AlGaAs Layers Research Articles

Detection wavelength and device performance tuning of InAs QDIPs with thin AlGaAs layers

QDIPs with thin inserted AlGaAs layers adjacent to the QDs were investigated for the tailoring of detection wavelength and device performance. Simple InAs/GaAs QDIPs and DWELL QDIPs with different insertion layer structure were studied. The thin AlGaAs layer is shown to effectively modify the electron wavefunction and associated confined state energies which lead to the change of the detection wavelength and the polarization dependent quantum efficiency. Furthermore, the dark current and conductive gain also change with different device structures. The insertion of AlGaAs layers provides an additional freedom of tuning the electronics states involved in the infrared detection and also enables the improvement of the absorption and device performance.

Numerical simulation of the effect of the Al molar fraction and thickness of an Al xGa 1− xAs window on the sensitivity of a p +–n–n + GaAs solar cell to 1 MeV electron irradiation

In this paper numerical simulation has been used to predict the effect of the thickness and aluminium (Al) mole fraction of an AlGaAs layer, used as a window for a p +–n–n + GaAs solar cell under AM0 illumination and exposed to 1 MeV electron irradiation. Such solar cells are used in satellites and undergo severe degradation in their performance due to induced structural defects. The irradiation-induced defects are modelled as energy levels in the energy gap of GaAs. To predict this effect, the spectral response is evaluated for different electron irradiation fluences for two types of cells. In the first a narrow Al 0.31Ga 0.69As window is a small part of the p + layer while in the second type the whole window is an Al x Ga 1− x As layer with a gradual Al mole fraction. The obtained results show that the Al x Ga 1− x As window with a gradual Al mole fraction improves the resistance of the solar cell to electron irradiation especially in the short wavelengths range.

Infrared p-i-n photodiodes based on InAs quantum dots grown on 20 nm patterned GaAs

We report on selective area growth of InAs quantum dots on GaAs substrates patterned with a hexagonal array of 20 nm pores using block copolymer lithography. We discuss the mechanisms of growth, highlighting the variation in the resulting morphology as a function of nucleation enhancing AlGaAs layers. We also evaluate the optoelectronic performance of p-i-n photodiodes based on single layer nanopatterned grown InAs quantum dot devices. At low to moderate reverse biases, we observe room temperature photoresponse in both near- and mid-IR regimes. At high biases, we observe strong avalanche effects in the mid-IR range with a gain factor of ∼4000.

Performance of Surface and Gate-Engineered AlGaAs ∕ InGaAs Pseudomorphic High-Electron Mobility Transistors

The epilayers in pseudomorphic high-electron mobility transistor (pHEMT) structures are grown by metallorganic chemical vapor deposition on GaAs substrates. A treatment with ammonium polysulfide to passivate the surface of AlGaAs barrier layer is performed. Then, the surface morphology of the sulfur-treated AlGaAs layers was investigated by atomic force microscopy. The chemical compositions of AlGaAs surfaces before and after S treatment are studied using X-ray photoelectron spectroscopy. Two metals (Au and ) are used as Schottky contacts on the gate. The passivated gate pHEMT outperforms the other three pHEMTs investigated in this work in both dc and high-frequency characteristics. The Schottky barrier height varies from for Au on the unpassivated device to for on the passivated device. Sulfur treatment and metallization yield the highest turn-on voltage and reverse breakdown voltage. An outstanding feature of this gate high-electron mobility transistor with passivation is its high . Furthermore, at , the passivated gate pHEMT has an exceptionally high -ratio of 3.95. Following full characterization of these transistors at dc and radio frequencies, these devices undergo high-temperature tests. Experimental data reveal remarkably favorable characteristics of the sulfur-treated pHEMT with a gate. Surface and gate engineering are applied to pseudomorphic heterostructures in the device, to achieve an unprecedented combination of high dc and high-frequency characteristics.

Layers of semiconductor nanostructure for image processing applications

In this paper a semiconductor tunneling nanostructure consisting of multiple layers of GaAs and AlGaAs along with an embedded quantum dot layer is analyzed as a cellular nonlinear network and applied for image processing applications. The logic cell for the structure consists of two resonant tunneling diodes connected in series through a quantum dot. Here the resonant tunneling diode is realized with multiple layers of two semiconductor materials with an embedded dot layer in between. The local interconnections of nanocells are achieved via tunneling between the neighboring quantum dots. Cells may be biased by the common column contacts, and only edge cells have individual I/O ports. Using approximate tunneling characteristics, we have calculated network dynamics and found procedures for useful logic functionality. Examples of image processing using these logic functions are presented.

Micron-scale confinement of light and current: single photon sources with oxide apertures

We report on the development of microcavity single photon sources in which optical and electrical confinement is achieved by means of a ring of low refractive-index aluminium oxide, buried within an etched mesa. The aluminium oxide is formed by the wet oxidation of aluminium rich AlGaAs layers. We demonstrate Purcell-enhanced emission from a quantum dot coupled to an oxide-confined cavity mode. When integrated into a diode architecture, the oxide annulus serves as a current aperture and electroluminesence is only observed from quantum dots in the confined current path. The oxide aperture does not impede the electrical properties of the diode, allowing the device to be driven using high frequency electrical pulses.

Off-axis electron holographic potential mapping across AlGaAs/AlAs/GaAs heterostructures

The electrostatic potential profile across AlGaAs/AlAs/GaAs heterostructures containing 1-μm-thick n-doped (or p-doped) AlGaAs layers is measured using off-axis electron holography. Simulations of the potential profiles assuming no unintentional impurities in the undoped regions of the samples show small discrepancies with experiment. Revised simulations reproduce the measurements accurately, when a p-layer with an 8.4×1011 cm−2 acceptor density is included at the buffer/substrate interface to simulate the presence of unintentional carbon impurities.

Fabrication of ohmic contact based on platinum to p-type compositionally graded AlGaAs layers

Novel metallization scheme was proposed for ohmic contact formation to compositionally graded p-type AlGaAs. A metal multilayers of Ti/Pt/Au, Pt/Ti/Pt/Au and Pt/Ti/Ni/Au were deposited by thermal evaporation using electron gun and resistance heater. The contacts were sequentially annealed by rapid thermall annealing system in N2 atmosphere at various temperatures (in the range from 350°C to 550°C). The duration of annealing step was 2 minutes. The as-deposited Pt/Ti/Pt/Au and Pt/Ti/Ni/Au multilayer metallizations had resistivities of 1.4·10-5 Ω·cm2 which have been gradually deteriorated after each subsequent annealing. The current-voltage characteristics of the ohmic contacts to compositionally graded p-type AlGaAs epitaxial layers were studied and discussed.

Nanometer-scale sharpness in corner-overgrown heterostructures

A corner-overgrown GaAs∕AlGaAs heterostructure is investigated with transmission and scanning transmission electron microscopy, demonstrating self-limiting growth of an extremely sharp corner profile 3.5nm wide. In the AlGaAs layers, we observe self-ordered diagonal stripes, precipitating exactly at the corner, which show increased Al content measured with x-ray spectroscopy. A quantitative model for self-limited growth is adapted to the present case of faceted molecular beam epitaxial growth, and the corner sharpness is discussed in relation to quantum confined structures. We note that corner overgrowth maintains nanometer sharpness after microns of growth, allowing corner-shaped nanostructures.

Structure-induced effects on the selective wet thermal oxidation of digital AlxGa1–xAs alloys

A thorough study of the selective wet oxidation in digital AlxGa1–xAs alloys is presented. We report experimental results and physical interpretation on the oxidation kinetics within those ranges of the AlGaAs composition (x = 0.95 to 1) and layer thickness (20 to 50 nm) of interest for oxide-aperture vertical-cavity surface-emitting laser (VCSEL) application. We demonstrate the high controllability of the oxidation reaction between different Al compositions; made different thanks to the use of digital alloys. Unlike standard alloys, we measured an invariability of the oxidation rates in the studied thickness range (20–50 nm), implying a better control of the fabrication process. The dependence of the reaction rate with the temperature is expressed as an Arrhenius law. Two activation energies (1.2 and 0.55 eV) have been derived for composition ranges of x = 0.95–0.98 and x = 0.99–1, respectively, revealing that two different mechanisms are involved depending on the Al content and the superlattice structure of the digitally-grown AlGaAs.

Oxide-apertured microcavity single-photon-emitting diodes—simultaneous confinement ofcurrent and light

We report on the development of a generation of microcavity single-photon sources inwhich an aluminium oxide aperture provides simultaneous confinement of the injectedcurrent and the optical mode. The aperture is formed by the wet oxidation of analuminium-rich AlGaAs layer. This approach allows a high quality cavity to be successfullyintegrated into an electrical device, from which enhanced photon emission is observedthrough the Purcell effect. The resulting source demonstrated an improved photoncollection efficiency and was shown to operate at repetition rates in excess of 0.5 GHz.

Surface-normal emission of a high-Q resonator using a subwavelength high-contrast grating

We report a novel high-quality (Q) factor optical resonator using a subwavelength high-contrast grating (HCG) with in-plane resonance and surface-normal emission. We show that the in-plane resonance is manifested is by a sharp, asymmetric lineshape in the surface-normal reflectivity spectrum. The simulated Q factor of the resonator is shown to be as high as 500,000. A HCG-resonator was fabricated with an InGaAs quantum well active region sandwiched in-between AlGaAs layers and a Q factor of >14,000 was inferred from the photoluminescence linewidth of 0.07 nm, which is currently limited by instrumentation. The novel HCG resonator design will serve as a potential platform for many devices including surface emitting lasers, optical filters, and biological or chemical sensors.

Effects of bias cooling on charge states in heterostructures embedding self-assembled quantum dots

Carrier transfer behavior has been investigated in selectively doped InGaAs/AlGaAs heterojunctions, in which a layer of self-assembled InAs quantum dots is embedded in close vicinity to a two-dimensional electron gas (2DEG). We applied the bias-cooling technique to derive information on the electron exchange properties from the bias dependence of the capacitance-voltage (CV) spectroscopy. Noise is observed in the CV results at large reverse biases after sufficiently negative bias cooling. The noise is temperature sensitive and can be eliminated when raising the temperature to 130 K. In combination with the experimental results in the control sample, we find that the noise induced by the bias cooling is most likely associated with the hot-electron trapping and detrapping in DX centers in the selectively doped AlGaAs layer. The effect of light illumination on the CV results supports our speculation on the noise origin. Furthermore, the variation in the 2DEG carrier concentration and mobility with bias-cooling processes has been discussed, which provides a direct insight into the vertical charge-transfer mechanism among the quantum dot related electronic states, DX centers, and 2DEG channel.

Improvement of the optical property and uniformity of self-assembled InAs/InGaAs quantum dots by layer-by-layer temperature and substrate rotation

The influence of layer-by-layer temperature and substrate rotation on the optical property and uniformity of self-assembled InAs/In 0.2Ga 0.8As/GaAs quantum dots (QDs) gown with an As 2 source was investigated. An improvement in the optical property of QDs was obtained by the precise control and optimization of growth temperature utilized for each layer, i.e., InAs QDs, InGaAs quantum wells, GaAs barriers and AlGaAs layers, respectively. By using a substrate rotation, the QD density increased from ∼1.4×10 10 to ∼3.2×10 10 cm −2 and its size also slightly increased, indicating a good quality of QDs. It is found that the use of an appropriate substrate rotation during growth improves the room-temperature (RT) optical property and uniformity of QDs across the wafer. For the QD sample with a substrate rotation of 6 rpm, the RT photoluminescence (PL) intensity is much higher and the standard deviation of RT-PL full-width at half-maximum is decreased by 35% compared to that grown without substrate rotation.

New approach to local anodic oxidation of semiconductor heterostructures

We have experimentally explored a new approach to local anodic oxidation (LAO) of a semiconductor heterostructures by means of atomic force microscopy (AFM). We have applied LAO to an InGaP/AlGaAs/GaAs heterostructure. Although LAO is usually applied to oxidize GaAs/AlGaAs/GaAs-based heterostructures, the use of the InGaP/AlGaAs/GaAs system is more advantageous. The difference lies in the use of different cap layer materials: Unlike GaAs, InGaP acts like a barrier material with respect to the underlying AlGaAs layer and has almost one order of magnitude lower density of surface states than GaAs. Consequently, the InGaP/AlGaAs/GaAs heterostructure had the remote Si– δ doping layer only 6.5 nm beneath the surface and the two-dimensional electron gas (2DEG) was confined only 23.5 nm beneath the surface. Moreover, InGaP unaffected by LAO is a very durable material in various etchants and allows us to repeatedly remove thin portions of the underlying AlGaAs layer via wet etching. This approach influences LAO technology fundamentally: LAO was used only to oxidize InGaP cap layer to define very narrow (∼50 nm) patterns. Subsequent wet etching was used to form very narrow and high-energy barriers in the 2DEG patterns. This new approach is promising for the development of future nano-devices operated both at low and high temperatures.

Preparation and characterization of multilayer AlGaAs/GaAs structures for photovoltaic application

In this paper, we present the preparation of multilayer AlGaAs/GaAs heterostructures for photovoltaic application. The structures developed consist of several layers grown on a highly conducting GaAs substrate: n-GaAs base, p-GaAs emitter embedded between two AlGaAs layers, and heavily doped p+GaAs capping contact layer. The second 0.03 μm thick ‘window’ AlGaAs layer has band gap energy of 2.1 eV and ensures penetration of the higher-energy photons in the active region of the p-n junction. The electrical and optical parameters of the multilayer heterostructure as well as the layers thickness are designed by numerical simulation using computer modeling. The optimized heterostructure has a back-surface-field AlGaAs layer of several microns and an ultra thin (20-40 nm) window layer providing the best conversion efficiency and a maximum spectral response in the range 300-900 nm.

Mesoscopic Structures for Microwave-THz Detection

Properties of microwave detectors of various design on the base of MBE grown GaAs and AlGaAs structures are discussed in this paper: simple asymmetrically shaped structures with heavily doped GaAs and AlGaAs layers of nanometric thickness as well as diodes with two-dimensional electron gas layers. Novel models of the detectors with partially gated two-dimensional electron gas layer as well as with small area GaAs/AlGaAs heterojuction are discussed to demonstrate difierent ways to increase the voltage sensitivity of the detectors of electromagnetic radiation in GHz{THz frequency range.

Ohmic contacts to pseudomorphic HEMTs with low contact resistance due to enhanced Ge penetration through AlGaAs layers

AuGe/Ni ohmic contacts are used as source and drain electrodes of pseudomorphic HEMTs (pHEMTs). High alloying temperatures are generally believed to be necessary to enhance penetration of the alloy materials through the AlGaAs layers in order to establish a very low resistance path for the source–drain currents to access the two-dimensional electron gas (2DEG) layer. Here we have performed alloying experiments in the temperature range of 390–450 °C, and the contact resistance was determined using transfer length method measurements. Germanium diffusion was studied using backside secondary ion mass spectrometry. During our study, we have observed that doping of the channel by germanium is possible even at lower temperatures. But alloying at lower temperatures does not appreciably enhance the concentration throughout the different device layers below the contact pads. Hence, unlike MESFET alloying, higher alloying temperatures are essential for increasing the doping concentration so as to reduce the contact resistance and overcome the resistance of the AlGaAs layers.

ENHANCED OPTICAL CHARACTERISTICS OF MODFET UNDER BACKSIDE ILLUMINATION

An enhanced optical response of the depletion mode AlGaAs / GaAs MODFET under backside optical illumination with DC light is presented. A device structure with fiber inserted into the substrate up to the GaAs layer is considered for direct illumination into the GaAs layer. The photoconductive effect in the GaAs layer which increases the 2DEG channel electron concentration and the photovoltaic effect which forward bias the AlGaAs – gate junction are considered. In addition the reflection effect of the contact metals which also increases the 2DEG channel electron concentration is also considered. The electrons generated in the GaAs layer and AlGaAs layer are collected in 2DEG, there by increasing the source to drain current and the photo generated holes are drift towards the semi insulating substrate and are capacitively coupled in to the grounded source. The I-V characteristics, transconductance, photovoltage and transfer characteristics have been evaluated and discussed . The I-V characteristics is compared with available experimental data at a particular gate source voltage with and without illumination showing good agreement . MODFET, photovoltage, internal reflection, transconductance.

Oxide–semiconductor micro-pillar cavities

We have investigated the wet oxidation of high aluminium-content AlGaAs layers to give a stable, low refractive index, insulating aluminium oxide (AlO x ), which we use to form high-index contrast Bragg mirrors. The AlO x /GaAs material combination achieves a ∼15% reflectivity at each interface and so not only are fewer mirror periods required to achieve a high reflectivity but also the penetration of light into the mirrors is reduced. Hence, a cavity bounded by these mirrors can confine photons within a very small volume, modifying the emission characteristics of a single quantum state in the cavity through the Purcell effect. Here, we present an experimental observation of a three-fold enhancement in spontaneous emission rate for a single InAs quantum dots in such a cavity.