DOI: 10.1021/cg900339u
Hydrothermal Growth of ZnO Single Crystals with High Carrier Mobility
2009, Vol. 9 4378–4383
Wenwen Lin, Dagui Chen, Jiye Zhang, Zhang Lin, Jiakui Huang, Wei Li, Yonghao Wang, and Feng Huang* Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China Received March 25, 2009; Revised Manuscript Received August 26, 2009
ABSTRACT: Thirty millimeter ZnO single crystals have been grown by the hydrothermal method with new mineralizers and a low-cost liner. A sharp X-ray rocking curve with the full width at half-maximum of 36 arcsec has been obtained for the (002) reflection, indicating a good crystallinity of the crystal. Low temperature photoluminescence spectrum revealed the crystal has a narrow and strong free exciton emission band (3.359 eV) at 10 K, with the absence of a green-yellow emission band that generally is induced by the impurities or lattice vacancies. Compared to ZnO crystals hydrothermally grown from conventional mineralizers (LiOH and KOH), this crystal has a unique feature that the room-temperature carrier mobility is close to the intrinsic value, with the carrier concentration maintained as high as 4.091016 cm-3. Two main donors which may relate to the H impurity were indicated. Analysis revealed that the diffusion of H in the ZnO lattice might be responsible for the decrease in conductivity of the ZnO crystal after annealing. The reason why the new hydrothermal method yielded low-resistance ZnO crystals could be interpreted as a much higher concentration of H impurity and much lower concentration of Li impurity incorporated into the ZnO lattice, compared with samples yielded by the conventional hydrothermal method. We anticipate the reported high-quality ZnO single crystals can not only be suitable objects for studying the intrinsic properties of ZnO, but also be potential substrates for fabricating ZnO light emitting diodes devices.
1. Introduction As one of the third generation semiconductors, ZnO has attracted considerable attention due to its promising applications in optoelectronic devices.1,2 Specifically, owing to the characteristics of a wide direct band gap (3.37 eV) and large exciton binding energy (60 meV) at room temperature, ZnO material becomes a potential candidate for preparing high luminous efficiency UV/visible light emitting diodes (LEDs) and laser diodes (LDs).3-5 ZnO single crystals can be grown via a chemical vapor transport (CVT) method,6-10 pressurized melt method,11 flux method,12-15 and hydrothermal method.16-19 The hydrothermal method, benefiting from a relatively low growth temperature and an approximate thermodynamic equilibrium growth condition, can produce ZnO crystals with low defect density and high crystallinity. Using KOH and LiOH as the mineralizers (here and after, referred to as conventional mineralizers), several research groups have grown 3 in. high-quality ZnO crystals successfully.18,19 However, the disadvantage of the conventional hydrothermal method lies in the basic mineralizers. Under basic solution, metal impurities from the autoclave body can be corroded easily and then incorporate into the lattice of ZnO crystals. For avoiding this, expensive noble metals such as Pt and Au were used as the liners of autoclaves. Even so, the impurities from the sintered ZnO strings, mineralizers, and liners, such as Li, K, Au, Pt, are still inevitable in the ZnO lattice.19 Possibly due to the impurities, ZnO crystals grown via the conventional hydrothermal method are found with low carrier mobility and low carrier concentration. *Corresponding author. E-mail:
[email protected]. pubs.acs.org/crystal
Published on Web 09/10/2009
The preparation of high-quality ZnO single crystals with near-intrinsic carrier mobility is essential for in-depth research on the ZnO material. The effect of each impurity element in the ZnO lattice can be more easily identified in high-quality crystals. Moreover, ZnO crystal with high carrier mobility can be used as a substrate to grow ZnO film and fabricate devices directly. It is believed that via the homoepitaxy method, the dislocation density and the nonradiative recombination efficiency of ZnO films can be reduced; thus the performance of LED can be improved. In this paper, we propose a new hydrothermal scheme. Aiming to suppress some impurities from incorporating into ZnO crystal, new mineralizers were designed. The selection of the new mineralizers was based on the chemical potential principle. Specifically, by using new mineralizers, we produce a circumstance that the chemical potential of some impurity metals in solution (μM,solution) were much lower than that in ZnO lattice (μM,ZnO). Thus, most impurity metals M can remain in the solution, while the incorporation of M into the ZnO crystal can be greatly inhibited.20 Since the new mineralizers can retain some impurities in solution, we can replace the inert noble metal autoclave liner with a low-cost autoclave liner. 2. Experimental Section 2.1. The Hydrothermal Growth of ZnO Crystals. Scheme 1 shows the scheme of the hydrothermal growth system. ZnO crystals were grown from an autoclave liner with a diameter of 66 mm, height of 1000 mm, capacity of 3400 mL, and baffle opening of 10%. The body of the autoclave was made of high strength Ni-Cr alloy, and the liner, the baffle, and the seeds holder were made of a low-cost material. The nutrient was prepared from 99.5% purity ZnO powder, which was sintered for 4 h at 900 °C in a platinum crucible r 2009 American Chemical Society
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Scheme 1. Scheme of the Hydrothermal Growth Systema
a
The ratio between the growth zone and dissolution zone is 3:1.
after being pressed. Over 100 seeding plates were suspended with Pt wires onto the seeds holder. A mixture of deionized water and the new mineralizer was added into the autoclave liner. Suitable quantity of distilled water was poured into the cavity between the liner and the wall of the autoclave for balancing the pressure of inner cavity and external cavity. The temperature of growth zone was 320 °C; the temperature difference between growth zone and dissolution zone was 15-20 °C, and the heating-up time is 20 h. At the liquid filling factor of 80%, the pressure can reach 100 MPa. The growth period was 30 days per run. By carefully adjusting the crystal growth parameters and the heating rate of the autoclave, spontaneous nucleation could be suppressed. 2.2. The Characterization of ZnO Crystals. X-ray Rocking Curve Measurement. The crystallinity of (0001) oriented ZnO crystals was investigated by rocking curve measurement using the (0002) reflection. A Philips MRD diffractometer with Cu KR radiation was employed in combination with a four crystal Ge (220) channel monochromator. A MTI sample was taken as the control sample for comparison in the measurement. All samples were polished before measurement. Impurity Analysis. Glow discharge mass spectrometry (GDMS) was used for determining impurity concentrations in ZnO crystals and sintered ZnO strings. The measurements were conducted on the Element GD (Thermal Fisher Corporation), and high-purity oxygen was used as discharge gas. Hall Measurements. The Hall measurements were performed on the HMS3000 instrument. The surface of the ZnO wafer (size: 10 101.0 mm, cut from as-grown ZnO crystal) was polished before measurements. Hall measurements were carried out at a magnetic field of 4500 Gauss and 10 mA direct current using the van der Pauw configuration, and the ohmic contacts were prepared by soldering In dots onto the ZnO samples. Liquid nitrogen was used as the refrigeration source, and the Hall measuring temperature range was from 79 to 300 K. Photoluminescence (PL) Spectra. The PL spectrum measurements were performed on a self-assembled apparatus in the Institute of Semiconductors, CAS, using a continuous-wave He-Cd laser (λexc =325 nm) as an excitation source. A CTI-22 vacuum cooling system was also used in the measurements to achieve low temperature down to 10 K. The resolution of the spectra was 1 nm. Optical Transmittance (OT) Spectra. The ZnO wafer (size as 10 100.5 mm) was polished and cleaned before the optical transmittance measurement. Another ZnO wafer (size as 10100.5 mm) purchased from Tokyo Denpa Corporation was selected as a control substance in the parallel experiment, and the wafer was polished via the same chemical-mechanical polishing (CMP) process. The optical transmittance spectra of the crystals were collected using a Perkin-Elmer Lambda-900 ultraviolet-visible (UV) spectrometer at room temperature. The range of incident light wavelength is 300-800 nm, and the spectrometer resolution is 1 nm. Annealing of Crystals. The ZnO wafer was put into a quartz tube, and then annealed at 900 °C under the flowing oxygen atmosphere for 2 h in a furnace. The Cutting, Polishing, Cleaning of Crystals. The WXD170 model to and fro diamond threadcutter and ZYP200 model polishing apparatus were used to cut and polish ZnO crystals. After being cut from the as-grown crystal, the ZnO slices were lapped successively with the polishing powder with grain sizes of 14 μm, 7 μm, 3.5 μm, and 0.5 μm, respectively, in order to remove the damaged layer
Figure 1. ZnO crystals hydrothermally grown with the new mineralizers (a) as-grown crystal (size as 30255 mm), (b) a wafer cut from (0001) sector (size as 10101 mm).
Scheme 2. Scheme of Growth Habit of ZnO Crystal in the New Hydrothermal Growing Environment
caused by the cutting process. Subsequently, the crystal was polished via chemical-mechanical polishing (CMP) which aimed to remove the damaged layer caused by the lapping process. The CMP process was conducted by using a silica-based alkaline slurry whose pH was 10. The last step was cleaning. The ZnO wafer was rinsed with ethanol and acetone in an ultrasonic vibrator for 10 min, then blown dry with nitrogen gas. Declarations. In this paper, the ZnO crystal grown by our group was called FJIRSM (Fujian Institute of Rearch on the Structure of Matter) ZnO for short. The as-supplied ZnO crystals from Tokyo Denpa Corporation were called TEW ZnO crystals, and as-supplied ZnO crystals from MTI Corporation were called MTI ZnO crystals. Conventional mineralizers refer to LiOH and KOH, and the hydrothermal method using conventional mineralizers for growing TEW and MTI ZnO crystal was called the conventional hydrothermal method.
3. Results 3.1. The Growth Habit of ZnO Single Crystal. With a 1 cm2 commercial light yellow ZnO wafer as a seeding plate, we obtained FJIRSM ZnO crystals as shown in Figure 1a. The
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newly grown ZnO layer is actually near colorless, as shown in the cut (0001) oriented ZnO wafer in Figure 1b. Scheme 2 shows the possible exposed sector of ZnO crystal during growth. The face of þc is terminated with Zn atoms, and -c is terminated with O atoms, and c-axis is the polar axis. The five exposing sectors named þc, -c, m, þp, and -p are correspondingly indexed as (0001), (0001), (1010), (1011), and (1011) plane, respectively. Table 1 lists the comparison of the growth rates of crystal faces under different hydrothermal conditions. It reveals that the growth of (1010) face is the fastest under the new mineralizers, while the growth of the (0001) face is the fastest under the conventional mineralizers. Generally, the growth rates for different crystal faces are low in the new hydrothermal system. 3.2. The Quality of ZnO Crystals. Figure 2 shows the X-ray rocking curve of FJIRSM and MTI ZnO crystals for (002) reflection, and the full width at half-maximum of FJIRSM and MTI samples are 36 arcsec and 18 arcsec, respectively. Table 2 illustrates the concentration of impurities M in FJIRSM ZnO crystal and ZnO sintered strings. Impurity analysis confirmed that the selection of the new mineralizers is applicable to suppress the impurities belonging to the main group, the first subgroup, and the second subgroup elements. Although ZnO raw material with purity as low as 99.5% was used as the nutrition, the concentrations of the corresponding elements in the ZnO crystal are quite low. We should mention that the suppressing effect of the new mineralizers to some transitional metals such as Fe, Cr, and Mn are not as strong as other metal elements; however, a higher purity ZnO sintered strings such as 99.99% ZnO can be used as nutrition supplier to avoid such a problem. 3.3. The Characterization Analysis of ZnO Single Crystals. Room Temperature Electrical Properties of As-Grown ZnO Crystal. Table 3 shows the room temperature (300 K) electrical properties of as-grown ZnO crystals grown from different methods. The FJIRSM ZnO crystal is found to be highly n-type. With the presence of high carrier concentration, the carrier mobility remains as high as 239 cm2/(V s), which is very close to the intrinsic value. (Albrecht et al. predicted that the intrinsic value is ∼300 cm2/(V s) using Monte Carlo simulations.22) This feature is similar to ZnO crystals grown by the CVT method. Interestingly, though the carrier concentration of FJIRSM ZnO is 3 orders of magnitude higher than ZnO crystals grown by the conventional hydrothermal method, the carrier mobility of FJIRSM ZnO can be still high. Temperature-Dependent Electrical Properties of As-Grown ZnO Crystal. Figure 3 shows the temperature dependence of resistivity, carrier concentration, and carrier mobility of FJIRSM ZnO crystal. With temperature increases from 79 to 151 K, the resistivity of the ZnO crystal first decreases
from 0.568 Ω 3 cm to a minimum value 0.297 Ω 3 cm, and then increases to 0.645 Ω 3 cm at 297 K. The carrier concentration of ZnO crystal increases with the increase of temperature, from 7.921015 cm-3 at 79 K to 4.221016 cm-3 at 297.3 K. The carrier mobility increases with the decrease of temperature, from 239 cm2/(V s) at 297 K to 1360 cm2/(V s) at 79 K. Limited by the refrigerating capacity, the carrier mobility still does not arrive at the maximum peak at 79 K. Table 4 shows the peak carrier mobility of ZnO crystals grown by different methods. By comparison, we found FJIRSM ZnO and CVT ZnO crystals have a much higher peak carrier mobility than the melt-grown and conventional hydrothermally grown crystals, which may due to the difference of defects in the materials. The data of carrier concentration vs inverse temperature was fitted using the classical equation.25 The fitting results show that ED1=47 ( 2 meV, ND1 = (4.2 ( 0.5)1016 cm-3, ED2=31 ( 5 meV, and ND2=(5.0 ( 1.3)1014 cm-3, where ED and ND denote donor ionization energy and donor concentration, respectively. It can be inferred that donor D1 is the dominate donor in high temperature because ND1 is 2 orders of magnitude higher than ND2. In conventional hydrothermally grown ZnO crystal, there is only one dominate donor with a much deeper donor ionization energy of 340 meV, which could be an antisite such as ZnO or other defects or impurities.26 It is clear that the crystal growth method has a strong influence on the types of lattice defects, resulting in different electrical properties of crystals. Optical Property of ZnO Crystals. Figure 4a shows the PL spectra of FJIRSM ZnO crystal vs TEW ZnO crystal (from Tokyo Denpa Corporation) at a temperature of 10 K. Both free exciton emission bands of FJIRSM ZnO crystal and TEW ZnO crystal are sharp. A yellow-green band is obviously observed in the TEW sample, while not found in the FJIRSM sample. Both emission bands of the two ZnO samples consist of a near band edge emission band (BX), binding exciton emission band (P1), the first phonon replica of free exciton emission band (P1 þ 1LO), and the second phonon replica of free exciton emission band (P1 þ 2LO). The emission band energies of the two samples in PL spectra are listed in Table 5 along with the calculated exciton binding energy and LO phonon energy. The yellow-green emission band is observed in the PL spectra of most currently available ZnO crystals regardless of their
Table 1. The Hydrothermal Growth Speed of ZnO Single Crystals under Different Mineralizers growth speed of crystal faces (mm/day) hydrothermal condition
(0001) (0001) (1010) references
under the new mineralizers 0.08 under the conventional mineralizers 0.25
0.06 0.08
0.15 0.08
21
Figure 2. X-ray rocking curve for (002) reflection of FJIRSM and MTI ZnO single crystals.
Table 2. Impurity Analysis of ZnO Sintered Strings and ZnO Crystals Determined by GDMS impurities
Li
K
Mg
Ca
Ba
B
Al
Ga
Si
Pb
P
Ag
Cd
ZnO sintered strings (ppm,wt) ZnO crystals (ppm,wt)
0.074
