ALBERT

Notes: The in situ, heteroepitaxial growth of Ge films grown on (100) n-GaAs, followed by in situ deposition of a Si3N4/Si insulator to form a Ge metal-insulator-semiconductor structure, is reported. The growth of the Ge, Si, and Si3N4 is carried out in an ultrahigh vacuum, chemical vapor deposition system which is vacuum connected to an adjacent III-V molecular beam epitaxy machine in which the GaAs is grown. The Ge is grown at low temperature (250 °C) using GeH4 and excited He from a remote plasma. After a rapid thermal annealing step, a marked reduction in hysteresis is observed in the capacitance-voltage characteristics and the density of interface trap states at the Si3N4/Si/Ge interface, as determined from the magnitude of the conductance peak, is found to decrease by a factor of 5, to 1×1011 eV−1 cm−2.

Notes: In the past several years, research in each of the wide-band-gap semiconductors, SiC, GaN, and ZnSe, has led to major advances which now make them viable for device applications. The merits of each contender for high-temperature electronics and short-wavelength optical applications are compared. The outstanding thermal and chemical stability of SiC and GaN should enable them to operate at high temperatures and in hostile environments, and also make them attractive for high-power operation. The present advanced stage of development of SiC substrates and metal-oxide-semiconductor technology makes SiC the leading contender for high-temperature and high-power applications if ohmic contacts and interface-state densities can be further improved. GaN, despite fundamentally superior electronic properties and better ohmic contact resistances, must overcome the lack of an ideal substrate material and a relatively advanced SiC infrastructure in order to compete in electronics applications. Prototype transistors have been fabricated from both SiC and GaN, and the microwave characteristics and high-temperature performance of SiC transistors have been studied. For optical emitters and detectors, ZnSe, SiC, and GaN all have demonstrated operation in the green, blue, or ultraviolet (UV) spectra.Blue SiC light-emitting diodes (LEDs) have been on the market for several years, joined recently by UV and blue GaN-based LEDs. These products should find wide use in full color display and other technologies. Promising prototype UV photodetectors have been fabricated from both SiC and GaN. In laser development, ZnSe leads the way with more sophisticated designs having further improved performance being rapidly demonstrated. If the low damage threshold of ZnSe continues to limit practical laser applications, GaN appears poised to become the semiconductor of choice for short-wavelength lasers in optical memory and other applications. For further development of these materials to be realized, doping densities (especially p type) and ohmic contact technologies have to be improved. Economies of scale need to be realized through the development of larger SiC substrates. Improved substrate materials, ideally GaN itself, need to be aggressively pursued to further develop the GaN-based material system and enable the fabrication of lasers. ZnSe material quality is already outstanding and now researchers must focus their attention on addressing the short lifetimes of ZnSe-based lasers to determine whether the material is sufficiently durable for practical laser applications. The problems related to these three wide-band-gap semiconductor systems have moved away from materials science toward the device arena, where their technological development can rapidly be brought to maturity.

Notes: Electrical properties of p+-Ge/N-AlGaAs (and N-GaAs) are studied as a function of temperature and current conduction mechanisms are outlined. Junctions with Ge grown on GaAs and AlGaAs show ideality factors of unity and 1.03 at room temperature, respectively. Temperature-dependent current-voltage (I-V) and room-temperature capacitance-voltage (C-V) characterization are employed to determine the built-in voltage (Vbi) of the two diode structures. For Ge/GaAs, a valence-band discontinuity of 0.49±0.05 eV is measured which is in good agreement with the value deduced from photoemission studies. Implications of p+-Ge base in AlGaAs/Ge/GaAs double-heterojunction bipolar transistors (DHBTs) are discussed.

Notes: The base-collector junction of GaAs/AlGaAs single heterojunction bipolar transistors has been observed to emit light at avalanche breakdown. The spectral distribution curve exhibits broad peaks at 2.03 and 1.43 eV, with the intensities dependent upon the reverse current. These observations suggest that electrons, excited to the upper conduction band by the field, lose their energy by impact ionizing electron-hole pairs and producing the 2.03 eV light, which corresponds to the threshold energy for electron impact ionization. The band-edge emission is the result of direct-gap free-carrier recombination and self-absorption of the high-energy transition.

Notes: Nonalloyed Al contacts were deposited by molecular beam epitaxy on both n- and p-type In0.53Ga0.47As layers prior to air exposure. These were shown to be ohmic, with specific contact resistances in the range of mid μΩ cm2 by the transmission-line model method. The thermal stability of these contacts was tested by annealing at temperatures between 350 and 450 °C for 30 min and at 300 °C for 500 h. Both experiments showed stable specific contact resistances.

Notes: We have fabricated 3 μm gate length self-aligned, depletion mode GaAs metal insulator semiconductor field effect transistors exhibiting transconductances, typically in the vicinity of 160 mS/mm. This achievement is attributed to the use of Si3N4 as the gate dielectric with a few monolayers of a Si/Ge interface layer between the GaAs channel layer and the insulator. The Si3N4/Si/Ge insulator structure is grown in situ using a plasma-enhanced chemical vapor deposition system which is connected by an ultrahigh vacuum transfer tube to an adjacent III-V molecular beam epitaxy system. Nearly ideal capacitance-voltage curves (compared to previous publications) suggest the existence of a high quality insulator/semiconductor interface. The lowest interface trap density that has been measured, as determined from the magnitude of the conductance peak is ∼2×1011 eV−1 cm−2.

Notes: Both AlxGa1−xAs/AlyGa1−yAs and GaAs pseudo-heterojunction bipolar transistors (HBTs) with lateral emitter resistors (laterally extended emitter ledge) are shown to exhibit base current ideality factors of 1.3 and 1.1, and uniform current gains of 10 and 5, respectively, even at current densities as low as 5×10−11 A/μm2. With this test structure, it is possible to separate the surface effects from the bulk effects. The structures investigated also allowed the confirmation that the emitter edge-thinning reported recently is very effective for reducing the surface recombination. The structures under investigation also show that GaAs HBTs can operate at collector currents in the microampere range or below.

Notes: A thermal-electrical model is presented to describe the negative output differential resistance in AlGaAs/GaAs heterojunction bipolar transistors. Included in this model are the base band-gap shrinkage due to heavy doping, temperature dependence of AlGaAs and GaAs band gaps, and valence band discontinuity. The experimental results unambiguously support the predictions of the model.