Search

GaN_1−xSb_x with x∼5%-7% is a highly mismatched alloy predicted to have favorable properties for application as an electrode in a photoelectrochemical cell for solar water splitting. In this study, we grew GaN_1−xSb_x under conditions intended to induce phase segregation. Prior experiments with the similar alloy GaN_1−xAs_x, the tendency of Sb to surfact, and the low growth temperatures needed to incorporate Sb all suggested that GaN_1−xSb_x alloys would likely exhibit phase segregation. We found that, except for very high Sb compositions, this was not the case and that instead interdiffusion dominated. Characteristics measured by optical absorption were similar to intentionally grown bulk alloys for the same composition. Furthermore, the alloys produced by this method maintained crystallinity for very high Sb compositions and allowed higher overall Sb compositions. This method may allow higher temperature growth while still achieving needed Sb compositions for solar water splitting applications.

We have analysed bismuth incorporation into GaN layers using plasma‐assisted molecular beam epitaxy (PA‐MBE) at extremely low growth temperatures of less than ∼100 °C under both Ga‐rich and N‐rich growth conditions. The formation of amorphous GaN1−xBixalloys is promoted by growth under Ga‐rich conditions. The amorphous matrix has a short‐range order resembling random crystalline GaN1−xBixalloys. We have observed the formation of small crystalline clusters embedded into amorphous GaN1−xBixalloys. Despite the fact that the films are pseudo‐amorphous we observe a well defined optical absorption edges that rapidly shift to very low energy of ∼1 eV.

We have investigated the growth of bulk, freestanding zincblende cubic GaN layers by plasmaassisted molecular beam epitaxy PAMBE. The PAMBE technique was used for bulk crystal growth and GaN layers up to 100 µm in thickness were produced. We have established that the best structural properties of freestanding zincblende GaN can be achieved with initiation under Garich conditions, but without Ga droplet formation. The procedure to produce freestanding bulk zincblende substrates from thick GaN layers grown on GaAs substrates has been developed. We have demonstrated the scalability of the process by growing freestanding GaN layers up to 3 in. in diameter. Growth of freestanding bulk GaN layers has allowed us to refine the values for the basic parameters of zincblende GaN. We have demonstrated also that the PAMBE process we have developed has allowed us to achieve freestanding AlxGa1−xN wafers.

We use cathodoluminescence and Raman spectroscopy to investigate GaN layers grown by plasma-assisted molecular beam epitaxy with different Bi fluxes. We find that the growth of GaN with an additional Bi flux favours the formation of submicron cubic domains, which results in layers with reduced crystalline quality and lower luminescence emission. We detect a Bi-related Raman feature at low frequencies that we tentatively assign to local Bi modes in GaN, which indicates that a fraction of Bi atoms incorporates into the GaN lattice. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Silicon migration during MBE growth of (Al, Ga)As and (Al, Ga)As/GaAs or AlAs/GaAs superlattices has been studied by electrochemical C-V and secondary ion mass spectrometry (SIMS) concentration-depth profiling. It is found to be concentration dependent, with no preferential migration towards or away from the growth front. At high concentrations, superlattice disordering during growth is observed using photovoltage and transmission electron microscopy (TEM) techniques. On the basis of C-V, SIMS and TEM data we propose that silicon migration occurs as the result of a concentration-dependent diffusion process. This is substantiated by measurements of the two-dimensional electron-gas mobility in selectively doped heterojunctions as a function of growth temperature and silicon concentration.

Beryllium diffusion during MBE growth of (Al, Ga)As layers, (Al, Ga)As/GaAs heterojunctions and GaAs/AlAs superlattices has been studied by electrochemical C-V and secondary ion mass spectrometry (SIMS) concentration profiling, in conjunction with transmission electron microscopy. Diffusion times were comparatively short since they were limited to part of the growth sequence, so non-equilibrium effects had a significant influence. The results are consistent with an interstitial-substitutional mechanism in which lattice site incorporation becomes more difficult with increasing band gap enthalpy. Incorporation involves a “kick-out” reaction which leads to the observed disordering of the superlattices.

The incorporation of Sn as a dopant in GaAs has been studied in the temperature range of 500°–650° C, over a wide range of Ga and As fluxes, the latter being incident as either As4 or As2 molecules. The results are explained in terms of a surface segregation model in which the behaviour at high growth temperatures (above ~600 °C) approaches thermal equilibrium, but growth at lower temperatures involves a kinetic limitation to the segregation process.

The desorption of Pb from MBE-grown {100} GaAs surfaces has been investigated using modulated-beam mass spectrometry, isothermal desorption rate measurements and temperature programmed thermal desorption spectroscopy. The results are in general agreement with a model in which desorption occurs from a disperse phase which is in equilibrium with adsorbate islands. From the independence of the activation energy of desorption on coverage from <10-3 s monolayers to several tens of monolayers, single atom desorption is the probable rate limiting step. An important conclusion is that it is not possible to perform a unique analysis of temperature programmed desorption data from condensed films without prior knowledge of the dependence of the desorption activation energy on coverage.

Molecular beam epitaxy (MBE) is a refined form of vacuum evaporation in which atomic and molecular beams impinge on a heated substrate under ultra high vacuum conditions. The technique evolved from surface kinetic studies on GaAs (1) and Si(2) and was subsequently used to grow films for device purposes. Because of the two complementary aspects of its development, many of the processes controlling both growth and dopant incorporation in MBE are quite well understood.

Far infrared spectroscopy experiments on the shallow donor states of GaAs:Si are presented. Transitions between shallow donor levels are induced by F.I.R. radiation from an optically pumped molecular gas laser. The measurements are performed by measuring the electrical conductivity of the GaAs:Si sample at fixed laser frequencies by sweeping an external applied magnetic field up to 12 T. In total about 30 different transitions are observed using radiation between 60 µm and 1.8 mm. In low magnetic fields and at high frequencies a series of photoconductivity signals are observed, which we ascribe to transitions from the ground state towards bound donor levels with hydrogen quantum numbers n, l=n-1, and m=n-1.

We have shown that a Ga1-xAlxAs/GaAs heterostructure can be used as a sensitive tunable detector of mm-wave/sub-mm-wave radiation. The mechanism for detection requires the application of a magnetic field varying from approximately 0.2T at 94GHz (3.2mm wavelength) to 6.2T at 2500GHz (119µm wavelength). The responsivity and N.E.P. at 3.2mm have been roughly estimated at 200V/W and 5×10-11W/?Hz respectively. The speed of such a detector could be several orders of magnitude greater than comparable InSb detectors.

We have used far-infrared oblique-incidence reflection spectroscopy to study bulk phonon polaritons, and attenuated total reflection (ATR) spectroscopy to study surface phonon polaritons, in long-period GaAs/AlxGa1-xAs and short-period GaAs/AlAs superlattices. Results on the former are in good agreement with an effective-medium bulk-slab model of the dielectric tensor of the superlattice; results on the latter are analysed in terms of a model that contains dielectric-tensor contributions from the confined optic phonons.

The refractive indices of short-period binary (AlAs)m(GaAs)n superlattices, wherem = n, were investigated by measuring the beam divergences of optical waveguides using these superlattices as the core material. It is demonstrated that the refractive index depends on the period of the superlattice and not simply on the average composition. The refractive index is shown to depend in a systematic way on the direct bandgap of the superlattice, although the relationship may not be quite the same as that for a random alloy.

We have studied charging effects in a lateral split-gate quantum dot defined by metal gates in the two dimensional electron gas (2 DEG) of a GaAs/AlGaAs heterostructure. The gate structure allows an independent control of the conductances of the two tunnel barriers separating the quantum dot from the two 2 DEG leads, and enables us to vary the number of electrons that are localized in the dot. We have measured Coulomb oscillations in the conductance and the Coulomb staircase in current-voltage characteristics and studied their dependence on the conductances of the tunnel barriers. We show experimentally that at zero magnetic field charging effects start to affect the transport properties when both barrier conductances are smaller than the first quantized conductance value of a point contact at 2e2/h. The experiments are described by a simple model in terms of electrochemical potentials, which includes both the discreteness of the electron charge and the quantum energy states due to confinement.

We have performed RF experiments on a lateral quantum dot defined in the two dimensional electron gas (2DEG) of a GaAs/AlGaAs heterostructure. The small capacitance of the quantum dot gives rise to single-electron charging effects, which we employed to realize a quantum dot turnstile device. By modulating the tunnel barriers between the quantum dot and the 2DEG leads with two phase-shifted RF signals, we pass an integer number of electrons through the quantum dot per RF cycle. This is demonstrated by the observation of quantized current plateaus at multiples ofef in current-voltage characteristics, wheref is the frequency of the RF signals. When an asymmetry is induced by applying unequal RF voltages, our quantum dot turnstile operates as a single-electron pump producing a quantized current at zero bias voltage.

Using far-infrared radiation from the free-electron laser at the University of California, Santa Barbara, high-power photoconductivity experiments were performed on n-type GaAs:Si. In order to distinguish among the various mechanisms of the observed photoconductivity, we simultaneously monitored the Hall voltage and the photocurrent created by the photoexcited electrons. In this way the number and mobility of the electrons contributing to the photoconductivity were determined. At 4.2 K and in weak magnetic fields, 200- and 295-μm radiation in high-power pulses of 2-μsec duration was used to excite donor electrons from the bound 1s ground state. The experiments show that the photoconductivity arises from electrons that are excited into the conduction band, where they are further heated by the radiation.