We have investigated photoluminescence (PL) properties of a high quality GaN thin film grown by metal organic vapor phase epitaxy under intense excitation conditions in a high temperature regime from 120K to room temperature. It is found that a PL band peculiar to intense excitation conditions appears with a threshold-like behavior. The energy spacing between the PL band at the threshold excitation power and the A exciton is proportional to temperature. The extrapolation of the linear dependence results in zero value of the energy spacing at absolute zero temperature. These PL profiles are specific to an emission process originating from exciton-electron scattering. Furthermore, we have demonstrated that the exciton-electron scattering process produces optical gain at room temperature from measurements of PL with a variable stripe-length method.
We have investigated photoluminescence (PL) properties of a high quality GaN thin film grown by metalorganic vapor phase epitaxy under intense excitation conditions in a wide temperature range from 10 to 300 K. It is found that there are two types of PL band peculiar to intense excitation conditions. In a low temperature region below 80 K, the exciton-exciton scattering dominates the PL, the so-called P emission.On the other hand, in a high temperature region above ~120 K, a PL band, which is different from the P emission, appears. The energy spacing between the new PL band and the fundamental A exciton linearly increases with an increase in temperature. In addition, the energy spacing is estimated to be zero at absolute zero temperature by extrapolation of the temperature dependence. These PL profiles indicate that the PL band observed in the high temperature regime originates from the exciton-electron scattering. . The well-known emission process is exciton-exciton scattering, the so-called P emission, in which one of two n = 1 excitons is scattered into a high-energy state with n ≥ 2, while the other is scattered into a photon-like state; the energy of which is lower than that of the n = 1 exciton state by the energy difference between the n = 1 and n ≥ 2 states. The P emission has been intensively studied in various wide-gap semiconductors: e.g., ), lightly-alloyed InGaN (Ref.[5]), CdS (Ref.[6]), ZnO (Ref. [7]), and CuI (Ref. [8]). On the other hand, there have been limited reports on the investigation of exciton-carrier scattering. In II-VI semiconductors [7,9,10] and cuprous halides [11], the characteristics of PL due to exciton-electron scattering were studied in a high temperature regime, which is called H emission. The exciton-electron scattering process is explained as follows. One electron around the bottom of the conduction band is scattered into a larger wavevector state, i.e., a hot electron, while one exciton in the first quantum (n = 1) state is scattered into a photon-like state; the energy of which is lower than that of the n = 1 exciton state by the energy difference between the initial and hot electron states. There are two specific features of the PL due to exciton-electron scattering. One is a low energy shift of the PL energy with an increase in excitation power because of an increase of effective temperature, which is similar to the P emission. The other is a linear temperature dependence of the en-
We demonstrate that photoluminescence‐excitation (PLE) spectroscopy is applicable to probe effects of the surface damages on the carrier transport in AlxGa1–xN/GaN heterostructures by systematically characterizing as‐grown and plasma‐exposed samples. The characterization of the surface morphology with atomic force microscopy clarifies that the plasma exposure modifies the atomic steps and pits on the AlxGa1–xN surface. The PLE spectrum of the as‐grown sample measured at the energy of the photoluminescence from the GaN layer shows a step rising from the AlxGa1–xN fundamental transition energy, which reflects the photogenerated‐carrier injection from the AlxGa1–xN layer to the GaN layer, while the rising step disappears in the plasma‐exposed sample. In contrast, the reflectance spectra are the same in the two samples; namely, the excitonic transition is hardly changed. Thus, it is concluded that PLE spectroscopy is highly sensitive to probe the carrier‐transport characteristics in the AlxGa1–xN/GaN heterostructure. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
We demonstrate that photoluminescence-excitation (PLE) spectroscopy can probe with high sensitivity the effects of plasma-induced surface damages on photogenerated-carrier-transport processes in AlxGa1−xN/GaN heterostructures, on the basis of systematic optical and structural characterization results for the as-grown reference sample and the plasma-exposed sample. It is found from the structural characterizations with atomic force microscopy that the plasma exposure remarkably modifies the atomic step boundaries and the pits on the AlxGa1−xN surface, which leads to a remarkable difference between the PLE spectra of the bound exciton photoluminescence from the underlying GaN layer in the two samples. The PLE spectrum of the reference sample shows a step rising from the AlxGa1−xN fundamental transition energy toward the high energy side, whereas the rising step disappears in the PLE spectrum of the plasma-exposed sample. In contrast, the reflectance characteristics are the same in the two samples; i.e., the excitonic transition itself is not influenced by the plasma exposure. The present findings indicate that the PLE spectral profile is sensitive to the change in efficiency of the photogenerated carrier injection from the AlxGa1−xN layer to the GaN layer. Thus, it is concluded that the PLE characterization is effective to probe the photogenerated-carrier transport in heterostructures.
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