Heterointerface control of ZnSe based II–VI laser diodes

applied

surface science Applied Surface Science 117/ 118 ( 1997) 7 19-724

Heterointerface

control of ZnSe based II-VI laser diodes

S. Itoh *, S. Tomiya, R. Imoto, A. Ishibashi Sony Corporation Research Center, Fujitsuka 174, Hodogaya-ku, Yokohama 240, Japan

Abstract The density of pre-existing crystal defects such as stacking faults is reduced less than 3 X lo3 cm-* when there is a GaAs buffer layer and when a GaAs layer is irradiated with Zn flux prior to ZnSe growth. This progress in controlling a GaAs/ZnSe heterointerface has made possible a ZnCdSe/ZnSSe/ZnMgSSe separate-confinement heterostructure laser diode with a lifetime of over 100 h at room temperature under CW conditions. We believe that we have entered into a stage where the operation of II-VI laser diodes is limited by recombination-enhanced defect reactions. We have observed surface roughening in ZnSe layers, as well as in ZnSSe and ZnMgSSe layers, all grown under II-rich conditions. The compositional modulation in ZnMgSSe is indicated to be caused by corrugations on the surface, but not by the instability of the material. Keywords: ZnMgSSe;

II-VI

laser diode; Wide bandgap;

Blue-green

1. Introduction Short wavelength light-emitting semiconductor devices have been studied intensively to realize fullcolor devices and the next generation of high density optical recording systems. &Se-based II-VI semiconductor systems provide the wide bandgap required for such blue-green light emitters. Since the first demonstration of ZnSe-based lasers by Haase et al. in 1991, rapid progress in the area of II-VI laser research has realized room-temperature continuouswave (CW) operation of blue-green laser diodes [l-4]. Further developments of the blue-green laser diodes have been demonstrated, including several hour CW operation and low threshold current pulsed operation [5-91. The device characteristics of II-VI laser diodes are becoming as good as those of wellestablished III-V laser diodes, except for device

* Corresponding author. Tel.: + 81-45-3536832; 3536905; e-mail: [email protected]. 0169-4332/97/$17.00 Copyright PII SO169-4332(97)00011-l

fax: + 81-45

laser diode; ABM; REDR

lifetime [lo]. Now, the device lifetime under CW operation exceeds 100 h [Ill. Incorporating Mg in Zn-chalcogenides has provided a wide range of II-VI materials lattice-matched to a GaAs substrate 1121. ZnMgSSe compounds have made room-temperature CW operation possible with a ZnCdSe/Zn(S)Se/ZnMgSSe separate-confinement heterostructure (SCH). This structure is now commonly used for ZnSe-based blue-green lasers. Several heterointerfaces are involved in the SCH, including the GaAs/ZnSe interface, the quantum well active layer and p-type ZnSe-ZnTe multiquantum well contact layers. Recent studies of degradation in II-VI light emitters have revealed that the degradation of devices is correlated to the crystal defects originating at the GaAs substrate/ZnSe epilayer interface during growth [13-151. The growth process of the GaAs/ZnSe heterointerface is now drawing much attention. The phase separation leading to periodic compositional modulation has been reported by Hua et al. in

0 1997 Elsevier Science B.V. All rights reserved.

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ZnMgSSe layers [16]. The modulation is related to the undulation of heterointerfaces in II-VI layers. We have observed surface roughening of a ZnSe layer which is grown under II-rich conditions. We think the compositional modulation in ZnMgSSe is related to surface roughening.

insulator

(1997) 719-724 Pd I Fft/ Au p-electrode

ZnTe : N layer ZnSe : N I ZnTe :N MQW ZnSe : N layer ZnSSe : N layer

ZnMgSSe : N cladding layer

2. Reducing stacking fault density at GaAs / II-VI interface

ZnSSe : N guiding layer ZnCdSe quantum well ZnSSe : Cl guiding layer

The schematic structure of the II-VI laser diodes is shown Fig. 1. A ZnCdSe quantum well is sandwiched between ZnSSe optical guiding layers. ZnMgSSe layers are used to give good carrier confinement. p-type ZnTe/ZnSe multi-quantum layers are grown to lower the operating voltage by reducing effective valence-band discontinuity between ZnSe and ZnTe. The lengthening of the device lifetime is now a major concern. Crystal defects, such as stacking faults and related threading dislocations, have been detected as dark spots in electroluminescence (EL) and identified as the origin of the (001) dark-line defects (DLDs) which limit the lifetime of laser diodes. Therefore, it is necessary to reduce grown-in crystal defects in the stripe region of laser diodes. Several groups have been investigating ways to reduce stacking faults originating at the GaAs/ZnSe heterointerface [17,18]. We have reported that the growth of the GaAs buffer layer prior to the growth of II-VI layers and the ZnSe buffer layer is indispensable in reducing the stacking faults to a defect density of lo5 cm-2 [19]. Because the surface roughness of GaAs could cause the generation of stacking faults, we have studied the microscopic surface morphology of GaAs substrates to develop a procedure to reduce the stacking-fault density. The surface after thermal oxide desorption of the (001)GaAs substrate observed using atomic force microscopy (AFM) is rough, as has been previously reported [20,21]. Fig. 2 is a typical thermally cleaned GaAs AFM image taken in air under contact mode conditions. The substrates were chemically etched, followed by thermal cleaning at 580°C under As, beam exposure in an MBE chamber for 20 min. A clear (2 X 4) structure was observed by means of reflection high-energy electron diffraction (RHEED)

ZnMgSSe : Cl cladding layer ZnSSe : Cl buffer layer ZnSe : Cl buffer layer GaAs

: Si buffer layer

GaAs

: Si substrate

In n-electrode Fig. 1. Schematic structure of a ZnCdSe/ZnSSe/ZnMgSSe gain-guided laser diode.

SCH

after the oxide desorption process. Large and deep holes with a density of lo*-10’ cm-’ have been observed. The depth of the holes exceeds 200 A. Surface morphology and the density of holes were nearly the same as without As, exposure. In both cases, we found the stacking-fault density in the II-VI layers was 106-lo7 cmd2. Although the hole density is higher than the stacking-fault density in the II-VI layers by two orders of magnitude, we believe that stacking-fault density is related to the microscopic surface morphology, because the growth conditions on the sharply sloped area of some holes are different from those on the flat and slightly sloped area of other holes. After the growth of the GaAs buffer layer with a thickness of 250 nm, only 4 to 8 monolayer-high steps0 can be observed with a step width of about 1000 A in the AFM image. The stacking-fault density in the II-VI layers grown on the GaAs buffer layer was reduced to a range of mid lo4 to lo5 cme2 in our laboratory, which led to device lifetimes of a few hours. Recent studies of the GaAs/ZnSe interface suggest that the generation of stacking faults is related to the surface chemistry of the interface atoms [ 18,221. Zn irradiation before ZnSe growth has been

S. Itoh et al./Applied

Fig. 2. Atomic force microscopy

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(AFh4) image of thermally

shown to be effective in reducing stacking-fault density. When (2 X 4)GaAs surface is irradiated with Zn before ZnSe growth, we obtain II-VI layers with a stacking-fault density of lower than lo4 cmP2. This procedure has reduced the previous stacking-fault density by one order of magnitude. However, the stacking-fault density in the range of lo3 cmm2 is still higher than a defect density of mid lo2 cm-’ in the GaAs substrate. The mechanism of the stacking fault formation is still not clear and needs to be investigated. However, we confirmed that the defect density in II-VI layers has been reduced to a point at which there are no pre-existing crystal defects in the stripe region. The structure of the laser diode is the same as is shown in Fig. 1 [ll]. The n-type dopine level was 2 X 1017 cme3. The p-type doping levels in the cladding layers and in the Zn,,Mg,.,S,.,,Se,.,, ZnSSE, ZnSe and ZnTe contact layers were 1.8 X 1017, 8 X 1017, 8 X 1017 and 1 X 1O’9 cmd3, respectively. The growth conditions of these layers were similar to those of LDs described elsewhere [19]. The laser diodes were mounted p-side down on a heatsink. High refractive facet coatings (70-90%) were used to reduce the threshold current. The threshold current Zth with a 600 pm cavity length under CW operation was 32 mA, which corre-

cleaned GaAs substrate with As, exposure in the MBE chamber.

sponds to a threshold current density Jth of 533 A/cm2. The threshold voltage was 11 V and the lasing wavelength is 514.7 nm under CW operation. The results of a lifetime test with two samples under CW operation in a 20°C ambient temperature are shown in Fig. 3, where the light output power of the laser diodes is set at 1 mW/facet. A 101.5 h device lifetime has been achieved, which surpasses the previous longest device lifetime by more than an order of magnitude. Since one crystal defect in the stripe can cause the propagation of secondary defects leading to a failure of the laser diode, devices with a defect-free stripe 60

,

I

,

,

8

,

RT ImW W=lOpm,L=600pm

0’ 0

’

’

’

’

’

J

20

40

60

60

100

120

Time (hour)

Fig. 3. Aging characteristics at room temperature.

of a laser diode under CW operation

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area are required for long lifetime of laser diodes. The dark-spot density in our laser diodes estimated from EL observation is lower than 3 X lo3 cm-*, which is sufficient to obtain laser diodes free of dark spots in their stripe region, even if there is current spreading at the stripe. Therefore, we consider that the failure of these devices is caused by recombination-enhanced defect reaction (REDR), not by the propagation of DLDs. A further experiment to clarify the degradation mechanism is now being performed. 3. Surface morphology of ZnSe and related epitaxial layers While the growth of II-VI materials has been studied intensively, its study is not as advanced as that of the growth of III-V materials. The microscopic surface structure of II-VI layers has received some attention [ 15,231. Periodic compositional modulation in ZnMgSSE epitaxial layers has been reported and shown to lead to corrugations at the top surface of epitaxial laser layers. This suggests that the ZnMgSSe layer is eventually decomposed because of its miscibility gap. However, the Extended X-ray absorption fine structure study suggests that the clustering of chalcogen atoms in both ZnSSe and ZnMgSSe is on a very small scale [24]. We have observed surface roughening in the ZnSe layer, as well as in ZnSSe and ZnMgSSe layers, all growth by MBE under II-rich conditions [25]. Fig. 4(a) shows the AFM image of a ZnSe layer.

The

(1997) 719-724

AFM

images

for ZnS,,,,Se,,,, and also shown in Fig. 4(b), Zn,,3Mgo.,,S,.,,Se,,*, (c). These three layers were grown on a semi-insulating (0Ol)GaAs substrate with elemental Zn, Se, Mg and a ZnS compound. The growth temperature was 280°C. During the growth, the RHEED pattern of the surface reconstruction was c(2 X 2) which indicated the surface was Zn-stabilized and that the growth was done under II-rich conditions. The thickness of films was about 1 pm. The images in Fig. 4 show the elongated corrugations in the [liO] direction on the sample surfaces. In the case of ZnSe, the corrugations have an average amplitude of 11 nm and an average period of 65 nm. The maximum amplitude exceeds 40 nm. Similar corrugations on the surface are observed in both the ZnSSe and the ZnMgSSe layers with average amplitudes of 14 nm for the ZnSSe and 4 nm for the ZnMgSSe. The average period of the corrugation for ZnSSe and ZnMgSSe is 61 and 48 nm, respectively. The surface roughening during growth seems to be related not to the lattice mismatching of the films to the GaAs substrate, but to the growth kinetics. Because we used a GaAs substrate, lattice mismatching of 0.26% exists between ZnSe and the GaAs substrate. Therefore, the ZnSe film suffers compressive strain, and a 0.07% lattice mismatch is found for the ZnSSe film by X-ray diffraction. No significant difference in surface morphology between ZnSSe and ZnSe can be observed. The ZnMgSSe has a 0.7% mismatch to the GaAs substrate. are

S. Itoh et al/Applied

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’ 200

Surface Science 117/118

I 250

300

350

400

Growth Temperature (C)

Fig. 5. Temperature

dependence

of average period of corrugations.

The growth temperature dependence of the average period of corrugation is shown in Fig. 5 for ZnS0.07Se0.9, films. The growth temperatures were from 250°C to 350°C. The Zn/Se ratio for growth was 0.48, determined by the nominal values of the beam flux gauge. The lattice constants of the films were controlled so they were ZnS0.07Se0.9, lattice-matched to the GaAs substrate by increasing the ZnS beam flux intensity as the growth temperature increased. The average period of corrugation increases as the growth temperature increases, but the average amplitude does not change. In the case of ZnSe, a similar temperature dependence of the average period is observed. Although the strain due to lattice mismatching in ZnSe is larger than that of ZnS,,,,Se,,,,, the average period of corrugation of both films is also about 60 nm at a growth temperature of 280°C and the temperature

(4

Fig, 6. Am 1.3.

image nf ZnSSe layers grown under II-rich conditions

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dependence is similar. The increase of the average period with an increase in growth temperature suggests that the surface diffusion length of adatoms is related to the corrugation. We compared the surface morphology of ZnSSe epitaxial film grown at a lower VI/II of 0.34 with that shown Fig. 6. The average period is 60 nm, which is almost the same as that of films grown at a VI/II of 0.48. However, the average amplitude is a little larger. At high VI/II in the range where the RHEED pattern exhibits (2 X 1) reconstruction of the Se-stabilized surface, we can observe rounded grains on the surface instead of corrugations, as shown in Fig. 6. Therefore, surface roughening during growth is related not to the lattice mismatching of the films to the GaAs substrate, but to the growth kinetics. The small amplitude and period of corrugation of ZnMgSSe may be due to the short diffusion length of Mg atoms on the surface because Mg atoms adhere more easily than Zn atoms. Phase separation has been observed in ZnMgSSe layers and may cause periodic compositional modulation. Wu et al. have suggested that ZnMgSSe is not stable and discomposes [23]. We think, however, that the strain field caused by corrugations on the surface affects the surface migration of Se and S atoms. We also think the difference of migration length or the desorption rate of S and Se may cause the phase separation in ZnMgSSe. A further experiment is required to clarify the mechanism of phase separation.

(IN

(a), and under VI-rich conditions

6). VI/II

ratio was (a) 0.34 and 6)

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4. Summary The density of pre-existing crystal defects such as stacking faults is reduced when there is a GaAs buffer layer on the GaAs substrate and when a GaAs layer is irradiated with Zn flux before ZnSe growth. A lifetime of over 100 h has been achieved at room temperature under CW conditions for a ZnCdSe/ZnSSe/ZnMgSSe separate-confinement heterostructure laser diode with a dark-spot density lower than 3 X lo3 cme2. We believe that we have entered into a stage in which the operation of II-VI laser diodes is limited by REDR. We have observed the surface roughening in the ZnSe layer, as well as in ZnSSe and ZnMgSSe layers, all grown by MBE under II-rich conditions. Compositional modulation in ZnMgSSe may be caused by the corrugations on the surface, but not by the instability of the material.

Acknowledgements The authors would like to acknowledge K. Nakano, H. Okuyama, H. Tsukamoto and E. Morita for helpful discussions. We would also like to thank Dr. T. Yamada for encouragement during the course of this work.

References [l] M.A. Haase, J. Qiu, J.M. DePuydt, H. Cheng, Appl. Phys. Lett. 59 (1991) 1272. [2] J.M. Gaines, R.R. Drenten, K.W. Haberem, T. Marshall, P. Mensz, J. Petruzzello, Appl. Phys. Lett. 62 (19931 2462. [3] S. Itoh, H. Okuyama, S. Matsumoto, N. Nakayama, T. Ohata, T. Miyajima, A. Ishibashi, K. Akimoto, Electron. Lett. 29 (1993) 766. [4] N. Nakayama, S. Itoh, T. Ohata, K. Nakano, H. Okuyama, M. Ozawa, A. Ishibashi, M. Ikeda, Y. Mori, Electron. Lett. 29 (1993) 1488. [5] N. Nakayama, S. Itoh, H. Okuyama, M. Ozawa, T. Ohata, K. Nakano, M. Ikeda, A. Ishibashi, Y. Mori, Electron. Lett. 29 (1993) 2194.

(1997) 719-724

[6] S. Itoh, N. Nakayama, S. Matsumoto, M. Nagai, K. Nakano, M. Ozawa, H. Okuyama, S. Tomiya, T. Ohata, M. Ikeda, A. Ishibashi, Y. Mori, Jpn. J. Appl. Phys. 33 (1994) L938. [7] D.C. Grillo, J. Han, M. Ringle, G. Hua, R.L. Gunshor, P. Kelkar, V. Kozlov, H. Jeon, A.V. Nurmikko, Electron. Lett. 30 (1994) 2131. [S] M.A. Haase, P.F. Baude, M.S. Hagedom, J. Qiu, J.M. DePuydt, H. Cheng, S. Guha, G.E. Hofler, B.J. Wu, Appl. Phys. Lett. 63 (1993) 2315. [9] K.-K. Law, P.F. Baude, T.J. Miller, M.A. Haase, G.M. Haugen, K. Smekalin, Electron. Lett. 32 (1996) 345. [lo] A. Ishibashi, J. Selected Topics Quantum Electron. 1 (1995) 741. [ 1 l] S. Taniguchi, T. Hino, S. Itoh, K. Nakano, N. Nakayama, A. Ishibashi, M. Ikeda, Electron. Lett. 32 (1996) 552. [12] H. Okuyama, K. Nakano, T. Miyajima, K. Akimoto, Jpn. J. Appl. Phys. 30 (1991) L1620. [13] S. Guha, J.M. DePuydt, J. Qiu, G.E. Hofler, M.A. Haase, B.J. Wu, H. Cheng, Appl. Phys. Lett. 63 (1993) 3023. 1141 G.C. Hua, N. Otsuka, D.C. Grillo, Y. Fan, J. Han, M.D. Ringle, R.L. Gunshor, M. Hovinen, A.V. Nurmikko, Appl. Phys. Lett. 65 (1994) 1331. [15] S. Tomiya, E. Morita, M. Ukita, H. Okuyama, S. Itoh, K. Nakano, A. Ishibashi, Appl. Phys. Lett. 66 (1995) 1208. [16] G.C. Hua, N. Otsuka, D.C. Grillo, J. Han, L. He, R.L. Gunshor, J. Cryst. Growth 138 (1994) 367. [ 171 L.H. Kuo, L. Salamanca-Riba, B.J. Wu, G.M. Haugen, J.M. DePuydt, G.E. Hofler, H. Cheng, J. Vat. Sci. Technol. B 13 (1995) 1694. [18] CC. Chu, T.B. Ng, J. Han, G.C. Hua, R.L. Gunshor, E. Ho, E.L. Warlick, L.A. Kolodziejski, A.V. Nurmikko, Appl. Phys. Lett. 69 (1996) 602. [19] S. Itoh, A. Ishibashi, J. Cryst. Growth 150 (1995) 701. [20] G.W. Smith, A.J. Pidduck, C.R. Whitehouse, J.L. Glasper, J. Spowart, J. Cryst. Growth 127 (1993) 966. [21] R. Hey, M. Wassermeier, J. Behrend, L. Daweritz, K. Ploog, H. Rabit, J. Cryst. Growth 154 (1995) 1. [22] L.H. Kuo, L. Salamanka-Riba, B.J. Wu, G. Hofler, J.M. DePuydt, H. Cheng, Appl. Phys. Lctt. 67 (1995) 3298. [23] B.J. Wu, J.M. DePuydt, G.M. Hofler, M.A. Haase, H. Cheng, S. Guha, J. Qiu, L.H. Kuo, L. Salamanca-Riba, Appl. Phys. Lett. 66 (1995) 3462. [24] T. Maruyama, T. Ogawa, K. Akimoto, Y. Kitajima, S. Itoh, A. Ishibashi, Jpn. J. Appl. Phys. 34 (1995) L539. [25] S. Tomiya, R. Minatoya, H. Tsukamoto, S. Itoh, K. Nakano, E. Morita, A. Ishibashi, M. Ikeda, 23rd Int. Conf. on Physics and Semiconductor 1996, TuP-48, Schematic structure Berlin, Germany.