Investigation of Pd/MoO_x/n-Si diodes for bipolar transistor and light-emitting device applications

Sub-stoichiometric molybdenum oxide (MoO x) has recently been investigated for application in high efficiency Si solar cells as a "hole selective"contact. In this paper, we investigate the electrical and light-emitting properties of MoO x-based contacts on Si from the viewpoint of realizing functional bipolar devices such as light-emitting diodes (LEDs) and transistors without any impurity doping of the Si surface. We realized diodes on n-type Si substrates using e-beam physical vapor deposition of Pd/MoO x contacts and compared their behavior to implanted p +n-Si diodes as a reference. In contrast to majority-carrier dominated conduction that occurs in conventional Schottky diodes, Pd/MoO x/n-Si diodes show minority-carrier dominated charge transport with I-V, C-V, and light-emitting characteristics comparable to implanted counterparts. Utilizing such MoO x-based contacts, we also demonstrate a lateral bipolar transistor concept without employing any doped junctions. A detailed C-V analysis confirmed the excessive band-bending in Si corresponding to a high potential barrier (> - > 0.90 V) at the MoO x/n-Si interface which, along with the observed amorphous SiO x(Mo) interlayer, plays a role in suppressing the majority-carrier current. An inversion layer at the n-Si surface was also identified comprising a sheet carrier density greater than 8.6 × 10 11 cm - 2, and the MoO x layer was found to be conductive though with a very high resistivity in the 10 4 ω-cm range. We refer to these diodes as metal/non-insulator/semiconductor diodes and show with our device simulations that they can be mimicked as high-barrier Schottky diodes with an induced inversion layer at the interface

Low-Frequency Noise Characterization of Ultra-shallow Gate N-channel Junction Field Effect Transistors

A recently developed technique for ultra shallow pn junction formation has been applied for the fabrication of ring-gate n-channel junction field effect devices (JFET) devices. Several different geometries, gate formation parameters and channel doping profiles have been realized and characterized with respect to I-V and C-V characteristics both on wafer and after packaging. Low-frequency noise measurements have been performed on packaged devices by mean of a cross-correlation scheme. Data have been compared both with that of similar devices fabricated in a standard process and with simulations results. The devices show good DC performance, and the transconductance values achieved, with respect of the channel dimensions, are notably high. No G-R noise was detected, but the devices yielded a high flicker noise component. This phenomenon is shown to be neither correlated to the device area nor to incidental trapping levels at the junction interface. It is therefore assumed that perimeter effects are decisive for the enhancement of the 1/f spectrum

Comparing current flows in ultrashallow pn-/Schottky-like diodes with 2-diode test method

-A 2-diode test structure is proposed and investigated for use with simple I-V measurements, giving an easy-to-process, fast turn-around-time method of comparing process-dependent current flows when developing ultrashallowl Schottky junction technologies. Differential diode current characteristics and collector currents obtained from lateral transistor operation of the same 2-diode test structure are used to reliably identify the diode type and variations in metal-Si interfacial properties, independent of parasitic leakage currents. The versatility of this method with respect to diode geometry and substrate doping is verified for the measurement of junction- and Schottky-like diodes formed by different chemical-vapor-deposition processes

Silicon drift detectors with the drift field induced by pureB-coated trenches

Junction formation in deep trenches is proposed as a new means of creating a built-in drift field in silicon drift detectors (SDDs). The potential performance of this trenched drift detector (TDD) was investigated analytically and through simulations, and compared to simulations of conventional bulk-silicon drift detector (BSDD) configurations. Although the device was not experimentally realized, the manufacturability of the TDDs is estimated to be good on the basis of previously demonstrated photodiodes and detectors fabricated in PureB technology. The pure boron deposition of this technology allows good trench coverage and is known to provide nm-shallow low-noise p+n diodes that can be used as radiation-hard light-entrance windows. With this type of diode, the TDDs would be suitable for X-ray radiation detection down to 100 eV and up to tens of keV energy levels. In the TDD, the drift region is formed by varying the geometry and position of the trenches while the reverse biasing of all diodes is kept at the same constant voltage. For a given wafer doping, the drift field is lower for the TDD than for a BSDD and it demands a much higher voltage between the anode and cathode, but also has several advantages: it eliminates the possibility of punch-through and no current flows from the inner to outer perimeter of the cathode because a voltage divider is not needed to set the drift field. In addition, the loss of sensitive area at the outer perimeter of the cathode is much smaller. For example, the simulations predict that an optimized TDD geometry with an active-region radius of 3100 µm could have a drift field of 370 V/cm and a photo-sensitive radius that is 500-µm larger than that of a comparable BSDD structure. The PureB diodes on the front and back of the TDD are continuous, which means low dark currents and high stability with respect to leakage currents that otherwise could be caused by radiation damage. The dark current of the 3100-µm TDD will increase by only 34% if an interface trap concentration of 1012 cm−2 is introduced to approximate the oxide interface degradation that could be caused during irradiation. The TDD structure is particularly well-suited for implementation in multi-cell drift detector arrays where it is shown to significantly decrease the cross-talk between segments. The trenches will, however, also present a narrow dead area that can split the energy deposited by high-energy photons traversing this dead area. The count rate within a cell of a radius = 300 µm in a multi-cell TDD array is found to be as high as 10 Mcps

PureB diode fabrication using physical or chemical vapor deposition methods for increased back-end-of-line accessibility

Several methods of depositing pure boron (PureB) layers on silicon are examined with respect to their potential for fabricating advanced PureB (photo)diodes with back-end-of-line (BEOL) CMOS compatibility. PureB devices were fabricated in two different batch furnace chemical-vapor deposition (CVD) systems or by electron-beam-assisted physical-vapor deposition (EBPVD), and their electrical characteristics were found to be comparable to those of devices previously fabricated using single-wafer CVD and molecular beam epitaxy (MBE) systems. For all methods, the material properties of the B-layers and the I-V characteristics of the PureB diodes follow the same temperature dependence over the range 50 °C–400 °C. This was also the case for the EBPVD layers which were deposited at 50 °C and then annealed at higher temperatures, instead of being deposited at these temperatures as for the other methods. At 400 °C, the ability to achieve an optimal suppression of the electron injection into the PureB anode regions, corresponding to an electron current density of ∼20 pA/cm2, was verified for all methods. The advantages and disadvantages of each deposition method is evaluated with respect to equipment availability, B-layer selectivity, conformality, and thickness control. The batch furnace systems could be attractive for high-volume production, but hardware improvements as discussed here would be needed to reduce the effects of gas depletion. On all points except conformality, EBPVD appears to be a very good option for fabricating nm-thin B-layers suitable for fabricating high-performance 400 °C PureB diodes

Random telegraph signal phenomena in ultra shallow p+n silicon avalanche diodes

An extensive time domain analysis of the random telegraph signal (RTS) phenomena in silicon avalanche diodes is presented. Experiments show two distinct types of RTSs classified herein, on the basis of the temporal behavior of the amplitude, as the “decaying” and the “constant” type. These RTSs are analyzed using a model for defects reported earlier, from which their ohmic series resistance and geometrical parameters have been estimated. The results indicate that breakdown of a relatively small area defect results in a “decaying” amplitude type of RTS, and breakdown of a relatively large area defect results in a “constant” amplitude type of RTS. These two types can be explained by the differences in the thermal resistance, which is higher for the former

Random telegraph signal phenomena in avalanche mode diodes: application to SPADs

The current-voltage (IV ) dependency of diodes close to the breakdown voltage is shown to be governed by Random Telegraph Signal (RTS) phenomena. We present a technology independent approach to accurately characterize the bias dependent statistical RTS properties and show that these can fully describe the steep IV -dependency in avalanche. The statistical properties also allow to more accurately describe e.g. the value of the self sustaining avalanche current that is crucial in designing optical detection systems using avalanche photo diodes or single photon avalanche diodes (SPADs). More accurate modelling is shown to allow improving on e.g. count rates, dead time and afterpulsing in quenching and recharge circuits for SPADs. Measurements are performed on diodes in a 140 nm SOI CMOS technology

Investigation of light-emission and avalanche-current mechanisms in PureB SPAD devices

The light emission from silicon PureB photodiodes was investigated in both forward- and avalanchemode operation and correlated to the presence of process-dependent defects that influence the reverse IV characteristics. As opposed to “defect-free” diodes with low dark currents and abrupt breakdown behavior, the diodes with defects had higher current levels and light-emitting spots appearing at voltages far below the breakdown voltage otherwise set by the implemented doping profiles. The role of such defect-related behavior for the application of the photodiodes as single-photon avalanche diodes (SPADs) and avalanche-mode light-emitting diodes (AMLEDs) is assessed in connection with the recent demonstration of these basic devices as both the light-emitting and light-detecting elements in optocoupler circuits integrated in CMOS for data transmission purposes