ternary phases of the type Pd/sub x/GaAs are also found to be the dominant products of the Pd/GaAs reaction. Conversely, only binary phases result from the Pt/GaAs reaction. These observations are used to construct isothermal sections of the M--Ga--As thin-film phase diagrams. The behavior of a thin (1--2 nm) native oxide--hydrocarbon layer during the Ni/GaAs, Pd/GaAs, and Pt/GaAs reactions is also investigated. Only the Ni/GaAs reaction is noticeably impeded in some regions by this intervening layer. In contrast, the Pd/GaAs and Pt/GaAs reactions tend to mechanically disperse the native oxide layers.

Effect of Si doping on the dislocation structure of GaN grown on the A‐face of sapphire

Transmission electron microscopy, x‐ray diffraction, low‐temperature photoluminescence, and Raman spectroscopy were applied to study stress relaxation and the dislocation structure in a Si‐doped GaN layer in comparison with an undoped layer grown under the same conditions by metalorganic vapor phase epitaxy on (11.0) Al2O3. Doping of the GaN by Si to a concentration of 3×1018 cm−3 was found to improve the layer quality. It decreases dislocation density from 5×109 (undoped layer) to 7×108 cm−2 and changes the dislocation arrangement toward a more random distribution. Both samples were shown to be under biaxial compressive stress which was slightly higher in the undoped layer. The stress results in a blue shift of the emission energy and E2 phonon peaks in the photoluminescence and Raman spectra. Thermal stress was partly relaxed by bending of threading dislocations into the basal plane. This leads to the formation of a three‐dimensional dislocation network and a strain gradient along the c axis of the layer.

DIFFUSION-INDUCED DISLOCATIONS IN SILICON

Plastic deformation produced by phosphorus diffusion into a (001) silicon surface has been studied by transmission electron microscopy. The lattice parameter differences in regions of steep solute concentration gradient are accommodated by a crossed grid of edge dislocations having Burgers vector a/2[110] and a/2[110]. The long edge dislocations end at nodes, which suggests that they are formed by dislocation reactions between pairs of dislocations that can glide into the crystal on {111} planes.

Structure and Origin of Stacking Faults in Epitaxial Silicon

Light optical and transmission electron microscopy on epitaxially deposited silicon crystals in [100], [110], and [111] orientation show that growth stacking faults are formed regardless of orientation. These faults occur in bundles of usually tetrahedral figures with the apices at the substrate‐film interface, and some faults may exist as closed intrinsic‐extrinsic pairs. The results suggest that faults are nucleated by oxygen contamination. Some evidence for impurity segregation to faults is also presented.Light optical and transmission electron microscopy on epitaxially deposited silicon crystals in [100], [110], and [111] orientation show that growth stacking faults are formed regardless of orientation. These faults occur in bundles of usually tetrahedral figures with the apices at the substrate‐film interface, and some faults may exist as closed intrinsic‐extrinsic pairs. The results suggest that faults are nucleated by oxygen contamination. Some evidence for impurity segregation to faults is also presented.

Microstructure of Ti/Al ohmic contacts for n-AlGaN

Transmission electron microscopy was employed to evaluate the microstructure of Al/Ti ohmic contacts to AlGaN/GaN heterostructure field-effect transistor structures. Contact resistance was found to depend on the structure and composition of the metal and AlGaN layers, and on atomic structure of the interface. A 15–25-nm-thick interfacial AlTi2N layer was observed at the contact-AlGaN interface. Formation of such nitrogen-containing layers appears to be essential for ohmic behavior on n-type III-nitride materials suggesting a tunneling contact mechanism. Contact resistivity was found to increase with Al fraction in the AlGaN layer.

STRESS INDUCED MOVEMENT OF CRYSTAL BOUNDARIES

Abstract The stress-induced movement of small angle boundaries in zinc crystals was investigated in the temperature range of 25° to 400°C. The dynamic behavior of the boundaries seemed to require that they consist of an array of edge dislocations of like sign distributed more or less uniformly over the plane of the boundary. The activation energy for the movement of the boundaries in the temperature range of 300 to 400°C was determined to be about 21,500 calories per mole. Attention was drawn to the close agreement between this value and those of the activation energies for self-diffusion, creep, and recovery in zinc crystals. A mechanism was suggested for the formation of a substructure in crystals plastically deformed at low temperatures which depended only on the stress-induced movement of small angle boundaries. This mechanism was based on observations that the rate of boundary motion decreased with increasing boundary angle. Small angle boundaries overtook and united with those of larger angle, forming slower moving, or even stationary substructure boundaries.

A comparative study of phase stability and film morphology in thin‐film M/GaAs systems (M=Co, Rh, Ir, Ni, Pd, and Pt)

To attain reproducible and stable contacts to compound semiconductor devices, it is necessary to achieve thermodynamically stable phases after the reaction of metals with the compound semiconductor. In this study, the final phases produced by the reactions between GaAs and thin metal films of Co, Rh, Ir, Ni, Pd, and Pt have been investigated. They are identified as MGa for M=Co, Rh, Ni, Pd, and Pt, monoarsenides of Co and Ni, diarsenides of Rh, Ir, Pd, and Pt, and Ir3Ga5. These phases, if deposited directly onto GaAs, will produce thermally stable contacts. In addition to the identification of these stable phases, analyses of the products of thin‐film M/GaAs reactions by transmission electron microscopy, x‐ray diffraction, and Rutherford backscattering spectrometry reveal the distribution, grain size, and crystallographic texture of these end phases. Trends in these observations across the six metal/GaAs reactions studied are explained by considering the effects of bond strength and the relative diffusivi...