Sulfur Passivation Research Articles

Fermi level depinning at the germanium Schottky interface through sulfur passivation

We demonstrate the depinning of Fermi level on both p- and n-type germanium after sulfur passivation by aqueous (NH4)2S treatment. Schottky contacts realized using metals with a wide range of work functions produce nearly ideal behavior confirming that the Fermi level is depinned. Examination of the passivated surface using x-ray photoelectron spectroscopy reveals bonding between Ge and sulfur. It is shown that good Ohmic contacts to n-type Ge and a hole barrier height (ϕBp) of 0.6 eV to p-type Ge can be achieved after this passivation treatment, with Zr Schottky contacts. This is the highest ϕBp reported so far.

III-V-semiconductor-on-insulator n-channel metal-insulator-semiconductor field-effect transistors with buried Al2O3 layers and sulfur passivation: Reduction in carrier scattering at the bottom interface

We have developed III-V-semiconductor-on-insulator (III-V-OI) structures on Si wafers with excellent bottom interfaces between In0.53Ga0.47As-OI channel layers and atomic-layer-deposited Al2O3 (ALD-Al2O3) buried oxides (BOXs). A surface activated bonding process and the sulfur passivation pretreatment have realized the excellent In0.53Ga0.47As-OI/ALD-Al2O3 BOX bottom interface properties. As a result, the III-V-OI n-channel metal-insulator-semiconductor field-effect transistors under the back-gate configuration showed the peak mobility of 1800 cm2/V s and the higher electron mobility than the Si universal one even in the high effective electric field range because of the reduction in the surface roughness and fixed charges.

Wet sulfur passivation of GaSb(1 0 0) surface for optoelectronic applications

A comparative analysis of the properties of the non-passivated and S-passivated GaSb(1 0 0) surfaces has been performed through PL, AFM and RHEED characterization. The samples treated with a 1 M Na 2S aqueous solution demonstrate an increase in the 5 K PL intensity. According to AFM data, the annealing of the S-passivated GaSb(1 0 0) leads to the formation of the clean flat (1 0 0) surface. Moreover, after annealing the PL intensity of the S-passivated GaSb(1 0 0) surfaces decreases by 20%, whereas for the non-passivated samples it drops by more than a factor of 4. The method of wet sulfur passivation has shown great effectiveness in pre-epitaxial processing for LPE and MBE growth of the GaSb-related materials for optoelectronics.

Sulfur passivation of InN surface electron accumulation

The effects of treatment with ammonium sulfide ((NH4)2Sx) solution on the electronic properties of InN surfaces have been investigated with high resolution x-ray photoemission spectroscopy. The valence band, In 3d, and N 1s x-ray photoemission spectra show that the surface Fermi level decreases by approximately 0.15 eV with (NH4)2Sx-treatment. This corresponds to a reduction of the downward band bending with the surface sheet charge density decreasing by 30%.

Investigations of the Surface Chemical Composition and Atomic Structure of ex-situ Sulfur Passivated Ge(100)

Using complementary surface analysis techniques, we have studied the chemical and structural properties of the S/Ge(100) passivation layer formed upon sulfidation in an aqueous (NH4)2S solution. Our experiments have revealed that only S-Ge bonds are formed upon the adsorption of S and that the surface is essentially free of other S-containing species (i.e. Sn or SOx). The passivation treatment was found to reduce, but not to fully inhibit Ge-O bonds on the surface. Moreover, we have shown that the top-layer of the passivated surface shows a high degree of disorder even though it is associated with a spotty (1x1) RHEED pattern. Based on our experimental findings, we conclude that the passivation layer can be best described as a thin amorphous GeSxOy film atop a partially ordered interfacial layer, a surface model that has been proposed previously in literature.

Comparative study of InGaP/GaAs heterojunction bipolar transistors (HBTs) with different base surface treatments

The interesting InGaP/GaAs heterojunction bipolar transistors (HBTs) with different surface passivations on the base surface are fabricated and studied. Experimentally, the HBT device with sulfur treatment passivation displays the lowest offset voltage. However, the device with a 0.02 μm-thick emitter ledge structure reveals better transistor behaviors such as higher current gain and lower base surface recombination current. In addition, it also exhibits improved thermal stability. For the reliability test, the device with a 0.02 μm-thick emitter ledge structure shows the best performance. Therefore, from experimental results, the HBT device performance could be improved by appropriate base surface treatments, e.g., sulfur passivation and emitter ledge structure.

Charge trapping and reliability characteristics of sputtered Y2O3 high-k dielectrics on N- and S-passivated germanium

We demonstrate the potential of sulfur passivation to improve the interface characteristics between germanium (Ge) and Y2O3 high-k gate dielectric. Effects of nitrogen (N) and sulfur (S) passivation of the Ge surface on the charge trapping and reliability properties of Y2O3/Ge gate stacks are studied in detail and the results are compared. Sulfur passivation of the Ge surface has been performed using both the wet sulfidation technique with aqueous ammonium sulfide and plasma sulfidation with H2S gas. N-passivation of Ge substrates has been performed in NO plasma for comparison. Ultrathin (∼15 nm) Y2O3 films are deposited on both the N- and S-passivated p-Ge (1 0 0) wafers. Y2O3 films on the S-passivated Ge surface show low fixed oxide charge and interface state density than what is achieved with N-passivation. The electrical characterization results of MOS capacitors with Y2O3 films reveal the potential of S-passivation for the fabrication of Y2O3/Ge gate stacks for Ge MOSFETs.

Advantages and disadvantages of sulfur passivation of InAs/GaSb superlattice waveguide photodiodes

In this work, the influence of ammonium sulfide (NH4)2S passivation on waveguide based mid-infrared InAs/GaSb superlattice photodetectors (2–5 µm wavelength) has been studied. The current–voltage characteristics for reverse as well as for forward bias of passivated samples have been examined. The advantages of this have been the reduction of the reverse leakage current and the increase of zero bias resistance. As a disadvantage the decrease of the photoresponsivity after sulfur passivation has been found. Furthermore, it has been observed that the passivation solution does not only passivate the surface of GaSb, but it also reacts with entire GaSb layers and can destroy the devices.

Improved carrier-transport properties of passivated CdMnTe crystals

By analyzing photoconductive decay curves, we compared the surface recombination velocities of semi-insulating CdMnTe:In crystals grown by the vertical Bridgman method with or without surface passivation. Sulfur passivation effectively prevents the formation of a conductive Te oxide layer on the CdMnTe surface and reduces the surface recombination velocities by about one third. We demonstrated, from IR observations of the distribution maps of Te precipitates, that their configuration affects the anomalous photoconductive decay curves and the gamma-ray spectrum in some areas of the CdMnTe crystal. Notably, not only the size but also the spatial configuration of the Te precipitates modulates the carrier-transport properties.

Comparative Studies on Temperature-Dependent Characteristics of Passivated In0.2Ga0.8AsSb ∕ GaAs High-Electron-Mobility Transistors

This work investigates improved performance of dilute-antimony-channel high-electron mobility transistors (HEMTs), grown by a molecular beam epitaxy system, with or silicon nitride passivations. With the advantageous dilute-antimony-channel design to effectively improve the carrier transport property and carrier confinement capability and the sulfur passivation technique to annihilate the surface states, the proposed -passivated HEMT has demonstrated superior amplification, breakdown, and power performances, with high-temperature threshold stability. It is suitable for high-gain and high-power millimeter-wave integrated circuit applications.

Fabrication of Self-aligned Enhancement-mode n-channel GaAs MOSFETs Employing a Wet Clean Process for GaAs Substrates

We report fabrication of self-aligned enhancement-mode GaAs MOSFETs employing a simple ex situ chemical clean. The role of precursor chemistry in removing GaAs native oxides as well as the impact of different ex situ chemical cleans on the properties of the interface between various atomic-layer-deposited high-k gate dielectrics and GaAs substrates were examined using x-ray photoelectron spectroscopy and Raman spectroscopy. The material characterization results indicate the effectiveness of ex situ sulfur passivation of GaAs surface prior to high-k deposition using ammonium sulfide solutions in improving the interface properties between high-k layers and GaAs substrates. Moreover, an appropriate choice of precursor chemistry for reduction of GaAs native oxides appears to play a crucial role in mitigating the Fermi level pinning at the interface. A maximum drive current of 4.5 uA/um at a gate overdrive of 2.5V was obtained for a MOSFET with a gate length of 20um.

A high Schottky barrier between Ni and S-passivated n-type Si(1 0 0) surface

By minimizing surface states with sulfur passivation, a record-high Schottky barrier is achieved with nickel on n-type Si(100) surface. Capacitance–voltage measurements yield a flat-band barrier height of 0.97eV. Activation-energy and current–voltage measurements indicate ∼0.2-eV lower barriers for the Ni/Si(100) junction. These results accompany a previously-reported record-high Schottky barrier of 1.1eV between aluminum and S-passivated p-type Si(100) surface. The operation of these metal/Si(100) junctions changes from majority-carrier conduction, i.e., a Schottky junction, to minority-carrier conduction, i.e., a p–n junction, with the increase in barrier height from 0.97eV to 1.1eV. Temperature-dependent current–voltage measurements reveal that the Ni/S-passivated n-type Si(100) junction is stable up to 110°C.

Gate-first inversion-type InP metal-oxide-semiconductor field-effect transistors with atomic-layer-deposited Al2O3 gate dielectric

We present n-channel enhancement-mode inversion-type indium phosphide (InP) metal-oxide-semiconductor field-effect transistors with atomic-layer-deposited Al2O3 gate dielectric. It has been found that applying sulfur passivation and postdeposition annealing in the process improves the drive current and subthreshold swing. Transistors on semi-insulating InP substrates show much higher drive current than the ones on p-type InP due to the asymmetric distribution of interface state along the bandgap between InP and Al2O3. The effects of transient and slow traps on the transistor performance have also been investigated using constant electrical stress measurements and pulse measurements.

First-principles study of sulfur passivation of GaP(001) surfaces at one-monolayer coverage

Using first-principles total energy method, we have studied the structural and electronic properties of Ga- and P-terminated GaP(001)(1×2) surfaces adsorbed with one monolayer of sulfur. It was found that the sulfur atoms prefer to occupy bridge sites and the periodicity becomes (1×1) on both Ga- and P-terminated surfaces. The S–Ga bond was confirmed to be stronger than the S–P bond. The electronic analysis showed that the surface state within the energy gap on the Ga-terminated GaP surface was noticeably reduced by the sulfur adsorption, while such reduction does not occur on the P-terminated surface due to the S–P antibonding state. The nearly filled S dangling bonds on the Ga-terminated surface make this surface resistant to contamination.

Metamorphic In0.53Ga0.47As Metal–Oxide–Semiconductor Structure on a GaAs Substrate with ZrO2 High-k Dielectrics

The electrical properties of metamorphic In0.53Ga0.47As metal–oxide–semiconductor capacitors with a 100-Å-thick ZrO2 layer as high-k dielectrics were investigated. The In0.53Ga0.47As surface was pretreated by either sulfur passivation or HCl cleaning before the ZrO2 deposition. Owing to the lower interface-state density after sulfidation, the sulfur-passivated capacitor exhibited better accumulation capacitance and strong inversion at capacitance–voltage measurement than the HCl-cleaned capacitor after post deposition annealing at 350 °C. On the basis of material analyses, the capacitors that subjected to underwent sulfur treatment were found to acquire a thin sulfur layer on the interface, which protects their surface from air exposure and prevents performance degradation.

The defect density of a SiNx/In0.53Ga0.47As interface passivated using (NH4)2Sx

Passivation of the electronic defect states at a SiNx/InGaAs interface has been achieved using (NH4)2Sx treatments of the InGaAs surface. The X-ray photoelectron spectroscopy technique was used to investigate the mechanism of sulfur passivation. The results indicate that sulfur treatment can effectively erase the native oxides, and S-In, S-Ga and S-As bondings are formed after sulfidation. The fabrication of Au/SiNx/InGaAs metal–insulator–semiconductor diodes has been achieved by depositing a layer of SiNx on (NH4)2Sx-treated n-InGaAs using the plasma enhanced chemical vapor deposition technique. The effect of passivation on the InGaAs surface before and after annealing was evaluated by current–voltage and capacitance–voltage measurements. The results indicate that the SiNx passivation layer exhibits good insulative properties. The annealing contributes to the decrease of the fixed charge density and the minimum surface state density, which are 4.5×1011 cm-2 and 3.92×1011 cm-2 eV-1, respectively. A 256×1 InP/InGaAs/InP heterojunction short-wavelength infrared detector, fabricated with the sulfidation plus a SiNx passivation layer, has shown a good response uniformity of 4.81%.

Effect of sulfur passivation on the InP surface prior to plasma-enhanced chemical vapor deposition of SiNx

The fabrication of Au/SiNx/InP metal–insulator–semiconductor (MIS) diodes has been achieved by depositing a layer of SiNx on the (NH4)2Sx-treated n-InP. The SiNx layer was deposited at 200 °C using plasma-enhanced chemical vapor deposition (PECVD). The effect of passivation on the InP surface before and after annealing was evaluated by current–voltage (I–V) and capacitance–voltage (C–V) measurements, and Auger electron spectroscopy (AES) analysis was used to investigate the depth profiles of several atoms. The results indicate that the SiNx passivation layer exhibits good insulative characteristics. The annealing process causes distinct inter-diffusion in the SiNx/InP interface and contributes to the decrease of the fixed charge density and minimum interface state density, which are 1.96 × 1012 cm−2 and 7.41 × 1011 cm−2 eV−1, respectively. A 256 × 1 InP/InGaAs/InP heterojunction photodiode, fabricated with sulfidation and SiNx passivation layer, has good response uniformity.

Optimization of surface passivation for InGaAs/InP pin photodetectors using ammonium sulfide

In this work, the effect of the process parameters during chemical and electrochemical passivation of InGaAs/InP PIN photodetectors using (NH4)2Sx (x >1) has been studied in detail. It has been observed that the time of passivation, temperature of the sulfide solution and illumination during electrochemical treatment play significant roles in the efficacy of the passivation process. These parameters therefore have to be carefully optimized in order to reduce the dark current of the detectors to a minimum value. The yield of the process is also found to improve with sulfur passivation.

Effects of Sulfur Passivation on Germanium MOS Capacitors With HfON Gate Dielectric

In this letter, we study the effects of sulfur (S) passivation, using aqueous ammonium sulfide ((NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S), on germanium (Ge) MOS capacitors with sputtered HfON as gate dielectric and TaN as metal-gate electrode. Compared with control samples, the S passivation can effectively reduce both equivalent oxide thickness and interface-state density. X-ray-photoelectron-spectroscopy analysis shows that (NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S treatment can reduce the Ge-O bonds on Ge surface. The thermal stability of the S passivation under different postmetal-annealing temperatures was also examined, and it was found that samples with (NH <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> ) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S treatment exhibit stable Ge/high-fc interface upon 550-deg C postmetal-deposition annealing, whereas interface quality degrades for those samples without S passivation.

Interface Characaterization in Nanoelectronics

A central requirement in the integration of III-V substrates in highly scaled CMOS gate stacks is to develop an ability to assure that we create a high capacitance, high mobility transistor structure with a low concentration of defects. In this contribution, recent studies of the compositional profile and interdiffusion of high-K gate stacks on III-V substrates are reviewed. Sulfur passivation methods have been explored to help ensure that the starting III-V surface (InGaAs in this case) has a low defect concentration. Diffusion processes associated with the introduction of oxygen-gettering metal gate materials will also be discussed.