D.E. Brodie

Effects of annealing ZnO films prepared by ion-beam-assisted reactive deposition

Annealing ZnO films prepared by ion-beam-assisted deposition (IBAD) in air above 773 K can change the resistivity from ≈10−4Ω cm to more than 106Ω cm, changing the films from a highly degenerate transparent material to a thermally activated semiconductor. No change in the transmission through the visible region of the spectrum (more than 90%) is observed but the optical band gap can change from ≈3.41 to ≈3.24 eV, as a result of a Burstein-Moss shift. However, for films that are not initially very transparent, the average transmission can be increased from ≈28% to more than 85%. High temperature annealing increases the average size of crystallites by ≈15%. After annealing, a large photoresponse is observed, with the conductivity increasing by more than a factor of 105 when the film is illuminated with UV light (≈4 × 1016 photons cm−2 s−1). The rise and decay time constants are very large, indicating that previously reported surface structural changes occur. These constants strongly depend on the intensity of the light and the ambient gases present. Any true photoconductivity present appears to be very small. Encapsulating the free surface of a ZnO film with a layer of amorphous SiNx can help to stabilize the film, and it decreases the photoresponse time as well as the magnitude of the photoresponse.

Photoconducting TiO2 prepared by spray pyrolysis using TiCl4

Abstract TiO 2 was prepared by the oxidation of TiCl 4 at relatively low deposition temperatures using spray pyrolysis. For a fixed carrier gas (N 2 ) flow rate, the substrate temperature is the critical parameter determining the film properties. Films made near 595 K contain only the anatase phase, adhere well to the glass substrate, and have a relative optical transmission of about 75% (between 450 and 850 nm), the value one calculates for the non-absorbing anatase phase in this region. These films have an absorption edge characteristic of a direct gap semiconductor with allowed transitions and optical gaps of 3.5–3.7 eV, depending on the porosity. At higher substrate temperatures (above 635 K), the resulting films have a cloudy appearance and they contain both the rutile and the anatase modifications, confirmed by X-ray diffraction. Lower temperature substrates reduce the deposition rate and yield more porous films. For the pure anatase films, the dark conductivities are about one order of magnitude larger, and the photoconductivity is about one order of magnitude smaller when measured with the sample in air, than those observed when the sample is in a vacuum. When the samples are in a vacuum, the conductivities of the clear films increase by about two orders of magnitude (from about 10 −10 to about 10 −8 S cm −1 ) when illuminated with light from an electrically programmable read-only memory eraser (mercury light), which gives an intensity of about 30 μW cm −2 at the sample. The photoresponse has two parts; the fast portion rises in less than 1 s to 99% of its final value, followed by a small slow rise portion. The slow response, which is more pronounced in the decay process, is interpreted as being due to the presence of surface trapping states.

Crystallite orientation and the related photoresponse of hexagonal ZnO films deposited by r.f. sputtering

Abstract Hexagonal ZnO films with different observed orientations were deposited on glass substrates at temperatures below 200°C by r.f. sputtering of a ZnO target using an oxygen and argon gas mixture. Orientations with the (002) planes of the crystallites parallel to the substrate, or with the (100) planes parallel to the substrate, or films in which the (101) peak is dominant in their X-ray diffraction spectra have been obtained by selecting the deposition parameters. The partial pressure of the oxygen in the sputtering gas, the distance between target and substrate, the substrate temperature during film nucleation and the input power each affect (to varying degrees) the observed crystallite orientation obtained. Polycrystalline ZnO films made in the presence of excess oxygen or at a high sputtering gas pressure tend to have the c axis parallel to the substrate and they exhibit a large photoresponse, while films sputter-deposited with a low partial pressure of oxygen tend to have the c axis perpendicular to the substrate and may have no detectable d.c. photoresponse. The availability of oxygen during sample preparation has a significant affect on the slow photoresponse of the resulting ZnO films.

Transparent conducting ZnO films deposited by ion-beam-assisted reactive deposition

Abstract Highly transparent conducting ZnO films have been deposited using ion-beam-assisted reactive vacuum deposition. The zinc deposition rate was controlled by adding gallium to the zinc in an open Al 2 O 3 crucible source. Oxygen was introduced into the system via a separate controlled leak and reacted with the zinc on the substrate. Mechanically stable polycrystalline conducting ZnO films having a preferred orientation were deposited with resistivities in the range from 4.0 × 10 -6 to 9.0 × 10 -6 Ω m, with carrier densities of more than 2 × 10 26 m -3 and Hall mobilities between 2.8 × 10 -3 and 4.0 × 10 -3 m 2 V -1 s -1 . The average transmission exceeded 90% for films 350 nm thick in the wavelength range of the visible spectrum.

Photoresponse of polycrystalline ZnO films deposited by r.f. bias sputtering

The photoresponse of polycrystalline ZnO films generally contains both a true photoconductivity and a contribution from surface structural changes which can alter the surface conductance via the chemisorption and photodesorption of an active ambient-gas component. The surface structural changes can increase the conductance by up to seven orders of magnitude for a 500 nm thick sample when illuminated with an intensity of 4 × 1016 photon cm−2 s−1 UV light, but the response times are very slow. Newly prepared samples exhibit a fast response characteristic such as a true photoconductivity, but after exposure to air for several months the slower response dominates. Thus if used as a photoconductor, oxygen adsorption or desorption from the surface of a ZnO film is particularly problematic, but some surface stability can be achieved by adding nitrogen to the surface layers of the ZnO film and these samples exhibit more normal and improved photoconducting behaviour.

The photoresponse of high resistance anatase TiO2 films prepared by the decomposition of titanium isopropoxide

High resistance films of the anatase phase of TiO2 have been prepared by spray pyrolysis starting with titanium isopropoxide Ti(OPri)4. The films were deposited on glass substrates held at 590 K. The TiO2 films were characterized structurally, optically, and electrically. The photoresponse of these films to prolonged UV radiation in a vacuum and subsequently subjected to O2, H2, water vapour, or air environments, has been investigated. When vacuum, O2 or air is present, the electrical and optoelectronic properties observed can be explained by the induced variations in the oxygen deficiency at the surface of the TiO2 film and the resulting formation or removal of an enhancement layer at the surface. An equivalent effect on the surface band bending can be induced by an absorbed species. For example, adsorbed water vapour can alter the surface conductance of the films and the result depends on the oxygen-to-titanium ratio present at the surface when the water vapour is introduced.

The effect on polycrystalline ZnO film surfaces due to an Ar plasma introduced by a vacuum gauge

Abstract A strong accumulation layer can be produced on the surface of polycrystalline ZnO films when the samples are exposed to ions from an inert gas discharge in Ar or Kr. A combination of surface conductance and surface potential measurements show that an accumulation layer is induced by the adsorption of charges from the inert gas plasma which neutralizes and removes chemisorbed oxygen atoms, and this can increase the conductance by more than 6 orders of magnitude, for 500 nm thick films. Electrical neutrality in the ZnO provides the electrons for the accumulation layer, either from the ohmic contacts or the plasma. This accumulation layer can be further enhanced by illumination with UV light.

GaN films prepared by ECR plasma-assisted deposition

An electron cyclotron resonance plasma-assisted deposition method has been used polycrystalline GaN films. These GaN films can have dark conductivities as low as 10−10 S cm−1 and they exhibit well-defined conductivity activation energies. The sample conductivity is decreased by surface adsorption of gases and it increases again when the adsorbed species is photodesorbed from the surface using UV light. As a result, the photoresponse is very slow, especially the decay process, which may require several days for a sample to recover when it is mounted in a vacuum. This recovery time can be reduced significantly if the sample temperature is increased or the sample is exposed to air. In this respect, these GaN films have a number of features in common with ZnO.

The effect of ion bombardment on some properties of a-Si0.8Ge0.2:H alloys prepared by ion beam-assited reactive evaporation

Amorphous hydrogenated silicon germanium thin films with 20% Ge content (a-Si0.8Ge0.2:H) were deposited by ion beam-assisted deposition (IBAD), using SiH4 as the source gas. Moderate ion energies of ∼ 125 to 135 eV and an ion-to-atom ratio of ∼1, produced samples with a reduced number of SiH2 and (SiH2)n bonding configurations, an increased hydrogen content and a decreased porosity compared with non-bombarded samples. This reduces the disorder in the a-SiGe network as well as the defect density. The preferential attachment coefficient for these films is near unity. The optical and electrical properties of the a-SiGe:H alloy have been studied as a function of the ion beam energy and the ion-to atom ratio at the substrate. The best films had a photosensitivity of σphσd = 1.7 × 104 and Eg = 1.65 eV. The ion beam increased the optical gap, lowers the dark conductivity and increases the electrical activation energy.

A low energy ion beam assisted deposition technique for realizing iso-type SiGe/Si hetero-interface diodes

Abstract Amorphous SiGe/Si iso-type diodes have been successfully fabricated at a low temperature of about 250°C, using a low energy ion-beam assisted vacuum co-evaporation technique. The physical characteristics of the prepared structure is studied using RBS, SIMS and high resolution TEM. The electrical characteristics of the iso-type diodes are studied at different ambient temperatures, which show an ideal behavior over a significant range of temperatures (90–200 K). An energy barrier of 0.38 V is extracted from the current-voltage study.