Growth and optical properties of III-nitride
semiconductors for deep UV (230–350 nm)
light-emitting diodes (LEDs) and laser diodes (LDs)
semiconductors for deep UV (230–350 nm)
light-emitting diodes (LEDs) and laser diodes (LDs)
Introduction
GaN and III-nitride compound semiconductors are attracting considerable attention for the realization of visible or ultraviolet (UV) laser diodes (LDs) and light-emitting diodes (LEDs). Blue LDs and violet, blue and green LEDs are already commercially available.1) In particular, AlGaN is expected to be applied in deep UV LDs and bright LEDs, because the direct transition energy can be adjusted between 6.2 eV (AlN) and 3.4 eV (GaN). Shows the relationship between the direct transition energy and the lattice constant of wurtzite (Al,Ga,In)N alloy.The emission wavelength range of GaN and related materials covers the red to deep UV region, which can be realized by excimer lasers, He-Cd laser or solid-state SHG lasers. Deep UV LDs or LEDs are useful for realizing large-capacity optical memories or longlifetime fluorescent light.Moreo ver, they are important in the biochemical and medical fields.Ho wever, there are some severe technical problems that prevent realization of UV optical devices.The most serious problems are difficulty in obtaining efficient UV emission from AlGaN quantum wells (QWs), in contrast to InGaN QWs,1) as well as the difficulty in achieving p-type doping in high-Al-content AlGaN. First, we studied the growth and optical properties of high- Al-content AlGaN.W e obtained single-peak emission of AlxGa1−xN over the entire compositional range, i.e., from GaN to AlN, from near the band edge.2) We obtained the shortest wavelength (208 nm) photoluminescence (PL) of a semiconductor from AlN grown on SiC.We also demonstrated the shortest wavelength efficient UV emission at 230–250nm from AlN(AlGaN) /AlGaN multi- (M-)QWs.3) The 200 nmband emissions from AlGaN QWs were as strong as those from commercially available InGaN QWs at 77 K,3) however, they were weak at room temperature (R.T.). For the purpose of obtaining R.T. bright UV emission and high hole conductivity of wide-bandgap AlGaN, we propose the use of the emission from a localized electron-hole pair in the In segregation region in InAlGaN quaternary.It was reported that the quantum-dot-like region formed by In segregation in InGaN QWs is very effective for the suppression of nonradiative recombination and that an InGaN QW emits well at room temperature.4,5) It was also reported that the In content segregation of more than 5% in InGaN is necessary for high-current injection devices such as LDs.W e need a high Al content ranging from 40 to 60% in order to achieve 300 nm-band-emitting InAlGaN quaternary with 5% In incorporation, because of the very large band bowing of InAlGaN.W e report on the growth and optical properties of InAlGaN quaternary and demonstrate R.T. intense deep-UV emission at 300–340nm from InxAlyGa1−x−yN quaternary QWs for the first time.6,7) As for the current injection of deep-UV-emitting QWs, we have already realized 333nm current injection emission using Mg-doped GaN/AlGaN superlattice (SL) hole conducting layers.8) We report on the possibility of p-type doping for wider bandgap AlGaN by introducing In.
Experimental details and discussions
Structures were grown at 76 Torr on the Si-face of an on-axis 6H-SiC(0001) substrate by low-pressure (76Torr) metalorganic vapor phase epitaxy (MOVPE).The layer structures consisting of GaN, AlxGa1−xN, InxGa1−xN or InxAlyGa1−x−yN were grown on a several-hundred-nm-thick AlxGa1−xN (x = 0.1–0.5) buffer layer which was grown on SiC under optimized growth conditions in order to achieve a flat surface suitable for the WQ layer growth and to reduce the threading dislocation density (TDD).As precursors, ammonia (NH3), trimethylaluminum (TMAl), trimethylgallium (TMGa), trimethlindium di-i-propyalamine adduct (TMI-adduct), tetraethylsilane (TESi), and bisethylcyclopentadienylmagnesium (BECp2Mg) were used with H2 and N2 carrier gas.The typical growth temperatures of GaN, AlxGa1−xN, InxGa1−xN, and InxAlyGa1−x−yN are 1000–1100 ◦C, 1050–1250 ◦C, 650–800 ◦C, and 800–870 ◦C, respectively. The typical growth rates of GaN, AlxGa1−xN, InxGa1−xN, and InxAlyGa1−x−yN are 2.4 μm/hour, 0.5– 2.4 μm/hour, 0.1 μm/hour and 0.12 μm/hour, respectively. First, we show the optical properties of AlGaN.Figure 2 shows the PL spectra of AlxGa1−xN films over the entire Al compositional range, i.e., from GaN to AlN, emitting from near the band edge measured at 77 K.The AlGaN alloy was grown directly on a very thin (∼5 nm) AlN layer deposited on SiC.The thickness of AlGaN film was approximately 250 and 400nm for AlN and Al0.11Ga0.89N, respectively.As seen, single-peak spectra were obtained for the entire Al compositional range emitting from near the band edge.2) We obtained the first PL emission of AlN and record the shortest wavelength (208 nm) PL of a semiconductor.The deep level yellow emission at approximately 500–550nm was negligible even for high-Al-content AlGaN, indicating the good crystal quality of AlGaN. Then, we fabricated several series of AlGaN MQW samples, consisting of various Al-content AlGaN barriers.Figure 3 shows the schematic layer structure and the PL spectra for various QW thicknesses of fabricated AlN/Al0.18Ga0.82N MQW samples.In order to achieve a flat surface suitable for the growth of AlGaN quantum wells, an approximately 250– 400 nm-thick AlN buffer layer was deposited.W e confirmed a step-flow grown surface by atomic force microscopy (AFM). The PL measurement was performed with excitation by a Xelamp light source (215 nm) measured at 77K.W e obtained single-peak intense PL emission from each MQW.The most efficient emission was obtained at a wavelength of 234 nm. This is the shortest intense UV emission of a semiconductor QW.The optimum value of well thickness was approximately 1.6 nm. The PL intensity of the MQWs was several tens of times higher than that of bulk AlGaN.The quantized level shift was clearly observed.A rapid reduction in the PL intensity with an increase in the well thickness was caused by a reduction in the radiative recombination probability due to a large piezoelectric field in the well.
The PL intensities were compared among AlN/AlGaN, Al- GaN/GaN and InGaN/InGaN MQWs under the same measurement conditions, as shown in Fig.4. We found that the PL intensity of 230 nm-band emission from AlGaNbased QWs was as strong as that of 420 nm-band emission from commercially available InGaN-based QWs, and much stronger than that from GaN-based QWs at 77K.Ho wever, at room temperature, the emissions from AlGaN and GaN QWs were much weaker than that from InGaN QWs.Th us, the next purpose was to obtain efficient UV emission at room temperature.
Structures were grown at 76 Torr on the Si-face of an on-axis 6H-SiC(0001) substrate by low-pressure (76Torr) metalorganic vapor phase epitaxy (MOVPE).The layer structures consisting of GaN, AlxGa1−xN, InxGa1−xN or InxAlyGa1−x−yN were grown on a several-hundred-nm-thick AlxGa1−xN (x = 0.1–0.5) buffer layer which was grown on SiC under optimized growth conditions in order to achieve a flat surface suitable for the WQ layer growth and to reduce the threading dislocation density (TDD).As precursors, ammonia (NH3), trimethylaluminum (TMAl), trimethylgallium (TMGa), trimethlindium di-i-propyalamine adduct (TMI-adduct), tetraethylsilane (TESi), and bisethylcyclopentadienylmagnesium (BECp2Mg) were used with H2 and N2 carrier gas.The typical growth temperatures of GaN, AlxGa1−xN, InxGa1−xN, and InxAlyGa1−x−yN are 1000–1100 ◦C, 1050–1250 ◦C, 650–800 ◦C, and 800–870 ◦C, respectively. The typical growth rates of GaN, AlxGa1−xN, InxGa1−xN, and InxAlyGa1−x−yN are 2.4 μm/hour, 0.5– 2.4 μm/hour, 0.1 μm/hour and 0.12 μm/hour, respectively. First, we show the optical properties of AlGaN.Figure 2 shows the PL spectra of AlxGa1−xN films over the entire Al compositional range, i.e., from GaN to AlN, emitting from near the band edge measured at 77 K.The AlGaN alloy was grown directly on a very thin (∼5 nm) AlN layer deposited on SiC.The thickness of AlGaN film was approximately 250 and 400nm for AlN and Al0.11Ga0.89N, respectively.As seen, single-peak spectra were obtained for the entire Al compositional range emitting from near the band edge.2) We obtained the first PL emission of AlN and record the shortest wavelength (208 nm) PL of a semiconductor.The deep level yellow emission at approximately 500–550nm was negligible even for high-Al-content AlGaN, indicating the good crystal quality of AlGaN. Then, we fabricated several series of AlGaN MQW samples, consisting of various Al-content AlGaN barriers.Figure 3 shows the schematic layer structure and the PL spectra for various QW thicknesses of fabricated AlN/Al0.18Ga0.82N MQW samples.In order to achieve a flat surface suitable for the growth of AlGaN quantum wells, an approximately 250– 400 nm-thick AlN buffer layer was deposited.W e confirmed a step-flow grown surface by atomic force microscopy (AFM). The PL measurement was performed with excitation by a Xelamp light source (215 nm) measured at 77K.W e obtained single-peak intense PL emission from each MQW.The most efficient emission was obtained at a wavelength of 234 nm. This is the shortest intense UV emission of a semiconductor QW.The optimum value of well thickness was approximately 1.6 nm. The PL intensity of the MQWs was several tens of times higher than that of bulk AlGaN.The quantized level shift was clearly observed.A rapid reduction in the PL intensity with an increase in the well thickness was caused by a reduction in the radiative recombination probability due to a large piezoelectric field in the well.
The PL intensities were compared among AlN/AlGaN, Al- GaN/GaN and InGaN/InGaN MQWs under the same measurement conditions, as shown in Fig.4. We found that the PL intensity of 230 nm-band emission from AlGaNbased QWs was as strong as that of 420 nm-band emission from commercially available InGaN-based QWs, and much stronger than that from GaN-based QWs at 77K.Ho wever, at room temperature, the emissions from AlGaN and GaN QWs were much weaker than that from InGaN QWs.Th us, the next purpose was to obtain efficient UV emission at room temperature.
Conclusions
We demonstrated intense UV emission at 230–350nm from QW structures consisting of III-nitride semiconductors grown by MOVPE.W e obtained 230 nm-band intense UV emission from AlN/AlxGa1−xN QWs at 77 K.The emission efficiency of AlGaN-based QW was as high as that of blue LEDs at a low temperature, however, at R.T., it was not significantly high.Then, we introduced In into AlGaN to improve R.T. emission efficiency by the radiative recombination of the localized electron-hole pair in In segregation regions.The efficiency of R.T. UV emission was dramatically enhanced by introducing several percent of In into AlGaN.W e obtained 320 nm-band R.T. intense emission from InAlGaN/InAlGaN
QWs.The emission intensity of the InAlGaN-based QW was as strong as that of commercially available InGaN QWs at R.T. We also achieved p-type doping into wide-bandgap Al- GaN using several methods and accomplished the first successful operation of a 330 nm-band LED.F rom these results, it was shown that the InAlGaN quaternary was very promising for use as active layers of 300–350 nm-band LDs or LE.
Bárbara Scarlett Betancourt Morales
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