GaAs Oxide Interface Research Articles

Interband resonances in the optical second-harmonic response of the (001) GaAso–xide interface

The optical second-harmonic (SH) signal generated at the GaAs–oxide interface on N+,P+, and semi-insulating GaAs (001) surfaces has been measured for SH photon energies between 2.7 and 3.5 eV. The magnitude and the phase of the interfacial SH response reveal the presence of two strong resonances at SH photon energies corresponding to the E1 and E1+Δ1 interband transitions at 2.9 and 3.2 eV. These resonances dominate the N+ and semi-insulating GaAs spectra but are absent for P+–GaAs. Evidence suggests that the resonances are related to the structure of the GaAs–oxide interface.

Real time in situ monitoring of surfaces during glow discharge processing: NH3 and H2 plasma passivation of GaAs

Numerous device applications of GaAs are hampered by poor electronic properties of GaAs surfaces and interfaces. Room temperature NH3 and H2 downstream plasma passivation of native oxide contaminated GaAs surfaces is investigated using attenuated-total-reflection (ATR) Fourier-transform-infrared spectroscopy (FTIR) and photoluminescence (PL). Using ATR FTIR concentrations of –As–O, –As–H, H–OH, and C–H bonds are monitored in situ and in real time during exposure of the GaAs surface to H (D) atoms from a microwave discharge through NH3 (ND3) and H2 (D2). Photoluminescence intensity from the GaAs is monitored simultaneously with the FTIR spectra and used as a measure of surface state reduction. At room temperature, H atoms produced from the discharge remove –As–O and C–H contaminants but not Ga2O3. The appearance of As–H bonds and corresponding increase in PL are delayed significantly from plasma initiation, an increase in –O–H concentration, and removal of –As–O bonds. Because the As antisite defects are concentrated at the GaAs–oxide interface, the rates of As–H formation and PL enhancement may be limited by diffusion of H through the oxide and water layer which forms on the surface. We find that the concentration of physisorbed H2O on the GaAs surface increases throughout passivation. There are two sources of water detected on the surface: (i) reduction of As–O bonds and (ii) reaction of H with the quartz reactor walls. Ga2O3 grows on the surface via oxidation of GaAs by the physisorbed water. We surmise that higher H concentration in NH3 plasmas results in faster water accumulation on the surface and trapping of the As–H species subsurface. In H2 plasma, water accumulation on the surface is slower and trapping of As–H is not observed.

Real-time, in situ monitoring of surface reactions during plasma passivation of GaAs

Real-time, in situ observations of surface chemistry during the remote plasma passivation of GaAs is reported herein. Using attenuated total reflection Fourier transform infrared spectroscopy, the relative concentrations of -As-O, -As-H, -H2O, and -CH2 bonds are measured as a function of exposure to the effluent from a microwave discharge through NH3, ND3, H2, and D2. The photoluminescence intensity (PL) from the GaAs substrate is monitored simultaneously and used qualitatively to estimate the extent of surface state reduction. It was found that, while the -CHx(x = 2,3) and -As-O concentrations are reduced rapidly, the rates at which the -As-H concentration and the PL intensity increase are relatively slow. The concentration of -H2O on the GaAs surface increases throughout the process as surface arsenic oxides and the silica reactor walls are reduced by atomic hydrogen. These observations suggest that removal of elemental As by reaction with H at the GaAs–oxide interface limits the passivation rate.

Electric-Field-Enhanced Emission from a Discrete Energy Level at the GaAs–Oxide Interface

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Observation of Fermi level pinning at the GaAs-plasma-oxide interface

Small-signal charge transient spectroscopy (CTS) measurements of GaAs MOS diodes with plasma oxide insulating layers point to two dominant trapping levels responsible for the steady-state Fermi level pinning at the GaAs-oxide interface. The Ec-0.70 eV discrete surface trap is identified with the defect level observed in fundamental studies of III-V surfaces; this defect is due to missing As. The second level, positioned at Ec-0.67 eV' has not been reported previously. It is argued that the 0.67 eV defect is characteristic of the transition layer between the moderately doped (ND approximately 1015-1016 cm-3) GaAs substrate and the oxide.

In/GaAs reaction: Effect of an intervening oxide layer

The effects of a thin (∼1.3 nm) intervening native GaAs oxide interface layer on the In/GaAs reaction have been investigated by comparing the reactions of In on GaAs substrates with the native oxide present or desorbed. Transmission electron microscopy of cross-sectional samples reveals that the thin native oxide layer, when present, disrupts the In/GaAs orientation relationship in as-deposited samples and prevents an extensive reaction between In and GaAs at 350 °C. These data also show that the In/GaAs reaction at 350 °C proceeds by dissolution of the GaAs substrate into the molten In followed by the subsequent nucleation and growth of epitaxial In1−xGaxAs particles with x≤0.2 or x≥0.8 only, indicating immiscibility. The In1−xGaxAs/GaAs interfaces are found to be structurally and compositionally abrupt to within ∼3 nm. The effects of the interfacial native oxide on the In/GaAs reaction and the suggested presence of an InAs-GaAs miscibility gap have implications for the fabrication of In/GaAs ohmic contacts by thermal reaction.

Rutherford backscattering analysis of gaAs-oxide interface

We have studied the behaviour of a gallium arsenide-anodic oxide heterostructure annealed under nitrogen with increasing temperatures. The As to Ga ratio evolution is measured using the technique of heavy ion Rutherford backscattering. These results are compared to photoluminescence measurements, which show that an annealing temperature of 550°C leads to the highest As superstoichiometry. Furthermore, we have investigated oxides created at higher temperatures.

Experimental Investigation of GaAs Surface Oxidation

ABSTRACTExperimental observations of the surface oxide chemistry of GaAs are reported for various commonly used chemical surface preparations. Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and ellipsometry were employed to obtain information regarding the stoichiometry, depth distribution, and oxide growth kinetics of thin surface oxides. Previous observations of the segregation in depth of Ga and As oxides are corroborated. Arsenic oxides tend to be found near the surface while Ga2O3 is found near the GaAs-oxide interface. The presence of elemental As was frequently detected at this interface, as well. Surfaces essentially free from oxide are shown to be produced by certain chemical treatments, and the state of the surface in the solution is inferred. It is shown that the GaAs surface oxide stoichiometry can undergo several changes in a short time when exposed to water and air. In addition, the characterization of the oxidation of GaAs by ozone in the presence of intense ultraviolet illumination is reported. The oxide primarily consists of Ga2O3 and exhibits an interesting growth kinetics: the thickness of the oxide proceeds at first linearly, then logarithmically, then parabolically. This behavior is explained in terms of various mechanisms which are dominant at different thicknesses of oxide.

Theory of GaAsoxide interface states

The discrete GaAsoxide interface states observed by Lagowski et al. can be explained by defects in the GaAs surface: The deep levels are in agreement with our predictions for surface antisite defects, and the shallow levels (observed for thick oxides) agree with our predictions for surface oxygen impurities on anion and cation sites.

The electronic structure of the GaAs-oxide interface

The electronic structure of the GaAs-oxide interface

Overcompensated surface layer in n-GaAs due to anodic oxidation

The effect of defect states at a GaAs–oxide interface on the electronic properties of n-type MOS structures was investigated. It is shown that these 0.65 eV discrete energy interface states are of the same nature as the defect levels formed near the surface due to the deposition of foreign atoms in a vacuum. The spatial distribution of these states in a region extending about 2000 Å below the GaAs surface is responsible for the anomalously low value fo the high frequency accumulation capacitance in n-type GaAs MOS structures, as well as for the long characteristic filling time of these states observed in DLTS measurements.

Ion beam characterization of the GaAs–GaAs oxide interface for plasma and anodic oxides

Ion beam channeling has been used to measure the effects of oxide growth and annealing on the substrate at the GaAs–GaAs oxide interface. Plasma and anodic oxides have been grown on 〈100〉 GaAs substrates, and the oxides were removed with an etch in hydrofluoric acid. The substrates were then examined with a beam of 1.5 MeV 3He particles channeled down the 〈100〉 axis. The resultant Ga and As surface peaks can be resolved with Rutherford backscattering and are a measure of the Ga and As at the substrate–oxide interface. Anodic oxides grown by the constant-current method (50 μA) show no change from virgin material at the interface; however, annealing of the oxide prior to its removal results in As enhancement at the surface (?2.0 monolayers). Plasma oxides grown by direct immersion in an rf plasma show As enhancement at the interface without annealing. When grown in a fluorinated plasma, the data show an enhanced Ga surface peak consistent with a thicker interface and modified composition of the substrate. The differential measurements were made with a precision of ±0.2 monolayer.

Gap states at the GaAs-natural oxide interface

The relaxation of surface conductivity changes during the application and after the removal of external electric fields was investigated for n-GaAs surfaces covered with natural oxide. The relaxation processes in the heterojunction system of the oxidized surface are governed by various excitation mechanisms between the depletion layer in the bulk and fast surface states at the GaAs-oxide interface. Analysis of the experimental results yields new data on the energetic distribution of the fast surface states which show that there is a set of empty states in the upper half of the bulk energy gap and a set of occupied states near the middle of the bulk energy gap.

Study of GaAs–oxide interface by transient capacitance spectroscopy: Discrete energy interface states

Interface states and bulk GaAs energy levels were simultaneously investigated in GaAs MOS structures prepared by anodic oxidation. These two types of energy levels were successfully distinguished by carrying out a comparative analysis of deep level transient capacitance spectra of the MOS structures and MS structures prepared on the same samples of epitaxially grown GaAs (n?1015 cm−3). The identification and study of the interface states and bulk levels was also pursued by investigating the transient capacitance spectra as a function of the filling pulse magnitude. It was found that in the GaAs–anodic oxide interface there are states present with a discrete energy (0.65 eV below the conduction band) rather than with a continuous energy distribution as previously reported. The value of the capture cross section of the interface states was found to be 1014 to 1015 cm2, which is far more realistic than the extremely large values of 10−8 to 10−9 cm2 reported on the basis of conductance measurements.