F. Mollot

Photoluminescence study of the interface in type II InAlAs–InP heterostructures

Spatially indirect radiative recombinations (type II) have been studied in InAlAs–InP heterostructures grown by gas source molecular beam epitaxy with emphasis on the direct (InAlAs grown on InP) or inverse (InP on InAlAs) interface composition profile. Based on the results of their injection-dependent energy, lifetime and polarization, a new transition scheme is proposed: type II transitions have a low injection limit between 1.27 and 1.28 eV, a long lifetime (τ>1 μs) and strongly shift towards higher energy when increasing the injection. The type II recombination is polarized, the direction of maximum intensity being correlated with the expected interface structure. Lower energy transitions (E⩽1.2 eV) indicate the presence of a well transition material at the interface: they should be better labeled as mixed type I–II. Previously published results are also reconsidered and seem to fit well within this model.

Kinetic model of element III segregation during molecular beam epitaxy of III‐III’‐V semiconductor compounds

Segregation of column III atoms during molecular beam epitaxy of III‐III’‐V semiconductor compounds causes nonabrupt interfaces and a surface composition different from the bulk one. To derive concentration profiles, a thermodynamical equilibrium model has been used for a long time. This model applies well to describe segregation processes at high growth temperatures, but fails in predicting concentration profile variations with substrate temperature. We have thus developed a kinetic model which correctly takes into account the evolution with the growth temperature. We apply this model to the case of indium segregation in the GaxIn1−xAs/GaAs system. The calculated indium concentration profiles are compared to those obtained with the thermodynamical equilibrium model. A kinetic limitation of segregation is shown to appear at low substrate temperatures and sufficiently high growth rates. This limitation is predicted to arise below 400 °C for a growth rate of 1 monolayer/s for In segregation in the GaxIn1−xA...

Detection of picosecond electrical pulses using the intrinsic Franz–Keldysh effect

We report time-resolved measurements of ultrafast electrical pulses propagating on a coplanar transmission line using the intrinsic Franz–Keldysh effect. A low-temperature-grown GaAs layer deposited on a GaAs substrate allows generation and also detection of ps pulses via electroabsorption sampling (EAS). This all-optical method does not require any external sampling probe. A typical rise time of 1.1 ps has been measured. EAS is a good candidate for use in THz characterization of ultrafast devices.

Generation and detection of terahertz pulses using post-process bonding of low-temperature-grown GaAs and AlGaAs

We present an electro-optical method to measure very high frequency characteristics of planar electronic devices. This method allows one to generate and detect subpicosecond electrical pulses on a coplanar stripline using photoconduction and electroabsorption sampling in transferred low-temperature-grown epitaxial layers. The epitaxial lifted-off films are directly van der Waals bonded on the transmission line under test. Good switching efficiency and short electrical rise time (<490 fs) are measured. A bandwidth of 2.5 THz with 60 dB of dynamic range is obtained. This confers to the technique a large field of applications in ultrahigh-speed electronic measurements.

Two-photon absorption in InP substrates in the 1.55μm range

The nonlinear optical absorption has been studied in three different InP substrates (semi-insulating Fe-doped, n-type S-doped, and p-type Zn-doped) by subpicosecond pump-probe differential transmission experiments at 1.6μm. A strong negative differential transmission peak is observed at zero delay, induced by an autocorrelation effect: it is related to two-photon absorption, but not to the occurrence of transitions involving any midgap level (noticeably the Fe-related one). The experimental two-photon absorption coefficient β for InP stands in the range between 24 and 33cm∕GW.

Interface quality and electron transfer at the GaInP on GaAs heterojunction

Hall measurements performed on Ga0.50In0.50P/In0.20Ga0.80As structures show abnormally low mobility both at room temperature and at 77 K, and too high electron densities which cannot be attributed to a normal two-dimensional electron gas in the channel. On the other hand, low temperature photoluminescence on asymmetrical AlGaAs/GaAs/GaInP quantum wells and x-ray photoemission spectroscopy measurements reveal the presence of arsenic atoms in the GaInP barrier. Using a one-dimensional Schrodinger–Poisson simulation with a nonabrupt interface model, we show that the presence of arsenic in GaInP leads to the formation of a parasitic GaInAsP well between the δ-doped layer and the channel, trapping the main part of transferred electrons. We experimentally show that the electron transfer can be drastically improved by inserting a thin AlInP layer at the interface. Insertion of at least six monolayers of AlInP is needed to recover a normal electron transfer as high as 2.1×1012 cm−2.

Electron lifetime of heavily Be-doped In0.53Ga0.47As as a function of growth temperature and doping density

The electron lifetime has been studied by a pump–probe optical transmission technique in heavily Be-doped InGaAs lattice matched to InP as a function of the growth temperature (350⩽Tg⩽500 °C) and doping (2×1019⩽[Be]⩽2.6×1020 cm−3). Reduction of the growth temperature to 350–400 °C induces the creation of electron recombining centers, efficient at the lowest doping studied here. But, for higher dopings, these defects have negligible effects compared to intrinsic Auger processes: the high diffusion of Be can thus be limited by growing heterostructures at reduced temperatures without compromising the electron lifetime. Subpicosecond electron lifetimes have been measured at the highest doping.

Long range gallium segregation in the AlAs layers of GaAs/AlAs superlattices

We demonstrate from Raman scattering on the AlAs‐type optical vibrations in GaAs/AlAs superlattices that small but significant amount of gallium atoms segregate in the AlAs layers over more than 10 monolayers from the AlAs on GaAs interface. We discuss the growth temperature dependence of this effect and its consequences for a global description of the interface roughness in this system.

Gunn oscillations up to 20 GHz optically induced in GaAs/AlAs superlattice

Direct observation of Gunn oscillations up to 20 GHz, induced by picosecond light pulses in an undoped GaAs/AlAs superlattice, is reported. They are obtained in the superlattice growth direction and from 7 K up to room temperature. The frequency is strongly dependent on the applied bias voltage and on the photoexcited carrier density. The oscillation frequency and the mode of operation are modeled by a classical numerical simulation.

dc and microwave negative differential conductance in GaAs/AlAs superlattices

Negative differential conductance (NDC) at 300 K in n+‐nn+‐GaAs/AlAs superlattice structures biased perpendicularly to the layers is demonstrated, and shown to be strongly enhanced at microwave frequencies close to the inverse transit time of electrons. The deduced electron velocities are in fair agreement with those independently determined in undoped superlattices where NDC was inhibited by the electric field nonuniformity. From the analysis of the experimental data, we show that NDC is a bulk superlattice effect, not related to ‘‘quantum defects,’’ e.g., enlarged barriers.