S.A. Clark

Nano-crystalline SnO2 gas sensor response to O2 and CH4 at elevated temperature investigated by XPS

Abstract The electronic changes occurring at the surface of a nano-crystalline SnO 2 sensor upon exposure to O 2 and CH 4 at elevated temperature have been investigated by X-ray photoelectron spectroscopy (XPS). Gas exposures and subsequent XPS scanning were conducted at 120 and 250 °C to simulate the working conditions of the sensor. Exposure to O 2 at 120 °C resulted in a 0.2 eV upward band bending while subsequent exposure to CH 4 resulted in a 0.1 eV downward band bending, as expected from oxidising and reducing gases, respectively. Similar changes in surface band bending were observed at 250 °C, although quantitative analysis suggests that oxygen absorption might be enhanced. The results clearly indicate the electronic nature of the gas sensing mechanism when exposed to O 2 and CH 4 and the very high sensitivity of the sensor.

Influence of premetallization surface treatment on the formation of Schottky Au-nGaN contacts

The influence of premetallization surface preparation on the structural, chemical, and electrical properties of Au–nGaN interfaces has been investigated by x-ray photoemission spectroscopy (XPS), current-voltage measurement (I-V) and cross-section transmission electron microscopy (TEM). XPS analysis showed that the three GaN substrate treatments investigated i.e., ex situ hydrofluoric acid etch, in situ anneal in ultrahigh-vacuum (UHV), and in situ Ga reflux cleaning in UHV result in surfaces increasingly free of oxygen contamination. XPS and TEM characterization of Au–nGaN formed after the three premetallization surface treatments show that HF etching and UHV annealing produce abrupt, well-defined interfaces. Conversely, GaN substrate cleaning in a Ga flux results in Au/GaN intermixing. I-V characterization of Au–nGaN contacts yields a Schottky barrier height of 1.25 eV with a very low-ideality factor and very good contact uniformity for the premetallization UHV anneal, while the Ga reflux cleaning resul...

Improvements to the Schottky barrier heights of intimate metal-InGaAs contacts by low temperature metallisation

One challenge in the processing of In 53 Ga 47 As/InP(100) heterostructures is the enhancement of the low Schottky barrier heights (Φ b ) typically exhibited by metal-nIn 53 Ga 47 As(100) contacts. Previous studies of the metallisation of chemically etched In 53 Ga 47 As surfaces have reported increases in Φ b if metallisation is performed at 77 K. This study extends this concept to probe the idealised case of the formation of metal contacts (Au-, Cu- and In-) to atomically clean nIn 53 Ga 47 As/InP(/I) at 80 K and room temperature (294 K). The in-situ current-voltage measurements of intimate Au and Cu contacts formed at both temperatures exhibited ohmic characteristics. However, intimate In contacts formed at 80 and 294 K showed diodic behaviour (Φ b 0.45 and 0.30 eV, respectively), with an improvement in the rectification when formed at 80 K. Conversely, In contacts to etched surfaces were ohmic. It is proposed that this behaviour is linked to the alteration of the structure and nature of the interface as a result of the various fabrication techniques. These results indicate the potential to select Φ b on In 53 Ga 47 As via a combination of cryogenic processing and surface preparation.

An investigation of the properties of intimate InInxGa1-xAs(100) interfaces formed at room and cryogenic temperatures

Abstract The electrical, chemical and structural properties of the interfaces formed at room and low temperatures, between In and atomically clean In 53 Ga 47 As/InP(100) have been studied. Current-voltage measurements indicate that diodes formed at 80 K exhibit significantly higher Schottky barriers (φ b = 0.45 eV) than diodes formed at 294 K (φ b = 0.30 eV). The reactions occurring during the formation of In In 53 Ga 47 As/InP(100) interfaces at room and low temperatures have been investigated using Soft X-Ray photoemission spectroscopy. Our results show that metallisation at room temperature results in a predominantly three dimensional mode of growth, accompanied by the out-diffusion of As. Low temperature (125 K) metallisation appears to reduce clustering and inhibit As out-diffusion. Examination of the resulting interfaces by transmission electron microscopy confirms the more uniform nature of the metal layers formed at low temperature. These observations, in conjunction with the barrier heights measured by the I-V technique, are discussed in the context of currently supported models of Schottky barrier formation.

The electronic and structural properties of the silicon-gallium arsenide(110) interface

Abstract The passivation properties of the Si GaAs(110) interface have been studied using scanning tunnelling microscopy/spectroscopy (STM/STS) and soft X-ray photoemission spectroscopy (SXPS). Silicon has been deposited at room temperature and STM images show the sub-monolayer growth of silicon islands on the GaAs substrate. The electrical properties of these islands together with the clean surface have been investigated using scanning tunnelling spectroscopy (STS). The spectroscopy clearly illustrates the difference in electrical properties between atomically flat regions of GaAs as compared to those containing defects or steps, i.e. where surface band bending occurs. We have investigated the use of sub-monolayer Si coverages to modify the electronic structure of the surface. Height variations of 3–4Aacross Si islands and 2Aacross steps on the GaAs surface have also been observed using the STM. STS spectra, collected simultaneously with the STM image, showed the Si to have semiconducting properties differing from that of crystalline Si and the GaAs substrate. Comparisons between the STM and STS results together with SXPS have provided a correlation between the structural, electrical and chemical nature of the Si/GaAs(110) interface.

MODIFICATION OF BAND OFFSETS BY A ZNSE INTRALAYER AT THE SI/GE(111) INTERFACE

In this letter, the use of an ordered ultrathin ZnSe dipole layer to significantly modify the band discontinuity at the Si/Ge(111)-c(2×8) heterojunction is reported. Soft x-ray photoemission spectroscopy (SXPS) was utilized to monitor the evolution of the interface. The ZnSe intralayer increased the valence band offset by ∼0.57 eV, as compared to a negligible valence band offset for the Si/Ge(111) junction. This dramatic modification is interpreted in terms of the charge transfer at the interface.

Ni∕Al0.2Ga0.8N interfacial reaction and Schottky contact formation using high quality epitaxial layers

The formation of the Ni∕Al0.2Ga0.8N Schottky contacts has been investigated by x-ray photoelectron spectroscopy. In situ scanning tunneling microscopy was used in parallel to investigate the morphology of the Ni covered surface after the last deposition. In the same way, results are presented through two perspectives: the intensity of core-level signals which give information on the growth mode, and the core-level binding energy positions which assess changes in electronic and chemical properties as a function of Ni coverage. Ni deposition on Al0.2Ga0.8N substrates follows the Stranski–Krastanov growth mode. It is suggested that Ni preferably reacts with the contaminants at the surface rather than with the epilayer itself. The Schottky barrier formation is discussed in terms of unified defect and metal-induced gap states models.

Effect of a ZnSe intralayer on the Si/Ge(111) heterojunction band offsets

Abstract Soft X-ray photoelectron spectroscopy (SXPS) was performed on Si/Ge(111)-c(2 × 8) heterojunctions. The effect of an ordered ultrathin (one monolayer) ionic dipole layer of ZnSe placed at the interface was studied. The formation of the interface was monitored via the evolution of the valence band edges and the movements of core levels. It was found that the ZnSe intralayer dramatically modified the valence band offset of the Si/Ge (111) junction. The valence band offset was increased by 0.57 ± 0.1 eV due to the presence of the ZnSe intralayer. This intralayer-induced modification of the valence band offset is interpreted in terms of the charge transfer at the interface.

Band engineering at the GaAsAlGaAs heterojunction using ultra-thin Si and Be dipole layers: a comparison of modification techniques

Abstract The control of semiconductor interfaces is essential to engineer new material properties for device applications. In this article we have considered the use of ultra-thin (1 monolayer) interfacial Si and Be dipoles layers to modify the band discontinuity present at the GaAs AlGaAs heterojunction. Soft X-ray photoelectron spectroscopy (SXPS) was performed at the Daresbury synchrotron radiation source (SRS) on samples previously grown by molecular beam epitaxy (MBE). Detailed deconvolution of the As 3d core level spectra enabled the valence band modification due to the presence of the interlayers to be extracted. The results of this study indicate the potential of this method to induce large valence band-offset modification (+0.4 eV for Si and −0.52 eV for Be) due to the presence of the dipole layers. The effect of any near interface doping by the Si and Be layers was considered by solving Poissons equation for these structures. Finally, the technique is compared to other band engineering methods, namely δ-doping and multi quantum barriers (MQB), to assess the potential and viability for use in real devices.

Photoemission study of the formation of intimate In–InGaAs(100) contacts at room and cryogenic temperatures

Previous current–voltage studies of In contacts deposited on atomically clean (intimate) In53Ga47As(100) have indicated the potential to “select” barrier heights in this materials system by cryogenic processing. Soft x-ray photoemission spectroscopy was used to determine the electronic and chemical nature of these interfaces, as a function of formation temperature. Metallization at room temperature results in a predominantly three-dimensional mode of growth, accompanied by the outdiffusion of As. Low temperature metallization appears to reduce clustering and inhibit As outdiffusion. It is proposed that the distribution of surface states and the fermi level pinning position are altered by the changes that occur in the geometry and bonding of the interface at low temperature.