Amphoteric Phosphorus Doping for Stable p‐Type ZnO

Abstract

Zinc oxide is a wide bandgap semiconductor with potential applications in optoelectronic devices. The greatest challenge for these applications, however, remains the fabrication of reliable and stable p-type ZnO thin films. Here we report stable phosphorus-doped p-type ZnO thin films grown on (0001) sapphire substrates by pulsed laser ablation. While as-deposited films all show n-type conductivity, films grown at 600°C become p-type after annealing in oxygen atmosphere with a resistivity of 4.9 × 10 1 X cm, a Hall mobility of 1 cm 2 V -1 s -1 , and a hole concentration of 1.3 × 10 17 cm -3 . Such p-type films have been stable under ambient conditions for 16 months so far without apparent degradation. Transmission electron microscopy reveals that the p-type films consist of a high density of dislocations, which enhance both the solubility of phosphorus and the formation of Zn vacancies to facilitate the n-to-p conversion of electrical conductivity. These studies provide microscopic evidence of the amphoteric nature of the phosphorus dopant in ZnO. There has recently been an increasing interest in ZnO for applications in optoelectronics such as light emitting diodes, ultraviolet (UV) lasers, and UV light detectors because of its wide bandgap (3.37 eV). In comparison with GaN, ZnO has some obvious advantages for optoelectronic applications due to the availability of single crystal substrates, relatively low growth temperatures (T G ), and a large exciton binding energy (∼ 60 meV).[1] Optically pumped excitonic lasing of ZnO thin films at room temperature (RT) has been reported. [2,3] Lasing effects in ZnO nanowire arrays have been demonstrated, [4] and electroluminescence (EL) has been observed at room temperature in thin-film ZnO homojunctions. [5][6][7] Although p-type ZnO thin films were reported by several groups, they showed high resistivity and/or poor stability and reproducibility. Thus, the greatest remaining challenge for ZnO optoelectronics is the reproducible fabrication of stable p-type ZnO thin films. Like many other II-VI semiconductors, ZnO has asymmetric doping limits: [8] it can be easily doped n-type, [9] but remains strongly resistant to p-type doping.[10] Though nitrogen is theoretically the most promising acceptor for ZnO, its low solubility and compensation by donors such as hydrogen [11] and Zn interstitials [12] are major obstacles.As alternatives to N, larger-size group V elements such as P, As, Sb and Bi have been widely studied. Puzzling observations of p-type conductivity in such materials have stimulated theoretical investigations into the electronic structure of the defects induced by P, As or Sb in ZnO. Limpijumnong et al. [13] predicted that under oxygen-rich growth conditions, a complex involving a group V antisite and two zinc vacancies (V Zn ) would have a low formation energy, and behave as a shallow acceptor with an ionization energy of 150-160 meV. Lee et al. used the same concept to study phosphorus complexes in ZnO.[14] One of the most important conclusions from these studies ...

The role of stacking faults and their associated 0.13 eV acceptor state in doped and undoped ZnO layers and nanostructures

“…Obviously, both transitions compete for free carriers and show total anti-correlation [2]. As an example, the findings on a (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) side facet are shown in Fig. 2.…”

“…As an alternative explanation we suggest, that a high number of SFs might be created due to the presence of group V atoms during growth, which (over-) compensate the n-type background conductivity and lead to a locally strongly fluctuating Fermi level. Indeed, recent TEM microscopy on such doped samples finds a high concentration of partial dislocations and stacking faults [11,12]. However, not only after group V doping this 3.314 eV PL band is found, but also after doping by Mn [13] or doping by Ag [14] introduces this defect band in PLpresumably due to stacking faults introduced in the crystal in high concentration.…”

“…(ii) Annealing converts back and forth p-type and n-type conductivity of samples (see, e.g., Ref. [27]). (iii) Hall measurements in van der Pauw configuration give contradictory results when permutating the connections.…”

Point defects in ZnO: an approach from first principles

“…Recently, ZnO has attracted renewed interest owing to applications in transparent electrodes, utilizing its high n-type conductivity and optical transparency (E g = 3.44 eV [3]). In addition, optoelectronic applications such as light-emitting diodes and ultraviolet lasers have been extensively explored using its p-n and p-i-n homojunctions [4][5][6][7][8] and p-n heterojunctions [9,10]. The quantum Hall effect has been observed recently in a high-mobility two-dimensional electron gas formed at ZnO/Mg x Zn 1−x O heterojunctions [11,12].…”

“…The use of homojunctions requires both p-and n-type doping, but ZnO is known to show a strong n-type preference. Although difficulty in p-type doping has been overcome in several studies [7,8,13,14], an efficient method of forming p-type ZnO has still been actively debated. Moreover, there exists controversy on the fundamental issues relevant to native defects and unintentional impurities, such as the sources of the n-type conductivity and non-stoichiometry of reduced ZnO.…”