J. C. Portal

An experimental determination of the effective masses for GaxIn1−xAsyP1−y alloys grown on InP

The band‐edge effective mass for conduction electrons in GaxIn1−xAsyP1−y has been determined for several different alloy compositions covering the complete range of alloys grown lattice‐matched on InP. Measurements show that the effective mass varies nearly linearly with alloy composition.

Sequential tunneling due to intersubband scattering in double‐barrier resonant tunneling devices

Magnetoquantum oscillations in the tunnel current of double‐barrier n‐GaAs/(AlGa)As/GaAs/(AlGa)As/GaAs resonant tunneling devices reveal evidence of sequential tunneling in the voltage range corresponding to the resonance when electrons tunnel into the second subband of the GaAs quantum well. The sequential tunneling arises from intersubband scattering between two quasi‐bound states of the well. Near this resonance, the charge buildup in the well can be estimated from the magnetoquantum oscillations.

Photoluminescence spectroscopy of self-assembled InAs quantum dots in strong magnetic field and under high pressure

We have investigated the photoluminescence spectrum of self-assembled InAs quantum dots embedded in a GaAs matrix in magnetic field B up to 23 T and under hydrostatic pressure up to 8 kbar. A strong anisotropy in the diamagnetic shift is found depending on whether B is applied parallel or perpendicular to the growth direction. In the former case, the spatial extent of the carrier wave function in the dot is estimated to be 60 A. The pressure coefficient for the dot emission line is (9.1±0.2) meV/kbar, about 20% smaller than for the Γ-point band gap in bulk GaAs.

Photoluminescence and impurity concentration in GaxIn1−xAsyP1−y alloys lattice‐matched to InP

The photoluminescence spectra of GaInAsP alloys are examined as a function of impurity concentration at several different temperatures and alloy compositions. The photoluminescence linewidth depends on both the impurity concentration and temperature, and our experimental results at 295 °K are described by the empirical relation: ND(ΔE) =9.1×1011 exp(ΔE/4.3 ×10−3) cm−3. It is proposed that photoluminescence linewidth can be used for a simple and rapid determination of the impurity concentration for n‐ and p‐type samples whose total impurity concentration lies in the range 1016 cm−3

Far infrared photoconductivity from majority and minority impurities in high purity Si and Ge

Abstract The effects of intrinsic illumination on the far infrared photoconductivity of high purity Si and Ge are studied. Such illumination permits the observation of both the majority and minority impurities. One of the donors in Ge is highly sensitive to heat treatment, a result suggesting that it arises from lattice defects rather than chemical impurities.

Reduced pressure chemical vapour deposition of SiGe virtual substrates for high mobility devices

We have studied the strain state, film and surface morphology of SiGe virtual substrates grown by reduced pressure chemical vapour deposition (RP-CVD). The macroscopic strain relaxation and the Ge composition of those virtual substrates have been estimated in high resolution x-ray diffraction, using Omega-2Theta scans around the (004) and (224) orders. Typically, linearly graded Si0.7Ge0.3 virtual substrates 5 ?m thick are 96% relaxed. From transmission electron microscopy, we confirm that the misfit dislocations generated to relax the lattice mismatch between Si and SiGe are mostly confined inside the graded layer. The threading dislocations density obtained for Ge concentrations of 20% and 27% is indeed typically of the order of (7.5 ? 2.5) ?105 cm?2. The surface roughness of the relaxed SiGe virtual substrates increases significantly as the Ge concentration approaches 30%. We find for the technologically important Ge concentration of 30% a surface root mean square roughness of 5 nm, with an undulation wavelength for the cross-hatch of the order of 1 ?m. We have also studied the electronic quality of our RP-CVD grown SiGe virtual substrates. We have grown a MODFET-like heterostructure for this purpose, with a buried tensile-strained Si channel 8 nm thick embedded inside SiGe 31%. We have obtained a well-behaved two-dimensional electron gas in the Si channel, with electron sheet densities and mobilities at 1.45 K of 5.7 ? 1011 cm?2 and 180?000 cm2 V?1 s?1, respectively.

Hydrostatic pressure control of the carrier density in GaAs / GaAlAs heterostructures; Role of the metastable deep levels

Abstract Hydrostatic pressure up to 18 kbar has been used to characterize GaAs/GaAlAs heterostructures manufactured using MOCVD and MBE techniques. The pressure dependence of the carrier concentration ns has been deduced from Hall measurements between 4.2 and 300 K and Shubnikov-De Haas experiments have been performed at helium temperature. The main result we have observed is a linear decrease of the carrier concentration as the pressure increases. We explain this behaviour taking into account the deep character of the impurity level in GaAlAs connected with the X minimum whose variation is found to be 11 meV kbar . When the Fermi energy is large enough compared to the difference of affinities ΔEc, a transition between a metallic to an insulating behaviour of the heterojunction is induced. Moreover, the metastable character of this impurity level, clearly pointed out by changing the cooling conditions must be taken into account. In conclusion, this kind of experiment, in which the carrier concentration may be varied, appears as a powerful tool to investigate the properties of the heterostructures. In the particular case of GaAs/GaAlAs the role of the metastable state is predominant in all the investigations.

An experimental determination of enhanced electron g-factors in GaInAs-A1InAs heterojunctions

Abstract Electron g-factors have been determined in GaInAs-AlInAs heterojunctions using the tilted field method. The g-factor is found to be enhanced with values of |g∗| up to 15.5, compared with a bulk g ∗ = - 3 . |g∗| increases with increasing spin splitting providing evidence that the enhancement is due to many-body effects dependent upon a difference in spin populations. The deduced value of |g∗| is also found to be dependant upon the position of the Fermi level relative to the spin split Landau levels.

FRACTIONAL QUANTUM HALL EFFECT MEASUREMENTS AT ZERO G FACTOR

Fractional quantum Hall effect energy gaps have been measured as a function of Zeeman energy. The gap at ν = 1/3 decreases as the g factor is reduced by hydrostatic pressure. This behavior is similar to that at ν = 1 and shows that the excitations are spinlike. At small Zeeman energy, the excitation is consistent with the reversal of 3 spins and may be interpreted as a small composite Skyrmion. At 20 kbar, where g has changed sign, the 1/3 gap appears to increase again.

First observation of the two-dimensional properties of the electron gas in Ga0.49In0.51P/GaAs heterojunctions grown by low pressure metalorganic chemical vapor deposition

We report the first observation of a two‐dimensional electron gas from Shubnikov–de Haas and quantum hall effect experiments in GaInP/GaAs heterostructures grown by low pressure metalorganic chemical vapor deposition. Angular‐dependent Shubnikov–de Haas measurements confirm two dimensionality of the system. Low‐temperature persistent photoconductivity was observed. Critical density at which the second electric subband starts to be populated was determined to be 7.3×1011 cm−2.We report the first observation of a two‐dimensional electron gas from Shubnikov–de Haas and quantum hall effect experiments in GaInP/GaAs heterostructures grown by low pressure metalorganic chemical vapor deposition. Angular‐dependent Shubnikov–de Haas measurements confirm two dimensionality of the system. Low‐temperature persistent photoconductivity was observed. Critical density at which the second electric subband starts to be populated was determined to be 7.3×1011 cm−2.