Two-step deposition of Al-doped ZnO on p-GaN to form ohmic contacts
© The Author(s). 2017
Received: 1 January 2017
Accepted: 18 July 2017
Published: 26 July 2017
Al-doped ZnO (AZO) thin films were deposited directly on p-GaN substrates by using a two-step deposition consisting of polymer assisted deposition (PAD) and atomic layer deposition (ALD) methods. Ohmic contacts of the AZO on p-GaN have been formed. The lowest sheet resistance of the two-step prepared AZO films reached to 145 Ω/sq, and the specific contact resistance reduced to 1.47 × 10−2 Ω·cm2. Transmittance of the AZO films remained above 80% in the visible region. The combination of PAD and ALD technique can be used to prepare p-type ohmic contacts for optoelectronics.
Nowadays GaN-based compound semiconductors have already achieved substantial progresses and have been comprehensively utilized in high temperature, high power, and high-frequency devices [1, 2], in which ohmic contacts are crucial for good device performance. So far, it is still very difficult to realize ohmic contacts to p-type GaN [3, 4]. Over a long period of time, oxidized Ni/Au , Ni/Pd/Au  and Pd/Ni  etc. are common solutions, although Au contacts are opaque, expensive and unstable at high temperature. Therefore, seeking an alternate that is thermally stable and transparent is imminent. Up till now, transparent conductive oxides (TCO) such as Al-doped ZnO (AZO) and Sn-doped In2O3 (ITO) have been widely used as the electrode materials. However, both tin and indium are costly and unfriendly to the environment. In contrast, AZO is promising due to its high-transparency, low-resistance, low-cost and non-toxicity [8–10]. It has been reported that AZO films can be prepared by many methods such as atomic layer deposition , sputtering , e-beam evaporation , pulsed laser deposition  and sol-gel . Due to the difference of the electron affinities between AZO (4.7 eV) and p-GaN (7.5 eV) , it is difficult to achieve ohmic contacts by directly depositing AZO onto GaN , although it was reported that after annealing the deposited AZO films on p-GaN resulted in ohmic behavior [17, 18]. To solve the problem, several kinds of interlayers have been introduced, e.g., NiO , Ag nanoparticles [19, 20], p-InGaN , Pt layer  and InON nanodots .
In this work, a two-step method was developed to achieve ohmic contacts between AZO and p-GaN. The first step is to grow AZO thin films as the interlayer by polymer assisted deposition (PAD). AZO films with different metal cation mole ratios of aluminum to zinc (nAl : nZn) were directly grown on p-GaN. The influence of different growing temperatures and annealing temperatures on the crystalline quality and the conductivity of the films were extensively studied. The second step is to grow AZO thin films by atomic layer deposition (ALD) on the top of the PAD-grown AZO. The AZO films show a favorable (002) orientation with good crystalline quality, a good ohmic behavior on p-GaN and high transmittance. PAD-AZO layer ensured ohmic contact while ALD-AZO layer decreased the specific contact resistance and the sheet resistance to make it usable.
PAD is a new chemical-solution deposition method developed in recent years and has been proven to be a practical method to grow metal oxide films with good crystalline quality on large scale of regular and irregular surfaces with very low cost [24–27]. PAD-AZO films (about 30 nm) were grown directly on p-GaN following the standard procedures of PAD method . The solution of the PAD-AZO films was prepared by blending two separate solutions of Zn and Al bound to polymers. The concentrations of Zn (3.06 × 10−4 mol/mL) and Al (7.41 × 10−5 mol/mL) in these two solutions were characterized by inductively coupled plasma-atomic emission spectrometer (ICP-AES), and the different volumes of the two solutions were mixed together, forming AZO precursors with different mole ratios of Al to Zn. The mixed solution was spin coated onto substrates at 3000 rpm for 40 s, and then preheated at 60 °C in air for 10 min on a hot plate. The films were then heated at 500, 600, 700 and 800 °C for 2 h in air. ALD method was used as the second step to increase the conductivity. The ALD-AZO films (about 120 nm) were deposited at 150 °C by using Beneq TFS-200, and the details of the ALD process can be found in our previous work [8–10]. The substrates in this experiment were p-GaN (the carrier concentration was about 1.2 × 1017 cm−3) and quartz glass. Surface topography was measured by atomic force microscopy (AFM, Bruker Multimode 8). Crystallinity and orientation of these films were measured by x-ray diffraction (XRD, Bede D1). Transmission of the films was measured by ultraviolet-visible spectrophotometer (UV-2550; Shimadzu, Kyoto, Japan). Electrical resistivity was measured by a hall measurement (Model 7707A, Lake Shore, USA) using a van der pauw geometry. The AZO films were etched by phosphoric acid for about 2 min (the etching speed was about 100 nm/min) with a 1 cm2 mask on them to form the square shape. After etching, four lead wires were connected onto the four square electrodes. Specific contact resistance and current-voltage (I-V) curves were measured by using circular transmission line model (CTLM) method. CTLM patterns were defined on substrate by using a standard photolithographic technique before grown.
Results and discussion
In this study we have successfully prepared AZO thin films on p-GaN by a combination of PAD and ALD method. The AZO thin films were (002) oriented, and highly transparent (about 80%) in the wavelength range of 400–700 nm. The optimal resistivity was 2.175 × 10−3 Ω·cm and the lowest specific contact resistance of the two-step deposited AZO film was about 1.47 × 10−2 Ω·cm2. Our results show that the two-step method can be used to prepare transparent and conducting AZO electrodes for industrial application.
This work is supported by the NSFC under Grant No. 11574235, the funding of Hubei Province No. 2015CFB418 and the funding of Jiangsu Province No. BK20151250. The authors would like to thank J. Yuan, H. H. Zheng, M. J. Zheng and J. Wu for technical support.
XS carried out the experiments and drafted the manuscript. GZZ, XW, CC participated in the design of the study, measurements and performed the analysis. HW conceived the study and participated in its design. CL supervised the overall study and polished the manuscript. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
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