Walter L. Bloss

High‐detectivity GaAs quantum well infrared detectors with peak responsivity at 8.2 μm

GaAs quantum well infrared detectors with peak responsivity at 8.2 μm and significant response beyond 10 μm have been demonstrated with detectivities of 4×1011 cm (Hz)1/2 /W at 6 K; this detectivity is the highest reported for a quantum well detector. The detectors comprised 50 GaAs quantum wells of width 40 A with an average Si doping density of 1×1018 cm−3 separated by 280‐A barriers of Al0.28Ga0.72As. In this design, the state to which electrons are excited by infrared absorption and from which they are subsequently collected lies in the continuum above the energy of the Al0.28Ga0.72As conduction‐band minimum. The maximum detector responsivity was mesured to be 0.34 A/W. The device dark current density is 5.5×10−6 A/cm2 with the detector biased for maximum detectivity (3.5 V), and the dark current remains constant with increasing temperature up to 50 K. The detector noise current was observed to be a constant fraction (70%) of the shot noise down to noise currents of 10−14 A/(Hz)1/2. A theoretical mode...

Electric field dependence of quantum‐well eigenstates

Two very accurate methods are developed, one based on the shooting method and the other on the relaxation method, for calculating the eigenenergies and eigenfunctions of states in a quantum well with an applied electric field. These methods, which give accuracies greater than 0.001 meV, are well controlled, give the quantum‐well eigenfunctions, and are easily applied to situations of varying potential and effective mass. Comparisons with the variational approach of Bastard and others are made. These techniques allow one to follow the development of the quantum‐well eigenstate outside the well and to determine the validity of the quasi‐bound state approximation. Recent results in the literature showing that the ground‐state hole eigenfunction becomes unbound at moderate electric fields are shown to be erroneous. Detailed calculations are presented for the electron (ground and first excited) and hole (ground) eigenstates of a quantum well with width 85 A, and barrier heights of 240 (conduction band) and 160...

Effects of Hartree, exchange, and correlation energy on intersubband transitions

We calculate the effects of the Hartree and the exchange‐correlation potential on the subband levels of a doped GaAs/Al x Ga1−x As quantum well within the local density approximation. The intersubband transition energy appropriate to infrared detectors is calculated including both exciton and depolarization shifts. These effects are all known to be very important in the Si inversion layer and are shown here to be significant for the intersubband transitions in dopedquantum wells. The effects of an applied electric field on these energy levels is explicitly included in the calculation. The Hartree potential is shown to effectively screen the electric field significantly reducing the Stark effect.

Neutron radiation effects in GaAs ion‐implanted metal‐semiconductor field‐effect transistors

Neutron radiation investigations have been carried out on ion‐implanted GaAs metal‐semiconductor field‐effect transistors (MESFETs). Device characteristics were measured before irradiation and following neutron irradiations with fluences from 5×1013 to 2×1015 n/cm2. At 5×1013 n/cm2, the device degradation is negligible, while at 2×1015 n/cm2 the threshold voltage shift was 0.7 V and the device transconductance was 30% of its original value. Degradation parameters needed to explain these results are larger than what has been previously reported; this discrepancy is, in part, due to the nonuniform doping profiles in the FET channel. The details of a new model applicable to nonuniform carrier distributions and to density dependent carrier removal rates are presented and results compared to those in the literature. A carrier removal rate of approximately 20 cm−1 is required to explain the degree of observed device degradation.

High energy neutron irradiation effects in GaAs modulation-doped field effect transistors (MOSFETs): threshold voltage

The effects of high energy neutrons on MODFETs were studied for fluences approaching 1*10/sup 15/ cm/sup -2/. Neutron-induced threshold-voltage shifts are described by application of a finite-temperature, strong-inversion, depletion-layer, charge-control model. The model indicates that the shifts are a consequence of electron trapping in the GaAs layer near the AlGaAs/GaAs interface. This allows a convenient parameterization of the neutron degradation by accounting for these trapped electrons as effective acceptors, defining an effective acceptor introduction rate, and applying the charge-control model to relate this introduction rate to the threshold voltage. The analysis shows that neutron degradation in these heterostructures is dominated by the change in the depletion layer charge and the shift in the Fermi level with neutron fluence. These dominant mechanisms depend on GaAs material parameters only. >

Characteristics of GaAs heterojunction FETs (HFETs) and source follower FET logic (SFFL) inverters exposed to high energy neutrons

GaAs heterojunction field-effect transistors (HFETs) and inverters and ring oscillators comprising HFETs have been exposed to neutron fluences of 5*10/sup 13/ n/cm/sup 2/ to 1*10/sup 15/ n/cm/sup 2/ to evaluate the characteristics of HFET integrated circuits in a high-energy-neutron environment. The HFETs exhibited preirradiation transconductances of approximately 120 mS/mm and threshold voltages of -0.82 V. The degree of transconductance degradation is similar to that observed in ion-implanted GaAs MESFETs, but the magnitude of the HFET threshold shift is significantly smaller. The neutron-induced threshold shift in GaAs HFETs has been modeled, including the effect of pinning the Fermi level at the semi-insulating boundary. Neutron bombardment of source-follower FET logic (SFFL) inverters and ring oscillators results in a reduction in high noise margin and an increase in low noise margin with 35% reduction in ring oscillator frequency at 1*10/sup 15/ n/cm/sup 2/. These results indicate that more complex HFET SFFL circuits should remain functional at high neutron fluences. >

Electric field dependence of the eigenstates of coupled quantum wells

We calculate the effects of the coupling between the eigenstates of two symmetric GaAs/ Alx Ga1−x As quantum wells separated by a thin barrier with an applied electric field using a very accurate numerical method that we have developed. Energy shifts with field and overlap integrals for the symmetric and antisymmetric electron and hole eigenstates are calculated. These results are compared to a single quantum well of equivalent dimensions. As suggested in the literature, the quantum‐confined Stark effect is shown to be significantly increased by the use of coupled quantum wells. Comparison with the experimental results of Chen et al. [Y. J. Chen, E. S. Koteles, B. S. Elman, and C. A. Armiento, Phys. Rev. B 36, 4562 (1987)] is made with surprisingly good agreement.

Density of states and linewidths of quantum wells with applied electric field

We calculate the density of states and corresponding linewidths for the quasi‐bound energy levels of a quantum well including both barriers of finite height and width, and an applied electric field. For a GaAs/AlxGa1−xAs quantum well with a well width of 85 A, barrier width of 50 A, and barrier height of 240 meV, we find a field‐induced linewidth for the excited state that increases from 2.9 meV at a field of 25 kV/cm to 9.5 meV at 150 kV/cm. This corresponds to a tunneling lifetime of 228 fs at 25 kV/cm decreasing to 69 fs at 150 kV/cm. The ground eigenstate has a narrow linewidth, less than 1 meV, up to a field of 150 kV/cm. The quasi‐bound approximation is shown to be applicable to the ground state, but not for the excited state.

Local plasmon modes of a semiconductor superlattice

The energy dispersion relation of the plasmon modes of an infinite periodic array of quantum wells where all the wells are doped with the same electron density except for one is calculated. In analogy to the local phonon modes of a vibrational lattice, we find that local plasmon modes exist. An explicit relation for the plasmon energy of the local mode is derived. Contrary to the surface plasmon mode of a semi‐infinite superlattice predicted by Giuliani and Quinn [Phys. Rev. Lett. 51, 919 (1983)], these local modes exist at all values of the wavevector. A low‐lying acoustic mode (ω≊qv) is found whose velocity is determined by the local density of the symmetry breaking well. Implications for obtaining amplification by drift instabilities in these structures are discussed.

Subthreshold I-V characteristics of AlGaAs/GaAs MODFETs: the role of unintentional acceptors

A strong-inversion depletion-layer model of threshold has been extended to describe subthreshold I-V characteristics in MODFETs. The results of this calculation yield the MODFET equivalent of the MOSFET charge sheet subthreshold model. For a typical molecular-beam-epitaxy-grown structure, the subthreshold current may differ by two orders of magnitude for a given gate voltage V/sub g/ and drain-to-source voltage V/sub ds/ as the acceptor doping varies from 10/sup 13/ to 10/sup 15/ cm/sup -3/. For these acceptor doping densities, the V/sub g/, for a given V/sub ds/, needed to maintain a constant subthreshold current varies by only approximately 0.1 V. If the acceptor density is increased to 10/sup 17/ cm/sup -3/, a large increase of approximately 0.8 V in the gate voltage is required to maintain a constant subthreshold current. These changes in subthreshold current with acceptor concentration in the bulk GaAs are significant and need to be included in an accurate MODFET model. >