Atomistic Modelling
In order to gain a deep insight into the properties of semiconductor materials and their nanostructures we make heavy use of computer modelling, often running on dedicated high performance clusters:
- We are experts in the development and use of empirical bond order potentials of the Abell-Tersoff type, which we use for modelling e.g. InAs/GaAs Quantum Dots for applications in the 1.3µm telecom frequency. We have recently modelled strain fields and studied segregation in InGaAsSb capping layers revealing interesting decompositions of the quaternary into the ternary alloy. This work is in collaboration with the Technical University of Eindhoven and the University of Sheffield.
- We also have a successful programme on using a semi-empirical scheme for the calculation of the dielectric response to strain in polar semiconductor alloys which has revealed the applicability of tuning the piezoelectric field from negative to positive in heterostructures such as Quantum Wells. The methodology relies upon calculating the elastic and dielectric properties of polar semiconductors using ab initio density functional theory and density functional perturbation theory. We make use of the plane-wave DFT code CASTEP implemented on the HPC facility at the University of Manchester.
- We are iminently starting a project on modelling of dilute magnetic semiconductors, mainly Mn in InGaAs. These materials are excellent candidates fro spintronics applications. We intend to evaluate their structural and magnetic properties in view of creating a suite of modelling tools, both empirical, semiempirical and ab initio to provide answers to the large experimental effort currently under way in the UK and elsewhere. This project is in collaboration with the STFC Daresbury laboratories and the University of Exeter.
Personnel: Max Migliorato, Vesel Haxha and Geoffrey Tse
Silvaco Modelling
Research is focused to develop predictive models for high speed devices using Silvaco, which provides a Virtual Wafer Fabrication (VWF) environment in which two or three dimensional device simulations can be performed using ATLAS. These physically-based simulations provide the opportunity to increase understanding of the effects of changes to the device’s physical structure, theoretical concepts and its general operation. It not only provides inexpensive, reliable and efficient prototyping, but also aids with the understanding of the underlying device physics.
Predictive modelling has been a long and on-going process within the M&N group whilst developing strong links with Silvaco. As a result of these links UoM hosted a Silvaco’s international workshop 2008 on semiconductor modelling at which UoM presented a number of papers.
Gunn Diode
Advanced step graded 77 GHz Gunn diode physical model has been successfully developed in Silvaco, ATLAS using VWF, the device structure is shown in Figure 1(a). The device is commercially manufactured by e2v Technologies limited for use in second harmonic mode 77GHz Intelligent Adaptive Cruise Control (ACC) systems for automobiles. The simulated forward and reverse-bias IV characteristics of the model are shown in Figure 1(b) and match extremely well with measured data, provided by e2v tech., thus validating the choice of the physical models and material parameters used. The results obtained so far prove that the modelling techniques used have the potential to provide predictive models for novel Gunn diodes operating as high power Terahertz sources.
GaAs/AlGaAs MODFET
GaAs/AlGaAs is one the most prevalent material systems for Modulation Doped Field Effect Transistors (MOSFETs) especially at very low temperatures because of its very high mobility under these conditions. Full physical model of a GaAs/AlGaAs MODEFET with 1μm gate length geometry is developed using 2-D ATLAS simulator and compared with measurements obtained from a similar MODEFET fabricated at the University of Manchester using Molecular Beam Epitaxy (MBE). Models taking into account the effect of transverse and lateral electric fields on mobility, deep-level traps, Fermi-level pinning, carrier generation/recombination and tunnelling have been included and these have led to excellent agreements between modelled and experimental values as shown below:
Pseudomorphic High Electron Mobility Transistors (pHEMT)
The InGaAs/InAlAs material system provides one of the highest transconductance pHEMT devices because of its large conduction band discontinuity, high electron mobility and very good carrier confinement in the channel. The device has been modelled in Silvaco and used extensively to study the interaction and effect of electric field on the release of carriers from deep-level traps of InAlAs buffer layer which are transferred to the channel. The kink phenomena has been successfully demonstrated that show reasonable agreement with the measured data for the device threshold voltage, i.e. Vth=-1.7 V. The drain current plots for different values of Vgs are comparable.
Single Heterojunction Bipolar Transistors (SHBTs)
InP/InGaAs SHBTs have shown excellent characteristics which make them extremely attractive for high frequency applications. For the next generation state-of-the-art devices, development tools are becoming increasingly necessary to fully characterise the physical phenomenon within a device. In this regard a two-dimensional DC physical model has been developed using Technology-Computer-Aided-Design (TCAD) within the SILVACO software package. The effect of the spacer layer on the turn-on voltage of device has been investigated in details. Excellent agreement between modelled and measured data is reported and physical insight into the working of the device gained. The effect of the spacer layer on the turn-on voltage and energy band diagram is studied. Further work on extending the DC simulations to RF simulations of HBTs is in progress.
Personnel: Professor Missous, Mr Mohammad Alduraibi, Mr Faisal Amir, Mr Shahzad Arshad, Mr Robert Knight, Mr Muhammad Mohiuddin, Mr Tauseef Tauqeer