Si-doped AlGaAs Research Articles

Sb-mediated growth of Si-doped AlGaAs by molecular-beam epitaxy

The Sb-mediated growth of Al0.65Ga0.35As is studied for Sb/III flux ratios from 0 to 2% and growth temperatures from 580 to 720 °C. The electrical properties and surface morphology are found to depend strongly on both the growth temperature and the Sb flux. As an isoelectronic dopant, Sb improves the conductivity of n-Al0.65Ga0.35As with the highest conductivities occurring at the highest growth temperatures. As a surfactant, Sb improves the surface morphology at all growth temperatures, with the most dramatic improvement occurring at 670 °C. The smoothest surface (0.2 nm rms roughness) was obtained at 700 °C using a Sb/III flux ratio of 0.02. Furthermore, we have demonstrated that the use of Sb during the molecular-beam-epitaxy growth of AlGaAs effectively eliminates the “forbidden temperature gap.”

Degradation and recovery of AlGaAs/GaAs p-HEMT irradiated by high-energy particle

Results of an extensive study on the irradiation damage and its recovery behavior resulting from thermal annealing in AlGaAs/GaAs pseudomorphic high electron mobility transistors (HEMTs) subjected to a 220-MeV carbon, 1-MeV electrons and 1-MeV fast neutrons are presented. The drain current and effective mobility decrease after irradiation, while the threshold voltage increases in positive direction. The decrease of the drain current and mobility is thought to be due to the scattering of channel electrons with the induced lattice defects and also to the decrease of the electron density in the two dimensional electron gas region. Isochronal thermal annealing shows that the device performance degraded by the irradiation recovers. The decreased drain current for output characteristics recovers by 75% of pre-rad value after 300°C thermal annealing for AlGaAs HEMTs irradiated by carbon particles with a fluence of 1×10 12 cm −2. The influence of the materials and radiation source on the degradation is also discussed with respect to the nonionizing energy loss. Those are mainly attributed to the difference of particle mass and the probability of nuclear collision for the formation of lattice defect in Si-doped AlGaAs donor layer. A comparison is also made with results obtained on irradiated InGaP/InGaAs p-HEMTs in order to investigate the effect of the constituent atom. The damage coefficient of AlGaAs HEMTs is also about one order greater than that of InGaP HEMTs for the same radiation source. The materials and radiation source dependence of performance degradation is mainly thought to be attributed to the difference of mass and the possibility of nuclear collision for the formation of lattice defects in Si-doped donor layer.

New type of persistent photoconductivity related to DX-center: the study of interband PPC in Si-doped AlGaAs

We studied the build-up of the persistent photoconductivity (PPC) in AlxGa1−xAs (x=0.15) doped with Si. The Hall electron concentration and the conductivity were measured at 77K as a function of time while the sample was illuminated by the monochromatic light. The case of excitation by photons with energy below the gap was compared to that by photons with energy just above the gap. The first mode leads to optical ionization of DX centers. The new mechanism of persistent emptying of DX centers, specific for inter-band PPC, is related to the capture of photo-created holes by negatively charged DX ions. Our results indicate the importance of effects of correlated space distribution of charges on impurity sites. The evolution of mobility during the build-up indicates that illumination with above-gap photons increases the degree of correlation, in contrast to the lower-energy ones. By analyzing the transients n(t) we conclude that the effective cross section for hole capture decreases dramatically during the build-up process. This indicates that the hole capture by DX− ions is strongly influenced by the Coulomb potential from the nearest impurity ions.

Heavy Si doping in AlGaAs near modulation-doped heterointerfaces

We propose that the local Fermi-level modulation that occurs at heterojunctions enables the achievement of a higher carrier concentration in a limited thin layer than that can be achieved in a thick layer. We studied heterojunctions consisting of Si-doped AlGaAs (Al=20%) (AlGaAs:Si)/InGaAs (In=20%) grown by metalorganic chemical vapor deposition to demonstrate effectiveness of our doping method. The thickness and doping concentration of the AlGaAs:Si were designed carefully not to form a neutral region in the structure. We obtained a maximum electron concentration Nd=2.4×1019 cm−3 at 300 K in AlGaAs:Si; this value is about seven times higher than that reported for AlGaAs:Si so far. We believe that the AlGaAs/InGaAs heterojunction positions the Fermi level in the AlGaAs:Si (near the heterojunction) far from the conduction-band minimum, and that this effectively suppresses creation of compensation centers in AlGaAs:Si, then all the Si donors in AlGaAs:Si are activated.

Spatially Resolved Optical Characterization of AlGaAs Layers Grown on Patterned Substrates

We have applied micro-photoluminescence (PL) spectroscopy to scan the Al concentration and the well width of an undoped GaAs/AlGaAs single quantum well (QW) and micro-Raman spectroscopy to scan the carrier concentration of a Si-doped AlGaAs layer along (100)/(111)A/(100) facet transitions. The QW and the doped layer were grown by MBE on patterned GaAs substrates exhibiting (100) ridges and (100) grooves separated by 5 μm wide (111)A facets. The width of the GaAs QW, obtained from the spectral position of the QW PL, decreases from 10 nm on the (100) planes to 8 nm on the (111)A facet. The Al concentration, derived from the spectral position of the AlGaAs-barrier PL, varies between 23.5 and 29.5% along the facet transition. The type and concentration of charge carriers, deduced from the spectral position and line shape of the coupled plasmon–LO-phonon modes varies from n = 1.8 × 1018 cm—3 on the (100) planes to n < 4 × 1017 cm—3 or p-type on the (111)A facet.

Electron mobility in Si-doped AlGaAs alloy and GaAs-AlAs pseudo-alloy

We compare the electrical properties of an AlGaAs thin layer and a GaAs-AlAs short-period superlattice, both being doped uniformly with silicon. The carrier thermal activation is interpreted within the framework of the multidonor splitting of the deep silicon state, the so-called DX centre. We focus on the scattering mechanisms involved in the two structures. In addition to classical scattering processes, alloy scattering is introduced to account for the behaviour of the Hall mobility with temperature in the alloy. We utilize the persistent electron photoionization effect in investigating the increase of the Hall mobility with the free carrier concentration. We find that interface roughness scattering cannot account alone for the experimental behaviour in the superlattice and we suggest that an additional anisotropic scattering process is probably responsible for this discrepancy owing to the two-semiconductor structure of the pseudo-alloy.

Minority carrier capture at DX centers in AlGaSb Schottky diodes

Hole capture at DX centers is evidenced in Schottky diodes made on Te-doped AlGaSb under forward bias conditions. Capacitance versus voltage measurements performed at low temperatures show that the occupancy of the DX centers is affected by sufficiently large forward biases. The current densities required to achieve such modifications are at least one order of magnitude smaller than those needed in Si-doped AlGaAs. Positive deep level transient spectroscopy signals are systematically detected in all the samples studied when forward-bias filling pulses are used. These observations are discussed in terms of minority carrier (hole) injection and subsequent capture by the DX centers.

Investigation of Si-doped p-type , quantum well infrared photodetectors and multiquantum wells grown on (3 1 1)A GaAs

We have studied two Si-doped p-type quantum well infrared photodetectors (QWIPs) using AlGaAs GaAs and AlGaAs InGaAs grown on (3 1 1)A GaAs. The Si-doped AlGaAs GaAs p-QWIP exhibits a symmetrical dark I–V characteristic at all the measured temperatures from 40 to 120 K. The strained p-type AlGaAs InGaAs QWIP exhibits a slightly asymmetrical dark I–V characteristic, but is markedly less asymmetrical than that doped with beryllium. The slight asymmetry in dark I–V characteristic and the large blue-shift in responsivity spectra may be due to the thickness modulation observed from TEM and the red-shift of PL peak energy, where PL peak energies from (3 1 1)A AlGaAs InGaAs multiple quantum wells are red-shifts of 7 and 22 meV to the side-by-side grown (1 0 0).

Photoluminescence investigation of p-type Si-doped AlGaAs grown by molecular beam epitaxy on (1 1 1)A, (2 1 1)A and (3 1 1)A GaAs surfaces

Abstract A systematic study of Si-doped AlGaAs grown on (1 1 1)A, (2 1 1)A, and (3 1 1)A GaAs surfaces by molecular beam epitaxy is reported. Both electrical and photoluminescence measurements have been carried out to investigate the physics of the Si incorporation. The optical identification of the incorporated impurities together with the determination of free carrier concentration and mobility allowed us to suggest a simple kinetic model for the Si incorporation in AlGaAs grown on substrates with different orientations.

A transient capacitance - voltage method for characterizing DX centres

A transient capacitance - voltage (TCV) method is proposed for measuring the net donor concentration, the free electron concentration and the space charge distribution of Schottky diodes prepared on Si-doped AlGaAs. The utility of this method is demonstrated by determining the binding energy of the DX centre from the temperature-dependent free electron concentration. Comparisons of the space charge distribution between theory and experiment are performed for the positive-U and negative-U model of the DX centres. The results obtained for the negative-U model are consistent with recent observations of multi-DX levels.

Metastable states of Si donors in AlxGa1−xAs

Donor related states in Si-doped AlGaAs with Al compositions ranging from 0.30 to 0.59 were investigated by capacitance measurements. In addition to the stable Si-DX state, two metastable states of the silicon donor were observed. Of the two metastable states, the shallower one is attributed to the X-conduction band related effective mass state arising from substitutional silicon on the group III site. The deeper one is proposed to be related to a donor configuration different from both the substitutional configuration and that of the DX state.

Growth of high-quality InGaP and application for modulation-doped structure by molecular beam epitaxy with a GaP source

High-quality In 0.48Ga 0.52P lattice-matched to GaAs has been successfully grown by solid-source molecular beam epitaxy (MBE). To avoid the problem associated with white phosphorus, a dimer phosphorus molecular beam was produced by the sublimation of GaP. The In 0.48Ga 0.52P layers have been characterized by double-crystal X-ray diffraction (DCXR), photoluminescence (PL) spectroscopy and Hall measurements. The full width at half maximum of the (004) DCXR peak from 1.7 μm thick layer is 13 arc sec, which is comparable to the smallest value ever reported. The PL linewidth at 10 K is 16 meV and the electron mobilities are comparable to similar InGaP/GaAs layers grown by gas-source MBE. To confirm the layer quality for device application, InGaP layers were used for pseudomorphic AlGaAs/InGaAs/GaAs modulation-doped field-effect transister (MODFET) structures. Since wet chemical etchants with pronounced selectivity between InGaP and GaAs are well established, InGaP layers are attractive for the device fabrication as etch-stopper. In addition, Si-doped InGaP can replace the Si-doped AlGaAs layer, which causes the problem of persistent photoconductivity (PPC) at low temperatures. We have grown three different samples to compare the properties of the MODFET structures. The insertion of the etch-stopper did not deteriorate the properties of the MODFET structures and the PPC effect was significantly improved, although the carrier concentration was reduced.

Lateral junctions of molecular beam epitaxial grown Si-doped GaAs and AlGaAs on patterned substrates

We investigated laterally aligned pn junctions formed by a one-step molecular beam epitaxial growth of Si-doped GaAs and AlGaAs on patterned substrates. Current-voltage (I-V) and cross-sectional electron beam induced current measurements suggested that the junctions’ configuration was rather nn−pn with a highly compensated n− region than simple npn configuration. Temperature dependence of the I-V curves was also examined and the breakdown mechanism was found to be avalanche multiplication. Transmission electron microscope measurement revealed that the critical angle of crystal plane which determines the conduction type of Si-doped GaAs is exactly 25°, i.e., the (311)A plane.

Thermally Stable Oxygen and Nitrogen Implant Isolation of C-Doped Al0.35Ga0.65As

ABSTRACTOxygen and nitrogen ion implantation have been applied, for the first time, to C-doped Al0.35Ga0.65As layers produce high resistivity regions (ps ≥ 1×1010 Ω/□) that are stable after annealing at 900 ºC. A dose threshold for stable compensation for both O and N ions was found above 8×1013 cm-2 for samples doped at 2×1018 cm-3. Although O implantation has been reported to form stable compensation in Si-doped and Be-doped AlGaAs, the ability of nitrogen implantation to produce thermally stable compensation has not been previously reported and may be due to a C-N complex. The existence of this C-N complex is supported by results for O- and N-implants into C-doped GaAs where N formed thermally stable compensation but O did not. Sheet resistance data versus anneal temperature and estimates of the depth of the defect levels are reported. This result will have application to heterojunction bipolar transistors and complementary heterostructure field effect transistor technologies that employ C-doped AlGaAs or GaAs layers along with high temperature post-isolation processing.

Enhancement of the Metal/Si-Doped AlGaAs Schottky Barrier Height by CH4/H2 Reactive Ion Etching

The damage introduced by CH4/H2 reactive ion etching (RIE) on Si-doped AlGaAs layers is studied by X-Ray Photoelectron Spectroscopy (XPS), Auger electron spectrocopy (AES) and electrical measurements on Schottky contacts. The XPS analysis of the surface stoichiometry after RIE exposure shows arsenic depletion and adsorbed carbon as the main characteristics. The carbon spectrum consists of a component due to atmospheric contamination and an additional photoelectron peak at 283 eV, which we correlate with the formation of Ga-C radicals at the AlGaAs surface during RIE. The reaction process at the Au/TiW/Ti/AlGaAs interface after RIE exposure and subsequent thermal annealing is monitored by AES. Also by this technique, carbon was detected at the Ti/AlGaAs interface and no interdiffusion was observed. The electrical behaviour of the contacts is characterized by capacitance-voltage (CV) and current-voltage (IV) measurements. Schottky barrier height, ideality factor and reverse breakdown were determined. The barrier height extracted from CV measurements of the samples exposed to RIE shows increased values (1.1 to 1.4 eV) compared to the reference samples (1.0 to 1.1 eV), depending on the aluminium concentration. The same behaviour was observed in the ideality factor. The results are explained by the formation of a p-n junction below the metal/AlGaAs barrier. Good agreement between experimental and theoretical values is found when the compensation of Si donors was taken into account.

Influence of CH4/H2 Reactive Ion Etching on Deep Levels in Si-Doped AlxGa1−xAs (x=0.25)

The effect of CH4/H2 reactive ion etching (RIE) on Si-doped AlxGa1−xAs (x=0.25) is studied by deep level transient spectroscopy (DLTS) and Hall measurements. After RIE exposure, the samples were annealed between 250 and 500°C in order to study the recovery kinetics of deep and shallow levels. Non-etched reference samples showed broad DLTS spectra which were deconvoluted in two different emission peaks. We assigned them to DX1 and DX2 centers. The different deep levels are characierized by different aluminium configurations, one or two aluminium atoms, surrounding the silicon donor which are responsible for the DX centers. After RIE exposure and subsequent thermal annealing, a third emission peak is observed. This emission is attributed to the DX3 center, which is characterized by three aluminium atoms neighbouring the silicon donor. The recovery activation energy is calculated based on first-order kinetics. The activation energies are found to be around 1.9 eV in all cases.Complementary Hall measurements as a function of temperature (4-300 K) were used to characterize the electron capture of deep levels in Si-doped AlGaAs exposed to CH4/H2 RIE. We observed that the samples exposed to RIE and annealed at temperatures higher than 400°C exhibit electron capture in the 120-150 K temperature range. On the other hand, samples annealed at lower temperatures, showed additional capture features between 200 and 230 K.

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.

Low threshold InGaAs strained quantum well laser with lateral npn current blocking structure grown by molecular beam epitaxy

A sophisticated current confinement structure using the amphoteric nature of MBE grown Si-doped GaAs and AlGaAs is demonstrated for an index-guided InGaAs strained quantum well laser. Lateral and vertical npn current blocking layers were formed for the first time by a selfaligned process. Devices showed threshold current of 10 mA and total slope efficiency of 0.56 under room temperature CW operation.

Longitudinal mode stability difference in Se- and Si-doped AlGaAs lasers

The longitudinal mode behavior of AlGaAs lasers with n-type cladding layers doped with Se and Si is reported. The longitudinal mode of the lasers with highly Se-doped cladding layer is stabilized, and large hysteresis of mode jump is observed as temperature changes. In the case of highly Si-doped cladding layers, however, the mode hops to the adjacent one and no hysteresis is observed. These phenomena are explained by the difference in thermal activation energy between Se- and Si-related DX centers.

Hydrogen Passivation and Reactivation of DX Centers in Se-Doped and Si-Doped AlGaAs - - A Comparison

ABSTRACTThe effect of hydrogenation on DX centers was evaluated for both Si- and Se-doped AlxGa1-xAs (x=0.26 and 0.23). MBE-grown AIGaAs:Si and MOCVD-grown AIGaAs:Se epilayers were hydrogenated with either monatomic hydrogen or deuterium from a remote plasma at 250°C for 1h. The passivation and subsequent reactivation kinetics were studied with C-V and DLTS techniques. Reactivation was investigated in the space-charge layer of Schottky diodes under different bias conditions. While the Group VI and Group IV deep donors respond similarly to passivation, they display significantly different reactivation kinetics, with thermal dissociation energies of 1.5 eV and 1.2 eV for Se-H and Si-H, respectively. These values are close to the energies previously determined for reactivation of the Si and Se shallow donors in both AIGaAs and GaAs. Therefore, they are not significantly dependent on the Al concentration (x &lt; 0.30) even for donors residing on the As sublattice. Our results are consistent with the Chang-Chadi model of DX centers.