JOURNAL OF APPLIED PHYSICS 101, 123103 共2007兲
Ultraviolet semiconductor laser diodes on bulk AlN Michael Kneissl,a兲 Zhihong Yang, Mark Teepe, Cliff Knollenberg, Oliver Schmidt, Peter Kiesel, and Noble M. Johnson Palo Alto Research Center, Incorporated, 3333 Coyote Hill Road, Palo Alto, California 94304
Sandra Schujman and Leo J. Schowalter Crystal IS, Incorporated, 70 Cohoes Avenue, Green Island, New York 12183
共Received 23 March 2007; accepted 30 April 2007; published online 20 June 2007兲 Current-injection ultraviolet lasers are demonstrated on low-dislocation-density bulk AlN substrates. The AlGaInN heterostructures were grown by metalorganic chemical vapor deposition. Requisite smooth surface morphologies were obtained by growing on near-c-plane AlN substrates, with a nominal off-axis orientation of less than 0.5°. Lasing was obtained from gain-guided laser diodes with uncoated facets and cavity lengths ranging from 200 to 1500 m. Threshold current densities as low as 13 kA/ cm2 were achieved for laser emission wavelengths as short as 368 nm, under pulsed operation. The maximum light output power was near 300 mW with a differential quantum efficiency of 6.7%. This 共first兲 demonstration of nitride laser diodes on bulk AlN substrates suggests the feasibility of using such substrates to realize nitride laser diodes emitting from the near to deep ultraviolet spectral regions. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2747546兴 I. INTRODUCTION
Semiconductor optoelectronic devices emitting in the ultraviolet 共UV兲 are of considerable interest for a number of applications that include water purification, compact bioagent detection systems,1 analytical devices for the biotechnology and medical fields, currency screening, UV curing, and rapid prototyping. The AlGaInN semiconductor materials system can access the entire desirable range of the ultraviolet spectrum. Substantial progress has been realized in the development of nitride-based UV light emitting diodes 共LEDs兲, with LEDs demonstrated throughout this range2–10 down to a wavelength of 210 nm.11 Despite findings from computational simulations indicating the absence of any fundamental barrier,12,13 the promise of UV nitride laser diodes has been largely unfulfilled. Several groups have reported near-UV laser diodes with InGaN, AlGaN and InAlGaN quantum-well active regions grown on sapphire or GaN substrates.14–20 Although progress has been made, the performance of GaNbased UV emitters is still inferior to their longer-wavelength counterparts, which points to the need for more suitable substrates that can access even shorter wavelengths. The problem with conventional GaN/ sapphire or bulk GaN is the lattice mismatch between AlGaN alloys necessary for UV emitters and the GaN-based templates. AlGaN layers grown on GaN are under tensile strain which results in the formation of microscopic cracks in the epitaxial layers at higher aluminum compositions or layer thicknesses. AlN is a nearideal substrate for nitride UV emitters, becoming an increasingly ideal choice as the emission is progressively shifted deeper into the UV through incorporation of higher Al compositions in the heterostructure. This article describes the dea兲
Present address: Institute for Solid State Physics, Technical University of Berlin, D-10623 Berlin, Germany. Electronic mail:
[email protected]
0021-8979/2007/101共12兲/123103/5/$23.00
sign, growth, fabrication, and operation of near-UV laser diodes on bulk AlN substrates, as the first step toward fulfilling the promise of deep-UV laser diodes. II. GROWTH AND PREPARATION OF SINGLE CRYSTAL BULK AlN SUBSTRATES
Bulk single crystal boules of AlN were grown by the sublimation-recondensation method.21–24 The bulk crystals were then oriented with respect to the c axis, sliced, and the substrates polished. The chemical-mechanical polishing process on the Al-polarity face of the polar substrates results in typical surface roughness in the order of 3 to 5 Å for a 10 m ⫻ 10 m area.25 In Fig. 1共a兲, the atomic force microscope scan of a 5 m ⫻ 5 m area is shown, in which atomic level steps can be clearly observed. The typical thickness of the AlN wafers after polishing was 400 m. X-ray diffraction rocking curves measured on bulk AlN wafers show a full width at half maximum 共FWHM兲 of less than 50 arcsec for the 共006兲 reflection. Detailed characterization of substrates used in the development of the laser devices described in this paper can be found in Refs. 22, 23, and 26. Prior to metalorganic chemical vapor deposition 共MOCVD兲 growth, the 共0001兲 AlN substrates were by cleaned using a sequence of wet chemical processes and dry etching. The bulk AlN substrates were first degreased in acetone and isopropanol. Subsequently the AlN wafers were dry etched with argon-ion sputtering, followed by wet chemical etching in a solution of H3PO4 : H2SO4 : H2O 共1 : 1 : 1兲. After the cleaning procedure the InAlGaN laser heterostructures were grown by MOCVD.27 The substrate temperatures during growth were 1100 °C for the GaN and AlGaN layers and 800 °C for the InGaN and InAlGaN layers. As organometallic precursors for the MOCVD growth TMGa 共trimethylgallium兲, TMAl 共trimethylaluminum兲, and TMIn 共trimethylindium兲 were used for the Group III elements and NH3 共ammonia兲 was used as the nitrogen source. Hydrogen and nitrogen were used as
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FIG. 1. 共Color online兲 共a兲 AFM image of the polished Al-face surface of a near on-axis 共0001兲 bulk AlN substrate. Atomic level steps are clearly seen in the 5 m ⫻ 5 m scan. 共b兲 AFM image of an AlN epilayer grown on near on-axis 共0001兲 AlN substrate. 共c兲 AFM images of an AlGaN layer grown on off-axis 共0001兲 AlN substrate, and 共d兲 nearly on-axis 共0001兲 AlN substrate.
carrier gases for the metalorganic sources. For the n-doping, 100 ppm SiH4 diluted in H2 was used, and Cp2Mg 共cyclopentadienylmagnesium兲 was used for p-doping. Growth of the laser heterostructure was initiated with the deposition of a 100-nm-thick AlN base layer. Figure 1共b兲 depicts an atomic force microscopy 共AFM兲 image of an AlN epi-layer grown on bulk AlN. It illustrates our general observation that the base layers have very smooth surfaces with step-flow growth and no obvious formation of dislocations. These findings were also confirmed with cross-sectional transmission electron microscopy. However, a fundamental problem that confronted our initial attempts to realize a laser diode on bulk AlN was the tendency for AlGaN films grown on nominally 共0001兲 AlN substrates to manifest step bunching for vicinal growth surfaces inclined 1°–4° off of the basal plane. This is illustrated in Fig. 1共c兲 with an AFM image of the AlGaN surface for a 1 m layer grown on nominally 共0001兲 AlN, with an uncertainty in the orientation of the substrate of ±4°. The figure reveals vicinal surfaces inclined at an angle of ⬃1°, step heights of 20–50 nm, and a root mean square 共rms兲 surface roughness of 9 nm. Between steps the surface is very smooth. As illustrated in Fig. 1共d兲, significantly improved surface morphology, with no step bunching, was obtained from near-c-plane AlN substrates, specifically, for off-axis 共mis-兲orientation of less than 0.5°. The AFM image also reveals a pit density of ⬃2 ⫻ 108 cm−2.
III. MOCVD GROWTH AND FABRICATION OF THE UV LASER HETEROSTRUCTURE
For growth of the UV laser heterostructures, a 1-m-thick undoped Al0.12Ga0.88N transition layer was deposited directly on the AlN base layer. The purpose of the transition layer is to release some of the strain originating from the large lattice mismatch between the AlGaN heterostructure and bulk AlN substrate. In the present structure the transition layer was grown with an abrupt interface and no composition gradient was used. As a consequence of the strain relaxation process during growth of the transition layer, an increased number of threading dislocations is observed in the AlGaN film, and these dislocations propagate through the laser heterostructure. Cross-sectional transmission electron microscopy micrograph images revealed threading dislocation densities in the mid-108 cm−2 range. Although this increase in dislocation density is not ideal, the materials quality was sufficient to allow laser operation. Further improvements in dislocation densities may be realized by optimizing the growth strategy for the transition layer, e.g., by utilizing a graded AlGaN interfaces or multiple AlGaN layers with stepped alloy compositions. Over the transition layer a 1-m-thick Si-doped Al0.12Ga0.88N currentspreading layer was deposited. This was followed by a Sidoped AlGaN/ AlGaN short-period-superlattice cladding layer 共with a total thickness of 500 nm and an average Al
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FIG. 2. 共Color online兲 共a兲 Schematic diagram of an InAlGaN heterostructure for a UV laser diode grown on a bulk AlN substrate. 共b兲 Photograph of fully processed UV laser diodes fabricated on a 1-cm-diameter wafer of bulk AlN. The wafer contains several hundred gain-guided laser diodes.
composition of 12%兲 and a 100-nm-thick Si-doped In0.01Al0.05Ga0.94N waveguide layer. Next, the active region was grown: this was comprised of five 30-Å-thick In0.01Ga0.99N quantum wells separated by 60 Å wide In0.01Al0.16Ga0.83N barriers. The laser structure was completed with the growth of a 20-nm-thick Mg-doped Al0.25Ga0.75N electron confinement layer, a 100-nm-thick Mg-doped Al0.05Ga0.95N waveguide layer, a 500-nm-thick Mg-doped AlGaN/ AlGaN short-period-superlattice cladding layer 共with an average Al composition of 12%兲, and finally a 20-nm-thick Mg-doped GaN contact layer. After completion of the MOCVD growth, gain-guided laser diode test structures were fabricated. The laser diode mirrors and mesas were formed by chemically assisted ion beam etching with cavity lengths ranging from 200 to 1500 m.28 Ti/ Al/ Ti/ Au metal contacts were then deposited on the exposed n-type AlGaN current spreading layer, for the lateral electrical connection, and Pd/ Ti/ Au metal contacts on the p-type GaN contact layer. The stripe width of the p-metal electrode ranged between 10 and 30 m. No facet coatings were applied to the laser mirrors. Figure 2共a兲 shows a schematic of the laser diode heterostructure. A photograph of fully processed UV laser diodes fabricated on a 1-cm-diameter wafer of bulk AlN is shown in Fig. 2共b兲. The wafer contains several hundred gain-guided laser diodes.
IV. CURRENT-INJECTION UV LASER DIODES ON AlN SUBSTRATES
Figure 3共a兲 shows the room-temperature spontaneous emission spectrum of a broad-area stripe laser operating below threshold. Operating simply as LEDs the devices nevertheless show very narrow emission spectra that are centered at 370 nm with a FWHM of only 6 nm; this indicates a homogenous quantum well composition and very little well width fluctuations. The transverse electric 共TE兲 and transverse magnetic 共TM兲 emission spectra from a 30⫻ 50 m2 laser diode operating above threshold is shown in Fig. 3共b兲, with the emission wavelength centered at 368.4 nm. We also determined that above threshold the laser light is strongly TE polarized with a TE/ TM polarization ratio greater than 150. Pulsed threshold current densities as low as 11.2 kA/ cm2 共pulse duration 100 ns, repetition frequency 3 kHz兲 were achieved for laser diodes emitting in the wavelength range between 368 and 373 nm, with cavity lengths ranging from 200 to 1500 m and uncoated mirror facets. A highresolution laser emission spectrum recorded slightly above threshold for a short-cavity device is shown in Fig. 4共a兲. It clearly resolves the multiple longitudinal Fabry-Perot modes in the laser cavity with a recorded FWHM of 0.15 Å for the individual modes, corresponding to the resolution limit of
FIG. 3. 共Color online兲 共a兲 Roomtemperature emission spectra for a 30 ⫻ 500 m2 InGaN MQW laser diode on AlN substrate for different pulsed current densities below threshold. 共b兲 TE and TM laser diode emission spectra for the same diode operating above threshold.
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FIG. 4. 共Color online兲 共a兲 Highresolution emission spectrum for a 20 ⫻ 300 m2 InGaN MQW laser diode operating slightly above threshold. 共b兲 Room-temperature pulsed light-output vs current 共L − I兲 characteristic for a 20⫻ 300 m2 laser diode emitting at 371 nm.
our measurement setup. Further confirmation of lasing is offered in Fig. 4共b兲. The graph shows the pulsed room temperature light-output versus current characteristic for a 20 ⫻ 300 m2 cavity device exhibiting clear nonlinear behavior in the L − I curve with the threshold current near 800 mA, which corresponds to a threshold current density of 13.3 kA/ cm2. The measured pulsed light output from the front facet approaches 300 mW, which corresponds to a differential quantum efficiency of 6.7%. The measured threshold current densities for these entry-level, near-UV lasers on AlN substrates are comparable to those from our first laser diodes on sapphire substrates, at the same wavelength, which exhibited threshold densities in the range of 10 − 15 kA/ cm2. Significantly lower threshold current densities are expected through further optimization of the laser heterostructure and MOVPE growth on AlN. For example, our lowest threshold current densities for laser diodes optimized on sapphire in the near UV are below 5 kA/ cm2,17 and it is expected that laser thresholds in the mid UV and deep UV would be in a similar range.12,13 We expect the benefits of AlN substrates to become increasingly more pronounced at shorter laser emission wavelengths as the Al compositions of the heterostructure approach that of AlN. V. OPTICALLY PUMPED UV LASER HETEROSTRUCTURES ON AlN
To further investigate the feasibility of deep-UV lasers on bulk AlN we fabricated undoped UV heterostructures for
optical pumping experiments. The laser heterostructure design was similar to the schematic shown in Fig. 2共a兲, except that all layers were left undoped and only a 100 nm thick AlGaN film was grown on top of the 共In兲AlGaN multiquantum well active region. The laser cavities were formed by cleaving 1000-m-wide bars from the AlN substrate. AFM images of the cleaved laser facets revealed extremely smooth surfaces with a rms roughness less than 0.5 nm. Figure 5共a兲 presents emission spectra obtained from a series of optically pumped laser devices with different InAlGaN and AlGaN MQW active regions grown on bulk AlN substrates. By adjusting the Al composition of the InAlGaN quantum wells and barriers, as well as the AlN mole fraction in the surrounding AlGaN cladding and waveguide layers, the emission wavelength was tuned between 355 and 308 nm. All spectra were measured above threshold with a pulsed 248 nm KrF excimer laser as excitation source 共10 ns pulse width, 100 Hz repetition frequency兲. Figure 5共b兲 shows the threshold power densities for various optically pumped laser heterostructures grown on sapphire 共red dots兲 and AlN substrates 共blue stars兲. Although we did not conduct a systematic comparison, we observed that the threshold power densities were consistently lower for lasers grown on AlN substrates as compared to lasers on sapphire, in particular for lasers emitting in the mid- to deep-UV wavelength range. We note that the measured threshold power density for devices on AlN emitting in the 320–330 nm range is ⬃190 kW/ cm2, which is comparable to what we have obtained for optically
FIG. 5. 共Color online兲 共a兲 Roomtemperature emission spectra obtained from a series of optically pumped laser devices with different InAlGaN and AlGaN MQW active regions grown on bulk AlN substrates. 共b兲 Threshold power densities for optically pumped lasers grown on sapphire 共red dots兲 and AlN substrates 共blue stars兲.
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pumped laser heterostructures on sapphire in the near UV and for near-UV heterostructures grown on sapphire that produced current-injection laser.19 The above-mentioned photopumping results establish that we have realized a viable optical cavity for UV laser diodes in the mid-UV range, although further improvements in the optical properties would certainly mitigate the electrical requirements for lasing under current-injection conditions. We attribute the steady increase in threshold power density with decreasing wavelength for lasers emitting below 320 nm merely to incomplete optimization of the laser structures in this wavelength range and not to a fundamental materials issue. It is expected that optimization of laser heterostructures grown on low dislocation density bulk AlN substrate can produce deep-UV lasers with even better performance than lasers in the near UV that have already been demonstrated. VI. SUMMARY AND CONCLUSIONS
We have described and demonstrated current-injection lasers fabricated on bulk AlN substrates, with emission wavelengths in the range of 368–372 nm and pulsed peak output power approaching 300 mW with a differential quantum efficiency of 6.7%. The laser emission is strongly TE polarized. We have also demonstrated optical pumping of AlGaInN heterostructures down to the shortest attempted laser wavelength at 308 nm, which demonstrates the feasibility of semiconductor laser diodes based on bulk AlN substrates with emission ranging from the near to deep UV. ACKNOWLEDGMENT
The work was partially supported by the Defense Advanced Research Projects Agency SUVOS program under SPAWAR Systems Center, Contract No. N66001-02-C-8017 monitored by Dr. H. Temkin. 1
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