The main concepts in ion scattering analysis of interfaces are reviewed and relevant literature is cited.

The atomic structure of semiconductor heterostructure interfaces and metalsemiconductor interfaces are best characterized by transmission electron microscopy (TEM). Both phase contrast TEM and structure factor contrast TEM are able to distinguish very small structural (two monolayers) or compositional (less than 0.2%) fluctuations at interfaces. Applications of these techniques to the study of the roughening transition temperature at the Gal−xAlxAs–GaAs and Ga1−xAlxAs–Ge interfaces grown by molecular beam epitaxy are presented. Minority carrier recombination at interfaces is characterized on a microscopic scale by low temperature cathodoluminescence. This technique is used to demonstrate the role of interfaces in gettering defects in Gal1−xAlxAs/GaAs heterostructures. Finally, the effects of interfacial strain in producing a localization of the luminescence in GaAs quantum well wire structures will be discussed.

The growth process of silver deposited onto the Si(111) 7 × 7 surface at room temperature was studied with medium energy ion scattering in combination with channelling. Coverages studied were in the range 0−8 × 1015 Ag atoms cm−2. The measurements show that silver forms three-dimensional islands. Although incorporation of small amounts of silicon in the silver islands (less than 8 × 10−4 Si atoms cm−2) cannot be excluded, no evidence was found for mixing between silver and silicon or for silicide formation.

We report surface extended X-ray absorption edge fine structure (SEXAFS) studies of the early stages of Schottky barrier formation. Results will be presented for aluminum, silver and palladium on Si(111) 7 × 7 in the range from ⅓ monolayer to five monolayers. The SEXAFS signal reveals the local structure around the metal atoms in terms of bond lengths and coordination numbers. Complex ordered and disordered surface structures are observed ranging from weak reaction between the components (e.g. silver clusters on silicon) to strong reaction (e.g. Pd2Si formation). At elevated temperatures the Ag–Si interface is particularly interesting and consists of a well-defined ordered interface layer (√3 × √3 Ag on Si(111)) with silver clusters growing on top. While most other surface structural techniques fail to yield complete information the power of SEXAFS as a local structural technique becomes most apparent for such complex systems. Structure determinations for some metalsemiconductor interfaces will be presented.

Pd2Si layers produced by evaporation or sputtering onto silicon substrates were examined by high resolution electron microscopy, microdiffraction, X-ray, energy loss and Auger spectroscopy. The Si-Pd2Si interfaces produced by evaporation were in all cases rougher and more polycrystalline than those produced by sputtering. X-ray microanalysis showed the predictable variation in palladium distribution across the interface but quantification did not produce the expected palladium–to–silicon ratios, primarily because of probe broadening and X-rayinduced fluorescence. Energy loss spectra showed plasmon energy shifts and changes in Si L edge shape due to bond formation with palladium. Auger data provided evidence for a small amount of oxygen at the Si-Pd2Si interface. Electrical measurements of the ideality factor for Schottky barriers made from these materials produced higher values for the rougher evaporation-formed interfaces consistent with interface-roughness-induced scattering and carrier recombination.

The ion beam mixing technique has been applied to the production of Al–Si thin alloy layers as an alternative method to thermal annealing. Both unimplanted deposited aluminum thin films on silicon substrates and films implanted with energetic xenon ions were studied by Rutherford backscattering, channeling, secondary ion mass spectroscopy, nuclear resonance profiling and scanning electron microscopy techniques. The results of these experiments indicate that (i) intermixing between aluminum and silicon became observable when the implantation dose of energetic xenon through the interface surpassed 2 × 1016 ions cm−2; (ii) intermixing is dependent on the dose but not on the dose rate of implantation; (iii) damage to the silicon substrate extended only to the region penetrated by implanted ions; (iv) the Al-Si alloy layer region is uniform in texture and no segregation can be observed. Moreover, the integrity of the alloy layer is retained for a long period of room temperature annealing.

Surfaces of two III–V semiconductor materials have been examined at 120 kV by reflection electron microscopy (REM) at high spatial resolution. Surface steps were found to give characteristic contrast due to several diffraction effects, one of which is interference between the Bragg (B) and Bragg–Laue (BL) reflected beam at a step edge. The electron diffraction pattern from a step showed split lines from B and BL reflections, which provides a method of accurately measuring the mean inner potential of these materials. Electron energy loss spectra showed large surface and bulk plasmon peaks which mainly limit the spatial resolution of REM. The penetration depth in REM is given.

Thin film amorphous W-Si and TiW-Si diffusion barriers have been studied on GaAs and InP surfaces for the purpose of establishing their reliability for ohmic contacts and Schottky barriers, particularly under high temperature stress. The amorphous films were formed by a new method in which alternate layers of tungsten or TiW and silicon were sputter deposited to a total thickness of about 1300 Å and subsequently annealed near the glass transition temperature Tg(≈ 500 °C). Electron channeling and reflection electron diffraction were used to determine the amorphous nature of the films as deposited and after 4 h anneals near Tg. The as-deposited films had interfacial amorphous regions with compositions determined by interfacial reactions during the sputtering process. As-deposited W-Si films showed a weak channeling pattern which came from the unreacted polycrystalline tungsten layers. From Auger electron spectroscopy (AES) sputter profiles, it was concluded that the amorphous regions were at the W-Si interfaces which had the required tungsten-to-silicon composition ratio. After annealing at 500 °C for 4 h, the films were completely amorphous with no marked evidence of crystallization, indicating interfacial reactions extended completely into the tungsten layers. High magnification scanning electron microscopy (by a factor of 20 000) examination of the films after annealing revealed smooth and continuous surfaces with no evidence of grain boundaries. Diffusion along grain boundaries between gold and GaAs or InP in these amorphous thin films was thus almost completely eliminated. Interdiffusion of gold in layered structures (e.g. Au/(W–Si)/GaAs) was studied by AES sputter profiling techniques. No interdiffusion of gold or GaAs was observed after 16 h anneals at 400 °C. With Au/(W-Si)/InP structures, no interdiffusion was observed after 8 h anneals at 450 °C. These results are significant improvements over those for previous polycrystalline diffusion barriers (e.g. TiPt) which degrade after 1 h at 350 °C. Based on the AES sputter profiles, the diffusion coefficients in W-Si amorphous thin films were found to be less than 3 × 10−18 cm2 s−1 at 400 °C for gold, gallium and arsenic and less than 6 × 1018 cm2 s−1 at 450 °C for gold, indium and phosphorus.

Backscattering measurements were performed to assess the stability of amorphous Ni-Nb for contacts of high temperature electronics. The interdiffusion of amorphous Ni-Nb and three semiconductors—silicon, GaAs and GaP—was measured to study the stability for primary metallization applications. Diffusion of gold with amorphous Ni-Nb and the same three semiconductors was also investigated in order to address diffusion barrier applications of amorphous metals. The results indicate that the use of amorphous Ni-Nb as a contact or a diffusion barrier could extend the useful operation temperature range for GaP devices to above 550°C.

We have studied reactively sputtered ZrN, the most thermally stable of the refractory metal nitrides, for its diffusion barrier properties in aluminum metallization schemes with Rutherford backscattering spectroscopy and transmission electron microscopy (TEM). We find this compound to be very effective against aluminum diffusion up to 500 °C, independently of substrate temperature during sputtering. The useful temperature range can be extended by 50 °C with proper preannealing prior to aluminum deposition. The TEM study of the ZrN grain size as a function of annealing temperature revealed that the grain size does not change significantly upon annealing and that the grains are relatively small even at the highest annealing temperatures (about 300 Å at 900 °C). In addition, for annealing temperatures of and below 500 °C large portions of ZrN films were found to be of either amorphous or extremely fine–grain material, thus inhibiting the diffusion along grain boundaries. The presence of Zr3Al4Si5 ternary compound in samples annealed at 600 °C, as determined by X-ray analysis, may suggest that the ZrN barrier fails by decomposition of the film by aluminum.

The effectiveness of a thin (360Å) layer of reactively sputtered TiN as a diffusion barrier between aluminum and two silicides (PtSi and CoSi2) was evaluated. The chemical composition, structural phases and electrical properties of silicide/Al and silicide/TiN/Al contacts to n-type silicon were studied by Rutherford backscattering spectroscopy, glancing angle X-ray diffraction and Schottky barrier height measurements respectively. The results show that TiN is an effective barrier in these two systems up to at least 450°C, the typical temperature at which aluminum contacts are sintered.

We used Rutherford backscattering and channeling and transmission electron microscopy (TEM) to study the epitaxial growth of BaF2 vacuum deposited onto InP(100), InP(111), Ge(100) and Ge(111). We observed no epitaxy in BaF2 on Ge(100). The other three cases all show epitaxy, with quality ranging from poor for BaF2 on InP(111) through fair for BaF2 on InP(100) to excellent for BaF2 on Ge(111). Epitaxial quality depends strongly on substrate temperature for BaF2 on Ge(l11). TEM analysis indicates that there are neither misfit dislocations nor coherence at the BaF2–Ge(111) interface in spite of the 9.1% lattice mismatch.

X-ray diffraction (pole figure) measurements were made on gold thin films (300–2300 Å) evaporated on GaAs{111} and GaAs{001} to investigate the heteroepitaxy and twinning of the gold films and the chemoepitaxy of Au7Ga2 on GaAs. For both GaAs orientations the epitaxial relationship (hkl)AU[hkl]Au/ (hkl)GaAs[hkl]GaAs was observed for all values of h, k and l. In addition, twin orientations were observed. For GaAs{111}, fourfold rotation twinning about the Au[l11] direction resulted in four azimuthal orientations: [112]Au // [112]GaAs, [112]Au// [112]GaAs, [112]Au// [110]GaAs, and [112]AU/ [110]GaAs. For GaAs{001}, reflection twinning with the four Au{111} planes as twinning planes resulted in four other orientations with Au(001) at approximately 71° from GaAs(001). The relative amounts of the twins depended on the substrate temperature, the film thickness and the deposition rate. Chemoepitaxy was observed for Au7Ga2 on GaAs{111} the epitaxial relationship was (111)Au/ (0110)Au7Ga2/(111)GaAsand Au7Ga2[0001] was off by 10° from GaAs[110], toward GaAs[011], i.e. Au7Ga2[0001] was off by 10° from Au[110], toward Au[011]. This relationship corresponds to a perfect lattice match between Au7Ga2 and gold.

The lattice structure of ZnSe grown on GaAs by a low temperature low pressure organometallic chemical vapor deposition (CVD) process was studied using high resolution transmission electron microscopy. The defect structure of ZnSe epitaxial layers and of their interface with the GaAs substrate was directly imaged in cross section for GaAs substrate surfaces in the <100> and <111> orientations. It is shown that the ZnSe layers grow indeed epitaxially. The ZnSe layers grown on GaAs(100) contain a large density of faulted loops which are extrinsic in nature, whereas the prevailing defects in ZnSe layers grown on GaAs(111)B substrates are microtwins and stacking faults parallel to the filmsubstrate interface. A possible connection between the observed defect structure and the reported photoluminescence and electrical transport properties of ZnSe layers grown by organometallic CVD is also discussed.

Lattice-matched Ga0.47In0.53As was epitaxially grown on InP substrates by the reaction of triethylgallium, triethylindium and arsine. The mobility and carrier concentration in these layers were determined by sequential etch and Hall effect measurements made on the grown layers. These measurements show a considerable fall–off in mobility in the vicinity of the interface, accompanied by a rapid increase in electron concentration. In situ chloride etching of the substrate, prior to Ga–In–As growth, is shown to reduce significantly but not eliminate these interface effects. In this paper we outline possible reasons for these effects, based on measurements made on films grown with and without substrate etching and also on measurements of the effect of etching on the substrate itself.

A transmission electron microscopy analysis has been made of the wet oxidation of polycrystalline silicon (poly–Si) films heavily doped with phosphorus in which the poly–Si–SiO2 interface has been given primary consideration. Using conventional and cross section transmission electron microscopy two distinct morphologies have been identified. Firstly, there is a region of enhanced oxidation at the poly–Si grain boundaries, with the concurrent formation of a new crystalline phase. Secondly, a significant roughening of the interface occurs, also associated with a crystalline phase. It is postulated that the formation of these features requires an inhomogenous distribution of the phosphorus dopant, which resulted from low temperature processing.

We have found that irradiation of a variety of thin film–substrate combinations by heavy ion beams at energies of mega-electronvolts per atomic mass unit will produce a remarkable enhancement in the adherence of the film. For example, gold films can be firmly attached to soft materials such as Teflon using a 1 MeV beam of protons (1014 cm−2) or helium ions (1013 cm−2) and to harder materials such as silicon (1015 cm−2), quartz (2×1015 cm−2) and tungsten (2×1014 cm−2) with 0.5 MeV a.m.u.−1 beams of fluorine or chlorine ions. In the case of metal films on semiconductors a low resistance contact results. The mixed layer at the interface is observed to be quite thin (approximately 50 Å or less); for silver on silicon electron diffraction and imaging studies of the interface region reveal the presence of crystalline silver compounds.

As+-ion-induced silicide formation in nickel thin films on silicon was investigated by Rutherford backscattering spectrometry and transmission electron microscopy. The emphasis was on the study of ion-beam-induced microstructural changes.

For 160 keV As+ implantation, amorphization of the interface occurred at a dose of 5 × 1014 cm−2. Ni2Si was found together with an amorphous layer after a 1 × 1015 cm−2 bombardment. For Ni/Si(100) the surface layer became completely amorphous after implantation to 5×1015 cm−2. Silicides were found after a 1×1016 cm−2 irradiation. The amorphous layer was not stable enough to withstand the enormous chemical driving force causing the formation of crystalline silicides as the composition ratio Nsi/NNi reached a critical value. A similar trend for ion-beam-induced reactions was found for 190 keV As+ implantation on Ni/Si(111) as for 160 keV implantation.

The results of post-implantation annealing showed major differences from those obtained for directly annealed samples.

The surface of ErSi2, formed by the reaction of thin erbium layers with a singlecrystal silicon substrate, is typically dominated by deeply penetrating regularly shaped pits. These pits are shown to have detrimental effects on the electronic performance of Schottky barrier diodes. Surface pits may be reduced in density or eliminated entirely (i) by the use of silicon substrate surfaces prepared under ultrahigh vacuum conditions prior to metal deposition, (ii) by means of ion irradiation techniques or (iii) by reacting erbium with an amorphous silicon (a-Si) layer. In this investigation, planar ErSi2 layers (pit free) are made using the third approach with a sample structure of a-Si/Er/c-Si where c-Si denotes crystalline silicon. The fast reaction between a-Si and erbium leads to a planar sample structure of ErSi2/c-Si with little or no reaction between erbium and the c–Si substrate. The electronic performance of pit–free ErSi2 diodes made in this manner is shown to be much superior to that of diodes made by reacting erbium with silicon substrates.

The formation of epitaxial CoSi2 on silicon by both conventional and two-step annealing of cobalt thin films on silicon was studied by transmission electron microscopy.

For two-step annealing, samples were first annealed at 350°C for 1 h, followed by high temperature annealing at 650–950°C for 1 h. The scheme was found to bevery effective in promoting the epitaxial growth of CoSi2 on silicon as well aseliminating faceted structures on Si(111). The results are discussed in terms of the driving away of impurities from the interfaces.