R.H. Williams

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.

Metal-semiconductor fluctuations on reconstructed Sn-Si(111) surfaces

We present soft X-ray photoelectron spectroscopy results from the T4single bondSn/Si(111)−(3×3)R30° (√3 for short) and the Sn/Si(111)−(2√3× 2√3)R30° (2√3 for short). Sn 4d spectra recorded using 110 eV photons reveal two components for the 2√3 phase with the smaller one shifted by 0.38 eV towards low kinetic energy relative to the larger one. The √3 phase on the other hand could only be fitted with three components. The intensity of the largest component decreases with increasing Sn coverage and almost disappears at 1.1 monolayers (i.e. corresponding to the 2√3). From our analysis, we find that in the √3 phase, 10% of the surface is covered by 2√3 reconstruction and 90% by √3 reconstruction. These results suggest metal-semiconductor fluctuations on the √3 reconstruction as well as for mixed √3 and 2√3 phases

Schottky barrier height enhancement on n-In0.53Ga0.47As using delta-doping

Abstract We describe the use of n- and p-type delta-doping layers in n-In 0.53 Ga 0.47 As for Schottky barrier enhancement. The delta-doping layers are introduced by MOCVD using silicon and zinc dopants respectively. From current-voltage characteristics of Au/In 0.53 Ga 0.47 As diodes measured at room temperature, we find substantial effective Schottky barrier height enhancement, with a best value of 0.69 eV and corresponding ideality factor of 1.06. Numerical modelling of Poissons equation for these structures predicts effective barrier heights in good agreement with those measured.

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.

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.

Ballistic electron emission microscopy of InAs grown on GaAs(100)

We report an investigation of barrier formation between InAs and GaAs interface. The barrier height has been found to decrease with the InAs thickness and the detailed variation is in accordance with the transition/relaxation of the InAs layer. Also, the transport properties at the interface have been studied, and the L valley transmission is not observed due to the requirement of lateral momentum conservation.

The control and modification of metal-semiconductor interfaces using multi quantum barriers

In this paper we report, for the first time, on the effect of placing a multi quantum barrier (MQB) near the gold-Al 0.3 Ga 0.7 As interface. A theoretical model has been developed to determine the effective barrier height for a given Schottky-MQB structure directly from the reflectivity calculated as a function of electron energy, RvE. The abruptness and shape of the RvE curve were used to characterise the enhancement efficiency owing to the presence of the MQB. Based on the results of this, a metal-Al 0.3 Ga 0.7 As sample containing an MQB structure (Al 0.3 Ga 0.7 As/GaAs) was designed and grown by molecular beam epitaxy. A standard metal-Al 0.3 Ga 0.7 As Schottky barrier (with no MQB), was also grown for comparison. Gold Schottky contacts were formed and the barrier height measured, using a current-voltage technique, in-situ to avoid degradation on exposure to air. In addition, capacitance-voltage, and ballistic electron emission microscopy were used to further characterise the interfaces ex-situ. The experimentally measured barriers of both samples were found to be in excellent agreement with the theoretical predictions from the reflectivity curves. The dependence of the barrier height enhancement on the structure and composition of the MQB is also discussed.

Use of ultrathin ZnSe dipole layers for band offset engineering at Ge and Si homo/heterojunctions

The ability to control semiconductor band discontinuities would allow solid devices to be specifically tailored so that efficiency and performance could be dramatically improved. This article reports the use of an ordered ZnSe monolayer to induce a valence band discontinuity at the Ge homojunction (0.38 eV), at the Ge–Si heterojunction (0.53 eV), and at the Si homojunction (∼0.2 eV). Soft x-ray photoemission was used to probe the interfaces as they were formed under ultrahigh vacuum conditions. The effect of overlayer band bending on the interpretation of band offset measurements is discussed. As the interfacial bonding and orientation of the dipole layer are key factors in determining the direction and magnitude of the band modification, x-ray standing wave measurements were performed on the Ge–ZnSe–Ge systems to identify the atomic structure of the junction. Se atoms were always found to bond to the Ge substrate in the a-top position, while the Zn atoms adopted the H3 sites, bonding to the overlayer. Th...