Andreas Bablich

Heterojunction Hybrid Devices from Vapor Phase Grown MoS2

We investigate a vertically-stacked hybrid photodiode consisting of a thin n-type molybdenum disulfide (MoS2) layer transferred onto p-type silicon. The fabrication is scalable as the MoS2 is grown by a controlled and tunable vapor phase sulfurization process. The obtained large-scale p-n heterojunction diodes exhibit notable photoconductivity which can be tuned by modifying the thickness of the MoS2 layer. The diodes have a broad spectral response due to direct and indirect band transitions of the nanoscale MoS2. Further, we observe a blue-shift of the spectral response into the visible range. The results are a significant step towards scalable fabrication of vertical devices from two-dimensional materials and constitute a new paradigm for materials engineering.

Optimizing the optical and electrical properties of graphene ink thin films by laser-annealing

We demonstrate a facile fabrication technique for graphene-based transparent conductive films. Highly flat and uniform graphene films are obtained through the incorporation of an efficient laser an ...

FIB-assisted a-SiGe:H/a-SiC:H alloy analysis for ultra-low biased multispectral pi x n sensors with enhanced color separation features and low reflective ZnO:Al back-contacts

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.

Systematic comparison of metal contacts on CVD graphene

An experimental study was conducted for forming high quality ohmic contacts to graphene. Metal contacts of platinum/gold (Pt/Au), nickel/gold (Ni/Au), palladium (Pd), Ni, and Au to monolayer chemical vapor deposited graphene were studied. The experimental data reveal that pure Au and Ni/Au provide highly reproducible low resistance ohmic contacts. The results presented in this work indicate potential contact metals suitable for high frequency electronic devices.

Spectral sensitivity of a graphene/silicon pn-junction photodetector

We investigate the optical properties of graphene-silicon Schottky barrier diodes composed of chemical vapor deposited (CVD) graphene on n- and p-type silicon (Si) substrates. The diodes fabricated on n-Si substrate exhibit better rectifying behavior compared to p-Si devices in the dark. An ultra-broadband spectral response is achieved for n-Si diodes. The results are compared with the spectral response of a molybdenum disulfide (MoS2) - p-type silicon photodiode.

Graphene / a-Si:H multispectral photodetectors

We demonstrate the integration of large area graphene transparent conductive electrodes in flexible amorphous silicon multispectral (MS) photodetectors (PD). These MS diodes show a bias dependent maximum of their spectral response between the ultraviolet (UV) and visual wavelength range. This ability to shift the response maximum by external bias without the use of filter-structures and the possibility to deposit the device structure directly onto flexible polyimide substrates gives these sensors wide applicability as one-pixel color detectors. Graphene MS PDs show an extended detection range well into the UV when compared to rigid devices with aluminum doped zinc oxide electrodes. The integration of bi-layer graphene contacts boosts the maximum spectral response up to 185 mAW-1, which is an improvement of 150 % compared to sensors with just one atom thick graphene contacts.

Flexible graphene-/a-Si:H multispectral photodetectors

Transparent conductive electrodes (TCEs) are ubiquitous and essential for photonic devices like solar cells, touchscreens and photodetectors. An increasing demand for TCEs may be met with graphene, which promises low production cost and an abundance of raw material and may enable flexible optoelectronic devices. We report amorphous silicon (a-Si:H) multispectral photodetectors with a bias-tunable maximum spectral response on rigid and flexible substrates with graphene TCEs. Electrical and optical measurements compare reasonably well to conventional devices with transparent conductive oxide (TCO) TCEs (here: ZnO:Al, [3]), reaching over 50% of their responsivity. Graphene enables flexible multispectral photodiodes, in contrast to the brittle ZnO:Al. A further decisive advantage of the graphene TCEs is a broader spectral response into the UV region, which is otherwise limited by the absorption in the ZnO:Al. Artifacts due to refraction in the 220 nm thick ZnO:Al are suppressed in the atomically thin graphene TCEs. Bilayer graphene (BLG) electrodes improve the responsivity considerably.

Chemical Vapor Deposited Graphene for Opto-Electronic Applications

In this work, fabrication and characterisation of graphene photodiodes and transfer length method structures is presented. Graphene growth is carried out using a thermal chemical vapor deposition process on copper foils and subsequently transferred onto silicon-dioxide/silicon substrate. Comparison of electrical and optical characteristics of the photodiodes, which are fabricated on both n-type and p-type silicon, is shown. The photodiodes fabricated on n-type silicon show good rectifying behaviour when compared with photodiodes fabricated on p-type silicon. Spectral response of graphene photodiodes is measured to be less than 0.2 mAW-1 which is attributed to the light absorbance of 2.3% for single layer graphene. Transfer length method device structures are also fabricated and contact resistance is calculated and plotted as a function of spacing between the contacts. The calculated contact resistance (RcW) is 0.87 kΩ.µm. The latter structures are also characterised under various ambient conditions, before and after annealing. The value of contact resistance reduces from 0.87 kΩ.µm to 0.75 kΩ.µm after annealing. This reduction is attributed to the improvement in bonding between graphene and metal. Measurements under vacuum show an increase in contact resistance which is attributed to the removal of adsorbed water molecules on the surface on graphene. The sheet resistivity of graphene is calculated to be between 1.17 kΩ/□ and 3.67 kΩ/□.

Systematic study of the palladium-graphene contact

We report a systematic study of the contact resistance present at the interface between palladium (Pd) and monolayer graphene measured at different conditions. Measurements in vaccum appear to increase the contact resistance. However, this is attributed to a shift of the charge neutrality point due to a reduction of random molecular doping and/or humidity. Post-processing rapid thermal annealing (RTA) was carried out to study its influence on the contact resistance. The contact resistance is reduced by approximately 50% after RTA at 450°C in hydrogen/argon (5%/95%) environment.