Table of Contents
CHAPTER: ION IMPLANTATION and SECONDARY ION MASS SPECTROMETRY of
COMPOUND SEMICONDUCTOR MATERIALS and DEVICES
Fig. 1. K implanted in Si
Fig. 2. Implanted Se isotope distribution
Fig. 3. Implanted GaAs MESFET: Be & Si
Fig. 4. B implant-isolated device
Fig. 5. 1% As stoichiometry difference
Fig. 6. Generic HBT: Si emitter Be base
Fig. 7. Redistributed Be base profiles
Fig. 8. Zn-doped HBT base
Fig. 9. C-doped HBT base
Fig. 10. Reverse polarity HBT
Fig. 11. Stacked device schematic
Fig. 12. SIMS profile of Fig. 11 structure
Fig. 13. Si in Fig. 11 device structure
Fig. 14. Be & Al in Fig. 11 device
Fig. 15. Different stacked device structure
Fig. 16. Be doping spikes in GaAs
Fig. 17. Si spikes in InGaAs
Fig. 18. Si spikes in a GaAs/AlGaAs structure
Fig. 19. Si profiles vs composition of a HEMT Schottky barrier
Fig. 20. Be & Si in a AlGaAs nipi structure
Fig. 21. Laser diode with graded Be
Fig. 22. Laser diode with Zn & Si
Fig. 23. Laser diode with graded Al
Fig. 24. InP/GaAs laser diode with graded As
Fig. 25. Undisordered superlattice
Fig. 26. Disordered superlattice (surface)
Fig. 27. In doping steps in HgCdTe by MBE
Fig. 28. As in Fig. 27 but temperature too high
Fig. 29. p/n structure in HgCdTe
Fig. 30. n/p/n structure in HgCdTe
Fig. 31. n/p/n structure in HgCdTe
Fig. 32. Impurities at interface in device structure
Fig. 33. Impurities in a quantum well
Fig. 34. HEMT metalization
Fig. 35. HEMT metalization
Fig. 36. HEMT metalization
Fig. 37. H in p- & n-GaAs with and without bias [Electron. Lett. 31, 496 (19950]
Fig. 38. Implanted N in ZnSe
Fig. 39. Implanted Cl in ZnSe
Fig. 40. Implanted Si in ZnSe
Fig. 41. Focused ion beam (FIB) implants
CHAPTER: ION IMPLANTATION and SECONDARY ION MASS SPECTROMETRY of
COMPOUND SEMICONDUCTOR MATERIALS and DEVICES
Ion implantation and secondary ion mass spectrometry (SIMS) are well established
technologies practiced in many laboratories around the world. Their application varies in
respect to the type of equipment and details of the practice of the two technologies.
Ion implantation is important to secondary ion mass spectrometry (SIMS) because it
provides SIMS with quantification standards. SIMS is important to ion implantation because it
provides isotope information for the SIMS quantification standards and depth profiles for all
elements in as-implanted and thermally processed materials, and for all elements as
intentional or unintentional impurities in all solid materials. SIMS is important to compound
semiconductor materials because it provides very sensitive quantitative analyses of impurities,
of layer structure, of doping densities, and of redistribution of dopants and impurities during
processing. This paper illustrates the current status of these technologies and experimental
results as examples of the two technologies.
The equipment and practice of the technology of SIMS used in this work are described
in reference 1. Implantation work was performed using a 400-kV custom ion mass
spectrometer used as an implanter. Mass separation is performed only on fully accelerated
ion beams, which are subsequently electrostatically scanned to produce uniform implantation,
and with crystalline samples tilted 7° to the beam axis to minimize channeling. Elements to be
implanted were ionized in an electron bombardment ion source described in reference 2. The
source materials used were the pure elements for elements whose vapor pressures are high
enough at temperatures up to about 800°C. Other elements were introduced into the source
as fluoride gases or by heating their chlorides to temperatures up to 800°C, for example the
rare earth elements. Annealing was carried out in a horizontal tube under flowing dry
nitrogen.
SIMS ISSUES
A brief description of SIMS: Secondary ion mass spectrometry is a sensitive analytical
technique (sputtering) that can detect all elements in all solid materials, in many cases with
detection limits from 1013 to 1016 cm-3, and to depths of many micrometers, which is
compatible with the depths of ion implantation and the thicknesses of electronics and
optoelectronics devices and other microelectronics structures. Mixing of atoms in the dynamic
surface being sputtered by the incident energetic ions and the equilibration/stabilization
time/depth are issues that must always be addressed. SIMS technology as used in this work
is described in detail in reference 1.
Depth resolution in SIMS profiles is important for defining the structure of sharp
interfaces and superlattices. Several factors affect depth resolution. Interface broadening is
caused by surface topography and nonuniformity of layer thickness, and by ion mixing, which
is caused by the penetration of the primary sputtering ions. If the layer thicknesses are
uniform and the surface topography is good, then the SIMS experimental conditions become
the determining factors. The mixing thickness decreases with decreasing primary ion energy
and increasing angle of incidence (to a point). Reducing the primary ion energy is necessary
to achieve the best depth resolution. Of the two most commonly employed primary ion
species, oxygen and cesium, oxygen produces less ion mixing. The lowest practical oxygen
ion bombardment energy is then used to produce the best depth resolution. For quadrupole
instruments this energy may be 1 keV or less. For CAMECA sector magnet instruments, this
energy is often 1.5 keV/O (3 keV for O2). The data shown here for superlattices was obtained
using this experimental condition. Another factor in achieving good depth resolution is
sputtering rate combined with secondary ion collection time. The sputtering rate and the
collection time must be small enough to define enough data points to define a layer or
interface accurately.
Often information is desired from SIMS profiling of unwanted impurity species
(elements) in various materials. Some of the common impurity species are from the ambient
vacuum or heated components of materials growth machines. The same ambient vacuum
species exist in a SIMS instrument. The lower the ambient vacuum, the lower the sputtered
secondary ion intensities of these species. The higher the sputtering rate during SIMS
profiling, the lower the adsorbed density of these ambient species and the lower their
sputtered secondary ion intensities. Thus, for the lowest backgrounds of these species, or
the best detection limits, the lowest practical vacuum and the highest practical sputtering rate
should be employed. It is noted that from the last issue discussed in the preceding paragraph,
that improved background and improved depth resolution cannot be achieved simultaneously
because they vary in opposite dependence on sputtering rate. Often separate profiles must
be measured at very different sputtering rates to achieve depth profiles with good depth
resolution and with good detection limits or backgrounds for certain species (elements).
These elements include H, C, N, O, Si (N2 and CO can produce signals that interfere with Si),
and all elements that may have a molecular interference when any of these elements are
combined with the masses of the matrix materials, which may be dozens of masses in some
cases.
The sputtering rate in SIMS depth profiling of multilayer/multimaterial structures is again
an important issue. For fixed SIMS profiling conditions, the sputtering rates of all materials are
different. Thus, the sputtering rates change whenever an interface between different
materials is crossed. The depth scale of a SIMS profile is usually obtained by measuring the
crater depth at the end of each profile. This depth divided by the sputtering time yields the
average sputtering rate. If this rate is applied uniformly to the profile, inaccurate layer
thicknesses result if different materials are sputtered. To obtain accurate layer thickness, the
sputtering rates of each and every material in the structure must be measured or otherwise
known (from other work or published sputtering rates -- for similar sputtering conditions). Then
the total depth profile must be divided into layers of each different material and the appropriate
sputtering rate applied to each layer.
INTERRELATIONSHIP BETWEEN ION IMPLANTATION AND SIMS
Figure 1 illustrates the combination of these two technologies via an implanted depth
distribution measured using SIMS -- for a SIMS quantification standard for K implanted in Si.
A quantification measure for SIMS called the relative sensitivity factor (RSF) is established by
setting the integral under the depth profile equal to the accurately measured fluence of the
implant [time integral of the current density (adjusted for the charge on the ion)]. This RSF
can then be used at any later time to quantify doping densities by measuring the ratio of the
intensity of the secondary impurity ions to the intensity of the same matrix ion for which the
RSF was measured,1 the matrix being one of the elements in any selected compound
semiconductor material in this case. Ion implanted standards were used to quantify the SIMS
depth profiles shown below to illustrate various applications in III-V and II-VI device and
materials technologies.
Figure 2 illustrates the use of SIMS to measure the distribution of the total implanted
fluence among isotopes of an implanted element when that element has several isotopes, and
the masses are greater than about 50 m/z. In the case of Fig. 2, the intended implanted mass
was 80Se, and the depth profile shows that about 93 % of the fluence was 80Se, 5 % was
78Se, and 2 % was 82Se. Quantification should be done by assigning 93% of the recorded
fluence to the RSF measured using 80Se.
DOPING USING ION IMPLANTATION
Ion implantation is used to dope a few specific kinds of III-V devices only, not many.
One class of devices that uses ion implantation doping extensively is MESFETs, especially in
GaAs. Zolper3 recently described the use of ion implantation in the fabrication of high speed
GaAs JFETs. Because of the statistical stopping process, the width of implantation-doped
regions is too great for devices that require narrow (thin) regions, such as HBTs and HEMTs.
A second issue is the difficulty of removing all of the damage created by the implantation
process from these materials, some of which cannot be raised to sufficiently high
temperatures. Implantation is generally not used to dope HgCdTe, an important narrow gap IIVI material (infrared detectors) because of the inability to remove the damage because of the
low melting temperature of this material. Implantation is sometimes used to dope ZnSe, a
wide gap II-VI material, which can more readily be annealed. Implantation is used to create
isolation regions in many devices, where selected area isolation is critical and
masking/lithography can be utilized in planar processing. Elements used for isolation include
H, He, B, O, F, and Ar. When implantation doping is employed, several energies are often
used to tailor the doping profile in depth and density. The Monte Carlo TRIM program4 can be
used to design appropriate implantation schedules. Measurements of electrical performance
parameters can be used to refine these implantation parameters and to tailor specific device
performance.
Figure 3 illustrates the application of implantation to the doping of GaAs MESFET
devices and the depth profiling of dopant impurities in GaAs via depth distributions for Si and
Be measured in the same depth profile using SIMS, for Cs primary ion bombardment and
negative secondary ions (Cs SIMS hereafter). In this generic example, Si is introduced to
create an n-type doped region at the surface, and a low density of the p-type compensating
dopant Be is placed on the tail of the Si depth distribution to cause a sharper n-type profile.
DEVICE ISOLATION USING ION IMPLANTATION
Many semiconductor devices are laterally isolated using broad area implantation and
masks fabricated using optical or electron beam lithography. The materials structures in these
devices may be grown using MBE or MOCVD or any one of various other techniques. Any of
a variety of elements that are not dopants may be implanted, which include H, for deep
isolation, and He, B, O, F, Ar, for shallower isolation, etc. Figure 4 shows a B depth profile
measured using SIMS in a commercial device (HBT) that is isolated using a B implant
processing step. The device is isolated from the surface down through the base.
MEASUREMENT OF STIOCHIOMETRY USING SIMS
SIMS can be used to measure 1% variations in stoichiometry among layers of a
material. Figure 5 illustrates the measurement of a 1% higher As concentration (intentionally
grown) in the central layer of a three-layer structure of GaAs grown using molecular beam
epitaxy (MBE) and measured using SIMS. This measurement demonstrates that a 1%
variation in As stoichiometry can be detected by measuring the As intensity through a device
structure. Statistically, the intensity of the As signal must be greater than 10 5 cts/s so that a
1% variation is statistically significant (n0.5/n<<1%).
DOPANT REDISTRIBUTION
SIMS can be used to measure the movement of dopants during growth and
redistribution of dopants during processing, for example Be and Zn. Figure 6 shows the depth
distributions of the n- and p-type dopants, Si and Be, in a generic heterojunction bipolar
transistor (HBT) measured using Cs SIMS. Figure 7 shows a set of depth profiles that
illustrates the manner in which Be (HBT base) can have undesired depth distributions caused
by a combination of high MBE growth temperature and high Be doping density. Such
measurements are used to establish a combination of MBE growth temperature and doping
density that will produce the desired controlled depth profile for the p-type dopant Be (in
combination with other differing materials composition that may be grown immediately
adjacent to the Be-doped region). Because of this tendency for Be to have an undesired depth
distribution and the need for sophisticated growth techniques to be employed to "keep it in
place", other p-type dopants have been studied as alternatives, for example Zn and C. Zn is
found to exhibit the same properties as Be (illustrated in Fig. 8). Carbon is a more "stable"
dopant, but can be amphoteric (exhibiting both n- and p-type behavior) and is sometimes
difficult to introduce using MBE, but is also introduced using metalorganic chemical vapor
deposition (MOCVD). Figure 9 shows a profile for a C-doped base region. Sometimes the
reversed polarity of HBT devices is desired, in which case the locations of n- and p-type
dopants are reversed; Figure 10 shows such a case, in which Be and Si are reversed
compared with Fig. 6.
VERTICALLY STACKED DEVICES
A more sophisticated application of these technologies is the vertical stacking of more
than one device structure during MBE growth, and the measurement (verification) of the
materials structure and the presence, location, and density of the dopants in such complicated
structures. One example is the stacking of an HBT on a HEMT (high electron mobility
transistor) on an RTD (resonant tunneling device), as is illustrated in Fig. 11. Figure 12 shows
a SIMS depth profile of such a device. The clarity is improved in Fig. 13 where only the profile
of Si is shown (two isotopes), and in Fig. 14 where Be and Al are profiled. Figure 15 shows a
different kind of multi level device structure.
PROCESSING ECONOMICS VIA SIMS
Another significant application is verification of epilayer growth before investment of 30
to 80K$ in processing (otherwise only to learn too late that the structures were not grown
exactly according to specifications or that dopants redistributed during growth). By knowing
the relative sputtering rates of the differing materials used in any selected device structure, the
thicknesses of the various layers of materials as grown using any growth technique can be
measured in the same depth profile in which are measured the doping densities and depth
distributions. All of the profiles shown so far illustrate this kind of analysis. When the depth
profiles are not as designed, the particular growth run is scrapped and no further cost is
wasted on subsequent processing.
DOPING SPIKES
Another application of SIMS is the measurement of the areal density of doping spikes
(deltas) grown in semiconductor device structures. While the energy/momentum aspects of
the SIMS sputtering process "spread out" the atoms that may be present in a partial
monolayer of an impurity "grown in" during MBE growth, the integral of all of the atoms in that
narrow depth profile of a grown in doping "spike" is an accurate measure of the areal density
of that spike. Figures 16, 17, and 18 illustrate such measurements for Be and Si in GaAs,
InGaAs, or AlGaAs structures (Cs SIMS).
SCHOTTKY BARRIER VARIATION
A more specialized measurement is illustrated in Fig. 19, where the variation of the Si
doping profile is shown as a function of the Schottky barrier composition in a HEMT device
structure.
NIPIs
Figure 20 shows how both the Si and Be doped regions can be measured in
GaAs/AlGaAs "nipi" structures in one depth profile (Cs SIMS).
LASER DIODES
A series of SIMS depth profiles of laser diodes is shown in Figs. 21 through 24. In
these devices, the first thick layer is p-type, doped with Be (Fig. 21, in which the Be is seen to
be graded) or with Zn (Fig. 22), the second thick layer is n-type, doped with Si (Figs 22 and
23), and the central quantum well may have various compositions, including InGaAs as in Fig.
22 and Fig. 23 (in which the Al in AlGaAs is seen to be graded on both sides). The structure
may be InP with a GaAs well with graded As composition as shown in Fig. 24 (with a GaAs
cap layer).
SUPERLATTICES AND DISORDERING
Superlattice disordering can be studied using SIMS, as illustrated in Figs. 25 and 26.
Figure 25 shows the definition of 66 periods of an undisordered GaAs/AlGaAs superlattice in
0.72 mm of depth, or about 5.5 nm per layer. In Fig. 26, the effect of an intentional
disordering of a GaAs/AlGaAs superlattice near the surface is seen; the layer thickness in this
case is about 12 nm.
II-VI DEVICES
A few examples of narrow gap II-VI device and test structures are shown in Figs. 27-31.
Figure 27 shows an O2 SIMS profile of a growth test structure designed to demonstrate the
capability to vary in a controlled manner the doping density of In HgCdTe MBE epilayers.
Figure 28 illustrates the degradation that results when the growth parameters are not
optimized in a similar structure. Doping profiles measured using O 2 SIMS are shown in Figs.
29, 30, and 31, respectively, for n-p and for p-n-p structures of two relative thicknesses in
HgCdTe.
ZnSe is representative of wide bandgap II-VI materials, and has been implanted with a
variety of elements to attempt to achieve electrical and optical activation. One such study was
carried out with P. Lowen and K. Jones at Univ. FL, which included implantation with N and
with Cl as potential n and p-type dopants followed by rapid annealing. "Although good
acceptor and donor activation were realized optically, no significant electrical activity was
observed." was the conclusion of those studies. SIMS results showed that the depth profiles
of N and Cl do not change with annealing for up to 10 s at 500 and 700°C, respectively. No
redistribution of N was observed for a 700°C/20 min anneal. The diffusivity of N in ZnSe was
shown to be less than 5x10-17 cm2/s at 700°C, suggesting that N diffuses substitutionally and
not interstitially. Figures 38 and 39 show profiles for N and Cl implanted in ZnSe, measured
using Cs SIMS. Figure 40 shows the profile of 2-MeV Si implanted in ZnSe (Cs SIMS).
IMPURITY INCORPORATION
SIMS can be used to measure the unintentional incorporation of elements (impurities)
during the materials growth process or other device processing steps. Common examples of
such elements are H, C, N, O, alkalis, and elements that create deep traps that can reduce the
lifetime of minority carriers in device structures, such as transition metals. Figure 32 illustrates
the measurement of C and O at the interfaces between different layers used to fabricate
devices. Figure 33 shows the detection of undesired Fe in the well of a III-V device structure.
METALIZATIONS
Metalization layers on III-V devices can be studied for composition and interdiffusion
using SIMS depth profiling. Figure 34 shows a Au/Ni-Ge metalization on a thin GaAs device
layer. Figure 35 shows a similar structure where the Ga has diffused out through both the NiGe and the Au layers, and the Ni-Ge has diffused out into the Au layer. Figure 36 shows a
structure that is Ti-Pt//Au/Ni-Ge on a thin GaAs layer. The Ga has diffused out to some
degree and the Ti and Pt have diffused into the Ge-Au, but not into the GaAs. The deep sides
of the SIMS profiles are "smeared" by the dynamic sputtering process, and some experienced
interpretation is required.
PASSIVATION
Hydrogen can be used intentionally to passivate device structures, or it may be
introduced unintentionally during processing and cause undesired passivation of electrical
characteristics of devices. Figure 37 illustrates the former case in which hydrogen (as 2H to
enhance SIMS detection) was introduced from a plasma at 250°C under 0 and 150 V bias into
both p- and n-type GaAs. Significant differences in diffusivity are observed, SIMS profiles
showing penetration up to 15 mm in this case (Cs SIMS).
FOCUSED ION BEAMS (FIB)
The final illustration (Fig. 41, O2 SIMS) is for a focused ion beam (FIB) implant of Ga
into Si to show the effect of the usually used nominally normal angle of incidence for FIB
implants in comparison with a "standard" 7° tilt (between incident beam and the <100> of the
crystal) and a normal incidence (0° tilt) implant performed in a custom ion mass spectrometer
"implanter". This profile could relate to the use of FIB Ga ion beams in GaAs technology as
well. One application of FIB technology is to remove material in small and specific locations in
microelectronics circuitry, sometimes to cut a section of vertical and lateral structure for
analyses using other diagnostic techniques, and sometimes to reroute or repair circuitry.
III-NITRIDES
Ion implantation is currently being studied as a technique to dope and isolate devices
and structures in the wide bandgap, high temperature III-nitride compound semiconductors
(AlN, GaN, AlGaN, InAlN, and InGaN). This work is not discussed here because it is the
specific subject of another paper, which includes doping by rare earth elements for
optoelectronics.
REFERENCES
1. R.G. Wilson, F.A. Stevie, and C.W. Magee, Secondary Ion Mass Spectrometry
[Wiley, New York, 1989]
2. R.G. Wilson, Ion Mass Spectra [Wiley, New York, 1974]
3. C.C. Zolper, A.G. Baca, M.E. Sherwin, and R.J. Shul (Sandia National Laboratory), Paper
No. 768 at 188th Electrochemical Society Meeting in Reno NV, May 1995
4. To obtain a copy of the TRIM program, contact J.F. Ziegler, IBM Watson Lab, Yorktown
Heights, NY 10598.
Figure captions
Fig. 1. K implanted in Si
Fig. 2. Implanted Se isotope distribution
Fig. 3. Implanted GaAs MESFET: Be & Si
Fig. 4. B implant-isolated device
Fig. 5. 1% As stoichiometry difference
Fig. 6. Generic HBT: Si emitter Be base
Fig. 7. Redistributed Be base profiles
Fig. 8. Zn-doped HBT base
Fig. 9. C-doped HBT base
Fig. 10. Reverse polarity HBT
Fig. 11. Stacked device schematic
Fig. 12. SIMS profile of Fig. 11 structure
Fig. 13. Si in Fig. 11 device structure
Fig. 14. Be & Al in Fig. 11 device
Fig. 15. Different stacked device structure
Fig. 16. Be doping spikes in GaAs
Fig. 17. Si spikes in InGaAs
Fig. 18. Si spikes in a GaAs/AlGaAs structure
Fig. 19. Si profiles vs composition of a HEMT Schottky barrier
Fig. 20. Be & Si in a AlGaAs nipi structure
Fig. 21. Laser diode with graded Be
Fig. 22. Laser diode with Zn & Si
Fig. 23. Laser diode with graded Al
Fig. 24. InP/GaAs laser diode with graded As
Fig. 25. Undisordered superlattice
Fig. 26. Disordered superlattice (surface)
Fig. 27. In doping steps in HgCdTe by MBE
Fig. 28. As in Fig. 27 but temperature too high
Fig. 29. p/n structure in HgCdTe
Fig. 30. n/p/n structure in HgCdTe
Fig. 31. n/p/n structure in HgCdTe
Fig. 32. Impurities at interface in device structure
Fig. 33. Impurities in a quantum well
Fig. 34. HEMT metalization
Fig. 35. HEMT metalization
Fig. 36. HEMT metalization
Fig. 37. H in p- & n-GaAs with and without bias [Electron. Lett. 31, 496 (19950]
Fig. 38. Implanted N in ZnSe
Fig. 39. Implanted Cl in ZnSe
Fig. 40. Implanted Si in ZnSe
Fig. 41. Focused ion beam (FIB) implants
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