Compact, highly accurate and fast optical systems for three-dimensional image acquisition are becoming increasingly important in various future relevant applications. A special optoelectronic detector that can be used for distance measurement is the Photonic Mixer Device (PMD). We developed a graphene-based PMD, which benefits from the advantages of graphene (high charge carrier mobility and broadband absorption spectrum) and results in a broadband PMD with enhanced depth resolution. Our graphene PMD has a mirror-symmetrical design and combines two photodetectors in such a way that they have a common source contact made of palladium and two separate drain contacts made of titanium. The charge carrier swing, which is the main feature of a standard PMD, is realized via two separate buried gate contacts. Two versions of the graphene PMD, one silicon-substrate based and the other glass-substrate based, were fabricated. The fabrication steps as well as measurements of the electrical behavior and the photocurrent of the manufactured graphene PMDs are presented.
In the recent past the generation and processing of multispectral data have had an immense impact on optical
characterization systems. A virtual test environment is used to examine which bands provide a high information density.
The photocurrent j = ∫ E(λ)*Sabs(λ)*r(λ) dλ was calculated for different light sources E, spectral response curves Sabs
(bands), and the reflectance r of whitish powder samples that were suspected to be dangerous or illegal. The multivariate
dataset will have to be determined whether we can gain any knowledge from this. The employed factor analysis is a
common method of the group of structure-discovering methods and provides good results in the discovery of connections
between parameters. It is particularly used if a variety of parameters must be reduced for some reason. For the
verification, a dimension of the external separation is defined. To carry out this an n-dimensional vector P must be
assigned to each measurement that is registered in the matrix M to determine the volume V of this dot cloud. The
dimension normalized volume is defined as ΔCL, where n is the quantity of employed bands. The reliability of the
complete measurement system is made by a membership function μ(P) comparable to the definitions from the area of the
fuzzy sets. The parameter μ indicates with which reliability a measured pattern P could be assigned to a sample S from a
dataset. The use of such optimized multispectral photodiodes would simplify and accelerate the identification of
potentially dangerous substances.
Common security CCD and CMOS imaging systems are not able to distinguish colorimetrically between dangerous
chemical substances, for example whitish powders [1]. Hydrogenated amorphous silicon (a-Si:H) with profiled bandgaps
can be found in solar cells to optimize the collection of incoming photons [2]. We developed multicolor photodiodes
based on a-Si:H with different spectral response characteristics for a reliable, fast, cheap and non-destructive
identification of potentially dangerous substances. Optical and I-V measurements were performed to explore the effect of
combining linearly graded a-SiC:H-/a-SiGe:H layers with low reflective aluminum doped zinc oxide (ZnO:Al) cathodes.
We determined absorption coefficients and mobility-lifetime products (μτ) of graded and non-graded absorbers to
calculate the penetration depth of photons at different energies into the device structure. This set of parameters enables
an optimization of the intrinsic layers so that charge accumulations are generated precisely at defined device depths.
Significant color separation improvements could be achieved by using ZnO:Al cathodes instead of commonly used
ZnO:Al/Chromium (Cr) reflectors. As a result, we obtained multicolor diodes with highly precise adjustment of the
spectral sensitivity ranging from 420 nm to 580 nm, reduced interference fringes and a very low reverse bias voltage of
-2.5 V maximum. Three terminal device architectures with similar absorbers exhibit a shift from 440 nm to 630 nm by
applying reverse voltages of, for instance, -11.5 V at 580 nm [3]. Present research efforts concentrate on further
improvements of the absorption region to reduce the bias without affecting the optical sensor performance, using
extensive bandgap engineering techniques.
Optical detection is an often used technique for recognition of potentially dangerous materials. Hydrogenated amorphous
silicon (a-Si:H) technology provides an inexpensive alternative material compared to crystalline silicon for being used in
photonic devices operating in the visible spectrum. Further materials' key benefits are the high light absorption, the
voltage-tunable spectral sensitivity and the high space efficiency. Present research efforts concentrate on the
determination of the color information in a-Si:H photodiodes. This work presents an approach to improve color
recognition of a-Si:H photodiodes by modifying the layer sequence.
The maximum of the spectral response (SR) of a single i-layer a-Si:H photodiode can be shifted by varying its bias
voltage. In this case, the shift is not more than some nanometers. Precise color recognition requires different SR maxima
(e.g. RGB-model). One possibility to accomplish a separation of the SR is to engineer the bandgap; another idea, which
is presented here, is based on a layer sequence modification. Normally, the SR at higher reverse bias voltages, with the
maximum at longer wavelengths, encloses that at lower voltages. Splitting the SR leads to an improvement of color
recognition and is achieved by depositing an additional interior anode. The SR maximum shift amounts to 100nm, from
570nm by contacting the interior anode, to 670nm at the top anode. Furthermore, the curves are clearly split. The
presented approach should lead to a tunable multi-spectral photodiode for high quality color recognition. Such a diode
can be used in photonic devices, e.g. for safety and security applications.
A fast and reliable detection of potentially dangerous substances has become very important in ensuring civilian security.
Currently, modern security systems have proven to be more effective on the basis that objects should be properly
characterized and identified. For instance, chemical tests are used to identify samples of whitish powder that is suspected
to be dangerous or illegal. Although these chemical tests are conducted very quickly, they are relatively expensive.
However, well established methods of optical characterization offer a suitable alternative. The demand for low-cost and
disposable devices have escalated the development of intelligent photodiodes, especially of tunable a-Si:H multispectral
photodiodes1. Our aim of reengineering is to develop the best match for the spectral response adjustment. Unfortunately,
it is not sufficient to optimize the spectral response only. The top down design flow begins with the calculation of the
photocurrent for different combinations of light sources, spectral responses and whitish powder samples to build up a
multivariate data set. The optimum combination is found at the point of intersection in the factor values in a 2-D
scattergram. It is therefore, required that the use optimized photodiodes would simplify and accelerate the identification
of potentially dangerous substances.
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