Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms
Metastable defects in SI-GaAs: Effect of high energy ion-irradiation
Introduction
Semi-insulating behavior of GaAs results from compensation of shallow acceptors by the intrinsic deep donor defect, EL2 [1]. Carbon is the main residual acceptor in GaAs, and present with a concentration of 1015–1016 cm−3, depending on the growth process. Since residual acceptors are compensated by EL2, the Fermi level gets pinned close to the mid-gap, resulting in semi-insulating (SI) GaAs (SI-GaAs). EL2 is always present in as-grown GaAs. The typical concentration of EL2 defects has been found to be 1015–1017 cm−3. The EL2 is the most extensively studied intrinsic point defect in GaAs because of two reasons. First: its application for obtaining SI-GaAs required for microwave devices and high-energy particle detectors and second: the understanding of the physics of this important point defect. It has several interesting properties and the most noticeable is its transition to a metastable state (EL2*) under photo-excitation at low temperature. In spite of extensive research work, the EL2 defect is not fully understood and the most contentious issues are the atomistic origin of EL2 and its photo-quenching (PQ) phenomena. One of the few things which is almost universally agreed upon is the involvement of an arsenic atom occupying a gallium site, and therefore, the discussion of EL2 properties evolves around the arsenic anti-site (AsGa) defect.
High energy electrons and ions are being used to study and engineer the defects in semi-conductors. In most cases, high-energy heavy ions damage the material by producing a large amount of extended defects, but high-energy light ions are suitable for producing and modifying the intrinsic point defects. The objective of this investigation is to study the modification created by 48 MeV Li ions in SI-GaAs. The range of 48 MeV Li ions is around 180 μm in GaAs, and within the initial 150 μm, defects are produced mainly by electronic energy loss processes. The defects produced by nuclear energy loss process is negligible. The thermally stimulated current spectroscopy (TSCS), which probes within a depth of 10 μm, has been used to study the defect modification. TSCS is the most useful characterization technique for high-resistive samples. This technique has been applied extensively to SI-GaAs [2], [3], [4]. The technique involves the low temperature filling of electron traps (above the Fermi-level EF), or hole traps (below EF), and then thermal emptying of these traps by slowly raising the temperature. A given trap will begin to emit at a characteristic temperature, with the emission rate increasing rapidly. However, the emission probability drops as the trap is depleted of electrons (or holes), so that the current goes through a peak. The approximate energy of the level associated with the peaks could be calculated considering a common approximation in the TSCS analysis. The energy E of a peak can be calculated [2] at the peak temperature Tm as E∼kTln(Tm4/β), where β is the temperature ramp applied during TSCS measurement.
There are few reports on radiation-induced defects in SI-GaAs, studied by TSCS. Look et al. [5] and Kuriyama et al. [6] have reported the generation and annihilation of new defect levels in SI-GaAs using high energy electrons and fast neutrons. Quenching phenomena of photo-conductance related to the EL2 defect in 15 MeV proton irradiated SI-GaAs have been studied by Kuriyama et al. [7]. From the observation of enhancement of PQ they have concluded that there is a transformation from non-quenchable EL2+ to quenchable EL20 defects. In contrast, in 24 GeV proton irradiated SI-GaAs, there is increase in the EL2+ fraction as shown in [8] using optical absorption spectroscopy. In this paper, we show (i) how the new metastable defects and the defects giving rise to shallow levels are annihilated and (ii) how the PQ is affected after the irradiation with 48 MeV Li ions.
Section snippets
Experimental details
Wafers of SI-GaAs with a resistivity of ∼106 Ω cm were used for these investigations. Samples with a size of 8 × 8 mm2 were cleaned and mounted on a thick copper ladder in the chamber and were irradiated at room temperature with 48 MeV Li ions of charge state 3+ using the 16 UD Pelletron at the Nuclear Science Centre, New Delhi. The ion beam current was between 1 and 2 particle nanoampere (pnA) and fluences were 1011, 1012 and 1013 ions/cm2. The samples were oriented at an angle 7° with respect to
Result
The TSCS results with 488 nm light illumination are shown in Fig. 2, for both unirradiated and irradiated samples. The results show some important modifications under irradiation. Following are the observations: (i) modifications of the defect levels from 20 to 150 K; (ii) accumulation of new deeper defect levels in the band gap at around 0.56 eV (the peak at 266 K) as seen in all three irradiated samples; (iii) appearance of a new deeper level at 0.6 eV (283 K) in the sample irradiated with a
Discussion
The probing depth of TSCS is approximately 10 nm. Using TRIM calculations, it can be shown that within this range the electronic energy loss is between 160 and 400 eV/nm, whereas the nuclear energy loss is only between 0.08 and 0.4 eV/nm. The lattice binding energy of Ga and As being 6 and 8 eV, respectively, it may be safely argued that the observed modifications are only due to the electronic energy loss.
In Fig. 2 the striking observation is the absence of all levels except one, at 0.23 eV,
Conclusion
The effect of irradiation of SI-GaAs with 48 MeV Li has been studied by TSCS and PQ measurements. The defects produced primarily by electronic energy loss have been compared at various fluences. Preferential creation and annihilation of some defects has been observed in the irradiated samples. An important conclusion is the correlation between the disappearance of metastable states and the reduction of pre-quenching photo-current in irradiated samples. The transformation of the EL2 defect to a
Acknowledgments
The authors are thankful to the scientific staff of the Nuclear Science Centre for their help during the implantation and to Mr. P.K. Sahoo of IIT Kanpur for doing the c-RBS measurements.
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Raman scattering by two-phonons and fractons in irradiated GaAs
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