SHEN Guiying ,ZHAO Youwen ,HE Jianjun

(1. Key Laboratory of Semiconductor Materials Science Beijing Key Laboratory of Low Dimensional Semiconductor Materials and Devices Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China; 2. College of Materials Science and Opto-electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China; 3. Rugao Institute of Compound Semiconductor Industry,Nantong 226500, China)

Abstract: Ion implantation induced damage in GaSb and its removal by rapid thermal annealing(RTA) have been investigated by Raman spectroscopy.The evolution of the Raman modes as a function of implantation fluence,annealing temperature and time has been analyzed.Results indicate that a lattice quality that is close to as-grown GaSb has been obtained by annealing the implanted samples at 500 ℃ for 45 s.However,consequent surface analyses by scanning electron microscope (SEM) and atomic force microscope(AFM) show that a heavily perturbed layer contains voids due to the outdiffusion of Sb atoms on the surface remains.Mechanism of the damage recovery and the structure of the implanted layer are discussed based on the experimental results.

Key words: ion implantation;Raman spectroscopy;GaSb;RTA

1 Introduction

GaSb has great application in a variety of optoelectronic devices such as lasers,photodetectors and photovoltaics in the mid-and long-wave infrared spectral range due to its high electron mobility and direct bandgap[1,2].Unintentionally doped GaSb system is alwaysp-type with a hole concentration of the order of 1016cm-3,regardless of the growth technique and conditions due to an intrinsic native defect[3].Carbon is an effectivep-type dopant in GaSb for its small diffusion coefficient and high solubility,so as to obtain high concentration and abrupt doping profiles[4].It has been reported that carbon acts as a hydrogenic shallow acceptor with binding energy in GaSb of 12.6 meV[5,6].It would be interesting to study the effect of carbon atoms as group dopant on the structure and properties of GaSb wafers due to the potential for high-speed device applications.

The implantation process is an effective method to govern the localized dopant profile that forms ohmic contacts and planarp-njunctions with high performance in the above devices[7].To date a lot of studies on GaSb implanted by different ions like Si+,S+,Mg+and Be+have been published[8-13],but there is a limited discussion on the case of carbon as an implanted ion.The essential issue about the implantation is the question of how implanted atoms affects the crystal structure of GaSb.And it is quite difficult to remove the lattice damage caused during implantation due to a limitation on the maximum annealing temperature[14,15],because GaSb has a low incongruent evaporation temperature (370 ℃).Hence,it is necessary to get a deep knowledge of the implantation behavior of GaSb and the lattice perfection during annealing.Raman spectroscopy is probably the most useful method available to obtain lattice damage and the recovery.Y K Suet alreported a better recovery of damage layer of GaSb implanted by Si is formed when the annealing temperature is 600 ℃ for 30 s[8].However,the surface characteristics of GaSb sample at high temperature have not been studied in detail.

In this work,it is reported a series of Raman spectroscopy of GaSb samples which have been implanted with carbon ions.Using room-temperature Raman spectroscopy,the effects of annealing time and temperature on the damage recovery of GaSb has been studied.The morphology of the implanted layer has been investigated by scanning electron microscope(SEM) and atomic force microscope (AFM).

2 Experimental

Un-doped GaSb (100) wafers with ap-type concentration of about 4.5×1016cm-3grown by the liquid encapsulated Czochralske (LEC) were used for implantation.GaSb wafers were sliced with the size of 2 cm×2 cm×0.05 cm and double-side polished.In the ion implantation process,graphite is used to produce the carbon gas source,in which an electron is stripped forming the atmosphere of C+.C+ion implantation was carried out at room temperature with the ion beam 7° off the normal incidence to avoid the channeling effects.It was determined that the dose of 4.6×1011,3.3×1012and 3.1×1013ions/cm2at an accelerating energy of 290 keV was used to produce a peak ion concentration in the range of 1016-1018cm-3with the implanted layer thickness exceeding 800 nm by using the Stopping and Range of Ions in Matter and Transport of Ions in Matter (SRIM/TRIM) program[16].The as-implanted samples were annealed by a rapid thermal annealing process (RTA) in the atmosphere of N2to active the implanted doping and remove the damage caused by the implantation.Several recipes for the rapid thermal annealing process were attempted by changing temperatures from 300 to 600 ℃ and the durations from 10 to 45 s.

Raman spectroscopy were obtained from the carbon-implanted side of the samples in backscattering geometry at room temperature using a 633 nm He-Ne laser at an incidence angle of 60°.For III-V compound semiconductors like GaSb and other zinc-blende structures,two Raman modes corresponding to the longitudinal optical (LO) and transverse optical (TO)phonons mode associated with the Brillouin zone centre exist in the Raman spectroscopy.In backscattering geometry,only LO phonons appear in the (100)direction by selection rules[17,18].The power density for all samples was fixed relatively low of 3.35 mW/cm2so as to prevent local heating of the samples and minimize the generation of photo-excited carriers.

3 Results and discussion

3.1 Implanted ion fluence dependence

Fig.1.shows Raman scattering spectra of nonimplanted GaSb and implanted carbon ions with various doses at 290 keV.The Raman spectroscopy of nonimplanted GaSb presents mainly two vibrational bands centered at around 236 and 157 cm-1corresponding to the LO and TO phonons mode.As mentioned above,only LO mode is allowed for (100) oriented material,and the TO mode is not allowed on the basis of Raman selection rules[8,19].However,a small peak due to the TO mode in the spectrum at 226 cm-1is observed here due to the slight deviation of the wafer from (100).This phenomenon has also been observed by other researchers[20].Two additional weak peaks appear at 164 and 113 cm-1at low wavenumber regime.The former corresponds to the T2mode of the point group Tdby the Raman selection rules[21].The latter at 113 cm-1is attributed to Sb(Eg) mode.For non-implanted GaSb,the existence of Sb(Eg) mode is attributed to the Sb-oxide on the surface reacting with GaSb substrate at room temperature forming additional Ga2O3and elemental Sb[22],associated with the reaction:

Fig.1 Raman spectroscopy for the GaSb samples: (a) non-implanted;implanted with 290 keV carbon ions with the fluence of (b)4.6×1011cm-2,(c) 3.3×1012cm-2 and (d) 3.1×1013cm-2

This reaction converts the Sb-oxides in the interfacial region and releases elemental deposits of Sb[23,24].As a result of the competition between thermodynamic and kinetic factors[25],the native oxides grown at room temperature contain both Ga-and Sb-oxides throughout the oxide film,with elemental Sb at the oxide-GaSb interface.Generally,the oxide layer covering the GaSb substrate possessed a layer thickness of 3-5 nm which is much lower than the penetration depth of Raman method[26-28].Hence,Sb(Eg)mode can be found in the Raman spectroscopy of nonimplantation GaSb.

After implantation with 4.6×1011and 3.3×1012cm-2,the Raman curves are similar to that of nonimplanted GaSb,which indicates little apparent damage forms during implantation below 3.3×1012cm-2.By increasing the fluence up to 3.1×1013cm-2,both Sb(Eg) andT2modes disappear,and the intensity of TO and LO mode decrease rapidly due to the disordering of the crystalline structure.Compared with as-grown GaSb with the LO mode at 236 cm-1the peaks of the as-implanted samples are red shift.Such behavior can be explained by the existence of a tensile stress effect[29].Carbon has a smaller covalent radius (0.77 Å)compared to Sb (1.4 Å),resulting in a concentrationdependent strain when carbon is present in the GaSb lattice.

3.2 Annealing temperature dependence

In order to study the degree of recovery from lattice damage with the fluence of 3.1×1013cm-2,the process of annealing with different temperature has been carried out.Fig.2.displays the Raman spectroscopy for carbon ion implanted GaSb after annealing.It is found that the Raman curve of the sample annealing at 300 ℃ is almost identical to that of as-implanted GaSb,which indicates no apparent recovery of crystal quality.For the annealing temperature above 400 ℃,the intensity of LO and TO modes increase slightly.However,the intensity of LO and TO modes is far below the level of non-implanted samples even after annealing at 600 ℃.This means that the recovery is still insufficient.In addition,Sb(Eg) and Sb (A1g) modes are observed at around 113 and 151 cm-1with the annealing temperature above 500℃.This can be explained as Sb-Sb vibration modes with aD3dsymmetry Raman tensor produced by the accumulation of Sb on the surface by the outdiffusion of Sb at high temperature[30,31].

It has been demonstrated that LO/TO intensity ratio is an index of the degree of disordering and depends on the annealing temperature[30].This is because the decay of the translational symmetry relaxes the momentum conservation,so that all phonons in the Brillouin zone participate in the first order Raman scattering[30].High LO/TO intensity ratio indicates the damage is largely recovered.The LO/TO intensity ratios as a function of annealing temperature has been calculated as shown in the inset of Fig.2.The LO/TO ratio can firstly increase and then decrease with the increasing of the annealing temperature.The highest value of the LO/TO ratio indicating the maximum recovery is achieved at 500 ℃ annealing.However,compared to the value of non-implanted sample,the LO/TO intensity ratios for the 300-600 ℃ annealing samples have extremely low values,which indicates that the recovery process of the crystallinity is incompleted with the annealing time of 10 s.

3.3 Annealing time dependence

In order to recover the damage furtherly,the annealing time has been extended at 500 ℃.The Raman intensity as a function of annealing time is shown in Fig.3.It is found that both Sb(A1g) and Sb(Eg)modes become stronger with the increase of annealing time due to the outdiffusion of Sb on the surface.The inset shows that the LO/TO ratios for 10-30 s annealing have nearly the same value indicating the same recovery level.The rapid increase of the LO/TO intensity ratios for 45 s annealing indicates the effective recovery process of the crystallinity.Hence,from the crystal quality point of view,annealing at 500 ℃ for 45 s generating the recovery of the disordering at the greatest extent.

Fig.3 Raman spectroscopy of GaSb samples: (a) non-implanted;(b)as-implanted;annealed after implantation at 500 ℃ for (c) 10 s,(d)20 s,(e) 30 s and (f) 45 s (The inset depicts the ratio of LO/TO as a function of annealing time)

The Raman analysis indicates that the RTA process at 500 ℃ for 45 s is successful in recovering the damage done to the lattice by implantation.However,the effects caused by the outdiffusion of Sb atoms for the long time annealing at high temperature on the surface are still unclear.Hence,SEM and AFM analysis on the implanted and annealed samples under above condition were conducted.Figs.4(a) and 4(b)depict the SEM images showing the surface and crosssectional morphology on GaSb sample implanted at 290 KeV carbon ions with the fluence of 3.1×1013cm-2.No visible damage was found on the surface and crosssection after implantation.Surface analysis in Fig.4(c)shows that dense voids with irregular shape scatter on the annealed surface,clearly indicating that annealing with long time at high temperature leads to significant surface degradation through Sb loss on the surface.Fig.4(d) shows the SEM image of the cross-sectional morphology on GaSb sample annealed at 500 ℃ for 45 s,from which the defect-layer with a characteristic of clouds can be observed.The formation of the cloud defects on cross section can be attributed to the accumulation of atomic defects such as Sb vacancies and interstitials created during annealing process.AFM image and the cross-sectional profile in Figs.5(a) and (b)show the three-dimensional structure of the voids and give the results that the voids have an average diameter of about 20 nm and a depth of 2-3 nm.Hence,the residual disorder persists after annealing in the form of voids due to the deviation from stoichiometric ratio.

Fig.4 SEM images for (a) surface and (b) cross-section of the GaSb sample implanted at 290 keV carbon ions with the fluence of 3.1×1013cm-2;SEM images for (c) surface and (d) crosssection of the implanted sample annealed at 500 ℃ for 45 s

The carbon ion fluence dependence of Raman spectroscopy shows the facts that severe damage appear in GaSb lattice with the implanted fluence above 3.1×1013cm-2at 290 keV.This is because the presence of tensile stress in the lattice after implantation,which increases with the implanted dose[19].The effective recovery of the damaged crystal structure can be obtained by annealing the implanted samples at 500 ℃ with a duration of 45 s.Combining with the results above,it can be concluded that the recovery process and the outdisffusion of Sb occur at the same time.Therefore,the recovery of the disordering is incompleted even by the best annealing process.Compared with other III-V compound semiconductors,GaSb has relatively low recovery efficiency mainly due to the outdiffusion of Sb.

4 Conclusions

GaSb samples implanted by carbon ions with a dose from 4.6×1011to 3.1×1013cm-2at 290 keV and subsequent damage recovery by RTA have been studied by Raman spectroscopy.It can be concluded that severe damage is produced in the implanted samples as the fluence reaches 3.1×1013cm-2.Damage recovery at the greatest extent can be obtained by annealing the implanted samples at 500 ℃ for 45 s.However,the outdiffusion of Sb atoms under this annealing condition degraded the surface quality.Therefore,even by the best annealing process the recovery of the disordering is incompleted.

Conflict of interest

All authors declare that there are no competing interests.