Semiconductors, Vol. 39, No. 2, 2005, pp. 174–176. Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 39, No. 2, 2005, pp. 189–191. Original Russian Text Copyright © 2005 by Avakyants, Bokov, Chervyakov.
ELECTRONIC AND OPTICAL PROPERTIES OF SEMICONDUCTORS
Photoreflection Studies of the Dopant Activation in InP Implanted with Be+ Ions L. P. Avakyants^, P. Yu. Bokov, and A. V. Chervyakov Moscow State University, Vorob’evy gory, Moscow, 119992 Russia ^e-mail:
[email protected] Submitted May 17, 2004; accepted for publication May 24, 2004
Abstract—Photoreflection spectroscopy is used to study the activation of impurity in InP crystals implanted with 100-keV Be+ ions at a dose of 1013 cm–2 and then subjected to thermal annealing for 10 s. After annealing at temperatures no higher than 400°C, lines characteristic of crystalline InP are not observed in the photoreflection spectrum, which indicates that the crystal lattice has become disordered as a result of the ion implantation. If the annealing temperature is in the range from 400 to 700°C, the lines related to the fundamental transition in InP (1.34 eV) and the transition between the conduction band and the subband, which has split off from the valence band owing to a spin–orbit interaction (1.44 eV), are observed in the spectrum, which indicates that the InP crystal structure is restored. The dopant is activated in samples annealed at 800°C, as indicated by the Franz– Keldysh oscillations observed in the corresponding photoreflection spectra. Free-carrier concentration is determined from the oscillation period and is found to be equal to 2.2 × 1016 cm–3. © 2005 Pleiades Publishing, Inc.
1. INTRODUCTION Indium phosphide (InP) is an important semiconductor material used in the fabrication of high-frequency field-effect transistors. It is valuable due to its high charge-carrier mobility and comparatively wide band gap (~1.35 eV at 300 K). Ion implantation and subsequent annealing is an efficient method of forming semiconductor layers with a specified dopant profile. The implantation of high-energy impurity ions into a crystal is accompanied by a disordering of the crystal lattice. In order to remove radiation defects and attain the electrical activation of impurities, one uses various types of annealing (conventional thermal, laser-related, and rapid thermal). Therefore, it is important to study the characteristics of ion-implanted layers before and after annealing with the aim of choosing annealing conditions that ensure the optimal activation of the impurity. In a number of studies (see, for example, [1]), the aforementioned characteristics have been studied using the spectroscopy of Raman scattering. This method provides data on both structural and electrical properties, since a longitudinal optical phonon is related to plasma oscillations in polar semiconductors. However, it is difficult to determine such an important parameter as the charge-carrier concentration from Raman scattering data on p-type semiconductors that include, in particular, InP doped with Be. This difficulty is caused by the weak dependence of the frequency of coupled phonon–plasmon modes on the charge-carrier concentration [2]. In this study, we use a contactless method of photoreflection spectroscopy to obtain data on the carrier concentration. This method provides a high sensitivity in studies of both the special features of the energy-band structure of a semiconductor and the built-
in electric fields whose magnitudes are controlled by the impurity distribution and concentration. 2. EXPERIMENTAL We studied samples of undoped n-InP (n ≈ 1016 cm–3) with a (100) surface orientation. After mechanical polishing and chemical etching, the semiconductor wafers were implanted with 100-keV beryllium ions at a dose of 1013 cm–2. The samples were then subjected to thermal annealing for 10 s at temperatures ranging from 300 to 800°C. The photoreflection spectra were measured using the system described previously in [3]. The spectral width of the monochromator slits amounted to 1 meV. The reflection was modulated using an He–Ne laser (the power was 10 mW and the wavelength, 632.8 nm) with a modulation frequency of 370 Hz. The spectra were measured at room temperature. 3. RESULTS AND DISCUSSION In Fig. 1, we show the photoreflection spectra of InP samples subjected to postimplantation thermal annealing at various temperatures. After annealing at temperatures from 400 to 700°C, the spectra included lines related to the fundamental transition in InP (Eg = 1.34 eV) and to the transition between the conduction band and the subband, which is split off from the valence band (Eg + ∆so = 1.44 eV) as a result of the spin–orbit interaction (see Fig. 2). As can be seen from Fig. 1, when the annealing temperature increases, the intensities of the observed lines also increase; simultaneously, the widths of the lines decrease. In addition, the line Eg shifts to lower energies. This effect can be related to restoration of the InP crystal structure.
1063-7826/05/3902-0174$26.00 © 2005 Pleiades Publishing, Inc.
∆R/R, arb. units
PHOTOREFLECTION STUDIES OF THE DOPANT ACTIVATION
Ec
f Eg
Eg + ∆so
Ed Ea
e
The photoreflection spectra of the samples annealed at temperatures from 400 to 700°C have a shape characteristic of a low-field case and can be described using the Aspnes formula [4] (1)
where A and ϕ are the amplitude and phase parameters, respectively; E is the photon-energy of the probe radiation; Ei is the energy position of the ith spectral feature; Γ is the phenomenological broadening parameter; and m is the parameter defined by the type of critical point and by the order of the derivative of the dielectric constant ε(E) with respect to energy. In the case under consideration, we have m = 2 [4]. We determined the main parameters of the spectral lines by approximating the low-field spectra with the sum of two expressions (1) for the lines Eg and Eg + ∆so, respectively. In Fig. 3, we show the position of the Eg line as a function of the annealing temperature. As can be seen, this line shifts to lower energies as the annealing temperature increases. Such shifts were previously observed [5] when studying the compensation of n-GaAs conductivity using an implantation of B+ ions. The observed shift of the Eg line cannot be related to either the dimensional effects or the stresses since each of the mentioned factors would make a corresponding contribution to the shift of the line Eg + ∆so (see Fig. 2). However, as the annealing temperature increases, a shift of the line Eg + ∆so is not observed. Apparently, the observed shift of the Eg line is caused by the appearance of acceptor levels near the valence-band top (Ea in Fig. 2) and to donor levels near the conduction-band bottom. In this case, the energy of the Eg transition should decrease by the acceptor-activation energy Ea as a result of the appearance of the acceptor levels. At the Vol. 39
No. 2
2005
Evl
Fig. 2. A schematic representation of the InP energy-band structure in the vicinity of the Γ point: Ec stands for the conduction band; Evh, for the heavy-hole subband of the valence band; Evl, for the light-hole subband of the valence band; ∆so, for the subband split off from the valence band owing to the spin–orbit interaction; and Ea and Ed are the acceptor and donor levels, respectively.
Fig. 1. The photoreflection spectra of InP ion-implanted with Be+ at a dose of 1013 cm–2 after subsequent 10-s thermal annealing at temperatures of (a) 300, (b) 400, (c) 500, (d) 600, (e) 700, and (f) 800°C.
SEMICONDUCTORS
Evh
∆ so
1.50 1.55 1.60 E, eV
∆R iϕ –m ------- ( E ) = Re [ Ae ( E – E i + Γ ) ], R
Eg
Eg + ∆ so
d c b a 1.25 1.30 1.35 1.40 1.45
175
same time, the appearance of these levels should not affect the energy of the transition Eg + ∆so. The Franz–Keldysh oscillations are observed near the fundamental-absorption edge in the photoreflection spectra of the sample annealed at a temperature of 800°C; i.e., the spectrum acquires a shape characteristic of the so-called midfield conditions and is abruptly shifted to lower energies (see Fig. 3), which indicates that the impurity is activated. In order to describe the midfield spectra using the Franz–Keldysh oscillations, we used the approximation [4] 3/2 2 បω – E ∆R π(d – 1) ------- ∝ cos --- -------------------g + -------------------- , 3 បΩ R 4
Eg, eV 1.36 1.35 1.34 1.33 1.32 1.31 1.30
300
400
500 600 700 800 Annealing temperature, °C
Fig. 3. The annealing-temperature dependence of the position of the spectral line that corresponds to the fundamental transition in InP (Eg).
(2)
AVAKYANTS et al.
176
fundamental-transition energy Eg for the sample under consideration. We thus found that Es = 50 kV/cm and Eg = 1.310 eV. It is well known [7] that the magnitude of the builtin electric field is controlled by the surface region of the space charge and depends on the charge-carrier concentration n and the surface potential Vs; i.e.,
Ej, eV 1.52 1.50 1.48 1.46 1.44 1.42 1.40 1.38 1.36 1.34 1.32
2en ( V s – kT /e ) E s = -----------------------------------εε 0
1
2
3
4
6
5
7
8
9 Fj
4. CONCLUSION
where បω is the energy of the probe-radiation photons, Eg is the fundamental-transition energy, and បΩ is the electrooptical energy equal to 2
2
2 1/3
.
(3)
Here, µ is the reduced interband effective mass 1 1 1 --- = ------- + ------- , µ m *e m *h
(4)
where m *e and m *h are the effective masses of the electron in the conduction band and hole in the valence band, respectively; Es is the electric field built into the semiconductor; and d is the dimensionality of the critical point. We have d = 3 for direct band-to-band transitions in InP. In this situation, the positions of the extrema in the oscillations (បω)j are given by ( បω ) j = បΩ ( F j ) + E g ,
j = 1, 2, 3,
(5)
where j – 1/2 F j = 3π ---------------2
2/3
.
(7)
,
where εε0 = 12.4 × 8.85 × 10–14 F/cm, Vs = 0.64 V is the surface potential, and ε = 12.4 is the relative static dielectric constant of InP (001) (the values of ε and Vs are taken from [6]). In this case, the value of the built-in electric field calculated using formula (7) corresponds to a charge-carrier concentration of 2.2 × 1016 cm–3.
Fig. 4. The dependence of the positions of extrema Ej in the Franz–Keldysh oscillations on Fj (6) for a sample annealed at a temperature of 800°C. The circles represent the extrema of the Franz–Keldysh oscillations and the solid line corresponds to the result of processing using the least-squares method.
e Es ប បΩ = --------------- 8µ
1/2
(6)
In Fig. 4, we show the positions of the extrema of the Franz–Keldysh oscillations Fj in relation to Fj (6). It can be seen from Fig. 4 that the dependence Ej(Fj) is adequately approximated using a straight line, which is in good agreement with (5) and (6). By assuming that the interband effective mass µ is equal to 0.071me [6] for transitions between the conduction band and the heavy-hole subband of the valence band, we were able to determine the values of the built-in field Es and the
We used photoreflection spectroscopy to study the activation of impurity in InP implanted with 100-keV Be+ ions at a dose of 1013 cm–2. The observed variations in the photoreflection spectra (an increase in the intensity of the lines and a decrease in their width) can be related to the restoration of the InP crystal structure. The appearance of the Franz–Keldysh oscillations in the photoreflection spectra indicates that the impurity activation occurs as a result of annealing at a temperature of 800°C. The charge-carrier concentration determined from the period of the Franz–Keldysh oscillations was found to be equal to 2.2 × 1016 cm–3 for a dose of the ionimplanted beryllium ions equal to 1013 cm–2. The results obtained show that photoreflection spectroscopy can be used to estimate the charge-carrier concentration and optimize the conditions of annealing ion-implanted InP. REFERENCES 1. L. P. Avakyants, V. S. Gorelik, and E. D. Obraztsova, J. Mol. Struct. 219, 141 (1991). 2. M. Gargouri, B. Prevot, and C. Schwab, J. Appl. Phys. 62, 3902 (1987). 3. L. P. Avakyants, P. Yu. Bokov, I. P. Kazakov, and A. V. Chervyakov, Vestn. Mosk. Univ., Ser. 3: Fiz. Astron. 4, 48 (2002). 4. D. E. Aspnes, Surf. Sci. 37, 418 (1973). 5. L. P. Avakyants, V. S. Gorelik, A. B. Korshunov, et al., Kratk. Soobshch. Fiz., No. 2, 17 (1999). 6. P. J. Hughes, B. L. Weiss, and T. J. S. Hosea, J. Appl. Phys. 77, 6472 (1995). 7. R. N. Bhattacharya, H. Shen, P. Parayanthal, et al., Phys. Rev. B 37, 4044 (1988).
Translated by A. Spitsyn SEMICONDUCTORS
Vol. 39
No. 2
2005
