N-face GaN epilayer grown on C-face SiC substrate by MOCVD

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(1. New Energy Source Institute, Shenyang Institute of Engineering, Shenyang 110136, China;2. School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, China)

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N-face GaN epilayer grown on C-face SiC substrate by MOCVD

SONGShiwei1,ZHANGDong1,2,ZHAOYan1,WANGCunxu1,KEYunjie1,LIYucai1,WANGJian1,WANGGang1,DINGYanbo1,WANGHan1,LIULiying1,GUORui1

(1. New Energy Source Institute, Shenyang Institute of Engineering, Shenyang 110136, China;2. School of Physics and Optoelectronic Technology, Dalian University of Technology, Dalian 116024, China)

The Ga-face GaN-based device exist a great polarization field, which cause carriers overflow and other problems to degrade the performance of GaN-based devices. However, N-face GaN can resolve these problems with the inversion of the polarization field. The N-face GaN epilayer was prepared on the C-face SiC substrate by MOCVD system, and the basic characters of the N-face GaN were investigated. N-face GaN has a rough surface, and exists a lot of edge dislocations and mixed dislocations. The yellow band is not observed at room temperature PL spectra despite a lot of Si-doped. After a phosphoric acid solution etching, a large number of Ga vacancies are produced on the surface of epilayer, and the corresponding yellow band appeared, which indicated the yellow band origins from Ga vacancy. After etching, GaN surface covered 12 pyramidal structures, which relaxes the tensile stress. In addition, as corrosion progresses, FWHM of low temperature PL spectra narrowed. Key words: N-face GaN; photoluminescence; gallium vacancy

N-face GaN; photoluminescence; gallium vacancy

0 Introduction

Recent years, GaN has been widely applied in the field of optoelectronic devices, such as LED, LD and other commercial applications. But the wurtzite GaN in the c-axis direction does not have the inversion symmetry,and Ga-plane GaN-based device exist a great polarization field, which cause carriers overflow and other problem to degrade the performance of GaN-based device. However, N-plane GaN can be reversed the polarization field, which aroused the interest of researchers.

In addition, N-plane GaN has many advantages: N-plane GaN-based HEMT has a smaller leakage current, lower dispersion and enhanced restrictions on carriers[1]; the gradient components of N-face p-type AlGaN induced polarization holes, increasing the hole injection efficiency[2]; Akyol designed a N-face double quantum well LED structure on a bulk N-face GaN, the injection current is more effective and the LED has a higher barrier, which does not require electron blocking layer to solve the problem of carrier overflow[3]. Generally, N-plane GaN is achieved by Al2O3substrate nitride or heavily Mg-doped[4]. Also, N-plane GaN can be achieved on C-plane SiC, only a few institutions to take research by MBE[5]. But very little research is taken about how to achieve N-plane GaN on the C-plane SiC substrate by MOCVD. In this letter, a preliminary study of N-plane GaN growth by MOCVD is carried out and its properties is studied.

1 Experiments

2 Results and discussions

2.1 X-Ray Diffraction (XRD) Analysis Results

Table 1 XRD of GaN epilayers

2.2 Scanning Electronic Microscopy (SEM) Analysis Results

Fig.1 SEM images of surface morphology after etching 10 min

Figure 1 shows the SEM images of N-plane GaN after 10 min H3PO4corrosion. From the images, a large number of pyramidal structures of 12 faces are formed. In the N-plane GaN, only one dangling bond of N atom point to the surface, thereby Ga atoms readily reacted with an acid solution to form a Ga2O3, and subsequently dissolved in an acid solution[6]. Phosphoric acid (H3PO4) played the role of a catalyst, but it is also Ga2O3dissolving agent. In other words, in the process of corrosion, Ga atoms are very susceptible to corrosion, as a result, the surface of N-plane GaN will form a large number of Ga vacancies.

2.3 Photoluminescence (PL) Analysis Results

Fig.2 Room temperature PL spectra of GaN epilayers

Fig.2 shows the room temperature (RT) PL spectra of the two samples. From the figure, an obvious yellow luminescence band located 2.2 eV is detected which is called the yellow band in PL spectra of Ga-plane GaN, while yellow band is not observed in the spectrum of N-plane GaN. The yellow band is widely present in bulk GaN, and GaN grown by MOCVD, MBE, HVPE, generally considered to be related to deep level of GaN. Neugebauer used first-principles calculations show that, due to the relatively small formation energy, gallium vacancy (VGa) with deep acceptor level and its related compound is associated with the origins of yellow[7]. Oila had taken the positive ions annihilation experiments to confirm that n-GaN generally have 1017～1018cm-3concentration of VGa[8]. In addition, Armitage shows that C related defects in GaN can cause yellow band[9]. Dislocations are also considered to cause yellow band[10]. In general, the content of C impurities in the N-plane GaN typically is much higher than Ga-face GaN, C impurity is difficult to become the origin of yellow band in GaN[11]. From the results of XRD, the threading dislocation density of N-plane GaN is much greater than that of Ga-face GaN, thus threading dislocations do not cause the yellow band. Thus we conclude that the yellow band of the sample is likely to come from the Ga vacancy.

Fig.3 Room temperature PL spectra of samples with different etch time

The RT-PL spectra of samples with different etch time is shown in Figure 3. After etching, RT-PL of N-plane GaN gradually appeared yellow band, and with the increase of etching time, the intensity of the yellow band enhances. Xu also found the same phenomenon, they found that after KOH etched, Ga vacancies of N-face GaN is up to 1020cm-3, resulting in surge of a yellow band intensities[7].

Table 2 shows the variation of D0X peaks and FWHM of D0X with etching time. With the increasing of etching time, peak slowly blue shift, indicating a certain stress relaxation. According to SEM image, a large number of 12 surface pyramid structure appeared, and with the corrosion process, the cones become isolated, and therefore relaxed of the GaN film tensile stress. Similarly, with the increasing of etching time, the FWHM of D0X emission peak decreases, indicating an improved optical quality. In general, the etching generally occurs at dislocations, which is a result of dislocation choice. Samples have smaller dislocation density after etching, and the FWHM of D0X peak has a great relationship with GaN quality, so that a smaller dislocation density of the sample has a smaller FWHM.

Table 2 Variation of D0X peaks and FWHM of D0X as a function of etching time

3 Conclusions

In this paper, a detailed study of the N-plane GaN epitaxial growth and properties is carried out. The growth of N-plane GaN is affected by VK mode, the epilayer exist a lot of twisted structure. And lots of black points and hexagonal structure are observed on the surface. After a hot phosphoric acid solution etching, the N-face GaN exhibits the morphology of 12 pyramidal structures, which results in a large number of Ga vacancies responsible for the “yellow band”. Also, the certain morphology relaxes the tensile stress of epilayer.

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1673-5862(2016)02-0140-04

C面SiC襯底上N面GaN的MOCVD制備及特性研究

宋世巍1, 張 東1,2, 趙 琰1, 王存旭1, 柯昀潔1, 李昱材1,王 健1, 王 剛1, 丁艷波1, 王 晗1, 劉莉瑩1, 郭 瑞1

(1. 沈陽工程學(xué)院 新能源學(xué)院, 沈陽 110136; 2. 大連理工大學(xué) 物理與光電工程學(xué)院, 遼寧 大連 116024)

Ga面GaN基器件結(jié)構(gòu)中存在巨大的極化場(chǎng),引起載流子溢流等問題,嚴(yán)重的損害了器件的性能。而N面GaN則可以使極化場(chǎng)反轉(zhuǎn),從而解決這些問題。詳細(xì)研究了在C面SiC襯底上N面GaN的MOCVD外延生長(zhǎng)和性質(zhì)。N面GaN表面非常粗糙,薄膜中刃位錯(cuò)和混合位錯(cuò)的含量較高。在Si摻雜的N面GaN的室溫PL譜中沒有觀測(cè)到黃帶的產(chǎn)生。利用熱磷酸溶液對(duì)N面GaN腐蝕,在外延膜的表面產(chǎn)生了大量的Ga空位,對(duì)應(yīng)的PL譜出現(xiàn)了黃帶,確定黃帶來源于Ga空位。經(jīng)過腐蝕后的GaN呈12面錐體形貌,錐體的形成可以弛豫薄膜中的張應(yīng)力,此外,隨著腐蝕的進(jìn)行,低溫PL譜的半峰寬變窄。

N面GaN; 光致發(fā)光; 鎵空位

TN 304.55Document code: A

10.3969/ j.issn.1673-5862.2016.02.003

材料科學(xué)