Very high carbon δdoping concentration in Al x Ga1−x As grown by metalorganic vapor phase epitaxy using trimethylaluminum as a doping precursor Our photo reflectance ͑PR͒ spectroscopy measurements of the ␦-doped GaAs film at 300 K reveal many Franz-Keldysh oscillations ͑FKOs͒ above the valence band edge, E 0 and the spin-orbit split energy, E 0 ϩ⌬ 0 , which enables us to determine the electric field strength from periods of FKOs provided reduced masses of the electron and holes are known. The reduced masses can be determined unambiguously at E 0 ϩ⌬ 0 , but not at E 0 , at which the heavy-and light-hole transitions are degenerate. However, the ambiguity at E 0 can be resolved by applying the fast Fourier transform to the PR spectrum to separate the contributions from the heavy and light holes.
The electroreflectance (ER) spectra of an undoped n+-type doped GaAs have been measured over a range of temperature from 25 to 400 K. Many Franz–Keldysh oscillations were observed above the band-gap energy, which enabled the electric field strength and, hence, also the Fermi level to be determined. The photovoltaic effect is shown to be negligible, even at the low temperature. The experiment shows that the Fermi level decreases with increasing temperature and has almost the same temperature dependence as the energy gap. It is pinned at about 0.63 of energy gap below the conduction band.
This study developed a novel type of normalization procedure for modulation reflectance spectroscopy experiments to obtain the relative change in the reflectance spectrum, ΔR/R. This technique uses a polymer-dispersed liquid crystal to ensure that the dc component of the signal from the detector remained constant by varying the intensity of the light striking the sample. This method is particularly useful for photoreflectance measurement, which may encounter background problems because of scattered pump light and/or photoluminescence. It does not require a change in the gain of the detector or the use of a variable neutral density filter mounted on a servo-motor.
We have studied the electronic properties of 5-doped AlQ48InQ»As/GaQ47InQ53As heterostructure samples by Shubnikovde Haas and quantum-Hall-effect measurements. The well-defined two-subband occupied two-dimensional electron gas was detected. From the Fermi level at these two subbands, we calculate the energy difference between the lowest two subbands' minima to be 54 meV. After illuminating the sample at low temperature, we did observe a persistent photoconductivity.The electron densities of the first and second subbands for the sample with preannealed substrate were increased by the illumination from 17.3 to 18.2X10" cm and 3.6 to 4. 1X10" cm, respectively. The onset of the second subband occurs at n Q = 10.3 X 10" cm . We also found that the concentration of deep donor traps is about 1.4X 10" cm and the deep donor traps are not affected by the preannealed substrate.We believe that the model to describe the persistent-photoconductivity effect in semiconductor heterostructure samples needs to be extended to those deep donor levels below Fermi energy and it does not have to be the DX center.
Photoreflectance spectroscopy of surface-intrinsic n ϩ-doped (s-in ϩ) GaAs has been measured at various power densities (P pu) of a pump beam. Many Franz-Keldysh oscillations ͑FKOs͒ were observed above the band-gap energy, which will enable the electric-field strength (F) to be determined from the periods of the FKOs. Field F thus obtained is subject to photovoltaic effects. In order to reduce the photovoltaic effects from the pump beam, P pu was kept below 10 W/cm 2 in the previous experiments. Here, we demonstrate that the built-in field can be determined at a larger P pu by using fast Fourier transform techniques.
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