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...
The growth mode of ZnO thin films can be well regulated in a molecular layer-by-layer growth by employing a ZnO buffer layer deposited on a lattice-matched ScAlMgO 4 substrate and annealed at high temperature. The annealed buffer layer has atomically flat surface and relaxed ͑strain-free͒ crystal structure. The intensity oscillation of reflection high-energy electron diffraction persisted for more than a 100-nm film deposition under optimized conditions on such a buffer layer. Thus prepared thin films show free exciton emissions in a 5 K photoluminescence spectrum and excited-state exciton resonance structures in a reflection spectrum, both indicating very high optical quality.
GaN/AlN heterostructures were grown on sapphire by reactive molecular beam epitaxy. The electrical and luminescent properties of the GaN have been studied. The GaN films on AlN have larger Hall mobilities and show more intense cathodoluminescence at the peak of 360 nm, compared with GaN films grown on sapphire. This suggests that the crystalline quality of the GaN is improved by making GaN/AlN heterostructures. The improvement is thought to result from the small mismatch of lattices and the small difference of thermal expansion coefficients between GaN and AlN.
Thin films of laser molecular-beam epitaxy grown n-type Ga-doped ZnO were investigated with respect to their optical properties. Intense room-temperature photoluminescence (PL) in the near-band edge (NBE) region was observed. Moreover, its broadening of PL band was significantly larger than predicted by theoretical results modeled in terms of potential fluctuations caused by the random distribution of donor impurities. In addition, the lineshape was rather asymmetrical. To explain these features of the NBE bands, a vibronic model was developed accounting for contributions from a series of phonon replicas.
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