Sulfur-passivated GaAs Research Articles

Electrical Properties of HfTiON Gate-Dielectric GaAs Metal-Oxide-Semiconductor Capacitor With AlON as Interlayer

High- k HfTiON gate-dielectric GaAs MOS capacitors with and without AlON as interfacial passivation layer (IPL) are fabricated and their interfacial and electrical properties are compared. It is found that low interface-state density ( 1.5×1012 cm-2eV-1 at midgap), small gate leakage current ( 1.3×10-4 A/cm2 at Vg=Vfb+1 V), small capacitance equivalent thickness (1.72 nm), large equivalent dielectric constant (25.6), and high device reliability can be achieved for the Al/HfTiON/AlON/GaAs MOS device. All of these should be due to the fact that the AlON IPL on sulfur-passivated GaAs can effectively reduce the density of defective states and unpin the Femi level at the AlON/GaAs interface, thus greatly improving the interfacial and electrical properties of the device.

Improved interfacial and electrical properties of GaAs metal-oxide-semiconductor capacitors with HfTiON as gate dielectric and TaON as passivation interlayer

The interfacial and electrical properties of sputtered HfTiON on sulfur-passivated GaAs with or without TaON as interfacial passivation layer (IPL) are investigated. Experimental results show that the GaAs metal-oxide-semiconductor capacitor with HfTiON/TaON stacked gate dielectric annealed at 600 °C exhibits low interface-state density (1.0 × 1012 cm−2 eV−1), small gate leakage current (7.3 × 10−5 A cm−2 at Vg = Vfb + 1 V), small capacitance equivalent thickness (1.65 nm), and large equivalent dielectric constant (26.2). The involved mechanisms lie in the fact that the TaON IPL can effectively block the diffusions of Hf, Ti, and O towards GaAs surface and suppress the formation of interfacial As-As bonds, Ga-/As-oxides, thus unpinning the Femi level at the TaON/GaAs interface and improving the interface quality and electrical properties of the device.

Heteroepitaxy of single-crystal LaLuO3 on GaAs(111)A by atomic layer deposition

We demonstrate that LaLuO3 films can be grown epitaxially on sulfur-passivated GaAs(111)A substrates by atomic layer deposition (ALD). Transmission electron microscopy and x-ray diffraction analyses reveal that the oxide film exhibits a cubic structure with a lattice mismatch of −3.8% relative to GaAs. The epitaxial layer has a high degree of crystalline perfection and is relaxed. Electrical characterizations performed on this structure show interfaces with a low interface state density of ∼7×1011 cm−2 eV−1. The measured dielectric constant is around 30, which is close to its bulk crystalline value. In contrast, ALD LaLuO3 is polycrystalline on GaAs(100) and amorphous on Si(111).

Interface and Electrical Properties of Atomic-layer-deposited HfAlO Gate Dielectric for N-channel GaAs MOSFETs

AbstractThe interface and electrical properties of HfAlO dielectric formed by atomic layer deposition (ALD) on sulfur-passivated GaAs were investigated. X-ray photoelectron spectroscopy (XPS) revealed the absence of arsenic oxides at the HfAlO/GaAs interface after dielectric growth and post-deposition annealing at 500 °C. A minimal increase in the amount of gallium oxides at the interface was detected between the as-deposited and annealed conditions highlighting the effectiveness of HfAlO in suppressing gallium oxide formation. An equivalent oxide thickness (EOT) of ∼ 2 nm has been achieved with a gate leakage current density of less than 10-4 A/cm2. These results testify a good dielectric interface with minimal interfacial oxides and open up potential for further investigation of HfAlO/GaAs gate stack properties to determine its viability for n-channel MOSFETs.

Band offsets of atomic-layer-deposited Al2O3 on GaAs and the effects of surface treatment

The metal gate/high-k dielectric/III-V semiconductor band alignment is one of the most technologically important parameters. We report the band offsets of the Al/Al2O3/GaAs structure and the effect of GaAs surface treatment. The energy barrier at the Al2O3 and sulfur-passivated GaAs interface is found to be 3.0±0.1 eV whereas for the unpassivated or NH4OH-treated GaAs is 3.6 eV. At the Al/Al2O3 interface, all samples yield the same barrier height of 2.9±0.2 eV. With a band gap of 6.4±0.05 eV for Al2O3, the band alignments at both Al2O3 interfaces are established.

Impact of elemental arsenic on electrical characteristics of metal-oxide-semiconductor capacitors on GaAs using atomic-layer deposited HfO2 gate dielectric

The interface between HfO2 and sulfur-passivated GaAs was analyzed after atomic-layer deposition (ALD) and postdeposition annealing (PDA) using x-ray photoelectron spectroscopy. The HfO2 ALD process resulted in elemental arsenic buildup at the interface. Electrical measurements confirmed that the elemental arsenic caused anomalously large values for equivalent oxide thickness (EOT), hysteresis, and frequency dispersion in accumulation. Arsenic outdiffusion after PDA lowered the EOT but increased the gate leakage. Annealing the (NH4)2S-treated GaAs prior to ALD yielded an EOT of 1.85nm and leakage of 6.6×10−4A∕cm2 at Vg=Vfb−1V. This modified passivation scheme looks promising for achieving a high-quality HfO2∕GaAs interface.

Surface reactions in atomic layer deposition of HfO 2, ZrO 2 and Al 2O 3 on hydroxylated and sulfur-passivated GaAs(1 0 0) surfaces: A comparative study by density functional theory

The surface reactions in atomic layer deposition (ALD) of HfO 2, ZrO 2 and Al 2O 3 on hydroxylated and sulfur-passivated GaAs surfaces are compared by using density functional theory. The HfCl 4 and ZrCl 4 half-reactions show large similarities in energetics and geometrical structure. However, both of them show large discrepancies with the Al(CH 3) 3 (TMA) half-reaction. Calculations find that it is more energetically favorable for the Al 2O 3 deposition than the HfO 2 and ZrO 2 deposition at the initial ALD stage. In addition, calculations find that although the GaAs passivation with sulfur helps to improve the interfacial properties, it is both kinetically and thermodynamically less favorable.

GaAs metal-oxide-semiconductor capacitors using atomic layer deposition of HfO2 gate dielectric: Fabrication and characterization

In this letter, we have investigated the physical and electrical characteristics of atomic layer deposition of HfO2 on GaAs substrates. X-ray photoelectron spectroscopy (XPS) analysis revealed no significant reduction of arsenic oxides upon deposition of HfO2 on GaAs using tetrakis(dimethyl-amino)hafnium [Hf(NMe2)4] as the metallic precursor. However, XPS confirmed the absence of arsenic oxides at the interface of HfO2 and sulfide-treated GaAs. High-resolution transmission electron microcopy analysis verified a smooth interface between HfO2 and sulfur-passivated GaAs. In addition, frequency dispersion behavior of capacitors on p-type GaAs substrates was remarkably improved by employing an appropriate surface chemical treatment.

Impact of surface chemical treatment on capacitance-voltage characteristics of GaAs metal-oxide-semiconductor capacitors with Al2O3 gate dielectric

The authors examine the impact of two different chemical surface treatment methods on capacitance-voltage characteristics of GaAs metal-oxide-semiconductor (MOS) capacitors using NH4OH and (NH4)2S prior to atomic layer deposition (ALD) of Al2O3. In both cases, x-ray photoelectron spectroscopy data confirm the removal of As2O3∕As2O6 upon Al2O3 deposition. However, Ga–O bonds appear to incorporate in the final gate stack at the Al2O3∕GaAs interface. MOS capacitors exhibit a steep transition from accumulation to depletion as well as very low leakage current density indicating high quality of ALD-Al2O3. The midgap interface trap density was evaluated to be (∼3–5)×1011∕cm2eV using the Terman method. In addition, quasistatic capacitance-voltage (C-V) measurement confirms the formation of true inversion layer in GaAs using both chemical treatment protocols. However, sulfur-passivated GaAs demonstrates better frequency dispersion behavior and slightly smaller capacitance equivalent thickness than hydroxylated GaAs. A statistical study substantiates the reproducibility of these results.

Electrical and Interfacial Characterization of Atomic Layer Deposited High- $\kappa$ Gate Dielectrics on GaAs for Advanced CMOS Devices

In this paper, electrical and interfacial properties of MOS capacitors with atomic layer deposited (ALD) Al2O3, HfO2, and HfAlO gate dielectrics on sulfur-passivated (S-passivated) GaAs substrates were investigated. HfAlO on p-type GaAs has shown superior electrical properties over Al2O3 or HfO2 on GaAs, and it is attributed to the reduction of the Ga-O formation at the interfacial layer. HfAlO on p-type GaAs exhibits the best electrical properties after postdeposition annealing (PDA) at 500degC. It is found that PDA, at above 500degC, causes a significant amount of Ga and As out-diffusion into the high-k dielectric, which degrades the interface, as well as bulk high-k properties.

Formation and properties of MnAs magnetic nanoscaled dots on sulfur-passivated semiconductor substrates

We have fabricated MnAs ferromagnetic nanoscaled dots on sulfur-passivated semiconductor substrates such as GaAs(0 0 1), GaAs(1 1 1)B, and InP(0 0 1) by low temperature molecular beam epitaxy (MBE). Atomic force microscopy (AFM) results reveal that the shape of MnAs dots depends on the surface energy of sulfur-passivated substrates. Extended X-ray absorption fine structure (EXAFS) and superconducting quantum interference device (SQUID) results suggest that MnAs dots on sulfur-passivated GaAs(0 0 1) are zb-type, while MnAs dots on sulfur-passivated GaAs(1 1 1)B and InP(0 0 1) are NiAs-type. We have found that the balance between substrate surface energy and lattice mismatch is essential to successfully fabricate zb-MnAs.

Enhancement of electron correlation in Co thin clusters grown onS∕GaAs(001)

An enhancement of the electron correlation effect by electron confinement is revealed in Co clusters formed on a sulfur passivated GaAs(001) surface. A satellite peak due to the strong electron correlation is observed at a binding energy of $5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ in the valence band spectrum of Co photoemission spectroscopy. The photon energy dependences of the intensity and the spin polarization of the satellite are essentially the same to those of the well-known Ni $6\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ satellite. This suggests that the satellite is caused by the relatively strong $3d\text{\ensuremath{-}}3d$ electron correlation effect of the Co cluster and less hybridization effect at the interface.

Interface formation of Mg with DiMePTCDI studied by Raman spectroscopy

Metal/organic interfaces play an important role in the performance of organic based devices. In this work the interface formation between Mg and N,N′-DiMethyl-3,4,9,10-Perylene Tetra Carboxylic DiImide (DiMePTCDI) films grown on sulphur-passivated GaAs (100) substrates is investigated in situ by Raman spectroscopy. The Raman spectra are taken in a backscattering geometry at room temperature under resonance conditions with the 488 nm (2.54 eV) Ar+ laser line. They reveal that when Mg is deposited onto a 15 nm DiMePTCDI layer the external phonon modes are preserved up to large metal coverages. Since these modes are a fingerprint of the molecular crystal, their preservation indicates a low diffusion of Mg into the DiMePTCDI layer. Concerning the internal molecular modes, the Mg deposition induces a breakdown of selection rules which is proposed to originate from a dynamical charge transfer between the DiMePTCDI molecules and the metal. Above 0.3 nm nominal thickness of Mg the line-shape of the molecular breathing mode at 221 cm–1 develops an asymmetric tail towards higher frequencies. A similar effect is observed for the bands that occur at 1291 cm–1 and 1606 cm–1 but the asymmetry appears at the low frequency side. This line-shape asymmetry is likely to be related to a Fano resonant coupling between the molecular vibrational modes and the electronic continuum of states of metallic clusters formed above 0.3 nm Mg coverage. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fluorescence EXAFS analysis of nanoscale zinc-blende MnAs dots grown on GaAs(001) by molecular beam epitaxy

In this work, geometric structures for ferromagnetic nanoscale zinc-blende MnAs dots grown on a sulfur-passivated GaAs(001) were investigated by using fluorescence EXAFS measurement. EXAFS measurements revealed that MnAs dots grown on GaAs(001) substrate did not form NiAs-type structure but zinc-blende structure. The MnAs bond length in the MnAs dots is about 2.48 Å which is close to that which was estimated from lattice constant of zinc-blende MnAs obtained by density-functional calculations. Existence of zinc-blende MnS (β-MnS) phase was also observed by EXAFS measurements.

Density-dependent electronic structure of zinc-blende-type MnAs dots on GaAs(001) studied byin situphotoemission spectroscopy

We report on the density dependence of the morphology and electronic structure of zinc-blende-type MnAs dots using in situ photoemission spectroscopy combined with molecular-beam epitaxy. Transmission electron microscopy images showed the zinc-blende-type crystalline structure of 10-nm diameter for each dot on sulfur-passivated GaAs(001) surface. The valence-band photoemission spectra of the MnAs dots were similar to those of the diluted magnetic semiconductor ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{As}$. With increasing dot density, the characteristic spectra of the zinc-blende-type MnAs persist but a weak Fermi edge appears, suggesting a metallic behavior as a result of percolation between the dots or the appearance of hexagonal MnAs as a minority phase.

Energy band dispersion in well ordered N,N′-dimethyl-3,4,9,10-perylenetetracarboxylic diimide films

The electronic properties of well ordered N,N′-dimethyl-3,4,9,10-perylenetetracarboxylic dimide (DiMe-PTCDI) films prepared on sulfur passivated GaAs(001) substrates were studied by means of photoemission spectroscopy. From the photon energy dependence of normal emission spectra an intermolecular energy band dispersion of about 0.2eV was determined for the highest occupied molecular orbital (HOMO). Simulation of the density of states reveals that the HOMO band has a single π -character. The observed energy band dispersion thus originates from the intermolecular π-π interaction and is modeled using the tight binding model. The analysis provides a value of 0.04eV for the transfer integral. The inner potential was treated as a fitting parameter such that the expected periodicity of the dispersion in the reciprocal space was obtained.

Chemical and electrical characterization of Gd2O3∕GaAs interface improved by sulfur passivation

We report the formation of a gate dielectric film on the sulfur passivated GaAs(001). The Gd2O3 films were deposited on the n-GaAs substrates using the electron-beam evaporation. The sulfur passivated GaAs metal-oxide-semiconductor diode exhibited the improved electrical properties compared to a similar GaAs diode that was cleaned with HCl including an enhanced conduction-band barrier and a reduced-leakage current. We investigated the relation between the interfacial bonds and the electrical properties by a systematic characterization using the photoemission spectroscopy. Sulfidation was shown to preserve a stable Gd2O3∕GaAs interface and to prevent an interfacial GaAs oxide formation. The stable interface and the passivated surface were caused by the bonding transition from As–S to Ga–S. The As-oxide was dissociated to form the interfacial Ga2O3 and to volatilize the elemental As in the Gd2O3 deposition, conducted without sulfidation. The interfacial oxides and excess As degraded the electrical properties and resulted in the decrease of the conduction-band offset. It was found that the Gd2O3 film was composed of two oxidation states: the amorphous Gd–O and the crystalline Gd2O3. The bonding transition with a substrate heating implies that the Gd2O3 film on the GaAs surface completely crystallizes above 400°C. Electrical properties were understood in relation to the band alignment and were estimated from the valence band and the core-level spectra analysis. The sulfidation introduced a shift of the Fermi level toward the conduction-band minimum on the GaAs surface and increased the conduction-band offset at the Gd2O3∕GaAs interface, followed by a saturated accumulation capacitance with a low-leakage current.

Density dependence of zinc-blende MnAs dots studied by X-ray absorption spectroscopy and X-ray magnetic circular dichroism

Abstract Zinc-blende type nanoscale MnAs dots were grown on a sulfur-passivated GaAs (0 0 1) surface by molecular beam epitaxy. We report on the dot density dependence of the X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) in the Mn 2p core-level region. The XAS spectrum of low-density MnAs dots showed “localized” behavior as in the case of (Ga,Mn)As, while the spectrum of high-density MnAs dots changed to metallic behavior. The magnetic-field dependence of the XMCD revealed ferromagnetic behavior in the case of high-density MnAs dots.

Sulphur-Passivated GaAs Investigation Using High Resolution X-Ray Diffractometry

The growth of a MoS2 layer on a GaAs(111) substrate produces a highly strained interface layer, which relaxes via generation of dislocations. Using high-resolution X-ray diffractometry in the triple-axis scheme we investigated this interfacial layer, which results from dipping an epi-ready GaAs wafer in ammonium sulphide, followed by deposition of an RF-sputtered MoS2 layer. The dislocation density revealed from the measurements is of the 106 cm-2. This suggests that high efficiency (~20%) MoS2/GaAs heterojunction photovoltaic devices are feasible.

Fabrication, magnetic properties, and electronic structures of nanoscale zinc-blende MnAs dots (invited)

Ferromagnetic nanoscale zinc-blende MnAs dots were successfully fabricated on a sulfur-passivated GaAs (001) surface by molecular-beam epitaxy. Transmission electron microscopy and selected area electron diffraction showed that the crystalline structure was not the same as that of bulk MnAs with NiAs-type hexagonal crystalline structure, but of zinc-blende type. In in situ photoemission spectroscopy of the zinc-blende MnAs dots, the Fermi edge was not clearly observed and the Mn 3d partial density of states was similar to that of the diluted ferromagnetic semiconductor Ga1−xMnxAs, which also supports the fabrication of zinc-blende MnAs in the nanoscale.