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STUDY OF DEEP LEVELS ASSOCEATED WITH Pt AND Pd CENTRES IN Si AND MID-GAP LEVEL IN GaP

Ali, Asghar (1993) STUDY OF DEEP LEVELS ASSOCEATED WITH Pt AND Pd CENTRES IN Si AND MID-GAP LEVEL IN GaP. PhD thesis, Quaid-i-Azam University, Islamabad .

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Abstract

Deep levels in Pt- and Pd- doped si and GaP green light emitting diodes (LED's), which are expected to be efficient recombination centres, were investigated. In si:Pt and GaP LED's the mid-gap levels were the focus of interest, while in si:Pd detailed characterization of all the Pd related levels was carried out. In the case of GaP, the samples were commercially fabricated LED's. pt and Pd was doped into silicon by solid state diffusion process carried out in Ar atmosphere, in sealed quartz ampoules, at 850°C and 950°C respectively. Deep level transient spectroscopy (DLTS) and single shot dark capacitance transient (SSDCT) measurements were used as complementary techniques for deep level characterization. Optical DLTS was also employed in some cases. To identify the origin of deep levels, samples were annealed 1sochronally from room temperature to -400 °C and exposed to a 5.48 MeV a-particle source (maximum dose of 3.8X1010 particles/cm2, in steps) to observe the response of the deep levels to these treatments. Post-irradiation annealing was also carried out. In GaP green LED's the DLTS measurements revealed one mid-gap level lying at Ec -1.02 eV. The transients obtained by SSDCT technique revealed the presence of two levels. The line-shape analysis of the DLTS spectra confirmed the results of SSDCT. The thermal activation energies obtained by SSDCT measurements were Ec -1.03 eV and Ec -0.93 eV for the two levels. The shallower level could possibly be a complex of PGa antisite defect with an unknown entity Y. The deeper level could be the analog of the well known level EL2 ( in GaAs) in GaP. In the case of Pt-doped silicon, the Pt-related mid-gap level was observed in our pt diffused samples. The thermal activation energy obtained was Ec -0.55 eV and capture cross-section measured at 279 K was 1.6x10-16 cm2. Our minority carrier injection DLTS and ODLTS measurements revealed that for this mid-gap levelσp>>σⁿ,which shows that it is a good candidate for the dominant recombination centre in si:Pt. For the other two well-known Pt-related levels, it was found that σp ≈σnfor the Ec -0.23eV acceptor and σ >σ for the Ev +0.32 eV donor level. In Pd doped si, four Pd-related levels were observed simultaneously in as-diffused (and quenched) samples for the first time. The activation energies found were E1 (Ec -0.18 eV), E2 (Ec -0.22 eV), E3 (Ec -0.37 eV) and E4 (Ec -0.59 eV). The detailed pre- and Post-irradiation annealing behaviour of the E1 and E2 levels negate a previous suggestion that these correspond to two configurations of Pd-vacancy complex which transform into each other at high temperatures. These two levels do, however, seem to be related to substitutional Pd. Different concentrations, concentration profiles and different responses to irradiation Prove that E3 and E4 have different origins which contradicts the hitherto held belief that they are different charge states of the same defect. α-irradiation introduced five expected levels in reference and Pd-doped samples. The presence of Pd, in general, suppresses the production of these levels. Combined with the observation that the Pd-related levels in general increase in concentration upon this treatment, it is concluded that the primary defects produced by a-irradiation make complexes with electrically inactive Pd atoms activating them electrically. In post-irradiation annealing of Pd-doped samples a new hole level H(α-Pd) emerged at Ev +0.27 eV which has been observed for the first time. This level could be a complex of Pd with some a-induced defect which needs annealing of 100 °C to become electrically active. Four new annealed-in levels appeared in post-irradiation annealing of reference samples (not reported for the virgin samples). These four levels were E12 (α)Ec -0.37 eV], E13(α)[Ec -0.31 eV], E14(α)[Ec -0.13 eV] and H5 (α) [Ev +0.16 eV]. These levels are suggested to be complexes of some quenched-in defects with radiation-induced defects which require annealing for becoming electrically active.

Item Type:Thesis (PhD)
Subjects:Physical Sciences (f) > Physics(f1)
ID Code:2592
Deposited By:Mr. Javed Memon
Deposited On:08 Jul 2009 10:30
Last Modified:08 Jul 2009 10:30

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