S.S. Li

Radiation damage and annealing in GaAs solar cells

Work on radiation damage and annealing of GaAs cells for space power applications is reviewed. The performance of cells before and after irradiation and after thermal annealing is correlated to the nature and density of defect centers identified using deep-level transient spectroscopy techniques. Two important deep levels are present in electron- and proton-irradiated GaAs cells (E/sub c/ -0.71 eV and E/sub v/ +0.71 eV). The data from periodic and continuous annealing experiments show that GaAs cells are more radiation resistant than silicon and that the damage can be annealed at temperatures as low as 200 degrees C. On the basis of these data, specific annealing schedules that can greatly enhance the end-of-life efficiency of the cells for specific missions can be designed for GaAs solar panels. >

A model for analyzing the interface properties of a semiconductor-insulator-semiconductor structure. I. Capacitance and conductance techniques

A model is presented for analyzing the interface properties of a semiconductor-insulator-semiconductor (SIS) capacitor structure. By introducing a coupling factor, conventional metal-oxide-semiconductor (MOS) capacitor theory is extended to analyze the interface properties of the film/buried-oxide/substrate interfaces of a silicon-on-insulator (SOI) material. This model was used to determine parameters such as doping concentration, buried oxide thickness, fixed oxide charge, and interface trap density from the SIMOX (separation by implantation of oxygen) based SIS capacitors. >

TP-A1 enhancement of effective barrier height in a Ti-silicon Schottky diode using low-energy ion implantation

Increasing the effective barrier height in a Ti-p-type silicon Schottky diode has been achieved by means of low-energy ion implantation to introduce a thin inversion layer on silicon substrate. It is shown theoretically that effective barrier height equal to the energy bandgap can be obtained in such structure if the thickness and dopant density of the implanted layer are properly chosen. Experimental results for several titanium (Ti) on phosphorus implanted p-type silicon Schottky diodes show that effective barrier heights were increased from 0.6 eV for the Ti-p Si Schottky diode to 0.96 eV for a Ti-n-p-Si Schottky diode with a phosphorus-implanted layer thickness of 400 A and dose of 1.26 × 1012cm-2. Good agreement is obtained between the calculated and the measured barrier height for several Ti-n-p silicon Schottky diodes.

A study of the surface passivation on GaAs and In/sub 0.53/Ga/sub 0.47/As Schottky-barrier photodiodes using SiO/sub 2/, Si/sub 3/N/sub 4/ and polyimide

Studies of surface passivation and antireflection coating on GaAs and In/sub 0.53/Ga/sub 0.47/As interdigitated metal-semiconductor-metal (IMSM) Schottky-barrier photodiodes have been made using SiO/sub 2/, Si/sub 3/N/sub 4/, and polyimide deposited films. The results show that Si/sub 3/N/sub 4/ on GaAs and polyimide on In/sub 0.53/Ga/sub 0.47/As are the most effective dielectric films for reducing the leakage current and reflection loss in these photodiodes. >

Deep-level defects, recombination mechanisms, and their correlation to the performance of low-energy proton-irradiated AlGaAs—GaAs solar cells

A detailed study of deep-level defects, recombination mechanisms, and performance characteristics in low-energy proton-irradiated AlGaAs-GaAs solar cells has been carried out for several proton energies (i.e., 50, 100, 200, and 290 keV) and fluences (i.e., 1010, 1011, 1012, and 1013p/cm2) using the deep-level transient spectroscopy (DLTS), current-voltage (I-V), capacitance-voltage (C-V), and the scanning-electron microscopy operating in electron-beam induced current (SEM-EBIC) methods. Important defect parameters such as densities and energy levels of defects, thermal emission rates and capture cross sections of electrons and holes in each trap level as well as hole diffusion lengths in n-GaAs LPE layer were determined from these measurements. It is shown that the forward bias dark current was dominated by the recombination of electron-hole pairs via the proton-induced deep-level defects in the junction space-charge region of the cell. The increase in the dark current is directly related to the increase of trap density with increasing proton fluence in the irradiated solar cells. A quantitative correlation between the measured defect parameters and the performance characteristics of the proton-irradiated AlGaAs-GaAs solar cells was obtained from this study.

Theoretical calculations of Debye length, built-in potential and depletion layer width versus dopant density in a heavily doped p-n junction diode

Abstract Theoretical calculations of Debye length, built-in potential, depletion layer width and capacitance as a function of dopant density in a heavily doped p - n junction diode are described in this paper. The heavy doping effects such as carrier degeneracy, dopant density-dependent dielectric constant and bandgap narrowing are accounted for by using the empirical approximation for the reduced Fermi-energy given by[1] and the dopant density dependent dielectric constant given by[2], as well as the bandgap narrowing model proposed by[3]. The results show that: (1) bandgap narrowing and carrier degeneracy have important effects on the junction built-in potential; (2) carrier degeneracy and dopant density-dependent dielectric constant are important to Debye length for the abrupt junction case, and (3) the dopant density-dependent dielectric constant is a key parameter which strongly affects the values of depletion layer width and depletion capacitance. These findings are important for modeling of heavily doped p - n junction devices in the VLSI applications.

A model for analyzing the interface properties of a semiconductor-insulator-semiconductor structure. II. Transient capacitance technique

For pt.I see ibid., vol.39, no.7, p.1740-46 (1992). A bias-scan deep-level transient spectroscopy (DLTS) method for measuring interface states in a semiconductor-insulator-semiconductor (SIS) capacitor structure is described. Introducing a charge coupling factor between the film/oxide (F/O) and the substrate/oxide (S/O) interfaces made it possible to extract interface trap densities from the measured transient signals. This technique was used to determine the interface trap densities and capture cross sections at both the F/O and S/O interfaces of SIMOX (separation by implantation of oxygen) based SIS capacitors. >

Study of deep-level defects and annealing effects in undoped and Sn-doped GaAs solar cells irradiated by one-MeV electrons

Abstract DLTS and C-V techniques have been employed to determine the defect energy levels and density, carrier capture cross sections, lifetimes and diffusion lengths in the Sn-doped and the undoped GaAs solar cells irradiated by one-MeV electrons under different electron fluences (10 14 to 10 16 cm −2 ), fluxes (2 × 10 9 , 4 × 10 10 e/cm 2 -s), and annealing conditions (150 ⩽ T ⩽ 230°C). The results show that density of both electron and hole traps will in general increase with incresing electron fluence and flux, and decrease with increasing annealing temperature and annealing time. Some distinct difference in defeat spectrum was observed in the undoped and the Sn-doped GaAs solar cells studied. The low temperature thermal annealing and the recombination enhanced annealing processes are found to be very effective in reducing the density of deep-level defects induced by one-MeV electrons. The results of our findings are discussed in detail in this paper.

Increased junction breakdown voltages in silicon-on-insulator diodes

A junction breakdown model and the results of PISCES II simulations are presented for silicon-on-insulator (SOI) devices. This model shows the dependence of breakdown voltage in fully depleted (FD) SOI diode on the backgate bias, the properties of the buried oxide layer, and the device parameters. Breakdown in a thin FD SOI diode is quite different from that observed in a thicker, partially depleted (PD) diode. The analysis is supported by breakdown voltage measurements of separation by implantation of oxygen (SIMOX)-based SOI diodes, the results of which suggest that body breakdown is dominant in FD SOI diodes, and the junction curvature effect is dominant in PD SOI diodes. Furthermore, the results also show that breakdown voltage in the FD SOI diode is higher than their bulk-silicon counterpart and can be further increased by applying the appropriate backgate bias. >

Retention-temperature and endurance characteristics of the MNOS capacitor with processing variations as parameter

Abstract The p+-gridded MNOS capacitor, which enables us to simulate the operational characteristics of the MNOS FET, was used to study the effects of temperature on the retention characteristics and of endurance stress on the Si/SiO2 interface-state density and retention characteristics of the MNOS capacitors fabricated under four different processing conditions. The differences in the retention characteristics of these devices were interpreted in terms of the chemical compositions and structures arising from the different processing conditions and were used to infer relative nitride trap depths in these devices. The increase in the Si/SiO2 interface-state density and the deterioration in the retention characteristics resulting from endurance stress were explained in terms of variations in chemical composition and structure.