J. A. Bardwell

Sampling depth of total electron and fluorescence measurements in Si L- and K-edge absorption spectroscopy

High resolution Si L-edge and K-edge X-ray absorption near edge structure (XANES) spectra for SiO2 on Si substrates have been recorded using total electron yield (TEY) and fluorescence yield (FY) techniques. The sampling depths of TEY and FY for Si L-edge and Si K-edge, respectively, have been investigated in the energy range 95–120 eV and 1830–1900 eV. The maximum sampling depth for TEY is found to be ∼ 5 nm for the Si L-edge and ∼ 70 nm for the K-edge regions. The FY sampling depth at the L-edge is ∼ 70 nm whereas for the K-edge, the sampling depth is several hundred nm. Based on these data, and using a theoretical model, electron escape depths for the TEY measurements in both energy ranges have been deduced.

Semi-insulating C-doped GaN and high-mobility AlGaN/GaN heterostructures grown by ammonia molecular beam epitaxy

A method of growing semi-insulating GaN epilayers by ammonia molecular beam epitaxy through intentional doping with carbon is reported. Thick GaN layers of high resistivity are an important element in GaN-based heterostructure field-effect transistors. A methane ion source was used as the carbon dopant source. The cracking of the methane gas by the ion source was found to be the key to the effective incorporation of carbon. High-quality C-doped GaN layers with resistivities greater than 106 Ω cm have been grown with high reproducibility and reliability. AlGaN/GaN heterostructures grown on the C-doped semi-insulating GaN-based layers exhibited a high-mobility two-dimensional electron gas at the heterointerface, with room-temperature mobilities typically between 1000 and 1200 cm2/V s, and liquid-nitrogen-temperature mobilities up to 5660 cm2/V s. The carrier density was almost constant, with less than 3% change over the measured temperature range.

Properties of carbon-doped GaN

The properties of carbon-doped GaN epilayers grown by molecular-beam epitaxy have been studied by temperature-dependent resistivity, Hall-effect measurements, x-ray diffraction, and by photoluminescence spectroscopy. Carbon doping was found to render the GaN layers highly resistive (>108 Ω cm) and quench the band edge excitonic emissions. Yellow luminescence is still present in carbon-doped GaN layers. The highly resistive state is interpreted as being caused by direct compensation by the carbon acceptors and by the consequently enhanced potential barrier at the subgrain boundaries. Evidence of dislocations joining to form potential barriers along the subgrain boundaries was observed in photoassisted wet etching experiments on electrically conducting GaN layers. GaN films grown on insulating carbon-doped base layers are of excellent transport and optical properties.

Ultraviolet photoenhanced wet etching of GaN in K2S2O8 solution

The mechanism of the UV photoenhanced wet etching of GaN is determined. The UV photoenhanced wet etching does not require an electrical contact to be made to the sample, and nitrides deposited on insulating substrates (such as sapphire) can be etched, unlike photoelectrochemical (PEC) wet etching. The present technique relies on adding an appropriate oxidizing agent, in this case, peroxydisulfate (S2O82−), to KOH solutions. In a similar mechanism to PEC wet etching, the regions of low defect density are preferentially etched, leaving regions of high electron recombination such as threading dislocations relatively intact. The threading dislocations may be physically broken off, either by stirring or by a postetch sonication of the sample in KOH solution. Smoothly etched surfaces can be obtained under the proper conditions. A noble metal mask acts in a catalytic manner, yielding etch rates approximately one order of magnitude greater than those observed using inert masks. The essential role of the free radi...

AlGaN/GaN Metal–Oxide–Semiconductor High-Electron Mobility Transistors Using Oxide Insulator Grown by Photoelectrochemical Oxidation Method

A photoelectrochemical oxidation method was used to directly grow oxide layer on AlGaN surface. The annealed oxide layer exhibited beta-Ga₂O₃ and alpha-Al₂O₃ crystalline phases. Using a photoassisted capacitance-voltage method, a low average interface-state density of 5.1 times 10¹¹ cm^-2. eV^-1 was estimated. The directly grown oxide layer was used as gate insulator for AlGaN/GaN MOS high-electron mobility transistors (MOS-HEMTs). The threshold voltage of MOS-HEMT devices is -5 V. The gate leakage currents are 50 and 2 pA at forward gate bias of VGS = 10 V and reverse gate bias of V_GS = -10 V, respectively. The maximum value of g_m is 50 mS/mm of VGs biased at -2.09 V.

Thin anodic oxides formed on GaAs in aqueous solutions

Thin (0–250 A) anodic oxides were formed on p‐GaAs (100) in aqueous solutions (borate buffer, pH 8.4 and 0.3 M NH4H2PO4, pH 4.4). The thickness, composition, and electrochemical behavior were characterized by x‐ray photoelectron spectroscopy, secondary ion mass spectrometry, ellipsometry, and electrochemical impedance spectroscopy. In borate buffer, oxide growth results in a two‐layer structure with an outer As2O3‐rich layer and ion transfer is mainly diffusion controlled. From the potential dependence of the film thickness a growth rate of 25 A/V was determined. Due to the formation of a Ga‐phosphate precursor layer in NH4H2PO4, films grow under field control which results in a time dependence of oxide thickness and composition. Under pseudosteady state conditions a growth rate of 28 A/V was obtained. Films formed in NH4H2PO4 have, except for the outer phosphate layer, a fairly uniform composition in depth with a significant enrichment of Ga2O3, and show a 40 times higher specific charge transfer resista...

The growth and stability of passive films

This paper will consider the growth and breakdown of passive oxide films on metals and alloys. Emphasis is placed on the use of surface-analytical techniques, particularly secondary ion mass spectrometry (SIMS) to characterize oxides formed on Fe and Fe-Cr alloys in18O-containing solutions, and to determine the oxides stability to subsequent air exposure. It is found that the stability of passive films towards air exposure decreases with increasing Cr content of the alloy. Films formed on Fe in the passive potential range in borate buffer solution are stable towardsex situ exposure, whereas films formed on Fe-26Cr alloys are not stable to air exposure after passivation at any potential in the passive region. This result has implications for examining the thickness and composition of passive films on high-Cr alloys usingex situ techniques. Passive films play a critical role in the initiation of pitting and the processes which influence film breakdown on Fe in Cl−-containing solutions are discussed. The results indicate that pitting is associated with a critical stage in the development of passive oxide films.

In Situ XANES Detection of Cr(VI) in the Passive Film on Fe‐26Cr

The passive film on sputter deposited thin film Fe-26Cr electrodes has been examined using the technique of in situ X-ray absorption near edge spectroscopy (XANES). During the X-ray spectroscopic measurements, the sample was maintained under electrochemical control in the pH 8.4 borate buffer electrolyte. The appearance of the distinctive Cr(VI) XANES pre-edge peak can be correlated with the transpassive wave in the cyclic voltammogram, and its disappearance with the corresponding reduction wave. The formation and reduction of Cr(VI) was reversible, and only small amounts were detected in the film. Cr(VI) in the passive film is not indefinitely stable, and it is completely absent after long periods of air exposure.

ac Impedance spectroscopy of the anodic film on zirconium in neutral solution

The frequency dependence of the impedance of the anodic film on Zr has been determined using ac impedance spectroscopy. This technique has been used to provide an in situ monitor of the thickness and resistance of the film during formation. The data suggest that the film can be modelled adequately by a parallel RC combination, with an additional series resistance corresponding to the solution resistance. A better fit was obtained using a constant phase element circuit, with a phase angle varying from 84° for thin films to 90° for thicker films. If φ = 84°, the thickness measured by a standard capacitance bridge would be expected to vary by a factor of 1.66 over a frequency range of 0.5–1000 Hz. The inverse of the capacitance (from fits to the parallel RC circuit) was observed to increase linearly with anodic charge for all solutions and charging rates used in this work. The efficiency of film formation calculated from this data was < 100% for charging rates of 0.05 and 0.5 mA cm−2, while it was close to 100% for a charging rate of 5 mA cm−2, in borate and phosphate solutions. The efficiency exceeded 100% in sulphate solution at the high charging rate, presumably due to incorporation of sulphate ions from the electrolyte. The parallel resistence of the anodic film on Zr increased with open circuit exposure time, while the capacitance was unchanged, suggesting that the film is becoming more defect-free, but not thickening during the open circuit exposure. The air-formed film on Zr after the HF etch was about 5 nm in thickness.

In Situ XANES Study of Galvanostatic Reduction of the Passive Film on Iron

In situ XANES (X-ray absorption near-edge structure) was used to study galvanostatic reduction of the passive film on iron. In a borate buffer, the film appears to be removed in a layer-by-layer fashion by reductive dissolution during the first potential arrest. A burst of dissolution takes place at the end of the arrest for slow reduction rates. During the course of reduction, the remaining film shows a composition change from ferric oxide toward Fe{sub 3}O{sub 4}. This may indicate that the film has an inner layer of Fe{sub 3}O{sub 4}. Galvanostatic reduction of the passive film grown on iron in 0.1M KOH leads to complete reduction of the passive film to ferrous oxide or hydroxide without any detectable dissolution.