AlOx Films Research Articles

High performance In2O3 thin film transistors using chemically derived aluminum oxide dielectric

We report high performance solution-deposited indium oxide thin film transistors with field-effect mobility of 127 cm2/Vs and an Ion/Ioff ratio of 106. This excellent performance is achieved by controlling the hydroxyl group content in chemically derived aluminum oxide (AlOx) thin-film dielectrics. The AlOx films annealed in the temperature range of 250–350 °C showed higher amount of Al-OH groups compared to the films annealed at 500 °C, and correspondingly higher mobility. It is proposed that the presence of Al-OH groups at the AlOx surface facilitates unintentional Al-doping and efficient oxidation of the indium oxide channel layer, leading to improved device performance.

Aluminum Oxide Deposited by Pulsed-DC Reactive Sputtering for Crystalline Silicon Surface Passivation

In this paper, we report on the surface passivation of crystalline silicon (c-Si) by pulsed-dc (p-dc) reactive-sputtered aluminum oxide (AlOx) films. For the activation of surface passivation, the films were subjected to post deposition annealing (PDA) in different ambients namely N2, N2 + O2, and forming gas (FG) in the temperature range of 420-520°C. The surface passivation was quantified by surface recombination velocity, which was correlated to the interface states at the silicon-dielectric interface and fixed charges in the dielectric. A good quality surface passivation with effective surface recombination velocity Seff of 41 cm · s-1 is obtained for PDA in N2 or N2 + O2 gas ambient. PDA in FG ambient at high temperature is found to degrade the passivation. The AlOx film annealed in FG ambient shows poorer thermal stability as compared with films annealed in the other two ambients. A clear path for further improvements in surface passivation quality of p-dc reactive sputter-deposited AlOx is suggested based on cross-sectional transmission electron microscopy and X-ray photoelectron spectroscopy analysis and electrical data.

Phosphonate- and Carboxylate-Based Self-Assembled Monolayers for Organic Devices: A Theoretical Study of Surface Binding on Aluminum Oxide with Experimental Support

We report a computational study on the chemical bonding of phosphonates and carboxylates to aluminum oxide surfaces and how the binding properties are related to the amount of water in the experimental environment. Two different surface structures were used in the calculations in order to model representative adsorption sites for the phosphonates and carboxylates and to account for the amorphous nature of the hydroxylated AlOx films in experiment. For the phosphonates, we find that the thermodynamically preferred binding mode changes between mono-, bi-, and tridentate depending on the surface structure and the amount of residual water. For the carboxylates, on the other hand, monodentate adsorption is always lower in energy at all experimental conditions. Phosphonates are more strongly bound to aluminum oxide than carboxylates, so that carboxylates can be replaced easily by phosphonates. The theoretical findings are consistent with those obtained in adsorption, desorption, and exchange reactions of n-alkyl phosphonic and carboxylic acids on AlOx surfaces. The results provide an atomistic understanding of the adsorption and help to optimize experimental conditions for self-assembly of organic films on aluminum oxide surfaces.

Resistance switching of thin AlOx and Cu-doped-AlOx films

Thin copper-doped aluminum oxide (Cu-doped-AlOx) and AlOx films of about 5nm thick were generated by oxidizing the surfaces of Al and Al-5wt.% Cu (Al-Cu) films in an oxygen plasma. According to X-ray photoelectron spectroscopy analyses, these two oxide films were found to be deficient in oxygen and had gradient concentrations of Al and O. The oxide films were employed as resistor layers sandwiched between an Al top electrode and an Al or Al-Cu bottom electrode to form resistive memory devices. The devices demonstrated unipolar resistance switching between high resistance state and low resistance state (LRS), and their resistance ratios measured at +0.2V were around 105. Furthermore, their current–voltage characteristics showed ohmic conduction with the resistance increasing with temperature, in LRS. Conductive filaments were thought to form inside the AlOx film and the Cu-doped AlOx film, causing resistive switching. The resistive memory device using the AlOx film had unstable switching behaviors during cyclic testing, whereas the device using the Cu-doped AlOx film demonstrated stable resistance switching during 100cycles of testing. The presence of the Cu ingredient in the AlOx film is likely to facilitate the formation and rupture of conductive filaments and induced stable resistance switching.

Resistive Random Access Memory Enabled by Carbon Nanotube Crossbar Electrodes

We use single-walled carbon nanotube (CNT) crossbar electrodes to probe sub-5 nm memory domains of thin AlOx films. Both metallic and semiconducting CNTs effectively switch AlOx bits between memory states with high and low resistance. The low-resistance state scales linearly with CNT series resistance down to ∼10 MΩ, at which point the ON-state resistance of the AlOx filament becomes the limiting factor. Dependence of switching behavior on the number of cross-points suggests a single channel to dominate the overall characteristics in multi-crossbar devices. We demonstrate ON/OFF ratios up to 5 × 10(5) and programming currents of 1 to 100 nA with few-volt set/reset voltages. Remarkably low reset currents enable a switching power of 10-100 nW and estimated switching energy as low as 0.1-10 fJ per bit. These results are essential for understanding the ultimate scaling limits of resistive random access memory at single-nanometer bit dimensions.

Growth and electrical properties of AlOx grown by mist chemical vapor deposition

Aluminum oxide (AlOx) thin films were grown using aluminum acetylacetonate (Al(acac)3) as a source solute by mist chemical vapor deposition (mist CVD). The AlOx thin films grown at temperatures above 400°C exhibited a breakdown field (EBD) over 6 MV/cm and a dielectric constant (κ) over 6. It is suggested that residual OH bonding in the AlOx thin films grown at temperatures below 375°C caused degradation of the breakdown field (EBD). With FC type mist CVD, the reaction proceeded efficiently (Ea = 22–24 kJ/mol) because the solvent, especially H2O, worked as a stronger oxygen source. The AlOx film could be grown at 450°C with a high deposition rate (23 nm/min) and smooth surface (RMS = 1.5 nm). Moreover, the AlOx thin films grown by mist CVD had excellent practicality as insulators because the gate leakage current (IG) of the oxide thin film transistor (TFT) with an IGZO/AlOx stack was suppressed below 1 pA at a gate voltage (VG) of 20 V.

Interfacial chemistry of hydrofluoric acid-treated In0.53Ga0.47As(100) during atomic layer deposition of aluminum oxide

Atomic layer deposition (ALD) of aluminum oxide on In0.53Ga0.47As(100) prepared with aqueous or gas phase hydrofluoric acid (HF) was followed using x-ray photoelectron spectroscopy after each pulse of trimethylaluminum (TMA, Al(CH3)3) and water at 170°C up to three cycles. On the surface prepared using aqueous HF, the primary oxide was As2O3, which was removed by the first TMA pulse depositing about two layers of AlOx and leaving residual As+1 oxide at the interface that persisted even after three complete ALD cycles. The aluminum oxide AlOx film was O deficient but was closed off, trapping the As+1 oxide at the interface. The C and O coverages were modulated by TMA and water pulses supporting the expected near 1 Å per cycle growth during the second and third ALD cycles. The larger absolute change in the C than the O coverage suggests that the reaction of TMA with O–H groups is more facile than the reaction of water with Al−CH3 moieties, explaining the less than monolayer growth per cycle intrinsic to ALD processes. On the surface prepared using gas phase HF, the fluorides and oxides of all substrate atoms were removed after one complete ALD cycle and a second TMA pulse, which also deposited about two layers of AlOx. Yet, the surface was passivated by the residual F left from the gas phase HF process and by the large increase in C deposited in the first TMA pulse. There is clear evidence for Al–F bonding, and the locations of the Al 2p peaks combined with the large peak widths indicate that the Al atoms deposited in a variety of local bonding environments. The film was not closed off by the first ALD cycle because F and C capped reaction sites, inhibiting further growth of aluminum oxide up to three ALD cycles.

Wet Chemical Oxidation of Silicon Surfaces Prior to the Deposition of All-PECVD AlO<sub>x</sub>/<i>a</i>-SiN<sub>x</sub> Passivation Stacks for Silicon Solar Cells

Aluminum oxide (AlOx) is currently under intensive investigation for use in surface passivation schemes in solar cells. AlOx films contain negative charges and therefore generate an accumulation layer on p-type silicon surfaces, which is very favorable for the rear side of p-type silicon solar cells as well as the p+-emitter at the front side of n-type silicon solar cells. However, it has been reported that quality of an interfacial silicon sub-oxide layer (SiOx), which is usually observed during deposition of AlOx on Silicon, strongly impacts the silicon/AlOx interface passivation properties [1]. The present work demonstrates that a convenient way to control the interface is to form thin wet chemical oxides of high quality prior to the deposition of AlOx/a-SiNx:H stacks by the plasma enhanced chemical vapor deposition (PECVD).

Spectroscopic analysis of Al and N diffusion in HfO2

X-ray photoelectron core level spectroscopy, secondary ion mass spectroscopy, spectroscopic ellipsometry, and extended x-ray absorption fine structure measurements have been employed to distinguish the effects of Al and N diffusion on the local bonding and microstructure of HfO2 and its interface with the Si substrate in (001)Si/SiOx/2 nm HfO2/1 nm AlOx film structures. The diffusion of Al from the thin AlOx cap layer deposited on both annealed and unannealed HfO2 has been observed following anneal in N2 and NH3 ambient. Both N2 and NH3 subsequent anneals were performed to decouple incorporated nitrogen from thermal reactions alone. Causal variations in the HfO2 microstructure combined with the dependence of Al and N diffusion on initial HfO2 conditions are presented with respect to anneal temperature and ambient.

Plasma‐enhanced Chemical Vapor Deposition of Aluminum Oxide Using Ultrashort Precursor Injection Pulses

AbstractAn alternative plasma‐enhanced chemical vapor deposition (PECVD) method is developed and applied for the deposition of high‐quality aluminum oxide (AlOx) films. The PECVD method combines a continuous plasma with ultrashort precursor injection pulses. We demonstrate that the modulation of the precursor flow in the reactor leads to enhanced control over plasma‐surface interactions. By variation of the time interval between the sequential Al(CH3)3 precursor injection pulses (10–50 ms) into the O2 plasma, the deposition rate (>30 nm · min−1) and material properties can be tailored. In situ diagnostics revealed that the deposition process is governed by fast precursor depletion and film growth directly after the precursor pulse. Subsequently, in the remainder of the interval between the precursor pulses, densification of the layer takes place under influence of the O2 plasma. The resulting AlOx films exhibit a low impurity content and refractive index >1.6 for optimized process settings. The films can be applied for effective surface passivation of Si as indicated by ultralow surface recombination velocities <1 cm · s−1. The structural properties and interface quality are quite similar as obtained for atomic layer deposited films. We anticipate that pulsed‐flow PECVD processes may also prove beneficial for the synthesis of other functional thin films.magnified image

Interface engineering for the passivation of c-Si with O3-based atomic layer deposited AlOx for solar cell application

We have investigated the effects of deposition temperature and post-annealing on the passivation performance of AlOx films deposited by O3-based atomic layer deposition for crystalline Si. We found that the dramatic enhancement in the passivation performance of room-temperature deposited AlOx films by post-annealing is due to the phase transformation of aluminum silicate to mullite in an AlOx interlayer and the resulting self-aligned AlOx/SiOx interface. This result is interesting for the fabrication of high-performance silicon solar cells with AlOx passivation layers.

Surface Recombination of Crystalline Silicon Substrates Passivated by Atomic-Layer-Deposited AlOx

AlOx films as passivation layers for p-type crystalline silicon were prepared by atomic layer deposition with ozone as an oxidant, and the effects of the AlOx film thickness and deposition temperature on the maximum recombination velocity (Smax) were evaluated. Smax is improved by increasing the layer thickness but saturates at a layer thickness of about 30 nm. In the case of samples deposited at room temperature, Smax is improved fivefold when the thickness is increased from 20 to 33 nm. Smax also improved as the deposition temperature was increased to 300 °C then deteriorated when it was further increased to 350 °C. After postdeposition annealing, we obtained an Smax of 8.5 cm/s.

SiOx interlayer to enhance the performance of InGaZnO-TFT with AlOx gate insulator

We have fabricated indium–gallium–zinc (IGZO) thin film transistor (TFT) using SiOx interlayer modified aluminum oxide (AlOx) film as the gate insulator and investigated their electrical characteristics and bias voltage stress. Compared with IGZO-TFT with AlOx insulator, IGZO-TFT with AlOx/SiOx insulator shows superior performance and better bias stability. The saturation mobility increases from 5.6 cm2/V s to 7.8 cm2/V s, the threshold voltage downshifts from 9.5 V to 3.3 V, and the contact resistance reduces from 132 Ωcm to 91 Ωcm. The performance improvement is attributed to the following reasons: (1) the introduction of SiOx interlayer improves the insulator surface properties and leads to the high quality IGZO film and low trap density of IGZO/insulator interface. (2) The better interface between the channel and S/D electrodes is favorable to reduce the contact resistance of IGZO-TFT.

Positron annihilation studies of the AlOx/SiO2/Si interface in solar cell structures

Film and film/substrate interface characteristics of 30 and 60 nm-thick AlOx films grown on Si substrates by thermal atomic layer deposition (ALD), and 30 nm-thick AlOx films by sputtering, have been probed using variable-energy positron annihilation spectroscopy (VEPAS) and Doppler-broadened spectra ratio curves. All samples were found to have an interface which traps positrons, with annealing increasing this trapping response, regardless of growth method. Thermal ALD creates an AlOx/SiOx/Si interface with positron trapping and annihilation occurring in the Si side of the SiOx/Si boundary. An induced positive charge in the Si next to the interface reduces diffusion into the oxides and increases annihilation in the Si. In this region there is a divacancy-type response (20 ± 2%) before annealing which is increased to 47 ± 2% after annealing. Sputtering seems to not produce samples with this same electrostatic shielding; instead, positron trapping occurs directly in the SiOx interface in the as-deposited sample, and the positron response to it increases after annealing as an SiO2 layer is formed. Annealing the film has the effect of lowering the film oxygen response in all film types. Compared to other structural characterization techniques, VEPAS shows larger sensitivity to differences in film preparation method and between as-deposited and annealed samples.

Excellent boron emitter passivation for high‐efficiency Si wafer solar cells using AlOx/SiNx dielectric stacks deposited in an industrial inline plasma reactor

ABSTRACTExcellent passivation of boron emitters is realised using AlOx/SiNx dielectric stacks deposited in an industrial inline plasma‐enhanced chemical vapour deposition reactor. Very low emitter saturation current density (J0e) values of 10 and 45 fA/cm2 are obtained for 180 and 30 Ω/sq planar p+ emitters, respectively. For textured p+ emitters, the J0e was found to be 1.5–2 times higher compared with planar emitters. The required thermal activation of the AlOx films is performed in a standard industrial fast‐firing furnace, making the developed passivation stack industrially viable. We also show that an AlOx thickness of 5 nm in the AlOx/SiNx stack is sufficient for obtaining a J0e of 18 fA/cm2 for planar 80 Ω/sq p+ emitters, which corresponds to a 1‐sun open‐circuit voltage limit of the solar cell of 736 mV at 25 °C. Copyright © 2012 John Wiley & Sons, Ltd.

Formation of Aluminum Oxide Films on Silicon Surface by Aluminum Evaporation in Oxygen Gas Atmosphere

ABSTRACTWe report formation of thin aluminum oxide AlOx films on the silicon surface by a simple method of Al metal evaporation in oxygen gas atmosphere. 520 μm thick 30-Ωcm p-type-silicon substrates with a top bare surface and a rear surface coated with 100 nm thick thermally grown SiO2 layers were prepared. AlOx films were formed on the top surfaces by Al metal evaporation up to 20 s in oxygen gas atmosphere at 0.8 Pa with a flow rate of 3 sccm. Samples were subsequently annealed with 9.0x105 Pa H2O vapor at 260°C for 3 h. Measurement of capacitance response to a modulation voltage at 500 kHz as a function of bias gate voltages C-V revealed that AlOx films had the effective oxide thickness ranging from 2.0 and 2.6 nm were formed. C-V measurements also revealed that negative fixed charges were accumulated with a density of 5x1012 cm-2 in AlOx films. Photo-induced carrier microwave absorption measurement resulted in a high minority carrier effective lifetime τeff of 3.6x10-4 s comparable to that of 4.1x10-4 s for thermally grown SiO2 passivation. Field effect passivation was probably caused by negative charges in AlOx so that the surface recombination velocity decreased to 70 cm/s. X-ray reflectivity analysis indicated that the interfacial layer like SiOx was formed between AlOx and Si substrate. High pressure H2O vapor heat annealing caused increase in the density and decrease in the thickness of AlOx layers, although it increased the density and thickness of the interfacial SiOx layer thickness. H2O vapor treatment is effective to improve the quality of nanometer thick AlOxlayer.

Comparison of ICP-AlOx and ALD-Al2O3 Layers for the Rear Surface Passivation of C-Si Solar Cells

The deposition rate of the standard (i.e. sequential) atomic layer deposition (ALD) process is very low compared to the plasma-enhanced chemical vapour deposition (PECVD) process. Therefore, as a short- and medium-term perspective, PECVD aluminium oxide (AlOx) films might be better suited for the implementation into industrial-type solar cells than ALD-Al2O3 films. In this paper, we report results achieved with a newly developed PECVD deposition process for AlOx using an inductively coupled plasma (ICP). We compare the results to high-quality ALD-Al2O3 films. We examine a stack consisting of a thin AlOx passivation layer and a PECVD silicon nitride (SiNy) capping layer. Surface recombination velocities below 9 cm/s were measured on low-resistivity (1.4 Ωcm) p-type crystalline silicon wafers passivated either by ICP-PECVD-AlOx films or by ALD-Al2O3 films after annealing at 425̊C. Both passivation schemes provide an excellent thermal stability during firing at 910̊C with SRVs below 12 cm/s for both, Al2O3/SiNy stacks and single Al2O3 layers. A fixed negative charge of −4×1012 cm−2 is measured for ICP-AlOx and ALD-Al2O3, whereas the interface state density is higher for the ICP-AlOx layer with values of 11.0×1011 eV−1cm−2 compared to 1.3×1011 eV−1cm−2 for ALD-Al2O3. Implemented into large-area screen-printed PERC solar cells, an independently confirmed efficiency of 20.1% for ICP-AlOx and an efficiency of 19.6% for ALD-Al2O3 are achieved.

High mobility, low voltage operating C60 based n-type organic field effect transistors

We report on C60 based organic field effect transistors (OFETs) that are well optimized for low voltage operation. By replacing commonly used dielectric layers by thin parylene films or by utilizing different organic materials like divinyltetramethyldisiloxane-bis(benzocyclo-butene) (BCB), low density polyethylene (PE) or adenine in combination with aluminum oxide (AlOx) to form a bilayer gate dielectric, it was possible to significantly increase the capacitance per unit area (up to two orders of magnitude). The assembly of metal-oxide and organic passivation layer combines the properties of the high dielectric constant of the metal oxide and the good organic–organic interface between semiconductor and insulator provided by a thin capping layer on top of the AlOx film. This results in OFETs that operate with voltages lower than 500mV, while exhibiting field effect mobilities exceeding 3cm2V−1s−1.

Ultralow surface recombination in p-Si passivated by catalytic-chemical vapor deposited alumina films

The passivation effects of AlOx films were investigated for p-type crystalline Si (c-Si) solar cells. The AlOx films were deposited on 10 Ωcm c-Si substrates by catalytic chemical vapor deposition (Cat-CVD) using tri-methyl aluminum (TMA) and O 2 at a film temperature of 230 °C. The surface recombination velocity (S 0) at the AlOx/Si interface was measured to be below 0.5 cm/s for AlOx films deposited with O 2/TMA gas flow-rate ratios of 15–35. This ultra low S 0 was achieved primarily by band bending due to the negative interface fixed-charge density (N f) of an order of 10 12 charges/cm 2. The decrease in interface trapping density D it in the negative fixed charge region assists in decreasing S 0.

Characterization of Interfaces between Chemically Cleaned or Thermally Oxidized Germanium and Metals

To implement a Ge channel in MOSFETs and to fulfill its potential, it is imperative to have a good understanding the natures of both metal/Ge and dielectric/Ge interfaces and to control them as precise as possible. It is well known that the Fermi-levels at any metal/Ge interfaces are pinned near the valence band edge of Ge. Recently, it has been reported that by inserting the GeOx and AlOx film into metal/Ge junction, the junction properties have been drastically changed from Schottky to Ohmic and Ohmic to Schottky characteristics on n-Ge and p-Ge, respectively. The interface properties remain a matter of research. In this work, we have characterized surface band bending and chemical bonding features of interfaces between chemically-cleaned or thermally-oxidized Ge and metals, and also examine their correlation with electrical properties.