E. S. Hellman

Growth of high Tc superconducting thin films using molecular beam epitaxy techniques

Thin films of the high‐temperature superconductor DyBa2Cu3O7−x have been grown on SrTiO3 substrates using molecular beam epitaxy techniques. Reflection high‐energy electron diffraction patterns observed during deposition indicate incomplete oxidation of copper and growth of oriented metallic copper microcrystals in a matrix of amorphous barium and dysprosium oxides. After post‐growth anneal the films exhibited sharp superconducting transitions with zero resistance observed as high as 89 K and critical current densities of 4.8×105 A/cm2 at 4.2 K and 3.9×104 A/cm2 at 77 K.

Infra-red transmission spectroscopy of GaAs during molecular beam epitaxy

Abstract Direct radiative heating of GaAs substrates for molecular beam epitaxy (MBE) has a very useful side-benefit: the infra-red light which heats the substrate can also serve as a light source for transmission spectroscopy. We demonstrate the use of in-situ transmission spectroscopy in two important applications, temperature measurement and growth rate measurement. The substrate temperature can be accurately determined by measuring the position of the band-gap absorption edge. The band gap of GaAs shifts about 50 mV per 100°C in the usual temperature range of MBE growth, and has been well characterized previously. We can measure the position of the absorption edge to better than 5 mV, so a temperature can be determined with an accuracy of better than 10°C. The precision of the measurement is ±2°C. We have measured GaAs substrate temperatures as low as 450°C, and the technique is easily extendable to much lower temperatures. We have used this technique to calibrate the thermocouple used to control our substrate heater during normal MBE growth. The growth rates of AlxGa1−xAs and GaAs can be determined by measuring the Fabry-Perot interference fringes resulting from thin layers. For a single AlxGa1−xAs layer, the amplitude of the fringes observed as a function of time is a good measure of the index difference between the layer and the substrate. From published data about the GaAs index at high temperature, we can get the AlxGa1−xAs index, and thus an estimate of the Al mole fraction of the layer. By counting the fringes as the layer is grown, we can determine the thickness of the layer. For a single layer of Al0.3Ga0.7As on GaAs, we observe fringes of magnitude 3.85%±0.62% at 1.468 μm. The optical thickness can be determined to within ±24 nm. For multi-layer structures, the variations of the transmittance with wavelength become large, so that the optical thickness of both GaAs and AlAs can be extracted from a single wavelength scan. An accurate determination of the refractive indices of these materials at high temperatures could make this technique very important for the reproducible growth of AlxGa1−xAs-GaAs heterostructures, because a high precision calibration could be done during each growth.

Molecular beam epitaxy of layered Dy‐Ba‐Cu‐O compounds

Heteroepitaxial Dy‐Ba‐Cu‐O films have been grown in situ on SrTiO3 substrates using an oxygen plasma beam and elemental source beams in a modified molecular beam epitaxy machine. By periodically shuttering the Dy and Ba beams during growth, flat surfaces of layered Dy‐Ba‐Cu‐O compounds have been obtained. Periodic oscillations in the intensity of the in situ reflection high‐energy electron diffraction pattern were observed during shuttered growths. Depending on growth conditions, the as‐grown layers have ranged from insulating to superconducting with onset temperatures above 60 K.

Elastic scattering centers in resonant tunneling diodes

The effect of impurities placed in the wells of double‐barrier resonant tunneling diodes on the current‐voltage characteristics was experimentally determined. Four different double‐barrier structures were grown by molecular beam epitaxy with n‐type, p‐type, undoped, and highly compensated doping in the center of the well. Resonant tunneling devices of various sizes were fabricated, and measured at 77 K. Systematic shifts in the peak position and peak to valley ratios were observed for the different dopant profiles. The shifts in peak position are correctly predicted by a ballistic model which includes the effects of band bending due to ionized impurities in the well. The doped devices showed a systematic decrease in the peak to valley ratio which is not predicted by the ballistic model. By scaling our results, it is apparent that in most cases unintentional background impurities are not sufficient to significantly degrade the current‐voltage characteristics of resonant tunneling diodes.

Molecular beam epitaxy of gallium arsenide using direct radiative substrate heating

Conventional mounting techniques in molecular beam epitaxy using indium to solder GaAs substrates to molybdenum blocks result in a rough back side which is incompatible with integrated circuit processing. We present the design of a new substrate holder which allows direct radiative heating of substrates without use of indium bonding. Temperature measurement and control are done by radiative coupling to a shielded thermocouple which sits just behind the GaAs wafer. Excellent temperature uniformity is achieved over a 3 in. wafer by a molybdenum holder ring design which minimizes contact with the substrate and holds the wafer against the back of the ring to eliminate shadowing of the substrate from the heater. No coating of the wafers is necessary for temperature control. Stress related defects and growth induced wafer warpage are not observed. Deep level transient spectroscopy, photoluminescence, and mobility measurements show the clear superiority of GaAs layers grown using direct radiative substrate heati...

Molecular‐beam epitaxy and deposition of high‐Tc superconductors

We have grown thin, highly oriented, polycrystalline DyBa2Cu3O7−x films using molecular‐beam epitaxy (MBE) techniques that show the onset of superconductivity at temperatures above 90 K and complete transitions at temperatures as high as 87 K. These films have critical current densities as high as 5×105 A/cm2 at 4.2 K. Films were grown in a modified Varian 360 MBE machine using effusion sources containing the metal constituents, along with a gaseous oxygen source. The early stages of deposition were monitored with reflection high‐energy electron diffraction (RHEED). The best films were obtained on SrTiO3 substrates at substrate temperatures of 600–750 °C. At these temperatures, the initial stage of growth is dominated by epitaxy of copper islands. At lower temperatures, the growth is amorphous, while at higher temperatures, copper may not be incorporated into the film. Copper incorporation is also affected by oxygen flux. In all cases, the films are semiconducting or insulating as grown, and become superc...

Reduction of the acceptor impurity background in GaAs grown by molecular beam epitaxy

We report very high purity, unintentionally doped n‐type GaAs with the lowest acceptor background of all reported molecular beam epitaxy (MBE) GaAs layers. The residual acceptor concentration is 2.4×1013 cm−3 and the residual donor concentration is 1.5×1014 cm−3, yielding a compensation ratio of Na/Nd=0.160. The measured Hall mobility is 163 000 cm2/V s at 77 K with a peak value of 216 000 cm2/V s at 45.9 K. The limitations of the Hall mobility at 77 K as a figure of merit are discussed and more accurate figures of merit are considered. The initial preparation of the MBE system and the growth conditions leading to the reduced incorporation of acceptor impurities are also presented.

GaAs with very low acceptor impurity background grown by molecular beam epitaxy

Abstract We report very high purity, MBE grown, unintentionally doped, n-type GaAs with an extremely low acceptor background. The residual acceptor concentration is 2.4×10 -3 and the residual donor concentration is 1.5×10 14 cm -3 , yielding a compensation ratio of N a / N d = 0.160. The measured Hall mobility is 163,000 cm 2 /V · at 77 K with a peak value of 216,000 cm 2 /V · at 45.9 K. The photolumminescence spectrum shows that this material is only lightly compensated and exhibits the narrow linwidths that are characteristic of high purity GaAs. Hall measurements show excellent electrical uniformity across the wafer, even at these low background doping.

One dimensional polaron effects and current inhomogeneities in sequential phonon emission

Abstract We have constructed a physical model to explain the tunneling current oscillations reported by Hickmott et. al. [Phys. Rev. Lett. 52 , 2053] for GaAs/AlGaAs heterostructures in high magnetic fields. We propose that the periodic structure observed is due to space charge which builds up in the undepleted layer when electrons enter it with energy just below the phonon emission threshold. Such electrons interact with the lattice to form polarons whose energy is pinned to the phonon energy, and thus have a very small group velocity. The polaron effect is strongly enhanced by the confinement of the electrons by the strong magnetic field. We infer from the current-voltage data that most of the tunneling current flows through a small area of the sample. The combined model gives reasonable quantitative agreement with experiment.

Polynomial kinetic energy approximation for direct-indirect heterostructures

Abstract The effective mass approximation, in which the band structure of a semiconductor is replaced by a simple parabolic dispersion relation for electrons, has worked suprisingly well for quantum calculations of electron eigenenergies and eigenstates in semiconductor heterostructures. It can be extended by systems with spacially varying effective mass by requiring wavefunction and particle flux continuity. However, for indirect heterostructures which include materials with electron bands of different symmetry, it fails to incorporate enough physics to give correct answers. An important example where effective mass calculations are inapplicable is the AlAs GaAs system, in which the conduction band minima occur at the Г and X points, respectively. The mixture of these two types of electrons in AlAs GaAs superlattices has only been calculated using tight-binding or pseudopotential methods, which are difficult to apply to a wide range of heterostructures. We have extended the spirit of effective mass calculations to a method applicable to indirect heterostructures. To do this, we write a Schrodinger equation in which the Hamiltonian is a nth degree polynomial in the gradient operator, ▽. For any energy, there exist n (complex) plane wave solutions. For spacially varying band structures, we can write a probability conserving Schrodinger equation which has a flux operator consistent with the usual interpretation of plane wave group velocities. The requirements imposed by this Schrodinger equation on the wavefunction and its derivatives allow matching of the plane wave solutions across heterojunctions. We have applied this method to AlAs GaAs double heterostructures, where we see interesting resonance and anti-resonance behaviors. The computational speed of our method will allow complicated structures, including compositional grading and electric fields, to be modeled on microcomputers.