P. R. Pukite

Damped oscillations in reflection high energy electron diffraction during GaAs MBE

Oscillations in the time evolution of electron diffraction during MBE growth of GaAs are shown to be related to periodic variations in the step distributions on GaAs surfaces during epitaxial growth. Unintentionally doped GaAs surfaces were first prepared by MBE. Then the Ga flux is interrupted until an instrument limited diffraction pattern was obtained. During this process the angular width of the specular beam was measured versus time. When the Ga flux there are intensity oscillations that are weak near the Bragg angle. At the Bragg angle, where the diffraction is insensitive to surface steps, the length of the specular RHEED streak does not change. At angles between the Bragg angles, where steps lengthen the streaks, there are periodic variations in the streak length. We interpret the results in terms of a model in which a partially completed surface has a step distribution with smaller average terrace lengths than a completed surface.

Multilayer step formation after As adsorption on Si (100): Nucleation of GaAs on vicinal Si

We have used reflection high‐energy electron diffraction to characterize the initial surface of misoriented Si (100) and then to follow the nucleation of GaAs. Measurement of the diffracted intensity along the length of the specular streak shows sharp structure due to an ordered array of steps. The initial surface contains monolayer steps. However, after exposing to an As4 flux above 650 °C, the surface morphology changes to multilayer steps with four times the original period. In contrast, below 650 °C, surface migration is inhibited and monolayer steps are retained. Subsequent growth of GaAs on either the monolayer‐ or multilayer‐stepped surfaces yields single domain films. However, GaAs grown on the monolayer steps is misoriented toward the (111)A while GaAs grown on the multilayer steps is misoriented toward the (111)B.

The dependence of RHEED oscillations on MBE growth parameters

Oscillations in both the intensity and width of reflection high‐energy electron diffraction beams that are measured during the MBE growth of GaAs are observed to depend upon most growth parameters. The envelope of the intensity oscillations, for example, depends upon the flux ratios, the substrate temperature, the sample misorientation, the scattering geometry, the sample flatness, and the flux uniformity. To separate these dependencies we have made RHEED measurements on small, exceedingly flat, near singular samples. The angular profiles of the specular beam were measured during the first two periods of oscillations. These data were analyzed in terms of the two‐level diffraction calculation of Lent and Cohen. The measured profiles are in excellent agreement with that calculation. For oscillations with a 26‐s period, moderate changes in the substrate temperature and As flux can cause significant changes in the pair correlation length on the surface.

Reflection high energy electron diffraction studies of epitaxial growth on semiconductor surfaces

Reflection high‐energy electron diffraction (RHEED) is so well suited to studies of epitaxial growth that it has become the standard in situ technique for monitoring the growth of III–V semiconductors during molecular beam epitaxy. The characteristic diffraction pattern for a smooth surface is a family of parallel streaks. The intensity along the length of these streaks is shown to be due primarily to the distribution of atomic steps over the surface. These steps are readily observable because RHEED is able to resolve order over long distances. During epitaxial growth synchronous oscillations in the intensity and width of the diffracted beams are observed. These are due to the competition between cluster nucleation and step propagation. The period of the oscillations is the time required to deposit a monolayer.

RHEED streaks and instrument response

Reflection high‐energy electron diffraction (RHEED) is so sensitive to surface morphology that it is difficult to separate the roles of instrument response and surface imperfection in the diffraction. To address this difficulty we have used MBE grown GaAs(001) as a test surface to study the angular dimensions of the diffracted beams. This is important if RHEED is to be useful as a quantitive probe of surface structure for in situ studies of crystal growth. The limitations placed by instrumental uncertainties on the maximum resolvable distance are estimated. Measurements of the angular length of the RHEED steaks versus angle of incidence are presented which show the changes expected from a combination of a uniformly broadened reciprocal lattice rod and an instrument limit due to angular uncertainties. Resolvable distances are obtained with RHEED that are much larger than those typically obtained with low‐energy electron diffraction.

Extrinsic effects in reflection high‐energy electron diffraction patterns from MBE GaAs

One uncontrolled parameter of MBE growth on GaAs is the orientation of the substrate. Typical material is oriented to within 1/2 ° of a low‐index bulk plane so that the resulting surface consists of a staircase with random terrace lengths and step heights. These steps are important since at room temperature they can orient epitaxial layers and at high temperatures the terrace lengths can be comparable to diffusion distances. Ordered step arrays can also be an important factor in the formation of the characteristic streaked RHEED pattern. We have measured the profiles of these streaks from different wafers with nominally GaAs(001) surfaces and have observed striking differences which we ascribe to ordered staircase steps due to crystal misorientation. Misorientations as small as 1 mrad have been found. Surfaces in which the misorientation from the (001) was about 6.5 mrad were studied extensively and a reciprocal lattice was constructed from a map of the diffracted intensity vs scattering angles. On surfaces misoriented by 1°, RHEED oscillations were found to be much weaker than on the 6.5 mrad surface. Differences in the oscillations when the beam was directed first down and then perpendicular to the staircase direction were also observed, but these cannot yet be separated from differences due to temperature and flux ratios.

Sensitive reflection high‐energy electron diffraction measurement of the local misorientation of vicinal GaAs surfaces

Reflection high‐energy electron diffraction (RHEED) is used to determine the local misorientation of vicinal, molecular beam epitaxy prepared GaAs surfaces. With the glancing angle of incidence fixed, the intensity along the (00) streak is measured for different crystal azimuths. The specular beam is observed to split by an amount that depends upon the scattering geometry and surface misorientation. The method is applied to surfaces misoriented with respect to low‐index bulk planes by an average polar angle of 2°, 1°, and 5 mrad. Local polar and azimuthal misorientations were determined to within 5% and 10°, respectively. The measurement shows that an important mechanism responsible for RHEED streaks is diffraction from ordered staircase steps.