Gallium concentration dependence of room-temperature near-band-edge luminescence in n-type ZnO:Ga

Abstract

We investigated the optical properties of epitaxial n-type ZnO films grown on lattice-matched ScAlMgO4 substrates. As the Ga doping concentration increased up to 6×10 20 cm −3 , the absorption edge showed a systematic blueshift, consistent with the Burstein-Moss effect. A bright near-bandedge photoluminescence (PL) could be observed even at room temperature, the intensity of which increased monotonically as the doping concentration was increased except for the highest doping level. It indicates that nonradiative transitions dominate at a low doping density. Both a Stokes shift and broadening in the PL band are monotonically increasing functions of donor concentration, which was explained in terms of potential fluctuations caused by the random distribution of donor impurities.PACS numbers: 78.55. Et, 81.15.Fg, 71.35.Cc, Optical properties of ZnO are currently subject of tremendous investigations, in response to the industrial demand for shortwavelength optoelectronics devices. Production of high-quality doped ZnO films is indispensable for the device application. Photoluminescence (PL) is a sensitive and non-destructive method, the results of which provide a good indicator of material quality. Impurity-doping, defect, and surface profile both have influence to its broadening, Stokes shift, and radiative efficiency. Room-temperature (RT) near-bandedge (NBE) luminescence has not been observed in donor-doped ZnO except for lightlydoped ones despite the long research history of this material as a transparent conductive window [1,2,3,4]. Indeed when ZnO:Al films were grown on lattice matched substrates, detectable NBE PL could be observed only at 5 K. As pointed out by Ko et al., oxidation of the Al during the growth owing to its high reactivity may be responsible for that. On the other hand, Ga is less reactive and more resistive to oxidation. The covalent bond lengths of Ga-O is slightly smaller than that of Zn-O, which will make the deformation of the ZnO lattice small even in the case of high Ga concentration [2]. In this publication, we report observation of the RT NBE luminescence from ZnO:Ga epitaxial layers. The radiative efficiency, threshold energy and the linewidth of the near-band-gap optical transition are investigated as a function of doping density of Ga.Ga-doped ZnO samples were grown by laser molecular-beam epitaxy on the (0001)-plane of a ScAlMgO 4 substrate. The samples were grown at temperatures of 650 to 680 • C. The Ga doping was varied to achieve doping densities in the range of 8 × 10 18 to 6 × 10 20 cm −3 [5]. We used Fig. 2 of Ref. 6 for the conversion from prescribed Ga concentration. The photoluminescence measurements were performed using an He-Cd laser, with emission at 325 nm. The luminescence from samples was dispersed in a 0.3 m spectrometer and detected by a charge-coupled device. Absorption was measured by using a UV/visible spectrometer (Shimadzu, UV2450) [4].Figure 1(a) shows room-temperature near-bandedge photoluminescence spectra (left-hand side) in n-type Ga-doped ZnO samples wit...

Solution-based synthesis of cobalt-doped ZnO thin films

“…Also, alluding forward to the PL data of section 3.3.2 it is interesting to note that, in the work by Makino et al [32], the shift of band gap to higher energies is not tracked by the centrewavelength of the PL, a feature which we also find here and in our previous work on similarly prepared nanowires [29].…”

“…This increase in band gap with increasing Co doping is in contrast to the decrease found in Co-doped ZnO films prepared by sol-gel [16] and hydrothermal [30] techniques, although in both cases the doping extended to higher levels than in this study. By contrast, but in common with the data presented here, an increase of band gap has been reported with increased concentration for the cases of Co [19,31] and Ga [32] dopants.…”

Ga-related photoluminescence lines in Ga-doped ZnO grown by plasma-assisted molecular-beam epitaxy

“…Ga-doped ZnO has been widely studied; [8][9][10][11][12][13][14] however, among the Ga-related excitonic transitions, only excitons bound to neutral Ga Zn donors ͑I 8 in the literature͒ have been commonly reported. This is in part due to the strong tendency for ionized donors to become neutral under light excitation because of the reaction D + + e − → D 0 .…”

Preparation of vertically aligned ZnO crystal rods in aqueous solution at external electric field

“…However, the peak at 380 nm is strongly suppressed in sample A. Emission peak at 380 nm is the near-band-edge photoluminescence and can be assigned to the donor electron and free hole recombination [21,22]. To explain the suppression of this peak, further study is needed but we could add as a comment here that different growth routes play an important role since sample B is expected to be grown in the environment rich of OH − species and H + trapped in ZnO crystal could compensate the concentration of donor defects in the crystals.…”