Realization of p-type conduction in undoped MgxZn1-xO thin films by controlling Mg content
TL;DR: In this article, the authors investigated the mechanism of transformation from n to p type and change of the carrier concentrations with Mg content, and investigated the photoluminescence and absorption measurements as well as first-principle calculation.
Abstract: Undoped MgxZn1−xO thin films with Mg content of 0⩽x⩽0.20 were grown on c-sapphire substrate by plasma-assisted molecular beam epitaxy. The MgxZn1−xO shows n-type conduction in Mg content of x⩽0.05, and the carrier concentration decreases slowly from 1018to1017cm−3 with increasing Mg content. However, as x⩾0.10, the MgxZn1−xO begins to show p-type conduction, and the carrier concentration goes down sharply to 1015cm−3 firstly and then increases slowly with increasing Mg content from 1015to1016cm−3. The mechanism of transformation from n to p type and change of the carrier concentrations with Mg content were investigated by photoluminescence and absorption measurements as well as first-principle calculation.
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TL;DR: In this paper, the authors discuss p-type ZnO materials: theory, growth, properties and devices, comprehensively, and summarize the growth techniques and properties of P-type materials.
329 citations
01 Jan 2013
TL;DR: In this article, the authors discuss p-type ZnO materials: theory, growth, properties and devices, comprehensively, and summarize the growth techniques for p- type ZnOs.
Abstract: Abstract In the past 10 years, ZnO as a semiconductor has attracted considerable attention due to its unique properties, such as high electron mobility, wide and direct band gap and large exciton binding energy. ZnO has been considered a promising material for optoelectronic device applications, and the fabrications of high quality p-type ZnO and p–n junction are the key steps to realize these applications. However, the reliable p-type doping of the material remains a major challenge because of the self-compensation from native donor defects (V O and Zn i ) and/or hydrogen incorporation. Considerable efforts have been made to obtain p-type ZnO by doping different elements with various techniques. Remarkable progresses have been achieved, both theoretically and experimentally. In this paper, we discuss p-type ZnO materials: theory, growth, properties and devices, comprehensively. We first discuss the native defects in ZnO. Among the native defects in ZnO, V Zn and O i act as acceptors. We then present the theory of p-type doping in ZnO, and summarize the growth techniques for p-type ZnO and the properties of p-type ZnO materials. Theoretically, the principles of selection of p-type dopant, codoping method and X Zn –2V Zn acceptor model are introduced. Experimentally, besides the intrinsic p-type ZnO grown at O-rich ambient, p-type ZnO (MgZnO) materials have been prepared by various techniques using Group-I, IV and V elements. We pay a special attention to the band gap of p-type ZnO by band-gap engineering and room temperature ferromagnetism observed in p-type ZnO. Finally, we summarize the devices based on p-type ZnO materials.
308 citations
TL;DR: In this paper, the room temperature ferromagnetism in band gap tunable MgxZn1−xO (x≤0.22) alloy thin films was investigated.
Abstract: We investigate the room temperature ferromagnetism in band gap tunable MgxZn1−xO (x≤0.22) alloy thin films and find that ferromagnetism is significantly enhanced in p-type MgxZn1−xO (x≥0.17) compared with the n-type counterparts (x≤0.15). Temperature-dependent photoluminescence measurements reveal the correlation between the p-type behavior, enhanced ferromagnetism, and zinc vacancies. First-principle calculations demonstrate that the formation energy of zinc vacancies decreases with the increasing Mg content and the zinc vacancies in MgxZn1−xO alloys stabilize the ferromagnetic coupling. Our results suggest a viable route to tune the magnetic properties of oxides through band gap and defect engineering.
93 citations
TL;DR: In this paper, the authors report on bandgap engineering of an emerging photovoltaic material of Cu 2Cdx Zn 1−x SnS 4 (CCZTS) alloy, which is a potentially suitable material to fabricate high efficiency multi-junction tandem solar cells with different bandgap-tailored absorption layers.
Abstract: We report on bandgap engineering of an emerging photovoltaic material of Cu 2Cdx Zn 1−x SnS 4 (CCZTS) alloy. CCZTS alloy thin films with different Cd contents and single kesterite phase were fabricated using the sol-gel method. The optical absorption measurements indicate that the bandgap of the kesterite CCZTS alloy can be continuously tuned in a range of 1.55–1.09 eV as Cd content varied from x = 0 to 1. Hall effect measurements suggest that the hole concentration of CCZTS films decreases with increasing Cd content. The CCZTS-based solar cell with x = 0.47 demonstrates a power conversion efficiency of 1.2%. Our first-principles calculations based on the hybrid functional method demonstrate that the bandgap of the kesterite CCZTS alloy decreases monotonically with increasing Cd content, supporting the experimental results. Furthermore, Cu 2ZnSnS4/Cu2CdSnS4 interface has a type-I band-alignment with a small valence-band offset, explaining the narrowing of the bandgap of CCZTS as the Cd content increases. Our results suggest that CCZTS alloy is a potentially suitable material to fabricate high-efficiency multi-junction tandem solar cells with different bandgap-tailored absorption layers.
83 citations
TL;DR: Schottky photodiodes based on Au-ZnMgO/sapphire were demonstrated covering the spectral region from 3.35 to 3.48 eV, with UV/VIS rejection ratios up to ∼105 and responsivities as high as 185 A/W as mentioned in this paper.
Abstract: Schottky photodiodes based on Au-ZnMgO/sapphire are demonstrated covering the spectral region from 3.35 to 3.48 eV, with UV/VIS rejection ratios up to ∼105 and responsivities as high as 185 A/W. Both the rejection ratio and the responsivity are shown to be largely enhanced by the presence of an internal gain mechanism, by which the compensated films become highly conductive as a result of illumination. This causes a large increase in the tunnel current through the Schottky barrier, yielding internal gains that are a function of the incident photon flux.
62 citations
References
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TL;DR: Wurtzitic ZnO is a widebandgap semiconductor which has many applications, such as piezoelectric transducers, varistors, phosphors, and transparent conducting films as discussed by the authors.
Abstract: Wurtzitic ZnO is a wide-bandgap (3.437 eV at 2 K) semiconductor which has many applications, such as piezoelectric transducers, varistors, phosphors, and transparent conducting films. Most of these applications require only polycrystalline material; however, recent successes in producing large-area single crystals have opened up the possibility of producing blue and UV light emitters, and high-temperature, high-power transistors. The main advantages of ZnO as a light emitter are its large exciton binding energy (60 meV), and the existence of well-developed bulk and epitaxial growth processes; for electronic applications, its attractiveness lies in having high breakdown strength and high saturation velocity. Optical UV lasing, at both low and high temperatures, has already been demonstrated, although efficient electrical lasing must await the further development of good, p-type material. ZnO is also much more resistant to radiation damage than are other common semiconductor materials, such as Si, GaAs, CdS, and even GaN; thus, it should be useful for space applications.
2,573 citations
TL;DR: In this paper, the authors used a new technique to fabricate p-type ZnO reproducibly, and showed high-quality undoped films with electron mobility exceeding that in the bulk.
Abstract: Since the successful demonstration of a blue light-emitting diode (LED)1, potential materials for making short-wavelength LEDs and diode lasers have been attracting increasing interest as the demands for display, illumination and information storage grow2,3,4. Zinc oxide has substantial advantages including large exciton binding energy, as demonstrated by efficient excitonic lasing on optical excitation5,6. Several groups have postulated the use of p-type ZnO doped with nitrogen, arsenic or phosphorus7,8,9,10, and even p–n junctions11,12,13. However, the choice of dopant and growth technique remains controversial and the reliability of p-type ZnO is still under debate14. If ZnO is ever to produce long-lasting and robust devices, the quality of epitaxial layers has to be improved as has been the protocol in other compound semiconductors15. Here we report high-quality undoped films with electron mobility exceeding that in the bulk. We have used a new technique to fabricate p-type ZnO reproducibly. Violet electroluminescence from homostructural p–i–n junctions is demonstrated at room-temperature.
1,964 citations
TL;DR: In this paper, the authors study the intrinsic defect physics of ZnO and find that ZnOs cannot be doped p type via native defects, despite the fact that they are shallow donors.
Abstract: ZnO typifies a class of materials that can be doped via native defects in only one way: either n type or p type. We explain this asymmetry in ZnO via a study of its intrinsic defect physics, including ${\mathrm{Zn}}_{\mathrm{O}},$ ${\mathrm{Zn}}_{i},$ ${\mathrm{V}}_{\mathrm{O}},$ ${\mathrm{O}}_{i},$ and ${V}_{\mathrm{Zn}}$ and n-type impurity dopants, Al and F. We find that ZnO is n type at Zn-rich conditions. This is because (i) the Zn interstitial, ${\mathrm{Zn}}_{i},$ is a shallow donor, supplying electrons; (ii) its formation enthalpy is low for both Zn-rich and O-rich conditions, so this defect is abundant; and (iii) the native defects that could compensate the n-type doping effect of ${\mathrm{Zn}}_{i}$ (interstitial O, ${\mathrm{O}}_{i},$ and Zn vacancy, ${V}_{\mathrm{Zn}}),$ have high formation enthalpies for Zn-rich conditions, so these ``electron killers'' are not abundant. We find that ZnO cannot be doped p type via native defects $({\mathrm{O}}_{i},{V}_{\mathrm{Zn}})$ despite the fact that they are shallow (i.e., supplying holes at room temperature). This is because at both Zn-rich and O-rich conditions, the defects that could compensate p-type doping ${(V}_{\mathrm{O}}{,\mathrm{}\mathrm{Zn}}_{i},{\mathrm{Zn}}_{\mathrm{O}})$ have low formation enthalpies so these ``hole killers'' form readily. Furthermore, we identify electron-hole radiative recombination at the ${V}_{\mathrm{O}}$ center as the source of the green luminescence. In contrast, a large structural relaxation of the same center upon double hole capture leads to slow electron-hole recombination (either radiative or nonradiative) responsible for the slow decay of photoconductivity.
1,724 citations
PARC1
TL;DR: This result indicates that the rate or self-diffusion depends strongly on the surface-annealing conditions and therefore the formation energy and hence the equilibrium concentration or the defects depends heavily on the atomic chemical potentials or As and Ga as well as the electron chemical potential.
Abstract: We calculate absolute formation energies of native defects in GaAs. The formation energy and hence the equilibrium concentration of the defects depends strongly on the atomic chemical potentials of As and Ga as well as the electron chemical potential. For example, the Ga vacancy concentration changes by more than ten orders of magnitude as the chemical potentials of As and Ga vary over the thermodynamically allowed range. This result indicates that the rate of self-diffusion depends strongly on the surface-annealing conditions.
1,294 citations
TL;DR: An N-doped p-type ZnO layer has been grown by molecular beam epitaxy on an Li-diffused, bulk, semi-insulating, N-O substrate as discussed by the authors.
Abstract: An N-doped, p-type ZnO layer has been grown by molecular beam epitaxy on an Li-diffused, bulk, semi-insulating ZnO substrate. Hall-effect and conductivity measurements on the layer give: resistivity=4×101 Ω cm; hole mobility=2 cm2/V s; and hole concentration=9×1016 cm−3. Photoluminescence measurements in this N-doped layer show a much stronger peak near 3.32 eV (probably due to neutral acceptor bound excitons), than at 3.36 eV (neutral donor bound excitons), whereas the opposite is true in undoped ZnO. Calibrated, secondary-ion mass spectroscopy measurements show an N surface concentration of about 1019 cm−3 in the N-doped sample, but only about 1017 cm−3 in the undoped sample.
1,237 citations