H. Ch. Alt

Photosensitivity of the 714 and 730 cm−1 absorption bands in semi‐insulating GaAs: Evidence for a deep donor involving oxygen

The optically induced metastable interconversion processes of the oxygen‐related vibrational modes at 730 and 714 cm−1, bands A and B, have been studied and compared with related properties of the EL2 defect. It is shown that the conversion from A to B is due to a charge exchange with the neutral EL2 level. The reverse process, recovery of band A, is observed after quenching of EL2. Conclusive evidence is presented that the two bands are associated with a donor in the upper half of the band gap. From Hall effect measurements, a trap energy of 0.43 eV below the conduction band is expected. A preliminary calibration of the absorption strength yields defect concentrations of ≊1015 cm−3 in some samples.

Infrared absorption study of nitrogen in N-implanted GaAs and epitaxially grown GaAs1−xNx layers

Fourier-transform infrared absorption measurements have been carried out in the two-phonon region of GaAs. Implantation of the nitrogen isotopes 14N and 15N, respectively, into bulk GaAs shows that a local vibrational mode at 471 cm−1 (14N) is due to isolated nitrogen. The band is also found in GaAs1−xNx(0<x<0.03) layers grown by solid-source molecular beam epitaxy. The strength of the band correlates quantitatively with the decrease of the lattice parameter determined by x-ray diffraction for x<0.01 and can be used for the assessment of the nitrogen fraction incorporated substitutionally on anion lattice sites.

Fine structure of the oxygen‐related local mode at 714 cm−1 in GaAs

Infrared absorption measurements on a new third state of the Ga‐O‐Ga center in GaAs are reported. This state gives rise to a local mode at 714 cm−1, B’, which is shifted from the band B to lower energy by 0.67 cm−1. The B’ state acts as an intermediate state for the photoinduced conversion from the 730 cm−1 band, A, to B. The kinetics is quantitatively described by a model considering successive capture of conduction‐band electrons, which have been photoexcited from the EL2 level. The reappearance of the band A after quenching of EL2 occurs through the corresponding sequence B→B’→A. The experimental results favor the interpretation of B’ in terms of a third charge state being not accessible at thermal equilibrium conditions.

High‐resolution imaging of the EL2 distribution in thin semi‐insulating GaAs wafers: A comparison with x‐ray topography

A detailed comparison of two‐dimensional near‐infrared absorption maps and x‐ray topographs of commercial semi‐insulating (100) GaAs wafers grown by the liquid‐encapsulated Czochralski technique is reported. The absorption measurements with a spatial resolution up to 100×100 μm2 were performed using a highly sensitive silicon diode array as the detector element. Fluctuations of the EL2 concentration up to 60% are found in conventional undoped wafers. In preannealed wafers, these fluctuations are reduced by a factor of 2, approximately. The spatial one‐to‐one correlation of the EL2 distribution with grown‐in dislocation networks, which is observed in both cases, is discussed. In indium‐doped wafers of low dislocation density, the homogeneity of EL2 is better than 5% in the central area of about 40 mm in diameter. By investigating the EL2 distribution near peripheral slip lines, it is definitely established that gettering is a relevant process leading to nonuniformities.

Distribution of the deep acceptor Fe in semi-insulating InP in both its charge states

The distribution of the compensating Fe acceptor in semi-insulating (SI) InP wafers is measured in both its charge states, (Fe2+) and (Fe3+). In the wafers studied the inhomogeneities of the material are due to the inhomogeneous distribution of the activated Fe, rather than to that of the residual shallow donors. According to the data, open questions exist with regard to the absolute values of the optical cross sections sigma p and sigma n of Fe in InP.

Annealing behavior of deep‐level defects in semi‐insulating gallium arsenide studied by photoluminescence, infrared absorption, and resistivity mapping

Deep‐level defects in as‐grown, ingot‐annealed, and wafer‐annealed samples of semi‐insulating gallium arsenide have been investigated by spatially resolved measurements of room‐temperature photoluminescence, infrared absorption, free‐carrier lifetime, and resistivity. High‐temperature ingot annealing mainly causes a homogenization of the EL2 distribution. Rapid cooling from a wafer annealing process at T>900 °C suppresses the formation of the previously lifetime‐limiting recombination center. After wafer annealing the EL2 defect may be the dominant recombination center, while in as‐grown and ingot‐annealed material lifetime is limited by a different trap. There is experimental evidence that this trap is related to the 0.8‐eV luminescence band and that its density is spatially anticorrelated to the EL2 distribution. Based on lifetime measurements and a correlation of EL2 and photoluminescence topographs, we developed a recombination model, which explains the relationship between defect densities, and photo...

Local mode spectroscopy of the carbon acceptor in GaAs: New experimental aspects

The temperature dependence of the local mode absorption of CAs at 580 cm−1 is reexamined using high‐resolution Fourier transform spectroscopy. A new carbon‐related line is detected at room temperature on the low‐energy side of the main carbon peak at 576.6 cm−1. It follows that the integrated absorption of the whole band is independent of temperature in the temperature range between 77 and 300 K. Calibration factors are given for 77 and 300 K, providing a consistent interpretation of previous data.

Calibration of the Fe2+ intracenter absorption in InP

A study is presented on the assessment of the Fe2+ concentration in Fe‐doped InP by the quantitative evaluation of the Fe2+ intracenter absorption at 2800–3200 cm−1. Based on a comparison with secondary‐ion mass spectrometry data from n‐type samples, a calibration factor is derived for the oscillator strength of the zero‐phonon lines at 10 K. The detection limit for 500 μm samples is ≊5×1013 cm−3.

Microscopic spatial fluctuations of the EL2 defect in semi‐insulating GaAs: Influence on bulk resistivity and correlation with implant activation

High‐resolution EL2 topography has been compared with the electrical characteristics of the bulk material and Si‐implanted layers by using resistivity and capacitance mapping. The typical cellular features are revealed in one‐to‐one correspondence. Microscopic variations of the bulk resistivity in the range 107<ρ<108 Ω cm can be explained quantitatively by fluctuations of the EL2 defect density and a constant carbon‐acceptor concentration. A higher activation of Si is observed in regions with a higher EL2 density, leading to an estimated influence on threshold voltages of −28 mV/1015 cm−3 EL2 variation.

Negative-U properties of off-centre substitutional oxygen in gallium arsenide

Recent infrared absorption studies of semi-insulating GaAs have revealed an electrically active, oxygen-related defect with remarkable spectroscopic and microscopic properties. This defect, the structural analogue of the oxygen vacancy centre in Si, occurs in three charge states, the zero-, one- and two-electron states. The experimental fingerprint for each charge state is the local mode frequency which shows a characteristic charge-state-induced shift. Dependent on the Fermi potential, at thermal equilibrium only the local modes corresponding to the zero- or the two-electron state are experimentally observable. The metastable one-electron state disproportionates spontaneously into the zero- and the two-electron states. The energy positions of the associated gap levels are at Ec-0.14 eV and at Ec-0.58 eV for the first and the second electron, respectively. These assignments are derived from the thermally activated decay of the local mode lines and, independently, from the threshold energies of the optical transitions sigma p0(1) and sigma n0(2). Through a comparative deep-level transient spectroscopy and infrared absorption study on neutron-transmutation-doped, n-type samples the second electron level is identified as the well known EL3 level.