Carrier Lifetime Control Research Articles

Irradiation of power semiconductor devices by high energy electrons: The Italian experience

Starting from 1982, research in the field of electron irradiation of power semiconductor devices has been conducted in Italy on experimental basis. The use of high energy electrons for carrier lifetime control in these devices has been widely investigated. In collaboration with the three major Italian power semiconductor firms, Ansaldo, IRCI and SGS-Thomson Microeletronics, series of irradiation tests have been performed on fast diodes and thyristors, GTO thyristors, power MOS transistors and insulated gate bipolar transistors in order to achieve modelling of the electrical parameters and specific adjustment of carrier lifetime. The radiation-induced defects in silicon have been studied and characterized by the DLTS technique. The optimum conditions for radiation processing of different types of fast switching power devices with medium and high blocking capabilities have been determined. Diodes and thyristors, in all power categories, which did not have the required switching and release times after diffusion, were properly adjusted by irradiation thus saving them from rejection. The results obtained have led the above mentioned firms to use this technology for series production in place of the conventional high-temperature heavy metal diffusion for lifetime control. Equipment and methods for series production have been developed, tested and put into operation.

MeV ion processing applications for industry

Ions beams with MeV energies produce a variety of interactions with matter, broadly classified as either electronic or nuclear. These interactions in turn lead to changes in the properties of the matter which may be beneficial or detrimental. In high technology industry, use is increasingly made of ion beam technologies to process novel materials. Typical applications include high energy implantation, in which the deposition of a specific element at depth within the structure of material is the required objective, and irradiation modification, in which the balance between the beneficial and the detrimental effects of the fast ion interactions is exploited. The basic principles behind MeV ion processing are described. Broad areas of application in industrial materials include effects in ion beam analysis, Thin Layer Activation for wear and corrosion measurement, carrier lifetime control in electronic devices, and the simulation of radiation damage effects in, for example, solar cells for spacecraft. New development areas are described in which subtle but potentially significant changes in the chemistry of surfaces and interfaces may be generated by exposure to MeV ion beams.

Minority carrier lifetime control in power semiconductor devices by fast neutron irradiation

A new technology for controlling reduction of minority carrier lifetime of power semiconductor devices was given in this paper, using fast neutron irradiation in nuclear reactor. The effects of high frequency thyristor by fast neutron irradiation is mainly discussed. Furthermore, a comparison is made with electron irradiated, 60Co-gamma irradiated and gold diffused devices. The conclusion shows, the trade-off of forward voltage drop and turn-off time of fast neutron irradiated devices is alike with gold diffused devices, while the leakage current is less than that of gold diffused devices, and not different from the electron or Gamma irradiated devices. Moreover, the deep energy levels of fast neutron irradiation in silicon was shown in this paper, and a single dominant level of Ec-0.44eV is identified for controlling minority carrier lifetime.

Opportunities in silicon photovoltaics and defect control in photovoltaic materials

Silicon is the dominant semiconductor material in use for terrestrial photovoltaic cells. The critical issues in cell design, materials specification and cell processing are reviewed. Specific attention is given to light trapping, minority carrier lifetime control and novel microstructures for enhancing photon absorption. Relevant topics in generic defect control are discussed with particular emphasis on compound semiconductor materials.

Novel electrical and annealing properties of defects in electron irradiated silicon p+-n junctions

Novel results on defect annealing behavior and minority-carrier lifetime control in electron irradiated silicon p+-n junctions are presented. Two mechanisms are found to be involved in the annealing process of the divacancies; one dominates in the lower temperature range (from 240 to 300 °C) and the other dominates in the higher temperature range (from 320 to 360 °C). A defect labeld as E3 with an energy level at 0.37 eV below the conduction band is found to be an efficient recombination channel responsible for minority carrier lifetime control. The activation energy for dissociation of the defect E3 obtained from the annealing study is 1.7 eV, and the frequency factor is 2.8×109 s−1. Annealing of electron irradiated samples at about 300 °C, or performing the electron irradiation at a similar high temperature is found to increase the concentration of the defect E3, and stabilize the carrier lifetime. These processes might be useful to improve the thermal stability of devices like high-voltage rectifiers and thyristors.

Current status and prospects of electron irradiation of power semiconductor devices in Italy

This report is a short review of the work done in Italy on the electron irradiation of power semiconductor devices. Electron irradiation for carrier lifetime control has been investigated using a 12 MeV linear accelerator. These studies includes: modeling of electrical semiconductor component parameters, specific adjustment of carrier lifetime, DLTS measurements to identify and characterize the radiation-induced defects. The optimum conditions for radiation processing of various types of power diodes and thyristors have been determined. Equipment and methods for series production have been developed and tested with pilot production quantities, then put into operation for mass production.

ChemInform Abstract: Control of Minority Carrier Lifetime by Gold Implantation in Semiconductor Devices.

ChemInform Abstract: Control of Minority Carrier Lifetime by Gold Implantation in Semiconductor Devices.

Carrier-lifetime control of photochemical dry etching using elevated impurity concentrations

Virtually total suppression of carrier-driven photochemical etching of semi-insulating GaAs, n-GaAs, and n+-GaAs has been achieved by indiffusion of Zn to a surface carrier concentration greater than 1020/cm3. This technique for photochemical reaction suppression can be used as a self-aligned etching technology for semiconductor device fabrication.

Control of Minority Carrier Lifetime by Gold Implantation in Semiconductor Devices

Silicon wafer and devices have been implanted with gold ions and thermally processed in the 1073–1243 K temperature range. Resistivity profiles, leakage currents in p‐n junctions, and recombination lifetime have been measured as a function of the Au‐implanted dose, annealing temperature, and time. The use of ion implantation for the predeposition of the Au atoms allows a good control of the lifetime in a rather wide range of values. In addition, it is shown that it is very easy to generate flat gold profiles across the silicon wafers.

Fast ion induced defects in silicon and semiconductor applications

A facility for irradiation of large areas with MeV ions is described. The system is primarily intended to be used for adhesion improvement of thin films and charge carrier lifetime control in semiconductors. In the latter application MeV protons and alphas are used to produce localized regions of low lifetime, since most of the lattice damage appears at the end of the ion range. The defects created by the irradiation appear as deep recombination levels in the silicon bandgap, thus reducing the lifetimes of electrons and holes. The defects are studied by resistivity profiling and deep level transient spectroscopy. The role of nuclear and electronic stopping in the defect creation is discussed.

Control of carrier lifetimes in PbTe doping superlattices

Following subnanosecond pulsed laser excitation, time-resolved photoconductivity has been measured in PbTe doping superlattices in the temperature range 50–200 K. The carrier lifetime changes drastically by about three orders of magnitude during the recombination processes indicating an instantaneous lifetime which varies from a ns time scale up to about 1 μs. The temporal evolution is explained by the initially complete flattening of the built-in superlattice electrostatic potential and its recovery with the decrease in the nonequilibrium carrier concentration. The temperature dependence of the recombination is determined by both the bulk band structure of PbTe and the superlattice potential. The effective lifetime initially increases with temperature due to a decrease in the electron-hole recombination by tunneling, but decreases at higher temperatures where a thermal activation process dominates.

Control of carrier lifetime in PbTe nipi-superlattices by external photoinjection

Photoexcitation of PbTe doping superlattices at different intensity levels can have a large influence on nonequilibrium carrier lifetime which in turn is determined both by PbTe bulk properties and details of the superlattice periodic potential. Such “tunability” in the recombination rate is examined theoretically in a model which considers details of carrier tunneling and thermal excitation in the superlattice as well as the role of Auger recombination as a decay channel at high densities. Good agreement is obtained with experimental studies of transient photoconductivity in which picosecond pulses from a Nd:YAG laser have been used to vary the effective lifetime from less than 2 nsec up to 10 usec. We also show how the presence of 300 K background radiation has an important influence on practical recombination rates.

Ion implantation for carrier lifetime control in silicon technology

Carrier lifetime characterizes the return to equilibrium in a semiconductor by recombination and generation of carriers. Its value affects the performance of many silicon devices. Ion implantation is an emerging method whereby lifetime can be reduced. A major advantage of this method is the possibility of three-dimentional localization of regions of low lifetime within devices and integrated circuits. Argon and proton implantation are discussed as examples of two different approaches to localization; one relies on thermal stability of damage, and the other on the nonuniform energy deposition profile of ions coming to rest in matter. Proton implantation and electron irradiation are compared in an application of lifetime control to insulated gate transistors. It is shown that device performance is improved with proton implantation if the appropriate energy is used.

Very High Speed Static Induction Thyristor

Characteristics of a newly developed static induction thyristor (SIThy) are described. The SIThy is irradiated by 2-MeV protons to improve the switching speed as a result of local carrier lifetime control. The characteristics of the proton irradiated SIThy are controlled by annealing conditions to obtain devices for various applications. The switching speed of the SIThy is very high; for example, at an anode current of 50 A, its rise time, storage time, and fall time are 100 ns, 60 ns, and 50 ns, respectively. Thus the newly developed SIThy is suitable for high-speed switching devices.

Localized lifetime control in insulated-gate transistors by proton implantation

The application of localized carrier lifetime control to power devices is explored. A new type of power device, the insulated-gate transistor, was chosen for this study. Proton implantation was used as an agent of lifetime control by varying energy (1 to 3.8 MeV) and dose. Most of the proton damage in silicon, and therefore most of the lifetime reduction, occurs near the proton end-of-range, resulting in a localized band of low lifetime. Proton energy determines the depth at which the region of minimum lifetime is placed within the device structure. The effect of localized lifetime reduction on forward voltage, turn-off time, and leakage current was studied. The results lead to an understanding of the proper choice of depth at which to place a region of low lifetime, and illustrate the advantages of localization. With this choice, a significantly better tradeoff curve of forward voltage versus turn-off time is shown to be realized with proton implantation compared to the unlocalized but common technique of electron irradiation.

High-resolution EM study of the gold silicon interface reaction

Gold occupies a key role in silicon solid state device technology, for contacts and carrier lifetime control. Gold is added to silicon by evaporation onto HF cleaned Si wafers, followed by annealing at high temperature. The Au-Si binary system is a simple eutectic, with very limited terminal solid solubility and no reported stable intermediate compounds. When Si wafers are HF cleaned and exposed to air, even for very short times, a native oxide film a few nm thickness will form, and be interposed between the Si substrate and the evaporated Au layer. The effect of this film on the interface reaction is of interest. Alternatively, conventional surface research on Au/Si interfaces formed by UHV in-situ cleaning, evaporation and annealing provided evidence for an amorphous Au-Si layer and metastable crystalline Au silicides at the position of the original Au/Si interface.

On the use of a microwave linear accelerator for control of carrier lifetime in electronic silicon devices

The possibility of using a 12 MeV linear accelerator for control of minority carrier lifetime in silicon power rectifiers and thyristors to be used at high frequencies has been investigated. Electron irradiation of these devices has been used in place of metallic diffusion and 60Co gamma irradiation. A comparison with the latter two techniques is made. The relation between absorbed dose and high and low injection lifetimes (τ HL and τ LL) has been studied for diodes and from the linear relationship 1 τ = 1 τ 0 + KD found, values for the damage constant have been calculated. The dependence of the stability of the electrical characteristics of the silicon devices after electron irradiation on the annealing process at 150, 200, and 360°C has been analyzed. The conclusions drawn from this study are that charge lifetime control in semiconductors is feasible by high energy electrons because electron irradiation provides the more precise, uniform, and reproducible method of lifetime control. In addition this technique is much better than the conventional metal diffusion or gamma irradiation in raising the quality and end-product rate of these devices.

Carrier Lifetime Reduction by Ion Implantation into Silicon

ABSTRACTIon implantation is emerging as a versatile tool for the control of carrier lifetime in silicon. We present results on the use of argon and proton implantations to reduce lifetime. Particle energies used imply submicron penetration depths into the silicon. We find that within the first micrometer from the surface, the generation lifetime can be reduced by several orders of magnitude, and varies inversely with dose. A change in doping concentration was found only at the highest dose used (1014 cm−2). Effects of the implantations on surface generation velocity and oxide charges were also studied.

Electron irradiation induced recombination centers in silicon-minority carrier lifetime control

Recombination centers introduced in silicon p+-n-n+structures by irradiation with 2-MeV electrons are studied by measuring minority carrier lifetime and annealing kinetics. The approximate location of these recombination centers in the forbidden gap and their densities are obtained by the thermally stimulated current method. The results identify one defect as a divacancy with an energy level of E v + 0.26 eV. Possible identities of other deep levels are discussed. The technique of minority carrier lifetime control by electron irradiation has been developed into a reliable manufacturing process for power devices.