Thin Solid Films 520 (2012) 3255–3258

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Effects of dose on activation characteristics of P in Ge

Mohammad Anisuzzaman ⁎, Taizoh SadohDepartment of Electronics, Kyushu University, 744 Motooka, Fukuoka 819–0395, Japan

⁎ Corresponding author. Tel.: +81 92 802 3737; fax:E-mail address: m_anisu@nano.ed.kyushu-u.ac.jp (M

0040-6090/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.tsf.2011.10.076

a b s t r a c t

a r t i c l e i n f o

Available online 29 October 2011

Keywords:GermaniumIon-implantationDopant activationSolid-phase epitaxial regrowth

Ion-implantation characteristics and dopant activation behavior of P in Ge have been investigated. A MonteCarlo simulation indicates a smaller projected range and consequently a smaller critical dose of amorphiza-tion for Ge compared to Si. The solid-phase epitaxial (SPE) regrowth characteristics of damaged layers forGe clearly depend on crystal orientation of the substrate in completely amorphized samples, while no orien-tation dependent regrowth is observed in the partially amorphized samples. These phenomena wereexplained on the basis of the damage cluster model. In addition, maximum carrier activation coincideswith the complete regrowth at annealing temperatures of 300–400 °C in completely amorphized samples.However, higher temperature annealing (500–550 °C) is necessary for maximum carrier activation in partial-ly amorphized samples, although SPE regrowth completes around 250–300 °C. Analysis of the temperaturedependence of carrier activation ratio in partially amorphized samples suggests that carrier-activation shouldbe mediated by vacancy-migration.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

In order to improve the processing power and packing density oflarge scale integrated circuits (LSI), Si transistors are being madeever smaller and innovative designs are being sought after all thetime. Nevertheless, it is inevitable that Si LSI technology will face itsscaling limit in the near future. To continue the development beyondthe scaling limit of Si LSI, it is necessary to develop the processingtechnology for alternative functional materials. Ge is considered as apotential material for the purpose mainly because it possesses muchhigher carrier mobility than Si [1,2]. It also gives the possibility of in-tegrating optoelectronic functionality on a single chip. Ge shows highabsorption coefficient in the infrared region and can potentially beused for fabricating photodetectors necessary for optical communica-tion [3]. In addition, the possible formation of a direct-transition bandstructure has been reported in tensile-strained Ge [4], which favorsthe application of Ge to light emitting diode. However, as yet, chal-lenges remain in every step of the processing technology.

In recent years, high quality single-crystalline Ge layers on insula-tor (GOI) have been achieved by the SiGe-mixing triggered rapidmelting growth technique [5–9]. The GOI structure reduces parasiticcapacitance and leakage current, which can significantly enhance Getransistor performance. One very important aspect for integratingsuch Ge transistors is the precise control of doping in the activelayers. For the purpose of doping into Ge, ion-implantation [10] andgas-phase doping techniques [11] are under intensive investigation.

+81 92 802 3724.. Anisuzzaman).

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Morii et al. achieved high concentration doping by a gas-phase tech-nique [11]. This technique is very useful to obtain source and drain re-gions with low parasitic resistance [11]. On the other hand, for precisetuning of the threshold voltage of transistors, the ion-implantationtechnique is necessary as it possesses greater controllability. Theregrowth of damaged regions introduced by dopant ion-implantationand the activation of dopants have been investigated by some researchgroups [12–15]. Most of these research groups concentrated their inter-est in achieving high dopant activation at a lower thermal budget usingrapid-thermal annealing (RTA). However, as the results are in the non-equilibrium, it is difficult to discuss the physical phenomena of carrieractivation based on RTA results. In addition, since vacancy-related de-fects induce shallow acceptor levels in Ge [16,17], recovery of the de-fects should significantly affect the doping efficiency. However, such aphenomenon has not been examined in the previous reports. Clarifica-tion of the effects of recovery of vacancy-related defects is important toestablish the doping technique using ion-implantation.

In this study, we have investigated the P-ion-implantationcharacteristics of Ge compared to Si and the physical phenomenainvolved in the activation of dopants in Ge, where dopant activationwas performed by furnace annealing.

2. Experimental procedure

We employed (100) and (111) oriented In-doped p-typeCzochralski-grown Ge wafers (thickness: 0.35 mm) with resistivityof 20 Ω cm and 1.7 Ω cm, respectively. A standard cleaning procedurewas performed before ion-implantation. Implantation of P+ ions wasperformed at an incident energy of 100 keV with different doses(5×1013, 1×1014, and 1×1015 cm−2), where the dose rate was

1014 10150

50

100

Si

Ge

Implantation Dose (cm-2)

Ge (111)Ge (100)

Cry

stal

linity

Fig. 2. Relation between crystallinity and implantation dose in as-implanted Ge and Si for100 keV P-ion-implantation (solid and dashed lines, respectively) and experimentallyobtained crystallinity in Ge (100) and (111).

3256 M. Anisuzzaman, T. Sadoh / Thin Solid Films 520 (2012) 3255–3258

1.6×1011 cm–2 s–1, and the surfaces of the samples were tilted at anangle of 7° from the incident direction of the ion beams to reducechanneling effects. All implantations were performed at room tem-perature. The dose values were selected to produce both partiallyand completely amorphized layers, while keeping the peak P concen-tration below the maximum chemical solubility (2×1020 cm–3) of Patoms in Ge, based on the result of a Monte Carlo simulation usingSRIM code [18] as explained later. Subsequently, the samples wereisochronally annealed at temperatures from 250 °C to 650 °C for30 min in N2 ambient. No capping layer was used during annealing.

Crystallinity of the implanted layers was evaluated by spectro-scopic ellipsometry by comparing the extinction coefficient spectraof the samples to those of a single-crystalline Ge substrate and afully amorphous Ge layer [19]. The crystallinity of 100% correspondsto single crystal Ge without damage, and that of 0% corresponds tocompletely amorphous Ge. The sheet resistance of the implantedlayers was evaluated by a four point probe technique.

3. Results and discussion

The concentration profile of P atoms implanted into a Ge substrateat an incident energy of 100 keV and with dose of 1×1015 cm–2 wasevaluated by a Monte Carlo simulation carried out using the SRIMcode [18]. The result is shown in Fig. 1, together with that into a Sisubstrate. The peak concentration of P atoms is 8×1019 cm–3 ,which is below the maximum chemical solubility (2×1020 cm–3)[13,14] of P atoms in Ge but slightly above the maximum electricalsolubility (7×1019 cm–3) [20]. The results also indicate a shorter pro-jected range (Rp) of P in Ge (95 nm) compared to that in Si (137 nm).Fig. 2 illustrates the relationship between implantation dose and theachieved crystallinity for Ge and Si substrates implanted with100 keV P-ions, also obtained from the Monte Carlo simulation. It isobserved that the achieved level of damage is greater in Gecompared to Si at any dosage. The critical dose for completeamorphization of Ge is indicated to be about 2×1014 cm–2 which islower than the critical dose for Si. The reason for the smaller criticaldose in Ge can be attributed to the smaller projected range of P inGe compared to Si. Because of the smaller range, the damage ismore extensive in the projected region and amorphization of thesubstrate is achieved more quickly. The simulation indicates acomplete amorphous layer of about 150 nm in the Ge substrate.

Based on the simulation results, we performed implantation withdose of 5×1013, 1×1014, and 1×1015 cm−2, where the low dose im-plantation (5×1013 and 1×1014 cm− 2) resulted in partial amorphi-zation of the Ge substrates, and the high dose (1×1015 cm−2)

0 100 200 300

1017

1018

1019

1020 95nm137nm

1x1015 cm-2

P C

once

ntra

tion

(cm

-3)

Depth (nm)

Ge

Si

Fig. 1. P concentration profiles in Ge and Si substrates (solid and broken lines, re-spectively) for 100 keV P-ion-implantation (dose: 1×1015 cm–2) obtained bySRIM simulation.

generated completely amorphized damaged layers. Measurementsfor crystallinity of the as-implanted samples were carried out usingspectroscopic ellipsometry. The crystallinity values, obtained fromthe extinction coefficient spectra [19], are plotted as a function of im-plantation dose in Fig. 2. It is found that the dose dependence of crys-tallinity shows a good agreement with the calculated curve obtainedfrom the SRIM simulation.

The solid-phase epitaxial (SPE) regrowth characteristics were in-vestigated with isochronal annealing of Ge (100) and (111) samplesimplanted with lower dose of 5×1013 cm–2, where only partial amor-phization is achieved, and a high dose of 1×1015 cm–2, where com-plete amorphization is achieved. The regrowth behavior is shown inFig. 3. For the sample with the low dose implantation, the recoveryof crystallinity shows almost no dependence on substrate orientation.High recovery (>80%) is achieved at annealing temperature as low as250 °C and completion of recovery is achieved at 300 °C. This orienta-tion independent regrowth can be explained with the damage clustermodel [21,22]. During low dose implantation, damage clusters areformed in the implanted region. These damage clusters are isolatedfrom each other, and upon annealing, crystal regrowth occurs in theclusters along every direction. Therefore, the dominance of growthalong any particular direction is not observable. On the other hand,in the high dose implanted samples, regrowth of the amorphous

0

50

100

Ge (100)

Ge (111)

0

50

100

As-implanted

a)

Dose: 5x1013 cm-2

Dose: 1x1015 cm-2

Annealing temperature (oC)

200 300 400 500 600

b)

Cry

stal

linity

Cry

stal

linity

Fig. 3. SPE regrowth behavior in Ge (100) and (111) samples implanted with 100 keVP-ions at doses of (a) 5×1013 and (b) 1×1015 cm–2.

1000/T (K-1)

Car

rier

Act

ivat

ion

Rat

io

Annealing Temperature (oC)

Dose: 5x1013 cm-2

Ea = 0.41 eV

1 1.2 1.4 1.6

0.1

0.4

0.6

0.8

1350450550650

Fig. 5. Arrhenius plot of dopant activation ratio for Ge(100) samples implanted with100 keV P-ions with a dose of 5×1013 cm–2.

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layer proceeds from the interface between the amorphous and thecrystalline regions. Since the SPE regrowth velocity depends on thecrystal orientation, i.e., faster regrowth velocity along (100) com-pared with that along (111), the regrowth for the (100) sample oc-curs at lower temperature compared with the (111) sample [23].The almost complete regrowth at 350 and 400 °C for (100) and(111) samples, respectively, quantitatively agrees with the reportedvalues of the SPE velocity [23].

The dependence of the sheet resistance (Rs) on annealing temper-ature for the Ge(100) samples with different doses is shown in Fig. 4.Theoretical minimum values, expected for full activation of P atoms,are also shown by the horizontal lines for the respective doses. Forboth doses, with increasing annealing temperature, the sheet resis-tance decreases and approaches the theoretical minimum value. Forthe high dose (1×1015 cm–2) sample, the sheet resistance decreasesto ~60 Ω/□ at 350 °C and shows saturation. The annealing tempera-ture (350 °C) well agrees with that for complete regrowth of thedamaged layer, as shown in Fig. 3(b). On the other hand, for thelow dose (5×1013 cm–2) implanted sample, the sheet resistance be-comes minimum at 550 °C. It is noted that the annealing temperature(550 °C) necessary to obtain the minimum sheet resistance is higherthan that (300 °C) for the complete recovery of the damaged regionsin the low dose sample, as shown in Fig. 3(a). It is also noticed thatthe sheet resistance increases slightly at temperatures above 550 °C.The slight increase in sheet resistance is probably due to out-diffusion and in-diffusion of P atoms [14,24]. The present sampleshad no capping layers. The addition of a capping layer can be effectivein suppressing the increase of sheet resistance due to out-diffusion ofdopants.

To clarify the mechanism for the discrepancy between the anneal-ing temperatures for the complete recovery of damage and the mini-mum sheet resistance observed in the low dose samples, weinvestigated dopant activation ratio as a function of the annealingtemperature. Fig. 5 shows the Arrhenius plot of the dopant activationratio, where the dopant activation ratio is defined by the equationa=Rs0/Rsexp, where Rsexp is the measured sheet resistance, and Rs0is the theoretical minimum sheet resistance assuming the full activa-tion of dopant atoms. The solid line in Fig. 5 has been obtained by fit-ting to the data at temperatures between 350 °C and 450 °C for thedose of 5×1013 cm–2. From this fitting, the value of the activation en-ergy (Ea) has been obtained as 0.41 eV. This value is close to the mi-gration energy (~0.44 eV) of vacancy in Ge [25,26]. This suggeststhat dopant activation is primarily mediated bymigration and annihila-tion of vacancy-related defects, which act as acceptors in crystallizedGe. On the other hand, the activation ratio at higher temperatures(>450 °C) saturates at around 1, because vacancies have been

Annealing Temperature (oC)

Rs (o

hm/s

qr)

5x1013 cm-2

1x1015 cm-2

300 400 500 6000

100

200

300

400

500

600

Theoretical minimum

Fig. 4. Annealing temperature dependence of sheet resistance for Ge(100) samplesimplanted with 100 keV P-ions at doses of 5×1013 and 1×1015 cm–2.

completely annihilated by annealing, as shown in Fig. 5. Suchvacancy-mediated activation characteristics are remarkably observedfor the low dose samples. In the low dose implanted case, isolated dam-age clusters are formed, and upon annealing, vacancies which compen-sate the implanted P donors are left. Because of the low concentration ofimplanted P-ions, higher temperature annealing is essential to annihilatevacancies completely and achieve maximum dopant activation. On thecontrary, in the high dose implanted case, almost complete activationof dopants coincides with the recrystallization of Ge, because of thehigh P concentration. These are important findings for the precisecontrol of the carrier concentration for threshold voltage tuning of Getransistors.

4. Conclusion

The ion-implantation characteristics of 100 keV P-ions into Gesubstrates were investigated with a range of doses (5×1013–

1×1015 cm–2). For Ge substrates compared to Si, a shorterprojected range and a resultant smaller critical dose for completeamorphization were found. The activation characteristics of theimplanted P atom in Ge were also investigated and compared withthe regrowth characteristics for low dose (5×1013 cm–2) and highdose cases (1×1015 cm–2). The regrowth characteristics ofamorphized layers indicated that isolated damage clusters wereformed at the low dose, while continuous amorphous layer wasformed at the high dose. For the high dose implanted samples, theactivation of dopant atoms coincided with the complete recovery ofthe amorphous layers at 350–400 °C. However, higher temperatureannealing at 550 °C was necessary to obtain the minimum sheetresistance for the low dose samples, where almost completerecovery of damage clusters occurred at a temperature as low as300 °C. From the detailed analysis of the carrier activation ratio, itwas suggested that the carrier activation process was mediated bythe migration of vacancy-related defects.

Acknowledgements

The authors would like to thank Prof. M. Miyao, Prof. T. Asano, andMr. T. Takao for valuable discussions and experimental supports. Partof this work was supported by a Grant-in-Aid for Scientific Researchfrom theMinistry of Education, Culture, Sports, Science, and Technologyin Japan.

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