InAlAs Surface Research Articles

Effect of in-situ Zn doping on suppression of phase separation in InxAl1−xAs epitaxial layer on InP(001) grown by MOCVD

The effect of Zn impurities on the surface morphology evolution and phase separation behavior of the InAlAs layer on InP(001) substrates is investigated using metal organic chemical vapor deposition (MOCVD). The phase separation of InAlAs occurs at a relatively low growth temperature (T < 620 °C), accompanied by surface roughening and bandgap lowering. According to the microstructural analyses, indium (In) and aluminum (Al) atoms have high reactivity with In-rich or Al-rich columns in a certain growth region. The addition of Zn impurities during InAlAs growth at the phase-separating temperature results in the suppression of phase separation, which can be attributed to a high probability of Zn incorporation into the sites, which generates an In-rich and Al-rich phase column in the phase-separated InAlAs layer. Based on atomic force microscopy (AFM) and high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) analysis, we classify four types of InAlAs surface morphology as a function of the growth temperature and flow rate of Zn impurities.

InAs quantum emitters at telecommunication wavelengths grown by droplet epitaxy

InAs quantum dots at telecommunication wavelengths are desired as single-photon sources, but a growth technique that enables wide control over quantum dot size, density, and morphology is needed. Droplet epitaxy is well suited for this purpose, but InAs nanostructures tend to form as rings on (001) InGaAs, InAlAs, and InP surfaces. In this work, we investigate how surface diffusion can be manipulated to grow quantum dots by molecular beam epitaxy without using high-index substrates or metamorphic buffers. First, surface diffusion characteristics of In on In0.52Al0.48As are compared to In and Ga on In0.53Ga0.47As. Then, a two-step arsenic exposure protocol is applied to modify the droplet crystallization step, resulting in a series of different nanostructure morphologies that have narrow-linewidth emission between 1200 and 1520 nm at 4 K. Ultimately, we show that controlling surface diffusion of the group-III species during growth is critical for achieving quantum dots appropriate for single-photon sources at telecommunication wavelengths.

Optical properties of native (anodic) layer on the InAlAs surface of different morphology

The influence of InAlAs/InP(001) heterostructures surface morphology on the optical properties (dispersive refractive index and extinction coefficient) of the anodic layer, formed by the oxidation in low-energy Townsend gas-discharge O2-Ar-containing plasma at room temperature, was studied using atomic force microscopy and ellipsometry methods. High resolution transmission electron microscopy and X-ray photoelectron spectroscopy were used to determine the anodic layer morphology and chemical composition. It is shown that the growth structural defects (pits) on the InAlAs surface, with a density 106–107 cm−2, do not significantly influence the optical properties of the amorphous anodic layers, mainly consisting of In2O3, Al2O3, As2O3 and elemental arsenic quite uniformly distributed over their depth and area. The thickness of thin (≤10 nm) oxide layers on InAlAs is measured at a high accuracy by the nondestructive ellipsometry method using the optical model of a single-layer isotropic film on an absorbing substrate.

Anodic layer formation on the InAlAs surface in Townsend gas-discharge plasma

The study of the InAlAs surface oxidation process in the Townsend gas-discharge plasma of gas-mixture Ar, O2, CF4 at room temperature was performed. The anodic layer thickness distribution, the morphology of InAlAs surface and anodic layer/InAlAs interface, as well as the layer chemical composition, were investigated by scanning ellipsometry, AFM, HREM and XPS methods. It was shown that the presented oxidation technique provides a controlled formation of non-fluorinated and fluorinated anodic layers on the InAlAs surface with a sharp interface and unchanged surface morphology. The non-fluorinated anodic layer mainly consists of semiconductor element oxides (In2O3, Al2O3, As2O3). The oxidation in the fluorine-containing medium leads to the formation of In, Al and As oxyfluorides.

About the nature of the barrier inhomogeneities at Au/Ti/n-InAlAs(001) Schottky contacts

A correlation study of InAlAs surface morphology by atomic force microscopy and a study on the temperature (78–380 K) dependence of Au/Ti/InAlAs(001) Schottky barriers' current-voltage characteristics were performed. It is shown that a change in the growth structural defect (pit) density on the InAlAs surface from 106 to 107 cm−2 practically does not affect the current-voltage dependence at temperatures above 200 K that is well described by the thermionic emission theory with the ideality factor and the barrier height close to 1.1 and 0.69 eV, respectively. At the same time, change in the pit density has a significant effect on the Schottky barrier parameters at temperatures below 200 K, which can be explained by the Tung model suggesting the presence of local inhomogeneities with a lower barrier height and different total occupied areas. Also, the areas at the pit defect periphery that can lead to effective barrier height lowering after the Schottky contact formation were revealed by Kelvin probe force microscopy. Based on the obtained data, it can be concluded that the growth defects on the InAlAs surface shaped as pits cause areas in Schottky contact with a lower barrier height.

Investigation of band alignment between InAlAs and atomic-layer-deposited HfO2/Al2O3 by X-ray photoelectron spectroscopy

Band alignments between atomic layer deposition (ALD) HfO2/Al2O3 double layers and InAlAs were investigated by angle-resolved X-ray photoelectron spectroscopy (AR-XPS). The relationship of As and Hf 4f peaks reducing with the decreasing take-off angle θ indicated upward band bending on the InAlAs surface and a potential gradient in the HfO2/Al2O3 layer. AR-XPS measurements were combined with numerical calculations. The valence band and conduction band discontinuity between HfO2/Al2O3 and InAlAs were 2.49 ± 0.2 eV and 1.86 ± 0.2 eV, respectively. A low leakage current density of 7.01 × 10−7 A cm−2 at 1 V and oxide trapped charge densities of 9.925 × 1011 cm−2 were obtained.

Barrier characteristics and interface properties of Au/Ti/n-InAlAs Schottky contacts

The InAlAs surface morphology (AFM), chemical composition (XPS) and the Ti/InAlAs interface structure (HREM), during the Au/Ti/n-In0.52Al0.48As/InP(001) mesa-structure Schottky barrier formation, were studied. The current-voltage (I-V) dependences analysis of the formed Schottky barrier in the temperature range of 100–380K was carried out. It was shown that the I-V dependences are well described by the thermionic emission theory at temperatures above 200K with the ideality factor and the barrier height close to 1.09 and 0.7eV, respectively. At temperatures below 200K, the I-V behavior is attributed to the Schottky barrier anomalies, which are explained by the existence of nanometer-sized patches with a low barrier height having a Gaussian distribution (Tung model). According to this model, the temperature dependences of the barrier height and ideality factor were described with the following parameters: the homogeneous barrier height of 0.88eV and standard deviation of 10−4cm2/3V1/3. In the analysis of the modified Richardson plot, the Richardson constant (10.7Acm−2K−2) and the area fraction (24%), occupied by the patches with a low barrier height and diameter ~80nm, were calculated. The patches appearance is explained by the presence of the spikes detected on the InAlAs surface by the AFM method.

Impact of SiNx passivation on the surface properties of InGaAs photo-detectors

We investigate surface passivation effects of SiNx films deposited by inductive coupled plasma chemical vapor deposition (ICPCVD) and plasma enhanced chemical vapor deposition (PECVD) technologies for InAlAs/InGaAs/InP photo-detectors. It is found that ICPCVD deposited SiNx film effectively reduces the densities of the interface states and slow traps near SiNx/InAlAs interface, which realize the small surface recombination velocity and low surface current for InAlAs/InGaAs/InP photo-detectors. By comparing C-V and XPS results, it is suggested that the trap density reduction by ICPCVD technology could be attributed to the disorder suppression on InAlAs surface due to the high density of SiNx film and less processing energy to the InAlAs surface.

Droplet epitaxy growth of telecom InAs quantum dots on metamorphic InAlAs/GaAs(111)A

We demonstrated the droplet epitaxial growth of InAs quantum dots on a GaAs(111)A substrate, which emitted at telecommunication wavelengths. A high-quality metamorphic In0.52Al0.48As layer was formed by inserting three monolayers of InAs between GaAs(111)A and InAlAs. InAs quantum dots were grown on the InAlAs surface by droplet epitaxy. They exhibited a laterally symmetrical shape owing to the C3v symmetry of the {111} surface. The photoluminescence signals of capped quantum dots indicated broadband spectra covering wavelengths from 1.3 to 1.55 µm. Thus, our dots are potentially useful for constructing entangled photon sources compatible with current telecommunication networks.

Chemical and electrical characterization of the HfO2/InAlAs interface

InAlAs has the potential to be used as a barrier layer in buried channel quantum well field effect transistor devices due to favorable lattice-matching and carrier confinement properties with InGaAs. Field effect device structures of this nature may also require a high-k oxide deposited on the InAlAs surface to reduce leakage current. This study investigates the impact of surface preparations and atomic layer deposition of HfO2 on these surfaces using x-ray photoelectron spectroscopy to analyse the chemical interactions taking place, as well as the electrical performance of associated capacitor devices. A large concentration of As related surface features is observed at the InAlAs surface, and is attributed to a large Dit response in electrical measurements.

Tuning Quantum Dot Luminescence Below the Bulk Band Gap Using Tensile Strain

Self-assembled quantum dots (SAQDs) grown under biaxial tension could enable novel devices by taking advantage of the strong band gap reduction induced by tensile strain. Tensile SAQDs with low optical transition energies could find application in the technologically important area of mid-infrared optoelectronics. In the case of Ge, biaxial tension can even cause a highly desirable crossover from an indirect- to a direct-gap band structure. However, the inability to grow tensile SAQDs without dislocations has impeded progress in these directions. In this article, we demonstrate a method to grow dislocation-free, tensile SAQDs by employing the unique strain relief mechanisms of (110)-oriented surfaces. As a model system, we show that tensile GaAs SAQDs form spontaneously, controllably, and without dislocations on InAlAs(110) surfaces. The tensile strain reduces the band gap in GaAs SAQDs by ~40%, leading to robust type-I quantum confinement and photoluminescence at energies lower than that of bulk GaAs. This method can be extended to other zinc blende and diamond cubic materials to form novel optoelectronic devices based on tensile SAQDs.

Arsenic-dominated chemistry in the acid cleaning of InGaAs and InAlAs surfaces

The surface cleaning of InGaAs and InAlAs is studied using synchrotron radiation photoelectron spectroscopy. Thermal annealing at 400°C cannot completely remove the native oxides from those surfaces. Elemental arsenic buildup is observed on both surfaces after acid treatment using HCl, HF, or H2SO4 solution, which is similar to acid-cleaned GaAs surface. Cleaned InGaAs surface is oxide-free but small amount of aluminum oxide remains on cleaned InAlAs surface. The common chemical reactions between III-As semiconductors and acid solutions are identified and are found to be dominated by arsenic chemistry.

Fabrication of InAs composite channel high electron mobility transistors by utilizing Ne-based atomic layer etching

High electron mobility transistors (HEMTs) with InAs∕InGaAs composite channel were fabricated by employing low damage, highly selective Ne-based atomic layer etching (ALET) for the dry gate recess process. Ne-based ALET exhibited very high etch selectivity of InP over InAlAs which is suitable for dry gate recess process removing InP etch stop layer on top of InAlAs Schottky barrier layer. The plasma induced damage on the exposed InAlAs surface due to the ALET was much lower than that due to the conventional Ar-based reactive ion etching (RIE), which was verified by atomic force microscopy and Hall measurements. For further comparison, dc characteristics were compared for the two types of 0.3μm HEMTs fabricated by utilizing ALET and conventional RIE during the dry gate recess processes.

Carrier dynamics in Ga0.53In0.47As∕InP near-surface quantum wells

Ga 0.53 In 0.47 As ∕ InP single-quantum wells with thin top barrier layers of InP and InAlAs have been studied by pump-probe experiments and temperature-dependent photoluminescence spectroscopy. The carrier lifetimes are shorter in samples with an InAlAs surface barrier than with an InP surface barrier. The decays consist in an early stage decay of typically 30 ps and an extended stage decay of several hundreds of picoseconds. This nonexponential relaxation is the result of thermally activated carrier escape, trapping, and recombination on the surface states. Two nonradiative surface states, at 11 and 68 meV from the ground state of the quantum well, were identified.

Hydrogen-induced changes in the breakdown voltage of InP HEMTs

In this work, electrical measurements show that the breakdown voltage, BV/sub DG/, of InP HEMTs increases following exposure to H/sub 2/. This BV/sub DG/ shift is nonrecoverable. The increase in BV/sub DG/ is found to be due to a decrease in the carrier concentration in the extrinsic portion of the device. We provide evidence that H/sub 2/ reacts with the exposed InAlAs surface in the extrinsic region next to the gate, changing the underlying carrier concentration. Hall measurements of capped and uncapped HEMT samples show that the decrease in sheet carrier concentration can be attributed to a modification of the exposed InAlAs surface. Consistent with this, XPS experiments on uncapped heterostructures give evidence of As loss from the InAlAs surface upon exposure to hydrogen.

Mechanical Stress Caused by Adsorption of O or N on Ga-terminated (100) GaAs Surface and InAl-terminated (100) InAlAs Surface: Degradation of Insulator/Semiconductor Interface

Mechanical stress at the surfaces of GaAs and InAlAs, caused by adsorption of O or N on their group III surfaces, was calculated using small cluster models and first-principles calculations. These elements adsorb at the bridge sites and give strong compressive stress to the crystal. Assuming that only the O and N at the interface generate the stress, i.e., the insulator does not give any stress, mechanical stress at the insulator/semiconductor interface were calculated. N at the insulator/GaAs interface does not give a strong stress, but O does. Calculation results were compared with various experimental results. The results do not contradict any of the examined experimental findings, and these findings are reasonably understandable when one takes the mechanical stress as a cause of the interface degradation.

Oxidation of InAlAs and Its Application to Gate Insulator of InAlAs/InGaAs Metal Oxide Semiconductor High Electron Mobility Transistor

InAlAs was oxidized by UV and ozone process. Nanometer-order thin-oxide layers were generated in proportion to the square root of the process period, although their correlation with the period is poor compared with that of InGaAs. Oxidation initially decreased photoluminescence intensity from the InAlAs surface and then gradually increased it when the process period exceeded 15 min until it reached 4 h (crystallographic-order degradation followed by recovery). Plasma nitridation of InAlAs exhibited a gradual and monotonic decrease in photoluminescence intensity. Lattice-matched InAlAs/InGaAs metal–oxide–semiconductor high electron mobility transistors were fabricated utilizing gate oxide layers which were formed by 4 h oxidation. A transconductance of 200 mS/mm was obtained using a 1.5-µm-gate-length device, in a high-forward-gate-bias region although hysteresis was observed in the current–voltage curve.

Intraband spectroscopy of self-organized InAs/InAlAs nanostructures grown on [formula omitted

InAs nanostructures on InAlAs/ InP(0 0 1) have been fabricated using Stranski–Krastanov growth mode. Depending on growth conditions, a full coverage of the InAlAs surface by either InAs strings of elongated dots or isolated dots can be achieved. Giant intraband absorptions are observed at mid-infrared wavelengths in both nanostructures. The intraband resonances are strongly polarized in the layer plane as a consequence of the quantum confinement along either [1 1 0] and [1 −1 0] directions.

Extremely flat growth-interrupted InAlAs surface grown on a [formula omitted]-oriented InP substrate by molecular beam epitaxy

We investigated flatness of growth-interrupted InAlAs and InGaAs surfaces using In 0.53Ga 0.47As/In 0.52Al 0.48As quantum wells (QWs) with well widths ( L w) of 0.6– 12 nm grown on a (4 1 1) A -oriented InP substrate by molecular beam epitaxy. In the (4 1 1) A QWs with growth interruption (GI) at the InGaAs/InAlAs interface (InAlAs surface), full width at half maximum (FWHM) of a photoluminescence peak of the 1.2-nm-thick InGaAs well was as narrow as 15.6 meV , which was almost the same as that of the corresponding well without GI ( 14.4 meV ). On the other hand, the FWHM of the (1 0 0) QWs with GI at the InAlAs surface was 24.1 meV ( L w =1.2 nm ), which was 55% larger than that of the (4 1 1) A QWs with GI at the InAlAs surface. This result indicates that the effectively atomically flat interfaces [ (4 1 1) A super-flat interfaces] are successfully kept in spite of introducing GI, while GI makes the (1 0 0) interface much rough.

Molecular-beam epitaxy growth of InGaAs–InAlAs high electron mobility transistors with enhanced electron densities and measurement of InAlAs surface potential

The electron densities in the channel of Si δ-doped InGaAs–InAlAs high electron mobility transistors grown on InP by molecular-beam epitaxy have been investigated by 1.4 K Shubnikov–de Haas (S–dH) measurements. Growth procedures have been developed that result in (i) minimized spreading of the Si δ doping, (ii) significantly improved transfer of electrons from the Si donors to the InGaAs channel, (iii) reduced parallel conduction associated with electrons remaining in the vicinity of the Si donors, and (iv) high sheet density-mobility products (nH×μH), important for a low access resistance into the channel. The Fermi energy of the free InAlAs surface has also been estimated by comparing self-consistent calculations of the channel electron density with the S–dH data from samples with progressively thinner InAlAs Schottky layers. The Fermi energy is found to be pinned at ∼0.6 eV below the conduction band edge, in agreement with the value deduced from photoreflectance spectrometry [J. S. Hwang, W. C. Hwang, Z. P. Yang, G. S. Chang, J. I. Chyi, and N. T. Yeh, Appl. Phys. Lett. 75, 2467 (1999)].