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Lowering of band-gap energy in heavily nitrogen-doped ZnSe
Journal of Crystal Growth 150 (1995) 797-802
ELSEVIER
Lowering of band-gap energy in heavily nitrogen-doped
ZnSe
Ziqiang
Okada
Zhu a, *>l, Kazuhisa
Takebayashi
a, Takafumi
Yao a, * )l, Yasumasa
b
aDepartment of Electrical Engineering, Hiroshima Unifier&, Higashi-Hiroshima 724, Japan b Electrotechnical Laboratory, Umezono, Tsukuba 305, Japan
Abstract This article reports the notable effect of N doping in ZnSe on the band-gap energy. The free the epilayers with different NA -No was measured by means of reflectance and photoreflectance The temperature dependence of reflectance, photoreflectance and photoluminescence spectra of was investigated in detail. It is found that the free exciton energy shifts to low energy side concentration, indicating the shrinkage of the band-gap energy due to N doping. The peak positions emissions shift to low energy side due to the shrinkage of energy gap. The lattice constant and the ZnSe were measured by means of X-ray diffraction (XRD). The origins for the shrinkage of energy on the basis of the optical and XRD results.
1. Introduction A deep N-associated donor with binding energy of 45-55 meV was found from recent optical studies of N-doped ZnSe [1,2], which has attributed to a complex of N-acceptor (Ns,) and Se vacancy [3]. Typically, photoluminescence (PL) spectra of N-doped ZnSe with NA - No < 1.0 x 1017 cm-3 show dominant emission between a shallow residual donor and an N-acceptor (DSAP) at about 2.693 eV, while the N-doped ZnSe epilayers with NA -No > 1.0 X 1017 cmp3 exhibit dominant emission between a deep N-associated donor and an N-acceptor (DdAP) at about 2.681 eV. The deep DAP emission energy
* Corresponding author. ’ Present address: Institute for Materials Research, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980, Japan.
exciton energy in spectroscopies. ZnSe : N epilayers with increasing N of near-band-edge strain in N-doped gap are discussed
shifts to a low energy side as [N] increases. In the case of extremely high nitrogen doping, the red shifts were reported to be 74 and 217 meV from the samples with [N] = 1.3 X 1019 and 1.5 x 10” cmp3 doped by RF method [4,51, and 82 meV from the sample with [N] = 2 x 10” cme3 doped by ECR [6] method. These facts strongly suggest decrease in the band-gap energy or the formations of the N-associated deep levels. The effect of N doping on the ZnSe lattice constant was investigated by Petruzzello et al using X-ray diffraction [7]. They indicated that the lattice constant decreases as the N concentration increases, indicating the generation of point defects induced by high N doping. In this article, we report the notable effect of N doping on the band-gap energy. The free exciton energy in the epilayers with different NA - No was measured by reflectance (R) and photore-
0022-0248/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0022-0248(94)00701-2
798
Z. Zhu et al. /Journal
of Crystal Growth 150 (1995) 797-802
flectance (PR) spectroscopies. It is found that the free exciton energy shifts to low energy side with increasing [N], indicating the shrinkage of the band-gap energy due to N doping. The temperature dependence of R, PR and PL spectra of ZnSe : N epilayers was investigated in detail. The positions of FA, DF, shallow DAP and deep DAP emissions were observed to shift to low energy side due to the shrinkage of energy gap. The correlation between the free exciton energy and lattice constant or the strain due to N doping is discussed on the basis of the optical and X-ray diffraction (XRD) study.
2. Experimental
---
procedure 2.77
The N-doped ZnSe films were grown on GaAs(100) semi-insulating (SI) substrates by MBE. The growth was carried out using high purity elemental Zn and Se and nitrogen gas (N,) as an acceptor dopant (6N grade). The substrate temperature was fixed at 250°C and the beam pressure ratio of Zn to Se was set at l/2. The growth rate was 0.65 mm/h and the thicknesses of the films were about 2.5 pm. The active nitrogen flux produced by a microwave plasma source was injected into the MBE chamber through a quartz glass pipe. The flow rate of N, was kept constant, at which the background pressure during growth was 2 x lo-” Torr. The nitrogen concentration incorporated into the epilayers was controlled by the microwave powers supplied to a plasma discharge cavity which varied from 50 to 180 w. The N-doped ZnSe samples used in the present study have different NA -No values varing from 5 x lOi to 2 x lOI cm-3, which are thought to be convenient for study of fundamental PL property and the measurements of reflectance and photoreflectance spectroscopies. NA -No was determined by C-V profiling using two coplanar Au electrodes as double Schottky barriers at 1 MHz. PL measurements were performed using the 3250 A line from a He-Cd laser with 10 mW as the excitation sources. Samples were attached to a cold finger in a closed-cycle cryostat. Continuous light from a halogen-tungsten lamp
2.81
2.79 Photon
energy
2.8:
(eV)
Fig. 1. Photoluminescence, photoreflectance and reflectance spectra at 14 K obtained from undoped MBE-grown ZnSe, N-doped ZnSe with NA - No = 8 X 10” cmm3, and chlorinedoped ZnSe with NA- No = 2~ 10’” cmm3, respectively.
was used as the probe beam for R and PR measurements. A chopped He-Cd laser beam was used in PR measurements to provide a repetitive modulation to the samples. The amount of residual strain in ZnSe: N epilayers with various [N] was directly measured with X-ray rocking curves using Cu K a , radiation diffracted by double-crystal monochromater. By measuring reflections from two lattice planes nonparallel to the surface, the in-plane and perpendicular lattice constants were determined. The strain was calculated from these lattice constants and the unstrained lattice constant of bulk ZnSe.
3. Experimental
results and discussion
Fig. 1 shows PL, PR and R spectra at 14 K obtained from undoped MBE-grown ZnSe, Ndoped ZnSe with NA -N, = 8 X 1016 cm-3, and chlorine-doped ZnSe with No - NA = 2 x 10” cmp3, respectively. The dips corresponding to the lower branch of free exciton energy due to strain were clearly observed at 2.801 eV in the R spec-
2. Zhu et al. /Journal
tra from the undoped and Cl-doped epilayers. The photoreflectance of the undoped epilayer yielded dips at 2.804 and 2.815 eV related to the upper branch of free exciton energy and excited exciton state (n = 2). The binding energy of free exciton is estimated to be 21 meV and the band gap energy 2.822 eV. However, both the reflectance and photoreflectance obtained from Ndoped epilayer yielded dips at 2.797 eV, indicating the decrease in the free exciton energy in N-doped ZnSe compared with undoped and Cldoped ZnSe. The free exciton energy is influenced by nitrogen concentration in ZnSe. Fig. 2 shows the R and PR spectra at 14 K obtained from ZnSe: N with various NA -No values ranged from 0.8 to 13 X 1016 cmp3. Both the reflectance and photoreflectance yielded dips at free exciton energies. The free exciton energy obtained from the reflection spectrum coincides with that from the
ZnSe:N
1 14K
1 Photon
energy
799
of Crystal Growth 150 (1995) 797-802
(eV)
Fig. 2. Photoreflectance and reflectance spectra at 14 K obtained from ZnSe: N with various NA - No values ranging from 0.8 to 13x lOI6 cm-3. The reflectance and photoreflectance yielded dips at free exciton energies.
He-Cd laser 1OmW Halogen lamp 14K NA-ND=lxlO”cm.:
%
I
2.: 75
I
I
2.80
2.85
Photon
energy
(eV)
Fig. 3. Photoreflectance spectrum of ZnSe: N with NA - No = 8~ lOI cmm3. Two dips related to the free excitons at the excited (2.811 eV) and ground states (2.797 eV) appear.
photoreflectance spectrum. The free exciton energy decreases from 2.802 to 2.790 eV when NA -N, increases from 0.8 to 13 X lOI cme3. For a specific sample with NA -No = 8 X 10’” cme3, we obtained the photoreflectance spectrum where two dips related to the free excitons at the excited (2.811 eV> and ground states (2.797 eV) appear, as shown in Fig. 3. Thus, the binding energy of free exciton was calculated to be 21 meV and the band gap energy 2.817 eV. The temperature dependence of the free exciton energy in ZnSe : N has been investigated. Fig. 4 shows the R and PR spectra obtained from a sample with NA -N, = 8 X lOI cmm3 at various temperatures. The dips related to free exciton energy appear in both R and PR spectra until 200 K. The free exciton energy obtained has been plotted against temperature [l], which can be well fitted by using empirical Varshni’s expression. The emission energies of shallow DAP, FA (free-electron-to-acceptor) and DF (donor-tofree-hole) obtained from PL spectra showed the same temperature dependence as that of the free exciton energy [l]. These facts confirm that the temperature dependence of the exciton energy obtained experimentally reflects that of the band gap energy of ZnSe : N. The free exciton energy decreases with increasing [N]. Accordingly, the DAP peak position
800
of Crystal Growth 150 (1995)
Z. Zhu et al. /Journal
N,- Nn=8x1016(cmJ)
ZnSe:N
797-802
le-Cd laser
ZnSe:F
OmW 14K
FmK
iL_
2.779eV ’ OOK_,&___ QOK
2
t
At*
2.736ev”
I
2.7
I
2.0 Photon
I n I
I
\I v,k-- ^I
I
I
2.7 energy (eV)
2.8
I
Fig. 4. Photoreflectance and reflectance spectra of 2nSe:N with NA - No = 8 x lOI cmm3 in a temperature range from 14 to 200 K. 2.
Photon
in PL spectra shifts to a low energy side. Fig. 5 shows the PL spectra obtained from samples grown at various microwave plasma powers ranging from 80 to 170 W, where the substrate temperature is kept at 250°C and the beam pressure ratio of Zn to Se was set at l/2. The shallow DAP emissions are dominant for the powers below 120 W, while the deep DAP emissions dominate the spectra for the powers above 150 W. As the power, i.e., the nitrogen concentration increases, both shallow and deep DAP emission energy shifts to low energy side. These results suggest decrease in the DAP emission energy due
free exciton energy, and several near-band-edge Power(W)
Undoped
0
_
1 2 3 4
80 100 100 120
8x 5x 8x 1x
10’5 1o16 1o16 10”
5 6
140 150
7
170
1.3 x 101’ 1 x 10’5 (High p) (High p)
(eV)
to lowering in band gap of ZnSe : N. For a comparison, the NA -No value, free exciton energy, and several near-band-edge emission energies are summarized in Table 1. The relationship between lattice constant and nitrogen concentration in the concentration range of 1 X 10” to 2 x 1019 cme3 was investigated by Petruzzello et al. using X-ray diffraction [7]. They indicated that the ZnSe lattice constant decreases
Sample
(cmm3)
energy
Fig. 5. Photoluminescence spectra from ZnSe: N epilayers grown at various microwave plasma powers ranged from 80 to 170 W.
Table 1 Value of NA -No
NA -No
:iT6
2.7
1
emission energies
E,, kV) 2.802 2.815 (n = 2) 2.802 2.798 2.797 2.796 2.811 (n = 2) 2.790 2.788
I, or If (eV) _
DAP (eV)
2.786 2.780 2.781 2.779
2.696 2.688 2.693 2.689
_ 2.770
2.670 2.671
_
2.658
_
Z. Zhu et al. /Journal
as the N concentration increases and that the reduction in lattice constant is greater than what can be explained by the shorter Zn-N bond distance of model predictions. The excess lattice contraction was attributed to the generation of point defects accompanying N doping. The strain relaxation due to N doping results in layers that contain residual compressive strain [7]. These phenomena were also observed in our samples. Fig. 6 shows the ZnSe lattice constants parallel (a,,) and perpendicular (a I> to the interface as a function of the free exciton, where the in-plane and perpendicular lattice constants were determined by measuring reflections from lattice planes of (001) and (117) nonparallel to the surface. The unstrained lattice constant of ZnSe : N (a,), strains parallel (E,,) and perpendicular (E I) to the interface were calculated from the measured inplane and perpendicular lattice constants, as shown in Fig. 7, using well-known relationships: a,=
[a,+2(C,*/C,,)a,,]/(I
+2C,,/C,,),
(1)
El/= (a,, - %)/a,,
(2)
El=
(3)
(a,
-%)/%.
801
of Crystal Growth 150 (1995) 797-802
Here, C,, and C,, are the elastic compliance constants, and ub is the unstrained lattice constant of bulk ZnSe. C,,, C,, and ub are assumed to be 8.10 X 10” dyn/cm*, 4.88 X 10” dyn/cm2 and 5.669424 A [8], respectively. When the nitrogen concentration increases, the in-plane lattice
5.671,
5.661 2.705
.
,
.
,
.
r
. 2.790 Free
10.1
l
2.795 exciton
energy
2.800
EL
’
-0.3 2.805
(eV)
Fig. 7. The lattice constant, strains versus the free exciton to energy. E,, and Ed are strains parallel and perpendicular the interface, respectively.
constants remarkably decreases due to compressive strain induced by N doping, while the perpendicular lattice constant slight inelastically increases, which results in the decrease in the unstrained lattice constant. The free exciton energy decreases with the unstrained lattice constant. These facts strongly suggest that the possible origins of shrinkage of the band-gap energy associated with N doping are due to (i) tailing of states induced by high N impurity, (ii) generation of defects, (iii) the residual strain, and (iv) the large difference in electronegativity between N and Se. Strong bowing in band-gap energy in GaPN [9] and ZnSeTe [lo] alloys due to the large difference in electronegativity between anions is reported.
4. Conclusion
0 al 5.655 2.785
.
l
energy
Fig. 6. The experimental ZnSe lattice and perpendicular (a,,) to the interface energy.
1
2.805
2.795 Free exciton
a,,
(eV)
constants parallel (a,,) versus the free exciton
We have investigated the effect of the N concentration in ZnSe on the band-gap energy. The free exciton energy in the epilayers with different NA --No was measured by reflectance spectra and photoreflectance spectra. It is found that the free exciton energy decreases with increasing the N concentration, indicating the shrinkage of the band-gap energy due to N doping. The energies of FA, DF, shallow DAP and deep DAP emissions shift to low energy side accompanying the shrinkage of energy gap.
802
Z. Zhu et al. /Journal
of Crystal Growth 150 (1995) 797-802
The correlation between the free exciton energy and lattice constant or the strain due to N doping is discussed on the basis of the XRD study. Since both the lattice constant and the free exciton energy decreases with increasing the nitrogen concentration, the shrinkage of the bandgap energy is correlated to be due to (i) tailing of states induced by high N impurity, (ii) generation of defects, (iii) residual strain, and (iv) the large difference in electronegativity between N and Se.
References [l] Z. Zhu, K. Takebayashi, K. Tanaka, T. Ebisutani, J. Kawamata and T. Yao, Appl. Phys. Lett. 64 (1994) 91.
[2] I.S. Hauksson, J. Simpson, S.Y. Wang, K.A. Prior and B.C. Cavenett, Appl. Phys. Lett. 61 (1992) 2208. [3] K.A. Prior, B. Murdin, C.R. Pidgeon, S.Y. Wang, I. Hauksson, J.T. Mullins, G. Horsburgh and B.C. Cavenett, J. Crystal Growth 138 (1994) 95. [4] J. Qiu, J.M. DePuydt, H. Cheng and M.A. Haase, Appl. Phys. Lett. 59 (1991) 2992. 151 T. Yao, T. Matsumoto, S. Sasaki, C.K. Chung, Z. Zhu and F. Nishiyama, J. Crystal Growth 138 (1994) 290. [6] S. Ito, M. Ikeda and K. Akimoto, Jap. J. Appl. Phys. 31 (1992) L1316. [7] J. Petruzzello, J. Gaines, P. van der Sluis, D. Olego and C. Ponzoni, Appl. Phys. Lett. 62 (1993) 1496. [8] T. Yao, Y. Okada, S. Matsui, K. Ishida and I. Fujimoto, J. Crystal Growth 81 (1987) 518. [9] X. Liu, S.G. Bishop, J.N. Bailargeon and K.Y. Cheng, Appl. Phys. Lett. 63 (1993) 208. [lo] T. Yao, M. Kato, J.J. Davies and H. Tanino, J. Crystal Growth 86 (1988) 552.
ELSEVIER
Lowering of band-gap energy in heavily nitrogen-doped
ZnSe
Ziqiang
Okada
Zhu a, *>l, Kazuhisa
Takebayashi
a, Takafumi
Yao a, * )l, Yasumasa
b
aDepartment of Electrical Engineering, Hiroshima Unifier&, Higashi-Hiroshima 724, Japan b Electrotechnical Laboratory, Umezono, Tsukuba 305, Japan
Abstract This article reports the notable effect of N doping in ZnSe on the band-gap energy. The free the epilayers with different NA -No was measured by means of reflectance and photoreflectance The temperature dependence of reflectance, photoreflectance and photoluminescence spectra of was investigated in detail. It is found that the free exciton energy shifts to low energy side concentration, indicating the shrinkage of the band-gap energy due to N doping. The peak positions emissions shift to low energy side due to the shrinkage of energy gap. The lattice constant and the ZnSe were measured by means of X-ray diffraction (XRD). The origins for the shrinkage of energy on the basis of the optical and XRD results.
1. Introduction A deep N-associated donor with binding energy of 45-55 meV was found from recent optical studies of N-doped ZnSe [1,2], which has attributed to a complex of N-acceptor (Ns,) and Se vacancy
* Corresponding author. ’ Present address: Institute for Materials Research, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai 980, Japan.
exciton energy in spectroscopies. ZnSe : N epilayers with increasing N of near-band-edge strain in N-doped gap are discussed
shifts to a low energy side as [N] increases. In the case of extremely high nitrogen doping, the red shifts were reported to be 74 and 217 meV from the samples with [N] = 1.3 X 1019 and 1.5 x 10” cmp3 doped by RF method [4,51, and 82 meV from the sample with [N] = 2 x 10” cme3 doped by ECR [6] method. These facts strongly suggest decrease in the band-gap energy or the formations of the N-associated deep levels. The effect of N doping on the ZnSe lattice constant was investigated by Petruzzello et al using X-ray diffraction [7]. They indicated that the lattice constant decreases as the N concentration increases, indicating the generation of point defects induced by high N doping. In this article, we report the notable effect of N doping on the band-gap energy. The free exciton energy in the epilayers with different NA - No was measured by reflectance (R) and photore-
0022-0248/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0022-0248(94)00701-2
798
Z. Zhu et al. /Journal
of Crystal Growth 150 (1995) 797-802
flectance (PR) spectroscopies. It is found that the free exciton energy shifts to low energy side with increasing [N], indicating the shrinkage of the band-gap energy due to N doping. The temperature dependence of R, PR and PL spectra of ZnSe : N epilayers was investigated in detail. The positions of FA, DF, shallow DAP and deep DAP emissions were observed to shift to low energy side due to the shrinkage of energy gap. The correlation between the free exciton energy and lattice constant or the strain due to N doping is discussed on the basis of the optical and X-ray diffraction (XRD) study.
2. Experimental
---
procedure 2.77
The N-doped ZnSe films were grown on GaAs(100) semi-insulating (SI) substrates by MBE. The growth was carried out using high purity elemental Zn and Se and nitrogen gas (N,) as an acceptor dopant (6N grade). The substrate temperature was fixed at 250°C and the beam pressure ratio of Zn to Se was set at l/2. The growth rate was 0.65 mm/h and the thicknesses of the films were about 2.5 pm. The active nitrogen flux produced by a microwave plasma source was injected into the MBE chamber through a quartz glass pipe. The flow rate of N, was kept constant, at which the background pressure during growth was 2 x lo-” Torr. The nitrogen concentration incorporated into the epilayers was controlled by the microwave powers supplied to a plasma discharge cavity which varied from 50 to 180 w. The N-doped ZnSe samples used in the present study have different NA -No values varing from 5 x lOi to 2 x lOI cm-3, which are thought to be convenient for study of fundamental PL property and the measurements of reflectance and photoreflectance spectroscopies. NA -No was determined by C-V profiling using two coplanar Au electrodes as double Schottky barriers at 1 MHz. PL measurements were performed using the 3250 A line from a He-Cd laser with 10 mW as the excitation sources. Samples were attached to a cold finger in a closed-cycle cryostat. Continuous light from a halogen-tungsten lamp
2.81
2.79 Photon
energy
2.8:
(eV)
Fig. 1. Photoluminescence, photoreflectance and reflectance spectra at 14 K obtained from undoped MBE-grown ZnSe, N-doped ZnSe with NA - No = 8 X 10” cmm3, and chlorinedoped ZnSe with NA- No = 2~ 10’” cmm3, respectively.
was used as the probe beam for R and PR measurements. A chopped He-Cd laser beam was used in PR measurements to provide a repetitive modulation to the samples. The amount of residual strain in ZnSe: N epilayers with various [N] was directly measured with X-ray rocking curves using Cu K a , radiation diffracted by double-crystal monochromater. By measuring reflections from two lattice planes nonparallel to the surface, the in-plane and perpendicular lattice constants were determined. The strain was calculated from these lattice constants and the unstrained lattice constant of bulk ZnSe.
3. Experimental
results and discussion
Fig. 1 shows PL, PR and R spectra at 14 K obtained from undoped MBE-grown ZnSe, Ndoped ZnSe with NA -N, = 8 X 1016 cm-3, and chlorine-doped ZnSe with No - NA = 2 x 10” cmp3, respectively. The dips corresponding to the lower branch of free exciton energy due to strain were clearly observed at 2.801 eV in the R spec-
2. Zhu et al. /Journal
tra from the undoped and Cl-doped epilayers. The photoreflectance of the undoped epilayer yielded dips at 2.804 and 2.815 eV related to the upper branch of free exciton energy and excited exciton state (n = 2). The binding energy of free exciton is estimated to be 21 meV and the band gap energy 2.822 eV. However, both the reflectance and photoreflectance obtained from Ndoped epilayer yielded dips at 2.797 eV, indicating the decrease in the free exciton energy in N-doped ZnSe compared with undoped and Cldoped ZnSe. The free exciton energy is influenced by nitrogen concentration in ZnSe. Fig. 2 shows the R and PR spectra at 14 K obtained from ZnSe: N with various NA -No values ranged from 0.8 to 13 X 1016 cmp3. Both the reflectance and photoreflectance yielded dips at free exciton energies. The free exciton energy obtained from the reflection spectrum coincides with that from the
ZnSe:N
1 14K
1 Photon
energy
799
of Crystal Growth 150 (1995) 797-802
(eV)
Fig. 2. Photoreflectance and reflectance spectra at 14 K obtained from ZnSe: N with various NA - No values ranging from 0.8 to 13x lOI6 cm-3. The reflectance and photoreflectance yielded dips at free exciton energies.
He-Cd laser 1OmW Halogen lamp 14K NA-ND=lxlO”cm.:
%
I
2.: 75
I
I
2.80
2.85
Photon
energy
(eV)
Fig. 3. Photoreflectance spectrum of ZnSe: N with NA - No = 8~ lOI cmm3. Two dips related to the free excitons at the excited (2.811 eV) and ground states (2.797 eV) appear.
photoreflectance spectrum. The free exciton energy decreases from 2.802 to 2.790 eV when NA -N, increases from 0.8 to 13 X lOI cme3. For a specific sample with NA -No = 8 X 10’” cme3, we obtained the photoreflectance spectrum where two dips related to the free excitons at the excited (2.811 eV> and ground states (2.797 eV) appear, as shown in Fig. 3. Thus, the binding energy of free exciton was calculated to be 21 meV and the band gap energy 2.817 eV. The temperature dependence of the free exciton energy in ZnSe : N has been investigated. Fig. 4 shows the R and PR spectra obtained from a sample with NA -N, = 8 X lOI cmm3 at various temperatures. The dips related to free exciton energy appear in both R and PR spectra until 200 K. The free exciton energy obtained has been plotted against temperature [l], which can be well fitted by using empirical Varshni’s expression. The emission energies of shallow DAP, FA (free-electron-to-acceptor) and DF (donor-tofree-hole) obtained from PL spectra showed the same temperature dependence as that of the free exciton energy [l]. These facts confirm that the temperature dependence of the exciton energy obtained experimentally reflects that of the band gap energy of ZnSe : N. The free exciton energy decreases with increasing [N]. Accordingly, the DAP peak position
800
of Crystal Growth 150 (1995)
Z. Zhu et al. /Journal
N,- Nn=8x1016(cmJ)
ZnSe:N
797-802
le-Cd laser
ZnSe:F
OmW 14K
FmK
iL_
2.779eV ’ OOK_,&___ QOK
2
t
At*
2.736ev”
I
2.7
I
2.0 Photon
I n I
I
\I v,k-- ^I
I
I
2.7 energy (eV)
2.8
I
Fig. 4. Photoreflectance and reflectance spectra of 2nSe:N with NA - No = 8 x lOI cmm3 in a temperature range from 14 to 200 K. 2.
Photon
in PL spectra shifts to a low energy side. Fig. 5 shows the PL spectra obtained from samples grown at various microwave plasma powers ranging from 80 to 170 W, where the substrate temperature is kept at 250°C and the beam pressure ratio of Zn to Se was set at l/2. The shallow DAP emissions are dominant for the powers below 120 W, while the deep DAP emissions dominate the spectra for the powers above 150 W. As the power, i.e., the nitrogen concentration increases, both shallow and deep DAP emission energy shifts to low energy side. These results suggest decrease in the DAP emission energy due
free exciton energy, and several near-band-edge Power(W)
Undoped
0
_
1 2 3 4
80 100 100 120
8x 5x 8x 1x
10’5 1o16 1o16 10”
5 6
140 150
7
170
1.3 x 101’ 1 x 10’5 (High p) (High p)
(eV)
to lowering in band gap of ZnSe : N. For a comparison, the NA -No value, free exciton energy, and several near-band-edge emission energies are summarized in Table 1. The relationship between lattice constant and nitrogen concentration in the concentration range of 1 X 10” to 2 x 1019 cme3 was investigated by Petruzzello et al. using X-ray diffraction [7]. They indicated that the ZnSe lattice constant decreases
Sample
(cmm3)
energy
Fig. 5. Photoluminescence spectra from ZnSe: N epilayers grown at various microwave plasma powers ranged from 80 to 170 W.
Table 1 Value of NA -No
NA -No
:iT6
2.7
1
emission energies
E,, kV) 2.802 2.815 (n = 2) 2.802 2.798 2.797 2.796 2.811 (n = 2) 2.790 2.788
I, or If (eV) _
DAP (eV)
2.786 2.780 2.781 2.779
2.696 2.688 2.693 2.689
_ 2.770
2.670 2.671
_
2.658
_
Z. Zhu et al. /Journal
as the N concentration increases and that the reduction in lattice constant is greater than what can be explained by the shorter Zn-N bond distance of model predictions. The excess lattice contraction was attributed to the generation of point defects accompanying N doping. The strain relaxation due to N doping results in layers that contain residual compressive strain [7]. These phenomena were also observed in our samples. Fig. 6 shows the ZnSe lattice constants parallel (a,,) and perpendicular (a I> to the interface as a function of the free exciton, where the in-plane and perpendicular lattice constants were determined by measuring reflections from lattice planes of (001) and (117) nonparallel to the surface. The unstrained lattice constant of ZnSe : N (a,), strains parallel (E,,) and perpendicular (E I) to the interface were calculated from the measured inplane and perpendicular lattice constants, as shown in Fig. 7, using well-known relationships: a,=
[a,+2(C,*/C,,)a,,]/(I
+2C,,/C,,),
(1)
El/= (a,, - %)/a,,
(2)
El=
(3)
(a,
-%)/%.
801
of Crystal Growth 150 (1995) 797-802
Here, C,, and C,, are the elastic compliance constants, and ub is the unstrained lattice constant of bulk ZnSe. C,,, C,, and ub are assumed to be 8.10 X 10” dyn/cm*, 4.88 X 10” dyn/cm2 and 5.669424 A [8], respectively. When the nitrogen concentration increases, the in-plane lattice
5.671,
5.661 2.705
.
,
.
,
.
r
. 2.790 Free
10.1
l
2.795 exciton
energy
2.800
EL
’
-0.3 2.805
(eV)
Fig. 7. The lattice constant, strains versus the free exciton to energy. E,, and Ed are strains parallel and perpendicular the interface, respectively.
constants remarkably decreases due to compressive strain induced by N doping, while the perpendicular lattice constant slight inelastically increases, which results in the decrease in the unstrained lattice constant. The free exciton energy decreases with the unstrained lattice constant. These facts strongly suggest that the possible origins of shrinkage of the band-gap energy associated with N doping are due to (i) tailing of states induced by high N impurity, (ii) generation of defects, (iii) the residual strain, and (iv) the large difference in electronegativity between N and Se. Strong bowing in band-gap energy in GaPN [9] and ZnSeTe [lo] alloys due to the large difference in electronegativity between anions is reported.
4. Conclusion
0 al 5.655 2.785
.
l
energy
Fig. 6. The experimental ZnSe lattice and perpendicular (a,,) to the interface energy.
1
2.805
2.795 Free exciton
a,,
(eV)
constants parallel (a,,) versus the free exciton
We have investigated the effect of the N concentration in ZnSe on the band-gap energy. The free exciton energy in the epilayers with different NA --No was measured by reflectance spectra and photoreflectance spectra. It is found that the free exciton energy decreases with increasing the N concentration, indicating the shrinkage of the band-gap energy due to N doping. The energies of FA, DF, shallow DAP and deep DAP emissions shift to low energy side accompanying the shrinkage of energy gap.
802
Z. Zhu et al. /Journal
of Crystal Growth 150 (1995) 797-802
The correlation between the free exciton energy and lattice constant or the strain due to N doping is discussed on the basis of the XRD study. Since both the lattice constant and the free exciton energy decreases with increasing the nitrogen concentration, the shrinkage of the bandgap energy is correlated to be due to (i) tailing of states induced by high N impurity, (ii) generation of defects, (iii) residual strain, and (iv) the large difference in electronegativity between N and Se.
References [l] Z. Zhu, K. Takebayashi, K. Tanaka, T. Ebisutani, J. Kawamata and T. Yao, Appl. Phys. Lett. 64 (1994) 91.
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