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The Structural and Electrical Characteristics of Silicon-Implanted Borosilicate Glass
View the table of contents for this issue, or go to the journal homepage for more 2002 Jpn. J. Appl. Phys. 41 L1379
(http://iopscience.iop.org/1347-4065/41/12A/L1379)
Part 2, No. 12A, 1 December 2002 c
2002 The Japan Society of Applied Physics
The Structural and Electrical Characteristics of Silicon-Implanted Borosilicate Glass
Gong-Ru LIN∗Institute of Electro-Optical Engineering, National Chiao Tung University, 1001 Ta Hsueh Rd., Hsinchu, Taiwan 300, R.O.C.
(Received May 29, 2002; revised manuscript received September 12, 2002; accepted for publication October 7, 2002)
The structural and electrical properties of silicon-implanted borosilicate glass (BSO:Si+) are studied. The nearly amorphous phase of as-implanted BSO:Si+ with a weak and broadened X-ray diffraction peak transforms into crystallite phases with associated peaks positioned at azimuth angles of 29◦and 14◦after thermal annealing. These peaks correspond to (111)-oriented Si nanocrystals of 0.6–0.8 nm size and the regrown (021)-oriented BSO host, respectively. The intensity of the photoluminescent (PL) peak of the BSO:Si+centered at 520 nm is found to decrease due to both the elimination of the radiative defects and the precipitation of Si nanocrystals, however, nanocrystal-related PL is not initiated even after low-temperature and long-term (> 4 h) annealing. Relatively high leakage current Schottky diodes with contact patterns for transmission line measurement (TLM) made on as-implanted BSO:Si+ reveal the defect-enhanced current transport mechanism. After annealing at 500◦C for 60 min or longer, the leakage current of the BSO:Si+ diode dramatically decreases by at least two orders of magnitude. The current–voltage analysis attributes the disappearance of the resonant tunneling behavior of the TLM diode made on as-implanted BSO:Si+with negative differential resistance to the annealing-induced reduction of radiative defect concentration. [DOI: 10.1143/JJAP.41.L1379]
KEYWORDS: silicon, implanted, borosilicate glass, nanocrystal, defects, negative differential resistance
Nanocrystallite Si semiconductor structure has recently been investigated for its potential application to direct-bandgap optoelectronics. Previously, considerable interest was focused on the porous Si1, 2)and Si3–7)ultrafine particles
or nanocrystals due to their optical properties. Earlier reports on the strong room-temperature luminescence in nanocrystal-lite5) and porous Si8) have stimulated the development and
application of such materials9) in light-emitting devices.10)
Later, Si nanocrystals were synthesized in the SiO2
matri-ces on silicon substrate via implantation and annealing pro-cesses.11)Such materials exhibit strong photo- and electrolu-minescence (PL and EL) in visible and near-infrared wave-lengths, which attracts much attention due to the potential ap-plications in electronic and optoelectronic devices.12–15)Most
of the studies on these materials discuss light-emitting proper-ties of the metal-oxide-semiconductor (MOS) structure with Si nanocrystals buried in the surface oxide layer on the silicon substrate. The combined carrier transport effects of the con-ventional MOS diode, the implanted Si-related defects, and the buried Si nanocrystals in the SiO2 layer on Si substrate
are thus complicated. In contrast, the electrical properties of the silicon nanocrystals embedded in the quartz or glass sub-strate, or the oxide film without semiconducting substrate are not frequently encountered. Borosilicate (BSO) glass was al-ways employed as the host of Si nanocrystals in previous stud-ies. The decreased PL intensity of the as-implanted or the an-nealed BSO:Si+with buried Si nanocrystals due to the exis-tence of p-type Boron states was reported. For future study, the competition between implanted Si-related radiative de-fects and boron-related non-radiative recombination centers and the photo-excited carrier dynamics in the BSO:Si+ sam-ples are of great interest. In this article, we report the for-mation of nanocrystallite Si semiconductor by multi-energy implantation of silicon ions into BSO glass substrate. The optical and electrical properties of the BSO:Si+ sample are characterized by using X-ray diffractometry (XRD), photo-luminescence (PL), transmission line measurement (TLM), and current–voltage (I –V ) analysis. The XRD and PL
analy-∗E-mail address: [email protected]
sis provide important information on the evolution of radia-tive defect density and the precipitation of Si nanocrystals in BSO:Si+ during annealing. The TLM and I –V charac-terizations of the Al-evaporated metal contacts made on the BSO:Si+substrate were used to investigate the resonant tun-neling behavior of the as-implanted BSO:Si+ with negative differential resistance, and the decreasing leakage current as-sociated with reduced defect concentration of BSO:Si+which was long-term annealed at 500◦C. Key parameters such as the defect-related PL wavelengths, the size and orientation of Si nanocrystals in BSO:Si+, the leakage current, the contact re-sistance, and the sheet resistivity of the TLM Schottky diodes made on such material are also determined.
The BSO:Si+ samples were prepared by implanting BSO glass with thickness of 125 ± 20 mm with 50, 100, and 200 keV silicon ions at the dosage of 1016ions/cm2. The
depth of implantation is theoretically estimated to be up to 350 nm below the surface by using the Monte-Carlo simula-tion program. Optimized annealing of the BSO:Si+at 500◦C for 30 min to obtain maximum PL signal was carried out ac-cording to the method of Rebohle et al.16)In our experiment,
the furnace-annealing process at 500◦C ranging from 30 to 120 min at 30 min increments was employed to modify the carrier transport property of the BSO:Si+samples. The struc-tural property of the deposited BSO:Si+was characterized by XRD measurements in theθ–2θ scanning mode with Cu-Kα radiation source (λ = 1.5418 ˚A). The spacing between the desired planes is calculated from Bragg’s law nλ = 2d sin θ, whereλ is the wavelength of Cu-Kα radiation, θ is the Bragg angle, and n is a positive integer. The PL spectra were taken at room temperature using a commercial fluorescence photo-spectrometer (Fluorolog ISO IOBINYUON-SPEX) with ex-citation wavelength of 270 nm. The 100-nm-thick aluminum-evaporated Schottky contact patterns for TLM analysis of the leakage current and the contact resistance of the BSO:Si+ diode were fabricated with contact size of 75× 50 µm2 and
spacing ranging from 2.5µm to 25 µm at 5 µm increment. After measuring the resistances of the BSO:Si+ diodes with different gap spacings, the specific contact resistivity, lateral
L1380 Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 2, No. 12A G.-R. LIN contact resistance, and sheet resistance can be evaluated from
the relationship Ri = 2Rc+ Rsli/w, where Ri is the
mea-sured resistance between the two contacts separated by li, Rc
is the contact resistance, and Rsis the sheet resistance of the
material. The plot of Ri as a function of li (with a slope of
Rs/w) has an intercept of −2Rcw/Rsat the x -axis and an
in-tercept of 2Rc at the y-axis. The specific contact resistivity
of the BSO:Si+ (ρc = ω2Rc2/Rs) can thus be obtained from
calculation of the experimental data.
It is found that the XRD diffraction peak of the as-implanted BSO:Si+ has split after annealing, as shown in Fig. 1. The as-implanted BSO:Si+ shows only a weak and broadened diffraction pattern due to presence of the sil-icon dioxide. After annealing, a relatively weak peak at
2θ = 28–29◦ is formed due to the recrystalization of
(111)-oriented Si matrices in the BSO:Si+. The precipita-tion of Si nanocrystals is believed to be initiated during the long-term and low-temperature annealing process, however, the nanocrystals have sub-nm diameters unlike typical Si nanocrystals (> 1.6 nm) formed under high temperature con-dition. The measurement of the size and density of consider-ably smaller Si nanocrystals in the BSO:Si+ thus relies on a high-resolution transmission electron microscopy. Nonethe-less, the size can still be evaluated by using Scherre’s rela-tion,17)size= 0.9λ/(β cos θ) nm, where λ is the wavelength
of the XRD source (0.15418 nm),β is the full-width at half maximum (FWHM) of the broadened XRD diffraction peak (in units of radian), and θ is the diffraction angle. In our BSO:Si+, the FWHM of the (111)-oriented Si-related XRD peak is about 1.75–2.27×10−1radian (10◦–13◦), which gives rise to (111)-oriented Si nanocrystals ranging from 0.63 to 0.82 nm in size. A relatively stronger diffraction peak located
at 2θ = 14◦ was also observed for the annealed BSO:Si+
sample, which is similar to that of (021)-oriented solid crys-talline SiO2phase according to the diffraction database.18)In
comparison, Sandhu et al.17)previously studied the effect of annealing on silicon-doped SiO2 film and reported relatively
similar XRD diffraction peaks. The authors attributed the ob-served peak centered at 2θ = 24◦with FWHM of 7.5◦to the Si-rich SiO2substrate. After annealing at 1100◦C for 1 h, the
original rocking-curve peak is split into two adjacent peaks
20 40 60 80
Intensity (arb. unit)
(b)
(a)
Angle (2 )
10 20 30 40 50 60
Sandhu et al. (ref. 17) Annealed As-implanted
Angle (2 )
Fig. 1. X-ray diffraction of (a) as-implanted and (b) annealed BSO:Si+ samples. The inset shows the results of silicon-doped BSO film grown on (100) silicon substrate for comparison.
and other sub-peaks centered at 20.5◦, 28.5◦, 47.3◦and 56◦. The former peak at 20.5◦ is also explained as the contribu-tion of the SiO2 matrices, and the latter three peaks are
at-tributed to solid crystalline silicon phases (silicon crystallites) with orientations of (111), (220), and (311), respectively.
The PL spectra of as-implanted and annealed BSO:Si+ samples revealing luminescent peaks centered at 515–520 nm (see Fig. 2) are similar to the PL spectra of SiO2:Si+samples
reported by other groups.19–22)For examples, Song et al.19)
have shown the PL band centered at about 470 nm for Si+ -implanted thermal oxide films with energy of 25 keV and dose of 1×1016cm−2after annealing at< 600◦C for 60 min.
How-ever, Schuppler et al.23) have observed that the average size
of Si nanocrystals responsible for the visible PL is far smaller than 1.3 nm. This corroborates the accuracy of the aforemen-tioned calculation that the PL peak shifts from 780 nm to
< 600 nm as the size of Si nanocrystals decreases from 3 nm
to < 1.4 nm as previously reported.24) The improvement of
the size of Si nanocrystals (larger than 1 nm) relies strictly on increasing the annealing temperature to 800◦C or higher. Nonetheless, most research has attributed the green or yel-low PL peaks to the contribution of non-bridging oxygen hole center (NBOHC, O3 ≡Si–O) or the ≡Si–Si≡ based
radia-tive defects rather than the Si nanocrystals in the Si-implanted SiO2samples.25) In addition, the PL spectra of BSO:Si+
af-ter long-af-term annealing process do not shift in wavelength but peak intensity decays, which may thus be interpreted as the elimination of the radiative defects. Although few reports have discussed similar phenomenon,25)it is believed that the
in-situ annealing process was concurrently initiated with the
implantation process, since the BSO glass was also heated during implantation owing to its smaller coefficient of ther-mal conductivity as compared to that of typical Si substrate. Therefore, the radiative defects in the BSO:Si+sample have already been activated during the implanting process. Further annealing simultaneously results in the reduction in density of these defects and the precipitation of sub-nm Si nanocrystals. In this case, such a decrease in PL intensity may qualitatively be interpreted as the modification in defect-assistant charac-teristics of the BSO:Si+furnace-annealed at 500◦C for longer durations. 300 400 500 600 700 800 0 2x105 4x105 6x105 8x105 1x106 as-implanted BSO:Si+ ref. 19 ref. 22 ref. 20 Intensity Wavelength (nm)
Fig. 2. Photoluminscence spectra of as-implanted BSO:Si+ substrate (solid line) and as-implanted SiO2:Si+on silicon substrate (dotted, dashed, and dash-dot-dot lines) measured by different research groups.
To characterize the electrical properties of the BSO:Si+ material, the leakage current of the Shottky diodes with TLM patterns (referred to as TLM diodes) of different spacings made on the BSO:Si+ and non-implanted BSO glass sub-strates were measured and plotted as a function of bias volt-age, as shown in Fig. 3. It is seen that the TLM diode with spacing of 2.5µm made on non-implanted BSO glass ex-hibits nearly insulating property with leakage current of less than 5 pA and breakdown voltage of up to 400 V, as shown in the inset of Fig. 3. As the spacing of the TLM patterns on glass increases up to 25µm, the leakage current becomes as small as 1 pA (which is another limitation of the sys-tem) at bias of larger than 500 V, and the breakdown volt-age further increases to 620 V. After implanting with sili-con ions, the current–voltage characteristics of the BSO:Si+ TLM diodes reveal completely different features. For ex-ample, the leakage current of the TLM diode with pattern spacing of 2.5µm made on the as-implanted BSO:Si+ sub-strate greatly increases (overshoots) up to 1.5 nA at bias of 80 V, however, it subsequently decays to 0.9 nA at higher bi-ases. In comparison, the current–voltage characteristic of the diode made on glass has a relatively linear relationship at bi-ases before breakdown. Such an overshooting phenomenon in current–voltage characteristics reveals the existence of neg-ative differential resistance (NDR) effect in BSO:Si+ sam-ples, which is repeatable for TLM diodes with different spac-ings. Note that the NDR effect has never been observed in samples with glass substrate and is primarily reported for the metal-semiconductor-metal (MSM) diode made on the silicon-implanted (or silicon-rich) BSO glass. In addition, the peak overshooting voltage tends to occur at higher biases for the diode with larger contact spacings. However, the elec-trodes of the BSO:Si+ diode with spacing of 2.5µm even-tually breakdown due to instantaneous air discharge as the bias increases to 330 V. The breakdown voltage of the as-implanted BSO:Si+ diode with contact spacing of 25µm can be as high as 550 V. The TLM results of as-implanted BSO:Si+ and BSO glass samples are shown in Fig. 4. It is found that the measured total resistance and contact resis-tance of the BSO glass substrate significantly decrease by at least 4 and 3 orders of magnitude, respectively, after Si-ion implantatSi-ion. For the BSO glass sample, we determined
0 100 200 300 400 1 2 (f) (e) (d) (c) (b) (a)
Dark current (nA)
Voltage (V) 0 100 200 300 400 0.0000 0.0025 0.0050 0.0075 Glass with 2.5 m Glass with 5 m
Dark current (nA)
Voltage (V)
Fig. 3. Measured current–voltage response of TLM diodes with contact spacing of (a) 2.5µm, (b) 5 µm, (c) 10 µm, (d) 15 µm, (e) 20 µm and (f) 25µm fabricated on as-implanted BSO:Si+substrate.
0 5 10 15 20 25 2x1014 4x1014 6x1014 8x1014 1x1015 1x1015 (b) (a)
(a) Glass Vbias= 200 volts
SiO 2 :Si + Resistance ( ) Glass Resistance ( ) Gap Spacing ( m) 0 2x1012 4x1012 6x1012 8x1012 1x1013 (b) As-implanted BSO:Si+ Vbias= 20 volts
Fig. 4. Measured resistance of TLM diodes made on BSO glass and as-implanted BSO:Si+substrates plotted as a function of contact spacing.
the contact and sheet resistances to be about 1.8 × 1014 and 2.3 × 1015/ at 20 V, respectively. After implantation of silicon ions, the contact resistance of the BSO:Si+ diode abruptly decreases to about 5.9 × 1010, and the sheet re-sistance of BSO:Si+ also decreases to 8.6 × 1012/ at
20 V. The specific contact resistivity ρc of the BSO glass
and as-implanted BSO:Si+ samples were calculated to be
7.7 × 108·cm2and 2.3 × 104·cm2, respectively.
In view of these electrical properties, we thus conclude that the electrical property of the BSO glass has been mod-ified from an insulating to a defect-assistant carrier trans-port process due to the silicon-ion implantation process. The huge leakage current of the BSO:Si+diode induced by Si im-plantation leads to nonlinear, overshooting, and rectified (or resonant tunneling) current–voltage characteristics. On the other hand, the leakage current of the TLM diode made on post-annealed BSO:Si+substrate was found to decrease by at least two orders of magnitude compared with that made on as-implanted BSO:Si+ substrate. The contact or sheet resis-tance of the TLM diode on BSO:Si+ substrate is recovered after annealing. The evaluated contact resistance, sheet re-sistance, specific contact resistivity, and breakdown voltage of the Shottky diodes made on BSO:Si+ and BSO glass are listed in Table I. Furthermore, the breakdown voltage of the BSO:Si+diode increases as the annealing time increases. Af-ter annealing at 500◦C, the breakdown voltage of BSO:Si+ TLM diode with 2.5µm spacing further increases from 354 V to 380 V as annealing time increases from 30 to 120 min, which is close to the value of the same device made on BSO glass. It is thus believed that the carrier transport mecha-nism of the as-implanted BSO:Si+has been degraded due to
Table I. The characteristic parameters of TLM diodes made on different substrates. Sample Rc() Rs(/) ρc(·cm2) VBreakdown Glass 1.76 × 1014 2.25 × 1015 7.744 × 108 390 As-implanted 5.9 × 1010 8.625 × 1012 2.27 × 104 330 BSO:Si+ Annealed BSO:Si+for 1.01 × 1014 1.46 × 1015 3.93 × 108 354 30 min
L1382 Jpn. J. Appl. Phys. Vol. 41 (2002) Pt. 2, No. 12A G.-R. LIN the elimination of implanted-related defects during long-term
thermal annealing. As the annealing time increases to more than 60 min, the leakage current of the BSO:Si+is reduced to a value equivalent to or even smaller than that of glass. This could be interpreted as a result of the reduction in density of defects in the BSO:Si+after annealing, and thus the contribu-tion of sub-nm Si nanocrystals buried in the BSO:Si+sample is negligible.
In conclusion, we have primarily demonstrated the charac-terization of silicon-ion-implanted BSO glass substrate. Elec-trical properties such as leakage current and breakdown volt-age, as well as other structural characteristics, and the TLM diodes with different Schottky-contact spacings fabricated on the BSO:Si+ materials have been measured and discussed. The XRD analysis reveals the transformation of a nearly amorphous structure of as-implanted BSO:Si+into a slightly crystallite surface associated with two relatively small peaks positioned at 2θ = 29◦ and 14◦, which corresponds to the formation of sub-nm Si nanocrystallite with (111) orienta-tion and recrystallized (021)-oriented SiO2 phase of BSO.
The continuous-wave PL indicate a luminescent peak at about 520 nm (∼ 2.5 eV) under the pumping wavelength of 270 nm (∼ 4.6 eV). The decrease of PL intensity corroborates the elimination of radiative defects in the annealed BSO:Si+ sam-ple. The significant leakage current of the defect-assistant car-rier transport in BSO:Si+measured by TLM analysis reveals an overshooting and rectified characteristic for the BSO:Si+ diode. The overshooting voltage increases as the gap spac-ing between contacts of TLM diodes increases, however, it diminishes after long-term annealing. This suggests that the effect of defects on the resonant tunneling behavior of the BSO:Si+diode is more pronounced than that of Si nanocrys-tals. The leakage current of the TLM diode made on annealed BSO:Si+substrate is further decreased by at least two orders of magnitude more than that made on as-implanted substrate. The current–voltage and breakdown analysis confirm the dis-appearance of the negative differential resistance property in BSO:Si+after annealing at 500◦C for longer than 60 min.
This work was supported in part by the National Science Council (NSC) of the Republic of China under grants NSC 89-2215-E-027-007 and NSC 90-2215-E-027-008. Technical support of this work by the Optical Science Center at National Central University is also appreciated. The author thanks Mr. Chin-Chia Hsu for obtaining most of the experimental data.
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