Channel Characteristics of Inas/Alsb Heterojunction Epitaxy: Comparative Study on Epitaxies with Diﬀerent Thickness of Inas Channel and Alsb Upper Barrier

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Article Channel Characteristics of InAs/AlSb Heterojunction Epitaxy: Comparative Study on Epitaxies with Diﬀerent Thickness of InAs Channel and AlSb Upper Barrier

He Guan 1, Shaoxi Wang 1,* , Lingli Chen 1, Bo Gao 2, Ying Wang 1 and Chengyu Jiang 1

1 School of Microelectronics, Northwestern Polytechnical University (NPU), Xi’an 710072, China; [email protected] (H.G.); [email protected] (L.C.); [email protected] (Y.W.); [email protected] (C.J.) 2 Xi’an Microelectronics Technology Institute, Xi’an 710000, China; bobbygoﬀ@foxmail.com * Correspondence: [email protected]

Received: 11 April 2019; Accepted: 9 May 2019; Published: 13 May 2019

Abstract: Because of the high electron mobility and electron velocity in the channel, InAs/AlSb high electron mobility transistors (HEMTs) have excellent physical properties, compared with the other traditional III-V semiconductor components, such as ultra-high cut-off frequency, very low power consumption and good noise performance. In this paper, both the structure and working principle of InAs/AlSb HEMTs were studied, the energy band distribution of the InAs/AlSb heterojunction epitaxy was analyzed, and the generation mechanism and scattering mechanism of two-dimensional electron gas (2DEG) in InAs channel were demonstrated, based on the software simulation in detail. In order to discuss the impact of different epitaxial structures on the 2DEG and electron mobility in channel, four kinds of epitaxies with different thickness of InAs channel and AlSb upper-barrier were manufactured. The samples were evaluated with the contact Hall test. It is found the sample with a channel thickness of 15 nm and upper-barrier layer of 17 nm shows a best compromised sheet carrier concentration of 2.56 1012 cm 2 and electron mobility of 1.81 104 cm2/V s, and a low × − × · sheet resistivity of 135 Ω/, which we considered to be the optimized thickness of channel layer and upper-barrier layer. This study is a reference to further design InAs/AlSb HEMT, by ensuring a good device performance.

Keywords: InAs/AlSb heterojunction; epitaxy; 2DEG; electron mobility

1. Introduction Compared with traditional III-V semiconductor materials such as GaAs, InP and GaN, antimonide-based compound semiconductors (ABCS) have higher electron mobility and electron velocity, which present them with broad prospects in ultra-high speed, low power consumption and low noise applications. InAs with a lattice constant of about 6.05 eV is a typical ABCS material. Its electron mobility is 3 and 5 times of that of GaAs and InP materials, respectively; its electron saturation drift velocity is about 5 times of that of GaAs and InP materials; its eﬀective mass of electrons is 1/3 of that of GaAs and InP materials; especially, its bandgap is only 0.35 eV, and is much more narrow than GaN, which makes InAs have excellent electrical properties at very low bias voltage. So InAs is often used as channel material for high-speed and low-power-consumption HEMT. As another typical antimony material, AlSb has little lattice mismatch with InAs, but its band gap of 1.27 eV with InAs can form very deep electron potential wells, and makes the InAs/AlSb heterogeneous structures possess a high concentration of two-dimensional electron gas density (2DEG).

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As another typical antimony material, AlSb has little lattice mismatch with InAs, but its band gapCoatings of 2019 1.27, 9 eV, 318 with InAs can form very deep electron potential wells, and makes the InAs/AlSb2 of 7 heterogeneous structures possess a high concentration of two-dimensional electron gas density (2DEG). Therefore, InAs/AlSb high electron mobility transistors (HEMTs) devices, with AlSb as a Therefore, InAs/AlSb high electron mobility transistors (HEMTs) devices, with AlSb as a barrier layer barrier layer and InAs as the channel layer, have excellent physical properties, such as ultra-high cut- and InAs as the channel layer, have excellent physical properties, such as ultra-high cut-off frequency, off frequency, very low power consumption and good noise performance, and present a very good very low power consumption and good noise performance, and present a very good application application prospect in analog/digital circuit, microwave field and space communication [1–5]. As prospect in analog/digital circuit, microwave field and space communication [1–5]. As we know, we know, the properties of HEMTs are fundamentally determined by the performance of epitaxial the properties of HEMTs are fundamentally determined by the performance of epitaxial materials. materials. So far, lots of research on III-V epitaxial materials focus on GaN/AlGaN, InAlAs/InGaAs, So far, lots of research on III-V epitaxial materials focus on GaN/AlGaN, InAlAs/InGaAs, etc. [6–8]. etc. [6–8]. However, studies on InAs/AlSb epitaxial materials are not broad, with few results [9–12]. However, studies on InAs/AlSb epitaxial materials are not broad, with few results [9–12]. Therefore, Therefore, a further research on the structures of epitaxial materials is significant to deeply a further research on the structures of epitaxial materials is significant to deeply understand the understand the performance of devices. In this paper, the structure and working principle of performance of devices. In this paper, the structure and working principle of InAs/AlSb HEMTs were InAs/AlSb HEMTs were studied, the energy band distribution of the InAs/AlSb heterojunction studied, the energy band distribution of the InAs/AlSb heterojunction epitaxy was analyzed, and the epitaxy was analyzed, and the generation mechanism and scattering mechanism of two-dimensional generation mechanism and scattering mechanism of two-dimensional electron gas (2DEG) in the InAs electron gas (2DEG) in the InAs channel were demonstrated by simulation in detail; additionally, channel were demonstrated by simulation in detail; additionally, four kinds of epitaxy samples with four kinds of epitaxy samples with different thicknesses of InAs channel and AlSb upper barrier were different thicknesses of InAs channel and AlSb upper barrier were fabricated and tested, in order to fabricated and tested, in order to discuss their channel characteristics. discuss their channel characteristics.

2.2. Simulation Simulation and and Principle Principle Analysis Analysis InIn order order toto studystudy the the working working principle principle and and performance performance of the of InAsthe InAs/AlSb/AlSb heterojunction heterojunction in detail, in detail,a simulation a simulation is applied is applied based onbased the on typical the typical device d structureevice structure as shown as shown in Figure in1 Figure[ 4,13, 141 [4,1]. A3 200,14]. nm A 200GaAs nm material GaAs material is settled is as settled substrate. as substrate. The lattice The constant lattice ofcon GaAsstant as of 5.653 GaAs Å as presents 5.653 Å a presents significant a significantdeviation fromdeviation the value from of the InAs value as 6.058 of InAs Å, leading as 6.058 to Å a, lattice leading mismatch to a lattice degree mismatch of 7.1%. degree This mismatch of 7.1%. Thismakes mismatch it easy tomakes generate it easy defects. to generate Therefore, defects. a 700 Therefore, nm AlGaSb a 700 layer nm with AlGaSb a lattice layer constant with a of lattice 6.1 Å, constantchosen asof a6.1 bu Åffer, chosen to release as a thebuffer mismatch, to release is settledthe mismatch on GaAs, is substrate.settled on GaAs Then ansubstrate. AlSb/InAs Then/AlSb an AlSb/InAs/AlSblaminated layer as laminated a quantum layer well structure as a quantum is settled well to form structure the down-barriers is settled/ channel to form/upper-barriers. the down- barriers/channel/upperInAs forms the channel-barriers. and the 2DEGInAs forms is located the channel on the sideand ofthe the 2DEG InAs is channel located near on the the side InAs of/AlSb the InAs channel near the InAs/AlSb (upper-barrier) heterojunction. A19n Si-doped3 InAs layer with a 1019 cm−3 (upper-barrier) heterojunction. An Si-doped InAs layer with a 10 cm− doping density is inserted into dopingthe AlSb density upper-barrier is inserted layer into in order the AlSb to make upper the- semiconductorbarrier layer in exhibit order asto an make n-type the semiconductor semiconductor to exhibitcompletely as an ionize, n-type in semiconductor order to form positiveto completely ionized ionize donors, in and order free to electrons. form positive Because ionized the band-gap donors andwidth free of electrons. InAs is much Because smaller the band than- AlSb,gap width leading of InAs electrons is much in thesmaller heavy than doped AlSb, n-type leading AlSb electrons to shift infrom the theheavy AlSb doped region, n-type where AlSb it is to with shift higher from energy,the AlSb to region the non-doped, where it InAsis with channel, higher where energy it, isto with the nonlower-doped electron InAs energy. channel This, where effect it makes is with the lower electrons electron accumulating energy. This on the effect opposite makes side the of electrons the InAs accumulatingchannel to form on 2DEG. the opposite Meanwhile, side of these the positiveInAs channel ionized to donors form 2DEG. are left Meanwhile, on the AlSb these side, makingpositive a ionizedspace charge donors zone are formedleft on the at theAlSb junction side, ma toking induce a space an electric charge field. zone Thisformed electric at the field junction causes to the induce band anto electric bend. field. This electric field causes the band to bend.

FigureFigure 1. 1. StructureStructure of of InAs/AlSb InAs/AlSb hig highh electron electron mobility mobility transistor transistor ( (HEMT).HEMT).

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Therefore, the modulation-doped structure formed by heavily doped n-type wide-band-gap Coatings 2019, 9, x FOR PEER REVIEW 3 of 7 semiconductor (AlSb region), and no-doping narrow-band-gap semiconductor (InAs region), can ensure Coatings 2019, 9, x FOR PEER REVIEW 3 of 7 the electronTherefore supply, the process modulation happening-doped in structure AlSb area, formed and by the heavily electron doped transport n-type process wide-band happening-gap in thesemiconductor InAsTherefore channel. , (AlSbthe These modulation region) two, processesand-doped no-doping structure are separated narrow formed-band in by space,-gap heavily semiconductor which doped can n- etypeff (InAsectively wide region)-band eliminate, -cangap the impactensuresemiconductor of ionized the electron impurity (AlSb supply region) scattering process, and during no happening-doping electrons narrow in AlSb transportation,-band area-gap, and semiconductor the and electron greatly (InAs transport improve region) process the, can electron mobilityhappeningensure and the 2DEG in electron the in InAs the supply channelchannel. process [Thes15– 17e happening ].two Finally, process in anes AlSb InAlAsare separated area protection, and in the space, electron layer which is settled transport can effectively to processreduce hole gate leakageeliminatehappening current, the in impactthe andInAs of an channel. ionized InAs layer Thes impurity ise two settled scattering process as thees duringare cap separated layer. electrons A heavyin space,transport doping whichation wascan, and appliedeffectively greatly on the improve the electron mobility and 2DEG in the channel [15–17]. Finally, an InAlAs protection layer ohmiceliminate contact region the impact of InAs of ionized cap layer impurity in order scattering to reduce during the contact electrons resistant transport [18ation]. A, Schottky and greatly contact isimprove settled tothe reduce electron hole mobility gate leakage and 2DEG current, in the and channel an InAs [1 5layer–17]. isFinally, settled a asn InAlAsthe cap protection layer. A heavy layer forms between the gate metal electrode and the InAlAs protection layer [19], to prevent the electrons dopingis settled was to reduceapplied hole on thegate ohmic leakage contact current, region and ofan InAs InAs caplayer layer is settled in order as theto reducecap layer. the A contact heavy continuallyresistantdoping spreading was[18]. applied A Schottky to on the the contact metal. ohmic forms Controlling contact between region the the of gate ga InAste biasmetal cap voltage layerelectrode in can order and change theto reduceInAlAs the Schottky theprotection contact barrier, thuslayer controllingresistant [19 ][,18 to]. the preventA Schottky 2DEG the density contact electrons in forms channel. continually between spreadingthe gate metal to the electrode metal. and Controlling the InAlAs the protection gate bias voltageInlayer this [1 can paper,9], tochange prevent we the used theSchottky Sentaurus electrons barrier, continually Technology thus contro spreading Computerlling the to2DEG the Aided density metal. Design Controllingin channel. (TCAD) the (2010 gate bias version, SynopsysvoltageIn Inc., this can Mountain paper,change w thee View, used Schottky Sentaurus CA, USA)barrier, Technology to thus study contro the Computerl generationling the 2DEG Aided mechanism density Design in ( andTCADchannel. scattering) ( 2010 version, mechanism of 2DEGSynopsysIn in this the Inc., paper, InAs Mountain w channele used View, Sentaurus [20]. CA In, Technology UStheA simulation,) to study Computer the the generation hydrodynamicAided Design mechanism (TCAD model ) and (2010 was scattering version, chosen for electronmechanismSynopsys transport, Inc., of 2DEG theMountain drift-di in the InAs View,ffusion channel CA model, US[20A]. was) In to the chosen study simulation, the for generation hole the hydrodynamic transport, mechanism the model high-field-saturation and was scattering chosen mobilityformechanism el wasectron chosen of transport, 2DEG to in be the the the InAs drift mobility channel-diffusion model, [20 ]. model In andthe wassimulation, the chosen SRH recombinationthe for hydrodynamic hole transport, module model the washigh was chosen-field chosen- to saturation mobility was chosen to be the mobility model, and the SRH recombination module was be thefor generation-recombination electron transport, the drift module.-diffusion The model simulated was chosen result forof the hole conduction transport, band the high of InAs-field/-AlSb chosensaturation to be mobility the generation was chosen-recombination to be the mobility module. model, The simulated and the SRHresult recombination of the conduction module band was of (upper-barrier) heterojunction is shown in Figure2. It is found that a triangle electronic potential well InAs/AlSbchosen to be(upper the generation-barrier) heterojunction-recombination is module.shown in The Figure simulated 2. It is resultfound o thatf the a conduction triangle electronic band of is indeedpotentialInAs/AlSb formed well (upper is near indeed-barrier) the formed heterojunction. heterojunction near the heterojunction. is AlSb shown with in Figure higher AlSb with2. barrier It ishigher found height barrier that worksa heighttriangle well works electronic to well limit the electronstopotential limit in the well electron InAs is indeed channels in the formed InAs in order nearchannel tothe form inheterojunction. order a high to form 2DEG. AlSb a high with 2DEG. higher barrier height works well to limit the electrons in the InAs channel in order to form a high 2DEG.

FigureFigure 2. Longitudinal 2. Longitudinal distribution distributionof of conductiveconductive band band in in heterojunction heterojunction zone zone from from simulation. simulation. Figure 2. Longitudinal distribution of conductive band in heterojunction zone from simulation. The keyThe parameterskey parameters of epitaxyof epitaxy such such as 2DEGas 2DEG density, density, electron electron mobility mobility and and electron electron velocityvelocity were simulatedwere Thesim as well.ulatedkey parameters Asas well. shown As of inshown epitaxy Figure in 3such Figure, the as maximum 3,2DEG the maximumdensity, electron electron electron concentration mobility concentration and in theelectron in channelthe channelvelocity reaches reacheswere sim toulated the 19order as well. of3 10 19As cm shown−3. The insheet Figure carrier 3, the concentration maximum electronwhich can concentration be obtained byin theintegrating channel to the order of 10 cm− . The sheet carrier concentration which can be obtained by integrating the thereaches curve to of the electron order of concentration 1019 cm−3. The is sheet generally carrier at concentration the order of 10 which1212 cm− can2.2 The be electronobtained mobility by integrating at the curve of electron concentration is generally at the order of 10 cm− . The electron mobility at the channelthe curve is ofsimulated electron asconcentration shown in Figure is generally 4. It is atfound the order the electron of 1012 cmmobility−2. The oelectronn the side mobility of the atInAs the channel is simulated as shown in Figure4. It is found the electron mobility on the side of the InAs channelchannel nearis simulated the AlSb as upper shown-barrier in Figure presents 4. It ais relativelyfound the high electron value mobility with the o norder the side of 10 of4 thecm2 /InAsV·s , channel near the AlSb upper-barrier presents a relatively high value with the order of 104 cm2/V s, whichchannel is nearobviously the AlSb higher upper than-barrier the other presents side of a therelatively InAs channel. high value This with is consistent the order with of 10 the4 cm region2/V·s , · whichofwhich is2DEG obviously is locationobviously higher that higher we than analyzed than the the other above. other side side of of the the InAs InAs channel. channel. This is consistent consistent with with the the region region of 2DEGof location 2DEG location that we that analyzed we analyzed above. above.

Figure 3. Distribution of electron concentration from simulation. FigureFigure 3. Distribution3. Distribution of of electron electron concentration from from simulation simulation..

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FigureFigure 4. 4.Distribution Distribution ofof electronelectron mobility mobility from from simulation. simulation. Figure 4. Distribution of electron mobility from simulation. As canAs becan seen be seen in Figurein Figure5, the5, the electronic electronic velocity velocity isis stable in in the the channel, channel, but but appears appears an increase an increase As can be seen in Figure 5, the electronic velocity is stable in the channel, but appears an increase in thein area the area of the of AlSbthe AlSb upper upper barrier barrier near near the the sourcesource and and drain. drain. This This is isdue due to the to thehigh high field ﬁeld strength strength in the area of the AlSb upper barrier near the source and drain. This is due to the high field strength formation between the ionized donors in the upper barrier and the electrode. Electron concentration formationformation between between the the ionized ionized donors donors in in the the upper upper barrier and and the the electrode. electrode. Electron Electron concentration concentration in this part is not large, but electrons would be instantly accelerated to higher speed under the high in thisin part this part is not is large,not large but, but electrons electrons would would be be instantlyinstantly accelerated accelerated to to higher higher speed speed under under the high the high field strength. ﬁeld strength.field strength.

Figure 5. Distribution of electron velocity from simulation. FigureFigure 5. 5.Distribution Distribution of of electronelectron velocity velocity from from simulation simulation.. 3. Experiments and Analysis 3. Experiments3. Experiments and and Analysis Analysis Four kinds of InAs/AlSb epitaxies with different thicknesses of InAs channel layer and AlSb FourFour kinds kinds of InAsof InAs/AlSb/AlSb epitaxies epitaxies with with didifferentﬀerent thickness thicknesseses of of InAs InAs channel channel layer layer and andAlSb AlSb upper-barrier layer were manufactured in this paper. Sample #1, with the structure in Figure 6a as upper-barrier layer were manufactured in this paper. Sample #1, with the structure in Figure 6a as upper-barrierthe same as layer the epitaxy were manufactured module in above in simulation this paper., is Sample to be the #1, reference with the structure structure: The in InAs Figure channel6a as the the same as the epitaxy module in above simulation, is to be the reference structure: The InAs channel sameis as set the with epitaxy 15 nm module, and thein AlSb above upper simulation, barrier is set is towith be 15 the nm reference (the part structure: below Si-doping The InAs InAs channel thin is is set with 15 nm, and the AlSb upper barrier is set with 15 nm (the part below Si-doping InAs thin set withfilm 15 is 5 nm, nm). and In order the AlSb to verify upper the barrierimpact of is channel set with thickness, 15 nm (the the sample part below #2, with Si-doping a reduced InAs channel thin ﬁlm film is 5 nm). In order to verify the impact of channel thickness, the sample #2, with a reduced channel is 5 nm).thickness In order of 12 tonm verify, is designed the impact as in Figure of channel 6b; in order thickness, to explore the samplethe impact #2, of with the thickness a reduced of channelthe thickness of 12 nm, is designed as in Figure 6b; in order to explore the impact of the thickness of the thicknessAlSb ofupper 12 nm, barrier is designed layer, 17 and as in 13 Figurenm upper6b; in barrier orders are to exploreapplied in the sample impact #3 of and the sample thickness #4, of AlSb upper barrier layer, 17 and 13 nm upper barriers are applied in sample #3 and sample #4, respectively, and their structures are as shown in Figure 6c,d. the AlSbrespectively, upper barrier and their layer, structures 17 and are 13 as nm shown upper in Figure barriers 6c,d are. applied in sample #3 and sample #4, respectively,Coatings and2019, 9 their, x FOR structures PEER REVIEW are as shown in Figure6c,d. 5 of 8

(a) (b) (c) (d)

FigureFigure 6. Epitaxy 6. Epitaxy structure structure diagram diagram of of 4 4 di differentﬀerent samples: samples: (a)( isa) for is forSample Sample #1, (b #1,) is for (b) Sample is for Sample#2, #2, (c) is for(c Sample) is for Sample #3, and #3, and (d) is(d) for is for Sample Sample #4. #4.

Molecular beam epitaxy (MBE) was chosen to grow the epitaxy in this study [21]. First, a semi- insulated GaAs substrate was treated by de-oxygenation, then a GaAs buffer layer with a thickness of 200 nm was grown at 610 degree centigrade. A 700 nm AlGaSb layer was grown on the GaAs epitaxial layer to be the buffer under 580 degrees centigrade. Subsequently, we cooled the temperature to 540 degrees centigrade to grow the AlSb/InAs/AlSb layer, including inserting a thin Si-doping InAs film with four atomic layer thickness in the AlSb upper barrier layer. Finally, the InAlAs protection layer and InAs cap layer was grown at 540 degree centigrade, as well. The surface is observed by atomic force microscope (AFM) with the test area of 10 μm × 10 μm. The AFM graph is shown in Figure 7. It is found that the surface roughness is good, and the Root-mean-square value of surface roughness (RMS) is tested as around 1.4 nm, as shown in Table 1. It indicates that the epitaxy is grown with a good compactness and uniformity.

(a) (b)

Figure 7. Atomic force microscope (AFM) test result: (a) Photograph of the surface; (b) Surface longitudinal photograph.

Table 1. RMA value of the four samples.

Sample Number RMS (nm) #1 1.432 #2 1.328 #3 1.435 #4 1.394

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(a) (b) (c) (d)

CoatingsFigure2019, 96., 318Epitaxy structure diagram of 4 different samples: (a) is for Sample #1, (b) is for Sample #2, 5 of 7 (c) is for Sample #3, and (d) is for Sample #4.

Molecular beam beam epitaxy epitaxy (MBE) (MBE) was was chosen chosen to grow to grow the epitaxy the epitaxy in this in study this [21study]. First, [21 a]. semi-insulated First, a semi- GaAsinsulated substrate GaAs wassubstrate treated was by treated de-oxygenation, by de-oxygenation, then a GaAs then bu aff erGaAs layer buffer with layer a thickness with a ofthickness 200 nm wasof 200 grown nm was at 610 grown degree at centigrade. 610 degree A centigrade. 700 nm AlGaSb A 700 layer nm wasAlGaSb grown layer on thewas GaAs grown epitaxial on the layer GaAs to beepitaxial the bu ff layerer under to be580 the degrees buffer centigrade. under 580 Subsequently, degrees centigrade. we cooled theSubsequently, temperature we to 540cool degreesed the centigradetemperature to to grow 540 thedegree AlSbs /centigradeInAs/AlSb layer,to grow including the AlSb/InAs/AlSb inserting a thin layer, Si-doping including InAs inserting film with a fourthin atomicSi-doping layer InAs thickness film with in the four AlSb atomic upper layer barrier thickness layer. Finally, in the the AlSb InAlAs upper protection barrier layerlayer. and Finally, InAs capthe layerInAlAs was protectio grownn at layer 540 degree and InAs centigrade, cap layer as was well. grown The surface at 540 degree is observed centigrade by atomic, as well. force The microscope surface (AFM) with the test area of 10 µm 10 µm. The AFM graph is shown in Figure7. It is found that the is observed by atomic force microscope× (AFM) with the test area of 10 μm × 10 μm. The AFM graph surfaceis shown roughness in Figure is7. good,It is found and thethat Root-mean-square the surface roughness value is ofgood surface, androughness the Root-mean (RMS)-square is tested value as aroundof surface 1.4 roughness nm, as shown (RMS in) Tableis tested1. It indicatesas around that 1.4 thenm epitaxy, as shown is grown in Tab withle 1. a It good indicates compactness that the andepitaxy uniformity. is grown with a good compactness and uniformity.

(a) (b)

Figure 7. 7. AtomicAtomic force force microscope (AFM) test result:result: ( (aa)) P Photographhotograph of the surface;surface; (b) Surface Surface longitudinal photograph.photograph.

Table 1. RMA value of the four samples. Table 1. RMA value of the four samples.

Sample NumberSample Number RMS (nm)RMS (nm) #1 #1 1.432 1.432 #2 #2 1.328 1.328 #3 #3 1.435 1.435 #4 #4 1.394 1.394

The samples were evaluated with the contact Hall test. Four indium points were added on the surface ofof thethe epitaxyepitaxy to to be be the the external external electrodes, electrodes, and and the the performance performance parameters parameters were were tested. tested The. The test datatest data of the of the four four samples samples are are summarized summarized in in Table Table2. It2. isIt is found found that that the the mobility mobility and and sheet sheet carrier carrier concentration varyvary obviously.obviously. TheThe samplesample #1 #1 presents presents a a sheet sheet carrier carrier concentration concentration of of 2.57 2.57 ×10 101212cm cm−22 × − and an electron mobilitymobility ofof 1.581.58 × 104 cm2//VV·ss.. The The s sampleample #2 #2 shows shows an an obviously obviously increased increased mobility × · of 1.71 × 1044 cmcm22/V/V·s.s. This This is is because because the the reduced reduced thickness thickness of of the the channel channel would would suppress the lattice × · mismatch dislocation, leading to a reduced interfacial scattering, thus the mobility of channel carriers is enhanced. However, itit showsshows thethe lowestlowest sheet sheet carrier carrier concentration concentration of of 2.29 2.29 ×10 101212cm cm−2,, because the × − reduced thicknessthickness ofof channel channel layer layer causes causes the the quantum quantum well well energy energy level level to increase, to increase and, quantumand quantum well depthwell depth to decrease, to decrease thus, the thus 2DEG the density 2DEG density is reduced. is reduced. The electron The electronmobility mobility of sample of #4 sample is relatively #4 is lowest,relatively the lowest, reason the is thatreason the is upper that the barrier upper layer barrier is much layer thinner, is much and thin thener scattering, and the ofscattering impurities of onimpurities the interface on the is enhanced, interface is so enhanced, the electron so velocity the electron is degraded. velocity Sampleis degraded #3 shows. Sample a compromised #3 shows a sheetcompromised carrier concentration sheet carrier concentration of 2.56 1012 ofcm 2.562 and× 1012 electron cm−2 and mobility electron of mobility 1.81 10 of4 1.81cm2 ×/V 10s,4 cm and2/V the·s , × − × · lowestand the sheet lowest resistivity sheet resist of 135ivityΩ/ of. 135 It is Ω found/□. It thatis found the 2DEG that the density 2DEG and density electron and mobility electron are mobility conflict parameters in general, i.e., an increased 2DEG density always accompanies a decreased electron mobility. This can be explained as following: The volume of the electrons penetrating to the AlSb/InAs Coatings 2019, 9, 318 6 of 7 interface would increase if the 2DEG density increases, which would induce the chaos and confusion of the lattice arrangement in the interface, thus making the interface roughness scattering effect more obvious. In this condition, electron momentum relaxation would occur at the interface, and the direction and speed of the electron motion would change. Therefore, the electron mobility decreases significantly. This indicates that it is difficult to obtain the most optimal value of 2DEG and electron mobility at the same time, and it is necessary to compromise the two parameters in real applications.

Table 2. Hall test result of the four kinds of samples.

Sample Sheet Carrier Concentration Electron Mobility Sheet Resistivity Number (cm 2) (cm2/V s) [Ω/] − · #1 2.57 1012 15776.53 154 × #2 2.29 1012 17101.36 159 × #3 2.56 1012 18088.44 135 × #4 2.81 1012 14312.70 155 ×

4. Conclusion The thickness of InAs channel and AlSb barrier layer obviously impact the performance of the InAs/AlSb heterojunction epitaxy. In general, the 2DEG density and electron mobility are conﬂict parameters. When the channel thickness is decreased from 15 to 12 nm, electron mobility is reduced from 1.77 104 to 1.51 104 cm2/V s. When the thickness of upper barrier is reduced from 15 to 13 nm, × × · the sheet carrier concentration increases from 2.57 1012 to 2.91 1012 cm 2; however, electron mobility × × − decreased obviously to 1.43 104 cm2/V s; conversely, when the thickness of the AlSb upper barrier × · layer is increased from 15 to 17 nm, the device shows a compromised sheet carrier concentration of 12 2 4 2 2.56 10 cm− and an electron mobility of 1.81 10 cm /V s, and a low sheet resistivity of 135 Ω/. × × · Therefore, we choose the channel thickness of 15 nm, and the upper-barrier layer of 17 nm, to be the optimized thickness, which is a reference to further design InAs/AlSb HEMT, by ensuring a good device performance.

Author Contributions: Formal analysis, investigation, and writing—original draft preparation, H.G.; software, L.C.; writing—review and editing, S.W., B.G., and Y.W.; data curation, C.J. Funding: This research was funded by “National science foundation of Shaanxi province, China, grant number 2018JQ6069” and “China Postdoctoral Science Foundation, grant number 2018M643733”. Acknowledgments: We are appreciated the support of Lv Hongliang from Xi Dian University of China on the experiment data. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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