Ohmic contact on a compound semiconductor layer

Abstract: The present invention provides an ohmic contact device which comprises : a first layer (2) made of a first compound semiconductor having a first energy band gap ; a superlattice (3) in contact with the first layer, the superlattice having modulation-periods comprising alternating a first very thin layer made of the first compound semiconductor and a second very thin layer made of a second compound semiconductor having a second energy band gap being smaller than the first energy band gap, thicknesses of the first very thin layers being gradually reduced from an interface of the first layer to an opposite interface and thicknesses of the second very thin layers are gradually increased from the interface of the first layer to the opposite interface ; a second layer (4) made of the second compound semiconductor in contact with the superlattice ; and a metal contact (5) in contact with the second layer.

Description: The invention relates to an ohmic contact on a compound semiconductor layer, and more particularly to a non-alloyed ohmic contact having a super-lattice structure for a compound semiconductor device. It is well known that parasitic resistances deteriorate the overall performances of semiconductor devices. One of the important causes of the parasitic resistances is the contact resistance such as a Schottky barrier which appears at the metal-semiconductor interface. The reduction of the potential barrier height is important to allow semiconductor devices exhibit such excellent performances as high speed and high frequency performances, particularly for submicron devices which utilize excellent properties of heterostructures such as HEMT (high electron mobility transistor) and HBT (heterojunction bipolar transistor). The scaling down of the device are being on the increase for an improvement in the performance and a high density of the integrated circuits. The importance and requirement of the reduction of the potential barrier or the contact resistance at the metal-semiconductor interface are on the increase, as both the scaling down of the device and thus the reduction of the device dimension and the improvements in the high speed and high frequency performances are increasingly required. Ohmic contacts are well known as an ideal contact free of the potential barrier and thus the metal-semiconductor contact resistance. Alloyed ohmic contacts have problems in the vertical and lateral diffusion of contact metal due to a high temperature processing. As the scaling down of the device is improved, such problems with the alloyed ohmic contacts are more considerable. That is why non-alloyed ohmic contacts tends to take an extremely important role. Some types of non-alloyed ohmic contacts on the compound semiconductor layer have been proposed. Typical non-alloyed ohmic contacts on the compound semiconductor layer such as GaAs layer will be described as the prior art. Low resistance non-alloyed ohmic contacts on an n-type GaAs layer are disclosed in Journal of Vacuum Science and Technology, 1981, Vol. 19. pp. 626-627, "Ohmic contacts to n-GaAs using graded band gap layers of Ga 1-x In x As gown by molecular beam epitaxy" J. M. Woodall et. al. The structure and property of the non-alloyed ohmic contacts will be described with reference to FIG. 1. FIG. 1 is a diagram illustrative of energy bands of the low resistance non-alloyed ohmic contacts on n-type GaAs layer. A layer 2 of GaAs doped with an n-type dopant is deposited on a substrate which is not illustrated. An epitaxial layer 6 of n-type Ga 1-x In x As is grown by molecular beam epitaxy on the n-type GaAs layer 2. The n-type Ga 1-x In x As layer 6 is graded in composition from x = 0 at the GaAs interface to 0.8 ≤ x ≤ 1 at opposite interface. A layer 4 of InAs doped with n-type dopant is formed on the n-type Ga 1-x In x As graded layer 6. A metal contact 5 is formed on the n-type InAs layer 4. From FIG. 1, it is understood that the conduction band profile of the contact across the interfaces of the above layers has no discontinuity nor potential barrier at each of the interfaces of the above layers. Such desirable conduction band profile being free of any discontinuity or potential barrier at the interfaces between the above layers is due to the existence of the n-type Ga 1-x In x As graded layer 6. This results in a nearly zero Schottky barrier height at the interfaces between the metal contact 5 and the n-type Ga 1-x In x As graded layer 4. Thus, such non-alloyed low resistance ohmic contact to the n-type GaAs layer is obtained. Further, another type of low resistance non-alloyed ohmic contacts on an n-type GaAs layer is disclosed in Applied Physics Letters, Vol. 53, pp-900, 1988, "Extremely low resistance non-allowed ohmic contacts on n-type GaAs using InAs/InGaAs and InAs/GaAs strained-layer superlattices", C. K. Peng, at. al. The structure and property of the non-alloyed ohmic contacts will be described with reference to FIG. 2. FIG. 2 is a diagram illustrative of energy bands of the low resistance non-alloyed ohmic contacts on n-type GaAs layer. A short-period n-type GaAs/InAs strained-layer superlattice 7 is grown by molecular beam epitaxy on a n-type GaAs layer 2. A layer 4 of n-type InAs is formed on the n-type GaAs/InAs strained-layer superlattice 7. A metal contact 5 is formed on the n-type InaAs layer 4. The strained-layer superlattice 7 comprises alternating n-type GaAs and n-type InAs layers. Each InAs layer forms a quantum well and each GaAs layer forms potential barrier. Electrons serving as carriers are confined in the periodical potential wells in the conduction band. In the superlattice, the thickness of the each GaAs layer and thus the width of the potential barrier is so thin that the resonant tunneling of electrons through the potential barrier occurs. Electron currents mainly due to the resonant tunneling flow across the interfaces of the superlattice structure. The electrons exhibiting the resonant tunneling across the superlattice structure experience nearly zero potential barrier thereby a low resistance ohmic contact is obtained. Another type of low resistance non-alloyed ohmic contacts on a p-type GaAs layer is disclosed in Electronics Letters 16th August 1990 Vol. 26, No. 17, "Very low resistivity ohmic contact to p-type GaAs using In x Ga 1-x As interlayer", P. L. Janega at al. The structure and property of the non-alloyed ohmic contacts will be described with reference to FIG. 3. FIG. 3 is a diagram illustrative of energy bands of the low resistance non-alloyed ohmic contacts on p-type GaAs layer. A layer 2' of GaAs doped with a p-type dopant is deposited on a substrate which is not illustrated. An epitaxial layer 6' of p-type Ga 1-x In x As is grown by molecular beam epitaxy on the p-type GaAs layer 2'. The p-type Ga 1-x In x As layer 6' is graded in composition from x = 0.07 at the GaAs interface to 0.5 at opposite interface. A layer 4' of InAs doped with p-type dopant is formed on the p-type Ga 1-x In x As graded layer 6'. The doping level of the p-type InAs layer is 1 X 10 19 atom/cm -3 . A metal contact 5 is formed on the n-type InAs layer 4. From FIG. 3, it is understood that the valence band profile across the interfaces of the above layers has no discontinuity nor potential barrier at each of the interfaces of the above layers. Holes serving as carriers across the interfaces of the layers experience a nearly zero potential barrier. Such desirable conduction band profile being free of any discontinuity or potential barrier at the interfaces between the above layers is due to the existence of the p-type Ga 1-x In x As graded layer 6'. This results in a 0.4 eV to 0.5 eV of the Schttoky barrier height at the interfaces between the metal contact 5 and the p-type InAs layer 4'. Thus, such non-alloyed low resistance ohmic contact to the p-type GaAs layer is obtained. Further, another type of low resistance non-alloyed ohmic contacts on a p-type GaAs layer has been known. The structure and property of the non-alloyed ohmic contacts will be described with reference to FIG. 4. FIG. 4 is a diagram illustrative of energy bands of the low resistance non-alloyed ohmic contacts on p-type GaAs layer. A short-period p-type GaAs/InAs strained-layer superlattice 7' is grown by molecular beam epitaxy on a p-type GaAs layer 2'. A layer 4' of p-type InAs is formed on the p-type GaAs/InAs strained-layer superlattice 7'. A metal contact 5 is formed on the p-type InAs layer 4'. The strained-layer superlattice 7' comprises alternating p-type GaAs and p-type InAs layers. Each InAs layer forms a quantum well and each GaAs layer forms potential barrier. Holes serving as carriers are confined in the periodical potential wells in the valence band. In the superlattice structure 7', the thickness of the each GaAs layer and thus the width of the potential barrier is so thin that the resonant tunneling of holes through the potential barrier occurs. Hole currents mainly due to the resonant tunneling flow across the interfaces of the superlattice structure. The holes exhibiting the resonant tunneling across the superlattice structure experience nearly zero potential barrier thereby a low resistance ohmic contact is obtained. Another type of low resistance non-alloyed ohmic contacts on a p-type GaAs layer is disclosed in IEEE Electron Devices Letters 1987 Vol. 8, No. 30, "An(Al,Ga)As/GaAs Heterostructure Bipolar Transistor with Nonalloyed Graded-Gap Ohmic Contacts to the Base and Emitter", M. A. Rao at al. The structure and property of the non-alloyed ohmic contacts will be described with reference to FIG. 5. FIG. 5 is a diagram illustrative of energy bands of the low resistance non-alloyed ohmic contacts on p-type GaAs layer. A layer 2' of GaAs doped with a p-type dopant is deposited on a substrate which is not illustrated. An epitaxial layer 6" of p-type GaAs 1-x Sb x is grown by molecular beam epitaxy on the p-type GaAs layer 2'. The p-type GaAs 1-x Sb x layer 6" is graded in composition from the GaAs interface to opposite interface. A layer 4" of GaSb doped with p-type dopant is formed on the p-type GaAs 1-x Sb x graded layer 6". The doping level of the GaSb layer 4" is 1 X 10 19 cm -3 . A metal contact 5 is formed on the n-type GaSb layer 4". From FIG. 5, it is understood that the valence band profile across the interfaces of the above layers has no discontinuity nor potential barrier at each of the interfaces of the above layers. Holes serving as carriers across the interfaces of the layers experience a nearly zero potential barrier. Such desirable conduction band profile being free of any discontinuity or potential barrier at the interfaces between the above layers is due to the existence of the p-type GaAs 1-x Sb x graded layer 6". This results in a 0.1 eV of the Schttoky barrier height at the interfaces between the metal contact 5 and the p-type GaSb layer 4". Thus, such non-alloyed low resistance ohmic contact to the p-type GaAs layer is obtained. Further, another type of low resistance non-alloyed ohmic contacts on a p-type GaAs layer has been known. The structure and property of the non-alloyed ohmic contacts will be described with reference to FIG. 6. FIG. 6 is a diagram illustrative of energy bands of the low resistance non-alloyed ohmic contacts on p-type GaAs layer. A short-period p-type GaAs/GaSb strained-layer superlattice 7" is grown by molecular beam epitaxy on a p-type GaAs layer 2'. A layer 4" of p-type GaSb is formed on the p-type GaAs/GaSb strained-layer superlattice 7". A metal contact 5 is formed on the p-type GaSb layer 4". The strained-layer superlattice 7" comprises alternating p-type GaAs and p-type GaSb layers. Each GaSb layer forms a quantum well and each GaAs layer forms potential barrier. Holes serving as carriers are confined in the periodical potential wells in the valence band. In the superlattice structure 7", the thickness of the each GaAs layer and thus the width of the potential barrier is so thin that the resonant tunneling of holes through the potential barrier occurs. Hole currents mainly due to the resonant tunneling flow across the interfaces of the superlattice structure. The hole exhibiting the resonant tunneling across the superlattice structure experience nearly zero potential barrier thereby a low resistance ohmic contact is obtained. The above ohmic contact using either the InGaAs or GaAsSb graded layer of p-type or n-type are, however, engaged with the following disadvantages which will be investigated. As described the above, the composition-graded layer providing the low contact resistance and thus a nearly zero Schottky barrier is grown by molecular beam epitaxy. The growing process of the composition-graded layer requires an extremely precise and continuous control of the flux intensity of a molecular beam for growing the composition-graded layer. That is why it is difficult to grow the available composition-graded InGaAs or GaAsSb layer by the molecular beam epitaxy. Particularly, it is extremely difficult to control the flux intensity of a molecular beam of Group-V elements such as As and Sb. The inferiority of the control of the flux intensity of a molecular beam results in an abrupt discontinuity in compositions of the composition-graded layer. Such abrupt discontinuity in the composition causes both high density stacking faults and high density misfit dislocations. This results in an enlargement of the contact resistance. If the over-shoot of the In flux appears in the latter half of the growing process of the composition-graded layer, the flux ratio of Group-V elements to Group-III elements are varied from the design value. This results in a roughness of the surface of the composition-graded layer, in addition the inferiority of the crystal quantity. This causes the enlargement of the contact resistance, for which reason it is no longer possible to obtain any excellent ohmic contact. Although the composition-graded layer is theoretically able to provide an ohmic contact between the metal and compound semiconductor layer, it is actually difficult to provide a desirable low resistive ohmic contact in the high yield and the high reliability due to the difficulty in the control of the flux intensity of composition for the growing of the composition-graded layer by the molecular beam epitaxy. If the organic metal chemical vapor deposition is used for growing the composition-graded layer in replacement of the molecular beam epitaxy, such problems as the above in the appearances of the staking faults and the misfit dislocations which causes the enlargement in the contact resistance. Therefore, similarly to the molecular beam epitaxy, the organic metal chemical vapor deposition is also not available to grow a high quality composition-graded layer for providing a desirable low resistance ohmic contact. Consequently, the composition-graded layer has the above mentioned problem in the inferiorities of the low contact resistance, the reliability and the yield of the ohmic contact. The above ohmic contact using either the InAs/GaAs or GaAs/GaSb strained-layer superlattice of p-type or n-type are also engaged with the following disadvantages which will be investigated with reference to FIG. 7. As illustrated in FIG. 7, the GaAs/InAs strained-layer superlattice 7 is formed on the GaAs buffer layer 2 overlying a semiconductor substrate 1. The InAs layer 4 overlays the GaAs/InAs strained-layer superlattice 7. The metal contact 5 is formed on the InAs layer 4. As described the above, the GaAs/InAs strained-layer superlattice 7 is grown by the molecular beam epitaxy. In this case, the growing process of the superlattice 7 by the molecular beam epitaxy is free from such difficulty as those in the precise and continuous control of the flux intensity of molecular beams. Namely, the growing process for the superlattice 7 has a facility as compared to the growing process for the composition-graded layer. According to the Heisenberg's uncertainty principle, it is impossible to specify a precise position of electrons or holes. The electron's position may thus be represented as a wave function of carriers and thus electrons and holes. The superlatice 7 has such a short period of the potential well and the potential barrier as to allow the resonant tunneling between adjacent potential wells. Actually, such short period of the superlattice 7 is generally smaller than a wave length of electrons. Such short period of the superlattice 7 also has respective width constancies of the potential barriers and potential wells. Although the carriers and thus electrons or holes experience a nearly zero potential barrier but only within the superlattice structure, the carriers and thus electrons or holes experience a relatively large potential barrier both at the interface between the superlattice structure 7 and the GaAs buffer layer 2 and at the interface between the superlattice structure and the InAs layer. Physically, the carriers and thus electrons or holes experience such a conduction band profile across the interfaces as that illustrated in FIG. 8 rather than the conduction band profile illustrated in FIG. 2. In this case, the effective energy band profile experienced by electrons is near an energy band profile as illustrated in FIG. 8 across the n-type GaAs layer 2 underlying an n-type In 0.5 Ga 0.5 As layer 10 underlying the n-type InAs layer 4 underlying the metal contact 5. The constancy of such short periodical superlattice provides the discontinuity in the conduction band at the superlattice structure. This results in an enlargement of the contact resistance. Further, the ohmic contact using the superlattice structure has another problems both in the misfit dislocation and in the staking fault due to the lattice mismatch. A lattice constant of InAs is larger by 7% than a lattice constant of GaAs. Such discontinuity of the lattice constants of the InAs/GaAs layers in the superlattice causes the lattice mismatch providing the misfit dislocation and the staking fault in the superlattice structure. The misfit dislocation and the staking fault form space charge regions around thereof. The space charge region due to a high density of the misfit dislocation and the staking fault in the superlattice structure results in a large internal resistance. Similarly, such misfit dislocation and the staking fault due to the lattice mismatch also appears both at the interface between the GaAs/InAs superlattice structure and the GaAs layer and at the interface between the GaAs/InAs superlattice structure and the InAs layer. Such misfit dislocation and the staking fault form space charge regions around thereof. The space charge region due to a high density of the misfit dislocation and the staking fault in the superlattice structure results in a large internal resistance. Of course, the p-type GaAs/InAs superlattice also has the above mentioned problems. Further, a difference in the lattice constants between GaAs and GaSb is 8%. Such discontinuity of the lattice constants of the InAs/GaAs layers in the superlattice causes the above problems. EP-A-0042066 discloses an ohmic contact device corresponding to the preamble of claim 1. US-A-4620206 describes a superlattice based device having a good negative resistance characteristic. It is therefore required to provide a novel ohmic contact structure on a compound semiconductor layer for a semiconductor device. Accordingly, it is a primary object of the present invention to provide a novel ohmic contact structure on a compound semiconductor layer. It is a further object of the present invention to provide a novel ohmic contact on a compound semiconductor layer, which has an extremely low contact resistance. It is a still further object of the present invention to provide a novel ohmic contact on a compound semiconductor layer, which is completely free from any potential barrier. It is a further more object of the present invention to provide a novel ohmic contact on a compound semiconductor layer, which is free from any misfit dislocation and staking fault due to the lattice mismatch. The above and other objects, features and advantages of the present invention will be apparent from the following descriptions. The present invention provides a novel ohmic contact device as outlined in the claims. Preferred embodiments of the present invention will hereinafter fully be described in detail with reference to the accompanying drawings. FIG. 1 is a diagram illustrative of energy band profiles of the conventional ohmic contact using the n-type composition-graded layer on the n-type GaAs layer. FIG. 2 is a diagram illustrative of energy band profiles of the conventional ohmic contact using the n-type strained-layer superlattice on the n-type GaAs layer. FIG. 3 is a diagram illustrative of energy band profiles of the conventional ohmic contact using the p-type composition-graded layer on the p-type GaAs layer. FIG. 4 is a diagram illustrative of energy band profiles of the conventional ohmic contact using the p-type strained-layer superlattice on the p-type GaAs layer. FIG. 5 is a diagram illustrative of energy band profiles of the conventional ohmic contact using the p-type composition-graded layer on the p-type GaAs layer. FIG. 6 is a diagram illustrative of energy band profiles of the conventional ohmic contact using the p-type strained-layer superlattice on the p-type GaAs layer. FIG. 7 is a cross sectional elevation view illustrative of the problems with the conventional ohmic contact using the n-type strained-layer superlattice on the n-type GaAs layer. FIG. 8 is a diagram illustrative of energy band profiles of the conventional ohmic contact using the n-type composition non-graded layer on the n-type GaAs layer. FIG. 9 is a cross sectional elevation view illustrative of a novel ohmic contact using n-GaAs/n-InAs modulation-period superlattice of a first embodiment according to the present invention. FIG. 10 is a diagram illustrative of energy band profiles of a novel ohmic contact using n-GaAs/n-InAs modulation-period superlattice of a first embodiment according to the present invention. FIG. 11 is a diagram illustrative of energy band profiles of a novel ohmic contact using n-GaAs/n-InAs modulation-period superlattice of a second embodiment according to the present invention. FIG. 12 is a diagram illustrative of energy band profiles of a novel ohmic contact using n-GaAs/n-InAs modulation-period superlattice of a third embodiment according to the present invention. The present invention provides a novel non-alloyed ohmic contact on a compound semiconductor layer. The novel non-alloyed ohmic contact includes a modulation-period superlattice structure which provides a nearly zero contact resistance. A first embodiment of the present invention will be described with reference to FIGS. 9 and 10. A structure of a novel ohmic contact on a compound semiconductor layer of the first embodiment will be described. A novel ohmic contact is formed on a GaAs compound semiconductor substrate 1. A layer 2 of GaAs doped with an n-type dopant is formed on the GaAs substrate 1. A supperlattice structure 3 which comprises modulation-periods of alternating n-type GaAs/InAs layers is grown on the n-type GaAs layer 3 by molecular beam epitaxy. A layer 4 of InAs doped with an n-type dopant is formed on the n-type GaAs/InAs modulation-period superlattice structure 3. A metal contact 5 is formed on the n-type InAs layer 4. As described the above, the n-type GaAs/InAs modulation-period superlattice structure 3 comprises the modulation-period of alternating the n-type GaAs layer and the n-type InAs layer. In the modulation-period superlattice 3, widths of the plural GaAs layers are so modulated as to be gradually reduced from 4 nanometers at an interface to the n-type GaAs layer 2 to 0.3 nanometers at an interface to the n-type InAs layer 4. In contrast, widths of the plural InAs layers are so modulated as to be gradually enlarged from 0.3 nanometers at the interface to the n-type GaAs layer 2 to 4 nanometers at the interface to the n-type InAs layer 4. Namely, the average composition of GaAs is gradually reduced from the n-type GaAs layer 2 to the n-type InAs layer 4. The average composition of InAs is gradually enlarged from the n-type GaAs layer 2 to the n-type InAs layer 4. Such modulation-period superlattice 3 has a total thickness of 50 nanometers. The n-type InAs layer 4 has a thickness of 50 nanometers. With respect to the doping level of the each layers, the n-type GaAs layer 2 has a dopant concentration of 2 X 10 18 cm -3 . The n-type InAs layer 4 has a high dopant concentration of 2 X 10 19 cm -3 . The n-type GaAs/InAs modulation-period superlattice structure 3 is subjected to such a modulation-doping as to have the dopant concentration exhibit a gradual increase from the dopant concentration of 2 X 10 18 cm -3 at the interface with the n-type GaAs layer 2 to the high dopant concentration of 2 X 10 19 cm -3 at the interface with the n-type InAs layer 4. The metal contact 5 comprises a layer of Ti of a thickness of 50 nanometers, a layer of Pt of a thickness of 50 nanometers overlaying the Ti layer and a layer of Au of a thickness of 300 nanometers overlaying the Pt layer. The above Ti, Pt and Au layers are sequentially formed by a lift-off process using vacuum evaporation. Properties of the ohmic contact including the n-type modulation-period superlattice to the n-type compound semiconductor layer of the first embodiment according to the present invention will be investigated with reference to FIG. 10 which is illustrative of an energy band profile across the interfaces of the each layer. With respect to the n-type GaAs layer 2, the conduction band edge exists below the Fermi level. Thus, the energy band gap also exists below the Fermi level. With respect to the n-type InAs layer 4, the conduction band edge exists below the Fermi level. Thus, the energy band gap also exists below the Fermi level. With respect to the n-type modulation-period superlattice 3, the n-type GaAs layers provide potential barriers. The n-type InAs layers provide potential wells. Each of the GaAs/InAs layers has such a very thin that electrons serving as carriers are able to exhibit resonant tunneling from a potential well to an adjacent potential well through a potential barrier. The widths of the potential wells are gradually increased from 0.3 nanometers at the interface with the n-type GaAs layer 2 to 4 nanometers at the interface with the n-type InAs layer 4. Contrary to the potential well widths, the widths of the potential barriers are gradually reduced from 4 nanometers at the interface with the n-type GaAs layer 2 to 0.3 nanometers at the interface with the n-type InAs layer 4. In each of the potential wells, a plurality of sub-bands or mini-bands separated by the mini-band gaps exist. Electrons are allowed to take only specified energy levels defined by plural sub-bands separated by the band gaps in the potential well. The sub-band energy levels are associated with the width of the potential well. A small width of the potential well does not allow the sub-band energy level at a deep point in the potential well. In contrast, a large width of the potential well allows the sub-band energy level at a deep point in the potential well. The widths of the sub-bands in the potential well are defined by a width of the potential barrier. The reduction of the width of the potential barrier makes the widths of the sub-bands enlarged. In the vicinity of the interface to the n-type GaAs layer 2, the sub-band energy levels available for electrons are limited. This does not allow many electrons to exist in the potential well, particularly at a deep point in the potential well. In contrast, in the vicinity of the interface to the n-type InAs layer 4, the sub-band energy levels available for electrons are not limited relatively. This allows many electrons to exist in the potential well. In the vicinity of the interface to the n-type GaAs layer 2, available energy levels for electrons are near to those in the n-type GaAs layer 2. In the vicinity of the interface to the n-type InAs layer 4, available energy levels for electrons are near to those in the n-type InAs layer 4. Electrons effectively experience such a smooth potential profile as that provided by a composition-graded layer. The effective potential profile experienced by electrons is completely free from any barrier due to the conduction band discontinuity. Accordingly, electrons will experience no potential barrier not only in the modulation-period superlattice 3 but also at the interface to either the n-type GaAs layer 2 or the n-type InAs layer 4. Such modulation-period superlattice 3 provides an extremely low resistance ohmic contact to the n-type GaAs compound semiconductor layer 2. On the other hand, as described above, the modulation-period superlattice 3 is grown by molecular beam epitaxy. The growing process for the modulation-period superlattice 3 is free from the necessity in the continuous and precise control of the flux intensity of molecular beams of In and Ga. This results in a facility in the growing process for the modulation-period superlattice 3. In the modulation-period superlattice 3, as described the above, the widths of the plural GaAs layers are so modulated as to be gradually reduced from the interface with the n-type GaAs layer 2 to the interface with the n-type InAs layer 4. The widths of the plural InAs layers are so modulated as to be gradually enlarged from the interface to the n-type GaAs layer 2 to the interface to the n-type InAs layer 4. Namely, the average composition of GaAs is gradually reduced from the n-type GaAs layer 2 to the n-type InAs layer 4. The average composition of InAs is gradually enlarged from the n-type GaAs layer 2 to the n-type InAs layer 4. The lattice constant is gradually increased from the n-type GaAs layer 2 to the n-type InAs layer 4. There exists no discontinuity in the lattice constant not only in the modulation-period superlattice 3 but also at the interfaces to the n-type GaAs layer 2 and the n-type InAs layer 4. There exists no misfit dislocation nor staking fault in such modulation-period superlattice 3. Accordingly, such ohmic contact including the modulation-period superlattice 3 is free from the rise of the contact resistance due to appearance of the misfit dislocation or staking fault. Such modulation-period superlattice 3 provides an extremely low resistance ohmic contact to the n-type GaAs compound semiconductor layer 2. The value of the contact resistance of the ohmic contact employing the modulation-period superlattice will be investigated as compared to the contact resistances of the conventional ohmic contact. In the prior arts, the conventional ohmic contact using the n-type InGaAs composition-graded layer has a contact resistance of 8.6 X 10 -8 ohms/cm 3 . Another types of the conventional ohmic contact using the n-type 2nm/2nm of GaAs/InAs composition-graded layer has a contact resistance of 1.5 X 10 -6 ohms/cm 3 . In contrast, indeed, the novel ohmic contact employing the p-type GaAs/InAs modulation-period superlattice 3 on the n-type GaAs layer provided by the first embodiment has an extremely low contact resistance of 5.8 X 10 -8 ohms/cm 3 . Alternatively, the n-type InAs layer 4 is replaceable with an n-type InGaAs layer under the condition of a large composition ratio of InAs to Ga. A second embodiment of the present invention will be described with reference to FIG. 11. A structure of a novel ohmic contact on a compound semiconductor layer of the second embodiment will be described. A novel ohmic contact is formed on a GaAs compound semiconductor substrate. A layer 2' of GaAs doped with a p-type dopant is formed on the GaAs substrate. A supperlattice structure 3' which comprises modulation-periods of alternating p-type GaAs/InAs layers is grown on the p-type GaAs layer 3' by molecular beam epitaxy. A layer 4' of InAs doped with an p-type dopant is formed on the p-type GaAs/InAs modulation-period superlattice structure 3'. A metal contact 5 is formed on the p-type InAs layer 4'. As described the above, the p-type GaAs/InAs modulation-period superlattice structure 3' comprises the modulation-periods of alternating the p-type GaAs layer and the p-type InAs layer. In the modulation-period superlattice 3', widths of the plural GaAs layers are so modulated as to be gradually reduced from 4 nanometers at an interface to the p-type GaAs layer 2' to 0.3 nanometers at an interface to the p-type InAs layer 4'. In contrast, widths of the plural InAs layers are so modulated as to be gradually enlarged from 0.3 nanometers at the interface to the p-type GaAs layer 2' to 4 nanometers at the interface to the p-type InAs layer 4'. Namely, the average composition of GaAs is gradually reduced from the p-type GaAs layer 2' to the p-type InAs layer 4'. The average composition of InAs is gradually enlarged from the p-type GaAs layer 2' to the p-type InAs layer 4'. Such modulation-period superlattice 3' has a total thickness of 50 nanometers. The p-type InAs layer 4' has a thickness of 50 nanometers. With respect to the doping level of the each layers, the p-type GaAs layer 2' has a dopant concentration of 2 X 10 18 cm -3 . The p-type InAs layer 4' has a high dopant concentration of 2 X 10 19 cm -3 . The p-type GaAs/InAs modulation-period superlattice structure 3' is subjected to such a modulation-doping as to have the dopant concentration exhibit a gradual increase from the dopant concentration of 2 X 10 18 cm -3 at the interface with the p-type GaAs layer 2' to the high dopant concentration of 2 X 10 19 cm -3 at the interface with the p-type InAs layer 4'. The metal contact 5 comprises a layer of Ti of a thickness of 50 nanometers, a layer of Pt of a thickness of 50 nanometers overlaying the Ti layer and a layer of Au of a thickness of 300 nanometers overlaying the Pt layer. The above Ti, Pt and Au layers are sequentially formed by a lift-off process using vacuum evaporation. Properties of the ohmic contact including the p-type modulation-period superlattice to the p-type compound semiconductor layer of the second embodiment according to the present invention will be investigated with reference to FIG. 11 which is illustrative of an energy band profile across the interfaces of the each layer. With respect to the p-type GaAs layer 2', the conduction band edge exists above the Fermi level. Thus, the valance band edge exists below but in the vicinity of the Fermi level. The Fermi level exists within the energy band gap but near the valence band edge. With respect to the p-type InAs layer 4', the valance band edge exists above the Fermi level. Thus, the energy band gap also exists above the Fermi level. With respect to the p-type modulation-period superlattice 3', the p-type GaAs layers provide potential barriers. The p-type InAs layers provide potential wells. Each of the GaAs/InAs layers has such a very thin that holes serving as carriers are able to exhibit resonant tunneling from a potential well to an adjacent potential well through a potential barrier. The widths of the potential wells are gradually increased from 0.3 nanometers at the interface with the p-type GaAs layer 2' to 4 nanometers at the interface with the p-type InAs layer 4'. Contrary to the potential well widths, the widths of the potential barriers are gradually reduced from 4 nanometers at the interface with the p-type GaAs layer 2' to 0.3 nanometers at the interface with the p-type InAs layer 4'. In each of the potential wells, a plurality of sub-bands or mini-bands separated by the mini-band gaps exist. Holes are allowed to take only specified energy levels defined by plural sub-bands separated by the band gaps in the potential well. The sub-band energy levels are associated with the width of the potential well. A small width of the potential well does not allow the sub-band energy level at a deep point in the potential well. In contrast, a large width of the potential well allows the sub-band energy level at a deep point in the potential well. The widths of the sub-bands in the potential well are defined by a width of the potential barrier. The reduction of the width of the potential barrier makes the widths of the sub-bands enlarged. In the vicinity of the interface to the p-type GaAs layer 2', the sub-band energy levels available for holes are limited. This does not allow many holes to exist in the potential well, particularly at a deep point in the potential well. In contrast, in the vicinity of the interface to the p-type InAs layer 4', the sub-band energy levels available for holes are not limited relatively. This allows many holes to exist in the potential well. In the vicinity of the interface to the p-type GaAs layer 2', available energy levels for holes are near to those in the p-type GaAs layer 2'. In the vicinity of the interface to the p-type InAs layer 4', available energy levels for holes are near to those in the p-type InAs layer 4'. Holes effectively experience such a smooth potential profile as that provided by a composition-graded layer. The effective potential profile experienced by holes is completely free from any barrier due to the valence band discontinuity. Accordingly, holes will experience no potential barrier not only in the modulation-period superlattice 3' but also at the interface to either the p-type GaAs layer 2' or the p-type InAs layer 4'. Such modulation-period superlattice 3' provides an extremely low resistance ohmic contact to the p-type GaAs compound semiconductor layer 2'. On the other hand, as described above, the modulation-period superlattice 3' is grown by molecular beam epitaxy. The growing process for the modulation-period superlattice 3' is free from the necessity in the continuous and precise control of the flux intensity of molecular beams of In and Ga. This results in a facility in the growing process for the modulation-period superlattice 3'. In the modulation-period superlattice 3', as described the above, the widths of the plural GaAs layers are so modulated as to be gradually reduced from the interface with the p-type GaAs layer 2' to the interface with the p-type InAs layer 4'. The widths of the plural InAs layers are so modulated as to be gradually enlarged from the interface to the p-type GaAs layer 2' to the interface to the p-type InAs layer 4'. Namely, the average composition of GaAs is gradually reduced from the p-type GaAs layer 2' to the p-type InAs layer 4'. The average composition of InAs is gradually enlarged from the p-type GaAs layer 2' to the p-type InAs layer 4'. The lattice constant is gradually increased from the p-type GaAs layer 2' to the p-type InAs layer 4'. There exists no discontinuity in the lattice constant not only in the modulation-period superlattice 3' but also at the interfaces to the p-type GaAs layer 2' and the p-type InAs layer 4'. There exists no misfit dislocation nor staking fault in such modulation-period superlattice 3'. Accordingly, such ohmic contact including the modulation-period superlattice 3' is free from the rise of the contact resistance due to appearance of the misfit dislocation or staking fault. Such modulation-period superlattice 3' provides an extremely low resistance ohmic contact to the p-type GaAs compound semiconductor layer 2'. The value of the contact resistance of the ohmic contact employing the modulation-period superlattice will be investigated. Indeed, the novel ohmic contact employing the p-type GaAs/InAs modulation-period superlattice 3' on the p-type GaAs layer provided by the second embodiment has an extremely low contact resistance of 1 X 10 -6 ohms/cm 3 . A third embodiment of the present invention will be described with reference to FIG. 12. A structure of a novel ohmic contact on a compound semiconductor layer of the third embodiment will be described. A novel ohmic contact is formed on a GaAs compound semiconductor substrate. A layer 2' of GaAs doped with a p-type dopant is formed on the GaAs substrate. A supperlattice structure 3" which comprises modulation-periods of alternating p-type GaAs/GaSb layers is grown on the p-type GaAs layer 3" by molecular beam epitaxy. A layer 4" of GaSb doped with an p-type dopant is formed on the p-type GaAs/GaSb modulation-period superlattice structure 3". A metal contact 5 is formed on the p-type GaSb layer 4". As described the above, the p-type GaAs/GaSb modulation-period superlattice structure 3" comprises the modulation-periods of alternating the p-type GaAs layer and the p-type GaSb layer. In the modulation-period superlattice 3", widths of the plural GaAs layers are so modulated as to be gradually reduced from 4 nanometers at an interface to the p-type GaAs layer 2' to 0.3 nanometers at an interface to the p-type GaSb layer 4". In contrast, widths of the plural GaSb layers are so modulated as to be gradually enlarged from 0.3 nanometers at the interface to the p-type GaAs layer 2' to 4 nanometers at the interface to the p-type GaSb layer 4". Namely, the average composition of GaAs is gradually reduced from the p-type GaAs layer 2' to the p-type GaSb layer 4". The average composition of GaSb is gradually enlarged from the p-type GaAs layer 2' to the p-type GaSb layer 4". Such modulation-period superlattice 3" has a total thickness of 50 nanometers. The p-type GaSb layer 4" has a thickness of 50 nanometers. With respect to the doping level of the each layers, the p-type GaAs layer 2' has a dopant concentration of 2 X 10 18 cm -3 . The p-type GaSb layer 4" has a high dopant concentration of 2 X 10 19 cm -3 . The p-type GaAs/GaSb modulation-period superlattice structure 3" is subjected to such a modulation-doping as to have the dopant concentration exhibit a gradual increase from the dopant concentration of 2 x 10 18 cm -3 at the interface with the p-type GaAs layer 2' to the high dopant concentration of 2 X 10 19 cm -3 at the interface with the p-type GaSb layer 4". The metal contact 5 comprises a layer of Ti of a thickness of 50 nanometers, a layer of Pt of a thickness of 50 nanometers overlaying the Ti layer and a layer of Au of a thickness of 300 nanometers overlaying the Pt layer. The above Ti, Pt and Au layers are sequentially formed by a lift-off process using vacuum evaporation. Properties of the ohmic contact including the p-type modulation-period superlattice to the p-type compound semiconductor layer of the third embodiment according to the present invention will be investigated with reference to FIG. 12 which is illustrative of an energy band profile across the interfaces of the each layer. With respect to the p-type GaAs layer 2', the conduction band edge exists above the Fermi level. Thus, the valance band edge exists below but in the vicinity of the Fermi level. The Fermi level exists within the energy band gap but near the valence band edge. With respect to the p-type GaSb layer 4", the valance band edge exists above the Fermi level. Thus, the energy band gap also exists above the Fermi level. With respect to the p-type modulation-period superlattice 3", the p-type GaAs layers provide potential barriers. The p-type GaSb layers provide potential wells. Each of the GaAs/GaSb layers has such a very thin that holes serving as carriers are able to exhibit resonant tunneling from a potential well to an adjacent potential well through a potential barrier. The widths of the potential wells are gradually increased from 0.3 nanometers at the interface with the p-type GaAs layer 2' to 4 nanometers at the interface with the p-type GaSb layer 4". Contrary to the potential well widths, the widths of the potential barriers are gradually reduced from 4 nanometers at the interface with the p-type GaAs layer 2' to 0.3 nanometers at the interface with the p-type GaSb layer 4". In each of the potential wells, a plurality of sub-bands or mini-bands separated by the mini-band gaps exist. Holes are allowed to take only specified energy levels defined by plural sub-bands separated by the band gaps in the potential well. The sub-band energy levels are associated with the width of the potential well. A small width of the potential well does not allow the sub-band energy level at a deep point in the potential well. In contrast, a large width of the potential well allows the sub-band energy level at a deep point in the potential well. The widths of the sub-bands in the potential well are defined by a width of the potential barrier. The reduction of the width of the potential barrier makes the widths of the sub-bands enlarged. In the vicinity of the interface to the p-type GaAs layer 2', the sub-band energy levels available for holes are limited. This does not allow many holes to exist in the potential well, particularly at a deep point in the potential well. In contrast, in the vicinity of the interface to the p-type GaSb layer 4", the sub-band energy levels available for holes are not limited relatively. This allows many holes to exist in the potential well. In the vicinity of the interface to the p-type GaAs layer 2', available energy levels for holes are near to those in the p-type GaAs layer 2'. In the vicinity of the interface to the p-type GaSb layer 4", available energy levels for holes are near to those in the p-type GaSb layer 4". Holes effectively experience such a smooth potential profile as that provided by a composition-graded layer. The effective potential profile experienced by holes is completely free from any barrier due to the valence band discontinuity. Accordingly, holes will experience no potential barrier not only in the modulation-period superlattice 3" but also at the interface to either the p-type GaAs layer 2' or the p-type GaSb layer 4". Such modulation-period superlattice 3" provides an extremely low resistance ohmic contact to the p-type GaAs compound semiconductor layer 2'. On the other hand, as described above, the modulation-period superlattice 3" is grown by molecular beam epitaxy. The growing process for the modulation-period superlattice 3" is free from the necessity in the continuous and precise control of the flux intensity of molecular beams of As and Sb. This results in a facility in the growing process for the modulation-period superlattice 3". In the modulation-period superlattice 3", as described the above, the widths of the plural GaAs layers are so modulated as to be gradually reduced from the interface with the p-type GaAs layer 2' to the interface with the p-type GaSb layer 4". The widths of the plural GaSb layers are so modulated as to be gradually enlarged from the interface to the p-type GaAs layer 2' to the interface to the p-type GaSb layer 4". Namely, the average composition of GaAs is gradually reduced from the p-type GaAs layer 2' to the p-type GaSb layer 4". The average composition of GaSb is gradually enlarged from the p-type GaAs layer 2' to the p-type GaSb layer 4". The lattice constant is gradually increased from the p-type GaAs layer 2' to the p-type GaSb layer 4". There exists no discontinuity in the lattice constant not only in the modulation-period superlattice 3" but also at the interfaces to the p-type GaAs layer 2' and the p-type GaSb layer 4". There exists no misfit dislocation nor staking fault in such modulation-period superlattice 3". Accordingly, such ohmic contact including the modulation-period superlattice 3" is free from the rise of the contact resistance due to appearance of the misfit dislocation or staking fault. Such modulation-period superlattice 3" provides an extremely low resistance ohmic contact to the p-type GaAs compound semiconductor layer 2'. The value of the contact resistance of the ohmic contact employing the modulation-period superlattice will be investigated. Indeed, the novel ohmic contact employing the p-type GaAs/GaSb modulation-period superlattice 3" on the p-type GaAs layer provided by the second embodiment has an extremely low contact resistance of 1 X 10 -7 ohms/cm 3 . As a modification of the metal contact, although in the above embodiments, the metal contact 5 comprises the Ti/Pt/Au layers, other metals such as tungsten silicide are also available as a material for the metal contact. The thickness and doping level of each of the layers may be so modified as to match variable conditions. Such desirable ohmic contacts provided by the above embodiments are of course applicable to contacts of electrical and optical devices, particularly high speed and high frequency heterostructure transistors such as the high electron mobility transistor (HEMT) and the heterojunction bipolar transistor (HBT). Such extremely low resistance ohmic contact provided by the embodiments allows the electrical and optical devices to exhibit excellent performances such as high speed and high frequency performances. Whereas modifications of the present invention will no doubt be apparent to a person having ordinary skill in the art, to which the invention pertains, it is to be understood that the embodiments shown and described by way of illustrations are by no means intended to be considered in a limiting sense. Accordingly, it is to be intended to cover by claims all modifications.

Claims: An ohmic contact device comprising: a first layer (2) made of a first compound semiconductor having a first energy band gap; a second layer (4) made of a second compound semiconductor; and a metal contact (5) in contact with said second layer, characterized by a superlattice (3) in contact and between said first and second layers, said superlattice having modulation-periods comprising alternating first very thin layers made of said first compound semiconductor and second very thin layers made of said second compound semiconductor having a second energy band gap being smaller than said first energy band gap, thicknesses of said first very thin layers being gradually reduced from 4 nm at an interface of said first layer to 0,3 nm at the opposite interface and thicknesses of said second very thin layers being gradually increased from 0,3 nm at said interface of said first layer to 4 nm at said opposite interface wherein said superlattice has a gradually graded dopant concentration profile from 2 x 10 18 cm -3 at said interface to said first layer to 2 x 10 19 cm -3 at said interface to said second layer. The ohmic contact device as claimed in claim 1, wherein the total thickness of said superlattice is 50 nanometers. The ohmic contact device as claimed in claim 1, wherein the thickness of said second layer is 50 nanometers. The ohmic contact device as claimed in claim 1, wherein said first compound semiconductor comprises GaAs doped with an n-type dopant; and said second compound semiconductor comprises InAs doped with an n-type dopant. The ohmic contact device as claimed in claim 1, wherein said first layer has a dopant concentration of 2 x 10 18 cm -3 ; and said second layer has a dopant concentration of 2 x 10 19 cm -3 . The ohmic contact device as claimed in claim 1 or 5, wherein said first compound semiconcductor comprises GaAs doped with a p-type dopant; and said second compound semiconductor comprises InAs doped with a p-type dopant. The ohmic contact device as claimed in claim 1 or 5, wherein said first compound semiconductor comprises GaAs doped with a p-type dopant; and said second compound semiconductor comprises GaSb doped with a p-type dopant. The ohmic contact device as claimed in claim 1, wherein said metal contact (5) comprises a first layer of Ti having a thickness of 50 nanometers, a second layer of Pt having a thickness of 50 nanometers overlying said first Ti layer and a third layer of Au having a thickness of 300 nanometers overlying said second Pt layer. The ohmic contact device as claimed in claim 1, wherein said metal contact comprises tungsten silicide.

Inventors: SHIRAISHI YASUSHI, SHIRAISHI, YASUSHI, Shiraishi, Yasushi, c/o NEC Corporation
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1993
Main classification: H01L29
IPC codes: H01L29, H01L21