Albert Hammerschmidt

Scanning tunneling microscopy based lithography employing amorphous hydrogenated carbon as a high resolution resist mask

Amorphous hydrogenated carbon (a–C:H) is introduced as a constituent of a two–layer resist system for lithography with a scanning tunneling microscope (STM) operating in air. The resist is made up of a thin electron sensitive and chemically amplified top resist (≤50 nm) and a–C:H as a thick conducting and etchable bottom resist. In this setup the bottom resist acts as the counter electrode allowing in principle operation on insulating substrates. We show that it is possible to generate structures with high aspect ratios by transfering the developed top resist patterns by means of oxygen reactive ion etching (RIE) into the bottom resist and halogen RIE into silicon substrates. Linewidths between 100 and 50 nm have been observed in the bottom resist as well as in the substrates.

Determination of thermal properties of hydrogenated amorphous carbon films via mirage effect measurements

Abstract We present a new photothermal method using the mirage effect to measure the thermal properties of thin film materials in a contactless, non-destructive way without the need for sample preparation. A complete theoretical treatment of the mirage signal generation in the case of an homogeneous N-layer system allows the thermal conductivity and the thermal diffusivity to be deduced from experimental data. We investigated the thermal properties of a wide variety of diamond-like or amorphous hydrogenated carbon films deposited in a capacitively coupled parallel plate r.f.-plasma reactor.

Electrical and optical properties of heterostructures made from diamond-like carbon layers on crystalline silicon

Abstract Thin films of amorphous, diamond-like carbon (DLC) were deposited in a capacitively coupled RF-plasma discharge on n- or p-type silicon substrates with different bias voltages, pressures and flow rates. The produced DLC/Si heterostructures show a broad variety of electrical and optical properties depending on the deposition parameters. The investigations concentrated on C—V measurements and on the photo-electrical properties of the DLC/Si heterostructure. The quantum efficiency of the heterostructure DLC on crystalline p-type silicon (specific resistance 10–20 Ω · cm) is 0.5 at a wavelength of 980 nm and a reverse bias of 2.7 V.

Gap‐state measurements on diamond‐like carbon films

A high frequency capacitance‐voltage method is used to measure the electronic gap‐state density in diamond‐like carbon (DLC) films. The gap‐state density is derived from the analysis of high frequency capacitance‐voltage characteristics of DLC on crystalline silicon (c‐Si) heterojunctions. Near the Fermi level the gap‐state density in the DLC thin film is obtained of the order 1016 cm−3 eV−1. Furthermore, the energy‐band diagram of a DLC/c‐Si heterojunction is evaluated, and the work function of DLC is derived to be 3.6 eV.

OBIC measurements on planar high voltage p+-n junctions with diamond-like carbon films as passivation layer

Abstract Diamond-like carbon (DLC) films (a-C:H) have been deposited on planar p+-n junctions in silicon for electroactive passivation. The films have been produced by an r.f. plasma discharge with various self-bias voltages. The DLC-coated devices show improved blocking characteristics compared with uncoated samples. In order to study the influence of DLC films on planar p+-n junctions, we have measured the local induced photoconductivity (OBIC, optical-beam-induced current). By using light with a wavelength of 500 nm, the space charge region near the interface has been investigated. We are able to correlate the observed changes in the surface space charge region due to the DLC coating with the improved blocking characteristics.

Diamond-like hydrogenated amorphous carbon films studied by X-ray emission spectroscopy

Abstract The occupied electronic states of diamond-like hydrogenated carbon (a-C:H) were studied by X-ray emission spectroscopy. We measured (i) the C K emission bands and (ii) the optical band gaps obtained from transmission measurements via the Tauc correlation. For comparison we also measured the C K emission bands of diamond, graphite, some samples of amorphous carbon (a-C) and fullerene C 60 . In addition ab initio calculations of the C K-emission bands of diamond (sp 3 tetrahedral CC 4 bonds) and graphite (sp 2 planar CC 3 + p 2 bonds) were performed. Excellent agreement was found between the measured and calculated spectra, thus enabling us to identify all observed features and to understand their origin. Additionally we observed that the relative intensities of shoulders in the C K-emission bands at 281.3 eV formed by p z -like electrons (η states in graphite) correlate with the widths of the optical band gaps.

Bilayer resist process for exposure with low-voltage electrons (STM-lithography)

Abstract With STM lithography employing a bilayer resist system, an electron sensitive top resist and a conductive bottom resist, it is possible to generate patterns with dimensions of 100 nm and less. Patterns with aspect ratios up to 8 at a width of 50 nm in flat silicon oxide surface have been achieved. We also demonstrate, that it is possible to operate on prepatterned substrates using a third planarizing resist layer. The exposure mechanism in our CARL top resist has been determined to work differently from the mechanism in the high electron energy regime. The low energy electrons directly cleave the t-butyl ester group. Chemical amplification was not observed. The maximum writing speed for complete exposure in the resist was 1–5 μm/s at 20 pA writing current.

Spatially resolved photocurrent measurements on a-C:H/c-Si and a-C:H/a-Si:H heterojunction photodiodes

Abstract Heterojunction photodiodes were prepared by r.f. plasma-enhanced chemical vapour deposition of a-C:H films on single crystalline p-type silicon substrates ( N A = 10 14 cm −3 , orientation 〈111〉) and a-Si:H substrates. We studied the quantum efficiency of devices working in photovoltaic and reverse-biased mode in the wavelength region between 350 and 1200 nm. Post-deposition treatment of a-C:H films with hydrogen at high pressures (up to 300 bar) and elevated temperatures (up to 650 K) changed the bulk properties of the a-C:H films significantly. Photoconductivity was observed within the films after the hydrogen processing. The responsivity of the photoconducting a-C:H films, however, is too small to account for the observed quantum efficiencies (~ 20–30%) of the devices. Multiplication of charge carriers is indicated by quantum efficiencies exceeding 100%, which are observed for reverse-biased devices after the hydrogen treatment. According to C-V measurements the formation of the heterojunction leads to an inversion layer on the silicon surface which is essential for properly working devices. In our paper we propose a model for the operation of photodiodes which is supported by optical beam-induced current measurements.

OBIC measurements on planar high-voltage p+-n junctions with diamond-like carbon films as passivation layer

Abstract Diamond-like carbon (DLC) films (a-C : H) were deposited on planar p+ -n junctions in silicon for electroactive passivation. The films were produced by an rf plasma discharge with different self-bias voltages. The influence of DLC films on planar p+ -n junctions was studied by measurements of the locally induced photoconductivity. Using light at different wavelengths the penetration depth was varied and different parts of the space-charge region were investigated.

Improved electronic properties of diamond-like carbon films by high pressure post-deposition treatment

Abstract Hydrogenated diamond-like carbon films prepared by capacitively coupled r.f.-enhanced chemical vapor deposition were exposed to hydrogen gas at elevated temperatures (up to 350 °C) and pressures (300 bar) for several hours. Structural and compositional changes cause strong changes in the electronic bulk and interface properties as can be derived from capacitance-voltage ( C-V ) measurements. The hydrogen content increases from 26.4 to 30.5 at.% (from ERDA), the IR sp 3 -CH ( x ) stretching modes indicate a higher concentration of chemically bonded hydrogen. The interpretation of the C-V curves shows a reduction of the interfacial density of states by one order of magnitude down to 3 × 10 11 cm −2 eV −1 . Diamond-like carbon/crystalline silicon heterojunction photodiodes treated in this way exhibit a quantum efficiency improved by a factor of ten.