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Far-infrared phonon combination bands in InP
a a a a a a Volume 54A, number!
PHYSICS LETTERS
11 August 1975
FAR-INFRARED PHONON COMBINATION BANDS IN InP R.F. KIRKMAN and R.A. STRADLING aarendon Liborotory, Oxford, UK Received 10 June 1975 Far-infrared phonon combination bands are studied in p-lnP at 4K. The 2TA (X) combination is of particular interest as providing an electronic energy relxation mechanism and the value of 16.92 ±0.05 meV is in good agreement with neutron, Raman and magnetophonon data.
There has recently been considerable interest in the influence of two-phonon processes on a variety of semiconductor optical and electron transport properties (see ref. [I] for a general review). Infrared measurements of the phonon combinations involved have generally been confined to the region above the Restrahl and the assignment of the different peaks has been somewhat uncertain (e.g. the differing interpretations given in earlier work with InP are discussed in ref. [2]). The development of techniques in the farinfrared now permits excellent signal-to-noise ratios in transmission experiments involving black-body sources down to energies of the order of 10 cm~. It is thus possible to observe low energy phonon combinations directly, thereby reducing the possibility of misinterpretation and improving the accuracy of measurement. In the course of a search for transitions involving holes bound to the acceptor states in InP, phonon combination bands below the Restrahl were clearly observed (see fig. 1). The present experiments were performed with a Beckmann RI I C photospectrometer and the radiation was detected with a cooled germanium bolometer. A typical transmission spectrum taken at 4 K and ratioed against a background spectrum measured with the germanium bolometer under identical conditions is shown in fig. 1. The low temperatures employed predude the possibility of difference frequencies being observed. The strong, sharp transmission minimum at 136.5 cm~ is believed to arise from 2TA(X) emission, and minima also occur near the predicted positions of other dipole-allowed phonon combinations, in particular the 2TA(L) at 108 cm~, 2LA(L) at 164 crn~ and TA(X)+ LA(X)at 192 cm~. The energy of the 2TA(X) combination (16.92 ±
10
8
~
2TA(X~
soho
120
140
200
220 crn~
Fig. 1. Transmission spectrum obtained from a p-lnP sample of thickness of 2 mm at 4K, ratioed against a background spectrum and normalised.
0.05 meV) corresponds well with the hot-electron magnetophon result of 16.7 ±0.3 meV [2] and the second order Raman scattering measurement of 16.6 ±0.2 meV [3]. The latter experiments were interpreted with the aid of neutron data which gave a value of 16.6 ±0.8 rneV. The combination of 2TA(X) is of particular interest as the TA branch is approximately flat for the last 50% of the zone along the X direction in Ill-V cornpounds. The low energy for the combination and the high combined density of states for the 2TA(X) modes gives rise to the pronounced well-resolved minimum in the far-infrared transmission shown in fig. I
31
Volume 54A~,number 1
PhYSICS LETTERS
as has previously been reported for GaAs by Slack and Roberts [41.The low energy and high density of states make emission of the 2TA(X) combination a significant mechanism for relaxation of the electronic energy in transport measurements. In such a process, the electron emits pairs of phonons of equal and opposite wavevector, and at electron teniperatures low compared with hwLo/k (where J1WLO is the energy of the LO phonon) this can dominate the normal relaxation by emission of single LO([’) phonons. This effect has been detected in hot electron magnetophonon experiments with lnSb, GaAs and lnP (see refs. [5, 6] for reviews) and has been discussed theoretically by a number of authors [e.g. 7, 8]. The agreement between the values for the 2TA(X) combination deduced from d.c. electrical measurements and optical measurements, demonstrated previously for InSb and GaAs and confirmed for InP in this letter is very satisfactory.
32
11 August 1975
We would like to thank Dr. P. Tufton for growing the p-type indium phosphide.
References Ill K.L. Ngai, Proc. Intern. Conf. on Physics of semiconductors (Stuttgart) p. 489 (1974). 121 L. Eaves et al., J. Phys. C 1(1974)1999. 131 G.F. Aifrey and P.1-I. Borcherds, J. Phys. C 5, 20 (1972) L2~/5.
141 GA. Slack and S. Roberts, Phys. Rev. B3 (1971) 2613. 151 R.A. Stradling, Proc. Intern. Conf. on Physics of semiconductors (Warsaw) p. 261 (1972). [61 P.G. Harper, J.W. Hodby and R.A. Stradling, Rep. Prog. Phys. 36(1973)1.
[71 G.P. Alidredge and F.J. Blatt, Ann. Phys. 45 (1967) 191. 181 K. Baumann, Acta Phys. Austr. 37 (1973) 350.
PHYSICS LETTERS
11 August 1975
FAR-INFRARED PHONON COMBINATION BANDS IN InP R.F. KIRKMAN and R.A. STRADLING aarendon Liborotory, Oxford, UK Received 10 June 1975 Far-infrared phonon combination bands are studied in p-lnP at 4K. The 2TA (X) combination is of particular interest as providing an electronic energy relxation mechanism and the value of 16.92 ±0.05 meV is in good agreement with neutron, Raman and magnetophonon data.
There has recently been considerable interest in the influence of two-phonon processes on a variety of semiconductor optical and electron transport properties (see ref. [I] for a general review). Infrared measurements of the phonon combinations involved have generally been confined to the region above the Restrahl and the assignment of the different peaks has been somewhat uncertain (e.g. the differing interpretations given in earlier work with InP are discussed in ref. [2]). The development of techniques in the farinfrared now permits excellent signal-to-noise ratios in transmission experiments involving black-body sources down to energies of the order of 10 cm~. It is thus possible to observe low energy phonon combinations directly, thereby reducing the possibility of misinterpretation and improving the accuracy of measurement. In the course of a search for transitions involving holes bound to the acceptor states in InP, phonon combination bands below the Restrahl were clearly observed (see fig. 1). The present experiments were performed with a Beckmann RI I C photospectrometer and the radiation was detected with a cooled germanium bolometer. A typical transmission spectrum taken at 4 K and ratioed against a background spectrum measured with the germanium bolometer under identical conditions is shown in fig. 1. The low temperatures employed predude the possibility of difference frequencies being observed. The strong, sharp transmission minimum at 136.5 cm~ is believed to arise from 2TA(X) emission, and minima also occur near the predicted positions of other dipole-allowed phonon combinations, in particular the 2TA(L) at 108 cm~, 2LA(L) at 164 crn~ and TA(X)+ LA(X)at 192 cm~. The energy of the 2TA(X) combination (16.92 ±
10
8
~
2TA(X~
soho
120
140
200
220 crn~
Fig. 1. Transmission spectrum obtained from a p-lnP sample of thickness of 2 mm at 4K, ratioed against a background spectrum and normalised.
0.05 meV) corresponds well with the hot-electron magnetophon result of 16.7 ±0.3 meV [2] and the second order Raman scattering measurement of 16.6 ±0.2 meV [3]. The latter experiments were interpreted with the aid of neutron data which gave a value of 16.6 ±0.8 rneV. The combination of 2TA(X) is of particular interest as the TA branch is approximately flat for the last 50% of the zone along the X direction in Ill-V cornpounds. The low energy for the combination and the high combined density of states for the 2TA(X) modes gives rise to the pronounced well-resolved minimum in the far-infrared transmission shown in fig. I
31
Volume 54A~,number 1
PhYSICS LETTERS
as has previously been reported for GaAs by Slack and Roberts [41.The low energy and high density of states make emission of the 2TA(X) combination a significant mechanism for relaxation of the electronic energy in transport measurements. In such a process, the electron emits pairs of phonons of equal and opposite wavevector, and at electron teniperatures low compared with hwLo/k (where J1WLO is the energy of the LO phonon) this can dominate the normal relaxation by emission of single LO([’) phonons. This effect has been detected in hot electron magnetophonon experiments with lnSb, GaAs and lnP (see refs. [5, 6] for reviews) and has been discussed theoretically by a number of authors [e.g. 7, 8]. The agreement between the values for the 2TA(X) combination deduced from d.c. electrical measurements and optical measurements, demonstrated previously for InSb and GaAs and confirmed for InP in this letter is very satisfactory.
32
11 August 1975
We would like to thank Dr. P. Tufton for growing the p-type indium phosphide.
References Ill K.L. Ngai, Proc. Intern. Conf. on Physics of semiconductors (Stuttgart) p. 489 (1974). 121 L. Eaves et al., J. Phys. C 1(1974)1999. 131 G.F. Aifrey and P.1-I. Borcherds, J. Phys. C 5, 20 (1972) L2~/5.
141 GA. Slack and S. Roberts, Phys. Rev. B3 (1971) 2613. 151 R.A. Stradling, Proc. Intern. Conf. on Physics of semiconductors (Warsaw) p. 261 (1972). [61 P.G. Harper, J.W. Hodby and R.A. Stradling, Rep. Prog. Phys. 36(1973)1.
[71 G.P. Alidredge and F.J. Blatt, Ann. Phys. 45 (1967) 191. 181 K. Baumann, Acta Phys. Austr. 37 (1973) 350.