J.M. Palau

Fermi level pinning by interface states: A calculation of the height and the shape of the Schottky barrier

Abstract A calculation of the potential distribution of the space charge region of a metal-semiconductor contact is proposed, which is based on the theories of Schottky and Bardeen. Inspired by the defect model of Spicer et al. , we assume that interface states are spread over several atomic layers inside the semiconductor. Thus the “neutral level” of Bardeen can be reached at the most buried states. Particular attention is paid to conditions for which such Fermi level pinning is possible.

Properties of the contact on ion cleaned n and p type silicon surfaces

Abstract Schottky contacts were produced by silver evaporation on Si(100) surfaces cleaned by ion sputtering and partial annealing. The samples work function were measured before and after metal deposition with the Kelvin method, in an experimental set-up which allowed a topografical study and direct comparison between n and p types. Clean surfaces with and without a residual layer of oxide was achieved and controlled by AES. It was found that the Fermi level of all the surfaces was pinned by donor states created by the bombardment and that there was no barrier on n type and an important surface barrier on p type. The diodes we obtained presented no barrier on n type and a rectifying contact on p type. So we deduced that the Schottky barrier is already fully formed before metal contact is achieved. Furthermore study of the electrical properties of the diodes had shown that the bombardment creates donor states responsible for the barrier and a perturbated layer with deep acceptor traps responsible for the current flow mechanism. A residual layer of oxide and a post annealing of the device did not noticeably change the Schottky barrier in the diodes achieved on p type but led to clearly differenciated performances for the diodes achieved on both p and n type substrate. So we therefore concluded that the characteristics of the deep acceptor traps of the superfacial layer are modified by the oxide and annealing, both of which on the other hand having no effect on the surface donor states.

Silver Schottky diodes on Kelvin, AES and LEED characterized (100)surfaces of GaAs cleaned by ion bombardment

Abstract Schottky contacts were produced by silver evaporation on GaAs surfaces cleaned by ion bombardment. The samples work function were measured before and after metal deposition with the Kelvin method in an experimental set up which allowed a topographical study and direct comparison between n and p types. Surfaces were controlled by AES and LEED. It was found that the Fermi level of all the surfaces was pinned in the midgap range and so there was, on both n and p types, an important surface barrier qVs whose value did not strongly depend on the surface preparations we used. The diodes we obtained also presented an important barrier φB for both n and p types. So it was deduced that diode barriers corresponded to those that were created by surface states and that metal deposition did not noticeably modify these states. On the other hand, surface preparations played an important role in the electrical performances of the diodes. It was deduced then that ion bombardment deteriorated the first atomic layers of the semiconductor.

Chemical etching and annealing induced GaAs(100) surface properties

Abstract We Study chemically cleaned or annealed GaAs(100) surfaces by using AES, work function (TCPD) and surface photovoltage topographies (TSPV). We show that whatever the etch, the Fermi level is pinned: acceptor and donor states are needed to explain these results which are analysed by comparing the models of Grant and of Spicer. However, some different properties are related to the surface preparation and treatment: electronic affinity or surface barrier height modifications, work function uniformities, surface composition, etc.

Electron beam effect on GaAs real surfaces and on AgGaAs schottky diodes

Abstract In this paper, we study by AES, work function and surface photovoltage topographies the properties of GaAs etched surfaces. After a characterization of the surface, we look at the modification induced by an electron beam and we show that the electrons induce large work function modifications. Schottky diodes are then produced on unbombarded areas and on electron-bombarded areas. We show that the electron beam induces noticeable modifications of the diodes properties. As suggested by the doping variations deduced from C ( V ) measurements, these modifications can result from donor states, induced by the beam, localized in a thin layer beneath the surface.

Calculation of the work function of a substrate-thin film heterojunction

Abstract We carried out a theoretical investigation of the work function of a semiconductor covered with a thin film. We assumed that the layer was also a semiconductor and that there were no surface and interface states. The structure can therefore be regarded as a simple heterojunction in which the space charge may extend over the whole thickness of the layer. The work function was calculated as a function of the film thickness.

The effect of surface preparation and properties on AgGaAs (100) Schottky diodes

Abstract Results are presented of experiments concerning electrical properties of chemically-etched GaAs surfaces in relation to their preparation and the properties of Ag Schottky contacts which have been achieved in situ. It is then tried to establish a link between the diodes and the initial surface properties. The results are analysed in terms of initial and induced defects.

Properties of InP(110) surfaces, InP(110)-Ag interfaces and InP(110)-Ag Schottky diodes: Contribution of the temperature

We present the results obtained on cleaved InP(110) surfaces by Auger electron spectroscopy (AES) and Kelvin measurements. We first study the electronic properties of the surface just after cleavage at room temperature and liquid nitrogen temperature. Then, we increase the temperature up to 670 K and follow the modification of the free surface stoichiometry by AES. We repeat the same experiments after the deposition of three Ag monolayers. From these experiments, we deduce information on the relative concentration of In−Ag−P in the surface layer. On the free surface the stoichiometry remains practically unchanged in the range 110–473 K. Above these temperatures, the stoichiometry is modified particularly above 480 K: the P concentration decreases and the In concentration increases. P desorption is yet important. Different results were obtained when the sample was Ag covered. At T =450 K the P concentration increases and the concentration of Ag decreases. There is an out-diffusion of P and in-diffusion of Ag which noticeably modify the layer stoichiometry. In the second part of the paper, we report results on Ag−InP(110) Schottky diodes obtained at room temperature and after annealing of the diodes. We compare the evolutions of the properties of the InP(110)-3 ML Ag interface with the evolution of the electrical properties of the diodes. Afterwards, the results are analysed in terms of interdiffusion and change in the stoichiometry.

Experimental indications that the perfectly cleaved surface of n-GaP may be unpinned

Abstract We report work functions and surface photovoltage results obtained on n-type gallium phosphide. Measurements have been performed with our topographic Kelvin probe that allows a detailed study of the effect of the cleavage faults. It is found that the surface potential is particularly sensitive to acceptor surface states associated with cleavage defects and, consequently, that band bending as high as 700 meV can be reached. But also band bending in the 100 meV range has been observed on the best areas, leading to a surface Fermi level position higher than the ones reported in previous papers. From our results, the assumption that the perfectly cleaved surface would be unpinned, as other III–V semiconductors do, is quite credible.