Surface layers capable of supporting optical modes at 10.6 microns have been produced in n-type GaAs wafers through 300 keV proton implantation. The dominant mechanism for this effect appears to be free carrier compensation. Characterization of the implanted layers by analysis of infrared reflectivity spectra and synchronous coupling at 10.6 microns produced results in good agreement with elementary models. These results of sample characterization by infrared reflectivity and by CO2 laser waveguiding as implanted are presented and evaluated

Jenkinson, H. A.

Larson, D. C.

English

NASA Technical Reports Server

CO LASER WAVEGUIDING IN PROTON IMPLANTED GaAs 
H. A. Jenkinson 
2 
US Army ARRADCOM, Dover, NJ 07801 
D. C.  Larson 
Drexel University, Philadelphia, PA 19104 
SUMMARY 
Surface layers capable of supporting optical modes at 10.6 microns have been 
produced in n-type GaAs wafers through 300 keV proton implantation. The dominant 
mechanism for this effect appears to be free carrier compensation. Characteriza- 
tion of the implanted layers by analysis of infrared reflectivity spectra and 
synchronous coupling at 10.6 microns produce results in good agreement with elemen- 
tary models. 
INTRODUCTION 
Integrated optical circuits have vast potential for use as sensing and signal 
processing elements in future Army fire control systems. One of the requirements 
for these systems is 24  hourfall weather operation. In  order to meet this require- 
ment, the Army has been emphasizing systems which operate at longer wavelengths, in 
particular, at the 10.6 micron CO laser line. Such systems, including CO laser 
radars, miss-distance sensors, an8 sensors for smart munitions, form a natural area 
of application for infrared integrated optical circuits. These circuits could be 
used to perform various functions including phased array transmission, heterodyne 
detection, parallel processing and optical correlation. 
2 
Methods which have been used to fabricate optical waveguides and components 
include thin film deposition, epitaxial growth, ion diffusion, and ion implantation. 
(ref. 1) The latter method is a particularly interesting technique that can pro- 
duce electronic, chemical, and optical changes. In crystalline semiconductor 
materials, ion implantation can alter the refractive index through various physical 
mechanisms such as damage, doping, stress, and free carrier compensation. However, 
it is possible, by careful design of the implantation conditions to accentuate one 
of these mechanisms and to minimize the effects of the others. The present exper- 
iments are centered on free carrier compensation of highly doped n-type GaAs through 
medium energy proton implantation. Although this technique has been used to produce 
optical waveguides for use in the near infrared (ref. 2), little work has been done 
to investigate this effect for use at 10.6 microns. 
This phenomenon can be illustrated by a simple dielectric model in which the 
dielectric function of GaAs is composed of Lorentz oscillator terms representing 
the lattice contribution (ref. 3) and a Drude term representing the depression of 
the dielectric function due to the presence of free carriers. (ref. 4 )  In the 
spectral region between the band gap and the IR phonon resonance, the dispersion 
due to the lattice contribution is much less than that due to the plasma for heavily 
doped material. Thus,to good approximation, the dielectric function for these 
23 1 
https://ntrs.nasa.gov/search.jsp?R=19820008039 2020-03-21T09:52:27+00:00Z
materials may be written, in terms of wavenumber 0, as 
where 
related to the carrier concentration N through 
is the high frequency dielectric constant of GaAs, taken to be 10.7. o2 is P 
Ne2 
where e is the electronic charge, m:k = .08m is the effective mass 
of electrons in GaAs, and is the vacuum permittivity. The damping 
factor, g, leads to losses and is related to the mobility, 1-1, of the material 
through 
where c is the speed 
centers are created in 
raising the refractive 
index is approximately 
of light. During ion implantation, electronic trapping 
the lattice which compensate the free carriers, eff.ectively 
index. Neglecting losses, the increase in the refractive 
(4) 
2 2 where OP,S is OP evaluated using the original carrier concentration Ns before 
implantation, and Nf 
implantation. 
is the carrier concentration remaining in the layer after 
Polished (100) wafers of silicon doped GaAs were used in these experiments. The 
These wafers were implanted free carrier concentration was approximately 4 ~ 1 O ~ ~ / c m ~ .
at room temperature with 300 keV protons to fluence levels from 1013 to 10 
The implantation was performed at the Naval Research Laboratory. According to the 
LSS projected range calculations, the compensated region is expected to be sharply 
defined at a depth of approximately 3 microns. (ref. 5) Such a compensation profile 
would be unsuitable for optical waveguiding. However, it is experimentally observed 
that the implantation produces a damage layer with more uniform optical properties 
extending from near the surface to the projected range. 
16 /cm2. 
INFRARED REFLECTIVITY 
Prior to waveguiding experiments the optical properties of the implanted sam- 
232  
ples were characterized by infrared reflectivity measurements. Differential reflec- 
tivity measurements, made over the spectral range of 4000 cm-1 to 800 cm-1, yielded 
interference fringes typical of a thin film/substrate structure, as illustrated in 
figure 1. The interesting features of these curves are the periodicity of the 
fringes and their increase in amplitude as the measurement progresses to shorter 
wavenumbers. A first-order model has been developed to aid in deducing the compen- 
sation and the thickness of the implanted layer from the reflectivity spectrum. 
Basically, losses are neglected and the implanted layer is considered to be a 
film of refractive index nf sitting atop a substrate whose index has been de- 
pressed a small amount An by its higher concentration of free carriers. The refer- 
ence sample is considered identical to the substrate. Expanding the relevant 
reflectivity equations in a McLaurin series expansion to first order in An and 
using equation (4) yields 
From this analysis, it can be shown that the thickness of the implanted layer can be 
determined by the separation S of the fringe minima, and the compensation X by the 
fringe amplitudes : 
nf (nZ-l)o2 
t = (2Snf)-l 7 x = 1 - N~/N, = c R ~ ,  max -1) (6) 
4o;, s 
The carrier concentration of the substrate may also be determined optically by 
measuring the reflectivity of an unimplanted sample and noting the wavenumber at 
which the reflectivity dips t o  a minimum at the edge of the plasma resonance. (ref. 
6) At this wavenumber, the dielectric function of the film is very nearly 1. Again 
neglecting losses, the equation expressing this condition is easily solved for the 
carrier concentration: 
The more heavily doped the sample is, the more the minimum moves to longer wave- 
numbers. 
This difference in wavenumber is easily resolved by a spectrophotometer. 
This technique is sensitive for doping levels on the order of 1018/cm3. 
For example, when N=2x10 18 /cm 3 , om=480 cm-1;whereas for N = 4 ~ 1 0 ~ ~ / c m ~ ,  Om = 679 cm-l. 
The measurements described above were made on a Perkin-Elmer Model 180 double 
beam infrared spectrophotometer fitted with dual specular reflectance attachments. 
As shown in table 1, the compensated layers are roughly 3 microns thick. Although 
the LSS theory predicts about 1 micron of penetration for each 100 keV of acceler- 
ator energy, some dose dependence is observed. The low values of the standard 
deviation, ot, are due t o  the periodicity of  the fringes, which in turn indicates 
the ion-implanted layers can be modeled as dielectric slabs. The determination of 
the compensation from the reflectivity data is less straight forward. In general, 
the fringe amplitudes build up at a different rate than can be accounted for by the 
dielectric model, even in its exact formalism. The values reported represent the 
average value obtained. The fractional compensation was about 41% for the 1013/cm2 
implant and began to show saturation with a dose of 1014/cm2. 
evident in the samples implanted to 
are larger in this case, due to the larger uncertainties in the reflectivities and 
the anomaly in the rate of fringe growth. The last two columns of table 1 show the 
Saturation is clearly 
The standard deviations, ox, and 1016/cm2. 
233 
calculated carrier concentration of the films and the refractive indices at 10.6 
microns, respectively. The refractive index of the sample implanted to 1016/cm2 is 
very close to the literature value of 3.275 for undoped GaAs at 10.6 microns. (ref.7) 
INFRARED OPTICAL WAVEGUIDING 
Since the reflectivity spectra are interpretable in terms of a single dielectric 
filmlsubstrate structure, the equations for a planar asymmetric dielectric waveguide 
were used as the waveguide model: (ref. 8) 
where k=2.rr/Xo, B/k=nfsinBf and the phase angles are defined by: 
The subscript i refers to either the cover layer o or to the substrate s. Equation 
(8) is the familiar eigenvalue condition which must be satisfied for optical wave- 
guiding to occur and is expressed in terms of the ratio Blk. 
interpreted as the effective refractive index of the waveguide for each mode and is 
restricted to values between n and n 
This factor can be 
f’ S 
Infrared optical waveguiding was achieved in these samples using a 1 watt C02 
laser operating at 10.6 microns. The radiation was focused through an f-10 ZnSe 
lens on a dove shaped germanium prism which effected coupling into and out of the 
waveguide. The emerging beam, totally internally reflected from the base of the 
prism, was imaged as a spot on a thermographic screen. Waveguiding was identified 
by an absorption notch in the reflected spot, indicating that radiation was removed 
from the incident beam. Measurements of the angles at which synchronous coupling 
was achieved were made for both s and p polarizations of the incident beam. From 
these angles, the effective indices of the guided modes were determined. As two 
modes were found, it was possible to numerically invert the mode equations to obtain 
the refractive index and thickness of the implanted layer. 
The results of preliminary waveguiding measurements at 10.6 microns are shown 
in table 2. Due to experimental limitations discussed below, synchronous coupling 
was only achieved in the two more heavily doped samples, and in those only the 
0-order modes were observed although it is believed those samples will also support 
the TE1 mode. 
The thickness obtained for sample 0519-7 is considerably larger than that 
obtained by IR reflectivity, although the values obtained for the refractive 
index compare well. With sample 0519-4 the agreement is better. The thickness 
obtained from the waveguide measurement is closer to that obtained by IR reflectivity 
while the values obtained for the refractive index are identical. 
Figures 2 and 3 present these results in a different format. These are com- 
puter generated plots of the predicted mode structure for a planar asymmetric 
dielectric waveguide using as inputs for nf and n 
reflectivity spectra. The TE modes are indicated by the solid curves, while the TM 
modes are represented by the dashed ones. 
values obtained from the IR 
S 
234 
A horizontal line has been drawn on each figure at the thickness determined by 
the IR fringe analysis. The effective indices of the modes supported by each sample 
would be expected to be at the intersection of the mode curves with these lines, as 
indicated by the A symbols on the figures. The corresponding points obtained by the 
synchronous coupling measurements are shown by the + symbols. 
For sample 0519-4 the measured modes are both obtained at larger values of @/k 
than expected and their separation is smaller, implying they intersect the mode 
curves at a larger value of thickness. That the waveguide data points remained on 
the mode curves indicates that the values of the refractive index obtained for the 
film by both methods are in excellent agreement. 
Another point to be made from figure 2 is that regardless of which value is 
used for the guide thickness, the TE1 mode should be supported by this sample. 
germanium prism used in these experiments had an angle of 5 4  degrees which limited 
the lowest value of @/k obtainable to about 2 . 7 .  Another prism is being prepared so 
the entire range of B/k will be accessible. If the TE mode is observed, the extra 
data point will provide more information about the actual refractive index profile. 
The 
1 
The last feature to be noted from this figure is that the mode curves are 
bounded, in P/k, from ns to nf. 
samples is 2.44  and is determined by the initial doping level of the GaAs. The 
refractive index of the film is determined by the carrier concentration remaining 
after implantation. It must therefore lie somewhere between 2.44 and 3 . 2 7 5 ,  the 
value for undoped GaAs. Thus as the implant dose is decreased, the mode curves are 
all squeezed to the left as the upper limit on B/k decreases. 
The refractive index of the substrate for all the 
Figure 3 presents the same data for sample 0519-7 .  The spacing between the 
TE and TM modes indicates the layer is considerably thicker than predicted by 
IROreflectivity. That the mode curves are displaced slightly to the left of the 
theoretical curves illustrates the lower value of nf obtained by the synchronous 
coupling measurements. This sample should also support the TE mode and possibly 
the TM1 mode. 
0 
1 
DISCUSS ION 
Several conclusions can be deduced from these results. The first of these is 
that proton implantation done at the moderate energy of 300 keV produces a damage 
layer which will support optical waveguiding at 10.6 microns. Although these layers 
are very thin relative to the wavelength used, the index change produced is so large 
that the waveguiding condition is easily satisfied. This layer also appears to have 
very uniform optical properties since empirical results are explainable in terms of 
elementary models. 
There is also very good agreement between the IR reflectivity results and the 
coupling measurements, indicating the experimentally easier reflectivity analysis 
has considerable predictive utility. Although it is not fully apparent from the 
data presented here, it has been found that these are probably complimentary 
techniques: IR reflectivity appears to be more sensitive to the thickness of the 
layer than to the refractive index while the coupling measurements yield greater 
uncertainty in the thickness of the layer and very small standard deviations in the 
refractive index. It is expected that the refractive index profile of ion-implanted 
waveguides will be shown to have a transition region between the guiding layer and 
the substrate rather than a sharp discontinuity as assumed in the models presented 
235 
here. The complimentarity of the measurement techniques, as well as the systematic 
differences in the experimental results, may be explained by the different way light 
experiences the transition region in each of these techniques. 
Finally it is noted that the effective indices of the guided modes are strongly 
dependent on dose, which is easily controlled. This suggests that structures re- 
quiring different velocities of propagation, such as delay lines, can be fabricated 
simultaneously on the same substrate through proper control of the ion beam. 
The measurements reported here were made on as-implanted samples. While 
current work is directed at completing these coupling experiments, the question of 
losses and post implantation processing to minimize them will be addressed. 
236 
REFERENCES 
1. 
2 .  
3 .  
4. 
5.  
6 .  
7 .  
8 .  
B a r n o s k i ,  M. K .  : I n t r o d u c t i o n  t o  I n t e g r a t e d  O p t i c s .  Plenum, NY , 1974.  
G a r m i r e ,  E . ,  S t o l l ,  H . ,  Yariv,  A . ,  and Elunsperger ,  R .  G . :  O p t i c a l  Waveguiding i n  
P ro ton - Implan ted  G a A s .  Appl .  Phys .  L e t t . ,  v o l .  21 ,  no .  3, 1 August 1972 ,  
pp.  87-88. 
Kacha re ,  A. H . ,  S p i t z e r ,  W .  G . ,  F r e d e r i c k s o n ,  3 .  E . ,  and  Euler ,  F.  K . :  Measure-  
ments  o f  Layer  T h i c k n e s s e s  and  R e f r a c t i v e  I n d i c e s  i n  High-Energy Ion - Implan ted  
G a A s  and  Gap. J .  Appl .  P h y s . ,  vox .  47 ,  no .  12, Dec. 1976 ,  pp .  5374-5381. 
Zavada ,  J .  M . ,  J e n k i n s o n ,  H .  A . ,  and G a v a n i s ,  T .  G . :  O p t i c a l  P r o p e r t i e s  o f  
P r o t o n  I m p l a n t e d  n - type  G a A s .  SPIE P r o c e e d i n g s ,  V o l .  276,  1981,  pp .  104-108. 
Gibbons ,  J .  F . ,  Johnson ,  W. S . ,  and M y l r o i e ,  S .  W . :  P r o j e c t e d  Range S t a t i s t i c s .  
Dowden, Hu tch inson  & Ross ,  I n c . ,  S t r o u d s b u r g ,  1975.  
Go ldsmi th ,  N . ,  and O s h i n s k y ,  W . :  Vapor-Phase S y n t h e s i s  and  E p i t a x i a l  Growth of 
G a l l i u m  A r s e n i d e .  RCA R e v i e w ,  v o l .  24,  Dec. 1963 ,  pp.  546-554. 
Cheo, P.  K . ,  and Wagner, R . :  I n f r a r e d  E l e c t r o o p t i c  Waveguides .  I E E E  J .  Quan t .  
E l e c t . ,  v o l .  QE-13, no .  4 ,  A p r i l  1977 ,  pp .  159-164. 
T i e n ,  P .  K . :  L i g h t  Waves i n  T h i n  F i lms  and I n t e g r a t e d  O p t i c s .  App l i ed  O p t i c s ,  
v o l .  10 ,  no .  11, Nov. 1971 ,  pp. 2395-2413. 
237 
TABLE 1.-CHARACTERIZATION OF SAMPLES BY INFRARED REFLECTIVITY 
AS-IMPLANTED: 300 keV H+ @ ROOM TEMPERATURE 
n 
(@10f6p) X Nf3 
SAMPLE DOSE t 0 X 0 
t 1 / cm2 um vm llcm 
0519-1 1013 2.64 .06 .41 .10 2.2 x 1018 2.79 
05 19-5 1 0 1 ~  3.05 .07 .57 .22 1.8 x 2.94 
0519-7 1015 3.35 .02 .93 .12 2.8 1 0 1 ~  3.22 
0519-4 1016 3.31 .01 .98 .19 6.8 x 3.26 
SUBSTRATE 3.8 x 2.44 
TABLE 2.-CHARACTERIZATION OF SAMPLES BY C02 LASER WAVEGUIDING 
AS-IMPLANTED: 300 keV H+ @ ROOM TEMPERATURE 
f SAMPLE DOSE MODE B /k t n 
1 I cm2 um 
3.071 
3.026 
0519-7 1015 4.54 3.20 TEO 
TMO 
3.087 
3.014 
05 19-4 1016 3.71 3.26 TEo 
0 
TM 
2 3 8  
0 
0 co 
0 
0 
N 
r 
0 
0 
2 
0 
0 
0 
N 
0 
0 
0 
Yl 
0 
0 
D 
rl. 
239 
0 I I I I 1 I I I I I I 
2.4 2.6 2 . 8  3.0 3.2 3 .4  
I 
EFFECTIVE INDEX, B/k 
I I I I I I 
Figure  2.- Comparison of waveguiding and r e f l e c t i v i t y  r e s u l t s  f o r  sample G a A s  
2 0519-4(1016 H+/cm ) .  Curves and p o i n t s  i n d i c a t e d  by t h e  symbol A are 
p r e d i c t i o n s  b a s e d  on I R  f r i n g e  a n a l y s i s .  R e s u l t s  of waveguiding exper iments  
are i n d i c a t e d  by t h e  + symbol. 
SAMPLE: 0519-7  
3 
h 
E 
3. 
v 
tn 
G 5  
E 
V 
H 
x 
E 
2 . 4  2.6 2.8 3.0 3 .2  3.4 
EFFECTIVE INDEX, @/k 
Figure  3 . -  Comparison of waveguiding and r e f l e c t i v i t y  r e s u l t s  f o r  sample G a A s  
15 + 2 0519-7(10 H / c m  ) . Curves and p o i n t s  i n d i c a t e d  by t h e  symbol A are 
p r e d i c t i o n s  b a s e d  on I R  f r i n g e  a n a l y s i s .  R e s u l t s  of waveguiding exper iments  
are i n d i c a t e d  by t h e  + symbol. 
240 


CO2 laser waveguiding in proton implanted GaAs

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