AlSb type-II cascade structures

Physica E 7 (2000) 80–83

www.elsevier.nl/locate/physe

Mid-infrared intersubband electroluminescence in InAs= GaSb= AlSb type-II cascade structures K. Ohtani ∗ , H. Ohno Laboratory for Electronic Intelligent Systems, Research Institute of Electrical Communication, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan

Abstract Mid-infrared intersubband light-emitting diodes based on InAs=GaSb=AlSb type-II cascade structure have been investigated. The observed emission energy is in good agreement with calculation based on the multi-band k · p theory. In contrast to interband cascade structures, dominant polarization of the emitted light is perpendicular to the quantum well layers. Structure c 2000 Published by Elsevier Science B.V. All rights dependence of intersubband electroluminescence is also presented. reserved. Keywords: Electroluminescence; Type-II InAs=GaSb=AlSb cascade structure; Antimonides

1. Introduction Using type-I quantum well (QW) structures, high-performance lasers utilizing intersubband transitions with a wide spectrum range were realized [1,2]. The wavelength range (3:4–17 m) covered by such lasers includes the atmospheric windows important for gas sensing and environment monitoring. Recently, high-power continuous-wave (CW) operation (∼ 5 and ∼ 8 m; ∼ 200 mW=facet at 80 K) was reported with high quantum eciency due to the carrier recycling called quantum cascading [3,4]. However, so far no room-temperature CW operation has been ∗ Corresponding author. Tel./fax: +81-22-217-5555. E-mail addresses: [email protected] (K. [email protected] (H. Ohno)

Ohtani),

reported presumably because of the severe sample heating due to the fast polar optical phonon scattering. Type-II InAs=GaInSb interband cascade lasers (ICL), proposed by Yang [5] and demonstrated by Lin et al. [6], utilizes interband transition to suppress the polar optical phonon scattering, while retaining the carrier recycling by the unique band alignment of InAs=GaSb heterojunction. Very recently, ICL with external eciency ¿450% has been reported [7]. Recent theoretical appraisal [8,9] of InAs= GaSb=AlSb type-II intersubband light emitter proposed earlier [10,11], small polar optical phonon scattering rate due to the small eective mass of InAs oers advantages over type-I intersubband light emitters. The type-II InAs=GaSb structure blocks electrons injected into the excited state in the InAs QW by the adjacent GaSb band gap as in the InAs=GaInSb

c 2000 Published by Elsevier Science B.V. All rights reserved. 1386-9477/00/$ - see front matter PII: S 1 3 8 6 - 9 4 7 7 ( 9 9 ) 0 0 2 8 2 - 9

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ICL, reducing the leakage current path present in type-I intersubband lasers. The injected electrons can be extracted eciently from the ground state after intersubband transition by interband tunneling through the InAs=GaSb broken gap heterojunction. Also the InAs=AlSb heterostructure has a wide range of tunability of intersubband transition energy because of the larger conduction band oset (∼ 1:35 eV) [12] compared to that of the GaInAs=AlInAs system (∼ 0:5 eV). Theoretical calculations indicate that optical gain of type-II intersubband laser is more than 50% higher than that of type-I [9] and the calculated threshold current can be as low as 750 A=cm2 at 300 K [8]. Since this value is a factor of 4 lower than the theoretical prediction for the 5 m type-I InGaAs=AlInAs intersubband lasers [8], it is a strong candidate for achieving room-temperature CW intersubband laser operation. Recently, we reported the rst observation of intersubband electroluminescence in InAs=GaSb=AlSb type-II quantum cascade structures [13]. Here we report the characteristics of intersubband electroluminescence, especially the structure dependence of intersubband electroluminescence. 2. Experimental The intersubband quantum cascade structures (ISBQC) were grown by molecular beam epitaxy system equipped with a compound As cell and a Sb cracker cell on undoped InAs (1 0 0) substrates. After growth of 700 nm Si-doped (3 × 1017 cm−3 ) n-type InAs as a bottom contact layer, 10 periods of injector structures and active layers were grown. The injector structure consisted of digitally graded InAs=AlSb superlattices in which the InAs layers were Si-doped to n = 2 × 1017 cm−3 . The active layer consisted of an InAs=GaSb coupled QW which was made of 10 ML AlSb barrier, X ML InAs quantum well, Y ML GaSb quantum well and 5 ML AlSb barrier. To investigate the structure dependence of intersubband electroluminescence, four samples ((A) X = 30 ML; Y = 25 ML (B) X = 33 ML; Y = 25 ML (C) X = 26 ML; Y = 25 ML (D) X = 33 ML; Y = 40 ML) were grown. After growth of the injector=active layer structures, 200 nm Si-doped (3 × 1017 cm−3 ) InAs layer was grown as a top contact layer.

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Fig. 1. Electroluminescence spectra at ∼ 77 K for injection current of 40 and 80 mA (sample (A)). Inset: the integrated electroluminescence intensity against injection current.

Grown sample was processed into 300 m × 300 m mesa structures by wet etching and photolithograpy. Non-alloyed Cr=Au ohmic contacts were deposited on both top and bottom contact layers. ◦ Sample edge was then polished 45 wedge for light emission. The electroluminescence measurement was performed with FT-IR spectrometer using lock-in detection technique [14]. A polarizer was inserted in the optical path to verify the polarization of the emission. Current pulse at 15 kHz with duty cycle of 50% was used for electroluminescence measurements. 3. Results and discussion Fig. 1 shows the electroluminescence spectra of sample (A) at ∼ 77 K under two dierent current biasing conditions. An emission peak was observed at 233 meV, corresponding to the wavelength of 5:6 m with full-width at half-maximum (FWHM) being ∼ 14 meV. The emission energy is in close agreement with the transition energy (218 meV) between E1 and E2 of InAs QW calculated using a multi-band k · p theory [15]. The inset of Fig. 1 shows the integrated electroluminescence intensity as a function of injection current at ∼ 77 K, where a linear relationship was observed, which is consistent with a spontaneous emission process with a constant emission energy.

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Fig. 2. (a). Polarized intersubband electroluminescence spectra at ∼ 77 K for injection current of 80 mA. (b) Polarized interband electrolumminescence spectra at ∼ 77 K for same injection current.

Fig. 2(a) and (b) show polarization resolved electroluminescence spectra; light polarized perpendicular or parallel to the quantum well layer. As shown in Fig. 2(a), the spectrum of intersubband cascade structure (sample (A)) is polarized perpendicular to the layer, which shows that the light emission is indeed from the intersubband optical transition. Fig. 2(b) shows the same measurements of an interband cascade structure (IBQC) (InAs 4:8 nm and ◦ GaSb 12 nm), collected from 45 wedge. The intensity of the two polarizations was almost the same and is in sharp contrast to the intersubband spectra. The selection rule derived from the multi-band k · p theory shows that the polarization characteristic to the intersubband transition is preserved in a low band-gap semiconductor such as InAs, where band mixing is appreciable, as long as the energy separation of E1 and E2 is small compared to the band gap [15,16]. This was veri ed by absorption measurements in AlSb=InAs=AlSb QWs [16]. Although the calculation based on the multi-band k · p theory shows that the light hole component in E1 state of the InAs well in sample A (AlSb=InAs=GaSb=AlSb) increases by a factor of 2 compared to a single AlSb=InAs=AlSb QW, the ratio of polarization perpendicular to the QW layer and parallel to the QW layer remains below the error level of the present experimental setup. Fig. 3(a) and (b) show the structure dependence of intersubband electroluminescence spectra under

Fig. 3. (a). InAs well width dependence of intersubband electroluminescence (X = 30 ML (sample (A)), X = 33 ML (sample (B)), X = 26 ML (sample (C)) at ∼ 77 K. (b) GaSb well width dependence of intersubband electroluminescence (Y = 25 ML (sample (B)), Y = 40 ML (sample (D)) at ∼ 77 K.

the same injection current (80 mA) at ∼ 77 K. The InAs well width dependence of electroluminescence is shown in Fig. 3(a). By decreasing the InAs well width, the emission peak energy shifts to the higher energy, because of the increase in the transition energy between the rst excited state and the ground state. On the other hand, the GaSb well width does not change the emission peak energy as shown in Fig. 3(b). These experimental results are again consistent with the observed emission being due to the intersubband optical transition in InAs QW. The temperature dependence of intersubband electroluminesence shows that the emission peak shifts to lower energy and the spectrum becomes slightly broader (by a factor of 1.5 between ∼ 77 and 300 K) as temperature increases. Typical full-width at half-maximum (FWHM) of an ISBQC sample at ∼ 77 K is in the range of 15 –25 meV and spectrum shape is nearly symmetric, whereas the IBQC samples show asymmetric and broader FWHM (35 –60 meV). 4. Conclusion We have investigated the electroluminescence characteristics of type-II InAs=GaSb=AlSb intersubband light-emitting diodes. The observed emission energies were in good agreement with the results of a multi-band k · p calculation. The spectrum was polar-

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ized perpendicular to QW layer, whereas the intensity of perpendicular and horizontal polarization was almost the same for the interband cascade structures. This electroluminescence polarization as well as the InAs and GaSb well widths dependence establish that the luminescence is from the intersubband optical transition in InAs QW. Acknowledgements The authors thank Y. Ohno, H.C. Liu and T. Dietl for helpful discussions. This work was partly supported by a Grant-in-Aid for Scienti c Research (A) (No. 11355012) from the Ministry of Education, Science, Sports and Culture, Japan and by ‘Research for the Future Program’ from the Japan Society for the promotion of Science (JSPS-RFTF 97P00202). References [1] J. Faist, F. Capasso, D.L. Sivco, A.L. Hutchinson, A.Y. Cho, Science 264 (1994) 553. [2] F. Capasso, J. Faist, C. Sirtori, A.Y. Cho, Solid State Commun. 102 (1997) 231.

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