phys. stat. sol. (a) 192, No. 1, 218–223 (2002) Optical Characterization of High Quality ZnTe Substrate K. Yoshino1) (a), A. Memon (a), M. Yoneta (b), A. Arakawa (c), K. Ohmori (b), H. Saito (b), and M. Ohishi (b) (a) Department of Electrical and Electronic Engineering, Miyazaki University, 1-1 Gakuen Kibanadai-nishi, Miyazaki 889-2192, Japan (b) Department of Applied Physics, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan (c) Innovative Materials Development Center, Nikko Materials Co. Ltd., 3-17-35 Niizo-Minami Toda, Saitama 335-8502, Japan (Received February 15, 2002; in revised form April 18, 2002; accepted April 20, 2002) PACS: 71.55.Gs; 78.55.Et; 82.80.Kq The photoluminescence (PL), optical reflectance, optical transmittance and piezoelectric photothermal spectra were successfully observed between liquid helium and room temperatures for high quality p-type P-doped ZnTe substrates. In the PL spectrum of the ZnTe substrate (1
1018 cm ––3), three distinct kinds of peaks which were due to the radiative recombination of the exciton bound to a neutral acceptor (I1) and free to a neutral acceptor (FA) with longitudinal optical phonon replicas were present at liquid helium temperature. The bandgap energy was estimated to be 2.394 eV by adding the reflection peak (2.381 eV) to the exciton binding energy. Furthermore, the activation energy of the phosphor acceptor impurity was estimated to be 65 meV. On the other hand, in the higher carrier concentration substrate (8
1018 cm ––3), the PL intensity was much smaller than in that of the lower carrier concentration substrate. This means that radiative recombination processes decrease with increasing carrier concentrations. Introduction ZnTe has been expected to be suitable for optical devices such as puregreen light emitting diodes (LEDs) and laser diodes (LDs) since it has the energy gap of 2.26 eV at room temperature and the band structure is of direct optical transition type. It is however difficult to realize p–n junctions because of the well-known compensation effect specific to II–VI materials. The p-type ZnTe can be easily obtained but the n-type ZnTe can not be realized because of self-compensation. Low resistivity ZnTe crystals can be utilized as lattice matching subtrates for these electroluminescence devices. Recently, ZnTe pure-green LEDs were successfully demonstrated by Sato et al. [1, 2]. They obtained high quality 80 mm diameter ZnTe crystals grown by the vertical gradient freezing (VGF) method without seed crystals [3–5], and fabricated p–n junctions by thermal diffusion of Al in p-type ZnTe substrates. Electroluminescence of 550 nm was clearly visualized under room light at room temperature. In order to develop ZnTe based LEDs, it is indispensable to clarify the property of n and p-type ZnTe. Furthermore, ZnTe also attracts our attention for electro-optic (EO) devices because of a relatively high EO coefficient and dielectric constant. A free space EO sampling technique for the coherent characterization of THz beams was developed and the con1 ) Corresponding author; Tel.: +81-985-587396; Fax: +81-985-587396; e-mail: yoshino@pem.miyazaki-u.ac.jp # WILEY-VCH Verlag Berlin GmbH, 13086 Berlin, 2002 0031-8965/02/19207-0218 $ 17.50þ.50/0 phys. stat. sol. (a) 192, No. 1 (2002) 219 version of a time-resolved far infrared image into a time-resolved optical image using a ZnTe sensor as demonstrated [3–5]. In the case of EO devices, high resistivity ZnTe is needed. A n-type impurity is conventionally doped in ZnTe to obtain high resistivity crystals. In this work, optical spectroscopy such as photoluminescence (PL), optical reflection, optical transmittance, and piezoelectric photothermal (PPT) spectroscopy, are investigated between liquid helium and room temperatures to study optical characterization of the p- type P-doped ZnTe substrates. Experimental Procedures ZnTe substrates were obtained from crystals grown by the VGF method [8]. B2O3 was used to prevent the evaporation of Zn and Te during crystal growth. Hole concentrations of more than about 1
1018 cm ––3 were obtained when ZnP2 was used as a dopant. The substrates were polished and etched in a 1 vol% Br2methanol solution for 5 min to remove the damaged surface layer. The etch pit density of the substrates were counted after etching in 1HF : 2H2O2 : 2H2O3 solution. The mean dislocation density of the substrates was about 4000–7000 cm ––2. The lowest dislocation density was less than 2000 cm ––2. The PL, optical reflection, and optical transmittance measurements were carried out between liquid helium and room temperatures. The sample was excited by a HeCd laser (325 and 441.6 nm) for the PL measurement and a halogen lamp for the optical reflection and transimittance measurements. The PL, optical reflection and transmittance spectra were measured by using a grating monochromator (Jobin-Yvon HR-1000) and a photomultiplier (Hamamatsu R2368). The PPT measurement was also carried out for the ZnTe substrates from liquid nitrogen and room temperatures under modulation frequencies between 100 and 800 Hz. The sample was mounted on the cold finger of a cryostat. The halogen lamp was used as the excitation light source. The PPT signals were detected by a piezoelectric transducer (PZT) that was attached directly to the rear surface of the sample with silver paste. The detailed experimental procedures have been reported in the previous paper [9]. Results and Discussion The PL and optical reflection spectra in the band-edge region at 4.2 K of the high quality ZnTe substrates (111) which have carrier concentrations of 1
1018 and 8
1018 cm ––3 are shown in Fig. 1. In the spectrum of the lower carrier concentration substrate (1
1018 cm ––3), three distinct kinds of peaks are present at 2.362, 2.329 and 2.304 eV. They are assigned to be the radiative recombination of an exciton bound to a neutral acceptor (I1) and one free to a neutral acceptor (FA) with longitudinal optical (LO) phonon replicas [10]. These peaks are not observed in undoped ZnTe substrates [5, 8]. Therefore, it is deduced that the origins of the peaks both are due to intentional doping by phosphorus. The bandgap energy (Eg) is estimated to be 2.394 eV by adding the reflection peak (2.381 eV) to the exciton binding energy (12.9 meV) [11]. This Eg is in good agreement with the value reported in the literature [12]. Therefore, the activation energy of the phosphor impurity is estimated to be 65 meV by subtracting Eg (to the peak energy) from the FA emission line (EFA). The activation energy of the phosphor impurity corresponds to that reported by Bhargava [13]. On the other hand, in the spectrum of the higher carrier concentration substrate (8
1018 cm ––3), the PL intensity is much smaller than that of a lower carrier concentra- 220 K. Yoshino et al.: Optical Characterization of High Quality ZnTe Substrate P-doped ZnTe (111) 4.2K N A-N D= RL PL&RL Intensity (arb. units) Fig. 1. PL and optical reflection spectra in the band-edge region at 4.2 K of high quality ZnTe substrates (111) which have carrier concentrations of 1
1018 and 8
1018 cm ––3 1× ×10 × 10 18 -3 cm N A-N D= PL 8× ×10 18 cm -3 N A-N D= 1× ×10 PL 2.20 2.25 2.30 2.35 2.40 2.45 18 cm -3 2.50 Photon Energy (eV) tion. This means that radiative recombination processes decrease with increasing carrier concentrations. Two peaks which are not observed in the substrate of a lower carrier concentration are slightly observed at 2.32 and 2.30 eV, which are due to free to bound emissions. The temperature dependence of the PL spectrum of the high quality ZnTe substrate (100) which has a carrier concentration of 1
1018 cm ––3 is shown in Fig. 2. It is clear that the PL emission is observed at 2.26 eV up to room temperature. This value is in P-doped ZnTe (100) N -N =1 ×10 PL Intensity (arb. units) A D 18 cm-3 × 10 300K ×2 80K Fig. 2. Temperature dependence of the PL spectrum of high quality ZnTe substrate (100) which has a carrier concentration of 1
1018 cm ––3 4.2K 2.0 2.1 2.2 2.3 Photon Energy (eV) 2.4 2.5 221 phys. stat. sol. (a) 192, No. 1 (2002) 60 Fig. 3. Optical transmittance spectra at room temperature of high quality ZnTe substrates (111) which have carrier concentrations of 1
1018 and 8
1018 cm ––3 P-doped ZnTe (111) RT Transmittance (%) 50 40 N A-N D= 1× ×10 18 cm -3 30 20 N A-N D= 10 8× ×10 18 cm -3 0 1.0 1.5 2.0 2.5 3.0 Photon Energy (eV) good agreement with the Eg at room temperature [14]. Therefore, it is deduced that this emission is due to the radiative carrier recombination process of the band to band transition. However, no PL emission is detected in the higher carrier concentration substrate (8
1018 cm ––3) at room temperature. Three distinct peaks are also observed at liquid nitrogen temperature, which can be observed at liquid helium temperature. However, peak shifts are very small from liquid helium to liquid nitrogen temperatures. The optical transmittance spectra at room temperature of high quality ZnTe substrates (111) which have carrier concentrations of 1
1018 and 8
1018 cm ––3, respectively, are shown in Fig. 3. It is clear that an optical transmittance in the sample with a carrier concentration of 8
1018 cm ––3 is low in comparison to that with 1
1018 cm ––3 in the region below Eg. This means that the sample of higher carrier concentration has a high absorption coefficient in comparison to that of lower carrier concentration. The PPT spectra of high quality ZnTe substrates (111) which have carrier concentrations of 1
1018 and 8
1018 cm ––3, respectively, are shown in Fig. 4 measured at room temperature. In the spectrum of the lower carrier concentration substrate (1
1018 cm ––3), signals are slightly observed in the lower photon energy region. On the other hand, signals are detected in the spectrum of the higher carrier concentration substrate (8
1018 cm ––3). This means that nonradiative recombination processes increase with increasing carrier concentrations. This may not be a conflict with the PL results since there is a typical functionality between the nonradiative and the radiative recombination processes. We do not have enough data to discuss this here. However, Kuwahata et al. [15] report that the PPT signal intensity at energies slightly higher than the Eg decrease for n-type and increase for p-type Si with increasing carrier concentration. The decrease and increase are considered to be due to the increase of free electrons at the bottom of the conduction band and to the increase of holes at the top of the valence band, respectively. The PPT signals are not observed for either type of sample above a carrier concentration of 1017 cm ––3. 222 K. Yoshino et al.: Optical Characterization of High Quality ZnTe Substrate Fig. 4. PPT spectra of high quality ZnTe substrates (111) which have carrier concentrations of 1
1018 and 8
1018 cm ––3, measured at room temperature P-doped ZnTe (111) PPT Intensity(arb. units) RT, 100HZ N A-N D= 8× ×10 18 cm -3 N A-N D= 1× ×10 0.50 18 cm -3 1.0 1.5 2.0 2.5 3.0 Photon Energy(eV) Conclusions The PL, PPT, optical reflection, and optical transmittance measurements were carried out between liquid helium and room temperatures for high quality p-type P-doped ZnTe substrates. In the PL spectrum of the lower carrier concentration substrate (1
1018 cm ––3), three distinct kinds of peaks which are assigned to be the radiative recombination of the I1, FA and FA-1LO emission bands are present at liquid helium temperature. It is deduced that the origins of the peaks are due to intentional doping by phosphorus because these peaks are not observed in undoped ZnTe substrates. The Eg is estimated to be 2.394 eV by adding the reflection peak (2.381 eV) to the exciton binding energy (12.9 meV). Furthermore, the activation energy of the phosphor impurity is estimated to be 65 meV by subtracting Eg from the peak energy of the FA emission band. On the other hand, in the spectrum of the higher carrier concentration substrate (8
1018 cm ––3), the PL intensity is much smaller than that of a lower carrier concentration. This means that radiative recombination processes decrease with increasing carrier concentrations. Acknowledgements The authors would like to thank Dr. K. Sato and Dr. T. Asahi of Nikko materials CO., LTD and Prof. T. Ikari of Miyazaki University for their fruitful discussion. This work was partly supported by The Saneyoshi Scholarship Foundation. References [1] A. Leitenstorfer, S. Hunsche, J. Shah, M. C. Nuss, and W. H. Knox, Appl. Phys. Lett. 74, 1516 (1999). [2] K. Sato, M. Hanafusa, A. Noda, A. Arakawa, M. Uchida, T. Asahi, and O. Oda, J. Cryst. Growth 214/215, 1080 (2000). [3] K. Sato, T. Asahi, M. Hanafusa, A. Noda, A. Arakawa, M. Uchida, O. Oda, Y. Yamada, and T. Taguchi, phys. stat. sol. (a) 180, 267 (2000). [4] Q. Wu, T. D. Hewitt, and X.-C. Zhang, Appl. Phys. Lett. 69, 1026 (1996). phys. stat. sol. (a) 192, No. 1 (2002) [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] 223 Q. Wu and X.-C. Zhang, Appl. Phys. Lett. 71, 1285 (1997). K. Sato, Y. Seki, Y. Matsuda, and O. Oda, J. Cryst. Growth 197, 413 (1998). T. Asahi, A. Arakawa, and K. Sato, J. Cryst. Growth 229, 74 (2001). A. Arakawa, T. Asahi, and K. Sato, phys. stat. sol. (b) 229, 11 (2002). K. Yoshino, H. Yokoyama, K. Maeda, T. Ikari, A. Fukuyama, P. J. Fons, A. Yamada, and S. Niki, J. Appl. Phys. 86, 4354 (1999). N. Nilloran, B. C. Cavenett, and P. J. Dean, Solid State Commun. 38, 739 (1981). H. P. Wanger, S. Lankes, K. Wolf, W. Kuhn, P. Link, and W. Gebhardt, J. Cryst. Growth 117, 290 (1992). R. E. Nahory and H. Y. Fan, Phys. Rev. 156, 825 (1957). R. Bhargava, J. Cryst. Growth 59, 15 (1982). A. Ebina and T. Takahashi, J. Cryst. Growth 59, 432 (1982). H. Kuwahata, N. Muto, and F. Uehara, Jpn. J. Appl. Phys. 39, 3169 (2000).