Semiconductors, Vol. 36, No. 5, 2002, pp. 481–486. Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 36, No. 5, 2002, pp. 513–518. Original Russian Text Copyright © 2002 by Kovalenko, Mironchenko, Shutov. ELECTRONIC AND OPTICAL PROPERTIES OF SEMICONDUCTORS Nonlinear Photoluminescence of Graded-Gap AlxGa1 – xAs Solid Solutions V. F. Kovalenko, A. Yu. Mironchenko, and S. V. Shutov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Kherson, 73008 Ukraine Submitted July 2, 2001; accepted for publication September 13, 2001 Abstract—Dependences of photoluminescence (PL) intensity for undoped and doped graded-gap AlxGa1 – xAs (x Շ 0.36) solid solutions on the excitation level J (1019 photon cm–2 s–1 Շ J Շ 1022 photon cm–2 s–1) were –1∇ Շ Շ investigated for various built-in electric fields E = e Eg (85 V/cm E 700 V/cm). It was found that the accelerating effect of the field E gives rise to a complex dependence of intensity of the edge PL (I) on the excitation level. The nonlinearity of the I(J) dependence is attributed to the contribution of the two-photon absorption of PL emission when the latter is reemitted. An optimal range of E values exists (120 V/cm Շ E Շ 200 V/cm). In this range, the contribution of two-photon absorption to the reemission process in undoped solid solutions is largest. © 2002 MAIK “Nauka/Interperiodica”. 1. INTRODUCTION 2. EXPERIMENTAL Generally, emission characteristics of graded-gap Undoped (n Շ 1016 cm–3) and Te-doped (n Ӎ 1018 cm–3) semiconductors under the accelerating effect of the epilayers of AlxGa1 – xAs solid solutions grown on GaAs –1∇ substrates from a limited volume of solution–melt were built-in electric field E = e Eg are determined by the drift of nonequilibrium charge carriers and the photon investigated. The composition of the layers varied drift, which is associated with the reemission of recom- along the growth axis. Specifically, the Al content was ∇ highest at the layer–substrate interface (x Ӎ 0.36) and bination radiation. Here, the Eg quantity is the band gap gradient. The contribution of these factors to the decreased to the surface, where x = 0. The layer compo- formation of luminescence properties is determined by sition varied linearly over the thickness in the region parameters of semiconductors [1]. Thus, for low E val- adjoining the substrate. This region makes approxi- mately 80% of the contribution to the total thickness d. ues, the photon drift of nonequilibrium charge carriers, ∇ which determines the formation of both the spectrum For various undoped structures, Eg varied within the range of 85 eV/cm Շ ∇E Շ 700 eV/cm. An increase and the intensity of radiative recombination, is domi- ∇ g nant [2–5]. Luminescent properties of semiconductors in Eg was attained by decreasing the thickness of the ∇ layers grown with the same Al content at the sub- with intermediate and large Eg, in which the drift of nonequilibrium charge carriers dominates in their strate–layer interface. The thickness of the solid-solu- tion layers investigated varied within the range of transport, are determined by the joint action of the drift µ Շ Շ µ ∇ of nonequilibrium charge carriers in the field E and ree- 16 m d 70 m. For doped layers, the Eg values mission. Much attention was paid to the investigation of were 150–170 eV/cm. the contribution of the former factor to the formation of The photoluminescence (PL) at 77 and 300 K was the emission spectrum for such semiconductors [1, 6, 7]. excited from the wide- and narrow-gap sides of the However, the influence of reemission on their lumines- epilayer (i.e., from the Emax and Emin sides, as is cence characteristics has been inadequately investi- g g gated. It is known [8, 9] that reemission with drift of shown in the inset to Fig. 1) using the angle laps of the structure [1]. The excitation was carried out using the nonequilibrium charge carriers in the field E leads to an λ µ increase in the external photon yield of luminescence. optical beam of an argon ( = 0.488–0.514 m) laser; the beam diameter was ~30 µm. The intensity of PL exci- However, the mechanism of reemission in such semi- 19 –2 –1 Շ Շ conductors has hardly been studied. tation J varied in the range of 10 photon cm s J 1022 photon cm–2 s–1. The PL spectra were recorded In this study, the specific features of reemission in using a Ge photodiode according to the conventional graded-gap AlxGa1 – xAs solid solutions with the drift procedure [1]. The external PL photon yield was esti- mechanism of transport of nonequilibrium charge car- mated using a calibrated Si photodiode. The effective riers are considered. These features were found from shift of nonequilibrium charge carriers l+ was deter- measurements of the dependence of their PL spectra on mined from the slope of the low-energy falloff of the the photoexcitation level. edge emission band [1]. 1063-7826/02/3605-0481 $22.00 © 2002 MAIK “Nauka/Interperiodica” 482 KOVALENKO et al. I+, IÐ, arb. units Eg 2 105 1', 4' 1Ð 4 10 max Eg m µ 3 min , 1 + 10 E l 4 g ~ 10 ~ AlxGa1 Ð xAs GaAs d z 0 1 4 100 2 3 ~ 10 ~ 3 4' 1.85 1' 102 1.75 , eV m 2' ν h 3' 1.65 101 2 1.55 1019 1020 1021 1022 J, photon/cmÐ2 sÐ1 1 1' Fig. 2. Effective shift of nonequilibrium charge carriers (1–3) 100 1.3 1.5 1.7 1.9 and location of the short-wavelength peak in the photolumi- ν nescence spectra (1'–3') for undoped epilayers in relation to h , eV the excitation intensity at T = 300 K. The built-in field E = (1, 1') 89, (2, 2') 160, and (3, 3') 256 V/cm. Fig. 1. Variation in the shape of the edge photoluminescence ∇ spectrum for the undoped epilayer with eE = Eg = 160 eV/cm in relation to the excitation level with the excitation from seen that, as J increases, the band is broadened to a low- max min the E side (1–4) and from the E side (1', 4') at T = energy region and the radiative recombination region g g shifts from the wide-gap region to the narrow-gap 300 K. J: (1, 1') 1019; (2) 1020; (3) 2.5 × 1021; and (4, 4') 22 –2 –1 region of the layer. This shift manifests itself in the shift 10 photon cm s . The variation in the band gap over the of the short-wavelength peak hν to the long-wave- epilayer thickness for undoped solid solutions and the m experimental layout are shown in the inset. The arrows indi- length region. The variation in the spectrum shape is cate the direction of photoluminescence excitation. Num- caused by an increase in the effective shift of nonequi- bering of arrows corresponds to numbering of the spectra librium charge carriers l+ with increasing J. The magni- (1–4, 1', 4'). ν tude of the shift of h m to the short-wavelength region is proportional to l+. These characteristics also depend on 3. RESULTS the built-in field E (Fig. 2). For high excitation intensities, ≈ the magnitude l+ in the layers with E 160–260 V/cm is The PL spectra of undoped solid solutions included largest. the edge emission band only. With the PL excitation max max If the Eg side is illuminated, the integrated inten- from the Eg side, i.e., with the accelerating effect of + the field E, the shape of PL spectra depended on the sity of the edge PL band IΣ side increases with increas- + magnitude of this field and excitation level J. For the ing J. The increase in IΣ follows the power law edge emission, the E dependence of its shape for a low PL excitation level (J Շ 2 × 1020 photon cm–2 s–1) was + m IΣ = CJ . (1) considered by us in [6]. The variation in the shape of the edge emission band for one of the undoped layers, Here, C is the proportionality coefficient, which depending on the excitation level, with the illumination accounts for the experimental layout, the angular distri- both from the Emax side (I+, curves 1–4) and from the bution of the PL intensity, the refractive index of the g semiconductor, the interaction of excitation radiation min – Eg side (I , curves 1', 4') is shown in Fig. 1. It can be with the semiconductor, the recombination rate, the SEMICONDUCTORS Vol. 36 No. 5 2002 NONLINEAR PHOTOLUMINESCENCE OF GRADED-GAP AlxGa1 – xAs SOLID SOLUTIONS 483 + Iλ, arb. units 2 2' 4 4 ' 10 1 3 λ max m 2 4 Σ 3 1 max 3' 10 m mΣ 1 1.4 1.6 1.8 hν, eV 102 2 6 0 3 ~ 1 ~ 101 1 1246 + 1 Iλ3 E, 102 V/cm + 2 100 2 + Iλ 104 Iλ1 2 , arb. units + 1 I 0 1.4 1.6 1.8 3 10Ð1 10 3 hν, eV 1019 1020 1021 1022 Ð2 Ð1 102 J, photon/cm s , arb. units Ð Σ I , 4 Fig. 4. Spectral dependences of photoluminescence inten- + Σ I + sity I on the excitation level for various sections of the 1 λ 10 5 edge photoluminescence band, as is shown in the inset, for the undoped layer of the solid solution with E Ӎ 180 V/cm, max and the dependence of the largest exponent mλ in rela- 0 10 19 20 21 22 tionship (1) as a function of energy of photons emitted in the 10 10 10 10 edge photoluminescence band at T = 300 K (curve 4).