Component simulation and circuit design
Parallel to the technological work, physical component models are created and permanently developed. The theoretical results are compared with the measurement data obtained in the laboratory in order to ultimately obtain reliable and, above all, scalable "tools" for the description of device behavior, which allow for a targeted optimization of the devices. In addition, methods are developed that allow to model the frequency-dependent component behaviour on the basis of electrical equivalent circuit diagrams in order to draw conclusions about the intrinsic behaviour and thus also about the technology. The information gained in this way forms the basis for the development of simpler models that can be used in various simulation environments and allow the simulation of complex circuits and also microwave circuits. Subsequently, the circuits developed and optimized in this way are realized in the technology and later measured in the laboratory.
Epitaxy of III/V semiconductors
For the epitaxial growth of compound semiconductors consisting of atoms of the III- (Ga, Al, In) and the V-major group (As, N, P), three metal-organic gas phase epitaxy systems from the company Aixtron (MOVPE) and a molecular beam epitaxy system (MBE) from the company Varian are used. The four systems are used for different semiconductor compounds and for different primary applications. Thus a wide range of applications is available, which is further extended by sample transfer between the systems under clean room conditions.
The following systems and their primary field of application are available:
- AIX 200 RF MOVPE system for the fabrication of III/As- and III/P-nanowire structures and experimental bonding of arsenide and nirtride compounds.
- 3 x 2" Showerhead MOVPE system for III/N semiconductors in the form of core-shell nanowires and other three-dimensional structures. Equipped with in situ measuring instruments.
- 3 x 3" Showerhead MOVPE system for InGaAs/InP-based layers for heterobipolar transistor. Equipped with in situ measuring instruments.
- 3 Inch Substrate Heater MBE for precision deposition of atomic layers from In(Ga)As, which are necessary for the production of resonant tunnel diodes.
The different semiconductor layers and 3D structures form the basis for all subsequent work in the own technology or are available to project partners and other cooperating institutes.
If you are interested in sample material or the joint development of epitaxial structures and projects, please contact us.
The semiconductor layers grown in epitaxy on a wafer are structured in the process technology with the so-called top-down method. This makes it possible to produce electronic components such as heterobipolar transistors (HBT), high-electron mobility transistors (HEMT) and resonant tunnel diodes (RTD) from a complex layer package. For this purpose, we have a wide range of semiconductor technology processes at our disposal. From lithography with electron beam and ultraviolet light, wet and dry chemical etching processes to (electro)chemical and physical deposition equipment. By repeating these processes, functional circuits or antennas for free space radiation can be produced around the components. These processes take place in our clean room to create ideal process conditions. For the analysis of surfaces and structures after individual process steps, we use field emission scanning electron beam microscopy (FSEM), atomic force microscopy (AFM) and soon also confocal microscopy for finest resolution.
The BHE has three different laboratories for the measurement and characterization of the electronic components manufactured in the clean room of the ZHO. On the one hand photoluminescence (PL) and electroluminescence (EL) measurements can be performed in the BHE and on the other hand high frequency (HF) and direct voltage (DC) measurements.
DC measurement technology
The DC measuring station can be used to measure the current-voltage characteristics characteristic of an electronic component. For example, the characteristic curve of a resonance tunnel diode (RTD) or the output characteristic of a hetero bipolar transistor (HBT) can be measured. For this purpose, the measuring station has four Source Measurement Units (SMUs), which can work both as source and as measuring device. These can be controlled by a control computer and thus the input current or voltage can be defined. Furthermore, due to its four SMUs, the measuring station offers the possibility to carry out four-point measurements in order to measure precisely the smallest resistances. Another typical measurement method in semiconductor technology, the so-called Transmission Line Measuremts (TLM) method, offers the possibility to measure contact resistances as well as area resistances of metal-semiconductor contacts.
HF measurement technology
BHE also has a measurement laboratory to characterize the high-frequency properties of the manufactured components. This RF test facility provides two Vector Network Analyzers (VNA). With these VNAs scattering parameter measurements can be performed. Scattering parameters are the reflection and transmission coefficients at an n-port (1-port: e.g. diode, 2-port: e.g. simple amplifier circuit built with a transistor). For this purpose, a wave is coupled in at the ports and the reflection as well as the transmission is measured in relation to the coupled wave. In the BHE, for example, the manufactured RTDs are measured at this measuring station. With these measurements an equivalent circuit diagram of the RTD can be derived to make this component accessible in circuit simulations. Furthermore, this laboratory has a terahertz free beam measuring station in which the radiation is guided in free space (air) via an arrangement of lenses and mirrors. This is used to characterize the properties of the manufactured RTDs as a detector and as an oscillator. In the case of the RTD detector the VNA is used as high frequency source. The source signal is coupled into the RTD via the lenses and mirrors and the voltage is measured. If the injected power is known, the spectral sensitivity of the detector can be determined. If the RTD is operated as an oscillator, the transmitted signal is coupled into the VNA via the test setup. The VNA now works as a spectrum analyzer and detects the frequency of the received signal. Thus the oscillation frequency of the RTD can be determined.
Highly sensitive THz detectors
A key component for the entry into the world of THz applications is the detection of THz radiation in addition to generation. For this purpose compact and very sensitive high-speed detectors, which work at room temperature, are urgently needed. The BHE department focuses on the design and characterization of semiconductor THz detectors. The Triple-Barrier Resonant Tunnel Diode (TB-RTD) is in the focus. The device is operated without supply voltage (zero-bias), which reduces the circuitry complexity. In addition, an additional noise source is not required, thus reducing the overall noise.
Systems with direct detection, built with one or more diodes, have the advantages of a wide bandwidth and a simple circuit layout. The RTD can be integrated into a planar circuit technology or an on-chip antenna. This allows an easy realization of an array architecture. For this reason RTD based detectors are attractive for compact systems.
The finished detector structure consists of a TB-RTD, which is monolithically integrated into an on-chip antenna. This chip is then placed on a hyper-hemispherical silicon lens to focus the incoming power of the THz signal. Different topologies of planar antennas will be pursued, with the focus on the widest possible signal detection at low input power.
Components based on resonant tunneling are the most compact and energy efficient semiconductor devices operating at THz frequencies. The Triple-Barrier Resonant Tunnel Diode (TB-RTD) has an asymmetric current-voltage characteristic with a strong non-linearity at the operating point without supply voltage. This characteristic makes the TB-RTD a promising device for high efficiency and low NEP (Noise-equivalent power) at THz frequencies compared to the commonly used III-V semiconductor Schottky barrier diodes.
Nanowires are structures with a high aspect ratio, due to their diameter in the range of some ten to a few hundred nanometers at a length of mostly several micrometers. They are produced by a "top-down" approach using etching processes from crystal layers or by a "bottom-up" approach epitaxially from the gas phase. The latter offers the advantage that lattice tensions can be relaxed within a few monolayers towards the lateral facets of the growing wire, thus enabling material combinations that are not possible in conventional layer growth due to large lattice mismatch. This and other nanowire-specific mechanisms lead to fewer defects in the crystal and thus potentially to more powerful electrical and electro-optical components.
Nanowires have a large surface area in relation to their volume, which offers advantages in terms of absorption capacity, especially when used as solar cells. The increased surface area is also advantageous for sensors. Further optoelectronic application examples are light-emitting diodes or photodetectors, but also switching field-effect and bipolar transistors or tunnel diodes are intensively researched as nanowire structures. Our research focus is both on the fabrication and analysis of axial and coaxial pn-junctions in the GaAs/InGaP material system, which resulted in the first demonstration of a nanowire heterostructure bipolar transistor, and on the fabrication and analysis of fast switching (> 1GHz) GaN-based nanowire LED arrays on silicon. Due to the three-dimensional nanowire geometry, crystal facets with special properties become accessible and a further degree of freedom for the optimization of existing and development of future devices is created.