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Effects of annealing treatment upon electrical and photoluminescence properties of phosphorus-doped ZnMgTe epilayers grown by metalorganic vapor phase epitaxy
Journal of Crystal Growth 370 (2013) 342–347
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Effects of annealing treatment upon electrical and photoluminescence properties of phosphorus-doped ZnMgTe epilayers grown by metalorganic vapor phase epitaxy Mitsuhiro Nishio, Keita Kai, Ryota Fujiki, Katsuhiko Saito n, Tooru Tanaka, Qixin Guo Department of Electrical and Electronic Engineering & Synchrotron Light Application Center, Saga University, Saga 840-8502, Japan
a r t i c l e i n f o
a b s t r a c t
Available online 3 August 2012
Post-annealing treatment in nitrogen gas flow has been carried out for P-doped Zn1 xMgxTe layer grown under a Te-rich or Te-poor condition by metalorganic vapor phase epitaxy. The electrical and photoluminescence properties of P-doped Zn1 xMgxTe layers are altered by annealing treatment. The post-annealing is very effective in obtaining p-type conductive Zn1 xMgxTe for the layer grown under a Te-poor condition. On the other hand, Zn1 xMgxTe layer is characterized by a high compensation ratio and DAP luminescence for the layer grown under a Te-rich condition, even after annealing treatment. Similar tendencies are also found in P-doped ZnTe layers. & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Doping A1. Metalorganic vapor phase epitaxy B2. Semiconducting II–VI materials B2. Semiconducting ternary compounds
1. Introduction Zn1 xMgxTe ternary alloy is a promising material for light emitting diodes (LEDs) with green or blue emission since the fundamental energy gap of Zn1 xMgxTe can be varied through Mg composition from 2.26 to 3.1 eV [1] or 3.2 eV [2]. Furthermore, Zn1 xMgxTe is expected to be one of the best materials as a cladding layer and a transparent substrate for improving the performance of ZnTe-based pure green LED because it is expected to form a type-I heterostructure with ZnTe [3]. We have already demonstrated a green ZnTe LED with a pn junction by means of thermal diffusion process of Al [4]. In order to improve the performance of LED, it is important to study growth and characterization of conductive p-type Zn1 xMgxTe crystals. Unfortunately, there are only a few works on Zn1 xMgxTe epitaxial layer grown by metalorganic vapor phase epitaxy (MOVPE) [3,5–10], which is a potential epitaxial growth technique for mass production. So far, nominally undoped Zn1 xMgxTe epitaxial layers have been successfully grown on (100) ZnTe substrates by MOVPE, and the growth parameters for control of Mg composition and for improvement of optical property were revealed [7–9]. Although intentional doping is essential for device applications, however, impurity doping into Zn1 xMgxTe layer by means of MOVPE has been hardly investigated except for our report [10]. As for ZnTe layer, on the other hand, trisdimethylaminophosphorus (TDMAP) is considered to be a suitable dopant source for achieving p-type doping over a wide range of carrier concentration with high
optical quality [11–15]. The effect of post-annealing treatment in nitrogen gas flow will be very effective for improving electrical and photoluminescence (PL) properties for P-doped ZnTe layers grown by MOVPE [11–13]. Then post-annealing treatment in nitrogen gas flow is expected to bring about p-type doping into Zn1 xMgxTe epitaxial layer effectively in addition to a use of TDMAP as dopant source. Very recently, we have obtained p-type Zn1 xMgxTe layers with relatively high carrier concentrations of 5 1016–2 1018 cm 3 based on this idea [10]. Therefore, annealing treatment after doping is expected to be promising means for improving the electrical and PL properties of p-type Zn1 xMgxTe layer. When the effective annealing for obtaining p-type conductive Zn1 xMgxTe and also P-doped ZnTe layers of high quality is considered, the data on the correlation between the annealing effects and growth conditions is important. However, it is ambiguous at present not only for P-doped Zn1 xMgxTe layer but also for P-doped ZnTe layer. In this paper, the effects of annealing treatment upon the electrical and PL properties are clarified for P-doped Zn1 xMgxTe layers grown at different transport rate ratios of VI-group–II-group, i.e., under a Te-rich or Te-poor condition. The electrical and PL properties of P-doped Zn1 xMgxTe layers may depend strongly upon such conditions as shown in previous report on P doping into ZnTe [14]. Furthermore, a comparison with P-doped ZnTe layer is made in order to get a guide to improve the crystal quality of P-doped Zn1 xMgxTe.
2. Experimental procedures n
Corresponding author. E-mail address: [email protected] (K. Saito).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.07.024
The growth conditions are summarized in Table 1. The as-grown P-doped Zn1 xMgxTe layers were prepared on semi-insulating
M. Nishio et al. / Journal of Crystal Growth 370 (2013) 342–347
343
Table 1 Growth and annealing conditions for P-doped Zn1 xMgxTe and ZnTe layers. Layer
P-doped Zn1 xMgxTe Te-rich condition
Growth conditions Substrate Growth pressure Transport rate (mmol/min) DMZn (MeCp)2Mg DETe TDMAP Substrate temperature (1C) Total flow rate of H2 carrier gas (sccm) Mg content x Remark Post-annealing conditions Ambient gas Annealing temperature Ta (1C) Duration time (h)
P-doped ZnTe Te-poor condition
Te-rich condition
Te-poor condition
Ga-doped semi-insulating Ga-doped (100) ZnTe substrates Atmospheric pressure 23 3.8 42 0.2 395 800 0.035 Sample 1
20 8 19 0.1 380 800 0.12 Sample 2
20 4 19 0.2 395 800 0.022 Sample 3
N2 100–400 3
N2 100–420 2
N2 420 2
84
368
100 3 500 4800 0
336 8 425 4800 0
N2 420 2
N2 420 2
T (K)
10 1017
1016 : Sample1 (Te-rich) : Sample2 (Te-poor) 1015
200
100
1018
Carrier concentration (cm–3)
Carrier concentration (cm–3)
1018
Mobility(cm2/Vs)
102
300
:Fitting curve
1017
1016
1015 As–grown
100 200 300 Annealing temperature (°C)
400
Fig. 1. The carrier concentration and mobility at room temperature for P-doped Zn1 xMgxTe layers as a function of annealing temperature.
1014 5
10 103/T (K–1)
Ga-doped (100) ZnTe substrates by the atmospheric pressure MOVPE system. Dimethylzinc (DMZn), diethyltelluride (DETe) and bismethylcyclopentadienyl-magnesium (MeCp)2Mg were used as the source materials. The flow rate of hydrogen carrier gas was 800 sccm. The transport rates of source materials were varied depending upon a Te-rich or Te-poor condition as shown in Table 1. The effect of annealing temperature upon the electrical properties at room temperature has been investigated using P-doped Zn1 xMgxTe layers grown under conditions as follows. For a Te-rich condition, the transport rates of DMZn, (MeCp)2Mg, DETe, TDMAP and substrate temperature were, respectively, 23, 3.8, 42, 0.2 mmol/min and 395 1C. For a Te-poor condition, on the other hand, those were 20, 8, 19, 0.1 mmol/min and 380 1C. As a result, Mg content of 0.035 was obtained for the former while that of 0.12 for the latter, due to the difference of substrate temperature [9]. Namely, the decrease of substrate temperature may bring about the reduction of the premature reaction between DETe and (MeCp)2Mg and lead to an increase of Mg content [7]. For a slight decrease of substrate temperature, further Te-rich circumstance may occur near the growing surface for Te-rich
Fig. 2. The temperature dependence of the carrier concentration for as-grown and annealed P-doped Zn1 xMgxTe layers.
condition while further Te-poor circumstance may happen for Tepoor condition owing to the premature reaction. In order to attain a low Mg content for a Te-poor condition, the (MeCp)2Mg transport rate was decreased and substrate temperature was increased. Under the conditions that the transport rates of DMZn, (MeCp)2Mg, DETe, TDMAP and substrate temperature were, respectively, 20, 4,19 and 0.2 mmol/min and 395 1C, P-doped Zn1 xMgxTe layer with a low Mg content of 0.022 was obtained. The use of Zn1 xMgxTe layer with a small Mg composition such as x¼0.035 or x¼0.022 facilitates the measurement of the electrical properties at low temperature even without heat treatment of electrode [15] as well as in case of ZnTe [16] when electroless Pd electrode was employed. In order to compare with P-doped ZnTe, the P-doped ZnTe layers grown under both Te-rich and Te-poor conditions in other growth system have
344
M. Nishio et al. / Journal of Crystal Growth 370 (2013) 342–347
Table 2 Best-fit parameters determined from Hall analysis for P-doped Zn1 xMgxTe layers. NA (cm 3)
VI/II ratio
ND (cm 3)
ND/NA
EA (meV)
Remark
1.8 1.8
(7.47 0.9) 1017 (4.97 0.4) 1017
(6.1 70.8) 1017 (3.8 70.4) 1017
0.82 0.78
217 0.6 247 0.5
As-grown
1.8
(6.47 0.6) 1017
(4.8 70.6) 1017
0.75
247 0.6
Annealed at 200 % C for 3 h
1.8
(7.77 0.4) 1017
(4.6 70.4) 1017
0.59
277 0.4
Annealed at 300 % C for 3 h
1.8
(4.6 708) 1018
(2.4 70.5) 1018
0.53
237 0.3
(1.3 70.05) 1017 (4.97 0.2) 1018
(1.0 70.4) 1015 (2.8 72.5) 1017
0.0079 0.057
477 0.3 207 0.3
Annealed at 400 % C for 3 h As-grown Annealed at 420 1C for 2 h
0.79 0.79
T (K) 300
Annealed :Sample1 (Te–rich) :Sample3 (Te–poor) As–grown :Sample1 (Te–rich) :Sample3 (Te–poor)
10 1
10
o o
2.3
2.2
4K Sample1
103
–1
102
10–2
×1
A
B
C B
C
B
C
A
×1 ×1
Ta = 400°c
A
×1
2.1
C
A
×1 Intensity (arb. units)
Resistivity (Ω cm)
102
o
Energy (eV) 2.4
2.5
100
Mobility (cm2/Vs)
103
200
o
Annealed at 100 % C for 3 h
A
C
B
300°c 200°c 100°c As–grown
A 10 5
Sample3
10
103/T (K–1) Fig. 3. The temperature dependences of resistivity and mobility for as-grown and annealed layers.
× 1/3 500
been investigated. The flow rate of hydrogen employed in this system was as high as 4.8 slm. For a Te-rich condition, the transport rates of DMZn, DETe and TDMAP and the substrate temperature were 84, 100 and 3 mmol/min and 500 1C. On the other hand, the transport rates of DMZn, DETe and TDMAP and the substrate temperature were 368, 336 and 8 mmol/min and 425 1C for a Te-poor condition. Although it seems that the growth conditions were different considerably between both cases, the as-grown P-doped ZnTe layers of relatively good quality are obtainable in each case under these conditions. The thicknesses of the layers used here were around 4–7 mm. The postannealing treatment was performed in nitrogen flow. The annealing conditions are also summarized in Table 1. The annealing temperature was varied from 100 1C to 400 or 420 1C in order to investigate the influence of annealing temperature. As shown in the table, annealing time for a Te-rich condition is longer than for a Te-poor condition in the case of P-doped Zn1 xMgxTe layer. This is due to the fact that Te-poor condition leads to effective annealing compared with Te-rich condition as described later. The composition of Mg was evaluated by energy dispersive X-ray (EDX) analysis. Hall-effect measurements were carried out using a system of ResiTest-8300 (LN) for the van der Pauw’s method. Electroless Pd electrode was used as ohmic contact. A magnetic field of 0.35 T was applied to the samples. For temperature-dependent measurements, the sample was placed in a liquid nitrogen cryostat that allows control of temperature in the range between 80 and 300 K. Furthermore, PL measurement at 4 K was performed. A 405 nm blue–violet laser diode was used for Zn1 xMgxTe as an excitation light source, while an Ar ion laser with a
C×1
520
540 560 Wavelength (nm)
Ta = 420°c
580
600
Fig. 4. PL spectra at 4 K of P-doped Zn1 xMgxTe layer annealed at different temperatures.
wavelength of 488 nm was employed for ZnTe. The laser power was attenuated by neutral density filters when the excitation intensity dependence of PL was investigated.
3. Results and discussion Fig. 1 shows the carrier concentration and mobility at room temperature for P-doped Zn1 xMgxTe layers with x¼0.035 corresponding to a Te-rich condition (sample 1) and x ¼0.12 corresponding to a Te-poor condition (sample 2) as a function of annealing temperature. The carrier concentration increases considerably for both samples when annealing temperature more than 300 1C is applied. Thus, it is confirmed that post-annealing is effective for enhancing P activation for Zn1 xMgxTe, similar to for P-doped ZnTe [11–13]. The carrier concentration increases with the annealing temperature and becomes saturated at 350 1C for Te-poor condition, while it does not show saturated tendency even at 400 1C for Te-rich condition. Furthermore, slight improvement of mobility somehow is found for Te-poor condition. Also, it is noted that the annealing time for sample 2 is shorter than for sample 1.
M. Nishio et al. / Journal of Crystal Growth 370 (2013) 342–347
These facts indicate that annealing treatment is more effective for sample 2 corresponding to a Te poor condition than sample 1 corresponding to a Te rich condition. Fig. 2 shows the temperature dependence of the carrier concentration for as-grown and annealed P-doped Zn1 xMgxTe layers. In the same figure, the temperature dependences of the carrier concentration with a small Mg content of x¼0.022 grown under a Te poor condition (sample 3) are shown to compare with sample 1. In sample 1, the carrier concentration increases over the entire temperature range when the annealing temperature is 300 1C, probably due to activation of the P atoms by annealing. The change of temperature dependence is found at a measurement temperature of about 200 K in sample 1 annealed at 400 1C, which may be associated with thermally activated transport in the valence band from two different acceptor levels rather than a conventional single acceptor level. On the other hand, the carrier concentration becomes increased by about two orders after annealing for sample 3. Since the annealing conditions for sample 3 is the same as for sample 2, it
seems that annealing treatment is effective for P-doped Zn1 xMgxTe layer grown under a Te poor condition, independent of Mg content. From the temperature dependence of carrier concentration, it seems that the activation energy of P-acceptor (EA) is almost independent of annealing temperature for sample 1 whereas a strong dependence found in the as-grown layer changes into moderate dependence after annealing for sample 3. The variation of activation energy may be related to change in the number of donor defects (ND) by analogy with the behavior of P-doped ZnTe, since EA decreases with the third root of ND in P-doped ZnTe [17]. In order to evaluate the degree of compensation, the curve fittings were made for each sample. The following well-known equation for a non-degenerate semiconductor dominated by a single acceptor center was used: pðp þ ND Þ=ðNA N D pÞ ¼ ðN v =gÞexpðEA =kTÞ where p is the carrier concentration, NA acceptor concentration, ND donor concentration, and g the degeneracy factor. Nv is given by 2(pkTm*h)3/2/h3 and m*h the density-of-state effective mass of holes.
Energy (eV) 2.42
2.40 A
345
Energy (eV) 2.44
2.38
2.28
A
Annealed
B
2.36
2.20
2.12 Annealed
C
Intensity (a.u.)
Intensity (a.u.)
B
512
516
520
524
100% 70 50 30 10 7 5 3
500
550
Wavelength (nm)
Wavelength (nm)
Energy (eV)
Energy (eV)
2.42
2.40
2.38
A
2.44
2.36
A
B
As–grown
Intensity (a.u.)
Intensity (a.u.)
B
510
600
515
520
Wavelength (nm)
525
500
2.28
2.20 C
2.12 As–grown
100% 70 50 30 7 3 0.7 550
600
Wavelength (nm)
Fig. 5. The change in bands A and B for as-grown and annealed P-doped Zn1 xMgxTe layers at various excitation powers. The layer grown under a Te-poor condition was annealed at 300 1C.
M. Nishio et al. / Journal of Crystal Growth 370 (2013) 342–347
The value of scattering parameter was taken to be equal to 1. The variation of effective mass of holes with Mg content is small enough in the range x¼0–0.11 [18] to be neglected. Then, the values of m*h and g are taken to be 0.56m0 and 4, respectively, from those reported in P-doped ZnTe [17]. Actually, a slight change of the effective mass with Mg content hardly influences the best-fit values of NA, ND and EA. After determining the best-fit values of NA, ND and EA using a least-squares method, a straight line relationship between ln p(p þND)/(NA ND p)T 3/2 and T was checked using the chosen values. The calculated curves are shown as solid lines in the figure. The closed agreement between the experimental and theoretical curves means the validity of the results of the analyses. The best-fit parameters are given in Table 2. For sample 1, it seems that a high NA is attainable as annealing temperature increases to 400 1C, similar to sample 3. It can be said from electrical analyses that a shallow acceptor is introduced by P doping for both samples. Also, the compensation ratio (ND/NA) decreases slightly for both samples as annealing temperature increases. However, compensation ratio is very high in sample 1. On the other hand, it is so low in sample 3, although it seems that ND becomes somehow increased by annealing. Fig. 3 shows the temperature dependences of resistivity and mobility for as-grown and annealed layers at 400 1C for sample 1 and 420 1C for sample 3. It seems that the resistivity varies with temperature depending on the carrier concentration behavior. Accordingly, post-annealing for P-doped Zn1 xMgxTe grown under a Te poor condition is effective for obtaining the layer with low resistivity. In sample 1, the temperature dependence of mobility for the as-grown layer seems to be almost the same as for annealed one. On the other hand, the temperature dependence mobility varies with annealing treatment in sample 3. A remarkable decrease in mobility can be seen with increasing annealing temperature in the low-temperature range of 80–120 K, where ionized impurity scattering plays an important role in most cases. However, it seems to be difficult to explain in terms of ionized impurity scattering why the mobility of sample 3 is lower than that of sample 1 in the low-temperature range of 80–120 K for the as-grown layers from the data on NA and ND summarized in Table 2. The number of deep defects contributing to neutral impurity scattering may be increased for the layer grown under a Te poor condition, although annealing treatment is very effective in enhancing the carrier concentration for such a layer. Fig. 4 shows PL spectra at 4 K of P-doped Zn1 xMgxTe layer annealed at different temperatures. P-doped Zn1 xMgxTe layer grown under a Te-rich condition, that is, sample 1 was used. The PL spectra of as-grown layers are dominated by strong emission band denoted by A, together with a weak band denoted by B located near band A and a broad band centered at around 550 nm. By the post-annealing treatment, the PL spectra were altered as shown in the figure. Especially, the deep broad band C becomes predominant in the spectrum at an annealing temperature of 400 1C. For samples 2 or 3 corresponding to a Te poor condition, on the other hand, the intensity of broad band C is very weak compared with that of band A or band B [10]. PL spectrum of sample 3 annealed at 420 1C is shown in the figure as an example of Te-poor condition. These facts are in conflict with the results on the growth of undoped Zn1 xMgxTe where a deep broad band like band C becomes predominant as VI/II transport rate ratio decreases [3,8] although the intensity ratio of the deep band to band A or band B decreases with decreasing substrate temperature [8] probably due to reduced efficiency of DETe pyrolysis. The broad band C may be associated with some defect accompanied by P doping. Further study on P doping into Zn1 xMgxTe is required in order to clarify origin of its broad band. Fig. 5 shows the change in bands A and B for as-grown and annealed P-doped Zn1 xMgxTe layers for Te-rich condition at
various excitation powers. The layer grown under a Te-rich condition, sample 1, was used. For both the as-grown and annealed layers, the peaks of band B and C shift to shorter wavelength side as the excitation power increases, which is a characteristic of a donor–acceptor pair (DAP) recombination emission. On the other hand, the peak for band A is independent of the excitation power. By analogy with those of P-doped ZnTe, the band A is assigned to be P acceptor-related excitonic emission and the band B is attributed to P acceptor related DAP luminescence. The band C may be related to a deep acceptor level as expected from the temperature dependence of carrier concentration for an annealing temperature of 400 1C. It is noticeable that the band B disappears and instead the emission band associated with free electron to bound acceptor (FB) transition appears with annealing in the case of a Te poor condition [10]. The emission band around 527 nm for sample 3 shown in Fig. 4 corresponds to FB transition. The fact that DAP related to band B is found even after annealing treatment may be closely connected with a high compensation ratio in sample 1. Thus, it is important to choose a Te-poor condition in MOVPE growth in order to improve the electrical and optical quality of P-doped Zn1 xMgxTe effectively by means of annealing treatment. It is useful that a comparison with P-doped ZnTe layers is made between Te-rich and Te-poor conditions in order to investigate whether or not the presence of Mg is important. Fig. 6 shows the temperature dependences of carrier concentration for as-grown and annealed P-doped ZnTe layers grown under Te-rich and Te-poor conditions. It is also clear in the case of P-doped ZnTe that the layer grown under a Te-poor condition rather than a Te-rich condition is favorable for obtaining the high carrier concentration with annealing treatment. Furthermore, a unique temperature dependence of carrier concentration is found for the layer grown under a Te rich condition, common to the case of P-doped Zn1 xMgxTe. The curve fittings were also made for P-doped ZnTe layers using the above equation for a non-degenerate semiconductor dominated by a single acceptor center. In Table 3, the best-fit parameters are summarized in P-doped ZnTe layer. In P-doped ZnTe layer grown
T (K) 300
200
100 :Annealed ZnTe (Te–rich) :Annealed ZnTe (Te–poor) :As–grown ZnTe (Te–rich) :As–grown ZnTe (Te–poor)
1018
Carrier concentration (cm–3)
346
1017
1016
–:Fitting curve 1015 10
5 103/T (K–1)
Fig. 6. The temperature dependences of carrier concentration for as-grown and annealed P-doped ZnTe layers grown under a Te rich and poor conditions.
M. Nishio et al. / Journal of Crystal Growth 370 (2013) 342–347
347
Table 3 Best-fit parameters determined from Hall analysis for P-doped ZnTe layers. VI/II ratio
NA (cm 3)
ND (cm 3)
ND/NA
EA (meV)
Remark
1.2 0.91 0.91
(8.07 0.5) 1017 (1.17 0.02) 1018 (3.97 0.1) 1018
(7.7 7 048) 1017 (1.97 0.2) 1017 (3.97 0.1) 1016
0.97 0.17 0.065
197 0.3 367 0.8 367 0.9
As-grown As-grown Annealed at 420 1C for 2 h
under a Te-poor condition, a high NA is attainable by annealing treatment, similar to the case of P-doped Zn1 xMgxTe. Both ND and compensation ratio are so small. As for P-doped ZnTe layer grown under a Te-rich condition, NA is as high as 8 1017 cm 3as well as in the case of P-doped Zn1 xMgxTe (see Table 2). Although it seems to be difficult to dope a Te lattice site with P under a Te rich condition essentially, P will be easily introduced into ZnTe and Zn1 xMgxTe during the growth probably due to relatively small atomic radius. However, the compensation ratio is as high as 0.97, in agreement with the case of P-doped Zn1 xMgxTe, probably due to influence of residual impurities or defects introduced simultaneously during the growth.
4. Summary The effects of annealing treatment upon the electrical and PL properties of P-doped Zn1 xMgxTe layers grown by MOVPE have been investigated. The post-annealing is effective for obtaining p-type conductive Zn1 xMgxTe, similar to the case of P-doped ZnTe. Especially, the electrical and PL properties for P-doped Zn1 xMgxTe layers grown under a Te-poor condition are improved effectively by post-annealing. On the other hand, P-doped Zn1 xMgxTe layers grown under a Te-rich condition shows a high compensation ratio and DAP luminescence. Similar tendencies are also found in P-doped ZnTe layers.
Acknowledgment This work is partially supported by a Grant-in-Aid for Scientific Research (C) (1) (No. 22560299) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Effects of annealing treatment upon electrical and photoluminescence properties of phosphorus-doped ZnMgTe epilayers grown by metalorganic vapor phase epitaxy Mitsuhiro Nishio, Keita Kai, Ryota Fujiki, Katsuhiko Saito n, Tooru Tanaka, Qixin Guo Department of Electrical and Electronic Engineering & Synchrotron Light Application Center, Saga University, Saga 840-8502, Japan
a r t i c l e i n f o
a b s t r a c t
Available online 3 August 2012
Post-annealing treatment in nitrogen gas flow has been carried out for P-doped Zn1 xMgxTe layer grown under a Te-rich or Te-poor condition by metalorganic vapor phase epitaxy. The electrical and photoluminescence properties of P-doped Zn1 xMgxTe layers are altered by annealing treatment. The post-annealing is very effective in obtaining p-type conductive Zn1 xMgxTe for the layer grown under a Te-poor condition. On the other hand, Zn1 xMgxTe layer is characterized by a high compensation ratio and DAP luminescence for the layer grown under a Te-rich condition, even after annealing treatment. Similar tendencies are also found in P-doped ZnTe layers. & 2012 Elsevier B.V. All rights reserved.
Keywords: A1. Doping A1. Metalorganic vapor phase epitaxy B2. Semiconducting II–VI materials B2. Semiconducting ternary compounds
1. Introduction Zn1 xMgxTe ternary alloy is a promising material for light emitting diodes (LEDs) with green or blue emission since the fundamental energy gap of Zn1 xMgxTe can be varied through Mg composition from 2.26 to 3.1 eV [1] or 3.2 eV [2]. Furthermore, Zn1 xMgxTe is expected to be one of the best materials as a cladding layer and a transparent substrate for improving the performance of ZnTe-based pure green LED because it is expected to form a type-I heterostructure with ZnTe [3]. We have already demonstrated a green ZnTe LED with a pn junction by means of thermal diffusion process of Al [4]. In order to improve the performance of LED, it is important to study growth and characterization of conductive p-type Zn1 xMgxTe crystals. Unfortunately, there are only a few works on Zn1 xMgxTe epitaxial layer grown by metalorganic vapor phase epitaxy (MOVPE) [3,5–10], which is a potential epitaxial growth technique for mass production. So far, nominally undoped Zn1 xMgxTe epitaxial layers have been successfully grown on (100) ZnTe substrates by MOVPE, and the growth parameters for control of Mg composition and for improvement of optical property were revealed [7–9]. Although intentional doping is essential for device applications, however, impurity doping into Zn1 xMgxTe layer by means of MOVPE has been hardly investigated except for our report [10]. As for ZnTe layer, on the other hand, trisdimethylaminophosphorus (TDMAP) is considered to be a suitable dopant source for achieving p-type doping over a wide range of carrier concentration with high
optical quality [11–15]. The effect of post-annealing treatment in nitrogen gas flow will be very effective for improving electrical and photoluminescence (PL) properties for P-doped ZnTe layers grown by MOVPE [11–13]. Then post-annealing treatment in nitrogen gas flow is expected to bring about p-type doping into Zn1 xMgxTe epitaxial layer effectively in addition to a use of TDMAP as dopant source. Very recently, we have obtained p-type Zn1 xMgxTe layers with relatively high carrier concentrations of 5 1016–2 1018 cm 3 based on this idea [10]. Therefore, annealing treatment after doping is expected to be promising means for improving the electrical and PL properties of p-type Zn1 xMgxTe layer. When the effective annealing for obtaining p-type conductive Zn1 xMgxTe and also P-doped ZnTe layers of high quality is considered, the data on the correlation between the annealing effects and growth conditions is important. However, it is ambiguous at present not only for P-doped Zn1 xMgxTe layer but also for P-doped ZnTe layer. In this paper, the effects of annealing treatment upon the electrical and PL properties are clarified for P-doped Zn1 xMgxTe layers grown at different transport rate ratios of VI-group–II-group, i.e., under a Te-rich or Te-poor condition. The electrical and PL properties of P-doped Zn1 xMgxTe layers may depend strongly upon such conditions as shown in previous report on P doping into ZnTe [14]. Furthermore, a comparison with P-doped ZnTe layer is made in order to get a guide to improve the crystal quality of P-doped Zn1 xMgxTe.
2. Experimental procedures n
Corresponding author. E-mail address: [email protected] (K. Saito).
0022-0248/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.07.024
The growth conditions are summarized in Table 1. The as-grown P-doped Zn1 xMgxTe layers were prepared on semi-insulating
M. Nishio et al. / Journal of Crystal Growth 370 (2013) 342–347
343
Table 1 Growth and annealing conditions for P-doped Zn1 xMgxTe and ZnTe layers. Layer
P-doped Zn1 xMgxTe Te-rich condition
Growth conditions Substrate Growth pressure Transport rate (mmol/min) DMZn (MeCp)2Mg DETe TDMAP Substrate temperature (1C) Total flow rate of H2 carrier gas (sccm) Mg content x Remark Post-annealing conditions Ambient gas Annealing temperature Ta (1C) Duration time (h)
P-doped ZnTe Te-poor condition
Te-rich condition
Te-poor condition
Ga-doped semi-insulating Ga-doped (100) ZnTe substrates Atmospheric pressure 23 3.8 42 0.2 395 800 0.035 Sample 1
20 8 19 0.1 380 800 0.12 Sample 2
20 4 19 0.2 395 800 0.022 Sample 3
N2 100–400 3
N2 100–420 2
N2 420 2
84
368
100 3 500 4800 0
336 8 425 4800 0
N2 420 2
N2 420 2
T (K)
10 1017
1016 : Sample1 (Te-rich) : Sample2 (Te-poor) 1015
200
100
1018
Carrier concentration (cm–3)
Carrier concentration (cm–3)
1018
Mobility(cm2/Vs)
102
300
:Fitting curve
1017
1016
1015 As–grown
100 200 300 Annealing temperature (°C)
400
Fig. 1. The carrier concentration and mobility at room temperature for P-doped Zn1 xMgxTe layers as a function of annealing temperature.
1014 5
10 103/T (K–1)
Ga-doped (100) ZnTe substrates by the atmospheric pressure MOVPE system. Dimethylzinc (DMZn), diethyltelluride (DETe) and bismethylcyclopentadienyl-magnesium (MeCp)2Mg were used as the source materials. The flow rate of hydrogen carrier gas was 800 sccm. The transport rates of source materials were varied depending upon a Te-rich or Te-poor condition as shown in Table 1. The effect of annealing temperature upon the electrical properties at room temperature has been investigated using P-doped Zn1 xMgxTe layers grown under conditions as follows. For a Te-rich condition, the transport rates of DMZn, (MeCp)2Mg, DETe, TDMAP and substrate temperature were, respectively, 23, 3.8, 42, 0.2 mmol/min and 395 1C. For a Te-poor condition, on the other hand, those were 20, 8, 19, 0.1 mmol/min and 380 1C. As a result, Mg content of 0.035 was obtained for the former while that of 0.12 for the latter, due to the difference of substrate temperature [9]. Namely, the decrease of substrate temperature may bring about the reduction of the premature reaction between DETe and (MeCp)2Mg and lead to an increase of Mg content [7]. For a slight decrease of substrate temperature, further Te-rich circumstance may occur near the growing surface for Te-rich
Fig. 2. The temperature dependence of the carrier concentration for as-grown and annealed P-doped Zn1 xMgxTe layers.
condition while further Te-poor circumstance may happen for Tepoor condition owing to the premature reaction. In order to attain a low Mg content for a Te-poor condition, the (MeCp)2Mg transport rate was decreased and substrate temperature was increased. Under the conditions that the transport rates of DMZn, (MeCp)2Mg, DETe, TDMAP and substrate temperature were, respectively, 20, 4,19 and 0.2 mmol/min and 395 1C, P-doped Zn1 xMgxTe layer with a low Mg content of 0.022 was obtained. The use of Zn1 xMgxTe layer with a small Mg composition such as x¼0.035 or x¼0.022 facilitates the measurement of the electrical properties at low temperature even without heat treatment of electrode [15] as well as in case of ZnTe [16] when electroless Pd electrode was employed. In order to compare with P-doped ZnTe, the P-doped ZnTe layers grown under both Te-rich and Te-poor conditions in other growth system have
344
M. Nishio et al. / Journal of Crystal Growth 370 (2013) 342–347
Table 2 Best-fit parameters determined from Hall analysis for P-doped Zn1 xMgxTe layers. NA (cm 3)
VI/II ratio
ND (cm 3)
ND/NA
EA (meV)
Remark
1.8 1.8
(7.47 0.9) 1017 (4.97 0.4) 1017
(6.1 70.8) 1017 (3.8 70.4) 1017
0.82 0.78
217 0.6 247 0.5
As-grown
1.8
(6.47 0.6) 1017
(4.8 70.6) 1017
0.75
247 0.6
Annealed at 200 % C for 3 h
1.8
(7.77 0.4) 1017
(4.6 70.4) 1017
0.59
277 0.4
Annealed at 300 % C for 3 h
1.8
(4.6 708) 1018
(2.4 70.5) 1018
0.53
237 0.3
(1.3 70.05) 1017 (4.97 0.2) 1018
(1.0 70.4) 1015 (2.8 72.5) 1017
0.0079 0.057
477 0.3 207 0.3
Annealed at 400 % C for 3 h As-grown Annealed at 420 1C for 2 h
0.79 0.79
T (K) 300
Annealed :Sample1 (Te–rich) :Sample3 (Te–poor) As–grown :Sample1 (Te–rich) :Sample3 (Te–poor)
10 1
10
o o
2.3
2.2
4K Sample1
103
–1
102
10–2
×1
A
B
C B
C
B
C
A
×1 ×1
Ta = 400°c
A
×1
2.1
C
A
×1 Intensity (arb. units)
Resistivity (Ω cm)
102
o
Energy (eV) 2.4
2.5
100
Mobility (cm2/Vs)
103
200
o
Annealed at 100 % C for 3 h
A
C
B
300°c 200°c 100°c As–grown
A 10 5
Sample3
10
103/T (K–1) Fig. 3. The temperature dependences of resistivity and mobility for as-grown and annealed layers.
× 1/3 500
been investigated. The flow rate of hydrogen employed in this system was as high as 4.8 slm. For a Te-rich condition, the transport rates of DMZn, DETe and TDMAP and the substrate temperature were 84, 100 and 3 mmol/min and 500 1C. On the other hand, the transport rates of DMZn, DETe and TDMAP and the substrate temperature were 368, 336 and 8 mmol/min and 425 1C for a Te-poor condition. Although it seems that the growth conditions were different considerably between both cases, the as-grown P-doped ZnTe layers of relatively good quality are obtainable in each case under these conditions. The thicknesses of the layers used here were around 4–7 mm. The postannealing treatment was performed in nitrogen flow. The annealing conditions are also summarized in Table 1. The annealing temperature was varied from 100 1C to 400 or 420 1C in order to investigate the influence of annealing temperature. As shown in the table, annealing time for a Te-rich condition is longer than for a Te-poor condition in the case of P-doped Zn1 xMgxTe layer. This is due to the fact that Te-poor condition leads to effective annealing compared with Te-rich condition as described later. The composition of Mg was evaluated by energy dispersive X-ray (EDX) analysis. Hall-effect measurements were carried out using a system of ResiTest-8300 (LN) for the van der Pauw’s method. Electroless Pd electrode was used as ohmic contact. A magnetic field of 0.35 T was applied to the samples. For temperature-dependent measurements, the sample was placed in a liquid nitrogen cryostat that allows control of temperature in the range between 80 and 300 K. Furthermore, PL measurement at 4 K was performed. A 405 nm blue–violet laser diode was used for Zn1 xMgxTe as an excitation light source, while an Ar ion laser with a
C×1
520
540 560 Wavelength (nm)
Ta = 420°c
580
600
Fig. 4. PL spectra at 4 K of P-doped Zn1 xMgxTe layer annealed at different temperatures.
wavelength of 488 nm was employed for ZnTe. The laser power was attenuated by neutral density filters when the excitation intensity dependence of PL was investigated.
3. Results and discussion Fig. 1 shows the carrier concentration and mobility at room temperature for P-doped Zn1 xMgxTe layers with x¼0.035 corresponding to a Te-rich condition (sample 1) and x ¼0.12 corresponding to a Te-poor condition (sample 2) as a function of annealing temperature. The carrier concentration increases considerably for both samples when annealing temperature more than 300 1C is applied. Thus, it is confirmed that post-annealing is effective for enhancing P activation for Zn1 xMgxTe, similar to for P-doped ZnTe [11–13]. The carrier concentration increases with the annealing temperature and becomes saturated at 350 1C for Te-poor condition, while it does not show saturated tendency even at 400 1C for Te-rich condition. Furthermore, slight improvement of mobility somehow is found for Te-poor condition. Also, it is noted that the annealing time for sample 2 is shorter than for sample 1.
M. Nishio et al. / Journal of Crystal Growth 370 (2013) 342–347
These facts indicate that annealing treatment is more effective for sample 2 corresponding to a Te poor condition than sample 1 corresponding to a Te rich condition. Fig. 2 shows the temperature dependence of the carrier concentration for as-grown and annealed P-doped Zn1 xMgxTe layers. In the same figure, the temperature dependences of the carrier concentration with a small Mg content of x¼0.022 grown under a Te poor condition (sample 3) are shown to compare with sample 1. In sample 1, the carrier concentration increases over the entire temperature range when the annealing temperature is 300 1C, probably due to activation of the P atoms by annealing. The change of temperature dependence is found at a measurement temperature of about 200 K in sample 1 annealed at 400 1C, which may be associated with thermally activated transport in the valence band from two different acceptor levels rather than a conventional single acceptor level. On the other hand, the carrier concentration becomes increased by about two orders after annealing for sample 3. Since the annealing conditions for sample 3 is the same as for sample 2, it
seems that annealing treatment is effective for P-doped Zn1 xMgxTe layer grown under a Te poor condition, independent of Mg content. From the temperature dependence of carrier concentration, it seems that the activation energy of P-acceptor (EA) is almost independent of annealing temperature for sample 1 whereas a strong dependence found in the as-grown layer changes into moderate dependence after annealing for sample 3. The variation of activation energy may be related to change in the number of donor defects (ND) by analogy with the behavior of P-doped ZnTe, since EA decreases with the third root of ND in P-doped ZnTe [17]. In order to evaluate the degree of compensation, the curve fittings were made for each sample. The following well-known equation for a non-degenerate semiconductor dominated by a single acceptor center was used: pðp þ ND Þ=ðNA N D pÞ ¼ ðN v =gÞexpðEA =kTÞ where p is the carrier concentration, NA acceptor concentration, ND donor concentration, and g the degeneracy factor. Nv is given by 2(pkTm*h)3/2/h3 and m*h the density-of-state effective mass of holes.
Energy (eV) 2.42
2.40 A
345
Energy (eV) 2.44
2.38
2.28
A
Annealed
B
2.36
2.20
2.12 Annealed
C
Intensity (a.u.)
Intensity (a.u.)
B
512
516
520
524
100% 70 50 30 10 7 5 3
500
550
Wavelength (nm)
Wavelength (nm)
Energy (eV)
Energy (eV)
2.42
2.40
2.38
A
2.44
2.36
A
B
As–grown
Intensity (a.u.)
Intensity (a.u.)
B
510
600
515
520
Wavelength (nm)
525
500
2.28
2.20 C
2.12 As–grown
100% 70 50 30 7 3 0.7 550
600
Wavelength (nm)
Fig. 5. The change in bands A and B for as-grown and annealed P-doped Zn1 xMgxTe layers at various excitation powers. The layer grown under a Te-poor condition was annealed at 300 1C.
M. Nishio et al. / Journal of Crystal Growth 370 (2013) 342–347
The value of scattering parameter was taken to be equal to 1. The variation of effective mass of holes with Mg content is small enough in the range x¼0–0.11 [18] to be neglected. Then, the values of m*h and g are taken to be 0.56m0 and 4, respectively, from those reported in P-doped ZnTe [17]. Actually, a slight change of the effective mass with Mg content hardly influences the best-fit values of NA, ND and EA. After determining the best-fit values of NA, ND and EA using a least-squares method, a straight line relationship between ln p(p þND)/(NA ND p)T 3/2 and T was checked using the chosen values. The calculated curves are shown as solid lines in the figure. The closed agreement between the experimental and theoretical curves means the validity of the results of the analyses. The best-fit parameters are given in Table 2. For sample 1, it seems that a high NA is attainable as annealing temperature increases to 400 1C, similar to sample 3. It can be said from electrical analyses that a shallow acceptor is introduced by P doping for both samples. Also, the compensation ratio (ND/NA) decreases slightly for both samples as annealing temperature increases. However, compensation ratio is very high in sample 1. On the other hand, it is so low in sample 3, although it seems that ND becomes somehow increased by annealing. Fig. 3 shows the temperature dependences of resistivity and mobility for as-grown and annealed layers at 400 1C for sample 1 and 420 1C for sample 3. It seems that the resistivity varies with temperature depending on the carrier concentration behavior. Accordingly, post-annealing for P-doped Zn1 xMgxTe grown under a Te poor condition is effective for obtaining the layer with low resistivity. In sample 1, the temperature dependence of mobility for the as-grown layer seems to be almost the same as for annealed one. On the other hand, the temperature dependence mobility varies with annealing treatment in sample 3. A remarkable decrease in mobility can be seen with increasing annealing temperature in the low-temperature range of 80–120 K, where ionized impurity scattering plays an important role in most cases. However, it seems to be difficult to explain in terms of ionized impurity scattering why the mobility of sample 3 is lower than that of sample 1 in the low-temperature range of 80–120 K for the as-grown layers from the data on NA and ND summarized in Table 2. The number of deep defects contributing to neutral impurity scattering may be increased for the layer grown under a Te poor condition, although annealing treatment is very effective in enhancing the carrier concentration for such a layer. Fig. 4 shows PL spectra at 4 K of P-doped Zn1 xMgxTe layer annealed at different temperatures. P-doped Zn1 xMgxTe layer grown under a Te-rich condition, that is, sample 1 was used. The PL spectra of as-grown layers are dominated by strong emission band denoted by A, together with a weak band denoted by B located near band A and a broad band centered at around 550 nm. By the post-annealing treatment, the PL spectra were altered as shown in the figure. Especially, the deep broad band C becomes predominant in the spectrum at an annealing temperature of 400 1C. For samples 2 or 3 corresponding to a Te poor condition, on the other hand, the intensity of broad band C is very weak compared with that of band A or band B [10]. PL spectrum of sample 3 annealed at 420 1C is shown in the figure as an example of Te-poor condition. These facts are in conflict with the results on the growth of undoped Zn1 xMgxTe where a deep broad band like band C becomes predominant as VI/II transport rate ratio decreases [3,8] although the intensity ratio of the deep band to band A or band B decreases with decreasing substrate temperature [8] probably due to reduced efficiency of DETe pyrolysis. The broad band C may be associated with some defect accompanied by P doping. Further study on P doping into Zn1 xMgxTe is required in order to clarify origin of its broad band. Fig. 5 shows the change in bands A and B for as-grown and annealed P-doped Zn1 xMgxTe layers for Te-rich condition at
various excitation powers. The layer grown under a Te-rich condition, sample 1, was used. For both the as-grown and annealed layers, the peaks of band B and C shift to shorter wavelength side as the excitation power increases, which is a characteristic of a donor–acceptor pair (DAP) recombination emission. On the other hand, the peak for band A is independent of the excitation power. By analogy with those of P-doped ZnTe, the band A is assigned to be P acceptor-related excitonic emission and the band B is attributed to P acceptor related DAP luminescence. The band C may be related to a deep acceptor level as expected from the temperature dependence of carrier concentration for an annealing temperature of 400 1C. It is noticeable that the band B disappears and instead the emission band associated with free electron to bound acceptor (FB) transition appears with annealing in the case of a Te poor condition [10]. The emission band around 527 nm for sample 3 shown in Fig. 4 corresponds to FB transition. The fact that DAP related to band B is found even after annealing treatment may be closely connected with a high compensation ratio in sample 1. Thus, it is important to choose a Te-poor condition in MOVPE growth in order to improve the electrical and optical quality of P-doped Zn1 xMgxTe effectively by means of annealing treatment. It is useful that a comparison with P-doped ZnTe layers is made between Te-rich and Te-poor conditions in order to investigate whether or not the presence of Mg is important. Fig. 6 shows the temperature dependences of carrier concentration for as-grown and annealed P-doped ZnTe layers grown under Te-rich and Te-poor conditions. It is also clear in the case of P-doped ZnTe that the layer grown under a Te-poor condition rather than a Te-rich condition is favorable for obtaining the high carrier concentration with annealing treatment. Furthermore, a unique temperature dependence of carrier concentration is found for the layer grown under a Te rich condition, common to the case of P-doped Zn1 xMgxTe. The curve fittings were also made for P-doped ZnTe layers using the above equation for a non-degenerate semiconductor dominated by a single acceptor center. In Table 3, the best-fit parameters are summarized in P-doped ZnTe layer. In P-doped ZnTe layer grown
T (K) 300
200
100 :Annealed ZnTe (Te–rich) :Annealed ZnTe (Te–poor) :As–grown ZnTe (Te–rich) :As–grown ZnTe (Te–poor)
1018
Carrier concentration (cm–3)
346
1017
1016
–:Fitting curve 1015 10
5 103/T (K–1)
Fig. 6. The temperature dependences of carrier concentration for as-grown and annealed P-doped ZnTe layers grown under a Te rich and poor conditions.
M. Nishio et al. / Journal of Crystal Growth 370 (2013) 342–347
347
Table 3 Best-fit parameters determined from Hall analysis for P-doped ZnTe layers. VI/II ratio
NA (cm 3)
ND (cm 3)
ND/NA
EA (meV)
Remark
1.2 0.91 0.91
(8.07 0.5) 1017 (1.17 0.02) 1018 (3.97 0.1) 1018
(7.7 7 048) 1017 (1.97 0.2) 1017 (3.97 0.1) 1016
0.97 0.17 0.065
197 0.3 367 0.8 367 0.9
As-grown As-grown Annealed at 420 1C for 2 h
under a Te-poor condition, a high NA is attainable by annealing treatment, similar to the case of P-doped Zn1 xMgxTe. Both ND and compensation ratio are so small. As for P-doped ZnTe layer grown under a Te-rich condition, NA is as high as 8 1017 cm 3as well as in the case of P-doped Zn1 xMgxTe (see Table 2). Although it seems to be difficult to dope a Te lattice site with P under a Te rich condition essentially, P will be easily introduced into ZnTe and Zn1 xMgxTe during the growth probably due to relatively small atomic radius. However, the compensation ratio is as high as 0.97, in agreement with the case of P-doped Zn1 xMgxTe, probably due to influence of residual impurities or defects introduced simultaneously during the growth.
4. Summary The effects of annealing treatment upon the electrical and PL properties of P-doped Zn1 xMgxTe layers grown by MOVPE have been investigated. The post-annealing is effective for obtaining p-type conductive Zn1 xMgxTe, similar to the case of P-doped ZnTe. Especially, the electrical and PL properties for P-doped Zn1 xMgxTe layers grown under a Te-poor condition are improved effectively by post-annealing. On the other hand, P-doped Zn1 xMgxTe layers grown under a Te-rich condition shows a high compensation ratio and DAP luminescence. Similar tendencies are also found in P-doped ZnTe layers.
Acknowledgment This work is partially supported by a Grant-in-Aid for Scientific Research (C) (1) (No. 22560299) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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