36 15th INTERNATIONAL CONFERENCE ON MICRO/NANOTECHNOLOGIES AND ELECTRON DEVICES EDM 2014

978-1-4799-4668-3/14/$31.00 ©2014 IEEE

Abstract – The PE NH3 pretreatment followed by the wet chemical cleaning of GaN surface in an aqueous solution on the base of NH4OH, HCl and HF prior to PE CVD SiNx:H passivation allowed to improve electrophysical characteristics MIS-structures formed on GaN.

Index Terms – gallium nitride, passivation, thin layers PE CVD SiNx:H.

I. INTRODUCTION

GALLIUM NITRIDE is of interest for high-power, high-frequency and high-temperature device applications

[1]. Surface cleaning has a significant influence on overall device quality. This includes removal of native oxides, organic contaminants, adsorbs molecules, and residual species. Increased requirements for operating parameters of devices based on complex semiconductors compounds requires the improvement in both manufacturing technology and the properties of dielectric layers and its interface with the semiconductor.

II. PROBLEM DEFINITION

The aim of this work is to investigate the effect of different methods of cleaning the gallium nitride surface prior to plasma enhanced chemical vapor deposition (PECVD) SiNx:H passivation on electrophysical characteristics MIS-structures formed on their basis.

III. THEORY

It’s known wet chemical cleaning of GaN surface with HCl and HF based solutions produced surface with the lowest levels of oxygen. However, residual F and Cl remain on the GaN surface [1]. It can be removed by a wet treatment in an aqueous solution of NH4OH [2]. Exposure to a low-power NH3 treatment prior to SiNx:H passivation show minimal gate lag an current collapse after extended dc bias operation for AlGaN-GaN HEMTs [3]. Layers of silicon nitride are the most popular dielectrics for semiconductors A3B5.

IV. EXPERIMENTAL RESULTS

1-µm GaN was grown by MBE on sapphire [4]. Ex situ SiNx:H passivation is used. The PE NH3 pretreatment and

the SiNx:H deposition were performed in PECVD system with a bottom electrode diameter of 8 in. The SiNx:H film was formed using a SiH4: NH3: Ar plasma recipe and power was set to 0.15 W/cm2. The substrate temperature was maintained at 2000C for both NH3 pretreatment and SiNx:H deposition processes. For the pretreatment, the chamber pressure was 0.3 Torr and the duration was 180 s. Power was set to 0.1 W/cm2. For measurement of capacitance-voltage (CV) dependencies the Ni contacts with thickness 0.1-µm were evaporated.

The methods were used for the study of the samples are as follows:• The thickness and the refractive index of layers were

determined by the ellipsometry method at wavelength of 0.6328 micrometer;

• IR spectra were recorded on Fourier spectrometer (BRUKER) with the permission of a 4 cm-1;

• High-frequency MIS CV techniques were used (10 kHz at temperature from 300 K to 473 K).

SiNx:H thickness variation over the substrate with a diameter of 5 cm was not exceed 2%.

CV characteristic of MIS-structures on the base of SiNx:H layers formed at different powers are presented in Fig. 1.

Fig. 1. Normalized high-frequency capacitance-voltage characteristics of MIS GaN/ SiNx:H, formed by various power densities.

Fig. 1. shows that the power W=0.15 W/cm2 is optimal for GaN surface passivation by SiNx:H. At this power concentration ratio N-H/ Si-H bonds is equal to 20, Table I.

Study of Gallium Nitride Surface Treatment Prior to PE CVD SiNx:H Passivation

Maria A. Parfenova, Dmitrii Yu. Protasov, Timur V. Malin, Artem E. NastoviakSvetlana F. Devyatova, Konstantin S. Zhuravlev

ISP SB RAS, Novosibirsk, Russia

3715th INTERNATIONAL CONFERENCE ON MICRO/NANOTECHNOLOGIES AND ELECTRON DEVICES EDM 2014

TABLE ITHE DEPENDENCE OF N-H AND SI-H BOND

CONCENTRATION IN PE SINX:H LAYERS ON POWER

Power, W/cm2 0.05 0.10 0.15 0.27

Concentration N-H bond, C.1021 cm-3 3.1 2.8 2.0 1.85

Concentration Si-H bond, C.1021 cm-3 0.04 0.06 0.1 0.18Ratio concentrationsN-H/ Si-H

77.5 47 20 10.3

It is known, that the treatment in hydrogen plasma leads to a strong hydrogen passivation of various defects [3]. Therefore pretreatment in PE NH3 before SiNx:H passivation was carried out. Fig. 2. presents capacitance-voltage characteristics of MIS GaN/ SiNx:H, depending on the plasma pretreatment and without.

Fig. 2. Capacitance-voltage characteristics of MIS GaN/ SiNx:H, depending on the plasma treatment and without.

Fig. 3. Capacitance-voltage characteristics MIS GaN/ SiNx:H for different wet treatment before passivation.

Exposition of the gallium nitride surface on air leads to surface oxidation. Known gallium oxides dissolve in hydrofluoric acid, so substrate after the degreasing treatment was performed in a dilute hydrofluoric acid. Such treatment increases the inclination CV, but there is a double threshold. Remains of fluoride on the surface were removed in aqueous ammonia. In Fig. 3. it can be seen that the step on the CV significantly decreased. After the surface treatment in a buffer solution a threshold disappears at CV and the need for further treatment in aqueous ammonia eliminates. The same effect is achieved and when processed in a mixture of HCl:HF:H2O (1:1:2).

As one can see from Fig. 1-3, the feature of CV is absent of inversion region at negative bias. This effect can be because of non-equilibrium depletion by minor charge carriers (in this case by holes) of near surface region of semiconductor. For checking of this assumption, the temperature dependencies of CV were measured in range 300-473 K (see Fig. 4). It was obtained that capacity in depletion region has a strong dependence from temperature. But the MIS simulation cannot explain such high increment of capacity by thermal generation of minor charge carriers. It can be assumed that such effects are induced by near-surface traps. Additional studies are required to resolve these features.

Fig. 4. Normalized high-frequency capacitance-voltage characteristics of MIS GaN/ SiNx:H at various temperatures.

V. DISCUSSION OF RESULTS

It is well known, that the oxidation of GaN surfaces lead to appearance of high density donor-like surface states [5]. This is playing an important role in physics of semiconductors devices on base of gallium nitride. As a particular, these donor-like surfaces states are considered at now as a most probably source for forming two dimensional electron gas in undoped AlGaN/GaN heterostructures [6]. The reduction of surface state density in a result of chemical treatment and passivation by SiNx:H layers lead to sufficient unpinning of Fermi level, and, as a consequence, to bigger modulation of MIS capacitance.

38 15th INTERNATIONAL CONFERENCE ON MICRO/NANOTECHNOLOGIES AND ELECTRON DEVICES EDM 2014

VI. CONCLUSION

In this work it was obtained that the optimal power at PECVD SiNx:H passivation of GaN is equal to 0.15 W/cm2. The HN3 plasma pretreatment leads to decrease of CV hysteresis. The wet treatment in a dilute hydrofluoric acid and aqueous ammonia provides the enhancement of MIS properties.

REFERENCES[1] King S.W., Barnak J.P., Bremser M.D., Tracy K.M., Ronning C.,

Davis R.F. «Cleaning of AlN and GaN surfaces» // Journal of Applied Physics, 1998. V. 84. No. 9. pp.5248-5260.

[2] Quay R. Gallium Nitride Electronics-M.: Technosfera,2011.- pp. 285. (in Russian)

[3] Edwards A.P., Mittereder J.A., Binari S. C., Katzer S., Storm D.F., Roussos J.A.// IEEE Electron Device Letters, 2005. V. 26, No. 4 pp. 225-227

[4] Zhuravlev K. S., Osinnykh I. V., Protasov D. Yu. et al, Phys. Status Sol. C 10(3), 315-318 (2013).

[5] Miao M.S., Weber J.R., and Van de Walle C. G. J. //Appl. Phys., 2010. V. 107, 123713.

[6] Ibbetson J.P., Fini P.T., Ness K.D. et al //Appl. Phys. Lett., 2010. V. 77. 250.

Maria A. Parfenova was born in 1993 in Biysk. Now she is 4th year student of Radiotechnical Electronics and Physics faculty of Novosibirsk State Technical University. Her interest focused of Study of gallium nitride surface treatment prior to PE CVD SiNx:H passivation.