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ZnSe-based laser diodes and p-type doping of ZnSe
Physica B 185 (1993) North-Holland
PHIllCA Ll
112-117
ZnSe-based K. Ohkawa,
laser diodes and p-type doping of ZnSe
A. Tsujimura,
Central Research Laboratories,
S. Hayashi, S. Yoshii and T. Mitsuyu
Matsushita Electric Industrial Co., Ltd., Moriguchi,
Osaka, Japan
Highly conductive p-type ZnSe layers have been grown by molecular beam epitaxy with nitrogen radical doping. Active nitrogens responsible for doping are N, metastables in the A3C: state. The free-hole concentration of N-doped ZnSe is of the order of 10” cm-j at room temperature. Laser diode action has been observed from ZnCdSe single quantum well structures grown on GaAs substrates without GaAs buffer layers. Coherent light was observed at 490-520 nm at 77 K. The minimum threshold current density was as low as 160 A/cm* under pulsed operation.
1. Introduction
ZnSe-based semiconductors such as ZnSe, ZnSSe and ZnCdSe are promising materials for light-emitting diodes (LEDs) and laser diodes (LDs) from green to violet [l]. Difficulties in amphoteric doping, especially p-type doping, of ZnSe had hindered the achievement of such devices. It is possible to obtain n-type ZnSe layers by doping with group III elements such as Ga [2]. However, electron concentration of Ga-doped ZnSe has been limited in the order of 1Or’ crnd3 so far. We have investigated Group VII elements as an n-type dopant, and attempted Cl doping in molecular beam epitaxy (MBE) [3] for the first time. We employed ZnCl, as the Cl source. Electron concentration of Cl-doped ZnSe layers exceeded 1019 cme3. Cl-doped ZnSe layers with a moderate doping level exhibited strong bandedge photoluminescence (PL) emission at room temperature. Intensity of the PL emission was four times greater than that from Ga-doped layers. These electrical and optical properties indicate that Cl-doped ZnSe layers have device quality. With respect to p-type doping, many attempts Correspondence to: K. Ohkawa, Matsushita Electric oratories, Moriguchi, Osaka 570, Japan. 0921-4526/93/$06.00
0
Central Research Industrial Co.,
1993 - Elsevier
Science
LabLtd..
Publishers
at the MBE process were made to incorporate shallow acceptors into ZnSe to realize p-type conduction. It was extremely difficult to grow reproducible p-type ZnSe. Nitrogen is the most promising element as a p-type dopant [4]. Doping of nitrogen acceptors in the MBE process is very difficult, because of the low sticking coefficient of N, and NH, molecules. In our previous work [S], we attempted ion doping with a high purity N: beam in order to enhance the sticking coefficient of nitrogen. N-doped ZnSe layers grown by this method exhibited good low temperature PL spectra; a dominant acceptor-bound exciton emission (I,) and well-suppressed other emissions. However, the electrical properties of the samples showed high resistivity. The crystallinity was degraded by ion damage in the case of heavy doping. We have renewed understanding of the importance of a damageless process coupled with enhancement of the sticking coefficient. Then we attempted nitrogen radical doping of ZnSe [6] for the first time. Reproducible p-type ZnSe layers are easily obtained by this technique. In the paper, we report the physics of radical doping and the properties of p-type ZnSe layers. Using amphoteric doping with Cl and active nitrogen, it is possible to fabricate optical devices such as blue LEDs [7] and LDs [S-lo]. The blue-green LDs previously reported [8,9] have ZnSe-based quantum well (QW) structures on
B.V. All rights
reserved
K. Ohkawa et al. I Z&e-based
GaAs substrates with GaAs buffer layers grown in an extra III-V MBE chamber. It is interesting to examine whether the GaAs buffer layer is necessary for the laser action. In this paper, we also report ZnSe-based LDs without GaAs buffer layers.
2. Nitrogen
113
laser diodes and p-type doping of ZnSe
3ol-----l
radical doping
Doping of nitrogen acceptors into ZnSe was made by a nitrogen radical beam during MBE growth. The beam source was mounted in the MBE chamber. The beam source operates by means of an electrical discharge created from inductively coupled radio-frequency (RF) excitation at 13.56MHz. Optical emission spectra obtained from N, and NH, plasma are shown in fig. 1. Every peak of the spectrum from the N, plasma has been assigned as the second (C311, --+ B311,) and the first (B311, --$ A3C:) positive emission bands of an N, molecule. These excited states of an N, molecule are shown in fig. 2. The Nl ionic radiative transition (B’C:+X*C, at 391 nm) was not observed except for the low pressure discharge. , N2
2nd
I
Cvl”
1
I
(a)
positive
-
N,
Bvle
,1
H. (b)
NH,
0
200
300
400
WAVELENGTH
Fig. 1. Optical NH, plasma.
emission
spectra
600
600
Fig. 2. Potential molecule.
(nm) from (a) N, plasma
and (b)
curves
for
4
3
DISTANCE
representative
6,
states
of a Nz
Strong H atomic lines (the Balmer series Ha,p,u) were observed in the spectrum of an NH, plasma. Other emission peaks shown in fig. l(b) are assigned as the first and second positive emission bands of an N, molecule. Nitrogen-related emissions in both cases are, therefore, N, molecular transitions (C311“-+B311,+A3C:). Considering the lifetime of each state in table 1, active nitrogens which can reach the substrate are only N, (A”C:) metastables. Park et al. [13] claim that N atoms are responsible for doping, since the color of the RF discharge is yellow. The yellow emission originates from the first positive transition of N, molecules. Furthermore, N atoms were not detected in the optical measurement. It is concluded that nitrogen species responsible for dop-
State
7
2
INTERNUCLEAR
Table 1 Lifetimes I
1
A?: B’rl C$
of spontaneous
of N, molecule
emission
for a N, molecule. Lifetime
Ref.
2.0 s 1.3 &S 40 ns
[Ill WI
WI
K. Ohkawa et al. I Z&e-hased
114
ing are presumably N, (A3Cl) metastables. Nakao et al. [14] have shown the mechanism of nitrogen radical doping. They indicate that the sticking coefficient of unexcited N, (X’C,‘) is low, but that of N, (A’C:) metastables is high. Using radical doping, the N concentration in the ZnSe layers reaches 1 X 10” cmp3 [6]. We used N, gas as the source of radical doping, since a broad deep-level emission was observed at around 2.15 eV in the case of NH, gas source [15]. We suppose that hydrogen is responsible for the 2.15eV deep level. The vibrational temperature of the N, metastables was about 4500 K which was evaluated from the second positive emission bands of the spectrum in fig. 1. It is natural to estimate its rotational temperature from classical equipartion of energy to be the same as the value of the kinetic energy. The kinetic energy equals the thermal energy from the temperature, around 1000 K, of the RF plasma cell. Thus it is estimated that N2 metastables have thermal energy (consists of kinetic, rotational and vibrational energies) of only 0.6 eV. The thermal energy is smaller than the Zn-Se bond energy, indicating that this radical doping is a low damage doping process. Figure 3 shows electrical properties of Ndoped ZnSe layers grown by radical doping. A typical structure of the samples is N-doped ZnSe ZnSe (1 km) /semi-insulating (2 km) / undoped GaAs substrate. The measurement was per-
laser diodes and p-type doping of ZnSe
formed by means of the Hall effect at 300 K. The highest carrier concentration of N-doped ZnSe is of the order of 10” cme3. Hole mobility depends on the N doping level; mobility decreases with increasing hole concentration. The theoretical limit of hole mobility for p-type ZnSe at 300 K is 110 cm*/(V s) [16]. Therefore the highest mobility of 86cm’/(V s) in fig. 3 indicates that the quality of our N-doped ZnSe layers is high. Figure 4 shows PL spectra (12 K) from lightly and heavily N-doped ZnSe layers on GaAs substrates. The lightly doped la er with hole concentration less than 1014cm- Yshows a dominant light-hole branch [17] of I, emission at 2.789 eV and weak donor-acceptor pair emission (DAP) at 2.694eV for the zero-phonon peak. The spectrum indicates that N2 metastables are incorporated as N acceptors in ZnSe and the donor concentration is very low. With increasing N acceptor concentration, the intensity of DAP emission increased and I, emission decreased. A typical PL spectrum from highly conductive ptype ZnSe grown by radical doping is shown in fig. 4(b). The heavily doped layer shows p-type conduction with hole concentration of 1.0 x 10” crne3 at 300 K. This p-type layer exhibits strong DAP emission at 2.679 eV. I, emission for
140/ ?
p-type
1201
300
ZnSe:NlGaAs K
I
I
t
440
460
500
480
l
0
I
10’5
I
1
CARRIER
I
CONCENTRATION
WAVELENGTH 10’8
10”
10’6
Fig. 3. Free-hole concentration ZnSe layers on GaAs substrates
I
(cm-31
and mobility at 3OOK.
of N-doped
(nm)
Fig. 4. PL spectra at 12 K from (a) lightly N-doped ZnSe layers with a hole concentration less than lOI cm ’ and (b) N-doped p-type ZnSe layers with a hole concentration of 1.0 X 10” cm-‘.
K. Ohkawa et al. I Z&e-based
the heavily doped layer was very weak, of the order of lo-‘, compared with the intensity of the DAP emission. Dominant DAP emission suggests that the p-type layer is compensated by donors. Donor species in p-type ZnSe grown by radical doping will be different from conventional donor elements such as Group III and VII, Since the conventional zero-phonon peak energy of DAP emission from N-doped ZnSe is around 2.70 eV [5]. The peak energy of DAP emission is given by the equation fi%‘U = E, -(E,
+ En) + e2/(41Tc,R)
115
laser diodes and p-type doping of ZnSe
(1)
where E,, E, and E, are the band-gap energy, acceptor ionization energy and donor ionization energy, respectively. E, for an N atom in the Se site (N,,) is 111 meV determined by Dean et al. [18]. The last term is the Coulomb term. Since the nitrogen concentration measured by secondary ion mass spectrometry is smaller than 101’ cme3, E, and E, may not be unchanged by the alloy effect. The decrease in DAP peak energy may be due to E, or the Coulomb term. The pair separation R decreases with increasing acceptor concentrations, and the Coulomb term will increase. The change in the Coulomb term, therefore, cannot be interpreted as the decrease of h%AP by 20 meV. The decrease would be ascribed to the donor ionization energy E,. Thus E, is as large as 50 meV. Such a deep donor is unknown so far since donor ionization energies of groups III and VII elements have 2.530meV Incorporated nitrogen may be responsible for the deep donors. There are a few possible donors: (1) an N donor in the Zn Site (N,,), (2) an N,,-N,, complex, (3) an Ns,-Vs, complex, (4) an interstitial N donor. Further study is necessary to find out the origin for the deep donors.
Zn,_,Cd,Se SQW with X= 0.2 or 0.3. The and valence band offsets for conduction Zn,_,Cd,Se (X = 0.3)/ZnSe are estimated to be AE, = 270 meV and AE, = 120 meV, respectively, by referring to the value in Ding’s report [19]. A laser device was fabricated by preparing a 1 mm long cleaved resonator structure with a 20-30 pm wide stripe electrode defining the top electrical contact. Silicon oxide (SiO,) was used as the insulating layer to define the current. Both end facets were uncoated. The device was mounted on Cu heat sinks with GaAs substrates down. Laser action was observed under pulsed current injection at 77 K. The current pulse duration was typically 1 t_~s,and a typical duty cycle was about 10-3. Spontaneous and stimulated emission spectra obtained from LDs with X= 0.2 and 0.3 are shown in fig. 5. A full width at half maximum
1
PULSED,
,
I
(9)
77K
4~3.4nm(2.5125eV) z z
I=1.26lth
5 z Ii
405
490
495
500
WAVELENGTH
I PULSED,
505
(nm)
I
I
I
it
77K
52o.lnm(2.3839eV)
IL
522.2nmC2.374eV) I=o.onl,h
3. ZnSe-based laser diodes
I=1.141th
x t
The laser structure is similar to that reported by Haase et al. [8]. The ZnCdSe SQW structures were grown directly on (10 0) n-GaAs substrates by MBE. There is no GaAs buffer layer, i.e. no extra III-V chamber. The active layer is
510
515
520
WAVELENGTH
525
530
x10
j 535
(nm)
Fig. 5. Spontaneous and stimulated emission spectra for LDs with (a) X= 0.2 and (b) X =0.3 under pulse current injection at 77 K.
K. Ohkawa et al. I ZnSe-based
116
laser diodes and p-type doping of ZnSe
20
0 0
100
200
300
400
served by spectroscopy of N, plasma in the beam source. Hole concentration of N-doped ZnSe layers exceeded 10” cmm3. DAP emission from the p-type layers appears at 2.68 eV in low temperature PL measurement. The peak energy is smaller than that from conventional N-doped ZnSe by 20 meV, and the difference may be ascribed to deep donors whose ionization energy is about 50 meV. We have shown that ZnSe-based LDs without GaAs buffer layers can emit coherent light under pulsed current injection at 77 K. The LDs with X = 0.3 operate with a very low threshold current of 160 A/cm’. It seems that lattice mismatch dislocations at the clad/waveguide interfaces are more important for the limitation of laser action.
CURRENT(mA) Fig. 6. The I-L characteristic under pulsed current injection
of a typical at 77 K.
LD
(X = 0.3)
Acknowledgements (FWHM) of spontaneous emission from the LD is about 5 nm (25 meV). A FWHM of stimulated emission is smaller than 0.08nm (0.4 meV). In laser action, the intensity of the electroluminescence was greatly increased, and a clear speckle pattern was observed. The laser beam has an elliptical far-field pattern, with a divergence of roughly 30” x 3”. The current versus output power from one end (Z-L) characteristics of a LD with X= 0.3 is shown in fig. 6. The maximum output power from one end was greater than 100 mW. The differential quantum efficiency measured was 39%. It is noted that the minimum threshold current density was as low as 160 A/cm’ where the total current was 38 mA in spite of uncoated facets. This threshold is half of that in the first report by Haase [S].
4. Conclusion Highly conductive p-type ZnSe layers were reproducibly grown by using nitrogen radical doping. The species responsible for doping are presumably N,(A3zl) metastables which are ob-
We wish to acknowledge the valuable contributions of H. Takeishi of Kagoshima Matsushita Electronics Co., Ltd.
References [I] R.N. Bhargava, J. Crystal Growth 117 (1992) 894. [2] T. Niina, T. Minato and K. Yoneda, Jpn. J. Appl. Phys. 21 (1982) L387. [3] K. Ohkawa, T. Mitsuyu and 0. Yamazaki, J. Appl. Phys. 62 (1987) 3216. [4] W. Stutius, J. Crystal Growth 59 (1982) 1. [S] K. Ohkawa, T. Mitsuyu and 0. Yamazaki, J. Crystal Growth 86 (1988) 329. [6] K. Ohkawa, T. Karasawa and T. Mitsuyu, in: Abstracts of 6th Int. Conf. on Molecular Beam Epitaxy, San Diego, 1990, PIII-21; J. Crystal Growth 111 (1991) 797. [7] K. Ohkawa, A. Ueno and T. Mitsuyu, Jpn. J. Appl. Phys. 30 (1991) 3873. [S] M.A. Haase, J. Qiu, J.M. DePuydt and H. Cheng, Appl. Phys. Lett. 59 (1991) 1272. [9] H. Jeon, J. Ding, W. Patterson, A.V. Nurmikko, W. Xie, D.C. Grillo, M. Kobayashi and R.L. Gunshor, Appl. Phys. Lett. 59 (1991) 3619. [lo] S. Hayashi, A. Tsujimura, S. Yoshii, K. Ohkawa and T. Mitsuyu, Jpn. J. Appl. Phys. 31 (1992) L1478. [ll] N.P. Carleton and 0. Oldenberg, .I. Chem. Phys. 36 (1962) 3460.
K. Ohkawa et al. / Z&e-based [12] W.A.
laser diodes and p-type doping of ZnSe
Fitzsimmons, L.W. Anderson, C.E. Riedhaush and J.M. Vrtilek, IEEE J. Quantum Electron. 12 (1976) 624. [13] R.M. Park, M.B. Troffer, C.M. Rouloeau, J.M. DePuydt and M.A. Haase, Appl. Phys. Lett. 57 (1990) 2127. [14] T. Nakao and T. Uenoyama, in: Extended Abstr. 1992 Int. Conf. on Solid State Devices and Materials, Tsukuba (1992) p. 336. [15] K. Ohkawa, T. Karasawa and T. Mitsuyu, Jpn. J. Appl. Phys. 30 (1991) L152.
117
[16] H.E. Ruda, J. Appl. Phys. 59 (1986) 3516. [17] K. Ohkawa, T. Mitsuyu and 0. Yamazaki, Phys. Rev. B 38 (1988) 12465. [18] P.J. Dean, W. Stutius, G.F. Neumark, B.J. Fitzpatrick and R.N. Bhargava, Phys. Rev. B 27 (1983) 2419. [19] J. Ding, N. Pelekanos, A.V. Nurmikko, H. Luo, N. Samarth and J. Furdyna, Appl. Phys. Lett. 57 (1990) 2885.
PHIllCA Ll
112-117
ZnSe-based K. Ohkawa,
laser diodes and p-type doping of ZnSe
A. Tsujimura,
Central Research Laboratories,
S. Hayashi, S. Yoshii and T. Mitsuyu
Matsushita Electric Industrial Co., Ltd., Moriguchi,
Osaka, Japan
Highly conductive p-type ZnSe layers have been grown by molecular beam epitaxy with nitrogen radical doping. Active nitrogens responsible for doping are N, metastables in the A3C: state. The free-hole concentration of N-doped ZnSe is of the order of 10” cm-j at room temperature. Laser diode action has been observed from ZnCdSe single quantum well structures grown on GaAs substrates without GaAs buffer layers. Coherent light was observed at 490-520 nm at 77 K. The minimum threshold current density was as low as 160 A/cm* under pulsed operation.
1. Introduction
ZnSe-based semiconductors such as ZnSe, ZnSSe and ZnCdSe are promising materials for light-emitting diodes (LEDs) and laser diodes (LDs) from green to violet [l]. Difficulties in amphoteric doping, especially p-type doping, of ZnSe had hindered the achievement of such devices. It is possible to obtain n-type ZnSe layers by doping with group III elements such as Ga [2]. However, electron concentration of Ga-doped ZnSe has been limited in the order of 1Or’ crnd3 so far. We have investigated Group VII elements as an n-type dopant, and attempted Cl doping in molecular beam epitaxy (MBE) [3] for the first time. We employed ZnCl, as the Cl source. Electron concentration of Cl-doped ZnSe layers exceeded 1019 cme3. Cl-doped ZnSe layers with a moderate doping level exhibited strong bandedge photoluminescence (PL) emission at room temperature. Intensity of the PL emission was four times greater than that from Ga-doped layers. These electrical and optical properties indicate that Cl-doped ZnSe layers have device quality. With respect to p-type doping, many attempts Correspondence to: K. Ohkawa, Matsushita Electric oratories, Moriguchi, Osaka 570, Japan. 0921-4526/93/$06.00
0
Central Research Industrial Co.,
1993 - Elsevier
Science
LabLtd..
Publishers
at the MBE process were made to incorporate shallow acceptors into ZnSe to realize p-type conduction. It was extremely difficult to grow reproducible p-type ZnSe. Nitrogen is the most promising element as a p-type dopant [4]. Doping of nitrogen acceptors in the MBE process is very difficult, because of the low sticking coefficient of N, and NH, molecules. In our previous work [S], we attempted ion doping with a high purity N: beam in order to enhance the sticking coefficient of nitrogen. N-doped ZnSe layers grown by this method exhibited good low temperature PL spectra; a dominant acceptor-bound exciton emission (I,) and well-suppressed other emissions. However, the electrical properties of the samples showed high resistivity. The crystallinity was degraded by ion damage in the case of heavy doping. We have renewed understanding of the importance of a damageless process coupled with enhancement of the sticking coefficient. Then we attempted nitrogen radical doping of ZnSe [6] for the first time. Reproducible p-type ZnSe layers are easily obtained by this technique. In the paper, we report the physics of radical doping and the properties of p-type ZnSe layers. Using amphoteric doping with Cl and active nitrogen, it is possible to fabricate optical devices such as blue LEDs [7] and LDs [S-lo]. The blue-green LDs previously reported [8,9] have ZnSe-based quantum well (QW) structures on
B.V. All rights
reserved
K. Ohkawa et al. I Z&e-based
GaAs substrates with GaAs buffer layers grown in an extra III-V MBE chamber. It is interesting to examine whether the GaAs buffer layer is necessary for the laser action. In this paper, we also report ZnSe-based LDs without GaAs buffer layers.
2. Nitrogen
113
laser diodes and p-type doping of ZnSe
3ol-----l
radical doping
Doping of nitrogen acceptors into ZnSe was made by a nitrogen radical beam during MBE growth. The beam source was mounted in the MBE chamber. The beam source operates by means of an electrical discharge created from inductively coupled radio-frequency (RF) excitation at 13.56MHz. Optical emission spectra obtained from N, and NH, plasma are shown in fig. 1. Every peak of the spectrum from the N, plasma has been assigned as the second (C311, --+ B311,) and the first (B311, --$ A3C:) positive emission bands of an N, molecule. These excited states of an N, molecule are shown in fig. 2. The Nl ionic radiative transition (B’C:+X*C, at 391 nm) was not observed except for the low pressure discharge. , N2
2nd
I
Cvl”
1
I
(a)
positive
-
N,
Bvle
,1
H. (b)
NH,
0
200
300
400
WAVELENGTH
Fig. 1. Optical NH, plasma.
emission
spectra
600
600
Fig. 2. Potential molecule.
(nm) from (a) N, plasma
and (b)
curves
for
4
3
DISTANCE
representative
6,
states
of a Nz
Strong H atomic lines (the Balmer series Ha,p,u) were observed in the spectrum of an NH, plasma. Other emission peaks shown in fig. l(b) are assigned as the first and second positive emission bands of an N, molecule. Nitrogen-related emissions in both cases are, therefore, N, molecular transitions (C311“-+B311,+A3C:). Considering the lifetime of each state in table 1, active nitrogens which can reach the substrate are only N, (A”C:) metastables. Park et al. [13] claim that N atoms are responsible for doping, since the color of the RF discharge is yellow. The yellow emission originates from the first positive transition of N, molecules. Furthermore, N atoms were not detected in the optical measurement. It is concluded that nitrogen species responsible for dop-
State
7
2
INTERNUCLEAR
Table 1 Lifetimes I
1
A?: B’rl C$
of spontaneous
of N, molecule
emission
for a N, molecule. Lifetime
Ref.
2.0 s 1.3 &S 40 ns
[Ill WI
WI
K. Ohkawa et al. I Z&e-hased
114
ing are presumably N, (A3Cl) metastables. Nakao et al. [14] have shown the mechanism of nitrogen radical doping. They indicate that the sticking coefficient of unexcited N, (X’C,‘) is low, but that of N, (A’C:) metastables is high. Using radical doping, the N concentration in the ZnSe layers reaches 1 X 10” cmp3 [6]. We used N, gas as the source of radical doping, since a broad deep-level emission was observed at around 2.15 eV in the case of NH, gas source [15]. We suppose that hydrogen is responsible for the 2.15eV deep level. The vibrational temperature of the N, metastables was about 4500 K which was evaluated from the second positive emission bands of the spectrum in fig. 1. It is natural to estimate its rotational temperature from classical equipartion of energy to be the same as the value of the kinetic energy. The kinetic energy equals the thermal energy from the temperature, around 1000 K, of the RF plasma cell. Thus it is estimated that N2 metastables have thermal energy (consists of kinetic, rotational and vibrational energies) of only 0.6 eV. The thermal energy is smaller than the Zn-Se bond energy, indicating that this radical doping is a low damage doping process. Figure 3 shows electrical properties of Ndoped ZnSe layers grown by radical doping. A typical structure of the samples is N-doped ZnSe ZnSe (1 km) /semi-insulating (2 km) / undoped GaAs substrate. The measurement was per-
laser diodes and p-type doping of ZnSe
formed by means of the Hall effect at 300 K. The highest carrier concentration of N-doped ZnSe is of the order of 10” cme3. Hole mobility depends on the N doping level; mobility decreases with increasing hole concentration. The theoretical limit of hole mobility for p-type ZnSe at 300 K is 110 cm*/(V s) [16]. Therefore the highest mobility of 86cm’/(V s) in fig. 3 indicates that the quality of our N-doped ZnSe layers is high. Figure 4 shows PL spectra (12 K) from lightly and heavily N-doped ZnSe layers on GaAs substrates. The lightly doped la er with hole concentration less than 1014cm- Yshows a dominant light-hole branch [17] of I, emission at 2.789 eV and weak donor-acceptor pair emission (DAP) at 2.694eV for the zero-phonon peak. The spectrum indicates that N2 metastables are incorporated as N acceptors in ZnSe and the donor concentration is very low. With increasing N acceptor concentration, the intensity of DAP emission increased and I, emission decreased. A typical PL spectrum from highly conductive ptype ZnSe grown by radical doping is shown in fig. 4(b). The heavily doped layer shows p-type conduction with hole concentration of 1.0 x 10” crne3 at 300 K. This p-type layer exhibits strong DAP emission at 2.679 eV. I, emission for
140/ ?
p-type
1201
300
ZnSe:NlGaAs K
I
I
t
440
460
500
480
l
0
I
10’5
I
1
CARRIER
I
CONCENTRATION
WAVELENGTH 10’8
10”
10’6
Fig. 3. Free-hole concentration ZnSe layers on GaAs substrates
I
(cm-31
and mobility at 3OOK.
of N-doped
(nm)
Fig. 4. PL spectra at 12 K from (a) lightly N-doped ZnSe layers with a hole concentration less than lOI cm ’ and (b) N-doped p-type ZnSe layers with a hole concentration of 1.0 X 10” cm-‘.
K. Ohkawa et al. I Z&e-based
the heavily doped layer was very weak, of the order of lo-‘, compared with the intensity of the DAP emission. Dominant DAP emission suggests that the p-type layer is compensated by donors. Donor species in p-type ZnSe grown by radical doping will be different from conventional donor elements such as Group III and VII, Since the conventional zero-phonon peak energy of DAP emission from N-doped ZnSe is around 2.70 eV [5]. The peak energy of DAP emission is given by the equation fi%‘U = E, -(E,
+ En) + e2/(41Tc,R)
115
laser diodes and p-type doping of ZnSe
(1)
where E,, E, and E, are the band-gap energy, acceptor ionization energy and donor ionization energy, respectively. E, for an N atom in the Se site (N,,) is 111 meV determined by Dean et al. [18]. The last term is the Coulomb term. Since the nitrogen concentration measured by secondary ion mass spectrometry is smaller than 101’ cme3, E, and E, may not be unchanged by the alloy effect. The decrease in DAP peak energy may be due to E, or the Coulomb term. The pair separation R decreases with increasing acceptor concentrations, and the Coulomb term will increase. The change in the Coulomb term, therefore, cannot be interpreted as the decrease of h%AP by 20 meV. The decrease would be ascribed to the donor ionization energy E,. Thus E, is as large as 50 meV. Such a deep donor is unknown so far since donor ionization energies of groups III and VII elements have 2.530meV Incorporated nitrogen may be responsible for the deep donors. There are a few possible donors: (1) an N donor in the Zn Site (N,,), (2) an N,,-N,, complex, (3) an Ns,-Vs, complex, (4) an interstitial N donor. Further study is necessary to find out the origin for the deep donors.
Zn,_,Cd,Se SQW with X= 0.2 or 0.3. The and valence band offsets for conduction Zn,_,Cd,Se (X = 0.3)/ZnSe are estimated to be AE, = 270 meV and AE, = 120 meV, respectively, by referring to the value in Ding’s report [19]. A laser device was fabricated by preparing a 1 mm long cleaved resonator structure with a 20-30 pm wide stripe electrode defining the top electrical contact. Silicon oxide (SiO,) was used as the insulating layer to define the current. Both end facets were uncoated. The device was mounted on Cu heat sinks with GaAs substrates down. Laser action was observed under pulsed current injection at 77 K. The current pulse duration was typically 1 t_~s,and a typical duty cycle was about 10-3. Spontaneous and stimulated emission spectra obtained from LDs with X= 0.2 and 0.3 are shown in fig. 5. A full width at half maximum
1
PULSED,
,
I
(9)
77K
4~3.4nm(2.5125eV) z z
I=1.26lth
5 z Ii
405
490
495
500
WAVELENGTH
I PULSED,
505
(nm)
I
I
I
it
77K
52o.lnm(2.3839eV)
IL
522.2nmC2.374eV) I=o.onl,h
3. ZnSe-based laser diodes
I=1.141th
x t
The laser structure is similar to that reported by Haase et al. [8]. The ZnCdSe SQW structures were grown directly on (10 0) n-GaAs substrates by MBE. There is no GaAs buffer layer, i.e. no extra III-V chamber. The active layer is
510
515
520
WAVELENGTH
525
530
x10
j 535
(nm)
Fig. 5. Spontaneous and stimulated emission spectra for LDs with (a) X= 0.2 and (b) X =0.3 under pulse current injection at 77 K.
K. Ohkawa et al. I ZnSe-based
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laser diodes and p-type doping of ZnSe
20
0 0
100
200
300
400
served by spectroscopy of N, plasma in the beam source. Hole concentration of N-doped ZnSe layers exceeded 10” cmm3. DAP emission from the p-type layers appears at 2.68 eV in low temperature PL measurement. The peak energy is smaller than that from conventional N-doped ZnSe by 20 meV, and the difference may be ascribed to deep donors whose ionization energy is about 50 meV. We have shown that ZnSe-based LDs without GaAs buffer layers can emit coherent light under pulsed current injection at 77 K. The LDs with X = 0.3 operate with a very low threshold current of 160 A/cm’. It seems that lattice mismatch dislocations at the clad/waveguide interfaces are more important for the limitation of laser action.
CURRENT(mA) Fig. 6. The I-L characteristic under pulsed current injection
of a typical at 77 K.
LD
(X = 0.3)
Acknowledgements (FWHM) of spontaneous emission from the LD is about 5 nm (25 meV). A FWHM of stimulated emission is smaller than 0.08nm (0.4 meV). In laser action, the intensity of the electroluminescence was greatly increased, and a clear speckle pattern was observed. The laser beam has an elliptical far-field pattern, with a divergence of roughly 30” x 3”. The current versus output power from one end (Z-L) characteristics of a LD with X= 0.3 is shown in fig. 6. The maximum output power from one end was greater than 100 mW. The differential quantum efficiency measured was 39%. It is noted that the minimum threshold current density was as low as 160 A/cm’ where the total current was 38 mA in spite of uncoated facets. This threshold is half of that in the first report by Haase [S].
4. Conclusion Highly conductive p-type ZnSe layers were reproducibly grown by using nitrogen radical doping. The species responsible for doping are presumably N,(A3zl) metastables which are ob-
We wish to acknowledge the valuable contributions of H. Takeishi of Kagoshima Matsushita Electronics Co., Ltd.
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