Title page for ETD etd-09102001-091811

Type of Document Dissertation
Author Songprakob, Wantana
URN etd-09102001-091811
Title Optical studies of highly-doped GaAs:C
Degree PhD
Department Physics
Advisory Committee
Advisor Name Title
Zallen, Richard H. Committee Chair
Di Ventra, Massimiliano Committee Member
Heflin, James R. Committee Member
Indebetouw, Guy J. Committee Member
Ritter, Alfred L. Committee Member
  • plasmon
  • intervalenceband transitions
  • Raman spectra
  • infrared spectroscopy
  • photoluminescence
Date of Defense 2001-08-31
Availability unrestricted
Infrared reflectivity and transmittance measurements (200=5000 cm^-1) were carried

out on heavily-doped GaAs:C films grown by molecular beam epitaxy. With increasing

carbon concentration, a broad reflectivity minimum develops in the

1000=3000 cm^-1 region and the one-phonon band near 270 cm^-1 rides on a progressively increasing

high-reflectivity background. An effective-plasmon/one-phonon dielectric

function with only two free parameters (plasma frequency ωp and damping constant

γp) gives a good description of the main features of the reflectivity spectra. The

dependence of effective plasma frequency on hole concentration p is linear. At each doping, the effective-plasmon damping

constant is large and corresponds to an optical hole mobility that is about half the

Hall mobility at that p. Secondary-ion mass spectroscopy and localized-vibrational-mode

measurements indicate that the Hall-effect-derived hole concentration is close

to the carbon concentration and that the Hall factor is close to unity, so that the Hall

mobility provides a good estimate of the actual dc mobility. Also, analysis shows that,

for our highly-doped samples, the observed dichotomy between the dc and infrared

mobilities is not a statistical-averaging artifact of the approximations involved in the

model. The explanation of the small infrared mobility resides in the influence of

intervalence band absorption on the effective-plasmon fit, which operationally defines

that mobility via the effective-plasmon damping.

The optical properties obtained with the use of the effective-plasmon model for

GaAs:C yield a phenomenological, approximate, overall picture of the infrared spectra.

But the neglect of intervalenceband transitions, for this p-type semiconductor,

is shown (in this dissertation) to be a serious drawback of this simple model. In

order to obtain the optical properties of GaAs:C in a model-independent way, and to

attempt to resolve the apparent dc/infrared mobility dichotomy, we made use of a

recently-developed spectroscopic-analysis procedure. Using direct numerical-solution

techniques for the reflectance (R) and transmittance (T) equations of a multilayer

structure, we analyzed our infrared R and T results for highly-doped films having

hole concentrations from 2×10^19 up to 1.4×10^20 cm^-3. The optical properties were

determined for photon energies from 0.07 to 0.6 eV, in which region plasmon (intraband)

and intervalenceband contributions are in competition. Our results for the

optical absorption coefficient resolve two separate peaks located (at high doping) at

about 0.1 and 0.2 eV. (The effective-plasmon model necessarily missed the two-peak

character of the actual absorption spectrum.)

By carrying out theoretical calculations of the intervalenceband (IVB) absorption

processes for our dopings, we identify the peak near 0.2 eV with light-hole

to heavy-hole IVB transitions, and we attribute the lower-energy peak to the hole

plasmon. Our experimental absorption spectra are very well described by a model

combining the intervalenceband contribution to the dielectric function with a plasmon

contribution. The hole-plasmon parameters omega_plasmon and gamma_plasmon that we obtain

for highly-doped p-GaAs yield an infrared mobility which (unlike the too-small IVB-entangled

infrared mobility implied by the use of the usual effective-plasmon model)

is in substantial agreement with the dc mobility. Therefore, in actuality, there is no

dc/infrared mobility discrepancy. The discrepancy implied by the use of the usual,

standard-operating-procedure, effective-plasmon model is a consequence of the inadequacy

of that model for p-type semiconductors exhibiting intervalenceband infrared


Raman-scattering measurements were carried out on the GaAs:C films. Only the

phononlike coupled plasmon-phonon mode is observed. The non-occurrence of the

plasmonlike mode is due to the large damping of the hole plasmon and the competition

with strong Raman scattering by intervalenceband transitions among the

heavy-hole, light-hole, and split-off bands. Analysis of the phononlike coupled mode,

within the framework of the wavevector-dependent Lindhard-Mermin dielectric function,

supports the hole properties that we determined by Hall and infrared studies.

Photoluminescence measurements showed that the split-off band also participates in

the photoluminescence of GaAs:C, giving rise to an above-bandgap emission band

corresponding to transitions from the conduction band to the split-off valence band.

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