jou rn al h om ep age: www.elsev ier .com/ locate /apsusc
spects of native oxides etching on n-GaSb(1 0 0) surface
. Cotirlan ∗, R.V. Ghita, C.C. Negrila, C. Logofatu, F. Frumosu, G.A. Lunguational Institute of Materials Physics, P.O. Box MG-7, Magurele, Bucharest, Romania
r t i c l e i n f o
rticle history:eceived 22 June 2015eceived in revised form8 November 2015ccepted 19 November 2015vailable online 29 November 2015
eywords:aSbative oxides
a b s t r a c t
Gallium antimonide (GaSb) is the basis of the most photovoltaic and thermophotovoltaic (TPV) systemsand its innovative technological aspects based on modern ultra-high vacuum techniques are in trend fordevice achievement. The real surface of GaSb is modified by technological processes that can conduceto problems related to the reproducible control of its surface properties. The GaSb surface is reactive inatmosphere due to oxygen presence and exhibits a native oxide layer. The evolution of native oxidesduring the ion sputtering, chemical etching and thermal annealing processes for preparing the surface ispresented in detailed way.
Ratios of surface constituents are obtained by Angle Resolved X-ray Photoelectron Spectroscopy(ARXPS). Moreover, Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS),
putteringtchingnnealingRXPS
Atomic Force Microscopy (AFM) and Low-Energy Electron Diffraction (LEED) are used for characteriza-tion. The surface stoichiometry is changed using a specific etchant (e.g. citric acid) at different etchingtime and is analyzed by ARXPS, SEM, EDS and AFM methods. The experimental results provide usefulinformation regarding surface native oxides characteristics on n-GaSb(1 0 0) to be taken into account fordevelopment of low resistance contacts for TPV devices based on GaSb alloy.
. Introduction
In this period of modern industry development that requiresifferent energy sources, a solution in good viability with environ-ental protection is the use of photovoltaic (PV) cells for direct
onversion of sun energy. The gallium antimonide (GaSb) is theasis of the most thermophotovoltaics (TPV) devices and its inno-ative technological aspects based on modern ultra-high vacuumechniques is in trend for device achievement that assures a goodfficiency on TPV market [1]. GaSb is an III-V semiconductor com-ound with zinc blende crystal structure which has an energy gapf 0.726 eV and it is worth to mention that the structure GaAs/GaSbad set a record for solar cell efficiency of 35% [2] opening a newra for PV applications. As was stated [3], the implementation ofaSb-based alloys in device technology is affected by the prob-
ems of reproducible control of their surface properties. In thiserspective, the real surfaces of GaSb are modified by technologicalrocesses presenting a surface damage region particularly after cut-ing, smoothing or mechanical polishing. The GaSb surface is moreeactive [3] than that of GaAs and oxidizes rapidly under atmo-
pheric conditions forming an oxide layer that is not self-limiting,table nor abrupt [4]. It is worth to mention that the chemical reac-ivity of antimonides is a distinct characteristic of these materials
[5]. The irreversible nature of the reactions suggests that oxygenatoms, rather than molecules, are involved in the chemical bond-ing. This paper presents data regarding the evolution of surfacenative oxides on n-GaSb(1 0 0) before and after Ar+ ion sputter-ing, respectively chemical etching in order to obtain uniform andstable contacts [6]. The composition of native oxides was put intoevidence qualitatively by an Energy-Dispersive X-ray Spectroscopy(EDS) analysis [7,8] and Angle Resolved X-ray Photoelectron Spec-troscopy (ARXPS) [9]. Maximum analysis depth for ARXPS is threeorders of magnitude less than in EDS. EDS is only for elementalanalysis, while XPS gives chemical state information [10]. Then,the samples were examined morphological by Scanning ElectronMicroscopy (SEM) [11], Atomic Force Microscopy (AFM) [12] andLow-Energy Electron Diffraction (LEED). Each etching step was ana-lyzed in connection with the evolution of surface stoichiometry fora contact aluminium epitaxial deposition, and finally, for a passiv-ation treatment of a photosensitive surface. The degree of noveltyfor this work is represented by a technology based on a new chem-ical etchant, Ar+ ion sputtering and thermal annealing used forremoving the native oxides in order to prepare rapidly the n-GaSbsurface required in a TPV device manufacturing process.
2. Experimental
The chemical composition of surface native oxides on n-GaSb(1 0 0) was examined by ARXPS measurements in a high
oxides presented a line centered on 31.78 eV for Sb substrate inGaSb and the line 34.30 eV for Sb in Sb oxide, with a compositionof 54.6% Sb in GaSb and 45.4% Sb in oxide. Considering that usuallythe Sb4d lines have a FWHM about 0.6÷0.7 eV, larger in oxide than
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acuum SPECS installation. The XR50 M monochromatic X-Rayource was operated at 300 W, 15 kV with a radiation energy of486.6 eV (Al K�) and a full width at half maximum (FWHM) of.3 eV. The spectrometer is based on a PHOIBOS 150 hemisphericalnalyzer with an ultimate resolution of 0.44 eV (defined as FWHMf recorded Ag3d5/2 spectral line). The C1s, O1s, Ga3d, Ga2p, Sb3d,b2p spectra have been recorded with 20 eV pass energy, and a stepf 0.05 eV. Each ARXPS measurements session has been followed by, 5, and respectively 10 min. of Ar+ sputtering using an IQE11/35
on gun operated at different accelerating voltages, from 500 V to000 V. The spectra were processed using Spectral Data ProcessorSDP) v.2.3 software. The spectra fitting were made using Voightunctions and Shirley background subtraction method. For quanti-ative analysis, the sensitivity factors provided by the spectrometer
anufacturer have been used. The relative error for calculationas estimated at 5% [13]. For chemical etching has been used a
olution of etchant HCl:H2O2:C6H8O7 (citric acid) at room tem-erature. Before chemical etching the samples were degreased inrichloroethylene and acetone. Then were exposed to a series oftchings with different periods. After each period the evolution ofurface oxides was examined by ARXPS analysis. The representativetages of etching process have been examined by SEM, EDS and AFMechniques. The scanning system was a Quanta Inspect F type cou-led with an EDAX/2001 facility for elemental analysis. The surfaceorphology of the etched samples was investigated by AFM with
MFP-3D SA microscope (Asylum Research) working in tappingode and using Olympus AC160 cantilevers (resonant frequency
20 kHz). Finally, the thermal annealing for oxide desorption on00/450 ◦C range in high vacuum SPECS installation was appliedn native GaSb surfaces in a similar manner to those of Si [14],e [15], GaAs [16]. By low-energy electron diffraction (LEED) was
nvestigated the reconstruction of semiconductor surface. The con-itions for development of good electrical and physical contactere shown for GaAs in Ref. [17] and for Si in Ref. [18].
. Results and discussion
A characteristic of GaSb compound is determined by therrangement of oxygen atoms on semiconductor surface. While Sbs octahedrally coordinated by oxygen, As on GaAs surface and Pn InP surface are tetrahedrally coordinated in their oxygen com-ounds. The composition of the oxide layers is determined byinetic factors (reaction rates, diffusion, dissolution, evaporation)nd thermodynamics [19]. As was expressed from the kinetics ofeaction between GaSb surface and oxygen, the oxidation showedhat it takes a course in two steps [20]:
GaSb + 3O2→ Ga2O3+Sb2O3 (1)
GaSb + Sb2O3→ Ga2O3 + 4Sb (2)
The different structure of semiconductor surface than that ofemiconductor bulk has an important influence on the electricalroperties of semiconductor device. In this perspective the studyf native oxides and the preparation methods of GaSb surface toliminate the contamination are necessary for the reproducibility ofurface properties. The experimental conditions involve the contactf surface with atmosphere in device technology, so that the surfacereparation usually takes place as a result of a controlled etchingrocess. The surface has been analyzed before Ar+ ion etching sim-
lar to [21,22] and before chemical etching [23]. The XPS spectrumf C1s line is fitted with three Gaussian peaks namely: 284.8 eVC C bond), 286.6 eV (C O bond) and 289.3 eV corresponds to more
omplex bonds like O C O or C O (Fig. 1).
The spectrum of Sb3d3/2 and Sb3d5/2 lines on as received sam-le of n-GaSb(1 0 0) covered with native oxides, measured in a XPSxperiment at take-off angle (TOA) of 90◦, is presented in Fig. 2.
Fig. 1. XPS spectrum of C1s before etching: peak A corresponds to C C, B to C O,C to C C or O C O .
The binding energy of the O1s core level partially overlaps with theoxidized Sb signal from the Sb3d3/2 core level from Sb2O3. Decon-volution of core levels is possible from the following constraints:the ratio of peak areas E and D should be maintained for B and A,furthermore the ratio of peak areas B/E and A/D must be 3/2.
The Sb3d5/2 centered on 527.85 eV corresponds to Sb fromGaSb substrate, the line 530.47 eV corresponds to Sb in Sb2O3,and 531.35 eV is related to O1s line. For Sb3d3/2 spectrum, theline 537.19 eV corresponds to Sb in GaSb, and the line 539.35 eVis related to Sb in oxide. The quantitative analysis stated a percent-age of 44.1% of Sb in GaSb and 55.9% Sb in oxide from the totalcontent of Sb.
In Fig. 3 is presented the Ga3d spectrum from the native oxidesof GaSb. The line corresponding to 19.05 eV is related to Ga fromGaSb substrate, and the line 20.35 eV is related to Ga oxide, mostprobably Ga2O3. The quantitative analysis indicates a percentageof 55.1% Ga in GaSb and 44.9% Ga in Ga oxide.
The Sb4d spectrum (Fig. 4) for the sample of GaSb with native
Fig. 2. XPS spectrum of Sb3d lines on n-GaSb: peak A is line Sb3d5/2 correspondingto Sb in GaSb, B is line Sb3d5/2 corresponding to Sb2O3, C is line corresponding toO1s, D is Sb3d3/2 line corresponding to Sb in GaSb, E is Sb3d3/2 line correspondingto Sb2O3.
C. Cotirlan et al. / Applied Surface Science 363 (2016) 83–90 85
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Fig. 3. XPS spectrum for Ga3d lines on n-GaSb: peak A corresponds to line Ga3d5/2 inGD
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Fig. 5. ARXPS spectra of as received sample in Sb3d range for TOA = 90◦ (A), 50◦ (B),20◦ (C), 10◦ (D).
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aSb, B corresponds to line Ga3d3/2 in GaSb, C corresponds to line Ga3d5/2 in Ga2O3, corresponds to Ga3d3/2 in Ga2O3.
n Sb from substrate, we applied the same constraints as for Sb3deconvolution, the ratio to be 3/2 between d5/2 and d3/2 peak areasoth for substrate lines and for oxide lines.
The ARXPS spectra for Sb at TOA: 90◦, 50◦, 20◦ and respectively0◦ are presented in Fig. 5. We can observe the evolution of theb3d lines for Sb in GaSb, and for Sb oxide that rise mostly fromb2O3.
The Sb4d ARXPS line (Fig. 6) shows a most intense signal fromhe surface TOA = 10◦ (D), an angle sensitive to surface compositionrom Sb oxide. The Sb signal from GaSb wafer is represented by ahoulder in right side of spectra (A).
A general remark from Figs. 5 and 6 with ARXPS spectra is thathe Sb3d in Sb2O3 and Sb4d in Sb2O3 lines become dominant forxternal layer of Sb oxide surface, therefore better observed atower TOA (TOA = 10◦), while the Sb3d and Sb4d signals from GaSbubstrate become weaker.
In Fig. 7 is presented the SEM image of n-GaSb sample covered
ith native oxides. As can be observed there exists a fine net-ork light colored granules with a medium dimension of 15 nm
hat related to a structured native oxides on the surface. In Fig. 8
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ig. 4. XPS spectrum for Sb4d line on n-GaSb: peak A corresponds to Sb4d5/2 in GaSb, corresponds to Sb4d3/2 in GaSb, C corresponds to Sb4d5/2 in Sb2O3, D correspondso Sb4d3/2 in Sb2O3.
Fig. 6. ARXPS spectra of as received sample in Sb4d range for TOA = 90◦ (A), 50◦ (B),20◦ (C), 10◦ (D).
Fig. 7. SEM image of n-GaSb(1 0 0) covered with native oxides.
86 C. Cotirlan et al. / Applied Surface Science 363 (2016) 83–90
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Fig. 8. AFM image of n-GaSb (1 0 0) covered with native oxides.
e present the AFM image of n-GaSb sample covered with nativexides.
As we observed the surface aspect is that of a conglomeratedtructure of Ga and Sb oxides that defines a roughness of the surfaceharacterized by a RMS of 1.854 nm.
The effect of Ar+ ion sputtering on native oxides for different vol-ages and sputter times is visible in ARXPS spectra, i.e., C1 (0.5 kV,1 = 3 min.); C2 (1 kV, t2 = 3 min.); C3 (1 kV, t3 = 5 min.); C4 (2 kV,4 = 10 min.).
In Fig. 9 is presented the XPS signal for a TOA = 20◦ for Sb3d, aignal sensitive to the surface composition. The signal for Sb oxidesresent on the as received sample starts to modify its intensityuring sputtering process, and at the last Ar+ sputtering (C4) itemains only the signal from Sb3d in the bulk of GaSb (∼537 eVnd ∼528 eV). In Fig. 10 is presented the XPS signal for TOA = 20◦
or Ga3d, following the evolution of Ga oxide.A strong signal from Ga oxide and a continuous decrease of
his signal during ion sputtering is present. The Ga oxide signalisappears after the fourth sputtering (C4) and the residual signalorresponds to Ga from GaSb (∼19 eV). The Sb4d XPS signal arising
rom Sb oxide at TOA = 20◦ is presented in Fig. 11.
In the evolution of Sb surface oxide during Ar+ ion sputteringeflected in Sb4d spectra is observed that after the third sputtering
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ig. 9. The evolution Sb oxides after Ar+ ion sputtering with different voltages andputter times reflected by Sb3d signal at TOA = 20◦: as received (A), and after sput-erings: C1 (B), C2 (C), C3 (D), C4 (E).
Fig. 10. The evolution of Ga oxide after ion sputtering for Ga3d signal at TOA = 20◦:as received (A), and after sputterings: C1 (B), C2 (C), C3 (D), C4 (E).
(C3) the Sb oxide vanished and it remains only the Sb4d signal fromGaSb (∼35 eV and ∼32 eV). The Sb oxides are the first ones to becleared off, and then are cleared off Ga oxides.
The SEM image of GaSb surface after the first ion sputtering (C1at 500 V, t1 = 3 min.) is presented in Fig. 12.
On the surface began a process of cleaning where the nativeoxides vanished, it remains a general aspect of bright points, but theclean surface of GaSb begins to be visible. In Fig. 13 is presented theAFM image with a better resolution for the GaSb surface partiallycovered with oxides.
The effect of ion sputtering is comprised in the network of brightconglomerates, the presence of the substrate of GaSb (the dark part)and an effect of increasing of roughness at the surface expressed byRMS = 2.8 nm. Likewise, it has been reported that HCl cleaning canremove the native oxides on GaSb [24].
Another n-GaSb (1 0 0) samples from the same wafer wereexposed to chemical etching time from 35 s to 300 s in a solutionof HCl:H2O2:C6H8O7 (citric acid) in a bath at room temperature.The evolution of native surface oxides was subjected to the XPS
analysis at a TOA = 90◦ (Fig. 14). The proportion of Sb in signalfrom GaSb substrate is 87.3%, and Sb from oxide is 12.7%. For Ga
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Fig. 11. The evolution of Sb oxide after ion sputtering for Sb4d signal at TOA = 20◦:as received (A), and after sputterings: C1 (B), C2 (C), C3 (D), C4 (E).
C. Cotirlan et al. / Applied Surface Science 363 (2016) 83–90 87
Fig. 12. SEM image of GaSb surface after ion sputtering (C1).
Fig. 13. AFM image of GaSb surface during ion etching.
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Fig. 14. Superposition of XPS spectra for Sb3d (a) and Sb4d (b) before and after chemica(E), C5 (F), C6 (G).
Fig. 15. XPS spectra for Ga3d before and after chemical etching at TOA = 90◦: asreceived (A), and after etchings: C1 (B), C2 (C), C3 (D), C4 (E), C5 (F), C6 (G).
signal the most part of the spectrum is represented by Ga fromGaSb substrate. There exist a small line of O2s related to Ga oxide.
The Sb3d spectra from Fig. 14(a) correspond to different stagesof etching namely: C1 (t1 ∼ 35 s), C2 (t2 ∼ 65 s), C3 (t3 ∼ 90 s), C4(t4 ∼ 120 s), C5 (t5 ∼ 180 s, C6 (t6 ∼ 300 s). The Sb4d spectra areshown in Fig. 14(b) for the same etching stages and for the sameXPS recording conditions. It was calculated that the Sb3d signalfrom GaSb is increasing and the signal from Sb oxide is decreasingfrom 91.5% to 8.5% in Fig. 14(a).
We observed in Fig. 14(b) that the more pronounced signal aris-ing from Sb4d oxides tends to reduce its intensity during chemicaletching. The signal has disappeared at C5, a fact that can estab-lish the experimental conditions for preparing the GaSb samplededicated to further contact technology.
The evolution of Ga native oxides on n-GaSb(1 0 0) as a result
of chemical etching is observed in the superposition presented inFig. 15. The Ga3d spectra from Fig. 15 are recorded after the samechemical etching stages as Sb3d and Sb4d lines. The Ga oxide sig-nal from GaSb surface disappears after C4, C5. The Ga3d line from
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(b) l etching at TOA = 90◦: as received (A), and after etchings: C1 (B), C2 (C), C3 (D), C4
88 C. Cotirlan et al. / Applied Surface Science 363 (2016) 83–90
ical etched surface (a) t = 90 s; (b) t = 25 s.
GFir
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Fig. 18. AFM image of n-GaSb(1 0 0) after 90 s chemical etching.
1.8
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Fig. 16. SEM images of GaSb chem
aSb substrate stands out after chemical etchings more prolonged.igs. 14 and 15 show that the Sb and Ga oxides had the same behav-or. Therefore, acid citric is appropriate as etchant for a rapid oxideemoval from the n-GaSb(1 0 0) surface.
The SEM image for a chemical etched surface is presented inig. 16(a) at the etching time of 90 s, and Fig. 16(b) for the etchingime of 25 s. There are presented some etching pits with a mediumiameter of ∼140 nm.
The SEM images from Fig. 16(a),(b) show the front face of chem-cal etched GaSb. The aspect of surface with veils is a result of anntense chemical attack that starts to destroy both the Sb nativexides and Ga native oxides.
From the EDS spectrum (Fig. 17) it can be observed that the Oignal is very low and the most intense signals are from SbL andaK lines. The presence of a small O K line after a chemical etching
ime of 25 s is a proof of an intense attack of native oxides startingrom C1 conditions.
In Fig. 18 is presented the AFM image of n-GaSb(1 0 0) surfacefter 90 s of chemical etching. There exists a strong attack of thetching solution at the surface. The network aspect is characterizedy valleys and hills. The roughness is high, i.e. RMS ∼ 13.96 nm.
Ratio of constituents from Fig. 19 shows that the native stoi-hiometry (Gat/Sbt) results from contributions of inner layers (justor TOA > 50◦), and interplay in depth between SbO > GaO, Ga/GaOnd Sb/SbO, where proportion of GaO tends to grow faster than
bO. Here, the SbO and GaO denote all the oxides of Sb, and Gaespectively (Fig. 20).
Fig. 17. EDS spectrum on n-GaSb (1 0 0) surface after 25 s etching time.
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Fig. 19. Review of XPS results for native oxides on GaSb surface.
C. Cotirlan et al. / Applied Surface Science 363 (2016) 83–90 89
AR c1 c2 c3 c4
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Fig. 20. Surface oxides evolution from as received (AR) probe to ion sputterings
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Fig. 22. Oxides desorption from GaSb surface after thermal treatment.
ample.
After first ion sputtering the stoichiometry Ga/Sb seems to beestored, but the oxides of Ga and Sb are still present on the sur-ace in approximately equal proportions. The ratio SbO/Sb becomeslmost zero after c3 sputtering, and GaO/Ga becomes almost zeroust after the last sputtering (Fig. 21).
The Sb/Ga ratio departs from stoichiometry if the chemical etch-ng time rises above 120 s, and oxides are still present on surfaceelow 120 s (Fig. 22).
The oxides of Sb from surface are removed by thermal treatmentetween 100/200 ◦C. Desorption of Ga oxides occurs at tempera-ures over 400 ◦C. The stoichiometry Sb/Ga is practically rebuilt onhe sample surface after 200 ◦C. We found similar with ref. [25] thats the Sb oxides are reduced on the surface over 100 ◦C, then theroportion of the Ga oxides increases, so that at 300 ◦C there is aaximum for GaO/Ga ratio. Likewise, ec.(2) of surface oxidation
hows conversion of Sb2O3 in Ga2O3.The pattern of glows from LEED (Fig. 23) shows reconstruction of
rystalline structure of the almost clean GaSb surface after thermal
reatment.
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5 Sbt/Gat GaO/Ga Sb O/Sb Sb/ Ga
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Fig. 21. Surface oxides evolution after chemical etchings.
Fig. 23. The LEED image for 80.3 eV electron energy at normal incidence.
4. Conclusions
The n-GaSb(100) surface is more reactive than GaAs [26] or GaP[27]. This surface oxidizes rapidly under atmospheric conditionsforming an oxide layer in the most part a mixture of Ga2O3 andSb2O3. The structure and the well-defined composition were stud-ied by ARXPS, SEM, AFM and LEED techniques. The presence of Sbexplains in part the high chemical reactivity of the surface. TheARXPS studies on Sb3d, Sb4d and Ga3d lines from GaSb are pre-sented in a detailed manner in order to understand the effects of Ar+
ion sputtering, chemical etching and thermal annealing. There wereestablished the experimental conditions in order to prepare a cleansurface of GaSb ready for carrying the ohmic contacts and Schottkybarrier. The Ar+ ion etching revealed an evolution of surface oxidesduring sputtering times with different accelerating voltages. It waspresented an extended study about etching times with a new chem-ical etchant in order to prepare n-GaSb surface. The stoichiometry
has not been maintained after ionic sputtering and chemical etch-ing, while the images for sample surfaces show that the oxideclusters remain and the roughness is high.
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The best solution to obtain a clean crystalline GaSb surface witheproducible stoichiometry for stable contacts was to complete ahemical etching with thermal treatment for oxide desorption. Thetoichiometry Sb/Ga is practically restaured on the sample surfacefter 200 ◦C. The chemical etching with time below 120 s followedy thermal annealing with temperatures in the range of 200/250 ◦Can be used for a good ohmic contact, but for a Schottky barrierhe temperature must be above 450 ◦C in order to obtain a cleaneconstructed surface.
cknowledgements
The authors are grateful to the Romanian Research ProgramPCCA 2013” for the financial support under the contract no.8/2014 and contract no.277/2014.
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