摘要:

CdS was deposited onto clean cleaved InP(110) by molecular beam epitaxy (MBE) using a growth rate of 0.2 monolayers/min and a substrate temperature of 440 K (510 K). Raman spectra were takeninsituof the clean InP surface and after each evaporation step using an Ar+ion laser as a light source. Due to this resonant excitation scattering signals originating from the CdS deposition are observed at coverages as low as 2 monolayers (ML). The number of phonon peaks observed and their selection rules reveal that the cubic modification is present. The spectra are dominated at all coverages by the longitudinal optical (LO) and 2LO phonon scattering intensities and the variation of the 2LO/LO intensity ratio with CdS deposition indicates changes in the electronic structure of the growing CdS. Another spectral feature in the Raman spectra is attributed to a chemically reacted layer at the interface most likely consisting of an In–S compound. The intensity of this feature is found to depend critically on the growth parameters, in particular the substrate temperature, but also on the operating time of the MBE cell. The amount of reaction at the interface also influences the critical CdS film thickness and the development of the 2LO/LO ratio. The results are discussed taking complementary photoluminescence, x‐ray diffraction, and photoemission data into account.

ISSN:1071-1023    

DOI:10.1116/1.585766

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Using a thermodynamic approach, we examine the role thermal expansion plays in determining the temperature dependence of band gaps and band offsets. Because of the large variation in the standard volume of electrons (or, equivalently, variation in conduction‐band deformation potential) with symmetry of the conduction band, most of the band‐gap temperature variation can be ascribed to the valence band for theXgap, while the variation is more equitably divided between the conduction and valence band for the Γ band gap. The particulars of the AlAs/GaAs system are examined in detail, where we find evidence thatXelectrons have a larger electron–phonon entropy than Γ electrons, and that the lowest conduction‐band offset exhibits considerable temperature variation.

ISSN:1071-1023    

DOI:10.1116/1.585767

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Ballistic electron emission microscopy (BEEM) and the conventionalI‐Vtechnique have been applied to study the band discontinuity at a relaxed InAs–GaAs heterojunction grown by molecular‐beam epitaxy (MBE). A conduction band discontinuity ΔEcof 0.72 eV has been determined from the measurements and from appropriate numerical modeling of the interface. The lateral variation of the conduction band discontinuity has been successfully probed with BEEM with a view to establishing the influence of misfit disclocations on the barrier height. Experiments are also reported where the conduction band offset has been significantly modified by appropriate δ doping close to the interface. Doping with Be has enabled values of the effective barrier as high as 1.2 eV to be obtained. These investigations are also well described by numerical modeling.

ISSN:1071-1023    

DOI:10.1116/1.585768

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Thin Si epitaxial layers (1–14 monolayers) were fabricated by molecular beam epitaxy on GaAs(001) and AlAs(001) substrates also obtained by molecular beam epitaxy on GaAs(001) wafers.Insitustudies by monochromatic x‐ray photoemission show initial layer‐by‐layer Si growth on both substrates with only minor Si indiffusion. Reflection high energy electron diffraction analysis shows good epitaxy with some indication of three‐dimensional growth at Si coverages higher than 4–8 monolayers. Comparison of our results with recent heterojunction theories suggests that the best predictions for the band offsets are obtained with the model solid approach using deformation potentials to describe the effect of strain. The Si epitaxial layers are found to remain stable upon growth of AlAs or GaAs layers on top of the Si layers.

ISSN:1071-1023    

DOI:10.1116/1.585725

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

We have used x‐ray photoelectron spectroscopy (XPS) to measure the valence band offsetinsitufor CdSe/ZnTe (100) heterojunctions grown by molecular‐beam epitaxy. XPS measurements were performed for films of CdSe (100) and ZnTe (100), and for heterojunctions consisting of either ∼25 Å of CdSe grown on ZnTe or ∼25 Å of ZnTe grown on CdSe. Observations of reflection high energy electron diffraction patterns indicated that CdSe films deposited on ZnTe were grown in cubic zinc blende form, rather than the natural wurtzite structure of CdSe. Our measurements yielded a CdSe/ZnTe valence band offset ΔEv=0.64±0.07 eV. The corresponding conduction band offset for CdSe/ZnTe is ΔEc=1.22±0.07 eV for room temperature band gaps for ZnTe and for cubic CdSe of 2.25 and 1.67 eV, respectively.

ISSN:1071-1023    

DOI:10.1116/1.585726

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The heterojunction between silicon(111) and zinc sulfide was studied using Auger electron spectroscopy, photoelectron spectroscopy, and low‐energy electron diffraction. Zinc sulfide layers were deposited onto cleaved Si(111) surfaces as well as Si(111)‐(7×7) wafers by molecular beam epitaxy. The overlayers exhibited fair crystalline quality, and the characteristic valence‐band spectrum of ZnS. The valence‐band offset between the two semiconductors was determined from the core and valence‐band spectra (ΔEv=−0.7 eV) and found to be much smaller than predicted. We attribute this disagreement, and the larger than usual scatter in our data, to the influence of interface dipoles in this polar interface, the density of which may partly be influenced by a varying amount of interface reaction.

ISSN:1071-1023    

DOI:10.1116/1.585727

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

We report a direct optical observation of process‐specific diffusion‐related deep levels associated with interdiffusion in AlGaAs/GaAs superlattice structures. We have used low‐energy cathodoluminescence spectroscopy (CLS) to investigate the formation and evolution of deep levels for intrinsic, under As overpressure, and Si induced layer intermixing. The spatial distribution of these deep levels strongly correlates with the extent of superlattice intermixing, as measured by secondary ion mass spectroscopy (SIMS) and photoluminescence spectroscopy (PLS). The measured cathodoluminescence emission energies and intensities reveal the important role of impurities in the mechanism of interdiffusion at III–V semiconductor superlattices. In particular, our experimental results strongly suggest that the larger interdiffusion rate of the Si induced layer intermixing process is related to the formation of a deep level associated with an optical emission at 1.3 eV. These results indicate the potential of the low‐energy cathodoluminescence technique for studying deep level formation and deep level inhomogeneities in semiconductor structures.

ISSN:1071-1023    

DOI:10.1116/1.585728

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

We present selected results from a theoretical study of the local density of states of imperfect quantum‐well structures. At the example of GaAs–AlGaAs, we investigate the validity of the virtual crystal approximation (VCA) for ternary alloys, show how quantum confinement can be evidenced in the local density of states (LDOS), discuss antisite defect levels in quantum wells, and finally, analyze the effects of interface roughness on the local density of states.

ISSN:1071-1023    

DOI:10.1116/1.585729

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

We have investigated the relationship between interface chemistry, structure, and surface electronic states in the band gap of Se and Te‐passivated GaAs(001). Metalorganic chemical vapor deposition was used to carry out the passivations. Chemical, structural, and electronic characterization of the surfaces was accomplished by means of x‐ray photoelectron spectroscopy and diffraction, and low‐energy electron diffraction. We find that the superior passivating capability of Se can be related to the structural quality of the interface created in the near‐surface region. Se undergoes extensive, but diffusion‐limited, anion exchange in the top five to seven anion layers. The majority of As atoms in the surface layer undergo anion exchange with Se, and the extent of anion exchange diminishes with increasing depth. Thus, a graded SexAs1−xGa/GaAs(001) heterojunction of high structural quality forms. In contrast, Te does not passivate GaAs(001), and this inability to passivate can also be related to interface structure. Te undergoes extensive anion exchange only in the surface layer; displacement of third‐layer As atoms appears to be limited to ∼10 at. %. Furthermore, the interface is partially disordered and ∼1 monolayer‐equivalent of unincorporated Te remains bound to the surface. The combined effect of these defects is to create a surface‐state density sufficient to induce significant band bending.

ISSN:1071-1023    

DOI:10.1116/1.585730

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Reflectance‐difference (RD) studies were performed on variously reconstructed (001) GaAs and AlAs surfaces. The spectra of the (2×4) and (4×2) reconstructions on (001) GaAs show prominent features due to electronic transitions between lone‐pair orbitals and dimer states, as previously identified by theoretical calculations. The spectra of thec(4×4) reconstructions on (001) GaAs and AlAs show similar features that we also interpret in terms of surface dimer excitations. These dimer features provide a capability of obtaining real‐time,insituinformation of dynamics on polar surfaces.

ISSN:1071-1023    

DOI:10.1116/1.585731

出版商:American Vacuum Society    

年代:1991

数据来源: AIP