SiGe Heterobipolar Transistors Research Articles

Impact of nickel silicide on SiGe BiCMOS devices

Nickel silicide (NiSi) can improve the RF performance of SiGe hetero bipolar transistors (HBT) compared to cobalt silicide (Heinemann et al 2016 IEDM Tech. Dig. 51–4). In this paper, the impact of different procedures to form NiSi on HBT and MOS devices of a 0.13 μm BiCMOS cobalt silicide technology is studied. The different NiSi formations are carried out by partly or fully Ni consumption (PC, FC) for low temperature furnace and low pressure anneals. Our investigations indicate, PC results in rough silicide surfaces and substrate interfaces, whereas FC leads to smooth surfaces and interfaces associated with lower resistivities. FC nickel silicidation at 300 °C and 450 °C exhibits an excessive NiSi growth on the STI edges of n doped source drain (N+SD) regions, reducing the breakdown voltage to substrate or p well. An enhanced NiSi growth is found for all investigated silicide schemes on narrow P+SD regions along polysilicon gates. The leakage current of these structures is caused by enlarged lateral silicidation towards the gates. The enhanced lateral NiSi growth could be suppressed by partly Ni silicidation with furnace anneals at 200 °C or 230 °C.

GaP collector development for SiGe heterojunction bipolar transistor performance increase: A heterostructure growth study

To develop a III/V wide bandgap collector concept for future SiGe heterobipolar transistor performance increase, a heterostructure growth study of GaP on pseudomorphic 4° off-oriented Si0.8Ge0.2/Si(001) substrates was performed. For pseudomorphic GaP/Si0.8Ge0.2/Si(001) heterostructure growth, critical thickness of GaP on Si and maximum thermal budget for GaP deposition were evaluated. A detailed structure and defect characterization study by x-ray diffraction, atomic force microscopy, and transmission electron microscopy is reported on single crystalline 170 nm GaP/20 nm Si0.8Ge0.2/Si(001). Results show that 20 nm Si0.8Ge0.2/Si(001) can be overgrown by 170 nm GaP without affecting the pseudomorphism of the Si0.8Ge0.2/Si(001) layer. The GaP layer grows however partially relaxed, mainly due to defect nucleation at the GaP/Si0.8Ge0.2 interface during initial island coalescence. The achievement of 2D GaP growth conditions on Si0.8Ge0.2/Si(001) systems is thus a crucial step for achieving fully pseudomorphic heterostructures. Anti-phase domain-free GaP growth is observed for film thicknesses beyond 70 nm.

Simulation of boron diffusion in Si and strained SiGe layers

Boron outdiffusion from the base into the emitter and collector caused by annealing in SiGe heterobipolar transistors (HBTs) has a serious influence on the transit frequency. One solution of the problem of boron outdiffusion is the creation of intrinsic spacers between the base, emitter and collector layers to prevent diffusion of boron across the heterointerface. For optimisation of SiGe HBT properties, several simulators are used. This paper presents a quantitative analysis of a SiGe HBT by process simulators SUPREM IV.GS and ISE TCAD-DIOS. Models for simulation of a boron-doped SiGe base of HBT are discussed and compared.

Resonance phase operation of a SiGe HBT

A novel operation mode—the so-called resonance phase operation—is demonstrated using a SiGe HBT (hetero-bipolar transistor). Transit time effects and coherent transport lead by proper design of the transistor to a phase shift larger than π . In this resonance phase mode, a current gain above 0 dB is achieved at frequency bands well above the transit frequency. We have designed and fabricated SiGe HBTs in order to investigate the resonance phase effect at frequencies below 50 GHz due to easier measurement technique. The transistors were fabricated using a low-temperature process. They showed a rather high breakthrough voltage of up to 20 V. The RF measurement showed the typical HBT behaviour up to the transit frequency f T. At higher frequencies the current gain H 21 rose again above 0 dB. We have thus demonstrated the existence of the resonance phase effect in a SiGe HBT.

Characterization of Ge gradients in SiGe HBTs by AES depth profile simulation

We show that AES depth profiling extended by a simple profile simulation technique allows characterization of details in the Ge concentration gradients for SiGe hetero-bipolar transistors (HBTs). Using the mixing-roughness-information depth (MRI) model to simulate the experimental data allows us to reveal concentration steps with a precision of about ±2 at.% and small deviations from linear concentration gradients. The obtainable high lateral resolution of AES facilitates an application for process optimization and control in small microelectronic structures.

Bandgap narrowing in strained SiGe on the basis of electrical measurements on Si/SiGe/Si hetero bipolar transistors

We have investigated the bandgap narrowing Δ E gSiGe of strained Si 1− x Ge x layers for 0.15< x<0.29. After MBE layer growth SiGe hetero bipolar transistors (HBTs) are processed by a minimal thermal budget technology to inhibit relaxation and to ensure sharp doping and Ge profiles. Δ E gSiGe is determined by electrical characterisation of the HBTs (gummel plot, temperature dependent measurements). The layer properties are measured by SIMS and TEM. The results show good agreement between room temperature and temperature dependent measurements improving earlier results (J.C. Sturm). This could be achieved by considering the correct temperature dependence of the minority carrier mobility. The obtained Δ E gSiGe is close to the room temperature results of Sturm (0.68 x) although a parabolic fit models our data best.

Fourier-transform infrared investigations of Si 1− yC y structures for hetero bipolar transistor applications

With the rapid proliferation of commercial SiGe hetero bipolar transistor (HBT) devices, incompatibilities with mainstream integration technologies become an important issue. A common problem in device processing is transient enhanced diffusion of boron out of the base region due to poly-emitter implantation and annealing. It has been shown that co-doping of the base with C can suppress the diffusion of B, but it is accompanied by strong C diffusion. By a combination of Fourier Transform Infrared spectroscopy (FTIR), X-ray diffraction and SIMS studies, we investigated in which form C is present in MBE grown Si 1− y C y layers and in SiGe:C:B HBTs with implanted poly-Si emitter. With the FTIR technique we are sensitive to substitutional C, coherent and incoherent SiC precipitates and some C-containing complexes. With these techniques we studied the time-scale on which the metastable C-containing layer relax by SiC formation.

Quantum study of ultra-thin-base SiGe heterojunction bipolar transistor

A quantum study has been carried out by solving the Schrö;dinger equation for the ultra-thin-base in SiGe heterobipolar transistor. The results are compared with those of the Boltzmann model.

Modelling of SiGe heterobipolar transistors: 200 GHz frequencies with symmetrical delay times

Our basic approach is to develop a symmetrical design with equal delay times for collector, base and the total load to simulate the high frequency behaviour of SiGe heterobipolar transistors (HBTs). On this base we have investigated the feasibility of SiGe HBTs with transit frequencies f T above 200 GHz. A symmetrical design reaching f T=208 GHz is presented. The dependence of the high frequency behaviour on Ge content and vertical transistor design is shown. Critical parameters like the maximum current density and the breakthrough voltage are considered. An analytical model is compared to numerical simulations and experimental data.

Effects of carbon on boron diffusion in SiGe: Principles and impact on bipolar devices

We show that the incorporation of low carbon concentration (&amp;lt;1020 cm−3) within the SiGe region of SiGe heterobipolar transistors (HBT) can significantly suppress boron outdiffusion caused by later processing steps. We were able to obtain fT/fmax ⩾50 GHz for a simple SiGe:C transistor with a box-shaped Ge profile. Comparing the high frequency performance of molecular beam epitaxy grown SiGe:C HBTs with identical SiGe HBTs, we found an increase in fT and fmax by a factor of more than 2. The static characteristics for SiGe:C HBTs demonstrate that the transistors should be suitable for circuit applications. Process margins for SiGe HBT technology are shown to be relaxed due to C incorporation. The dramatic reduction of B diffusion in C-rich Si under conditions of point defect equilibrium is attributed to a reduction of the concentration of interstitials available for the B diffusion.

Carbon-containing group IV heterostructures on Si: properties and device applications

We review some important material properties of Si 1− y C y and Si 1− x− y Ge x C y layers grown pseudomorphically on Si(001). This new material might overcome some of the limitations of strained Si 1− x Ge y and open new fields for device applications of heteroepitaxial Si-based systems. In addition, we demonstrated incorporating low carbon concentrations (<10 20 cm −3) into the SiGe region of a heterobipolar transistor (HBT) can significantly suppress boron outdiffusion caused by later processing steps. The static characteristics demonstrate that the transistors should be suitable for circuit applications. Comparing the high-frequency performance of MBE-grown SiGe:C HBTs with identically SiGe HBTs we found an increase in f T and f max by a factor of more than 2 for our chosen SiGe profile. This indicates that adding carbon enabled one to use implantation steps without affecting the boron profile, that is, it offers wider latitude in process margins.

High Resolution Determination of the Ge Depth Profile in SiGe Heterobipolar Transistor Structures by X-Ray Diffractometry

X-ray double crystal diffractometry is used to measure the rocking curve (RC) of SiGe heterobipolar transistor structures using CuKα 400 reflection. The depth profile of the Ge concentration is determined by simulation of RCs with a semi-kinematical theory and fitting the calculated RCs to the experimental one. The problem of achievable accuracy in the determination of the main structural parameters, Si cap layer thickness, total SiGe layer thickness, plateau layer thickness, and the maximum Ge content, is discussed in detail for three different samples by analyzing sets of independent simulations with slightly modified starting conditions. Using only one fitting procedure, it is possible to determine the thickness of the Si cap layer and the total thickness of the SiGe layer with an error of less than ±0.5 nm. The accuracy of the plateau thickness and of the maximum Ge content depends on the total SiGe layer thickness. Errors for the plateau layer thickness of less than ±2 nm and for the maximum Ge content of less than 0.5% are possible.

Characterization of SiGe HBT-structures by double-and triple-crystal diffractometry

X-ray double- and triple-crystal diffractormetry was used to measure the rocking curves of typical SiGe heterobipolar transistor structures. The depth profile of the Ge content can be determined by simulation of the rocking curves with an accuracy of about 1 nm for the thickness of the Si-cap layer and the total thickness of the SiGe layer, and an absolute error in the maximum Ge content of less than 0.5%. Comparison of experimental and simulated curves allows a fast qualitative assessment of the structural perfection of the layer system. It is doubtful that profile of boron doping inside the SiGe layer can be characterized by X-ray diffraction methods.

Si1−x−yGexCy: a new material for future microelectronics?

The paper summarizes a few basic properties of SiGe showing that SiGe is an interesting material for high speed electronics. The advantage of using heterostructures in silicon-based technologies is demonstrated by taking SiGe heterobipolar transistors (HBTs) as an example. First results obtained with very fast and low-noise HBTs are briefly mentioned. The paper is concluded by a short discussion of a few optoelectronic properties observed in various Si/Ge, Si/Si1−xGex and Si1−x−yGexCy strained-layer superlattices and quantum wells with particular emphasis on electroluminescence properties and the benefit of these materials for new buffer concepts.

Unambiguous determination of crystal-lattice strains in epitaxially grown SiGe/Si multilayers

A new method for unambiguous reconstruction of crystal-lattice strains in epitaxially grown layers from high-resolution x-ray diffraction data is proposed. The technique uses x-ray diffracted intensity profiles collected for two different radiation wavelengths. We enhance the theory for the previously developed algorithm for model-independent determination of crystal-lattice strain profiles in single crystals with epitaxially grown top-surface layers. The method relies on the retrieval of the scattered x-ray wave phase from its intensity profile via a logarithmic Hilbert transform. This phase-retrieval technique is always associated with the problem of complex polynomial root finding. A practical procedure for the mapping of complex polynomial roots is proposed to distinguish true and virtual zeros. This allows the phase of the diffracted x-ray wave to be retrieved unambiguously. The method was applied to determine physical dimensions and concentration composition of a Si/Si1−xGex/Si alloy multilayer structure typical for SiGe heterobipolar transistor device.

High speed SiGe heterobipolar transistors

SiGe heterobipolar transistors are near the point of commercial availability. They will be inserted in mobile phones in the 0.9 to 2.4 GHz range and in wireless local area networks using the 2.4 to 5.8 GHz band. The advantage of SiGe HBTs is not only their excellent high frequency and noise performance with a maximum frequency of oscillation up to 120 GHz and 0.9 dB noise figure at 10 GHz, but the compatibility of SiGe to standard silicon technology. Fabrication processes and results of a research-like SiGe HBT and a possible production type HBT version are described. In addition, some circuit demonstrators for SiGe ICs will be presented.

CoSi 2 and TiSi 2 for [formula omitted] heterodevices

Novel Si SiGe heterodevices with enhanced performance demand low thermal budget contact formation. CoSi 2 and TiSi 2 can meet this requirement. The investigation of self-aligned rapid thermal processes for the formation of CoSi 2 (at 600–650 °C) and TiSi 2 (at 650–750 °C) and their application to n-channel hetero field effect transistors and heterobipolar transistors (HBTs) is described and effects on the device performance are presented. Specific resistances of around 20 μΩ cm for CoSi 2 and 17 μΩ cm for TiSi 2 could be confirmed for the Si/SiGe heterosystem. Contact resistances on n +-phosphorous implanted layers below 6 μΩ cm 2 have been derived from TLM structures. The silicide formation is severely affected by SiGe. Therefore the compound formation of CoSi 2 in the Co Si and the Co Si 1 − xGe x system has been investigated and compared as a function of annealing time in a temperature range from 400 to 600 °C. X-ray diffraction measurements on SiGe layers confirmed the formation of a mixture of CoSi and Co germanides with higher resistivity due to the interfacial reaction and the accumulation of Ge at the interface, which may act as a diffusion barrier. The choice of a Si cap above SiGe, whose thickness was properly adjusted to the material consumption correlated with the resulting silicide thickness, is proposed. The process and the performance of SiGe HBTs could be improved by the introduction of a Ti salicide process: base resistances yield values around 20 Ω (for an emitter area of 0.8 × 5 μm 2). Nearly ideal Gummel plots confirm the advantage of using TiSi 2.

SiGe HBTs and HFETs

SiGe is just on an upswing. Attractive potentials can be foreseen and outstanding performance was even demonstrated, e.g. high frequencies above 100 GHz, low noise below 0.5 dB at 2 GHz, high transconductances around 400 mS/mm. A further driving force for the growing engagement with SiGe is its basic compatibility to standard Si-technology. Though most of the results stem from discrete devices SiGe is already going to be produced. The first devices will be SiGe-heterobipolar transistors (HBT). Targeted applications are converters or mobile communication systems. The status of our devices is reviewed here. In long term the SiGe hetero fieldeffect transistor (HFET) will become another candidate creating a new advanced generation in mainstream CMOS, s.c. SiGe Hetero CMOS (HCMOS). Our status of SiGe HFETs and its potential for HCMOS is presented also.

Growth of 100 GHz SiGe-Heterobipolar Transistor (HBT) Structures

The transit frequency f T of SiGe-heterobipolar transistors (HBT's) was increased from 20 GHz to 100 GHz. This was mainly achieved by thickness reduction of the double heterojunction SiGe-base from 65 nm to 25 nm. The complete vertical structure of the SiGe-HBT's (collector, base, emitter, emitter contact) was grown in one run by Si molecular beam epitaxy (Si-MBE). The growth temperature was varied from 650° C at the collector side to 325° C at the emitter contact side. The different n-type doping levels (1017/ cm3, 1018/ cm3, 1020/ cm3) were obtained by applying three different Sb-doping techniques (secondary implantation, adatom pre build-up, low temperature doping). The p-type base was doped with boron. The doping level in the base (6×1019/ cm3) exceeded the emitter doping level by a factor of 30 (doping level inversion).

Challenges for a Si/Ge heterodevice technology

Abstract Previous results reported for Si-based devices like the SiGe heterobipolar transistor (SiGe HBT), the n- and p-channel SiGe modulation doped field effect transistor (SiGe MODFET or MOSFET) and SiGe optoelectronic devices point to the outstanding potential of this novel heterosystem. Enhanced speed, distinctly above 100 GHz can be envisaged, high gains or transconductances and low noise. This performance will extend the application area of the conventional Si-microelectronic. The technology for Si/Ge heterodevices is on a minor development level. Inspite of a basic compatibility with Si-technology, SiGe introduces additional constraints, mainly due to a large lattice mismatch and to the requirement of nm-thin layers with atomically sharp junctions. Here actual Si/Ge technologies will be reviewed covering growth, layout aspects and device processes. Effects on the device performance will be discussed. Challenging technological activities concerning thermal budget, implantation, etching, passivation wait for a solution.