Femtosecond differential transmission measurements on low temperature GaAs metal–semiconductor–metal structures

Keil, U. D.; Hvam, Jørn Märcher; Tautz, S.; Dankowski, S. U.; Kiesel, P.; Döhler, G. H.

doi:10.1063/1.119310

Cited by 19 publications

(5 citation statements)

References 12 publications

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“…For first characterization, differential transmission measurements are performed on a LT GaAs layer separated from the substrate by epitaxial lift-off. 10 These measurements show that the carrier life time increases to about 800 fs for T a ϭ600°C and is below our temporal resolution limit of about 200 fs for T a ϭ525°C. Other important measures of the substrate are the conductivity and the breakdown field.…”

Section: B Probe Tip Design and Optimizationmentioning

confidence: 99%

Fiber Coupled Ultrafast Scanning Tunneling Microscope

Keil¹,

Hvam²

1996

EQEC 96 1996 European Quantum Electronic Conference EQEC-96

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We report on a scanning tunneling microscope with a photoconductive gate in the tunneling current circuit. The tunneling tip is attached to a coplanar transmission line with an integrated photoconductive switch. The switch is illuminated through a fiber which is rigidly attached to the switch substrate. By using a firmly attached fiber we achieve an excellent reproducibility and unconstrained positioning of the tip. We observe a transient signal with 2.9 ps pulse width in tunneling mode and 5 ps in contact mode. The instrument is applied to investigating the mode structure on a coplanar waveguide. The measurements show that the probe works as a transient voltage detector in contact and a capacitively coupled transient field detector in tunneling mode. We do not measure the transient voltage change in the ohmic tunneling current. In this sense, the spatial resolution for propagating electrical pulses is better in contact mode than in tunneling mode.

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Section: B Probe Tip Design and Optimizationmentioning

confidence: 99%

Fiber Coupled Ultrafast Scanning Tunneling Microscope

Keil¹,

Hvam²

1996

EQEC 96 1996 European Quantum Electronic Conference EQEC-96

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“…3,13 A curve obtained with the fastscanning technique for a central laser energy of 1.545 eV, i.e., 25 meV above the band edge at 8 K, is shown in Fig. 1.…”

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confidence: 99%

Energy resolved ultrafast relaxation dynamics close to the band edge of low-temperature grown GaAs

Segschneider

Dekorsy

Kurz

et al. 1997

Applied Physics Letters

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We investigate the relaxation dynamics of photogenerated carriers in low-temperature grown GaAs by femtosecond pump-probe measurements. The carrier dynamics in the vicinity of the band edge is disentangled in a two-color technique. The filling of shallow bound states close beneath the band edge is resolved. A temporal delay in the occupation of these states as well as a large optical nonlinearity points towards microscopic potential fluctuations forming these states. © 1997 American Institute of Physics. ͓S0003-6951͑97͒00345-8͔In the last few years the interest in low-temperature grown ͑LT͒ semiconductors rapidly increased because of their unique electrical and optical properties and the resulting wide range of optoelectronic applications. The most promising material is LT GaAs grown at temperatures T g ϭ180°Ϫ300°C by molecular beam epitaxy ͑MBE͒. After annealing at T a ϭ400°Ϫ800°C LT GaAs exhibits an extraordinary high resistance in conjunction with a relatively high photocarrier mobility. 1 Important for THz devices are ultrashort carrier lifetimes in LT GaAs, which can be tailored into the sub-picosecond range. 2-4 All these properties are assigned to the large excess of As ͑1%-2%͒ incorporated during the growth process. In postgrowth annealed LT GaAs, As precipitates of a few nm in diameter nucleate. 5 The density of precipitates and point defects can be tailored with T g and T a and hence the electronic and optic properties can differ largely as a function of these temperatures. Two competing models have been proposed for explaining the properties of LT GaAs. In the defect model, EL2-like point defects stemming from excess As play the most important role. 6 The alternative model assigns the properties of LT GaAs to the formation of As-clusters in a buried Schottkybarrier model. 7 Ultrashort carrier lifetimes are attributed to either trapping in point defects or in As clusters.Presently, LT GaAs is used in photomixing, 8 ultrafast photoconductive switches or substrate material for THz antennas. 9,10 For an optimum performance of these devices the lifetime of photogenerated carriers is crucial. The device bandwidth should be closely related to 1/ , where is the lifetime of mobile carriers. However, a distinct difference is often observed between and the device bandwidth, which can be more than a factor of 2 smaller than 1/ . On the one hand, this difference certainly is influenced by the capacity and the impedance matching of the device, on the other hand carriers trapped in shallow potentials may be reexcited thermally or by the THz electric fields, thus reducing the optimum performance concerning bandwidth and noise.In this letter, we carefully inspect the dynamics of photogenerated carriers close to the band edge in thin LT GaAs films. Two color time-resolved transmission changes allow to resolve the carrier relaxation close to the band edge with high energetic and temporal resolution. We obtain information on shallow bound sub-band-gap states and point out their relevance for the optoelectronic properties...

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“…A carrier lifetime of 400 fs has been determined by differential transmission and a photoconductive switch response time of 800 fs by electro-optic sampling measurements. 7 Two observations deviate from the simple picture considering a photocurrent driven by the applied field: the signal does not vanish for V b ϭ0 V and for V b Ͼ0 V the initial positive peak is followed by a small negative component.…”

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confidence: 53%

Transient measurements with an ultrafast scanning tunneling microscope on semiconductor surfaces

Keil¹,

Jensen²,

Hvam³

1998

Applied Physics Letters

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We demonstrate the use of an ultrafast scanning tunneling microscope on a semiconductor surface. Laser-induced transient signals with 1.8 ps rise time are detected. The investigated sample is a low-temperature grown GaAs layer placed on a sapphire substrate with a thin gold layer that serves as a bias contact. For comparison, the measurements are performed with the tip in contact to the sample as well as in tunneling above the surface. In contact and under bias, the transient signals are identified as a transient photocurrent. An additional signal is generated by a transient voltage induced by the nonuniform carrier density created by the absorption of the light (photo Dember effect). The transient depends in sign and in shape on the direction of optical excitation. This signal is the dominating transient in tunneling mode. The signals are explained by a capacitive coupling across the tunneling gap.

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Femtosecond differential transmission measurements on low temperature GaAs metal–semiconductor–metal structures

Cited by 19 publications

References 12 publications

Fiber Coupled Ultrafast Scanning Tunneling Microscope

Fiber Coupled Ultrafast Scanning Tunneling Microscope

Energy resolved ultrafast relaxation dynamics close to the band edge of low-temperature grown GaAs

Transient measurements with an ultrafast scanning tunneling microscope on semiconductor surfaces

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