Recent progress of growth techniques of bulk GaN crystals is remarkable for realizing GaN-based power switching devices with a high breakdown voltage. Point defects play important role to characterize the high quality GaN, because structural defects like threading dislocations (TDs) and stacking faults are nearly disappeared. We have been investigating the relation between the near-band-edge (NBE) photoluminescence (PL) lifetime observed at room temperature and the concentration of intrinsic nonradiative recombination centers (NRCs) in n-GaN, of which origins are point defect complexes containing Ga vacancy, by combining positron annihilation spectroscopy (PAS) and time-resolved PL methods [S. F. Chichibu, et al., APL86, 021914 (2005)]. However, PAS becomes less sensitive below the concentration of Ga vacancy ([VGa]) of 10^16 cm^-3. Thus, the development of an alternative way to detect such dilute point defects (<10^16 cm^-3) in high quality GaN crystals is essential.
In this presentation, we will show the quantification of absolute quantum efficiency of radiation (AQE) by employing the omnidirectional PL (ODPL) technique to determine internal quantum efficiency (IQE) of the emission in GaN crystals with different excitation conditions. A high AQE of 8.22% corresponding to IQE of 70.9% was measured at room temperature for the NBE emission of a freestanding-GaN crystal grown by hydride vapor phase epitaxy on a GaN seed crystal manufactured with the acidic ammonothermal method, when cw photo pumping density was 66 W/cm^2.
Positron annihilation is a non-destructive technique for investigating vacancy-type defects in condensed matter. When a
positron is implanted into a sample, it annihilates with an electron and emits two 511-keV γ quanta. From measurements
of Doppler broadening spectra of the annihilation radiation and positron lifetimes, one can detect point defects such as
monovacancies, vacancy clusters, and vacancy-impurity complexes. The regions sampled can range from the surface to a
depth on the order of microns. In the present study, we have used the positron annihilation technique to study
relationship between the impurity doping and vacancies in GaN. Defects in ion-implanted GaN and their annealing
properties were studied. The defects introduced by the implantation were identified as divacancies, and the defect
reaction during isochronal annealing were found to depend on ion spices. A relationship between intra-4f transitions of
Er and vacancies was studied. A correlation between the defect concentration and the PL intensity was observed. We will
demonstrate that the positron annihilation technique is sensitive to vacancy-type defects in GaN, and it can contribute to
the development of optical and electronic devices based such materials.
Thin films of ZnO and Mg<sub>x</sub>Zn<sub>1-x</sub>O were epitaxially grown on Zn-polar ZnO substrates by plasma assisted
molecular beam epitaxy. The miscut of c-plane ZnO substrates toward the [1-100] axis direction leads to a flat substrate
surface with straight step edges. The growth mode of epitaxial ZnO films significantly depended on the growth
temperature, and a substrate temperature over 800°C was needed for flat film surfaces with monolayer-height steps.
Photoluminescence (PL) peak originating from the n = 2 state of A-free excitons was observed at 12 K for the ZnO films
grown under stoichiometric and O-rich growth conditions. Mg<sub>x</sub>Zn<sub>1-x</sub>O films were also fabricated under Zn-rich
conditions. The film surface exhibited a step-and-terrace structure. The effective PL lifetime of Mg<sub>0.08</sub>Zn<sub>0.92</sub>O film was as
long as 1.9 ns, which is the highest value ever reported, presumably due to a high purity level of the film.
We perform degenerate four-wave-mixing (FWM) studies of GaN excitons especially for an understanding of the strain-fields in the heteroepitaxial films. The shifts of exciton energies and their beating oscillation variations highlight the biaxial strain, allowing for a precise determination of the strain parameters.
The uniaxial strain field can be characterized by the polarization dependence of FWM, which shows distinct polarizations and energy variations depending on the sample and its position. The minimum changes of the polarized FWM intensity and exchange energy splittings correspond to a uniaxial strain of 5.0 × 10<sup>-5</sup>, which currently gives a lower resolution limit of this technique and is comparable with that of conventional X-ray diffraction.
In the time-evolutions, we investigate the strain effects on the phase of the quantum beats (QBs), giving insight into the excitons interactions. By using time-resolved FWM, difference between two-types of exciton transitions is identified. In addition, coherent manipulations of QBs are successfully realized in the FWM with a Michelson interferometer.
Time-integrated and spectrally-resolved four-wave mixing (FWM) has been used to study dephasing dynamics of excitons in a free-standing bulk ZnO. Clear FWM signals due to A⟩Γ<sub>5</sub>- and BΓ<sub>5</sub>-excitions have been observed. We discuss the dephasing dynamics based on the polariton dispersion and four-particle Coulomb correlations.
Bound and unbound biexcitons in a free-standing bulk GaN are investigated by time-integrated and spectrally-resolved four-wave mixing measurements, where the formation of hetero-biexcitons that consist of <i>A</i> and <i>B</i> excitons (<i>XX<sub>AB</sub></i>) as well as <i>A</i>-biexcitons (<i>XX<sub>AA</sub></i>) and their unbound biexciton (<i>XX</i>*<i><sub>AA</sub></i>) are clearly observed. The FWM spectra and delay-time dependence are explained qualitatively and the interaction between <i>A</i>- and <i>B</i>-excitons gives rise to the phase shifts of the quantum beating and the energy shifts of the spectra, which is considered as the effect of the unbound state of <i>XX<sub>AB</sub></i> (i.e. <i>XX</i>*<i><sub>AB</sub></i>). The unbound A-biexciton (<i>XX</i>*<i><sub>AA</sub></i>). Is also observed clearly in spectral and temporal domain and is found to play an important role in FWM signals for all polarizations.
Static, field-mounted and time-resolved spectroscopic measurements were carried out to compare the electronic structures between AlGaN/GaN binary and GaN/InGaN ternary single quantum wells (SQWs). The internal field exits across the quantum well (QW) naturally induces quantum-confined Stark effects, namely the redshift of the QW resonance energy and separation of electron-hole wavefunction overlap. Thus AlGaN/GaN SQWs exhibited a weak luminescence peak due to the presence of nonradiative channels. However, optical absorption and degenerate pump-probe measurements revealed that excitonic character still remains for the thin QWs having the well width nearly the same as the bulk free exciton Bohr radius even under high electric field as high as 0.73 MV/cm. A slightly In-alloyed InGaN SQW exhibited bright luminescence peak in spite of the pronounced effective bandgap inhomogeneity in the QW, which was confirmed by the point excitation and monochromatic cathodoluminescence mapping methods to have the lateral potential interval smaller than 40 nm. Therefore the light emitting area of the potential minima has the size defined as 'quantum-disk'. Carriers generated in the InGaN QWEs are effectively localized in these regions to form localized QW excitons exhibiting highly efficient spontaneous emissions.