Band Alignment Parameters of Al2O3/InSb Metal-Oxide-Semiconductor Structure and Their Modification with Oxide Deposition Temperatures

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Band Alignment Parameters of Al2O3/InSb Metal–Oxide–Semiconductor Structure and Their

Modification with Oxide Deposition Temperatures

View the table of contents for this issue, or go to the journal homepage for more 2013 Appl. Phys. Express 6 061202

(http://iopscience.iop.org/1882-0786/6/6/061202)

Band Alignment Parameters of Al

₂

O

₃

/InSb Metal–Oxide–Semiconductor Structure

and Their Modification with Oxide Deposition Temperatures

Hai Dang Trinh1;2
, Minh Thuy Nguyen2, Yueh Chin Lin1, Quoc Van Duong2, Hong Quan Nguyen1, and Edward Yi Chang1;3

1_{Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan} 2_{Department of Physics, Hanoi National University of Education, 136 Xuan Thuy, Cau Giay, Hanoi, Vietnam} 3_{Department of Electronic Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan}

E-mail: trinhhaidang@gmail.com

Received April 6, 2013; accepted May 12, 2013; published online May 28, 2013

From the Fowler–Nordheim (FN) current–voltage (I–V ) characteristic and X-ray photoelectron spectroscopy (XPS) analysis, the conduction band offset of2:73  0:1 eV and the valence band offset of 3:76  0:1 eV have been extracted for the atomic-layer-deposition (ALD) Al2O3/InSb structure. By these analyses, the parameters of an Al2O3film including bandgap, electron affinity, and electron effective mass are also deduced. The capacitance–voltage andI–V characteristics of ALD Al2O3/InSb at different deposition temperatures indicate the modification of the Fermi level in InSb to 0.09 eV lower than that in metal side of the sample deposited at 250C as compared to the samples deposited at lower temperatures. # 2013 The Japan Society of Applied Physics

I

II–V compounds have been widely studied for future scaling down of low power, high speed field-effect transistors (FETs) due to their high electron mobility and saturation velocity. Among them, InSb has the highest electron mobility (7:7  104cm2V1s1) and high hole mobility (840 cm2V1s1) that has potential for both high speed n- and p-FETs.1–3) Due to its narrow bandgap, InSb has been suggested to be applicable to quantum-well (QW),2,3) nanowire (NW),4–6) or tunnel (T) FET architec-tures.7) For these kinds of application the determination of band alignment between InSb and the barrier layers (high-bandgap semiconductors or high-k dielectrics) is of critical importance. By using internal photoemission (IPE) measure-ment, the conduction band offset (EC) at the atomic-layer-deposition (ALD) Al₂O₃/(100)InSb interface was deter-mined to be2:9  0:1 eV.8)In the present work, we further investigate the band alignment of ALD Al₂O₃/(100)InSb structure including conduction and valence band offsets by using Fowler–Nordheim (FN) current–voltage (I–V) char-acteristic and X-ray photoelectron spectroscopy (XPS) analysis. The EC value determined from the FN char-acteristic is consistent with that determined by the IPE method.8)Since the characteristics of high-k/InSb structure are very sensitive to the thermal process due to the low thermal budget in InSb,1,9,10)the effect of oxide deposition temperatures on the band alignment modification of Al₂O₃/ InSb is also investigated.

The wafers used in this work were n-type (100)InSb substrates with a donor concentration of2:2  1016cm3 at room temperature (determined by Hall measurement). After decreasing in acetone and iso-propanol, the samples were dipped in diluted HCl (4%) solution for removing native oxides. The samples were then loaded into the ALD chamber (Cambridge NanoTech Fiji 202 DSC) for Al₂O₃ deposition using trimethylaluminium (TMA) and water as precursors. In the ALD chamber, 10 pulses of TMA/Ar were used to further removing native oxides before the deposition of 100 CYC (9:2 nm) Al₂O₃. The use of in-situ TMA pre-cleaning before oxide deposition was proved to improve the Al₂O₃/ InGaAs, Al₂O₃/InAs, Al₂O₃/InSb interfaces by self-clean-ing effect.11–16)Beside the effect on the reduction of III–V native oxides, the reduction of dangling bonds at Al₂O₃/ InSb interface after using some pulses of TMA/Ar is also

expected. For the fabrication of metal–oxide–semiconductor capacitors (MOSCAPs), Ni/Au gate metal was formed via photolithography/e-beam evaporation/lift-off process. Finally, the Au/Ge/Ni/Au was deposited for back side ohmic contact followed by post metal annealing at 200C in N₂ for 30 s.

The valence band offset (EV) of Al2O3/InSb was determined for the sample deposited at 200C by using XPS measurement via the following formula:17–19)

EV¼ ECLþ ðEInSbIn4d E InSb VBMÞ  ðE AlO Al2p E AlO VBMÞ; ð1Þ where ECL¼ EAlO=InSbAl2p  EAlO=InSbIn4d is the energy difference between Al 2p and In 4d core levels measured in 2 nm Al₂O₃/InSb interface, while EInSbIn4d E

InSb

VBM and E AlO Al2p EAlOVBM are the differences between valence band maximum (VBM) energies and corresponding In 4d core level in InSb and Al 2p core level in 9.2-nm-thick Al₂O₃. XPS measure-ments were performed in a commercial Microlab 350 XPS system equipped with an Al K source in an ultrahigh-vacuum chamber (5  109Torr) with a 60 take-off angle. The In 4d, Al 2p core levels and VBM spectra of the InSb, Al₂O₃/InSb interface and Al₂O₃ are shown in Fig. 1. The core levels were determined by using XPSPEAK software package (version 4.1) with Gaussian–Lorentz line shape and a Shirley background. The uncertainty of the core positions is 0.05 eV. The VBM positions were determined by the linear extrapolation of the leading valence band edge on the semiconductor and oxide. The uncertainty of determining VBM positions was also 0.05 eV. From the measurement values shown in Fig. 1 and Eq. (1), the value of EV 3:76  0:1 eV is extracted.

Figure 2(a) shows the current density–voltage (J–V) characteristics of the MOSCAP samples that were deposited at 150, 200, and 250C. I–V measurement was performed using a Keithley 4200 analyzer. All the samples exhibit a low leakage current with a breakdown field of above 7.2 MV/cm. In the FN regime, the current density can be expressed as20) JFN¼ m m
q3 162_hBE2exp  4pffiffiffiffiffiffiffiffi2m
3qh 3=2B E ! ; ð2Þ where m, m
, q, h, E, and Bare the electron mass, electron effective mass in Al₂O₃ oxide, elementary charge, Plank’s

Applied Physics Express 6 (2013) 061202

061202-1 # 2013 The Japan Society of Applied Physics

constant, oxide electrical field (E ¼ Vg=tox, Vg: gate bias and tox: oxide thickness), and tunneling barrier height, respec-tively. According to Eq. (2), the plot of ln JFN=E2 versus 1=E (so-called FN plot) should be linear with slope S given by S ¼d½lnðJFN=E 2_Þ dð1=EÞ ¼  4pffiffiffiffiffiffiffiffi2m
3qh 3=2B : ð3Þ The barrier heights can be expressed as þB ¼ S  under forward gate bias and B ¼ m  under reverse gate bias, where S, m, and  are semiconductor electron affinity, metal work function and oxide electron affinity, respectively. The FN plots of the three samples are shown in Figs. 2(b)– 2(d). It can be seen that the curves exhibit a linear rela-tionship at high electrical field, indicating the dominance of the FN mechanism. The slopes under forward and reverse bias (Sþ and S, respectively) can be extracted from these data and the values of m
, þB, and  can be determined using the following formulas:

m
m ¼ 9q2_h2 32m ðS2=3   S2=3þ Þ3 ðm SÞ3 ; þB ¼ 3qhS þ 4pffiffiffiffiffiffiffiffi2m
 ₂₌₃ ;  ¼ ðS þBÞ: ð4Þ

By taking the values of Ni work function m¼ 5:05 eV21) and InSb electron affinity S¼ 4:6 eV,1) the extracted parameters of the MOS structures are listed Table I. As

shown in the table, the extracted values of the two samples deposited at 150 and 200C are very similar. The con-duction band offset EC values are found to be 2.71 and 2.77 eV, respectively. These values are in agreement and very close to that reported by Chou et al. by using the IPE method.8) The values of m
and  are about 0:2{0:22m0 and 1.8–1.9 eV, respectively. For the sample deposited at 250C,EC, m
, and  are 3.84 eV, 0:092m₀, and 1.21 eV, respectively, which are very different from those of the other samples.

To find out the reason for this difference, the capacitance– voltage (C–V ) measurement was performed using an HP4284A meter for further study. Figure 3(a) shows the C–V curves at 1 kHz of the three samples. While the C–V curves of the two samples deposited at 150 and 200C are very similar, that of the sample deposited at 250C shifts positively toV ¼ 0:09 V. In the previous work, we found that there was an In, Sb out diffusion layer with a thickness of 1.5 nm at the Al₂O₃/InSb interface when the sample was deposited at 250C [see the transmission electron micros-copy (TEM) image of the sample with 7.5 nm ALD Al₂O₃/ InSb at 250C in Fig. 3(b)].9)This out diffusion layer might cause a negative fixed charge which results in lowering the Fermi level in InSb side to ¼ 0:09 eV below that in the metal side [Fig. 3(b)]. The root cause of negative charge layer needs further investigating but it may attribute to the dominant diffusion of Sb at low thermal temperature (a)

(b)

(c)

Fig. 1. XPS spectra of (a) In 4d core level and valence band of InSb surface (after HCl cleaning), (b) Al 2p and In 4d core levels at the 2 nm Al2O3/InSb interface, and (c) Al 2p and valence band of Al2O3film.

(a) (b)

(c) (d)

Fig. 2. (a) I–V characteristics of samples; (b) and (c) FN plots of samples deposited at 150 and 200C; (d) FN plots after revision of samples deposited at 250C.

Table I. Band parameters of the samples extracted by FN characteristics. þB (eV) B (eV) m
_=m 0 _(eV) Sample at 150C 2.71 3.16 0.20 1.89 Sample at 200C 2.77 3.22 0.22 1.83 Sample at 250C (before revision) 3.39 3.84 0.092 1.21 Sample at 250C (after revision) 2.71 3.16 0.21 1.89 H. D. Trinh et al. Appl. Phys. Express 6 (2013) 061202

process as compared to the diffusion of In.10) Due to this modification, the determination of band alignment param-eters of the sample deposited at 250C was not accurate. We then revised the electrical field for the FN plot of this sample by adding a revisional factorV:

E ¼Vg V tox

: ð5Þ

The FN plot after revision is also linear as indicated in Fig. 2(d). The band alignment parameters were extracted again and interestingly, all the parameters became very similar to those of others samples (see Table I). By this revision, one can conclude that the conduction band offset of Al₂O₃/InSb is EC 2:73  0:1 eV. Combining this value of conduction band offset with the valence band offset EV extracted above and the InSb band gap of 0.17 eV at room temperature,1)_{the band gap of the Al}

2O3 film can be deduced to be Eg  6:66  0:1 eV. This value is consistent with that reported by Huang et al. (6:8  0:1 eV) by using energy loss spectra analysis.19)

In conclusion, the band alignment parameters of Al₂O₃/ InSb structures have been evaluated by using XPS analysis and FN characteristics. The conduction band offsetEC 2:73  0:1 eV, and valence band offset EV 3:76  0:1 eV are extracted. For the ALD Al₂O₃ film parameters, an energy band gap of 6:66  0:1 eV, an electron affinity of 1.8–1.9 eV, and an electron effective mass of 0:2{0:22m₀ were deduced. The I–V and C–V analyses also indicated the band modification of the sample deposited at 250C caused by the In, Sb out diffusion layer at the Al₂O₃/InSb interface. These results would be useful for the future study of high-k/InSb MOS devices.

Acknowledgments The authors would like to thank to NCTU-UCB I-RiCE Program, Taiwan National Science Council (contract No. NSC-102-2911-I-009-301), and Vietnam National Foundation for Science and Technology Development (contract No. 103.02-2011.12) for providing support.

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H. D. Trinh et al. Appl. Phys. Express 6 (2013) 061202