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Abstract
We report a thorough study of InGaN quantum wells spatially modified by varying the local misorientation of the GaN substrate prior to the epitaxial growth of the structure. More than 25 nm shift of emission wavelength was obtained, which is attributed to indium content changes in the quantum wells. Such an active region is promising for broadening of the emission spectrum of (In,Al,Ga)N superluminescent diodes. We observed that the light intensity changes with misorientation, being stable around 0.5° to 2° and decreasing above 2°. This relation can be used as a base for future device designing.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
InGaN quantum wells grown on (0001) c direction GaN substrates are the heart of most of the commercially available nitride laser diodes (LDs) or light emitting diodes (LEDs), which are now widely present in our everyday lives. The wide range of applications, present and emerging, include solid state lighting [1–3], optical data storage [4,5], displays [6], projection [7,8], car headlights [9,10], visible light communication [11–13], atom cooling in atomic clocks [14], and more [15].
Still, there are applications in which the requirements are not fulfilled by LDs nor LEDs. A good example is projection, where a good light beam is needed, but the high time coherence of LDs produces unwanted interference effects (speckles) which lower the quality of projected image. The problem can be solved by proper processing of the light, but a more straightforward approach is using a superluminescent diode (SLD) – a device characterized by a good light beam but wide emission spectrum. Superluminescent diodes are semiconductor light emitters similar to laser diodes in terms of epitaxial structure, emission from a small aperture, and the amplification of light by stimulated emission. However, the devices do not lase, which makes them an interesting alternative light source that can be easily coupled to a fiber, but also is characterized by low temporal coherence [16–22]. The applications of such nitride emitters include, optical coherence tomography (OCT) [23], visible light communication [24,19,21], or image projection [17]. In applications like OCT, the demand for optical power of the device is quite moderate (tens of mW [23]), but the width of the emission spectrum is wished to be as large as possible. To meet these expectations, in the (In,Al,Ga)As material system various spectral broadening approaches were proposed. Usually they are based on fabricating the active region as a stack of different quantum wells. This variation may be introduced through the change in width, indium content or introduction of various quantum dots inside [25–28]. However, in case of nitride material the low hole mobility leads to possible uneven pumping of the quantum wells, which disturbs the spectrum. Also, this kind of structure is meant to suffer from the reabsorption of light by the regions having lower bandgap (optical mode is much bigger than the width of the active layers). In the (In,Al,Ga)N material system, where superluminescent diodes are still a rather novel type of device, the spectral broadening is studied in a limited number of reports [29–31]. Our proposed solution to this problem, introduced in [29,30], is based on fabricating a profile of indium content along the device waveguide in order to induce the change in local emission wavelength. This is achieved by fabricating a misorientation angle profile on the substrate prior to the epitaxial growth. A detailed description is introduced in the next paragraph. Our approach allowed to fabricate SLD with 15.5 nm wide emission spectrum centered at around 412 nm. This is an interesting result when compared with the typically reported spectral width for blue-violet emitters of between around 3 and 9 nm [17,18,21,32]. However, we were not able to fully exploit the potential of our method and observed serious drop in the optical power [29,30]. The present work is devoted to the detailed characterization of InGaN quantum wells with a spatial variation of indium content and finding the limits of the proposed modification method.
The approach we are applying to change the local indium content in the InGaN quantum wells, is based on the change of misorientation of a GaN substrate, prior to the epitaxy. Misorientation angle is defined as the angle between the atomic planes of the crystal and its surface (also called as vicinal, miscut or off-cut angle). The patterning of the crystal substrate is obtained by means of multilevel photolithography and dry-etching. This method was successfully applied in earlier work [29,30,33,34]. Although the misorientation of the GaN substrate is one of the widely known parameters that strongly influence the layers grown on it, there are not many studies available which report how the indium incorporation changes with the misorientation angle. A modification of indium amount is observed for various kinds of substrates: GaN [35], n-polar GaN [36], m-plane GaN [37,38], sapphire [39], Si [40]. But there are also reports showing that the indium content change due to misorientation change is not present on m-plane GaN [41,42]. Sarzynski et al. [35] explained that the change on the amount of incorporated indium is related to the speed of flowing monolayer steps during the epitaxial growth. It was shown that the vertical growth rate does not depend on the misorientation [43], which means that on a smaller misorientation the atomic steps should move faster. The faster-moving steps may entrap the indium atoms more effectively before they desorb from the crystal surface. The dependence of indium incorporation efficiency on the growth rate was demonstrated by Keller at al. [44] for the growth on sapphire substrates. This suggests that higher misorientation regions should be characterized by a lowered amount of incorporated indium. It is also important to note, that the change of substrate misorientation is usually accompanied by a change in the morphology of the InGaN layer [33]. For lowest misorientation the monolayer steps are very wavy or we even observe hillocks on the crystal surface. Increasing of the misorientation leads to smoothing of the monolayer steps, but at a high-enough value, the steps bunch together forming step bunches and reducing the quality of the surface. The presence of step bunching was identified as a factor determining the indium incorporation efficiency [45]. The difference in indium incorporation on bunched steps and the neighboring areas was clearly observed by Jiang et al. [46]. The change of indium incorporation is also predicted and observed between various crystallographic directions [47–50], however in this work we examine only small deviations from c-plane – below 4°.
2. Sample fabrication
The modified active region studied in this work was fabricated according to the scheme presented in Fig. 1. Positive photoresist AZ9260 was spin-coated onto a bulk GaN substrate to form a 5 µm thick layer, which was then illuminated in a Microtech Laser Writer according to a dedicated design. The shape was calculated based on the initial misorientation of the substrate (measured by x-ray diffraction) in order to obtain a set of areas with different misorientation profiles or with uniform misorientation. Typically, the areas had the shape of 40 × 40 µm squares. Figure 1 shows an example of the 3D shape characterized by a linear change of height in both x and y directions that should lead to a linear profile of misorientation along the pattern diagonal (more details are presented in further paragraphs). After the photolithography and development of the resist a 3D structure was obtained. Next, we transferred this shape onto the substrate by Inductively Coupled Plasma Reactive Ion Etching. This was followed by mechanochemical polishing (MChP) in order to prepare the sample for the epitaxial growth, but the polishing parameters were chosen so that the patterns of the crystal surface are not destroyed. The epitaxial structure was grown in a metalorganic vapour phase epitaxy (MOVPE) close coupled showerhead Aixtron reactor, according to the scheme presented in the Tab. 1. The structure has two quantum wells (QW) grown above a layer set simulating the bottom part of a superluminescent diode – the InGaN waveguide and a thinned AlGaN graded index cladding [51].
Fig. 1. Scheme of the GaN substrate patterning approach. In the illumination pattern, the brighter the region, the higher the light dose.
Table 1. Epitaxial structure of the studied sample.
3. Study of the pattern shape by a laser microscope
The shape of the sample surface was examined using a laser microscope (3D profiler) Keyence VK 9510. Due to the high noise of the height values, the obtained data were first fitted in the area of the examined surface region. In order to allow for more flexibility during the fitting, the area was divided into subareas of 5 µm x 5 µm, which were fitted separately. An example of the expected 3D structure and the result of measurement together with a fit is presented in Fig. 2. The quality of the fit was studied by calculating the height difference between the fit and experimental data – an example is shown in Fig. 3. This comparison shows a very uniform match and is characterized by the standard deviation of around 0.03 µm. The topmost region of the structure, around the (40,40) corner is not visible, as it shows a plateau which appeared during the epitaxial growth. This area may be a consequence of the expansion of 0001 crystallographic plane. The presence of this area is not a serious problem from the point of view of device fabrication. The superluminescent diode waveguide can be fabricated beside or below the plateau region and the width of the slope can be made significantly larger than the plateau width (e.g. 80 µm).
Fig. 2. Example of the shape of the structure according to the design (a) and a corresponding real shape obtained on the sample (b). The flat corner in the (40,40) area is a plateau which always appears around the highest point of the structure during epitaxial growth. Color scale in (a) represents height.
Fig. 3. Example of the height distance between the measured data and the fitted plane. The total height difference shown in Fig. 2(b) is around 0.9 µm.
After the fitting, a set of vectors normal to the subareas of the fitted plane was calculated. To quantify the local misorientation we use a single angle determined in a plane containing two normal vectors: an atomic plane (c-plane) normal and local, fitted surface normal. The orientation of the atomic planes was assumed based on an XRD study performed on the substrate prior to the patterning. To check the correspondence between the designed and the real surface shape, we used data from the laser lithography of the pattern. The light dose information, estimated during the pattern preparation, was converted to the height data using a calibration pattern present on the studied sample.
The presented results, Fig. 4, prove that the shape of the fabricated structure is very similar to the expectation. The range of misorientation calculated for the real structure is smaller but comparable with the original design. The missing area of the highest misorientation, upper right corner of the graph Fig. 4(b), is related to the presence of a plateau in the real structure. Another difference between the designed pattern and the real structure is the vertical shift of the area with lowest misorientation from around (0,0) to (0,10). The reason for this discrepancy is not clear.
Fig. 4. Comparison of the misorientation angle maps obtained for (a) the calibrated design of the structure and (b) the experimental data calculated based on laser microscope measurement. The results in (b) are presented omitting the plateau.
4. Transmission Electron Microscopy (TEM) study
In order to directly check if the modification of local misorientation of the substrate is indeed correlated with the changes of the indium content, we performed a TEM study of quantum well width and Energy-dispersive X-ray Spectroscopy in the TEM system (TEM-EDS) measurement of In composition on the patterned sample. A comparison of indium content and width of the top quantum well is presented in Fig. 5, along with the height scan obtained by the laser profilometer. Both measurements were done along the diagonal of the square test pattern, from the high misorientation region to the low misorientation region. The indium content shows a smooth change that seems clearly correlated with the height profile of the examined area. In contrast, the quantum well width does not show a close association with the sample shape. We also did not observe the deterioration of structural quality with changing misorientation angle.
Fig. 5. Comparison of the indium content of InGaN quantum wells (a) and width of the quantum wells (b) together with the local height measured along a chosen line in the area of changing misorientation. The error bar of the In content is estimated as the standard deviation of the Ga content in GaN layers of the structure measured in 21 points (0.015). However it needs to be emphasized, that the In content values are underestimated due to the spread of the electron beam to the surrounding GaN layers (around 15 nm). QW width estimation error is approximately equal to the thickness of one monolayer of the material (0.26 nm).
5. Atomic Force Microscopy (AFM) study
The quality of the material after the epitaxy was also checked by AFM, Fig. 6. For this test, areas with intentionally uniform misorientation were chosen. The value of the misorientation was estimated using the laser microscope. The scan of the low misorientation region shows monolayer steps, proving the good quality of the sample preparation. In case of the large misorientation angle, it was impossible to observe single-monolayer steps as the terrace width corresponding to a tilt of 2.71° is as narrow as ∼ 5 nm. However, it is observed that the sample surface is smooth.
Fig. 6. Atomic force microscopy scans of the sample after the epitaxy in regions with intentionally uniform misorientation of (a) 0.72° and (b) 2.71°.
6. Photoluminescence study
Next, the optical properties of the patterned sample were studied using micro-photoluminescence (µPL) mapping. The system used for the experiment is presented in Fig. 7. Excitation is performed using a continuously operated laser diode emitting at 375 nm, which can selectively excite InGaN layers. The beam profile is shaped by a set of two irises. The fluorescence image of the excited sample can be obtained through a CCD camera. Sample is placed on a motorized xy precision translation stage. The exciting laser is focused to a spot of approximately 2.5 µm diameter by a 0.42 NA objective. The light emitted by the sample is guided to a 560 mm long spectrometer equipped with a 200 grooves/mm grating with a blazed wavelength of 430 nm. The measurements were done at room temperature. Assuming the absorption coefficient of InGaN as 105 cm−1, we estimate the percentage of the excitation light absorbed by the active layers as 4%.
Prior to the TEM study shown in Fig. 5 the sample was examined by µPL mapping. A comparison of the indium content profile and the peak emission wavelength is presented in Fig. 8. A good correlation of both types of data is observed suggesting that the modification of emission wavelength is directly related to the local change of indium content. The µPL mapping was performed using 6.1 kW/cm2 of excitation power density.
Fig. 8. Profile of indium content obtained from TEM-EDS measurement compared with µPL measurement done along approximately the same scanning line. The positions from 75 to 90 µm correspond to a plateau on the sample surface that is observed as a constant local emission wavelength and nearly constant indium content.
The PL maps obtained studying the area introduced in Fig. 4 are presented in Fig. 9. The presented map of central emission wavelength was calculated as a center of gravity of the local spectra (sum of products of wavelength and intensity for each point of the spectrum divided by the sum of all intensity values). The local central emission wavelength measured on the studied area spans from around 415 nm to 440 nm, which means that a shift of emission of 25 nm was obtained. The change of the wavelength is rather smooth and similar to the misorientation distribution. The area characterized by the longest emission wavelength does not appear at the corner of the map but is shifted vertically, which agrees with the shift of the lowest misorientation angle region observed in Fig. 4(b). The peak intensity map shows a rather smooth distribution, with a similar value for a wide range of misorientations (from around 0.5° to 2°) and emission wavelengths (from around 422 to 440 nm). This area is marked on the central wavelength map by the red dashed line. The intensity is clearly reduced for the area characterized by highest misorientation, which suggests a practical limit for emission modification by substrate misorientation. In case of applying this method in fabrication of broad-emission superluminescent diodes, the problem of intensity drop may be mitigated by expanding the short-wavelength emitting region.
Fig. 9. Maps of emission wavelength (a) and peak intensity (b) of the area with the shape introduced in Fig. 4. The µPL measurement was performed with the excitation power density of 3.06 kW/cm2.
The local µPL spectra, shown in Fig. 10(a), have similar, single-peak shapes for all the central emission wavelengths. This proves the good quality of the active region fabricated on the area with changing misorientation. The spectra tend to broaden towards the longer emission wavelengths, which is presented also in Fig. 10(b). The spectral width ranges from 12.5 to 18.5 nm; in the energy units from 90 to 120 meV. The reason for this change is not clear at this point, but the FWHM modification may be a simple consequence of the decrease of electron effective mass with increasing indium content. It is also possible that the inhomogeneity of the QWs increases for higher indium content, but it is not certain as the In contents used in this experiment are relatively small (the longest emission wavelength is below 450 nm).
Fig. 10. Comparison of the local microphotoluminescence spectra obtained within the measurement of the maps presented in Fig. 9(a) and map of the full width at half maximum of the local PL spectra (b).
It is interesting to compare the emission wavelength map with the misorientation angle map. Figure 11 presents a peak wavelength map of the area introduced in Fig. 9 together with the corresponding misorientation map. The distributions of both parameters are quite similar, with distinctive features appearing at similar positions. This further proves the close relation of the structure shape and the emission wavelength. Also, the comparison suggests that the a and m crystallographic directions (on the graph: horizontal and vertical direction, respectively) show a very similar dependence of emission wavelength on misorientation angle. This suggests, that the change of indium incorporation may indeed be a mechanism independent on the crystallographic direction.
Fig. 11. Comparison of the peak emission wavelength map of the studied area with the map of the misorientation angle.
The central emission wavelength map introduced in Fig. 9 was measured at a moderate excitation power density of about 3 kW/cm2. The study was also performed for higher and lower excitation and the results are compared in Fig. 12, which presents the relation between the central emission wavelength and the peak intensity for the data corresponding to the three maps. The comparison shows, that the emission range of the studied area depends on the excitation power. Such a change can be a consequence of either the screening of Quantum Confined Stark Effect (QCSE) by the photo-excited carriers or the filling of energy bands. The long-wavelength end of the distribution shows a more significant blue-shift than the short-wavelength end. This agrees with both mechanisms. QWs with higher indium content are more strained and as a result the QCSE and carrier screening are more pronounced. On the other hand, the higher indium content is usually accompanied by increased potential fluctuations and deeper tail states, leading to larger emission shift. Assuming the carrier lifetime of 1 ns, we estimated the excited carrier density as approximately 5·1018 cm−3, 5·1017 cm−3, and 5·1016 cm−3. Consequently, the emission shift between the highest and middle excitation power is probably dominated by the QCSE. In case of the emission shift from low to middle excitation, the observed effect may be also influenced by filling effects.
Fig. 12. Correlation of local central emission wavelength and the light intensity measured in the studied area. The relation was measured under three excitation power densities: 0.31, 3.06, and 30.56 kW/cm2.
Additionally, as it was observed in Fig. 9, the intensity of the emission is quite stable in a wide range of wavelengths which is advantageous from the point of view of using misorientation change in device fabrication. However, near the short wavelength end of the distribution, the intensity starts to drop. This may be related to the decreasing depth of the quantum well in this region, which promotes carrier escape. Figure 13 shows the normalized data presented in Fig. 12 together with integrated intensity correlation with central emission wavelength. Each set of data was divided by the maximal value of intensity for the purpose of convenient comparison in linear scale. For the integrated intensity plots, the drop of intensity with decreasing wavelength is observed in the whole wavelength range, which is consistent with local µPL spectra presented in Fig. 10.
Fig. 13. Correlation of local central emission wavelength with (a) peak PL intensity and (b) integrated PL intensity of the corresponding spectrum measured in the studied area. The distributions were divided by the maximum value for the purpose of convenient comparison. The relation was measured under three excitation power densities: 0.31, 3.06, and 30.56 kW/cm2.
7. Time-resolved µPL measurement
We also performed room-temperature time-resolved µPL measurements at chosen points along the diagonal of the described region, characterized by different peak emission wavelengths. The excitation was performed by second harmonic generated from a Ti: sapphire laser. The excitation wavelength was 375 nm, frequency was 4 MHz, peak power density around 0.36 kW/cm2, while the decay of the system response was around 1.2 ns. Figure 14 presents the corresponding time decays, obtained by integrating the emission spectra measured at different positions. It was observed that shorter emission wavelength (larger misorientation) areas are characterized by a faster decay. The time decays were fitted with a double exponential function and showed a similar change of values for both time constants. If we assume that the studied decays are dominated by nonradiative recombination processes, then the findings may suggest that nonradiative recombination is suppressed in the area with higher indium content [52,53]. In such a case, the increase of the decay time with peak wavelength should be accompanied by the increase of the integrated PL intensity, which is in fact observed in Fig. 13(b). However, it is also equally possible that the increased lifetime in the long wavelength region may be related to the more pronounced QCSE at regions with higher indium content. The increased electron and hole wavefunction separation may lead to the decreased rate of Shockley-Read-Hall type of non-radiative recombination, which would be observed as increased lifetime for long wavelength emission [54,55]. In such a case, the difference in PL decay is an inherent property of the structure. Alternatively, the excitation power density used for this experiment may have been not low enough which would lead to a significant contribution from radiative recombination. However, in such a case, the time-decay results are not supported by the intensity relation observed in Fig. 13. In the present moment it is difficult to decide which mechanism is dominant in the observed time decays. However still, the devices can be designed in a way, which balances the smaller intensity by longer amplification path.
Fig. 14. Comparison of the time decay measured for different points in the studied pattern (introduced in Fig. 9) and the time constants obtained through a double-exponential fit. The legend of (a) describes the peak emission wavelength of the particular measurement point estimated around t = 0, and the same peak wavelength values are the horizontal axis of (b).
8. Uniform misorientation
The examined sample included also several areas with intentionally uniform misorientation towards m-direction and having the shape of 40 × 40 µm squares. For those samples, the measurement of the misorientation angle had smaller uncertainty, which is why they were used for estimation of the dependence of emission wavelength on misorientation angle. The obtained relation is presented in Fig. 15 and has a continuous and rather linear shape, which is useful for designing a device based on the proposed technology. The presented results were obtained for moderate excitation power of around 12.8 kW/cm2 under the CW excitation condition. The shift of central emission wavelength was obtained in a wide range between 409 nm and 446 nm, which opens the possibility to fabricate SLDs with above 30 nm of spectral width.
Fig. 15. Relation between the PL emission wavelength and substrate misorientation angle as well as PL peak intensity and substrate misorientation angle, obtained using areas with uniform misorientation.
The approach of SLD spectral broadening described in [29,30] is based on combining together light generated at different positions of the waveguide. With a proper substrate design, a profile similar to what is observed in Fig. 15 should be fabricated along the waveguide. The length of the high misorientation regions should be increased to balance the reduced intensity with longer amplification path. In such a case all the local spectral contribution should be preserved while travelling in the waveguide. As a result, we can expect to fabricate an SLD with a spectral width similar to the central emission wavelength range observed in Fig. 15. However, it should be noted that at higher excitation of the device, the screening of QCSE may lead to the reduction of the observed spectral width of the SLD spectrum, similarly to what is observed in Fig. 12.
9. Summary
In summary, we have studied the modification of the local emission wavelength (spectrum) of an InGaN quantum well by the changes of bulk GaN substrate local misorientation. The fabrication was achieved by using multilevel photolithography and dry etching. After epitaxial growth by MOVPE, the sample surface shape was studied using a laser microscope and the obtained structures proved to be similar to the initial design. The optical properties of the sample were studied by microphotoluminescence mapping, revealing a wide range of emission wavelengths in an area of the misorientation change. The range can even reach 25 nm, but the value depends on the excitation power density. Most probably, excited carriers screen the quantum confined Stark effect, which results in a blue-shift of the emission. The change of the emission wavelength is accompanied by a change in the light intensity. The highest intensity values are observed for the longest wavelengths (low misorientation). But, in the middle of the wavelength range, a stable intensity region is observed that covers around 17 nm. This property is beneficent from the point of view of device fabrication using substrate patterning. Time-resolved PL measurements suggest that the drop of intensity may be related to the difference in defect related nonradiative recombination. A set of data obtained for areas differing by misorientation value show, that a shift of emission of even 35 nm can be obtained by the proposed method. The presented results prove that the profiling of substrate misorientation is an efficient and well-controlled method for engineering of indium content in InGaN layers.
Funding
Japan Society for the Promotion of Science (JP15H05732, JP16H02332, JP16H06426); Fundacja na rzecz Nauki Polskiej (TEAM TECH/2017-4/24).
Disclosures
The authors declare no conflicts of interest.
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