Search

Acceptor doping of MBE grown Ga0.47In0.53As on InP:Fe substrates utilizing manganese is investigated as an alternative for overcoming problems related to beryllium doping. The incorporation behaviour of manganese is analyzed in detail with respect to its application in optoelectronic device structures. Special emphasis is put on low-level and maximum-level doping relevant for use in buffer and contact layers, respectively. The dependence of activation energy and the degree of ionization on acceptor concentration is determined. At high doping levels the free-hole concentration is markedly lower than the doping concentration. It is attributed to the diffusion of acceptor species across the heterointerface into the substrate. Manganese diffusion is demonstrated to be an important effect for the interpretation of the measurement results. Manganese doping is applied in p+/n/p--layer structures for junction field effect transistor applications. The intended abruptness of acceptor profiles is deteriorated by diffusion of manganese in the Ga0.47In0.53As material. The degraded device characteristics obtained are attributed to the acceptor diffusion behaviour established.

The persistent photoconductivity effect in Si-dopedn-AlxGa1-xAs layers grown by molecular beam epitaxy on (100)GaAs substrates has been investigated by detailed Halleffect and capacitance measurements at 10–300 K. In the alloy composition range 0.25xGa1-xAs are governed by a deep electron trap having an emission barrier of 0.34–0.40 eV (depending on the doping concentration), as determined by admittance measurements. The concentration of deep electron traps, deduced from low-temperature capacitance measurements, is found to coincide with the amount of persistent photoconductivity observed in the material. Consequently, the earlier proposed population of two-dimensional subbands at the AlxGa1-xAs/GaAs-substrate hetero-interface, i.e. charge separation bymacroscopic barriers, can not account for the measured high overall number of persistent photoexcited carriers. Instead, the vanishing small capture rates of photoexcited electrons result frommicroscopic capture barriers. The dominant deep electron trap, which we attribute to deep donor-type (DX) centers, is found to be homogeneously distributed throughout the AlxGa1-xAs layer depth. From our Hall effect measurements a trap depth of 0.05–0.12 eV (depending on the doping concentration) below the conduction band is derived. The capture barrier is thus in the order of 0.30 eV. This value is in excellent agreement with data obtained from liquid phase epitaxially grown Si-dopedn-AlxGa1-xAs.

We present a new type of an artificial periodic semiconductor structure with tunable carrier concentrations and tunable energy gap which has been prepared by molecular beam epitaxy (MBE). The structure consists of a periodic sequence of thin (50 < d < 3000 ) n(Si)- and p(Be)-doped GaAs layers separated possibly by thin intrinsic (i-) layers called a “NIPI” structure. The space charge field of the ionised impurities varying in the direction of layer sequence produces a periodic ‘parallel’ modulation of the energy bands which determines the unusual electrical and optical properties of the material. The 1-dimensional periodic potential induces a splitting of the conduction and valence bands into sub-bands similar to those in the well-known compositional superlattices. In contrast to the GaAs/AlxGa1−xAs superlattice, however, GaAs doping superlattices exhibit an ‘indirect energy gap in real space’, with the electron and hole states spatially separated by half a period. Deviations from thermal equilibrium are thus quasi-stable, and excess-carrier lifetimes become very large. In this new class of semiconductor materials we demonstrate the tunability of conductivity, absorption coefficient, and luminescence by external voltage and by optical excitation.

Capacitance and Hall effect measurements in the temperature range 10-300 K were performed to evaluate the deep and shallow level characteristics of Si-doped n-AlxGa-xAs layers with 0 × 0.4 grown by molecular beam epitaxy. For alloy compositions × 0.3 the overall trap concentration was found to be less than 10−2of the carrier concentration. In this composition range the transport properties of the ternary alloy are comparable to those of n-GaAs:Si except for lower electron mobibities due to alloy scattering. With higher Al content one dominant electron trap determines the overall electrical properties of the material, and in n-Al0.35Ga0.65As:Si the deep trap concentration is already of the order of the free-carrier concentration or even higher. For the composition × = 0.35 ± 0.02 the influence of growth temperature and of Si dopant flux intensity on the deep trap concentration, on shallow and deep level activation energy, and on carrier freeze-out behaviour was studied and analyzed in detail. Our admittance measurements clearly revealed that the previously assumed deepening of the shallow level in n-AlxGa1-xAs of alloy composition close to the direct-indirect cross-over point does actuallynotexist. In this composition range an increase of the Si dopant flux leads to a reduction of the thermal activation energy for electron emission from shallow levels due to a lowering of the emission barrier by the electric field of the impurities. The increasing doping flux also enhances the concentration of the dominant electron trap strongly, thus indicating a participation of the dopant atoms in the formation of deep donor-type (D,X) centers. These results are in excellent agreement with the model first proposed by Lang et al. for interpretation of deep electron traps in n-AlxGa1-xgrown by liquid phase epitaxy.

The persistent photoconductivity effect in Si-doped n-AlGaAs layers grown by molecular beam epitaxy on (100)GaAs substrates has been investigated by detailed Halleffect and capacitance measurements at 10-300 K. In the alloy composition range 0.25< x <0.40 the electrical properties of n-AlGaAs are governed by a deep electron trap having an emission barrier of 0.34-0.40 eV (depending on the doping concentration), as determined by admittance measurements. The concentration of deep electron traps, deduced from low-temperature capacitance measurements, is found to coincide with the amount of persistent photoconductivity observed in the material. Consequently, the earlier proposed population of two-dimensional subbands at the AlGaAs/GaAs-substrate hetero-interface, i.e. charge separation by macroscopic barriers, can not account for the measured high overall number of persistent photoexcited carriers. Instead, the vanishing small capture rates of photoexcited electrons result from microscopic capture barriers. The dominant deep electron trap, which we attribute to deep donor-type ( DX) centers, is found to be homogeneously distributed throughout the AlGaAs layer depth. From our Hall effect measurements a trap depth of 0.05-0.12 eV (depending on the doping concentration) below the conduction band is derived. The capture barrier is thus in the order of 0.30 eV. This value is in excellent agreement with data obtained from liquid phase epitaxially grown Si-doped n-AlGaAs. [ABSTRACT FROM AUTHOR]