Free Carrier Absorption in P-Type Epitaxial Si and GaAs Films for far-Infrared Detection
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Table 1:Characteristics of thin films. All samples are doped p type with Boron for Si and Beryllium for GaAs.
sample MT324F (Si) MT325F (Si) MT326F (Si) MT327F (Si) MT328F (Si) GSA (GaAs) GSB (GaAs) GSC (GaAs)
thickness (/Zm) 0.68 0.73 1.37 2.46 2.42 0.10 0.10 0.10
Eoncentration P (cm- 3 ) 5.5x1019 1.5x 1019 7.1x 101' 1.9x 1018 8.1 x 1017 1.0X 1018 5.0x 1018 2.Ox 1019
mobility /i (cm 2 /Vs) 29 42 55 110 126 -
similar to the high dopant level average values reported in the literature[7]. To determine the infrared absorption of the samples, the transmission and reflectance were measured using a Perkin-Elmer, system 2000, Fourier transform infrared spectrometer (FTIR) and a Si composite bolometer detector. The measurements were performed at room temperature with a resolution of 2 cm-1 and no changes are expected at low temperatures. Both the transmission and reflectance measurements were made using a normal incidence geometry with light incident on the doped layer surface. A 2.5 lm Mylar Pellicle film was used as a beamsplitter by placing it at 450 with the incident light in the reflectance measurements. THEORY The absorption (A) in thin films is determined from the transmission (T) and reflection (R) in conjunction with the expression A=I-T-R
(1)
and further subtraction of the absorption of the substrates. The dielectric constant of the thin films is derived from the frequency-dependent conductivity for free carriers by 1 Oiwr
(2)
where c0 is the dc conductivity and r is the relaxation time, which is independent of frequency w in the semiclassical transport theory. Since our main interest is in the FIR range (_Ž 50 lm), the other contributions, e.g., intervalence band transitions and lattice vibrations, have been ignored. Using the measured values of mobility A for Si samples in Table I, the relaxation time was determined by the relation u= er/m;, where mn=0.37m0 [5] is the heavy-hole effective mass in Si, m0 is the free electron mass, and e is the magnitude of the electron charge. For GaAs, the relaxation time 1.7x10-1 4 s is used based on the measured mobility of 60 cm 2 /Vs for similar doping level of 2.5 x 101s cm-3[6]. This means that no free parameters are used to fit the experimental data with the modeling results. The absorption depends not only on the real part of the refractive index, but also on its imaginary part, which is proportional to the absorption coefficient (a) defined by ae = 2kq
200
(3)
60-
Exp.
Theory
E
Experimental FIR Figure 1: free hole absorption in two pSi (MT325F and MT328F) and one GaAs (GSC) thin films at room temperature (solid curves). dashed curves are the the-
0)
105 (b/
100
0 ()MT324F
j -hkL
70 jThe
"oretical results
Z 0
40-
0 0 MT325F
_ ry
0 OJ)
MT328F
100 200 WAVELENGTH (tm) -
--
20-
M
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