Electrical Characterization of Femtosecond Laser Activated Ge….51

mobility of the samples shown in the figures are also shown as the insets of Fig. 3-8 to 3-11.

Here we are not going to measure the activated dopant profile by SRP. The resolution of SRP is about 7-8 nm, which is not precise enough for the shallow junction herein. The activation parameters and measurement results are summarized in Table 3-2.

The inset of Fig. 3-8 shows that the sheet resistance decreased because of irradiated with more laser pulses is identical to the improvement of hall mobility. As the irradiated laser pulses increased from 5 to 20, the corresponding activated dopant concentration slightly decreased from 3.01×10¹⁴ to 1.26×10¹⁴ atoms/cm² could be imputed to the dopant diffusion. Despite the reduced active concentration, one possible explanation for the improvement of sheet resistance and mobility could be the improvement in crystal quality.

Here we measured a highest mobility of 993 cm²/Vs.

As the laser fluence increased from 35 to 45.5 mJ/cm² (see Fig. 3-9), the activated dopant concentration slightly decreased from 3.01×10¹⁴ to 2.42×10¹⁴ atoms/cm² due to increased activation fluence. The corresponding huge improvement of electrical characteristics (815-1410 cm²/Vs) may mainly attribute to the improvement of crystal quality [18].

The best activation rates which correspond to the lowest sheet resistance of the

samples in Fig. 3-9 are ~7%. The profile of P in Fig. 3-8 and 3-9 exhibits a concentration peat of about 2×10²¹ atoms/cm², which is already above the maximum equilibrium solubility of P in germanium and would not improve the sheet resistance [25]. The highest hall mobility is even higher than that of ion-implanted Ge samples activated by RTA (~ 920 cm²/Vs) [18].

The peak concentration of boron atoms enormously increased from 4.97×10²¹ to 1.17×10²² atoms/cm³ at a depth of ~ 6 nm as the diffusion enhanced by irradiated with more laser pulses (see Fig. 3-10). Since the solid solubility of boron in Ge is ~ 5×10¹⁸ atoms/cm³, more atoms piled up at that depth is not able to further increase B activated concentration which is just 1.41×10¹⁴ atoms/cm² [15]. Thus the incremental of mobility from 121 to 330 cm²/Vs could be attributed to the better crystallinity.

The activation of boron atoms with 5 laser pulses and various laser fluences show better performance in both electrical characteristics and diffusion suppression (Fig. 3-11).

Mobility increased from 107 to 426 cm²/Vs due to increasing irradiated laser fluence from 28 to 35mJ/cm². The improvement of sheet resistance and mobility, however, could be attributed to the incremental of activated carrier concentration from 1.05×10¹⁴ to 5.25×10¹⁴ atoms/cm² and crystallinity improvement. The highest activation rates of the dopants here is

~ 22%. This value is slightly lower than our previous results of dopant activation in Si with implantation energy of 25 keV.

3.4 Summary

Femtosecond laser annealing (FLA) was employed for activation of P - and B -implanted silicon with negligible dopant diffusion. Preamorphization by implantation, commonly used in conventional activation schemes for minimizing the diffusion of dopant during annealing, was found not to be required. We found that dopant profiles in FLA-activated samples essentially duplicate those of as-implanted ones even for junctions

as deep as 100 nm below the surface. The measured sheet resistances and activation efficiencies of P- and B-implanted samples were in the range of 100-400 Ω/ and 28-35 % respectively.

Moreover, thermal-energy-assisted dopant diffusion by heating was observed for substrate temperature as low as 100 °C (Fig. 3-3). The shallow activated-depth feature associated with FLA reduces the separation between end-of-range defects and high-concentration portion of dopants. This generates a steep interstitial gradient responsible for observed B and P uphill diffusion at a depth of about 60 nm below the surface.

Excellent dopant profile controlling of n-type and p-type dopants on Ge substrate by femtosecond laser annealing was demonstrated for the first time. The fast delivery of photon energy, non-thermal melting, is the key feature of such intense ultrafast laser annealing process. Significant dopant loss, which is frequently observed in RTA and laser thermal activation, can be eliminated thus. Shallow junction formation (17, 40 nm) and high mobility (425, 1410 cm²/Vs) of BF2- and P-implanted samples are both reached by activated with appropriate laser fluence and laser pulses. This high mobility could be attributed to the high activation rate or the improvement of crystallinity.

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Figures

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 10¹⁶

Fig. 3-1 SIMS profiles of as implanted B-doped samples, with (sample A) and without PAI (sample B) as well as BF2+-implanted samples (sample C). SIMS profiles for all B-doped layers activated by FLA at different substrate temperature are also shown.

Fig. 3-2 SIMS profiles for BF2+-implanted layers activated by either FLA or ELA at different substrate temperatures. SRP profiles for BF2+-implanted layers activated by FLA at different temperature are also shown.

0 50 100 150 200

10¹⁶ 10¹⁷ 10¹⁸ 10¹⁹ 10²⁰ 10²¹

ELA (SIMS)

FLA (SIMS) FLA (SRP)

as-implanted ELA at 24 ^oC ELA at 200 ^oC FLA at 24 ^oC FLA at 100 ^oC FLA at 200 ^oC FLA at 24 ^oC by SRP FLA at 200 ^oC by SRP (sample D)

concentration (atoms/cm-3 )

depth (nm)

Fig. 3-3 SIMS and SRP profiles for P-implanted layers activated by FLA at different substrate temperatures. SIMS profiles for P-implanted layers activated by ELA at different substrate temperatures are also shown. The dopant depth was designed to be the same as for BF2+-implanted samples.

Fig. 3-4 AFM result of FLA-activated B-implanted Si.

Fig. 3-5 AFM result of FLA-activated P-implanted Si.

Fig. 3-6 Sheet resistance of sample C activated with different overlapping of neighboring pulses and different substrate temperature.

0 20 40 60 80

Fig. 3-7 Sheet resistance of sample D activated with different overlapping of neighboring pulses and different substrate temperature.

Fig. 3-8 Phosphorous SIMS profiles of the samples annealed with different number of laser pulses. The inset shows the corresponding sheet resistance and mobility of

0 20 40 60 80

Fig. 3-9 Phosphorous SIMS profiles of the samples annealed with various laser fluences.

The inset shows the corresponding sheet resistance and mobility of the samples.

Fig. 3-10 Boron SIMS profiles of the samples annealed with various number of laser pulses. The inset shows the corresponding sheet resistance and mobility of the samples.

Fig. 3-11. Boron SIMS profiles of the samples annealed with various laser fluences. The inset shows the corresponding sheet resistance and mobility of the samples.

0 10 20 30

Tables

B-implanted a-Si (without PAI)

B-implanted Si

(with PAI) BF2+

-implanted Si P-implanted Si

Si PAI parameters 50 KeV 5×10¹⁵/cm²

Table. 3-1 Implantation parameters, FLA-activation and ELA-activation conditions for three B-doped and P-doped layers. Sheet resistance for doped layers activated by FLA with those activated by ELA methods in this work and reported in Refs.

6, 7 and 11 are listed for comparison. Dopant depth is defined as the distance from the surface, at which the dopant concentrations drop to 10¹⁸/cm³.

Fluence used for

FLA (mJ/cm²)

Sheet resistance

of FLA activated

samples (Ω/)

Mobility (cm²/Vs)

Activated carrier Concentration (activation rate)

(atoms/cm²)

Junction depth

(nm)

BF₂-implanted

Ge ~ 32-37 ~ 182-304 ~ 122-427 ~1.05×10¹⁴-5.25×10¹⁴

(~22%) ~ 17

P-implanted

Ge ~ 35-49 ~ 22-24 ~ 814-1410 ~2.42×10¹⁴-3.01×10¹⁴

(~7%) ~ 40

Table.3-2 Activation fluence and electrical characteristics of B-doped and P-doped layers activated by FLA.

Chapter 4

Time-Resolved THz Spectroscopy of Femtosecond-Laser-Annealed Amorphous Silicon

4.1 Introduction

With the demand of larger display area and high pixel density of thin film transistor liquid crystal display (TFT-LCD), high mobility TFTs are required for the pixel driver of TFT-LCD in order to shorten the charging time of pixel electrodes. Low temperature poly silicon (LTPS) technology has been studied for the purpose of driver integration at periphery of active matrix liquid crystal display [1]. Because of its better crystalline quality than a-Si, especially the grain size, poly-Si attracted great attention for last two decades [2-5]. TFTs fabricated by poly-Si show higher carrier mobility and better electrical characteristics.

The defects, such as dangling-bond defects in grain boundary or strained defects of poly-Si [6], can reduce the carrier transition speed and lead to the leakage current, which in turn degenerates the performance of TFTs [6-7]. Typical channel length of TFTs are about 5- 10 μm, which ideally should be close to or smaller than the grain size of poly-Si in order to leave a single grain in the channel area [8-9]. Laser- or furnace-annealed LTPS fabrications have been employed to obtain larger grain sizes of poly-Si [5]. Since the grain size of poly-Si is one of the key features to affect electrical characteristics of TFTs such as mobility, it is important to examine the grain size of poly-Si before TFT-fabrication. Traditionally, the grain size is examined by SEM. However, a destructive sample preparation, such as Secco etching is required for SEM. Besides, SEM is limited for the observation of tiny area and is not easy to offer the information about the uniformity of poly-Si in large area. Although Hall

measurements can measure the mobility of semiconductors, however, it is also not easy to measure the mobility of intrinsic poly-Si.

Recently, the spectroscopic technique using pulsed THz radiation, called ”terahertz time-domain spectroscopy (THz-TDS)”, has been developed, by taking advantage of short pulses of broadband THz radiation. And THz-TDS is a non-destructive method to measure the carrier concentration and mobility of doped semiconductotrs. Many researches have been performed on a variety of gases, liquids, dielectric materials and semiconductors by THz-TDS. For example, in 1990, D. Grischkowsky et al. studied the THz-TDS with the dielectric materials, such as quartz and sapphire, and semiconductors, like silicon and GaAs.

They discovered that different carrier concentrations affect the absorption characteristics of the samples in the THz frequency range. The Drude Model could be used to link the frequency-dependent dielectric response to the material’s free-carrier dynamics properties [10].

In this chapter, the carrier mobility of FLA poly-Si is measured by optical-pump–THz-probe (OPTP) technique [11]. We also measured the temporal evolution of far-infrared conductivity and refractive index of FLA poly-Si. This technique is contact-free, therefore, damage-free. The quality of poly-Si samples annealed at various pump fluence was directly identified by the OPTP technique.

4.2 Generation and Detection of Terahertz Radiation

4.2.1 Surge Current

4.2.1.1 Surface Depletion Field

In semiconductors with a wide bandgap, such as GaAs (Eg = 1.43 eV) or InP (Eg = 1.34 eV), the surface bands of a semiconductor lie within its energy bandgap, and thus Fermi-level pinning occurs, leading to band bending and formation of a depletion region,

where the surface built-in field exists [12]. When a laser beam excites the semiconductor surface, the photo-generated electrons and holes are accelerated in opposite directions under the surface-depletion field, so that a surge current is formed in the direction normal to the surface.

The direction and magnitude of the surface depletion field depend on the dopants, impurity species and the position of the surface states relative to the bulk Fermi level.

Generally, the energy band of n-type semiconductors bends upward (Fig. 4-1(a)) and the energy band bends downward for p-type semiconductors (Fig. 4-1(b)). The surface built-in field in the p-type semiconductor drives the photogenerated carriers, which is the transient surge current. In the n-type semiconductor, the transient surge current is driven in the opposite direction compared to the p-type semiconductor, as shown in Fig. 4-1(a). In the far-field approximation, the emitted THz-radiation-field amplitude, ETHz(t), is proportional to the time derivative of the surge current, J(t):

t t t J

E_THz

∂

∝ ∂ ( ) )

( . (1) The unambiguous evidence for distinguishing the main mechanism of THz emission from the semiconductor surface is the polarity of the THz waveform between the n-type and p-type semiconductor. When the surface depletion field is the dominant mechanism for the surge current, the polarity of the THz waveform is opposite between the n-type and p-type semiconductors.

4.2.1.2 Photo-Dember Effect

The narrow-bandgap InAs and InSb are very interesting materials because of their high electron mobilities: ≈ 30000 cm²/Vs for InAs and ≈ 76000 cm²/Vs for InSb, respectively.

Recently, InAs attracts much attention as an efficient THz emitter since a significant enhancement of THz emission from InAs has been observed under magnetic fields [13]. The

effect of surface depletion field is not so large for the narrow-bandgap semiconductors because of their small bandgap energies.

The absorption depth of a narrow bandgap semiconductor surface photoexcited by near infrared light (hν =1.5 eV) is very thin (≈100 nm) [14], and the excess energy of the photoexcited carrier is very large. THz generation from the narrow-bandgap semiconductors is mainly due to the Photo-Dember effect, which is known to generate current or voltage in semiconductors attributed to the difference of the electron and hole diffusion velocities.

The diffusion current due to the Photo-Dember effect after photoexcitation near a semiconductor surface is illustrated in Fig. 4-2. Because the electron mobility is always larger than the hole mobility, the direction of diffusion current induced by the Photo-Dember effect is in the same way for each kind of semiconductor and irrespective of the doping type (n or p). Therefore, the THz waveform emitted from the surface surge current due to the Photo-Dember effect will show the same polarity for n-type and p-type semiconductors.

The diffusive currents of the electrons (J ) and holes (_n J ) are, respectively, described _p by the following equations [15],

x and holes, D and _e D are the diffusion coefficient of electrons and holes, respectively. The _h diffusion coefficient D is defined by the Einstein relation, D=k_bTμ/e, where k is the _b Boltzman constant, T is the temperature of the corresponding carrier, and μ is the mobility of electrons or holes. The THz radiation from the Dember current J_dif =J_n +J_pis proportional to the difference in the mobility for the electrons and holes, and the gradient of the carrier density.

Narrow bandgap semiconductors should have the ability to create a large Photo-Dember field due to the large electron mobility and large excess carrier energies.

Moreover, the Photo-Dember field in the narrow bandgap semiconductors is further enhanced by the small absorption depth.

4.2.2 Free Space Electro-Optics Sampling

The coherent detection of a THz-pulse beam with EO crystal is based on the linear EO effect (Pockels effect) [16]. The incident THz-pulse beam modifies the refractive index ellipsoid (or birefringence) of the EO crystal giving rise to a phase retardation of the linearly polarized optical probe beam. By monitoring the phase retardation, the field strength of the THz pulse is detected. A pellicle beam splitter combines the THz beam and the probe beam so that both may copropagate. The polarization of both the THz and optical probe beams are aligned parallel to the [1, −1, 0] direction of a (110) oriented ZnTe sensor crystal. Following the sensor crystal, a quarter-wave plate is used to afford a π/4 optical bias to the probe beam, which allows the system to be operated in the linear range. A Wollaston polarizer is used to convert the THz-radiation-field induced phase retardation of the probe beam into an intensity modulation between the two mutually orthogonal linearly polarized beams. A pair of Si p-i-n photodiodes connected in a balanced circuit is used to detect the optical intensity modulation.

The difference signal of p-i-n photodiodes is fed to a lock-in amplifier referenced at a frequency at which the pumping optical beam is chopped to generate the THz radiation.

Because of the instantaneous response of the Pockels effect, the EO crystal acts well as a sampling detector. Hence, this method is called EO sampling. The detection with an EO crystal is becoming popular due to its broad-bandwidth capability and ease of implementation. In the EO detection there is a clear trade-off between the sensitivity and frequency response that is determined by the choice of crystal and its thickness. A thicker

crystal produces a greater interaction length, but on the other hand it reduces the detection bandwidth due to group-velocity mismatch between THz beam and probe beam. In addition, the EO technique is very sensitive to the laser noise and to low-frequency mechanical and

crystal produces a greater interaction length, but on the other hand it reduces the detection bandwidth due to group-velocity mismatch between THz beam and probe beam. In addition, the EO technique is very sensitive to the laser noise and to low-frequency mechanical and

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