JP2000068528A - Contact between electrode and semiconductor, and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a Schottky barrier from existing when an electrode layer is directly formed on an III-V group semiconductor layer, and to provide a semiconductor element having excellent contact between the III-V group semiconductor layer and the electrode layer. SOLUTION: By arranging an inclined band gap layer 24 between an III-V group semiconductor layer 22 and an electrode layer 23, a more excellent state of contact is obtained between the above-mentioned semiconductor layer 22 and the electrode 23. The band gap of th inclined band gap layer 24 located on the interface of the III-V group semiconductor layer 22 is substantially equal to the band gap of the III-V group semiconductor layer. The band gap of the inclined band gap layer 24 decreases more away from the interface of the III-V group semiconductor layer, and the band gap substantially becomes zero or a negative value on the interface of the inclined band gap layer 24 and the electrode layer 23. By feeding the inclined band gap layer 24, formation of a Schottky barrier generated when the electrode layer 23 is directly formed on the III-V group semiconductor layer 22 can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electrode and a III-V
The contact with the group III semiconductor layer. In particular, the present invention relates to the contact of a metal electrode with a group III nitride semiconductor layer such as GaN. The invention also relates to a method for making such a contact.

[0002]

2. Description of the Related Art As is well known, when a semiconductor layer is brought into contact with a metal layer, a potential barrier is formed at the interface between the metal and the semiconductor. This barrier is called a Schottky barrier.

FIG. 1 shows a Schottky barrier between a metal layer 1 and an n-type GaAs layer 2. As can be understood from FIG. 1, the Fermi level in the metal layer 1 is the same as the Fermi level in the GaAs layer, and the conduction band edge and the valence band edge of the GaAs layer are at the interface with the metal layer. Make a turn around. In FIG. 1, the line labeled “E _C ” represents the lower end of the conduction band (ie, conduction band edge), and the line labeled “E _V ” represents the upper end of the valence band (ie, valence band edge). ing. The Fermi level is indicated by “E _F ”.

[0004] A potential barrier is formed at the interface due to the discontinuity between the Fermi level of the metal layer and the conduction band edge of the GaAs layer. This barrier has a level of about half the band gap of the semiconductor layer. is there. In the case of a Schottky barrier formed by a GaAs layer, the potential barrier is about 0.7 eV. This is because the band gap of GaAs is 1.43 eV at 300K.

[0005] Group III nitrides have a large band gap. For example, the band gap of GaN is 3.5 eV
It is. Contact between the metal and the III-nitride layer typically has a large resistance due to the large Schottky barrier that exists between the metal and the semiconductor layer (the potential barrier is about half the semiconductor band gap). Level
The Schottky barrier is larger than in GaAs). This problem is particularly acute when making contacts to p-type GaN. This is because the Fermi level in the p-type semiconductor is close to the valence band edge.

[0006] There have been many attempts to reduce the contact resistance due to the Schottky barrier at the interface between the metal and the semiconductor. One way is to use metal and n
2a, 2b and 2c show the case of the interface with the GaAs.

First, an n-type InAs layer 3 is arranged adjacent to the metal layer 1. Since n-type InAs has a Fermi level fixed at the conduction band edge, there is no potential difference in the conduction band at the interface with the metal layer 1 and a good n-type contact is formed on the metal layer 1. . This is shown in FIG. 2a.

However, if the n-type GaAs layer 2 is
When arranged adjacent to the nAs layer 3, as shown in FIG. 2b, discontinuity may occur in the conduction band at the interface between the InAs layer 3 and the GaAs layer 2. To remove this discontinuity at the conduction band edge, Ga _1-x In _x As, as shown in FIG.
The layer 4 is disposed between the InAs layer 3 and the GaAs layer 2 (hereinafter, Ga _1-x In _x As is referred to as “GaInAs” for convenience, and other ternary compounds are similarly referred to in the present specification. ). The GaInAs layer 4 has an inclined band gap. That is, while the band gap at the interface with the InAs layer 3 matches the band gap of InAs,
At the interface with the aAs layer 2, the band gap is G
It matches the band gap of aAs. This slant band gap eliminates discontinuities in the conduction band. The gradient band gap is obtained by changing the molar fraction of In from x = 0 at the interface of the GaAs layer to x = 1 at the interface of the InAs layer 3 when the GaInAs layer is grown.

As mentioned above, the contact between the metal and the group III nitride layer usually has a high resistance. In particular, most prior art metal-to-GaN contacts do not provide ohmic contact due to the high height of the Schottky barrier and the inability to do very high doping of GaN. This contact resistance is one of the factors limiting the performance of blue-emitting light emitting diodes (LEDs) and laser diodes (LDs) based on Group III nitrides such as GaN. S. Nakamura et al., "Appl.
Phys. Blue LED and LD describe the Lett ", pp .72,211 (1988), has a threshold current density of about 4 kA / cm ² at a wavelength of 397 nm, it has a threshold voltage of about 5V, the performance of these laser diodes Is
Deterioration is caused by resistance heat at the contact between the metal and the semiconductor due to the presence of the Schottky barrier.

Other known methods of making contact with the GaN layer include using a suitable metal (such as chromium, nickel, gold, titanium, or platinum), or using an alloy of two or more of these metals. There is a way. For example, US Pat. No. 5,652,434 discloses a method using an alloy of gold and nickel to make contact with p-type GaN. Here, the nickel has a contact surface with the GaN layer (in initial contact),
A gold layer is formed over the nickel layer. The two metals can be alloyed before forming the contact, or can be annealed after being deposited as separate layers. The use of such alloys to form contacts with semiconductor layers is not limited to GaN-based
It has also been commonly used to form contacts with other III-V compounds such as As. Alloy contacts generally have lower contact resistance than contacts formed of a single metal,
Requires more process steps than contacts made of a single metal.

A problem in forming a low-resistance contact with a semiconductor layer having a large band gap in the semiconductor layer is as follows.
This also occurs when the device is manufactured using a group VI semiconductor material. One solution is F. Hiei's "Elect"
onic Letters "Vol.29,878
Page (1993), which is shown in FIG.
Shown in

FIG. 3 shows a valence band structure in contact with the p-type ZnSe layer 7 using the ZnTe / ZnSe multiple quantum well (MQW) layer 6 and the p-type ZnTe layer 5. Since the thickness of the quantum well in the MQW layer 6 decreases as the distance from the ZnTe layer 5 increases, each quantum well has a thickness of Zn.
The Te layer 5 has a binding energy level at an energy substantially equal to the energy of the conduction band edge. Carrier is M
It can be implanted into the ZnSe layer 7 by a resonant tunneling process through the bound state of the quantum well in the QW layer 6.

Forming a contact as shown in FIG. 3 in a GaN layer involves substantial difficulties. This is because the multiple quantum well structure has (I
This is because it is necessary to include an nGa) N layer. (InG
a) Since the N layer does not grow at the same substrate temperature as GaN,
It is necessary to repeatedly change the substrate temperature during growth,
It is difficult to do this accurately.

In addition, the contact shown in FIG. 3 is such that the different layers have a limited level (c
dependent level). Obtaining this when making contacts of the type shown in FIG. 3 in a GaN layer requires very tight control of the In mole fraction (and layer width) of the layer, which is substantially achieved. Have difficulty.

In US Pat. No. 5,366,927, p
It discloses the use of an HgSe layer to make contact with a type ZnSe layer. This patent also discloses that the HgSe layer and the Z
An inclined band gap Zn (Te, S
e) or providing a graded bandgap (Zn, Hg) Se layer. The graded bandgap layer reduces contact resistance in a manner similar to that shown in FIGS. 2a, 2b and 2c. This patent describes a HgSe layer,
It teaches that the lattices of the graded bandgap layer and the ZnSe layer should be matched.

US Pat. No. 5,670,798 discloses Ga
In order to improve electron injection and hole injection between the N layer and the (AlGa) N cladding layer, a gradient band gap (A
It discloses the use of an lGa) N layer. Without a graded bandgap layer, there is a heterobarrier at the GaN / (AlGa) N interface, which is used to "obscure" the barrier. This graded bandgap layer does not reduce the resistance between the GaN layer of the device and the metal contacts anyway.

US Pat. No. 5,679,965 discloses providing a graded bandgap layer or MQW layer between a p-type AlN layer and a p-type GaN layer. The graded bandgap layer provides a heterojunction band offset between these layers.
offset) is used to reduce or eliminate
It does not reduce the resistance between the metal contact layer and the semiconductor layer.

FIG. 4 shows a conventional technique for reducing the contact resistance. This is N. E. FIG. Lin's "Appl"
ied Physics Letters "Vol.
64, 2557 (1994). In this prior art device, a short-period superlattice (SPS) layer 8 including alternating InN layers and GaN layers is provided between a metal contact layer 9 and a GaN semiconductor layer 10. This again has the disadvantage that different layers must be grown at different substrate temperatures, complicating the growth process.

US Pat. No. 5,689,123 describes II
It suggests that the group IV arsenide nitride material system is suitable for producing LEDs capable of coloring over the entire visible wavelength range. The basic principle of the operation of these LEDs is shown with reference to FIG. FIG. 5 shows band gap energies of some semiconductor compounds.

FIG. 5 is a graph showing G according to the ratio of arsenic to nitrogen.
a (As, N) sled with a strong band gap (bowi)
ng) changes. In fact, this band gap is largely negative, indicating that the ternary compound has semimetallic properties. As can be seen from FIG. 5, as the ratio of arsenic to nitrogen decreases, the bandgap changes rapidly, ranging from 0 to about 3.5 eV (corresponding to blue to pale emission). Have. U.S. Pat. No. 5,689,
No. 123 requires Ga (A) in this range to achieve a band gap of any value from 0 to 3.5 eV.
s, N).

Kondow et al., "Japanese"
Journal of Applied Physi
cs ", Vol. 33, L1056 (1994), also suggests using a III-V group arsenide nitride material system. And from 1.3 μm to 1.
It is suggested that an (InGa) (AsN) laser diode operating in the 5 μm wavelength range but lattice matched to the GaAs substrate is obtained.

A conventional (InGa) N laser diode is disclosed in, for example, S.M. Nakamura and G.M. Faso
1 "The Blue Laser Diode
e ", Springer, 1997. Examples of these known laser diodes are shown in FIGS.

FIG. 10 shows an (InGa) N laser diode grown on a sapphire substrate 12.
A GaN buffer layer 13 is grown on the substrate 12 and n-type Ga
An N layer 14 is grown on the buffer layer 13. n-type Ga
The N layer has a reduced thickness portion, and the n-type electrode 11 is formed in the reduced thickness portion.

On the portion where the thickness of the n-type GaN is not reduced, the n-type (InGa) N layer 15 and the n-type (AlGa)
N layer 16, n-type GaN layer 17, (InGa) N quantum well active layer 18, p-type (AlGa) N layer 19, p-type GaN layer 20, p-type (AlGa) N layer 21, and p-type GaN cap layer 22 grow sequentially. Finally, the p-type GaN layer 2
2, a p-type electrode 23 is arranged.

Therefore, in the LD of FIG. 10, since both the n-type electrode 11 and the p-type electrode 23 are arranged on the GaN layer, it can be understood that a large Schottky barrier is formed on both electrodes.

FIG. 11 shows another conventional (InGa) N laser diode. This is because silicon carbide (S
iC) Growing on substrate 12 'and n-type electrode 11
It differs from the laser diode shown in FIG. 10 in that it is formed below 2 '. Although the n-type electrode 11 of the laser diode shown in FIG. 11 is not arranged on the GaN layer, the p-type electrode 23 is arranged on the p-type GaN layer 22 so that the Schottky barrier is reduced to the p-type electrode 23. And p
(When an electrode is arranged on the p-type GaN layer, a large potential barrier is formed as described above).

[0027]

SUMMARY OF THE INVENTION The present invention has been achieved in view of the above, and an object of the present invention is to dispose an inclined band gap layer between a III-V semiconductor layer and an electrode layer. As a result, the Schottky barrier generated when the electrode layer is formed directly on the III-V semiconductor layer is prevented,
It is to obtain better contact between the II-V semiconductor layer and the electrode layer.

[0028]

A semiconductor device according to the present invention comprises a first semiconductor layer, a second semiconductor layer disposed on the first semiconductor layer, and an electrode layer disposed on the second semiconductor layer. Wherein the second semiconductor layer has an inclined band gap,
The band gap decreases as the distance from the interface between the first semiconductor layer and the second semiconductor layer decreases, and the first semiconductor layer has
A layer of a III-V semiconductor, which achieves the above objects.

The second semiconductor layer may be a III-V semiconductor layer.

The band gap of the second semiconductor layer may be substantially zero or a negative value at an interface between the second semiconductor layer and the electrode layer.

[0031] The band gap of the second semiconductor layer may be substantially equal to the band gap of the first semiconductor layer at an interface between the second semiconductor layer and the first semiconductor layer.

[0032] The first semiconductor layer may be a group III nitride.

[0033] The first semiconductor layer may be GaN.

[0034] The second semiconductor layer may be a GaAs _1-X N X.

The x is equal to about 0.8 at the interface between the second semiconductor layer and the electrode. The second semiconductor layer may be a GaP _1-y N _y.

[0036] The y may be substantially equal to 0.75 at the interface between the second semiconductor layer and the electrode.

[0037] The thickness of the second semiconductor layer may be between about 10 nm and about 200 nm.

[0038] The first semiconductor layer may be a GaAs layer.

[0039] The second semiconductor layer may be GaAs _1-X N _X.

[0040] The x may be about 0.1 at an interface between the second semiconductor layer and the electrode layer.

[0041] The thickness of the second semiconductor layer may be between about 20 nm and about 100 nm.

[0042] The first semiconductor layer may be doped p-type.

[0043] The first semiconductor layer may be doped n-type.

The device may be a light emitting diode.

[0045] The element may be a laser diode.

The first portion of the second semiconductor layer has a first thickness, and the second portion of the second semiconductor layer has a second thickness greater than the first thickness. And the electrode layer comprises the second
Disposed on the first portion of the semiconductor layer, the device further comprising a third semiconductor layer disposed on the second portion of the second semiconductor layer, wherein the band of the second semiconductor layer A gap is substantially equal to a band gap of the first semiconductor layer at an interface between the second semiconductor layer and the first semiconductor layer, and the band gap of the second semiconductor layer is equal to a band gap of the second semiconductor layer. At the interface between the first portion and the electrode layer, the band gap of the second semiconductor layer is substantially zero or a negative value, and the band gap of the second semiconductor layer is substantially equal to the negative value. 3
At the interface with the semiconductor layer, it may be substantially equal to the band gap of the third semiconductor layer.

[0047] The first semiconductor layer and the third semiconductor layer may be formed of the same semiconductor material.

A method for forming an electrical contact to a first semiconductor layer according to the present invention is characterized in that the first semiconductor layer is a III-V semiconductor layer and a second semiconductor layer is disposed on the first semiconductor layer. And arranging an electrode layer on the second semiconductor layer, wherein the second semiconductor layer includes the second semiconductor layer and the first semiconductor layer.
The above object is achieved by forming the structure so that the band gap decreases as the distance from the interface with the semiconductor layer increases.

The operation will be described below.

According to a first aspect of the present invention, a first semiconductor layer includes:
A second semiconductor layer disposed on the first semiconductor layer; and an electrode layer disposed on the second semiconductor layer, wherein the second semiconductor has an inclined band gap, and the band gap is a first semiconductor layer. To provide a semiconductor device, wherein the first semiconductor layer is a layer of a group III-V semiconductor, decreasing as the distance from the interface between the semiconductor layer and the second semiconductor layer increases. The second semiconductor layer is also III
-V group semiconductor.

Providing the second semiconductor layer with an inclined band gap prevents the formation of a Schottky barrier generated when the electrode layer is directly disposed on the first semiconductor layer, or prevents any potential barrier formed from being formed. Reduce height at least. This reduces contact resistance, thereby reducing heat generated during operation of the device. The alloying or annealing steps required to form the alloy contacts are eliminated.

The band gap of the second semiconductor layer can be substantially zero or a negative value at the interface between the second semiconductor layer and the electrode layer. The band gap of the second semiconductor layer may be substantially equal to the band gap of the first semiconductor layer at the interface between the second semiconductor layer and the first semiconductor layer. FIG. 6A is a schematic diagram of a band gap showing contact between the metal and the GaN layer in this case, and FIG. 6B is a schematic diagram showing current-voltage characteristics in the contact of FIG. 6A. The potential barrier shown in FIG. 1 is removed, and the current-voltage characteristics are linear characteristics indicating that the contact is an ohmic contact.

The first semiconductor layer may be a group III nitride or GaN. In the latter case, the second semiconductor layer is Ga
As _1-X N _X may be used. As a result, a low-resistance ohmic contact is formed in the GaN layer.

The value of x may be substantially equal to 0.8 at the interface between the second semiconductor layer and the electrode. This means that the band gap of the second semiconductor layer is almost zero at the interface between the electrode and the second semiconductor layer. This is shown in FIG. 7 (from US Pat. No. 5,689,123), which shows that the band gap of GaAs _1-X N _X depends on the nitrogen mole fraction (x).

[0055] The second semiconductor layer may also be GaP _1-y N _y . Also in this case, a low-resistance ohmic contact can be formed in the GaN layer.

The value of y can be substantially equal to 0.75 at the interface between the second semiconductor layer and the electrode. This is the second
This means that the band gap of the semiconductor layer is substantially zero at the interface between the electrode and the second semiconductor layer.

[0057] The thickness of the second semiconductor layer can be between about 10 nm and about 200 nm. A thickness in this range means that the composition of the second semiconductor layer is gradually changing, so that dislocations are unlikely to be introduced into the second semiconductor layer.

The first semiconductor layer may be a GaAs layer. In this case, the second semiconductor layer may be a GaAs _1-x N _x layer. Thereby, a low-resistance ohmic contact can be formed in the GaAs layer. Compared to the prior art contact shown in FIG. 2c, one less layer is required, which simplifies the growth process.

The value of x can be about 0.1 at the interface between the second semiconductor layer and the electrode. This means that the band gap of the second semiconductor layer is approximately zero at the interface between the electrode and the second semiconductor layer.

The thickness of the second semiconductor layer is about 20 nm and about 1
00 nm. A thickness in this range means that the composition of the second semiconductor layer is gradually changing, so that dislocations are unlikely to be introduced into the second semiconductor layer.

The first semiconductor layer is doped p
Or doped n-type.

The semiconductor device can be a light emitting diode or a laser diode.

[0063] A first portion of the second semiconductor layer may have a first thickness, a second portion of the second semiconductor layer may have a second thickness greater than the first thickness, The electrode layer is the first of the second semiconductor layers.
The element is further disposed on a portion of the second semiconductor layer.
May include a third semiconductor layer on a portion of the second semiconductor layer, wherein a band gap of the second semiconductor layer may be substantially equal to a band gap of the first semiconductor layer at an interface between the second semiconductor layer and the first semiconductor layer; The band gap of the second semiconductor layer may be substantially zero or a negative value at the interface between the first portion of the second semiconductor layer and the electrode layer, and the band gap of the second semiconductor layer may be At the interface between the second portion of the layer and the third semiconductor layer, it may be substantially equal to the band gap of the third semiconductor layer. The first semiconductor layer and the third semiconductor layer include:
It may be formed from the same semiconductor material. This allows the present invention to be applied to the contact between the n-type electrode and the n-type GaN layer of the prior art laser diode shown in FIG. 11 as described below.

A second aspect of the present invention is a method for forming an electrical contact with a first semiconductor layer, the method comprising:
A layer of an IV group semiconductor, the method comprising disposing a second semiconductor layer on the first semiconductor layer and disposing an electrode layer on the second semiconductor layer, wherein the second semiconductor layer comprises a second semiconductor layer. It is an object of the present invention to provide a method in which the band gap is reduced as the distance from the interface between the semiconductor layer and the first semiconductor layer increases.

[0065]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 12 shows a first embodiment of the present invention.
This shows an (InGa) N laser diode grown on a sapphire substrate. This LD is similar to the laser diode shown in FIG.
Between the N layer 22 and the p-type electrode 23, p-type Ga (As,
N) An inclined band gap layer 24 is provided. The Ga (As, N) layer 24 has a monotonically changing band gap from near the GaN band gap at the interface with the GaN layer 22 to near zero or a negative value at the interface with the p-type electrode 23. . When the band gap of the gradient band gap layer 24 at the interface with the p-type electrode 23 is zero and equal to the band gap of GaN at the interface with the GaN layer 22, the band of the gradient band gap layer 24 and the p-type GaN layer 22 The structure is as shown in FIG.

In order to obtain a zero band gap in the Ga (As, N) gradient band gap layer, the nitrogen mole fraction of the gradient band gap layer 24 is set to about 0 from the value 1 at the interface with the p-type GaN layer 22. .8.
This includes, for example, metal organic chemical vapor deposition (MOCVD),
Molecular beam epitaxy (MBE) or gaseous feed (ga
It can also be performed using any conventional epitaxial growth technique such as a s source molecular beam epitaxy (GSMBE) method.

FIG. 8 shows that the lattice constant of GaAs _1-X N _X depends on the nitrogen mole fraction x (from US Pat. No. 5,689,123). This means that as x decreases from 1.0 to 0.8, the wurtzite lattice parameter of GaAs _1-X N _X increases from about 3.15 ° to about 3.35 °, or about 6.4% increase. It indicates that you want to.

The slope of the band gap need not be linear.

It is preferable that the composition of the gradient band gap layer is changed as gradually as possible. This is to minimize the risk of dislocations or other crystal defects in the graded bandgap layer. If the graded layer can be realized with a thickness between about 10 nm and about 200 nm, this thickness is suitable for avoiding dislocations.

The laser diode shown in FIG.
It has a Ga (As, N) gradient band gap layer 24
However, the same effect is obtained by the p-type GaN layer 22 and the p-type electrode 23.
Using a p-type Ga (P, N) gradient band gap
It can also be achieved by doing This is shown in FIG. 9 (US
(From Patent No. 5,689,123). FIG. 9 shows GaP
_1-XN_XBand gap depends on nitrogen mole fraction x
Is shown. Ga (P, N) inclined band gear
In the case where a top layer is used,
About 10 nm and about 200 nm
The thickness may be between 10 nm and 10 nm.

In order to achieve a zero band gap in the Ga (P, N) layer, the phosphorus mole fraction at the interface with the p-type electrode 23 is about 0.25 (that is, the nitrogen mole fraction is 0.75).
(At the interface with the p-type GaN layer 22, the phosphorus mole fraction is zero, corresponding to a nitrogen mole fraction of one).

FIG. 13A shows a second embodiment of the present invention. This is similar to the first embodiment, except that n-type Ga
Second graded bandgap layer 2 disposed in N layer 14
5 is provided. Since the n-type electrode 11 is disposed on the second inclined band gap layer 25, the n-type electrode 11 and the n-type Ga that existed in the laser diode shown in FIG.
The Schottky barrier to the N layer 14 can be removed.

Since the second graded bandgap layer 25 is grown in the n-type GaN contact layer 14, the laser diode will have a "lower" n-type GaN contact layer 14 ', a second n-type Ga (As, N 1.) Includes a graded bandgap layer 25 and an "top" n-type GaN contact layer 14 ".

The second graded bandgap layer 25 initially grows to a thickness h ₂ , but a portion of the graded bandgap layer 25 is reduced to a thickness h ₁ using, for example, any conventional etching process. Is done. The band gap of the second graded bandgap layer 25 in the thickness h _1, GaN
Is substantially reduced from the bandgap of the n-type electrode 11 to near zero or a negative value.
It is close to zero or negative at the interface with.

The band gap of the inclined band cap layer 25 then increases again, and again at the thickness h ₂ of GaN
Becomes substantially equal to the band gap. If inclined band gap layer is formed by Ga (As, N), arsenic mole fraction is about 0.2 (or greater in a zero thickness h ₁ at the interface between the lower GaN contact layer 14 ' increases in value), must be reduced to zero again in the subsequent thickness h _2.

When etching the inclined band gap layer 25, it is important to carefully control the etching depth so that the etched thickness h ₁ matches the zero band gap portion of the inclined region. One suitable technique is dry etching of the material. If the etching rate is known, it is possible to stop at or near the desired thickness h ₁ corresponding to the zero band gap portion of the second graded band gap layer.

In practice, the second graded bandgap layer 2
5 is etched after depositing layers 14 "to 24. A second graded bandgap layer 25 is applied to layer 1
Etching before depositing the 4 "to layer 24 is possible in principle but difficult.

Like the first gradient band gap layer 24, the second gradient band gap layer can be formed of Ga (P, N) instead of Ga (As, N).

FIG. 13B shows the upper contact layer 14 'and the lower contact layer 1 in the case of a Ga (As, N) gradient band gap layer.
FIG. 13B is a schematic view of the band structure of 4 ”and the second inclined bandgap layer 25. In the embodiment of FIG. 13B, the bandgap of the second inclined bandgap layer has a thickness h.
It is reduced to zero at ₁ but may alternatively be reduced to a negative value at this thickness.

FIG. 14 shows a third embodiment of the present invention. In this embodiment, the graded bandgap layer 24
Are used in the laser diode shown in FIG. The third embodiment is similar to the first embodiment of the present invention, but uses a SiC substrate 12 ′, and the n-type electrode is arranged on the bottom surface of the SiC substrate 12 ′. I have.

FIG. 14 shows that the gradient bandgap layer 24 is formed of Ga (As, N), but may be formed of Ga (P, N) instead.

FIG. 15 shows a fourth embodiment of the present invention. This is similar to the embodiment of FIG. 14, but the laser diode is grown on a GaN substrate 12 "".

The above embodiments relate to the problem of improving the contact between the metal electrode and the III-V nitride semiconductor layer having a wide band gap. However, the invention can also be applied to improve the contact between the metal electrode and the GaAs layer. This is because, as shown in FIG.
_This is because the dependence of the band gap of _1-X N _X has a similar behavior at low values of x (ie, low nitrogen mole fraction) as a value of x near 1. FIG. 7 shows that the band gap decreases as nitrogen is introduced into GaAs, reaching zero at a nitrogen mole fraction of about 0.1. This makes it possible to form an inclined bandgap layer between the GaAs layer and the metal electrode, thereby reducing the contact resistance between the metal electrode and the GaAs layer.

FIG. 16 is a schematic diagram of a laser diode based on the (InGa) (AsN) system. It comprises a multiple quantum well active region 30 formed from GaAs and (InGa) (As, N) quantum wells. On its upper surface, a GaAs layer 31, a p-type (AlGa) As layer 32, a p-type GaAs layer 33, and a gradient band gap Ga
An (As, N) layer 34 and a p-type electrode 35 are sequentially arranged. Under the active region 30, a GaAs layer 36 and an n-type (A
1Ga) As layer 37, n-type GaAs layer 38, and n-type electrode 39 are sequentially arranged.

The inclined band gap layer 34 is made of p-type GaAs
It has a band gap that decreases from the band gap of GaAs at the interface with the s layer 33 to zero or a negative value at the interface with the p-type electrode 35. This means that as layer 34 grows, the nitrogen mole fraction in layer 34 is reduced to p-type Ga
This is achieved by increasing from zero at the interface with the As layer 33 to about 0.1 or more at the interface with the p-type electrode 35.

As in the other embodiments described above, it is desirable not to give any unnecessary distortion between the gradient band gap layer 34 and the p-type GaAs layer 33. This can be achieved, for example, by gradually inclining the bandgap of the graded bandgap layer from a thickness of about 20 nm to about 100 nm.

Although the invention has been described with reference to illustrative embodiments, the invention is not limited to the specific embodiments described herein. For example, the present invention relates to the aforementioned Ga
It is not limited to the two examples of (P, N) and Ga (As, N). S. Sakai et al., Theoretical Study, “Japanese
Journal of Applied Physi
cs ", Vol 32, p. 4413 (1993),
The difference in electronegativity between various III-nitrides and other III-V compounds results in large bandgap negative warpage in forming appropriate ternary or quaternary alloys.
Suggests. For example, FIG. 5 shows GaN-AlAs,
GaN-AlP, GaN-InP, GaN-InAs,
This shows that the bandgap warpage between GaN-GaSb and GaN-AlSb is all negative. As a result, it is possible to mix nitrogen with another group V element group to make the energy gap zero.
As long as nothing prevents the successful growth of these alloys, each of these combinations mentioned above can be used as the graded contact area of the present invention to make contact with the GaN layer. Similarly, FIG. 5 suggests that bandgap warpage between GaAs and InSb is also negative, so this system can also be used to form contacts with an AlAs layer according to the present invention.

[0089]

As described above, according to the present invention, by disposing the gradient band gap layer between the III-V semiconductor layer and the electrode layer, the electrode layer is formed on the III-V semiconductor layer. It prevents the Schottky barrier that occurs when formed directly and provides better contact between the III-V semiconductor layer and the electrode layer.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing the formation of a Schottky barrier at an interface between a metal layer and an n-type GaAs layer.

FIG. 2 (a) is a diagram showing one prior art method for removing the Schottky barrier of FIG. 1; FIG. 2B is a diagram showing one of the prior art methods for removing the Schottky barrier of FIG. FIG. 2C is a diagram showing one prior art method for removing the Schottky barrier of FIG.

FIG. 3 is a schematic diagram of a valence band structure in contact with n-type ZnSe according to a conventional technique.

FIG. 4 is a schematic band diagram of a prior art contact between a metal layer and a GaN layer using an InN / GaN short-period superlattice layer.

FIG. 5 is a schematic diagram of bandgap energy versus lattice constant for some III-V alloys and compounds.

FIG. 6a is a schematic diagram of a band gap in the contact of the present invention.

6b is a schematic diagram of the current-voltage characteristic at the contact of FIG. 6a.

FIG. 7 shows the bandgap energy of GaAsN as a function of nitrogen mole fraction.

FIG. 8 is a diagram illustrating lattice parameters of a GaAsN alloy as a function of a nitrogen mole fraction.

FIG. 9 corresponds to FIG. 7, but shows the case of a GaPN alloy.

FIG. 10 shows a conventional (InG) film grown on a sapphire substrate.
a) Schematic sectional view of an N laser diode.

FIG. 11 is a schematic sectional view of a conventional (InGa) N laser diode grown on a silicon carbide (SiC) substrate.

FIG. 12 is a schematic cross-sectional view of an (InGa) N laser diode formed on a sapphire substrate, including a metal-semiconductor contact according to the present invention.

FIG. 13a is a schematic cross-sectional view of a (InGa) N laser diode grown on a sapphire substrate, including a p-type metal-semiconductor contact and an n-type metal-semiconductor contact according to the present invention.

13b is a partial schematic view of the band gap of the laser diode of FIG. 13a.

FIG. 14 is a schematic cross-sectional view of an (InGa) N laser diode grown on a silicon carbide (SiC) substrate, including a metal-semiconductor contact according to the present invention.

FIG. 15 shows Ga including metal-semiconductor contacts according to the present invention.
FIG. 3 is a schematic sectional view of an (InGa) N laser diode grown on an N substrate.

FIG. 16 shows Ga including metal-semiconductor contacts according to the present invention.
FIG. 3 is a schematic cross-sectional view of a (InGaNAs) laser diode grown on an As substrate.

[Explanation of symbols]

Reference Signs List 11 n-type electrode 12 sapphire substrate 13 GaN buffer 14 n-type GaN 15 n-type (InGa) N 16 n-type (AlGa) N 17 n-type GaN 18 (InGa) N-QW 19 p-type (AlGa) N 20 p-type GaN 21 p-type (AlGa) N 22 p-type GaN 23 p-type electrode 24 p-type Ga (As, N) or p-type Ga (P, N)
Graded layer

Claims (22)

[Claims] 1. A semiconductor device comprising: a first semiconductor layer; a second semiconductor layer disposed on the first semiconductor layer; and an electrode layer disposed on the second semiconductor layer. A semiconductor device having an inclined band gap, wherein the band gap decreases as the distance from the interface between the first semiconductor layer and the second semiconductor layer increases, and the first semiconductor layer is a layer of a III-V semiconductor. 2. The semiconductor device according to claim 1, wherein the second semiconductor layer is a III-V semiconductor layer. 3. The semiconductor according to claim 1, wherein the band gap of the second semiconductor layer is substantially zero or a negative value at an interface between the second semiconductor layer and the electrode layer. element. 4. The semiconductor device according to claim 1, wherein said band gap of said second semiconductor layer is substantially equal to a band gap of said first semiconductor layer at an interface between said second semiconductor layer and said first semiconductor layer. Or the semiconductor device according to 3. 5. The semiconductor device according to claim 1, wherein said first semiconductor layer is a group III nitride. 6. The semiconductor device according to claim 5, wherein said first semiconductor layer is GaN. 7. The semiconductor device according to claim 6, wherein said second semiconductor layer is GaAs _1-X N _X. 8. The semiconductor device according to claim 7, wherein x is equal to about 0.8 at an interface between the second semiconductor layer and the electrode. 9. The semiconductor device according to claim 6, wherein said second semiconductor layer is GaP _1-y N _y . 10. The semiconductor device according to claim 9, wherein y is substantially equal to 0.75 at an interface between the second semiconductor layer and the electrode. 11. The thickness of the second semiconductor layer is about 10n.
11. The semiconductor device according to claim 7, 8, 9, or 10, which is between m and about 200 nm. 12. The semiconductor device according to claim 1, wherein said first semiconductor layer is a GaAs layer. 13. The semiconductor device according to claim 12, wherein said second semiconductor layer is GaAs _1-X N _X. 14. The semiconductor device according to claim 13, wherein x is about 0.1 at an interface between the second semiconductor layer and the electrode layer. 15. The thickness of the second semiconductor layer is about 20 n.
15. The method according to claim 13, wherein the distance is between m and about 100 nm.
A semiconductor device according to item 1. 16. The semiconductor device according to claim 1, wherein said first semiconductor layer is doped p-type. 17. The semiconductor device according to claim 1, wherein the first semiconductor layer is doped n-type. 18. The semiconductor device according to claim 1, wherein said device is a light emitting diode. 19. The semiconductor device according to claim 1, wherein said device is a laser diode. 20. A first portion of the second semiconductor layer has a first thickness, and a second portion of the second semiconductor layer has a second thickness greater than the first thickness. A third semiconductor layer comprising: the electrode layer disposed on the first portion of the second semiconductor layer; and the element disposed on the second portion of the second semiconductor layer. Wherein the band gap of the second semiconductor layer is substantially equal to the band gap of the first semiconductor layer at an interface between the second semiconductor layer and the first semiconductor layer; Is substantially zero or a negative value at the interface between the first portion of the second semiconductor layer and the electrode layer, and the band gap of the second semiconductor layer is 2
The semiconductor device according to claim 1, wherein an interface between the second portion of the semiconductor layer and the third semiconductor layer is substantially equal to a band gap of the third semiconductor layer. 21. The semiconductor device according to claim 2, wherein the first semiconductor layer and the third semiconductor layer are formed of the same semiconductor material.
The semiconductor element according to 0. 22. A method for forming an electrical contact to a first semiconductor layer, wherein the first semiconductor layer is a III-V semiconductor layer, the method comprising: forming a second semiconductor layer on the first semiconductor layer. Arranging a layer and arranging an electrode layer on the second semiconductor layer, wherein the second semiconductor layer moves away from an interface between the second semiconductor layer and the first semiconductor layer, A method formed to reduce the bandgap.