GaAsN Layers Research Articles

Relationship between the structural and electronic properties of n-GaAs layer grown on rough Si substrate by molecular beam epitaxy

The relationship between the electronic and structural quality of n–GaAs layer grown by Molecular Beam Epitaxy (MBE) on rough silicon surface substrate (RSi) was investigated by Reflection High–Energy Electron Diffraction (RHEED), Time–Resolved Photoluminescence (TRPL), High–Resolution X–Ray Diffractometer (HRXRD) and photocurrent spectroscopy. The results demonstrate a high correlation between the defect density and the minority carrier lifetime, showing that the defect density is a strong cause for trapping the minority carriers. Atomic Force Microscopy (AFM) and HRXRD analysis show a flattening of GaAs film on rough Si substrate. The minority carrier lifetime is enhanced when the GaAs layer is grown on RSi. This enhancement was explained by the minimisation of the defect density when using the rough surface. The improvement of minority carrier lifetime is at the origin of enhancement of photo–carrier collection.

Effects of a key deep level and interface states on the performance of GaAsN solar cells: a simulation analysis

To explore the origin of low conversion efficiency of GaAsN solar cells, the effects from a key deep level (E1) at about 0.3–0.4 eV below the conduction band and interface states between the GaAsN emitter and GaAs front surface field layer were investigated by numerical simulation. Our results show that a high deep level concentration in GaAsN layers and a large interface recombination velocity are necessary to fit the experimental data. Interface recombination plays an important role in reducing the quantum efficiency in the short-wavelength region. Its effect can be weakened by applying a thin emitter and a high doping level in the GaAs surface field layer. The abundant existence of E1 is responsible for the high contribution of Shockley–Read–Hall recombination from the space-charged region. Its density should be reduced to <1015 cm−3 for higher conversion efficiency.

Schottky contacts to high-resistivity epitaxial GaAs layers for detectors of particles and X- or γ-ray photons

The electrical characteristics of Ti and Pt Schottky contacts to epitaxial n-GaAs layers with the charge-carrier concentration <1012 cm−3 for detectors of particles and X- or γ-ray photons are studied. It is shown that it is preferable to use a diffusion-based theory of charge transport in calculations of the parameters of Schottky contacts to thick high-resistivity lightly compensated GaAs layers. The calculated barrier heights were 0.84 and 0.87 eV for the Ti and Pt contacts, respectively. The fabricated samples of the surface-barrier detectors featured a linear response in the studied range of energies from 6 to 140 keV for γ-ray photons and from 4 to 8 MeV for α particles; the efficiency of charge collection was close to 100% and the energy resolution was high at room temperature.

Ultrafast optical response originating from carrier-transport processes in undoped GaAs/n-type GaAs epitaxial structures

We have investigated ultrafast optical responses of undoped GaAs/n-type GaAs (i-GaAs/n-GaAs) epitaxial structures at room temperature using a reflection-type pump-probe technique. The built-in electric field in the i-GaAs layer is controlled by its thickness. It is found that the decay time of a photoexcitation-induced reflectivity change in a sub-picosecond range decreases with an increase in the built-in electric field strength. The observed optical response is related to the transport process of photogenerated carriers from the i-GaAs layer to the n-GaAs layer. The shortest response time about 60 fs demonstrates that the i-GaAs/n-GaAs structure is useful for ultrafast optical applications.

Impact of rough silicon buffer layer on electronic quality of GaAs grown on Si substrate

The electronic and the structural properties of n-GaAs layers grown on rough surface of silicon substrate by molecular beam epitaxy (MBE) has been investigated by photoluminescence (PL), time resolved photoluminescence (TRPL) and high resolution X-ray diffraction (HRXRD). The relationship between electronic and structural properties of the n-GaAs layer was checked, showing that the defect density is a strong cause for trapping the minority carriers. The impact of introducing intermediate rough silicon layer between silicon substrate and n-GaAs layer on the electronic properties was observed, showing that the structure grown on rough Si involves higher lifetime than those developed on flat silicon substrate. Such structure could be used for economic solar cells fabrication.

Photoluminescence of i-Ga x In1 − x As/n-GaAs heterostructures containing a random InAs quantum dot array

i-GaxIn1 − xAs/n-GaAs heterostructures containing InAs quantum dots have been grown by ion beam deposition and their photoluminescence has been studied. The photoluminescence spectrum of the heterostructures contains three characteristic peaks. The peak at 1.1 eV has a large width and is due to the array of InAs quantum dots of different sizes, randomly arranged on the surface of the i-GaxIn1 − xAs layer. From the position of another peak (hν = 0.94 eV), we have evaluated the composition of the GaxIn1 − xAs solid solution: x = 0.64. The third photoluminescence peak corresponds to the intrinsic absorption edge of the n-GaAs substrate. The key features of luminescence photoexcitation in the heterostructures are discussed. We show that the Ga0.64In0.36As quantum well may accumulate not only photogenerated electrons but also electrons that come from a thin interfacial n-GaAs layer through ballistic transport.

Ohmic contacts to n-GaAs nanowires

We report on the technology and the electrical properties of two different contact systems on n-GaAs nanowires. Annealed Ge/Ni/Ge/Au and Pd/Ge/Au multilayer metallization were investigated. Rapid thermal annealing at temperatures common for identical contact systems on n-GaAs layers is found to be crucial due to an enhanced out-diffusion of the Ga component into the Au contact layer. The maximum annealing temperatures ensuring intact nanowires are 320 °C for Ge/Ni/Ge/Au and 280 °C for Pd/Ge/Au. The fabricated Pd/Ge/Au contacts reveal a specific contacts resistance of 2.77 × 10−7 Ωcm2, which is about one order of magnitude lower compared to the values of Ge/Ni/Ge/Au and also lower than Pd/Ge/Au contacts on bulk material (1.2 × 10−6 Ωcm2).

Properties of GaAsN layers grown from melt containing Li3N as flux for enhancing nitrogen dissolution

Dilute GaAsN layers are grown by the liquid phase epitaxy technique from a GaAs + Ga + GaN melt with up to 2 mol% Li3N added to the same to act as a flux to promote nitrogen dissolution in Ga. X-ray diffraction study indicates a nitrogen content of 0.9 at.% in the material. 10 K photoluminescence measurements indicate a band gap reduction of 130 meV in the as-grown layers which increases to 150 meV after a high-temperature anneal. Energy dispersive x-ray measurements indicate incorporation of Li in the material and local vibrational modes related to both nitrogen and Li are observed through Raman spectroscopy. Furthermore, a photoluminescence peak at 1.33 eV is suggested to be due to Li.

Transport of nitrogen atoms during liquid phase epitaxial growth of InAsN and GaAsN

We have used a one-dimensional diffusion model to investigate the transport of nitrogen during a liquid phase epitaxial growth of InAsN layers from Bi and In solvents and GaAsN layers from Ga solvent. The concentration profile of nitrogen near the growing interface has been obtained for the materials for different melt supercooling and cooling rates. Experimental results on the growth of InAsN layers from Bi solutions using the optimum growth parameters suggested by the study are shown.

Enhanced 1.3 µm luminescence from InGaAs self-assembled quantum dots with a GaAsN strain-compensating layer

We have studied room temperature (RT) photoluminescence (PL) emission and strain characteristics of InGaAs quantum dots (QDs) grown on GaAs using metal organic vapor phase epitaxy. The InGaAs QDs were covered by a 10 nm thick GaAsN layer having a nitrogen composition of either 0.3% or 0.9% and a following 50 nm thick GaAs layer. The RT-PL emission intensity of InGaAs QDs has been increased with reduced linewidth for 1.3 µm device application by using GaAsN strain-compensating layers. The Raman spectra of InGaAs QDs covered by GaAsN0.009 show the InGaAs QD phonon frequency shift toward that of InGaAs surface QDs, which gives clear evidence for strain compensation.

Comparative Studies on InGaP/GaAs Heterostructure-Emitter Bipolar Transistors with Tunneling- and Superlattice-Confinement Structures

This article first compares the difference of InGaP/GaAs heterostructure-emitter bipolar transistors (HEBTs) with tunneling- and superlattice-confinement structures at room temperature. In the two devices, the insertion of a thin n-GaAs layer at base-emitter junction helps to eliminate the potential spike and a relatively low collector-emitter offset voltage of about 40 mV is observed. The difference of tunneling mechanism will be investigated. Experimentally, a maximum current gain of 285 (176) and a current gain cutoff frequency of 26.4 (16.3) GHz are achieved for the tunneling (superlattice) device. As comparing the superlattice device, the tunneling device shows higher current gain at small B-E bias, while it has smaller current gain at large B-E bias.

The effect of different treatments on fractal forms of gallium arsenide surface

The surface of epitaxial gallium arsenide (as-grown and subjected to standard chemical treatments for producing metal-GaAs electrical contacts) has been studied by atomic force microscopy. It was shown that the surface relief can be characterized by fractal relief in the local approximation. As-grown epitaxial n-GaAs layers have lower values of the mean relief irregularity and roughness and reveal high surface homogeneity. The most marked relief changes are observed due to plasmachemical deposition of silicon dioxide and its removal in a buffered etch and deionized water rinse. The following finishing treatments make the surface more smooth and homogeneous, especially in ammonia-water mixture, which gives almost an ideal (Gaussian) distribution of irregularities and phase contrast in most cases. The fractal dimensionality of the studied surfaces changes in the relatively narrow limits D f = 2.5−2.8. The values of the local approximation limit L determining the boundary of the applicability of the fractal approach when describing the surfaces change from 0.039 to 10.99 μm depending on the kind of treatment.

Growth of n-GaAs layer on a rough surface of p-Si substrate by molecular beam epitaxy (MBE) for photovoltaic applications

The n-GaAs layer was successfully grown on (1 0 0) p-type silicon (p-Si) substrate by molecular beam epitaxy (MBE) using rough surface buffer layer (RSi) to reduce the tensile stress in GaAs layer grown on Si substrate. We have reviewed the initial stage and the recombination of GaAs epitaxial layer grown on rugged silicon substrate (RSi). Two dimensional reconstructions were observed for 2 μm GaAs layer by reflection high-energy electron diffraction (RHEED). High resolution X-ray diffractometer (HRXRD) has been achieved, showing the better relaxation and high quality of n-GaAs layer deposited on p-RSi substrate. The effects of rapid thermal annealing (RTA) on crystal structure of GaAs on Si were studied by analyzing the linewidth of the rocking curve related to the samples. Photocurrent measurements indicate potential application of this structure for photovoltaics.

Study of melt-grown GaAsN and InGaAsN epitaxial layers

This paper presents investigation of electronic transport properties of GaAsN and InGaAsN epitaxial layers with low nitrogen content, the so called dilute nitrides, grown by liquid-phase epitaxy (LPE). The layers up to 2 microns thick have been grown from Ga- and In-rich melt at different initial epitaxy temperatures in the range 660–620 °C. Polycrystalline GaN has been used as a source for nitrogen. As grown, unintentionally doped GaAsN and InGaNAs are n-type with free carrier concentrations one order of magnitude higher than those for the reference nitrogen free undoped GaAs and InGaAs layers. Lattice matched to GaAs substrate InGaAsN layers exhibit Hall mobility values higher than 2000 V/cm2.s

Determination of the fractal dimension for the epitaxial n-GaAs surface in the local limit

Atomic-force microscopy studies of epitaxial n-GaAs surfaces prepared to deposit barrier contacts showed that major relief for such surfaces is characterized by a roughness within 3–15 nm, although “surges” up to 30–70 nm are observed. Using three independent methods for determining the spatial dimension of the surface, based on the fractal analysis for the surface (triangulation method), its section contours in the horizontal plane, and the vertical section (surface profile), it was shown that the active surface for epitaxial n-GaAs obeys all main features of behavior for fractal Brownian surfaces and, in the local approximation, can be characterized by the fractal dimension D f slightly differing for various measuring scales. The most accurate triangulation method showed that the fractal dimensions for the studied surface of epitaxial n-GaAs for measurement scales from 0.692 to 0.0186 μm are in the range D f = 2.490−2.664. The real surface area S real for n-GaAs epitaxial layers was estimated using a graphical method in the approximation δ → 0 δ is the measurement scale parameter). It was shown that the real surface area for epitaxial n-GaAs can significantly (ten times and more) exceed the area of the visible contact window.

Photomagnetic effect in In(Ga)As/GaAs heteronanostructures with quantum dots and wells

The influence of quantum-size layers (InAs quantum dots, In0.2Ga0.8As quantum wells, and combined quantum-well/quantum-dot layers) and heteroepitaxial passivation of surface by an In0.5Ga0.5P layer on the photomagnetic effect in epitaxial n-GaAs layers has been investigated.

Bandedge absorption of GaAsN films measured by the photothermal deflection spectroscopy

The optical absorption of GaAsN films grown by molecular beam epitaxy on GaAs substrates is measured using the mirage effect photothermal deflection spectroscopy (PDS). The PDS spectra were fitted with a modified Fernelius model, which takes into account multiple reflections within the GaAsN layer and GaAs substrate. This allowed the extraction of bandedge parameters for a series of GaAsN films with N content varying from 0.24% to 1.4% N. All films show a clear Urbach absorption edge with a composition-dependent bandgap consistent with literature and Urbach slope parameters roughly 3 times larger than GaAs values.

Lateral Spin Diffusion Probed by Two-Color Hanle-MOKE Technique

We report on all optical spatially resolved spin diffusion experiments in an unstrained, unbiased n-GaAs layer. Optical pump and probe intensities are varied over a wide range to study the impact of optical disturbance on spin transport. Both quantities have a considerable influence on the measured spin diffusion length and spin lifetime. Furthermore, an effective spin diffusion coefficient was obtained as a function of temperature.

Calculation of electric-field strength profiles in GaAs:Cr epitaxial-diffusion structures

The electric-field strength profiles are calculated for the structures used in X-ray detectors produced by diffusion of the deep acceptor Cr impurity into an n-GaAs epitaxial layer. The effects of diffusion temperature, external bias, and the ratio of electron and hole emission coefficients on the distribution of electric-field strength in the structure are examined.

Enhancement of terahertz electromagnetic wave emission from an undoped GaAs∕n-type GaAs epitaxial layer structure

We have investigated the emission of the terahertz electromagnetic wave from an undoped GaAs (200nm)∕n-type GaAs (3μm) epitaxial layer structure (i-GaAs∕n-GaAs structure), where the doping concentration of the n-GaAs layer is 3×1018cm−3. It is found that the first-burst amplitude of terahertz wave of the i-GaAs∕n-GaAs sample is remarkably larger than that of a n-GaAs crystal, which means that the i-GaAs layer enhances the terahertz emission intensity. The first-burst amplitude of the i-GaAs∕n-GaAs sample, by tuning the pump-beam energy to the higher energy side, exceeds that of an i-InAs crystal that is known as one of the most intense terahertz emitters. We, therefore, conclude that the i-GaAs∕n-GaAs structure is useful to obtain intense terahertz emission.