P-type Surface Research Articles

Hydrogen sensitive gas sensor based on porous silicon/TiO 2−x structure

Porous silicon (PS) layer was formed by electrochemical anodization on a p-type Si surface. Thereafter, n-type TiO 2− x thin film was deposited onto the PS surface by electron-beam evaporation. Pt catalytic layer and Au electrical contacts for further measurements were deposited onto the PS/TiO 2− x structure by ion-beam sputtering. Current–voltage characteristic, sensitivity to different concentration of hydrogen and resistance change of obtained structures versus time were examined. Results of measurements have shown that the current–voltage characteristics of structures are similar to that of diode. High sensitivity to hydrogen of obtained structures was also detected. Note that all measurements were carried out at room temperature.

Fermi-level pinning and charge neutrality level in germanium

The Schottky barrier height in metal/Ge contacts shows weak dependence on the metal work function indicating strong Fermi-level pinning close to the Bardeen limit. The pinning factor S is about 0.05 and the charge neutrality level (CNL) is only about 0.09eV above the top of the valence band. Because of this, the Fermi level in Ge lies higher than CNL in most cases of interest so that unpassivated acceptorlike gap states at the interface are easily filled, building up a net negative fixed charge. This could prevent efficient inversion of a p-type Ge surface in a metal-oxide-semiconductor structure.

Structural and Electronic Properties of Oxygen-Adsorbed Diamond (100) Surface

The structural and electronic properties of diamond (100) surface, with hydrogen and oxygen terminations, have been studied using density functional theory. The energy calculations indicate that a hydroxyl is the most stable structure on the oxygenation surface. The surface properties with different terminations have been investigated by means of density of states, charges, bond population, and differential charge density maps. A novel explanation is proposed for the deterioration or disappearance of p-type surface conductivity of diamond with surface oxygenation from the view of electron transfer. The results not only support that hydrogen is a prerequisite factor for surface conductivity, but also explain very recent experimental observations.

Enhanced Light Output in Roughened GaN-Based Light-Emitting Diodes Using Electrodeless Photoelectrochemical Etching

We have demonstrated enhanced output power from roughened GaN-based light-emitting diodes (LEDs) by using electrodeless photoelectrochemical etching with a chopped source (ELPEC-CS etching). It was found that the 20-mA output power of the ELPEC-CS treated LED (with roughened surfaces on the top p-type and bottom n-type GaN surface as well as the mesa sidewall) was 1.41 and 2.57 times as high as those LEDs with a roughened p-type GaN surface and a conventional surface, respectively. The light output pattern of the ELPEC-CS treated LED was five times greater than the conventional LED at 0deg which was caused by the roughened GaN surface that improved the light extraction efficiency of the LED

Fabrication and Investigation of Nanostructures on Transition Metal Dichalcogenide Surfaces Using a Scanning Tunneling Microscope

Nanometer-scale holes have been fabricated on the surfaces of the semiconducting transition metal dichalcogenides (TMDCs) molybdenum ditelluride (MoTe2) and molybdenum disulfide (MoS2) by applying voltage pulses from the tip of a scanning tunneling microscope (STM) operating in ultrahigh vacuum (UHV). It was found that the tip geometry (tip shape and sharpness) influences the formation and structure of the atomic-scale nanostructures. Threshold voltage ranges for the surface modification of MoTe2 (3.0 +/- 0.3 V) and MoS2 (3.4 +/- 0.3 V) were determined. Negative sample voltage pulses applied to a p-type MoTe2 surface produced much larger and deeper nanometer-scale holes when compared with those produced by positive voltage pulses. The existence of threshold voltages and the pulse polarity dependence of nanostructure fabrication suggests that an electric field evaporation mechanism is applicable. Support for this mechanism was obtained by nanostructuring metallic TMDC NbSe2, where both the produced features and the threshold voltages (3.0 +/- 0.3 V) were similar for both positive and negative voltage pulses.

Induced changes in surface band bending of n-type and p-type AlGaN by oxidation and wet chemical treatments

The surface chemistry and electrical properties of p-type and n-type AlGaN surfaces were studied via x-ray photoelectron spectroscopy before and after oxidation and wet chemical treatments. Shifts in the surface Fermi level were measured with the change in onset of the valence-band spectra. Oxidation and HF and (NH4)2Sx treatments on p-type AlGaN (n-type AlGaN) led to an increase (the reduction) in the surface band bending due to more N vacancies and N vacancies being occupied by S (i.e., donorlike states) than Al vacancies and Ga vacancies (i.e., acceptorlike states) near the p-type AlGaN (n-type AlGaN) surface region. The changes in surface chemistry indicate that oxidation and wet chemical treatments alter the surface state density through the formation of more donorlike states.

Formation and clustering of surface vacancies under electronic excitation on semiconductor surfaces

We review the morphologic changes of Si(1 1 1)-(7×7) and InP(1 1 0)-(1×1) surfaces under repeated irradiation with laser pulses of intensities well below thresholds of melting and ablation. Direct imaging of the irradiated surfaces by means of scanning tunneling microscopy (STM) has revealed the excitation-induced electronic bond breaking, with efficiencies strongly site-dependent and highly superlinear with respect to the excitation intensity, resulting in formation of monovacancies and clustering of pre-existing vacancies. On Si(1 1 1)-(7×7), formation of monovacancies is predominantly induced under repeated irradiation, and the rate is constant up to the vacancy concentration of 10% relative to the total adatom sites. On the other hand, STM images of irradiated InP(1 1 0)-(1×1) have shown the preferential removal of the surface P atoms and prominent Fermi-level effects on the morphology of laser-induced vacancies: vacancy clusters are efficiently formed on n-type surface, while monovacancies are exclusively formed on p-type surface. The surface-dependent morphology of laser-induced vacancies can be explained by non-linear localization of surface holes at vacancy-sites based on the structural and electronic properties of the two surfaces.

Transfer doping of diamond: The use of [formula omitted] and [formula omitted] to effect p-type surface conductivity

Ab initio density-functional theory is used to determine the suitability for transfer doping of diamond surfaces with various hydrogen and oxygen terminations. Transfer doping of the hydrogenated (0 0 1) diamond surface with C 60 is investigated, and electron transfer from diamond to C 60 (leaving a hole layer near the diamond surface) is predicted for a C 60 coverage of about one monolayer. The efficiency of electron transfer is expected to increase for higher coverages. It is also shown that the greater electron affinity of fluorinated C 60 is likely to enhance the effect, in that C 60 F 36 can extract electrons from hydrogenated diamond at much lower coverages. The consequences of oxygenation of the diamond surface on the transfer-doping properties of C 60 are also investigated.

Diamond Based Ion-Sensitive Field Effect Transistors for Cellular Biosensing

ABSTRACTThe use of a chemically inert and essentially biocompatible material for cellular biosensing is an attractive idea. In this context we have studied the operation of diamond-based ion-sensitive field effect transistors (ISFETs) within solutions of varying pH, and alkali-halide concentrations. In particular, we report the use of an inexpensive diamond substrate material, often referred to as ‘black’ diamond. pH sensitivity was observed when devices based on hydrogen-terminated p-type surfaces where employed, provide some surface oxidation was performed prior to their use. Variation in the threshold voltage for ISFET operation of the order of 20mV/pH unit was determined. In terms of operation in potassium iodide solution, we have shown that the device is shows selective sensitivity to the iodide species, despite the equi-molar presence of both K+ and I- species. The origin of this selectivity is discussed.

Comparison of Efficiencies of Different Surface Passivations Applied to Crystalline Silicon

In this work the efficiencies of different surface passivation techniques are compared. This paper emphasizes on the passivation provided by SiNx:H layers that is commonly used in photovolaic industry as surface passivation and anti reflection layer. The method used to evaluate the surface recombination velocity is detailed and discussed. It is shown that light phosphorus diffusion at 850°C – 20 min provides good surface passivation of n-type silicon surface and noticeable passivation of p-type, that can be improved by SiNx:H Layer.

Surface channel effect on [formula omitted]-ray response functions of coaxial germanium detectors

A surface channel formed in a coaxial germanium detector degrades the pulse-height of output signals. The pulse-height degradation gives rise to an enhancement of low pulse-height events in γ -ray response functions at the cost of the full-energy peak. This phenomenon was closely examined for a twin coaxial-germanium detector. It was found that imperfect charge collection is primarily caused by p-type surface channels and supplemented by the n + –i and i– p + boundaries, producing low-energy events both in the valley between the full-energy peak and the Compton edge and on the hill below the Compton edge. It is shown that experimental response functions of the twin Ge can be reproduced with high precision by incorporating the imperfect charge collection in Monte Carlo simulations in a phenomenological way.

Characterization of locally modified diamond surface using Kelvin probe force microscope

The surface potential difference between an H-terminated surface and a locally oxidized diamond surface produced by an atomic force microprobe was investigated using a Kelvin probe force microscope. The potential of the H-terminated diamond surface was observed to be ∼0.1 V higher than that of the oxidized diamond surface. The surface potential difference can be interpreted in terms of the positions of the vacuum level, the Fermi level, and the conduction and valence band edges, when negative electron affinity and p-type surface conduction are assumed on the H-terminated diamond surface. The surface dipole induced by the electronegativity differences between the surface atoms of the diamond affects the difference in the surface potential between the two surfaces.

Influence of stoichiometry of direct plasma-enhanced chemical vapor deposited SiNx films and silicon substrate surface roughness on surface passivation

In this article, we report on the use of direct plasma-enhanced chemical vapor deposited silicon nitride (SiNx) films deposited at low excitation frequency (440 kHz) on low-resistivity (1.5 Ω cm) p-type Czochralski silicon substrate surfaces with different textures, to elucidate the influence of microroughness of the substrate surface on the surface-passivating properties of thin SiNx films. Whereas flat surfaces get the best passivation from Si-rich SiNx films, the optimum passivation shifts towards stoichiometric nitride as the microroughness increases, which points to the increasing relative importance of a charge-induced field effect. When short high-temperature (firing) treatments are applied upon passivation layer deposition, the process window to yield good surface passivation broadens, although very Si-rich films tend to suffer from blistering.

Change in Electrical Resistance of Diamond Surface Conductive Layer due to Acid Mist

Experimentally obtained results of the change in electrical resistance of the p-type diamond surface conductive layer (PSCL) are simulated using a rate equation. According to a model that simulates the process responsible for the decrease in electrical resistance caused by acid mist containing oxonium ions (H3O+), H3O+ ions react with hydrogen atoms terminating dangling bonds on the diamond surface and cause the creation of holes in diamond films. The rate equation describing these hole creation processes is set up and time-dependent electrical resistance is simulated. The simulated results are in agreement with the experimental results.

Terahertz emission from p-lnAs due to the instantaneous polarization

Terahertz radiation from differently doped n- and p-type InAs crystal surfaces was investigated by time-resolved measurement. Large increase of the emitted terahertz power has been observed for p-InAs samples with the p-doping levels of approximately 1016–1017cm−3. This increase was explained by a large surface depletion layer and an electric-field-induced optical rectification effect in this layer.

Electrical characterization of B10 doped diamond irradiated with low thermal neutron fluence

A sample of B10 isotope doped diamond was neutron irradiated to a thermal fluence of 1.3×1019 neutron cm−2. The diamond sample was cooled continuously during irradiation in a nuclear reactor. Li7 is formed by nuclear transmutation reaction from B10. Characterization for electrical conductance in the temperature range of 160 K<T<600 K was performed on an unirradiated B10 doped sample and the B10 doped and irradiated sample. The unirradiated diamond sample showed p-type conductance at higher temperature (T>200 K) and p-type surface conductance at lower temperature (T<200 K). The irradiated sample showed decreasing conductance below 230 K and increasing conductance above 230 K with increasing temperature. Furthermore, the conductance showed a decrease above 400 K followed by an increase above 500 K. The observed behavior below 400 K with increase in temperature is interpreted in terms of compensation of surface p-type carriers by n-type bulk carriers generated from Li7 that is formed by nuclear transmutation reaction from B10 atoms. Also, compensation of n-type carriers from Li7 by p-type carriers from B10 is used to interpret the conductance above 400 K. A low concentration of radiation induced defects, absence of defect complexes, and the low activation energy of n-type Li7 are thought responsible for the observed variation of conductance in the irradiated diamond. The present results illustrate that neutron transmutation from B10 doped diamond is a useful method to achieve n-type conductivity in diamond.

Reduction of ohmic contact on p-GaN with surface treatment using Cl2 inductively coupled plasma

A reduction of Ni/Au ohmic contact on p-type GaN is obtained with surface treatment on GaN films using Cl2 inductively coupled plasma (ICP). X-ray photoelectron spectroscopy (XPS) shows the modifying of the surface atomic ratio after the Cl2 ICP treatment. The atomic ratio of nitrogen to gallium becomes larger after the Cl2 ICP treats samples for 5 s. It suggests that GaClx and/or GaOx is formed and then removed in the boiling HCl solution. The Ga vacancies at the p-type GaN surface are therefore produced and act as acceptors for holes. It leads to the reduction of the contact resistance through the decrease of the resistivity for the conduction of holes.

Electronic Properties of GaN (0001) – Dielectric Interfaces

The characteristics of clean n- and p-type GaN (0001) surfaces and the interface between this surface and SiO 2, Si 3 N 4, and HfO 2 have been investigated. Layers of SiO 2, Si3 N 4, or HfO 2 were carefully deposited to limit the reaction between the film and clean GaN surfaces. After stepwise deposition, the electronic states were measured with x-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). A valence band offset (VBO) of 2.0±0.2 eV with a conduction band offset (CBO) of 3.6±0.2 eV was determined for the GaN / SiO 2 interface. The large band offsets suggest SiO 2 is an excellent candidate for passivation of GaN . For the GaN / Si 3 N 4, interface, type II band alignment was observed with a VBO of -0.5±0.2 eV and a CBO of 2.4±0.2 eV . While Si3 N 4 should passivate n-type GaN surfaces, it may not be appropriate for p-type GaN surfaces. A VBO of 0.3±0.2 eV with a CBO of 2.1±0.2 eV was determined for the annealed GaN / HfO 2 interface. An instability was observed in the HfO 2 film, with energy bands shifting ~0.4 eV during a 650°C densification anneal. The electron affinity measurements via UPS were 3.0, 1.1, 1.8, and 2.9±0.1 eV for GaN , SiO 2, Si 3 N 4, and HfO 2 surfaces, respectively. The deduced band alignments were compared to the predictions of the electron affinity model and deviations were attributed to a change of the interface dipole. Interface dipoles contributed 1.6, 1.1 and 2.0±0.2 eV to the band alignment of the GaN / SiO 2, GaN / Si 3 N 4, and GaN / HfO 2 interfaces, respectively. It was noted that the existence of Ga-O bonding at the heterojunction could significantly affect the interface dipole, and consequently the band alignment in relation to the GaN .

Structural and optical properties of p-InP(1 0 0) anodized in halogenic acids

Nanometer sized pores were formed for the first time on p-type InP surfaces by electrochemical anodic polarization in HF and HBr solutions in the dark. The surface structures were characterized with scanning electron microscopy (SEM), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and photo-luminescence (PL) measurements. It is found that the surface morphology strongly depends on the type of halogen acid present in the electrolyte, and that the anodized samples emit photoluminescence in the visible wavelength range.

Low resistivity contacts to p‐type GaN by plasma treatment

A N2-based plasma has been applied to a p-type GaN surface in order to improve contact resistivity. It has been found that the dc bias voltage of the plasma, which dictates the energies of incident ions, plays an important role in the effectiveness of the treatment. A reduction of contact resistivity has been achieved with a plasma of −75 V dc bias voltage. Excessive ion energies result in a deterioration of the contacts, as a result of plasma damage. The result is supported by XPS data, where a decrease of contact resistivity is linked to a shift of the binding energy of the Ga 3d core level towards lower energies. Such a shift indicates a reduction of the surface band bending coincident with an decrease of the surface barrier height, giving rise to a decrease contact resistance. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)