H. Thomas

Cardiff University, Cardiff, Wales, United Kingdom

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Publications (37)51.78 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: The reliability of AlGaInP double-heterostructure (DH) light-emitting diodes (LEDs) operating typically at 600 nm has been studied. To investigate degradation, accelerated aging at ambient temperatures of 50, 75 and 125°C has been carried out for over 5000 h. The activation energy of homogeneous degradation was determined to be 0.8 eV and an extrapolated half-life in excess of 106 h was estimated at an ambient temperature of 20°C. Copyright © 2000 John Wiley & Sons, Ltd.
    Quality and Reliability Engineering 01/2000; 16(1):45-49. · 0.99 Impact Factor
  • Y H Aliyu, D V Morgan, H Thomas
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    ABSTRACT: A technique for the rapid evaluation of the optical performance of wafers grown for fabricating visible-light-emitting diodes (LEDs) is presented. The technique is simple, non-destructive and can be used to qualify wafers prior to the relatively expensive device-processing stage. The technique resolves the problem resulting from the lack of correlation between photoluminescence measurements on as-grown materials and electroluminescence on the final structures. The lack of such correlation has been a pertinent problem for the material growth industry. The materials used for this investigation are those grown specifically for the fabrication of high-performance LEDs.
    Measurement Science and Technology 12/1998; 8(4):437. · 1.44 Impact Factor
  • D W Davies, D V Morgan, H Thomas
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    ABSTRACT: The effect of simultaneously depositing In metallization on n-GaAs using a novel ion-assisted deposition (IAD) technique has been investigated. Using current-voltage, specific contact resistance and secondary ion mass spectroscopy measurements, the contacts were compared to conventional contacts fabricated using thermal evaporation. Ion-mixed contacts fabricated with an ion dose of 2.8 × 1018 ion cm-2 and ion energy of 1 keV exhibited a lower specific contact resistance of 3 × 10-6 cm-2 at a lower annealing temperature of 375 °C compared to conventionally fabricated contacts. For all ion doses the annealing time and temperature which gave the minimum specific contact resistance remained unchanged. SIMS analysis also confirmed that the In deposited using the ion deposition technique formed a graded junction prior to annealing. After annealing both ion-mixed and thermally evaporated contacts had formed the graded junction and were electrically comparable.
    Semiconductor Science and Technology 12/1998; 14(7):615. · 1.92 Impact Factor
  • J K Luo, H Thomas, N M Pearsall
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    ABSTRACT: Plasma-etched p-type indium phosphide has been studied by I-V, C-V and deep level transient spectroscopy in the form of Schottky diodes. Plasma etching caused an increase in the barrier height of Schottky diodes. Four majority hole defects were found to be induced by plasma etching, and were distributed within 5000 AA of the surface. Two dominant hole defects H3 and H4 were simply annealed at approximately 100 degrees C for 10 minutes. These defects were found to be similar to those induced in InP by high energy electron or proton irradiation. This implies that sputtering and related pre-processing techniques may induce severe problems for surface devices due to the creation of defects.
    Semiconductor Science and Technology 12/1998; 7(1):168. · 1.92 Impact Factor
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    ABSTRACT: The electrical properties of as-grown low temperature (LT) GaAs grown at 200-300 degrees C have been investigated in the temperature range of 100-400 K. It was found that the resistivity of the LT GaAs layer increased as the growth temperature was increased from 200 degrees C to 300 degrees C. Correspondingly, over the same growth temperature range, the breakdown field decreased from 320 kV cm-1 to 80 kV cm-1, yet was more than one order of magnitude higher than that of semi-insulating GaAs. The breakdown voltage VBD was found to increase as the measurement temperature was decreased, differing from the behaviour of conventional avalanche breakdown. The transport properties of LT GaAs were characterized by hopping conduction at low electric field and low temperature. The particular properties of the as-grown LT GaAs layers suggest a useful application as in insulator in GaAs field effect transistors (FETS). The predominant ohmic behaviour resulting from hopping conduction, produces a uniform field in the material, which prevents the breakdown at the gate edge of the FETS or at the surface. The high breakdown strength suggests enhanced performance in power devices.
    Semiconductor Science and Technology 12/1998; 9(12):2199. · 1.92 Impact Factor
  • H Thomas, J K Luo
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    ABSTRACT: Admittance spectroscopic assessment of Au/p-InP structures has revealed two new hole defects HD1 and HD2, following 1 MeV electron radiation of fluence 1016 cm-2. These defects were found to be different from the previously reported hole defects H3 and H4, and were responsible for the removal of most carriers, hence the increase of the series resistance and the degradation of solar cell efficiency. The defect HD1 showed a recovery threshold annealing temperature of approximately 70 degrees C and was completed by 100 degrees C after 10 minutes annealing. This annealing behaviour corresponded to the recovery of the removed carriers.
    Semiconductor Science and Technology 12/1998; 8(4):608. · 1.92 Impact Factor
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    ABSTRACT: Low-temperature (LT) grown GaAs in either an as-grown or annealed condition has been used as a gate dielectric for GaAs field effect transistors (FET). It was found that both types of LT GaAs layers can effectively passivate the device surface states, eliminate hysteresis and light induced effects, and dramatically increase the breakdown voltage, , between source and drain contacts of the FETs. was found to increase on cooling the as-grown LT GaAs FET with a low activation energy, typically for the corresponding reverse bias gate current. However, for the annealed LT GaAs FET, decreased with decreasing temperature and the fixed reverse bias gate current showed a large activation energy . A large gate-lag and kink effect was observed from the annealed LT GaAs FETs, but was insignificant for the as-grown LT GaAs FETs. The superior performance of the as-grown LT GaAs FETs is attributed to the high degree of hopping conductivity found, and the high breakdown characteristic of the as-grown LT GaAs. The high density of defects and the trap-like nature of the As/GaAs clusters in annealed LT GaAs layer are believed to be responsible for the transient behaviour and large values of activation energy found in the annealed LT GaAs FETs.
    Semiconductor Science and Technology 12/1998; 11(9):1333. · 1.92 Impact Factor
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    ABSTRACT: The effects of growth temperature and subsequent annealing temperatures on the electrical properties of the low temperature (LT) grown GaAs have been investigated. It was found that the resistivity of the as‐grown LT‐GaAs layer increased with increasing growth temperature, but was accompanied by a reduction of breakdown voltage over the same temperature range. Thermal annealing of the samples caused the resistivity to rise exponentially with increasing annealing temperature T A , giving an activation energy of E A =2.1 eV. The transport of the LT‐GaAs layers grown at T g ≤250 °C was found to be dominated by hopping conduction in the entire measurement temperature range (100–300 K), but following annealing at T A ≳500 °C, the resistivity‐temperature dependence gave an activation energy of ∼0.7 eV. The breakdown voltage V BD , for as‐grown LT‐GaAs was enhanced on lowering the measurement temperature, but conversely, decreased over the same temperature range following annealing at T A ≳500 °C. The hopping conduction between arsenic defects, or arsenic clusters in annealed samples, is believed to be responsible for the observed electrical breakdown properties. Since the resistivities of the as‐grown LT‐GaAs layers are dependent, solely, on the excess arsenic, which in turn depends on the growth temperature, then the resistivities obtained can be used as a measure of the growth temperature. © 1996 American Institute of Physics.
    Journal of Applied Physics 05/1996; · 2.21 Impact Factor
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    ABSTRACT: Low temperature (LT-) grown GaAs has been used as a dielectric in a metal/ dielectric/semiconductor structure, and its capacitance behavior has been investigated by C–VB} and admittance spectroscopy. The C-VB} measurement revealed a barrier height of 0.40 eV at the interface of the LT- and n-GaAs. The capacitance-temperature profile shows a step decrease in capacitance, accompanied by a maximum conductance as the measurement temperature was decreased. The detailed investigation shows that this anomalous C–T behavior is caused by the increase of resistivity of the LT-GaAs, which leads to the formation of a metal/insulator/semiconductor structure at low temperature. This result has an impact on the application of the LT-GaAs, because it introduces a frequency dispersion to the device characteristics.
    Journal of Electronic Materials 01/1996; 25(12):1832-1836. · 1.64 Impact Factor
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    ABSTRACT: Reactive thermally evaporated transparent conducting indium tin oxide (ITO) layers are used as window material and for current spreading layers on AlGaInP light emitting diodes (LEDs). The sheet resistance of the ITO films deposited is of the order of 4.5 Ω/□ with up to 90% transmission in the visible region of the spectrum. In all cases, the devices incorporating the ITO layer gave a marginally greater output (10%) than the standard non-ITO devices
    Electronics Letters 01/1996; · 1.04 Impact Factor
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    ABSTRACT: Low turn-on voltage AlGaInP/GaInP surface emitting LEDs have been achieved using a thermally evaporated transparent conducting Indium Tin Oxide (ITO) layer. The devices have lower forward series resistance (1-3) ohms compared to standard Au/Zn devices which have a series resistance greater than 5 ohms. The ITO/AlGaInP LEDs emit orange light, with a peak wavelength of 600 nm and full width at half maximum (FWHM) of 15 nm. A forward voltage of typically 1.70 V at 20 mA was obtained. Variations in the thicknesses of the cladding and GaAs cap layer thicknesses did not cause any significant change in the device turn-on voltages. Evidence of reduced junction heating has been observed
    High Performance Electron Devices for Microwave and Optoelectronic Applications, 1995. EDMO., IEEE 1995 Workshop on; 12/1995
  • Y.H. Aliyu, D.V. Morgan, H. Thomas
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    ABSTRACT: Reactive thermally evaporated transparent conducting Indium-Tin-Oxide (ITO) layers are used as window material and current spreading layer on AlGaInP Light Emitting Diodes (LEDs). The sheet resistance of the ITO films deposited is of the order 4.5 ohm/sq with up to 90% transmission in the visible region of the spectrum. Optimum thickness of the p and n-type AlGaInP cladding and the top p<sup>+</sup>GaAs epitaxial layers were determined. The LEDs fabricated emit light with a peak wavelength (λp) at 600 nm and Full Width at Half Maximum (FWHM) of 15 nm. A forward voltage of 1.71 V at 20 mA was obtained. In all cases the devices incorporating the ITO layer gave a marginally greater output (10%) than the standard non-ITO devices
    High Speed Semiconductor Devices and Circuits, 1995. Proceedings., IEEE/Cornell Conference on Advanced Concepts in; 09/1995
  • H. Thomas, J. K. Luo
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    ABSTRACT: Electrical characterization has been carried out on electron irradiated InP grown by metal‐organic chemical‐vapor deposition (MOCVD) and liquid encapsulated Czochralski (LEC), through I–V (–T), C–V, deep level transient spectroscopy (DLTS) and admittance spectroscopy measurements and the resistance to electron radiation for these two materials has been compared. It was found that MOCVD‐InP was more resistant to electron radiation than LEC‐InP, as demonstrated by the lower carrier removal rate and change of series resistance in the MOCVD‐InP diodes as a result of electron radiation. The introduction rates for the dominant hole defects H3 and H4 and for additional electron defect states were found to be similar for both materials, but were insufficient to explain the degree of degradation of solar cell efficiency incurred by these known defects. A new defect, HD1, has been found to be responsible for the high carrier removal rate and the introduction of a large series resistance which accounts for the difference of radiation hardness between these two materials. The results again show that the dominant irradiation defects in InP are not the defects H3 and H4 as is usually accepted, but the new found defect HD1, which was undetected by the DLTS technique. © 1995 American Institute of Physics.
    Journal of Applied Physics 02/1995; · 2.21 Impact Factor
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    ABSTRACT: Low temperature grown GaAs has been fabricated containing a limited amount of excess arsenic. The material has a low conductivity in the order of 100KΩ cm, due to hopping in a deep donor band. This σ-LT-GaAs was grown reproducibly by using the lattice mismatch as the primary parameter for substrate temperature calibration. Breakdown fields, in the order of 100kV/cm, are observed for planar structures and increased at low measurement emperatures. Low hopping conductivity and high breakdown field are also observed in the lossy dielectric metal-insulator-semiconductor field-effect transistor device using σ-LT-GaAs as a surface layer. The record radio frequency power density of 4.0W/mm at 77K is extracted from the dc output characteristics.
    Journal of Electronic Materials 01/1995; 24(7):913-916. · 1.64 Impact Factor
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    ABSTRACT: The post‐growth annealing effects on the electrical properties of low temperature (LT‐) GaAs grown by molecular beam epitaxy have been investigated. It was found that the resistivity of the LT‐GaAs layer increased exponentially with annealing temperature T A , resulting in an activation energy of 2.1 eV. This activation energy is related to the activation energy of arsenic precipitation. Based on hopping conduction theory, an As cluster density N T , has been estimated from the resistivities of the LT‐GaAs layers. The change of density of arsenic clusters with T A , was found to be of the form N T =N T0  exp(-T/T 0 ), in agreement with values obtained by transmission electron microscopy measurements. The breakdown voltage of the LT‐GaAs layer remained almost unchanged as T A was increased up to 650 °C, but the breakdown characteristic became soft. The formation of As clusters is held responsible for the soft breakdown of the LT‐GaAs layer after annealing.
    Applied Physics Letters 07/1994; · 3.52 Impact Factor
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    ABSTRACT: The effect of thermal treatment on the electrical properties of Al <sub>0.48</sub>In<sub>0.52</sub>As/InP materials grown by molecular beam epitaxy (MBE) has been investigated using Schottky barrier structures. It was found that the intrinsic defects incorporated in MBE-AlInAs layers cause an excess tunnelling current at forward and reverse bias, and are responsible for the low barrier height of Schottky diodes. Thermal annealing at T<sub>A</sub>>500°C annihilates the major defects in the AlInAs layer, which results in a reduction of the excess tunnelling current and enhancement of the barrier height. These results therefore, demonstrate a method to overcome the shortcomings introduced by defects and to improve the electrical properties of MBE-AlInAs materials
    Indium Phosphide and Related Materials, 1994. Conference Proceedings., Sixth International Conference on; 04/1994
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    ABSTRACT: An as-grown GaAs layer grown by molecular beam epitaxy at low temperature (LT-) has been used as an insulator for power GaAs field effect transistors (MISFETs). It was found that the LT-GaAs layer successfully passivates the surface, of GaAs-FETs, eliminates the hysterisis of drain-source current, and improves the breakdown behaviour. The breakdown voltage, V<sub>BD</sub>, for MISFETs is as high as 40-50 V for 2 μm spacing between the gate and drain contacts, and is independent of the doping-thickness product. V<sub>BD</sub> also exhibits an increase on decrease of temperature, and suggests a high breakdown voltage in the high frequency domain. It is the particular electrical properties of the as-grown LT-GaAs layer which are found to be responsible for the superior properties of the LT-GaAs MISFETs. The LT-GaAs layer exhibits ohmic behaviour which leads to a linear distribution of electric field between the gate and drain. The dominant hopping conduction eliminates the concentration of injected electrons, while the high breakdown field of the LT-GaAs layer ensures a high breakdown voltage of LT-GaAs MISFETs
    Power Semiconductor Devices and ICs, 1994. ISPSD '94. Proceedings of the 6th International Symposium on; 01/1994
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    ABSTRACT: Al 0.48 In 0.52 As layers grown on n<sup>+</sup>‐InP substrates by molecular beam epitaxy (MBE) and metal‐organic chemical‐vapor deposition (MOCVD), as a function of substrate temperature, have been characterized by current‐voltage‐temperature, capacitance‐voltage, deep‐level transient spectroscopy, and admittance spectroscopy measurements. It was found that for diodes formed on MOCVD‐AlInAs the current in forward bias is dominated by thermionic emission and the reverse current by recombination through the residual midgap states; whereas, for MBE‐AlInAs diodes, the respective currents are dominated by defect‐assisted tunneling at low forward and reverse biases. Schottky barrier heights were found to decrease with decreasing growth temperature. Three defect levels E1, E2, and E3 were observed in both material systems, and their densities were found to increase rapidly from ∼10<sup>12</sup> to ∼10<sup>16</sup> cm<sup>-3</sup> as the growth temperature decreased from 740 to 500 °C. The decrease of barrier height and the appearance of the defect‐assisted tunneling current at low bias were found to correlate with the increase of defect density. The high density of defects may be responsible for the low barrier heights and higher leakage currents previously observed in AlInAs/InGaAs/InP high electron mobility transistors though MOCVD growth provided high quality AlInAs layers.
    Journal of Applied Physics 01/1994; · 2.21 Impact Factor
  • J. K. Luo, H. Thomas
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    ABSTRACT: The influence of the deposition of indium tin oxide (ITO) on the electrical properties at n- and p-InP junctions have been investigated by current-voltage-temperature and capacitance-voltage measurements. It was found that the formation of the ITO layer on n-type InP substrates causes the reduction of the barrier height and subsequently forms an ohmic contact. The ITO layer on p-type substrates increases the barrier height by 200–300 meV, and causes a defect-assisted tunneling at low forward bias. The results, therefore, can be explained by the introduction of process induced donor-like defects, with the formation of a thin n+-layer in the near-surface, decreasing the barrier height for n-substrates and increasing the barrier height for p-substrates. These results support the buried n+/p-junction model for ITO/p-InP solar cell structures.
    Journal of Electronic Materials 10/1993; 22(11):1311-1316. · 1.64 Impact Factor
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    ABSTRACT: Changes in the performance of epitaxial InP homojunction solar cells after irradiation and annealing are discussed, for 1 MeV electron and 1 MeV proton irradiation. Material samples have been investigated in order to characterise the irradiation induced defects. Admittance spectroscopy has identified a defect, HD1, which cannot be observed by DLTS and which is believed to influence carrier concentration in the p-type material. The relationship of the defect characterisation to thermal annealing of the cells, both at room and elevated temperature, is considered
    Photovoltaic Specialists Conference, 1993., Conference Record of the Twenty Third IEEE; 06/1993

Publication Stats

128 Citations
51.78 Total Impact Points

Institutions

  • 1998–2000
    • Cardiff University
      • School of Engineering
      Cardiff, Wales, United Kingdom
  • 1988–1998
    • University of Wales
      Cardiff, Wales, United Kingdom
  • 1991
    • The Newcastle upon Tyne Hospitals NHS Foundation Trust
      Newcastle-on-Tyne, England, United Kingdom