Yellow luminescence and persistent photoconductivity of undoped n-type GaN

Abstract

Deep-level defect-related optical properties of undoped n-type GaN grown by metalorganic chemical vapor deposition are investigated using photoluminescence (PL), optical absorption (OA), photoconductivity (PC), and persistent photoconductivity (PPC) measurements. From the temperature dependence of the PL and OA, we find that the yellow luminescence (YL) is due to shallow-to-deep donor recombination. PL, PC, and PPC results manifest a strong correlation in properties related to deep levels. Samples which emit YL exhibit a PC peak at 1.9 eV due to the photoionization of deep levels as well as to the persistent photoconductivity effect, whereas samples with no YL have no PC peak in the forbidden gap and no PPC at any photon energy, suggesting a common origin. Furthermore, two types of PPC behavior were observed depending on the sample quality: typical stretched exponential decay in relatively thick samples and photocurrent quenching and a subsequent reduction of the dark current in thin samples. An explanation of the latter phenomenon based on photoinduced metastable electron traps in a highly defective layer near the interface is suggested from the temporal behavior of the PC. These traps seem to disappear slowly after the illuminating light is turned off.

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“…The PPC threshold energy was observed at ~1.6 eV. The obtained value is in a good agreement with the threshold energy reported for GaN (1.1-2.3 eV) [12,14,26].…”

“…The sharp feature observed at 2.1 eV (the PC resonance peak [17]) in the spectral response is due to the photoionization of the deep states [12]. It was reported earlier [12] that the PC peak observed in the spectral range from 1.8 to 2.3 eV in GaN is always accompanied by the yellow luminescence (YL) (broad the YL band with a maximum at about 2.2 eV) [15,[21][22][23]). Recent experiments evidence that YL is associated with the transitions between a shallow donor and AX state in GaN [15,22,23].…”

“…The spectral responsivity below 4.1 eV is associated with the various defect-to-band transitions [17]. The sharp feature observed at 2.1 eV (the PC resonance peak [17]) in the spectral response is due to the photoionization of the deep states [12]. It was reported earlier [12] that the PC peak observed in the spectral range from 1.8 to 2.3 eV in GaN is always accompanied by the yellow luminescence (YL) (broad the YL band with a maximum at about 2.2 eV) [15,[21][22][23]).…”

“…On the other hand, both localized deep acceptor (AX) and DX states have been also reported as an origin of the PPC effect in GaN epilayers [11][12][13]. The former appears to be a source of the well-known yellow luminescence (YL) phenomenon [12,14,15].…”

Trap-related photoconductivity in ZnO epilayers

“…The oxygen flow rate was maintained at 4 sccm, with ECR power of 270 W, while the Zn flux was maintained at a beam equivalent pressure of 5 3 10 ÿ7 torr. The MBE material (Received August 1, 2005; accepted October 12,2005) was determined to grow epitaxially based on in-situ monitoring by reflection high energy electron diffraction, where the sapphire substrate pattern was observed to transition to a streaky ZnO pattern. The layer thickness for the ZnO samples was determined to be between 0.5 mm and 1.5 mm using optical reflectance.…”