Super widegap nitride semiconductors for UV lasers

Transcription

1 (Registration number: 2001MB047) Super widegap nitride semiconductors for UV lasers Research Coordinator Fernando A. Ponce Research Team Members Hiroshi Amano David Cherns Isamu Akasaki Arizona State University: U.S.A. Meijo University: JAPAN University of Bristol: United Kingdom Meijo University: JAPAN DurationApril, 2001 March, 2004 Abstract This report summarizes the result of the program done by Meijo-ASU-Bristol team. A team of Meijo-ASU-Bristol has achieved tremendous improvement of the understanding and to control optical and electrical properties of AlGaN and AlGa(In)N alloys, exhibiting each ability for growth in Meijo University and characterization in Arizona State University and University of Bristol of these new semiconductors. One of the most tremendous results achieved in this program is to achieve world s shortest wavelength laser diode on the sapphire substrate. The results obtained by this team will surely lead to the quick development of the semiconductor in the UV region by group III nitrides, which had been already realized in the visible short wavelength region. Keywords: AlGaN, UV, Laser diode, Dislocation, Facet controlled epitaxial growth 1. Introduction There exists much current interest in the development of high efficiency light emitting devices emitting in the ultraviolet range of the spectrum. One main objective is the realization of white light sources based on near UV emitters coupled with phosphors. Another important objective is the production of UV emitters for various applications, such as UV for medical applications, biological sensors, and light source for high-density optical storage systems. There are several challenges that one encounters when attempting to grow device structures that are totally transparent to the peak emission wavelength. It is difficult to grow thick epilayers with high aluminum content necessary for the fabrication of efficient UV LEDs and LDs. At the present time, only violet and blue laser diodes based on GaInN quantum wells have been achieved with long-life times. LDs in the UV and VUV region had not yet been successfully manufactured, even in the pulsed mode. The wavelength of efficient UV-LEDs achieved to

2 date is longer than 363 nm. The main source of the problem lies in the growth of specular AlGaN. At the microscopic scale, it has been observed that the microstructure becomes smaller with increasing AlN molar fraction. Threading dislocations and point defects act as strong non-radiative recombination centers, which significantly reduced the UV light emitting efficiencies. Theoretical investigations also suggest that donor and acceptor impurity levels becomes deeper and deeper with increasing AlN molar fraction, making it very difficult to achieve highly conductive n-type and p-type AlGaN films. The combination of these factors creates a large barrier for the fabrication of UV lasers based on these materials. In this report we present recent results from collaborative work sponsored by NEDO, on the impact of microstructure on the electrical and optical properties of AlGaN epilayers. Our research has centered on four important issues: (a) Effect of the substrate on the growth of AlGaN; (b) lateral overgrowth of AlGaN alloys; (c) the detailed nature of extended defects such as dislocations; and (d) microstructural effects of doping of AlGaN. Finally we have demonstrated the shortest wavelength LDs ever achieved on the sapphire substrate. 2. Experimental All the samples were grown by organometallic vapor phase epitaxial method at Meijo University, while their microstructures were observed at Arizona State University and University of Bristol. Details of the experimental conditions were given at the next section. 3. Results and Discussions 3.1 New substrate GaN and sapphire have large differences in terms of lattice constants and thermal expansion coefficients, and epitaxial layers of GaN on sapphire have high-densities of dislocations and typically exhibit large residual stresses. Nevertheless, the epitaxy of GaN layers seems to benefit from the existence of high dislocation densities. For high efficiency UV LEDs, however, the materials used in the epitaxial device structure should be transparent to the emitted light. This means that GaN base layer should be avoided. High quality AlGaN layers grown directly on sapphire have been difficult to achieve. Dislocations appear to behave differently in the presence of significant concentrations of aluminum. For this reason, it will be important to develop alternative substrates that are well matched to the base AlGaN layers. ZrB 2 has been found to be a suitable candidate for the growth of AlGaN. It has a primitive hexagonal lattice structure (AlB 2 -type). The lattice constants are a=3.170 Å and c=3.530 Å respectively, indicating only 0.6% mismatch with GaN. Their thermal expansion coefficients on the basal plane are also comparable (5.9 vs. 5.6x10-6 K -1 ). ZrB 2 is perfectly lattice matched to Al x Ga 1-x N with x=26%, suggesting that the substrate has a potential application in high-performance UV-LEDs. High quality GaN grown on ZrB 2 has been grown by metal-organic vapor phase epitaxy [1, 2]. A low temperature AlN nucleation layer was used for achieving two-dimensional growth.

3 Figure 1 shows a lattice image at the interface of AlN/ZrB 2, taken using a 400kV microscope with a point resolution around 1.7 Å. The lattice image is projected along [1120] zone axis of AlN or ZrB 2. The image shows only a part of the substrate and of the AlN nucleation layer. The main GaN layer was grown over the AlN layer. An unintentionally grown intermediate layer is evident at the interface as indicated by two white bars on the left of Fig. 1. This 2 nm thick intermediate layer has been identified as a cubic phase, observed here on the <101> projection. The lattice constant of the cubic phase layer is estimated to be a= Å=4.6 Å. The image contrast of the cubic layer is close to the substrate, suggesting that it may result from phase transformation of the hexagon-structure substrate. The phase transformation can be triggered by the diffusion of nitrogen into the substrate, transforming ZrB 2 into the cubic phase Zr x B y N z. The lattice constant of Zr x B y N z is in the range of 4.58 Å ~ 4.65 Å, a good match with the observed value. In high quality samples, the insertion of this cubic phase layer does not disturb the lattice coherency at the interface. To the right of Fig. 1, the continuity of atomic planes across the interface is indicated by a vertical white line. Counting the vertical planes of AlN and ZrB 2 reveals that misfit dislocations are created at the interface to relax the -2% lattice mismatch. AlN [0001] AlN 54.7 o 70.5 o 2.65Å Zr x N y B z 2.74Å ZrB 2 Fig. 1. Lattice image at the AlN/ZrB 2 interface showing an intermediate cubic phase layer with its {111} planes coherently matched to the {1100} of AlN and ZrB Epitaxial Lateral Overgrowth of AlGaN Epitaxial lateral overgrowth (ELO) is often used to reduce the defect density in GaN. In traditional ELO, an initial layer of GaN is first grown. SiO 2 masks of various geometries are used, followed by more GaN growth. The interruption of growth is not convenient. Direct growth directly onto a striped sapphire offers an interesting alternative. This method is especially beneficial in the case of AlGaN, which suffers from polycrystalline deposits on ELO masks. One such growth is shown in Fig. 2 where an Al 0.03 Ga 0.97 N layer has been grown on a striped sapphire substrate [3].

4 Fig. 2. Cross-section TEM image in tandem with a schematic of the AlGaN stripes grown on patterned sapphire. The layer is 6 µm thick and contains a Mg concentration of 9x10 19 cm -3. The geometry of the specimen is shown in cross-section in a diagram in Fig. 2, together with a cross-section TEM image. The cross-section TEM in Fig. 3 shows two distinct regions in the specimen. The region to the left of the dashed line is grown directly on the sapphire substrate in the [0001] direction and the region to the right of the dashed line grows laterally and (b) 70 nm (a) 1µ m Fig. 3. (a) Cross-section TEM image showing the defect density reduction in the overgrowth region. The boundary between the high and low defect region is delineated. (b) High magnification of region in (a) showing detail of pyramidal defects. Dislocations and pyramidal defects are not observed in the overgrowth region. overhangs the trenches in the substrate. The material grown directly on the sapphire substrate has a dislocation density of cm -2, and is decorated with pyramidal defects with a density of cm -3. The laterally grown material is defect free and has no pyramidal defects. Cathodoluminescence (CL) spectra were taken in cross-section over a few periods of the ELO structure at 5K. The CL spectra in Fig. 4 exhibits near-band-edge emission at 354 nm and a magnesium related donor-acceptor pair band at ~380 nm. The spatial variation of the luminescence was obtained from monochromatic CL images at

5 354 nm and 380 nm, as shown in figures 5b and c respectively. The intensity of the near-band-edge emission is much stronger in the laterally grown regions. Comparing the near-band-edge CL image to the TEM image, we observe that the defect free, laterally grown region, correlates well with the region of high emission intensity. This suggests that the highly defective region has a large density of non-radiative recombination centers that affect the near-band-edge emission. The donor-acceptor-pair band emission is observed in both the [0001] and laterally grown regions, suggesting that the Mg-acceptor is present in both regions. CL Intensity (counts) 354 nm ~380 nm (a) Fig. 5. Light emission observed in cross-section by cathodoluminescence. (a) Schematic of sample in cross-section. Monochromatic CL images taken at (b) 354 nm and (c) 380 nm show the spatial variation of the near-band-edge and the donor-acceptor-pair emissions, respectively Wavelength (nm) Fig. 4. CL spectrum taken at 5K from a cross-section specimen. The main peaks consist of near-band-edge emission (354 nm) and donor-acceptor-pair emission (380 nm). (b) (c) More detailed TEM studies reveal that, in the seed region, threading dislocations of edge and mixed type are highly decorated with pyramidal defects, whereas regions within 50nm of these dislocations are relatively free of defects [3, 4]. This implies segregation of Mg to the dislocation cores, a result now confirmed directly by electron energy loss spectroscopy (EELS) [4]. High-resolution studies of the edge and mixed dislocations, in a near end-on geometry, show that the dislocations have open cores, i.e. are nanopipes. Fig. 6 shows an example where the core diameter is around 1nm, the Burgers circuit confirming the presence of an edge component of the Burgers vector. The occurrence of open core edge and mixed dislocations in Mg-doped materials is in contrast to our previous observations on undoped and Si-doped material, where nanopipes have been identified as open core screw dislocations [5].

6 Fig.6: (0001) lattice image of an end-on dislocation in a plan view sample of Mg-doped Al 0.03 Ga 0.97 N c.f. Figs 1-4. The Burgers circuit reveals that this hollow core dislocation has an edge component of the Burgers vector. 3.3 Minimization of dislocations in GaN/AlGaN layers It is becoming clear that for UV LEDs and detectors defects in the semiconductors play a negative role. This has been thought to be related to the absence of compositional inhomogeneities typical of the InGaN alloy system. Therefore, minimization of dislocations in epilayers is an important issue for this set of materials. There have been several attempts at minimizing the dislocation density. Epitaxial lateral overgrowth has been attempted using many types of geometrical configurations [6]. The introduction of intermediate AlN layers have shown to be effective in reducing cracks and screw dislocations [7]. Recent studies have shown that GaN/AlGaN layers provide an effective means for the reduction of edge dislocations. The compressive interfacial stress originating from the different lattice parameter between GaN and AlGaN produces a force that leads to the formation of edge dislocation dipoles. The dipoles terminate at the interface and thus the subsequent layer has a substantially lower edge dislocation density [8]. The experiment was performed on GaN layers of differing thickness grown on thick Al 0.28 Ga 0.72 N on sapphire. X-ray diffraction measurements indicated that the residual compressive strain in the GaN layers decreased from 5.6x10-6 to 2.3x10-6 when the GaN layer thickness was increased from 40nm to 1130nm. Observations by transmission electron microscopy indicated that the density of threading dislocations is reduced with increasing GaN layer thickness. The plan-view TEM image in Fig. 6 shows that the dislocations are closely aligned with [0001] in the AlGaN layer, but that some migrate laterally, often traveling along the AlGaN/GaN interface until annihilating with a dislocation of opposite Burgers vector to form a dipole. The dipole formation is not limited to the AlGaN/GaN interface, but also occurs in the upper GaN layer. The reduction in defect density with increasing GaN layer thickness is attributed to the compressive strain relaxation in the GaN layer with increasing layer thickness. The method of relaxation of the compressive strain with layer thickness is attributed to lateral migration of threading edge-type dislocations and their reaction to form dipole loops terminating at the hetero-interface or in its vicinity. A similar mechanism for dislocation annihilation, this time of screw dislocations, has been observed to occur by silicon delta-doping. Screw dislocations terminating at the growth surface are associated with an atomic step ledge that winds itself around the dislocation as the film grows. During growth interruption, the ledge minimizes its length, and in the presence of other dislocations with opposite Burgers vector, it can form dislocation dipole

7 loops. A burst of silicon (delta-doping) creates silicon atoms that diffuse on the surface and decorate the ledges. Once the growth is resumed, the decorated ledges are pinned, and the dislocation loop is buried, with a net result of a significant reduction in the density of screw dislocations [9]. Fig.7: Plan view TEM micrograph of a 90 nm thick GaN layer on AlGaN. The sample is tilted so that threading dislocations aligned along the c-axis have traces that run left to right. The image reveals changes in line direction as dislocations turn over at the GaN/AlGaN interface, sometimes forming dipoles which reduce the threading dislocation density. 0.1 µm 3.4 Fabrication of UV LD [10] Figure 8 shows the device p-gan structure of UV-LD with a SiO 2 p-al 0.18 Ga 0.82 N cladding layer separated confinement p-al 0.25 Ga 0.75 N blocking layer heterostructure of a GaN/AlGaN MQW active layer grown on the i-al 0.08 Ga 0.92 N guide layer GaN/Al 0.08 Ga 0.92 N MQW low-dislocation-density AlGaN. i-al 0.08 Ga 0.92 N guide layer n-al 0.18 Ga 0.82 N contact layer All nitride layers in this device were epitaxially grown on a n-al 0.18 Ga 0.82 N cladding GaN sapphire substrate by Sapphire substrate LT-AlN interlayer LT-buffer layer organometallic vapor phase Fig.8 Schematic structure of UV-LD grown on the epitaxy. After depositing the low-dislocationdensity AlGaN. LT-buffer layer with the thickness of about 20 nm at 500, 3 µm-thick GaN was grown at 1,100. Grooves along the < 1100 > direction were formed by conventional photolithography and Cl 2 reactive ion etching (RIE). The width, spacing and depth of the grooves were 20 µm, 10µm and 1.5 µm, respectively. Then LT-AlN with the thickness of 20 nm, which is effective for suppressing crack generation, was deposited at 500on the GaN surface with the periodic grooves. A 4 µm-thick AlGaN layer was grown at 1,100. Combining the LT-AlN interlayer technique and heteroepitaxial ELO yielded the crack-free, low-threading-dislocation AlGaN on the grooves. The active region of the UV-LD was aligned on this low-threading-dislocation-density Al 0.18 Ga 0.82 N layer. The thickness of both the unintentionally doped Al 0.18 Ga 0.82 N layer and the Si-doped n-al 0.18 Ga 0.82 N contact layer with electron

8 3000 concentration of 2 x10 18 cm was 4 RT, Pulse Duty 0.1% 2500 current injection: 200mA µm each. Then, an unintentionally doped Al 0.08 Ga 0.92 N guide layer 2000 (120 nm), three pairs of GaN (3 nm)/al 0.08 Ga 0.92 N:Si (8 nm) MQW 1500 active layer, an unintentionally 1000 doped Al 0.08 Ga 0.92 N guide layer (120 nm), a p-type Al 0.25 Ga 0.75 N ( nm) blocking layer, a p-al 0.18 Ga 0.82 N (700 nm) cladding Wavelength[nm] layer and a p-gan (20 nm) contact layer were successively stacked. Fig.9 Electroluminescence spectrum at room temperature under the pulse current condition of 200 ma. The ridge waveguide structure was formed by RIE. The laser cavity mirrors were formed by cleaving. The width of the ridge and a cavity length were 5.5 µm and 500 µm, respectively. A Ni/Pt/Au was evaporated on to the p-gan contact layer, while a Ti/Al was deposited on to the n- Al 0.18 Ga 0.82 N contact layer. Electrical and optical characterizations were performed at room temperature under the pulsed conditions with 1 µs width and 1 khz repetition which correspond to the duty ratio of 0.1%. The operating voltage at the injection current of 200mA is 10.4 V. The reason for this high operational voltage was originated from the high resistivity of the p-al 0.18 Ga 0.82 N cladding layer. Figure 9 shows the electroluminescence spectra of an UV-LD with an injection currents of 200 ma. The peak spontaneous emission at nm with full-width at half maximum of 6.0 nm was observed when the injection current was 100 ma. Upon increasing the injection current up to 200 ma, a strong and sharp lasing spectrum distinctly appeared at the wavelength of nm. The corresponding current density at 200mA is 7.3 ka/cm 2. The lasing wavelength of nm is, to our knowledge, the shortest ever reported for nitride-based LDs. Intensity[a.u.] 4. Summary Microstructure of AlGaN grown by organometallic vapor phase epitaxy was studied in detail. Epitaxial lateral overgrowth was found to be effective in obtaining low dislocation density AlGaN. Effect of Si doping and Mg doping on the microstructure of AlGaN was also characterized. Over doping of Mg was found to form inversion domains with inverted triangular form, by which electrical properties were degraded. Based on these findings, we have fabricated UV-LD having GaN/AlGaN MQW on thick, crack-free, low-dislocation density AlGaN grown by the combination of the HELO and LTinterlayer. The lasing wavelength was nm under pulsed current injection at room temperature, which is the shortest wavelength ever reported. We also demonstrated ZrB 2 as a brand-new candidate for the growth of AlGaN. Careful preparation of the surface before the growth leads to the high quality and low dislocation density AlGaN on this brand-new substrate.

9 Acknowledgement The member of Meijo-ASU-Bristol Nitride team (Fernando Ponce, Hiroshi Amano, David Cherns, Isamu Akasaki) greatly acknowledge NEDO for their financial support. References [1] S. Kamiyama, S. Takanami, Y. Tomida, K. Iida, T. Kawashima, S. Fukui, M. Iwaya, H. Kinoshita, T. Matsuda, T. Yasuda, H. Amano, I. Akasaki, Phys. Stat. Sol. (a) 200, 67(2003). [2] R. Liu, A. Bell, F. A. Ponce, S. Kamiyama, H. Amano, and I. Akasaki. Appl. Phys. Let. 81, 3182 (2002). [3] A. Bell, R. Liu, F. A. Ponce, H. Amano, I. Akasaki, D. Cherns. Appl. Phys. Let. 82, 349 (2003). [4] D. Cherns, M. Q. Baines, Y. Q. Wang, F. A. Ponce, H. Amano, and I. Akasaki. Phys. Stat. Sol. 234, 850 (2002). [5] D. Cherns, Y. Q. Wang, R. Liu, and F. A. Ponce. Appl. Phys. Let. 81, 4541 (2002). [6] K. Hiramatsu, J. Phys.: Condes. Matter 13, 6961 (2001). [7] H. Amano and I. Akasaki, J. Cryst. Growth 223, 83 (2001). [8] S. L. Sahonta, M. Q. Baines, D. Cherns, H. Amano, and F. A. Ponce. Phys. Stat. Sol. B 234, 952 (2002). [9] O. Contreras, F. A. Ponce, J. Christen, A. Dadgar, and A. Krost. Appl. Phys. Lett. 81, 4712 (2002). [10] K.Iida, T. Kawashima, A. Miyazaki, H. Kasugai, S. Mishima, A. Honshio, Y. Miyake, M. Iwaya, S. Kamiyama, H. Amano, I. Akasaki, Jpn. J. Appl. Phys., 43, L499 (2004). The list of the most important papers and patents from the project Papers [1] R. Liu, A. Bell, F. A. Ponce, S. Kamiyama, H. Amano, I. Akasaki, Atomic arrangement at the AlN/ZrB 2 interface, Appl. Phys. Lett. 81, 3182 (2002). [2] H. Amano, A. Miyazaki, K. Iida, T. Kawashima, M. Iwaya, S. Kamiyama, I. Akasaki, R. Liu, A. Bell, F. A.. Ponce, Defect and stress control of AlGaN for fabrication of high performance UV light emitters, Phys. Stat. Sol. (a), 201, 2679 (2004). [3] D. Cherns, S.-L. Sahonta, R. Liu, F. A. Ponce, H. Amano and I. Akasaki, The generation of misfit dislocations in facet-controlled growth of AlGaN/GaN films, Appl. Phys. Lett., 85, **** (2004). (in print) Presentations [1] F. A. Ponce, Microstructural issues in UV light emitting nitride semiconductors, First Asia-Pacific Workshop on Widegap Semiconductors (APWS-2003), Awaji island, Japan (2003). [2] H. Amano, Critical issues for achieving high efficiency/high power nitride-based UV devices, International Symposium on Blue Lasers and Light Emitting Diodes 2004, Gyounjyu, Korea (2004). [3] D. Cherns, TEM, CL and electron holography of dislocations in GaN, Int Congress on electron Microscopy, Durban, South Africa (2002).