J Mater Sci: Mater Electron (2009) 20:S264–S267 DOI 10.1007/s10854-008-9571-y
Post-annealing effect upon electrical and optical properties of MOVPE grown P-doped ZnTe homoepitaxial layers Katsuhiko Saito Æ Kouji Yamaguchi Æ Tooru Tanaka Æ Mitsuhiro Nishio Æ Qixin Guo Æ Hiroshi Ogawa
Received: 20 July 2007 / Accepted: 4 January 2008 / Published online: 20 January 2008 Ó Springer Science+Business Media, LLC 2008
Abstract The effect of post-annealing treatment upon the electrical and optical properties of phosphorus-doped ZnTe homoepitaxial layers grown by metalorganic vapour phase epitaxy using tris-dimethylaminophosphorus (TDMAP) has been investigated. After the annealing treatment in N2 atmosphere, all the layers exhibited not only improvement of the optical properties but also enhancement of the carrier concentration by one order. Almost linear relationship between the carrier concentration and transport rate of TDMAP is obtainable by the post-annealing treatment. The reversible change of PL properties by alternate annealing treatment in H2 and in N2 was also revealed.
1 Introduction ZnTe is a II-VI compound semiconductor with a direct transition band gap of 2.26 eV at room temperature. This material is promising for pure green light emitting diodes (LEDs). Phosphorus (P) is considered to be a suitable p-type dopant for ZnTe LEDs, since the pure green electroluminescence is obtainable by a thermal diffusion of Al into Pdoped p-type ZnTe bulk crystal grown by the Bridgman method [1]. As for metalorganic vapour phase epitaxy (MOVPE), which is promising growth technique for mass K. Saito (&) T. Tanaka H. Ogawa Synchrotron Light Application Center, Saga University, 1 Honjo, Saga 840-8502, Japan e-mail:
[email protected] K. Yamaguchi M. Nishio Q. Guo Faculty of Science and Engineering, Department of Electrical and Electronic Engineering, Saga University, 1 Honjo, Saga 840-8502, Japan
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production, we have successfully grown P-doped ZnTe homoepitaxial layers with a carrier concentration of up to 1.3 9 1018 cm-3 using tris-dimethylaminophosphorus (TDMAP) as a P source [2], although they show only donor–acceptor pair emission (DAP) in the photoluminescence (PL) spectrum at 4.2 K. For these layers, we have found that the post-annealing treatment in N2 atmosphere is very effective for improving the optical properties, i.e. DAP emission vanishes and instead free-to-bound transition emission and broadened acceptor-related excitonic emission appear [3]. Therefore, the post annealing treatment is expected to be promising means for improving the electrical properties of P-doped ZnTe layer. It is important to determine suitable post-annealing conditions such as annealing temperature, duration time and ambient gas by measuring the electrical properties of P-doped ZnTe layers together with their optical properties. Also, it is necessary to clarify the post-annealing effect upon electrical and optical properties of P-doped ZnTe layers grown at various TDMAP transport rates in order to obtain the P-doped ZnTe layer of good quality and also establish the optimum TDMAP transport rate. First, we determine a suitable annealing condition in N2 atmosphere by studying the influence of annealing temperature and duration time upon the electrical properties. Second, the effect of annealing in H2 atmosphere and alternate annealing in H2 and in N2 is described. Finally, the annealing effect upon the carrier concentration and the intensity of the band-edge emission at room temperature is clarified as a function of TDMAP transport rate.
2 Experimental details The as-grown P-doped ZnTe homoepitaxial layers were prepared on semi-insulating Ga-doped (100) ZnTe
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3 Results and discussion
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10 1018 1
10-1 1017
as-grown
300
400
500
Resistivity (Ωcm)
Carrier Concentration (cm-3)
1019
Mobility (cm2/Vs)
Figure 1 shows a typical relationship between the electrical properties of P-doped ZnTe layers and annealing temperature in N2 atmosphere for the duration time of 2 h. The carrier concentration increases and the resistivity decreases considerably with increasing of the annealing temperature up to 400 °C and then becomes saturated. On the other hand, the mobility is almost unchanged. Figure 2 shows a typical relationship between the electrical properties of P-doped ZnTe layers and annealing time in N2 atmosphere. The annealing temperature was fixed at 400 °C. After the annealing for 2 h, the values of the carrier concentration and resistivity are 1.4 9 1018 cm-3 and 0.08 X cm, respectively, whereas those before the annealing are 4.6 9 1017 cm-3 and 0.2 X cm. In the range of 2–7 h, the electrical properties are not changed. Thus, a choice of
Annealing Temperature (oC)
Fig. 1 Typical relationship between the electrical properties of Pdoped ZnTe layer and annealing temperature in N2 atmosphere for 2h
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10 1018 1
10-1 10
17
0
2
4 Annealing Time (h)
6
Resistivity (Ωcm)
Carrier Concentration (cm-3)
substrates by the atmospheric pressure MOVPE system with vertical reactor. DMZn and DETe were used as the source materials, while TDMAP as a dopant source. Hydrogen was employed as a carrier gas. The substrate temperature, the transport rates of both DMZn and DETe and the total flow rate of hydrogen were fixed at 420 °C, 90 lmol/min and 800 sccm, respectively. The dopant transport rate was varied from 0.1 to 3 lmol/min. The thicknesses of the layers were fixed at around 3 lm. The electrical properties of the layers were determined by van der Pauw’s method using electroless Pd electrode as ohmic contact. The intensity of the band-edge emission was estimated from the PL spectrum at room temperature, using a 488 nm Ar+ laser as an excitation light source. For some samples, the low temperature optical properties were studied by PL measurement at 4.2 K.
Mobility (cm2/Vs)
J Mater Sci: Mater Electron (2009) 20:S264–S267
Fig. 2 Typical relationship between the electrical properties of P-doped ZnTe layer and annealing time in N2 atmosphere at 400 °C
420 °C and 2 h is sufficient for improving the electrical property of P-doped ZnTe layer. Second, we investigated the effect of alternate annealing in H2 and in N2 upon the PL properties at 4.2 K and room temperature. Here, H2 annealing under the same condition as N2 annealing was applied in order to investigate the influence of ambient gas. Figure 3 shows low temperature PL spectra of layers grown at TDMAP transport rate of 0.8 lmol/min after various annealing treatments: (a) as-grown, (b) annealed in H2, (c) annealed in N2 after being annealed in H2, (d) annealed in H2 after the treatment (c), and (b0 ) annealed in N2. The PL spectrum of as-grown layer is characterized by dominant DAP whose zero phonon line exists at 2.329 eV and considerably weak P acceptorrelated excitonic emission observed at 2.375 eV (Fig. 3a). The PL spectrum of the layer annealed in H2 is also dominated by DAP (Fig. 3b). However, the spectrum of the layer annealed in N2 after being annealed in H2 is considerably altered (Fig. 3c). DAP vanishes and instead free-to-bound transition emission (FB) and broadened acceptor-related excitonic emission (Ia) appear. The spectrum is almost the same as that of the layer annealed in N2 atmosphere (Fig. 3b0 ) given in Ref. [3]. Figure 4 shows the PL spectra at 300 K of the same layers in Fig. 3. All the spectra exhibit only band-edge emission at around 2.26 eV with no deep broad band in the region between 1.77 and 2.06 eV (not shown), i.e. the feature of the spectrum is independent of the annealing treatment. For annealed layers, the intensity of the band-edge emission is enhanced and the degree of the enhancement depends on the final annealing atmosphere. The intensity of the layers after N2 annealing is about 2-times higher than that after H2 annealing, which is consistent with the change in the low temperature PL spectrum. These results indicate that the annealing in N2 atmosphere is available to improve or recover the PL properties of P-doped ZnTe layer not only for as-grown but also after H2 annealed. In Figs. 3d and 4d,
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4.2 K
Ia
R.T.
FB
(b')
(b') (d)
(d)
PL Intensity (a.u.)
PL Intensity (a.u.)
DAP
(c)
(c) DAP
(b)
(b)
(a) 2.26
(a)
Ia
2.30
2.34
2.1
2.38
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Photon Energy (eV)
Fig. 3 PL spectra at 4.2 K of P-doped ZnTe layers after various treatments: (a) as-grown, (b) annealed in H2, (c) annealed in N2 after being annealed in H2, (d) annealed in H2 after the treatment (c), and (b0 ) annealed in N2
2.3
Photon Energy (eV) Fig. 4 PL spectra at 300 K of P-doped ZnTe layers after various treatments: (a) as-grown, (b) annealed in H2, (c) annealed in N2 after being annealed in H2, (d) annealed in H2 after the treatment (c), and (b0 ) annealed in N2 1019
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annealed in N2 Carrier Concentration (cm-3)
low temperature and room temperature PL spectra of the layer re-annealed in H2 after the treatment (c) are shown, respectively. It is noticeable that FB and Ia vanish and again DAP with zero phonon line at 2.324 eV becomes dominant again in the PL spectra at 4.2 K and the intensity of the band-edge emission reduces to the value as low as that of H2 annealed layer. From these experimental results, it can be said that the change of the PL properties of Pdoped ZnTe layer is reversible phenomenon. Figure 5 shows the carrier concentration of as-grown and N2 annealed layers as a function of TDMAP transport rate. For as-grown layers, as shown by open circles, the carrier concentration increases monotonically with increasing TDMAP transport rate up to 2 lmol/min and then becomes saturated at the value of approximately 2 9 1017 cm-3. The similar tendency of the saturation is reported in the previous study [2]. For P-doped ZnTe layer annealed in N2, as indicated by filled circles in the same figure, all the layers exhibit enhancement of the carrier
1018
annealed in H2
as-grown
1017
1016
0
0.5
1.0 1.5 2.0 2.5 TDMAP Transport Rate (µmol/min)
3.0
Fig. 5 Relationship between the carrier concentration and TDMAP transport rate for as-grown, N2 annealed and H2 annealed P-doped ZnTe layers
concentration by one order and almost a linear relationship between the carrier concentration and TDMAP transport rate is obtained. Thus, it is concluded that the post-
J Mater Sci: Mater Electron (2009) 20:S264–S267
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transport rate of 0.8 lmol/min, respectively. Almost the same increase in carrier concentration is obtained, in spite of the fact that the PL spectrum at 4.2 K is dominated by DAP as shown in Fig. 3b. On the other hand, the intensity of edge emission is also enhanced by H2 annealing, but the value is about half as much as that after N2 annealing. Therefore, hydrogen may partially relate to the formation of donor and non-radiative recombination center, which may induce DAP at low temperature and the reduction of the edge emission intensity at 300 K, although P atom may displace into regular lattice site of Te during thermal annealing as well as in the case of N2 annealing.
R.T.
PL Intensity (a.u.)
annealed in N2
annealed in H2 as-grown
0
0.2
0.4 0.6 0.8 10 1.2 TDMAP Transport Rate (µmol/min)
1.4
1.6
Fig. 6 Relationship between the intensity of the band-edge emission at 300 K and TDMAP transport rate for as-grown, N2 annealed and H2 annealed P-doped ZnTe layers
annealing treatment in N2 atmosphere is very effective for improving not only the optical property as reported previously [3] but also the electrical property of P-doped ZnTe layer grown at any TDMAP transport rate. The maximum carrier concentration reaches 4.9 9 1018 cm-3 at a TDMAP transport rate of 2 lmol/min. It should be noted that this value is 20-times higher than that of the as-grown layer. Figure 6 shows the relationship between the intensity of the band-edge emission at 300 K and TDMAP transport rate for as-grown and N2 annealed P-doped ZnTe layers. For as-grown layers, the intensity of the edge emission increases monotonically with increasing TDMAP transport rate. This is due to the increase of the carrier concentration as shown in Fig. 5, since the edge emission intensity increases with the increase of holes thermally excited from the acceptor level. After the annealing in N2 atmosphere, a large enhancement is observed for all the layers. The edge emission intensity increases monotonically with increasing TDMAP transport up to 0.8 lmol/min, which is also due to the increase of the carrier concentration by the annealing in N2. On the other hand, when TDMAP transport rate becomes higher than 0.8 lmol/min the edge emission intensity decreases. This is probably due to the inevitable deterioration of the crystalline quality caused by high concentration of P in ZnTe layer. Therefore, it can be said that a transport rate of 0.8 lmol/min is an optimum one for improving P-doped ZnTe layer. In Figs. 5 and 6, the carrier concentration and the edge emission intensity of the layer annealed in H2 are also plotted by open triangle for the layer grown at TDMAP
4 Conclusions We have investigated the effect of post-annealing treatment upon the electrical and optical properties of P-doped ZnTe homoepitaxial layers grown by MOVPE using TDMAP. It was confirmed that the N2 annealing at 420 °C and 2 h is sufficient for improving the electrical property of P-doped ZnTe layer. The reversible change of PL properties by alternate annealing treatment in H2 and in N2 was revealed. By the annealing treatment in N2 atmosphere, all the layers exhibited enhancement of the carrier concentration by one order and almost linear relationship between the carrier concentration and TDMAP transport rate was obtained. Thus, it is concluded that the post-annealing treatment in N2 atmosphere is very effective for improving not only the optical property but also the electrical property of P-doped ZnTe layer grown at any TDMAP transport rate. The main reason of the improvements in carrier concentration at room temperature may be due to the displacement of P atom into regular lattice site of Te during thermal annealing, although hydrogen may partially relate to the formation of donor and non-radiative recombination center. Through this study, the maximum carrier concentration of 4.9 9 1018 cm-3 was obtained. Acknowledgements This study was partly supported by Industrial Technology Research Grant Program in 2005 from New Energy and Industrial Technology Development Organization (NEDO) of Japan.
References 1. T. Tanaka, Y. Kume, M. Nishio, Q. Guo, H. Ogawa, A. Yoshida, Jpn. J. Appl. Phys. 42, L362 (2003) 2. T. Tanaka, M. Nishio, K. Hayashida, K. Fujimoto, Q. Guo, H. Ogawa, J. Cryst. Growth 298, 437 (2007) 3. K. Saito, K. Fujimoto, K. Yamaguchi, T. Tanaka, M. Nishio, Q. Guo, H. Ogawa, Phys. Stat. Sol. (b) 244, 1634 (2007)
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