JP2005051235A - N-type ohmic electrode for n-type group iii nitride semiconductor, semiconductor light-emitting element having same, and n-type ohmic electrode forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an n-type ohmic electrode structure suitable for use on an n-type group III nitride semiconductor layer, a forming method necessary for achieving a low contact resistance, and to provide a semiconductor light-emitting element with its electrical characteristics improved by using them. <P>SOLUTION: The region for the electrode to be in contact with the surface of the n-type group III nitride semiconductor layer is formed of an La/Al alloy film, preferably, an LaAl<SB>2</SB>alloy film. The La/Al alloy film is formed at ≤300°C for the lanthanum to contain abundantly in the junction region between the alloy and the semiconductor. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an n-type ohmic electrode provided in contact with the surface of an n-type group III nitride semiconductor layer, a semiconductor light emitting device including the same, and a method for forming an n-type ohmic electrode.

Conventionally, for example, a group III nitride semiconductor represented by the general formula Al _x Ga _y In _z N (0 ≦ x, y, z ≦ 1, x + y + z = 1) has been used for short-wavelength visible light in a blue band or a green band. Is used to form a barrier layer such as an n-type clad layer (see, for example, Non-Patent Document 1). In addition, in an electron mobility field effect transistor, there is an example in which an electron supply layer is made of n-type aluminum nitride / gallium (compositional formula Al _x Ga _y N: 0 ≦ X ≦ 1) (for example, non-patent) Reference 1). A semiconductor light emitting element such as a light emitting diode (abbreviation: LED) or a laser diode (abbreviation: LD) is brought into contact with the surface of the n-type group III nitride semiconductor layer to form an n-type ohmic contact electrode (n-type ohmic contact). Electrode). A field effect transistor (abbreviation: FET) is manufactured by, for example, providing an n-type ohmic electrode in direct contact with the surface of an electron supply layer or active layer made of an n-type group III nitride semiconductor. .

  Conventionally, an n-type ohmic electrode for an n-type group III nitride semiconductor is, for example, titanium (element symbol: Ti) (for example, refer to Patent Document 1) or aluminum (element symbol: Al) to n-type ohmic electrode. An example of constituting an electrode is disclosed (for example, see Patent Document 2).

Japanese Patent No. 2783349 JP 7-45867 A Akazaki Isao, "III-V compound semiconductor", first edition, Baifukan Co., Ltd., May 20, 1994, Chapter 13

  However, in a compound semiconductor LED in which an n-type ohmic electrode made of a conventional metal material is provided on an n-type group III nitride semiconductor layer, further reduction of the forward voltage (so-called Vf) is desired. ing. Further, in the FET, it is desired to further reduce the drain resistance in order to suppress deterioration of element characteristics due to an increase in thermal resistance. In order to improve the characteristics of the compound semiconductor light emitting device or FET, therefore, it is necessary to construct an n-type ohmic electrode from a material that provides a low contact resistance for the n-type group III nitride semiconductor. In particular, the present invention provides an n-type ohmic electrode for improving the characteristics of a compound semiconductor light-emitting device, a compound light-emitting device including the same, and a method for forming the n-type ohmic electrode.

In order to solve the above problems, the present invention has the following configuration.
(1) An n-type ohmic electrode for n-type group III nitride semiconductor provided in contact with the surface of an n-type group III nitride semiconductor layer, wherein the n-type ohmic electrode layer is made of aluminum (element symbol: An n-type ohmic electrode for an n-type group III nitride semiconductor, characterized by comprising an alloy of Al) and lanthanum (element symbol: La) or lanthanum.
(2) The lanthanum content in the n-type ohmic electrode layer is 10% by mass or more on the surface in contact with the n-type group III nitride semiconductor layer. electrode.
(3) The lanthanum content in the n-type ohmic electrode layer is less than 10% by mass in a region 30 nm or more away from the junction interface with the n-type group III nitride semiconductor layer (2) An n-type ohmic electrode as described in 1.
(4) The n-type ohmic electrode according to (3) above, wherein the surface of the n-type ohmic electrode layer opposite to the surface in contact with the n-type group III nitride semiconductor layer is made of aluminum.

(5) An n-type group III nitride semiconductor layer and a p-type compound semiconductor layer provided on one surface of the crystal substrate, and a light emitting layer between the n-type and p-type compound semiconductor layers. In the semiconductor light emitting device configured by providing an ohmic contact electrode to the laminated structure, the n-type ohmic electrode provided in contact with the n-type group III nitride semiconductor layer is a lanthanum / aluminum alloy layer. A semiconductor light-emitting element comprising a lanthanum layer.
(6) The n-type ohmic electrode is characterized in that the surface side in contact with the n-type group III nitride semiconductor layer is composed of a lanthanum-aluminum alloy layer or a lanthanum layer, and the opposite surface side is composed of an aluminum layer. The semiconductor light-emitting device according to (5) above.
(7) The n-type ohmic electrode has a lanthanum content of 10% by mass or more at the junction interface with the n-type group III nitride semiconductor layer, and a lanthanum content of 10 nm in a region away from the junction interface by 30 nm or more. The semiconductor light-emitting device according to (5) or (6), wherein the semiconductor light-emitting device is composed of a lanthanum-aluminum alloy layer that is less than mass%.

(8) A method of forming an n-type ohmic electrode, wherein the lanthanum / aluminum alloy layer constituting the n-type ohmic electrode is formed using a lanthanum / aluminum alloy (composition formula: LaAl ₂ ) as a raw material.
(9) The n-type group III nitride semiconductor layer is set to 300 ° C. or lower, and a lanthanum-aluminum alloy layer is bonded to the surface of the n-type group III nitride semiconductor layer. The lanthanum content is 10 mass at the bonding interface. The n-type ohmic electrode is formed from a lanthanum-aluminum alloy layer having a lanthanum content of less than 10% by mass in a region separated by 30 nm or more from the bonding interface. Of forming an n-type ohmic electrode.
(10) An n-type group III nitride semiconductor layer and a p-type compound semiconductor layer provided on one surface of the crystal substrate, and a light emitting layer between the n-type and p-type compound semiconductor layers. In the method of manufacturing a semiconductor light emitting device in which an ohmic contact electrode is provided on the laminated structure, a lanthanum / aluminum alloy layer constituting the n-type ohmic electrode provided in contact with the n-type group III nitride semiconductor layer, A method of manufacturing a semiconductor light emitting device, comprising forming a lanthanum-2 aluminum alloy (composition formula: LaAl ₂ ) as a raw material.
(11) The n-type group III nitride semiconductor layer is set to 300 ° C. or lower, and a lanthanum-aluminum alloy layer is bonded to the surface of the n-type group III nitride semiconductor layer. The lanthanum content is 10 mass at the bonding interface. The n-type ohmic electrode is formed from a lanthanum-aluminum alloy layer having a lanthanum content of less than 10% by mass in a region 30% or more away from the bonding interface. Manufacturing method of the semiconductor light-emitting device.

  According to the present invention, since the n-type ohmic electrode provided in contact with the surface of the n-type group III nitride semiconductor layer is made of an alloy of aluminum and lanthanum, a low contact resistance electrode can be formed. The supplied device driving current can be efficiently converted for light emission, and therefore a compound semiconductor light emitting device with high emission intensity can be provided.

An n-type group III nitride semiconductor layer for providing an n-type ohmic electrode can be formed by a halogen method, a hydride method, or a MOCVD (metal organic chemical vapor deposition) method. It can also be formed by molecular beam epitaxy (see J. Solid State Chem., 133 (1997), pages 269-272). The carrier concentration of the n-type group III nitride semiconductor layer is preferably 1 × 10 ¹⁸ cm ⁻³ or more and the resistivity (specific resistance) is preferably 5 × 10 ⁻² Ω · cm or less. The layer thickness of the n-type group III nitride semiconductor layer is suitably 50 nm or less at which a continuous film can be obtained and 5000 nm or less at which no significant cracks are observed. The carrier concentration is preferably substantially constant in the layer thickness direction of the n-type group III nitride semiconductor layer or increases toward the surface of the same layer.

The n-type ohmic electrode made of a lanthanum-aluminum alloy according to the present invention has, for example, a composition formula of LaAl ₄ (lanthanum content = 56.3 mass%), LaAl ₂ (lanthanum content = 73.0 mass%), and A lanthanum-aluminum alloy expressed as LaAl (lanthanum content = 83.7% by mass) can be used as a raw material. Among them, lanthanum-2 aluminum alloy (composition formula: LaAl ₂ ) has a higher lanthanum content than LaAl ₄ and has a melting point that can be sufficiently deposited by ordinary vacuum deposition means. Available as In LaAl, a lanthanum / aluminum alloy film having a stable lanthanum / aluminum composition ratio is used, although the lanthanum content is higher than LaAl ₂ as in the case of 3 lanthanum / 2 aluminum (composition formula: La ₃ Al ₂ ). It cannot be formed.

In order to obtain an n-type ohmic electrode with better contact properties, it is not necessary to intentionally heat the n-type group III nitride semiconductor layer during deposition. The temperature of the group III nitride semiconductor layer at the time of deposition is preferably 50 ° C. or less. When the n-type group III nitride semiconductor layer is formed by heating to a temperature exceeding 300 ° C., a lanthanum-aluminum alloy layer having a low lanthanum content is formed at the junction interface with the n-type semiconductor layer. The contact resistance of the n-type ohmic electrode increases as the lanthanum-aluminum alloy layer having a small lanthanum content is formed in the region near the bonding interface. When the lanthanum content of the lanthanum / aluminum alloy layer in the region near the bonding interface is less than 10% by mass, an n-type ohmic electrode having a contact resistance of 0.1 Ω · cm ⁻² or less cannot be formed stably. The aluminum-lanthanum alloy layer containing 10% by mass or more of lanthanum is preferably provided in a region within 30 nm from the junction interface with the n-type group III nitride semiconductor layer toward the surface of the alloy layer. On the contrary, in a region away from the bonding interface by 30 nm or more, the lanthanum content is preferably less than 10% by mass. The layer in contact with the surface of the n-type group III nitride semiconductor layer may be composed of lanthanum alone, that is, a layer having a lanthanum mass content of 100% by mass.

The lanthanum / aluminum alloy forming the n-type ohmic electrode preferably has a layer thickness that can cover the surface of the n-type group III nitride semiconductor layer without any gap. In general, the thickness is preferably 50 nm or more. In the case of constituting a connection pad electrode, a metal film is further laminated on the lanthanum-aluminum alloy film. The metal film provided by being bonded to the lanthanum / aluminum alloy film that contacts the surface of the n-type group III nitride semiconductor layer is preferably composed of aluminum or titanium. The thickness of the pedestal electrode is suitably about 1 μm to about 5 μm. For example, an example in which an Al film having a thickness of about 1.5 μm is bonded to a LaAl ₂ alloy film having a layer thickness of 0.3 μm to form a pedestal electrode having a total layer thickness of 1.8 μm. Can be mentioned. For example, after depositing an aluminum layer on a lanthanum-aluminum alloy layer, applying so-called alloying heat treatment at a temperature exceeding 300 ° C. is inconvenient for constituting a good n-type ohmic electrode. This is because the lanthanum element cannot be accumulated in a preferable region near the junction interface with the n-type group III nitride semiconductor layer due to the thermal diffusion of lanthanum due to the heat treatment at a high temperature.

  The lanthanum-aluminum alloy film provided in contact with the surface of the n-type group III nitride semiconductor layer has the effect of providing an n-type ohmic electrode with low contact resistance.

(First embodiment)
The present invention will be specifically described by taking as an example a case where an n-type ohmic electrode is provided from a lanthanum / aluminum alloy film on the surface of an n-type group III nitride semiconductor layer.

An n-type GaN layer doped with silicon (Si) was vapor-phase grown by reduced pressure MOCVD using trimethylgallium (molecular formula: (CH ₃ ) ₃ Ga) and ammonia (molecular formula: NH ₃ ) as raw materials. The n-type GaN layer had a carrier concentration of 9 × 10 ¹⁹ cm ⁻³ and a layer thickness of about 3.2 μm. After the surface of the GaN layer was treated with an aqueous ammonium fluoride (NH ₄ F) solution, a lanthanum / aluminum alloy film was deposited on the surface.

While maintaining the n-type GaN layer at room temperature (˜23 ° C.), the lanthanum / aluminum alloy layer was deposited on the surface of the GaN layer by using a LaAl ₂ alloy as a deposition source and by a normal vacuum deposition means. The layer thickness of the lanthanum / aluminum alloy layer was about 0.5 μm. FIG. 1 shows the results of elemental analysis in the junction region between an n-type GaN layer and a lanthanum / aluminum alloy layer measured using an energy dispersive X-ray microanalyzer (EDX) attached to an electron microscope. From the analysis results, a lanthanum / aluminum alloy layer containing about 15% by mass of lanthanum in the vicinity of the junction interface with the n-type GaN layer by depositing a lanthanum / aluminum alloy layer on the GaN layer at room temperature. It was found that a layer was formed. Further, the region where the lanthanum / aluminum alloy layer containing lanthanum exceeding 10 mass% was formed was a region of about 8 nm from the junction interface with the n-type GaN layer toward the surface side of the alloy layer. From that, the content of lanthanum decreased from 10% by mass toward the region on the surface side of the alloy layer as the thickness of the alloy layer increased. In the surface region of the alloy layer, lanthanum was hardly contained, and it was composed mainly of aluminum.

  FIG. 2 shows a current-voltage (IV) characteristic between electrodes made of a lanthanum / aluminum alloy film formed on the surface of the n-type GaN layer. In the positive and negative low voltage regions of ± 50 millivolts (mV), the current increases linearly in direct proportion to the voltage. Further, even in a voltage region of ± 5 V or less, the current increased linearly as the applied voltage increased. From the IV characteristics, it was shown that a good n-type ohmic electrode was formed for the n-type GaN layer by the lanthanum-aluminum alloy layer.

FIG. 3 shows IV characteristics measured by changing the distance (L) between the lanthanum and aluminum alloy electrodes to 0.25 mm, 0.50 mm, 1.0 mm, and 2.0 mm. From the electrode spacing dependence of the resistance value obtained from the I-V characteristic, contact resistance was calculated in accordance with TLM (T ransmission L ine M ode ) theory became ^{1.6 × 10 -2 Ω · cm -2} . In this calculation, the width of the electrode that is actually effective for current flow was taken into consideration.

(Second embodiment)
While maintaining the n-type GaN layer at 350 ° C., a lanthanum / aluminum alloy layer was deposited on the surface of the GaN layer. FIG. 4 shows the results of Auger electron spectroscopy analysis of a lanthanum / aluminum alloy layer formed by changing only the deposition temperature with the first example. Since the alloy source (LaAl ₂ ) of the lanthanum / aluminum alloy layer was deposited at a high temperature, it was observed that lanthanum accumulated in the surface region of the alloy layer. On the other hand, in the region near the junction interface with the n-type GaN layer, at least in the region within 30 nm from the junction interface to the alloy layer side, lanthanum is hardly contained, and aluminum whose mass content is less than 5% is contained. It became the main layer.

  The IV characteristics of the lanthanum-aluminum alloy layer deposited at high temperature are shown in FIG. From the IV characteristics, it was shown that a high resistance was already generated in a low voltage region of ± 1V. From the comparison with the IV characteristics of the lanthanum / aluminum alloy layer deposited at room temperature shown in FIG. 2, in the lanthanum / aluminum alloy film formed at a high temperature in this example, the n-type GaN layer was It has been shown to result in an n-type ohmic electrode with inferior ohmic characteristics than the first example. Similar IV characteristics exhibiting non-rectifying properties were remarkably exhibited when the lanthanum-aluminum alloy layer was formed at a high temperature exceeding 300 ° C.

(Third embodiment)
The present invention will be described in detail by taking as an example a case where a compound semiconductor LED is constructed using an n-type ohmic electrode made of a lanthanum-aluminum alloy.

  FIG. 6 is a schematic cross-sectional view of the LED 1A described in this example. FIG. 7 shows a schematic plan view of the LED 1A.

On a (0001) -sapphire (Al ₂ O ₃ ) substrate 101, an Si-doped n-type GaN layer (carrier concentration = 6 × 10 ¹⁸ cm ⁻³ , layer thickness = 4.2 μm) 102, indium (In) composition ratio An n-type gallium nitride indium (Ga _0.90 In _0.10 N) layer 103 having a multiphase structure composed of a plurality of different domains or phases, an undoped p-type boron phosphide (BP) layer ( (Carrier concentration = 1 × 10 ¹⁹ cm ⁻³ , layer thickness = 0.6 μm) 104 were sequentially laminated to form a laminated structure 1B.

Next, patterning was applied to the p-type BP layer 104 which is the outermost layer of the multilayer structure 1B using a known photolithography technique. Next, it was processed into the shape of the element by a general plasma etching means using chlorine gas (Cl ₂ ). Due to this processing, a part of the p-type BP layer 104 was deleted as shown in FIGS. On the surface of the n-type GaN layer 102 exposed by plasma etching, a lanthanum / aluminum alloy layer 105a and an aluminum layer 105b were continuously deposited by a known vacuum deposition means. Next, the stacked metal layers (105a, 105b) were patterned to form a rectangular n-type ohmic electrode 105. The lanthanum / aluminum alloy layer 105a was formed on the n-type GaN layer 102 at room temperature using a lanthanum / aluminum alloy (LaAl ₂ ) as an evaporation source. The layer thickness of the lanthanum / aluminum alloy layer 105a was 100 nm. The layer thickness of the aluminum layer 105b was 1.5 μm. On the other hand, a p-type ohmic electrode 106 having a multilayer structure of titanium (Ti) 106a and gold (Au) 106b deposited by an electron beam evaporation method was formed on the surface of the p-type BP layer. Both the ohmic electrodes 105 and 106 were not subjected to alloying treatment.

  A double hetero (DH) in which an n-type GaN layer 102 having a (0001) crystal plane as a surface is used as a lower cladding layer, and a p-type (111) -BP layer 104 oriented in the (111) crystal orientation is used as an upper cladding layer. An element driving current was passed through the LED 1A having a junction structure in the forward direction. The emission center wavelength when a forward current of 20 mA was passed between the n-type and p-type ohmic electrodes 105 and 106 was 440 nm. The forward voltage was 3.4 V, and the reverse voltage was 8.3 V when the reverse current was 10 μA.

It is a figure which shows the elemental-analysis result of the lanthanum aluminum layer described in 1st Example. It is a figure which shows the electric current-voltage characteristic of the electrode as described in 1st Example. It is a figure which shows the change of the electric current-voltage characteristic according to the distance (L) between the electrodes as described in 1st Example. It is a figure which shows the element distribution inside the lanthanum aluminum alloy layer deposited at the high temperature described in the second embodiment. It is a figure which shows the current-voltage characteristic of the lanthanum aluminum alloy layer as described in 2nd Example. It is a cross-sectional schematic diagram of LED as described in 3rd Example. It is a plane schematic diagram of LED as described in FIG.

Explanation of symbols

1A LED
1B Laminated structure 101 Sapphire substrate 102 n-type GaN layer (lower cladding layer)
103 n-type GaInN light emitting layer 104 p-type BP layer (upper clad layer)
105 n-type ohmic electrode 105a lanthanum / aluminum alloy layer 105b aluminum layer 106 p-type ohmic electrode 106a titanium layer 106b gold (Au) layer

Claims (11)

  An n-type ohmic electrode for n-type group III nitride semiconductor application provided in contact with the surface of an n-type group III nitride semiconductor layer, the n-type ohmic electrode layer being made of aluminum (element symbol: Al) An n-type ohmic electrode for an n-type group III nitride semiconductor, characterized by comprising an alloy of lanthanum (element symbol: La) or lanthanum.   2. The n-type ohmic electrode according to claim 1, wherein a content of lanthanum in the n-type ohmic electrode layer is 10% by mass or more on a surface in contact with the n-type group III nitride semiconductor layer.   The lanthanum content in the n-type ohmic electrode layer is less than 10% by mass in a region 30 nm or more away from the junction interface with the n-type group III nitride semiconductor layer. Ohmic electrode.   The n-type ohmic electrode according to claim 3, wherein a surface of the n-type ohmic electrode layer opposite to a surface in contact with the n-type group III nitride semiconductor layer is made of aluminum.   A stacked structure comprising an n-type group III nitride semiconductor layer and a p-type compound semiconductor layer provided on one surface of a crystal substrate, and a light emitting layer between the n-type and p-type compound semiconductor layers In a semiconductor light emitting device configured by providing an ohmic contact electrode on a body, the n-type ohmic electrode provided in contact with the n-type group III nitride semiconductor layer is a lanthanum / aluminum alloy layer or a lanthanum layer A semiconductor light emitting device comprising:   6. The n-type ohmic electrode is composed of a lanthanum-aluminum alloy layer or a lanthanum layer on the surface side in contact with the n-type group III nitride semiconductor layer, and an opposite surface side is composed of an aluminum layer. The semiconductor light-emitting device described in 1.   The n-type ohmic electrode has a lanthanum content of 10% by mass or more at the junction interface with the n-type group III nitride semiconductor layer, and the lanthanum content is less than 10% by mass in a region 30 nm or more away from the junction interface. 7. The semiconductor light emitting device according to claim 5, wherein the semiconductor light emitting device is composed of a lanthanum / aluminum alloy layer. A method of forming an n-type ohmic electrode, comprising forming a lanthanum / aluminum alloy layer constituting an n-type ohmic electrode using a lanthanum / aluminum alloy (composition formula: LaAl ₂ ) as a raw material.   The n-type group III nitride semiconductor layer is set to 300 ° C. or less, and a lanthanum-aluminum alloy layer is bonded to the surface of the n-type group III nitride semiconductor layer, and the lanthanum content at the bonding interface is 10% by mass or more. The n-type ohmic electrode according to claim 8, wherein an n-type ohmic electrode is formed from a lanthanum-aluminum alloy layer having a lanthanum content of less than 10% by mass in a region separated by 30 nm or more from the bonding interface. Electrode formation method. A stacked structure comprising an n-type group III nitride semiconductor layer and a p-type compound semiconductor layer provided on one surface of a crystal substrate, and a light emitting layer between the n-type and p-type compound semiconductor layers In the method of manufacturing a semiconductor light emitting device in which an ohmic contact electrode is provided on a body, a lanthanum / aluminum alloy layer constituting an n-type ohmic electrode provided in contact with the n-type group III nitride semiconductor layer A method for producing a semiconductor light emitting device, comprising forming an aluminum alloy (composition formula: LaAl ₂ ) as a raw material.   The n-type group III nitride semiconductor layer is set to 300 ° C. or less, and a lanthanum-aluminum alloy layer is bonded to the surface of the n-type group III nitride semiconductor layer, and the lanthanum content at the bonding interface is 10% by mass or more. 11. The semiconductor light emitting device according to claim 10, wherein an n-type ohmic electrode is formed from a lanthanum-aluminum alloy layer having a lanthanum content of less than 10 mass% in a region away from the bonding interface by 30 nm or more. Manufacturing method.

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