Variable Temperature Hall Measurements Research Articles

The growth of shallow high mobility two-dimensional electron gas structures

GaAs/AlAs heterostructures are described in which the two-dimensional electron gases (2DEGs) are in close proximity to the crystalline surface. Measurements are presented from samples in which the 2DEGs were 15 and 25 nm below the GaAs/vacuum interface. The sheet carrier concentrations were 6.7x10 11 and 3.4x10 11 cm −2 and Hall mobilities were 5x10 5 and 1.2x10 6 cm 2 V −1 s −1, respectively, at 4 K. The two samples were examined by transmission electron microscopy. Hall measurements are given both as a function of temperature and as a function of surface Schottky gate voltage at 4 K. A simple capacitor model was used to relate the sheet carrier concentration versus gate voltage dependence to the depth of the 2DEG below the gate. Variable temperature Hall measurements suggest that ionized impurity scattering is less significant than phonon scattering at 4 K.

Hall analysis of the electrical properties of n-type AlxGa1-xAs grown by MOVPE

Variable temperature Hall measurements were used to study the electrical properties of undoped and Se-doped AlxGa1-xAs (0 <x < 0.4) layers grown by metalorganic vapour phase epitaxy (MOVPE). It is shown that the deep donor activation energy measured in undoped AlGaAs exhibits a similar dependency upon composition as that reported for Si-doped AlGaAs grown by MBE. For AlxGa1-xAs, doping with selenium is found to reduce the activation energy from 66 meV (forn = 4.1 x 1016/cm3), to 9 meV (forn= 1.6 × 1018/cm3).

Determination of acceptor ionisation energies in CdxHg1-xTe

Variable temperature Hall measurements have been performed on p-type CdxHg1-xTe(CMT) with 0.18<x<0.32 grown by both Bridgman and modified Bridgman (accelerated crucible rotation technique, ACRT) techniques. Samples of material deliberately doped with acceptor impurities as well as undoped samples were studied in the as-grown state and after low-temperature (<400 degrees C) annealing. Carrier freeze out was observed in most samples and values for the ionisation energy Ea determined. For undoped material Ea was found to increase with increasing x and with decreasing acceptor concentration Na. The doped samples measured after isothermal annealing in a mercury rich atmosphere, to remove metal vacancies, were found to have lower Ea values that their undoped counterparts of similar x and Na. The Ea values obtained are compared with other values found in the literature. In the bulk-grown material it is not possible to distinguish between impurities simply from their Ea values but the technique can be used to differentiate material dominated by defects from that controlled by impurities. It is also concluded that Na, x and the degree of compensation need to be specified when quoting values for Ea in undoped and doped material.

The dynamic gradient freeze growth of (Ti + Zn) doped semi-insulating InP

High quality single crystals of semi-insulating InP were grown by the electrodynamic gradient freeze technique. The material is co-doped with the shallow acceptor zinc and the deep donor titanium. Resistivities in the 10 5−10 6 Ω cm range were measured with corresponding mobilities of 2500 cm 2 V −1s −1. Variable temperature Hall measurements resulted in an activation energy 0.61 ± 0.03 eV below the conduction band. The dislocation density is less than 500 cm −2 and is the lowest ever reported for comparably sized semi-insulating InP bulk crystals. Precipitates containing titanium and phosphorus were found when the titatium concentration exceeded 3 × 10 16 cm −3. A distribution coefficient of 4 × 10 −4 was measured for titanium in InP. The precipitates have been identified for the first time as hexagonal TiP using X-ray powder diffraction.

Assessment of n-type GaAs by the Hall effect

Variable temperature (4–300 K) Hall measurements were used to determine the donor and acceptor concentrations in high-purity epitaxial GaAs grown by metalorganic chemical vapor deposition, molecular-beam epitaxy, and vapor-phase epitaxy. It is found that in many cases the onset of impurity conduction at low temperatures limits the successful determination of the impurity concentrations in the material. Furthermore, attention is drawn to the fact that experimentally determined compensation ratios may differ appreciably from those obtained by the much used method of comparing a single 77 K measurement with theoretically calculated tables.

The temperature dependence of the anomalous Hall effects in p-type HgCdTe

The anomalous Hall effects of narrow-band-gap p-type HgCdTe, which manifest themselves as negative Hall coefficients at low temperatures, have caused serious problems in material characterization in the past two decades. These phenomena are now widely recognized as being caused by the inverted surface effect describable by Petritz’s two-layer model [Phys. Rev. 110, 1254 (1958)]. We report results from variable-temperature Hall measurements on p-type HgCdTe liquid-phase epitaxy and bulk slices with bare, anodically sulfidized, and anodically oxidized surfaces. We show that the Hall anomalies can be eliminated by depositing an anodic sulfide layer on the surface and subsequently can be restored by removing the sulfide layer. Based on Petritz’s two-layer model, we were able to use the same set of bulk transport parameters and different sets of surface transport parameters to fit the experimental temperature-dependent Hall coefficient and Hall mobility curves of the same sample with surfaces that have been subjected to different chemical treatments. It was demonstrated that the Hall anomalies occurred when the n-type surface conductivity increased relative to the p-type bulk conductivity. The surface conductivity is mainly controlled by the density of fixed positive surface charges which was found to be larger on an anodically oxidized surface and smaller on an anodically sulfidized surface than on a bare surface.

Effect of the InP doping density on the electrical properties of the two-dimensional electron gas in LPE-grown modulation-doped heterostructures

In 0.53Ga 0.47As/ n-InP modulation-doped heterostructures have been grown by liquid phase epitaxy. Shubnikov-de Haas and quantum Hall effect measurements were made for the confirmation of the existence of the two-dimensional electron gas (2DEG). Mobility enhancements were observed at low temperatures from the variable-temperature Hall measurements. When the doping density of InP layer was varied from 2.1 × 10 17 to the 5.2 × 10 18 cm −3, the sheet electron density and mobility of 2DEG at 10K were changed from 6.52 × 10 11 to 1.01 × 10 12 cm −2 and 49,500-43,600 cm 2/V-s, respectively. From the recordings of high-field Hall plateaus, the channel densities were 4.03 × 10 11 and 6.03 × 10 11 cm −2 with doping densities of 2.1 × 10 17 and 8.1 × 10 17 cm −3, respectively. The corresponding Fermi energies were 20.1 and 29.9 meV measured from the lowest energy level in the potential well.

Quantum size effect in δ-doped AlGaAs heterostructures

We report a new structure formed by δ doping the barrier of an AlGaAs heterostructure. Our structure contains a pair of electronically coupled quantum systems that develop from the quantum size effect occurring in the barrier and at the heterointerface. Compared to conventional homogeneous doping, we find enhancements in interface density resulting in δ-doped structures for all spacer thicknesses. A mobility of 1.9×106 cm2/V s is observed at a spacer layer thickness of 360 Å. Systematic dependences of interface mobility and density on spacer thickness are deduced from the quantum Hall effect and variable temperature Hall measurements.

Experimental and theoretical characterization of delta-doped quantum well field-effect transistors grown by gas-source molecular-beam epitaxy

An experimental and theoretical study has been made of a delta-doped quantum well field-effect transistor, ( delta QWFET), that realizes for the first time the high two-dimensional electron gas concentration, transconductance, saturation current density, and DC gain, and the very low output conductance that are expected for this device. The structure is made unique by the presence of quantum size effects that occur in both delta-doped barriers and the quantum well. The quantum size effects in the barriers and well can thus be tailored independently to suit specific FET requirements while still maintaining the advantage of a spatially confined electron gas. A complete DC characterization of this structure has been carried out using capacitance-voltage, Shubnikov-de Haas, and variable-temperature Hall measurements. A model incorporating quantum size effects has been developed and used as a basis for calculations. >

Depletion corrections in variable temperature Hall measurements

The decrease in the measured Hall free-electron concentration with decreasing temperature near 300 K is often observed for thin high-purity GaAs layers. This has previously been interpreted as electron freezeout on deep donor sites. However, it can be quantitatively described by the decrease in carrier concentration per unit area associated with increasing surface and interface depletion region thicknesses. It is shown that when these depletion regions are included in the analysis of the Hall-effect data, the measured free-electron freezeout behavior can be accurately described by a simple shallow donor. If necessary, a deep donor may be included in the modeling. The results agree with the observed temperature variation of the capacitance-voltage (C-V) profiling data.

The activation energy of copper shallow acceptors in mercury cadmium telluride

The thermal activation energy (E0) of isolated copper shallow acceptors in bulk Hg0.8Cd0.2Te has been determined using variable-temperature Hall measurements on samples with 77-K carrier concentrations ranging between 1015 and 1017 cm−3. It was found that the decrease of the activation energy with the increasing hole concentration can be adequately described by the screening of the acceptor potential by free carriers. The measured E0 (11.5 meV) also agrees well with that predicted by the effective-mass theory.