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doi:10.1016/j.jc

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Journal of Crystal Growth 298 (2007) 375–378

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The reactive ion etching characteristics of AlGaN/GaN SLs andetch-induced damage study of n-GaN using Cl2/SiCl4/Ar plasma

Rui Li, Tao Dai, Ling Zhu, Huapu Pan, Ke Xu, Bei Zhang, Zhijian Yang, Guoyi Zhang,Zizhao Gan, Xiaodong Hu�

Research Center for Wide Gap Semiconductors of Peking University, State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, School of

Physics, Peking University, 100871 Beijing, PR China

Available online 26 December 2006

Abstract

In this study, the etching characteristics of AlxGa1�xN/GaN superlattices with x ¼ 0.11 and 0.21 of Al and n-GaN with Cl2/SiCl4/Ar

plasma using reactive ion etching (RIE) system were investigated. By varying gas ratio and rf power, it was found that SiCl4 is an

effective getter to remove residual oxygen in the chamber and has a strong physical sputtering effect to remove the oxide layer during the

etching, and a nearly nonselective smooth etching of AlxGa1�xN/GaN SLs with the high etch rate of 220 nm/min could be obtained.

X-ray photoelectron spectroscopy (XPS) and Hall measurements were employed together to reveal the correlation between stoichiometry

and electrical changes of n-GaN induced by plasma etching. Combining with N2O plasma post-etch treatments to restore etched surfaces,

those results suggested that oxygen not only influences morphology of the Al-containing samples, but also electrical properties of n-GaN

by changing the status of oxygen-related defects, which may play crucial roles in determining the nature of the damage.

r 2006 Elsevier B.V. All rights reserved.

PACS: 52.77.�j

Keywords: A1. Defects; A1. Etching; A1. Surfaces; A3. Superlattices; B2. Semiconducting III–V materials; B3. Laser diodes

1. Introduction

Due to the inert chemical nature and strong bond energyof group III nitrides, Cl2-based dry etching techniques areindispensable for reliable pattern transfer in the semicon-ducting III–V materials. However, slow etch rates withrough morphology of AlxGa1�xN layers resultant fromaluminum oxidation have often been observed [1–3]. Tosome extent, these problems were resolved by adding BCl3or CH4 gas to Cl2 plasma [2,4]. Some articles reported thatGaN and AlxGa1�xN etched using high-density plasmaetch systems including inductively coupled plasma (ICP) inCl2/Ar plasma on the Si or Ge wafer could also get smoothmorphology [3,5]. So far, an impediment still remainsnonselective etching among various binary and ternary

e front matter r 2006 Elsevier B.V. All rights reserved.

rysgro.2006.11.018

ing author. Tel.: +8610 62767621; fax: +86 10 62752585.

esses: ruili79924@gmail.com (R. Li), huxd@pku.edu.cn

nitrides. Extensive studies are required in this area sincemany optoelectronic devices, especially laser diodes, arebased on heterostructures of AlxGa1�xN and GaN layers.For a ridge structure laser, etched surfaces need be smoothin order to get better oxidation of the structure andprecisely control the residual thickness of upper waveguidelayer [6]. An etching process that is highly anisotropic withetch rates independent of layer compositions is mostfavorable in that circumstance. In this paper, we reporton the etching characteristics of high Al-containingAlxGa1�xN/GaN SLs and n-GaN with Cl2/SiCl4/Arplasma using reactive ion etching (RIE) system. Whereasthe conduction properties of n-GaN are governed by eitherintrinsic or extrinsic defects [7,8], and plasma exposuretypically alters the conduction properties at the surfaceregion by the creation of nonstoichiometric surface andconsequent formation of near-surface lattice defects. Etch-induced damage study and N2O plasma post-etch treat-ments of n-GaN are also conducted to examine the

ARTICLE IN PRESSR. Li et al. / Journal of Crystal Growth 298 (2007) 375–378376

influence of the plasma exposure on the carrier concentra-tion at the surface of n-GaN, on which the Ohmic contactswould be formed.

Fig. 1. The influence of SiCl4 gas flow on the etch rate.

2. Experiments

In this study, a 2.5-mm thick undoped n-GaN, two 120pairs of Mg-doped Al0.11Ga0.89N/GaN and Al0.21Ga0.79N/GaN (25/25 A) SLs layers grown on sapphire substrates bymetal organic chemical vapor deposition (MOCVD) wereused. The mole fraction of Al in the SLs layers wasdetermined by X-ray diffraction. All the samples wererinsed in a H2SO4:H2O2 (1:1) solution at 125 1C to removeorganic contamination and in a HCl:H2O2 (1:1) solution at75 1C to remove oxide from the surface. Then, 420 nm thickSiO2 layers were deposited on the samples as a mask layerby ANELVA L-450P PECVD system. The mask patternwas defined using conventional photolithography and wetetched by buffered oxide etchant (BOE). The etchingprocesses were conducted using a commercial Load-lockedANEVAL L-451D-L RIE system in which samples weremounted on a 4-in Si wafer.

Unless otherwise noted, RIE plasma parameters used inthis study were: 30 sccm total gas flow of Cl2/SiCl4 with anadditional 5 sccm Ar for plasma stabilization, 1min 20 s or3min 30 s of etch time and 2Pa total process pressure. Inorder to investigate the influence of plasma compositionand rf power on the etch characteristics, two series ofexperiments were conducted as follows: First, the rf power(13.56MHz) was held at 70W, varying the SiCl4 flow from0 to 30 sccm. Second, the rf power was changed from 40 to110W, with the gas ratio of Cl2:SiCl4:Ar equal to 26:4:5.After the etching process, etch rates were calculated fromthe step heights obtained from alpha-Step IQ surfaceprofiler before and after the removal of the SiO2 with BOE.The depth was measured at least three points on eachsample to avoid incorrect data. A root mean square (rms)surface roughness of 3� 3 mm area was examined using anatomic force microscope (AFM) in the tapping mode.

To recover the etched surface of n-GaN, N2O plasmapost-etch treatments were carried out using PECVD underconditions listed in Table 1. Carrier concentration andmobility of the original, etched and post-etch-treatedsamples were measured using indium dots as Ohmiccontacts at the corners of the square samples in van derPauw mode. Before the etch process, the contacts of theoriginal sample were removed in a hot H2SO4:H2O2 (1:1)solution. Before post-etch treatments, the etched sample of10� 10mm was split into two small pieces of 5� 5mm

Table 1

Post-etch treatments of n-GaN etched at Cl2:SiCl4:Ar ¼ 26:4:5, power 70W, p

Process no. Gas flow (sccm) Pressure (Pa) rf power

A 150 100 35

B 100 70 35

named (A) and (B) and the indium dots on them werescratched away leaving surfaces intact. According to theetch rate mentioned later, the etched thickness of �0.3 mmwas considered for those etched and treated ones in theHall analyses. Besides, X-ray photoelectron spectroscopy(XPS) and room temperature photoluminescence (PL)measurements were also performed to study stoichiometryand optical changes.

3. Results and discussion

The dependence of the etch rates on the gas flow of SiCl4at fixing power of 70W is shown in Fig. 1. As a whole, theetch rates of all samples monotonically decrease as gas flowof SiCl4 increases. Compared with the region from 8 to16 sccm, the etch rates of all samples in the region from 16to 30 sccm decrease drastically to 20.6–29.5 nm/min. Thiscan be due to the reduction of active Cl radicals and Cl+

ions in the chamber, an evidence of the predominant role ofCl radical species in III–V material etching. It is wellaccepted that rough morphology of etched AlxGa1�xN ismainly caused by micromask effect of aluminum oxidewith huge bond strength of Al–O (21.2 eV/atom) [9,10].Previous research has reported that GeClx

+, SiClx+ have a

strong sputtering effect on GaN etching [5]. Consistently,as shown in Fig. 2, 2 sccm of SiCl4 is enough to scavenge alloxygen and prevent oxidation of aluminum for bothAlxGa1�xN/GaN SLs layers, while the presence of SiCl4in the plasma is insignificant for the smooth etching ofGaN.

ressure 2 Pa, time 1min 20 s

(W) Self-bias (V) Temperature (1C) Time (min)

0 300 10

0 300 10

ARTICLE IN PRESSR. Li et al. / Journal of Crystal Growth 298 (2007) 375–378 377

Fig. 3 presents etch rates of n-GaN, AlxGa1�xN/GaN(x ¼ 0.11 and 0.21) SLs layers, and the SiO2 mask as afunction of the rf power. With the Cl2:SiCl4:Ar gas ratioheld at 26:4:5, the etch rates of all samples monotonicallyincrease as rf power is elevated from 40 to 110W. In thelow rf power region from 40 to 70W, etch rates increase asthe rf power increases due to higher concentrations ofreactive Cl radicals, which increases the chemical compo-nent of the etch mechanism and higher ion flux, whichincreases the bond breaking and sputter desorption

Fig. 2. The influence of SiCl4 gas flow on the RMS roughness.

Table 2

XPS and Hall data of the original, etched and N2O plasma-treated samples

Sample Ga (%) N (%) C (%) O (%) Cl (%)

Original 46.01 32.31 16.13 5.55 —

Etched 47.65 25.40 13.53 10.11 3.30

(A) 30.23 23.43 19.98 26.36 —

(B) 33.15 11.76 25.41 29.69 —

Fig. 3. The influence of rf power on the etch rate.

efficiency of the etch. And the etching is nearly nonselectivefor all samples. At high rf power region above 70W, etchrates have also been observed to stabilize or decrease dueeither to saturation of reactive species at the surface orsputter desorption of reactive species from the surfacebefore the reactions occur. The slower etch rates of Al-containing layers are probably related to the higher bondenergy of AlN (11.52 eV/atom) compared with that of GaN(8.92 eV/atom) [9,10].According to the etch results mentioned above, we chose

the optimized condition of Cl2:SiCl4:Ar 26:4:5, power 70Wand pressure 2 Pa for the damage study of n-GaN. Thesurface stoichiometries of n-GaN samples were studied bycomparing the Ga (3d), N (1s), C (1s), O (1s), and Cl (2p)peaks in the XPS spectra. From the Ga 3d:N 1s ratioshown in Table 2, it is evident that N atoms arepreferentially removed by the plasma etchant resulting ina gallium-rich surface, which would give rise to the creationof donors such as VN and/or ON at the surface region [11].However, carrier concentration decreases to 83.4% of theoriginal one indicating the creation of acceptors withincrease of O concentration has occurred simultaneously,which can mainly be attributed to newly formed VGa–ON

complexes, a deep level acceptor in n-GaN [12]. Due to thelarger bond strength of Ga–O (14.7 eV/atom) comparedwith that of Ga–N (8.92 eV/atom), O atom is more likely tooccupy dangling-bond states of Ga-rich etched surfaceduring surface reconstruction, besides the plasma can alsopromote VGa–ON complexes formation by exposing theinner parts of epitaxial layer with high density of intrinsicVGa during the etching [13]. The evidence of the formationof VGa–ON complexes (not shown) is provided by theintegrated PL intensity ratio of the band-edge and yellowluminescence transitions (IBE/IYE) [14] decreases from 0.28to 0.16. It can be further proved by the FWHM of the Ga3d core peaks broadened from 1.258 to 1.422 eV after etch.Since the VGa–ON complex is doubly charged while theVGa–(ON)2 complex is singly charged, it causes the multiplesplitting of the core level as the system has unpairedelectrons in the valence band [15]. As for mobility, theresidual carrier concentration of original sample at roomtemperature is in the range of 1018 cm�3 and the mobility is212 cm2/ns. This mobility value indicates that the n-GaNlayer is highly compensated [16]. In typical semiconductorswhere the predominant scattering mechanism is the ionizedimpurity scattering, the mobility increases as the freecarrier concentration, n ¼ (Nd

+–Na�) is reduced and vice

versa. Those changes are consistent with the results of

Ga/N ratio Ga/O ratio m (cm2/ns) n (� 1018 cm�3)

1.424 8.306 212 4.549

1.876 4.713 241 3.795

1.290 1.147 222 4.272

2.819 1.117 226 3.880

ARTICLE IN PRESS

Fig. 4. XPS spectra of Ga (3d) core peak of the original, etched and N2O

plasma-treated n-GaN samples.

R. Li et al. / Journal of Crystal Growth 298 (2007) 375–378378

oxygen interaction with the n-GaN during etch process(Fig. 4).

In order to recover the damaged surface of n-GaN, (A)and (B) underwent two different N2O plasma post-etchtreatments of A and B listed in Table 1. To ensure thecredibility of the data, the measured m and n for (A) and (B)before treatments are 262 cm2/ns, 3.633� 1018 cm�3 and233 cm2/ns, 3.753� 1018 cm�3, respectively. By comparingXPS data of the two treated samples, the peak positions ofGa 3d shift from 20.45 eV of the etched to 20.20 eV of (B)and 20.15 eV of (A), indicating that the damage conditionof the etched surface is modified under the energetic ionbombardments. As Ga 3d:N 1s ratio concerned, the furtherloss of nitrogen in (B) probably due to the formation ofvolatile products would increase VN concentration accord-ing to conventional wisdom, while (A) has an excessnitrogen supply on the nitrogen-deficient surface, whichhas an adverse effect upon it. However, the carrierconcentration of (A) increases up to 94% higher than thatof (B) up to 85% of original sample, indicating that thestatus of oxygen-related defects within two samples mustdiffer from each other. The discrepancies in the resultscould probably be attributed to the competition among theacceptors of VGa–ON complexes and the donor of ON

except commonly believed VN.

4. Conclusion

From our current studies, it was found that SiCl4 as anoxygen getter within plasma has a strong physical sputtering

effect, which has a great impact on the surface morphologyof Al-containing layers. By optimizing radio-frequencypower and plasma composition to reach a compromisebetween chemical and physical mechanism, a nearlynonselective etch with smooth morphology can be obtained.Under such condition, the etch rate is �220 nm/min for theAl0.21Ga0.79N/GaN SLs layers, the highest etch rate everknown. Hall measurements and XPS analysis of etched andpost-etch N2O plasma-treated n-GaN samples stronglysuggested that oxygen not only influences morphology ofthe Al-containing layers, but also the electrical properties ofn-GaN at the surface by altering the status of its relateddefects during plasma exposure.

Acknowledgments

This work was supported by the National HighTechnology Program of China under Grant Nos.2002CB613505, 2005AA31G020; the National NaturalScience Foundation of China under Grant Nos.60477011, 60476028, 60406007, 60607003, 60577030,60276010; Beijing City Science and Technology Projectunder Grant No. H030430020230.

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