AVS2013 Session EM-ThP: Electronic Materials and Processing Poster Session

Silicon Rich oxides have been considered as a material to overcome the drawbacks of silicon to achieve optical functions. Various techniques can be used to produce it, including Hot-fire Chemical Vapor Deposition (HFCVD). Obtain silicon compatible material with an optimal photoluminescence response is important for to have optoelectronic functions. The SiOx has shown to have a very good photoluminescence response and is also compatible with silicon technology. In this works, SiO_x Films were obtained by hot fire chemical vapor deposition (HF-CVD) with different deposit conditions. The hydrogen flows were changing of: 50, 75, 100, and 125 and 150 sccm, and the range of deposit temperature were from 800 to 1000°C. The compositional and optical properties of the SiOx films were obtained using fourier transformed infrared spectroscopy (FTIR), photoluminescence (PL), Transmittance and perfilometer. The IR absorption spectrum shows the presence of three typical Si–O–Si vibrations modes in SiO₂ also the change of the morphology. At 75 and 100sccm showed the most intense PL. Transmittance spectra showed a wavelength shift of the absorption border from 350 to 450nm. We have analyzed the dependence of PL on the composition of non-stoichiometric SiOx films.

Water uptake in porous low-k dielectrics has become a significant challenge for both back-end-of-the-line integration and circuit reliability. The influence of absorbed water on the mechanical properties of PECVD organosilicate glasses (SiCOH) was investigated with nanoindentation. The roles of physisorbed (α-bonded)and chemisorbed (β-bonded) water were examined separately through annealing at different temperatures. Nanoindentation measurements were made on dehydrated organosilicate glass (SiCOH) during exposure to different humidity conditions. The indentation results at shallow depth show that for as-deposited SiCOH, the elastic modulus and hardness were all intimately linked to the concentration of the absorbed water in the dielectric bulk and qualitatively held the same evolution mechanism with in-diffused water concentration. After annealing, the water-concentration-related film mechanical property changes were shown to be reversible. UV curing of SiCOH was shown to lower water-induced mechanical property variation effectively by depopulating the hydrophilic chemical groups in SiCOH. A high-load indentation test shows that in-diffusion of water at the film/substrate interface can degrade the hardness of a SiCOH/Si film stack significantly, while showing less effect on the elastic modulus.

This work was supported by the Semiconductor Research Corporation under Contract 2012-KJ-2359 and by the National Science Foundation under Grant CBET-1066231.

For the development of high speed III-V-based metal-oxide-semiconductor field-effect transistors (MOSFETs) in replacement of current Si-based devices, high-k gate dielectric is the most crucial component and, especially, atomic layer deposition (ALD) technique has been acknowledged to be the ultimate solution in acquiring high electrical quality such as low leakage current and equivalent oxide thickness (EOT). In order to achieve a sub-1nm EOT value, many researchers have tried to introduce TiO₂ film due to its much higher k value than Al₂O₃ and HfO₂ [1, 2]. However, one major drawback in adopting TiO₂ is relatively high leakage current due to its small band gap (~3.1 eV) [1]. As an effort to reduce the leakage current, recently, Kim et al. employed Al-doped TiO₂ (ATO) films for memory capacitor application and demonstrated promising electrical performance [1]. More recently, Mahata et al. tried (TiO₂)_x(Al₂O₃)_1-x alloy film on In_0.53Ga_0.47As as a gate insulator [2].

In this study, we deposited various ATO films on S-passivated p-type GaAs substrates and characterized their electrical properties, while varying the Al₂O₃ doping concentration and film thickness. The nanolaminating ALD process for the formation of ATO films utilized trimethylaluminum, titanium tetrakis-isopropoxide, and H₂O precursors at a deposition temperature of 250 °C. After MOS capacitor fabrication, various electrical properties including capacitance-voltage, interface state density, and leakage current were evaluated. The dependency of these electrical properties on the ATO film composition will be discussed and correlated with various interface and film characterization results.

[1] S. K. Kim, G.-J. Choi, S. Y. Lee, M. Seo, S. W. Lee, J. H. Han, H.-S. Ahn, S. Han, and C. S. Hwang, Adv. Mater. 20, 1429 (2008).

[2] C. Mahata, S. Mallik, T. Das, C. K. Maiti, G. K. Dalapati, C. C. Tan, C. K. Chia, H. Gao, M. K. Kumar, S. Y. Chiam, H. R. Tan, H. L. Seng, D. Z. Chi, and E. Miranda, Appl. Phys. Lett. 100, 062905 (2012).

Transparent conductive oxides (TCO) have become increasingly important in a large variety of applications due to demands as transparent and conductive materials. Applications of these devices include thin-film solar cells, display devices, optoelectronic devices, polymer-inorganic composite solar cells, gas sensors, and frost-resistant surfaces [1-4]. ITO is an n-type semiconductor where indium oxide (In₂O₃) has been doped with tin oxide in order to improve many of the material’s properties, including its electrical conductivity [5]. Also, FTO is an ideal candidate for applications requiring TCO due to its ability to adhere strongly to glass, resistance to physical abrasion, chemical stability, high optical visible transparency, and electrical conductivity. FTO is frequently used as an alternative to ITO when chemical and electrical stability at elevated temperatures is required for device fabrication or application. FTO is more thermally stable because it does not depend on oxygen vacancies to provide charge carriers. As a result, when exposed to elevated temperatures, FTO does not experience the characteristic decrease in conductivity seen in the ITO system. Therefore, as device size continues to decrease, the potential use of nanoscaled structures of these TCOs grows, and a study for these TCO´s is necessary. In this work, Spray pyrolysis automatized technique was used to deposit FTO and ITO films on glass corning. This technique allows us to obtain a better control on thin FTO and ITO films, with different properties just by varying the concentration of the fluorine. The structural and optical properties of the FTO and ITO films were obtained using X-ray Diffraction and scanning electronic microscopy, photoluminescence (PL), Transmittance and perfilometer. The diffractograms shows the presence of preferential planes of the FTO and ITO. SEM images showed the formation of different agglomerates. All films showed different intensities PL and transmittance spectra showed a wavelength shift of the absorption border. With Hall Effect we obtain some electrical properties.

Acknowledgements: This work has been partially supported by CONACyT-154725 and VIEP-BUAP-2013.

Poly(vinylidene fluoride) has been studied extensively owing to both its ferroelectric properties and its usefulness as a porous membrane. Its large polarization response makes PVDF a promising material for electrical applications in digital memory, super capacitors, and electrocaloric coolers. These and other applications of PVDF require high quality thin films. While there are different ways to deposit thin polymer films, spin coating is particularly attractive due to its affordability and ubiquitous use in industry. Understanding the processes occurring under varying conditions during spin coating of PVDF is therefore important. Humid environments have been shown to affect the morphology and crystallinity of spin coated PVDF films. In this presentation we use in situ optical reflectance to present evidence that film formation during spin coating in a humid environment is controlled by liquid-liquid and solid-liquid phase separation similar to the demixing seen in the vapor induced phase separation and immersion techniques used to create PVDF membranes. This enables real-time identification of the film formation processes, suggesting that monitoring of optical reflectivity is a basis for process control during spin coating and hence tailoring of film morphology for specific applications.

Semipolar GaN semiconductors have become significantly important owing to their potential applications to high-efficiency optoelectronic and electronic devices, especially for the light-emitting diodes (LEDs) with a quantum-confined Stark effect free by eliminating the polarization-induced internal electric fields in the active regions. Another feature of semipolar GaN planes is weaker chemical inertness against chemical solutions compared to the polar (0001) c-plane, indicating that wet etching can be practically used. Indeed, several previous studies have revealed that the wet etching of semipolar GaN planes can form peculiar surface textures such as striated trigonal prisms by exposing specific crystallographic planes. These features are quite interesting in terms of the fabrication of more energy-efficient and cost-competitive LEDs. For example, first, the mesa of LEDs having semipolar orientations can be made using a wet etching process instead of conventional dry etching. Second, the crystallographic surface textures formed after wet etching can act as excellent light scatters or exits for the guided mode caused by the large mismatch of the refractive indices of GaN and the environment (air), implying that the extraction efficiency can be significantly improved. Furthermore, wet etched surfaces were found to be much more efficient in making ohmic contacts, as will be discussed in our study for the first time. These findings suggest that wet etching can be practically used in the fabrication of LEDs. In this regard, we demonstrate the first semipolar GaN-based LEDs fabricated by a wet etching process, which led to better device performance than reference LEDs.

It was shown that the wet etching of semipolar GaN was a reaction-limited process with an E_a of 11.3 kcal mol^-1, exhibiting etching rates as high as 530 Å/min under 4M-KOH solution at 90°C. In particular, the wet etched surfaces exhibited the typical trigonal prism cell structure with a (0001) c-plane and [10-10] m-planes. Notably, the ohmic contact could be better formed on the wet etched surface, which could be attributed to the enhanced carrier transport through local shallow barriers associated with the exposed crystallographic structure. The LEDs fabricated by wet etching showed excellent output performance at 1.89 times higher than that of the reference LEDs. This could be due to the role of the textured surfaces. These findings suggest that wet chemical etching can be potentially important to semipolar or nonpolar-plane LEDs, which open new ways to achieve brighter and cost-competitive lighting engines for solid state lighting.

Zinc tungstate (ZnWO₄) has recently gained much attention for its possible uses in photocatalysis and photovoltaic applications [1,2]. While many studies have focused on the fabrication and characterization of nanoscale structures of this material, few have investigated the properties of zinc tungstate thin films. Fewer have explored the properties of non-stoichiometric zinc tungstate materials.

The current study presents preliminary results on the structural, optical, and electronic properties of non-stoichiometric zinc tungstate thin-films fabricated using reactive DC sputter deposition. Such Zn_1-xW_xO₄ have been investigated in the parameter space from x = 0 to 0.5. The initial results of these studies suggest that the properties of zinc tungstate are strongly dependent on the quantity of W incorporated into the film. Therefore, the physical properties of zinc tungstate may be tailored for specific applications by employing such non-stoichiometric materials.

References:

[1] C. J. Spengler and S. O’Hara, Applied Optics, 3,1084-1085 (1964)

[2] J. Lin, J. Lin, and Y. Zhu, Inorg. Chem., 46, 8372−8378 (2007)

The dc characteristics as well as critical voltage of the drain-voltage electrical step-stress of AlGaN/GaN high electron mobility transistors (HEMTs) were measured prior to and post 5 MeV proton irradiation at doses from 10⁹ to 2×10¹⁴ cm^-2 to evaluate the feasibility of AlGaN/GaN HEMTs for space applications, which need to demonstrate radiation hardness of various irradiations. On-chip transmission line method (TLM) was used to extract contact and sheet resistances. The threshold of contact resistivity (R_C) and transfer resistance (R_T) degradation was at a proton dose of 5×10⁹ cm^-2, however, the threshold for sheet resistance (R_S) degradation was much higher at 2×10¹³ cm^-2 as compared to those for R_T and R_C. For the dc characteristics, minimal degradations of saturation drain current (I_DSS), transconductance (g_m), electron mobility, and sheet carrier concentration were observed for the samples irradiated with proton dose below 2×10¹³ cm^-2, while the reduction of these parameters were 15%, 9%, 41% and 16.6%, respectively, for the device irradiated with 2×10¹⁴ cm^-2 of protons. Drain breakdown voltage (V_BR) and of critical voltage (V_cri) unexpectedly increased 37% and 45%, respectively for the devices irradiated with 2×10¹⁴ cm^-2 of protons. Gate and drain pulse measurement were also conducted to study the trap characteristics. Both measurements showed apparent reduction of I_DSS for the HEMTs irradiated at proton doses higher than 2×10¹² cm^-2, which meant more traps were introduced for the implanted HEMTs. The improvements of drain breakdown voltage (V_BR) and critical voltage (V_cri) were attributed to the modification of the depletion region due to the introduction of a higher density of defects after irradiation at a higher dose.

Recently the digital memory industry has exploited chalcogenide materials for their distinct phase-dependent electrical properties, where the large resistivity difference between crystalline and amorphous states is used to create efficient memory platforms.

Digital applications primarily require large DC on/off ratios; RF switches on the other hand require very low on-resistance (R_on) and off-capacitance (C_off). These requirements are often mutually exclusive, where gains in one come at the expense of the other. Chalcogenide phase change materials represent a unique solution to this dilemma. With PCM devices, not only is it possible to obtain a very low R_on concomitant with very low C_off, but one can also achieve zero prime power consumption during steady state operation.

This presentation will detail the development of phase change materials that enable world class RF switch performance. Specifically a chalcogenide inline phase change switch (IPCS) utilizing GeTe has been fabricated for the first time utilizing a 3^rd terminal for controlling an independent resistive heater to accomplish the thermal actuation of the phase change material. An on-state resistance of 6 Ω (0.06 Ω-mm), with an off-state capacitance and resistance of 33fF and 1 MΩ were measured, respectively, resulting in an RF switch figure-of-merit cut-off frequency (F_co) of 0.7 THz and a switching on/off ratio of 10⁵. The power required to change the GeTe from amorphous to crystalline was as low as 0.5W, with zero power consumption during steady state operation, making it a non-volatile RF switch. To the authors’ knowledge, this is the first reported implementation of an RF phase change switch in a traditional, 3-terminal, in-line configuration.

In order to achieve such a high level of performance, significant material development efforts were undertaken. GeTe films were deposited using the DC magnetron sputtering technique and a power-pressure matrix was utilized to minimize sheet resistivity (R_s) and maximize the on-off ratio. Morphological features were evaluated using XRD, SEM and FIB-based cross sections. Electrical properties of as-deposited GeTe films were evaluated using 4-point probe measurements and a heated stage. The combined effect of sputtering power and pressure on film morphology and material properties will be presented.

In addition to wide ranging morphological and R_s variations with deposition parameters, it was found that the sputtering process significantly affects the chemical susceptibility of the films. An optimized fabrication scheme will be presented, in which high quality GeTe films were capable of being processed without a capping layer.

To determine the possible current-conduction mechanisms (CCMs) at low temperatures, Au/n-GaAs SBDs were fabricated and their current-voltage (I-V) characteristics were carried under room temperatures (80-300 K). The ideality factor (n) and apparent barrier height (Φ_bo) values were found as 5.200, 0.198 eV at 80 K and 1.256, 0.613 eV at 300 K, respectively. These results indicated that possible CCM is Thermionic Field Emission (TFE) rather than Thermionic Emission (TE) or other mechanisms. While the value of Φ_bo increases with increasing temperature, nΦ_bo decreases. The negative coefficient of Φ_bo and BH (0K) values were found as -13x10^-4 eV/K and 1.12 eV, respectively. This -13x10^-4 eV/K value is close to negative temperature coefficient bandgap of GaAs (-5.405x10^-4 eV/K). On the other hand, the high value of n especially at low temperatures cannot be also explained only by TFE theory. Such behavior of n and Φ_bo with temperature was explained on the basis of single Gaussian distribution (GD) of the BHs. As a result to avoid these nonideal behavior of forward bias I-V characteristics, the fabrication process should be carried out in very clear room at high vacuum, annealing of the samples, and use to a a thin interfacial with high dielectric constant.

Keywords: I-V-T Characteristics; Possible current-conduction mechanisms; Barrier inhomogeneity; Gaussian distribution of BH;