GB2424313A - Ohmic contacts for n-type AlInGaN - Google Patents

Ohmic contacts for n-type AlInGaN Download PDF

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GB2424313A
GB2424313A GB0505360A GB0505360A GB2424313A GB 2424313 A GB2424313 A GB 2424313A GB 0505360 A GB0505360 A GB 0505360A GB 0505360 A GB0505360 A GB 0505360A GB 2424313 A GB2424313 A GB 2424313A
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layer
semiconductor
contact
refractory metal
molybdenum
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Valerie Bousquet
Stuart Hooper
Matthias Kauer
Christoph Zellweger
Jonathan Hefferman
Katherine Johnson
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Sharp Corp
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Sharp Corp
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    • 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
    • H01L21/28—Manufacture of electrodes on semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/268
    • H01L21/283—Deposition of conductive or insulating materials for electrodes conducting electric current
    • H01L21/285—Deposition of conductive or insulating materials for electrodes conducting electric current from a gas or vapour, e.g. condensation
    • H01L21/28506—Deposition of conductive or insulating materials for electrodes conducting electric current from a gas or vapour, e.g. condensation of conductive layers
    • H01L21/28575—Deposition of conductive or insulating materials for electrodes conducting electric current from a gas or vapour, e.g. condensation of conductive layers on semiconductor bodies comprising AIIIBV compounds
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
    • H01L29/40—Electrodes ; Multistep manufacturing processes therefor
    • H01L29/43—Electrodes ; Multistep manufacturing processes therefor characterised by the materials of which they are formed
    • H01L29/45—Ohmic electrodes
    • H01L29/452—Ohmic electrodes on AIII-BV compounds
    • 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
    • H01L33/02—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 characterised by the semiconductor bodies
    • H01L33/26—Materials of the light emitting region
    • H01L33/30—Materials of the light emitting region containing only elements of Group III and Group V of the Periodic Table
    • H01L33/32—Materials of the light emitting region containing only elements of Group III and Group V of the Periodic Table containing nitrogen
    • 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
    • H01L33/36—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 characterised by the electrodes
    • H01L33/40—Materials therefor
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S2304/00—Special growth methods for semiconductor lasers
    • H01S2304/02—MBE
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/04—Processes or apparatus for excitation, e.g. pumping, e.g. by electron beams
    • H01S5/042—Electrical excitation ; Circuits therefor
    • H01S5/0425—Electrodes, e.g. characterised by the structure
    • H01S5/04252—Electrodes, e.g. characterised by the structure characterised by the material
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S5/00—Semiconductor lasers
    • H01S5/30—Structure or shape of the active region; Materials used for the active region
    • H01S5/32—Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures
    • H01S5/323—Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser
    • H01S5/32308—Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser emitting light at a wavelength less than 900 nm
    • H01S5/32341—Structure or shape of the active region; Materials used for the active region comprising PN junctions, e.g. hetero- or double- heterostructures in AIIIBV compounds, e.g. AlGaAs-laser, InP-based laser emitting light at a wavelength less than 900 nm blue laser based on GaN or GaP

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  • General Physics & Mathematics (AREA)
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Abstract

Refractory metal ohmic contacts to n-type AlInGaN semiconductor materials are formed by depositing a refractory metal layer and annealing at a temperature of / 800{C and at a pressure of & 10<-3> mbarr for a period of time greater than one hour. The refractory metals Mo, Ta, W, Re, Rh, Ru, Os, Ir, Nb, Hf and V are used.

Description

Forming a Contact, and Manufacture of a Semiconductor Device The present
invention relates to a method of forming a contact on a semiconductor waler and in particular to forming a contact on a semiconductor wafer in a nitride materials system such as, fbr example, the (Al,Ga,In)N materials system. The invention also relates to a method of manufacturing a semiconductor device that incorporates the method of forming a contact; the invention may be applied to growth of for example, a transistor, or a semiconductor light-emitting device such as a semiconductor laser diode (LD) or a light-emitting diode.
The (Al,Ga,In)N material system includes materials having the general formula AlGa1niN where 0 = x = 1, 0 = y = I, and x + y = 1. In this application, a member of the (Al,Ga,In)N material system that has nonzero mole fractions of aluminium, gallium and indium will be referred to as AIGaInN, a member that has a zero aluminium mole fraction but that has non-zero mole fractions of gallium and indium will he referred to as InGaN, a member that has a zero indium mole fraction but that has nonzero mole fractions of gallium and aluminium will he referred to as A1GaN, and so on.
There is currently considerable interest in fabricating semiconductor light-emitting devices in the (Al,Ga,In)N material system since devices fabricated in this system can emit light in the blue-violet wavelength range of the spectrum (corresponding to wavelengths in the range of approximately 380-450nm).
Semiconductor light-emitting devices fabricated in the (Al,Ga,In)N materials system are described, for example, by S.Nakamura et al in "Jap. J. Appi. Phys." Vol. 35, ppL74- L76 (1996). They arc also described in US patent No. 5 777 350, which teaches use of the metal-organic chemical vapour deposition (MOCVD) growth technique to fabricate light-emitting devices in the (Al,Ga,ln)N materials system. MOCVD (also known as metal-organic vapour phase epitaxy or MOVPE) takes place in an apparatus which is commonly at atmospheric pressure but sometimes at a slightly reduced pressure of typically about 10 kPa. Ammonia and the species providing one or more Group III elements to be used in epitaxial growth are supplied substantially parallel to the surface of a substrate upon which epitaxial growth is to take place, thus forming a boundary layer adjacent to and flowing across the substrate surface. It is in this gaseous boundary layer that decomposition to form nitrogen and the oilier elements to be epitaxially deposited takes place so that the epitaxial growth is driven by gas phase equilibria.
Another known semiconductor growth technique is molecular beam epitaxy (MBE). In contrast to MOCVD, MBE is carried out in a high vacuum environment. In the case of MBE as applied to the (Al,Jn,Ga)N system, an ultra-high vacuum (UHV) environment, typically around 1 x lO Pa, is used. A nitrogen precursor is supplied to the MBE chamber by means of a supply conduit and species providing aluminium, gallium and/or indium, and possibly also a suitable dopant species, are supplied from appropriate sources within heated effusion cells titled with controllable shutters to control the amounts of the species supplied into the MBE chamber dLlring the epitaxial growth period. The shutter-control outlets from the effusion cells and the nitrogen supply conduit face the surface of the substrate upon which epitaxial growth is to take place.
The nitrogen precursor and the species supplied from the effusion cells travel across the MBE chamber and reach the substrate where epitaxial growth takes place in a manner which is driven by the deposition kinetics.
Figure 1 is a schematic illustration of a semiconductor light-emitting device, in this case a semiconductor laser device. The device comprises a substrate 1, and a semiconductor layer structure 2 grown over a first surface of the substrate 1. In the example of figure 1, the semiconductor layer structure 2 comprises a first cladding region 3, an active region for light emission 4, and a second cladding region 5. The first cladding region 3 and the second cladding region 5 are generally of different conductivity types to one another, and in figure 5 the first cladding region 3 is shown as n-type and the second cladding region 5 is shown as p-type. The substrate is also shown as n-type in figure 1.
The cladding regions 3,5 and the active region 4 are each represented as a single layer in figure 1, for simplicity of description. In general, however, one or more of the cladding regions 3,5 and the active region 4 may be constituted of a plurality of semiconductor layers. When a laser diode having the genera] structure of figure 1 is fabricated in the (Al,Ga,ln)N materials system, the cladding regions 3,5 and the active region 4 may each be formed of one or more (Al,Ga,In)N layers.
The layer structure 2 may comprise more regions than the cladding regions 3,5 and the active region 4. For example, optical guiding regions may be provided to produce a laser of the well-known "separate confinement helerostructure" type. a buffer layer may be grown between the substrate 1 and the first cladding laycr 3, and/or a cap layer niay be grown over the upper surface of the second cladding layer 6.
In order to allow an electrical current to be passed through the active region 4 to enable the generation of light, the laser diode of figure 1 is provided with a first contact 6 on the n-side of the active region 4 and a second contact 7 on the p-side of the active region 4. The first contact 6 is known as an "n-contact" and the second contact 7 is known as an "p-contact" since they are positioned on the n-side and the p-side of the active region - respectively. In the laser (bode of figure 1, the n-contact is provided on the opposite surface of the substrate 1 to the surface on which the layer structure 2 is provided (the surface of the substrate on which the n-contact 6 is provided is generally referred to as the "back side" of the substrate. The p-contact 7 is provided on the upper surface of the second cladding layer 5.
In a "ridge" laser in which the upper layers of the layer structure 2 are reduced in lateral width to define a "ridge", as shown in figure 1, the pcontact 7 has a smaller area than the n-contact 6. Conductive bonding pads 8 that are in electrical contact with the p- contact 7 are therefore provided so that an external wire can be connected to a bonding pad 8 rather than directly to the p-contact 7.
It is important that the n- and p-contacts provide good electrical contact to the substrate or to the upper cladding layer. If there is a high electrical resistance between the n- contact and the substrate, or between the p-contact and the second cladding layer, it will require a high applied voltage to drive the device with a current that is sufficiently high to allow laser oscillation to take place. Furthermore, a high resistance at one or both contacts will lead to excessive generation of heat during operation olihe device.
A contact to a semiconductor layer is generally formed by depositing a metal or alloy of metals on the semiconductor layer and subsequently annealing the metal or alloy. The standard method of forming a contact to an (Al,Ga,ln)N layer is to deposjt an alloy of titanium and aluminium on the (Al,Ga,ln)N layer followed by annealing. This produces a contact having good, ohmic characteristics. Where a titanium/aluminium contact is provided as the n-type contact oii a device such as the device shown in figure 1, the titanium and aluminium are generally deposited in a separate step after growth of the semiconductor layers.
In MBE growth of nitride semiconductors over a transparent substrate such as sapphire, SiC or GaN, it is known to provide a layer of molybdenum, typically a few hundred nm thick, on the back face of the substrate. The molybdenum layer extends over substantially the entire area of the substrate, and is used to heat the substrate during the MBE growth of layers over the substrate; heat from an external source is transmitted through the molybdenum layer by conduction, and so is distributed over the area of the substrate to evenly heat the substrate. As an example, R. C Powell et al. describe, in AppI. Phys.Lett. 60, p. 2505 (1992), that the deposition of a 0.5p.m thick layer of molybdenum on the back surface of a sapphire substrate before the MBE growth of GaN over the substrate improves the growth temperature of the GaN. Although it might appear at first sight that such a molybdenum heating layer could form the n-type contact to the GaN substrate of a device such as that shown in figure 1, the electrical characteristics of the molybdenum layer have been much worse than the characteristics of a titanium/aluminium contact, and in particular have been highly non-ohmic, and it has not been possible to use the molybdenum layer as the n-contact. It is therefore usual to remove the molybdenum layer once the MBE growth process is complete and apply a titanium/aluminium contact to the back face of the substrate. Furthermore, many of the reports of use of a molybdenum heating layer relate to MBE growth of riitrides over a sapphire substrate. A sapphire substrate is electrically insulating, and it is not Possible to deposit the n-contact on the back face of a sapphire substrate; if a sapphire substrate is used, the n-type contact must he provided on part of the front face of the substrate.
Molybdenum is not deposited over the back face of a GAN substrate in the growth of an (ln,Ga,AI)N layer structure over a GaN substrate by MOCVD. The substrate is heated in a different way in MOCVD growth compared to MI3E growth, and there is no need to apply the molybdenum layer to conduct heat over the substrate.
it is expected that a molybdenum layer on a GaN substrate would exhibit non-ohmic behaviour, since molybdenum has a workfunction that is significantly higher than the workfunction of titanium or other metals commonly used as n-type contacts. It is known that metals with a high workfunction make rectifying contacts ("Schottky contacts") to an n-type semiconductor layer.
E.C. Piquette et al. disclose, in Mat. Res. Soc. Synip. Proc. Vol 482, p1089 (1998), a study on the use of different metals to form a contact to an n-type GaN layer.
Molybdenum is one of the metals included in the study, and they describe the use of a - I SOnm thick molybdenum layer as a contact to an n-type GaN layer. They anneal the molybdenum layer at 700 C for 30s under an atmosphere of argon in a rapid thermal annealing (RTA) oven. The study concludes that, for a contact to n-type GaN, a molybdenum contact has worse electrical properties than a conventional Ti/Al contact.
There have been reports on the use of molybdenum as a contact to other materials. As an example, S.S.Cohen et al. disclose, in AppI Phys. Lctt. Vol. 46, p. 657 (1985), making a contact to a silicon layer by depositing molybdenum (Mo) on the surface of the silicon layer, and annealing the molybdenum at 700 C. K.L. Moazed et al. disclose, in J Appl. Phys. Vol. 68, p. 2246 (1990), making a contact to a boron-doped diamond substrate by depositing a molybdenum layer having a thickness of l0-25nrn and annealing at 950 C for 8-10 minutes; they report that this annealing time gives the lowest contact resistance and that the contact resistance increases if the annealing lime is made greater than 10 minutes. However, neither of these reports specifically addresses making a contact to a nitride semiconductor layer such as an (Al,Ga,ln)N layer.
US patent No. 6 281 526 discloses forming a contact to an InAIGaN layer by disposing a molybdenum layer over the InAIGaN layer, subjecting the molybdenum layer to ion implantation, and annealing the molybdenum layer to activate the implanted impurities.
The need to carry out the ion implantation increases the processing time required to deposit the contact.
US patent No. 6 521 998 describes the use of metal-nitride electrode to form a Schottky contact to a Ill-V nitride semiconductor layer such as GaN. The metal nitride is deposited by, for example, sputtering nitrogen at a metallic target, and has a thickness of 10-200nm. The resultant contact is thermally stable and is not affected by armealing at 500-800 C. The metal of the metal-nitride electrode is selected from group IVa metals such as titanium and zirconium, Va group metals such as vanadium, niobium and tantalum, from VIa group metals such as chromium, molybdenum and tungsten, or from alloys thereof.
US patent 6 521 998 also describes, as a comparative example, deposition of a titanium film onto a GaN layer. After annealing at 500 C for 10 minutes, a mixed GaTiN layer is formed between the titanium film and the GaN layer, and an ohmic contact to the GaN layer is formed.
A first aspect of the present invention provides a method of forming a contact on a nitride semiconductor, the method comprising the steps of: a) depositing a layer containing a refractory metal over a surface of a nitride semiconductor; and b) annealing the layer containing the refractory metal at a temperature of at least 800 C at a pressure of 1 x l03rnbarr or below.
The method of the invention uses a higher annealing temperature than the prior art methods of making contacts to nitride semiconductor layers. (Although MOCVD growth over a GaN substrate has been carried out at high temperatures, a molybdenum heating layer is not provided on the (iaN substrate in M()CVD growth, as stated above.) It has been found that, presumably as a result of the higher annealing temperature, the layer of refractory metal forms a good, substantially ohmic contact to the nitride semiconductor layer. The electrical characteristics of a contact obtained by the method of the invention are cotnparablc to the electrical characteristics of a conventional Ti/Al contact. The invention thus allows a good quality n-type contact to be obtained using a layer of refractory metal and without carrying out an ion implantation step, SO that the time required to form a contact by a method of the invention is significantly shorter than the time required to form a contact by the method of US patent No. 6 281526.
The layer of refractory metal may be used as an electrode without any additional processing steps. An external lead may be attached to the layer of refractory metal, for example using a conductive paste such as silver paint or a conductive resin.
The term "refractory metal" as used herein denotes a metal melting at a temperature above 1 850 C, namely one of tungsten (chemical symbol W), rhenium (Re), osmium - (Os), tantalum (Ta), molybdenum (Mo), iridium (Ir), niobium (Nb), ruthenium (Ru), hafnium (Hf), rhodium (Rh) and vanadium (V). The refractory metals are generally considered to be metals which have higher melting points than the common alloying bases iron, nickel and zinc, and one common interpretation of the term "refractory metals" is that it includes all metals having a melting point that is higher than the melting point of titanium (melting point 1660 C) and zirconium (1850 C).
Step (b) may be carried out at a temperature of at least 900 C.
Step (b) may have a duration of at least 60 minutes.
Step (a) may comprise depositing the layer containing the refractory metal with a thickness of at least 500nm. This allows the layer to be used as a heating layer during the subsequent growth of semiconductor layers over the nitride semiconductor wafer.
Heat from an external heat source is applied to the molybdenum layer, and is transmitted over the area of the wafer by conduction through the molybdenum layer.
Moreover, if a device structure is grown over the substrate, an electrode of this thickness of a refractory metal can act as a good heat sink for heat generated in the device (luring its operation.
A second aspect of' the invention provides a niethod of manufacturing a semiconductor device, the method comprising forming a contact on a first surface of a nitride semiconductor by a method of the first aspect and subsequently depositing a plurality of semiconductor layers over a second surface of the nitride semiconductor, at least one of the semiconductor layers being deposited at a temperature of at least 800 C and at a pressure of 1 x i0 rnbarr or below, whereby the layer containing the refractory metal is annealed during the deposition of' the plurality of semiconductor layers. Where the semiconductor layers arc deposited by MBE, for example, the growth temperature for at least some of the semiconductor layers is likely to be at least 800 C, the semiconductor layers are deposited under high vacuum condition, and the overall time required to deposit the semiconductor layers is likely to he at least one hour - so that the conditions required to deposit the semiconductor layers are the same as the required annealing condition for the electrode. (in practice the required growth temperature for each of the semiconductor layers will depend on the composition of the layer, and so will vary from one layer to another. The growth temperature required for deposition of one or more of the semiconductor layers may therefore be less than 800 C. This does not affect the invention, provided that sufficient of the layers arc grown at a growth temperature of over 800 C such that the growth temperature is 800 C or above for at least one hour.) Thus, the layer of refractory metal may serve more than one purpose. It can be used to heat the substrate during deposition of the semiconductor layers and, because of its good electrical characteristics, may also he used as the n-type contact in the final devices. The invention thus avoids the need to remove the molybdenum heating layer and apply a titanium/aluminium contact, and thereby simplifies the manufacturing process. The layer of refractory metal may also act as a heat sink in a finished device.
Moreover, the layer of refractory metal is annealed during the process of depositing the semiconductor layers and there is no need for a separate annealing step. Omission of the separate annealing step also significantly reduces the overall processing time required to produce the light-emitting device.
One or more of the semiconductor layers may he deposited at a temperature of at least 900 C.
A third aspect of the present invention provides a method of manufacturing a semiconductor device, the method comprising the steps of: depositing a plurality of semiconductor layers over a first surface of a nitride semiconductor; and subsequently depositing a contact on a second surface of the nitride semiconductor by a method of the first aspect. In some cases ii is necessary for a semiconductor substrate to he reduced in thickness after a layer structure has been grown over the substrate. This aspect of the invention allows the nitride semiconductor to be reduced in thickness after the semiconductor layers have been deposited. A refractory metal may then be deposited on the opposite side of the thinned nitride semiconductor (i.e., on the "back side" of the thinned nitride semiconductor) to form a contact to the nitride semiconductor.
In a method of this aspect, the growth of the semiconductor layers is canied out before the layer containing refractory metal has been deposited. A separate step of annealing the layer containing refractory metal is thus required in this aspect. The layer of refractory metal is not used as a heating layer during the growth in this aspect, and ii is therefore riot necessary for the layer of refractory metal to have a thickness of 500nm or more - a relatively thin layer of refractory metal may be used.
Each semiconductor layer may be a nitride semiconductor layer. Each semiconductor layer may be an (Al,Ga,In)N layer.
The plurality of semiconductor layers may constitute a semiconductor laser diode structure, or they may constitute a light-emitting diode structure.
The nitride semiconductor may be (Al,Ga,In)N.
The nitride semiconductor may he doped n-type.
The layer containing a refractory metal may he a layer of molybdenum, a layer of tantalum or a layer of tungsten.
A fourth aspect of the present invention provides a semiconductor lightemitting device manufactured by a method of the second or third aspect. The semiconductor light- emitting device may be a semiconductor laser diode or a light-emitting diode.
Preferred embodiments of the present invention will now he described by way of example with reference to the accompanying figures, in which: Figure 1 is a schematic sectional view of a semiconductor laser diode; Figure 2 shows current-voltage characteristics for a light-emitting diodes with (i) a Ti/Al contact and (ii) a molybdenum contact; Figure 3 shows the current voltage characteristic of a molybdenum contact to an n- GaN layer; Figure 4 is a schematic block flow diagram of an embodiment of the present invention; and Figure 5 is a schematic block flow diagram of another embodiment of the present invention.
Figure 4 illustrates the principal steps of one embodiment of the present invention.
Tnitially, at step 1 a layer of a refractory metal is deposited over a surface of a nitride semicoiiductor wafer. In one implementation of this embodiment the refractory metal is molybdenum, and the nitride semiconductor wafer is a GaN wafer, but the invention is not limited to these two specific materials.
it will be assumed that the nitride semiconductor wafer is intended to form the substrate I of a semiconductor device, for example, a lightemitting device such as a laser diode similar to that shown in Figure 1. The nitride semiconductor wafer is therefore preferably doped n-type; silicon, oxygen, carbon, germanium and tin arc examples of suitable dopants. The nitride semiconductor wafcr may have been grown by any conventional growth process such as, for example, MBE and MOCVD.
The layer of molybdenum or other refractory metal is preferably deposited with a thickness of 500nm or more. This gives the layer of molybdenum or other refractory metal a high thennal conductance and so allows the layer to be used as a heating layer during the subsequent growth of semiconductor layers over the nitride semiconductor wafer. A further advantage of depositing the layer of molybdenum or other refractory metal to have a high thermal conductance is that the molybdenum layer can act as a heat sink in the finished device.
Next, at step 2, the nitride semiconductor wafer is introduced into the growth chamber of an MBE reactor. As is well known, MIlE is performed under high vacuum conditions, and the pressure in the growth chamber is in particular maintained below io3 mbarr.
Next, at step 3, a plurality of semiconductors layers are grown over a second surface of the nitride semiconductor wafer. The surface of the nitride semiconductor wafer on which the molybdenum layer was deposited will form the back side of the substrate 1 of Figure 1, and the semiconductor layers deposited in step 3 are deposited over a surface of the nitride semiconductor wafer that is opposite to the surface on which the molybdenum layer was deposited. The semiconductor layers deposited in step 3 are selected to have appropriate thicknesses and compositions such that the structure of a light-emitting device is grown in step 3. The semiconductor layer structure grown in step 3 may be, for example, a light-emitting device structure such as a light-emitting diode structure or a laser diode structure similar to that shown in Figure 1. The invention is not limited to growth of a light-emitting device, and the semiconductor layer structure may alternatively be, for example, a transistor structure. Each semiconductor layer deposited in step 3 may be a nitride semiconductor layer, and particuLarly preferably may be an (Al, Ga,ln)N layer.
The layer of molybdenum or other refractory metal may he used to heat the semiconductor wafer during the MBF growth process, provided that the layer is sufficiently thick to act as good conductor of heat. Heat from an external heat source may be applied to the layer of refractory metal, and transmitted over the area of the wafer by conduction through the layer.
The growth of an (Al,Ga,ln)N layer by MBE is preferably carried out at a growth temperature of 800 C or more, and is more preferably out at a growth temperature in the range 900 C to 950 C (although the preferred growth temperature of a layer depends on its composition, so that the growth temperature is typically changed after the growth of one layer is completed and before the growth of another layer starts). Furthermore, the time required to grow the layer structure of a light-emitting device is likely to take at least 1 hour. The effect of step 3, therefore, is that the niolybdenurn layer is exposed to a temperature of at least 800C as a consequence of one or more layers of the layer structure being grown at a temperature of 800 C or above (and to a temperature of 900 C or above if one or more of the layers of the layer structure is grown at 900 C or above) fbr at least one hour. I)uring step 3 the molybdenum layer is thus annealed at a temperature of 800 C, and probably at at least 900 C, fbr at least 1 hour, under high vacuum conditions. (As noted above, the growth temperature of one or more layers of the layer structure may be less than 800 C, depending on the composition of the layers.
In particular, the active region of an (Al,Ga,In)N light-emitting device may be grown at temperatures below 800 C. This does not affect the invention, provided that the total time for which the growth temperature is greater than 800 C is sufficient to anneal the molybdenum layer, and preferably is at least an hour.) As a result, it is not necessary to provide a separate annealing step for the molybdenui-n layer.
The MBE growth of an (Al,Ga,In)N layer structure by MBE at growth temperatures of 800 C' or above is described in, for example, GB-A-2 392 169, to which attention is directed.
Next, at stcp 4, the resultant layer structure is removed from the growth chamber of the MBE reactor, and at step 5 is processed to produce individual light-emitting devices.
The Further processing of step 5 may include, for example, deposition of a p-contact over the upper surface of the semiconductor layer structure grown in step 3, arid dicing the semiconductor sample into individual devices. These steps are conventional, and will not he described here. Step 5 may also include attaching an external lead to the layer of refractory metal, for example using a conductive paste such as silver paint or a conductive resin. For example, if an individLial device obtained by dicing the semiconductor structure is mounted in a package at step 5, the n-type electrode of the device (which is constituted by the portion of the layer of refractory metal included in the device) may be electrically connected to the n-terminal of the package by an external lead. The layer of refractory metal may he used as au electrode without any additional processing steps.
- In the growth of the layers in step 3, the layers are grown over a surface of the nitride semiconductor wafer that is opposite to the surface on which the molybdenum layer was deposited in step 1. The molybdenum layer thus acts as the n-contact in the finished device, and corresponds to the n-contact 6 shown in Figure 1.
Results of the method of the present invention are shown in Figure 2. Figure 2 shows the current-voltage characteristics of a light-emitting diode manufactured by the method of the present invention, using a molybdenum layer as the n-contact. These results are shown by the square data points in Figure 2. Figure 2 also shows for comparison, the current-voltage characteristics of a light-emitting diode having a conventional Ti/Al n- contact - these are shown by the circular data points in Figure 2. The results of Figure 2 were obtained for light-emitting diodes fabricated in (Al,Ga,ln)N material system, and the molybdenum contact was deposited by a method as shown in Figure 4 in which the contact is annealed during deposition of the semiconductor layer structure. The thickness of the molybdenum layer was approximately 2tm,the growth temperature was maintained at at least 800 C for approximately 6 hours, and the growth process was carried out at a pressure of approximately lft4mbarr.
It can he seen that the light-emitting diode with the molybdenum ncontact deposited by the method of the present invention has comparable electrical characteristics to the comparative light-emitting diode with a conventional Ti/Al n-contact. This indicates that the contact of the invention, of molybdenum or other refractory metal, may be used in place of a conventional Ti/Al contact.
Figure 3 shows the current-voltage characteristic of a contact made by the method of the present invention. To obtain the results of Figure 3, a molybdenum layer was deposited with a thickness of 2im on an n-type GaN layer. The current-voltage characteristic of the contact as deposited (that is, without any annealing having been carried out) is shown as the square data points in Figure 3, and it can he seen that the contact as deposited has a relatively high electrical resistance and that a voltage of even 2.5V produces a very low current density.
- The data points shown as circles in Figure 3 illustrate the currentvoltage characteristics of the molybdenum n-contact afler the contact has been annealed at 800 C R)r 1 minute under a nitrogen atmosphere. It can be seen that these data points are virtually coincident with the data points obtained for the contact as deposited, showing that this annealing step has had no effect on the current-voltage characteristics of the contact.
Figure 3 also shows the current-voltage characteristics of the contact after it had been annealed at a temperature of 950 C for 1 hour under vacuum conditions (i.e. at a pressure of l0 mbarr or below). These results are denoted by the lines labelled "c" in Figure 3, and it can be seen that this annealing step had a dramatic effect on the current- voltage characteristics of the contact. The current-voltage characteristic is now extremely steep, and an applied voltage of approximately 0. 1 V produces a high current density through the contact. Furthermore, it can be seen that the characteristics are substantially ohmic, in that application of a negative voltage produces a current that is equal in the magnitude and opposite in sign to the current produced by a positive voltage.
While the layer of refractory metal is preferably annealed for at least an hour, a shorter annealing lime can he used. Annealing the layer of refractory metal for 45 minutes or more, or even for 30 minutes or more, should provide a contact with acceptable electrical characteristics, although these shorter annealing times are likely to lead to a contact that is less ohmic and has a higtier contact resistance than a contact obtained by annealing for an hour or longer.
The method described in Figure 4 has the advantage that, not only is a low resistance, ohmic n-contact obtained, but the method does not require a separate annealing step so that the overall processing time is significantly reduced. In the manufacture of some semiconductor devices, however, it is necessary to deposit a layer structure over a substrate, and then reduce the thickness of the substrate after the layer structure has been deposited. The method of Figure 4 cannot be applied in such a case, since the layer of refractory metal would he removed when the thickness of the substrate was reduced.
Figure 5 shows a further embodiment of the present invention, which is suitable Ibr use in a case where the substrate must he red need in thickness after the layer structure has been grown on the substrate. In this embodiment, a nitride semiconductor wafer, for example a GaN wafer, is initially introduced into the growth chamber of an MBE reactor at step 1 0, and semiconductor layers are grown over the nitride semiconductor wafer at step 11. On completion of the growth of semiconductor layers in step 11, the wafer and layer structure are removed from the growth chamber at step 12. Steps 10, 11 and 12 of the Figure 5 correspond to step 2, 3 and 4 of Figure 4 respectively, and will not be described further.
Next, at step 13, the thickness of the nitride semiconductor wafer is reduced, by removing material from the surface of the nitride semiconductor wafer opposite to the surface over which the semiconductor layers were deposited in step 11. The thickness of the nitride semiconductor wafer may he reduced using any suitable technique including, for example, a mechanical process such as grinding or a chemical process such as etching.
When the thickness of the nitride semiconductor wafer has been reduced to the desired value, a layer of refractory metal, for example molybdenum, is deposited, at step 14, over the back surface of the wafer. Step 14 of the method of Figure 5 corresponds generally to step 1 of Figure 1 and the description of the deposition of the molybdenum layer will not he repeated.
In the embodiment of figure 5 the layer of molybdenum or other refractory metal is not used to heat the substrate during the growth of the layer structure, since the layer of molybdenum or other refractory metal is not deposited until after the layer structure has been grown. It is therefore possible to deposit a thin layer of molybdenum or other refractory metal to form the n-contact in this embodiment. However, if it is desired for the layer of molybdenum or other refractory metal to act as a heat sink in operation of the finished device, the layer of molybdenum or other refractory metal must he made sufficiently thick so as to have a good thermal conductance.
The molybdenum layer is then annealed, at step 15. As previously described, step 15 consists of annealing the molybdenum layer for at least 1 hour, at a temperature of 8OOC or above, under high vacuum conditions. The effect of the annealing step is, as shown in Figure 3, to produce an ohmic contact to the nitride semiconductor wafer.
Finally, the semiconductor wafer is subject to further processing steps at step 16 to produce individual light-emitting devices. Step 16 corresponds generally to step 5 of Figure 4, and will not be further described.
it should be noted that the steps shown in Figure 4 and 5 show only the principal steps of two embodiments of the methods. In a practical implementation there are likely to be further steps such as, for example, cleaning the nitride semiconductor wafer before depositing the refractory metal at step 1 of Figure 4 or step 14 of Figure 5 or before growing the semiconductor layers at step 3 of Figure 4 or step 11 of Figure 5. These further steps are not related to the concept of the present invention, and have therefore been omitted from Figures 4 and 5.
In the embodiments of Figure 4 and 5 the semiconductor wafer is processed at step 5 or step 16 with the layer of refractory metal in situ. Since the semiconductor layers arc grown on the opposite side of the wafer to the side on which the layer of refractory metal is deposited, the layer of refractory metal will be behind the substrate during the processing steps, and so is shielded by the substrate. The layer of refractory metal is therefore not signiflcanlly affected during the processing steps.
In embodiments of the present invention in which the refractory metal is deposited to a thickness of at least SOOnm, the n-contact in the resultant device thus has a thickness of at least 500nm. The n-contact is thus able to act as an effective heat sink, and can remove from the device heat generated within the device (luring its operation.
The invention is not hinited to the preferred embodiments described above, hut may be implemented in other ways. For example, the invention is not limited to deposition of a molybdenum layer, but a layer of any refractory metal may be used. In particular, the invention may he implemented by (lepositing a layer of tungsten or tantalum instead of a layer of molybdenum.
Furthermore, the layer containing a refractory metal may consist of a mixture or an alloy of two or more refractory metals. For example, a mixed layer of tungsten and molybdenum, or a mixed layer of tantalum and molybdenum could be used.
The invention has been described with reference to manufacture of a lightemitting device. However, the invention is not limited to use in manufacture of a light-emitting device, and may be generally applied to form the n-type contact to any device fabricated in the (Al,Ga,In)N material system including, for example, an (Al,Ga,ln)N transistor.
In the embodiments described above the nitride semiconductor wafer is a GaN wair.
The invention is not limited to use of a GaN wafer, but may in general he implemented with any n-type (Al,Ga,ln)N semiconductor wafer.
The nitride semiconductor wafer will typically have a thickness of more than I 00tm, to ensure that the resultant devices have sufficient structural strength. II the nitride semiconductor waler has a thickness of less than I 00.tm it becomes very fragile, and it would be difficult to grow a layer structure over the substrate without breaking the substrate. Jiowever, the invention may in principal be performed with a nitride semiconductor wafer of any thickness.
In the method of figure 5, the initial thickness of the nitride semiconductor wafer is preferably over I 00im, but the thickness of the wafer can be reduced to I 00tm or below in step 13.
In the description of the embodiment of Figure 4, an M BE growth process is used to grow the semiconductor layers in step 3. This embodiment of the invention is not in principle limited to MBE growth, and any growth process that can be carried out at - 800 C or above in the high-vacuum conditions required in step 3 of Figure 4 may be used in place of MBE.
In tile embodiment of Figure 5, the semiconductor growth process (step 11) does not constitute part of the annealing process of the layer of refractory metal. Step 11 of Figure 5 is therefore not restricted to a high-vacuum growth process such as MBE, but may be carried out using any known semiconductor growth process including, for example MOCVD. (It should he noted that the method of Figure 4, in which the semiconductor layers are grown over the substrate after the layer of refractor metal has been deposited, may he in principal he carried out using any semiconductor growth technique. If, however, the semiconductor growth technique used in step 3 is not a high-vacuum growth technique, or is carried out below 800 C, the layer of refractory metal will not be annealed during step 3, and a separate annealing step would then be required after step 3.)

Claims (19)

  1. CLAIMS: 1. A method of forming a contact on a nitride semiconductor, the
    method comprising the steps of: a) depositing a layer containing a refractory metal over a surface of a nitride semiconductor; and h) annealing the layer containing the refractory metal at a temperature of at least 800 C at a pressure of I x l03mbarr or below.
  2. 2. A method as claimed in claim 1 wherein step (a) comprises depositing the layer containing the refractory metal with a thickness of at least SOOnm.
  3. 3. A method as claimed in claim I or 2 wherein step (b) has a duration of at least minutes.
  4. 4. A method as claimed in any preceding claim wherein step (h) is carried out at a temperature of at least 900 C.
  5. 5. A method of manufacturing a semiconductor device, the method comprising forming a contact on a first surface of a nitride semiconductor by a method as defined in claim 1, 2 or 3 wherein step (b) comprises depositing a plurality of semiconductor layers over a second surface of the nitride semiconductor, at least one of the semiconductor layers being deposited at a temperature of at least 800 C and at a pressure of I x I 0 rnbarr or below, whereby the layer containing the refractory metal is annealed during the deposition of the plurality of semiconductor layers.
  6. 6. A method as claimed in claim 5 wherein at least one of the semiconductor layers is deposited at a temperature of at least 900 C.
  7. 7. A method of manufacturing a semiconductor device, the method comprising the steps of: depositing a plurality of semiconductor layers over a first surface of a nitride semiconductor; and subsequently depositing a contact on a second surface of the nitride semiconductor by a method as defined in claim 1,2,3 or 4.
  8. 8. A method as claimed in claim 5, 6 or 7 wherein each semiconductor layer is a nitride semiconductor layer.
  9. 9. A method as claimed in claim 8 wherein each semiconductor layer is an (AI,Ga,In)N layer.
  10. 10. A method as claimed in any of claims 5 to 9 wherein the plurality of semiconductor layers constitute a semiconductor laser diode structure.
  11. 11. A method as claimed in any of claims 5 to 9 wherein the plurality of semiconductor layers constitute a light-emitting diode structure.
  12. 12. A method as claimed in any preceding claim wherein the nitride semiconductor is (Al,Ga,ln)N.
  13. 13. A method as claimed in any preceding claim wherein the nitride semiconductor is doped n-type.
  14. 14. A method as claimed in any preceding claim wherein the Layer containing a refractory metal is a layer of molybdenum.
  15. 15. A method as claimed in any of claims I to 13 wherein the layer containing a refractory metal is a layer of tantalum.
  16. 16. A method as claimed in any of claims I to 13 wherein the layer containing a refractory metal is a layer of tungsten.
  17. 17. A semiconductor light-emitting device manufactured by a method as defined in any of claims 5 to 16.
  18. 18. A semiconductor laser diode device manufactured by a method as defined in claim 10.
  19. 19. A semiconductor light-emitting diode manulactured by a method as defined in clainis 11
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Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JP2002151737A (en) * 2000-11-08 2002-05-24 Nichia Chem Ind Ltd Gallium nitride compound semiconductor device
US6410460B1 (en) * 2000-05-15 2002-06-25 Ramot University Authority For Applied Research And Industrial Development Ltd. Technology for thermodynamically stable contacts for binary wide band gap semiconductors

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6410460B1 (en) * 2000-05-15 2002-06-25 Ramot University Authority For Applied Research And Industrial Development Ltd. Technology for thermodynamically stable contacts for binary wide band gap semiconductors
JP2002151737A (en) * 2000-11-08 2002-05-24 Nichia Chem Ind Ltd Gallium nitride compound semiconductor device

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
"Effects of vacuum annealing on electrical properties of GaN contacts" I Fujimoto et al, J. Electronic Materials, Vol 32, No 9, pp 957-963, 2003. *

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