Hui Jin Looi

High carrier mobility in polycrystalline thin film diamond

Polycrystalline diamond films have been found to display p-type surface conductivity. No bulk impurity is added to the films; the p-type characteristics of the undoped diamond are thought to be due to a surface or near surface hydrogenated layer. Carrier concentrations within the range 1017–1019 cm−3 have been measured; control over the carrier concentration can be achieved by annealing the “as-grown” films in air. For a given annealing temperature a stable carrier concentration arises. The Hall carrier mobility has been explored and a value of >70 cm2/Vs has been measured for a film with a carrier concentration of ∼5×1017 cm−3, the highest reported for polycrystalline thin film diamond and equivalent to boron doped single crystal diamond.

AN INSIGHT INTO THE MECHANISM OF SURFACE CONDUCTIVITY IN THIN FILM DIAMOND

Abstract Diamond containing hydrogen at or near the surface displays p-type conductivity. The origin of this effect has been controversial. We have used I–V, Hall effect, SIMS, Raman, UPS and XPS to study hydrogenated polycrystalline CVD diamond films. The direct formation of acceptor states by hydrogen, which resides within the top 20 nm of the film, is the origin of the carriers present rather than surface band bending. Up to 1019 holes cm−3 can be measured and mobilities as high as 70 cm2 Vs−1 recorded. H-termination of the surface is important for the formation of high quality metal-diamond interfaces.

The effect of hydrogen on the electronic properties of CVD diamond films

Abstract Thin film diamond, grown by chemical vapour deposition, have been found to display p-type surface conductivity. No bulk impurity is added to the films; the p-type characteristics of the undoped diamond are thought to be due to a surface or near surface hydrogenated layer. Carrier concentrations within the range 10 17 –10 19 cm −3 have been measured; control over the carrier concentration can be achieved by annealing the ‘as-grown’ films in air. For a given annealing temperature a stable carrier concentration arises. The Hall carrier mobility has been measured and a value of >n70 cm 2 /Vs has been found for a film with a carrier concentration of ~ 5 × 10 17 cm −3 , the highest reported for polycrystalline thin film diamond and equivalent to boron doped single crystal diamond. I–V characteristics for Al-based Schottky diodes are good (ideality factor 1.1) and show no breakdown at reverse bias levels greater than 100 V, suggesting this form of diamond is well suited to electronic applications.

High-performance metal-semiconductor field effect transistors from thin-film polycrystalline diamond

Abstract Polycrystalline CVD diamond films with a near-surface hydrogenated layer have been used to form the first normally off enhancement mode MESFET structures from this material. A room temperature transconductance of 0.14 mS mm −1 has been measured, the highest yet reported for a transistor structure made from polycrystalline material. The devices fully turn off, display current saturation and have a low gate leakage. Aluminium forms a near-ideal Schottky barrier on this material (SBH ∼0.98 eV, ideality 1.1) and was used as the gate metallisation within the MESFET. Optimised forms of these structures would appear to offer a commercially viable route to high-performance, diamond-based electronic circuits.

Enhancement mode metal-semiconductor field effect transistors from thin-film polycrystalline diamond

Polycrystalline CVD diamond films with a near surface hydrogenated layer have been used to form the first normally off enhancement mode MESFET structures from this material. A room temperature transconductance of 0.14 mS/mm has been measured, the highest yet reported for a transistor structure made from polycrystalline material. The devices fully turn off, display saturation and have a low gate leakage current. Al forms a near ideal Schottky barrier on this material (SBH /spl sim/0.98 eV, ideality <1.1) and was used as the gate metallization within the MESFET. Optimized forms of these structures would appear to offer a commercially viable route to high-performance diamond based electronic circuits.

Metal–semiconductor–metal photodiodes fabricated from thin-film diamond

A metal–semiconductor–metal Schottky barrier photodetector has been fabricated on a “hydrogen-doped” surface-conducting chemical vapor deposition (CVD) diamond. The device is fabricated in one step by forming two back-to-back aluminum Schottky diodes on the p-type surface. This simple process is compatible with previously reported metal–semiconductor field-effect transistor fabrication on this type of CVD diamond and offers the prospect of the monolithic integration of a ultraviolet detector and active circuitry. Preliminary electrical and optical characteristics of the device have been measured, including the spectral response over the range 180–800 nm. The device exhibits a linear response with the applied optical power at 220 nm, operates at a bias of only 2 V, and shows visible blind characteristics, with a spectral discrimination of three orders of magnitude as determined from the ratio of 200–550 nm responses.

A thin-film diamond phototransistor

A phototransistor fabricated from thin-film diamond is reported. Polycrystalline diamond grown by chemical vapor deposition, which is p-type by virtue of near-surface hydrogen, has been used to realize optically activated metal–semiconductor field-effect transistors (FETs). Devices with thin (30 nm) Al Schottky gates and Au source and drain contacts operate as effective enhancement-mode metal–semiconductor field-effect transistors at room temperature; illumination of an electrically isolated gate leads to increased channel current, although saturation is still evident. At deep UV wavelengths (<220 nm), a photodetector gain of around 4 has been measured; the mechanism of operation has been identified as photodiode-like turn-on followed by FET amplification.

AN OPTICALLY ACTIVATED DIAMOND FIELD EFFECT TRANSISTOR

Abstract Thin film diamond photodetectors are one of the most promising classes of diamond devices for commercial exploitation. Already, photoconductive devices which display extremely high levels of selectivity between deep UV and visible light, allied to good sensitivity, are becoming available. However, more advanced device designs are required if high speed operation is to be achieved alongside high sensitivity. Phototransistors are ideally suited to this application, but until recently room temperature operation of diamond field effect transistors (FETs) was not possible. This has changed with the emergence of p-type hydrogenated diamond and this paper describes the fabrication and operation of the first MESFET based diamond photodetectors (OPFETs) to be made. Optical modulation has been demonstrated and the wavelength selectivity measured. Devices respond significantly more to UV light than visible wavelengths, with gain levels of around 4. The mechanism(s) of operation are discussed.