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(Received 1 November 2010; accepted 8 January 2011; published online 16 March 2011). Electrical and physical characteristics of the atomic layer deposited ...
JOURNAL OF APPLIED PHYSICS 109, 064101 (2011)

Atomic layer deposited beryllium oxide: Effective passivation layer for III-V metal/oxide/semiconductor devices J. H. Yum,1,2,a) T. Akyol,1 M. Lei,3 T. Hudnall,4 G. Bersuker,2 M. Downer,3 C. W. Bielawski,4 J. C. Lee,1 and S. K. Banerjee1

1 Microelectronics Research Center, Department of Electrical and Computer Engineering, The University of Texas, Austin, Texas 78758, USA 2 Sematech, 2706 Montopolis Dr. Austin, Texas 78741, USA 3 Department of Physics, The University of Texas, Austin, Texas 78758, USA 4 Department of Chemistry, The University of Texas, Austin, Texas 78758, USA

(Received 1 November 2010; accepted 8 January 2011; published online 16 March 2011) Electrical and physical characteristics of the atomic layer deposited beryllium oxide (BeO) grown on the Si and GaAs substrates were evaluated as a barrier/passivation layer in the III-V devices. Compared to Al2O3, BeO exhibits lower interface defect density and hysteresis, and smaller frequency dispersion and leakage current density at the same effective oxide thickness, as well as an excellent self-cleaning effect. These dielectric characteristics combined with its advantageous intrinsic properties, such as high thermal stability, large energy band-gap(10.6 eV), effective diffusion barrier, and low intrinsic structural defects, make BeO an excellent candidate for the C 2011 American Institute of interfacial passivation layer applications in the channel III-V devices. V Physics. [doi:10.1063/1.3553872]

I. INTRODUCTION

High electron mobility in the III-V materials attracted significant attention to them for a possible application as a channel material in metal/oxide/semiconductor (mos) transistors. One of the main challenges is that the III-V metal oxide semiconductor field effect transistors (MOSFETs) is generally lack of thermodynamically stable insulators of high electrical quality, which would passivate the interface states at the dielectric/substrate interface and unpin the Fermi level. To address this issue, various dielectric, such as Si/SiO2, (Ref. 1) Ge, (Ref. 2) SiGe, SiN and Al2O3, (Ref. 3) were considered as an interface passivation layer (IPL). Atomic layer deposited (ALD) Al2O3 has demonstrated superior IPL characteristics versus other considered candidates due to its high dielectric constant and interface quality. However, defect density in Al2O3 is still too high even as several cleaning methods such as NH4OH, (NH4)2S and F treatment have been developed,4 which limits performance of the III-V MOSFETs. On the basis of above requirements, beryllium oxide (BeO) was selected as a potential IPL for the III-V MOS devices. BeO has excellent thermal stability in contact with both Si and III-V substrates, in agreement with the value of its Gibbs free energy.5 The energy band-gap of the bulk BeO is 10.6eV,7 which is among the largest and its dielectric constant is around 6.8,8 which is very close to that of the ALD Al2O3 (about 7.1). Beryllium (its atomic number is 4) does not have p orbitals resulting in its small atomic radius. Beryllium oxide is well known to act as a strong diffusion barrier due to its very small Be-O length and dense structure with small interstitial spaces as well as a strong covalent bonding due to a similar electronegativity of Be and O. BeO causes low remote phonon scattering in the channel due to its a)

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compared to other dielectrics. The high phonon energy also leads to high thermal conductivity in BeO compared to other dielectrics. The thermal conductivity of BeO at room temperature is 300 W  m1K1 (while for alumina, it is 35 W  m1K1), which can lead to efficient heat dissipation near the MOSFET drain edge. II. EXPERIMENT AND MEASUREMENT

To grow a thin film of BeO, several physical vapor deposition (PVD) methods including reactive sputtering and E-beam evaporation were attempted but all were unsuccessful due to high diffusivity of Beryllium atom when their kinetic energy is high. Instead of the physical deposition, which has high kinetic energy, a chemical vapor deposition is employed in this study. We report the first chemical vapor deposition route using ALD to deposit BeO on GaAs. Considering selfcleaning effect on GaAs and ALD precursor properties, a dimethylberyllium, Be(CH3)2, whose sublimation temperature is 120  130  C at 0.2  0.3torr,9 was chosen as a viable BeO precursor. Unfortunately, only two organometallic BeO precursors, beryllium chloride (BeCl2) and beryllium acetylacetonate, (Be(acac), acac ¼ CH3COCHCOCH3), are commercially available, both of which are not suitable Be(CH3)2 substitutes for ALD. The high sublimation temperature of BeCl2 (>200  C) precludes the use of the Cambridge ALD system for chemical vapor deposition which is restricted to an upper temperature limit of 200  C. Subsequently, a BeCl2 pulse could not be observed even if heating was carried out at low pressure (0.2–0.3 Torr). In contrast, Be(acac) has a low sublimation temperature (100  C), but exhibited insufficient reactivity with water to afford BeO in the vapor phase. As a result, the desired BeO precursor (Be(CH3)2) was synthesized from BeCl2 via Grignard metathesis,10 and subsequently utilized for the first time as an ALD precursor. Methyl

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FIG. 1. 1H NMR spectrum of dimethylberyllium for Be precursor (C6D6).

magnesium bromide (CH3MgBr, 2.5 M in diethyl ether (Et2O)) was added drop wise over a period of 10 min to a rapidly stirred solution of BeCl2 in anhydrous Et2O cooled to – 30  C. After stirring the resulting slurry for 12 h, the solvent was removed under reduced pressure. The residual colorless solid was then extracted with benzene and filtered to remove precipitated magnesium salts. Subsequent removal of the residual benzene under reduced pressure afforded Be(CH3)2 solvated with approximately 2.5 molar equivalents of Et2O (see Fig. 1 for the 1H NMR spectrum (C6D6) of this material). As expected, the protons assigned to the methyl groups of Be(CH3)2 were strongly shielded (d ¼ 0.45 ppm) due to the inverse polarity of the Be-CH3 bonds (Be is more electropositive than C and H). Multiple rounds of sublimation and preheating below the sublimation temperature were successful in removing the residual Et2O which resulted in improved resulted in improved ALD BeO gate dielectrics on III-V and Si, as for the first time demonstrated below. Electrical characterization was performed on the MOS capacitors which were fabricated on both p- type Si and GaAs substrates each with doping concentrations of around 5  1017/cm3. After hydrofluoric acid (HF) surface cleaning, 5  10 nm ALD BeO was deposited at 200  C using a Nano Cambridge ALD module. BeO was deposited using dimethyl beryllium and water for reagents. For control samples, ALD Al2O3 MOS capacitors were also fabricated using trimethyl aluminum (TMA), and water under the same conditions. Physical thickness was measured by ellipsometry with various wavelengths and vertical angles between 45  75 and confirmed by TEM (not shown here). Post-deposition annealing (PDA) in the range of 500  C to 600  C was performed by rapid thermal annealing in N2 ambient for 30 sec – 3 min, ˚ ) as the followed by the reactively sputtered TaN (2000 A gate electrode. After patterning and etching, post metal-deposition annealing (PMA) was done at 400  C, 3 min in the forming gas ambient. The leakage current and capacitance were measured using a Keithley semiconductor parameter analyzer and HP4284A LCR meter with frequencies varying from 1 MHz to 1 kHz, respectively. Electrical characterization was performed on MOS capacitors. Effective oxide thickness (EOT) values were extracted from the C-V data

FIG. 2. (Color online) (a) C-V and (b) I-V characteristics of BeO and Al2O3 on p-GaAs(100) substrate with 3.9 nm EOT. Inset of (a) shows frequency dispersion at 1 MHz and 1 kHz for each oxide.

using the NCSU CVC program (simulation software developed by North Carolina State University). To estimate the conduction band offset of BeO gate dielectric, a second harmonic generation (SHG) characterization11 was performed using 720  820 nm wave length with 70  220 mW laser beam intensity and 20 micron focal length.

FIG. 3. (Color online) SHG results of as-deposited BeO(47.8A) and Al2O3 (48.5A) on p-Si(100).

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FIG. 4. (Color online) (a) Physical thickness vs. EOT, (b) Hysteresis, (c) Frequency dispersion vs. PDA temperature, (d) Dit of BeO and Al2O3 MOS capacitors. Frequency dispersion is capacitance difference between 1 MHz and 10 kHz. Hysteresis is voltage difference at half of Cox.

III. RESULTS AND DISCUSSION

Figure 2(a) shows the C-V characteristics of the MOS capacitors with 3.9 nm EOT of Al2O3 and BeO. The inset demonstrates the frequency dispersion between 1 MHz and

1 kHz. The leakage current density versus gate voltage for the same device is shown in Fig. 2(b). As expected, at high electric fields, BeO exhibits lower leakage current than Al2O3 due to its larger energy bandgap.

FIG. 5. (Color online) (a) After HF clean, (b) HF clean and Al2O3 growth, (c) HF clean and BeO growth. Ga 2p, As 3d, and Al 2s or Be 1s are measured in the energy range of 1121  1114 eV, 46  39 eV, 122  111 eV, respectively.

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self-cleaning properties comparable with the Al precursor. It appears that the Be precursor efficiently absorbs oxygen from the GaAs native oxide, possibly because the number of Be precursor molecules per unit area that adhere to the surface may be larger than in the case of the Al precursor. Figure 6 shows AFM images of 5 nm ALD BeO and Al2O3 respectively grown on GaAs surfaces with the HF treatment. As shown in Fig. 6, BeO deposited surface exhibits a low root mean square roughness of 0.194 nm. It may be due to a smaller length of the Be-O bond. BeO and Al2O3 have mol volumes of 8.28 cm3/mol and 25.81 cm3/mol, respectively. IV. CONCLUSIONS

FIG. 6. (Color online) AFM images of 5 nm ALD BeO and Al2O3 grown on GaAs after HF treatment.

Figure 3 shows SHG results for the p-Si gate stack. Since the transition from three-photon to two-photon process takes place at around 1.67 eV, one can estimate the BeO conduction band offset with Si to be of 2  1.67 eV – 1.1 eV (Si energy bandgap) ¼ 2.24 eV. It is larger than that of Al2O3 (1.9 eV before annealing, 1.98 eV after annealing). Although it is not possible to do a similar SHG study on GaAs because it is not a centro-symmetric material like Si, one may expect, based on the Anderson affinity rule, a conduction band offset between BeO and GaAs to be higher than that of Al2O3 in III-V. Figure 4(a) shows the extraction of the dielectric constant from a physical thickness versus EOT plot. The dielectric constants of BeO (k ¼ 6.67  6.83) and Al2O3 (k ¼ 6.56  7.12) are quite close. ALD Al2O3 has smaller dielectric constant compared to bulk Al2O3 value (k ¼ 9.1) because of its nonstoichiometry and interfacial defects. But BeO is less oxygen deficient,12 so the stoichiometry of ALD BeO may be better than that of Al2O3. Figures 4(b), 4(c) and 4(d) show the hysteresis, frequency dispersion and interface defect density (Dit), respectively. BeO shows lower hysteresis, frequency dispersion, and Dit as compared to those of Al2O3. Dit of BeO and Al2O3 at flat-band is 2E13 cm2 eV1 and 4E13 cm2 eV1, respectively. Dit of BeO is approximately two times lower. We have observed similar results on In53GaAs. Therefore, it can be expected that the electron mobility of III-V MOSFETs with BeO IPL will be improved. X-ray photoelectron spectroscopy (XPS) data of BeO grown on a GaAs substrate are shown in Fig. 5. The Ga-O and As-O signals are reduced after the Be precursor deposition. It demonstrates that the BeO precursor has excellent

In summary, we have performed a systematically comparison of the interface quality of BeO and Al2O3 dielectrics using C-V, I-V, SHG, XPS and atomic force microscopy (AFM) characterization of MOS capacitors. The ALD BeO results in a lower gate leakage current at the same equivalent EOT, around 0.3 eV higher conduction band offset, approximately 50% lower Dit value, at least 20 mV lower hysteresis, at most 2  4% frequency dispersion at flatband capacitance, 10% lower RMS and good self-cleaning effect compared to Al2O3. Multiple rounds of sublimation and preheating of a novel Be precursor allowed to reach a sufficiently high quality of the BeO, which can be further improved by increasing the purity of the Be precursor. ACKNOWLEDGMENTS

This work was supported in part by DARPA, Micron Foundation, Robert Welch Foundation grant F-1038, and NSF grant DMR-0706227. 1

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