X-ray-photoelectron-diffraction investigation of strain at the Si/Ge(001) interface

Abstract

We describe a structural determination of pseudomorphic Si on Ge(OOl) by means of x-ray-photoelectron diffraction. By comparing the measured angular coordinates of the forward-scattering-induced [001] peak with those calculated by means of single-scattering theory, we find that the lattice constant perpendicular to the interface is 5.34 ±0.04 A. This value exceeds that predicted by elastic theory by 0.08 A.The effects of strain on the band offset at a semiconductor heterojunction are significant, yet they are not completely understood and are often ignored. l Experimental determination of tetragonal distortion in strained semiconductor overlayers has proven to be an exceedingly difficult task, largely because the layer of interest is often only a few atomic spacings in thickness. Thus, theoretical rather than experimental means have often been used to determine the strain. Both classical elastic theory and full quantum-mechanical calculations have been employed. However, there are very few experimental values of the lattice constant normal to the interface, a±, with which to compare theoretical results.In this Letter, we describe the use of x-ray photoelectron diffraction (XPD) in conjunction with low-energy electron diffraction (LEED) and high-energy-resolution x-ray photoelectron spectroscopy (XPS) to characterize a lattice-mismatched heterojunction, Si/Ge(001), prepared with different amounts of strain and intermixing. We have determined a± by comparison of experimental XPD angular distributions with those calculated via single-scattering theory 2 for various amounts of strain, and have assessed the effects of strain and intermixing on core-level binding energies.All experiments were performed in a system of three interconnecting ultrahigh vacuum chambers equipped for sample preparation, molecular-beam epitaxial (MBE) growth, and interface characterization by means of XPS, XPD, and LEED. 3 Scanned-angle XPD measurements were carried out with a full angle of acceptance of 5°, and with a precision and accuracy of ±0.2°. Highenergy-resolution XPS measurements were made using a monochromatic Al Ka x-ray source, yielding a total energy resolution of 0.74 eV, as judged by the full width at half maximum of the Au 4/7/2 peak from a clean Au surface. The binding energy of the Au 4/7/2 peak was monitored frequently during the course of the investigation to insure that the spectrometer electronics were sufficiently stable to insure highly accurate core-level binding energies. These measurements yielded a constant value of 84.00 ±0.01 eV. Ge(001) surfaces were prepared by growing 150-200 A of epitaxial Ge at a temperature of 350 °C on clean, semi-insulating GaAs substrates cut to within ±0.5° of (001). Thin epilayers of Si were then grown at temperatures of 335 °C, which is near the minimum temperature required for good epitaxy and laminar growth of up to four monolayers (ML) of Ge on Si(OOl), 4 and 450 °C, which is ~50°C above the onset temperature for intermixing. 5 Thicknesses were determined by means of calibrated qua...

Breakdown of continuum elasticity theory in the limit of monatomic films

“…The In-As bonds directly at the InAs/GaAs interface are thus stretched in order to conserve their bulk bond length [16]. The tendency towards conservation of the unstrained bulk bond length has in fact been suggested for rather different chemical environments [17][18][19][20][21]. Thus, while films of a few ML thickness are well described by macroscopic arguments, the atomic configuration at the interface can be understood only by considering the local properties of the crystal lattice, namely, the chemical bond.…”

Strain Relaxation inInAs/GaAs(111)AHeteroepitaxy

“…On the other hand, information about the strain relaxation in the direction normal to the surface has so far been lacking. Although it has recently been demonstrated that high-energy x-ray photoelectron diffraction can be used to directly determine strain in the surface-normal direction at a strained, latticemismatched heterojunction [1,2], no method has so far been used to observe how the strain in this direction is relaxed as growth proceeds.This Letter reports the first in situ measurements of strain in the surface-normal direction during heteroepitaxy. For this purpose, we have used rocking-curve analysis of reflection high-energy electron diffraction (RHEED) based on dynamical diffraction theory, which has been used to determine atomic structures of crystal surfaces [3][4][5][6][7].…”

Epitaxial film crystallography by high-energy Auger and X-ray photoelectron diffraction

“…In this case multiple-scattering-induced defocusing effects along [011] are particularly strong, as can be seen by comparing the calculated (singlescattering) peak intensity with experiment [36][37][38]. The reconstruction evidently has [85]. The presence eta substantial peak at 0 = 90 ° for the strained Si overlayer indicates agglomeration as a result of the high surface free energy of Si.…”

“…Nevertheless, an a~ value of 5.38 _ 0-08/~ can be derived from the R-factor curve. See [85] for more details.…”

“…The critical thickness of either semiconductor on the other is about 4-5 ML. Chambers and Loebs [85] have used XPD to determine the perpendicular lattice constant a± in the strained overlayer Downloaded by [Cornell University Library] at 05: 53 19 November 2014 for the two systems. The approach is the same as that illustrated in section 4.1.2, namely accurate measurement of the forward-scattering peak along [011] and fitting to calculated angular distributions employing single-scattering theory.…”