Volume 90, Issue 8, 15 October 2001
Index of content:
- DEVICE PHYSICS (PACS 85)
90(2001); http://dx.doi.org/10.1063/1.1402670View Description Hide Description
Porous silicon(PS) based structures with PS layer of different thickness were prepared on moderate- and high-resistivity substrates. Measurements of current–voltage characteristics and impedance at various temperatures were used for the investigation of the electrical properties of these structures. Electrical properties of the structures with relatively thin (1 μm) PS layer significantly differ from those of thick structures. The exponential forward bias dependencies for thin structures spread over several orders of magnitude with a low value of quality factor (close to 2) and have activation temperature dependencies with an activation energy equal to half the c-Si band gap. The reverse current has a square root dependence on the reverse bias voltage and the activation energy is equal to half the c-Si band gap. Therefore, it was concluded that the reverse and forward currents in thin PS-based device structures were determined by the generation and recombination of carriers in the depletion region of the c-Si substrate. It was shown that a large area spreading current exists in structures made on highly resistive substrates, which appears to be due to a highly conductive inverse (n-type) layer formed in the p-Si substrate at the heterojunction. The spreading effect leads to high reverse currents and high capacitance of the device structures made on highly resistive substrates.
90(2001); http://dx.doi.org/10.1063/1.1403665View Description Hide Description
GaN/InGaN light emitting diodes(LEDs) grown on sapphire substrates have current transport along the lateral direction due to the insulating nature of the substrate. The finite resistance of the n-type GaN buffer layer causes the pn junction current to be nonuniform and “crowd” near the edge of the contact. The current-crowding effect is analyzed both theoretically and experimentally for p-side-up mesa structure GaN/InGaN LEDs. The calculation yields an exponential decay of the current distribution under the p-type contact with a characteristic current spreading length, It is shown that GaN/InGaN LEDs with high p-type contact resistance and p-type confinement layer resistivity have a relatively uniform current distribution. However, as the p-type GaN conductivity and p-type ohmic contact conductivity is improved, significant current crowding near the contact edge will occur. The current crowding effect is analyzed experimentally in GaN/InGaN LEDs emitting in the blue spectral range. Experimental results show the light intensity decreasing with distance from the contact edge. A current spreading length of is found, in good agreement with theory.
Determination of traps in poly(p-phenylene vinylene) light emitting diodes by charge-based deep level transient spectroscopy90(2001); http://dx.doi.org/10.1063/1.1403669View Description Hide Description
Charge-based deep level transient spectroscopy has been used to study the defect states that exist within poly(p-phenylene vinylene) (PPV), a semiconducting polymer with a band gap of about 2.4 eV. The technique allows the determination of activation energies, capture cross sections, and trap concentrations. In some circumstances, it is also possible to distinguish between minority and majority carrier traps. The structures investigated here consisted of indium–tin–oxide (ITO)/PPV/MgAg light emitting diode(LED) devices. Two types of trapping centers were found. The first type has activation energies in the range 0.49–0.53 eV and capture cross sections on the order of It shows a Poole–Frenkel, field assisted–emission process. This level has been identified as a bulk acceptor-like majority carrier (i.e., hole) trap. The second type has activation energies in the range 0.40–0.42 eV and capture cross sections on the order of This level has been identified as a minority carrier (i.e., electron) trap. This second trap type is therefore expected to limit minority carrier injection into the PPV layer within the LED, and hence reduce electroluminescence under forward bias conditions.
90(2001); http://dx.doi.org/10.1063/1.1403683View Description Hide Description
We simulated capacitance–voltage curves of and also (virtual semiconductor) metal–insulator–semiconductor (MIS)capacitors and compared them with experimental curves of a structure. The experimental curves of the MIS capacitors are not in agreement with the simulated curves of the and MIS capacitors, but are in agreement with those of the MIS capacitors, where is a virtual semiconductor with or This indicates that the structure is somewhat like a narrow band gap material with The comparison yields strong support for our theoretical energy band of the MIS structure based on quantum well confinement. A depletion mode MIS field-effect-transistor (MISFET) is successfully fabricated with transconductance of 85 mS/mm, and an inversion mode MISFET is fabricated with transconductance of 0.05 mS/mm. The small transconductance for the inversion mode MISFET is ascribed to strong scattering due to confinement of electrons in the Si quantum well.