AlGaAs Films Research Articles

Study of the free carrier characteristics and surface morphology of AlGaAs/GaAs thin films deposited using MOCVD

Ultra-low loss optical thin films find broad applications in fields such as vertical-cavity surface-emitting lasers and optical atomic clocks. The main optical losses in AlGaAs/GaAs distributed Bragg reflectors (DBRs) prepared using metal-organic chemical vapor deposition (MOCVD) arise from absorption loss caused by free carriers within the layers and scattering loss caused by surface roughness. In this study, we fabricated AlGaAs and GaAs single-layer thin films with varying Al compositions on substrates of three crystal orientations and under different V/III ratios. The dependence of carrier concentration and surface morphology on different substrates and growth conditions was investigated. Thin films grown on substrates with three different crystal orientations exhibited three distinct growth modes (step-flow mode, SK mode, and FM mode). The impact of the V/III ratio on the growth mode was found to be complex. Higher V/III ratios resulted in poorer morphology for films grown on (100) substrates, while better morphology was observed on (211) B substrates. Furthermore, the surface morphology of films grown on (100) 15° off substrates showed less sensitivity to changes in the V/III ratio. With increasing Al composition, the carrier concentration of the films significantly increased. Elevating the V/III ratio proved effective in suppressing the incorporation of carbon, thereby reducing the carrier concentration of AlGaAs films. GaAs films exhibited a low carrier concentration at an appropriate V/III ratio. Additionally, the distinct abilities of different substrates to adsorb impurities exerted a significant impact on the carrier concentration of the films. This study demonstrates that, under optimal conditions, it is feasible to fabricate AlGaAs/GaAs Bragg mirrors with low carrier concentration and relatively small roughness on (100) 15° off substrates.

Metal-Semiconductor AsSb-Al0.6Ga0.4As0.97Sb0.03 Metamaterial.

AlGaAsSb and AlGaAs films as thick as 1 μm with Al content as high as 60% were successfully grown by low-temperature (200 °C) MBE. To overcome the well-known problem of growth disruption due to a high aluminum content and a low growth temperature, we applied intermittent growth with the temperature elevation to smooth out the emerging roughness of the growth front. Post-growth annealing of the obtained material allowed us to form a developed system of As or AsSb nanoinclusions, which occupy 0.3-0.6% of the material volume. While the As nanoinclusions are optically inactive, the AsSb nanoinclusions provide a strong optical absorption near the band edge of the semiconductor matrix due to the Fröhlich plasmon resonance. Owing to the wider bandgap of the grown Al0.6Ga0.4As0.97Sb0.03 compound, we have expanded the spectral range available for studying the Fröhlich plasmon resonance. The grown metamaterial represents an optically active medium of which the formation process is completely compatible with the epitaxial growth technology of semiconductors.

Direct Observation of Dopants Distribution and Diffusion in GaAs Planar Nanowires with Atom Probe Tomography

Intentional and unintentional doping in semiconductor nanowires undoubtedly have significant impact on the device performance. However, spatially resolved precise determination of dopant concentration is challenging due to insufficient sensitivity and resolution of conventional techniques. In this paper, quantitative 3D distribution of Si and Zn dopants in planar GaAs nanowires and their interface with AlGaAs film underneath are obtained by using a unique atom probe tomography technique, providing critical insights for the growth and potential applications of these nanowires.

Structural characterization of AlGaAs:Si/GaAs (631) heterostructures as a function of As pressure

AlGaAs:Si/GaAs heterostructures were grown on (631) and (100) GaAs substrates and studied as a function of the As cell beam equivalent pressure. High-resolution x-ray diffraction patterns showed that the highest quality AlGaAs epitaxial layers were grown at PAs = 1.9 × 10−5 for (100)- and PAs = 4 × 10−5 mbar for (631)-oriented substrates. Raman spectroscopy revealed higher crystalline quality for films grown on (631) oriented substrates. The GaAs- and AlAs-like modes of the AlGaAs(631) films exhibited increased intensity ratios between the transverse optical phonons and longitudinal optical phonons with increasing PAs, whereas the ratios were decreased for the (100) plane. This is in agreement with the selection rules for (631) and high-resolution x-ray diffraction observations. Anisotropy and surface corrugation of the AlGaAs(631) films also were characterized using atomic force microscopy and Raman spectroscopy.

AlGaAs film growth using thermionic vacuum arc (TVA) and determination of its physical properties

In this research, an AlGaAs film was deposited on a microscope slide by means of the thermionic vacuum arc (TVA) technique which is a novel plasma production technique. AlGaAs structures were grown by this deposition technique for the first time and this process occurred in a very short period of time. In order to characterize the produced film, nano-structural, nano-mechanical, optical, and surface properties were determined by field emission scanning electron microscope (FESEM), atomic force microscope (AFM), X-ray diffractometer (XRD) and interferometer. According to the results of the measurements, the mean thickness value of the produced film was obtained as 1.8 μm. The band gap value was determined as 2eV from the Kubelka-Munk plot. The refractive index value was obtained as approximately 3.4. Hardness value was determined as 2 GPa from the Oliver-Pharr method. All these values are consistent with the reported values in the literature for the AlGaAs films produced by different methods. TVA technique appeared as a suitable and promising technique for the production of AlGaAs films.

Si-doped AlGaAs/GaAs(6 3 1)A heterostructures grown by MBE as a function of the As-pressure

The effects of doping with silicon (Si) AlGaAs layers grown by molecular beam epitaxy on GaAs (631)-oriented substrates as a function of the arsenic pressure (PAs) is presented and compared with layers grown on (100) oriented substrates. The surface texture of the AlGaAs (631) films is composed by nanogrooves, whose dimensions depend on PAs. On the contrary, the MBE growth on the (100) plane resulted on rough surfaces, without evidence of formation of terraces. Mobility and carrier density of AlGaAs:Si layers grown on substrates (631) were studied as a function of PAs. The doping type conversion from p-type to n-type as a function of the As pressure is corroborated for high index samples. All the films grown on (100) exhibited silicon n-type doping. These observations were related with the amphotericity of Si, where it acts as a donor impurity occupying Al or Ga-sites or as an acceptor when it takes an As-site, depending on the competition that the Si atoms encounters with As for any of these sites. The acceptor and donor lines close to the AlGaAs transition observed by photoluminescence spectroscopy (PL) were affected by the incorporation of Si. When increasing PAs the energy of the main PL peak is redshifted for n-type AlGaAs layers, but it is shifted back towards high energy once the conduction type conversion takes place. X-ray diffraction patterns revealed high crystalline quality for samples grown at the highest PAs.

Study of p-type AlGaAs film grown on sapphire substrate using GaAs buffer layer

The characteristics of AlGaAs films grown directly on a lattice-mismatched Al2O3 substrate were investigated. Metal organic chemical vapor deposition based AlGaAs films were grown on a transparent Al2O3 substrate, using a GaAs film as the buffer layer for the post-growth of the AlGaAs film. It was found that a higher Al composition in AlGaAs film on the Al2O3 substrate was observed when GaAs buffer layer was thermally treated. Notably, a resistivity of 0.018Ω/cm was obtained from the AlGaAs film grown on the Al2O3 substrate, almost equivalent to that for GaAs substrate. These results thus support the position that AlGaAs films grown on a transparent Al2O3 substrate can be attractive for use in solar cells or light-emitting diodes.

Quantum Hall Effect in AlGaAs and Graphite Quantum Dots

Abstract. The 30 nm wide quantum wells on a 4x4 mm2 piece of GaAs/AlGaAs are formed when the layers of GaAs are deposited on AlGaAs films. The two-dimensional density of electrons is 3x1011 cm-2 and the mobility is 32x106 cm2/Vs. In such a sample the Hall resistivity as a function of magnetic field is not a linear function. Hence a suitable theory to understand the Hall effect is formulated. We find that there are phase transitions as a function of temperature. There are lots of fractions of charge which are explained on the basis of spin and orbital angular momentum of the electron. The nano meter size films of graphite also show that the Hall resistivity is non-linear and shows steps as a function of magnetic field. We make an effort to understand the steps in the Hall effect resistivity of graphite with quantum wells formed on the surface. It is found that the fractions are in four categories, (i) the principal fractions which are determined by spin and orbital angular momenta, (ii) the resonances which occur at the difference between two values such as =1-2, (iii) two-particle states which occur at the sum of the two frequencies and (iv) there are clusters of electrons localized in some areas of the sample. The spin in the clusters is polarized so that it becomes NS which is not 1/2 but depends on the number N, of electrons in a cluster.

Preproduction prediction of the optical normalized reflectance transient of MOVPE-grown AlGaAs films using transfer matrix method

This paper reported on the prediction and analysis of the optical normalized reflectance (NR) transient of AlGaAs multi layers and Distributed Bragg Reflector (DBR) using transfer matrix method (TMM). The simulation result correlated well with the measured NR transient of grown samples. Deviations of AlxGa1⿿xAs composition between the expected and real grown sample were predicted successfully. Smaller optical oscillation amplitude in the first DBR layer was predicted and could be used as an indication of Al composition. Special characteristics of NR transient within the first three pairs of DBR were also clarified using the calculated transmissivity changing with the growth thickness. TMM simulation of NR transient was thus shown to be a convenient and reliable pre-production technique, also not restricted to the AlGaAs material.

The effect of the inserted AlGaAs films on the behaviors of InAs quantum dot detector

Two InAs quantum dot samples have been grown by the solid source molecular beam epitaxy (MBE) and fabricated to detectors. AlGaAs thin films have been inserted into the source region for one of the two devices. The structural features of the two samples have been studied by using the transmission electron microscope (TEM). The photoelectric properties of them have been measured by the photoluminescence (PL) and photocurrent (PC) spectra. The experimental results indicated that the AlGaAs films have profound effects on the properties of the detector. According to the calculations based on effective mass approximation, the origins of the photocurrent peaks of the two devices have been identified.

Effects of different masks on GaAs microtips grown by selective liquid-phase epitaxy

We investigate the dependence of selective liquid-phase epitaxial growth of GaAs microtips on different masks, including SiO 2 films deposited by liquid-phase deposition and magnetron sputtering separately, aluminium film prepared by vacuum evaporation, and AlGaAs film grown by liquid-phase epitaxy. Scanning electron microscopy is employed to characterize the surface morphologies of the GaAs microtips. The results indicate that well-distributed microtips with high quality can be achieved using either SiO 2 film deposited by magnetron sputtering or aluminium film prepared by vacuum evaporation as mask.

Terahertz absorption in AlGaAs films and detection using heterojunctions

HEterojunction Interfacial Workfunction Internal Photoemission (HEIWIP) detectors using AlGaAs as both the emitter and the barrier material with different Al fractions for the two layers are demonstrated. The extension of the HEIWIP concept to wavelengths longer than 110 μm in the GaAs/AlGaAs system requires the use of AlGaAs as the emitter material to reduce the barrier height. The p-type doping produces an offset in the valance band between doped and undoped material. The Al fraction difference then gives a valance band offset in the opposite direction, which reduces the total offset. The FIR absorption up to ∼400 μm for AlGaAs films with different Al fractions and doping are presented. The absorption in the films with low Al fraction (1%) shows little variation from comparable GaAs films while for 20% Al, the absorption is reduced. The spectral results on a device with 12% Al emitters and 11% Al barriers have shown a response of 0.6 A/W at 30 μm with D ∗ = 3 × 10 10 Jones measured at 5 K. The low responsivity is due to the reduced number (3) of emitters in the test device, and when scaled for the number of emitters this is comparable to results obtained from GaAs/AlGaAs HEIWIP detectors. Based on these results, a design for a 300 μm detector is presented and potential difficulties in growth and fabrication such as dopant migration are discussed.

Synthesis of GaAs quantum dots on Si-layers on AlGaAs films grown on GaAs(100)

In this work we report a novel method for obtaining GaAs quantum dots (QDs) by molecular beam epitaxy (MBE) on an AlGaAs underlying film. We propose to use a Si monolayer (ML) grown on AlGaAs in order to induce a three-dimensional nucleation during the GaAs overgrowth. We observe that when 2.1 MLs of GaAs are grown on 1 ML of Si, the GaAs is self-assembled in islands. Islands of 15-Å height and a lateral size of 200 Å were clearly observed. 77 K photoluminescence and photoreflectance spectra of capped GaAs dots showed an additional signal at ∼1.88 eV, presumably related to the islands emission, thereby verifying an efficient quantum confinement.

Photoresponse on Surface Acoustic Wave Devices with Compound Semiconductor and LiNbO3 Structures

We studied the photoresponse on a surface acoustic wave (SAW) device coupled with AlGaAs film. The experimental results indicate that the transferred carriers by SAW are optically generated electrons rather than optically generated holes. The electric field due to SAW propagation reduces the recombination of the optically generated electrons and holes. We were able to observed a modulated SAW signal due to the nonlinear interaction between SAW and optically induced carriers. The photoresponse due to the interaction between SAW and optically induced carriers shows a high sensitivity for the light intensity. The main results are (1) the control of SAW by optically induced carriers, and (2) the control of the optically induced carriers by SAW. We expect that the characteristics of the photoresponse will be applicable to the fabrication of functional optical-SAW coupled devices.

Next generation adduct purification techniques for low oxygen content metal alkyls

A new synthesis and adduct purification route has been developed for Group III metal alkyls which avoids the use of ether solvents. The R3M compound is synthesized in a trialkylamine solvent such as NEt3 which leads to adducts of the type R3Ga(NEt3). The NEt3 ligand can be displaced by the addition of involatile tertiary amines (L) containing 2, 4, or 6 nitrogen donor sites to give adducts of the type (R3Ga)xL(x=2,4,6), from which base free R3Ga compounds are obtained by mild thermal dissociation. The synthesis, characterization and thermal dissociation of these adducts is described and brief growth data are given for AlGaAs films grown by CBE using triisopropyl gallium prepared by this new route

Effects of atomic hydrogen in molecular beam epitaxy of Al(Ga)As

The effects of atomic hydrogen on the growth of Al(Ga)As by molecular beam epitaxy (MBE) have been investigated. A decrease in Al content was observed for AlGaAs films grown by atomic hydrogen-assisted MBE (H-MBE) compared to conventional MBE-grown films. This tendency was consistent with the results of decreased AlAs growth rates as determined by in situ reflection high-energy electron diffraction (RHEED). Further, the increase in the mass signal of AlH 2 + by quadruple mass spectrometer was observed during H-MBE growth. In H-MBE, it is thought that enhanced Al re-evaporation from the growth surface arises via formation of Al hydrides during growth. The observed difference of Al re-evaporation between vicinal A-surface and B-surface has been discussed, which is ascribed to the decreased growth rate and Al content.

Iterative solution of the inverse problem of dynamic diffraction in inhomogeneous crystals

One of the central problems in solid-state physics is the investigation of the spatial distributions of structural characteristics of crystals using x-ray diffraction. This problem has taken on crucial significance in the last 20‐25 years in connection with the development of new semiconductor devices. Despite the large number of publications devoted to this subject ~see, for example, Refs. 1‐10!, a generally accepted, universal method for solving the inverse problem of x-ray diffraction has still not been developed. Therefore, the search continues for new avenues for progress in this area of solidstate physics. In Ref. 8 we developed a method for solving the inverse problem within the kinematic approximation. Alongside some other approaches, this method has been used to obtain the spatial structural characteristics of a thin graded singlecrystal AlGaAs film on a thick GaAs substrate. 10 However, this method is not applicable to relatively thick heteroepitaxial systems, since the kinematic ~Born! approximation is valid only for thin layers. Therefore, the purpose of the present work is to develop a more general approach to the solution of the inverse problem within an iterative procedure. 2. BASIC EQUATIONS

Absorption enhancement in ultra-thin textured AlGaAs films

We have studied light randomization and the absorption enhancement in textured ultra-thin Al x Ga 1− x As films, with a thickness corresponding to a few optical wavelengths. A correlation between the degree of light randomization and trapping, with the scale length of the texturization geometry was found. The observed absorption enhancement corresponds to 90% of the best possible theoretical value, or 90% light randomization. A modified photon gas model is proposed to calculate the light trapping and absorption at the band edge in the textured thin films.

Selectively dry-etched n +-GaAs/AlGaAs/n-InGaAs doped-channel FETs by using a CHF 3+BCl 3 plasma

The gate recess, which involves removing the n +-cap layer before putting the Schottky gate, is the most critical procedure in the fabrication of heterostructure field effect transistors (HFETs). Excellent etching depth control is required in order to obtain a uniform threshold voltage distribution, which is essential for high performance integrated circuit fabrication. A newly developed high selective reactive ion etching (RIE) based on CHF 3+BCl 3 plasma has been studied for the gate-recess-technique in GaAs/AlGaAs heterostructure FETs, where the non-volatile AlF 3 compound is found to prevent the further etching in AlGaAs films. By changing the mixing ratio between chlorine- (BCl 3) and fluorine-based (CHF 3) gases, an etching selectivity of about 40 between the GaAs and AlGaAs materials was obtained. Using this optimum condition for later gate recessing, we have successfully fabricated the AlGaAs/InGaAs/GaAs heterostructure doped-channel FET. Device characteristics and the RIE associated damages have also been evaluated.

High-density inductively coupled plasma etching of GaAs/AlGaAs in BCl3/Cl2/Ar: A study using a mixture design experiment

Inductively coupled plasma etching of GaAs/AlGaAs was investigated in BCl3/Cl2/Ar using a mixture design experiment. According to the model extracted from the experiment, the etch rate was linearly proportional to the gas flows, and the process was reactant or diffusion limited. Etch rates from 200 to over 3000 nm/min were obtained. Equirate etch was observed for AlGaAs films with different aluminum content in the entire gas composition range. A quadratic dependence was observed for the etch rates of the resist mask with the gas flows. Etched profiles ranged from positively sloped to vertical to negatively sloped depending on the gas composition. Smooth etched surfaces and mirror quality smooth sidewalls were obtained.