J.W. Cook

Publications (11)15.47 Total impact

• Article: Quantum Hall effect and setback modulation doping in HgTe–CdTe heterostructures

  C. A. Hoffman, J. R. Meyer, F. J. Bartoli, Y. Lansari, J. W. Cook, J. F. Schetzina
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  ABSTRACT: HgTe–CdTe superlattices with electron concentrations up to 3×10¹⁷ cm^-3 have been grown by photoassisted molecular‐beam epitaxy with controlled indium doping of the CdTe barriers. Magnetotransport measurements indicate that the low‐temperature electron mobility is relatively independent of donor concentration at large N D , in contrast to the Hg 1-x Cd x Te alloy system in which μ n strongly decreases with N D . Also studied are the first Hg‐based heterostructures with setback doping, i.e., with an undoped spacer layer between the donors in the middle of the barriers and the electrons in the quantum wells. A 43 Å setback is found to produce a factor of 2 mobility increase over any measured previously in heavily doped samples, and an 81 Å setback leads to further enhancement of μ n . All superlattices with doping levels ≥8×10¹⁵ cm^-3 display the quantum Hall effect. Quantization of the Hall conductivity in multiples of ≊N W e²/h indicates participation by nearly all of the N W periods in the superlattice, implying that the controlled doping is extremely uniform.
  Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 08/1992; · 1.34 Impact Factor
• Article: Setback modulation doping of HgTe‐CdTe multiple quantum wells

  C. A. Hoffman, J. R. Meyer, F. J. Bartoli, Y. Lansari, J. W. Cook, J. F. Schetzina
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  ABSTRACT: Photoassisted molecular beam epitaxy has been used to achieve the first setback modulation doping of HgTe‐CdTe heterostructures. A 43 Å setback is found to yield a factor‐of‐2 increase of the mobility over any measured previously for intentionally doped samples, and an 81 Å setback leads to further enhancement of the mobility.
  Applied Physics Letters 06/1992; · 3.84 Impact Factor
• Article: Quantum Hall effect in HgTe–CdTe superlattices with strong three‐dimensional dispersion

  C. A. Hoffman, J. R. Meyer, F. J. Bartoli, Y. Lansari, J. W. Cook, J. F. Schetzina
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  ABSTRACT: We have studied the quantum Hall effect in modulation‐doped HgTe–CdTe superlattices with electron concentrations in the range 8 × 10¹⁵ ≤ n ≤ 3 × 10¹⁷ cm^-3. Since the quantum Hall phenomenon should in principle be restricted to two‐dimensional systems, it is not surprising that the plateaus in the Hall conductivity (at multiples of ≊ 200e²/h in the 200‐period superlattices) and minima in the diagonal conductivity are most pronounced in samples with relatively thick barriers, i.e., when the energy bands have little dispersion in the growth direction. However, well‐resolved quantum Hall features are also observed in thinner‐well superlattices having miniband widths of up to 17 meV. While present phenomenological models can account for this as long as the Landau level spacing is large compared to the miniband width, it is surprising that broad lowest‐order plateaus are also evident in data for two superlattices having little or no gap between adjacent levels. Possible implications of this finding are discussed.
  Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 04/1992; · 1.34 Impact Factor
• Article: Shubnikov–de Haas oscillations and quantum Hall effect in modulation‐doped HgTe–CdTe superlattices

  C. A. Hoffman, J. R. Meyer, D. J. Arnold, F. J. Bartoli, Y. Lansari, J. W. Cook, J. F. Schetzina
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  ABSTRACT: We have investigated quantum oscillations in the magneto‐transport properties of HgTe–CdTe superlattices grown by molecular‐beam epitaxy. Modulation doping was achieved by incorporating either indium donors or arsenic acceptors into the CdTe barriers. In a p‐type sample, quantized plateaus were observed in the Hall conductivity down to i=3 conduction channels. Since the structure contained 200 periods, this implies that the quantized holes populated only a small fraction of the total superlattice volume. A mixed conduction analysis of the nonoscillating component of magneto‐transport data provided confirming evidence for the presence of a two‐dimensional hole gas with the appropriate density in addition to the superlattice holes. Previous reports of the quantum Hall effect in HgTe–CdTe also yielded i far less than the total number of superlattice wells. In contrast, an n‐type sample from the present study displayed a single quantum Hall plateau at i≊140, indicating that in this case most of the 200 superlattice periods contributed to the conduction. We argue that this represents the first observation of the quantum Hall effect associated with carriers distributed throughout the interior of a HgTe–CdTe superlattice.
  Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 06/1991; · 1.34 Impact Factor
• Article: Nonlinear optical coefficients of narrow‐gap semiconductors

  E. R. Youngdale, C. A. Hoffman, J. R. Meyer, F. J. Bartoli, J. W. Han, J. W. Cook, J. F. Schetzina, M. A. Engelhardt, E. W. Niles, H. Hochst
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  ABSTRACT: Nondegenerate four‐wave mixing experiments have been performed at CO 2 laser wavelengths on HgTe–CdTe superlattices and α‐Sn 1-x Ge x films grown by molecular beam epitaxy. In HgTe–CdTe, the magnitude, temperature dependence, and laser‐intensity dependences of χ⁽³⁾ are found to vary systematically with superlattice energy gap. A theoretical model for nonlinear optical coefficients in narrow‐gap semiconductors gives excellent agreement with the experimental results when the nonlinearity is assumed to be due to optical modulation of the free carrier temperature. In the first investigation of α‐Sn 1-x Ge x as a nonlinear optical material, we find that the measured χ⁽³⁾’s are quite large (up to 8×10^-⁵ esu), particularly in heavily strained films with compositions up to x=0.08. The mechanism responsible for the nonlinearity in that system is unclear at present.
  Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 04/1990; · 1.25 Impact Factor
• Article: Field‐effect transistors in Hg1-xCdxTe grown by photoassisted molecular beam epitaxy

  D. L. Dreifus, R. M. Kolbas, J. W. Han, J. W. Cook, J. F. Schetzina
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  ABSTRACT: The first demonstration of Hg 1-x Cd x Te metal–insulator‐semiconductor field‐effect transistors prepared using n‐type layers grown by photoassisted molecular beam epitaxy is reported. These transistor structures were processed by means of a new low‐temperature (≪60 °C) semiconductor device processing technology developed at North Carolina State University, and differ significantly from structures previously reported in that (1) a depletion region instead of a surface inversion layer is used as the principle for device operation, (2) larger x‐value Hg 1-x Cd x Te is employed as the active region of the device, and (3) the transistor structures are fabricated without the use of ion implantation and thermal annealing processing steps. In addition, the low‐temperature processing technology is compatible with the future development of superlattice and multilayer device structures consisting of Hg‐based alloys, in which mercury interdiffusion at heterointerfaces can significantly alter the material properties. Depletion and enhancement‐mode operation have been observed and digital inverter circuits have been characterized at speeds of 1 MHz.
  Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 04/1990; · 1.25 Impact Factor
• Article: Nonlinear optical coefficients of Hg‐based superlattices

  E. R. Youngdale, C. A. Hoffman, J. R. Meyer, F. J. Bartoli, X. Chu, J. P. Faurie, J. W. Han, J. W. Cook, J. F. Schetzina
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  ABSTRACT: The first measurement of the third‐order nonlinear optical susceptibility χ⁽³⁾ in Hg‐based narrow‐gap semiconductor superlattices is reported. Four‐wave mixing experiments were performed at CO 2 laser wavelengths on a series of HgTe/CdTe and Hg 1-x Zn x Te/CdTe superlattices whose energy band gaps are approximately zero at T=0 K. The nonlinearity is believed to due to modulation of the free‐carrier temperature and density by the optical beams. Theoretical estimates of the third‐order nonlinear susceptibility due to this mechanism are in good agreement with the experimental results.
  Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 04/1989; · 1.25 Impact Factor
• Article: Magneto‐optical properties of a small‐gap HgTe/CdTe superlattice

  R. J. Wagner, J. M. Perez, J. R. Meyer, Jeong W. Han, J. W. Cook, J. F. Schetzina
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  ABSTRACT: Far‐infrared magneto‐optical experiments have been performed on a p‐type HgTe/CdTe superlattice with a small effective band gap. The angular dependence of the hole cyclotron resonance spectra reveals that the effective mass is two orders of magnitude heavier in the growth direction than transverse to it. These results provide evidence for a large valence‐band offset. In addition, a tentative identification of hole spin resonance has been made. Evidence is presented which shows that the superlattice band structure can be substantially modified by the application of modest magnetic fields.
  Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 04/1989; · 1.25 Impact Factor
• Article: Properties of HgTe–Hg0.15Cd0.85Te superlattices grown by molecular‐beam epitaxy

  Y. Lansari, J. W. Han, S. Hwang, L. S. Kim, J. W. Cook, J. F. Schetzina, J. N. Schulman, N. Otsuka
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  ABSTRACT: The structural and optical properties of a series of molecular‐beam epitaxy (MBE) grown HgTe–Hg 0.15 Cd 0.85 Te superlattices (SL) have been studied. Vertical cross‐section transmission electron microscopy studies show no evidence of Hg interdiffusion, confirming the high quality of these multilayered structures. Transmittance and reflectance measurements were performed on each sample and analyzed to obtain optical absorption coefficients. The absorption coefficient versus photon energy spectra show consecutive rises and plateaus characteristic of two‐dimensional quantum structures. A two‐band tight‐binding model was used to analyze the absorption data and identify the quantum transitions. The observation of both light‐hole and heavy‐hole transitions allowed the determination of both the superlattice band gap and the valence‐band offset. The offset was found to be 300 meV (±20 meV) for the superlattices studied, which extrapolates to 350 meV (±20 meV) for the HgTe–CdTe binary interface.
  Journal of vacuum science & technology. B, Microelectronics and nanometer structures: processing, measurement, and phenomena: an official journal of the American Vacuum Society 04/1989; · 1.34 Impact Factor
• Article: Free‐carrier transport in HgTe/CdTe superlattices

  C. A. Hoffman, J. R. Meyer, E. R. Youngdale, J. R. Lindle, F. J. Bartoli, J. W. Han, K. A. Harris, J. W. Cook, J. F. Schetzina
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  ABSTRACT: Magnetotransport and phototransport measurements on molecular‐beam epitaxially grown HgTe/CdTe superlattices are discussed. A detailed analysis of the magnetic‐field‐dependent Hall data yields that mixed conduction by more than one carrier species is significant at all temperatures.The T dependence of the intrinsic carrier density has been used to characterize the band gap and the product of the electron and hole density‐of‐states effective masses. Hole mobilities ≫10⁵ cm²/V s are reported, as well as the first observation of hole freezeout in a HgTe/CdTe superlattice. Photo‐Hall measurements employing CO 2 laser irradiation to generate excess electrons and holes are shown to yield minority electron mobilities and free‐carrier lifetimes.
  Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 08/1988; · 1.25 Impact Factor
• Article: Transport characterization of HgTeCdTe superlattice band structure

  J.R. Meyer, C.A. Hoffman, F.J. Bartoli, J.W. Han, J.W. Cook, J.F. Schetzina, J.N. Schulman
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  ABSTRACT: An extensive study of electron and hole transport in MBE-grown Hg-based superlattices is discussed. Temperature-dependent free carrier mobilities and densities have been obtained from a mixed-conduction analysis of the Hall and resistivity data as a function of magnetic field. Photo-Hall measurements using high-intensity CO2 laser excitation have yielded the first temperature-dependent energy relaxation times for Hg-based superlattices.
  Superlattices and Microstructures.