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One of the first reports of a radiation-induced phase change from the crystalline to amorphous state was observed in naturally occurring minerals containingα-emitting radioactivity (1). The term “metamictisation” from the Greek meta (mixed or between) and miktos (compounded) was coined to describe the amorphization of the crystal structure of a mineral due to radiation from contained or nearby radioactive atoms. This was observed soon after von Laue’s discovery of x-rays when mineralogists noted the loss of crystalline structure in minerals exposed to radioactivity.

Reflection high energy electron diffraction (RHEED) uses electrons of energy from a few keV to ∼100 keV directed at low angles of incidence (0.01 to 0.1 rad) with respect to a surface. RHEED is a fairly old technique. There are several examples1,2 of its use in the 1930’s and it was extensively employed for corrosion studies and epitaxial growth in the 1950’s and 1960’s3–6. However the enthusiasm which existed around 1970 for performing a complete surface structure determination using the low energy electron diffraction technique deflected interest away from RHEED. One of the reasons for this was the sensitivity of RHEED to topography, which the surface scientist took to be a serious disadvantage! The recent revival of interest in RHEED stems from its wide use in monitoring the films and layers grown by molecular beam epitaxy (MBE) particularly of the III–V semiconductors7–9. The technology which has developed for MBE of the III–Vs has now been applied to the growth of II–VIs, group IV, various metals and even oxides. It now seems very likely that the molecule-by-molecule or atom-by-atom approach which is central to the MBE concept can be applied to grow any natural occuring material and to grow completely new “artificial” crystal structures. This has already been achieved for atomic layer superlattices of the III–V systems and the group IV system.

For the case of Rutherford scattering the rule of reversibility (reciprocity theorem) has been proved to hold quantitatively. Thus the enhancement of Rutherford scattering of an incident beam parallel to a major axis or plane is equivalent to Kikuchi excess bands normally observed from crystals a few extinction distances or less in thickness. The contrast of the excess bands decreases drastically by increasing the foil thickness due to the influence of multiple scattering (absorption). It will be shown that the experimental thickness dependence for the investigated crystals can be scaled by a mean absorption distance l=(Nσ)−1, where N is the density of scatterers and σ the total cross section for incoherent scattering.

A relationship has been derived between the constants for equilibria involving graphite and diamond, and some kinetic aspects of the crystallization are discussed. Autoepitaxial films of diamond have been produced that resemble natural diamond in very many aspects.

The critical voltages for systematic reflections and splits of Kikuchi lines were measured using a high-voltage electron microscope to investigate the atomic temperature factors in cubic crystals. The split of the Kikuchi line at the intersection with the forbidden 222 Kikuchi line as well as the critical voltage of the 333 reflection for Si and Ge decreased steeply with temperature. The temperature dependence showed that the anharmonic contribution to the atomic-temperature factor for Si and Ge is extremely weak in the temperature range 300 approximately 1078 K. On the contrary, the B factors obtained from the measured critical voltages for Al, Cu, and Fe varied nonlinearly with temperature, suggesting the importance of the anharmonic effect in the vibration of atoms. The observed temperature dependence of the critical voltages for the metals were compared with calculations based on harmonic, quasi-harmonic, and anharmonic approximations. The quasi-harmonic approximation that takes into account the thermal expansion modification reproduces well the observed values for Fe but not those for Al and Cu. The effect of intrinsic anharmonic vibration should be considered for reproducing the results for Al and Cu. Fitting the measured critical voltages with the calculated ones, we estimated the values for coefficients of the isolated atom potentials. The results are in good agreement with those obtained by neutron and X-ray diffraction.

CHNIQUES. The polishing processes which to date produced the most defect-free surfaces are described. The surface crystallinities were investigated by high energy electron diffraction (HEED). Clearly defined Kikuchi lines were observed indicating that if there is a disturbed surface layer, its thickness is less than 100A. The effectiveness of the hydrogen peroxide based cleaning procedure was investigated by Auger electron spectroscopy. A fraction of a monolayer of carbon was the only observable contaminant on the surface after this procedure was used. Atomically clean quartz surfaces could be produced by ion bombardment cleaning, but the ion bombardment itself produced some undesired side effects. The effects of the electron beam and of the ion beam are discussed. (Modified author abstract)

The attachment consists of a modified airlock section, specimen stage and specimen holder. Normal microscope facilities are maintained, and an outlet provided for radiation generated within the specimen. A Le Poole lens incorporated in the airlock section allows the illuminating spot to be varied between 0.25 μ and 200 μ. This small illuminating spot has been used for both micro-diffraction studies, and microanalysis of normal transmission specimens.

A convenient and rapid slide rule method for solving electron diffraction patterns is described.

Determination of topography of crystallite orientations is an important technique of investigation of polycrystalline materials. A system for creating orientation maps using transmission electron microscope (TEM) Kikuchi patterns and Convergent beam electron diffraction patterns is presented. The orientation maps are obtained using a step-by-step beam scan on a computer-controlled TEM equipped with a CCD camera. At each step, acquired diffraction patterns are indexed and orientations are determined. Although, the approach used is similar to that applied in SEM/electron back scattered diffraction (EBSD) orientation imaging setups, the TEM-based system considerably differs from its SEM counterpart. The main differences appear due to specific features of TEM and SEM diffraction patterns. Also, the resulting maps are not equivalent. On these generated by TEM, the accuracy of orientation determination can be better than 0.1°. The spatial resolution is estimated to be about 10 nm. The latter feature makes the TEM orientation mapping system an important tool for studies at fine scale unreachable by SEM/EBSD systems. The automatic orientation mapping is expected to be a useful complement of the conventional TEM contrast images. The new technique will be essential for characterization of fine structure materials. To illustrate that, example maps of an aluminum sample produced by severe plastic deformation are included.

We have grown epitaxial ZnSe films on (001)GaAs substrates at 300 °C by pulsed laser deposition (PLD). Before the growth, thin buffer layers of GaAs are also grown by PLD at 300 °C. While the pattern of reflection high energy electron diffraction (RHEED) of the buffer layers is spotty, the pattern of the ZnSe films subsequently grown is streaky, and shows distinct Kikuchi lines and bands. The x‐ray rocking curve width of the films is as narrow as 150 arcsec. Photoluminescence (PL) at 10 K of the films shows free and bound excitons, donor‐acceptor pairs (DAP), and is free of any deep level emissions, indicating good crystalline quality of the films. Scanning electron microscopy (SEM) shows that the particulate number density of the films is only about 1 particulate per 400 μm2.

We analyse the signal formation process for scanning electron microscopic imaging applications on crystalline specimens. In accordance with previous investigations, we find nontrivial effects of incident beam diffraction on the backscattered electron distribution in energy and momentum. Specifically, incident beam diffraction causes angular changes of the backscattered electron distribution which we identify as the dominant mechanism underlying pseudocolour orientation imaging using multiple, angle-resolving detectors. Consequently, diffraction effects of the incident beam and their impact on the subsequent coherent and incoherent electron transport need to be taken into account for an in-depth theoretical modelling of the energy- and momentum distribution of electrons backscattered from crystalline sample regions. Our findings have implications for the level of theoretical detail that can be necessary for the interpretation of complex imaging modalities such as electron channelling contrast imaging (ECCI) of defects in crystals. If the solid angle of detection is limited to specific regions of the backscattered electron momentum distribution, the image contrast that is observed in ECCI and similar applications can be strongly affected by incident beam diffraction and topographic effects from the sample surface. As an application, we demonstrate characteristic changes in the resulting images if different properties of the backscattered electron distribution are used for the analysis of a GaN thin film sample containing dislocations.

We present a detailed description of ultrafast electron diffraction and its applications to study photoinduced molecular dynamics at single crystal surfaces. Experimental investigations for a new design of an ultrashort pulsed laser activated electron gun ((tau) < 5 ps) for time- resolved surface analysis are described. In addition, a novel electron detection and image analysis system, as it applies specifically to time-resolved reflection high-energy electron diffraction in the multiple-shot operation, are reviewed. The total experimental temporal resolution is discussed in terms of the electron pulse width and the time difference between an electron scattered at the front edge of the sample to an electron scattered at the trailing edge of the sample.

Exceptionally highly crystalline sp3-bonded boron nitride showing Kikuchi lines in transmission electron diffraction was formed by pulsed laser vaporization deposition assisted with synchronously pulsated reactive plasma. Transmission electron microscopic observations indicated that a fluid-state precursor material was generated by the high-power nanosecond laser at 193 nm. Other sp2- and sp3-hybridized nanostructures with exceptionally wide variations in density, reflecting the original large density fluctuations, suggested that the precursor material was formed in the vicinity of the thermodynamic critical point (CP). These results as well as the exceptionally fast growth rates indicated a new mode of crystal growth, namely, the direct condensation of nonequilibrium-phase crystals from the fluid state near the CP.

A technique is described for the projection of diagrams at an oblique angle using simple equipment. The technique permits in situ identification of diffraction patterns in the electron microscope.

It has been demonstrated that specimen contamination in convergent-beam diffraction operation can be prevented by maintaining the specimen temperature between -110 °C and -165 °C, without the use of especially high or clean vacuum conditions. At these temperatures, surface migration of molecules causing contamination is evidently inhibited. Precautions to prevent deposition from the vapour phase both before and after cooling are also required.

An immersion lens in a TEM allows diffracted beams from an extended area of the sample to be sharply focussed. However, the tangential (rotational) divergence caused by the electrons spiraling in the magnetic field does not allow Kikuchi lines from an extended area to be focussed. This is because the diffracted beams are at fixed orientations with respect to the incident beam whereas the orientations of the Kikuchi lines are fixed to the crystal. Kikuchi lines from a small area can be focussed and are in focus precisely in the back focal plane, which makes them a useful reference by which to focus correctly the diffraction lens and to set the height of the objective aperture.

Chill-cast Al6CuLi3 was studied together with roller-quenched Al4Mn and melt-spun Al7Mn2. With convergent beam electron diffraction it was established that Al6CuLi3 and Al4Mn exhibited a pentagon as the dominant Kikuchi band pattern while Al7Mn2 showed a decagon. The quasi-crystalline m3¯5¯ and the crystalline Im3 structures seem to occur simultaneously.

Contrast behavior of the Laue case Kikuchi pattern in low and medium energy reflection electron diffractions is investigated theoretically. The roles of the normal and abnormal absorption effects in determining the contrast behavior are presented.

The density and heat of crystallization of rapidly quenched Mn14Al86 were measured and found to be 3.287±0.038 g/cm3 and 16.2 cal/g, respectively. Transmission electron microscopy revealed nodules of the icosahedral phase with lattice fringes. No Kikuchi lines were observed in either selected area or convergent beam diffraction.

SINGLE crystal layers of silicon up to 50µ thick have been deposited on to a heated single crystal silicon substrate. Reflexion electron diffraction of the deposited layer showed Kikuchi lines indicating a high order of crystalline perfection. PN junctions with a maximum reverse breakdown of voltage in excess of 300 V. have been made. The conventional bevelling and staining techniques1 showed a sharp junction between the deposited layer and the substrate. Maximum resistivities of 10 Ω-cm. for P-type and 3 Ω-cm. for N-type layers have been obtained.

N-PATTERNS of spots from thin crystals, and Kikuchi-lines from comparatively thick ones, have been observed by several workers1 as electron diffraction patterns from mica. In this study a flake of mica having various thicknesses was stuck on a mesh of 0.1 mm. diameter by means of methyl alcohol and was then placed on the projection lens of an electron microscope so that electron micrographs and electron-diffraction patterns of the mica might be taken. First the electron image of the mica (Fig. la) was observed, then the incident electron beam was converged on it (the least area illuminated by the electrons was a circle of about 0.01 mm. diameter), and was moved horizontally to the part of the mica to be observed having the desired thickness. Then the electric current through the projection lens was switched off, and afterwards the electron diffraction pattern of one part of the mica (Fig. 1 b) was observed. The thickness of each part of the mica was determined relatively by comparing the intensities of the transmitted electrons.

THE N pattern of electron diffraction was first obtained from a thin piece of mica. It has not been obtained for many substances because of the technical difficulty of preparing single crystal films of suitable thickness for electron penetration.

The X-ray atomic scattering factors f ; X for the 110 reflection of V and Cr have been determined by high voltage electron diffraction, and the results are compared with theoretical values.

An annular semiconductor diode detector was used in a Cambridge Stereoscan electron microscope to resolve beam‐orientation‐dependent backscattered electron images not only as bands but also as extremely complicated defect and excess line patterns with angular resolution ≤ 5 × 10−4 rad. The resultant patterns are similar in appearance to transmission and reflection Kikuchi electron diffraction patterns.

The design and construction of two additions to the Metropolitan-Vickers Type E.M.3 electron microscope which are useful in the study of the electron diffraction by single crystals are described. One addition, an adjustable rectangular diffraction aperture, is used to reduce the diffraction area selected at the specimen to a minimum, while the other, two pairs of deflector plates at right angles to which saw-tooth waveform voltages are applied, is useful in the accurate measurement of the intensities in single crystal patterns.

WE have developed a method for studying the structure of thin films during the earliest stages of growth. The method is based on small angle electron diffraction1–3 and is more sensitive than in situ electron microscopy.

AN improved system for scanning electron diffraction has recently been developed in this laboratory. The main improvements on the earlier system of Grigson1,2 are greater stability and sensitivity of the diffraction instrument and a more accurate display ; moreover, provision is made for evaporating specimens, so that intensity profiles from a growing film are obtained ; and the diffraction patterns may be scanned in two dimensions.

It is shown that the intensity of the Kikuchi line of the second order reflection 220 in electron diffraction patterns from a thin single-crystal of Fe–20 at.% Al alloy gradually changes with the accelerating voltage for incident electrons and vanishes at about 340 kV owing to the systematic dynamical interactions combined with the relativistic change of electron mass.

FOR thin films to be of use, in microelectronics, for example, they must be deposited on bulk substrates. Little is known, however, of the nucleation and structure of very thin films deposited in this way, because the usual methods of study, electron diffraction or microscopy in transmission are not applicable. If the film is polycrystalline and thinner than 100 A, reflexion electron diffraction can provide crystallographic information; if the film is a single crystal film then low energy electron diffraction is also possible. To study the film in the early stages of formation it must be deposited inside the diffraction camera. Rather few in situ studies by reflexion electron diffraction have been reported1,2, perhaps because conventional reflexion cameras suffer severe disadvantages compared with transmission cameras. The chief disadvantages are that (a) large numbers of secondary and loss electrons are produced which obscure the elastically scattered electrons carrying the erystallographic information; the total intensity of the loss electrons usually exceeds that of the elastically diffracted ones; (b) the effects of contamination are troublesome unless partial pressures of contaminant gases are below 5 × 10−9 torr; (c) the intensities are greatly affected by the surface condition of the specimen and by small changes of the angle of incidence of the electron beam.

A description of a computer program written in Fortran IV for the production of arbitrarily oriented stereographic projections for any crystal system is given. Subroutines exist for plotting zones, simulated Kikuchi line patterns, locus of points of equi-distance from poles and/or directions and for facilitating the construction of inverse pole figures.

Doubling of Kikuchi lines inside strong bands has been reported previously in electron diffraction patterns from MgO. The `doubling' is now explained as being due to two different lines, which at the particular wavelength employed in the experiment are very close. Owing to strong enhancement from multiple beam interactions inside the band, one of the lines is visible only in this region. Further examples of such doubling are shown in patterns from MgO and natural spinel. The observed patterns are compared with theoretical calculations.

The accelerating voltage Ec at which a second order Kikuchi line vanishes was measured for 220 of iron (305 kV), and 222 of nickel (295 kV) and aluminium (430 kV). A measured value of Ec made it possible to determine accurately the X-ray atom form factor fX for the first order reflection assuming those for the second and higher order reflections.

In established parallel-beam electron-diffraction practice, it is known that reversal of contrast from excess to defect occurs in Kikuchi bands when the crystal thickness is increased. A similar effect is observed when the aperture angle of the incident electrons is increased.

DURING the past decade there have been considerable developments in high voltage electron microscopes and diffraction cameras which work reliably at voltages above 300 kV. These instruments are designed for studies of “thick” specimens—foils which it is practicable to prepare by cutting or thinning bulk materials. Metal specimens thinned from bulk are usually several thousand angstroms thick and, if examined at 100 kV, electrons which have been inelastically scattered fog the image or diffraction pattern. By working at much higher voltages the “contrast” of thick specimens is enhanced.

Kikuchi patterns have been observed as an etched, or re-sputtered, pattern in the sputtered deposit obtained by electron irradiation of sodium chloride.

THE bubble model developed by Bragg and Nye1 gives a pictorial representation corresponding, with a high degree of probability, to the arrangement of atomic sites within a crystalline material. In order, however, to make further progress with the help of this model, it is desirable to produce an experimental technique which will enable some correspondence between the bubble model and the actual degree of order in the crystal to be established.

IT is well known that a cross-grating pattern of spots is produced when a fast electron beam is incident, at a small angle, on the etched surface of a single crystal. Two explanations have been given of this phenomenon. Thomson1 showed that the effect would arise if the electrons passed through small projections on the surface, while Germer2 attributed it to distortion of the crystal lattice. When etched single crystals of zincblende and galena were studied by Tillman3, he found Kikuchi lines to be present in addition to the cross-grating and, as has been pointed out previously4, the presence and sharpness of Kikuchi lines may be taken as a criterion of the degree of perfection of the crystal lattice.

A specimen stage for electron diffraction and electron shadow microscopy has been constructed, to enable a holder containing two transmission specimens to be inserted into, or withdrawn from, the specimen chamber without breaking the vacuum in the system. The time required to change specimens is less than one minute. Controls are provided to give specimens two independent translations in a plane normal to the electron beam and a rotation about the axis of the holder.

Changes in intensity profiles of Kikuchi lines obtained in reflection electron diffraction from NaCl freshly-cleaved surfaces were measured as functions of exposure times to electron beams. Data are presented in graphical form. Reduced data from microphotometer traces of photographic exposures were accurate within plus or minus 3%. The increase was about 5% after 3 min of electron bombardment at 3.