1. Introduction

Nitride-based ultraviolet laser diodes have considerably interest for a number of applications including chemical analysis, medical devices, bio-agent detection and material processing. The laser diodes promise higher power, high efficiency, smaller size, and less cost than currently used bulky ultraviolet lasers. In nitride-based light emitting diodes (LEDs), the emission wavelengths were already expanded down to 210 nm, and high external quantum efficiency of over 10% was achieved even at a short-wavelength of 278 nm [1,2]. By optical pumping, laser actions of nitride materials were also demonstrated in 200-nm-band [3–5]. In contrast, progress in developing short-wavelength ultraviolet laser diodes has been still limited in 330-nm-band, because laser diodes require a more complex structure, thicker layers and higher crystalline quality than LEDs [6–18]. In order to shift in the lasing wavelength toward shorter ultraviolet wavelength, more effective carrier recombination in active region and the laser-diode structure composed of relatively low AlN mole fractions in AlGaN layers are indispensable.

Due to the strong internal electric field in AlGaN heterostructure, the spatial separation between electron and hole wavefunctions leads to the decrease of electron-hole recombination probability. Narrowing the well width in quantum well is expected to be an effective way to increase the wavefunctions overlap. In addition, that leads to the shift of the emission wavelength toward shorter wavelength region as a secondary effect. Hirayama et al. reported the effects of ultraviolet LEDs [19].

AlGaN layers with higher AlN mole fractions are favorable for the layer structure design of short-wavelength ultraviolet laser diodes. The AlGaN layers with relatively high AlN mole fraction were used for the underlying and the cladding layers of the previously reported ultraviolet laser-diodes [9, 12, 13]. Generally, the output power of ultraviolet laser diodes and LEDs decrease with the increase of AlN mole fractions in the AlGaN layers. Because the AlGaN layers have serious problems due to lack of suitably lattice-matched substrates. The growth of AlGaN layers with higher AlN mole fractions on lattice-mismatched substrates causes dislocations and eventually crack-generation because of tensile strain and thermal mismatch. Moreover the increase of the AlN mole fractions in p-type AlGaN layers leads to the decrease of hole concentration, due to the poor crystalline quality and the high activation energy of Mg-acceptor.

From these points of view, it is important to demonstrate the laser operation of ultraviolet laser diodes in which the quantum well consists of quite narrow well width and/or AlN mole fractions of AlGaN layers are relatively low. In this paper, we report on the laser operation of a short-wavelength ultraviolet laser diode with multiple-quantum-wells (MQWs) composed of GaN well layers. The laser operation has been achieved in 340-nm-band far from the wavelength corresponding to GaN band gap under the pulsed current mode at room temperature. This record is the shortest lasing wavelength ever reported for a semiconductor laser composed of binary compound well layer. Moreover, the device has been realized on the Al_0.2Ga_0.8N underlying layer. The AlN mole fraction of the underlying layer is 0.1 lower than that of the underlying layer of the previously reported 342 nm laser diode [12]. We believe that these results provide a potential breakthrough to next stage for a shorter-wavelength ultraviolet laser diode.

2. Experimental

Figure 1 shows a schematic illustration of the layer structure of the ultraviolet laser diode. Material growth was carried out by metalorganic vapor phase epitaxy. The laser structure is similar to the previously reported ones [12, 13, 16]. An Al_0.2Ga_0.8N underlying layer was grown on a c-sapphire substrate by a hetero-facet-controlled epitaxial lateral overgrowth technique [20, 21]. The laser structure consists of a 2.8-μm-thick n-Al_0.2Ga_0.8N n-contacting layer, a 0.6-μm-thick n-Al_0.2Ga_0.8N cladding layer, a 120-nm-thick Al_0.1Ga_0.9N guiding layer, GaN/Al_0.1Ga_0.9N MQWs, a 120-nm-thick Al_0.1Ga_0.9N guiding layer, an AlGaN electron-blocking layer, a 0.5-μm-thick p-Al_0.2Ga_0.8N cladding layer, and a 25-nm-thick p-GaN contacting layer.

 

Fig. 1 Schematic illustration of the layer structure of ultraviolet laser diode.

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The laser stripes with a width of 3 μm were defined by lithography. Both of the formation of ridge structure and the exposure of n-contacting layer were carried out by dry etching. Au and Al based contacting pads were deposited onto the p- and n-contacting layers, respectively. Cavity facets were also formed by dry etching for the laser diodes with a cavity length of 500 μm. No reflective coating was employed for the cavity facets. The GaN well width was estimated to be around 1 nm from the growth rate, the photoluminescence measurement, and the theoretical calculation of a potential well model. On the negative side, the optical confinement of the laser diodes are relatively lower than that of the previously reported ones, because of the narrower well width despite the same number of quantum wells in the MQWs.

3. Results and discussion

Figure 2 shows a series of lasing spectra of the device. These spectra were measured under the pulsed current mode with a pulse duration of 10 ns and a repetition frequency of 5 kHz at room temperature. The device starts lasing at a threshold current of approximately 700 mA and exhibits sharp lasing emissions around a wavelength of 345 nm above the threshold.We have realized laser operation of the laser diode composed of GaN well layers in 340-nm-band, the shortest wavelength ever reported for a semiconductor laser diode with binary compound well layer. We already reported the GaN/AlGaN MQW laser diodes with the similar layer structure, but with a wider GaN well width of 3 nm [9]. Those previous laser diodes lased in the peak wavelength range from 355.4 to 361.6 nm under a 100-ns-pulse condition at room temperature. There is a clear correlation between the emission wavelength and the well width. The relation of the wavelength and the well width approximately agrees with the theoretical calculation of the simple potential well model. At a current of 914 mA, the device exhibits the sharp spectrum with a full-width at half-maximum of 0.5 nm, which is slightly larger than the resolution limit of the spectrometer. The emission spectrum is thought to be a multimode one. The peak wavelength shift of 0.7 nm above the threshold is comparable to that of the previously reported 342 nm laser diode [12]. The wavelength shift is caused principally by the self-heating, the band-gap renormalization, the band filling (Burstein-Moss shift), and the screening of quantum-confined Stark effect (QCSE). The remarkable blue-shift tendencies were observed in the polar (c-plane) nitride-based laser diodes compared to the non-polar (m-plane) ones [22, 23]. In the case of polar nitride-based laser diodes, due to spontaneous and piezoelectric polarizations, the QCSE is thought to be the dominant effect on the shift to shorter wavelength. Similarly in this device, a relatively strong screening of the internal electric field by free carriers can be anticipated around the threshold even in the narrower well width.

 

Fig. 2 Series of lasing spectra of the device operating above threshold.

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The corresponding threshold current density is estimated to be 47 kA/cm², which is still higher than 8.7 kA/cm² of the previously reported 342 nm laser diode. The threshold current density is sensitive to multiple factors including, but not limited to, the carrier recombination rate, the optical confinement in the active region, the reflectivity and/or the surface profile of cavity mirrors, and the ridge-waveguide geometry. It would be considered that the high threshold current density is mainly due to decreased optical and carrier confinements in the narrow wells.

Figure 3 shows transverse electric (TE) and transverse magnetic (TM) spectra operating above the threshold. Excellent TE/TM ratio has been also confirmed due to the dominance of TE optical gain in the GaN/AlGaN MQWs as well as the high reflectivity for the TE mode by the cavity facets [3, 12, 13]. The light output power and the other detail characteristics will be described later [24].

 

Fig. 3 Transverse electric (TE) and transverse magnetic (TM) spectra operating above threshold.

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By increasing the AlN mole fraction in each layer of the laser diodes, it would be possible to shift the lasing wavelength into shorter wavelength region. Based on the results that have been demonstrated in this work, we expect further shift in the lasing wavelength of the previous reported 336 nm laser diode [13] into 320-nm-band or shorter wavelength band by applying both the narrower well width and the AlN mole fraction margin.

4. Summary

We report on the laser operation of a short-wavelength ultraviolet laser diode with MQWs composed of GaN well layers. The laser operation has been achieved in 340-nm-band far from the wavelength corresponding to GaN band gap under the pulsed current mode at room temperature. This record is the shortest lasing wavelength ever reported for a semiconductor laser composed of not only the GaN well layer but also the binary compound well layer. Moreover, the device has been realized on the Al_0.2Ga_0.8N underlying layer. The AlN mole fraction of the underlying layer is 0.1 lower than that of the underlying layer of previously reported 342 nm laser-diode. The device configuration provides another approach toward shorter ultraviolet wavelength of nitride-based laser diodes with the difficulty of higher AlN mole fractions.