Major Research Areas

Investigation into the complexities in active region of quantum well laser diode arrays
1. Introduction : The photonic devices based on III-V semiconductors started instinctive development after the invention of the light emitting diode (LED) and laser diode (LD). Subsequently, in the last five decades LEDs and LDs remains the cardinal components, both in traditional and modern applications which includes telecommunication, data storage & processing, optical sensors & sensing networks, image processing, high efficient low power lightening, display & cinema, materials processing, 3D printing, molecular & bio photonics etc [1]. Even pioneers in this field report that the laser diodes are the future for lighting and have imperative advantages over LED because of higher efficiency [2]. In spite of numerous applications of these devices, several newer challenges are also arising, such as high power with controlled wavelength and low divergence, wide wavelength tuning for advanced material processing, monolithic integration on Si and low threshold high efficiency spin laser [3-7]. The success secrets of these devices predominantly depends on the efficient radiative recombination of the carriers into an optically active region of the forward-biased p-in junction diode. Fig. 1a shows the schematic of forward-biased p-i-n junction diode in which the conduction &valence band edges E , E are plotted as c v solid lines. The dashed lines represents Fermi energy E , F that splits into quasi-Fermi energy E and E in the Fc Fv undoped transition region, where holes and electrons recomibines. In this region, inversion condition, E - Fc E > ħ >E can be easily achieved because the quasi- Fv g Fermi energy are inside the bands. This optically active region consist of quantum wells (QWs) and by tuning its structural parameters one can control the properties of the devices. Thus, in order to meet the newer challenges, one need to understand the complexities of the materials used in the QWs. Some of these complexities are distinguishable under extreme conditions, such as at very high injection current, the emission wavelength may start shifting to higher wavelength. This is mainly due to excess internal heating caused by the nonradiative recombination and the migration of charge carrier into the localized states and subsequent radiative recombination via those states. The charge carriers localized in these states have different effective mass and therefore can affect the laser diode properties including the operating wavelength, threshold current density and electrical to optical conversion efficiency etc [4, 8]. Thus to meet the present requirements, both the basic studies on QW and engineering on laser diode structures are the prime focus of laser diode community.

Schematic of laser diode structures grown by using MOVPE at RRCAT (a) Forward-biased p-i-n junction (b) Description of multilayer, front & rear facets coated with single & multilayer stacks of ZrO & 2 ZrO /SiO (not to scale).

The recent investigations on Bragg-like waveguide resulted an ultra-narrow circular beam for edge emitting laser diodes[9]. Similarly, investigations on new sets of multilayer materials ZrO2 and ZrO2/SiO2 used for facet coting, assured to have increase damage threshold of these laser diode array[10]. Further, very recent innovations in QW intermixing and asymmetric waveguides resulted unique high power, brightness, and reliability which eliminated the problems associated with catastrophic optical mirror damage[11]. In India, in-spite of some early achievements, progress in this field is slow, both in research &technology front, due to several limitations and unavailability of semiconductor resources. In-spite of these facts, we at RRCAT have put strong efforts & developed high power QW lasers in the wavelength range of 670 to 1000 nm. The active regions of these laser diodes mainly consists of AlGaAs/GaAs, GaAs/InGaAs, & AlGaAs/GaAsP QWs [10, 12-13].
One such QW laser diodes arrays with ~ 980 nm emission wavelength, delivered 23.5 W peak power under pulsed operation and 3 W, CW power. However the operating wavelength shifts towards higher wavelength with the increase of injection curent. In order to understand this process and for meeting the future requirement of the department, we have investigated the ultralow disorders and effective mass parameters for these QWs and active region of laser diode structures using temperature, excitation power and magnetic field dependent photoluminescence (PL). Here in this article, we summarise the progress of high power laser diode arrays development work at RRCAT. Subsequently, we also discuss the complexities of the active region of QW laser diode arrays with a particular aim of the development of fiber lasers.

2. Experimental Details:
The epitaxial structure of semiconductor QWs and laser diodes are grown on nominally(001) oriented n+- GaAs substrate (carrier density ~ 1×1018 cm-3) by using horizontal low-pressure metal organic vapor phase epitaxy (MOVPE) reactor (AIX-200). Trimethyl Gallium (TMGa), Trimethyl Indium (TMIn) and TrimethylAluminium (TMAl) are used as precursors for group III elements while 100% Arsine (AsH ) is used for 3 group V element. Dimethyl Zinc (DMZn) and 2% SiH 4 in H are used as the dopant source for p- and n-type 2 doping respectively. The single QW structures of AlGaAs/GaAs, GaAs/InGaAs, InP/InAsP and AlGaAs/GaAsP are grown and their basic properties are investigated using numerous techniques [10,12-21]. The complete laser diode structures are also grown which consist of AlGaAs based cladding layer, graded refractive index layer, waveguide layer and the active region embedded with double quantum wells of above materials(Fig.1b). The grown laser diode structures are processed through conventional optical lithography, nand p- type metal contact formation by e-beam/thermal evaporation, lift-off process, and rapid thermal annealing. After making a smooth walled mesastructure using H PO :CH OH:H O etchant solution, 3 4 3 2 2 electrical isolation and side-wall passivation is realized by SiO layer deposition between the metal stripes. 2 Finally, the structure is thinned down to ~140 m and several laser elements of 500 m cavity length and 100 m strip width are cleaved. Subsequently, antireflection and high reflection coatings of ZrO based multilayers are 2 carried out on the laser diodes using e-beam evaporation. The low deposition temperature is preferred to avoid any deleterious diffusion of materials in the active region of laser diodes. Surface and interface properties of single and multilayer facet coatings of ZrO and ZrO /SiO are investigated by optical and X- 2 2 2 ray reflectivity (at Indus-1) [10]. Subsequently, the laser diode characteristics are evaluated in the light output power versus current (L-I) testing setup that is equipped with precision pulsed laser diode driver, power meter, spectrometer, mounting stage, microscope etc. Further, the die bonding process is optimized with two different types of solder materials namely Indium preform (soft solder) and Gold-Tin (AuSn-hard solder). The LD devices are bonded on gold plated Copper and KOVAR substrates. Finally, semiconductor laser diode arrays are die bonded and packaged in a water cooled micro channel based gold coated assembly. The photographs of processed laser diode arrays along with gold coated assembly are shown in Fig.2.

Photograph of laser diode arrays packaged in the microchannel assembly by RRCAT.

as ultralow disorders and effective mass in the QWs & LD structures the contactless Magneto-PL experiments are performed (Fig.3). In this setup, sample is kept in a holder of variable temperature inserts (VTI) which is immersed inside a Dewar of the thermostat where the lowest temperature of 1.2 K with mK accuracy can be achieved. High magnetic field up to 8T is achieved by helical shaped niobium-titanium superconducting magnet. Figure 3(a) show the photograph of superconducting magnet. The laser light is passed through band pass filter to remove any unwanted fluorescence lines from the diode-pumped solid-state lasers, followed by neutral-density filters to control the laser excitation power. The laser excitation power is kept at minimum levels to reduce the intensity dependent effects, such as saturation of energy levels, linewidth broadening, temperature rise etc. The chopped laser light is focused into an optical fiber using gold coated GaAs mirror.The excitation laser beam is guided through an optical fiber having ~ 400 μm diameter with ~ 3 meter length. Samples kept at 1.2K are excited by the laser light that is guided with the help of a

Magneto-PL setup at RRCAT (a) Superconducting magnet, (b) PL with fiber coupling, (c) Electronic controls, (d) Schematic of Magneto-PL setup.

fiber and the same fiber is used to collect the PL signal. The PL signal after suitable filters is dispersed by monochromator and detected by Si/Ge photodiode using lock-in amplifier technique. Figures 3(b) & (c) show the optical and electrical data processing arrangement for magneto-PL experiment. In this experiment, the sample is mounted horizontally in the VTI assembly and field is applied along the growth direction, also the direction of light, known as Faraday geometry, and excitonic properties are probed in the plane of the sample, shown in Fig. 3(d). Under this condition, the magnetic field driven confinement of charge carrier at high field produce discrete harmonic oscillator like Landau energy levels which blue shift with field. From the analysis of PL line width and the field dependent blue shift microscopic parameters are obtained. The details are given in next section.

3. Results and Discussion:
A. Structural Analysis and Limitations:
In-depth structural parameters, such as layer thickness, composition and strain, interface roughness of the grown structures are determined from high resolution x-ray diffraction (HRXRD), transmission electron microscopy (TEM) and x-ray reflectivity (XRR) by Hard/soft x-ray beam of CuK lab α source/synchrotron radiation source at Indus-1 synchrotron beamline facility, respectively [10,12-23]. It is concluded from the structural analysis that the semiconductor heterojunction and quantum structures are of excellent quality. However, all these techniques do not predict directly the influence of ultralow disorder on the opto-electronic properties of the material. In contrast, contactless PL and surface photo-voltage (SPV) spectroscopy are more effective techniques for studying the optical quality and also the defects. Superiority of these contactless techniques lies in their simple and non-destructive nature. Localization of charge carrier and their dynamics can be understood from the conventional emission and absorption of photon based PL and SPV techniques.

B. Photo luminescence &Surface Photo Voltage Analysis:
In order to investigate the emission wavelength and carrier recombination mechanism in the grown structures, intensity and temperature dependent PL measurements are performed. PL spectra of several QWs are shown in Fig.4a. The origin of PL emission is explained by the theoretically calculated values obtained by solving the Schrodinger equation using finite difference approach within an envelope-function formalism framework[24]. It is observed that for the same power density of excitation the relative recombination efficiency of AlGaAs/GaAs QW sample is high compared to other QW materials.

a) 10K PL spectra of QWs, Temperature dependent PL spectra of QWs b) AlGaAs/GaAs c) InP/InAsP, Power and temperature dependent energy band gap variation for AlGaAs/GaAs &InP/InAsP QWs.

The recombination efficiency variations among the QW materials may be due the combined effect of several factors including effective mass, wave function overlap, influence of atomic irregularity, effect of barrier layer, different magnitude of photon absorptions etc. It is also observed that almost all the QW samples show monotonic decrease in transition energy with rise in temperature. However at low temperatures, these samples show different temperature dependent behavior (Fig 4b & c). This behavior is different for different level of illumination power (Fig 4d). The samples with ternary materials in QWs mostly show anomalous S (red-blue-red) shaped energy versus temperature behavior, while samples with binary material in the QW region does not show such behavior. The S-shaped temperature dependence of ground state transition energy is explained by considering thecarrier localization in the band-tail states at low temperatures where the ground state energy follows the temperature dependence of the localization states. One therefore sees a normal temperature induced bandgap shrinkage (red shift) up to the critical temperature which is mainly governed by the localization energy. At a critical temperature, excitons having sufficient thermal energy enabling their delocalization which leads to the dominance of band to band exciton feature. Because of this, the energy of ground state feature increases (blue shift) until the localized exciton feature starts to weaken upto critical temperature (Fig4d).Above this temperature, excitons are in thermal equilibrium and once again one sees the usual temperature induced bandgap shrinkage (red shift) up to room temperature. At reasonably high excitation intensities, a large numbers of electron-hole pairs are generated which lead to the saturation of band tail states that reduces the effect of carrier localization because of the limited density of localized states. Under such condition, though the band to band recombination dominates over the localized states recombination but they influence the luminescence spectra significantly. In such a situation conventional models of Varshni, Viña and Passler break down where Dixit et al.[12] have proposed a phenomenological model for evaluating the material parameters and localization energy of carriers trapped in the band-tail states as given below,

where E Peakis the PL peak energy at a given PL temperature, n and n are the associated weighting 1 2 factors which tells if either the transitions are band-toband dominated or the localized exciton dominated at energy E QW(0) and E (0) respectively [12]. Here, F(T) is g t the temperature dependent term which takes into account the electron-phonon interaction and thermal expansion of the lattice parameters. If the fraction (p) is defined in terms of weighting factors as p= n /(n +n ) 2 1 2 then equation 1 can be rewritten as,

The material parameters (α, β) and localization energy (ΔE = E QW(0) - E ) for all the QW samples showing such g t behavior can be obtained. Here, α tells the disorders or entropy of the system while β is related to Debye or phonon temperature. Thus for ternary QWs system using above process one can estimate the trap energy values and materials parameters which can also give information of disorders in the systems. It is further explained how the magnitude of charge carrier localization energy influences the critical temperature of S-shaped temperature dependent energy variation. It is also proposed a methodology to extract the value of carrier localization energy directly from temperature dependent SPV and PC [12,14-15].This information can play a pivotal role in defining the operating temperature range of absorption based quantum structures devices, where such S-shaped behaviour of excitonic transition energy is observed.

C. Magneto-PL analysis: In order to investigate the microscopic properties of exciton, such as effective mass, binding energy, Bohr radius and ultralow disorders for semiconductor quantum structures, contactless Magneto-PL measurement are performed. In the Faraday geometry, the excitonic properties are probed in the plane of the sample. It is observed that the spectrum of all the samples show blue shift with applied magnetic field due to the predominant contributions of diamagnetic and Landau shifts (Fig. 5 a & b). The magnetic field dependent diamagnetic blue shift of PL spectra is proportional to the square of B (B <B ) [16]

However, at relatively higher magnetic field, excitons perform cyclotronic motion, having very small radius (at B = 8 T radius r ≈ 90.7 Å), being centered on their center of mass motion. This magnetic field driven confined motion (in x-y plane) of charge carrier is responsible for the formation of discrete Landau levels. In this region (B ≥ Bc), blue shift in energy levels become proportional to the applied field B [16].

Here, R &r are the dimensionality factor & Bohr radius D B respectively, &α, β are the proportionality constants. μ* is the reduced effective mass of exciton and can be expressed as, 1/μ* = 1/m * +1/m *, with m * &m * are the effective mass of electron &hole, respectively

a) Magneto-PL spectra of GaAs/AlGaAsQW b) Blue shift of PL peak energy as a function of B.

The magnetic field, above which magnetic energy becomes dominant over Columbic energy, is termed as critical magnetic field (B = μ*2e3/16π2ε 2ε 2ħ 3), and is c 0 r theoretically estimated as 4.9 T for GaAs. Here, ε = r 13.18 is the dielectric constant of GaAs, ε is the 0 permittivity of free space and ħ is the (Planck's constant)/2π. Thereafter, excitonic model is used to estimate the reduced effective mass of excitons for all QWs using the Magneto-PL spectra. Significant increase of effective mass is observed for the confined exciton in narrow QWs. The foremost reason behind such an observation is due to the induced nonparabolicity in bands, which varies with the thickness of the QWs [16]. In order to obtain such results on the laser diode structure, we have performed similar experiment and observed interesting results which are discussed below.

a) Magneto-PL spectra of GaAs/InGaAs Laser diode structure b) Blue shift of PL peak energy as a function of B.

It is observed that the luminescence spectrum of LD structure at low temperature show strong influence of carrier localization (data not shown here). Fig 6 a & b show the magneto-PL spectrum of laser diode structure. The transition energy of localized peak (e -hh b) and 1 1 band to band (e -hh ) peak blue shift with applied 1 1 magnetic field. The shift for localized peak (e -hh b) is 1 1 significantly higher (10 meV) then band to band (e -hh ) 1 1 peak (3 meV). This observation confirms that charge carrier gets redistributed among the localized states and give rise to higher energy shift with field. Similarly, under the extreme perturbation, like high electric field or current carriers gets redistributed among these states and therefore may affect the operating wavelength of the devices and their significance is discussed in the next section.

D. Emission spectra, L-I characteristics & output parameters of laser diode arrays:
Initially after processing, all the laser diode structures are tested under pulsed operation at room temperature. Lasing action is observed with the range of threshold current density varied from 100-300A/cm2. The emission spectrum of laser diode is varied by changing the material of the active region i.e. the QWs.

a) Emission spectrum of laser diodes with different active regions developed at RRCAT b).The CW L–I characteristics of facet coated single element laser diode, inset shows the L–I characteristics with and without facet coating & corresponding emission spectrum of laser diodes.

Figure 7 a shows the longitudinal spectrum of five laser diodes operating at different wavelengths. Figure 7b shows the optical output power vs. current (L-I) characteristics of lasers diodes with and without AR/HR coatings (inset of the figure). The output power of laser diodes after facet coatings is also measured from the front (AR side) and rear facets (HR side) which have the reflectivity values of 2% and 90% respectively. The increase in slope efficiency of about 1.82 times confirms the suitability of ZrO2 facet coating because it is closely matching with the reported results of the best facet coated laser diodes with other materials. Inset of Fig. 7b also shows the emission spectra of laser diode without and with facets coating. It is noted that the position of

a) L-I characteristics of laser diode arrays &its emission is shown in the inset photograph b)Emission spectrum of laser diode arrays at different power level, c) shows the actual photograph of a 980 nm laser diode array under operation.

emission spectrum of the facet coated surfaces are nearly similar to that of uncoated surface. The small blue shift of ~ 1 nm may be due to the variation of the operating temperature of the Peltier cooler assembly. Typically the rate of change of wavelength with temperature in the semiconductor laser diode is 0.3 nm/K. Subsequently, these lasers, die bonded p-side down with indium preform on a gold plated copper package, were also operated under continuous wave (CW) mode of operation. The total output power from the facet coated laser diode was ~ 0.75 W at ~3.3A injection current with the differential quantum efficiency 65% and the maximum wall plug efficiency ~ 52%.

Finally, the laser diode arrays emitting at ~ 980 nm consists of more than 10 elements are bonded and packaged on water cooled micro-channel laser mounting assembly. These laser diode arrays were successfully operated at 23.5 W peak power with 5.65 ms pulse width at 1 Hz rep rate (Fig. 8 a, b & c). These laser diode arrays also delivered 3 W output power under CW mode.

E. Applications of indigenously developed Laser & Diode Arrays:
One of such bonded and packaged laser diode is also used as a pump source for exciting PL of InAsP/InP QW structure to study the electronic transitions. Figure 9 shows the electronic transitions of InAsP/InP QWs having two different thickness. Similarly, emission light of the laser diode arrays is coupled to 18 meter long, 400 m diameter, fiber for demonstrating the pumping in a fiber media (Fig 9b). Fluorescence at around 1080 nm was clearly recorded at the output of the core region of 18m long fiber (Fig 9c). However, due to absorption band mismatch between the fiber and emission spectrum substantial power is leaked out. In order to reduce this mismatch, we have cooled these laser diode devices by the circulation of chilled water at ~ 5 C in the microchannel assembly. A shift of the lasing wavelength (980 nm) due to junction heating is brought down to < 4 nm while operating them up to 20W peak power under quasi CW operation.

a) PL of InAsP/InP QW structure obtained by using indigenously developed RRCAT laser b) Schematic of experimental setup for pumping optical fiber c) CW emission spectrum and photograph at the output of 18m long fiber

However the emission wavelength is still higher than the desired one in order to match with the absorption band of optical fiber. Thus it is concluded that device heating alone is not the actual driving force for the wavelength shifting. Carrier recombination from the localized states may be responsible for such large shift and this is also evident from the results of magneto- PL experiments.

4. Conclusion:
In summary, we have developed high power QW laser diodes in the wavelength range of 670 to 1000 nm. Several issues related to epitaxial growth, device processing, facet coating, bonding & packaging and testing have been addressed. Significance of the charge carrier localization on the microscopic parameters are explained from the temperature, power and magnetic field dependent photoluminescence. Properties of new sets of multilayer materials ZrO2 and ZrO2/SiO2 are investigated for facet coating which assured to have increased the damage threshold of laser diode arrays. The processed laser diode emitting at ~ 980nm, is operated under CW mode with total output power of 0.75 W from the facet coated laser diode with ~ 52% wall plug efficiency. Subsequently, laser diode arrays are also developed which delivered 23.5 W peak power under pulsed operation and 3 W CW power. These bonded and packaged laser diode can be used for investigating the material parameters and the same is proven by using it asa pump source in the photoluminescence experiments. Further, for high power application, such as using it as pump source for fiber laser, significant shift in the emission wavelength is observed and it is explained due to carrier localization and heating of the device. The heating of the device is substantially controlled by the efficient cooling. The charge carrier localization in the disorder states will be removed by more controlled growth of the structures. Successful implementation of these steps will ensure an efficient coupling of the emission light with fiber for making fiber lasers.

Acknowledgments:
Authors acknowledge Shri U. K. Ghosh, Shri A. Jaiswal and Shri G. Jayaprakash for the technical support. Authors also acknowledge Dr. Aparna Chakrabarti, Dr. Tapas Ganguli, Dr. B. N. Upadhaya, Dr. Suparna Pal, Dr. S. D Singh, Dr. R Jangir and Dr. S. M. Oak for their useful contributions in this work. Authors also acknowledge Shri P. K. Kush for providing adequate liquid helium during Magneto-PL experiment. We also acknowledge colleagues from RRCAT & TIFR for many useful discussions. Authors also acknowledge Dr. P. A. Naik, Director RRCAT, & Dr. P. D. Gupta, VC of HBNI for their constant support during the course of this work.