IMPACT-2013

Multiple Quantum Well Based Lasing Nanostructures: A Review

1Syed Gulraze Anjum, 2Mohammad Jawaid Siddiqui

Deptt. of Electronics Engg., Z.H.C.E.T, A.M.U, Aligarh, U.P, India 1sganjum@gmail.com, 2mjs_siddiqui@rediffmail.com

Abstract— In this paper we have reviewed recent experimental and Simulated results on multiple quantum well based lasing nanostructures. GaN/AlGaN and indium free AlGaN multiple quantum well based laser diodes are demonstrated. It is found that by controlling the well widths of GaN/AlGaN MQW laser diode, there is a limit to shift the lasing wavelength toward deeper ultraviolet region while indium free AlGaN MQW laser diode can have lasing emission at even shorter ultraviolet wavelength. Optimization of active layer structures for GaN/AlGaN MQW laser diodes are also discussed. Keywords: Multiple quantum well, Laser, GaN/AlGaN MQW laser.

I. INTRODUCTION The semiconductor laser development has been a popular topic for longer than half a century [10]. With the advent of commercial optical fiber, Laser Diode radiation properties such as brightness, directivity, narrow spectral width, and coherence made them the best light sources for long-distance fiber optic communication links [6]. Over the years, the requirements for long-haul system capacity have continued growth, as has the need to improve laser-diode quality. In response, quantum-well based laser diodes with extremely narrow spectral width (in the order of tenths of a nanometer) have been developed [6]. Numerous designs have been researched, most of them were just dying out, and only a few remain as successful products. Within the last couple of decades there has been increasing interest in materials with nanometer-scale dimensions. Dramatic progress in the development of nanoscale crystal growth and fabrication technologies has driven the miniaturization of semiconductor laser diodes, a trend also motivated by the desire to achieve greater colour range, higher optical gain, and lower lasing threshold. Advances in material growth technologies, particularly molecular beam epitaxy, metal-organic chemical vapour deposition, and a suite of chemical synthesis techniques, make the fabrication of high quality nanometre scale semiconductor structures possible [12]. Thanks to the quantum size effects that drastically change the energy spectra of confined electrons in reduced dimensions, the population inversion necessary for lasing action occurs more

efficiently as the active semiconductor gain medium is scaled down from the bulk to the nanometre scale. Consequently, semiconductor lasers built with nanoscale active media are expected to exhibit extraordinary properties such as great colour range, high optical gain, and low lasing threshold [12]. Using heterojunctions in lasers was first proposed in 1963 when Herbert Kroemer, a prominent scientist in this field, suggested the idea that population inversion could be greatly increased by incorporating heterostructures [4]. By incorporating a smaller direct band gap material like GaAs between two larger band gap layers like AlAs, carriers can be confined so that lasing can occur at room temperature with low threshold currents. It took many years for the material science of heterostructure fabrication to catch up with Kroemer's ideas but now it is up to the industry standard. It was later discovered that the band gap could be controlled by using advantage of the quantum size effects in quantum well heterostructures. Semiconductor diode lasers used in CD, DVD players, laser printer and fiber optic transceivers are manufactured using alternating layers of various III-V and II-VI compound semiconductors to form lasing heterostructures [13]. The semiconductor laser diode has a number of unique advantages when compared against other coherent light sources, such as high efficiency, robustness, high speed, higher reliability and potentially lower cost [11].

II. WORKING OF A LASER

The laser is a device that amplifies light by means of the stimulated emission of radiation [5]. As we know that electrons and holes are injected by forward current into an active area, where they recombine, and that each recombination results in the radiation of a photon. Its active area is much smaller in size (thickness) results in a much higher current density and, thus, a much more intensive recombination process occurs [6]. A large number of electrons injected into a small area leads to population inversion, to stimulated emission and, when gain exceeds loss, to laser action—the generation of monochromatic, coherent, powerful light. When the current density becomes sufficient enough to create population inversion and the threshold condition is

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reached (where gain equals loss), the diode acts like a laser. For lasing action, we need to have more electrons at the higher-energy conduction band than at the lower-energy valence band. This situation is called population inversion as we know that, normally, the valence band is much more heavily populated than the conduction band. To achieve this population inversion, high-density forward current is passed through the small active area. Population inversion is a necessary condition to get a lasing effect in the laser diode because the greater the number of excited electrons, the greater the number of stimulated photons that can be radiated. In other words, the emission intensity will be higher as well. Finally, the number of excited electrons determines the gain of a semiconductor diode [6]. On the other hand, a laser diode also introduces some loss. There are two main loss mechanisms. First, many photons are absorbed within the semiconductor material before they can escape to create radiation. Secondly, mirrors at the both ends, do not reflect back 100% of the incident photons. At the starting of the lasing process, the number of photons continues to grow at a nuclear-explosion rate; but, as the process continues, the more photons that are stimulated, the greater the number lost [6]. Fortunately, loss is a constant for a given diode, but gain can be changed. Increasing gain is done by increasing the forward current. Eventually gain becomes equal to loss, a situation called the threshold condition. The corresponding forward current is called the threshold current as shown in Fig. 1. At this point, a semiconductor diode starts to act like a laser. As we continue to increase the forward current the number of emitted stimulated photons continues to increase, which means the intensity of the output light also continues to increase. When the current density becomes sufficient enough to create population inversion and the threshold condition is reached (where gain equals loss), the diode starts to act like a laser. Taking all the above considerations into account, we conclude that a semiconductor diode works like a laser if the following conditions are met: Population inversion, Stimulated emission, Positive feedback [6].

Fig. 1. Input – Output characteristic [6].

III. QUANTUM WELL LASER DIODE

For making lasing action more efficient, a special fabrication technique is utilized to form an especially thin active region, one on the order of 4nm to 20nm of thickness [6]. Such devices are called quantum-well (QW) laser diodes. The quantum-well technique changes the density of energy levels available for electrons and holes. The result is a much larger optical gain. The main advantages of a quantum-well based laser diode are more efficient current-to-light conversion efficiency, better confinement of the output beam, and the potential to radiate a variety of wavelengths [6]. From a practical point of view, these advantages of the quantum-well structure dramatically reduce the threshold current and increase the chance of changing radiating wavelengths by varying the thickness of an active layer. Fig. 2 presents the concept of quantum-well structures. Quantum-well based laser diodes are available with the following structures: single quantum well (SQW), multiple quantum well (MQW), and graded-index separate-confinement heterostructure (GRINSCH) [6]. A MQW based laser provides powerful radiation (up to 100mW). Latest advances include strained quantum-well active media. By introducing a controlled strain of an active layer, a designer may control the quantum-well width and the potential barrier height. This results in the possibility of enhancing the properties of a semiconductor laser diode: controlling its wavelength, reducing its threshold current, and increasing laser efficiency [6].

Fig. 2. Schematics of structure and energy band diagrams of quantum well laser diodes: (a) A SQW laser diode (b) A MQW laser diode (c) A graded index separate confinement heterostructure [6].

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IV. GaN/AlGaN MQW LASER DIODE

AlGaN based laser diodes for the short wavelength ultraviolet region has already reported [1]. Group III nitride materials are one of the most promising candidates for producing such devices. Ultraviolet laser diodes and LEDs with GaN, AlGaN or AlGaInN active layer can provide light emission at a wavelength shorter than 365nm corresponding to the GaN band gap of 3.4eV. To shift the lasing wavelength in shorter wavelength region, laser diode must have high AlN mole fractions in AlGaN layers in the layer structure. At high AlN mole fraction, AlGaN layers growth on the lattice mismatched substrate (SiC, GaN) results in dislocations and because of tensile strain there is crack formation in the epitaxial layers. To meet the criteria of laser operation, high crystalline quality (low dislocation density) AlGaN materials with high AlN mole fractions are needed. In the paper [1], authors have presented the room temperature operation of GaN/AlGaN and indium free AlGaN multiple quantum well (MQW) laser diode under pulsed current mode. Low dislocation density AlGaN films with AlN mole fractions of 20%-30% on sapphire substrates are successfully grown using hetero facet controlled epitaxial lateral overgrowth method. On the low dislocation density Al0.2Ga0.8N and Al03Ga0.7N films, GaN/AlGaN and indium free AlGaN multiple quantum well (MQW) laser have been fabricated respectively. GaN/AlGaN MQW laser diode lased in the range 359.6nm and 354.4nm. Threshold current density of 8kAcm-2, output power as high as 80mW and differential external quantum efficiency of 17.4% are achieved. AlGaN based laser diodes grown on high quality AlGaN films presented in paper [1] are useful for future development of laser diodes emitting much shorter wavelengths. In the paper [1], authors fabricated GaN/AlGaN MQW laser diodes, the active region consists of GaN quantum wells and AlGaN barrier layers are grown on Al0.2Ga0.8N films. The layer structure of Al0.2Ga0.8N film was made up of an n-Al0.2Ga0.8N contacting layer, n-Al0.2Ga0.8N cladding layer, Al0.1Ga0.9N guiding layer, GaN/AlGaN MQWs, an Al0.1Ga0.9N guiding layer, a p-Al0.2Ga0.8N cladding layer and a p-GaN contacting layer. A p-AlGaN electron blocking layer with higher Al content than that of the cladding layer was included into the waveguide on the MQWs. After the 5µm laser strip formation and exposing the n-Al0.2Ga0.8N contacting layer by the conventional dry etching technique, a

Ti/Al contacting pad was deposited on the exposed n-Al0.2Ga0.8N contacting layer. Ni/Au contacting pad was deposited on the p-GaN contacting layer. Facets of the laser cavities were formed by dry etching process. Ultimately successful fabrication of GaN/AlGaN MQW laser diodes on high quality Al0.2Ga0.8N film without any crack has carried out. The optical characteristics of GaN/AlGaN MQW laser diodes are also investigated in paper [1]. Room temperature emission spectra of the GaN/AlGaN MQW laser diode below and above the threshold current is investigated. It has been found that a very weak spontaneous emission below the threshold current and sharp lasing emissions at a wavelength of 359.6nm is obtained above the threshold current of 247mA [1]. These emissions were measured with pulse duration of 100ns and a repetition frequency of 5kHz under pulse current mode of operation. Light output-current (L-I) characteristic was also measured at room temperature under same pulse current mode of operation [1]. This compound semiconductor laser diode exhibits a nonlinear behaviour in the L-I characteristic having pulse output powers of more than 80mW from one side of the cavity facets [1]. For different GaN well widths, the lasing spectra of GaN/AlGaN MQW laser diodes are found to be 359.6nm and 354.4nm emission wavelengths [1].

V. INDIUM FREE AlGaN MQW LASER

Limited wavelength region of light emission of GaN/AlGaN MQW laser diode due to intrinsic band gap of GaN wells even with indium free active layer. In contrast AlGaN MQW laser diode provides the ability to access shorter wavelengths by adding AlN to the active layers with indium free design. To open the way for semiconductor laser diodes lasing at deep ultraviolet wavelengths [1], authors have fabricated indium free AlGaN laser diodes. On Al0.3Ga0.7N film AlGaN MQW laser diodes are fabricated. On Al0.3Ga0.7N film, the layer structure consists of a 2.8 µm thick n-Al0.3Ga0.7N contacting layer, a 600nm thick n-Al0.3Ga0.7N cladding layer, a 90nm AlGaN guiding layer, AlGaN MQWs, a 120nm thick AlGaN guiding layer, a 20nm thick p-AlGaN electron blocking layer, a 500nm thick p-Al0.3Ga0.7N cladding layer and a 25nm thick p-GaN contacting layer[1]. Lasing spectra for the AlGaN MQW laser diodes with different AlN mole fractions in the AlGaN wells being 4%, 5%, 6% are found to be 342.3nm, 339.5nm, 336nm emission wavelengths under pulsed current mode of device operation with a pulse duration of 10ns, 5kHz frequency. Authors of paper [1] have achieved 336nm wavelength of lasing emission, which is the shortest wavelength ever for a

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semiconductor laser diode. By controlling the well widths of GaN/AlGaN MQW laser diode, there is a limit to shift the lasing wavelength toward deeper ultraviolet region. While AlGaN MQW laser diode can have lasing emission at even shorter ultraviolet wavelength. The L-I characteristics of AlGaN MQW laser diodes exhibit different nonlinear behaviour at emission wavelengths of 342nm and 336nm under the same mode of operating condition [1].The AlGaN MQW and GaN/AlGaN compound semiconductor laser diodes are useful optical sources of ultraviolet coherent light which can be used to access any ultraviolet wavelengths by varying the MQW configuration, different from conventional bulky ultraviolet lasers that cover only a discrete range of wavelengths [1].

VI. OPTIMIZATION OF ACTIVE LAYER STRUCTURES FOR GaN/AlGaN MQW LASER DIODES

Quantum-well number and quantum-barrier aluminium composition effect on threshold current of ultraviolet GaN/AlGaN MQW laser diodes are theoretically studied in detail [2]. To achieve high performance ultraviolet laser diodes, systematic and compact theoretical modeling is a necessary approach to enhance existing laser structures and understand internal physical processes, which gives timely and efficient guidance to provide the optimal structure design and device parameters. They focus their study on GaN/AlGaN based system and systematically study the effects of quantum well number, quantum barrier aluminium composition, and quantum well thickness on ultraviolet GaN/AlGaN multiple quantum well based laser performance [2]. Furthermore, how the different physical mechanisms effect the threshold properties is shown in their study as well [2]. The laser output power of the GaN/AlGaN based laser diode structure as a function of input current when the number of quantum wells changes from one to five [2]. The simulation results indicate that the best laser performance is observed when the number of quantum wells is three and the worst laser performance is obtained when the number of quantum well is one [2]. In order to further study the effects of quantum-barrier aluminium composition on threshold current of the GaN/AlGaN based laser diodes. The threshold current values of the GaN/AlGaN laser diodes with different barrier aluminum compositions are plotted when the number of quantum well changes from one to five is shown in Fig. 3. According to the simulation results, optimal barrier aluminum composition is found to be about

10%–12% for the GaN/AlGaN quantum-well based lasers [2]. Lower and higher aluminium compositions in AlGaN barrier/confining layer result in larger threshold current values. Furthermore, the optimized number of quantum wells for GaN/AlGaN laser diodes is found to be three [2]. It has already been presented the theoretical simulation to investigate the effects of quantum-well number, quantum-barrier alumiinum composition, and quantum-well thickness on the GaN/AlGaN multiple-quantum-well based laser performance. The relations between the electron leakage currents and the active region structures, for different numbers of quantum well and aluminium compositions in the barrier/confining layers are studied and analyzed [2]. The simulation results show that, among the active layer structures under study, lower threshold current can be obtained when the number of quantum wells is two or three and the aluminium composition in barrier/confining layer is near about 10%–12%. There are five different effects which cause this result [2]. First, the severe electron leakage current is found because of the lower barrier aluminium composition and fewer number of quantum wells. Second, the obvious non-uniform distribution of electron carriers is found because of the higher barrier aluminium composition and the more number of quantum wells. Third, the higher density of positive polarization charges at the interface between the AlxGa1-xN barrier layer and the Al0.25Ga0.75N electronic blocking layer with decreasing barrier aluminium composition is also another main factor which enhances the electron leakage current. Fourth, the charge density at the interface between AlxGa1-xN barrier layer and GaN quantum well layer enhances with the barrier aluminium composition, which decreases the photon emission rate. Fifth, the optical confinement factor decreases with the increasing aluminium composition in AlGaN barrier layer, which leads to the larger threshold current. Therefore, the GaN/AlGaN MQW based laser diode with an active layer of two or three quantum wells and x = 0.10–0.12 in the AlxGa1-xN barrier/confining layer has found to be the optimized active layer structure due to the competition of these five internal physical mechanisms [2].

Fig. 3. Threshold current vs Number of quantum wells for different Al composition [2].

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Fig. 4. Series of lasing spectra of the device operating beyond threshold [7]. By increasing the AlN mole fraction of each layer of the laser diodes, it would be a chance to shift the lasing wavelength into shorter ultraviolet wavelength region [7, 9]. It has been already reported that the laser diode composed of GaN well layers operated in 340 nm wavelength, the shortest wavelength ever reported for a semiconductor laser diode with any binary compound well layer [7]. The laser structure composed of a 2.8μm thick n-Al0.2Ga0.8N n-contacting layer, a 0.6μm thick n-Al0.2Ga0.8N cladding layer, a 120nm thick Al0.1Ga0.9N guiding layer, GaN/Al0.1Ga0.9N MQWs, a 120-nm-thick Al0.1Ga0.9N guiding layer, an AlGaN electron blocking layer, a 0.5μm thick p-Al0.2Ga0.8N cladding layer and a 25nm thick p-GaN contacting layer [7]. A series of lasing spectra of a laser diode composed of GaN well layers is shown in the Fig. 4 under the pulse current mode with pulse duration of 10ns and a frequency of 5kHz at room temperature [7]. The device starts lasing at a threshold current of nearly 700mA and exhibits sharp lasing emissions around a wavelength of 345nm above the threshold [7]. At a current of 914mA, the laser exhibits the sharp spectrum. They also reported the GaN/AlGaN MQW based laser diodes having similar layer structure but with wider GaN well width of 3nm, operating in 355.4nm to 361.6nm wavelength range under 100ns pulse condition at room temperature [8]. An ultraviolet 336nm AlGaN MQW based laser diode is already demonstrated [9]. In addition, the behavior of modal gain with current density for SQW and MQWs based GRIN InGaAlAs/InP lasing nano- heterostructures have been studied [14].

VII. CONCLUSION

The GaN/AlGaN and AlGaN multiple quantum well lasers are useful optical sources in ultraviolet range which can be used to access any ultraviolet wavelengths by designing multiple quantum well configuration, unlike the latest used bulky ultraviolet lasers that operate in only a discrete range of

wavelength. To open the way for semiconductor laser diodes lasing at deep ultraviolet wavelengths, Indium free AlGaN laser diodes have been reported. The study of ultraviolet GaN/AlGaN multiple quantum well laser diodes show that the lower threshold current can be achieved when the number of quantum wells is two or three and the aluminium composition in the barrier layer is found to be 10%-12%.

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