N. G. Paulter

Carrier lifetimes in ion-damaged GaAs

Photoluminescence excitation correlation spectroscopy has been used to measure the dependence of carrier lifetime on the H+ ion implantation dose in GaAs. For doses greater than 1×10^12 cm^−2 the carrier lifetime is inversely proportional to the ion dose. The minimum lifetime measured was 0.6±0.2 ps for a dose of 1×10^14 cm^−2. Most important, there is no sign of saturation of carrier lifetime with ion dose down to this lifetime, thus still shorter lifetimes should be achievable with increased ion dose.

Optoelectronic measurements of picosecond electrical pulse propagation in coplanar waveguide transmission lines

Observations are presented concerning the effects of coplanar waveguide transmission lines on the propagation of picosecond electrical pulses using an optoelectronic time-domain measurement technique. Effects of various test structure design factors such as substrate thickness, thickness of transmission line metallization, discontinuity spacing, ground plane width, pulser/sampler line length, and pulser/sampler geometry on picosecond electrical pulse propagation in microwave/millimeter wave coplanar waveguide transmission lines are discussed, and schemes for minimizing the adverse effects of each of the above factors are provided. >

Observed circuit limits to time resolution in correlation measurements with Si‐on‐sapphire, GaAs, and InP picosecond photoconductors

We report cross‐correlation measurements of the response of photoconductor pulsers and sampling gates excited by a femtosecond laser. The photoconductors were fabricated in microstrip transmission line structures on Si‐on‐sapphire, semi‐insulating GaAs, and semi‐insulating InP wafers. The photoconductor sampling gates were ion beam damaged to produce short carrier lifetimes (<3 ps in one case). Damage was introduced with 6 MeV 20Ne on the Si‐on‐sapphire, 2 MeV 2H on the GaAs, and 2 MeV 4He on the InP. Doses in the range 1012–1015 cm−2 were used. Our results show circuit limits to the time resolution in correlation measurements from two sources: (a) RC time constants due to photoconductor gap capacitance and transmission line characteristic impedance and (b) dispersion in microstrip transmission lines.

Excitation and Fe concentration dependences in the impulse photoconductance of InP:Fe

We report impulse response measurements on InP:Fe photoconductors excited by laser and electron beam radiation. Measurements are reported on crystals with Fe concentrations from 2×1015 cm−3 to 4×1016 cm−3 and with excited electron‐hole‐pair densities of ∼1012 cm−3 and 9×1017 cm−3. Measured signal decays are purely exponential in character, and decay times are inversely related to Fe concentration. No long‐lived tails are observed. Decay times show no dependence on excitation level for excited carrier concentrations that are well above and well below the Fe concentrations. The magnitude of the photoresponse indicates that electrons and not holes are the primary current carriers. The data suggest that for impulse excitation photoconductance decay in InP:Fe is due to trap‐assisted recombination of electrons and holes at the Fe sites, with a rate determined by the species with the slower capture rate.

Integrated picosecond photoconductors produced on bulk Si substrates

We report optoelectronic cross‐correlation measurements of the response of photoconductor pulsers and sampling gates formed on Si wafers. These photoconductors were fabricated with standard integrated circuit fabrication techniques followed by shadow‐masked ion beam irradiation. Successful ion beam irradiations were performed with 2 MeV 2H, 6 MeV He, and 30 MeV O with doses of 1015 ion/cm2. Deep damage was necessary to eliminate long‐lived background currents in the cross correlations. Carrier lifetimes of 96, 47, and 29 ps were observed in photoconductors with carrier mobilities of ∼250 cm2/Vs.

Fabrication of high-speed GaAs photoconductive pulse generators and sampling gates by ion implantation

An ion-implantation technique used to create high-speed photoconductive devices in semi-insulating gallium arsenide (GaAs) is described. The effects of electrical contacts, GaAs substrate material, and various implant parameters on device performance are presented. The best measured performance characteristics of sampled (correlation) waveforms are: full-width-at-half-maximum of 4.5 ps, rise time (10 to 90% of full amplitude) of 3.2 ps, and signal-to-noise ratio of approximately 50 dB (integration time is 10 ms). >

Photoconductor pulse generators and sampling gates for characterization of high-speed devices and transmission lines

We describe photoconductive semiconductor devices developed for application in diagnostics of high-speed electronic devices and circuits. Both pulse generation and sampling functions are provided by these ultrafast photoconductors. The photoresponse of different semiconductor materials (GaAs, InP, Si) that have been ion bombarded (Ar, H, He, Ne, 0, Si) was investigated and characterized. Response times as short as 1 picosecond have been observed. High frequency propagation characteristics of microstrip and coplanar waveguide transmission lines have been studied and modelled. Application of this measurement technique to the characterization of a microwave GaAs transistor is presented.

GaAs Photoconductors to Characterize Picosecond Response in GaAs Integrated Devices and Circuits

Optoelectronic techniques that use femtosecond lasers and promise the precise measurement of transient response in high-speed electronic devices and circuits have been under study for several years. We have investigated on-wafer electrical-impulse generation and sampling using femtosecond-laser-excited GaAs photoconductors. This approach is applicable to any transmission line structure, it is directly integrable, noninvasive, jitter-free, and it is applicable to both microwave and digital circuits.

On-Chip, Picosecond, Electrical-Characterization Measurements For Si Integrated Circuits

We have developed integrated photoconductors for picosecond pulse generation and sampling on Si integrated circuits. Here we describe the device implementation.

InP:Fe Picosecond Photoconductors

We have measured the impulse response of InP:Fe photoconductors to mode-locked dye laser. excitation. Bias, excitation, and Fe concentration dependence of the impulse response are reported. Linearity of response over two decades in both bias and excitation and independence of transient-response pulse width to both bias and excitation are demonstrated. Correlation is observed between Fe concentration and transient response pulse width. Finally, optical electronic autocorrelation of impulse response consistent with the sampling oscilloscope measurements is reported.