J. O. McCaldin

Diffusivity and Solubility of Si in the Al Metallization of Integrated Circuits

Si was diffused along the evaporated Al layer of an integrated-circuit structure at temperatures between 360 and 560 °C, and the resulting concentration profile analyzed by electron microprobe. The Si solubility was found to agree with literature values for Si in wrought Al. The Si diffusivity was found to be substantially enhanced, however, probably due to a high density of imperfections in the evaporated Al film. Our measured diffusivities indicate an activation energy EA ~= 0. 8 eV, about 40% less than the value for Si in wrought Al.

Crystallization of Ge and Si in metal films. I

The behavior of amorphous Si in contact with Ag films and Ge in contact with Al films has been studied at temperatures well below those at which any liquid phase is present. MeV 4He+ ion backscattering techniques, transmission electron diffraction, scanning electron microscopy, and electron microprobe analysis have been used. We find that of the many possible reactions which carry the amorphous Si or Ge into their crystalline forms the reaction predominating under our experimental conditions consists of dissolution, diffusion, and crystal growth. During isothermal heat treatment, the semiconductor film is dissolved into the metal film where it diffuses and precipitates as crystalline Si or Ge. These processes are solid-solid reactions, since this behavior is observed over temperatures of 300°C to as low as 100°C for Ge/Al, compared to the 424°C eutectic in this system. In Si/Ag, this behavior was observed from 700 to 400°C, compared with the 840°C eutectic.

Solid-phase epitaxial growth of Si mesas from Al metallization

Si epitaxial growth from solution in solid Al onto crystal Si substrates was studied by scanning electron microscopy. Growth in reentrant corners of the substrate was found to be favored over growth onto a flat surface. For this reason, the smaller-diameter oxide cuts used in integrated-circuit fabrication, in which no portion of the exposed substrate Si is far from a reentrant corner, are favored sites for growth. Si growth readily fills in such oxide cuts forming mesa structures potentially useful in device construction. The probable cause for such preferential growth was indicated in pressure experiments which show that regions in the solid Al under relatively less compression are favored locations for growth.

Band Offsets in Semiconductor Heterojunctions

Publisher Summary This chapter presents an overview of several theoretical approaches and experimental measurement techniques for determining band offset values and discuss the experimental and theoretical data reported for a number of specific heterojunction systems. It also evaluates the credibility and accuracy of the experimental measurements and provides a tabulation of reliable band offset values for as many heterojunctions as possible. Among the most important physical parameters for a given heterojunction system are the conduction- and valence-band offsets; indeed, the quality and even the feasibility of heterojunction device concepts often depend crucially on the values of these band offsets. The band offset is defined simply as the discontinuity in the band edge at the interface between two semiconductors. A number of current theories seem to yield band offset values in reasonable agreement with experiment, even though the physical ideas underlying these theories can be quite different. These ideas include electron affinities, Schottky barrier heights, bulk band structures on the same energy scale, and the definition of effective midgap energies corresponding to charge neutrality for each bulk constituent.

Schottky‐based band lineups for refractory semiconductors

An overview is presented of band alignments for small-lattice parameter, refractory semiconductors. The band alignments are estimated empirically through the use of available Schottky barrier height data, and are compared to theoretically predicted values. Results for tetrahedrally bonded semiconductors with lattice constant values in the range from C through ZnSe are presented. Based on the estimated band alignments and the recently demonstrated p-type dopability of GaN, we propose three novel heterojunction schemes which seek to address inherent difficulties in doping or electrical contact to wide-gap semiconductors such as ZnO, ZnSe, and ZnS.

Growth and characterization of ZnTe films grown on GaAs, InAs, GaSb, and ZnTe

We report the successful growth of ZnTe on nearly lattice-matched III-V buffer layers of InAs (0.75%), GaSb (0.15%), and on GaAs and ZnTe by molecular beam epitaxy. In situ reflection high-energy electron diffraction measurements showed the characteristic streak patterns indicative of two-dimensional growth. Photoluminescence measurements on these films show strong and sharp features near the band edge with no detectable luminescence at longer wavelengths. The integrated photoluminescence intensity from the ZnTe layers increased with better lattice match to the buffer layer. The ZnTe epilayers grown on high-purity ZnTe substrates exhibited stronger luminescence than the substrates. We observe narrow luminescence linewidths (full width at half maximum ~ 1–2 A) indicative of uniform high quality growth. Secondary-ion mass spectroscopy and electron microprobe measurements, however, reveal substantial outdiffusion of Ga and In for growths on the III-V buffer layers.

n-CdSe/p-ZnTe based wide band-gap light emitters: Numerical simulation and design

The only II‐VI/II‐VI wide band‐gap heterojunction to provide both good lattice match and p‐ and n‐type dopability is CdSe/ZnTe. We have carried out numerical simulations of several light emitter designs incorporating CdSe, ZnTe, and Mg alloys. In the simulations, Poisson’s equation is solved in conjunction with the hole and electron current and continuity equations. Radiative and nonradiative recombination in bulk material and at interfaces are included in the model. Simulation results show that an n‐CdSe/p‐ZnTe heterostructure is unfavorable for efficient wide band‐gap light emission due to recombination in the CdSe and at the CdSe/ZnTe interface. An n‐CdSe/Mg_(x)Cd_(1−x)Se/p‐ZnTe heterostructure significantly reduces interfacial recombination and facilitates electron injection into the p‐ZnTe layer. The addition of a Mg_(y)Zn_(1−y)Te electron confining layer further improves the efficiency of light emission. Finally, an n‐CdSe/Mg_(x)Cd_(1−x)Se/Mg_(y)Zn_(1−y)Te/p‐ZnTe design allows tunability of the wavelength of light emission from green into the blue wavelength regime.

Proposal and verification of a new visible light emitter based on wide band gap II‐VI semiconductors

We propose a new device structure for obtaining visible light emission from wide band gap semiconductors. This heterojunction structure avoids ohmic contacting problems by using only the doping types which tend to occur naturally in II-VI semiconductors, while using a novel injection scheme to obtain efficient minority carrier injection into the wider band gap semiconductor. To verify this proposal we have fabricated green light emitting structures using n-CdSe and p-ZnTe regions separated by a graded MgxCd1-xSe injection region. Room temperature electroluminescence spectra from these devices demonstrate the effectiveness of the injection scheme, while the current-voltage characteristics show the merits of avoiding difficult ohmic contacts. We further show how the structure can be extended to blue wavelengths and beyond by opening up the band gap of the ZnTe recombination region with a MgyZn1-yTe alloy.

Measurement of the valence band offset in novel heterojunction systems: Si/Ge (100) and AlSb/ZnTe (100)

We have used x-ray photoelectron spectroscopy to measure the valence band offset in situ for strained Si/Ge (100) heterojunctions and for AlSb/ZnTe (100) heterojunctions grown by molecular-beam epitaxy. For the Si/Ge system, Si 2p and Ge 3d core level to valence band edge binding energies and Si 2p to Ge 3d core level energy separations were measured as functions of strain, and strain configurations in all samples were determined using x-ray diffraction. Our measurements yield valence band offset values of 0.83±0.11 eV and 0.22±0.13 eV for Ge on Si (100) and Si on Ge (100), respectively. If we assume that the offset between the weighted averages of the light-hole, heavy-hole, and spin-orbit valence bands in Si and Ge is independent of strain, we obtain a discontinuity in the average valence band edge of 0.49±0.13 eV. For the AlSb/ZnTe (100) heterojunction system, we obtain a value of –0.42±0.07 eV for the valence band offset. Our data also suggest that an intermediate compound, containing Al and Te, is formed at the AlSb/ZnTe (100) interface.

Measurement of the CdSe/ZnTe valence band offset by x‐ray photoelectron spectroscopy

We have used x-ray photoelectron spectroscopy (XPS) to measure the valence band offset in situ for CdSe/ZnTe (100) heterojunctions grown by molecular-beam epitaxy. XPS measurements were performed for films of CdSe (100) and ZnTe (100), and for heterojunctions consisting of either ~25 A of CdSe grown on ZnTe or ~25 A of ZnTe grown on CdSe. Observations of reflection high energy electron diffraction patterns indicated that CdSe films deposited on ZnTe were grown in cubic zinc blende form, rather than the natural wurtzite structure of CdSe. Our measurements yielded a CdSe/ZnTe valence band offset DeltaEv=0.64±0.07 eV. The corresponding conduction band offset for CdSe/ZnTe is DeltaEc=1.22±0.07 eV for room temperature band gaps for ZnTe and for cubic CdSe of 2.25 and 1.67 eV, respectively.