Andrey Kosarev

MICROBOLOMETERS FABRICATED WITH SURFACE MICROMACHINING WITH a-Si-Ge: H THERMO-SENSING FILMS

In this work we present the process flow for the fabrication of un-cooled IR detectors employing surface micro-machining techniques over silicon substrates. These detectors are based on thin films deposited by plasma at low temperatures. The thermo sensing film used is an intrinsic a-SixGe1-x:H film, which has demonstrated a very high temperature coefficient of resistance (TCR), and a moderated resistivity, these properties are better than those of the a-Si:H intrinsic film, which is commonly used in commercial IR devices. Two device configurations have been designed and fabricated, labeled planar and sandwich. The former is the configuration commonly used in commercial micro-bolometers, while the latter is proposed in order to reduce the high cell resistance observed in this kind of devices, without the necessity of doping the intrinsic film, which results in a decrement of the TCR and therefore in responsivity. Finally some performance characteristics of the devices studied are discussed in comparison with data reported in literature.

Un-Cooled Microbolometers with Amorphous Germanium-Silicon (a-GexSiy:H) Thermo-Sensing Films

Silicon integrated circuits (IC) in conjunction with the micro-machining technology for thin films, have opened new ways for the development of low cost and reliable night vision systems based on thermal detectors. Among the thermal detectors used as pixels on IR focal plane arrays, the microbolometer appears as one of them. A microbolometer is a device in which the IR transduction is performed through a change in the resistivity of its thermosensing material, due to the heating effect caused by the absorbed radiation. Among the requirements for the materials used as thermo-sensing layer in microbolometers it can be mentioned a high activation energy (Ea), high temperature coefficient of resistance (TCR), low noise, and compatibility with standard CMOS fabrication processes. A variety of materials have been used as thermo-sensing elements in microbolometers, as vanadium oxide (VOx) (B. E. Cole, 1998, 2000), metals (A. Tanaka, 1996), polycrystalline (S. Sedky, 1998) and amorphous semiconductors (A. J. Syllaios, 2000).

Study of electrical conductivity of PEDOT:PSS at temperatures >300 K for hybrid photovoltaic applications

Temperature dependence of DC conductivity σDC(T) of spin-coated poly (3,4 ethylenedioxythiophene): poly(4-styrenesulfonate) (PEDOT:PSS) films has been studied. σDC(T) was measured in films deposited from mixtures of PEDOT/PSS: H₂O (1:0.5), PEDOT:PSS (without dilution), PEDOT/PSS: Isopropyl Alcohol (IPA) (1:0.5) and PEDOT/PSS:IPA (1:1). Room temperature conductivity σ_RT of PEDOT/PSS films is enhanced from 4.46×10^-5 S/cm to 6.07×10^-4 S/cm after dilutions with H₂O and IPA (1:1), respectively. Experimental data is fitted with two different transport models: thermal activated conduction and variable range hopping (VRH) model. It is found that σ_DC(T) of PEDOT/PSS:IPA (1:1) sample can be described by the one-dimensional (1D) VRH model with an pre-exponential factor σ₀=28.5 S/cm and material-dependent parameter T₀=39,204 K.

Effect of hydrogen dilution on morphology and electronic properties of silicon-germanium films deposited by RF plasma discharge

In this work we study the influence of hydrogen dilution on morphological properties and electronic of SiGe:H films. Surface morphology measured by Atomic Force Microscopy (AFM) technique and temperature dependence of conductivity, together with conductivity measurements in dark and under AM1.5 illumination at room temperature were used for characterization. Dark conductivity and Fermi energy varied from 10^-8 Ω^-1cm^-1 to 10^-4 Ω^-1cm^-1 and from 0.45 to 0.08 eV, respectively, with changing hydrogen dilution from R_H =9 to.80.

SIMS analysis of atomic composition of silicon-germanium films deposited by RF plasma discharge

In this work we study the atomic composition in SiGe:H films as a function of hydrogen dilution. Silane and germane fluxes are kept constant while hydrogen flow is varying to achieve dilutions up to 80 times. Solid content of principle elements as well as contaminants are determined by SIMS. For low dilutions (from 9 to 30), germanium solid content strongly depends on hydrogen dilution since it varies from 0.485 to 0.675. Higher presence of hydrogen in the mixture does not change germanium content which remains close to the value of 0.69.

Electronic properties of Ge y Si 1−y :H films deposited by LF PECVD at low temperatures

This paper reports the study of Ge<inf>y</inf>Si<inf>1−y</inf>:H films deposited at temperatures in the range of T<inf>d</inf>= 70 to 300 °C. The films were grown in capacitive low-frequency (f=110 KHz) discharge from Si:H<inf>4</inf> and Ge:H<inf>4</inf> feed gases diluted with H<inf>2</inf>. Other parameters were as follow: hydrogen flow Q<inf>H2</inf>= 3750 sccm, silane flow Q<inf>SiH4</inf>=50 sccm, germane flow Q<inf>GeH4</inf>= 500 sccm hydrogen dilution ratio R= Q<inf>H2</inf>/(Q<inf>SiH4</inf>+Q<inf>GeH4</inf>)=75, discharge power W= 300 Watt and pressure P= 0.76 Torr. The deposition rate of the films was varied not monotonically in the range of Td from 70 to 300 °C. Hydrogen bonding was characterized by Fourier transform infrared (FTIR) spectroscopy. The electrical parameters were extracted from the measurements of temperature dependence of conductivity σ(T). Activation energy and room temperature conductivity of the films were observed in the range of the values E<inf>a</inf>=0.27 to 0.37 eV and σ<inf>RT</inf>=5.7 × 10⁻⁵ to 9.6 × 10⁻⁴ Ω⁻¹ cm⁻¹, respectively. The electronic properties characterized by different electrical and optical measurements showed optimal properties within the deposition temperature range T<inf>d</inf>∼160 °C to 220 °C

Nano-structured GeySi1-y:H Films Deposited by Low Frequency Plasma for Photovoltaic Application

In this work we present the study of fabrication, Ge incorporation, structure and electronic properties of nano-structured Ge y Si 1-y :H films with y>0.5 prepared by low frequency (LF) PECVD. Ge y Si 1-y :H films were deposited by LF PECVD at a frequency f= 110 kHz from SiH 4 +GeH 4 +H 2 gas mixture. SiH 4 and GeH 4 flows were varied to fabricate the films in wide range of 0 H =20 to 80. Structure of the films was studied by AFM and SEM with consequent image processing to extract statistical parameters such as grain distribution and mean values. Composition of the films was characterized by SIMS and EDX. Electronic properties were characterized by temperature dependence of conductivity, spectral dependence of optical absorption. Sub-gap absorption was characterized by Urbach energy, E U; and defect absorption, α D . We observed grain like nano-structure with Gauss distribution of grain diameters by both AFM and SEM measurements. The most interesting films had mean grain diameter = 24.0±0.7 nm, dispersion D=11.0±0.2 nm and fill factor FF=0.313, Ge content y=0.96-0.97(by SIMS and EDS). These films showed also the lowest values of Urbach energy E U = 0.030 eV and low defect absorption α D = 5×10 2 cm −1 (at photon energy hv = 1.04 eV) indicating on low density of localized states in mobility gap. Doped films have been also fabricated and studied. Finally we shall discuss application of the above films in photovoltaic devices.

Boron Incorporation and Its Effect on Electronic Properties of Ge:H Films Deposited by LF Plasma

Boron (B) doping of plasma deposited silicon films have been widely studied and applied in many devices, while B-doping of germanium has been poorly reported in literature. We have reported previously about Ge:H films with low density of localized states deposited by LF plasma with optimal hydrogen dilution. This work is devoted to a study of boron incorporation and its effect on electronic properties in Ge:H films. The films were obtained by low frequency (LF) plasma deposition from GeH4+SiH4 +B2H6 mixture diluted with hydrogen. The deposition parameters were as follow: substrate temperature Ts = 300 oC, discharge frequency f= 110 kHz, pressure P= 0.6 Torr, power W= 300 W, germane flow QGeH4= 50 sccm, silane flow, hydrogen flow QH2=3500 sccm, diborane flow was varied in the range of QB2H6=0 to 20 sccm providing boron concentration in gas phase in the range of Y=0 to 4%. Composition of the films was characterized by SIMS profiling. Hydrogen bonding was studied by FTIR. Temperature dependence of conductivity measured in DC regime in vacuum thermostat was employed to study carrier transport. Optical measurements provided optical gap, sub-gap absorption and refraction index. Boron incorporation in solid film demonstrated fast increase in the range of Y = 0 to 1.4% and then increase became slower. Hydrogen concentration in the films was determined by absorption of Ge-H stretching mode at k ≈ 1870 cm −1 and it showed weak increase with change of Y from 0 to 4%. Activation energy of conductivity increased in the range of Y = 0 to 1.5% suggesting a compensation of electron conductivity, reaching maximum value E a =0.5 eV (corresponding approximately to E g /2) at Y= 1.5%. Then E a reduced to minimum value E a = 0.27 eV at Y= 3.5% showing a trend to saturation with further Y increase. This behavior is related to change of charge transport from electron to intrinsic at Y= 1.5% and further to hole transport.

Un-cooled Micro-bolometer with Sandwiched Thermo-sensing Layer Based on Ge Films Deposited by Plasma

An un-cooled micro-bolometer with thermo-sensing layer sandwiched between two electrodes has been fabricated and characterized. The micro-bolometer has a “bridge” configuration for providing sufficient thermo-isolation of the thermo-sensing layer. Surface micro-machining techniques were used for its fabrication onto a silicon wafer. The support layer is of SiN and the thermo-sensing layer is a-Ge:H,F film, both have been deposited by low frequency PE CVD technique. The active area of the thermo-sensing layer is AB = 70x66 m 2 . The temperature dependence of conductivity (T), current-voltage characteristics I(U), spectral noise density and thermal response time have been measured in order to characterize its performance characteristics. The measured activation energy of the thermo-sensing layer is Ea = 0.34 eV providing a thermal coefficient of resistance = 0.043 K -1 . The pixel resistance is in the range of RB = (1-30)x10 5 Ohm. Measured current and voltage responsivities are in the range of ℜI = 0.314 AW -1 and ℜU = (1-2)10 5 VW -1 , respectively. The estimated value of the detectivity is in the range of D * = (1-40)x10 8 cmHz 1/2 W -1 and the obtained response time is = 1x10 -4 sec. The performance characteristics obtained in this micro-bolometer with sandwiched thermo-sensing layer make it promising for further development of IR imaging systems.

AZO/PEDOT:PSS Polymer Frontal Interface Deposited on Flexible Substrates for a-Si:H Photovoltaic Applications

Thin-film hybrid organic-inorganic photovoltaic structures based on hydrogenated silicon (Si:H), poly(3,4ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) polymer Al-doped ZnO (AZO) films deposited on different types of flexible substrates have been fabricated and investigated. The compatibility of the polymer and inorganic materials regimes and deposition techniques used for device fabrication has been demonstrated on flexible substrates. Morphological characteristics of transparent Al-doped ZnO (AZO) films deposited on substrates have been measured by atomic force microscopy. Electronic characteristics of the fabricated photovoltaic structures have been measured and analyzed for different thicknesses of the transparent electrodes and different substrate types. Photovoltaic hybrid structure on polyethylene naphthalate (PEN) substrate showed the best characteristics: short circuit current density Jsc = 9.79 mA/cm2, open circuit voltage Uoc = 565 mV, and PCE η = 1.3%. To analyze the mechanisms governing the device performance, short circuit current density spectral dependence of the devices fabricated on different types of flexible substrates has been measured. As demonstrated by our analysis, the structures on PEN substrates, besides better substrate transmittance, also show better junction properties.