Liquid phase deposited SiO2 on GaN

Liquid phase deposited SiO

2

on GaN

H.R. Wu

, K.W. Lee

, T.B. Nian

, D.W. Chou

, J.J. Huang Wu

, Y.H. Wang

,

M.P. Houng

, P.W. Sze

, Y.K. Su

, S.J. Chang

, C.H. Ho

, C.I. Chiang

,

Y.T. Chern

, F.S. Juang

, T.C. Wen

, W.I. Lee

, J.I. Chyi

a_{Institute of Microelectronics, Department of Electrical Engineering, National Cheng-Kung University, Tainan 701, Taiwan} b_{Department of Electrical Engineering, Kau-Yuan Institute of Technology, Kaoshiung, Taiwan}

c_{Materials R&D Center, Chung-Shan Institute of Science and Technology, Tao-Yuan, Taiwan} d_{Department of Electro-Optics Engineering, National Huwei Institute of Technology, Yunlin, Taiwan}

e_{Institute of Electro-Physics, National Chaio Tung University, Hsinchu, Taiwan} f_{Department of Electrical Engineering, National Central University, Chungli, Taiwan} Received 5 March 2002; received in revised form 22 October 2002; accepted 29 October 2002

Abstract

An efficient and low cost approach to deposit uniform silicon dioxide layers on GaN by liquid phase deposition (LPD) near room temperature are described and discussed. The process is simple. GaN wafers are immersed into a H2SiF6and H3BO3solution to form the

silicon dioxide layers. The deposition conditions and the properties of the SiO2films will be characterized.

© 2002 Elsevier Science B.V. All rights reserved.

Keywords: Liquid phase deposition; Silicon dioxide

1. Introduction

Recently, research on GaN-based materials has attracted not only its use in the light emitting devices, but also in the power applications. The GaN-based FETs are attrac-tive in power amplification and switching under high power and high temperature application [1,2]. This is due to the wide band gap (3.4 eV), high breakdown voltage (>5× 106V cm−1), unique chemical and thermal stability pos-sessed by GaN. Although GaN FETs and AlGaN/GaN het-erostructure FETs have been reported [3–5], the Schottky gate barrier still limits the swing voltage and gate leakage which can be solved by metal-oxide-semiconductor(MOS) structures. Efforts have been made in pursuit of high qual-ity gate insulators, such as SiO2, Si3N4etc.[6,7], and pho-toanodic oxide have been developed[8]. The reliable oxide (insulator) layers on GaN, however, are very few reported to be seen on GaN MOSFET. Recently, Ren et al. [9,10]

reported using e-beam evaporation in a molecular beam epi-taxy (MBE) chamber to deposit Ga2O3(Gd2O3) as the gate dielectrics for GaN MOSFET. However, the required sys-tems and procedures are complicated.

∗_{Corresponding author. Fax:+886-6-2080598.}

E-mail address: [email protected] (Y.H. Wang).

Liquid phase deposition (LPD) process, a low tempera-ture, low cost and reliable method, has been used to de-posit high quality insulators on Si and GaAs material system with promising results[11]. However, its exploration to GaN materials is not developed. In this work, LPD process will be employed for the insulating dielectrics on GaN with the supersaturated hydrofluosilicic acid (H2SiF6) and the boric acid (H3BO3) aqueous solution. The properties of the ox-ide films will be characterized. The selective deposition on GaN, photoresistor, and metals will also be described.

2. Experimental

The experimental setup for preparing LPD silicon oxide is shown inFig. 1, it only consists of a temperature controller, substrate holder in the saturated solution. The preparation of the saturated solution and the depositing flowchart is illus-trated inFig. 2. The GaN samples, grown on c-plane sap-phire substrates, were all prepared by MOCVD. The LPD technique utilizes supersaturated hydrofluosilicic acid aque-ous solution (H2SiF6) as a source liquid and boric acid aque-ous solution (H3BO3) as a deposition rate controller. The chemical reaction in the aqueous solutions of hexafluorosili-cic acid (H2SiF6) can be expressed in the following. H + H2SiF6+ 2H2O↔ 6HF + SiO2↓ (1) 0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.

Fig. 1. The setup of the LPD system.

When the aqueous solution of boric acid (H3BO3) is dipped into the solution, H3BO3 reacts with HF and they generate boron tetrafluoride ions (BF4−). The chemical re-action is shown in the following.

H3BO3+ 4HF ↔ BF4−+ H3O++ 2H2O (2) While a wafer is dipped into a HF solution saturated by SiO2, the chemical reaction ofEq. (1)is in a balanced con-dition. The addition of H3BO3 consumes HF through the reaction ofEq. (2)and reduces the concentration of HF solu-tion inEq. (1). The reduction of HF in dipping solution will shift the equilibrium to the right of reaction (1). Therefore, addition of H3BO3 shifts the equilibrium to the deposition of SiO2 on substrates. The mechanism is similar to that of SiO2on Si.

The chemical composition of silicon oxide was studied by Fourier transform infrared (FTIR) and X-ray

photoelec-Fig. 2. The flowchart for depositing SiO2 films on GaN layers.

Electrical properties of the SiO2 layers on MOS struc-tures were characterized by HP4280A and HP4156B for capacitance–voltage (C–V) and current–voltage (I–V) mea-surements. The LPD-silicon oxide films worked as the insulator and Al metal gates were evaporated through a shadow mask. Ohmic contact to the n-GaN was Ti/Al/Au (25/100/100 nm). Photo-enhanced chemical etching per-forms the device isolation.

3. Results and discussion

Fig. 3shows the deposition rate of SiO2on GaN at 40◦C

[12]with the concentration of H2SiF6and H3BO3at 0.4 and 0.01 M, respectively. Also shown is the deposition rate on GaAs for comparison. Due to the transparency of the GaN substrate to He-Ne laser, thickness of the oxide was deter-mined by Dektak through the selective deposition of oxides on photoresistor instead of by Ellipsometer as appeared in GaAs. The thickness of the oxide was consistent with those measured by SEM. The oxidation rate is about 50 nm h−1. There is almost no difference between the deposition rate on GaN and that of GaAs. The LPD-silicon oxide deposition rate first increases with H3BO3concentration, and then be-comes saturated. After reaching a maximum, the deposition rate decreases again. This is explained by the variation of pH values, and the role of H3BO3, as shown in the figure. The increase of pH value from−0.38 (starting solution), −0.09

Fig. 3. SiO2 deposition rate on GaN and GaAs as a function of time.

to 0.35 marks the reaction ofEq. (1)to suit the SiO2 depo-sition. At the third hour, the pH value decreases to 0.26 and the thickness of the oxide is almost kept constant. It means that the reaction is balanced, for example, the deposition rate is equal to the etching back rate. After this, the etching rate is larger than that of deposition, resulting in the decrease of oxide thickness. By adding H3BO3to adjust the pH value, constant deposition rate is obtained. Otherwise, etchback is to be observed. The growth solution becomes turbid if large amount of H3BO3 is added into an extremely low H2SiF6 concentration[13,14].

Typical Fourier transform infrared spectroscopy spectra ranging from 400 to 1400 cm−1deposited LPD-SiO2films on GaN; Si and GaAs (100) substrates are shown inFig. 4. The transmission bands are almost the same. The peaks around 455, 810 and 1090 cm−1are attributed to Si–O rock-ing, Si–O bending and Si–O stretching vibration, respec-tively. The LPD-SiO2 film is abundant in Si–O–Si bonds and has an orderly silica network and consequently good chemical stability. Another main transmission band around 935 cm−1is found in the LPD-SiO2spectra and may be at-tributed to Si–F stretching vibration. The fluorine (F)

con-Fig. 4. FTIR spectra for 50 and 100 nm thick SiO2 on GaN. Also shown are the LPD-SiO2 on Si and GaAs substrates for comparison.

Fig. 5. The XPS data of Si 2p core level of LPD-SiO2on GaN substrate prepared at T = 40◦C. Also shown is the theoretical fitting curve for comparison.

tained in the Si–F bond must be incorporated from the H2SiF6solution. The oxide perhaps is more accurate to be considered as SiOF instead of SiO2. However, as the result of secondary ion mass spectrometer depth profiles and the Auger analysis, F is not found in SiO2on GaN due to de-tection limitation.

XPS was performed with a Mg K␣ X-ray source. The electron analyzer normal collects Si 2p, C 1s, and O 1s core levels to the surface.Fig. 5shows the Si–O bonding spectra fitting to the Gaussian distribution function. The Si 2p line through SiO2/GaN is separated into three species, that is, el-emental Si (binding energy: 98.5 eV), the oxidized Si (SiO2) (103.4 eV, FWHM: 2.1 eV) and SiO_x. As the results show, the composition of the oxide films might then be attributed to SiO2.

Fig. 6 shows the surface morphologies of the oxides on GaN and Al-metal. Smooth oxide surface can be seen only on GaN. This provides potential device applications. Also shown inFig. 6(b) is the AFM analysis for a 70 nm thick oxide. The root mean square (RMS) surface roughness is 5.2 nm, which is high as compared to the SiO2 on Si (0.2 nm). Similar work is also seen on GaAs surface which is perhaps due to the polar substrate.

As shown inFig. 7, the typical LPD-SiO2on GaN break-down field for various thickness of oxide of 50, 70, 85 and 100 nm are 3.42, 3.8, 5.1 and 7 MV cm−1, respectively. It also shows the comparison of LPD-SiO2on GaAs and Si at thickness of 27 and 24 nm, respectively. The carrier trans-ports deduced from the log I versus V0.5 for low and high electrical field corresponded to the Schottky emission and Poole–Frenkel conduction, respectively. At an electric field of 1 MV cm−1, the corresponding leakage current densities ranged from 10−4 to 10−5A cm−2, which are higher than those on GaAs or Si wafers. This is in consistence with the rougher surface of SiO2on GaN layer. After annealing oxide at 900◦C in N2O for 20 min, the leakage current densities can be lowered to less than 10−7A cm−2.

Fig. 6. The SEM photograph and AFM 3-D image of LPD oxide surface (a) The smooth selective LPD oxide surface on GaN. (b) The AFM surface morphology on a 70 nm thick oxide with a RMS roughness value of 5.2 nm.

The typical C–V characteristics measured by 4280 A at 1 MHz are shown inFig. 8. The deduced interface trap den-sity with 2.8 × 1011cm−2eV−1 for an oxide thickness of 50 nm on GaN can be obtained[15]. After annealing oxide at 900◦C in N2O for 20 min, the interface trap density can be lowered to less than 1.9. × 1011cm−2eV−1. It also in-dicates that the interface trap density is further reduced to less than 1011cm−2eV−1upon suitable annealing processes. However, there is still much room to improve the film qual-ity as compared to SiO2/Si interface. Although the proper-ties of SiO2on GaN are not as good as those on Si or GaAs, however, it remains fine for device applications. Details of the LPD oxide gated GaN MOSFETs and AlGaN/GaN het-erostructure MOSFETs are seen elsewhere[16,17].

Fig. 7. The log I–V characteristics for the 50, 70, 85 and 100 nm thick SiO2 on GaN. Also shown are the SiO2 on Si and GaAs substrates for comparison.

Fig. 8. The measured and ideal C–V characteristic for a 50 nm thick LPD-SiO2 on GaN MOS diode. The deduced interface state density is about 2.8 × 1011_cm−2_eV−1_.

4. Conclusion

The liquid phase deposited SiO2on GaN surface has been demonstrated and characterized. The leakage current, break-down field and interface trap has also been discussed. As the results show, the proposed method is likely to have the potential for device application.

Acknowledgements

This work was supported in part by the National Sci-ence Council of Republic of China under the contracts No: NSC90-2215-E-006-013, NSC91-2215-E006-017, NSC91-2215-E244-001 and A-91-E-FA08-1-4. Thanks are

to the support from Chen Jieh-Chen Scholarship Founda-tion, Tainan, Taiwan.

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