JPH0472740A - Ohmic electrode - Google Patents

Abstract

PURPOSE:To make it possible to obtain an ohmic electrode, whose internal resistance is small and moreover, whose contact resistance can be made small, by a method wherein while the compositional ratios of indium in n-type InxGa1-x As layers and n-type InYGa1-Y as layers hold a constant relation, the compositional ratios of indium are gradually increased from the side of a GaAs substrate to the side of the surface and the surface is constituted of a superllatice layer, which consists of both n-type layers, and an electrode metal film formed on the superllatice layer. CONSTITUTION:An n-type InxGa1-x As layer 2 and an n-type In YGa1-Y As layer 3 are alternately laminated on a GaAs substrate and while both of compositional ratios X and Y of indium in those layers hold a relation of X<Y, the compositional ratios of indium are gradually increased from the side of the substrate toward the side of the surface and the surface is constituted of a superllatice layer 4, which consists of the n-type InxGa1-x As layers 2 and the n-type InYGa1-Y As layers 3, and an electrode metal film 6 formed on the layer 4. As an ohmic electrode has a structure, in which the compositional ratios of In in the layers 2 and 3 are gradually increased from the lower side of the layer 4 toward the surface, a large potential barrier does not exist and as a current flows as a tunnel current in the layer 4, the ohmic electrode having a small internal resistance is obtained. Moreover, as a compositional ratio of In in an n-type InGaAs layer 5, in which a com-positional ratio of In is large and which comes into contact to the electrode metal film 6, can be made sufficiently small, a potential barrier in the interface between the film 6 and the layer 5 can be made small and the contact resistance of the ohimc electrode can be made very small.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor device, and particularly to an ohmic electrode for n-type GaAs.

[Conventional technology]

As a low-resistance non-alloy ohmic electrode for n-type GaAs, an InGaAs substrate with an In composition ratio of n-type InGaAs gradually increasing from the substrate side to the surface is used.
An As graded composition layer is formed, and the surface is coated with n having a high In composition ratio.
A structure in which a type InG'aAs layer is formed and an electrode metal is provided on the layer is described in the Journal of Vacuum Science and Technology by Woodal et al.
m 5science and Technology)
It is proposed in Vol. 19, p. 626 ('1981).

An energy band diagram for this electrode structure is shown in FIG. In Inx Ga1-XA55 with a high In composition ratio, the forbidden band width is very small compared to GaAs 1.
When heavily doped to n-type, the potential barrier at the interface with metal 6 becomes extremely small. Furthermore, in this structure, since GaAs and InGaAs are connected by the graded composition layer 7, there is no potential barrier between the GaAs and InGaAs layers, and an ohmic electrode with low resistance can be obtained. In FIG. 4, solid lines indicate the control of the molecular beam flux intensities of In and Ga when such an InGaAs gradient composition layer is formed by the molecular beam growth method (hereinafter referred to as MBE method). In other words, by increasing the temperature of the In molecular beam cell, I
An InGaAs graded composition layer is formed by increasing the n flux intensity and conversely decreasing the Ga flux intensity by lowering the temperature of the Ga molecular beam cell.

Also, instead of the InGaAs graded composition layer, GaAs
Electrode structures using superlattice layers have also been described, for example, by Peng et al. in Applied Physics Letters.
etters), Vol. 53, p. 900 (1988). An energy band diagram of the electrode structure is shown in FIG. When the period of the superlattice is short, n-type GaA
Since current flows as a tunnel current through the superlattice layer 10 consisting of the s-layer 8 and the n-type InAs layer 9, the internal resistance is small.
An ohmic electrode with very low resistance can be obtained.

Furthermore, GaAs/InAs superlattice layer is replaced with GaAs/InAs superlattice layer.
/The method using InGaAs superlattice layer is
This is proposed in Japanese Patent Publication No. 18965/1999. In this case, the composition of the InGaAs layer is constant or the I
FIG. 6 shows an energy band diagram when the In composition is increased, and the N composition may be gradually increased from the bottom to the surface. According to this method, the n-type GaAs layer 11 and the n-type
The potential barrier existing in the superlattice layer 13 consisting of the type I nGaAs layer 12 is the GaAs/InAs superlattice layer 1.
It is expected that an ohmic electrode that is smaller and has lower resistance than the case of 0 can be obtained.

[Problem to be solved by the invention]

In the ohmic electrode as described in the conventional example, Ga
Difficulties arose due to the large difference in lattice constant between As and InAs, about 7%. In other words, there is a problem in that dislocations due to lattice constant differences are likely to occur in the graded composition layer or superlattice layer, and a depletion layer spreads around the dislocations, increasing internal resistance. In order to suppress the occurrence of dislocations, it was necessary to reduce the film thickness to about 300 angstroms in the case of a gradient composition layer, and to smooth the compositional change. In order to form such a structure using the MEB method, the temperature of the In molecular beam cell is increased to increase the In flux intensity, and conversely, the temperature of the Ga molecular beam cell is increased, as shown by the solid line in Figure 4. Lower Ga
Flux intensity must be reduced. However, it is difficult to smoothly control the molecular beam flux intensity in a short period of time, and a designed compositional layer cannot be formed, resulting in the formation of parts with steep compositional changes, which tend to cause dislocations. In addition, the In flux overshooted and the ratio of the molecular beam fluxes of In and As changed from the designed value, which also caused a decrease in the crystallinity of InGaAs. An example of an actual flux change is shown in FIG. 4 by a broken line. Because of these many problems, it has not been possible to freely design the structure and thickness of the InGaAs layer.

In the method of using a GaAs/InAs superlattice layer instead of a graded composition layer, there is no need to change the molecular beam flux intensities of In and Ga in the MBE method. However, because the difference in lattice constant between InAs and GaAs is as large as approximately 7%, the critical film thickness at which dislocations do not form is extremely thin at 5 to 10 angstroms, and to form a dislocation-free superlattice layer, InAs is used.
The growth conditions for a superlattice layer such as that required for a superlattice layer of about one molecular layer/one molecular layer of GaAs are extremely difficult, and it has been extremely difficult to grow a superlattice layer free of dislocations.

In addition, in the case of using a GaAs/InGaAs superlattice layer, the difference in lattice constant is smaller than in the case of a GaAs/InAs superlattice layer, and the critical film thickness at which dislocations do not occur is thicker, making it easier to form a superlattice layer. However, in order to lower the potential barrier at the interface between the electrode metal and the InGaAs layer, it is necessary to
The In composition of GaAs must be increased, resulting in a superlattice layer with a large difference in lattice constants.

An object of the present invention is to provide an ohmic electrode which eliminates the drawbacks of the prior art.

[Means to solve the problem]

The ohmic electrode of the present invention is an electrode formed on GaA's, in which n-type Inx Ga1-x As and n-type Iny Ga and As are alternately laminated, and their indium A superlattice layer in which the composition ratios X and Y both gradually increase from the GaAs side to the surface side while keeping the relationship X<Y, and the surface is made of n-type InYGa, -7As, and the superlattice layer. and an electrode metal formed thereon.

[Effect]

FIG. 1 is an energy band diagram of the ohmic electrode of the present invention. Because the structure has a structure in which the In composition of each layer gradually increases from the bottom to the surface of the superlattice, there is no large potential barrier, and current flows through the superlattice layer as a tunnel current, resulting in an ohmic film with low internal resistance. A sexual electrode is obtained.

Furthermore, since the In composition ratio of InGaAs in contact with the electrode metal can be made sufficiently small, the potential barrier at the interface with the electrode metal can be made small, and the contact resistance can be made very small.

Furthermore, since there is no interface with a large lattice constant difference in the superlattice layer, dislocations do not occur even if the period of the superlattice layer is lengthened, and the superlattice layer can be easily formed. For example, Ga
The critical thickness of In,,3Ga(,7As) on As is about 30 angstroms. Therefore, the thickness of the superlattice layer can be changed freely without generating dislocations.

〔Example〕

Next, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 is an energy band diagram for explaining one embodiment of the ohmic electrode of the present invention. n-type I nXGa1-, (As layer 2 and n-type I r
A superlattice layer 4 is formed by alternately stacking ly Ga1-v As layers 3. The period of the superlattice is 20 angstroms/20 angstroms, and the total thickness of the superlattice layer 4 is 400 angstroms. n-type I nX
The In composition ratio X of the Ga1-x As layer 2 increases from 0 to 0.75 from the GaAs active layer 1 side toward the surface, and the n-type I
The structure has a structure in which the nYGat-yAsABO3n composition ratio similarly increases from 0.5 to 0°8.6 Furthermore, the superlattice layer 4
For example, as an InGaAs layer with a high In composition ratio, for example, In6. B Ga □, 2 A 8 layers 5 are formed with a thickness of 200 angstroms. The doping concentration is 2 x 1018 cm-3 in the n-type GaAs working layer 1, and gradually increases in the superlattice layer 4.
o, 2 A 2 X 1019 cm- in 8 layers 5
It is doped at a high concentration of 3. I nr3. B
For example, T i /P t on the GaQ2A s layer 5
The electrode metal 6 is formed using a metal multilayer film made of /Au. The potential barrier between the Ino, BGaO, 2As layer 5 and the electrode metal 6 is theoretically almost equal to 0, and a sufficiently low resistance ohmic contact can be achieved without alloying the electrode metal and the semiconductor layer by heat treatment. is obtained.

In this structure, the interface with the largest lattice constant difference is Ga
At the interface between the As active layer 1 and the superlattice layer 4, the difference in lattice constant is still about 3.5%, and even if the period is increased to 20 angstroms/20 angstroms, the generation of dislocations can be suppressed.

FIG. 2 shows an embodiment of the present invention in which I
In order to explain the manufacturing method of an ohmic electrode when one n molecular beam cell and two Ga molecular beam cells are used.
and FIG. 6 is a diagram showing molecular beam flux control of Ga.

The substrate temperature during MBE growth was kept constant at 500°C, and the As flux was 5. It is assumed that Ox is constant at 10"" Torr. After growing the n-type GaAs active layer 1, a superlattice layer 4 is grown. During the growth of the superlattice layer 4, the temperature of the first Ga cell was varied from 800°C to 740°C, during which the Ga flux was varied from 3.4 × 10-'Torr to 1.7X10-
'Decrease to Torr. The second Ga cell has a temperature of 74
The temperature was varied from 0℃ to 710℃, during which the Ga flux was changed from 1.7X10-'Torr to 6.8X10-8T.
decreases to orr. For example, the In cell temperature is 600°C.
to 640℃, during which the Ip flux was changed from 7.5X10-8Torr to 1.5XIO-'Torr.
increases to r. The period of the superlattice is 20 angstroms to 720 angstroms, and the total thickness of the superlattice layer 4 is 400 angstroms. n-type I nX Ga
p-)< The In composition ratio X of the As layer 2 gradually increases from, for example, 0 to 0.5 from the active layer side toward the surface, and
YGa, -.

Similarly, the In composition ratio of the As layer 3 increases from 0.5 to 0.8. Furthermore, I no on the superlattice layer 4
, scao,2 As layer 5 is formed to have a thickness of, for example, 200 angstroms. In the n-type GaAs active layer 1, 2-
Si doping of X 1018 am-3 is performed, and the doping concentration is gradually increased in the superlattice layer 4, and
1,g 2×1019 cm in Gao2As layer 5
It is doped at a high concentration of -3.

On top of that, for example, Ti (500 angstroms)/P
Electrode metal 6 is formed using a metal multilayer film consisting of t (500 angstroms)/A u (300 angstroms).

The ohmic electrode of the embodiment of the present invention can be manufactured by the manufacturing process described above. In and Ga in Figure 2
As shown in the molecular beam flux control diagram, since two cells are used and are alternately opened and closed, there is no need to rapidly change the cell temperature during superlattice layer growth, and the molecular beam flux can be controlled. It's easy. In other words, In during superlattice layer growth
The temperature control of the two molecular beam cells of Ga and Ga is on the order of several tens of degrees Celsius, which is easier to control than in the conventional example shown in FIG. 4, which requires control over 100 degrees Celsius. In this case, the In flux does not need to be changed greatly and may be constant. In this way, the intensity of each molecular beam flux can be easily controlled, and with the structure of the present invention, an ohmic electrode can be manufactured as designed.

In addition, in this embodiment, there is one In cell and two Ga cells.
I used two ■cells, one Ga cell, or In
Two cells and two Ga cells may be used, and the substrate temperature, each molecular beam cell temperature, each molecular beam flux intensity, In composition, film thickness, doping concentration, etc. do not need to be the values shown here. do not have. In a normal MEB device, providing two molecular beam cells with the same source does not pose any particular problem.

〔Effect of the invention〕

As explained above, in the ohmic electrode of the present invention,
As shown in the energy band diagram in Figure 1, the In composition has a structure in which the In composition gradually increases from the bottom to the surface of the superlattice, so there is no large potential barrier, and Because the current flows as a tunnel current,
An ohmic electrode with low internal resistance can be obtained. Furthermore, since the In composition ratio of InGaAs in contact with the electrode metal can be made sufficiently small, the potential barrier at the interface with the electrode metal can be made small, and the contact resistance can be made very small.

Furthermore, since there is no interface with a large lattice constant difference in the superlattice layer, no dislocation occurs even if the period of the superlattice layer is lengthened, and the superlattice layer can be easily formed. Therefore, the thickness of the superlattice layer can be freely changed without generating dislocations.

[Brief explanation of the drawing]

Figure 1 is an energy band diagram for explaining one embodiment of the present invention, and Figure 2 is an MB diagram for explaining its manufacturing method.
It is a figure showing each flux control in E method. Third
The figure is an energy band diagram for explaining the first conventional example, and FIG. 4 is an energy band diagram for explaining the manufacturing method of the conventional example.
FIG. 6, which is a diagram showing each flux control in the BE method, is an energy band diagram for explaining the second conventional example. FIG. 6 is an energy band diagram for explaining the third conventional example. 1-.-n-type GaAs operating layer, 2-n-type InXGat-
XAs layer, 3-/-n-type InyGa1-yAs layer,
4--- n-type I nX cl-X As/n-type I
nY Ga1-yAs superlattice layer, 5... n-type In GaAs layer with high In composition ratio, 6... electrode metal, 7-
・-n-type InGaAs gradient composition layer, 8-=n-type GaAs layer, 9-'.n-type InAs layer, 10=-n-type GaAs/n-type InAs superlattice layer, 11 ・
n-type GaAs layer, 12/n-type InGaAs layer,
13-n-type G a As / n-type In Ga
A s superlattice layer.

Claims (1)

[Claims] On the GaAs substrate, n-type In_XGa_1_-_XA
s and n-type In_YGa_1_-_YAs are alternately stacked, and both of their indium composition ratios X and Y are
< Y gradually increases from the substrate side to the surface side while maintaining the relationship, and the surface is made of n-type In_YGa_1_-_
An ohmic electrode comprising a superlattice layer made of YAs and an electrode metal formed on the superlattice layer.