The influence of defects on device performance of MBE-grown Si homojunction and strained Si1-xGex/Si heterostructures

Different Si homojunction and strained Si_1-xGe_x/Si heterojunction diodes and bipolar transistors have been fabricated by Si-MBE. The effect of annealing on Si homojunction diodes and transistors are studied. It is found that annealing generally improves the Si device performance, such as the ideality factor and breakdown characteristics. The influence of⁶⁰Co γ irradiation on the Si_1-xGe_x/Si diode performances are investigated by studying the temperature dependence of their electrical characteristics, and the results are correlated with the quality of the MBE-films. γ irradiation causes a drop in material conductivity due to the generation of atom-displacement defects in the whole volume of the wafers and increases the defect density at hetero-interfaces. The forward I-V curves of Si_1-xGe_x/Si devices may shift towards lower or higher voltages, depending on the film quality and the irradiation dose. The increase of defect density in strained Si_1-xGe_x/Si films appears to occur easier for the films with lower quality. Electrical measurements and calculations show that the defect-associated tunneling process is important in current transport for these MBE grown Si homojunction and strained Si_1-xGe_x/Si heterojunction devices, which have initially medium film quality or have been treated by irradiation.     

Determination of bandgap narrowing and parasitic energy barriers inSiGe HBT's integrated in a bipolar technology

This paper describes a method for characterizing the bandgap narrowing and parasitic energy barrier in SiGe heterojunction bipolar transistors (HBTs), fabricated using a single-polysilicon self-aligned bipolar process. From a comprehensive study of the temperature dependence of the collector current, the bandgap narrowing in the base due to germanium has been dissociated from that due to the heavy dopant concentration. The same approach has been used to characterize the height and width of parasitic energy barriers which appear when boron out-diffusion from the SiGe base is present. The method has been applied to SiGe heterojunction bipolar transistors fabricated using a single polysilicon, self-aligned, bipolar process, as well as mesa transistors. The experimental results show that small geometry transistors have degraded collector currents due to boron out-diffusion around the perimeter of the emitter. This behavior has been explained by accelerated boron diffusion due to point defects generated during the extrinsic base implant. The values of undoped SiGe spacer thickness needed to suppress the parasitic energy barrier are described. Finally, high-frequency results are reported, which correlate the frequency transition to these parasitic energy barriers     

Novel Cu/Cr/Ge/Pd Ohmic Contacts on Highly Doped n-GaAs

The thermal stability of the Cu/Cr/Ge/Pd/n⁺-GaAs contact structure was evaluated. In this structure, a thin 40 nm layer of chromium was deposited as a diffusion barrier to block copper diffusion into GaAs. After thermal annealing at 350°C, the specific contact resistance of the copper-based ohmic contact Cu/Cr/Ge/Pd was measured to be (5.1 ± 0.6) × 10⁻⁷ Ω cm². Diffusion behaviors of these films at different annealing temperatures were characterized by metal sheet resistance, X-ray diffraction data, Auger electron spectroscopy, and transmission electron microscopy. The Cu/Cr/Ge/Pd contact structure was very stable after 350°C annealing. However, after 400°C annealing, the reaction of copper with the underlying layers started to occur and formed Cu₃Ga, Cu₃As, Cu₉Ga₄, and Ge₃Cu phases due to interfacial instability and copper diffusion.     

Rapid thermal chemical vapor deposition of in-situ boron doped polycrystalline SIxGe1-x

In-situ doped polycrystalline Si_xGe_1-x (x = 0.7) alloys were deposited by rapid thermal chemical vapor deposition (RTCVD) using the reactive gases SiH₂Cl₂, GeH₄ and B₂H₆ in a H₂ carrier gas. The depositions were performed at a total pressure of 4.0 Torr and at temperatures 600° C, 650° C and 700° C and different B₂H₆ flow rates. The conditions were chosen to achieve high doping levels in the deposited films. Our results indicate negligible effect of B₂H₆ flow on the deposition rate. The depositions follow an Arrhenius type behavior with an activation energy of 25 kcal/mole. Boron incorporation in the films was found to follow a simple kinetic model with higher boron levels at lower deposition rates and higher B₂H₆ flow rates. As-deposited resistivities as low as 2 mΩ-cm were obtained. Rapid thermal annealing (RTA) in the temperature range 800-1000° C was found to reduce the resistivity only marginally due to the high levels of boron activation achieved during the deposition process. The results indicate that polycrystalline Si_xGe_1-x films can be deposited by RTCVD with resistivities comparable to those reported for in-situ doped polysilicon.     

Silicon molecular beam epitaxy: Highlights of recent work

This paper describes highlights of recent work in Si MBE directed at device application of the technique. Topics include studies of doping by solid sources and co-ion-implantation, reduction of particle and metal induced defect levels, gas source MBE and recent results on high speed Ge _x Si_1-x heterojunction bipolar transistors.     

Advances in remote plasma-enhanced chemical vapor deposition for low temperature In situ hydrogen plasma clean and Si and Si1-xGex epitaxy

Remote plasma-enhanced chemical vapor deposition (RPCVD) is a low temperature growth technique which has been successfully employed inin situ remote hydrogen plasma clean of Si(100) surfaces, silicon homoepitaxy and Si_{1- x}Ge_x heteroepitaxy in the temperature range of 150–450° C. The epitaxial process employs anex situ wet chemical clean, anin situ remote hydrogen plasma clean, followed by a remote argon plasma dissociation of silane and germane to generate the precursors for epitaxial growth. Boron doping concentrations as high as 10²¹ cm^?3 have been achieved in the low temperature epitaxial films by introducing B₂H₆/He during the growth. The growth rate of epitaxial Si can be varied from 0.4Å/min to 50Å/min by controlling therf power. The wide range of controllable growth rates makes RPCVD an excellent tool for applications ranging from superlattice structures to more conventional Si epitaxy. Auger electron spectroscopy analysis has been employed to confirm the efficacy of this remote hydrogen plasma clean in terms of removing surface contaminants. Reflection high energy electron diffraction and transmission electron microscopy have been utilized to investigate the surface structure in terms of crystallinity and defect generation. Epitaxial Si and Si_1-xGe_x films have been grown by RPCVD with defect densities below the detection limits of TEM (~10⁵ cm^-2 or less). The RPCVD process also exploits the hydrogen passivation effect at temperatures below 500° C to minimize the adsorption of C and 0 during growth. Epitaxial Si and Si_1-xGe_x films with low oxygen content (~3 × 10₁₈ cm^-3) have been achieved by RPCVD. Silicon and Si/Si_1-xGe_x mesa diodes with boron concentrations ranging from 10¹⁷ to 10¹⁹ cm^-3 in the epitaxial films grown by RPCVD show reasonably good current-voltage characteristics with ideality factors of 1.2-1.3. A Si/Si_1-xGe_x superlattice structure with sharp Ge transitions has been demonstrated by exploiting the low temperature capability of RPCVD.In situ plasma diagnostics using single and double Langmuir probes has been performed to reveal the nature of the RPCVD process.     

Photoluminescence spectra of Si1-xGex/Si quantum well structures grown by three different techniques

Photoluminescence (PL) spectra of Si_1-xGe_x/Si multiple quantum wells have been measured at 4.2 K for the samples grown by three different techniques; conventional molecular beam epitaxy (MBE), gas-source MBE, and ultra high vacuum chemical vapor deposition (UHV-CVD). Only in the case of conventional MBE, strong emission bands appear about 80 meV below the band gap of Si_1-xGe_x. These strong emission bands disappear after the annealing at 800° C. From the dependence of the PL intensity on the excitation power, strong emission is considered to be due to some recombination center. On the other hand, in the case of gas-source MBE and UHV-CVD, the strong emission bands are undetectable, although the band-edge PL lines of Si_1-xGe_x are clearly observed. There is no significant change in the PL spectra after the annealing. The origin of the strong emission band is considered to be defects which are characteristic of conventional MBE.     

Role of a Si0.95Ge0.05 epilayer cap on boron diffusion in silicon under inert and dry oxidizing ambient annealing

Silicon (Si) and Si with a 60 nm Si_0.95Ge_0.05 epilayer cap (Si_0.95Ge_0.05/Si) were implanted with 60 keV, 1×10¹³ cm⁻² boron (B) followed by annealing in nitrogen (N₂) or dry oxygen (O₂) in two different anneal conditions. B⁺implantation energy and dose were set such that the B peak is placed inside Si in Si_0.95Ge_0.05/Si samples and concentration independent B diffusion is achieved upon annealing. For samples annealed above 1075 °C, Ge diffusing from the Si_0.95Ge_0.05 epilayer cap in Si_0.95Ge_0.05/Si samples reached the B layer inside Si and resulted in retarded B diffusion compared to the Si samples. For annealing done at lower temperatures, diffusion of Ge from Si_0.95Ge_0.05 epilayer cap does not reach the B layer inside Si. Thus B diffusion profiles in the Si and Si_0.95Ge_0.05/Si samples appear to be similar. B diffusion in dry oxidizing ambient annealing of Si_0.95Ge_0.05/Si samples further depends on the nature of Si_0.95Ge_0.05 oxidation which is set by the duration and the thermal budget of the oxidizing anneal.     

Materials and technology issues for SiGe heterojunction bipolar transistors

This paper reviews progress in SiGe heterojunction bipolar technology, with particular emphasis on the influence of materials issues on the technology performance. Low-temperature epitaxy and transient enhanced diffusion are identified as the main materials issues. Three epitaxy approaches are described, namely differential epitaxy, selective epitaxy, and combined selective and non-selective epitaxy. Differential epitaxy has the advantage of a simple growth process, selective epitaxy the advantage that an extrinsic base implant is not needed, and combined expitaxy the advantage that the Si collector and SiGe base can be grown in a single step. Transient enhanced diffusion of the boron in the base is shown to be a constraint on transistor performance, and the incorporation of a carbon concentration of ≈1×10²⁰ cm⁻³ in the base is shown to ease this constraint. Preliminary results on SiGe HBTs on wafer bonded SOI substrates are reported. The SOI transistors have characteristics comparable to those of bulk transistors, but the advantage of a gain that is a factor of approximately two higher.     

Fe and Ti implants in In0.52Al0.48As

Single (200 keV) and multiple energy Fe implants in n-type and Ti implants in p-type material were performed in In_0.52Al_0.48As at both room temperature and 200°C. For the Fe implants, the secondary ion mass spectrometry profiles showed a severe out-diffusion for all rapid thermal annealing schemes used, independent of the implantation temperature. The Fe implant peaks observed after annealing, at 0.8Rp, Rp+ΔRp and 2Rp (where Rp and ΔRp are range and straggle, respectively) depth locations in other In-based compounds like InP and InGaAs were not observed here. On the contrary, Ti implants showed only a slight in- and out-diffusion for both room temperature and 200°C implants as in the case of InP and InGaAs. The Rutherford backscattering measurements on the annealed samples implanted at 200°C showed a crystal quality similar to that of the virgin material. The resistivity of all the samples after annealing was higher than 10⁶ Ω-cm.     

Influence of the oxygen content in SiGe on the parameters of Si/SiGe heterojunction bipolar transistors

We demonstrate peak f_T and f_max of 50 GHz for heterojunction bipolar transistors (HBTs) with an oxygen concentration in the epitaxial SiGe base layer of about 10²⁰ cm⁻³. These f_T/f_max values are over 10 GHz higher than for identically processed HBTs with an O content of only 10¹⁸ cm⁻³. This is due to reduced transient enhanced diffusion of boron in the O-rich layers. However, the base carrier lifetimes are reduced by the high oxygen content. We show that ideal base current characteristics and a low 1/fnoise level can be obtained despite this effect by localizing the emitter-base space-charge region outside the O-rich layer.     

Nickel silicidation techniques for strained Si1?xGex, Si1?x?yGexCy, and Si1?yCy alloys material-device applications

A nickel silicide process for Si_1-xGe_x, Si_1-x-yGe_xC_y, and Si_1-yC_y alloy materials compatible with Si technology has been developed. Low-resistivity-phase (12–20 μΘ cm) nickel silicides have been obtained for these alloys with different low sheet-resistance temperature windows. The study shows that thin (15–18 nm) silicide layers with high crystalline quality, smooth silicide surface, and smooth interface between silicide and the underlying material are achievable. The technique could be used to combine the benefits of Ni silicide and Si_1-xGe_x, Si_1-x-yGe_xC_y, and Si_1-yC_y alloys. The technique is promising for Si or Si_1-xGe_x, Si_1-x-yGe_xC_y, and Si_1-yC_y alloy-based metal-oxide semiconductor, field-effect transistors (MOSFETs) or other device applications.     

The effect of hydrogen on boron diffusion in SiO2

The diffusivity of boron in silicon dioxide may be increased by the introduction of hydrogen into the annealing atmosphere. In this paper we report on the diffusion characteristics of boron ion-implanted into thermally grown SiO₂. A sensitive technique was used in which the boron atoms redistributed into the substrate are characterized by electrical methods. The diffusivity of boron in thermal SiO₂ was measured over the temperature range of 950-1150°C with hydrogen partial pressure from 0 to 0.2 atm. It was found that the diffusion coefficient of boron in oxide at 1150° C increases as the square root of the hydrogen partial pressure. At fixed pressure the temperature dependence of the diffusion coefficient obeys a single-activation-energy exponential rule. At 0.1 atm partial pressure of H₂ the activation energy is 3.0 eV and the preexponential factor is 6 x 10⁵ [cm²/sec.].     

Fabrication and electrical characterization of p-NiPc/n-Si heterojunction

Nickel-phthalocyanine (NiPc) thin film was prepared by thermal evaporation method on n-Si single-crystal substrate to fabricate p-NiPc/n-Si heterojunction. The electrical transport properties of the p-NiPc/n-Si heterojunctions were investigated by temperature-dependent current-voltage (I-V) measurements and room temperature capacitance-voltage (C-V) measurements. The temperature-dependent I-V characteristics revealed that the forward conduction was determined by thermionic-emission and space-charge-limited current (SCLC) mechanisms at low and high voltage, respectively⋅ On the other hand, the reverse current is limited by the carrier generation process. The 1/C²-V plot indicated the junction was abrupt and the junction built-in potential was 0.61 V at room temperature.     

Microstructural analysis of a Au/Pt/Pd/Zn ohmic contact to an AlGaAs/GaAs heterojunction bipolar transistor

Gold-based ohmic contacts, incorporating Pt, Pd, and Zn layers, to AIGaAs/GaAs heterojunction bipolar transistors (HBTs) have been characterized using transmission electron microscopy (TEM). The metallization was deposited onto a 30 nm graded emitter layer of n-type Al_xGa_1−xAs, which was on a 30 nm emitter layer of n-type Al_0.3Ga_0.7As, with the aim of contacting the underlying 80 nm thick graded base layer of p-type Al_xGa_1−xAs. Metal layers were deposited sequentially using electron beam evaporation and the resultant metallizations were annealed at temperatures ranging from 250-500°C for up to several minutes. A minimum contact resistance of ≈8.5 × 10⁻⁷ Ω-cm² was achieved, which corresponded to the decomposition of ternary phases at the metallization/semiconductor interface, to binary phases, i.e., PdGa and PtAs₂. Long term stability tests were done on the optimum contacts. Anneals at 270°C for up to four weeks in duration produced virtually no change in microstructure, with the exception of some outward diffusion of Ga and As.     

Strain relaxation and dopant distribution in the rapid thermal annealing of Co with Si/Si1−xGex/Si heterostructure

The reaction of cobalt with the Si-sacrificial cap in the strained Si/Si_1−xGe_x/Si MBE grown heterostructure was studied. The Si-cap is added to prevent the relaxation of the SiGe and to guarantee uniform and reliable silicidation reaction. The Si_1−xGe_x epilayer, with Ge content between 18 and 28 at%, was highly B doped, while the Si-cap was undoped or B doped either during growth or by ion implantation. Cobalt evaporation was followed by rapid thermal annealing at 450–700°C for 30 sec in N₂ or Ar+10%H₂. When the silicide penetrated the Si-cap/Si_1−xGe_x interface, noticeable out-diffusion of Ge and B to the surface was observed. In spite of the presence of the Si-cap significant strain relaxation was observed in three cases: (1) in the implanted samples, although the implantation was confined to the Si-cap, (2) when the Co layer was too thick, such that the silicide penetrated the SiGe layer and (3) when the Ge content in the SiGe layer was relatively high (27.5%).     

Interfacial reactions and electrical properties of Hf/p-Si0.85Ge0.15

Interfacial reactions and electrical properties of Hf/p-Si_0.85Ge_0.15 as a function of the annealing temperature were studied. Hf₃(Si_1−xGe)₂ and Hf(Si_1−xGe)₂ were initially formed at 500°C and 600°C, respectively. At temperatures above 400°C, Ge segregation out of the reacted layers associated with strain relaxation of the unreacted Si_0.85Ge_0.15 films appeared. At 780°C, agglomeration occurred in the Hf(Si_1−xGe_x)₂ films. All the as-deposited and annealed Hf/p-Si_0.85Ge_0.15 samples showed the formation of an ohmic contact. The lowest specific contact resistance around 10⁻⁵ ω cm² could be obtained for the Hf₃ (Si_1−xGe_x)₂ contacts to p-Si_0.85Ge_0.15 formed at 500°C. Below 500°C, the decrease of specific contact resistance with the annealing temperature is mainly caused by the formation of Hf₃(Si_1−xGe_x)₂ and an interfacial Ge-rich layer between the Hf₃(Si_1−xGe_x)₂ and unreacted Si_0.85Ge_0.15 films, while above 600°C, the increase of specific contact resistance may be due to the formation of Hf(Si_1−xGe_x)₂ and SiC as well as the roughness of the Hf(Si_1−xGe_x)₂ films.     

Effects of annealing on the hydrogen concentration and the performance of InGaP/InGaAsN/GaAs heterojunction bipolar transistors

The effects of rapid thermal annealing (RTA) on InGaP/InGaAsN heterojunction bipolar transistors (HBTs) with a carbon-doped base have been studied. The hydrogen and nitrogen concentrations in the base, as well as the direct current (DC) and radio frequency (RF) device performance, were studied as a function of the annealing temperature. A 30-sec anneal at 650°C and 700°C under N₂ ambient effectively eliminates hydrogen from the base. As the annealing temperature is increased, the base sheet resistance decreases, and the corresponding maximum frequency of oscillation increases. For all annealing temperatures studied, we found degradation in the DC gain, presumably caused by the increase of nitrogen concentration in the base region.     

Thermal activation of shallow boron-ion implants

Boron implanted into n-type Si at 10¹⁵ cm⁻² dose and energies from 500 eV to 1 keV was activated by annealing in nominally pure N₂ and in N₂ with small admixtures of O₂. Effective process times and temperatures were derived by thermal activation analysis of various heating cycles. The lowest thermal budgets used “spike anneals” with heating rates up to 150°C/sec, cooling rates up to 80°C/sec, and minimal dwell time at the maximum temperature. Dopant activation was determined by sheet electrical transport measurements. Surface oxidation was characterized by film thickness ellipsometry. P-n junction depths were inferred from analysis of sheet electrical transport measurements and secondary ion mass spectroscopy profiles. Boron activation increases with boron diffusion from the implanted region. Electrical activation has a thermal activation energy near 5 eV, while boron diffusion has an activation energy near 4 eV. Surface oxide can retard boron diffusion into the ambient for high-temperature anneals.     

Characterization of the effectiveness of carbon incorporation inSiGe for the elimination of parasitic energy barriers in SiGe HBTs

An electrical method is applied to SiGe and SiGe:C heterojunction bipolar transistors (HBTs) to extract the bandgap narrowing in the base layer and to characterize the presence of parasitic energy barriers in the conduction band, arising from boron transient enhanced out-diffusion from the SiGe layer. It is shown that a background carbon concentration within the base (≈10²⁰ cm^-3) eliminates parasitic energy barriers at the collector/base junction, and hence shows that transient enhanced diffusion of boron from the base has been completely suppressed