AlGaAs Surface Research Articles

Nanometre spaced electrodes on a cleaved AlGaAs surface

We present a novel technique for fabricating nanometre spaced metal electrodes on asmooth crystal cleavage plane with precisely predetermined spacing. Our method does notrequire any high-resolution nanolithography tools, all lateral patterning being based onconventional optical lithography. Using molecular beam epitaxy we embedded athin gallium arsenide (GaAs) layer in between two aluminium gallium arsenide(AlGaAs) layers with monolayer precision. By cleaving the substrate an atomicallyflat surface is obtained exposing the AlGaAs–GaAs sandwich structure. Afterselectively etching the GaAs layer, the remaining AlGaAs layers are used as asupport for deposited thin film metal electrodes. We characterized these coplanarelectrodes by atomic force microscopy and scanning electron microscopy; thisrevealed clean, symmetric and macroscopically flat surfaces with a maximumcorrugation of less than 1.2 nm. In the case of a device with a 20 nm thick GaAslayer the measured electrode distance was 22.5 nm with a maximum deviation ofless than 2.1 nm. To demonstrate the electrical functionality of our device wepositioned single colloidal gold nanoparticles between the electrodes by an alternatingvoltage trapping method; this resulted in a drop of electrical resistance from∼11 G Ω to∼1.5 k Ω at 4.2 K. The device structure has large potential for the manipulation of nanosized objectslike molecules or more complex aggregates on flat surfaces and the investigation of theirelectrical properties in a freely suspended configuration.

Degradation mechanisms of GaAs PHEMTs in high humidity conditions

We have studied the degradation mechanisms of AlGaAs/InGaAs pseudomorphic HEMTs (PHEMTs) under high humidity conditions (85 °C, 85% relative humidity). The degraded samples under high humidity conditions show a decrease in maximum drain current ( I max) and a positive shift in threshold voltage ( V th). Cross-sectional transmission electron microscopy (TEM) images from the deteriorated devices reveal an existence of damaged recess surface region and a peeling of a passivation film (SiN x ). The secondary ion mass spectrometry (SIMS) depth profile at the interface between the passivation film and AlGaAs surface also indicates the diffusion of gallium (Ga), arsenic (As) and aluminum (Al) into the passivation film. The degradation of PHEMTs arises from mainly two mechanisms: (1) the positive shift in V th due to stress change under the gate caused by the peeling of passivation films, and (2) the decrease in I max due to the net carrier concentration reduction of the AlGaAs carrier supply layer caused by the combination of surface degradation at the AlGaAs recess regions and diffusion of Ga, As and Al at the interface between the passivation film and AlGaAs surface. A special treatment just prior to the deposition of SiN x films on the devices effectively suppresses the degradation of PHEMTs under high humidity conditions without degradation of the high frequency performance.

New optoelectronic integrated device for optically controlled microwave oscillators

A new optoelectronic integrated device for the design of optically tuned microwave oscillators is presented. The device consists of an n-GaAs optically controlled field effect transistor (OPFET) and a p-AlGaAs/p-GaAs/n+-AlGaAs surface emitting double heterostructure light emitting diode (DH-LED) which are developed vertically on a p+-GaAs subtrate and separated by a semi-insulating (SI) GaAs layer. This new device is referred to as a light source integrated OPFET (LSI-OPFET). The proposed device structure is such that optical radiation generated by the LED is fed back into the OPFET as back illumination. The level of back illumination to the OPFET may be controlled by simply changing the bias current of the LED. The back illumination is used as the control signal to the OPFET to change certain parameters of the device such as the transconductance, gate–source and gate–drain capacitances etc. This makes the LSI-OPFET device suitable for the design of optically controlled microwave amplifiers and oscillator circuits. The proposed device is demonstrated in optically tuned common-source and common-drain microwave oscillators in which the frequency of oscillation may be controlled by the level of back illumination from the LED into the OPFET.

Self-assembled GaAs/AlGaAs quantum dots by molecular beam epitaxy and in situ AsBr3 etching

We report on a new method to produce self-assembled, unstrained, GaAs/AlGaAs quantum dots (QDs) with large confinement energy. First we create nanoholes on a GaAs surface by growing InAs islands on GaAs(0 0 1), overgrowing them with GaAs and etching the surface in situ with AsBr 3 gas. Then we transfer the holes to an AlGaAs surface, fill them with GaAs and overgrow them with AlGaAs. In this way we obtain GaAs inclusions in an AlGaAs matrix. We investigate the optical properties of such QDs by photoluminescence spectroscopy and their morphology by atomic force microscopy. We show that the QD size can be tuned and emission with inhomogeneous broadening down to 8.9 meV can be achieved.

Deep‐level defects induced by Ar‐ion beam etching in n‐type AlxGa1‐xAs grown by MBE

AbstractResults of the electrical study of the damage caused by Ar‐ion beam etching in MBE grown AlxGa1‐xAs : Si laser diode material are reported. This study has involved measurements of capacitance as a function of voltage and temperature as well as deep level transient spectroscopy measurements on Schottky‐barriers diodes. The Schottky contacts were fabricated conventionally by depositing Au in a vacuum onto the etched AlGaAs surface. For a reference the Schottky diodes of the best quality were produced in‐situ by evaporation of Al on the top of the epi‐layer at the end of the MBE growth run. On the basis of the results of a comparative study deep‐level defects related to the subsurface damaged layer have been recognised. Emission signatures as well as depth profiles of the defect concentration have been studied and analysed. Moreover, DLTS spectra revealed the presence of a well‐known DX(Si) centre and M‐type defect related level attributed to a vacancy and impurity complex. The presence of an additional deep‐level defect located in the lower half of the energy gap that was induced by dry etching is also suggested. An extra defect considered has the high enough concentration to disturb carrier spatial profiles and DX centre characteristics. It is suggested that it may be responsible for degradation effects in GaAs/AlGaAs heterostructure lasers, which fabricating involves dry etching.

Polarization conversion in the reflectivity properties of photonic crystal waveguides

Strong resonant polarization conversion is observed in the reflectivity properties of one-dimensional (1-D) lattices of air trenches deeply etched in AlGaAs surface waveguides. The symmetry properties and the magnitudes of the observed effects are found to be in good agreement with the results of scattering matrix calculations.

Formation of Defects in MBE Re-Grown GaAs Films on GaAs/AlGaAs Heterostructures

ABSTRACTIn the present work, we examine the formation of defects on the sidewall slope in 1 μm - thick GaAs layers regrown on GaAs/AlGaAs heterostructures. Site-specific TEM specimens of sidewall slopes are obtained using focused ion beam combined with lift-out method. TEM analysis shows planar defects, such as stacking faults and microtwins, dislocations and large twinned areas, nucleating on the AlGaAs surfaces. SIMS and EDX reveal an increase in carbon and oxygen at the interface. The defect density increased with Al content exceeding 1010 cm-2 on Al0.4Ga0.6As. The defect formation is related to the oxidation of Al-containing surfaces.

Distinctly different two-dimensional ordering alignments of InGaAs island arrays on GaAs(3 1 1)B and AlGaAs(3 1 1)B surfaces

By using the atomic force microscopy image, fast Fourier transformation and auto-correlation spectrum analysis, we have systematically investigated the spatial alignment of self-assembled In 0.4Ga 0.6As islands on the GaAs(3 1 1)B and the AlGaAs(3 1 1)B surfaces grown by atomic hydrogen assisted molecular beam epitaxy . Surprisingly, it is revealed that InGaAs island arrays are aligned in very different patterns on these two surfaces. They are aligned in a well-defined square two-dimensional (2D) lattice when formed on GaAs surface, but in a nearly perfect hexagonal 2D lattice when formed on AlGaAs surface, despite the fact that both AlGaAs and GaAs have the same lattice mismatch with InGaAs. This striking difference is preliminarily related to the enhanced bond strength of Al in surface, which not only changes the surface energy, but also slows down column III atom migration on the growing surface.

Local-field effects in reflectance anisotropy spectra of oxidized (001) GaAs and AlGaAs surfaces

Oxidized (001) surfaces of both GaAs crystals and Ga 0.7Al 0.3As alloys are found to exhibit characteristic reflectance anisotropy (RA) spectra in the range 1.5–5.5 eV. A microstructure model of GaAs/oxide interface is proposed to treat the observed spectra within a quasi-microscopic theory of reflectance anisotropy developed for multilayer dielectric arrangements. Quantitative agreement between measured and calculated RA spectra shows the principal spectral features to be related to local-field effects at the GaAs/oxide interface, which is typical of A 3B 5 compounds.

Conductance gap anomaly in scanning tunneling spectra of MBE-Grown (0 0 1) surfaces of III–V compound semiconductors

A scanning-tunneling-spectroscopy (STS) study was performed on MBE-grown (001) surfaces of GaAs, Al0.3Ga0.7As and In0.53Ga0.47As in an ultrahigh vacuum (UHV) STS system to clarify microscopic behavior of surface states causing Fermi level pinning on these III–V compound semiconductor surfaces. On all the sample surfaces, there existed spots which showed anomalous STS spectra showing conductance gaps much larger than the energy gap of the material. The rates of finding such spots as well as the magnitudes of the anomalous conductance gap were strongly material-dependent, increasing in the order of InGaAs, GaAs and AlGaAs. Scanning tunneling microscope (STM) images under low-positive sample biases showed dark areas which gradually decreased with the increase of the positive sample bias, and correlated with the spatial variation of conductance gaps of the STS spectra. On the basis of a detailed computer simulation, the conductance gap anomaly is explained by a tip-induced local charging of surface states where the apparent gap width depends on surface state distribution shape and density. The result shows that an extremely high density of surface states exist on the AlGaAs surface, but not so much on the InGaAs surface with the GaAs surface in between.

Advanced selective dry etching of GaAs/AlGaAs in high density inductively coupled plasmas

We report a breakthrough for selective etching of GaAs over AlxGa1−xAs, x=0.2, layer with a high density plasma source. This result is particularly important for III–V devices such as heterojunction bipolar transistors (HBTs) or high electron mobility transistors (HEMTs). For example, fabrication of HBTs requires a process for selective etching of a GaAs contact layer while stopping on AlGaAs layer. Inductively coupled plasma (ICP) etching with BCl3/SF6/N2/He chemistries showed extremely high selectivity of GaAs over AlGaAs (>200:1) and a photoresist (>10:1). This process also produced excellent sidewall passivation on GaAs with reasonably high rate (>1500 Å /min.). Both scanning electron microscope and atomic force microscope data showed AlGaAs etch stop layer was quite smooth after processing. We found that He played a key role in enhancing selectivity and obtaining smooth AlGaAs surfaces. When used with resist masks, addition of N2 into BCl3/SF6 plasma helped formation of passivation on the sidewall and maintained high anisotropy. An optimized condition with BCl3/SF6/N2/He ICP plasmas showed excellent pattern transfer into GaAs with high rate, anisotropy, and selectivity.

Self-assembling nanostructures and atomic layer precise etching in molecular beam epitaxy

We report on the preparation of 10 nm lateral size semiconductor structures based on island formation in strained layer growth in molecular beam epitaxy. Red light emitting InP quantum dot injection lasers are presented. They contain densely stacked layers of self-assembled InP quantum dots embedded in a Ga 0.51In 0.49P wave guide layer. In the second part of this contribution we report on a new atomic layer precise etching technique in MBE, which allows improved interface control for the preparation of semiconductor nanostructures. The etching process involves AsBr 3 exposure of a GaAs or AlGaAs surface. Switching between atomic layer precise growth and etching is possible within a few seconds.

Computer analysis of photon-induced non-equilibrium phenomena at Si and AlGaAs surfaces

A theoretical analysis of the surface recombination, surface band bending and photoluminescence (PL) under strongly absorbed light illumination was performed for n-type Si and AlGaAs surfaces. A very strong dependence of the so-called effective surface recombination velocity ( S eff), surface photovoltage (SPV) and photoluminescence quantum efficiency ( Y PL) on both the excitation intensity and surface Fermi level position was found from rigorous computer calculations. The initial surface band bending was varied from accumulation to inversion by introducing the surface fixed charge ( Q FC) in the Si case, and from weak to strong depletion by increasing the surface state density ( N SS0), in the AlGaAs case. It was revealed that S eff can be significantly reduced (almost four orders of magnitude on Si surface) by shifting the surface Fermi level towards band edges. On the other hand, a deep quenching of PL efficiency was observed for the values of N SS around midgap above 5×10 10 cm −2 eV −1 on AlGaAs surface. In addition, a comparison of simulated Y PL dependencies versus photo-excitation with the experimental data obtained for MBE-grown AlGaAs layers passivated by ultrathin Si interface control layer was performed. This technique of passivation is known to reduce the interface state density down to the range of 10 10 cm −2 eV −1.

Arsenic-free high-temperature surface cleaning of molecular beam epitxy (MBE)-grown AlGaAs layer with new passivation structure

To facilitate an MBE regrowth on an AlGaAs epilayer surface which is easily oxidized in the atmosphere, a passivation layer was provided on the top of the AlGaAs surface. This layer was either a GaAs single layer or a GaAs/ultra-thin AlGaAs/GaAs double-hetero (DH) structure layer grown at the final stage of the first MBE, and it was sublimated during the arsenic-free high-temperature surface cleaning which was carried out at the first stage of MBE regrowth. The cleaned AlGaAs surface was examined by Auger electron spectroscopy. It exhibited an intensity ratio of oxygen to Al, Ga and As much lower for the DH passivated AlGaAs than for the AlGaAs passivated by a GaAs layer. Large electron mobility was systematically obtained for the high electron mobility structure grown on the cleaned AlGaAs.

Self-ordered nanostructures grown by organometallic chemical vapor deposition on V-grooved substrates: experiments and Monte-Carlo simulations

The growth the GaAs/GaAlAs quantum wires by organometallic chemical vapor deposition on V-grooved substrates relies on the formation of a self-limiting AlGaAs surface profile and the thickness modulation in the form of a crescent of the GaAs layer. In order to gain understanding on the growth process at the atomic level, we developed a two-dimensional Monte-Carlo simulation based on the solid-on-solid model. In good agreement with experimental results, our kinetic model shows that the self-limited profile results from the competition between the growth rate anisotropy on the different facets of the groove and the surface diffusion of adatoms. The predictions of the growth modeling are experimentally employed to control at the nanometer scale the shape of quantum wires using successive changes in the growth conditions. This understanding of the growth mechanisms opens the way to an accurate control of the quantum confinement in quantum wires.

Electronic properties of Al xGa 1− xAs surface passivated by ultrathin silicon interface control layer

The photoluminescence surface state spectroscopy (PLS 3) method was applied to a study of the surface state distribution ( N SS), effective surface recombination velocity ( S eff), electron ( E Fn) and hole ( E Fp) quasi-Fermi levels and band bending ( V S) on the Al 0.33Ga 0.67As surface air-exposed and passivated by the Si interface control layer (ICL) technique. Using the detailed measurements of the PL quantum efficiency for different excitation intensities, combined with the rigorous computer simulations of the bulk and surface recombination processes, the behavior and correlation among the surface characteristics under photo-excitation was determined. The present analysis indicated that forming of a Si 3N 4/Si ICL double layer (with a monolayer level control) on AlGaAs surface reduces the minimum interface state density down to 10 10 cm −2 eV −1 and surface recombination velocity to the range of 10 4 cm/s under low excitations.

Resonant coupling of near-infrared radiation to photonic band structure waveguides

Sharp resonance features are observed in the polarized reflectivity spectra of semiconductor photonic crystals fabricated by deep periodic patterning of AlGaAs surface waveguides. Both one- (1-D) and two-dimensional (2-D) lattices are studied by angular dependent reflectivity. By comparison with theory we show that the sharp features in reflectivity arise from resonant coupling of the external radiation to the folded band structure of the photonic crystal waveguides. Wavevector selective coupling to heavy photon states at the edge of the photonic Brillouin zone is demonstrated for the 1-D lattices. In the case of the 2-D lattices we observe polarization mixing of the photonic hands. Theoretical reflectivity spectra were obtained from a numerical solution of Maxwells equations for the patterned waveguide and were found to be in very good agreement with experiment.

In situ etching of GaAs by AsCl 3 for regrowth on AlGaAs in metalorganic vapor-phase epitaxy

In situ etching of GaAs with AsCl 3 in metalorganic vapor-phase epitaxy (MOVPE) reactor was studied with in situ reflectometry as a function of the AsCl 3 flow rate, substrate temperature, and AsH 3 flow rate during etching. A smooth and uniform etched surface was successfully achieved at substrate temperatures above 680°C. The etch rate was found to be proportional to the flow rate of AsCl 3, and exhibited Arrhenius temperature dependence with an activation energy of ∼1 eV. The etching was also found to be very selective towards the Al composition in AlGaAs, and stopped abruptly on AlGaAs. This process has applications for MOVPE regrowth on an AlGaAs surface after a selective in situ etch of the GaAs cap layer.

Formation of self-assembled GaAs/AlGaAs quantum dots by low-temperature epitaxy

We report the direct formation of self-assembled GaAs/AlGaAs quantum dots by low-temperature molecular beam epitaxy. To drive a three dimensional growth mode, the (1×1) AlGaAs surface was exposed alternately to the Ga and As sources. The resulting GaAs nanocrystals having {111} facets were clearly identified by high-resolution transmission electron microscopy. The emission spectra also confirmed the formation of dots. The transition to a three-dimensional growth mode is attributed to the limited surface migration of Ga adatoms on the AlGaAs surface, which has excess As at low substrate temperature.

Photoluminescence characterization of air exposed AlGaAs surface and passivated ex situ by ultrathin silicon interface control layer

Using the photoluminescence surface state spectroscopy (PLS 3) technique, attempts were made to determine the surface state density ( N ss) distribution on Al x Ga 1− x As ( x≈0.3) surfaces passivated by the Si interface control layer (ICL) technique. Air-exposed AlGaAs epitaxial wafers which are technologically important for fabrication of various devices were passivated ex situ by forming a SiO 2/Si 3N 4/Si ICL/AlGaAs structure after the HCl treatment and their photoluminescence behavior was investigated in detail. The result of the PLS 3 analysis indicated that Si ICL-based passivation reduces the minimum interface state density value down to 10 10 cm −2 eV −1 range. Some indication was also obtained that further improvements are possible by using electron cyclotron resonance (ECR)-enhanced N 2 plasma for Si 3N 4/Si ICL interface formation.