Reduction Of Interface State Density Research Articles

Reduction of interface-state density in 4H–SiC n-type metal–oxide–semiconductor structures using high-temperature hydrogen annealing

The effects of hydrogen annealing on capacitance–voltage (C–V) characteristics and interface-state density (Dit) of 4H–SiC metal–oxide–semiconductor (MOS) structures have been investigated. The Dit was reduced to as low as 1×1011 eV−1 cm−2 at Ec−E=0.6 eV using hydrogen annealing above 800 °C, where Ec−E is the energy level from the conduction-band edge. Secondary ion mass spectroscopy and Dit analysis revealed that Dit decreased with the increase of hydrogen concentration accumulated at the SiO2/4H–SiC interface. The interface states at SiO2/4H–SiC are thought to be originated from the dangling bonds of C atoms as well as Si atoms, because Dit decreases as the hydrogen annealing temperature increases and saturates around 800 °C. This high-temperature hydrogen annealing is useful for accumulation-type SiC metal–oxide–semiconductor field-effect transistors, which have n-type MOS structures to reduce the Dit.

Effect of GaAs surface pretreatment on electrical properties of MBE-ZnSe/GaAs substrate interfaces

Interface properties of MBE-grown ZnSe/GaAs substrate systems formed on variously pretreated GaAs surfaces, which include standard chemically etched (5H2SO4:1H2O2: 1H2O), (NH4)2Sx-, NH4I-, and HF-pretreated surfaces, are investigated by capacitance-voltage (C-V) and deep level transient spectroscopy (DLTS) measurements. A HF-pretreated and annealed ZnSe/p-GaAs sample showed marked reduction of interface state density, Nss, with Nss,min below 4 x 1011cm-2 eV-1 near Ec- EFS= 1.0 eV. The value is about one order of magnitude smaller than that of the standard chemically etched interface, and comparable to (NH4)2Sx- pretreated interface. Nevertheless, C-V characteristics of ZnSe/nGaAs samples, which were measured for the first time, indicate that interface Fermi level, EFS, is not completely unpinned due to the interface states located above the midgap. A consistent result was obtained by DLTS method in determining EFS position. The influence of Nss distribution on vertical current conduction is also analyzed. It is found that U-shaped interface states with Nss(E) > 1 x 1013 cm-2 eV-1 above the midgap may cause an excess voltage drop larger than a few volts at the interface.

Reduced interface state density after photocurrent oscillations and electrochemical hydrogenation of n-Si(111): A surface photovoltage investigation

The electrochemical H-termination process of n-Si(111) surfaces in aqueous 0.1 M NH4F pH 4.0 solution was optimized for a two step procedure consisting of the surface smoothing during oxidation in the photocurrent oscillating potential region followed by the oxide etching and passivation of the surface atoms with hydrogen. The hydrogen termination was monitored in situ measuring the dark current transient and evaluated using pulsed surface photovoltage technique. An unusually low density of interface states it=1×1010 eV−1 cm−2 was obtained by the hydrogen termination on n-Si(111) surfaces following this procedure with better results than applying an electropolishing treatment.

Novel Surface Passivation Scheme for Compound Semiconductor Using Silicon Interface Control Layer and Its Application to Near-Surface Quantum Wells

A novel passivation scheme using an ultrathin molecular beam epitaxy (MBE) silicon interface control layer (ICL) and an ultrathin silicon nitride layer is investigated for use in the passivation of compound semiconductor quantum structures. Process optimization was performed by in-situ X-ray photoelectron spectroscopy (XPS) and electrical characterization of the insulator-semiconductor interface formed on In0.53Ga0.47As. A drastic reduction of interface state density into the 1010 cm-2·eV-1 range was obtained under optimum condition. The passivation scheme was then applied to passivation of Al0.3Ga0.7As/GaAs/Al0.3Ga0.7As near-surface quantum wells. Remarkable recovery of photoluminescence (PL) intensity was observed upon application of the present passivation scheme to the barrier surface, which was consistent with the reduction of surface states.

Study of surface stoichiometry and luminescence efficiency of near-surface quantum wells treated by hydrogen ions and atomic hydrogen

A near-surface quantum well (QW) structure has been used as an effective probe of surface states before and after different hydrogen treatments. By correlating the surface composition with the luminescence efficiency of the near-surface QW, we find that the surface passivation is dominated by the defect density of the interface between the AlGaAs surface barrier and the overlying oxide. A complete recovery or further enhancement of luminescence can be readily achieved by treatments using hydrogen ions. However, atomic hydrogen at low exposures is not capable of modifying that interface, and is not effective in passivation. These results corroborate a passivation mechanism which involves removal of As from the interface between the AlGaAs surface barrier and the overlying oxide, and the reduction of interface state density.

Improvements on Electrical Properties of Ultra-Thin Silicon Oxides Grown by Microwave Afterglow Oxygen Plasma

ABSTRACTFundamental characteristics such as the oxide breakdown fields, oxide charges and interface state density of various ultra-thin silicon oxides (≤ 8 nm) grown by microwave plasma afterglow oxidation at low temperatures (400 °C and 600 °C) were investigated. The effective Oxide charge density of 600 °C as-grown oxide was as low as 6×1010 cm-2. The breakdown fields of the oxides were further enhanced and the interface state densities were reduced by employing fluorination (HF soaked) and low temperature N2O plasma annealing. The breakdown field of the thin oxide grown at 600 °C with 15 min N2O plasma annealing was 12 MV/cm. The reduction of interface state density was about 35% for 600 °C fluorinated oxide. When integrated with poly-gate process, the interface state density was as low as 5×1010 cm-2eV-1.

Interface profile optimization in novel surface passivation scheme for InGaAs nanostructures using Si interface control layer

This paper attempts to control and optimize the interface atomic profiles of a novel surface passivation scheme for InGaAs nanostructures, using a silicon interface control layer (ICL). An in-situ x-ray photoelectron spectroscopy characterization technique was used to establish a process sequence that satisfies the conditions of maintenance of pseudomorphic matching to InGaAs, prevention of direct oxidation of InGaAs, and formation of a good SiO2/Si interface with minimal suboxide components. It is shown that the above conditions can be satisfied by a new process that is a formation of the thermal SiO2 at the SiO2-Si interface by repetition of deposition/oxidation/annealing cycle. A large reduction of interface state density (Nss) was realized by the optimization of the new process, resulting in a minimum Nss of 4 × 1011 cm−2 eV−1. The silicon ICL technique was successfully applied to the passivation of InGaAs wire structures.

Improved surface properties of InP through chemical treatments

Chemical treatment is a very effective method for passivation of semiconductor surfaces. HF and sulfide (Na2S⋅9H2O) pretreatments of InP have been shown to improve the properties of BaF2/InP interface significantly. The interface state density as obtained from C–V (1 MHz) measurements of metal-insulator–semiconductor structures was found to be reduced from 5.8×1010 cm−2 eV−1 to 2.1×1010 cm−2 eV−1 after HF treatment. The reduced interface state density resulted in increased photoluminescence intensity. X-ray photoelectron spectroscopy studies revealed that the formation of InF3 and P2S3 after HF and sulfide treatments, respectively, are responsible for better interfacial behavior.

Enhancement of SiO2/GaAs interface properties by electron cyclotron resonance plasma-enhanced chemical vapor deposition and Ga outdiffusion control

High quality electron cyclotron resonance plasma-enhanced chemical vapor deposition SiO2 film and annealing are investigated in an effort to enhance SiO2/GaAs interface properties. Dramatic reduction of interface state density is achieved by reducing nitrogen and hydrogen impurities in the SiO2 film and optimizing the annealing temperature. A minimum interface state density of 3×1010 eV−1 cm−2 is obtained in SiO2/GaAs annealed at 690 °C for 30 min. Secondary-ion mass spectroscopy data suggest that the reduction in interface state density may be related to the amount of Ga outdiffusion into the SiO2 film.

ZnSe/GaAs heterovalent interfaces: interface microstructure versus electrical properties

Epitaxial ZnSe/epitaxial GaAs interfaces are formed by molecular beam epitaxy and evaluated by several techniques including capacitance-voltage ( C-V) measurements. The GaAs surface stoichiometry is systematically varied prior to the nucleation of ZnSe. A dramatic reduction of interface state density occurred when the GaAs epilayer is made As deficient. The ZnSe/GaAs interfaces exhibiting low interface state densities are associated with the presence of an interfacial layer of zincblende Ga 2Se 3. In situ X-ray photoelectron spectroscopy (XPS) is used to study the nature of the bonding at the interfacial layer. The character of Se 3d core level features from the interfacial region and from separately grown Ga 2Se 3 epilayers support the identification of the interfacial layer as Ga 2Se 3.

Plasma-deposited germanium nitride gate insulators for indium phosphide metal-insulator-semiconductor field-effect transistors

Germanium nitride (Ge3N4) films were deposited on indium phosphide (InP) compound semiconductor substrates using plasma-enhanced chemical vapor deposition. The depositions were performed in a capacitively coupled parallel-plate reactor using two different processes. In the first process, germane (GeH4), nitrogen (N2), and ammonia (NH3) reactant gases were used. In the second process, germane and pure nitrogen were used as reactant gases. The films were deposited using 13.56 MHz rf excitation to a typical thickness of 800 Å with a refractive index of 2.11–2.16. The breakdown field strength of the films was greater than 106 V/cm. Auger electron spectroscopy did not indicate significant chemical composition differences between the two deposition processes. In order to study the feasibility of plasma-deposited germanium nitride as a gate insulator, metal-insulator-semiconductor field-effect transistors (MISFETs) with 2-μm channel lengths were fabricated on InP. The device transconductance and threshold voltage for the GeH4/N2/NH3 process were 17 mS/mm and −3.6 V, respectively. The drain-source breakdown voltages were greater than 10 V. The GeH4/N2/NH3 deposition process was preferred over the GeH4/N2 process because of enhanced thickness uniformity, increased deposition rate, increased resistivity, reduced interface state density, and comparable MISFET electrical characteristics.

Temperature Dependent Schottky Contacts to InP and GaAs

ABSTRACTThe barrier height of a Pd/n-InP diode was found to be increased from 0.48 to 0.96eV with the substrate temperature decreased from 300 to 77K during metal deposition. The leakage current density was reduced by more than six order of magnitude. It is obvious that the interface Fermi-level position lies well outside the variance associated with Fermi-level pinning. The barrier height for the Au/n-GaAs diode was found to be increased by about 0.25 eV with low temperature deposition and the leakage current reduced by more than five orders of magnitude. The mechanism responsible for the ultrahigh barrier height obtained at low substrate temperature was investigated by Raman spectroscopy, current voltage temperature measurement, deep level transient spectroscopy, and electroreflectance technique. The metal-insulator-semiconductor (MIS)-like structure formed at low substrate temperature and the reduction of interface state density may be the main reason for the dramatic enhancement of Schottky barrier height.

Effect of GaAs surface reconstruction on interface state density of epitaxial ZnSe/epitaxial GaAs heterostructures

Epitaxial ZnSe/epitaxial GaAs interfaces have been grown using molecular beam epitaxy and evaluated by capacitance–voltage measurements and transmission electron microscopy. The GaAs surface stoichiometry was systematically varied prior to the nucleation of ZnSe. A significant reduction of interface state density occurred when the GaAs epilayer surface was As deficient. The resulting interface state densities of as-grown structures are comparable to values obtained with (Al,Ga)As/GaAs interfaces.

Influence of GaAs surface stoichiometry on the interface state density of as-grown epitaxial ZnSe/epitaxial GaAs heterostructures

Epitaxial ZnSe/epitaxial GaAs interfaces have been formed by molecular beam expitaxy and evaluated by several techniques including capacitance-voltage measurements. In the study reported here, the GaAs surface stoichiometry was systematically varied prior to the nucleation of ZnSe. A dramatic reduction of interface state density occurred when the GaAs epilayer was made As deficient. The resulting interface state densities of as-grown structures are comparable to values obtained with (Al,Ga)As/GaAs interfaces.

Spectroscopic and electrical studies of GaAs metal–oxide semiconductor structures

Recent studies of plasma-enhanced chemical vapor deposition of SiO2 upon Si epitaxial layers grown in situ upon epitaxial GaAs layers, with the deposited interfacial Si of order 10 Å, strongly suggest that greatly reduced state densities at SiO2/GaAs interfaces have been attained with this approach. We report spectroscopic ellipsometry results for molecular-beam epitaxy (MBE) Si/GaAs samples both before and after SiO2 deposition, as well as before and after annealing. The ellipsometric results indicate that the SiO2/GaAs interface is <9 Å thick. The amount of unreacted silicon ranged from 3±3 to 0.4±0.2 Å equivalent thickness for the best fit cases. Ellipsometric results for samples without the deposited SiO2 suggest of order 0.4-Å unoxidized silicon at the interface. Results of x-ray photoelectron spectroscopy (XPS) studies of the samples (without deposited SiO2) suggest that all silicon is being oxidized prior to SiO2 deposition for at least some of the ‘‘unpinned’’ samples. Raman studies find no Si–Si bonds for some of the unpinned samples. Our data strongly suggest that the reduced interface state density is not dependent upon the presence of unoxidized silicon, at least in the amount required to form the Si/GaAs heterojunction previously assumed in explaining this improved interface. We further report preliminary electrical data for a sample prepared with no silicon deposition; indentical heat treatments on this sample gave similar results, i.e., reduced interface states for n-GaAs metal–oxide semiconductor capacitors.

Effect of GaAs Surface Stoichiometry on The Interface of As-Grown Epitaxial ZnSe/Epitaxial GaAs Heterostructures

ABSTRACTIn the study reported here, the GaAs surface stoichiometry was systematically varied prior to the nucleation of ZnSe to form epitaxial ZnSe/epitaxial GaAs interfaces. The structures were grown by molecular beam epitaxy and evaluated by several techniques including capacitance-voltage (C-V) measurements. A dramatic reduction of interface state density occurred when the GaAs epilayer was made As deficient. The resulting interface state densities of as-grown structures are comparable to values obtained with (Al,Ga)As/GaAs interfaces.

SiO2/InP interfaces with reduced interface state density

By employing a new InP surface preparation procedure based upon KOH/methanol, the interface state density of plasma-enhanced chemically vapor deposited SiO2/InP structures has been significantly reduced. Capacitance–voltage characteristics of these structures exhibit unusual nonequilibrium behavior which appears to be assoicated with the formation of a p-type inversion layer and with the presence of interface traps with very slow response times. This surface preparation procedure yields an InP native oxide which has no detectable In2O3 and is contaminated with K.

The correlation of channel mobility with interface state measurements on InP MOSFET structures

The channel mobilities of InP MOSFETS fabricated using three different types of insulator (viz. Al 2O 3, as deposited SiO 2 and annealed SiO 2.) have been measured. The interface state densities for each type of insulator-InP interface have been determined by the conductance technique. The results have been compared and a strong correspondence has been found between low interface state density and high channel mobility. A drastic reduction in interface state density was produced by annealing the SiO 2 insulator at high temperature and this process also produced transistors with the highest mobility.

Passivation of gallium arsenide by reactively sputtered gallium nitride thin films

An attempt has been made to correlate the interface state characteristics of the GaN/GaAs MIS structure with the composition at the interface as obtained from Auger spectroscopy. The considerably reduced interface state density of nitrogen annealed samples is due to the reduction of the oxygen content and the corresponding increase of the nitrogen content at the GaAs/GaN interface. The interface state density for this MIS structure shows a two peaked distribution and is characterised by a much lower interface state density in the upper half of the band gap in comparison with native oxides and other deposited insulators.

InP–SiO2 m.i.s. structure with reduced interface state density near conduction band

HCl addition to the SiO 2 c.v.d. process was found to be an efficient method for producing n--InP m.i.s. structures with low density of interface states near conduction band. Remaining deep interface donor states approximately 0.5 eV below conduction band restrict the steady-state bias range in h.f. applications.