David Krix

Inductively coupled plasma reactive ion etching of bulk ZnO single crystal and molecular beam epitaxy grown ZnO films

Dry etching processes for bulk-single crystal zinc oxide (ZnO) and molecular beam epitaxy (MBE) grown ZnO have been investigated using inductively coupled plasma (ICP) of CH4 and SiCl4 based plasma chemistry. The CH4-based chemistry showed a higher etch rate than the SiCl4 based chemistry, presumably due to the formation of highly volatile metal organic zinc compound. The influence of base pressure, radio frequency table power, and ICP power on etch rate was studied. Auger electron spectroscopy has been employed to examine the surface stoichiometry of etched ZnO using both plasma chemistries. Furthermore, with optimized process parameters, the effect of plasma etching on the optical properties of MBE grown ZnO film is studied. An enhancement of the band edge luminescence along with almost complete suppression of defect level luminescence in hydrogen-containing plasma treated ZnO film has been observed.

Varying the Schottky barrier of thin film Mg∕H:p-Si(111) contacts: Properties and applications

Different types of Mg∕p-Si diodes were prepared by thermal evaporation of thin Mg layers on H-passivated Si(111) surfaces at low temperatures followed by various annealing procedures. After deposition at 190K, the current-voltage characteristics of the diodes are typical for interfaces with lateral inhomogeneity and high ideality factors. The homogeneous barrier height is measured as 0.8eV. This value is much larger than expected from the metal-induced gap states and electronegativity concept. The deviation is explained by interface dipoles due to a monatomic hydrogen layer between Mg and Si. Hydrogen is removed by annealing the diodes to room temperature. This procedure leads to the formation of an intermediate Mg silicide layer and reduces the homogeneous barrier height to 0.69eV. The annealed diodes show excellent current-voltage properties. Applications of the diodes for chemical sensing such as chemicurrents and for internal photoemission are discussed.

Ultrathin K∕p-Si(001) Schottky diodes as detectors of chemically generated hot charge carriers

Ultrathin potassium layers were deposited on hydrogen passivated Si(001)−1×1 surfaces at a temperature of 120K in the thickness range from submonolayers up to 3nm. They were investigated with Auger spectroscopy, work function, and current-voltage measurements. After the formation of a wetting layer, potassium deposition leads to island growth. The surface hydrogen atoms are removed by the adsorption process. By attaching an electrical contact to the thin film, the current-voltage characteristics of the Schottky diodes could be determined. The analysis yields a homogeneous Schottky barrier height of 0.55eV for K∕p-Si(001) diodes in agreement with the metal induced gap state model. Exposing the diodes to molecular oxygen generates a strong chemicurrent signal which first increases with exposure, passes a maximum, and drops exponentially. The chemicurrent transient is attributed to a nucleation and growth formation of oxide islands and gives strong evidence for the existence of precurser states prior to oxid...

Chemicurrent measurements using alkali metal covered Pd /p- Si (001) Schottky diodes

Thin-film Pd∕p-Si(001) Schottky diodes were fabricated and electrically characterized. Alkali metal films (K and Na) were deposited on these devices. The work function change during the deposition was studied using a Kelvin probe. The K films were subsequently oxidized at low oxygen pressures and the formation of an oxide layer was monitored by measuring the work function. Nonadiabatic contributions to the reaction leading to internal exoemission currents were observed. The chemicurrents during K oxidation show two distinct maxima that are attributed to the nucleation and growth of the respective oxide phases.

Chemical interaction of H and D atoms with Ag/H:p-Si"111… thin film diodes

During exothermic reactions of atomic hydrogen or deuterium on a silver surface hot charge carriers are produced which have been observed by using Ag∕p-Si(111) Schottky diodes. Thin film devices provide a means to bring a charge detector as close to the reaction site as the mean free path of the charge carriers. In the case of a p-doped substrate the Schottky barrier works as a high-pass energy filter for hot holes. The authors have therefore produced large area Schottky diodes with film thicknesses of up to 30nm varying the thickness to rule out any influence of this device parameter. Those diodes were then exposed to beams of hydrogen atoms and deuterium atoms produced in a hot capillary source. Gas exposures do not affect the Schottky barrier height significantly. While exposing the samples to defined atom fluxes, the closed-loop current was monitored in real time. It shows that the current is proportional to the flux of atoms impinging on the surface. The authors have found hydrogen to generate 3.7 ti...

Internal detection of surface plasmon coupled chemiluminescence during chlorination of potassium thin films

The interaction of chlorine with potassium surfaces is a prototype reaction with a strong non-adiabatic energy transfer leading to exoemission and chemiluminescence. Thin film K/Ag/p-Si(111) Schottky diodes with 8 nm potassium on a 5-200 nm thick Ag layer are used as 2π-photodetectors for the chemiluminescence during chlorination of the K film at 110 K. The observed photocurrent shows a sharp maximum for small exposures and decreases gradually with the increasing chloride layer. The time dependence can be explained by the reaction kinetics, which is governed initially by second-order adsorption processes followed by an electric field-assisted diffusion. The detector current corresponds to a yield of a few percent of elementary charge per reacting chlorine molecule and is orders of magnitude larger than for external detection. The photoyield can be enhanced by increasing the Ag film thickness. For Ag films of 30 and 50 nm, the yield exhibits a maximum indicating surface plasmon coupled chemiluminescence. Surface plasmon polaritons in the Ag layer are excited by the reaction and decay radiatively into Si leading to the observed currents. A model calculation for the reverse process in attenuated total reflection is applied to explain the observed current yield maxima.

Low temperature diffusion of Li atoms into Si nanoparticles and surfaces

The diffusion of Li atoms deposited on hydrogen-passivated Si(001) surfaces, chemically oxidized Si(001) surfaces, Si nanoparticle films, and thick SiO2 layers is investigated with electron-beam induced Auger electron spectroscopy. The nanoparticles exhibit an average diameter of 24 nm. The Li metal film is evaporated at a sample temperature below 120 K. The reappearance of the Si substrate Auger signal as a function of time and temperature can be measured to study the Li diffusion into the bulk material. Values for the diffusion barrier of 0.5 eV for H:Si(001) and 0.3 eV for the ox-Si(001) and Si nanoparticle films are obtained. The diffusion of the Li atoms results in the disruption of the crystalline Si surfaces observed with atomic force microscopy. Contrasting to that, the Si nanoparticle films show less disruption by Li diffusion due to filling of the porous films detected with cross section electron microscopy. Silicon dioxide acts as a diffusion barrier for temperatures up to 300 K. However, the e...

Non-adiabatic processes in the charge transfer reaction of O2 molecules with potassium surfaces without dissociation.

Thin potassium films grown on Si(001) substrates are used to measure internal chemicurrents and the external emission of exoelectrons simultaneously during adsorption of molecular oxygen on K surfaces at 120 K. The experiments clarify the dynamics of electronic excitations at a simple metal with a narrow valence band. X-ray photoemission reveals that for exposures below 5 L almost exclusively peroxide K2O2 is formed, i.e., no dissociation of the molecule occurs during interaction. Still a significant chemicurrent and a delayed exoelectron emission are detected due to a rapid injection of unoccupied molecular levels below the Fermi level. Since the valence band width of potassium is approximately equal to the potassium work function (2.4 eV) the underlying mechanism of exoemission is an Auger relaxation whereas chemicurrents are detected after resonant charge transfer from the metal valence band into the injected level. The change of the chemicurrent and exoemission efficiencies with oxygen coverage can be deduced from the kinetics of the reaction and the recorded internal and external emission currents traces. It is shown that the non-adiabaticity of the reaction increases with coverage due to a reduction of the electronic density of states at the surface while the work function does not vary significantly. Therefore, the peroxide formation is one of the first reaction systems which exhibits varying non-adiabaticity and efficiencies during the reaction. Non-adiabatic calculations based on model Hamiltonians and density functional theory support the picture of chemicurrent generation and explain the rapid injection of hot hole states by an intramolecular motion, i.e., the expansion of the oxygen molecule on the timescale of a quarter of a vibrational period.