Schottky junction device

Abstract: A Schottky junction device having a Schottky junction of a semiconductor and a metal and a superlattice on the interface of the semiconductor and the metal, wherein the upper edge of the valence band of said superlattice gradually varies.

Description: Detailed Description of the Invention [The Field of Industrial Use] The present invention relates to a Schottky junction device having a Schottky junction of a semiconductor and a metal. [Prior Art] As well known in the field of semiconductor technology, In 0.53 Ga 0.47 As lattice-match to InP and has a number of remarkable properties including a high mobility in a low electric field, a high saturation velocity, a band gap as small as 0.74eV and the ability to absorb light with a 1.3 to 1.65µm wavelength region be used for optical telecommunications. In an attempt to prepare semiconductor devices that can fully exploit these properties, there have been proposed MESFETs and photo-diodes having a Schottky junction of In 0.53 Ga 0.47 As and a metal. However, these Schottky junction devices are accompanied by disadvantages such as a low barrier height and a high leakage current. Techniques for solving these problems are discussed in Reference Documents 1 through 4 listed below. [Reference Document 1] D. V. Morgan et al., Electron. Lett. 14,737 (1978). This paper proposes the use of an SiO 2 layer on the interface of a Schottky junction. [Reference Document 2] J. Barrard et al., IEEE Electron Device Lett. EDL-1,174 (1980). The above identified document proposes the use of an n-In 0.52 Al 0.48 As layer as a contact layer of a Schottky junction. [Reference Document 3] D. H. Lee et al., Appl. Phys. Lett. vol 54 (19) (1989) pages 1863-1865. This document, or Reference Document 3, discusses the use of a graded superlattice consisted of In 0.53 Ga 0.47 As/In 0.52 Al 0.48 As on the interface of a Schottky junction. As illustrated in Figs. 3(a) and 3(b) of the accompanying drawings, a device disclosed in Reference Document 3 comprises an Au/Ge-Ni ohmic electrode 11, n + -InP substrate 12, a 0.5µm thick n + -In 0.53 Ga 0.47 As layer 13, a 1.5µm thick n-In 0.53 Ga 0.47 As layer 14, a graded superlattice 15 and an Au Schottky electrode 16. The graded superlattice 15 of the above described device is formed by alternately laying In 0.53 Ga 0.47 As film layers 15a and In 0.52 Al 0.48 As film layers 15b to produce a multilayer structure and the thickness of the In 0.53 Ga 0.47 As film layers 15a and that of the In 0.52 Al 0.48 As film layers 15b are gradually increased in opposite directions. With a device of Reference Document 3, the height of the Schottky barrier for the Au Schottky electrode is improved to show a value of approximately 0.71eV from the ordinary value of 0.2eV. for InGaAs bulk layer. This improvement in the height of Schottky barrier is explained in terms of the effective band gap of the contact layer which is increased due to the average composition of the graded superlattice. [Reference Document 4] U.S. Patent Application No. 08/057,757 filed May 7, 1993 based on Japanese Patent Application No. 143420 filed May 8, 1992, Irikawa and Iga. The above document discloses the use of an InGaAs/InAlAs multiple quantum barrier layer on the interface of a Schottky junction. With a device of Reference Document 4, the height of the Schottky barrier relative to the Au Schottky electrlode is improved to show a value of approximately 1.71eV from the ordinary value of 0.2eV. [Problems to be Solved by the Invention] When a graded superlattice or a multiple quantum barrier layer is used on the interface of a Schottky junction, the upper end of the valence band of such a superlattice is varied to show a rugged profile. When holes run from the semiconductor toward the metal in a Schottky junction device having such a superlattice, they are apt to be piled up on the protruding areas of the upper end of the valence band caused by the valence band discontinuity each having a value of ΔEv to obstruct any quick response actions of the device. [Object of the Invention] In view of the above identified technological problems, it is therefore an object of the present invention to provide a Schottky junction device having a superlattice arranged on the Schottky interface in order to hold a high Schottky barrier and at the same time showing a high speed responsivity by resolving the phenomenon of hole-piling up due to the valence band edge discontinuity, while maintaining the height of the Schottky barrier. [Summary of the Invention] According to the present invention, there is provided a Schottky junction device, comprising a Schottky junction formed by a semiconductor and a metal element with a superlattice disposed at the interface between the semiconductor and metal element, characterised in that the superlattice comprises a plurality of alternating tensile-strained and compressive-strained layers, the tensile-strained layers comprising GaInAs and/or GaAsSb and the compressive strained layers comprising AlInAs, and in that the compositions of the successive tensile-strained layers and of the successive compressive strainer layers change progressively such that the upper edge of the valence band of the superlattice changes gradually to thereby progressively increase the bandgap towards the metal element of the Schottky junction, the change in the upper edge of the valence band of the superlattice, from one said layer to the next, being no greater than 15 meV. Said change in the upper end of the valence band of said superlattice may be in the form of a stepped profile, or a continuous e.g. curved profile. Said superlattice may be on an InP substrate. [Function] A Schottky junction device according to the invention has a superlattice arranged at the interface of a Schottky junction of a semiconductor and a metal and the upper edge of the valence band of said superlattice changes gradually in a predetermined sense. Such a superlattice shows nob only a low leak current and is free from the phenomenon of hole pile-up because the upper edge of the valence band does not have any protruding areas. Hence, the device shows an excellent high speed response. [Brief Description of the Drawings] Fig. 1(a) is a schematic sectional view of a preferred embodiment of Schottky junction device of the invention, which is a Schottky diode. Fig. 1(b) is a schematic illustration of the energy band diagram of the superlattice of the embodiment of Schottky junction device of Fig. 1(a). Fig. 2 is a graph showing the relationship between the lattice constant and the valence band energy of a III-V group semiconductor. Fig. 3(a) is a schematic sectional view of a conventional Schottky diode. Fig. 3(b) is a schematic illustration of the energy band diagram of the superlattice of the Schottky diode of Fig. 3(a). [Embodiment] Now, the present invention will be described by referring to the accompanying drawings that illustrate a preferred embodiment of the invention. Fig. 1(a) schematically illustrate the embodiment, which is a semiconductor device comprising a Schottky diode and realized by modifying a device disclosed in Reference Document 3 as cited above. The device of Fig. 1(a) comprises an Au/Ge-Ni ohmic electrode 1, an n + InP substrate 2, a 0.5µm thick n + -In 0.53 Ga 0.47 As layer 3, a 1.5 thick n-In 0.53 Ga 0.47 As layer 4, a superlattice 5 and an Au Schottky electrode 8. As clearly shown in Fig. 1(b), the superlattice 5 of the above device is formed by alternately growing compressive-strained layers 6a, 6b, ... and tensile-strained layers 7a, 7b, ... to produce a multilayer structure between the n-In 0.53 Ga 0.47 As layer 4 and the Au Schottky electrode 8, the total number of the compressive and tensile-strained layers being fifteen, each having a thickness of approximately 4nm. Said multilayer structure is so arranged that the strains of the compressive-strained layers 6a, 6b, ... and that of the tensile-strained layers 7a, 7b, ... are compensated so that the strain produces no dislocations in the superlattice 5. Referring to Fig. 1(b), it will be seen that each of the valence band edge discontinuities ΔEv has a value of 15meV to show a stepped profile at the upper edge of the valence band. In general, the valence band energy levels and the lattice constants of GaAsSb, GaInAs, AlInAs or AlAsSb show a relationship as described in detail in Reference Document 5 as listed below and illustrated in Fig. 2 [if AlAs is used as reference]. [Reference Document 5] F. L. Schuermeyer et al., Appl. phys. Lett. 55, 1877 (1989). The compositions of each of the compressive-strained layers 6a, 6b, ... and the tensile-strained layers 7a, 7b, ... of a Schottky junction device according to the invention are defined in terms of the relationship as described in Reference Document 5. More specifically, the compressive-strained layer 6a which is held in contact with the n-In 0.53 Ga 0.47 As layer 4 that lattice-matches with InP has a composition of Al 0.25 In 0.75 As and the tensile-strained layer 7a arranged next to the compressive-strained layer 6a has a composition of GaAs 0.75 Sb 0.24 , where ΔEv between the layers 6a and 7a having a value of 15meV. Then, each of the remaining layers is so formed as to show 15meV for ΔEv by modifying the composition of the compressive-strained layers 6b, ... in the direction from InAs to AlAs along Line L 1 and that of the tensile-strained layers 7b, ... in the direction from GaSb to GaAs along Line L 2 . With such an arrangement, the compressive-strained layers 6a, 6b, ... having a lattice constant greater than that of InP are subjected to an in-plane compressive strain, whereas the tensile-strained layers 7a, 7b, ... having a lattice constant smaller than that of InP are subjected to an in-plane tensile strain. Further, in order to avoid the introduction of the dislocation, the compressive-strained layers and the tensile-strained layers are balanced by the same amount. Note that InGaAs is used in place of GaAsSb for the tensile-strained layers 7n, 7o, ... arranged behind the tensile-strained layer 7m which has a composition of GaAs 0.8 Sb 0.2 . That is, the composition of the tensile-strained layer 7n will be In 0.4 Ga 0.6 As and the composition of the following tensile-strained layers will be modified in the direction from InAs to GaAs along Line L 3 . Finally, an n-In 0.52 Al 0.48 As layer 9 that lattice-matches to InP is laid thereon to contact with the Au Schottky electrode 8. Each of the compressive-strained layers 6a, 6b, ... and tensile-strained layers 7a, 7b, ... may, for instance, have a doping concentration of n to 5x10 15 cm -3 . The reason why the value of ΔEv is set to 15meV is that the phenomenon of hole-piling up can be effectively prevented from occurring in the valence band by holding the value of ΔEv to be not greater than 15 meV. The thermal energy level of kT, to be 26meV in ambient temperature. The Schottky junction devices according to the invention are produced by e.g., MOCVD, MBE, gas source MBE method and the like, of the methods excellent in the controlling of the extremely thin films. The change in the valence band edge of the superlattice of the Schottky junction device according to the invention may show a continuously graded or curved profile. When a multiple quantum barrier layer as disclosed in Reference Document 4 is used for the superlattice of a device according to the invention, the compositions or the materials of the layers need to be modified and made to be same as those of the above embodiment. The Schottky junction technology as disclosed in this patent specification may be used not only on InP substrates but also on GaAs substrates. Additionally, the technology as disclosed in this patent specification may be applied not only to Schottky diodes (including photo-diodes) but also to SIS-type FETs and HEMTs. [Advantages of the Invention] Since a Schottky junction device having a Schottky junction between a semiconductor and a metal and a superlattice on the interface between the semiconductor and the metal is characterized in that the upper edge of the valence band of said superlattice is changed gradually in a predetermined sense, the phenomenon of hole pile-up at the upper edge of the valence band is avoided while maintaining the height of the Schottky barrier. Consequently, such a Schottky junction device shows an excellent high speed performance.

Claims: A Schottky junction device, comprising a Schottky junction formed by a semiconductor (4) and a metal element (8) with a superlattice (5) disposed at the interface between the semiconductor (4) and metal element (8), characterised in that the superlattice (5) comprises a plurality of alternating tensile-strained and compressive-strained layers, the tensile-strained layers (6a,6b,6c...) comprising GaInAs and/or GaAsSb and the compressive strained layers (7a,7b,7c....) comprising AlInAs, and in that the compositions of the successive tensile-strained layers and of the sucessive compressive strainer layers change progressively such that the upper edge of the valence band of the superlattice (5) changes gradually to thereby progressively increase the bandgap towards the metal element (8) of the Schottky junction, the change in the upper edge of the valence band of the superlattice, from one said layer to the next, being no greater than 15 meV. A Schottky junction device according to claim 1, characterised in that the upper edge of the valence band of the superlattice (5) changes in a stepped profile. A Schottky junction device according to claim 1, characterised in that the upper edge of the valence band of the superlattice (5) changes in a continuous profile. A Schottky junction device according to claim 3, characterised in that the upper edge of the valence band of the superlattice (5) changes in a curved profile. A Schottky junction device according to any preceding claim, characterised in that the superlattice (5) is formed on an InP substrate (2).

Inventors: HIRAYAMA YOSHIYUKI, IRIKAWA MICHINORI, SHIMIZU HITOSHI, IRIKAWA, MICHINORI, SHIMIZU, HITOSHI
Applicants: FURUKAWA ELECTRIC CO LTD, THE FURUKAWA ELECTRIC CO., LTD.
Application year: 1993
Main classification: H01L29
IPC codes: H01L31, H01L29