Synthesis and properties of zinc-nitrogen compounds for the MOVPE of p-type ZnSe

......... CRYSTAL GROWTH

ELSEVIER

Journal of Crystal Growth 170 (1997) 144-148

Synthesis and properties of zinc-nitrogen compounds for the MOVPE of p-type ZnSe U.W. Pohl a,*, S. Freitag b, j. Gottfriedsen b, W. Richter a, H. Schumann b Technische Universit?# Berlin, lnstitut fiir Festk6rperphysik, Sekr. PN 6-1, Hardenbergstrasse 36, D-10623 Berlin, German)' b Technische Universit~t Berlin, lnstitutfiirAnorganische undAnalytische Chemie, Sekr. C2, Strasse des 17. Juni 137, D-10623 Berlin, German),

Abstract

Novel nitrogen-based compounds for p-type doping of ZnSe have been studied. Photoluminescence spectra of epilayers doped with synthesized zinc amides Zn(NRR')2 and with corresponding amines HNRR' (R and R' are organic ligands) indicate an insufficient stability of the Zn-N bond preventing effective doping of the tested zinc amides. Doping efficiency is improved by replacing t-butyl groups (-CMe 3) of the ligands by trimethylsilyl groups (-SiMe3). Nitrogen incorporation under usual growth conditions remained, however, too low for device applications.

1. Introduction

For the effective p-type doping of ZnSe epilayers nitrogen has proven to be the most suitable dopant at present. In molecular beam epitaxy acceptor activation up to 1018 cm -3 is readily achieved [1]. In the metalorganic vapor phase epitaxy (MOVPE), however, activation levels obtained are significantly lower. The use of ammonia (NH 3) as nitrogen source in early doping studies required high decomposition temperatures and resulted in only poor doping [2]. Furthermore, hydrogen has been claimed to induce a passivation of incorporated N acceptors [3,4]. Therefore, nitrogen containing compounds without N - H bonds like tertiary amines where all three hydrogens

* Corresponding author. Fax: +49 30 314 22064; E-mail: [email protected].

are replaced by organic ligands were applied in doping studies [5]. Recent work indicates that the lack of N - H bonds within the N precursor is not a suitable criterium, A comparison of triallylamine (Allyl3N) and monoallylamine (AllylNH 2) showed that only the latter leads to effective N incorporation and to activation of N acceptors [6]. Likewise no N incorporation was found using triethylamine (Et3N) [7] while application of compounds like dimethylamidolithium (M%NLi) [8] leads to significant incorporation and activation. The present work intends to provide a first step in the qualification of novel nitrogen-based compounds for ZnSe doping. Zinc amides which have a direct Z n - N bond are compared with the corresponding secondary amines with epilayers grown under comparable conditions. Since precursors compatible to standard processes are desirable growth was performed applying usual parameters without photoassistance.

0022-0248/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved PII S0022-0248(96)00649- 5

U.W. Pohl et al. /Journal of Crystal Growth 170 (1997) 144-148

2. Experimental procedure

Table 1 Studied nitrogen and zinc-nitrogen compounds

Nr.

\ / Znj N

1

@©

[9]

[10]

C0n

_3

I

/ \

[lO]

_5

t18]

_s [21] Z [17]

~N/~

b.p.

[oc] [oc]

Zn[(CH2)3NMe2]2 39

{Zn[(CH2)3]2NMe}2

71

Zn[MeN(CH2)2NMe2]2 117

I

N.-"~

Zn{N[(CH2)~NEt2]2}2 104

~"--~--"'-~

~L~N__Zn__N X

Zn[N(CMe3)(CHMe2)];

167

×

\// X N--Zn--N / SI / X \\/

36

Zn[N(CMe3)SiMe3]z

--L x

~

NH

70 (10-2

52 Zn[N(SiMez)~2

(10 .2 mbar

HN(CHMe2)2

84

HN(CMe3)(CHMe2)

98

HN(CH=CHMe2)=

135

3. Synthesis of nitrogen and zinc-nitrogen compounds Three kinds of nitrogen-containing compounds were studied, namely amines, zinc amines and zinc amides. Amines were purchased, dried with Call 2 and cleaned by distillation before filling into bubblers. While amines generally build monomers with reasonable vapor pressures, organozinc compounds

NH

11 /?i

MOCVD growth was performed at 340°C under reduced pressure of 100 mbar using diethylzinc (EteZn) or the adduct dimethylzinc-triethylamine (Me2Zn-Et3N) and ditertiarybutylselen (tBu2Se) as precursors. The tBu2Se flow rate was fixed at 18 p,m o l / m i n at a low V I / I I ratio of 0.3 to facilitate substitutional doping at a selenium site. Pd-diffused hydrogen or, for comparison, nitrogen purified by gettering were used as carrier gas. These conditions are close to standard MOVPE parameters and have not been optimized with respect to the individual nitrogen compounds tested in this comparative study. Despite a somewhat deteriorated structural quality of some epilayers growth parameters remained unchanged for all nitrogen dopants. The effect of doping was studied by near band edge photoluminescence at low temperatures. The I1 line connected to the radiative recombination of excitons bound to neutral acceptors and the donoracceptor pair (DAP) recombination provide sensitive monitors of acceptor activation. The samples were cooled in a steady flow cryostat and excited by the 325 nm radiation of a HeCd laser with about 1 W / c m 2. The luminescence was spectrally selected by a double monochromator with 2 × 0.85 m focal length and detected by a cooled GaAs photomultiplier tube.

mbar

\//

Si\ / ai /N--Zn--N\ / / al x / \ Sl '-.

i ( NH

t_Q

m.p.

\ /

r--"\S-1 t,.,N / \ N / '

[10]

4

Formula

Structure

145

\NH /

HN(SiMe3)~

125

Note to Table 1: 1, 2: zinc amines with coordinative bonding to zinc; 3, 4: zinc amides with intramolecular stabilization; 5, 6, 7: zinc amides with bulky ligands; 8, 9, 10, 11: secondary amines without zinc. Lines denote bondings to carbon if no atom is displayed, m.p. and b.p. denote melting and boiling point, respectively; Refs. [9,10,16,17,21] refer to first synthesis.

146

U. W. Pohl et al. / Journal of Co,sml Growth 170 (1997) 144-148

tend to form oligomers or polymers with very low vapor pressures. To attain monomer species with larger volatility the synthesized organozinc amines were stabilized by intramolecular coordination to zinc by five-membered chelate rings via nitrogen. Organozinc amides were either stabilized in the same way or contained bulky ligands like t-butyl. The general principle of the synthesis is given by the metal exchange reaction

-...w

1

~

"~

4. Doping effects of amines and zinc amides In a study of novel precursors purity is a crucial problem. The effect of purification of a nitrogen precursor on the optical properties of doped ZnSe epilayers is shown in Fig. 1. The luminescence of ZnSe doped with a not highly purified precursor (i.e. only double distilled after synthesis) is typically dominated by a bright green-yellow emission. The luminescence increases with the flux of the nitrogen precursor and can be assigned to the impurity-induced so-called copper-green CUg emission. In addi-

1

D A P II n r

CUg

as synthesized

II oc

SA

S

X I~[Iv

' X IvI'nl

2,00

2LiR + ZnC12 --+ ZnR 2 + 2LiCl $. In an analogous manner asymmetric zinc amides ZnRR' were synthesized in two steps. The syntheses were performed in diethylether or toluene solvent under argon ambient. The studied compounds are listed in Table 1. The asymmetric zinc amides ZnRR' with R = MeN(CH2)2NMe 2 in a five-membered ring formation and R' = methyl (Me), t-butyl, phenyl and neopentyl all built dimers with melting points above 100°C (not listed in Table 1). Due to the low volatility they have not been considered further. As to the zinc amines X-ray diffraction proved the formation of monomer species for some compounds like compound 1 the synthesis of which has already been reported [9]. Compound 2 has been synthesized for the first time [10]. Unfortunately, all synthesized zinc amines were solid at RT with still low vapor pressures below 0.1 mbar. In addition, doping tests with the zinc amines compound 1 and 2 indicated that thermal decomposition behavior seems unfavorable for nitrogen incorporation into ZnSe epilayers due to the weak coordinative Z n - N bond. Doping studies therefore concentrated on amines without zinc and symmetric zinc amides.

.r

ZnSe:Zn(N(SiMe3)2)2

1600

2000

2400

Energy(meV)

2800

Fig. 1. Effect of precursor purification on the optical properties of" a doped ZnSe layer.

tion, a strong Q-DAP series appears at higher energies. It is assigned to Ga or CI donors and residual Li acceptors unintentionally introduced by the doping precursor. After additional triple distillation of the precursor (which required some patience due to the restricted stability of the compound and the low vapor pressure) the spectrum changes completely showing basically bound exciton lines connected to structural defects. The luminescence is nearly identical to that of an undoped ZnSe layer grown under the same conditions indicating a negligible nitrogen doping effect of the used compound as discussed below. Emission lines I , and Y (also denoted Y0 in literature), visible in more detail in the insert of Fig. 1, are known to originate from extended structural defects being independent of impurity incorporation [11]. This seems also to hold for the line denoted I v near 2.735 eV and the deep S-DAP series [12] both being as well observed in undoped ZnSe epilayers. The appearance of these luminescences is connected to the rough morphology of the epilayer resulting from the low V I / I I chosen by purpose. Emission line X shows a sample dependent fine structure indicating different centers leading to such an emission. Its transition is close to the DAP band induced by oxygen incorporation as found in ZnSe epilayers grown by MBE with ZnO doping [13]; phonon coupling of the X emitting center is, however, significantly weaker. A source of oxygen in our samples might be the ether solvent used in our synthesis. In the synthesis of A1 precursors in diethylether (Et,O)

U.W. Pohl et al. / Journal of Crystal Growth 170 (1997) 144-148

; : a

°

ZnSe:HN(SiMe3)2

>

b

-

iN

2780

~.

2800 Energy (meV)

2820

Fig. 2. (a) Effect of increasing dopant flux of amine 8 on the excitonic emission. (b) Increase of I N bound exciton line induced by doping with amine 11 under increasing dopant flux ON.

for I I I - V MOVPE oxygen incorporation has been found to result from the solvent [14]. Therefore, we also performed syntheses using the oxygen-free solvent toluene. The formation of centers creating X lines could, however, not be suppressed under the given growth conditions. In the following, secondary amines HNRR' (usually with R = R') are compared with the corresponding zinc amides Zn(NRR') 2. The observed doping effects of the three amines 8, 9 and 10 were quite similar. At low dopant flux the excitonic spectrum is virtually identical to that of undoped epilayers. Besides the strain split free light and heavy hole exciton recombinations X ~h and X hh and corresponding transitions of the n = 2 excited state distinct bound exciton lines appear (Fig. 2a). The structures I" with a binding energy E B = 6.5 meV to Xhh and 12 with E B = 5.5 meV to X~h originate from donor bound excitons. The shoulder at the low energy side of 12 with E B = 8 meV with respect to X~h denoted 13 is usually interpreted in terms of excitons bound to ionized donors D +, although this attribution is not unequivocal. For the samples shown in Fig. 2a an alternative assignment of 13 line to the very shallow oxygen acceptor which is reported to have a comparable binding energy [13], can be ruled out due to the missing corresponding DAP transitions. Line 13 appears especially when Et2Zn is used as Zn precursor while its strength is very low when Me2Zn-Et3N with high purity is used. With increasing dopant flux QN a decrease of the relative intensity of the 13

147

emission line was found (Fig. 2a). No nitrogen-related I 1 line was observed, the X lines remained unchanged. This result was unexpected, since at least compound 8 diisopropylamine has been reported to induce a quite efficient nitrogen incorporation with [N] = 10 ~s cm 3 and activation under photo-assisted MOVPE as demonstrated by a working laser diode [15]. The chosen growth conditions obviously did not lead to nitrogen incorporation. The quite strong 13 line is attributed to a considerable degree of compensation; its relative decrease under dopant flux may be caused by additionally introduced donor species. Amine 9 was now used as a ligand to form the organozinc amide 5 which has been synthesized for the first time [16]. Doping behavior of amide 5 was found to be similar to that of the amines shown in Fig. 2a. The comparable doping effect indicates an insufficient stability of the Z n - N bond. A weak Z n - N bond can result in decomposition by bond cleavage and hence the formation of free amine ligands. As a next step ligands containing silicon were introduced into the nitrogen precursors, namely the trimethylsilyl ( - S i M % ) instead of the t-butyl ( - C M e 3) group. In analogy to experiences with amides of group III A metals an increase of volatility might occur. Furthermore, since the N - S i bond is weaker than the N - C bond, a cleavage of complete ligands should be less probable. For the zinc amide 7 (Zn(N(SiMe3)2) 2) synthesis [17] and application as nitrogen dopant [18] have already been reported. The success of doping remained, however, unclear. Applying standard growth parameters we could not find a significant doping effect. Very recent investigations performed with the same precursors and growth temperature demonstrated that nitrogen incorporation becomes efficient only at a very low V I / I I ratio of 0.005 [19]. Organozinc amides 5 and 7 thus appear not quite suitable for doping under usual growth conditions. The zinc amide with amine 8 as ligands has not been investigated further due to its high melting point above 300°C. Finally, we tried amine 11 (bistrimethylsilylamine) which is basically the ligand of zinc amide 7. With this compound a significant doping effect was observed (Fig. 2b). The Ij line which increases with the dopant flux has a binding energy of 11 meV with respect to Xlh. Application of Hayne's rule E u / E A

148

U. W. Pohl et aL / Journal of Crystal Growth 170 (1997) 144-148

......

e,°--A

t .............

oAP//5° K1 /

.

. I I

= °

'2 I

Kt II o,,,

I

,.,o

'

"

,

i

.

.

2600

':ill illI ~I

/IrAk^ H~k~ 1/11' 6QN

~Y

.

.

i

.

.

.

.

i

.

.

device applications so far. Up to now only photoassistance enables reasonable acceptor doping and activation above 1017 cm -3 [20]. The preliminary results encourage nevertheless doping with suitable nitrogen compounds and optimization of growth conditions also without photo-assistance.

.

2650 2700 Energy (meV)

.

i

.

2750

.

.

.

i

References

"

,

2800

Fig. 3. Donor-acceptor pair emission and free to bound emission of ZnSe doped with compound 11 depending on dopant flux QN and temperature (insert). Bars denote LO phonon coupling.

0.1 as a reasonable estimate yields an acceptor ionization energy of 110 meV demonstrating substitutional nitrogen incorporation. In addition, the nitrogen acceptor related DAP appears, see Fig. 3. The high energy series originates from free to bound eA° transitions as proven by the temperature behavior. The occurrence of strong eA° transitions indicates the ionization of residual neutral donors by acceptors and consequently a relative decrease of the D°A° donor-acceptor pair transitions. Under the given growth conditions the incorporation remained, however, below SIMS detection limit of 2 × 1017 c m - 3 .

5. Conclusion A number of amines and zinc amides have been studied with respect to p-type doping of ZnSe grown under standard conditions. A comparison of zinc amides Zn(NRR') 2 and corresponding secondary amines HNRR' which are basically the ligands of the zinc amides indicates an insufficient Z n - N bond strength for effective nitrogen incorporation. Substitution of t-butyl by trimethylsilyl groups within the amine rest NRR' induces more appropriate decomposition properties. In this way nitrogen incorporation under standard growth conditions was achieved using bistrimethylsilylamine (compound 11 in Table 1). Doping efficiency appears, however, too low for

[1] E. Kurtz, E. Einfeldt, J. Niirnberger, S. Zerlauth, D. Hommel and G. Landwehr, Phys. Status Solidi (b) 187 (1995) 393. [2] A. Ohki, N. Shibata and S. Zembutsu, Jpn. J. Appl. Phys. 27 (1988) L909. [3] A. Wolk, J.W. Ager III, K.J. Duxstad, E.E. Hailer, N.R. Taskar, D.R. Dorman and D.J. Olego, Appl. Phys. Lett. 63 (1993) 2756. [4] A. Kamata, H. Mitsuhashi and H. Fujita, Appl. Phys. Lett. 63 (1993) 3353. [5] Y. Tanaka, S. Komatsu, M. Kobayashi and A. Yoshikawa, Int. Symp. on Laser and Light Emitting Diodes, Chiba, Japan (1996), Ext. Abstr. p. 425. [6] W. Gebhardt, B. Hahn, T. Reisinger, M.J. Kastinger and M. Deufel, Int. Syrup. on Laser and Light Emitting Diodes, Chiba, Japan (1996), Ext. Abstr. p. 33. [7] A. Kamata, J. Crystal Growth 145 (1994) 557. [8] K. Yanashima, K. Koyanagi, K. Hara, J. Yoshino and H. Kukimoto, J. Crystal Growth 124 (1992) 616. [9] H.K. Hofstee, J. Boersma, J.D. van der Meulen and G.J.M. van der Kerk, J. Organomet. Chem. 153 (1978) 245. [10] S. Freitag, PhD Thesis, D83, Technical University of Berlin (1995). [11] K. Shahzad, J. Petruzello, D.J. Olego, D.A. Cammack and J.M. Gaines, Appl. Phys. Lea. 57 (1990) 2452. [12] P.J. Dean, Phys. Status Solidi (a) 81 (1984) 625. [13] K. Akimoto, T. Miyajima and Y. Mori, Phys. Rev. B 39 (1989) 3138. [14] R.W. Freer, T. Martin, P.A. Lane, C.R. Whitehouse, TJ. Whitaker, M. Houlton, P.DJ. Calcott, D. Lee, A.C. Jones and S.A. Rushworth, J. Crystal Growth 150 (1995) 539. [15] A. Toda, T. Margalith, D. Imanishi and A. lshibashi, Electron. Lett. 31 (1995) 1921. [16] J. Gottfriedsen, MSc Thesis, Technical University of Berlin (1996). [17] H. Biirger, W. Sawodny and U. Wannagat, J. Organomet. Chem. 3 (1965) 113. [18] W.S. Rees and D.M. Green, Mater. Res. Soc. Proc. 242 (1992) 281. [19] W. Taudt, S. Lampe, F. Sauerl~inder, J. S511ner, H. Hamadeh, M. Heuken, A.C. Jones, S. Rushworth, P. O'Brien and M.A. Malik, J. Crystal Growth 169 (1996) 243. [20] Sz. Fujita, T. Asano, K. Maehara, T. Toiyo and Sg. Fujita, J. Crystal Growth 138 (1994) 737. [21] W.S. Rees, D.M. Green and W. Hesse, Polyhedron 11 (1992) 1697.