poly-Si Emitter Research Articles

Recombination behavior and contact resistance of n+ and p+ poly-crystalline Si/mono-crystalline Si junctions

We present an investigation of the electrical characteristics – recombination and contact resistance – of poly-crystalline (poly) Si/mono-crystalline (c) Si junctions and of the influence of the interfacial oxide between the poly-Si and the c-Si on these characteristics. In particular, we compare thermally grown oxides with different thickness values with wet chemically grown oxides. Both n- and p-type poly-Si emitters are investigated. For one combination (n-type poly-Si, thermal oxide), we compare planar and textured surfaces.For all oxide types investigated, we achieve combinations of low recombination current densities <20fA/cm² and low specific contact resistances <0.1Ωcm². The corresponding implied open-circuit voltages measured on our test structures are 732mV (n-type poly-Si) and 711mV (p-type poly-Si). By applying these poly-Si layers on a solar cell structure, we achieve an open-circuit voltage of 714mV and a series resistance of 0.6Ωcm².

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We present an analytical model for the current transport in polycrystalline (poly)Si/interfacial oxide/monocrystalline ( c)-Si base junctions, which consistently describes the symmetrical behavior of an n+ poly-Si emitter/ p c-Si base and p+ poly-Si emitter/ n c-Si base configuration. Our model is focused on a regime within which the current transport is possibly dominated by a flow through oxide pinholes rather than by tunneling. For an emitter region assumed to form underneath the interfacial oxide by diffusion of dopants from the poly-Si into the c-Si, we calculate the minority charge carrier distribution and the resistance implied for majority charge carriers. With reasonable parameters, our model simultaneously reproduces the experimentally observed low emitter saturation current densities and low junction resistances values. Our model provides a plausible explanation for the high current gain observed in p-n-p and n-p-n bipolar transistors featuring a poly-Si emitter. In principle, the obtained correlation between recombination current and series resistance is analogous to the situation in a base region of a solar cell with local rear contacts. Thus, a poly-Si/ c-Si junction can be explained within the framework of a classical p-n junction picture for a passivated, locally contacted emitter.

DC and low-frequency-noise characterization of epitaxially grown raised-emitter SiGe HBTs

DC and low-frequency-noise characteristics of SiGe HBTs with a raised-emitter structure, fabricated by epitaxial growth of phosphorous-doped Si layers, were investigated. Experimental results indicate unexpected emitter-size dependencies of both base current and low-frequency noise, because mono–poly interfacial native oxides close to the intrinsic emitter-base junction are localized at the emitter periphery. The raised mono-Si emitter SiGe HBT with a scaled emitter exhibits low-frequency noise that is about ten times smaller than a conventional poly-Si emitter SiGe HBT.

Hydrogen-induced dopant neutralisation in p-type AIC poly-Si seed layers functioning as buried emitters in ALICE thin-film solar cells on glass

Poly-Si thin-film solar cells fabricated on glass superstrates via the aluminium-induced crystallisation solid-phase epitaxy (ALICE) method, where an aluminium-induced crystallisation (AIC) poly-Si seed layer is thickened via solid-phase epitaxy (SPE) of a-Si:H, are investigated. For comparison, solar cells made by solid-phase crystallisation (SPC) of a-Si:H are also investigated. In both cases, the a-Si:H films are deposited by plasma-enhanced chemical vapour deposition (PECVD) vapour. The structural quality, as determined by Raman and ultraviolet (UV) reflectance measurements, of ALICE diodes is found to be superior to that of SPC diodes. Sheet resistance profiling of n-type ALICE and p-type SPC diodes reveals that the Al doping of the AIC poly-Si layer, which is designed to function as a p + emitter in n-type ALICE diodes, is almost completely neutralised during the high-temperature hydrogenation process. In contrast, the n + (phosphorus) emitter of the p-type SPC diodes is not neutralised appreciably by the same hydrogenation process. To overcome the problem with Al-doped emitters, an ALICE structure with an n + AIC poly-Si emitter and a p-type base (glass/SiN/n +p −p +) is suggested and experimentally investigated. The doping in the n + emitter of ALICE cells is found to be essentially unaffected by the hydrogenation process. Open-circuit voltages of up to 480 mV are achieved for p-type ALICE cells with an n + AIC poly-Si emitter. These voltages seem to be the highest reported so far for poly-Si thin-film solar cells on glass featuring an AIC poly-Si seed layer.

Promoting emitter diffusion process and optimization of vertical profiles for high-speed SiGe HBT/BiCMOS

A high-temperature anneal-resistant process, which enables high-speed SiGe HBTs to embed scaled CMOS, is optimized in SiGe BiCMOS technology. This process, called promoting emitter diffusion (PED), is based on enhanced phosphorous diffusion from poly-Si emitter electrodes at high temperature to fabricate thin base layers and shorten the base transit time. By investigating the dependence of high-frequency performance on diffusion temperature, as-grown base layer thickness, and Si cap thickness, the methodology for PED optimization was yielded. In addition, this PED process is effective in reducing an extrinsic base resistance due to deep boron diffusion from poly-Si base electrodes. This indicates that the PED process is very effective at improving the tradeoff relationship between cutoff frequency f/sub T/ and maximum oscillation frequency f/sub max/ in self-aligned SiGe HBTs using selective epitaxial growth. As a consequence, both f/sub T/ and f/sub max/ of more than 200GHz were successfully obtained.

Investigation of Various Metal Silicide Field Emitters and Their Application to Field Emission Display

Silicidation of tip surface of gated poly-Si field emitters was carried out to improve the electron emission behavior. Mo, Co, and Ti layers with a thickness of 25 nm were deposited through the gate opening using directional electron beam evaporation. Silicide layers were formed by subsequent rapid thermal annealing under environment. Compared with emission characteristics of poly-Si emitters, those of Mo- and Co-silicided poly-Si (polycide) emitters showed enhanced electron emission efficiency, that is, lower turn-on voltage and higher emission current. On the other hand, the emission characteristics of Ti-polycide emitters were remarkably degraded, which was due to the oxide formation on tip surface. A strong oxygen peak on the Ti-silicided surface was found from the spectrums of XPS and SIMS, indicating oxidation or oxygen-related contamination. A field emission display prototype with 25 × 25 pixels was successfully demonstrated using Mo-polycide emitter arrays. The prototype adopting low voltage anode scheme of ZnO:Zn phosphor produced maximum brightness of 70 cd/m2 at an anode voltage of 200 V. © 2002 The Electrochemical Society. All rights reserved.

Feasibility study of 25 V SiGe RF-power transistors for cellular base station output amplifiers

For 2+ GHz-bandwidth applications, the commonly used bipolar Si RF-power transistors in the output amplifiers in cellular base stations for mobile communications are pushed to the limit of performance. The objective of this work was to study the feasibility of using SiGe/Si grown on pre-processed Si wafers for power-HBTs operating at 25 V for improved performance. The large size HBT devices (2×0.1 mm 2) were processed using an existing 100 mm poly-Si emitter RF power BJT technology with Au metallization and some necessary modified steps for SiGe implementation. The base layers with designed Ge and B profiles were deposited either by MBE or CVD. The devices showed very high BV cbo (>80 V) with very low leakage currents. The current gain was very stable over a wide I C range, and weakly influenced by the environmental temperature. At 2 GHz, the CW output power of 20 W (at 25 V) was obtained with an efficiency of 68% in class AB operation. The long-term temperature stability was excellent. SiGe RF power-HBTs could be operated at full output power for an extended time without any external temperature bias compensation, which is virtually impossible with conventional Si-BJTs.

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.

A low-complexity 62-GHz fT SiGe heterojunction bipolar transistor process using differential epitaxy and in situ phosphorus-doped poly-Si emitter at very low thermal budget

A low-complexity SiGe heterojunction bipolar transistor process based on differential epitaxy and in situ phosphorus doped polysilicon emitter technology is described. Silane-based chemical vapor deposition at reduced pressure was used for low-temperature SiGe epitaxy. Following SiGe epitaxy, the process temperature budget was kept very low with 900°C for 10 s as the highest temperature step. A very high current gain of almost 2000 and cut-off frequency of 62 GHz were achieved for a uniform 12% Ge profile. The breakdown voltage BV CEO and forward Early voltage were equal to 2.9 and 6.5 V, respectively.

A high-speed low 1/f noise SiGe HBT technology using epitaxially-aligned polysilicon emitters

A 200 mm 0.35 /spl mu/m silicon-germanium heterojunction bipolar transistor (SiGe HBT) technology involving epitaxially-aligned polysilicon emitters is described. The devices are shown to combine the high speed performances typical for poly-Si emitter SiGe base devices (f/sub max/ up to 70 GHz) and the low 1/f noise properties of monocrystalline emitter structures (noise figure-of-merit KB as low as 7.2/spl times/10/sup -10/ /spl mu/m/sup 2/). Statistical current gain data are used to demonstrate the manufacturability of this innovative SiGe HBT technology.

Investigation of field emission characteristics for Si-base materials: Titanium silicide, poly-Si, and single crystal Si

We have fabricated poly-Si, Si, and Ti silicide field emitter arrays employing in situ vacuum encapsulated lateral field emitter structures and investigated the field emission characteristics such as turn-on voltage, emission current density, and the stability of the emission current. Although poly-Si and Si emitters have almost identical turn-on voltages, Si emitters have a sharper turn on than poly-Si emitters due to their uniform surface. The current densities of poly-Si and Si emitters are 0.47 and 0.43 μ A/tip, respectively, at an anode to cathode voltage of 90 V. The turn-on voltage and current density of Ti silicide emitters are about 31 V and 1.81 μ A/tip at VAK of 90 V. The data of the normalized current fluctuations indicate that the Ti silicide emitters have the most stable current. The experimental results show that Ti silicide is the most promising among these three materials due to its low work function, uniform surface, and stable characteristics.

A vacuum encapsulated lateral FED with various cathode materials

We have fabricated poly-Si, Si, and Ti-silicide field emitter arrays employing in-situ vacuum encapsulated lateral field emitter structures and investigated the field emission characteristics such as turn-on voltage, emission current density, and the stability of the emission current. Although poly-Si and Si emitter exhibit almost identical turn-on voltages, the Si emitter shows a sharp turn-on characteristic compared with the poly-Si emitter. It may be caused by the uniform surface of the Si emitter. The current densities of poly-Si, and Si emitter are 0.47, 0.43 µA/tip respectively when the anode to cathode voltage is 90 V. The turn-on voltage and current density of the Ti-silicide emitter are about 31V, and 1.81 µA/tip at a VAK of 90 V. The normalized current fluctuations shows that the Ti-silicide emitter exhibits the most stable current.Our experiment shows that Ti-silicide is most promising among those three cathode materials due to its low work function, uniform surface and stable characteristics.

An advanced PNP bipolar transistor design for a low-power and very-high-performance quarter-micron CBiCMOS process

This paper describes an advanced PNP bipolar transistor which has been designed by using the mixed two-dimensional device/circuit simulation (CODECS) [1] for a low-power and very-high-performance 0.25 μm complementary BiCMOS (CBiCMOS) device. The optimized PNP structure has a 30-nm-wide emitter, a 39-nm-wide intrinsic base region, a maximum cut-off frequency of 14 GHz and a current gain of 16 (without poly-Si emitter effect). A high performance and limits in terms of delay for pull-down of 0.25 μm CBiCMOS were obtained and compared to those offered by BiCMOS and complementary metal-oxide semiconductor circuits at different power supplies and charge capacitance. An improvement of 1.5 × at 1 pF, 1.6 × at 0.6 pF and 2 × at 0.2 pF over BiCMOS has been achieved.

Improvement of electron emission efficiency and stability by surface application of molybdenum silicide onto gated poly-Si field emitters

As an approach to improve electron field emission and its stability, molybdenum (Mo) silicide formation on n/sup +/ polycrystalline silicon (poly-Si) emitters has been investigated. Mo silicide was produced by direct metallurgical reaction, namely, deposition of Mo and subsequent rapid thermal annealing. The surface morphologies and emission properties of Mo-silicided poly-Si (Mo-polycide) emitters have been examined and compared with those of poly-Si emitters. While anode current of 0.1 /spl mu/A per tip could be obtained at the gate voltage of 82 V from poly-Si emitters, the same current level was measured at 72 V from Mo-polycide emitters. In addition, the application of Mo silicide onto poly-Si emitters reduced the emission current fluctuation considerably. These results show that the polycide emitters can have potential applications in vacuum microelectronics to obtain superior electron emission efficiency and stability.

Surface application of molybdenum silicide onto gated poly-Si emitters for enhanced field emission performance

This article investigates the merits of molybdenum (Mo) silicide formation on gated polycrystalline silicon (poly-Si) field emitters. Metal silicides are promising materials for field emission cathode due to their high electrical and thermal conductivity and high-temperature stability. In our experiment, Mo silicide was produced by direct metallurgical reaction, that is, deposition of Mo and subsequent rapid thermal annealing. The surface morphologies and field emission properties of Mo-silicided poly-Si (Mo-polycide) emitters have been examined and compared with those of pure poly-Si emitters. Field emission from these Mo-polycide emitters exhibited significant enhancement compared with pure poly-Si emitters in both total emission current and stability. The reason for this improved electron emission efficiency and stability could be explained by the smaller work function and better surface inertness of Mo-polycide emitters than those of poly-Si emitters.

Comparison of Lateral Field Emitter Characteristics for Titanium Silicide, Poly-Si, and Single Crystal Si Tip

AbstractWe have fabricated poly-Si, Si, and Ti-silicide field emitter arrays employing in-situ vacuum encapsulated lateral field emitter structures and investigated the field emission characteristics such as turn-on voltage, emission current density, and the stability of emission current. Although poly-Si and Si emitter have almost identical turn-on voltages, Si emitter has a sharper turn-on than poly-Si emitter due to its uniform surface. The current densities of poly-Si, and Si emtter are 0.47, 0.43 μA/tip respectively at anode to cathode voltage of 90 V. The turn-on voltage and current density of Ti-silicide emitter are about 31 V, and 1.81 μA/tip at VAK of 90 V. The data of the normalized current fluctuations indicate that Ti-silicide emitter has the most stable current.Our experiment shows that Ti-silicide is most promising among these three materials due to its low work function, uniform surface, and the stable characteristics.

Tem Studies of Polysilicon Emitter Bipolar Materials &amp; Devices: Increased Interfacial Oxide Break-Up and Polysilicon Regrowth and Decreased Emitter Resistance by Fluorine Implantation

AbstractThe role of fluorine (F) in the break-up of the native oxide and the regrowth of As doped poly-Si layers on unpatterned Si wafers and on patterned regions of Si device wafers at temperatures of 900°C to 1000°C are investigated by TEM and by the fabrication of npn poly-Si emitter bipolar devices. Results for unpatterned wafers with F show i) a 950°C dopant drive-in anneal causes oxide break-up and regrowth after a time suitable for the fabrication of devices, ii) a pre-anneal, before the As implant, further enhances the break-up and regrowth and iii) there is an optimum F dose of 5×1015/cm2. Based on these results poly-Si emitter bipolar devices were fabricated using F=5×1015/cm2, a pre-anneal and a 900°C As drive-in anneal. The results establish quantitatively the relationship between the interface structures and the specific emitter resistance, i.e. with no F there is no break-up or regrowth and the emitter resistance is high (114Ωμm2) while with F there is break-up and regrowth and the emitter resistance is low (17Ωμm2).

New Lateral Field Emitter Arrays Inherently Integrated with Thin Film Transistor

AbstarctWe have fabricated a new three-terminal lateral field emitter structure in which the anode current is limited by the channel current of undoped region. The new device exhibits an excellent stability of the emission current. The field emission characteristics of fabricated device have two modes. In the first mode below 89 V, the mechanism of emission is identical to that of conventional poly-Si emitters and, in the second mode above 89 V, the emission current is limited by the inversion charges in the channel, so stable anode current is maintained. Furthermore, the fabrication process of the device is very simple.

Hardness assurance testing of bipolar junction transistors at elevated irradiation temperatures

The effect of dose rate on radiation-induced current gain degradation was quantified for radiation-hardened poly-Si emitter n-p-n bipolar transistors over the range of 0.005 to 294 rad(Si)/s. Degradation increases sharply with decreasing dose rate and saturates near 0.005 rad(Si)/s. The amount of degradation enhancement at low dose rates decreases monotonically with total dose. In addition, the effect of ambient temperature on radiation-induced gain degradation at 294 rad(Si)/s was investigated over the range of 25 to 240/spl deg/C. Degradation is enhanced with increasing temperature while simultaneously being moderated by in situ annealing, such that, for a given total dose, an optimum irradiation temperature for maximum degradation results. The optimum irradiation temperature decreases logarithmically with total dose and, for a given dose, is smaller than optimum temperatures reported previously for p-n-p devices. High dose rate irradiation at elevated temperatures is less effective at simulating low dose rate degradation for the n-p-n transistor than for the p-n-p transistors. However, additional degradation of the n-p-n device at elevated temperatures is easily obtained using overtest. Differences in the radiation responses of the device types are attributed to the relative effects of oxide trapped charge on gain degradation. High dose rate irradiation near 125/spl deg/C is found to be suitable for the hardness assurance testing of these devices provided a design margin of at least two is employed.

Process design and emission properties of gated n+ polycrystalline silicon field emitter arrays for flat-panel display applications

The gated n+ polycrystalline silicon (poly-Si) field emitter arrays (FEAs) have been designed and successfully fabricated on an oxidized silicon wafer for large display applications. The proposed structure of the FEAs eliminates the difficulty of having the cathode electrode (n+ diffusion layer)-to-cathode electrode isolation, which is common to crystalline silicon (c-Si) field emitter arrays. Compared with c-Si field emitters, poly-Si emitters showed poor uniformity in device structure such as emitter shape and gate hole, which was thought to be due to the variation of the grain size of poly-Si and the oxide thickness associated with grain boundaries of poly-Si in the sharpening oxidation step. The anode current of 0.1 μA/tip was measured at the gate bias of 82 V from poly-Si emitters with gate hole diameter of 1.2 μm under the vacuum pressure of 3×10−9 Torr. The same anode current was obtained at 80 V from c-Si emitters with the gate hole diameter of 1.6 μm. The gate leakage current for both the c-Si FEA and the poly-Si FEA was much less than 1% of the anode current. Long-term fluctuation characteristics in emission current of the poly-Si FEA are also studied under different sample treatments. In spite of much room for improvement, the emission characteristics of poly-Si FEAs are outstanding in terms of emission current and gate leakage current, compared with those of previously reported poly-Si FEAs.