Fabrication and Characterisation of Short Gate Length Heterojunction Field Effect Transistors

This thesis is concerned with the fabrication and characterisation of short gate length heterojunction field effect transistors (HFETs). Devices with gate lengths in the range 80-200nm were fabricated on three different material structures containing two dimensional electron gases (2DEGs). The layer structures were based on: i) A10.25GaAs/GaAs with an A10.25GaAs back confining barrier 300A from the 2DEG ii) pseudomorphic A10.3GaAs/In0.15GaAs/GaAs with a 150A In0.15GaAs channel layer iii) pseudomorphic In0.52A1As/In0.65GaAs/InP with a 100A In0.65GaAs channel layer Magnetoresistance studies showed the 2DEGs of the three materials had very different transport properties. This permitted an investigation of the dependence of high frequency device performance on material structure to be performed. To investigate the dependence of gate resistance on device performance, HFET's with conventional and T-gate structures were fabricated. The 80nm footprint T-gate process developed in the course of this work reduced the gate resistance by a factor of five compared with conventional 80nm footprint structures. High frequency characterisation of devices up to 60GHz showed the following main results: i) 80nm gate length In0.52A1As/In0.65GaAs/InP HFETs with rf transconductances up to 1100mS/mm. This translates to an effective channel velocity of 2.4x105ms-1. ii) 80nm devices with fT's of up to 275GHz were fabricated on the InAlAs/InGaAs/InP layer structure. Such fT's were nearly twice those of similar gate length devices fabricated on both the AlGaAs/GaAs and AlGaAs/InGaAs/GaAs structures. iii) From the Fr measurements, the effective carrier velocity in the device channel was extracted. Effective velocities in excess of 2.0x105ms-1 were extracted for the InAlAs/InGaAs/InP devices, indicating significant velocity overshoot in the channel of this layer structure. The large indium content of the channel gives a large F-L valley energy separation whilst reducing the electron effective mass. Both these effects increase the probability of velocity overshoot, and are most probably the cause of the large effective velocities deduced for the In0.65GaAs channel devices. There was no conclusive evidence of overshoot in devices fabricated on either the AlGaAs/InGaAs/GaAs or AIGaAs/GaAs structures. iv) For the materials of this study, it was deduced that effective velocity was the dominant transport property in determining device fr at a given gate length. Neither the low field mobility or 2DEG carrier concentration were found to govern device fT v) Both device DC and RF output resistance can be increased by increasing the potential barrier below the 2DEG and thus improving electron confinement to the channel. vi) The 80nm footprint T-gate structure increases device gain by up to 6dB at 60GHz compared to a conventional 80nm gate device. vii) Although the fT's of the InAlAs/InGaAs/InP HI-ETs were much larger than those of the AlGaAs/GaAs and AlGaAs/InGaAs/GaAs HFET's, the tax of conventional gate structure devices fabricated on all three materials were around 80GHz. The fmax of 80nm T-gate InAlAs/InGaAs/InP devices was 180GHz, clearly showing that gate resistance dominates short gate length device high frequency gain

Plasma Processing of III-V Materials for Energy Efficient Electronics Applications

This paper reviews some recent activity at the James Watt Nanofabrication Centre in the University of Glasgow in the area of plasma processing for energy efficient compound semiconductor-based transistors. Atomic layer etching suitable for controllable recess etching in GaN power transistors will be discussed. In addition, plasma based surface passivation techniques will be reviewed for a variety of compound semiconductor materials ((100) and (110) oriented InGaAs and InGaSb)

Initial Investigation on the Impact of In Situ Hydrogen Plasma Exposure to the Interface Between Molecular Beam Epitaxially Grown P-Ga_0.7In_0.3Sb (100) and Thermal Atomic Layer Deposited (ALD) Al₂O₃

This work presents, to the best of the authors knowledge, the first experimental findings on the impact of in situ H<sub>2</sub> plasma exposure to the electrical properties of the interface between p-type Ga<sub>0.7</sub>In<sub>0.3</sub>Sb and atomic layer deposited Al<sub>2</sub>O<sub>3</sub>. The effects of trimethyl aluminium (TMA) exposure prior to Al<sub>2</sub>O<sub>3</sub> deposition, and of a post gate metal forming gas anneal (FGA) are also investigated. The control sample, which was subjected to an ex situ HCl clean prior to ALD only, demonstrated a capacitance modulation of 36.29 % before FGA. This degraded for samples exposed to the H<sub>2</sub> plasma for all plasma powers investigated. TMA exposure offered no improvement, and significantly increased the frequency dispersion in accumulation for all samples. A post gate metal FGA at 350 °C for 15 minutes was found to substantially improve the interface quality, with the capacitance modulation, frequency dispersion in accumulation and dC/dV improving by as much as 190 %, 91 %, and 170 % respectively

Terahertz Monolithic Integrated Circuits (TMICs) Array Antenna Technology On GaN-on-Low Resistivity Silicon Substrates

In this paper, we have demonstrated a viable microstrip array patch antenna technology for the first time on GaN-on-low resistivity silicon (LR-Si) substrates (ρ <; 40 Ω.cm) at H-band frequencies (220-325 GHz). The developed technology is compatible with standard MMIC technology with no requirement for high temperature processes. To mitigate the losses presented by the substrate and to enhance the performance of the integrated array antenna at THz frequencies, the driven patch is shielded by silicon nitride and gold layer in addition to a layer of benzocyclobutene (BCB). The demonstrated 4×1 array integrated antenna showed a measured resonance frequency in agreement with our simulation at 0.27 THz; a measured S11 as low as -41 dB was obtained. A directivity, gain and radiation efficiency of 11.2 dB, 5.2 dB, and 20% respectively was observed from the 3D EM model for a 5 μm BCB inset. To the authors' knowledge, this is the first demonstration of a THz integrated microstrip array antenna for TMIC technology; this developed technology is promising for high performance III-V electronic material on low resistivity/high dielectric substrates

Multispectral mid-infrared light emitting diodes on a GaAs substrate

We have designed, simulated, and experimentally demonstrated four-colour mid-infrared (mid-IR) Light Emitting Diodes (LEDs) integrated monolithically into a vertical structure on a semi-insulating GaAs substrate. In order to finely control the peak wavelength of the emitted mid-IR light, quantum well (QW) structures based on AlInSb/InSb/AlInSb are employed. The completed device structure consists of three p-QW-n diodes with different well widths stacked on top of one bulk AlInSb p-i-n diode. The epitaxial layers comprising the device are designed in such a way that one contact layer is shared between two LEDs. The design of the heterostructure realising the multispectral LEDs was aided by numerical modelling, and good agreement is observed between the simulated and experimental results. Electro-Luminescence measurements, carried out at room temperature, confirm that the emission of each LED peaks at a different wavelength. Peak wavelengths of 3.40 μm, 3.50 μm, 3.95 μm, and 4.18 μm are observed in the bulk, 2 nm, 4 nm, and 6 nm quantum well LEDs, respectively. Under zero bias, Fourier Transform Infrared photo-response measurements indicate that these fabricated diodes can also be operated as mid-IR photodetectors with an extended cut-off wavelength up to 4.6 μm

Optical properties of refractory metal based thin films

There is a growing interest in refractory metal thin films for a range of emerging nanophotonic applications including high temperature plasmonic structures and infrared superconducting single photon detectors. We present a detailed comparison of optical properties for key representative materials in this class (NbN, NbTiN, TiN and MoSi) with texture varying from crystalline to amorphous. NbN, NbTiN and MoSi have been grown in an ultra-high vacuum sputter deposition system. Two different techniques (sputtering and atomic layer deposition) have been employed to deposit TiN. We have carried out variable angle ellipsometric measurements of optical properties from ultraviolet to mid infrared wavelengths. We compare with high resolution transmission electron microscopy analysis of microstructure. Sputter deposited TiN and MoSi have shown the highest optical absorption in the infrared wavelengths relative to NbN, NbTiN or ALD deposited TiN. We have also modelled the performance of a semi-infinite metal air interface as a plasmonic structure with the above mentioned refractory metal based thin films as the plasmonic components. This study has implications in the design of next generation superconducting nanowire single photon detector or plasmonic nanostructure based devices

A sub-critical barrier thickness normally-off AlGaN/GaN MOS-HEMT

A new high-performance normally-off gallium nitride (GaN)-based metal-oxide-semiconductor high electron mobility transistor that employs an ultrathin subcritical 3 nm thick aluminium gallium nitride (Al0.25Ga0.75N) barrier layer and relies on an induced two-dimensional electron gas for operation is presented. Single finger devices were fabricated using 10 and 20 nm plasma-enhanced chemical vapor-deposited silicon dioxide (SiO2) as the gate dielectric. They demonstrated threshold voltages (Vth) of 3 and 2 V, and very high maximum drain currents (IDSmax) of over 450 and 650 mA/mm, at a gate voltage (VGS) of 6 V, respectively. The proposed device is seen as a building block for future power electronic devices, specifically as the driven device in the cascode configuration that employs GaN-based enhancement-mode and depletion-mode devices

Low off-state Leakage Currents in AlGaN/GaN High Electron Mobility Transistors By Employing A Highly Stressed SiNx Surface Passivation Layer

In this study, the impact of the stress in SiNx surface passivation layers on off-state drain and gate leakage currents and off-state breakdown voltage in AlGaN/GaN High Electron Mobility Transistors (HEMTs) is assessed. The SiNx films were deposited at room temperature by inductively coupled plasma chemical vapour deposition (ICP-CVD). Compared to unpassivated devices, the off-state drain and gate leakage currents of AlGaN/GaN HEMTs is increased by up to 2 orders of magnitude for a 200 nm thick SiNx passivation layer with 309 MPa compressive stress. The use of a bilayer SiNx passivation scheme comprising 70 nm SiNx with 309 MPa compressive stress followed by 130 nm SiNx with 880 MPa compressive stress resulted in off-state drain and gate leakage currents reduced by up to 1 order of magnitude when compared to unpassivated devices

Investigation of mid-infrared AlInSb LEDs with an n-i-p structure

We report on the investigation on mid-infrared AlInSb LEDs with an n-i-p structure. Compared to the conventional AlInSb LEDs with a p-i-n structure, a better current spreading corresponding to a uniform current distribution in the active region is expected in the n-i-p structure because of a high electron mobility in the n-type AlInSb material. The output optical power of laterally injected LEDs were investigated as a function of the device geometry by COMSOL simulations and confirmed by experimental results

Design and characterisation of titanium nitride sub-arrays of kinetic inductance detectors for passive terahertz imaging

We report on the investigation of titanium nitride (TiN) thin films deposited via atomic layer deposition (ALD) for microwave kinetic inductance detectors (MKID). Using our in-house ALD process, we have grown a sequence of TiN thin films (thickness 15, 30, 60 nm). The films have been characterised in terms of superconducting transition temperature Tc , sheet resistance Rs and microstructure. We have fabricated test resonator structures and characterised them at a temperature of 300 mK. At 350 GHz, we report an optical noise equivalent power NEPopt≈2.3×10−15 W/√Hz , which is promising for passive terahertz imaging applications