Oxides of GaAs grown using various oxidation processes were analyzed with X-ray photoelectron spectroscopy (XPS). Oxides investigated were the native (naturally grown or exposed to air), the chemical (grown in boiling deionized water) and the thermal (at 350°C and 500°C in dry oxygen) ones. With the use of a spectral deconvolution technique, all types of suboxides of both As and Ga including elemental arsenic were observed in addition to well-known As2O3 and Ga2O3. Elemental arsenic is considered to be one of the oxidized forms of GaAs. As2O5 was observed in the thermal oxides. In the chemical oxide and the native oxide grown in short exposure to air, elemental arsenic is the main component of oxide, while As2O3 is the dominant species in more highly oxidized films such as the thermal oxide. XPS data suggest that oxidation of As bonded in GaAs proceeds as GaAs→elemental As (As0)→As2O (As1+)→AsO (As2+)→As2O3 (As3+)→As2O5 (As5+). Oxidation of Ga bonded in GaAs advances as GaAs→Ga2O (Ga1+)→GaO (Ga2+)→Ga2O3 (Ga3+). Angle-resolved XPS measurements and semiquantitative analyses of these data were performed and an effective thickness of each oxide was also derived with simplified assumptions. The native and the chemical oxides were nearly stoichiometric. However, the thermal oxide was substantially Ga-rich due to desorption and evaporation of As2O3 from the surface during oxidation.

Using energy dispersive diffraction techniques employing synchrotron radiation we measured the effect of pressure on the lattice parameters of La2Cu04. During compression the c/a ratio remains constant up to about 150 kbar showing that the effect of pressure is isotropic, in contrast with the magnetic properties which are highly twodimensional. The results agree well with the predictions of band-structure calculations, suggesting that the bonding is governed by the La and 0 atoms and not by the Cu-02 layers.

A density distribution of the interface states in GaAs Schottky barrier was derived for the first time from observed nonideal I-V characteristics of a GaAs Schottky barrier with an oxidized interface. With increasing forward bias voltage, the ideality factor increases and then decreases after passing a maximum. Fermi level of the interface states shifts with the applied bias in the interfacial layer model adopted for the analysis. The obtained energy level of the interface states is in agreement with a previously reported value. However, the absolute magnitude of the state density is quite small compared with that obtained from the weak dependence of the barrier height on metal work functions. Implications of this result are discussed.

No abstract

A new theory is developed for nonideal J-V characteristics of Schottky barriers with an interfacial layer. This theory is based on the model that nonideal characteristics are due to changes of population in the interface states under applied bias and accompanying changes of the barrier height. The population in the interface states is expressed by the Fermi level, which can be determined by analyzing experimental results. The J-V characteristics are obtained from the flow of carriers into and out of the interface. Tunneling through the interfacial layer constitutes the bottleneck for the carrier flow. Under forward bias, the carrier concentration ns at the interface is proved to be in thermal equilibrium with the bulk. Under reverse bias, ns is in local thermal equilibrium with the interface states. This theory is applied to an undoped a-Si:H Schottky barrier without introducing any ambiguous quantities. The experimental ideality factor, its dependence on temperature and voltage, and current density are quantitatively explained. By analyzing experimental results, the following behaviors are disclosed. The Fermi level of the interface states is significantly lower than the bulk Fermi level at low forward bias, but it approaches the bulk Fermi level with increasing forward-bias voltage. As for the reverse characteristics, the decrease of the barrier height is proportional to √V in the present sample for applied voltage V. For electrons in the interface states, the probability of tunnel transition to the metal is small compared with that of communication with the conduction band.