Passivation Of Interface States Research Articles

Improved passivation performance of Al2O3 interlayer/MoOX thin films continuously grown via atomic layer deposition

Atomic-layer-deposited MoOx was investigated as a hole-selective layer in Si heterojunction solar cells. MoOx grown on Si through atomic layer deposition (ALD) produce numerous sub-oxidation states at the MoOx/Si interface. The passivation of interface states remains an important problem for hole-selective contact Si solar cells. We improved the passivation of interface defects in the ALD-MoOx/Si structure by introducing ultrathin ALD-Al2O3 at 443 K for continuous processing with ALD-MoOx. Consequently, the carrier lifetime and implied open-circuit voltage increased from 22 μs to 215 μs and 568 mV to 637 mV, respectively. The introduction of ultrathin ALD-Al2O3 significantly reduced the number of oxidation states that acted as defects. The best passivation was observed up to 907 μs and 676 mV after post-deposition annealing at 623 K. The ALD-MoOX film prepared by introducing ultrathin Al2O3 is a promising hole-selective contact layer.

(Invited, Digital Presentation) Interface Charge Trapping and Scattering in SiC MOSFET Channels

The channel conductivity of SiC MOSFETs is severely limited by charge trapping at interface states and mobility limiting scattering processes. In this talk, the status of interface state passivation for both n- and p-channel 4H-SiC MOSFETs will be presented. First, results from capacitance-based methods to extract interface state density (Dit) across the entire 4H-SiC band gap for thermally grown nitrided silicon dioxide gate dielectrics will be presented, emphasizing the critical importance of the SiO2(N)/SiC interface composition and nitrogen bonding. Next, the scattering mechanisms for channel electrons and holes in these devices will be analyzed and contrasted by Hall measurements. [1] The transport will be characterized as a function of temperature and substrate-bias within an effective transverse electric field model. The final part of the talk will focus on latest development on deposited dielectrics for 4H-SiC. Deposited dielectrics are an attractive alternative to thermal oxidation as it minimizes the release of carbon and the perturbation of surface crystal structure and opens the possibility of ‘high-k’ dielectrics with higher permittivity than SiO2. Herein, the key importance of 4H-SiC surface control prior to dielectric deposition will highlighted using results obtained for atomic layer deposition (ALD) of Aluminum Oxide (Al2O3) [2]. To this end, a systematic variation of SiC surface termination with processes involving oxygen, hydrogen and nitrogen will be presented. The composition of the deposited films and the interfacial bonding will be characterized for various deposition processes and correleated with electrical measurements . It will be demonstrated that surface nitridation followed by H2 annealing prior to ALD results in a factor of 2 higher channel mobility than typical NO annealed thermal SiO2. This is due to the formation of stable sub-nm SiON layers that passivates the surface and enables the formation of high-quality interfaces between SiC and deposited dielectric. In addition, the results indicate that clean Si-H terminated surfaces formed by hydrogen etching and annealing, prior to dielectric deposition leads to uniform nucleation of Al2O3. The reduction of Dit is also accompanied by improvements in oxide charge trapping and bias instability, but dielectric leakage current is significantly higher for Al2O3 compared to SiO2 due to bulk trap-assisted tunneling in Al2O3. The reliability is a significant challenge that need to be overcome to exploit the enhanced mobilities and make deposited dielectrics competitive with traditional nitrided SiO2.Acknowledgments: The authors acknowledge support from the US Army Research Laboratory (grant ARMY-W911NF-18-2-0160) and US Department of Energy (subcontracted to National Renewable Energy Laboratory, grant NREL-AHL-9-92632-01). The authors also acknowledge the support Dr. Leonard C. Feldman (Auburn, Rutgers) and Mr. Hengfei Gu (Rutgers) for help with XPS measurements. The authors also deeply thank Ms. T Isaacs-Smith, Ms. Lu Wang (Auburn), Dr. K. Ramadoss and Dr. D. Morisette (Purdue Univ.) for useful discussions and help with fabrication.[1] S. Das et al., J. Appl. Phys. 130, 225701, (2021).[2] I. U. Jayawardhena et al., J. Appl. Phys. 129, 75702, (2021) Figure 1

Lifetime degradation of n-type Czochralski silicon after hydrogenation

Hydrogen plays an important role in the passivation of interface states in silicon-based metal-oxide semiconductor technologies and passivation of surface and interface states in solar silicon. We have shown recently [Vaqueiro-Contreras et al., Phys. Status Solidi RRL 11, 1700133 (2017)] that hydrogenation of n-type silicon slices containing relatively large concentrations of carbon and oxygen impurity atoms {[Cs] ≥ 1 × 1016 cm−3 and [Oi] ≥ 1017 cm−3} can produce a family of C-O-H defects, which act as powerful recombination centres reducing the minority carrier lifetime. In this work, evidence of the silicon's lifetime deterioration after hydrogen injection from SiNx coating, which is widely used in solar cell manufacturing, has been obtained from microwave photoconductance decay measurements. We have characterised the hydrogenation induced deep level defects in n-type Czochralski-grown Si samples through a series of deep level transient spectroscopy (DLTS), minority carrier transient spectroscopy (MCTS), and high-resolution Laplace DLTS/MCTS measurements. It has been found that along with the hydrogen-related hole traps, H1 and H2, in the lower half of the gap reported by us previously, hydrogenation gives rise to two electron traps, E1 and E2, in the upper half of the gap. The activation energies for electron emission from the E1 and E2 trap levels have been determined as 0.12, and 0.14 eV, respectively. We argue that the E1/H1 and E2/H2 pairs of electron/hole traps are related to two energy levels of two complexes, each incorporating carbon, oxygen, and hydrogen atoms. Our results show that the detrimental effect of the C-O-H defects on the minority carrier lifetime in n-type Si:O + C materials can be very significant, and the carbon concentration in Czochralski-grown silicon is a key parameter in the formation of the recombination centers.

Bias temperature stress induced hydrogen depassivation from Al2O3/InGaAs interface defects

We study the reliability of Al2O3/InGaAs metal-oxide-semiconductor gate stacks by investigating the effect of bias temperature stress on the charge trap density at the Al2O3/InGaAs interface and in the bulk oxide. Under extended negative biasing at 100 °C, the gate stacks display a notable increase in the interface trap density (Dit), but little change in the border trap density. This phenomenon is more prominent for samples exposed to a H2/N2 forming gas anneal (FGA) than for the as-deposited samples. Negative gate bias applied during 100 °C thermal stress negates the FGA-induced passivation of interface states and causes convergence of the Dit of the post-FGA and as-deposited gate stacks with increasing biasing time. This appears to be caused by hydrogen depassivation of interface traps under bias temperature stress, which is further supported by an observed hydrogen isotope effect when comparing the rate of Dit increase after annealing in hydrogenated versus deuterated forming gas. A N2 anneal control experiment also indicates that the stability of the interface trap density of post-FGA Al2O3/InGaAs gate stacks is more strongly influenced by the behavior of hydrogen at the interface than by the thermal treatment effect of the anneal.

Chemical Modification of Semiconductor Surfaces for Molecular Electronics.

Inserting molecular monolayers within metal/semiconductor interfaces provides one of the most powerful expressions of how minute chemical modifications can affect electronic devices. This topic also has direct importance for technology as it can help improve the efficiency of a variety of electronic devices such as solar cells, LEDs, sensors, and possible future bioelectronic ones. The review covers the main aspects of using chemistry to control the various aspects of interface electrostatics, such as passivation of interface states and alignment of energy levels by intrinsic molecular polarization, as well as charge rearrangement with the adjacent metal and semiconducting contacts. One of the greatest merits of molecular monolayers is their capability to form excellent thin dielectrics, yielding rich and unique current-voltage characteristics for transport across metal/molecular monolayer/semiconductor interfaces. We explain the interplay between the monolayer as tunneling barrier on the one hand, and the electrostatic barrier within the semiconductor, due to its space-charge region, on the other hand, as well as how different monolayer chemistries control each of these barriers. Practical tools to experimentally identify these two barriers and distinguish between them are given, followed by a short look to the future. This review is accompanied by another one, concerning the formation of large-area molecular junctions and charge transport that is dominated solely by molecules.

Total ionizing dose response of fluorine implanted Silicon-On-Insulator buried oxide

A comparison is made of the behavior of silicon on insulator buried oxides and wet thermal oxides before and after fluorine implantation and irradiation. Before irradiation, the electrical characteristics of the thermal oxide and buried oxide are significantly different. The fluorine implantation in the smart-cut® buried oxide results in a large negative threshold shift due to the trapping of positive charges. These charges are associated with positively charged fluorine ions on implantation and are trapped at pre-existing trap sites, particularly at the bonding interface, and at additional defects caused by the ion implantation damage. This shift is absent in the wet thermal oxide. After Co60 irradiation up to 500Krad(Si), the negative flatband and threshold voltage shift in the fluorine implanted buried oxide is larger than in the unimplanted buried oxide indicating that any potential positive effect of fluorine on the passivation of interface states is more than offset by the additional trapping sites created during implantation. These results demonstrate that in the design of a transistor utmost care must be taken to prevent any fluorine being implanted into the buried oxide.

Characterization of Al2O3 surface passivation of silicon solar cells

A study of the passivation of silicon surface by aluminum oxide (Al2O3) is reported. A correlation of fixed oxide charge density (Qf) and interface trap density (Dit) on passivation efficiency is presented. Low surface recombination velocity (SRV) was obtained even by as-deposited Al2O3 films and this was found to be associated to the passivation of interface states. Fourier transfer infrared spectroscopy spectra show the existence of an interfacial silicon oxide thin layer in both as-deposited and annealed Al2O3 films. Qf is found positive in as-deposited films and changing to negative upon subsequent annealing, providing thus an enhancement of the passivation in p-type silicon wafers, associated to field effects. Secondary ion mass spectrometry analysis confirms the correlation between Dit and hydrogen concentration at the Al2O3/Si interface. A lowest SRV of 15cm/s was obtained after an anneal at 400°C in nitrogen atmosphere.

Improvement of Minority Carrier Lifetime by HCN Treatments

We have developed a method of improving minority carrier lifetime by use of HCN treatments. Metal contaminants, Fe, Ni, and Cu, are introduced during slicing Si ingots to produce Si wafers, and also during alkaline etching processes for removal of a damaged layer and formation of mat-textured surfaces. The concentrations of metal contaminants are greatly decreased by cleaning with HCN solutions having pH of 12, while they are decreased only slightly by pH 9 HCN solutions. Two kinds of metal contaminants, i.e., contaminants present deeply in saw damaged regions and on the surface due to re-adsorption during alkaline etching, can be removed by pH 12 HCN solutions while only the latter can be by pH 9 HCN solutions. The minority carrier lifetime is increased by removal of a damaged layer by alkaline etching but decreased below that of as-sliced wafers after formation of mat-textured surfaces. The lifetime is increased by the HCN treatment and it is attributed to i) removal of metal contaminants and ii) passivation of interface states by the formation of Si-CN bonds from Si dangling bonds.

Mechanisms of Schottky Barrier Control on n-Type Germanium Using Ge3N4 Interlayers

The influence of a few monolayers of crystalline and amorphous Ge3N4 on the Schottky barrier height of n-type Ge has been investigated. Low temperature capacitance–voltage measurements are used to accurately determine the barrier height. Both amorphous and epitaxial Ge3N4 effectively eliminate pinning of the Fermi level at the metal/Ge interface. Metal/Ge3N4/n-Ge contacts therefore show a linear dependence of the Schottky barrier height with metal work function. Our results indicate that the Fermi level unpinning is achieved mainly due to the passivation of interface states related to defects at the metal/Ge interface. Aluminum on amorphous and epitaxial Ge3N4 delivers barrier heights of 0.09 ± 0.05 and 0.0 ± 0.1 eV, respectively, resulting in Ohmic behavior. The formation of epitaxial Ge3N4 requires temperatures above 600°C, whereas amorphous layers can be formed at much lower temperatures. Amorphous Ge3N4 can therefore be used to form Ohmic contacts at a low thermal budget.

Detection of Ga suboxides and their impact on III-V passivation and Fermi-level pinning

The passivation of interface states remains an important problem for III-V based semiconductor devices. The role of the most stable bound native oxides GaOx (0.5≤x≤1.5) is of particular interest. Using monochromatic x-ray photoelectron spectroscopy in conjunction with controlled GaAs(100) and InGaAs(100) surfaces, a stable suboxide (Ga2O) bond is detected at the interface but does not appear to be detrimental to device characteristics. In contrast, the removal of the Ga 3+ oxidation state (Ga2O3) is shown to result in the reduction of frequency dispersion in capacitors and greatly improved performance in III-V based devices.

Impact of Device Layout and Annealing Process During the Passivation of Interface States in Presence of Silicon Nitride Layers

This paper reports the experimental evidence of anomalous electrical characteristics of large test structures for the characterization of both silicon-oxide-nitride-oxide-silicon (SONOS) and MOS gate stacks featuring nitride caps. The anomaly has been studied on devices featuring different layouts and it has been attributed to the property of silicon nitride layers to block the diffusion of hydrogen used for the passivation of the Si/SiO2 interface dangling bonds. Since the hydrogen passivation can occur only from the lateral sides of the device, our findings imply restrictions on the dimensions and on the layout of the test structures used to study the electrical properties of the gate stacks in SONOS or in large MOS devices featuring protective nitride caps.

Structure and photoelectrical properties of SiO2/Si/SiO2 single quantum wells prepared under ultrahigh vacuum conditions

SiO2/Si/SiO2 single quantum wells (QWs) were prepared under ultrahigh vacuum conditions in order to study their structural, chemical and photoelectrical properties with respect to a possible application in photovoltaic devices. Amorphous silicon (a-Si) layers (thickness <10nm) were deposited onto quartz glass (SiO2) substrates and subsequently oxidized with neutral atomic oxygen at moderate temperatures of 600°C. Under these conditions, the formation of suboxides is mostly suppressed and abrupt Si/SiO2 interfaces are obtained. Crystallization of a-Si QWs requires temperatures as high as 1000°C resulting in a nanocrystalline structure with a small amorphous fraction. The spectral dependence of the internal quantum efficiency of photoconductivity correlates well with the nanocrystalline structure and yields mobility lifetime products of <10−7cm2V−1. This rather low value points towards a strong influence of Si/SiO2 interface states on the carrier mobility and the carrier lifetime in Si QWs. Electronic passivation of interface states by subsequent hydrogen treatment in forming gas enhances the internal quantum efficiency by nearly one order of magnitude.

The Mechanism of Interface State Passivation by NO

Preliminary results of a systematic theoretical study on the reactions of NO with a model 4H-SiC/SiO2 interface are presented. We show, that nitridation is a complex process, in which the balance between various mechanisms depends on doping and temperature. For weakly doped (1015-16 cm-3) n-type SiC, the crucial effect is an additional oxidation without creation of excess carbon at the interface.

Thermal annealing effects on the interface state density of metal-oxide-semiconductor capacitors with electron cyclotron resonance plasma enhanced chemical vapor deposition Silicon dioxide

Silicon dioxide films (SiO 2), deposited at room temperature by electron cyclotron resonance (ECR) plasma reactor from a gas phase combination of O 2, SiH 4 and He, present excellent structural and electrical properties. However, when fabricating field effect devices it is also crucial to minimize the defect density at the semiconductor/insulator interface. We show that the interface state density, investigated in Al/SiO 2/Si MOS capacitors, can be substantially reduced performing post-deposition annealing. In particular we studied the effects of annealing temperature and time in different gas ambient: vacuum, nitrogen and forming gas (5% H 2 + N 2). We found that interface state passivation mainly occurs when thermal annealing is performed after Al-contact deposition and that it is quite insensitive to the annealing atmosphere. The present results clearly suggest that the hydrogen passivation mechanism is driven by the H-containing species present in the film and a possible mechanism to explain the results is proposed.

Deep-Etched Native-Oxide-Confined High-Index-Contrast AlGaAs Heterostructure Lasers With 1.3 $\mu$m Dilute-Nitride Quantum Wells

Using a modified, O2-enhanced nonselective wet thermal oxidation process, deep-etched ridge waveguides in AlGaAs heterostructures containing lambda = 808 nm InAlGaAs single quantum well or aluminum-free lambda = 1.3 mum GaAsP/InGaAsN dilute nitride multi-quantum-well active regions have been directly oxidized to effectively provide simultaneous electrical isolation, interface state passivation, and sidewall roughness reduction. The resulting high- index-contrast (HIC) ridge waveguide (RWG) diode lasers show improved performance relative to conventional shallow-etched devices owing to both strong optical confinement and the complete elimination of current spreading, with 5 mum stripe width dilute- nitride devices showing up to a 2.3 times threshold reduction and strong index guiding for kink-free operation. Oxidation of an AlGaAs graded-index separate confinement heterostructure is studied for varying O2 concentrations, and the interface passivation effectiveness of the native oxide is studied through comparison with deposited SiO2 and via a study of the stripe-width dependence of internal quantum efficiency and modal loss. The HIC RWG structure is shown to enable the operation of half-racetrack-ring- resonator lasers with a bend radius as small as r = 6 mum.

Electrically Active Interface Defects in the (100)Si/SiOx/HfO2/TiN System: Origin, Instabilities and Passivation

An analysis of the origin and passivation of interface states in (100)Si/SiOx/HfO2/TiN capacitor structures is presented. For high-k gate/metal gate capacitors which exhibit relatively high interface state densities (> 1x1011cm-2) the dominant interfacial defects are silicon dangling bond (Pbo) centres. For (100)Si/SiOx/HfO2/TiN capacitors which experience no high temperature thermal budget following HfO2/TiN gate formation (T<600{degree sign}C), the devices exhibit instabilities, where the interface state densities are modified during electrical measurements. The origin of this instability is studied. The response of the interface state density to rapid thermal annealing (30s) in N2 over the temperature range 600-900{degree sign}C is presented. In addition, results are presented for interface state passivation in forming gas (0.5H2/0.95N2) from 350-550{degree sign}C for (100)Si/SiOx/HfO2/TiN gate stacks with no post deposition annealing following TiN gate formation and for devices following a 900{degree sign}C, 30s N2 RTA.

Pt Schottky diode gas sensors formed on GaN and AlGaN/GaN heterostructure

Exposure of Pt/GaN and Pt/AlGaN/GaN Schottky diodes to H 2 gas at moderately high temperatures around 100 °C resulted in marked increase of forward and reverse currents. Increase was much larger in the Pt/AlGaN/GaN diode than in the Pt/GaN diode. Rapid turn-on responses and somewhat slower turn-off responses were observed with reproducible response magnitudes. A rigorous computer simulation of I– V curves indicated that current changes are due to changes in the Schottky barrier height caused either by H-induced formation of interfacial dipole or by hydrogen passivation of interface states.

Electrical and radiation assisted passivation of Ta2O5/Si interface

An unusual interface state passivation effect has been observed in Al/Ta2O5/Si capacitors. The effect is found for negative bias stress fields ∼−0.4 MV cm−1, it is also observed for positive bias fields if the structures are simultaneously x irradiated. The passivation relaxes if the structure is heated to moderate temperatures (⩽70 °C) for relatively short times (∼tens of minutes). Bias stress measurements also evidence the presence of positive and negative trapped charge in the Ta2O5 films in the “as-deposited” state.

Effect of gate oxidation method on electrical properties of metal-oxide-semiconductor field-effect transistors fabricated on 4H-SiC C(0001̄) face

The effect of gate oxidation method on the electrical properties of metal-oxide-semiconductor field-effect transistors (MOSFETs) fabricated on 4H-SiC C(0001̄) face has been investigated. In the case of SiC MOSFETs fabricated by dry gate oxidation, the peak value of field-effect mobility (μFE) is 16.3 cm2/V s. On the other hand, pyrogenic gate oxidation and pyrogenic gate oxidation followed by H2 postoxidation annealing (POA) considerably decreased the interface trap density (Dit) and the threshold voltage, and markedly improved the μFE. The depth profiles of hydrogen density were measured using secondary ion mass spectroscopy. These verified that pyrogenic gate oxidation increases hydrogen density at the SiO2/SiC interface compared to dry gate oxidation, and that the pyrogenic gate oxidation followed by H2 POA increases considerably it. It is thought that the Dit reduction might be caused by the passivation of interface states by –H or –OH. The peak value of μFE for SiC MOSFETs fabricated by pyrogenic gate oxidation followed by H2 POA is 111 cm2/V s, which is much higher than that of SiC MOSFETs fabricated on a Si(0001) face. Therefore, the 4H-SiC C(0001̄) face is suitable for the fabrication of SiC power MOSFETs.

Defect introduction during E-beam metal deposition on bulk n-type 6H–SiC

Deep level transient spectroscopy is employed in order to assess whether E-beam metal deposition on bulk n-type 6H–SiC effects any changes to the underlying material. Near-surface E-beam-related damage is shown to be device specific. The EB1-related defect at Ec − (0.346 ± 0.007) eV with σna = (5.4+1.5−1.2) × 10−16 cm2 and the EB2-related defect at Ec − (0.47 ± 0.01) eV with σna = (2.2+0.5−1.7) × 10−15 cm2 are detected in several devices, independently of the metal used for deposition. It is shown that the broad EB3 peak, detected in a set of E-beam fabricated devices, corresponds to the p5-related band of interface states also detected in thermally metallized devices. It is shown that neither the EB3 nor the p5 peak can be detected in environmentally aged E-beam fabricated devices. This is attributed to interface state passivation via in-diffusion of atmospheric oxygen.