Thermodynamic Analysis and Single Crystal Growth of ZnSe from ZnSe-N-H-Cl System Changyou Liu and Wanqi Jie* College of Materials Science and Engineering, Northwestern Polytechnical UniVersity, Xi’an 710072, China
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 10 3532–3536
ReceiVed April 6, 2007; ReVised Manuscript ReceiVed May 23, 2008
ABSTRACT: The thermodynamic properties of the transporting reactions in the closed-tube ZnSe-N-H-Cl system are analyzed by solving a set of equations with numerical methods, where NH4Cl serves as the transport agent. The results show that ZnSe-NH4Cl system has the properties of high total pressure and low transporting reaction equilibrium constant. The decomposition of hydrogen selenide determines the difference between the partial pressures of ZnCl2 and H2Se, which slows down the growth rate. This difference can be adjusted by adding extra Se and the hydrogen (H2) produced through decomposition of NH3. The ZnCl2 and a little extra Se should be added into the ZnSe-NH4Cl system to improve the utilizing efficiency of H2 and the concentration of practical transporting components of ZnCl2 and H2Se. A ZnSe single crystal as large as 20 × 15 × 8 mm3 was obtained by using ZnCl2 · 2NH4Cl as the transport agent with a little extra Se at about 1300 K. The experimental results about the diffusion-limited transport rate are well coincided to the thermodynamical analysis.
1. Introduction ZnSe is a direct-transition-type compound semiconductor with a bandgap energy of 2.7 eV at room temperature, which has been identified as an important contender for the fabrication of blue light-emitting diode (LED),1 laser diode (LD),2 ZnSe-based mixed-color LEDs,3 and nonlinear optoelectronic devices4 and so on. The preparation of high-quality ZnSe substrates is crucial to promote the practical application of the materials. Due to the twins and the high density of dislocations in ZnSe crystals grown from the melt, many others methods, such as physical vapor transport (PVT),5-7 chemical vapor transport (CVT),8-12 and solid state recrystallization (SSR) 13,14 are applied to obtain perfect ZnSe single crystals. In the CVT method with low temperature, iodine,8 hydrogenchloride,10 hydrogen,12 ammoniumchloride,11,15 hydrogen/water9 have been used as transport agents. The chemical transportion processes in ZnSe-I2 system have been studied carefully by experiments and computations. However, from the point of view of nucleation, the transport agent HCl is more suitable for the growth of ZnSe single crystals than I2.11 Recently, our group has obtained high-quality ZnSe single crystals by chemical transport reactions using ZnCl2 · 3NH4Cl as the transport agent.15 However, we found that the reproducibility was not satisfactory when crystal growth runs were carried out with large size ampoules. Therefore, the main aim of the present paper is to investigate the thermodynamic properties of ZnSe-N-H-Cl system in the closed-tube. We report the results of ZnSe single crystal growth under the optimized parameters.
2. Thermodynamics Analysis In our analysis on the growth system of ZnSe with NH4Cl as the transport agent, all the thermodynamics data are taken from the handbook edited by Ye et al.16 Ammonium chloride nearly completely dissociates into N2, H2, and HCl at high temperature.17-22 The ammonium has partial pressure at least 4-5 orders lower than those of N2 and H2 at about 1060 * To whom correspondence should be addressed. Tel.: +86-29-88495414. Fax: +86-29-88495414. E-mail address:
[email protected].
K,22 so it will be ignored in our analysis. The gases of HCl and H2 can be used as the transport agents. The chemical reactions relevant in ZnSe-N-H-Cl system are shown as follows:
ZnSe(s) + 2HCl(g) ) ZnCl2(g) + H2Se(g)
(1)
ZnSe(s) + H2(g) ) Zn(g) + H2Se(g)
(2)
where the gas of hyhdrogen selenide further dissociates into hydrogen and selenium dimmer at the temperature above 433 K,23 that is:
H2Se(g) ) H2(g) + 0.5Se2(g)
(3)
The temperature dependence of the equilibrium constants (Kp) of eqs 1–3 were calculated using thermodynamic data in Table 1 from Ye et al.16 The results are shown in Figure 1. According to the relative thermodynamic data, the reaction of Zn(g) with HCl(g) carries very rapidly in the same environment, which is described by eq 4.
Zn(g) + 2HCl(g) ) ZnCl2(g) + H2(g)
(4)
So, it is believed that the transport reactions are dominated by HCl. Furthermore, among reactions above, the eq 1 can be obtained by the combining eqs 2 and 4. Thus, only eqs 1 and 3 are independent and need to be considered. The equilibrium constant of eq 3 is larger than that of eq 1, which shows that eq 1 is the lowest step reaction among the chemical transport reactions. Due to eq 3, the dimmer (Se2) exists in the vapor phase. In the equilibrium, the chalcogens (Se) are dominated by dimmer Se2 in the vapor phase. The molecular ZnSe (g) and all other possible chalcogen monomers or polymers should also present, but in the range of temperature and the pressures involved in the research, their concentrations are very low, and can be ruled out. Therefore, only ZnCl2, HCl, H2Se, H2, Se2 and N2 will be included in our analysis. The nitrogen is considered as the inertia gas, and eqs 1 and 3 are considered for the partial pressure calculation.
10.1021/cg070337d CCC: $40.75 2008 American Chemical Society Published on Web 09/06/2008
Thermodynamic Analysis and Single Crystal Growth of ZnSe
Crystal Growth & Design, Vol. 8, No. 10, 2008 3533
Table 1. Thermodynamics Data16 ∆Hf,298° /Jmol
-1
ZnSe(s) HCl(g) ZnCl2(g) H2Se(g) H2(g) Zn(g) Se2(g)
-158992 -92048 -224295 (1005 K) 29288 0 148735 (1180 K) 138185
S298° /JK-1mol-1
Cp /JK-1mol-1
70.291 186.774 346.181 (1005 K) 218.823 130.583 189.461 (1180 K) 246.865
50.166 + 5.774 × 10-3 T 26.527 + 4.602 × 10-3 T+1.088 × 105 T-2 60.250 + 0.837 × 10-3 T 31.757 + 14.644 × 10-3 T - 1.297 × 105 T-2 27.280 + 3.264 × 10-3T + 0.502 × 105 T-2 20.786 44.601 - 2.657 × 10-3 T - 2.502 × 105 T-2 (298-985 K) 44.601-2.657 × 10-3 T - 2.481 × 105 T-2 (985-2000 K)
3. Partial Pressures Calculation and Discussion The partial pressures of all the volatile species in equilibriums at given temperature (T) and total pressure (Ptotal) can be calculated by solving the following set of equations by numerical methods:
PN2+PHCl+PZnCl2+PH2Se+PH2+PSe2)Ptotal
(5)
PZnCl2PH2Se⁄(PHCl)2)KP1
(6)
PH2PSe20.5 ⁄ (PH2SeP00.5) ) KP3
(7)
PHCl+2PH2Se+2PH2)PH
(8)
PHCl+2PZnCl2)PCl
(9)
(PHCl+2PH2Se+2PH2) ⁄ (PHCl+2PZnCl2) ) [H ⁄ Cl]
(10)
Equation 5 is simply Dalton’s law. The N2 pressure is constant at the given temperature and the concentration of NH4Cl. Equations 6 and 7 are the equilibrium conditions, where P0 is the atmosphere pressure at the standard state. Equations 8 and 9 give the conservation conditions of the atomic composition in the systems. PH and PCl are the pressure formats at the conservation conditions of the atomic composition for H and Cl atoms, respectively. Equation 10 is the confining condition according to the additives, for example, the value of [H/Cl] can be 4:1, 12:5, 8:4, 4:(1 + 2x) for NH4Cl, ZnCl2 · 3NH4Cl, ZnCl2 · 2NH4Cl, and xZnCl2 · NH4Cl, respectively. 3.1. Partial Pressures versus NH4Cl Concentration and Temperature. The variations of partial pressures with temperature and concentration of NH4Cl are depicted in Figures 2 and 3. As shown in Figure 2, the partial pressures of ZnCl2 and H2Se are much smaller than those of H2, HCl, N2, and the total pressure. The difference between the partial pressures of ZnCl2 and H2Se increases with the increase of temperature. The pressure of ZnCl2 is higher than that of H2Se due to the de-
composition of hydrogen selenide as described by eq 3. The partial pressure of Se2 is about 1 order lower than those of ZnCl2 and H2Se. In Figure 3, the curves show that the calculated partial pressures of all components except Se2 increase obviously with the increase of NH4Cl concentration. The partial pressure of Se2 is inclined to a plateau due to the effect of equilibrium conditions according to eqs 1 and 3. The difference between the partial pressures of ZnCl2 and H2Se seems to decrease, but obvious difference still exists when we notice that the curves are logarithmically plotted. The related data are listed in the Table 2. The difference between the partial pressures of ZnCl2 and H2Se, i.e. ∆P ) PZnCl2 - PH2Se, increases gradually against NH4Cl concentration. However, the relative difference between them, R (%) ) ∆P/PZnCl2, decreases dramatically with the increase of NH4Cl concentration. The total pressure ranges from about 2.7 to 30 atm as the concentration of NH4Cl increases from 0.5 to 5.0 mg/mL.
Figure 2. Partial pressures as the function of temperature (0.5 mg/mL NH4Cl).
Figure 3. Partial pressures as the function of NH4Cl concentration (T ) 1200 K). Table 2. Partial Pressures of ZnCl2 and H2Se at Different NH4Cl Concentrations at 1200 K
Figure 1. Temperature dependence of the equilibrium constants, KP. (1) for reaction ZnSe(s) + 2HCl(g) ) ZnCl2(g) + H2Se(g); (2) for reaction ZnSe(s) + H2(g) ) Zn(g) + H2Se(g); (3) for reaction H2Se(g) ) H2(g) + 0.5Se2(g).
concentration /mg · mL-1
PZnCl2 /Pa
PH2Se /Pa
∆P /Pa
R (%)
0.5 1.0 2.0 5.0
5324.03 10037.53 19340.96 47105.60
3873.77 8341.75 17471.11 45103.98
1450.26 1695.78 1869.85 2001.62
27.24 16.89 9.67 4.25
3534 Crystal Growth & Design, Vol. 8, No. 10, 2008
Figure 4. Partial pressures corresponding to the Se excess in the source (0.5 mg/mL NH4Cl).
In view of the kinetic processes of crystal growth, the vapor phase crystal growth is considered to be a diffusion-limited transport process in the case that the total pressure varies in the range of 0.1-10 atm at small temperature difference between the two ends. The inert and the excess majority components constitute a diffusion barrier through which the minority components have to diffuse.24,25 The diffusion coefficients are inverse proportion to the total pressure. They are comparatively small and roughly similar when the total pressure is above about 3 atm.26 The ZnSe-N-H-Cl growth system has a comparatively higher total pressure. The sum of partial pressures of N2, H2, and HCl is about 40 times higher than that of partial pressures of ZnCl2. Hereby it is believed that the molecules of ZnCl2 or H2Se diffuse at the same transport rate from the source end to the growth end. The transport components of ZnCl2 and H2Se are removed in stoichiometric proportion from the vapor by the growing crystal. The growth rate is the greatest when the two components be completely depleted,24,25 that is, ∆P ) 0. Therefore, the large difference between the partial pressures of ZnCl2 and H2Se goes against improving crystal growth rate. The positive ∆P (∆P > 0) is not favor to obtain a large growth rate. Therefore, low temperature and high NH4Cl concentration should be adopted in ZnSe-N-H-Cl growth system. However, low temperature and high concentration will result in two problems. First, low temperature means that the transporting chemical reaction rate is low, which results in the decrease of growth rate, even stopping crystal growth. Second, high concentration means that the total pressure is very high. The growth rate is lower at higher concentration. Furthermore, the quartz ampule should be avoided to explore at the high total pressure. Hereby, we prefer to choose the case that the crystal growth is carried out at higher temperature and moderate concentration, for example, about 1300 K and 0.5-2.0 mg/mL NH4Cl. The difference between the partial pressures of ZnCl2 and H2Se could be adjusted through shifting the reaction equilibrium direction of eq 3 by adding extra selenium element. It is well-known that the deviation from stoichiometry has more significant effects on the growth in closed tubes than uncertainties in the temperature. In fact, it is the problem of deviation from stoichiometry that the materials release the metal component into the vapor, Zn from ZnSe, while its composition deviates from exact stoichiometry to Se-rich side. Figure 4 shows the partial pressures of ZnCl2 and H2Se change with the excess of selenium (cm-3) in source materials (2 cm3) in case of its composition deviating from exact stoichiometry. For the Se excess concentration below 1017/cm3, the partial pressure change is not obvious. While for the Se excess concentration above 1017/cm3, the curve shows that the partial pressure change is strongly dependent on the deviation. However, in vapor
Liu and Jie
Figure 5. Partial pressures as the functions of temperature with extra Se addition.
Figure 6. Partial pressures as the function of extra Se addition at NH4Cl concentration of 0.5 mg/mL at 1300 K.
growth the source materials usually need to be pretreated by heating under high temperature or sublimation under dynamic evacuating. The amount of Zn released from pretreated materials will be so low that it will be ignored in the pressure analysis in the next discussions. 3.2. Adjustment of the Partial Pressures of ZnCl2 and H2Se. The imbalance between the partial pressures of ZnCl2 and H2Se can be adjusted by adding extra Se, which will shift the equilibrium of eq 3 to the left. In order to get ∆P ) 0, the pressure of Se2, PSe2, can be determined by resolving eq 3.
PSe2)(KP3PZnCl2)2⁄(PH2(d))2
(11)
where the symbols of H2(d) and PH2(d) represent the hydrogen and its pressure derived from the decomposition of NH3, respectively. For ∆P ) 0, the calculated partial pressures against temperature with a little extra Se were plotted in Figure 5, which is very similar to Figure 2 except that ZnCl2 and H2Se have the same the partial pressure. At fixed temperature 1300 K and the NH4Cl concentration of 0.5 mg/mL, the partial pressures are plotted in Figure 6, which shows that PH2Se, PSe2, and PHCl increase gradually while ZnCl2 and PH2 reduce gently with the increase of additional Se2. The N2 pressure is a constant at the given temperature. After the partial pressure of H2Se reaches to that of ZnCl2, the difference between them becomes negative (∆P < 0). This result implies that we could improve the utilizing efficiency of H2(d). The negative difference can be adjusted by adding ZnCl2. This also means that we can improve the virtual total concentration of HCl in the closed-tube. The calculated results are similar to those in Figure 2 and Figure 5 except that the pressure of H2 deduces gradually. To show the effect of adjustment, the partial pressures of ZnCl2, Se2, and H2Se are displayed in the same figure, as shown in Figure 7. The labeled 2_ ZnCl2 curve is located between curves labeled by 1_ ZnCl2 and 1_ H2Se, which indicates that the adjustment of the partial
Thermodynamic Analysis and Single Crystal Growth of ZnSe
Crystal Growth & Design, Vol. 8, No. 10, 2008 3535
Figure 7. Partial pressures of ZnCl2, Se2 and H2Se at different conditions with NH4Cl concentration of 0.5 mg/mL 1_ NH4Cl, 2_ NH4Cl + Se, 3_ NH4Cl + Se + ZnCl2 (about 0.6 mg/mL).
Figure 9. Partial pressures as the functions of ZnCl2 at 1300 K (Extra Se added and 0.5 mg/mL NH4Cl).
Figure 8. Partial pressures in ZnSe-HCl system as the functions of temperature (HCl concentration is tantamount to 0.5 mg/mL NH4Cl).
Figure 10. Pictures of as-grown single crystals In (a) unseeded ampule (b) polycrystalline-seeded ampule.
pressures of ZnCl2 and H2Se can be achieved by adding a little extra Se. The curve labeled by 3_ ZnCl2 locates above the curve labeled by 2_ ZnCl2, which shows that the utilizing efficiency of H2(d) is improved by adding ZnCl2 and extra Se. 3.3. Effect of H2 Derived from the Decomposition of NH3. According to eq 3, H2(d) can adjust the equilibrium to shift to the left. As the result, H2Se will be increased. The calculated partial pressures in ZnSe-HCl system are displayed in Figure 8. As compared with ZnSe-N-H-Cl system shown in Figure 2, the difference between the partial pressures of ZnCl2 and H2Se in Figure 8 is obviously larger. But this kind effect of H2(d) is limited by the equilibrium condition and H2(d) is considered not to take part in transport reaction. The partial pressures of ZnCl2 and H2Se are much lower than the total pressure. They can not be augmented even if a little extra Se is added, which only reduces the partial pressure of ZnCl2 and increases that of H2Se. It is a practical application to improve transport component concentration as high as possible under the condition without dramatic augment of total pressure. As discussed above, if we add ZnCl2 and extra Se into the system at the same time, it means the total concentration of HCl increases according to eqs 1 and 3, and then H2(d) is considered to take part in transport reaction. In turn, the partial pressures of ZnCl2 and H2Se are increased at the same time. The partial pressures depending on the amount of ZnCl2 with a little Se are calculated, which results are shown in Figure 9.
The chemical agents NH4Cl (4N) and ZnCl2 (PT) were purchased from the Kermel Ltd.,Tianjian, China. For operation convenience, instead of NH4Cl and ZnCl2, the synthesized ammonium zinc chloride (AZC, II type, ZnCl2 · 2NH4Cl) was used as the transport agent. 4.2. Growth of single crystals. Eight to 10 g of ZnSe polycrystalline was filled in a quartz ampule with a length of 80 mm and a diameter of 15-20 mm with ZnCl2 · 2NH4Cl (1.0 mg/mL NH4Cl) and/ or a few milligrams of Se. The ampule was placed into a double-zone vertical electrical furnace controlled by Eurotherm controllers with the accuracy of (0.1 K. A reverse temperature profile was developed across the ampule over 10 h to clean microcrystals near the growth end. The temperature of the source end was maintained at 1270-1300 K and the temperature difference was controlled in the range of 5-7 K. After 12-48 days, the ampule was cooled down to the room temperature at the rate of 20 K/h.
4. Experiments 4.1. Preparation of the Source Materials. ZnSe polycrystalline was synthesized by direct reaction of the constituent elements of nominal purity 6N5 Zn and 5N Se (EMEI Semiconductor Materials Ltd.) in the sealed quartz ampule. The synthesized polycrystalline was pretreated by sublimation and then sintered the powder materials to form 0.8-1.0 mm grains which were used for the single crystal growth.
5. Results Figure 10a shows the ZnSe single crystal grown for 12 days in an unseeded tip ampule. The crystal has developed wellcomplete faces of {111} identified by XRD. The X-ray rocking curve was measured by the X’Pert Pro double crystal spectrometer. The ZnSe {111} plane diffraction curve shows a Gaussian distribution with an FWHM of about 60 s., which indicated that the grown crystal has a good crystallization quality. Figure 10b is the picture of the ZnSe single crystal grown in a polycrystalline seeded ampule in order to avoid nucleation on the wall. As shown in Figure 10b, large bulk ZnSe single crystal was obtained after growing for 48days. The investigations of transport rate were carried using the polycrystalline seeded ampules with various tip angles. The values of average transport rate are obtained simply by dividing the weight of the grown crystal by the growth time. Figure 11 is a curve of the transport rate obtained in the ampules with about 50 ° tip angle, which indicates that the growth is cline to a plateau with the increase of growth time. However, it needs long continuing time to get the datum of steady transport rate.
3536 Crystal Growth & Design, Vol. 8, No. 10, 2008
Liu and Jie
adding extra Se and the hydrogen derived from the decomposition of NH3. The ZnCl2 and a little extra Se are added into the ZnSe-N-H-Cl system to improve the utilizing efficiency of H2(d) and the total concentration of HCl. (3) Large-sized ZnSe single crystals have been grown using ZnCl2 · 2NH4Cl as the transport agent with a little extra Se at about 1300 K. The experimental results about the diffusionlimited transport rate are well coincided to theoretical analysis. Acknowledgment. We are grateful for the financial support of the National Science Foundation of China. Figure 11. Transport rate against to growth time (about 50 ° tip angle).
Figure 12. Transport rates under various conditions(NH4Cl concentration is tantamount to 1.0 mg/mL) (a) ZnCl2•2NH4Cl, (b) ZnCl2• 2NH4Cl+Se.
Therefore, we experiment for 2 weeks in the ampules with the same tip angle (about 60 °) to obtain the transport rates under various conditions, as shown in Figure 12. The growth rate using ZnCl2•2NH4Cl as transport agent is much smaller than that obtained under the same conditions with a little extra Se. This result well agrees with our thermodynamic analysis. When only ZnCl2•2NH4Cl is introduced into the growth tube, the difference between the partial pressures of ZnCl2 and H2Se increases while the partial pressure of H2Se decreases, and accordingly the transport rate slows down.
6. Conclusions (1) The thermodynamic properties of the transporting reactions in ZnSe-NH4Cl CVT system are analyzed by solving sets of equations with numerical methods. The results show that ZnSe-NH4Cl system has the properties of high total pressure and low transporting reaction equilibrium constant. It is suggested that crystal growth should be carried out at higher temperature and moderate concentration. (2) The decomposition of hydrogen selenide determines the difference between the partial pressures of ZnCl2 and H2Se. To improve the growth rate, this difference can be adjusted by
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CG070337D
