atomic nitrogen doping in p-znse with high activation ratio using a high-power plasma source

ELSEVIER Journal of Crystal Growth 184/185 (1998) 41 l-414

Atomic nitrogen doping in p-ZnSe with high activation ratio using a high-power plasma source

K. Kimuraa,*,‘,2, S. Miwaa*133, C.G. JiP4, L.H. Kuo~,~~~, T. Yasuda”,4, A. Ohtake”,‘, K. Tanakaas4, T. YaoaT4,6, H. Kobayashib

a Joint Research Center,for Atom Technolog?, (JRCAT). l-l-4 Higashi, Tsukuba 305, Japan

b Sonv Corporation Research Center, 134 Goudo-cho, Hodogaya-ku, Yokohama 240, Japan

Abstract

The p-type doping in ZnSe molecular-beam epitaxial growth was studied using a high-power plasma source. The effect of growth conditions on p-ZnSe : N was investigated. An activation ratio of almost 100% with net acceptor concentration (NA-ND) of around 1 x 10” cme3 was reproducibly achieved. The 4.2 K PL spectrum of p-ZnSe : N with high NA-ND ( - 1 x 1Or8 cm-3) shows high crystal quality. cm 1998 Elsevier Science B.V. All rights reserved.

PACS: 61.72.V~; 78.55.Et; 81.05.D~; 81.15.Hi

Keywor& Molecular-beam epitaxy (MBE); Nitrogen doping; Plasma source; ZnSe

* Corresponding author. Tel.: + 81 6 466 5593; fax: + 81 6

466 5733; e-mail: [email protected].

’ Also with Angstrom Technology Partnership (ATP),

Tsukuba 305, Japan.

’ Present address, Basic High-Technology Laboratories,

Sumitomo Electric Industries Ltd., l-l-3 Shimaya, Konohana-

ku, Ohsaka 554, Japan.

3 Present address: Sony Corporation Research Center, 134,

Goudo-cho, Hodogaya-ku, Yokohama 240, Japan.

4 Also with National Institute for Advanced Interdisciplinary

Research (NAIR), Tsukuba 305, Japan. ’ Present address: Super Epitaxial Products Inc. 9160 Rumsey

Rd. # B2, Columbia, MD 21045, USA.

‘Also with Institute for Materials Research, Tohoku Univer-

sity, Sendai 980, Japan.

Since the success of the nitrogen plasma doping method, several kinds of nitrogen plasma sources have been used to dope p-type into ZnSe grown by molecular-beam epitaxy (MBE) [l-3]. However, maximum net acceptor concentration (NA-No) has still been limited to 1 x 10” cmm3 in ZnSe and only low activation ratio ((NAPND)/N) has been obtained with a high N,-Nn value [l]. One of the

most important growth parameters is the excited chemical species in the nitrogen plasma. The plasma source generates excited neutral nitrogen molecules and nitrogen ions together with excited neutral nitrogen atoms. It was confirmed that

0022-0248/98/%19.00 kc? 1998 Elsevier Science B.V. All rights reserved

PII SOO22-0248(97)00678-7

412 K. Kimura et al. 1 Journal of Crystal Growth 184/185 (1998) 411-414

nitrogen ions decrease the activation ratio in p-ZnSe : N MBE layers and the activation ratio was greatly improved by removing nitrogen ions emitted from a plasma source [4]. Excited nitrogen molecules must be decomposed into nitrogen atoms before being incorporated at Se lattice sites and the binding energy of nitrogen molecules is very large. Therefore, the generation of nitrogen atoms is the important clue to over- come the compensation problem in wide gap II-VI compounds.

We have developed a high-power plasma source for nitrogen doping in ZnSe MBE growth and obtained high activation ratio ( - 60%) even at high NA-ND such as 1.2 x 1018 cmm3 [S]. However, there still remains carrier compensation presumably due to nitrogen ions and/or excited neutral nitrogen molecules. Quite recently, we have improved the high-power plasma source to eliminate nitrogen ions and excited neutral nitrogen molecules in the nitrogen plasma flux and obtained high activation ratio of almost 100% with NA-ND of - 1 x 10” cmm3 for p-Z&Se : N MBE layers [6].

In this work, we have studied p-type doping of ZnSe using the modified high-power plasma source. The effects of growth conditions, such as growth rate, Se/Zn flux ratio (Ps,/Pz,) and growth temperature (Tsub), on electrical and optical properties of p-ZnSe : N have been investigated.

The epitaxial growth was carried out using a dual growth chamber MBE system. Details of the high-power plasma source (Taisei Industry Co., Japan), used in the present experiment, have been reported elsewhere [6]. Nitrogen-doped ZnSe was grown on the GaAs buffer layer ( - 0.5 urn thick) under the same plasma operating condition, such as a RF power of 1.5 kW and a nitrogen gas flow rate of 1.5 seem. The length and the diameter of the orifice between the plasma discharge tube and the MBE chamber are 10 and 1 mm, respectively. The total thickness of the ZnSe layers ranged from 0.6 to 0.9 urn.

Electrical properties were characterized by the capacitance-voltage (C-V) method at 10 kHz and room temperature, using double-Schottky Au con- tacts on the ZnSe epitaxial layer. Nitrogen atomic concentration was measured by secondary-ion mass spectrometry (SIMS). The detection limit and

GROWTH RATE (pm/h)

Fig. 1. Growth rate dependence of net acceptor concentration

(0) and nitrogen atomic concentration (0) of p-Z&e : N.

error of SIMS are 8 x lOi cme3 and + 20%, respectively. Optical properties were characterized by photoluminescence (PL) spectroscopy at 4.2 K using a He-Cd laser operated at 325 nm.

Fig. 1 shows N,-Nn values of p-ZnSe : N as a function of the growth rate (closed circles). Tsub and Ps,/Pz, were 250°C and 2 + 0.2, respectively. The NAPND value increases as the growth rate decreases from 0.59 urn/h, then saturates at around 0.30 urn/h. The nitrogen atomic concentration (open circle) increases monotonically with the decrease of growth rate. The activation ratio of p- ZnSe : N grown at the growth rate of 0.38 urn/h is almost loo%, while that of a layer grown at 0.14 urn/h is around 50%, which could be at- tributed to the fact that as the number of nitrogen atoms introduced into the ZnSe layer increases relatively to the growth rate, excess nitrogen atoms are presumably introduced into Zn substitutional sites and/or interstitial sites of nearest-neighbor sites of nitrogen at Se substitutional sites and make compensating centers such as Ns,-Nz,, Zn-(Ns,)n [7] and Ns,-Ni [S].

Fig. 2 shows 4.2 K PL spectra of p-ZnSe : N layers grown with various growth rates. The spectrum of p-ZnSe : N grown at the growth rate of 0.59 urn/h shows a deep acceptor-bound exciton emission (If) and deep donor-acceptor pair (DdAP) emission with phonon replicas. Shallow donor-acceptor pair (D’AP) emission is observed at the higher energy side of the DdAP emission peak. As the growth rate increases, the D”AP emission peak

K. Kimura et al. /Journal of Crystal Growth 1841185 (1998) 411-414 413

ok’ p-Z&e : N rf:lSkW Tub : 250 % se / zn : 2.0 f0.2

G. Ft. :0.59/1 m/h N~-No:6.3xlO~~cmJ

0. R. :0.39,~n,& NA. ND: 1.0 x 10’8 cm4

WAVELENGTH (nm)

Fig. 2. 4.2 K PL spectra of p-Z&e : N layers grown with vari-

ous growth rates.

disappears and bound exciton emission peaks be- come weak and finally disappear for the layer with the activation ratio of 50%. In spite of the high NAPND value of - lOi cmp3, the 1’: peak is still observed in the spectrum of a p-ZnSe : N layer grown at 0.38 urn/h. These results for p-ZnSe : N layers with a high activation ratio of - 100% show good crystal quality [9].

Fig. 3 shows NA-ND values (closed circles) and nitrogen atomic concentration (open circles) of p-ZnSe : N as a function of PsJPz, at Tsub = 250°C. The Zn flux intensity was 4.334.7 x 10-l Torr and the Se flux intensity was changed from 6.4 x lo-’ to 1.6 x 10m6 Torr. The surface reconstruction in the initial growth stage of ZnSe was observed by reflection high-energy electron diffraction (RHEED). A (2 x 1) surface recon-

struction pattern was observed at P,,/P,, of 3.1 and 3.7, a c(2 x 2) surface reconstruction pattern appeared at Ps,/Pz, of 1.4 and a mixed structure of (2 x 1) with a weak ~(2 x 2) RHEED pattern was observed at P,,/P,, of 2.0. The NA-ND value

lOlB

10”

1 ’ . ._ i -- I’rf:1.5kW 14

-1 .._.. -_ ._..._ .._ .:1 .!.I I:: .: :. . . . -:- I ,., ,I

1 2 3 4 5

Pse / PZ”

Fig. 3. Se/Zn flux ratio (Ps,/Pz,) dependence of net acceptor

concentration (0) and nitrogen atomic concentration (0) of

p-ZnSe : N.

‘“180 200 220 240 260 280 300 320

Tsub (“c)

Fig. 4. Growth temperature (Tsub.) dependence of net acceptor

concentration (0) and nitrogen atomic concentration (0) of

p-ZnSe : N.

increases as Ps,/Pz, decreases from 3.7 to 2.0 and then saturates with the decrease of Ps,IPz,, while the nitrogen atomic concentration increases monotonically as Ps,/Pz, decreases from 3.7 to 1.4. The activation ratio of p-ZnSe : N layers grown at Ps,/Pz, of over 2.0 is around 100% and that for Ps,IPz, of 1.4 is 60%.

Fig. 4 shows NAPND values of p-ZnSe : N as a function of Tsub (closed circles). The P,,/Pz, was 2.1 + 0.1. The N*-N, value increases gradually with the decrease of Tsub from 280 to 210°C. The NA-ND value decreases quickly for Tsub above 280°C and under 210°C. The nitrogen atomic concentration (open circles) increases with the decrease

of Tsub, presumably due to an enhancement in the sticking coefficient of nitrogen atom on the ZnSe

414 K. Kimura et al. /Journal of Crystal Growth 1841185 (1998) 411-414

Fig. 5. Net acceptor concentration versus nitrogen atomic con-

centration. Open circles are results of this study and triangles are

our previous results [S]. Symbols for additional data from other

works using conventional RF plasma sources are as following:

Kurtz et al. [11]: (0). Ohkawa et al. [12]: (A) and Qiu et al. [l]:

(H).

growing surface at lower temperatures [lo]. The activation ratio of p-ZnSe : N grown above 225°C is about 90%, while that of p-ZnSe : N grown at 200°C is about 60%, presumably due to the degradation of crystal quality and/or the increase of nitrogen sticking coefficient.

The NA-No values of p-ZnSe : N layers grown under the optimized growth conditions are plotted in Fig. 5 against the nitrogen atomic concentration (open circles). Our previous results using the high- power plasma source before being modified are shown in Fig. 5 in open triangles [6]. Some data from previous work using conventional plasma sources are plotted in Fig. 5 for comparison [1,11,12]. Obviously, maximum activation ratio is attained by the modified high-power plasma source even for high NA-Nn around 1018 cm-3. These results suggest that the modified high-power plasma source can reduce the quantity of nitrogen ions and excited neutral nitrogen molecules, which are believed to enhance the carrier compensation

[6], and supply predominantly nitrogen atoms, which are effectively incorporated into the Se substitutional sites without causing degradation in crystal quality. Therefore, almost 100% activation ratio for p-ZnSe : N MBE layers with high NAPND of - 1 x 10” cmp3 is obtained under the optimized growth conditions.

In conclusion, the effect of growth conditions of nitrogen-doped ZnSe layers grown by MBE using a modified high-power plasma source was studied. The NA-ND value was controlled from 1.7 x 10” cmm3 to 1.1 x lOi cm-3 under various growth conditions. High activation ratio of almost 100% with high NA-ND values were obtained with high reproducibility under the optimized growth condition. The 4.2 K PL spectrum of p-ZnSe : N of - 10” cme3 grown under optimized growth con-

ditions showed bound exciton emission and well- resolved DdAP and phonon replicas which are indicative of high crystal quality.

This study, partly supported by the New Energy and Industrial Technology Development Organi- zation, was performed at JRCAT under the research agreement between NAIR and ATP.

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atomic nitrogen doping in p-znse with high activation ratio using a high-power plasma source

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