Gallium Arsenide (GaAs) - Courses

Gallium Phosphide (GaP) 447
data from Pikhtin and Yas'kov [15] because the n-type sample was monocrystalline,
not intentionally doped, with the lowest carrier concentration
(1.15 x 10 17 cm- 3 ). Besides, T spectra were obtained with a double-beam
spectrophotometer, and R was checked against accurate measurements of n.
Near V T the reported values (labeled kr) were obtained from Eq. (1).
As for the 4-23-pm range, Table IV contains experimental values of n
deduced from the positions of interference peaks in T [5,7J. The thickness
of the samples and the interference order were accurately determined by
comparing, in the 0.5-4-pm range, the results obtained with this method and
with the more precise results obtained by using the prism method. The data
from Abagyan et at. [5] and Pikhtin et at. [7] agree to within the 0.1 %, with
an error [7] of ±0.2% at 4 pm and of ±0.47% at 23 pm. The samples were
n type, undoped, and high resistivity (N < 5 x 10 16 cm - 3). Measurements
performed on low-resistivity samples, doped both n- and p-type up to 1.7 X
10 18 cm - 3 have shown that for )" :;::: 3-pm doping reduces n [5, 7]. For this
reason we have disregraded older data for n [22, 23] that are '" 2% lower.
The absorption coefficient between 1 and 12 pm is free-carrier-like; it typically
increases with wavelength and with free-carrier concentration. Besides,
all the n-type samples show a broad absorption band in the 1-4-pm region
that does not occur in p-type material. In this region there are many measurements
of a [8, 12, 15] on crystals doped with different impurities and
with carrier densities ranging from 10 17 to 10 18 cm - 3. We have tabulated
those from Pikhtin and Yas'kov [15] on the basis of the same reasons that
determined the choice in the 12-20-pffi region.
In the 0.53-4-pm range, where GaP is nearly transparent, n measurements
were carried out by the method of prism minimum deviation [7, 9, 13] and
by interference spacing [11] at 2.24 eV (0.5536 pm) and 2.62 eV (0.4733 pm).
The scattering of n data from different authors is within ± 0.0025. In Pikhtin
et at. [7] the absolute values of n were determined to within ±0.0025; in
Bond [9J n was measured to the fourth decimal. In Nelson and Turner [13J
the uncertainty in the individual value was ±0.0002; however, the value
obtained for different prisms differed by substantially more than this. The
differences are believed to be real and to reflect the variation that can be
expected from different samples of GaP. To include the value for all of the
prisms, an uncertainty of ±0.0012 would have to be assigned to the n values
from 0.545 to 0.70 pm in Table IV. These variations were not correlated with
the level of doping, which varied from no intentional doping to 10 18 cm - 3
with the type of dopant (Te, Sn, Zn) or the method of crystal growth (highpressure
boat growth, floating zone, or solution growth).
The intrinsic electronic absorption edge of single crystals was presented
and analyzed by Spitzer et al. [8] at 300 K, by Dean and Thomas [10J between
1.6 and 300 K, and by Pikhtin and Yas'kov [14J between 4.2 and
500 K. The results agree qualitatively, although there is some quantitative
difference. In Table IV we have reported in the 0.54-0.57-pm range the values
from Dean and Thomas [lOJ that are a little lower than those reported