Compound Semiconductor Surfaces Research Articles

Erratum: Efficient electronic passivation scheme for computing low-symmetry compound semiconductor surfaces in density-functional theory slab calculations [Phys. Rev. Materials 5 , 044605 (2021)

Erratum: Efficient electronic passivation scheme for computing low-symmetry compound semiconductor surfaces in density-functional theory slab calculations [Phys. Rev. Materials <b>5</b> , 044605 (2021)

Efficient electronic passivation scheme for computing low-symmetry compound semiconductor surfaces in density-functional theory slab calculations

Removing artificial bands from the back side of surface slabs with pseudohydrogen atoms has become the method of choice to boost the convergence of density-functional theory (DFT) surface calculation with respect to slab thickness. In this paper we apply this approach to semipolar compound semiconductor surfaces, which have recently become attractive for device applications. We show that approaches employing saturation of dangling bonds by pseudohydrogen atoms alone are inadequate to properly passivate the surfaces, remove spurious surface states from the fundamental band gap, and achieve flat band conditions in the slab. We propose and successfully apply to technologically interesting semipolar wurtzite surfaces of III-N, III-V, and II-VI semiconductors a reconstruction-inspired passivation scheme that utilizes native anions to passivate cation dangling bonds and pseudohydrogen atoms to obey the electron counting rule and compensate for polarization-induced surface-bound charges. This scheme is generic and robust and can be straightforwardly implemented in DFT investigations of low-symmetry surfaces as well as in high-throughput and machine learning studies.

Electronically Controlled Chemical Stability of Compound Semiconductor Surfaces.

Effects of a humid environment on the degradation of semiconductors were studied to understand the role of the surface charge on material stability. Two distinctly different semiconductors with the Fermi level stabilization energy EFS located inside the conduction band (CdO) and valence band (SnTe) were selected, and effects of an exposure to 85 °C and 85% relative humidity conditions on their electrical properties were investigated. Undoped CdO films with bulk Fermi level EF below EFS and positively charged surface are very unstable. The stability greatly improves with doping when EF shifts above EFS, and the surface becomes negatively charged. This charge-controlled reactivity is further confirmed by the superior stability of undoped p-type SnTe with EF above EFS. These distinct reactivities are explained by the surface attracting either the reactive OH- or passivating H+ ions. The present results have important implications for understanding the interaction of semiconductor surfaces with water or, in general, ionic solutions.

Reactivity studies and structural properties of Al on compound semiconductor surfaces

The authors studied the structural properties of Al on III-V semiconductors (InAs, GaAs, and InGaAs) with the aim of creating smooth and abrupt interfaces. Growth conditions, such as the residual As content and the presence of intermediate layers, affect the structural properties of the Al and the underlying semiconductor. The authors find that an ultrathin layer of AlAs on (001) InAs drastically reduces the interface reaction and improves the epitaxial growth of Al. No such layer is necessary for interface reaction mitigation for Al deposited onto InGaAs or GaAs. The crystal orientation of Al planes grown on (001) InAs is [110], but is [111] on InGaAs or GaAs. The authors discuss the significance of the results for realization of structures for proximity superconductivity.

Toward the Surface Preparation of InGaAs for the Future CMOS Integration

The integration of III-V and Ge materials on Si surface causes many issues with complexity such as lattice mismatch with silicon. In particular, the surface preparation and passivation of InGaAs is very challenging, because the formation of InGaAs/high-K interface is important, but not well understood. For the systematical study of InGaAs surface during wet processes, the effect of various wet etching processes on the surfaces of binary III-V compound semiconductors (GaAs, InAs, GaSb and InSb) was studied from the viewpoints of surface oxidation, material loss (dissolution), and passivation. Based on that, further effort to understand the surface reactions on ternary InGaAs compound semiconductor was made. In addition, process sequential effect on the InGaAs surface was investigated.

Effect of WET treatment on Group III-V Compound Semiconductor Surface

In this work, we investigated interfacial properties of InP, which is a typical group III-V compound used for semiconductors, by using a chemical-treated metal oxide semiconductor (MOS) capacitor. The interfacial properties of InP is more affected by interface state density than the surface roughness and is greatly affected by In2O3in particular. Additionally, we evaluated In2O3growth during 24-hour rinsing and air exposure and found that In2O3on an InP surface grows larger during rinsing than during air exposure. To reduce In2O3, the rinse needs to be optimized.

Stability of wurtzite semipolar surfaces: Algorithms and practices

A complete knowledge of absolute surface energies with any arbitrary crystal orientation is important for the improvements of semiconductor devices because it determines the equilibrium and nonequilibrium crystal shapes of thin films and nanostructures. It is also crucial in the control of thin film crystal growth and surface effect studies in broad research fields. However, obtaining accurate absolute formation energies is still a huge challenge for the semi-polar surfaces of compound semiconductors. It mainly results from the asymmetry nature of crystal structures and the complicated step morphologies and related reconstructions of these surface configurations. Here we propose a general approach to calculate the absolute formation energies of wurtzite semi-polar surfaces by first-principles calculations, taking GaN as an example. We mainly focused on two commonly seen sets of semi-polar surfaces: a-family (11-2X) and m-family (10-1X). For all the semi-polar surfaces that we have calculated in this paper, the self-consistent accuracy is within 1.5 meV/{\AA}^2. Our work fills the last technical gap to fully investigate and understand the shape and morphology of compound semiconductors.

Competing charge density wave and antiferromagnetism of metallic atom wires in GaN( 101¯0 ) and ZnO( 101¯0 )

Low-dimensional electron systems often show a delicate interplay between electron-phonon and electron-electron interactions, giving rise to interesting quantum phases such as the charge density wave (CDW) and magnetism. Using the density-functional theory (DFT) calculations with the semilocal and hybrid exchange-correlation functionals as well as the exact-exchange plus correlation in the random-phase approximation (EX + cRPA), we systematically investigate the ground state of the metallic atom wires containing dangling-bond (DB) electrons, fabricated by partially hydrogenating the GaN(10${\overline{1}}$0) and ZnO(10${\overline{1}}$0) surfaces. We find that the CDW or antiferromagnetic (AFM) order has an electronic energy gain due to a band-gap opening, thereby being more stabilized compared to the metallic state. Our semilocal DFT calculation predicts that both DB wires in GaN(10${\overline{1}}$0) and ZnO(10${\overline{1}}$0) have the same CDW ground state, whereas the hybrid DFT and EX+cRPA calculations predict the AFM ground state for the former DB wire and the CDW ground state for the latter one. It is revealed that more localized Ga DB electrons in GaN(10${\overline{1}}$0) prefer the AFM order, while less localized Zn DB electrons in ZnO(10${\overline{1}}$0) the CDW formation. Our findings demonstrate that the drastically different ground states are competing in the DB wires created on the two representative compound semiconductor surfaces.

Surface reconstruction of InAs (001) depending on the pressure and temperature examined by density functional thermodynamics

A detailed understanding of the atomic configuration of the compound semiconductor surface, especially after reconstruction, is very important for the device fabrication and performance. While there have been numerous experimental studies using the scanning probe techniques, further theoretical studies on surface reconstruction are necessary to promote the clear understanding of the origins and development of such subtle surface structures. In this work, therefore, a pressure-temperature surface reconstruction diagram was constructed for the model case of the InAs (001) surface considering both the vibrational entropy and configurational entropy based on the density functional theory. Notably, the equilibrium fraction of various reconstructions was determined as a function of the pressure and temperature, not as a function of the chemical potential, which largely facilitated the direct comparison with the experiments. By taking into account the entropy effects, the coexistence of the multiple reconstructions and the fractional change of each reconstruction by the thermodynamic condition were predicted and were in agreement with the previous experimental observations. This work provides the community with a useful framework for such type of theoretical studies.

Massive-scale molecular dynamics of ion-irradiated III–V compound semiconductors at the onset of nanopatterning

Large-scale atomistic computational modeling is critical to developing a fundamental understanding of the driving mechanisms behind ion beam-induced pattern formation on III–V compound semiconductor surfaces. Existing theoretical efforts fail to account for the existence of compositional depth profiles, as observed in experiments, which evolve over time and contribute to the development of lateral compositional instabilities. Another critical knowledge gap in existing theories is the a priori selection of ion-driven compositional mechanisms, so that different models predict the same surface morphology for radically different surface compositions. Atomistic simulations can elucidate key ion-induced compositional mechanisms leading to nonequilibrium compositional depth profiles. Such simulations must be performed on large length scales to enable connection of depth compositional mechanisms to lateral instabilities which ultimately drive patterning. To address these essential knowledge gaps, a 100×100nm2 GaSb surface was constructed for molecular dynamics simulations with altered composition based on experimental data at the pattern threshold fluence. The surface was then bombarded with 500eV Kr+ ions to a differential fluence of 8.4×1013cm−2. In Ga-enriched (Sb-enriched) regions, clusters of Ga (Sb) are observed to form. While the Ga clusters remain amorphous, Sb clusters rapidly crystallize. The crystallization is not directly ion-induced but rather an intrinsic material behavior of pure Sb in response to the ion-induced compositional instability in the surface. To elucidate the ion-driven compositional changes leading to this state, 25×25nm2 “pristine” 50–50 GaSb crystalline surfaces were irradiated with 500eV Ne+, Ar+, and Kr+ to fluences on the order of 1016cm−2. The ion-induced displacement cascade tends to lead to formation of Sb nanoclusters, while Ga rarely forms clusters but readily bonds to the Sb clusters. While initially the clustering behavior is the same for all ion species, for continued Kr+ irradiation the clusters become less prevalent within the material surface.

Quantitative Analysis of Trace Metallic Contamination on III-V Compound Semiconductor Surfaces

We studied the detection by TXRF of several transition metals on the surface of III-V materials for high mobility channel. It has been found that the lower limits of detection of some transition metals on the surface of III-V materials become higher than that on the Si surface because the sum peaks or Raman scattering peaks as well as the fluorescent X-ray main signals from the materials themselves partially cover those from the transition metals

Non-conformal decoration of semiconductor nanowire surfaces with boron nitride (BN) molecules for stability enhancement: degradation-resistant Zn3P2, ZnO and Mg2Si nanowires.

A simple and reliable strategy for stabilizing the surfaces of compound semiconductors was presented. The strategy involved decorating the surfaces of compound semiconductor nanowires non-conformally with small molecules of boron nitride (BN). More specifically, Zn3P2, ZnO and Mg2Si nanowires, highly useful in energy conversion device fabrication (e.g., photovoltaics and thermoelectrics), have been stabilized against air- and acid-assisted degradation by decorating their surfaces with small molecules of BN. It is believed that the decoration of the nanowire surfaces with BN molecules made the nanowire surfaces non-wettable to water and aqueous acid solutions, and thereby imparted them enhanced resistance against water- and acid-assisted degradation. This procedure did not alter the bandgap of the nanowires. Moreover, this procedure aided in retaining the electrical conduction between the nanowire interfaces when the nanowires are assembled into mats or pellets. This strategy solves one of the primary bottlenecks in the widespread use of nanowires in energy conversion device fabrication, namely their stability. It is believed that this strategy is applicable for stabilizing other compound semiconductor nanowires, including nitrides, sulfides, silicides and antimonides.

Formation and evolution of ripples on ion-irradiated semiconductor surfaces

We have examined the formation and evolution of ripples on focused-ion-beam (FIB) irradiated compound semiconductor surfaces. Using initially normal-incidence Ga+ FIB irradiation of InSb, we tuned the local beam incidence angle (θeff) by varying the pitch and/or dwell time. For single-pass FIB irradiation, increasing θeff induces morphological evolution from pits and islands to ripples to featureless surfaces. Multiple-pass FIB irradiation of the rippled surfaces at a fixed θeff leads to island formation on the ripple crests, followed by nanorod (NR) growth. This ripple-NR transition provides an alternative approach for achieving dense arrays of NRs.

Universal mechanism for ion-induced nanostructure formation on III-V compound semiconductor surfaces

We have examined the formation of nanostructures on ion-irradiated compound semiconductor surfaces. We computed the ion doses needed to fully deplete group V elements from the surfaces. These group V depletion doses are in good agreement with the measured threshold ion doses for nucleation of group III-rich nanostructures on a wide variety of III-V compound semiconductor surfaces. Since the group V depletion doses decrease with increasing sputtering yield, these results suggest a universal nanostructure formation mechanism which depends upon the total sputtering yield of each III-V compound.

Scanning Probe Microscopy Imaging before and after Atomic Layer Oxide Deposition on a Compound Semiconductor Surface

The megasonic cleaning efficiency is evaluated as a function of the angle of incidence of acoustic waves on a Si wafer. Acoustic Schlichting streaming alone is not able to remove nanoparticles smaller than 400 nm. It is shown that oscillating or collapsing behavior of bubbles are responsible for removing nanoparticles smaller than 400 nm during a cleaning process with ultrasound. Optimal particle removal efficiency is obtained around the angle of acoustic transmission of the silicon wafer.

Device quality Sb-based compound semiconductor surface: A comparative study of chemical cleaning

We have studied the surface cleaning of Sb-based compound semiconductors using HF, NH4OH, and HCl cleans and the metal–oxide–semiconductor (MOS) capacitors fabricated subsequently. GaSb, InGaSb, and AlGaSb surfaces are investigated using low-energy radiation from the synchrotron. Capacitance–voltage (C–V) and photoluminescence measurements are carried out on capacitors made with Al2O3 from atomic layer deposition and corroborated with the results from synchrotron spectroscopy. Excellent C–V characteristics with a mid-band-gap interface state density of 3 × 1011/cm2eV are obtained on samples with the HCl clean. This is consistent with the finding that only the HCl acid clean is able to remove the native oxides present on GaSb and InGaSb surfaces, and produce clean and stable surfaces suitable for MOSFET development. Complete removal of AlOx on the AlGaSb surface was not possible using chemical cleaning. Termination of AlGaSb with two monolayers of GaSb is proposed as a solution.

Atomic imaging of the monolayer nucleation and unpinning of a compound semiconductor surface during atomic layer deposition

The reaction of trimethyl aluminum on the group III rich reconstructions of InAs(0 0 1) and In(0.53)Ga(0.47)As(0 0 1) is observed with scanning tunneling microscopy/spectroscopy. At high coverage, a self-terminated ordered overlayer is observed that provides the monolayer nucleation density required for subnanometer thick transistor gate oxide scaling and removes the surface Fermi level pinning that is present on the clean InGaAs surface. Density functional theory simulations confirm that an adsorbate-induced reconstruction is the basis of the monolayer nucleation density and passivation.

Scanning Probe Microscopy Imaging of Nucleation and Electronic Structure Passivation during Atomic Layer Deposition on a Compound Semiconductor Surface

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Effects of screening of the interaction potential on the mobility characteristics of degenerate surface layers in compound semiconductors at low lattice temperatures

The rates of scattering of the conduction electrons in degenerate two-dimensional electron gas in the surface of compound semiconductors at low lattice temperatures have been obtained for interaction with the piezoelectric and deformation potential acoustic phonons, under different prevailing conditions. The calculations have been carried out taking due account of the screening of the interaction potential at low temperatures where again the phonon energy cannot be neglected in comparison to the average thermal energy of the electrons and, as a result, the equipartition approximation for the phonon distribution can hardly be valid. The scattering rates thus obtained for inversion layers in GaAs and ZnO are found to depend upon the carrier energy, the lattice temperature and the level of degeneracy in quite involved manners, which are very different from what follows if one makes the simplifying approximation of negligible phonon energy or disregards the effects of screening. The mobility characteristics are then obtained using these scattering rates. The results show how the screening of the interaction potential and the finite energy of the intravalley acoustic and piezoelectric phonons significantly change the mobility characteristics of the degenerate surface layers at low lattice temperatures. The inadequacies of the present theory are pointed out and recommendations for possible refinements are discussed.

Ab initio-based approach to structural change of compound semiconductor surfaces during MBE growth

Phase diagrams of GaAs and GaN surfaces are systematically investigated by using our ab initio-based approach in conjunction with molecular beam epitaxy (MBE). The phase diagrams are obtained as a function of growth parameters such as temperature and beam equivalent pressure (BEP). The versatility of our approach is exemplified by the phase diagram calculations for GaAs(0 0 1) surfaces, where the stable phases and those phase boundaries are successfully determined as functions of temperature and As 2 and As 4 BEPs. The initial growth processes are clarified by the phase diagram calculations for GaAs(1 1 1)B-(2×2). The calculated results demonstrate that the As-trimer desorption on the GaAs(1 1 1)B-(2×2) with Ga adatoms occurs beyond 500–700 K while the desorption without Ga adatoms does beyond 800–1000 K. This self-surfactant effect induced by Ga adsorption crucially affects the initial growth of GaAs on the GaAs(1 1 1)B-(2×2). Furthermore, the phase diagram calculations for GaN(0 0 0 1) suggests that Ga adsorption or desorption during GaN MBE growth can easily change the pseudo-(1×1) to the (2×2)-Ga via newly found (1×1) and vice versa. On the basis of this finding, the possibility of ghost island formation during MBE growth is discussed.