INFRAREDPHYSICS &TECHNOLOGY ELSEVIER
Infrared Physics & Technology 37 (1996) 509-512
Photoluminescence studies of CulnSe 2 W.Z. Shen a, S.C. Shen a, y. Chang a, W.G. Tang a, L.S. Yip h, W.W. Lam I. Shih b
a National LaboratoryJbr Infrared Physics, Shanghai Institute of Technical Physics, Shanghai 200083, China b Department of Electrical Engineering, McGill University, Montreal, PQ H3A 27"8, Canada Received 29 August 1995
Abstract Photoluminescence measurements have been performed on a p-type CulnSe 2 polycrystalline sample. The dependences of the luminescence spectra on temperature and excitation density are studied in detail. In addition to the observation of donor-acceptor pair emissions at low temperatures, the bound electron-to-localized hole transitions were identified at high temperatures based on a model of thermal activation of localized hole states within the density-of-states tail inside the fundamental gap. PACS: 78.55.Hx, 78,20.- e
1. Introduction Recently, the chalcopyrite-type ternary semiconductor CuInSe 2 has received considerable attention mainly due to its potential application in solar cell and photodiode technologies [1,2]. The optical and electrical properties of CuInSe 2 have been shown to be strongly dependent on both intrinsic defects and extrinsic doping. Photoluminescence (PL) measurements are a standard technique to observe defects and impurities in semiconductors. The luminescence spectra of CuInSe 2 are mainly dominated by donor-acceptor (D°A°) pair recombinations . The study of these luminescence processes can not only show the content and behavior of defects and impurities in semiconductors, but also improve the operational characteristics of semiconductor devices based on radiative recombinations. In this paper, we report, for the first time, the evolution and identification in CulnSe 2 of bound electron-to-localized hole transitions at high temperatures assuming the hole localization within the den-
sity-of-states tail inside the fundamental gap, in addition to the observation of D°A° pair recombination at low temperatures.
2. Experiment details The CulnSe 2 sample used in the PL measurement was grown by the gradient-freezing method and was prepared by cleaving. The infrared PL measurement was performed on a Nicolet 800 Fourier transform infrared spectrometer with an Ar-ion (514.5 nm) laser as the excitation source and a liquid-nitrogencooled InSb detector for receiving the PL signal. The optical measurement was made at the resolution of 4 c m - i.
3. Results and discussion Fig. 1 shows the temperature-dependent PL spectra of the polycrystalline CulnSe 2 sample under the
1350-4495/96f$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 1 3 5 0 - 4 4 9 5 ( 9 5 ) 0 0 1 24-7
W.Z. Shen et al./ lnfrared Physics & Technology 37 (1996) 509-512
same laser excitation density ( I 0) of 5.0 W / c m 2. At 4 K, the luminescence spectrum was dominated by a single broad peak (labeled A) with a full-width at half-maximum (FWHM) of ~ 36.0 meV, accompanied by a luminescence shoulder (labeled C) at its low energy side. The broad peak A exhibits most of the characteristics of D°A° recombination: (i) The peak shifts to higher energy as the excitation density is increased. The D°A° pair peak energy strongly depends on the separation ( r ) of the D°A° pair involved in the recombination. As the excitation density increases, the average separation decreases, resulting in a blue shift of the luminescence peak energy. (ii) The PL spectrum shows asymmetry of the line shape with an abrupt decrease on the high energy side. The ionized D+A - pair cannot bind free carriers when r is less than the effective Bohr radius of the bound carriers, therefore, an abrupt decrease of the recombination probability of the high energy is expected . (iii) The integrated intensity of the luminescence peak ( I p L ) w a s found to obey: IpL 13[ I ~ w i t h Ot = 0 . 7 0 -I- 0 . 1 0 , a n d gradually saturated under higher excitation density. The sublinear increase and the saturation of luminescence intensity i
T -- 5 0 K ,
(/3 1 0 4 C
2; ~o ~ ©
>~ lO 2 Z LJ ~--
C_ 10 o 0
Fig. 2. Arrhenius plot of log(lpL) as a function of inverse temperature for the CulnSe 2 sample under the laser excitation density of 5.0 W / c m 2. The solid line is fitted to the experimental data by using Eq. (1).
provide additional evidence for the D°A° pair recombination, which can be seen by some simple rate equation arguments . With increasing temperature, the integrated luminescence intensity decreases indicating the presence of nonradiative recombination mechanisms. The log of the measured integrated intensity of PL peak A is plotted as a function of inverse temperature in Fig. 2. The sample clearly shows a temperature-dependent behavior characterized by two temperature regimes, corresponding to two thermally activated nonradiative recombination mechanisms. The solid line is the best fit to the experimental data with a model involving two nonradiative recombination processes described by [6,7]:
EL + C 2 exp
IpL=A l + C I exp
Ei',!EACY (rneV) Fig. 1. Temperature-dependent PL spectra of CulnSe 2 sample under the same laser excitation density of 5.0 W / c m 2. Each spectrum set has been shifted up by a constant for clarity.
( )1-' - ~
where A, C l and C 2 are constants, E l and E 2 are the thermal activation energies, K B is the Boltzmann constant, and T is the sample temperature. The best fit yields A, C L, C 2 of 1000.0, 15.0, 7 × l0 s, and activation energies E L, E 2 of 16.5 + 0.5 meV, 162.0
W.Z. Shen et a l . / lnfrared Physics & Technology 37 (1996) 509-512
___0.5 meV, respectively. The value of E~ is in excellent agreement with the energy difference of luminescence peaks A and C of ~ 17.2 meV. The activation energy E 2 is consistent with the acceptor (copper vacancy) binding energy of 160 meV . Therefore, we can conclude that the dominant decrease in luminescence in the CulnSe 2 system is mainly due to the thermal ionization of the holes above 50 K, and thermalization of another defect-related transition (structure C) at low temperatures, which is also in agreement with the experimental fact that the luminescence shoulder at the low energy side of peak A disappears at 50 K (see Fig. 1). The exact transition of the luminescence structure C is not clear now, but it is under further investigation. In addition, the energy position of peak A does not shift with temperature within the uncertainty of the measurement, revealing that the electron and hole involved in the recombination must be bound and the binding energies are large, which is also the distinguishing characteristics of D°A° recombination. The distinguishing feature in Fig. 1 is that another luminescence structure (labeled B) appears gradually with the increase of temperature, which causes the symmetric PL line shape at 50 K: the luminescence intensity decreases smoothly at high energy side. Its integrated intensity also displays a sublinear increase with the laser excitation density, however, its luminescence intensity relative to peak A increases with temperature, together with the blue shift of the peak energy. Such a temperature dependence of the luminescence spectrum in CulnSe 2 samples has not been reported before. The fundamental energy gap in CulnSe 2 structures decreases from 40 K to room temperature . Furthermore, the energy separation of luminescence peak B and the energy gap at 150 K is ~ 43 meV, corresponding well with the donor (selenium vacancy) binding energy of 45 meV . Based on the above two facts, we explain our experimental results in the frame of recombination transfer between localized hole states within the density-ofstates tail inside the fundamental gap, and the luminescence structure B is attributed to a bound electron-to-localized hole transition. The schematic representation of the luminescence transitions in CuInSe 2 is shown in Fig. 3. Since CuInSe 2 is the I - I I I - V I 2 ternary compounds analog of the pseudobinary I I - V I semicon-
E ~ / C B
.. . . . . . . . . . . . . Acceptor -~,- ........... Localized
Density o f states Fig. 3. Schematic representation of the PL transitions in CulnSe 2.
ductors, such as HgCdTe, ZnCdSe , local composition fluctuations should exist, which result in a finite density-of-states with an exponential energy dependence inside the fundamental gap, similar to the case of I I - V I alloys [10,11]. The existence of a band tail is further confirmed by absorption measurement where clearly shows the strong band tail absorption below the energy gap. In our p-type CulnSe/ sample, the ionized holes can be localized at material defects and impurities, and the influence of the tail states in the conduction band is negligible due to the large difference of electron and hole effective masses. With increasing temperature, the localized hole states are transferred from deeper localized states to shallower localized states due to the thermal activation, resulting in the blue shift of the luminescence peak. Numerically calculation shows [10,11] that the luminescence peak can shift to high energy at a rate of 3.5 ~ 4.0 K B. With the correction of the band gap red shift of ~ 1.0 K B, the calculated shift is consistent with the experimental value of ~ 2.8 K B. On the other hand, the population of extended hole states increases with temperature, and the nonradiative recombination lifetime will also increase, explaining the observed increase of the luminescence intensity relative to that of structure A. Of course, the luminescence intensities of both A and B, in general, decrease with increasing temperature due to the presence of nonradiative recombination. By further rising the temperature, the localized hole states will be transferred to the valence band of
W.Z. Shen et al. / Infrared Physics & Technology 37 (1996) 509-512
CulnSe 2. And at 150 K, the luminescence peak B is mainly evolved into a bound electron-to-free hole transition, based on its energy position described above. Further evidence of the above origin of peak B at 150 K is demonstrated in Fig. 1 which shows the line shape fit of the high energy side of peak B. The triangular curve is the calculated line shape of bound electron-to-free hole transition (lex) given by the following equation : /'ex( ~ ¢.0) = B
(KBT) 3 × exp
Its temperature and laser-excitation-density dependence have been analyzed on the basis of a model of thermal activation of localized hole states within the density-of-states tail inside the fundamental gap. With increasing temperature, the localized hole states are transferred from deeper localized states to shallower localized states due to the thermal activation, resulting in the blue shift of the luminescence peak. At 150 K, the luminescence peak B evolves into a bound electron-to-free hole transition. References
/~0)-- ( E g - E D )
where B, Eg, E D are the amplitude, band gap energy, and donor binding energy of CulnSe 2, respectively. The best fit of the high energy tail of the peak B is found with E D of 49.6 meV, also in agreement with the donor binding energy of ~ 45 meV .
In conclusion, from the analysis of temperaturedependent PL spectra of a p-type CulnSe 2 polycrystalline sample grown by gradient-freezing technique, we have identified the D°A° pair recombination dominated at low temperatures and the bound electron-to-localized hole transition at high temperatures.
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