摘要:

Porous is comprised of nanometer‐scale crystallites of Si which, on exposure to the atmosphere, become surrounded by thin oxide layers. It has been proposed that the red room temperature luminescence in this material is primarily due to defects at or near the interface between the crystallites and the thin oxide. We report here the observation of two electron spin resonance (ESR) active oxygen‐centric defects; one in nominally unoxidized porous Si and the second in material which has been oxidized for a short period of time. The first is a variant of the thermal donors observed in crystalline Si and the second is the EX center observed in thin layers of SiO2on crystalline Si. These two centers are thought to have similar cores, a Si vacancy surrounded by four Si–O linkages, with the primary difference in the surrounding lattice (Si vs SiO2). The ESR intensity of the EX center is related to the photoluminescence intensity and may be directly involved in the luminescence process.

ISSN:0734-211X    

DOI:10.1116/1.587873

出版商:American Vacuum Society    

年代:1995

数据来源: AIP

摘要:

The deposition of thin Ce films on Si(111) and subsequent or simultaneous annealing leads to the formation of various structurally, electronically, and chemically different CeSixphases, as revealed by scanning‐tunneling microscopy, low‐energy electron diffraction, and photoelectron spectroscopy. Several phases can coexist on the same sample, and with increasing annealing temperatures larger homogeneous domains are observed. This behavior is assigned to a mismatch of the lattices of the Si‐rich CeSixbulk compounds with that of the Si(111) substrate surface, resulting at temperatures above 700 °C in the formation of a thin film of hexagonal CeSi1.67, which does not exist as a bulk compound.

ISSN:0734-211X    

DOI:10.1116/1.587874

出版商:American Vacuum Society    

年代:1995

数据来源: AIP

摘要:

The influence of hydrogen adsorption on the surface order, dielectric function, and dielectric anisotropy was investigated for the three main reconstructions of the GaAs(100) surface [c(4×4), (2×4), (4×2)] by low‐energy electron diffraction, spectroscopic ellipsometry, and reflectance difference spectroscopy. For all reconstructions, low hydrogen exposure removes the surface dimers, whereas surface roughening is observed at high hydrogen exposures. However, detailed differences can be resolved depending on the surface reconstruction. On the As‐richc(4×4) surface, the outermost As‐dimers are removed at low H‐exposures leaving behind a still As‐rich surface of (1×1) symmetry. Thereafter, more As is removed and a new surface anisotropy develops due to the exposed Ga‐dimers arranged in a mixture of (1×2) and (√2×√2)‐reconstructions. For H‐adsorption on the (2×4)‐ and (4×2)‐surfaces, in contrast, only the subsequent reduction of the surface reconstruction is observed with increasing exposure, but no structural rearrangement. The removal of the As‐ [Ga‐] dimers modifies the surface symmetry from (2×4) [(4×2)]through (1×4) [(4×1)] to (1×1). Finally, for large exposures, inhomogeneous surface etching by hydrogenation leads to rough, disordered surfaces for all three reconstructions.

ISSN:0734-211X    

DOI:10.1116/1.587875

出版商:American Vacuum Society    

年代:1995

数据来源: AIP

摘要:

The growth of indium‐layers in the monolayer regime on As‐rich (2×4)/c(2×8)‐ and Ga‐rich (4×2)/c(8×2)‐GaAs(100) surfaces has been investigated by scanning tunneling microscopy (STM), low‐energy electron diffraction (LEED), Auger electron spectroscopy (AES) and reflectance anisotropy spectroscopy (RAS). Clean (2×4)‐ and (4×2)‐reconstructed surfaces were prepared in UHV by thermal desorption of a protective arsenic layer deposited on top of GaAs(100) surfaces grown in a molecular beam epitaxy (MBE)‐system. For the (2×4) reconstruction, the STM images show As‐dimer rows consisting mostly of two dimers per unit cell in the outermost layer. Besides a large number of kinks in the dimer rows, a surface roughness corresponding to about three bilayer steps (8.4 Å) is observed due to multiple layer nucleation during MBE growth. After deposition of indium and subsequent annealing to approximately 450 °C a well ordered reconstruction withc(8×2)‐symmetry is established. The STM images show a pattern consisting of kink free, alternating straight and broken rows oriented along the [110]‐direction attributed to In‐dimer rows. Remarkably, the surface roughness of these In‐terminated surfaces is strongly reduced. For the Ga‐terminated (4×2) surface reconstruction, STM results show a surface terminated with Ga‐dimer rows, free of kinks, which appears flat over large areas similar to the In‐terminated surface. After deposition of indium and annealing to 450 °C, LEED, RAS and STM reveal a surface structure very similar to that formed on the (2×4)‐surface. Thus we conclude that in both cases a structure with In‐dimer‐rows on top of an As‐layer (second layer) is formed. The smoothing of the surface roughness is a consequence of the larger mobility of the group III surface atoms, following the As desorption.

ISSN:0734-211X    

DOI:10.1116/1.587876

出版商:American Vacuum Society    

年代:1995

数据来源: AIP

摘要:

The room temperature chemisorption of gaseous NH3onp‐type GaAs(110) has been studied with scanning tunneling microscopy (STM). Atomic‐resolution images confirm that the reaction is nondissociative and occurs between lone‐pair electrons of the free molecule and cation dangling bonds at the surface. Because the adsorption complex is uncharged, the microscopic dipole screening produced by a single, tilted ammonia molecule is directly observed. The results agree completely with the predictions of Lewis acid‐base chemistry.

ISSN:0734-211X    

DOI:10.1116/1.587877

出版商:American Vacuum Society    

年代:1995

数据来源: AIP

摘要:

We report here our investigations upon both strained and relaxed In0.35Ga0.65As/AlAs interfaces using the ballistic electron emission microscopy (BEEM) and current–voltage (I–V) techniques. Six samples, of different InGaAs thickness and doping density, were grown by molecular beam epitaxy and were designed to assess the importance of strain relaxation/dislocations in barrier formation. In the heavily dislocated situation (thicker InGaAs film) the obtained barrier most likely resulted from Fermi level pinning. However, this was obviously not the case for both the pseudomorphic and lightly dislocated situations (thinner InGaAs films). It was also noted that the measured barriers varied more widely over space in the lightly dislocated layer than that in the pseudomorphic case and that the simultaneously obtained topographic and BEEM images points to a better interface in the pseudomorphic layer than that in the partially relaxed situation.

ISSN:0734-211X    

DOI:10.1116/1.587878

出版商:American Vacuum Society    

年代:1995

数据来源: AIP

摘要:

We present reflection high energy electron diffraction, secondary ion mass spectroscopy, scanning tunneling microscopy and x‐ray photoelectron spectroscopy studies of the abruptness of InAs–GaSb interfaces. We find that the interface abruptness depends on growth order: InAs grown on GaSb is extended, while GaSb grown on InAs is more abrupt. We first present observations of the interfacial asymmetry, including measurements of band alignments as a function of growth order. We then examine more detailed studies of the InAs–GaSb interface to determine the mechanisms causing the extended interface. Our results show that Sb incorporation into the InAs overlayer and As exchange for Sb in the GaSb underlayer are the most likely causes of the interfacial asymmetry.

ISSN:0734-211X    

DOI:10.1116/1.587879

出版商:American Vacuum Society    

年代:1995

数据来源: AIP

摘要:

Transmission electron microscopy is used to investigate the structure of the crystalline defects in degraded and undegraded ZnSe‐based films/GaAs heterostructures. In degraded areas, dark line defects (DLDs) originate from the preexisting or grown‐in defects (i.e., stacking faults and misfit dislocations) during device operation and make the laser diode fail. From our observation, the nucleation of DLDs is based on the initial emission of a mobile faulted defect from the preexisting grown‐in defects. Along the trace of the faulted defects, [100] dark line defects form. We also investigated the origins and mechanisms for the generation of stacking faults and misfit dislocations. The generation of stacking faults is strongly related to the doping concentration, substrate surface stoichiometry, and growth mode of the films. The vacancy contained Ga–Se interfacial layers are thought to be sources for the generation of Frank‐type stacking faults. In addition, Shockley partial dislocations form due to island coalescence of the ZnSxSe1−x/ZnSe epilayers on GaAs substrates.Insituelectron beam heating studies were carried out to realize the mechanism of strain relaxation and formation of 60°‐type misfit dislocations in the II–VI/GaAs interface from the dissociation of Frank‐type partial dislocations. The generation of screw‐type interfacial dislocations was also observed. This takes place by gliding the threading segments of the Shockley partial dislocations toward the ZnSxSe1−x/GaAs interface on {111}‐type planes.

ISSN:0734-211X    

DOI:10.1116/1.587880

出版商:American Vacuum Society    

年代:1995

数据来源: AIP

摘要:

We have used low energy cathodo‐ and photoluminescence spectroscopies to characterize the formation of charge states deep within the ZnSe band gap and localized near the ZnSe/GaAs interface. These deep levels are a common feature of the ZnSe/GaAs heterostructure and depend sensitively on growth conditions and post‐growth annealing. Here we reportinsitumeasurements of deep level formation near the ZnSe/GaAs interface during post‐growth annealing and their dramatic dependence on the initial epitaxical growth conditions. We used 1–2 keV electron beam energies in tandem with HeCd and HeNe excitation to obtain emission spectra characteristic of the free ZnSe surface, the ZnSe bulk, and the ZnSe/GaAs(100) interface. Annealing in ultrahigh vacuum in the 300–500 °C range produces a new emission feature at 1.9–2.0 eV whose intensity increases exponentially with temperature, exhibiting an activation energy of 1.10±0.08 eV. This feature appears enhanced (suppressed) in structures fabricated in Se(Zn)‐rich growth, and its intensity may therefore be related to the Zn vacancy concentration or the Ga indiffusion. These results emphasize the importance of local composition in the formation of interface deep level defects and their subsequent effects on luminescence efficiency and heterojunction stability.

ISSN:0734-211X    

DOI:10.1116/1.587881

出版商:American Vacuum Society    

年代:1995

数据来源: AIP

摘要:

ZnTe/CdSe superlattices are II–VI heterostructures with a lattice mismatch less than 0.3%, so strain effects are expected to be negligible for ideal structures except for the two interface layers that consist of ZnSe and CdTe. Recent experiments indicated the exchange of whole atomic layers across the interfaces in ZnTe/CdSe superlattices despite the fact that more atomic layers near the interface would become strained. To understand such exchanges, first‐principles pseudopotential studies of several different interface structures were performed. Total energies, band offsets, and band structures of superlattices with different atomic rearrangements at interfaces are studied and compared. Our results indicate that the exchange of cation atomic layers across an anion atomic layer in these superlattices is energetically favorable.

ISSN:0734-211X    

DOI:10.1116/1.587882

出版商:American Vacuum Society    

年代:1995

数据来源: AIP