摘要:

In this paper, we present a study of structural changes which may occur as a function of surface composition within a single surface reconstruction. We examine in detail the GaAs{001}(2×4) surface by measuring the ion beam desorbed Ga+ion angular distributions as a function of As coverage. The distributions of these ions, initiated by 3‐keV Ar+ion bombardment at normal incidence, are measured as a function of azimuthal and polar angles of ejection from the molecular‐beam‐epitaxy‐grown (2×4) surface. The distributions display extreme anisotropy, indicative of a specific collision sequence which results from the covalent nature of the GaAs crystal. More subtle blocking and channeling effects are also found in the angular pattern which allow qualitative analysis of the (2×4) surface structure. These results agree with the currently accepted model of missing As dimers for this reconstruction, which appear as open channels parallel to the 〈011̄〉 direction on the surface. The As coverage of the (2×4) surface was modified by molecular beam deposition of either As or Ga. These results indicate structural variations within the range of As and Ga coverages which are considered to yield the (2×4) reconstruction. The excess Ga atoms appear to adsorb in the fourfold hollow sites on top of the As dimers and at least 0.25 monolayer of Ga can be deposited before the surface reconstructs to a (3×1) symmetry. It is proposed that this adsorbed Ga may destabilize the (2×4) surface by opening the dimers, and thus initiating the reconstruction. Excess As atoms appear to adsorb in the fourfold hollow sites of the open channels on the (2×4) surface. The As coverage of the (2×4) surface was also modified by ultrahigh vacuum annealing. Angle‐resolved secondary ion distributions and reflection high‐energy electron diffraction patterns indicate that the surface reconstructs. It is concluded that the use of molecular beam deposition to vary the surface As coverage may be more effective than the thermal desorption of As2from a prepared surface.

ISSN:1071-1023    

DOI:10.1116/1.585732

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

A multichamber ultrahigh vacuum (UHV) scanning tunneling microscope (STM) system which includes a molecular‐beam epitaxy (MBE) growth chamber, as well as a STM chamber, has been constructed for the investigation of processed GaAs surfaces. We observed MBE‐grown GaAs surfaces using this system. Samples were grown on GaAs (001) surfaces and cooled to certain temperatures in an As4flux before being transferred into an UHV. STM images ofc(4×4), 2×4, and mixedc(4×2) and 2×2 structures were obtained for the samples cooled to 330, 570, and 470 °C in an As4flux, respectively. The mixedc(4×2) and 2×2 structure seems to be formed by desorption of As atoms from thec(4×4) surface in an UHV. Also, a new reconstructed structure which has a 2.8‐nm periodicity along both the [110] and [11̄0]directions was observed. It seems to be a local structure which has a larger arsenic dimer density than that of thec(4×4) structure.

ISSN:1071-1023    

DOI:10.1116/1.585733

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The dynamical force fields of the clean GaAs(110) surface, an isolated Sb chain, and the GaAs(110)–p(1×1)–Sb(1‐ML) interface have been computed utilizing tight‐binding total energy models that have been used successfully to describe the atomic and electronic structure of the clean and adsorbed cleavage faces of the tetrahedrally coordinated compound semiconductors. Since the main consequences of the different chemical bonding in these two cases are manifested in changes in the force field associated with the dynamics of the top‐layer atoms, we explore these consequences using a restricted dynamical model in which only the top‐layer atoms are allowed to vibrate. The resulting vibrational energies are in remarkably good agreement with experimental measurements, and hence afford a vehicle to obtain quantitative relationships between the nature of the surface chemical bonds and the vibrational energies of the surface atomic species.

ISSN:1071-1023    

DOI:10.1116/1.585734

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The soft x‐ray standing‐wave technique has been used to study ordered monolayers of Sb on GaAs(110) and InP(110). Using the back‐reflection diffraction geometry from (220) planes, we determine the perpendicular distances of Sb atoms to the substrate and compare these with theoretical calculations and elastic low‐energy electron diffraction determinations. The various models of Sb chemisorption are evaluated on the basis of our data.

ISSN:1071-1023    

DOI:10.1116/1.585735

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

We studied the Bi/InP(110) interface for 0.35‐ and 0.9‐monolayer (ML) Bi coverages by photoemission extended x‐ray absorption fine‐structure (PEXAFS). P 2pPEXAFS data were acquired. The data were analyzed by Fourier filtering followed by phase analysis using a novel curve‐fitting procedure in which theE0is also floated. For 0.9‐ML Bi/InP(110), the results show that Bi grows epitaxially and the P–Bi bond length is 2.42±0.05 Å. The first P–In nearest neighbor distance is determined as 2.46±0.05 Å, which is almost equal to the P–In bond length for the clean InP(110) surface and the bond length is 3% contracted in comparison to its value for bulk InP. Note that the surface states for the clean InP(110) surface are pushed out of the band gap due to surface relaxation. Hence, the interface states due to the atomic geometries of the substrate at the interface may not influence Schottky barrier formation to cause Fermi‐level pinning. The P–P and P–Bi bond lengths in the second near‐neighbor distance were determined as 4.17±0.06 and 4.26±0.06 Å, respectively.

ISSN:1071-1023    

DOI:10.1116/1.585736

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

We present evidence supporting the formation of a new, (2×8) surface reconstruction on Si1−xGexalloys grown on (100) Si substrates by molecular‐beam epitaxy. Surfaces of Si1−xGexalloys were studied using reflection high‐energy electron diffraction (RHEED) and low‐energy electron diffraction (LEED) techniques. RHEED patterns from samples with Ge concentrations,x, falling within the range 0.10–0.30 and grown at temperatures between 350 and 550 °C, exhibitn/8 fractional‐order diffraction streaks in addition to the normal (2×1) pattern seen on (100) Si. The presence of fractional‐order diffracted beams is indicative of an eight‐fold‐periodic modulation in electron scattering factor across the alloy surface. LEED patterns from surfaces of samples grown under similar conditions are entirely consistent with these results. In addition, the LEED patterns support the conclusion that the modulation is occurring in the direction of the dimer chains of a (2×1) reconstruction. We have examined the thermal stability of the (2×8) reconstruction and have found that it reverts to (2×1) after annealing to 700 °C and reappears after the sample temperature is allowed to cool below 600 °C. Such behavior suggests that the reconstruction is a stable, ordered phase for which the pair‐correlation function of surface Ge atoms exhibits an eightfold periodicity in the ‘‘1’’ direction of a Si‐like (2×1) reconstruction. We also present a simulation in the kinematic approximation, confirming the validity of our interpretation of these findings.

ISSN:1071-1023    

DOI:10.1116/1.585737

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The surface and interface structures of molecular beam epitaxially grown CoSi2films on Si(111) have been studied by scanning tunneling microscopy and by transmission electron microscopy, respectively. All surfaces are found to be inhomogeneous, exhibiting (2×1) and (2×2) reconstructed domains along with unreconstructed areas, depending on their stoichiometry. All of them could be imaged with atomic resolution. The surface step structure and the formation of pinholes have been examined for a wide range of growth conditions. Evidence is presented for micron‐scale surface diffusion of Si on CoSi2at temperatures as low as 800 K. The interface step structure, studied by transmission electron microscopy, has been found to depend critically on the details of the growth procedure.

ISSN:1071-1023    

DOI:10.1116/1.585738

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The intensity behavior of the specular beam in reflection high‐energy electron diffraction (RHEED) from GaAs(111)B grown by molecular‐beam epitaxy (MBE) is investigated for various growth and diffraction conditions. The temporal behavior during the initial growth of a buffer layer is examined at a fixed diffraction condition. Intensity increase is observed during and after the initial stages of buffer layer growth and found to saturate after about 80 monolayer growth. Intensity oscillations are seen starting at different moments of initial growth, the earliest observed after only 14 monolayer growth. These results are used to guide and control GaAs(111)B growths with mirrorlike surface morphology.

ISSN:1071-1023    

DOI:10.1116/1.585739

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

A novel technique is described to enhance the doping in semiconductors by the application of external electric fields during crystal growth. We show that this technique can enhance the doping efficiency, and suppress self‐compensation processes in novel growth techniques such as molecular‐beam epitaxy (MBE). An obvious application of this technique is to enhance the doping of wide band gap II–VI semiconductors, where doping in bothn‐ andp‐types is usually not possible to achieve because of extensive self‐compensation. The physics of the electric field assisted doping process can be described in two parts. First, the external electric field produces a change in the band bending at the growth surface and alters the carrier concentrations near the surface region. This influences doping near the surface region. Second, this enhanced surface doping concentration can be kinetically buried by low temperature growth processes. In our calculations, the dopants are modeled as charged, mobile species that are free to diffuse and drift under electric fields. In the case of MBE growth, we solve for the equilibrium of these species in a moving coordinate frame that travels with the growth front. We have specifically applied our analysis to Li donors inn‐type ZnTe. Our results indicate that excellent improvements in the doping concentrations could be obtained under normal MBE growth conditions, with the application of substantial electric fields. We expect that our analysis, and the proposed electric field assisted doping technique will play an important role in the effort to overcome compensation, and achieve selective doping in wide band gap II–VI semiconductors.

ISSN:1071-1023    

DOI:10.1116/1.585740

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Annealed GaAs layers grown at low temperatures (180–300 °C) by molecular‐beam epitaxy (MBE) were studied by transmission electron microscopy (TEM). These layers were used as buffers for a field‐effect transistor (FET) device structure and effectively eliminated sidegating effects. All these layers were found to contain As precipitates. Precipitate size and separation between them differed from sample to sample. The smallest precipitates were coherent ‘‘pseudocubic.’’ Larger precipitates had hexagonal structure. The distance between precipitates was estimated to be in the range of 10–40 nm. These results are consistent with the buried Schottky model by Warrenetal., although other explanations cannot be excluded at present.

ISSN:1071-1023    

DOI:10.1116/1.585741

出版商:American Vacuum Society    

年代:1991

数据来源: AIP