摘要:

An extensive theoretical study has been made on the factors that influence the formation of native defects and extrinsic impurity incorporation in semiconductors. A complete set of reaction equations within equilibrium statistical mechanics is used to predict defect concentrations of intrinsic and extrinsic defects. The competing reactions involving host atoms include vacancies, antisites, and interstitial defects and the extrinsic impurity reactions include anion and cation site substitutional and interstitial site impurities. The extrinsic and intrinsic reactions form a coupled system. Examples are given to illustrate how various factors influence defect abundances, and such factors include temperature, stoichiometry, the host material, and the chemical potential of the electron/hole system. It is found that extrinsic impurities have a profound effect on native defect abundances. The defect formation energies used in the theory are obtained from a recently developed pseudoatomic‐orbital scheme using the local density approximation and pseudopotentials.

ISSN:0734-211X    

DOI:10.1116/1.584243

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

The advanced unified defect model (AUDM) for GaAs proposed in this paper can be looked upon as a refinement of the unified defect model (UDM) proposed in 1979 to explain Fermi level pinning on 3–5 compounds due to metals or nonmetals. The refinement lies in identifying the defect producing pinning at 0.75 and 0.5 eV above the valence band maximum as the AsGaantisite. Since the AsGaantisite is a double donor, a minority compensating acceptor is necessary. This is tentatively identified as the GaAsantisite. The concentration of As excess or deficiency due to processing or reactions at interfaces is particularly emphasized in this model. A wide range of experimental data is discussed in terms of this model and found to be in agreement with it. This includes the original data on which the UDM was based as well as more recent data including Fermi level pinning on the free‐GaAs(100) molecular‐beam epitaxy surface, Schottky barrier height for thick (∼1000 Å) Ga films on GaAs, and the LaB6Schottky barrier height on GaAs (including thermal annealing effects). Of particular importance is the ability of this model to explain the changes in Schottky barrier height for Al and Au on GaAs due to thermal annealing and to relate these changes to interfacial chemistry.

ISSN:0734-211X    

DOI:10.1116/1.584244

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

The amphoteric native defect model of the Schottky barrier formation is used to analyze the Fermi level pinning at metal/semiconductor interfaces for submonolayer metal coverages. It is assumed that the energy required for defect generation is released in the process of surface back‐relaxation. Model calculations for metal/GaAs interfaces show a weak dependence of the Fermi level pinning on the thickness of metal deposited at room temperature. This weak dependence indicates a strong dependence of the defect formation energy on the Fermi level, a unique feature of amphoteric native defects. This result is in very good agreement with experimental data. It is shown that a very distinct asymmetry in the Fermi level pinning onp‐ andn‐type GaAs observed at liquid nitrogen temperatures can be understood in terms of much different recombination rates for amphoteric native defects in those two types of materials. Also, it is demonstrated that the Fermi level stabilization energy, a central concept of the amphoteric defect system, plays a fundamental role in other phenomena in semiconductors such as semiconductor/semiconductor heterointerface intermixing and saturation of free carrier concentration.

ISSN:0734-211X    

DOI:10.1116/1.584246

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

We have used soft x‐ray photoemission spectroscopy measurements to demonstrate that metal–GaAs interfaces can exhibit relatively unpinned Fermi level (Ef) movements. For clean GaAs (100) surfaces obtained by molecular‐beam epitaxy (MBE) growth and thermal decapping of a protective As overlayer, the metals Au, Al, Cu, and In produce a 0.6–0.7‐eV range ofEfstabilization. For a given metal, this stabilization occurs at the same energies forn‐type andp‐type GaAs. Furthermore,Efmovements are metal‐dependent and can evolve over multimonolayer coverages. These results are examined with respect to pronounced differences in semiconductor quality between MBE versus melt‐grown GaAs and in the context of previous GaAsEfmeasurements. A self‐consistent analysis of the junction electrostatics accounts for the functional dependence of barrier height on metal work function. The results highlight the importance of bulk quality as well as interface specific phenomena in controlling the Schottky barrier formation at metal/III–V compound semiconductor interfaces.

ISSN:0734-211X    

DOI:10.1116/1.584247

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

During the formation of metal–semiconductor contacts two principally different types of electronic states are effective in determining the surface position of the Fermi level within the semiconductor band gap. These are, first, adatom‐related surface states of donor character and, second, the continuum of adsorbate‐induced interface states which result from the tailing of the metal wave functions into the virtual gap states of the semiconductor band structure. The first type of state is observed at submonolayer coverages either after depositions at low temperatures or even at room temperature when a cation exchange occurs. Each metal adatom contributes one of those surface donors and their energy levels are linearly correlated with the first ionization energies of the metal atoms. The second type of state exists at interfaces under metallic islands and continuous metallic films. The charge transferred across the interface by these tails of the metal wave functions is explained by the difference in electronegativities of metal and semiconductor in analogy to the concept of the partial ionic character of covalent bonds.

ISSN:0734-211X    

DOI:10.1116/1.584248

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

In order to assess the role of metallic screening at the metal–semiconductor interface, we have investigated the effects of Ag deposition on the band bending of GaAs(110) surfaces previously ‘‘pinned’’ with vanadium atoms. Their coverages ranged from 0.05 to 1.5 monolayers (ML), which positioned the surface Fermi level (EF)∼0.85–0.6 eV above the valence band maximum (VBM). For subsequent depositions of Ag additional changes inEFof up to 200 meV towards the VBM were observed on bothn‐ andp‐type GaAs. The onset of these changes, which occurs for mean Ag coverages of ∼0.1 ML, coincides with the appearance of metallic characteristics in the Ag clusters. These changes inEFcan be explained in terms of a new model in which the interface impurity levels in the band gap of the semiconductor become delocalized through interaction with states of the metal overlayer. A functional expression is developed which relates the Fermi level to the metal and semiconductor parameters, and to an effective charge in each delocalized level. These levels, actually broadened resonances of the impurity‐metal system, accommodate the charge densities required to equalizeEFthroughout the system. In analogy to a partially filled band,EFis determined by the average effective charge residing in each resonance. The resonances are nearly neutral and can accept or donate electronic charge as needed to compensate for the conductivity type of the semiconductor. Calculated changes ofEFbased on this model agree well with the experimental observations.

ISSN:0734-211X    

DOI:10.1116/1.584249

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

Experimental and theoretical determinations of the valence band offsets at heterojunctions between tetragonal semiconductors are reviewed. The physical mechanisms underlying the offset are illustrated on the basis of midgap level theories, in particular the dielectric midgap energy version (DME). Results obtained with the DME method for several nearly lattice‐matched heterojunctions are presented and critically compared with experimental data and the results of other theoretical evaluations.

ISSN:0734-211X    

DOI:10.1116/1.584250

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

A tight‐binding treatment of (100) heterojunctions with common anions is performed. A new zero‐dipole approximation is presented together with a first‐order correction that explicitly depends upon the static macroscopic dielectric constant. The difference between the zero‐charge and zero‐dipole approximations is discussed. These two approximations are then compared to a complete self‐consistent calculation and shown to provide band offsets within 0.1 eV of the exact value. The corrected zero‐dipole treatment gives a much better accuracy of order of 0.02 eV. Agreement with experiment is very good for the three lattice matched systems: AlAs–GaAs, CdTe–HgTe, and AlSb–GaSb.

ISSN:0734-211X    

DOI:10.1116/1.584251

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

The Ge/GaAs heterojunction valence band discontinuity (ΔEv) is observed to depend on crystallographic orientation. We explore theoretically the effect of interface geometry on ΔEv. Interface geometries with unphysical charge accumulation are excluded. Allowed geometries with the same minimum sums of independent bond energies occur with positive, negative, and zero interface dipole giving a statistical average of zero dipole. Interbond interactions (metallization) yield different energies for geometries with unequal dipoles. Metallization energies are calculated for the smallest interface cells needed to sample positive, negative, and zero dipoles. A statistical average of the interface cells at the Ge/GaAs growth temperature predicts a dipole shift for each crystallographic orientation. The relative ordering of the predicted polar interface ΔEv’s agrees with experiment [i.e., ΔEv(1̄1̄1̄)As>ΔEv(100)>ΔEv(111) Ga] while the range of values from the model calculation is about a factor of 2 smaller than is experimentally observed.

ISSN:0734-211X    

DOI:10.1116/1.584252

出版商:American Vacuum Society    

年代:1988

数据来源: AIP

摘要:

Simulations have been performed for dimers and diatomic molecules adsorbing on the (110) surface of GaAs. Al2usually remains together after adsorption, indicating a tendency toward cluster formation. In one run, this dimer broke up, but the energy was found to be substantially higher; i.e., the dissociated atoms are metastable with respect to the metallic cluster. On the other hand, O2tends to dissociate immediately upon adsorption, in ways that suggest the beginning of oxide formation. Molecular chemisorption of O2was observed in one run, but found to be metastable.

ISSN:0734-211X    

DOI:10.1116/1.584253

出版商:American Vacuum Society    

年代:1988

数据来源: AIP