WO2018196948A1 - Interlayer barrier - Google Patents

Interlayer barrier Download PDF

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Publication number
WO2018196948A1
WO2018196948A1 PCT/EP2017/059633 EP2017059633W WO2018196948A1 WO 2018196948 A1 WO2018196948 A1 WO 2018196948A1 EP 2017059633 W EP2017059633 W EP 2017059633W WO 2018196948 A1 WO2018196948 A1 WO 2018196948A1
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Prior art keywords
layer
heterostructure
substrate
gan
exclusion
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PCT/EP2017/059633
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French (fr)
Inventor
Jr-Tai CHEN
Original Assignee
Swegan Ab
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Priority to PCT/EP2017/059633 priority Critical patent/WO2018196948A1/en
Priority to EP17723024.0A priority patent/EP3616243A1/en
Publication of WO2018196948A1 publication Critical patent/WO2018196948A1/en

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Definitions

  • the present disclosure relates to a heterostructure for semiconductor devices and to a method for producing the same.
  • WO201 6155794A1 discloses Al x Gai- x N/GaN heterostructu res that are suitable for manufacturing high electron mobility (HEMT) devices.
  • materials comprising indium such as InAIN and/or InAIGaN, generally expressed as ln x Al y Gai-x- y N.
  • An object of the present invention is to provide an improved ln x Al y Ga-i-x- y N/GaN heterostructure, and in particular an ln x Al y Ga-i-x- y N/GaN
  • a heterostructure for producing a high electron mobility transistor (HEMT), comprising a GaN a buffer layer comprising a buffer portion proximally to a substrate and a channel portion distally from the substrate, an AlxiGai-xi N exclusion layer, wherein 0.3 ⁇ x1 ⁇ 1 .0, preferably 0.5 ⁇ x1 ⁇ 1 .0, most preferably 0.7 ⁇ x1 ⁇ 1 .0, formed on the GaN buffer layer, and an ln y AI X 2Gai-x2- y N barrier layer, wherein 0 ⁇ y ⁇ 0.25, preferably 0.10 ⁇ y ⁇ 0.22, most preferably 0.15 ⁇ y ⁇ 0.20, and
  • An Alx3Gai-x3N interlayer wherein 0 ⁇ x3 ⁇ 0.5, preferably 0 ⁇ x3 ⁇ 0.3, most preferably 0 ⁇ x3 ⁇ 0.1 , is sandwiched between the exclusion layer and the barrier layer.
  • the heterostructure presents a 2DEG electron mobility of 1800- 2300 cm 2 /Vs, preferably 1900-2200 cm 2 /Vs, most preferably 2000-2100 cm 2 /Vs.
  • Heterostructure is herein defined as a structure comprising a stack of two or more different epitaxial materials.
  • formed on may be construed as “formed directly on”, alternatively this may be interpreted as there can be one or more additional layers which do not affect the function of the heterostructure.
  • 2DEG electron mobility is meant the mobility an electron gas is free to move in two dimensions, but being tightly confined in the third dimension.
  • Heterostructures based on the above mentioned materials and having such high mobility have, as far as is known to the inventors, not previously been available.
  • An advantage of this heterostructure is that it has higher electron mobility as compared to prior art ln x Al y Gai-x- y N/GaN heterostructures.
  • the Alx3Gai-x3N, interlayer may have a thickness of 0.20 to 5 nm, preferably 0.20 to 3 nm, most preferably 0.20 to 1 nm.
  • the interlayer may be as thin as possible while the electron mobility enhancement is sustained. 0.20 nm is the thickness close to a monolayer of the Alx3Gai-x3N interlayer.
  • the exclusion layer may have a thickness of 0.5 to 2.5 nm, preferably 0.8 to 2 nm, most preferably 1 .0 to 1 .5 nm.
  • the barrier layer may have a thickness of 2 to 12 nm, preferably 3 to 10 nm, most preferably 4 to 8 nm.
  • the buffer layer may have a thickness of 1 .0 to 4.0 ⁇ , preferably 1 .5 to 3.0 ⁇ , most preferably 1 .8 to 2.0 ⁇ .
  • the heterostructure may further comprise a substrate and an AIN nucleation layer formed on the substrate, wherein the GaN buffer layer may be formed on the AIN nucleation layer.
  • the substrate may be a SiC, sapphire, Si, GaN, AIN or diamond substrate.
  • the nucleation layer may have a thickness of 5 to 200 nm, preferably 20 to 100 nm, most preferably 30 to 60 nm.
  • the heterostructure may further comprise a passivation layer formed on the barrier layer.
  • the passivation layer may be a GaN or a SiN layer.
  • the passivation layer may have a thickness of 1 to 20 nm, preferably 1 .5 to 15 nm, most preferably 2 to 10 nm.
  • the sheet resistance of the heterostructure may be 100 to 350 ⁇ , preferably 150 to 300 ⁇ , most preferably 180 to 250 ⁇ .
  • the heterostructure may present a 2DEG density of 0.8E+12 cm -2 to
  • a HEMT device comprising the heterostructure.
  • a method of producing a heterostructure in a MOCVD reactor comprises flowing Ga and N precursor gases into the MOCVD reactor forming a GaN buffer layer, flowing Al and N precursor gases into the MOCVD reactor forming an AlxiGai-xi N exclusion layer on the GaN buffer layer, flowing In, Al, Ga and N precursor gases into the MOCVD reactor forming an ln y AI X 2Gai-x2- y N barrier layer on the AlxiGai-xi N exclusion layer.
  • the method comprises flowing Ga, Al and N precursor gases into the MOCVD reactor for forming a AI X 3Gai-x3N interlayer on the Alxi Gai-xi N exclusion layer prior to forming the ln y AI X 2Gai-x2- y N barrier layer, wherein forming the buffer layer and the exclusion layer comprise steps of: stopping at least the Ga precursor flow in the forming of the buffer layer, waiting 1 -20 seconds, preferably 2-15 seconds or 3-10 seconds,
  • AIN growth rate of less than 100 nm/h is achieved for a time sufficient to form less than 100 % of a monolayer of the AIN layer, preferably less than 80 % of a monolayer of the AIN layer, most preferably less than 60 % of a monolayer of the AIN layer.
  • the method may further comprise, when forming the exclusion layer, a step of increasing the exclusion layer growth rate to 100-300 nm/h, preferably 100-200 nm/h, most preferably 100-150 nm/h subsequently to the step of applying the Al precursor pre-flow.
  • the increase of exclusion layer growth may be made by an increase of a second flow rate of the Al precursor.
  • heterostructure may be 10 to 1000 mbar, perferably 30 to 200 mbar, most preferably 50 to 100 mbar.
  • heterostructure may be 950 to 1 150 °C, perferably 1000 to 1 100 °C, most preferably 1020 to 1080 °C.
  • the method may further comprise a step of decreasing a temperature in the MOCVD reactor from 950-1 150 °C to 650- 950°C, preferably to 700- 900°C, most preferably to 750-850°C prior to forming the barrier layer 1 6.
  • the method may further comprise etching away the interlayer by 20 to 100 %, preferably by 30 to 80 %, most preferably 40 to 60 % of the total thickness of the interlayer, the etching being performed during the step of decreasing the temperature, and optionally also during an additional waiting period prior to a next processing step.
  • the interlayer may be etched by H2 in the MOCVD reactor and/or thermally etched.
  • the method may comprise an initial step of providing a substrate and arranging the substrate in the MOCVD reactor.
  • the method may further comprise forming a nucleation layer on the substrate.
  • a buffer layer may then be formed on the nucleation layer.
  • a nucleation layer may not be needed if using a GaN or a AIN substrate.
  • GaN or AIN may be grown onto the GaN or AIN substrate to obtain a clean surface.
  • Such an additional layer of GaN or AIN may be about 100 to 300 nm.
  • a method of producing a HEMT device comprises the steps of providing a source, a drain, and a gate, contact onto the barrier layer of the heterostructure.
  • the method of producing a HEMT device may comprise an additional step of providing an insulating layer between the gate contact and the barrier layer.
  • the method of producing a HEMT device may comprise the steps of providing a heterostructure, providing an insulating layer onto the barrier layer, providing a gate metal onto the insulating layer, providing a source and a drain contact such that they extend into the channel portion of the buffer layer.
  • the method of producing a HEMT device may further comprise an additional step of providing an AIGaN back barrier layer between the buffer portion and channel portion of the buffer layer.
  • Fig. 1 a-1 b schematically illustrate embodiments of an ln y AI X 2Gai-x2- yN/GaN heterostructure as disclosed herein.
  • Fig. 2a-2d schematically illustrates examples of HEMT devices comprising the heterostructure of Fig. 1 a or 1 b.
  • ln y AI X 2Gai-x2- y N/GaN heterostructures may be used in semiconductor devices, such as in high electron mobility transistor (HEMT) devices.
  • HEMT high electron mobility transistor
  • Fig. 1 a schematically illustrates an example of such a heterostructure.
  • 1 a comprises, when viewed from bottom to top, an optional substrate 1 1 , an AIN nucleation layer 12, a GaN buffer layer 13, having a buffer portion 13a and a channel portion 13b, an AlxiGai-xiN exclusion layer 14, an AI X 3Gai-x3N interlayer 15, an ln y AI X 2Gai-x2- y N barrier layer 1 6 and optionally a GaN or SiN passivation layer 17.
  • a SiC substrate may be used due to its high thermal conductivity properties in order to efficiently extract heat and to minimize temperature rise in the semiconductor device.
  • the SiC polytype may be 4H or 6H.
  • a Si, sapphire, GaN or AIN substrate may be used.
  • a nucleation layer 12 of AIN may be deposited onto the substrate.
  • the AIN substrate may function as a combined substrate and nucleation layer and hence an additional AIN layer may not be added.
  • typically a layer of AIN may be grown onto the substrate to obtain a fresh AIN surface.
  • GaN may be grown onto the substrate to obtain a fresh GaN surface.
  • the purpose of the AIN nucleation layer is to compensate for the lattice mismatch between the substrate 1 1 and the GaN buffer layer 13, and to obtain high quality epitaxial growth of the GaN buffer layer on the substrate.
  • the nucleation layer enables growth of the GaN buffer layer 13 since GaN does not nucleate two-dimensionally on some substrates, such as SiC, by changing the surface potential, such that GaN can be grown.
  • the buffer layer 13 of GaN comprises a buffer portion 13a proximally to the substrate and a channel portion 13b distally from the substrate.
  • the buffer portion 13a may be doped by acceptor-like impurities like carbon or iron.
  • the acceptor-like impurities compensate the free electrons to obtain semi-insulating properties.
  • the dopant concentration of carbon or iron may preferably be higher than 5- 10 16 cm -3 , more preferably higher than 1 ⁇ 10 17 cm -3 , most preferably higher than 1 ⁇ 10 18 cm -3 .
  • a part of the buffer portion may also comprise aluminum to form an AIGaN back barrier layer.
  • the channel portion 13b may be unintentionally doped, which means that no intentional doping is used.
  • the amount of e.g. carbon in the channel portion may be varied by varying the pressure and/or the temperature in the MOCVD reactor.
  • the purpose of the buffer portion 13a GaN buffer layer 13 is to develop the structure quality by a thick layer growth and to exhibit a semi-insulating property.
  • the channel portion 13b of the GaN buffer layer prevents possible trapping effects and ionized impurity scattering that adversely affects 2DEG density and mobility.
  • the amount of residual impurites in the channel portion 13b of the GaN buffer layer 13 such as oxygen, silicon and/or carbon, may be low, typically of about less than 5- 10 16 cm -3 , i.e. close to the detection limit of Secondary Ion Mass Spectroscopy (SIMS).
  • SIMS Secondary Ion Mass Spectroscopy
  • the purpose of the AlxiGai-xi N exclusion layer 14 is to reduce the alloy and interface scatterings, thus enhancing the 2DEG mobility.
  • the introduction of an AI X 3Gai-x3N interlayer 15 between the exclusion layer 14 and the indium comprising barrier layer 1 6, see Fig. 1 a, may prevent the AlxGai-xN exclusion layer from relaxation during a growth interruption step (see discussion on growth below).
  • laxation is meant that the biaxial lattice constant of the AIGaN layer becomes shorter than the biaxial lattice constant of the GaN buffer.
  • the interlayer may protect the exclusion layer from
  • barrier layer 1 6 comprising indium such as ln y Alx2Gai-x2- y N
  • a lattice matched condition can be achieved between the barrier layer 1 6 and the buffer layer 13, when the percentage of indium in the barrier layer is about 17 to 18% and of Al is about 82 to 83 %.
  • the lattice match means that ideally no strain is built in the between ln y Alx2Gai-x2- y N/GaN heterostructure.
  • An indium comprising barrier layer, i.e. an ln y Alx2Gai -x2- y N/GaN heterostructure may be more thermally stable as compared to AIGaN/GaN or AIN/GaN heterostructures.
  • the purpose of the optional passivation layer 17 of GaN or SiN is to stabilize the surface conditions of the HEMT structure since the surface conditions influence the 2DEG density.
  • the 2DEG density may be increased or decreased, but the 2DEG mobility will not change much.
  • the heterostructure may be deposited by Metal Organic Chemical Vapor Deposition (MOCVD), which also is known as Metal Organic Vapor Phase Epitaxy (MOVPE).
  • MOCVD Metal Organic Chemical Vapor Deposition
  • MOVPE Metal Organic Vapor Phase Epitaxy
  • MOCVD, or MOVPE is a chemical vapor deposition method in which a solid material is deposited onto a substrate by chemical reactions of vapor phase precursors. The method is mainly used for growing complex semiconductor multilayer structures.
  • the precursors are metal-organic compounds, typically in combination with a hydride gas such as ammonia, N h .
  • Precursors used for the AIN nucleation layer 12 growth may be N hta, and TMAI, trimethylaluminum, Al2(CH3)6 or triethylaluminium, TEA,
  • Precursors used for GaN buffer layer 13 growth may be TMGa, trimethylgallium, i.e. Ga(CH3)3 or TEGa, triethylgallium Ga(C2Hs)3, and ammonia, N hta.
  • Precursors used for growth of the Alxi Gai -xi N exclusion layer 14 and the Alx3Gai -x3N interlayer 15 growth may be TMGa or TEGa, and TMAI or TEA, and NH 3 .
  • Precursors used for growth of the ln y AI X 2Gai -x2- y N barrier layer 1 6 may be e.g. TMIn, trimethylindium, i.e. ln(CH3)3, TMAI or TEA and TMGa or TEGa.
  • Precursors used for growth of an SiN passivation layer may be SiH 4 and NH 3 .
  • Precursors used for growth of an GaN passivation layer may be TMGa or TEGa and NH 3 .
  • the precursors are transported, often by means of a carrier gas, such as H2, N2 or a mixture of N2 and H2, into a MOCVD reactor, in which at least one substrate or sample (e.g. a part of a HEMT structure) is placed.
  • a carrier gas such as H2, N2 or a mixture of N2 and H2
  • MOCVD reactor in which at least one substrate or sample (e.g. a part of a HEMT structure) is placed.
  • the flow rate of the carrier gas, flowing through a precursor (e.g. TMAI or TMGa) bubbler, may be about 70 ml/min for TMAI, about 50 ml/min for TMGa and 70 ml/min for TMIn.
  • the precursor flow merges with a main carrier gas flow, which may be on the order of 30 l/min for further transport to the reactor.
  • Reactions of the precursors form reactive intermediates and by- products take place on or in near vicinity of the substrate(s).
  • the reactants are adsorbed on the substrate, forming a thin film layer and finally byproducts are transported away from the substrate, by means of pumping on the MOCVD reactor.
  • the pressure in a MOCVD reactor upon thin film growth normally ranges from a few mbars up to atmospheric pressure.
  • the reactor chamber may be of either cold-wall or hot-wall type.
  • a cold-wall reactor the substrate is typically heated from below while the reactor walls are kept cooler than the substrate.
  • a hot-wall reactor the entire reactor chamber is heated, i.e. both the substrate and the reactor.
  • the steps of growing of an ln y AI X 2Gai-x2- y N/GaN heterostructure by MOCVD will now be described in detail.
  • the sample e.g. a part of a HEMT structure or a substrate, onto which the heterostructure is to be grown is inserted into a MOCVD reactor (for details about the MOCVD method, see above).
  • the pressure in the MOCVD reactor may be the same during production of all layers in the heterostructure described below. Typically, the pressure in the MOCVD reactor may be about 50 mbar upon production of the heterostructure.
  • the pressure in the MOCVD reactor may be increased during production of the channel portion 13b of the buffer layer 13.
  • the pressure in the MOCVD reactor may be about 60 mbar upon production of the channel portion 13b of the buffer layer.
  • the temperature in the MOCVD reactor may be the same upon growth of the nucleation layer 12, exclusion layer 14 and interlayer 15. As an example the temperature may be about 1050 °C in the MOCVD reactor.
  • the temperature in the MOCVD reactor may be increased.
  • the temperature may be increased to about 1060 °C.
  • the temperature may be decreased as compared to the temperature upon growth of the layers discussed above.
  • the temperature in the MOCVD reactor may be about 950 to 650 °C upon growth of the barrier layer and the passivation layer.
  • the Ga, Al and In precursors may be transported to the MOCVD reactor by at least one carrier
  • the presursors may be provided at room temperature.
  • at least one of the precursors may be heated in order to increase the vapor pressure and hence the growth of the different layers.
  • room temperature is meant a temperature of 1 6 °C to 27 °C.
  • the flow of the precursors and/or the carrier gases may be controlled by at least one mass flow controller which may be situated between each of the precursor containers and the MOCVD reactor.
  • the control of providing the presursors to the MOCVD reactor may be performed by opening or closing at least one valve situated between each of the precursor containers and the MOCVD reactor. The opening and closing may be performed manually or by computer control.
  • vent line bypassing a main run line.
  • a pressure balance may be provided between the vent line and the run line to avoid flow bursts when gas is switched into the main carrier flow.
  • the selected flow rates and the times the precursors are provided into the MOCVD reactor is dependent on many different parameters, such as the MOCVD reactor size, the sample/substrate size, the distance between the gas outlet of the precursor and the sample/substrate, the background pressure in the MOCVD reactor etc.
  • the flow rate and time the precursor gas is provided may vary in different experimental set-ups.
  • the skilled person may generally be expected to be able to provide a layer of a predetermined thickness, composition and quality.
  • Nh During all process steps below a flow of Nh may be provided.
  • the flow rate of Nh may be kept constant during all process steps.
  • the flow rate of Nhh may be decreased upon production of the AIN nucleation layer growth.
  • the flow rate of Nhh may be about 2 l/min for production of the buffer layer 13, exclusion layer 14, interlayer 15, barrier layer 1 6 and passivation layer 17.
  • the flow rate of Nhta may be 0.5 to 1 .0 l/min for production of the nucleation layer 12.
  • the steps of producing the heterostructure may be performed in the following order: Firstly, an optional nucleation layer 1 2 may be grown onto the substrate 1 1 .
  • the flow rate may be 0.8 ml/min of TMAI and 0.5 l/min of Nhta resulting in a growth rate of about 360 nm/min.
  • the purity of TMAI may be more than 5N, i.e. 99.999%.
  • the precursors for production of the GaN buffer layer may be a Ga precursor, e.g. TMGa, and ammonia, NHs.
  • the purity of TMGa may be more than 5N, i.e. 99.999%.
  • the start of providing the Ga precursor and the Nhh may take place simultaneously.
  • Nhh may be provided into the reactor before the start of providing the Ga preucursor.
  • the Ga precursor e.g. TMGa may be provided to the reactor at a first flow rate of e.g. 3.2 ml/min.
  • the flow rate of Nhh may be 2 l/min as mentioned above.
  • the buffer layer comprises two different portions, the buffer portion 1 3a and the channel portion 1 3b, respectively.
  • the pressure and temperature in the MOCVD reactor are increased, resulting in a decrease of residual carbon in the channel portion.
  • the temperature in the MOCVD reactor may be increased to about 1 060 °C and/or the pressure in the MOCVD may be increased to about 60 mbar durning production of the channel portion 1 3b of the buffer layer 1 3.
  • the growth rate of the GaN buffer portion 1 3a may be about 1 200 nm/h.
  • the flow rate of the channel portion 1 3b may be about 600 nm/h by a reduction of the flow rate of the Ga precursor.
  • the buffer portion, T a may have a thickness of about 1 .8 ⁇ .
  • the channel portion may have a thickness, Tt>, of about 100 to 200 nm.
  • the buffer layer may also comprise a so-called back barrier layer.
  • the concentration of Al of the AIGaN back barrier layer may be higher than 3 %, preferably higher than 5 % and most preferably higher than 8 %.
  • the thickness of the back barrier layer may be 3 nm to 1 000 nm, more preferably 3 nm to 800 nm, most preferably 3 nm to 500 nm.
  • the back barrier layer may be produced by provision of an Al precursor upon growth of the buffer layer.
  • the flow rates of Ga and NH3 are the same as described above for growth of the buffer layer, for example >0.02 ml/min.
  • the GaN layer growth is stopped, by (substantially) stopping the provision of the Ga precursor to the MOCVD reactor. This may be performed by switching the flow of the Ga precursor into the vent line.
  • the flow of NH3 may be kept at the same flow rate as a above, i.e. 2 l/min.
  • an optional step of waiting for about 1 to 20 seconds, normally 3 to 5 seconds, may be performed.
  • the time of waiting is highly dependent on the design of the reactor, such as the reactor size. The longer waiting, the larger risk of thermal or chemical etching by H2 of the channel portion 13b of the buffer layer 13.
  • the purpose of this step of waiting is to let a portion of residual Ga be flushed away from the MOCVD reactor by means of pumping.
  • the step of stopping the Ga precursor and waiting for some time before growth of the AlxiGai-xi N exclusion layer is commenced, is referred to as a “sharpening", which provides a more distinct, “sharper”, transition between the buffer layer 13 and the exclusion layer 14.
  • a Al precursor e.g. TMAI
  • TMAI TMAI
  • the pre-flow of the Al precursor is provided in order to facilitate a steep Al content transition from GaN to AIGaN in a transition zone, but not complete, formation of an AIN layer on the GaN layer.
  • the time and flow rate of the pre-flow of the Al precursor may be selected such that less than one monolayer of AIN is formed, preferably less than 90 % of a monolayer, less than 75 % of a monolayer or less than 50 % of a monolayer.
  • the definition of "monolayer of AIN" is that the surface is covered by one molecular layer of AIN.
  • One monolayer of AIN has a thickness of about 0.2 nm.
  • the first flow rate, the so-called pre-flow, of the Al precursor may be 0.2 ml/min and the flow rate of Nhh may be the same as above, i.e. about 2 l/min.
  • This step (i.e. pre-flow of Al) may be carried out for about 20 to 30 seconds.
  • the steps of stopping the Ga precursor flow and providing the Al precursor for 20 to 30 seconds is in this disclosure referred to as an
  • the AlxiGai-xiN exclusion layer 14 is grown.
  • no Ga precursor may be provided since there is residual Ga in the MOCVD reactor from the GaN buffer layer 13 growth.
  • the flow of the Al precursor, the so-called “second flow” may be kept at the same flow rate as the so-called pre-flow of Al precursor.
  • the second flow of Al may be increased as compared to the pre-flow of Al.
  • the flow rate of Al provided at this step may be the same as above, i.e. 0.2 ml/min.
  • an AI X 3Gai-x3N interlayer 15 is grown on the exclusion layer 14.
  • a layer of GaN is preferred as interlayer 15.
  • the flow rate of TMGa may be 1 .0 ml/min and the flow rate of Nhta may be 2 l/min upon GaN interlayer growth.
  • a portion of the interlayer may disapear due to due to thermal etching during the temperature decrease performed before growth of the barrier layer 1 6 as discussed below.
  • a interlayer of about 4 nm is grown and about 2 nm of this layer may be thermally and/or chemically etched away.
  • the entire interlayer may also function as a sacrifice layer.
  • the temperature in the MOCVD reactor may be decreased.
  • the temperature may be decreased by turning off (or decreasing) the power provided by the power supply for heating the MOCVD reactor.
  • the flow rate of TMAI may be 0.5 ml/min
  • the flow rate of TMIn may be 1 .25 ml/min
  • the flow rate of Nhta may be 2 l/min for production of the barrier layer.
  • the flow rates of TMGa may be 3.2 ml/min and Nhta may be 2 l/min.
  • the flow rate of 250 ppm SiH 4 may be 250 ml/min and Nhta may be 0.5 ml/min.
  • the temperature upon growth of the passivation layer may the same as upon growth of the barrier layer, i.e. about 800 °C.
  • the two samples have the same thicknesses and compositions of the barrier layer 1 6 and the exclusion layer 14.
  • the first sample is grown according to the method descibred above but without insertion of an interlayer 15.
  • the second sample is grown according to the method described above and with an AI X 3Gai-x3N interlayer 15 sandwiched between the ln y AI X 2Gai-x2- y N barrier layer 16 and the AlxiGai-xi N exclusion layer 14. Both samples are grown using the sharpening technique (sharpening step) as discussed above.
  • Table 1 Example of two samples, sample 1 without an interlayer and sample 2 with an interlayer, respectively. As can be seen, sample 2 (with interlayer) has a significantly higher 2DEG electron mobility as compared to sample 1 (without interlayer).
  • the 2DEG density of sample 2 is lower as compared to the 2DEG density of sample 1 since the distance from the surface to the channel portion of the buffer layer is increased by adding the GaN interlayer. It is not clear how the decreased 2DEG density affects the a HEMT device performance, since the 2DEG electron mobility is increased.
  • the sheet resistance of sample 2 is slightly increased as compared to sample 1 since the increased 2DEG electron mobility does not compensate for its decreased 2DEG density.
  • sample 2 The increased 2DEG electron mobility of sample 2 is due to the combination of the addition of the AI X 3Gai-x3N interlayer 15 and the
  • Fig. 2a schematically illustrates a HEMT device 2 comprising, from bottom to top, a SiC substrate 1 1 , an AIN nucleation layer 12, a GaN buffer layer 13 with a buffer portion 13a and a channel portion 13b, an AIN exclusion layer 14, a GaN interlayer 15 (as discussed above, this layer may be an Alx3Gai-x3N layer, although GaN is preferred), an indium comprising barrier layer, ln y AI X 2Gai-x2- y N,1 6. On top of the barrier layer 16, source 18, gate 19 and drain 20 contacts are deposited.
  • the HEMT device 2' illustrated in Fig. 2b has the same structure as the HEMT device shown in Fig. 2a with the addition that it has an insulating layer 22 between the source 18 and drain 20 contacts and below the gate contact 19.
  • the insulating layer 22 may be SiN x or an oxide layer such as AI2O3.
  • Fig. 2c another example of a HEMT device 2" is illustrated.
  • This device has the same structure as the device in Fig. 2b but with the difference that the source 18 and drain 20 contacts extend into the layers of the device, at least partially into the channel portion 13b of the buffer layer 13. Hence the channel portion 13b, the exclusion layer 14, the interlayer 15, the barrier layer 1 6 and the insulating layer 22 are located between the source 18 and drain 20 contacts.
  • This method of contacting the HEMT device may be referred to as "recess contact method".
  • Fig. 2d schematically illustrates yet another example of a HEMT device 2"'.
  • This device has the same structure as the device is Fig. 2a with the addition that a so-called back barrier layer 21 is added between the channel portion 13b and the buffer portion 13a of the buffer layer 13.
  • the back barrier layer may be an AIGaN layer.

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Abstract

The present document discloses a heterostructure, for producing a high electron mobility transistor (HEMT). The heterostructure comprises a GaN a buffer layer with a buffer portion proximally to a substrate and a channel portion distally from the substrate, an Alx1Ga1-x1N exclusion layer, wherein 0.3≤x1≤1.0, preferably 0.5≤x1≤1.0, most preferably 0.7≤x1≤1.0, formed on the GaN buffer layer, and an lnyAIx2Ga1-x2-yN barrier layer, wherein 0≤y≤0.25, preferably 0.10≤y≤0.22, most preferably 0.15≤y≤0.20, and 0.70≤x2≤1.0, preferably 0.75≤x2≤0.9, most preferably 0.80≤x2≤0.85. The heterostructure has an AIx3Ga1-x3N interlayer, wherein 0≤x3≤0.5, preferably 0≤x3≤0.3, most preferably 0≤x3≤0.1, sandwiched between the exclusion layer and the barrier layer. The heterostructure presents a 2DEG electron mobility of 1800-2300 cm2/Vs, preferably 1900-2200 cm2/Vs, most preferably 2000-2100 cm2/Vs.

Description

INTERLAYER BARRIER
Technical Field
The present disclosure relates to a heterostructure for semiconductor devices and to a method for producing the same.
Background
WO201 6155794A1 discloses AlxGai-xN/GaN heterostructu res that are suitable for manufacturing high electron mobility (HEMT) devices.
In order to achieve higher thermal stability of the heterostructure it may be preferred to use materials comprising indium, such as InAIN and/or InAIGaN, generally expressed as lnxAlyGai-x-yN.
Prior art lnxAlyGai-x-yN have, however, at best, been able to achieve 2DEG electron mobilities in the range 1400-1800 cm2/Vs (see e.g. Wang et al, IEEE Electron Device Letters, Vol. 32, No. 9, 1215 (201 1 )) .
There is thus a need for improved lnxAlyGa-i-x-yN/GaN heterostructu res with higher electron mobility.
Summary
An object of the present invention is to provide an improved lnxAlyGa-i-x- yN/GaN heterostructure, and in particular an lnxAlyGa-i-x-yN/GaN
heterostructure which is improved in terms of one or more of the above mentioned properties.
The invention is defined by the appended independent claims, with embodiments being set forth in the appended dependent claims, in the following description and in the drawings.
According to a first aspect, there is provided a heterostructure, for producing a high electron mobility transistor (HEMT), comprising a GaN a buffer layer comprising a buffer portion proximally to a substrate and a channel portion distally from the substrate, an AlxiGai-xi N exclusion layer, wherein 0.3<x1 <1 .0, preferably 0.5<x1 <1 .0, most preferably 0.7<x1 <1 .0, formed on the GaN buffer layer, and an lnyAIX2Gai-x2-yN barrier layer, wherein 0 <y≤0.25, preferably 0.10≤y<0.22, most preferably 0.15≤y<0.20, and
0.70≤x2<1 , preferably 0.75≤x2<0.9, most preferably 0.80≤x2<0.85. An Alx3Gai-x3N interlayer, wherein 0≤x3<0.5, preferably 0≤x3<0.3, most preferably 0≤x3<0.1 , is sandwiched between the exclusion layer and the barrier layer. The heterostructure presents a 2DEG electron mobility of 1800- 2300 cm2/Vs, preferably 1900-2200 cm2/Vs, most preferably 2000-2100 cm2/Vs.
"Heterostructure" is herein defined as a structure comprising a stack of two or more different epitaxial materials.
By "formed on" may be construed as "formed directly on", alternatively this may be interpreted as there can be one or more additional layers which do not affect the function of the heterostructure.
By "2DEG electron mobility" is meant the mobility an electron gas is free to move in two dimensions, but being tightly confined in the third dimension.
Heterostructures based on the above mentioned materials and having such high mobility have, as far as is known to the inventors, not previously been available.
An advantage of this heterostructure is that it has higher electron mobility as compared to prior art lnxAlyGai-x-yN/GaN heterostructures.
The Alx3Gai-x3N, interlayer may have a thickness of 0.20 to 5 nm, preferably 0.20 to 3 nm, most preferably 0.20 to 1 nm.
The interlayer may be as thin as possible while the electron mobility enhancement is sustained. 0.20 nm is the thickness close to a monolayer of the Alx3Gai-x3N interlayer.
The exclusion layer may have a thickness of 0.5 to 2.5 nm, preferably 0.8 to 2 nm, most preferably 1 .0 to 1 .5 nm.
The barrier layer may have a thickness of 2 to 12 nm, preferably 3 to 10 nm, most preferably 4 to 8 nm.
The buffer layer may have a thickness of 1 .0 to 4.0 μηπ, preferably 1 .5 to 3.0 μηπ, most preferably 1 .8 to 2.0 μηι. The heterostructure may further comprise a substrate and an AIN nucleation layer formed on the substrate, wherein the GaN buffer layer may be formed on the AIN nucleation layer.
The substrate may be a SiC, sapphire, Si, GaN, AIN or diamond substrate.
The nucleation layer may have a thickness of 5 to 200 nm, preferably 20 to 100 nm, most preferably 30 to 60 nm.
The heterostructure may further comprise a passivation layer formed on the barrier layer.
The passivation layer may be a GaN or a SiN layer.
The passivation layer may have a thickness of 1 to 20 nm, preferably 1 .5 to 15 nm, most preferably 2 to 10 nm.
The sheet resistance of the heterostructure may be 100 to 350 Ω, preferably 150 to 300 Ω, most preferably 180 to 250 Ω.
The heterostructure may present a 2DEG density of 0.8E+12 cm-2 to
2.5E+13 cm-2, preferably 1 E+13 cm-2 to 2.2E+13 cm-2, most preferably 1 .2E+13 cm"2 to 2.0E+13 cm"2.
According to a second aspect there is provided a HEMT device comprising the heterostructure.
According to a third aspect a method of producing a heterostructure in a MOCVD reactor is provided. The method comprises flowing Ga and N precursor gases into the MOCVD reactor forming a GaN buffer layer, flowing Al and N precursor gases into the MOCVD reactor forming an AlxiGai-xi N exclusion layer on the GaN buffer layer, flowing In, Al, Ga and N precursor gases into the MOCVD reactor forming an lnyAIX2Gai-x2-yN barrier layer on the AlxiGai-xi N exclusion layer. The method comprises flowing Ga, Al and N precursor gases into the MOCVD reactor for forming a AIX3Gai-x3N interlayer on the Alxi Gai-xi N exclusion layer prior to forming the lnyAIX2Gai-x2-yN barrier layer, wherein forming the buffer layer and the exclusion layer comprise steps of: stopping at least the Ga precursor flow in the forming of the buffer layer, waiting 1 -20 seconds, preferably 2-15 seconds or 3-10 seconds,
subsequently applying a pre-flow of Al precursor such that an AIN growth rate of less than 100 nm/h is achieved for a time sufficient to form less than 100 % of a monolayer of the AIN layer, preferably less than 80 % of a monolayer of the AIN layer, most preferably less than 60 % of a monolayer of the AIN layer.
The method may further comprise, when forming the exclusion layer, a step of increasing the exclusion layer growth rate to 100-300 nm/h, preferably 100-200 nm/h, most preferably 100-150 nm/h subsequently to the step of applying the Al precursor pre-flow. The increase of exclusion layer growth may be made by an increase of a second flow rate of the Al precursor.
The pressure in the MOCVD reactor upon growth of the
heterostructure may be 10 to 1000 mbar, perferably 30 to 200 mbar, most preferably 50 to 100 mbar.
The temperature in the MOCVD reactor upon growth of the
heterostructure may be 950 to 1 150 °C, perferably 1000 to 1 100 °C, most preferably 1020 to 1080 °C.
The method may further comprise a step of decreasing a temperature in the MOCVD reactor from 950-1 150 °C to 650- 950°C, preferably to 700- 900°C, most preferably to 750-850°C prior to forming the barrier layer 1 6.
The method may further comprise etching away the interlayer by 20 to 100 %, preferably by 30 to 80 %, most preferably 40 to 60 % of the total thickness of the interlayer, the etching being performed during the step of decreasing the temperature, and optionally also during an additional waiting period prior to a next processing step.
The interlayer may be etched by H2 in the MOCVD reactor and/or thermally etched.
The method may comprise an initial step of providing a substrate and arranging the substrate in the MOCVD reactor.
The method may further comprise forming a nucleation layer on the substrate. A buffer layer may then be formed on the nucleation layer.
A nucleation layer may not be needed if using a GaN or a AIN substrate. However, GaN or AIN may be grown onto the GaN or AIN substrate to obtain a clean surface. Such an additional layer of GaN or AIN may be about 100 to 300 nm. According to a fourth aspect a method of producing a HEMT device is provided, which comprises the steps of providing a source, a drain, and a gate, contact onto the barrier layer of the heterostructure.
The method of producing a HEMT device may comprise an additional step of providing an insulating layer between the gate contact and the barrier layer.
The method of producing a HEMT device may comprise the steps of providing a heterostructure, providing an insulating layer onto the barrier layer, providing a gate metal onto the insulating layer, providing a source and a drain contact such that they extend into the channel portion of the buffer layer.
The method of producing a HEMT device may further comprise an additional step of providing an AIGaN back barrier layer between the buffer portion and channel portion of the buffer layer.
Description of the Drawings
Fig. 1 a-1 b schematically illustrate embodiments of an lnyAIX2Gai-x2- yN/GaN heterostructure as disclosed herein.
Fig. 2a-2d schematically illustrates examples of HEMT devices comprising the heterostructure of Fig. 1 a or 1 b.
Detailed Description
The concept disclosed herein will now be explained more in detail. First a heterostructure for semiconductor devices is discussed and thereafter a method for producing the heterostructure is described. Finally,
characterization results of heterostructures produced by this method are discussed.
Heterostructure
As discussed above, lnyAIX2Gai-x2-yN/GaN heterostructures may be used in semiconductor devices, such as in high electron mobility transistor (HEMT) devices. Fig. 1 a schematically illustrates an example of such a heterostructure. The heterostructure in Fig. 1 a comprises, when viewed from bottom to top, an optional substrate 1 1 , an AIN nucleation layer 12, a GaN buffer layer 13, having a buffer portion 13a and a channel portion 13b, an AlxiGai-xiN exclusion layer 14, an AIX3Gai-x3N interlayer 15, an lnyAIX2Gai-x2-yN barrier layer 1 6 and optionally a GaN or SiN passivation layer 17.
As substrate 1 1 , a SiC substrate may be used due to its high thermal conductivity properties in order to efficiently extract heat and to minimize temperature rise in the semiconductor device. The SiC polytype may be 4H or 6H. As non-limiting examples of alternatives, a Si, sapphire, GaN or AIN substrate may be used.
A nucleation layer 12 of AIN may be deposited onto the substrate. Alternatively, if using an AIN substrate, the AIN substrate may function as a combined substrate and nucleation layer and hence an additional AIN layer may not be added. However, if using an AIN substrate, typically a layer of AIN may be grown onto the substrate to obtain a fresh AIN surface. The same applies if using a GaN substrate, GaN may be grown onto the substrate to obtain a fresh GaN surface.
The purpose of the AIN nucleation layer is to compensate for the lattice mismatch between the substrate 1 1 and the GaN buffer layer 13, and to obtain high quality epitaxial growth of the GaN buffer layer on the substrate.
The nucleation layer enables growth of the GaN buffer layer 13 since GaN does not nucleate two-dimensionally on some substrates, such as SiC, by changing the surface potential, such that GaN can be grown.
The buffer layer 13 of GaN comprises a buffer portion 13a proximally to the substrate and a channel portion 13b distally from the substrate.
The buffer portion 13a may be doped by acceptor-like impurities like carbon or iron. The acceptor-like impurities compensate the free electrons to obtain semi-insulating properties. The dopant concentration of carbon or iron may preferably be higher than 5- 1016 cm-3, more preferably higher than 1■ 1017 cm-3, most preferably higher than 1■ 1018 cm-3. A part of the buffer portion may also comprise aluminum to form an AIGaN back barrier layer.
The channel portion 13b may be unintentionally doped, which means that no intentional doping is used.
However, the amount of e.g. carbon in the channel portion may be varied by varying the pressure and/or the temperature in the MOCVD reactor.
The purpose of the buffer portion 13a GaN buffer layer 13 is to develop the structure quality by a thick layer growth and to exhibit a semi-insulating property.
The channel portion 13b of the GaN buffer layer prevents possible trapping effects and ionized impurity scattering that adversely affects 2DEG density and mobility.
The amount of residual impurites in the channel portion 13b of the GaN buffer layer 13 such as oxygen, silicon and/or carbon, may be low, typically of about less than 5- 1016 cm-3, i.e. close to the detection limit of Secondary Ion Mass Spectroscopy (SIMS).
The purpose of the AlxiGai-xi N exclusion layer 14 is to reduce the alloy and interface scatterings, thus enhancing the 2DEG mobility.
The introduction of an AIX3Gai-x3N interlayer 15 between the exclusion layer 14 and the indium comprising barrier layer 1 6, see Fig. 1 a, may prevent the AlxGai-xN exclusion layer from relaxation during a growth interruption step (see discussion on growth below).
By "relaxation" is meant that the biaxial lattice constant of the AIGaN layer becomes shorter than the biaxial lattice constant of the GaN buffer.
Moreover, the interlayer may protect the exclusion layer from
unintentional thermal and/or chemical etching during the growth interuption.
One major advantage of using a barrier layer 1 6 comprising indium, such as lnyAlx2Gai-x2-yN, is that a lattice matched condition can be achieved between the barrier layer 1 6 and the buffer layer 13, when the percentage of indium in the barrier layer is about 17 to 18% and of Al is about 82 to 83 %. The lattice match means that ideally no strain is built in the between lnyAlx2Gai-x2-yN/GaN heterostructure. An indium comprising barrier layer, i.e. an lnyAlx2Gai -x2-yN/GaN heterostructure may be more thermally stable as compared to AIGaN/GaN or AIN/GaN heterostructures.
By the use of an indium comprising barrier layer, a lattice matched or a nearly lattice matched lnyAIX2Gai -x2-yN/GaN heterostructure can be realized, while the high 2DEG density can still be obtained due to the high
spontaneous polarization of the lnyAIX2Gai -x2-yN barrier layer.
The purpose of the optional passivation layer 17 of GaN or SiN is to stabilize the surface conditions of the HEMT structure since the surface conditions influence the 2DEG density. By the use of a GaN or SiN
passivation layer the 2DEG density may be increased or decreased, but the 2DEG mobility will not change much.
Method for growth of the heterostructure
The heterostructure may be deposited by Metal Organic Chemical Vapor Deposition (MOCVD), which also is known as Metal Organic Vapor Phase Epitaxy (MOVPE). MOCVD, or MOVPE, is a chemical vapor deposition method in which a solid material is deposited onto a substrate by chemical reactions of vapor phase precursors. The method is mainly used for growing complex semiconductor multilayer structures.
In MOCVD, the precursors are metal-organic compounds, typically in combination with a hydride gas such as ammonia, N h .
Precursors used for the AIN nucleation layer 12 growth may be N hta, and TMAI, trimethylaluminum, Al2(CH3)6 or triethylaluminium, TEA,
Figure imgf000010_0001
Precursors used for GaN buffer layer 13 growth may be TMGa, trimethylgallium, i.e. Ga(CH3)3 or TEGa, triethylgallium Ga(C2Hs)3, and ammonia, N hta.
Precursors used for growth of the Alxi Gai -xi N exclusion layer 14 and the Alx3Gai -x3N interlayer 15 growth may be TMGa or TEGa, and TMAI or TEA, and NH3.
Precursors used for growth of the lnyAIX2Gai -x2-yN barrier layer 1 6 may be e.g. TMIn, trimethylindium, i.e. ln(CH3)3, TMAI or TEA and TMGa or TEGa. Precursors used for growth of an SiN passivation layer may be SiH4 and NH3.
Precursors used for growth of an GaN passivation layer may be TMGa or TEGa and NH3.
The precursors are transported, often by means of a carrier gas, such as H2, N2 or a mixture of N2 and H2, into a MOCVD reactor, in which at least one substrate or sample (e.g. a part of a HEMT structure) is placed.
The flow rate of the carrier gas, flowing through a precursor (e.g. TMAI or TMGa) bubbler, may be about 70 ml/min for TMAI, about 50 ml/min for TMGa and 70 ml/min for TMIn.
The precursor flow merges with a main carrier gas flow, which may be on the order of 30 l/min for further transport to the reactor.
Flow rates of the precursor gases are discussed below.
Reactions of the precursors form reactive intermediates and by- products take place on or in near vicinity of the substrate(s). The reactants are adsorbed on the substrate, forming a thin film layer and finally byproducts are transported away from the substrate, by means of pumping on the MOCVD reactor.
The pressure in a MOCVD reactor upon thin film growth normally ranges from a few mbars up to atmospheric pressure.
The reactor chamber may be of either cold-wall or hot-wall type. In a cold-wall reactor the substrate is typically heated from below while the reactor walls are kept cooler than the substrate. In contrast, in a hot-wall reactor the entire reactor chamber is heated, i.e. both the substrate and the reactor.
For growth of the heterostructure discussed in this disclosure, a hot- wall VP508GFR, Aixtron reactor was used. The background pressure of the reactor is lower than -1 - 10"4 mbar. (Refs: Doping of Al-content AIGaN grown by MOCVD, PhD thesis, D. Nilsson, 2014 and Wikipedia).
Growth of the heterostructure
The steps of growing of an lnyAIX2Gai-x2-yN/GaN heterostructure by MOCVD will now be described in detail. The sample, e.g. a part of a HEMT structure or a substrate, onto which the heterostructure is to be grown is inserted into a MOCVD reactor (for details about the MOCVD method, see above).
The pressure in the MOCVD reactor may be the same during production of all layers in the heterostructure described below. Typically, the pressure in the MOCVD reactor may be about 50 mbar upon production of the heterostructure.
As an exception, the pressure in the MOCVD reactor may be increased during production of the channel portion 13b of the buffer layer 13. The pressure in the MOCVD reactor may be about 60 mbar upon production of the channel portion 13b of the buffer layer.
The temperature in the MOCVD reactor may be the same upon growth of the nucleation layer 12, exclusion layer 14 and interlayer 15. As an example the temperature may be about 1050 °C in the MOCVD reactor.
Upon growth of the channel portion 13b of the buffer layer 13, the temperature in the MOCVD reactor may be increased. As an example the temperature may be increased to about 1060 °C.
Upon growth of the barrier layer 1 6 and the passivation layer 17 the temperature may be decreased as compared to the temperature upon growth of the layers discussed above. As an example the temperature in the MOCVD reactor may be about 950 to 650 °C upon growth of the barrier layer and the passivation layer.
During the production steps described below, the Ga, Al and In precursors may be transported to the MOCVD reactor by at least one carrier
Figure imgf000012_0001
The presursors may be provided at room temperature. As an alternative, at least one of the precursors may be heated in order to increase the vapor pressure and hence the growth of the different layers.
By "room temperature" is meant a temperature of 1 6 °C to 27 °C.
The flow of the precursors and/or the carrier gases may be controlled by at least one mass flow controller which may be situated between each of the precursor containers and the MOCVD reactor. The control of providing the presursors to the MOCVD reactor may be performed by opening or closing at least one valve situated between each of the precursor containers and the MOCVD reactor. The opening and closing may be performed manually or by computer control.
To eliminate build-up of gas bursting into the MOCVD reactor the flow of the precursors may be directed into a secondary line bypassing a main run line. This secondary line is called "vent line". A pressure balance may be provided between the vent line and the run line to avoid flow bursts when gas is switched into the main carrier flow.
Below, examples of flow rates of the precursors used upon production of the different heterostructure layers are discussed. The selected flow rates and the times the precursors are provided into the MOCVD reactor is dependent on many different parameters, such as the MOCVD reactor size, the sample/substrate size, the distance between the gas outlet of the precursor and the sample/substrate, the background pressure in the MOCVD reactor etc. Hence, the flow rate and time the precursor gas is provided may vary in different experimental set-ups.
The skilled person may generally be expected to be able to provide a layer of a predetermined thickness, composition and quality.
During all process steps below a flow of Nh may be provided. The flow rate of Nh may be kept constant during all process steps.
As an exception, the flow rate of Nhh may be decreased upon production of the AIN nucleation layer growth.
As an example the flow rate of Nhh may be about 2 l/min for production of the buffer layer 13, exclusion layer 14, interlayer 15, barrier layer 1 6 and passivation layer 17.
As an example the flow rate of Nhta may be 0.5 to 1 .0 l/min for production of the nucleation layer 12.
The steps of producing the heterostructure may be performed in the following order: Firstly, an optional nucleation layer 1 2 may be grown onto the substrate 1 1 . As an example the flow rate may be 0.8 ml/min of TMAI and 0.5 l/min of Nhta resulting in a growth rate of about 360 nm/min.
The purity of TMAI may be more than 5N, i.e. 99.999%.
Then a buffer layer of GaN 1 3 is grown. The precursors for production of the GaN buffer layer may be a Ga precursor, e.g. TMGa, and ammonia, NHs. The purity of TMGa may be more than 5N, i.e. 99.999%.
The start of providing the Ga precursor and the Nhh may take place simultaneously. As an alternative, Nhh may be provided into the reactor before the start of providing the Ga preucursor.
As an example, the Ga precursor, e.g. TMGa may be provided to the reactor at a first flow rate of e.g. 3.2 ml/min. The flow rate of Nhh may be 2 l/min as mentioned above.
As discussed above, the buffer layer comprises two different portions, the buffer portion 1 3a and the channel portion 1 3b, respectively. During the production of the channel portion 13b, at least one, preferably both, of the pressure and temperature in the MOCVD reactor are increased, resulting in a decrease of residual carbon in the channel portion. The temperature in the MOCVD reactor may be increased to about 1 060 °C and/or the pressure in the MOCVD may be increased to about 60 mbar durning production of the channel portion 1 3b of the buffer layer 1 3.
Under those conditions, the growth rate of the GaN buffer portion 1 3a may be about 1 200 nm/h. The flow rate of the channel portion 1 3b may be about 600 nm/h by a reduction of the flow rate of the Ga precursor.
The buffer portion, Ta, may have a thickness of about 1 .8 μηι. The channel portion may have a thickness, Tt>, of about 100 to 200 nm.
The buffer layer may also comprise a so-called back barrier layer. The concentration of Al of the AIGaN back barrier layer may be higher than 3 %, preferably higher than 5 % and most preferably higher than 8 %. The thickness of the back barrier layer may be 3 nm to 1 000 nm, more preferably 3 nm to 800 nm, most preferably 3 nm to 500 nm. The back barrier layer may be produced by provision of an Al precursor upon growth of the buffer layer. The flow rates of Ga and NH3 are the same as described above for growth of the buffer layer, for example >0.02 ml/min.
After a desired thickness of the GaN buffer layer has been reached, the GaN layer growth is stopped, by (substantially) stopping the provision of the Ga precursor to the MOCVD reactor. This may be performed by switching the flow of the Ga precursor into the vent line. The flow of NH3 may be kept at the same flow rate as a above, i.e. 2 l/min.
After (substantially) stopping the provision of the Ga precursor, an optional step of waiting for about 1 to 20 seconds, normally 3 to 5 seconds, may be performed. The time of waiting is highly dependent on the design of the reactor, such as the reactor size. The longer waiting, the larger risk of thermal or chemical etching by H2 of the channel portion 13b of the buffer layer 13. The purpose of this step of waiting is to let a portion of residual Ga be flushed away from the MOCVD reactor by means of pumping.
The step of stopping the Ga precursor and waiting for some time before growth of the AlxiGai-xi N exclusion layer is commenced, is referred to as a "sharpening", which provides a more distinct, "sharper", transition between the buffer layer 13 and the exclusion layer 14.
Then a Al precursor, e.g. TMAI, may be provided to the MOCVD reactor at a first flow rate, a so-called pre-flow of Al.
The pre-flow of the Al precursor is provided in order to facilitate a steep Al content transition from GaN to AIGaN in a transition zone, but not complete, formation of an AIN layer on the GaN layer.
Preferably, the time and flow rate of the pre-flow of the Al precursor may be selected such that less than one monolayer of AIN is formed, preferably less than 90 % of a monolayer, less than 75 % of a monolayer or less than 50 % of a monolayer. The definition of "monolayer of AIN" is that the surface is covered by one molecular layer of AIN. One monolayer of AIN has a thickness of about 0.2 nm.
Examples of different flow rates and flow times will be as set forth below. As an example, the first flow rate, the so-called pre-flow, of the Al precursor may be 0.2 ml/min and the flow rate of Nhh may be the same as above, i.e. about 2 l/min.
This step (i.e. pre-flow of Al) may be carried out for about 20 to 30 seconds. The steps of stopping the Ga precursor flow and providing the Al precursor for 20 to 30 seconds is in this disclosure referred to as an
"sharpening step".
After this so-called "sharpening step" the AlxiGai-xiN exclusion layer 14 is grown. During the growth of the exclusion layer no Ga precursor may be provided since there is residual Ga in the MOCVD reactor from the GaN buffer layer 13 growth. The flow of the Al precursor, the so-called "second flow" may be kept at the same flow rate as the so-called pre-flow of Al precursor. Alternatively, the second flow of Al may be increased as compared to the pre-flow of Al. As an example the flow rate of Al provided at this step may be the same as above, i.e. 0.2 ml/min.
Then an AIX3Gai-x3N interlayer 15 is grown on the exclusion layer 14. Typically a layer of GaN is preferred as interlayer 15. As an example, in the MOCVD reactor herein, the flow rate of TMGa may be 1 .0 ml/min and the flow rate of Nhta may be 2 l/min upon GaN interlayer growth.
A portion of the interlayer may disapear due to due to thermal etching during the temperature decrease performed before growth of the barrier layer 1 6 as discussed below. Typically a interlayer of about 4 nm is grown and about 2 nm of this layer may be thermally and/or chemically etched away. The entire interlayer may also function as a sacrifice layer.
Then a barrier layer 16 is grown on the interlayer 15. Before starting the growth of the barrier layer, the temperature in the MOCVD reactor may be decreased. The temperature may be decreased by turning off (or decreasing) the power provided by the power supply for heating the MOCVD reactor.
For the MOCVD reactor herein, it typically takes about 20 min to decrease the temperature to about 800 °C. As an example the flow rate of TMAI may be 0.5 ml/min, the flow rate of TMIn may be 1 .25 ml/min and the flow rate of Nhta may be 2 l/min for production of the barrier layer.
Finally an optional passivation of GaN or SiN may be grown on the barrier layer.
As an example for growth of an GaN passivation layer, the flow rates of TMGa may be 3.2 ml/min and Nhta may be 2 l/min.
As an example for growth of an SiN passivation layer the flow rate of 250 ppm SiH4 may be 250 ml/min and Nhta may be 0.5 ml/min.
The temperature upon growth of the passivation layer may the same as upon growth of the barrier layer, i.e. about 800 °C.
Results
In table 1 two different heterostructure samples are discussed. The two samples have the same thicknesses and compositions of the barrier layer 1 6 and the exclusion layer 14. The first sample is grown according to the method descibred above but without insertion of an interlayer 15. The second sample is grown according to the method described above and with an AIX3Gai-x3N interlayer 15 sandwiched between the lnyAIX2Gai-x2-yN barrier layer 16 and the AlxiGai-xi N exclusion layer 14. Both samples are grown using the sharpening technique (sharpening step) as discussed above.
Figure imgf000017_0001
Table 1 . Example of two samples, sample 1 without an interlayer and sample 2 with an interlayer, respectively. As can be seen, sample 2 (with interlayer) has a significantly higher 2DEG electron mobility as compared to sample 1 (without interlayer).
Typically prior art heterostructures show a 2DEG electron mobility of about 1500 cm2/Vs (see e.g. Wang et al, IEEE Electron Device Letters, Vol. 32, No. 9, 1215 (201 1 )).
As can be seen in Table 1 , the 2DEG density of sample 2 is lower as compared to the 2DEG density of sample 1 since the distance from the surface to the channel portion of the buffer layer is increased by adding the GaN interlayer. It is not clear how the decreased 2DEG density affects the a HEMT device performance, since the 2DEG electron mobility is increased.
As also is seen in Table 1 , the sheet resistance of sample 2 is slightly increased as compared to sample 1 since the increased 2DEG electron mobility does not compensate for its decreased 2DEG density.
The increased 2DEG electron mobility of sample 2 is due to the combination of the addition of the AIX3Gai-x3N interlayer 15 and the
sharpening interface technique used upon growth of the interface between the buffer layer 13 and exclusion layer 14.
Examples of HEMT structures
In Figs. 2a-2d four different HEMT devices comprising the
heterostructure as discussed above is shown.
Fig. 2a schematically illustrates a HEMT device 2 comprising, from bottom to top, a SiC substrate 1 1 , an AIN nucleation layer 12, a GaN buffer layer 13 with a buffer portion 13a and a channel portion 13b, an AIN exclusion layer 14, a GaN interlayer 15 (as discussed above, this layer may be an Alx3Gai-x3N layer, although GaN is preferred), an indium comprising barrier layer, lnyAIX2Gai-x2-yN,1 6. On top of the barrier layer 16, source 18, gate 19 and drain 20 contacts are deposited.
The HEMT device 2' illustrated in Fig. 2b has the same structure as the HEMT device shown in Fig. 2a with the addition that it has an insulating layer 22 between the source 18 and drain 20 contacts and below the gate contact 19. The insulating layer 22 may be SiNx or an oxide layer such as AI2O3. In Fig. 2c, another example of a HEMT device 2" is illustrated. This device has the same structure as the device in Fig. 2b but with the difference that the source 18 and drain 20 contacts extend into the layers of the device, at least partially into the channel portion 13b of the buffer layer 13. Hence the channel portion 13b, the exclusion layer 14, the interlayer 15, the barrier layer 1 6 and the insulating layer 22 are located between the source 18 and drain 20 contacts. This method of contacting the HEMT device may be referred to as "recess contact method".
Fig. 2d schematically illustrates yet another example of a HEMT device 2"'. This device has the same structure as the device is Fig. 2a with the addition that a so-called back barrier layer 21 is added between the channel portion 13b and the buffer portion 13a of the buffer layer 13. The back barrier layer may be an AIGaN layer.

Claims

1 . A heterostructure (1 ), for producing a high electron mobility transistor (HEMT), comprising:
a GaN a buffer layer (13) comprising a buffer portion (13a) proximally to a substrate (1 1 ) and a channel portion (13b) distally from the substrate, an AlxiGai-xi N exclusion layer (14), wherein 0.3≤x1 <1 .0, preferably 0.5<x1 <1 .0, most preferably 0.7<x1 <1 .0, formed on the GaN buffer layer, and an lnyAlx2Gai-x2-yN barrier layer (1 6), wherein 0≤y<0.25, preferably 0.10≤y<0.22, most preferably 0.15<y<0.20, and 0.70≤x2<1 .0, preferably 0.75≤x2<0.9, most preferably 0.80≤x2<0.85,
characterized by
an AlxsGai-xsN interlayer (15), wherein 0≤x3<0.5, preferably 0≤x3<0.3, most preferably 0≤x3<0.1 , sandwiched between the exclusion layer (14) and the barrier layer (1 6),
said heterostructure presenting a 2DEG electron mobility of 1800-2300 cm2/Vs, preferably 1900-2200 cm2/Vs, most preferably 2000-2100 cm2/Vs.
2. The heterostructure (1 ) according to claim 1 , wherein the Alx3Gai-x3N, interlayer (15) has a thickness of 0.20 to 5 nm, preferably 0.20 to 3 nm, most preferably 0.20 to 1 nm.
3. The heterostructure (1 ) according to any of the preceding claims, wherein the exclusion layer (14) has a thickness of 0.5 to 2.5 nm, preferably 0.8 to 2 nm, most preferably 1 .0 to 1 .5 nm.
4. The heterostructure (1 ) according to any of the preceding claims, wherein the barrier layer (16) has a thickness of 2 to 12 nm, preferably 3 to 10 nm, most preferably 4 to 8 nm.
5. The heterostructure (1 ) according to any of the preceding claims, wherein the buffer layer (13) has a thickness of 1 .0 to 4.0 μηπ, preferably 1 .5 to 3.0 μηπ, most preferably 1 .8 to 2.0 μηι.
6. The heterostructure (1 ) as claimed in any of the preceding claims, further comprising a substrate (1 1 ) and an AIN nucleation layer (12) formed on the substrate, wherein the GaN buffer layer (13) is formed on the AIN nucleation layer.
7. The heterostructure (1 ) according to claim 6, wherein the substrate (1 1 ) is a SiC, sapphire, Si, GaN, AIN or diamond substrate.
8. The heterostructure (1 ) according to any of claims 6 or 7, wherein the nucleation layer (12) has a thickness of 5 to 200 nm, preferably 20 to 100 nm, most preferably 30 to 60 nm.
9. The heterostructure (1 ) as claimed in any of the preceding claims, further comprising a passivation layer (17) formed on the barrier layer (1 6).
10. The heterostructure (1 ) as claimed in claim 9, further wherein the passivation layer (17) is GaN or SiN.
1 1 . The heterostructure (1 ) according to any of claims 9 or 10, wherein the passivation layer (17) has a thickness of 1 to 20 nm, preferably 1 .5 to 15 nm, most preferably 2 to 10 nm.
12. The heterostructure (1 ) according to any of the preceding claims, wherein the sheet resistance is 150 to 350 Ω, preferably 1 70 to 300 Ω, most preferably 180 to 250 Ω.
13. The heterostructure (1 ) according to any of the preceding claims, wherein the heterostructure presenting a 2DEG density of 0.8E+12 cm-2 to 2.5E+13 cm-2, preferably 1 E+13 cm-2 to 2.2E+13 cm-2, most preferably 1 .2E+13 cm"2 to 2.0E+13 cm"2.
14. A HEMT device (2, 2', 2", 2"') comprising the heterostructure (1 ) according to any of the preceding claims.
15. Method of producing a heterostructure (1 ) in a MOCVD reactor, comprising:
flowing Ga and N precursor gases into the MOCVD reactor forming a GaN buffer layer (13),
flowing Al and N precursor gases into the MOCVD reactor forming an AlxiGai-xi N exclusion layer (14) on the GaN buffer layer,
flowing In, Al, Ga and N precursor gases into the MOCVD reactor forming an lnyAIX2Gai-x2-yN barrier layer (1 6) on the AlxiGai-xi N exclusion layer (14),
characterised by
flowing Ga, Al and N precursor gases into the MOCVD reactor for forming a AIX3Gai-x3N interlayer (15) on the AlxiGai-xi N exclusion layer prior to forming the lnyAIX2Gai-x2-yN barrier layer,
wherein forming the buffer layer and exclusion layers comprise steps of:
stopping at least the Ga precursor flow in the forming of the buffer layer,
waiting 1 -20 seconds, preferably 2-15 seconds or 3-10 seconds, subsequently applying Al precursor flow such that an AIN layer growth rate of less than 100 nm/h is achieved for a time sufficient to form less than 100 % of a monolayer of the AIN layer, preferably less than 80 % of a monolayer of the AIN layer, most preferably less than 60 % of a monolayer of the AIN layer.
16. The method according to claim 15, wherein forming the exclusion layer (14) comprises a step of increasing the exclusion layer growth rate to 100-300 nm/h, preferably 100-200 nm/h, most preferably 100-150 nm/h subsequently to the step of applying the Al precursor flow.
17. The method according to any of claims 15 to 17, wherein a pressure in the MOCVD reactor upon growth of the heterostructure is 10 to 1000 mbar, perferably 30 to 200 mbar, most preferably 50 to 100 mbar.
18. The method according to any of claims 15 to 17, wherein a temperature in the MOCVD reactor upon growth of the heterostructure is 950 to 1 150 °C, perferably 1000 to 1 100 °C, most preferably 1020 to 1080 °C.
19. The method according to claim 15 to 18, comprising a step of decreasing a temperature in the MOCVD reactor from 950-1 150 °C to 650- 950°C, preferably to 700-900°C, most preferably to 750-850°C prior to forming the barrier layer (1 6).
20. The method according to claim 19, further comprising etching away the interlayer (15) by 20 to 100 %, preferably by 30 to 80 %, most preferably 40 to 60 % of the total thickness of the interlayer, said etching being performed during said step of decreading the temperature, and optionally also during an additional waiting period prior to a next processing step.
21 . Method as claimed in any one of claims 15 to 20, wherein the method comprises an initial step of providing a substrate and arranging the substrate in the MOCVD reactor.
22. The method as claimed in claim 21 , further comprising forming a nucleation layer (12) on the substrate (1 1 ), and wherein the buffer layer (13) is formed on the nucleation layer.
23. Method of producing a HEMT device (2, 2', 2", 2"') comprising the steps of
providing a heterostructure (1 ) according to claim 1 , and providing a source (18), a gate (19) and a drain (20) contact onto the barrier layer (1 6).
24. The method of producing a HEMT device (2, 2', 2", 2"') according to claim 22, comprising an additional step of providing an insulating layer (22) between the gate contact (19) and the barrier layer (1 6).
25. The method of producing a HEMT device (2, 2', 2", 2"') according to claim 22 comprising the steps of
providing a heterostructure (1 ) according to claim 1 ,
providing an insulating layer (22) onto the barrier layer (1 6), providing a gate contact (19) onto the insulating layer,
providing a source (18) and a drain (20) contact such that they extend into the channel portion (13b) of the buffer layer (13).
26. The method of producing a HEMT device (2, 2', 2", 2"') according to claim 22, comprising an additional step of providing an AIGaN back barrier layer (21 ) between the buffer portion (13a) and channel portion (13b) of the buffer layer (13).
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Citations (2)

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WO2013120990A1 (en) * 2012-02-17 2013-08-22 Isos Technologies Sarl Hemt heterostructure and a method of hemt manufacturing
WO2016155794A1 (en) 2015-03-31 2016-10-06 Erik Janzén Heterostructure and method of its production

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WO2013120990A1 (en) * 2012-02-17 2013-08-22 Isos Technologies Sarl Hemt heterostructure and a method of hemt manufacturing
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