The role of the growth temperaturefor the SiN interlayer deposition in GaN

Tim B�ttcher*, Jens Dennemarck, Roland Kr�ger, Stephan Figge, and Detlef Hommel

Institute of Solid State Physics, University of Bremen, P.O. Box 330440, 28334 Bremen, Germany

Received 15 November 2002, revised 30 November 2002, accepted 2 December 2002Published online 20 October 2003

PACS 68.37.Lp, 68.55.Ln, 81.05.Ea, 81.15.Gh

The application of SiN interlayers in GaN-based structures for the annihilation of threading dislocationscaptivates by its simplicity and the possibility to use it. However, since the metalorganic vapor phaseepitaxy (MOVPE) of the group-III nitrides happens in a hydrogen-containing atmosphere, the surfacedecomposes during the SiN deposition. This work addresses the impact of the SiN growth with respectto temperature and surface dose. In particular, a quantitative analysis of the surface coverage is pre-sented and the successive overgrowth with GaN is discussed in view of threading dislocation densityand island growth mode.

# 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction The use of SiN as a micromask in GaN structures to reduce the threading disloca-tion density was first reported by Venn�gu�s et al. for the application on sapphire [1]. Later it wasdemonstrated to work on GaN films as well, although Si was already used before as an antisurfactantfor the growth of GaN quantum dots on AlGaN [2–5]. The transition to the Volmer-Weber growthmode is explained by a reduction of the sticking coefficient of the precursors on the SiN. The SiNforms a mask with nano-holes, through which GaN starts to grow. No growth on the masked regionswas observed [3]. If the growth is continued, the SiN is laterally overgrown. It was observed bytransmission electron microscopy (TEM), that many threading dislocations change their line directionat the SiN layer. The preferential incorporation of Si at the dislocation cores could be the reason forthis, as nitrogen dangling bonds are available at these sites. Accordingly, the core sites are overgrownand the dislocations are bent into the (0001) plane, where they can annihilate with dislocations havingan opposite Burgers vector. As a result, the dislocations form dislocation loops, which do not continuein the growth direction [2, 3].

Utilizing metalorganic vapor phase epitaxy (MOVPE), the SiN is grown on GaN films by supplyingammonia and silane to the surface at elevated temperatures. The silane is usually supplied from thesame source as used for the n-type doping, i.e. it is delivered in a low concentration diluted in hydro-gen. This requires long deposition times, such that the epitaxial surface can be affected by the decom-position in a hydrogen-containing atmosphere [6]. In order to avoid this surface degradation, we stu-died the deposition and the overgrowth of the SiN above and below the decomposition temperature ofGaN.

2 Experimental All structures studied in this work were grown by MOVPE on sapphire substrates,using ammonia and trimethylgallium as the source materials and hydrogen as the carrier gas. TheSiH4 was provided in a concentration of 10 ppm diluted in hydrogen. The growth runs were moni-

* Corresponding author: e-mail: boett@physik.uni-bremen.de, Phone: +49 421 218 4524, Fax: +49 421 218 4581

phys. stat. sol. (c) 0, No. 7, 2039–2042 (2003) / DOI 10.1002/pssc.200303370

# 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

tored by monochromatic normal-incidence reflectance measurement in order to obtain information onsurface roughening and the coalescence process following the SiN deposition. Details of the growthprocess are published elsewhere [7]. Using the reflectance measurements a GaN decomposition tem-perature of 860 �C was found for a pressure of 100 Torr of pure hydrogen. Accordingly, the SiH4

treatment in the structures was performed either at a temperature of 800 �C or at 1030 �C. In particu-lar, the deposition times were 200 s and 600 s, corresponding to a total amount of silane of 0.15 mmoland 0.45 mmol, respectively.

The threading dislocation and the interface structure were characterized by TEM in plan-view andcross-section geometry. To study the surface morphology and the SiN coverage, the growth was inter-rupted after the SiH4 treatment and analyzed by atomic force microscopy (AFM) and X-ray photoelec-tron spectroscopy (XPS).

3 Surface and interface structure The morphologies of the GaN surfaces treated by 0.45 mmolSiH4 at 800 �C and 1030 �C, respectively, are shown in Fig. 1. The surface is smooth for the deposi-tion at 800 �C, i.e. it only shows terraces with monolayer steps. In contrast to this, the SiN depositionat 1030 �C caused the formation of weakly faceted islands on the surface. Since the island volumeexceeds the volume of the deposited SiN by an order of magnitude, those features are not formed bypure SiN.

This is also evident from the cross-section TEM images of overgrown structures shown in Fig. 2,where the SiN interface appears as a faint line in both samples. If one compares the interface struc-tures of the two samples, one finds a perfectly smooth interface for the deposition at 800 �C, whilethe interface appears to be faceted for the treatment at 1030 �C. The height difference across theinterface amounts to roughly 40 nm as indicated by arrows, which is in good agreement with the dataobtained from AFM. The facets might be f1011g planes, which form the least stable surface in GaN[8]. Thus, the islands found after SiN deposition at 1030 �C presumably form through the combina-tion of decomposition in a hydrogen-containing atmosphere and the surface stabilization by SiN. Ifthe SiN preferentially nucleates on the top of the islands, then the decomposition would deepen the

2040 T. B�ttcher et al.: The role of the growth temperature

Fig. 1 AFM images of the GaN surfacetreated by 0.45 mmol SiH4 at 800 �C and1030 �C, respectively.

Fig. 2 Cross-section TEM image of the SiN interface region inovergrown structures. The SiN interlayer was generated by supply-ing 0.45 mmol SiH4 at 800 �C and 1030 �C, respectively.

# 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

trenches in between, such that the surface roughness increases with deposition time. This was con-firmed by AFM scans, where the rms roughness increased from 4.2 nm to 10.6 nm, when the deposi-tion time was increased from 200 s to 600 s.

4 Surface coverage In order to estimate the SiN surface coverage after the SiH4 treatment, thesamples were analyzed by XPS. Since the Si 2p peak is largely affected by the chemical bonds to thenitrogen atoms, the Si 2s peak was chosen for the quantitative analysis. To ensure that surface oxidiza-tion does not affect the signal, samples were transferred under oxygen-free contitions into the vacuumchamber.

The XPS spectra are depicted in Fig. 3, where the data of the samples exposed to 0.45 mmol SiH4

is shown together with a spectrum of a bare GaN surface as a reference. As can be expected from thestructural data, the Si 2s signal is rather weak compared to the Ga 3s peak. The Si 2s signal increaseswith deposition temperature and deposition time. The quantitative analysis was performed followingthe work of Cumpson and Seah [9]. To determine the area of the Si 2s peak, the spectrum of the GaNreference sample was used as the background signal and the Si 2s signal was added as a Gaussianpeak, using a numerical least-squares fit. The resulting surface coverages are listed in Table 1.

For all samples, a surface coverage well below a monolayer was determined. This supports themodel of Tanaka et al., who suggested that the Si atoms mainly occupy the sites at the atomic stepson the surface, thereby inhibiting the step-flow growth mode of GaN [3]. As can be expected, the SiNthickness increases almost linearly with the SiN deposition time. Furthermore, it also increases withthe deposition temperature in spite of the enhanced surface decomposition, which is presumablycaused by a more efficient formation reaction of the SiN.

phys. stat. sol. (c) 0, No. 7 (2003) / www.physica-status-solidi.com 2041

Table 1 The SiN surface coverage obtained from the quantitative XPS analysis.

TSiN (�C) SiH4 dose (mmol) SiN thickness (ML)

800 0.15 0.037800 0.45 0.068

1030 0.15 0.0451030 0.45 0.241

0 1000 2000

Time (s)

Ref

lect

ance

(ar

b.u.

)

SiN

SiN

800 °C

1030 °C

Fig. 3 XPS spectra of the samples dosed with0.45 mmol of SiH4 together with the spectrum ofa bare GaN surface.

Fig. 4 Reflectance transients of structures grown withdifferent SiN deposition temperatures. The SiH4 dose was0.45 mmol.

# 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

5 Overgrowth and threading dislocation densities The overgrowth of the GaN treated with SiH4

at either 800 �C or 1030 �C is illustrated in Fig. 4. Here, the in-situ reflectance transients of thestructures dosed with 0.45 mmol of SiH4 are shown. Following the SiN deposition, the reflectancesignal drops for both SiN deposition temperatures, indicating the transition to the island growth modeinduced by the SiN. During the SiN deposition, the surface roughening is directly visible for thetemperature of 1030 �C. In this case the reflectance signal is first reversed compared to the signalduring GaN growth due to the decomposition. It is then superimposed by diffuse scattering caused bythe increasing surface roughness. In comparison, the surface is not affected during deposition at800 �C. The island growth mode succeeding the SiN deposition is clearly more pronounced for theincreased deposition temperature, which is caused by the higher SiN surface coverage as well as bythe increased roughness of the epitaxial surface. However, since the latter roughness scales well belowthe roughness during the overgrowth process, it can be expected that the SiN coverage has the maininfluence.

This is in accordance with the threading dislocation densities after the overgrowth, which weredetermined by plan-view TEM and by defect-sensitive wet chemichal etching. For the SiN depositionat 1030 �C an edge type threading dislocation density of 2 � 108 cm�2 was found compared to4 � 108 cm�2 at a SiN deposition temperature of 800 �C. Below the SiN layer, the edge type thread-ing dislocation density was 1 � 109 cm�2 for both samples.

6 Summary The growth of SiN used for the in-situ reduction of the threading dislocation density inGaN was studied with respect to the deposition temperature and the deposition time. It was demon-strated that the SiN-treated surface degrades, if the decomposition temperature of GaN is exceeded.This is presumably caused by the accompanying etching of the underlying GaN. XPS measurementsallowed a quantitative analysis of the surface coverage, which revealed a surface occupation wellbelow a monolayer. The SiN deposition was more efficient at 1030 �C, such that the successive coa-lescence process was delayed as well and the threading dislocation density in the overgrown structureswas reduced by almost an order of magnitude.

Acknowledgements The authors would like to thank C. Kruse and A. Ueta for support with the XPS measure-ments. The work was supported by the Deutsche Forschungsgemeinschaft under contract number HO 1388/13-3and by the European Union within the framework of the DENIS project.

References

[1] P. Venn�gu�s, B. Beaumont, S. Haffouz, M. Vaille, and P. Gibart, J. Cryst. Growth 187, 167 (1998).[2] H. Lahr�che, P. Venn�gu�s, B. Beaumont, and P. Gibart, J. Cryst. Growth 205, 245 (1999).[3] S. Tanaka, M. Takeuchi, and Y. Aoyagi, Jpn. J. Appl. Phys. (Part 2) 39, L831 (2000).[4] Y. Ohuchi, K. Tadatomo, H. Nakayama, N. Kaneda, T. Detchprohm, K. Hiramatsu, and N. Sawaki, J. Cryst.

Growth 170, 325 (1997).[5] S. Tanaka, I. Suemune, P. Ramvall, and Y. Aoyagi, phys. stat. sol. (b) 216, 431 (1999).[6] D. Koleske, A. Wickenden, R. Henry, J. Culbertson, and M. Twigg, J. Cryst. Growth 223, 466 (2001).[7] S. Figge, T. B�ttcher, S. Einfeldt, and D. Hommel, J. Cryst. Growth 221, 262 (2000).[8] P. Fini, X. Wu, E. Tarsa, Y. Golan, V. Srikant, S. Keller, S. DenBaars, and J. Speck, Jpn. J. Appl. Phys.

(Part 1) 37, 4460 (1998).[9] P. Cumpson and M. Seah, Surface and Interface Analysis 25, 430 (1997).

2042 T. B�ttcher et al.: The role of the growth temperature

# 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim