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Abstract
We report on the results of the investigation of the influence of an additional a-directed off-cut of the substrate on the opto-electrical properties of the laser diodes grown on bulk GaN with initial misorientation 0.3° towards the m-direction. The investigated a-directed off-cut is varied from 0° up to 1° by means of multilevel laser lithography and dry etching. The results show that the increase of the a-directed off-cut causes the decrease of internal optical losses and increase of injection efficiency. In consequence, the devices fabricated on the wafer areas characterized by higher a-directed off cut are characterized by the lower threshold current, and higher slope and wall-plug efficiencies.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
Corrections
Szymon Stanczyk, Anna Kafar, Krzysztof Gibasiewicz, Szymon Grzanka, Iryna Levchenko, and Piotr Perlin, "Influence of the a-directed off-cut on the opto-electrical properties of the laser diodes grown on the 0.3° misoriented m-directed GaN substrate: erratum," Opt. Mater. Express 12, 2594-2594 (2022)https://opg.optica.org/ome/abstract.cfm?uri=ome-12-7-2594
1. Introduction
It was quite recently demonstrated by our group that the misorientation of the GaN substrate affects the free hole concentration in Mg doped p-type GaN layers. This effect occurs without the change of the magnesium content in the GaN:Mg layer [1,2]. This effect was explained by the different concentration of the compensating/passivating centers for different off-cuts of the substrates. It was also shown by other group that the substrate misorientation towards a-plane significantly affects the incorporation of the carbon impurities in GaN p-type layers [2]. In both, above cases, the MOVPE GaN:Mg layers were characterized by the lower resistivity when grown on highly misoriented substrates (0.6° or more). It is well known that the growth of III-N layers on the low (< 0.2°) or non-misoriented substrates may lead to poor quality of the GaN or InGaN due to formation of hillocks, which destabilize the step-flow growth mode. On the other hand, the usage of highly misoriented substrates may lead to deterioration of the layer’s morphology by step condensation and step bunching (> 0.6°) [2,3]. Also, it was shown that misorientation of the substrate affects the incorporation of Indium. For example, the use of the 0.2° misoriented substrate towards m-plane results in the growth of InGaN layer with 17% of In, whereas for the same growth conditions on the wafer with 2.5° off-cut we obtain InGaN layer with only 7% of the incorporated indium [3,4]. Recently, it was reported that the misorientation of the substrate affects not only the incorporation of the indium inside the QWs, but also the incorporation of the In and Al in InAlGaN layers (AlGaN cladding, electron blocking layer and InGaN waveguides) in a laser-like structure [5].
The desired off-cut angle of the whole substrate is usually is obtained by the use of the mechanochemical polishing [6,7]. Unfortunately, due to technology of GaN wafer fabrication (wafer to wafer method), polishing it under large angle leads to a huge material loss and is impractical. However, the desired vicinal angle can be also obtained by the use of the multilevel laser lithography combined with dry etching, which can be used to create patterning on the surface of the substrate [8,9]. However, this approach has two main limitations. The first is that the change of the vicinal angle can be done only locally. The second is that the maximal available vicinal angle depends on the thickness of the photoresist used for the patterning and the width of the modified area. Assuming a linear change in the resist thickness in the direction of misorientation change, the value of the off-cut angle can be simply estimated as:
where hmax and hmin are the maximal and minimal resist thickness in the modified area and d is the width in this area. It is easy to see that for a fixed resist thickness change, increasing of the width of the modified area will reduce the available range of the off-cut angles. This size limit may seem disadvantageous, but the local character of the change of the vicinal angles allows for the fabrication of the GaN substrate with different vicinal angles across one wafer and thus, simultaneous grow of the laser diode structure on areas with different misorientations. Also, the quality of the lithography plays a crucial role, because any imperfection will be translated onto the substrate.From a practical point of view, the substrate miscut is not only important for stabilizing the step-flow growth needed to achieve good morphology and high quantum efficiency [10] or to reduce the GaN:Mg resistivity but also the spatial variation of the vicinal angle across the wafer can be used for the fabrication of the emitters with new functionality. An example of such devices is a superluminescent diode characterized by 15 nm broad spectrum, thanks to the use of gradually changing miscut along the device’s waveguide [11,12]. Another example of the devices where the spatial variation of the vicinal angle was used were the multicolor laser diode arrays, where each of the emitters in array were grown on area of the substrate with slightly different local vicinal angle [8,9], which resulted in the fabrication of the laser diode array emitting laser light with several different wavelengths. In both of mentioned emitters the variable off-cut angle was used to control In composition of the active layer. However, there are no regular studies of the influence of the substrate off-cut on important optoelectrical laser diodes parameters like internal optical losses, differential gain or injection efficiency.
2. Substrate preparation and laser fabrication
The investigated laser diodes were grown on patterned GaN substrate, where the initial off-cut angle of the virgin substrate was 0.3° towards m-plane. The substrate patterning process was carried on by spin coating the surface of the substrate with 5 µm thick positive photoresist and then by the use of laser writer, expose the photoresist with different doses of light. The exposed photoresist was then developed. As solubility of the photoresist depends on the exposure dose, we finally obtain the variable thickness coating. After the development of such exposed photoresist, the newly formed surface of the photoresist was translated onto the substrate surface by the use of Inductively Coupled Plasma Reactive Ion Etching (ICP RIE). More information about the creation of such patterned substrate with the use of the laser lithography can be found in the work of Kafar et al. [11] or Perlin et al. [13]. Thanks to such processing, the additional slopes were created with the vicinal angles 0.2°, 0.4°, 0.6°, 0.8° and 1° towards a-plane. The width of each of the slopes was approximately 200 µm. Also, the flat surfaces were created (with no additional vicinal angle) as a reference laser diode. However, it is worth to point out that the reference diodes are still susceptible to the imperfections of this lithography method, as their area of the substrate was covered by a flat resist layer and dry-etched as well.
The profile scan of the surface of the patterned substrate is shown in Fig. 1(a) and the scheme of the single laser diode chip grown and processed on such patterned substrate is shown in Fig. 1(b).
Fig. 1. (a) Scan of the surface of the patterned substrate measured across the wafer (perpendicular to the optical axis/ridge waveguide). In (a) the inset graph shows, as an example, the profile of the surface of the patterned substrate with fabricated additional off-cut 0.8° towards a-plane. Figure (b) shows the scheme of single laser diode chip grown on patterned substrate, to visual how such a device is made and in which direction the optical mode is propagating, with respect to the pattern.
After the substrate patterning, the wafer was placed in MOVPE reactor and overgrown with the full laser structure. To avoid the interaction between the steps flow between the neighboring chips with different additional off-cuts, we separated them by 3 µm and deep trenches (which can be seen in Fig. 1(a)). The direction of the flow of the steps is – from the point of view of the laser – from the back to the front. The structure consisted of 2.8 µm thick bottom AlGaN:Si cladding layer, 350 nm of bottom graded-index AlGaN:Si cladding, 150 nm InGaN:Si bottom waveguide layer, a two InGaN QWs interspersed by three GaN QBs, 150 nm InGaN waveguide inside which, after first 50 nm, the 10 nm of Al0.14Ga0.86N:Mg electron blocking layer was grown. Then the 100 nm of graded-index upper AlGaN:Mg cladding layer was grown, 540 nm upper AlGaN:Mg cladding layer finished with 45 nm of highly doped GaN:Mg subcontact layer. The concept of the graded-index (Al,In)GaN type of structure was introduced in the work of Stanczyk et al. [14–16]. The applied growth conditions were identical as for the standard, unpatterned, plain substrate with the off-cut of 0.3° towards m-plane, with the targeted lasing wavelength at 440 nm. After the growth, the wafer was processed to obtain laser diodes with 750 µm long resonator and 3 µm wide ridge, which is placed in the middle of the slope and perpendicular to it (as shown in Fig. 1(b)). The laser diode facet formation was realized by scribing and cleaving of the bars towards easy cleavage plane (m-plane), which provides almost atomically flat/perfect mirrors. Also, because the additional off-cut is made in a- direction, it does not affect the quality of the facets. After the dicing the laser diode chips were left uncoated and mounted directly into the TO 5.6 housings. The laser diodes were left uncoated for more accurate estimation of the reflectivity of the mirrors and thus, more accurate estimation of the net gain values and internal optical losses.
3. Experimental setup and measurement results
After the processing and mounting into TO 56 housings, the laser diodes were connected to the proper fixture and placed on a thermoelectric cooler (TEC), which was stabilizing the housing at room temperature (RT). The devices were driven under continuous wave operation (RT) by the dual current-temperature controller (Thorlabs ITC4005QCL). The light emitted from the laser diodes was collected and collimated by convex-convex aspheric lens with large numerical aperture of NA = 0.6. The L-I-V characteristics were measured by an optical power sensor (Thorlabs S121C) connected to an optical power meter (Thorlabs PM320E). The high-resolution spectra (and gain spectra) were measured with FHR1000 Horiba Jobin Yvon high-resolution spectrometer with 1 m long monochromator, a 3600 groove per mm diffraction grating and a Synapse 2048 × 512 CCD camera. Such a setup provides a high resolution of around 5.6 pm for 450 nm wavelength, which is almost one order of magnitude lower than the distance between Fabry-Pérot oscillations for 750 µm long cavity (∼45 pm). The results of the measurement of the L-I characteristics are shown in Fig. 2.
Fig. 2. Light-current characteristics (a) and estimated threshold current and slope efficiency (b) of the laser diodes grown on patterned substrate with different additional misorientation towards a-plane. In (b) the dashed lines are linear fits of the data.
The results of light-current characteristics show that with the increase of the additional vicinal angle the threshold current of the laser diodes is decreasing. The 2-fold reduction of the threshold current was observed for the highest (1 deg) additional miscut angle towards a-plane. Also, the slope efficiency is increasing with the increase of the additional off-cut (almost 40% in the investigated range of the additional off-cut angles). For the laser diodes grown on the highest investigated additional off-cut the slope efficiency is 0.75 W/A, which is a very good result as for the uncoated, 750 µm long device. The improvement of both, the threshold current and slope efficiency suggests that the internal optical losses and/or injection efficiency is improving with the increase of the additional vicinal angle. To investigate that, the gain spectra were measured, which results are shown in Fig. 3. Because the epitaxial structure changes with the change of the additional off-cut, to calculate the gain spectra we used the effective refractive index which was obtained from the Fabry-Pérot mode spacing which was estimated from EL spectra.
Fig. 3. Increase of the gain with the current measured for the laser diodes grown on patterned substrate with different additional misorientation towards a-plane (a). Also, in (a), the inset graph shows the gain spectrum of the laser diode grown on the highest additional off-cut. Figure (b) shows the measured internal optical losses and estimated injection efficiency as a function of additional off-cut. In (b) lines shows linear fits.
The gain measurement shows that the differential gain increases and internal optical losses decrease with the increase of the additional off-cut, which explains the change of the threshold current. The decrease of the internal optical losses with the increase of the additional off-cut might suggest that the Mg concentration is decreasing – and thus the absorption – with the increase of the off-cut. Although, the investigated structures were not measured by SIMS technique for estimating the Mg concentration, in literature there are evidences that the Mg concentration should not change within the investigated range of the misorientation (not for the a-directed change [2], not in m-directed change [17]). Therefore, we assume that the change of the internal optical losses is not due to the change of the Mg concentration with the misorientation.
As the slope efficiency as well as the internal optical losses were measured, and the mirror losses could be estimated (the devices were uncoated), the injection efficiency could be calculated [18], which is shown in Fig. 3(b) (red dots). The improvement of the differential gain below the threshold current, the injection efficiency above the threshold and internal optical losses with the increase of the additional off-cut angle might suggest that the quality of the growth is improving with the increase of the additional a-directed misorientation. This is interesting, because in work of Jiang et al. [2] the deterioration of the morphology was observed for the 0.6° or higher off-cut angles. However, in the mentioned paper, the investigated samples were grown on sapphire with different c-planes misorientations towards m-plane and thus the GaN was grown with different misorientations towards the a-axis. In this work, the lasers were grown on bulk GaN with the initial misorientation of 0.3° towards the m-axis and the additional miscut was perpendicular to the existing one (towards a). Therefore, the samples were grown on the mixture of the m- and a- off-cut – not on the pure a- or m- off-cut. This makes the studied samples too different and not comparable with Jiang et al. [2]. Even though, we did not measure the AFM scans (because we did not want to break the fabrication chain which could increase the risk of the contamination or damage of the wafer) or XRD rocking curves for each additional off-cuts (because the size of the areas which have additional off-cuts is too small to perform such measurement), we performed a series of investigations of the structural and morphology change with the change of the misorientation on similar laser-like structures [12,19,20]. Those mentioned structures differ from the presented in this paper in such a way that the growth was stopped 30 nm above last grown QW. In such approach the morphology of the surface should more reflect the quality of the QW than if it would be measured after the whole laser diode structure was grown. The results presented in works of Kafar et al. [12,19,20] support that the quality of the morphology (from AFM scans) and QWs (from STEM images) improves with the increase of the off-cut and that the high misorientation regions are characterized by lower rate of non-radiative recombination processes (from the time-resolved photoluminescence measurements). Therefore, the results shown in References 12, 19 and 20 supports strongly that the observed improvement of the laser diode parameters might be from the improvement of the structural quality with the increase of the misorientation.
Another difference, when comparing with Reference 2, is observed when analyzing the influence of the additional misorientation on operating voltage, which can be seen in Fig. 4. In our experiment, the voltage is not changing with the change of the additional off-cut – only voltage at threshold is decreasing with the increase of the additional misorientation. But this behavior is attributed rather to the reduction of the threshold current. The threshold current must decrease with the decrease of the internal optical losses and increase of the differential gain. This result is unexpected in the light of earlier observation that the GaN:Mg resistance decreases with miscut angle. On the other hand, the laser diodes are more complicated structures, than the previous reported simple GaN layers for which the decrease of the resistance with the increase of the misorientation was observed. For example, for highly efficient laser diodes, the self-heating is less important which leads to higher operating voltage (p-GaN resistance decreases substantially with the temperature). Nevertheless, even for 2 mA of operating current, for which the thermal effects are negligible, no clear tendency can be seen.
Fig. 4. Voltage-current characteristics measured for laser diodes grown on different additional off-cuts (a) and value of the operating voltage at several chosen currents.
One can clearly see that the wall plug efficiency (WPE) is increasing with the increasing additional off-cut. Figure 5(a) shows that the laser diodes grown on high additional off-cuts have more than two times larger WPE in the measured range than the lasers grown on low additional off-cuts, where at the same time all of the lasers experience the same operating voltage. This suggest that the whole WPE improvement comes from the improvement of the injection efficiency and internal losses, which suggests that the quality of the growth improves but without the improvement of the electrical resistance of the p-type layers. Also, in Fig. 5(b), it can be seen that for the current of 2 mA, the wavelength of peak of the electroluminescence is non-linearly but consistently shortening with the increase of the additional off-cut, which is somewhat expected and consisted with the work of Sarzynski et al. [3] for similar growth temperature of 780°C (here of the QWs). The change of the wavelength with the increasing vicinal angle at threshold current is not equally consistent as for 2 mA. The lasing wavelength is increasing with the increase of the off-cut angle up to 0.4° and above it is decreasing with the increase of the off-cut. This behavior is caused by the interplay between the decrease of the wavelength caused by the increasing miscut and with the reduced blue shift of the emission related to quantum confined Stark effect screening [21,22]. Therefore, we suggest that the EL spectra measured at low current are better indicators of the change of the emission wavelength with the change of the misorientation. Moreover, the change of the wavelength at 2 mA is less than 10 nm and at threshold is less than 6 nm. Therefore, such difference between the wavelength is too small to play a noticeable role in the change of the wall plug efficiency on its own. Also, the vales of the full width at half maximum estimated from the electroluminescence spectra taken from the laser diodes at very low current – which is shown in Fig. 6 – can be used as an indicator of the quality of the morphology of the QWs.
Fig. 5. Wall plug efficiency measured for the laser diodes grown on different additional off-cuts (a) and the wavelength of the peak of the electroluminescence of the lasers grown on different additional off-cuts measured at 2 mA and threshold current.
Fig. 6. (a) The electroluminescence spectra of the laser diodes grown on different additional off-cut measured for the current of 2 mA. The fillings in EL spectra are due to the dense Fabry-Pérot oscillations. (b) Values of the full width at half maximum of the spectra presented in Fig. 1(a).
The results of the full width at half maximum (FWHM) show that it is decreasing with the increase of the additional off-cut, which suggest that the morphology of the QWs is improving. This is in correspondence to the results shown in reference 20, which investigates the quality of the QWs morphology of the bulk structure – not on the laser diode chips. However, in case of the laser diodes, the value of the FWHM can be also affected by the value of the net gain. If the differential gain is improving with the off-cut angle, it is expected that the FWHM will be decreasing.
4. Conclusions
In this paper we showed the influence of the additional a-directed misorientation, patterned on 0.3° m-directed misoriented substrate, on optoelectrical properties of the laser diodes. The results show that with the increase of the additional miscut in the range from 0° up to 1° the internal optical losses, differential gain and injection efficiency are improving, resulting in lower threshold current and higher slope efficiency, which results in the increase of the wall plug efficiency. Beside the previous mentioned reports on the improvement of the resistance of the p-type in a- or m- directed misorientation, here where the m- and a- misorientation is mixed, we did not observe the drop of the voltage with the increase of the additional misorientation. We attribute this behavior (constant voltage, increase of the differential gain and injection efficiency and decrease of the internal optical losses) to the improvement of the growth quality with the increase of the off-cut, but without the impact on the incorporation of the compensating donors. Also, the results in the paper show that the misorientation of the substrate is a very important parameter which has great impact on the final optoelectrical performance of the laser diodes.
Funding
Narodowe Centrum Nauki (TEAM TECH/2017-4/24); Narodowe Centrum Badań i Rozwoju (1/POLBER-1/2014, 1/POLBER-3/2018, WPC1/DEFEGaN/2019).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but maybe obtained from the authors upon reasonable request.
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