E. Bunte

Silicon-based micro-Fourier spectrometer

A novel Fourier spectrometer based on a partly transparent thin-film detector in combination with a tunable silicon micromachined mirror was developed. The operation principle based on the detection of an intensity profile of a standing-wave by introducing a partly transparent detector in the standing-wave. Varying the position of the mirror results in a phase shift of the standing-wave and thus in a change of the optical intensity profile within the detector. The photoelectric active region of the sensor is thinner than the wavelength of the incoming light, so that the modulation of the intensity leads to the modulation of the photocurrent. The spectral information of the incoming light can be determined by the Fourier transform of the sensor signal. Based on the linear arrangement of the sensor and the mirror, the spectrometer facilitates the realization of one- and two-dimensional arrays of spectrometers combining spectral and spatial resolution. The operation principle of the spectrometer will be described and the influence of the detector design on the spectrometer performance will be discussed. A spectral resolution of down to 6 nm was achieved under real-time imaging conditions.

Influence of contact effect on the performance of microcrystalline silicon thin-film transistors

Microcrystalline silicon thin-film transistors were prepared by plasma-enhanced chemical vapor deposition at substrate temperatures below 200°C. The transistors exhibit electron mobilities of 38cm2∕Vs, threshold voltages in the range of 2V, and subthreshold slopes of 0.3V∕decade. Despite the realization of transistors with high carrier mobility, contact effects limit the performance of the transistors. The influence of the drain and source contacts on device parameters including the mobility, the threshold voltage, and the subthreshold slope will be discussed in detail.

Standing-wave interferometer

An interferometric position sensor was developed using the concept of sampling a standing wave. Interference of a standing wave created in front of a plane mirror can be detected by thin, partly transparent sensors based on amorphous silicon. The optical thickness of the absorption layer is thinner than the wavelength λ of the incident light. Detection of minima and maxima of the standing wave can be used to determine the relative displacement of the plane mirror and the detector. For determination of bidirectional fringe counting, two detectors with a certain phase shift were introduced into the standing wave. An integrated solution of two stacked n-i-p diodes and a phase shifter will be presented. The operation principle of the device will be demonstrated by measured Lissajous figures.

Standing wave detection by thin transparent n-i-p diodes of amorphous silicon

Abstract Interferences of a standing wave created in front of a plane mirror can be detected by thin transparent sensors based on a n–i–p layer sequence with an optical thickness of the i-layer thinner than the wavelength λ of the incident light. The detection of the minima and maxima of a standing wave can be used to determine the relative displacement of the plane mirror towards the detector. The optoelectronic properties of thin transparent detectors are studied regarding reflection and transmission, capacitance, and ability to distinguish between the minima and maxima of the standing wave. The developed sensors with a high transmittance consist of thin diodes of amorphous silicon (a-Si:H) and/or silicon carbide (a-SiC:H) with an optical thickness of λ/2 embedded between two transparent conductive oxide layers with an optical thickness of 3λ/4. The highest yield and the best device performance are achieved for detectors with an absorber layer of a-SiC:H.

Influence of low temperature thermal annealing on the performance of microcrystalline silicon thin-film transistors

Top-gate staggered microcrystalline silicon thin-film transistors (μc-Si:H TFTs) were prepared by plasma enhanced chemical vapor deposition at temperatures below 200°C. The μc-Si:H TFTs exhibit high effective electron mobilities (device mobilities) of up to 35cm2∕Vs for long channel devices. Due to the high carrier mobility μc-Si:H TFTs are promising devices for large area electronics such as organic light-emitting diode displays or radio frequency identification devices. The fabrication process of the μc-Si:H TFTs is similar to the fabrication process of amorphous silicon thin-film transistors, which facilitates an easy transfer of the technology to industry. In this paper, the influence of postfabrication low temperature thermal annealing (150°C) on the device properties of top-gate staggered μc-Si:H TFTs is investigated. Low temperature thermal annealing reduces the device threshold voltage and subthreshold slope. Furthermore, the annealing step results in an increase of the effective mobility for long...

Observation of the Evolution of Etch Features on Polycrystalline ZnO:Al Thin-Films

The transparent conducting oxide (TCO) ZnO:Al is often used as the window layer and a source of light trapping in thin-film silicon solar cells. Light scattering in sputtered zinc oxide is achieved by wet chemical etching, which results in craters distributed randomly over the ZnO surface. To gain a better understanding of the etching process on ZnO thin films, a method for atomic force microscope (AFM) realignment between etching steps is developed. Using this method, the evolution of the HCl etch on a polycrystalline ZnO thin-film is observed. Results showed that this observation method did not modify the etching behavior, nor did stopping and restarting the etching change the points of attack, indicating that the points of HCl attack are built into the films as they are grown. Additionally, we investigated the evolution of the HCl etch on a ZnO surface previously etched in KOH, and found that the etch sites for both the acidic and basic solution are identical. We conclude that “peculiar” defects, which induce accelerated etching, are built into the film during growth, and that these defects can extend part or all the way though the thin-film in a similar way as screw dislocations in single crystalline ZnO.

Optimization of phase-sensitive transparent detector for length measurements

A phase selective partly transparent detector (PSTD) enables length measurement with nm-accuracy by sampling an optical standing wave. The PSTD consists of two transparent n-i-p photodiodes of amorphous silicon (a-Si:H) which are embedded between three transparent conductive oxide (TCO) layers. The two photodiodes measure the intensity of an optical standing wave by means of absorption layers with thicknesses below 50 nm and thus, provide two photocurrents which are proportional to the intensity at their individual positions. For an optimization of the device performance, simulations based on a standard electromagnetic formalism were performed. The considered thin-film structure is a glass/TCO/n-i-p/TCO/n-i-p/TCO layer sequence. The aim was to design a layer stack which avoids significant distortions of the standing wave while the phase shift between the photocurrents approximately amounts to 90/spl deg/, since this will minimize the measurement error. The comparison of experimentally determined and simulated data shows that a further adjustment of the fabricated PSTD into an ideal thickness scheme is necessary to enhance the device performance.

Preparation and topography analysis of randomly textured glass substrates

Randomly textured glass sheets in combination with a transparent conductive layer are promising as front contacts for silicon based thin-film solar cells. The authors have developed a novel method to create randomly textured glass surfaces. For the fabrication a wet chemically textured zinc oxide (ZnO) film is used as three dimensional etch mask and its surface topography is transferred to the glass substrate by ion beam etching. The typical texture of the sputtered and etched ZnO film exhibits craterlike features, which can be tuned by a variation of the initial ZnO film thickness and the etching time. Thus, the surface features of the resulting rough glass can be varied within a wide range. Its topography exhibits a maximum root mean square roughness of more than 200 nm with a lateral correlation length around 1500 nm. Microcrystalline silicon solar cells on the textured glass substrates show an increase of the short circuit current density by 36% compared to cells on flat glass. This proves the improve...

Fabrication of Anodic Aluminum Oxide Templates with Small Interpore Distances

The influence of the electropolishing and anodization voltage on surface morphology has been carefully studied for fabrication of ordered anodic aluminum oxide (AAO) templates. In accordance with that in the anodization experiment, the size of small patterns on the foil surface formed from the electropolishing treatment increases with voltage. Using a combined method of small-voltage electropolishing and anodization, we have fabricated well-ordered templates with much smaller interpore distances compared with that under normal-voltage fabrication conditions. The Ni nanowire arrays with two small diameters are electrodeposited through the above templates, exhibiting different magnetic properties. This also helps us to clarify the inner structure of this kind of templates.

Standing-wave spectrometer

A standing-wave sensor was developed which facilitates the miniaturization of Fourier spectrometers down to the micrometer scale. The spectrometer concept is based on sampling a standing wave by an ultrathin and partially transmissive sensor. The active region of the sensor has a thickness of 30nm–40nm. The standing wave is created in front of a tunable mirror. Varying the position of the mirror results in a phase shift of the standing wave, a modulation of the intensity profile within the ultrathin sensor, and thus, in a modulation of the photocurrent. The spectral information of the incoming light can be determined by the Fourier transform of the sensor signal. The operation principle of the spectrometer is described and the influence of the device design on the spectral resolution of the spectrometer is discussed. Due to the simple linear setup of the sensor and the tunable mirror, the realization of one- and two-dimensional spectrometer arrays is feasible.