An InAs/high-k/low-k structure: Electron transport and interface analysis

We fabricated and investigated an InAs/high-k/low-k structure in comparison with an InAs/low-k structure, where the former and the latter are respectively obtained by bonding of InAs/Al2O3/AlN and InAs on low-k flexible substrates (FS). The InAs/high-k/low-k (InAs/Al2O3/AlN/FS) exhibits electron mobilities immune to interface fluctuation scattering, whereas this scattering is serious for the InAs/low-k (InAs/FS). Moreover, we find that electron sheet concentrations in the InAs/high-k/low-k are significantly higher than those in the InAs/low-k. From InAs/Al2O3 interface analysis by energy-dispersive X-ray spectroscopy and electron energy-loss spectroscopy, we find that the higher electron concentrations can be attributed to natural modulation doping from Al2O3 to InAs.

Considering these problems, in this work, we fabricated and investigated an InAs/high-k/lowk structure, where a thin high-k insulator layer between InAs and the low-k FS can be beneficial to suppress the interface fluctuation and to improve the heat release capability, almost keeping the merit of the low parasitic capacitance of the FS.We employed Al 2 O 3 /AlN as a high-k insulator layer, where k ∼ 9 and κ ∼ 30 W/m-K for Al 2 O 3 , and k ∼ 9 and κ ∼ 300 W/m-K for AlN, to obtain the InAs/high-k/low-k (InAs/Al 2 O 3 /AlN/FS).Electron transport properties of the InAs/high-k/low-k were investigated in comparison with those of the InAs/low-k, for InAs film thicknesses from 10 nm to ∼ 150 nm.As a result, we find that the InAs/high-k/low-k exhibits electron mobilities immune to interface fluctuation scattering.We also find that electron sheet concentrations in the InAs/high-k/lowk are significantly higher than those in the InAs/low-k.Interface analysis by energy-dispersive X-ray spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) for the InAs/Al 2 O 3 interface a Author to whom correspondence should be addressed; electronic mail: tosikazu@jaist.ac.jp 2158-3226/2017/7(5)/055303/8 7, 055303-1 © Author(s) 2017 indicates that the higher electron concentrations can be attributed to natural modulation doping from Al 2 O 3 to InAs.

II. SAMPLE FABRICATION
The InAs/high-k/low-k and InAs/low-k structures were fabricated as shown in the top of Fig. 1.Using a heterostructure, InAs device layer (500 nm thickness)/ AlAs sacrificial layer (4 nm thickness)/ InAs buffer layer (2500 nm thickness)/ GaAs(001), we carried out ELO, [14][15][16][17] separation of the InAs device layer attached to an adhesive sheet.The InAs device layer was transferred onto an intermediate support, a sapphire(0001) coated by resists, followed by removal of the adhesive sheet and InAs surface cleaning using phosphoric acid.For the InAs/high-k/low-k structure, high-k insulator deposition on the InAs was carried out; we deposited Al 2 O 3 (50 nm thickness) by atomic layer deposition (ALD) using trimethylaluminum and H 2 O, and AlN (30 nm thickness) by electron cyclotron resonance (ECR) sputtering deposition using an Ar-N 2 plasma and an AlN target.The InAs/Al 2 O 3 /AlN was separated from the intermediate support, followed by 'inverted' VWB on the low-k FS, PET coated by bisazide-rubber, to obtain the InAs/high-k/low-k (InAs/Al 2 O 3 /AlN/FS).The reasons of employing Al 2 O 3 /AlN as a high-k insulator layer are as follows.If we employ a single layer deposition of Al 2 O 3 or AlN, we observe convex or concave sample warpage during VWB as shown in Fig. 2, probably owing to a strain during the deposition, which makes the process quite difficult.On the other hand, employing the Al 2 O 3 /AlN layer is advantageous to suppress sample warpage as shown in Fig. 2, owing to strain balancing, and consequently helpful for easiness of the VWB process.The thicknesses of Al 2 O 3 and AlN are optimized; we found that, for the 50-nm-thick Al 2 O 3 , the 30-nm-thick AlN leads to an almost flat sample profile.In addition, the ALD deposition of Al 2 O 3 on InAs is suitable to avoid interface fluctuations, while the ECR sputtering deposition of AlN causes damages of the InAs surface.5][16][17] Nomarski optical microscope images of the Hall-bar devices are shown in the bottom of Fig. 1, where the differential interference contrasts indicate same smooth surfaces for the InAs/high-k/low-k and the InAs/low-k.The Hall-bar devices enable us to characterize electron transport properties of the InAs/high-k/low-k and the InAs/low-k for several InAs channel thicknesses from 10 nm to ∼ 150 nm, where the 'inverted' VWB is advantageous to obtain a high crystal quality after the thinning according to the growth-direction dislocation distribution. 20

III. ELECTRON TRANSPORT PROPERTIES
From room-temperature measurements of the Hall-bar devices, as shown in Fig. 3, we electron µ and electron sheet concentrations n s as functions of the InAs channel thickness d, where the error bars of d come from thickness measurements. 15The InAs/low-k for d 15 nm exhibits µ rapidly decreasing with decrease in d, µ ∝ d γ (γ 5-6) attributed to serious interface fluctuation scattering or thickness fluctuation scattering; 15,[21][22][23][24][25][26] when there is a bonding interface fluctuation, the confinement potential fluctuates with a thickness fluctuation in the Cartesian coordinate, leading to µ ∝ d γ behavior.On the other hand, for the InAs/high-k/low-k, we do not observe µ ∝ d γ behavior, indicating that the InAs/high-k/low-k exhibits µ immune to interface fluctuation scattering even for d 10 nm.This can be attributed to the fact that the interface fluctuation of the InAs/Al 2 O 3 obtained by ALD is smaller than that of the InAs/FS obtained by VWB.The InAs/low-k for d 15 nm exhibits µ dominated by Coulomb scattering, where µ slowly decreases with decrease in d. 15 The InAs/high-k/low-k exhibits similar µ attributed to Coulomb scattering, but slightly lower than that of the InAs/low-k.This suggests more Coulomb scattering centers near the interface for the InAs/high-k/low-k, as discussed later.We also find that n s in the InAs/high-k/low-k is significantly higher (10 12 cm −2 order) with a smaller dispersion than in the InAs/low-k (10 11 cm −2 order).The observed higher n s with the smaller dispersion suggests that electrons are supplied from Al 2 O 3 to InAs through the InAs/Al 2 O 3 interface.

IV. INTERFACE ANALYSIS
In order to examine the possibility of the electron supply from Al 2 O 3 to InAs, we carried out InAs/Al 2 O 3 interface analysis by using a scanning transmission electron microscope (STEM) with an acceleration voltage of 120 kV. Figure 4  Moreover, in the STEM, we carried out EELS near the InAs/Al 2 O 3 interface.Figure 5 shows EELS spectra around the O-K edge for x = 1.5-+3.5 nm.While clear O-K edge peaks at 541 eV are observed for positive x, clear satellite peaks at 532 eV are observed only for x 1.5 nm. Figure 6 shows (a) O-K edge peak and (b) satellite peak intensities as functions of the position x, which can be also fitted by ]]/2 using the error function.For comparison, the EDX O-Kα intensity is also shown.We find the onset position x 0 0 for the O-K edge peak, which is consistent with the EDX O-Kα intensity, while x 0 1.3 nm for the satellite peak.In Al 2 O 3 , there are inevitable oxygen-vacancies, which act as donors. 27,28It has been reported that, non-ionized oxygen-vacancy donors (occupied by electrons) in Al 2 O 3 can cause the satellite peak, while ionized oxygen-vacancy donors (unoccupied by electrons) do not give the satellite peak. 29Therefore, we conclude that oxygen-vacancy donors are ionized near the InAs/Al 2 O 3 interface (x 1.3 nm), while not ionized for the Al 2 O 3 -inside (x 1.3 nm).This indicates that natural modulation doping takes place at the InAs/Al 2 O 3 interface; oxygen-vacancy donors near the interface supply electrons from Al 2 O 3 to InAs leading to the higher n s , and become ionized donors.The ionized donors act as Coulomb scattering centers near the interface, giving the slightly lower µ.The situation is similar to the natural modulation doping taking place at InAs/AlSb interfaces, 30,31 where deep donors due to antisite defects in AlSb supply electrons from AlSb to InAs, leading to a high electron concentration in InAs.

V. POISSON-SCHR ÖDINGER CALCULATION
In order to confirm the natural modulation doping picture quantitatively, we carried out Poisson-Schrödinger calculation. 32Figure 7 shows examples of the calculated energy band profile and electron distribution at 300 K, where we plot the conduction band bottom energy E c , the valence band top energy E v , the oxygen-vacancy donor level E D , the Fermi energy E F , and the electron concentration ρ (in the unit of cm 3 ), for (a) InAs/Al 2 O 3 and (b) InAs/FS modeled as free-standing InAs.The energy gaps of Al 2 O 3 and InAs are set to be 6.8 eV and 0.37 eV, respectively, with the conduction band offset of 3.0 eV. 33,34We employ Fermi level pinning at the InAs surface E F E c = 56 meV, giving 10 11 cm −2 order electron sheet concentrations for the InAs/FS.As shown in Fig. 7(a), we observe an electron accumulation in InAs near the InAs/Al 2 O 3 interface, and a depletion region in Al 2 O 3 .We obtain a depletion length in Al 2 O 3 x dep 1.3 nm in good agreement with the EELS result, assuming 1.6 × 10 19 cm −3 oxygen-vacancy donors in Al 2 O 3 with a level of E c E D = 2.6 eV, located ∼ 4 eV above the valence band top. 35Moreover, from the calculation, we obtain n s as a function of d for the InAs/Al 2 O 3 , as shown in the black solid curve of Fig. 3.We find good agreement between the calculated and experimental results, which supports the natural modulation doping picture.

VI. SUMMARY
In summary, we investigated the InAs/high-k/low-k (InAs/Al 2 O 3 /AlN/FS) in comparison with the InAs/low-k (InAs/FS).While interface fluctuation scattering is serious for the InAs/low-k, we find that the InAs/high-k/low-k exhibits electron mobilities immune to this scattering, attributed to small InAs/Al 2 O 3 interface fluctuation obtained by ALD.We also find higher electron sheet concentrations in the InAs/high-k/low-k than in the InAs/low-k.From EDX and EELS, we conclude that natural modulation doping takes place at the InAs/Al 2 O 3 interface leading to the higher electron concentrations, as supported by Poisson-Schrödinger calculation.

FIG. 2 .
FIG. 2. (Left) Schematic warpage during VWB in the case of employing a single layer deposition of Al 2 O 3 or AlN.(Right) Employing the Al 2 O 3 /AlN layer is advantageous to suppress warpage.

FIG. 3 .
FIG. 3. Electron mobilities µ and electron sheet concentrations n s as functions of the InAs channel thickness d for the InAs/high-k/low-k and InAs/low-k structures.The blue solid line for µ shows µ ∝ d γ (γ 5.2).The black solid curve for n s is obtained by Poisson-Schrödinger calculation.
FIG. 4. (a) A HAADF STEM image near the InAs/Al 2 O 3 interface.(b) EDX maps obtained in STEM for In-Lα, As-L, Al-Kα, and O-Kα near the InAs/Al 2 O 3 interface.(c) EDX intensities as functions of the position x.

FIG. 6 .
FIG. 6.(a) O-K edge peak and (b) satellite peak intensities as functions of the position x, with the EDX O-Kα intensity.

FIG. 7 .
FIG. 7. Examples of the calculated energy band profile and electron distribution at 300 K for (a) InAs/Al 2 O 3 and (b) InAs/FS, showing the conduction band bottom energy E c , the valence band top energy E v , the oxygen-vacancy donor level E D , the Fermi energy E F , and the electron concentration ρ (in the unit of cm 3 ).