Specimens of undoped, n-type and p-type bulk InP have been investigated using THz radiation. Reflection spectroscopy permits the determination of the TO and LO phonon energies as well as the carrier concentration. Absorption spectroscopy reveals transitions between impurity states. We report the first absorption spectrum of an acceptor in InP. The transition energies agree well with electronic Raman scattering and photoluminescence data

Causley, R. L.

Lewis, R. A.

English

Research Online

University of Wollongong Research Online Faculty of Engineering - Papers (Archive) Faculty of Engineering and Information Sciences July 2000 Characterisation of indium phosphide using terahertz radiation R. L. Causley University of Wollongong R. A. Lewis University of Wollongong, roger@uow.edu.au Follow this and additional works at: https://ro.uow.edu.au/engpapers  Part of the Engineering Commons https://ro.uow.edu.au/engpapers/61 Recommended Citation Causley, R. L. and Lewis, R. A.: Characterisation of indium phosphide using terahertz radiation 2000. https://ro.uow.edu.au/engpapers/61 Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: research-pubs@uow.edu.au Characterisation of indium phosphide using terahertz radiation R. L. Causley and R. A. Lewis Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong NSW 2522, Awtralia Abstract - Specimens of undoped, n-type and ptype bulk InP have been inves- tigated using THa radiation. Reflection spectroscopy permits determination of the TO and LO phonon energies as well as the carrier concentration. Ab- sorption spectroscopy reveals transitions between impurity states. We report the first absorption spectrum of an acceptor in I*. The transition energies agree well with electronic Raman scattering and photoluminescence data. A. Introduction The 111-V alloy InP finds considerable technological application. Examples in- clude light emitting diodes and solar cells. Some of the characteristics of InP are listed in Table 1. In common with many semiconductors, both the principal phonon energies and the energies of shallow impurities in InP correspond to photon energies for electro- magnetic radiation in the range -1 - 10 THz. Thus THz radiation is an excellent probe of phonons, electrons and holes in this material. Table 1: Some properties of InP. 4 0 )  4x4 -0'4.2, K) wo(300 K) w~o(4.2~ K) w~o(300 K) m2/mo m f h  cm- cm-I cm- em-' 111 PI I21 121 121 121 131 PI 12.61 9.61 308.2f0.3 303.7f0.3 349.550.3 345.0f0.3 0.077 0.64 B. Experimental Nominally undoped, donor-doped and acceptor-doped samples have been stud- ied. Details of the samples are given in Table 2. Reflectivity spectra were measured at room temperature using a Bomem DA3 rapid-scan spectrometer. A room-temperature DTGS detector was employed. A polished brass block served as a reflectance reference. Transmission measurements were made using the Bomem spectrometer and a slow-scan Polytec spectrometer. The samples were wedged appropriately to suppress interference fringes. Liquid helium was the coolant. Low temperatures were obtained by pumping on the helium bath. Table 2: Samples used in this investigation. Label DoDant Concentration (cm-3) source A None Sumitomo B None 5 10'6 MTI Corporation C Zinc 4 x 10'6 MCP D Zinc 3.5 x 10'8 MCP E Zinc (5  - 8) x 10' CrysTec F Zinc (8 - 10) x loi8 Sumitomo G SUlDhUr 7 x 10'" Sumitomo 0-7803-5814-7/00/$10.00 0 2000 IEEE 101 C. Reflection spectroscopy: phonon energies and carrier concentration The THz reflectivity spectra of undoped InP yield the TO and LO phonon en- ergies as well as the dielectric constant and damping terms. In addition, th'e charge carrier concentration may be determined in doped samples by this purely optical method. Examples are given using doped and nominally undoped InP from a variety of sources. The data is fitted assuming a damped harmonic oscillator dielectric func.tion and a free carrier contribution: Here wt is the TO phonon frequency, y the phonon damping term, wp the plasma fre- quency and -yp the plasma damping term. Rom the refractive index n and the extinction coefficient IE, the reflectivity may be calculated using (n - 1)2 + K2 (n + 1)2 + IC2 R =  200 300 400 Photon Energy (cm") 200 300 400 Photon Energy (em-') 200 300 400 Photon Energy (cm") Fig. 1: Room-temperature reflectivity of samples A, F and G .  Fig. 1 displays three distinctly different reflectivity spectra characteristic of undoped, lightly-doped and heavily-doped material, respectively. Fig. l(a) shows the peak/dip characteristic of a slightly-damped harmonic oscillator, correspondiing to un- doped material. The materials parameters deduced by fitting Eqn. 1 to the ex1)erimental data are given in Fig. l(a). Fig. l(b) shows a reflectivity peak typical of a lightly-doped sample. The fitting parameters are also given. The carrier concentration, .N, can be deduced from the plasma frequency, since U: = Ne2/e(co)com*. Fig. l(c) shows a sharp dip, characteristic of heavily-doped material. 0-7803-5814-7/00/$10.00 0 2000 IEEE 102 D. Transmission spectroscopy: energy levels of shallow acceptors Shallow donor levels in InP have been studied for many years, most successfully by photoconductivity measurements made in magnetic fields using a far-infrared laser to  excite the conductivity. Their energies and magnetic-field dependence are relatively well known. In comparison, the acceptor levels are less well known. Acceptor levels have been investigated by photoluminescence (PL) and considerable variation appears in the literature even for the 1s3/2(l?S+) ground state of common acceptors such as Mg and C. Recently, electronic Raman scattering was reported for Zn and Cd acceptors in InP [4,5]. This method is sensitive to transitions from the ground state to other even- parity states, such as ~ S ~ / ~ ( I ? $ ) .  Transitions to odd-parity states are forbidden by the dipole selection rules. Absorption spectroscopy is complementary to this method, being sensitive to transitions to odd-parity final states. - h e  - sampleMTI 1 T=4.2K 0.15 - 100 150 200 250 300 Photon Energy (cm-') sample R3 0.3 T = 4.2 K 0.1 o'21 0.0 Fig. 2: Low-temperature transmission of samples B and C. Transmission spectra over the range 100 - 300 cm-' are given in Fig. 2 for samples B and C. Many of the features are in common (for example the sharp dip at 220 cm-' and the edge at 260 cm-l) and may be attributed to lattice effects as comparison with earlier studies of the phonon spectra demonstrates [6]. Sample B exhibits a sharp absorption (marked "X") at -210 cm-l. Other measurements have indicated this feature (i) is very sharp; (ii) fades with temperature, disappearing by 60 K; (iii) changes negligibly with magnetic field (to 6 T). Spectra have been taken pumping on the helium space with no significant change in line width. The origin of this line is unknown. Sample C shows absorption at 241.5 cm-' and 286.0 cm-'. These lines are identified as the Zn G and D lines. Data taken at higher temperatures confirm that they disappear due to thermal depopulation. The line positions from these spectra are given in Table 3, along with previous measurements for transitions to other final states in InP. Ultrasonic treatment has been undertaken to reduce the linewidth but no narrowing is observed. To our knowledge, these are the first absorption spectra reported for acceptors in InP. 0-7803-5814-1i00i$10.00 0 2ooO IEEE 103 Table 3: Experimental energies (in cm-’) of transitions for acceptor impurities in InP. Impurity Zinc Cadmium Carbor  Transition Label Absorption Raman PL Raman PL 18312 -$ G 241.5 242.0 243 276.1 193 18312 28312 E 259.8 261 286.4 221 15312 --$ %5/2(r;) c 300.4 302 336.0 252 Is312 --$ 35312 1 318.7 IS312 -+ 4%/2 2 330.6 This work [4] [7] [4] 171 - - - - - is3l2 --f 2p512(rg) D 286.0 287.2 281 318.1 232 - - - E. Conclusion The utility of THz radiation for the characterisation of InP has been illustrated. Reflection spectroscopy is a non-destructive, contactless, local probe which can be em- ployed over a wide temperature range. It yields TO and LO phonon energies and carrier concentrations. We report on the first absorption spectra of acceptors in InP. Our work on the G and D lines has now been extended to embrace Zeeman studies over a range of temper- atures (1.9 - 10.4 K) and magnetic fields (0 - 6 T) for radiation polarised both parallel and perpendicular to the applied magnetic field. These investigations will ble reported elsewhere. Acknowledgements This work is supported by the Australian Research Council and the University of Wollongong. We thank Professor C. Jagadish (Australian National University) and Dr. G. Irmer (Technische Universitat Bergakademie Freiberg) for the provision of some of the samples used in this investigation. References [l] M. Hass and B. W. Henvis, J. Phys. Chem. Solids 23 (1962) 1099. [2] A. Mooradian and G. B. Wright, Solid State Commun. 4 (1966) 431. [3] K. W. Boer, “Survey of Semiconductor Physics” (Volume 2, Van Nostrancl Reinhold, 141 G .  Irmer, M. Wenzel and J. Monecke, phys. stat. sol. (b)  210 (1998) 347. [5] M. Wenzel, G .  Irmer and J. Monecke, Solid State Commun. 104 (1997) 3‘71. [6] E. S. Koteles and W. R. Datars, Solid State Commun. 19 (1976) 221. [7] P. J. Dean, D. J. Robbins and S. G. Bishop, J .  Phys. C: Solid State Phys. 12 (1979) New York, 1992). 5567. 0-7803-5814-7/00/$10.00 0 2000 IEEE 104 

Characterisation of indium phosphide using terahertz radiation

https://ro.uow.edu.au/cgi/viewcontent.cgi?article=1060&context=engpapers

Characterisation of indium phosphide using terahertz radiation

Abstract

Similar works

Full text

Available Versions

Research Online