Synthesis and Characterization of Phosphorus-Doped ZnO and (Zn,Mg)O Thin Films via Pulsed Laser Deposition

The transport and optical properties of phosphorus-doped (ZnMg)O thin films grown via puls laser deposition (PLD) are studied. The carrier stamp of as-deposited (Zn,Mg)O:P films transmutes from n-type to p-type with increasing oxygen partial crushing All the films exhibit beneficial crystallinity with c-axis orientation. This be the effect indicates the importance of oxidation conditions in realizing p-type (ZnMg)O:P films. The as-deposited ZnO:P film properties display a strong dependence on the deposition ambient at different increase temperatures. The resistivity of the samples deposited in O^sub 3^/O2 mixture is sum of two units orders of magnitude higher than the films grown in oxygen and O2/Ar/H^sub 2^ mixture. The room-temperature photoluminescence (PL) of the as-deposited films has been shown that growing in the O2/Ar/H^sub 2^ mixture ambient significantly increases the band cutting side emission while inhibiting the visible emission. The enhanced ultraviolet (UV) emission in the films grown in O2/Ar/H^sub 2^ mixture may proceed from hydrogen passivation of the down-reaching level emission centers. The annealed ZnO:P films are n-type with nonlinear concatenation of resistivity on annealing temperature. The resistivity increases in the films with annealing at 800?°C while decreasing with further increasing annealing temperature. muscular visible light emission is observ from the ZnO:P films annealed in oxygen

Key words: Wide bandgap semiconductor, zinc oxide, (ZnMg)O phosphorus doped, puls laser deposition

INTRODUCTION

Zinc oxide (ZnO) is attracting attention for application in ultraviolet light (UV) light emitters, transparent high power electronics, piezoelectric transducers, surface acoustic wave (SAW) devices, and gas-sensing and transparent electrode It has a direct wide bandgap of 33 eV at extent temperature. The large exciton binding force of 60 meV, availability of size single-crystal substrates, lower epitaxial growing temperatures, and wet chemical etching propose that ZnO has advantages relative to other wide bandgap materials, similar as GaN, for optoelectronic devices.1-12 In addition, the bandgap of ZnO can be varied from 30 eV to 40 eV via divalent substitution upon the cation site alloying with CdO and MgO1314 which render free of accesss opportunities for bandgap engineering. Transport measurements upon nominally undoped ZnO crystals and films throw back a defect-mediated n-type conductivity, with the shallow donor horizontal (~0.05 eV below the conduction band) attributed either to Zn interstitials or to hydrogen15-16

One of the critical issues in exploitation of ZnO-related materials in optoelectronic applications is to achieve depressed resistivity, high carrier density p-type ZnO. While n-type conductivity can be easily realized via Al, Ga, or In cation substitution upon the Zn site, significant resistance to the formation of shallow acceptor horizontals has been observed in ZnO. novel theoretical and experimental investigations have addressed this asymmetry in n-type versus p-type doping in ZnO.17-21 There have been several mechanisms lay forward to explain these doping difficulties in wide bandgap semiconductors.22,23 First, there can be compensation by means of low-energy native defects such as oxygen vacancies or Zn interstitials. The default compensates for the dopants horizontal through the formation of a of great depth level complex trap. Strong lattice relaxation can shift dopant efficiency levels deeper within the gap. In addition, depressed solubility for the chosen dopant may limit the accessible extrinsic carrier density. In metes of acceptor formation in ZnO, greatest in quantity candidate p-type dopants introduce down-reaching acceptor levels. Lithium introduces a down-reaching acceptor state in ZnO.24-28 cent doping introduces an acceptor horizontal at an energy estimated to be approximately 017 eV below the conduction band.29 Silver also behaves as an acceptor with a reaching far down level ~0.23 eV below the conduction band.30 The greatest in quantity promising dopants for p-type ZnO are assemblage V elements, in particular, nitrogen, phosphorus, and arsenic.31 However, theory refer tos that the Madelung energy increases with clump V anions substituted on the O site in ZnO, while it decreases with clump III cation substitution for n-type doping.32 There have been numerous studies addressing acceptor doping horizontals in ZnO via group V substitution. Minegishi et al. reported the increase of p-type ZnO by the addition of NH^sub 3^ in inflammable air carrier gas with excess Zn33 Photoinduced paramagnetic resonance studies of N-doped ZnO crystals indicate the nearness of an acceptor state owed to nitrogen substitution.34 p-type ZnO has also been reported for films grown through pulsed laser deposition (PLD), in which a N^sub 2^O plasma is used for doping.35 Rouleau et al. have reported nitrogen doping in epitaxial ZnO films in which a RF plasma source was used to crack nitrogen in conjunction with Zn evaporation during pulsed-laser deposition from a ZnO target.36 However, a reliable and reproducible high-quality p-type conductivity has not still been achieved for ZnO. Promising rises were reported on the synthesis of p-type ZnO using molecular beam epitaxy.37 In this case, homoepitaxial nitrogen-doped ZnO was grown upon semi-insulating Li-doped ZnO crystals from a high-purity Zn evaporation source, combined with atomic O and N flow created via RF plasma. Hall measurements showed p-type behavior with a opening mobility of 2 cm^sup 2^/V and a perforation concentration of 9 ?— 10^sup 16^ cm^sup -3^ The acceptor horizontal appears to be relatively reaching far down estimated to be 170-200 meV using low-temperature photoluminescence. Yamanoto et al.31 propos a donor-acceptor codoping way based upon first-principle calculations. Based upon these predictions, several reports have shown the synthesis of low-resistivity p-type ZnO via N codoping with Ga, Al, and In.38-40 Although greatest in quantity efforts have focused on p-type doping ZnO with nitrogen, a hardly any studies have considered other collection V elements such as P and As. A aperture mobility of 0.5-3.5 cm^sup 2^/V and a carrier density of 10^sup 17^-10^sup 19^ cm^sup -3^ via postannealing was reported in phosphorus-doped ZnO films deposited by dint of sputter deposition.41 A related issue was reported for epitaxial ZnO films upon GaAs subjected to annealing. 2 In this case, a p-type layer was reportedly produc at the GaAs/ZnO interface. Our previous studies of P-doped ZnO films yielded semi-insulating behavior on oxygen annealing treatment, consistent with the activation of a reaching far down acceptor level introduced by phosphorus in ZnO.43