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SEMI M19-91 © SEMI 1991, 1996 2 scattering , and other related ph enomena. Thus, based on producer ex perience, the m inimum value was s et at 3500 cm 2 /V-s. For n-type materials, the mobility ranges were selected to re…
SEMI M19-91 © SEMI 1991, 19961
SEMI M19-91
SPECIFICATION FOR ELECTRICAL PROPERTIES OF BULK GALLIUM
ARSENIDE SINGLE CRYSTAL SUBSTRATES
1 Introduction
For the specification, three principal types of material
were identified: semi-insulating, n-type, and p-type.
This encompasses the full range of conductivity
characteristics for Gallium Arsenide (GaAs). Section 3
considers subclasses of these characteristics defining
the species which may be used for producing the
conductivity type. For semi-insulating material, special
cases have been isolated.
Undoped, Grade A1 represents those materials which
are of high resistivity and stable following growth,
without necessitating any additional annealing or
processing. The Grade A2 material, “high purity”,
requires additional thermal processing following
growth to bring the resistivity to a level > 10
7
Ω-cm. At
this time, most producers have indicated that the
majority of the Grade A2 ingots have resistivity
characteristics which rise into the acceptable range after
an appropriate thermal cycle. However, this
phenomenon is sensitive to the details of the time-
temperature cycle, and thus, such an increase cannot be
guaranteed in every application. Thus, the label “Grade
A2” is used to denote this material.
Chromium doping produces a high resistivity material.
However, due to the rapid diffusion of Cr during
processing and the propensity for surface accumulation,
it is not as well suited to processing as grades 3.A.1 and
3.A.2, thus we assign this material “grade A”.
Iso-electric dopant additions do not appear to affect the
resistivity significantly, but rather, permit the reduction
of dislocation generation in the final product. Thus, this
material is classified Grade A. While In is the most
effective hardening agent, Al, P, and Sb are also viable
species. They are included for completeness and to
reduce the likelihood of document revision at a later
date. The Grade B specification encompasses those
materials where the impurity and point defect densities
are not quite in the proper balance, but the resistivity is
suitable for less-stringent applications.
For conducting material, the best characterized, non-
transition metal species have been included as n-type
dopants; for p-type, a broader range of species has been
included, as transition metals are useful acceptor
impurities.
Section 4 defines the resistivity and stability of semi-
insulating material as specified in Section 3. A stringent
limit was placed on the Grade A2 material to minimize
the likelihood that this material converts to low
resistivity upon annealing. The issue of surface
conversion in Cr-doped material was dealt with by
eliminating a requirement for n-type characteristics
following annealing.
The resistivity ranges for n-type conducting material
were determined from the carrier concentration table
(Section 5) and mobility values, using the analysis of
Walukiewicz et al, J. Appl. Phys. 50 (1979) 899. This
places a constraint on the permissible compensation
ratio, selected to be in the range of approximately 0.0 to
0.7. The specification was designed then to exclude
abnormally poor crystals. As limited data and demand
exist for p-type material, there are no ranges specified
at this writing. Should a significant body of data evolve,
this specification may be designed and balloted as
appropriate.
Section 5 deals with impurity concentrations, net
electron yields and implicitly, with point defect
concentrations. For semi-insulating material of high
quality, the impurities are, at the present time, not
reliably measurable (carbon is the notable exception).
Thus the “unspecified” nomenclature. For types A-3
and A-4, the determination of the amount of the
relevant impurity species is given to the appropriate
party, the user, and producer, respectively.
For n-type materials, the ranges were selected to define
a high purity regime (n ≤ 4 × 10
16
cm
-3
), an intermediate
regime where donor density is not more than ~10 times
the typical deep level density; the transition region
wherein deep levels are suppressed (range B-3); a
highly doped range (> 10 ppm net electron
concentration yield); and “saturation” doping range
where the crystal growth process and thermodynamics
determine the limits of impurity incorporation and
electron yield.
In Section 6, the electron mobility values and ranges are
stated. For semi-insulating materials, the values that
have been determined by consensus are 5000 cm
2
/V-s
for Grade A1 and 6000 cm
2
/V-s. In the interest of
harmony, and to prevent further debilitating
discussions, the authors and participants at the SEMI
meetings, and responses from producers have been used
to set this value at 5000 cm
2
/V-s. The producers
indicated that the Grade A2 material nearly always
exceeds 6000 cm
2
/V-s, and thus this value was adopted.
For chromium doped materials, the mobility is less
predictable and, therefore, is negotiated between the
user and producer. In-doped materials have consistently
lower mobilities, resulting from strain-effects,
SEMI M19-91 © SEMI 1991, 1996 2
scattering, and other related phenomena. Thus, based on
producer experience, the minimum value was set at
3500 cm
2
/V-s.
For n-type materials, the mobility ranges were selected
to represent compensation ratios in the range of
approximately 0.0 to 0.7 for a given net electron
concentration. P-type material is not yet characterized
sufficiently to warrant a specification.
2 Scope
This document specifies the characteristics and ranges
of electrical properties for bulk GaAs crystals and
substrate wafers. For high resistivity and n-type
conducting materials, the permissible growth conditions
or impurity species are stated; the electrical properties
corresponding to the appropriate range(s) are noted.
Due to the limited understanding, experience and
demand for p-type materials only guidelines are
provided in this specification at the present writing.
3 Conductivity Type
A. Semi-insulating, n-type
B. Conducting, n-type
C. Conducting, p-type
4 Dopant Species
A. 1. Undoped; Grade A1
2. Undoped; Grade A2
3. Chromium; Grade A
4. Isoelectronic impurity; Grade A Dopant
specified by producer: In, Al, P, Sb
5. Undoped; Grade B
B. Dopant specified by user: Si, S, Se, Te, Sn
C. Dopant specified by user: Zn, Cd, Be, Mn, Fe, Co,
Mg
NOTE See Sections 4 and 5 below for grade definition.
5 Resistivity at 300K
A. Semi-insulating, n-type*
B. 1. ≥ 1 × 10
7
Ω-cm, n-type before and after anneal
2. ≥ 5 × 10
6
Ω-cm becoming ≥ 1 × 10
7
Ω-cm after
anneal n-type before and after anneal
3. ≥ 1 × 10
7
Ω-cm, n-type before anneal
4. ≥ 1 × 10
7
Ω-cm, n-type before and after anneal
5. ≥ 5 × 10
6
Ω-cm, n-type before and after anneal
1 × 10
7
Ω-cm = 1.6 × 10
8
Ω/❐; 5 × 10
6
Ω-cm = 8 × 10
7
Ω/❐;
* 1 × 10
6
Ω-cm = 1.6 × 10
7
Ω/❐ based on 625 µm
wafer thickness
C. Conducting, n-type
D. 1. ≥ 0.0026 Ω-cm (at n – 4 × 10
16
cm
-3
; µ = 6000
cm
2
/V-s
2. ≤ 0.06 Ω-cm
3. ≤ 0.042 Ω-cm
4. ≤ 0.012 Ω-cm
5. < 5.2 10
-3
Ω-cm
6. to be determined between user and producer
NOTE Ranges overlap due to the interrelationship of ρ, n,
and µ.
E. Ranges to be determined between the user and
producer.
6 Free Carrier or Impurity Concentration
A. 1. Unspecified impurity concentration
2. Unspecified impurity concentration
3. Cr concentration range in atomic ppm specified
by user
4. Isoelectronic impurity concentration range in
atomic ppm specified by producer
5. Unspecified impurity concentration
6. To be determined between user and producer
B. 1. ≤ 4 × 10
16
cm
-3
2. > 4 × 10
16
– 1 × 10
17
cm
-3
3. > 1 × 10
7
– 5 × 10
17
cm
-3
4. > 5 × 10
7
– 3 × 10
18
cm
-3
5. > 3 × 10
18
cm
-3
C. Carrier concentration agreed upon between user and
producer
7 Electron Mobility, 300K (determined by Hall
Effect)
A. 1 ≥ 5000 cm
2
/V-s
2. ≥ 6000 cm
2
/V-s
SEMI M19-91 © SEMI 1991, 19963
3. Concentration dependent: to be agreed upon
between user and producer
4. ≥ 3500 cm
2
/V-s
5. Unspecified
B. 1. ≥ 4000 cm
2
/V-s
2. ≥ 2500 cm
2
/V-s
3. ≥1500 cm
2
/V-s
4. ≥ 1000 cm
2
/V-s
5. ≥ 400 cm
2
/V-s
6. to be determined between user and producer
C. To be agreed upon between user and producer
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