A. Denisenko

RF performance of surface channel diamond FETs with sub-micron gate length

Abstract Diamond hydrogen-induced surface channel field effect transistors (FETs) were fabricated with gate lengths down to 0.2 μm in part using electron beam lithography. Down-scaling of the gate-length resulted in both improved DC- and RF characteristics, especially for a 0.2-μm gate length in a maximum output current of I D max =360 mA/mm with a peak transconductance of 148 mS/mm. The optimum cut-off frequencies were f T =11.5 GHz, f max(MAG) =31.7 GHz and f max(U) =40.2 GHz. A maximum drain voltage of 68 V was obtained before pattern-related destructive breakdown occurred. This allows estimation of the RF power handling capability to above 3.0 W/mm. The data are the highest reported for diamond FETs. Scaling of the device parameters with gate length allows to estimate a velocity-limited maximum output current to 750 mA/mm and an f T above 20 GHz at 0.1-μm gate length. At high drive, current drift and current compression is observed in the quasi-DC output characteristics as well as in first microwave large signal measurements. These instabilities seem at present to be the main hurdle of hydrogen-induced surface channel FETs in high power microwave applications.

Diamond diodes and transistors

Over the past few years a variety of diamond electron devices have been fabricated, analysed and simulated. This includes Schottky diodes on boron-doped p+ diamond substrates, boron/nitrogen pn-junction diodes, bipolar transistors based on this pn-junction and field effect transistors (FETs) with boron delta-doped channels and hydrogen-related surface conductive layers. Many of the fabricated devices considered here represent the current state-of-the-art in this field. This includes the operation of diamond Schottky diodes at temperatures of up to 1000 °C, as well as diamond FET devices with a cut-off frequency of 30 GHz and channel current densities of 300 mA mm−1. Simulations show that diamond boron delta-doped FETs might yield an RF-output power density of up to 30 W mm−1.

Size-Dependent Electrocatalytic Activity of Gold Nanoparticles on HOPG and Highly Boron-Doped Diamond Surfaces

Gold nanoparticles were prepared by electrochemical deposition on highly oriented pyrolytic graphite (HOPG) and boron-doped, epitaxial 100-oriented diamond layers. Using a potentiostatic double pulse technique, the average particle size was varied in the range from 5 nm to 30 nm in the case of HOPG as a support and between <1 nm and 15 nm on diamond surfaces, while keeping the particle density constant. The distribution of particle sizes was very narrow, with standard deviations of around 20% on HOPG and around 30% on diamond. The electrocatalytic activity towards hydrogen evolution and oxygen reduction of these carbon supported gold nanoparticles in dependence of the particle sizes was investigated using cyclic voltammetry. For oxygen reduction the current density normalized to the gold surface (specific current density) increased for decreasing particle size. In contrast, the specific current density of hydrogen evolution showed no dependence on particle size. For both reactions, no effect of the different carbon supports on electrocatalytic activity was observed.

The electronic surface barrier of boron-doped diamond by anodic oxidation

It was shown that a strong anodic oxidation of 100-oriented diamond induces the electronic surface states, which pin the surface Fermi level at about 3.6 eV above the valence-band maximum. The characteristics of the electronic surface barrier were evaluated from the analysis of boron-doped diamond electrodes and correlated with the four-point probe measurements of an oxidized diamond resistor with a boron delta-doped channel. The same evaluation procedure applied to the case of a wet chemical oxidation yielded a surface barrier of 1.9 eV, which is consistent with the data in the literature. The characteristics of the 3.6 eV barrier by the anodic oxidation remained stable after subsequent chemical treatments even at elevated temperatures, and were also not degraded in air for a long time. The x-ray photoemission spectroscopy study showed that the anodic oxidation generates complex oxygen functionalities, like polycarbonate groups, and also C-O-C bridging bond structures with possible contribution of an add...

pH sensing by surface-doped diamond and effect of the diamond surface termination

Abstract For the first time, pH sensing of liquid solutions by a free diamond surface is demonstrated using ungated FET structures. The surface channel is in direct contact with the liquid, forming a liquid gate. The sensors tested were acidic and basic water solutions with pH varying from 1 to 13. Three different types of surface channel were tested: (a) p-type boron-doped channel /hydrogen-terminated surface; (b) p-type hydrogen-induced surface channel; and (c) boron-doped channel/oxygen-terminated surface. The surface termination determined the pH response. For the H-terminated surface (a, b), the surface channel was gradually depleted with increasing pH. No charge transfer across the diamond surface to reach equilibrium between the semiconductor and liquid solution was observed. It is proposed that CH surface bonds pin the Fermi level at the valence band edge due to a high density of states. Shifting the surface state level into the bandgap results in channel depletion. The effect is believed to be related to interaction between the CH surface dipoles and negatively charged radicals in solution. The oxygen termination (c) resulted in a high density of surface states at approximately 1.7 eV above the valence band. No pH sensitivity was observed, thus the energy level of the state is not moved.

P-channel InGaN-HFET structure based on polarization doping

A p-channel GaN-based heterostructure field-effect transistor (HFET) concept based on a two-dimensional hole gas (2DHG) induced by polarization doping is presented. The structure employed is a GaN-InGaN-GaN heterostructure without external acceptor doping. The p-channel 2DHG characteristics are verified by operation at low temperature (20 K) and capacitance-voltage profiling. Hall measurements result in a positive Hall-coefficient and indicate a 2DHG hole mobility of approximately 700 cm/sup 2//Vs at 66 K. For an optimum structure supported by extrinsic doping a simulated output current of approximately 100 mA/mm is predicted for a gate length of 0.5 /spl mu/m.

Catalytic activity of platinum nanoparticles on highly boron-doped and 100-oriented epitaxial diamond towards HER and HOR

Platinum nanoparticles supported on boron-doped single-crystalline diamond surfaces were used as a model system to investigate the catalytic activity with respect to the influence of particle morphology, particle density and surface preparation of the diamond substrates. We report on the preparation, characterization and activity regarding hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) of these Pt/diamond electrodes. Two kinds of diamond layers with boron doping above 10(20) cm(-3) were grown epitaxially on (100)-oriented diamond substrates; post-treatments of wet chemical oxidation and radio frequency (rf) oxygen plasma treatments were applied. Electrochemical deposition of Pt was performed using a potentiostatic double-pulse technique, which allowed variation of the particle size in the range between 1 nm and 15 nm in height and 5 nm and 50 nm in apparent radius, while keeping the particle density constant. Higher nucleation densities on the plasma processed surface at equal deposition parameters could be related to the plasma-induced surface defects. Electrochemical characterization shows that the platinum particles act as nanoelectrodes and form an ohmic contact with the diamond substrate. The catalytic activity regarding HER and HOR of the platinum nanoparticles exhibits no dependence on the particle size down to particle heights of ∼1 nm. The prepared Pt on diamond(100) samples show a similar platinum-specific activity as bulk platinum. Therefore, while keeping the activity constant, the well-dispersed particles on diamond offer an optimized surface-to-material ratio.

On the ion-sensitivity of H-terminated surface channel devices on diamond

The transient behavior of ungated ion-sensitive FET-like structures with no reference gate electrode on H-terminated diamond surfaces is studied in various pH solutions. The surface conductive channel of these devices is formed by hydrogen termination of the diamond layers selectively grown on single crystal and polycrystalline diamond substrates. Two types of the ISFET-like structures are used in the experiments: (a) large area devices, where the as-formed surface channel is not subjected to further processing and (b) lithography-patterned (micron-size) ISFET-like structures with the surface channel formed by openings in polyimide passivation via chemical etching. The response of the devices to solutions with different pH was found to be strongly dependent upon the type of substrate and the channel fabrication technology. Reproducible response is observed only for large area devices on single crystal substrates. The response of the large area devices on polycrystalline diamond is less reproducible and is characterized by a large (several hours) relaxation time to restore the initial conductivity. The structures patterned by lithography on polycrystalline diamond undergo fast degradation, once immersed into solution. Then, no ion-sensitivity and no recovery are observed.

Surface damages in diamond by Ar/O2 plasma and their effect on the electrical and electrochemical characteristics of boron-doped layers

Epitaxial single crystal and boron-doped diamond layers were exposed to reactive ion etching in Ar/O2 plasma (rf power of 25 W and self-bias of 100 V); and the electrical, structural, and electrochemical characteristics of the exposed surface were investigated. Angle-resolved x-ray photoemission spectroscopy (XPS) measurements revealed a nonuniform layer of amorphous carbon at the exposed surface with an average thickness of approximately 4 nm, as confirmed also by atomic force microscopy profiling of selectively etched areas. On highly boron-doped diamond, the plasma-induced damages resulted also in a nonconductive surface layer. This damaged and insulating surface layer remained resistant to graphite-etching chemicals and to rf oxygen plasma but it was removed completely in microwave hydrogen plasma at 700 °C. The surface characteristics after the H-plasma process followed by wet chemical oxidation were restored back to the initial state, as confirmed by XPS. Such “recovery” treatment had been applied t...

Effect of surface quality on ion sensitivity of H-terminated diamond

Results on the electrochemical behavior of H-terminated diamond electrode and ion sensitive FET (ISFET)-like structures in various aqueous solutions are still difficult to reproduce and in part contradictory. It seems therefore that the material quality and device preparation still have a major impact. This contribution tries to address some of theses problems, which is firstly the effect of surface contamination caused by resist processes, secondly the effect of high drain bias applied to ungated ISFET samples and thirdly the question of visualization of charge transfer.