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Optical Properties of Si-DOPED AlxGa1−xN/AlyGa1−yN (x=0.24−0.53, y=0.11) Multi-Quantum-Well Structures

Published online by Cambridge University Press:  13 June 2014

H. Hirayama  and
Y. Aoyagi
H. Hirayama
Affiliation:
The Institute of Physical and Chemical Research (RIKEN) 2-1 Hirosawa, Wako-shi, Saitama, 351-0198, Japan, hirayama@postman.riken.go.jp
Y. Aoyagi
Affiliation:
The Institute of Physical and Chemical Research (RIKEN) 2-1 Hirosawa, Wako-shi, Saitama, 351-0198, Japan, hirayama@postman.riken.go.jp

Abstract

We demonstrate strong ultraviolet (UV) (280-330nm) photoluminescence (PL) emission from multi-quantum-well (MQW) structures consisting of AlGaN active layers fabricated by metal-organic chemical-vapor-deposition (MOCVD). Si-doping is shown to be very effective in order to enhance the PL emission of AlGaN QWs. We found that the optimum values of well thickness and Si-doping concentration of AlxGa1−xN/AlyGa1−yN (x=0.24−0.53, y=0.11) MQW structure for efficient emission were approximately 3nm and 2×1019cm−3, respectively. In addition, the PL intensities of AlGaN, GaN and InGaN quantum well structures are compared. We have found that the PL emission at 77K from a Al0.53Ga0.47N/Al0.11Ga0.89N MQW is as strong as that of InGaN QWs.

Type
Research Article
Information
Materials Research Society Internet Journal of Nitride Semiconductor Research , Volume 4 , Issue S1 , 1999 , pp. 405 - 410
Copyright
Copyright © 1999 Materials Research Society

Introduction

GaN and related nitrides are currently of great interest for the application to optical devices operating in the visible and ultraviolet (UV) energy range. Blue laser diodes (LDs) and bluegreen light-emitting diodes (LEDs) have been developed in recent years [Reference Nakamura, Senoh, Nagahama, Iwasa, Yamada, Matsushita, Sugimoto and Kiyoku1-Reference Nakamura, Senoh, Nagahama, Iwasa, Yamada, Matsushita, Sugimoto and Kiyoku3]. High-power, long-lifetime InGaN multi-quantum well (MQW) lasers have been demonstrated [Reference Nakamura, Senoh, Nagahama, Iwasa, Yamada, Matsushita, Sugimoto and Kiyoku1].

The AlGaN alloy is a useful material for optical devices operating in the UV, because direct transition emission can be adjusted between 3.4eV (GaN) and 6.2eV (AlN). The wide transition range of AlGaN covers the entire lasing wavelength range covered by UV gas and solid state lasers, for example, XeCl(308nm) and KrF(248nm) excimer lasers or N2(337nm), He-Cd(325nm), SHG-Ar(257nm) lasers. UV semiconductor lasers are attractive in comparison with gas lasers because of small size, long lifetime, high efficiency and continuous-wave (CW) operation. CW-UV lasers using AlGaN materials are believed to replace UV gas lasers in the near future. For the realization of UV semiconductor lasers, several technical problems such as current injection through high Al content AlGaN crystals or efficient UV emission from AlGaN QW structures must be solved. Especially, the realization of high optical gain from AlGaN in UV emission range is most important for the use as the active region of UV lasers.

Many research groups have obtained a strong emission of InGaN QWs when the In content is 10-20%. However the emission efficiency of binary GaN is much weaker than that of InGaN and actually not useful for active region in a semiconductor laser. The mechanism of the efficient emission in InGaN alloy in comparison with GaN has been investigated using Nichia’s samples [Reference Chichibu, Azuhata, Sota and Nakamura4,Reference Narukawa5]. It was reported that the quantum efficiency of InGaN-based quantum well lasers is enhanced by the effect of localized excitons in nano-scale In segregated (In-rich) regions of the quantum well [Reference Narukawa5]. The efficient emission of InGaN is observed even when the In incorporation is only a few percent.

On the other hand, AlGaN QW structure with respect to strong UV emission has not yet been well investigated and its optical property is still unknown even though it is very important material in order to realize UV (especially for wavelength shorter than 360nm) optical devices. Recently, we fabricated AlGaN QW and quantum dot (QD) structures for the purpose of intense UV emission [Reference Hirayama and Aoyagi6].

In this work, we report on the AlxGa1−xN/AlyGa1−yN (x=0.24−0.53, y=0.11) MQW structures fabricated by metal-organic chemical-vapor-deposition (MOCVD) and demonstrate an intense UV (280-330nm) photoluminescence (PL). We also compare the PL intensity between AlGaN, GaN and InGaN QW structures.

Experiments and Discussions

The structures were grown at 76 torr on the Si-face of an on-axis 6H-SiC(0001) substrate, by a conventional horizontal-type MOCVD system. As precursors ammonia (NH3), trimethylaluminum (TMAl), trimethylgallium (TMGa), and tetraethylsilane (TESi) were used with H2 as carrier gas. N2 gas was independently supplied by a separate line and mixed with the H2 just before the substrate susceptor. Typical gas flows were 2 standard liters per minute (SLM), 2 SLM, and 0.5 SLM for NH3, H2, and N2, respectively. The molar fluxes of TMGa and TMAl for the growth of AlxGa1−xN (x=0.11−0.53) were 38μmol/min and 2.6−45μmol/min, respectively. At this condition, the growth rate was approximately 2.5μm/h. The substrate temperature measured with a thermocouple located at the substrate susceptor during the growth was 1140°C for all layer.

At first, we investigated the growth condition of high Al content AlGaN alloy. Figure 1 shows the 77K PL spectra of AlGaN films grown directory on very thin (∼10nm) AlN buffer layer. The thickness of all the AlGaN film was approximately 400nm. As seen in Fig. 1, single peak emission was obtained for Al contents of 0.11-0.53. The phonon-replica peaks, seen at the low energy side of each spectra, confirms the good crystal quality of the AlGaN. The typical value of full width at half maximum (FWHM) of the spectrum was 20meV for AlxGa1−xN (x=0.10−0.12) and 65meV for AlxGa1−xN (x=0.30-0.60) at 77K. For an Al content of 0.6, an additional emission peak around the wavelength of 290nm probably originating from defects was observed besides the 264nm peak. Therefore, the highest Al content we used in this experiment was 0.53.

Fig. 1. 77K PL spectra of AlxGa1−xN (x=0.11−0.60) films grown on the 6H-SiC substrates.

Figure 2 shows schematic layer structure of the fabricated (a) Al0.53Ga0.47N/Al0.11Ga0.89N and (b) Al0.24Ga0.76N/Al0.11Ga0.89N MQW sample. In order to achieve a flat surface suitable for the growth of AlGaN quantum well, an approximately 400nm thick Al0.53Ga0.47N buffer layer for sample (a), and 100nm-thick Al0.24Ga0.76N followed by an 300nm-thick Al0.35Ga0.65N buffer layer for sample (b) were deposited. The buffer layer was found to provide a step-flow grown surface as confirmed by atomic force microscopy (AFM). After that, for sample (a), 5-layer MQW structure consisting of 2.7-6.7nm-thick Al0.11Ga0.89N wells and 8nm-thick undoped Al0.53Ga0.47N barrier layers, and 20nm-thick Al0.53Ga0.47N capping layer were grown. Also, for sample (b), 6-layer MQW structure consisting of 2-5nm-thick Si-doped (undoped) Al0.11Ga0.89N wells and 6nm-thick undoped Al0.24Ga0.76N barrier layers, and 10nm-thick Al0.24Ga0.76N capping layer were grown. The well and barrier thickness is estimated simply from the growth rate of bulk.

Fig. 2. Schematic layer structure of (a) Al0.53Ga0.47N/Al0.11Ga0.89N and (b) Al0.24Ga0.76N/Al0.11Ga0.89N MQW sample.

Figure 3 shows PL spectra measured at 77K of the undoped Al0.53Ga0.47N/Al0.11Ga0.89N 5-layer MQW structures excited with an Ar-SHG laser (257nm) for various well thickness. The spectra of the Al0.53Ga0.47N bulk without Al0.11Ga0.89N well is also shown for comparison of emission intensity. As seen in Fig. 3, the peak wavelength of QW emission shifts from 344nm to 271nm as the well thickness decreases from 6.7nm to 2.7nm. We attribute these shift to the increased quantization energy in the QWs. We cannot see the emission from barrier or capping layers for each MQW spectrum, which indicates that the emission from the quantum well is efficient. The well emission intensity is strongest for a well thickness of 3.3nm. The emission peak wavelength of 400nm-thick Al0.53Ga0.47N buffer layer from the sample without QW is slightly longer than that of 2.7nm-thick MQW structures. This may be because that the barrier bandgap is extended due to the strain compensation between barrier and well. The rapid reduction of the PL intensity with the increase of the well thickness may be caused by a reduction of the radiative recombination probability due to a separation of electron and hole wave-functions in the large piezoelectric field of the well [Reference Nardelli, Rapcewicz and Bernholc7]. The reduction of the emission intensity for the thin well may be mainly due to the increase of nonradiative recombination on the hetero-interfaces between well and barrier.

Fig. 3. PL spectra measured at 77K of the undoped Al0.53Ga0.47N/Al0.11Ga0.89N 5-layer MQW structures excited with Ar-SHG laser (257nm) for various well thickness.

Figure 4 shows the room temperature PL spectra of the undoped Al0.24Ga0.76N/Al0.11Ga0.89N 6-layer MQW structures excited with a XeCl excimer laser for various well thickness. The peak wavelength of QW emission shifts from 344nm to 323nm as the well thickness decreases from 5nm to 2nm. The 321nm weak emission originates from Al0.24Ga0.76N barrier layers. The dependence of the PL intensity on the QW thickness was similar to that obtained for Al0.53Ga0.47N/Al0.11Ga0.89N MQWs. The optimized well thickness was also around 3nm.

Fig. 4. Room temperature PL spectra for various well thickness from the undoped Al0.24Ga0.76N/Al0.11Ga0.89N 6-layer MQW structures excited with a XeCl excimer laser.

Figure 5 shows PL spectra measured at 77K from 3nm thick Si-doped and undoped Al0.24Ga0.76N /Al0.11Ga0.89N 6-layer MQWs excited with a He-Cd laser. Si-doping was used only in the QW layers. The doping concentration was changed from 8×1018 to 5×1019cm−3. The emission intensity increases drastically by Si-doping. The PL intensity is enhanced by 2-3 times with a Si-doping concentration of 2×1019cm−3, as seen in Fig 5. We can see phonon replica peaks on the low energy side of main peaks for a doping concentration below 2×1019cm−3. We assume that the screening of piezoelectric field with doping is causing the PL intensity enhancement, as reported in the case of InGaN QWs[Reference Chichibu, Cohen, Mack, Abare, Kozodoy, Minsky, Fleischer, Keller, Bowers, Mishra, Coldren, Clarke and DenBaars8].

Fig. 5. PL spectra measured at 77K from 3nm thick Si-doped and undoped Al0.24Ga0.76N /Al0.11Ga0.89N 6-layer MQWs excited with a He-Cd laser.

Finally, we will compare the emission intensity of AlGaN, GaN and InGaN QWs. Figure 6 shows the PL emission spectra measured at 77K of Al0.53Ga0.47N/Al0.11Ga0.89N 5-layer MQW, Al0.24Ga0.76N/Al0.11Ga0.89N 6-layer MQW, Al0.12Ga0.88N/GaN 5-layer MQW and In0.02Ga0.98N/In0.20Ga0.80N single-QW structures. All samples are undoped with optimized well thickness. All samples were excited with Ar-SHG laser with the same excitation condition. As can be seen in Fig. 6, the 280nm emission of the Al0.53Ga0.47N/Al0.11Ga0.89N MQW is as strong as that of the In0.02Ga0.98N/In0.20Ga0.80N QW and much stronger than those of the Al0.24Ga0.76N/Al0.11Ga0.89N or Al0.12Ga0.88N/GaN MQWs at 77K. However, the temperature dependence of PL intensity was strongest for Al0.53Ga0.47N/Al0.11Ga0.89N MQW and the emission intensity was much reduced at room temperature. We believe that the emission mechanism of AlGaN QWs is much different from that of InGaN QWs. We suggest that the strong UV emission from AlGaN QWs originates in the confinement states which is stable only at low temperature, though, at this moment we don’t know the exact mechanism.

Fig. 6. PL emission spectra of Al0.53Ga0.47N/Al0.11Ga0.89N, Al0.24Ga0.76N/Al0.11Ga0.89N, Al0.12Ga0.88N/GaN MQW and In0.02Ga0.98N/In0.20Ga0.80N single-QW structures.

Conclusion

We have demonstrated strong UV (280-330nm) PL emission from AlGaN MQW structures fabricated by MOCVD. Si-doping was shown to be very effective in order to enhance the emission of AlGaN QWs. We investigated systematically the optimized well thickness and the Si-doping concentration of AlxGa1−xN/AlyGa1−yN (x=0.24−0.53, y=0.11) MQW structure with respect to efficient emission and found that the optimum values were approximately 3nm and 2×1019cm−3, respectively. The, PL intensities of AlGaN, GaN and InGaN quantum well structures were compared. We found that the emission at 77K from a Al0.53Ga0.47N/Al0.11Ga0.89N MQW was as strong as that from the InGaN QWs.

Footnotes

MRS Internet J. Nitride Semicond. Res. 4S1, G3.74(1999)

References

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Figure 0

Fig. 1. 77K PL spectra of AlxGa1−xN (x=0.11−0.60) films grown on the 6H-SiC substrates.

Figure 1

Fig. 2. Schematic layer structure of (a) Al0.53Ga0.47N/Al0.11Ga0.89N and (b) Al0.24Ga0.76N/Al0.11Ga0.89N MQW sample.

Figure 2

Fig. 3. PL spectra measured at 77K of the undoped Al0.53Ga0.47N/Al0.11Ga0.89N 5-layer MQW structures excited with Ar-SHG laser (257nm) for various well thickness.

Figure 3

Fig. 4. Room temperature PL spectra for various well thickness from the undoped Al0.24Ga0.76N/Al0.11Ga0.89N 6-layer MQW structures excited with a XeCl excimer laser.

Figure 4

Fig. 5. PL spectra measured at 77K from 3nm thick Si-doped and undoped Al0.24Ga0.76N /Al0.11Ga0.89N 6-layer MQWs excited with a He-Cd laser.

Figure 5

Fig. 6. PL emission spectra of Al0.53Ga0.47N/Al0.11Ga0.89N, Al0.24Ga0.76N/Al0.11Ga0.89N, Al0.12Ga0.88N/GaN MQW and In0.02Ga0.98N/In0.20Ga0.80N single-QW structures.