Koshi Ando

DEEP LEVEL ANALYSIS OF RADIATION-INDUCED DEFECTS IN SI CRYSTALS AND SOLAR CELLS

Deep level transient spectroscopy (DLTS) analysis of radiation-induced defects in p-type Si crystals and solar cells has been carried out to clarify the mechanism of the anomalous degradation of Si n+–p–p+ structure space cells induced by high-energy, high-fluence electron/proton irradiations. A large concentration of a minority-carrier trap with an activation energy of about 0.18 eV has been observed in irradiated p-Si using DLTS measurements, as well as the majority-carrier traps at around Ev+0.18 eV and Ev+0.36 eV, Correlations between DLTS data and solar-cell properties for irradiated and annealed Si diodes and solar cells have shown that type conversion of p-Si base layer from p-type to n-type is found to be mainly caused by introduction of the 0.18 eV minority-carrier trap center, that is, this center acts as a deep-donor center. The Ev+0.36 eV majority-carrier trap center is thought to also act as a recombination center that decreases minority-carrier lifetime (diffusion length). Moreover, origins ...

The effect of oxygen on the properties of AlGaAs solar cells grown by molecular‐beam epitaxy

The effect of oxygen, which is unintentionally incorporated into AlGaAs films during epitaxial growth, on AlGaAs solar cell characteristics is investigated using molecular‐beam epitaxy. It is found that oxygen concentration strongly influences minority‐carrier diffusion length and that solar cell efficiency greatly decreases as oxygen concentration increases. A model to describe the effect of oxygen on solar cell characteristics is suggested.

Recombination enhanced defect reactions in 1 MeV electron irradiated p InGaP

Direct recombination enhanced annealing of the radiation-induced defect H2 in p InGaP has been observed by deep level transient spectroscopy (DLTS). Detailed analysis of the annealing data at zero and reverse bias shows that annealing rates are independent of the defect charge state or this defect interacts with the two bands, i.e., is a recombination center trapping alternatively an electron, then a hole. An experiment based on minority carrier capture on a majority trap by the double carrier pulse DLTS technique further supports the evidence that H2 has a large minority carrier capture cross section and is an efficient nonradiative recombination center. Recombination-enhanced defect annealing rates obeys a simple Arrhenius law with an activation enthalpy of 0.51±0.09 eV, in contrast to athermal processes observed in GaP. Detailed analysis of results reveals that the mechanism involved in the minority carrier injection annealing of the H2 defect is energy release mechanism in which enhancement is induced...

Nitrogen-doped p-type ZnSe films grown by MOVPE

Abstract Nitrogen doped ZnSe layers have been grown on (100) GaAs substrates by metalorganic vapor phase epitaxy using NH 3 as the doping material. The N-doped layers exhibit strong free to acceptor transition emission at 77 K and an acceptor bound-exciton emission line at 15 K. The layers with a high doping condition exhibit p-type conduction with resistivities from 10 2 to 10 3 ω cm, whereas undoped layers show high resistivity of about 10 5 ω cm. The N-doped layers have carrier concentrations in the order of 10 14 cm −3 and mobilities from 20 to 50 cm 2 /V·s. The p-type conduction has been confirmed by the current-voltage characteristics of N-doped ZnSe heterojunctions formed on GaAs substrates.

Stable avalanche-photodiode operation of ZnSe-based p+–n structure blue-ultraviolet photodetectors

A high-field operation of p+–n structure blue-ultraviolet photodetectors has been studied using ZnSe-based II–VI wide-band-gap compound semiconductors grown by molecular-beam epitaxy (MBE). With an improved crystal quality on macrodefects (dislocations and stacking faults) during an initial MBE growth as well as an optimized device structure including a complete superlattice ohmic-contact layer, a stable high electric-field operation up to 8×105 V/cm is established. It is demonstrated that the p+–n ZnSe photodiodes have shown a stable avalanche-photodiode operation with a large avalanche gain of G=60 in the blue-ultraviolet region at room temperature.

Deep hole trap properties of p‐type ZnSe grown by molecular beam epitaxy

The characteristics of deep hole traps in p‐type ZnSe are studied by means of a transient capacitance spectroscopy technique. p‐type ZnSe layers were grown by molecular beam epitaxy using radical N2 (nitrogen) doping. A major deep hole trap with a thermal hole activation energy of ΔE=720±30 meV is detected for two different sample structures: (a) n+‐p ZnSe diodes and (b) Au‐p ZnSe double‐Schottky diodes. The trap concentration tends to increase as the net acceptor (N) concentration increases. The deep hole trap has revealed an exponential temperature dependence of hole capture rates, indicating a strong electron‐lattice coupling in carrier capture/emission processes.

Microscopic defect induced slow-mode degradation in II–VI based blue–green laser diodes

Abstract We have studied the microdefect induced degradation mode in long-lifetime blue–green laser diodes (LDs) and light emitting diodes (LEDs) based on II–VI wide bandgap semiconductors. Microscopic deep defect centers in the LDs and LEDs are detected using mainly DLTS technique, coupled with ICTS methods. It is evidenced that a slow-mode degradation, commonly observed in dislocation-free LD devices, is caused by the generation and enhancement of microscopic deep centers during the device aging process. One possible degradation mechanism with a “carrier removal effect” is presented.

Comparison of the effects of electron and proton irradiation on n+–p–p+ silicon diodes

We have carried out an investigation of n+–p–p+ silicon diodes after irradiation with 1 MeV electrons and 10 MeV protons and subsequently after annealing. The effects upon the material and device parameters of samples irradiated with different particles are compared by expressing the particle fluence in terms of an effective absorbed dose of 1 MeV electrons. Although the spectrum of defects (observed by deep-level transient spectroscopy) introduced by 1 MeV electrons and 10 MeV protons was slightly different, the total defect introduction rate per effective 1 MeV electron dose was similar, as was the effect upon the device parameters. After irradiation with high fluences of electrons or protons, the effective carrier concentration in the base of the diodes was reduced dramatically, an effect referred to as “carrier removal.” The effects of carrier removal upon the device parameters, in particular, the series resistance and saturation current, are discussed in detail. In addition, the relative importance o...

Acceptor compensation mechanism by midgap defects in nitrogen‐doped ZnSe films

A new carrier compensation mechanism has been investigated in nitrogen‐doped ZnSe films grown by low‐pressure metalorganic vapor phase epitaxy. Photoconduction spectrum measurements have revealed the existence of deep defect levels located just above the center of the band gap in N‐doped, highly compensated films. These results combined with photo‐Hall measurements (under infrared illumination) have revealed that these midgap defect levels act as deep donors, and play an important role in carrier compensation in shallow acceptor doped ZnSe films.

Explanation for carrier removal and type conversion in irradiated silicon solar cells

Heavy doses of radiation in space can cause the failure of n+/p/p+ silicon solar cells due to the gradual introduction of compensating defects into the base layer of the diode. In this letter, we show that the radiation-induced defects, which play the most important role in this process, referred to as “carrier removal,” are probably minority-carrier traps at an energy level approximately 0.18 eV below the conduction band. We conclude that these defects must be positively charged before electron capture, and therefore, act as donor centers which compensate the p-type base layer.