52 research outputs found
Temperature dependence of minority and majority carrier mobilities in degenerately doped GaAs
Measured minority and majority carrier mobility temperature dependencies in heavily doped n- and p-GaAs are compared. Majority carrier mobilities in heavily doped GaAs are essentially temperature ~T! independent while minority carrier mobilities exhibit a roughly 1/T dependence. Majority carrier freezeout, which reduces both majorityâminority carrier and ionized impurity scattering, is shown not to be responsible for the 1/T minority carrier mobility dependence. The difference in minority and majority carrier mobility T dependencies is explained in terms of the increased degree of degeneracy of majority carriers with decreased temperature, which decreases majorityâminority carrier scattering
Minority Hole Mobility in n+ GaAs
The minority hole diffusivity, or equivalently the hole mobility, was measured in n+GaAs with the zeroâfield timeâofâflight technique. The minority hole mobility was measured for the donor doping range of 1.3Ă1017 cmâ3 to 1.8Ă1018 cmâ3 and was found to vary from 235 to 295 cm2/Vâs. At the lower doping level, the minority hole mobility is comparable to the corresponding majority hole mobility, but at 1.8Ă1018 cmâ3 the minority hole mobility was 30% higher than the majority carrier hole mobility. These results have important implications for the design of devices such as solar cells and pnpâheterojunction bipolar transistors
Characterization of photon recycling in thin crystalline GaAs light emitting diodes
Gallium arsenide light emitting diodes (LEDs) were fabricated using molecular beam epitaxial films on GaAs substrates and removed by epitaxial lift-off (ELO). Lifted off devices were then mounted on a Si wafer using a Pd/Au/Cr contact layer, which also served as a back surface reflector. Devices were characterized by electrical and optical measurements, and the results for devices on the GaAs substrate were compared to those for EL0 devices. EL0 LEDs coated with a ZnS/MgF2 antireflection coating exhibited an optical output that was up to six times that of LEDs on GaAs substrates. At the same time, the measured current-voltage characteristics of the EL0 devices displayed a lower IZ = 1 current component. EL0 LEDs with efficiencies up to 12.5% were realized. We attribute these results to photon recycIing enhanced by the back-surface reflector in the EL0 LEDs. The luminescence versus current and current versus voltage characteristics of the LEDs were analyzed to obtain the nonradiative minority carrier lifetimes and the photon recycling factors. The results demonstrate that the measured characteristics are well described by photon recycling theory. EL0 LEDs may prove useful for characterizing recombination processes in LEDs, and thin-crystalline structures could provide substantial efficiency enhancements for LEDs and solar cells
Effect of impurity trapping on the capacitanceâvoltage characteristics of nâGaAs/NâAlGaAs heterojunctions
We have studied the capacitance-voltage (C- V) characteristics of Schottky barriers on inverted nGaAs/ N-AIGaAs and normal N-AIGaAs/n-GaAs heterojunctions. Impurities introduced during film growth produced a negative sheet charge of 6.0 X 10 II cm -2 at the interface of the inverted n-GaAs/N-AIGaAs heterojunction. The effectiveness of GaAs quantum wells in trapping these impurities was investigated. GaAs quantum wells 20 A wide were placed in intervals of 2500 A for the first 0.75 pm of the AIGaAs layer; in the last 0.25 pm, the periodicity of the quantum wells was progressively decreased by half with the last quantum well placed at about 160 A from the GaAs/ AIGaAs interface. The resulting measured interface charge concentration of 4.4 X 1010 cm -2 is more than a magnitude lower than measured before the use of the quantum wells and is essentially at the limit of the accuracy of the C-V technique for this structure
Transistor-Based Studies of Heavy Dop-ing Effects in n-GaAs
The n2ieDp product (where n2ie is the np product and Dp is the minority hole mobility) in heavily doped nâGaAs has been measured by electrical characterization of pânâp GaAs homojunction transistors with base dopings ranging from approximately 1Ă1017 to 9Ă1018 cmâ3. The measured n2ieDp product decreases as the doping density increases. These results suggest that nie is roughly constant with doping density, in sharp contrast to the large increase observed for pâtype GaAs. This work shows that when designing GaAs bipolar devices, it is important to consider the large difference in effective band gap between n+ and p+ regions
Effective Bandgap Shrinkage in GaAs
Electrical measurements of the equilibrium np product (n2ie) in heavily doped nâ and pâGaAs were performed. The n2ieDproduct (where D is the diffusivity) was measured by fitting the collector currentâvoltage characteristic of a homojunction bipolar transistor to an ideal diode equation modified to account for transport in thin base transistors.The n2ie product was then extracted from n2ieD by utilizing diffusivity results obtained with the zeroâfield timeâofâflight technique. Our results show significant effective bandâgap shrinkage in heavily doped pâGaAs, and very little effective bandâgap shrinkage in heavily doped nâGaAs. At extremely heavy dopings, an effective bandâgap widening is observed for both nâ and pâGaAs and is attributed to the effects of degeneracy
Proposed Structure for Large Quantum Interference Effects
In this letter we propose and analyze a new semiconductor structure that can be fabricated by present day technology and can lead to large quantum interference effects with potential device applications
Experimental determination of the effects of degenerate Fermi statistics on heavily pâdoped GaAs
The effects of degenerate Fermi statistics on electron injection currents for p+âGaAs grown by molecular beam epitaxy are presented. To achieve Be dopant concentrations of greater than 8Ă1019 cmâ3, the substrate temperature during growth was reduced to approximately 450â°C from the usual 600â°C. In this heavily doped material, we measure unexpectedly large electron injectioncurrents which are interpreted in terms of an effective narrowing of the band gap. At extremely heavy doping densities, the Fermi level pushes into the valence band and degenerate Fermi statistics must be taken into account. For doping concentrations greater than 1Ă1020 cmâ3, effects due to degenerate Fermi statistics oppose the bandâgap shrinkage effects; consequently, a reduction in the electron injection currents is observed. The result is a substantial reduction in gain for AlGaAs/GaAs heterostructure bipolar transistors when the base is doped above 1020 cmâ3
Ultra narrow AuPd and Al wires
In this letter we discuss a novel and versatile template technique aimed to
the fabrication of sub-10 nm wide wires. Using this technique, we have
successfully measured AuPd wires, 12 nm wide and as long as 20 m. Even
materials that form a strong superficial oxide, and thus not suited to be used
in combination with other techniques, can be successfully employed. In
particular we have measured Al wires, with lateral width smaller or comparable
to 10 nm, and length exceeding 10 m.Comment: 4 pages, 4 figures. Pubblished in APL 86, 172501 (2005). Added
erratum and revised Fig.
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