Journal of Luminescence 43 (1989) 121—124 North-Holland, Amsterdam
EXCITONIC PHOTOLUMINESCENCE PROPERTIES OF InSe Kazuaki IMAI Department of Electronic Engineering Hokkaido Institute of Technology, Sapporo 006, Japan Received 19 July 1988 Revised 3 January 1989 Accepted 10 January 1989
High quality InSe with low concentration of lattice defects indicates three sharp emission lines in the region of the band edge below 40 K. The intensity of the exciton—neutral donor complex (D°, x) shows a strong temperature dependence. Its activation energy is 5 meV at the most. The half width of the exciton line is 0.5 meV at 0 K. The remaining line is due to the split exciton line.
1. Introduction InSe is one of the Ill—VI compound layer-type semiconductors. This material is an anisotropic semiconductor and applicable to devices using its high optical sensitivity and stable surface. The bond between the layers is the weak Van der Waals type. Each of the stacked layers consists of fourfold sheets with such ordering as Se! In/ In/ Se, and the bonds between them are covalent, Cleaved surfaces are chemically stable. Many authors investigated the photoluminescence of InSe [1—6].In these papers, one or two excitomc emission line(s) appeared in the region of the gap energy. The half width (FWHM) of the line(s) was wide compared with kT. We reported  previously that the crystal with a nearly-perfect cleaved surface indicates three emission lines above 1.330 eV at 14 K. The FWHM of the three lines are about kT. The emission structure below 1.330 eV is duemechanical to imperfections such as native at defects and/or damages introduced cleavage of the crystal. We evaluated the quality of the crystal surface using the ion channeling measurements in our report. Cingolani et al.  observed two broad emission peaks in the region of the band edge below 40 K. They assigned the origin of the emission peak with the lower energy to a direct exciton bound to
a neutral donor and the peak with the higher energy to a direct exciton. No investigation on the three emission peaks of the high-quality crystal has been reported in detail. In this paper, the temperature dependence of the three lines is measured using nearly perfect crystals. In particular, we point out that the ternperature dependence of the emission intensity of the strongest line indicates an activation type below 40 K. Two out of three lines have a similar temperature dependence.
2. Experimental The InSe crystals were grown by the nonstoichiometric Bridgman method from In1 ~Se0% melt . The crystals grown by this method were n-type and their crystal symmetry was C~ (3R y-type). The samples were cut 2out from the c plane and ingots. 3 mm The typical size was 7 X 5 mm thickness. We pasted the sample on a 1 cm2 Cu plate with an In—Hg alloy and annealed this at 150°Cunder 10 ~ TOrr for 2 h to evaporate Hg. We cleaved carefully the c plane of the stuck sample. The quality of the cleaved surfaces was evaluated by Rutherford backscattering (RBS)  and ion blocking pattern  measurements using 1 MeV He + ions. We can evaluate the spatial distri-
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/ Excitonic photoluminescence properties of InSe
bution of defects in the surface region within 1 p.m from the RBS spectra. The Cu plate with the sample was pasted by Thermal Compound (Wakefield Co., Ltd.) on the sample holder in a cryostat. We used the He—Ne laser with an output power of 7 mW to achieve optical excitation. The diameter of the focused laser beam on the sample surface was about 0.1 mm. The emission light was introduced into a Perkin—Elmer E-1 monochromator and detected by HTV-316 photomultiplier tube with S-i surface. The wavelength resolution was about 0.08 nm in the region of our measurements (900 980 nm). The standard lock-in technique was employed. For measuring the temperature dependence of the emission intensity, the optical apparatus was so arranged that the detected intensity became strongest at 5.2 K. While the temperature was increasing, the optical system was not moved artificially. Therefore, it is considered that the results on the intensity in this work involve the error due to the shift of the optical axis which comes of the thermal expansion of the cryostat. We ascertained that the three lines do not correspond with the spin orbit coupling of the excitons .
Fig. 1. Photoluminescence spectrum at 5.2 K. The C line corresponds to the n =1 free exciton line.
of ‘A depending on the samples is within 10% in fig. 2. It means that the ‘A of all samples used in this work obey the similar function of temperature. The peak heights of B and C increase and
3. Results and discussion
;.~ As mentioned above, the emission strength below 1.330 eV reduces with decreasing defect concentration in the surface region (about 1 p.m deep). Figure 1 shows the photoluminescence spectrum in the region of the band edge at 5.2 K. We label the emission lines located at 1.333, 1.3345 and 1.336 eV A, B and C, respectively. In comparison with the results reported by Voitchovsky et al.  and by Camassel et al. , we can assign the C line to the free exciton peak. Figure 2 shows the temperature dependence of the peak intensity of the emission lines. The vertical axis is normalized by the peak height of A at 5.2 K for all samples. The error bar in the low temperature region shows an amount of scatter depending on the samples. As the temperature increases, the peak height of A, ‘A’ reduces exponentially and vanishes at about 50 K. The scatter
~ A I
0.1 0.2 l/T (K~)
o.c Ternperature(K) 150 Fig. 2. Peak intensity of the three lines vs temperature. The intensity is normalized to unity by the maximum of A at 5.2 K. The inserted figure is log intensity vs. 1/T.
Excitonic photoluminescence properties of InSe
have the highest intensity at about 20 K, whereupon they decrease monotonously. The FWHM of the three lines increases with increasing temperature. The B and C lines can no longer be distinguished at about 50 K. At room temperature, the emission of the mixture survives as a broad peak. The A line is due to the exciton—neutral donor complex (D°, x) , so that the temperature dependence of the emission intensity ‘A is the activation type, described by
~ ri;—~ ~
~i30 c a
Here I~is the emission intensity at T = 0 K, C0 is a temperature-independent constant and Ea is the thermal binding energy. The log plot of ‘A vs. 1/T is inserted in fig. 2. Ea is given by the slope of the straight-line portion of the figure as 5 meV. Including the error due to the thermal expansion of the cryostat as stated above Ea is ~ 5 meV. (D°, x) decays thermally through several processes . Ea is the dissociation energy for dissociating the complex to the exciton and the neutral donor. The value of Ea is close to the energy difference (3 meV) between the A and C lines as shown in fig. 1. The binding energy of the n = 1 exciton (C line) is 14.5 meV . Even if we disregard the interaction energy between the exciton and the donor, the impurity level exists at 18 meV at most from the bottom of the conduction band. The origin of the B line is the splitting of the exciton line due to stacking faults. The temperature dependence of the B line is similar to that of the C line. The number of atomic displayed rows due to the stacking faults in the presentis samples is 2. The number measured less than rows/cm along the 1016 <241> axis  in the hexagonal system. This value is a limit of our ion channeling measurement . However, it it considered that InSe includes the e phase as the stacking fault. The stacking faults in y-InSe are possibly responsible for the $ (2H type and D 6h symmetry) or e (2H, D3h) phase at the fault plane. The $ phase is constructed by rotation of part of the crystal against motheris crystal around the by c axis by 60°. Thethee phase constructed easily shifting
100 200 Temperature (K)
Fig. 3. Peak energy (a) and FWHM (b) of the C line as a function of temperature. The solid lines are the best fitted results using eqs. (2) and (3).
the layer along the c plane by ~ 0) a, where a is the lattice constant perpendicular to the c axis. In GaSe, the energy of the exciton lines for y and e are very close to each other . The temperature dependence of the peak energy and FWHM of the C line are shown in fig. 3. Figure 3(a), the temperature dependence of the peak energy, is differdnt from the result of the absorption measurements . Since the electron phonon coupling of InSe is weak , a line shape function S(hco E~) of the exciton absorption band is a Lorentzian curve . Here, E~ is the exciton energy at 0 K. The photon energy of the emission line does not agree with that of the absorption line by the exciton distribution (Boltzman the width of to S( the ho, E’). Since thefunction) emissionover is proportional product of the Boltzmann function and S( h~ Er), the energy at the peak of the emission line, h ~peak’ ~ —
(E~+ z~) kT+ [(kT)2 —
(2) where hi’ is FWHM, self energy (— 2 and L% N the the phonon occupa_N(q~0)(hca~0)i/ tion number). From fig. 3(b), the FWHM is about
/ Excitonic phozoluminescence properties of InSe
2kT. The peak energy of the emission line is
lowered by about kT from the absorption peak. With hwLo = 14 meV in eq. (2), the best fit in fig. 3(a) is obtained as a solid line. This value agrees with the value derived from the absorption measurement  and is also confirmed by the photoluminescence investigation. FWHM relates to the reciprocal of the life time. It gives tentative criteria for judgment of the crystal quality. The value of FWHM of the C line, hi’, is 0.6 meV at 5.2 K. hi’ is the sum of contributions from (1) imperfections, stress and/or impurities, hFimp, and (2) the numbers of interacting phonons of LA and LO modes, hFPh. Then, in our temperature region
hT’imp + hi’Ph,
2n(q~A) =hFimp+ B(q~~)I + IB(qLO) I2n(q 10),
2(kT/hcoL~) =hFjmp+ + B(qLB(q~A)I 2exp(—h~LO/kT).
(3) 0)~ Here the coefficients I B(q) 2 are proportional to the electron lattice matrix elements and n is the phonon occupation number. In fig. 3(b), the solid line is determined by parameter fitting as hr’tm~= 0.5 meV, B(qL~)I 2/hWLA = 1, I B(q~O)I = 10.5 meV and hwLo = 14 meV. The value of hl7jmp, 0.5 meV, is smallest in past reports. The third term cannot be ignored for low hw LO’ The temperature dependence of three excitonic emission hnes has been measured in detail. Though (D°, x) of InSe has a very high emission efficiency at low temperature, it decays with increasing temperature at an activation energy of 5 meV. The layer type crystal includes native stacking faults because of the remarkable structure. The B line appears as the individual level with decreasing the defect concentration. The reason why the B line could not be observed in the past reports is that the defect density was large cornpared with the crystals in this work. The emission
peaks were indistinguishable because of wide FWHM. The exciton emission of the high quality InSe has an FWHM of 0.5 meV at 0 K. Our detailed results become visible only by scrupulous crystal treatments.
Acknowledgements The author wishes to thank Professors Y. Abe of Hokkaido University and K. Kumazaki of the Hokkaido Institute of Technology for many discussions. The present work was supported in part by scientific research grants from the Suhara Memorial Foundation, the Japan Private School Promotion Foundation and the HIT Foundation.
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