Stephen E. Ralph

Trap-enhanced electric fields in semi-insulators : the role of electrical and optical carrier injection

We report extremely large field enhancement near the anode of an electrically biased metal/semi‐insulator/metal structure. The large anode field results from a trap‐enhanced space‐charge region and is large enough to cause injection of holes at the anode. Our numerical simulations confirm this interpretation and show that for typical semi‐insulating GaAs, large trap‐enhanced fields (TEF) are to be expected. The TEF effect, contrary to that observed in doped materials, is enhanced by optical injection of carriers near the anode, and can be exploited for the efficient generation of ultrafast THz radiation.

Carrier dynamics of low-temperature-grown GaAs observed via THz spectroscopy

Subpicosecond electron lifetimes in low-temperature-grown GaAs are unambiguously demonstrated via far infrared terahertz spectroscopy. A systematic study of low-temperature-grown GaAs, as-grown and annealed, reveal carrier lifetimes to be directly related to the excess arsenic incorporation and anneal conditions. Contrary to previous observations, electron lifetimes of 600 fs (200 fs) are found in 0.25% (0.5%) excess arsenic GaAs. We attribute the observed differences to the far infrared interaction and the use of dilute photoexcitation densities which eliminate both band-edge resonance and high carrier densities effects. A simple model is developed to determine the relative electron mobility and to interpret the results. Additionally, time resolved differential spectroscopy reveals Drude-like behavior of the free carrier conductivity within 1 ps of excitation.

Ultrafast carrier dynamics and optical nonlinearities of low‐temperature‐grown InGaAs/InAlAs multiple quantum wells

Low‐temperature‐grown Be‐doped In0.53Ga0.47As/In0.52Al0.48As multiple quantum wells are investigated via wavelength‐dependent time‐resolved nonlinear absorption measurements. Annealed Be‐doped material, in contrast to annealed undoped material, is found to retain the carrier lifetime reduction induced by low‐temperature growth in this narrow‐gap material system. This is attributed to Be–As complexes which, in addition to producing high resistivity material, provide anneal–stable trap states. We also report that ultrafast band‐edge and photoinduced absorption effects can produce subpicosecond absorption recovery in material exhibiting much longer (20 ps) defect‐meditated carrier trapping.

THz spectroscopy and source characterization by optoelectronic interferometry

We demonstrate a new type of THz optoelectronic interferometer, by fully characterizing a recently developed THz source to beyond 6 THz, and by measuring the absorption coefficient of high‐resistivity GaAs from 1 to 5 THz. The two source THz interferometer is driven with two 4 mW beams of 60 fs dye‐laser pulses and produces interferograms with exceptional signal‐to‐noise ratios.

Trapping and recombination dynamics of low-temperature-grown InGaAs/InAlAs multiple quantum wells

We have observed a long-lived residual photoconductivity in low-temperature-grown (LT) InGaAs. These results have significant consequences for devices comprised of LT-InGaAs, other defect moderated materials, and standard-temperature-grown InGaAs. Our investigation utilizes time-resolved terahertz conductivity to quantify the trapping and recombination rates of LT Be-doped In0.53Ga0.47As/In0.52Al0.48As multiple quantum wells and bulk InGaAs. It is found that Be doping reduces the residual photoconductivity and increases the initial electron trapping rate. These results are in contrast to those observed via transient absorption studies, which suggest that these systems have returned to equilibrium after the initial transient. Furthermore, a 600 °C anneal increases both the trapping and recombination rate in all Be-doped samples.

Terahertz spectroscopy of optically thick multilayered semiconductor structures

Using freely propagating terahertz radiation, we have measured the complex dielectric constant of optically thick layered materials from 0.2 THz (6.6 cm−1) to 6 THz (200 cm−1). Transmission measurements of a CdTe–adhesive–Si structure have been successfully fitted to a theoretical model over the measurement range. The accuracy of the theoretical fit shows that the technique of time-domain spectroscopy offers advantages over other spectroscopic methods in the extreme far infrared below 200 cm−1. The signal-to-noise capability of our terahertz-spectroscopy technique permits accurate measurement of power transmission coefficients less than 0.001 (absorption coefficients >5000 cm−1) and index variations larger than λ(dn/dλ) > 44, as demonstrated by the accurate fit of our data through the Reststrahlen region of CdTe.

Large area, low voltage, transit time limited InGaAs metal semiconductor metal photodetectors

We demonstrate the efficacy of a method for reducing the effects of charge associated with the InP substrate‐InGaAs interface. By incorporating an AlInAs buffer layer we have dramatically improved the performance of InGaAs metal‐semiconductor‐metal‐photodetectors grown by molecular beam epitaxy. The buffer layer reduces the parasitic capacitances and also reduces the carrier concentration in the absorbing InGaAs layer. This allows complete depletion at low bias, giving rise to a more uniform field strength. The relatively large, 50‐μm‐diam detectors, operating at 5 V, have an extremely linear response, and a 3 dB power bandwidth of ∼30 GHz. Low frequency gain was not observed for bias voltages to 20 V.

Optical properties of GaN epitaxial films grown by low‐pressure chemical vapor epitaxy using a new nitrogen source: Hydrazoic acid (HN3)

We report results of our growth and characterization of GaN films using low‐pressure chemical vapor epitaxy with a new nitrogen source, hydrazoic acid (HN3). This growth technique allows for low‐temperature deposition, low III/V ratios, and increased deposition rates (up to ∼2–3 μm/h). The deposited films show Ga:N atomic ratios of 1±0.25 based on our x‐ray photoelectron spectroscopy analyses, and the He(II) UPS (ultraviolet photoelectron spectroscopy) spectra compare favorably with the semi‐ab initio calculations for the GaN valence bands and with the reported UPS data for single crystal GaN films. X‐ray and Raman spectra show deposited films crystallized in the expected wurtzite structure. We find these epitaxial films to be efficient light emitters in the blue or yellow region of the spectrum, depending upon growth conditions. Our photoluminescence time‐decay kinetics confirm the excitonic nature of the blue emission. Lastly, far infrared time‐domain spectroscopy shows the low carrier concentration of ...

Temporal and spectral characteristics of back-illuminated InGaAs metal-semiconductor-metal photodetectors

We report dramatic differences in the impulse response and wavelength dependence of back versus top illuminated In/sub 0.53/Ga/sub 0.47/As planar metal-semiconductor-metal devices. Via direct measurement of transit-time limited devices we identify the mechanisms involved and thereby allow the optimum design of multi-Gbit, high responsivity back-illuminated devices. We show that responsivities greater than 0.8 A/W are achievable with >8 GHz bandwidth for 50-/spl mu/m-diameter devices.

Hole dominated transport in InGaAs metal semiconductor metal photodetectors

We report the direct measurement of the intrinsic photocurrent response of both top and back illuminated planar metal–semiconductor–metal structures. We directly observe the temporal dynamics of the hole transport dependence on applied bias and the initial spatial distribution using a near infrared tunable femtosecond light source and electrically biased structures. The increased hole transit time of back illuminated structures can be completely understood in terms of the hole velocity and the initial spatial distribution of the carriers. Additionally, we report the fastest directly measured 50 μm diameter InGaAs photodetector with a 26 ps full width at half maximum.