Information is given in viewgraph form. The advantages of the Si blocked impurity band (BIB) detector invented by M. D. Petroff and M. G. Stabelbroek are noted: smaller detection volume leading to a reduction of cosmic ray interference, extended wavelength response because of dopant wavefunction overlap, and photoconductive gain of unity. It is argued that the stated advantages of Si BIB detectors should be realizable for Ge BIB detectors. Information is given on detector development, subtrate choice and preparation, wafer polising, epitaxy, characterization of epi layers, and preliminary Ge BIB detector test results

Baumann, H.

Beeman, J. W.

Haller, E. E.

Hansen, W. L.

Luke, P. N.

Lutz, M.

Rossington, C. S.

Wu, I. C.

English

NASA Technical Reports Server

N90-21321
IR Detector Technology Workshop
Feb. 7 - 9, 1989
NASA Ames Research Center
GERMANIUM BLOCKED
IMPURITY BAND (BIB) DETECTORS
E. E. Hailer1,2
H. Baumannl,2
J. Beeman2
W. L. Hansen2
P. N.Luke2
M. Lutzl,2
C. S. Rossingtonl,2,3
I. C. Wul,2
1University of California at Berkeley
2Lawrence Berkeley Laboratory, UCB
3Present address: Hewlett-Packard, Opto-Electronic Division,
350 W. Trimble Rd., San Jose, CA 95131
85
https://ntrs.nasa.gov/search.jsp?R=19900012005 2020-03-19T23:18:04+00:00Z
L LT.F.,I tT 
1. Introduction
2. Ge BIB
3. Ge BIB Detector Development
3.1.Epitaxial Blocking Layer Devices
3.1.1. Ge epitaxy
3.1.2.Characterization of epi layers
3.1.3. Preliminary detector test results
3.2. Ion Implanted BIB Detectors
4. Conclusions
86
1. INTRODUCTION
• Extrinsic, photoconductive semiconductor detectors
cover the infrared spectrum from a few ktm up to 250
[tm.
• Photoconductors exhibit high responsivity and low noise
equivalent power.
• The Si blocked impurity band (BIB) detector invented
by M. D. Petroff and M. G. Stapelbroek has a number
of advantages over standard bulk photoconductors.
These include:
smaller detection volume leading to a reduction of
cosmic ray interference
extended wavelength response because of dopant
wavefunction overlap
- photoconductive gain of unity
87
2 •
• The success of Si BIB detectors has been a strong
incentive for the development of Ge BIB detectors.
• The advantages of Si BIB detectors stated above should,
in principle, be realizable for Ge BIB detectors.
• If Ge BIB detectors can be made to work out to 250 _tm
with high responsivity and sufficiently low dark current,
they could replace stressed Ge:Ga photoconductors.
• Can the dark current be reduced to acceptable levels?
88
/ on,act
Contact, I
41- Doped infrared-
active material,
~lmm thick.
14 -3
Ge:Ga 2x10 cm
Figure l(a). Schematic of conventional detector.
II
Contact
Contact
Pure epilayer,/
// _ ~ 5 _m thick.
i 16 -3
Ge:Ga 2x10 cm
Doped infrared-active
substrate, .,. 50 _m thick.
Figure l(b). Schematic of BIB detector.
89
t-
O
o--
E
0
i--
0
0
0
-2
20
18--
16-
14--
Contacts--..%
Infrared-active material
d
m
0.1 um
_._ mm v
Figure 2(a). Doping levels in a conventional Ge detector.
20-
O
.--
E 18
¢D
tj
E
O
(.,)
_16--
0
I14
Contacts
Infrared-active_
substrate
i
50 I_m _ 5 _:n
0.1 um
Figure 2 (b). Doping levels in a Ge BIB detector.
9O
p-_
I1I--I- 'i!
i
Doped Layer
p p-_
I :rll-- (a) Biased detector
4.
P
4. + +
U (b) Space charge, p,in detector
E
x
(c) Electric field. E, in detector
Potential Energy
Conduc_
+ I (d) PolentJa[ energy diagram
tar biased detector. ,shown with
photoionization of a hole from ai
J A- IM=_'_"_"_ neutral aceeptor in the impurily
, _".3,,,_, band to the valence band
Valence Band I
I_ x
Fig. 3. Schematics of space charge, electric field and potential energy for a
reverse biased p-type BIB detector.
91
3. Ge BIB DETECTOR DEVELOPMENT
3.1.Epitaxial Blocking Layer Devices
3.1.1. Ge epitaxy
• Whereas Si epitaxy techniques have been developed to a
very high degree of perfection, Ge epitaxy has been
attempted only on a few occasions.
• Ge chemistry is very different from Si chemistry.
• Ultra-pure Ge compounds [Ge(CH3)4, Ge(C2H5)4] are
being developed for III-V semiconductor technology.
They may be useful to Ge epitaxy.
92
Substrate choice and preparation
• We have used a number of different crystals with
various crystallographic orientations in the develop-
ment of Ge epitaxy:
n-type wafers (~1011 cm -3) are used for the electrical
characterization of the epitaxial layers which are
typically p-type because of residual copper contam-
ination (junction isolation).
p-type wafers (~10 is cm -3) are used for I-V compari-
son tests with conventional photoconductors.
p-type wafers (~2 x 1016 cm -3, low compensation)
are used for Ge BIB detectors.
93
• Wafer polishing process:
- mechanical planar lapping with alumina slurry.
mechano-chemical polishing with syton containing
H202.
brief etch in HNO3:HF (3:1) followed by soak in HF
(1% in H20) to remove oxides.
• Epitaxy:
first experiments with atmospheric pressure vapor
phase epitaxy (VPE). Disadvantage: high substrate
temperature, H2 diluted feed gas (contamination,
diffusion of dopants into the blocking layer).
current experiments are performed with low pressure
VPE. Advantage: low substrate temperature.
94
EQI
c_
L
_N
oI
E_
_6
95
3.1.2.Characterization of epi layers
• Optical micrographs
• Variable temperature Hall effect and resistivity
• Rutherford backscattering (channeling) spectrometry
(RBS)
• Secondary ion spectrometry (SIMS)
• Spreading resistance measurements
96
t._
E
"E
o
N
.N
I,,,,
o
L.
P--,
om
97
ORIGINAL PAGE'
BLACK AND WHITE PH_,,J/0_tRAPH
ORIGINAEPAGE
BLACK AND WHITE PHOTOGRAPH
0.03 mm
(100) N D- N A = 1 x 1014 / cm 3
580°C; 5 sccm GeH4,
with H 2 reduction step,
polycrystalline deposition
0.03 mm
I.__J
(113) ND-N A = 2x 1014/cm 3
580°C ; 5sccm GeH 4,
with H 2 reduction step,
no growth (etching)
0.03 mm
I I
Fig. 6.
(113) ND-NA=5X 1011/cm 3
550°C; 10sccmGeH 4,
no H 2 reduction step,
single crystal deposition
Optical micrographs of Ge epi layers.
98
18
10
17
10
1016
CO 015I 1
E
O
v
C
0 1014
-.4
O
L
C 01301
U
C
0
l.J
1012
10 11
i0 I°
0.0
Holl EFFect
I I I 1
550A +
550-01 *
EGap Ge (700 meV)
÷
C u "/° (Ev + 44 meV)
I I
16.0 24.0
IO001T (K-l)
I
32. 0 0
Fig. 7. Variable temperature Hall effect measurements of a Ge epilayer on
an n-type [113] substrate. The hole freeze-out curves indicate a light
copper contamination. The two curves (+, *) are measurements of
the same sample and demonstrate reproducibility.
99
'2
C)
C)
C)
A
V
73NNVH3/SINn03 od
100
10 :'
g
i
I0 z°-
09
E
0
<
C
0 10 11
e_
13
t-
_)
0
C
0
0 10 la
=m
0.0
Clean #.1 W/. Plasma Etch
Cleon.#2 W/ Plasma Etch
Leak In Reactor During Dep.
No Plasma Etch
I
I
I I
I I
I I
I I
I I
I I
I I
I I
I I
I I
I
t -4
/
I fl 11\
I / t I\
I I I I I I I I U | I U I I I I I I I | I I I I I I I I I | I I I I I I I I I |
0.5 1.0 1.5 2.0
Depth (/_m)
Fig. 9. SIMS of LPVPE Epi Films:
Oxygen Concentration
101
4Cleon _1 WW//Plosmo Etchl l t
Leok in Reoctor During Dep.
No Plosmo Etch
0.0 0.5 1.0 1.5 2.0
Depth (p,m)
Fig. lo. SIMS of LPVPE Epi Films:
Corbon Concentrotion
102
3.1.3. Preliminary
• Responsivity
• Dark current
detector test results
103
uDepth (microns)
o
_ _0_l_°lJ Iml0Iilll l_l_lI_i_ _l_._01_ WliV_gJI Wl°0_Wl
Dep_ (m|crona)
b
Fig. 11. Spreading resistance as a function of depth from the epilayer sur-
face for: (a) an area of epilayer close to the leading edge of the
wafer in II-16 where the growth rate was ~ 0.06 _tmmin-l, and
(b) an area of epilayer farthest from the leading edge of the same
wafer where the growth rate was ~ 0.02 _mmin-l. The slight rise
in resistivity at the very surface is due to the native oxide.
104
OH
W'-"
<:
.4..J
O3
t-
O
O3t
_0
rF,-
I
o
41..,.,
13-3
13-2
I I I I I1111 I I I a Ilil[ i I I I alia I I I I I illl'rl
10 BicllO'2 V"s (m) 10' 10'
Fig. 12. Responsivity as a function of bias for detectors 13-2 and 13-3 at 2.3
K under reverse bias. The substrate material is moderately doped
(5 x 1015 cm-3). Such material exhibits hopping conduction but
does not have extended wavelength spectral response. Tests were
performed with a narrow band filter at _ = 98.9 _tm.
105
aI
c-
C)
L...¢_
_T
f'_ O'
°
T
O
T=..'
1 I I I I I III I I I I I Ill I I I I I I Ill I
Bias (m
Fig. 13. Dark current as a function of detector bias for detector 13-2 with
an epilayer and for the same "detector" without an epilayer at 2.3 K
under reverse bias. Below a bias of ~ 100 mV, the blocking layer
effectively reduces hopping conduction in this moderately doped
material (NA - ND = 5 x 1015 cm-3).
106
Ol
1
CD
_T
o'
v--:
I
O'
T-.. °
I I I I llll I i I I I iili I I I I I IIII I I I I I Ilii I
lO Bios_°(_v)lO+ 1o,
Fig. 14. Dark current as a function of bias for detectors 13-2 and 13-3 at
2.3 K under reverse bias.
107
3.2.Ion Implanted BIB Detectors
• Concept:
In case pure and structurally perfect epitaxial layers
are hard to produce, we can resort to implantation of
dopants into an ultra-pure crystal.
• Low energy B+-implantation tests:
- three B + energies: 150keV, 95 keV, 50keV form
a 0.4 _tm thick layer with NA = 3.5 x 1016 cm-3.
- annealing at 400°C for one hour in argon.
extended wavelength response.
responsivity = 0.5 A/W, dark current < 10 -14 A, at
bias = 100 mV and T = 2.0 K. NEP = 4 x 10 "16
w/4h .
108
--!10 -1_
2
_ 10 -2_
C12
2.0 K
<...
.,,.,4.2 K
_ -
"." ,.2-}
/ 4.2 K/"I" /'
®
I I I I I IIII I I I I I I III I I I I I I III
10 IVQ)2BIAS(
i
10 -13__.
10 -I_
7
10 -15
Fig. 15. Responsivity of (]
energy B+-implent
depth = 0.6 /_m,
Xf, ter = 98.9 H.m,
Ge BIB detector, low
[ 1 X 10 cm
fchopper-- 2:3 Hz
109
,m
E
J5
<
(/)
E
O
r_
rY
I00
80
6O
4O
2O
0
0
Wave Numbers (cm -1)
1O0 200 300 400
I I I I I I I I I | I I I l I I I I I I ! ! I 1 I 1 I I I I I I I I I I I I I
t t
250 150/_m
ri_. ]4. Spectral response of Ge BIB detector,
low energy 13+-implantation type.
110
* High energy B+-implantation tests:
14 implant energies up to 4 MeV doubly charged
boron ions lead to a 5 ktm thick layer with NA =
1 x 1016 cm "3.
Variable temperature Hall effect and resistivity mea-
surements indicate full activation of shallow accep-
tor dopant B. No deep levels are detectable after
annealing. Below 15 K, hopping conduction
becomes dominant.
Infrared transmission measurements and device tests
are in progress.
111
10 19_
r
ulO ".
c"
o
&_
0
_0
I0 17
0
-4--'
0
&-
c-
Q)
0
"10 ,6_
0
l--
0
10,5 /
0
_- P+ contact layer
sum
I I I I I I I ! I I | I I I I I I i I I I I I I li I I I
2 4 6
Depth (j_,m)
Fig. 17. Ge BIB, high energy ion implant profile
112
CO
I
E
U
10
18
v
17
10
C
0 10 16
,_
0
L
C
0
0
C
0
U
15
I0
14
10
0.0
Hall
I l !
HV-OI
HV-400
÷
÷
_,_ Shallow Acceptor
Freeze-Out
4-
÷ \*
4- _ 4-
4- 4" 4" 41.
\
4-
el-
I I I I
20. 0 40. 0 150. 0 80. 0 100. 0
IOOO/T (K -1)
Fig. 18. Free carrier freeze-out of high energy B+-implanted layer. Before
annealing (+), the slope of the freeze-out curve is steeper than after
annealing (,). The latter slope corresponds to ~ 10.4 meV, the
binding energy of shallow boron acceptors. Below 15 K, hopping
conduction becomes dominant.
113
104
103
102
E
(J
_" 101
>,,
.¢-I
>
.¢'4
(/) 0
•,I 10
tO
0
CK
-1
lO
-2
10
.i, 4.'1"
"4"
÷
4-
4-
4-
4- .it.
++.*
t,++
I I I
41,._
÷,_"
÷
I
HV-O1 +
HV-400 *
.#FI- ÷ ÷
÷
,.,IP
I 1 I I
0. 0 100. 0 200. 0 300. 0 400. 0 500. 0
IO00/T (K -1)
Fig. 19. Resistivity as a function of inverse temperature of the high energy
B+-implant layer before (+) and after (,) annealing. Hopping
conduction becomes dominant below 15 K.
114
4. CONCLUSIONS
• A LPVPE technique for the low temperature growth of
epitaxial Ge layers has been developed.
• Hall effect and resistivity measurements indicate that
the epi layers are lightly p-type due to residual copper
contamination.
• First generation Ge BIB detectors made with moderately
doped substrates (5 x 101s cm-3) exhibit effective
blocking of the hopping current.
• First generation Ge BIB detectors exhibit responsivities
around 1 A/W.
• Second generation devices using low pressure VPE are
being processed.
• Ion implanted active layers are tested.
• It is currently not known what temperatures will be
required to reduce the dark current down to levels which
are acceptable for SIRTF applications.
115



Germanium Blocked Impurity Band (BIB) detectors

Germanium Blocked Impurity Band (BIB) detectors

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