B. Theys

Effects of intentionally introduced hydrogen on the electrical properties of ZnO layers grown by metalorganic chemical vapor deposition

Unintentionally doped ZnO layers grown epitaxially on a sapphire substrate have been exposed either to a hydrogen or deuterium plasma. Secondary ion mass spectroscopy measurements performed subsequently showed a rapid diffusion of hydrogen in these layers. Furthermore, the presence of hydrogen in the ZnO samples is found to be responsible for nearly a factor of 3 increase in the free electron concentration. This effect is attributed to the hydrogen passivation of compensating acceptor impurities present in the as-grown ZnO layers.

Hydrogen-acceptor interactions in diamond

Abstract Hydrogen-acceptor interactions are investigated in boron-doped diamond through deuterium diffusion experiments followed by SIMS measurements and through infrared absorption spectroscopy. From deuterium diffusion, we show that BD interactions can be properly demonstrated in low compensation B-doped homoepitaxial layers. However, the presence of defects in such layers strongly affects this interaction. The degree of passivation of boron acceptors by deuterium depends on the diffusion temperature. At 550°C or below, the B and D concentrations exactly match giving rise to a complete disappearance of the absorption bands related to the electronic transitions of neutral boron acceptors. Under thermal annealing above 500°C, (B,D) pairs dissociate and neutral boron acceptors recover. At deuterium diffusion temperatures of 700°C, the B passivation is absent.

Hydrogen in Monocrystalline CVD Boron Doped Diamond

The diffusion properties of hydrogen have been investigated in homoepitaxial layers of CVD boron doped diamond as a function of the diffusion temperature, the boron dopant concentration and the nature of the hydrogen source (rf or microwave plasma). Preliminary hydrogen effusion experiments on these hydrogenated samples are also reported. For both kinds of plasma we show that, below diffusion temperatures of 500 °C, the hydrogen diffusion is limited by trapping on boron acceptors. The onset of dissociation of B, H pairs occurs at 550 °C which implies a significantly higher stability of these pairs in diamond compared to silicon. However, a higher hydrogen diffusion activation energy is found in the microwave plasma exposed samples compared with the rf plasma exposed samples together with the absence of the subsurface hydrogen accumulation layer which is observed after rf plasma. Hydrogen effusion experiments show that in the rf plasma exposed samples the main effusion peak at 720 °C is attributed to the subsurface hydrogen accumulation layer. Another effusion peak at 850 °C is ascribed to the desorption of hydrogen adsorbed at the surface.

Improved performance of InSe‐based photoelectrochemical cells by means of a selective (photo)electrochemical etching

It is shown that the performance of photoelectrochemical cells based on the lamellar material InSe can be considerably improved by means of a selective (photo)electrochemical etching. Whereas the cleavage Van de Waals plane (⊥ to c axis) shows little improvement, the photcurrent in the ∥ face (parallel to the c axis) is doubled (30 mA cm−2 under AM1 illumination). For n‐type InSe a reverse bias (+1.5 V versus standard calomel electrode SCE) was employed during the photoetching, p‐InSe electrodes were electrochemically etched by applying a forward bias (+1.5 V). In both cases, surface holes carry out the selective corrosion of the semiconductor surface which is another manifestation for the asymmetry played by holes and electrons on semiconductor surfaces. It is hoped that this finding will pave the way for the construction of high‐efficiency solar cells based on a thin film made of lamellar materials.

Hydrogen diffusion and stability in polycrystalline CVD undoped diamond

Abstract Diffusion profiles and effusion experiments performed on post-hydrogenated (deuterated) CVD diamond layers (grain size 2 and 0.2 μm) are reported in order to study the configurations and stability of hydrogen bonding in polycrystalline undoped CVD diamond. Deuterium is used as a tracer to improve the hydrogen detection limit. The diamond layers are first annealed at 1200°C in order to out-diffuse hydrogen present in the as-grown sample. Then the samples are exposed either to a radiofrequency plasma or a microwave plasma and the deuterium diffusion profiles are analyzed by secondary ion mass spectrometry. For r.f. and microwave plasma, the diffusion profiles are explained in term of trapping on plasma-induced defects near the surface and/or on inter- and intra-granular defects. The mean free paths of deuterium and capture radius of traps are calculated by fitting the deuterium diffusion profiles and depend on the grain sizes. Some CVD diamond layers are deposited using a gas mixture (CH 4 +D 2 ) and a deuterium concentration of 3×10 19 cm −3 , originating from the vector gas, is found in these as-grown samples. The stabilities of deuterium bonding in as-grown and post-deuterated samples are compared.

Hydrogenation of InAs on GaAs heterostructures

Atomic hydrogen has been introduced from a plasma source into InAs layers grown by molecular beam epitaxy on GaAs substrates. It is shown that hydrogen diffuses very fast into this material. The presence of hydrogen modifies the electronic transport properties, the near‐band‐edge luminescence spectra, and the far‐infrared reflectivity spectra. The most striking effect is that, unlike other III‐V compounds, the free‐carrier density increases by one order of magnitude after hydrogenation. These phenomena are reversible and thermal annealing restores the original properties of the samples. Finally, models are proposed to explain the experimental results.

Characterization of hydrogenated GaAs/AlGaAs multiple quantum well structures

We report on the photoluminescence properties and the hydrogen depth distributions of plasma treated GaAs/AlGaAs multiple quantum well (MQW) structures. Specimens grown by molecular beam epitaxy were exposed to a deuterium plasma under different temperature‐time conditions. Photoluminescence measurements were made at 4.2 K, using low and high excitation powers, on the hydrogenated samples and on untreated partners. A decrease in the linewidth of the free exciton and an increase in the peak intensity were observed in specimens following plasma treatment. In general, each of the hydrogenated MQW specimens displayed an increase in luminescence efficiency and a diminution of impurity‐related peaks after hydrogenation. Secondary ion mass spectrometry measurements yielded depth distributions for 2H and Al atoms. In samples having the best luminescence, the 2H was nearly constant throughout the MQW region, at about 1018 cm−3.

Optical waveguides formed by deuterium passivation of acceptors in Si doped p‐type GaAs epilayers

We report on the use of plasma hydrogenation of Si doped, p‐type GaAs crystalline samples to form infrared waveguides through acceptor passivation. Epilayers grown by liquid phase epitaxy were exposed to a deuterium plasma for ninety minutes at three different temperatures. Secondary‐ion mass spectrometry (SIMS) analysis indicated that the deuterium concentrations in the crystals after plasma exposure were nearly equal to the acceptor level and extended to depths between 2.0 and 4.0 μm. Reflectivity measurements showed that the epilayers had passivated regions whose thicknesses corresponded to those determined by SIMS analysis. Laser coupling experiments at 1.15 μm showed optical waveguiding in each sample and lowest propagation losses were on the order of 35 dB/cm. At a wavelength of 1.523 μm, only the sample processed at the highest temperature exhibited laser guiding and losses were considerably higher.

Evidence of hydrogen–boron interactions in diamond from deuterium diffusion and infrared spectroscopy experiments

Abstract We report on the first experimental evidence of hydrogen–boron interactions in boron-doped diamond. Deuterium diffusion studies in homoepitaxial B-doped diamond films reveal that hydrogen diffusion is limited by the B concentration and is characterized by a low effective diffusion activation energy. Infrared spectroscopy experiments show that boron acceptor electronic transitions disappear under hydrogenation. These results are consistent with hydrogen ionization and diffusion of fairly mobile H+ which form pairs with B−.

Channel optical waveguides formed by deuterium passivation in GaAs and InP

Channel optical waveguides have been formed on both GaAs and InP substrates utilizing deuterium passivation of the surface to provide planar confinement and etching to provide lateral confinement. Design criteria were established for obtaining single mode channel waveguides for the present case of small index changes and thick surface layers associated with deuterium passivation. Planar and channel waveguide operations were demonstrated and channel waveguide propagation losses have been measured. For GaAs channel waveguides, optical loss was measured as a function of channel waveguide width, ranging from 3 to 9 μm, with a minimum loss found for a width of 6.0 μm. Channel waveguide losses as low as 12.7 dB/cm for GaAs and 6.0 dB/cm for InP have been measured at λ=1.3 μm. For InP this loss value is close to the limiting value imposed by free carrier absorption in the semiconductor region below the passivated region. Since the waveguide loss due to free carriers can be reduced by increasing waveguide confine...