A. L. Powell

Techniques to minimize DX center deleterious effects in III‐V device performance

The properties of deep donor states (DX centers) in III‐V alloys are discussed in relation to their influence on device characteristics and performance. The techniques to avoid or minimize such deleterious effects in AlGaAs‐based devices are discussed, along with their physical basis, and some guidelines for improved III‐V device design are established. New results about the benefits of proper donor selection, the role of In alloying, the advantage of δ doping in layers and in modulation‐doped devices, and the use of AlInAs and InGaP as alternative wide band‐gap III‐V alloys are presented.

Impact ionization of Se-related DX centers in AlGaAs

The ionization processes of Se‐related DX centers have been studied in AlGaAs Schottky diodes under high reverse bias conditions. A spectroscopy technique that provides directly the free‐electron concentration has been used. Besides the well known thermal and optical electron emission processes, a new mechanism, attributed to an impact ionization process of DX centers, is described, and its kinetics is analyzed.

A comparison of dopant type and aluminium mole fraction on persistent photoconductivity in HEMT structures grown by MOVPE

The authors report the results of an investigation of persistent photoconductivity (PPC) in AlxGa1-xAs/GaAs HEMT structures grown by MOVPE, and the effects of different dopants and AlxGa1-xAs compositions. Temperature- and time-dependent Hall measurements for combinations of selenium-/silicon-doped, x=0.32/x=0.45 AlxGa1-xAs/GaAs layers are presented, allowing comparison of the factors influencing PPC. From their results they conclude that the amount of PPC exhibited by MOVPE grown HEMT structures is significantly reduced by using Se as the dopant, as opposed to Si. The PPC of the Se-doped samples also shows a virtual independence to AlxGa1-xAs composition. The relative stability of these Se-doped AlxGa1-xAs/GaAs structures at cryogenic temperatures should correspond to a significant reduction in the amount of drain-current collapse seen in HEMTS at low temperatures.

Persistent decrease of dark conductivity due to illumination in AlGaAs/GaAs modulation‐doped heterostructures

We report on a persistent decrease of the dark conductivity in AlGaAs/GaAs heterostructures due to illumination. The decrease was observed for photon energies between 0.7 and 1.15 eV and larger than 1.4 eV in the temperature range 170<T<300 K. Using proper bias conditions the dark conductivity after illumination can be 20% smaller than the dark conductivity in thermal equilibrium. The studies have been performed on samples with different doping species and compositions. A possible model for the observed behavior is discussed.

Deep level transient spectroscopy assessment of Silicon contamination in AlGaAs layers grown by metalorganic vapor phase epitaxy

A systematic silicon contamination has been detected by deep level transient spectroscopy in undoped and n-type doped (Te, Se, Sn) AlGaAs layers, grown in two different metalorganic vapor phase epitaxy reactors. DX center generation by substitutional donors, with very specific capture and emission thermal barriers (fingerprints), is the key to unambiguously identifying their presence, with detection limits well below the standard secondary ion mass spectroscopy capability. We comment on the potential sources of Si contamination (most common in this epitaxial technique), and on the relevance of such contamination to interpreting correctly experimental data related to the microscopic structure of DX centers.

DX center electron occupancy under hydrostatic pressure in Si-doped Iny(Ga1-xAlx)1-yAs alloys

DX centers in In‐mixed AlGaAs alloys are analyzed by deep level transient spectroscopy and capacitance vs temperature measurements. The addition of In to Si‐doped AlGaAs, with x=0.21 and 0.30, shifts the Si‐DX center to a shallower position. Under hydrostatic pressure, DX centers deepen again into the band gap. The DX center shift, and consequently, the reduction of the DX center electron occupancy, when In is added, is due to an increase of the Γ to L energy difference. In terms of band‐gap energy and DX center depth, adding 1% In is equivalent to a 1% Al reduction. Then, In mixing does not offer any new benefit to minimize DX center effects in AlGaAs‐based heterojunction devices.

DX centre characterization in Se-doped AlGaAs under hydrostatic pressure

Se-related DX centres in AlGaAs alloys have been characterized by deep-level transient spectroscopy techniques under hydrostatic pressure. The thermal-emission energy, Ee=0.24 +or-0.02 eV, is constant throughout the alloy range considered (0.29<x<0.77), and independent of the applied pressure. The thermal-capture barrier energy follows a V-shaped curve as a function of the alloy composition, with Ecmin=0.13 eV for Al compositions around 38%, where the Se-De centre electron occupancy is a maximum. Emission spectra show up to three peaks generated by three discrete emission rates. Their pressure dependence suggests the existence of three close discrete DX levels originating from changes of the donor local environment. These results confirm the model proposed by Chadi and Chang (1988, 1989) for Se-related DX centres (group-VI donors). As compared with the Si-DX centre properties quantitative differences originate from a shallower energy position of Se-Dx centres in the gap, owing to smaller values of the thermal emission and capture barriers, 200 meV and 70 meV lower, respectively.

Defect characterization in GaAlInAs alloys

Deep level transient spectroscopy (DLTS) and capacitance‐voltage measurements at various temperatures have been used to characterize defects in Si‐doped (Ga1−xAlx)1−yInyAs materials for x=0.3 and different values of y (0, 0.005, and 0.07). We only detect DX centers, those associated with the doping impurity (Si), but also others associated with Te and, eventually, Sn not introduced intentionally. When the experimental conditions are chosen to obtain exponential transients, the shape of the DLTS spectrum and its variation with the filling pulse duration can be accounted for by this contamination; i.e., no sign of the so‐called alloying effect is detected.

Li doping of GaAlAs

A series of unintentionally doped Ga1−xAlxAs epitaxial layers grown by molecular beam epitaxy, having various Al compositions (0.15, 0.18, and 0.29), have been Li diffused at 300 °C. Capacitance–voltage techniques show that the initial n‐type doping concentrations (of the order of 1016 cm−3) increase after diffusion by typically a factor 5 to 10 demonstrating that isolated Li interstitials do exist and behave as donors. However, secondary‐ion mass spectroscopy measurements show that the material contains 1019–1020 Li cm−3, which indicates that a large fraction of the Li impurities is not electrically active. Thus Li also produces defects as revealed by the fact that free electrons are frozen in the diffused layers below 77 K. Search for the existence of Li associated DX centers deep defects related to the donor impurities has been performed by deep level transient spectroscopy. Only the DX centers present before diffusion, i.e., associated with residual donor impurities, are detected in the layer of 0.29 ...