Comparison of trimethylgallium and triethylgallium as “Ga” source materials for the growth of ultrathin GaN films on Si (100) substrates via hollow-cathode plasma- assisted atomic layer deposition Mustafa AlevliAli Haider, Seda Kizir, Shahid A. Leghari, and Necmi Biyikli Citation: Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 34, 01A137 (2016); doi: 10.1116/1.4937725 View online: http://dx.doi.org/10.1116/1.4937725 View Table of Contents: http://avs.scitation.org/toc/jva/34/1 Published by the American Vacuum Society Articles you may be interested in Substrate temperature influence on the properties of GaN thin films grown by hollow-cathode plasma-assisted atomic layer deposition Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 34, 01A12501A125 (2015); 10.1116/1.4936230 Atomic layer deposition of GaN at low temperatures Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 30, 01A12401A124 (2011); 10.1116/1.3664102 Low-temperature self-limiting atomic layer deposition of wurtzite InN on Si(100) Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 6, 045203045203 (2016); 10.1063/1.4946786 Kinetics of thermal decomposition of triethylgallium, trimethylgallium, and trimethylindium adsorbed on GaAs(100) Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 9, (1998); 10.1116/1.577146 Substrate impact on the low-temperature growth of GaN thin films by plasma-assisted atomic layer deposition Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 34, 041511041511 (2016); 10.1116/1.4953463 Demonstration of flexible thin film transistors with GaN channels Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 109, 233504233504 (2016); 10.1063/1.4971837 Comparison of trimethylgallium and triethylgallium as “Ga” source materials for the growth of ultrathin GaN films on Si (100) substrates via hollow-cathode plasma-assisted atomic layer deposition Mustafa Alevlia) Department of Physics, Marmara University, Goztepe€ Kadıkoy,€ 34722 Istanbul,_ Turkey Ali Haider, Seda Kizir, Shahid A. Leghari, and Necmi Biyiklib) Institute of Materials Science and Nanotechnology, Bilkent University, Bilkent, 06800 Ankara, Turkey and National Nanotechnology Research Center (UNAM), Bilkent University, Bilkent, 06800 Ankara, Turkey (Received 5 September 2015; accepted 30 November 2015; published 21 December 2015) GaN films grown by hollow cathode plasma-assisted atomic layer deposition using trimethylgal- lium (TMG) and triethylgallium (TEG) as gallium precursors are compared. Optimized and satu- rated TMG/TEG pulse widths were used in order to study the effect of group-III precursors. The films were characterized by grazing incidence x-ray diffraction, atomic force microscopy, x-ray photoelectron spectroscopy, and spectroscopic ellipsometry. Refractive index follows the same trend of crystalline quality, mean grain, and crystallite sizes. GaN layers grown using TMG precur- sor exhibited improved structural and optical properties when compared to GaN films grown with TEG precursor. VC 2015 American Vacuum Society.[http://dx.doi.org/10.1116/1.4937725] I. INTRODUCTION reduces the performance of GaN-based devices.5 When epi- The enormous potential of gallium nitride (GaN) based taxial growth of GaN directly takes place on Si at high growth electronics and optoelectronics is still being limited by the temperatures, surface atoms will easily react with nitrogen, absence of commercially available native substrates with which forms SiNx interfacial layer and considerably hinders large diameters at reasonable cost levels. Instead, GaN de- the successive GaN epitaxy. In order to overcome these prob- vice layers are most widely grown on non-native substrate lems, metal-organic vapor deposition (MOCVD) and molecu- materials like sapphire and SiC. Compared to Si substrates, lar beam epitaxy runs at reduced substrate temperatures have both these materials perform short at the available wafer size been carried out. However, those GaN films show poor crystal and materials cost. Therefore, researchers have paid more quality due to the substantially low kinetic energy which is required for the thermodynamically favorable migration of attention to the growth of GaN films on various unconven- 5 tional oxide and metallic substrates. Besides, many research- precursors over the substrate surface. In order to overcome the limitations due to high substrate temperatures and the need ers have devoted significant effort in the growth of GaN 6 films on Si substrates, which is an ongoing active research for complex buffer layers, among alternative approaches for area with promising results and even commercial success.1–3 low-temperature growth of GaN include physical vapor depo- The integration of GaN-based RF-microwave and optoelec- sition (PVD) techniques (sputtering and pulsed laser deposi- tronic devices with highly advanced CMOS electronic back- tion) as well as a more-recently developed chemical vapor deposition technique: plasma-assisted atomic layer deposition bone might significantly increase the functionality, perform- 7–9 ance, and application spectrum of III-nitride devices.4 Most (PA-ALD). When compared with the alternative low- of the GaN-on-Si research has been focused on the develop- temperature PVD techniques, PA-ALD stands out in terms of ment of GaN layers on Si (111) substrates due to the three- large-area uniformity, submonolayer precision thickness con- fold surface symmetry and lowest amount of lattice trol, and ultimate conformality. Moreover, PA-ALD growth of mismatch of this particular plane. GaN layers grown on Si GaN films might be optimized in a self-limiting growth re- (111) using various buffer layers showed low impurity con- gime, where precursors do not need high kinetic energy for centrations and decent electrical properties; however, Si migration, making PA-ALD a powerful method over other (100) is still known as the standard substrate for mainstream low-temperature growth techniques. These features make PA- 3 ALD method suitable for deposition on temperature sensitive Si-based CMOS technology. Therefore, the development of 10 high-quality GaN on Si (100) is necessary for the integration device layers and flexible polymer substrates as well. of GaN HEMTs with advanced Si electronics and III-nitride PA-ALD might be a potential alternative pathway for light emitting diodes with enhanced light output. On the low-temperature deposition of GaN due its unique character- other hand, the mismatch in thermal expansion coefficients istics including uniformity, conformality, good reproducibil- between Si (100) and GaN causes large tensile stress in as- ity, precise thickness, and stoichiometry control of the films. grown films during cool-down period from the typical high Partially reflecting this emerging potential, an increasing growth temperatures (1000 C) and results in cracks which number of notable contributions appeared on the low- temperature (<300 C) growth of GaN films using PA-ALD a)Electronic mail: [email protected] with improved crystalline quality and electrical properties 6,11,12 b)Electronic mail: [email protected] recently. In those reports, PA-ALD of GaN using 01A137-1 J. Vac. Sci. Technol. A 34(1), Jan/Feb 2016 0734-2101/2016/34(1)/01A137/6/$30.00 VC 2015 American Vacuum Society 01A137-1 01A137-2 Alevli et al.: Comparison of TMG and TEG 01A137-2 trimethylgallium (TMG)11/triethylgallium (TEG),6,12 and N2/H2 plasma as the precursors were reported. However, no study exists yet which compares the influence of these gal- lium precursors on the optical and structural properties of PA-ALD grown GaN films. In this work, we report a comparative study on GaN thin films grown by a specific PA-ALD technique, namely, hollow-cathode plasma-assisted atomic layer deposition, using TMG and TEG as group-III precursors. The GaN films analyzed were deposited using N2/H2 plasma as the nitrogen source at 200 C substrate temperature. II. EXPERIMENT GaN samples were grown on Si (100) substrates at 200 C using a modified Fiji F200-LL ALD reactor (Ultratech/ Cambridge Nanotech, Inc.) in which quartz-based ICP source was replaced with a stainless steel hollow-cathode FIG. 1. (Color online) TEG and TMG saturation curves at 200 C. The N2/H2 plasma source (Meaglow, Inc.). For comparison purpose, the flow rate and flow duration were kept constant at 50 þ 50 sccm and 40 s, films were grown with identical parameters except for Ga respectively. precursor pulse durations due to the vapor pressure differ- ence of precursors. Ga precursor exposure doses were and TEG doses, respectively. Both saturation curves are indi- adjusted in order to achieve the self-limiting growth scheme, cators of self-limited growth. The difference in GPC values which is an evidence of ideal atomic layer deposition (ALD) might be attributed to the different physical and chemical growth mechanism.11 Each ALD cycle consisted of four properties (sticking coefficient, nucleation behavior, steric steps: 0.03 s TMG/0.5 s TEG, 10 s Ar purge, 40 s 50 þ 50 hindrance effect, etc.) of group-III precursor molecules.13 sccm 300 W N2/H2 plasma, 10 s Ar purge, repeated for 1750 In order to identify the crystalline nature of the GaN films, cycles in order to deposit TEG-grown GaN films and 2727 GIXRD measurements were performed. As revealed by their cycles in order to deposit TMG-grown GaN films. GIXRD patterns, GaN films showed single-phase polycrystal- The film thicknesses and optical constants were measured line character with hexagonal wurtzite crystallite structure by spectroscopic ellipsometer (V-VASE, J. A. Woollam Co., due to the existence of multiple GaN diffraction planes. The Inc.) in the wavelength range of 300–1000 nm at various same reflection peaks were observed for both samples and incidence angles. Crystalline phases of the GaN films were were referred to h-GaN (002) and h-GaN (103). No other studied with grazing-incidence x-ray diffraction (GIXRD) phase mixing was detected (i.e., cubic). The use of different measurements (PANalytical X‘Pert PRO MRD diffractome- metal alkyl precursors affected neither peak positions nor the ter) in H-2H mode with Cu Ka radiation.