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AC photoconductivity of amorphous GaP diodes made by reactive evaporation under implantation of hydrogen ions
Journal of Non-Crystalline Solids 97&98 (1987) 1347-1350 North-Holland, Amsterdam
1347
AC PHOTOCONDUCTIVITY OF AMORPHOUS GaP DIODES MADE BY REACTIVE EVAPORATION UNDER IMPLANTATION OF HYDROGEN IONS
Masami ONUKI, Koji TSUBUSAKI and Hiroshi KUBOTA Department of Electrical Engineering and Computer Science, Kumamoto University, Kurokami 2, Kumamoto 860, Japan
The gap states and mobility gap in a-GaP:H films have been determined by measuring the optical absorption spectrum, and the amplitude and phase lag of the ac photocurrent, under periodically chopped illumination and a d c voltage, vs. the wavelength. Two kinds of the gap states due to defects other than dangling bonds are proposed, which are concerned with P-vacancies and/or Ga-Ga wrong bonds and Ga-vacancies and/or P-P wrong bonds. The mobility gap is estimated to be 2.6 eV at 90 K and 2.2 eV at 300 K.
1. INTRODUCTION Although works on the optical absorption spectra in far-, near-infrared,
and
visible
range
for amorphous hydrogenated GaP (a-GaP:H) films made by different methods have been carried out in order to make clear atomistic and electronic structures of the material, a few papers have been published on electronic transport in a-GaP:H films, because of difficulties due to many defects preventing electronic transport in the films.
Defects are assumed to concern, 1) dangling bonds
on Ga- and P-sites, 2) P-vacancies and/or Ga-Ga wrong bonds, and 3) Ga-vacancies and/or P-P wrong bonds. This paper describes the optical absorption and transport of photocarriers in a-GaP:H films with different values of the mole ratio of Ga to P, r, where films were deposited by electron beam evaporation under implanting hydrogen ions.
From results on the optical absorption, transition of elec-
trons between the gap states is suggested, which is ineffective to induce a photocurrent. The amplitude and phase lag of the ac photocurrent, under periodically chopped illumination of 270 Hz and a dc voltage, vs. the wavelength have been measured at 90 and 300 K to determine the gap states and mobility gap, and so the above results were useful to analyze the sign ofphotocarriers excited by different wavelengths.l ),2) 2. HYDROGEN REACTIVE EVAPORATION Films of a-GaP:H have been made so far by techniques of GD-MOCVD 3), sputtering4),s), and flash evaporation 6).
In this work, we adopted the technique of hydrogen-reactive evaporation 7)
using an ion gun, since this method is excellent in evaluating amounts of hydrogen ions implanted during deposition and in protecting films in growth from oxidation. coated by ITO.
Substrates used are glass plates
The mole ratio and density of hydrogen atoms in films were controlled by varying
the evaporation speed and hydrogen ion current.
The value of r was between 0.2 and 2, and the
maximum content of hydrogen due to our ion gun was estimated to about 40 mole % in calculation. 0022-3093/87/$03.50 ©Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)
M. Onuki et al. / A C photoconductioity of amorphous GaP diodes
1348
The growth rate of films was about 100 A/min and the thickness of them ranged from 1500 to 2500 A.
The other electrode was made of evaporated A1 of 2 X 9 mm 2.
A broad band of
far-infrared absorption between 650 and 550 cm 1 , due to wagging vibrations of H at P-H, Ga-H, and Ga-H-Ga bonds s), was observed for our films made by hydrogen-reactive evaporation, where KBr crystals were used as substrates in this case. 3. OPTICAL ABSORPTION AND AC PHOTOCONDUCTIVITY The optical threshold energy, Eo, was estimated from the optical absorption spectrum using the ordinary method.
The spectra for different films are shown in FIG. 1, where Curve A, B, B', C, and
C' correspond to the film of r = 1.3 and I b = 2.5#A/cm 2 , that o f t = 0.8 and lb = 5pA/cm ~ , that of r = 0.8 and Ib = 3.5#A/cm 2, that or r = 0.5 and lb = 2.3/~A/cm ~, and that of r = 0.5 and 10 = 0, respectively, where I b is the ion current density during deposition.
Hydrogen atoms contained in
films play a role to enhance the value of Eo, comparing Curve C with C' and Curve B with B'.
The
threshold energy of the film with r < 1 is larger than that with r > 1, when both rdms contain nearly equal density of hydrogen atoms, (see Curve A and C),
When a film deposited at 300 K on a sub-
~trate was annealed at 473 K for an hour, the value ofE o was increased a little. The value of Eo at 300 K ranged from 0.5 to 1.6 eV for different films.
T=300 K
[ Ibul/cm2
/a
/B'~
1031- b 1.3
/c
8O0
M, /
f
//
--~
//
//
~-b
,/"
/
/.Ui ,,-----/
l Jl/J, li ';Tf==92070Hz
""
O' '1111 1.0
0
/
/ 200
2.5 1.2
032
c'0.4
n 2.0
3.0
Photon Energy (eV) FIGURE 1 Optical absorption spectra for different films, where a is the absorption coefficient and l~o the photon energy.
1.0
2.0 3.0 PhotonEnergy(eV)
4.0
FIGURE 2 Ac photoconductivity vs. wavelength for different films.
M. Onuki et al. / A C photoconductioity of amorphous GaP diodes
1349
Results of the ac p h o t o c o n d u c t i v i t y vs. the wavelength at 90 K are shown in Fig.2, where the photoconductivity is normalized for a constant incident power and film data are tabulated in the figure.
Curve a, b, c, and c' correspond to the film of r = 1.0 and Ib = l p A / c m 2 , that of r = 1.3 and
Ib = 2.5pA/cm 2 , that o f r = 0.32 and I~ = 1.2 /aA/cm 2, and that of r = 0.4 and I b = 0, respectively.
C.B.
Comparing Curve c with c', the
photoconductivity is enhanced by implanting hydrogen
ions during deposition.
_ _
difference of curve shapes between Curve a
Mobility
I
D°
Edge
i
The
F.L.
and b around 1.8 eV may be due to the gap states
/~
D,°
!T4 i i
concerned with P-vacancies (and/or
6
Ga-Ga wrong bonds), denoted as D~, because
Mobility Edge
V.B.
the film of curve b with r > I contains much higher d e n s i t y o f D~ than the film of Curve a with r = 1.
A bell shape band around 1.5 eV
FIGURE 3 Different electronic transitions.
in Curve c may be attributed to the gap states concerned
with
Ga-vacancies (and/or P-P
wrong bonds), denoted as D~, because of
180
, ~A/cmZ
10~ c
o~
r=0.9 I b--.2.5pNcm2
-lo
E
102 e.-
m 90 o o ca-10
'
OOK
O T=3OOK • T=90K f =270Hz
.Ic (3_
0 I
1.0
I
I
2.0 3.0 Photon Energy (eV)
FIGURE 4 Amplitude of ac photocurrent vs. p h o t o n energy for a t'tim.
I
40
1.0
i
L
2.0
J
=
3.0
Photon Energy (eV) FIGURE 5 Phase lag of ac photocurrent vs. p h o t o n energy for the same film shown in Fig.4.
410
1350
large amount
M. Onuki et al. / A C photoconductwity of amorphous GaP diodes
of D~ in the film of Curve c with r < 1.
Since GaP is ionic in part, P-vacancies
without trapped electrons, D~, are positively charged and Ga-vacancies without trapped holes, D~, negatively charged.
A band around 1.8 eV may be due to D~ ~ D~ + e, T 2 in FIG.3, and that
around 1.5 eV, D~ -~ D~ + h, shown as T3, considering results on the phase lag of the ac photocurrent. Here e and h mean a free electron and hole, respectively.
The optical threshold may be concerned
with transition of electrons from D~ states to neighboring D~ states, shown as T~. The amplitude and phase lag of the ac photocurrent vs. the wavelength at 90 and 300 K for a film of r = 0.9 and Ib = 2.5/aA/cm 2 are shown in FIG.4 and 5, respectively.
Oheda suceeded to relate
data of the lag vs. the wavelength with mechanisms of excitation and transport of photocarriers for a-As2 Se 31), and Yamaguchi and Morigaki for a-Si:H 2).
Applying similar analysis to the results at
90 K of FIG.4 and 5, the photocurrent created by photons around 1.5 eV and that around 1.8 eV may be attributed to transports of free holes and electrons, respectively, since the phase lag at the region above 1.8 eV is decreased with the increase of the photon energy. From results in FIG.5, the mobility gap is estimated as 2.6 eV at 90 K and 2.2 eV at 300 K, which corresponds to the band-to-band transition, T4 in FIG.3. Different transitions of electrons are proposed and shown in FIG.3, where T 2 and T3 lead to the photocurrents at red and near-infrared wavelengths, respectively, T4 to the photocurrent at visible ones, and T 1 contributes only to the optical absorption without a photocurrent. AKNOWLEDGEMENT This work was in part supported by Hoso Bunka Foundation. The authors should like to express their thanks to Dr. M. Yoshiyama of Matsushita Electric Industrial Co., LTD., for supplying data by an X-ray microanalyzer, and to Messrs. K. Tsuji and I. Nojiri for their assistances in experiments. REFERENCES 1) H. Oheda, Solid State Commun. 33(1970) 203 2) M. Yamaguchi and K. Morigaki, Jpn. J. Appl. Phys. 20(1981) 677. 3) J.C. Knight and R. A. Lujan, J. Appl. Phys. 49 (1978) 1291. 4) K. Kumabe and N. Matsumoto, Jpn. J. Appl. Phys. 18 (1979) 1789. 5) Z.P. Wang, L. Ley and M. Cardona, Physica 117B & 118B (1983) 968. 6) A. Gheorghiu and M. They, Philos. Mag. B44 (1981) 285. 7) M. Shindo, S. Sato, I. Myokan, S. Mano and T. Shibata, J. Non-Cryst. Solids 59 & 60 (1983) 747.
1347
AC PHOTOCONDUCTIVITY OF AMORPHOUS GaP DIODES MADE BY REACTIVE EVAPORATION UNDER IMPLANTATION OF HYDROGEN IONS
Masami ONUKI, Koji TSUBUSAKI and Hiroshi KUBOTA Department of Electrical Engineering and Computer Science, Kumamoto University, Kurokami 2, Kumamoto 860, Japan
The gap states and mobility gap in a-GaP:H films have been determined by measuring the optical absorption spectrum, and the amplitude and phase lag of the ac photocurrent, under periodically chopped illumination and a d c voltage, vs. the wavelength. Two kinds of the gap states due to defects other than dangling bonds are proposed, which are concerned with P-vacancies and/or Ga-Ga wrong bonds and Ga-vacancies and/or P-P wrong bonds. The mobility gap is estimated to be 2.6 eV at 90 K and 2.2 eV at 300 K.
1. INTRODUCTION Although works on the optical absorption spectra in far-, near-infrared,
and
visible
range
for amorphous hydrogenated GaP (a-GaP:H) films made by different methods have been carried out in order to make clear atomistic and electronic structures of the material, a few papers have been published on electronic transport in a-GaP:H films, because of difficulties due to many defects preventing electronic transport in the films.
Defects are assumed to concern, 1) dangling bonds
on Ga- and P-sites, 2) P-vacancies and/or Ga-Ga wrong bonds, and 3) Ga-vacancies and/or P-P wrong bonds. This paper describes the optical absorption and transport of photocarriers in a-GaP:H films with different values of the mole ratio of Ga to P, r, where films were deposited by electron beam evaporation under implanting hydrogen ions.
From results on the optical absorption, transition of elec-
trons between the gap states is suggested, which is ineffective to induce a photocurrent. The amplitude and phase lag of the ac photocurrent, under periodically chopped illumination of 270 Hz and a dc voltage, vs. the wavelength have been measured at 90 and 300 K to determine the gap states and mobility gap, and so the above results were useful to analyze the sign ofphotocarriers excited by different wavelengths.l ),2) 2. HYDROGEN REACTIVE EVAPORATION Films of a-GaP:H have been made so far by techniques of GD-MOCVD 3), sputtering4),s), and flash evaporation 6).
In this work, we adopted the technique of hydrogen-reactive evaporation 7)
using an ion gun, since this method is excellent in evaluating amounts of hydrogen ions implanted during deposition and in protecting films in growth from oxidation. coated by ITO.
Substrates used are glass plates
The mole ratio and density of hydrogen atoms in films were controlled by varying
the evaporation speed and hydrogen ion current.
The value of r was between 0.2 and 2, and the
maximum content of hydrogen due to our ion gun was estimated to about 40 mole % in calculation. 0022-3093/87/$03.50 ©Elsevier Science Publishers B.V. (North-HoUand Physics Publishing Division)
M. Onuki et al. / A C photoconductioity of amorphous GaP diodes
1348
The growth rate of films was about 100 A/min and the thickness of them ranged from 1500 to 2500 A.
The other electrode was made of evaporated A1 of 2 X 9 mm 2.
A broad band of
far-infrared absorption between 650 and 550 cm 1 , due to wagging vibrations of H at P-H, Ga-H, and Ga-H-Ga bonds s), was observed for our films made by hydrogen-reactive evaporation, where KBr crystals were used as substrates in this case. 3. OPTICAL ABSORPTION AND AC PHOTOCONDUCTIVITY The optical threshold energy, Eo, was estimated from the optical absorption spectrum using the ordinary method.
The spectra for different films are shown in FIG. 1, where Curve A, B, B', C, and
C' correspond to the film of r = 1.3 and I b = 2.5#A/cm 2 , that o f t = 0.8 and lb = 5pA/cm ~ , that of r = 0.8 and Ib = 3.5#A/cm 2, that or r = 0.5 and lb = 2.3/~A/cm ~, and that of r = 0.5 and 10 = 0, respectively, where I b is the ion current density during deposition.
Hydrogen atoms contained in
films play a role to enhance the value of Eo, comparing Curve C with C' and Curve B with B'.
The
threshold energy of the film with r < 1 is larger than that with r > 1, when both rdms contain nearly equal density of hydrogen atoms, (see Curve A and C),
When a film deposited at 300 K on a sub-
~trate was annealed at 473 K for an hour, the value ofE o was increased a little. The value of Eo at 300 K ranged from 0.5 to 1.6 eV for different films.
T=300 K
[ Ibul/cm2
/a
/B'~
1031- b 1.3
/c
8O0
M, /
f
//
--~
//
//
~-b
,/"
/
/.Ui ,,-----/
l Jl/J, li ';Tf==92070Hz
""
O' '1111 1.0
0
/
/ 200
2.5 1.2
032
c'0.4
n 2.0
3.0
Photon Energy (eV) FIGURE 1 Optical absorption spectra for different films, where a is the absorption coefficient and l~o the photon energy.
1.0
2.0 3.0 PhotonEnergy(eV)
4.0
FIGURE 2 Ac photoconductivity vs. wavelength for different films.
M. Onuki et al. / A C photoconductioity of amorphous GaP diodes
1349
Results of the ac p h o t o c o n d u c t i v i t y vs. the wavelength at 90 K are shown in Fig.2, where the photoconductivity is normalized for a constant incident power and film data are tabulated in the figure.
Curve a, b, c, and c' correspond to the film of r = 1.0 and Ib = l p A / c m 2 , that of r = 1.3 and
Ib = 2.5pA/cm 2 , that o f r = 0.32 and I~ = 1.2 /aA/cm 2, and that of r = 0.4 and I b = 0, respectively.
C.B.
Comparing Curve c with c', the
photoconductivity is enhanced by implanting hydrogen
ions during deposition.
_ _
difference of curve shapes between Curve a
Mobility
I
D°
Edge
i
The
F.L.
and b around 1.8 eV may be due to the gap states
/~
D,°
!T4 i i
concerned with P-vacancies (and/or
6
Ga-Ga wrong bonds), denoted as D~, because
Mobility Edge
V.B.
the film of curve b with r > I contains much higher d e n s i t y o f D~ than the film of Curve a with r = 1.
A bell shape band around 1.5 eV
FIGURE 3 Different electronic transitions.
in Curve c may be attributed to the gap states concerned
with
Ga-vacancies (and/or P-P
wrong bonds), denoted as D~, because of
180
, ~A/cmZ
10~ c
o~
r=0.9 I b--.2.5pNcm2
-lo
E
102 e.-
m 90 o o ca-10
'
OOK
O T=3OOK • T=90K f =270Hz
.Ic (3_
0 I
1.0
I
I
2.0 3.0 Photon Energy (eV)
FIGURE 4 Amplitude of ac photocurrent vs. p h o t o n energy for a t'tim.
I
40
1.0
i
L
2.0
J
=
3.0
Photon Energy (eV) FIGURE 5 Phase lag of ac photocurrent vs. p h o t o n energy for the same film shown in Fig.4.
410
1350
large amount
M. Onuki et al. / A C photoconductwity of amorphous GaP diodes
of D~ in the film of Curve c with r < 1.
Since GaP is ionic in part, P-vacancies
without trapped electrons, D~, are positively charged and Ga-vacancies without trapped holes, D~, negatively charged.
A band around 1.8 eV may be due to D~ ~ D~ + e, T 2 in FIG.3, and that
around 1.5 eV, D~ -~ D~ + h, shown as T3, considering results on the phase lag of the ac photocurrent. Here e and h mean a free electron and hole, respectively.
The optical threshold may be concerned
with transition of electrons from D~ states to neighboring D~ states, shown as T~. The amplitude and phase lag of the ac photocurrent vs. the wavelength at 90 and 300 K for a film of r = 0.9 and Ib = 2.5/aA/cm 2 are shown in FIG.4 and 5, respectively.
Oheda suceeded to relate
data of the lag vs. the wavelength with mechanisms of excitation and transport of photocarriers for a-As2 Se 31), and Yamaguchi and Morigaki for a-Si:H 2).
Applying similar analysis to the results at
90 K of FIG.4 and 5, the photocurrent created by photons around 1.5 eV and that around 1.8 eV may be attributed to transports of free holes and electrons, respectively, since the phase lag at the region above 1.8 eV is decreased with the increase of the photon energy. From results in FIG.5, the mobility gap is estimated as 2.6 eV at 90 K and 2.2 eV at 300 K, which corresponds to the band-to-band transition, T4 in FIG.3. Different transitions of electrons are proposed and shown in FIG.3, where T 2 and T3 lead to the photocurrents at red and near-infrared wavelengths, respectively, T4 to the photocurrent at visible ones, and T 1 contributes only to the optical absorption without a photocurrent. AKNOWLEDGEMENT This work was in part supported by Hoso Bunka Foundation. The authors should like to express their thanks to Dr. M. Yoshiyama of Matsushita Electric Industrial Co., LTD., for supplying data by an X-ray microanalyzer, and to Messrs. K. Tsuji and I. Nojiri for their assistances in experiments. REFERENCES 1) H. Oheda, Solid State Commun. 33(1970) 203 2) M. Yamaguchi and K. Morigaki, Jpn. J. Appl. Phys. 20(1981) 677. 3) J.C. Knight and R. A. Lujan, J. Appl. Phys. 49 (1978) 1291. 4) K. Kumabe and N. Matsumoto, Jpn. J. Appl. Phys. 18 (1979) 1789. 5) Z.P. Wang, L. Ley and M. Cardona, Physica 117B & 118B (1983) 968. 6) A. Gheorghiu and M. They, Philos. Mag. B44 (1981) 285. 7) M. Shindo, S. Sato, I. Myokan, S. Mano and T. Shibata, J. Non-Cryst. Solids 59 & 60 (1983) 747.