J. Przybytek

Determining Curie temperature of (Ga,Mn)As samples based on electrical transport measurements: Low Curie temperature case

In this paper, we show that the widely accepted method of the determination of Curie temperature (TC) in (Ga,Mn)As samples, based on the position of the peak in the temperature derivative of the resistivity, completely fails in the case of non-metallic and low-TC unannealed samples. In this case, we propose an alternative method, also based on electric transport measurements, which exploits temperature dependence of the second derivative of the resistivity upon magnetic field.

Search for free holes in InN:Mg-interplay between surface layer and Mg-acceptor doped interior

We measured lateral ac transport (up to 20 MHz), thermopower, as well as resistivity and Hall effect in InN:Mg samples with various Mg content. The sign of the Hall effect for all the samples was negative (electrons), however, the thermopower (α) measurements have shown the p-type sign of α for moderate Mg content—in the window centered around 1×1019 cm−3. Further overdoping with Mg yields donor type of defects and the change of thermoelectric power sign. The ac measurements performed as a function of frequency revealed that in both samples exhibiting and nonexhibiting p-type sign of thermopower, the n-type inversion layer at the surface does not prevent the electric contact to the bulk layer. Therefore we conclude that the n-type Hall effect invariably reported for all the Mg-doped samples originates from electron domination in mobility-weighted contributions of both types of carriers.

Hydrostatic pressure study of the paramagnetic-ferromagnetic phase transition in (Ga,Mn)As

The effect of hydrostatic pressure on the paramagnetic-ferromagnetic phase transition has been studied in (Ga,Mn)As. The variation in the Curie temperature (T-C) with pressure was monitored by two transport methods: (1) measurement of zero-field resistivity versus temperature rho(T) and (2) dependence on temperature of the Hall voltage hysteresis loop. Two specimens of different resistivity characteristics were examined. The measured pressure-induced changes in T-C were relatively small (on the order of 1 K/GPa) for both samples, however they were opposite for the two.

Low-frequency noise in AlGaAs/InGaAs/GaAs Hall micromagnetometers

We report on studies aimed at understanding and improving the intrinsic noise of high-performance sensors using a 2D electron gas channel confined by a quantum well in the pseudomorphic AlGaAs/InGaAs/GaAs heterostructure. MIS gated and ungated Hall sensors shaped as a Greek cross with dimensions ranging from 100 μm down to submicrometer range have been investigated. At room temperature the predominant low frequency Hall voltage noise originates from the ensemble of trapping/detrapping events occurring within the continuum of GaAs surface states. Its power spectral density can be deduced from independent measurements of the interface trap density-of-states by applying Shockley-Read-Hall dynamics and the Fluctuation-Dissipation Theorem. In fact, theoretical spectra calculated without any adjustable fitting parameter coincide closely with the experimentally measured ones. At cryogenic temperature this interface traps noise freezes out, thus revealing a much weaker intrinsic background noise with 1/f spectrum. For small sensors the intrinsic 1/f noise converts into one or a few lorentzians due to the action of individual random telegraph signals (RTS). For Hall crosses with an intersection of 4x4μm2, we find statistically less than 1 fluctuator per each decade of time constant at 77 K. Due to the random distribution of the elementary fluctuators, some of these small Hall crosses may show less low-frequency noise than much larger 60x60μm2 sensors.

LF noise in cross Hall effect devices: geometrical study

The aim of this work was to optimize the shape of the cross-shaped Hall devices with regard to noise generated on sensing contacts. We have performed systematical experimental and numerical study of the influence of the geometry of the cross shaped Hall devices on the electrical low frequency noise measured on the sensing and driving current contacts. Based on our numerical calculations of the current density distribution (Finite Elements Method) we have designed several samples with various geometry/shape and dimensions in the 10 μm-200 μm range. Hall devices were fabricated on GaAs-based pseudomorphic heterostructures. Presented experimental results are in good agreement with our calculations of the noise power density based on electrical network theory and references therein. These results enable us to optimize the geometry of the devices giving us in the best case the reduction of the low frequency noise power density by ca -10 dB as compared to a standard Greek cross of identical size. This purely geometrical effect is independent of the sample physical structure and depends only on the sample shape. These results can be applied to any planar Si or III-V based semiconductor Hall device.

Hydrostatic pressure study of indium DX-like centers in MBE-grown CdTe and CdMnTe layers

We present Hall effect and resistivity measurements as functions of hydrostatic pressure performed on MBE-grown CdTe and CdMnTe layers doped with indium. Our data give evidence that indium impurities introduce to the layers a localized resonant state lying about 130 meV above the bottom of the conduction band and having a DX-like character. The experimental data are analyzed using both positive and negative U models of the In charge state. Both models reasonably reproduce the experimental behaviour.

Spatial correlations of In-donor charges in CdTe layers

Abstract We present experimental evidence that indium donors in CdTe localize electrons in a spatially correlated manner. We have studied the Hall mobility, μ H , as a function of the electron concentration, n H , at T = 77 K. Three methods leading to changes of n H have been used (i) high pressure freeze-out of electrons onto localized states of In-donors, (ii) the persistent photoconductivity effect, and (iii) a subsequent annealing of the sample. Each method causes a related modification in a degree of spatial correlations in the arrangement of donor charges. As a consequence, different values of μ H correspond to the same value of n H .

Hydrostatic-pressure-induced changes of magnetic anisotropy in (Ga, Mn)As thin films

The impact of hydrostatic pressure on magnetic anisotropy energies in (Ga, Mn)As thin films with in-plane and out-of-plane magnetic easy axes predefined by epitaxial strain was investigated. In both types of sample we observed a clear increase in both in-plane and out-of-plane anisotropy parameters with pressure. The out-of-plane anisotropy constant is well reproduced by the mean-field p-d Zener model; however, the changes in uniaxial anisotropy are much larger than expected in the Mn-Mn dimer scenario.

Noise signatures of metastable resistivity states in ferromagnetic insulating manganite

Pronounced noise signatures enabling one to discriminate metastable resistivity states in La0.86Ca0.14MnO3 single crystals have been observed. The normalized noise spectra for metastable resisitivity differ both in shape and magnitude, indicating that the metastable state is associated with transition of the electronic system into another local minimum of the potential landscape. Such scenario is consistent with freezing of the electronic system into a Coulomb glass state.

High frequency cut-off in 1/f conductivity noise of hole-doped La1−x Ca x MnO3 manganite single crystals

High frequency bias and temperature-dependent Lorentzian cut-off has been observed in the 1/f spectra of the conductivity fluctuations in low hole-doped ferromagnetic insulating La1−x Ca x MnO3 manganite at low temperatures. The cut-off frequency depends on dc current bias and temperature. The high frequency cut-off has been tentatively associated with intrinsic limits of the appearance of 1/f noise in the hopping regime of the Coulomb glass state. The assumption is validated by the fact that the Efros–Shklovskii temperature , estimated from the fit of the model to the experimentally measured temperature dependence of the cut-off frequency, has the same value as the temperature evaluated independently from the temperature dependence of the resistivity in the corresponding temperature range.