The Solid State Devices and Ion Beam Technology (SSD/IBT) Research Group


INTRODUCTION

Various new projects began during the year, including SERC sponsored work on FeSi2 and SiGe alloys, and the JESSI project involving the use of the 2 MV implanter to study high energy implants in silicon for CMOS circuits. The alloys and compounds based on silicon may be particularly important for optoelectronics applications, which is a topic becoming of increasing importance in our research.

We are also very pleased to report that a new project has recently begun sponsored by Matsushita Electric Works Co, who have an office on the Research Park. The contract which supports a study of light emission from microcrystalline silicon, is initially for two years. The contract allows Mr Takuya Komoda, the Manager of MEW, UK, to register as a post graduate student and to spend about half of his time working in the University. It also supports a full time, research studentship over three years.

During the past year, the group has made significant progress in the utilisation of the new HVEE 2 MV high energy implanter. There have, however, been numerous unexpected breakdowns and stoppages, which have delayed our programme of development. But, despite these setbacks, many SERC customers have been serviced and various development projects have been completed. Further developments and improvements remain a high priority for the coming year. In March 1993, the second Facility Workshop was held at which about 60 attendees were present. This year the programme was different from last year with five invited review papers and twelve short presentations as introductions to some of the twenty posters. The five reviews, which were all associated with the uses of ion beams in the fabrication of devices and integrated circuits, and in materials science, were presented by Professor M J Kelly (Surrey), Professor J Robertson (Edinburgh), Dr J Marsh (Glasgow), Professor A R Peaker (UMIST) and Dr P L F Hemment (Surrey). The format of invited speakers, short presentations and posters is likely to be maintained for next year's Workshop planned for March 1994. Thanks go to Chris Jeynes for his hard work in organising the event.

The group received a visit from an SERC panel in April to discuss the latest roll-on application to support the Central Facility. The panel was very pleased with our achievements over the past two years and have recommended continued support, although, due to the financial situation at the SERC, the details of the award will not be announced until after the Autumn, 1993 round of submissions. The roll-on date has, therefore, been delayed by six months, until the 1st February 1994.

In October 1992, Michael Kelly was appointed Professor based in the Department of Physics, but with the remit to work closely with our Department and with the Department of Materials Science and Engineering. It is expected that his very positive interaction with the SSD & IBT Research Group will

be extremely rewarding during the coming years. We congratulate Mike on his recent election to the Fellowship of the Royal Society.

Also during the past year: Bernard Weiss and Roger Webb were promoted to Reader and Senior Lecturer, respectively; and Peter Hemment was appointed Chairman of E-MRS Network 3: Ion Beam Processing of Semiconductors.

The group has continued to receive visitors from collaborating institutions around the world, including China, Russia, Germany, Spain, Yugoslavia, India. Most of these collaborations are supported by the British Council, the Royal Society and the European Community.

The following pages describe briefly much of the current research work of the group. Further information can be obtained from Professor Sealy.


THE UNIVERSITY OF SURREY ION BEAM FACILITY FOR MICROELECTRONICS

Aims:

The Facility, which is an integral part of the Solid State Devices and Ion Beam Technology research group, was established by the SERC in 1978 and currently services about thirty UK University research projects.

The accelerators and associated characterization and processing facilities are available to users authorised by the SERC to have access to the Surrey Facility. A total of 2500 hours of beam time per annum is available to such users. The time used is mostly associated with work on semiconductors, (75%), the remainder being used to support studies of polymers, metals and ion beam analysis of non-semiconducting materials. In addition, we offer our facilities to industry, either as a service or via collaborative projects.

Facilities:

Three implanters cover the energy range 25 keV to 4 MeV for many ions, including the major dopants for silicon and III-V compound semiconductors and many transition and rare earth metals. A variety of sample holders are available to accommodate samples of arbitrary shape and for wafers up to 6" in diameter. Facilities also exist to perform implantsinto samples over the temperature range 80 K to 1200 K.

Our new HVEE 2 MV van de Graaff implanter, installed in the Spring of 1991, has serviced a variety of SERC customers as well as European collaborators. However, although we have made good progress, there have many difficult and unexpected problems to overcome before the machine will be available for turn key operation.

The 500 kV implanter has remained our workhorse and achieves an up time approaching 90%, whilst the 400 kV high current implanter is utilised about 50% of the available time being used primarily for ion beam synthesis of silicides. A 2 MV van de Graaff accelerator is available for ion beam analysis, with most work requiring Rutherford backscattering (RBS) and channelling analysis. However, also available is particle induced X-ray excitation analysis (PIXE) using a focused beam having a diameter down to about a 10 um. Light mass elements can be detected using nuclear reaction analysis (NRA) or elastic recoil detection analysis (ERDA).

An extensive range of processing and characterization equipment is also available, including rapid thermal annealing, film deposition, device fabrication, and optical and electrical assessment.

Access to the Facility

The UK academic community may freely access the Facility in three different ways.
  1. limited access via the pump-priming scheme. To do this, it is necessary to contact Mr E G Jones, the Microelectronics Facilities Coordinator at RAL, Didcot, Oxon, OX11 0QX.
  2. SERC supported students can obtain access in their own right.
  3. major users can gain access to the Facility by applying for a research grant using the RG2 form and append a form F11 requesting access to the Facility. The latter form requires the signature of Dr Jeynes prior to submission. Again Mr Jones can advise on the administrative details.

Potential users should discuss their requirements with the Senior Liaison Fellow, Dr C Jeynes.

Funding: SERC


OPTOELECTRONIC DEVICES

Optoelectronics is an important technology which has a profound impact on the performance of future systems.

Aims:

The aim of our research is the development of novel devices for high speed signal processing and sensor applications. This research is device based although it often includes the modelling and characterization of material properties. The work naturally falls into two parts, devices based on III-V quantum well (QW) structures and Si based materials.

Achievements:

Description:

III-V Devices and Structures

QW disordering, which involves interdiffusion across the well/barrier interface, modifies the optical and electrical properties of the structure. Since this process may be localised, using implantation for example, where the presence of specific impurities cause a significant acceleration of the process, selected parts of the sample may be disordered. In this way the optical properties of such structures (including the absorption coefficient, refractive index and Stark shift) may be modified to form a disorder delineated stripe optical waveguide. However, the variation of the optical properties of such structures as a function of the extent of the disordering needs to be known to enable devices with the required characteristics to be designed. A computer model has been developed for the optical properties of single QW structures, as a function of the properties of both lattice matched and strain layer QW structures, which has been able to explain optical waveguide results obtained by workers at BellCore. This model is being developed to model the properties of multiple QW structures, which are more commonly found in practical devices. Photoreflection is being used to verify this model. Results of the thermal stability of single AlGaAs/GaAs quantum well structures has been found to agree with the model and experiments are underway to verify the model for impurity induced disordering of single and multiple quantum well structures. Subsequently this process will be used to optimise the characteristics of optoelectronic devices.

Si Based Optoelectronic Devices

Optoelectronics in silicon based materials is a rapidly expanding subject of increasing importance. Work in this are includes the development of waveguides and devices in SIMOX structures, SiGe heterostructures and in planar SiO2 structures.

The aim of the SIMOX work is to develop a generic, low cost integrated optics technology primarily aimed at sensor systems, although other applications are clearly possible. Thus far we have investigated planar and rib waveguide structures, and optical modulators. The research has demonstrated that guiding structures can be realised that have losses that are experimentally indistinguishable from those of pure silicon. Furthermore, optical phase modulators have been designed, with figures of merit which significantly exceed those of other technologies for some applications. The modulators are based upon refractive index changes due to injection of free carriers. It is vital to maximise the interaction between the propagating optical mode and the injected carriers in order to produce an efficient device. One example is a modulator and that has been designed to maximise this interaction in a relatively large waveguide (several um in cross-section), resulting in a device that can efficiently act as a phase modulator, but is also compatible with other optical devices with similar dimensions (see Figure 1.1).

The UV photosensitivity of proton implanted SiO2 doped with germanium which may be produced at selective locations within a sample has been determined. The presence of hydrogen creates absorption bands around 212 and 240 nm which are bleached by exposure to 249 nm excimer laser pulses. The removal of these absorption bands causes a change in the refractive index of the material at longer wavelengths and this change in refractive index is described by the Kramers Kronig transformation. The relationship between the implantation process, the post implantation annealing and the absorption spectra of the material have been determined. In addition, the waveguide loss of rib waveguides at 1.5 um has been found to be ~2 dB/cm. Experiments to characterise waveguides produced by this process are underway.

SiGe is of interest because its bandgap wavelength covers the useful optical communications wavelengths for optical fibre systems so that, by locally changing the composition by ion beam synthesis for example, devices for signal processing and detecting/generating optical signals could be fabricated on a single substrate. The optical properties of planar SiGe/Si heterostructures have been measured as a function of wavelength for Ge concentrations ranging from 1.3% to 33% with the lowest losses of ~1 dB/cm being obtained for 1.3% Ge at 1.523 um.

3 x 3 optical couplers have been investigated for applications in fibre optic gyroscopes. The gyroscope configuration that contains the 3 x 3 coupler is potentially a low cost configuration that relies on the characteristics of the coupler for suitable performance. In particular stability of amplitude and phase are crucial. A series of couplers have been experimentally evaluated, and compared with the literature, from which it has been established that excess loss of optical couplers is not a simple parameter, and as such is often misunderstood. An alternative matrix representation has therefore been developed for one family of couplers.

SCAN THIS PHOTO IN.

Figure 1.1. Proposed Modulator Geometry.

Funding:

SERC, US Army, Royal Society of London, British Telecom Laboratories, Bookham Technology, DRA, NATO.


MODELLING ION IMPLANTED GaAs

Aims:

The aim of this work is to study the mechanisms by which impurities become electrically activated following implantation into GaAs. This includes a determination of the annealing kinetics and accurate measurements of carrier concentration and mobility profiles after annealing.

Achievements:

Description:

Work has continued on this project via collaborations with the Universities of Lisbon and Bonn and also via interactions with CERN. In particular, progress has been made in predicting the atomic concentration profile from measurements of carrier concentration and mobility for silicon implants into GaAs.

The samples used in this study had an approximately uniform atomic profile, achieved using multiple implants of silicon at energies of 100 keV, 330 keV and 900 keV. The dose was chosen so that the atomic concentration was about 1 x 10 [-18] cm [-3] over a depth of about one micron. This was done in order that carrier concentrations would not vary significantly with depth following annealing. Also, it was important to abtain carrier concentrations of the order of 10 [-18] cm [-3] so that the effect of the zero bias depletion region at the air-semiconductor interface could be ignored.

After implantation, samples were encapsulated with silicon nitride and rapid thermally annealed between 800 deg.C and 950 deg.C for times of 5 to 30 seconds. Electrical profiles were measured using the automated Hall apparatus. The electron concentration profiles have three shallow peaks corresponding to the three ion energies used. However, there is very poor agreement between the peak positions and the ion ranges calculated using LSS theory. In contrast, an extremely good fit between theory and experiment occurs when TRIM is used.Profiles of the total concentration of ionised impurities were obtained for each sample using published tables of electron concentration and mobility as a function of compensation ratio. These profiles are identical in shape to the atomic profiles calculated using TRIM, but the concentration is of the order of 50% higher than the TRIM profiles, assuming the doses implanted are accurate. In order to check this, atomic profiles will be measured using an alternative technique such as SIMS.

This work is being extended to include the use of the electrochemical C-V technique to produce reliable carrier concentration depth profiles. These, together with the theoretical atomic profile and tabulated values of carrier concentration and mobility as a function of compensation ratio may be used to estimate the mobility profile. Results will be compared with data from Hall effect and resistivity measurements which we feel are more reliable, and, of course produce measured values of mobility.

Funding: EC, SERC


INTERDIFFUSION IN III-V HETEROSTRUCTURES

Aims:

The aim of this work is to characterize the interdiffusion in III-V heterojunction systems and to understand the mechanisms by which interdiffusion occurs.

Achievements:

Description:

Work has been extended in this area through collaborations with the Forschungszentrum Jülich who are performing TEM analysis on diffused samples, and with Université Montpellier II, who are using reflectance spectroscopy to study the higher confined states in diffused InGaAs/GaAs and GaAs/AlGaAs quantum wells. In addition we are also collaborating with British Telecom and BNREurope Ltd. to study interdiffusion in the InGaAsP material system.

Using ion implantation to introduce known quantities of the lattice constituents Ga and As into InGaAs/GaAs quantum wells has helped to show that the diffusion is governed by thermally created vacancies. This was achieved by studying the time dependence of the diffusion following ion implantation, and the variations in this dependence with different implanted ions. In order to compare the interdiffusion on the group III and group V sublattices, quantum wells grown in the InGaAsP material system were supplied by both British Telecom and BNR Europe Ltd. Different samples were designed, each of which had only a concentration gradient on either the group III or group V sublattice, so that the diffusion in the two sublattices could be measured independently. Using these materials we have shown that the diffusion on both sublattices are controlled by vacancies on each sublattice, and that the activation energy for interdiffusion is the same for both sublattices. However, for the group V sublattice the prefactor for diffusion is two orders of magnitude greater than that measured for group III diffusion in the InGaAs/GaAs system. In addition we have found that for anneal temperatures below 700deg.C there is a low activation energy process controlling the diffusion on the group V sublattice which we believe to be due to grown-in vacancies. This low activation energy process is important as it is responsible for significant diffusion occurring on the group V sublattice during the growth of devices such as lasers. This diffusion during growth can result in changes in the operating wavelength of semiconductor lasers from their designed values.

Rutherford backscattering and channelling of 1.5 MeV helium ions has also been used to study the diffusion in GaAsSb/GaAs quantum wells. Measurements of the GaAsSb/GaAs material system suggest that the diffusion is not a simple Fickian process, but that the diffusion is strongly dependent on composition.

Funding: SERC, DTI


ION BEAM SYNTHESIS OF SILICIDES

Aims:

This research is aimed at developing silicide technology for the next generation of integrated and optoelectronic circuits. It uses high dose ion implantation (ion beam synthesis) to fabricate high quality, heteroepitaxial Si/silicide/Si structures.

Achievements:

Over the last year the major achievements of this project have been:

Description:

Ion beam synthesis (IBS) is a technique which involves the implantation of high doses of energetic ions into a target (usually silicon). By implanting at elevated temperatures >300deg.C the target retains its crystallinity. The energy of the ion beam together with the intimate mixture of the target and implanted atoms allows the thermodynamic barrier to compound formation to be surmounted at much lower temperatures than for normal equilibrium growth. Moreover, the synthesised layer grows within the target facilitating the growth of heteroepitaxial structures. After implantation the layers are annealed to remove the residual radiation damage and to redistribute the implanted species into a well defined layer.

CoSi2:

Heteroepitaxial Si/CoSi2/Si are produced in silicon (n-type) by high dose cobalt implantation and subsequent annealing. These structures have applications in high speed interconnect technology, novel bipolar devices and infrared detectors. The resistivity of the layers at room temperature is ~10 u[[Omega]]cm. A Schottky barrier height of 0.64 + 0.01 eV with an ideality factor close to unity is recorded indicating that thermionic emission is the dominant conduction mechanism across the barrier. Implantation and diffusion of n-type dopants (As, P, Sb) causes a reduction in the barrier height by the formation of an n + region adjacent to the lower CoSi2 interface, facilitating tunnelling through the barrier. The implantation and diffusion of Sb causes an order of magnitude reduction in the defect density this is either a consequence of amorphisation and regrowth or a reduction of the interfacial mismatch by the presence of Sb atoms at the interface.

FeSi2:

FeSi2 can exist in two stable phases. Below 950deg.C the semiconducting [[beta]] phase is formed whereas above 950deg.C the metallic [[alpha]] phase is present. Experimental measurements on IBS [[beta]]FeSi2 layers indicate a direct band gap of 0.86 eV. This means that the material has the potential of being integrated into silicon optoelectronic devices. Photoluminescence measurements show a decrease in FWHM with increasing annealing time, a minimum of 3 meV is recorded after 920deg.C, 18 hours. Preliminary studies also indicate that the band-gap can be engineered by the implantation of small doses of cobalt. Moreover, when higher doses of Co are implanted prior to Fe the FeSi2 layer grows epitaxially using the CoSi2 layer as a seed.

SCAN IN THIS PHOTO

Figure 1.2. Direct bandgap measurement in [[beta]]FeSi2.

Funding: SERC (2 grants)


ION BEAM SYNTHESIS OF DIELECTRICS - SOI/SIMOX TECHNOLOGY

Aims:

The aims are threefold:

The latter has involved the preparation of substrates at Surrey, the fabrication of MOS structures at Southampton and detailed evaluation by colleagues at Liverpool.

Achievements:

Description:

The development of SOI/SIMOX technology has continued as a major activity during the year and the work received a boost by the award of a nine month extension to the current collaborative information engineering directorate project IED 1777.

Surrey has a unique capability to prepare thin film SIMOX structures using low energy O + implantation (40 keV to 90 keV) with pre- and background wafer heating up to a temperature of 680deg.C. In order to evaluate these structures a comprehensive study of process induced defects in this material and in SOI structures formed by sacrificial thermal oxidation of silicon overlayer in thick film SIMOX wafers has been undertaken. To do so a new defect etchant based upon K2Cr2O7 and Cu(NO2)3 has been developed which enables extended defects to be delineated in silicon films as thin as 500 Å. By using both defect etching andplan view transmission electron microscopy it has been possible to identify small stacking fault tetrahedra at the Si/SiO2 interface and to follow the evolution of oxidation induced stacking faults during sacrificial oxidation. A model has been proposed to describe the evolution of these defects which is based upon the capture of silicon interstitials emitted during thermal oxidation by the dislocations associated with the vacancy type tetrahedra. As these extended defects (in sacrificially thinned SIMOX) will have an adverse effect upon processing yields it is argued that the direct formation of thin film structures by low energy O + implantation is to be preferred and, potentially, is a lower cost process due to the benefits of scaling the O + implantation equipment.

Isotope marker experiments involving the implantation and SIMS depth profiling of 18 O in SIMOX structures has enabled the mass transport and isotope exchange between the buried oxide layer and SiO2 cap during high temperature annealing to be quantified. For example it is found that in thin film (1000 Å) structures up to 40% of the oxygen can exchange between cap and buried oxide layer. This mass transport, which is driven by a process of diffusion controlled precipitate growth (Oswald ripening), can be so pronounced in thin film structures that all of the implanted oxygen is gettered by the capping layer thus putting tight constraints on the processing window.

Funding: SERC, CAPS, Brazil


GROUP IV SEMICONDUCTOR HETEROSTRUCTURES

Aims:

Achievements:

Description:

The ability to form group IV semiconductor heterostructures makes possible the realisation of advanced devices with performances superior to comparable devices in bulk silicon. A number of new projects have been initiated in order to exploit these opportunities.

High quality, low defect density Si/SiGe/Si structures have been prepared by high dose Ge + ion implantation followed by amorphisation of the complete structure by Si + self ion implantation and low temperature regrowth. As regrowth is initiated deep within the substrate, end of range defects (EOR) are located remote from the active volume. Also the alloy/Si interfaces are graded and cross-sectional TEM analysis has revealed no extended defects in the vicinity of these interfaces. This low thermal budget process is known by the acronym "EPIFAB" (epitaxial fabrication across phase boundaries).

Thermal oxidation of SiGe alloy is problematic due to different diffusivities and heats of formation of the oxides. Detailed studies of dry and wet oxidation of Si0.5Ge0.5 material has led to physical models which describe the mass transport of Ge during the movement of the alloy/oxide interface. Oxide layers with compositions from SiO2 to a mixture of (SiO2 + GeO2) can be formed depending upon the choice of processing conditions.

The electrical characteristics of heterojunction bipolar transistors and test structures fabricated at Southampton (Dr P Ashburn) in strained SiGe MBE and CVD substrates have been determined. The laterial non-uniformity of the transistor parameters has been correlated with process related geometric non-uniformities. Junction ideality factors have been determined both for e/b and b/c junctions from measured Gummel plots.

A new grant has been awarded by the SERC involving studies of dopant activation and control of strain in SiGe HBTs with the aim of enhancing the manufacturability of these devices. This is a collaborative project with GPS, Cheney Manor.

Funding: SERC, British Council and EC (ERASMUS)


NON-DESTRUCTIVE CHARACTERIZATION OF MATERIALS AND DEVICES USING MICROSCOPE-SPECTROPHOTOMETRY

Aims:

To apply the technique of microscope-spectrophotometry (MSP) to the non-destructive characterization of semiconducting materials and devices. This involves the measurement of specular, near normal incidence reflectance, to which theoretical models are applied. These are fitted and optimised to the experimental reflectance data, so that structural and chemical parameters for the materials can be elucidated.

Achievements:

Description:

MSP has been used to characterize mineral specimens for many years, however, the technique has only been applied to the semiconductor industry in its most simplistic form. This has meant that until now, very simple structures or only superficial layers have been characterized using this technique. Using the theoretical models that we have developed has meant that MSP can now be applied to complex three dimensional, multi-layer structures. This is achieved by creating a theoretical optical model from the optical parameters of the constituents within the structure. This model is then fitted to the experimental reflectance data and optimised using a numerical inversion technique which varies the structural dimensions. Chemical parameters are varied semi-automatically. Using this technique, we have successfully measured and non destructively characterized a variety of complex multi-layer semiconducting materials.

Funding: SERC


COMPUTER SIMULATION OF DYNAMIC IMPLANTATION PROFILES IN SINGLE CRYSTAL MATERIALS

Aims:

The aim of this project is to produce a simulator to accurately predict the range profiles of the major dopants of silicon at intermediate dose levels.

Achievements:

Description:

A computer simulation package, CRYSTAL, is now complete, which is capable of predicting implantation range distributions of intermediate ion doses in single crystal and damaged silicon. Range profiles of low dose ions in single crystals are relatively simple to calculate, as the target structure does not change during the calculation. Range profiles of high dose ions in single crystals can often be simplified by assuming that the majority of the ions are implanted into amorphous material as the target is progressively damaged during the implantation. However, in the case of intermediate doses the rate at which the target becomes amorphous is very important in terms of the shape of the final profile; CRYSTAL is now capable of predicting these effects.

The program has been further modified to simulate the effects of a realistic surface topography. Input to the program, describing this topography, is available from the process simulator COMPOSITE and profile data can be returned to COMPOSITE for more accurate modelling. Full effects of surface sputtering and redeposition can be taken into account during the simulations.

Funding: SERC, DTI


MOLECULAR DYNAMICS MODELLING OF PARTICLE INTERACTIONS WITH SURFACES

Aims:

The aims of this study are to produce realistic simulations of the behaviour of surfaces under particle bombardment, making as few assumptions as possible about the system.

Achievements:

Description:

In the molecular dynamics simulator we make as few assumptions as possible. For example, we make assumptions about the interatomic forces based on sound experimental observation, but otherwise solve only Newton's laws of motion to calculate the atomic motions. We use a many body potential which gives the correct elastic constants for the medium we are simulating and correct bond angles for the multimeters. This ensures accurate modelling of both continuum and particulate properties. We make the assumption that the movements and forces are suitably high that quantum mechanical effects can be ignored.

Work currently is concerned with molecular impacts on graphite, diamond and silicon surfaces. Due to the weak inter-planar bonding of graphite the surface reacts in a completely different fashion to bombardment of silicon or diamond.

Funding: SERC


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7th March 1994