Deep levels induced by focused ion implantation in GaAs

Nuclear Instruments and Methods in Physics Research B21 (1987) 151-154 North-Holland, Amsterdam

151

D E E P L E V E L S I N D U C E D BY F O C U S E D I O N I M P L A N T A T I O N

IN G a A s

Y o s h i h i k o Y U B A , T a k a y u k i Y A N O , T o m o h i r o I S H I D A , Kenji G A M O a n d Susumu N A M B A

Faculty of Engineering Science, Osaka Universi(v, Toyonaka, Osaka 560, Japan

Deep level centers associated with defects induced by 100 keV Si ion implantation using focused or flood beam have been investigated by means of DLTS and C-V carrier profiling. It was observed that three different electron traps with an activation energy between 0.25 and 0.5 eV were generated irrespective of ion beam type. No difference in the species of induced defect center was noticed. Induced defects were detected in the region far beyond the ion range in both implantations. It was found that residual defects had a little higher concentration and deeper distribution in the FIB implantation in comparison with the conventional flood beam implantation.

1. Introduction There has been an increasing interest in focused ion beam (FIB) implantation, which adds a unique feasibility of a maskless processing to the well-established various advantages of ion implantation technology. An intrinsic feature of FIB implantation is a high ion current density which results from a unique liquid metal ion source with a high brightness and a fine beam focusing and this may give rise to different implantation- and annealing- characteristics from those usually observed in the conventional flood beam implantation. In FIB implanted Si, it has been observed that amorphous layers are generated at lower dose and recrystallize at lower temperature [1]. On the contrary, it has been reported that in GaAs FIB implantation induces less lattice disorders as compared with the conventional flood beam implantation from the characterization using Raman spectroscopy [2]. Some differences have also been noticed in the activation process and the distribution of FIB implanted impurities in GaAs [3,4]. For example, FIB implanted Si in GaAs has a significantly deeper profile and the electronic excitation during the FIB implantation is considered to enhance the impurity migration [4]. In this paper, we will present a characterization of induced defects with a deep level in FIB implanted GaAs and compare that characterization with that of conventional flood beam implanted GaAs in order to investigate a possible difference in defect behavior due to the high current density effect.

2. Experimentals GaAs (100) oriented wafers used in the present experiments were obtained from an n-type Si-doped HB0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

grown single crystal with a carrier concentration of 1 × 1017//cm3. They were implanted with 100 keV Si ions at room temperature by use of our focused ion beam system [5]. We employed various doses ranged from 1011 to 1013/cm 2. A typical beam diameter was about 1/~m and a beam current density was 1.6 m A / c m 2 in the present experiment. For comparison, some wafers from the same crystal were also implanted with a flood ion beam by use of a conventional ion implanter at the same conditions except for the dose rate or the ion current density. In this implantation, an ion current density was between 50 and 500 n A / c m 2. Annealing was performed in a flowing pure hydrogen atmosphere at various temperatures up to 600 ° C. Deep levels associated with implantation induced defects were characterized by DLTS. A differential C - V method using a bias modulation technique was applied for the measurements of free carrier and defect concentrations. Samples for these measurements were Schottky barriers formed on the implanted surface by vacuum deposition of A1 through a Mo shadow mask. Ohmic contacts on the backside were formed by depositing and alloying A u - G e before implantation.

3. Results and discussion Fig. 1 shows typical DLTS spectra for samples FIB implanted to a dose of 1011//cm 2 before and after annealing. In the as-implanted samples, two peaks labeled L-1 and L-2 were observed clearly at temperatures around 170 and 250 K. In the as-received samples, we detected two different deep electron traps EL2 and EL6 [6] around 180 and 380 K, which are typical grown-in defects in HB grown GaAs. The dominant L-2 center was different from two grown-in defects in the II. CHALLENGES/EMERGING TECHNOLOGIES

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Y. Yuba et a L / Deep levels induced by ion implantation

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Table 1 Summary of trap parameters

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Fig. 1. DLTS spectra of the samples implanted by using 100 keV Si focused ion beams to a dose of 10U/cm2 before and after annealing. Rate window is 72/s.

substrate and therefore, it was considered to be induced by the implantation. The L-1 center appeared at a similar temperature where EL6 was observed. However, the comparison of the spectrum suggested that this center was associated with implantation induced damages. We have previously resolved a defect center, probably the same as L-l, in proton irradiated GaAs and observed that it disappears after annealing around 250 ° C. [71 Another center L-3 was resolved in the FIB implanted sample after annealing at 300 ° C and seemed to be induced by the implantation since the similar center was not observed in the substrate. We considered that this center existed in the as-implanted samples but was masked by a broad tail of the L-1 center since we observed previously in the proton as-implanted sample a similar center which increased with a dose [7]. The same centers L-l, L-2 and L-3 were also detected in the FIB implanted samples with doses of 10t2 and 1013/cm2 although a relative intensity in each spectrum was different. We obtained the characteristic parameters of the observed defects from the temperature dependence of thermal emission rate and the results were summarized

in table 1. The parameters for the L-2 center were tentative since their determination had some ambiguity as described later. Fig. 2 showed DLTS spectra for the samples implanted with a flood Si ion beam to a dose of 1011/cm2 before and after annealing. The spectrum of the asimplanted sample was very similar to that of the FIB implanted sample suggesting the presence of L-1 and L-2 centers. Also after annealing at 300 ° C, we observed L-3 center. It was found from the comparison of the DLTS spectrum and trap parameters that both implantations using FIB and flood beam induced the same defects with deep levels and so no clear evidence that the defect introduction depended on the ion beam type used was obtained. We recognized no significant difference in annealing behavior for the samples implanted by two different methods from the results shown in figs. 1 and 2. The annealing at temperatures up to 300 ° C reduced the L-1 center considerably as observed previously [7] and the L-3 center became evident. After annealing at 500 ° C, the L-3 center disappeared completely in the 1011/cm2 implanted samples but a trace of it was observed in the high dose sample. Also the dominant center L-2 still remained though its signal intensity decreased to lower than that of the grown-in defect EL-2. The peak position of the L-2 center seemed to shift to higher temperature with increasing annealing temperature as can be seen in figs. 1 and 2 and also was observed to depend on the applied bias during the measurement. This center had a broad structure in contrast with a usual deep level observed in GaAs such as EL2 and EL6 and also a theoretical line shape. It is difficult to understand these unusual properties of the L-2 center from the contribution of the single isolated deep center. Therefore, it seems natural that this center is composed of some different deep centers with similar energy levels. We have previously investigated a similar broad center in Ar ion implanted GaAs and suggested presumably that it has two or more different components taking into account its annealing behavior [8]. A similar broad peak labeled U-band has been observed by DLTS in the samples irradiated by neutrons and B ions [9].

Y. Yuba et al. / Deep levels induced b.v ion implantation

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I 400

Fig. 2. DLTS spectra of the samples implanted by using 100 keV Si conventional flood ion beams to a dose of 10n/cm 2 before and after annealing. Rate window is 72/s.

153

been shown that deep levels affect the free carrier concentration determined by the differential C / V technique [12,13]. If a d C / d V is measured at a frequency sufficiently higher than the thermal emission rate of the deep level, we can obtain the carrier concentration corresponding to the shallow level concentration but if not so, an apparent free carrier concentration includes the contribution of the deep level in addition to the shallow levels [13]. Therefore, it is allowed to determine the deep level concentration from the difference of above two C - V carrier profiles. We realized this measurement by changing the sample temperature rather than the modulation frequency for d C / d V determination taking into consideration the temperature dependence of thermal emission rate. Typically we used an AC bias of 100 m V p - p at 300 Hz. Fig. 3 shows deep level concentration profiles of the FIB and flood beam implanted samples with a dose of 10P-/cm 2. These concentrations were determined from the carrier profiles measured at 110 and 350 K and corresponded to the total concentration of three deep centers L-l, L-2 and L-3 detected by DLTS. We obtained nearly identical carrier profiles in the flood beam implanted sample after annealing at 500 ° C from the measurements at two temperatures and the estimated defect concentration was less than 5 × 1015/cm3. It was clear that the defects with an appreciable concentration

C O N V E N T I O N A L ION B E A M • ANNEALED 30~C

Initially this has been considered to be associated with a band of defects but recently it has been proposed that EL2 center surrounded by other shallower electron traps such as EL6 is responsible for the U-band and that the continuous change of emission kinetics from EL2 due to a phonon assisted tunneling can give rise to a broad DLTS structure [10]. Some experimental evidences supporting this model have been reported [11]. In any case the identification of this dominant defect center still remains to be clarified. Other two deep levels detected did not show such an unusual behavior though a minor shift of the peak position probably due to the influence of the L-2 center was observed. It was expected that the defect distribution could give a cue to gain insight into the possible difference in induced defects depending on beam current density. However, because of the problems described above, an estimation of deep level concentration from a standard DLTS analysis seemed to have some uncertainty. Therefore, for the concentration measurement of defect with deep level in the present study, we used a differential C - V carrier profiling using a bias modulation technique where the differential coefficient of C - V curve i.e. d C / d V was determined at a fixed frequency. It has

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_o

f
3

2

1

0

I 5000

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i

DISTANCE FROM SURFACE

I 7000 (A)

Fig. 3. Depth distribution of defects in 100 keV Si ion implanted GaAs measured by the differential C-V profiling using a bias modulation technique. Si ion dose was 1012/cm2. Annealing was performed for 10 rain at each temperature. II. CHALLENGES/EMERGING TECHNOLOGIES

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Y. Yuba et al. / Deep levels induced by ion implantation

were distributed over the region fairly deeper than the LSS projected range (850 ,~) in both" samples and this was consistent with a general observation of the enhanced diffusion of defects in ion implanted GaAs. It was found that defects in FIB implanted samples had a higher concentration and a deeper profile in comparison with those in the flood beam implanted samples. This seemed not to be the case for the sample annealed at 300 o C but since the carriers were heavily compensated in the surface side of the measured region, residual defects were expected to be higher in the FIB implanted sample. The same conclusion was suggested from the investigation of the C-I/characteristics for these samples. Our results seemed to differ from the defect characterization in FIB implanted GaAs reported previously [2] but the reason is not clear at present. The deep level defects changed their profiles probably due to the different thermal stability of the three centers and showed a gradual decrease with increasing annealing temperature. After annealing at 500°C, defects with a concentration of about 5 × 101S/cmBor less in the flood beam implanted samples and around 1 × 1016/cm3 in FIB implanted samples still remained. We observed that the free cartier concentration could not recover to that of the as-received sample, suggesting the presence of the large amount of compensation centers with a level below the mid gaps not covered by the present characterization.

4. Conclusions We have investigated deep levels associated with defects induced by FIB and flood beam implantations by means of DLTS and C- V carrier profiling. It is found that the species of induced defects are the same in both implantations. But the FIB implantation seems to induce more defects with somewhat deeper distributions than the flood beam implantation. We

consider that as to induced defect there are no serious problems inherent to FIB implantation in its application to the usual field of ion implantation. The authors would like to thank H. Yamazaki of N T T Atsugi Electrical Communication Laboratories for the Si implantation using a conventional implanter. They also thank M. Urai, D. Takehara and K. Kawasaki for their help in the FIB implantation.

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