AlOx Films Research Articles

Low surface recombination velocity in n-Si passivated by catalytic-chemical vapor deposited alumina films

The surface recombination velocity (S0) in n-type Si (n-Si) wafers has been reduced below 0.1cm/s by dint of positive fixed charges created by alumina (AlOx) films deposited at a film-temperature of 230°C by catalytic chemical vapor deposition (Cat-CVD) using trimethyl aluminum (TMA) and O2. Positive fixed charges of the order of 1012charges/cm2 can be created in AlOx films deposited under O2/TMA flow-rate ratios in the range of 3.5–6.5. The extremely small S0 has been confirmed to be obtainable mainly due to a band bending effect brought about by the positive charges. The polarity and amount of the fixed charges can be determined by the flow-rate ratio of O2/TMA.

Reduction of surface recombination velocity by rapid thermal annealing of p-Si passivated by catalytic-chemical vapor deposited alumina films

Excellent electrical-passivation of p-type Si (p-Si) in Si solar cells has been achieved by post-deposition rapid annealing of aluminum oxide (AlOx) films prepared by catalytic chemical vapor deposition (Cat-CVD) using trimethyl aluminum (TMA) and O2. Extremely small surface recombination velocity of below 0.1cm/s has been obtained at post-deposition annealing temperatures in the range of 350–400°C for an annealing time of 2min. The reduction of surface recombination velocity has been attributed to band bending induced by a fixed negative charge density of 5×1011charges/cm2 and an additional small interface trapping density of around 1010cm−2eV−1.

Silicon Surface Passivation by Sputtered Aluminium Oxide: Influence of Annealing Temperature and Ambient Gas

Aluminium oxide (AlOx) films have achieved excellent passivation on crystalline-silicon surfaces. Irrespective of the deposition method, an anneal is required to activate the passivation. In this work, the reaction kinetics of the annealing process for radiofrequency reactively-sputtered AlOx is investigated. The effectiveness of the activation anneal critically depend on the ambient gas. A gas mixture of N2 and H2 is found to be the best annealing ambient for sputtered Al2O3 films, leading to an effective surface recombination velocity Seff of ∼5 cm/s. The experiments indicate that the surface passivation anneal follows an activation energy EA close to 1.1 eV.

Comparative study of amorphous indium gallium zinc oxide thin film transistors passivated by sputtered non-stoichiometric aluminum and titanium oxide layers

The amorphous indium gallium zinc oxide thin film transistors (a-IGZO TFTs) passivated by sputtered non-stoichiometric aluminum oxide (AlOx) and titanium oxide (TiOx) layers were comparatively investigated in this letter. The devices with TiOx passivation layers exhibited better long-time storage stability compared with those passivated by AlOx films. Although both a-IGZO TFTs with AlOx and TiOx showed quite stable properties for negative bias-stress tests, the device passivated by TiOx film exhibited abnormal Vth shift during positive bias-stress tests. A novel passivation-layer structure consisting of both AlOx (~15nm, bottom side) and TiOx film (~85nm, top side) was proposed, which could perform better than either sing layers and might be preferred in mass production of a-IGZO TFTs.

Chemical Composition and Surface Roughness of AlOx-Controlled Activity of Pt/AlOx Thin Film Catalysts for Methanol Oxidation Reaction

Chemical composition and surface roughness of AlOx films were facilely controlled by manipulating O2/Ar ratio during aluminum oxide sputter-deposition process. Pt/AlOx catalyst synthesized with O2/Ar ratio of 2 % exhibits the highest roughness (8.55 nm) and lowest O/Al ratio (0.11), which showed the best catalytic activity with a methanol conversion rate of 92.5 % and a remarkable temperature rise of 12 °C induced by exothermic reaction at 100 °C. It was demonstrated that Pt/AlOx catalysts had significant higher activity for methanol oxidation when the AlOx was nonstoichiometric compared with its stoichiometric counterpart (Al2O3). The outstanding catalytic performance and novel nanostructure of the deposited Pt/AlOx thin film catalyst endow it with great potential for developing next-generation energy conversion devices and combustible gas sensors.

Scratch resistance and durability enhancement of bulk heterojunction organic photovoltaics using ultra-thin alumina layers

Atomic layer deposition (ALD) was studied for ultra-thin (≤100nm) barrier coating deposition onto organic photovoltaics (OPVs) to enhance device lifetime. Herein, we report the first known investigation of the mechanical characteristics of AlOx encapsulated OPVs using nanoindentation and scratch testing with deposition temperature as a tunable parameter. The higher organic content in the AlOx film, grown at lower temperatures, enhanced the interfacial bonding at the AlOx–OPV interface and provided modulus and hardness performance exceeding that of a common polymer encapsulant. Furthermore, AlOx׳s organic content impeded fracture propagation, confirming the durability enhancement associated with this approach.

Light Wavelength Effects on Charge Trapping and Detrapping of AlOx Embedded ZrHfO High-K Stack

When the MOSFET with the SiO2 gate dielectric is scaled down to the nanosize, the leakage current becomes very large and the dopant can easily diffuse to the channel region to degrade the device's performance and reliability [1]. To solve these problems, a physically thicker high dielectric constant (high-k) film with the same equivalent oxide thickness (EOT) as the SiO2 film is used. For the conventional floating gate (FG) memory device, a poly-Si layer is embedded in the silicon dioxide (SiO2) gate dielectric to trap charges. Because of the large band offsets between Si and SiO2, a large operating voltage is required to achieve the acceptable write/erase efficiency and the tunnel oxide layer has to be thick enough to retain the trapped charges [2]. Since the high-k dielectric has lower band offsets with Si than the SiO2 film has, it can be used as the tunnel oxide to solve the above power consumption and charge retention problems. Separately, it has been proved that the Zr-doped HfO2 (ZrHfO) has better bulk and interface properties, such as a higher crystallization temperature, lower interfacial layer thickness, and lower interface density of states, than the undopd HfO2 has [3]. Also, the memory functions of the nc-ZnO or nc-MoOx embedded capacitor are affected by light exposure [4,5]. In this study, MOS capacitors made of the AlOx embedded ZrHfO high-kgate dielectric are fabricated and studied for the light wavelength effects on the memory functions.

Nonvolatile memory devices with AlOx embedded Zr-doped HfO2 high-k gate dielectric stack

MOS capacitors with the ZrHfO-AlOx-ZrHfO gate dielectric structure have been fabricated and studied for memory functions. Without the embedded AlOx film, the capacitor traps negligible amount of charges. With the embedded AlOx layer, the sample traps a large amount of holes or negligible amount of electrons depending on the polarity of the applied gate voltage. The charge retention and frequency dispersion measurements show that some of the holes are loosely retained at the tunnel ZrHfO/AlOx interface and some are deeply retained to the AlOx related sites. The leakage current–voltage curve further confirms the charge trapping capability of the AlOx embedded sample. Nearly half of the originally trapped holes can be retained in the memory device for more than 10 years. The AlOx embedded ZrHfO high-k stack is a good gate dielectric for the nonvolatile memory device.

Enhanced endurance reliability and low current operation for AlOx/HfOx based unipolar RRAM with Ni electrode

A 1-nm-thick AlOx layer is adopted to successfully eliminate soft-errors and reduce the operation current in HfOx based unipolar resistive random access memory (RRAM) with Ni as top electrode. Ni/HfOx/TiN RRAMs with one transistor–one resistor configuration during repetitive programming/erasing suffer serious soft-errors. The AlOx layer can share the applied voltage and suppress the soft-errors in the AlOx/HfOx based RRAM in high resistance state. The AlOx layer also modifies the conductive filaments in the devices with the low resistance to avoid the unexpected resistive switching. The forming/SET voltage, the high resistance state, and the on/off ratio in the Ni/AlOx RRAMs are larger than those of Ni ones. In particular, the soft-errors in the devices with ramped voltage verification are eliminated. The intermediate AlOx film is also beneficial for the reduced operation current of the device, which is as low as 10μA with a pulse width of 40ns.

Mist chemical vapor deposition of aluminum oxide thin films for rear surface passivation of crystalline silicon solar cells

We show the promising potential of mist chemical vapor deposition (CVD), a nonvacuum and solution-based technology, for the growth of an AlOx film as a rear surface passivation layer of crystalline silicon (c-Si) solar cells, supported by its safety, low process cost, and low energy consumption for the growth. The AlOx layer grown at 350 °C with aluminum acetylacetonate and ozone, without a successive annealing process, showed an excellent surface passivation property with the negative fixed charge density of about (1–2) × 1012 cm−2, as highlighted by the surface recombination velocity of ∼10 cm/s.

Hybrid aluminum and indium conducting filaments for nonpolar resistive switching of Al/AlOx/indium tin oxide flexible device

The nonpolar resistive switching characteristics of an Al/AlOx/indium tin oxide (ITO) device on a plastic flexible substrate are investigated. By analyzing the electron diffraction spectroscopy results and thermal coefficient of resistivity, it is discovered that the formation of aluminum and indium conducting filaments in AlOx film strongly depends on the polarity of the applied voltage. The metal ions arising from the Al and ITO electrodes respectively govern the resistive switching in corresponding operation polarity. After 104 times of mechanical bending, the device can perform satisfactorily in terms of resistance distribution, read sequence of high and low resistive states, and thermal retention properties.

Densification of Thin Aluminum Oxide Films by Thermal Treatments

Thin AlOx films were grown on 4H-SiC by plasma-assisted atomic layer deposition (ALD) and plasma assisted electron-beam evaporation at 300°C. After deposition, the films were annealed in nitrogen at temperatures between 500°C and 1050°C. The films were analyzed by X-ray reflectivity (XRR) and atomic force microscopy (AFM) in order to determine film thickness, surface roughness and density of the AlOx layer. No differences were found in the behavior of AlOx films upon annealing for the two different employed deposition techniques. Annealing results in film densification, which is most prominent above the crystallization temperature (800°C). In addition to the increasing density, a mass loss of ~5% was determined and attributed to the presence of aluminum oxyhydroxide in as deposited films. All changes in film properties after high temperature annealing appear to be independent of the deposition technique.

AlOx prepared by atomic layer deposition for high efficiency-type crystalline silicon solar cell

The influence of atomic layer deposition parameters on the negative charge density in AlOx film is investigated by the corona-charge measurement. Results show that the charge density can reach up to −1.56 × 1012 cm−2 when the thickness of the film is 2.4 nm. The influence of charge density on cell conversion efficiency is further simulated using solar cell analyzing software (PC1D). With AlOx passivating the rear surface of the silicon, the cell efficiency of 20.66% can be obtained.

Structure Analyses of Room Temperature Deposited AlOx Passivation Films for Crystalline Silicon Solar Cells

We studied the structure of ozone-based atomic layer deposited aluminium oxide (AlOx) films as a passivation layer for p-type crystalline silicon (c-Si) solar cells and focused on the differences in the structure by the production conditions of AlOx films. Carbon (C)-related groups such as methyl, hydroxyl, and carboxyl groups which originate from the aluminium source, trimethylaluminium, were only found in the AlOx film deposited at room temperature (RT-sample). By post-deposition thermal annealing (PDA), the C-related groups were desorbed from the film and a part of their space remained as voids. The C-related groups were not found in the films deposited at 200 or 300 °C (heated-samples) since they were desorbed during the deposition. Even though C-related groups did not exist in the both RT- and heated-samples after PDA, the structure of the AlOx film of the RT-sample was different from that of the heated-sample.

Passivation of boron-doped р+-Si emitters in the (p+nn+)Si solar cell structure with AlOx grown by ultrasonic spray pyrolysis

AlOx/(p+nn+)Cz–Si/IFO and ITO/AlOx/(p+nn+)Cz–Si/IFO solar cell structures have been fabricated from n-type Czochralski (Cz) silicon wafers through boron and phosphorus diffusion for producing the p+-Si emitter and n+-Si layer, respectively. The In2O3:F (IFO), AlOx, and In2O3:Sn (ITO) films have been grown by ultrasonic spray pyrolysis at 475°С, 330°С, and 375°С, respectively. The AlOx film thickness was varied in the range 27–108nm, the annealing time at 330°С in an Ar+5% O2 atmosphere containing vapor of a 2M H2O solution in methanol – in the range 0–18min, and the sheet resistance of the p+-Si emitter – in the range 28–133Ω/□ (varied through layer-by-layer chemical etching). We have studied the internal quantum efficiency (IQE) spectrum, photocurrent JIQE of the structures (evaluated from the IQE spectrum), as well as their photovoltage and pseudo-fill factor evaluated from Suns–Voc measurements. The results demonstrate that the level of p+-Si surface passivation increases with increasing AlOx film thickness and as a result of annealing. The optimal sheet resistance of the emitter is ∼65Ω/□. The pseudo-efficiency of the optimized ITO/AlOx/(p+nn+)Cz–Si/IFO structures was 20.2% under front illumination.

Ultra-thin alumina layer encapsulation of bulk heterojunction organic photovoltaics for enhanced device lifetime

Successful organic photovoltaic (OPV) device fabrication is contingent on selecting an effective encapsulation barrier layer to preserve device functionality by inhibiting atmosphere-induced degradation. In this work, ultra-thin AlOx layers are deposited by atomic layer deposition (ALD) to encapsulate pre-fabricated OPV devices. A summary of ALD recipe effects (temperature, cycling time, and number of cycles) on AlOx film growth and device longevity is presented. First, AlOx film growth on the hydrophobic OPV surface is shown to occur by a 3D island growth mechanism with distinct nucleation and cluster growth regions before coalescence of a complete encapsulation layer with a thickness ⩾7nm by 500cycles. Encapsulated device performance testing further demonstrates that reducing ALD processing temperature to 100°C minimizes OPV phase segregation and surface oxidation loss mechanisms as evidenced by improved short circuit current and fill factor retention when compared with the conventional 140–150°C range. Ultra-thin AlOx encapsulation by ALD provides significant device lifetime enhancement (∼30% device efficiency after 2000h of air exposure), which is well beyond other ALD-based encapsulation works reported in the literature. Furthermore, the interfacial bonding strength at the OPV–AlOx interface is shown to play a crucial role in determining film failure mode and therefore, directly impacts ultimate device lifetime.

Feasibility of using sputtered AlOx film as gate insulator for high performance InGaZnO-TFTs

We reported on the electrical and surface characteristics of radio frequency (rf) AlOx film and their applications in InGaZnO-based thin film transistors (TFTs). By optimizing the rf power, AlOx film with the low leakage current density and smooth surface was obtained. The leakage current density for the 180W AlOx film was observed to be ∼2.0×10−9A/cm2 at electrical field strength of 2MV/cm. The root mean square (rms) roughness of 180W AlOx film was about 1.36nm. InGaZnO-TFTs with different AlOx insulators were fabricated. The InGaZnO-TFT with 180W AlOx insulator exhibits a field-effect mobility of 6.9cm2/Vs, a threshold voltage of 4.2V, an on/off ratio of 2.7×107, and a much smaller Vth shift of 6.6V for 20V bias stress duration of 10,800s. The improvement of InGaZnO-TFT with 180W AlOx film is attributed to smooth surface of AlOx film and smaller trap charges. The results indicate that AlOx is a promising candidate insulator for InGaZnO-TFTs.

Influence of precursor gas ratio and firing on silicon surface passivation by APCVD aluminium oxide

AbstractUsing a high throughput, in‐line atmosphere chemical vapor deposition (APCVD) tool, we have synthesized amorphous aluminum oxide (AlOx) films from precursors of trimethyl‐aluminum (TMA) and O2, yielding a maximum deposition 150 nm min–1 per wafer. For p‐type crystalline silicon (c‐Si) wafers, excellent surface passivation was achieved with the APCVD AlOx films, with a best maximum effective surface recombination velocity (Seff,max) of 8 cm/s following a standard industrial firing step. The findings could be attributed to the existence of large negative charge (Qf ≈ –3 × 1012 cm–2) and low interface defect density (Dit ≈ 4 × 1011 eV–1 cm–2) achieved by the films. This data demonstrates a high potential for APCVD AlOx to be used in high efficiency, low cost industrial solar cells. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-performance nonvolatile Al/AlOx/CdTe:Sb nanowire memory device

Here we demonstrate a room temperature processed nonvolatile memory device based on an Al/AlOx/CdTe:Sb nanowire (NW) heterojunction. Electrical analysis shows an echelon hysteresis composed of a high-resistance state (HRS) and a low-resistance state (LRS), which can allow it to write and erase data from the device. The conductance ratio is as high as 106, with a retention time of 3 × 104 s. Moreover, the SET voltages ranged from +6 to +8 V, whilst the RESET voltage ∼0 V. In addition, flexible memory nano-devices on PET substrate with comparable switching performance at bending condition were fabricated. XPS analysis of the Al/AlOx/CdTe:Sb NW heterojunction after controlled Ar+ bombardment reveals that this memory behavior is associated with the presence of ultra-thin AlOx film. This Al/AlOx/CdTe:Sb NW heterojunction will open up opportunities for new memory devices with different configurations.

Plasma hydrogenated, reactively sputtered aluminium oxide for silicon surface passivation

AbstractThe intentional addition of hydrogen during reactive sputtering of AlOx films has led to a dramatic improvement of the surface passivation of crystalline silicon wafers achieved with this technique. The 5 ms effective minority carrier lifetime measured on 1.5 Ω cm n‐type CZ silicon wafers is close to the 6 ms of a control wafer coated by atomic layer deposition (ALD) of AlOx. Hydrogen‐sputtered films also provide excellent passivation of 1 Ω cm p‐type silicon, as demonstrated by an effective lifetime of 1.1 ms. It is likely that the improved passivation is related to the formation of an interfacial silicon oxide layer, as indicated by FTIR measurements. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)