Amorphous Semiconductor Thin Film Research Articles

Low Contact Resistance via Quantum Well Structure in Amorphous InMoO Thin Film Transistors

The amorphous oxide semiconductor (AOS) thin film transistor (TFT) shows promise for use in advanced integrated circuits, such as 2T0C dynamic random-access memory, due to its excellent electronic performance and ability to be fabricated at low temperatures. Nevertheless, the high contact resistance between the metal and AOS restricts the applicability of AOS-TFT. This study demonstrates the achievement of a reduced contact resistance in InMoO (IMO) transistors by using a MoOx interlayer during fabrication. Increasing the oxygen concentration alters the band structure of MoOx and creates a graded Mo-MoOx-IMO structure with a pronounced quantum well at the interlayer between the metal and channel. Consequently, the quantum well’s ability to attract electrons and shape the band edge suppresses the Fermi-level pinning effect, ultimately leading to the establishment of an ohmic contact. The optimized MoOx interlayer showed a significant improvement in contact resistance (∼400%) through the adjustment of oxygen content during annealing procedures. This finding suggests that it is an attractive approach to provide excellent source/drain contacts in future ultra-scaled amorphous oxide semiconductor thin-films.

Structure and optical properties of Si thin film improved by metal Cd nanolayers

Enhancing the characteristics of amorphous semiconductor thin films is imperative for a multitude of applications. The present study examines the impact of Cd nanolayers deposited on a thin film of amorphous Si. The X-ray test showed that the thin silicon film had less of an amorphous structure after a nanolayer of cadmium was added. There has also been the appearance of new phases, and as the thickness of the Cd nanolayer increases, so does the intensity of these phases. Using a field emission scanning electron microscope, it was seen that nanoparticles were developed and subsequently transformed into clusters as the thickness of the Cd nanolayer grew. The absorbance was maximized at 750 nm and near-infrared region after depositing Cd nanolayers, while transmittance was reduced, especially at 100 nm thicknesses. The energy gap was reduced, with a decrease from 5.1 to 1.8 electron volts (eV). However, an increase in the band tails was also noted, rising from 0.7 to 4.9 eV. An increase in the values of the refractive index (n) and extinction coefficient (k) was observed following the deposition of Cd nanolayers of different thicknesses.

Accurate determination of band tail properties in amorphous semiconductor thin film with Kelvin probe force microscopy

The disordered microscopic structure of amorphous semiconductors causes the formation of band tails in the density of states (DOS) that strongly affect charge transport properties. Such band tail properties are crucial for understanding and optimizing thin-film device performance with immense relevance for large area electronics. Among the available techniques to measure the DOS, Kelvin Probe Force Microscopy (KPFM) is exceptional as it enables precise local electronic investigations combined with microscopic imaging. However, a model to interpret KPFM spectroscopy data on amorphous semiconductors of finite thickness is lacking. To address this issue, we provide an analytical solution to the Poisson equation for a metal–insulator–semiconductor junction interacting with the atomic force microscope tip. The solution enables us to fit experimental data for semiconductors with finite thickness and to obtain DOS parameters, such as band tail width, doping density, and flat band potential. To demonstrate our method, we perform KPFM experiments on Indium–Gallium–Zinc Oxide (IGZO) thin-film transistors (IGZO-TFTs). DOS parameters compare well with values obtained with photocurrent spectroscopy. We demonstrate the relevance of the developed method by investigating the impact of ionizing radiation on DOS parameters and TFT performance. Our results provide clear evidence that the observed shift in threshold voltage is caused by static charge in the gate dielectric, leading to a shift in flat band potential. Band-tails and doping density are not affected by the radiation. The developed methodology can be easily translated to different semiconductor materials and paves the way for quantitative microscopic mapping of local DOS parameters in thin-film devices.

Application of the Holomorphic Tauc-Lorentz-Urbach Function to Extract the Optical Constants of Amorphous Semiconductor Thin Films

The Tauc–Lorentz–Urbach (TLU) dispersion model allows us to build a dielectric function from only a few parameters. However, this dielectric function is non-analytic and presents some mathematical drawbacks. As a consequence of this issue, the model becomes inaccurate. In the present work, we will adopt a procedure to conveniently transform the TLU model into a self-consistent dispersion model. The transformation involves the integration of the original TLU imaginary dielectric function ϵ2 by using a Lorentzian-type function of semi-width, Γ. This novel model is analytic and obeys the other necessary mathematical requirements of the optical constants of solid-state materials. The main difference with the non-analytic TLU model occurs at values of the photon energy near or lower than that of the bandgap energy (within the Urbach absorption region). In particular, this new model allows us to reliably extend the optical characterization of amorphous-semiconductor thin films within the limit to zero photon energy. To the best of our knowledge, this is the first time that the analytic TLU model has been successfully used to accurately determine the optical constants of unhydrogenated a-Si films using only their normal-incidence transmission spectra.

Multilayer Crossbar Array of Amorphous Metal-Oxide Semiconductor Thin Films for Neuromorphic Systems

A multilayer crossbar array has been developed using amorphous metal-oxide semiconductor (AOS) thin films and implemented into a neuromorphic system. The multilayer structure can be realized, because the AOS thin films can be deposited by a simple sputtering method without heat treatment, which does not damage the underlying structures. First, Au thin films are deposited by vapor evaporation as electrodes, an amorphous In-Ga-Zn-O (α-IGZO) thin film is deposited by a sputtering method as a conductance change layer, these processes are repeated, and a multilayer crossbar array is completed, where each of the three conductance change layers is sandwiched between the electrodes. Next, the multilayer crossbar array is implemented into a neuromorphic system with modified Hebbian learning, which enables autonomous learning without control circuitry, and an associative memory function is confirmed, which guarantees the possibility of further advanced functions. These results lead to astronomical large-scale integration (LSI) of synaptic elements in neuromorphic systems in the future.

Monolithic low-temperature fabrication of amorphous indium oxide thin-film transistors utilizing a selective hydrogen doping technique

Selective doping is a key technology for integrated circuit (IC) fabrication since it enables ultrahigh density transistors with various electrical characteristics to be integrated on a single substrate. Also, for vertical IC integration, low-processing temperature compatibility is an essential feature since the annealing of additional ICs fabricated on top should not compromise the characteristics of underlying ICs. For amorphous oxide semiconductor (AOS) thin-films, which are actively being pursued for vertical integration, a selective doping process combined with a low temperature activation thermal annealing process has not been demonstrated. In this Letter, we demonstrate a low-temperature selective doping process that enables selective multi-level n-type doping of insulating amorphous In2O3 (a-In2O3) thin-films. By applying multiple H plasma processes on lithographically defined areas, a monolithic planar In2O3 inverter circuit was demonstrated. Also, the doping process was used to fabricate and demonstrate the operation of an In2O3 thin-film transistor with a 30 nm spaced channel, showing that our process may be used for back-end-of-line vertical integration of AOS devices.

PRISA: a user-friendly software for determining refractive index, extinction co-efficient, dispersion energy, band gap, and thickness of semiconductor and dielectric thin films

A user-friendly software PRISA has been developed to determine optical constants (refractive index and extinction co-efficient), dispersion parameters (oscillator energy and dispersion energy), absorption co-efficient, band gap and thickness of semiconductor and dielectric thin films from measured transmission spectrum, only. The thickness, refractive index, and extinction co-efficient of the films have been derived using Envelope method proposed by Swanepoel. The absorption co-efficient in the strong absorption region is calculated using the method proposed by Connel and Lewis. Subsequently, both direct and indirect bandgap of the films is estimated from the absorption co-efficient spectrum using Tauc plot. The software codes are written in Python and the graphical user interface is programmed with tkinter package of Python. It provides convenient input and output of the measured and derived data. The software has a feature to cross check the results by retrieving transmission spectrum using the values of refractive index, extinction co-efficient, and thickness obtained from Envelope method. The performance of the software is verified by analyzing numerically generated transmission spectra of a-Si:H amorphous semiconductor thin films, and experimentally measured transmission spectra of electron beam evaporated HfO2 dielectric thin films as examples. PRISA is found to be much simpler and accurate as compared to the other freely available softwares. To help researchers working on thin films, the software is made freely available at https://www.shuvendujena.tk/download.

Amorphous metal oxide semiconductor thin film, analog memristor, and autonomous local learning for neuromorphic systems

Artificial intelligence is a promising concept in modern and future societies. Presently, software programs are used but with a bulky computer size and large power consumption. Conversely, hardware systems named neuromorphic systems are suggested, with a compact computer size and low power consumption. An important factor is the number of processing elements that can be integrated. In the present study, three decisive technologies are proposed: (1) amorphous metal oxide semiconductor thin films, one of which, Ga–Sn–O (GTO) thin film, is used. GTO thin film does not contain rare metals and can be deposited by a simple process at room temperature. Here, oxygen-poor and oxygen-rich layers are stacked. GTO memristors are formed at cross points in a crossbar array; (2) analog memristor, in which, continuous and infinite information can be memorized in a single device. Here, the electrical conductance gradually changes when a voltage is applied to the GTO memristor. This is the effect of the drift and diffusion of the oxygen vacancies (Vo); and (3) autonomous local learning, i.e., extra control circuits are not required since a single device autonomously modifies its own electrical characteristic. Finally, a neuromorphic system is assembled using the abovementioned three technologies. The function of the letter recognition is confirmed, which can be regarded as an associative memory, a typical artificial intelligence application.

(Invited) Atomic Layer Deposited Amorphous Igzo Semiconductor with Vertical Composition Engineering for High Mobility Field Effect Transistor

Amorphous oxide semiconductors have been widely studied for the potential use in flat-panel displays such as active matrix liquid crystal display (LCD) and Organic light emitting didoes (OLEDs). Since reporting amorphous InGaZnO semiconductor thin film transistor (TFT) in 2003 & 2004, many multi-component oxide semiconductors have been intensively investigated and developed by physical vapor deposition (PVD) method. Very recently, the sputtered InGaZnO TFTs are already adopted in mass-production to fabricate AMOLED TVs. Also, semiconductor industries have looked at metal oxide semiconductor thin films as an alternative Si channel layer because this semiconductor exhibited extremely low-off current level (below 10-18A/um). It is the unique property to decrease power consumption for various semiconductor devices. However, there remain several problems such as enhancing field effect mobility, composition uniformity, bias-temperature stability, and p-type oxide semiconductor in order to go further for the future device applications.In this talk, I will introduce various oxide semiconductor materials with atomic layer deposition (ALD) methods. Unlike PVD, ALD method has unique properties, including controlling atomic-level-thickness, manipulating atomic-level-composition control, and depositing impurity-free & uniform films. The key materials such In2O3, ZnO, SnO2, and Ga2O3 will be summarized and then I will show the combinatorial research works in terms of amorphous InGaZnO semiconductor thin film transistors. Generally, electrical properties of amorphous oxide semiconductors are strongly dependent on each element composition. The indium (In) rich InGaZnO exhibited a high mobility of 50cm2/V.sec. However, the In-rich InGaZnO TFTs have the demerit of negative threshold voltage owing to intrinsically high carrier density. I will present that In-rich IGZO TFTs with vertical stacked InOx, ZnOx and GaOx atomic layers show excellent performances such as saturation mobility of ~ 74cm cm2/V.sec, threshold voltage of -1.3V, on/off ratio of 8.9 × 108, subthreshold swing of 0.26V/decade, and hysteresis of 0.2V, while keeping a reasonable carrier density of ~1017 cm-3. Flexible ALD InGaZnO TFTs on polymer substrates were also discussed depending on gate insulator materials, in terms of device performances. ALD technique will be a very promising candidate to improve oxide semiconductor materials as well as various device applications (Display & Semiconductor Industry) because it has not only unique nano-growth mechanism but also ultimate uniform property.

Photoconductive laser spectroscopy as a method to enhance defect spectral signatures in amorphous oxide semiconductor thin-film transistors

Defects in semiconductor thin-films often leave optical spectral signatures that can be used for their identification. In this letter, we report on spectrally resolved photoconductivity measurements of amorphous oxide semiconductor thin-film transistors. In contrast to previously reported photoconductive spectroscopy measurements recorded using spectrally filtered broadband light sources, we used a wavelength tunable picosecond laser to illuminate the thin-film. We extracted the absorption coefficient as a function of wavelength from the photocurrent measurement and showed that it followed the typical characteristic behaviour previously reported for amorphous oxide semiconductor thin-films. However, in addition, we observed several sharp spectral peaks in the photoconductivity spectrum which can be associated with sub-bandgap defects. These enhanced peaks are not normally visible in previously reported photoconductivity spectra. Furthermore, we show that we can control the sensitivity of our measurement by changing the applied gate bias voltage when the thin-films were fabricated into transistors. The enhancement achieved by using the wavelength tunable laser makes this a particularly sensitive characterisation tool and can additionally be used to discriminate between defects which have been incorporated after device fabrication.

The effect of Se/Te ratio on transient absorption behavior and nonlinear absorption properties of CuIn0.7Ga0.3(Se1−xTex)2 (0 ≤ x ≤ 1) amorphous semiconductor thin films

The characterization of the CuInSe2 (CIS), CuInGaSe (CIGS) and CuGaSe2 (CGS) based semiconductor thin films are very important role for solar cell and various nonlinear optical applications. In this paper, the amorphous CuIn0.7Ga0.3(Se1−xTex)2 semiconductor thin films (0 ≤ x ≤ 1) were prepared with 60 nm thicknesses by using vacuum evaporation technique. The nonlinear absorption properties and ultrafast transient characteristics were investigated by using open aperture Z-scan and ultrafast pump-probe techniques. The energy bandgap values were calculated by using linear absorption spectra. The bandgap values are found to be varying from 0.67 eV to 1.25 eV for CuIn0.7Ga0.3Te2, CuIn0.7Ga0.3Se1.6Te0.4, CuIn0.7Ga0.3Se0.4Te1.6 and CuIn0.7Ga0.3Se2 thin films. The energy bandgap values decrease with increasing telluride (Te) doping ratio in mixed CuIn0.7Ga0.3(Se1−xTex)2 films. This affects nonlinear characteristics and ultrafast dynamics of amorphous thin films. Ultrafast pump-probe experiments indicated that decreasing of bandgap values with increasing the Te amount switches from the excited state absorption signals to ultrafast bleaching signals. Open aperture Z-scan experiments show that nonlinear absorption properties enhance with decreasing bandgaps values for 65 ps pulse duration at 1064 nm. Highest nonlinear absorption coefficient was found for CuIn0.7Ga0.3Te2 thin film due to having the smallest energy bandgap.

Stress originating from nanovoids in hydrogenated amorphous semiconductors

Structural inhomogeneities in the form of voids of nanometer sizes (nanovoids) have long been known to be present in hydrogenated amorphous semiconductors (Si, Ge). The physical and electrical properties of hydrogenated amorphous semiconductors can be pronouncedly influenced by the presence and characteristics of such nanovoids. In this work, by measuring in situ the intrinsic stress developments during deposition of pure, amorphous and of hydrogenated amorphous semiconductor (Si, Ge) thin films, under the same conditions in ultrahigh vacuum and on a comparative basis, a major source of tensile stress development could be ascribed to the occurrence of nanovoids in a-Si:H and a-Ge:H. The measurements allowed a quantitative evaluation of the surface stress acting along the surface of the nanovoids: 1.1–1.9 N/m for a-Si:H and 0.9–1.9 N/m for a-Ge:H.

Effects of Al grain size on metal-induced layer exchange growth of amorphous Ge thin film on glass substrate

Metal-induced layer exchange (MILE) has attracted increasing attention as a way to lower the crystallization temperature of amorphous semiconductor thin films on insulating substrates. This paper demonstrates that the quality of the catalytic Al layer strongly influences the growth properties in the MILE of amorphous Ge. The growth velocity of the MILE significantly decreases with increasing the deposition temperature of Al (TAl: RT–200°C), while the grain size of crystallized Ge becomes maximum (28μm) at TAl=100°C. This behavior is attributed to the Al grain size depending on TAl, which influences both the nucleation frequency and the lateral growth velocity of Ge in Al. These findings give new insight into MILE for fabricating high-quality semiconductor thin films at low temperatures on inexpensive substrates.

Unified Analytic Model for Current–Voltage Behavior in Amorphous Oxide Semiconductor TFTs

We present a simple and semi-physical analytical description of the current-voltage characteristics of amorphous oxide semiconductor thin-film transistors in the above-threshold and sub-threshold regions. Both regions are described by single unified expression that employs the same set of model parameter values directly extracted from measured terminal characteristics. The model accurately reproduces measured characteristics of amorphous semiconductor thin film transistors in general, yielding a scatter of <;4%.

Solution processed amorphous InGaZnO semiconductor thin films and transistors

Amorphous indium gallium zinc oxide (a-IGZO) semiconductor thin films and transistors were deposited on alkali-free glasses by the sol–gel route. The atomic ratio of In:Ga:Zn in the solution was 0.7:0.3:1. In this study, the effects of annealing temperature on the structural, surface condition, optical transmittance, and electrical resistivity of a-IGZO semiconductor thin films were investigated. GIXRD measurements and TEM-NBD analysis indicated that all annealed IGZO thin films had an amorphous phase structure. The dried IGZO sol–gel films annealed at a temperature higher than 425°C had a flat surface and exhibited high transparency (>89%) in the visible region. According to results from TGA, FT-IR and XPS, the residual organic compounds in the dried IGZO sol–gel films were completely removed at the annealing temperatures higher than 450°C. Therefore, we chose the 450°C annealed thin film as the active channel layer in the bottom-gate, bottom-contact (BGBC) thin-film transistor (TFT) in the present study. Current–voltage (I–V) characteristics of the 450°C annealed a-IGZO TFT revealed that it operated in n-type behavior with a positive threshold voltage (enhancement mode).

Unified Subthreshold Coupling Factor Technique for Surface Potential and Subgap Density-of-States in Amorphous Thin Film Transistors

We report a unified subthreshold coupling factor technique for a simultaneous extraction of the surface potential $(\psi_{\rm S})$ and the subgap density-of-states [DOS: $g(E)$ ] over the bandgap in amorphous semiconductor thin film transistors (TFTs). It is fully based on the experimental gate bias-dependent coupling factor $[m(V_{\rm GS})]$ under subthreshold bias. Through the proposed technique only with current-voltage data under subthreshold operation, a unified extraction of the DOS with a consistent mapping of the gate bias $(V_{\rm GS})$ to the subgap energy is obtained. Applying to amorphous InGaZnO TFTs, $g(E)$ is obtained to be a superposition of two exponential functions with $N_{\rm TA}=1.62\times 10^{17}~{\rm eV}^{-1}~{\rm cm}^{-3}$ and $kT_{\rm TA}=0.026~{\rm eV}$ for the tail states while $N_{\rm DA}=6.5\times 10^{16}~{\rm eV}^{-1}~{\rm cm}^{-3}$ and $kT_{\rm DA}=0.22~{\rm eV}$ for the deep states.

Analysis and evaluation techniques of amorphous oxide semiconductor thin films

Analysis and evaluation techniques of amorphous oxide semiconductor thin films

Modeling of spiking analog neural circuits using organic semiconductor thin film transistors with silicon oxide nitride semiconductor gates

This paper uses the results of the characterization of amorphous semiconductor thin film transistors (TFTs) with the quasi-permanent memory structure referred to as silicon oxide nitride semiconductor (SONOS) gates, to model spiking neural circuits. SONOS gates were fabricated and characterized. In addition, MOSFETs using organic copper phthalocyanine (CuPc) were fabricated with these SONOS gates to demonstrate proof of concept performance. Analog spiking circuits were then modeled using these low performance TFTs to demonstrate the general suitability of organic TFTs in neural circuits. The basic circuit utilizes a standard comparator with charge and discharge circuits. A simple Hebbian learning circuit was added to charge and discharge the SONOS device. The use of these elements allows for the design and fabrication of high-density 3-dimensional circuits that can achieve the interconnect density of biological neural systems.

Local structure and local conduction paths in amorphous (In,Ga,Hf)–ZnO semiconductor thin films

The local structure and local conduction paths of Ga–In–Zn–O (GIZO) and Hf–In–Zn–O (HIZO) amorphous thin films were investigated by the extended X-ray absorption fine structure (EXAFS). We found that the local hindrance paths of In–Ga and In–Hf exist in the conduction paths of amorphous GIZO and HIZO semiconductor thin films, respectively.

Determination of the Optical Bandgap of Thin Amorphous (SiC)&lt;sub&gt;1-x&lt;/sub&gt;(AlN)&lt;sub&gt;x&lt;/sub&gt; Films

Amorphous wide bandgap semiconductor thin films of the pseudobinary compound (SiC)1-x(AlN)x were grown by radio frequency dual magnetron sputtering on CaF2, MgO and glass substrates. We performed isochronical annealing steps up to 500°C. The optical bandgap is determined for each composition from spectroscopic transmission measurement in two different ways: according to Tauc and using the (αhν)2 plot. The dependence of the optical bandgap on the composition x can be described by Vegard’s empirical law for alloys.