M. Buchanan

Negative capacitance effect in semiconductor devices

Nontrivial capacitance behavior, including a negative capacitance (NC) effect, observed in a variety of semiconductor devices, is discussed emphasizing the physical mechanism and the theoretical interpretation of experimental data. The correct interpretation of NC can be based on the analysis of the time-domain transient current in response to a small voltage step or impulse, involving a self-consistent treatment of all relevant physical effects (carrier transport, injection, recharging, etc.). NC appears in the case of the nonmonotonic or positive-valued behavior of the time-derivative of the transient current in response to a small voltage step. The time-domain transient current approach is illustrated by simulation results and experimental studies of quantum well infrared photodetectors (QWIPs). The NC effect in QWIPs has been predicted theoretically and confirmed experimentally. The huge NC phenomenon in QWIPs is due to the nonequilibrium transient injection from the emitter caused by the properties of the injection barrier and the inertia of the QW recharging.Nontrivial capacitance behavior, including a negative capacitance (NC) effect, observed in a variety of semiconductor devices, is discussed emphasizing the physical mechanism and the theoretical interpretation of experimental data. The correct interpretation of NC can be based on the analysis of the time-domain transient current in response to a small voltage step or impulse, involving a self-consistent treatment of all relevant physical effects (carrier transport, injection, recharging etc.). NC appears in the case of the non-monotonic or positive-valued behavior of the time-derivative of the transient current in response to a small voltage step. The time-domain transient current approach is illustrated by simulation results and experimental studies of quantum well infrared photodetectors (QWIPs). The NC effect in QWIPs has been predicted theoretically and confirmed experimentally. The huge NC phenomenon in QWIPs is due to the non-equilibrium transient injection from the emitter caused by the properties of the injection barrier and the inertia of the QW recharging.

Transparent and highly conductive films of ZnO prepared by rf reactive magnetron sputtering

Highly conductive films of zinc oxide have been prepared by reactive rf magnetron sputtering from an oxide target. Film conductivities ranging from ∼10−8 Ω−1 cm−1 to 5×102 Ω−1 cm−1 can be obtained depending on the sputter conditions. Films with sheet resistivities of 85 Ω/⧠ showed little absorption and ∼90% transmission between λ = 4000→8000 A. A second low power discharge at the substrate is used to initiate growth of the highly conducting material on room‐temperature substrates. Thus, during the deposition of insultating ZnO, turning on this second discharge causes the deposition to ’’switch’’ from low conductivity to high conductivity material. This is of particular interest in the fabrication of semiconductor‐insulator‐semiconductor solar cells where precise control over the thickness of the insulating layer is necessary and where a highly transparent and conductive window‐junction layer is required.

Preparation of conducting and transparent thin films of tin‐doped indium oxide by magnetron sputtering

High‐quality 800‐A‐thick films of tin‐doped indium oxide have been prepared by magnetron sputtering. It is shown that films with low resistivity (∼4×10−4 Ω cm) and high optical transmission (≳85% between 4000 and 8000 A) can be prepared on low‐temperature (40–180 °C) substrates with O2 partial pressures of (2–7)×10−5 Torr.

Resonant tunneling in Si/Si1−xGex double‐barrier structures

Resonant tunneling of holes has been observed for the first time in double‐barrier diodes with strained Si1−xGex quantum wells formed between unstrained Si barriers. Negative differential resistance with a peak‐to‐valley ratio in current of 1.8 at 77 K and of 2.2 at 4.2 K has been exhibited by a sample with a 3.3‐nm‐wide Si0.79Ge0.21 well between 6.0 nm Si barriers. The positions of the current peaks are in reasonable agreement with calculations of the positions of heavy‐hole levels in the quantum well.

How good is the polarization selection rule for intersubband transitions

Using GaAs based quantum well infrared photodetectors (QWIPs) with either GaAs or InGaAs wells, we experimentally investigate the accuracy of the polarization selection rule for conduction band intersubband transitions. We employ a device structure and a light coupling geometry where the parasitic light scattering is negligible. The experiments imply that the selection rule is followed to an accuracy of 0.2% for a 8.1 μm QWIP with GaAs wells; this degrades to 3% for a 4.6 μm QWIP with In0.1Ga0.9As wells.

Segregation of Si δ doping in GaAs‐AlGaAs quantum wells and the cause of the asymmetry in the current‐voltage characteristics of intersubband infrared detectors

Dopant segregation in the well region of a multiple quantum well intersubband photodetector can cause an asymmetry in the observed forward and reverse current‐voltage characteristics. We compensate for the segregation by shifting the position of the Si δ doping in the well and model the effect with good agreement for a range of shift values. For samples grown at a substrate temperature of 605 °C, we find that the observed behavior is best described by assuming that the Si δ‐doping profile smears in the growth direction resulting in an asymmetric broadening of about 27 A.

NEGATIVE CAPACITANCE OF GAAS HOMOJUNCTION FAR-INFRARED DETECTORS

Bias, frequency and temperature-dependent capacitance characteristics of p-GaAs homojunction interfacial work-function internal photoemission (HIWIP) far-infrared detectors are reported. A strong negative capacitance phenomenon has been observed. Unlike in other devices, even up to 1 MHz in HIWIP, the negative capacitance value keeps increasing with frequency, giving a stronger effect. The origin of this effect is believed to be due to the carrier capture and emission at interface states. Fitting data based on charging-discharging current and the inertial conducting current model show good agreement with the experimental observations.

Multicolor voltage-tunable quantum-well infrared photodetector

A novel concept for a quantum-well infrared photodetector (QWIP) with a spectral response peak tunable by an external voltage is described and tested experimentally. This multicolor detector structure is made by stacking conventional (one-color) QWIPs, separated by thin, heavily doped layers ( approximately=90 nm in the test structure). The most important feature is that externally applied DC voltage is distributed among the different one-color QWIPs according to their DC resistances. Each one-color QWIP therefore turns on sequentially in the order determined by its resistance.<<ETX>>

Unusual capacitance behavior of quantum well infrared photodetectors

We report experimental and simulation results of capacitance of quantum well infrared photodetectors (QWIPs). We found that the QWIP capacitance displays unusual behavior as a function of voltage and frequency, deviating far from the constant geometric capacitance value. At high voltages, capacitance starts with a negative value at low frequencies, increases above zero with frequency, and eventually decays to the geometric capacitance value. The magnitude of negative capacitance exceeds the geometric capacitance by more than two orders of magnitude. Negative capacitance arises when the transient current in response to a voltage step is nonmonotonic with time. Simulation shows that this effect is due to nonequilibrium transient electron injection from the emitter resulting from the properties of the injection barrier and inertia of the QW recharging processes.

QUANTUM-WELL INTERMIXING FOR OPTOELECTRONIC INTEGRATION USING HIGH ENERGY ION IMPLANTATION

The technique of ion‐induced quantum‐well (QW) intermixing using broad area, high energy (2–8 MeV As4+) ion implantation has been studied in a graded‐index separate confinement heterostructure InGaAs/GaAs QW laser. This approach offers the prospect of a powerful and relatively simple fabrication technique for integrating optoelectronic devices. Parameters controlling the ion‐induced QW intermixing, such as ion doses, fluxes, and energies, post‐implantation annealing time, and temperature are investigated and optimized using optical characterization techniques such as photoluminescence, photoluminescence excitation, and absorption spectroscopy.