blue-green light emitting diode structures with ZnSe-based wide-gap semiconductors

¸

applied surface science Applied Surface Science 79/80 (1994) 27{) 274

ELSEVIER

Photo-assisted metalorganic vapor-phase epitaxy of blue/blue-green light emitting diode structures with ZnSe-based wide-gap semiconductors S h i z u o F u j i t a *, T a k e h a r u

Asano, Kensaku

Maehara,

Shigeo Fujita

Department of Electrical Engineering, Kyolo Unil,ersity, Kyoto 600-01..lapan

(Received 13 October 1993:accepted for publication ~ January" 1994)

Abstract Successful growth of well-defined ZnCdSe/ZnSc quantum well structures and acccptor doping in ZnSc arc demonstrated based on photo-assisted metalorganic vapor-phase epitaxy (MOVPE), which utilizes photocatalytic growth processes at the growing surface. Blue/blue-green electroluminescence from quantum wells in a double heterostructure diode suggests promising potential of this technique for fabrication of short wavelength optoelectronic devices such as light emitting diodes and lasers.

1. Introduction Recent great success of blue/blue-green light emitting diodes (LEDs) and laser diodes (LDs) with wide-gap I I - V I semiconductor quantum wells (QWs) grown by molecular beam epitaxy (MBE) strongly encourages the growth by metalorganic vapor-phase epitaxy (MOVPE) which is believed to be more suitable for mass production and for fabrication of novel laser structures. For this purpose, the problems in M O V P E growth still lay in structural control as well as p-type conductivity control. In this paper, we propose that photo-assisted MOVPE, which utilizes photocatalytic reactions [1,2], is a suitable growth technique in order to overcome these problems. Blue/blue-green electroluminescence (EL) from QWs, together with

* Corresponding author. Fax: (+ 81) 75 753 5898.

experimental results on photopumped lasing and acceptor doping, will demonstrate promising potentials of this technique for future applications to optoelectronic devices such as LEDs and LDs.

2. Advantages of photo-assisted MOVPE In M O V P E growth of ZnSe-based wide-gap semiconductors, alkyl-compounds such as dimethylzinc (DMZn) have been commonly used as group-II precursors. However, for fabrication of well-defined structures, we must pay attention to the appropriate choice of group-VI precursors. Use of hydrides such as H2Se results in epitaxial growth at the temperature as low as 300-350°C [3], but heavy occurrence of gas phase reactions with alkylzinc tends to obstruct uniform growth and smooth interfaces of QWs [4]. On the other hand, if alky[-compounds such as dimethylselenium (DMSe) are used, gas phase reactions can

0169-4332/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0169-4332(94)00062-6

Sz. Fujita et al. /Applied Surface Science 79/80 (1994) 270-274

be eliminated [5], and successful growth of highly planer epilayers compared to those grown with hydrides have been demonstrated [6]. However, in order to obtain the reasonable growth rate, e.g., 1 ~ m / h , the growth temperature should be as high as 450-500°C, which seems to be against the effective doping, high quality epilayers with low defect density, and abrupt interfaces. With photo-assisted MOVPE, the growth temperature using alkyl-chalcogens has successfully been lowered to be about 300-400°C [1,2]. Different from the photo-irradiation effects during the growth of I I I - V compounds and Si-related semiconductors, this technique utilizes photocatalytic reactions at the growth surface, where the carriers generated by above bandgap irradiation enhance the alkyl elimination from precursors (probably from alkyl-zinc [7,8]) chemisorbed at the growth surface [2]. p-type doping control is an another serious problem in MOVPE. Doping of Li has achieved the carrier concentration up to 9 × 1017 cm -3 [9], but Li is not always a stable dopant. Nitrogen doping, which was succeeded in MBE, has also been investigated by MOVPE, but the carrier concentration has been lower than the order of 1016 cm -3 [10-12]. We also recognize that the photo-irradiation is effective for acceptor doping, because (i) low growth temperature brings in more sticking of acceptor impurities, (ii) reduction of compensating defects and activation of acceptor impurities under photo-irradiation are theoretically predicted [13,14], and (iii) photocatalytic reactions may enhance the decomposition of a dopant precursor if it contains alkyls. From these points of view, photo-assisted M O V P E is expected to be promising both for fabrication of well-defined QWs and for conductivity control.

ZnCdSe SQW

271

ZnSe

ZnSe

ZnSe

ZnSe

GaAssub.

GaAssub.

(a) SQW

(b) MQW

ZnCdSe/ ZnSe MQW

Fig. 1. Examples of simple double heterostructure for LEDs and LDs with ZnCdSe/ZnSe single QW (SQW) or multiple QW (MQW).

mium (DMCd) and diethylselenium (DESe). Irradiation source was a 500 W xenon lamp, and minimum wavelength of the irradiation light was limited at 310 nm (4 eV) by inserting a filter in the irradiation path, in order to eliminate direct photolitic decomposition of source precursors in gas phase. The irradiation intensity on the sample was 45-80 m W / c m 2, where the growth rate was proportional to the intensity. Under the above growth conditions, the growth rate at the temperature of 350°C was 1 - 2 / . , m / h , which is reasonable for practical growth of total structures. However, we found that the composition and structure control in the QW was slightly inferior under irradiation compared to without irradiation; although we could not speculate the reason - the growth rate of 1-2 t x m / h might be too fast for the growth of QWs. Therefore, the QWs were grown without irradiation. The growth rate without irradiation was as low as 400-800 A / h , but was thought to be sufficient or preferable for the growth of QWs composed with thin ( < 100 A.) layers. o

4. Photopumped iasing o

3. Epitaxial growth Examples of the simple double heterostructure for LEDs and LDs are shown in Fig. 1. For the growth of these heterostructures, source precursors used were diethylzinc (DEZn), dimethylcad-

For the ZnCdSe(100 A ) / Z n S e single QW grown by the above procedure, an example of 4.2 K PL spectrum is shown in Fig. 2. The sharp and strong peak near 450 nm originated from the ZnCdSe well layer, while peaks near 443 nm are band edge (free exciton and donor-bound exciton) emissions in the ZnSe barrier layers. The

272

Sz. Fujita et al. /Applied Surface Science 79 / 80 (1994) 2 70- 2 74

4.2K

448.9nm FWHM=13.5meV

c~

._1

13440

460 Wavelength (nm)

480

the Cd composition in the Q W was 0.18, and the emission wavelength and the threshold were 497 nm and 150 k W / c m 2, respectively, at 15(1 K. C o m p a r e d to these, in our experiments the sample possessed lower Cd composition in the Q W and the excitation was done with shorter wavelength. Nevertheless, the threshold at 150 K was found to be comparable, i.e., 140 k W / c m e, suggesting that the well-defined heterostructures can be grown by photo-assisted MOVPE.

Fig. 2.4.2 K PL spectrum of Z n C d S e / Z n S e single QW.

5. Acceptor doping F W H M was sufficiently small, i.e., 13.5 meV. Cd composition in the well layer estimated from the PL peak position was 0.05. P h o t o p u m p e d lasing characteristics were investigated for the single Q W sample. An excitation source was a N 2 gas laser emitting 337 nm in wavelength. Fig. 3 shows the emission spectra measured at 77 K as a function of the excitation power intensity. Above threshold, Ith = 58 k W / c m 2, spectrum narrowing was observed, which indicates stimulated emission. Stimulated emission was observed up to 200 K, where lth = 280 k W / c m 2. In a MBE-grown Z n C d S e / Z n S e single QW, Ding et al. [15] reported p h o t o p u m p e d lasing using a dye laser as an excitation source. Here,

77K

A t--

In the previous studies, ZnSe layers grown under photo-irradiation tended to show n-type conductivity [16,17], probably because of high incorporation of halogen impurities in alkylzinc precursors [18]; this p h e n o m e n o n has seriously obstructed the effective acceptor doping. Recently, on the other hand, we could grow high resistive layers using commercially available precursors, probably because of improved purity. 4.2 K PL spectrum was dominated by free exciton emission rather than donor-bound excitonic emissions. Tertiarybutylamine (t-BuNH 2) was selected as a source precursor for nitrogen acceptor doping. With the flow rate of D E Z n and DMSe of 9.4 and 72 p . m o l / m i n , respectively, the crystalline quality was not seriously degraded at the t-BuNH 2 flow rate up to 150 # m o l / m i n . Fig. 4 shows 4.2 K PL spectra of non-doped and nitrogen-doped ZnSe. The thickness of ZnSe

--q-. E× t{l×

~~

Ith

440 450 460 470 Wavelength (nm) Fig. 3. Photopumped emission spectra at 77 K below and above the lasing threshold, l~h = 58 k W / c m 2.

non-doped To:350oc ~ 4,2K

' " ~DDAP [t-BNH2]= "~-~ xl00 / ~9.0'umol/min 440

460 480 500 Wavelength (nm)

Fig. 4. 4.2 K PL spectra from non-doped and nitrogen-doped ZnSe layers grown at 350°C.

273

Sz. Fujita et al. /Applied Surface Science 79/80 (1994) 270-274

epilayer was 1 Ixm. For the nitrogen-doped ZnSe ( Z n S e : N ) , the spectrum is dominated by broad d o n o r - a c c e p t o r pair (DAP) emission whose peak appears at around 463 nm, which is similar to MBE-grown ZnSe : N showing low resistive p-type conductivity [19,20]. The secondary ion mass spectroscopy (SIMS) and capacitance-voltage ( C - V ) measurements of the Schottky contacts revealed the nitrogen and the net acceptor concentrations as 5 × 10 [7 and 2 × 1017 cm -3, respectively. To the best of our knowledge, this is one of the highest net acceptor concentration in MOVPE-grown ZnSe : N. At the growth t e m p e r a t u r e of 400°C, D A P emission was hardly observed. This indicates that low t e m p e r a t u r e growth brought by photo-irradiation is a key for effective acceptor doping. EL was measured for a Z n C d S e / Z n S e multiple QW (MQW) diode with the conventional stripe-shaped electrode laser structure, as shown in Fig. 5. The cavity length and the electrode width were 1 m m and 20/xm, respectively. Examples of E L spectra measured at 77 K are shown in Fig. 5. Here, the peak at around 460 nm (by about 15 nm longer than the bandgap) is attributed to the emission from the p-type Z n S e : N due to D A P recombination, as specu-

Au

~ M Q n-ZnSe n-GaAs (sub)

-i

¢./)

W ] I ]In

77K CW

8mA 4mA

t'-.I LLI

400 '

'

'

'4"0'5 ~ ' ' 500 I , Wavelength (nm)

,

,

,

550

Fig. 5. 77 K EL spectra of a MQW diode, whose structure is also shown in the figure. Here, the diode possess

ZnCdSe/ZnSe MQW sandwiched by nitrogen-doped ZnSe (p-ZnSe) and gallium-doped ZnSe (n-ZnSe) as cladding layers.

"->.

77K CW

"6

'"1

440

,J,

,,

460 480 500 Wavelength (nm) Fig. 6. 77 K EL spectra from one of the MQW diodes which exhibited extremely intense luminescence and a narrow emission line above a certain current value.

lated from 4.2 K PL shown in Fig. 4. On the other hand, the peak at 496 nm is from the QWs. With the increase of current, the 460 nm peak decreases and the 496 nm peak increases superlinearly; at 8 mA, blue-green emission was observed by naked eyes under normal room lighting conditions. This behavior of these two peaks with current is not clearly explained at the present stage. Since ZnSe-based diodes commonly possess various problems, such as Schottky-like A u / p - Z n S e contacts, low hole concentrations in p-ZnSe layers at 77 K due to deep aeceptor levels, or defect states in layers and at interfaces, detailed discussions on EL spectra will be disclosed by taking into account of the contribution of these problems on carrier transport. One of the M Q W diodes showed extremely strong EL, whose EL spectra are shown in Fig. 6. The broad peak at 5 m A seems to be again attributed to the D A P recombination in the pZnSe layer. At 10 mA, the narrow emission line appeared at 461 nm; this might be characterized by symptoms of stimulated emission in the MQW. Here, it should be noted that the wavelength of 461 nm corresponds to the bandgap energy of Zn0.93Cd0.07Se at 77 K. The Cd composition and thickness of constituent layers in the M Q W have not been measured for this sample, but the assumption of stimulated emission in the M Q W seems to be reasonable from the wavelength. However, 10 m A is too small as a lasing threshold compared to MBE-grown lasers; hence the emis-

274

Sz. Fujita et al. /Applied Surface Science 79/80 (1994) 270-274

sion seems to be characterized as what we call filament emission, where emission occurs only from a limited area under the stripe-shaped electrode.

References [I] Sg. Fujita, A. Tanabe, T. Sakamoto. M. lscmura and Sz. Fujita, Jpn. J. Appl. Phys. 26 119871 L2000. [2] Sz. Fujita and Sg. Fnjita, J. Cryst. Growth 117 (1992) 67, and references therein.

6. Conclusions ZnSe-based b l u e / b l u e - g r e e n light emitting device structures with well-defined QWs could be grown by photo-assisted M O V P E at low temperature, e.g., 350°C, using an alkyl precursor combination where the gas-phase premature reaction hardly occurs. Photopumped lasing was observed up to 200 K, despite low barrier heights between well and barrier layers in the present samples. Doping of nitrogen from t-BuNH 2 resulted in a net acceptor concentration of 2 x l017 cm 3 which was successfully brought by low temperature growth due to photo-irradiation. The double heterostructure samples grown in this manner showed strong blue/blue-green EL at 77 K. Photo-irradiation seems to give a breakthrough for fabricating short wavelength region optoelectronic devices with wide gap II-VI semiconductors.

Acknowledgements This work was supported in part by a Grant-inAid on Priority-Area Research on the Photo-Excited Process from the Ministry of Education, Science and Culture, Japan.

[3] W. Stutius, Appl. Phys. Lett. 33 119781 656. [4] P.J. Parbrook, P.J. Wright, B. Cockaync. A.C.. Cullis, I3. l-|enderson and K.P. O'Donnell, J, ('ryst. Growth 106 (1990) 503. [5] 1I. Milsuhashi, 1. Mitsuishi and H. Kukimoto. ,I. ('ryst. Growth 77 (1986) 219. [6] P.J. Parbrook, A. Kamata and T. Ucmoto. Jpn. J. App]. Phys. 32 (1993) 669. [7] Sz. Fujila, S. | | i r a t a and Sg. Fuiita, Jpn. J. Appl. Phys. 3(I ( 1991 ) L507. [8] Sz. Fujita, S. Hirata and Sg. Fujita, J. ('rysl. Growth 115 (1991) 269. [9] T. Yasuda, I. Mitsuishi and H. Kukirnoto, Appl. Phys. Lett. 52 (1988) 57. [10] A. Ohki, N. Shibata and S. Zembutsu, Jpn. J. Appl. Phys. 27 (1988) Lg0tL [11] I. Suemune, K. Yamada, H. Masato, T. Kanda, 5. Kan and M. Yamanishi, Jpn. J. Apph Phys. 27 (1988) L2195. [12] N.R. Taskar, B.A. Khan, D.R. Dorman and K, Shahzad, Appl. Phys. Lett. 62 (1993) 270. [13] M. lchimura, T. Wada, Sz. Fujita and Sg. Fujita. Jpn. J. Appl. Phys. 30 (1991) 3475. [14] M. lchimura, T. Wada, Sz. Fujita and Sg. Fujita, J. Cryst. Growth 117 (1992) 699. [15] J. Ding, 1t. Jeon, A.V. Nurmikko, 1I. Luo, N. Samarth and J.K. Furdyna, Appl. Phys. Lctt. 57 (19911) 2756. [16] Sz. Fujita, A. Tanabe, T. Kinoshita and Sg. Fujita, .I. C~,st. Growth 101 (1990) 48. [17] T. Yasuda, Y. Koyama, J. Wakilani, J. Yoshino and 11. Kukimoto, Jpn. J. Appl. Phys. 28 (1989) L1628. [18] H. Kukimoto, J. Cryst. Growth 101 (199(/) 953. [19] J. Qiu, J.M. DePuydt, H. Cheng and M.A. llaasc. App[. Phys. Len. 59 (1991) 2992. [2(t] l.S. ttauksson, J. Simpson, S.Y. Wang, K.A. Priof and B.C. Cavenett, Appl. Phys. Lctt. 61 (1992) 22(18.