The emission wavelength of the semiconductor lasers developed to date is mainly in the near infrared (IR). Efficient semiconductor light sources for the longer wavelength (> 3 μm) IR spectrum region are in growing demand for many military and commercial applications. Conventionally, narrow band-gap semiconductor materials, primarily IV-VI lead salts, [1,2] are employed for such long wavelength IR emitters. However, the rather poor quality of these narrow band-gap materials limits their performance. The demonstration of a quantum cascade (QC) laser, [3] based on intersubband transitions in semiconductor quantum well (QW) staircase structures made of relatively wide band-gap III-V compounds, [4–6] opened a new door towards efficient long-wavelength IR light sources. In contrast to conventional diode lasers, the wavelength of intersubband lasers is determined by the small energy separation of conduction subbands arising from quantum confinement in QWs rather than the band-gap of the material. Therefore, the lasing wavelength can in principle be tailored over a wide spectral range from mid-IR to sub-millimeter by merely changing the QW layer thickness. Another distinct feature of the intersubband QC laser is that each injected electron is reused with the possibility of generating an additional photon as it cascades down a step of the energy staircase. The recent demonstrations of room temperature high power, and distributed feedback (DFB) QC lasers at λ ~ 5 and 8 μm, [7–9] and a long wavelength (~11 μm) QC laser operating up to 200 K, [10] indicate the great potential of the QC configuration.

An alternative approach, utilizing optical transitions between an electron state in the conduction band and a hole state in the valence band in a staircase of Sb-based type-II QW structures as originally proposed by Yang, [11] represents another fundamentally new class of semiconductor long-wavelength IR light sources for practical applications. The so-called interband cascade lasers (or type-II QC lasers) retain the advantages of cascade injection and the ability to tailor the emission wavelength, [11–13] while circumventing the fast phonon scattering loss of the type-I intersubband QC laser, and possibly leading to higher radiative efficiency. Following the observation of mid-IR electroluminescence from interband cascade devices, [14–16] the first interband cascade laser based on InAs/GaInSb/AlSb type-II QW structures was demonstrated at temperatures up to 170 K in early 1997. [17] The latest demonstration of large peak output powers (e.g. ~0.5 W/facet at a emitting wavelength of ~3.9 μm) and high differential external quantum efficiencies (each injected electron can generate ~1.3 photons) from interband cascade lasers with improved designs and material quality indicates a significant improvement in a short period of time. [18,19] In this work, we report the observation of electroluminescence in the 10-15 μm IR spectrum from a type-II interband cascade structure as shown in Fig. 1.

 

Fig. 1 Interband cascade structure based on InAs/GaInSb/AlSb type-II QWs under a forward bias.

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The interband cascade light emitting diode (LED) comprises 10 repeated periods of active regions separated by n-type doped injection regions which serve both as collectors for the preceding active regions and emitters for the following ones. The active region comprises coupled broken-gap InAs/Ga(In)Sb type-II QWs as shown in Fig. 1, in which the wavefunctions of the two transition states E_e and E_h are mainly confined in different layers; thus their energy levels can be adjusted fairly independently. Therefore, the emission wavelength of type-II QC devices can be tailored over a wide spectrum from the mid- to far-IR (2.5-50 μm) by simply changing QW layer thickness and composition. The injection region (dotted lines in Fig. 1) consists of digitally graded InAs/Al(In)Sb multilayers in which the InAs layers are Si doped at 5 × 10¹⁷ cm^-3. The whole multilayer structure is strain-balanced and lattice-matched to the GaSb substrate. Under a forward bias, electrons are injected from an emitter into the level E_e which is in the band-gap region of the adjacent GaInSb layer. Since the electrons at the level E_e are effectively blocked from directly tunneling out by the GaInSb, AlSb and GaSb layers, they tend to relax to the hole state E_h in the adjacent valence-band QW, resulting in the emission of photons as shown in Fig. 1. Electrons at state E_h can then cross the thin AlSb barrier and GaSb layer by tunneling and scattering into the conduction band of the collector because of a strong spatial interband coupling (a unique feature of type-II heterostructure), and are ready for the next interband transition – leading to sequential photon emission with a possible quantum efficiency greater than the conventional limit of unity.

The device sample was grown in a Riber 32 MBE system on a rotating p-type GaSb substrate. The substrate temperature was kept at about 440°C which was adjusted for the growths of the InAs, GaSb and AlSb layers. The V/III beam equilibrium pressure (BEP) ratio for InAs layer was about 3, and was about 2 for GaSb. Reflection high energy electron diffraction (RHEED) was used to monitor the growth in situ. The RHEED pattern was 2×1 for InAs layers and 1×3 for Ga(In)Sb and AlSb layers. Automated shutter sequencing was employed at interface interruptions to ensure interface quality. After growth, the wafer was annealed at an elevated substrate temperature to improve material quality. High resolution double crystal X-ray diffraction experiment was then used to characterize the material quality.

 

Fig. 2 Electroluminescence spectra of the type-II QC LED at 77K. Inset: the optical output power-current and the current-voltage characteristics.

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Then the sample was made into devices with a mesa size of 0.4 × 0.9 mm². Current pulses 1.5 μs long were injected into the QC device at 20 kHz repetition rate (3% duty-cycle). The light emitted from the surface was collected by a f/2 ZnSe lens and monitored by a HgCdTe detector. As shown in Fig. 2, the luminescence spectrum of the cascade device at 77 K shows peaks at a wavelength of ~11 μm under various driving currents ranging from 0.1A to 1A. The full width at half-maximum (FWHM) of the peaks under different injection currents are all in the 23-26 meV range. The inset shows the current-voltage characteristics as well as the optical output power as a function of the driving current. The peak output power is about 50 nW under a driving current of 1 A. We have corrected for all the known losses due to reflection by the lens and dewar window; however, we have not corrected for the finite collection cone solid angle because the radiation pattern is not known accurately. The estimated collection efficiency is about 1 × 10^-3. The observed power is considerably higher than the maximum power reported for intersubband QC LEDs, [20–22] suggesting an improved radiative efficiency. In contrast to the previously reported type-II QC LEDs with a similar design but emitting at shorter wavelengths (~ 6-7 μm), [15] the blue shift of electroluminescent peak expected from the Stark effect was essentially unobservable from this device. This is not currently understood, but bears some similarity to intersubband QC devices at different emission wavelengths. [10,20,21] The current-voltage characteristics (Fig. 2, inset) observed from this device seems to indicate that leakage current is more serious in this long-wavelength IR LED compared to previously reported cascade LEDs. [15]

The electroluminescence has also been observed at room temperature, as shown in Fig. 3. At room temperature, the spectrum becomes broader, and the emission peak wavelength red shifts to ~14 μm, due to the decrease of the band-gap. To the best of our knowledge, the observed emission wavelength from this type-II QC device is the longest ever reported in any compound semiconductor light emitting device for room temperature operation, demonstrating the unique capability of this Sb-family type-II heterostructure material system to be tailored for the desired wavelength.

 

Fig. 3 Electroluminescence spectrum at 300K under 1A driving current.

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In summary, we have demonstrated electroluminescence in the 10-15 μm spectrum range from Sb-based type-II interband cascade devices, with a measured optical power up to 50 nW. With the addition of cladding layers and a waveguide cavity, as well as improvements in the growth process and design, interband cascade lasers operating at the long wavelength IR spectrum (8-15μm) are possible.