WO2006066962A2 - Selective oxidation and selective etching of allnn layers for manufacturing group iii nitride semiconductor devices - Google Patents

Selective oxidation and selective etching of allnn layers for manufacturing group iii nitride semiconductor devices Download PDF

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WO2006066962A2
WO2006066962A2 PCT/EP2005/013966 EP2005013966W WO2006066962A2 WO 2006066962 A2 WO2006066962 A2 WO 2006066962A2 EP 2005013966 W EP2005013966 W EP 2005013966W WO 2006066962 A2 WO2006066962 A2 WO 2006066962A2
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group iii
layer
iii nitride
layers
oxidized
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WO2006066962A3 (en
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Hans-Joerg Buehlmann
Julien Dorsaz
Jean-François Carlin
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Ecole Polytechnique Federale De Lausanne
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/005—Processes
    • H01L33/0062—Processes for devices with an active region comprising only III-V compounds
    • H01L33/0066—Processes for devices with an active region comprising only III-V compounds with a substrate not being a III-V compound
    • H01L33/007—Processes for devices with an active region comprising only III-V compounds with a substrate not being a III-V compound comprising nitride compounds
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
    • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
    • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
    • H01L21/31—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to form insulating layers thereon, e.g. for masking or by using photolithographic techniques; After treatment of these layers; Selection of materials for these layers
    • H01L21/32—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to form insulating layers thereon, e.g. for masking or by using photolithographic techniques; After treatment of these layers; Selection of materials for these layers using masks
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02107—Forming insulating materials on a substrate
    • H01L21/02109—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates
    • H01L21/02112—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer
    • H01L21/02172—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer the material containing at least one metal element, e.g. metal oxides, metal nitrides, metal oxynitrides or metal carbides
    • H01L21/02175—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer the material containing at least one metal element, e.g. metal oxides, metal nitrides, metal oxynitrides or metal carbides characterised by the metal
    • H01L21/02178—Forming insulating materials on a substrate characterised by the type of layer, e.g. type of material, porous/non-porous, pre-cursors, mixtures or laminates characterised by the material of the layer the material containing at least one metal element, e.g. metal oxides, metal nitrides, metal oxynitrides or metal carbides characterised by the metal the material containing aluminium, e.g. Al2O3
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02107—Forming insulating materials on a substrate
    • H01L21/02225—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer
    • H01L21/02227—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a process other than a deposition process
    • H01L21/0223—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a process other than a deposition process formation by oxidation, e.g. oxidation of the substrate
    • H01L21/02233—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a process other than a deposition process formation by oxidation, e.g. oxidation of the substrate of the semiconductor substrate or a semiconductor layer
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02107—Forming insulating materials on a substrate
    • H01L21/02225—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer
    • H01L21/02227—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a process other than a deposition process
    • H01L21/0223—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a process other than a deposition process formation by oxidation, e.g. oxidation of the substrate
    • H01L21/02233—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a process other than a deposition process formation by oxidation, e.g. oxidation of the substrate of the semiconductor substrate or a semiconductor layer
    • H01L21/02241—III-V semiconductor
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02107—Forming insulating materials on a substrate
    • H01L21/02225—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer
    • H01L21/02227—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a process other than a deposition process
    • H01L21/02258—Forming insulating materials on a substrate characterised by the process for the formation of the insulating layer formation by a process other than a deposition process formation by anodic treatment, e.g. anodic oxidation

Definitions

  • This invention relates to methods for manufacturing Group III nitride devices for optoelectronic and electronic applications.
  • Nitride compounds emerged in recent years as the leading III-V materials for blue to near ultraviolet optoelectronics, as well as for high power electronics. Nitride compounds exhibit an enhanced hardness and chemical inertness compared to other III-V semiconductors, which represents a unique advantage for high temperature operation and device reliability, but also introduces new processing challenges.
  • Selective oxidation is a widely used technique in the AlAs/GaAs system, as it allows, for instance, successful current confinement schemes in GaAs-based vertical cavity surface emitting lasers (VCSELs), high reflectivity distributed Bragg reflectors (DBRs) with broad stop- bands suitable for VCSEL applications, improved light confinement in planar waveguides, or insulating buffer layers for field effect transistors (FETs).
  • VCSELs vertical cavity surface emitting lasers
  • DBRs distributed Bragg reflectors
  • FETs field effect transistors
  • the selective oxidation technique consists in the insertion of pure AlAs or Al-rich AlGaAs layers in the device during the epitaxial growth, which do not affect the structural quality of the device, since AlAs is nearly lattice-matched to GaAs. Trenches or mesas are then etched in order to give access to the layers from the sidewalls.
  • AlAs layers are oxidized in wet atmosphere at ⁇ 450 °C and converted into "AlO x " oxide, while other layers are left unaffected.
  • the oxidation depth of such laterally oxidized layers can be precisely controlled.
  • the main properties of these oxidized layers are their electrically insulating behavior and their much lower refractive index (less than 1.8) as compared with AlAs ( ⁇ 2.9 at 1.4 eV, 300 K).
  • AlN, and Al-rich AlGaN alloys exhibit a significant lattice mismatch to GaN and induce tensile strain in the structure, which prevents their use as sacrificial layers for selective oxidation.
  • a method for including an oxide region in a layered structure being grown epitaxially on a substrate such as a group III-V, Si or sapphire substrate is provided which method comprises the steps of epitaxially forming a group III nitride precursor layer and selectively oxidizing the precursor layer, thereby forming the oxide region.
  • the inventive method permits selective oxidation of a group III nitride layer that is inserted as a group III nitride precursor layer such as AlInN for example into a multilayer structure during its growth. Subsequently the precursor layer is transformed selectively into a layer including oxidized regions such as AlInO x . The transformation can be achieved without noticeable change in volume. Moreover, the insertion of the precursor layer e.g. in a Group III nitride heterostructure can be performed without a degradation of structural quality. After the group III nitride precursor layer device-quality layers can be obtained by subsequent growth steps.
  • the layer including selectively oxidized regions may be used, for example, for a current confinement in group III nitride semiconductor devices. According to one preferred embodiment the selective oxidizing is controlled in depth.
  • the method includes a step of selectively dissolving the oxidized region.
  • a step of selectively dissolving the oxidized region By removing and particularly by etching oxidized regions air-gap structures such as free-standing GaAs membranes can be obtained. This permits to obtain well-defined structures including undercuts of a well-defined depth.
  • a step of annealing the oxide region is provided if the oxide is to remain in the structure. The annealing step improves the optical and electrical properties of the structure.
  • the step of selectively oxidizing the Group III precursor layer includes an electrochemical selective oxidation for example by means of an electrolytic solution.
  • this step permits to perform oxidation of the Group III precursor layer by a room temperature electrochemical process. Any additional processing such as additional illumination of the structure is not necessary.
  • this oxidation does not involve any noticeable change in volume in the structure.
  • the electro-chemical selective oxidation may include an electrolyte comprising an amino- polycarboxylic acid and in particular nitrilotriacetic acid (NTA).
  • NTA nitrilotriacetic acid
  • the electro-chemical selective oxidation process is carried out by applying a small current density to the sample. Since the process is highly selective surrounding layers are not affected.
  • the method includes adapting the electrolytic solution so that the oxide is dissolved or etched during the electrochemical oxidation step. Thereby, a direct electrolytic etching is performed which is still selective.
  • the group III nitride precursor layer comprises AlInN, in particular Al(I -x) In x N with 0.25>x>0.1, or this alloy with adjunction of Ga.
  • This composition provides layers with a lattice constant near to the lattice constant of GaN and which are moderately strained, the lattice mismatch being within [-1%....1%].
  • Some strain may however be present in the AlInN precursor layer, by adjusting its indium content, in order to improve its oxidation rate.
  • some difference in strain / indium content can be used to provide oxidation selectivity between the AlInN precursor layer and other AlInN layers possibly present in the layered structure.
  • the growth of all layers may be performed by metalorganic vapor phase epitaxy (MOVPE) or other known semiconductor growth techniques such as MBE.
  • MOVPE metalorganic vapor phase epitaxy
  • the method includes the formation of a superlat- tice and in particular of a Group III nitride superlattice.
  • a Group III nitride pre- cursor layer of AlInN can be replaced by a short period superlattice of AlInN / GaN having a layer thickness of for example 2nm.
  • the method may include according to a further preferred embodiment the insertion of a composition-graded Group III nitride layer such as a layer of (Ali.yIn y )i -x Ga x N having a varying Ga content x as a function of the layer thickness.
  • This layer can be provided at least at one of the interfaces between the Group III nitride layer and Group III nitride precursor layer.
  • the method may include a step of entirely oxidizing and dissolving or etching the Group III nitride precursor layer.
  • the Group III nitride precursor layer can serve as a sacrificial layer which after its removal permits to separate a substrate from a thin-film device such as a LED.
  • a layered semiconductor structure including a GaN compatible substrate such as a group IH-V, Si or sapphire substrate, at least one layer of Group III nitride such as GaN and a Group III nitride precursor layer including a selectively oxidized region or selectively etched region.
  • the Group III nitride precursor layer can be partially or entirely oxidized or etched.
  • a partially oxidized or etched Group III nitride precursor layer can be used in group III nitride semiconductor devices such as VCSELs and LEDs for providing a lateral current confinement.
  • An oxidized Group III nitride precursor layer can also be used in high contrast DBRs and waveguides e.g.
  • VCSELs and RCLEDs can include an oxidized DBR or a DBR having an air-gap or may additionally comprise an oxidized layer to improve current confinement.
  • an oxidized layer according to the invention can be used as a highly resistive buffer layer in HEMTs or FETs to achieve better insulating properties such as a higher electrical resistivity and to avoid leakage currents.
  • group III nitride MOEMs the inventive method can be used to form air-gap structures by oxidizing and dissolving or etching oxidized regions.
  • the layered structure can include a Group III quantum well structure such as an InGaN/GaN quantum well structure.
  • the structure includes a superlattice and in particular a short period Group III nitride precursor / Group III nitride superlattice such as a short period superlattice of AlInN / GaN.
  • the layer thickness may comprise for example 2nm.
  • the structure includes a composition-graded Group III nitride layer such as a layer of (Al 1-y In y )i -x Ga x N comprising a varying x-value over its thickness at least at one the interfaces between the Group III nitride and Group III nitride precursor layer.
  • the composition-graded layer permits to achieve a better current confinement. Furthermore, the mechanical strength of the layer including the oxide region or oxidized region can be improved.
  • the structure does not comprise a substrate.
  • This type of structure can be advantageous for instance for thin-film LEDs and can be achieved by removing a previously grown thin-film III-N structure.
  • the layered structure according to the invention may be part of a semiconductor device such as a LED (light emitting device), VCSEL (vertical cavity surface emitting laser), RCLED (resonant cavity LED), HEMT (high electron mobility transistor) or FET (field effect transistor) or a MOEM (micro-opto-electro mechanical) device or part of a waveguide or DBR.
  • a semiconductor device such as a LED (light emitting device), VCSEL (vertical cavity surface emitting laser), RCLED (resonant cavity LED), HEMT (high electron mobility transistor) or FET (field effect transistor) or a MOEM (micro-opto-electro mechanical) device or part of a waveguide or DBR.
  • Fig. 1 shows an example of a layered structure including the semiconductor structure according to one embodiment of the present invention
  • Fig. 2 shows a schematic view of the oxidation setup according to one embodiment
  • Fig. 3a-f show examples of oxidized and non-oxidized LED structures according to one embodiment of the invention; wherein (a) and (b) are taken after the oxidation step, (c) and (d) are taken at the end of the processing, and (e) and (f) with an operating current of about 0.5 mA; and Fig. 4 shows I-V characteristics of a non-oxidized LED, a fully oxidized LED and a
  • FIG. 1 shows a schematic cross section of the structure used according to one embodiment.
  • a 50 nm thick Al O g 2 In O i 8 N layer 1 is introduced above the «-doped region 3 of an InGaN/GaN quantum well (QW) based LED grown on a 2 inch sapphire substrate 5.
  • QW quantum well
  • the AlInN layer 1 is non intentionally doped but its donor concentration measured by C-V analysis is estimated to 10 18 cm "3 due to high residual doping.
  • the «-doped GaN region 3 is 2 ⁇ m thick with a Si doping level of ⁇ 3 xlO 18 cm “3
  • the p-doped GaN region 7 is 200 nm thick with a hole concentration of ⁇ 2 ⁇ l ⁇ 17 cm “3 after a Mg activation thermal treatment of 10 minutes at 800 0 C under N 2 atmosphere.
  • the active region features a single 3 nm thick InGaN QW 9, grown 10 nm above the AlInN layer 1, without any AlGaN electron blocking layer. This basic design was chosen to limit the amount of tensile strain generated in the cap layer, which could otherwise induce strain issues during the oxidation process and possibly damage the device.
  • a 400 nm SiO 2 mask 11 is deposited on the wafer by standard photolithography techniques and square mesas of various sizes (28-400 ⁇ m) are etched by Cl 2 / Ar reactive ion etching to give access to the n-GaN bottom layer 3 and the AlInN layer 1 side- wall.
  • the SiO 2 cap layer 1 1 can be removed or possibly kept in place to protect the top surface of the mesa and avoid oxidation of the AlInN layer 1 from the surface through defects.
  • a small piece of indium 13 is alloyed to the tt-GaN layer 3.
  • the preparation of the electrolyte 23 is done by adjusting a 0.1 M solution of ni- trilotriacetic acid (NTA) by addition of potassium hydroxyde to a pH-value between 8 and 12 depending on the layer stack and the respecting doping levels.
  • Another way of preparation of the electrolyte 23 is to dissolute nitrilotriacetic acid (NTA) in a 0.3 M solution of potassium hydroxide in order to reach a pH value of 8.5.
  • the anodic oxidation is performed at room temperature without illumination in a small beaker 15 fitted with a platinum cathode 17, as described in Fig. 2.
  • Typical oxidation rates of the AlInN layer 1 vary between 5 and 20 ⁇ m per hour and depend on the applied current density, composition/strain of the AlInN layer, crystalline quality and doping levels in the structure. The process is highly selective and the surrounding GaN layers 3, are kept unaffected. Besides, no significant change in the volume of the oxidized AlInN layer 1 was detected.
  • the oxidation extends laterally 22 ⁇ m inside the structure, leaving 70 ⁇ m ⁇ 70 ⁇ m mesas with a 26 ⁇ m ⁇ 26 ⁇ m square opening, whereas 28 ⁇ m ⁇ 28 ⁇ m mesas are fully oxidized.
  • the oxidation time was reduced and, as a result, the oxidation front extends only 10.5 ⁇ m within the structure leaving a small circular aperture at the center of 28 ⁇ m ⁇ 28 ⁇ m mesas (see Fig. 3).
  • the aperture diameter is about 5-7 ⁇ m which is the diameter needed for current confinement in nitride VCSELs.
  • the third area was left non-oxidized for the sake of comparison.
  • the processing of the LEDs involved the following steps: removal of both the indium contact 13 and the protective SiO 2 cap layer 1 1 by wet etching in a HF solution, deposition of a Ti/Al/Ni/Au (10/300/10/200 nm) ⁇ -contact (not shown) by electron beam evaporation and of a semitransparent Ni/ Au (10/10 nm) />-contact (not shown) that is further oxidized during 5 min at 550 °C in an O 2 atmosphere.
  • An insulating SiO 2 dielectric layer (not shown) is deposited on the layer and eventually opened to deposit large Ti/Au (10/300 nm) contact pads.
  • Figure 3 shows a comparison of optical microscope images of an oxidized 28 ⁇ m ⁇ 28 ⁇ m LED device and a nonoxidized one at different stages of the process. Images (a) and (b) are taken just after the oxidation step, (c) and (d) at the end of the processing, while (e) and (f) show the LEDs operating with an injected current of about 0.5 mA.
  • the effect of the oxidation aperture is clearly seen on these pictures: for the oxidized device, carriers are confined within the 5 ⁇ m aperture and light is emitted only in this area (guided light is also noticed on the mesa sides). On the other hand, light is emitted over the full mesa for the nonoxidized device due to lateral diffusion of the carriers below the top contact. The insulating behavior of the oxide layer is also observed on the electrical characteristics of the devices. The reduction of the current aperture leads to an increase in the differential resistance of the LEDs.
  • the anodically formed oxide material is insulating and permits to provide a current confinement scheme in a LED device. This technique is useful for several applications in nitride optoelectronics and electronics.

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Abstract

A method for including an oxide region in a layered structure being grown epitaxially on a substrate, comprising the steps of epitaxially forming a Group III-nitride precursor layer, and selectively oxidizing the precursor layer, thereby forming the oxide region.

Description

Selective oxidation and selective etching of AHnN layers for manufacturing Group HI nitride semiconductur devices
This invention relates to methods for manufacturing Group III nitride devices for optoelectronic and electronic applications.
Group III nitride semiconductors emerged in recent years as the leading III-V materials for blue to near ultraviolet optoelectronics, as well as for high power electronics. Nitride compounds exhibit an enhanced hardness and chemical inertness compared to other III-V semiconductors, which represents a unique advantage for high temperature operation and device reliability, but also introduces new processing challenges.
In particular, a significant number of well established processing methods for classical III-V compounds do not apply to group-Ill nitride semiconductors, which makes the fabrication of advanced devices technically harder. This is especially the case for selective oxidation.
Selective oxidation is a widely used technique in the AlAs/GaAs system, as it allows, for instance, successful current confinement schemes in GaAs-based vertical cavity surface emitting lasers (VCSELs), high reflectivity distributed Bragg reflectors (DBRs) with broad stop- bands suitable for VCSEL applications, improved light confinement in planar waveguides, or insulating buffer layers for field effect transistors (FETs). This technique is described for example in the article "Oxidation kinetics of AlAs and (AlGa)As layers in GaAs-based diode laser structures: comparative analysis of available experimental data" by W. Nakwaski et al., Semicond. Sci. Technol. 19, 333 (2004).
Basically, the selective oxidation technique consists in the insertion of pure AlAs or Al-rich AlGaAs layers in the device during the epitaxial growth, which do not affect the structural quality of the device, since AlAs is nearly lattice-matched to GaAs. Trenches or mesas are then etched in order to give access to the layers from the sidewalls. AlAs layers are oxidized in wet atmosphere at ~ 450 °C and converted into "AlOx" oxide, while other layers are left unaffected. The oxidation depth of such laterally oxidized layers can be precisely controlled. The main properties of these oxidized layers are their electrically insulating behavior and their much lower refractive index (less than 1.8) as compared with AlAs (~2.9 at 1.4 eV, 300 K).
Among Group III— nitrides, AlN, and Al-rich AlGaN alloys exhibit a significant lattice mismatch to GaN and induce tensile strain in the structure, which prevents their use as sacrificial layers for selective oxidation.
It is the object of the present invention to provide a method that allows selective oxidation of group III nitride layers in a semiconductor structure as well as a corresponding semiconductor structure including an oxidized region. This object is achieved by a method according to claim 1 and a layered semiconductor structure according to claim 8. Preferred embodiments of the invention are defined in the dependent claims.
According to the invention a method for including an oxide region in a layered structure being grown epitaxially on a substrate such as a group III-V, Si or sapphire substrate is provided which method comprises the steps of epitaxially forming a group III nitride precursor layer and selectively oxidizing the precursor layer, thereby forming the oxide region.
The inventive method permits selective oxidation of a group III nitride layer that is inserted as a group III nitride precursor layer such as AlInN for example into a multilayer structure during its growth. Subsequently the precursor layer is transformed selectively into a layer including oxidized regions such as AlInOx. The transformation can be achieved without noticeable change in volume. Moreover, the insertion of the precursor layer e.g. in a Group III nitride heterostructure can be performed without a degradation of structural quality. After the group III nitride precursor layer device-quality layers can be obtained by subsequent growth steps. The layer including selectively oxidized regions may be used, for example, for a current confinement in group III nitride semiconductor devices. According to one preferred embodiment the selective oxidizing is controlled in depth.
According to another embodiment the method includes a step of selectively dissolving the oxidized region. By removing and particularly by etching oxidized regions air-gap structures such as free-standing GaAs membranes can be obtained. This permits to obtain well-defined structures including undercuts of a well-defined depth. According to a further preferred embodiment, a step of annealing the oxide region is provided if the oxide is to remain in the structure. The annealing step improves the optical and electrical properties of the structure.
According to yet another embodiment, the step of selectively oxidizing the Group III precursor layer includes an electrochemical selective oxidation for example by means of an electrolytic solution. Advantageously, this step permits to perform oxidation of the Group III precursor layer by a room temperature electrochemical process. Any additional processing such as additional illumination of the structure is not necessary. Advantageously, this oxidation does not involve any noticeable change in volume in the structure.
The electro-chemical selective oxidation may include an electrolyte comprising an amino- polycarboxylic acid and in particular nitrilotriacetic acid (NTA). The electro-chemical selective oxidation process is carried out by applying a small current density to the sample. Since the process is highly selective surrounding layers are not affected.
According to another preferred embodiment the method includes adapting the electrolytic solution so that the oxide is dissolved or etched during the electrochemical oxidation step. Thereby, a direct electrolytic etching is performed which is still selective.
Preferably, the group III nitride precursor layer comprises AlInN, in particular Al(I -x) In x N with 0.25>x>0.1, or this alloy with adjunction of Ga. This composition provides layers with a lattice constant near to the lattice constant of GaN and which are moderately strained, the lattice mismatch being within [-1%....1%]. This permits the growth of the group III nitride precursor layer without structural degradation due to strain. Some strain may however be present in the AlInN precursor layer, by adjusting its indium content, in order to improve its oxidation rate. Furthermore, some difference in strain / indium content can be used to provide oxidation selectivity between the AlInN precursor layer and other AlInN layers possibly present in the layered structure. The growth of all layers may be performed by metalorganic vapor phase epitaxy (MOVPE) or other known semiconductor growth techniques such as MBE.
According to a further preferred embodiment the method includes the formation of a superlat- tice and in particular of a Group III nitride superlattice. For example, a Group III nitride pre- cursor layer of AlInN can be replaced by a short period superlattice of AlInN / GaN having a layer thickness of for example 2nm.
In order to improve the mechanical strength and electrical characteristics of the structure, the method may include according to a further preferred embodiment the insertion of a composition-graded Group III nitride layer such as a layer of (Ali.yIny)i-xGaxN having a varying Ga content x as a function of the layer thickness. This layer can be provided at least at one of the interfaces between the Group III nitride layer and Group III nitride precursor layer.
According to yet another embodiment it is provided that the method may include a step of entirely oxidizing and dissolving or etching the Group III nitride precursor layer. In this case the Group III nitride precursor layer can serve as a sacrificial layer which after its removal permits to separate a substrate from a thin-film device such as a LED.
According to the invention, also a layered semiconductor structure is provided, including a GaN compatible substrate such as a group IH-V, Si or sapphire substrate, at least one layer of Group III nitride such as GaN and a Group III nitride precursor layer including a selectively oxidized region or selectively etched region. The Group III nitride precursor layer can be partially or entirely oxidized or etched. A partially oxidized or etched Group III nitride precursor layer can be used in group III nitride semiconductor devices such as VCSELs and LEDs for providing a lateral current confinement. An oxidized Group III nitride precursor layer can also be used in high contrast DBRs and waveguides e.g. for a better light confinement due to an increased refractive index contrast. VCSELs and RCLEDs can include an oxidized DBR or a DBR having an air-gap or may additionally comprise an oxidized layer to improve current confinement. Furthermore, an oxidized layer according to the invention can be used as a highly resistive buffer layer in HEMTs or FETs to achieve better insulating properties such as a higher electrical resistivity and to avoid leakage currents. In group III nitride MOEMs the inventive method can be used to form air-gap structures by oxidizing and dissolving or etching oxidized regions.
According to a preferred embodiment, the layered structure can include a Group III quantum well structure such as an InGaN/GaN quantum well structure. According to a further preferred embodiment the structure includes a superlattice and in particular a short period Group III nitride precursor / Group III nitride superlattice such as a short period superlattice of AlInN / GaN. The layer thickness may comprise for example 2nm.
According to still another embodiment the structure includes a composition-graded Group III nitride layer such as a layer of (Al1-yIny)i-xGaxN comprising a varying x-value over its thickness at least at one the interfaces between the Group III nitride and Group III nitride precursor layer. The composition-graded layer permits to achieve a better current confinement. Furthermore, the mechanical strength of the layer including the oxide region or oxidized region can be improved.
According to still another embodiment the structure does not comprise a substrate. This type of structure can be advantageous for instance for thin-film LEDs and can be achieved by removing a previously grown thin-film III-N structure.
The layered structure according to the invention may be part of a semiconductor device such as a LED (light emitting device), VCSEL (vertical cavity surface emitting laser), RCLED (resonant cavity LED), HEMT (high electron mobility transistor) or FET (field effect transistor) or a MOEM (micro-opto-electro mechanical) device or part of a waveguide or DBR.
Further objects, characteristics and advantages of the present invention will become apparent from the following description taken in conjunction with the appended drawings in which:
Fig. 1 shows an example of a layered structure including the semiconductor structure according to one embodiment of the present invention;
Fig. 2 shows a schematic view of the oxidation setup according to one embodiment;
Fig. 3a-f show examples of oxidized and non-oxidized LED structures according to one embodiment of the invention; wherein (a) and (b) are taken after the oxidation step, (c) and (d) are taken at the end of the processing, and (e) and (f) with an operating current of about 0.5 mA; and Fig. 4 shows I-V characteristics of a non-oxidized LED, a fully oxidized LED and a
LED with a 5 μm diameter aperture. In the following the invention will be described by means of an exemplary embodiment of a method of fabricating a layered semiconductor structure according to the present invention.
The fabrication method includes the insertion during the MOVPE growth of an AlInN inter- layer nearly lattice matched to GaN, which will be subsequently oxidized. Figure 1 shows a schematic cross section of the structure used according to one embodiment. A 50 nm thick AlO g2InO i8N layer 1 is introduced above the «-doped region 3 of an InGaN/GaN quantum well (QW) based LED grown on a 2 inch sapphire substrate 5. The AlInN layer 1 is non intentionally doped but its donor concentration measured by C-V analysis is estimated to 1018 cm"3 due to high residual doping. The «-doped GaN region 3 is 2 μm thick with a Si doping level of ~3 xlO18 cm"3, and the p-doped GaN region 7 is 200 nm thick with a hole concentration of ~2 χlθ17cm"3 after a Mg activation thermal treatment of 10 minutes at 800 0C under N2 atmosphere. The active region features a single 3 nm thick InGaN QW 9, grown 10 nm above the AlInN layer 1, without any AlGaN electron blocking layer. This basic design was chosen to limit the amount of tensile strain generated in the cap layer, which could otherwise induce strain issues during the oxidation process and possibly damage the device.
After the growth step, a 400 nm SiO2 mask 11 is deposited on the wafer by standard photolithography techniques and square mesas of various sizes (28-400 μm) are etched by Cl2/ Ar reactive ion etching to give access to the n-GaN bottom layer 3 and the AlInN layer 1 side- wall. The SiO2 cap layer 1 1 can be removed or possibly kept in place to protect the top surface of the mesa and avoid oxidation of the AlInN layer 1 from the surface through defects. For current access during the anodic oxidation process, a small piece of indium 13 is alloyed to the tt-GaN layer 3.
Preferably the preparation of the electrolyte 23 is done by adjusting a 0.1 M solution of ni- trilotriacetic acid (NTA) by addition of potassium hydroxyde to a pH-value between 8 and 12 depending on the layer stack and the respecting doping levels. Another way of preparation of the electrolyte 23 is to dissolute nitrilotriacetic acid (NTA) in a 0.3 M solution of potassium hydroxide in order to reach a pH value of 8.5. The anodic oxidation is performed at room temperature without illumination in a small beaker 15 fitted with a platinum cathode 17, as described in Fig. 2. During the oxidation process an effective current density between 2 and 20 mA/cm2 is applied from a current source 19 to the sample 25 and the resulting voltage increase is recorded by a recorder 21. In the ideal case an immediate threshold voltage of ~3 V is established, followed by a smooth linear voltage rise over time, the latter resulting from the progressive increase in the length of the formed oxide. Typical oxidation rates of the AlInN layer 1 vary between 5 and 20 μm per hour and depend on the applied current density, composition/strain of the AlInN layer, crystalline quality and doping levels in the structure. The process is highly selective and the surrounding GaN layers 3, are kept unaffected. Besides, no significant change in the volume of the oxidized AlInN layer 1 was detected.
In the first area investigated, the oxidation extends laterally 22 μm inside the structure, leaving 70 μm χ70 μm mesas with a 26 μm χ26 μm square opening, whereas 28 μm χ28 μm mesas are fully oxidized. In the second area, the oxidation time was reduced and, as a result, the oxidation front extends only 10.5 μm within the structure leaving a small circular aperture at the center of 28 μm χ28 μm mesas (see Fig. 3). The aperture diameter is about 5-7 μm which is the diameter needed for current confinement in nitride VCSELs. Finally, the third area was left non-oxidized for the sake of comparison. The processing of the LEDs involved the following steps: removal of both the indium contact 13 and the protective SiO2 cap layer 1 1 by wet etching in a HF solution, deposition of a Ti/Al/Ni/Au (10/300/10/200 nm) ^-contact (not shown) by electron beam evaporation and of a semitransparent Ni/ Au (10/10 nm) />-contact (not shown) that is further oxidized during 5 min at 550 °C in an O2 atmosphere. An insulating SiO2 dielectric layer (not shown) is deposited on the layer and eventually opened to deposit large Ti/Au (10/300 nm) contact pads.
Figure 3 shows a comparison of optical microscope images of an oxidized 28 μm χ28 μm LED device and a nonoxidized one at different stages of the process. Images (a) and (b) are taken just after the oxidation step, (c) and (d) at the end of the processing, while (e) and (f) show the LEDs operating with an injected current of about 0.5 mA. The effect of the oxidation aperture is clearly seen on these pictures: for the oxidized device, carriers are confined within the 5 μm aperture and light is emitted only in this area (guided light is also noticed on the mesa sides). On the other hand, light is emitted over the full mesa for the nonoxidized device due to lateral diffusion of the carriers below the top contact. The insulating behavior of the oxide layer is also observed on the electrical characteristics of the devices. The reduction of the current aperture leads to an increase in the differential resistance of the LEDs.
This effect is shown on Fig. 4, which compares in curves 27, 29 and 31 the I— V characteristics of a nonoxidized LED (curve 27), an oxidized LED (curve 29) with a 5 μm diameter aperture and a fully oxidized LED (curve 31). A dramatic increase of the differential resistance from curve 27 to curve 31 with decreasing current aperture is noticed. However, it should be noted that a small current is still flowing through the completely oxidized device under a high applied voltage (curve 31).
In summary, a technique which allows for selective oxidation of lattice-matched AlInN layers is provided. The anodically formed oxide material is insulating and permits to provide a current confinement scheme in a LED device. This technique is useful for several applications in nitride optoelectronics and electronics.
It will be clear to the person skilled in the art that the above embodiment may be altered in many ways without departing from the scope of the invention. The features disclosed in the description, the claims and the drawings can be of importance for the present invention either alone or in any combination thereof.

Claims

1. A method for including an oxide region in a layered structure being grown epitaxially on a substrate, comprising the steps of:
(i) epitaxially forming a Group Ill-nitride precursor layer (1), and
(ii) selectively oxidizing the precursor layer, (1) thereby forming the oxide region.
2. The method of claim 1, wherein oxidizing is controlled in depth.
3. The method of claim 1 or 2, further comprising selectively dissolving the oxide region so as to form an air-gap structure.
4. The method of one of the preceding claims, further comprising annealing the oxide region.
5. The method of one of the preceding claims, wherein step (ii) comprises electrochemical selective oxidation.
6. The method of one of the preceding claims, wherein the Group Ill-nitride precursor layer (1) comprises AlInN, in particular Al(I -x) In x N with 0.25>x>0.1, or this alloy with adjunction of Ga.
7. The method one of the preceding claims, wherein the Group Ill-nitride precursor layer (1) is moderately strained, with a lattice mismatch within [-1%....1%].
8. A layered semiconductor structure including a substrate (5); at least one layer of Group III nitride (3, 7); and a Group Ill-nitride precursor layer (1) comprising an oxidized or etched region.
9. Layered structure of claim 8, further including a short period Group III nitride precursor / Group III nitride superlattice.
10. Layered structure of claim 8 or 9, further including a composition-graded Group III nitride layer and in particular a layer of (Ali.yIny)1-xGaxN at least at one the interfaces between the Group III nitride (3, 7) and Group III nitride precursor layers (1).
11. Semiconductor device, including a layered structure of one of claims 8 to 10.
PCT/EP2005/013966 2004-12-24 2005-12-23 Selective oxidation and selective etching of allnn layers for manufacturing group iii nitride semiconductor devices WO2006066962A2 (en)

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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2454655A (en) * 2007-11-09 2009-05-20 Sharp Kk Nitride structures with AlInN current confinement layers

Citations (2)

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Publication number Priority date Publication date Assignee Title
EP1235279A2 (en) * 2001-02-27 2002-08-28 Matsushita Electric Industrial Co., Ltd. Semiconductor device using nitride compound and method for fabricating the same
US20040188693A1 (en) * 2003-03-25 2004-09-30 Matsushita Electric Industrial Co., Ltd. Semiconductor device and method for fabricating the same

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1235279A2 (en) * 2001-02-27 2002-08-28 Matsushita Electric Industrial Co., Ltd. Semiconductor device using nitride compound and method for fabricating the same
US20040188693A1 (en) * 2003-03-25 2004-09-30 Matsushita Electric Industrial Co., Ltd. Semiconductor device and method for fabricating the same

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2454655A (en) * 2007-11-09 2009-05-20 Sharp Kk Nitride structures with AlInN current confinement layers

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