Experiments and simulation on diffusion and

JOURNAL OF APPLIED PHYSICS 108, 024903 共2010兲

Experiments and simulation on diffusion and activation of codoped with arsenic and phosphorous germanium P. Tsouroutas,1 D. Tsoukalas,1,2,a兲 and H. Bracht3 1

Department of Applied Physics, School of Applied Sciences, National Technical University of Athens, 15780 Zographou, Greece 2 Institute of Microelectronics, National Center for Scientific Research “Demokritos,” 15310 Aghia Paraskevi, Greece 3 Institute of Materials Physics, University of Muenster, Wilhelm-Klemm-Strasse 10, D-48149 Muenster, Germany

共Received 24 February 2010; accepted 26 May 2010; published online 27 July 2010兲 We report arsenic and phosphorus diffusion experiments and activation related phenomena in codoped germanium substrates utilizing conventional thermal annealing. Chemical profiles were obtained by secondary ion mass spectroscopy, sheet resistance was estimated by the Van der Pauw method. Our study covers the temperature range from 600 to 750 ° C. We accurately described the dopant profiles with a quadratic dependence of the dopants diffusion coefficient on the free electron concentration. In our simulations we considered the dopant pile-up near the surface and dopant loss owing to outdiffusion during the annealing. Although the double donor codoping technique exhibited no advantage over monodoping with P concerning the level of activation and junction depth, it was interesting to observe the different diffusion behavior of the two dopants. Whereas the diffusion of As indicates a retardation under codoping the diffusion of P remains either unaffected or is slightly enhanced by codoping. The activation level of the codoped samples remains lower compared to the respective monodoped samples, except for the highest annealing temperature. © 2010 American Institute of Physics. 关doi:10.1063/1.3456998兴 I. INTRODUCTION

New materials as well as innovative device design are currently examined as we reach the theoretical limit for downscaling complementary metal-oxide semiconductor technology 共CMOS兲. In this context germanium is a promising candidate as a future semiconductor material offering attractive physical properties compared to silicon such as higher carrier mobility.1–4 Additionally the Ge smaller band gap is more compliant with the operating voltage scaling as specified by the international technology roadmap for semiconductors while the lower melting point makes feasible the fabrication of Ge based devices with clearly lower thermal budget processes. This is compatible with the thermal stability requirements in integrating novel high-k dielectrics and metal gate electrodes into advanced transistors. However, critical issues concerning diffusion and activation of dopants that will determine the feasibility of germanium as the next generation semiconductor material need to be addressed. Recent reports for the p-type dopant, that is mostly boron, have shown the realization of p-type metal-oxide semiconductor 共PMOS兲 devices exhibiting both well-behaved p+n shallow junction and high activation,5–8 due to low diffusivity 共and in some cases even without annealing9兲 and high solid solubility limit. On the other hand, for the respective n-type metaloxide semiconductor 共NMOS兲 devices implementing the most common n-type dopants such as P, As, and Sb these requirements have not yet been met satisfactorily. Although modern annealing methods such as laser and flash annealing a兲

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show promising results in dopant diffusion and activation10,11 more work need to be done for the realization of competent NMOS devices. For the case of phosphorus and arsenic, possible reasons for these poor-quality n+p junctions are the high P and even higher As diffusivity,6,12,13 the difference between chemical solubility limit 关estimated approximately for P at 2 ⫻ 1020 cm−3 and for As 8 ⫻ 1019 cm−3 at 600 ° C 共Refs. 14 and 15兲兴 and the electrical activation 关reported for P at 5 – 6 ⫻ 1019 cm−3 and for As 1 – 3 ⫻ 1019 cm−3 共Refs. 6, 14, 16, and 17兲兴 as well as the severe dopant loss observed during annealing especially for the case of P.6,12,14,16,18–21 The bigger atomic size of Sb creates an irreversible damage to the Ge crystal during ion implantation14,20 and thus it is avoided as a donor dopant. It is reported in the literature that the diffusion of donor dopants in Ge is mediated through the formation of complexes with vacancies 共V兲.12,13,22–24 Also in recent works it is predicted by density functional theory 共DFT兲 calculations that donor atoms can either form complexes with vacancies 共As2V, P2V, As4V, etc.兲25 or, as it was concluded from the analysis26 of experimental profiles of isotopically controlled multilayer structures doped with carbon27 form clusters which might lead to a deactivation of the dopants similar to what happens to silicon.28–30 Therefore an engineering of the vacancy concentration seems to be needed in an attempt to both limit the diffusion depth and increase the electrically active concentration of the donor dopants. In a recent report electronic structure calculations predict that in a double donor 共P and As兲 coimplant, As atoms can suppress the concentration of the free vacancies by forming complexes AsnVm due to a higher binding energy between As and V compared

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© 2010 American Institute of Physics

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to the respective binding energy between P and V.31 The lower V concentration may suggest a limited diffusion depth as well as less deactivated P atoms. The present work investigates experimentally the diffusion and electrical activation of double donor doped 共P and As兲 Ge substrates. In order to better understand the influence of the codoping on the dopant diffusion behavior we have also conducted monodoping experiments with P and As of similar implanted doses and annealing conditions. The main part of the paper is then devoted to the estimation of P and As diffusivity through simulations of experimental profiles under both monodoping and codoping conditions. For that purpose, and similar to our previous report,32 we have made use of the commercial simulator TSUPREM IV from SYNOPSYS taking into account not only the Fermi level dependence of the diffusion coefficient but also the dopant loss phenomenon and the dopant pile-up observed during diffusion in the near surface area. The estimation of the dopants’ electrical activation was done by means of four point probe Van der Pauw measurements. II. EXPERIMENTAL DETAILS

In our study substrate wafers of Ga-doped p-type Ge of 共100兲 orientation with a resistivity ⬎20 ⍀ cm were used. We have designed two codoping procedures that we call concurrent and successive codoping, respectively. In addition we have taken the opportunity to study systematically the diffusion of P and As when they diffuse alone 共monodoping兲. Concurrent codoping. At the concurrent codoping condition P and As atoms were implanted at energies 30 keV and 65 keV, respectively, with the same dose 5 ⫻ 1014 cm−2. In this way the peak concentration of the dopants is approximately at the same depth from the surface. Thus, the influence of the surface is expected to be similar during annealing. The implantation was performed at room temperature with no special surface preparation or cleaning in advance. In order to minimize the channeling effect the ion beams were tilted by 7° to the substrate surface normal during implantation. After implantation the wafer was dipped into 10% HF in order to obtain a clean surface. Subsequently, a 60 nm thick silicon nitride 共Si3N4兲 capping layer was deposited on the surface of the wafer by sputtering at 200 ° C. After the nitride deposition diffusion anneals in a conventional resistance furnace were performed at four temperatures 600, 650, 700, and 750 ° C for 30 min in nitrogen ambient. Successive codoping. During the successive codoping we first implanted two Ge wafers with As at an energy of 65 keV with two different doses 1 ⫻ 1015 and 2 ⫻ 1015 cm−2. After the deposition of silicon nitride under the same conditions as above these two Ge wafers were preannealed at 650 ° C for 15 min. After removal of the nitride both wafers were implanted with P at 30 keV with the dose 1 ⫻ 1015 cm−2. Subsequently, a silicon nitride layer was again deposited under the above mentioned conditions and conventional furnace anneals at four temperatures 600, 650, 700, and 750 ° C for 30 min in nitrogen ambient were performed. The implantation was carried out at room temperature with no special surface preparation or cleaning in advance. To

minimize channeling effects the ion beam was tilted by 7° to the substrate surface normal during implantation. Prior to the deposition of silicon nitride the two wafers were dipped into 10% HF to obtain a clean surface. Monodoping. For comparison we also performed monodoping experiments. The sample preparation was similar to the preparation of the codoping samples except that we used four doses for dopant implantation. The doses for both dopants were 5 ⫻ 1014, 1 ⫻ 1015, 2 ⫻ 1015, and 3 ⫻ 1015 cm−2 at 30 keV and 65 keV for P and As, respectively. The same conditions as above were chosen for the nitride deposition and the furnace annealing.

III. RESULTS AND DISCUSSION A. Methodology for the simulation and the activation

To simulate the experimental profiles the technology computer-aided design 共TCAD兲 simulator TSUPREM IV from SYNOPSYS was used. Our aim during simulation was to capture not only the dopant loss phenomenon but also the dopant pile-up occurring very close to the surface. For this reason the pile-up model proposed by Normand et al.,33 that assumes the existence of dopant traps near the surface, has been incorporated. Based on the McNabb and Foster model it is considered that the traps are capable of capturing and releasing dopant atoms assuming local equilibrium. The dose loss phenomenon was simulated by depositing a virtual oxide onto the germanium substrate and by controlling the segregation coefficient between them. In this way we succeeded in confining the surplus dose into the oxide. Then by stripping the oxide we could simulate the dopant dose that has remained in the substrate. The maximum electrical activation was set at 6 ⫻ 1019 cm−3 and 1 ⫻ 1019 cm−3 for P and As, respectively, for the entire temperature range. These values were estimated from the monodoping case and are in good agreement with literature. We considered full activation for both dopants below the aforementioned threshold and ignored any possible formation of complexes with vacancies or clustering. The two diffusion’s fluxes incorporated into the simulator were34 JAs = − hAs · DAs · − 1兲 · DAs ·

JP = − hP · DP · − 1兲 · DP ·

⳵ CAs − 共hAs ⳵x

⳵ CP ⳵x

共1兲

for Arsenic,

⳵ CP − 共hP ⳵x ⳵ CAs ⳵x

for Phosphorus,

共2兲

where the terms hAs and hP describe the diffusion enhancement due to the electric field hAs = 1 +

CAs 2ni

冋冉

CAs + CP 2ni

冊 册 2

+1

−1/2

for Arsenic, 共3兲

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hP = 1 +

CP 2ni

冋冉

CP + CAs 2ni

冊 册 2

−1/2

+1

for Phosphorus, 共4兲

DP and DAs are the extrinsic diffusion coefficients for phosphorus and arsenic, respectively, that can be described by the following equation: D = Do + D− ·

冉冊

冉冊

n n + D−− · ni ni

2

,

共5兲

where n represents the free electron concentration, ni is the intrinsic carrier concentration, Do, D−, and D−− are the intrinsic diffusivities of donors due to positively, neutral, and negatively charged donor-vacancy pairs, respectively, and CP and CAs is the dopant concentration of phosphorus and arsenic, respectively. In accordance with previous reports6,12,13 the vacancy mechanism was considered to mediate donor dopant diffusion in Ge. In particular, we assumed singly negatively charged mobile donor-vacancy pairs and doubly negatively charged vacancies. The charge difference of two between the mobile dopant-vacancy pair and the substitutional donor predicts a donor diffusion coefficient for extrinsic doping that depends on the square of the free electron concentration n 共see, e.g., Ref. 35兲. This vacancy mechanism provides satisfactory fits to the diffused profiles when implementing only the D−− term. For this reason the neutral 共Do兲 and singly charged 共D−兲 terms were set to zero. For more details concerning the method used to simulate the dopant profiles we refer to our previous report.32 Contributions to the donor dopant diffusion coefficient associated with positively charged and neutral dopant-vacancy pairs were not taken into account. These contributions, respectively, would predict donor dopant diffusion coefficients that are independent of n and linear dependent on n.35 In order to estimate the electrical activation we performed sheet resistance measurements using the four point Van der Pauw method. We also estimated the theoretical minimum sheet resistance 共Rs兲, considering the whole chemical profile electrically active, through the following equation: Rs =

1 兰x0jq · ␮共n兲 · n共x兲 · dx

,

共6兲

where q is the elementary electron charge. The mobility ␮共n兲 as function of electron concentration was calculated using the model of Fistul36

␮n ⬇ 1.52 ⫻ 1010 · n−0.4 .

共7兲

The distribution n共x兲 of the free electron concentration is approximated by the dopant profiles measured with secondary ion mass spectroscopy 共SIMS兲. From the comparison of the measured and the minimum theoretical sheet resistance we determined the level of the donor activation.

FIG. 1. 共Color online兲 Intrinsic diffusion coefficients of phosphorus 共a兲 and arsenic 共b兲 in germanium determined from simulations of the experimental donor profiles obtained under monodoping conditions 共symbols兲. The solid lines represent extrapolations of the diffusion coefficients reported in the literature, Refs. 6, 12, and 13, that are shown for comparison.

B. Diffusion

Monodoping. Before we present our results for the codoping diffusion experiments we first describe the diffusion behavior of the dopants under monodoping conditions. In Fig. 1 we show the dopant diffusivities for intrinsic conditions that were determined from the simulations of the donor diffusion profiles obtained after annealing. Since the simulations were very sensitive to the value of the intrinsic diffusivity the respective error was largely function of SIMS accuracy and in any case small enough to be shown on logarithmic scale. Figure 1 demonstrates that the diffusion of P under monodoping conditions is independent of the implantation dose. All values are consistent within the experimental accuracy. This is not the case for As, especially at the lowest temperature 共600 ° C兲. We observe a gradual reduction in the diffusivity as the implanted As dose increases. However, as the annealing temperature increases the diffusivity shows similar values independently of the implanted dose. The formation of clustering of As atoms with vacancies forming immobile and electrically inactive clusters 共presumably As4V兲 at high concentrations can influence the number of available vacancies for As diffusion. This appears in agreement with the literature37 where it is reported that when the arsenic concen-

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FIG. 2. 共Color online兲 Diffusion profiles of 共a兲 P 共symbols兲 and 共b兲 As measured with SIMS after 30 min for all annealing temperatures. The solid lines are simulations of P and As diffusion for the concurrent codoping case.

tration is above 1 ⫻ 1020 cm−3 clustering occurs due to the lower solid solubility of As in germanium compared to phosphorus. The clustering process influences more the higher implanted doses, thus the reduction in the diffusivity is more prominent in that case. These phenomena seem to fade out as the annealing temperature increases. A plausible explanation is that at elevated temperatures the arsenic clusters tend to dissolve. For the P case similar clustering of P atoms with vacancies 共presumably P4V too兲 is happening to a smaller extent due to the higher solid solubility in Ge compared to As. These P clusters seem to dissolve completely even at low annealing temperatures.37 The reduction in As diffusivity with increasing implantation dose could be also due to Ge interstitial injection from extended defects formed during high dose implantation. In order to confirm this argument microstructural investigations by means of transmission electron microscopy would be necessary. The solid lines present results from the literature for the temperature range investigated for comparison. For a detailed discussion about the divergence of the intrinsic diffusivities’ values we refer to our previous report.32 In the following we describe the diffusion of P and As for the concurrent and successive codoping conditions. We start with the concurrent case. Concurrent codoping. Figure 2共a兲 illustrates diffusion profiles of P measured with SIMS after annealing at temperatures between 600 and 750 ° C. The solid lines show the simulations of P diffusion under the concurrent codoping conditions. Figure 2共b兲 shows the respective experimental diffusion profiles of As together with the corresponding simulation. The intrinsic diffusion coefficients of P and As extracted from the simulation of the dopant diffusion under the con-

FIG. 3. 共Color online兲 Temperature dependence of the intrinsic diffusion coefficient of phosphorus 共a兲 and arsenic 共b兲 under concurrent codoping conditions 共filled circles兲 in comparison to data for monodoping conditions 共filled squares兲 that represent the same total implantation dose of 1 ⫻ 1015 at/ cm2. The solid lines indicate data that are expected from the results reported in the literature 关Chui et al. 共Ref. 6兲, Carroll and Koudelka 共Ref. 12兲, and Brotzmann and Bracht 共Ref. 13兲兴.

current codoping conditions are illustrated in Fig. 3. For comparison we also show the intrinsic diffusivities from the monodoping case and the expected diffusion coefficients of P and As according to the results of Chui et al.,6 Carroll and Koudelka,12 and Brotzmann and Bracht.13 It is noted that the total dose of the displayed data for the monodoping case equals the total dose for the concurrent codoping 共i.e., 1 ⫻ 1015 at/ cm2兲. Whenever we compare the diffusivities for both dopants for monodoping and codoping conditions we will match the initial doses for better comparison. The comparison of the P diffusion data for the concurrent codoping and monodoping case 关Fig. 3共a兲兴 shows that the P diffusion coefficient is not affected by the presence of As. On the other hand the presence of P atoms clearly affects the diffusion of As 关Fig. 3共b兲兴. The reduction in the As diffusion coefficient for codoping compared to monodoping amounts to 40% to 60% for the temperature range investigated. The solid lines present literature data at the temperature range used and serve only for comparison. Successive codoping. In this section we present our results for the second type of codoping experiments. First the

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FIG. 4. 共Color online兲 SIMS concentration profiles of 共a兲 P and 共b兲 As 共symbol兲 obtained after diffusion annealing for 30 min for all annealing temperatures for the lower initial dose of As 共1 ⫻ 1015 cm−2兲. The solid lines are best simulations of diffusion under successive codoping conditions.

results are summarized that refer to the lower initial dose of As 共1 ⫻ 1015 at/ cm2兲. Figure 4共a兲 illustrates SIMS concentration profiles of P that were obtained after diffusion annealing for successive codoping. The solid lines are the corresponding best results of the diffusion simulation. Figure 4共b兲 shows the respective SIMS concentration profiles of As and the corresponding best simulations. The intrinsic diffusion coefficients of phosphorus and arsenic obtained from the simulation of the experimental profiles for the successive codoping case are summarized in Fig. 5. In contrast to the concurrent codoping case, the diffusion of P seems to be affected by the presence of As atoms under successive codoping conditions. For the lowest annealing temperature the diffusivity is similar to the monodoping case. But with increasing annealing temperature the diffusion coefficient of P exhibits a clear enhancement ranging from 35% to 55% compared to the monodoping case with initial dose 2 ⫻ 1015 at/ cm2. The diffusion of As under successive codoping also behaves different compared to the concurrent codoping case. For the lowest annealing temperature the diffusion of As is enhanced by approximately 200% compared to the respective monodoping case. At 700 ° C the two diffusivities are similar and for the highest temperature the diffusion coefficient for successive codoping is approximately 45% lower than the value for the monodoping case. We attribute the enhancement of As diffusion at the lower temperatures to the absence of As clusters. The low initial As implanted dose 共1 ⫻ 1015 at/ cm2兲 and the subsequent preannealing has reduced the concentration of As well below the value 1

FIG. 5. 共Color online兲 Temperature dependence of the intrinsic diffusion coefficient of phosphorus 关共a兲, filled circles兴 and arsenic 关共b兲, filled circles兴 for successive codoping conditions as extracted from the simulations of the experimental profiles shown in Figs. 5 and 6. The intrinsic diffusion coefficients for monodoping conditions and an equal dose of 2 ⫻ 1015 at/ cm2 共filled squares兲 are shown for comparison as well as data for P and As diffusion in our temperature range that are expected according to the results of Chui et al. 共Ref. 6兲, Carroll et al. 共Ref. 12兲, and Brotzmann et al. 共Ref. 13兲.

⫻ 1020 at/ cm2, where the clustering seems to occur. On the contrary, the initial dose of 2 ⫻ 1015 at/ cm2 for the monodoping case is high enough to allow As clustering and thus retard the dopant diffusion. With the exception of the As diffusion enhancement at low temperatures the diffusion behavior of As under successive codoping conditions is similar to the concurrent codoping case, i.e., As diffusion is retarded in the presence of P. Next we present the results that refer to the higher initial dose of As 共2 ⫻ 1015 at/ cm2兲. Figure 6共a兲 shows the corresponding SIMS concentration profiles of P after diffusion annealing for 30 min at the various temperatures for the successive codoping case. The corresponding concentration profiles of As with the best simulations are displayed in Fig. 6共b兲. The intrinsic diffusion coefficients determined from the simulations of the dopant profiles shown in Figs. 6共a兲 and 6共b兲 for successive codoping are illustrated in Fig. 7. The comparison of Figs. 5 and 7 reveals the same trends in the diffusion behavior of P and As for successive codoping

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FIG. 6. 共Color online兲 Concentration profiles of 共a兲 P and 共b兲 As 共symbols兲 measured with SIMS after diffusion annealing for 30 min for all annealing temperatures under successive codoping for the higher initial dose of As 共2 ⫻ 1015 cm−2兲. The solid lines are best simulations of the P and As diffusion behavior.

with both lower and higher initial As dose. The diffusion of P for the higher initial As dose is again enhanced at the two higher temperatures compared to the monodoping case but this time to a smaller extent compared to the case of successive codoping with a lower initial As dose. More specific there is an enhancement by 35% for the temperature of 700 ° C and 17% for the 750 ° C compared to the monodoping case and an initial dose of 3 ⫻ 1015 at/ cm2. On the other hand As exhibits again a higher diffusivity under successive codoping at the lowest temperature that exceeds the value for the respective monodoping by 72%. However, the enhancement is smaller 共compared with 200% in the lower dose兲 probably due to some clustering that might have occurred since the As concentration in this case is higher due to higher implanted dose. With increasing annealing temperature the diffusion of As shows a gradual reduction reaching a value for 650 ° C that is similar to the monodoping case and finally is smaller by 40% compared to the monodoping case for the two higher temperatures. Overall the diffusion behavior of P and As under the two conditions of successive codoping are very similar. Phosphorus diffusion exhibits a small enhancement as the annealing temperature increases while As diffusion exhibits a retardation at higher temperatures. The different diffusion behavior of As at 600 ° C can be explained with a more pronounced clustering in case of monodoping conditions compared to the codoping case. In comparison to concurrent codoping the diffusion behavior of As under successive codoping is similar only for higher temperatures while the behavior of P diffusion under successive codoping seems only to be slightly different to that for concurrent codoping.

FIG. 7. 共Color online兲 Temperature dependence of the intrinsic diffusion coefficient of P 关共a兲, filled circles兴 and As 关共b兲, filled circles兴 obtained for successive codoping. Diffusion coefficients determined for monodoping with an equal total dose of 3 ⫻ 1015 at/ cm2 are shown for comparison 共filled squares兲. Solid lines indicate literature data of Chui et al. 共Ref. 6兲, Carroll et al. 共Ref. 12兲, and Brotzmann et al. 共Ref. 13兲, for the intrinsic diffusion coefficient of P 共a兲 and As 共b兲 that are expected for our temperature range.

These findings may imply that PV pairs are bonded tighter than AsV pairs. However, a recent report from Chroneos et al.31 predicts exactly the opposite via ab initio calculation. In addition, it is predicted that at low temperatures 共below 400 ° C兲 the diffusion of P should be retarded. Our lowest annealing temperature was 600 ° C so we could not validate the prediction. Nevertheless at higher temperatures we observed the opposite trend. Taking into account the small difference in the binding energies of the two pairs, which is of the order of 0.1 eV, and the accuracy of the experimental diffusion study it remains difficult to verify a clear impact of As doping on P diffusion. C. Electrical activation

Monodoping. The four point Van der Pauw method was employed to measure the sheet resistance of the monodoped samples. The maximum activation level of P and As monodoped samples was estimated by defining a concentration threshold on SIMS profiles assuming that below this threshold all dopants are electrically active. By matching the calculated sheet resistance from the levelled SIMS profiles with the respective experimental value we found maximum acti-

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FIG. 8. 共Color online兲 Temperature dependence of the sheet resistance measured by means of the four point Van der Pauw method from the concurrent codoped samples. The extracted sheet resistance from levelled SIMS profiles, assuming a fixed maximum electrical activation of 6 ⫻ 1019 cm−3 for P and 1 ⫻ 1019 cm−3 for As dopant, is shown for comparison.

vation levels comparing well with literature reported data. The P maximum electrical concentration used was 6 ⫻ 1019 cm−3 and for As was 1 ⫻ 1019 cm−3. Concurrent codoping. The four point Van der Pauw method was used to measure the sheet resistance of the codoped samples. For both codoped cases 共concurrent and successive兲 we have considered a fixed maximum electrical activation assuming that the above estimated values for the monodoping case are valid for both codoping cases. Figure 8 compares the calculated values of sheet resistance with the experimental ones. Figure 8 shows that the aforementioned maximum electrical activation for P and As monodoped cases is not increased by the concurrent codoping method for the temperature range investigated. The same figure demonstrates that P and As in the codoped samples are in fact activated to a lower level at low annealing temperatures reaching the maximum electrical activation above 700°. In summary it seems that the concurrent codoping is not more advantageous compared to monodoping. We need to anneal the codoped samples at high temperatures to reach the activation level of the monodoped samples. Successive codoping. Following the same procedure as in concurrent codoping case we calculated the sheet resistance values from the SIMS profiles imposing again the fixed maximum electrical activation estimated from the monodoping case. Figure 9 illustrates these calculated in comparison with the experimentally determined sheet resistance values for both initial As doses 1 ⫻ 1015 cm−2 关Fig. 9共a兲兴 and 2 ⫻ 1015 cm−2 关Fig. 9共b兲兴. For comparison we have maintained the same scaling in both Figs. 8 and 9. Figures 9共a兲 and 9共b兲 show that the estimated maximum electrical activation from P and As monodoped samples is still not increased by the use of successive codoping for both initial As doses within the temperature range investigated. More particularly Fig. 9共a兲 demonstrates that the codoped samples with the lower initial As dose 共1 ⫻ 1015 cm−2兲 are in fact activated to a lower level for the low annealing temperatures reaching the maximum electrical activation above 700°. However, at the elevated initial As dose 共2 ⫻ 1015 cm−2兲 关Fig. 9共b兲兴 the electrical activation of

FIG. 9. 共Color online兲 Temperature dependence of the sheet resistance measured by means of the four point Van der Pauw method from the successive codoped samples for initial implanted As doses of 共a兲 1 ⫻ 1015 cm−2 and 共b兲 2 ⫻ 1015 cm−2. The extracted sheet resistance from levelled SIMS profiles, assuming a fixed maximum electrical activation of 6 ⫻ 1019 cm−3 for P and 1 ⫻ 1019 cm−3 for As dopant, is shown for comparison.

the dopants seems to reach the maximum level even at low annealing temperatures. Despite this high electrical activation at lower temperatures, the maximum estimated activation level of monodoping is still not reached even for the highest annealing temperature. The activation level seems to saturate around the maximum active concentration reported for P and As. Accordingly during concurrent and successive codoping we observe almost the same trend in the temperature dependence of the activation level. Both codoped conditions do not enhance the level of donor activation compared to monodoping conditions. IV. CONCLUSIONS

In summary the results from the codoping experiments did not seem to have an advantage over the P monodoping case in terms of shallower junction depths and electrical activation. Nevertheless, the different behavior observed for P and As diffusion under codoping conditions is interesting especially since it comes to an opposite conclusion on the stability of As-vacancy and P-vacancy pairs than recent theoretical calculations.31 However, the predicted differences in the binding energy of the dopant-defect pairs are quite small and consequently cannot be conclusive. Our work clearly shows that the availability of vacancies in Ge cannot be affected by a codoping strategy of P and As in a way to simultaneously suppress donor dopant diffusion effectively and increase the level of dopant activation efficiently. Another

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strategy is to suppress the formation of the donor-vacancy pairs, which are the mobile entities mediating donor diffusion in Ge, by the formation of Ge interstitials as was recently shown by generation of interstitials through Ge irradiation38 leading to retardation of P diffusion. However, it still remains unsolved in that case whether also the level of activation increases. ACKNOWLEDGMENTS

Two of the authors 共P.T., D.T.兲 would like to acknowledge financial support from Greek GSRT and ST Microelectronics through the PENED program and also Dr. Mario Barozzi from FBK Trento for SIMS measurements through ANNA project. H.B. acknowledges financial support from the Deutsche Forschungsgemeinschaft under Contract No. BR 1520/6-2. We also thank Dr. A. Chroneos for stimulating discussions. 1

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