J.P.R. David

Publications (209)292.23 Total impact

• Article: Impact Ionization Coefficients in 4H-SiC by Ultra-Low Excess Noise Measurement

  J. E. Green, Wei Sun Loh, A. R. J. Marshall, B. K. Ng, R. C. Tozer, J. P. R. David, S. I. Soloviev, P. M. Sandvik
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  ABSTRACT: Photomultiplication and excess noise measurements have been undertaken on two 4H-SiC avalanche photodiodes (APDs) using 244-nm light and 325-nm light. The structures are APDs with separate absorption and multiplication regions having multiplication regions of 2.74 and 0.58 μm, respectively. Pure injection conditions in the thicker device permit the measurement of pure-hole-initiated photomultiplication and an excess noise factor. Ionization coefficients for both carrier types have been extracted from these data using a local model. The use of the excess noise factor to infer the value of the less readily ionizing coefficient α from pure hole injection measurements is more robust than direct extraction from mixed injection measurements. This is because mixed injection introduces uncertainty in the generation profile. We report a significant reduction of the electron ionization coefficient α at low fields. The more readily ionizing hole coefficient β remains very similar to prior work.
  IEEE Transactions on Electron Devices 04/2012; 59(4):1030 - 1036. · 2.32 Impact Factor
• Article: Short-Wave Infrared GaInAsSb Photodiodes Grown on GaAs Substrate by Interfacial Misfit Array Technique

  K.C. Nunna, Siew Li Tan, C.J. Reyner, A.R.J. Marshall, Baolai Liang, A. Jallipalli, J.P.R. David, D.L. Huffaker
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  ABSTRACT: We report GaInAsSb-based p - i - n photodiodes operating in the 2-2.4- #x03BC;m wavelength range grown on GaAs (100) substrates using the interfacial misfit (IMF) array technique. A zero-bias dynamic-resistance-area product of 260 #x03A9;cm² and a room temperature peak responsivity of 0.8 A/W (at 2 #x03BC;m) with an estimated maximum detectivity (D*) of 3.8 #x00D7;10¹⁰ cm Hz^1/2 W^-1 is obtained in the photodiodes at -0.2 V. These preliminary results of the IMF-based GaInAsSb detectors are comparable to similar detectors grown on native GaSb substrates demonstrating the potential of the IMF array growth mode to realize high-quality Sb-based infrared detectors on GaAs substrates.
  IEEE Photonics Technology Letters 02/2012; 24(3):218-220. · 2.19 Impact Factor
• Article: Effects of Dead Space on Avalanche Gain Distribution of X-Ray Avalanche Photodiodes

  R. B. Gomes, C. H. Tan, J. E. Lees, J. P. R. David, J. S. Ng
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  ABSTRACT: A random path length model for X-ray avalanche photodiodes (APDs) has been used to assess the effects of carriers' dead spaces and injection position on the avalanche gain distributions. Significant carrier's dead space, typically found in submicrometer avalanche regions, is found to reduce spread in the avalanche gain distribution, which is consistent with the reduced excess noise factors observed in conventional APDs. Mixed carrier-type injection increases the spread in the gain distribution along the avalanche region. The model was validated by comparing the results with experimental X-ray spectra from a $hbox{GaAs}/ hbox{Al}_{0.8}hbox{Ga}_{0.2}hbox{As}$ separate absorption and multiplication APD. Good agreement was achieved between measured and simulated data. Avalanche multiplication shifted the detected photo peak away from the system noise, thereby improving its energy resolution and signal-to-noise ratio.
  IEEE Transactions on Electron Devices 01/2012; 59(4):1063-1067. · 2.32 Impact Factor
• Article: Non-stoichiometric GaAsBi/GaAs (100) molecular beam epitaxy growth

  F. Bastiman, A. R. B. Mohmad, J. S. Ng, J. P. R. David, S. J. Sweeney
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  ABSTRACT: The growth of III-V bismuthides is complicated by the low incorporation efficiency of Bi in GaAs, leading either to the formation of metallic Bi droplets or low layer composition fractions. Typically growth is performed between 280 and 350 °C and at near stoichiometric Ga:As fluxes in order to encourage Bi incorporation. However most work reported to date also utilises As2 as the As overpressure constituent. It is found in this work that growth with As4 allows high Bi composition films with the standard 1:20 Ga:As4 beam equivalent pressure ratio (BEPR) utilised for higher temperature buffer layer growth. The Bi fraction versus Bi:As4 BEPR is found to be initially linear, until a maximum value is obtained for a given temperature after which the continued oversupply of Bi results in the formation of droplets.
  Journal of Crystal Growth 12/2011; 338:57-61. · 1.73 Impact Factor
• Article: InAs avalanche photodiodes for X-ray detection

  R B Gomes, C H Tan, P J Ker, J P R David, J S Ng
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  ABSTRACT: In this work we present the first InAs avalanche photodiode (APD) for X-ray detection and demonstrate the improved energy resolution due to its avalanche gain. Under the radiation from a 55Fe radioactive X-ray source the full width at half maximum (FWHM) at 5.9 keV peak reduces as the avalanche gain increases. The FWHM drops from 2.02 keV at an avalanche gain of 1.58 to 950 eV at a gain of 5.3. The FWHM was larger than the fano-limited value due to the noise from accompanying electronics and the increased leakage current introduced by the diode packaging.
  Journal of Instrumentation 12/2011; 6(12):P12005. · 1.87 Impact Factor
• Article: Development of high temperature AlGaAs soft X-ray photon counting detectors

  J E Lees, A M Barnett, D J Bassford, J S Ng, C H Tan, N Babazadeh, R B Gomes, P Vines, J P R David, R D McKeag, D Boe
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  ABSTRACT: New types of detectors based on the wide band gap material AlGaAs have been developed for soft X-ray spectroscopy applications. We report on the spectroscopic performance of simple p-i-n diodes and avalanche photodiodes (APDs). A number of diode types with different layer thicknesses have also been characterised. X-ray spectra from 55Fe and 109Cd radioactive sources show these diodes can be used for spectroscopy with promising energy resolution (1.0–1.25 keV) over a -30 to +90 °C temperature range. The temperature dependence of the avalanche multiplication process at soft X-ray energies in Al0.8Ga0.2As APDs was also investigated at temperatures from -20 to +80 °C. The temperature dependence of the pure electron initiated multiplication factor (Me) and the mixed carrier initiated avalanche multiplication factor (Mmix) were extracted from the X-ray spectra. The experimental results are compared with a spectroscopic Monte Carlo model for Al0.8Ga0.2As diodes from which the temperature dependence of the pure hole initiated multiplication factor (Mh) is determined. Monte Carlo simulations for the avalanche gain of absorbed X-ray photons have also been developed to study the relationship between avalanche gain and energy resolution for semiconductor X-ray avalanche photodiodes. The model showed that the distribution of gains, which directly affects the energy resolution, depends on the number of injected electron-hole pairs (and hence the photon energy), the relationship between the two ionization coefficients, and the overall mean gain. Our model showed that the conventional notion of APD gains degrading energy resolution significantly is incomplete. We compare the Monte Carlo simulations with experimental data from a number of different Al0.8Ga0.2As diodes.
  Journal of Instrumentation 12/2011; 6(12):C12007. · 1.87 Impact Factor
• Article: Impact Ionization Coefficients in

  J.S.L. Ong, J.S. Ng, A.B. Krysa, J.P.R. David
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  ABSTRACT: Electron and hole multiplication characteristics have been measured on a series of Al_0.52In_0.48P p⁺-i-n⁺ and n⁺-i-p⁺ homojunction diodes with nominal avalanche region thicknesses ranging from 0.22 to 1.03 μm. From these, the electron and hole impact ionization coefficients are deduced over an electric-field range from 530 to 990 kV/cm. The results suggest that the electron ionization coefficient is larger than the hole ionization coefficient, particularly at lower electric fields. Extremely low dark currents were obtained, and there was no evidence of any tunneling dark current even at electric fields of approximately 1.0 MV/cm. From these ionization coefficients, we deduce that the breakdown voltage in Al_0.52In_0.48P is almost 2.5× greater than that of GaAs and 4.5× lower than that of GaN.
  IEEE Electron Device Letters 12/2011; · 2.85 Impact Factor
• Article: Temperature Dependence of Leakage Current in InAs Avalanche Photodiodes

  Pin Jern Ker, A.R.J. Marshall, A.B. Krysa, J.P.R. David, Chee Hing Tan
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  ABSTRACT: Measurement and analysis of the temperature dependence of bulk and surface leakage currents in InAs avalanche photodiodes have been performed between 77 K and 290 K. At unity gain, SU-8 passivated InAs photodiodes have low dark current densities of 100 mA/cm² at 290 K and 150 nA/cm² at 77 K. An avalanche multiplication factor of 25 was measured at 13 V and 19.5 V at 290 K and 77 K, respectively. The photodiodes exhibit dynamic resistance-area products, calculated at 0.1 V of 34 Ω-cm² at 290 K and 910 MΩ-cm² at 77 K. Our analysis showed that between the temperatures of 200 K and 290 K, the bulk leakage current is proportional to n _i² whereas the surface leakage current is proportional to n _i from 77 K to 290 K, where n _i is the intrinsic carrier concentration. The activation energies deduced were 0.36 eV and 0.18 eV suggesting diffusion dominated bulk current and generation and recombination dominated surface current.
  IEEE Journal of Quantum Electronics 09/2011; · 1.88 Impact Factor
• Article: GaInNAsSb/GaAs Photodiodes for Long-Wavelength Applications

  Siew Li Tan, Shiyong Zhang, Wai Mun Soong, Yu Ling Goh, L.J.J. Tan, Jo Shien Ng, J.P.R. David, I.P. Marko, A.R. Adams, S.J. Sweeney, J. Allam
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  ABSTRACT: GaInNAsSb p-i-n photodetectors on GaAs substrates capable of detecting wavelengths up to 1550 nm with a reduced dark current are presented in this letter. Responsivities of 0.18 A/W at 1300 nm and 0.098 A/W at 1550 nm were achieved in devices with a ~0.5-μm-thick GaInNAsSb p-i-n epitaxial layer with 10% In, 4.08% N, and 4.4% Sb. The absorption coefficient (α) spectra show that α is intrinsically higher than that of the indirect-gap Ge layer but ~2.6 times lower than that reported for a In_0.53Ga_0.47As epitaxial layer at 1550 nm. The dark currents of the GaInNAsSb devices are found to be lower than not only those of the GaInNAs devices of a similar energy gap but also the state-of-the-art Ge/Si avalanche photodiodes. The lower dark currents in the GaInNAsSb devices compared with the GaInNAs devices can possibly be attributed to the reduction of defects in the Sb-containing epitaxial layer.
  IEEE Electron Device Letters 08/2011; · 2.85 Impact Factor
• Article: Avalanche Multiplication and Excess Noise in InAs Electron Avalanche Photodiodes at 77 K

  A.R.J. Marshall, P. Vines, P.J. Ker, J.P.R. David, Chee Hing Tan
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  ABSTRACT: The findings of a study of impact ionization, avalanche multiplication and excess noise in InAs avalanche photodiodes at 77 K are reported. It is shown that hole impact ionization is negligible in practical devices which continue to operate as electron avalanche photodiodes, as they do at room temperature. A new electron ionization coefficient capable of modeling multiplication at 77 K is presented and it is shown that significant multiplication can be achieved in practical devices without excessive tunneling currents. The characteristic changes observed between room temperature and 77 K are discussed. This paper helps to demonstrate the potential for practical InAs electron avalanche photodiodes, operating cooled.
  IEEE Journal of Quantum Electronics 07/2011; · 1.88 Impact Factor
• Article: Avalanche Gain and Energy Resolution of Semiconductor X-ray Detectors

  Chee Hing Tan, R.B. Gomes, J.P.R. David, A.M. Barnett, D.J. Bassford, J.E. Lees, Jo Shien Ng
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  ABSTRACT: Realistic Monte Carlo simulations for the avalanche gain of absorbed X-ray photons were carried out in a study of the relationship between avalanche gain and energy resolution for semiconductor X-ray avalanche photodiodes (APDs). The work explored how the distribution of gains, which directly affects the energy resolution, depends on the number of injected electron-hole pairs (and, hence, the photon energy), the relationship between ionization coefficients, and the mean gain itself. We showed that the conventional notion of APD gains significantly degrading energy resolution is incomplete. If the X-ray photons are absorbed outside the avalanche region, then high avalanche gains with little energy resolution penalty can be achieved using dissimilar ionization coefficients. However, absorption of X-ray photons within the avalanche region will always result in broad gain distribution (degrading energy resolution), unless electrons and holes have similar ionization coefficients.
  IEEE Transactions on Electron Devices 07/2011; · 2.32 Impact Factor
• Source

  Available from: Thomas Edwin Vandervelde

  Article: Noise, Gain, and Responsivity in Low-Strain Quantum Dot Infrared Photodetectors With up to 80 Dot-in-a-Well Periods

  P. Vines, C.H. Tan, J.P.R. David, R.S. Attaluri, T.E. Vandervelde, S. Krishna
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  ABSTRACT: We present a systematic study of noise, gain, responsivity, and specific detectivity D* , in a series of low-strain dot-in-a-well (DWELL) quantum dot infrared photodetectors (QDIPs). The lattice-matched GaAs quantum wells and AlGaAs barriers in these devices prevent the accumulation of excessive strain and allow the growth of up to 80 DWELL periods. We show that the photoconductive gain in these QDIPs is inversely proportional to the number of periods, while the total quantum efficiency is proportional to the number of periods, meaning that the responsivity remains constant at a given mean electric field as the number of periods is varied. The dark current in each QDIP was also found to be constant at a given mean electric field.
  IEEE Journal of Quantum Electronics 06/2011; · 1.88 Impact Factor
• Article: Photoluminescence investigation of high quality GaAs1−xBix on GaAs

  A. R. Mohmad, F. Bastiman, J. S. Ng, S. J. Sweeney, J. P. R. David
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  ABSTRACT: Photoluminescence (PL) of GaAs0.97Bi0.03 alloy was measured over a wide range of temperatures and excitation powers. Room temperature PL with peak wavelength of 1038 nm and full-width-half-maximum of 75 meV was observed which is relatively low for this composition. The improved quality is believed due to reduced alloy fluctuations by growing at relatively high temperature. The temperature dependence of PL peak energy indicated significant exciton localization at low temperatures. Furthermore, the band gap temperature dependence was found to be weaker than GaAs. An analysis of dominant carrier recombination mechanism(s) was also carried out indicating that radiative recombination is dominant at low temperature.
  Applied Physics Letters 03/2011; 98(12):122107-122107-3. · 3.84 Impact Factor
• Article: InAlAs Avalanche Photodiode With Type-II Superlattice Absorber for Detection Beyond 2

  D.S.G. Ong, Jo Shien Ng, Yu Ling Goh, Chee Hing Tan, Shiyong Zhang, J.P.R. David
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  ABSTRACT: An avalanche photodiode (APD) using a InAlAs multiplication region and a type-II InGaAs/GaAsSb superlattice as the absorber (both lattice matched to InP) is reported. An optical and electrical characterization of the photodiode is performed. The APD exhibited an absorption cutoff wavelength of 2.5 μm, which is expected from the InGaAs/GaAsSb superlattice. A responsivity of 0.47 A/W (without gain) for the APD at a 2004-nm wavelength was demonstrated. The APD breakdown voltage showed a weak temperature dependence of ~40 mV/K, as a result of the excellent temperature stability in InAlAs.
  IEEE Transactions on Electron Devices 03/2011; · 2.32 Impact Factor
• Source

  Available from: unm.edu

  Article: Sensitivity of High-Speed Lightwave System Receivers Using InAlAs Avalanche Photodiodes

  D.S.G. Ong, M.M. Hayat, J.P.R. David, Jo Shien Ng
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  ABSTRACT: Calculations based on a rigorous analytical model are carried out to compare the sensitivity of optical receivers that use InP and In_0.52Al_0.48As avalanche photodiodes (APDs). The model includes the effects of intersymbol interference, tunneling current, avalanche noise and its correlation with the stochastic avalanche duration, dead space, and transimpedance amplifier noise. For a 10-Gb/s system with a bit-error rate of 10^-12, the optimum receiver sensitivity predicted for In_0.52Al_0.48As and InP APDs is -28.6 and -28.1 dBm, respectively, corresponding to a reduction of 11% in optical signal power for receivers using In_0.52Al_0.48As APDs. Thus, considering overall receiver sensitivity, the improvement offered by In_0.52Al_0.48As APDs over InP is modest.
  IEEE Photonics Technology Letters 03/2011; · 2.19 Impact Factor
• Article: Dark Current Mechanism in Bulk GaInNAs Lattice Matched to GaAs

  L.J.J. Tan, Wai Mun Soong, J.P.R. David, Jo Shien Ng
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  ABSTRACT: The reverse-bias current-voltage characteristics of a series of Ga_1-xln_xN_yAs_1-y diodes with bandgap of between 0.87 and 1.04 eV are reported. At low bias, diffusion and generation-recombination currents are dominant at high and low temperatures, respectively. At high reverse bias, the dark current is insensitive to changes in temperature, which is indicative of tunneling current mechanisms. We also observe an exponential dependence of the dark current with the electric field in the mid-bias range for all our diodes, which may be explained by the Poole-Frenkel effect.
  IEEE Transactions on Electron Devices 02/2011; · 2.32 Impact Factor
• Article: Electroluminescence Studies of Modulation p-Doped Quantum Dot Laser Structures

  N.F. Hasbullah, M. Hopkinson, R.R. Alexander, R.A. Hogg, J.P.R. David, T.J. Badcock, D.J. Mowbray
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  ABSTRACT: Electroluminescence (EL) measurements have been performed on InAs/InGaAs/GaAs quantum dot (QD) structures with varying amounts of modulation p-doping. Temperature-dependent EL measurements show a reduction of the integrated EL intensity (IEL) with increasing temperature but with the size of this reduction decreasing with increasing doping level. An increase in the activation energy controlling the EL quenching is found with increasing p-doping. This is attributed to an increased coulombic attraction between the extrinsic holes and injected electrons. At room temperature and low injection current, a superlinear dependence of the IEL on the injection current is observed. This superlinearity decreases as the p-doping increases and this behavior indicates a reduction in the amount of nonradiative recombination. This reduction is believed to be caused by the saturation of nonradiative centers and/or reduced escape of electrons to the GaAs barrier due to the increased confinement potential.
  IEEE Journal of Quantum Electronics 01/2011; · 1.88 Impact Factor
• Conference Proceeding: Avalanche gain distribution of X-ray avalanche photodiodes

  R.B. Gomes, C.H. Tan, J.E. Lees, J. P.R. David, J.S. Ng
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  ABSTRACT: Realistic Monte Carlo simulations have been performed to study the direct effect of avalanche gain distribution on the energy resolution of X-ray APDs. The dependence of the gain distribution on the incident photon energy, detector material pair creation energy, ionization coefficient ratio, dead space, position of charge injection and the mean gain itself are analyzed. The results suggest that the distribution of avalanche gain narrows significantly when the number of carriers generated per photon increases. Initiation of the impact ionization process with carriers’ with the higher ionization coefficient as well as the presence of dead space were also found to narrow avalanche gain distribution.
  Nuclear Science Symposium and Medical Imaging ConferenceNuclear Science Symposium and Medical Imaging Conference, Valencia, Spain; 01/2011
• Conference Proceeding: Development of AlGaAs avalanche diodes for soft X-ray photon counting

  J. E. Lees, A. M. Barnett, D. J. Bassford, J. S. Ng, C. H. Tan, J. P. R. David, N. Babazadeh, R. B. Gomes, P. Vines, R. D. McKeag, D. Boe
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  ABSTRACT: We report on the performance of avalanche photodiodes (APDs) based on the wide band gap material AlGaAs which have been developed for soft X-ray spectroscopy applications. A number of diode types with different layer thicknesses have been characterised. The temperature dependence of the avalanche multiplication process at soft X-ray energies in Al_0.8Ga_0.2As APDs was investigated at temperatures from +80°C to -20°C. X-ray spectra from a ⁵⁵Fe radioactive source show these diodes can be used for spectroscopy with promising energy resolution (0.9-2.5keV) over a wide temperature range. The temperature dependence of the pure electron initiated multiplication factor (M_e) and the mixed carrier initiated avalanche multiplication factor (M_mix) were experimentally measured. The experimental results are compared with a spectroscopic Monte Carlo model for Al_0.8Ga_0.2As diodes from which the temperature dependence of the pure hole initiated multiplication factor (M_h) is determined.
  Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), 2011 IEEENuclear Science Symposium and Medical Imaging Conference (NSS/MIC), 2011 IEEE; 01/2011
• Conference Proceeding: GaAs P-I-N Diode for Room Temperature Soft X-Ray Photon Counting

  J. S. Ng, P. Vines, R.B. Gomes, N. Babazadeh, J. E. Lees, J.P.R. David, C. H. Tan
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  ABSTRACT: GaAs p-i-n diodes for room temperature soft X-ray photon counting were produced using a custom-designed wafer. The wafer was grown by molecular beam epitaxy on n+ GaAs substrate. It consists of a 300nm n-GaAs layer, 3 μm of i-GaAs layer, and 210nm of p-GaAs layer (top). The p-GaAs layer was thin to minimise loss of X-ray generated electrons to recombination. Circular mesa diodes were fabricated from the wafer using standard photolithography and chemical etching, which used phosphoric acid and sufuric acid solutions sequencially. The mesas were passivated using SU-8, a material normally used as negative photoresist. The chemical etchants and mesa passivation material were chosen following trials that used reverse leakage current of the diodes as assessment criteria. Metal contacts to the p- and the n-layers were deposited prior to mesa etch and passivation. Four different sizes of diodes were available at the end of the fabrication process, although only two of the larger-sized diodes were wire-bonded in the subsequent packaging into TO-cans. Reverse current-voltage characteristics of the diodes were measured at room temperature before and after the TO-can packaging. Comparison of the data showed increase in both dark current magnitude and variation after the packaging step, which has yet to be optimized. Nevertheless X-ray photon counting tests of these diodes at room temperature were carried out using a 185 MBq 55Fe radioisotope source with characteristic peaks at 5.9 and 6.4 keV. The TO-can was placed ~ 2 cm away from the source. The spectra obtained from some of the diodes show a peak ~ 6 keV. The detrimental effects of the diodes dark current on the spectra were observed. The higher the diodes dark current, the broader the main peak at ~ 6 keV. The results however clearly show that epitaxially grown GaAs mesa diodes are promising for room temperature soft X-ray photon counting.
  Nuclear Science Symposium and Medical Imaging ConferenceNuclear Science Symposium and Medical Imaging Conference, Valencia, Spain; 01/2011