Shu Chang

Low‐temperature formation of metal/molecular‐beam epitaxy‐GaAs(100) interfaces: Approaching ideal chemical and electronic limits

We report soft x‐ray photoemission studies of metal/molecular‐beam epitaxy (MBE)‐GaAs(100) interfaces formed at low temperature. Our results indicate that rectifying barrier heights are proportional to the metal work function in accordance with Schottky’s original description of metal–semiconductor contacts. These results confirm the predictions of a self‐consistent model of metal–semiconductor interfaces, and suggest that metal‐induced gap states and native defect mechanisms are not major factors in determining the Fermi level energy at ‘‘ideal’’ interfaces. We attribute deviations from the ideal Schottky limit behavior observed for interfaces formed at room temperature to metallization‐induced atomic relaxations (rather than electronic relaxations) occurring at metal–semiconductor contacts. We present a useful methodology for analyzing electronic properties at metal–semiconductor interfaces. The pronounced differences in barrier height formation between MBE vs melt‐grown GaAs can evidence the role of de...

Microscopic control of semiconductor surface oxidation

We examined the effect of ultrathin (0.1–10 A) chromium overlayers on the reactivity with oxygen of Si(111) and GaAs(110) cleavage surfaces. Synchrotron radiation photoemission shows that for Cr coverages below a critical threshold coverage the overlayer does not affect substantially the oxygen adsorption rate. For chromium coverage above threshold the overlayer sharply enhances the oxygen adsorption kinetics so that most semiconductor atoms in the surface and near surface region appear oxidized at activated oxygen exposures as low as 100 L. The critical threshold coverage corresponds to the onset of reactive interdiffusion at the Si(111)–Cr and GaAs(110)–Cr interfaces. Therefore we suggest that ultrathin Si–Cr and As–Cr reacted phases created at the surface act as activation layers for semiconductor oxidation.

Overlayer‐induced enhanced oxidation of GaAs surfaces

Thin metal overlayers deposited on GaAs(110) cleavage surfaces enhance the surface oxidation rate several orders of magnitude above the clean surface value. The magnitude of this effect depends on the chemistry of the overlayer, on the local morphology of the overlayer–semiconductor interface region, and on the nature of the gaseous reactants. We present a synchrotron radiation photoemission investigation of the GaAs surface interaction with oxygen in the presence of Ag, Au, and Cr overlayers, and of the GaAs–H2O reaction in the presence of Cr overlayers. We find that the promotion effect is maximum in the presence of Cr overlayers, and that the nature of the surface reaction products changes if oxygen or water is employed. In particular, in the case of oxygen, Ga and As‐oxide phases are found with high atomic oxidation states present. If water is employed, only Ga oxide/hydroxyl phases are found at the surface, on top of a Cr–As subsurface layer that remains largely unaffected by oxidation.

Inhomogeneous and wide range of barrier heights at metal/molecular‐beam epitaxy GaAs(100) interfaces observed with electrical measurements

In situ internal photoemission, current–voltage, and capacitance–voltage measurements of electronic barrier heights at metal/molecular‐beam epitaxy GaAs(100) interfaces indicate near‐ideal behavior and large barrier heights for Au contacts but nonideal characteristics and barrier height inhomogeneity for Al contacts. The observed Au barrier heights of 0.98–1.10 eV are comparable among the three techniques employed and relatively uniform across the interface. Measurements indicate that Al barrier heights are laterally inhomogeneous. Analysis of internal photoemission results for the Al interface using a model of two parallel conduction channels yields good agreements with the experimental results. Fitting the model to the experimental results for different diodes prepared on the same surface yields a low electronic barrier of 0.37±0.05 eV and a high barrier of 0.72±0.04 eV. The former is consistent with our previous soft x‐ray photoemission measurements and the latter agree with the value of reported Fermi...

Increased range of Fermi‐level stabilization energy at metal/melt‐grown GaAs(100) interfaces

We report a room temperature soft x‐ray photoemission spectroscopic study of electronic barrier formation and chemistry at Al, Ag, and Au contacts with horizontal Bridgman‐grown GaAs(100) surfaces, prepared by etching, heat‐cleaning in situ, and As‐capping. As reported previously, we observe a pronounced chemical reaction with formation of dissociated Ga for Al, a minimal interface reaction for Ag, and interdiffusion for Au. However, we find significant differences in the interface electronic barrier heights between the present metal/melt‐grown GaAs(100) contacts and those made on cleaved (110) surfaces as well as on melt‐grown (100) surfaces. We obtain Schottky barrier heights of 0.62, 0.85, and 1.03 eV respectively for Al, Ag, and Au/GaAs interfaces covering a range of 0.41eV—in contrast to the 0.2–0.3 eV range obtained earlier for both cleaved (110) and heat‐cleaned (100) surfaces as well as the 0.7 eV range observed for metal/molecular beam epitaxy GaAs(100) interfaces. This intermediate behavior unde...

Local stoichiometry and atomic interdiffusion during reactive metal/mercury–cadmium–telluride junction formation

Synchrotron-radiation photoemission studies of Ag, Ge, and Sm overlayers on Hg1-xCdxTe (110) surfaces are summarized. These metals exhibit widely different interface reactivity with Hg/sub 1-x/Cd/sub x/Te and yield a range of different interface morphologies. To assess the relative importance of the microscopic driving forces that determine the local composition at the interface and in the semiconductor near-surface region, the authors present a systematic comparison the data with calculated thermodynamic parameters such as the cation-metal heat of solution, the heat of alloying from Miedemas semi-empirical model, and the metal/telluride formation enthalpy.

Electronic structure of ternary semimagnetic semiconductors

The electronic structure of the ternary semiconductor alloys Hg1−xMnxSe, Cd1−xMnxSe, and Zn1−xMnxSe was examined through synchrotron radiation photoemission studies of single crystals cleaved in situ for 0≤x≤20%. Comparison with the parent binary compounds HgSe, CdSe, and ZnSe and resonant photoemission at the 3p–3d transition energy indicate that a Mn‐derived density of states feature appears 3.5±0.1 eV below the valence band maximum and exhibits elemental 3d character with no evidence of important hybridization effects. The constant binding energy of the Mn 3d states in all of the compounds explored forces a reevaluation of existing models of bonding in ternary magnetic semiconductors.

A photoemission survey of the electronic properties of ternary semimagnetic semiconductor alloys

We review the results of a number of recent synchrotron radiation photoemission studies of the electronic structure of concentrated ternary semimagnetic semiconducting alloys. The materials examined include Cd1−xMnxS, Cd1−xMnxSe, Cd1−xMnxTe , Zn1−xMnxSe, and the narrow‐gap Hg1−xMnxTe, Hg1−xMnxSe, and Hg1−xFexSe. Photon‐energy‐dependent photoemission methods are being used to probe the elemental and orbital contribution to the valence states. The new information is stimulating novel theoretical work on the electronic structure and lattice stability of these semiconductors.

Surface and interface states for GaAs(100) (1×1) and (4×2)‐c(8×2) reconstructions

Low energy cathodoluminescence spectroscopy measurements of GaAs(100) surfaces prepared by thermal desorption of an As passivation layer reveal deep level transitions localized at the clean surfaces and metallized interfaces. These surface and interface state features extend from 0.7 to 1.3 eV and exhibit subtle differences between the As‐rich (1×1) and Ga‐rich (4×2)‐c(8×2) reconstructions. Both Au deposition and subsequent annealing induce additional deep level emissions which appear relatively unchanged between these two reconstructions. In contrast, Al deposition introduces new features which depend significantly upon starting surface stoichiometry and reconstruction. We discuss the formation and energies of these states in relation to reported variations in Fermi level stabilization. We conclude that surface stoichiometry and atomic bond configuration are a significant factor in formation and evolution of electrically active, deep level metal‐GaAs(100) interface states.

Temperature‐dependent formation of interface states and Schottky barriers at metal/molecular‐beam epitaxy GaAs(100) junctions

We report temperature‐dependent cathodoluminescence spectroscopy (CLS) and internal photoemission spectroscopy (IPS) studies of metal/molecular‐beam epitaxy GaAs(100) interfaces. For Au, Cu, and Al deposited at 90 K on clean, ordered GaAs(100) surfaces under ultrahigh vacuum conditions, we observe low intensity, metal‐induced optical transitions centered at 0.85 eV. Upon room temperature (RT) annealing, this emission feature increases in intensity, corresponding to an increased density of midgap states. For Al, an additional excitation‐energy‐dependent emission is evident at 1.2 eV due to states even more localized near the interface. RT IPS measurements for metals deposited on GaAs at 90 K give well‐defined Schottky barriers. We obtain stable and reproducible Schottky barrier heights of 1.05 and 0.93 eV for Au/GaAs and Cu/GaAs, respectively. For Al/GaAs, we obtain a Schottky barrier height as low as 0.3 eV with significant, time‐dependent variations up to 0.41 eV. After 400 °C annealing, the Schottky bar...